http://physrev.physiology.org Physiological Reviews RSS feed -- current issue 1522-1210 Apr 1 2008 12:00:00:000AM Physiological Reviews 0031-9333 http://physrev.physiology.org/icons/banner/title.gif http://physrev.physiology.org http://physrev.physiology.org/cgi/content/abstract/88/2/333?rss=1 This review attempts to organize the different aspects of nicotinic transmission in the context of nonsynaptic interactions. Nicotinic acetylcholine receptors (nAChRs) dominantly operate in the nonsynaptic mode in the central nervous system despite their ligand-gated ion-channel nature, which would otherwise be better suited for fast synaptic transmission. This fast form of nonsynaptic transmission, most likely unique to nAChRs, represents a new avenue in the communication platforms of the brain. Cholinergic messages received by nAChRs, arriving at a later phase following synaptic activation, can interfere with dendritic signal integration. Nicotinic transmission plays a role in both neural plasticity and cellular learning processes, as well as in long-term changes in basic activity through fast activation, desensitization of receptors, and fluctuations of the steady-state levels of ACh. ACh release can contribute to plastic changes via activation of nAChRs in neurons and therefore plays a role in learning and memory in different brain regions. Assuming that nAChRs in human subjects are ready to receive long-lasting messages from the extracellular space because of their predominantly nonsynaptic distribution, they offer an ideal target for drug therapy at low, nontoxic drug levels.

]]> 2008-04-07 info:doi/10.1152/physrev.00040.2006 American Physiological Society 2 88 349 2008-04-01 333 Articles http://physrev.physiology.org/cgi/content/abstract/88/2/351?rss=1 CLC-0 and cystic fibrosis transmembrane conductance regulator (CFTR) Cl^– channels play important roles in Cl^– transport across cell membranes. These two proteins belong to, respectively, the CLC and ABC transport protein families whose members encompass both ion channels and transporters. Defective function of members in these two protein families causes various hereditary human diseases. Ion channels and transporters were traditionally viewed as distinct entities in membrane transport physiology, but recent discoveries have blurred the line between these two classes of membrane transport proteins. CLC-0 and CFTR can be considered operationally as ligand-gated channels, though binding of the activating ligands appears to be coupled to an irreversible gating cycle driven by an input of free energy. High-resolution crystallographic structures of bacterial CLC proteins and ABC transporters have led us to a better understanding of the gating properties for CLC and CFTR Cl^– channels. Furthermore, the joined force between structural and functional studies of these two protein families has offered a unique opportunity to peek into the evolutionary link between ion channels and transporters. A promising byproduct of this exercise is a deeper mechanistic insight into how different transport proteins work at a fundamental level.

]]> 2008-04-07 info:doi/10.1152/physrev.00058.2006 American Physiological Society 2 88 387 2008-04-01 351 Articles http://physrev.physiology.org/cgi/content/abstract/88/2/389?rss=1 The dramatic increase in the prevalence of obesity and its strong association with cardiovascular disease have resulted in unprecedented interest in understanding the effects of obesity on the cardiovascular system. A consistent, but puzzling clinical observation is that obesity confers an increased susceptibility to the development of cardiac disease, while at the same time affording protection against subsequent mortality (termed the obesity paradox). In this review we focus on evidence available from human and animal model studies and summarize the ways in which obesity can influence structure and function of the heart. We also review current hypotheses regarding mechanisms linking obesity and various aspects of cardiac remodeling. There is currently great interest in the role of adipokines, factors secreted from adipose tissue, and their role in the numerous cardiovascular complications of obesity. Here we focus on the role of leptin and the emerging promise of adiponectin as a cardioprotective agent. The challenge of understanding the association between obesity and heart failure is complicated by the multifaceted interplay between various hemodynamic, metabolic, and other physiological factors that ultimately impact the myocardium. Furthermore, the end result of obesity-associated changes in the myocardial structure and function may vary at distinct stages in the progression of remodeling, may depend on the individual pathophysiology of heart failure, and may even remain undetected for decades before clinical manifestation. Here we summarize our current knowledge of this complex yet intriguing topic.

]]> 2008-04-07 info:doi/10.1152/physrev.00017.2007 American Physiological Society 2 88 419 2008-04-01 389 Articles http://physrev.physiology.org/cgi/content/abstract/88/2/421?rss=1 Calcium is the most universal signal used by living organisms to convey information to many different cellular processes. In this review we present well-known and recently identified proteins that sense and decode the calcium signal and are key elements in the nucleus to regulate the activity of various transcriptional networks. When possible, the review also presents in vivo models in which the genes encoding these calcium sensors-transducers have been modified, to emphasize the critical role of these Ca²⁺-operated mechanisms in many physiological functions.

