The plasma membrane of Xenopus laevis oocytes contains voltage-dependent anion-selective porin channels

The functions of voltage-dependent anion channels in plants

Voltage-dependent anion channels (VDACs), known as outer mitochondrial membrane proteins, are present in all eukaryotic cells. In mammals, they are now recognized to play crucial roles in the regulation of metabolic and energetic functions of mitochondria as well as in mitochondria-mediated apoptosis, in association with various proteins and non-protein modulators. Although there is much less information available for plant than for animal VDACs, their similar electrophysiological and topological properties suggest that some common functions are conserved among eukaryotic VDACs. Recently, it has been revealed that plant VDACs also have various important physiological functions not only in developmental and reproductive processes, but also in biotic and abiotic stress responses, including programmed cell death. In this review, we summarize recent findings about the sequence motifs, localization, and function of plant VDACs and discuss these results in the light of recent advances in research on animal VDACs.

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.

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.

VDAC, a multi-functional mitochondrial protein regulating cell life and death

Research over the past decade has extended the prevailing view of the mitochondrion to include functions well beyond the generation of cellular energy. It is now recognized that mitochondria play a crucial role in cell signaling events, inter-organellar communication, aging, cell proliferation, diseases and cell death. Thus, mitochondria play a central role in the regulation of apoptosis (programmed cell death) and serve as the venue for cellular decisions leading to cell life or death. One of the mitochondrial proteins controlling cell life and death is the voltage-dependent anion channel (VDAC), also known as mitochondrial porin. VDAC, located in the mitochondrial outer membrane, functions as gatekeeper for the entry and exit of mitochondrial metabolites, thereby controlling cross-talk between mitochondria and the rest of the cell. VDAC is also a key player in mitochondria-mediated apoptosis. Thus, in addition to regulating the metabolic and energetic functions of mitochondria, VDAC appears to be a convergence point for a variety of cell survival and cell death signals mediated by its association with various ligands and proteins. In this article, we review what is known about the VDAC channel in terms of its structure, relevance to ATP rationing, Ca(2+) homeostasis, protection against oxidative stress, regulation of apoptosis, involvement in several diseases and its role in the action of different drugs. In light of our recent findings and the recently solved NMR- and crystallography-based 3D structures of VDAC1, the focus of this review will be on the central role of VDAC in cell life and death, addressing VDAC function in the regulation of mitochondria-mediated apoptosis with an emphasis on structure-function relations. Understanding structure-function relationships of VDAC is critical for deciphering how this channel can perform such a variety of functions, all important for cell life and death. This review also provides insight into the potential of VDAC1 as a rational target for new therapeutics.

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.

The maxi-anion channel: a classical channel playing novel roles through an unidentified molecular entity

The maxi-anion channel is widely expressed and found in almost every part of the body. The channel is activated in response to osmotic cell swelling, to excision of the membrane patch, and also to some other physiologically and pathophysiologically relevant stimuli, such as salt stress in kidney macula densa as well as ischemia/hypoxia in heart and brain. Biophysically, the maxi-anion channel is characterized by a large single-channel conductance of 300-400 pS, which saturates at 580-640 pS with increasing the Cl(-) concentration. The channel discriminates well between Na(+) and Cl(-), but is poorly selective to other halides exhibiting weak electric-field selectivity with an Eisenman's selectivity sequence I. The maxi-anion channel has a wide pore with an effective radius of approximately 1.3 nm and permits passage not only of Cl(-) but also of some intracellular large organic anions, thereby releasing major extracellular signals and gliotransmitters such as glutamate(-) and ATP(4-). The channel-mediated efflux 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, well-defined properties and physiological/pathophysiological significance of this classical channel, the molecular entity has not been identified. Molecular identification of the maxi-anion channel is an urgent task that would greatly promote investigation in the fields not only of anion channel but also of physiological/pathophysiological signaling in the brain, heart and kidney.

