Biomedical subjects

Azad Bonni

Publications and source records attributed to Azad Bonni.

25 records · Page 2Linked to original sources

PubMed2003

Apoptosis induced by p75NTR overexpression requires Jun kinase-dependent phosphorylation of Bad.

The p75 neurotrophin receptor (p75NTR), a member of the tumor necrosis factor receptor superfamily, facilitates apoptosis during development and after injury to the CNS. The signaling cascades activated by p75NTR that result in apoptosis remain poorly understood. In this study, we show that overexpression of p75NTR in primary cortical neurons, in pheochromocytoma cell line (PC12) cells, and in glioma cells results in activation of Jun kinase (JNK), accumulation of cytochrome c within the cytosol, and activation of caspases 9, 6, and 3. To link p75NTR-dependent JNK activation to mitochondrial cytochrome c release, regulation of BH3-domain-only family members was examined. Transcription of BH3-domain-only family members was not induced by p75NTR, but p75NTR-dependent JNK activation resulted in phosphorylation and oligomerization of the BH3-domain-only family member Bad. Loss of function experiments using Bad dominant negatives or RNA interference demonstrated a requirement for Bad in p75NTR-induced apoptosis. Together, these studies provide the first data linking apoptosis induced by p75NTR to the phosphorylation of BH3-domain-only family members.

Animals↗

PubMed2003

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.

Age Factors↗

PubMed2003

The E2F-Cdc2 cell-cycle pathway specifically mediates activity deprivation-induced apoptosis of postmitotic neurons.

Neuronal apoptosis plays a critical role in the normal development of the mammalian brain and is thought to contribute to the pathogenesis of several neurologic disorders. However, the intracellular mechanisms underlying apoptosis of neurons remain incompletely understood. In the present study, we characterized a cell-cycle-based mechanism by which neuronal activity deprivation induces apoptosis of postmitotic neurons. Activity deprivation, but not growth factor withdrawal, was found to induce Cdc2 expression and consequent Cdc2-mediated apoptosis in granule neurons of the developing rat cerebellum. We found that activity deprivation induces cdc2 transcription in neurons via an E2F-binding element (EBE) within the cdc2 promoter. The transcription factor E2F1 that is expressed in granule neurons was found in DNA binding assays to bind to the EBE of the cdc2 gene. In chromatin immunoprecipitation analysis, endogenous E2F1 forms a complex with the promoter of the endogenous cdc2 gene in granule neurons, indicating that endogenous E2F1 is poised to activate transcription of the endogenous cdc2 gene in neurons. Consistent with this conclusion, a dominant interfering form of E2F, when expressed in granule neurons, blocked activity deprivation-induced cdc2 transcription. In other experiments, we found that the expression of E2F1 in granule neurons induces Cdc2 expression and promotes neuronal apoptosis via the activation of Cdc2. Remarkably, in contrast to inducing the E2F-mediated expression and activation of Cdc2 in granule neurons, activity deprivation fails to stimulate the expression of E2F-target genes that trigger DNA synthesis and replication. Together, our findings define a novel apoptotic mechanism whereby E2F selectively couples an activity deprivation-induced signal to cdc2 transcription in the absence of stimulating DNA synthesis and thus culminating in Cdc2-mediated apoptosis of postmitotic neurons.

Animals↗

PubMed2003

Neurodegeneration: A non-apoptotic role for AIF in the brain.

A recent report provides new insights into the mechanisms of neurodegeneration. In a mouse mutant with progressive cerebellar and retinal degeneration, the expression of the apoptotic protein AIF was found to be downregulated leading paradoxically to apoptosis of neurons that is associated with an imbalance in free radical metabolism and cell cycle re-entry.

Animals↗

PubMed2002

RNA interference reveals a requirement for myocyte enhancer factor 2A in activity-dependent neuronal survival.

RNA interference (RNAi) provides a powerful method of gene silencing in eukaryotic cells, including proliferating mammalian cells. However, the utility of RNAi as a method of gene knock-down in primary postmitotic mammalian neurons remained unknown. Here, we asked if RNAi might be utilized to allow the assessment of the biological function of a specific gene in the nervous system. We employed a U6 promoter-driven DNA template approach to induce hairpin RNA-triggered RNAi to characterize the role of the transcription factor myocyte enhancer factor 2A (MEF2A) in the neuronal activity-dependent survival of granule neurons of the developing rat cerebellum. We found that the expression of MEF2A hairpin RNAs leads to the efficient and specific inhibition of endogenous MEF2A protein expression in primary cerebellar granule neurons. We also found that RNAi of MEF2A reduces significantly MEF2 response element-mediated transcription in granule neurons and inhibits activity-dependent granule neuron survival. Taken together, our RNAi experiments have revealed that MEF2A plays a critical role in activity-dependent neuronal survival. In addition, our findings indicate that RNAi does operate in postmitotic mammalian neurons and thus offers a rapid genetic method of studying gene function in the development and function of the mammalian nervous system.

Animals↗

PubMed2002

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.

3T3 Cells↗

PubMed2002

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.

14-3-3 Proteins↗