]]> 2008-04-07 info:doi/10.1152/physrev.00041.2005 American Physiological Society 2 88 449 2008-04-01 421 Articles http://physrev.physiology.org/cgi/content/abstract/88/2/451?rss=1 This review focuses on the intricate properties of the glomerular barrier. Other reviews have focused on podocyte biology, mesangial cells, and the glomerular basement membrane (GBM). However, since all components of the glomerular membrane are important for its function, proteinuria will occur regardless of which layer is affected by disease. We review the properties of endothelial cells and their surface layer, the GBM, and podocytes, discuss various methods of studying glomerular permeability, and analyze data concerning the restriction of solutes by size, charge, and shape. We also review the physical principles of transport across biological or artificial membranes and various theoretical models used to predict the fluxes of solutes and water. The glomerular barrier is highly size and charge selective, in qualitative agreement with the classical studies performed 30 years ago. The small amounts of albumin filtered will be reabsorbed by the megalin-cubulin complex and degraded by the proximal tubular cells. At present, there is no unequivocal evidence for reuptake of intact albumin from urine. The cellular components are the key players in restricting solute transport, while the GBM is responsible for most of the resistance to water flow across the glomerular barrier.

]]> 2008-04-07 info:doi/10.1152/physrev.00055.2006 American Physiological Society 2 88 487 2008-04-01 451 Articles http://physrev.physiology.org/cgi/content/abstract/88/2/489?rss=1 To migrate, a cell first extends protrusions such as lamellipodia and filopodia, forms adhesions, and finally retracts its tail. The actin cytoskeleton plays a major role in this process. The first part of this review (sect. ii) describes the formation of the lamellipodial and filopodial actin networks. In lamellipodia, the WASP-Arp2/3 pathways generate a branched filament array. This polarized dendritic actin array is maintained in rapid treadmilling by the concerted action of ADF, profilin, and capping proteins. In filopodia, formins catalyze the processive assembly of nonbranched actin filaments. Cell matrix adhesions mechanically couple actin filaments to the substrate to convert the treadmilling into protrusion and the actomyosin contraction into traction of the cell body and retraction of the tail. The second part of this review (sect. iii) focuses on the function and the regulation of major proteins (vinculin, talin, tensin, and -actinin) that control the nucleation, the binding, and the barbed-end growth of actin filaments in adhesions.

]]> 2008-04-07 info:doi/10.1152/physrev.00021.2007 American Physiological Society 2 88 513 2008-04-01 489 Articles http://physrev.physiology.org/cgi/content/abstract/88/2/515?rss=1 Neurons have complex and often extensively elongated processes. This unique cell morphology raises the problem of how remote neuronal territories are replenished with proteins. For a long time, axonal and presynaptic proteins were thought to be exclusively synthesized in the cell body, which delivered them to peripheral sites by axoplasmic transport. Despite this early belief, protein has been shown to be synthesized in axons and nerve terminals, substantially alleviating the trophic burden of the perikaryon. This observation raised the question of the cellular origin of the peripheral RNAs involved in protein synthesis. The synthesis of these RNAs was initially attributed to the neuron soma almost by default. However, experimental data and theoretical considerations support the alternative view that axonal and presynaptic RNAs are also transcribed in the flanking glial cells and transferred to the axon domain of mature neurons. Altogether, these data suggest that axons and nerve terminals are served by a distinct gene expression system largely independent of the neuron cell body. Such a local system would allow the neuron periphery to respond promptly to environmental stimuli. This view has the theoretical merit of extending to axons and nerve terminals the marginalized concept of a glial supply of RNA (and protein) to the neuron cell body. Most long-term plastic changes requiring de novo gene expression occur in these domains, notably in presynaptic endings, despite their intrinsic lack of transcriptional capacity. This review enlightens novel perspectives on the biology and pathobiology of the neuron by critically reviewing these issues.

]]> 2008-04-07 info:doi/10.1152/physrev.00051.2006 American Physiological Society 2 88 555 2008-04-01 515 Articles http://physrev.physiology.org/cgi/content/abstract/88/2/557?rss=1 Telomeres play a central role in cell fate and aging by adjusting the cellular response to stress and growth stimulation on the basis of previous cell divisions and DNA damage. At least a few hundred nucleotides of telomere repeats must "cap" each chromosome end to avoid activation of DNA repair pathways. Repair of critically short or "uncapped" telomeres by telomerase or recombination is limited in most somatic cells and apoptosis or cellular senescence is triggered when too many "uncapped" telomeres accumulate. The chance of the latter increases as the average telomere length decreases. The average telomere length is set and maintained in cells of the germline which typically express high levels of telomerase. In somatic cells, telomere length is very heterogeneous but typically declines with age, posing a barrier to tumor growth but also contributing to loss of cells with age. Loss of (stem) cells via telomere attrition provides strong selection for abnormal and malignant cells, a process facilitated by the genome instability and aneuploidy triggered by dysfunctional telomeres. The crucial role of telomeres in cell turnover and aging is highlighted by patients with 50% of normal telomerase levels resulting from a mutation in one of the telomerase genes. Short telomeres in such patients are implicated in a variety of disorders including dyskeratosis congenita, aplastic anemia, pulmonary fibrosis, and cancer. Here the role of telomeres and telomerase in human aging and aging-associated diseases is reviewed.

