Cascade of caspase activation in potassium-deprived cerebellar granule neurons: Targets for treatment with peptide and protein inhibitors of apoptosis

Cerebellar granule neurons (CGN) cultured in the presence of serum and depo larizing potassium concentrations undergo apoptosis when switched to serum- free medium containing physiological potassium concentrations. Here we show that processing of the key protease, caspase-3, depends on the activation of caspase-9, but not of caspase-8. Selective peptide inhibitors of caspase -9 block processing of caspase-3 and caspase-8 and inhibit apoptosis, where as a selective inhibitor of caspase-8 blocks neither processing of caspase- 3 nor cell death. The data obtained with peptide inhibitors were confirmed by adenovirally mediated ectopic expression of the cytokine response modifi er A (crmA), the baculovirus protein p35, and the X chromosome-linked inhib itor of apoptosis (XIAP). Further, caspase-8-activating death receptors do not mediate apoptosis in CGN and potassium withdrawal-induced apoptosis evo lves unaltered in gld or lpr mice, which harbor mutations in the CD95/CD95 ligand system. Thus, neuronal apoptosis triggered by potassium deprivation is death receptor-independent but involves the mitochondrial pathway of cas pase activation.