CRITICAL ROLE OF EUKARYOTIC TRANSLATION INITIATION FACTOR 4G IN MEDIATING ISCHEMIA-INDUCED NEURONAL DEATH

Stroke is the third leading cause of death in the United States and the second leading cause of death in the world. Despite the epidemiological significance of this disease, there are few treatment options. The purpose of this dissertation is to expand the understanding of underlying mechanisms mediating neuronal death caused by stroke, or cerebral ischemia. Two major metabolic disturbances occur due to ischemia—persistent protein synthesis inhibition and secondary energy depletion. All ischemia-affected neurons experience protein synthesis inhibition. However, neurons that recover protein synthesis live, while neurons that fail to recover die. This makes protein synthesis a robust predictor of neuronal death. However, the underlying mechanisms of persistent protein synthesis inhibition remain unknown. The hypothesis of this dissertation is that persistent protein synthesis inhibition is caused by activation of the calcium-sensitive protease calpain, which degrades eukaryotic translation initiation factor (eIF) 4G. Inhibition of calpain or overexpression of eIF4G results in increased protein synthesis and increased neuronal viability following the in vitro model of ischemia oxygen glucose deprivation in rat primary cortical neurons. Importantly, the neuroprotective effect of preservation of eIF4G is only partly due to its restoration of protein synthesis. Potential protein synthesis-independent mechanisms eIF4G-mediated protection are discussed. Neurons subjected to ischemia suffer an initial loss of energy in the form of ATP, which returns to baseline within fifteen minutes of restoration of blood flow. However, ischemia-sensitive neurons undergo secondary energy depletion prior to delayed neuronal death. The cause of secondary energy failure is hypothesized to be due to DNA recognition enzyme poly(ADP)-ribose polymerase (PARP)-1 depletion of the energy substrate NAD+. Evidence is presented linking PARP-1 activation to mitochondrial calcium dysregulation with subsequent calpain activation and apoptosis-inducing factor release. The results of these two findings are discussed in depth and future experiments are outlined. The potential of role of eIF4G in mitochondrial biogenesis, inhibition of autophagy and prevention of secondary energy loss is postulated. The research presented in this dissertation provides a novel perspective regarding the mechanisms underlying delayed neuronal death and may eventually lead to the development of clinically applicable neuroprotective strategies