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Membrane Pore: Reviews

(227 References)

Talley, E. M., J. E. Sirois, et al. (2003). "Two-pore-Domain (KCNK) potassium channels: dynamic roles in neuronal function." Neuroscientist 9(1): 46-56.

            Leak K+ currents contribute to the resting membrane potential and are important for modulation of neuronal excitability. Within the past few years, an entire family of genes has been described whose members form leak K+ channels, insofar as they generate potassium-selective currents with little voltage- and time-dependence. They are often referred to as "two-pore-domain" channels because of their predicted topology, which includes two pore-forming regions in each subunit. These channels are modulated by a host of different endogenous and clinical compounds such as neurotransmitters and anesthetics, and by physicochemical factors such as temperature, pH, oxygen tension, and osmolarity. They also are subject to long-term regulation by changes in gene expression. In this review, the authors describe multiple roles that modulation of leak K+ channels play in CNS function and discuss evidence that members of the two-pore-domain family are molecular substrates for these processes.

Rehling, P., N. Pfanner, et al. (2003). "Insertion of hydrophobic membrane proteins into the inner mitochondrial membrane--a guided tour." J Mol Biol 326(3): 639-57.

            Only a few mitochondrial proteins are encoded by the organellar genome. The majority of mitochondrial proteins are nuclear encoded and thus have to be transported into the organelle from the cytosol. Within the mitochondrion proteins have to be sorted into one of the four sub-compartments: the outer or inner membranes, the intermembrane space or the matrix. These processes are mediated by complex protein machineries within the different compartments that act alone or in concert with each other. The translocation machinery of the outer membrane is formed by a multi-subunit protein complex (TOM complex), that is built up by signal receptors and the general import pore (GIP). The inner membrane houses two multi-subunit protein complexes that each handles special subsets of mitochondrial proteins on their way to their final destination. According to their primary function these two complexes have been termed the pre-sequence translocase (or TIM23 complex) and the protein insertion complex (or TIM22 complex). The identification of components of these complexes and the analysis of the molecular mechanisms underlying their function are currently an exciting and fast developing field of molecular cell biology.

Montoya, M. and E. Gouaux (2003). "Beta-barrel membrane protein folding and structure viewed through the lens of alpha-hemolysin." Biochim Biophys Acta 1609(1): 19-27.

            The beta-barrel is a transmembrane structural motif commonly encountered in bacterial outer membrane proteins and pore-forming toxins (PFTs). Alpha-hemolysin (alphaHL) is a cytotoxin secreted by Staphylococcus aureus that assembles from a water-soluble monomer to form a membrane-bound heptameric beta-barrel on the surface of susceptible cells, perforating the cell membranes, leading to cell death and lysis. The mechanism of heptamer assembly, which has been studied extensively, occurs in a stepwise manner, and the structures of the initial, monomeric form and final, membrane-embedded pore are known. The toxin's ability to assemble from an aqueous, hydrophilic species to a membrane-inserted oligomer is of interest in understanding the assembly of PFTs in particular and the folding and structure of beta-barrel membrane proteins in general. Here we review the structures of the monomeric and heptamer states of LukF and alphaHL, respectively, the mechanism of toxin assembly, and the relationships between alphaHL and nontoxin beta-barrel membrane proteins.

Jena, B. P. (2003). "Fusion pore or porosome: structure and dynamics." J Endocrinol 176(2): 169-74.

            Electrophysiological measurements on live secretory cells almost a decade ago suggested the presence of fusion pores at the cell plasma membrane. Membrane-bound secretory vesicles were hypothesized to dock and fuse at these sites, to release their contents. Our studies using atomic force microscopy on live exocrine and neuroendocrine cells demonstrate the presence of such plasma membrane pores, revealing their morphology and dynamics at near nm resolution and in real time.

Zeuthen, T. and N. MacAulay (2002). "Passive water transport in biological pores." Int Rev Cytol 215: 203-30.

            Three kinds of membrane proteins have been shown to have water channels properties: the aquaporins, the cotransporters, and the uniports. A molecular-kinetic description of water transport in pores is compared to analytical models based on macroscopic parameters such as pore diameter and length. The use and limitations of irreversible thermodynamics is discussed. Experimental data on water and solute permeability in aquaporins are reviewed. No unifying transport model based on macroscopic parameters can be set up; for example, there is no correlation between solute diameter and permeability. Instead, the influence of hydrogen bonds between solute and pore, and the pH dependence of permeability, point toward a model based upon chemical interactions. The atomic model for AQP1 based on electron crystallographic data defines the dimensions and chemical nature of the aqueous pore. These structural data combined with quantum mechanical modeling and computer simulation might result in a realistic description of water transport. Data on water and solute permeability in cotransporters and uniports are reviewed. The function of these proteins as substrate transporters involves a series of conformational changes. The role of conformational equilibria on the water permeability will be discussed.

Zagoskin, P. P. and E. M. Khvatova (2002). "[Mitochondrial diseases--a new branch of the modern medicine]." Vopr Med Khim 48(4): 321-36.

            The review highlights current aspects of a large group of diseases the main pathogenetic element of which is an inherited or acquired disturbance of gene expression of nuclear or mitochondrial genome encoding mitochondrial proteins. The recent data on mutant genetic loci specific to the most wide spread mitochondrial diseases are considered. The steps of pathogenesis, include the mutations of nuclear or mitochondrial genes, disturbances of mitochondrial protein synthesis, dissipation of proton membrane potential, opening of a permeability transition pore, releasing of procaspases, cytochrome c, and other proapoptotic molecules, and finally chromatin fragmentation and apoptotic cell death. We discuss the possible reasons of polysymptomatic character and different variants of mitochondrial disease manifestations on the basis of the phenomenon of mitochondrial DNA heteroplasmy and metabolic compensation of the genetic defects. Modern biochemical methods of a mitochondrial disease diagnostics: (PCR-amplification, polarographic research of mitochondrial respiration and oxidative phosphorylation, analysis and monitoring of metabolites in biological liquids) are characterized. The basic principles and perspectives of the treatment of mitochondrial diseases, (gene therapy, correction of metabolic disorders, application of antioxidants and neuropeptides) are described.

Zagon, I. S., M. F. Verderame, et al. (2002). "The biology of the opioid growth factor receptor (OGFr)." Brain Res Brain Res Rev 38(3): 351-76.

            Opioid peptides act as growth factors in neural and non-neural cells and tissues, in addition to serving for neurotransmission/neuromodulation in the nervous system. The native opioid growth factor (OGF), [Met(5)]-enkephalin, is a tonic inhibitory peptide that plays a role in cell proliferation and tissue organization during development, cancer, cellular renewal, wound healing, and angiogenesis. OGF action is mediated by a receptor mechanism. Assays with radiolabeled OGF have detected specific and saturable binding, with a one-site model of kinetics. Subcellular fractionation studies show that the receptor for OGF (OGFr) is an integral membrane protein associated with the nucleus. Using antibodies generated to a binding fragment of OGFr, this receptor has been cloned and sequenced in human, rat, and mouse. OGFr is distinguished by containing a series of imperfect repeats. The molecular and protein structure of OGFr have no resemblance to that of classical opioid receptors, and have no significant homologies to known domains or functional motifs with the exception of a bipartite nuclear localization signal. Immunoelectron microscopy and immunocytochemistry investigations, including co-localization studies, have detected OGFr on the outer nuclear envelope where it interfaces with OGF. The peptide-receptor complex associates with karyopherin, translocates through the nuclear pore, and can be observed in the inner nuclear matrix and at the periphery of heterochromatin of the nucleus. Signal transduction for modulation of DNA activity is dependent on the presence of an appropriate confirmation of peptide and receptor. This report reviews the history of OGF-OGFr, examines emerging insights into the mechanisms of action of opioid peptide-receptor interfacing, and discusses the clinical significance of these observations.

Wirtz, M., S. Yu, et al. (2002). "Template synthesized gold nanotube membranes for chemical separations and sensing." Analyst 127(7): 871-9.

            We have developed a new class of synthetic membranes that consist of a porous polymeric support that contains an ensemble of gold nanotubes that span the thickness of the support membrane. The support is a commercially-available microporous polycarbonate filter with cylindrical nanoscopic pores. The gold nanotubes are prepared via electroless deposition of Au onto the pore walls; i.e., the pores acts as templates for the nanotubes. We have shown that by controlling the Au deposition time, Au nanotubes that have effective inside diameters of molecular dimensions (< 1 nm) can be prepared. These membranes are a new class of molecular sieves and can be used to separate both small molecules and proteins on the basis of molecular size. In addition, the use of these membranes in new approaches to electrochemical sensing is reviewed here. In this case, a current is forced through the nanotubes, and analyte molecules present in a contacting solution phase modulate the value of this transmembrane current.

Weiger, T. M., A. Hermann, et al. (2002). "Modulation of calcium-activated potassium channels." J Comp Physiol A Neuroethol Sens Neural Behav Physiol 188(2): 79-87.

            Potassium currents play a critical role in action potential repolarization, setting of the resting membrane potential, control of neuronal firing rates, and regulation of neurotransmitter release. The diversity of the potassium channels that generate these currents is nothing less than staggering. This diversity is generated by multiple genes (as many as 100 and perhaps more in some creatures) encoding the pore-forming channel alpha subunits, alternative splicing of channel gene transcripts, formation of heteromultimeric channels, participation of auxiliary (non-pore-forming) beta and other subunits, and modulation of channel properties by post-translational modifications and other mechanisms. Prominent among the potassium channels are several families of calcium activated potassium channels, which are highly selective for potassium ions as their charge carrier, and require intracellular calcium for channel gating. The modulation of one of these families, that of the large conductance calcium activated and voltage-dependent potassium channels, has been especially widely studied. In this review we discuss a few selected examples of the modulation of these channels, to illustrate some of the molecular mechanisms and physiological consequences of ion channel modulation.

Waterhouse, N. J., J. E. Ricci, et al. (2002). "And all of a sudden it's over: mitochondrial outer-membrane permeabilization in apoptosis." Biochimie 84(2-3): 113-21.