]]> 2008-04-07 info:doi/10.1152/physrev.00026.2007 American Physiological Society 2 88 579 2008-04-01 557 Articles http://physrev.physiology.org/cgi/content/abstract/88/2/581?rss=1 Mitochondria play an important role in cell death and cardioprotection. During ischemia, when ATP is progressively depleted, ion pumps cannot function resulting in a rise in calcium (Ca²⁺), which further accelerates ATP depletion. The rise in Ca²⁺ during ischemia and reperfusion leads to mitochondrial Ca²⁺ accumulation, particularly during reperfusion when oxygen is reintroduced. Reintroduction of oxygen allows generation of ATP; however, damage to the electron transport chain results in increased mitochondrial generation of reactive oxygen species (ROS). Mitochondrial Ca²⁺ overload and increased ROS can result in opening of the mitochondrial permeability transition pore, which further compromises cellular energetics. The resultant low ATP and altered ion homeostasis result in rupture of the plasma membrane and cell death. Mitochondria have long been proposed as central players in cell death, since the mitochondria are central to synthesis of both ATP and ROS and since mitochondrial and cytosolic Ca²⁺ overload are key components of cell death. Many cardioprotective mechanisms converge on the mitochondria to reduce cell death. Reducing Ca²⁺ overload and reducing ROS have both been reported to reduce ischemic injury. Preconditioning activates a number of signaling pathways that reduce Ca²⁺ overload and reduce activation of the mitochondrial permeability transition pore. The mitochondrial targets of cardioprotective signals are discussed in detail.

]]> 2008-04-07 info:doi/10.1152/physrev.00024.2007 American Physiological Society 2 88 609 2008-04-01 581 Articles http://physrev.physiology.org/cgi/content/abstract/88/2/611?rss=1 Mitochondria contain their own genetic system and undergo a unique mode of cytoplasmic inheritance. Each organelle has multiple copies of a covalently closed circular DNA genome (mtDNA). The entire protein coding capacity of mtDNA is devoted to the synthesis of 13 essential subunits of the inner membrane complexes of the respiratory apparatus. Thus the majority of respiratory proteins and all of the other gene products necessary for the myriad mitochondrial functions are derived from nuclear genes. Transcription of mtDNA requires a small number of nucleus-encoded proteins including a single RNA polymerase (POLRMT), auxiliary factors necessary for promoter recognition (TFB1M, TFB2M) and activation (Tfam), and a termination factor (mTERF). This relatively simple system can account for the bidirectional transcription of mtDNA from divergent promoters and key termination events controlling the rRNA/mRNA ratio. Nucleomitochondrial interactions depend on the interplay between transcription factors (NRF-1, NRF-2, PPAR, ERR, Sp1, and others) and members of the PGC-1 family of regulated coactivators (PGC-1, PGC-1β, and PRC). The transcription factors target genes that specify the respiratory chain, the mitochondrial transcription, translation and replication machinery, and protein import and assembly apparatus among others. These factors are in turn activated directly or indirectly by PGC-1 family coactivators whose differential expression is controlled by an array of environmental signals including temperature, energy deprivation, and availability of nutrients and growth factors. These transcriptional paradigms provide a basic framework for understanding the integration of mitochondrial biogenesis and function with signaling events that dictate cell- and tissue-specific energetic properties.

]]> 2008-04-07 info:doi/10.1152/physrev.00025.2007 American Physiological Society 2 88 638 2008-04-01 611 Articles http://physrev.physiology.org/cgi/content/abstract/88/2/639?rss=1 This article reviews the current state of knowledge about the bestrophins, a newly identified family of proteins that can function both as Cl^– channels and as regulators of voltage-gated Ca²⁺ channels. The founding member, human bestrophin-1 (hBest1), was identified as the gene responsible for a dominantly inherited, juvenile-onset form of macular degeneration called Best vitelliform macular dystrophy. Mutations in hBest1 have also been associated with a small fraction of adult-onset macular dystrophies. It is proposed that dysfunction of bestrophin results in abnormal fluid and ion transport by the retinal pigment epithelium, resulting in a weakened interface between the retinal pigment epithelium and photoreceptors. There is compelling evidence that bestrophins are Cl^– channels, but bestrophins remain enigmatic because it is not clear that the Cl^– channel function can explain Best disease. In addition to functioning as a Cl^– channel, hBest1 also is able to regulate voltage-gated Ca²⁺ channels. Some bestrophins are activated by increases in intracellular Ca²⁺ concentration, but whether bestrophins are the molecular counterpart of Ca²⁺-activated Cl^– channels remains in doubt. Bestrophins are also regulated by cell volume and may be a member of the volume-regulated anion channel family.

