Regulation of metabolite flux through voltage-gating of VDAC channels

Assessing the role of residue E73 and lipid headgroup charge in VDAC1 voltage gating

The voltage-dependent anion channel (VDAC) is the most abundant protein of the mitochondrial outer membrane (MOM) where it regulates transport of ions and metabolites in and out of the organelle. VDAC function is extensively studied in a lipid bilayer system that allows conductance monitoring of reconstituted channels under applied voltage. The process of switching from a high-conductance state, open to metabolites, to a variety of low-conducting states, which excludes metabolite transport, is termed voltage gating and the mechanism remains poorly understood. Recent studies have implicated the involvement of the membrane-solvated residue E73 in the gating process through β-barrel destabilization. However, there has been no direct experimental evidence of E73 involvement in VDAC1 voltage gating. Here, using electrophysiology measurements, we exclude the involvement of E73 in murine VDAC1 (mVDAC1) voltage gating process. With an established protocol of assessing voltage gating of VDACs reconstituted into planar lipid membranes, we definitively show that mVDAC1 gating properties do not change when E73 is replaced by either a glutamine or an alanine. We further demonstrate that cholesterol has no effect on mVDAC1 gating characteristics, though it was shown that E73 is coordinating residue in the cholesterol binding site. In contrast, we found a pronounced gating effect based on the charge of the phospholipid headgroup, where the positive charge stimulates and negative charge suppresses gating. These findings call for critical evaluation of the existing models of VDAC gating and contribute to our understanding of VDAC's role in control of MOM permeability and regulation of mitochondrial respiration and metabolism.

Structure of voltage-dependent anion channel-tethered bilayer lipid membranes determined using neutron reflectivity

Neutron reflectivity (NR) has emerged as a powerful technique to study the structure and behavior of membrane proteins at planar lipid interfaces. Integral membrane proteins (IMPs) remain a significant challenge for NR owing to the difficulty of forming complete bilayers with sufficient protein density for scattering techniques. One strategy to achieve high protein density on a solid substrate is the capture of detergent-stabilized, affinity-tagged IMPs on a nitrilotriacetic acid (NTA)-functionalized self-assembled monolayer (SAM), followed by reconstitution into the lipids of interest. Such protein-tethered bilayer lipid membranes (ptBLMs) have the notable advantage of a uniform IMP orientation on the substrate. Here, NR is used to provide a structural characterization of the ptBLM process from formation of the SAM to capture of the detergent-stabilized IMP and lipid reconstitution. The mitochondrial outer-membrane voltage-dependent anion channel (VDAC), which controls the exchange of bioenergetic metabolites between mitochondria and the cytosol, was used as a model β-barrel IMP. Molecular dynamics simulations were used for comparison with the experimental results and to inform the parameters of the physical models describing the NR data. The detailed structure of the SAM is shown to depend on the density of the NTA chelating groups. The relative content of detergent and protein in surface-immobilized, detergent-stabilized VDAC is measured, while the reconstituted lipid bilayer is shown to be complete to within a few percent, using the known atomic structure of VDAC. Finally, excess lipid above the reconstituted bilayer, which is of consequence for more indirect structural and functional studies, is shown to be present.

Lipid composition and salt concentration as regulatory factors of the anion selectivity of VDAC studied by coarse-grained molecular dynamics simulations

The voltage-dependent anion channel (VDAC) is a mitochondrial outer membrane protein whose fundamental function is to facilitate and regulate the flow of metabolites between the cytosol and the mitochondrial intermembrane space. Using coarse-grained molecular dynamics simulations, we investigated the dependence of VDAC selectivity towards small inorganic anions on two factors: the ionic strength and the lipid composition. In agreement with experimental data we found that VDAC becomes less anion selective with increasing salt concentration due to the screening of a few basic residues that point into the pore lumen. The molecular dynamics simulations provide insight into the regulation mechanism of VDAC selectivity by the composition in the lipid membrane and suggest that the ion distribution is differently modulated by POPE compared to the POPC bilayer. This occurs through the more persistent interactions of acidic residues located at both rims of the β-barrel with head groups of POPE which in turn impact the electrostatic potential and thereby the selectivity of the pore. This mechanism occurs not only in POPE single component membranes but also in a mixed POPE/POPC bilayer by an enrichment of POPE over POPC lipids on the surface of VDAC. Thus we show here that computationally-inexpensive coarse-grained simulations are able to capture, in a semi-quantitative way, essential features of VDAC anion selectivity and could pave the way toward a molecular level understanding of metabolite transport in natural membranes.

VDAC electronics: 5. Mechanism and computational model of hexokinase-dependent generation of the outer membrane potential in brain mitochondria

Glycolysis plays a key role in brain energy metabolism. The initial and rate-limiting step of brain glycolysis is catalyzed mainly by hexokinase I (HKI), the majority of which is bound to the mitochondrial outer membrane (MOM), mostly through the mitochondrial inter-membrane contact sites formed by the voltage-dependent anion channel (VDAC, outer membrane) and the adenine nucleotide translocator (ANT, inner membrane). Earlier, we proposed a mechanism for the generation of the mitochondrial outer membrane potential (OMP) as a result of partial application of the inner membrane potential (IMP) to MOM through the electrogenic ANT-VDAC-HK inter-membrane contact sites. According to this previous mechanism, the Gibbs free energy of the hexokinase reaction might modulate the generated OMP (Lemeshko, Biophys. J., 2002). In the present work, a new computational model was developed to perform thermodynamic estimations of the proposed mechanism of IMP-HKI-mediated generation of OMP. The calculated OMP was high enough to electrically regulate MOM permeability for negatively charged metabolites through free, unbound VDACs in MOM. On the other hand, the positive-inside polarity of OMP generated by the IMP-HKI-mediated mechanism is expected to protect mitochondria against elevated concentrations of cytosolic Ca²⁺. This computational analysis suggests that metabolically-dependent generation of OMP in the brain mitochondria, controlled by many factors that modulate VDAC1-HKI interaction, VDAC's voltage-gating properties and permeability, might represent one of the physiological mechanisms of regulation of the brain energy metabolism and of neuronal death resistance, and might also be involved in various neurodegenerative disorders, such as Alzheimer's disease.

Interaction of the cryptic fragment of myelin basic protein with mitochondrial voltage-dependent anion-selective channel-1 affects cell energy metabolism

In demyelinating nervous system disorders, myelin basic protein (MBP), a major component of the myelin sheath, is proteolyzed and its fragments are released in the neural environment. Here, we demonstrated that, in contrast with MBP, the cellular uptake of the cryptic 84-104 epitope (MBP84-104) did not involve the low-density lipoprotein receptor-related protein-1, a scavenger receptor. Our pull-down assay, mass spectrometry and molecular modeling studies suggested that, similar with many other unfolded and aberrant proteins and peptides, the internalized MBP84-104 was capable of binding to the voltage-dependent anion-selective channel-1 (VDAC-1), a mitochondrial porin. Molecular modeling suggested that MBP84-104 directly binds to the N-terminal α-helix located midway inside the 19 β-blade barrel of VDAC-1. These interactions may have affected the mitochondrial functions and energy metabolism in multiple cell types. Notably, MBP84-104 caused neither cell apoptosis nor affected the total cellular ATP levels, but repressed the aerobic glycolysis (lactic acid fermentation) and decreased the l-lactate/d-glucose ratio (also termed as the Warburg effect) in normal and cancer cells. Overall, our findings implied that because of its interactions with VDAC-1, the cryptic MBP84-104 peptide invoked reprogramming of the cellular energy metabolism that favored enhanced cellular activity, rather than apoptotic cell death. We concluded that the released MBP84-104 peptide, internalized by the cells, contributes to the reprogramming of the energy-generating pathways in multiple cell types.

