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Archives of cardiovascular diseases
Volume 109, n° 3
pages 207-215 (mars 2016)
Doi : 10.1016/j.acvd.2015.10.004
Received : 27 April 2015 ;  accepted : 9 October 2015
Nicotinamide adenine dinucleotide homeostasis and signalling in heart disease: Pathophysiological implications and therapeutic potential
Homéostasie et signalisation du nicotinamide adénine dinucléotide dans les pathologies cardiaques : implications physiopathologiques et potentiel thérapeutique
 

Mathias Mericskay
 CNRS UMR8256–Inserm U1164, Biology of Adaptation and Ageing, Institute of Biology Paris-Seine, University Pierre-and-Marie-Curie Paris 6, 7, quai Saint-Bernard, 75005 Paris, France 

Summary

Heart failure is a highly morbid syndrome generating enormous socio-economic costs. The failing heart is characterized by a state of deficient bioenergetics that is not currently addressed by classical clinical approaches. Nicotinamide adenine dinucleotide (NAD+/NADH) is a major coenzyme for oxidoreduction reactions in energy metabolism; it has recently emerged as a signalling molecule with a broad range of activities, ranging from calcium (Ca2+) signalling (CD38 ectoenzyme) to the epigenetic regulation of gene expression involved in the oxidative stress response, catabolic metabolism and mitochondrial biogenesis (sirtuins, poly[adenosine diphosphate-ribose] polymerases [PARPs]). Here, we review current knowledge regarding alterations to myocardial NAD homeostasis that have been observed in various models of heart failure, and their effect on mitochondrial functions, Ca2+, sirtuin and PARP signalling. We highlight the therapeutic approaches that are currently in use or in development, which inhibit or stimulate NAD+-consuming enzymes, and emerging approaches aimed at stimulating NAD biosynthesis in the failing heart.

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Résumé

L’insuffisance cardiaque est un syndrome hautement morbide qui génère un coût socio-économique considérable. Le cœur insuffisant est caractérisé par un état de déficit bioénergétique qui n’est pas directement adressé par les thérapies les plus communément utilisées en clinique à l’heure actuelle. Le nicotinamide adénine dinucléotide (NAD+/NADH) est un coenzyme majeur des réactions d’oxydo-réduction du métabolisme énergétique qui a récemment émergé comme une molécule de signalisation avec un large spectre d’action allant de la signalisation Ca2+ (CD38) à la régulation épigénétique de l’expression des gènes de la biogenèse mitochondriale et du métabolisme catabolique et de la résistance au stress oxydant (sirtuines, PARP). Dans cette revue, nous reprenons l’état des connaissances actuelles sur les altérations de l’homéostasie du NAD qui ont été observées dans différents modèles d’insuffisance cardiaque et leur impact sur les fonctions mitochondriales et les fonctions de signalisation Ca2+, sirtuines et PARP. Nous mettons en avant les molécules thérapeutiques déjà utilisées ou en cours de développement qui inhibent ou stimulent les enzymes hydrolysant le NAD+ et les approches nouvelles visant à stimuler les voies de biosynthèse du NAD+.

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Keywords : Heart failure, Energy metabolism, Nicotinamide adenine dinucleotide, Sirtuin, PARP, CD38

Mots clés : Insuffisance cardiaque, Métabolisme énergétique, Nicotinamide adénine dinucléotide, Sirtuine, PARP, CD38

Abbreviations : ADP, ADPR, ART, ATP, Ca2+, cADPR, INa , NA, Na+, NAD, NAD+, NADH, NADP, NADPH, NAM, Nampt, Nmrk, Nmnat, NR, PARP, PBEF, PGC, ROS, SIRT, TRPM2


Energy failure in heart failure

The population is ageing worldwide, leading to a higher prevalence of age-related cardiovascular and metabolic diseases, such as hypertension, coronary heart diseases and type 2 diabetes mellitus, all of which raise the risk of developing heart failure. Despite improvements in heart failure therapy in the last two decades, patients still have a poor quality of life, repeated hospitalizations and a reduced life expectancy. The last decade of research has shown that mitochondrial dysfunctions, leading to bioenergetics defects and increased production of reactive oxygen species (ROS), are key players in the process of cardiac ageing and the development of heart failure [1].

