A reduced form of nicotinamide riboside defines a new path for NAD+ biosynthesis and acts as an orally bioavailable NAD+ precursor

NAD+ bioavailability mediates PARG inhibition-induced replication arrest, intra S-phase checkpoint and apoptosis in glioma stem cells

Elevated expression of the DNA damage response proteins PARP1 and poly(ADP-ribose) glycohydrolase (PARG) in glioma stem cells (GSCs) suggests that glioma may be a unique target for PARG inhibitors (PARGi). While PARGi-induced cell death is achieved when combined with ionizing radiation, as a single agent PARG inhibitors appear to be mostly cytostatic. Supplementation with the NAD⁺ precursor dihydronicotinamide riboside (NRH) rapidly increased NAD⁺ levels in GSCs and glioma cells, inducing PARP1 activation and mild suppression of replication fork progression. Administration of NRH+PARGi triggers hyperaccumulation of poly(ADP-ribose) (PAR), intra S-phase arrest and apoptosis in GSCs but minimal PAR induction or cytotoxicity in normal astrocytes. PAR accumulation is regulated by select PARP1- and PAR-interacting proteins. The involvement of XRCC1 highlights the base excision repair pathway in responding to replication stress while enhanced interaction of PARP1 with PCNA, RPA and ORC2 upon PAR accumulation implicates replication associated PARP1 activation and assembly with pre-replication complex proteins upon initiation of replication arrest, the intra S-phase checkpoint and the onset of apoptosis.

Temporal dynamics of base excision/single-strand break repair protein complex assembly/disassembly are modulated by the PARP/NAD+/SIRT6 axis

Assembly and disassembly of DNA repair protein complexes at DNA damage sites are essential for maintaining genomic integrity. Investigating factors coordinating assembly of the base excision repair (BER) proteins DNA polymerase β (Polβ) and XRCC1 to DNA lesion sites identifies a role for Polβ in regulating XRCC1 disassembly from DNA repair complexes and, conversely, demonstrates Polβ’s dependence on XRCC1 for complex assembly. LivePAR, a genetically encoded probe for live-cell imaging of poly(ADP-ribose) (PAR), reveals that Polβ and XRCC1 require PAR for repair-complex assembly, with PARP1 and PARP2 playing unique roles in complex dynamics. Further, BER complex assembly is modulated by attenuation/augmentation of NAD⁺ biosynthesis. Finally, SIRT6 does not modulate PARP1 or PARP2 activation but does regulate XRCC1 recruitment, leading to diminished Polβ abundance at sites of DNA damage. These findings highlight coordinated yet independent roles for PARP1, PARP2, and SIRT6 and their regulation by NAD⁺ bioavailability to facilitate BER.

Koczor et al. use quantitative confocal microscopy to characterize DNA-damage-induced poly(ADP-ribose) (PAR) formation and assembly/disassembly kinetics in human cells. These studies highlight the coordinated yet independent roles for XRCC1, POLΒ, PARP1, PARP2, and SIRT6 (and regulation by NAD⁺) to facilitate BER/SSBR protein complex dynamics.

Sex-specific alterations in NAD+ metabolism in 3xTg Alzheimer's disease mouse brain assessed by quantitative targeted LC-MS

Levels of nicotinamide adenine dinucleotide (NAD+) are known to decline with age and have been associated with impaired mitochondrial function leading to neurodegeneration, a key facet of Alzheimer's disease (AD). NAD+synthesis is sustained via tryptophan-kynurenine (Trp-Kyn) pathway as de novo synthesis route, and salvage pathways dependent on the availability of nicotinic acid and nicotinamide. While being currently investigated as a multifactorial disease with a strong metabolic component, AD remains without curative treatment and important sex differences were reported in relation to disease onset and progression. The aim of this study was to reveal the potential deregulation of NAD+metabolism in AD with the direct analysis of NAD+precursors in the mouse brain tissue (wild type (WT) versus triple transgenic (3xTg) AD), using a sex-balanced design. To this end, we developed a quantitative liquid chromatography-tandem mass spectrometry (LC-MS/MS) method, which allowed for the measurement of the full spectrum of NAD+precursors and intermediates in all three pathways. In brain tissue of mice with developed AD symptoms, a decrease in kynurenine (Kyn) versus increase in kynurenic acid (KA) levels were observed in both sexes with a significantly higher increment of KA in males. These alterations in Trp-Kyn pathway might be a consequence of neuroinflammation and a compensatory production of neuroprotective kynurenic acid. In the NAD+ salvage pathway, significantly lower levels of nicotinamide mononucleotide (NMN) were measured in the AD brain of males and females. Depletion of NMN implies the deregulation of salvage pathway critical for maintaining optimal NAD+ levels and mitochondrial and neuronal function.

A Method to Monitor the NAD+ Metabolome-From Mechanistic to Clinical Applications

Nicotinamide adenine dinucleotide (NAD+) and its reduced form (NADH) are coenzymes employed in hundreds of metabolic reactions. NAD+ also serves as a substrate for enzymes such as sirtuins, poly(ADP-ribose) polymerases (PARPs) and ADP-ribosyl cyclases. Given the pivotal role of NAD(H) in health and disease, studying NAD+ metabolism has become essential to monitor genetic- and/or drug-induced perturbations related to metabolic status and diseases (such as ageing, cancer or obesity), and its possible therapies. Here, we present a strategy based on liquid chromatography-tandem mass spectrometry (LC-MS/MS), for the analysis of the NAD+ metabolome in biological samples. In this method, hydrophilic interaction chromatography (HILIC) was used to separate a total of 18 metabolites belonging to pathways leading to NAD+ biosynthesis, including precursors, intermediates and catabolites. As redox cofactors are known for their instability, a sample preparation procedure was developed to handle a variety of biological matrices: cell models, rodent tissues and biofluids, as well as human biofluids (urine, plasma, serum, whole blood). For clinical applications, quantitative LC-MS/MS for a subset of metabolites was demonstrated for the analysis of the human whole blood of nine volunteers. Using this developed workflow, our methodology allows studying NAD+ biology from mechanistic to clinical applications.