Sequence diversity of tubulin isotypes in regulation of the mitochondrial voltage-dependent anion channel

The microtubule protein tubulin is a heterodimer comprising α/β subunits, in which each subunit features multiple isotypes in vertebrates. For example, seven α-tubulin and eight β-tubulin isotypes in the human tubulin gene family vary mostly in the length and primary sequence of the disordered anionic carboxyl-terminal tails (CTTs). The biological reason for such sequence diversity remains a topic of vigorous enquiry. Here, we demonstrate that it may be a key feature of tubulin's role in regulation of the permeability of the mitochondrial outer membrane voltage-dependent anion channel (VDAC). Using recombinant yeast α/β-tubulin constructs with α-CTTs, β-CTTs, or both from various human tubulin isotypes, we probed their interactions with VDAC reconstituted into planar lipid bilayers. A comparative study of the blockage kinetics revealed that either α-CTTs or β-CTTs block the VDAC pore and that the efficiency of blockage by individual CTTs spans 2 orders of magnitude, depending on the CTT isotype. β-Tubulin constructs, notably β3, blocked VDAC most effectively. We quantitatively described these experimental results using a physical model that accounted only for the number and distribution of charges in the CTT, and not for the interactions between specific residues on the CTT and VDAC pore. Based on these results, we speculate that the effectiveness of VDAC regulation by tubulin depends on the predominant tubulin isotype in a cell. Consequently, the fluxes of ATP/ADP through the channel could vary significantly, depending on the isotype, thus suggesting an intriguing link between VDAC regulation and the diversity of tubulin isotypes present in vertebrates.

ER-Mitochondria Microdomains in Cardiac Ischemia-Reperfusion Injury: A Fresh Perspective

The mitochondrial and endoplasmic reticulum (ER) homeostasis is pivotal to the maintenance of an array of physiological processes. The physical contact and association between ER and mitochondria, known as the ER–mitochondria microdomains or mitochondria-associated ER membrane (MAM), temporally and spatially regulates the mitochondria/ER structure and function. More evidence suggests a role for MAMs in energy production, cellular contraction and mobility, and normal extracellular signal transmission. In pathological states, such as cardiac ischemia–reperfusion (I/R injury), this ER–mitochondria microdomains may act to participate in the cellular redox imbalance, ER stress, mitochondrial injury, energy deletion, and programmed cell death. From a therapeutic perspective, a better understanding of the cellular and molecular mechanisms of the pathogenic ER–mitochondria contact should help to identify potential therapeutic target for cardiac I/R injury and other cardiovascular diseases and also pave the road to new treatment modalities pertinent for the treatment of reperfusion damage in clinical practice. This review will mainly focus on the possible signaling pathways involved in the regulation of the ER–mitochondria contact. In particular, we will summarize the downstream signaling modalities influenced by ER–mitochondria microdomains, for example, mitochondrial fission, mitophagy, calcium balance, oxidative stress, and programmed cell death in details.

Nutrition and Training Influences on the Regulation of Mitochondrial Adenosine Diphosphate Sensitivity and Bioenergetics

Since the seminal finding almost 50 years ago that exercise training increases mitochondrial content in skeletal muscle, a considerable amount of research has been dedicated to elucidate the mechanisms inducing mitochondrial biogenesis. The discovery of peroxisome proliferator-activated receptor γ co-activator 1α as a major regulator of exercise-induced gene transcription was instrumental in beginning to understand the signals regulating this process. However, almost two decades after its discovery, our understanding of the signals inducing mitochondrial biogenesis remain poorly defined, limiting our insights into possible novel training modalities in elite athletes that can increase the oxidative potential of muscle. In particular, the role of mitochondrial reactive oxygen species has received very little attention; however, several lifestyle interventions associated with an increase in mitochondrial reactive oxygen species coincide with the induction of mitochondrial biogenesis. Furthermore, the diminishing returns of exercise training are associated with reductions in exercise-induced, mitochondrial-derived reactive oxygen species. Therefore, research focused on altering redox signaling in elite athletes may prove to be effective at inducing mitochondrial biogenesis and augmenting training regimes. In the context of exercise performance, the biological effect of increasing mitochondrial content is an attenuated rise in free cytosolic adenosine diphosphate (ADP), and subsequently decreased carbohydrate flux at a given power output. Recent evidence has shown that mitochondrial ADP sensitivity is a regulated process influenced by nutritional interventions, acute exercise, and exercise training. This knowledge raises the potential to improve mitochondrial bioenergetics in the absence of changes in mitochondrial content. Elucidating the mechanisms influencing the acute regulation of mitochondrial ADP sensitivity could have performance benefits in athletes, especially as these individuals display high levels of mitochondria, and therefore are subjects in whom it is notoriously difficult to further induce mitochondrial adaptations. In addition to changes in ADP sensitivity, an increase in mitochondrial coupling would have a similar bioenergetic response, namely a reduction in free cytosolic ADP. While classically the stoichiometry of the electron transport chain has been considered rigid, recent evidence suggests that sodium nitrate can improve the efficiency of this process, creating the potential for dietary sources of nitrate (e.g., beetroot juice) to display similar improvements in exercise performance. The current review focuses on these processes, while also discussing the biological relevance in the context of exercise performance.

VDAC1 as Pharmacological Target in Cancer and Neurodegeneration: Focus on Its Role in Apoptosis

Cancer and neurodegeneration are different classes of diseases that share the involvement of mitochondria in their pathogenesis. Whereas the high glycolytic rate (the so-called Warburg metabolism) and the suppression of apoptosis are key elements for the establishment and maintenance of cancer cells, mitochondrial dysfunction and increased cell death mark neurodegeneration. As a main actor in the regulation of cell metabolism and apoptosis, VDAC may represent the common point between these two broad families of pathologies. Located in the outer mitochondrial membrane, VDAC forms channels that control the flux of ions and metabolites across the mitochondrion thus mediating the organelle's cross-talk with the rest of the cell. Furthermore, the interaction with both pro-apoptotic and anti-apoptotic factors makes VDAC a gatekeeper for mitochondria-mediated cell death and survival signaling pathways. Unfortunately, the lack of an evident druggability of this protein, since it has no defined binding or active sites, makes the quest for VDAC interacting molecules a difficult tale. Pharmacologically active molecules of different classes have been proposed to hit cancer and neurodegeneration. In this work, we provide an exhaustive and detailed survey of all the molecules, peptides, and microRNAs that exploit VDAC in the treatment of the two examined classes of pathologies. The mechanism of action and the potential or effectiveness of each compound are discussed.

