Delayed hippocampal neurodegeneration after transient global
ischemia is mediated, at least in part, through the activation of terminal
caspases, particularly
caspase-3, and the subsequent proteolytic degradation of critical cellular
proteins.
Caspase-3 may be activated by the membrane receptor-initiated caspase-8-dependent extrinsic pathway and the mitochondria-initiated caspase-9-dependent intrinsic pathway; however, the precise role of these deduced apoptosis-signaling pathways in activating
caspase-3 in ischemic neurons remains elusive. The authors cloned the
caspase-9 gene from the rat brain and investigated its potential role in mediating ischemic neuronal death in a rat model of transient global
ischemia.
Caspase-9 gene expression and
protease activity were extremely low in the adult brain, whereas they were developmentally upregulated in newborn rats, especially at postnatal 12 weeks, a finding consistent with the theory of an essential role for
caspase-9 in neuronal apoptosis during brain development. After 15-minute transient global
ischemia,
caspase-9 was overexpressed and proteolytically activated in the hippocampal CA1 neurons at 8 to 72 hours of reperfusion. The temporal profile of
caspase-9 activation coincided with that of
cytochrome c release and
caspase-3 activation, but preceded CA1 neuronal death. Immunoprecipitation experiments revealed that there was enhanced formation of Apaf-1/
caspase-9 complex in the hippocampus 8 and 24 hours after
ischemia. Furthermore, intracerebral ventricular infusion of the relatively specific
caspase-9 inhibitor N-benzyloxycarbonyl-Leu-Glu-His-Asp-fluoro-methylketone before
ischemia attenuated caspase-3-like activity and significantly enhanced neuronal survival in the CA1 sector. In contrast, inhibition of
caspase-8 activity had no significant effect on
caspase-3 activation or neuronal survival. These results suggest that the caspase-9-dependent intrinsic pathway may be the primary mechanism responsible for the activation of
caspase-3 in ischemic hippocampal neurons.