Preclinical and clinical evidence of NAD+ precursors in health, disease, and ageing

NAD⁺ is a fundamental molecule in human life and health as it participates in energy metabolism, cell signalling, mitochondrial homeostasis, and in dictating cell survival or death. Emerging evidence from preclinical and human studies indicates an age-dependent reduction of cellular NAD⁺, possibly due to reduced synthesis and increased consumption. In preclinical models, NAD⁺ repletion extends healthspan and / or lifespan and mitigates several conditions, such as premature ageing diseases and neurodegenerative diseases. These findings suggest that NAD⁺ replenishment through NAD⁺ precursors has great potential as a therapeutic target for ageing and age-predisposed diseases, such as Alzheimer's disease. Here, we provide an updated review on the biological activity, safety, and possible side effects of NAD⁺ precursors in preclinical and clinical studies. Major NAD⁺ precursors focused on by this review are nicotinamide riboside (NR), nicotinamide mononucleotide (NMN), and the new discovered dihydronicotinamide riboside (NRH). In summary, NAD⁺ precursors have an exciting therapeutic potential for ageing, metabolic and neurodegenerative diseases.

The balance between NAD+ biosynthesis and consumption in ageing

Nicotinamide adenine dinucleotide (NAD⁺) is a vital coenzyme in redox reactions. NAD⁺ is also important in cellular signalling as it is consumed by PARPs, SARM1, sirtuins and CD38. Cellular NAD⁺ levels regulate several essential processes including DNA repair, immune cell function, senescence, and chromatin remodelling. Maintenance of these cellular processes is important for healthy ageing and lifespan. Interestingly, the levels of NAD⁺ decline during ageing in several organisms, including humans. Declining NAD⁺ levels have been linked to several age-related diseases including various metabolic diseases and cognitive decline. Decreasing tissue NAD⁺ concentrations have been ascribed to an imbalance between biosynthesis and consumption of the dinucleotide, resulting from, for instance, reduced levels of the rate limiting enzyme NAMPT along with an increased activation state of the NAD⁺-consuming enzymes PARPs and CD38. The progression of some age-related diseases can be halted or reversed by therapeutic augmentation of NAD⁺ levels. NAD⁺ metabolism has therefore emerged as a potential target to ameliorate age-related diseases. The present review explores how ageing affects NAD⁺ metabolism and current approaches to reverse the age-dependent decline of NAD⁺.

NAD+ Degrading Enzymes, Evidence for Roles During Infection

Declines in cellular nicotinamide adenine dinucleotide (NAD) contribute to metabolic dysfunction, increase susceptibility to disease, and occur as a result of pathogenic infection. The enzymatic cleavage of NAD⁺ transfers ADP-ribose (ADPr) to substrate proteins generating mono-ADP-ribose (MAR), poly-ADP-ribose (PAR) or O-acetyl-ADP-ribose (OAADPr). These important post-translational modifications have roles in both immune response activation and the advancement of infection. In particular, emergent data show viral infection stimulates activation of poly (ADP-ribose) polymerase (PARP) mediated NAD⁺ depletion and stimulates hydrolysis of existing ADP-ribosylation modifications. These studies are important for us to better understand the value of NAD⁺ maintenance upon the biology of infection. This review focuses specifically upon the NAD⁺ utilising enzymes, discusses existing knowledge surrounding their roles in infection, their NAD⁺ depletion capability and their influence within pathogenic infection.

Precursor comparisons for the upregulation of nicotinamide adenine dinucleotide. Novel approaches for better aging

Nicotinamide adenine dinucleotide (NAD) is a coenzyme found in every human cell and regulates a number of systems across multiple cellular compartments and tissue types via an endogenous and exogenous influence. NAD levels are demonstrated to decline with age and therefore measures to counteract the waning of NAD have been devised. A number of NAD precursor candidates such as nicotinamide riboside (NR), nicotinamide mononucleotide (NMN), the reduced form of nicotinamide mononucleotide (NMNH), nicotinic acid (NA) nicotinamide (NAM), and dihydronicotinamide riboside (DNR) increase NAD levels in vitro and in vivo. This discussion will focus on the precursors NR, NMN, NMNH, and DNR in the upregulation of NAD. There are many publications on NAD precursors as it has become popular for human consumption in recent years due to its vital importance to the general consumer. However, there is no consensus between researchers and this was the aim of this review, to determine and discuss their areas of agreement versus disagreement, to highlight the gaps in research, and to give recommendations for future work. Bioavailability and potency of NR, NMNH, NMN, and DNR is also examined on the light of the most recent literature.

Risks and rewards of targeting NAD+ homeostasis in the brain

Strategies to correct declining nicotinamide adenine dinucleotide (NAD⁺) levels in neurological disease and biological ageing are promising therapeutic candidates. These strategies include supplementing with NAD⁺ precursors, small molecule activation of NAD⁺ biosynthetic enzymes, and treatment with small molecule inhibitors of NAD⁺ consuming enzymes such as CD38, SARM1 or members of the PARP family. While these strategies have shown efficacy in animal models of neurological disease, each of these has the mechanistic potential for adverse events that could preclude their preclinical use. Here, we discuss the implications of these strategies for treating neurological diseases, including potential off-target effects that may be unique to the brain.

