Each mammalian mitochondrial outer membrane porin protein is dispensable: effects on cellular respiration.
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Voltage-dependent anion channels (VDACs, also known as mitochondrial porins) are small pore-forming proteins of the mitochondrial outer membrane found in all eukaryotes. Mammals harbor three distinct VDAC isoforms, with each protein sharing 65-70% sequence identity. Deletion of the yeast VDAC1 gene leads to conditional lethality that can be partially or completely complemented by the mammalian VDAC genes. In vitro, VDACs conduct a variety of small metabolites and in vivo they serve as a binding site for several cytosolic kinases involved in intermediary metabolism, yet the specific physiologic role of each isoform is unknown. Here we show that mouse embryonic stem cells lacking each isoform are viable but exhibit a 30% reduction in oxygen consumption. VDAC1 and VDAC2 deficient cells exhibit reduced cytochrome c oxidase activity, whereas VDAC3 deficient cells have normal activity. These results indicate that VDACs are not essential for cell viability and we speculate that reduced respiration in part reflects decreased outer membrane permeability for small metabolites necessary for oxidative phosphorylation. (+info)
Evidence for secretory pathway localization of a voltage-dependent anion channel isoform.
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Voltage-dependent anion channels (VDACs) are pore-forming proteins (porins) that form the major pathway for movement of adenine nucleotides through the outer mitochondrial membrane. Electrophysiological studies indicate that VDAC-like channel activity is also prevalent in the cell membranes of many mammalian cells. However, the multitopological localization of porins outside the mitochondrion has remained an extremely controversial issue. Herein, we show that usage of two alternative first exons of the murine VDAC-1 gene leads to expression of two porins differing within their N termini. One porin (plasmalemmal VDAC-1) harboring a hydrophobic leader peptide is primarily targeted through the Golgi apparatus to the cell membrane. In contrast, the second isoform lacking the N-terminal leader (mitochondrial VDAC-1) is translocated more efficiently into the outer mitochondrial membrane. Thus, our data provide unique genetic evidence in favor of a multitopological localization of a mitochondrial porin. (+info)
Bax and Bcl-xL independently regulate apoptotic changes of yeast mitochondria that require VDAC but not adenine nucleotide translocator.
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Mitochondria play an essential role in apoptosis by releasing apoptogenic molecules such as cytochrome c and AIF, and some caspases, which are all regulated by Bcl-2 family proteins. Pro-apoptotic Bax and Bak have been shown to induce cytochrome c release and loss of membrane potential (Deltapsi) leading to AIF release in the isolated mitochondria. We have previously shown that Bax and Bak open the voltage-dependent anion channel (VDAC) allowing cytochrome c to pass through the channel, and Bcl-xL closes the channel. However, it has been reported that it is adenine nucleotide translocator (ANT) with which Bax/Bcl-xL interacts that modulate the channel activity. Here, we investigated the role of ANT and VDAC in the changes of isolated mitochondria triggered by Bax and by chemicals that induce permeability transition (PT). In rat and yeast mitochondria, Bax did not affect the ADP/ATP exchange activity of ANT. VDAC-deficient but not ANT-deficient yeast mitochondria showed resistance to cytochrome c release, Deltapsi loss, and swelling caused by Bax and PT inducers. Bcl-xL showed similar inhibition of all these changes in ANT-deficient and wild type yeast mitochondria. Furthermore, Bax induces cytochrome c release in wild type yeast cells but not VDAC1-deficient yeast cells. These data indicate that VDAC, but not ANT, is essential for apoptotic mitochondrial changes. The data also indicate that Bcl-xL and Bax possess an ability to regulate mitochondrial membrane permeability independently of other Bcl-2 family members. (+info)
Characterization of porin isoforms expressed in tumor cells.
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Mitochondria from malignant tumor cell lines show a higher capability for hexokinase binding than those from normal liver. To explore possible differences in hexokinase binding sites of mitochondria between tumor cells and normal liver, we characterized porin isoforms expressed in tumor cells. Cloning experiments on the three porin isoforms, VDAC1, VDAC2 and VDAC3 from malignant tumor cell line AH130 clearly showed that their primary structures were completely identical to those of the corresponding VDACs of normal liver cells. Possible expression of the fourth porin isoform in AH130 cells was excluded by degenerate primer-based RT-PCR. However, the transcript levels of the three VDAC isoforms in AH130 cells were significantly higher than those in normal liver. These results suggest that the high hexokinase-binding capability of malignant tumor cell mitochondria was not due to any structural difference, but due to a quantitative difference in binding sites. (+info)
Altered mitochondrial sensitivity for ADP and maintenance of creatine-stimulated respiration in oxidative striated muscles from VDAC1-deficient mice.
