Visualizing VDAC1 in live cells using a tetracysteine tag

The voltage-dependent anion channel 1 (VDAC1) is a crucial gatekeeper in the outer mitochondrial membrane, controlling metabolic and energy homeostasis. The available methodological approaches fell short of accurate visualization of VDAC1 in living cells. To permit precise VDAC1 imaging, we utilized the tetracysteine (TC)-tag and visualized VDAC1 dynamics in living cells. TC-tagged VDAC1 had a cluster-like distribution on mitochondria. The labeling of TC-tagged VDAC1 was validated with immunofluorescence. The majority of VDAC1-TC-clusters were localized at endoplasmic reticulum (ER)-mitochondria contact sites. Notably, VDAC1 colocalized with BCL-2 Antagonist/Killer (BAK)-clusters upon apoptotic stimulation. Using this new tool, we were able to observe VDAC1-TC at mitochondrial fission sites. These findings highlight the suitability of the TC-tag for live-cell imaging of VDAC1, shedding light on the roles of VDAC1 in cellular processes.

Decoding Cancer through Silencing the Mitochondrial Gatekeeper VDAC1

Mitochondria serve as central hubs for regulating numerous cellular processes that include metabolism, apoptosis, cell cycle progression, proliferation, differentiation, epigenetics, immune signaling, and aging. The voltage-dependent anion channel 1 (VDAC1) functions as a crucial mitochondrial gatekeeper, controlling the flow of ions, such as Ca2+, nucleotides, and metabolites across the outer mitochondrial membrane, and is also integral to mitochondria-mediated apoptosis. VDAC1 functions in regulating ATP production, Ca2+ homeostasis, and apoptosis, which are essential for maintaining mitochondrial function and overall cellular health. Most cancer cells undergo metabolic reprogramming, often referred to as the "Warburg effect", supplying tumors with energy and precursors for the biosynthesis of nucleic acids, phospholipids, fatty acids, cholesterol, and porphyrins. Given its multifunctional nature and overexpression in many cancers, VDAC1 presents an attractive target for therapeutic intervention. Our research has demonstrated that silencing VDAC1 expression using specific siRNA in various tumor types leads to a metabolic rewiring of the malignant cancer phenotype. This results in a reversal of oncogenic properties that include reduced tumor growth, invasiveness, stemness, epithelial-mesenchymal transition. Additionally, VDAC1 depletion alters the tumor microenvironment by reducing angiogenesis and modifying the expression of extracellular matrix- and structure-related genes, such as collagens and glycoproteins. Furthermore, VDAC1 depletion affects several epigenetic-related enzymes and substrates, including the acetylation-related enzymes SIRT1, SIRT6, and HDAC2, which in turn modify the acetylation and methylation profiles of histone 3 and histone 4. These epigenetic changes can explain the altered expression levels of approximately 4000 genes that are associated with reversing cancer cells oncogenic properties. Given VDAC1's critical role in regulating metabolic and energy processes, targeting it offers a promising strategy for anti-cancer therapy. We also highlight the role of VDAC1 expression in various disease pathologies, including cardiovascular, neurodegenerative, and viral and bacterial infections, as explored through siRNA targeting VDAC1. Thus, this review underscores the potential of targeting VDAC1 as a strategy for addressing high-energy-demand cancers. By thoroughly understanding VDAC1's diverse roles in metabolism, energy regulation, mitochondrial functions, and other cellular processes, silencing VDAC1 emerges as a novel and strategic approach to combat cancer.

Study on the mechanism of arsenic-induced renal injury based on SWATH proteomics technology

BACKGROUND

Arsenic (As) poisoning is a worldwide endemic disease affecting thousands of people. As is excreted mainly through the renal system, and arsenic has toxic effects on the kidneys, but the mechanism has not been elucidated. In this study, the molecular basis of arsenic's nephrotoxicity was studied by using a high-throughput proteomics technique.

METHODS

Eight SD (Sprague-Dawley) rats, half male and half female, were fed an As diet containing 50 mg/kg NaAsO2. Age- and sex-matched rats fed with regular chow were used as controls. At the end of the experiment (90 days), kidney tissue samples were collected and assessed for pathological changes using hematoxylin-eosin staining. Proteomic methods were used to identify alterations in protein expression levels in kidney tissues, and bioinformatic analyses of differentially expressed proteins between arsenic-treated and control groups were performed. The expression of some representative proteins was validated by Western blot analysis.

RESULTS

NaAsO2 could induce renal injury. Compared with the control group, 112 proteins were up-regulated, and 46 proteins were down-regulated in the arsenic-treated group. These proteins were associated with the electron transport chain, oxidative phosphorylation, mitochondrial membrane, apoptosis, and proximal tubules, suggesting that the mechanisms associated with them were related to arsenic-induced kidney injury and nephrotoxicity. The expressions of Atp6v1f, Cycs and Ndufs1 were verified, consistent with the results of omics.

CONCLUSION

These results provide important evidence for arsenic-induced kidney injury and provide new insights into the molecular mechanism of arsenic-induced kidney injury.

Structure and Gating Behavior of the Human Integral Membrane Protein VDAC1 in a Lipid Bilayer

The voltage-dependent anion channel (VDAC), the most abundant protein in the outer mitochondrial membrane, is responsible for the transport of all ions and metabolites into and out of mitochondria. Larger than any of the β-barrel structures determined to date by magic-angle spinning (MAS) NMR, but smaller than the size limit of cryo-electron microscopy (cryo-EM), VDAC1’s 31 kDa size has long been a bottleneck in determining its structure in a near-native lipid bilayer environment. Using a single two-dimensional (2D) crystalline sample of human VDAC1 in lipids, we applied proton-detected fast magic-angle spinning NMR spectroscopy to determine the arrangement of β strands. Combining these data with long-range restraints from a spin-labeled sample, chemical shift-based secondary structure prediction, and previous MAS NMR and atomic force microscopy (AFM) data, we determined the channel’s structure at a 2.2 Å root-mean-square deviation (RMSD). The structure, a 19-stranded β-barrel, with an N-terminal α-helix in the pore is in agreement with previous data in detergent, which was questioned due to the potential for the detergent to perturb the protein’s functional structure. Using a quintuple mutant implementing the channel’s closed state, we found that dynamics are a key element in the protein’s gating behavior, as channel closure leads to the destabilization of not only the C-terminal barrel residues but also the α2 helix. We showed that cholesterol, previously shown to reduce the frequency of channel closure, stabilizes the barrel relative to the N-terminal helix. Furthermore, we observed channel closure through steric blockage by a drug shown to selectively bind to the channel, the Bcl2-antisense oligonucleotide G3139.