Amyloid-Beta and Phosphorylated Tau Accumulations Cause Abnormalities at Synapses of Alzheimer's disease Neurons

Amyloid-beta (Aβ) and hyperphosphorylated tau are hallmark lesions of Alzheimer's disease (AD). However, the loss of synapses and dysfunctions of neurotransmission are more directly tied to disease severity. The role of these lesions in the pathoetiological progression of the disease remains contested. Biochemical, cellular, molecular, and pathological studies provided several lines of evidence and improved our understanding of how Aβ and hyperphosphorylated tau accumulation may directly harm synapses and alter neurotransmission. In vitro evidence suggests that Aβ and hyperphosphorylated tau have both direct and indirect cytotoxic effects that affect neurotransmission, axonal transport, signaling cascades, organelle function, and immune response in ways that lead to synaptic loss and dysfunctions in neurotransmitter release. Observations in preclinical models and autopsy studies support these findings, suggesting that while the pathoetiology of positive lesions remains elusive, their removal may reduce disease severity and progression. The purpose of this article is to highlight the need for further investigation of the role of tau in disease progression and its interactions with Aβ and neurotransmitters alike.

Bcl-xL knockout attenuates mitochondrial respiration and causes oxidative stress that is compensated by pentose phosphate pathway activity

Bcl-x_L is an anti-apoptotic protein that localizes to the outer mitochondrial membrane and influences mitochondrial bioenergetics by controlling Ca²⁺ influx into mitochondria. Here, we analyzed the effect of mitochondrial Bcl-x_L on mitochondrial shape and function in knockout (KO), wild type and rescued mouse embryonic fibroblast cell lines. Mitochondria of KO cells were more fragmented, exhibited a reduced ATP concentration, and reduced oxidative phosphorylation (OXPHOS) suggesting an increased importance of ATP generation by other means. Under steady-state conditions, acidification of the growth medium as a readout for glycolysis was similar, but upon inhibition of ATP synthase with oligomycin, KO cells displayed an instant increase in glycolysis. In addition, forced energy production through OXPHOS by replacing glucose with galactose in the growth medium rendered KO cells more susceptible to mitochondrial toxins. KO cells had increased cellular reactive oxygen species and were more susceptible to oxidative stress, but had higher glutathione levels, which were however more rapidly consumed under conditions of oxidative stress. This coincided with an increased activity and protein abundance of the pentose phosphate pathway protein glucose-6-phosphate dehydrogenase, which generates NADPH necessary to regenerate reduced glutathione. KO cells were also less susceptible to pharmacological inhibition of the pentose phosphate pathway. We conclude that mitochondrial Bcl-x_L is involved in maintaining mitochondrial respiratory capacity. Its deficiency causes oxidative stress, which is associated with an increased glycolytic capacity and balanced by an increased activity of the pentose phosphate pathway.

Electro-pharmacological profiles of two brain mitoplast anion channels: Inferences from single channel recording

We have characterized the conduction and blocking properties of two different chloride channels from brain mitochondrial inner membranes after incorporation into planar lipid bilayers. Our experiments revealed the existence of channels with a mean conductance of 158 ± 7 and 301 ± 8 pS in asymmetrical 200 mM cis/50 mM trans KCl solutions. We determined that the channels were ten times more permeable for Cl⁻ than for K⁺, calculated from the reversal potential using the Goldman-Hodgkin-Katz equation. The channels were bell-shaped voltage dependent, with maximum open probability 0.9 at ± 20 mV. Two mitochondrial chloride channels were blocked after the addition of 10 µM DIDS. In addition, 158 pS chloride channel was blocked by 300 nM NPPB, acidic pH and 2.5 mM ATP, whereas the 301 pS chloride channel was blocked by 600 µM NPPB but not by acidic pH or ATP. Gating and conducting behaviors of these channels were unaffected by Ca²⁺. These results demonstrate that the 158 pS anion channel present in brain mitochondrial inner membrane, is probably identical to IMAC and 301 pS Cl channel displays different properties than those classically described for mitochondrial anion channels.

Mitochondrial VDAC1: A Key Gatekeeper as Potential Therapeutic Target

Mitochondria are the key source of ATP that fuels cellular functions, and they are also central in cellular signaling, cell division and apoptosis. Dysfunction of mitochondria has been implicated in a wide range of diseases, including neurodegenerative and cardiac diseases, and various types of cancer. One of the key proteins that regulate mitochondrial function is the voltage-dependent anion channel 1 (VDAC1), the most abundant protein on the outer membrane of mitochondria. VDAC1 is the gatekeeper for the passages of metabolites, nucleotides, and ions; it plays a crucial role in regulating apoptosis due to its interaction with apoptotic and anti-apoptotic proteins, namely members of the Bcl-2 family of proteins and hexokinase. Therefore, regulation of VDAC1 is crucial not only for metabolic functions of mitochondria, but also for cell survival. In fact, multiple lines of evidence have confirmed the involvement of VDAC1 in several diseases. Consequently, modulation or dysregulation of VDAC1 function can potentially attenuate or exacerbate pathophysiological conditions. Understanding the role of VDAC1 in health and disease could lead to selective protection of cells in different tissues and diverse diseases. The purpose of this review is to discuss the role of VDAC1 in the pathogenesis of diseases and as a potentially effective target for therapeutic management of various pathologies.

The mitochondrial VDAC of bean seeds recruits phosphatidylethanolamine lipids for its proper functioning

The voltage-dependent anion-selective channel (VDAC) is the main pathway for inorganic ions and metabolites through the mitochondrial outer membrane. Studies recently demonstrated that membrane lipids regulate its function. It remains, however, unclear how this regulation takes place. In this study, we show that phospholipids are key regulators of Phaseolus VDAC function and, furthermore, that the salt concentration modulates this regulation. Both selectivity and voltage dependence of Phaseolus VDAC are very sensitive to a change in the lipid polar head from PC to PE. Interestingly enough, this dependence is observed only at low salt concentration. Furthermore, significant changes in VDAC functional properties also occur with the gradual methylation of the PE group pointing to the role of subtle chemical variations in the lipid head group. The dependence of PcVDAC gating upon the introduction of a small mole fraction of PE in a PC bilayer has prompted us to propose the existence of a specific interaction site for PE on the outer surface of PcVDAC. Eventually, comparative modeling and molecular dynamics simulations suggest a potential mechanism to get insight into the anion selectivity enhancement of PcVDAC observed in PE relative to PC.

