Caspase-3 expression by cerebellar granule neurons is regulated by calciumand cyclic AMP

Caspase-3 enzyme activity is induced, and cell death follows, when cerebell ar granule neurons (CGNs) from 8-day-old rats are transferred from an extra cellular concentration of 25 mM K+ (25 mM [K+](e)) to 5 mM [K+](e). Death o f these neurons is diminished by an inhibitor of caspase-3 but not by an in hibitor of caspase-1. Actinomycin D and cycloheximide inhibit induction of caspase-3 and prevent death. Experiments in which CGN intracellular Ca2+ co ncentration ([Ca2+](i)) was manipulated by either changing [K+](e) or addin g a voltage-gated Ca2+ channel antagonist or a Ca2+ ionophore to the medium showed that caspase-3 mRNA rises 2.5-fold when [Ca2+](i) is diminished fro m 300 to 150 nM, with a corresponding rise in peak caspase enzyme activity. Whereas the caspase-3 mRNA level does not rise further with a still greate r diminution in [Ca2+](i), peak caspase enzyme activity continues to increa se, reaching sevenfold induction when [Ca2+](i) is reduced to 55 nM. In CGN s in which [Ca2+](i) is set at 55 nM by incubation in 5 mM [K+](e), treatme nt with forskolin or dibutyryl 3',5'-cyclic adenosine-5'-monophosphate dela ys caspase-3 induction and diminishes death but does not alter [Ca2+](i). W e conclude that, in immature CGNs, both caspase-3 transcription and the sub sequent processing of caspase-3 are induced by a fall in [Ca2+](i). Elevati ng cyclic AMP content delays caspase-3 induction by a mechanism that does n ot require an increase in [Ca2+](i).