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Instituto de Biofísica da Universidade Federal do Rio de Janeiro, Rio de Janeiro; Ludwig Institute for Cancer Research, Hospital Alemão Oswaldo Cruz, São Paulo; Programa de Farmacologia Molecular, Universidade Federal de Minas Gerais, Belo Horizonte; Centro de Memória, Pontifica Universidade Católica do Rio Grande do Sul, Porto Alegre; Hospital A. C. Camargo and Faculdade de Medicina, University of São Paulo, São Paulo, Brazil
ABSTRACT I. INTRODUCTION: A BRIEF ACCOUNT OF PRION PATHOLOGY A. The Prion Diseases B. The Prion Concept C. Discovery of the Cellular Prion Protein D. Conformational Conversion and Gain of Function in Prion Diseases E. The Loss-of-Function Hypothesis F. What Function? II. STRUCTURAL AND MOLECULAR BIOLOGY OF THE PRION PROTEIN A. Structure of PrPC B. Structure and Regulation of the Prnp Gene C. Expression and Distribution of PrPC D. PrPC Ligands E. Copper and PrPC III. SUBCELLULAR ENVIRONMENT AND TRAFFICKING OF THE PRION PROTEIN A. The Subcellular Environment of PrPC B. Trafficking of PrPC to the Cell Surface C. Endocytosis and Internalization of PrPC D. Modulation of the PrPC Endocytic Cycle IV. FUNCTIONS OF THE PRION PROTEIN IN THE NERVOUS SYSTEM A. The First Knockouts and the Lack of Phenotype B. Neurodegeneration in PrP-null mice C. The Knockout Paradox, Doppel gene, and neurodegeneration D. Neurodegeneration in Other Transgenic Mice E. Role of PrPC in Behavior F. PrPC and the Sleep-Wakefulness Cycle G. PrPC and memory H. PrPC and Alzheimer's disease I. PrPC, synaptic activity, and neuronal excitability V. IMMUNOMODULATORY FUNCTIONS OF THE PRION PROTEIN A. Evidence for a Role of PrPC in the Immune System B. Lymphoid Cells C. Antigen-Presenting Cells/Mononuclear Phagocytes D. PrPC in inflammatory responses E. Implications of PrPC Functions in the Immune System VI. ROLES OF THE PRION PROTEIN BEYOND THE NERVOUS AND IMMUNE SYSTEMS A. Neuromuscular Junction and Muscle B. Liver C. Pituitary VII. ROLES OF THE PRION PROTEIN ON THE CELL LIFE CYCLE A. PrPC and Cell Proliferation B. PrPC and Cell Differentiation C. PrPC in Cell Death and Survival VIII. SIGNALING MEDIATED BY THE PRION PROTEIN A. Approaches to PrPC-mediated signal transduction B. Signaling Dependent on PrPC-Caveolin Interaction C. PrPC, cAMP, and protein kinase A D. PrPC and MAP Kinases E. PrPC and Non-Receptor Tyrosine Kinases F. PrPC and Calcium G. PrPC and Protein Kinase C H. PrPC and the PI 3-Kinase/Akt Pathway I. Implications of Current Evidence on PrPC-Mediated Signaling IX. THE PRION PROTEIN AS A DYNAMIC CELL SURFACE PLATFORM FOR THE ASSEMBLY OF SIGNALING MODULES X. CONCLUSION AND FUTURE DIRECTIONS GRANTS ACKNOWLEDGMENTS REFERENCES
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I. INTRODUCTION: A BRIEF ACCOUNT OF PRION PATHOLOGY |
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Prion diseases correspond to anatomo-pathologically defined transmissible spongiform encephalopathies (TSEs) of an infectious, genetic, or sporadic nature and are characterized by neurodegeneration and protein aggregation. These diseases include kuru and Creutzfeldt-Jakob disease (CJD) in humans, scrapie in sheep and bovine spongiform encephalopathy (BSE), also known as "mad cow disease," among others. The diseases can affect subjects at distinct age groups, course with a variety of motor or cognitive symptoms, and although their prevalence is relatively low, TSEs are still incurable and invariably fatal (262).
The pathogenesis of prion diseases is attributed to major changes in the metabolism of the cellular prion protein (PrPC). Current understanding of TSEs evolved from the concept of the "prion," that is, a proteinaceous, nucleic acid-free, infectious particle (427).
Studies of kuru among the Fore people of Papua-New Guinea indicated that this spongiform encephalopathy was transmitted through ritual cannibalism. Indeed, injection of brain tissue from deceased patients into the brains of chimpanzees induced a similar disease, whereas the incidence of kuru sharply declined following the end of ritual cannibalism among the Fore. Scientists quickly noted the similarities among kuru, scrapie, as well as CJD (428).
The scrapie agent resisted doses of radiation that easily inactivated both viruses and bacteria (7), and the profile of sensitivity of the scrapie agent to various chemicals differed from both viruses and viroids, suggesting that the infectious agent might not depend on nucleic acids (24).
Further studies demonstrated that a protein unusually resistant to proteolysis was required for infectivity of diseased brain extracts (429), whereas no compelling evidence is available of the need for other components, especially nucleic acids. Therefore, although formal proof is still lacking, the prevailing hypothesis is that transmission of TSEs relies on a single protease-resistant protein component of the prion (427), which has received several denominations, such as PrPSc, for scrapie; PrPres, due to its protease resistance; or PrP*, due to the uncertainties of whether the whole or only part of this component is actually pathogenic (5, 60).
C. Discovery of the Cellular Prion Protein
Sequencing of a microheterogeneous protein fraction of 27–30 kDa (PrP 27–30), produced by limited proteolysis of PrPSc, allowed the identification of a cellular gene (Prnp), that encodes a protein known as the cellular prion protein (PrPC), or simply prion protein (33, 429). Both the infective protein component of scrapie and PrPC share the amino acid sequence and are encoded by a single-copy gene where the entire open reading frame is located in one exon (20, 396). This suggested that the distinctive properties of either the infective or the cellular prion proteins are determined by posttranslational modifications, and most research in the field was focused on the mechanisms by which PrPC converts into PrPSc.
D. Conformational Conversion and Gain of Function in Prion Diseases
Following a brief flirt with the hypothesis that glycosylation imparts either normal or pathogenic properties to the prion protein (195, 503), structural studies of both PrP 27–30 and PrPC led to the current view that PrPSc is an anomalous conformer of PrPC, which results from the conversion of a predominantly 
-helical structure into a predominantly β-sheet structure (18, 81, 169, 407). The latter explains the tendency of PrPSc to form compact, protease-resistant, protein aggregates, which in turn accumulate within the brain (452).
Although several models have been proposed to account for the formation of PrPSc aggregates, the basic proposal is that, following either infection with PrPSc or conversion of PrPC into PrPSc associated with certain mutations thought to destabilize the protein (100, 221), binding of PrPSc to PrPC leads to further conversion, thus resulting in accumulation of PrPSc at the expense of the normal PrPC (Fig. 1). This hypothesis is consistent with the progressive nature of all variants of the prion diseases, as well as with the resistance of Prnp knockout mice to infection with scrapie brain extracts. It is also thought to underlie the predominant sporadic forms, in which pathogenesis might start with spontaneous conversion of a fraction of PrPC by hitherto unknown reasons (153, 174, 564).
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E. The Loss-of-Function Hypothesis
Despite compelling evidence for conformational conversion in the course of the diseases, it is still not clear what leads to the accumulation and cytotoxicity of the pathological conformer. For example, although it is widely assumed that accumulation of PrPSc causes neurodegeneration, systematic examination of the brains of deceased patients revealed no spatial correlation between apoptosis and deposits of PrPSc (96, 133, 185). Accumulated PrPSc within PrPC-expressing tissue grafted into the brains of Prnp-knockout mice does not damage the neighboring PrPC-null tissue (40), and progressive accumulation of PrPSc in glial cells around PrPC-null neurons does not induce cell death in the knockout neurons, also arguing against a direct cytotoxic effect of PrPSc (326–328).
Thus the pervasive gain-of-toxic-function hypothesis is still unproven, and current models assume that PrPSc propagates at the expense of depletion of PrPC (564), which warrants an examination of the hypothesis that loss of function of PrPC (469), or of neurochemical systems associated with PrPC, contributes to the pathogenesis of TSEs (Fig. 1). Critical appraisal of loss-of-function components in prion diseases is, nonetheless, hampered by the controversies surrounding the physiological functions of PrPC.
Full appreciation of the functional roles of PrPC has likely been abashed by the striking lack of phenotype reported for the first Prnp knockout mouse (63). The contrast between the undisturbed phenotype and the spectacular change in the sensitivity to disease in these mice (63, 430, see below) greatly advanced the field of prion pathology, whereas physiological roles of PrPC were largely neglected.
Nonetheless, data on functional consequences of interfering with PrPC have accumulated over time, and the multiplicity of seemingly unrelated findings, as well as the nature of this particular protein, warrant an attempt at extracting and consolidating the major themes.
The purpose of this article is a comprehensive review of the evidence for physiological functions of PrPC. This subject has, in recent years, been approached from molecular, cellular, and systems view points, each of which constrains the possible answers to the following question: What is(are) the major function(s) of the cellular prion protein?
We approach the subject by first discussing the structure, expression, regulation, molecular interactions, and subcellular trafficking of PrPC. Then, functional roles detected in the nervous and immune systems, as well as in other organs, are discussed at the systems level. Finally, the evidence for functional roles of PrPC is examined at the level of cellular physiology. The main hypothesis examined in the context of this review is that the prion protein is a dynamic cell surface platform for the assembly of signaling modules, based on which selective molecular interactions and transmembrane signaling translate into wide-range consequences upon both physiology and behavior.
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II. STRUCTURAL AND MOLECULAR BIOLOGY OF THE PRION PROTEIN |
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Following the cleavage of a 22-amino acid (aa) signal peptide, most of mammalian PrPC is exported to the cell surface as an N-glycosylated, glycosylphosphatidylinositol (GPI)-anchored protein of 208–209 aa. The tridimensional structure of the prion protein was determined by nuclear magnetic resonance at acid pH (71, 213, 307, 311, 444, 445, 590), and its major features are briefly discussed here.
PrPC contains an NH2-terminal flexible, random coil sequence of 
100 aa, and a COOH-terminal globular domain of about another 100 aa. The globular domain of human PrPC is arranged in three -helices corresponding to aa 144–154, 173–194, and 200–228, interspersed with an antiparallel β-pleated sheet formed by β-strands at residues 128–131 and 161–164. A single disulfide bond is found between cysteine residues 179 and 214. The NH2-terminal flexible tail comprises approximately residues 23–124, and a short flexible COOH-terminal domain corresponds to residues 229–230 (590). The structure of the globular half of human PrPC is identical to various other mammals, as expected from a high degree of sequence identity (307, 311). Notably, despite the low sequence identity between PrPC in chicken, turtle, or frog, and the mammalian proteins, the major structural features of PrPC are remarkably preserved in those nonmammalian species (71). In addition, both tertiary structure as well as the thermal stability of PrPC isolated from the brain of healthy calf were indistinguishable from bacterially expressed, recombinant bovine PrPC (213, 307).
Full-length PrPC is found in non-, mono-, or diglycosylated forms, corresponding to the variable occupancy of residues Asn-181 and Asn-197 in human PrPC and Asn-180 and Asn-196 in mice (195). A rather large variety of N-glycans were found attached to both full-length and truncated PrPC (408, 457), which may be differentially distributed in various areas of the central nervous system (CNS) (27, 124).
The role of glycosylation was addressed both regarding the susceptibility to conformational conversion, as well as upon the diversity of TSEs (278). As for functional properties of PrPC, less is known. The nearly identical tridimensional structures of both recombinant PrPC and the glycosylated PrPC isolated from calf brain indicate that neither the attached N-glycans nor most of the GPI anchor impart major structural features by themselves (213).
However, molecular dynamic simulations suggest that some attached N-glycans may modulate PrPC stability (128, 139, 598), although experimental evidence is still lacking (213). Nevertheless, protein glycosylation affects other protein properties, such as intracellular traffic and ligand binding (299, 413, 504), the latter of which may be modulated both by subtle effects on protein structure as well as by steric hindrance (32, 70, 237, 243, 393, 513). Indeed, glycosylation reportedly affects the recognition of various species of PrPC by monoclonal antibodies in both the brain and in other cells (27, 289), and differing responses to certain monoclonal antibodies were described for cells bearing distinctly glycosylated forms of PrPC (365). Analogous selectivity may impinge on the binding of PrPC physiological ligands.
In turn, experimental data suggest that membrane attachment through the GPI anchor (502), as well as other PrPC-membrane interactions, may modulate the structure of the protein. An early study showed that PrPC tends to interact with lipid vesicles containing acidic lipids, depending on pH. This interaction induced a slight, but consistent, change in the circular dichroism (CD) spectrum of the full-length protein, and a robust change in the CD spectrum of an NH2-terminal fragment, upon binding to phosphatidylserine-containing liposomes, but not to vesicles composed of phosphatidylcholine alone (369). A similar change in CD spectrum was also found upon binding of a phospholipid-conjugated full-length PrPC to raftlike membranes (see sect. IIIA) containing phosphatidylcholine, phosphatidylethanolamine, sphingomyelin, cerebrosides, and cholesterol, but not to lipid vesicles containing phosphatidylcholine and cholesterol only (137). Molecular dynamics simulations suggest that, although secondary structure is largely preserved, the NH2-terminal domain of PrPC can interact with membrane lipids and even traverse the surface of the membrane (128). A recent study employed another phospholipid-conjugated PrPC and showed that, despite a large difference between the CD spectra of this form when compared with anchorless PrPc, insertion into model lipid membranes restored the secondary structure of the protein, as shown by attenuated infrared spectroscopy (205). The data, therefore, indicate that the GPI-mediated anchoring of PrPC into membranes has but little effect on secondary structure. It remains to be tested whether GPI-anchoring modulates other properties, such as biological activity, as shown for GPI-anchored fibroblast growth factor (263). Interestingly, anchorless PrPC undergoes an unusual amyloid conversion into PrPres and aggravates disease induced in mice expressing GPI-anchored PrPC, but does not mediate clinical symptoms in infected mice lacking GPI-anchored PrPC (91).
Analysis of the effect of pH on the structure of the prion protein may also be relevant to physiological functions of PrPC. Both biophysical methods and selective antibody binding to exposed epitopes showed that the structure of PrPC is sensitive to pH (6, 128, 212, 341, 519). This is particularly evident for the NH2-terminal flexible domain, upon which some degree of structure, as well as changes in antibody binding were imparted by acid pH. This may be relevant both to the binding of PrPC partners as well as to binding affinity, due to the continuous subcellular trafficking of PrPC (see below). In addition, local pH modulation by a putative electrostatic potential formed by highly charged glycoconjugates at the cell surface (90, 158), including the glycosylated residues in PrPC itself, may also contribute to the structure and biological functions of PrPC in situ.
Thus further work on both the structure of PrPC in situ as well as on the effects of local microenvironmental cues upon protein structure is likely to shed light on functional properties of the prion protein.
B. Structure and Regulation of the Prnp Gene
Whereas the structure and regulation of the Prnp gene is important for epidemiological, diagnostic and/or therapeutic considerations, the current review focuses on aspects that may be relevant for physiological functions of PrPC.
The Prnp gene contains either three (in rat, mouse, bovine, sheep) or two exons (in hamster, humans, tamar wallaby), of which a single exon codes for PrPC protein (95, 210, 425, 432, 459, 569). Control of Prnp gene expression has been attributed to sequences within the 5'-flanking region, within the first intron, and to 3'-untranslated sequences. The Prnp promoter did not display methylation either in nerve growth factor (NGF)-treated and untreated PC12 cells or in the rat brain (593).
No TATA box was identified linked to the Prnp gene, whereas a CCAAT element was found in most cases. Analysis of the 5'-flanking region indicated that strong promoter activity required sequences within 80–150 bp upstream of exon 1 (22, 231, 324, 460). Nonetheless, in bovine Prnp, the proximal promoter sequence between –88 and –30 within the 5'-flanking region required the simultaneous presence of a region of 770 bp within intron 1 to drive reporter activity, indicating an interaction between promoter and intronic sequences (231). In the mouse Prnp gene, two equally strong promoter sequences upstream of both exon 1 and of exon 2, as well as a suppressor sequence within intron 1, were shown to control promoter activity in transfected neuroblastoma cells (22).
Four conserved motifs of as yet unknown significance were also detected upstream of the Prnp gene in mouse, human, and sheep (281, 569), but not in the tamar wallaby (425). In addition, several insertions, including retrotransposons, were described within the Prnp gene and its promoter sequences (281).
Differing from the Prnp ORF, the degree of homology of potential promoter sequences among various mammalian species is variable. For example, whereas bovine and sheep 5'-flanking regions share 89% homology, the bovine 5'-flanking sequence displays only 46–62% homology with the corresponding sequences from rodent and human Prnp (231).
Evidence for posttranscriptional control includes the finding, within the Prnp 3'-untranslated region, of conserved nuclear-specific and maturation-specific polyadenylation signal sequences (180, 425). More importantly, two distinct transcripts encoding the same Prnp ORF were found in sheep tissues, produced by alternative polyadenylation. Their relative levels differed in either brain or spleen, and also among ovine, caprine, and bovine tissues, and affected the level of synthesis of PrPC. Still, no alternative polyadenylation was found in either human or mouse tissues (180). An additional significant finding of the latter study was the discrepant levels of translation of one of the alternative transcripts between sheep brain and neuroblastoma cells (180).
Prnp is often labeled as a housekeeping gene, based on the absence of a TATA box, presence of CpG islands, and the identification of several Sp1 binding sites (20, 22, 432, 460). However, evidence that transcription of Prnp is modulated by chromatin structure (68), as well as the potential binding sites for many transcription factors, indicate that Prnp expression likely depends on a variety of cellular factors. Notwithstanding some species variation, the following elements were reported, in addition to Sp1, both in the 5'-flanking region and within the first intron: AP1, AP2, MZF-1, MEF2, MyT1, Oct-1, NFAT, POZ (BCL6); RP58 (ZNF238); NEUROG1; EGR4, Oct-1/Oct-2, NF-IL6, MyoD, p53, HSE, MRE, MLS, but not CRE, NF-
B or OTF-I (231, 324, 425, 460, 470, 490, 541, 569). Polymorphism of the bovine promoter modulated the expression of the Prnp gene, depending on interaction of transcription factors RP58 and Sp1 (470). It was, however, claimed that activity of a promoter sequence including exon 1 and upstream was independent on the level of Sp1 found in several transfected cell lines (515).