Current therapies for heart failure are mainly based on the reduction of heart rate (beta-blockers) and cardiac workload and remodelling (angiotensin-converting enzyme inhibitors, mineralocorticoid receptor antagonists, vasodilators), which contribute to spare energy consumption, but do not address the issue of deficient energy production in heart failure. The decline in energy production and usage capacity observed in the failing heart is caused by a global alteration in bioenergetics systems, including deficient creatine kinase-mediated energy transfer systems, decreased fatty acid β-oxidation and decreased mitochondrial oxidative phosphorylation capacities [2]. The exact causes of this decline in energy production in the failing heart are far from being completely understood, but are thought to be associated with the repression of important transcriptional regulators of metabolic pathways and mitochondrial biogenesis, including peroxisome proliferator-activated receptor gamma coactivator 1-α and β (PGC1α and β) and oestrogen-related receptor-α and γ [3]. Alteration of calcium (Ca2+) handling systems in failing cardiomyocytes may also affect mitochondrial Ca2+ load and oxidative phosphorylation capacities. A common endpoint of these metabolic alterations is that the myocardium becomes energy starved, which is reflected in an alteration to the homeostasis of high-energy metabolites: first the major energy reserve compound phosphocreatine declines (reduced phosphocreatine/adenosine triphosphate [ATP] ratio), then the ATP/adenosine diphosphate (ADP) ratio and finally the total ATP concentration are progressively reduced [2].

Nicotinamide adenine dinucleotide (NAD) at the crossroad between energy metabolism, cell signalling and epigenetics
NAD coenzyme functions

While the alteration in ATP nucleotide homeostasis has been studied extensively in the context of cardiac ageing and heart failure, much less is known about the homeostatic regulation of NAD, despite its major role as a coenzyme of oxidoreduction reactions in energy metabolism. Oxidation of glucose and fatty acids lead to the reduction of oxidized NAD (NAD+) to NADH. Glycolysis occurring in the cytosol produces two NADH molecules. This cytosolic NADH is converted back to NAD+ by the malate dehydrogenase that initiates the transfer of reducing equivalents to the mitochondria though the malate–aspartate shuttle system. On the other hand, fatty acid β-oxidation takes place inside the mitochondria, and generates one flavin adenine dinucleotide-2 (FADH2) molecule, one NADH molecule and one acetyl coenzyme A molecule for each cycle of cleavage of two carbons. For instance, a 16-carbon long palmitate molecule generates seven NADH and FADH2 molecules and eight acetyl coenzyme A molecules. As the Krebs cycle produces three NADH molecules and only one FADH2 molecule per cycle, NADH is the major electron donor to the electron transport chain in the mitochondria. In addition to this major function in energy metabolism, NAD is also the precursor of nicotinamide adenine dinucleotide phosphate (NADP), by means of NAD kinase-mediated phosphorylation or the mitochondrial nicotinamide nucleotide transhydrogenase. NADP in its reduced form (NADPH) is the essential coenzyme of the enzymatic pathways dedicated to the detoxification of ROS, such as the glutathione and thioredoxin reductase systems. In all these functions as a coenzyme for oxidoreductases of the energy metabolism, the recycling of NAD+ and NAD does not modify the total pool of NAD. However, several cellular pathways exist that are net consumers of NAD+.

NAD consumption by signalling pathways

Indeed, NAD+ has emerged in the recent years as an important signalling molecule that is used by different pathways involved in the regulation of energy metabolism (sirtuins), the response to oxidative stress (poly[ADP-ribose (ADPR)] polymerase [PARP], sirtuins) and Ca2+ signalling (CD38 ectoenzyme), setting NAD+ as a major regulatory hub directly interfacing with energy metabolism and cellular functions (Figure 1).



Figure 1




Figure 1. 

