The older we grow, the weaker our muscles get, riddling old age with frailty and physical disability. But this doesn’t only affect the individual, it also creates a significant burden on public healthcare. And yet, research efforts into the biological processes and biomarkers that define muscle aging have not yet defined the underlying causes.
Now, a team of scientists from lab of Johan Auwerx at EPFL’s School of Life Sciences looked at the issue through a different angle: the similarities between muscle aging and degenerative muscle diseases.
They have discovered protein aggregates that deposit in skeletal muscles during natural aging, and that blocking this can prevent the detrimental features of muscle aging. The study is published in Cell Reports.
“During age-associated muscle diseases, such as inclusion body myositis (IBM), our cells struggle to maintain correct protein folding, leading these misfolded proteins to precipitate and forming toxic protein aggregates within the muscles,” explains Auwerx.
“The most prominent component of these protein aggregates is beta-amyloid, just like in the amyloid plaques in the brains of patients with Alzheimer’s disease.”
In the study, the scientists identify amyloid-like protein aggregates in aged muscles from different species, from the nematode C. elegans all the way to humans. In addition, they also found that these aggregates also impair mitochondrial function.
Although aggregated proteins have been suggested to contribute to brain aging, this is the first time that they have been shown to contribute to muscle aging and to directly damage mitochondria.
“These abnormal proteotoxic aggregates could serve as novel biomarkers for the aging process, beyond the brain and muscle,” says Auwerx.
But can the formation of the protein aggregates be reversed?
To answer this, the researchers fed worms the vitamin nicotinamide riboside and the antitumor agent Olaparib, both of which boost the levels of nicotinamide adenine dinucleotide (NAD+), a biomolecule that is essential for maintaining mitochondrial function, and whose levels decline during aging.
In the worms, the two compounds turned on the defense systems of the mitochondria, even when provided at advanced age. Turning on the so-called “mitochondrial quality control system” reduced the age-related amyloid protein aggregates and improved the worms’ fitness and lifespan.
The scientists then moved on to human muscle tissue, taken from aged subjects and IBM patients. Turning on the same mitochondrial quality control systems produced similar improvements in protein and mitochondrial homeostasis.
The encouraging results led the researchers to test nicotinamide riboside in aged mice. The treatment also activated the mitochondrial defense systems and reduced the number and size of amyloid aggregates in different skeletal muscle tissues.
“Drugs that boost mitochondrial quality control could therefore be tested in the clinic to reverse these age-related proteotoxic aggregates and rejuvenate tissues,” says Mario Romani, the first author of the study.
In recent years, interest in NAD+ biology has been gaining momentum, revealing critical insights into its roles in numerous physiological processes and underlining the beneficial effects of supplementation with its precursors. Moreover, accumulating evidence indicates that a decrease in NAD+ levels contributes to the development of age-associated pathophysiology [1,2,3].
The systemic NAD+ decrease is caused by both lowered rates of biosynthesis and increased use of NAD+. The immense demand for NAD+ is caused by its importance in cellular oxidation–reduction reactions, including the majority of catabolic and anabolic reactions, such as glycolysis, fatty acid β-oxidation, tricarboxylic acid cycle, synthesis of fatty acids, cholesterol, steroids, etc. [4,5,6].
Additionally, NAD+-consuming enzymes, such as sirtuins, poly-ADP-ribose polymerases (PARPs), cADP-ribose synthases (CD38/157 ectoenzymes) [7,8,9] and mono-ADP-ribose transferases (ARTs) contribute to an overall depletion of NAD+.
Biosynthesis can compensate somewhat to the depleted levels of NAD+ via de novo synthesis from tryptophan (Trp) or in the salvage pathways from four other precursors, nicotinamide (NAM), nicotinic acid (NA), nicotinamide riboside, and nicotinamide mononucleotide (NMN).
While de novo synthesis from Trp is carried out in an eight-step pathway, the salvageable precursors NA and NAM require only three (Preiss–Handler pathway) and two steps, respectively (Figure 1). Nicotinamide riboside (NR) is an additional salvageable NAD+ precursor with a two-step  or three-step pathway  to form NAD+ (Figure 1).
In mammals, the most common precursor is NAM, which can further be used to form NMN by the rate-limiting enzyme, phosphoribosyltransferase (NAMPT) . In the final step, NMN is converted to NAD+ by NMN/NaMN adenylyltransferases (NMNATs) [13,14].
As a function of the aging process and/or overnutrition, NAD+ content and NAMPT expression are found to decline in multiple tissues [3,15,16,17], while the maintenance of NAD+ levels relies on diverse biosynthetic routes and precursors in different tissues [18,19]. Nevertheless, the decreased expression of NAMPT enzyme is one of the major causes of the NAD+ decline over age [15,20].