Reduced nicotinamide mononucleotide is a new and potent NAD+ precursor in mammalian cells and mice

Nicotinamide adenine dinucleotide (NAD⁺ ) homeostasis is constantly compromised due to degradation by NAD⁺ -dependent enzymes. NAD⁺ replenishment by supplementation with the NAD⁺ precursors nicotinamide mononucleotide (NMN) and nicotinamide riboside (NR) can alleviate this imbalance. However, NMN and NR are limited by their mild effect on the cellular NAD⁺ pool and the need of high doses. Here, we report a synthesis method of a reduced form of NMN (NMNH), and identify this molecule as a new NAD⁺ precursor for the first time. We show that NMNH increases NAD⁺ levels to a much higher extent and faster than NMN or NR, and that it is metabolized through a different, NRK and NAMPT-independent, pathway. We also demonstrate that NMNH reduces damage and accelerates repair in renal tubular epithelial cells upon hypoxia/reoxygenation injury. Finally, we find that NMNH administration in mice causes a rapid and sustained NAD⁺ surge in whole blood, which is accompanied by increased NAD⁺ levels in liver, kidney, muscle, brain, brown adipose tissue, and heart, but not in white adipose tissue. Together, our data highlight NMNH as a new NAD⁺ precursor with therapeutic potential for acute kidney injury, confirm the existence of a novel pathway for the recycling of reduced NAD⁺ precursors and establish NMNH as a member of the new family of reduced NAD⁺ precursors.

Biochemical Mechanisms Associating Alcohol Use Disorders with Cancers

Simple Summary

Of all yearly deaths attributable to alcohol consumption globally, approximately 12% are due to cancers, representing approximately 0.4 million deceased individuals. Ethanol metabolism disturbs cell biochemistry by targeting the structure and function of essential biomolecules (proteins, nucleic acids, and lipids) and by provoking alterations in cell programming that lead to cancer development and cancer malignancy. A better understanding of the metabolic and cell signaling realm affected by ethanol is paramount to designing effective treatments and preventive actions tailored to specific neoplasias.

Abstract

The World Health Organization identifies alcohol as a cause of several neoplasias of the oropharynx cavity, esophagus, gastrointestinal tract, larynx, liver, or female breast. We review ethanol’s nonoxidative and oxidative metabolism and one-carbon metabolism that encompasses both redox and transfer reactions that influence crucial cell proliferation machinery. Ethanol favors the uncontrolled production and action of free radicals, which interfere with the maintenance of essential cellular functions. We focus on the generation of protein, DNA, and lipid adducts that interfere with the cellular processes related to growth and differentiation. Ethanol’s effects on stem cells, which are responsible for building and repairing tissues, are reviewed. Cancer stem cells (CSCs) of different origins suffer disturbances related to the expression of cell surface markers, enzymes, and transcription factors after ethanol exposure with the consequent dysregulation of mechanisms related to cancer metastasis or resistance to treatments. Our analysis aims to underline and discuss potential targets that show more sensitivity to ethanol’s action and identify specific metabolic routes and metabolic realms that may be corrected to recover metabolic homeostasis after pharmacological intervention. Specifically, research should pay attention to re-establishing metabolic fluxes by fine-tuning the functioning of specific pathways related to one-carbon metabolism and antioxidant processes.

NAD+ homeostasis in human health and disease

Depletion of nicotinamide adenine dinucleotide (NAD+ ), a central redox cofactor and the substrate of key metabolic enzymes, is the causative factor of a number of inherited and acquired diseases in humans. Primary deficiencies of NAD+ homeostasis are the result of impaired biosynthesis, while secondary deficiencies can arise due to other factors affecting NAD+ homeostasis, such as increased NAD+ consumption or dietary deficiency of its vitamin B3 precursors. NAD+ depletion can manifest in a wide variety of pathological phenotypes, ranging from rare inherited defects, characterized by congenital malformations, retinal degeneration, and/or encephalopathy, to more common multifactorial, often age-related, diseases. Here, we discuss NAD+ biochemistry and metabolism and provide an overview of the etiology and pathological consequences of alterations of the NAD+ metabolism in humans. Finally, we discuss the state of the art of the potential therapeutic implications of NAD+ repletion for boosting health as well as treating rare and common diseases, and the possibilities to achieve this by means of the different NAD+ -enhancing agents.

Evolving concepts in NAD+ metabolism

NAD(H) and NADP(H) have traditionally been viewed as co-factors (or co-enzymes) involved in a myriad of oxidation-reduction reactions including the electron transport in the mitochondria. However, NAD pathway metabolites have many other important functions, including roles in signaling pathways, post-translational modifications, epigenetic changes, and regulation of RNA stability and function via NAD-capping of RNA. Non-oxidative reactions ultimately lead to the net catabolism of these nucleotides, indicating that NAD metabolism is an extremely dynamic process. In fact, recent studies have clearly demonstrated that NAD has a half-life in the order of minutes in some tissues. Several evolving concepts on the metabolism, transport, and roles of these NAD pathway metabolites in disease states such as cancer, neurodegeneration, and aging have emerged in just the last few years. In this perspective, we discuss key recent discoveries and changing concepts in NAD metabolism and biology that are reshaping the field. In addition, we will pose some open questions in NAD biology, including why NAD metabolism is so fast and dynamic in some tissues, how NAD and its precursors are transported to cells and organelles, and how NAD metabolism is integrated with inflammation and senescence. Resolving these questions will lead to significant advancements in the field.

