Conrad C. Alano

NAD+ depletion is necessary and sufficient for poly(ADP-ribose) polymerase-1-mediated neuronal death.

Poly(ADP-ribose)-1 (PARP-1) is a key mediator of cell death in excitotoxicity, ischemia, and oxidative stress. PARP-1 activation leads to cytosolic NAD+ depletion and mitochondrial release of apoptosis-inducing factor (AIF), but the causal relationships between these two events have been difficult to resolve. Here, we examined this issue by using extracellular NAD+ to restore neuronal NAD+ levels after PARP-1 activation. Exogenous NAD+ was found to enter neurons through P2X7-gated channels. Restoration of cytosolic NAD+ by this means prevented the glycolytic inhibition, mitochondrial failure, AIF translocation, and neuron death that otherwise results from extensive PARP-1 activation. Bypassing the glycolytic inhibition with the metabolic substrates pyruvate, acetoacetate, or hydroxybutyrate also prevented mitochondrial failure and neuron death. Conversely, depletion of cytosolic NAD+ with NAD+ glycohydrolase produced a block in glycolysis inhibition, mitochondrial depolarization, AIF translocation, and neuron death, independent of PARP-1 activation. These results establish NAD+ depletion as a causal event in PARP-1-mediated cell death and place NAD+ depletion and glycolytic failure upstream of mitochondrial AIF release.

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

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.

Tricarboxylic Acid Cycle Substrates Prevent PARP-Mediated Death of Neurons and Astrocytes

The DNA repair enzyme, poly(ADP-ribose) polymerase-1 (PARP1), contributes to cell death during ischemia/reperfusion when extensively activated by DNA damage. The cell death resulting from PARP1 activation is linked to NAD+ depletion and energy failure, but the intervening steps are not well understood. Because glycolysis requires cytosolic NAD+, the authors tested whether PARP1 activation impairs glycolytic flux and whether substrates that bypass glycolysis can rescue cells after PARP1 activation. PARP1 was activated in mouse cortical astrocyte and astrocyte-neuron cocultures with the DNA alkylating agent, N-methyl-N ′-nitro-N-nitrosoguanidine (MNNG). Studies using the 2-deoxyglucose method confirmed that glycolytic flux was reduced by more than 90% in MNNG-treated cultures. The addition of 5 mmol/L of α-ketoglutarate, 5 mmol/L pyruvate, or other mitochondrial substrates to the cultures after MNNG treatment reduced cell death from approximately 70% to near basal levels, while PARP inhibitors and excess glucose had negligible effects. The mitochondrial substrates significantly reduced cell death, with delivery delayed up to 2 hours after MNNG washout. The findings suggest that impaired glycolytic flux is an important factor contributing to PARP1-mediated cell death. Delivery of alternative substrates may be a promising strategy for delayed treatment of PARP1-mediated cell death in ischemia and other disorders.

Neuronal Sirt3 Protects against Excitotoxic Injury in Mouse Cortical Neuron Culture

Background Sirtuins (Sirt), a family of nicotinamide adenine nucleotide (NAD) dependent deacetylases, are implicated in energy metabolism and life span. Among the known Sirt isoforms (Sirt1-7), Sirt3 was identified as a stress responsive deacetylase recently shown to play a role in protecting cells under stress conditions. Here, we demonstrated the presence of Sirt3 in neurons, and characterized the role of Sirt3 in neuron survival under NMDA-induced excitotoxicity. Methodology/Principal Findings To induce excitotoxic injury, we exposed primary cultured mouse cortical neurons to NMDA (30 µM). NMDA induced a rapid decrease of cytoplasmic NAD (but not mitochondrial NAD) in neurons through poly (ADP-ribose) polymerase-1 (PARP-1) activation. Mitochondrial Sirt3 was increased following PARP-1 mediated NAD depletion, which was reversed by either inhibition of PARP-1 or exogenous NAD. We found that massive reactive oxygen species (ROS) produced under this NAD depleted condition mediated the increase in mitochondrial Sirt3. By transfecting primary neurons with a Sirt3 overexpressing plasmid or Sirt3 siRNA, we showed that Sirt3 is required for neuroprotection against excitotoxicity. Conclusions This study demonstrated for the first time that mitochondrial Sirt3 acts as a prosurvival factor playing an essential role to protect neurons under excitotoxic injury.

Differences among cell types in NAD(+) compartmentalization: a comparison of neurons, astrocytes, and cardiac myocytes.