The requirement of this enzyme can be bypassed with the direct conversion of NR to NMN by two nicotinamide ribose kinases, NMRK1 and NMRK2 (also known as NRK1 and NRK2) . This also circumvents the requirement of energetically costly PRPP (phosphoribosyl pyrophosphate. Figure 2) and the feedback inhibition by NAD+ .
Alternatively, NR can be turned into NAM by purine nucleoside phosphorylase (NP), which is subsequently converted to NAD+ via NMN by NMNAT (Figure 1). Hence, the utilization of NR depends on the expression of either the Nrk pathway or NP combined with the Nampt pathway.
The importance of NAD+ is reflected through the activity of NAD+-depleting enzymes, the mediators of aging, which are mostly induced by stress factors, such as DNA damage, oxidative stress, and inflammation.
The major downstream mediators are sirtuins, the NAD+-dependent deacetylases/deacylases. Sirtuins are conserved regulators of aging and longevity in diverse organisms, and regarded as the master switches of metabolism  due to their numerous regulatory functions in metabolism, DNA repair, stress response, chromatin remodeling and circadian rhythm. (Table 1) [2,23].
Together with sirtuins, PARPs use NAD+ to produce a chain of ADP-ribose (ADPR) molecules. PARP1 and PARP2 respond to DNA breaks in the nucleus and facilitate the process of DNA repair . As DNA damage accumulates over time, the activation of PARPs increase, which in turn, lowers the activity of SIRT1 due to both substrate competition and PARP2′s ability to bind to the promoter of Sirt1 and repress its expression .
Furthermore, the content of the primary NADase in mammals, CD38, increases with age. This enzyme uses NAD+ to produce and hydrolyze the Ca2+-mobilizing second messenger, cADP-ribose [25,26,27]. CD38 can also degrade the NAD+ intermediates, NR and NMN [28,29], which further decreases the content of NAD+ .
The effect of CD38 on NAD+ content was demonstrated in CD38-deficient mice, whose NAD+ levels remain high. This preserves mitochondrial respiration and metabolic function with age . Moreover, the inhibition of CD38 can increase NAD+ levels and improve glucose and lipid metabolism .
Apart from the ectoenzymes CD38 and CD157, SARM1 (sterile alpha and Toll/interleukin-1 receptor motif-containing 1) is an additional NAD+-depleting enzyme that uses NAD+ to promote axonal degeneration after injury and thereby decreases its overall content .
Central role of sirtuins in DNA repair, cell cycle control and mitochondria function (adapted from Meng et al. ).
|SIRT1||Nucleus, Cytosol||DNA repair||Glucose metabolism, differentiation, insulin secretion, neuroprotection, vascular protection|
|SIRT2||Cytosol, Nucleus||Cell cycle||Adipose tissue development and functionality, blood glucose homeostasis, modulation of peripheral myelination|
|SIRT3||Mitochondria, Nucleus, Cytosol||Mitochondrial metabolism||ATP homeostasis, ROS detoxification, tumor suppression, DNA repair, neuroprotection, apoptosis suppression|
|SIRT4||Mitochondria||Mitochondrial metabolism||Insulin secretion, DNA repair, apoptosis suppression|
|SIRT5||Mitochondria, Cytosol, Nucleus||Mitochondrial metabolism||Urea cycle, ketone body formation, nitrogenous waste management, ROS detoxification|
|SIRT6||Nucleus (Chromatin)||DNA repair||Telomerase protection, genome stability, cholesterol homeostasis, glycolysis and gluconeogenesis|
|SIRT7||Nucleus (Nucleolus)||rRNA transcription, cell cycle||Cardiac protection|
The immense involvement of NAD+ in all these processes suggests a significant potential for the treatment of various pathophysiological conditions with supplementation of NAD+ precursors. However, multiple options exist for supplementation, and the optimal precursor and dosage are still unclear with regard to the specific condition.
For the baseline requirements of NAD+ synthesis, dietary tryptophan or less than 20 mg of niacin (NA, NAM) daily are sufficient. However, growing evidence demonstrates that substantially greater rates of NAD+ synthesis exhibit numerous beneficial or even therapeutic effects, which can be achieved by supplementation of its intermediates.
These intermediates are found in a wide variety of foods, including meat, eggs, dairy, certain vegetables and whole wheat [35,36]. Specifically, NR is the third discovered NAD+ precursor that naturally occurs in milk and is already available as a nutraceutical. Oral supplementation with NR has been shown to increase NAD+ levels in multiple tissues, along with increased SIRT activity [10,11], improved mitochondrial function , and regenerative potential of stem cells .