Multiple roles of oxidized nicotinamide adenine dinucleotide (NAD+) in energy metabolism and cell signalling. A. Skeletal formula of NAD+ showing the site of reduction that gives rise to the reduced form (NADH) in oxidoreduction reactions. The boxes indicate the nicotinamide (NAM) and adenosine diphosphate-ribose (ADPR) moieties that are released after cleavage by NAD+-consuming enzymes. B. NAD+ and its vitamin B3 precursors NAM and nicotinamide riboside (NR) can be found in the extracellular compartment. NAD+ synthetic pathways are highlighted in green and consuming pathways are highlighted in red. NAD+ present in food is broken down into nicotinamide mononucleotide (NMN), NAM or NR components. NMN is converted to NR by CD73 5′-ectonucleotidase. NR can enter the cells through nucleoside transporters. The NAD+ biosynthetic pathways are initiated by the nicotinamide phosphoribosyl transferase (Nampt) and nicotinamide riboside kinase (Nmrk) enzymes forming NMN, followed by the nicotinamide mononucleotide adenylyl transferase (Nmnat) enzymes fusing an NMN to an ADP moiety to form NAD+. The NAD+ coenzyme is reduced to NADH during glycolysis, fatty acid β-oxidation (FAO) and mitochondrial oxidative phosphorylation, and is the precursor of oxidized/reduced NAD phosphate (NADP+/NADPH) in the cytosol and mitochondria. NAD+ is cleaved by enzymes, such as sirtuins (SIRT) and poly(ADPR) polymerases (PARP), involved in gene regulation for oxidative stress resistance and mitochondrial biogenesis. NAD is also used by ADP-ribosylases, such as ADP-ribosyl transferase C1 (ART1), located at the membrane. CD38 cleaves NAD+ to generate cyclic ADPR (cADPR) and ADPR second messengers or nicotinic acid adenine dinucleotide phosphate (NAADP) from NADP. The second messengers are involved in calcium (Ca2+) mobilization from the extracellular compartment (Trpm2) and intracellular stores, notably the sarcoplasmic reticulum, through the activation of the ryanodin receptor (RyR) or the lysosomal stores. Ac: acetyl; ATP: adenosine triphosphate; CoA: coenzyme A; ETC: electron transport chain; FoxO: forkhead box O; GSH: glutathione; Idh2: isocitrate dehydrogenase 2; NADK: NAD kinase; Nnt: nicotinamide nucleotide transhydrogenase; NOX: NADPH oxidase; Nrt1: nitrate transporter 1; PGC: peroxisome proliferator-activated receptor gamma coactivator; ROS: reactive oxygen species; Trpm2: transient receptor potential cation channel, subfamily M, member 2; TXN: thioredoxin.

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NAD+-dependent sirtuin deacetylases

Sirtuins function as metabolic regulators in response to energy stress, through the stimulation of mitochondrial biogenesis and oxidative phosphorylation genes [4]. Sirtuins are enzymes (SIRT1 to 7) that cleave NAD into nicotinamide (NAM) and ADPR moieties (Figure 1A) to perform different types of post-translational modifications on cellular proteins. In one type of reaction, sirtuins remove acetyl (SIRT1, SIRT2, SIRT3, SIRT6, SIRT7), succinyl (SIRT5) or lipid (SIRT6, SIRT7) groups that have been covalently linked to lysine residues on cellular proteins, modifying their charge and activity [4]. In that case, ADPR is used as the acceptor of the removed group. Alternatively, in the mono-ADP-ribosylation reaction performed by some sirtuins (SIRT4, SIRT6, SIRT7), the ADPR moiety is transferred to arginine residues on target proteins, modifying their charges and activities. SIRT1, SIRT6 and SIRT7 are predominantly nuclear, and exert their functions through a direct effect on transcription factors involved in the regulation of metabolism, autophagy and cell survival. SIRT3 and SIRT5 reside in the mitochondrial matrix, and enhance the activity of enzymes involved in the Krebs cycle and oxidative phosphorylation metabolism [4]. One of the most extensively studied sirtuins is SIRT1. SIRT1 plays a role in chromatin remodelling and gene expression by deacetylating histones as well as transcription factors, including forkhead box O (FoxO) factors, p53 and PGC1α, which regulate autophagy, survival and metabolic pathways. SIRT1-mediated deacetylation of the PGC1α transcription factor stimulates its activity and increases mitochondrial biogenesis [5]. Resveratrol, a SIRT1 activator, improves mitochondrial function in cardiac and skeletal muscles [5, 6]. A moderate level of SIRT1 overexpression in the myocardium protects the heart against ageing and oxidative stress [7], as well as against ischaemia/reperfusion injury [8]. Other sirtuins, including SIRT3, SIRT4, SIRT6 and SIRT7, have also been shown to play a protective role in the heart by targeting mitochondrial or nuclear proteins [4]. Notably, SIRT3 plays an essential role in counteracting inhibitory non-enzymatic acetylation of mitochondrial proteins caused by the excess of free acetyl coenzyme A, observed in the setting of a high-fat diet [9]. SIRT3 overexpression in transgenic mice protects the heart from pressure overload stress [10]. Angiotensin 2 and phenylephrine agonists have been shown to lower NAD+ concentrations in the heart, and NAD+ administration was able to blunt left ventricular hypertrophy in a SIRT3-dependent manner [11].

