Studies on human porin XXII: cell membrane integrated human porin channels are involved in regulatory volume decrease (RVD) of HeLa cells

Opening up of plasmalemma type-1 VDAC to form apoptotic "find me signal" pathways is essential in early apoptosis - evidence from the pathogenesis of cystic fibrosis resulting from failure of apoptotic cell clearance followed by sterile inflammation

Cell membrane-standing type-1 VDAC is involved in cell volume regulation and thus apoptosis. The channel has been shown to figure as a pathway for osmolytes of varying classes, ATP included. An early event in apoptotic cell death is the release of "find me signals" by cells that enter the apoptotic process. ATP is one of those signals. Apoptotic cells this way attract phagocytes for an immunologically silent cell clearance. Thus, whenever apoptosis fails by a blockade of plasmalemma type-1 VDAC processes of sterile inflammation must be assumed for cell elimination. This is evident from a close look on the pathogenetic process of cystic fibrosis (CF). However, in normal airway epithelia two different anion channels cooperate to guarantee an appropriate volume of airway surface liquid (ASL) necessary for surface clearing: the cystic fibrosis conductance regulator (CFTR) and the outwardly rectifying chloride channel (ORCC) complex also called "alternate chloride channel" and under the control of the CFTR. There are arguments, that type-1 VDAC forms the channel part of the ORCC complex, and it has been shown that CFTR and type-1 VDAC co-localize in the apical membranes of human surface respiratory epithelium. In cystic fibrosis, the central cAMP-dependent regulation of ion and water transport via functional CFTR is lost. Here, CFTR molecules do not reach the apical membranes of airway epithelia anymore or work in an insufficient way, respectively. In addition, type-1 VDAC is no longer available to work as a "find me signal" pathway. In consequence, clearing away of apoptotic cells is blocked. There are experimental data on the channel characteristics of type-1 VDAC under the anion channel blocker DIDS (4,4-diisothiocyanato-stilbenedisulphonic acid) that argue in favor of this hypothesis. Together, type-1 VDAC should be kept as a "find me signal" pathway, which may give way to several classes of such signals.

New findings concerning vertebrate porin II--on the relevance of glycine motifs of type-1 VDAC

New findings concerning vertebrate porin part I was published in 1997, then summarizing early data and reflections regarding the molecular structure of vertebrate voltage-dependent anion-selective channels, VDAC/eukaryotic porin, and the extra-mitochondrial expression pattern of human type-1 VDAC. Meanwhile, endeavors of different laboratories confirmed and widened this beginning by encircling the function of the channels. Regarding the function of mitochondrial outer membrane-standing VDACs the channels are established parts of the intrinsic apoptotic pathway and thus therapeutic targets in studies on several diseases: cancer, Alzheimer's disease, Down Syndrome, Parkinson's disease, Amyotrophic Lateral Sclerosis, cystic fibrosis and malaria. Regarding cell membrane-integrated type-1 VDAC it has been documented by different approaches that this porin channel is engaged in cell volume regulation, trans-membrane electron transport and apoptosis. Furthermore, new data insinuate a bridging of extrinsic and intrinsic apoptotic pathways, putatively gaining relevance in Alzheimer research. Mammalian type-1 VDAC, a β-barrel, is basically built up by nineteen β-sheets connected by peptide stretches of varying lengths. The molecule also comprises an N-terminal stretch of some twenty amino acids which, according to biochemical data, traverses the channel lumen towards the cytosolic surface of outer mitochondrial membranes or the plasma lemma, respectively and works as voltage sensor in channel gating. In artificial lipid bilayers VDACs figure as anion or cation-channels, as VDACs are permeable to both cations and anions, with voltage shifts changing the relative permeability. Type-1 VDAC carries several motifs where glycine residues are in critical positions. Motifs of this type, on the on hand, are established nucleotide binding sites. On the other hand, the GxxxG motifs are also discussed as relevant peptide dimerization/aggregation/membrane perturbation motifs. Finally, GxxxG motifs bind cholesterol. Type-1 VDAC shows one such GxxxG motif at the proximal end of its N-terminal voltage sensor while amyloid Aβ peptides include three of them in series. Noteworthy, two additional may be modified versions, GxxxGxG and GxxGxxxG, are found on β-sheet 19 or 9, respectively. Recent data have allowed speculating that amyloid Aβ induces apoptosis via opening type-1 VDAC in cell membranes of hypo-metabolic neurons, a process most likely running over life time--as leaves fall from trees in the tropics--and ending in Alzheimer's disease whenever critical brain regions are affected. The expression of GxxxG motifs on either reactant under consideration is in line with this model of Alzheimer's disease pathogenesis, which clearly differs from the amyloid Aβ cascade theory, and which can, furthermore, be understood as a basic model for apoptosis induction. However, to assume randomly distributed interactions of body wide found amyloid Aβ peptides with the N-terminal voltage sensors of ubiquitously expressed cell membrane-standing human type-1 VDAC opens up a new view on Alzheimer's disease, which might even include a clue on systemic aspects of the disease. While elaborating this concept, my focus was at first only on the GxxxG motif at the proximal end of the N-terminal voltage sensor of type-1 VDAC. Here, I include a corresponding sequence stretch on the channel's β-sheet 19, too.