Furthermore, NR is currently regarded as a favorable precursor since it has not been implicated to have serious side effects or flushing as opposed to other NAD+ precursors [14,39]. NR chloride has been given GRAS status (generally regarded as safe) which further supports its rapid implementation as a drug-like therapy. However, to adapt NR technologies for therapeutic use, it is necessary to determine oral availability, therapeutic dosage, and utilization in different tissues.
Human digestion and the microbiome  additionally play important roles in NR metabolism, which has yet to be characterized in detail. In this regard, we have summarized the available data from human and animal studies on NR effects on metabolic, neurodegenerative, and cardiovascular disorders, along with the advantages of NR over other available NAD+ precursors.
We also considered immunomodulatory effects in this review, as recent data indicates that NR can support the treatment of infections, including those caused by SARS-CoV-2 that has resulted in the recent Coronavirus pandemic. The current state of NR research including its bioavailability, safety, and effects on oxidative stress and longevity is described in this paper.
Effects of NR on Metabolism and Age-Associated Pathophysiology
Effects on Insulin Sensitivity, Liver Health and Other Metabolic Functions
Along with its intermediates, NAD+ plays an essential role in metabolism, and evidence suggests that an increase in NAD+ levels can exhibit ameliorating effects on metabolic disorders, such as type 2 diabetes (T2D), metabolic syndrome, and nonalcoholic fatty liver disease (NAFLD) [14,41,42]. Furthermore, NR is one of the NAD+ intermediates that also serves as a precursor of NADH, as well as hepatic NADP+ and NADPH . Since the hepatic NAD+ metabolome is regarded as a function of prediabetic (PD) and T2D mouse models, NADP+ and NADPH can be used to assess the progression of the disease.
Namely, both NADP+ and NADPH are important for resistance to oxidative stress, while NADPH is thought to be the major contributor to insulin resistance . The significantly decreased levels of hepatic NADP+ and NADPH in PD and T2D mice are restored with supplementation of NR .
In mice models, NR can increase NAD+ metabolism and thereby improve glucose tolerance, reduce weight gain, and exhibit neuroprotective effects against diabetic neuropathy and liver steatosis . Similarly, in mice models of high-fat-induced obesity, a dose of dietary NR as small as 400 mg/kg/day was shown to improve insulin sensitivity and protect mice from weight gain .
However, these results have not yet been replicated in humans, as 12-week supplementation of NR with a dose of 2000 mg/day was not able to improve insulin sensitivity and other metabolic parameters in insulin-resistant, obese men . Additional research is required to determine the long-term effects of NR on insulin sensitivity . On the other hand, there is evidence that NR administration in mice models increases the activity of SIRT1, an important factor in the prevention of T2D and preservation of insulin sensitivity [37,44].
Moreover, SIRT1 inhibits effects of oxidative stress in T2D mice , promotes glucose-stimulated insulin secretion from pancreatic β-cells [46,47], and protects against insulin resistance in peripheral tissues , whilst SIRT1 overexpression promotes fatty acid oxidation and inhibits lipogenesis, protecting the liver from steatosis. In addition, sirtuins are downregulated in NAFLD patients, a state characterized by liver steatosis .
NAFLD is widely considered a hepatic manifestation of metabolic syndrome as it is found to be associated with T2D, obesity, and insulin resistance . Via activation of SIRT1 and additional factors associated with cholesterol homeostasis, NR could potentially reduce cholesterol levels and improve liver health [51,52,53,54].
Fat accumulation is also potentially reduced through a mechanism that involves the induction of the mitochondrial unfolded protein response . In regenerating liver, NR reduces lipid accumulation, promotes hepatocyte replication, and increases hepatic ATP content leading to a faster regaining of liver weight in mice .
Moreover, dietary supplementation of NR was able to restore NAD+ levels caused by impaired biosynthesis in a mouse model of hepatocellular carcinoma, and thereby prevent both DNA damage and tumorigenesis . Altogether, there is sufficient evidence indicating that increasing NAD+ content with NR supplementation can be considered a promising therapeutic strategy for metabolic dysfunctions, including T2D and NAFLD.
Effects on Cardiovascular Diseases
Disruption of NAD+ homeostasis due to mitochondrial dysfunction is central in the development of cardiac hypertrophy and heart failure (HF) and has been reported in several models of HF including pressure overload, myocardial infarction, and angiotensin II infusion [57,58,59,60,61,62].
