Pre-B-cell colony-enhancing factor exerts a neuronal protection through its enzymatic activity and the reduction of mitochondrial dysfunction in in vitro ischemic models

Possible involvement of NAMPT in neuronal survival in cerebral ischemic injury under high-glucose conditions through the FoxO3a/LC3 pathway

The incidence of cerebral infarction triggered by abnormal glucose tolerance has increased; however, the relationship between glucose concentration in the brain and the detailed mechanism of post ischemic cell death remains unclear. Nicotinamide phosphoribosyltransferase (NAMPT), an adipocytokine, is the rate-limiting enzyme for NAD+ synthesis in the salvage pathway. Although NAMPT activation prevents neuronal injury, the relationship between NAMPT activity, glucose metabolism disorders, and cerebral ischemia-induced neuronal cell death is unknown. In this study, we determined changes in NAMPT on cerebral ischemic injuries with diabetes using a db/db mouse model of type 2 diabetes and then identified the underlying mechanisms using Neuro2a cells. The expression of inflammatory cytokine mRNAs was increased in db/db and db/+ middle cerebral artery occlusion and reperfusion (MCAO/R) mice. Although NeuN-positive cells were decreased after MCAO/R, the number of NAMPT and NeuN double-positive cells in NeuN-positive neuronal cells increased in db/db MCAO/R mice. Next, the role of NAMPT in Neuro2a cells under conditions of high glucose (HGC) and oxygen-glucose deprivation (OGD), which mimics diabetes-complicated cerebral infarction, was examined. Treatment with P7C3-A20, a NAMPT activator, suppressed the decrease in cell viability caused by HGC/OGD; however, there were no significant differences in the levels of cleaved caspase-3 and Bax proteins. Moreover, increased FoxO3a and LC3-II levels after HGC/OGD were inhibited by P7C3-A20 treatment. Our findings indicate that NAMPT activation is associated with neuronal survival under ischemic conditions with abnormal glucose tolerance through the regulation of FoxO3a/LC3.

Importance of NAD+ Anabolism in Metabolic, Cardiovascular and Neurodegenerative Disorders

The role of nicotinamide adenine dinucleotide (NAD+) in ageing has emerged as a critical factor in understanding links to a wide range of chronic diseases. Depletion of NAD+, a central redox cofactor and substrate of numerous metabolic enzymes, has been detected in many major age-related diseases. However, the mechanisms behind age-associated NAD+ decline remains poorly understood. Despite limited conclusive evidence, supplements aimed at increasing NAD+ levels are becoming increasingly popular. This review provides renewed insights regarding the clinical utility and benefits of NAD+ precursors, namely nicotinamide (NAM), nicotinic acid (NA), nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN), in attenuating NAD+ decline and phenotypic characterization of age-related disorders, including metabolic, cardiovascular and neurodegenerative diseases. While it is anticipated that NAD+ precursors can play beneficial protective roles in several conditions, they vary in their ability to promote NAD+ anabolism with differing adverse effects. Careful evaluation of the role of NAD⁺, whether friend or foe in ageing, should be considered.

Metabolomic and transcriptional profiling reveals bioenergetic stress and activation of cell death and inflammatory pathways in vivo after neuronal deletion of NAMPT

Nicotinamide phosphoribosyltransferase (NAMPT) is the rate-limiting enzyme in the NAD+ salvage pathway. Our previous study demonstrated that deletion of NAMPT gene in projection neurons using Thy1-NAMPT-/- conditional knockout (cKO) mice causes neuronal degeneration, muscle atrophy, neuromuscular junction abnormalities, paralysis and eventually death. Here we conducted a combined metabolomic and transcriptional profiling study in vivo in an attempt to further investigate the mechanism of neuronal degeneration at metabolite and mRNA levels after NAMPT deletion. Here using steady-state metabolomics, we demonstrate that deletion of NAMPT causes a significant decrease of NAD+ metabolome and bioenergetics, a buildup of metabolic intermediates upstream of glyceraldehyde 3-phosphate dehydrogenase (GAPDH) in glycolysis, and an increase of oxidative stress. RNA-seq shows that NAMPT deletion leads to the increase of mRNA levels of enzymes in NAD metabolism, in particular PARP family of NAD+ consumption enzymes, as well as glycolytic genes Glut1, Hk2 and PFBFK3 before GAPDH. GO, KEGG and GSEA analyses show the activations of apoptosis, inflammation and immune responsive pathways and the inhibition of neuronal/synaptic function in the cKO mice. The current study suggests that increased oxidative stress, apoptosis and neuroinflammation contribute to neurodegeneration and mouse death as a direct consequence of bioenergetic stress after NAMPT deletion.

Lipopolysaccharide-Induced Microglial Neuroinflammation: Attenuation by FK866

Alleviating microglia-mediated neuroinflammation bears great promise to reduce neurodegeneration. Nicotinamide phosphoribosyltransferase (NAMPT) may exert cytokine-like effect in the brain. However, it remains unclear about role of NAMPT in microglial inflammation. Also, it remains unknown about effect of NAMPT inhibition on microglial inflammation. In the present study, we observed that FK866 (a specific noncompetitive NAMPT inhibitor) dose-dependently inhibited lipopolysaccharide (LPS)-induced proinflammatory mediator (interleukin (IL)-6, IL-1β, inducible nitric oxide synthase, nitric oxide and reactive species) level increase in BV2 microglia cultures. FK866 also significantly inhibited LPS-induced polarization change in microglia. Furthermore, LPS significantly increased NAMPT expression and nuclear factor kappa B (NF-κB) phosphorylation in microglia. FK866 significantly decreased NAMPT expression and NF-κB phosphorylation in LPS-treated microglia. Finally, conditioned medium from microglia cultures co-treated with FK866 and LPS significantly increased SH-SY5Y and PC12 cell viability compared with conditioned medium from microglia cultures treated with LPS alone. Our study strongly indicates that NAMPT may be a promising target for microglia modulation and NAMPT inhibition may attenuate microglial inflammation.

FK866 alleviates cerebral pyroptosis and inflammation mediated by Drp1 in a rat cardiopulmonary resuscitation model

OBJECTIVES

Dynamin-related protein 1 (Drp1) mediates mitochondrial fission and triggers NLRP3 inflammasome activation. FK866 (a NAMPT inhibitor) exerts a neuroprotective effect in ischemia/reperfusion injury through the suppression of mitochondrial dysfunction. We explored the effects of FK866 on pyroptosis and inflammation mediated by Drp1 in a cardiac arrest/cardiopulmonary resuscitation (CA/CPR) rat model.