Aspirin induces cell death by directly modulating mitochondrial voltage-dependent anion channel (VDAC)

Aspirin induces apoptotic cell death in various cancer cell lines. Here we showed that silencing of VDAC1 protected HeLa cells from aspirin-induced cell death. Compared to the wild type cells, VDAC1 knocked down cells showed lesser change of mitochondrial membrane potential (Δψ_m), upon aspirin treatment. Aspirin augmented ATP and ionomycin-induced mitochondrial Ca²⁺ uptake which was abolished in VDAC1 knocked down cells. Aspirin dissociated bound hexokinase II (HK-II) from mitochondria. Further, aspirin promoted the closure of recombinant human VDAC1, reconstituted in planar lipid bilayer. Taken together, these results imply that VDAC1 serves as a novel target for aspirin. Modulation of VDAC1 is possibly associated with the cell death and anticancer effects of aspirin.

Alterations in Ca2+ Signalling via ER-Mitochondria Contact Site Remodelling in Cancer

Inter-organellar contact sites establish microdomains for localised Ca²⁺-signalling events. One of these microdomains is established between the ER and the mitochondria. Importantly, the so-called mitochondria-associated ER membranes (MAMs) contain, besides structural proteins and proteins involved in lipid exchange, several Ca²⁺-transport systems, mediating efficient Ca²⁺ transfer from the ER to the mitochondria. These Ca²⁺ signals critically control several mitochondrial functions, thereby impacting cell metabolism, cell death and survival, proliferation and migration. Hence, the MAMs have emerged as critical signalling hubs in physiology, while their dysregulation is an important factor that drives or at least contributes to oncogenesis and tumour progression. In this book chapter, we will provide an overview of the role of the MAMs in cell function and how alterations in the MAM composition contribute to oncogenic features and behaviours.

Clinical implication of voltage-dependent anion channel 1 in uterine cervical cancer and its action on cervical cancer cells

Two-dimensional gel electrophoresis and liquid chromatography-tandem mass spectrometry were performed to investigate the influence of human nonmetastatic clone 23 type 1 (nm23-H1), a metastasis-associated gene on proteomic alterations in cancer cells of the uterine cervix. It was validated by RT-PCR and Western blot analysis. The expression of voltage-dependent anion channel 1 (VDAC1) was increased in nm23-H1 gene silenced SiHa or CaSki cervical cancer cells. The clinical implication was shown that cervical cancer tissues with positive VDAC1 immunoreactivity exhibited deep stromal invasion (>10 mm in depth) and large tumor size (> 4 cm in diameter). Cervical cancer patients with positive VDAC1 immunoreactivity displayed higher recurrence and poorer overall survival than those with negative VDAC1. Silencing of VDAC1 reduced cell proliferation and migratory ability. Mitochondrial membrane potential was decreased and reactive oxygen species generation was increased in the VDAC1 gene-silenced cervical cancer cells. Cell cycle progression and autophagy were not changed in VDAC1 silencing cells. The cytotoxicity of cisplatin was significantly enhanced by knockdown of cellular VDAC1 and the compounds that interfere with hexokinase binding to VDAC. Therapeutic strategies may be offered using VDAC1 as a target to reduce cell growth and migration, enhance the synergistic therapeutic efficacy of cisplatin and reduce cisplatin dose-limiting toxicity.

Voltage Dependence of Conformational Dynamics and Subconducting States of VDAC-1

The voltage-dependent anion channel 1 (VDAC-1) is an important protein of the outer mitochondrial membrane that transports energy metabolites and is involved in apoptosis. The available structures of VDAC proteins show a wide β-stranded barrel pore, with its N-terminal α-helix (N-α) bound to its interior. Electrophysiology experiments revealed that voltage, its polarity, and membrane composition modulate VDAC currents. Experiments with VDAC-1 mutants identified amino acids that regulate the gating process. However, the mechanisms for how these factors regulate VDAC-1, and which changes they trigger in the channel, are still unknown. In this study, molecular dynamics simulations and single-channel experiments of VDAC-1 show agreement for the current-voltage relationships of an "open" channel and they also show several subconducting transient states that are more cation selective in the simulations. We observed voltage-dependent asymmetric distortions of the VDAC-1 barrel and the displacement of particular charged amino acids. We constructed conformational models of the protein voltage response and the pore changes that consistently explain the protein conformations observed at opposite voltage polarities, either in phosphatidylethanolamine or phosphatidylcholine membranes. The submicrosecond VDAC-1 voltage response shows intrinsic structural changes that explain the role of key gating amino acids and support some of the current gating hypotheses. These voltage-dependent protein changes include asymmetric barrel distortion, its interaction with the membrane, and significant displacement of N-α amino acids.

The porin VDAC2 is the mitochondrial platform for Bax retrotranslocation

The pro-apoptotic Bcl-2 protein Bax can permeabilize the outer mitochondrial membrane and therefore commit human cells to apoptosis. Bax is regulated by constant translocation to the mitochondria and retrotranslocation back into the cytosol. Bax retrotranslocation depends on pro-survival Bcl-2 proteins and stabilizes inactive Bax. Here we show that Bax retrotranslocation shuttles membrane-associated and membrane-integral Bax from isolated mitochondria. We further discover the mitochondrial porin voltage-dependent anion channel 2 (VDAC2) as essential component and platform for Bax retrotranslocation. VDAC2 ensures mitochondria-specific membrane association of Bax and in the absence of VDAC2 Bax localizes towards other cell compartments. Bax retrotranslocation is also regulated by nucleotides and calcium ions, suggesting a potential role of the transport of these ions through VDAC2 in Bax retrotranslocation. Together, our results reveal the unanticipated bifunctional role of VDAC2 to target Bax specifically to the mitochondria and ensure Bax inhibition by retrotranslocation into the cytosol.

In silico construction of HK2-VDAC1 complex and investigating the HK2 binding-induced molecular gating mechanism of VDAC1

Hexokinase 2 (HK2) binds to Voltage-Dependent Anion Channel 1 (VDAC1) on mitochondrial outer membrane (MOM) to facilitate a preferential access of ATP to HK2 for glycolysis, in order to maintain a constant energy source for cell proliferation in cancer especially. While previous studies have discovered that the VDAC1 N-terminal helix is responsible for regulating molecules from within mitochondria to cytoplasm, the molecular mechanism of how HK2 is able to regulate the ATP access remains elusive. We hereby propose a model for the HK2-VDAC1 association. The model is then subjected to molecular dynamics (MD) simulations, where we probe the effect of HK2 binding on the mobility of the VDAC1 N-terminal helix. Results from the simulations show that HK2 binding restricts the movement of the VDAC1 N-terminal helix. As a result, VDAC1 is kept in the open state most of the time and probably allows a constant supply of ATP to HK2 for glycolysis.

