Physiol. Rev. 83: 1359-1400, 2003;
doi:10.1152/physrev.00007.2003
0031-9333/03 $15.00
Plasma Membrane Channels Formed by Connexins: Their Regulation and Functions
JUAN C. SÁEZ,
VIVIANA M. BERTHOUD,
MARÍA C. BRAÑES,
AGUSTÍN D. MARTÍNEZ and
ERIC C. BEYER
Departamento de Ciencias Fisiológicas, Pontificia Universidad Católica de Chile, Santiago, Chile; and Department of Pediatrics, University of Chicago, Chicago, Illinois
 |
ABSTRACT
|
|---|
Sáez, Juan C., Viviana M. Berthoud, María C. Brañes, Agustín D. Martínez, and Eric C. Beyer. Plasma Membrane Channels Formed by Connexins: Their Regulation and Functions. Physiol Rev 83: 1359-1400, 2003; 10.1152/physrev.00007.2003.—Members of the connexin gene family are integral membrane proteins that form hexamers called connexons. Most cells express two or more connexins. Open connexons found at the nonjunctional plasma membrane connect the cell interior with the extracellular milieu. They have been implicated in physiological functions including paracrine intercellular signaling and in induction of cell death under pathological conditions. Gap junction channels are formed by docking of two connexons and are found at cell-cell appositions. Gap junction channels are responsible for direct intercellular transfer of ions and small molecules including propagation of inositol trisphosphate-dependent calcium waves. They are involved in coordinating the electrical and metabolic responses of heterogeneous cells. New approaches have expanded our knowledge of channel structure and connexin biochemistry (e.g., protein trafficking/assembly, phosphorylation, and interactions with other connexins or other proteins). The physiological role of gap junctions in several tissues has been elucidated by the discovery of mutant connexins associated with genetic diseases and by the generation of mice with targeted ablation of specific connexin genes. The observed phenotypes range from specific tissue dysfunction to embryonic lethality.
|
I. GENERAL COMMENTS
|
|---|
In this review, we provide a brief historical background regarding gap junction structure and physiology and then concentrate on reviewing several areas of active research in the field of gap junctions including studies of the molecular composition and biochemical regulation of these channels, recent studies of hemichannels, and investigations of the role of gap junction channels and hemichannels in various normal physiological processes and diseases in vertebrates.
Due to limitations of space, some of the earlier investigations and some specific areas of gap junction biology that have shown significant advance in recent years (e.g., nervous system) have not been considered in detail in the current review. The reader is referred to previous reviews for more detailed information on earlier studies (35, 48, 66, 67, 190, 539, 551). Several reviews have been published recently on gap junction structure (534, 548, 665), trafficking of protein subunits (154, 314, 317, 666), channel gating (36, 59, 108), pharmacological regulation (45, 499, 550), and functional features of the channels (353, 443, 540, 614). Other reviews have covered the roles and regulation of gap junctions in development (338, 352) and in various mature tissues and organs, including vascular tissue (60, 91), urogenital smooth muscle (269), the heart (50) and cardiovascular system (128), pancreas (377), endocrine glands (395), the nervous system (65, 122, 123, 168, 546, 552), and the immune system (11, 502, 503). Other reports describe the regulation and role of gap junctions in tissue injury (120) and the involvement of gap junctional communication in the phenotypes observed in connexin knock-out animals and/or different diseases (134, 540, 647, 662).
|
II. BACKGROUND
|
|---|
A. Intercellular Communication
The first indications that a pathway for direct cell-to-cell communication might exist substantially preceded discovery of the gap junction structure. Weidmann (637), working with strips of myocardium, found that the space constant for the spread of current extends beyond the expected value for a single Purkinje fiber. He suggested that this phenomenon might be explained by the existence of a low-resistance intercellular pathway. Stronger evidence supporting the existence of such a pathway was provided by the discovery of electrical transmission at the giant crayfish motor synapses (170). Later, this type of pathway was demonstrated in other excitable tissues (22, 34, 162) and in nonexcitable cell types (184, 356, 432, 468, 482). Today, it is accepted that cells of most invertebrate and vertebrate tissues can communicate with their neighbors via a low-resistance intercellular pathway. In vertebrates, only a few cell types do not form gap junctions in their fully differentiated state, including red blood cells, spermatozoa, and skeletal muscle. Nevertheless, the progenitors of these cells do express gap junctions (98, 386, 469, 495).
B. Gap Junction Structure
Electron microscopy studies provided evidence for a structure responsible for intercellular electrical transmission. Robertson (491) demonstrated the structural units of the neuronal junction. In studies of excitable tissues, a close apposition between the plasma membranes of adjacent cells was observed when current transfer occurred, but it was absent when electrical transmission was not detectable (22, 440, 482). This intercellular junction has been given several names, including nexus, macula communicans, and gap junction. The designation "gap junction" from the work of Revel and Karnovsky (481) has prevailed despite the contradiction between the function and the morphological characteristics that it denotes.
The structure of gap junctions and their constituent channels have been significantly elucidated. In transmission electron micrographs of ultrathin tissue sections, gap junctions appear as regions where the plasma membranes of adjacent cells closely approach each other, but appear to be separated by a small gap of 2-3 nm (33, 481, 491) (Fig. 1A). Electron micrographs of freeze-fracture replicas of vertebrate junctions show 8.5- to 9.5-nm particles on the P face either free or in plaque-like arrays with complementary pits on the E face (Fig. 1C). In many cases, these particles (termed connexons) are regularly ordered in a hexagonal pattern that has allowed detailed structural studies by other techniques. Studies of gap junctions using atomic force microscopy (AFM) (226, 318) show a dense packing of particles with a center-to-center distance of 9-10 nm and a similar long-range hexagonal packing. The particles contain a porelike structure with a diameter of 2 nm, a depth of 1 nm, and a width of 3.8 nm based on freeze-fracture and AFM analyses (226, 318, 481). X-ray diffraction examination of isolated gap junction membranes (78, 362) and Fourier analysis of low-dose electron micrographs (595) show that each particle or connexon within a gap junction forms a cylinder with an apparent aqueous pore in the center. The wall of the connexon is formed of six protein subunits (362). Other studies suggest that these subunits may move with respect to each other and control channel opening (594) (Fig. 3, inset 1).
View larger version (123K):
[in this window]
[in a new window]
|
FIG. 1. Ultrastructure of gap junction plaques and channels. A: electron micrograph of a thin section of a rat liver showing a long stretch of plasma membrane apposition corresponding to a gap junction between two hepatocytes (arrows). Inset corresponds to a magnified view of a gap junction region showing the heptalaminar structure. B: immunogold labeling of connexin (Cx) 43 in an annular gap junction from a rat Leydig cell. C: freeze-fracture view of a gap junction plaque in rat hepatocytes; the protoplasmic (P) and exoplasmic (E) faces are indicated. D: dimensions of gap junction channels estimated from X-ray diffraction studies. Magnification bar: 600 nm in A, inset; 200 nm in B; 0.1 nm in C. [Modified from Perkins et al. (451).]
|
|
View larger version (53K):
[in this window]
[in a new window]
|
FIG. 3. Diagram illustrating the pathways from transcription to degradation of connexins. Nascent connexin mRNAs leave the nucleus and are translated in the rough endoplasmic reticulum (ER). A newly synthesized connexin (Cx) is folded and then assembled with other connexins into hemichannels. While most Cx26 and only some Cx43 hemichannels are inserted directly into the plasma membrane, most hemichannels made of other connexins, including Cx43 and to a minor extent Cx26, follow the exocytic/secretory pathway [i.e., they go through the Golgi apparatus (GA) and are transported in vesicles to the plasma membrane along microtubules]. Hemichannels can stay free on the cell surface or they can dock with an hemichannel at sites of cell-cell contact to form a gap junction channel. Gap junction (GJ) channels may cluster to form a GJ plaque. 1: Hemichannels are at a steady-state between two conformational states (open and closed). 2: Connexin topology at the plasma membrane. 3: En face view of a growing GJ plaque. Gap junction channels or hemichannels may be ubiquitinated (Ub). Internalization of a gap junctional plaque with formation of an annular ring is depicted on the cell on the left; this ring interacts with actin filaments and moves toward lysosomes and proteasomes where its protein components are degraded. Misfolded connexins or ubiquitinated hemichannels that might be degraded by the proteasome are shown on the top right corner on the cell on the right.
|
|
The structure of the gap junction channel has been refined in recent years (Fig. 1D). Using gap junctions isolated from cells expressing a recombinant truncated gap junction protein, Unger et al. (592) demonstrated the dodecameric structure of the channel at a resolution of 7 Å in the membrane plane and 21 Å in the vertical direction; each hemichannel contains 24 
-helices corresponding to the four transmembrane domains of the six protein subunits (592, 593). These structures were also suggested by circular dichroism (77) and X-ray patterns obtained from oriented gap junctions (577). The outer diameter of the hemichannel is 
70 Å at the intracellular region and 50 Å at the extracellular regions giving the complete channel an hour glass-like appearance. The channel pore narrows from 40 Å diameter at the cytoplasmic side to 15 Å at the extracellular side of the bilayer and then widens to 25 Å in the extracellular region (451, 592, 593). The connexon interfaces within a channel must be tightly bound to form a tight seal. Thus the docking domain provides a high resistance to the leakage of current carrying ions between the channel lumen and the extracellular space. Recently, conformational changes of the cytoplasmic and extracellular surfaces in gap junction plaques containing channels made of native connexin26, a protein subunit, have been imaged using AFM (394). The carboxy terminus reversibly collapses when increased forces are applied to the AFM stylus. The presence of Ca2+ in the buffer solution reduces the diameter of the extracellular channel entrance from 1.5 to 0.6 nm, a conformational change fully reversible and specific among the divalent cations. Calcium induces the formation of microdomains, and the plaque height increases in 0.6 nm (from 17.4 to 18 nm), indicating that Ca2+ induces conformational changes affecting the structure of connexin26 membrane channels (394).
|
III. MOLECULAR COMPONENTS OF GAP JUNCTIONS
|
|---|
A. Gap Junction Proteins
Gap junction-enriched preparations, similar to those utilized for structural studies, were used for biochemical identification of protein components; initially, it was thought that all gap junctions contained the same protein. Evidence for differences in gap junction proteins came from the differing electrophoretic mobilities (21-70 kDa) of bands detected by SDS-PAGE in gap junction-enriched preparations from different tissues (211, 219, 285, 366). In support of this, analysis of nonproteolized heart gap junctions by electron microscopy showed a fuzzy interface that was absent in isolated liver gap junctions or proteolyzed heart gap junctions (365, 535). Comparison of heart and liver gap junctional proteins by two-dimensional peptide mapping of proteolytic fragments followed by high-pressure liquid chromatography revealed differences between these proteins (200). Furthermore, microsequencing of the amino-terminal regions of the hepatic and cardiac proteins revealed both similarities and dissimilarities (414). Subsequently, sequencing of a lens gap junction protein showed that it differed from both the liver and heart proteins (284).
B. The Family of Chordate Gap Junction Protein Subunits
The production of antibodies to the main protein in isolated liver gap junction preparations, and the synthesis of oligonucleotide probes based on the amino-terminal sequence information, allowed the isolation of rat and human cDNA clones encoding a liver gap junction protein of 32 kDa (300, 442). Proof that this protein produces gap junction channels came from its expression in Xenopus oocytes (138, 566, 639). It became evident that there was a family of subunit gap junction proteins when Beyer et al. (52) isolated cDNA clones encoding a related, but clearly different, polypeptide of 43 kDa corresponding to a rat heart gap junction protein, which they named connexin43 (Cx43). While connexins were originally denoted according to the tissue of origin or the apparent size of a polypeptide as determined by SDS-PAGE, it soon became clear that such designations were inappropriate, because many of these proteins are not uniquely expressed in a single tissue (52), and their mobilities on SDS-PAGE may vary with electrophoresis conditions (196). Therefore, to distinguish members of this family, a standard nomenclature was needed. The most widely used system uses the word connexin (often abbreviated Cx) followed by a suffix indicating the predicted molecular mass of the polypeptide connexin (in kilodaltons) to distinguish different protein subunits (52, 53). In some cases, a prefix is added to indicate the species of origin. The finding that two (or more) connexins may have similar molecular mass has led to the use of a decimal point to distinguish them (e.g., mCx30.3 or mCx31.1). Some confusion has also been created when orthologous and functionally homologous connexins have different molecular masses (e.g., rat Cx46 vs. bovine Cx44 vs. chicken Cx56). An alternative (but also problematic and less widely used) nomenclature in which connexins are classified in subclasses according to amino acid sequence homology has also been proposed (423, 486).
Subsequently, a number of cloning strategies including hybridization screening of genomic and cDNA libraries at reduced stringency and use of the polymerase chain reaction with degenerate/consensus primers have been successfully applied to identify additional members of the connexin family (for recent review, see Ref. 653).
C. Connexin Genes
Recently, the screening of the mouse and human genomic databases has revealed 19 and 20 connexin genes in the mouse and human genome, respectively (55, 359, 417, 653). A dendrogram for all identified human connexins is shown in Figure 2A. The genes for various connexins have been localized to human (32, 92, 161, 214, 233, 271, 417, 654) and mouse (9, 93, 207, 215, 233, 521) chromosomes. Connexin genes are found in many different chromosomes, but there is a cluster of connexins in a region of mouse chromosome 4 (214) which is syntenic to a region in human chromosome 1. The genes for many connexins have been cloned, and the gene structure of other connexins has been partially assessed by DNA blotting and by the polymerase chain reaction (for a review, see Ref. 653). The available information indicates that many connexin genes have a similar organization, and virtually all have only single copies in the haploid genome (653). The sequence similarities between connexins and their similar gene structures suggest that they have arisen by gene duplication. In humans, a Cx43 pseudogene has been identified (163).
View larger version (15K):
[in this window]
[in a new window]
|
FIG. 2. Phylogenetic tree and gene structure of connexins. A phylogenetic tree for the connexin family is shown in A. The tree was constructed by aligning the protein sequences using CLUSTAL W and the DRAWGRAM representation (http://workbench.sdsc.edu). Because no sequence corresponding to human Cx33 has been deposited in the databases, and it is not identifiable in available genomic sequence, all sequences used for the aligment are from human origin with the exception of rat Cx33. (The accession numbers for the connexin sequences are as follows: Cx25, AJ414563; Cx26, NM_004004; Cx30, AJ005585; Cx30.2, AC004977; Cx30.3, BC034709; Cx31, NM_024009; Cx31.1, AF052693; Cx31.3, AF503615; Cx31.9, AF514298 and NM_152219; Cx32, NM_000166; rat Cx33, M76534; Cx36, NM_020660; Cx37, AF132674; Cx40, AF151979; Cx40.1, AJ414564; Cx43, AF151980; Cx45, NM_005497; Cx46, AF075290; Cx46.6, NM_020435; Cx50, AF217524; Cx59, AF179597; Cx62, NM_032602). A diagram of the structure of connexin genes is shown in B. B, top: most connexin genes contain two exons and an intron of variable length; the first exon contains only 5'-untranslated region (UTR) and the second exon contains the full coding region and 3'-UTR. B, bottom: in some connexin genes (e.g., Cx34.7, Cx36), the coding region is interrupted by an intron. Untranslated and translated regions are depicted as boxes in dark gray and light gray, respectively. The genomic DNA sequences not present in the mRNA are represented by the black lines. Other exceptions to this gene structure are described in the text.
|
|
Most connexin genes have a first exon containing only 5'-untranslated (UTR) sequences and a large second exon containing the complete coding region as well as all remaining untranslated sequences (Fig. 2B). Exceptions to this gene structure are the Cx32, Cx36, and Cx45 genes. The human (and rodent) Cx32 gene contains alternative first exons (containing only 5'-UTR) whose use is tissue specific (411, 545, 547); some bovine Cx32 transcripts have three exons, two of which contain only 5'-UTR (135). The Cx36 (and the Morone americana Cx34.7) gene contains two exons, both of which contain untranslated and translated sequences. The coding region is interrupted by an intron (32, 97, 422, 423, 544). The Cx45 gene contains three exons; exons 1 and 2 contain only 5'-UTR and exon 3 contains the full coding region and 3'-UTR (19, 242); all Cx45 transcripts examined contain exons 2 and 3, while some also contain exon 1 (242).
D. Transcriptional Regulation and Gene Promoters
In various cell types, different treatments are known to increase the levels of connexin mRNA due to enhanced transcription. For example, cAMP induces elevation of the Cx43 transcription rate in Morris hepatoma cells (380). Moreover, the transcription rates of Cx32 and Cx26, but not Cx43, are upregulated by glucocorticoids in liver-derived cells (479), and the phorbol ester [12-O-tetradecanoylphorbol-13-acetate (TPA)] induces transcriptional up-regulation of Cx26 in human immortalized MCF-10 mammary epithelial cells (340). Also, the large increase of Cx43 in myometrium before parturition is at least in part due to increased transcription (433, 486). Thus knowledge of connexin gene structure and identification of regulatory elements will allow us to understand the transcriptional regulation of connexin expression.
The basal promoter of the rat Cx32 gene was first localized to a region -179 to 0 or -134 to 0, immediately upstream of the first exon (17). The mouse Cx32 gene contains two putative binding sites for the transcription factor HNF-1 and consensus motifs for NF-1 as well as NF
B within the 680 bp upstream of the main transcription start site (215). Cx32 cDNA clones from rat or mouse liver, sciatic nerve, and embryonic stem cells differ (545). They correspond to different splicing variants with a different 5'-UTR or exon 1, each with its own promoter region (545).
The human Cx32 gene can be transcribed from two tissue-specific promoters, P1 and P2, leading to the production of two mRNAs with different 5'-UTRs through the use of alternative exons (411). The P1 promoter is functional in liver and pancreas and is located more than 8 kb upstream of the translation start codon, while the P2 promoter is functional in nerve cells and is located 497 bp upstream from the translation start codon (411). Cx32 expression in vitro is strongly activated by direct binding of SOX10, a common transcription factor for other myelin genes (56). Methylation of the Cx32 (and Cx43) promoter at Msp I/Hpa II restriction sites correlates well with cell type-dependent decreased transcriptional activity (459).
Sequence analysis of mouse Cx37 gene has revealed that the regions flanking exon I contain a consensus "TATA box" 43 bp 5' from the transcription start site preceded by several putative transcription factor binding sites and a 282-bp truncated L1Md transposon (529).
Determination of the structure and sequence of the Cx43 gene and analysis of trans-activation have elucidated some of the hormonal and pharmacological regulation of Cx43 expression. Sullivan et al. (565) and Yu et al. (674) cloned the mouse and rat Cx43 genes, identified the transcriptional start site, and showed that there were a number of putative transcription factor binding sites within the 5'-flanking region including a TATA box, AP-1, AP-2, Sp1, CRE sites, and half-palindromic estrogen response elements. The basal promoter is contained within 110 bp 5' to the start site (88). DNase I footprinting assays have confirmed the use of AP-1 (-44 to -36) and Sp1 (-77 to -69 and -59 to -48) consensus sequences; moreover, mutation of this AP-1 site abolishes phorbol ester-induced transactivation of Cx43 gene transcription in uterine smooth muscle cells (178). Transfection studies using Cx43 constructs linked to luciferase have shown the responsiveness of Cx43 transcription to estrogen, parathyroid hormone, and prostaglandin (94, 674). cAMP, which can also increase Cx43 expression (380), may serve as the mediator of the parathyroid hormone and prostaglandin effects (519) through the Cx43 promoter cAMP response elements (18). The mechanism of the estrogen-induced response is not so clear; Piersanti and Lye (460) have suggested that it occurs indirectly through production of c-fos, which interacts with the AP-1 sites. Electrophoretic mobility shift assays have demonstrated c-jun and Sp1 binding at the AP1 and Sp1 sites, respectively (141). In positions -480 and -464 of the rat gene, there is a thyroid hormone binding sequence that is able to activate the promoter (561).
Rat Cx43 and Cx30 transcription may be transactivated by the Wnt-1 signaling pathway, probably through TCF/LEF binding elements (376, 600). These are HMG box transcription factors that bind to 
-catenin in response to activation of the Wnt-1 signaling pathway, and not through a cAMP-induced pathway (600). Functional analysis demonstrated that mouse Cx43 promoter constructs could be activated in zebrafish embryonic neural crest cells (87), suggesting that the sequence motifs and transcriptional regulation involved in targeting Cx43 expression to neural crest cells are evolutionarily conserved in zebrafish and mouse embryos.
The Cx40 mRNA sequence is identical in all tissues studied (198). Thus transcriptional regulation may account for the differences in levels of Cx40 transcripts detected during development and among different tissues. Transient transfection studies have demonstrated that the basal promoter activity of the Cx40 gene lies within 300 bp upstream of the transcription start site and that a strong negative regulatory element is present in the intron in the region from +100 to +297 bp (530). Moreover, potential transcription binding sites include AP-1, AP-2, Sp1, TRE, p53, and a TATA box located near exon 1 (530). Transcription of Cx40 is transactivated by Tbx5 (T-box transcription factor 5) and the homeodomain transcription factor Nkx2-5 (62). Heterozygous and homozygous Tbx5-deficient mice have similar features to the human Holt-Oram syndrome (62), and it has been suggested that decreased Cx40 transcription may contribute to the cardiac conduction defects observed in this syndrome.
The mouse Cx26 promoter contains at least two transcription start sites and is located in a very GC-rich region without a TATA box, reminiscent of promoters of housekeeping genes (215). Consensus sequences for a metal response element, NFB, and six GC boxes were found within 600 bp upstream of the Cx26 transcription start sites in mouse and human DNA (215, 276). Moreover, the human sequence contains a YY1-like binding site and a consensus mammary gland factor binding site (276). The region -140 to -113 of the Cx26 promoter includes an Sp binding site overlapping with an AP-2 binding site which are responsible for the upregulation of Cx26 in the myometrium during pregnancy and in the mammary gland during lactation (590).
The Cx31 gene promoter region contains five putative binding sites for the GATA transcription factor, a NFB element, a CAAT box, and E-box/E-box related sequences (172). Interestingly, if part of the intron sequence is deleted, the promoter activity is lost (172).
E. Posttranscriptional Regulation of Connexin mRNA Levels
The abundance of mRNA can be affected by conditions that increase or reduce its stability. The 5'-UTR of the Cx43 mRNA contains a strong internal ribosome entry site that confers increased transcription levels (518; for a review, see Ref. 638).
Elevated cAMP concentrations increase the levels of Cx32 mRNA in primary cultures of rat hepatocytes without detectable changes in the transcriptional rate, suggesting an enhanced Cx32 mRNA stability (505). Moreover, during liver regeneration, the reduction in Cx26 and Cx32 mRNA occurs without changes in transcriptional rate, suggesting a posttranscriptional mechanism (293). In this system, the administration of cycloheximide, a protein synthesis inhibitor, completely inhibits the disappearance of Cx26 mRNA, but not of Cx32 mRNA, suggesting the participation of protein factors in destabilization of the Cx26 mRNA (293). The reduction in liver Cx32 steady-state mRNA levels induced by bacterial lipopolysaccharide appears to occur at the posttranscriptional level, since the rate of degradation of this message is higher than the rate of transcription of the gene (185).
F. Connexin Topology and Secondary Structure
The amino acid sequences derived from each cloned cDNA have been used to predict the structure of connexins. Hydropathy plots of several connexins predict the presence of four hydrophobic domains, with a carboxy-terminal hydrophilic tail. In addition, three hydrophilic domains separate the hydrophobic regions (52, 220, 442) (Fig. 3, inset 2). The possible connexin membrane topologies derived from these data were tested by examining the protease sensitivity of isolated liver and heart gap junctions. Antisera raised against synthetic oligopeptides representing various segments of Cx26, Cx32, and Cx43 were used to map the connexin topology. The amino terminus, the carboxy terminus, and a loop in the middle of the protein are all located on the cytoplasmic side of the junctional membrane, and the predicted extracellular connexin domains can be detected on the extracellular surfaces of the junctional membranes (218, 382, 471, 663, 679, 682) (Fig. 3, inset 2). The four transmembrane spanning regions and the extracellular loops contain many identical residues or conservative substitutions among the different connexins. In contrast, the cytoplasmic domains are unique to each connexin; the cytoplasmic loop and the carboxy terminus vary extensively in length and amino acid composition.
Sequence analysis and expression of wild-type or mutant connexins are approaches that are actively being used to determine residues within these sequences that may be responsible for various channel properties. The transmembrane regions must be responsible for spanning the plasma membrane and forming the aqueous pore. Three transmembrane regions M1, M2, and M4 contain mainly hydrophobic amino acid residues, while M3 also contains a number of charged (positive and negative) amino acid residues. Therefore, M3 has been modeled as an amphipathic -helix, and it has been suggested that its hydrophilic face might line the pore of the gap junction channel (35). Structural studies (593) indicate that regions of two -helices line the inner wall of the channel. Scanning cysteine mutagenesis of Cx46 hemichannels in which amino acid residues from M1 and M3 were replaced by cysteines indicated that both transmembrane regions contribute to the lining of the pore (681). Scanning cysteine mutagenesis of Cx32 channels suggests that M2 also contributes to the lining of the pore (543).
The extracellular regions must be responsible for the "docking" of connexons which occurs by collision of two connexons, each provided by each of two adjacent cells. This process apparently occurs through noncovalent forces (180). White and co-workers (643, 648) demonstrated the importance of the second extracellular loop (E2) in determining which connexins can interact to form heterotypic channels. Synthetic peptides homologous to the extracellular loop 2 of Cx40 or Cx43 selectively block the function of the corresponding connexin (304). Werner et al. (640) showed that site-directed mutagenesis of amino acid residues within the extracellular loops that differ among connexins alter but do not prevent gap junctional communication. Biochemical studies have shown that cysteines in the extracellular loops participate in forming intramolecular but not intermolecular disulfide bonds (252, 475). Mutation of any of these cysteines to serines prevents gap junction channel formation (109, 110), emphasizing the importance of the secondary and tertiary structures imposed on the connexins by the disulfide bonds. Alteration of the position of the cysteines in Cx32 can seriously impair the ability to form functional channels (165); these data have been used to model this region as adopting a -sheet conformation. The cysteine residues involved in formation of the intramolecular disulfide bonds in Cx43 have been studied using site-directed mutagenesis (580); however, these differ from those proposed for Cx32 (165). In expression systems, different connexins form channels with differing properties including unitary conductance, selectivity/permeability, and gating/regulation (23, 73, 142, 144, 162, 292, 367, 389, 391, 617-620). Some of the unique properties must be conferred by the unique portions of the connexin sequences (many of which are located within or near the cytoplasm). The main differences in the primary sequence between two closely related connexins, Cx26 and Cx30, are located in the cytoplasmic loop and carboxy-terminal domains. Exchange of one or both of these domains has demonstrated that the cytoplasmic domains interfere directly or indirectly with the permeability, conductance, and voltage gating of the channels in HeLa cells transfected with Cx26/30 chimeras (367).
|
IV. BIOCHEMISTRY
|
|---|
A. Phosphorylation of Connexins
All studied connexins with the exception of Cx26 (508, 585) are phosphoproteins (for previous reviews, see Refs. 100, 322, 506). Connexin phosphorylation has been implicated in the regulation of intercellular communication through a number of mechanisms, including connexin biosynthesis, trafficking, assembly, membrane insertion, channel gating, internalization, and degradation (16, 42, 44, 146, 217, 229, 320, 330, 390, 401, 426, 470, 567, 586, 670, 669). It is likely that some connexin phosphorylation occurs before insertion into the plasma membrane, but some must occur at the plasma membrane to affect gating. Nevertheless, the specific phosphorylation sites and functional role for most of them have not been elucidated.
The most extensively studied connexin in terms of modification by protein kinases and phosphatases is Cx43. This protein contains several consensus sites for phosphorylation in the carboxy terminus, several of which have been identified as target sites for specific protein kinases. Fusion proteins containing the cytoplasmic tail of Cx43 or synthetic peptides corresponding to the deduced sequences in this region are phosphorylated by protein kinase C (PKC), mitogen-activated protein kinase (MAPK), and cdc2 kinase (323, 507, 575, 634). The identified phosphorylation sites are Ser-368 and Ser-372 phosphorylated by PKC (507) and Ser-255, Ser-279, and Ser-282 phosphorylated by MAPK (633, 634). Although phosphorylated Cx43 can be found at the plasma membrane (400), phosphorylation of Cx43 could also happen in intracellular compartments (103, 315).
Epidermal growth factor (EGF)-, vascular endothelial growth factor (VEGF)-, fibroblast growth factor 2 (FGF-2)-, and platelet-derived growth factor (PDGF)-induced Cx43 phosphorylation can be mediated by intracellular kinase pathways that activate MAPK also termed ERK1/2 (132, 229, 230, 263, 564, 664) (Fig. 4). In some systems, this effect appears to occur in a PKC-independent manner (449), but in other systems it can be prevented by PKC inhibition, but not by inhibition of the ERK1/2 pathway (133). In T51B cells, this effect requires the activation of both PKC and ERK1/2 (229). MAPK activation may also mediate the induction of Cx43 phosphorylation by lysophosphatidic acid (LPA) (221). The observations are consistent with the variations in crosstalk between different protein kinase-dependent pathways in different cell types (e.g., differences in the relative activity of each pathway and different subcellular compartmentalization of different protein kinases). It has been confirmed that Cx43 is a MAPK substrate in vivo and that phosphorylation on Ser-255, Ser-279, and/or Ser-282 is sufficient to disrupt gap junctional intercellular communication (634). Accordingly, 3T3 A31 fibroblasts transfected with a carboxy-terminal truncated Cx43 remain highly coupled after treatment with PDGF (388). Nevertheless, other studies suggest that other unknown factors besides phosphorylation are required for disruption of Cx43-mediated cell-to-cell communication (231, 232). The consensus sites for MAPK and cdc-2 kinase present in Cx43 are apparently shared, since the two-dimensional tryptic maps of the carboxy terminus phosphorylated by MAPK or cdc-2 kinase appear identical (507). Yet, comparison of two-dimensional phosphotryptic peptide analysis of the p34(cdc2)/cyclin B kinase-phosphorylated carboxy terminus and of the P3 phosphoform of Cx43 immunoprecipitated from mitotic cells reveals several differences. This suggests that in vivo Cx43 phosphorylation would involve the participation of other protein kinase(s), which may be activated by the p34(cdc2)/cyclin B kinase (321).
View larger version (34K):
[in this window]
[in a new window]
|
FIG. 4. Schematic representation of possible activation routes of different protein kinases involved in Cx43 phosphorylation. Epidermal growth factor (EGF) binding to its receptor promotes receptor dimerization, autophosphorylation, and interaction with Grb2 adaptor protein and SOS guanine nucleotide exchange protein. Membrane-bound SOS triggers the transformation of ras-GDP (p21ras-GDP) to the ras-GTP activated form (p21ras-GTP). Lysophosphatidic acid (LPA) receptor (LBP) is coupled to G1 protein that also activates ras. EGF- or LPA-induced p21ras-GTP phosphorylates raf-kinase (c-raf) leading to its activation. c-raf leads to MEK1/2 activation, which in turn leads to activation of mitogen-activated protein kinase (MAPK). EGF or LPA-activated MAPK phosphorylates Cx43 on serine residues. Binding of other ligands (L) to their membrane receptors (R) induces activation of Gq proteins, which induce activation of phospholipase C (PLC). PLC converts phosphatidylinositol 4,5-bisphosphate (PIP2) to diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3). DAG activates protein kinase C (PKC), which in turn can phosphorylate MEK1/2 and MAPK leading to their activation. The IP3-induced Ca2+ release from intracellular stores in the ER also promotes PKC activation. PKC and PKC-activated MAPK can directly phosphorylate Cx43 on serine residues. Phosphorylation of Cx43 on tyrosine residues occurs by the expression of v-Src tyrosine kinase. v-Src interacts directly with Cx43 through its SH3 and SH2 domains catalyzing the phosphorylation of two tyrosine residues. [Modified from Warn-Cramer et al. (633).]
|
|
Numerous reports have described changes in the state of phosphorylation of Cx43 through PKC-dependent pathways. The pattern of Cx43 phosphorylation and the functional consequences are cell type dependent (for reviews, see Refs. 322, 506). Ser-368 in Cx43 appears to be an important site that may underlie the cellular uncoupling that follows TPA treatment, since mutation of Ser-368 partially prevents this effect (323, 349).
The products of various oncogenes are tyrosine kinases that disrupt gap junctional communication by phosphorylation of tyrosine residues in Cx43. Oncogene products with tyrosine kinase activity phosphorylate Cx43 in vivo (102, 160, 302) and in vitro (357). Similarly, c-Src seems to be involved in loss of gap junctional communication induced when ligands bind to c-Src-associated receptors (466). The interaction with v-Src is mediated by a proline-rich motif of Cx43, Pro-274-Pro-284 (264). Moreover, in vitro studies indicate that Cx43 is a substrate for v-Src tyrosine kinase and that phosphorylation of Cx43 occurs in Tyr-247 and Tyr-265 (345). Expression in Cx43-null cells of Cx43 constructs in which Tyr-247 and/or Tyr-265 were replaced with phenylalanines suggests that phosphorylation of both tyrosine residues is required for the complete v-Src-induced disruption of gap junction channel communication (345).
In rat (but not human) Cx43, Ser-257 is flanked by a proline and a lysine, making it a possible site for phosphorylation by cGMP-dependent protein kinase. Accordingly, the extent of phosphorylation of rat Cx43, but not of human Cx43, expressed in SkHep1 cells is increased by 8-bromo-cGMP (306).
