Preventing NAD+ depletion protects neurons against excitotoxicity: Bioenergetic effects of mild mitochondrial uncoupling and caloric restriction

Neurons are excitable cells that require large amounts of energy to support their survival and functions and are therefore prone to excitotoxicity, which involves energy depletion. By examining bioenergetic changes induced by glutamate, we found that the cellular nicotinamide adenine dinucleotide (NAD⁺) level is a critical determinant of neuronal survival. The bioenergetic effects of mitochondrial uncoupling and caloric restriction were also examined in cultured neurons and rodent brain. 2, 4-dinitrophenol (DNP) is a chemical mitochondrial uncoupler that stimulates glucose uptake and oxygen consumption on cultured neurons, which accelerates oxidation of NAD(P)H to NAD⁺ in mitochondria. The NAD⁺-dependent histone deacetylase sirtulin 1 (SIRT1) and glucose transporter 1 (GLUT1) mRNA are upregulated mouse brain under caloric restriction. To examine whether NAD ⁺ mediates neuroprotective effects, nicotinamide, a precursor of NAD⁺ and inhibitor of SIRT1 and poly (ADP-ribose) polymerase 1 (PARP1) (two NAD⁺-dependent enzymes), was employed. Nicotinamide attenuated excitotoxic death and preserved cellular NAD⁺ levels to support SIRT1 and PARP 1 activities. Our findings suggest that mild mitochondrial uncoupling and caloric restriction exert hormetic effects by stimulating bioenergetics in neurons thereby increasing tolerance of neurons to metabolic stress.