Nicotinamide adenine dinucleotide (NADNicotinamide adenine dinucleotide (NAD ϩ ) is an essential redox molecule and a key player in several signaling pathways that govern fundamental biological processes (1, 2). In the redox reactions, a hydride equivalent is reversibly transferred at the nicotinamide moiety, resulting in a switch between oxidized (NAD ϩ ) and reduced (NADH) forms of the nucleotide. Although the redox reactions are critical for efficient mitochondrial metabolism, they are not accompanied by any net consumption of the nucleotide. On the contrary, NAD ϩ -dependent signaling processes lead to its degradation.Three distinct families of enzymes consume NAD ϩ as substrate: poly(ADP-ribose) polymerases (PARPs), 2 ADP-ribosyl cyclases (CD38 and CD157), and sirtuins (SIRT1-7) (3-5). PARPs hydrolyze NAD ϩ and transfer the ADP-ribose moiety of NAD ϩ to a receptor amino acid, building poly(ADP-ribose) polymers. PARPs regulate DNA damage repair, tumorigenesis, cell differentiation, and metabolism (3, 6, 7). CD38 is a ubiquitously expressed multifunctional enzyme that catalyzes the production of second messengers (like cyclic ADP-ribose) (8), which act as potent intracellular calcium-mobilizing agents to control cell cycle, insulin signaling, and microglial activation (8 -10). Sirtuins are a highly conserved family of proteins capable of catalyzing NAD ϩ -dependent deacylation and mono-(ADP-ribosyl)ation reactions (11). Sirtuin activation has been shown to modulate mitochondrial biogenesis and all major mitochondrial processes, including the tricarboxylic acid cycle, fatty acid metabolism, oxidative phosphorylation, and antioxidant response (4,(12)(13)(14)(15). Because all of the above NAD ϩ -consuming enzymes generate nicotinamide (NAM) as a byproduct, mammalian cells have evolved an NAD ϩ salvage pathway capable of resynthesizing NAD ϩ from NAM (16). Although NAD ϩ synthesis can occur from L-tryptophan (kynurenine pathway), nicotinic acid (Preiss-Handler pathway), or nicotinamide riboside (NR) (17-19), the salvage pathway appears to account for the majority of NAD ϩ synthesis in mammalian cells. The enzyme nicotinamide phosphoribosyltransferase (NAMPT) catalyzes the conversion of NAM and 5Ј-phosphoribosyl 1-pyrophosphate to nicotinamide mononucleotide (NMN); subsequently nicotinamide mononucleotide * This study was supported by National Institutes of Health Grants NS089640 and GM103542. The authors declare that they have no conflict of interest. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. 20,21). NAMPT is the rate-limiting enzyme in this pathway. Accordingly, overexpression of NAMPT, but not NMNATs, increases NAD levels (22-24).adenylyltransferases (NMNATs) transfer adenine from ATP to NMN to generate NAD ϩ (All the different types of NAD ϩ -consuming reactions have been described in the mitochondria, but in general NAMPT appears to be absent from the mitochondrial compartment (22, 24 -26), and the ori...

Amyotrophic lateral sclerosis (ALS) is characterized by the progressive degeneration of motor neurons in the spinal cord, brain stem, and motor cortex. The molecular mechanism underlying the progressive degeneration of motor neuron remains uncertain but involves a non-cell autonomous process. In acute injury or degenerative diseases astrocytes adopt a reactive phenotype known as astrogliosis. Astrogliosis is a complex remodeling of astrocyte biology and most likely represents a continuum of potential phenotypes that affect neuronal function and survival in an injury-specific manner. In ALS patients, reactive astrocytes surround both upper and lower degenerating motor neurons and play a key role in the pathology. It has become clear that astrocytes play a major role in ALS pathology. Through loss of normal function or acquired new characteristics, astrocytes are able to influence motor neuron fate and the progression of the disease. The use of different cell culture models indicates that ALS-astrocytes are able to induce motor neuron death by secreting a soluble factor(s). Here, we discuss several pathogenic mechanisms that have been proposed to explain astrocyte-mediated motor neuron death in ALS. In addition, examples of strategies that revert astrocyte-mediated motor neuron toxicity are reviewed to illustrate the therapeutic potential of astrocytes in ALS. Due to the central role played by astrocytes in ALS pathology, therapies aimed at modulating astrocyte biology may contribute to the development of integral therapeutic approaches to halt ALS progression.

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A growing body of evidence suggests the beneficial role of enhancing NAD availability in models of neurodegeneration. The challenge ahead is to establish the value and safety of the long-term use of these strategies for the treatment of neurodegenerative diseases. Antioxid. Redox Signal. 28, 1652-1668.

Spike frequency adaptation (SFA or accommodation) and calcium‐activated potassium channels that underlie after‐hyperpolarization potentials (AHP) regulate repetitive firing of neurons. Precisely how neuromodulators such as dopamine from the ventral tegmental area (VTA) regulate SFA and AHP (together referred to as intrinsic inhibition) in the prefrontal cortex (PFC) remains unclear. Using whole cell electrophysiology, we measured intrinsic inhibition in prelimbic (PL) layer 5 pyramidal cells of male adult rats. Results demonstrate that bath application of dopamine reduced intrinsic inhibition (EC 50: 25.0 μmol/L). This dopamine action was facilitated by coapplication of cocaine (1 μmol/L), a blocker of dopamine reuptake. To evaluate VTA dopamine terminals in PFC slices, we transfected VTA dopamine cells of TH::Cre rats in vivo with Cre‐dependent AAVs to express channelrhodopsin‐2 (ChR2) or designer receptors exclusively activated by designer drugs (DREADDS). In PFC slices from these animals, stimulation of VTA terminals with either blue light to activate ChR2 or bath application of clozapine‐N‐oxide (CNO) to activate Gq‐DREADDs produced a similar reduction in intrinsic inhibition in PL neurons. Electrophysiological recordings from cells expressing retrograde fluorescent tracers showed that this plasticity occurs in PL neurons projecting to the accumbens core. Collectively, these data highlight an ability of VTA terminals to gate intrinsic inhibition in the PFC, and under appropriate circumstances, enhance PL neuronal firing. These cellular actions of dopamine may be important for dopamine‐dependent behaviors involving cocaine and cue‐reward associations within cortical–striatal circuits.