METHODS

Healthy male Sprague-Dawley rats were subjected to 7 min CA by trans-esophageal electrical stimulation followed by CPR. The surviving rats were treated with FK866 (a selective inhibitor of NAMPT), Mdivi-1 (Drp1 inhibitor), FK866 + Mdivi-1, or vehicle and then underwent 24 h reperfusion. Hematoxylin and eosin staining and immunohistochemistry (to detect NSE) were used to evaluate brain injury. We performed immunofluorescent staining to analyze NLRP3 and GSDMD expression in microglia or astrocytes and western blot to determine expression of NLRP3, IL-1β, GSDMD, Drp1, and Mfn2. Transmission electron microscopy was used to observe mitochondria.

RESULTS

FK866 significantly decreased pathological damage to brain tissue, inhibited the activation of NLRP3 in microglia or astrocytes, downregulated the expression of NLRP3, IL-1β, GSDMD, p-Drp1 protein, upregulated Mfn2 and improve mitochondrial morphology.

CONCLUSIONS

Our results demonstrated that FK866 protects the brain against ischemia-reperfusion injury in rats after CA/CPR by inhibiting pyroptosis and inflammation mediated by Drp1.

Fasting- and ghrelin-induced food intake is regulated by NAMPT in the hypothalamus

AIM

Neurons in the arcuate nucleus of the hypothalamus are involved in regulation of food intake and energy expenditure, and dysregulation of signalling in these neurons promotes development of obesity. The role of the rate-limiting enzyme in the NAD⁺ salvage pathway, nicotinamide phosphoribosyltransferase (NAMPT), for regulation energy homeostasis by the hypothalamus has not been extensively studied.

METHODS

We determined whether Nampt mRNA or protein levels in the hypothalamus of mice were affected by diet-induced obesity, by fasting and re-feeding, and by leptin and ghrelin treatment. Primary hypothalamic neurons were treated with FK866, a selective inhibitor of NAMPT, or rAAV carrying shRNA directed against Nampt, and levels of reactive oxygen species (ROS) and mitochondrial respiration were assessed. Fasting and ghrelin-induced food intake was measured in mice in metabolic cages after intracerebroventricular (ICV)-mediated FK866 administration.

RESULTS

NAMPT levels in the hypothalamus were elevated by administration of ghrelin and leptin. In diet-induced obese mice, both protein and mRNA levels of NAMPT decreased in the hypothalamus. NAMPT inhibition in primary hypothalamic neurons significantly reduced levels of NAD⁺ , increased levels of ROS, and affected the expression of Agrp, Pomc and genes related to mitochondrial function. Finally, ICV-induced NAMPT inhibition by FK866 did not cause malaise or anhedonia, but completely ablated fasting- and ghrelin-induced increases in food intake.

CONCLUSION

Our findings indicate that regulation of NAMPT levels in hypothalamic neurons is important for the control of fasting- and ghrelin-induced food intake.

The effect of NAMPT deletion in projection neurons on the function and structure of neuromuscular junction (NMJ) in mice

Nicotinamide adenine dinucleotide (NAD⁺) plays a critical role in energy metabolism and bioenergetic homeostasis. Most NAD⁺ in mammalian cells is synthesized via the NAD⁺ salvage pathway, where nicotinamide phosphoribosyltransferase (NAMPT) is the rate-limiting enzyme, converting nicotinamide into nicotinamide mononucleotide (NMN). Using a Thy1-Nampt^−/− projection neuron conditional knockout (cKO) mouse, we studied the impact of NAMPT on synaptic vesicle cycling in the neuromuscular junction (NMJ), end-plate structure of NMJs and muscle contractility of semitendinosus muscles. Loss of NAMPT impaired synaptic vesicle endocytosis/exocytosis in the NMJs. The cKO mice also had motor endplates with significantly reduced area and thickness. When the cKO mice were treated with NMN, vesicle endocytosis/exocytosis was improved and endplate morphology was restored. Electrical stimulation induced muscle contraction was significantly impacted in the cKO mice in a frequency dependent manner. The cKO mice were unresponsive to high frequency stimulation (100 Hz), while the NMN-treated cKO mice responded similarly to the control mice. Transmission electron microscopy (TEM) revealed sarcomere misalignment and changes to mitochondrial morphology in the cKO mice, with NMN treatment restoring sarcomere alignment but not mitochondrial morphology. This study demonstrates that neuronal NAMPT is important for pre-/post-synaptic NMJ function, and maintaining skeletal muscular function and structure.

NAD+ therapy in age-related degenerative disorders: A benefit/risk analysis

Nicotinamide adenine dinucleotide (NAD+) is an essential pyridine nucleotide that is present in all living cells. NAD+ acts as an important cofactor and substrate for a multitude of biological processes including energy production, DNA repair, gene expression, calcium-dependent secondary messenger signalling and immunoregulatory roles. The de novo synthesis of NAD+ is primarily dependent on the kynurenine pathway (KP), although NAD+ can also be recycled from nicotinic acid (NA), nicotinamide (NAM) and nicotinamide riboside (NR). NAD+ levels have been reported to decline during ageing and age-related diseases. Recent studies have shown that raising intracellular NAD+ levels represents a promising therapeutic strategy for age-associated degenerative diseases in general and to extend lifespan in small animal models. A systematic review of the literature available on Medline, Embase and Pubmed was undertaken to evaluate the potential health and/or longevity benefits due to increasing NAD+ levels. A total of 1545 articles were identified and 147 articles (113 preclinical and 34 clinical) met criteria for inclusion. Most studies indicated that the NAD+ precursors NAM, NR, nicotinamide mononucleotide (NMN), and to a lesser extent NAD+ and NADH had a favourable outcome on several age-related disorders associated with the accumulation of chronic oxidative stress, inflammation and impaired mitochondrial function. While these compounds presented with a limited acute toxicity profile, evidence is still quite limited and long-term human clinical trials are still nascent in the current literature. Potential risks in raising NAD+ levels in various clinical disorders using NAD+ precursors include the accumulation of putative toxic metabolites, tumorigenesis and promotion of cellular senescence. Therefore, NAD+ metabolism represents a promising target and further studies are needed to recapitulate the preclinical benefits in human clinical trials.