NAD+ Metabolism, Metabolic Stress, and Infection

Nicotinamide adenine dinucleotide (NAD⁺) is an essential metabolite with wide-ranging and significant roles in the cell. Defects in NAD⁺ metabolism have been associated with many human disorders; it is therefore an emerging therapeutic target. Moreover, NAD⁺ metabolism is perturbed during colonization by a variety of pathogens, either due to the molecular mechanisms employed by these infectious agents or by the host immune response they trigger. Three main biosynthetic pathways, including the de novo and salvage pathways, contribute to the production of NAD⁺ with a high degree of conservation from bacteria to humans. De novo biosynthesis, which begins with l-tryptophan in eukaryotes, is also known as the kynurenine pathway. Intermediates of this pathway have various beneficial and deleterious effects on cellular health in different contexts. For example, dysregulation of this pathway is linked to neurotoxicity and oxidative stress. Activation of the de novo pathway is also implicated in various infections and inflammatory signaling. Given the dynamic flexibility and multiple roles of NAD⁺ intermediates, it is important to understand the interconnections and cross-regulations of NAD⁺ precursors and associated signaling pathways to understand how cells regulate NAD⁺ homeostasis in response to various growth conditions. Although regulation of NAD⁺ homeostasis remains incompletely understood, studies in the genetically tractable budding yeast Saccharomyces cerevisiae may help provide some molecular basis for how NAD⁺ homeostasis factors contribute to the maintenance and regulation of cellular function and how they are regulated by various nutritional and stress signals. Here we present a brief overview of recent insights and discoveries made with respect to the relationship between NAD⁺ metabolism and selected human disorders and infections, with a particular focus on the de novo pathway. We also discuss how studies in budding yeast may help elucidate the regulation of NAD⁺ homeostasis.

A metabolomic endotype of bioenergetic dysfunction predicts mortality in critically ill patients with acute respiratory failure

Acute respiratory failure (ARF) requiring mechanical ventilation, a complicating factor in sepsis and other disorders, is associated with high morbidity and mortality. Despite its severity and prevalence, treatment options are limited. In light of accumulating evidence that mitochondrial abnormalities are common in ARF, here we applied broad spectrum quantitative and semiquantitative metabolomic analyses of serum from ARF patients to detect bioenergetic dysfunction and determine its association with survival. Plasma samples from surviving and non-surviving patients (N = 15/group) were taken at day 1 and day 3 after admission to the medical intensive care unit and, in survivors, at hospital discharge. Significant differences between survivors and non-survivors (ANOVA, 5% FDR) include bioenergetically relevant intermediates of redox cofactors nicotinamide adenine dinucleotide (NAD) and NAD phosphate (NADP), increased acyl-carnitines, bile acids, and decreased acyl-glycerophosphocholines. Many metabolites associated with poor outcomes are substrates of NAD(P)-dependent enzymatic processes, while alterations in NAD cofactors rely on bioavailability of dietary B-vitamins thiamine, riboflavin and pyridoxine. Changes in the efficiency of the nicotinamide-derived cofactors' biosynthetic pathways also associate with alterations in glutathione-dependent drug metabolism characterized by substantial differences observed in the acetaminophen metabolome. Based on these findings, a four-feature model developed with semi-quantitative and quantitative metabolomic results predicted patient outcomes with high accuracy (AUROC = 0.91). Collectively, this metabolomic endotype points to a close association between mitochondrial and bioenergetic dysfunction and mortality in human ARF, thus pointing to new pharmacologic targets to reduce mortality in this condition.

Advances in NAD-Lowering Agents for Cancer Treatment

Nicotinamide adenine dinucleotide (NAD) is an essential redox cofactor, but it also acts as a substrate for NAD-consuming enzymes, regulating cellular events such as DNA repair and gene expression. Since such processes are fundamental to support cancer cell survival and proliferation, sustained NAD production is a hallmark of many types of neoplasms. Depleting intratumor NAD levels, mainly through interference with the NAD-biosynthetic machinery, has emerged as a promising anti-cancer strategy. NAD can be generated from tryptophan or nicotinic acid. In addition, the "salvage pathway" of NAD production, which uses nicotinamide, a byproduct of NAD degradation, as a substrate, is also widely active in mammalian cells and appears to be highly exploited by a subset of human cancers. In fact, research has mainly focused on inhibiting the key enzyme of the latter NAD production route, nicotinamide phosphoribosyltransferase (NAMPT), leading to the identification of numerous inhibitors, including FK866 and CHS-828. Unfortunately, the clinical activity of these agents proved limited, suggesting that the approaches for targeting NAD production in tumors need to be refined. In this contribution, we highlight the recent advancements in this field, including an overview of the NAD-lowering compounds that have been reported so far and the related in vitro and in vivo studies. We also describe the key NAD-producing pathways and their regulation in cancer cells. Finally, we summarize the approaches that have been explored to optimize the therapeutic response to NAMPT inhibitors in cancer.

Hypoxia ameliorates brain hyperoxia and NAD+ deficiency in a murine model of Leigh syndrome

Leigh syndrome is a severe mitochondrial neurodegenerative disease with no effective treatment. In the Ndufs4-/- mouse model of Leigh syndrome, continuously breathing 11% O2 (hypoxia) prevents neurodegeneration and leads to a dramatic extension (~5-fold) in lifespan. We investigated the effect of hypoxia on the brain metabolism of Ndufs4-/- mice by studying blood gas tensions and metabolite levels in simultaneously sampled arterial and cerebral internal jugular venous (IJV) blood. Relatively healthy Ndufs4-/- and wildtype (WT) mice breathing air until postnatal age ~38 d were compared to Ndufs4-/- and WT mice breathing air until ~38 days old followed by 4-weeks of breathing 11% O2. Compared to WT control mice, Ndufs4-/- mice breathing air have reduced brain O2 consumption as evidenced by an elevated partial pressure of O2 in IJV blood (PijvO2) despite a normal PO2 in arterial blood, and higher lactate/pyruvate (L/P) ratios in IJV plasma revealed by metabolic profiling. In Ndufs4-/- mice, hypoxia treatment normalized the cerebral venous PijvO2 and L/P ratios, and decreased levels of nicotinate in IJV plasma. Brain concentrations of nicotinamide adenine dinucleotide (NAD+) were lower in Ndufs4-/- mice breathing air than in WT mice, but preserved at WT levels with hypoxia treatment. Although mild hypoxia (17% O2) has been shown to be an ineffective therapy for Ndufs4-/- mice, we find that when combined with nicotinic acid supplementation it provides a modest improvement in neurodegeneration and lifespan. Therapies targeting both brain hyperoxia and NAD+ deficiency may hold promise for treating Leigh syndrome.

