Evidence for identity between the hexokinase-binding protein and the mitochondrial porin in the outer membrane of rat liver mitochondria

E as in Enigma: The Mysterious Role of the Voltage-Dependent Anion Channel Glutamate E73

The voltage-dependent anion channel (VDAC) is the main passageway for ions and metabolites over the outer mitochondrial membrane. It was associated with many physiological processes, including apoptosis and modulation of intracellular Ca²⁺ signaling. The protein is formed by a barrel of 19 beta-sheets with an N-terminal helix lining the inner pore. Despite its large diameter, the channel can change its selectivity for ions and metabolites based on its open state to regulate transport into and out of mitochondria. VDAC was shown to be regulated by a variety of cellular factors and molecular partners including proteins, lipids and ions. Although the physiological importance of many of these modulatory effects are well described, the binding sites for molecular partners are still largely unknown. The highly symmetrical and sleek structure of the channel makes predictions of functional moieties difficult. However, one residue repeatedly sticks out when reviewing VDAC literature. A glutamate at position 73 (E73) located on the outside of the channel facing the hydrophobic membrane environment was repeatedly proposed to be involved in channel regulation on multiple levels. Here, we review the distinct hypothesized roles of E73 and summarize the open questions around this mysterious residue.

Insulin, Muscle Glucose Uptake, and Hexokinase: Revisiting the Road Not Taken

Research conducted over the last 50 yr has provided insight into the mechanisms by which insulin stimulates glucose transport across the skeletal muscle cell membrane Transport alone, however, does not result in net glucose uptake as free glucose equilibrates across the cell membrane and is not metabolized. Glucose uptake requires that glucose is phosphorylated by hexokinases. Phosphorylated glucose cannot leave the cell and is the substrate for metabolism. It is indisputable that glucose phosphorylation is essential for glucose uptake. Major advances have been made in defining the regulation of the insulin-stimulated glucose transporter (GLUT4) in skeletal muscle. By contrast, the insulin-regulated hexokinase (hexokinase II) parallels Robert Frost's "The Road Not Taken." Here the case is made that an understanding of glucose phosphorylation by hexokinase II is necessary to define the regulation of skeletal muscle glucose uptake in health and insulin resistance. Results of studies from different physiological disciplines that have elegantly described how hexokinase II can be regulated are summarized to provide a framework for potential application to skeletal muscle. Mechanisms by which hexokinase II is regulated in skeletal muscle await rigorous examination.

Hexokinase 1 cellular localization regulates the metabolic fate of glucose

The product of hexokinase (HK) enzymes, glucose-6-phosphate, can be metabolized through glycolysis or directed to alternative metabolic routes, such as the pentose phosphate pathway (PPP) to generate anabolic intermediates. HK1 contains an N-terminal mitochondrial binding domain (MBD), but its physiologic significance remains unclear. To elucidate the effect of HK1 mitochondrial dissociation on cellular metabolism, we generated mice lacking the HK1 MBD (ΔE1HK1). These mice produced a hyper-inflammatory response when challenged with lipopolysaccharide. Additionally, there was decreased glucose flux below the level of GAPDH and increased upstream flux through the PPP. The glycolytic block below GAPDH is mediated by the binding of cytosolic HK1 with S100A8/A9, resulting in GAPDH nitrosylation through iNOS. Additionally, human and mouse macrophages from conditions of low-grade inflammation, such as aging and diabetes, displayed increased cytosolic HK1 and reduced GAPDH activity. Our data indicate that HK1 mitochondrial binding alters glucose metabolism through regulation of GAPDH.

Homocysteine-Thiolactone Modulates Gating of Mitochondrial Voltage-Dependent Anion Channel (VDAC) and Protects It from Induced Oxidative Stress

The gating of the Voltage-Dependent Anion Channel (VDAC) is linked to oxidative stress through increased generation of mitochondrial ROS with increasing mitochondrial membrane potential (ΔΨ_m). It has been already reported that H₂O₂ increases the single-channel conductance of VDAC on a bilayer lipid membrane. On the other hand, homocysteine (Hcy) has been reported to induce mitochondria-mediated cell death. It is argued that the thiol-form of homocysteine, HTL could be the plausible molecule responsible for the alteration in the function of proteins, such as VDAC. It is hypothesized that HTL interacts with VDAC that causes functional abnormalities. An investigation was undertaken to study the interaction of HTL with VDAC under H₂O₂ induced oxidative stress through biophysical and electrophysiological methods. Fluorescence spectroscopic studies indicate that HTL interacts with VDAC, but under induced oxidative stress the effect is prevented partially. Similarly, bilayer electrophysiology studies suggest that HTL shows a reduction in VDAC single-channel conductance, but the effects are partially prevented under an oxidative environment. Gly172 and His181 are predicted through bioinformatics tools to be the most plausible binding residues of HTL in Rat VDAC. The binding of HTL and H₂O₂ with VDAC appears to be cooperative as per our analysis of experimental data in the light of the Hill-Langmuir equation. The binding energies are estimated to be - 4.7 kcal mol^-1 and - 2.8 kcal mol^-1, respectively. The present in vitro studies suggest that when mitochondrial VDAC is under oxidative stress, the effects of amino acid metabolites like HTL are suppressed.

Historical Perspective of Pore-Forming Activity Studies of Voltage-Dependent Anion Channel (Eukaryotic or Mitochondrial Porin) Since Its Discovery in the 70th of the Last Century

Eukaryotic porin, also known as Voltage-Dependent Anion Channel (VDAC), is the most frequent protein in the outer membrane of mitochondria that are responsible for cellular respiration. Mitochondria are most likely descendants of strictly aerobic Gram-negative bacteria from the α-proteobacterial lineage. In accordance with the presumed ancestor, mitochondria are surrounded by two membranes. The mitochondrial outer membrane contains besides the eukaryotic porins responsible for its major permeability properties a variety of other not fully identified channels. It encloses also the TOM apparatus together with the sorting mechanism SAM, responsible for the uptake and assembly of many mitochondrial proteins that are encoded in the nucleus and synthesized in the cytoplasm at free ribosomes. The recognition and the study of electrophysiological properties of eukaryotic porin or VDAC started in the late seventies of the last century by a study of Schein et al., who reconstituted the pore from crude extracts of Paramecium mitochondria into planar lipid bilayer membranes. Whereas the literature about structure and function of eukaryotic porins was comparatively rare during the first 10years after the first study, the number of publications started to explode with the first sequencing of human Porin 31HL and the recognition of the important function of eukaryotic porins in mitochondrial metabolism. Many genomes contain more than one gene coding for homologs of eukaryotic porins. More than 100 sequences of eukaryotic porins are known to date. Although the sequence identity between them is relatively low, the polypeptide length and in particular, the electrophysiological characteristics are highly preserved. This means that all eukaryotic porins studied to date are anion selective in the open state. They are voltage-dependent and switch into cation-selective substates at voltages in the physiological relevant range. A major breakthrough was also the elucidation of the 3D structure of the eukaryotic pore, which is formed by 19 β-strands similar to those of bacterial porin channels. The function of the presumed gate an α-helical stretch of 20 amino acids allowed further studies with respect to voltage dependence and function, but its exact role in channel gating is still not fully understood.