Subcellular NAMPT-mediated NAD+ salvage pathways and their roles in bioenergetics and neuronal protection after ischemic injury

NAD⁺ is a cofactor required for glycolysis, tricarboxylic acid cycle, and complex I enzymatic reaction. In mammalian cells, NAD⁺ is predominantly synthesized through the salvage pathway, where nicotinamide phosphoribosyltransferase (NAMPT) is the rate-limiting enzyme. Previously, we demonstrated that NAMPT exerts a neuroprotective effect in ischemia through the suppression of mitochondrial dysfunction. Mammalian cells maintain distinct NAD⁺ pools in the cytosol, mitochondria, and nuclei. However, it is unknown whether mitochondria have an intact machinery for NAD⁺ salvage, and if so, whether it plays a dominant role in bioenergetics, mitochondrial function, and neuronal protection after ischemia. Here, using mouse primary cortical neuron and cortical tissue preparations, and multiple technologies including cytosolic and mitochondrial subfractionation, viral over-expression of transgenes, molecular biology, and confocal microscopy, we provided compelling evidence that neuronal mitochondria possess an intact machinery of NAMPT-mediated NAD⁺ salvage pathway, and that NAMPT and nicotinamide mononucleotide adenylyltransferase 3 (NMNAT3) are localized in the mitochondrial matrix. By knocking down NMNAT1-3 and NAMPT with siRNA, we found that NMNAT3 has a larger effect on basal and ATP production-related mitochondrial respiration than NMNAT1-2 in primary cultured neurons, while NMNAT1-2 have a larger effect on glycolytic flux than NMNAT3. Using an oxygen glucose deprivation model, we found that mitochondrial, cytoplasmic, and non-subcellular compartmental over-expressions of NAMPT have a comparable effect on neuronal protection and suppression of apoptosis-inducing factor translocation. The current study provides novel insights into the roles of subcellular compartmental NAD⁺ salvage pathways in NAD⁺ homeostasis, bioenergetics, and neuronal protection in ischemic conditions.

Targeting NAMPT as a therapeutic strategy against stroke

Stroke is the second and the leading most common cause of death in the world and China, respectively, but with few effective therapies. Nicotinamide phosphoribosyltransferase (NAMPT) is the rate-limiting enzyme for nicotinamide adenine dinucleotide (NAD) salvage synthesis in mammals, thereby influencing NAD-dependent enzymes and constituting a strong endogenous defence system against various stresses. Accumulating in-vitro and in-vivo studies have demonstrated the neuroprotective effect of NAMPT in stroke. Here, we review the direct evidence of NAMPT as a promising target against stroke from five potential therapeutic strategies, including NAMPT overexpression, recombinant NAMPT, NAMPT activators, NAMPT enzymatic product nicotinamide mononucleotide (NMN), and NMN precursors nicotinamide riboside and nicotinamide, and describe the relevant mechanisms and limitations, providing a promising choice for developing novel and effective therapeutic interventions against ischaemic and haemorrhagic stroke.

Nicotinamide adenine dinucleotide emerges as a therapeutic target in aging and ischemic conditions

Nicotinamide adenine dinucleotide (NAD⁺) has been described as central coenzyme of redox reactions and is a key regulator of stress resistance and longevity. Aging is a multifactorial and irreversible process that is characterized by a gradual diminution in physiological functions in an organism over time, leading to development of age-associated pathologies and eventually increasing the probability of death. Ischemia is the lack of nutritive blood flow that causes damage and mortality that mostly occurs in various organs during aging. During the process of aging and related ischemic conditions, NAD⁺ levels decline and lead to nuclear and mitochondrial dysfunctions, resulting in age-related pathologies. The majority of studies have shown that restoring of NAD⁺ using supplementation with intermediates such as nicotinamide mononucleotide and nicotinamide riboside can be a valuable strategy for recovery of ischemic injury and age-associated defects. This review summarizes the molecular mechanisms responsible for the reduction in NAD⁺ levels during ischemic disorders and aging, as well as a particular focus is given to the recent progress in the understanding of NAD⁺ precursor's effects on aging and ischemia.

Deletion of Nampt in Projection Neurons of Adult Mice Leads to Motor Dysfunction, Neurodegeneration, and Death

Intracellular nicotinamide phosphoribosyltransferase (iNAMPT) is the rate-limiting enzyme of the mammalian NAD⁺ biosynthesis salvage pathway. Using inducible and conditional knockout (cKO) mice, we show that Nampt gene deletion in adult projection neurons leads to a progressive loss of body weight, hypothermia, motor neuron (MN) degeneration, motor function deficits, paralysis, and death. Nampt deletion causes mitochondrial dysfunction, muscle fiber type conversion, and atrophy, as well as defective synaptic function at neuromuscular junctions (NMJs). When treated with nicotinamide mononucleotide (NMN), Nampt cKO mice exhibit reduced motor function deficits and prolonged lifespan. iNAMPT protein levels are significantly reduced in the spinal cord of amyotrophic lateral sclerosis (ALS) patients, indicating the involvement of NAMPT in ALS pathology. Our findings reveal that neuronal NAMPT plays an essential role in mitochondrial bioenergetics, motor function, and survival. Our study suggests that the NAMPT-mediated NAD⁺ biosynthesis pathway is a potential therapeutic target for degenerative MN diseases.

Effects of intermedin on autophagy in cerebral ischemia/reperfusion injury

OBJECTIVE

This study aimed to evaluate the effects of intermedin (IMD) on autophagy in cerebral ischemia/reperfusion (I/R) injury (CIRI).

METHODS

Sixty rats were randomly averaged into four groups: sham, ischemia/reperfusion (I/R), IMD, and 3-methyladenine (3-MA). In the sham group, the right common carotid artery, external carotid artery, and internal carotid artery were detached, and no monofilament was inserted. In the other groups, two hours after cerebral ischemia, the rats were injected through the lateral ventricle with normal saline for I/R group, IMD for the IMD group, and 3-MA for the 3-MA group for 24h. The cerebral injury was assessed by evaluation of neurological function, hematoxylin and eosin (H&E) staining, and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining. The expressions of autophagy associated proteins, such as microtubule-associated protein 1 light chain 3 (LC3), Beclin1, and sequestosome 1 (P62) were analyzed using immunohistochemistry staining and western blot. Meanwhile, transmission electron microscopy was used to investigate the ultrastructure of the brains.