            Identification of pro-apoptotic activities for a variety of proteins normally resident in the mitochondrial inter-membrane space has substantiated the role of mitochondria as integral to the apoptotic process. Cytochrome c is involved in apoptosome formation and caspase activation, SMAC/Diablo deregulates the inhibitor of apoptosis proteins, apoptosis-inducing factor may play a role in chromatin condensation and release of other proteins such as adenylate kinase may adversely affect cellular metabolism and contribute to the death of a cell if the downstream apoptotic pathway is blocked. It is still unclear how these proteins are released from the mitochondria. Recent advances in our knowledge of mitochondrial outer-membrane permeabilization and the consequences of this event on mitochondria will be discussed.

Wasada, T. (2002). "Adenosine triphosphate-sensitive potassium (K(ATP)) channel activity is coupled with insulin resistance in obesity and type 2 diabetes mellitus." Intern Med 41(2): 84-90.

            ATP sensitive potassium (K(ATP)) channels reside in the plasma membrane of many excitable cells such as pancreatic beta-cells, heart, skeletal muscle and brain, where they link cellular metabolic energy to membrane electrical activity. They are composed of two subunits, K+ ion selective pore (Kir) and sulfonylurea receptor (SUR). In addition to the central role of pancreatic beta-cell K(ATP) channels in glucose-mediated insulin secretion, several lines of evidence support the hypothesis that K(ATP) channels modulate glucose transport in the insulin target tissues. Inhibition of K(ATP) channels by glibenclamide or gliclazide or an increase in intracellular ATP during hyperglycemia (glucose effect) or exercise facilitates glucose utilization, while activation of the channels by potassium channel openers, hypothermia (cardiac surgery), or ischemic damage (myocardial and brain infarction) reduces glucose uptake induced by insulin or hyperglycemia. Because insulin action has been known to depend on the energy level of the target cells, K(ATP) channel may function as an effector in this respect. It is now evident that long chain acyl-CoA esters, metabolically active forms of fatty acids, are the most potent and physiologically important activator of K(ATP) channels. Thus, I suppose that the sustained activation of K(ATP) channels by long chain fatty acyl-CoA seems to be a missing link between lipotoxicity and insulin resistance in obesity and type 2 diabetes mellitus.

Walter, L., H. Miyoshi, et al. (2002). "Regulation of the mitochondrial permeability transition pore by ubiquinone analogs. A progress report." Free Radic Res 36(4): 405-12.

            The permeability transition pore (PTP) is a mitochondrial inner membrane Ca2+-sensitive channel that plays a key role in different models of cell death. In a series of recent studies we have shown that the PTP is modulated by quinones, and we have identified three functional classes: (i) PTP inhibitors; (ii) PTP inducers; and (iii) PTP-inactive quinones that compete with both inhibitors and inducers. Here, we review our current understanding of pore regulation by quinones, and present the results obtained with a new series of structural variants. Based on the effects of the compounds studied so far, we confirm that minor structural changes profoundly modify the effects of quinones on the PTP. On the other hand, quinones with very different structural features may have qualitatively similar effects on the PTP. Taken together, these results support our original proposal that quinones affect the PTP through a common binding site whose occupancy modulates its open-closed transitions, possibly through secondary changes of the Ca2+-binding affinity.

Unwin, N. (2002). "Structure of the acetylcholine-gated channel." Novartis Found Symp 245: 5-15; discussion 15-21, 165-8.

            The structure of the acetylcholine-gated channel, trapped in open as well as closed states, has been analysed by electron crystallographic methods. The channel has large vestibules extending from the membrane which shape the acetylcholine-binding pockets and facilitate selective transport of cations across a narrow membrane-spanning pore. When acetylcholine enters these pockets it triggers a concerted conformational change that opens the pore by destabilizing a gate in the middle of the membrane made by a ring of pore-lining alpha-helical segments. The alternative 'open' configuration of porelining segments reshapes the lumen and creates new surfaces, allowing the ions to pass through. Recent results, at nearly 4A resolution, have defined more precisely the structure of the pore and the design of the vestibular entrances.

Tsujimoto, Y. and S. Shimizu (2002). "The voltage-dependent anion channel: an essential player in apoptosis." Biochimie 84(2-3): 187-93.

            The increase of outer mitochondrial membrane permeability is a central event in apoptotic cell death, since it releases several apoptogenic factors such as cytochrome c into the cytoplasm that activate the downstream destructive processes. The voltage-dependent anion channel (VDAC or mitochondrial porin) plays an essential role in the increase of mitochondrial membrane permeability, and it is regulated by the Bcl-2 family of proteins via direct interaction. Anti-apoptotic Bcl-2 family members close the VDAC, whereas some (but not all) pro-apoptotic members interact with the VDAC to generate a protein-conducting channel through which cytochrome c can pass. Although the VDAC is directly involved in the apoptotic increase of mitochondrial membrane permeability and is known to be a component of the permeability transition pore complex, its role in the regulation of outer membrane permeability can be separated from the occurrence of permeability transition events, such as mitochondrial swelling followed by rupture of the outer mitochondrial membrane. The VDAC not only interacts with Bcl-2 family members, but also with other proteins, and probably acts as a convergence point for a variety of life-or-death signals.

Tsujimoto, Y. (2002). "Bcl-2 family of proteins: life-or-death switch in mitochondria." Biosci Rep 22(1): 47-58.

            An increase in the permeability of outer mitochondrial membrane is central to apoptotic cell death, and results in the release of several apoptogenic factors such as cytochrome c into the cytoplasm to activate downstream destructive programs. The voltage-dependent anion channel (VDAC or mitochondrial porin) plays an essential role in disrupting the mitochondrial membrane barrier and is regulated directly by members of the Bcl-2 family proteins. Anti-apoptotic Bcl-2 family members interact with and close the VDAC, whereas some, but not all, proapoptotic members interact with VDAC to open protein-conducting pore through which apoptogenic factors pass. Although the VDAC is involved directly in breaking the mitochondrial membrane barrier and is a known component of the permeability transition pore complex, VDAC-dependent increase in outer membrane permeability can be independent of the permeability transition event such as mitochondrial swelling followed by rupture of the outer mitochondrial membrane. VDAC interacts not only with Bcl-2 family members but also with proteins such as gelsolin, an actin regulatory protein, and appears to be a convergence point for a variety of cell survival and cell death signals.

Tani, T., T. Tominaga, et al. (2002). "Development of periodic tension in the contractile vacuole complex membrane of paramecium governs its membrane dynamics." Cell Biol Int 26(10): 853-60.

            The contractile vacuole complex is a membrane-bound osmoregulatory organelle of fresh water protozoa such as Paramecium. In Paramecium it consists of a central vacuole (the contractile vacuole) and 5-10 arms that radially extend from the vacuole into the cytosol (the radial arms). Excess cytosolic water, acquired osmotically, is segregated by the radial arms and enters the vacuole, so that the vacuole swells (the fluid-filling phase). The vacuole then rounds (the rounding phase) and the radial arms sever from the vacuole. The vacuole membrane then fuses with the plasma membrane at the pore region and the pore opens. The vacuole shrinks as its fluid is discharged through the pore (the fluid-discharging phase). The pore closes when the fluid has been discharged. The radial arms then reattach to the vacuole, so that the vacuole swells again as the fluid enters from the arms (the next fluid-filling phase). We found that the vacuole continued to show rounding and slackening even after it together with a small amount of cytosol had been isolated from the cell. Using a microcantilever placed on the surface of the vacuole the tension of the in vitro vacuole increased to 5 x 10(-3)N m(-1) as the vacuole rounds, and its lowest value was 1 x 10(-4)N m(-1) during slackening. We propose a hypothesis that an increase in the spontaneous curvature of the organelle's membrane leads to an increase in membrane tension and thus to the vacuole's rounding, severing of the radial arms from the vacuole, and opening of the pore. Conversely, a decrease in the spontaneous curvature accompanied by a decrease in membrane tension could lead to the closing of the pore and reattachment of the radial arm at the start of the fluid-filling phase.

Slaugenhaupt, S. A. (2002). "The molecular basis of mucolipidosis type IV." Curr Mol Med 2(5): 445-50.

            Mucolipidosis Type IV (MLIV) is a lysosomal storage disorder that is characterized by severe neurologic and ophthalmologic abnormalities. It is a progressive disease that usually presents during the first year of life with mental retardation, corneal opacities, and delayed motor milestones. First described in 1974, MLIV is a rare autosomal recessive disease and the majority of patients diagnosed to date are of Ashkenazi Jewish descent. MLIV was originally classified as a lysosomal storage disorder due to the abnormal accumulation of mucopolysaccharides and lipids. Extensive studies in MLIV cells, however, have shown that the abnormal storage is due to a defect in the late endocytic pathway. Positional cloning led to the recent discovery of a novel gene on human chromosome 19, MCOLN1, that is mutated in MLIV. To date 14 independent mutations have been reported in MCOLN1, with two mutations accounting for 95% of the Ashkenazi Jewish MLIV alleles. The identification of the MLIV gene has led to a simple tool for definitive diagnosis and will permit carrier screening in the Ashkenazi Jewish population. MCOLN1 is a new member of the transient receptor potential (TRP) cation channel gene family. The protein encoded by MCOLN1, mucolipin-1, has six predicted transmembrane domains and a putative channel pore. The identification of mutations in MCOLN1 represents the first example of a neurological disease caused by a TRP-related channel. While the function of mucolipin-1 is currently unknown, homology to the TRP superfamily and the recent description of the C. elegans mucolipin-1 homolog allow us to begin to speculate about the role of mucolipin-1 in diverse cellular processes.

Sansom, M. S., P. Bond, et al. (2002). "Water in ion channels and pores--simulation studies." Novartis Found Symp 245: 66-78; discussion 79-83, 165-8.