Expression of both PrPC mRNA and protein are developmentally regulated, increasing postnatally with distinct time courses for various regions of the hamster, rat, and mouse brains (280, 293, 330, 349, 355). In addition, induction of Prnp mRNA expression in the mouse was first detected between embryonic days 8.5 and 9, coinciding with the transition from anaerobic to aerobic metabolism (355).
Consistent with developmental regulation, injections of NGF into the brains of neonatal hamsters upregulated Prnp mRNA together with activity of choline acetyltransferase in regions that contain NGF-responsive cholinergic neurons (364). Neuronal-like differentiation of PC12 cells induced by either interleukin (IL)-6 or NGF was accompanied by increased expression of Prnp mRNA (279). The induction of Prnp by NGF was shown to depend on opposing effects of either the mitogen-activated protein (MAP) kinase or the PI 3-kinase/Akt pathways and occurred in fibroblasts without simultaneous differentiation, indicating that the effects on Prnp are not a trivial consequence of the NGF-induced differentiation (593). Expression of Prnp mRNA and PrPC protein also showed signs of differential regulation along two diverging differentiation pathways of a neuroectodermal cell line (375).
Treatment with cytokines of various neural-like cell lines, as well as teratocarcinoma-derived differentiated neurons, also indicated that distinct cell types respond differently to growth factors. Thus SK-N-SH neuroblastoma cells upregulated Prnp mRNA in response to IL-1β, tumor necrosis factor (TNF)-, and phorbol ester and downregulated Prnp mRNA upon treatment with interferon (IFN)-
. In contrast, no response to these cytokines was detected in IMR-32 neuroblastoma, U-373MG astrocytoma, or teratocarcinoma-derived differentiated neurons (474).
In contrast to neuronal-like differentiation in vitro, the mRNA for both glial fibrillary acidic protein (GFAP) and PrPC in cultured astrocytes correlated during proliferation, but, contrary to GFAP, Prnp transcription did not respond to several growth factors (78, 279). Therefore, increased levels of expression of Prnp correlate with brain development and neuronal differentiation, but apparently not with astroglial differentiation.
Still related with development and tissue differentiation, Prnp mRNA in cell cultures of perodontal ligament (PDL), a neural crest-derived connective tissue, was upregulated in response to MRP8 and downregulated by platelet-derived growth factor (PDGF), both of which are chemotactic for PDL cells (261). Differentiation-related regulation of Prnp expression was also reported in pancreatic insulin-producing beta cells, following treatment with growth hormone and dexamethasone (16).
Stress modulates the expression of Prnp. In neuroblastoma cells, heat shock increased both PrPC mRNA and protein (491), together with Hsp70 (490). Both hyperbaric oxygen and hypoglycemia upregulated the Prnp, 70-kDa heat shock protein (Hsp70), and JNK genes, and the activation of the Prnp promoter depended on the presence of one of the two heat shock elements identified in its sequence (489, 493), similar to neuroblastoma cells subject to hypoxia-reoxygenation (492). Although the activation of HSE may be consequent to phosphorylation of HSF1 by either JNK or Erk under hyberbaric oxygen or hypoxia-reoxygenation paradigms, respectively (490, 492), direct evidence for this particular chain of events is lacking.
Also related to stress, inflammation both in skin and in gastrointestinal epithelium led to upregulation of PrPC (405, 406). Altered redox states correlated with the content of PrPC in prostate tumor spheroids, possibly related to a reactive role of PrPC to oxidative stress (477; see sect. IIE). Nonetheless, Prnp mRNA was not quantified; thus it is unclear whether any of these conditions actually induces increased gene expression or, alternatively, prevents PrPC degradation.
Chronic copper overload resulted in upregulation of Prnp in two lines of fibroblasts from mutant mice which accumulate abnormally high levels of copper in normal culture medium (10). Hippocampal and cortical neurons in culture upregulated Prnp transcription in response to copper, and a reporter vector driven by the Prnp promoter was activated by copper when transfected into PC12 cells, but not into C6 glioma cells. Responses both to copper and to cadmium depend on the presence of a putative metal responsive element (MLS1), although transcription factor MTF-1 is not involved (541).
Expression of Prnp is also regulated among immune cells. Both PrPC mRNA and protein were downregulated upon differentiation of bone marrow cells along the granulocyte lineage, an effect also found following retinoic acid-induced granulocyte differentiation of the HL60 leukemia cell line (132). More recently, it was shown that the effects of retinoic acid on PrPC mRNA and protein depend on protein synthesis, but could be dissociated from the retinoic acid-induced differentiation of the cell line (458).
Apart from direct evidence such as that obtained for heat shock or metal responsive elements (493, 541), the roles of most of the potential regulatory elements identified in Prnp are still unknown. Transduction of regulatory signals generated by phorbol ester or certain growth factors and cytokines upon immune or neural cells (135, 404, 474) may be mediated by AP-1, NF-IL6, and NFAT elements (324, 425). Proliferation or differentiation signals may relate, for example, with MZF-1, MyT1, and MyoD binding sites, while tissue- and cell-specific expression (78, 180, 279, 474) may depend on elements such as MEF2 or Oct-1 (425). Little data are available to identify the chain of events linking most cellular responses with Prnp expression. Understanding of PrPC function, particularly in signal transduction, will require further functional analysis of the Prnp promoter.
Several studies reported altered expression or surface exposure of PrPC without examination of Prnp mRNA levels (135, 289, 315, 404). However, a significant C1q-dependent upregulation of PrPC protein was induced in follicular dendritic cells of spleen germinal centers, by intravenous immunization of mice with either preformed immune complexes or vesicular stomatitis virus. In this case there was no change in mRNA content, indicating that the PrPC was modulated posttranscriptionally (308).
C. Expression and Distribution of PrPC
The prion protein is highly expressed within the nervous system, although its content varies among distinct brain regions, among differing cell types, and among neurons with distinct neurochemical phenotypes. Various cellular components of the immune system, in the bone marrow, blood, and peripheral tissues, also express substantial amounts of PrPC. Finally many other organs and tissues present PrPC expression (Table 1).
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Although mRNA and protein levels usually correlate well, one particular study revealed marked disparities between PrP mRNA, shown by in situ hybridization, and PrPC protein, revealed by immunohistochemistry (150). Here, PrPC-negative dopaminergic neurons of the substantia nigra, olfactory glomeruli, and locus coeruleus displayed a high PrP mRNA content. Also, within the deep cerebellar nuclei, all neurons expressed high levels of PrP mRNA, but only interneurons contained high levels of PrPC (150).
The reason for this disparity between mRNA and protein levels of PrPC within the CNS is not known. Notably, in a simultaneous study from the same group, employing the same techniques, mRNA and protein correlated well in both peripheral neurons and nonnervous tissues (151). The authors suggested that an exceptionally fast turnover of PrPC in cerebellar granule cells may explain the disparity. This, however, does not fit studies of PrPC turnover in vitro, because primary cultures of granule cells and of splenocytes showed similar half-lives for PrPC (410), whereas there was no disparity between mRNA and protein levels in the spleen (151). The in vitro turnover study (410) also indicated that primary culture cells degraded PrPC more rapidly than cell lines, but overexpressing transgenic tissue was employed in this investigation. Therefore, further studies of the turnover of PrPC in distinct neuronal populations, as well as other cell types, are warranted.
The studies of the mouse brain and peripheral nervous system (150, 151) employed a range of antibodies raised against epitopes along the whole PrPC molecule, coupled to carrier by disuccimidyl-glutarate, which cross-links free amide groups in a reaction similar to that of glutaraldehyde. The authors aimed at detecting PrPC cross-linked to its normal positions at the cell surface, which is deemed to occur in glutaraldehyde-fixed tissue (150). This procedure highlights the additional caveat, that fixation and handling may bias the detection of protein at the surface of expressing cells due to the continuous trafficking and turnover of PrPC (see sect. III).
Reports in the immune system also harbor some controversy. Thus a study of mouse lymphoid tissue employing flow cytometry with the 6H4 monoclonal antibody (270) differed from data obtained by immunohistochemistry (151) as to the presence of PrPC in spleen cells, peritoneal cells, peripheral blood leukocytes, and intestinal intraepithelial lymphocytes, suggesting that maturing mouse immunocytes either downregulate PrPC to a level undetectable by flow cytometry or, alternatively, mask the 6H4 epitope through posttranslational changes in PrPC. On the other hand, a study of bovine lymphoid tissue also by flow cytometry with the 6H4 antibody confirmed the negative results in mouse for a variety of T cells and myeloid cells, but PrPC was detected in lymphocytes and, to a lesser extent, in monocytes and even polymorphonuclear cells (403). It is not clear whether and which of these distinct findings are related to species differences, degree of surface exposure of PrPC, or technical artifacts.
An important additional finding is that the expression of PrPC depends on both maturation and cellular activation in various cell types. Thus PrPC is developmentally regulated in both the differentiating brain and muscle (53, 330) and in maturing granulocytes (77, 132). Also, PrPC is upregulated by activation of T cells, monocytes, and dendritic cells (135, 289, 337), and intracellular PrPC migrates to the cell surface upon activation of platelets (193, 203, 505). Therefore, heterogeneous expression of PrPC among various cell types, organs, and species may be additionally imparted by conditions such as the raising of laboratory mice in specific pathogen free (SPF) conditions and differential exposure of distinct animal cohorts to inflammatory/stress inducers.
The overall evidence for heterogeneous, cell type-specific, and regulated expression of PrPC is relevant both for the understanding of pathogenesis as well as for the interpretation of physiological functions of PrPC. It should be carefully taken into account when examining transgenic and PrP-null mice, in which the expression of PrPC is evenly modulated in all organs and tissues, as well as in cases when multiple cell types and systems may be involved, such as complex behavior mediated by interacting neural pathways and diverse neurochemical systems, or the cross-talk of inflammatory and neurodegenerative effects.
Many reports are available on putative ligands of the prion protein, most of which aimed at the identification of the elusive "protein X," believed to be a necessary component of the pathogenic conformational conversion (427, 528). Findings have been reviewed, usually grouping ligands of PrPC together with ligands of PrPSc (282, 333). Table 2 shortlists data related to candidate physiological ligands of PrPC.
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Both the GASP and the Fbx protein families are related to mechanisms of protein trafficking and turnover. The GASP family comprises several proteins, many of which are predominantly expressed in the CNS and associate with COOH-terminal cytoplasmic tails of various GPCRs, including glutamate, acetylcholine, and neuropeptide receptors (204, 494). The founding member of the family is required for the choice between recycling and degradation of the 
-opioid receptor (573). It is, nonetheless, unclear whether the topology of these proteins may allow control over the traffic of PrPC.
F-box-only proteins are responsible for substrate recognition by the SCF complex, a major type of E3 ubiquitin ligase, and therefore regulate protein stability and degradation (206). The highest degree of homology in our BLAST search of clone 12b2 was found with the F-box-only protein Fbxo2 (Fig. 2C), which binds specifically to N-linked high-mannose oligosaccharides and is associated with the ubiquitination of N-glycosylated proteins retrotranslocated from the endoplasmic reticulum to the cytosol by the quality control mechanisms (588). Interestingly, the high-mannose, immature form of PrPC that precedes complex glycosylation appears to be the most sensitive to conformational conversion into PrPSc (580), and therefore, turnover mechanisms dependent on the binding of PrPC to Fbxo2 may be key components of prion disease pathology.
Binding domains along the whole extent of the PrPC molecule have been identified for a number of ligands (Fig. 3). It should, however, be noted that the techniques used for those studies were quite variable, and many interactions detected by screening methods have yet to be confirmed by biochemical and cell biological approaches. In particular, some putative ligands appear not to be accessible from the usual topology of PrPC, which constitutes a critical question to be addressed by future studies.
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With the use of a yeast two-hybrid approach, followed by cell-binding experiments, it was shown that PrPC binds the 37-kDa laminin receptor precursor (LRP), and/or the 67-kDa laminin receptor (LR), both of which have been identified as receptors for distinct viruses. Two binding sites each in both PrPC and LRP were identified. One set of binding sites (hamster PrPC144–179; LRP161–179) provided direct interaction, and another (PrPC53–93; LRP undetermined, either between aa 101–160 or 180–295) underlies heparan sulfate proteoglycan (HSPG)-mediated interaction (171, 226, 443). PrPC binds both heparin and heparan sulfate through at least three domains, one of which coincides with the HSPG-dependent LRP-binding site PrPC53–93 (216, 408, 557). Importantly, all interactions of PrPC with LRP/LR were independent of either the laminin-specific integrin VLA-6 or galectin-3 (171, 226). In turn, LRP161–179 binds both PrPC and laminin (443), and PrPC173–182 binds laminin itself (103), at the sequence RNIAEIIKDI (known as the LN -1 peptide) of the conserved COOH-terminal laminin chain, with functional consequences (183, see sects. IVG and VIIB).
In turn, in situ cross-linking of N2a cells in the presence of low concentrations of formaldehyde resulted in a high-molecular-weight complex composed of PrPC coupled to various isoforms of neural cell adhesion molecule (NCAM). The binding site in NCAMs was found in β-strands C and C' within the two consecutive fibronectin type III domains, whereas the binding site in PrPC was located in the NH2 terminus, helix A (residues 144–154) and the adjacent loop region of PrPC (479). PrPC and NCAM also coimmunoprecipitated from brain tissue (471).
A distinct approach led to the discovery of a strong interaction between PrPC and the cochaperone Hsp70/Hsp90 organizing protein/Stress-induced protein 1 (hop/STI1). Complementary hydropathy theory (42, 264) led to the design of a putative PrPC-ligand peptide which proved to actually bind PrPC. An antibody raised against the designed peptide recognized a single PrPC-binding protein in brain extracts (338), identified as hop/STI-1 (591). Hop/STI-1 is an adaptor and modulator of the activities of Hsp70 and Hsp90 and may have additional roles in cell metabolism (394, 589 for reviews).
Although heat shock proteins and cochaperones are mainly intracellular, hop/STI-1 has also been located extracellularly. Thus hop/STI-1 is secreted by certain tumor cell lines, together with several chaperones (141), as well as by primary astrocyte cultures (F. R. S. Lima, C. P. Arantes, A. G. Muras, R. Nomizo, R. R. Brentani, and V. R. Martins, unpublished results). These data are consistent with a physiological interaction of hop/STI1 with PrPC at the cell surface.
Finally, recent studies have implicated the low-density lipoprotein receptor-related protein 1 (LRP1) with endocytic trafficking of PrPC (525). Indirect evidence suggested that LRP1 may bind PrPC, although from the published data it is unclear whether direct binding of the two proteins actually takes place, or, alternatively, may be mediated by glycosaminoglycans or other intermediates. Nonetheless, cross-linking, coimmunoprecipitation, and direct binding experiments support the hypothesis that LRP1 does actually bind the NH2-terminal domain of PrPC (R. J. Morris, personal communication).
Copper constitutes a special case of confirmed PrPC ligand, the physiological role of which is a matter of controversy. Early work showed that the NH2-terminal PrPC59–91 domain, which contains four or five repeats of eight aa residues (PHGGGWGQ), binds Cu2+ (52, 214, 215). Coordination of Cu2+ likely occurs with residues HGGGW, and imidazole coordination has been identified (12, 64, 361, 546); amide-nitrogen coordination from the Gly residues was also detected (64, 361), making the coordination site likely composed of three nitrogen and one oxygen atoms (546; see Ref. 359 for review).
In vitro, the octarepeats appear to strictly prefer Cu2+ over Cu+ and other metal ions (509, 546), but lower affinity binding of other metals has also been detected in full-length PrPC (48, 236; see Refs. 94, 560 for reviews). Cooperative binding of four Cu2+ was shown in these octarepeats. However, some studies reported positive cooperativity (52, 65, 167, 576), whereas others have shown negative cooperativity with a Hill coefficient of 0.7 (552).
A range of binding affinities for interactions of PrPC and Cu2+ were estimated, mostly within the low micromolar range (359). Binding of Cu2+ to PrPC was reportedly the most effective at neutral pH, with reduced affinity at lower pH (363, 576, but see Ref. 567). This is consistent with a transport or sink function of PrPC, because the protein could bind Cu2+ at the cell surface and release it inside acidic endosomal vesicles (283, 361, 414, 417, 576; see sect. IIID). Nevertheless, direct evidence that PrPC does in fact transport Cu2+ is still lacking.
On the other hand, other Cu2+ coordination sites were identified at His-96 (236) and His-111 (245) in human PrPC, and it is possible that up to six Cu2+ can be present in a fully copper-loaded PrPC (245). Moreover, recent experiments have indicated that, either at low Cu2+ occupancy or at low pH, there is a distinct coordination mode involving multiple histidine imidazole groups (567, 568). These can bind two Cu2+ with an estimated affinity in the nanomolar range, which has been proposed to be the biologically relevant mode of Cu2+ coordination, given the affinity of other proteins for copper (567).
Binding of Cu2+ changes PrPC conformation (236, 362, 436, 437, 509, 546, 584), and folding of recombinant PrPC is reportedly affected by Cu2+ (57, 245, 436, 584), although the physiological consequences of this change are not understood. Nonetheless, evidence has been reported that Cu2+ facilitates PrPC self-association (567, 568).
Many reports showed a protective role of PrPC against oxidative damage, which is perhaps the most widely accepted PrPC function (52, 56, 202, 260, 438, 574, 583). Remarkably, loss of antioxidant defenses was suggested to play a major role in scrapie-infected cells (357) and prion diseases (9, 188, 189, 582).
Reactive oxygen species (ROS) increase lipid peroxidation and protein oxidation, leading to cellular damage. Free Cu2+ is highly cytotoxic, and must be minimized, because redox reactions of Cu2+ generate ROS (28). The antioxidant activity of PrPC has been attributed to copper binding to the octarepeat region (47). For example, cultured PrP-null neurons were more vulnerable than wild type (WT) when exposed to high levels of Cu2+, an effect that was blocked by treating cells with a synthetic PrPC59–91 peptide (55). Moreover, PrPC can also be oxidized by Cu2+, which could contribute to alterations in physiological functions and increased aggregation (381). Interestingly, anchoring the copper-binding NH2 terminus of PrPC to the plasma membrane using a signal peptide diminished oxidative responses in cells without affecting many aspects of PrPC metabolism (595).
Copper content of brain membranes was reportedly lower in PrP-null mice, when compared with WT (46, 52, 202). Others, however, have failed to find decreased copper content in membranes from PrP-null mice (550), albeit some evidence exists that synaptosomal, rather than total brain, content of copper is decreased (46). Controlling the levels of PrPC in an inducible cell system allowed regulation of Cu2+ binding to membranes, but not Cu2+ transport (438). Consistently, Cu2+ transport in synaptosomes appears to be independent of the expression of PrPC (172).