Activation of the nuclear enzyme poly(ADP‐ribose)‐1 leads to the death of neurons and other types of cells by a mechanism involving NAD+ depletion and mitochondrial permeability transition. It has been proposed that the mitochondrial permeability transition (MPT) is required for NAD+ to be released from mitochondria and subsequently consumed by PARP‐1. In the present study we used the MPT inhibitor cyclosporine‐A (CsA) to preserve mitochondrial NAD+ pools during PARP‐1 activation and thereby provide an estimate of mitochondrial NAD+ pool size in different cell types. Rat cardiac myocytes, mouse cardiac myocytes, mouse cortical neurons, and mouse cortical astrocytes were incubated with the genotoxin N‐methyl‐N′‐nitro‐N‐nitrosoguanidine (MNNG) in order to activate PARP‐1. In all four cell types MNNG caused a reduction in total NAD+ content that was blocked by the PARP inhibitor 3,4‐dihydro‐5‐[4‐(1‐piperidinyl)butoxy]‐1(2H)‐isoquinolinone. Inhibition of the mitochondrial permeability transition with cyclosporine‐A (CsA) prevented PARP‐1‐induced NAD+ depletion to a varying degree in the four cell types tested. CsA preserved 83.5% ± 5.2% of total cellular NAD+ in rat cardiac myocytes, 85.7% ± 8.9% in mouse cardiac myocytes, 55.9% ± 12.9% in mouse neurons, and 22.4% ± 7.3% in mouse astrocytes. CsA preserved nearly 100% of NAD+ content in mitochondria isolated from these cells. These results confirm that it is the cytosolic NAD+ pool that is consumed by PARP‐1 and that the mitochondrial NAD+ pool is consumed only after MPT permits mitochondrial NAD+ to exit into the cytosol. These results also suggest large differences in the mitochondrial and cytosolic compartmentalization of NAD+ in these cell types.

Mitochondrial permeability transition and calcium dynamics in striatal neurons upon intense NMDA receptor activation

Deregulation of the intracellular Ca2+ homeostasis by NMDA receptor activation leads to neuronal cell death. Induction of the mitochondrial permeability transition pore (MPT) by Ca2+ is a critical event in mediating cell death. In this study, we used fluorescent Ca2+ indicators to investigate the effect of high concentrations of NMDA on cytosolic and mitochondrial Ca2+ concentrations ([Ca2+]c and [Ca2+]m, respectively) in cultured striatal neurons. Exposure to NMDA resulted in an immediate, sustained increase in [Ca2+]c followed by a secondary increase in [Ca2+]c. This second increase of [Ca2+]c was prevented by pretreatment with N‐methyl‐valine‐4‐cyclosporin (NMV‐Cys). Exposure of neurons to NMDA also resulted in an increase in [Ca2+]m that was followed by a precipitous decrease in the rhod‐2 signal. This decrease followed the time frame of the secondary increase in [Ca2+]c. Preincubation of the neurons with NMV‐Cys prevented the decrease in rhod‐2 fluorescence. These dynamic changes in the rhod‐2 signal and [Ca2+]m in response to NMDA were confirmed by using confocal microscopy. The presented results indicate that MPT can be detected in living neurons using fluorescent Ca2+ indicators, which would allow the study of the physiological role of MPT in cell death.

High-density lipoprotein determines adult mouse cardiomyocyte fate after hypoxia-reoxygenation through lipoprotein-associated sphingosine 1-phosphate

The lipid mediator sphingosine 1-phosphate (S1P) confers survival benefits in cardiomyocytes and isolated hearts subjected to oxidative stress. High-density lipoprotein (HDL) is a major carrier of S1P in the serum, but whether HDL-associated S1P directly mediates survival in a preparation composed exclusively of cardiomyocytes has not been demonstrated. Accordingly, we tested the hypothesis that signal activation and survival during simulated ischemia-reperfusion injury in response to HDL require lipoprotein-associated S1P. As a model, we used adult mouse cardiomyocytes subjected to hypoxia-reoxygenation. Cells were treated or not with autologous mouse HDL, which significantly increased myocyte viability as measured by trypan blue exclusion. This survival effect was abrogated by the S1P(1) and SIP(3) receptor antagonist VPC 23019. The selective S1P(3) antagonist CAY10444, the G(i) antagonist pertussis toxin, the MEK (MAPK/ERK) kinase inhibitor PD-98059, and the phosphoinositide-3 kinase inhibitor wortmannin also inhibited the prosurvival effect of HDL. We observed that HDL activated both Akt (protein kinase B) and the MEK1/2-ERK1/2 pathway and also stimulated phosphorylation of glycogen synthase kinase-3beta. ERK1/2 activation was through an S1P(1) subtype receptor-G(i) protein-dependent pathway, whereas the activation of Akt was inhibited by CAY10444, indicating mediation by S1P(3) subtype receptors. We conclude that HDL, via its cargo of S1P, can directly protect cardiomyocytes against simulated oxidative injury in the absence of vascular effects and that prosurvival signal activation is dependent on both S1P(1) and S1P(3) subtype receptors.

