Intracellular Bax translocation after transient cerebral ischemia: Implications for a role of the mitochondrial apoptotic signaling pathway in ischemic neuronal death

Activation of terminal caspases such as caspase-3 plays an important role i n the execution of neuronal cell death after transient cerebral ischemia. A lthough the precise mechanism by which terminal caspases are activated in i schemic neurons remains elusive, recent studies have postulated that the mi tochondrial cell death-signaling pathway may participate in this process. T he bcl-2 family member protein Bar is a potent proapoptotic molecule that, on translocation from cytosol to mitochondria, triggers the activation of t erminal caspases by increasing mitochondrial membrane permeability and resu lting in the release of apoptosis-promoting factors, including cytochrome c . In the present study, the role of intracellular Bar translocation in isch emic brain injury was investigated in a rat model of transient focal ischem ia (30 minutes) and reperfusion (1 to 72 hours). Immunochemical studies rev ealed that transient ischemia induced a rapid translocation of Bar from cyt osol to mitochondria in caudate neurons, with a temporal profile and region al distribution coinciding with the mitochondrial release of cytochrome c a nd caspase-9. Further, in postischemic caudate putamen in vivo and in isola ted brain mitochondria in vitro, the authors found enhanced heterodimerizat ion between Bar and the mitochondrial membrane permeabilization-related pro teins adenine nucleotide translocator (ANT) and voltage-dependent anion cha nnel. The ANT inhibitor bongkrekic acid prevented Bar and ANT interactions and inhibited Bax-triggered caspase-9 release from isolated brain mitochond ria in vitro. Bongkrekic acid also offered significant neuroprotection agai nst ischemia-induced caspase-3 and caspase-9 activation and cell death in t he brain. These results strongly suggest that the Bar-mediated mitochondria l apoptotic signaling pathway may play an important role in ischemic neuron al injury.