Abstract

The c-Jun N-terminal kinase (JNK) signaling pathway plays a critical role in mediating apoptosis in the nervous system; however, the mechanisms by which JNK triggers neuronal apoptosis remain incompletely understood. Recent studies suggest that in addition to inducing transcription of pro-apoptotic genes, JNK also directly activates the cell death machinery. Here, we report that JNK catalyzed the phosphorylation of the BH3-only protein Bcl-2 interacting mediator of cell death (Bim_EL) at serine 65, both in vitro and in vivo. The JNK-induced phosphorylation of Bim_EL at serine 65 promoted the apoptotic effect of Bim_EL in primary cerebellar granule neurons. We also characterized the role of the JNK-Bim_EL signaling pathway in apoptosis that was triggered by overexpression of the p75 neurotrophin receptor (p75^NTR). We found that activation of p75^NTR induced the JNK-dependent phosphorylation of endogenous Bim_EL at serine 65 in cells. The genetic knockdown of Bim_EL by RNA interference or the expression of a dominant interfering form of Bim_EL significantly impaired the ability of activated p75^NTR to induce apoptosis. Together, these results suggest that JNK-induced phosphorylation of Bim_EL at serine 65 mediates p75^NTR-induced apoptosis. Our findings define a novel mechanism by which a death-receptor pathway directly activates the mitochondrial apoptotic machinery.