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

Caspase-3 enzyme activity is induced, and cell death follows, when cerebellar granule neurons (CGNs) from 8-day-old rats are transferred from an extracellular concentration of 25 mM K⁺ (25 mM [K⁺](e)) to 5 mM [K⁺](e). Death of these neurons is diminished by an inhibitor of caspase-3 but not by an inhibitor of caspase-1. Actinomycin D and cycloheximide inhibit induction of caspase-3 and prevent death. Experiments in which CGN intracellular Ca²⁺ concentration ([Ca²⁺](i)) was manipulated by either changing [K⁺](e) or adding 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 from 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 greater diminution in [Ca²⁺](i), peak caspase enzyme activity continues to increase, reaching sevenfold induction when [Ca²⁺](i) is reduced to 55 nM. In CGNs in which [Ca²⁺](i) is set at 55 nM by incubation in 5 mM [K⁺](e), treatment with forskolin or dibutyryl 3',5'-cyclic adenosine-5'-monophosphate delays caspase-3 induction and diminishes death but does not alter [Ca²⁺](i). We conclude that, in immature CGNs, both caspase-3 transcription and the subsequent processing of caspase-3 are induced by a fall in [Ca²⁺](i). Elevating cyclic AMP content delays caspase-3 induction by a mechanism that does not require an increase in [Ca²⁺](i).