NAD+ and NAFLD - caution, causality and careful optimism

The prevalence of non-alcoholic fatty liver disease (NAFLD) is increasing worldwide, and new treatments are sorely needed. Nicotinamide adenine dinucleotide (NAD⁺ ) has been proposed as a potential target to prevent and reverse NAFLD. NAD⁺ is an important redox factor for energy metabolism and is used as a substrate by a range of enzymes, including sirtuins (SIRT), which regulates histone acetylation, transcription factor activity and mitochondrial function. NAD⁺ is also a precursor for reduced nicotinamide adenine dinucleotide phosphate (NADPH), which is an important component of the antioxidant defense system. NAD⁺ precursors such as nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN) are available as over-the-counter dietary supplements, and oral supplementation with these precursors increases hepatic NAD⁺ levels and prevents hepatic lipid accumulation in pre-clinical models of NAFLD. NAD⁺ precursors have also been found to improve hepatic mitochondrial function and decrease oxidative stress in pre-clinical NAFLD models. NAD⁺ repletion also prevents NAFLD progression to non-alcoholic steatohepatitis (NASH), as NAD⁺ precursor supplementation is associated with decreased hepatic stellate cell activation, and decreased fibrosis. However, initial clinical trials have only shown modest effects when NAD⁺ precursors were administrated to people with obesity. We review the available pre-clinical investigations of NAD⁺ supplementation for targeting NAFLD, and discuss how data from the first clinical trials can be reconciled with observations from preclinical research.

Chemical and Biochemical Reactivity of the Reduced Forms of Nicotinamide Riboside

All life forms require nicotinamide adenine dinucleotide, NAD⁺, and its reduced form NADH. They are redox partners in hundreds of cellular enzymatic reactions. Changes in the intracellular levels of total NAD (NAD⁺ + NADH) and the (NAD⁺/NADH) ratio can cause cellular dysfunction. When not present in protein complexes, NADH and its phosphorylated form NADPH degrade through intricate mechanisms. Replenishment of a declining total NAD pool can be achieved with biosynthetic precursors that include one of the reduced forms of nicotinamide riboside (NR⁺), NRH. NRH, like NADH and NADPH, is prone to degradation via oxidation, hydration, and isomerization and, as such, is an excellent model compound to rationalize the nonenzymatic metabolism of NAD(P)H in a biological context. Here, we report on the stability of NRH and its propensity to isomerize and irreversibly degrade. We also report the preparation of two of its naturally occurring isomers, their chemical stability, their reactivity toward NRH-processing enzymes, and their cell-specific cytotoxicity. Furthermore, we identify a mechanism by which NRH degradation causes covalent peptide modifications, a process that could expose a novel type of NADH-protein modifications and correlate NADH accumulation with "protein aging." This work highlights the current limitations in detecting NADH's endogenous catabolites and in establishing the capacity for inducing cellular dysfunction.

Reduced Nicotinamide Mononucleotide (NMNH) Potently Enhances NAD+ and Suppresses Glycolysis, the TCA Cycle, and Cell Growth

Decreased cellular NAD⁺ levels are causally linked to aging and aging-associated diseases. NAD⁺ precursors in oxidized form such as nicotinamide mononucleotide (NMN) and nicotinamide riboside (NR) have gained much attention and been well studied for their ability to restore NAD⁺ levels in model organisms. Less is known about whether NAD⁺ precursors in reduced form can also efficiently increase the tissue and cellular NAD⁺ levels and have different effects on cellular processes than NMN or NR. In the present study, we developed a chemical method to produce dihydronicotinamide mononucleotide (NMNH), which is the reduced form of NMN. We demonstrated that NMNH was a better NAD⁺ enhancer than NMN both in vitro and in vivo, mediated by nicotinamide mononucleotide adenylyltransferase (NMNAT). Additionally, NMNH increased the reduced NAD (NADH) levels in cells and in mouse livers. Metabolomic analysis revealed that NMNH inhibited glycolysis and the TCA cycle. In vitro experiments demonstrated that NMNH induced cell cycle arrest and suppressed cell growth. Nevertheless, NMNH treatment did not cause an observable difference in mouse weight. Taken together, our work demonstrates that NMNH is a potent NAD⁺ enhancer and suppresses glycolysis, the TCA cycle, and cell growth.