To the knowledge of the 20GYGFG24 sequence stretch of type-1 VDAC: to understand why BCl-XL B4 domain peptides keep HeLa cells closed in hypotonic surroundings

Type-1 VDAC/porin, as a part of its voltage sensor, includes a GxxxG motif sequence that has been shown to work as an ATP-binding site. The motif has also been demonstrated to function as an aggregation/membrane perturbation sequence that opens VDAC in the plasmalemma of neuronal cells in experiment on apoptosis induction. Here it is discussed how type-1 VDAC channels at the cell surface of HeLa cells in hypotonic surroundings might be kept closed after pre-incubation with BCl-XL B4 domain peptides.

Plasmalemmal VDAC controversies and maxi-anion channel puzzle

The maxi-anion channel has been observed in many cell types from the very beginning of the patch-clamp era. The channel is highly conductive for chloride and thus can modulate the resting membrane potential and play a role in fluid secretion/absorption and cell volume regulation. A wide nanoscopic pore of the maxi-anion channel permits passage of excitatory amino acids and nucleotides. The channel-mediated release of these signaling molecules is associated with kidney tubuloglomerular feedback, cardiac ischemia/hypoxia, as well as brain ischemia/hypoxia and excitotoxic neurodegeneration. Despite the ubiquitous expression and physiological/pathophysiological significance, the molecular identity of the maxi-anion channel is still obscure. VDAC is primarily a mitochondrial protein; however several groups detected it on the cellular surface. VDAC in lipid bilayers reproduced the most important biophysical properties of the maxi-anion channel, such as a wide nano-sized pore, closure in response to moderately high voltages, ATP-block and ATP-permeability. However, these similarities turned out to be superficial, and the hypothesis of plasmalemmal VDAC as the maxi-anion channel did not withstand the test by genetic manipulations of VDAC protein expression. VDAC on the cellular surface could also function as a ferricyanide reductase or a receptor for plasminogen kringle 5 and for neuroactive steroids. These ideas, as well as the very presence of VDAC on plasmalemma, remain to be scrutinized by genetic manipulations of the VDAC protein expression. This article is part of a Special Issue entitled: VDAC structure, function, and regulation of mitochondrial metabolism.

Apoptogenic interactions of plasmalemmal type-1 VDAC and Aβ peptides via GxxxG motifs induce Alzheimer's disease - a basic model of apoptosis?

Human type-1 porin/VDAC (voltage-dependent anion channel) carries a GxxxG motif in its N-terminal part, traversing the β-barrel, while the Alzheimer's disease (AD) relevant amyloid peptides Aβ1-42 and Aβ1-40 show a series of corresponding motifs close to their C-terminus. GxxxG motifs are established as aggregation/membrane perturbation motifs. These peptide primary structure data support a proposal I recently made on the basis of a synopsis of recent literature. Accordingly, amyloid Aβ, cut from APP by beta-secretase BACE1 and gamma-secretase, has been insinuated to induce Alzheimer's disease via apoptosis by opening type-1 porin/VDAC in cell membranes of hypometabolic neuronal cells. Considering the ubiquitous expression modus of APP, beta- and gamma-secretases and type-1 VDAC/eukaryotic porin a basic model of apoptosis might be given.