Furthermore, a shift from fatty acid oxidation and oxidative phosphorylation to other forms of substrate metabolism (glycolysis and ketone oxidation) often occurs in the development of HF [63,64], while the NAD+/NADH ratio also decreases . The change in oxidative-reductive capacity further increases cardiac susceptibility to stress.
The level of protein hyperacetylation also increases, driven by decreasing NAD+-dependent deacetylation both in mouse models of hypertrophy and in human patients with ischemic HF or dilated cardiomyopathy (DCM) [64,65]. NR supplementation normalizes the myocardial NAD+/NADH ratio and exhibits protective effects in adverse cardiac remodeling, while long-term supplementation increases nucleocytoplasmic protein acetylation by stimulating citrate and acetyl-CoA metabolism and antioxidant gene expression . By improving NAD+ homeostasis and activating NMRK2, NR can further prevent the deterioration of cardiac function and adverse remodeling, which are both early and persistent events in a mouse model of DCM leading to HF.
In several models of cardiac injuries, the NAMPT enzyme is repressed , while Nmrk2 expression is robustly upregulated . A similar shift was observed in human failing heart in models of cardiomyopathy . Hence, it was proposed that the activation of the NMRK2 pathway represents a common adaptive mechanism in the failing heart, while the Nmrk2 gene can be activated in response to NAMPT inhibition.
Furthermore, the shift from NAMPT to NMRK2 for NAD+ synthesis is an energy-sparing mechanism that may be favored, since NMN synthesis from NR by NMRK enzymes requires a single ATP, while synthesis from NAM by NAMPT requires more than three ATP equivalents (Figure 2). Although the NMRK2 pathway is activated in the HF mice, the myocardial NAD+ level is depressed, which suggests that circulating and tissue levels of NAD+ precursors are insufficient to sustain cardiac NAD+ synthesis on a regular rodent diet.
This supports the interest in NR supplementation to improve this condition. Moreover, a strong beneficial effect of NR was discovered in mouse models of HF with the preservation of cardiac function and a limitation of cardiac remodeling, that was associated with maintained NAD+ levels in the heart. This further suggests that oral NR supplementation is a powerful approach to preserve cardiac function and limit remodeling in DCM.
NAD+ precursor supplementation also has the potential to protect against adverse cardiac remodeling by additional mechanisms of activating SIRTs (Table 2) and maintaining Ca2+ homeostasis . Namely, in vivo activation of SIRT1 protects against cardiac hypertrophy, metabolic dysregulation, and cardiac inflammation in a mouse model of cardiac hypertrophy, and exhibits protective effects in other models of cardiac dysfunction [68,69,70,71].
In addition, SIRT2 and SIRT6 have emerged as prominent cardioprotective SIRTs [72,73], as deficiency of SIRT2 intensified cardiac hypertrophy in aged mice and mice stressed with angiotensin II , while a loss of SIRT6 in mice resulted in the development of cardiac hypertrophy and HF . The activity of mitochondrial SIRTs is implicated in cardiac remodeling and the development of HF, including SIRT3 which appears to be required for maintaining cardiac function .
Data further suggests NR as a favored NAD+ precursor in mitochondria , while in vivo NR effects have been interpreted as dependent on mitochondrial sirtuin activities [37,77]. However, the importance of nucleocytosolic targets should not be excluded [1,78]. NR-induced reduction of systolic blood pressure (SBP) and aortic stiffness , two clinically important risk indicators of cardiovascular function and health [79,80], may occur due to nuclear and cytosolic SIRT1 activation (Table 1).
Namely, NAD+ is an obligate substrate for the deacetylase SIRT1, which has been implicated in the maintenance of healthy vascular function [81,82,83]. Although NAD+ involvement in numerous physiological responses is not yet fully understood, documented health benefits in mice models validate the increasing interest in the translation of NR to therapy for cardiovascular diseases, especially in HF and cardiac hypertrophy.
The roles of sirtuins in heart failure development acquired from experiments on knock-out and transgenic mice (adapted from Pillai et al., ).
|SIRT1||Heart cell growth and development, mediation of cardiac hypertrophy, protection from ischemic injury; partial deficiency protects from pressure overload-induced hypertrophy and failure|
|SIRT2||Mediating ischemic injury due to attenuated programmed apoptosis|
|SIRT3||Protection from age-induced hypertrophy, fibrosis and contractile dysfunction, prevents susceptibility to cardiac hypertrophic stimuli|
|SIRT6||Protection from cardiac hypertrophy and heart failure|
|SIRT7||Protection from spontaneous cardiac hypertrophy and inflammatory cardiomyopathy|
|SIRT1||Low to moderate overexpression attenuates age-dependent decline in cardiac functions in mice, while high overexpression induces cardiac hypertrophy and heart failure|
|SIRT3||Cardiac-specific overexpression protects the heart from hypertrophic stimuli by preserving mitochondrial function|
|SIRT6||Cardiac-specific overexpression protects the heart from hypertrophic stimuli by blocking activation of Akt signaling at the level of chromatin|
Effects on Neurodegenerative Disorders
Neurodegenerative disorders are associated with DNA damage and oxidative stress, which accumulate with age  leading to impaired mitochondrial function . In addition, NAD+ depletion was observed during the aging process in multiple animals, including humans, and is considered the major risk factor for Alzheimer’s disease (AD) .
