Nicotinamide Prevents NAD+ Depletion and Protects Neurons Against Excitotoxicity and Cerebral Ischemia: NAD+ Consumption by SIRT1 may Endanger Energetically Compromised Neurons

Abstract

Neurons require large amounts of energy to support their survival and function, and are therefore susceptible to excitotoxicity, a form of cell death involving bioenergetic stress that may occur in several neurological disorders including stroke and Alzheimer's disease. Here we studied the roles of NAD + bioenergetic state, and the NAD + -dependent enzymes SIRT1 and PARP-1, in excitotoxic neuronal death in cultured neurons and in a mouse model of focal ischemic stroke. Excitotoxic activation of NMDA receptors induced a rapid decrease of cellular NAD(P)H levels and mitochondrial membrane potential. Decreased NAD + levels and poly (ADP-ribose) polymer (PAR) accumulation in nuclei were relatively early events (<4 h) that preceded the appearance of propidium iodide-and TUNEL-positive cells (markers of necrotic cell death and DNA strand breakage, respectively) which became evident by 6 h. Nicotinamide, an NAD + precursor and an inhibitor of SIRT1 and PARP1, inhibited SIRT1 deacetylase activity without affecting SIRT1 protein levels. NAD + levels were preserved and PAR accumulation and neuronal death induced by excitotoxic insults were attenuated in nicotinamide-treated cells. Treatment of neurons with the SIRT1 activator resveratrol did not protect them from glutamate/NMDA-induced NAD + depletion and death. In a mouse model of focal cerebral ischemic stroke, NAD + levels were decreased in both the contralateral and ipsilateral cortex 6 h after the onset of ischemia. Stroke resulted in dynamic changes of SIRT1 protein and activity levels which varied among brain regions. Administration of nicotinamide (200 mg/kg, i.p.) up to 1 h after the onset of ischemia elevated brain NAD + levels and reduced ischemic infarct size. Our findings demonstrate that the NAD + bioenergetic state is critical in determining whether neurons live or die in excitotoxic and ischemic conditions, and suggest a potential therapeutic benefit in stroke of agents that preserve cellular NAD + levels. Our data further suggest that, SIRT1 is linked to bioenergetic state and stress responses in neurons, and that under conditions of reduced cellular energy levels SIRT1 enzyme activity may consume sufficient NAD + to nullify any cell survival-promoting effects of its deacetylase action on protein substrates.

Age-related changes in AMP-activated protein kinase after stroke

“…In the present study, SIRT1 expression was unaffected by age and ischemic injury in vehicle-treated mice. This is in contrast with a previous report that found elevated cortical SIRT1 protein levels 3 h after stroke (Liu et al 2009a). This same study found a decrease at 6 h in SIRT1 protein, and differences may reflect the timing of evaluation (at 4 and 24 h in our study).…”

“…AMPK enhances SIRT1 activity by increasing cellular NAD+ levels (Canto et al 2009). Although it has been suggested that increased SIRT1 activity leads to extension of life span (Howitz et al 2003;Wood et al 2004), what role SIRT1 plays in the response to ischemic brain insults is unknown (Raval et al 2008;Liu et al 2009a). In the present study, SIRT1 expression was unaffected by age and ischemic injury in vehicle-treated mice.…”

Identification of the Aryl Hydrocarbon Receptor Target Gene TiPARP as a Mediator of Suppression of Hepatic Gluconeogenesis by 2,3,7,8-Tetrachlorodibenzo-p-dioxin and of Nicotinamide as a Corrective Agent for This Effect

“…This unorthodox hypothesis is consistent with our findings and the suggestion of others that in a variety of contexts, effects of NAM on NAD ϩ synthesis may overcome sirtuin inhibition with resulting net activation of NAD ϩ -dependent sirtuin activity (27, 40 -42). It has been reported, for example, that NAM improved suppression of neuronal SIRT1 deacetylase activity by G/N glutamate (43). A possible additional explanation of this observation is that NAM has suppressive effects on acetyltransferase activity, although this will require further investigation.…”

Microglial Activation in Stroke: Therapeutic Targets

“…206,207 Cerebral ischemia results in altered SIRT1 protein and activity levels in brain. 208 Sirtuin activity is reduced within the first 6 hours after ischemic injury, but is then is increased at 12 and 24 hours. Resveratrol, a potent SIRT1 activator, has neuroprotective effects in cerebral ischemia, 209 -211 but whether this effect is mediated through actions on microglia remains to be established.…”