NAD+-dependent ADPRs

NAD+ is also used as an ADPR donor by a large family of enzymes known collectively as ADP-ribosyl transferases (ARTD1 to 18 [previously known as the PARPs] and ARTC1 to 5) [12]. PARP1 (ARTD1 in the new nomenclature) is a major NAD-consuming enzyme in the cell, which recognizes DNA lesions induced by an excess of ROS. When activated, PARP1 catalyzes the formation of long polymers of ADPR (parylation) on itself and partner proteins, to recruit the machinery of DNA repair enzymes. Mild activation of PARP1 is protective, but overactivation can deplete the cellular pool of NAD and alter SIRT1 activity, which led to cardiomyocyte death in a mouse model of pressure overload hypertrophy [13, 14]. Interestingly, SIRT1 was shown to deacetylate and repress PARP1 [13]. As both enzymes use NAD+ for their catalytic activity, they appear to be involved in competitive pathways to decipher cell fate in a stress situation. PARP1 is also involved in the regulation of energy metabolic pathways, as it was recently shown to bind to peroxisome proliferator-activated receptor-γ nuclear receptor, enhancing ligand binding and co-factor exchange in adipocytes [15]. Other ART enzymes are less well characterized, but the ectoenzyme ART1 (ARTC1 in the new nomenclature), for instance, has been shown to ADP-ribosylate a number of membrane proteins, including integrin α7 in skeletal muscle [16]. Interestingly, this modification was shown to enhance the adhesion of integrin α7 to the laminin protein in the extracellular matrix [16], which may be of importance in the highly active cardiac muscle.

NAD+-dependent Ca2+ signalling

A major enzyme involved in NAD hydrolysis is the CD38 ectoenzyme that generates NAM, ADPR and cyclic ADPR (cADPR); ADPR and cADPR act as second messengers in calcium signalling [17]. CD38 can also generate the nicotinic acid adenine dinucleotide phosphate (NAADP) derivative of NADP involved in lysosomal Ca2+ mobilization [17]. The activity of the ryanodine receptor is stimulated by cADPR in cardiac myocytes [18], and cADPR increases the frequency of Ca2+ sparks that are essential for the Ca2+-induced Ca2+ release mechanism at the basis of cardiac rhythmic contractility [19]. The cADPR second messenger is also required for the angiotensin II-induced sustained Ca2+ rise that is observed after the initial rapid transient Ca2+ elevation triggered via the inositol trisphosphate receptor [20]. Some authors, however, found that connexin 43 hemichannels could be an alternative route of entry for cADPR [21], and that NAD+ may also enter directly by these channels [22].