Volume-sensitive chloride channels involved in apoptotic volume decrease and cell death

Apoptosis is an essential process in organ development, tissue homeostasis, somatic cell turnover, and the pathogenesis of degenerative diseases. Apoptotic cell death occurs in response to a variety of stimuli in physiological and pathological circumstances. Efflux of K(+) and Cl(-) leads to apoptotic volume decrease (AVD) of the cell. Both mitochondrion-mediated intrinsic, and death receptor-mediated extrinsic, apoptotic stimuli have been reported to rapidly activate Cl(-) conductances in a large variety of cell types. In epithelial cells and cardiomyocytes, the AVD-inducing anion channel was recently determined to be the volume-sensitive outwardly rectifying (VSOR) Cl(-) channel which is usually activated by swelling under non-apoptotic conditions. Blocking the VSOR Cl(-) channel prevented cell death in not only epithelial and cardiac cells, but also other cell types, by inhibiting the induction of AVD and subsequent apoptotic events. Ischemia-reperfusion-induced apoptotic death in cardiomyocytes and brain neurons was also prevented by Cl(-) channel blockers. Furthermore, cancer cell apoptosis induced by the anti-cancer drug cisplatin was recently found to be associated with augmented activity of the VSOR Cl(-) channel and to be inhibited by a Cl(-) channel blocker. The apoptosis-inducing VSOR Cl(-) channel is distinct from ClC-3 and its molecular identity remains to be determined.

Studying the mechanosensitivity of voltage-gated channels using oocyte patches

The mechanosensitivity of voltage-gated (VG) channels is of biophysical, physiological. and pathophysiological interest. Xenopus oocytes offer a critical advantage for investigating the electrophysiology of recombinant VG channels subjected to membrane stretch, namely, the ability to monitor macroscopic current from membrane patches. High-density channel expression in oocytes makes for macroscopic current in conventional-size, mechanically sturdy patches. With the patch configuration, precisely the same membrane that is voltage-clamped is the membrane subjected to on-off stretch stimuli. With patches, meaningful stretch dose responses are possible. Experimental design should facilitate within-patch comparisons wherever possible. The mechanoresponses of some VG channels depend critically on patch history. Methods for minimizing and coping with interference from endogenous voltage-dependent and stretch-activated endogenous channels are described.

ATP release via anion channels

ATP serves not only as an energy source for all cell types but as an 'extracellular messenger' for autocrine and paracrine signalling. It is released from the cell via several different purinergic signal efflux pathways. ATP and its Mg(2+) and/or H(+) salts exist in anionic forms at physiological pH and may exit cells via some anion channel if the pore physically permits this. In this review we survey experimental data providing evidence for and against the release of ATP through anion channels. CFTR has long been considered a probable pathway for ATP release in airway epithelium and other types of cells expressing this protein, although non-CFTR ATP currents have also been observed. Volume-sensitive outwardly rectifying (VSOR) chloride channels are found in virtually all cell types and can physically accommodate or even permeate ATP(4-) in certain experimental conditions. However, pharmacological studies are controversial and argue against the actual involvement of the VSOR channel in significant release of ATP. A large-conductance anion channel whose open probability exhibits a bell-shaped voltage dependence is also ubiquitously expressed and represents a putative pathway for ATP release. This channel, called a maxi-anion channel, has a wide nanoscopic pore suitable for nucleotide transport and possesses an ATP-binding site in the middle of the pore lumen to facilitate the passage of the nucleotide. The maxi-anion channel conducts ATP and displays a pharmacological profile similar to that of ATP release in response to osmotic, ischemic, hypoxic and salt stresses. The relation of some other channels and transporters to the regulated release of ATP is also discussed.