Cell-free electrophysiology of human VDACs incorporated into nanodiscs: An improved method

Voltage-dependent anion-selective channel (VDAC) is one of the main proteins of the outer mitochondrial membrane of all eukaryotes, where it forms aqueous, voltage-sensitive, and ion-selective channels. Its electrophysiological properties have been thoroughly analyzed with the planar lipid bilayer technique. To date, however, available results are based on isolations of VDACs from tissue or from recombinant VDACs produced in bacterial systems. It is well known that the cytosolic overexpression of highly hydrophobic membrane proteins often results in the formation of inclusion bodies containing insoluble aggregates. Purification of properly folded proteins and restoration of their full biological activity requires several procedures that considerably lengthen experimental times. To overcome these restraints, we propose a one-step reaction that combines in vitro cell-free protein expression with nanodisc technology to obtain human VDAC isoforms directly integrated in a native-like lipid bilayer. Reconstitution assays into artificial membranes confirm the reliability of this new methodological approach and provide results comparable to those of VDACs prepared with traditional protein isolation and reconstitution protocols. The use of membrane-mimicking nanodisc systems represents a breakthrough in VDAC electrophysiology and may be adopted to further structural studies.

Functional Characterization of VDACs in Grape and Its Putative Role in Response to Pathogen Stress

Voltage-dependent anion channels (VDACs) are the most abundant proteins in the mitochondrial outer membranes of all eukaryotic cells. They participate in mitochondrial energy metabolism, mitochondria-mediated apoptosis, and cell growth and reproduction. Here, the chromosomal localizations, gene structure, conserved domains, and phylogenetic relationships were analyzed. The amino acid sequences of VDACs were found to be highly conserved. The tissue-specific transcript analysis from transcriptome data and qRT-PCR demonstrated that grapevine VDACs might play an important role in plant growth and development. It was also speculated that VDAC3 might be a regulator of modulated leaf and berry development as the expression patterns during these developmental stages are up-regulated. Further, we screened the role of all grape VDACs' response to pathogen stress and found that VDAC3 from downy mildew Plasmopara viticola-resistant Chinese wild grapevine species Vitis piasezkii "Liuba-8" had a higher expression than the downy mildew susceptible species Vitis vinifera cv. "Thompson Seedless" after inoculation with P. viticola. Overexpression of VpVDAC3 resulted in increased resistance to pathogens, which was found to prevent VpVDAC3 protein accumulation through protein post-transcriptional regulation. Taken together, these data indicate that VpVDAC3 plays a role in P. viticola defense and provides the evidence with which to understand the mechanism of grape response to pathogen stress.

Alpha-Synuclein and Mitochondrial Dysfunction in Parkinson's Disease: The Emerging Role of VDAC

Alpha-Synuclein (αSyn) is a protein whose function is still debated, as well as its role in modulation of mitochondrial function in both physiological and pathological conditions. Mitochondrial porins or Voltage-Dependent Anion Channel (VDAC) proteins are the main gates for ADP/ATP and various substrates towards the organelle. Furthermore, they act as a mitochondrial hub for many cytosolic proteins, including αSyn. This review analyzes the main aspects of αSyn-mitochondria interaction, focusing on the role of VDAC and its emerging involvement in the pathological processes.

CD73 Ectonucleotidase Restrains CD8+ T Cell Metabolic Fitness and Anti-tumoral Activity

CD39 and CD73 are ectoenzymes that dephosphorylate ATP into its metabolites; ADP, AMP, and adenosine, and thus are considered instrumental in the development of immunosuppressive microenvironments. We have previously shown that within the CD8+ T cell population, naïve and memory cells express the CD73 ectonucleotidase, while terminally differentiated effector cells are devoid of this enzyme. This evidence suggests that adenosine might exert an autocrine effect on CD8+ T cells during T cell differentiation. To study the possible role of CD73 and adenosine during this process, we compared the expression of the adenosinergic signaling components, the phenotype, and the functional properties between CD73-deficient and WT CD8+ T cells. Upon activation, we observed an upregulation of CD73 expression in CD8+ T cells along with an upregulation of the adenosine A2A receptor. Interestingly, when we differentiated CD8+ T cells to Tc1 cells in vitro, we observed that these cells produce adenosine and that CD73-deficient cells present a higher cytotoxic potential evidenced by an increase in IFN-γ, TNF-α, and granzyme B production. Moreover, CD73-deficient cells presented a increased glucose uptake and higher mitochondrial respiration, indicating that this ectonucleotidase restrict the mitochondrial capacity in CD8+ T cells. In agreement, when adoptively transferred, antigen-specific CD73-deficient CD8+ T cells were more effective in reducing the tumor burden in B16.OVA melanoma-bearing mice and presented lower levels of exhaustion markers than wild type cells. All these data suggest an autocrine effect of CD73-mediated adenosine production, limiting differentiation and cytotoxic T cells' metabolic fitness.

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.

Mild depolarization of the inner mitochondrial membrane is a crucial component of an anti-aging program

The mitochondria, organelles that produce the largest amounts of ATP and reactive oxygen species (mROS) in living cells, are equipped with a universal mechanism that can completely prevent mROS production. This mechanism consists of mild depolarization of the inner mitochondrial membrane to decrease the membrane potential to a level sufficient to form ATP but insufficient to generate mROS. In short-lived mice, aging is accompanied by inactivation of the mild depolarization mechanism, resulting in chronic poisoning of the organism with mROS. However, mild depolarization still functions for many years in long-lived naked mole rats and bats.

The mitochondria of various tissues from mice, naked mole rats (NMRs), and bats possess two mechanistically similar systems to prevent the generation of mitochondrial reactive oxygen species (mROS): hexokinases I and II and creatine kinase bound to mitochondrial membranes. Both systems operate in a manner such that one of the kinase substrates (mitochondrial ATP) is electrophoretically transported by the ATP/ADP antiporter to the catalytic site of bound hexokinase or bound creatine kinase without ATP dilution in the cytosol. One of the kinase reaction products, ADP, is transported back to the mitochondrial matrix via the antiporter, again through an electrophoretic process without cytosol dilution. The system in question continuously supports H⁺-ATP synthase with ADP until glucose or creatine is available. Under these conditions, the membrane potential, ∆ψ, is maintained at a lower than maximal level (i.e., mild depolarization of mitochondria). This ∆ψ decrease is sufficient to completely inhibit mROS generation. In 2.5-y-old mice, mild depolarization disappears in the skeletal muscles, diaphragm, heart, spleen, and brain and partially in the lung and kidney. This age-dependent decrease in the levels of bound kinases is not observed in NMRs and bats for many years. As a result, ROS-mediated protein damage, which is substantial during the aging of short-lived mice, is stabilized at low levels during the aging of long-lived NMRs and bats. It is suggested that this mitochondrial mild depolarization is a crucial component of the mitochondrial anti-aging system.