Nicotinamide Mononucleotide Attenuates Renal Interstitial Fibrosis After AKI by Suppressing Tubular DNA Damage and Senescence

Acute kidney injury (AKI) is a worldwide health problem currently lacking therapeutics that directly promote renal repair or prevent the occurrence of chronic fibrosis. DNA damage is a feature of many forms of kidney injury, and targeting DNA damage and repair might be effective strategies for kidney protection in AKI. Boosting nicotinamide adenine dinucleotide (NAD⁺) levels is thought to have beneficial effects on DNA damage repair and fibrosis in other organs. However, no kidney-related studies of such effects have been performed to date. Here, we have shown that NMN (an NAD⁺ precursor) administration could significantly reduce tubular cell DNA damage and subsequent cellular senescence induced by hydrogen peroxide and hypoxia in human proximal tubular cells (HK-2 cells). The DNA damage inhibition, antiaging and anti-inflammatory effects of NMN were further confirmed in a unilateral ischemia-reperfusion injury (uIRI) mouse model. Most importantly, the antifibrosis activity of NMN was also shown in ischemic AKI mouse models, regardless of whether NMN was administered in advance or during the recovery phase. Collectively, these results suggest that NMN could significantly inhibit tubular cell DNA damage, senescence and inflammation. NMN administration might be an effective strategy for preventing or treating kidney fibrosis after AKI.

Enzymology of extracellular NAD metabolism

Extracellular NAD represents a key signaling molecule in different physiological and pathological conditions. It exerts such function both directly, through the activation of specific purinergic receptors, or indirectly, serving as substrate of ectoenzymes, such as CD73, nucleotide pyrophosphatase/phosphodiesterase 1, CD38 and its paralog CD157, and ecto ADP ribosyltransferases. By hydrolyzing NAD, these enzymes dictate extracellular NAD availability, thus regulating its direct signaling role. In addition, they can generate from NAD smaller signaling molecules, like the immunomodulator adenosine, or they can use NAD to ADP-ribosylate various extracellular proteins and membrane receptors, with significant impact on the control of immunity, inflammatory response, tumorigenesis, and other diseases. Besides, they release from NAD several pyridine metabolites that can be taken up by the cell for the intracellular regeneration of NAD itself. The extracellular environment also hosts nicotinamide phosphoribosyltransferase and nicotinic acid phosphoribosyltransferase, which inside the cell catalyze key reactions in NAD salvaging pathways. The extracellular forms of these enzymes behave as cytokines, with pro-inflammatory functions. This review summarizes the current knowledge on the extracellular NAD metabolome and describes the major biochemical properties of the enzymes involved in extracellular NAD metabolism, focusing on the contribution of their catalytic activities to the biological function. By uncovering the controversies and gaps in their characterization, further research directions are suggested, also to better exploit the great potential of these enzymes as therapeutic targets in various human diseases.

Equilibrative Nucleoside Transporters Mediate the Import of Nicotinamide Riboside and Nicotinic Acid Riboside into Human Cells

Nicotinamide riboside (NR), a new form of vitamin B3, is an effective precursor of nicotinamide adenine dinucleotide (NAD⁺) in human and animal cells. The introduction of NR into the body effectively increases the level of intracellular NAD⁺ and thereby restores physiological functions that are weakened or lost in experimental models of aging and various pathologies. Despite the active use of NR in applied biomedicine, the mechanism of its transport into mammalian cells is currently not understood. In this study, we used overexpression of proteins in HEK293 cells, and metabolite detection by NMR, to show that extracellular NR can be imported into cells by members of the equilibrative nucleoside transporter (ENT) family ENT1, ENT2, and ENT4. After being imported into cells, NR is readily metabolized resulting in Nam generation. Moreover, the same ENT-dependent mechanism can be used to import the deamidated form of NR, nicotinic acid riboside (NAR). However, NAR uptake into HEK293 cells required the stimulation of its active utilization in the cytosol such as phosphorylation by NR kinase. On the other hand, we did not detect any NR uptake mediated by the concentrative nucleoside transporters (CNT) CNT1, CNT2, or CNT3, while overexpression of CNT3, but not CNT1 or CNT2, moderately stimulated NAR utilization by HEK293 cells.

Dihydronicotinamide riboside: synthesis from nicotinamide riboside chloride, purification and stability studies

In the present work, we describe an efficient method for scalable synthesis and purification of 1,4-dihydronicotinamide riboside (NRH) from commercially available nicotinamide riboside chloride (NRCl) and in the presence of sodium dithionate as a reducing agent. NRH is industrially relevant as the most effective, synthetic NAD⁺ precursor. We demonstrated that solid phase synthesis cannot be used for the reduction of NRCl to NRH in high yield, whereas a reduction reaction in water at room temperature under anaerobic conditions is shown to be very effective, reaching a 55% isolation yield. For the first time, by using common column chromatography, we were able to highly purify this sensitive bio-compound with good yield. A series of identifications and analyses including HPLC, NMR, LC-MS, FTIR, and UV-vis spectroscopy were performed on the purified sample, confirming the structure of NRH as well as its purity to be 96%. Thermal analysis of NRH showed higher thermal stability compared to NRCl, and with two major weight losses, one at 218 °C and another at 805 °C. We also investigated the long term stability effects of temperature, pH, light, and oxygen (as air) on the NRH in aqueous solutions. Our results show that NRH can be oxidized in the presence of oxygen, and it hydrolyzed quickly in acidic conditions. It was also found that the degradation rate is lower under a N₂ atmosphere, at lower temperatures, and under basic pH conditions.

A convenient and scalable method for synthesis of dihydronicotinamide riboside (NRH) from the commercially available nicotinamide riboside chloride (NRCl) is elaborated as well as a fast purification method that led to high purity NRH.