VDACs: An Outlook on Biochemical Regulation and Function in Animal and Plant Systems

The voltage-dependent anion channels (VDACs) are the most abundant proteins present on the outer mitochondrial membrane. They serve a myriad of functions ranging from energy and metabolite exchange to highly debatable roles in apoptosis. Their role in molecular transport puts them on the center stage as communicators between cytoplasmic and mitochondrial signaling events. Beyond their general role as interchangeable pores, members of this family may exhibit specific functions. Even after nearly five decades of their discovery, their role in plant systems is still a new and rapidly emerging field. The information on biochemical regulation of VDACs is limited. Various interacting proteins and post-translational modifications (PTMs) modulate VDAC functions, amongst these, phosphorylation is quite noticeable. In this review, we have tried to give a glimpse of the recent advancements in the biochemical/interactional regulation of plant VDACs. We also cover a critical analysis on the importance of PTMs in the functional regulation of VDACs. Besides, the review also encompasses numerous studies which can identify VDACs as a connecting link between Ca2+ and reactive oxygen species signaling in special reference to the plant systems.

Post-Translational Modification Analysis of VDAC1 in ALS-SOD1 Model Cells Reveals Specific Asparagine and Glutamine Deamidation

Mitochondria from affected tissues of amyotrophic lateral sclerosis (ALS) patients show morphological and biochemical abnormalities. Mitochondrial dysfunction causes oxidative damage and the accumulation of ROS, and represents one of the major triggers of selective death of motor neurons in ALS. We aimed to assess whether oxidative stress in ALS induces post-translational modifications (PTMs) in VDAC1, the main protein of the outer mitochondrial membrane and known to interact with SOD1 mutants related to ALS. In this work, specific PTMs of the VDAC1 protein purified by hydroxyapatite from mitochondria of a NSC34 cell line expressing human SOD1G93A, a suitable ALS motor neuron model, were analyzed by tryptic and chymotryptic proteolysis and UHPLC/High-Resolution ESI-MS/MS. We found selective deamidations of asparagine and glutamine of VDAC1 in ALS-related NSC34-SOD1G93A cells but not in NSC34-SOD1WT or NSC34 cells. In addition, we identified differences in the over-oxidation of methionine and cysteines between VDAC1 purified from ALS model or non-ALS NSC34 cells. The specific range of PTMs identified exclusively in VDAC1 from NSC34-SOD1G93A cells but not from NSC34 control lines, suggests the appearance of important changes to the structure of the VDAC1 channel and therefore to the bioenergetics metabolism of ALS motor neurons. Data are available via ProteomeXchange with identifier .

VDAC1 at the Intersection of Cell Metabolism, Apoptosis, and Diseases

The voltage-dependent anion channel 1 (VDAC1) protein, is an important regulator of mitochondrial function, and serves as a mitochondrial gatekeeper, with responsibility for cellular fate. In addition to control over energy sources and metabolism, the protein also regulates epigenomic elements and apoptosis via mediating the release of apoptotic proteins from the mitochondria. Apoptotic and pathological conditions, as well as certain viruses, induce cell death by inducing VDAC1 overexpression leading to oligomerization, and the formation of a large channel within the VDAC1 homo-oligomer. This then permits the release of pro-apoptotic proteins from the mitochondria and subsequent apoptosis. Mitochondrial DNA can also be released through this channel, which triggers type-Ι interferon responses. VDAC1 also participates in endoplasmic reticulum (ER)-mitochondria cross-talk, and in the regulation of autophagy, and inflammation. Its location in the outer mitochondrial membrane, makes VDAC1 ideally placed to interact with over 100 proteins, and to orchestrate the interaction of mitochondrial and cellular activities through a number of signaling pathways. Here, we provide insights into the multiple functions of VDAC1 and describe its involvement in several diseases, which demonstrate the potential of this protein as a druggable target in a wide variety of pathologies, including cancer.

Cysteine Oxidations in Mitochondrial Membrane Proteins: The Case of VDAC Isoforms in Mammals

Cysteine residues are reactive amino acids that can undergo several modifications driven by redox reagents. Mitochondria are the source of an abundant production of radical species, and it is surprising that such a large availability of highly reactive chemicals is compatible with viable and active organelles, needed for the cell functions. In this work, we review the results highlighting the modifications of cysteines in the most abundant proteins of the outer mitochondrial membrane (OMM), that is, the voltage-dependent anion selective channel (VDAC) isoforms. This interesting protein family carries several cysteines exposed to the oxidative intermembrane space (IMS). Through mass spectrometry (MS) analysis, cysteine posttranslational modifications (PTMs) were precisely determined, and it was discovered that such cysteines can be subject to several oxidization degrees, ranging from the disulfide bridge to the most oxidized, the sulfonic acid, one. The large spectra of VDAC cysteine oxidations, which is unique for OMM proteins, indicate that they have both a regulative function and a buffering capacity able to counteract excess of mitochondrial reactive oxygen species (ROS) load. The consequence of these peculiar cysteine PTMs is discussed.

T-2 toxin cytotoxicity mediated by directly perturbing mitochondria in human gastric epithelium GES-1 cells

T-2 toxin is one of the most toxic trichothecenes and harmful to human health and animal husbandry. The mechanism underlying its growth suppression remains unclear, especially for mitochondrial damage in human gastric epithelial cells. In the present study, we investigated cell death caused by T-2 toxin in a human gastric epithelial cell line (GES-1) and the possible mechanism of T-2-induced cytotoxicity. T-2 strongly reduced the viability of GES-1 cells in a time- and dose-dependent manner within a small range of concentrations. However, when the concentrations of T-2 were >40 nM, there was no concentration dependence, only time dependence. Moreover, T-2 induced apoptosis, with the activation of caspase-3 in GES-1 and mitochondrial membrane potential (MMP) decrease and cytochrome c release. T-2 also resulted in the accumulation of reactive oxygen species (ROS) and DNA damage with a positive signal of p-H2A.X in GES-1 cells. While T-2 caused a MMP decrease, DNA damage and cell death were not blocked by pretreatment with 3 mM glutathione (GSH), a typical scavenger of ROS. The induction of mitochondrial permeability transition pore (mPTP) regulators voltage-dependent anion channel (VDAC1) and cyclophilin D (CypD) were also observed in T-2-treated cells. Interestingly, cyclosporine A (CsA), a CypD inhibitor, significantly reversed the drop in MMP and the DNA damage, as well as ROS accumulation caused by T-2. Additionally, GES-1 cell death could also be protected to some extent by 4, 4'-diisothiocyanatostilbene-2, 2'-disulfonic acid (DIDS), an inhibitor of VDAC1, especially the combination of CsA and DIDS, and 3 mM GSH could further enhance the effect of CsA + DIDS on cell viability. In conclusion, our present findings indicate that the T-2 induced MMP decrease, DNA damage and cell death, as well as ROS accumulation in GES-1 cells, starts with T-2 directly perturbing the mitochondria triggering ROS generation by acting on CypD and VDAC1. This study presents a new viewpoint for evaluating the toxicity of T-2 toxin.