Lactate Buildup at the Site of Chronic Inflammation Promotes Disease by Inducing CD4+ T Cell Metabolic Rewiring

Accumulation of lactate in the tissue microenvironment is a feature of both inflammatory disease and cancer. Here, we assess the response of immune cells to lactate in the context of chronic inflammation. We report that lactate accumulation in the inflamed tissue contributes to the upregulation of the lactate transporter SLC5A12 by human CD4+ T cells. SLC5A12-mediated lactate uptake into CD4+ T cells induces a reshaping of their effector phenotype, resulting in increased IL17 production via nuclear PKM2/STAT3 and enhanced fatty acid synthesis. It also leads to CD4+ T cell retention in the inflamed tissue as a consequence of reduced glycolysis and enhanced fatty acid synthesis. Furthermore, antibody-mediated blockade of SLC5A12 ameliorates the disease severity in a murine model of arthritis. Finally, we propose that lactate/SLC5A12-induced metabolic reprogramming is a distinctive feature of lymphoid synovitis in rheumatoid arthritis patients and a potential therapeutic target in chronic inflammatory disorders.

Hepatic HKDC1 Expression Contributes to Liver Metabolism

Glucokinase (GCK) is the principal hexokinase (HK) in the liver, operating as a glucose sensor to regulate glucose metabolism and lipid homeostasis. Recently, we proposed HK domain-containing 1 (HKDC1) to be a fifth HK with expression in the liver. Here, we reveal HKDC1 to have low glucose-phosphorylating ability and demonstrate its association with the mitochondria in hepatocytes. As we have shown previously that genetic deletion of HKDC1 leads to altered hepatic triglyceride levels, we also explored the influence of overexpression of HKDC1 in hepatocytes on cellular metabolism, observing reduced glycolytic capacity and maximal mitochondrial respiration with concurrent reductions in glucose oxidation and mitochondrial membrane potential. Furthermore, we found that acute in vivo overexpression of HKDC1 in the liver induced substantial changes in mitochondrial dynamics. Altogether, these findings suggest that overexpression of HKDC1 causes mitochondrial dysfunction in hepatocytes. However, its overexpression was not enough to alter energy storage in the liver but led to mild improvement in glucose tolerance. We next investigated the conditions necessary to induce HKDC1 expression, observing HKDC1 expression to be elevated in human patients whose livers were at more advanced stages of nonalcoholic fatty liver disease (NAFLD) and similarly, found high liver expression in mice on diets causing high levels of liver inflammation and fibrosis. Overall, our data suggest that HKDC1 expression in hepatocytes results in defective mitochondrial function and altered hepatocellular metabolism and speculate that its expression in the liver may play a role in the development of NAFLD.

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.

Voltage-Dependent Anion Channel 1 Interacts with Ribonucleoprotein Complexes To Enhance Infectious Bursal Disease Virus Polymerase Activity

Infectious bursal disease virus (IBDV) is a double-stranded RNA (dsRNA) virus. Segment A contains two overlapping open reading frames (ORFs), which encode viral proteins VP2, VP3, VP4, and VP5. Segment B contains one ORF and encodes the viral RNA-dependent RNA polymerase, VP1. IBDV ribonucleoprotein complexes are composed of VP1, VP3, and dsRNA and play a critical role in mediating viral replication and transcription during the virus life cycle. In the present study, we identified a cellular factor, VDAC1, which was upregulated during IBDV infection and found to mediate IBDV polymerase activity. VDAC1 senses IBDV infection by interacting with viral proteins VP1 and VP3. This association is caused by RNA bridging, and all three proteins colocalize in the cytoplasm. Furthermore, small interfering RNA (siRNA)-mediated downregulation of VDAC1 resulted in a reduction in viral polymerase activity and a subsequent decrease in viral yield. Moreover, overexpression of VDAC1 enhanced IBDV polymerase activity. We also found that the viral protein VP3 can replace segment A to execute polymerase activity. A previous study showed that mutations in the C terminus of VP3 directly influence the formation of VP1-VP3 complexes. Our immunoprecipitation experiments demonstrated that protein-protein interactions between VDAC1 and VP3 and between VDAC1 and VP1 play a role in stabilizing the interaction between VP3 and VP1, further promoting IBDV polymerase activity.IMPORTANCE The cellular factor VDAC1 controls the entry and exit of mitochondrial metabolites and plays a pivotal role during intrinsic apoptosis by mediating the release of many apoptogenic molecules. Here we identify a novel role of VDAC1, showing that VDAC1 interacts with IBDV ribonucleoproteins (RNPs) and facilitates IBDV replication by enhancing IBDV polymerase activity through its ability to stabilize interactions in RNP complexes. To our knowledge, this is the first report that VDAC1 is specifically involved in regulating IBDV RNA polymerase activity, providing novel insight into virus-host interactions.

Phospho-ubiquitin-PARK2 complex as a marker for mitophagy defects

The E3 ubiquitin ligase PARK2 and the mitochondrial protein kinase PINK1 are required for the initiation of mitochondrial damage-induced mitophagy. Together, PARK2 and PINK1 generate a phospho-ubiquitin signal on outer mitochondrial membrane proteins that triggers recruitment of the autophagy machinery. This paper describes the detection of a defined 500-kDa phospho-ubiquitin-rich PARK2 complex that accumulates on mitochondria upon treatment with the membrane uncoupler CCCP. Formation of this complex is dependent on the presence of PINK1 and is absent in mutant forms of PARK2, whereby mitophagy is also arrested. These results signify a functional signaling complex that is essential for the progression of mitophagy. The visualization of the PARK2 signaling complex represents a novel marker for this critical step in mitophagy and can be used to monitor mitophagy progression in PARK2 mutants and to uncover additional upstream factors required for PARK2-mediated mitophagy signaling.

Hexokinase I N-terminal based peptide prevents the VDAC1-SOD1 G93A interaction and re-establishes ALS cell viability

Superoxide Dismutase 1 mutants associate with 20–25% of familial Amyotrophic Lateral Sclerosis (ALS) cases, producing toxic aggregates on mitochondria, notably in spinal cord. The Voltage Dependent Anion Channel isoform 1 (VDAC1) in the outer mitochondrial membrane is a docking site for SOD1 G93A mutant in ALS mice and the physiological receptor of Hexokinase I (HK1), which is poorly expressed in mouse spinal cord. Our results demonstrate that HK1 competes with SOD1 G93A for binding VDAC1, suggesting that in ALS spinal cord the available HK1-binding sites could be used by SOD1 mutants for docking mitochondria, producing thus organelle dysfunction. We tested this model by studying the action of a HK1-N-terminal based peptide (NHK1). This NHK1 peptide specifically interacts with VDAC1, inhibits the SOD1 G93A binding to mitochondria and restores the viability of ALS model NSC34 cells. Altogether, our results suggest that NHK1 peptide could be developed as a therapeutic tool in ALS, predicting an effective role also in other proteinopathies.

