Apaf-l-deficient fog mouse cell apoptosis involves hypopolarization of the mitochondrial inner membrane, ATP depletion and citrate accumulation

To explore how the intrinsic apoptosis pathway is controlled in the spontaneous fog （forebrain overgrowth） mutant mice with an Apafl splicing deficiency, we examined spleen and bone marrow cells from Apafl＋/＋ （＋/＋） and Apafl^fog/fog （fog/fog） mice for initiator caspase-9 activation by cellular stresses. When the mitochondrial inner membrane potential （△ψm） was disrupted by staurosporine, ＋/＋ cells but not fog/fog cells activated caspase-9 to cause apoptosis, indicating the lack of apoptosome （apoptosis protease activating factor 1 （Apaf-l）/cytochrome c/（d）ATP/procaspase-9） function in fog/fog cells. However, when a marginal （-20%） decrease in △ψm was caused by hydrogen peroxide （0.1 mM）, peroxynitrite donor 3-morpholinosydnonimine （0.1 mM） and UV-C irradiation （20 J/m^2）, both ＋/＋ and fog/fog cells triggered procaspase-9 auto-processing and its downstream cascade activation. Supporting our previous results, procaspase-9 pre-existing in the mitochondria induced its auto-processing before the cytosolic caspase activation regardless of the genotypes. Cellular ATP concentration significantly decreased under the hypoactive △ψm condition. Furthermore, we detected accumulation of citrate, a kosmotrope known to facilitate procaspase-9 dimerization, probably due to a feedback control of the Krebs cycle by the electron transfer system. Thus, mitochondrial in situ caspase-9 activation may be caused by the major metabolic reactions in response to physiological stresses, which may represent a mode of Apaf-l-independent apoptosis hypothesized from recent genetic studies.