Pentenediol-Type Compounds Specifically Bind to Voltage-Dependent Anion Channel 1 in Saccharomyces cerevisiae Mitochondria

Voltage-dependent anion channel 1 (VDAC1) situated in the outer mitochondrial membrane regulates the transfer of various metabolites and is a key player in mitochondria-mediated apoptosis. Although many small chemicals that modulate the functions of VDAC1 have been reported to date, most, if not all, of them cannot be regarded as specific reagents due to their interactions with other transporters or enzymes. By screening our chemical libraries using isolated Saccharomyces cerevisiae mitochondria, we found pentenediol (PTD)-type compounds (e.g., PTD-023) as new specific inhibitors of VDAC1. PTD-023 inhibited overall ADP-uptake/ATP-release reactions in isolated mitochondria at a single digit μM level. To identify the binding position of PTDs in VDAC1 by visualizing PTD-bound peptides, we conducted ligand-directed tosyl (LDT) chemistry using the synthetic LDT reagent t-PTD-023 derived from the parent PTD-023 in combination with mutagenesis experiments. t-PTD-023 made a covalent bond predominantly and subsidiarily with nucleophilic Cys210 and Cys130, respectively, indicating that PTDs bind to the region interactive with both residues. Site-directed mutations of hydrogen bond-acceptable Asp139 and Glu152 to Ala, which were selected as potential interactive partners of the critical pentenediol moiety based on the presumed binding model of PTDs in VDAC1, resulted in a decrease in susceptibility against PTD-023. This result strongly suggests that PTDs bind to VDAC1 through a specific hydrogen bond with the two residues. The present study is the first to demonstrate the binding position of specific inhibitors of VDAC1 at the amino acid level.

Preserving Insulin Secretion in Diabetes by Inhibiting VDAC1 Overexpression and Surface Translocation in β Cells

Type 2 diabetes (T2D) develops after years of prediabetes during which high glucose (glucotoxicity) impairs insulin secretion. We report that the ATP-conducting mitochondrial outer membrane voltage-dependent anion channel-1 (VDAC1) is upregulated in islets from T2D and non-diabetic organ donors under glucotoxic conditions. This is caused by a glucotoxicity-induced transcriptional program, triggered during years of prediabetes with suboptimal blood glucose control. Metformin counteracts VDAC1 induction. VDAC1 overexpression causes its mistargeting to the plasma membrane of the insulin-secreting β cells with loss of the crucial metabolic coupling factor ATP. VDAC1 antibodies and inhibitors prevent ATP loss. Through direct inhibition of VDAC1 conductance, metformin, like specific VDAC1 inhibitors and antibodies, restores the impaired generation of ATP and glucose-stimulated insulin secretion in T2D islets. Treatment of db/db mice with VDAC1 inhibitor prevents hyperglycemia, and maintains normal glucose tolerance and physiological regulation of insulin secretion. Thus, β cell function is preserved by targeting the novel diabetes executer protein VDAC1.

Hydrogen-rich saline protects rat from oxygen glucose deprivation and reperusion-induced apoptosis through VDAC1 via Bcl-2

BACKGROUND

Hydrogen is received as an inert gas that thought to be non-functional in vivo previously. Recently, emerging evidences showed that in ischemia/reperfusion (IR) condition, hydrogen reduced cellular reactive oxygen species (ROS) production and ameliorated cell apoptosis. However, the underlying mechanism of hydrogen on IR-induced apoptosis remains elusive. Here we tried to unravel the mode of action of hydrogen with rat adrenal medulla cell line PC-12 in vitro.

METHODS

The mitochondrial functions before and after oxygen glucose deprivation and reperfusion (OGD/RP) were determined with corresponding dyes. The expression of Bcl-2, Bax, VDAC1, cytochrome c and caspase 9 was detected using qRT-PCR and Western Blotting method. Then Bcl-2 inhibitor, AB-199, was applied to investigate the role of Bcl-2 in OGD/RP-induced cell apoptosis. Finally, we manipulated the expression of VDAC1 with plasmids transfection to understand the effects of VDAC1 on Bcl-2-mediated anti-apoptosis in OGD/RP.

RESULTS

In this study, we demonstrated that hydrogen-rich saline (HRS) reduced OGD/RP-mediated neuronal loss by stimulating the expression of Bcl-2, which suppressed the activity of VDAC1. Consequently, HRS maintained the mitochondrial functions, restrained the release of cytochrome c and caspase 9 activation, resulting in ameliorated cell viability.

CONCLUSIONS

HRS ameliorated OGD/RP-induced PC-12 cell apoptosis and provided a novel treatment option for ischemia.

Dysfunction in the mitochondrial Fe-S assembly machinery leads to formation of the chemoresistant truncated VDAC1 isoform without HIF-1α activation

Biogenesis of iron-sulfur clusters (ISC) is essential to almost all forms of life and involves complex protein machineries. This process is initiated within the mitochondrial matrix by the ISC assembly machinery. Cohort and case report studies have linked mutations in ISC assembly machinery to severe mitochondrial diseases. The voltage-dependent anion channel (VDAC) located within the mitochondrial outer membrane regulates both cell metabolism and apoptosis. Recently, the C-terminal truncation of the VDAC1 isoform, termed VDAC1-ΔC, has been observed in chemoresistant late-stage tumor cells grown under hypoxic conditions with activation of the hypoxia-response nuclear factor HIF-1α. These cells harbored atypical enlarged mitochondria. Here, we show for the first time that depletion of several proteins of the mitochondrial ISC machinery in normoxia leads to a similar enlarged mitochondria phenotype associated with accumulation of VDAC1-ΔC. This truncated form of VDAC1 accumulates in the absence of HIF-1α and HIF-2α activations and confers cell resistance to drug-induced apoptosis. Furthermore, we show that when hypoxia and siRNA knock-down of the ISC machinery core components are coupled, the cell phenotype is further accentuated, with greater accumulation of VDAC1-ΔC. Interestingly, we show that hypoxia promotes the downregulation of several proteins (ISCU, NFS1, FXN) involved in the early steps of mitochondrial Fe-S cluster biogenesis. Finally, we have identified the mitochondria-associated membrane (MAM) localized Fe-S protein CISD2 as a link between ISC machinery downregulation and accumulation of anti-apoptotic VDAC1-ΔC. Our results are the first to associate dysfunction in Fe-S cluster biogenesis with cleavage of VDAC1, a form which has previously been shown to promote tumor resistance to chemotherapy, and raise new perspectives for targets in cancer therapy.

Hypoxic-induced truncation of voltage-dependent anion channel 1 is mediated by both asparagine endopeptidase and calpain 1 activities

The voltage-dependent anion channel 1 (VDAC1), an outer mitochondria membrane (OMM) protein, serves as a mitochondrial gatekeeper, mediating the transport of nucleotides, Ca²⁺ and other metabolites across the OMM. VDAC1 also plays a central role in mitochondria-mediated apoptosis by facilitating the release of apoptotic proteins and by association with both pro- and anti-apoptotic proteins. Tumor cells, which are constantly exposed to hypoxic conditions, affect the cell via the transcription factor hypoxia-inducible factor (HIF) that induces transcriptional activity. In cultured cells and in lung cancer patients, hypoxia induces VDAC1 truncation at the C-terminus (VDAC1-ΔC). However, the molecular mechanisms involved in VDAC1-ΔC formation are unknown. Here, we show that hypoxia-induced VDAC1-ΔC formation is inhibited by the Ca²⁺ chelator BAPTA-AM, by calpain inhibitor-1, by inhibitor of the asparagine endopeptidase (AEP) and by si-RNA targeting HIF1-α or Ca²⁺-activated protease calpain-1 expression but not that of calpain-2. Finally, VDAC1-ΔC expressed in bacteria and reconstituted into a planar lipid bilayer exhibited decreased channel conductance relative to the full-length protein, yet retained voltage-dependent conductance. These findings suggest that hypoxia, acting via HIF-1α expression, leads to VDAC1 cleavage involving the activation of calpain 1 and AEP.

