Neuronal cell death can be divided into three major morphological subtypes: apoptotic, autophagic, and necrotic. In contrast to necrotic death, both apoptotic and autophagic neuronal death involve highly regulated, energy-dependent molecular processes. Apoptosis is characterized by cellular shrinkage, chromatin condensation and nuclear fragmentation. The predominant morphological feature of autophagic cell death is the presence of numerous double-membrane vesicular structures (autophagosomes) that envelope cytoplasmic proteins and organelles. In studies supported by this grant, we have identified an apoptotic pathway wherein an increased ratio of Bax:Bcl-XL leads to formation of a multimolecular complex between cytochrome c, APAF-1, and caspase-9 resulting in downstream activation of caspase-3 and apoptosis. The molecular pathways regulating autophagic death are less well-defined. We hypothesize that the intracellular balance of pro- and anti-apoptotic Bc1-2 family members regulates both caspase-dependent and -independent neuronal death pathways. We further propose that stimulus-specific changes in the mitochondrial permeability transition pore, coupled with the availability of effector caspases, determine whether apoptotic and/or autophagic death pathways are engaged. To test these hypotheses, we will perform a series of focused studies on mice carrying targeted gene disruptions of bax, bcl-x, apaf-l, caspase-9, and caspase-3. In the nematode C. elegans, the homologues of these molecules form a linear apoptotic pathway; however, definitive evidence for such a pathway in the mammalian nervous system is lacking and can only be obtained by analyzing gene-disrupted mice. We will use these mice to investigate the significance of Bc1-2 and caspase family members in both in vivo and in vitro models of apoptotic and autophagic cell death. These experiments will yield significant new insights into the molecular pathways regulating neuronal cell death during development and neuropathological conditions.
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