Voltage-dependent anion channels 1 and 2 are expressed in porcine oocytes

The eukaryotic VDAC (voltage-dependent anion channel) is a pore-forming protein originally discovered in the outer membrane of mitochondria. It has been established as a key player in mitochondrial metabolism and ion signalling. In addition, in recent years, it has also been proposed that VDAC is present in extra-mitochondrial membranes, and it has been related to cytoskeletal structures. However, little is known about the presence and intracellular localization of VDAC subtypes in mammalian gametes. In the present study, we confirm the synthesis of VDAC1 and 2 subtypes in GV (germinal vesicle) and MII (meiosis II) stage porcine oocytes as well as their protein expression. A shift in the abundance of immunoreactive 32 kDa VDAC protein between GV and MII stage oocytes was observed with anti-VDAC2 antibody. Furthermore, subcellular localization by confocal laser microscopy demonstrated fluorescent labelling of VDAC1 over the entire oocyte surface, suggesting the presence of VDAC1 in the porcine oocyte plasma membrane and around the cortical area. Anti-VDAC2 immunostaining yielded ring-like clusters of structures distributed on the cortical area in some GV, but not in MII, stage oocytes. These results are the first data obtained for VDAC in mammalian female gametes and provide the basis for studying protein–protein interactions, distribution and possible functions of VDAC subtypes during maturation and fertilization of mammalian oocytes.

Neuroendocrine differentiation of LNCaP cells suggests: VDAC in the cell membrane is involved in the extrinsic apoptotic pathway

The primary structure of native human type-1 voltage dependent anion-selective channel/porin was presented twenty years ago, so was first data on its extra-mitochondrial expression in cell membranes of lymphocytes. Then, the channel had already entered cancer research as the docking molecule for hexokinase at outer mitochondrial membrane. Cell membrane standing porin met the cancer field only four years ago, when it was reported that normal and cancerous prostate cells from a single patient differed in the expression level of the channel. Meanwhile studies on a role of VDAC in cell differentiation, apoptosis, cancer and even pharmacology increase, mostly focused on porin in the outer membrane of mitochondria, but sometimes also pointing to the channel in the plasmalemma, e.g. prostate cancer cells on their way to neuroendocrine-differentiation. The synopsis presented discusses some recent papers on this issue, and argues in favor of considering voltage dependent anion-selective channel-cored volume regulated anion channel complexes in studies focused on apoptosis and cancer, where the channel might be part of the extrinsic cell death pathway. In this context, heed should also be given to the interaction of extra-mitochondrial porin and estrogen receptors alpha in cell membrane caveolae. Finally, it is insinuated to search for natural antibodies against type-1 porin, what in combination with other established markers might help in early diagnosis of prostate cancer.

Localisation and function of voltage-dependent anion channels (VDAC) in bovine spermatozoa

Sperm motility, regulation of cell volume, sperm capacitation, acrosome reaction and tight binding of spermatozoa to the zona pellucida are crucial events in the process of fertilisation. Voltage-dependent anion channels (VDAC) are highly conserved pore-forming proteins implicated in apoptosis, metabolite transport between mitochondria and cytosol, energy metabolism, and cell volume regulation in somatic cells. Several studies have demonstrated the presence of VDAC in cell compartments other than mitochondria. In previous studies using immunofluorescence, we were able to localise VDAC2 and VDAC3 in outer dense fibres of the bovine sperm flagellum. Furthermore, we described the presence of VDAC2 in the head of bovine sperm. In the present study, we confirm the localisation of VDAC2 in the acrosomal region of bovine spermatozoa using immunoelectron microscopy. After incubation with anti-VDAC antibodies raised against each VDAC isoform, bovine spermatozoa showed an increased loss of the acrosomal cap, noticeable changes in the surface of the head, coiled tails and an increased cell volume. The incubation of bovine spermatozoa with anti-VDAC antibodies might lead to alteration of the intracellular ion concentration that causes changes in the cell volume, followed by destabilization of the cytoskeleton and, finally, to loss of the acrosomal cap.

Ca2+-mediated Potentiation of the Swelling-induced Taurine Efflux from HeLa Cells: On the Role of Calmodulin and Novel Protein Kinase C Isoforms

The present work sets out to investigate how Ca(2+) regulates the volume-sensitive taurine-release pathway in HeLa cells. Addition of Ca(2+)-mobilizing agonists at the time of exposure to hypotonic NaCl medium augments the swelling-induced taurine release and subsequently accelerates the inactivation of the release pathway. The accelerated inactivation is not observed in hypotonic Ca(2+)-free or high-K(+) media. Addition of Ca(2+)-mobilizing agonists also accelerates the regulatory volume decrease, which probably reflects activation of Ca(2+)-activated K(+) channels. The taurine release from control cells and cells exposed to Ca(2+) agonists is equally affected by changes in cell volume, application of DIDS and arachidonic acid, indicating that the volume-sensitive taurine leak pathway mediates the Ca(2+)-augmented taurine release. Exposure to Ca(2+)-mobilizing agonists prior to a hypotonic challenge also augments a subsequent swelling-induced taurine release even though the intracellular Ca(2+)-concentration has returned to the unstimulated level. The Ca(2+)-induced augmentation of the swelling-induced taurine release is abolished by inhibition of calmodulin, but unaffected by inhibition of calmodulin-dependent kinase II, myosin light chain kinase and calcineurin. The effect of Ca(2+)-mobilizing agonists is mimicked by protein kinase C (PKC) activation and abolished in the presence of the PKC inhibitor Gö6850 and following downregulation of phorbol ester-sensitive PKC isoforms. It is suggested that Ca(2+) regulates the volume-sensitive taurine-release pathway through activation of calmodulin and PKC isoforms belonging to the novel subclass (nPKC).