NAD+ metabolism and its roles in cellular processes during ageing

Nicotinamide adenine dinucleotide (NAD⁺) is a coenzyme for redox reactions, making it central to energy metabolism. NAD⁺ is also an essential cofactor for non-redox NAD⁺-dependent enzymes, including sirtuins, CD38 and poly(ADP-ribose) polymerases. NAD⁺ can directly and indirectly influence many key cellular functions, including metabolic pathways, DNA repair, chromatin remodelling, cellular senescence and immune cell function. These cellular processes and functions are critical for maintaining tissue and metabolic homeostasis and for healthy ageing. Remarkably, ageing is accompanied by a gradual decline in tissue and cellular NAD⁺ levels in multiple model organisms, including rodents and humans. This decline in NAD⁺ levels is linked causally to numerous ageing-associated diseases, including cognitive decline, cancer, metabolic disease, sarcopenia and frailty. Many of these ageing-associated diseases can be slowed down and even reversed by restoring NAD⁺ levels. Therefore, targeting NAD⁺ metabolism has emerged as a potential therapeutic approach to ameliorate ageing-related disease, and extend the human healthspan and lifespan. However, much remains to be learnt about how NAD⁺ influences human health and ageing biology. This includes a deeper understanding of the molecular mechanisms that regulate NAD⁺ levels, how to effectively restore NAD⁺ levels during ageing, whether doing so is safe and whether NAD⁺ repletion will have beneficial effects in ageing humans.

Dihydronicotinamide riboside promotes cell-specific cytotoxicity by tipping the balance between metabolic regulation and oxidative stress

Nicotinamide adenine dinucleotide (NAD⁺), the essential cofactor derived from vitamin B3, is both a coenzyme in redox enzymatic processes and substrate in non-redox events; processes that are intimately implicated in all essential bioenergetics. A decrease in intracellular NAD⁺ levels is known to cause multiple metabolic complications and age-related disorders. One NAD⁺ precursor is dihydronicotinamide riboside (NRH), which increases NAD⁺ levels more potently in both cultured cells and mice than current supplementation strategies with nicotinamide riboside (NR), nicotinamide mononucleotide (NMN) or vitamin B3 (nicotinamide and niacin). However, the consequences of extreme boosts in NAD⁺ levels are not fully understood. Here, we demonstrate the cell-specific effects of acute NRH exposure in mammalian cells. Hepatocellular carcinoma (HepG3) cells show dose-dependent cytotoxicity when supplemented with 100–1000 μM NRH. Cytotoxicity was not observed in human embryonic kidney (HEK293T) cells over the same dose range of NRH. PUMA and BAX mediate the cell-specific cytotoxicity of NRH in HepG3. When supplementing HepG3 with 100 μM NRH, a significant increase in ROS was observed concurrent with changes in the NAD(P)H and GSH/GSSG pools. NRH altered mitochondrial membrane potential, increased mitochondrial superoxide formation, and induced mitochondrial DNA damage in those cells. NRH also caused metabolic dysregulation, altering mitochondrial respiration. Altogether, we demonstrated the detrimental consequences of an extreme boost of the total NAD (NAD⁺ + NADH) pool through NRH supplementation in HepG3. The cell-specific effects are likely mediated through the different metabolic fate of NRH in these cells, which warrants further study in other systemic models.

Immunotherapy of COVID-19 with poly (ADP-ribose) polymerase inhibitors: starting with nicotinamide

COVID-19 induces a proinflammatory environment that is stronger in patients requiring intensive care. The cytokine components of this environment may determine efficacy or otherwise of glucocorticoid therapy. The immunity modulators, the aryl hydrocarbon receptor (AhR) and the nuclear NAD+-consuming enzyme poly (ADP-ribose) polymerase 1 (PARP 1) may play a critical role in COVID-19 pathophysiology. The AhR is overexpressed in coronaviruses, including COVID-19 and, as it regulates PARP gene expression, the latter is likely to be activated in COVID-19. PARP 1 activation leads to cell death mainly by depletion of NAD+ and adenosine triphosphate (ATP), especially when availability of these energy mediators is compromised. PARP expression is enhanced in other lung conditions: the pneumovirus respiratory syncytial virus (RSV) and chronic obstructive pulmonary disease (COPD). I propose that PARP 1 activation is the terminal point in a sequence of events culminating in patient mortality and should be the focus of COVID-19 immunotherapy. Potent PARP 1 inhibitors are undergoing trials in cancer, but a readily available inhibitor, nicotinamide (NAM), which possesses a highly desirable biochemical and activity profile, merits exploration. It conserves NAD+ and prevents ATP depletion by PARP 1 and Sirtuin 1 (silent mating type information regulation 2 homologue 1) inhibition, enhances NAD+ synthesis, and hence that of NADP+ which is a stronger PARP inhibitor, reverses lung injury caused by ischaemia/reperfusion, inhibits proinflammatory cytokines and is effective against HIV infection. These properties qualify NAM for therapeutic use initially in conjunction with standard clinical care or combined with other agents, and subsequently as an adjunct to stronger PARP 1 inhibitors or other drugs.

NAD+ metabolism: pathophysiologic mechanisms and therapeutic potential

Nicotinamide adenine dinucleotide (NAD+) and its metabolites function as critical regulators to maintain physiologic processes, enabling the plastic cells to adapt to environmental changes including nutrient perturbation, genotoxic factors, circadian disorder, infection, inflammation and xenobiotics. These effects are mainly achieved by the driving effect of NAD+ on metabolic pathways as enzyme cofactors transferring hydrogen in oxidation-reduction reactions. Besides, multiple NAD+-dependent enzymes are involved in physiology either by post-synthesis chemical modification of DNA, RNA and proteins, or releasing second messenger cyclic ADP-ribose (cADPR) and NAADP+. Prolonged disequilibrium of NAD+ metabolism disturbs the physiological functions, resulting in diseases including metabolic diseases, cancer, aging and neurodegeneration disorder. In this review, we summarize recent advances in our understanding of the molecular mechanisms of NAD+-regulated physiological responses to stresses, the contribution of NAD+ deficiency to various diseases via manipulating cellular communication networks and the potential new avenues for therapeutic intervention.