Genetic demonstration that the plasma membrane maxianion channel and voltage-dependent anion channels are unrelated proteins

The maxianion channel is widely expressed in many cell types, where it fulfills a general physiological function as an ATP-conductive gate for cell-to-cell purinergic signaling. Establishing the molecular identity of this channel is crucial to understanding the mechanisms of regulated ATP release. A mitochondrial porin (voltage-dependent anion channel (VDAC)) located in the plasma membrane has long been considered as the molecule underlying the maxianion channel activity, based upon similarities in the biophysical properties of these two channels and the purported presence of VDAC protein in the plasma membrane. We have deleted each of the three genes encoding the VDAC isoforms individually and collectively and demonstrate that maxianion channel (approximately 400 picosiemens) activity in VDAC-deficient mouse fibroblasts is unaltered. The channel activity is similar in VDAC1/VDAC3-double-deficient cells and in double-deficient cells with the VDAC2 protein depleted by RNA interference. VDAC deletion slightly down-regulated, but never abolished, the swelling-induced ATP release. The lack of correlation between VDAC protein expression and maxianion channel activity strongly argues against the long held hypothesis of plasmalemmal VDAC being the maxianion channel.

Wide nanoscopic pore of maxi-anion channel suits its function as an ATP-conductive pathway

The newly proposed function of the maxi-anion channel as a conductive pathway for ATP release requires that its pore is sufficiently large to permit passage of a bulky ATP(4-) anion. We found a linear relationship between relative permeability of organic anions of different size and their relative ionic mobility (measured as the ratio of ionic conductance) with a slope close to 1, suggesting that organic anions tested with radii up to 0.49 nm (lactobionate) move inside the channel by free diffusion. In the second approach, we, for the first time, succeeded in pore sizing by the nonelectrolyte exclusion method in single-channel patch-clamp experiments. The cutoff radii of PEG molecules that could access the channel from intracellular (1.16 nm) and extracellular (1.42 nm) sides indicated an asymmetry of the two entrances to the channel pore. Measurements by symmetrical two-sided application of PEG molecules yielded an average functional pore radius of approximately 1.3 nm. These three estimates are considerably larger than the radius of ATP(4-) (0.57-0.65 nm) and MgATP(2-) (approximately 0.60 nm). We therefore conclude that the nanoscopic maxi-anion channel pore provides sufficient room to accommodate ATP and is well suited to its function as a conductive pathway for ATP release in cell-to-cell communication.

The large cytoplasmic loop of the glucose transporter GLUT1 is an essential structural element for function

Alanine scanning mutagenesis and the introduction of deletions and insertions were used to address the role of the large cytoplasmic loop in 2-deoxy-D-glucose (2-DOG) uptake by GLUT1 expressed in Xenopus oocytes. Alanine scanning mutagenesis of 29 amino acid residues that are identical or homologous in GLUT1 to GLUT4 demonstrated that the transport activities of only a few variants were affected. Progressive truncation of the loop by six deletions leaving intact 59 (delta236-241), 49 (delta231-246), 39 (delta226-251), 28 (delta221-257), 18 (delta216-262), or 10 (delta213-267) amino acid residues resulted in a progressive decrease in 2-DOG uptake. Compared with wild-type GLUT1 the uptake rates varied between 33% for the delta236-241 mutant and 4% for the delta213-267 mutant. Insertional mutagenesis using hexaalanine or hexaglycine to fill in the deletion 236D-241L restored 2-DOG uptake to 73% of wild-type GLUT1 in the case of hexaalanine, whereas hexaglycine insertion was without effect. Confocal laser microscopy demonstrated that a deletion of six amino acid residues did not influence the expression level in the plasma membrane (delta236-241 mutant), whereas the plasma membrane fluorescence of the delta213-267 mutant was comparable with that of water-injected Xenopus oocytes. Computer-aided secondary structure prediction of the loop suggested that it consists of a long alpha-helix bundle interrupted or kinked by the highly conserved glycine-233.