Ion channels and membrane rafts in apoptosis

Ion channels have been demonstrated to be a central element in the induction and the execution of apoptosis. In particular, mitochondrial ion channels, including not only the permeability transition pore but also a mitochondrial, ATP-sensitive (mKATP) channel as well as a mitochondrial calcium-activated potassium channel are involved critically in apoptotic changes in mitochondria. Ion channels in the cell membrane that are altered by induction of apoptosis include potassium, chloride and calcium channels. The Kv1.3 potassium channel belongs to the best-characterized ion channels involved in apoptosis and a genetic model of cells deficient for Kv1.3 has indicated a critical role for Kv1.3, at least in some forms of apoptosis. The mechanisms regulating ion channels during apoptosis are, however, still poorly defined. Recent studies have suggested a function for distinct membrane domains, termed rafts, in the cell membrane for the regulation of ion channels during apoptosis. Small sphingolipid- and cholesterol-enriched membrane domains are modified by many apoptotic stimuli to form large ceramide-enriched membrane platforms. These platforms serve to cluster receptor molecules, to re-organize intracellular signalling molecules including ion channels, to bring ion channels into close contact with their regulators and/or to separate proteins from a specific ion channel. Finally, the lipid composition of the cell membrane might be involved directly in ion channel regulation.

Extracellular ATP as a signaling molecule for epithelial cells

The charge of this invited review is to present a convincing case for the fact that cells release their ATP for physiological reasons. Many of our "purinergic" colleagues as well as ourselves have experienced resistance to this concept, because it is teleologically counter-intuitive. This review serves to integrate the three main tenets of extracellular ATP signaling: ATP release from cells, ATP receptors on cells, and ATP receptor-driven signaling within cells to affect cell or tissue physiology. First principles will be discussed in the Introduction concerning extracellular ATP signaling. All possible cellular mechanisms of ATP release will then be presented. Use of nucleotide and nucleoside scavengers as well as broad-specificity purinergic receptor antagonists will be presented as a method of detecting endogenous ATP release affecting a biological endpoint. Innovative methods of detecting released ATP by adapting luciferase detection reagents or by using "biosensors" will be presented. Because our laboratory has been primarily interested in epithelial cell physiology and pathophysiology for several years, the role of extracellular ATP in regulation of epithelial cell function will be the focus of this review. For ATP release to be physiologically relevant, receptors for ATP are required at the cell surface. The families of P2Y G protein-coupled receptors and ATP-gated P2X receptor channels will be introduced. Particular attention will be paid to P2X receptor channels that mediate the fast actions of extracellular ATP signaling, much like neurotransmitter-gated channels versus metabotropic heptahelical neurotransmitter receptors that couple to G proteins. Finally, fascinating biological paradigms in which extracellular ATP signaling has been implicated will be highlighted. It is the goal of this review to convert and attract new scientists into the exploding field of extracellular nucleotide signaling and to convince the reader that extracellular ATP is indeed a signaling molecule.

Anti-porin antibodies prevent excitotoxic and ischemic damage to brain tissue

The mitochondrial permeability transition (MPT) is a converging event for different molecular routes leading to cellular death after excitotoxic/oxidative stress, and is considered to represent the opening of a pore in the mitochondrial membrane. There is evidence that the outer mitochondrial membrane protein porin is involved in the MPT and apoptosis. We present here a proof-of-principle study to address the hypothesis that anti-porin antibodies can prevent excitotoxic/ischemia-induced cell death. We generated anti-porin antibodies and show that the F(ab)(2) fragments penetrate living cells, reduce Ca(2+)-induced mitochondrial swelling as other MPT blockers do, and decrease neuronal death in dissociated and organotypic brain slice cultures exposed to excitotoxic and ischemic episodes. These observations present direct evidence that anti-porin antibody fragments prevent cell damage in brain tissue, that porin is a crucial protein involved in mitochondrial and cell dysfunction, and that it is conceivable that antibodies can be used as therapeutic agents.

