NAD⁺ as a metabolic link between DNA damage and cell death

Abstract

DNA damage occurs in ischemia, excitotoxicity, inflammation, and other disorders that affect the central nervous system (CNS). Extensive DNA damage triggers cell death and in the mature CNS, this occurs primarily through activation of the poly(ADP-ribose) polymerase-1 (PARP-1) cell death pathway. PARP-1 is an abundant nuclear enzyme that, when activated by DNA damage, consumes nicotinamide adenine dinucleotide (NAD)+ to form poly(ADP-ribose) on acceptor proteins. The mechanisms by which PARP-1 activation leads to cell death are not understood fully. We used mouse astrocyte cultures to explore the bioenergetic effects of NAD+ depletion by PARP-1 and the role of NAD+ depletion in this cell death program. PARP-1 activation was induced by the DNA alkylating agent, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), using medium in which glucose was the only exogenous energy substrate. PARP-1 activation led to a rapid but incomplete depletion of astrocyte NAD+, a near-complete block in glycolysis, and eventual cell death. Repletion of intracellular NAD+ restored glycolytic function and prevented cell death. The addition of non-glucose substrates to the medium, pyruvate, glutamate, or glutamine, also prevented astrocyte death after PARP-1 activation. These studies suggest PARP-1 activation leads to rapid depletion of the cytosolic but not the mitochondrial NAD+ pool. Depletion of the cytosolic NAD+ pool renders the cells unable to utilize glucose as a metabolic substrate. Under conditions where glucose is the only available metabolic substrate, this leads to cell death. This cell death pathway is particularly germane to brain because glucose is normally the only metabolic substrate that is transported rapidly across the blood-brain barrier.

Prevention of Acute/Severe Hypoglycemia-Induced Neuron Death by Lactate Administration

“…In settings where glucose is the chief metabolic substrate, this leads to mitochondrial dysfunction and cell death (Alano et al, 2004;Ying et al, 2003;Zong et al, 2004). Interestingly, repletion of NAD + after PARP-1 activation has been shown to restore glycolytic capacity and prevent cell death (Ying et al, 2003(Ying et al, , 2005 in primary neuronal cultures. Similarly, providing nonglucose substrates such as pyruvate and a-ketoglutarate that can be metabolized without the need for cytosolic NAD + also preserves cell viability, even when delivered hours after PARP-1 activation (Alano et al, 2004;Ying et al, 2002).…”

Altering DNA base excision repair: Use of nuclear and mitochondrial‐targeted N‐methylpurine DNA glycosylase to sensitize astroglia to chemotherapeutic agents

“…However, the increased presence of these pathway intermediates in nuclear DNA resulting from increased MPG activity proved to be cytotoxic in astrocytes treated with MNU. One possible mechanism that could be responsible for the increase in cell killing is the activation of Poly-ADP Ribose Polymerase (PARP) with the subsequent depletion of NAD within the cells (Ying et al, 2005). PARP is abundant in the nucleus and is activated by SSB, which are generated at a 4-fold greater rate in NucMPG astrocytes.…”

Pyruvate Improves Recovery after PARP-1-Associated Energy Failure Induced by Oxidative Stress in Neonatal Rat Cerebrocortical Slices

“…Additionally, NAD + restoration adds to energy depletion, because four ATP molecules are required to resynthesize one NAD + (Zhang et al, 1994). The futile-cycle hypothesis is supported by results of monolayer neuron and astrocyte cell culture experiments (Ying et al, 2005(Ying et al, , 2002, which found that PARP hyperactivation by N-methyl-N-nitrosoguanidine or H 2 O 2 led to cytosolic NAD + depletion, glycolytic arrest, and cell death from energy failure. More significantly, cell death in those studies was prevented or reversed by administration of PARP inhibitors or tricarboxylic acid cycle (TCA) substrates such as pyruvate, glutamine, and a-ketoglutarate.…”