Outer mitochondrial membrane permeabilization during apoptosis triggers caspase-independent mitochondrial and caspase-dependent plasma membrane potential depolarization: a single-cell analysis

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Abstract
Little is known about the temporal relationship between mitochondrial and plasma membrane potential changes and outer mitochondrial membrane permeabilization during apoptosis. Confocal imaging of breast carcinoma and HeLa cells stably transfected with cytochrome-C-GFP demonstrated that mitochondria rapidly depolarized after the release of the fusion protein into the cytosol. Of note, mitochondria did not completely depolarize but established a new steady-state level that could be further dissipated by treatment with the protonophore carbonyl cyanide p-trifluoromethoxy-phenylhydrazone. Treatment with the FOF1-ATP-synthase inhibitor oligomycin likewise induced a collapse of this steady-state level, suggesting that FOF1-ATP-synthase reversal maintained mitochondrial potential after outer mitochondrial membrane permeabilization. Treatment with a broad spectrum caspase inhibitor failed to inhibit the partial depolarization of mitochondria during apoptosis, yet potently abolished the activation of effector caspases detected by fluorescence resonance energy transfer analysis in the same experiment. Interestingly, the onset of mitochondrial depolarization was always coupled with a depolarization of the plasma membrane potential. This was associated with the degradation of the regulatory Na+/K+-ATPase β-subunit, and both events were blocked by caspase inhibition. Our results demonstrate that outer mitochondrial membrane permeabilization coordinates the depolarization of both membrane potentials during apoptosis.