CD38 hydrolysis of NAD+ also generates ADPR, which, together with cADPR, is a known activator of the TRPM2 channel, a member of the M-family of transient receptor potential channels that are permeable to Ca2+ [23, 24]. Interestingly, Ca2+ entry via TRPM2 is essential for cardiac myocyte bioenergetics maintenance in the context of ischaemia-reperfusion injury or doxorubicin cardiomyopathy [25], suggesting that TRPM2 is a potential link between NAD+ and Ca2+ signalling for cardiomyocyte survival.

CD38 was once thought to essentially hydrolyze extracellular NAD+, but recent data have shown that the enzyme exists in two conformations, one with the catalytic side on the outer side of the membrane and one with the catalytic side facing the cytosol, hence it is able to hydrolyze the intracellular pool of NAD+ [17].

CD38-deficient mice display a general increase in NAD+ tissue concentration, although results discrepancy was notable for the heart between the study of Aksoy et al., which reported a 30-fold increase [26], and the study of Young et al. [27], which reported no change. Myocardial contractility, contraction and relaxation velocities are significantly enhanced in male CD38-deficient mice [28]. Altogether, these studies suggest that CD38 is an important regulator of the balance of extra- and intracellular NAD homeostasis, and establish a potential link between NAD+ and Ca2+ signalling, although further research is needed to fully understand this connection.

Alteration of the NAD+/NADH ratio and NAD loss in ageing and heart failure

Tissue concentrations of NAD+ decline in different organs, including the heart, liver, kidney and lungs, during ageing in the rat [29]. The decline was most pronounced in the heart, with a loss of 70% of NAD+ between the ages of 3 and 24months, compensated by a 50% increase in NADH concentrations, so that the change was mainly at the level of the NAD+/NADH ratio, which was strongly reduced from 0.7 to 0.1.

Alterations in NAD concentrations and the NAD+/NADH ratio have been involved in multiple pathogenic mechanisms leading to heart failure – notably in links with the impact of Ca2+ signalling in mitochondria. An original report by De Lisa et al. showed that Ca2+ overload of isolated rat heart mitochondria resulted in a profound decrease in their NAD+ content [30]. In this study, 30minutes of ischaemia in isolated hearts led to a 30% decrease in NAD+ both in mitochondria and at the tissue level. This loss was due to increased hydrolysis of mitochondria-released NAD+ by an unidentified glucohydrolase, and was worsened by reperfusion. Interestingly, the permeability transition pore inhibitor cyclosporin A preserved NAD+ concentrations in the mitochondria and protected the heart from reperfusion damage, suggesting that NAD+ transits through this complex.

In a guinea pig model of non-ischaemic heart failure obtained by ascending aorta constriction, the elevated cytosolic concentration of sodium (Na+), which is characteristic of failing cardiomyocytes, reduced the mitochondrial Ca2+ concentration by accelerating Ca2+ efflux via mitochondrial Na+/Ca2+ exchange [31]. In turn, because Ca2+ normally stimulates enzymes of the Krebs cycle involved in NADH production, low mitochondrial Ca2+ concentrations decreased the mitochondrial NADH content and bioenergetic capacities in failing cardiomyocytes.

Conversely, the same group showed that the NADH/NAD+ redox state could modulate the Na+ current (INa ) in the heart. A mutation in the gene encoding the glycerol-3-phosphate dehydrogenase 1-like protein leads to abnormally elevated NADH concentrations, and was shown to cause Brugada syndrome. Elevated NADH concentrations were shown to inhibit the INa through increased ROS signalling, mediated by NADPH oxidase and protein kinase C-mediated inhibition of the Nav 1.5 channel encoded by the SCN5A gene [32]. In this study, NAD+ perfusion restored the NADH/NAD+ redox state and had antiarrhythmic effects on isolated SCN5A -deficient mouse hearts – the more classical model of Brugada syndrome. The same authors showed, in a mouse model of hypertensive heart failure obtained by unilateral nephrectomy and deoxycorticosterone acetate pellet implantation, that NADH concentrations measured by metabolite extractions and colourimetric assay were increased [33], unlike that observed in the guinea pig model cited above [31]. Differences in the method of NADH quantification and the stage of heart failure in different animal models may account for this discrepancy. In the study by Liu et al. [33], the INa was decreased in failing cardiomyocytes, and treating the cells with NAD+ or mitoTEMPO (a mitochondria-targeted antioxidant) restored INa levels. The NAD+ effect was dependent on CD38 activity to allow entry of NAD+ into the cell. As CD38 is a major NAD+ hydrolase, we hypothesize that an intermediate step of intracellular NAD+ regeneration from NAM would be required; this hypothesis has not been tested as yet. Correlating with their mouse studies, these authors also showed that NAD+ perfusion of isolated human failing heart improved conduction velocity, consistent with a positive effect on Nav 1.5 activity [33].