Some studies have shown that activation of cAMP-dependent protein kinase (cAMP-dPK) leads to a rapid augmentation in Cx43 phosphorylation as detected by immunoblotting (193) and increased intercellular communication (69, 186). However, direct phosphorylation of Cx43 by cAMP-dPK has not been documented. The polypeptides corresponding to the carboxy terminus of Cx43 are not phosphorylated by cAMP-dPK (509, 575). Moreover, activation of cAMP-dPK does not change the 32P incorporation into Cx43 transfected into fibroblasts derived from Cx43-null mice (575).
Cx32 is phosphorylated by cAMP-dPK, PKC, Ca2+/calmodulin-dependent kinase II, and EGF receptor (130, 508, 511, 569, 570, 586). In rat hepatocytes, the effect of cAMP-dPK is temporally correlated with increased junctional conductance (511). Similarly, in the human colonic cell line T84, a rapid (<20 min) increase in intercellular communication mediated by Cx32 gap junctions is induced by an increase in intracellular cAMP concentration (84). Functional correlates of Cx32 phosphorylation by EGF receptor or Ca2+/calmodulin-dependent protein kinase have not been reported.
The stoichiometry of Cx32 phosphorylation in vitro by PKC or cAMP-dPK is between 0.1 and 1 mol phosphate/mol Cx32 (508, 511, 570), suggesting that not every channel or not every subunit within a channel is phosphorylated. The stoichiometry of in vivo phosphorylation and identification of the cell compartment(s) in which Cx32 phosphorylation occurs have not been reported. Cx32 is phosphorylated in Ser-233 by both cAMP-dPK and PKC (508). It is likely that PKC phosphorylates other amino acid residues that are not phosphorylated by cAMP-dPK (570). Increased incorporation of 32P into Cx32 is observed when isolated liver gap junctions (previously phosphorylated by cAMP-dPK) are incubated with PKC (570); moreover, two-dimensional tryptic phosphopeptide maps of Cx32 immunoprecipitated from hepatocytes treated with PKC activators show a higher number of 32P-labeled peptides than the maps of Cx32 obtained from cells treated with cAMP-dPK activators (508). Phosphorylation of Ser-233 is not required for formation of functional channels (640), but its possible role in other functions has not been studied. The tyrosine residues phosphorylated after activation of EGF receptor (130) have not been identified.
Lens connexins are phosphoproteins, and changes in their state of phosphorylation correlate with changes in cell-cell communication (39, 248, 514, 670). In ovine lens, Cx49 is phosphorylated by casein kinase I (89), and inhibition of casein kinase I increases intercellular communication between cultured lens cells (90). Moreover, Cx45.6 is phosphorylated at Ser-363 by casein kinase II; this phosphorylation inhibits the cleavage of the connexin carboxy terminus mediated by caspase-3-like protease (669). Cx46 is phosphorylated in serine and threonine by PKC in the lens cortex (514). Two phosphorylation sites have been identified in Cx56, Ser-118 and Ser-493. These sites are phosphorylated under basal conditions in lentoid-containing cultures. Upon activation of PKC, phosphorylation is enhanced in Ser-118 (39). Phosphorylation of Ser-118 correlates with a reduction in intercellular communication (39). The Cx56 phosphorylation and reduced intercellular communication induced by a tumor promoter phorbol ester appear to be mediated by PKC-
(44).
Phosphorylation of other connexins (e.g., Cx31, Cx37, Cx40, and Cx45) in transfected cells has been demonstrated directly by incorporation of 32P into the protein or shift in their electrophoretic mobilities after treatments that increase protein kinase activity (313, 328, 583, 584, 610, 613). Mouse and human Cx37 expressed in cell lines are phosphorylated mainly in serine residues (328, 584). Mouse Cx45 is detected as a doublet that is reduced to a single 46-kDa band after treatment with alkaline phosphatase (613). After mutation or deletion of nine serine residues in the carboxy terminus of Cx45, phosphorylation is decreased by 90%, indicating that those serine residues represent main phosphorylation sites in the protein (217). Activation of cAMP-dPK leads to an electrophoretic mobility shift of human Cx40 that correlates with increased cell-to-cell communication (610). However, the identification of amino acid residues with functional significance and dissection of protein kinases responsible for their phosphorylation await further studies.
Finally, treatment of some cells with phosphatase inhibitors leads to increased Cx43 phosphorylation, suggesting the involvement of phosphoprotein phosphatases 2A and 2B (42, 104, 342). The phosphatases that mediate dephosphorylation of other connexins have not been reported.
B. Other Protein Modifications
The amino termini of several connexins contain potential consensus N-glycosylation sites, but they are not utilized (474, 630), presumably because they are located in the cytoplasm and therefore are not accessible to the glycosylating enzymes. Fatty acylation of connexins has been suggested by metabolic labeling studies (585), but in-depth studies have not been published.
Modification of Cx43 by ubiquitination has been demonstrated (309, 312, 501) (Fig. 3). The residue to which ubiquitin molecules are added has not been identified, but it is likely to be a lysine.
Cleavage of the carboxy terminus of lens fiber-specific connexins has been proposed to occur naturally during maturation of lens fiber cells (283). Studies of the ovine Cx50 have identified calpain as the enzyme that removes a 32-kDa portion of the carboxy tail of Cx50 (344, 669, 670). However, a similar phenomenon may not occur in chicken lenses, since cleavage of Cx56 was not observed when lens cortex and lens nucleus homogenates were prepared using EDTA (40).
C. Interaction and Colocalization of Connexins With Other Proteins
Connexins may interact with other proteins within the cell. Because gap junction isolation methods involve the use of relatively harsh detergents, interacting proteins do not copurify with connexins. However, other strategies have suggested some possible associated proteins. Blotting experiments have suggested Cx32 could bind calmodulin (604, 682). In addition, Cx43 contains SH2 domains that may be important for its interaction with tyrosine kinases such as pp60v-src (264, 358). Recent studies have suggested an association of Cx43, Cx45, Cx46, and Cx31.9 with the peripheral membrane protein ZO-1 (181, 310, 416, 417, 581) (Fig. 3), which was originally identified as a component of tight junctions. Binding of Cx32, but not Cx26, to another tight junction protein, occludin, has also been demonstrated by coimmunoprecipitation experiments (288, 289). The expression of Cx32, but not Cx32 truncated at position 220, wild-type Cx26, or Cx43, in Cx32-null hepatocytes enhances the expression of the tight junction proteins claudin, occludin, and ZO-1, which colocalize with Cx32, suggesting that the Cx32-mediated intercellular communication participates in the formation of tight junctions (290). With the use of a yeast two-hybrid protein interaction screen, the interaction of Cx43 through its carboxy terminus with the second PDZ domain of the ZO-1 protein has been demonstrated (181). The v-Src-induced disruption of gap junction plaques may occur through interaction of v-Src with ZO-1 (579).
Connexins appear to interact with caveolin-1, a structural protein of lipid raft domains (112, 520); this interaction seems to be cell type specific, and its functional significance is unknown. Colocalization of connexins with other membrane proteins, including FGF receptors (268), aquaporins (476), or cytoplasmic proteins such as calmodulin (549) and cytoskeletal proteins (182) has been demonstrated.
|
V. FORMATION AND DEGRADATION OF GAP JUNCTIONS
|
|---|
A. Connexin Biosynthesis and Gap Junction Assembly
The development of specific anti-connexin antibodies has made possible in vitro studies of connexin biosynthesis by metabolic labeling and immunoprecipitation (reviewed in Refs. 314, 317). The biosynthetic studies have been most extensive for Cx43. Pulse-chase studies show that Cx43 is initially synthesized as a 40- to 42-kDa polypeptide that is subsequently posttranslationally modified to forms with slightly slower mobilities on SDS-PAGE due to phosphorylation of serine residues (102, 160, 399, 400). Multiple phosphates are added to Cx43, and at least some of this phosphorylation occurs in a serine-rich sequence near the carboxy terminus. Phosphorylation of Cx43 to yield a phosphoform (P1) occurs soon after translation. Inhibition of protein trafficking with monensin or brefeldin A reveals that partial phosphorylation of Cx43 occurs before its exit from the Golgi apparatus (315, 470).
The location at which connexins oligomerize into connexons is connexin-type dependent (116, 129, 370, 402, 474); as examples, it appears that Cx32 assembles in the endoplasmic reticulum (ER) (or ER/Golgi intermediate compartment) while Cx43 assembles in the trans-Golgi network (116, 129, 370, 402, 474). With the use of an in vitro cell-free transcription/translation system, it has been demonstrated that Cx26 hemichannels are integrated directly into plasma membranes in a posttranslational manner (5). In cells expressing two connexins, it is possible to find hemichannels formed by both connexins (heteromeric) (43, 246, 355, 372, 556) or hemichannels constituted by one or the other protein (homomeric). In studies performed in liver, oligomeric intermediates of Cx26 are detected in an ER/Golgi intermediate compartment while oligomers containing both Cx26 and Cx32 are preferentially detected in a Golgi membrane fraction (129). This might have implications for the formation of heteromeric connexons (see below). Newly synthesized connexins appear to be transported to the plasma membrane through two different pathways: one sensitive to brefeldin A (a drug that disrupts the Golgi compartment) and one sensitive to nocodazole (a drug that disassembles microtubules) (179, 370).
Connexons are inserted into the plasma membrane in a closed configuration (Fig. 3, inset 1) perhaps at regions of cadherin/catenin-mediated cell adhesion or near tight junctions (169). Formation of gap junctions requires appropriate cell adhesion, especially that mediated by Ca2+-dependent molecules (cadherins) (258, 400). Musil et al. (400) have demonstrated that Cx43 localizes intracellularly and not at gap junctional plaques in the noncommunicating cell lines L929 and S180. The Cx43 in these cells is incompletely phosphorylated. However, transfection of the S180 cells with the cell adhesion molecule LCAM (E-cadherin) restores gap junctional communication, phosphorylation of Cx43 (to a "P2" form), and presence of gap junction plaques. These findings suggest a relation between the ability of cells to more extensively phosphorylate Cx43 and the ability to form communicating junctions. They also suggest a hierarchy of events in the formation of intercellular junctions: a cell adhesion event is required before formation of gap junction plaques. Similar observations regarding the requirement of cadherin-mediated cell adhesion for development of gap junctions have been made in epidermal cells (258). Cell-to-cell contact sites mediated by the cadherin-catenin complex also seem to act as foci for gap junction formation as shown by colocalization of connexins with E-cadherin or -catenin during gap junction formation in regenerating liver (169). Moreover, antibodies to A-CAM (N-cadherin) or peptides representing extracellular domains of Cx43 inhibit gap junction formation in Novikoff hepatoma cells (381). However, the effects of increased cell adhesion seem to be cell type specific, since expression of exogenous cadherin decreased gap junctional communication between mouse L cells but increased it in Morris hepatoma cells (632). A role for Ca2+-independent cell adhesion molecules (e.g., N-CAM) has also been suggested (270).
Incorporation of connexons made of Cx43 into gap junction plaques correlates with resistance to solubilization in Triton X-100 and with additional phosphorylation (401). However, some studies suggest that phosphorylation may not be an absolute requirement for assembly or insertion of connexins into the plasma membrane. In exogenous expression systems, Cx26, a nonphosphorylated protein, is able to induce junctional currents (23). Moreover, truncated Cx43 or Cx32 constructs devoid of some phosphorylatable serine residues from their carboxy termini can still induce intercellular coupling (162, 351, 640). In addition, phosphorylation of Cx43 can occur in Madin-Darby canine kidney cells in the absence of Ca2+-dependent cell adhesion molecule activity and does not correlate with existence of intercellular coupling (42).
Formation of gap junction plaques requires clustering of gap junction channels. Increased levels of cAMP induce clustering of Cx43 gap junction channels and enhanced gap junction plaque growth through a mechanism that seems to be cell type dependent. In some cell types, this process depends on polymerized actin while in others it depends on intact microtubules and trafficking through intracellular membrane compartments (158, 256, 445, 631). It has recently been suggested that the carboxy terminus of Cx43 (including Ser-364) plays a role in the cAMP-induced clustering of gap junction channels (575). The differences observed between cell types might reflect differences in the source from which additional connexons come (i.e., a pool at or near the plasma membrane or a pool at earlier locations within the biosynthetic/exocytic pathway).
Connexins that are coexpressed in the same cell can localize to the same gap junction plaque (404, 413, 428, 585, 115, 228, 660, 668) or to distinct membrane domains (203, 553), suggesting the presence of a sequence motif in connexins that determines its topographic fate. Although differences in primary sequence between connexins might explain the delivery of hemichannels to specific plasma membrane domains, these sequences have not been identified for most connexins. It has been reported that the transmembrane domains, but none of the cytoplasmic domains, are required for the delivery of Cx32 to its specific destination in cultured epithelial cells (337). How gap junction channels aggregate and form small junctional plaques is not yet resolved. In ultrastructural studies, small particle aggregates have been found within particle poor, flattened regions of the plasma membrane as junctions are beginning to form between reaggregated cells (255). Growth of gap junctions occurs by incorporation of additional gap junction channels to the plasma membrane followed by their incorporation to the periphery of existing gap junction plaques (174, 331). Imaging of living cells transfected with fluorescent protein-tagged connexins suggests that connexins are delivered to the gap junction plaque in cytoplasmic vesicles (260) that move along microtubules (331). Large junctional plaques can form by coalescence of small junctional plaques (227, 282).
Some incompletely resolved issues are as follows: how connexins are brought together to assemble into connexons, how these connexons are incorporated into the plasma membrane, how gap junctional channels become part of a plaque, and which other proteins/factors participate in these processes. It is possible that no other factors are necessary for channel or plaque formation because purified connexons tend to aggregate in filaments and sheets (557).
B. Gap Junction and Connexin Degradation
Proteins in gap junctions are apparently rather shortlived. Based on in vivo labeling, Fallon and Goodenough (157) estimated that the half-life of a major polypeptide in liver gap junctions was in the range of 5 h. Similarly, pulse-chase experiments have shown that connexins have half-lives of 1-3 h in cultured cells (102, 115, 316, 586). A similar half-life has been measured for Cx43 in intact lens and heart (29, 399). However, at least some of the lens fiber connexins are more stable, with half-lives of 2-3 days or more (38, 247).
Morphological and biochemical approaches have recently been applied to begin to elucidate the mechanisms of gap junction degradation (reviewed in Ref. 49). Electron microscopy studies have implied endosomal internalization of entire junctions forming "annular gap junctions" (Figs. 1B and 3) followed by degradation in lysosomal, multivesicular body, or autophagosomal compartments (260, 325, 326, 375, 397, 408, 447, 489, 533, 616). In vivo, these structures have been most frequently found following pathological insults such as ischemia or during tissue remodeling. Cell fractionation and immunoelectron microscopy studies have shown an association between connexins and lysosomes in some cultured cells (515, 616). Murray et al. (398) found that in SW-13 adrenal cortical tumor cells, many of the cytoplasmic annular Cx43 gap junction profiles are associated with myosin II, but not with tubulin, or vimentin-containing fibers. Disruption of microfilaments resulted in a decrease in the number and an increase in the size of these annular gap junctions, suggesting a role for myosin-containing cytoskeletal elements in annular gap junction turnover (327, 398). A recombinant Cx43 containing tetracysteine tags was recently used to demonstrate that older Cx43 is removed from the center of plaques into pleiomorphic vesicles of widely varying sizes (174).
Recently, investigators have used biochemical or immunochemical techniques to study proteolysis of the subunit gap junction proteins. Studies of Cx43 proteolysis in E36 Chinese hamster ovary cells have shown the major involvement of the ubiquitin-proteasome pathway in Cx43 degradation. In these cells, treatment with the protease inhibitor N-acetyl-leucyl-leucyl-norleucinal (ALLN) induces accumulation of Cx43 and a significant prolongation of its half-life. Also, Cx43 degradation is abolished when ts20 Chinese hamster ovary cells, which contain a thermolabile ubiquitin activating enzyme, E1, are incubated at the restrictive temperature (309). Immunofluorescence studies in both primary cultures of cardiac myocytes and in established cell lines have shown that Cx43 gap junction plaques as well as the Cx43 polypeptide exhibit rapid turnover rates. The rapid disappearance of Cx43 staining can be blocked by inhibitors of either the proteasome or the lysosome, implicating both proteolytic systems in gap junction degradation (260, 311, 312). These studies have also shown that connexin degradation could be stimulated by cellular stress such as heat shock (311). It is possible that the proteasome is mainly involved in degradation of misfolded connexins (as occurs for many other proteins); this process is called ER-associated degradation and serves as a quality control check point. Misfolded connexins likely are dislocated from the ER and then degraded by the proteasome. The involvement of this pathway during connexin biosynthesis has been recently demonstrated for wild-type Cx43 (612). The participation of both proteasomal and lysosomal pathways in the degradation of Cx43 in the heart has also been demonstrated (29).
The published studies suggest that the preference of one degradative pathway over the other is cell type dependent and that within the same cell type the degradative pathway chosen might depend on the metabolic/pathological state of the cell. The experiments do not allow differentiation between the possibilities of a sequential (e.g., the carboxy tail of Cx43 is degraded in the proteasome while the core of the protein embedded in the membrane is degraded by the lysosome) or parallel (Cx43 can be degraded by the proteasome or the lysosome) pathway for degradation of connexins.
|
VI. HEMICHANNELS
|
|---|
Although for several decades hemichannels found at nonjunctional membranes were thought to remain permanently closed to avoid cell death, data reported during the last decade have documented the existence of regulatable hemichannel opening in cultured cells. Nevertheless, it is still not completely known whether cell surface hemichannels have different permeability properties than gap junction channels formed by the same connexin type. Moreover, it is unknown whether physiological stimuli may induce brief hemichannel openings that allow transfer of small molecules between the intracellular and extracellular compartments.
A. Opening and Closing Hemichannels
The opening of connexin hemichannels was first described based on the swelling and death of Xenopus oocytes injected with the RNA of Cx46 (444). Shortly thereafter, DeVries and Schwartz (125) reported that cultured solitary horizontal cells of the catfish retina express an endogenous current mediated by hemichannels that open upon reduction of the extracellular [Ca2+]. The current is reduced by external [Ca2+] higher than 1 mM, treatment with dopamine, or a weak acid and coincides with blockade of Lucifer yellow uptake. Subsequently, the electrophysiological characterization of macroscopic and single hemichannel currents have been reported for various connexins exogenously expressed in Xenopus oocytes or mammalian cell lines (137, 139, 205, 291, 341, 462, 471, 562, 589, 597, 599).
Opening of hemichannels formed by different connexin types is rather infrequent or absent under resting conditions but can be regulated by several factors. Low extracellular [Ca2+] enhances the opening of hemichannels in the Xenopus oocyte expression system (137, 139), in Novikoff cells (341), in some cardiac myocytes (291), in astrocytes (225, 562), and in transfected human osteo-blast-like cells (492). In addition, opening of Cx43 hemichannels has been induced by mechanical stimulation in cultured astrocytes (562). Agonists of hemichannel opening have also been described. The hemichannel-mediated currents expressed by retinal horizontal cells bathed in low extracellular [Ca2+] are enhanced by the antimalaric drugs quinidine or quinine (363). Quinine-induced dye uptake revealing opening of Cx43 hemichannels has also been observed in astrocytes (562), but not in Cx43 RNA-injected Xenopus oocytes (644). The quinine-induced hemichannel opening cannot be explained solely by the quinine-induced alkalinization because it is still observed using 80 mM buffered HEPES in the recording patch pipette (131). Alendronate, a drug used widely in the treatment of bone diseases, induces opening of hemichannels formed at least by Cx43 (462).
Lens connexins, including rat Cx46, bovine Cx44, and chicken Cx56, form functional hemichannels in Xenopus oocytes bathed in low or even normal extracellular [Ca2+] (137, 139, 205, 444). However, lens fibers have high input resistance, suggesting the absence of open hemichannels at the plasma membrane under physiological conditions. This apparent controversy might be explained by differences in the posttranscriptional modification of these connexins in oocytes with respect to that in lens fibers. These connexins are phosphoproteins and are detected as multiple bands in immunoblots of lens homogenates (40, 205, 444), but when expressed in Xenopus oocytes, they show the same electrophoretic mobilities as in vitro translated proteins (137, 205, 444). Sheep lens fibers containing Cx49 show an increase in dye coupling when treated with a casein kinase I inhibitor, suggesting that phosphorylation of the lens fiber connexin by the endogenous casein kinase I activity leads to closure of gap junction channels (89, 90). A similar mechanism might help in keeping lens fiber hemichannels closed to maintain the high input resistance of lens cells.
Opening of Cx43 hemichannels induced by low extracellular [Ca2+] can be blocked by activation of a PKC-dependent pathway (341, 350), suggesting the involvement of protein phosphorylation as a gating mechanism that maintains the hemichannels closed. Likewise, the membrane current mediated by Cx46 hemichannels expressed in Xenopus oocytes is greatly reduced by activation of a PKC-dependent mechanism (245, 412). Moreover, Cx43 is a substrate for MAPK (633). MAPK can be activated by tyrosine kinase receptors, such as the EGF receptor, or through transduction pathways that activate PKC leading to direct or indirect phosphorylation of Cx43 via PKC or MAPK as proposed for Cx43 forming gap junction channels (Fig. 4). A similar mechanism might operate on Cx43 hemichannels. In support of this hypothesis, it is known that hemichannels of MAPK-phosphorylated Cx43 reconstituted in lipid vesicles remain preferentially closed, but dephosphorylated Cx43 forms opening hemichannels (280). Nevertheless, it is difficult to conceive that under resting conditions MAPK would keep the hemichannels closed without affecting gap junctional communication. Alternatively, Cx43 forming hemichannels could be phosphorylated by a protein kinase activity with several isoenzymes that show cellular compartmentalization (e.g., PKC). Fibroblasts from Cx43-null mice transfected with Cx43 mutated at Ser-368, a change that confers resistance to PKC-induced closure of gap junction channels (323), show a decrease in hemichannel opening after treatment with a phorbol ester to activate PKC (349).
Closure of hemichannels has been observed after extracellular application of lanthanide cations, La3+ (99, 251, 280, 291), or Gd3+ (562), or treatment with gap junction channel blockers, such as octanol, heptanol, carbenoxolone, oleamide, halothane, and 18-- and 18--glycyrrhetinic acid. Moreover, closure of hemichannels formed by Cx30, Cx44, Cx46, Cx50, or Cx56 can be induced by hyperpolarization of the plasma membrane (137, 139, 205, 587, 599). In horizontal cells bathed in Ca2+-free saline and exposed to positive holding potentials, open hemichannels are closed by retinoic acid (678). With the use of cysteine replacement mutagenesis, the voltage gate in Cx46 hemichannels has been localized extracellularly to the amino acid residue at position 35 (453). Extracellular acidification also closes hemichannels formed by various connexins (28, 588, 676). The membrane current mediated by hemichannels is greatly reduced by 1-2 mM extracellular Ca2+ (137, 139, 454). It has been proposed that Ca2+ induces a regional closure of the pore (454). Mg2+ can partially substitute for the Ca2+ blocking effect on Cx46 hemichannels (139).
The conductance of homomeric hemichannels formed by Cx30, Cx32, Cx43, Cx45, Cx46, and Cx50 has been measured using voltage clamp in the cell-attached or the excised-patch configuration (140, 149, 587, 597, 599). The unitary conductance determined for homomeric connexin hemichannels ranges from 31 to 352 pS (140, 149, 587, 599).
In support of the existence of conduits permeable to small molecules, it has been demonstrated that dextran polymers of high molecular weight conjugated to different fluorophores are not taken up by cells expressing hemichannels (99, 562). Moreover, it has been demonstrated that Cx43 hemichannels are permeable to Lucifer yellow, ethidium bromide, carboxyfluorescein, 7-hydroxy-coumarin-3-carboxylic acid, and fura 2 (99, 291, 341, 350, 462, 562, 607). Direct or indirect measurements have demonstrated the permeability of Cx43 hemichannels to other small molecules [e.g., NAD+, ATP and inositol trisphosphate (IP3)] (15, 63, 64, 492, 562). Similarly, cells expressing Cx32 release ATP, suggesting that Cx32 hemichannels are permeable to this nucleotide (15).
Alterations in hemichannel features by genetic mutation have also been documented. Two mutations linked to congenital cataracts, Cx46Asn63Ser and Cx46 frame-shift 380, were impaired in their ability to form functional hemichannels (438). Studies on Cx50 have revealed that the His176Gln mutant is oocyte lethal and that the His161Asn mutant does not form detectable hemichannels (28). Moreover, the double mutant Cx50His161Asn/His176Gln neither forms hemichannels nor kills the oocytes (28). Hemichannels formed by a CMTX-associated Cx32 mutant, Cx32Ser85Cys, show increased currents compared with those made of wild-type Cx32 (2). A chimeric connexin consisting of Cx32 where the first extracellular loop sequence is replaced by the corresponding Cx43 sequence induces a membrane conductance in single Xenopus oocytes (455). Finally, some carboxy-terminal mutations of Cx32, including some of those identified in CMTX, prevent the formation of functional connexons (79).
B. Hemichannel Functions
Recent reports indicate that hemichannel opening could play relevant functions under physiological and pathological conditions. Quist et al. (471) proposed that Cx43 hemichannels regulate the cell volume in response to changes in extracellular physiological [Ca2+] in an isosmotic situation. In addition, Cx43 hemichannels mediate the release of NAD+ to the extracellular medium in fibroblasts (63, 64). This provides an answer to the old paradox of having intracellular NAD+, the substrate for CD38, an ectoenzyme that catalyzes the conversion of NAD+ to cADPR (a potent physiological ligand for the ryanodine receptor found in the ER). CD38 also mediates the uptake of cADPR. A paracrine intercellular Ca2+ signaling through Cx43 hemichannel-mediated NAD+ efflux has been proposed to enhance cell proliferation and shorten the S phase of the cell cycle of NIH 3T3 fibroblasts (167). The existence of an autocrine cADPR-induced calcium release that regulates glutamate and GABA release has been demonstrated in hippocampal astrocytes (621). Furthermore, NAD+ efflux mediated by astrocytic Cx43 hemichannels provides a paracrine signal mechanism that results in a delayed calcium transient in neuronal cells in coculture, which is strongly inhibited by glutamate receptor blockers (621). Moreover, mechanical stimulation elicits release of ATP through Cx43 hemichannels in astrocytes and could mediate the propagation of Ca2+ signals for intercellular communication in astrocytes and other nonexcitable cells (562).
The detection of Cx26 hemichannels on the dendrite surface of retinal horizontal cells has been demonstrated by immunofluorescence (262, 243). It has been proposed that opening of Cx26 hemichannels would lead to depolarization of cell dendrites mediating a negative feedback mechanism that modulates the activity of Ca2+ channels and subsequent glutamate release from cones (262). Massive opening of hemichannels made of Cx43, the most ubiquitous connexin, has been demonstrated in astrocytes and cardiomyocytes subjected to metabolic inhibition to mimic ischemia (99, 251, 291, 339). The metabolic inhibition-induced opening of Cx43 hemichannels would speed up mechanisms leading to cell death (99). Moreover, the increased opening of hemichannels made of a CMTX-associated Cx32 mutant (Cx32Ser85Cys) could be deleterious for glial cells and normal neural function (2).
|
VII. GAP JUNCTIONS IN CARDIOVASCULAR, DIGESTIVE, REPRODUCTIVE, AND IMMUNE SYSTEMS
|
|---|
A. Cardiovascular System
Gap junctions between cardiac myocytes are found in specialized plasma membrane regions known as the intercalated disks adjacent to adherens junctions (which facilitate mechanical coupling between cells). Gap junction channels in the heart appear to play a critical role by allowing the intercellular passage of current-carrying ions, which facilitates action potential propagation. Differences in the abundance, size, and location of gap junctions in different cardiac regions (117, 118) may contribute to differences in their properties of electrical conduction. As examples, Saffitz and colleagues (360, 512) have shown that cardiac tissues that differ in their anisotropy of conduction (infarct border zones vs. normal myocardium or crista terminalis vs. ventricular myocardium) show marked differences in their relative abundance of "end-to-end" versus "side-to-side" gap junctions. Moreover, because gap junction channels made of different connexins have different conductance and gating properties, differences in expression patterns of connexins may also contribute to differences in cardiac conduction.
Several connexins including Cx43, Cx40 (or its ortholog, chicken Cx42), and Cx45 are expressed in cardiac tissues (266, 267), but they have different distributions in specialized cardiac tissues. Cx43 appears to be the major connexin in the working myocardium of the ventricle (117, 118, 605). In atrial myocytes, Cx43 is coexpressed with Cx40 (117, 118, 191, 512, 625). Studies of cardiac tissues from several different species (human, dog, cow, rabbit) have shown that Cx43 is rare or absent in cells of the specialized conducting regions of the sinus and atrioventricular nodes (14, 117, 118, 307, 430, 431). However, some studies have found Cx43-expressing cells that might form specialized pathways for electrical conduction out of nodal zones (307, 622, 623). The expression of Cx40 in the heart is more restricted than that of Cx43. Cx40 is abundant in atrial myocytes and cells of the His-Purkinje system and is also found in cells of the sinoatrial and atrioventricular nodes (24, 117, 191, 199, 266). Although Cx40 is not present in adult mammalian ventricular myocytes, its ortholog (Cx42) is abundant in the ventricular myocardium of the developing chicken heart (384). Cx45 expression is detectable in cells of the atria, ventricle, sinus and atrioventricular nodes, and His bundle (117, 266). The absolute amounts of different connexins in cardiac myocytes have not been determined; rather, different abundances have been considered based on the intensity of immunostaining. This has led to some debate regarding the significance of levels of Cx45 in ventricular myocardium (101, 253).
Gap junctions are also present in distributions that link all of the different cell types in the walls of blood vessels. Gap junctions may play multiple roles in facilitating interactions between these cells (as recently reviewed in Refs. 420, 532); most prominently, endothelial cell gap junctions may be critical for the propagation of vasomotor signals (523). In general, all types of vessel wall cells express Cx43, in vivo and in vitro. In vivo, Cx40 mRNA or protein has been demonstrated in large and small vessel endothelium and in smooth muscle from several species but not others. Cx40 is a major gap junction protein of endothelial cells of the adult vasculature in most organs (346, 606), but it shows heterogeneous expression along the vasculature (528). Other than expression in the ovary and perhaps in some lung cells, Cx37 expression appears to be virtually exclusively limited to endothelial cells (478), but there may be substantial variability in expression in vivo and in culture, depending on vascular bed type and species. The differential distribution of Cx37 and Cx43 suggests that they are involved in more dynamic processes than Cx40. However, interpretations of their patterns of expression along vessel walls are controversial. Cx43 is highly localized to sites of disturbed flow in rat aortic endothelium, but Cx37 and Cx40 are more uniformly distributed (173). While arterial smooth muscle cells abundantly express Cx43, they may also express Cx40 (346) and Cx45 (222, 287, 298). The patterns of expression of connexins among endothelial and smooth muscle cells are modulated under pathological conditions including early human coronary atherosclerosis or arterial wall injury (54, 208, 209, 305, 464, 465, 667).
Myocardial gap junctions and connexins may be critical for the coordinated electrical activation of the heart. A variety of alterations in their number and distribution have been observed in diseased hearts (265, 532). Disorganization of gap junctions and/or reduced expression of Cx43 have been observed and may explain increased arrhythmogenesis in human and animal ventricular myocardium associated with ischemia or infarction (452, 531). Decreased Cx43 at plasma membrane appositions has been found between cardiac myocytes infected with Trypanosoma cruzi, the Chagas disease agent, suggesting a potential cause for cardiac conduction disturbances observed in affected individuals (71). Levels of Cx40 increase 3.1-fold, and those of Cx43 decrease 3.3-fold in the myocardium of hypertensive rats (24). In contrast, Cx40 levels were reduced, and its distribution was altered without any changes in Cx43 in association with models of atrial fibrillation (603).
Recently, molecular biology approaches have been used to alter the expression of cardiovascular connexins to elucidate their functions. Reaume et al. (477) used homologous recombination to produce Cx43-null mice. These animals survive until birth and look essentially normal. At birth, they have beating hearts, but they appear cyanotic and die shortly thereafter, apparently due to a lack of blood flow to their lungs due to obstruction of the right ventricular outflow tract. Thus it appears that the presence of Cx43 is not an absolute requirement for the heart to beat or for the anatomic development of the heart. Perhaps other coexpressed connexins (such as Cx45) can explain the coupling of Cx43-null ventricular myocytes, but the amount of Cx45 is also diminished in Cx43-deficient mice (253).
The importance of Cx43 for normal cardiac conduction and velocity has been elucidated by studies of mice that are heterozygous for the knock-out genotype (Cx43+/-). These mice appear phenotypically normal and reproduce normally. However, epicardial conduction is slowed in the ventricle (but not the atrium) (202, 576). In addition, the QRS complex of the electrocardiogram is widened, implying ventricular conduction delay (202, 576). Finally, tissue-specific knock-out strategies have been used to produce mice with cardiac-specific loss of Cx43; these animals show normal heart structure and contractile function, but they develop sudden cardiac death due to spontaneous ventricular arrhythmia by 2 months of age (206). It has been recently suggested that mutations in Cx43 associated with oculodentodigital dysplasia might be responsible for the cardiac conduction abnormalities observed in some of the families (446).