VDAC1 at the crossroads of cell metabolism, apoptosis and cell stress

This review presents current knowledge related to VDAC1 as a multi-functional mitochondrial protein acting on both sides of the coin, regulating cell life and death, and highlighting these functions in relation to disease. It is now recognized that VDAC1 plays a crucial role in regulating the metabolic and energetic functions of mitochondria. The location of VDAC1 at the outer mitochondrial membrane (OMM) allows the control of metabolic cross-talk between mitochondria and the rest of the cell and also enables interaction of VDAC1 with proteins involved in metabolic and survival pathways. Along with regulating cellular energy production and metabolism, VDAC1 is also involved in the process of mitochondria-mediated apoptosis by mediating the release of apoptotic proteins and interacting with anti-apoptotic proteins. VDAC1 functions in the release of apoptotic proteins located in the mitochondrial intermembrane space via oligomerization to form a large channel that allows passage of cytochrome c and AIF and their release to the cytosol, subsequently resulting in apoptotic cell death. VDAC1 also regulates apoptosis via interactions with apoptosis regulatory proteins, such as hexokinase, Bcl2 and Bcl-xL, some of which are also highly expressed in many cancers. This review also provides insight into VDAC1 function in Ca²⁺ homeostasis, oxidative stress, and presents VDAC1 as a hub protein interacting with over 100 proteins. Such interactions enable VDAC1 to mediate and regulate the integration of mitochondrial functions with cellular activities. VDAC1 can thus be considered as standing at the crossroads between mitochondrial metabolite transport and apoptosis and hence represents an emerging cancer drug target.

Voltage-Dependent Anion Channel 1 As an Emerging Drug Target for Novel Anti-Cancer Therapeutics

Cancer cells share several properties, high proliferation potential, reprogramed metabolism, and resistance to apoptotic cues. Acquiring these hallmarks involves changes in key oncogenes and non-oncogenes essential for cancer cell survival and prosperity, and is accompanied by the increased energy requirements of proliferating cells. Mitochondria occupy a central position in cell life and death with mitochondrial bioenergetics, biosynthesis, and signaling are critical for tumorigenesis. Voltage-dependent anion channel 1 (VDAC1) is situated in the outer mitochondrial membrane (OMM) and serving as a mitochondrial gatekeeper. VDAC1 allowing the transfer of metabolites, fatty acid ions, Ca²⁺, reactive oxygen species, and cholesterol across the OMM and is a key player in mitochondrial-mediate apoptosis. Moreover, VDAC1 serves as a hub protein, interacting with diverse sets of proteins from the cytosol, endoplasmic reticulum, and mitochondria that together regulate metabolic and signaling pathways. The observation that VDAC1 is over-expressed in many cancers suggests that the protein may play a pivotal role in cancer cell survival. However, VDAC1 is also important in mitochondria-mediated apoptosis, mediating release of apoptotic proteins and interacting with anti-apoptotic proteins, such as B-cell lymphoma 2 (Bcl-2), Bcl-xL, and hexokinase (HK), which are also highly expressed in many cancers. Strategically located in a “bottleneck” position, controlling metabolic homeostasis and apoptosis, VDAC1 thus represents an emerging target for anti-cancer drugs. This review presents an overview on the multi-functional mitochondrial protein VDAC1 performing several functions and interacting with distinct sets of partners to regulate both cell life and death, and highlights the importance of the protein for cancer cell survival. We address recent results related to the mechanisms of VDAC1-mediated apoptosis and the potential of associated proteins to modulate of VDAC1 activity, with the aim of developing VDAC1-based approaches. The first strategy involves modification of cell metabolism using VDAC1-specific small interfering RNA leading to inhibition of cancer cell and tumor growth and reversed oncogenic properties. The second strategy involves activation of cancer cell death using VDAC1-based peptides that prevent cell death induction by anti-apoptotic proteins. Finally, we discuss the potential therapeutic benefits of treatments and drugs leading to enhanced VDAC1 expression or targeting VDAC1 to induce apoptosis.

VDAC3 As a Potential Marker of Mitochondrial Status Is Involved in Cancer and Pathology

VDAC3 is the least known isoform of the mammalian voltage-dependent anion selective channels of the outer mitochondrial membrane. It has been recently shown that cysteine residues of VDAC3 are found over-oxidized. The VDAC3 cysteine over-oxidation was associated with the oxidizing environment and the abundance of reactive oxygen species (ROS) in the intermembrane space. In this work, we have examined the role of VDAC3 in general pathogenic mechanisms at the basis of mitochondrial dysfunction and involving the mitochondrial quality control. Many of the diseases reported here, including cancer and viral infections, are often associated with significant changes in the intracellular redox state. In this sense, VDAC3 bearing oxidative modifications could become marker of the oxidative load in the mitochondria and part of the ROS signaling pathway.

Mitochondrial VDAC2 and cell homeostasis: highlighting hidden structural features and unique functionalities

Voltage-dependent anion channels (VDACs) are the gateway to mitochondrial processes, interlinking the cytosolic and mitochondrial compartments. The mitochondrion acts as a storehouse for cytochrome c, the effector of apoptosis, and hence VDACs become intricately involved in the apoptotic pathway. Isoform 1 of VDAC is abundant in the outer mitochondrial membrane of many cell types, while isoform 2 is the preferred channel in specialized cells including brain and some cancer cells. The primary role of VDACs is metabolite flux. The pro- and anti-apoptotic role of VDAC1 and VDAC2, respectively, are secondary, and are influenced by external factors and interacting proteins. Herein, we focus on the less-studied VDAC2, and shed light on its unique functions and features. VDAC2, along with sharing many of its functions with VDAC1, such as metabolite and Ca²⁺ transport, also has many delineating functions. VDAC2 is closely engaged in the gametogenesis and steroidogenesis pathways and in protection from oxidative stress as well as in neurodegenerative diseases like Alzheimer's and epilepsy. A closer examination of the functional pathways of VDACs indicates that the unique functions of VDAC2 are a result of the different interactome of this isoform. We couple functional differences to the structural and biophysical evidence obtained for the VDACs, and present a testament of why the two VDAC isoforms with >90% sequence similarity, are functionally diverse. Based on these differences, we suggest that the VDAC isoforms now be considered as paralogs. An in-depth understanding of VDAC2 will help us to design better biomolecule targets for cancer and neurodegenerative diseases.