Recently, a model of cardiomyopathy induced by deletion of a complex I subunit of the mitochondrial electron transport chain was also shown to decrease the NAD+/NADH ratio in the mitochondrial matrix by accumulation of NADH, leading to diminished SIRT3 activity and hyperacetylation of mitochondrial proteins [34]. Treatment of these mice with nicotinamide mononucleotide (NMN), as a precursor of NAD+, helped to restore normal mitochondrial permeability, transition pore sensibility, ROS concentrations and deacetylation of mitochondrial proteins. Finally, several types of mouse models of chronic heart failure, including pressure overload hypertrophy induced by transverse aorta constriction and ischaemia-reperfusion injury, were shown to display globally reduced myocardial NAD+ concentrations in the heart [13, 35, 36]. However, it should be noted that in most of these studies, NAD+ and NADH were quantified based on metabolite extraction in acidic buffer for NAD+ and basic buffer for NADH, which gives an indication of the raw steady-state concentration of each metabolite, but should not be considered to directly reflect the redox state of the cell. When sodium hydroxide basic buffer is used to extract NADH, it is known to liberate a high amount of protein-bound NADH, which is acceptable for estimating the total concentration of NADH, but does not correspond to the free NADH concentration in the cytosol. The latter defines the redox state and is about 2 orders of magnitude lower than the free NAD+ concentration. A new magnetic resonance-based in vivo NAD assay has been designed recently, and is capable of non-invasively assessing NAD+ and NADH content [37]. This technology was applied to the analysis of NAD+ and NADH concentrations in the human brain, and showed a significant decline in total NAD concentration and the NAD+/NADH ratio between individuals aged 20years and 75years. No doubt this technology applied to cardiac tissue will be extremely powerful for better defining alterations in NAD homeostasis, given how efficient 31P-magnetic resonance spectroscopy has proved to be in characterizing energy failure in heart failure.

Stimulation of the NAD biosynthetic pathways in heart failure

Altogether, these studies showing that NAD+ is lost in pathological conditions have raised interest in the pathways linked to NAD biosynthesis [38]. NAD+ can be derived from deamidated precursors, such as tryptophan, through the kyurenine pathway, and nicotinic acid (NA), a vitamin B3 (niacin) precursor of NAD+. In the mouse heart, this “deamidated precursors” pathway makes a limited contribution [39]. The main source of NAD+ precursors seems to be the amidated vitamin B3 nicotinamide (NAM) and nicotinamide riboside (NR) (Figure 1B, green pathway). NAD+ contained in food is essentially hydrolyzed into these two precursors in the intestinal lumen before reaching the circulation and being distributed to the body [40].