MicroRNA-7 downregulates the oncogene VDAC1 to influence hepatocellular carcinoma proliferation and metastasis

Recent studies have been shown that voltage-dependent anion channel 1 (VDAC1) plays an important role in carcinogenesis. However, its molecular biological function in hepatocellular carcinoma (HCC) has not been entirely clarified. This study investigated the expression of VDAC1 in HCC and its prognostic value for HCC patients. Furthermore, we also identify the relevant VDAC1 direct target. Western blot, real-time quantitative PCR (qRT-PCR), and immunohistochemical (IHC) staining were performed to detect the expression of VDAC1 in HCC. Furthermore, the relationship between the VDAC1 level and clinicopathological features and prognostic values was explored. The effects of VDAC1 on HCC cell proliferation, migration, and invasion were also investigated in vitro. Predicted target gene of VDAC1 was determined by dual-luciferase reporter assay, qRT-PCR, and Western blot analyses. Our results revealed elevated VDAC1 messenger RNA (mRNA) (P = 0.0020) and protein (P = 0.0035) expression in tumor tissue samples compared with paired adjacent non-tumorous tissue samples. High VDAC1 expression was correlated with distant metastasis (P = 0.025), differentiation (P = 0.002), and advanced tumor stage (P = 0.004) in HCC patients. Kaplan-Meier survival analysis demonstrated that high expression of VDAC1 was significantly correlated with a poor prognosis for HCC patients (P < 0.001). The multivariate analysis revealed that VDAC1 expression was an independent prognostic factor of the overall survival rate of HCC patients. Furthermore, knockdown of VDAC1 inhibits HCC cell proliferation, migration, and invasion in vitro. Moreover, further study revealed that miR-7 was a putative target of VDAC1. Our study suggested that miR-7 suppressed the expression of VDAC1. VDAC1 plays an important role in tumor progression and may be used as a potential role in the prognosis of HCC patients.

Identification of a mitochondrial-binding site on the N-terminal end of hexokinase II

Hexokinase II (HKII) is responsible for the first step in the glycolysis pathway by adding a phosphate on to the glucose molecule so it can proceed down the pathway to produce the energy for continuous cancer cell growth. Tumour cells overexpress the HKII enzyme. In fact, it is the overexpression of the HKII enzyme that makes the diagnosis of cancer possible when imaged by positron emission tomography (PET). HKII binds to the voltage-dependent anion channel (VDAC) located on the mitochondrial outer membrane (MOM). When bound to the MOM, HKII is blocking a major cell death pathway. Thus, HKII is responsible for two characteristics of cancer cells, rapid tumour growth and inability of cancer cells to undergo apoptosis. One method to identify novel compounds that may interfere with the HKII-VDAC-binding site is to create a molecular model using the crystal structure of HKII. However, the amino acid(s) responsible for HKII binding to VDAC are not known. Therefore, a series of truncations and point mutations were made to the N-terminal end of HKII to identify the binding site to VDAC. Deletions of the first 10 and 20 amino acids indicated that important amino acid(s) for binding were located within the first 10 amino acids. Next, a series of point mutations were made within the first 10 amino acids. It is clear from the immunofluorescence images and immunoblot results that mutating the fifth amino acid from histidine to proline completely abolished binding to the MOM.

The proteomic 2D-DIGE approach reveals the protein voltage-dependent anion channel 2 as a potential therapeutic target in epithelial thyroid tumours

We investigated the role of VDAC2 in human epithelial thyroid tumours using proteomic 2D-DIGE analysis and qRT-PCR. We found a significant up-regulation of VDAC2 in thyroid tumours and in thyroid tumour cell lines (TPC-1 and CAL-62). We did not detect overexpression of VDAC2 in a normal thyroid cell line (Nthy-ori 3-1). Silico analysis revealed that two proteins, BAK1 and BAX, had a strong relationship with VDAC2. BAK1 gene expression showed down-regulation in thyroid tumours (follicular and papillary tumours) and in TPC-1 and CAL-62 cell lines. Transient knockdown of VDAC2 in TPC-1 and CAL-62 promoted upregulation of the BAK1 gene and protein expression, and increased susceptibility to sorafenib treatment. Overexpression of the BAK1 gene in CAL-62 showed lower sorafenib sensitivity than VDAC2 knockdown cells. We propose the VDAC2 gene as a novel therapeutic target in these tumours.

Targeting hexokinase II to mitochondria to modulate energy metabolism and reduce ischaemia-reperfusion injury in heart

Mitochondrially bound hexokinase II (mtHKII) has long been known to confer cancer cells with their resilience against cell death. More recently, mtHKII has emerged as a powerful protector against cardiac cell death. mtHKII protects against ischaemia-reperfusion (IR) injury in skeletal muscle and heart, attenuates cardiac hypertrophy and remodelling, and is one of the major end-effectors through which ischaemic preconditioning protects against myocardial IR injury. Mechanisms of mtHKII cardioprotection against reperfusion injury entail the maintenance of regulated outer mitochondrial membrane (OMM) permeability during ischaemia and reperfusion resulting in stabilization of mitochondrial membrane potential, the prevention of OMM breakage and cytochrome C release, and reduced reactive oxygen species production. Increasing mtHK may also have important metabolic consequences, such as improvement of glucose-induced insulin release, prevention of acidosis through enhanced coupling of glycolysis and glucose oxidation, and inhibition of fatty acid oxidation. Deficiencies in expression and distorted cellular signalling of HKII may contribute to the altered sensitivity of diabetes to cardiac ischaemic diseases. The interaction of HKII with the mitochondrion constitutes a powerful endogenous molecular mechanism to protect against cell death in almost all cell types examined (neurons, tumours, kidney, lung, skeletal muscle, heart). The challenge now is to harness mtHKII in the treatment of infarction, stroke, elective surgery and transplantation. Remote ischaemic preconditioning, metformin administration and miR-155/miR-144 manipulations are potential means of doing just that.

Targeting hexokinase II to mitochondria to modulate energy metabolism and reduce ischaemia-reperfusion injury in heart

Mitochondrially bound hexokinase II (mtHKII) has long been known to confer cancer cells with their resilience against cell death. More recently, mtHKII has emerged as a powerful protector against cardiac cell death. mtHKII protects against ischaemia-reperfusion (IR) injury in skeletal muscle and heart, attenuates cardiac hypertrophy and remodelling, and is one of the major end-effectors through which ischaemic preconditioning protects against myocardial IR injury. Mechanisms of mtHKII cardioprotection against reperfusion injury entail the maintenance of regulated outer mitochondrial membrane (OMM) permeability during ischaemia and reperfusion resulting in stabilization of mitochondrial membrane potential, the prevention of OMM breakage and cytochrome C release, and reduced reactive oxygen species production. Increasing mtHK may also have important metabolic consequences, such as improvement of glucose-induced insulin release, prevention of acidosis through enhanced coupling of glycolysis and glucose oxidation, and inhibition of fatty acid oxidation. Deficiencies in expression and distorted cellular signalling of HKII may contribute to the altered sensitivity of diabetes to cardiac ischaemic diseases. The interaction of HKII with the mitochondrion constitutes a powerful endogenous molecular mechanism to protect against cell death in almost all cell types examined (neurons, tumours, kidney, lung, skeletal muscle, heart). The challenge now is to harness mtHKII in the treatment of infarction, stroke, elective surgery and transplantation. Remote ischaemic preconditioning, metformin administration and miR-155/miR-144 manipulations are potential means of doing just that.

Hexokinase II integrates energy metabolism and cellular protection: Akting on mitochondria and TORCing to autophagy

Accumulating evidence reveals that metabolic and cell survival pathways are closely related, sharing common signaling molecules. Hexokinase catalyzes the phosphorylation of glucose, the rate-limiting first step of glycolysis. Hexokinase II (HK-II) is a predominant isoform in insulin-sensitive tissues such as heart, skeletal muscle, and adipose tissues. It is also upregulated in many types of tumors associated with enhanced aerobic glycolysis in tumor cells, the Warburg effect. In addition to the fundamental role in glycolysis, HK-II is increasingly recognized as a component of a survival signaling nexus. This review summarizes recent advances in understanding the protective role of HK-II, controlling cellular growth, preventing mitochondrial death pathway and enhancing autophagy, with a particular focus on the interaction between HK-II and Akt/mTOR pathway to integrate metabolic status with the control of cell survival.