RESULTS

IMD could reduce neuron cell damage and infarction formation and has a protective effect against CIRI as 3-MA. The levels of LC3II/LC3I and Beclin1 were significantly decreased and the P62 level was significantly higher in the IMD group compared with I/R group, which is similar to the effect of 3-MA on CIRI.

CONCLUSIONS

IMD has a similar effect as 3-MA, can reduce pathological neuronal injury and protect the brain against CIRI in rats by attenuating the effects of autophagy. Our findings provide evidence for IMD's protective effects in relation to ischemic cerebrovascular diseases.

Beneficial Effects of Delayed P7C3-A20 Treatment After Transient MCAO in Rats

Despite ischemic stroke being the fifth leading cause of death in the USA, there are few therapeutic options available. We recently showed that the neuroprotective compound P7C3-A20 reduced brain atrophy, increased neurogenesis, and improved functional recovery when treatment was initiated immediately post-reperfusion after a 90-min middle cerebral artery occlusion (MCAO). In the present study, we investigated a more clinically relevant therapeutic window for P7C3-A20 treatment after ischemic stroke. MCAO rats were administered P7C3-A20 for 1 week, beginning immediately or at a delayed point, 6 h post-reperfusion. Delayed P7C3-A20 treatment significantly improved stroke-induced sensorimotor deficits in motor coordination and symmetry, as well as cognitive deficits in hippocampal-dependent spatial learning, memory retention, and working memory. In the cerebral cortex, delayed P7C3-A20 treatment significantly increased tissue sparing 7 weeks after stroke and reduced hemispheric infarct volumes 48 h after reperfusion. Despite no reduction in striatal infarct volumes acutely, there was a significant increase in spared tissue volume chronically. In the hippocampus, only immediately treated P7C3-A20 animals had a significant increase in tissue sparing compared to vehicle-treated stroke animals. This structural protection translated into minimal hippocampal-dependent behavioral improvements with delayed P7C3-A20 treatment. However, all rats treated with delayed P7C3-A20 demonstrated a significant improvement in both sensorimotor tasks compared to vehicle controls, suggesting a somatosensory-driven recovery. These results demonstrate that P7C3-A20 improves chronic functional and histopathological outcomes after ischemic stroke with an extended therapeutic window.

Nicotinamide riboside, a form of vitamin B3, protects against excitotoxicity-induced axonal degeneration

NAD⁺ depletion is a common phenomenon in neurodegenerative pathologies. Excitotoxicity occurs in multiple neurologic disorders and NAD⁺ was shown to prevent neuronal degeneration in this process through mechanisms that remained to be determined. The activity of nicotinamide riboside (NR) in neuroprotective models and the recent description of extracellular conversion of NAD⁺ to NR prompted us to probe the effects of NAD⁺ and NR in protection against excitotoxicity. Here, we show that intracortical administration of NR but not NAD⁺ reduces brain damage induced by NMDA injection. Using cortical neurons, we found that provision of extracellular NR delays NMDA-induced axonal degeneration (AxD) much more strongly than extracellular NAD⁺ Moreover, the stronger effect of NR compared to NAD⁺ depends of axonal stress since in AxD induced by pharmacological inhibition of nicotinamide salvage, both NAD⁺ and NR prevent neuronal death and AxD in a manner that depends on internalization of NR. Taken together, our findings demonstrate that NR is a better neuroprotective agent than NAD⁺ in excitotoxicity-induced AxD and that axonal protection involves defending intracellular NAD⁺ homeostasis.-Vaur, P., Brugg, B., Mericskay, M., Li, Z., Schmidt, M. S., Vivien, D., Orset, C., Jacotot, E., Brenner, C., Duplus, E. Nicotinamide riboside, a form of vitamin B₃, protects against excitotoxicity-induced axonal degeneration.

Pre-B-cell colony-enhancing factor protects against apoptotic neuronal death and mitochondrial damage in ischemia

We previously demonstrated that Pre-B-cell colony-enhancing factor (PBEF), also known as nicotinamide phosphoribosyltransferase (NAMPT), the rate-limiting enzyme in mammalian NAD⁺ biosynthesis pathway, plays a brain and neuronal protective role in ischemic stroke. In this study, we further investigated the mechanism of its neuroprotective effect after ischemia in the primary cultured mouse cortical neurons. Using apoptotic cell death assay, fluorescent imaging, molecular biology, mitochondrial biogenesis measurements and Western blotting analysis, our results show that the overexpression of PBEF in neurons can significantly promote neuronal survival, reduce the translocation of apoptosis inducing factor (AIF) from mitochondria to nuclei and inhibit the activation of capase-3 after glutamate-induced excitotoxicity. We further found that the overexpression of PBEF can suppress glutamate-induced mitochondrial fragmentation, the loss of mitochondrial DNA (mtDNA) content and the reduction of PGC-1 and NRF-1 expressions. Furthermore, these beneficial effects by PBEF are dependent on its enzymatic activity of NAD⁺ synthesis. In summary, our study demonstrated that PBEF ameliorates ischemia-induced neuronal death through inhibiting caspase-dependent and independent apoptotic signaling pathways and suppressing mitochondrial damage and dysfunction. Our study provides novel insights into the mechanisms underlying the neuroprotective effect of PBEF, and helps to identify potential targets for ischemic stroke therapy.