It was also reported that cultured cells derived from PrP-null mice were more susceptible to oxidative damage and had reduced superoxide dismutase (SOD) activity, when compared with WT (56). Moreover, recombinant PrPC refolded in the presence of Cu2+ was shown to have SOD activity (57). However, other authors found neither decreased SOD activity in PrP-null mice (550) nor SOD activity in recombinant PrPC (462). Careful analysis comparing possible SOD activity in recombinant PrPC with the activity of the SOD enzyme SOD-1 also failed to confirm any such activity for PrPC (246). In addition, experiments using genetically modified mice, as well as crosses between PrPC overexpressing mice and SOD-deficient and overexpressing mice, argue against such a role for PrPC in vivo (227).
Alternatively, it is possible that the binding of Cu2+ induces endocytosis of PrPC (see sect. IIID) as a signal for triggering antioxidative defense (542). Reduced levels of glutathione reductase and other antioxidant enzymes have also been detected in PrPC-deficient neurons (574). Conversely, increased PrPC expression appears to augment such antioxidant defenses (438), and both oxidative stress and copper exposure appear to increase PrPC expression (54, 541). These changes may constitute alternative mechanisms of PrPC-dependent resistance to oxidative stress.
Therefore, although the binding of copper to PrPC appears to impart cellular resistance to oxidative stress, the mechanisms are still controversial.
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III. SUBCELLULAR ENVIRONMENT AND TRAFFICKING OF THE PRION PROTEIN |
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Notwithstanding minor truncated and transmembrane forms (see sect. IIIB), the GPI-anchored form constitutes almost all of PrPC found during its normal life cycle (524). Similar to other GPI-anchored proteins, PrPC molecules are found attached to low-density, detergent-insoluble membrane domains (DRM), rich in cholesterol and sphingolipids (382, 524).
DRMs are biochemically fractionated entities, usually equated with isolated lipid rafts and/or liquid ordered domains (370). The exact meaning of the terms lipid raft and membrane raft, as used over the years, is somewhat uncertain, and there is variable acceptance of distinct methods for preparing DRMs as representative of specific membrane domains (292, 370, 378). Membrane rafts have recently been consensually defined as small (10–200 nm), heterogeneous, highly dynamic, sterol- and sphingolipid-enriched domains that compartmentalize cellular processes and can sometimes be stabilized to form larger platforms through protein-protein and protein-lipid interactions (422).
Despite these uncertainties, evidence points to a critical role of distinctive membrane domains upon the biology of the cell surface. Membrane rafts are involved in pathogen invasion, regulation of protein and lipid sorting, as well as in cellular signaling (45, 380, 415, 481, 537). The latter category highlights the dynamic nature of membrane signaling domains, which were shown to assemble from otherwise separate units upon engagement of surface proteins (273, 495).
Raft association appears to control the distribution of mature PrPC among very distinct regions of the plasma membrane, as shown by the extensive redistribution of PrPC at the surface of cultured neurons upon cholesterol or sphingolipid depletion (166). PrPC coimmunoprecipitated with GM1 from neuroblastoma, and with GM3 from lymphoblastoid cells, both of which are markers of membrane rafts in their respective cell types (344). However, controversial findings were reported as both of the nature of PrPC-containing rafts as well as of the distribution of PrPC between raft and nonraft membrane domains.
In cultured dorsal root ganglia neurons, dissociated from 3- to 4-wk-old rats or Thy-1 transgenic mice, immunohistochemical labeling, detergent solubilization, and immunoaffinity purification led to the conclusion that PrPC-containing rafts are distinct from those containing Thy-1 (318). Further characterization of DRMs isolated from whole rat brain supported this hypothesis and showed that PrPC DRMs contain significantly more unsaturated, longer chain lipids than Thy-1 DRMs, as well as other differences in lipid composition, consistent with the distinct solubility of the two GPI-anchored membrane proteins in nonionic detergents (61), as well as with the heterogeneity of membrane rafts (421).
Western blots of DRMs from rat cerebellar granule cells, followed by confocal microscopy of cultured neurons and coimmunoprecipitation, showed that, although both PrPC and the acylated non-receptor tyrosine kinase Fyn are associated with DRMs, these two proteins in general do not colocalize in the membrane, whereas PrPC associates with the growth-associated GAP43 protein (38).
In contrast to both studies above, no distinction between PrPC, Thy-1, and Fyn-containing rafts was achieved in another study of DRMs isolated from rat cerebellar granule cells (304). It is unclear why studies employing a similar cell culture procedure achieved such distinct results.
In addition, association of PrPC with lipid rafts is dynamic. A large fraction of the protein is found in nonraft membrane, on its way to coated pits (518; see sect. IIIC). Conversely, PrPC can be recruited from a large pool of nonraft to raft membrane domains upon engagement of lymphoblastoid cells by cross-linked antibodies (225).
B. Trafficking of PrPC to the Cell Surface
Experiments using brefeldin A showed that Golgi passage is needed for PrPs to reach the plasma membrane (520) and that exit of PrPC from the Golgi and passage by the cell surface are required for the generation of de novo PrPSc (36, 82, 520). However, only recently has progress been made in the understanding of initial events of PrPC synthesis, raft insertion and localization (Fig. 4).
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Physiological roles of the transmembrane forms of PrP are not clear. However, transgenic mice expressing CtmPrP develop neurological illness and neuronal death that resembles certain prion diseases (508). Since CtmPrP accumulates in the Golgi apparatus in these mice, there is a possible role for Golgi dysfunction in the neuropathology of mice that express PrPC mutants that favor CtmPrP (507). It is noteworthy that the transmembrane domain in membrane inserted PrPs is similar to the region that binds certain PrPC ligands, such as hop/STI1 (92, 591). This suggests that the membrane occlusion of this site may influence cell physiology, which would be consistent with the purported importance of PrPC:hop/STI1 interaction (see sects. VII and VIII).
The significance of variable amounts of cytosolic PrPC, detected usually after proteasome inhibition or overexpression of transfected Prnp, but also found in a subset of cerebellar neurons, is still debatable. It is not clear why WT PrPC is occasionally retained in the cytosol, nor whether it serves any physiological function or, alternatively, may be associated with neuronal pathogenesis (72, 145, 312, 314, 360, 399, 555, 556). Indeed, in human neurons in culture, cytosolic PrPC is not toxic and may, instead, have antiapoptotic functions (456). Recent work by Leblanc and collaborators (244) suggests that lack of translocation to cytosol for some PrP mutants affects their anti-Bax activity (244). Remarkably, cytosolic expression of PrP mutants that show defective anti-Bax activity restored the antiapoptotic function of the mutants, suggesting that these mutations may affect the ability of PrPC to move to the cytosol, rather than affecting a direct antiapoptotic function of PrPC (244). Although cytosolic PrPC has been attributed to ER retrotranslocation (313), others have found that cytosolic PrPC contains the NH2-terminal signal peptide, indicating that this subcellular location may be exclusively related to abortive translocation into the ER (72, 134, 145).
The GPI anchor is added in the ER, after removal of a COOH-terminal peptide signal. The core GPI anchor added to the immature PrPC is then processed (502), by a hitherto poorly understood mechanism.
Association of PrPC with lipid rafts reportedly starts early within the ER, and immature (endoglycosidase H-sensitive) and mature (endo H-resistant) PrPC are differentially sensitive to cholesterol or sphingolipid depletion, suggesting that maturation of PrPC along the ER-Golgi-plasma membrane pathway is associated with a changing association with distinct membrane rafts (472). Results of cholesterol depletion also suggested that raft association is required for correct folding of PrPC (472), as well as for the export of the protein to the Golgi and proper glycosylation (73, 472). Moreover, PrPC that does not associate with rafts in the ER appears to undergo conformational changes that modify protease sensitivity, indicating that the immature protein may be misfolded and subject to the ER quality control mechanism (472).
The glycosylation patterns of PrPC may also affect protein trafficking and biophysical features. This was tested using transgenic mice expressing PrPC mutants that were monoglycosylated either on Asn-180 or Asn-196, or nonglycosylated (74). Neurons from mice that produce only nonglycosylated PrPC presented a shift in the localization of the protein, with accumulation in the Golgi (74, 581). Interestingly, no overt phenotype, neurological signs, or neurodegeneration were found in these mice, indicating that simple accumulation of PrPC in the Golgi may not necessarily be deleterious.
C. Endocytosis and Internalization of PrPC
Initial interest on trafficking of PrPC was raised by the finding that PrPSc (or PrPres) was derived from a GPI-anchored cellular precursor, which journeys to the cell surface and to endosomal vesicles prior to conversion (38, 82). Indeed, evidence for endosomal conversion of PrPC into protease-resistant forms has received considerable attention in recent years (see, for example, Ref. 80).
Many GPI-anchored proteins are sequestered into intracellular compartments (346, 347, 384, 412). Earlier work showed that chicken PrPC transfected into a neuroblastoma cell line cycled between the plasma membrane and endosomal compartments with a t1/2 of 20–30 min (486). In neurons, endogenous PrPC appears to internalize as fast as classical membrane receptors, such as the transferrin receptor, with a t1/2 of 3–5 min (518).
A controversy regarding the role of clathrin- and non-clathrin-coated vesicles in the internalization of PrPC (424) was fueled by the idea that GPI-anchored proteins use a lipid raft-based mechanism for internalization (250), because they lack transmembrane domains able to interact with clathrin adaptor proteins. Moreover, localization of both PrPC and PrPSc in rafts, or caveolae-like domains, was also reported and led to the contention that clathrin-coated vesicles did not participate in PrPC trafficking (250, 544). Evidence for PrPC internalization via non-clathrin-coated vesicles was, indeed, obtained in nonneural cells and also in glia (335, 419).
A recent electron microscopic study of PrPC internalization in CHO cells used protein A-gold to follow PrPC internalization into caveolae, since the authors indicated that protein A-gold, but not protein A-FITC, binds PrPC in transfected cells (419). However, this unconventional way to follow PrPC trafficking, added to the use of nonneural cells, precludes generalization. Since GPI-anchored proteins may follow distinct endocytic routes depending on both the cell type and membrane environment (147), the results obtained in CHO cells likely do not apply to other specialized cell types, such as neurons. In fact, despite its GPI-dependent raft association, sequestered PrPC was soluble in nonionic detergents, a common criterion for residence outside raft membranes. Therefore, rather than internalization via rafts, PrPC appears to move to nonraft membrane to be internalized (518).
Although PrPC lacks the expected intracellular internalization signals, mounting evidence supports a major role of clathrin-coated vesicles in the internalization of PrPC (Fig. 4). The pioneer evidence that chicken PrPC is internalized via clathrin-coated vesicles (485) has now been confirmed and extended to mammalian PrPC (518, 525, 526). Cell surface biotinylation, live cell microscopy, GFP-tagged PrPs, and electron microscopy support the view that clathrin-coated vesicles and classical endosomal organelles are involved in endocytosis of PrPC (59, 321, 485, 518, 526). Dominant negative approaches indicated a role for the activities of dynamin (321) and clathrin in the internalization of PrPC in distinct cell lines (526).
A mechanism was identified by which raft-associated PrPC can be sequestered by clathrin-coated vesicles. It was proposed that an NH2-terminal, positively charged domain of PrPC (KKRPKP) is responsible, perhaps by interacting with a transmembrane protein, for the constitutive endocytosis of PrPC by clathrin-coated vesicles (518). Remarkably, a number of reports indicated a role for the NH2-terminal region of PrPC upon endocytosis and cellular trafficking (283, 392, 488), and this basic region of the protein has been previously implicated in the binding of negatively charged proteoglycans, which are thought to modulate PrPC sequestration (80).
The hypothesis that GPI-anchored PrPC may "piggy-back" on an integral membrane protein had long been raised (485), similar to the mechanism of interaction between the GPI-anchored urokinase receptor and a low-density lipoprotein receptor-related protein (LRP1),1 a transmembrane protein that can interact with adaptor proteins (112, 119, 391). Indeed, recent experiments indicated that LRP1 may participate in clathrin-mediated PrPC endocytosis, because knock-down of LRP1, but not LRP1b, reduced internalization of PrPC (525; see also Ref. 371).
In addition to these findings, blockade of PrPC internalization by antibodies to the LRP in various cells, as well as transfection of deleted mutants, suggests that LRP and/or the laminin receptor may play a major role in the subcellular traffic of PrPC (170, 171). Nonetheless, the latter experiments were done with a recombinant soluble PrPC, rather than the GPI-anchored protein, and LRP was able to mediate internalization of only part of the membrane-bound recombinant protein (171), which is consistent with the participation of other cell surface proteins in the trafficking of PrPC.
High extracellular levels of Cu2+ induce the endocytosis of PrPC to intracellular organelles and the Golgi (59, 283, 414, 417). It was reported that Cu2+-induced endocytosis of PrPC expressed in neuroblastoma cells caused its movement from raft to nonraft membrane regions. The KKRP motif was shown to be important for endocytosis, supporting earlier findings for the constitutive endocytosis of endogenous PrPC (526). Interestingly, this KKRP motif is not essential for the lateral displacement of PrPC to nonraft membrane, indicating that this movement occurs prior to PrPC endocytosis (526). It was also suggested that Cu2+ may destabilize a putative PrPC interaction within rafts, rather than inducing PrPC to interact with a nonraft protein (526).
Further analysis along these lines should allow the understanding of the fundamental mechanisms through which the GPI-anchored PrPC may be internalized by clathrin-coated vesicles.
D. Modulation of the PrPC Endocytic Cycle
In addition to copper (see above), both sulfated glycans and suramin, compounds that affect scrapie replication, may also induce internalization of PrPC (173, 258, 487).
It was proposed that changes in PrPC conformation induced by suramin and Cu2+ may lead to internalization (258). Whether copper-induced endocytosis of PrPC is an acceleration of a physiological mechanism is unclear, although many of its features are similar to those of the constitutive internalization of PrPC, such as the involvement of clathrin and the need for exit from lipid rafts (518, 526).
If exit from lipid rafts, rather than misfolding, is the trigger for PrPC internalization, engagement of PrPC with ligands that affect its lateral distribution may also induce endocytosis. PrPC appears to reside in the most external shell of the rafts (318, 371), the protein remains only for a relatively short period of time at the cell surface (486; reviewed in Ref. 424), and indeed, PrPc cross-linking in lymphoid cells induced endocytosis (512). However, it is still unknown whether the endocytic cycle of PrPC may depend on a physiological ligand.
It is likely that cytosolic factors that participate in clathrin-mediated endocytosis, such as the small GTP binding Rab proteins (596), may have a key role in controlling PrPC trafficking (Fig. 4). Rab 5 is a candidate (321), but Rab 7-positive organelles (late endosomes and lysosomes), which participate in PrPSc invasion of cells, may also play a role (320). Other Rab proteins, such as Rab 6, involved with retrograde transport of PrPSc to the ER, may also regulate trafficking of PrPC (26).
Proteolytic cleavage of PrPC occurs during its cycle between intracellular compartments and the cell surface, and clipping usually removes the NH2-terminal region, involved in endocytosis (see above), and possibly also in raft localization (551). Thus the processed COOH-terminal domain of the protein accumulates at the cell surface (488) and may affect sensitivity to cell death (517).
Major unanswered questions remain as to the physiological regulation of the endocytic cycle of PrPC. This subject merits further attention, especially because of its possible involvement in physiological PrPC-mediated signaling mechanisms (see below). An important issue refers to the fate of recycled PrPC, following each round of internalization. A small fraction of endocytosed PrPC is degraded by lysosomes, but large fractions return to the cell surface. In addition, work originally intended to understand the intercellular transfer of PrPSc has led to evidence that part of the recycled PrPC may be secreted to the extracellular medium associated with exosomes (144, 423, 447). These membrane-bound vesicular structures are derived from multivesicular bodies formed within the endosomal system and have been associated with transfer of molecules, for example, between immune cells (see Ref. 423 for review). This may allow bidirectional cellular interactions, mediated either by the binding of extracellular ligands (sect. IID) to cell-surface PrPC, as well as the binding of secreted PrPC to cell-surface ligands (see sect. IX).
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IV. FUNCTIONS OF THE PRION PROTEIN IN THE NERVOUS SYSTEM |
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Molecular cloning of the cDNA coding the entire ORF of mouse Prnp (305) and the use of homologous recombination allowed the generation of mice in which the Prnp gene was deleted. All PrP-null animals generated to date established a tight correlation between the presence of PrPC and the sensitivity to prion infection, and support the model of propagation of PrPSc by co-option of PrPC. This has been largely discussed in the literature (566 for review) and is outside the scope of this article.
The first PrP-null mouse strain was produced in a mixed C57BL/6J x 129/Sv(ev) background, by replacement of codons 4–187 with a neomycin phosphotransferase (neo) expression cassette (63). These animals, designated Prnp0/0 or Zurich I (ZrchI), had no gross anatomical abnormalities in the brain, nor in skeletal muscle and visceral organs. Morris' water maze, Y maze discrimination, and a test that measures the animal's ability to compare the efficiency of distinct problem-solving strategies showed no defects in behavior or learning (63). Unfortunately, the mixed background has been a barrier for the interpretation of several more recent experiments, particularly those related to behavior.
A second line of PrP-null mice was produced by interruption of the Prnp ORF at position 93 (a Kpn 1 site) and introduction of a neo cassette (331). These mice are known as Npu, Prnp–/– Edinburgh (Edbg), and are under 129/Ola genetic background. In agreement with the previously generated ZrchI mice, the Edbg mice did not present any developmental alterations.
Taken together, the first results obtained with two distinct PrP-null mouse strains suggested that either PrPC is unnecessary for normal development, or its absence is compensated by a redundant protein which maintains an important phenotype (63, 331).
B. Neurodegeneration in PrP-null mice
In sharp contrast to the preceding strains, PrP-null mice were generated that developed normally but underwent severe ataxia and Purkinje cell degeneration at advanced ages. This was first observed in Prnp-ablated mice now known as Nagasaki (Ngsk), in which besides the Prnp ORF, 0.9 kb of intron 2, 10 bp of the 5'-noncoding region, and 0.45 kb of the 3'-noncoding region were also replaced by a neo cassette (461). Since the observed phenotype was abolished by reintroduction of Prnp, it was attributed to the absence of PrPC (389).
The same Prnp deletion approach and replacement by another resistance marker, the hypoxanthine phosphoribosyl transferase gene, led to the same degenerative phenotype in mice now known as Rcm0 (366). Notably, similar results were obtained when, in addition to the Prnp ORF, 0.26 kb of intron 2, 10 bp of the 5'-noncoding region, the 3'-noncoding region of exon 3 plus 0.6 kb of the adjacent region, were deleted in Zurich II (ZrchII) mice (453).
C. The Knockout Paradox, Doppel gene, and neurodegeneration
The constructions used to generate animals that suffered cerebellar degeneration and ataxia differ from those that lacked phenotypic effects (63). In the former, large portions of the ORF flanking regions were removed. The remarkably distinct phenotypes of Ngsk, Rcm0, and ZrchII mice, when compared with ZrchI and Npu, were obviously associated with the deletion approach (563).