Novel Compounds Targeting the Mitochondrial Protein VDAC1 Inhibit Apoptosis and Protect against Mitochondrial Dysfunction

Apoptosis is thought to play a critical role in several pathological processes, such as neurodegenerative diseases (i.e. Parkinson's and Alzheimer's diseases) and various cardiovascular diseases. Despite the fact that apoptotic mechanisms are well defined, there is still no substantial therapeutic strategy to stop or even slow this process. Thus, there is an unmet need for therapeutic agents that are able to block or slow apoptosis in neurodegenerative and cardiovascular diseases. The outer mitochondrial membrane protein voltage-dependent anion channel 1 (VDAC1) is a convergence point for a variety of cell survival and death signals, including apoptosis. Recently, we demonstrated that VDAC1 oligomerization is involved in mitochondrion-mediated apoptosis. Thus, VDAC1 oligomerization represents a prime target for agents designed to modulate apoptosis. Here, high-throughput compound screening and medicinal chemistry were employed to develop compounds that directly interact with VDAC1 and prevent VDAC1 oligomerization, concomitant with an inhibition of apoptosis as induced by various means and in various cell lines. The compounds protected against apoptosis-associated mitochondrial dysfunction, restoring dissipated mitochondrial membrane potential, and thus cell energy and metabolism, decreasing reactive oxidative species production, and preventing detachment of hexokinase bound to mitochondria and disruption of intracellular Ca²⁺ levels. Thus, this study describes novel drug candidates with a defined mechanism of action that involves inhibition of VDAC1 oligomerization, apoptosis, and mitochondrial dysfunction. The compounds VBIT-3 and VBIT-4 offer a therapeutic strategy for treating different diseases associated with enhanced apoptosis and point to VDAC1 as a promising target for therapeutic intervention.

Role of cysteines in mammalian VDAC isoforms' function

In this mini-review, we analyze the influence of cysteines in the structure and activity of mitochondrial outer membrane mammalian VDAC isoforms. The three VDAC isoforms show conserved sequences, similar structures and the same gene organization. The meaning of three proteins encoded in different chromosomes must thus be searched for subtle differences at the amino acid level. Among others, cysteine content is noticeable. In humans, VDAC1 has 2, VDAC2 has 9 and VDAC3 has 6 cysteines. Recent works have shown that, at variance from VDAC1, VDAC2 and VDAC3 exhibit cysteines predicted to protrude towards the intermembrane space, making them a preferred target for oxidation by ROS. Mass spectrometry in VDAC3 revealed that a disulfide bridge can be formed and other cysteine oxidations are also detectable. Both VDAC2 and VDAC3 cysteines were mutagenized to highlight their role in vitro and in complementation assays in Δporin1 yeast. Chemico-physical techniques revealed an important function of cysteines in the structural stabilization of the pore. In conclusion, the works available on VDAC cysteines support the notion that the three proteins are paralogs with a similar pore-function and slightly different, but important, ancillary biological functions. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.

A computational study of ion current modulation in hVDAC3 induced by disulfide bonds

The human VDAC channel exists in three isoforms characterized by high sequence homology and structural similarity. Yet the function and mode of action of hVDAC3 are still elusive. The presence of six surface cysteines exposed to the oxidizing environment of the mitochondrial inter-membrane space suggests the possible establishment of intramolecular disulfide bonds. Two natural candidates for disulfide bridge formation are Cys2 and Cys8 that, located on the flexible N-terminal domain, can easily come in contact. A third potentially important residue is Cys122 that is close to Cys2 in the homology model of VDAC3. Here we analyzed the impact of SS bonds through molecular dynamics simulations of derivatives of hVDAC3 (dubbed SS-2-8, SS-2-122, SS-8-122) including a single disulfide bond. Simulations showed that in SS-8-122, the fragment 1-7 crosses the top part of the barrel partially occluding the pore and causing a 20% drop of conductance. In order to identify other potential channel-occluding disulfide bonds, we used a set of neural networks and structural bioinformatics algorithms, after filtering with the steric constraints imposed by the 3D-structure. We identified other three species, namely SS-8-65, SS-2-36 and SS-8-36. While the conductance of SS-8-65 and SS-2-36 is about 30% lower than that of the species without disulfide bonds, the conductance of SS-8-36 was 40-50% lower. The results show how VDAC3 is able to modulate its pore size and current by exploiting the mobility of the N-terminal and forming, upon external stimuli, disulfide bridges with cysteine residues located on the barrel and exposed to the inter-membrane space.

N-helix and Cysteines Inter-regulate Human Mitochondrial VDAC-2 Function and Biochemistry

Human voltage-dependent anion channel-2 (hVDAC-2) functions primarily as the crucial anti-apoptotic protein in the outer mitochondrial membrane, and additionally as a gated bidirectional metabolite transporter. The N-terminal helix (NTH), involved in voltage sensing, bears an additional 11-residue extension (NTE) only in hVDAC-2. In this study, we assign a unique role for the NTE as influencing the chaperone-independent refolding kinetics and overall thermodynamic stability of hVDAC-2. Our electrophysiology data shows that the N-helix is crucial for channel activity, whereas NTE sensitizes this isoform to voltage gating. Additionally, hVDAC-2 possesses the highest cysteine content, possibly for regulating reactive oxygen species content. We identify interdependent contributions of the N-helix and cysteines to channel function, and the measured stability in micellar environments with differing physicochemical properties. The evolutionary demand for the NTE in the presence of cysteines clearly emerges from our biochemical and functional studies, providing insight into factors that functionally demarcate hVDAC-2 from the other VDACs.

Succination is Increased on Select Proteins in the Brainstem of the NADH dehydrogenase (ubiquinone) Fe-S protein 4 (Ndufs4) Knockout Mouse, a Model of Leigh Syndrome

Elevated fumarate concentrations as a result of Krebs cycle inhibition lead to increases in protein succination, an irreversible post-translational modification that occurs when fumarate reacts with cysteine residues to generate S-(2-succino)cysteine (2SC). Metabolic events that reduce NADH re-oxidation can block Krebs cycle activity; therefore we hypothesized that oxidative phosphorylation deficiencies, such as those observed in some mitochondrial diseases, would also lead to increased protein succination. Using the Ndufs4 knockout (Ndufs4 KO) mouse, a model of Leigh syndrome, we demonstrate for the first time that protein succination is increased in the brainstem (BS), particularly in the vestibular nucleus. Importantly, the brainstem is the most affected region exhibiting neurodegeneration and astrocyte and microglial proliferation, and these mice typically die of respiratory failure attributed to vestibular nucleus pathology. In contrast, no increases in protein succination were observed in the skeletal muscle, corresponding with the lack of muscle pathology observed in this model. 2D SDS-PAGE followed by immunoblotting for succinated proteins and MS/MS analysis of BS proteins allowed us to identify the voltage-dependent anion channels 1 and 2 as specific targets of succination in the Ndufs4 knockout. Using targeted mass spectrometry, Cys(77) and Cys(48) were identified as endogenous sites of succination in voltage-dependent anion channels 2. Given the important role of voltage-dependent anion channels isoforms in the exchange of ADP/ATP between the cytosol and the mitochondria, and the already decreased capacity for ATP synthesis in the Ndufs4 KO mice, we propose that the increased protein succination observed in the BS of these animals would further decrease the already compromised mitochondrial function. These data suggest that fumarate is a novel biochemical link that may contribute to the progression of the neuropathology in this mitochondrial disease model.

