Zinc-induced cortical neuronal death: Contribution of energy failure attributable to loss of NAD(+) and inhibition of glycolysis

Excessive zinc influx may contribute to neuronal death after certain insult s, including transient global ischemia. In light of evidence that levels of intracellular free Zn2+ associated with neurotoxicity may be sufficient to inhibit glyceraldehyde-3-phosphate dehydrogenase (GAPDH), experiments were performed looking for reduced glycolysis and energy failure in cultured mo use cortical neurons subjected to lethal Zn2+ exposure. As predicted, cultu res exposed for 3-22 hr to 40 mu M Zn2+ developed an early increase in leve ls of dihydroxyacetone phosphate (DHAP) and fructose 1,6-bisphosphate (FBP) and a progressive loss of ATP levels, followed by neuronal cell death; fur thermore, addition of the downstream glycolytic substrate pyruvate to the b athing medium attenuated the fall in ATP and neuronal death. However, an alternative to direct Zn2+ inhibition of GAPDH was raised by th e observation that Zn2+ exposure also induced an early decrease in nicotina mide-adenine dinucleotide (NAD(+)) levels, an event itself capable of inhib iting GAPDH. Favoring this indirect mechanism of GAPDH inhibition, the neur oprotective effects of pyruvate addition were associated with normalization of cellular levels of NAD(+), DHAP, and FBP. Zn2+ induced neuronal death w as also attenuated by addition of the energy substrate oxaloacetate, the ac tivator of pyruvate dehydrogenase, dichloroacetate, or the inhibitors of NA D(+) catabolism, niacinamide or benzamide. Acetyl carnitine, alpha-keto but yrate, lactate, and beta-hydroxy-butyrate did not attenuate Zn2+ induced ne urotoxicity, perhaps because they could not regenerate NAD(+) or be used fo r energy production in the presence of glucose.