In fact, it was demonstrated that ataxia in these animals was caused by overexpression of a protein that received the name Doppel, Dpl, coded by the gene Prnd, located 16 kb downstream of Prnp (366). The Prnd promoter is weak in the brain, but when the Prnp deletion extends to exon 3, its splice acceptor site is lost, leading to exon skipping and the generation of a chimeric mRNA containing the first two noncoding Prnp exons plus the Dpl-encoding Prnd exon. Thus Dpl becomes controlled by the Prnp promoter and therefore highly expressed in brain (288, 366).
Dpl is overexpressed in Ngsk, Rcm0, and Zrch II mice, but not in Zrch I or Npu animals (366). The ectopic expression of Dpl in Zrch I mice caused ataxia and degeneration of cerebellar granule and Purkinje cells, and its levels were inversely correlated with the onset of disease (367). Indeed, these results showed that ectopic expression of Dpl, rather than the absence of PrPC, caused neurodegeneration. Notably, the reintroduction of Prnp gene in mice overexpressing Dpl in the brain rescued the phenotype (367, 389). Thus Dpl neurotoxicity is counteracted by PrPC, but the mechanisms underlying this antagonism remain elusive (reviewed in Ref. 23).
D. Neurodegeneration in Other Transgenic Mice
Neurodegeneration is not uniquely associated with overexpression of Dpl (Fig. 5). Transgenic mice overexpressing PrPC were generated from various constructions. The first was a large cosmid containing the three exons and two introns of the mouse Prnpb allele (108Phe, 189Val) and encompasses 6 kb of the upstream sequence plus 18 kb of the 3'-downstream sequence (572). A second construction, named half-genomic DNA, was derived from the previous cosmid by deleting intron 2 and replacing the exon 3 by the Prnpa allele (108Leu, 189Thr) and the 18 kb of the 3'-sequence by a 2.2 kb downstream sequence from the Prnpa locus (146). Mice overexpressing WT PrPC coded by the first construction developed ataxia, hindlimb paralysis, and tremors, while those expressing high levels of PrPC from the half-genomic construction had no obvious phenotype (146, 527, 570).
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The distinct phenotypes of these mice may be due to the allelic nature of the two transgenes. It is known that alleles Prnpa and Prnpb have been associated with either long or short disease incubation times, respectively (571). Mouse PrPC coded by alleles Prnpa and Prnpb contain Leu or Phe at codon 108, respectively. Both amino acids are nonpolar and neutral, but Phe holds a longer lateral chain than Leu. On the other hand, Prnpa allele codes for polar Thr at position 189, whereas Val in allele Prnpb is nonpolar. The chemical properties of these amino acids could possibly affect either the interaction between PrPC and its cellular ligands (see sect. IID) or recognition of PrPC by proteases, leading to changes in cellular processes such as sensitivity to cell death (517). These possibilities deserve further investigation in the context of PrPC physiology.
The half-genomic PrP vector was also used to clone deletion mutants of the Prnpa allele, which were then expressed as transgenes in Zrch I Prnp0/0 animals (Fig. 5). Interestingly, although the reexpression of either full-length PrPC or mutants with deleted aa 32–80, 32–93, or 32–106 in Prnp0/0 mice did not cause an altered phenotype, deletion of aa 32–121, or 32–134, led to both severe ataxia and apoptosis in the cerebellum of relatively young animals (484). This latter phenotype was rescued by introducing one copy of a WT Prnp gene (484).
The phenotype observed in mice expressing PrP
32–134 resembles animals that overexpress Dpl. Since the globular domains of PrPC and Dpl are similar, and both Dpl and the PrPC deletion mutants lack the flexible NH2-terminal domain, it was suggested that the mechanisms of neurodegeneration by both Dpl and PrP32–134 might be the same (565). Remarkably, however, while Dpl leads to massive degeneration of cerebellar Purkinje cells (461), PrP32–134 caused degeneration in the granule cell layer (484).
The half-genomic construction partially mimics the endogenous expression pattern of PrPC, but it promotes high expression of PrPC in granule cells, while expression in Purkinje cells is undetectable (93, 146). On the other hand, Dpl under control of the Prnp promoter is highly expressed in Purkinje cells, in contrast to lower expression in granule cells (288, 453). Thus specificity of neuronal death induced either by truncated PrPC in granule cells, or by Dpl in Purkinje cells, may be associated with their differing levels of expression in distinct neurons. This hypothesis is supported by the elegant demonstration that truncated PrP32–134 specifically targeted to Purkinje cells of ZrchI Prnp0/0 mice led to their selective degeneration, whereas the granule layer was unaffected (148).
Mice expressing either Dpl, or one of the truncated forms PrP32–121 or PrP32–134, presented severe demyelination and axon loss in the spinal cord and cerebellar white matter (439). The leukodystrophy, but not the cerebellar granule cell degeneration (484), was rescued by oligodendrocyte-specific expression of PrPC. On the other hand, neuron-specific expression of PrPC partially rescued cerebellar granule cell degeneration but not demyelination. Thus white matter disease and cerebellar granule cell degeneration in these mice are distinct, and endogenous expression of PrPC in both neurons and glia was required for complete reversion of the degenerative phenotype (439).
In recent studies, transgenic mice expressing truncated PrP105–125 (Fig. 5) developed an extremely severe illness within 2 wk of birth, with decreased body size and weight, immobility, impaired righting, myoclonus, and tremor and died within 1 mo. Histopathology demonstrated cerebellar atrophy, severe loss of cerebellar granule cells, gliosis, and astrocytic hypertrophy. Remarkably, older symptomatic mice expressing the truncated protein associated with the Tga20 allele (which overexpresses PrPC) in hemizygosis, presented vacuolar degeneration of the white matter of both the spinal cord and the brain, but did not undergo cerebellar degeneration (287).
Simultaneously, mice expressing a truncated PrP94–134 form (Fig. 5) showed extensive central and peripheral myelin degeneration and developed early ataxia, which progressed to spastic paraparesis and full paraplegia (21). Interestingly, this lethal phenotype was not associated with the presence of pathological aggregates, altered PrPC glycosylation, subcellular mislocalization, or inappropriate membrane topology. The phenotype was rescued dose dependently either by WT PrPC or a PrPC lacking all octarepeats. Mice expressing a smaller truncation PrP114–121 showed no phenotype either in the presence or absence of WT PrPC. Interestingly, however, the expression of PrP114–121 ameliorated the phenotype caused by truncated PrP32–134, but enhanced the degenerative effect of PrP94–134 (21).
The mechanisms of neurodegeneration mediated by the expression of Dpl, PrP32–134, PrP94–134, and PrP105–125 are still elusive. An early hypothesis (484, 565) was that PrPC binds to a putative ligand, inducing specific cell signaling that mediates neuronal survival. In the absence of PrPC, a redundant homolog molecule, with lower affinity for the putative PrPC ligand, would replace PrPC, therefore explaining the lack of neurodegeneration in PrP-null mice. On the other hand, NH2-terminal truncated PrPC or Dpl molecules might compete with this PrP homolog for the putative ligand without eliciting survival signals (484, 565). Alternatively, truncated PrP and Doppel may activate pathways of programmed cell death that are normally kept quiescent either by PrPC or by its functional homolog (148). The recent studies of centrally truncated PrP transgenics (21, 285) led to distinct hypotheses, based on the putative interaction of truncated PrPC with a single PrPC receptor.
One proposal suggests that myelin integrity depends on dimerization of PrPC, leading to dimerization of the putative PrPC receptor. Dimers of truncated PrP32–134 would trap the putative receptor in a dominant-negative form, which fails to induce signals for myelin maintenance. The presence of the octarepeat domain in the truncated PrP94–134 is thought to generate a more stable dominant-negative form of the receptor, causing more severe pathology than truncated PrP32–134 (21).
Another hypothesis is that the more severe neurodegeneration in mice expressing the truncated PrP105–125, when compared with PrP32–134 (285), is associated with a higher affinity of the former for a putative receptor. In normal conditions this receptor would transduce neuroprotective signals, but its altered conformation when coupled with the centrally truncated PrPC, would lead to neurotoxic signals. The authors also proposed that the so-called neurotoxic peptide PrPC105–125 may compete with PrPC for this receptor, thereby inducing toxicity (285). However, this seems unlikely in view of the apparent requirement that the neurotoxic peptide must be aggregated to produce toxicity (92, 140).
Interestingly, while PrP105–125 in PrPC-null background produced degeneration particularly in the cerebellum (285), PrP114–121 in the same background was innocuous (21). Thus PrPC domains from aa 105–114 and 121–125 may exert important modulatory activity. Moreover, the most prominent phenotype observed in animals expressing truncated PrP105–125 was cerebellar cell death, whereas PrP93–134 caused massive myelin degeneration, indicating that the PrPC domains including amino acids 93–104 and 126–134 may have an impact on specific mechanisms underlying myelin integrity.
The idea of a PrPC homolog/redundant molecule which mediates neuronal survival in the CNS (484) is plausible, but its identity is unknown. Notably, in the peripheral nervous system, PrPC mediates neuronal differentiation through its interaction with vitronectin, which is compensated by integrins in PrP-null cells (192). Nevertheless, there is as yet no evidence for a role of integrins in compensatory mechanisms associated with either neuronal survival or myelin integrity in PrP-null neurons.
As for the elusive PrPC receptor alluded to in the recent studies (21, 285), among the various PrPC ligands identified to date (see sect. IID), at least hop/STI1, the interaction of which with PrPC mediates neuroprotection (92, 591, see sect. VIIC), binds to the domain PrPC113–128, which is deleted in the truncated molecules with neurodegenerative activity. However, there is no evidence that hop/STI1 is the putative PrPC ligand/receptor proposed in the different models (21, 285, 484). In addition, the vitronectin binding site in PrPC maps between aa 105–119, which are also within the domain the deletion of which promotes neurodegeneration. Nonetheless, the role of PrPC-vitronectin interaction in neuronal survival was not tested (192). It is, in fact, unlikely that a single PrPC-binding molecule may explain the degenerative phenotypes induced by all the truncated forms described to date. A conservative hypothesis is that various truncated PrPC forms may lead to distinctive patterns of cell surface protein complexes formed by its ligands, which differ from the protein complexes, that a full-length, normal PrPc molecule may be able to sustain (see sect. IX).
One of the first phenotypes attributed to the lack of PrPC was the impaired motor coordination of aged Ngsk PrP-null mice, which correlated with the loss of cerebellar Purkinje cells (253). However, these findings are associated with increased expression of Dpl in the brain (see above), rather than loss of function of PrPC.
On the other hand, ZrchI Prnp0/0 mice showed a slight increase in locomotor activity during exploration of a novel environment (451). These mice also showed reduced hyperlocomotor responses to the NMDA receptor antagonist MK-801, but normal responses to either amphetamine or caffeine, when compared with WT. This suggested that the lack of PrPC leads to functional changes in the glutamatergic system, whereas both dopaminergic and adenosinergic systems are preserved (102).
Under acute stress, such as restraint or electric foot shock, ZrchI Prnp0/0 mice showed reduced levels of anxiety, when compared with WT controls (386). Anxiety is accompanied by a characteristic set of behavioral and physiological responses that tend to protect the individual from danger and is taken as part of a universal mechanism of adaptation to adverse conditions (reviewed in Ref. 187). The lower levels of anxiety in Prnp0/0 animals suggest that PrPC may help the adaptation to stress at the systems level (386).
F. PrPC and the Sleep-Wakefulness Cycle
The circadian clock relies on molecular clocks and their key regulated genes. Mutations or polymorphisms in human clock genes are linked with several sleep-phase syndromes (194). Several studies are consistent with a role of PrPC in the control of the sleep-wakefulness cycle.
Fatal familial insomnia (FFI) is clinically characterized by progressive insomnia, dysautonomia, and motor signs and was associated with severe atrophy of the anterior ventral and mediodorsal thalamic nuclei (309). The brains of most members of an affected family showed gliosis of the cerebral cortex, a moderate degree of cerebellar atrophy, and severe atrophy of the inferior olivary nuclei. One case also showed spongiform degeneration of the cerebral cortex. These findings raised the possibility that FFI belongs to the group of prion diseases (329). In fact, protease-resistant PrP was found in FFI patients (353), and the disease was successfully transmitted to experimental animals, confirming that FFI is an infectious cerebral amyloidosis (523). In addition, all affected members of the kindred had a point mutation in PRNP (the human PrPC gene) codon 178 that substitutes asparagine for aspartic acid. Linkage analysis showed a close relationship between the point mutation and the disease (353), which was confirmed in a second kindred (352).
Nonetheless, substitution of Asn for Asp at codon 178 also segregates with CJD (177, 178). Interestingly, both FFI and familial CJD segregate with a combination of the Asn-178 mutation and the Met-Val polymorphism at codon 129. The Met-129/Asn-178 allele segregated with FFI, whereas the Val-129/Asn-178 allele segregated with familial CJD. Thus a common polymorphism determines disease phenotypes linked to the same pathogenic mutation (179).
Altered circadian rhythms and sleep patterns were found in PrP-null mice, raising the hypothesis that FFI resulted from PrPC loss of function. Both ZrchI Prnp0/0 and WT (C57Bl/6 x 129Sv) presented similar motor activity under light-dark conditions (12/12 h). However, a longer and more stable period of circadian motor activity was found in Prnp0/0 mice under constant darkness, when compared with WT (535). Overexpression of PrPC in transgenic Tga20 mice (146) restored the WT phenotype (535). Similar results were obtained with Npu mice (Edbg Prnp–/–), when compared with their 129/Ola controls (535), strongly indicating that the changes in PrP-null mice are due to PrPC depletion, instead of genetic background. Interestingly, PrPC mRNA is regulated in a marked circadian manner, in both the suprachiasmatic nuclei and other forebrain regions (69).
Sleep is regulated as a function of prior wakefulness (34, 35), and electroencephalogram (EEG) slow-wave activity (SWA) is used as an index of sleep intensity (34). PrP-null mice from both ZrchI and Npu strains presented a lower amount of SWA during the episodes of non-rapid-eye-movement (NREM) sleep, whereas rapid-eye-movement (REM) sleep was unaffected (222, 534).
Mice show an increase of SWA after sleep deprivation (reviewed in Ref. 533). Notably, the increase of SWA during recovery sleep was twice as long in both ZrchI and Npu mice than in their respective WT (222, 534). Although these results are difficult to interpret, they suggested that PrP-null animals may have a low sleep pressure, leading to more frequent interruptions of sleep and reduced SWA (222, 534, 535). Tga20 PrPC-overexpressing mice presented sleep fragmentation and a response to sleep deprivation similar to WT, indicating that reexpression of PrPC rescued the altered phenotype of PrP-null mice (534, 535).
The EEG of mice shows regional differences during recovery from sleep deprivation (223). In a further experiment, Npu Prnp–/– mice presented a larger increase of SWA than WT in the occipital, but not in frontal derivation. The difference between both genotypes appeared after the waking-NREM sleep transition, indicating that PrPC is not required for the transition to sleep (224). It was suggested that the differing sleep EEG between WT and PrP-null mice reflects either a direct influence of PrPC on neurotransmission, or its protective role upon synapses (224). It is also possible that PrP-null mice are more susceptible to stress related to sleep deprivation, or even that the absence of PrPC in the neuronal membranes may affect the generation of slow waves (222).
Besides FFI, intractable insomnia was reported in patients with a mutation at codon 200 responsible for a familial form of CJD (85, 521), in up to 45% of the patients with confirmed sporadic CJD (255, 529, 547), and in iatrogenic CJD (68). Sleep alterations may have been underestimated in prion diseases, and these signs may reflect a similar underlying mechanism in both CJD and FFI, associated with prominent histopathological changes in cortical areas, but independent on selective neuronal loss in thalamic nuclei (276).
The first experiments in PrP-null ZrchI mice failed to demonstrate altered memory (63). However, a role of PrPC in memory retention was postulated, based on studies of PrP-null Ngsk mice (388). These animals presented impairment in latent learning at 24 wk of age, with normal learning ability and short-term memory (STM) retention, but a significant deficit in long-term memory (LTM) consolidation. Again, however, the high levels of Dpl (366) preclude an association of this deficit with PrPC loss of function.
However, although Zrch I mice presented both normal STM and LTM at the age of 3–4 mo (105, 451), memory impairment was observed at 9 mo (105). Npu PrPC null mice in either pure 129/Ola background or a mixed 129/Ola x C57Bl/10 background also exhibited impaired hippocampal-dependent spatial learning, while nonspatial learning remained intact. These deficits were rescued when PrPC was selectively reexpressed in neurons, indicating that they were caused by loss of function of neuronal PrPC (116).
Studies of genetically modified animals are controversial, largely because the gene deletion approach, genetic background and compensatory mechanisms, may all influence cognitive performance. Thus the use of pharmacological approaches offers distinct advantages for evaluating the roles of selected molecules upon cognitive processes, especially in memory consolidation (234).
Intrahippocampal infusion of an antagonist antibody to PrPC impaired both STM and LTM retention in old rats, when compared with age-matched animals infused with an irrelevant IgG (105). Moreover, infusion of antibodies against either PrPC or laminin (LN), or of peptides representing their respective binding sites (see sect. IID), impaired memory consolidation.
In rats trained in one-trial inhibitory avoidance, memory retention was impaired by an immediate, posttraining intra-CA1 infusion of either PrPC173–192, a peptide that contains the LN binding site or of anti-PrPC antibodies that inhibit PrPC-LN interaction. The amnesic effects of both anti-PrPC antibodies and PrPC173–192 were reversed by coinfusion of the LN -1 peptide, that is the PrPC-binding site in LN (see sect. IID). In addition, both anti-LN and anti-PrPC antibodies also inhibited the activation of hippocampal PKA and ERK1/2, two kinases that mediate the upregulation of signaling pathways needed for the effective consolidation of inhibitory avoidance memory. Remarkably, LN -1 peptide reversed the PKA and ERK1/2 inhibition induced by anti-PrPC antibodies (103). These findings show that hippocampal PrPC plays a critical role in memory processing through interaction with LN and suggest that this role is mediated by PKA and ERK1/2 (103).
The mechanism by which PrPC-LN interaction modulates memory is still unknown. However, a plausible hypothesis is based on the interaction of LN with the tissue type plasminogen activator (tPA)/plasmin proteolytic cascade. On the one hand, laminins stimulate neurite outgrowth, and the most abundant laminin isoform in the hippocampus is LN-10 (5β11) (230), which is produced and secreted by neurons. These cells bind to LN-10 through integrin 3β1 (390), as well as through PrPC (103). The PrPC-binding domain maps to the COOH-terminal domain of the laminin -1 chain, and only PrPC binds to this domain, through which it is able to promote neurite outgrowth (183). The same LN domain was also shown to promote neurite outgrowth, although the cell surface ligand was ignored (294).