Current state of theoretical and experimental studies of the voltage-dependent anion channel (VDAC)

Voltage-dependent anion channel (VDAC), the major channel of the mitochondrial outer membrane provides a controlled pathway for respiratory metabolites in and out of the mitochondria. In spite of the wealth of experimental data from structural, biochemical, and biophysical investigations, the exact mechanisms governing selective ion and metabolite transport, especially the role of titratable charged residues and interactions with soluble cytosolic proteins, remain hotly debated in the field. The computational advances hold a promise to provide a much sought-after solution to many of the scientific disputes around solute and ion transport through VDAC and hence, across the mitochondrial outer membrane. In this review, we examine how Molecular Dynamics, Free Energy, and Brownian Dynamics simulations of the large β-barrel channel, VDAC, advanced our understanding. We will provide a short overview of non-conventional techniques and also discuss examples of how the modeling excursions into VDAC biophysics prospectively aid experimental efforts. This article is part of a Special Issue entitled: Membrane Proteins edited by J.C. Gumbart and Sergei Noskov.

Overexpression of human SOD1 in VDAC1-less yeast restores mitochondrial functionality modulating beta-barrel outer membrane protein genes

Cu/Zn Superoxide Dismutase (SOD1), the most important antioxidant defense against ROS in eukaryotic cells, localizes in cytosol and intermembrane space of mitochondria (IMS). Several evidences show a SOD1 intersection with both fermentative and respiratory metabolism. The Voltage Dependent Anion Channel (VDAC) is the main pore-forming protein in the mitochondrial outer membrane (MOM), and is considered the gatekeeper of mitochondrial metabolism. Saccharomyces cerevisiae lacking VDAC1 (Δpor1) is a very convenient model system, since it shows an impaired growth rate on non-fermentable carbon source. Transformation of Δpor1 yeast with human SOD1 completely restores the cell growth deficit in non-fermentative conditions and re-establishes the physiological levels of ROS, as well as the mitochondrial membrane potential. No similar result was found upon yeast SOD1 overexpression. A previous report highlighted the action of SOD1 as a transcription factor. Quantitative Real-Time PCR showed that β-barrel outer-membrane encoding-genes por2, tom40, sam50 are induced by hSOD1, but the same effect was not obtained in Δpor1Δpor2 yeast, indicating a crucial function for yVDAC2. Since the lack of VDAC1 in yeast can be considered a stress factor for the cell, hSOD1 could relieve it stimulating the expression of genes bringing to the recovery of the MOM function. Our results suggest a direct influence of SOD1 on VDAC.

The VDAC channel: Molecular basis for selectivity

The voltage dependent anion-selective channel, VDAC, is the major permeability pathway by which molecules and ion cross the mitochondrial outer membrane. This pathway has evolved to optimize the flow of these substances and to control this flow by a gating process that is influenced by a variety of factors including transmembrane voltage. The permeation pathway formed through the membrane by VDAC is complex. Small ion flow is primarily influenced by the charged surface of the inner walls of the channel. Channel closure changes this landscape resulting in a change from a channel that favors anions to one that favors cations. Molecular ions interact more intimately with the inner walls of the channel and are selected by their 3-dimensional structure, not merely by their size and charge. Molecular ions typically found in cells are greatly favored over those that are not. For these larger structures the channel may form a low-energy translocation path that complements the structure of the permeant. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.

A novel SOD mimic with a redox-modulating mn (II) complex, ML1 attenuates high glucose-induced abnormalities in intracellular Ca2+ transients and prevents cardiac cell death through restoration of mitochondrial function

A key contributor to the pathophysiology of diabetic cardiomyopathy, mitochondrial superoxide can be adequately countered by Mn-superoxide dismutase, which constitutes the first line of defense against mitochondrial oxidative stress. Our group has recently synthesized low molecular weight SOD mimics, demonstrating superior protection against oxidative damages to kidney cells. In the current study, we sought to evaluate the protective effect of the SOD mimic ML1 against high glucose induced cardiomyopathy in diabetes. Mechanistic studies using rat cardiac myoblast H9c2 showed that ML1 markedly inhibited High Glucose (HG) induced cytotoxicity. This was associated with increased Mn-SOD expression along with decreased mitochondrial [Formula: see text], ONOO- and Ca²⁺ accumulation, unveiling its anti-oxidant potentials. ML1 also attenuated HG-induced loss of mitochondrial membrane potential (ΔΨ_m) and release of cytochrome c, suggesting that ML1 effectuates its cytoprotective action via the preservation of mitochondrial function. In an ex-vivo model normal adult rat ventricular myocytes (ARVMs) were isolated and cultured in either normal glucose (5.5 mmol/l glucose) or HG (25.5 mmol/l glucose) conditions and the efficiency of ML-1 was analyzed by studying contractile function and calcium indices. Mechanical properties were assessed using a high-speed video-edge detection system, and intracellular Ca²⁺ transients were recorded in fura-2-loaded myocytes. Pretreatment of myocytes with ML1 (10 nM) ameliorated HG induced abnormalities in relaxation including depressed peak shortening, prolonged time to 90% relenghthening, and slower Ca²⁺ transient decay. Thus, ML1 exhibits significant cardio protection against oxidative damage, perhaps through its potent antioxidant action via activation of Mn-SOD.

Antifungal drug itraconazole targets VDAC1 to modulate the AMPK/mTOR signaling axis in endothelial cells

Itraconazole, a clinically used antifungal drug, was found to possess potent antiangiogenic and anticancer activity that is unique among the azole antifungals. Previous mechanistic studies have shown that itraconazole inhibits the mechanistic target of rapamycin (mTOR) signaling pathway, which is known to be a critical regulator of endothelial cell function and angiogenesis. However, the molecular target of itraconazole that mediates this activity has remained unknown. Here we identify the major target of itraconazole in endothelial cells as the mitochondrial protein voltage-dependent anion channel 1 (VDAC1), which regulates mitochondrial metabolism by controlling the passage of ions and small metabolites through the outer mitochondrial membrane. VDAC1 knockdown profoundly inhibits mTOR activity and cell proliferation in human umbilical vein cells (HUVEC), uncovering a previously unknown connection between VDAC1 and mTOR. Inhibition of VDAC1 by itraconazole disrupts mitochondrial metabolism, leading to an increase in the cellular AMP:ATP ratio and activation of the AMP-activated protein kinase (AMPK), an upstream regulator of mTOR. VDAC1-knockout cells are resistant to AMPK activation and mTOR inhibition by itraconazole, demonstrating that VDAC1 is the mediator of this activity. In addition, another known VDAC-targeting compound, erastin, also activates AMPK and inhibits mTOR and proliferation in HUVEC. VDAC1 thus represents a novel upstream regulator of mTOR signaling in endothelial cells and a promising target for the development of angiogenesis inhibitors.

VDAC3 gating is activated by suppression of disulfide-bond formation between the N-terminal region and the bottom of the pore

The voltage-dependent anion channels (VDACs), VDAC1, VDAC2, and VDAC3, are pore-forming proteins that control metabolite flux between mitochondria and cytoplasm. VDAC1 and VDAC2 have voltage-dependent gating activity, whereas VDAC3 is thought to have weak activity. The aim of this study was to analyze the channel properties of all three human VDAC isoforms and to clarify the channel function of VDAC3. Bacterially expressed recombinant human VDAC proteins were reconstituted into artificial planar lipid bilayers and their gating activities were evaluated. VDAC1 and VDAC2 had typical voltage-dependent gating activity, whereas the gating of VDAC3 was weak, as reported. However, gating of VDAC3 was evoked by dithiothreitol (DTT) and S-nitrosoglutathione (GSNO), which are thought to suppress disulfide-bond formation. Several cysteine mutants of VDAC3 also exhibited typical voltage-gating. Our results indicate that channel gating was induced by reduction of a disulfide-bond linking the N-terminal region to the bottom of the pore. Thus, channel gating of VDAC3 might be controlled by redox sensing under physiological conditions.