]]> 2008-04-07 info:doi/10.1152/physrev.00022.2007 American Physiological Society 2 88 672 2008-04-01 639 Articles http://physrev.physiology.org/cgi/content/abstract/88/2/673?rss=1 Prion diseases are transmissible spongiform encephalopathies (TSEs), attributed to conformational conversion of the cellular prion protein (PrP^C) into an abnormal conformer that accumulates in the brain. Understanding the pathogenesis of TSEs requires the identification of functional properties of PrP^C. Here we examine the physiological functions of PrP^C at the systemic, cellular, and molecular level. Current data show that both the expression and the engagement of PrP^C with a variety of ligands modulate the following: 1) functions of the nervous and immune systems, including memory and inflammatory reactions; 2) cell proliferation, differentiation, and sensitivity to programmed cell death both in the nervous and immune systems, as well as in various cell lines; 3) the activity of numerous signal transduction pathways, including cAMP/protein kinase A, mitogen-activated protein kinase, phosphatidylinositol 3-kinase/Akt pathways, as well as soluble non-receptor tyrosine kinases; and 4) trafficking of PrP^C both laterally among distinct plasma membrane domains, and along endocytic pathways, on top of continuous, rapid recycling. A unified view of these functional properties indicates that the prion protein is a dynamic cell surface platform for the assembly of signaling modules, based on which selective interactions with many ligands and transmembrane signaling pathways translate into wide-range consequences upon both physiology and behavior.

]]> 2008-04-07 info:doi/10.1152/physrev.00007.2007 American Physiological Society 2 88 728 2008-04-01 673 Articles http://physrev.physiology.org/cgi/content/abstract/88/2/729?rss=1 The importance of β-adrenergic signaling in the heart has been well documented, but it is only more recently that we have begun to understand the importance of this signaling pathway in skeletal muscle. There is considerable evidence regarding the stimulation of the β-adrenergic system with β-adrenoceptor agonists (β-agonists). Although traditionally used for treating bronchospasm, it became apparent that some β-agonists could increase skeletal muscle mass and decrease body fat. These so-called "repartitioning effects" proved desirable for the livestock industry trying to improve feed efficiency and meat quality. Studying β-agonist effects on skeletal muscle has identified potential therapeutic applications for muscle wasting conditions such as sarcopenia, cancer cachexia, denervation, and neuromuscular diseases, aiming to attenuate (or potentially reverse) the muscle wasting and associated muscle weakness, and to enhance muscle growth and repair after injury. Some undesirable cardiovascular side effects of β-agonists have so far limited their therapeutic potential. This review describes the physiological significance of β-adrenergic signaling in skeletal muscle and examines the effects of β-agonists on skeletal muscle structure and function. In addition, we examine the proposed beneficial effects of β-agonist administration on skeletal muscle along with some of the less desirable cardiovascular effects. Understanding β-adrenergic signaling in skeletal muscle is important for identifying new therapeutic targets and identifying novel approaches to attenuate the muscle wasting concomitant with many diseases.

]]> 2008-04-07 info:doi/10.1152/physrev.00028.2007 American Physiological Society 2 88 767 2008-04-01 729 Articles http://physrev.physiology.org/cgi/content/abstract/88/2/769?rss=1 Most synaptic inputs are made onto the dendritic tree. Recent work has shown that dendrites play an active role in transforming synaptic input into neuronal output and in defining the relationships between active synapses. In this review, we discuss how these dendritic properties influence the rules governing the induction of synaptic plasticity. We argue that the location of synapses in the dendritic tree, and the type of dendritic excitability associated with each synapse, play decisive roles in determining the plastic properties of that synapse. Furthermore, since the electrical properties of the dendritic tree are not static, but can be altered by neuromodulators and by synaptic activity itself, we discuss how learning rules may be dynamically shaped by tuning dendritic function. We conclude by describing how this reciprocal relationship between plasticity of dendritic excitability and synaptic plasticity has changed our view of information processing and memory storage in neuronal networks.

]]> 2008-04-07 info:doi/10.1152/physrev.00016.2007 American Physiological Society 2 88 840 2008-04-01 769 Articles