An endothelial cell-specific Cx43 knock-out mouse shows hypotension and bradycardia (343). The hypotensive and bradycardic phenotypes correlate with elevated plasma levels of nitric oxide and angiotensins I and II, suggesting the importance of Cx43 gap junctions for maintenance of vascular tone. An extensive series of transgenic experiments (using wild-type or mutant Cx43) by Lo et al. (354) have emphasized the role of Cx43 expressed by cardiac neural crest cells for the proper development of the heart. For example, Huang et al. (235) have shown that Cx43 gene dosage is critical for the function of neural crest cells during cardiac development.
The importance of Cx40 in conduction through the specialized cardiac conduction system is emphasized by the analysis of Cx40-null mice. These animals develop to adulthood and reproduce normally, suggesting that Cx40 is not necessary for cardiac development, since the cardiac structure and chambers appear normal. However, electrocardiograms show wide and bizarre QRS complexes (47, 281, 542, 611) consistent with a right bundle branch block pattern (likely due to the absence of this connexin from the atrioventricular conduction pathways). These animals may also have defects in propagation of endothelium-dependent vasodilator responses (127). Cx40-deficient mice retain gap junctional communication between aortic endothelial cells, although with altered characteristics with respect to wild-type mice; moreover, Cx37 and Cx43 are upregulated and redistributed (297), suggesting a compensatory mechanism. Cx40- (and Cx37-)null animals exhibit no obvious vascular abnormalities.
Cx45-deficient mice die during embryogenesis due to abnormalities of vascular development and cardiogenesis (298, 299). Endothelial cell development and positioning are normal, but further formation of vascular trees is impaired and the smooth muscle layer of major arteries fails to develop. The hearts of these animals are dilated, and apoptosis is observed in almost all tissues by embryonic day 9.5.
B. Digestive System
1. Gastrointestinal tract
Gap junctions, connexins, and functional intercellular coupling have been identified in multiple portions of the gastrointestinal tract. The distribution of different connexins in the various organs and tissues of the digestive system is shown in Table 1 based on a compilation of diverse published studies. Along the gastrointestinal tract, Cx26 and Cx32 are found in many of the epithelial cells, whereas Cx43 is found in most of the smooth muscle cells.
Within the intestine, gap junctions are present in the circular muscle layer (reviewed in Ref. 114) where they allow the formation of a muscular syncytium that can synchronize contractile activity. Intestinal smooth muscle cells are electrically coupled (1, 21, 143, 177, 212, 572), and they show intercellular passage of microinjected dyes (675). In contrast, gap junctions appear to be absent in the longitudinal muscle layer (212), which may explain why the contraction of canine duodenal circular muscle correlates much better with spike potentials than does contraction of longitudinal muscle.
Cells in different layers of the intestine may show electrical (143, 572) but not dye coupling (675) with each other. This coupling might be mediated indirectly through interstitial cells and fibrocytes (572). Gap junctions between interstitial cells of Cajal of the guinea pig ileum have recently been functionally demonstrated (31). Functional gap junctions between cells of Cajal and smooth muscle cells have also been demonstrated (348), but their role in the transmission of contraction signals is still a matter of debate (113). Recently, reduced Cx43 expression has been observed in tissues from patients with Hirschprung's disease (410), suggesting that impaired intercellular communication between interstitial cells of Cajal and smooth muscle cells may be partly responsible for the motility dysfunction in this disease.
Several studies suggest a role for gap junctions in gastric ulcers or cancers. The amount of Cx32 and Cx43 is reduced in the mucosa surrounding a chronic gastric ulcer, but it reverts to normal after antiulcer treatment (383, 427). Blockade of rat gastric gland gap junctions with 18-glycyrrhetinic acid prevents dye coupling, calcium wave propagation, and acid secretion (472). Moreover, gap junction blockade (induced with octanol) significantly inhibits the recovery of gastric mucosa from acid-induced injury (568). It has also been proposed that inhibition of gap junctions weakens the barrier function of gastric mucosa and subsequently contributes to the damaged barrier function following ischemia-reperfusion (241). Moreover, analysis of tissue specimens from gastric cancers has shown loss of Cx43 expression (427, 591).
2. Accessory glands
Different cell types in the salivary glands contain different connexins (Table 1). During the ontogeny of the rat submandibular gland, Cx43 appears in myoepithelial cells on gestational day 17, and Cx32 appears in acinar cells on day 18 (237). The differential distribution of connexins suggests that Cx26- and Cx32-containing junctions participate in the secretory function of acinar cells and Cx43 participates in the contractile function of myoepithelial cells.
In the pancreas, connexin expression and gap junction function are very different in the exocrine and endocrine cells. Gap junctions are abundant, and Cx26 and Cx32 are found in acinar cells (Table 1). As in other secretory organs, it is believed that intercellular communication may enhance secretion by coordinating the response of coupled cells within an acini to a secretagogue (378, 558). Functional gap junctions do not appear to be necessary for zymogen secretion, since dissociated pancreatic acinar cells show the same rate of exocytosis as intact acini (72). However, connexin expression may be necessary for proper control of secretory function, since pancreatic amylase secretion is increased in Cx32-deficient mice (81). Gap junction function in pancreatic duct cells may also contribute to the pancreatic dysfunction observed in cystic fibrosis patients. Chloride currents evoked by cAMP analogs increase intercellular cell coupling, and intercellular communication is impaired when mutant cystic fibrosis transmembrane conductance regulator (CFTR) is expressed (83), but this effect is reversed by transfection of wild-type CFTR (82).
In the endocrine islets of the pancreas, several connexins including Cx26, Cx43, and Cx45 have been found (Table 1), but Cx36 now appears to be most involved in insulin secretion. In Cx36-null mice, the development and differentiation of -cells are normal, but insulin secretion in response to a glucose challenge is reduced (70). While in vitro studies suggest that Cx43 expression is necessary for insulin production (627), Cx43-null mouse embryos have normal endocrine pancreatic ultrastructure, insulin content, and secretion (86). The connexin type or abundance may also be crucial for the normal islet function, since insulin secretion in response to glucose is reduced in transgenic mice with targeted expression of Cx32 in pancreatic -cells despite increased levels of insulin, improved electrical synchronization, and increased intracellular calcium levels (85). It has been suggested that gap junctions between islet cells are important for synchronizing glucose-induced intracellular calcium oscillations (259). However, published evidence contradicts this hypothesis, since these calcium waves are not affected by gap junction blockers and may be observed in the absence of cell contacts (46).
Rodent hepatocytes are morphologically and metabolically heterogeneous, yet electrical coupling between hepatocytes occurs over a relative long distance within the liver (0.5 mm, 25 hepatocytes), which accounts for the similarity in membrane potentials among hepatocytes (334). Consistent with the notion that gap junctions provide a pathway for synchronization of electrical responses, the glucagon-induced hyperpolarization is higher in periportal (Z1 zone) than in pericentral (Z3 zone) hepatocytes in isolated liver perfused with octanol, a gap junction blocker (334). Moreover, dye coupling (119, 407) and ligand-induced Ca2+ waves (393, 406, 490) have been demonstrated in intact rat liver. Therefore, gap junctions between hepatocytes allow propagation of electrotonic potentials and Ca2+ waves as well as the intercellular exchange of small molecules by simple diffusion (Fig. 5).
View larger version (56K):
[in this window]
[in a new window]
|
FIG. 5. Transfer of three different intercellular signals through gap junctions. A and B: a phase-contrast micrograph of a pair of superior cervical ganglion neurons is shown in A. Each neuron is impaled with a microelectrode; one electrode is used to inject current into one of the cells and the voltage deflections generated in each cell are recorded. An example of one of those recordings is shown in B. This cell-cell signaling corresponds to an electrotonic potential that propagates from one cell to another in milliseconds or less. C and D: phase-contrast (C) and fluorescent (D) images of a cluster of MDCK cells in culture showing intercellular transfer of Lucifer yellow. This process occurs by simple diffusion through gap junctions and takes from several seconds to a couple of minutes. F-L: levels of intracellular free [Ca2+] measured by ratio imaging after loading the cells with fura 2. E: Nomarski image of a hepatocyte triplet showing the microelectrode used for microinjections into the top cell. F-H: intercellular propagation of a Ca2+ wave generated by the intracellular microinjection of IP3. The increase in intracellular free [Ca2+] in adjacent cells occurred sequentially and was not due to Ca2+ diffusion because the increase in free [Ca2+] occurred in discrete cellular regions. I-L: intracellular free [Ca2+] measured in cells bathed in Ca2+-free extracellular medium after Ca2+ was injected into the cell with an electrode positioned as drawn. A localized increase in intracellular free [Ca2+] was evident in the right side of the lower cell. This occurred 2 s after Ca2+ microinjection and before visualization of Ca2+ diffusion into the neighboring cell, suggesting that the microinjected Ca2+ induced the generation of a diffusible Ca2+-releasing agent that permeated through gap junctions to the adjacent cell. This process is regenerative and takes a few seconds. Magnification bar: 20 µm in A, 45 µm in E-L; 60 µm for C and D. [E-L modified from Sáez et al. (504).]
|
|
In rodent liver, the propagation of vasopressin-induced Ca2+ waves occurs in the absence of extracellular Ca2+ (490), suggesting the participation of IP3-induced Ca2+ release from intracellular stores instead of Ca2+ influx. These Ca2+ waves generated by a subthreshold concentration of vasopressin propagate from terminal hepatic venules (THV) to terminal portal venules (TPV), a direction opposite to the blood flow (Fig. 6), ruling out the involvement of an ATP-mediated paracrine intercellular communication mechanism at the apical region of the cells. Moreover, perfusion of liver with ATP causes rapid transient elevations in intracellular free [Ca2+] in single hepatocytes or groups of hepatocytes throughout the lobule without a specific anatomic location for ATP-susceptible cells (393). In contrast, studies in rat hepatocyte triplets or quadruplets (578) or in perfused liver (527) have demonstrated that Ca2+ waves induced by vasopressin start at cells bearing high vasopressin receptor density, as proposed previously (41).
View larger version (53K):
[in this window]
[in a new window]
|
FIG. 6. The heterogeneous distribution of membrane receptors determines the site of origin of Ca2+ waves that propagate across liver acini. Top: schematic representation of a liver section showing a hepatic acinus composed of hepatocytes located between two terminal hepatic venules (THV). A portal triad constituted of a terminal portal venule (TPV), an hepatic arteriole (HA), and a bile duct (BD) is depicted in the center of the acinus. Z1, Z2, and Z3 denote liver zones specialized in different metabolic pathways of the hemiacinus; Z1 hepatocytes are more gluconeogenic, Z3 cells are more glycolytic, and Z2 cells show an intermediate activity of both metabolic pathways. Middle: the gradients of oxygen tension ([O2], light blue), the density of vasopressin receptors (V1R, yellow), and the constant density of inositol trisphosphate receptors (IP3R, gray) throughout the acinus are represented. Bottom: diagram of Ca2+ wave propagation in a sinusoid. An hepatocyte triplet formed by representative cells of each zone shows a gradient of V1 (yellow rectangles) receptor densities. Ca2+ waves generated by a subthreshold concentration of vasopressin (V) are born in Z3 and propagate toward Z1. In each cell, IP3 induces Ca2+ release from ER stores. The increase in [Ca2+] activates phospholipase C (PLC), which generates IP3 that permeates through gap junctions, thus leading to a regenerative wave of IP3/Ca2+ that propagates to the adjacent cell. BC denotes bile cannaliculi next to gap junction channels (illustrated by the 4 green cylinders located at cellular interfaces).
|
|
The propagation of Ca2+ waves would require as mentioned above a regenerative system for increases in intracellular [Ca2+] such as Ca2+-activated phospholipase C and IP3 receptors (Fig. 6). It has been demonstrated that IP3 also permeates hepatocyte gap junctions (504) and that coordination of Ca2+ oscillations is fully dependent on the intercellular diffusion of IP3 and not Ca2+ between neighboring hepatocytes (95, 136). Immunofluorescence studies have demonstrated the presence of protein Gq, phospholipase C, and IP3 receptors in hepatocytes (223, 582). The importance of gap junctional communication and IP3-sensitive Ca2+ stores in coordinating increases in intracellular [Ca2+] between epithelial cells from a community with an heterogeneous density of vasopressin receptors has been demonstrated recently (336).
While nonparenchymal cells of the liver express Cx43, hepatocytes express Cx26 and Cx32 (41, 188, 413, 585, 680). In hepatocytes, these connexins form homotypic and heterotypic gap junction channels as demonstrated by single-channel analysis in cells of Cx32-null and wild-type mice (598). Cx26 and Cx32 gap junction channels show differences in permeability and charge selectivity when expressed in HeLa cells (73, 144). Hepatocytes from Cx32-null mice show a strong reduction in IP3 permeability compared with wild-type hepatocytes (419). Likewise, the intercellular propagation of IP3-induced Ca2+ waves is three- to fourfold more efficient in Cx32 than in Cx26 transfected HeLa cells (418).
In support of the notion that hepatocytes require gap junctional communication for an efficient metabolic response, it has been reported that Cx32-null mice show close to a 70% reduction in nerve stimulation-induced glucose release compared with wild-type mice (409). Similarly, Cx32-null mice show a drastic reduction in glucagon- and norephinephrine-induced glucose release (563). Moreover, with the use of dissociated hepatocytes as well as reaggregated hepatocytes and gap junction blockers, a reduction of 70% in vasopressin-induced glycogenolysis has been observed (153). In addition to their involvement in liver metabolic responses, gap junctions participate in bile flow because 18-glycyrrhetinic acid exacerbates the decrease induced by vasopressin (407). Cx32-null mice also show a dilated bile canaliculi, and the nerve-dependent decrease in hepatic bile flow is attenuated drastically (574).
A reduced number of gap junctions between hepatocytes occurs in liver cirrhosis and in chronic viral hepatitis (661), two pathological conditions likely to be associated with an inflammatory response. During liver inflammation, Cx26 and Cx32 are downregulated (119, 188), a process that can be mimicked by soluble factors released by activated Kupffer cells (188). The inflammatory response is a factor common to diverse pathologies; thus reduced gap junctional communication might contribute to the metabolic malfunctioning of the liver observed in various diseases. In addition, several studies have demonstrated the role of gap junction in cell growth and tumor promotion. In this respect, the incidence of liver tumors in Cx32-null mice is much higher than in wild-type animals (156, 573). The Cx32 deficiency enhances the growth of liver tumors irrespective of the genetic background of the mouse strain used (385).
C. Reproductive System
1. Gap junctions in the female reproductive system
Gap junctions interconnect cells in several different tissues of the female reproductive system. In the ovarian follicle, they are important for coordinating functions of granulosa or stromal cells and for permitting interactions of the developing oocyte with the surrounding follicular cells. In the myometrium, they allow the electrical synchronization of muscle cells to facilitate contractions in labor.
Ultrastructural studies have established the presence of gap junctions between rat follicular cells (6, 183). Since this observation, several connexins have been identified in ovarian cells from different mammalian species, including Cx26, Cx32, Cx30.3, Cx37, Cx40, Cx41, Cx43, Cx45, Cx57, and Cx60 (51, 192, 195, 240, 254, 368, 374, 421, 428, 541, 596, 672). Cx43 is very abundant and appears to be the primary connexin connecting the granulosa cells (51). Expression of Cx43 is evident in ovaries from sexually mature animals, but not from fetuses (374). A cycle of Cx43 expression has been detected during follicular growth; Cx43 abundance is elevated before ovulation and decreases when levels of luteinizing hormone are high and during follicular atresia (194, 650, 651). However, other investigators have detected gap junctions and Cx43 in the corpus luteum (275, 374). Thus ovarian Cx43 is developmentally and hormonally regulated. Two mechanisms may be involved in the inhibition of ovarian Cx43 gap junction function during the maturation of the rat oocyte: gating of gap junction channels due to increased Cx43 phosphorylation (192) and inhibition of Cx43 expression (193). Lawrence et al. (333) established the functional ability of gap junctions between granulosa cells to pass cAMP-dependent signals. Analyses of ovaries from Cx43 mutant or null mice support a critical role for Cx43 gap junctions between granulosa cells in the regulation of granulosa cell development. When these ovaries are studied in vitro or after grafting beneath the kidney capsule of adult females, folliculogenesis can reach only the initial steps (i.e., multilaminar follicles are absent in ovaries from Cx43-null mice but present in ovaries from wild-type mice) (4, 261). Similarly, maturation of bovine oocytes is partially impaired by adenoviral transfection of cumulusoocyte complexes with an antisense Cx43 cDNA (626).
Gap junctions also link the growing female gamete with its surrounding somatic cells in developing follicles (13). This heterologous intercellular communication allows the cumulus cells to maintain the meiotic arrest of the oocyte, since meiotic maturation resumes if intercellular communication is disrupted (183). There may be a reciprocal transfer of signals through gap junctions between somatic cells and the oocyte in which follicular cells provide signals for oocyte growth and the oocyte provides regulatory signals for folliculogenesis (61, 148, 601, 602). It is likely that cAMP is the maturation inhibitory signal that is passed through gap junctions from the granulosa cells to the oocyte (333, 636). Calcium signals are also generated in cumulus cells and transported toward the oocyte in response to ATP (635), but they may not be related to oocyte maturation. Cx37 is a critical component of these heterocellular interactions, since this protein is found between the developing oocyte and surrounding cumulus cells. Female Cx37-null mice are infertile and produce numerous inappropriate corpus lutea (541). Follicle development arrests at the type 4 preantral stage, and oocyte growth ceases at a diameter that is only 74% of control size; the oocytes arrest in a G2 state and cannot enter M phase (initiate meiotic maturation) unless treated with a phosphatase inhibitor (74).
In the uterus, gap junction structures are present connecting the smooth muscle cells of the myometrium and connecting the epithelial cells of the endometrium. Dramatic changes in the abundance of gap junctions and intercellular communication precede the onset of labor and likely facilitate the organization of uterine contraction during parturition (176). Cx43 is abundantly found between the myometrial cells (51) and may be the most important connexin for the regulation of uterine activity associated with labor. However, multiple connexins have been identified in these cells including Cx26, Cx40, and Cx45 (7, 433). Cx43 is colocalized with Cx40 and with Cx45 in myometrial smooth muscle (7, 278, 279). Cx26 has been identified in the epithelium of the implantation chamber of the pregnant, but not in nonpregnant or pseudo-pregnant rabbits (655). Cx26, Cx43, and Cx45 show different temporal and cell type-specific patterns of expression during pregnancy and postpartum (51, 441, 485, 656).
The expression of myometrial Cx43 and Cx26 is hormonally regulated during the onset of labor. Rather than the individual hormone levels, it appears that the increased ratio of estrogen to progesterone is the most important factor. At term, Cx43 mRNA increases (by transcriptional regulation as discussed above) and Cx43 protein trafficking is altered (201, 213, 487). However, the abundance of Cx43 in other organs is minimally affected (486).
In the human, rat, mouse, and hamster oviduct, Cx43 and Cx26 have been identified between epithelial cells, and Cx43 has been found between smooth muscle cells (216). The abundance of connexins correlated with sexual maturity, and the levels of Cx43 correlated with high levels of estrogens (216).
2. Gap junctions in the male reproductive system
Several connexin mRNAs have been found in seminiferous tubules by RT-PCR analysis (488). Immunohistochemical studies have demonstrated that Cx43 is the most prominent connexin in testes. Cx43 expression begins at early stages of mouse embryonic gonads and increases with time in Leydig and Sertoli cells (448, 450). A continuous increase in Cx43 expression is observed postnatally until adulthood, when Cx43 expression in Sertoli cells depends on the stage of the seminiferous epithelium (26, 58, 489). In rat testes, Cx33 colocalizes with Cx43 in Sertoli-Sertoli gap junction plaques (571). The expression of Cx33 is delayed with respect to that of Cx43 appearing at postnatal day 15; Cx33 accumulates at Sertoli cell interfaces until stage VII, the stage at which its expression decreases together with that of Cx43 (571). Consistent with these observations, in situ dye coupling has been detected between Leydig cells, between peritubular cells, between Sertoli cells, and between Sertoli and basal germ cells in rodent species (26, 489). Electrical coupling has been demonstrated between Leydig cells (615). Cx26 and Cx32 have not been found in Leydig cells (615) but are present in the apical regions of the seminiferous epithelia of mature rats (58, 489). Analysis of testes from neonatal Cx43-null mice demonstrate that Cx43 is required for germ line expansion, but not for steroidogenesis (261, 463, 494). In human testis, Cx43 is first expressed during puberty (560).
Connexin expression in the rat testis and epididymis is hormonally regulated. Human chorionic gonadotropin administration in vivo and in vitro diminishes Cx43 mRNA and protein in rat Leydig cells (673). Cx43 is differentially expressed in an androgen-dependent manner by different cells of the epididymis (107). Also, retinoid X receptor (RXR)-deficient mice exhibit abnormal spermatogenesis due to altered Sertoli cell function and present decreased levels of Cx43 transcripts (26). This suggests that retinoids, through the RXR receptors, could be involved in the control of Cx43 gene expression in Sertoli cells. As described above, the expression of Cx43 can occur under the control of c-jun transcriptional factors (460). It has also been observed that jun-d-null mice are viable but infertile; they present impaired spermatogenesis that correlates with a drastic reduction or abolishment of Cx43 expression (25). In agreement with these findings, immunohistochemical studies have shown a direct correlation between severe spermatogenic impairment in men and loss of Cx43 immunoreactivity in seminiferous tubules (560). Thus gap junctions in testicular cells may play a role in gonad development and spermatogenesis, and their contribution may be critical to male fertility.
D. Immune System
Morphological and functional demonstration of gap junctions and connexin identification in cells of the immune system (published between 1972 and 1999) has been reviewed previously (11, 502, 503). This section summarizes the latest reports with particular emphasis on conditions that control the expression of connexins in different cells and their possible functional roles in particular steps of the immune response.
With the use of in vivo and in vitro approaches, Cx43 and gap junctional communication have been identified in the bone marrow between stromal cells and between stromal and hematopoietic cells (295). It has been hypothesized that gap junctions participate in hematopoiesis, and therefore, they may also play a role in leukemia (387, 497). However, Cx43-mediated communication between stromal and hematopoietic stem cells may not be an absolute requirement (496).
Gap junctions may provide a direct signaling pathway for transmigration, angiogenic, and metastatic processes. Recently, it has been demonstrated that HTLV-1-transformed lymphocytes (which are related to T-cell leukemia-lymphoma) form functional gap junctions with endothelial cells in vitro (145). Nevertheless, inhibition of gap junctions formed between human lymphocytes and endothelial cells does not abolish lymphocyte transmigration through a human umbilical vein endothelial cell monolayer (434). But, these cells do not form a tight paracellular seal that would offer resistance for cell transmigration. Yet, gap junction blockers (octanol or 18-glycyrrhetinic acid) reduce by 60% the number of human monocytes that transmigrate across a blood-brain barrier model (151).
In primary lymph nodes, functional coupling between thymocytes as well as between thymocytes and thymic epithelial cells has been demonstrated (10, 75). These cell junctions are at least in part composed of Cx43 (10). In secondary lymph organs, Cx43 has also been detected in human follicular dendritic cells (294), and Cx40 and Cx43 are expressed in human tonsil-derived T and B lymphocytes (436). Moreover, dye coupling between human cultured dendritic cells and B lymphocytes (296) and Cx43 immunoreactivity between halogeneic mouse Langerhans cells and T lymphocytes in culture (510) have been observed, suggesting the possible role of gap junctions in antigen presentation and/or lymphocyte proliferation. Moreover, synthetic peptides with sequence corresponding to a region of the extracellular loop 1 of connexins, but not unrelated peptides, reduce the proliferation response of concanavalin A-stimulated lymphocytes (510). In addition, the inhibition of B-lymphocyte gap junctions with connexin-mimetic peptides or 18-glycyrrhetinic acid drastically reduces the secretion of immunoglobulins and the expression of interleukin-10 mRNA, suggesting that intercellular communication mediated through gap junctions favors these functions (435).
Epithelial and endothelial cell barriers, members of the immune system, express connexins and form homocellular gap junctions (329, 346, 347, 532, 608) and could form heterocellular gap junctions with migratory cells of the immune system (204, 244, 369). Nevertheless, circulating nonactivated leukocytes, including monocytes, polymorphonuclear cells, and lymphocytes, do not express, at least, Cx32, Cx40, or Cx43 (11, 57, 244, 465). Consistent with the lack of connexin expression, gap junctional communication has not been detected between freshly isolated monocytes (12, 465), lymphocytes (510), or polymorphonuclear cells (57). However, dye coupling and Cx40 and Cx43 have been detected in T and/or B lymphocytes freshly isolated from human blood (436), raising controversy about connexin expression in these cells. Species differences or isolation procedures, some of which are known to induce cell activation, might explain these differences.
Mediators of inflammation induce opposite effects on gap junction expression in leukocytes and endothelial and parenchymal cells. In vitro treatment with tumor necrosis factor (TNF)- plus interferon (IFN)- induces expression of Cx43 in human monocytes (151) and rodent microglia (152). TNF- plus other yet unidentified factor(s) released from activated endothelial cells induce Cx40 and Cx43 in human polymorphonuclear cells (57). Moreover, cerebral and hepatic fixed macrophages, microglia, and Kupffer cells express at least Cx43 at inflammatory foci or after treatment with proinflammatory agents (152, 188, 305, 465). Moreover, expression of Cx43 and Cx32 by murine bone marrow cultured mast cells and the growth factor-independent murine mast cell line C57 has been documented (624). Nevertheless, the possible functional role of these connexins remains unknown.
In agreement with the requirement for an increase in intracellular calcium concentration for activation of cells of the immune system, it has been found that a calcium ionophore induces the expression of Cx43 in cultured rat microglia (371). The increased levels of at least Cx43 by monocytes/macrophages and microglia occur with an increase in the levels of Cx43 mRNA (371, 465), suggesting activation of Cx43 mRNA transcription. In contrast to the findings in myeloid and lymphoid cells, the expression of connexins in endothelial (234, 609) and epithelial cells such as hepatocytes (188) is reduced by cytokines.
Further studies may help elucidate the mechanisms by which soluble factors control the expression of connexins and thus the formation of homocellular and heterocellular gap junctions between cells of the immune system. The possible functional role of hemichannels in the immune response has not been reported but deserves to be studied.
|
VIII. GAP JUNCTION CHANNELS AND DISEASE
|
|---|
Gap junctions have been implicated in many cellular processes, but the lack of specific blockers has limited the study of their physiological role in vivo. New information on the physiological roles of vertebrate connexins has emerged from genetic studies. Mutations in connexin genes correlate with a variety of human diseases, including demyelinating neuropathies, deafness, epidermal diseases, and lens cataracts. Genetic modification of connexins in mice has provided valuable information on connexin function and significance of their diversity. In addition, targeted ablation of mouse connexin genes has revealed basic insights into the functional diversity of the connexin gene family.
A. Connexin Mutations Related to Human Peripheral Neuropathy
The first disease demonstrated to rely on a connexin mutation was the X-linked form of Charcot-Marie-Tooth disease (CMTX), a demyelinating syndrome associated with Schwann cell defects that produce progressive degeneration of peripheral nerves. Cx32 protein is located in the paranodal loops and Schmidt-Lantermann incisures in myelinating Schwann cells (516) (Fig. 7). Cx32 channels form intracellular connections between adjacent loops of noncompacted myelin in one cell (reflexive gap junctions). These autocellular junctions provide a radial diffusion pathway between the Schwann cell nucleus and adaxonal membranes that is 300 times shorter than the corresponding distance along the circumferential membrane (20). Communication through these junctions likely provides metabolic and nutritional support for these bits of cytoplasm. More than 160 CMTX-associated mutations in the Cx32 gene have been identified since 1993 (3, 37). Cx32 mutants identified in patients with CMTX include missense, frameshift, deletion, and nonsense mutations. Some of these mutations lead to complete loss of function with no expression of functional channels (68, 79, 124, 425, 429, 480, 671). Mutations within the noncoding region of the Cx32 gene have also been reported; some lead to a reduced level or lack of Cx32 mRNA (239, 403), while another affects mRNA translation (236). It has been described that SOX-10, a transcriptional factor responsible for the expression of Cx32 and other myelin proteins, fails to activate the mutated Cx32 promoter (56). Other mutant forms of Cx32 fail to leave the Golgi apparatus, leading to an abnormal accumulation in the cytoplasm (124). Other Cx32 mutants form functional channels, but their voltage-gating properties are abnormal, thus impeding normal intercellular communication (79, 124, 425, 429, 480). Yet, one mutation restricts the channel pore diameter, possibly affecting the type of signaling molecules that cross between adjacent loops of myelin (425). A mutant Cx32 (Ser85Cys) that forms functional gap junction channels shows increased hemichannel currents with respect to wild-type Cx32, a phenomenon that has been ascribed to increased open probability (2). In this case, the loss of ionic gradients and small metabolites and the increased influx of Ca2+ through putative Cx32 hemichannels might damage Schwann cells leading to the diseased state (2). Rat Schwann cells also express Cx29, but its pattern of distribution differs from that of Cx32 (8). In addition, low levels of Cx43 have been detected in peripheral myelin (364). Despite the expression of other connexin types in myelinating cells, dysfunctions due to Cx32 mutations predominate. Neuropathies due to mutations in other connexins may remain unrecognized if they are lethal due to abnormalities caused in the other tissues where they are expressed.
View larger version (80K):
[in this window]
[in a new window]
|
FIG. 7. Myelin structure and location of connexin32 gap junction channels. The structure of peripheral myelin is shown at the top. The cytoplasm of a Schwann cell wraps several times around a nerve fiber (axon) forming the myelin sheath; several Schwann cells form myelin around the same axon, and the space not enwrapped by myelin is called the node of Ranvier. An enlarged view of myelin is diagramed at the bottom (left, longitudinal section; right, cross section); gap junction channels (=) formed between adjacent plasma membranes of the enwrapping layers that originate from the same Schwann cell connect the incisures of Schmidt-Lanterman. [Modified from White and Paul (647).]
|
|
One intriguing aspect of the observed phenotype in individuals carrying Cx32 mutants is that functional abnormalities are restricted to the peripheral nervous system. It is well documented that Cx32 is a major component of gap junctions in the liver and many other cell types in different organs such as neurons, follicular cells, pancreatic acinar cells, lacrimal acinar cells, and oligodendrocytes (67). Deletion of Cx32 in mice leads not only to neuropathy but also to reduced nerve stimulation- and hormone-induced hepatic glucose release (409, 563) and reduced bile flow (574) following sympathetic stimulation, reduced fluid secretion by lacrimal glands (628), and an increased amylase secretion from the exocrine pancreas (81). The absence of Cx32 results in a distinct pattern of gene dysregulation in Schwann cells (415). However, Cx32-null mice show a normal organization of central nervous system myelin (517). This might result in part from the distribution of Cx32 in the central nervous system (i.e., mainly in oligodendrocyte cell bodies and proximal processes, but not in paranodes) and from myelin differences between the parasympathetic nervous system and the CNS (i.e., oligodendrocytes in central nervous system wrap around axons less times, so the width of the myelin sheath is smaller). Alternatively, the functional role of Cx32 might be performed by other connexins (e.g., Cx45 in the rat) that are coexpressed with Cx32 in oligodendrocytes (301). In fact, some human Cx32 mutations produce subclinical evidence of CNS dysfunction. These mutations, at least in vitro, do not produce a trans-dominant negative effect over Cx45 communication-competent HeLa cells (286), which would be in agreement with the possibility of Cx45 compensating for the lack of Cx32.
B. Connexin Mutations Related to Deafness and Skin Disorders
In mammals, Cx26 and Cx30 are expressed by nonsensory epithelial cells among which hair cells are dispersed, and by connective tissue cells at more distal locations from hair cells (271, 277, 332). Connective tissue cells, fibrocytes, and spiral ligaments and spiral limbs, express at least Cx31 (658).
Congenital deafness is a very frequent disorder occurring in 1 in 1,000 live births. Autosomal recessive (DFNB1) and autosomal dominant (DFNA3) forms of genetic deafness have been associated with more than 50 mutations within the coding region of Cx26 [reviewed by Kelsell et al. (273), White (641), and Lefebvre and van de Water (335)]. (For an extensive list, visit the Connexins and Deafness Homepage: http//www.iro.es/cx26deaf.htlm.) Recessive mutations frequently result in a severely truncated Cx26 that is unlikely to retain any channel activity (76, 121, 150, 272, 677). In contrast, dominant mutations encode full-length products containing nonconservative amino acid substitutions (80, 274, 392, 484, 646). The Cx26Met34Thr mutant (274) was previously described as a dominant negative, and in vitro data demonstrated that the wild-type Cx26 could not form functional channels when coexpressed with the mutant Cx26 (646). However, some controversy about the interpretation of these results has been raised by the frequent finding that individuals heterozygous for the Cx26Met34Thr mutation have normal hearing (121, 272, 522). Although the precise role of Cx26 in the etiology of nonsyndromic deafness remains unknown, it has been proposed that junctional communication influences the ionic environment of the inner ear sensory epithelia. Thus gap junctions would be involved in recirculation of K+ from the interstitial space through the syncytium formed by cochlear supporting cells (166, 257, 424), similar to the spatial buffering of K+ by astrocytes in the CNS. The use of Cx26-null mice to investigate the role of Cx26 in the inner ear was not possible because of its embryonic lethality (171). The role of Cx26 in cochlear function and cell survival has been demonstrated recently after tissue-specific ablation of Cx26 (96). In ear-specific homozygous Cx26(OtogCre)-null mice, the inner ear develops normally, but after the onset of hearing, cell death is evident extending to the cochlear epithelial network and sensory hair cells. These mice have hearing impairment, but no vestibular dysfunction. Because cell death initially affects the supporting cells of the inner hair cell, it has been suggested that the inner hair cell response to sound stimulation could be triggering the observed cell death.
