Mitochondrial outer membrane permeability to metabolites influences the onset of apoptosis
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Apoptosis is a process in multicellular organisms to signal and induce death of specific cells, while avoiding inflammatory reactions. It is an important way to recycle the materials of unwanted cells and maintain cell balance. The execution phase of apoptosis can be initiated by proteins released from mitochondria (such as cytochrome c). Results reported here are consistent with this release being influenced by changes in the mitochondrial outer membrane permeability to metabolites. Phosphorothioate oligonucleotides induce cell death and block VDAC, a protein in the mitochondrial outer membrane that facilitates metabolite flow. These properties seem to be linked in that both require the phosphorothioate modification, both are enhanced by an increase in oligonucleotide length, and both are insensitive to nucleotide sequence. VDAC reconstituted into planar phospholipid membranes is blocked by phosphorothioate oligonucleotides with a 1:1 stoichiometry. They block the pore of the channel through interacting with the inner wall of the pore. The rate of binding occurs at a 100 μs scale but the binding is usually unstable. However, some conformational change stabilizes the complex resulting in long-term complete blockage of VDAC. In mitochondria, this blockage interferes with metabolite flow and inhibits the respiration of mitochondria. It is very specific for VDAC at sub-micromolar concentrations of phosphorothioate oligonucleotide and under these conditions there is minimal effect on enzymatic processes in the mitochondrial inner membrane. The ability of PorB from Neisseria meningitidis to inhibit apoptosis by moving to the mitochondrial outer membrane, was investigated in light of VDAC's role in apoptosis. PorB is unable to alter VDAC's gating properties but does allow ATP to cross membranes. Thus it may restore metabolite flux when VDAC channels close early in apoptosis. Attempts to test this in yeast were not successful. VDAC gating influences transmembrane Ca2+ flux. The closed states favor calcium permeation and the open state limits calcium flux. In mitochondria this gating could influence the rate of Ca2+-dependent mitochondrial swelling and subsequent cytochrome c release. Thus, the mitochondrial outer membrane permeability regulated by VDAC gating may play an important role in mitochondrial function and control of apoptosis.