Calpain is a Ca2+-dependent cysteine protease with multiple substrates, including caspases and the cytoskeletal protein fodrin. Increased calpain activity has been observed during the early hours of reperfusion after global ischemia in selectively vulnerable regions (58-60). A delayed increase in activity can also occur in hippocampus (59,61). In dendrites, calpain is associated with NMDA receptors whose activity can normally modulate cytoskeletal reorganization during synaptic plasticity (62). After ischemia, degradation of fodrin is ameliorated by NMDA receptor antagonists and calpain inhibition (58). Abnormalities in dendritic morphology and synaptic organization during reperfusion (63) might contribute to abnormal Ca2+ signaling and electrophysiologic function, at least temporarily, despite overall recovery of cellular energy metabolism. Calpain may play a role in (i) initially downregulating caspase-3 activity, (ii) later augmenting apoptotic signaling through actions on Bid and several downstream caspases (64), and (iii) cleaving apoptosis-inducing factor (AIF) (65). Other actions of calpain include cleavage of the Na+/Ca2+ exchanger (57), which will retard extrusion of Ca2+ once the Na+ gradient is restored during early reperfusion. Therefore, calpain can control the ischemic injury cascade at multiple sites.
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