            The microscopic properties of water in narrow pores are relevant to the function of ion channels and related membrane transport proteins. The emergence of several high-resolution structures allows one to perform molecular dynamics simulation studies of water in such pores. Simulations of bundles of parallel alpha-helical peptides (e.g. alamethicin) have enabled development of methodologies and concepts appropriate to such investigations. In the narrow channels formed by such bundles, water molecules exhibit reduced rotational and translation motion. This reduction in water mobility may be a general property of narrow pores. We have used simplified channel models to explore the role of hydrophobicity/hydrophilicity in the entry of water into pores. Narrow pores with a hydrophobic lining, although physically open, may not admit water molecules, acting as a 'hydrophobic gate' that prevents water and ion permeation. Such a gate can be opened either by widening the pore or making its lining more polar. Simulations have been used to explore the behaviour of water in GlpF, a member of the aquaporin family of water pores, and OmpA, a bacterial outer membrane protein. Preliminary results suggest that a continuous water wire is not formed within the amphipathic GlpF pore. Simulations of OmpA, in which polar residues line the channel, indicate that a small conformational change in one of the channel lining side chains may open the channel. In summary, comparison of the behaviour of water in different narrow transmembrane pores suggests that an amphipathic pore is ideal for water permeation, and that either a highly hydrophobic pore lining or a charged pore-lining region can act as a gate.

Saiz, L. and M. L. Klein (2002). "Computer simulation studies of model biological membranes." Acc Chem Res 35(6): 482-9.

            This Account is focused on computer simulation studies of model biological membrane systems with potential applications in biomedical research. In the past decade, classical molecular dynamics has provided novel insights into the properties of model biomembrane systems, including the nature of the DNA-lipid interactions, the effect of pore-forming transmembrane peptides on the lipid environment, and the partitioning of volatile anesthetic molecules. Such simulations, employing full atomic detail, are typically restricted to systems of dimensions less than approximately 10 nm. Simplified models of the coarse-grain type have been intended to bridge the gap between full atomistic detail and the mesoscopic (micron) regime. The use of such models is illustrated with the example of anesthetics in a phospholipid bilayer.

Russo, L. M., G. L. Bakris, et al. (2002). "Renal handling of albumin: a critical review of basic concepts and perspective." Am J Kidney Dis 39(5): 899-919.

            Biochemical and physiological processes that underlie the mechanism of albuminuria are completely reassessed in this article in view of recent discoveries that filtered proteins undergo rapid degradation during renal passage and the resulting excreted peptide fragments are not detected by conventional urine protein assays. This means that filtered protein and/or albumin levels in urine have been seriously underestimated. The concept that albuminuria is a result of changes in glomerular permeability is questioned in light of these findings and also in terms of a critical examination of charge selectivity, shunts, or large-pore formation and hemodynamic effects. The glomerulus appears to function merely in terms of size selectivity alone, and for albumin, this does not change significantly in disease states. Intensive albumin processing by a living kidney occurs through cellular processes distal to the glomerular basement membrane. Failure of this cellular processing primarily leads to albuminuria. This review brings together recent data about urinary albumin clearance and current knowledge of receptors known to process albumin in both health and disease states. We conclude with a discussion of topical and controversial issues associated with the proposed new understanding of renal handling of albumin.

Ruiz Gomez, M. J., A. Souviron Rodriguez, et al. (2002). "[P-glycoprotein, a membrane pump that represents a barrier to chemotherapy in cancer patients]." An Med Interna 19(9): 477-85.

            Multidrug resistance (MDR) in oncology is considered to be the main cause of chemotherapy failure in the treatment of patients with cancer. The resistance mechanism consists in decrease intracellular drug accumulation by P-glycoprotein (Gp-P) overexpression. This protein acts as a drug-extracting pump that needs energy in the process. The efflux takes place by mean of a pore in the cell membrane that consist in twelve segments. The activity of this pump is regulated by protein kinase C and shows homology with other transport systems. The analysis of the presence of Gp-P and the characterization of MDR phenotype in biopsy material could be important in the overcome of the resistance to cancer chemotherapy.

Roux, B. (2002). "Computational studies of the gramicidin channel." Acc Chem Res 35(6): 366-75.

            Ion channels are highly specific membrane-spanning protein structures which serve to facilitate the passage of selected ions across the lipid barrier. In the past decade, molecular dynamics simulations based on atomic models and realistic microscopic interactions with explicit solvent and membrane lipids have been used to gain insight into the function of these complex systems. These calculations have considerably expanded our view of ion permeation at the microscopic level. This Account will mainly focus on computational studies of the gramicidin A channel, one of the simplest and best characterized molecular pore.

Raulin, J. (2002). "Human immunodeficiency virus and host cell lipids. Interesting pathways in research for a new HIV therapy." Prog Lipid Res 41(1): 27-65.

            It has been reported in the literature that biological membranes arising from HIV-induced cell fusion, as well as syncytium formation between infected and non-infected cells and those involved in transduction, viral DNA nuclear import and virion budding from the host cell, are all made of proteins, a phospholipid (P) bilayer and cholesterol (C). However, the P/C molar ratio is higher in the retroviral envelope than in the plasma membrane where they originate, and higher than in the nuclear envelope. Mechanisms are described which elucidate this puzzling fact, as well as cholesterol-dependent leakage and pore formation during cell fusion. Fatty acylation of viral and host cell proteins is required to direct them to membranes. Detergent-insoluble microdomains enriched in cholesterol and sphingolipids, termed either DIGs (detergent-insoluble glycolipid-enriched complexes), DRMs (detergent resistant membranes), TIFFs (Triton-insoluble floating fractions) or GEMs (glycolipid-enriched membranes), function as platforms for attachment of proteins in the process of signal transduction. HIV-SUgp120 (HIV-surface glycoprotein), T-cell receptor (TCR)-CD4+ and co-receptors promote aggregation of these lipid "rafts" which concentrate the Src family tyrosine kinases SFKs (PTK, Lyn, Fyn, Lck), GPI (glycosyl phosphatidylinositol)-anchored proteins, and phosphatidylinositol kinases PI(3)K and PI(4)K, inducing cell signalling. HIV-SUgp120 transduces the activation signal and provokes the formation of polyunsaturated fatty acid (PUFA) metabolites, i.e. the prostaglandin PGE2 suppressor of immune function and inhibitor of cytotoxic T-lymphocyte (CTL) proliferation, while PGB2 activates SFKs and increases mRNA expression, as well as NFkappaB (nuclear transcription factor) translocation to nucleus. HIV nuclear import, DNA integration, chromatin template capacity may be mediated by the lipid environment. The lipid-enriched microdomains from which HIV-1 buds, may explain the high level of cholesterol and sphingolipids in the viral envelope, since host cell rafts become a viral coat. HIV-1 infection induces alteration of cellular lipids: (1) shift in phospholipid synthesis to neutral lipids associated with the viral load, polyunsaturated fatty acid (PUFA) peroxidation, and n-3 deficiency with deregulation of cytokines and PPAR-gamma (peroxisome proliferator-activated receptor-gamma), and (2) alloimmune phospholipid antibody production in which antibodies to cardiolipin and to phosphatidylserine are most prevalent, due to the destruction of mitochondrial membranes and progression of lymphocyte apoptosis. The current highly active anti-retroviral therapy, including both viral reverse transcriptase (RT) inhibitors (NRTIs and NNRTIs, nucleoside and non-nucleoside RT inhibitors) and protease inhibitors (PIs), induces side-effects in the long term. Lipodystrophy (LD), consists of peripheral lipoatrophy associated with central fat accumulation (called "crixbelly" and "buffalo hump"), insulin resistance, elevation of very low density lipoproteins, decrease in high density lipoproteins and inhibition of adipocyte differentiation. LD syndrome appears to be induced by PIs that inhibit GLUT4, glucose transporter isoform, and by NRTIs which provoke mitochondrial failure. New therapeutic strategies assessed: (1) inhibition of the viral integrase and/or HIV entry into cells through natural products or their derivatives, (2) inhibition of HIV-1 entry into macrophages pretreated with Gram-negative bacterial lipopolysaccharide, (3) vaccination with multi-lipopeptides, i.e. sequences of HIV-1 peptides with CD4+ T-cell and B-cell epitopes, modified by adding a lipid tail to one end, which produce HIV-specific CTL and multispecific immune responses in most of the vaccinated subjects and (4) stimulation of antiviral drug activity with lipid-prodrugs targeting viral RT, polymerase, integrase, or aspartyl-protease.

Phenix, B. N. and A. D. Badley (2002). "Influence of mitochondrial control of apoptosis on the pathogenesis, complications and treatment of HIV infection." Biochimie 84(2-3): 251-64.

            HIV infection is inexorably linked with disordered regulation of apoptosis, and consequent alterations in mitochondrial homeostasis, resulting in CD4 T cell death and enhanced susceptibility to opportunistic infections and malignancies. Effective treatment of HIV reverses the changes in mitochondrial homeostasis and apoptosis, and enhances immunocompetence. This review will summarize current knowledge of: i) the associations of apoptosis with HIV disease progression; ii) mechanisms of enhanced apoptosis in HIV infection; iii) putative role of apoptosis in HIV complications; iv) direct effects of HIV therapies on mitochondria and apoptosis; and finally v) treatment strategies for HIV based upon modifying the apoptotic response.

Pfanner, N. and A. Chacinska (2002). "The mitochondrial import machinery: preprotein-conducting channels with binding sites for presequences." Biochim Biophys Acta 1592(1): 15-24.

            Mitochondrial preproteins with amino-terminal presequences must cross two membranes to reach the matrix of the organelle. Both outer and inner membranes contain hydrophilic high-conductance channels that are responsible for selective translocation of preproteins. The channels are embedded in dynamic protein complexes, the TOM complex of the outer membrane and the TIM23 complex of the inner membrane. Both channel-forming proteins, Tom40 and Tim23, carry specific binding sites for presequences, but differ in their pore size and response to a membrane potential. Studies with the TOM machinery show that other subunits of the translocase complex also provide specific binding sites for preproteins, modulate the channel activity and are critical for assembly of the channel.

Parone, P. A., D. James, et al. (2002). "Mitochondria: regulating the inevitable." Biochimie 84(2-3): 105-11.

            Apoptosis is a form of programmed cell death important in the development and tissue homeostasis of multicellular organisms. Abnormalities in cell death control can lead to a variety of diseases, including cancer and degenerative disorders. Hence, the process of apoptosis is tightly regulated through multiple independent signalling pathways that are initiated either from triggering events within the cell or at the cell surface. In recent years, mitochondria have emerged as the central components of such apoptotic signalling pathways and are now known to control apoptosis through the release of apoptogenic proteins. In this review we aim to give an overview of the role of the mitochondria during apoptosis and the molecular mechanisms involved.

