Studies on human porin XVIII: the multicompartment effector ruthenium red reduces the voltage dependence of human VDAC in planar lipid bilayers

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.

Modulation of the voltage-dependent anion channel (VDAC) by glutamate

The voltage-dependent anion channel (VDAC), also known as mitochondrial porin, is a large channel permeable to anions, cations, ATP, and other metabolites. VDAC was purified from sheep brain synaptosomes or rat liver mitochondria using a reactive red-agarose column, in addition to the hydroxyapatitate column. The red-agarose column allowed further purification (over 98%), concentration of the protein over ten-fold, decreasing Triton X-100 concentration, and/or replacing Triton X-100 with other detergents, such as Nonidet P-40 or octylglucoside. This purified VDAC reconstituted into planar-lipid bilayer, had a unitary maximal conductance of 3.7 +/- 0.1 nS in 1 M NaCl, at 10 mV and was permeable to both large cations and anions. In the maximal conducting state, the permeability ratios for Na(+), acetylcholine(+), dopamine,(+) and glutamate(-), relative to Cl(-), were estimated to be 0.73, 0.6, 0.44, and 0.4, respectively. In contrast, in the subconducting state, glutamate(-) was impermeable, while the relative permeability to acetylcholine(+) increased and to dopamine(+) remained unchanged. At the high concentrations (0.1-0.5 M) used in the permeability experiments, glutamate eliminated the bell shape of the voltage dependence of VDAC channel conductance. Glutamate at concentrations of 1 to 20 mM, in the presence of 1 M NaCl, was found to modulate the VDAC channel activity. In single-channel experiments, at low voltages (+/-10 mV), glutamate induced rapid fluctuations of the channel between the fully open state and long-lived low-conducting states or short-lived closed state. Glutamate modification of the channel activity, at low voltages, is dependent on voltage, requiring short-time (20-60 sec) exposure of the channel to high membrane potentials. The effect of glutamate is specific, since it was observed in the presence of 1 M NaCl and it was not obtained with aspartate or GABA. These results suggest that VDAC possesses a specific glutamate-binding site that modulates its activity.

Calcium binding and translocation by the voltage-dependent anion channel: a possible regulatory mechanism in mitochondrial function

Mitochondria play a central role in energy metabolism, Ca(2+) signalling, aging and cell death. To control cytosolic or mitochondrial Ca(2+) concentration, mitochondria possess several Ca(2+)-transport systems across the inner membrane. However, the pathway for Ca(2+) crossing the outer membrane has not been directly addressed. We report that purified voltage-dependent anion channel (VDAC) reconstituted into lipid bilayers or liposomes is highly permeable to Ca(2+). VDAC contains Ca(2+)-binding sites that bind Ruthenium Red (RuR), La(3+) and that RuR completely closed VDACs in single or multichannel experiments. Energized, freshly prepared mitochondria accumulate Ca(2+) (500-700 nmol/mg of protein), and subsequently released it. The release of Ca(2+) is accompanied by cyclosporin A-inhibited swelling, suggesting activation of permeability transition pore (PTP). RuR and ruthenium amine binuclear complex, when added to mitochondria after Ca(2+) accumulation has reached a maximal level and before PTP is activated, prevented the release of Ca(2+) and the accompanied mitochondrial swelling. RuR also prevented PTP opening promoted by atractyloside, an adenine nucleotide translocase inhibitor. These results suggest that VDAC, located in the mitochondrial outer membrane, controls Ca(2+) transport into and from the mitochondria, and that the inhibition of Ca(2+) uptake by RuR and La(3+) may result from their interaction with VDAC Ca(2+)-binding sites. Inhibition of PTP opening or assembly by RuR and ruthenium amine binuclear complex suggest the involvement of VDAC in PTP activity and/or regulation. The permeability of VDAC to Ca(2+) and its binding of Ca(2+), suggest that VDAC has a role in regulation of the mitochondrial Ca(2+) homoeostasis.

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.

Studies on human porin XIX: zinc ions reduce the voltage-dependence of human type-1 porin integrated in artificial planar lipid bilayers; barium ions do not

We report on the first data concerning the effects of ionic zinc or barium on human type-1 porin. The channel enriched as mature protein from B-lymphocyte crude membranes was integrated into artificial planar lipid bilayers and both agonists were applied as their chloride salts at 100 microM. Relative conductance was measured from -80 to 80 mV. The presence of Zn2+ resulted in a distinct reduction of the voltage dependence of the channel, the effect appearing to be symmetric. The addition of Ba2+ did not produce any measurable effect on human porin. According to our knowledge we present the first results on the interaction of bivalent metal ions and eukaryotic type-1 porin. The putative physiological relevance of the data is discussed.