In Saccharomyces cerevisiae fructose-1,6-bisphosphate contributes to the Crabtree effect through closure of the mitochondrial unspecific channel

In Saccharomyces cerevisiae addition of glucose inhibits oxygen consumption, i.e. S. cerevisiae is Crabtree-positive. During active glycolysis hexoses-phosphate accumulate, and probably interact with mitochondria. In an effort to understand the mechanism underlying the Crabtree effect, the effect of two glycolysis-derived hexoses-phosphate was tested on the S. cerevisiae mitochondrial unspecific channel (ScMUC). Glucose-6-phosphate (G6P) promoted partial opening of ScMUC, which led to proton leakage and uncoupling which in turn resulted in, accelerated oxygen consumption. In contrast, fructose-1,6-bisphosphate (F1,6BP) closed ScMUC and thus inhibited the rate of oxygen consumption. When added together, F1,6BP reverted the mild G6P-induced effects. F1,6BP is proposed to be an important modulator of ScMUC, whose closure contributes to the "Crabtree effect".

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.

Cardiac-specific hexokinase 2 overexpression attenuates hypertrophy by increasing pentose phosphate pathway flux

BACKGROUND

The enzyme hexokinase-2 (HK2) phosphorylates glucose, which is the initiating step in virtually all glucose utilization pathways. Cardiac hypertrophy is associated with a switch towards increased glucose metabolism and decreased fatty acid metabolism. Recent evidence suggests that the increased glucose utilization is compensatory to the down-regulated fatty acid metabolism during hypertrophy and is, in fact, beneficial. Therefore, we hypothesized that increasing glucose utilization by HK2 overexpression would decrease cardiac hypertrophy.

METHODS AND RESULTS

Mice with cardiac-specific HK2 overexpression displayed decreased hypertrophy in response to isoproterenol. Neonatal rat ventricular myocytes (NRVMs) infected with an HK2 adenovirus similarly displayed decreased hypertrophy in response to phenylephrine. Hypertrophy increased reactive oxygen species (ROS) levels, which were attenuated by HK2 overexpression, thereby decreasing NRVM hypertrophy and death. HK2 appears to modulate ROS via the pentose phosphate pathway, as inhibition of glucose-6-phosphate dehydrogenase with dehydroepiandrosterone decreased the ability of HK2 to diminish ROS and hypertrophy.

CONCLUSIONS

These results suggest that HK2 attenuates cardiac hypertrophy by decreasing ROS accumulation via increased pentose phosphate pathway flux.

ATP produced by oxidative phosphorylation is channeled toward hexokinase bound to mitochondrial porin (VDAC) in beetroots (Beta vulgaris)

Mitochondrial porins or voltage-dependent anion channels (VDAC) are the main route for solute transport through outer mitochondrial membranes (OMM). In mammals, hexokinase (HK) binds to VDAC, which allows the channeling of ATP synthesized by oxidative phosphorylation toward HK. In plants, although HK has been found associated with OMM, evidence for an interaction with VDAC is scarce. Thus, in this work, we studied the physical and functional interaction between these proteins in beetroot mitochondria. To observe a physical interaction between HK and VDAC, OMM presenting HK activity were prepared from purified mitochondria. Protein complexes were solubilized from OMM with mild detergents and separated by centrifugation in glycerol gradients. Both HK activity and immunodetected VDAC were found in small (9S-13S) and large (>40S) complexes. OMM proteins were also separated according to their hydropathy by serial phase partitioning with Triton X-114. Most of HK activity was found in hydrophobic fractions where VDAC was also present. These results indicated that HK could be bound to VDAC in beetroot mitochondria. The functional interaction of HK with VDAC was demonstrated by observing the effect of apyrase on HK-catalyzed glucose phosphorylation in intact mitochondria. Apyrase, which hydrolyzes freely soluble ATP, competed efficiently with hexokinase for ATP when it was produced outside mitochondria (with PEP and pyruvate kinase), but not when it was produced inside mitochondria by oxidative phosphorylation. These results suggest that HK closely interacts with VDAC in beetroot mitochondria, and that this interaction allows the channeling of respiratory ATP toward HK through VDAC.

Physiologic functions of cyclophilin D and the mitochondrial permeability transition pore

This review focuses on the role of cyclophilin D (CypD) as a prominent mediator of the mitochondrial permeability transition pore (MPTP) and subsequent effects on cardiovascular physiology and pathology. Although a great number of reviews have been written on the MPTP and its effects on cell death, we focus on the biology surrounding CypD itself and the non-cell death physiologic functions of the MPTP. A greater understanding of the physiologic functions of the MPTP and its regulation by CypD will likely suggest novel therapeutic approaches for cardiovascular disease, both dependent and independent of programmed necrotic cell death mechanisms.  

Hexokinase II knockdown results in exaggerated cardiac hypertrophy via increased ROS production

Hexokinase-II (HKII) is highly expressed in the heart and can bind to the mitochondrial outer membrane. Since cardiac hypertrophy is associated with a substrate switch from fatty acid to glucose, we hypothesized that a reduction in HKII would decrease cardiac hypertrophy after pressure overload. Contrary to our hypothesis, heterozygous HKII-deficient (HKII(+/-)) mice displayed increased hypertrophy and fibrosis in response to pressure overload. The mechanism behind this phenomenon involves increased levels of reactive oxygen species (ROS), as HKII knockdown increased ROS accumulation, and treatment with the antioxidant N-acetylcysteine (NAC) abrogated the exaggerated response. HKII mitochondrial binding is also important for the hypertrophic effects, as HKII dissociation from the mitochondria resulted in de novo hypertrophy, which was also attenuated by NAC. Further studies showed that the increase in ROS levels in response to HKII knockdown or mitochondrial dissociation is mediated through increased mitochondrial permeability and not by a significant change in antioxidant defenses. Overall, these data suggest that HKII and its mitochondrial binding negatively regulate cardiac hypertrophy by decreasing ROS production via mitochondrial permeability.

Phylogenetic and coevolutionary analysis of the β-barrel protein family comprised of mitochondrial porin (VDAC) and Tom40

Beta-barrel proteins are the main transit points across the mitochondrial outer membrane. Mitochondrial porin, the voltage-dependent, anion-selective channel (VDAC), is responsible for the passage of small molecules between the mitochondrion and the cytosol. Through interactions with other mitochondrial and cellular proteins, it is involved in regulating organellar and cellular metabolism and likely contributes to mitochondrial structure. Tom40 is part of the translocase of the outer membrane, and acts as the channel for passage of preproteins during their import into the organelle. These proteins appear to share a common evolutionary origin and structure. In the current study, the evolutionary relationships between and within both proteins were investigated through phylogenetic analysis. The two groups have a common origin and have followed independent, complex evolutionary pathways, leading to the generation of paralogues in animals and plants. Structures of diverse representatives were modeled, revealing common themes rather than sites of high identity in both groups. Within each group, intramolecular coevolution was assessed, revealing a new set of sites potentially involved in structure-function relationships in these molecules. A weak link between Tom40 and proteins related to the mitochondrial distribution and morphology protein, Mdm10, was identified. This article is part of a Special Issue entitled: VDAC structure, function, and regulation of mitochondrial metabolism.