Evidence of Distinct Channel Conformations and Substrate Binding Affinities for the Mitochondrial Outer Membrane Protein Translocase Pore Tom40

Nearly all mitochondrial proteins are coded by the nuclear genome and must be transported into mitochondria by the translocase of the outer membrane complex. Tom40 is the central subunit of the translocase complex and forms a pore in the mitochondrial outer membrane. To date, the mechanism it utilizes for protein transport remains unclear. Tom40 is predicted to comprise a membrane-spanning β-barrel domain with conserved α-helical domains at both the N and C termini. To investigate Tom40 function, including the role of the N- and C-terminal domains, recombinant forms of the Tom40 protein from the yeast Candida glabrata, and truncated constructs lacking the N- and/or C-terminal domains, were functionally characterized in planar lipid membranes. Our results demonstrate that each of these Tom40 constructs exhibits at least four distinct conductive levels and that full-length and truncated Tom40 constructs specifically interact with a presequence peptide in a concentration- and voltage-dependent manner. Therefore, neither the first 51 amino acids of the N terminus nor the last 13 amino acids of the C terminus are required for Tom40 channel formation or for the interaction with a presequence peptide. Unexpectedly, substrate binding affinity was dependent upon the Tom40 state corresponding to a particular conductive level. A model where two Tom40 pores act in concert as a dimeric protein complex best accounts for the observed biochemical and electrophysiological data. These results provide the first evidence for structurally distinct Tom40 conformations playing a role in substrate recognition and therefore in transport function.

Metabolic Reprogramming Is Required for Myofibroblast Contractility and Differentiation

Contraction is crucial in maintaining the differentiated phenotype of myofibroblasts. Contraction is an energy-dependent mechanism that relies on the production of ATP by mitochondria and/or glycolysis. Although the role of mitochondrial biogenesis in the adaptive responses of skeletal muscle to exercise is well appreciated, mechanisms governing energetic adaptation of myofibroblasts are not well understood. Our study demonstrates induction of mitochondrial biogenesis and aerobic glycolysis in response to the differentiation-inducing factor transforming growth factor β1 (TGF-β1). This metabolic reprogramming is linked to the activation of the p38 mitogen-activated protein kinase (MAPK) pathway. Inhibition of p38 MAPK decreased accumulation of active peroxisome proliferator-activated receptor γ coactivator 1α in the nucleus and altered the translocation of mitochondrial transcription factor A to the mitochondria. Genetic or pharmacologic approaches that block mitochondrial biogenesis or glycolysis resulted in decreased contraction and reduced expression of TGF-β1-induced α-smooth muscle actin and collagen α-2(I) but not of fibronectin or collagen α-1(I). These data indicate a critical role for TGF-β1-induced metabolic reprogramming in regulating myofibroblast-specific contractile signaling and support the concept of integrating bioenergetics with cellular differentiation.

Dual mechanism of ion permeation through VDAC revealed with inorganic phosphate ions and phosphate metabolites

In the exchange of metabolites and ions between the mitochondrion and the cytosol, the voltage-dependent anion channel (VDAC) is a key element, as it forms the major transport pathway for these compounds through the mitochondrial outer membrane. Numerous experimental studies have promoted the idea that VDAC acts as a regulator of essential mitochondrial functions. In this study, using a combination of molecular dynamics simulations, free-energy calculations, and electrophysiological measurements, we investigated the transport of ions through VDAC, with a focus on phosphate ions and metabolites. We showed that selectivity of VDAC towards small anions including monovalent phosphates arises from short-lived interactions with positively charged residues scattered throughout the pore. In dramatic contrast, permeation of divalent phosphate ions and phosphate metabolites (AMP and ATP) involves binding sites along a specific translocation pathway. This permeation mechanism offers an explanation for the decrease in VDAC conductance measured in the presence of ATP or AMP at physiological salt concentration. The binding sites occur at similar locations for the divalent phosphate ions, AMP and ATP, and contain identical basic residues. ATP features a marked affinity for a central region of the pore lined by two lysines and one arginine of the N-terminal helix. This cluster of residues together with a few other basic amino acids forms a "charged brush" which facilitates the passage of the anionic metabolites through the pore. All of this reveals that VDAC controls the transport of the inorganic phosphates and phosphate metabolites studied here through two different mechanisms.

Molecular mechanism of mitochondrial calcium uptake

Mitochondrial calcium uptake plays a critical role in various cellular functions. After half a century of extensive studies, the molecular components and important regulators of the mitochondrial calcium uptake complex have been identified. However, the mechanism by which these protein molecules interact with one another and coordinate to regulate calcium passage through mitochondrial membranes remains elusive. Here, we summarize recent progress in the structural and functional characterization of these important protein molecules, which are involved in mitochondrial calcium uptake. In particular, we focus on the current understanding of the molecular mechanism underlying calcium through two mitochondrial membranes. Additionally, we provide a new perspective for future directions in investigation and molecular intervention.

Lipid bilayer-bound conformation of an integral membrane beta barrel protein by multidimensional MAS NMR

The human voltage dependent anion channel 1 (VDAC) is a 32 kDa β-barrel integral membrane protein that controls the transport of ions across the outer mitochondrial membrane. Despite the determination of VDAC solution and diffraction structures, a structural basis for the mechanism of its function is not yet fully understood. Biophysical studies suggest VDAC requires a lipid bilayer to achieve full function, motivating the need for atomic resolution structural information of VDAC in a membrane environment. Here we report an essential step toward that goal: extensive assignments of backbone and side chain resonances for VDAC in DMPC lipid bilayers via magic angle spinning nuclear magnetic resonance (MAS NMR). VDAC reconstituted into DMPC lipid bilayers spontaneously forms two-dimensional lipid crystals, showing remarkable spectral resolution (0.5-0.3 ppm for (13)C line widths and <0.5 ppm (15)N line widths at 750 MHz). In addition to the benefits of working in a lipid bilayer, several distinct advantages are observed with the lipid crystalline preparation. First, the strong signals and sharp line widths facilitated extensive NMR resonance assignments for an integral membrane β-barrel protein in lipid bilayers by MAS NMR. Second, a large number of residues in loop regions were readily observed and assigned, which can be challenging in detergent-solubilized membrane proteins where loop regions are often not detected due to line broadening from conformational exchange. Third, complete backbone and side chain chemical shift assignments could be obtained for the first 25 residues, which comprise the functionally important N-terminus. The reported assignments allow us to compare predicted torsion angles for VDAC prepared in DMPC 2D lipid crystals, DMPC liposomes, and LDAO-solubilized samples to address the possible effects of the membrane mimetic environment on the conformation of the protein. Concluding, we discuss the strengths and weaknesses of the reported assignment approach and the great potential for even more complete assignment studies and de novo structure determination via (1)H detected MAS NMR.

