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 Zn ²⁺ ([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 Zn ²⁺ 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.