Some Cx26 mutations are also linked to skin disorders, such as palmoplantar keratoderma (164, 210, 273, 484). Mutations of Cx31 and Cx30 are also associated with both deafness and epidermal disorders (197, 319, 483, 657). Recently, it has been shown that a new Cx26 mutation (Cx26
E42) associated with auditory and skin disorders inhibited the intercellular conductance in paired Xenopus oocytes when coexpressed with wild-type Cx43 (498). These results suggest that dominant negative Cx26 mutations can produce a disease phenotype in the skin if they interfere with the function of wild-type Cx43 also found in the same cells.
C. Connexin Mutations Related to Human Cataracts
The eye lens is an avascular organ formed by an epithelial cell layer covering the anterior surface of the organ and a large mass of fiber cells which form the bulk of the organ. The fiber cells contain few organelles, yet they must survive for the life span of the organism. Because there is no blood supply to the organ, most cells receive their nutrients by diffusion from the aqueous humor in which the organ is suspended. Thus it has been hypothesized that these cells are maintained by fluid, ion, nutrient, and metabolite movement through an extensive network of gap junctions (189). Mathias et al. (373) have shown the presence of circulating currents in the lens that would serve to drive water and ions between lens cells. Several connexins have been identified in the lens. While epithelial cells express Cx43 (51) and Cx50 (also known as MP70) (111), fiber cells express Cx46 and Cx50 in mammals (285, 444, 642) or Cx56 and Cx45.6 in chickens (40, 250, 500) (Fig. 8). The existence of gap junctions connecting epithelial and fiber cells has been demonstrated with the dye coupling technique in the normal adult rat lens (473). Accordingly, Cx43/Cx50 double knock-out mice do not show dye coupling between epithelial and fiber lens cells (649).
View larger version (40K):
[in this window]
[in a new window]
|
FIG. 8. Diagram of the eye lens. The regions of expression of different connexins are indicated. The symbols "<" or ">" indicate the relative levels of expression of connexins in the adult mammalian lens. Cx46 and its chicken ortholog, Cx56, have also been reported to be present in embryonic lens epithelium (249, 493). In this region, levels of Cx46/Cx56 are much lower than those of Cx43. In mouse embryonic lens cells, expression levels of Cx50 are higher than those of Cx46. In the chicken embryo, Cx56 immunoreactivity in epithelial cells seems to be more abundant than that of Cx45.6, the chicken ortholog of Cx50.
|
|
Point mutations of the Cx50 and Cx46 genes have been identified in patients with inherited zonular pulverulent cataract (361, 536). When expressed in Xenopus oocytes, Cx46 and Cx50 mutations do not form functional channels (437, 438). Similarly, the No2 mouse, which develops congenital cataracts, carries a mutation within the Cx50 sequence (559). Both Cx50-null and No2 mice develop cataracts and exhibit microphtalmia (559, 646), suggesting involvement of Cx50 in lens development. In Xenopus oocytes, this mouse Cx50 mutant does not form functional gap junctions (659). Recently published data indicate that Cx43 and Cx50 are not required for prenatal lens development but both are required for organ homeostasis (649). Cx46 knock-out mice exhibit normal lens growth and development, but progressive cataractogenesis is evident 3 wk after birth (187). Because microphtalmia is present in Cx50- but not in Cx46-null mice, it has been proposed that an early postnatal growth signal may propagate through Cx50 but not Cx46 gap junction channels (647). Because Cx46- and Cx50-null mice develop lens opacities, it is likely that channels made of a single connexin type cannot compensate for the functional role of gap junctions when both connexin types are expressed. It remains to be determined whether heteromeric gap junction channels made of Cx46 and Cx50 have different permeability than homomeric Cx46 or Cx50 channels.
|
IX. PHYLOGENY
|
|---|
While functionally equivalent intercellular channels and gap junction structures are found in many lower multicellular organisms, no connexin genes have been identified in any invertebrate organism (456). Invertebrate gap junctions are made of protein subunits termed innexins which lack sequence homology with connexins, but do exhibit similar membrane topology (175, 554). A phylogenetic study on connexins in working myocardium of chordates has demonstrated positive immunoreactivity in the myocardium of Ascidian, a primitive chordate (30). In turn, innexin family members have been detected in a variety of animal phyla, including Platyhelminthes, Nematoda, Arthropoda, Mollusca, and Chordata. The finding of two human innexin homologs would imply that intercellular communication might not only depend on the distribution and activity of connexin-containing gap junctions, but also on that of innexins (439).
With the use of the Xenopus oocyte expression system, some of these proteins have been shown to form functional gap junctions (324, 458) with pH and voltage gating somewhat similar to those of vertebrate gap junctions (324). In the Caenorhabditis elegans genome, 25 innexins have been identified. Similar to vertebrates, C. elegans tissues and single cells express more than one innexin; some innexins are expressed in many tissues while others show a more restricted pattern of expression (555). In the fruitfly Drosophila, specific innexins play a relevant role in the development of the gastrointestinal tract (27) and the nervous system (105). Innexins are not interchangeable in the development of neural function in the Drosophila visual system (106). Cloning of an innexin expressed in annelida has been recently described (467). Reviews on gap junction in invertebrates have been reported (457, 555). The possibility of generating specific mutants both in Drosophila melanogaster and C. elegans will be an important approach to learn about the functional significance of gap junctions in different tissues of invertebrates.
|
X. PERSPECTIVES
|
|---|
The introduction of new techniques and approaches has increased our knowledge of gap junctions at several levels. It is anticipated that in the near future they will lead to an understanding of the multiplicity of different connexins and which of their functions are unique or common to other members of the family. Connexin mutants will allow identification of the molecular determinants of gating (e.g., voltage, pH, Ca2+, other chemicals), permeability (to ions and molecules of different size), oligomerization with other connexins, docking of hemichannels, and interactions with other proteins. Structural studies of gap junctions at even higher resolution may elucidate the molecular events leading to closing and opening of gap junction channels or hemichannels. The use of cell type-specific promoters to knock out connexins in different cells or at different times during development will clarify their roles. Conceivably, molecules that modify channel/hemichannel behavior (including channel blockers) in a connexin type-specific manner will be identified, allowing their use for pharmacological (and maybe therapeutic) purposes. Genetic studies may uncover additional diseases associated with other connexins. Finally, the possible physiological role(s) of hemichannels may be determined.
|
NOTE ADDED IN PROOF
|
|---|
Söhl et al. (544a) have suggested that the human ortholog of mouse Cx29 corresponds to Cx30.2, while Altevogt et al. (8) have suggested it corresponds to Cx31.3 due to the presence of an intron within the carboxy terminus.
|
ACKNOWLEDGMENTS
|
|---|
We thank Helmuth Sánchez for the artwork of Figure 3.
This work was supported by Fondo Nacional de Investigación Científica y Tecnológica Grants 8990008 and 1030945 (to J. C. Sáez), National Institutes of Health Grants HL-45466 and EY-08368 (to E. C. Beyer), and a postdoctoral fellowship award from the American Heart Association (to A. D. Martínez).
Address for reprint requests and other correspondence: J. C. Sáez, Departamento de Ciencias Fisiológicas, Pontificia Universidad Católica de Chile, Alameda 340, Santiago, Chile (E-mail: jsaez{at}genes.bio.puc.cl).
|
REFERENCES
|
|---|
- Abe Y and Tomita T. Cable properties of smooth muscle. J Physiol 196: 87-100, 1968.[Abstract/Free Full Text]
- Abrams CK, Bennett MV, Verselis VK, and Bargiello TA. Voltage opens unopposed gap junction hemichannels formed by a connexin 32 mutant associated with X-linked Charcot-Marie-Tooth disease. Proc Natl Acad Sci USA 99: 3980-3984, 2002.[Abstract/Free Full Text]
- Abrams CK, Oh S, Ri Y, and Bargiello TA. Mutations in connexin32: the molecular and biophysical bases for the X-linked form of Charcot-Marie-Tooth disease. Brain Res Rev 32: 203-214, 2000.[Medline]
- Ackert CL, Gittens JE, O'Brien MJ, Eppig JJ, and Kidder GM. Intercellular communication via connexin43 gap junctions is required for ovarian folliculogenesis in the mouse. Dev Biol 233: 258-270, 2001.[Web of Science][Medline]
- Ahmad S and Evans WH. Post-translational integration and oligomerization of connexin 26 in plasma membranes and evidence of formation of membrane pores: implications for the assembly of gap junctions. Biochem J 365: 693-699, 2002.[Web of Science][Medline]
- Albertini DF and Anderson E. The appearance and structure of intercellular connections during the ontogeny of the rabbit ovarian follicle with particular reference to gap junctions. J Cell Biol 63: 234-250, 1974.[Abstract/Free Full Text]
- Albrecht JL, Atal NS, Tadros PN, Orsino A, Lye SJ, Sadovsky Y, and Beyer EC. Rat uterine myometrium contains the gap junction protein connexin45, which has a differing temporal expression pattern from connexin43. Am J Obstet Gynecol 175: 853-858, 1996.[Web of Science][Medline]
- Altevogt BM, Kleopa KA, Postma FR, Scherer SS, and Paul DL. Connexin29 is uniquely distributed within myelinating glial cells of the central and peripheral nervous systems. J Neurosci 22: 6458-6470, 2002.[Abstract/Free Full Text]
- Al-Ubaidi MR, White TW, Ripps H, Poras I, Avner P, Gomes D, and Bruzzone R. Functional properties, developmental regulation, and chromosomal localization of murine connexin36, a gap-junctional protein expressed preferentially in retina and brain. J Neurosci Res 59: 813-826, 2000.[Web of Science][Medline]
- Alves LA, Campos de Carvalho AC, Lima EOC, Rocha e Souza CM, Dardene M, Spray DC, and Savino W. Functional gap junctions in thymic epithelial cells are formed by connexin 43. Eur J Immunol 25: 431-437, 1995.[Web of Science][Medline]
- Alves LA, Campos de Carvalho AC, and Savino W. Gap junctions: a novel route for direct cell-cell communication in the immune system? Immunol Today 19: 269-275, 1998.[Web of Science][Medline]
- Alves LA, Coutinho-Silva R, Persechini PM, Spray DC, Savino W, and Campos de Carvalho AC. Are there functional gap junctions or junctional hemichannels in macrophages? Blood 88: 328-334, 1996.[Abstract/Free Full Text]
- Anderson E and Albertini DF. Gap junctions between the oocyte and companion follicle cells in the mammalian ovary. J Cell Biol 71: 680-686, 1976.[Abstract/Free Full Text]
- Anumonwo JM, Wang HZ, Trabka-Janik E, Dunham B, Veenstra RD, Delmar M, and Jalife J. Gap junctional channels in adult mammalian sinus nodal cells: immunolocalization and electrophysiology. Circ Res 71: 229-239, 1992.[Abstract/Free Full Text]
- Arcuino G, Lin JH, Takano T, Liu C, Jiang L, Gao Q, Kan J, and Nedergaard M. Intercellular calcium signaling mediated by point-source burst release of ATP. Proc Natl Acad Sci USA 99: 9840-9845, 2002.[Abstract/Free Full Text]
- Asamoto M, Oyamada M, el-Aoumari A, Gros D, and Yamasaki H. Molecular mechanisms of TPA-mediated inhibition of gap-junctional intercellular communication: evidence for action on the assembly or function but not the expression of connexin 43 in rat liver epithelial cells. Mol Carcinog 4: 322-327, 1991.[Web of Science][Medline]
- Bai S, Spray DC, and Burk RD. Identification of proximal and distal regulatory elements of the rat connexin32 gene. Biochim Biophys Acta 1216: 197-204, 1993.[Medline]
- Bailey J, Phillips RJ, Pollard AJ, Gilmore K, Robson SC, and Europe-Finner GN. Characterization and functional analysis of cAMP response element modulator protein and activating transcription factor 2 (ATF2) isoforms in the human myometrium during pregnancy and labor: identification of a novel ATF2 species with potent transactivation properties. J Clin Endocrinol Metab 87: 1717-1728, 2002.[Abstract/Free Full Text]
- Baldridge D, Lecanda F, Shin CS, Stains J, and Civitelli R. Sequence and structure of the mouse connexin45 gene. Biosci Rep 21: 683-689, 2001.[Web of Science][Medline]
- Balice-Gordon RJ, Bone LJ, and Scherer SS. Functional gap junctions in the Schwann cell myelin sheath. J Cell Biol 142: 1095-1104, 1998.[Abstract/Free Full Text]
- Barr L, Berger W, and Dewey MM. Electrical transmission at the nexus between smooth muscle cells. J Gen Physiol 51: 347-368, 1968.[Abstract/Free Full Text]
- Barr L, Dewey MM, and Berger W. Propagation of action potentials and the structure of the nexus in the cardiac muscle. J Gen Physiol 48: 797-823, 1965.[Abstract/Free Full Text]
- Barrio LC, Suchyna T, Bargiello TA, Xu LX, Roginski RS, Bennett MV, and Nicholson BJ. Gap junctions formed by connexins 26 and 32 alone and in combination are differently affected by applied voltage. Proc Natl Acad Sci USA 88: 8410-8414, 1991.[Abstract/Free Full Text]
- Bastide B, Neyses L, Ganten D, Paul M, Willecke K, and Traub O. Gap junction protein connexin40 is preferentially expressed in vascular endothelium and conductive bundles of rat myocardium and is increased under hypertensive conditions. Circ Res 73: 1138-1149, 1993.[Abstract/Free Full Text]
- Batias C, Defamie N, Lablack A, Thepot D, Fenichel P, Segretain D, and Pointis G. Modified expression of testicular gap-junction connexin 43 during normal spermatogenic cycle and in altered spermatogenesis. Cell Tissue Res 298: 113-121, 1999.[Web of Science][Medline]
- Batias C, Siffroi JP, Fenichel P, Pointis G, and Segretain D. Connexin43 gene expression and regulation in the rodent seminiferous epithelium. J Histochem Cytochem 48: 793-805, 2000.[Abstract/Free Full Text]
- Bauer R, Lehmann C, and Hoch M. Gastrointestinal development in the Drosophila embryo requires the activity of innexin gap junction channel proteins. Cell Commun Adhes 8: 307-310, 2001.[Medline]
- Beahm DL and Hall JE. Hemichannel and junctional properties of connexin 50. Biophys J 82: 2016-2031, 2002.[Web of Science][Medline]
- Beardslee MA, Laing JG, Beyer EC, and Saffitz JE. Rapid turnover of connexin43 in the adult rat heart. Circ Res 83: 629-635, 1998.[Abstract/Free Full Text]
- Becker DL, Cook JE, Davies CS, Evans WH, and Gourdie RG. Expression of major gap junction connexin types in the working myocardium of eight chordates. Cell Biol Int 22: 527-543, 1998.[Web of Science][Medline]
- Belzer V, Kobilo T, Rich A, and Hanani M. Intercellular coupling among interstitial cells of Cajal in the guinea pig small intestine. Cell Tissue Res 307: 15-21, 2002.[Web of Science][Medline]
- Belluardo N, Trovato-Salinaro A, Mudo G, Hurd YL, and Condorelli DF. Structure, chromosomal localization, and brain expression of human Cx36 gene. J Neurosci Res 57: 740-752, 1999.[Web of Science][Medline]
- Benedetti EL and Emmelot P. Electron microscopic observations on negatively stained plasma membranes isolated from rat liver. J Cell Biol 26: 299-305, 1965.[Free Full Text]
- Bennett MV. Physiology of electrotonic junctions. Ann NY Acad Sci 137: 509-539, 1966.[Web of Science][Medline]
- Bennett MV, Barrio LC, Bargiello TA, Spray DC, Hertzberg E, and Sáez JC. Gap junctions: new tools, new answers, new questions. Neuron 6: 305-320, 1991.[Web of Science][Medline]
- Bennett MV and Verselis VK. Biophysics of gap junctions. Semin Cell Biol 3: 29-47, 1992.[Medline]
- Bergoffen J, Scherer SS, Wang S, Scott MO, Bone LJ, Paul DL, Chen K, Lensch MW, Chance PF, and Fischbek KH. Connexin mutations in X-linked Charcot-Marie-Tooth disease. Science 262: 2039-2042, 1993.[Abstract/Free Full Text]
- Berthoud VM, Bassnett S, and Beyer EC. Cultured chicken embryo lens cells resemble differentiating fiber cells in vivo and contain two kinetic pools of connexin56. Exp Eye Res 68: 475-484, 1999.[Web of Science][Medline]
- Berthoud VM, Beyer EC, Kurata WE, Lau AF, and Lampe PD. The gap-junction protein connexin 56 is phosphorylated in the intracellular loop and the carboxy-terminal region. Eur J Biochem 244: 89-97, 1997.[Web of Science][Medline]
- Berthoud VM, Cook AJ, and Beyer EC. Characterization of the gap junction protein connexin56 in the chicken lens by immunofluorescence and immunoblotting. Invest Ophthalmol Vis Sci 35: 4109-4117, 1994.[Abstract/Free Full Text]
- Berthoud VM, Iwanij V, Garcia AM, and Sáez JC. Connexins and glucagon receptors during development of rat hepatic acinus. Am J Physiol Gastrointest Liver Physiol 263: G650-G658, 1992.[Abstract/Free Full Text]
- Berthoud VM, Ledbetter ML, Hertzberg EL, and Sáez JC. Connexin43 in MDCK cells: regulation by a tumor-promoting phorbol ester and Ca2+. Eur J Cell Biol 57: 40-50, 1992.[Web of Science][Medline]
- Berthoud VM, Montegna EA, Atal N, Aithal NH, Brink PR, and Beyer EC. Heteromeric connexons formed by the lens connexins, connexin43 and connexin56. Eur J Cell Biol 80: 11-19, 2001.[Web of Science][Medline]
- Berthoud VM, Westphale EM, Grigoryeva A, and Beyer EC. PKC isoenzymes in the chicken lens and TPA-induced effects on intercellular communication. Invest Ophthalmol Vis Sci 41: 850-858, 2000.[Abstract/Free Full Text]
- Bertram JS. Carotenoids and gene regulation. Nutr Rev 57: 182-191, 1999.[Web of Science][Medline]
- Bertuzzi F, Davalli AM, Nano R, Socci C, Codazzi F, Fesce R, Di Carlo V, Pozza G, and Grohovaz F. Mechanisms of coordination of Ca2+ signals in pancreatic islet cells. Diabetes 48: 1971-1978, 1999.[Abstract]
- Bevilacqua LM, Simon AM, Maguire CT, Gehrmann J, Wakimoto H, Paul DL, and Berul CI. A targeted disruption in connexin40 leads to distinct atrioventricular conduction defects. J Interv Card Electrophysiol 4: 459-567, 2000.[Web of Science][Medline]
- Beyer EC. Gap junctions. Int Rev Cytol 137C: 1-37, 1993.[Medline]
- Beyer EC and Berthoud VM. Gap junction synthesis and degradation as therapeutic targets. Curr Drug Targets 3: 409-416, 2002.[Web of Science][Medline]
- Beyer EC, Davis LM, Saffitz JE, and Veenstra RD. Cardiac intercellular communication: consequences of connexin distribution and diversity. Braz J Med Biol Res 28: 415-425, 1995.[Web of Science][Medline]
- Beyer EC, Kistler J, Paul DL, and Goodenough DA. Antisera directed against connexin43 peptides react with a 43-kD protein localized to gap junctions in myocardium and other tissues. J Cell Biol 108: 595-605, 1989.[Abstract/Free Full Text]
- Beyer EC, Paul DL, and Goodenough DA. Connexin43: a protein from rat heart homologous to a gap junction protein from liver. J Cell Biol 105: 2621-2629, 1987.[Abstract/Free Full Text]
- Beyer EC, Paul DL, and Goodenough DA. Connexin family of gap junction proteins. J Membr Biol 116: 187-194, 1990.[Web of Science][Medline]
- Blackburn JP, Peters NS, Yeh HI, Rother YS, Green CR, and Severs NJ. Upregulation of connexin-43 gap junctions during early stages of human coronary atherosclerosis. Arterioscler Thromb Vasc Biol 15: 1219-1228, 1995.[Abstract/Free Full Text]
- Bondarev I, Vine A, and Bertram JS. Cloning and functional expression of a novel human connexin-25 gene. Cell Commun Adhes 8: 167-171, 2001.[Medline]
- Bondurand N, Girard M, Pingault V, Lemort N, Dubourg O, and Goosens M. Human connexin 32, a gap junction protein altered in the X-linked form of Charcot-Marie-Tooth disease, is directly regulated by the transcription factor SOX10. Hum Mol Genet 10: 2783-2795, 2001.[Abstract/Free Full Text]
- Brañes MC, Contreras JE, and Sáez JC. Activation of human polymorphonuclear cells induces formation of functional gap junctions and expression of connexins. Med Sci Monit 8: BR313-BR323, 2002.[Medline]
- Bravo-Moreno JF, Díaz-Sánchez V, Montoya-Flores JG, Lamoyi E, Sáez JC, and Pérez-Armendáriz EM. Expression of connexin43 in mouse Leydig, Sertoli, and germinal cells at different stages of postnatal development. Anat Rec 264: 13-24, 2001.[Medline]
- Brink PR. Gap junction channel gating and permselectivity: their roles in co-ordinated tissue function. Clin Exp Pharmacol Physiol 23: 1041-1046, 1996.[Web of Science][Medline]
- Brink PR. Gap junctions in vascular smooth muscle. Acta Physiol Scand 164: 349-356, 1998.[Web of Science][Medline]
- Brower PT and Schultz RM. Intercellular communication between granulosa cells and mouse oocytes: existence and possible nutritional role during oocyte growth. Dev Biol 90: 144-153, 1982.[Web of Science][Medline]
- Bruneau BG, Nemer G, Schmitt JP, Charron F, Robitaille L, Caron S, Conner DA, Gessler M, Nemer M, Seidman CE, and Seidman JG. A murine model of Holt-Oram syndrome defines roles of the T-box transcription factor Tbx5 in cardiogenesis and disease. Cell 106: 709-721, 2001.[Web of Science][Medline]
- Bruzzone S, Franco L, Guida L, Zocchi E, Contini P, Bisso A, Usai C, and De Flora A. A self-restricted CD38-connexin 43 cross-talk affects NAD+ and cyclic ADP-ribose metabolism and regulates intracellular calcium in 3T3 fibroblasts. J Biol Chem 276: 48300-48308, 2001.[Abstract/Free Full Text]
- Bruzzone S, Guida L, Zocchi E, Franco L, and De Flora A. Connexin 43 hemi channels mediate Ca2+-regulated transmembrane NAD+ fluxes in intact cells. FASEB J 15: 10-12, 2001.[Free Full Text]
- Bruzzone R and Ressot C. Connexins, gap junctions and cell-cell signalling in the nervous system. Eur J Neurosci 9: 1-6, 1997.[Medline]
- Bruzzone R, White TW, and Goodenough DA. The cellular Internet: on-line with connexins. Bioessays 18: 709-718, 1996.[Web of Science][Medline]
- Bruzzone R, White TW, and Paul DL. Connections with connexins: the molecular basis of direct intercellular signaling. Eur J Biochem 238: 1-27, 1996.[Web of Science][Medline]
- Bruzzone R, White TW, Scherer SS, Fischbeck KH, and Paul DL. Null mutations of connexin32 in patients with X-linked Charcot-Marie Tooth disease. Neuron 13: 1253-1260, 1994.[Web of Science][Medline]
- Burt JM and Spray DC. Ionotropic agents modulate gap junctional conductance between cardiac myocytes. Am J Physiol Heart Circ Physiol 254: H1206-H1210, 1988.[Abstract/Free Full Text]
- Calabrese A, Guldenagel M, Charollais A, Mas C, Caton D, Bauquis J, Serre-Beinier V, Caille D, Söhl G, Teubner B, Le Gurun S, Trovato-Salinaro A, Condorelli DF, Haefliger JA, Willecke K, and Meda P. Cx36 and the function of endocrine pancreas. Cell Commun Adhes 8: 387-391, 2001.[Web of Science][Medline]
- Campos de Carvalho AC, Masuda MO, Tanowitz HB, Wittner M, Goldenberg RC, and Spray DC. Conduction defects and arrhythmias in Chagas' disease: possible role of gap junctions and humoral mechanisms. J Cardiovasc Electrophysiol 5: 686-698, 1994.[Medline]
- Campos-Toimil M, Edwardson JM, and Thomas P. Acetylcholine-induced zymogen granule exocytosis: comparison between acini and single pancreatic acinar cells. Pancreas 24: 179-183, 2002.[Web of Science][Medline]
- Cao F, Eckert R, Elfgang C, Nitsche JM, Snyder SA, Hülser DF, Willecke K, and Nicholson BJ. A quantitative analysis of connexin-specific permeability differences of gap junctions expressed in HeLa transfectants and Xenopus oocytes. J Cell Sci 111: 31-43, 1998.[Abstract]
- Carabatsos MJ, Sellitto C, Goodenough DA, and Albertini DF. Oocyte-granulosa cell heterologous gap junctions are required for the coordination of nuclear and cytoplasmic meiotic competence. Dev Biol 226: 167-179, 2000.[Web of Science][Medline]
- Carolan EJ and Pitts JD. Some murine thymic lymphocytes can form gap junctions. Immunol Lett 13: 255-260, 1986.[Medline]
- Carrasquillo MM, Zlotogora J, Barges S, and Chakravarti A. Two different connexin 26 mutations in an inbred kindred segregating non-syndromic recessive deafness: implications for genetic studies in isolated populations. Hum Mol Genet 6: 2163-2172, 1997.[Abstract/Free Full Text]
- Cascio M, Gogol E, and Wallace BA. The secondary structure of gap junctions. Influence of isolation methods and proteolysis. J Biol Chem 265: 2358-2364, 1990.[Abstract/Free Full Text]
- Caspar DL, Goodenough DA, Makowski L, and Phillips WC. Gap junction structures. I. Correlated electron microscopy and X-ray diffraction. J Cell Biol 74: 605-628, 1977.[Abstract/Free Full Text]
- Castro C, Gómez-Hernández JM, Silander K, and Barrio LC. Altered formation of hemichannels and gap junction channels caused by C-terminal connexin-32 mutations. J Neurosci 19: 3752-3760, 1999.[Abstract/Free Full Text]
- Chaib H, Lina-Granade G, Guilford P, Plauchu H, Levilliers J, Morgon A, and Petit C. A gene responsible for a dominant form of neurosensory non-syndromic deafness maps to the NSRD1 recessive deafness gene interval. Hum Mol Genet 3: 2219-2222, 1994.[Abstract/Free Full Text]
- Chanson M, Fanjul M, Bosco D, Nelles E, Suter S, Willecke K, and Meda P. Enhanced secretion of amylase from exocrine pancreas of connexin32-deficient mice. J Cell Biol 141: 1267-1275, 1998.[Abstract/Free Full Text]
- Chanson M, Scerri I, and Suter S. Defective regulation of gap junctional coupling in cystic fibrosis pancreatic duct cells. J Clin Invest 103: 1677-1684, 1999.[Web of Science][Medline]
- Chanson M and Suter S. Regulation of gap junctional communication in CFTR-expressing pancreatic epithelial cells. Pflügers Arch 443: S81-S84, 2001.[Medline]
- Chanson M, White MM, and Garber SS. cAMP promotes gap junctional coupling in T84 cells. Am J Physiol Cell Physiol 271: C533-C539, 1996.[Abstract/Free Full Text]
- Charollais A, Gjinovci A, Huarte J, Bauquis J, Nadal A, Martin F, Andreu E, Sánchez-Andrés JV, Calabrese A, Bosco D, Soria B, Wollheim CB, Herrera PL, and Meda P. Junctional communication of pancreatic beta cells contributes to the control of insulin secretion and glucose tolerance. J Clin Invest 106: 235-243, 2000.[Web of Science][Medline]
- Charollais A, Serre V, Mock C, Cogne F, Bosco D, and Meda P. Loss of alpha 1 connexin does not alter the prenatal differentiation of pancreatic beta cells and leads to the identification of another islet cell connexin. Dev Genet 24: 13-26, 1999.[Web of Science][Medline]
- Chatterjee B, Li YX, Zdanoiwicz M, Sonntag JM, Chin AJ, Kozlowski DJ, Valdimarsson G, Kirby ML, and Lo CW. Analysis of Cx43alpha1 promoter function in the developing zebrafish embryo. Cell Commun Adhes 8: 289-292, 2001.[Web of Science][Medline]
- Chen ZQ, Lefebvre D, Bai XH, Reaume A, Rossant J, and Lye SJ. Identification of two regulatory elements within the promoter region of the mouse connexin 43 gene. J Biol Chem 270: 3863-3868, 1995.[Abstract/Free Full Text]
- Cheng HL and Louis CF. Endogenous casein kinase I catalyzes the phosphorylation of the lens fiber cell connexin49. Eur J Biochem 263: 276-286, 1999.[Web of Science][Medline]
- Cheng HL and Louis CF. Functional effects of casein kinase I-catalyzed phosphorylation on lens cell-to-cell coupling. J Membr Biol 181: 21-30, 2001.[Web of Science][Medline]
- Christ GJ, Spray DC, El-Sabban M, Moore LK, and Brink PR. Gap junctions in vascular tissues. Evaluating the role of intercellular communication in the modulation of vasomotor tone. Circ Res 79: 631-646, 1996.[Abstract/Free Full Text]
- Church RL, Wang JH, and Steele E. The human lens intrinsic membrane protein MP70 (Cx50) gene: clonal analysis and chromosome mapping. Curr Eye Res 14: 979-981, 1995.[Medline]
- Cicirata F, Parenti R, Spinella F, Giglio S, Tuorto F, Zuffardi O, and Gulisano M. Genomic organization and chromosomal localization of the mouse Connexin36 (mCx36) gene. Gene 251: 123-130, 2000.[Medline]
- Civitelli R, Ziambaras K, Warlow PM, Lecanda F, Nelson T, Harley J, Atal N, Beyer EC, and Steinberg TH. Regulation of connexin43 expression and function by prostaglandin E2 (PGE2) and parathyroid hormone (PTH) in osteoblastic cells. J Cell Biochem 68: 8-21, 1998.[Web of Science][Medline]
- Clair C, Chalumeau C, Tordjmann T, Poggioli J, Erneux C, Dupont G, and Combettes L. Investigation of the roles of Ca2+ and InsP(3) diffusion in the coordination of Ca2+ signals between connected hepatocytes. J Cell Sci 114: 1999-2007, 2001.[Abstract/Free Full Text]
- Cohen-Salmon M, Ott T, Michel V, Hardelin JP, Perfettini I, Eybalin M, Wu T, Marcus DC, Wangemann P, Willecke K, and Petit C. Targeted ablation of connexin26 in the inner ear epithelial gap junction network causes hearing impairment and cell death. Curr Biol 12: 1106-1111, 2002.[Web of Science][Medline]
