Survival- and death-promoting events after transient cerebral ischemia: Phosphorylation of Akt, release of cytochrome c, and activation of caspase-like proteases

Release of cytochrome c (cyt c) into cytoplasm initiates caspase-mediated a poptosis, whereas activation of Akt kinase by phosphorylation at serine-473 prevents apoptosis in several cell systems. To investigate cell death and cell survival pathways, the authors studied release of cyt c, activation of caspase, and changes in Akt phosphorylation in rat brains subjected to 15 minutes of ischemia followed by varying periods of reperfusion, The authors found by electron microscopic study that a portion of mitochondria was swo llen and structurally altered, whereas the cell membrane and nuclei were in tact in hippocampal CA1 neurons after 36 hours of reperfusion. In some neur ons, the pattern of immunostaining for cyt c changed from a punctuate patte rn, likely representing mitochondria, to a more diffuse cytoplasmic localiz ation at 36 and 48 hours of reperfusion as examined by laser-scanning confo cal microscopic study. Western blot analysis showed that cyt c was increase d in the cytosolic fraction in the hippocampus after 36 and 48 hours of rep erfusion. Consistently, caspase-3-like activity was increased in these hipp ocampal samples. As demonstrated by Western blot using phosphospecific Nit antibody, phosphorylation of Akt at serine-473 in the hippocampal region wa s highly increased during the first 24 hours but not at 48 hours of reperfu sion. The authors conclude that transient cerebral ischemia activates both cell death and cell survival pathways after ischemia. The activation of Nit during the first 24 hours conceivably may be one of the factors responsibl e for the delay in neuronal death after global ischemia.