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.

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.

Generator-specific targets of mitochondrial reactive oxygen species

To understand the role of reactive oxygen species (ROS) in oxidative stress and redox signaling it is necessary to link their site of generation to the oxidative modification of specific targets. Here we have studied the selective modification of protein thiols by mitochondrial ROS that have been implicated as deleterious agents in a number of degenerative diseases and in the process of biological aging, but also as important players in cellular signal transduction. We hypothesized that this bipartite role might be based on different generator sites for "signaling" and "damaging" ROS and a directed release into different mitochondrial compartments. Because two main mitochondrial ROS generators, complex I (NADH:ubiquinone oxidoreductase) and complex III (ubiquinol:cytochrome c oxidoreductase; cytochrome bc1 complex), are known to predominantly release superoxide and the derived hydrogen peroxide (H2O2) into the mitochondrial matrix and the intermembrane space, respectively, we investigated whether these ROS generators selectively oxidize specific protein thiols. We used redox fluorescence difference gel electrophoresis analysis to identify redox-sensitive targets in the mitochondrial proteome of intact rat heart mitochondria. We observed that the modified target proteins were distinctly different when complex I or complex III was employed as the source of ROS. These proteins are potential targets involved in mitochondrial redox signaling and may serve as biomarkers to study the generator-dependent dual role of mitochondrial ROS in redox signaling and oxidative stress.

The mitochondrial voltage-dependent anion channel 1 in tumor cells

VDAC1 is found at the crossroads of metabolic and survival pathways. VDAC1 controls metabolic cross-talk between mitochondria and the rest of the cell by allowing the influx and efflux of metabolites, ions, nucleotides, Ca2+ and more. The location of VDAC1 at the outer mitochondrial membrane also enables its interaction with proteins that mediate and regulate the integration of mitochondrial functions with cellular activities. As a transporter of metabolites, VDAC1 contributes to the metabolic phenotype of cancer cells. Indeed, this protein is over-expressed in many cancer types, and silencing of VDAC1 expression induces an inhibition of tumor development. At the same time, along with regulating cellular energy production and metabolism, VDAC1 is involved in the process of mitochondria-mediated apoptosis by mediating the release of apoptotic proteins and interacting with anti-apoptotic proteins. The engagement of VDAC1 in the release of apoptotic proteins located in the inter-membranal space involves VDAC1 oligomerization that mediates the release of cytochrome c and AIF to the cytosol, subsequently leading to apoptotic cell death. Apoptosis can also be regulated by VDAC1, serving as an anchor point for mitochondria-interacting proteins, such as hexokinase (HK), Bcl2 and Bcl-xL, some of which are also highly expressed in many cancers. By binding to VDAC1, HK provides both a metabolic benefit and apoptosis-suppressive capacity that offer the cell a proliferative advantage and increase its resistance to chemotherapy. Thus, these and other functions point to VDAC1 as an excellent target for impairing the re-programed metabolism of cancer cells and their ability to evade apoptosis. Here, we review current evidence pointing to the function of VDAC1 in cell life and death, and highlight these functions in relation to both cancer development and therapy. In addressing the recently solved 3D structures of VDAC1, this review will point to structure-function relationships of VDAC as critical for deciphering how this channel can perform such a variety of roles, all of which are important for cell life and death. Finally, this review will also provide insight into VDAC function in Ca2+ homeostasis, protection against oxidative stress, regulation of apoptosis and involvement in several diseases, as well as its role in the action of different drugs. We will discuss the use of VDAC1-based strategies to attack the altered metabolism and apoptosis of cancer cells. These strategies include specific siRNA able to impair energy and metabolic homeostasis, leading to arrested cancer cell growth and tumor development, as well VDAC1-based peptides that interact with anti-apoptotic proteins to induce apoptosis, thereby overcoming the resistance of cancer cell to chemotherapy. Finally, small molecules targeting VDAC1 can induce apoptosis. VDAC1 can thus be considered as standing at the crossroads between mitochondrial metabolite transport and apoptosis and hence represents an emerging cancer drug target. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Thiol-based redox proteins in abscisic acid and methyl jasmonate signaling in Brassica napus guard cells

Reversibly oxidized cysteine sulfhydryl groups serve as redox sensors or targets of redox sensing that are important in various physiological processes. However, little is known about redox-sensitive proteins in guard cells and how they function in stomatal signaling. In this study, Brassica napus guard-cell proteins altered by redox in response to abscisic acid (ABA) or methyl jasmonate (MeJA) were identified by complementary proteomics approaches, saturation differential in-gel electrophoresis and isotope-coded affinity tagging. In total, 65 and 118 potential redox-responsive proteins were identified in ABA- and MeJA-treated guard cells, respectively. All the proteins contain at least one cysteine, and over half of them are predicted to form intra-molecular disulfide bonds. Most of the proteins fall into the functional groups of 'energy', 'stress and defense' and 'metabolism'. Based on the peptide sequences identified by mass spectrometry, 30 proteins were common to ABA- and MeJA-treated samples. A total of 44 cysteines were mapped in the identified proteins, and their levels of redox sensitivity were quantified. Two of the proteins, a sucrose non-fermenting 1-related protein kinase and an isopropylmalate dehydrogenase, were confirmed to be redox-regulated and involved in stomatal movement. This study creates an inventory of potential redox switches, and highlights a protein redox regulatory mechanism in ABA and MeJA signal transduction in guard cells.

Cysteine residues impact the stability and micelle interaction dynamics of the human mitochondrial β-barrel anion channel hVDAC-2

The anti-apoptotic 19-stranded transmembrane human voltage dependent anion channel isoform 2 (hVDAC-2) β-barrel stability is crucial for anion transport in mitochondria. The role of the unusually high number of cysteine residues in this isoform is poorly understood. Using a Cys-less construct of hVDAC-2, we haveinvestigated the contribution of cysteines to channel function, barrel stability and its influence on the strength of protein-micelle interactions. We observe that despite the overall preservation in barrel structure upon cysteine mutation, subtle local variations in the mode of interaction of the barrel with its refolded micellar environment arise, which may manifest itself in the channel activity of both the proteins.Fluorescence measurements of the Trp residues in hVDAC-2 point to possible differences in the association of the barrel with lauryldimethylamine oxide (LDAO) micelles. Upon replacement of cysteines in hVDAC-2, our data suggests greater barrel rigidity by way of intra-protein interactions. This, in turn, lowers the equilibrium barrel thermodynamic parameters in LDAOby perturbingthe stability of the protein-micelle complex. In addition to this, we also find a difference in the cooperativity of unfolding upon increasing the LDAO concentration, implying the importance of micelle concentration and micelle-protein ratios on the stability of this barrel. Our results indicate that the nine cysteine residues of hVDAC-2 are the key in establishing strong(er) barrel interactions with its environment and also impart additional malleability to the barrel scaffold.