- Condorelli DF, Parenti R, Spinella F, Trovato-Salinaro A, Belluardo N, Cardile V, and Cicirata F. Cloning of a new gap junction gene (Cx36) highly expressed in mammalian brain neurons. Eur J Neurosci 10: 1202-1208, 1998.[Web of Science][Medline]
- Constantin B, Cronier L, and Raymond G. Transient involvement of gap junctional communication before fusion of newborn rat myoblasts. C R Acad Sci III 320: 35-40, 1997.[Medline]
- Contreras JE, Sánchez HA, Eugenín EA, Speidel D, Theis M, Willecke K, Bukauskas FF, Bennett MVL, and Sáez JC. Metabolic inhibition induces opening of unapposed connexin 43 gap junction hemichannels and reduces gap junctional communication in cortical astrocytes in culture. Proc Natl Acad Sci USA 99: 495-500, 2002.[Abstract/Free Full Text]
- Cooper CD, Solan JL, Dolejsi MK, and Lampe PD. Analysis of connexin phosphorylation sites. Methods 20: 196-204, 2000.[Web of Science][Medline]
- Coppen SR, Kodama I, Boyett MR, Dobrzynski H, Takagishi Y, Honjo H, Yeh HI, and Severs NJ. Connexin45, a major connexin of the rabbit sinoatrial node, is co-expressed with connexin43 in a restricted zone at the nodal-crista terminalis border. J Histochem Cytochem 47: 907-918, 1999.[Abstract/Free Full Text]
- Crow DS, Beyer EC, Paul DL, Kobe SS, and Lau AF. Phosphorylation of connexin43 gap junction protein in uninfected and Rous sarcoma virus-transformed mammalian fibroblasts. Mol Cell Biol 10: 1754-1763, 1990.[Abstract/Free Full Text]
- Cruciani V, Kaalhus O, and Mikalsen SO. Phosphatases involved in modulation of gap junctional intercellular communication and dephosphorylation of connexin43 in hamster fibroblasts: 2B or not 2B? Exp Cell Res 252: 449-463, 1999.[Web of Science][Medline]
- Cruciani V and Mikalsen SO. Stimulated phosphorylation of intracellular connexin43. Exp Cell Res 251: 285-298, 1999.[Web of Science][Medline]
- Curtin KD, Zhang Z, and Wyman RJ. Gap junction proteins expressed during development are required for adult neural function in the Drosophila optic lamina. J Neurosci 22: 7088-7096, 2002.[Abstract/Free Full Text]
- Curtin KD, Zhang Z, and Wyman RJ. Gap junction proteins are not interchangeable in development of neural function in the Drosophila visual system. J Cell Sci 115: 3379-3388, 2002.[Abstract/Free Full Text]
- Cyr DG, Hermo L, and Laird DW. Immunocytochemical localization and regulation of connexin43 in the adult rat epididymis. Endocrinology 137: 1474-1484, 1996.[Abstract]
- Dahl G. Where are the gates in gap junction channels? Clin Exp Pharmacol Physiol 23: 1047-1052, 1996.[Web of Science][Medline]
- Dahl G, Levine E, Rabadan-Diehl C, and Werner R. Cell/cell channel formation involves disulfide exchange. Eur J Biochem 197: 141-144, 1991.[Web of Science][Medline]
- Dahl G, Werner R, Levine E, and Rabadan-Diehl C. Mutational analysis of gap junctional formation. Biophys J 62: 172-182, 1992.[Web of Science][Medline]
- Dahm R, van Marle J, Prescott AR, and Quinlan RA. Gap junctions containing alpha8-connexin (MP70) in the adult mammalian lens epithelium suggests a re-evaluation of its role in the lens. Exp Eye Res 69: 45-56, 1999.[Web of Science][Medline]
- Daniel EE, Jury J, and Wang YF. nNOS in canine lower esophageal sphincter: colocalized with Cav-1 and Ca2+-handling proteins? Am J Physiol Gastrointest Liver Physiol 281: G1101-G1114, 2001.[Abstract/Free Full Text]
- Daniel EE, Thomas J, Ramnarain M, Bowes TJ, and Jury J. Do gap junctions couple interstitial cells of Cajal pacing and neuro-transmission to gastrointestinal smooth muscle? Neurogastroenterol Motil 13: 297-307, 2001.[Web of Science][Medline]
- Daniel EE and Wang YF. Gap junctions in intestinal smooth muscle and interstitial cells of Cajal. Microsc Res Tech 47: 309-320, 1999.[Web of Science][Medline]
- Darrow BJ, Laing JG, Lampe PD, Saffitz JE, and Beyer EC. Expression of multiple connexins in cultured neonatal rat ventricular myocytes. Circ Res 76: 381-387, 1995.[Abstract/Free Full Text]
- Das Sarma J, Wang F, and Koval M. Targeted gap junction protein constructs reveal connexin-specific differences in oligomerization. J Biol Chem 277: 20911-20918, 2002.[Abstract/Free Full Text]
- Davis LM, Kanter HL, Beyer EC, and Saffitz JE. Distinct gap junction protein phenotypes in cardiac tissues with disparate conduction properties. J Am Coll Cardiol 24: 1124-1132, 1994.[Abstract]
- Davis LM, Rodefeld ME, Green K, Beyer EC, and Saffitz JE. Gap junction protein phenotypes of the human heart and conduction system. J Cardiovasc Electrophysiol 6: 813-822, 1995.[Web of Science][Medline]
- De Maio A, Gingalewski C, Theodorakis NG, and Clemens MG. Interruption of hepatic gap junctional communication in the rat during inflammation induced by bacterial lipopolysaccharide. Shock 14: 53-59, 2000.[Web of Science][Medline]
- De Maio A, Vega VL, and Contreras JE. Gap junctions, homeostasis, and injury. J Cell Physiol 191: 269-282, 2002.[Web of Science][Medline]
- Denoyelle F, Weil D, Maw MA, Wilcox SA, Lench NJ, Allen Powell DR, Osborn AH, Dahl HH, Middleton A, Houseman MJ, Dodé C, Marlin S, Boulila-ElGaied A, Grati M, Ayadi H, Ben Arab S, Bitoun P, Lina-Granade G, Godet J, Mustapha M, Loiselet J, El-Zir E, Aubois A, Joannard A, Levillers J, Garabédian EN, Mueller RF, McKinaly Gardner RJ, and Petit C. Prelingual deafness: high prevalence of a 30delG mutation in the connexin 26 gene. Hum Mol Genet 6: 2173-2177, 1997.[Abstract/Free Full Text]
- Dermietzel R. Gap junction wiring: a new principle in cell-to-cell communication in the nervous system? Brain Res Rev 26: 176-183, 1998.[Medline]
- Dermietzel R and Spray DC. Gap junctions in the brain: where, what type, how many and why? Trends Neurosci 16: 186-192, 1993.[Web of Science][Medline]
- Deschenes SM, Walcott JL, Wexler TL, Scherer SS, and Fischbeck KH. Altered trafficking of mutant connexin32. J Neurosci 17: 9077-9084, 1997.[Abstract/Free Full Text]
- DeVries SH and Schwartz EA. Hemi-gap-junction channels in solitary horizontal cells of the catfish retina. J Physiol 445: 201-230, 1992.[Abstract/Free Full Text]
- Dewey MM and Barr L. Intercellular connection between smooth muscle cells: the nexus. Science 137: 670-672, 1962.[Abstract/Free Full Text]
- De Wit C, Roos F, Bolz SS, Kirchhoff S, Kruger O, Willecke K, and Pohl U. Impaired conduction of vasodilation along arterioles in connexin40-deficient mice. Circ Res 86: 649-655, 2000.[Abstract/Free Full Text]
- Dhein S. Gap junction channels in the cardiovascular system: pharmacological and physiological modulation. Trends Pharmacol Sci 19: 229-241, 1998.[Medline]
- Diez JA, Ahmad S, and Evans WH. Assembly of heteromeric connexons in guinea-pig liver en route to the Golgi apparatus, plasma membrane and gap junctions. Eur J Biochem 262: 142-148, 1999.[Web of Science][Medline]
- Diez JA, Elvira M, and Villalobo A. The epidermal growth factor receptor tyrosine kinase phosphorylates connexin32. Mol Cell Biochem 187: 201-210, 1998.[Medline]
- Dixon DB, Takahashi K, Bieda M, and Copenhagen DR. Quinine, intracellular pH and modulation of hemi-gap junctions in catfish horizontal cells. Vision Res 36: 3925-3931, 1996.[Web of Science][Medline]
- Doble BW, Chen Y, Bosc DG, Litchfield DW, and Kardami E. Fibroblast growth factor-2 decreases metabolic coupling and stimulates phosphorylation as well as masking of connexin43 epitopes in cardiac myocytes. Circ Res 79: 647-658, 1996.[Abstract/Free Full Text]
- Doble BW, Ping P, and Kardami E. The epsilon subtype of protein kinase C is required for cardiomyocyte connexin-43 phosphorylation. Circ Res 86: 293-301, 2000.[Abstract/Free Full Text]
- Donaldson P, Eckert R, Green C, and Kisler J. Gap junction channels: new roles in disease. Histol Histopathol 12: 219-231, 1997.[Web of Science][Medline]
- Duga S, Asselta R, Del Giacco L, Malcovati M, Ronchi S, Tenchini ML, and Simonic T. A new exon in the 5' untranslated region of the connexin32 gene. Eur J Biochem 259: 188-196, 1999.[Medline]
- Dupont G, Tordjmann T, Clair C, Swillens S, Claret M, and Combettes L. Mechanism of receptor-oriented intercellular calcium wave propagation in hepatocytes. FASEB J 14: 279-289, 2000.[Abstract/Free Full Text]
- Ebihara L, Berthoud VM, and Beyer EC. Distinct behavior of connexin56 and connexin46 gap junctional channels can be predicted from the behavior of their hemi-gap-junctional channels. Biophys J 68: 1796-1803, 1995.[Web of Science][Medline]
- Ebihara L, Beyer EC, Swenson KI, Paul DL, and Goodenough DA. Cloning and expression of a Xenopus embryonic gap junction protein. Science 243: 1194-1195, 1989.[Abstract/Free Full Text]
- Ebihara L and Steiner E. Properties of a nonjunctional current expressed from a rat connexin46 cDNA in Xenopus oocytes. J Gen Physiol 102: 59-74, 1993.[Abstract/Free Full Text]
- Ebihara L, Xu X, Oberti C, Beyer EC, and Berthoud VM. Co-expression of lens fiber connexins modifies hemi-gap-junctional channel behavior. Biophys J 76: 198-206, 1999.[Medline]
- Echetebu CO, Ali M, Izban MG, MacKay L, and Garfield RE. Localization of regulatory protein binding sites in the proximal region of human myometrial connexin 43 gene. Mol Hum Reprod 5: 757-766, 1999.[Abstract/Free Full Text]
- Eckert R, Dunina-Barkovskaya A, and Hülser D. Biophysical characterization of gap-junction channels in HeLa cells. Pflügers Arch 424: 335-342, 1993.[Web of Science][Medline]
- Elden L and Bortoff A. Electrical coupling of longitudinal and circular intestinal muscle. Am J Physiol Gastrointest Liver Physiol 246: G618-G626, 1984.[Abstract/Free Full Text]
- Elfgang C, Eckert R, Lichtenberg-Frate H, Butterweck A, Traub O, Klein RA, Hülser DF, and Willecke K. Specific permeability and selective formation of gap junction channels in connexin-transfected HeLa cells. J Cell Biol 129: 805-817, 1995.[Abstract/Free Full Text]
- El-Sabban ME, Merhi RA, Haidar HA, Arnulf B, Khoury H, Basbous J, Nijmeh J, de The H, Hermine O, and Bazarbachi A. Human T-cell lymphotropic virus type 1-transformed cells induce angiogenesis and establish functional gap junctions with endothelial cells. Blood 99: 3383-3389, 2002.[Abstract/Free Full Text]
- Elvira M, Diez JA, Wang KKW, and Villalobo A. Phosphorylation of connexin-32 by protein kinase C prevents its proteolysis by mu-calpain and m-calpain. J Biol Chem 268: 14294-14300, 1993.[Abstract/Free Full Text]
- Endo K, Watanabe S, Nagahara A, Hirose M, and Sato N. Restoration of gap junctions in the regenerative process of ethanol-induced gastric mucosal injury. J Gastroenterol Hepatol 10: 589-594, 1995.[Medline]
- Eppig JJ. A comparison between oocyte growth in coculture with granulosa cells and oocytes with granulosa cell-oocyte junctional contact maintained in vitro. J Exp Zool 209: 345-353, 1979.[Web of Science][Medline]
- Eskandari S, Zampighi GA, Leung DE, Wright EM, and Loo DD. Inhibition of gap junction hemichannels by chloride channel blockers. J Membr Biol 185: 93-102, 2002.[Web of Science][Medline]
- Estivill X, Fortina P, Surrey S, Rabionet R, Melchionda S, D'Agruma L, Mansfield E, Rappaport E, Govea N, Mila M, Zelante L, and Gasparini P. Connexin-26 mutations in sporadic and inherited sensorineural deafness. Lancet 351: 394-398, 1998.[Web of Science][Medline]
- Eugenín EA, Brañes MC, Berman JW, and Sáez JC. TNF- and IFN- induce connexin-43 expression and formation of gap junctions between human monocytes/macrophages that enhance physiological responses. J Immunol 170: 1320-1328, 2003.[Abstract/Free Full Text]
- Eugenín EA, Eckardt D, Theis M, Willecke K, Bennett MV, and Sáez JC. Microglia at brain stab wounds express connexin 43 and in vitro form functional gap junctions after treatment with interferon- and tumor necrosis factor-. Proc Natl Acad Sci USA 98: 4190-4195, 2001.[Abstract/Free Full Text]
- Eugenín EA, González H, Sáez CG, and Sáez JC. Gap junctional communication coordinates vasopressin-induced glycogenolysis in rat hepatocytes. Am J Physiol Gastrointest Liver Physiol 274: G1109-G1116, 1998.[Abstract/Free Full Text]
- Evans WH, Ahamad S, Díez J, George CH, Kendall JM, and Martin PE. Trafficking pathways leading to the formation of gap junctions. In: Novartis Foundation Symposium. Gap Junction-Mediated Intercellular Signaling in Health and Disease, edited by Cardew G. London: Wiley, 1999, vol. 219, p. 44-54.
- Evans WH and Martin PE. Gap junctions: structure and function. Mol Membr Biol 19: 121-136, 2002.[Web of Science][Medline]
- Evert M, Ott T, Temme A, Willecke K, and Dombrowski F. Morphology and morphometric investigation of hepatocellular preneoplastic lesions and neoplasms in connexin32-deficient mice. Carcinogenesis 23: 697-703, 2002.[Abstract/Free Full Text]
- Fallon RF and Goodenough DA. Five hour half-life of mouse liver gap junction protein. J Cell Biol 90: 521-526, 1981.[Abstract/Free Full Text]
- Faucheux N, Dufresne M, and Nagel MD. Organization of cyclic AMP-dependent connexin 43 in Swiss 3T3 cells attached to a cellulose substratum. Biomaterials 23: 413-421, 2002.[Medline]
- Fiertak A, Semik D, and Kilarski WM. Immunohistochemical analysis of connexin26 and 43 expression in the mouse alimentary tract. Folia Biol (Krakow) 47: 5-11, 1999.[Medline]
- Filson AJ, Azarnia R, Beyer EC, Loewenstein WR, and Brugge JS. Tyrosine phosphorylation of a gap junction protein correlates with inhibition of cell-to-cell communication. Cell Growth Differ 1: 661-668, 1990.[Abstract]
- Fishman GI, Eddy RL, Shows TB, Rosenthal L, and Leinwand LA. The human connexin gene family of gap junction proteins: distinct chromosomal locations but similar structures. Genomics 10: 250-256, 1991.[Web of Science][Medline]
- Fishman GI, Moreno AP, Spray DC, and Leinwand LA. Functional analysis of human cardiac gap junction channel mutants. Proc Natl Acad Sci USA 88: 3525-3529, 1991.[Abstract/Free Full Text]
- Fishman GI, Spray DC, and Leinwand LA. Molecular characterization and functional expression of the human cardiac gap junction channel. J Cell Biol 111: 589-598, 1990.[Abstract/Free Full Text]
- Fitzgerald DA and Verbov JL. Hereditary palmoplantar keratoderma with deafness. Br J Dermatol 134: 939-942, 1996.[Medline]
- Foote CI, Zhou L, Zhu X, and Nicholson BJ. The pattern of disulfide linkages in the extracellular loop regions of connexin 32 suggests a model for the docking interface of gap junctions. J Cell Biol 140: 1187-1197, 1998.[Abstract/Free Full Text]
- Forge A, Becker D, Casalotti S, Edwards J, Evans WH, Lench N, and Souter M. Gap junctions and connexin expression in the inner ear. In: Novartis Foundation Symposium. Gap Junction-Mediated Intercellular Signaling in Health and Disease, edited by Cardew G. London: Wiley, 1999, vol. 219, p. 134-150.
- Franco L, Zocchi E, Usai C, Guida L, Bruzzone S, Costa A, and De Flora AG. Paracrine roles of NAD+ and cyclic ADP-ribose in increasing intracellular calcium and enhancing cell proliferation of 3T3 fibroblasts. J Biol Chem 276: 21642-21648, 2001.[Abstract/Free Full Text]
- Froes M and Menezes J. Coupled heterocellular arrays in the brain. Neurochem Int 41: 367-375, 2002.[Web of Science][Medline]
- Fujimoto K, Nagafuchi A, Tsukita S, Kuraoka A, Ohokuma A, and Shibata Y. Dynamics of connexins, E-cadherin and alpha-catenin on cell membranes during gap junction formation. J Cell Sci 110: 311-322, 1997.[Abstract]
- Furshpan EJ and Potter DD. Transmission at the giant motor synapses of the crayfish. J Physiol 145: 289-325, 1959.[Free Full Text]
- Gabriel HD, Jung D, Butzler C, Temme A, Traub O, Winterhager E, and Willecke K. Transplacental uptake of glucose is decreased in embryonic lethal connexin26-deficient mice. J Cell Biol 140: 1453-1461, 1998.[Abstract/Free Full Text]
- Gabriel HD, Strobol B, Hellmann P, Buettner R, and Winterhager E. Organization and regulation of the rat Cx31 gene. Implication for a crucial role of the intron region. Eur J Biochem 268: 1749-1759, 2001.[Medline]
- Gabriels JE and Paul DL. Connexin43 is highly localized to sites of disturbed flow in rat aortic endothelium but connexin37 and connexin40 are more uniformly distributed. Circ Res 83: 636-643, 1998.[Abstract/Free Full Text]
- Gaietta G, Deernick TJ, Adams SR, Bouver J, Tour O, Laird DW, Sosinsky GE, Tsien RY, and Ellisman MH. Multicolor and electron microscopic imaging of connexin trafficking. Science 296: 503-507, 2002.[Abstract/Free Full Text]
- Ganfornina MD, Sánchez D, Herrera M, and Bastiani MJ. Developmental expression and molecular characterization of two gap junction channel proteins expressed during embryogenesis in the grasshopper Schistocerca americana. Dev Genet 24: 137-150, 1999.[Medline]
- Garfield RE, Sims S, and Daniel EE. Gap junctions: their presence and necessity in myometrium during parturition. Science 198: 958-960, 1977.[Abstract/Free Full Text]
- Garfield RE, Thilander G, Blennerhassett MG, and Sakai N. Are gap junctions necessary for cell-to-cell coupling of smooth muscle?: an update. Can J Physiol Pharmacol 70: 481-490, 1992.[Medline]
- Geimonen E, Jiang W, Ali M, Fishman GI, Garfield RE, and Andersen J. Activation of protein kinase C in human uterine smooth muscle induces connexin-43 gene transcription through an AP-1 site in the promoter sequence. J Biol Chem 271: 23667-23674, 1996.[Abstract/Free Full Text]
- George CH, Kendall JM, and Evans WH. Intracellular trafficking pathways in the assembly of connexins into gap junctions. J Biol Chem 274: 8678-8685, 1999.[Abstract/Free Full Text]
- Ghoshroy S, Goodenough DA, and Sosinsky GE. Preparation, characterization, and structure of half gap junctional layers split with urea and EGTA. J Membr Biol 146: 15-28, 1995.[Web of Science][Medline]
- Giepmans BN and Moolenaar WH. The gap junction protein connexin43 interacts with the second PDZ domain of the zona occludens-1 protein. Curr Biol 8: 931-934, 1998.[Web of Science][Medline]
- Giepmans BN, Verlaan I, Hengeveld T, Janssen H, Calafat J, Falk MM, and Moolenar WH. Gap junction protein connexin-43 interacts directly with microtubules. Curr Biol 11: 1364-1368, 2001.[Web of Science][Medline]
- Gilula NB, Epstein ML, and Beers WH. Cell-to-cell communication and ovulation. A study of the cumulus-oocyte complex. J Cell Biol 78: 58-75, 1978.[Abstract/Free Full Text]
- Gilula NB, Reeves OR, and Steinbach A. Metabolic coupling, ionic coupling and cell contacts. Nature 235: 262-265, 1972.[Medline]
- Gingalewski C, Wang K, Clemens MG, and De Maio A. Posttranscriptional regulation of connexin 32 expression in liver during acute inflammation. J Cell Physiol 166: 461-467, 1996.[Web of Science][Medline]
- Godwin AJ, Green LM, Walsh MP, McDonald JR, Walsh DA, and Fletcher WH. In situ regulation of cell-cell communication by the cAMP-dependent protein kinase and protein kinase C. Mol Cell Biochem 127- 128: 293-307, 1993.
- Gong X, Li E, Klier G, Huang Q, Wu Y, Lei H, Kumar NM, Horwitz J, and Gilula NB. Disruption of alpha3 connexin gene leads to proteolysis and cataractogenesis in mice. Cell 91: 833-843, 1997.[Web of Science][Medline]
- González HE, Eugenín EA, Garcés G, Solís N, Pizarro M, Accatino L, and Sáez JC. Regulation of hepatic connexins in cholestasis: possible involvement of Kupffer cells and inflammatory mediators. Am J Physiol Gastrointest Liver Physiol 282: G991-G1001, 2002.[Abstract/Free Full Text]
- Goodenough DA. Lens gap junctions: a structural hypothesis for nonregulated low-resistance intercellular pathways. Invest Ophthalmol Vis Sci 18: 1104-1122, 1979.[Abstract/Free Full Text]
- Goodenough DA, Goliger JA, and Paul DL. Connexins, connexons, and intercellular communication. Annu Rev Biochem 65: 475-502, 1996.[Web of Science][Medline]
- Gourdie RG, Severs NJ, Green CR, Rothery S, Germroth P, and Thompson RP. The spatial distribution and relative abundance of gap-junctional connexin40 and connexin43 correlate to functional properties of components of the cardiac atrioventricular conduction system. J Cell Sci 105: 985-991, 1993.[Abstract]
- Granot I, Bechor E, Barash A, and Dekel N. Connexin43 in rat oocytes: developmental modulation of its phosphorylation. Biol Reprod 66: 568-573, 2002.[Abstract/Free Full Text]
- Granot I and Dekel N. Phosphorylation and expression of connexin-43 ovarian gap junction protein are regulated by luteinizing hormone. J Biol Chem 269: 30502-30509, 1994.[Abstract/Free Full Text]
- Granot I and Dekel N. Developmental expression and regulation of the gap junction protein and transcript in rat ovaries. Mol Reprod Dev 47: 231-239, 1997.[Web of Science][Medline]
- Grazul-Bilska AT, Redmer DA, Bilski JJ, Jablonka-Shariff A, Doraiswamy V, and Reynolds LP. Gap junctional proteins, connexin 26, 32, and 43 in sheep ovaries throughout the estrous cycle. Endocrine 8: 269-279, 1998.[Web of Science][Medline]
- Green CR, Harfst E, Goudie RG, and Severs NJ. Analysis of the rat liver gap junction protein: clarification of anomalies in its molecular size. Proc R Soc Lond B Biol Sci 233: 165-174, 1988.[Medline]
- Grifa A, Wagner CA, D'Ambrosio L, Melchionda S, Bernardi F, López-Bigas N, Rabionet R, Arbones M, Monica MD, Estivill X, Zelante L, Lang F, and Gasparini P. Mutations in GJB6 cause nonsyndromic autosomal dominant deafness at DFNA3 locus. Nat Genet 23: 16-18, 1999.[Web of Science][Medline]
- Groenewegen WA, van Veen TA, van der Velden HM, and Jongsma HJ. Genomic organization of the rat connexin40 gene: identical transcription start sites in heart and lung. Cardiovasc Res 38: 463-471, 1998.[Abstract/Free Full Text]
- Gros D, Jarry-Guichard T, Ten Velde I, de Maziere A, van Kempen MJ, Davoust J, Briand JP, Moorman AF, and Jongsma HJ. Restricted distribution of connexin40, a gap junctional protein, in mammalian heart. Circ Res 74: 839-851, 1994.[Abstract/Free Full Text]
- Gros DB, Nicholson BJ, and Revel JP. Comparative analysis of the gap junction protein from rat heart and liver: is there a tissue specificity of gap junctions? Cell 35: 539-549, 1983.[Medline]
- Grümmer R, Chwalisz K, Mulholland J, Traub O, and Winterhager E. Regulation of connexin26 and connexin43 expression in rat endometrium by ovarian steroid hormones. Biol Reprod 51: 1109-1116, 1994.[Abstract]
- Guerrero PA, Schuessler RB, Davis LM, Beyer EC, Johnson CM, Yamada KA, and Saffitz JE. Slow ventricular conduction in mice heterozygous for a connexin43 null mutation. J Clin Invest 99: 1991-1998, 1997.[Web of Science][Medline]
- Guerrier A, Fonlupt P, Morand I, Rabilloud R, Audebet C, Krutocskikh V, Gros D, Rousset B, and Munari-Silem Y. Gap junctions and cell polarity: connexin32 and connexin43 expressed in polarized thyroid epithelial cells assemble into separate gap junctions, which are located in distinct regions of the lateral plasma membrane domain. J Cell Sci 108: 2609-2617, 1995.[Abstract]
- Guinan EC, Smith BR, Davies PF, and Pober JS. Cytoplasmic transfer between endothelium and lymphocytes: quantitation by flow cytometry. Am J Pathol 132: 406-409, 1988.[Abstract]
- Gupta VK, Berthoud VM, Atal N, Jarillo JA, Barrio LC, and Beyer EC. Bovine connexin44, a lens gap junction protein: molecular cloning, immunologic characterization, and functional expression. Invest Ophtalmol Vis Sci 35: 3747-3758, 1994.[Abstract/Free Full Text]
- Gutstein DE, Morley GE, Tamaddon H, Vaidya D, Schneider MD, Chen J, Chien KR, Stuhlmann H, and Fishman GI. Conduction slowing and sudden arrhythmic death in mice with cardiac-restricted inactivation of connexin43. Circ Res 88: 333-339, 2001.[Abstract/Free Full Text]
- Haefliger JA, Bruzzone R, Jenkins NA, Gilbert DJ, Copeland NG, and Paul DL. Four novel members of the connexin family of gap junction proteins. Molecular cloning, expression, and chromosome mapping. J Biol Chem 267: 2057-2064, 1992.[Abstract/Free Full Text]
- Haefliger JA and Meda P. Chronic hypertension alters the expression of Cx43 in cardiovascular muscle cells. Braz J Med Biol Res 33: 431-438, 2000.[Web of Science][Medline]
- Haefliger JA, Polikar R, Schnyder G, Burdet M, Sutter E, Pexieder T, Nicod P, and Meda P. Connexin37 in normal and pathological development of mouse heart and great arteries. Dev Dyn 218: 331-344, 2000.[Web of Science][Medline]
- Heathcote K, Syrris P, Carter ND, and Paton MA. A connexin 26 mutation causes a syndrome of sensorineural hearing loss and palmoplantar hyperkeratosis (MIM 148350). J Med Genet 37: 50-51, 2000.[Abstract/Free Full Text]
- Henderson D, Eibl H, and Weber K. Structure and biochemistry of mouse hepatic gap junctions. J Mol Biol 132: 193-218, 1979.[Web of Science][Medline]
- Henderson RM, Duchon G, and Daniel EE. Cell contacts in duodenal smooth muscle layers. Am J Physiol 221: 564-574, 1971.[Free Full Text]
- Hendrix EM, Myatt L, Sellers S, Russell PT, and Larsen WJ. Steroid hormone regulation of rat myometrial gap junction formation: effects on cx43 levels and trafficking. Biol Reprod 52: 547-560, 1995.[Abstract]
- Hennemann H, Dahl E, White JB, Schwarz HJ, Lalley PA, Chang S, Nicholson BJ, and Willecke K. Two gap junction genes, connexin 31.1 and 30.3, are closely linked on chromosomes 4 and preferentially expressed in skin. J Biol Chem 267: 17225-17233, 1992.[Abstract/Free Full Text]
- Hennemann H, Kozjek G, Dahl E, Nicholson B, and Willecke K. Molecular cloning of mouse connexins26 and -32: similar genomic organization but distinct promoter sequences of two gap junction genes. Eur J Cell Biol 58: 81-89, 1992.[Web of Science][Medline]
- Hermoso M, Sáez JC, and Villalón M. Identification of gap junctions in the oviduct and regulation of connexins during development and by sexual hormones. Eur J Cell Biol 74: 1-9, 1997.[Medline]
- Hertlein B, Butterweck A, Haubrich S, Willecke K, and Traub O. Phosphorylated carboxy terminal serine residues stabilize the mouse gap junction protein connexin45 against degradation. J Membr Biol 162: 247-257, 1998.[Web of Science][Medline]
- Hertzberg EL, Disher RM, Tiller AA, Zhou Y, and Cook RG. Topology of the Mr 27,000 liver gap junction protein. Cytoplasmic localization of amino- and carboxyl termini and a hydrophilic domain which is protease-hypersensitive. J Biol Chem 263: 19105-19111, 1988.[Abstract/Free Full Text]
- Hertzberg EL and Gilula NB. Isolation and characterization of gap junctions from rat liver. J Biol Chem 254: 2138-2147, 1979.[Free Full Text]
- Heynkes R, Kozjek G, Traub O, and Willecke K. Identification of a rat liver cDNA and mRNA coding for the 28 kDa gap junction protein. FEBS Lett 205: 56-60, 1986.[Medline]
- Hill CS, Oh SY, Schmidt SA, Clark KJ, and Murray AW. Lysophosphatidic acid inhibits gap-junctional communication and stimulates phosphorylation of connexin-43 in WB cells: possible involvement of the mitogen-activated protein kinase cascade. Biochem J 303: 475-479, 1994.[Web of Science][Medline]
- Hill CE, Rummery N, Hickey H, and Sandow SL. Heterogeneity in the distribution of vascular gap junctions and connexins: implications for function. Clin Exp Pharmacol Physiol 29: 620-625, 2002.[Web of Science][Medline]
- Hirata K, Pusl T, O'Neill AF, Dranoff JA, and Nathanson MH. The type II inositol 1,4,5-trisphosphate receptor can trigger Ca2+ waves in rat hepatocytes. Gastroenterology 122: 1088-1100, 2002.[Web of Science][Medline]
- Hirono C, Shiba Y, and Kanno Y. Different localizations of 21 and 27 kDa gap-junction proteins in rat salivary glands. Histochem Cell Biol 103: 39-46, 1995.[Medline]
- Hofer A and Dermietzel R. Visualization and functional blocking of gap junction hemichannels (connexons) with antibodies against external loop domains in astrocytes. Glia 24: 141-154, 1998.[Web of Science][Medline]
- Hoh JH, Sosinsky GE, Revel JP, and Hansma PK. Structure of the extracellular surface of the gap junction by atomic force microscopy. Biophys J 65: 149-163, 1993.[Medline]
- Holm I, Mikhailov A, Jillson T, and Rose B. Dynamics of gap junctions observed in living cells with connexin43-GFP chimeric protein. Eur J Cell Biol 78: 856-866, 1999.[Web of Science][Medline]
- Honjo H, Boyett MR, Coppen SR, Takagishi Y, Opthof T, Severs NJ, and Kodama I. Heterogeneous expression of connexins in rabbit sinoatrial node cells: correlation between connexin isotype and cell size. Cardiovasc Res 53: 89-96, 2002.[Abstract/Free Full Text]
- Hossain MZ, Ao P, and Boynton AL. Platelet-derived growth factor-induced disruption of gap junctional communication and phosphorylation of connexin43 involves protein kinase C and mitogen-activated protein kinase. J Cell Physiol 176: 332-341, 1998.[Web of Science][Medline]
- Hossain MZ, Ao P, and Boynton AL. Rapid disruption of gap junctional communication and phosphorylation of connexin43 by platelet-derived growth factor in T51B rat liver epithelial cells expressing platelet-derived growth factor receptor. J Cell Physiol 174: 66-77, 1998.[Web of Science][Medline]
