The role of BCL-X(L) in regulating mitochondrial function and neuronal cell death

Programmed cell death is a fundamental process that is essential for normal development and tissue homeostasis. As the central stage of regulatory mechanisms and a crucial commitment point to death, mitochondrial involvement in apoptosis controls the release of apoptogenic factors that activate downstream death machinery. Mounting evidence also indicates an integrated relationship between the physiological function of mitochondria and their role in cell death regulation. BCL-2 family proteins are core regulators of apoptosis. Their role in regulating the mitochondrial cell death pathway is well established, yet the exact biochemical mechanisms are still unknown.; My thesis work primarily focused on analyzing the action of BCL-X L, a member of the BCL-2 family proteins, on mitochondrial functions in neurons. The crucial role of BCL-XL in cell death regulation of the central nervous system has been indicated by the massive immature neuronal cell death in the BCL-XL null mice, yet analysis of the mechanisms is restricted by the early embryonic lethality. To circumvent this limitation, I specifically ablated the bcl-x gene in the post-mitotic neurons of the cerebral cortex and hippocampus using a NEX promoter-driven Cre/loxP system, obtained both viable mice and in vitro neuronal cultures, and identified defects in mitochondrial morphology and distribution, as well as altered mitochondrial metabolic function in BCL-XL-deficient neurons that are present before apoptosis initiation. Therefore, these studies provide the first available information for the function of endogenous BCL-XL as an integrated component regulating mitochondrial physiological functions.; I also studied the mechanisms that can regulate the functional transition of BCL-2 family proteins from anti- to pro-death during apoptosis. The role of caspase cleavage of BCL-XL and BCL-2 during cell death was addressed in vivo by generating caspase-resistant BCL-XL and BCL-2 knock-in mice. In addition, a BH3-only BCL-2 protein BAD was found to have anti-death function that is regulated by caspase cleavage and alternative splicing.; My work suggests the role of BCL-2 family proteins in the regulation of apoptosis is not an isolated function that is only activated when cells challenged with death stimuli, instead it is closely coupled to their roles in regulating the normal cellular physiological functions.