Characterization of a neurotrophin signaling mechanism that mediates neuron survival in a temporally specific pattern.

The temporally specific nature of neurotrophic factor-induced responses is a general feature of mammalian nervous system development, the mechanisms of which remain to be elucidated. We characterized a mechanism underlying the temporal specificity by which BDNF selectively promotes the survival of newly generated, but not mature, granule neurons of the mammalian cerebellum. We found that BDNF specifically induces the extracellular signal-regulated kinase 5 (ERK5)-myocyte enhancer factor (MEF2) signaling pathway in newly generated granule neurons and thereby induces transcription of neurotrophin-3 (NT-3), a novel gene target of MEF2. Inhibition of endogenous ERK5, MEF2, or NT-3 in neurons by several approaches including disruption of the NT-3 gene in mice revealed a requirement for the ERK5-MEF2-NT-3 signaling pathway in BDNF-induced survival of newly generated granule neurons. These findings define a novel mechanism that underlies the antiapoptotic effect of neurotrophins in a temporally defined pattern in the developing mammalian brain.

Cdc2 phosphorylation of BAD links the cell cycle to the cell death machinery.

A mechanism that triggers neuronal apoptosis has been characterized. We report that the cell cycle-regulated protein kinase Cdc2 is expressed in postmitotic granule neurons of the developing rat cerebellum and that Cdc2 mediates apoptosis of cerebellar granule neurons upon the suppression of neuronal activity. Cdc2 catalyzes the phosphorylation of the BH3-only protein BAD at a distinct site, serine 128, and thereby induces BAD-mediated apoptosis in primary neurons by opposing growth factor inhibition of the apoptotic effect of BAD. The phosphorylation of BAD serine 128 inhibits the interaction of growth factor-induced serine 136-phosphorylated BAD with 14-3-3 proteins. Our results suggest that a critical component of the cell cycle couples an apoptotic signal to the cell death machinery via a phosphorylation-dependent mechanism that may generally modulate protein-protein interactions.

JNK phosphorylation and activation of BAD couples the stress-activated signaling pathway to the cell death machinery.

The c-Jun N-terminal kinase (JNK) signaling pathway plays a critical role in mediating apoptosis in the developing and mature organism. The JNK signaling pathway is thought to induce apoptosis via transcription-dependent and transcription-independent mechanisms that remain to be elucidated. In this study, we report a novel mechanism by which the JNK signaling pathway directly activates a component of the cell death machinery. We have found that JNK catalyzes the phosphorylation of the BH3-only protein BAD at the distinct site of serine 128 in vitro. Activation of the JNK signaling pathway induces the BAD serine 128 phosphorylation in vivo, including in primary granule neurons of the developing rat cerebellum. The JNK-induced BAD serine 128 phosphorylation promotes the apoptotic effect of BAD in primary neurons by antagonizing the ability of growth factors to inhibit BAD-mediated apoptosis. These findings indicate that BAD is a novel substrate of JNK that links the stress-activated signaling pathway to the cell death machinery.