Volume-dependent ATP-conductive large-conductance anion channel as a pathway for swelling-induced ATP release

In mouse mammary C127i cells, during whole-cell clamp, osmotic cell swelling activated an anion channel current, when the phloretin-sensitive, volume-activated outwardly rectifying Cl(-) channel was eliminated. This current exhibited time-dependent inactivation at positive and negative voltages greater than around +/-25 mV. The whole-cell current was selective for anions and sensitive to Gd(3)+. In on-cell patches, single-channel events appeared with a lag period of approximately 15 min after a hypotonic challenge. Under isotonic conditions, cell-attached patches were silent, but patch excision led to activation of currents that consisted of multiple large-conductance unitary steps. The current displayed voltage- and time-dependent inactivation similar to that of whole-cell current. Voltage-dependent activation profile was bell-shaped with the maximum open probability at -20 to 0 mV. The channel in inside-out patches had the unitary conductance of approximately 400 pS, a linear current-voltage relationship, and anion selectivity. The outward (but not inward) single-channel conductance was suppressed by extracellular ATP with an IC(50) of 12.3 mM and an electric distance (delta) of 0.47, whereas the inward (but not outward) conductance was inhibited by intracellular ATP with an IC(50) of 12.9 mM and delta of 0.40. Despite the open channel block by ATP, the channel was ATP-conductive with P(ATP)/P(Cl) of 0.09. The single-channel activity was sensitive to Gd(3)+, SITS, and NPPB, but insensitive to phloretin, niflumic acid, and glibenclamide. The same pharmacological pattern was found in swelling-induced ATP release. Thus, it is concluded that the volume- and voltage-dependent ATP-conductive large-conductance anion channel serves as a conductive pathway for the swelling-induced ATP release in C127i cells.

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.

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

Cell volume regulation receives increasing attention not only as the basis of regulatory volume increase or regulatory volume decrease (RVD) of cells in surroundings of changing osmolarity, but also appears to be relevant in cell proliferation, differentiation, and apoptosis. A central event in RVD is the opening of a volume-sensitive chloride/anion channel(s), and blocking this pathway would abolish RVD. This is shown here with monoclonal mouse anti-human type-1 porin antibodies, proving that porin is involved in this process. HeLa cells preincubated with these antibodies dramatically increase their volume within about 1 min after a hypotonic stimulus by 70 mM NaCl Ringer solution, but do not move back toward their starting volume, thus indicating abolished RVD. Corresponding effects are induced by the established anion channel inhibitor DIDS. Video camera monitoring of cell size over time was used as a direct and noninvasive approach. We had already accumulated evidence that plasmalemma integrated eukaryotic porin channels form chloride/anion channels in this cell compartment and that they are involved in cell volume regulation. Finally, the present data again demonstrate the suitability of our anti-porin antibodies in physiological studies.

Studies on human porin XXI: gadolinium opens Up cell membrane standing porin channels making way for the osmolytes chloride or taurine-A putative approach to activate the alternate chloride channel in cystic fibrosis

We recently proposed that cell-membrane-integrated vertebrate porin/voltage-dependent anion-selective channel (VDAC) forms part of the outwardly rectifying chloride channel (ORCC) complex that may be involved in volume regulation. The results we present here support this thesis. According to light scattering measurements micromolar concentrations of Gd(3+) induce cell swelling of human healthy and cystic fibrosis (CF) B-lymphocyte cell lines in isotonic Ringer solution. In high-potassium Ringer solution additional swelling is observed. Gd(3+) induces excessive cell swelling of cell lines in hypotonic Ringer solutions, containing 70 mM NaCl or 135 mM taurine, respectively. The gadolinium effect is lost when NaCl is replaced by Na-gluconate. Using video camera monitoring we show that HeLa cells also swell in micromolar concentrations of Gd(3+) in isotonic taurine Ringer solution. The dose-dependent effect of the agonist was always blocked by extracellular application of anti-human type-1 porin antibodies. Together with data on a decreasing effect of micromolar amounts of gadolinium on the voltage dependence of reconstituted human porin the results prove the involvement of porin channels in the swelling behavior in different cell lines. As a mechanism we propose that ionic gadolinium opens up plasmalemma-integrated porin channels, chloride or taurine then following their concentration gradients into the cells. Furthermore, our data argue for a single pathway for inorganic and organic osmolytes during regulatory volume decrease after cell swelling. There is indirect evidence that porin forms part of the cystic fibrosis relevant ORCC channel. Gadolinium thus may work to open the alternate chloride channel in CF.