Panchal, R. G., M. L. Smart, et al. (2002). "Pore-forming proteins and their application in biotechnology." Curr Pharm Biotechnol 3(2): 99-115.

            Proteins and peptides that form membrane-spanning pores and channels comprise a diverse class of molecules ranging from short peptides that are unregulated and create non-selective pathways to large ion channel proteins that are highly regulated and exhibit exquisite selectivity for particular ions. The diversity of regulation and selectivity, together with recent advances in protein "re-engineering" technology, provide a strong framework on which to build custom molecules with wide-ranging biotechnological application. Here we review a selection of pore-forming peptides and proteins from a number of different species to highlight their structural and functional diversity. The current and potential uses of native and re-engineered molecules are discussed together with a novel strategy to re-engineer alpha-hemolysin to create targeted and regulable cell-killing agents termed proimmunolysins. Numerous pore-forming peptides are currently in development as antimicrobial agents with potential application as anti-tumorigenic agents. In addition to their roles as biotherapeutic agents, pore-forming proteins are also being developed as biosensors for a range of different analytes. Recent examples of this technology include the use of alpha-hemolysin with an adapter molecule to create sensors for organic molecules and gramicidin as a general-purpose sensor for a range of analytes. These approaches promise to deliver a configurable binding site for analytes encoded in a readily measured electrical signal. The number of applications for pore-forming molecules is sure to grow in both quantity and diversity with increased knowledge of the fundamental structure and function of pores.

Otomo, S. (2002). "[Hair growth effect of minoxidil]." Nippon Yakurigaku Zasshi 119(3): 167-74.

            The length and size of hair are depend on the anagen term in its hair cycle. It has been reported that the some cell growth factors, such as VEGF, FGF-5S, IGF-1 and KGF, induce the proliferation of cells in the matrix, dermal papilla and dermal papillary vascular system and increase the amount of extra cellular matrix in dermal papilla and then maintain follicles in the anagen phase. On the other hand, negative factors, like FGF-5, thrombospondin, or still unknown ones, terminate the anagen phase. If the negative factors become dominant against cell proliferation factors according to fulfilling some time set by the biological clock for hair follicles, TGF beta induced in the matrix tissues evokes apoptosis of matrix cells and shifts the follicles from anagen to catagen. Androgenetic alopecia is caused by miniaturizing of hair follicles located in the frontal or crown part of scalp and are hereditarily more sensitive to androgen. In their hair cycles, the androgen shortens the anagen phase of follicles and shifts them to the catagen phase earlier than usual. The mode of action of hair growth effect of minoxidil is not completely elucidated, but the most plausible explanation proposed here is that minoxidil works as a sulfonylurea receptor (SUR) activator and prolongs the anagen phase of hair follicles in the following manner: minoxidil (1) induces cell growth factors such as VEGF, HGF, IGF-1 and potentiates HGF and IGF-1 actions by the activation of uncoupled SUR on the plasma membrane of dermal papilla cells, (2) inhibits of TGF beta induced apoptosis of hair matrix cells by opening the Kir 6.0 channel pore coupled with SUR on the mitochondrial inner membrane, and (3) dilates hair follicle arteries and increases blood flow in dermal papilla by opening the Kir 6.0 channel pore coupled with SUR on the plasma membrane of vascular smooth muscle cells.

O'Connell, A. D., M. J. Morton, et al. (2002). "Two-pore domain K+ channels-molecular sensors." Biochim Biophys Acta 1566(1-2): 152-61.

            Two-pore domain K(+) (K2P) channels have been cloned from a variety of species and tissues. They have been characterised biophysically as a 'background' K(+)-selective conductance and are gated by pH, stretch, heat, coupling to G-proteins and anaesthetics. Whilst their precise physiological function is unknown, they are likely to represent an increasingly important family of membrane proteins.

Noebels, J. L. (2002). "Sodium channel gene expression and epilepsy." Novartis Found Symp 241: 109-20; discussion 120-3, 226-32.

            Na+ channelopathies that prolong membrane depolarization lead to neuronal bursting, abnormal network synchronization, and various patterns of episodic neurological disorders, including epilepsy. Two distinct pathways exist for generating epileptic phenotypes based on inherited disorders of voltage-gated Na+ ion channels. The first pathway is direct, involving mutations in genes encoding the pore-forming alpha1 and regulatory beta subunits of the channel that directly alter current amplitude or kinetics. These mutations favour repetitive firing and network hyperexcitability, although often the circuits most vulnerable to functional alterations are not easy to identify and the emergent clinical phenotypes are difficult to predict. The second pathway involves mutation of other genes that lead to downstream modifications in Na+ channel expression. Two clinically relevant examples of localization-related vulnerability in brain are described that illustrate how specific phenotypes arise from both direct and secondary pathways. Selective expression of the cardiac SCN5A channel within limbic regions of brain may explain why mutation of the gene for this tetrodotoxin-insensitive current may be associated with seizures. Ectopic expression of type II Na+ channels along axonal internodes in hypomyelinated brain may reveal why deletion of the myelin basic protein gene leads to subcortical seizure patterns. Analysis of these models offers insight into developmental processes that control the cellular expression and plasticity of Na+ channel genes, and will help to clarify mechanisms of hereditary Na+ channel-based epileptogenesis.

Nerbonne, J. M. and W. Guo (2002). "Heterogeneous expression of voltage-gated potassium channels in the heart: roles in normal excitation and arrhythmias." J Cardiovasc Electrophysiol 13(4): 406-9.

            In the mammalian myocardium, there are marked regional differences in action potential waveforms and frequency dependences. This heterogeneity impacts the normal dispersion of ventricular repolarization and appears to reflect the differential expression of voltage-gated K+ channels. Multiple types of voltage-gated K+ currents have been distinguished in mammalian ventricles and, in many cases, the K+ (Kv) channel pore-forming (alpha) and accessory (beta) subunits encoding these channels have been identified. In the diseased myocardium, remodeling of voltage-gated K+ currents occurs, influencing propagation and rhythmicity, effects that can lead to increased dispersion of ventricular repolarization and create substrates for reentrant arrhythmias. Targeting the K+ channels that function to maintain the normal dispersion of ventricular repolarization could be effective in treating cardiac arrhythmias.

Montal, M. and S. J. Opella (2002). "The structure of the M2 channel-lining segment from the nicotinic acetylcholine receptor." Biochim Biophys Acta 1565(2): 287-93.

            The structures of functional peptides corresponding to the predicted channel-lining M2 segment of the nicotinic acetylcholine (AChR) were determined using solution NMR experiments on micelle samples, and solid-state NMR experiments on bilayer samples. The AChR M2 peptide forms a straight transmembrane alpha-helix, with no kinks. M2 inserts in the lipid bilayer at an angle of 12 degrees relative to the bilayer normal, with a rotation about the helix long axis such that the polar residues face the N-terminus of the peptide, which is assigned to be intracellular. A molecular model of the AChR channel pore, constructed from the solid-state NMR 3-D structure of the AChR M2 helix in the membrane assuming a pentameric organization, results in a funnel-like architecture for the channel with the wide opening on the N-terminal intracellular side. A central narrow pore has a diameter ranging from about 3.0 A at its narrowest, to 8.6 A at its widest. Nonpolar residues are predominantly on the exterior of the bundle, while polar residues line the pore. This arrangement is in fair agreement with evidence collected from permeation, mutagenesis, affinity labeling and cysteine accessibility measurements. A pentameric M2 helical bundle may, therefore, represent the structural blueprint for the inner bundle that lines the channel of the nicotinic AChR.

Mitra, A. K., G. Ren, et al. (2002). "The architecture of a water-selective pore in the lipid bilayer visualized by electron crystallography in vitreous ice." Novartis Found Symp 245: 33-46; discussion 46-50; 165-8.

            The water-selective pathway through aquaporin 1 (AQP1) membrane channel has been visualized by fitting an atomic model to a 3.7 A resolution three-dimensional density map. This map was determined by analysing images and electron diffraction patterns of lipid-reconstituted two-dimensional crystals of AQP1 preserved in vitrified buffer in the absence of any additive. The aqueous pathway in a monomer is characterized by a size-selective, to approximately 4.0 +/- 0.5 A wide pore that spans a length of to approximately 18 and bends by to approximately 25 degrees as it traverses the bilayer. This narrow pore is connected by wide, funnel-shaped openings at the extracellular and cytoplasmic faces, and is outlined mostly by hydrophobic residues interspersed with short stretches of polar amino acids, which results in relatively inert pathway conducive to diffusion-limited water flow. Although not visible at the current resolution, the 3D structure suggests putative binding sites for water molecules in the size-selective pore.

Maisterrena, B., K. Fiaty, et al. (2002). "Formulation of a coupled mechanism between solute diffusion, phosphatase-kinase reactions and membrane potentials for the primary active transport of phosphorylated substrates through biological membranes." Prog Biophys Mol Biol 80(3): 109-37.

            Coupled interrelations occurring between a phosphatase/kinase reaction sequence acting in unstirred layers and on both sides of a charged biomembrane pore structure are presented as a plausible kinetic model for the primary active transport of phosphorylated molecules. Simulations conducted at the cell level and with credible numerical values demonstrate that the enzymes positions strongly regulate the membrane permeability for the transported substrate. Depending on both the enzymes positions (more or less far from the membrane) and the membrane charges, the membrane may appear either impervious, either permeable or able to actively transport a phosphorylated substrate. Globally all happens as if, in function of the enzymes positions, a permanent pore may be regulated, changing from a more closed to a more open conformation.

Lim, M. L., M. G. Lum, et al. (2002). "On the release of cytochrome c from mitochondria during cell death signaling." J Biomed Sci 9(6 Pt 1): 488-506.