The role of VDAC in cell death: friend or foe?

As the voltage-dependent anion channel (VDAC) forms the interface between mitochondria and the cytosol, its importance in metabolism is well understood. However, research on VDAC's role in cell death is a rapidly growing field, unfortunately with much confusing and contradictory results. The fact that VDAC plays a role in outer mitochondrial membrane permeabilization is undeniable, however, the mechanisms behind this remain very poorly understood. In this review, we will summarize the studies that show evidence of VDAC playing a role in cell death. To begin, we will discuss the evidence for and against VDAC's involvement in mitochondrial permeability transition (MPT) and attempt to clarify that VDAC is not an essential component of the MPT pore (MPTP). Next, we will evaluate the remaining literature on VDAC in cell death which can be divided into three models: proapoptotic agents escaping through VDAC, VDAC homo- or hetero-oligomerization, or VDAC closure resulting in outer mitochondrial membrane permeabilization through an unknown pathway. We will then discuss the growing list of modulators of VDAC activity that have been associated with induction/protection against cell death. This article is part of a Special Issue entitled: VDAC structure, function, and regulation of mitochondrial metabolism.

Clotrimazole induces a late G1 cell cycle arrest and sensitizes glioblastoma cells to radiation in vitro

Tumor cells are characterized by their high rate of glycolysis and clotrimazole has been shown to disrupt the glycolysis pathway thereby arresting the cells in the G1 cell cycle phase. Herein, we present data to support our hypothesis that clotrimazole arrests tumor cells in a radiosensitizing, late G1 phase. The effects of clotrimazole were studied using the glioblastoma cell line, U-87 MG. Flow cytometry was used to analyze cell cycle redistribution and induction of apoptosis. Immunoblots were probed to characterize a late G1 cell cycle arrest. Nuclear and cytoplasmic fractions were collected to follow the clotrimazole-induced translocation of hexokinase II. Clonogenic assays were designed to determine the radiosensitizing effect by clotrimazole. Our studies have shown a dose-dependent and time-dependent clotrimazole arrest in a late G1 cell cycle phase. Concurrent with the late G1 arrest, we observed an overexpression of p27 along with a decreased expression of p21, cyclin-dependent kinase 1, cyclin-dependent kinase 4, and cyclin D. Clotrimazole induced the translocation of mitochondrial-bound hexokinase II to the cytoplasm and the release of cytochrome c into the cytoplasm. Clotrimazole-induced apoptosis was enhanced when combined with radiation. Clotrimazole was shown to sensitize tumor cells to radiation when the cells were irradiated for 18 h post-clotrimazole treatment. The disruption of the glycolysis pathway by clotrimazole leads to cell cycle arrest of U-87 MG cells in the radiosensitizing late G1 phase. The use of clotrimazole as a radiosensitizing agent for cancer treatment is novel and may have broad therapeutic applications.

Reduction in hexokinase II levels results in decreased cardiac function and altered remodeling after ischemia/reperfusion injury

RATIONALE

Cardiomyocytes switch substrate utilization from fatty acid to glucose under ischemic conditions; however, it is unknown how perturbations in glycolytic enzymes affect cardiac response to ischemia/reperfusion (I/R). Hexokinase (HK)II is a HK isoform that is expressed in the heart and can bind to the mitochondrial outer membrane.

OBJECTIVE

We sought to define how HKII and its binding to mitochondria play a role in cardiac response and remodeling after I/R.

METHODS AND RESULTS

We first showed that HKII levels and its binding to mitochondria are reduced 2 days after I/R. We then subjected the hearts of wild-type and heterozygote HKII knockout (HKII(+/)⁻) mice to I/R by coronary ligation. At baseline, HKII(+/)⁻ mice have normal cardiac function; however, they display lower systolic function after I/R compared to wild-type animals. The mechanism appears to be through an increase in cardiomyocyte death and fibrosis and a reduction in angiogenesis; the latter is through a decrease in hypoxia-inducible factor-dependent pathway signaling in cardiomyocytes. HKII mitochondrial binding is also critical for cardiomyocyte survival, because its displacement in tissue culture with a synthetic peptide increases cell death. Our results also suggest that HKII may be important for the remodeling of the viable cardiac tissue because its modulation in vitro alters cellular energy levels, O₂ consumption, and contractility.

CONCLUSIONS

These results suggest that reduction in HKII levels causes altered remodeling of the heart in I/R by increasing cell death and fibrosis and reducing angiogenesis and that mitochondrial binding is needed for protection of cardiomyocytes.

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

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

Outer membrane VDAC1 controls permeability transition of the inner mitochondrial membrane in cellulo during stress-induced apoptosis

Voltage-dependent anion channel (VDAC)1 is the main channel of the mitochondrial outer membrane (MOM) and it has been proposed to be part of the permeability transition pore (PTP), a putative multiprotein complex candidate agent of the mitochondrial permeability transition (MPT). Working at the single live cell level, we found that overexpression of VDAC1 triggers MPT at the mitochondrial inner membrane (MIM). Conversely, silencing VDAC1 expression results in the inhibition of MPT caused by selenite-induced oxidative stress. This MOM-MIM crosstalk was modulated by Cyclosporin A and mitochondrial Cyclophilin D, but not by Bcl-2 and Bcl-X(L), indicative of PTP operation. VDAC1-dependent MPT engages a positive feedback loop involving reactive oxygen species and p38-MAPK, and secondarily triggers a canonical apoptotic response including Bax activation, cytochrome c release and caspase 3 activation. Our data thus support a model of the PTP complex involving VDAC1 at the MOM, and indicate that VDAC1-dependent MPT is an upstream mechanism playing a causal role in oxidative stress-induced apoptosis.

Regulation of hexokinase binding to VDAC

Hexokinase isoforms I and II bind to mitochondrial outer membranes in large part by interacting with the outer membrane voltage-dependent anion channel (VDAC). This interaction results in a shift in the susceptibility of mitochondria to pro-apoptotic signals that are mediated through Bcl2-family proteins. The upregulation of hexokinase II expression in tumor cells is thought to provide both a metabolic benefit and an apoptosis suppressive capacity that gives the cell a growth advantage and increases its resistance to chemotherapy. However, the mechanisms responsible for the anti-apoptotic effect of hexokinase binding and its regulation remain poorly understood. We hypothesize that hexokinase competes with Bcl2 family proteins for binding to VDAC to influence the balance of pro-and anti-apoptotic proteins that control outer membrane permeabilization. Hexokinase binding to VDAC is regulated by protein kinases, notably glycogen synthase kinase (GSK)-3beta and protein kinase C (PKC)-epsilon. In addition, there is evidence that the cholesterol content of the mitochondrial membranes may contribute to the regulation of hexokinase binding. At the same time, VDAC associated proteins are critically involved in the regulation of cholesterol uptake. A better characterization of these regulatory processes is required to elucidate the role of hexokinases in normal tissue function and to apply these insights for optimizing cancer treatment.

