The pyridine nucleotides NAD and NADP play vital roles in metabolic conversions as signal transducers and in cellular defence systems. Both coenzymes participate as electron carriers in energy transduction and biosynthetic processes. Their oxidized forms, NAD + and NADP + , have been identified as important elements of regulatory pathways. In particular, NAD + serves as a substrate for ADP-ribosylation reactions and for the Sir2 family of NAD + -dependent protein deacetylases as well as a precursor of the calcium mobilizing molecule cADPr (cyclic ADP-ribose). The conversions of NADP + into the 2 -phosphorylated form of cADPr or to its nicotinic acid derivative, NAADP, also result in the formation of potent intracellular calcium-signalling agents. Perhaps, the most critical function of NADP is in the maintenance of a pool of reducing equivalents which is essential to counteract oxidative damage and for other detoxifying reactions. It is well known that the NADPH/NADP + ratio is usually kept high, in favour of the reduced form. Research within the past few years has revealed important insights into how the NADPH pool is generated and maintained in different subcellular compartments. Moreover, tremendous progress in the molecular characterization of NAD kinases has established these enzymes as vital factors for cell survival. In the present review, we summarize recent advances in the understanding of the biosynthesis and signalling functions of NAD(P) and highlight the new insights into the molecular mechanisms of NADPH generation and their roles in cell physiology.

NAD is a vital molecule in all organisms. It is a key component of both energy and signal transduction--processes that undergo crucial changes in cancer cells. NAD(+)-dependent signalling pathways are many and varied, and they regulate fundamental events such as transcription, DNA repair, cell cycle progression, apoptosis and metabolism. Many of these processes have been linked to cancer development. Given that NAD(+)-dependent signalling reactions involve the degradation of the molecule, permanent nucleotide resynthesis through different biosynthetic pathways is crucial for incessant cancer cell proliferation. This necessity supports the targeting of NAD metabolism as a new therapeutic concept for cancer treatment.

Increased somatic mitochondrial DNA (mtDNA) mutagenesis causes premature aging in mice, and mtDNA damage accumulates in the human brain with aging and neurodegenerative disorders such as Parkinson disease (PD). Here, we study the complete spectrum of mtDNA changes, including deletions, copy-number variation and point mutations, in single neurons from the dopaminergic substantia nigra and other brain areas of individuals with Parkinson disease and neurologically healthy controls. We show that in dopaminergic substantia nigra neurons of healthy individuals, mtDNA copy number increases with age, maintaining the pool of wild-type mtDNA population in spite of accumulating deletions. This upregulation fails to occur in individuals with Parkinson disease, however, resulting in depletion of the wild-type mtDNA population. By contrast, neuronal mtDNA point mutational load is not increased in Parkinson disease. Our findings suggest that dysregulation of mtDNA homeostasis is a key process in the pathogenesis of neuronal loss in Parkinson disease.

NAD is a vital redox carrier, and its degradation is a key element of important regulatory pathways. NAD-mediated functions are compartmentalized and have to be fueled by specific biosynthetic routes. However, little is known about the different pathways, their subcellular distribution, and regulation in human cells. In particular, the route(s) to generate mitochondrial NAD, the largest subcellular pool, is still unknown. To visualize organellar NAD changes in cells, we targeted poly-(ADP-ribose) polymerase activity into the mitochondrial matrix. This activity synthesized immunodetectable poly(ADPribose) depending on mitochondrial NAD availability. Based on this novel detector system, detailed subcellular enzyme localizations, and pharmacological inhibitors, we identified extracellular NAD precursors, their cytosolic conversions, and the pathway of mitochondrial NAD generation. Our results demonstrate that, besides nicotinamide and nicotinic acid, only the corresponding nucleosides readily enter the cells. Nucleotides (e.g. NAD and NMN) undergo extracellular degradation resulting in the formation of permeable precursors. These precursors can all be converted to cytosolic and mitochondrial NAD. For mitochondrial NAD synthesis, precursors are converted to NMN in the cytosol. When taken up into the organelles, NMN (together with ATP) serves as substrate of NMNAT3 to form NAD. NMNAT3 was conclusively localized to the mitochondrial matrix and is the only known enzyme of NAD synthesis residing within these organelles. We thus present a comprehensive dissection of mammalian NAD biosynthesis, the groundwork to understand regulation of NAD-mediated processes, and the organismal homeostasis of this fundamental molecule.NAD is an essential electron carrier and a key molecule of signaling pathways (1-4). In bioenergetic pathways, NAD is reversibly converted between its oxidized (NAD ϩ ) and reduced (NADH) states, which would not require continuous regeneration. Indeed, when the principal pathway of NAD ϩ synthesis from nicotinic acid (NA) 2 had been established (5), the "case" was nearly closed, because neither additional roles of NAD nor a regulatory importance of its synthesis were suspected. Meanwhile, discoveries of signaling processes in which NAD ϩ is degraded have dramatically changed this view. Signaling conversions of NAD ϩ include the cleavage to nicotinamide (Nam), which is recycled into NAD ϩ synthesis, and a concomitant reaction of the remaining ADP-ribose moiety. NAD ϩ -dependent deacetylases (members of the sirtuin family) and mono-ADP-ribosyltransferases control life span, the biological clock, insulin secretion, and key metabolic enzymes (6 -9). In addition, NAD ϩ represents the substrate for poly(ADP-ribosylation) to regulate DNA repair, transcription, telomerase activity, and chromatin dynamics (10 -12). NAD ϩ is also the precursor of cyclic ADP-ribose and NAADP, potent agents to mobilize calcium from intracellular stores (13). This multitude of NAD ϩ -degrading reactions clearly necessitates permanent re...

