Decrease of PKC precedes other cellular signs of calpain activation in area CA1 of the hippocampus after transient cerebral ischemia.

One of the specific features of severe brain injury is an activation of calcium-dependent proteolysis by calpains. We have observed a significant increase of activity as early as 3 h after the insult in a well defined model of delayed ischemic neuronal death in gerbil hippocampus. At 24 h, the enzymatic activity transiently normalized, then increased again, following the place and time of selective cellular death in the CA1 region of hippocampus. The enhanced postischemic proteolysis resulted in concomitant cleavage of calpain-specific endogenous substrates like protein kinase C (PKC), fodrin and microtubule-associated protein-2 (MAP2). These effects were also time-dependent and restricted to the vulnerable, CA1 pyramidal neurons-containing the dorsal part (DP) of the hippocampus. We have also characterized the postischemic changes of six different isoforms of PKC. The vulnerable dorsal part of the hippocampus, but not its relative resistant abdominal part (AbP), exhibited a loss of PKCalpha, beta, gamma, and delta isoforms as early as 3 h after ischemic insult. However, at this time, solely in the soluble fraction of homogenate. Later (72 h), a further loss of the enzyme proteins, comprised the particulate fraction as well and resulted in an about 50% decrease of total PKCs in the vulnerable DP region. In the case of PKCalpha, the immunostaining pattern showed, in addition to the disappearance of the enzyme from the injured area, an extensive translocation into nuclei of the survived, ischemia-resistant neurones. The early decreases of PKC isoforms in the cytosol paralleled the transient calpain activation at 3h postischemia but substantially preceded the proteolysis of any other classical calpain substrates, such as fodrin and MAP2, being evidenced not earlier than 48-72 h after the insult and restricted also to the vulnerable dorsal part. In conclusion, our results of the time-dependent effects of transient global cerebral ischemia on the calpain activity, levels and localization of its several substrates suggest, that calpain-mediated proteolysis is specifically involved in the early (induction) as well as in the late (execution) phases of delayed ischemic neuronal death in the CA1 hippocampus.