Voltage-dependent anion-selective channel (VDAC) interacts with the dynein light chain Tctex1 and the heat-shock protein PBP74

The voltage-dependent anion-selective channel 1 (VDAC1), i.e. eukaryotic porin, functions as a channel in membranous structures as described for the outer mitochondrial membrane, the cell membrane, endosomes, caveolae, the sarcoplasmatic reticulum, synaptosomes, and post-synaptic density fraction. The identification of VDAC1 interacting proteins may be a promising approach for better understanding the biological context and function of the channel protein. In this study human VDAC1 was used as a bait protein in a two-hybrid screening, which is based on the Sos recruitment system (SRS). hVDAC1 interacts with the dynein light chain Tctex-1 and the heat-shock protein peptide-binding protein 74 (PBP74)/mitochondrial heat-shock protein 70 (mtHSP70)/glucose-regulated protein 75 (GRP75)/mortalin in vivo. Both interactions were confirmed by overlay-assays using recombinant partner proteins and purified hVDAC1. Indirect immunofluorescence on HeLa cells indicates a co-localisation of hVDAC1 with the dynein light chain and the PBP74. In addition, HeLa cells were transfected transiently with enhanced green fluorescent protein (EGFP)-hVDAC1 fusion proteins, which also clearly co-localise with both proteins. The functional relevance of the identified protein interactions was analysed in planar lipid bilayer (PLB) experiments. In these experiments both recombinant binding partners altered the electrophysiological properties of hVDAC1. While rTctex-1 increases the voltage-dependence of hVDAC1 slightly, the rPBP74 drastically minimises the voltage-dependence, indicating a modulation of channel properties in each case. Since the identified proteins are known to be involved in the transport or processing of proteins, the results of this study represent additional evidence of membrane-associated trafficking of the voltage-dependent anion-selective channel 1.

Extracellular nucleotide signaling along the renal epithelium

During the past two decades, several cell membrane receptors, which preferentially bind extracellular nucleotides, and their analogs have been identified. These receptors, collectively known as nucleotide receptors or "purinergic" receptors, have been characterized and classified on the basis of their biological actions, their pharmacology, their molecular biology, and their tissue and cell distribution. For these receptors to have biological and physiological relevance, nucleotides must be released from cells. The field of extracellular ATP release and signaling is exploding, as assays to detect this biological process increase in number and ingenuity. Studies of ATP release have revealed a myriad of roles in local regulatory (autocrine or paracrine) processes in almost every tissue in the body. The regulatory mechanisms that these receptors control or modulate have physiological and pathophysiological roles and potential therapeutic applications. Only recently, however, have ATP release and nucleotide receptors been identified along the renal epithelium of the nephron. This work has set the stage for the study of their physiological and pathophysiological roles in the kidney. This review provides a comprehensive presentation of these issues, with a focus on the renal epithelium.

Human voltage-dependent anion-selective channel expressed in the plasmalemma of Xenopus laevis oocytes

Recent studies indicate a plasmalemmal localisation of eukaryotic porin, i.e. voltage-dependent anion-selective channel (VDAC), and there is evidence that the channel in this cell compartment is engaged in cell volume regulation. Until recently, others and we have used immuno-topochemical and biochemical methods to demonstrate the integration of the channel into the cell membrane and endoplasmic reticulum of vertebrate cells. In the present study, we used molecular biological methods to induce the heterologous expression of tagged human type-1 porin in oocytes of Xenopus laevis and to illustrate its appearance at the plasma membrane of these cells. Applying confocal fluorescent microscopy, green fluorescent protein attached to the C-terminus of porin could clearly be recorded at the cell surface. N-terminal green fluorescent protein-porin fusion proteins remained in the cytoplasm, indicating a strong influence of the porin N-terminus on protein trafficking to the plasma membrane. FLAG-tagged porin was also expressed in frog oocytes. Here, plasmalemmal expression was observed using anti-FLAG M2 monoclonal antibodies and gold-conjugated secondary antibodies, followed by silver enhancement through scanning electron microscopy. In contrast to the EGFP-porin fusion protein, the influence of the small FLAG-epitope (8 amino acids) did not prevent plasmalemmal expression of N-terminally tagged porin. These results indicate the definite expression of human type-1 porin in the plasma membrane of Xenopus oocytes. They thus corroborate our early data on the extra-mitochondrial expression of the eukaryotic porin channel and are essential for future electrophysiological studies on the channel.