Need for NAD+: Focus on Striated Muscle Laminopathies

Laminopathies are a heterogeneous group of rare diseases caused by genetic mutations in the LMNA gene, encoding A-type lamins. A-type lamins are nuclear envelope proteins which associate with B-type lamins to form the nuclear lamina, a meshwork underlying the inner nuclear envelope of differentiated cells. The laminopathies include lipodystrophies, progeroid phenotypes and striated muscle diseases. Research on striated muscle laminopathies in the recent years has provided novel perspectives on the role of the nuclear lamina and has shed light on the pathological consequences of altered nuclear lamina. The role of altered nicotinamide adenine dinucleotide (NAD⁺) in the physiopathology of striated muscle laminopathies has been recently highlighted. Here, we have summarized these findings and reviewed the current knowledge about NAD⁺ alteration in striated muscle laminopathies, providing potential therapeutic approaches.

Implications of NAD+ boosters in translational medicine

Nicotinamide adenine dinucleotide (NAD⁺ ) is an essential metabolite in energy metabolism as well as a co-substrate in biochemical reactions such as protein deacylation, protein ADP-ribosylation and cyclic ADP-ribose synthesis mediated by sirtuins, poly (ADP-ribose) polymerases (PARPs) and CD38. In eukaryotic cells, NAD⁺ is synthesized through three distinct pathways, which offer different strategies to modulate the bioavailability of NAD⁺ . The therapeutic potential of dietarily available NAD⁺ boosters preserving the NAD⁺ pool has been attracting attention after the discovery of declining NAD⁺ levels in ageing model organisms as well as in several age-related diseases, including cardiometabolic and neurodegenerative diseases. Here, we review the recent advances in the biology of NAD⁺ , including the salubrious effects of NAD⁺ boosters and discuss their future translational strategies.

Nicotinamide Riboside-The Current State of Research and Therapeutic Uses

Nicotinamide riboside (NR) has recently become one of the most studied nicotinamide adenine dinucleotide (NAD+) precursors, due to its numerous potential health benefits mediated via elevated NAD+ content in the body. NAD+ is an essential coenzyme that plays important roles in various metabolic pathways and increasing its overall content has been confirmed as a valuable strategy for treating a wide variety of pathophysiological conditions. Accumulating evidence on NRs' health benefits has validated its efficiency across numerous animal and human studies for the treatment of a number of cardiovascular, neurodegenerative, and metabolic disorders. As the prevalence and morbidity of these conditions increases in modern society, the great necessity has arisen for a rapid translation of NR to therapeutic use and further establishment of its availability as a nutritional supplement. Here, we summarize currently available data on NR effects on metabolism, and several neurodegenerative and cardiovascular disorders, through to its application as a treatment for specific pathophysiological conditions. In addition, we have reviewed newly published research on the application of NR as a potential therapy against infections with several pathogens, including SARS-CoV-2. Additionally, to support rapid NR translation to therapeutics, the challenges related to its bioavailability and safety are addressed, together with the advantages of NR to other NAD+ precursors.

The 4 D's of Pellagra and Progress

Nicotinamide homeostasis is a candidate common denominator to explain smooth transitions, whether demographic, epidemiological or economic. This 'NAD world', dependent on hydrogen-based energy, is not widely recognised as it is neither measured nor viewed from a sufficiently multi-genomic or historical perspective. Reviewing the importance of meat and nicotinamide balances during our co-evolution, recent history suggests that populations only modernise and age well with low fertility on a suitably balanced diet. Imbalances on the low meat side lead to an excess of infectious disease, short lives and boom-bust demographics. On the high side, meat has led to an excess of degenerative, allergic and metabolic disease and low fertility. A 'Goldilocks' diet derived from mixed and sustainable farming (preserving the topsoil) allows for high intellectual capital, height and good health with controlled population growth resulting in economic growth and prosperity. Implementing meat equity worldwide could lead to progress for future generations on 'spaceship' earth by establishing control over population quality, thermostat and biodiversity, if it is not already too late.

Nicotinamide riboside-amino acid conjugates that are stable to purine nucleoside phosphorylase

The nutraceutical Nicotinamide Riboside (NR), an efficacious biosynthetic precursor to NAD, is readily metabolized by the purine nucleoside phosphorylase (PNP). Access to the PNP-stable versions of NR is difficult because the glycosidic bond of NR is easily cleaved. Unlike NR, NRH, the reduced form of NR, offers sufficient chemical stability to allow the successful functionalisation of the ribosyl-moiety. Here, we report on a series of NRH and NR derived amino acid conjugates, generated in good to excellent yields and show that O5'-esterification prevents the PNP-catalyzed phosphorolysis of these NR prodrugs.

NRH salvage and conversion to NAD+ requires NRH kinase activity by adenosine kinase

Dihydronicotinamide riboside (NRH) has been suggested to act as a precursor for the synthesis of NAD⁺, but the biochemical pathway converting it has been unknown. Here, we show that NRH can be converted into NAD⁺ via a salvage pathway in which adenosine kinase (ADK, also known as AK) acts as an NRH kinase. Using isotope-labelling approaches, we demonstrate that NRH is fully incorporated into NAD⁺, with NMNH acting as an intermediate. We further show that AK is enriched in fractions from cell lysates with NRH kinase activity, and that AK can convert NRH into NAD⁺. In cultured cells and mouse liver, pharmacological or genetic inhibition of AK blocks formation of reduced nicotinamide mononucleotide (NMNH) and inhibits NRH-stimulated NAD⁺ biosynthesis. Finally, we confirm the presence of endogenous NRH in the liver with metabolomics. Our findings establish NRH as a natural precursor of NAD⁺ and reveal a new route for NAD⁺ biosynthesis via an NRH salvage pathway involving AK.