Novel subcellular localization for α-synuclein: possible functional consequences

α-synuclein (α-syn) is one of the genes that when mutated or overexpressed causes Parkinson’s Disease (PD). Initially, it was described as a synaptic terminal protein and later was found to be localized at mitochondria. Mitochondria-associated membranes (MAM) have emerged as a central endoplasmic reticulum (ER) subcellular compartments where key functions of the cell occur. These domains, enriched in cholesterol and anionic phospholipids, are where calcium homeostasis, lipid transfer, and cholesterol metabolism are regulated. Some proteins, related to mitochondrial dynamics and function, are also localized to this area. Several neurodegenerative diseases have shown alterations in MAM functions and resident proteins, including Charcot Marie-Tooth and Alzheimer’s disease (AD). We have recently reported that MAM function is downregulated in cell and mouse models of PD expressing pathogenic mutations of α-syn. This review focuses on the possible role of α-syn in these cellular domains and the early pathogenic features of PD that could be explained by α-syn-MAM disturbances.

Magic angle spinning nuclear magnetic resonance characterization of voltage-dependent anion channel gating in two-dimensional lipid crystalline bilayers

The N-terminus of the voltage-dependent anion channel (VDAC) has been proposed to contain the mechanistically important gating helices that modulate channel opening and closing. In this study, we utilize magic angle spinning nuclear magnetic resonance (MAS NMR) to determine the location and structure of the N-terminus for functional channels in lipid bilayers by measuring long-range ¹³C–¹³C distances between residues in the N-terminus and other domains of VDAC reconstituted into DMPC lipid bilayers. Our structural studies show that the distance between A14 C_β in the N-terminal helix and S193 C_β is ∼4–6 Å. Furthermore, VDAC phosphorylation by a mitochondrial kinase at residue S193 has been claimed to delay mitochondrial cell death by causing a conformational change that closes the channel, and a VDAC-Ser193Glu mutant has been reported to show properties very similar to those of phosphorylated VDAC in a cellular context. We expressed VDAC-S193E and reconstituted it into DMPC lipid bilayers. Two-dimensional ¹³C–¹³C correlation experiments showed chemical shift perturbations for residues located in the N-terminus, indicating possible structural perturbations to that region. However, electrophysiological data recorded on VDAC-S193E showed that channel characteristics were identical to those of wild type samples, indicating that phosphorylation of S193 does not directly affect channel gating. The combination of NMR and electrophysiological results allows us to discuss the validity of proposed gating models.

Markov chain Monte Carlo based analysis of post-translationally modified VDAC gating kinetics

The voltage-dependent anion channel (VDAC) is the main conduit for permeation of solutes (including nucleotides and metabolites) of up to 5 kDa across the mitochondrial outer membrane (MOM). Recent studies suggest that VDAC activity is regulated via post-translational modifications (PTMs). Yet the nature and effect of these modifications is not understood. Herein, single channel currents of wild-type, nitrosated, and phosphorylated VDAC are analyzed using a generalized continuous-time Markov chain Monte Carlo (MCMC) method. This developed method describes three distinct conducting states (open, half-open, and closed) of VDAC activity. Lipid bilayer experiments are also performed to record single VDAC activity under un-phosphorylated and phosphorylated conditions, and are analyzed using the developed stochastic search method. Experimental data show significant alteration in VDAC gating kinetics and conductance as a result of PTMs. The effect of PTMs on VDAC kinetics is captured in the parameters associated with the identified Markov model. Stationary distributions of the Markov model suggest that nitrosation of VDAC not only decreased its conductance but also significantly locked VDAC in a closed state. On the other hand, stationary distributions of the model associated with un-phosphorylated and phosphorylated VDAC suggest a reversal in channel conformation from relatively closed state to an open state. Model analyses of the nitrosated data suggest that faster reaction of nitric oxide with Cys-127 thiol group might be responsible for the biphasic effect of nitric oxide on basal VDAC conductance.

Modulation of the mitochondrial voltage dependent anion channel (VDAC) by curcumin

Voltage dependent anion channel (VDAC) of mitochondria plays a crucial role in apoptosis. Human VDAC-1, reconstituted in planar lipid bilayer showed reduced conductance when treated with curcumin. Curcumin interacts with residues in the α helical N-terminus of VDAC and in the channel wall, as revealed by molecular docking, followed by mutational analysis. N-terminus mimicking peptide showed conformational changes in circular dichroism, upon curcumin treatment. We propose that the interaction of curcumin with amino acids in N-terminus and in channel wall fixes the α helix in closed conformation. This restricts its movement which is required for the opening of the channel.

Mitochondrial targets for volatile anesthetics against cardiac ischemia-reperfusion injury

Mitochondria are critical modulators of cell function and are increasingly recognized as proximal sensors and effectors that ultimately determine the balance between cell survival and cell death. Volatile anesthetics (VA) are long known for their cardioprotective effects, as demonstrated by improved mitochondrial and cellular functions, and by reduced necrotic and apoptotic cell death during cardiac ischemia and reperfusion (IR) injury. The molecular mechanisms by which VA impart cardioprotection are still poorly understood. Because of the emerging role of mitochondria as therapeutic targets in diseases, including ischemic heart disease, it is important to know if VA-induced cytoprotective mechanisms are mediated at the mitochondrial level. In recent years, considerable evidence points to direct effects of VA on mitochondrial channel/transporter protein functions and electron transport chain (ETC) complexes as potential targets in mediating cardioprotection. This review furnishes an integrated overview of targets that VA impart on mitochondrial channels/transporters and ETC proteins that could provide a basis for cation regulation and homeostasis, mitochondrial bioenergetics, and reactive oxygen species (ROS) emission in redox signaling for cardiac cell protection during IR injury.

Role of mitochondria in mutant SOD1 linked amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease with an adult onset characterized by loss of both upper and lower motor neurons. In ~10% of cases, patients developed ALS with an apparent genetic linkage (familial ALS or fALS). Approximately 20% of fALS displays mutations in the SOD1 gene encoding superoxide dismutase 1. There are many proposed cellular and molecular mechanisms among which, mitochondrial dysfunctions occur early, prior to symptoms occurrence. In this review, we modeled the effect of mutant SOD1 protein via the formation of a toxic complex with Bcl2 on mitochondrial bioenergetics. Furthermore, we discuss that the shutdown of ATP permeation through mitochondrial outer membrane could lead to both respiration inhibition and temporary mitochondrial hyperpolarization. Moreover, we reviewed mitochondrial calcium signaling, oxidative stress, fission and fusion, autophagy and apoptosis in mutant SOD1-linked ALS. Functional defects in mitochondria appear early before symptoms are manifested in ALS. Therefore, mitochondrial dysfunction is a promising therapeutic target in ALS.

