Thesis Abstract

Cellular demise is a well controlled and complex process. The role of apoptosis and their contribution to disease pathogenesis are well known. Yet, emerging evidence indicates that apoptosis interplay with autophagy may be more complex than previously thought. Autophagy may serve a pro-survival or cyto-destructive role depending on the cellular context and type of stressor. In this context the requirement for PKC-delta; in MA and ischemia-induced apoptotic cell death is briefly discussed.

In the first portion of my thesis I investigated the cell death mechanism(s) underlying methamphetamine induced dopaminergic neurodegeneration. Oxidative stress has been shown to trigger mitochondrial damage and the induction of autophagy, a protein clearance system. We and others have previously demonstrated the induction of autophagy/apoptosis in MA-treated N27 cells; however the precise cellular mechanisms underlying MA-induced stimulation of autophagy/apoptosis remain unclear. The present study explores the following issues: (1) Does mitochondrial impairment precede autophagic induction. If so, does it occur independent of or in association with UPS dysfunction? (2) Does PKC-delta;, an oxidative stress sensor kinase, play a role in the mechanism of MA-induced cell death? If so, does it activate caspase-3 dependent cell death events? (3) What is the exact contribution of autophagy, is it cytoprotective or prodeath mechanism? Taken together, our results suggest that MA-induced dopaminergic neurotoxicity is likely caused, at least in part, by pronounced proteolytic cleavage of PKC-delta; and associated oxidative cell signaling events. Additionally, MA, through the induction of mitochondrial stress, activates autophagy as a cytoprotective mechanism to minimize the magnitude of neuronal cell death.

In the second portion of my thesis I evaluated the role of PKC-delta; dependent cell death signaling events in striatal neuronal degeneration. Striatal ischemia is also often associated with oxidative stress associated cell death following reperfusion post cerebral ischemia. Using an OGD model of ischemia we demonstrated the occurrence of apoptotic cell death in mouse primary striatal neurons. As a key detector of oxidative stress, depolarization of mitochondria membrane potential and cytochrome c release into the cytoplasm are signs of mitochondrial impairment. Additionally, downstream events involve caspase-3 cleavage and PKC-delta; activation. Caspase-3 mediated proteolytic cleavage of PKC-delta; in the striatum as a result of ischemic insult is a novel finding which suggests PKC-delta; plays a regulatory role in striatal neuronal cell death. This finding is further supported from our data showing that cell death is attenuated in PKC-delta; knockout mice. Previous studies have shown neuron subtype specific susceptibility to ischemic insult in the striatum. Our investigation reveals that GABAergic neurons are more vulnerable to cell death during striatal ischemia, confirming different neuronal subtypes exhibit varying degrees of vulnerability to ischemic damage.

In conclusion, despite the use of two different types of cellular injury models, namely MA and OGD-induced neurotoxicity, our findings underscore the importance of caspase-3 mediated PKC-delta; dependent cell death events in apoptotic cell death. This study demonstrates that PKC-delta; might represent a common cell death mechanism, facilitating neuronal demise following exposure to diverse cell stressors that are capable of causing apoptotic cell death.