Survival- and Death-Promoting Events after Transient Cerebral Ischemia: Phosphorylation of Akt, Release of Cytochrome C, and Activation of Caspase-Like Proteases

Abstract

Release of cytochrome c (cyt c) into cytoplasm initiates caspase-mediated apoptosis, 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 swollen and structurally altered, whereas the cell membrane and nuclei were intact in hippocampal CA1 neurons after 36 hours of reperfusion. In some neurons, the pattern of immunostaining for cyt c changed from a punctuate pattern, likely representing mitochondria, to a more diffuse cytoplasmic localization at 36 and 48 hours of reperfusion as examined by laser-scanning confocal microscopic study. Western blot analysis showed that cyt c was increased in the cytosolic fraction in the hippocampus after 36 and 48 hours of reperfusion. Consistently, caspase-3-like activity was increased in these hippocampal samples. As demonstrated by Western blot using phosphospecific Akt antibody, phosphorylation of Akt at serine-473 in the hippocampal region was highly increased during the first 24 hours but not at 48 hours of reperfusion. The authors conclude that transient cerebral ischemia activates both cell death and cell survival pathways after ischemia. The activation of Akt during the first 24 hours conceivably may be one of the factors responsible for the delay in neuronal death after global ischemia.

Hypoxic neuronal necrosis: Protein synthesis-independent activation of a cell death program

“…Because all mitochondrial toxins used in this study cause energy failure, which in turn induces calcium influx into mitochondria, this is one possible mechanism of mitochondrial dysfunction, which would explain why activation of DNA repair enzymes can paradoxically enhance cell death (31), presumably through energy depletion, and would imply that other types of mitochondrial malfunction may well trigger similar cascades. This hypothetical mechanism would also explain the relationship between energy failure and excitotoxicity, both of which might trigger calcium accumulation and cytochrome c release, and the reports of a necrotic morphology (7,8) and an apoptotic-like biochemistry (32)(33)(34)(35) in pathological situations characterized by depletion of energy reserves, such as cerebral ischemia.…”

Apoptotic Cell Death Following Traumatic Injury to the Central Nervous System

“…Recent studies have also provided additional evidence of caspase-3 activation in other models of cerebral ischemia following strokes in adults and neonates. This includes the mitochondrial release of cytochrome c and activation of caspase-9, a proximal caspase in the caspase-3 apoptotic cascade (Gillardon et al, 1997;Cheng et al, 1998;Namura et al, 1998;Pulera et al, 1998;Kiprianova et al, 1999;Krajewski et al, 1999;Ouyang et al, 1999;Velier et al, 1999;. Several recent studies using caspase inhibitors provide support of these original observations, and demonstrate that injury severity can affect the efficacy of caspase inhibitors in reducing cell death.…”

rAAV8-733-Mediated Gene Transfer of CHIP/Stub-1 Prevents Hippocampal Neuronal Death in Experimental Brain Ischemia

“…29 The PI3K-AKT pathway has been attributed a major role in the protection of neurons from caspase-mediated cell death after brain ischemia. 30 Phosphorylation of AKT at serine 473 was increased after several minutes of reperfusion and was sustained for days following ischemia. 8 In turn, it was shown that CHIP promotes downregulation and degradation of phosphatase and tensin homolog (PTEN), a negative modulator of AKT phosphorylation, which increases the levels of p-AKT.…”