Influence of protein-micelle ratios and cysteine residues on the kinetic stability and unfolding rates of human mitochondrial VDAC-2

Delineating the kinetic and thermodynamic factors which contribute to the stability of transmembrane β-barrels is critical to gain an in-depth understanding of membrane protein behavior. Human mitochondrial voltage-dependent anion channel isoform 2 (hVDAC-2), one of the key anti-apoptotic eukaryotic β-barrel proteins, is of paramount importance, owing to its indispensable role in cell survival. We demonstrate here that the stability of hVDAC-2 bears a strong kinetic contribution that is dependent on the absolute micellar concentration used for barrel folding. The refolding efficiency and ensuing stability is sensitive to the lipid-to-protein (LPR) ratio, and displays a non-linear relationship, with both low and high micellar amounts being detrimental to hVDAC-2 structure. Unfolding and aggregation process are sequential events and show strong temperature dependence. We demonstrate that an optimal lipid-to-protein ratio of 2600∶1 – 13000∶1 offers the highest protection against thermal denaturation. Activation energies derived only for lower LPRs are ∼17 kcal mol⁻¹ for full-length hVDAC-2 and ∼23 kcal mol⁻¹ for the Cys-less mutant, suggesting that the nine cysteine residues of hVDAC-2 impart additional malleability to the barrel scaffold. Our studies reveal that cysteine residues play a key role in the kinetic stability of the protein, determine barrel rigidity and thereby give rise to strong micellar association of hVDAC-2. Non-linearity of the Arrhenius plot at high LPRs coupled with observation of protein aggregation upon thermal denaturation indicates that contributions from both kinetic and thermodynamic components stabilize the 19-stranded β-barrel. Lipid-protein interaction and the linked kinetic contribution to free energy of the folded protein are together expected to play a key role in hVDAC-2 recycling and the functional switch at the onset of apoptosis.

Modulation of human mitochondrial voltage-dependent anion channel 2 (hVDAC-2) structural stability by cysteine-assisted barrel-lipid interactions

Human mitochondrial voltage-dependent anion channel 2 (hVDAC-2), the most predominant isoform seen in brain mitochondria, is not only crucial for cell survival but is also implicated in Alzheimer disease. The abundance of cysteines in this isoform is particularly fascinating, as hVDAC-1 cysteines have no associated functional role. We report a detailed biophysical examination of a Cys-less mutant of hVDAC-2, and its behavioral comparison with the wild type protein. Our findings suggest that cysteine mutation results in the formation of a better barrel at the expense of weakened protein-lipid interactions. The wild type protein displays stronger lipid association, despite being less structured. A reversal in behavior of both proteins is observed in the case of chemical denaturation, with the Cys-less mutant exhibiting lowered unfolding free energies. In bicellar systems comprising 14-C phosphocholines, we observe that protein-lipid interactions are weakened in both constructs, resulting in barrel structure destabilization. Our biochemical and biophysical studies together reveal key structural roles for the cysteine residues. We find that minor conformational variations in local residues are sufficient to define the membrane protein dynamics in hVDAC-2. Such subtle sequence variations contribute to differential stability of VDACs and may have implications in their in vivo regulation and recycling.

Substitution of transmembrane domain Cys residues alters pH(o)-sensitive anion transport by AE2/SLC4A2 anion exchanger

AE2/SLC4A2 is the most widely expressed of the Na(+)-independent SLC4 Cl(-)/HCO3 (-) exchangers and is essential for postnatal survival, but its structure remains unknown. We have generated and expressed a mouse AE2 construct devoid of transmembrane domain cysteine (Cys) residues, mAE2Cys-less, to enhance the utility of Cys-substitution mutagenesis for structural and structure-function studies of mAE2. mAE2Cys-less expressed in Xenopus oocytes exhibited partial reduction of stilbene disulfonate-sensitive anion exchange activity. This activity was independent of the mAE2 N-terminal cytosolic domain and was accompanied by near-normal surface expression, without change in K 1/2 for extracellular Cl(-). mAE2Cys-less exhibited wildtype activation of anion exchange by hypertonicity and by NH4Cl, and wildtype inhibition of anion exchange by acidic intracellular pH (pHi) in the absence of NH4 (+). However, inhibition of anion exchange by extracellular pH (pHo) exhibited an alkaline shifted pHo(50) value of at least 0.6-0.7 pH units. Although SO4 (2-) transport by mAE2Cys-less resembled wildtype mAE2 in its stimulation by acidic pHo, the absence of transmembrane domain Cys residues abrogated activation of oxalate transport by acidic pHo. The contrasting enhancement of SO4 (2-) transport by alkaline pHo in the mAE1 anion translocation pathway mutant E699Q (Am J Physiol Cell Physiol 295: C302) was phenocopied by the corresponding mutant E1007Q in both AE2 and AE2Cys-less. However, the absence of transmembrane domain Cys residues exacerbated the reduced basal anion transport function exhibited by this and other missense substitutions at AE2 residue E1007. AE2Cys-less will be a valuable experimental tool for structure-function studies of the SLC4 gene family, but its utility for studies of AE2 regulation by extracellular pH must be evaluated in the context of its alkaline-shifted pHo sensitivity, resembling that of AE2 gastric parietal cell variant AE2c1.

Structure-based analysis of VDAC1: N-terminus location, translocation, channel gating and association with anti-apoptotic proteins

Structural studies place the VDAC1 (voltage-dependent anion channel 1) N-terminal region within the channel pore. Biochemical and functional studies, however, reveal that the N-terminal domain is cytoplasmically exposed. In the present study, the location and translocation of the VDAC1 N-terminal domain, and its role in voltage-gating and as a target for anti-apoptotic proteins, were addressed. Site-directed mutagenesis and cysteine residue substitution, together with a thiol-specific cross-linker, served to show that the VDAC1 N-terminal region exists in a dynamic equilibrium, located within the pore or exposed outside the β-barrel. Using a single cysteine-residue-bearing VDAC1, we demonstrate that the N-terminal region lies inside the pore. However, the same region can be exposed outside the pore, where it dimerizes with the N-terminal domain of a second VDAC1 molecule. When the N-terminal region α-helix structure was perturbed, intra-molecular cross-linking was abolished and dimerization was enhanced. This mutant also displays reduced voltage-gating and reduced binding to hexokinase, but not to the anti-apoptotic proteins Bcl-2 and Bcl-xL. Replacing glycine residues in the N-terminal domain GRS (glycine-rich sequence) yielded less intra-molecular cross-linked product but more dimerization, suggesting that GRS provides the flexibility needed for N-terminal translocation from the internal pore to the channel face. N-terminal mobility may thus contribute to channel gating and interaction with anti-apoptotic proteins.