Glucose phosphorylation and mitochondrial binding are required for the protective effects of hexokinases I and II

Alterations in glucose metabolism have been demonstrated for diverse disorders ranging from heart disease to cancer. The first step in glucose metabolism is carried out by the hexokinase (HK) family of enzymes. HKI and II can bind to mitochondria through their N-terminal hydrophobic regions, and their overexpression in tissue culture protects against cell death. In order to determine the relative contributions of mitochondrial binding and glucose-phosphorylating activities of HKs to their overall protective effects, we expressed full-length HKI and HKII, their truncated proteins lacking the mitochondrial binding domains, and catalytically inactive proteins in tissue culture. The overexpression of full-length proteins resulted in protection against cell death, decreased levels of reactive oxygen species, and possibly inhibited mitochondrial permeability transition in response to H(2)O(2). However, the truncated and mutant proteins exerted only partial effects. Similar results were obtained with primary neonatal rat cardiomyocytes. The HK proteins also resulted in an increase in the phosphorylation of voltage-dependent anion channel (VDAC) through a protein kinase Cepsilon (PKCepsilon)-dependent pathway. These results suggest that both glucose phosphorylation and mitochondrial binding contribute to the protective effects of HKI and HKII, possibly through VDAC phosphorylation by PKCepsilon.

A study on the two binding sites of hexokinase on brain mitochondria

Background

Type I hexokinase (HK-I) constitutes the predominant form of the enzyme in the brain, a major portion of which is associated with the outer mitochondrial membrane involving two sets of binding sites. In addition to the glucose-6-phosphate (G6P)-sensitive site (Type A), the enzyme is bound on a second set of sites (Type B) which are, while insensitive to G6P, totally releasable by use of high concentrations of chaotropic salts such as KSCN. Results obtained on release of HK-I from these "sites" suggested the possibility for the existence of distinct populations of the bound enzyme, differing in susceptibility to release by G6P.

Results

In the present study, the sensitivity of HK-I toward release by G6P (2 mM) and a low concentration of KSCN (45 mM) was investigated using rat brain, bovine brain and human brain mitochondria. Partial release from the G6P-insensitive site occurred without disruption of the mitochondrial membrane as a whole and as related to HK-I binding to the G6P-sensitive site. While, as expected, the sequential regime release-rebinding-release was observed on site A, no rebinding was detectable on site B, pre-treated with 45 mM KSCN. Also, no binding was detectable on mitochondria upon blocking site A for HK-I binding utilizing dicyclohexylcarbodiimide (DCCD), followed by subsequent treatment with KSCN. These observations while confirmed the previously-published results on the overall properties of the two sites, demonstrated for the first time that the reversible association of the enzyme on mitochondria is uniquely related to the Type A site.

Conclusion

Use of very low concentrations of KSCN at about 10% of the level previously reported to cause total release of HK-I from the G6P- insensitive site, caused partial release from this site in a reproducible manner. In contrast to site A, no rebinding of the enzyme takes place on site B, suggesting that site A is 'the only physiologically-important site in relation to the release-rebinding of the enzyme which occur in response to the energy requirements of the brain. Based on the results presented, a possible physiological role for distribution of the enzyme between the two sites on the mitochondrion is proposed.

High-level expression, refolding and probing the natural fold of the human voltage-dependent anion channel isoforms I and II

The voltage-dependent anion channel (VDAC) is the major protein found in the outer membrane of mitochondria. The channel is responsible for the exchange of ATP/ADP and the translocation of ions and other small metabolites over the membrane. In order to obtain large amounts of pure and suitably folded human VDAC for functional and structural studies, the genes of the human isoforms I and II (HVDAC1 and HVDAC2) were cloned in Escherichia coli. High-level expression led to inclusion body formation. Both proteins could be refolded in vitro by adding denatured protein to a solution of zwitterionic or nonionic detergents. A highly efficient and fast protocol for refolding was developed that yielded more than 50 mg of pure human VDACs per liter of cell culture. The native and functional state of the refolded porins was probed by Fourier transform infrared spectroscopy to determine the secondary structure composition and by electrophysiological measurements, demonstrating the pore-forming activity of HVDAC1. Furthermore, binding of HVDAC1 to immobilized ATP was demonstrated. Limited proteolysis of HVDAC1 protein embedded in detergent micelles in combination with matrix-assisted laser desorption ionization mass spectrometric analysis was applied to identify micelle-exposed regions of the protein and to develop an improved topology model. Our analysis strongly suggests a 16-stranded, antiparallel beta-barrel with one large and seven short loops and turns. Initial crystallization trials of the protein yielded crystals diffracting to 8 Angstrom resolution.

The permeability transition pore in cell death

The permeability transition pore (PT-pore) is a multi-component protein aggregate in mitochondria that comprises factors in the inner as well as in the outer mitochondrial membrane. This complex has two functions: firstly, it regulates the integration of oxidative phosphorylation into the cellular energy household and secondly, it induces cell death when converted into an unspecific channel. The latter causes a collapse of the mitochondrial membrane potential and activates a chain of events that culminate in the demise of the cell. It has been controversial for some time whether the PT-pore is causative for or only amplifies a signal of cell death but novel results confirm a central role of this protein complex for cell death induction. While a considerable body of data exist on its subunit composition, recent genetic knock-out experiments suggest that the identity of the core factors of the PT-pore is still unresolved. Moreover, accumulating evidence point to a much more complex composition of this protein complex than anticipated. Here, we review the current knowledge of its subunit composition, the evidence of a role in cell death, and we propose a model for the activation of the PT-pore for cell death.

The evolutionary history of mitochondrial porins

BACKGROUND

Mitochondrial porins, or voltage-dependent anion-selective channels (VDAC) allow the passage of small molecules across the mitochondrial outer membrane, and are involved in complex interactions regulating organellar and cellular metabolism. Numerous organisms possess multiple porin isoforms, and initial studies indicated an intriguing evolutionary history for these proteins and the genes that encode them.

RESULTS

In this work, the wealth of recent sequence information was used to perform a comprehensive analysis of the evolutionary history of mitochondrial porins. Fungal porin sequences were well represented, and newly-released sequences from stramenopiles, alveolates, and seed and flowering plants were analyzed. A combination of Neighbour-Joining and Bayesian methods was used to determine phylogenetic relationships among the proteins. The aligned sequences were also used to reassess the validity of previously described eukaryotic porin motifs and to search for signature sequences characteristic of VDACs from plants, animals and fungi. Secondary structure predictions were performed on the aligned VDAC primary sequences and were used to evaluate the sites of intron insertion in a representative set of the corresponding VDAC genes.

CONCLUSION

Our phylogenetic analysis clearly shows that paralogs have appeared several times during the evolution of VDACs from the plants, metazoans, and even the fungi, suggesting that there are no "ancient" paralogs within the gene family. Sequence motifs characteristic of the members of the crown groups of organisms were identified. Secondary structure predictions suggest a common 16 beta-strand framework for the transmembrane arrangement of all porin isoforms. The GLK (and homologous or analogous motifs) and the eukaryotic porin motifs in the four representative Chordates tend to be in exons that appear to have changed little during the evolution of these metazoans. In fact there is phase correlation among the introns in these genes. Finally, our preliminary data support the notion that introns usually do not interrupt structural protein motifs, namely the predicted beta-strands. These observations concur with the concept of exon shuffling, wherein exons encode structural modules of proteins and the loss and gain of introns and the shuffling of exons via recombination events contribute to the complexity of modern day proteomes.