Background: Nuclear and cytosolic poly(ADP-ribose) metabolism is established but debated in mitochondria. Results: Novel mitochondrial and cytosolic poly(ADP-ribose) glycohydrolase splice variants are inactive for poly(ADP-ribose) degradation. Conclusion: Degradation of mitochondrial matrix-accumulated poly(ADP-ribose) can be catalyzed only by ADP-ribosylhydrolase 3, whereas small poly(ADP-ribose) glycohydrolase isoforms may have functions different from poly(ADP-ribose) degradation. Significance: Important insights into the regulation of subcellular poly(ADP-ribose) metabolism are provided.

BackgroundWhether antidiabetic glitazone drugs protect against Parkinson's disease remains controversial. Although a single clinical trial showed no evidence of disease modulation, retrospective studies suggest that a disease‐preventing effect may be plausible. The objective of this study was to examine if the use of glitazone drugs is associated with a lower incidence of PD among diabetic patients.MethodsWe compared the incidence of PD between individuals with diabetes who used glitazones, with or without metformin, and individuals using only metformin in the Norwegian Prescription Database. This database contains all prescription drugs dispensed for the entire Norwegian population. We identified 94,349 metformin users and 8396 glitazone users during a 10‐year period and compared the incidence of PD in the 2 groups using Cox regression survival analysis, with glitazone exposure as a time‐dependent covariate.ResultsGlitazone use was associated with a significantly lower incidence of PD compared with metformin‐only use (hazard ratio, 0.72; 95% confidence interval, 0.55‐0.94; P = 0.01).ConclusionsThe use of glitazones is associated with a decreased risk of incident PD in populations with diabetes. Further studies are warranted to confirm and understand the role of glitazones in neurodegeneration. © 2017 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society

Mitochondrial metabolism is intimately connected to the universal coenzyme NAD. In addition to its role in redox reactions of energy transduction, NAD serves as substrate in regulatory reactions that lead to its degradation. Importantly, all types of the known NAD-consuming signalling reactions have been reported to take place in mitochondria. These reactions include the generation of second messengers, as well as posttranslational protein modifications such as ADP-ribosylation and protein deacetylation. Therefore, the availability and redox state of NAD emerged as important factors in the regulation of mitochondrial metabolism. Molecular mechanisms and targets of mitochondrial NAD-dependent protein deacetylation and mono-ADP-ribosylation have been established, whereas poly-ADP-ribosylation and NAD-derived messenger generation in the organelles await in-depth characterization. In this review, we highlight the major NAD-dependent reactions occurring within mitochondria and describe their metabolic and regulatory functions. We also discuss the metabolic fates of the NAD-degradation products, nicotinamide and ADPribose, and how the mitochondrial NAD pool is restored.

Interest in the modulation of nicotinamide adenine dinucleotide (NAD) metabolome is gaining great momentum because of its therapeutic potential in different human disorders. Suppression of nicotinamide salvage by nicotinamide phosphoribosyl transferase (NAMPT) inhibitors, however, gave inconclusive results in neoplastic patients because several metabolic routes circumvent the enzymatic block converging directly on nicotinamide mononucleotide adenylyl transferases (NMNATs) for NAD synthesis. Unfortunately, NMNAT inhibitors have not been identified. Here, we report the identification of Vacor as a substrate metabolized by the consecutive action of NAMPT and NMNAT2 into the NAD analog Vacor adenine dinucleotide (VAD). This leads to inhibition of both enzymes, as well as NAD-dependent dehydrogenases, thereby causing unprecedented rapid NAD depletion, glycolytic block, energy failure, and necrotic death of NMNAT2-proficient cancer cells. Conversely, lack of NMNAT2 expression confers complete resistance to Vacor. Remarkably, Vacor prompts VAD formation and growth suppression in NMNAT2-positive neuroblastoma and melanoma xenografts. Our data show the first evidence of harnessing the entire nicotinamide salvage pathway for antimetabolic strategies.