Human Serum Albumin and its N-Terminal Tetrapeptide (DAHK) Block Oxidant-Induced Neuronal Death

Background and Purpose— Studies using animal models of stroke have shown that human serum albumin (HSA) significantly ameliorates cerebral ischemic injury after both transient and permanent ischemia, even when administered after the onset of ischemia or reperfusion. The mechanism of this effect remains uncertain, and prior studies suggest both indirect hemodynamic and direct cytoprotective effects. HSA is a potent antioxidant, in part because of its strong copper-binding capacity. Here we examined the effect of HSA on oxidant-induced neuronal death in a cortical cell culture system. Methods— Murine cortical cultures were exposed to oxidative stress generated by hydrogen peroxide and by a mixture of copper plus ascorbic acid. We examined the ability of HSA and a tetrapeptide occupying its N-terminus (DAHK) to prevent neuronal death after these challenges. Results— H2O2 and CuCl2/ascorbic acid were used at concentrations that, in the absence of HSA, killed >90% of the neurons. HSA provided complete protection at a concentration of 37.5 &mgr;mol/L and 50% protection at 3.75 &mgr;mol/L. The copper-binding tetrapeptide DAHK had nearly identical potency and efficacy. HSA and DAHK were also equally effective in preventing neuronal death induced by CuCl2/ascorbic acid. Conclusions— HSA has potent antioxidant properties, probably due to binding of copper and other transition metals. HSA extravasation into ischemic brain may provide neuroprotection by limiting metal-catalyzed oxidant stress. The tetrapeptide DAHK may be an effective, small-molecular-weight alternative to HSA as a therapeutic agent for stroke.

Minocycline protects cardiac myocytes against simulated ischemia-reperfusion injury by inhibiting poly(ADP-ribose) polymerase-1

There is an increase in reactive oxygen and nitrogen species in cardiomyocytes during myocardial ischemia/reperfusion injury. This leads to oxidative DNA damage and activation of nuclear repair enzymes such as poly(ADP-ribose) polymerase-1 (PARP-1). PARP-1 activation promotes DNA repair under normal conditions. However, excessive activation of PARP-1 leads to cell death. We report that PARP-1 enzymatic activity is directly inhibited by minocycline, and we propose that one mechanism of minocycline cardioprotection is the result of PARP-1 inhibition. Using cultured adult rat cardiac myocytes, we evaluated the mechanism of minocycline protection in which PARP-1 activation was induced by simulated ischemia/reperfusion injury using oxygen-glucose deprivation. We found an increase in reactive oxygen species production, PARP-1 activation, and PARP-1-mediated cell death after simulated ischemia/reperfusion. Cell death was significantly reduced by the PARP inhibitors 3, 4-dihydro-5-[4-(1-piperidinyl)butoxy]-1(2H)-isoquinolinone (10 μM) and PJ-34 (500 nM) or by minocycline (500 nM). Cellular NAD+ depletion and poly(ADP-ribose) formation, which are biochemical markers of PARP-1 activation, were also blocked by minocycline. Finally, simulated ischemia/reperfusion led to induction of the mitochondrial permeability transition, which was prevented by minocycline. Therefore, we propose that the protective effect of minocycline on cardiac myocyte survival is the result of inhibition of PARP-1 activity.

Cardiomyocyte S1P1 receptor-mediated extracellular signal-related kinase signaling and desensitization.

We examined the ability of sphingosine-1-phosphate (S1P) to desensitize extracellular signal-related kinase (ERK), a mitogen-activated protein kinase linked to antiapoptotic responses in the heart. In isolated adult mouse cardiomyocytes, S1P (10 nM-5 μM) induced ERK phosphorylation in a time- and dose-dependent manner. S1P stimulation of ERK was completely inhibited by an S1P1/3 subtype receptor antagonist (VPC23019), by a Gi protein inhibitor (pertussis toxin) and by a mitogen-activated protein kinase/ERK kinase inhibitor (PD98059). A selective S1P3 receptor antagonist (CAY10444) had no effect on S1P-induced ERK activation. The selective S1P1 agonist SEW2871 also induced ERK phosphorylation. Activation of ERK by restimulation with 100 nM S1P was suppressed after 1 hour of preincubation with 100 nM S1P but recovered fully the next day, suggesting receptor recycling. Similar results were obtained in protein kinase Cϵ-null cardiomyocytes. Treatment with the nonselective S1P receptor agonist FTY720 for 1 hour also reduced phospho-ERK expression in response to subsequent S1P stimulation. In contrast to S1P, some desensitization to FTY720 persisted after overnight exposure. Cell death induced by hypoxia/reoxygenation was reduced by pretreatment with exogenous S1P. This enhanced survival was abrogated by pretreatment with PD98059, VPC23019, or pertussis toxin. Thus, exogenous S1P induces rapid and reversible S1P1-mediated ERK phosphorylation. S1P-induced adult mouse cardiomyocyte survival requires ERK activation mediated via an S1P1-Gi pathway.