            Mitochondria play key roles in apoptosis, a central step being the release of cytochrome c (cyt c) across the outer mitochondrial membrane into the cytoplasm. We review this process in terms of the influences that induce mitochondria to release cyt c, the possible mechanisms of such release and the downstream consequences for caspase activation. The contributions of members of the Bcl-2 family in regulating mitochondrial activities relevant to apoptotic signaling are considered. Antiapoptotic members, such as Bcl-2 itself, are antagonistic to other family members, which prominently include Bax amongst a host of other proapoptotic proteins homologous to Bcl-2. Focus is placed on technical methods of determining cyt c release, which encompass cell fractionation, biochemistry, immunochemistry and confocal microscopy [including observations of release in real time using cyt c-green fluorescent protein (GFP) fusion proteins]. The advantages and potential pitfalls of the various approaches are discussed, with some emphasis on the use of cyt c-GFP fusions and the determination of the characteristics of the putative outer membrane pore through which cyt c and other mitochondrial proteins with proapoptotic functions may pass. The richness of this field relating to mitochondria and cell death is brought out by consideration of studies carried out in mammalian and yeast cells.

Law, R. J. and M. S. Sansom (2002). "Water transporters: how so fast yet so selective?" Curr Biol 12(7): R250-2.

            A high-resolution X-ray structure of an aquaporin has revealed water molecules bound within the transmembrane pore and provided new clues to the mechanisms of rapid water transport and high selectivity in this important class of membrane proteins.

Lang, T. and R. Jahn (2002). "Fusion of vacuoles--where are the membranes, and where are the holes?" Dev Cell 2(3): 257-9.

            In the February 8th issue of Cell, Wang et al. report the surprising finding that vacuolar fusion occurs at the periphery of the contact area of the vacuoles and not by the expansion of a central fusion pore. During fusion, a disk of boundary membrane is excised and left behind within the fused vacuoles.

Lacy, D. B. and R. J. Collier (2002). "Structure and function of anthrax toxin." Curr Top Microbiol Immunol 271: 61-85.

            Anthrax toxin is a binary A-B toxin comprised of protective antigen (PA) and two enzymatic moieties, edema factor (EF) and lethal factor (LF). In the presence of a host cell-surface receptor, PA can mediate the delivery of EF and LF from the extracellular milieu into the host cell cytosol to effect toxicity. In this delivery, PA undergoes multiple structural changes--from a monomer to a heptameric prepore to a membrane-spanning heptameric pore. The catalytic factors also undergo dramatic structural changes as they unfold to allow for their translocation across the endosomal membrane and refold to preserve their catalytic activity within the cytosol. In addition to these gross structural changes, the intoxication mechanism depends on the ability of PA to form specific interactions with the host cell receptor, EF, and LF. This chapter presents a review of experiments probing these structural interactions and rearrangements in the hopes of gaining a molecular understanding of toxin action.

Kuvichkin, V. V. (2002). "DNA-lipid interactions in vitro and in vivo." Bioelectrochemistry 58(1): 3-12.

            The data on lipid-nucleic interactions and their role in vitro and in vivo are presented. The results of study of DNA-lipid complexes in absence and in presence of divalent metal cations (triple complexes) are discussed. The triple complexes represent the generation of cellular structures such as pore complexes of eucaryotes and "Bayer's junctions" of procaryotes. The participation of triple complexes in the formation of structure of bacterial and eucaryotic nucleoid and nuclear matrix is analysed. A model of formation of triple complexes and cellular structures and their role in DNA-lipid interactions are discussed.

Kemp, P. J., A. Lewis, et al. (2002). "Airway chemotransduction: from oxygen sensor to cellular effector." Am J Respir Crit Care Med 166(12 Pt 2): S17-24.

            The process of sensing, transducing, and acting on environmental cues is critical to normal physiologic function. Furthermore, dysfunction of this process can lead to the development of disease. This is especially true of the homeostatic mechanisms that have evolved to maintain the carriage of O2 to respiring tissues during acute hypoxic challenge. During periods of reduced O2 availability, three major mechanisms act conjointly to increase ventilation and optimize the ventilation-perfusion ratio throughout the lung by directing pulmonary blood flow to better ventilated areas of the lung. These mechanisms are as follows: (1) increased carotid sinus nerve discharge rate to the respiratory centers of the brain, (2) intrinsic hypoxic vasoconstriction of pulmonary resistance vessels, and (3) potential local and central modulation via stimulation of neuroepithelial bodies of the lung. The key to the rapid response to the O2 signal is the ability of each of these tissues to sense acutely the changes in PO2, to transduce the signal, and for cellular effectors to initiate compensatory mechanisms that will offset rapidly the reduction in PO2 before O2 availability to tissues is compromised. This review concentrates on the signal transduction mechanism that links altered PO2 to depolarization in the recently proposed airway chemosensory element, the neuroepithelial body (and its immortalized cellular counterpart, the H146 cell line), and discusses the pertinent similarities and differences that exist between airway, carotid body, and pulmonary arteriolar O2 sensing.

Jena, B. P. (2002). "Fusion pore in live cells." News Physiol Sci 17: 219-22.

            Earlier electrophysiological measurements on live secretory cells suggested the presence of fusion pores at the plasma membrane, where secretory vesicles fuse to release vesicular contents. Recent studies using atomic force microscopy demonstrate for the first time the presence of the fusion pore and reveal its morphology and dynamics at near-nanometer resolution and in real time.

Isom, L. L. (2002). "Beta subunits: players in neuronal hyperexcitability?" Novartis Found Symp 241: 124-38; discussion 138-43, 226-32.

            Voltage-gated Na+ channels are glycoprotein complexes responsible for initiation and propagation of action potentials in excitable cells such as central and peripheral neurons, cardiac and skeletal muscle myocytes, and neuroendocrine cells. Mammalian Na+ channels are heterotrimers, composed of a central, pore-forming a subunit and two auxiliary beta subunits. The a subunits form a gene family with at least 10 members. Mutations in alpha subunit genes have been linked to paroxysmal disorders such as epilepsy, long QT syndrome, and hyperkalaemic periodic paralysis in humans, and motor endplate disease and cerebellar ataxia in mice. Three genes encode Na + channel beta subunits with at least one alternative splice product. A mutation in the beta1 subunit gene has been linked to generalized epilepsy with febrile seizures plus type 1 (GEFS+1) in a human family with this disease. Na+ channel beta subunits are multifunctional. They modulate channel gating and regulate the level of channel expression at the plasma membrane. More recently, they have been shown to function as cell adhesion molecules in terms of interaction with extracellular matrix, regulation of cell migration, cellular aggregation, and interaction with the cytoskeleton. Structure-function studies have resulted in the preliminary assignment of functional domains in the beta1 subunit. A Na+ channel signalling complex is proposed that involves beta subunits as channel modulators as well as cell adhesion molecules, other cell adhesion molecules such as neurofascin and contactin, RPTPbeta, and extracellular matrix molecules such as tenascin.

Isom, L. L. (2002). "The role of sodium channels in cell adhesion." Front Biosci 7: 12-23.

            Voltage-gated sodium channels are unique in that they combine action potential conduction with cell adhesion. Mammalian sodium channels are heterotrimers, composed of a central, pore-forming alpha subunit and two auxiliary beta subunits. The alpha subunits are members of a large gene family containing the voltage-gated sodium, potassium, and calcium channels. Sodium channel alpha subunits form a gene subfamily with at least eleven members. Mutations in sodium channel alpha subunit genes have been linked to paroxysmal disorders such as epilepsy, long QT syndrome (LQT), and hyperkalemic periodic paralysis in humans, and motor endplate disease and cerebellar ataxia in mice. Three genes encode the sodium channel beta subunits with at least one alternative splice product. Unlike the pore-forming alpha subunits, the sodium channel beta subunits are not structurally related to beta subunits of calcium and potassium channels. Sodium channel beta subunits are multifunctional. They modulate channel gating and regulate the level of channel expression at the plasma membrane. We have shown that beta subunits also function as cell adhesion molecules (CAMs) in terms of interaction with extracellular matrix molecules, regulation of cell migration, cellular aggregation, and interaction with the cytoskeleton. A mutation in SCN1B has been shown to cause GEFS+1 epilepsy in human families. We propose that the sodium channel signaling complex at nodes of Ranvier involves beta subunits as channel modulators as well as CAMs, other CAMs such as neurofascin and contactin, RPTPbeta, and extracellular matrix molecules such as tenascin. Finally, we explore other subunits of voltage-gated ion channels as potential CAM candidates.

Humlova, Z. (2002). "Protooncogene bcl-2 in process of apoptosis. Review article." Sb Lek 103(4): 419-25.

            The process of apoptosis is genetically regulated form of cell death, which is tightly connected, with maintaining of tissue homeostasis in multicellular organisms. Mitochondria play a key role in this process being involved in ATP synthesis, production of oxygen free radicals, control of Ca2+ ions, extrusion of apoptogenic molecules such as cytochrome c, apoptosis inducing factor, Smac/DIABLO protein and several procaspases. Changes in the flux of ions and water across the inner mitochondrial membrane characterize the early phase of apoptosis, during which an increase in matrix volume may precede a collapse of mitochondrial membrane potential (delta psi m). These changes are suppressed by Bcl-2/Bcl-XL facilitated by Bax, and mediated at least by so-called permeability transition pore complex which is one of possible mechanisms involved in mitochondrial membrane permeabilization (MMP).

Horn, R. (2002). "Molecular basis for function in sodium channels." Novartis Found Symp 241: 21-6; discussion 26-33, 226-32.

            Na+ channels earned their unique role in excitable cells because of two functional properties, finely honed by evolution. The first is their exquisite sensitivity to small changes of membrane potential: a depolarization of only 10 mV can increase open probability by as much as two orders of magnitude. The second is the rapidity with which they respond to changes of membrane potential: their gates begin to open tens of microseconds after a depolarization. These features are built into two sets of moving parts: voltage sensors that respond directly to changes of membrane potential, and gates that open and close in response to voltage sensor movement. We have explored these movements using a combination of electrophysiology, site-directed mutagenesis, cysteine accessibility scanning and photoactivated cross-linking using a bifunctional cysteine reagent. The main voltage sensors of Na+ channels are four homologous S4 segments, each of which has a unique functional role. These transmembrane segments are almost completely surrounded by hydrophilic crevices. The membrane electric field moves these positively charged helices through a short, hydrophobic 'gating pore'. The minimum contact between an S4 segment and its gating pore insure that a small movement can rapidly move several of its charged residues across the electric field.