The Role of Nicotinamide Phosphoribosyltransferase in Cerebral Ischemia

As recombinant tissue plasminogen activator is the only drug approved for the clinical treatment of acute ischemic stroke, there is an urgent unmet need for novel stroke treatments. Endogenous defense mechanisms against stroke may hold the key to new therapies for stroke. A large number of studies suggest that nicotinamide phosphoribosyl-transferase (NAMPT is an attractive candidate to improve post-stroke recovery. NAMPT is a multifunctional protein and plays important roles in immunity, metabolism, aging, inflammation, and stress responses. NAMPT exists in both the intracellular and extracellular space. As a rate-limiting enzyme, the intracellular form (iNAMPT catalyzes the first step in the biosynthesis of nicotinamide adenine dinucleotide (NAD from nicotinamide. iNAMPT closely regulates energy metabolism, enhancing the proliferation of endothelial cells, inhibiting apoptosis, regulating vascular tone, and stimulating autophagy in disease conditions such as stroke. Extracellular NAMPT (eNAMPT is also known as visfatin (visceral fat-derived adipokine and has pleotropic effects. It is widely believed that the diverse biological functions of eNAMPT are attributed to its NAMPT enzymatic activity. However, the effects of eNAMPT on ischemic injury are still controversial. Some authors have argued that eNAMPT exacerbates ischemic neuronal injury non-enzymatically by triggering the release of TNF-α from glial cells. In addition, NAMPT also participates in several pathophysiological processes such as hypertension, atherosclerosis, and ischemic heart disease. Thus, it remains unclear under what conditions NAMPT is beneficial or destructive. Recent work using in vitro and in vivo genetic/ pharmacologic manipulations, including our own studies, has greatly improved our understanding of NAMPT. This review focuses on the multifaceted and complex roles of NAMPT under both normal and ischemic conditions.

Targeting Nicotinamide Phosphoribosyltransferase as a Potential Therapeutic Strategy to Restore Adult Neurogenesis

Adult neurogenesis is the process of generating new neurons throughout life in the olfactory bulb and hippocampus of most mammalian species, which is closely related to aging and disease. Nicotinamide phosphoribosyltransferase (NAMPT), also an adipokine known as visfatin, is the rate-limiting enzyme for mammalian nicotinamide adenine dinucleotide (NAD) salvage synthesis by generating nicotinamide mononucleotide (NMN) from nicotinamide. Recent findings from our laboratory and other laboratories have provided much evidence that NAMPT might serve as a therapeutic target to restore adult neurogenesis. NAMPT-mediated NAD biosynthesis in neural stem/progenitor cells is important for their proliferation, self-renewal, and formation of oligodendrocytes in vivo and in vitro. Therapeutic interventions by the administration of NMN, NAD, or recombinant NAMPT are effective for restoring adult neurogenesis in several neurological diseases. We summarize adult neurogenesis in aging, ischemic stroke, traumatic brain injury, and neurodegenerative disease and review the advances of targeting NAMPT in restoring neurogenesis. Specifically, we provide emphasis on the P7C3 family, a class of proneurogenic compounds that are potential NAMPT activators, which might shed light on future drug development in neurogenesis restoration.

Analysis of the nicotinamide phosphoribosyltransferase family provides insight into vertebrate adaptation to different oxygen levels during the water-to-land transition

One of the most important events in vertebrate evolutionary history is the water-to-land transition, during which some morphological and physiological changes occurred in concert with the loss of specific genes in tetrapods. However, the molecular mechanisms underlying this transition have not been well explored. To explore vertebrate adaptation to different oxygen levels during the water-to-land transition, we performed comprehensive bioinformatics and experimental analysis aiming to investigate the NAMPT family in vertebrates. NAMPT, a rate-limiting enzyme in the salvage pathway of NAD+ biosynthesis, is critical for cell survival in a hypoxic environment, and a high level of NAMPT significantly augments oxidative stress in normoxic environments. Phylogenetic analysis showed that NAMPT duplicates arose from a second round whole-genome duplication event. NAMPTA existed in all classes of vertebrates, whereas NAMPTB was only found in fishes and not tetrapods. Asymmetric evolutionary rates and purifying selection were the main evolutionary forces involved. Although functional analysis identified several functionally divergent sites during NAMPT family evolution, in vitro experimental data demonstrated that NAMPTA and NAMPTB were functionally conserved for NAMPT enzymatic function in the NAD+ salvage pathway. In situ hybridization revealed broad NAMPTA and NAMPTB expression patterns, implying regulatory functions over a wide range of developmental processes. The morpholino-mediated knockdown data demonstrated that NAMPTA was more essential than NAMPTB for vertebrate embryo development. We propose that the retention of NAMPTB in water-breathing fishes and its loss in air-breathing tetrapods resulted from vertebrate adaptation to different oxygen levels during the water-to-land transition.

Nampt/PBEF/visfatin serum levels: a new biomarker for retinal blood vessel occlusions

The main objective of the study was to quantify serum levels of nicotinamide phosphoribosyltransferase (Nampt/pre-B-Cell colony-enhancing factor 1/visfatin) in subjects with a history of retinal vascular occlusions (RVOs), disease conditions characterized by pronounced ischemia, and metabolic energy deficits. A case-control study of 18 subjects with a history of RVO as well as six healthy volunteers is presented. Serum Nampt levels were quantified using a commercially available enzyme-linked immunosorbent assay kit. Serum Nampt levels were 79% lower in patients with a history of RVO compared with that in healthy volunteers (P<0.05). There was no statistically significant difference among the types of RVOs, specifically branch retinal vein occlusions (n=7), central retinal vein occlusions (n=5), hemiretinal vein occlusions (n=3), and central retinal artery occlusions (n=3; P=0.69). Further studies are needed to establish the temporal kinetics of Nampt expression and to determine whether Nampt may represent a novel biomarker to identify at-risk populations, or whether it is a druggable target with the potential to ameliorate the long-term complications associated with the condition, ie, macular edema, macular ischemia, neovascularization, and permanent loss of vision.

Signaling pathways leading to ischemic mitochondrial neuroprotection

There is extensive evidence that ischemic/reperfusion mediated mitochondrial dysfunction is a major contributor to ischemic damage. However data also indicates that mild ischemic stress induces mitochondrial dependent activation of ischemic preconditioning. Ischemic preconditioning is a neuroprotective mechanism which is activated upon a brief sub-injurious ischemic exposure and is sufficient to provide protection against a subsequent lethal ischemic insult. Current research demonstrates that mitochondria are not only the inducers of but are also an important target of ischemic preconditioning mediated protection. Numerous proteins and signaling pathways are activated by ischemic preconditioning which protect the mitochondria against ischemic damage. In this review we examine some of the proteins activated by ischemic precondition which counteracts the deleterious effects of ischemia/reperfusion thereby maintaining normal mitochondrial activity and lead to ischemic tolerance.

