Regulation of neuronal apoptosis

During development, neonatal sympathetic (SCG) neurons require target-derived nerve growth factor (NGF) for survival; in its absence, the neurons undergo programmed cell death resulting in apoptosis. This death, which is blocked by KCl and cAMP, requires de novo protein synthesis and expression of the proapoptotic BCL-2 protein BAX. Since BAX levels remain unchanged during trophic factor deprivation (TFD), BAX must be regulated posttranslationally in neonatal SCG neurons.; We found that NGF deprivation in sympathetic neurons induces two parallel processes: (1) a protein synthesis-dependent, caspase-independent translocation of BAX from the cytosol to mitochondria, followed by mitochondrial membrane integration and loss of cytochrome c (cyt c); and, (2) the development of competence-to-die, which requires neither macromolecular synthesis nor BAX expression. Activation of both pathways is required for caspase activation and apoptosis in immature sympathetic neurons. BCL-2 acts at mitochondria to prevent BAX-mediated cyt c release; whereas NGF, KCl, or cAMP aborts the apoptotic program at multiple checkpoints.; The gene products responsible for BAX-dependent cyt c release remain unknown. We found that BIM, a BH3-only proapoptotic BCL-2 protein, is one such molecule. TFD induces expression of BIM_EL, an integral mitochondrial membrane protein that functions upstream of the BAX/BCL-2 and caspase checkpoints. Bim deletion confers protection against developmental and induced neuronal apoptosis in both central and peripheral populations, but only transiently, suggesting that BIM—and perhaps other BH3-only proteins—serve partially redundant functions upstream of BAX-mediated cyt c release. However, expression alone does not guarantee functional redundancy among BCL-2 proteins.; The dependence on trophic support decreases dramatically with maturation in several neuronal populations, including SCG neurons. Analyses of the biochemical and molecular events induced by trophic deprivation in immature and mature sympathetic neurons indicate that maturation aborts the cell death pathway at a point that is mechanistically indistinguishable from Bax deletion. NGF withdrawal in mature sympathetic neurons does not induce the translocation of either BAX or cyt c. Moreover, mature neurons do not develop competence-to-die with cytoplasmic accumulation of cyt c. Therefore, inhibition of both BAX-dependent cyt c release and the development of competence-to-die contributes to the loss of trophic factor dependence associated with neuronal maturation.