Poly(ADP-ribose) glycohydrolase mediates oxidative and excitotoxic neuronal death

Excessive activation of poly(ADP-ribose) polymerase 1 (PARP1) leads to NAD( +) depletion and cell death during ischemia and other conditions that gener ate extensive DNA damage. When activated by DNA strand breaks, PARP1 uses N AD+ as substrate to form ADP-ribose polymers on specific acceptor proteins. These polymers are in turn rapidly degraded by poly(ADP-ribose) glycohydro lase (PARG), a ubiquitously expressed exo- and endoglycohydrolase. In this study, we examined the role of PARG in the PARP1-mediated cell death pathwa y. Mouse neuron and astrocyte cultures were exposed to hydrogen peroxide, N -methyl-D-aspartate (NMDA), or the DNA alkylating agent, N-methyl-N'-nitro- N-nitrosoguanidine (MNNG). Cell death in each condition was markedly reduce d by the PARP1 inhibitor benzamide and equally reduced by the PARG inhibito rs gallotannin and nobotanin B. The PARP1 inhibitor benzamide and the PARG inhibitor gallotannin both prevented the NAD+ depletion that otherwise resu lts from PARP1 activation by MNNG or H2O2. However, these agents had opposi te effects on protein poly(ADP-ribosyl)ation. Immunostaining for poly(ADP-r ibose) on Western blots and neuron cultures showed benzamide to decrease an d gallotannin to increase poly(ADP-ribose) accumulation during MNNG exposur e. These results suggest that PARG inhibitors do not inhibit PARP1 directly , but instead prevent PARP1-mediated cell death by slowing the turnover of poly(ADP-ribose) and thus slowing NAD+ consumption. PARG appears to be a ne cessary component of the PARP-mediated cell death pathway, and PARG inhibit ors may have promise as neuroprotective agents.