On the other hand, tPA is induced as an immediate-early gene during long-term potentiation (LTP) (434). LTP in the CA1 region of the hippocampus is strongly associated with one-trial (passive) avoidance learning (233, 575; see sect. IVI), which is impaired in PrP-null mice. There is a clear correlation between the level of expression of tPA and the intensity of LTP, since mice deficient in tPA have a reduction in LTP (159, 220), whereas tPA overexpressors show enhanced LTP (317). Notably, LN is a substrate of the tPA/plasmin proteolytic cascade (89). It is, therefore, possible that partial LN degradation, consequent to physiological stimuli which elicit enhanced tPA activity, releases the -1 chain peptide, which then binds to PrPC initiating signal transduction. This hypothetical chain of events may be a major component of the PrPC/LN-mediated memory effects described above.
In turn, the binding of hop/STI1 to PrPC largely induced retention of STM and consolidation of LTM, acting on the CA1 region of the hippocampus (104). The inhibition of this interaction by intrahippocampal infusion of antibodies against PrPC or STI1 binding sites disrupted both STM and LTM of one-trial avoidance. Furthermore, infusion of PrPC peptide 106–126, which competes for PrPC-STI1 interaction, also inhibited memory consolidation. This effect was not due to peptide toxicity, since memory retention was completely restored 24 h after treatment. Strikingly, the peptide hop/STI1230–245, which contains the PrPC binding site, had a potent enhancing effect in memory retention, which was counteracted by peptide PrPC106–126. Taken together, these results demonstrate that interaction of PrPC with hop/STI1 is critical to consolidate both STM and LTM and suggest a potential use of the peptide hop/ST11230–245 as a therapeutic tool (104).
Thus at least two definite molecular interactions of PrPC with hippocampal cell surface proteins, hop/STI1 and LN, can mediate effects of PrPC on memory consolidation, and it is likely that PrPC modulates memory retention through both these interactions.
Further support of the hypothesis that PrPC plays important roles in memory and cognition is found in human subjects. The presence of Val at codon 129 of PRNP in at least one allele was associated with worsened cognitive performance in elderly subjects (30, 247), with early cognitive decline (117), and with faster decline of intellectual abilities in Downs syndrome patients (126). Conversely, healthy young adults expressing Met at codon 129 in at least one allele exhibit better LTM than those with Val in this codon, albeit STM was unaffected (409). Thus polymorphisms at codon 129, a site that is highly important for the protein structure (418), seems to be strongly related with cognitive performance.
H. PrPC and Alzheimer's disease
Interest in relating prion diseases with Alzheimer's disease (AD) stems mainly from their common association with misfolded peptides (2). Early work identified amyloid precursor-like protein 1 (APLP1) as a putative ligand of PrPC (586), but to date this has not been confirmed. Other possible links relate to the fact that both PrPC and the APLP1 close relative amyloid precursor protein (APP) bind copper (49, 83) and share common mechanisms of proteolytic processing (98, 209), although the significance of these findings for physiological functions is obscure.
Neurological studies provided evidence of linkage of variant alleles of PRNP with AD. The data refer to possible correlations of the polymorphism at codon 129 with either the risk of AD or the cognitive decline of these patients.
No association between the PRNP 129 polymorphism and the risk of AD was reported in sporadic AD patients from Spain, Italy, or Japan (76, 110, 127, 398). In contrast, Dutch (129) and Polish (176) patients presenting the 129 allele in homozygosis, particularly 129VV, had a higher risk of early-onset AD when compared with heterozygous. The same was found for German 129MM carriers (446).
The conflicting results may be due to a strong effect of ethnicity on a link between PRNP polymorphisms and genetic susceptibility to AD (127). Indeed, meta-analysis of the studies above indicated that Caucasian subjects homozygous at codon 129 had a 1.3-fold increased risk of developing AD, compared with heterozygous. In addition, AD patients carrying at least one V allele presented a small acceleration in their cognitive decline compared with MM carriers (76, 127).
Brains from patients with either AD (142, 548) or Downs syndrome (351) showed an accumulation of PrPC as globular deposits, which also stained with antibodies to Aβ and to SOD1, suggesting that PrPC and SOD1 accumulate at Aβ plaques (351). This could result from a response of PrPC to oxidative stress induced by the Aβ plaque. Since Val at codon 129 is both linked to enhanced cognitive decline (see above) and correlates with the presence and density of Aβ-associated lesions (29), it is possible that neuroprotective effects mediated by PrPC are less effective when the protein has a Val at codon 129 (351).
Unfortunately, the reports of human subjects are too preliminary to establish a causal chain among the expression of PrPC, the polymorphism in codon 129, neuroprotection, and memory-associated synaptic plasticity (see Ref. 233). However, investigation of both the proteolytic processing and the properties of distinct PrPC polymorphisms, as well as their possible links with amyloid precursor protein are warranted.
In fact, recent studies demonstrated that expression of PrPC inhibits the cleavage of APP by β-secretase (BACE1), thereby reducing the formation of the amyloid β-peptides, the major constituents of the senile plaque (411). This effect was abolished in PrPC molecules lacking the NH2-terminal KKRP (23–26) residues, previously associated with PrPC endocytosis (518). In addition, modifications of PrPC that cause its exclusion from cholesterol lipid rafts, such as tethering the PrPC NH2 terminus to the membrane, deletion of the GPI anchor signal peptide or anchoring PrPC by a transmembrane domain, also blocked the inhibitory effect of PrPC on β-secretase activity. These results suggest that PrPC endocytosis mediated by the KKRP domain, and its localization within specific membrane domains, are necessary for the inhibitory effect of PrPC upon β-secretase. Nonetheless, two PrPC mutants associated with familial forms of human prion diseases, PG14 with an extra nine copies of the octarepeat and A116V (equivalent to human A117V), also lost the ability to modulate β-secretase. Since there is no indication that these two mutants are excluded from cholesterol lipid rafts, nor that they have altered endocytosis, additional factors may contribute to the effect of PrPC on β-secretase activity. Moreover, scrapie-infected mice presented higher β-secretase activity when compared with uninfected animals, indicating that PrPC loses its normal function on the modulation of β-secretase activity either when holding specific mutations associated with prion diseases or when it is converted to PrPSc (411). This is important, because mice infected with PrPSc presented much higher levels of Aβ1–42, the peptide associated with AD. It would be interesting to infect APP knockout mice with PrPSc, to test whether the increased production of Aβ1–42 may have a role in the cognitive decline in prion diseases.
In brains from PrP-null mice (Edbg), the levels of both peptides Aβ1–42 and Aβ1–40 were significantly higher than in wild-type animals, consistent with the proposed inhibitory role of PrPC on β-secretase activity (411). On the other hand, a study using a double transgenic mouse expressing both mutant amyloid precursor protein and overexpressed Syrian hamster PrPC indicated that PrPC enhances the formation of amyloid plaques without causing any significant alteration in the levels of either Aβ1–42 or Aβ1–40 (482). These mice expressed endogenous PrPC and presumably the maximum inhibition of β-secretase activity, although this has not yet been tested in Tg20 mice that overexpress PrPC. It has, nonetheless, been suggested that overexpression of hamster PrPC does not cause any further β-secretase inhibition, whereas it is possible that PrPC has a secondary effect on Aβ aggregation (411).
The effect of PrPC polymorphisms at codon 129 on β-secretase activity was also addressed in mice whose endogenous Prnp was replaced by human PRNP. The amount of the peptide Aβ1–42 was similar between MM or VV homozygous mice, while peptide Aβ1–40 tended to be slightly more abundant in MM mice compared with VV animals (411). The relevance of these data for possible effects of PrPC polymorphisms on AD await further studies, particularly in view of the evidence for a small acceleration in the cognitive decline of patients carrying the VV polymorphism at codon 129, when compared with MM carriers (76, 127).
The data from Parkin et al. (411) raise important questions related to AD and suggest that modulation of the role of PrPC upon β-secretase activity may provide a novel therapeutic approach for AD (411). Furthermore, these results add to the evidence for physiological roles of PrPC (as discussed in the present review), in cautioning against interventions in TSEs that focus on either the blockade or the inactivation of PrPC.
I. PrPC, synaptic activity, and neuronal excitability
The evidence for significant roles of PrPC in a variety of behavioral processes, and particularly in memory, raises the question of whether and how does the prion protein affect synaptic mechanisms and neuronal excitability.
The presence of PrPC in both pre- (202) and postsynaptic structures (191) raised the hypothesis that PrPC plays a role in neuronal communication. In addition, some PrPC glycoforms can be selectively transported along axons, suggesting that these glycoforms may be specifically presynaptic (450).
A functional role of PrPC in synapses was suggested on the basis of the ability of PrPC to bind copper (see sect. IIE), because nerve endings release copper into the synaptic cleft upon depolarization (252). It was proposed that presynaptic PrPC may buffer Cu2+ levels in the synaptic cleft and ensure its transport back into the presynaptic cytosol. This would also protect synapses from the ROS generated by Fenton-type redox reactions. In addition, the buffering of Cu2+ by PrPC could play a role in calcium homeostasis, because copper, at the concentrations found in the synaptic cleft, reduces Ca2+ influx through voltage-gated calcium channels (VGCC) (542).
LTP has long been associated with learning and memory. Recently, it was shown that LTP in the hippocampal CA1 area is at the root of memory formation of one-trial inhibitory (passive) avoidance in the rat (233, 575). Synaptic plastic changes in the basolateral amygdala, the entorhinal, parietal and cingulate cortex, and probably others, are also required for memory consolidation of this task (233). However, CA1 LTP clearly stands out as one crucial component of memory consolidation (575). LTP in other hippocampal subregions, such as the dentate gyrus, or in extrahippocampal brain structures, has frequently been ascribed roles in learning, but their molecular basis is distinct from that of CA1 LTP (325), and evidence for their role in learning is not as compelling as that of the role of CA1 LTP in one-trial avoidance.
In early studies, impaired LTP was found in tissue slices from PrP-null mice, when compared with WT at physiological temperature (108, 332, 577). This was associated with reduced GABAA receptor-mediated fast inhibition (108) and was rescued by a transgene encoding PrPC (577). Differing from these data, no deficits were found in cell excitability, synaptic inhibition, reversal potential for inhibitory postsynaptic potentials or LTP, when hippocampal slices from Prnp0/0 Zrch I animals were examined at room temperature (302). Also, outside-out membrane patches of cerebellar Purkinje cells from these PrP-null mice displayed no change of GABAA receptors (200). Recently, however, it was shown that the levels of both posttetanic potentiation and LTP in the CA1 region of aged PrP-null mice were significantly reduced, when compared with younger animals, a result that was attributed to increased levels of oxidative stress in aged animals (118).
Contrasting with the impaired CA1 LTP in PrP-null mice, hippocampal slices from ZrchI animals demonstrated increased excitability of the dentate gyrus (323). Thus physiological effects of PrPC in the dentate gyrus may differ from those in CA1. Still, these data do not challenge the hypothesis that altered LTP underlies the memory loss observed in PrP-null animals. First, dentate gyrus LTP obeys mechanisms very different from those of CA1 LTP. Second, the latter, but not the former, is unequivocally linked to memory formation processes (233, 575).
PrP-null mouse neurons reportedly had lower input resistances but lacked a late IAHP, which may have opposing consequences on neuronal excitability (107). Remarkably, ectonucleotidase activity is diminished in synaptosomes from PrP-null animals, when compared with WT, thus attenuating the hydrolysis of ADP and decreasing the synaptic levels of the anticonvulsant neurotransmitter adenosine (416). Moreover, although synaptosomes from PrP-null animals show normal glutamate uptake (530), this function is impaired in cortical astrocytes (50), which may lead to increased glutamate concentration at the synaptic cleft and higher neuronal excitability. Thus the loss of PrPC may have either positive or negative effects on neuronal excitability and synaptic transmission, probably as a function of the affected region of the brain, as well as the type of response and intervening neurotransmitter systems elicited by diverse patterns of stimulation.
The afterhyperpolarization potentials (IAHP) depend on calcium influx through L-type VGCC. Interestingly, Ca2+ homeostasis is altered in cells from PrP-null animals (199), and patch-clamp studies of cerebellar Purkinje cells showed a significant correlation between PrPC expression and the maximal amplitude of Ca2+-activated K+ currents. However, the absence of PrPC reportedly did not alter the K+ channels directly responsible for the slow IAHP observed in neurons from PrP-null mice, but rather modulates K+ channels indirectly by reducing Ca2+ influx through L-type VGCC channels (161).
Conditional knockouts, in which the expression of PrPC is abolished at 12 wk of age, also presented a slow afterhyperpolarization in hippocampal CA1 cells, and thus increased neuronal excitability. Because PrPC was deleted only in adult life, this phenotype was caused by neuronal dysfunction, rather than by a developmental deficit (328). In keeping with this observation, the impairment of Ca2+-activated K+ currents previously found in PrP-null mice was rescued in mice where PrPC had been reintroduced (201). Interestingly, a transgene encoding a mutated PrPC at codon 200, which is the most common mutation associated with inherited prion disease, was also able to rescue the normal IAHP (13), indicating that this particular protein domain is not responsible for sustaining normal neuronal excitability.
Increased neuronal excitability in PrP-null hippocampus is also consistent with the lower threshold found in ZrchI mice, compared with WT, to seizures induced by either a single convulsant or repeated subconvulsant (kindling) doses of pentylenetetrazol, and to status epilepticus induced by kainic acid or pilocarpine (553). Similarly, both Npu and conditional postnatal PrPC knockouts also presented a lower seizure threshold than WT (M. C. Landenberger, C. Fagundes, R. Walz, B. Chesebro, G. Mallucci, and V. R. Martins, unpublished results). The expression of a single Prnp allele in heterozygous animals was sufficient to rescue the normal phenotype. Notably, Tga20 mice, which express 10 times more PrPC than WT (146), are highly resistant to seizures induced by either kainic acid or pentylenetetrazol (Landenberger et al., unpublished results). These data demonstrate that expression of PrPC is directly correlated with a higher resistance to seizures.
The higher excitability of PrP-null mice is also consistent with anatomical observations, specifically of the mossy fiber pathway within the hippocampus. Timm-stained brain slices from PrP-null mice exhibited more granules than controls in the granule cell layer, the inner molecular layer of the dentate gyrus, and the infrapyramidal region of CA3 (107). The observed reorganization of neuronal circuitry is similar to the mossy fiber collateral and terminal sprouting seen in temporal lobe epilepsy related to hippocampal sclerosis and may represent an "epileptic neuronal network."
Thus the combined data demonstrate that PrPC significantly modulates neuronal excitability and synaptic activity, which likely constitute the neural basis for at least some of the systemic brain functions attributed to PrPC.
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V. IMMUNOMODULATORY FUNCTIONS OF THE PRION PROTEIN |
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Despite the lack of both overt peripheral inflammatory signals, and of antibodies against the PrPC in the course of TSE, substantial amplification of the infectious agent takes place within lymphoid compartments in early stages of the diseases. In addition, both the state of the immune system and conditions that challenge innate immune defenses affect the susceptibility and the course of experimental prion infection (4, 111). Similar to the CNS, PrPC in immune cells has been usually studied as far as either pathogenesis or treatment are concerned. Here we examine evidence for immunomodulatory functions of PrPC in physiological context.
A. Evidence for a Role of PrPC in the Immune System
No gross defects were found in PrP-null mice in either numbers or maturation of cell components of the immune system, including hematopoietic stem cells, nor in the expression of critical cell surface antigens (see Ref. 232 for review). This suggests that immunomodulatory functions of PrPC are either subtle, or may be relevant only in particular contexts. Indeed, it was argued that expression of PrPC in hematopoietic progenitors and mitotic lymphocytes might be due to high rates of membrane turnover or cell division (151).
A preliminary caveat is that the expression of PrPC is nonhomogeneous both across species and among subsets and states of maturation of immune cells (Table 1). As a rule, the expression of PrPC is downregulated with maturation of granulocytes and upregulated with maturation of myeloid antigen-presenting cells (APC). However, among lymphoid cells, maturation was accompanied by downregulation of PrPC compared with precursor cells in mice, but apparently upregulated, or at least maintained at elevated levels in mature human lymphoid cells (see references in Ref. 232).
PrPC affects the ability of long-term hematopoietic stem cells (HSC) to sustain self-renewal under stressful conditions (597). It was not resolved whether proliferation or, alternatively, resistance to cell death, underlies the enhanced self-renewal ability of HSC to reconstitute bone marrow. Nonetheless, these results support the hypothesis that PrPC modulates the responses of immune cell precursors to extracellular factors and highlight an important role of PrPC in the immune system. Previous studies had, however, already pointed at this direction.
Early work showed that activation increased PrPC at the surface of both human lymphocytes and lymphoid cell lines, and anti-PrPC antibodies prevented concanavalin A (ConA)-induced proliferation (77). Subsequent studies confirmed the upregulation of PrPC following stimulation with multivalent lectins (19, 289, 315) and showed reduced mitogen-induced proliferation in PrP-null murine splenic lymphocytes (315). Other reports contained conflicting results about differential responses of either WT or PrP-null splenocytes to mitogens (17, 19, 301, 348), but PrPC transfected into PrP-null murine splenocytes sustained an increased ConA-induced proliferative response, compared with the parental PrP-null splenocytes transfected with an empty plasmid (17).
Interestingly, supernatants of transfected PrPC-expressing splenocytes contained higher amounts of both IL-2 and IL-4, compared with PrP-null splenocytes following stimulation with ConA, which requires the T-cell receptor (TCR) complex, but not after stimulation with phorbol ester plus ionomycin, which bypasses the TCR (17). Previous work had shown that removal of GPI-anchored proteins prevented T-cell activation by ConA, but not direct activation via the TCR (426, 532). The overall data suggest that PrPC cooperates with the TCR in the activation of T cells induced by a cross-linking lectin.
Confocal microscopy of CEM-T human lymphoid cells showed colocalization at the level of the plasma membrane, of PrPC with the Src-family non-receptor tyrosine kinase Fyn, but not Src itself. A polyclonal anti-PrPC antibody coimmunoprecipitated Fyn, and conversely, anti-Fyn pulled down PrPC in both resting and CD28/CD3 stimulated cells. Following stimulation, anti-PrPC also coimmunoprecipitated the Syk family tyrosine kinase ZAP-70, which binds both CD3
chains and CD45-associated phosphatase upon TCR stimulation (343). These data support the interpretation that PrPC associates with the TCR complex and with at least some of the latter's intracellular downstream effectors.