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Voltage-dependent anion channels (VDACs) form the main pathway for metabolites across the mitochondrial outer membrane. The mouse vdac1 gene has been disrupted by gene targeting, and the resulting mutant mice have been examined for defects in muscle physiology. To test the hypothesis that VDAC1 constitutes a pathway for ADP translocation into mitochondria, the apparent mitochondrial sensitivity for ADP (Km(ADP)) and the calculated rate of respiration in the presence of the maximal ADP concentration (Vmax) have been assessed using skinned fibers prepared from two oxidative muscles (ventricle and soleus) and a glycolytic muscle (gastrocnemius) in control and vdac1(-/-) mice. We observed a significant increase in the apparent Km((ADP)) in heart and gastrocnemius, whereas the V(max) remained unchanged in both muscles. In contrast, a significant decrease in both the apparent Km((ADP)) and V(max) was observed in soleus. To test whether VDAC1 is required for creatine stimulation of mitochondrial respiration in oxidative muscles, the apparent Km((ADP)) and Vmax were determined in the presence of 25 mm creatine. The creatine effect on mitochondrial respiration was unchanged in both heart and soleus. These data, together with the significant increase in citrate synthase activity in heart, but not in soleus and gastrocnemius, suggest that distinct metabolic responses to altered mitochondrial outer membrane permeability occur in these different striated muscle types. (+info)
VDAC2 (porin-2) expression pattern and localization in the bovine testis.
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In this study, sequencing of voltage-dependent anion channel 2 (VDAC2, porin-2) cDNA from bovine testis is reported. High identity to the murine, rabbit, and human subtypes at both the nucleotide and amino acid levels is demonstrated. mRNA analysis revealed expression of VDAC2 in bovine testis, whereas high levels of VDAC2 proteins were found in late spermatocytes, spermatids, and spermatozoa. In contrast, VDAC1 (porin-1) is exclusively localized in Sertoli cells. The possible role of testicular VDAC2 in providing energy metabolites and in germ cell apoptosis is discussed. (+info)
Immotile sperm and infertility in mice lacking mitochondrial voltage-dependent anion channel type 3.
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Voltage-dependent anion channels (VDACs), also known as mitochondrial porins, are small channel proteins involved in the translocation of metabolites across the mitochondrial outer membrane. A single channel-forming protein is found in yeast, whereas higher eukaryotes express multiple VDACs, with humans and mice each harboring three distinct channels (VDAC1-3) encoded by separate genes. To begin to assess the functions of each of the three isoforms, the VDAC3 gene was inactivated by targeted disruption in embryonic stem cells. Here we show that mice lacking VDAC3 are healthy, but males are infertile. Although there are normal sperm numbers, the sperm exhibit markedly reduced motility. Structural defects were found in two-thirds of epididymal axonemes, with the most common abnormality being loss of a single microtubule doublet at a conserved position within the axoneme. In testicular sperm, the defect was only rarely observed, suggesting that instability of a normally formed axoneme occurs with sperm maturation. In contrast, tracheal epithelial cilia showed no structural abnormalities. In addition, skeletal muscle mitochondria were abnormally shaped, and activities of the respiratory chain complexes were reduced. These results demonstrate that axonemal defects may be caused by associated nonaxonemal components such as mitochondrial channels and illustrate that normal mitochondrial function is required for stability of the axoneme. (+info)
Retinal voltage-dependent anion channel: characterization and cellular localization.
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PURPOSE: To characterize and localize retinal voltage-dependent anion channel (VDAC) and to understand its possible contribution to mitochondrial function and dysfunction. METHODS: VDAC was characterized by a method involving purification from isolated mitochondria and reconstitution into a planar lipid bilayer (PLB). The permeability transition pore (PTP) was monitored by Ca(2+) accumulation in isolated mitochondria and swelling of mitochondria. Localization was studied by immunocytochemistry and in situ hybridization. RESULTS: Retinal VDACs exhibited the electrophysiological fingerprint of the VDAC superfamily. It had a maximal chord conductance of 3.7 +/- 0.1 nanosiemens (nS) in 1 M NaCl, and a voltage-dependent conductance that was highest at transmembrane potential close to zero. It was modulated by glutamate, which decreased the channel's open probability, and by La(3+) and ruthenium amine binuclear complex (Ru360), which closed the channel. Energized and freshly prepared retinal mitochondria accumulated Ca(2+) that is inhibited by La(3+) ruthenium red and Ru360. Subsequent to Ca(2+) accumulation, mitochondria released the accumulated Ca(2+), probably through activation of the PTP. Ru360 inhibited Ca(2+) release and mitochondrial swelling. VDAC was present in mitochondria of all retinal cell types: photoreceptor, bipolar, horizontal, amacrine, and ganglion cells. Most cells primarily expressed VDAC-1, but they also expressed VDAC-2 and -3. CONCLUSIONS: These results suggest that VDAC is involved in PTP activity and/or regulation and thus is an important player in retinal degeneration associated with PTP-mediated mitochondrial dysfunction. (+info)