Regulation of NAD biosynthetic enzymes modulates NAD-sensing processes to shape mammalian cell physiology under varying biological cues

In addition to its role as a redox coenzyme, NAD is a substrate of various enzymes that split the molecule to either catalyze covalent modifications of target proteins or convert NAD into biologically active metabolites. The coenzyme bioavailability may be significantly affected by these reactions, with ensuing major impact on energy metabolism, cell survival, and aging. Moreover, through the activity of the NAD-dependent deacetylating sirtuins, NAD behaves as a beacon molecule that reports the cell metabolic state, and accordingly modulates transcriptional responses and metabolic adaptations. In this view, NAD biosynthesis emerges as a highly regulated process: it enables cells to preserve NAD homeostasis in response to significant NAD-consuming events and it can be modulated by various stimuli to induce, via NAD level changes, suitable NAD-mediated metabolic responses. Here we review the current knowledge on the regulation of mammalian NAD biosynthesis, with focus on the relevant rate-limiting enzymes. This article is part of a Special Issue entitled: Cofactor-dependent proteins: evolution, chemical diversity and bio-applications.

NAMPT inhibitor and metabolite protect mouse brain from cryoinjury through distinct mechanisms

Nicotinamide phosphoribosyltransferase (NAMPT) is the key enzyme in the biosynthesis of nicotinamide adenine dinucleotide (NAD). In the brain, NAMPT is primarily expressed in neurons and can prevent neuronal degeneration. NAMPT is also highly expressed in inflammatory cells, and is responsible for their activation. Since inflammation following traumatic brain injury enhances neuronal damage, we assessed the effects of nicotinamide mononucleotide (NMN), the direct NAMPT metabolite, and FK866, a potent NAMPT inhibitor, on brain injury in a cryoinjury mouse model. Twenty-four hours after brain cryoinjury, the density of neuron and the level of NAD decreased. Both NMN and FK866 alleviated the neuronal loss and decreased the lesion volume. NMN prevented the cryoinjury-induced decrease of NAD level, and FK866 decreased it further. On day 14 after cryoinjury, further neuronal loss occurred, astrocytes and Iba1-positive macrophage/microglia activated, and the NAD level increased. At this time-point, NAMPT expression was strongly induced in Iba1-positive macrophages/microglia in the lesion core. NMN and FK866 also alleviated the neuronal loss and decreased the lesion volume. In addition, FK866 significantly attenuated the activation of astrocytes and Iba1-positive macrophages/microglia, and decreased the NAD, while NMN had no such effects. Taken together, both FK866 and NMN attenuate traumatic brain injury. However, FK866 acts via the inhibition of the NAMPT activity in inflammatory cells resulting in the inhibition of inflammation, whereas NMN is effective via replenishing NAD.

The effects of NAD+ on apoptotic neuronal death and mitochondrial biogenesis and function after glutamate excitotoxicity

NAD+ is an essential co-enzyme for cellular energy metabolism and is also involved as a substrate for many cellular enzymatic reactions. It has been shown that NAD+ has a beneficial effect on neuronal survival and brain injury in in vitro and in vivo ischemic models. However, the effect of NAD+ on mitochondrial biogenesis and function in ischemia has not been well investigated. In the present study, we used an in vitro glutamate excitotoxicity model of primary cultured cortical neurons to study the effect of NAD+ on apoptotic neuronal death and mitochondrial biogenesis and function. Our results show that supplementation of NAD+ could effectively reduce apoptotic neuronal death, and apoptotic inducing factor translocation after neurons were challenged with excitotoxic glutamate stimulation. Using different approaches including confocal imaging, mitochondrial DNA measurement and Western blot analysis of PGC-1 and NRF-1, we also found that NAD+ could significantly attenuate glutamate-induced mitochondrial fragmentation and the impairment of mitochondrial biogenesis. Furthermore, NAD+ treatment effectively inhibited mitochondrial membrane potential depolarization and NADH redistribution after excitotoxic glutamate stimulation. Taken together, our results demonstrated that NAD+ is capable of inhibiting apoptotic neuronal death after glutamate excitotoxicity via preserving mitochondrial biogenesis and integrity. Our findings provide insights into potential neuroprotective strategies in ischemic stroke.

A rise in NAD precursor nicotinamide mononucleotide (NMN) after injury promotes axon degeneration

NAD metabolism regulates diverse biological processes, including ageing, circadian rhythm and axon survival. Axons depend on the activity of the central enzyme in NAD biosynthesis, nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2), for their maintenance and degenerate rapidly when this activity is lost. However, whether axon survival is regulated by the supply of NAD or by another action of this enzyme remains unclear. Here we show that the nucleotide precursor of NAD, nicotinamide mononucleotide (NMN), accumulates after nerve injury and promotes axon degeneration. Inhibitors of NMN-synthesising enzyme NAMPT confer robust morphological and functional protection of injured axons and synapses despite lowering NAD. Exogenous NMN abolishes this protection, suggesting that NMN accumulation within axons after NMNAT2 degradation could promote degeneration. Ectopic expression of NMN deamidase, a bacterial NMN-scavenging enzyme, prolongs survival of injured axons, providing genetic evidence to support such a mechanism. NMN rises prior to degeneration and both the NAMPT inhibitor FK866 and the axon protective protein Wld^S prevent this rise. These data indicate that the mechanism by which NMNAT and the related Wld^S protein promote axon survival is by limiting NMN accumulation. They indicate a novel physiological function for NMN in mammals and reveal an unexpected link between new strategies for cancer chemotherapy and the treatment of axonopathies.