- Hossain MZ, Jagdale AB, Ao P, and Boynton AL. Mitogen-activated protein kinase and phosphorylation of connexin43 are not sufficient for the disruption of gap junctional communication by platelet-derived growth factor and tetradecanoylphorbol acetate. J Cell Physiol 179: 87-96, 1999.[Web of Science][Medline]
- Hossain MZ, Jagdale AB, Ao P, Kazlauskas A, and Boynton AL. Disruption of gap junctional communication by the platelet-derived growth factor is mediated via multiple signaling pathways. J Biol Chem 274: 10489-10496, 1999.[Abstract/Free Full Text]
- Hsieh CL, Kumar NM, Gilula NB, and Francke U. Distribution of genes for gap junction membrane channel proteins on human and mouse chromosomes. Somat Cell Mol Genet 17: 191-200, 1991.[Web of Science][Medline]
- Hu J and Cotgreave IA. Differential regulation of gap junctions by proinflammatory mediators in vitro. J Clin Invest 99: 2312-2316, 1997.[Web of Science][Medline]
- Huang GY, Wessels A, Smith BR, Linask KK, Ewart JL, and Lo CW. Alteration in connexin 43 gap junction gene dosage impairs conotruncal heart development. Dev Biol 198: 32-44, 1998.[Web of Science][Medline]
- Hudder A and Werner R. Analysis of a Charcot-Marie-Tooth disease mutation reveals an essential internal ribosome entry site element in the connexin-32 gene. J Biol Chem 275: 34586-34591, 2000.[Abstract/Free Full Text]
- Ihara A, Muramatsu T, and Shimono M. Expression of connexin 32 and 43 in developing rat submandibular salivary glands. Arch Oral Biol 45: 227-235, 2000.[Web of Science][Medline]
- Iino S, Asamoto K, and Nojyo Y. Heterogeneous distribution of a gap junction protein, connexin43, in the gastroduodenal junction of the guinea pig. Auton Neurosci 93: 8-13, 2001.[Web of Science][Medline]
- Ionasescu VV, Searby C, Ionasescu R, Neuhaus IM, and Werner R. Mutations of the noncoding region of the connexin32 gene in X-linked dominant Charcot-Marie-Tooth neuropathy. Neurology 47: 541-544, 1996.[Abstract/Free Full Text]
- Itahana K, Tanaka T, Morikazu Y, Komatu S, Ishida N, and Takeya T. Isolation and characterization of a novel connexin gene, Cx-60, in porcine ovarian follicles. Endocrinology 139: 320-329, 1998.[Abstract/Free Full Text]
- Iwata F, Joh T, Ueda F, Yokoyama Y, and Itoh M. Role of gap junctions in inhibiting ischemia-reperfusion injury of rat gastric mucosa. Am J Physiol Gastrointest Liver Physiol 275: G883-G888, 1998.[Abstract/Free Full Text]
- Jacob A and Beyer EC. Mouse connexin 45: genomic cloning and exon usage. DNA Cell Biol 20: 11-19, 2001.[Web of Science][Medline]
- Janssen-Bienhold U, Schultz K, Gelhaus A, Schmidt P, Ammermuller J, and Weiler R. Identification and localization of connexin26 within the photoreceptor-horizontal cell synaptic complex. Vis Neurosci 18: 169-178, 2001.[Web of Science][Medline]
- Jara PI, Boric MP, and Sáez JC. Leukocytes express connexin-43 after activation with lipopolysaccharide and appear to form gap junctions with endothelial cells after ischemia-reperfusion. Proc Natl Acad Sci USA 92: 7011-7015, 1995.[Abstract/Free Full Text]
- Jedamzik B, Marten I, Ngezahayo A, Ernst A, and Kolb HA. Regulation of lens rCx46-formed hemichannels by activation of protein kinase C, external Ca2+ and protons. J Membr Biol 173: 39-46, 2000.[Web of Science][Medline]
- Jiang JX and Googenough DA. Heteromeric connexons in lens gap junction channels. Proc Natl Acad Sci USA 93: 1287-1291, 1996.[Abstract/Free Full Text]
- Jiang JX and Goodenough DA. Phosphorylation of lens-fiber connexins in lens organ cultures. Eur J Biochem 255: 37-44, 1998.[Web of Science][Medline]
- Jiang JX, Paul DL, and Goodenough DA. Posttranslational phosphorylation of lens fiber connexin46: a slow occurrence. Invest Ophthal Visual Sci 34: 3558-3565, 1993.[Abstract/Free Full Text]
- Jiang JX, White TW, and Goodenough DA. Changes in connexin expression and distribution during chick lens development. Dev Biol 168: 649-661, 1995.[Web of Science][Medline]
- Jiang JX, White TW, Goodenough DA, and Paul DL. Molecular cloning and functional characterization of chick lens fiber connexin 45.6. Mol Biol Cell 5: 363-373, 1994.[Abstract]
- John SA, Kondo R, Wang SY, Goldhaber JI, and Weiss JN. Connexin-43 hemichannels opened by metabolic inhibition. J Biol Chem 274: 236-240, 1999.[Abstract/Free Full Text]
- John SA and Revel JP. Connexon integrity is maintained by non-covalent bonds: intramolecular disulfide bonds link the extracellular domains in rat connexin-43. Biochem Biophys Res Commun 178: 1312-1318, 1991.[Web of Science][Medline]
- Johnson CM, Kanter EM, Green KG, Laing JG, Betsuyaku T, Beyer EC, Steinberg TH, Saffitz JE, and Yamada KA. Redistribution of connexin45 in gap junctions of connexin43-deficient hearts. Cardiovasc Res 53. 921-935, 2002.[Abstract/Free Full Text]
- Johnson ML, Redmer DA, Reynolds LP, and Grazul-Bilska AT. Expression of gap junctional proteins connexin 43, 32, and 26 throughout follicular development and atresia in cows. Endocrine 10: 43-51, 1999.[Web of Science][Medline]
- Johnson R, Hammer M, Sheridan J, and Revel JP. Gap junction formation between reaggregated Novikoff hepatoma cells. Proc Natl Acad Sci USA 71: 4536-4540, 1974.[Abstract/Free Full Text]
- Johnson RG, Meyer RA, Li XR, Preuss DM, Tan L, Grunenwald H, Paulson AF, Laird DW, and Sheridan JD. Gap junctions assemble in the presence of cytoskeletal inhibitors, but enhanced assembly requires microtubules. Exp Cell Res 275: 67-80, 2002.[Web of Science][Medline]
- Johnstone BM, Patuzzi R, Syka J, and Sykova E. Stimulus-related potassium changes in the organ of Corti of guinea-pig. J Physiol 408: 77-92, 1989.[Abstract/Free Full Text]
- Jongen WM, Fitzgerald DJ, Asamoto M, Piccoli C, Slaga JT, Gros D, Takeichi M, and Yamasaki H. Regulation of connexin 43-mediated gap junctional intercellular communication by Ca2+ in mouse epidermal cells is controlled by E-cadherin. J Cell Biol 114: 545-555, 1991.[Abstract/Free Full Text]
- Jonkers FC, Jonas JC, Gilon P, and Henquin JC. Influence of cell number on the characteristics and synchrony of Ca2+ oscillations in clusters of mouse pancreatic islet cells. J Physiol 520: 839-849, 1999.[Abstract/Free Full Text]
- Jordan K, Chodock R, Hand AR, and Laird DW. The origin of annular junctions: a mechanism of gap junction internalization. J Cell Sci 114: 763-773, 2001.[Abstract]
- Juneja SC, Barr KJ, Enders GC, and Kidder GM. Defects in the germ line and gonads of mice lacking connexin43. Biol Reprod 60: 1263-1270, 1999.[Abstract/Free Full Text]
- Kamermans M, Fahrenfort I, Schultz K, Janssen-Bienhold U, Sjoerdsma T, and Weiler R. Hemichannel-mediated inhibition in the outer retina. Science 292: 1178-1180, 2001.[Abstract/Free Full Text]
- Kanemitsu MY and Lau AF. Epidermal growth factor stimulates the disruption of gap junctional communication and connexin43 phosphorylation independent of 12-O-tetradecanoylphorbol 13-acetate-sensitive protein kinase C: the possible involvement of mitogen-activated protein kinase. Mol Biol Cell 4: 837-848, 1993.[Abstract]
- Kanemitsu MY, Loo LW, Simon S, Lau AF, and Eckhart W. Tyrosine phosphorylation of connexin 43 by v-Src is mediated by SH2 and SH3 domain interactions. J Biol Chem 272: 22824-22831, 1997.[Abstract/Free Full Text]
- Kanno S and Saffitz JE. The role of myocardial gap junctions in electrical conduction and arrhythmogenesis. Cardiovasc Pathol 10: 169-177, 2001.[Web of Science][Medline]
- Kanter HL, Laing JG, Beau SL, Beyer EC, and Saffitz JE. Distinct patterns of connexin expression in canine Purkinje fibers and ventricular muscle. Circ Res 72: 1124-1131, 1993.[Abstract/Free Full Text]
- Kanter HL, Saffitz JE, and Beyer EC. Cardiac myocytes express multiple gap junction proteins. Circ Res 70: 438-444, 1992.[Abstract/Free Full Text]
- Kardami E, Stoski RM, Doble BW, Yamamoto T, Hertzberg EL, and Nagy JI. Biochemical and structural evidence for the association of basic fibroblast growth factor with cardiac gap junctions. J Biol Chem 266: 19551-19557, 1991.[Abstract/Free Full Text]
- Karicheti V and Christ GJ. Physiological roles for K+ channels and gap junctions in urogenital smooth muscle: implications for improved understanding of urogenital function, disease and therapy. Curr Drug Targets 2: 1-20, 2001.[Medline]
- Keane RW, Mehta PP, Rose B, Honig LS, Loewenstein WR, and Rutishauser U. Neural differentiation, N-CAM-mediated adhesion, and gap junctional communication in neuroectoderm. A study in vitro. J Cell Biol 106: 1307-1319, 1988.[Abstract/Free Full Text]
- Kelley PM, Abe S, Askew JW, Smith SD, Usami S, and Kimberling WJ. Human connexin 30 (GJB6), a candidate gene for nonsyndromic hearing loss: molecular cloning, tissue-specific expression, and assignment to chromosome 13q12. Genomics 62: 172-176, 1999.[Web of Science][Medline]
- Kelley PM, Harris DJ, Comer BC, Askew JW, Fowler T, Smith SD, and Kimberling WJ. Novel mutations in the connexin 26 gene (GJB2) that cause autosomal recessive (DFNB1) hearing loss. Am J Hum Genet 62: 792-799, 1998.[Web of Science][Medline]
- Kelsell DP, Di WL, and Houseman MJ. Connexin mutations in skin disease and hearing loss. Am J Hum Genet 68: 559-568, 2001.[Web of Science][Medline]
- Kelsell DP, Dunlop J, Stevens HP, Lench NJ, Liang JN, Parry G, Mueller RF, and Leigh IM. Connexin 26 mutations in hereditary non-syndromic sensorineural deafness. Nature 387: 80-83, 1997.[Medline]
- Khan-Dawood FS, Yang J, and Dawood MY. Expression of gap junction protein connexin-43 in the human and baboon (Papio anubis) corpus luteum. J Clin Endocrinol Metab 81: 835-842, 1996.[Abstract]
- Kiang DT, Jin N, Tu ZJ, and Lin HH. Upstream genomic sequence of the human connexin26 gene. Gene 199: 165-171, 1997.[Web of Science][Medline]
- Kikuchi T, Kimura RS, Paul DL, and Adams JC. Gap junctions in the rat cochlea-immunohistochemical and ultrastructural analysis. Anat Embryol (Berl) 191: 101-118, 1995.[Medline]
- Kilarski WM, Dupont E, Coppen S, Yeh HI, Vozzi C, Gourdie RG, Rezapour M, Ulmsten U, Roomans GM, and Severs NJ. Identification of two further gap-junctional proteins, connexin40 and connexin45, in human myometrial smooth muscle cells at term. Eur J Cell Biol 75: 1-8, 1998.[Web of Science][Medline]
- Kilarski WM, Rothery YS, Roomans GM, Ulmsten U, Rezapour M, Stevenson S, Coppen SR, Dupont E, and Severs NJ. Multiple connexins localized to individual gap-junctional plaques in human myometrial smooth muscle. Microsc Res Tech 54: 114-122, 2001.[Medline]
- Kim DY, Kam Y, Koo SK, and Joe CO. Gating connexin 43 channels reconstituted in lipid vesicles by mitogen-activated protein kinase phosphorylation. J Biol Chem 274: 5581-5587, 1999.[Abstract/Free Full Text]
- Kirchhoff S, Kim JS, Hagendorff A, Thonnissen E, Kruger O, Lamers WH, and Willecke K. Abnormal cardiac conduction and morphogenesis in connexin40 and connexin43 double-deficient mice. Circ Res 87: 399-405, 2000.[Abstract/Free Full Text]
- Kistler J, Bond J, Donaldson P, and Engel A. Two distinct levels of gap junction assembly in vitro. J Struct Biol 110: 28-38, 1993.[Medline]
- Kistler J and Bullivant S. Protein processing in lens intercellular junctions: cleavage of MP70 to MP38. Invest Ophthalmol Vis Sci 28: 1687-1692, 1987.[Abstract/Free Full Text]
- Kistler J, Christie D, and Bullivant S. Homologies between gap junction proteins in lens, heart and liver. Nature 331: 721-723, 1988.[Medline]
- Kistler J, Kirkland B, and Bullivant S. Identification of a 70,000-D protein in lens membrane junctional domains. J Cell Biol 101: 28-35, 1985.[Abstract/Free Full Text]
- Kleopa KA, Yum SW, and Scherer SS. Cellular mechanisms of connexin32 mutations associated with CNS manifestations. J Neurosci Res 68: 522-534, 2002.[Web of Science][Medline]
- Ko YS, Coppen SR, Dupont E, Rother YS, and Severs NJ. Regional differentiation of desmin, connexin43, and connexin45 expression patterns in rat aortic smooth muscle. Arterioscler Thromb Vasc Biol 21: 355-364, 2001.[Abstract/Free Full Text]
- Kojima T, Kokai Y, Chiba H, Yamamoto M, Mochizuki Y, and Sawada N. Cx32 but not Cx26 is associated with tight junctions in primary cultures of rat hepatocytes. Exp Cell Res 263: 193-201, 2001.[Web of Science][Medline]
- Kojima T, Sawada N, Chiba H, Kokai Y, Yamamoto M, Urban M, Lee GH, Hertzberg EL, Mochizuki Y, and Spray DC. Induction of tight junctions in human connexin 32 (hCx32)-transfected mouse hepatocytes: connexin 32 interacts with occludin. Biochem Biophys Res Commun 266: 222-229, 1999.[Web of Science][Medline]
- Kojima T, Spray DC, Kokai Y, Chiba H, Mochizuki Y, and Sawada N. Cx32 formation and/or Cx32-mediated intercellular communication induces expression and function of tight junctions in hepatocytic cell line. Exp Cell Res 276: 40-51, 2002.[Web of Science][Medline]
- Kondo RP, Wang SY, John SA, Weiss JN, and Goldhaber JI. Metabolic inhibition activates a non-selective current through connexin hemichannels in isolated ventricular myocytes. J Mol Cell Cardiol 32: 1859-1872, 2000.[Web of Science][Medline]
- Koval M, Geist ST, Westphale EM, Kemendy AE, Civitelli R, Beyer EC, and Steinberg TH. Transfected connexin45 alters gap junction permeability in cells expressing endogenous connexin43. J Cell Biol 130: 987-995, 1995.[Abstract/Free Full Text]
- Kren BT, Kumar NM, Wang SQ, Gilula NB, and Steer CJ. Differential regulation of multiple gap junction transcripts and proteins during rat liver regeneration. J Cell Biol 123: 707-718, 1993.[Abstract/Free Full Text]
- Krenacs T and Rosendaal M. Immunohistological detection of gap junctions in human lymphoid tissue: connexin43 in follicular dendritic and lymphoendothelial cells. J Histochem Cytochem 43: 1125-1137, 1995.[Abstract]
- Krenacs T and Rosendaal M. Connexin43 gap junctions in normal, regenerating, and cultured mouse bone marrow and in human leukemias: their possible involvement in blood formation. Am J Pathol 152: 993-1004, 1998.[Abstract]
- Krenacs T, van Dartel M, Lindhout E, and Rosendaal M. Direct cell/cell communication in the lymphoid germinal center: connexin43 gap junctions functionally couple follicular dendritic cells to each other and to B lymphocytes. Eur J Immunol 27: 1489-1497, 1997.[Web of Science][Medline]
- Krüger O, Beny JL, Chabaud F, Traub O, Theis M, Brix K, Kirchhoff S, and Willecke K. Altered dye diffusion and upregulation of connexin37 in mouse aortic endothelium deficient in connexin40. J Vasc Res 39: 160-172, 2002.[Web of Science][Medline]
- Krüger O, Plum A, Kim JS, Winterhager E, Maxeiner S, Hallas G, Kirchhoff S, Traub O, Lamers WH, and Willecke K. Defective vascular development in connexin 45-deficient mice. Development 127: 4179-4193, 2000.[Abstract]
- Kumai M, Nishii K, Nakamura K, Takeda N, Suzuki M, and Shibata Y. Loss of connexin45 causes a cushion defect in early cardiogenesis. Development 127: 3501-3512, 2000.[Abstract]
- Kumar NM and Gilula NB. Cloning and characterization of human and rat liver cDNAs coding for a gap junction protein. J Cell Biol 103: 767-776, 1986.[Abstract/Free Full Text]
- Kunzelmann P, Blumcke I, Traub O, Dermietzel R, and Willecke K. Coexpression of connexin45 and -32 in oligodendrocytes of rat brain. J Neurocytol 26: 17-22, 1997.[Web of Science][Medline]
- Kurata WE and Lau AF. p130gag-fps disrupts gap junctional communication and induces phosphorylation of connexin43 in a manner similar to that of pp60v-src. Oncogene 9: 329-335, 1994.[Web of Science][Medline]
- Kuraoka A, Yamanaka I, Miyahara A, Shibata Y, and Uemura T. Immunocytochemical studies of major gap junction proteins in rat salivary glands. Eur Arch Otorhinolaryngol 251: S95-S99, 1994.[Medline]
- Kwak BR and Jongsma HJ. Selective inhibition of gap junction channel activity by synthetic peptides. J Physiol 516: 679-685, 1999.[Abstract/Free Full Text]
- Kwak BR, Mulhaupt F, Veillard N, Gros DB, and Mach F. Altered pattern of vascular connexin expression in atherosclerotic plaques. Arterioscler Thromb Vasc Biol 22: 225-230, 2002.[Abstract/Free Full Text]
- Kwak BR, Sáez JC, Wilders RW, Chanson M, Fishman GL, Hertzberg EL, Spray DC, and Jongsma HJ. Effects of cGMP-dependent phosphorylation on rat and human connexin43 gap junction channels. Pflügers Arch 430: 770-778, 1995.[Web of Science][Medline]
- Kwong KF, Schuessler RB, Green KG, Laing JG, Beyer EC, Boineau JP, and Saffitz JE. Differential expression of gap junction proteins in the canine sinus node. Circ Res 82: 604-612, 1998.[Abstract/Free Full Text]
- Kyoi T, Ueda F, Kimura K, Yamamoto M, and Kataoka K. Development of gap junctions between gastric surface mucous cells during cell maturation in rats. Gastroenterology 102: 1930-1935, 1992.[Web of Science][Medline]
- Laing JG and Beyer EC. The gap junction protein connexin43 is degraded via the ubiquitin proteasome pathway. J Biol Chem 270: 26399-26403, 1995.[Abstract/Free Full Text]
- Laing JG, Manley-Markowski RN, Koval M, Civitelli R, and Steinberg TH. Connexin45 interacts with zonula occludens-1 and connexin43 in osteoblastic cells. J Biol Chem 276: 23051-23055, 2001.[Abstract/Free Full Text]
- Laing JG, Tadros PN, Green K, Saffitz JE, and Beyer EC. Proteolysis of connexin43-containing gap junctions in normal and heat-stressed cardiac myocytes. Cardiovasc Res 38: 711-718, 1998.[Abstract/Free Full Text]
- Laing JG, Tadros PN, Westphale EM, and Beyer EC. Degradation of connexin43 gap junctions involves both the proteasome and the lysosome. Exp Cell Res 236: 482-492, 1997.[Web of Science][Medline]
- Laing JG, Westphale EM, Engelmann GL, and Beyer EC. Characterization of the gap junction protein, connexin45. J Membr Biol 139: 31-40, 1994.[Web of Science][Medline]
- Laird DW. The life cycle of a connexin: gap junction formation, removal, and degradation. J Bioenerg Biomembr 28: 311-318, 1996.[Web of Science][Medline]
- Laird DW, Castillo M, and Kasprzak L. Gap junction turnover, intracellular trafficking, and phosphorylation of connexin43 in brefeldin A-treated rat mammary tumor cells. J Cell Biol 131: 1193-1203, 1995.[Abstract/Free Full Text]
- Laird DW, Puranam KL, and Revel JP. Turnover and phosphorylation dynamics of connexin43 gap junction protein in cultured cardiac myocytes. Biochem J 273: 67-72, 1991.[Web of Science][Medline]
- Laird DW and Sáez JC. Posttranscriptional events in expression of gap junctions. In: Advances in Molecular Cell Biology, edited by Hertzberg EL. Greenwich: JAI, 2000, vol 18, p. 99-128.
- Lal R, John SA, Laird DW, and Arnsdorf MF. Heart gap junction preparations reveal hemiplaques by atomic force microscopy. Am J Physiol Cell Physiol 268: C968-C977, 1995.[Abstract/Free Full Text]
- Lamartine J, Munhoz Essenfelder G, Kibar Z, Lanneluc I, Caallouet E, Laoudj D, Lemaitre G, Hand C, Hayflick SJ, Zonana J, Aantonarakis S, Radhakrishma U, Kelsell DP, Christianson AL, Pitaval A, Der Kaloustian V, Fraser C, Blanchet-Bardon C, Rouleau GA, and Waksman G. Mutations in GJB6 cause hidrotic ectodermal dysplasia. Nat Genet 26: 142-144, 2000.[Web of Science][Medline]
- Lampe PD. Analyzing phorbol ester effects on gap junctional communication: a dramatic inhibition of assembly. J Cell Biol 127: 1895-1905, 1994.[Abstract/Free Full Text]
- Lampe PD, Kurata WE, Warn-Cramer BJ, and Lau AF. Formation of a distinct connexin43 phosphoisoform in mitotic cells is dependent upon p34cdc2 kinase. J Cell Sci 111: 833-841, 1998.[Abstract]
- Lampe PD and Lau AF. Regulation of gap junctions by phosphorylation of connexins. Arch Biochem Biophys 384: 205-215, 2000.[Web of Science][Medline]
- Lampe PD, TenBroek EM, Burt JM, Kurata WE, Johson RG, and Lau AF. Phosphorylation of connexin43 on serine368 by protein kinase C regulates gap junctional communication. J Cell Biol 149: 1503-1512, 2000.[Abstract/Free Full Text]
- Landesman Y, White TW, Starich TA, Shaw JE, Goodenough DA, and Paul DL. Innexin-3 forms connexin-like intercellular channels. J Cell Sci 112: 2391-2396, 1999.[Abstract]
- Larsen WJ. Biological implications of gap junction structure, distribution and composition: a review. Tissue Cell 15: 645-671, 1983.[Medline]
- Larsen WJ and Tung HN. Origin and fate of cytoplasmic gap junctional vesicles in rabbit granulosa cells. Tissue Cell 10: 585-598, 1978.[Web of Science][Medline]
- Larsen WJ, Tung HN, Murray SA, and Swenson CA. Evidence for the participation of actin microfilaments and bristle coats in the internalization of gap junction membrane. J Cell Biol 83: 576-587, 1979.[Abstract/Free Full Text]
- Larson DM, Seul KH, Berthoud VM, Lau AF, Sagar GD, and Beyer EC. Functional expression and biochemical characterization of an epitope-tagged connexin37. Mol Cell Biol Res Commun 3: 115-121, 2000.[Medline]
- Larson DM and Sheridan JD. Junctional transfer in cultured vascular endothelium. II. Dye and nucleotide transfer. J Membr Biol 83: 157-167, 1985.[Web of Science][Medline]
- Lau AF, Kanemitsu MY, Kurata WE, Danesh AL, and Boynton AK. Epidermal growth factor disrupts gap-junctional communication and induces phosphorylation of connexin43 on serine. Mol Biol Cell 3: 865-874, 1992.[Abstract]
- Lauf U, Giepmans BN, Lopez P, Braconnot S, Chen SC, and Falk MM. Dynamic trafficking and delivery of connexons to the plasma membrane and accretion to gap junctions in living cells. Proc Natl Acad Sci USA 99: 10446-10451, 2002.[Abstract/Free Full Text]
- Lautermann J, ten Cate WJ, Altenhoff P, Grummer R, Traub O, Frank H, Jahnke K, and Winterhager E. Expression of the gap-junction connexins 26 and 30 in the rat cochlea. Cell Tissue Res 294: 415-420, 1998.[Web of Science][Medline]
- Lawrence TS, Beers WH, and Gilula NB. Transmission of hormonal stimulation by cell-to-cell communication. Nature 272: 501-506, 1978.[Medline]
- Lee SM and Clemens MG. Subacinar distribution of hepatocyte membrane potential response to stimulation of gluconeogenesis. Am J Physiol Gastrointest Liver Physiol 263: G319-G326, 1992.[Abstract/Free Full Text]
- Lefebvre PP and van de Water TR. Connexins, hearing and deafness: clinical aspects of mutations in the connexin 26 gene. Brain Res Rev 32: 159-162, 2000.[Medline]
- Leite MF, Hirata K, Pusl T, Burgstahler AD, Okazaki K, Ortega JM, Goes AM, Prado MA, Spray DC, and Nathanson MH. Molecular basis for pacemaker cells in epithelia. J Biol Chem 277: 16313-16323, 2002.[Abstract/Free Full Text]
- Leube RE. The topogenic fate of the polytopic transmembrane proteins, synaptophysin and connexin, is determined by their membrane-spanning domains. J Cell Sci 108: 883-894, 1995.[Abstract]
- Levin M. Isolation and community: a review of the role of gapjunctional communication in embryonic patterning. J Membr Biol 185: 177-192, 2002.[Web of Science][Medline]
- Li F, Sugishita K, Su Z, Ueda I, and Barry WH. Activation of connexin-43 hemichannels can elevate [Ca2+]i and [Na+]i in rabbit ventricular myocytes during metabolic inhibition. J Mol Cell Cardiol 33: 2145-2155, 2001.[Web of Science][Medline]
- Li GY, Lin HH, Tu ZJ, and Kiang DT. Gap junction Cx26 gene modulation by phorbol esters in benign and malignant human mammary cells. Gene 209: 139-147, 1998.[Web of Science][Medline]
- Li H, Liu TF, Lazrak A, Peracchia C, Goldberg GS, Lampe PD, and Johnson RG. Properties and regulation of gap junctional hemichannels in the plasma membranes of cultured cells. J Cell Biol 134: 1019-1030, 1996.[Abstract/Free Full Text]
- Li WE and Nagy JI. Connexin43 phosphorylation state and intercellular communication in cultured astrocytes following hypoxia and protein phosphatase inhibition. Eur J Neurosci 12: 2644-2650, 2000.[Web of Science][Medline]
- Liao Y, Day KH, Damon DN, and Duling BR. Endothelial cellspecific knockout of connexin 43 causes hypotension and bradycardia in mice. Proc Natl Acad Sci USA 98: 9989-9994, 2001.[Abstract/Free Full Text]
- Lin JS, Fitzgerald S, Dong Y, Knight C, Donaldson P, and Kistler J. Processing of the gap junction protein connexin50 in the ocular lens is accomplished by calpain. Eur J Cell Biol 73: 141-149, 1997.[Web of Science][Medline]
- Lin R, Warn-Cramer BJ, Kurata WE, and Lau AF. v-Src-mediated phosphorylation of connexin43 on tyrosine disrupts gap junctional communication in mammalian cells. Cell Commun Adhes 8: 265-269, 2001.[Medline]
- Little TL, Beyer EC, and Duling BR. Connexin43 and connexin40 gap junctional proteins are present in arteriolar smooth muscle and endothelium in vivo. Am J Physiol Heart Circ Physiol 268: H729-H739, 1995.[Abstract/Free Full Text]
- Little TL, Xia J, and Duling BR. Dye tracers define differential endothelial and smooth muscle coupling patterns within the arteriolar wall. Circ Res 76: 498-504, 1995.[Abstract/Free Full Text]
- Liu LW, Farraway L, Berezin I, and Huizinga JD. Interstitial cells of Cajal: mediators of communication between circular and longitudinal muscle layers of canine colon. Cell Tissue Res 294: 69-79, 1998.[Web of Science][Medline]
- Liu TF and Johnson RG. Effects of TPA on dye transfer and dye leakage in fibroblast transfected with a connexin 43 mutation at ser368. Methods Find Exp Clin Pharmacol 21: 387-390, 1999.[Medline]
- Liu TF, Paulson AF, Li HY, Atkinson MM, and Johnson RG. Inhibitory effects of 12-O-tetradecanoylphorbol-13-acetate on dye leakage from single Novikoff cells and on dye transfer between reaggregated cell pairs. Methods Find Exp Clin Pharmacol 19: 573-577, 1997.[Web of Science][Medline]
- Liu S, Taffet S, Stoner L, Delmar M, Vallano ML, and Jalife J. A structural basis for the unequal sensitivity of the major cardiac and liver gap junctions to intracellular acidification: the carboxyl tail length. Biophys J 64: 1422-1433, 1993.[Medline]
- Lo CW. The role of gap junction membrane channels in development. J Bioenerg Biomembr 28: 379-385, 1996.[Web of Science][Medline]
- Lo CW. Genes, gene knockouts, and mutations in the analysis of gap junctions. Dev Genet 24: 1-4, 1999.[Web of Science][Medline]
- Lo CW, Waldo KL, and Kirby ML. Gap junction communication and the modulation of cardiac neural crest cells. Trends Cardiovasc Med 9: 63-69, 1999.[Web of Science][Medline]
- Locke D, Perusinghe N, Newman T, Jayatilake H, Evans WH, and Monaghan P. Developmental expression and assembly of connexins into homomeric and heteromeric gap junction hemichannels in the mouse mammary gland. J Cell Physiol 183: 228-237, 2000.[Web of Science][Medline]
- Loewenstein WR and Kanno Y. Studies on an epithelial (gland) cell junction. I. Modification of cell permeability. J Cell Biol 22: 565-586, 1964.[Abstract/Free Full Text]
- Loo LW, Berestecky JM, Kanemitsu MY, and Lau AF. pp60src-mediated phosphorylation of connexin 43, a gap junction protein. J Biol Chem 270: 12751-12561, 1995.[Abstract/Free Full Text]
- Loo LW, Kanemitsu MY, and Lau AF. In vivo association of pp60v-src and the gap-junction protein connexin 43 in v-src-transformed fibroblasts. Mol Carcinog 25: 187-195, 1999.[Web of Science][Medline]
- Lu C, Zhang H, Deng H, Xia K, Pan Q, and Xia J. Molecular cloning of the human CX 58 gene. Zhonghua Yi Xue Yi Chuan Xue Za Zhi 19: 95-99, 2002.[Medline]
- Luke RA and Saffitz JE. Remodeling of ventricular conduction pathways in healed canine infarct border zones. J Clin Invest 87: 1594-1602, 1991.[Web of Science][Medline]
- Mackay D, Ionides A, Berry V, Moore A, Bhattacharya S, and Shiels A. A new locus for dominat "zonular pulverulent" cataract, on chromosome 13. Am J Hum Genet 60: 1474-1478, 1997.[Web of Science][Medline]
- Makowski L, Caspar DLD, Phillips WC, and Goodenough DA. Gap junction structures. II. Analysis of the X-ray diffraction data. J Cell Biol 74: 629-645, 1977.[Abstract/Free Full Text]
- Malchow RP, Quian H, and Ripps H. A novel action of quinine and quinidine on the membrane conductance of neurons from the vertebrate retina. J Gen Physiol 104: 1039-1055, 1994.[Abstract/Free Full Text]
- Mambetisaeva ET, Gire V, and Evans WH. Multiple connexin expression in peripheral nerve, Schwann cells, and Schwannoma cells. J Neurosci Res 57: 166-175, 1999.[Web of Science][Medline]
- Manjunath CK, Goings GE, and Page E. Cytoplasmic surface and intramembrane components of rat heart gap junctional proteins. Am J Physiol Heart Circ Physiol 246: H865-H875, 1984.[Abstract/Free Full Text]
- Manjunath CK, Goings GE, and Page E. Proteolysis of cardiac gap junctions during their isolation from rat hearts. J Membr Biol 85: 159-168, 1985.[Medline]
- Manthey D, Banach K, Desplantez T, Lee CG, Kozak CA, Traub O, Weingart R, and Willecke K. Intracellular domains of mouse connexin26 and -30 affect diffusional and electrical properties of gap junction channels. J Membr Biol 181: 137-148, 2001.[Web of Science][Medline]
- Manthey D, Bukauskas F, Lee CG, Kozak CA, and Willecke K. Molecular cloning and functional expression of the mouse gap junction gene connexin-57 in human HeLa cells. J Biol Chem 274: 14716-14723, 1999.[Abstract/Free Full Text]
- Martin CA, Homaidan FR, Palaia T, Burakoff R, and El-Sabban ME. Gap junctional communication between murine macrophages and intestinal epithelial cell lines. Cell Commun Adhes 5: 437-449, 1998.