NAM is converted into NMN by nicotinamide phosphoribosyl transferase (Nampt), which transfers an alpha-d-5-phosphoribosyl-1-pyrophosphate to the NAM ring and consumes one ATP molecule in the process, through transient autophosphorylation of its histidine 247 residue [41]. NMN is then condensed with the ADP moiety of an ATP molecule by the Nmnat enzymes (Nmnat 1 to 3) to form NAD+. Because NAM is the by-product of all enzymatic activities hydrolyzing NAD (sirtuins, PARP, CD38), Nampt is a key enzyme for the regeneration of the NAD+ pool in the cell. Nampt has been found to be repressed in different models of heart failure, including pressure overload and ischaemia-reperfusion [13, 35]. Nampt plays a crucial role in the maintenance of the myocardial NAD+ pool and SIRT1 activity in the heart. Hsu et al. reported that transgenic overexpression of Nampt in the mouse heart or NAD+ supplementation in mice had a protective role against ischaemia-reperfusion, notably through restoration of the autophagic flux, with no deleterious impact on cardiac functions at baseline [35]. However Pillai et al. showed that overexpression of Nampt, possibly at higher levels than in the study of Hsu et al., can trigger cardiac hypertrophy, whereas mice with a half dose of Nampt (Nampt+/−) were protected against agonist (isoproterenol and angiotensin II)-induced hypertrophy, showing that NAD+ production is an important intermediate in this process [42]. Interestingly, the Nampt complementary DNA open reading frame was found to be identical to a cytokine named pre-B-cell colony-enhancing factor (PBEF), and was recently reidentified as a hormone named visfatin, reported to exert insulin-mimetic effects, but also pro-inflammatory roles in different contexts [43]. Hence, these studies have revealed that Nampt can exist in two forms, intracellular Nampt and excreted Nampt (visfatin). The existence of these two forms of Nampt led to the hypothesis that Nampt is centrally involved in a systemic regulatory network that regulates NAD+ concentrations in the different organs, a model for which the term “NAD world” was coined by Imai [44]. However, it is still not clear at this stage whether excreted Nampt can effectively synthesize NMN in the extracellular compartment and, in fact, recent evidence suggests that this is not the case, at least in human plasma [45]. So, excreted Nampt/PBEF/visfatin may function mainly as a cytokine binding to a yet unknown receptor(s) [43]. The study of Pillai et al. [42] showed that Nampt can also be excreted by cardiomyocytes and, in vitro, excreted Nampt added to the culture medium triggered hypertrophy of cultured cardiomyocytes.

Inhibitors of Nampt catalytic activity, such as FK866, have been evaluated in clinics in the context of cancer therapy, because Nampt is found to be overexpressed in different tumour cells [43]. However, its central role in NAD biosynthesis and its impact on mitochondrial functions should raise concerns about the potential cardiotoxicity of these compounds, as with other chemotherapies.

NR is a more recently characterized NAD+ precursor, which can be found in milk [38]. Belenky et al. showed that NR promotes yeast replicative longevity through a Sir2 (SIRT1 homologue) pathway [46]. This group cloned the two mammalian homologues of nicotinamide riboside kinase (Nmrk1 and 2) that phosphorylate NR to form NMN. The role of Nmrk enzymes has not been addressed in mammals so far. Interrogation of Gene Expression Omnibus (GEO) dataset profiles revealed that Nmrk1 is ubiquitously expressed, while Nmrk2 appears to be specific to striated muscle tissue (skeletal and cardiac). Interestingly, the Nmrk2 open reading frame corresponds to the sequence of a muscle integrin binding protein previously shown to bind α7β1 integrin heterodimers in a C2C12 myoblast cell line, and to inhibit the deposition of laminin in the extracellular matrix [47]. This suggests a potential link between the function of integrin binding and the function of NAD biosynthesis for Nmrk2/muscle integrin binding protein. One link could be signalling mechanisms related to the ADP-ribosylation of integrin α7 by ARTC1 [16], which require local NAD+ hydrolysis at the membrane, although this hypothesis remains to be tested.

NR enters into yeast cells through the nucleoside transporter Nrt1 – without a clearly identified homologue in humans [48] – or through Fun26, a homologue of human equilibrative nucleoside transporter [49]. In addition, when NMN is given to the cells to stimulate NAD synthesis, it may, in fact, be first transformed into NR by the CD73 ecto-5′-nucleotidase, to allow entry into the cell [50].