Sirtuins (SIRTs) are NAD+‐dependent deacylases that play a key role in transcription, DNA repair, metabolism, and oxidative stress resistance. Increasing NAD+ availability regulates endogenous SIRT activity, leading to increased resistance to oxidative stress and decreased mitochondrial reactive oxygen production in multiple cell types and disease models. This protection, at least in part, depends on the activation of antioxidant mitochondrial proteins. We now show that increasing total NAD+ content in astrocytes leads to the activation of the transcription factor nuclear factor, erythroid‐derived 2, like 2 (Nfe212 or Nrf2) and up‐regulation of the antioxidant proteins heme oxygenase 1 (HO‐1) and sulfiredoxin 1 (SRXN1). Nrf2 activation also occurs as a result of SIRT6 overexpression. Mutations in Cu‐Zn superoxide dismutase 1 (SOD1) cause familial forms of amyotrophic lateral sclerosis (ALS). Astrocytes isolated from mutant human SOD1‐overexpressing mice induce motor neuron death in coculture. Treatment with nicotinamide mononucleotide or nicotinamide riboside increases total NAD+ content in ALS astrocytes and abrogates their toxicity toward cocultured motor neurons. The observed neuroprotection depends on SIRT6 expression in astrocytes. Moreover, overexpression of SIRT6 in astrocytes by itself abrogates the neurotoxic phenotype of ALS astrocytes. Our results identify SIRT6 as a potential therapeutic target to prevent astrocyte‐mediated motor neuron death in ALS.—Harlan, B. A., Pehar, M., Killoy, K. M., Vargas, M. R. Enhanced SIRT6 activity abrogates the neurotoxic phenotype of astrocytes expressing ALS‐linked mutant SOD1. FASEB J. 33, 7084–7091 (2019). http://www.fasebj.org

Dopamine neurons in the ventral tegmental area (VTA) influence learned behaviors and neuropsychiatric diseases including addiction. The stress peptide corticotrophin-releasing factor (CRF) contributes to relapse to drug and alcohol seeking following withdrawal, although the cellular actions are poorly understood. In this study, we show that presynaptic CRF type 1 receptors (CRF-R1) potentiate GABA release onto mouse VTA dopamine neurons via a PKC-Ca signaling mechanism. In naive animals, activation of CRF-R1 by bath application of CRF or ethanol enhanced GABA inhibitory postsynaptic currents (IPSCs). Following 3 days of withdrawal from four weekly cycles of chronic intermittent ethanol (CIE) vapor exposure, spontaneous IPSC frequency was enhanced while CRF and ethanol potentiation of IPSCs was intact. However, withdrawal for 3 weeks or more was associated with reduced spontaneous IPSC frequency and diminished CRF and ethanol responses. Long-term withdrawal was also accompanied by decreased sensitivity to the CB1 receptor agonist WIN55212 as well as greatly enhanced sensitivity to the CB1 antagonist AM251. Inclusion of BAPTA in the internal recording solution restored the responsiveness to CRF or ethanol and reduced the potentiating actions of AM251. Together, these data suggest that GABA inhibition of VTA dopamine neurons is regulated by presynaptic actions of CRF and endocannabinoids and that long-term withdrawal from CIE treatment enhances endocannabinoid-mediated inhibition, thereby suppressing CRF facilitation of GABA release. Such findings have implications for understanding the impact of chronic alcohol on stress-related, dopamine-mediated alcohol-seeking behaviors.

Fatty acid binding proteins (FABPs) are key regulators of lipid metabolism, energy homeostasis, and inflammation. They participate in fatty acid metabolism by regulating their uptake, transport, and availability of ligands to nuclear receptors. In the adult brain, FABP7 is especially abundant in astrocytes that are rich in cytoplasmic granules originated from damaged mitochondria. Mitochondrial dysfunction and oxidative stress have been implicated in the neurodegenerative process observed in amyotrophic lateral sclerosis (ALS), either as a primary cause or as a secondary component of the pathogenic process. Here we investigated the expression of FABP7 in animal models of human superoxide dismutase 1 (hSOD1)-linked ALS. In the spinal cord of symptomatic mutant hSOD1-expressing mice, FABP7 is upregulated in gray matter astrocytes. Using a coculture model, we examined the effect of increased FABP7 expression in astrocyte-motor neuron interaction. Our data show that FABP7 overexpression directly promotes an NF-κB-driven pro-inflammatory response in nontransgenic astrocytes that ultimately is detrimental for motor neuron survival. Addition of trophic factors, capable of supporting motor neuron survival in pure cultures, did not prevent motor neuron loss in cocultures with FABP7 overexpressing astrocytes. In addition, astrocyte cultures obtained from symptomatic hSOD1-expressing mice display upregulated FABP7 expression. Silencing endogenous FABP7 in these cultures decreases the expression of inflammatory markers and their toxicity toward cocultured motor neurons. Our results identify a key role of FABP7 in the regulation of the inflammatory response in astrocytes and identify FABP7 as a potential therapeutic target to prevent astrocyte-mediated motor neuron toxicity in ALS.