Poly(ADP-ribose) polymerase-1 protects neurons against apoptosis induced by oxidative stress

In neurons, DNA is prone to free radical damage, although repair mechanisms preserve the genomic integrity. However, activation of the DNA repair system, poly(ADP-ribose) polymerase (PARP-1), is thought to cause neuronal death through NAD+ depletion and mitochondrial membrane potential (delta psi(m)) depolarization. Here, we show that abolishing PARP-1 activity in primary cortical neurons can either enhance or prevent apoptotic death, depending on the intensity of an oxidative stress. Only in severe oxidative stress does PARP-1 activation result in NAD+ and ATP depletion and neuronal death. To investigate the role of PARP-1 in an endogenous model of oxidative stress, we used an RNA interference (RNAi) strategy to specifically knock down glutamate-cysteine ligase (GCL), the rate-limiting enzyme of glutathione biosynthesis. GCL RNAi spontaneously elicited a mild type of oxidative stress that was enough to stimulate PARP-1 in a Ca2+-calmodulin kinase II-dependent manner. GCL RNAi-mediated PARP-1 activation facilitated DNA repair, although neurons underwent delta psi(m) loss followed by some apoptotic death. PARP-1 inhibition did not prevent delta psi(m) loss, but enhanced the vulnerability of neurons to apoptosis upon GCL silencing. Conversely, mild expression of PARP-1 partially prevented to GCL RNAi-dependent apoptosis. Thus, in the mild progressive damage likely occur in neurodegenerative diseases, PARP-1 activation plays a neuroprotective role that should be taken into account when considering therapeutic strategies.