An alternative mechanism linking PrPC with T-cell function was based on the result that IL-2 production was delayed in PrP-null splenocytes stimulated by ConA in the presence of a Cu2+ chelator. The authors suggested that rapid Cu2+ transport by PrPC may be involved in T-cell activation (269), but alternative explanations for the role of Cu2+ may apply (see sect. IIE).
ConA and PHA are tetrameric at physiological pH (358, 554), and therefore, their effects likely rely on lateral reorganization of surface glycoproteins, which may include not only the TCR and CD3, but also PrPC. The requirement for lateral reorganization is also apparent in other PrPC-mediated responses. In CEM-T cells, PrPC antibodies modulated both thapsigargin-induced Ca2+ release and entry through store-operated Ca2+ channels, with the most pronounced effects occurring upon specific PrPC cross-linking with either avidin or polyclonal antibodies, which likely promote substantial lateral movement of PrPC (225). The authors suggested that a physiological ligand associated with PrPC-mediated T-cell responses would be the most efficient if multivalent (225). However, the effects described may be related to particular experimental conditions, which promote the concentration of PrPC molecules dragging a particular set of components of a multi-protein cell surface signaling complex (e.g., Ref. 339).
Indeed, cross-linking of PrPC in Jurkat T cells and peripheral blood T lymphocytes produced both cocapping of PrPC with the raft-associated proteins reggie-1 and reggie-2 (flotillin-2 and flotillin-1, respectively), as well as a transient Ca2+ signal and phosphorylation of Erk1/2, which were somewhat distinct from similar signals induced by TCR/CD3 stimulation. In addition, PrPC cross-linking also dragged the Src-family kinases Fyn and Lck, as well as CD3 and LAT to the cap, consistent with recruitment of a functional TCR complex, followed by endocytosis of PrPc together with the reggies (512).
These results show that cross-linked PrPC can promote the assembly of TCR complex components within rafts, and this dynamic behavior of a set of cell surface, transmembrane, and juxtamembrane molecules may constitute the basis of several PrPC-mediated T-cell responses described above. Responses induced by the multivalent lectin PHA were selectively blocked with antibodies to certain epitopes in the PrPC molecule, but not to others (289). This may be due to selectively interfering with the interaction of PrPC and other cell surface components necessary for the assembly of the TCR complex in specific experimental context.
C. Antigen-Presenting Cells/Mononuclear Phagocytes
Despite the focused interest on APC in prion infectivity (4, 316), few studies have dealt with physiological roles of PrPC in dendritic cells (DCs) and macrophages. PrPC is expressed in myeloid cells (19, 66, 135, 151, 240, 301, 337). Increased expression of PrPC consequent to DC maturation paralleled that of the MHC class II and costimulatory molecules, as well as the production of IL-12 induced by either TLR4 or TLR9 ligands (337). PrPC also colocalized with MHC class II in monocyte-derived DCs (66). However, PrP-null mice also showed increased expression of MHC class II and costimulatory molecules, upon lipopolysaccharide (LPS)-induced maturation of DCs in vitro (19). Thus PrPC is not required for maturation of DCs.
Recently, the behavior of interacting DCs and T cells was examined in the context of an allogeneic mixed lymphocyte reaction (MLR) and a peptide-MHC driven proliferative T-cell response (19). Confocal microscopy of T/DC contact points in the latter case showed accumulation of PrPC at the contact zones, but no colocalization with CD3, LFA-1, or LAT, all of which reside at various points at the immunological synapse, nor with CD43 or CD90/Thy-1. The absence of PrPC in the DCs abrogated the proliferative T-cell response. Antibodies to PrPC also prevented T-cell proliferation in both types of response involving WT DCs and either WT or PrP-null T cells. The same was observed in a peptide-MHC driven response in vivo. In contrast, the absence of PrPC in the T cells did not prevent their APC-induced proliferation, but antibodies to PrPC blocked the T-cell response in MLR between PrP-null DCs and WT T cells.
These experiments are arguably more representative of physiologically relevant functions of PrPC than studies employing multivalent lectins and cross-linking antibodies. Furthermore, both the APC and the target T cells were examined in the context of the immunological synapse, including in vivo responses (see Refs. 136, 298 for reviews). Comparing the effects of either intact Ig or Fab fragments of the PrPC antibody indicated that their effect on the DCs was not due to cross-linking of PrPC and supports the hypothesis that PrPC in DCs is a positive regulator of the immunological synapse. In turn, the antibody effect on the T cells, coupled to normal response of PrP-null T cells, suggests that the antibody to PrPC had an agonist effect on the latter cell type, although it is not clear whether this may depend on cross-linking of PrPC.
In summary, this study (19) provided strong evidence for an important role of PrPC upon the interaction of APC and T cells, although its molecular basis is still unclear. Notably, inhibition of T-cell proliferation by intact antibody to PrPC is consistent with evidence that antibody-mediated mobilization of GPI-anchored proteins on T cells may provide a negative signal for clonal expansion (336, 585), rather than with the assembly of the TCR complex signaling pathways promoted by cross-linking antibodies to PrPC (512), or the agonist effects of GPI-anchored proteins on T-cell responses suggested by enzymatic removal of the former (426, 532).
As for macrophages, it was reported that PrPC is required for macrophage invasion by the Gram-negative bacteria Brucella abortus, through interaction with bacterially secreted Hsp60 (558, 559). However, these findings have been challenged, because blocking antibodies to PrPC did not affect macrophage invasion by Brucella, there was no difference in the infection of WT or PrP-null macrophages or mice, and no specific binding was demonstrated between Brucella and recombinant PrPC (149). Therefore, a role of PrPC on the interaction of microorganisms with macrophages (3) is dubious.
In a distinct approach, a soluble PrPC-Fc fusion protein was applied to the P388D-1 mouse macrophage-like cell line. The fusion protein bound to the cell surface and induced tyrosine phosphorylation, as well as phosphorylation of the Erk and Akt kinases (268). Nonetheless, P388D-1 is a triploid, highly unstable cell line derived from a lymphoblastic pre-B lymphoma, with a high degree of genomic instability and abundant cytogenetic aberrations (106). It is therefore of little value as a surrogate macrophage/monocyte of physiological relevance. Investigation of a more conventional macrophage cell line, or indeed of primary macrophages, may provide more reliable evidence on the possible effects of extrinsic PrPC on the latter.
We examined the role of PrPC in the phagocytosis of apoptotic cells by peritoneal macrophages. Macrophages from PrP-null mice exhibited higher rates of phagocytosis in vitro of a variety of apoptotic cell types than WT macrophages. Elimination of GPI-anchored proteins from the cell surface of WT macrophages rendered these cells as efficient as PrP-null macrophages (120).
PrPC on the macrophage surface seemed enough for the distinct responses, since apoptotic leukocytes derived from WT or PrP-null mice were phagocytosed with similar efficiency, but PrP-null macrophages were more effective in phagocytosing either type of leukocyte (Fig. 6). Furthermore, both the phagocytosis of retinal apoptotic bodies by PrP-null Muller glial cells in situ and of zymosan by peritoneal PrP-null macrophages in vivo were higher than WT (120). These data show that PrPC is a negative modulator of phagocytosis, which may help balance the threshold of phagocytic responses (see Ref. 121 for review).
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Following intraperitoneal injection of zymosan, there was reduced leukocyte infiltration, and few polymorphonuclear cells were recruited to the peritoneum of PrP-null mice at 6 h after injection, in sharp contrast to the robust PMN infiltrates in WT (120). A similar result followed intraperitoneal injection of LPS (R. Mariante, A. Nobrega, and R. Linden, unpublished results). In addition, preliminary experiments employing a model of Fas-dependent induced pulmonary silicosis (37) showed that PrP-null mice undergo reduced leukocyte infiltration in the bronchoalveolar liquid, reduced body weight loss, and reduced pulmonary fibrosis, compared with WT after intratracheal instillation of silica (Fig. 7).
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E. Implications of PrPC Functions in the Immune System
Despite the involvement of specific immune cell types in the accumulation of PrPSc in peripheral lymphoid compartments at early stages of prion diseases (see Ref. 4 for review), no attention has been paid to whether PrPC is depleted in the immune cells and possible consequences for immune responses. The data reviewed above show that PrPC may play important roles in the development and maintenance of the immune system, as well as in specific cellular immunological responses. Time is ripe for examining possible loss-of-immune-function components of prion diseases, at least in the context of peripheral infection. Moreover, changes in the behavior of immune cells and associated deregulation of cytokines and chemokines may in itself have an impact on neurodegeneration (67, 75).
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VI. ROLES OF THE PRION PROTEIN BEYOND THE NERVOUS AND IMMUNE SYSTEMS |
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A. Neuromuscular Junction and Muscle
No major abnormalities of neuromuscular function had been described in early work with PrP-null mice (41). However, a forced swimming test, which measures muscle activity and resistance to fatigue, showed impairment of locomotor activity under extreme exercise conditions, thus suggesting a role for PrPC in muscle physiology (387). Differing from previously reported mitochondrial abnormalities (356), no change in mitochondrial respiration was found in PrP-null mice (303). The finding of functional impairment only under stress (387) is similar to other changes observed in PrP-null mice (217, 334, 386, 501, 562). However, the authors cautioned that the impairment of locomotor activity in the forced swimming test was inferred from behavioral parameters, and not from direct assessment of muscular function. Thus, alternatively, these results might be an indication of modifications in either motor coordination or in anxiety levels, leading to less effective swimming and earlier fatigue (386).
Older mice overexpressing the Prnpb allele suffered from a profound, gene dose-dependent, necrotizing myopathy of skeletal muscle, but not cardiac or smooth muscle. Little PrPres, probably representing aggregation, was accumulated, but PrPres did not associate with onset of disease. Therefore, the myopathy is a consequence of overexpression of PrPC, and not of PrPres (570). Conversely, transgenic mice overexpressing the Prnpa allele presented no noticeable myopathy (146), although muscular function was not assessed.
The PrP alleles (Prnpa and Prnpb) described above differ at codons 108 and 189 (see sect. IVD). It is not known whether these amino acid changes affect PrPC function, in particular when the protein is overexpressed. However, if the myopathies observed in mice expressing Prnpb are in fact related to altered PrPC function, then the frequency and risk of PRNP polymorphisms in human myopathies warrant further investigation.
Expression of PrPC is increased in sporadic and hereditary inclusion body myositis and myopathy, polydermatomyositis, and neurogenic muscle atrophy (15, 592). Moreover, a uniform pattern of increased PrPC expression was described in a series of muscular disorders (267). The PrP was proteinase-K sensitive, which rules out PrPSc toxicity. Interestingly, both glycoform profile and size of PrPC in normal muscle are distinct from human brain (592).
Based on these findings, it was suggested that PrPC may have a general stress-response effect in neuromuscular disorders (267). This hypothesis is supported by the accumulation of PrPC in muscle fibers of an experimental model of chloroquine-induced myopathy (162).
In addition, PrPC was upregulated when myotubes differentiate from immortalized C2C1 murine myoblasts (53). PrPC content progressively increased during maturation of myocytes in primary cultures of skeletal muscle, attributed to both transcriptional and posttranslational changes. Fast muscle fibers present a higher concentration of PrPC than slow fibers, also consistent with a role of PrPC in skeletal muscle physiology (340).
A severe dilated cardiomyopathy has also been described in a patient diagnosed as sporadic CJD, and a heart biopsy contained evidence of the presence of PrPSc. Since no other cause was found, it was suggested that the disease derived from the accumulation of PrPSc into the heart (14). However, the possibility of PrPC loss of function cannot be excluded (31). Recently, disease-associated PrP was also detected in cardiac myocytes of elk and white-tailed deer infected with chronic wasting disease (CWD), but heart physiology was not evaluated (241).
These data raise the hypothesis that PrPC may have important functions in both skeletal and cardiac muscle. Analysis of functional parameters, such as contractile function and neuromuscular junction properties, in both constitutive and conditional PrP-null mice and muscle cells, and particularly under conditions of stress (386), should help clarify this issue.
PrPC is barely detectable in the liver (see sect. IIC), but increased levels of both PrPC mRNA and protein were detected during hepatic stellate cell activation (228). These cells undergo activation and proliferation under stress, a process that is closely related to liver fibrosis (160). PrPC is particularly detected in the fibrous septa of carbon tetrachloride-damaged liver and in sinusoidal linings of common bile duct-ligated liver. Thus it was suggested that PrPC may be involved in the response to oxidative stress and in the development of liver fibrosis (228).
Functions of PrPC in neuroendocrine secretion were addressed in melanotrope cells of the intermediate pituitary of Xenopus laevis (538). When animals are exposed to a dark background, these cells produce high levels of proopiomelanocortin (POMC) (239). POMC is processed into several bioactive peptides, including -melanophore-stimulating hormone (-MSH), the release of which causes melanin dispersion and darkening of the skin (239, 454). Thus this model allows the physiological manipulation of biosynthetic and secretory activities of melanotrope cells.
PrPC was detected both throughout embryonic development and in adult frogs, but its expression was not affected during background adaptation, indicating that it does not affect POMC biosynthesis (538). Xenopus transgenesis was used to overexpress PrPC under control of the POMC promoter specifically in the melanotrope cells. Subcellular structure, POMC synthesis and processing, and the secretion of POMC-derived products remained unaltered within the PrPC-overexpressing cells (538, 539). However, secretion of POMC was reduced when cells expressed PrPC for long periods (540). Remarkably, electron microscopy revealed the induction of lysosomes taking up POMC secretory granules (crinophagy), which is probably responsible for the reduced POMC levels. Thus, in neuroendocrine cells from Xenopus, PrPC modulates the secretory pathways inducing crinophagy (540).
Functional properties of PrPC in mammalian pituitary have not been addressed to date, but its role in POMC secretion deserves further investigation, due to the importance of the melanocortin system particularly in the regulation of feeding behavior and weight homeostasis.
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VII. ROLES OF THE PRION PROTEIN ON THE CELL LIFE CYCLE |
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Early embryonic and progressive expression of PrPC mRNA and/or protein is generally interpreted as evidence for developmental roles of PrPC (279, 293, 330, 349, 355, 467). During embryogenesis, as well as in continually renewing cell populations of the adult organism, developmentally regulated molecules may be involved in mechanisms related to cell proliferation, differentiation, and programmed cell death. Each of these cellular responses is reviewed in turn.
A. PrPC and Cell Proliferation
Expression of PrPC also correlated positively with the rate of proliferation in both the subventricular zone and the dentate gyrus in adult mice. There were no morphometric differences among the brains of mice containing no, normal, or overexpressed PrPC (506), indicating that the PrPC-dependent increased cell production is compensated by other events to define the final cellular composition of the brain (578). Interestingly, however, PrPC in the proliferating regions of the adult brain was found restricted to postmitotic neurons (506). This suggests an indirect effect on the proliferation of the underlying mitotic precursors, a possibility that must be entertained when dealing with complex tissues.
Nonetheless, the expression of PrPC at the surface of certain cells confers higher rates of mitogen-induced proliferation (315, 348), which may be due to enhanced sensitivity to growth factors present in the cellular microenvironment, as well as changes in the downstream cell cycle machinery.
Based on an array of 597 mouse DNA clones, several cell cycle-related genes, such as cyclin D1, Eps8, and CD44, were reportedly downregulated, whereas certain other genes associated with growth factor-related signal transduction were upregulated in PrP-null compared with WT cells (473). In contrast, Northern blots of brain extracts showed a substantially lower level of expression of both Eps8 and CD44 in PrP-null, when compared with WT, whereas cyclin D1 displayed no difference, and the expression of the growth-factor related genes was similar in both WT and PrP-null brain extracts (473). It is, therefore, possible that effects of altered Prnp gene expression on the levels of expression of cell cycle-related genes may contribute to the control of cell proliferation by PrPC in a cell type-specific way.
B. PrPC and Cell Differentiation
Data on this subject are somewhat contradictory. For example, both PrPC mRNA and protein were downregulated along progressive differentiation of granulocytes derived either from bone marrow precursors or from retinoic acid-treated HL60 leukemia cells (132). In contrast, PrPC mRNA was upregulated upon both growth arrest and terminal differentiation of murine erythroleukemia cells, as well as human neuroectodermal and myoid cell lines (181).
Studies of the expression of PrPC in developing neurons also provided conflicting results. In paraformaldehyde-fixed sections of developing hamster brain, monoclonal antibody 3F4 initially labeled mainly axons. Then, at later stages, staining shifted to synaptic beds, suggesting that PrPC is involved in axon growth or guidance, followed by a role in synaptogenesis, both of which are major differentiation events of neuronogenesis (467). In cultures of multipotent neural precursors, PrPC content positively correlated with differentiation, as judged by reduced nestin and increased MAP2 (506).
In contrast, PrPC was equally distributed in both dendrites and axons of cultured hippocampal neurons, mostly within intracellular compartments in early stages. Then, PrPC evolved to a restricted distribution at the axonal surface, allegedly at stages when axon growth has stopped, with no colocalization with the synaptic marker synaptophysin (166). These controversial findings highlight the difficulties in attributing function to PrPC on the basis of its selective distribution alone. Stronger evidence arises from comparative studies of either mice or cells expressing variable amounts of PrPC, as well as on experimental studies of the effects of soluble PrPC and its ligands on cell differentiation.
Early work had shown that the expression of PrPC affects morphological differentiation of hippocampal neurons in vitro. Thus both at short-term and in response to stimulation with a phorbol ester, neurons derived from Ngsk PrP-null mice grew shorter neurites than WT neurons, which was reverted by transfection of the Prnp gene (274). In recent work, the acquisition of MAP2 also correlated with PrPC content among neural precursors of WT, knockout, and overexpressing mice (506).
Recombinant PrPC binds to both the hippocampal neuropil and to cerebellar granule cells in histoblots of WT, PrP-null, and PrPC-overexpressing mouse brain (284), as well as to primary cultures of neurons from either WT or PrP-null mice (171). Cultured granule cells from the mouse cerebellum treated with either a substrate of PrPC-Fc or a feeding layer of PrPC-expressing, transfected CHO cells, responded with neurite outgrowth, which also occurred with mouse embryonic hippocampal neurons treated with PrPC-Fc. Both WT and PrP-null neurons were equally sensitive to the effect of PrPC (86, 471).
Further evidence that trans-acting PrPC induces neurite outgrowth was obtained in cultures of neurons derived from the hippocampus of rat embryos. Soluble, recombinant PrPC induced vigorous neurite outgrowth, both dendritelike and, particularly, a robust single axonlike process per neuron, and enhanced synaptogenesis, recognized as overlapping staining for both synaptophysin and the postsynaptic protein PSD-95, among a richly interweaving network of processes (249). Interestingly, immunostaining of the exogenous protein applied to the cultures indicated that the neurite-promoting effect was not associated with internalization of PrPC.
