Apoptosis-inducing factor (AIF), a mitochondrial oxidoreductase, is released into the cytoplasm to induce cell death in response to poly(ADP-ribose) (PAR) polymerase-1 (PARP-1) activation. How PARP-1 activation leads to AIF release is not known. Here we identify PAR polymer as a cell death signal that induces release of AIF. PAR polymer induces mitochondrial AIF release and translocation to the nucleus. PAR glycohydrolase, which degrades PAR polymer, prevents PARP-1-dependent AIF release. Cells with reduced levels of AIF are resistant to PARP-1-dependent cell death and PAR polymer cytotoxicity. These results reveal PAR polymer as an AIF-releasing factor that plays important roles in PARP-1-dependent cell death.excitotoxicity ͉ poly(ADP-ribose) polymerase

Excessive activation of the nuclear enzyme, poly(ADP-ribose) polymerase-1 (PARP-1) plays a prominent role in various of models of cellular injury. Here, we identify poly(ADP-ribose) (PAR) polymer, a product of PARP-1 activity, as a previously uncharacterized cell death signal. PAR polymer is directly toxic to neurons, and degradation of PAR polymer by poly(ADP-ribose) glycohydrolase (PARG) or phosphodiesterase 1 prevents PAR polymer-induced cell death. PARP-1-dependent, NMDA excitotoxicity of cortical neurons is reduced by neutralizing antibodies to PAR and by overexpression of PARG. Neuronal cultures with reduced levels of PARG are more sensitive to NMDA excitotoxicity than WT cultures. Transgenic mice overexpressing PARG have significantly reduced infarct volumes after focal ischemia. Conversely, mice with reduced levels of PARG have significantly increased infarct volumes after focal ischemia compared with WT littermate controls. These results reveal PAR polymer as a signaling molecule that induces cell death and suggests that interference with PAR polymer signaling may offer innovative therapeutic approaches for the treatment of cellular injury.excitotoxicity ͉ poly(ADP-ribose) glycohydrolase ͉ poly(ADP-ribose) polymerase ͉ stroke P oly(ADP-ribose) polymerase-1 (PARP-1) is an abundant nuclear protein that is involved in the DNA base excision repair system, where it is potently activated by DNA strand nicks and breaks (1, 2). Using NAD ϩ as a substrate, PARP-1 builds up homopolymers of ADP ribose units on various nuclear proteins including histones, DNA polymerases, topoisomerases, DNA ligase-2, transcription factors (3, 4), and PARP-1 itself (5, 6). Although the exact physiologic function of PARP-1 is not completely understood, in some tissues it plays an important role in DNA repair and genomic stability (5,7,8). Poly(ADP-ribose) (PAR) catabolism and metabolism is a dynamic process, with PAR glycohydrolase (PARG) playing the major role in the degradation of the polymer (9).Recent studies using pharmacologic inhibition of PARP or genetic KO of PARP-1 indicate that PARP-1 plays a dramatic and significant role in cellular injury after stroke, trauma, ischemiareperfusion of the heart, spleen, skeletal muscle, and retina, arthritis, ␤-islet cytotoxicity͞diabetes mellitus, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of Parkinson's disease, experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis, endotoxic shock, multiple-system organ failure, and liver damage (for review, see refs. 1 and 10). PARP-1 activation also plays a prominent role in NMDA excitotoxicity, because PARP-1 KO mice are remarkably resistant both in vitro and in vivo to the excitotoxic effects of glutamate and NMDA (11,12). A cell-suicide hypothesis has been proposed (1,2,13,14) to explain the actions of PARP-1 in mediating cell death. However, studies in mice lacking PARG suggest that PAR polymer formed during the activation of PARP-1 might play a role in PARP-1-dependent cell death. PARG KO mice die at embryoni...

The mitochondrial protein apoptosis-inducing factor (AIF) plays a pivotal role in poly(ADP-ribose) polymerase-1 (PARP-1)-mediated cell death (parthanatos), during which it is released from the mitochondria and translocates to the nucleus. Here, we show that AIF is a high affinity poly(ADP-ribose) (PAR)–binding protein and that PAR binding to AIF is required for parthanatos both in vitro and in vivo. AIF bound PAR at a site distinct from AIF’s DNA binding site and this interaction triggered AIF release from the cytosolic side of the mitochondrial outer membrane. Mutation of the PAR binding site in AIF did not affect its NADH oxidase activity, its ability to bind FAD or DNA, or its ability to induce nuclear condensation. However, this AIF mutant was not released from mitochondria and did not translocate to the nucleus or mediate cell death following PARP-1 activation. These results suggest a mechanism for PARP-1 to initiate AIF-mediated cell death and indicate that AIF’s bioenergetic cell survival-promoting functions are separate from its effects as a mitochondrially-derived death effector. Interference with the PAR-AIF interaction or PAR signaling may provide unique opportunities for preventing cell death following activation of PARP-1.