- Martin PE, Blundell G, Ahmad S, Errington RJ, and Evans WH. Multiple pathways in the trafficking and assembly of connexin 26, 32 and 43 into gap junction intercellular communication channels. J Cell Sci 114: 3845-3855, 2001.[Abstract/Free Full Text]
- Martínez AD, Eugenín EA, Brañes MC, Bennett MV, and Sáez JC. Identification of second messengers that induce expression of functional gap junctions in microglia cultured from newborn rats. Brain Res 943: 191-201, 2002.[Medline]
- Martínez AD, Hayrapetyan V, Moreno AP, and Beyer EC. Connexin43 and connexin45 form heteromeric gap junction channels in which individual components determine permeability and regulation. Circ Res 90: 1100-1107, 2002.[Abstract/Free Full Text]
- Mathias RT, Rae JL, and Baldo GJ. Physiological properties of the normal lens. Physiol Rev 77: 21-50, 1997.[Abstract/Free Full Text]
- Mayerhofer A and Garfield RE. Immunocytochemical analysis of the expression of gap junction protein connexin 43 in the rat ovary. Mol Reprod Dev 41: 331-338, 1995.[Web of Science][Medline]
- Mazet F, Wittenberg BA, and Spray DC. Fate of intercellular junctions in isolated adult rat cardiac cells. Circ Res 56: 195-204, 1985.[Abstract/Free Full Text]
- McGrew LL, Takemaru K, Bates R, and Moon RT. Direct regulation of the Xenopus engrailed-2 promoter by the Wnt signaling pathway, and a molecular screen for Wnt-responsive genes, confirm a role for Wnt signaling during neural patterning in Xenopus. Mech Dev 87: 21-32, 1999.[Web of Science][Medline]
- Meda P. Gap junction involvement in secretion: the pancreas experience. Clin Exp Pharmacol Physiol 23: 1053-1057, 1996.[Web of Science][Medline]
- Meda P, Bruzzone R, Knodel S, and Orci L. Blockage of cell-to-cell communication within pancreatic acini is associated with increased basal release of amylase. J Cell Biol 103: 475-483, 1986.[Abstract/Free Full Text]
- Meda P, Pepper MS, Traub O, Willecke K, Gros D, Beyer E, Nicholson B, Paul D, and Orci L. Differential expression of gap junction connexins in endocrine and exocrine glands. Endocrinology 133: 2371-2378, 1993.[Abstract/Free Full Text]
- Mehta PP, Yamamoto M, and Rose B. Transcription of the gene for the gap junctional protein connexin43 and expression of functional cell-to-cell channels are regulated by cAMP. Mol Biol Cell 3: 839-850, 1992.[Abstract]
- Meyer RA, Laird DW, Revel JP, and Johnson RG. Inhibition of gap junction and adherens junction assembly by connexin and A-CAM antibodies. J Cell Biol 119: 179-189, 1992.[Abstract/Free Full Text]
- Milks LC, Kumar NM, Houghten R, Unwin N, and Gilula NB. Topology of the 32-kD liver gap junction protein determined by site directed antibody localizations. EMBO J 7: 2967-2975, 1988.[Web of Science][Medline]
- Mine T, Yusuda H, Kataoka A, Tajima A, Nagasawa J, and Takano T. Human chronic gastric ulcer and connexin. J Clin Gastroenterol 21: S104-S107, 1995.[Medline]
- Minkoff R, Rundus VR, Parker SB, Beyer EC, and Hertzberg EL. Connexin expression in the developing avian cardiovascular system. Circ Res 73: 71-78, 1993.[Abstract]
- Moennikes O, Buchmann A, Ott T, Willecke K, and Schwarz M. The effect of connexin32 null mutation on hepatocarcinogenesis in different mouse strains. Carcinogenesis 20: 1379-1382, 1999.[Abstract/Free Full Text]
- Mok BW, Yeung WS, and Luk JM. Differential expression of gap-junction gene connexin 31 in seminiferous epithelium of rat testes. FEBS Lett 453: 243-248, 1999.[Medline]
- Montecino-Rodríguez E and Dorshkind K. Regulation of hematopoiesis by gap junction-mediated intercellular communication. J Leukoc Biol 70: 341-347, 2001.[Abstract/Free Full Text]
- Moorby CD and Gherardi E. Expression of a Cx43 deletion mutant in 3T3 A31 fibroblasts prevents PDGF-induced inhibition of cell communication and suppresses cell growth. Exp Cell Res 249: 367-376, 1999.[Web of Science][Medline]
- Moreno AP, Campos de Carvalho AC, Verselis V, Eghbali B, and Spray DC. Voltage-dependent gap junction channels are formed by connexin32, the major gap junction protein of rat liver. Biophys J 59: 920-925, 1991.[Medline]
- Moreno AP, Fishman GI, and Spray DC. Phosphorylation shifts unitary conductance and modifies voltage dependent kinetics of human connexin43 gap junction channels. Biophys J 62: 51-53, 1992.[Medline]
- Moreno AP, Laing JG, Beyer EC, and Spray DC. Properties of gap junction channels formed of connexin 45 endogenously expressed in human hepatoma (SKHep1) cells. Am J Physiol Cell Physiol 268: C356-C365, 1995.[Abstract/Free Full Text]
- Morle L, Bozon M, Alloisio N, Latour P, Vanderberghe A, Plauchu H, Collet L, Edery P, Godet J, and Lina-Granade G. A novel C202F mutation in the connexin26 gene (GJB2) associated with autosomal dominant isolated hearing loss. J Med Genet 37: 368-370, 2000.[Abstract/Free Full Text]
- Motoyama K, Karl IE, Flye MW, Osborne DF, and Hotchkiss RS. Effect of Ca2+ agonists in the perfused liver: determination via laser scanning confocal microscopy. Am J Physiol Regul Integr Comp Physiol 276: R575-R585, 1999.[Abstract/Free Full Text]
- Muller DJ, Hand GM, Engel A, and Sosinsky GE. Conformational changes in surface structures of isolated connexin 26 gap junctions. EMBO J 21: 3598-3607, 2002.[Medline]
- Munari-Silem Y and Rousset B. Gap junction-mediated cell-to-cell communication in endocrine glands. Molecular and functional aspects: a review. Eur J Endocrinol 135: 251-264, 1996.[Abstract/Free Full Text]
- Muramatsu T, Hashimoto S, and Shimono M. Differential expression of gap junction proteins connexin32 and 43 in rat submandibular and sublingual glands. J Histochem Cytochem 44: 49-56, 1996.[Abstract]
- Murray SA, Larsen WJ, Trout J, and Donta ST. Gap junction assembly and endocytosis correlated with patterns of growth in a cultured adrenocortical tumor cell (SW-13). Cancer Res 41: 4063-4074, 1981.[Abstract/Free Full Text]
- Murray SA, Williams SY, Dillard CY, Narayanan SK, and Mc-Cauley J. Relationship of cytoskeletal filaments to annular gap junction expression in human adrenal cortical tumor cells in culture. Exp Cell Res 234: 398-404, 1997.[Web of Science][Medline]
- Musil LS, Beyer EC, and Goodenough DA. Expression of the gap junction protein connexin43 in embryonic chick lens: molecular cloning, ultrastructural localization, and post-translational phosphorylation. J Membr Biol 116: 163-175, 1990.[Web of Science][Medline]
- Musil LS, Cunningham BA, Edelman GM, and Goodenough DA. Differential phosphorylation of the gap junction protein connexin43 in junctional communication-competent and -deficient cell lines. J Cell Biol 111: 2077-2088, 1990.[Abstract/Free Full Text]
- Musil LS and Goodenough DA. Biochemical analysis of connexin43 intracellular transport, phosphorylation, and assembly into gap junctional plaques. J Cell Biol 115: 1357-1374, 1991.[Abstract/Free Full Text]
- Musil LS and Goodenough DA. Multisubunit assembly of an integral plasma membrane channel protein, gap junction connexin43, occurs after exit from the ER. Cell 74: 1065-1077, 1993.[Web of Science][Medline]
- Musso M, Balestra P, Bellone E, Cassandrini D, Di Maria E, Doria LL, Grandis M, Mancardi GL, Schenone A, Levi G, Ajmar and Mandich PF. The D355V mutation decreases EGR2 binding to an element within the Cx32 promoter. Neurobiol Dis 8: 700-706, 2001.[Web of Science][Medline]
- Nagy JI, Li X, Rempel J, Stelmack G, Patel D, Staines WA, Yasumura T, and Rash JE. Connexin26 in adult rodent central nervous system: demonstration at astrocytic gap junctions and colocalization with connexin30 and connexin43. J Comp Neurol 441: 302-323, 2001.[Web of Science][Medline]
- Nakamura K, Kuraoka A, Kawabuchi M, and Shibata Y. Specific localization of gap junction protein, connexin45, in the deep muscular plexus of dog and rat small intestine. Cell Tissue Res 292: 487-494, 1998.[Web of Science][Medline]
- Nathanson M, Burgstahler AD, Mennone A, Fallon MB, González CB, and Sáez JC. Ca2+ waves are organized among hepatocytes in the intact organ. Am J Physiol Gastrointest Liver Physiol 269: G167-G171, 1995.[Abstract/Free Full Text]
- Nathanson MH, Ríos-Vélez L, Burgstahler AD, and Mennone A. Communication via gap junctions modulates bile secretion in the isolated perfused rat liver. Gastroenterology 116: 1176-1183, 1999.[Web of Science][Medline]
- Naus CC, Hearn S, Zhu D, Nicholson BJ, and Shivers RR. Ultrastructural analysis of gap junctions in C6 glioma cells transfected with connexin43 cDNA. Exp Cell Res 206: 72-84, 1993.[Web of Science][Medline]
- Nelles E, Bützler C, Jung D, Temme A, Gabriel HD, Dahl U, Traub O, Stümpel F, Jungermann K, Zielasek J, Toyka KV, Dermietzel R, and Willecke K. Defective propagation of signals generated by sympathetic nerve stimulation in the liver of conexin32-deficient mice. Proc Natl Acad Sci USA 93: 9565-9570, 1996.[Abstract/Free Full Text]
- Nemeth L, Maddur S, and Puri P. Immunolocalization of the gap junction protein Connexin43 in the interstitial cells of Cajal in the normal and Hirschsprung's disease bowel. J Pediatr Surg 35: 823-828, 2000.[Medline]
- Neuhaus IM, Bone L, Wang S, Ionasescu V, and Werner R. The human connexin32 gene is transcribed from two tissue-specific promoters. Biosci Rep 16: 239-248, 1996.[Web of Science][Medline]
- Ngezahayo A, Zeilinger C, Todt II, Marten II, and Kolb H. Inactivation of expressed and conducting rCx46 hemichannels by phosphorylation. Pflügers Arch 436: 627-629, 1998.[Medline]
- Nicholson BJ, Dermietzel R, Teplow D, Traub O, Willecke K, and Revel JPP. Two homologous protein components of hepatic gap junctions. Nature 329: 732-734, 1987.[Medline]
- Nicholson BJ, Gros DB, Kent SB, Hood LE, and Revel JP. The Mr 28,000 gap junction proteins from rat heart and liver are different but related. J Biol Chem 260: 6514-6517, 1985.[Abstract/Free Full Text]
- Nicholson SM, Gomes D, De Nechaud B, and Bruzzone R. Altered gene expression in Schwann cells of connexin32 knockout animals. J Neurosci Res 66: 23-36, 2001.[Web of Science][Medline]
- Nielsen PA, Baruch A, Giepmans BN, and Kumar NM. Characterization of the association of connexins and ZO-1 in the lens. Cell Commun Adhes 8: 213-217, 2001.[Web of Science][Medline]
- Nielsen PA, Beahm DL, Giepmans BN, Baruch A, Hall JE, and Kumar NM. Molecular cloning, functional expression, and tissue distribution of a novel human gap junction forming protein, connexin-31.9. Interaction with zona occludens protein-1. J Biol Chem 277: 38272-38283, 2002.[Abstract/Free Full Text]
- Niessen H, Harz H, Bedner P, Kramer K, and Willecke K. Selective permeability of different connexin channels to the second messenger inositol 1,4,5-trisphosphate. J Cell Sci 113: 1365-1372, 2000.[Abstract]
- Niessen H and Willecke K. Strongly decreased gap junctional permeability to inositol 1,4,5-trisphosphate in connexin32 deficient hepatocytes. FEBS Lett 466: 112-114, 2000.[Web of Science][Medline]
- Nishii K, Kumai M, and Shibata Y. Regulation of the epithelialmesenchymal transformation through gap junction channels in heart development. Trends Cardiovasc Med 11: 213-218, 2001.[Web of Science][Medline]
- Nuttinck F, Peynot N, Humblot P, Massip A, Dessy F, and Flechon JE. Comparative immunohistochemical distribution of connexin 37 and connexin 43 throughout folliculogenesis in the bovine ovary. Mol Reprod Dev 57: 60-66, 2000.[Web of Science][Medline]
- O'Brien J, Al-Ubaidi MR, and Ripps H. Connexin35: a gap junctional protein expressed preferentially in the skate retina. Mol Biol Cell 7: 233-243, 1996.[Abstract]
- O'Brien J, Bruzzone R, White TW, Al-Ubaidi MR, and Ripps H. Cloning and expression of two related connexins from the perch retina define a distinct subgroup of the connexin family. J Neurosci 18: 7625-7637, 1998.[Abstract/Free Full Text]
- Oesterle EC and Dallos P. Intracellular recordings from supporting cells in the guinea pig cochlea: DC potencials. J Neurophysiol 64: 617-636, 1990.[Abstract/Free Full Text]
- Oh S, Ri Y, Bennett MV, Trexler EB, Verselis VK, and Bargiello TA. Changes in permeability caused by connexin 32 mutations underlie X-linked Charcot-Marie-Tooth disease. Neuron 19: 927-938, 1997.[Web of Science][Medline]
- Oh SY, Grupen CG, and Murray AW. Phorbol ester induces phosphorylation and down-regulation of connexin 43 in WB cells. Biochim Biophys Acta 1094: 243-245, 1991.[Medline]
- Ohkusa T, Fujiki K, Tamura Y, Yamamoto M, and Kyoi T. Freeze-fracture and immunohistochemical studies of gap junctions in human gastric mucosa with special reference to their relationship to gastric ulcer and gastric carcinoma. Microsc Res Tech 31: 226-233, 1995.[Medline]
- Okuma A, Kuraoka A, Iida H, Inai T, Wasano K, and Shibata Y. Colocalization of connexin 43 and connexin 45 but absence of connexin 40 in granulosa cell gap junctions of rat ovary. J Reprod Fertil 107: 255-264, 1996.[Abstract/Free Full Text]
- Omori Y, Mesnil M, and Yamasaki H. Connexin 32 mutations from X-linked Charcot-Marie-Tooth disease patients: functional defects and dominant negative effects. Mol Biol Cell 7: 907-916, 1996.[Abstract]
- Oosthoek PW, Viragh S, Lamers WH, and Moorman AFM. Immunohistochemical delineation of the conduction system. II. The atrioventricular node and the Purkinje fibers. Circ Res 73: 482-491, 1993.[Abstract/Free Full Text]
- Oosthoek PW, Viragh S, Mayen AEM, van Kempen MJA, Lamers WH, and Moorman AFM. Immunohistochemical delineation of the conduction system. I. The sinoatrial node. Circ Res 73: 473-481, 1993.[Abstract/Free Full Text]
- Orkand RK, Nicholls JG, and Kuffler SWK. Effect of nerve impulses on the membrane potential of glial cells in the central nervous system of amphibia. J Neurophysiol 29: 788-806, 1966.[Free Full Text]
- Orsino A, Taylor CV, and Lye SJ. Connexin-26 and connexin43 are differentially expressed and regulated in the rat myometrium throughout late pregnancy and with the onset of labor. Endocrinology 137: 1545-1553, 1996.[Abstract]
- Oviedo-Orta E, Errington RJ, and Evans WH. Gap junction intercellular communication during lymphocyte transendothelial migration. Cell Biol Int 26: 253-263, 2002.[Web of Science][Medline]
- Oviedo-Orta E, Gasque P, and Evans WH. Immunoglobulin and cytokine expression in mixed lymphocyte cultures is reduced by disruption of gap junction intercellular communication. FASEB J 15: 768-774, 2001.[Abstract/Free Full Text]
- Oviedo-Orta E, Hoy T, and Evans WH. Intercellular communication in the immune system: differential expression of connexin40 and 43, and perturbation of gap junction channel functions in peripheral blood and tonsil human lymphocyte subpopulations. Immunology 99: 578-590, 2000.[Web of Science][Medline]
- Pal JD, Berthoud VM, Beyer EC, Mackay D, Shiels A, and Ebihara L. Molecular mechanism underlying a Cx50-linked congenital cataract. Am J Physiol Cell Physiol 276: C1443-C1446, 1999.[Abstract/Free Full Text]
- Pal JD, Liu X, Mackay D, Shiels A, Berthoud VM, Beyer EC and Ebihara L. Connexin46 mutations linked to congenital cataract show loss of gap junction channel function. Am J Physiol Cell Physiol 279: C596-C602, 2000.[Abstract/Free Full Text]
- Panchin Y, Kelmanson I, Matz M, Lukyanov K, Usman N, and Lukyanov S. A ubiquitous family of putative gap junction molecules. Curr Biol 29: R473-R474, 2000.
- Pappas GD, Asada Y, and Bennett MVL. Morphological correlates of increased coupling resistance at an electrotonic synapse. J Cell Biol 49: 173-188, 1971.[Abstract/Free Full Text]
- Pauken CM and Lo CW. Nonoverlapping expression of Cx43 and Cx26 in the mouse placenta and decidua: a pattern of gap junction gene expression differing from that in the rat. Mol Reprod Dev 41: 195-203, 1995.[Medline]
- Paul DL. Molecular cloning of cDNA for rat liver gap junction. J Cell Biol 103: 123-134, 1986.[Abstract/Free Full Text]
- Paul DL. New functions for gap junctions. Curr Opin Cell Biol 7: 665-672, 1995.[Web of Science][Medline]
- Paul DL, Ebihara L, Takemoto LJ, Swenson KI, and Goodenough DA. Connexin46, a novel lens gap junction protein, induces voltage-gated currents in nonjunctional plasma membrane of Xenopus oocytes. J Cell Biol 115: 1077-1089, 1991.[Abstract/Free Full Text]
- Paulson AF, Lampe PD, Meyer RA, TenBroek E, Atkinson MM, Walseth TF, and Johnson RG. Cyclic AMP and LDL trigger a rapid enhancement in gap junction assembly through a stimulation of connexin trafficking. J Cell Sci 113: 3037-3049, 2000.[Abstract]
- Paznekas WA, Boyadjiev SA, Shapiro RE, Daniels O, Wollnik B, Keegan CE, Innis JW, Dinulos MB, Christian C, Hannibal MC, and Jabs EW. Connexin 43 (GJA1) mutations cause the pleiotropic phenotype of oculodentodigital dysplasia. Am J Hum Genet 72: 408-418, 2003.[Web of Science][Medline]
- Pelletier RM. Cyclic modulation of Sertoli cell junctional complexes in a seasonal breeder: the mink (Mustela vison). Am J Anat 183: 68-102, 1988.[Web of Science][Medline]
- Pelletier RM. The distribution of connexin 43 is associated with the germ cell differentiation and with the modulation of the Sertoli cell junctional barrier in continual (guinea pig) and seasonal breeders' (mink) testes. J Androl 16: 400-409, 1995.[Abstract/Free Full Text]
- Pelletier DB and Boynton AL. Dissociation of PDGF receptor tyrosine kinase activity from PDGF-mediated inhibition of gap junctional communication. J Cell Physiol 158: 427-434, 1994.[Web of Science][Medline]
- Pérez-Armendáriz EM, Lamoyi E, Mason JI, Cisneros-Armas D, Luu-The V, and Bravo Moreno JF. Developmental regulation of connexin 43 expression in fetal mouse testicular cells. Anat Rec 264: 237-246, 2001.[Medline]
- Perkins GA, Goodenough DA, and Sosinsky GE. Formation of the gap junction intercellular channel requires a 30 degree rotation for interdigitating two apposing connexons. J Mol Biol 277: 171-177, 1998.[Web of Science][Medline]
- Peters NS. Myocardial gap junction organization in ischemia and infarction. Microsc Res Tech 31: 375-386, 1995.[Web of Science][Medline]
- Pfahnl A and Dahl G. Localization of a voltage gate in connexin46 gap junction hemichannels. Biophys J 75: 2323-2331, 1998.[Medline]
- Pfahnl A and Dahl G. Gating of cx46 gap junction hemichannels by calcium and voltage. Pflügers Arch 437: 345-353, 1999.[Web of Science][Medline]
- Pfahnl A, Zhou XW, Werner R, and Dahl G. A chimeric connexin forming gap junction hemichannels. Pflügers Arch 433: 773-779, 1997.[Web of Science][Medline]
- Phelan P, Bacon JP, Davies JA, Stebbings LA, Todman MG, Avery L, Baines RA, Barnes TM, Ford C, Hekimi S, Lee R, Shaw JE, Starich TA, Curtin KD, Sun YA, and Wyman RJ. Innexins: a family of invertebrate gap-junction proteins. Trends Genet 14: 348-349, 1998.[Web of Science][Medline]
- Phelan P and Starich TA. Innexins get into the gap. Bioessays 23: 388-396, 2001.[Web of Science][Medline]
- Phelan P, Stebbings LA, Baines RA, Bacon JP, Davies JB, and Ford C. Drosophila Shaking-B proteins forms gap junctions in paired Xenopus oocytes. Nature 391: 181-184, 1998.[Medline]
- Piechocki MP, Burk RD, and Ruch RJ. Regulation of connexin32 and connexin43 gene expression by DNA methylation in rat liver cells. Carcinogenesis 20: 401-406, 1999.[Abstract/Free Full Text]
- Piersanti M and Lye SJ. Increase in messenger ribonucleic acid encoding the myometrial gap junction protein, connexin-43, requires protein synthesis and is associated with increased expression of the activator protein-1, c-fos. Endocrinology 136: 3571-3578, 1995.[Abstract]
- Pinero GJ, Parker S, Rundus V, Hertzberg EL, and Minkoff R. Immunolocalization of connexin 43 in the tooth germ of the neonatal rat. Histochem J 26: 765-770, 1994.[Medline]
- Plotkin LI, Manolagas SC, and Bellido T. Transduction of cell survival signals by connexin-43 hemichannels. J Biol Chem 277: 8648-8657, 2002.[Abstract/Free Full Text]
- Plum A, Hallas G, Magin T, Dombrowski F, Hagendorff A, Schumacher B, Wolpert C, Kim J, Lamers WH, Evert M, Meda P, Traub O, and Willecke K. Unique and shared functions of different connexins in mice. Curr Biol 10: 1083-1091, 2000.[Web of Science][Medline]
- Polacek D, Bech F, McKinsey JF, and Davies PF. Connexin43 gene expression in the rabbit arterial wall: effects of hypercholesterolemia, balloon injury and their combination. J Vasc Res 34: 19-30, 1997.[Web of Science][Medline]
- Polacek D, Lal R, Volin MV, and Davies PF. Gap junctional communication between vascular cells. Induction of connexin43 messenger RNA in macrophage foam cells of atherosclerotic lesions. Am J Pathol 142: 593-606, 1993.[Abstract]
- Postma FR, Hengeveld T, Alblas J, Giepmans BN, Zondag GC, Jalink K, and Moolenaar WH. Acute loss of cell-cell communication caused by G protein-coupled receptors: a critical role for c-Src. J Cell Biol 140: 1199-1209, 1998.[Abstract/Free Full Text]
- Potenza N, del Gaudio R, Rivieccio L, Russo GM, and Geraci G. Cloning and molecular characterization of the first innexin of the phylum annelida-expression of the gene during development. J Mol Evol 54: 312-321, 2002.[Medline]
- Potter DD, Furshpan EJ, and Lennox ES. Connections between cells of the developing squid as revealed by electrophysiological methods. Proc Natl Acad Sci USA 55: 328-336, 1966.[Free Full Text]
- Proulx A, Merrifield PA, and Naus CC. Blocking gap junctional intercellular communication in myoblasts inhibits myogenin and MRF4 expression. Dev Genet 20: 133-144, 1997.[Web of Science][Medline]
- Puranam KL, Laird DW, and Revel JP. Trapping an intermediate form of connexin43 in the Golgi. Exp Cell Res 206: 85-92, 1993.[Web of Science][Medline]
- Quist AP, Rhee SK, Lin H, and Lal R. Physiological role of gap-junctional hemichannels. Extracellular calcium-dependent isosmotic volume regulation. J Cell Biol 148: 1063-1074, 2000.[Abstract/Free Full Text]
- Radebold K, Horakova E, Gloeckner J, Ortega G, Spray DC, Vieweger H, Siebert K, Manuelidis L, and Geibel JP. Gap junctional channels regulate acid secretion in the mammalian gastric gland. J Membr Biol 183: 147-153, 2001.[Web of Science][Medline]
- Rae JL, Bartling C, Rae J, and Mathias RT. Dye transfer between cells of the lens. J Membr Biol 150: 89-103, 1996.[Web of Science][Medline]
- Rahman S, Carlile G, and Evans WH. Assembly of hepatic gap junctions. Topography and distribution of connexin 32 in intracellular and plasma membranes determined using sequence-specific antibodies. J Biol Chem 268: 1260-1265, 1993.[Abstract/Free Full Text]
- Rahman S and Evans WH. Topography of connexin32 in rat liver gap junctions. Evidence for an intramolecular disulfide linkage connecting the two extracellular peptide loops. J Cell Sci 100: 567-578, 1991.[Abstract/Free Full Text]
- Rash JE and Yasumura T. Direct immunogold labeling of connexins and aquaporin-4 in freeze-fracture replicas of liver, brain, and spinal cord: factors limiting quantitative analysis. Cell Tissue Res 296: 307-321, 1999.[Web of Science][Medline]
- Reaume AG, de Sousa PA, Kulkarni S, Langille BL, Zhu D, Davies TC, Juneja SC, Kidder GM, and Rossant J. Cardiac malformation in neonatal mice lacking connexin43. Science 267: 1831-1834, 1995.[Abstract/Free Full Text]
- Reed KE, Westphale EM, Larson DM, Wang HZ, Veenstra RD, and Beyer EC. Molecular cloning and functional expression of human connexin37, an endothelial cell gap junction protein. J Clin Invest 91: 997-1004, 1993.[Web of Science][Medline]
- Ren P, de Feijter AW, Paul DL, and Ruch RJ. Enhancement of liver cell gap junction protein expression by glucocorticoids. Carcinogenesis 15: 1807-1813, 1994.[Abstract/Free Full Text]
- Ressot C, Gomes D, Dautigny A, Pham-Dinh D, and Bruzzone R. Connexin32 mutations associated with X-linked Charcot-Marie-Tooth disease show two distinct behaviors: loss of function and altered gating properties. J Neurosci 18: 4063-4075, 1998.[Abstract/Free Full Text]
- Revel JP and Karnovsky MJ. Hexagonal array of subunits in intercellular junctions of the mouse heart and liver. J Cell Biol 33: C7-C12, 1967.[Free Full Text]
- Revel JP, Yee AG, and Hudspeth AJ. Gap junctions between electrotonically coupled cells in tissue culture and in brown fat. Proc Natl Acad Sci USA 68: 2924-2927, 1971.[Abstract/Free Full Text]
- Richard G, Smith LE, Bailey RA, Itin P, Hohl D, Epstein EH Jr, DiGiovanna JJ, Compton JG, and Bale SJ. Mutations in the human connexin gene GJB3 cause erythrokeratodermia variabilis. Nat Genet 20, 366-369, 1998.[Web of Science][Medline]
- Richard G, White TW, Smith LE, Bailey RA, Compton JG, Paul DL, and Bale SJ. Functional defects of Cx26 resulting from a heterozygous missense mutation in a family with dominant deafmutism and palmoplantar keratoderma. Hum Genet 103: 393-399, 1998.[Web of Science][Medline]
- Risek B and Gilula NB. Spatiotemporal expression of three gap junction gene products involved in fetomaternal communication during rat pregnancy. Development 113: 165-181, 1991.[Abstract]
- Risek B, Guthrie S, Kumar N, and Gilula NB. Modulation of gap junction transcript and protein expression during pregnancy in the rat. J Cell Biol 110: 269-282, 1990.[Abstract/Free Full Text]
- Risek B, Klier FG, Phillips A, Hahn DW, and Gilula NB. Gap junction regulation in the uterus and ovaries of immature rats by estrogen and progesterone. J Cell Sci 108: 1017-1032, 1995.[Abstract]
- Risley MS. Connexin gene expression in seminiferous tubules of the Sprague-Dawley rat. Biol Reprod 62: 748-754, 2000.[Abstract/Free Full Text]
- Risley MS, Tan IP, Roy C, and Sáez JC. Cell-, age- and stage-dependent distribution of connexin43 gap junctions in testes. J Cell Sci 103: 81-96, 1992.[Abstract/Free Full Text]
- Robb-Gaspers LD and Thomas AP. Coordination of Ca2+ signaling by intercellular propagation of Ca2+ waves in the intact liver. J Biol Chem 270: 8102-8107, 1995.[Abstract/Free Full Text]
- Robertson JD. The occurrence of a subunit pattern in the unit membranes of the club endings in Mauthner cell synapses in goldfish brains. J Cell Biol 19: 201-221, 1963.[Abstract/Free Full Text]
- Romanello M and D'Andrea P. Dual mechanism of intercellular communication in HOBIT osteoblastic cells: a role for gap-junctional hemichannels. J Bone Miner Res 16: 1465-1476, 2001.[Web of Science][Medline]
- Rong P, Wang X, Niesman I, Wu Y, Benedetti LE, Dunia I, Levy E, and Gong X. Disruption of Gja8 (alpha8 connexin) in mice leads to microphthalmia associated with retardation of lens growth and lens fiber maturation. Development 129: 167-174, 2002.[Abstract/Free Full Text]
- Roscoe WA, Barr KJ, Mhawi AA, Pomerantz DK, and Kidder GM. Failure of spermatogenesis in mice lacking connexin43. Biol Reprod 65: 829-838, 2001.[Abstract/Free Full Text]
- Rosendaal M, Green CR, Rahman A, and Morgan D. Up-regulation of the connexin43+ gap junction network in haemopoietic tissue before the growth of stem cells. J Cell Sci 107: 29-37, 1994.[Abstract]
- Rosendaal M and Jopling C. Hemopoietic capacity of connexin43 wild-type and knock-out fetal liver cells not different on wild type stroma. Blood 101: 2996-2998, 2003.[Abstract/Free Full Text]
- Rosendaal M and Krenacs TT. Regulatory pathways in blood-forming tissue with particular reference to gap junctional communication. Pathol Oncol Res 6: 243-249, 2000.[Medline]
- Rouan F, White TW, Brown N, Taylor AM, Lucke TW, Paul DL, Munro CS, Uitto J, Hodgins MB, and Richard G.. Trans-dominant inhibition of connexin-43 by mutant connexin-26: implications for dominant connexin disorders affecting epidermal differentiation. J Cell Sci 114: 2105-2113, 2001.[Abstract/Free Full Text]
- Rozental R, Srinivas M, and Spray DC. How to close a gap junction channel. Efficacies and potencies of uncoupling agents. Methods Mol Biol 154: 447-476, 2001.[Medline]
- Rup DM, Veenstra RD, Wang HZ, Brink PR, and Beyer EC. Chick connexin-56, a novel lens gap junction protein. Molecular cloning and functional expression. J Biol Chem 268: 706-712, 1993.[Abstract/Free Full Text]
- Rutz ML and Hülser DF. Supramolecular dynamics of gap junctions. Eur J Cell Biol 80: 20-30, 2001.[Web of Science][Medline]
- Sáez JC, Araya R, Brañes MC, Concha M, Contreras JE, Eugenín EA, Martínez AD, Palisson F, and Sepúlveda MA. Gap junctions in inflammatory responses: connexins, regulation and possible functional roles. In: Current Topics in Membranes. Molecular Basis of Cell Communication in Health and Disease, edited by Perrachia C. San Diego: Academic, 2000, vol 49, p. 555-579.
- Sáez JC, Brañes MC, Corvalán LA, Eugenín EA, González H, Martínez AD, and Palisson F. Gap junctions in cells of the immune system: structure, regulation and possible functional roles. Braz J Med Biol Res 33: 447-455, 2000.[Web of Science][Medline]
- Sáez JC, Connor JA, Spray DC, and Bennett MVL. Hepatocyte gap junctions are permeable to the second messenger inositol 1,4,5-trisphosphate and calcium ions. Proc Natl Acad Sci USA 86: 2708-2712, 1989.[Abstract/Free Full Text]
- Sáez JC, Gregory WA, Watanabe T, Dermietzel R, Hertzberg EL, Reid L, Bennett MVL, and Spray DC. cAMP delays disappearance of gap junctions between pairs of rat hepatocytes in primary culture. Am J Physiol Cell Physiol 257: C1-C11, 1989.[Abstract/Free Full Text]
- Sáez JC, Martínez AD, Brañes MC, and González HE. Regulation of gap junctions by protein phosphorylation. Braz J Med Biol Res 31: 593-600, 1998.[Web of Science][Medline]
- Sáez JC, Nairn AC, Czernik AJ, Fishman GI, Spray DC, and Hertzberg EL. Phosphorylation of connexin43 and the regulation of neonatal rat cardiac myocyte gap junctions. J Mol Cell Cardiol 29: 2131-2145, 1997.[Web of Science][Medline]
- Sáez JC, Nairn AC, Czernik AJ, Spray DC, Hertzberg EL, Greengard P, and Bennett MV. Phosphorylation of connexin 32, a hepatocyte gap-junction protein, by cAMP-dependent protein kinase, protein kinase C and Ca2+/calmodulin-dependent protein kinase II. Eur J Biochem 192: 263-273, 1990.[Web of Science][Medline]
- Sáez JC, Nairn AC, Czernik AJ, Spray DC, and Hertzberg EL. Rat heart connexin43: regulation by phosphorylation in heart. Gap junctions. In: Progress in Cell Research, Gap Junctions, edited by Hall JE, Zampighi GA, and Davis RM. Amsterdam: Elsevier Science, 1993, vol. 39, p. 263-269.
- Sáez JC, Sepúlveda MA, Araya R, Sáez CG, and Palisson F. Concanavalin A-activated lymphocytes form gap junctions that increase their rate of DNA replication. In: Gap Junctions, edited by Werner R. Amsterdam: IOS, 1998, p. 372-381.