NA was one of the first niacins to be used as a supplement to show protection of the heart in stress conditions [51]. NA derivatives, such as acipimox, were used efficiently in clinics for the treatment of hyperlipidaemia in type 2 diabetes mellitus patients, although some rebound effects on free fatty acid concentrations in the blood occurred after a few days of treatment, and a “flushing” effect was induced by acipimox, which was difficult for patients to bear, lowering the prospects of clinical use [52]. Nevertheless, acipimox was shown to improve oxidative metabolism in the skeletal muscle of type 2 diabetes mellitus patients [53]. NR supplementation in mice was shown to have a similar impact on oxidative metabolism, without some of the limitations of NA, and allowed the mice to be partially resistant to high-fat diet-induced obesity [54]. More recently, NR was shown to stimulate mitochondrial biogenesis in the skeletal muscle of mouse models of mitochondrial diseases [55, 56]. So far, the impact of NR supplementation on cardiac functions is unknown.

Conclusion

NAD+ has emerged as a central regulator of energy metabolism, through both its direct role as a coenzyme in oxidoreduction reactions of glycolysis, fatty acid β-oxidation and oxidative phosphorylation, and its multiple facets as a signalling molecule connecting Ca2+ signalling to mitochondrial functions and the transcription of genes involved in metabolic and oxidative stress resistance. Hence, we propose that acting on NAD bioavailability and usage in the failing heart may have a strong impact on the evolution of the disease (Figure 2). Several drugs already available in clinics or in development are, in fact, dealing with the rate of NAD consumption, such as the PARP inhibitors, which could be useful as inhibitors of cell death and inflammation in cardiovascular diseases [57]. Alternatively, available inhibitors or therapeutic antibodies targeting CD38 could be tested for their ability to raise the concentration of NAD+ [58, 59], considering that male CD38-deficient mice present improved cardiac contractility [28].



Figure 2


Figure 2. 

Therapeutic potential of compounds modulating oxidized nicotinamide adenine dinucleotide (NAD+) homeostasis and signalling in heart failure. Vitamin B3 (nicotinic acid [NA] and NA derivatives, such as acipimox, nicotinamide [NAM] and nicotinamide riboside [NR]) and nicotinamide mononucleotide (NMN) can be used to stimulate NAD+ synthesis and oxidative metabolism. NAM is not only a precursor of NAD+ but also an inhibitor of sirtuins, so its use maybe counterproductive. Poly(adenosine diphosphate-ribose) polymerase (PARP) inhibitors (e.g. olaparib, veliparib, niraparib, L-2286 and AG-690/11026014) can limit the high NAD+ consumption by PARP1 and have been shown to be beneficial in preclinical models. Alternatively, inhibitors of the other major NAD+ hydrolase CD38 (e.g. 4-amino-8-quinoline carboxamide compounds, daratumumab [HuMax®-CD38; Genmab, Copenhagen, Denmark] a human immunoglobulin G1κ monoclonal antibody) could help to maintain NAD concentrations in the myocardium, although they have not been tested in preclinical models of heart failure as yet. Sirtuins consume NAD+, but at moderate level and, overall, their action is thought to be protective in the context of pathological cardiac remodelling. This hypothesis is supported by the beneficial action of sirtuin activators on cardiovascular health (e.g. resveratrol, SRT1460, SRT1720, SRT2183, STAC-5, STAC-9, STAC-10). Importantly, a beneficial side effect of sirtuin-1 (SIRT1) activators could be repression of deleterious PARP1 activity. Ca2+: calcium.

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As the rate of NAD+ consumption is likely to be increased in the failing heart in the face of PARP activation by oxidative stress and SIRT1 activation by energetic stress, strategies that aim to push the rate of NAD+ biosynthesis by niacin supplementation could be interesting alternatives to restore bioenergetics capacities in the heart.

Finally, the fact that NAD+ is also regulated at the systemic level and can be found in serum raises the possibility of using circulating concentrations of NAD+ or NAD+ metabolites as biomarkers of energetic defects in cardiovascular diseases. In line with this hypothesis, NAD+ concentrations have been found to be decreased in multiple sclerosis patients, in correlation with the severity of the disease [60].

Disclosure of interest

The author declares that he has no competing interest.


Acknowledgements

We thank Association Française Contre les Myopathies for supporting related projects, and Zhenlin Li, Anne Garnier, Renée Ventura-Clapier, Antoine Muchir and colleagues for their support and fruitful discussions.

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