Imaging of mitochondrial Ca2+ dynamics in astrocytes using cell-specific mitochondria-targeted GCaMP5G/6s: mitochondrial Ca2+ uptake and cytosolic Ca2+ availability via the endoplasmic reticulum store

Mitochondrial Ca(2+) plays a critical physiological role in cellular energy metabolism and signaling, and its overload contributes to various pathological conditions including neuronal apoptotic death in neurological diseases. Live cell mitochondrial Ca(2+) imaging is an important approach to understand mitochondrial Ca(2+) dynamics. Recently developed GCaMP genetically-encoded Ca(2+) indicators provide unique opportunity for high sensitivity/resolution and cell type-specific mitochondrial Ca(2+) imaging. In the current study, we implemented cell-specific mitochondrial targeting of GCaMP5G/6s (mito-GCaMP5G/6s) and used two-photon microscopy to image astrocytic and neuronal mitochondrial Ca(2+) dynamics in culture, revealing Ca(2+) uptake mechanism by these organelles in response to cell stimulation. Using these mitochondrial Ca(2+) indicators, our results show that mitochondrial Ca(2+) uptake in individual mitochondria in cultured astrocytes and neurons can be seen after stimulations by ATP and glutamate, respectively. We further studied the dependence of mitochondrial Ca(2+) dynamics on cytosolic Ca(2+) changes following ATP stimulation in cultured astrocytes by simultaneously imaging mitochondrial and cytosolic Ca(2+) increase using mito-GCaMP5G and a synthetic organic Ca(2+) indicator, x-Rhod-1, respectively. Combined with molecular intervention in Ca(2+) signaling pathway, our results demonstrated that the mitochondrial Ca(2+) uptake is tightly coupled with inositol 1,4,5-trisphosphate receptor-mediated Ca(2+) release from the endoplasmic reticulum and the activation of G protein-coupled receptors. The current study provides a novel approach to image mitochondrial Ca(2+) dynamics as well as Ca(2+) interplay between the endoplasmic reticulum and mitochondria, which is relevant for neuronal and astrocytic functions in health and disease.

Protein kinase C epsilon regulates mitochondrial pools of Nampt and NAD following resveratrol and ischemic preconditioning in the rat cortex

Preserving mitochondrial pools of nicotinamide adenine dinucleotide (NAD) or nicotinamide phosphoribosyltransferase (Nampt), an enzyme involved in NAD production, maintains mitochondrial function and confers neuroprotection after ischemic stress. However, the mechanisms involved in regulating mitochondrial-localized Nampt or NAD have not been defined. In this study, we investigated the roles of protein kinase C epsilon (PKCɛ) and AMP-activated protein kinase (AMPK) in regulating mitochondrial pools of Nampt and NAD after resveratrol or ischemic preconditioning (IPC) in the cortex and in primary neuronal-glial cortical cultures. Using the specific PKCɛ agonist ψɛRACK, we found that PKCɛ induced robust activation of AMPK in vitro and in vivo and that AMPK was required for PKCɛ-mediated ischemic neuroprotection. In purified mitochondrial fractions, PKCɛ enhanced Nampt levels in an AMPK-dependent manner and was required for increased mitochondrial Nampt after IPC or resveratrol treatment. Analysis of intrinsic NAD autofluorescence using two-photon microscopy revealed that PKCɛ modulated NAD in the mitochondrial fraction. Further assessments of mitochondrial NAD concentrations showed that PKCɛ has a key role in regulating the mitochondrial NAD(+)/nicotinamide adenine dinucleotide reduced (NADH) ratio after IPC and resveratrol treatment in an AMPK- and Nampt-dependent manner. These findings indicate that PKCɛ is critical to increase or maintain mitochondrial Nampt and NAD after pathways of ischemic neuroprotection in the brain.

Extracellular visfatin has nicotinamide phosphoribosyltransferase enzymatic activity and is neuroprotective against ischemic injury

AIM

Visfatin, a novel adipokine, is predominantly produced by visceral adipose tissue and exists in intracellular and extracellular compartments. The intracellular form of visfatin is proved to be nicotinamide phosphoribosyltransferase (NAMPT) and exhibits neuroprotection through maintaining intracellular NAD(+) pool. However, whether extracellular form of visfatin has NAMPT activity and the effect of extracellular visfatin in cerebral ischemia are unknown.

METHODS AND RESULTS

Plasma concentrations of visfatin, NAD(+) , and ATP were increased in mice upon cerebral ischemia. Cultured glia, but not neuron, was able to secrete visfatin. Oxygen-glucose deprivation (OGD) stress increased the secretion of visfatin from glia. Extracellular recombinant mouse wild-type visfatin, but not mouse H247A-mutant enzymatic-dead visfatin, had NAMPT enzymatic function in vitro. Treatment of wild-type visfatin, but not H247A-mutant enzymatic-dead visfatin, significantly attenuated detrimental effect of OGD on the cell viability and apoptosis in both cultured mouse neuron and glia. Treatment of neutralizing antibody, abolished the protective effect of extracellular visfatin on cell viability, but failed to block the antiapoptotic effect of extracellular visfatin. At last, we observed that plasma visfatin concentrations decreased in 6-month-old but not 3-month-old SHR-SP compared with that in age-matched Wistar-Kyoto rats. Inhibition of NAMPT enzymatic function of visfatin (by FK866) accelerated the occurrence of stroke in SHR-SP.

CONCLUSIONS

Extracellular visfatin has NAMPT enzymatic activity and maybe be neuroprotective just as intracellular visfatin in cerebral ischemic injury.

Resistin and visfatin in steatotic and non-steatotic livers in the setting of partial hepatectomy under ischemia-reperfusion

BACKGROUND & AIMS

This study examined whether the regulation of resistin and visfatin could reduce damage and improve regeneration in both steatotic and non-steatotic livers undergoing partial hepatectomy under ischemia-reperfusion, a procedure commonly applied in clinical practice to reduce bleeding.

METHODS

Resistin and visfatin were pharmacologically modulated in lean and obese animals undergoing partial hepatectomy under ischemia-reperfusion.

RESULTS

No evident role for these adipocytokines was observed in non-steatotic livers. However, obese animals undergoing liver surgery showed increased resistin in liver and plasma, without changes in adipose tissue, together with visfatin downregulation in liver and increment in plasma and adipose tissue. Endogenous resistin maintains low levels of visfatin in the liver by blocking its hepatic uptake from the circulation, thus regulating the visfatin detrimental effects on hepatic damage and regenerative failure. Indeed, the administration of anti-resistin antibodies increased hepatic accumulation of adipocyte-derived visfatin, exacerbating damage and regenerative failure. Interestingly, treatment with anti-visfatin antibodies protected steatotic livers, and similar results were obtained with the concomitant inhibition of resistin and visfatin. Thus, when visfatin was inhibited, the injurious effects of anti-resistin antibodies disappeared. Herein we show that upregulation of visfatin increased NAD levels in the remnant steatotic liver, whereas visfatin inhibition decreased them. These later observations suggest that visfatin may favour synthesis of NAD instead of DNA and induces alterations in amino acid metabolism-urea cycle and NO production, overall negatively affecting liver viability.

