Induction and modulation of cerebellar granule neuron death by E2F-1

Growing evidence suggests that certain cell cycle regulators also mediate neuronal death. Of relevance, cyclin D1-associated kinase activity is increased and the retinoblastoma protein (Rb), a substrate of the cyclin D1-Cdk4/6 complex, is phosphorylated during K+ deprivation-evoked death of cerebellar granule neurons (CGNs), Cyclin-dependent kinase (CDK) inhibitors block this death, suggesting a requirement for the cyclin D1/Cdk4/6-Rb pathway. However, the downstream target(s) of this pathway are not well defined. The transcription factor E2F-1 is regulated by Rb and is reported to evoke death in proliferating cells when overexpressed. Accordingly, we examined whether E2F-1 was sufficient to evoke death of CGNs and whether it was required for death evoked by low K+. We show that adenovirus-mediated expression of E2F-1 in CGNs results in apoptotic death, which is independent of p53, dependent upon Bax, and associated with caspase 3-like activity. In addition, we demonstrate that levels of E2F-1 mRNA and protein increase during K+ deprivation-evoked death. The increase in E2F-1 protein is blocked by the CDK inhibitor flavopiridol, Finally, E2F-1-deficient neurons are modestly resistant to death induced by low K+. These results indicate that E2F-1 expression is sufficient to promote neuronal apoptosis and that endogenous E2F-1 modulates the death of CGNs evoked by low K+.