These data strongly indicate that the binding of PrPC to target cells triggers neuronal differentiation. Notwithstanding the possibility that either NCAM or the laminin receptor precursor may underlie these effects, the true cell surface receptor complex involved in the transduction of the PrPC-induced differentiation signals remains unidentified.
In turn, identified ligands of PrPC have been shown to induce PrPC-dependent neuronal differentiation. This is the case for laminin, the cochaperone hop/STI1, and vitronectin.
Process outgrowth from either NGF-primed PC12 cells or mouse hippocampal neurons was selectively blocked by either antibodies to PrPC or by chromophore-assisted laser inactivation of cell surface PrPC. In addition, laser ablation of PrPC led to retraction of outgrown laminin-induced processes in PC12, and laminin-induced differentiation was abrogated in hippocampal neurons from Prnp0/0 mice (183, 184). The PrPC-mediated effect of laminin is selectively induced by the LN -1 peptide, which contains the PrPC-binding site in LN (see sect. IID). Two important pending issues are 1) whether PrPC-sensitive morphological differentiation is strictly an adhesion effect of laminin (184), and 2) whether neurite outgrowth induced by the PrPC-laminin interaction is accompanied by phenotypic differentiation into either dendrites or axons, such as demonstrated for soluble PrPC (249).
Similarly, the recombinant cochaperone hop/STI1 induced neuritogenesis in cultured hippocampal neurons from WT, but not from Prnp0/0 mouse embryos, an effect that was abolished by antibodies against either PrPC or hop/STI1 and was mimicked by a peptide corresponding to the hop/STI1 domain (aa 230–245) that binds PrPC (306).
Finally, vitronectin, another component of the extracellular matrix, promoted axonal growth in dorsal root ganglion neurons (DRG) from WT mouse embryos, an effect that was mimicked by vitronectin peptide 307–320, which contains the PrPC binding site. In DRGs from two distinct PrPC-null mice strains, vitronectin, but not vitronectin peptide 307–320, induced axonal growth. Functional assays demonstrated that PrPC-null DRG neurons are more responsive than WT neurons to Arg-Gly-Asp peptide, which is the vitronectin binding site for integrins. PrPC-null DRG also presented a higher activity of the integrin vβ3 (192). This, together with changes in the basal activity of several signal transduction pathways demonstrated in PrP-null mice (see sect. VIII, C and D), provides evidence for compensatory mechanisms upon constitutive abrogation of PrPC.
In summary, the overall data show that both PrPC and at least some of its ligands can induce neuronal differentiation and suggest that PrPC may also mediate the differentiation of nonneuronal cells.
C. PrPC in Cell Death and Survival
Early evidence that expression of PrPC modulates the sensitivity to cell death was obtained in cell lines established from hippocampal neurons derived from either WT or Ngsk PrP-null mice by retroviral-mediated transfection of the simian virus 40 (SV40) large-T antigen. PrP-null cells died quickly upon serum deprivation, whereas their WT counterparts survived. Resistance to cell death was conferred by transfection of knockout cells with Prnp, similar to the effect of Bcl-2 transfection (274). In either WT or mock-transfected cells, serum deprivation led to p53 and Bax upregulation, cleavage of both caspase-3 and poly(ADP-ribose) polymerase (PARP), decreased Bcl-2 and mitochondrial cytochrome c, increased levels of mitochondrial calcium, and lowered mitochondrial membrane potential. In contrast, transfection of knockout cells with a Prnp expression vector prevented all those typical apoptotic changes (259).
In cultured human fetal neurons microinjected with a variety of cDNAs, apoptosis induced by injection of Bax was prevented by coinjection of Prnp, similar to the well-known protective effect of Bcl-2. Prnp antisense treatment did not by itself induce cell death, but enhanced the degenerative effect of Bax. Interestingly, whereas interfering with the traffic of secreted proteins along the Golgi abolished the protection conferred by Prnp microinjection, expression of a soluble form of PrPC lacking the GPI anchor was as effective as regular GPI-anchored PrPC (39). The latter result is consistent with the increased survival conferred by trans-interacting PrPC upon both cerebellar granule cells and mouse hippocampal neurons (86, 249). A protective effect was also demonstrated for cytosolic PrPC (456).
Ectopic expression of mammalian Prnp in yeast also prevented Bax-induced cell death (286). However, whereas deletion of the octapeptide repeats abrogated the protective effect of PrPC in human neurons (39), it did not affect yeast. Also contrary to human neurons, cytosolic PrPC failed to provide protection against Bax-induced cell death in yeast (286). It should be noticed that cell death associated with prion diseases may not be restricted to a single Bax-dependent mode, among a variety of available cell death pathways (190). For example, evidence was provided that cell death associated with NH2-terminal truncated forms of PrPC may be either Bax dependent or Bax independent (285).
Analysis of human neurons, mouse neuronal cell lines, and MCF7 breast cancer cells showed that PrPC delays the conformational change of Bax required for inducing the mitochondrial release of cytochrome c. Bax-induced events downstream of mitochondria were unaffected by PrPC, and experiments with isolated mitochondria suggested that PrPC does not interact directly with Bax. In addition, PrPC did not affect cell death induced by either Bak or truncated Bid, nor by staurosporine or thapsigargin in MCF7 cells (455).
It has also been reported that PrPC protects against ethanol-induced, Bax-mediated brain cell death in vivo. However, the results were not consistently related to the level of expression of PrPC, and the differences reported were not statistically significant (164). Transient protection of axotomized facial motoneurons was reported for PrPC overexpressing mice. Nonetheless, motoneuron loss in both WT and PrP-null mice was similar, and there were no differences among the three genotypes following facial nerve transection in adult mice (115).
Other studies, however, provided stronger evidence that the expression of PrPC has a neuroprotective role upon hypoxic-ischemic insult to the brain in vivo. Thus PrPC was upregulated in the infarcted brain within a day of permanent focal cerebral ischemia in mice (350, 561), as well as within the penumbra region following transient focal ischemia in rats (492). Increased numbers of PrPC-immunoreactive profiles were found both at the penumbra of hypoxic damage in adult human brain and in cases of perinatal hypoxic-ischemic injury, accompanied by increased in situ hybridization signals (350). Brains of PrP-null mice suffered more extensive damage than WT following both permanent and transient ischemia (466, 562), and a gene dose-response, protective effect was observed among WT, homozygous, and heterozygous PrP-null mice (350, 501). However, PrPC above normal levels did not provide additional protection in the cerebral cortex, and the rate of cell death within the striatum was the same among WT, knockout, and overexpressing mice (501). Finally, overexpression of PrPC following intracerebral transduction of an adenoviral vector reduced infarct volume and improved neurobehavioral signals after cerebral ischemia in rats (492).
PrPC also protects against apoptosis in cancer cells. Samples of gastric adenocarcinoma contained increased expression of PrPC, which correlated with both histopathological differentiation and tumor progression. Modulation of the expression of PrPC in a gastric cancer cell line confirmed that PrPC blocked apoptosis, accompanied by upregulation of Bcl-2 and downregulation of both p53 and Bax (291). Also, the expression of both PrPC mRNA and protein is severalfold higher in TNF-resistant than in TNF-sensitive MCF7 breast carcinoma cells. Consistently, gene transfer of PrPC in both TNF-sensitive MCF7 and BT20 cell lines conferred resistance to TNF-induced cell death (130).
In sharp contrast to the data supporting a protective role for PrPC, transgenic overexpression of PrPC led to a gene dose-dependent, unprovoked neurodegenerative phenotype (570). In line with these results, ectopic expression of PrPC potentiated staurosporine-stimulated, caspase 3-dependent apoptotic cell death in both the HEK293 and in an inducible PrPC-transfected cell line derived from rabbit kidney epithelial cells. The proapoptotic effect of PrPC was potentiated by proteasome inhibitors and abrogated by sequestration of cell surface PrPC with antibodies (400).
Work was extended (401, 402) with the use of a cell line derived by sequential oncogenic retroviral infection of neocortical neuronal precursors with SV40 large-T antigen, followed by simultaneous v-Src and v-Myc (97), HEK293, and cultured neurons derived from WT and Prnp–/– mice. Both overexpression of PrPC in the cell lines as well as its endogenous expression in WT neurons, compared with knockouts, enhanced basal and staurosporine-induced toxicity and caspase-3 activation by staurosporine, ceramide, and etoposide, an effect dependent on p53. Conversely, Prnp antisense diminished staurosporine-induced apoptosis (401, 402).
Notwithstanding, increased cellular content of PrPC in MCF7 cells did not affect staurosporine-induced cell death (455), the rates of either basal or anisomycin-induced cell death were similar in retinal explants from either WT or Prnp0/0 mice (92, 591), and both basal and the increased survival induced by soluble PrPC were indistinguishable between WT and PrP-null cerebellar granule neurons (86). Thus conditions for either cytoprotective or proapoptotic effects of modulating the expression of PrPC may vary among distinct cells and tissues.
Certain experimental preparations offer distinct operational advantages, which may be, nonetheless, accompanied by critical caveats. Immortalized cell lines express disregulated genes directly or indirectly associated with mechanisms of cell death, some of which may either coincide with or affect essential intermediaries of PrPC-associated effects. For example, a critical interplay of the oncogenes src and myc affects the transactivation of p53-regulated genes in the context of cell death (545), which might affect the p53-dependent degenerative effects of PrPC in a v-src/v-myc transformed cell line (97, 402). Similar considerations may apply to the HEK293 cell line, originally derived by adenoviral-induced transformation (182), which shows a rather distinctive gene expression profile, similar to other transformed cell lines, and quite different from nontransformed human embryonic kidney and other cells (251), which in turn may affect other studies of PrPC-modulated cell death (401, 402). The role played by these departures from physiological conditions on the effects of modulating the expression of PrPC is unknown.
It is likely that the roles of PrPC on the sensitivity to cell death may differ among cell lines, between either isolated cells or complex tissues, as well as depending on the availability of PrPC ligands. In fact, contradictory functions of cell surface molecules involved in neurodegeneration are not limited to PrPC. For example, both excitotoxic and neuroprotective roles have been demonstrated for NMDA-type glutamate receptors (see Refs. 295, 476 for reviews), at least in part depending on neurotransmitter concentration (448). Moreover, the role of the low-affinity NGF receptor p75NGFR, whether neuroprotective or proapoptotic, depends on its association with distinct partners, most notably the high-affinity TrkA neurotrophin receptor (see Refs. 84, 391 for reviews).
Indeed, both a PrPC-binding peptide and the PrPC-binding hop/STI1 cochaperone protected undifferentiated postmitotic cells in retinal explants, as well as dissociated hippocampal neurons subject to induced cell death. Neuroprotection depended on the binding of specific domains in both proteins: STI1230–245 and PrPC113–128 (92, 306, 591). Other studies also suggest that interaction of hop/STI1 with PrPC induces cytoprotective signals in a PrPC-transfected neuronal cell line, through regulation of SOD activity involving the NH2-terminal domain of PrPC (463–465).
In vivo cross-linking of PrPC with two monoclonal antibodies targeting PrPC epitopes within the range of aa 95–105 induced cell death within the hippocampus and cerebellum of WT mice, whereas both their monovalent Fab fragments and an antibody targeting aa 133–157 were innocuous (497). The authors suggested that dimerization of PrPC initiates an apoptotic cascade, possibly through an as yet unidentified secondary molecule. Interruption of PrPC-mediated survival signaling produced by association of PrPC with another molecule was dismissed based on the lack of an effect of the Fab fragments. The latter argument, however, is far from compelling, since the breaking of sustained ligand-induced signaling cannot be excluded amidst the widespread effects of antibody cross-linking, particularly the extensive lateral reorganization of cell surface molecules (e.g., Ref. 512; see sect. VB).
Sensitivity to cell death is critically dependent on cell-cell interactions, and the mechanisms of cell death or survival of individual cells in a complex, richly interconnecting tissue such as the brain are strongly dependent on the integrity of the tissue (295, 296, 440). Thereby, studies of isolated cells, such as cell lines or cultures of dissociated tissues, must be viewed with caution, because of the intrinsic rupture of histotypical modulators of cell metabolism (208, 297, 372).
A final conclusion about the roles of PrPC expression, soluble PrPC and PrPC ligands on cell death requires further analysis in preparations more akin to their physiological context. Still, the available data indicate that PrPC is a critical element of the network that controls the sensitivity to programmed cell death in both the nervous system and among other cell types. The outcome of the engagement of PrPC on either cell death or survival is likely dictated by its available ligands, which in turn determine the array of intervening signaling pathways.
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VIII. SIGNALING MEDIATED BY THE PRION PROTEIN |
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Activation of certain signal transduction pathways by engagement of PrPC was demonstrated through direct measurements. In other cases, evidence was inferred from the effect of pharmacological or molecular inhibitors on cellular responses that depend on PrPC. The latter approach is less reliable, because it may, in fact, disclose signaling pathways networked to the downstream responses, but not directly activated by engagement of PrPC. This is particularly critical in the case of neurotrophic responses, given the evidence that cell death may be triggered by single events superimposed on a metabolic state controlled by multiple mechanisms (99).
Engagement of PrPC with either ligands or antibodies, as well as exposure of cells to recombinant PrPC, have been used in various experimental designs (Fig. 8). The results must be interpreted with caution, due to the singularities of either cross-linking or non-cross-linking ligands (e.g., Ref. 19) and the possibility of either blocking or agonist effects of antibodies (e.g., Ref. 496). Other studies compared signaling intermediates between WT and PrP-null cells or mice, which may be affected by compensatory responses to deletion of PrPC. Although the work provided evidence that PrPC relates to a variety of signaling pathways, fundamental questions remain unanswered as to how the GPI-anchored PrPC may transfer signals across the plasma membrane.
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B. Signaling Dependent on PrPC-Caveolin Interaction
A cell line (1C11) derived from transformation of F9 embryonic carcinoma cells with SV40 T antigen, which can be induced to differentiate along either serotonergic or catecholaminergic phenotypes, was used to test for PrPC-mediated signaling. Cross-linking of PrPC induced activation of the Fyn non-receptor tyrosine kinase, which was abolished by antibodies to caveolin-1, and reduced by antibodies to clathrin. The effect was restricted to neurites and growth cones of neurochemically differentiated cells (374).
This suggested that caveolin mediates signaling triggered by engagement of PrPC in cells that express both proteins. However, the functional significance of this interaction is likely not universal. Caveolin has been demonstrated in neurons after a few weeks in culture (62, 165) and in brain extracts (165, 536), and deletion of the cav-1 gene produces neurological deficits in mice (536). However, it is still not known whether mature neurons in situ express caveolin (277). It was suggested that, in the absence of caveolin, interaction of PrPC with gangliosides might be responsible for transferring signals across the membrane (343).
Moreover, caveolins are predicted to attach to the plasma membrane through a hydrophobic hairpin, without traversing the bilayer to the extracellular side (101, 579) and, therefore, can interact directly only with the GPI anchor, not with the polypeptide chain of PrPC. Caveolins bind cholesterol strongly (379), which is required for membrane incorporation of the protein (290), and transport cholesterol from the ER to membrane caveolae (101), but caveolin has not been shown to bind the stearic acid chains of the GPI anchor of PrPC (502).
Caveolin-mediated signaling induced by PrPC cross-linking may, thus, require an intermediate component. NCAM, engagement of which induces phosphorylation of Fyn, is a candidate PrPC-binding transmembrane, signaling protein (471; see sect. VIIIE). Nonetheless, biochemical and coimmunoprecipitation procedures indicated that DRMs from growth cones are heterogeneous with regard to their content of distinct NCAM species, and that caveolin segregates from phosphorylated Fyn (196).
Overall, current data fail to support a simple model of signaling from cross-linked PrPc to Fyn through caveolin. An alternative mechanism is that cross-linking of PrPC induces a rearrangement of membrane rafts through lateral movement of distinct domains (345, 524), which may involve caveolin in some cells, and then, probably through a transmembrane protein such as NCAM, may couple the GPI-anchored prion protein with the acylated non-receptor tyrosine kinase on the cytoplasmic leaflet of the bilayer.
C. PrPC, cAMP, and protein kinase A
Direct evidence that PrPC mediates activation of the cAMP/protein kinase A (PKA) pathway was obtained with a peptide that both binds PrPC (338) and induces neuroprotection in retinal tissue (92). Exposure of developing retinal tissue from WT, but not from PrP-null mice, to the PrPC-binding peptide increased intracellular cAMP and led to activation of PKA. Neuroprotection induced by both the PrPC-binding peptide, as well as by an analogous peptide corresponding to the PrPC-binding domain of protein hop/STI1 (591), was abrogated by blockade of the PKA pathway (Ref. 92 and unpublished data). Protection by hop/STI1 of hippocampal neurons in culture was also blocked by a PKA inhibitor (306). Interestingly, the basal levels of both intracellular cAMP and PKA activity are higher in PrP-null retinal tissue than in WT (92), which likely represents a compensatory response to the lack of PrPC.
A PKA inhibitor also blocked both neurite outgrowth and neuronal survival of cerebellar granule cells, induced by trans-interacting PrPC (86). However, it was not reported whether the trans-interacting PrPC leads to increased cAMP. In contrast, a PKA inhibitor had no effect on axon outgrowth induced by recombinant PrPC upon embryonic hippocampal neurons in culture (249).
It is unclear how the engagement of PrPC or its presence in the extracellular environment leads to increased cAMP. Adenylyl cyclase is commonly targeted by heterotrimeric G proteins engaged by GPCR (248, 310). Both GPCR and G proteins might serve as intermediates in PrPC-mediated production of cAMP. Indeed, heterotrimeric G proteins reportedly associate with certain GPI-anchored proteins in membrane rafts (300, 397, 498, 499), but no such evidence is available for PrPC. The identification of a GASP as a PrPC ligand (see sect. IID) may be of significance, but the topologies of PrPC and GASP are not consistent with direct interaction.
In the 1C11 cell line differentiated along the serotonergic phenotype, ligation of PrPC with antibodies partially blocked the reduction in cAMP content induced by agonist binding to 5-HT1B/D receptors, an effect attributed to interference with a complex cross-talk among various 5-HT receptor types (376). This indicates that PrPC can modulate the production of cAMP triggered by engagement of GPCR. Still, it is not known whether PrPC and the 5-HT receptors cohabit the same membrane compartments, nor whether they undergo physical interaction. In this study, as in many others, the PrPC antibodies induce clustering of the protein (365), which may underlie the observed effects, but raises some concern as to physiological relevance.