CONCLUSIONS

Our results indicate the clinical potential of visfatin blocking-based therapies in steatotic livers undergoing partial hepatectomy with ischemia-reperfusion.

Cerebral ischemia is exacerbated by extracellular nicotinamide phosphoribosyltransferase via a non-enzymatic mechanism

Intracellular nicotinamide phosphoribosyltransferase (iNAMPT) in neuron has been known as a protective factor against cerebral ischemia through its enzymatic activity, but the role of central extracellular NAMPT (eNAMPT) is not clear. Here we show that eNAMPT protein level was elevated in the ischemic rat brain after middle-cerebral-artery occlusion (MCAO) and reperfusion, which can be traced to at least in part from blood circulation. Administration of recombinant NAMPT protein exacerbated MCAO-induced neuronal injury in rat brain, while exacerbated oxygen-glucose-deprivation (OGD) induced neuronal injury only in neuron-glial mixed culture, but not in neuron culture. In the mixed culture, NAMPT protein promoted TNF-α release in a time- and concentration-dependent fashion, while TNF-α neutralizing antibody protected OGD-induced, NAMPT-enhanced neuronal injury. Importantly, H247A mutant of NAMPT with essentially no enzymatic activity exerted similar effects on ischemic neuronal injury and TNF-α release as the wild type protein. Thus, eNAMPT is an injurious and inflammatory factor in cerebral ischemia and aggravates ischemic neuronal injury by triggering TNF-α release from glia cells, via a mechanism not related to NAMPT enzymatic activity.

Change in plasma visfatin level after severe traumatic brain injury

Higher plasma visfatin concentration has been associated with ischemic stroke. Thus, we sought to investigate change in plasma visfatin level after traumatic brain injury and to evaluate its relation with disease outcome. Seventy-six healthy controls and 98 patients with acute severe traumatic brain injury were recruited. Twenty-seven patients (27.6%) died and 48 patients (49.0%) suffered from unfavorable outcome (Glasgow outcome scale score of 1-3) in 6 months. On admission, plasma visfatin level was increased in patients than in healthy controls and was highly correlated with Glasgow Coma Scale score. A multivariate analysis identified plasma visfatin level as an independent predictor for 6-month mortality and unfavorable outcome. According to receiver operating characteristic curve analysis, the predictive value of the plasma visfatin concentration was similar to Glasgow Coma Scale score's. In a combined logistic-regression model, visfatin did not improve the predictive value of Glasgow Coma Scale score. Thus, increased plasma visfatin level is associated with 6-month clinical outcomes after severe traumatic brain injury.

Nicotinamide phosphoribosyltransferase may be involved in age-related brain diseases

Nicotinamide phosphoribosyltransferase (NAMPT) is a key enzyme for nicotinamide adenine dinucleotide (NAD) biosynthesis, and can be found either intracellularly (iNAMPT) or extracellularly (eNAMPT). Studies have shown that both iNAMPT and eNAMPT are implicated in aging and age-related diseases/disorders in the peripheral system. However, their functional roles in aged brain remain to be established. Here we showed that upon aging, NAMPT level increased in serum but decreased in brain, decreased in cortex and hippocampus but remained unchanged in cerebellum and striatum in brain, and increased in microglia but likely decreased in neuron. Accordingly, total NAD (tNAD) level significantly decreased in hippocampus, cerebellum and striatum in aged brain. Application of recombinant NAMPT, mimicking the elevated serum NAMPT level, enhanced the susceptibility of cerebral endothelial cells to ischemic injury, while inhibition of iNAMPT by FK866, a specific inhibitor, reduced intracellular NAD level and induced neuronal death. Taken together, we have revealed a region- and cell-specific change of NAMPT level in brain and serum upon aging, deduced its potential consequences, which suggests that NAMPT is a regulatory factor in aging and age-related brain diseases.

Pleiotropic functions of pre-B-cell colony-enhancing factor (PBEF) revealed by transcriptomics of human pulmonary microvascular endothelial cells treated with PBEFsiRNA

This study profiled transcriptomes of human pulmonary microvascular endothelial cells (HMVEC-L) treated with pre-B-cell colony-enhancing factor (PBEF) siRNA or scrambled RNA to gain insight into transcriptional regulations of PBEF on the endothelial function using the Affymetrix GeneChips HG-U133 plus 2. Several important themes are emerged from this study. First, PBEF affected expressions of multiple genes in the endothelium. Expression of 373 genes was increased and 64 genes decreased by at least 1.3-fold in the PBEFsiRNA-treated HMVEC-L versus the scramble RNA control. Second, the microarray results confirmed previous reports of PBEF-mediated gene expressions in some pathways but provided a more complete repertoire of molecules in those pathways. Third, most of the affected canonical pathways have not previously been reported to be PBEF responsive. Fourth, network analysis supports that PBEF has pleiotropic functions. Our first transcriptome analysis of human pulmonary microvascular endothelial cells treated with PBEFsiRNA has provided important insights into the transcriptional regulation of gene expression in HMVEC-L cells by PBEF. Further in-depth analysis of these transcriptional regulations may shed light on molecular mechanisms underlying PBEF-mediated endothelial functions and dysfunctions in various diseases and provide new leads of therapeutic targets to those diseases.

Visfatin/NAMPT: a multifaceted molecule with diverse roles in physiology and pathophysiology

Visfatin/NAMPT (nicotinamide phosphoribosyltransferase) is a protein with several suggested functions. Although the first discovery of this molecule as a pre-B-cell colony-enhancing factor suggested primarily a cytokine function, its rediscovery as the key enzyme in nicotinamide adenine dinucleotide generation has considerably widened its potential biological activities. Although originally thought to be produced in adipose tissue (i.e., adipocytes and infiltrating macrophages), its production seems to involve other cells and tissues such as skeletal muscle, liver, immune cells, cardiomyocytes, and the brain. Visfatin/NAMPT has both intracellular and extracellular effects influencing several signaling pathways. Its broad spectrum of effects is mirrored by its potential involvement in a wide range of disorders including human immunodeficiency virus infection, septicemia, myocardial failure, atherosclerosis, metabolic disorders, inflammatory diseases, malignancies, and neurodegenerative disorders and aging. Moreover, studies on visfatin/NAMPT in atherosclerotic disorders suggest a rather complex role of this molecule in pathophysiology, potentially mediating both adaptive and maladaptive responses.