Regulation of apoptosis at the point of cytochrome c in disease models

Apoptosis is a tightly regulated genetic process which governs the ability of a cell to undergo death under various developmental and pathological stimuli. The apoptotic pathway is essential not only for the development of organisms but also for maintaining homeostasis. However, in pathological conditions, aberrant regulation of apoptosis can cause disarray of normal cell physiology.;In the mitochondrial-dependent, intrinsic apoptotic pathway, death signals converge to the mitochondria where Bcl-2 family members regulate the release of cytochrome c from the mitochondria. Cytosolic cytochrome c binds to Apaf-1 and induces the formation of the apoptosome complex, which in turn activates caspases that are responsible for the execution of cell death.;Despite having similar core apoptotic components, postmitotic neurons are found to have a more stringent regulation on apoptosis as compared to mitotic cells. It is beneficial for postmitotic neurons to evolve mechanisms that restrict apoptosis because of their limited regenerative potential and their need to last the lifetime of the organism. Here, I demonstrate the differential sensitivity of mitotic primary brain tumors and post-mitotic non-malignant neural tissues to the activation of apoptosis at the point of cytochrome c. Cytochrome c induces rapid caspase activation in brain tumor tissue but not the surrounding normal neural tissues. This difference in response to cytochrome c-mediated death is attributed to differential expression of Apaf-1. In addition, this work suggests that direct activation of apoptosis at the point of cytochrome c can be utilized as an adjuvant treatment for various brain tumors.;In this work, I also show that the lack of XIAP in postmitotic neurons make them more vulnerable to a mitochondrial damaging stimulus in an animal model of ALS (amyotrophic lateral sclerosis). This result strengthens and validates the role of endogenous XIAP as a safety brake in postmitotic neurons that prevent unwanted caspase activation and induction of cell death in situations of accidental cytochrome c release secondary to mitochondrial damage. Moreover, this study suggests that mutations in XIAP that reduce its caspase inhibition function can be a risk factor to the development of neurodegenerative diseases in humans.