VDAC1 serves as a mitochondrial binding site for hexokinase in oxidative muscles

Voltage-dependent anion channels (VDACs), also known as mitochondrial porins, are the main pathway for metabolites across the mitochondrial outer membrane and may serve as binding sites for kinases, including hexokinase. We determined that mitochondria-bound hexokinase activity is significantly reduced in oxidative muscles (heart and soleus) in vdac1(-/-) mice. The activity data were supported by western blot analysis using HK2 specific antibody. To gain more insight into the physiologic mean of the results with the activity data, VDAC deficient mice were subjected to glucose tolerance testing and exercise-induced stress, each of which involves tissue glucose uptake via different mechanisms. vdac1(-/-) mice exhibit impaired glucose tolerance whereas vdac3(-/-) mice have normal glucose tolerance and exercise capacity. Mice lacking both VDAC1 and VDAC3 (vdac1(-/-)/vdac3(-/-)) have reduced exercise capacity together with impaired glucose tolerance. Therefore, we demonstrated a link between VDAC1 mediated mitochondria-bound hexokinase activity and the capacity for glucose clearance.

Prediction of protein orientation upon immobilization on biological and nonbiological surfaces

We report on a rapid simulation method for predicting protein orientation on a surface based on electrostatic interactions. New methods for predicting protein immobilization are needed because of the increasing use of biosensors and protein microarrays, two technologies that use protein immobilization onto a solid support, and because the orientation of an immobilized protein is important for its function. The proposed simulation model is based on the premise that the protein interacts with the electric field generated by the surface, and this interaction defines the orientation of attachment. Results of this model are in agreement with experimental observations of immobilization of mitochondrial creatine kinase and type I hexokinase on biological membranes. The advantages of our method are that it can be applied to any protein with a known structure; it does not require modeling of the surface at atomic resolution and can be run relatively quickly on readily available computing resources. Finally, we also propose an orientation of membrane-bound cytochrome c, a protein for which the membrane orientation has not been unequivocally determined.

Mitochondrial hexokinases, novel mediators of the antiapoptotic effects of growth factors and Akt

Cell survival has been closely linked to both trophic growth factor signaling and cellular metabolism. Such couplings have obvious physiologic and pathophysiologic implications, but their underlying molecular bases remain incompletely defined. As a common mediator of both the metabolic and anti-apoptotic effects of growth factors, the serine/threonine kinase Akt - also known as protein kinase B or PKB - is capable of regulating and coordinating these inter-related processes. The glucose dependence of the antiapoptotic effects of growth factors and Akt plus a strong correlation between Akt-regulated mitochondrial hexokinase association and apoptotic susceptibility suggest a major role for hexokinases in these effects. Mitochondrial hexokinases catalyse the first obligatory step of glucose metabolism and directly couple extramitochondrial glycolysis to intramitochondrial oxidative phosphorylation, and are thus well suited to play this role. The ability of Akt to regulate energy metabolism appears to have evolutionarily preceded the capacity to control cell survival. This suggests that Akt-dependent metabolic regulatory functions may have given rise to glucose-dependent antiapoptotic effects that evolved as an adaptive sensing system involving hexokinases and serve to ensure mitochondrial homeostasis, thereby coupling metabolism to cell survival. We hypothesize that the enlistment of Akt and hexokinase in the control of mammalian cell apoptosis evolved as a response to the recruitment of mitochondria to the apoptotic cascade. The central importance of mitochondrial hexokinases in cell survival also suggests that they may represent viable therapeutic targets in cancer.

Hunting interactomes of a membrane protein: obtaining the largest set of voltage-dependent anion channel-interacting protein epitopes

The identification of epitopes involved in protein-protein interactions is essential for understanding protein structure and function. Large scale efforts, although identifying the interactions, did not always yield these epitopes, could not confirm most of the known interactions, and seemed particularly unsuccessful for native intrinsic membrane proteins. We have developed a fluidics-based approach (non-steady-state kinetics) to obtain the broadest set of the epitopes interacting with a given target and applied it to a phage display methodology optimized for membrane proteins. Phages expressing a liver cDNA library were screened against a membrane protein (voltage-dependent anion channel) reconstituted into liposomes and captured on a chip surface. The controlled fluidics was obtained by a surface plasmon resonance (SPR) device that combined the advantages of working with minute reaction volumes and non-equilibrium conditions. We demonstrated selective enrichment of binders and could even select for different binding affinities by fractionation of the selected outputs at various elution times. With voltage-dependent anion channel as bait (a mitochondrial channel critical for cellular metabolism and apoptosis) we found at least 40% of its already reported ligands and independently confirmed 55 novel functional interactions, some of which fully blocked the channel. This highly efficient approach is generally applicable for any protein and could be automated and scaled up even without the use of a SPR device. The epitopes directly identified by this method are useful not only for unraveling interactomes but also for drug design and therapeutics.

Functional coupling between nucleoside diphosphate kinase of the outer mitochondrial compartment and oxidative phosphorylation

In rat liver mitochondria all nucleoside diphosphate kinase of the outer compartment is associated with the outer surface of the outer membrane (Lipskaya, T. Yu., and Plakida, K. N. (2003) Biochemistry (Moscow), 68, 1136-1144). In the present study, three systems operating as ADP donors for oxidative phosphorylation have been investigated. The outer membrane bound nucleoside diphosphate kinase was the first system tested. Two others employed yeast hexokinase and yeast nucleoside diphosphate kinase. The two enzymes exhibited the same activity but could not bind to mitochondrial membranes. In all three systems, muscle creatine phosphokinase was the external agent competing with the oxidative phosphorylation system for ADP. Determination of mitochondrial respiration rate in the presence of increasing quantities of creatine phosphokinase revealed that at large excess of creatine phosphokinase activity over other kinase activities (of the three systems tested) and oxidative phosphorylation the creatine phosphokinase reaction reached a quasi-equilibrium state. Under these conditions equilibrium concentrations of all creatine phosphokinase substrates were determined and K(eq)app of this reaction was calculated for the system with yeast hexokinase. In samples containing active mitochondrial nucleoside diphosphate kinase the concentrations of ATP, creatine, and phosphocreatine were determined and the quasi-equilibrium concentration of ADP was calculated using the K(eq)app value. At balance of quasi-equilibrium concentrations of ADP and ATP/ADP ratio the mitochondrial respiration rate in the system containing nucleoside diphosphate kinase was 21% of the respiration rate assayed in the absence of creatine phosphokinase; in the system containing yeast hexokinase this parameter was only 7% of the respiration rate assayed in the absence of creatine phosphokinase. Substitution of mitochondrial nucleoside diphosphate kinase with yeast nucleoside diphosphate kinase abolished this difference. It is concluded that oxidative phosphorylation is accompanied by appearance of functional coupling between mitochondrial nucleoside diphosphate kinase and the oxidative phosphorylation system. Possible mechanisms of this coupling are discussed.