Poly(ADP-ribose) Polymerase-1-mediated Cell Death in Astrocytes Requires NAD+ Depletion and Mitochondrial Permeability Transition

Abstract

Extensive activation of poly(ADP-ribose) polymerase-1 (PARP-1) by DNA damage is a major cause of caspase-independent cell death in ischemia and inflammation. Here we show that NAD+ depletion and mitochondrial permeability transition (MPT) are sequential and necessary steps in PARP-1-mediated cell death. Cultured mouse astrocytes were treated with the cytotoxic concentrations of N-methyl-N′-nitro-N-nitrosoguanidine or 3-morpholinosydnonimine to induce DNA damage and PARP-1 activation. The resulting cell death was preceded by NAD+ depletion, mitochondrial membrane depolarization, and MPT. Sub-micromolar concentrations of cyclosporin A blocked MPT and cell death, suggesting that MPT is a necessary step linking PARP-1 activation to cell death. In astrocytes, extracellular NAD+ can raise intracellular NAD+ concentrations. To determine whether NAD+ depletion is necessary for PARP-1-induced MPT, NAD+ was restored to near-normal levels after PARP-1 activation. Restoration of NAD+ enabled the recovery of mitochondrial membrane potential and blocked both MPT and cell death. Furthermore, both cyclosporin A and NAD + blocked translocation of the apoptosis-inducing factor from mitochondria to nuclei, a step previously shown necessary for PARP-1-induced cell death. These results suggest that NAD+ depletion and MPT are necessary intermediary steps linking PARP-1 activation to AIF translocation and cell death.