- Sáez JC, Spray DC, Nairn AC, Hertzberg E, Greengard P, and Bennett MV. cAMP increases junctional conductance and stimulates phosphorylation of the 27-kDa principal gap junction polypeptide. Proc Natl Acad Sci USA 83: 2473-2477, 1986.[Abstract/Free Full Text]
- Saffitz JE, Kanter HL, Green KG, Tolley TK, and Beyer EC. Tissue-specific determinants of anisotropic conduction velocity in canine atrial and ventricular myocardium. Circ Res 74: 1065-1070, 1994.[Abstract/Free Full Text]
- Saitoh M, Oyamada M, Oyamada Y, Kaku T, and Mori M. Changes in the expression of gap junction proteins (connexins) in hamster tongue epithelium during wound healing and carcinogenesis. Carcinogenesis 18: 1319-1328, 1997.[Abstract/Free Full Text]
- Saleh SM, Takemoto LJ, Zoukhri D, and Takemoto DJ. PKC-gamma phosphorylation of connexin 46 in the lens cortex. Mol Vis 7: 240-246, 2001.[Web of Science][Medline]
- Sasaki T and Garant PR. Fate of annular gap junctions in the papillary cells of the enamel organ in the rat incisor. Cell Tissue Res 246: 523-530, 1986.[Medline]
- Scherer SS, Deschenes SM, Xu YT, Grinspan JB, Fischbeck KH, and Paul DL. Conexin32 is a myelin-related protein in the PNS and CNS. J Neurosci 15: 8281-8294, 1995.[Abstract]
- Scherer SS, Xu YT, Nelles E, Fischbeck K, Willecke K, and Bone LJ. Connexin32-null mice develop demyelinating peripheral neuropathy. Glia 24: 8-20, 1998.[Web of Science][Medline]
- Schiavi A, Hudder A, and Werner R. Connexin43 mRNA contains a functional internal ribosome entry site. FEBS Lett 464: 118-122, 1999.[Web of Science][Medline]
- Schiller PC, Mehta PP, Roos BA, and Howard GA. Hormonal regulation of intercellular communication: parathyroid hormone increases connexin 43 gene expression and gap-junctional communication in osteoblastic cells. Mol Endocrinol 6: 1433-1440, 1992.[Abstract/Free Full Text]
- Schubert AL, Schubert W, Spray DC, and Lisanti MP. Connexin family members target to lipid raft domains and interact with caveolin-1. Biochemistry 41: 5754-5764, 2002.[Medline]
- Schwarz HJ, Chang YS, Hennemann H, Dahl E, Lalley PA, and Willecke K. Chromosomal assignments of mouse connexin genes, coding for gap junctional proteins, by somatic cell hybridization. Somat Cell Mol Genet 18: 351-359, 1992.[Web of Science][Medline]
- Scott DA, Kraft ML, Stone EM, Sheffield VC, and Smith RJ. Connexin mutations and hearing loss. Nature 391: 32, 1998.[Medline]
- Segal SS and Duling BR. Conduction of vasomotor responses in arterioles: a role for cell-to-cell coupling? Am J Physiol Heart Circ Physiol 256: H838-H845, 1989.[Abstract/Free Full Text]
- Seki K and Komuro T. Further observations on the gap-junction-rich cells in the deep muscular plexus of the rat small intestine. Anat Embryol 197: 135-141, 1998.[Medline]
- Seki K and Komuro T. Immunocytochemical demonstration of the gap junction proteins connexin 43 and connexin 45 in the musculature of the rat small intestine. Cell Tissue Res 306: 417-422, 2001.[Web of Science][Medline]
- Serre-Beinier V, Le Gurun S, Belluardo N, Trovato-Salinaro A, Charollais A, Haefliger JA, Condorelli DF, and Meda P. Cx36 preferentially connects beta-cells within pancreatic islets. Diabetes 49: 727-734, 2000.[Abstract]
- Serrière V, Berthon B, Boucherie S, Jacquemin E, Guillon G, Claret M, and Tordjmann T. Vasopressin receptor distribution in the liver controls calcium wave propagation and bile flow. FASEB J 15: 1484-1486, 2001.[Free Full Text]
- Seul KH and Beyer EC. Heterogeneous localization of connexin40 in the renal vasculature. Microvasc Res 59: 140-148, 2000.[Web of Science][Medline]
- Seul KH and Beyer EC. Mouse connexin37: gene structure and promoter analysis. Biochim Biophys Acta 1492: 499-504, 2000.[Medline]
- Seul KH, Tadros PN, and Beyer EC. Mouse connexin40: gene structure and promoter analysis. Genomics 46: 120-126, 1997.[Web of Science][Medline]
- Severs NJ. The cardiac muscle cell. Bioessays 22: 188-199, 2000.[Web of Science][Medline]
- Severs NJ, Rothery S, Dupont E, Coppen SR, Yeh HI, Ko YS, Matsushita T, Kaba R, and Halliday D. Immunocytochemical analysis of connexin expression in the healthy and diseased cardiovascular system. Microsc Res Tech 52: 301-322, 2001.[Web of Science][Medline]
- Severs NJ, Shovel KS, Slade AM, Powell T, Twist VW, and Green CR. Fate of gap junctions in isolated adult mammalian cardiomyocytes. Circ Res 65: 22-42, 1989.[Abstract/Free Full Text]
- Shibata Y, Kumai M, Nishii K, and Nakamura K. Diversity and molecular anatomy of gap junctions. Med Electron Microsc 34: 153-159, 2001.[Medline]
- Shibata Y, Manjunath CK, and Page E. Differences between cytoplasmic surfaces of deep-etched heart and liver gap junctions. Am J Physiol Heart Circ Physiol 249: H690-H693, 1985.[Abstract/Free Full Text]
- Shiels A, Mackay D, Ionides A, Berry V, Moore A, and Bhattacharya S. A missense mutation in the human connexin50 gene (GJA8) underlies autosomal dominant "zonular pulverulent" cataract, on chromosome 1q. Am J Hum Genet 62: 526-532, 1998.[Web of Science][Medline]
- Shimono M, Muramatsu T, Ihara A, Enokiya Y, Hashimoto S, and Inoue T. Connexins in the developing salivary glands. Eur J Morphol 36 Suppl.: 112-117, 1998.[Medline]
- Shimono M, Young Lee C, Matsuzaki H, Ishikawa H, Inoue T, Hashimoto S, and Muramatsu T. Connexins in salivary glands. Eur J Morphol 38: 257-261, 2000.[Medline]
- Simon AM. Gap junctions: more roles and new structural data. Trends Cell Biol 9: 169-170, 1999.[Web of Science][Medline]
- Simon AM and Goodenough DA. Diverse functions of vertebrate gap junctions. Trends Cell Biol 8: 477-483, 1998.[Web of Science][Medline]
- Simon AM, Goodenough DA, Li E, and Paul DL. Female infertility in mice lacking connexin 37. Nature 385: 525-529, 1997.[Medline]
- Simon AM, Goodenough DA, and Paul DL. Mice lacking connexin40 have cardiac conduction abnormalities characteristic of atrioventricular block and bundle branch block. Curr Biol 8: 295-298, 1998.[Web of Science][Medline]
- Skerrett IM, Aronowitz J, Shin JH, Cymes G, Kasperek E, Cao FL, and Nicholson BJ. Identification of amino acid residues lining the pore of a gap junction channel. J Cell Biol 159: 349-360, 2002.[Abstract/Free Full Text]
- Söhl G, Degen J, Teubner B, and Willecke K. The murine gap junction gene connexin36 is highly expressed in mouse retina and regulated during brain development. FEBS Lett 428: 27-31, 1998.[Web of Science][Medline]
- Söhl G, Eiberger J, Jung YT, Kozak CA, and Willecke K. The mouse gap junction gene connexin 29 is highly expressed in sciatic nerve and regulated during brain development. Biol Chem 382: 973-978, 2001.[Web of Science][Medline]
- Söhl G, Gillen C, Bosse F, Gleichmann M, Muller HW, and Willecke K. A second alternative transcript of the gap junction gene connexin32 is expressed in murine Schwann cells and modulated in injured sciatic nerve. Eur J Cell Biol 69: 267-275, 1996.[Web of Science][Medline]
- Söhl G, Guldenagel M, Traub O, and Willecke K. Connexin expression in the retina. Brain Res Rev 32: 138-145, 2000.[Medline]
- Söhl G, Theis M, Hallas G, Brambach S, Dahl E, Kidder G, and Willecke K. A new alternatively spliced transcript of the mouse connexin32 gene is expressed in embryonic stem cells, oocytes, and liver. Exp Cell Res 266: 177-186, 2001.[Web of Science][Medline]
- Sosinsky GE. Molecular organization of gap junction membrane channels. J Bioenerg Biomembr 28: 297-309, 1996.[Web of Science][Medline]
- Sotkis A, Wang XG, Yasumura T, Peracchia LL, Persechini A, Rash JE, and Peracchia C. Calmodulin colocalizes with connexins and plays a direct role in gap junction channel gating. Cell Commun Adhes 8: 277-281, 2001.[Web of Science][Medline]
- Spray DC. Physiological and pharmacological regulation of gap junction channels. In: Molecular Mechanisms of Epithelial Cell Junctions: From Development to Disease, edited by Citi S. Austin, TX: Landes, 1994, p. 195-215.
- Spray DC. Molecular physiology of gap junction channels. Clin Exp Pharmacol Physiol 23: 1038-1040, 1996.[Web of Science][Medline]
- Spray DC and Dermietzel R. Gap Junctions in the Nervous System. New York: Chapman and Hall, 1996.
- Spray DC, Moreno AP, Kessler JA, and Dermietzel R. Characterization of gap junctions between cultured leptomeningeal cells. Brain Res 568: 1-14, 1991.[Web of Science][Medline]
- Starich T, Lee RY, Panzarella C, Avery L, and Shaw JE. eat-5 and unc-7 represent a multigene family in Caenorhabditis elegans involved in cell-cell coupling. J Cell Biol 134: 537-548, 1996.[Abstract/Free Full Text]
- Starich T, Sheehan M, Jadrich J, and Shaw J. Innexins in C. elegans. Cell Commun Adhes 8: 311-314, 2001.[Medline]
- Stauffer KA. The gap junction proteins beta 1-connexin (connexin-32) and beta 2-connexin (connexin-26) can form heteromeric hemichannels. J Biol Chem 270: 6768-6772, 1995.[Abstract/Free Full Text]
- Stauffer KA, Kumar NM, Gilula NB, and Unwin N. Isolation and purification of gap junction channels. J Cell Biol 115: 141-150, 1991.[Abstract/Free Full Text]
- Stauffer PL, Zhao H, Luby-Phelps K, Moss RL, Star RA, and Muallem S. Gap junction communication modulates [Ca2+]i oscillations and enzyme secretion in pancreatic acini. J Biol Chem 268: 19769-19775, 1993.[Abstract/Free Full Text]
- Steele EC Jr, Lyon MF, Favor J, Guillot PV, Boyd Y, and Church RL. A mutation in the connexin 50 (Cx50) gene is a candidate for the No2 mouse cataract. Curr Eye Res 17: 883-889, 1998.[Web of Science][Medline]
- Steger K, Tetens F, and Bergmann M. Expression of connexin 43 in human testis. Histochem Cell Biol 112: 215-220, 1999.[Web of Science][Medline]
- Stock A and Sies H. Thyroid hormone receptors bind to an element in the connexin43 promoter. Biol Chem 381: 973-979, 2000.[Web of Science][Medline]
- Stout CE, Costantin JL, Naus CC, and Charles AC. Intercellular calcium signaling in astrocytes via ATP release through connexin hemichannels. J Biol Chem 277: 10482-10488, 2002.[Abstract/Free Full Text]
- Stumpel F, Ott T, Willecke K, and Jungermann K. Connexin 32 gap junctions enhance stimulation of glucose output by glucagon and noradrenaline in mouse liver. Hepatology 28: 1616-1620, 1998.[Web of Science][Medline]
- Suárez S and Ballmer-Hofer K. VEGF transiently disrupts gap junctional communication in endothelial cells. J Cell Sci 114: 1229-1235, 2001.[Abstract]
- Sullivan R, Ruangvoravat C, Joo D, Morgan J, Wang BL, Wang XK, and Lo CW. Structure, sequence and expression of the mouse Cx43 gene encoding connexin 43. Gene 130: 191-199, 1993.[Web of Science][Medline]
- Swenson KI, Jordan JR, Beyer EC, and Paul DL. Formation of gap junctions by expression of connexins in Xenopus oocyte pairs. Cell 57: 145-155, 1989.[Web of Science][Medline]
- Swenson KI, Piwnica-Worms H, McNamee H, and Paul DL. Tyrosine phosphorylation of the gap junction protein connexin43 is required for the pp60v-src-induced inhibition of communication. Cell Regul 1: 989-1002, 1990.[Web of Science][Medline]
- Takahashi N, Joh T, Yokohama Y, Seno K, Nomura T, Ohara H, Ueda F, and Itoh M. Importance of gap junction in gastric mucosal restitution from acid-induced injury. J Lab Clin Med 136: 93-99, 2000.[Medline]
- Takeda A, Hashimoto E, Yamamura H, and Shimazu T. Phosphorylation of liver gap junction protein by protein kinase C. FEBS Lett 210: 169-172, 1987.[Web of Science][Medline]
- Takeda A, Saheki S, Shimazu T, and Takeuchi N. Phosphorylation of the 27-kDa gap junction protein by protein kinase C in vitro and in rat hepatocytes. J Biochem 106: 723-727, 1989.[Abstract/Free Full Text]
- Tan IP, Roy C, Sáez JC, Sáez CG, Paul DL, and Risley MS. Regulated assembly of connexin33 and connexin43 into rat Sertoli cell gap junctions. Biol Reprod 54: 1300-1310, 1996.[Abstract]
- Taylor AB, Kreulen D, and Prosser CL. Electron microscopy of the connective tissues between longitudinal and circular muscle of small intestine of cat. Am J Anat 150: 427-441, 1977.[Web of Science][Medline]
- Temme A, Buchmann A, Gabriel HD, Nelles E, Schwarz M, and Willecke K. High incidence of spontaneous and chemically induced liver tumors in mice deficient for connexin32. Curr Biol 7: 713-716, 1997.[Web of Science][Medline]
- Temme A, Stumpel F, Sohl G, Rieber EP, Jungermann K, Willecke K, and Ott T. Dilated bile canaliculi and attenuated decrease of nerve-dependent bile secretion in connexin32-deficient mouse liver. Pflügers Arch 442: 961-966, 2001.[Web of Science][Medline]
- TenBroek EM, Lampe PD, Solan JL, Reynhout JK, and Johnson RG. Ser364 of connexin43 and the upregulation of gap junction assembly by cAMP. J Cell Biol 155: 1307-1318, 2001.[Abstract/Free Full Text]
- Thomas SA, Schuessler RB, Berul CI, Beardslee MA, Beyer EC, Mendelsohn ME, and Saffitz JE. Disparate effects of deficient expression of connexin43 on atrial and ventricular conduction: evidence for chamber-specific molecular determinants of conduction. Circulation 97: 686-691, 1998.[Abstract/Free Full Text]
- Tibbitts TT, Caspar DLD, Phillips WC, and Goodenough DA. Diffraction diagnosis of protein folding in gap junction connexons. Biophys J 57: 1025-1036, 1990.[Medline]
- Tordjmann T, Berthon B, Claret M, and Combettes L. Coordinated intercellular calcium waves induced by noradrenaline in rat hepatocytes: dual control by gap junction permeability and agonist. EMBO J 16: 5398-5407, 1997.[Web of Science][Medline]
- Toyofuku T, Akamatsu Y, Zhang H, Kuzuya T, Tada M, and Hori M. c-Src regulates the interaction between connexin-43 and ZO-1 in cardiac myocytes. J Biol Chem 276: 1780-1788, 2001.[Abstract/Free Full Text]
- Toyofuku T, Yabuki M, Otsu K, Kuzuya T, Hori M, and Tada M. Intercellular calcium signaling via gap junction in connexin-43-transfected cells. J Biol Chem 273: 1519-1528, 1998.[Abstract/Free Full Text]
- Toyofuku T, Yabuki M, Otsu K, Kuzuya T, Hori M, and Tada M. Direct association of the gap junction protein connexin-43 with ZO-1 in cardiac myocytes. J Biol Chem 273: 12725-12731, 1998.[Abstract/Free Full Text]
- Tran D, Stelly N, Tordjmann T, Durroux T, Dufour MN, Forchioni A, Seyer R, Claret M, and Guillon G. Distribution of signaling molecules involved in vasopressin-induced Ca2+ mobilization in rat hepatocyte multiplets. J Histochem Cytochem 47: 601-616, 1999.[Abstract/Free Full Text]
- Traub O, Butterweck A, Elfgag C, Hertlein B, Balzer K, Gregs U, Hafemann B, and Willecke K. Immunochemical characterization of connexin31, -37, -40, -43, and -45 in cultured primary cells, transfected cell lines and and murine tissues. In: Progress in Cell Research, Intercellular Communication Through Gap Junctions, edited by Kano K, Shiba Y, Shibata Y, and Shimazu T. Amsterdam: Elsevier Science, 1995, vol. 4, p. 343-347.
- Traub O, Hertlein B, Kasper M, Eckert R, Krisciukaitis A, Hülser D, and Willecke K. Characterization of the gap junction protein connexin37 in murine endothelium, respiratory epithelium, and after transfection in human HeLa cells. Eur J Cell Biol 77: 313-322, 1998.[Web of Science][Medline]
- Traub O, Look J, Dermietzel R, Brummer F, Hülser D, and Willecke K. Comparative characterization of the 21-kD and 26-kD gap junction proteins in murine liver and cultured hepatocytes. J Cell Biol 108: 1039-1051, 1989.[Abstract/Free Full Text]
- Traub O, Look J, Paul D, and Willecke K. Cyclic adenosine monophosphate stimulates biosynthesis and phosphorylation of the 26 kDa gap junction protein in cultured mouse hepatocytes. Eur J Cell Biol 43: 48-54, 1987.[Web of Science][Medline]
- Trexler EB, Bennett MV, Bargiello TA, and Verselis VK. Voltage gating and permeation in a gap junction hemichannel. Proc Natl Acad Sci USA 93: 5836-5841, 1996.[Abstract/Free Full Text]
- Trexler EB, Bukauskas FF, Bennett MV, Bargiello TA, and Verselis VK. Rapid and direct effects of pH on connexins revealed by the connexin46 hemichannel preparation. J Gen Physiol 113: 721-742, 1999.[Abstract/Free Full Text]
- Trexler EB, Bukauskas FF, Kronengold J, Bargiello TA, and Verselis VK. The first extracellular loop domain is a major determinant of charge selectivity in connexin46 channels. Biophys J 79: 3036-3051, 2000.[Web of Science][Medline]
- Tu ZJ, Pan W, Gong Z, and Kiang DT. Involving AP-2 transcription factor in connexin 26 up-regulation during pregnancy and lactation. Mol Reprod Dev 59: 17-24, 2001.[Medline]
- Uchida Y, Matsuda K, Sasahara K, Kawabata H, and Nishioka M. Immunohistochemistry of gap junctions in normal and diseased gastric mucosa of humans. Gastroenterology 109: 1492-1496, 1995.[Web of Science][Medline]
- Unger VM, Kumar NM, Gilula NB, and Yeager M. Projection structure of a gap junction membrane channel at 7 Å resolution. Nat Struct Biol 4: 39-43, 1997.[Web of Science][Medline]
- Unger VM, Kumar NM, Gilula NB, and Yeager M. Three-dimensional structure of a recombinant gap junction membrane channel. Science 283: 1176-1180, 1999.[Abstract/Free Full Text]
- Uwin PN and Ennis PD. Calcium-mediated changes in gap junction structure: evidence from the low angle X-ray pattern. J Cell Biol 97: 1459-1466, 1983.[Abstract/Free Full Text]
- Uwin PN and Zampighi G. Structure of the junction between communicating cells. Nature 283: 545-549, 1980.[Medline]
- Valdimarsson G, De Sousa PA, and Kidder GM. Coexpression of gap junction proteins in the cumulus-oocyte complex. Mol Reprod Dev 36: 7-15, 1993.[Medline]
- Valiunas V. Biophysical properties of connexin-45 gap junction hemichannels studied in vertebrate cells. J Gen Physiol 119: 147-164, 2002.[Abstract/Free Full Text]
- Valiunas V, Niessen H, Willecke K, and Weingart R. Electrophysiological properties of gap junction channels in hepatocytes isolated from connexin32-deficient and wild-type mice. Pflügers Arch 437: 846-856, 1999.[Web of Science][Medline]
- Valiunas V and Weingart R. Electrical properties of gap junction hemichannels identified in transfected HeLa cells. Pflügers Arch 440: 366-379, 2000.[Web of Science][Medline]
- Van der Heyden MA, Rook MB, Hermans MM, Rijksen G, Boonstra J, Defize LH, and Destree OH. Identification of connexin43 as a functional target for Wnt signalling. J Cell Sci 111: 1741-1749, 1998.[Abstract]
- Vanderhyden BC, Caron PJ, Buccione R, and Eppig JJ. Developmental pattern of the secretion of cumulus-expansion enabling factor by mouse oocytes and the role of oocytes in promoting granulosa cell differentiation. Dev Biol 140: 307-317, 1990.[Web of Science][Medline]
- Vanderhyden BC, Cohen JN, and Morley P. Mouse oocytes regulate granulosa cell steroidogenesis. Endocrinology 133: 423-426, 1993.[Abstract/Free Full Text]
- Van der Velden HMW and Jongsma HJ. Cardiac gap junctions and connexins: their role in atrial fibrillation and potential as therapeutic targets. Cardiovasc Res 54: 270-279, 2002.[Abstract/Free Full Text]
- Van Eldik LJ, Hertzberg EL, Berdan RC, and Gilula NB. Interaction of calmodulin and other calcium-modulated proteins with mammalian and arthropod junctional membrane proteins. Biochem Biophys Res Commun 126: 825-832, 1985.[Web of Science][Medline]
- Van Kempen MJ, Fromaget C, Gros D, Moorman AF, and Lamers WH. Spatial distribution of connexin43, the major cardiac gap junction protein, in the developing and adult rat heart. Circ Res 68: 1638-1651, 1991.[Abstract/Free Full Text]
- Van Kempen MJ and Jongsma HJ. Distribution of connexin37, connexin40 and connexin43 in the aorta and coronary artery of several mammals. Histochem Cell Biol 112: 479-486, 1999.[Web of Science][Medline]
- Vanoye CG, Vergara LA, and Reuss L. Isolated epithelial cells from amphibian urinary bladder express functional gap junctional hemichannels. Am J Physiol Cell Physiol 276: C279-C284, 1999.[Abstract/Free Full Text]
- Van Rijen H, van Kempen MJ, Analbers LJ, Rook MB, van Ginneken AC, Gros D, and Jongsma HJ. Gap junctions in human umbilical cord endothelial cells contain multiple connexins. Am J Physiol Cell Physiol 272: C117-C130, 1997.[Abstract/Free Full Text]
- Van Rijen HV, van Kempen MJ, Postma S, and Jongsma HJ. Tumour necrosis factor alpha alters the expression of connexin43, connexin40, and connexin37 in human umbilical vein endothelial cells. Cytokine 10: 258-264, 1998.[Web of Science][Medline]
- Van Rijen HV, van Veen TA, Hermans MM, and Jongsma HJ. Human connexin40 gap junction channels are modulated by cAMP. Cardiovasc Res 45: 941-951, 2000.[Abstract/Free Full Text]
- Van Rijen HV, van Veen TA, van Kempen MJ, Wilms-Schopman FJ, Potse M, Krueger O, Willecke K, Opthof T, Jongsma HJ, and de Bakker JM. Impaired conduction in the bundle branches of mouse hearts lacking the gap junction protein connexin40. Circulation 103: 1591-1598, 2001.[Abstract/Free Full Text]
- VanSlyke JK and Musil LS. Dislocation and degradation from the ER are regulated by cytosolic stress. J Cell Biol 157: 381-394, 2002.[Abstract/Free Full Text]
- Van Veen TA, van Rijen HV, and Jongsma HJ. Electrical conductance of mouse connexin45 gap junction channels is modulated by phosphorylation. Cardiovasc Res 46: 496-510, 2000.[Abstract/Free Full Text]
- Van Veen TA, van Rijen HVM, and Opthof T. Cardiac gap junction channels: modulation of expression and channel properties. Cardiovasc Res 51: 217-229, 2001.[Abstract/Free Full Text]
- Varanda WA and Campos de Carvalho AC. Intercellular communication between mouse Leydig cells. Am J Physiol Cell Physiol 267: C563-C569, 1994.[Abstract/Free Full Text]
- Vaughan DK and Lasater EM. Acid phosphatase localization in endocytosed horizontal cell gap junctions. Vis Neurosci 8: 77-81, 1992.[Web of Science][Medline]
- Veenstra RD, Wang HZ, Beblo DA, Chilton MG, Harris AL, Beyer EC, and Brink PR. Selectivity of connexin-specific gap junctions does not correlate with channel conductance. Circ Res 77: 1156-1165, 1995.[Abstract/Free Full Text]
- Veenstra RD, Wang HZ, Beyer EC, and Brink PR. Selective dye and ionic permeability of gap junction channels formed by connexin45. Circ Res 75: 483-490, 1994.[Abstract/Free Full Text]
- Veenstra RD, Wang HZ, Beyer EC, Ramanan SV, and Brink PR. Connexin37 forms high conductance gap junction channels with subconductance state activity and selective dye and ionic permeabilities. Biophys J 66: 1915-1928, 1994.[Medline]
- Veenstra RD, Wang HZ, Westphale EM, and Beyer EC. Multiple connexins confer distinct regulatory conductance properties of gap junctions in developing heart. Circ Res 71: 1277-1283, 1992.[Abstract/Free Full Text]
- Verderio C, Bruzzone S, Zocchi E, Fedele E, Schenk U, De Flora A, and Matteoli M. Evidence of a role for cyclic ADP-ribose in calcium signalling and neurotransmitter release in cultured astrocytes. J Neurochem 78: 646-657, 2001.[Web of Science][Medline]
- Verheijck EE, van Kempen MJ, Veereschild M, Lurvink J, Jongsma HJ, and Bouman LN. Electrophysiological features of the mouse sinoatrial node in relation to connexin distribution. Cardiovasc Res 52: 40-50, 2001.[Abstract/Free Full Text]
- Verheule S, van Kempen MJ, Postma S, Rook MB, and Jongsma HJ. Gap junctions in the rabbit sinoatrial node. Am J Physiol Heart Circ Physiol 280: H2103-H2115, 2001.[Abstract/Free Full Text]
- Vliagoftis H, Hutson AM, Mahmudi-Azer S, Kim H, Rumsaeng V, Oh CK, Moqbel R, and Metcalfe DD. Mast cells express connexins on their cytoplasmic membrane. J Allergy Clin Immunol 103: 656-662, 1999.[Medline]
- Vozzi C, Dupont E, Coppen SR, Yeh HI, and Severs NJ. Chamber-related differences in connexin expression in the human heart. J Mol Cell Cardiol 31: 991-1003, 1999.[Web of Science][Medline]
- Vozzi C, Formenton A, Chanson A, Senn A, Sahli R, Shaw P, Nicod P, Germond M, and Haefliger JA. Involvement of connexin 43 in meiotic maturation of bovine oocytes. Reproduction 122: 619-628, 2001.[Abstract]
- Vozzi C, Ullrich S, Charollais A, Philippe J, Orci L, and Meda P. Adequate connexin-mediated coupling is required for proper insulin production. J Cell Biol 131: 1561-1572, 1995.[Abstract/Free Full Text]
- Walcott B, Moore LC, Birzgalis A, Claros N, Valiunas V, Ott T, Willecke K, and Brink PR. Role of gap junctions in fluid secretion of lacrimal glands. Am J Physiol Cell Physiol 282: C501-C507, 2002.[Abstract/Free Full Text]
- Wang Y and Daniel EE. Gap junctions in gastrointestinal muscle contain multiple connexins. Am J Physiol Gastrointest Liver Physiol 281: G533-G543, 2001.[Abstract/Free Full Text]
- Wang Y, Mehta PP, and Rose B. Inhibition of glycosylation induces formation of open connexin-43 cell-to-cell channels and phosphorylation and triton X-100 insolubility of connexin-43. J Biol Chem 270: 26581-26585, 1995.[Abstract/Free Full Text]
- Wang Y and Rose B. Clustering of Cx43 cell-to-cell channels into gap junction plaques: regulation by cAMP and microfilaments. J Cell Sci 108: 3501-3508, 1995.[Abstract]
- Wang Y and Rose B. An inhibition of gap-junctional communication by cadherins. J Cell Sci 110: 301-309, 1997.[Abstract]
- Warn-Cramer BJ, Cottrell GT, Burt JM, and Lau AF. Regulation of connexin-43 gap junctional intercellular communication by mitogen-activated protein kinase. J Biol Chem 273: 9188-9196, 1998.[Abstract/Free Full Text]
- Warn-Cramer BJ, Lampe PD, Kurata WE, Kanemitsu MY, Loo LWM, Eckhart W, and Lau AF. Characterization of the mitogen-activated protein kinase phosphorylation sites on the connexin-43 gap junction protein. J Biol Chem 271: 3779-3786, 1996.[Abstract/Free Full Text]
- Webb RJ, Bains H, Cruttwell C, and Carroll J. Gap-junctional communication in mouse cumulus-oocyte complexes: implications for the mechanism of meiotic maturation. Reproduction 123: 41-52, 2002.[Abstract]
- Webb RJ, Marshall F, Swann K, and Carroll J. Follicle-stimulating hormone induces a gap junction-dependent dynamic change in [cAMP] and protein kinase A in mammalian oocytes. Dev Biol 246: 441-454, 2002.[Web of Science][Medline]
- Weidmann S. The electrical constants of Purkinje fibers. J Physiol 118: 348-360, 1952.[Free Full Text]
- Werner R. IRES elements in connexin genes: a hypothesis explaining the need for connexins to be regulated at the translational level. IUBMB Life 50: 173-176, 2000.[Web of Science][Medline]
- Werner R, Levine E, Rabadan-Diehl C, and Dahl G. Formation of hybrid cell-cell channels. Proc Natl Acad Sci USA 86: 5380-5384, 1989.[Abstract/Free Full Text]
- Werner R, Levine E, Rabadan-Diehl C, and Dahl G. Gating properties of connexin32 cell-cell channels and their mutants expressed in Xenopus oocytes. Proc R Soc Lond B Biol Sci 243: 5-11, 1991.[Medline]
- White TW. Functional analysis of human Cx26 mutations associated with deafness. Brain Res Rev 32: 181-183, 2000.[Medline]
- White TW, Bruzzone R, Goodenough DA, and Paul DL. Mouse Cx50, a functional member of the connexin family of gap junction proteins, is the lens fiber protein MP70. Mol Biol Cell 3: 711-720, 1992.[Abstract]
- White TW, Bruzzone R, Wolfram S, Paul DL, and Goodenough DA. Selective interactions among the multiple connexin proteins expressed in the vertebrate lens: the second extracellular domain is a determinant of compatibility between connexins. J Cell Biol 125: 879-892, 1994.[Abstract/Free Full Text]
- White TW, Deans MR, O'Brien J, Al-Ubaidi MR, Goodenough DA, Ripps H, and Bruzzone R. Functional characteristics of skate connexin35, a member of the gamma subfamily of connexins expressed in the vertebrate retina. Eur J Neurosci 11: 1883-1890, 1999.[Web of Science][Medline]
- White TW, Deans MR, Kelsell DP, and Paul DL. Connexin mutations in deafness. Nature 394: 630-631, 1998.[Medline]
- White TW, Goodenough DA, and Paul DL. Targeted ablation of connexin50 in mice results in microphthalmia and zonular pulverulent cataracts. J Cell Biol 143: 815-825, 1998.[Abstract/Free Full Text]
- White TW and Paul DL. Genetic diseases and gene knockouts reveal diverse connexin functions. Annu Rev Physiol 61: 283-310, 1999.[Web of Science][Medline]
- White TW, Paul DL, Goodenough DA, and Bruzzone R. Functional analysis of selective interactions among rodent connexins. Mol Biol Cell 6: 459-470, 1995.[Abstract]
- White TW, Sellito C, Paul DL, and Goodenough DA. Prenatal lens development in connexin43 and connexin50 double knockout mice. Invest Ophthalmol Vis Sci 42: 2916-2923, 2001.[Abstract/Free Full Text]
- Wiesen JF and Midgley AR Jr. Changes in expression of connexin 43 gap junction messenger ribonucleic acid and protein during ovarian follicular growth. Endocrinology 133: 741-746, 1993.[Abstract/Free Full Text]
- Wiesen JF and Midgley AR Jr. Expression of connexin 43 gap junction messenger ribonucleic acid and protein during follicular atresia. Biol Reprod 50: 336-348, 1994.[Abstract]
- Wilgenbus KK, Kirkpatrick CJ, Knuechel R, Willecke K, and Traub O. Expression of Cx26, Cx32 and Cx43 gap junction proteins in normal and neoplastic human tissues. Int J Cancer 51: 522-529, 1992.[Web of Science][Medline]
- Willecke K, Eiberger J, Degen J, Eckardt D, Romualdi A, Guldenagel M, Deutsch U, and Sohl G. Structural and functional diversity of connexin genes in the mouse and human genome. Biol Chem 383: 725-737, 2002.[Web of Science][Medline]
- Willecke K, Jungbluth S, Dahl E, Hennemann H, Heynkes R, and Grzeschik K-H. Six genes of the human connexin gene family coding for gap junctional proteins are assigned to four different human chromosomes. Eur J Cell Biol 53: 275-280, 1990.[Web of Science][Medline]
- Winterhager E, Brummer F, Dermietzel R, Hülser DF, and Denker HW. Gap junction formation in rabbit uterine epithelium in response to embryo recognition. Dev Biol 126: 203-211, 1988.[Medline]
- Winterhager E, Grummer R, Jahn E, Willecke K, and Traub O. Spatial and temporal expression of connexin26 and connexin43 in rat endometrium during trophoblast invasion. Dev Biol 157: 399-409, 1993.[Web of Science][Medline]
- Xia JH, Liu CY, Tang BS, Pan Q, Huang L, Dai HP, Zhang BR, Xie W, Hu DX, Zheng D, Shi XL, Wang DA, Xia K, Yu KP, Liao XD, Feng Y, Yang YF, Xiao JY, Xie DH, and Huang JZ. Mutations in the gene encoding gap junction protein beta-3 associated with autosomal dominant hearing impairment. Nat Genet 20: 370-373, 1998.[Web of Science][Medline]
- Xia AP, Ikeda K, Katori Y, Oshima T, Kikuchi T, and Takasaka T. Expression of connexin 31 in the developing mouse cochlea. Neuroreport 11: 2449-2453, 2000.
Top
Next