Holaska, J. M., K. L. Wilson, et al. (2002). "The nuclear envelope, lamins and nuclear assembly." Curr Opin Cell Biol 14(3): 357-64.

            The nuclear lamina is composed of both A- and B-type lamins and lamin-binding proteins. Many lamin-binding proteins are integral proteins of the inner nuclear membrane. Lamins and inner nuclear membrane proteins are important for a variety of cell functions, including nuclear assembly, replication, transcription, and nuclear integrity. Recent advances in the field in the past year include the identification of a family of spectrin-repeat-containing inner nuclear membrane proteins and other novel inner-membrane proteins, and the discovery of a nuclear membrane fusion complex. There is also growing evidence that A- and B-type lamins and their binding partners have distinct roles during nuclear assembly and interphase.

Hoffmann, A., U. Pag, et al. (2002). "Combination of antibiotic mechanisms in lantibiotics." Farmaco 57(8): 685-91.

            Recent studies on the mode of action have revealed exciting features of multiple activities of nisin and related lantibiotics making these peptides interesting model systems for the design of new antibiotics (Molec. Microbiol. 30 (1998) 317; Science 286 (1999) 2361; J. Biol. Chem. 276 (2001) 1772.). In contrast to other groups of antibiotic peptides, the lantibiotics display a substantial degree of specificity for particular components of bacterial membranes. Mersacidin and actagardine were shown to bind with high affinity to the lipid coupled peptidoglycan precursor, the so-called lipid II, which prevents the polymerisation of the cell wall monomers into a functional murein sacculus. The lantibiotics nisin and epidermin also bind tightly to this cell wall precursor; however, for these lantibiotics the binding of lipid II has two consequences. Like with mersacidin blocking of lipid II inhibits peptidoglycan biosynthesis; in addition, lipid II is used as a specific docking molecule for the formation of pores. This combination of lethal effects explains the potency of these peptides, which are active in nanomolar concentration. Other type-A lantibiotics are believed to also use docking molecules for pore formation, although identification of such membrane components has not yet been achieved.

Hoenderop, J. G., B. Nilius, et al. (2002). "Molecular mechanism of active Ca2+ reabsorption in the distal nephron." Annu Rev Physiol 64: 529-49.

            The identification of the epithelial Ca(2+) channel (ECaC) complements the group of Ca(2+) transport proteins including calbindin-D28K, Na(+)/Ca(2+) exchanger and plasma membrane Ca(2+)-ATPase, which are co-expressed in 1,25(OH)2D3- responsive nephron segments. ECaC constitutes the rate-limiting apical entry step in the process of active transcellular Ca(2+) transport and belongs to a superfamily of Ca(2+) channels that includes the vanilloid receptor and transient receptor potential channels. This new Ca(2+) channel consists of six transmembrane-spanning domains, including a pore-forming hydrophobic stretch between domain 5 and 6. The C- and N-terminal tails contain several conserved regulatory sites, implying that the channel function is modulated by regulatory proteins. The distinctive functional properties of ECaC include a constitutively activated Ca(2+) permeability, a high selectivity for Ca(2+), hyperpolarization-stimulated and Ca(2+)-dependent feedback regulation of channel activity, and 1,25(OH)2D3-induced gene activation. This review covers the distinctive properties of this new highly Ca(2+)-selective channel and highlights the implications for active transcellular Ca(2+) reabsorption in health and disease.

Heuck, A. P. and A. E. Johnson (2002). "Pore-forming protein structure analysis in membranes using multiple independent fluorescence techniques." Cell Biochem Biophys 36(1): 89-101.

            A large number of transmembrane proteins form aqueous pores or channels in the phospholipid bilayer, but the structural bases of pore formation and assembly have been determined experimentally for only a few of the proteins and protein complexes. The polypeptide segments that form the transmembrane pore and the secondary structure that creates the aqueous-lipid interface can be identified using multiple independent fluorescence techniques (MIFT). The information obtained from several different, but complementary, fluorescence analyses, including measurements of emission intensity, fluorescence lifetime, accessibility to aqueous and to lipophilic quenching agents, and fluorescence resonance energy transfer (FRET) can be combined to characterize the nature of the protein-membrane interaction directly and unambiguously. The assembly pathway can also be determined by measuring the kinetics of the spectral changes that occur upon pore formation. The MIFT approach therefore allows one to obtain structural information that cannot be obtained easily using alternative techniques such as crystallography. This review briefly outlines how MIFT can reveal the identity, location, conformation, and topography of the polypeptide sequences that interact with the membrane.

Henter, J. I. (2002). "Biology and treatment of familial hemophagocytic lymphohistiocytosis: importance of perforin in lymphocyte-mediated cytotoxicity and triggering of apoptosis." Med Pediatr Oncol 38(5): 305-9.

            Familial hemophagocytic lymphohistiocytosis (FHL) is, without treatment, an invariably fatal disease of infancy and early childhood characterized by fever, hepatosplenomegaly, pancytopenia, and a widespread accumulation of T-lymphocytes and macrophages. During recent years, the diagnosis and the survival as well as the understanding of the disease have improved dramatically. Recent studies suggest that FHL is caused by impaired lymphocyte-mediated cytotoxicity and defective triggering of apoptosis, and that the symptoms are mediated by a pro-inflammatory hypercytokinemia. Moreover, specific genetic alterations, mutations in the perforin gene, have been revealed in FHL patients. Perforin, which normally is secreted from cytotoxic T-lymphocytes and natural killer (NK) cells upon conjugation between effector and target cells, is able to insert into the membrane of the target cell. It there polymerizes to form a cell death-inducing pore through which toxic granzymes may enter the cell and trigger apoptosis. The establishment of perforin deficiency as a cause of the rapidly fatal disease FHL has demonstrated the essential role of perforin in human immune homeostasis.

Heales, S. J. and J. P. Bolanos (2002). "Impairment of brain mitochondrial function by reactive nitrogen species: the role of glutathione in dictating susceptibility." Neurochem Int 40(6): 469-74.

            Mitochondrial enzymes involved in energy metabolism display varying degrees of sensitivity towards reactive nitrogen species such as peroxynitrite (ONOO-). With regards to the electron transport chain, cytochrome oxidase appears particularly sensitive. Inhibition of this component may lead to an increase in mitochondrial superoxide formation, exacerbation of cellular oxidative stress and further mitochondrial damage. Impairment of the electron transport chain may lead to a loss of membrane potential, ATP deficiency, opening of the permeability transition pore and the release of factors capable of initiating apoptosis. Reduced glutathione will react, via a number of diverse reactions, with reactive nitrogen species and hence is capable of limiting mitochondiral damage. Loss of brain glutathione may therefore be an important factor in those neurological conditions in which there is evidence of excessive nitric oxide formation and mitochondrial damage.

Hanlon, M. R. and B. A. Wallace (2002). "Structure and function of voltage-dependent ion channel regulatory beta subunits." Biochemistry 41(9): 2886-94.

            Voltage-dependent K(+), Ca(2+), and Na(+) channels play vital roles in basic physiological processes, including management of the action potential, signal transduction, and secretion. They share the common function of passively transporting ions across cell membranes; thus, it would not be surprising if they should exhibit similarities of both structure and mechanism. Indeed, the principal pore-forming (alpha) subunits of each show either exact or approximate 4-fold symmetry and share a similar transmembrane topology, and all are gated by changes in membrane potential. Furthermore, these channels all possess an auxiliary polypeptide, designated the beta subunit, which plays an important role in their regulation. Despite considerable functional semblences and abilities to interact with structurally similar alpha subunits, however, there is considerable structural diversity among the beta subunits. In this review, we discuss the similarities and differences in the structures and functions of the beta subunits of the voltage-dependent K(+), Ca(2+), and Na(+) channels.

Halestrap, A. P., G. P. McStay, et al. (2002). "The permeability transition pore complex: another view." Biochimie 84(2-3): 153-66.

            Mitochondria play a critical role in initiating both apoptotic and necrotic cell death. A major player in this process is the mitochondrial permeability transition pore (MPTP), a non-specific pore, permeant to any molecule of < 1.5 kDa, that opens in the inner mitochondrial membrane under conditions of elevated matrix [Ca(2+)], especially when this is accompanied by oxidative stress and depleted adenine nucleotides. Opening of the MPTP causes massive swelling of mitochondria, rupture of the outer membrane and release of intermembrane components that induce apoptosis. In addition mitochondria become depolarised causing inhibition of oxidative phosphorylation and stimulation of ATP hydrolysis. Pore opening is inhibited by cyclosporin A analogues with the same affinity as they inhibit the peptidyl-prolyl cis-trans isomerase activity of mitochondrial cyclophilin (CyP-D). These data and the observation that different ligands of the adenine nucleotide translocase (ANT) can either stimulate or inhibit pore opening led to the proposal that the MPTP is formed by a Ca-triggered conformational change of the ANT that is facilitated by the binding of CyP-D. Our model is able to explain the mode of action of a wide range of known modulators of the MPTP that exert their effects by changing the binding affinity of the ANT for CyP-D, Ca(2+) or adenine nucleotides. The extensive evidence for this model from our own and other laboratories is presented, including reconstitution studies that demonstrate the minimum configuration of the MPTP to require neither the voltage activated anion channel (VDAC or porin) nor any other outer membrane protein. However, other proteins including Bcl-2, BAX and virus-derived proteins may interact with the ANT to regulate the MPTP. Recent data suggest that oxidative cross-linking of two matrix facing cysteine residues on the ANT (Cys(56) and Cys(159)) plays a key role in regulating the MPTP. Adenine nucleotide binding to the ANT is inhibited by Cys(159) modification whilst oxidation of Cys(56) increases CyP-D binding to the ANT, probably at Pro(61).

Goldmacher, V. S. (2002). "vMIA, a viral inhibitor of apoptosis targeting mitochondria." Biochimie 84(2-3): 177-85.