Reduced VDAC1 protects against Alzheimer's disease, mitochondria, and synaptic deficiencies

The objective of this study was to elucidate the effect of VDAC1 on Alzheimer's disease (AD)-related genes, mitochondrial activity, and synaptic viability. Recent knockout studies of VDAC1 revealed that homozygote VDAC1 knockout (VDAC1-/-) mice exhibited disrupted learning and synaptic plasticity, and in contrast, VDAC1+/- mice appeared normal in terms of lifespan, fertility, and viability relative to wild-type mice. However, the effects of reduced VDAC1 on mitochondrial/synaptic genes and mitochondrial function in AD-affected neurons are not well understood. In the present study, we characterized mitochondrial/synaptic and AD-related genes and mitochondrial function in VDAC1+/- mice and VDAC1+/+ mice. We found reduced mRNA levels in the AD-related genes, including AβPP, Tau, PS1, PS2, and BACE1; increased levels of the mitochondrial fusion genes Mfn1, Mfn2; reduced levels of the fission genes Drp1 and Fis1; and reduced levels of the mitochondrial permeability transition pore genes VDAC1, ANT, and CypD in VDAC1+/- mice relative to VDAC1+/+ mice. Hexokinase 1 and 2 were significantly upregulated in the VDAC+/- mice. The synaptic genes synaptophysin, synapsin 1 and 2, synaptobrevin 1 and 2, neurogranin, and PSD95 were also upregulated in the VDAC1+/- mice. Free radical production and lipid peroxidation levels were reduced in the VDAC1+/- mice, and cytochrome oxidase activity and ATP levels were elevated, indicating enhanced mitochondrial function in the VDAC1+/- mice. These findings suggest that reduced VDAC1 expression, such as that we found in the VDAC1+/- mice, may be beneficial to synaptic activity, may improve function, and may protect against toxicities of AD-related genes.

Acidification asymmetrically affects voltage-dependent anion channel implicating the involvement of salt bridges

The voltage-dependent anion channel (VDAC) is the major pathway for ATP, ADP, and other respiratory substrates through the mitochondrial outer membrane, constituting a crucial point of mitochondrial metabolism regulation. VDAC is characterized by its ability to "gate" between an open and several "closed" states under applied voltage. In the early stages of tumorigenesis or during ischemia, partial or total absence of oxygen supply to cells results in cytosolic acidification. Motivated by these facts, we investigated the effects of pH variations on VDAC gating properties. We reconstituted VDAC into planar lipid membranes and found that acidification reversibly increases its voltage-dependent gating. Furthermore, both VDAC anion selectivity and single channel conductance increased with acidification, in agreement with the titration of the negatively charged VDAC residues at low pH values. Analysis of the pH dependences of the gating and open channel parameters yielded similar pKa values close to 4.0. We also found that the response of VDAC gating to acidification was highly asymmetric. The presumably cytosolic (cis) side of the channel was the most sensitive to acidification, whereas the mitochondrial intermembrane space (trans) side barely responded to pH changes. Molecular dynamic simulations suggested that stable salt bridges at the cis side, which are susceptible to disruption upon acidification, contribute to this asymmetry. The pronounced sensitivity of the cis side to pH variations found here in vitro might provide helpful insights into the regulatory role of VDAC in the protective effect of cytosolic acidification during ischemia in vivo.

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

Abnormal alpha-synuclein reduces nigral voltage-dependent anion channel 1 in sporadic and experimental Parkinson's disease

Both the misfolding of α-synuclein and mitochondrial dysfunction are considered two major contributors to Parkinson's disease (PD). However, the relationship between the two in normal and PD states remains unclear. Here, we report that voltage-dependent anion channel 1 (VDAC1), a major component of the outer mitochondrial membrane known to regulate mitochondrial functions, is down-regulated in response to α-synuclein accumulation and aggregation. Stereological analysis revealed that 58.33% of the neurons were VDAC1 immunoreactive in the remaining neuromelanin laden neurons in the PD group while 87.48% of the nigral neurons were VDAC1 immunoreactive in the age-matched control group. The relative levels of VDAC1 were significantly decreased in PD nigral neurons when compared to age-matched controls. In PD, this decrease was significantly greater in nigral neurons with α-synuclein inclusions. VDAC1 was observed in fibers with granular α-synuclein but not in fibers with aggregated α-synuclein. Viral vector-mediated overexpression of mutant human α-synuclein (A30P) in rats resulted in significantly decreased VDAC1 in nigral neurons and striatal fibers. These results indicate that mitochondrial function associated with VDAC1 is decreased in sporadic and experimental PD, and this decrease is associated with α-synuclein accumulation and aggregation.

Origin of ion selectivity in Phaseolus coccineus mitochondrial VDAC

The mitochondrial voltage-dependent a nion-selective channel (VDAC) is the major permeation pathway for small ions and metabolites. Although a wealth of electrophysiological data has been obtained on different VDAC species, the physical mechanisms of their ionic selectivity are still elusive. We addressed this issue using electrophysiological experiments performed on plant VDAC. A simple macroscopic electrodiffusion model accounting for ion diffusion and for an effective fixed charge of the channel describes well its selectivity. Brownian Dynamics simulations of ion permeation performed on plant and mammalian VDACs point to the role of specific charged residues located at about the middle of the pore.

Measurement of mitochondrial Ca2+ transport mediated by three transport proteins: VDAC1, the Na+/Ca2+ exchanger, and the Ca2+ uniporter

Ca(2+) is a ubiquitous cellular signal, with changes in intracellular Ca(2+) concentration not only stimulating a number of intercellular events but also triggering cell death pathways, including apoptosis. Mitochondrial Ca(2+) uptake and release play pivotal roles in cellular physiology by regulating intracellular Ca(2+) signaling, energy metabolism and cell death. Ca(2+) transport across the inner and outer mitochondrial membranes is mediated by several proteins, including channels, antiporters, and a uniporter. In this article, we present the background to several methods now established for assaying mitochondrial Ca(2+) transport activity across both mitochondrial membranes. The first of these is Ca(2+) transport mediated by the outer mitochondrial protein, the voltage-dependent anion-selective channel protein 1 (VDAC1, also known as porin 1), both as a purified protein reconstituted into a planar lipid bilayer (PLB) or into liposomes and as a mitochondrial membrane-embedded protein. The second method involves isolated mitochondria for assaying the activity of an inner mitochondrial membrane transport protein, the mitochondrial Ca(2+) uniporter (MCU) that transports Ca(2+) and is powered by the steep mitochondrial membrane potential. In the event of Ca(2+) overload, this leads to opening of the mitochondrial permeability transition pore (MPTP) and cell death. The third method describes how Na(+)-dependent mitochondrial Ca(2+) efflux mediated by mitochondrial NCLX, a member of the Na(+)/Ca(2+) exchanger superfamily, can be assayed in digitonin-permeabilized HEK-293 cells. The Ca(2+)-transport assays can be performed under various conditions and in combination with inhibitors, allowing detailed characterization of the transport activity of interest.

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.