Notwithstanding, distinct members of the adenylyl cyclase family include not only transmembrane, but also soluble forms, and both classes are regulated by additional factors, such as calcium and calmodulin (113, 248). Calcium-stimulated isoforms were identified in both adult and developing rodent retina (1, 441), and both the mRNA for a calcium-stimulated adenylyl cyclase and the calcium-induced accumulation of cAMP were detected, albeit at low levels, in the cerebellum (342), at a time compatible with the reported cAMP/PKA-dependent responses of cerebellar granule cells induced by trans-interacting PrPC (86). Thus it cannot be ruled out that PrPC-dependent activation of the cAMP/PKA pathway is mediated by either voltage-gated or store-operated (capacitative) calcium channels at the plasma membrane (113). Finally, a direct interaction of PrPC with transmembrane forms of adenylyl cyclase also cannot be excluded.
In summary, although PrPC clearly modulates the cAMP/PKA pathway with significant biological consequences, the mechanism of signal transfer across the plasma membrane remains unknown. An additional caveat must be raised, in that biochemical measurements of cAMP, as well as pharmacological inhibitors of PKA, override subcellular compartmentalization, a particularly relevant issue due to the importance of both PrPC localization and trafficking, as well as of differential localization of adenylyl cyclases for their physiological functions (113, 248, 522).
Treatment of developing retinal tissue with PrPC-binding peptides led to activation of Erk (92), and the same occurred in cultured hippocampal neurons treated with either hop/STI1 or with the PrPC-binding peptide hop/STI1230–245 (306). In the latter cells, neuronal differentiation induced by hop/STI1 was abrogated by an Erk inhibitor, differing from axon outgrowth induced by recombinant PrPC upon embryonic rat hippocampal neurons, in which Erk inhibition had no effect (249). No Erk activation was detected in PrP-null retina or hippocampal neurons treated with hop/STI1. Interestingly, basal Erk phosphorylation was higher in PrP-null retinae and hippocampal neurons than in WT (92, 306), a result that was also found in extracts from either total brain or cultured cerebellar cells (51) and is apparent in studies of splenocytes from either genotype (348). This increased basal activity of Erk may also represent a compensatory response to the deletion of Prnp.
Antibody-induced clustering of PrPC led to phosphorylation of Erk1/2, but neither of Jun NH2-terminal kinase nor of the stress-activated p38 kinase, in the 1C11 cell line and both its differentiated derivatives. Similar Erk activation was found in the mouse neurohypothalamic GTI-7 cell line, the lymphoid BW5147 cell line, and the Jurkat T cell line but not in GPI-deficient cells, primary embryonic cortical or hippocampal neuron cultures, and rat neuroblastoma B104 cells (365, 480, 512). Erk phosphorylation was prevented by blocking the activity of NADPH oxidase, which also follows PrPC cross-linking, thus indicating that the production of ROS may be an intermediate step in the PrPC-dependent activation of Erk (480). In the same study, phorbol ester also led to activation of the NADPH oxidase-Erk cascade, although the interpretation that activation of protein kinase C by engagement of PrPC is an intermediate step in this response is not warranted (254). In turn, both ROS production and Erk phosphorylation in 1C11-derived differentiated cells were prevented by PP2, consistent with differentiation-dependent intermediation of Fyn kinase (480).
The fact that pharmacological inhibition of NADPH oxidase did not completely block Erk phosphorylation in the differentiated 1C11 derivatives was taken as evidence for an additional NADPH oxidase-independent pathway (480). Indeed, a specific antagonist of the epidermal growth factor (EGF) receptor blocked Erk phosphorylation induced by PrPC cross-linking in the GTI-1 cell line, suggesting that PrPC clustering leads to transactivation of the EGF receptor, which might then lead to Erk phosphorylation (365). However, NADPH oxidase inhibition completely abolished Erk phosphorylation in the same cell line (480), and EGF receptor ligation also leads to the production of ROS (531).
On the other hand, a PrPC-Fc fusion protein was shown to induce phosphorylation of Erk1/2 in the macrophage-like P388D-1 cell line, which was reduced by either the Src-family kinase inhibitor protein phosphatase 1 or by the phosphatidylinositol (PI) 3-kinase inhibitor wortmannin (268). Although the use of a control Fc domain allowed the extraction of PrPC-derived responses, it was not clear whether Fc receptors in the macrophage-like cells may have interfered with PrPC binding to cell surface partners.
Thus both engagement of PrPC at the cell surface as well as exposure to extracellular PrPC induce Erk activation, and expression of PrPC affects the basal level of Erk activity. A canonical pathway of Erk activation includes the activation of the Ras GTPases, following binding of the SH2-containing Grb2 adaptor protein to phosphorylated cytoplasmic domains of receptor tyrosine kinases. PrPC bound Grb in a two-hybrid screen (500), but the topology of both proteins seems incompatible with a functional role for this binding. Adding to the canonical pathway, an active cAMP/PKA pathway may either activate or inhibit the Erk pathway, changes in intracellular calcium (see sect. VIIIF) can indirectly modulate Erk activity, and protein kinase C (see sect. VIIIG) can lead to Erk activation by either a Ras-dependent mechanism or directly through activation of Raf-1 (see Refs. 186, 310, 510 for reviews). Therefore, the activation of Erk1/2 may be secondary to various upstream events. Still, the question of how PrPC interacts with any or all of these upstream intracellular signaling components remains unsolved.
E. PrPC and Non-Receptor Tyrosine Kinases
As mentioned above, antibody cross-linking of PrPC in differentiated 1C11 cells led to phosphorylation of Fyn (374), and the Fyn kinase inhibitor PP2 abrogated downstream phosphorylation of Erk (480). Other studies provided evidence that cross-linking of PrPC recruits non-receptor tyrosine kinases, such as Fyn and Lck, to PrPC-containing membrane rafts (512), but their state of activation was not reported.
Axon outgrowth induced by recombinant PrPC upon embryonic rat hippocampal neurons in culture was impaired by both the general Src kinase family inhibitor herbimycin, as well as by the Fyn inhibitor PP2 (249). In contrast, PrPC cross-linking in CEM-T lymphoid cells produced phosphorylation of a Src-family kinase distinct from Fyn (225). Biochemical studies of cerebellar granule cells also failed to confirm an association of PrPC with Fyn (38). In addition, exposure of a macrophage cell line to PrPC-Fc induced phosphorylation of the non-receptor tyrosine kinases Syk and Pyk2, as well as the adaptor protein Cbl, but Fyn was not affected (268).
Thus it appears that recruitment and activation of non-receptor tyrosine kinases by PrPC may be a component of PrPC-mediated signaling, but the recruited and/or activated kinases appear to vary either among distinct cell types or depending on particular conditions.
As for the signal transfer from PrPC to Fyn, functional studies provided strong evidence that PrPC is able to recruit and stabilize NCAMs into lipid rafts upon stimulation of NCAM, which appears to be required for the ensuing activation of Fyn by the complex formed by NCAM and the receptor type protein phosphatase (471). Nonetheless, since PrPC and Fyn appear to reside in distinct membrane domains (38), this interpretation requires either fusion of membrane domains or lateral trafficking of protein components.
Reduced calcium influx through VGCC was found both in cultured cerebellar granule cells and in hippocampal neurons in brain slices of PrP-null, compared with WT mice (161, 199), and exposure of WT cerebellar granule cells to PrPC reduced VGCC (266). Cells transfected with Prnp presented an increased agonist-induced calcium influx through the plasma membrane, while reducing the release of Ca2+ from the ER and Ca2+ uptake by mitochondria (44, 259). In contrast, ConA-induced calcium fluxes were similar in both WT and PrP-null splenocytes (348).
Engagement of PrPC by specific antibodies increased cytosolic calcium in CEM-T cells (225). More importantly, cross-linking of PrPC in Jurkat cells induced a transient Ca2+ signal, which is itself necessary for the ensuing PrPC capping events, because coincubation with the Ca2+ chelator BAPTA-AM prevented the recruitment of PrPC and other proteins into the caps (512; see sect. VB). These results show that an early Ca2+ signal induced by PrPc cross-linking feeds back into further events dependent on lateral redistribution of cell surface proteins, including the lateral trafficking of PrPC itself.
Thus PrPC is associated with calcium-mediated cellular events, and calcium channels may be transmembrane partners of PrPC-mediated signaling. However, no evidence is available to date of direct physical interaction of PrPC with calcium channels at the plasma membrane.
Protein kinase C (PKC) comprises various subfamilies, either calcium dependent or independent (125). A few studies provided evidence that PrPC may lead to activation of PKC. Thus both proteins were recovered from the same fractions of immunoprecipitated DRMs (38), and axon outgrowth induced by soluble PrPC in embryonic rat hippocampal neurons in vitro was blocked by a wide-spectrum PKC inhibitor (249).
In splenocytes, the level of lectin-induced phosphorylation of the calcium-dependent PKC- and PKC-β forms, but not the calcium-independent PKC- form, were reduced in PrP-null cells, compared with WT. This was not observed in response to phorbol ester, nor were there differences between lectin-induced calcium fluxes in PrP-null and WT splenocytes. These data indicate that the presence of PrPC affects signaling related to phosphorylation of some forms of PKC, which may be related to their function in the transduction of signals initiated at the level of the plasma membrane (348). The link between PrPC and PKC remains, however, undetermined.
H. PrPC and the PI 3-Kinase/Akt Pathway
The activity of PI 3-kinase (PI3-K) was higher in the brains of WT than PrP-null mice, and in neural cell lines transfected with Prnp compared with parental cells. In Prnp-transfected cell lines, the activity of PI3-K as well as cytoprotection conferred by the latter against oxidative stress were abrogated by either copper chelation or deletion of the PrPC NH2-terminal octarepeat domain, suggesting that this copper-binding domain plays a major role in the effect of PrPC upon PI3-K (543). In other studies, phosphorylation of Akt was reduced in the brains of PrP-null mice, compared with WT, both in control conditions and early after an ischemic insult (562).
Conversely, PI3-K inhibitors partially blocked the axon outgrowth of hippocampal neurons induced by recombinant PrPC (249), as well as neuronal survival, but not neurite outgrowth induced by PrPC-Fc upon cerebellar granule cells (86). In the macrophage-like P388D-1 cell line, this same PrPC-Fc fusion protein induced a slightly higher level of Akt phosphorylation than control Fc fragment, and Akt phosphorylation was completely blocked by Src-kinase inhibitor protein phosphatase 1 (268). Interestingly, in the latter study, both a PI3-K inhibitor diminished Erk phosphorylation as well as an Erk inhibitor inhibited Akt phosphorylation induced by the fusion protein, suggesting reciprocal interactions of the two pathways upon PrPC-mediated signaling.
I. Implications of Current Evidence on PrPC-Mediated Signaling
Modern approaches to cell signaling emphasize networks, rather than independent pathways (122, 238, 431). However, studies of PrPC-mediated signaling have usually been directed at individual responses (Fig. 8). When several signals were induced, little effort was made to examine their interdependency, and in certain cases, the sequence of activation of signaling cascades was assumed, but not adequately proven.
Further analysis of signaling mediated by the prion protein is likely to unravel the mechanisms by which modulation of expression, engagement, or exposure to soluble PrPC trigger proliferative, differentiating, or death/survival responses, as well as other effects on cell metabolism, such as modulation of responses to oxidative stress, synaptic modulation, and immunomodulation. These, in turn, will likely explain the systems-level functions of PrPC. An integrated systems approach may help defining signaling patterns generated by multiple pathways.
Nevertheless, the major conundrum of signal transfer from PrPC to intracellular compartments still remains unsolved and may benefit from novel approaches. In particular, although the need for transmembrane signaling partners of PrPC has long been recognized, the evidence that PrPC binds a variety of ligands, several of which may fulfill the requirement for a signaling intermediate and others may bridge PrPC with further transmembrane partners (see below), likely requires the abandonment of the idea that a single intermediate molecule may explain the multiple PrPC-mediated signals already demonstrated in many distinct models.
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IX. THE PRION PROTEIN AS A DYNAMIC CELL SURFACE PLATFORM FOR THE ASSEMBLY OF SIGNALING MODULES |
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Some basic features of PrPC favor such a dynamic scaffolding of multicomponent complexes. Distal to the lipid bilayer, specific PrPC domains bind various proteins, glycosaminoglycans, and metal ions (see sect. IID, Fig. 3). Current data preclude the accreditation of a major, let alone a single relevant ligand. Since the binding domains of PrPC are distinct for at least some of these ligands, and the affinity constants for confirmed ligands are variable, each cell likely interacts with a distinct set of partners through PrPC, depending on both the level of expression of the latter and the characteristics of the immediate microenvironment (Fig. 9). The evidence for potential involvement of PrPC in multiple signaling pathways (sect. VIII, Fig. 8) also cautions against the interpretation that a single PrPC ligand may explain complex phenotypes associated with various manipulations of either the expression or the structure of PrPC (21, 287, 484).
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In addition, the fast trafficking of PrPC between the cell surface and endosomal compartments (sect. IIIC; Fig. 4), as well as the effect of local charged residues at the cell surface (90), impart a continuous variation of local pH, which may also affect the array of complexed PrPC ligands (Fig. 9), as shown for other ligand-receptor pairs (319). The arrays of ligands selectively available within endosomal compartments may have critical consequences for PrPC-mediated functions, as shown for other signaling pathways (218, 219). This may explain, for example, the dependence on endocytosis shown for PrPC-mediated cell death (516), as well as for activation of the Erk pathway by the PrPC ligand hop/STI1 (8).
Therefore, the ligand binding-dependent movement of PrPC along a changing microenvironment (sect. III, C and D), continuously feeds back into the composition of the protein complexes assembled at differing points along the protein's trafficking trajectory. It follows that PrPC partners, and thus PrPC-mediated signaling, may change significantly along trafficking pathways. Interestingly, dependency on both lateral and endocytic trafficking on PrPC function may even explain certain discrepancies reported at the systems level, such as that between LTP in brain slices studied at either physiological or room temperature (108, 302, 332, 577), due to the effect a difference of 15°C may have on the rate of protein trafficking and raft organization (229, 433, 594).
These properties confer a high degree of complexity to the functions of PrPC at the cellular and molecular level (Fig. 9). To date, little data are available on the interaction of confirmed PrPC ligands, but further elements are superimposed by higher order interactors, such as laminin and the LRP, both of which bind PrPC (171, 183) and each other (79), or laminin-binding integrins, which extend the range of influence of PrPC. Notably, the binding site in LRP for both laminin and PrPC is the same (443), and the binding sites in PrPC for both LRP and laminin partially overlap (103, 183, 226; see Fig. 3), which implies an intricate kinetics of binding among the three proteins at the cell surface, as well as challenging structural requirements.
Engagement of GPI-anchored PrPC, as well as treatment of cells with soluble PrPC, often activate similar signaling pathways (sect. VIII) and produce similar biological effects, such as cell differentiation or survival (sect. VII). Although it is not clear whether soluble PrPC occurs in physiological conditions, both direct contacts between PrPC-expressing cells as well as secretion of PrPC associated with exosomes (see sect. IIIB) may represent a physiological counterpart of the soluble PrPC used in experiments. This is also consistent with the idea that the polypeptide chain of PrPC may scaffold multicomponent complexes. Thus both cross-linking agents, such as lectins or antibodies, as well as PrPC added to the extracellular medium, increase the concentration of PrPC locally at the cell surface, with consequences for the binding of ligands with distinct affinity constants. The latter is also a function of the level of expression of PrPC, which may vary for individual cells depending on cell differentiation or environmental conditions. If, indeed, PrPC partners may vary qualitatively as a function of expression of PrPC, then data on various transgenic animals, as well as on distinct preparations of transfected cells, should be viewed with additional caution.
Dragging of PrPC by an extracellular matrix molecule such as laminin should bring PrPC closer to another ligand, the laminin receptor, and similar effects would be expected from other PrPC ligands, such as hop/STI1, with regard to the latter's own additional ligands. The case of hop/STI1 is somewhat similar to the soluble PrPC, in that it may serve as a bridge between transmembrane and/or cell surface molecules, including GPI-anchored PrPC itself. Importantly, the arrays of putative ligands identified to date for PrPC (sect. IID), as well as for its direct ligands, such as hop/STI1 (394), were derived from experimental approaches aimed at each individual protein. It is possible that binding of PrPC to its ligands may allow the co-option of novel higher-order ligands into the complex, for example, as a consequence of changes in the structure of each or both binding partners (e.g., Ref. 11). Indeed, biophysical methods show that the structure of PrPC is affected upon binding of its ligand hop/STI1 (S. A. Romano, Y. Cordeiro, L. M. Trambaioli, D. Foguel, and R. Linden, unpublished results).
These considerations suggest that PrPC may, indeed, function at the cell surface as a dynamic platform for the assembly of various signaling modules (Fig. 9). This may explain why so many individual signal transduction pathways have already been shown to underlie PrPC-mediated effects (sect. VIII), as well as the multiplicity of seemingly unrelated functions in various cell types.
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X. CONCLUSION AND FUTURE DIRECTIONS |
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Notwithstanding, there is now overwhelming evidence that PrPC is involved in a variety of important physiological properties at the systemic, cellular, and molecular levels, within the nervous and immune systems, and probably in other organs. A particularly recurrent theme among these properties appears to be the processing of either systemic or cellular stress and danger signals, but the growing literature on both transgenic and pharmacological/biochemical approaches to PrPC suggests that many additional putative systemic functions are likely to emerge in the near future.
In turn, the physiological approach to PrPC bears upon the fundamental understanding of membrane biology and signal transduction. The current review suggests that a fresh perspective on functions of PrPC depends on further studies of 1) the properties of PrPC ligands, including the latter's' own ligand profiles; 2) the structural basis of PrPC-ligand interactions and their consequences; 3) the regulation of the expression of PrPC in physiological context; 4) the effects of PrPC ligands on the trafficking of PrPC; and 5) the consequences of molecular trafficking on PrPC-based multicomponent complexes and signal transduction.
Experiments along these lines should provide a clearer picture as to why PrPC differentially affects such varied functional properties at the systems level, how PrPC-mediated cellular responses explain systemic effects, which and how signaling pathways underlie responses at the cellular level, and the molecular and structural interactions associated with PrPC functions. The latter are particularly important for the eventual modulation of physiological functions of PrPC.
The generalized view of PrPC as a dynamic platform for the assembly of signaling modules at the cell surface (Fig. 9) extends the notion of scaffolding proteins involved in the assembly of intracellular pathways of signal transduction (43, 168, 265, 435, 483, 522). On the basis of this concept, further studies of PrPC-mediated molecular interactions and transmembrane signaling should help clarify the most important questions within the framework of physiological functions of the prion protein. Explaining these properties in the light of the loss-of-function hypothesis may have a decisive impact on the design of novel therapeutic approaches for prion diseases.
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ACKNOWLEDGMENTS |
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Address for reprint requests and other correspondence: R. Linden, Instituto de Biofísica da UFRJ, Centro de Ciências da Saúde, bloco G, Cidade Universitária, 21949-900, Rio de Janeiro, RJ, Brasil (e-mail: rlinden{at}biof.ufrj.br).
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