            Human cytomegalovirus encodes a powerful cell death suppressor vMIA (viral mitochondria-localized inhibitor of apoptosis), also known as pUL37x1. vMIA, a product of the immediate early gene UL37 exon 1, is predominantly localized in mitochondria, where it appears to form a complex with adenine nucleotide translocator, believed to be a component of the mitochondrial transition pore complex. vMIA suppresses apoptosis by blocking permeabilization of the mitochondrial outer membrane. Expression of vMIA protects cells against apoptosis triggered by diverse stimuli, including ligation of death receptors, exposure to certain cytotoxic drugs, and infection with an adenovirus mutant deficient in E1B19K. Deletion mutagenesis of vMIA revealed two domains that are necessary and, together, sufficient for its anti-apoptotic activity. The first domain contains a mitochondrial targeting signal. The function of the second domain is still unknown. vMIA does not share any significant amino acid sequence homology with Bcl-2, and, unlike Bcl-2 or Bcl-x(L), it does not bind BAX or VDAC. These structural and functional differences between vMIA and Bcl-2 suggest that vMIA represents a separate class of cell death suppressors. Experiments with vMIA-deficient CMV (human cytomegalovirus) mutants provide strong evidence that the anti-apoptotic function of vMIA is required to prevent CMV-induced apoptosis, and is necessary for viral replication. In addition to vMIA, UL37 encodes two longer splice-variant proteins, gpUL37 and GP37(M). Biological functions of these proteins have not yet been identified, and may be unrelated to their anti-apoptotic activity. The identification of vMIA and the finding that its anti-apoptotic function is required for CMV replication provides a rationale for the development of anti-CMV pharmaceuticals that would inactivate vMIA and thus restore apoptosis in cells infected with CMV.

Gilbert, R. J. (2002). "Pore-forming toxins." Cell Mol Life Sci 59(5): 832-44.

            Pore-forming toxins are widely distributed proteins which form lesions in biological membranes. In this review, bacterial pore-forming toxins are treated as a paradigm and discussed in terms of the structural principles on which they work. Then, a large family of bacterial toxins, the cholesterol-binding toxins, are analyzed in depth to provide an overview of the processes involved in pore formation. The ways in which the cholesterol-binding toxins (cholesterol-dependent cytolysins) interact with membranes and form pores, the structure of the monomeric soluble and oligomeric pore-forming states, and the effects of the toxin on membrane structure are discussed. By surveying the range of work which has been done on cholesterol-binding toxins, a working model is elaborated which reconciles two current, apparently diametrically opposed, models for their mechanism.

Gerke, V. and S. E. Moss (2002). "Annexins: from structure to function." Physiol Rev 82(2): 331-71.

            Annexins are Ca2+ and phospholipid binding proteins forming an evolutionary conserved multigene family with members of the family being expressed throughout animal and plant kingdoms. Structurally, annexins are characterized by a highly alpha-helical and tightly packed protein core domain considered to represent a Ca2+-regulated membrane binding module. Many of the annexin cores have been crystallized, and their molecular structures reveal interesting features that include the architecture of the annexin-type Ca2+ binding sites and a central hydrophilic pore proposed to function as a Ca2+ channel. In addition to the conserved core, all annexins contain a second principal domain. This domain, which NH2-terminally precedes the core, is unique for a given member of the family and most likely specifies individual annexin properties in vivo. Cellular and animal knock-out models as well as dominant-negative mutants have recently been established for a number of annexins, and the effects of such manipulations are strikingly different for different members of the family. At least for some annexins, it appears that they participate in the regulation of membrane organization and membrane traffic and the regulation of ion (Ca2+) currents across membranes or Ca2+ concentrations within cells. Although annexins lack signal sequences for secretion, some members of the family have also been identified extracellularly where they can act as receptors for serum proteases on the endothelium as well as inhibitors of neutrophil migration and blood coagulation. Finally, deregulations in annexin expression and activity have been correlated with human diseases, e.g., in acute promyelocytic leukemia and the antiphospholipid antibody syndrome, and the term annexinopathies has been coined.

Gentschev, I., G. Dietrich, et al. (2002). "The E. coli alpha-hemolysin secretion system and its use in vaccine development." Trends Microbiol 10(1): 39-45.

            Many Gram-negative bacteria use a type I secretion system to translocate proteins, including pore-forming toxins, proteases, lipases and S-layer proteins, across both the inner and outer membranes into the extracellular surroundings. The Escherichia coli alpha-hemolysin (HlyA) secretion system is the prototypical and best characterized type I secretion system. The structure and function of the components of the HlyA secretion apparatus, HlyB, HlyD and TolC, have been studied in great detail. The functional characteristics of this secretion system enable it to be used in a variety of different applications, including the presentation of heterologous antigens in live-attenuated bacterial vaccines. Such vaccines can be an effective delivery system for heterologous antigens, and the use of a type I secretion system allows the antigens to be actively exported from the cytoplasm of the bacterial carrier rather than only becoming accessible to the host immune system after bacterial disintegration.

Friberg, H. and T. Wieloch (2002). "Mitochondrial permeability transition in acute neurodegeneration." Biochimie 84(2-3): 241-50.

            Acute neurodegeneration in man is encountered during and following stroke, transient cardiac arrest, brain trauma, insulin-induced hypoglycemia and status epilepticus. All these severe clinical conditions are characterized by neuronal calcium overload, aberrant cell signaling, generation of free radicals and elevation of cellular free fatty acids, conditions that favor activation of the mitochondrial permeability transition pore (mtPTP). Cyclosporin A (CsA) and its analog N-methyl-valine-4-cyclosporin A (MeValCsA) are potent blockers of the mtPTP and protect against neuronal death following excitotoxicity and oxygen glucose deprivation. Also, CsA and MeValCsA diminish cell death following cerebral ischemia, trauma, and hypoglycemia. Here we present data that strongly imply the mtPT in acute neurodegeneration in vivo. Compounds that readily pass the blood-brain-barrier (BBB) and block the mtPT may be neuroprotective in stroke.

Firth, J. A. (2002). "Endothelial barriers: from hypothetical pores to membrane proteins." J Anat 200(6): 541-8.

            The anatomical counterpart of the physiologically defined small pore system of capillary endothelia has proved difficult to establish. In non-brain continuous capillaries, the contributions of caveolar and transmembrane pathways are likely to be small and paracellular clefts are probably the dominant routes. Analogy with epithelial paracellular pathways suggests that tight junctions may be the most restrictive elements. However, structural features of tight junction-based models are incompatible with physiological data; it is more likely that the tight junction acts as a shutter limiting the available cleft area. Proposed molecular sieves elsewhere in the paracellular pathway include the glycocalyx and the cadherin-based complexes of the adherens junctions. The molecular architecture of tight junctions and adherens junctions is moderately well defined in terms of molecular species, and there are differences at both sites between the endothelial and epithelial spectra of protein expression. However, definition of the size-restricting pore remains elusive and may require structural biology approaches to the spatial arrangements and interactions of the membrane molecular complexes surrounding the endothelial paracellular clefts.

Findlay, J. B. and M. A. Harrison (2002). "A protein chemical approach to channel structure and function: the proton channel of the vacuolar H(+)-ATPase." Novartis Found Symp 245: 207-18; discussion 218-22, 261-4.

            The vacuolar H(+)-ATPase provides a channel through which protons can be pumped across the bilayer. It is a complex assembly of about 20 subunits made up from 13 different polypeptide chains. The proton channel is located in the bilayer and therefore must be formed from one or both of the two intramembraneous subunits, called in yeast Vphlp (100 kDa) and Vma3p (16 kDa). Electron microscopy and the use of water soluble and hydrophobic chemical probes in conjunction with mutagenesis to cysteine or glutamic acid residues, suggest that the membrane sector consists of a single Vphlp subunit in association with a hexameric complex of the four-helical bundle Vma3p subunit. This hexamer encloses a large central pore lined by the first transmembrane helix. This pore appears to be impermeable, however; instead, a glutamic acid residue critical to transport function is located on the outside of the hexamer, deeply buried in the membrane and accessible to probes and inhibitors resident in the hydrophobic phase of the bilayer. The stoichiometry and chemistry of inhibitor binding supports the postulate that the mechanism of action involves rotation of the hexamer in the plane of the bilayer. Mutagenesis data on the Vphlp subunit indicate that it is vital to proton transport. It is likely, therefore, that the proton channel is formed at the interface of the Vphlp and Vma3p subunits, the protons moving via a network of interacting charged amino acid side-chains.

Estevez, R. and T. J. Jentsch (2002). "CLC chloride channels: correlating structure with function." Curr Opin Struct Biol 12(4): 531-9.

            CLC chloride channels form a large gene family that is found in bacteria, archae and eukaryotes. Previous mutagenesis studies on CLC chloride channels, combined with electrophysiology, strongly supported the theory that these channels form a homodimeric structure with one pore per subunit (a'double-barrelled' channel), and also provided clues about gating and permeation. Recently, the crystal structures of two bacterial CLC proteins have been obtained by X-ray diffraction analysis. They confirm the double-barrelled architecture, and reveal a surprisingly complex and unprecedented channel structure. At its binding site in the pore, chloride interacts with the ends of four helices that come from both sides of the membrane. A glutamate residue that protrudes into the pore is proposed to participate in gating. The structure confirms several previous conclusions from mutagenesis studies and provides an excellent framework for their interpretation.

Deutsch, C. (2002). "Potassium channel ontogeny." Annu Rev Physiol 64: 19-46.

            Potassium channels are multi-subunit complexes, often composed of several polytopic membrane proteins and cytosolic proteins. The formation of these oligomeric structures, including both biogenesis and trafficking, is the subject of this review. The emphasis is on events in the endoplasmic reticulum (ER), particularly on how, where, and when K(+) channel polypeptides translocate and integrate into the bilayer, oligomerize and fold to form pore-forming units, and associate with auxiliary subunits to create the mature channel complex. Questions are raised with respect to the sequence of these events, when biogenic decisions are made, models for integration of K(+) channel transmembrane segments, crosstalk between the cell surface and ER, and recognition of compatible partner subunits. Also considered are determinants of subunit composition and stoichiometry, their consequence for trafficking, mechanisms for ER retention and export, and sequence motifs that direct channels to the cell surface. It is these mechanistic issues that govern the differential distributions of K(+) conductances at the cell surface, and hence the electrical activity of cells and tissues underlying both the physiology and pathophysiology of an organism.

