Zinc neurotoxicity is dependent on intracellular NAD+ levels and the sirtuin pathway

Zinc neurotoxicity has been demonstrated in ischemic, seizure, hypoglycemic, and trauma-induced neuronal death where Zn2+ is thought to be synaptically released and taken up in neighbouring neurons, reaching toxic concentrations. We previously demonstrated that toxicity of extracellular Zn2+ depended on entry, elevation in intracellular free Zn2+ ([Zn2+](i)), a reduction in NAD(+) and ATP levels, and dysfunction of glycolysis and cellular metabolism. We suggested that PARP-1 activation alone can not explain this loss of neuronal NAD(+). NAD(+) was recently demonstrated to permeate neurons and glia, and we have now shown that exogenous NAD(+) can reduce Zn2+ neurotoxicity, and 3-acetylpyridine, which generates inactive NAD(+), potentiated Zn2+ neurotoxicity. Sirtinol and 2-hydroxynaphthaldehyde, inhibitors of the sirtuin pathway (SIRT proteins are NAD(+)-catabolic protein deacetylases), attenuated both acute and chronic Zn2+ neurotoxicity. Resveratrol and fisetin (sirtuin activators) potentiated NAD(+) loss and Zn2+ neurotoxicities. Furthermore, neuronal cultures derived from the Wld(s) mouse, which overexpress the NAD(+) synthetic enzyme nicotinamide mononucleotide adenyl transferase (NMNAT-1), had reduced sensitivity to Zn2+ neurotoxicity. Finally, nicotinamide was demonstrated to attenuate CA1 neuronal death after 10 min of global ischemia in rat even if administered 1 h after the insult. Together with previous data, these results further implicate NAD(+) levels in Zn2+ neurotoxicity.