Neuronal damage following stroke or neurodegenerative diseases is thought to stem in part from overexcitation of N-methyl-D-aspartate (NMDA) receptors by glutamate. NMDA receptors triggered neurotoxicity is mediated in large part by activation of neuronal nitric oxide synthase (nNOS) and production of nitric oxide (NO). Simultaneous production of superoxide anion in mitochondria provides a permissive environment for the formation of peroxynitrite (ONOO-). Peroxynitrite can damage DNA leading to strand breaks and activation of poly(ADP-Ribose) Polymerase (PARP). We have found that PARP activation plays a key role in NMDA excitotoxicity, and experimental models of stroke and Parkinson’s disease. The mechanisms of PARP mediated neuronal death are just being revealed. While decrements in ATP and NAD are readily observed following PARP activation, it is not yet clear whether loss of ATP and NAD contribute significantly to the neuronal death cascade. We observe cytochrome c release and caspase-3 activation; however, broad-spectrum caspase inhibitors are ineffective in preventing neuronal cell death. Thus, caspase activation is not a primary trigger for neuronal excitotoxic cell death. Due to the morphology of dying neurons following exposure to NMDA and the caspase independence of NMDA-triggered, PARP-dependent excitotoxicity, a putative role for the mitochondrial-associated protein, apoptosis-inducing factor (AIF), was investigated. The time course of AIF translocation is concomitant with PARP activation. Neutralizing antibodies to AIF are sufficient to prevent NMDA neurotoxicity. Our data suggest that PARP activation in the nucleus triggers cross talk between nuclear and mitochondrial proteins that result in AIF release from mitochondria and subsequent neuronal cell death. References: 1.Yu SW et al: Mediation of Poly(ADP-Ribose) Polymerase-1 Dependent Cell Death by Apoptosis Inducing Factor. Science, 2002; 297: 259-263 2.Cregan SP et al: Apoptosis-inducing factor is involved in the regulation of caspase-independent neuronal cell death. J Cell Biol, 2002; 5; 158(3): 507-17 3.Mandir AS et al: Poly (ADP-ribose) Polymerase Activation Mediates MPTP-induced Parkinsonism. Proc Natl Acad Sci USA, 1999; 96: 5774-5779 4.Eliasson MJL et al: Poly(ADP-Ribose) Polymerase Gene Disruption Renders Mice Resistant to Cerebral Ischemia. Nature Med, 1997; 3: 1089-1095

Keywords: apoptosis-inducing factor (AIF), caspase, excitotoxicity, Mitochondria, Stroke, Parkinson’s disease, nitric oxide (NO)