Delcour, A. H. (2002). "Structure and function of pore-forming beta-barrels from bacteria." J Mol Microbiol Biotechnol 4(1): 1-10.

            Crystallographic studies of the past ten years have revealed that many outer membrane proteins and bacterial toxins are constructed on the beta-barrel motif. Two structural classes can be identified. The first class, represented by the porins, includes monomeric or multimeric proteins where each beta-barrel is formed from a single polypeptide. The second class features proteins where the beta-barrel is itself a multimeric assembly, to which each subunit contributes a few beta-strands. In addition to structural investigations, much work has also been devoted to the functional aspects of these proteins, and to the relationships between structure and function. Here we present a review of the structural and the functional properties of some of the best-studied examples of these various classes of proteins, namely the general-diffusion, specific and ligand-gated porins, multidrug efflux proteins and the staphylococcal toxin alpha-hemolysin.

Crompton, M., E. Barksby, et al. (2002). "Mitochondrial intermembrane junctional complexes and their involvement in cell death." Biochimie 84(2-3): 143-52.

            Mitochondria establish contact sites between the inner and outer membranes. The contact sites are held together by junctional complexes of the adenine nucleotide translocase (ANT; inner membrane) and the voltage-dependent anion channel (VDAC; outer membrane). The junctional complexes act as multifunctional recruitment centres, binding a range of proteins according to the function to be executed. Some of these, involving kinases and enzymes of lipid transfer, are readily understood as ongoing functions in energy and lipid metabolism. But the roles of other proteins recruited to the junctional complexes are less well defined. Here, we focus on the complexes formed with Bax and with cyclophilin-D, and their possible roles in apoptotic and necrotic cell death. We have isolated both types of complexes using glutathione-S-transferase fusion proteins of Bax and of cyclophilin-D. The VDAC/ANT/cyclophilin-D complex reconstitutes Ca(2+)- and cyclosporin A-sensitive permeability transition pore activity when incorporated into proteoliposomes. The complex forms readily in the absence of factors required for pore opening in isolated mitochondria, suggesting that these factors act on the preexisting complex, rather than drive its assembly, and that the complex is a physiological entity in healthy cells.

Cornelis, G. R. (2002). "The Yersinia Ysc-Yop virulence apparatus." Int J Med Microbiol 291(6-7): 455-62.

            The Yop virulon is an integrated system allowing extracellular Yersinia adhering at the surface of a target cell to inject an array of bacterial effectors into the eukaryotic cytosol. It consists of a type III secretion apparatus, called the Ysc injectisome and an array of proteins secreted by this apparatus, called Yops. The injectisome is made of about 25 Ysc proteins. The proximal part of the injectisome resembles the basal body of the flagellum while the most distal part is made of a secretin and a small needle protruding from the bacterial surface. Three of the Yops, namely YopB, YopD and LcrV, are required for the translocation of the others across the target cell membrane. They form some kind of a pore in the target cell membrane. Four Yop effectors, YopE, YopT, YpkA and YopH disturb the cytoskeleton dynamics by targeting monomeric GTPases of the Rho family. YopP downregulates the onset of the inflammatory response by blocking the NF-kappaB and MAPK pathways. Strong arguments indicate that it is a SUMO protease. Finally, YopM has been shown to travel to the nucleus of the target cell.

Cooper, M. E., P. Mundel, et al. (2002). "Role of nephrin in renal disease including diabetic nephropathy." Semin Nephrol 22(5): 393-8.

            Nephrin, a newly described protein, has been localized to the slit membrane between adjacent podocytes of the glomerulus. Its discovery followed the demonstration of the gene NPHS1 and its mutation, resulting in the absence of the protein product, nephrin, in the congenital nephrotic syndrome of the Finnish type. The link between permutations in nephrin expression and proteinuria has been shown in animal models by using neutralizing antibodies or studying mice with inactivation of the nephrin gene. Moreover, the expression of nephrin has been shown to be reduced in various animal models of proteinuric renal disease. The relationship between changes in nephrin expression and proteinuric renal disease in humans is not fully elucidated, with a reduction in expression of this protein reported in a range of renal diseases. Diabetic nephropathy, one of the major causes of end-stage renal disease, is associated with substantial proteinuria and in experimental models with a reduction in slit pore density. In experimental models of diabetes, nephrin expression has been described as being transiently increased in the first 8 weeks of diabetes, followed in longer-term studies with reduced nephrin expression in association with increasing proteinuria. An angiotensin II-receptor blocker has been shown to prevent depletion in glomerular nephrin expression in the diabetic kidney. Human studies in both type 1 and type 2 diabetes suggest down-regulation of nephrin expression in the diabetic kidney and it has been postulated that these changes may play a role in the pathogenesis of diabetic nephropathy, specifically the development of proteinuria in this condition. Although there are other proteins involved in the structure of the epithelial podocyte and specifically the slit pore, nephrin seems to play a pivotal role in preventing passage of protein through the glomerular barrier. Furthermore, it is suggested that the antiproteinuric effects of inhibition of the renin-angiotensin system may partly relate to the effects of these agents on nephrin expression.

Choe, S., S. Cushman, et al. (2002). "Excitability is mediated by the T1 domain of the voltage-gated potassium channel." Novartis Found Symp 245: 169-75; discussion 175-7, 261-4.

            The T1 domain of voltage-gated K+ (Kv) channel is the N-terminal cytoplasmic part of the channel preceding the transmembrane pore domain of the channel. Several crystal structures of the T1 domain show that the four T1 subunits are arranged in a rotationally symmetric tetramer. The subunit interface of the T1 domain encodes the assembly specificity of intact functional Kv channels. Along the fourfold symmetry axis of the T1 tetramer, a water-filled cavity exists. K+ ions, however, do not pass through this T1 cavity. Instead, they are believed to enter the transmembrane pore through four identical inter-subunit spaces created between the membrane-facing C-terminal side of the T1 tetramer and the inner leaflet of the membrane. Several point mutations have been introduced into the putative membrane-facing region of the T1 tetramer. These mutations led to a systematic change of the channel's voltage sensitivity. Such functional change was accompanied by a distinct structural change in the C-terminal membrane-facing side of the T1 tetramer. Interestingly, a similar structural alteration that renders the channel more excitable is also induced by the binding of a cytoplasmic protein Kv beta subunit. Within this conformationally flexible part of the T1 tetramer, non-Shaker type Kv channel subunits invariably contain one Zn2+ per subunit. With the Kv4.2 T1, we demonstrated that the tetramer can be reversibly converted to monomers by chelating zinc away from the protein. The rate of removal of Zn2+ is pH-dependent. The structural ability of the T1 tetramer to alter conformation could be an essential property to mediate and process protein protein interaction events in the cytoplasm to control excitability of intact full-length Kv channels.

Catterall, W. A. (2002). "Molecular mechanisms of gating and drug block of sodium channels." Novartis Found Symp 241: 206-18; discussion 218-32.

            Voltage-gated Na+ channels are composed of an alpha subunit of 260 kDa associated with beta subunits of 33-36 kDa. Alpha subunits have four homologous domains (I to IV) containing six transmembrane alpha helices (S1-S6). The S4 segments serve as voltage sensors and move outward to initiate activation. The S5 and S6 segments and the short membrane-associated loops between them form the pore. Fast inactivation is mediated by closure of an inactivation gate formed by the intracellular loop between domains III and IV. The 3-D structure of the inactivation gate has been determined bv NMR spectroscopy, revealing the conformation of the pore-blocking IFM motif. Peptide scorpion toxins that alter gating of Na+ channels bind to the extracellular ends of the IIS4 and IVS4 segments, trap them in either an activated or non-activated position, and thereby selectively alter channel activation or inactivation. Voltage sensor-trapping may be a general mechanism of toxin action on voltage-gated ion channels. Local anaesthetics block the pore of Na+ channels by binding to a receptor site in segment S6 in domains III and IV. Anticonvulsants and antiarrhythmic drugs also interact with this site. A high-affinity Na+ channel blocker has recently been developed with this site as its target. The emerging knowledge of the molecular mechanisms of Na+ channel gating and drug block may allow development of novel therapeutics for epilepsy, cardiac arrhythmia and persistent pain syndromes.

Capener, C. E., H. J. Kim, et al. (2002). "Ion channels: structural bioinformatics and modelling." Hum Mol Genet 11(20): 2425-33.

            Ion channels are membrane proteins of key physiological and pharmacological importance. As is the case for many integral membrane proteins, X-ray structures are known for a few bacterial channels, yet structures of human homologues are required for analysis of channel-associated diseases and for drug design. Homology modelling can be used to help remedy this deficit. In combination with molecular dynamics simulations and associated calculations, modelling provides a powerful approach to understanding structure/function relationships in human ion channels. Modelling techniques have been applied to two classes of potassium channels: voltage-gated (Kv) and inward rectifier (Kir) channels. Kir channel models, based on the structure of the bacterial channel KcsA, have been used as a starting point for detailed simulation studies that have increased our understanding of ion permeation and selectivity mechanisms. The transmembrane domain of GluR0, a bacterial homologue of mammalian glutamate receptors, also may be modelled using the KcsA structure as a template. Models of the nicotinic acetylcholine receptor may be constructed in a modular fashion. The snail acetylcholine-binding protein provides a template for the extracellular ligand-binding domain. The transmembrane pore region can be modelled on the basis of NMR structures of the pore-lining M2 helix.

Bowry, S. K. (2002). "Dialysis membranes today." Int J Artif Organs 25(5): 447-60.

            In recent years, hemodialytic therapies have evolved from the simple, diffusion-dependent removal of small molecular weight substances from blood to advanced therapy modalities involving the convective removal of larger uremic sloutes. The clinical benefits of removal of substances such as beta2-microglobulin (beta2-m) have been reported by several authors: elimination of large-molecular weight "uremic toxins" is now widely accepted as being beneficial to the overall quality of life of patients. This trend would not have been possible without parallel technical developments, especially that of new membranes having more open pore structures resulting in higher sieving coefficients and incre