3-Bromopyruvate: targets and outcomes

The pyruvate mimetic 3-bromopyruvate (3-BP) is generally presented as an inhibitor of glycolysis and has shown remarkable efficacy in not only preventing tumor growth, but even eradicating existant tumors in animal studies. We here review reported molecular targets of 3-BP and suggest that the very range of possible targets, which pertain to the altered energy metabolism of tumor cells, contributes both to the efficacy and the tumor specificity of the drug. Its in vivo efficacy is suggested to be due to a combination of glycolytic and mitochondrial targets, as well as to secondary effects affecting the tumor microenvironment. The cytotoxicity of 3-BP is less due to pyruvate mimicry than to alkylation of, e.g., key thiols. Alkylation of DNA/RNA has not been reported. More research is warranted to better understand the pharmacokinetics of 3-BP, and its potential toxic effects to normal cells, in particular those that are highly ATP-/mitochondrion-dependent.

Mechanisms of altered redox regulation in neurodegenerative diseases--focus on S--glutathionylation

SIGNIFICANCE

Neurodegenerative diseases are characterized by progressive loss of neurons. A common feature is oxidative stress, which arises when reactive oxygen species (ROS) and/or reactive nitrogen species (RNS) exceed amounts required for normal redox signaling. An imbalance in ROS/RNS alters functionality of cysteines and perturbs thiol-disulfide homeostasis. Many cysteine modifications may occur, but reversible protein mixed disulfides with glutathione (GSH) likely represents the common steady-state derivative due to cellular abundance of GSH and ready conversion of cysteine-sulfenic acid and S-nitrosocysteine precursors to S-glutathionylcysteine disulfides. Thus, S-glutathionylation acts in redox signal transduction and serves as a protective mechanism against irreversible cysteine oxidation. Reversal of protein-S-glutathionylation is catalyzed specifically by glutaredoxin which thereby plays a critical role in cellular regulation. This review highlights the role of oxidative modification of proteins, notably S-glutathionylation, and alterations in thiol homeostatic enzyme activities in neurodegenerative diseases, providing insights for therapeutic intervention.

RECENT ADVANCES

Recent studies show that dysregulation of redox signaling and sulfhydryl homeostasis likely contributes to onset/progression of neurodegeneration. Oxidative stress alters the thiol-disulfide status of key proteins that regulate the balance between cell survival and cell death.

CRITICAL ISSUES

Much of the current information about redox modification of key enzymes and signaling intermediates has been gleaned from studies focused on oxidative stress situations other than the neurodegenerative diseases.

FUTURE DIRECTIONS

The findings in other contexts are expected to apply to understanding neurodegenerative mechanisms. Identification of selectively glutathionylated proteins in a quantitative fashion will provide new insights about neuropathological consequences of this oxidative protein modification.

The role of lipids in VDAC oligomerization

Evidence has accumulated that the voltage-dependent anion channel (VDAC), located on the outer membrane of mitochondria, plays a central role in apoptosis. The involvement of VDAC oligomerization in apoptosis has been suggested in various studies. However, it still remains unknown how exactly VDAC supramolecular assembly can be regulated in the membrane. This study addresses the role of lipids in this process. We investigate the effect of cardiolipin (CL) and phosphatidylglycerol (PG), anionic lipids important for mitochondria metabolism and apoptosis, on VDAC oligomerization. By applying fluorescence cross-correlation spectroscopy to VDAC reconstituted into giant unilamellar vesicles, we demonstrate that PG significantly enhances VDAC oligomerization in the membrane, whereas cardiolipin disrupts VDAC supramolecular assemblies. During apoptosis, the level of PG in mitochondria increases, whereas the CL level decreases. We suggest that the specific lipid composition of the outer mitochondrial membrane might be of crucial relevance and, thus, a potential cue for regulating the oligomeric state of VDAC.

Structure-based analysis of VDAC1 protein: defining oligomer contact sites

The outer mitochondrial membrane protein, the voltage-dependent anion channel (VDAC), is increasingly implicated in the control of apoptosis. Oligomeric assembly of VDAC1 was shown to be coupled to apoptosis induction, with oligomerization increasing substantially upon apoptosis induction and inhibited by apoptosis blockers. In this study, structure- and computation-based selection of the predicated VDAC1 dimerization site, in combination with site-directed mutagenesis, cysteine replacement, and chemical cross-linking, were employed to identify contact sites between VDAC1 molecules in dimers and higher oligomers. The predicted weakly stable β-strands were experimentally found to represent the interfaces between VDAC1 monomers composing the oligomer. Replacing hydrophobic amino acids with charged residues in β-strands 1, 2, and 19 interfered with VDAC1 oligomerization. The proximity of β-strands 1, 2, and 19 within the VDAC1 dimer and the existence of other association sites involving β-strand 16 were confirmed when a cysteine was introduced at defined positions in cysteineless VDAC1 mutants, together with the use of cysteine-specific cross-linker bis(maleimido)ethane. Moreover, the results suggest that VDAC1 also exists as a dimer that upon apoptosis induction undergoes conformational changes and that its oligomerization proceeds through a series of interactions involving two distinct interfaces. Dissection of VDAC1 dimerization/oligomerization as presented here provides structural insight into the oligomeric status of cellular VDAC1 under physiological and apoptotic conditions.

Dominant-negative VDAC1 mutants reveal oligomeric VDAC1 to be the active unit in mitochondria-mediated apoptosis

Mitochondria play a central role in the intrinsic pathway of apoptosis. Oligomerization of the mitochondrial protein VDAC1 (voltage-dependent anion channel 1) has been proposed to play a role in apoptosis in various studies. In the present study, we have generated dimeric fusion proteins consisting of tandem-linked wild-type and RuR (Ruthenium Red)-insensitive mutant VDAC1 monomers and studied the capacity of RuR to protect against apoptosis, as induced by various means. Fusion proteins composed of wild-type and/or E72Q-VDAC1 were successfully expressed in T-REx-293 cells. Bilayer-reconstituted dimeric rVDAC1 (rat VDAC1) functions as a channel-forming protein, showing typical voltage-dependence conductance, but with a unitary conductance higher than that of monomeric VDAC. As with wild-type VDAC1, overexpression of either the wild-type or mutated VDAC1 dimeric fusion protein induced apoptotic cell death. In addition, as shown previously, the anti-apoptotic effect of RuR was not observed in cells expressing E72Q-VDAC1, despite endogenous VDAC1 being present in these cells. Similar RuR insensitivity governed the VDAC1 fusion proteins comprising the E72Q mutation in either the first, second or both VDAC1 monomers of the same dimer. RuR-mediated protection against apoptosis in T-REx-293 cells, as induced by staurosporine, was observed in cells expressing VDAC1 or dimeric wild-type VDAC1. However, RuR offered no protection against staurosporine-induced apoptosis in cells expressing E72Q-VDAC1 or E72Q-containing dimeric VDAC1. These results suggest that E72Q-VDAC1 has a dominant-negative effect and implies that VDAC1 homo-oligomerization, involving intermolecular interactions, might be involved in the apoptotic process.