When administered in AD mouse models, NR displays beneficial effects on both oxidative stress and DNA repair by increasing NAD+ levels . Additionally, NR can improve other aspects of AD neuropathology including pTau, amyloid-β, neurogenesis, neuroinflammation, hippocampal synaptic plasticity, and cognition [89,90].
Specifically, NR treatment reduce neuroinflammation and amyloidogenesis in the whole brain of high-fat diet (HFD)-fed mice, by decreasing amyloid-β levels and several inflammatory markers (NLRP3, CASP1, IL-1, TNF-α, and IL-6) . Since brain inflammation is closely related to cognitive impairment [92,93,94], cognitive function and recognition memory could be attenuated by NR treatment in only 6 weeks .
Furthermore, increased PARylation, another hallmark of AD, could be decreased in AD mice with NR supplementation . Increased PARylation has also been detected in several other neurodegenerative disorders that involve DNA repair defects, including Cockayne syndrome, xeroderma pigmentosum and ataxia-telangiectasia.
Nevertheless, PARP-mediated NAD+ depletion was recently confirmed to play a major role in the pathogenesis of these disorders. Despite the underlying DNA repair deficiencies, NR could dramatically improve the phenotype of each of these conditions in mouse models, while prolonging survival by more than three times in ataxic mice [95,96].
Another early event that occurs in acute cerebral injury and in chronic neurodegenerative diseases, including Alzheimer’s and Parkinson’s diseases, is axonal degeneration [97,98,99,100]. In such cases, axonal degeneration is caused by excitotoxicity, which is another feature implicated in most neurodegenerative disorders that affect the central nervous system.
Interestingly, a strong NAD+ depletion in neurons has been revealed during excitotoxicity [101,102,103,104], whereas mice injected with NR were protected from excitotoxicity-induced axonal degeneration . Among three tested NAD+ precursors (NA, NAM, and NR) including NAD+, only NR could prevent axonal degeneration by altering the local NR metabolism within the axon [105,106].
Namely, NR prevented nuclear condensation and axonal degeneration in neurons of Nmrk2-KO mice, by inducing Nmrk1 , which suggests that Nmrk1 might be the key mediator of the neuroprotective activity of NR. This neuroprotective effect depends on both mitochondrial and axonal NAD+ content .
At present, two possible mechanisms of neuroprotection have been proposed: increasing mitochondrial NAD+ to support SIRT3  and preserving axonal NAD+ to supply damage-induced SARM1 activation . Via SIRT3 activation, a NR-induced increase in NAD+ levels can have additional beneficial and possible therapeutic effects. Specifically, NR has been shown to prevent noise-induced hearing loss and neurite retraction from hair cells in the inner ear through a SIRT3-dependent mechanism .
Moreover, SIRT3 and 5 are both critical for the health of retinal photoreceptors . SIRT3 activity is sensitive to the reduction of NAD+ content that has been detected in multiple disorders with retinal degeneration, including age-associated dysfunction, diabetic retinopathy, and light-induced degeneration in mice . This suggests a potential therapeutic treatment with NR for a variety of disorders that include photoreceptor degeneration.
Furthermore, a general decrease in NAD+ levels has been observed in neuromuscular diseases which are often caused by inherited mutations that lead to progressive skeletal muscle weakness and degeneration . Increasing NAD+ content with NR can stimulate energy production and improve mitochondria function.NR has been shown to have therapeutic effects in several muscle disorders in mouse models.
Although supplementation with NR could not correct the underlying genetic defects, it could improve mitochondrial abundance and function in two different mitochondrial myopathies [109,110]. Furthermore, NR could reverse the progressive wasting syndrome in skeletal muscle in mice lacking Nampt whilst restoring endurance, in as little as 1 week of treatment . Accordingly, NR was found to confer increased endurance and improved cold tolerance in the HFD-fed mice. However, whether NR has significant benefits in lean, healthy muscle is less clear, as only a nonsignificant trend toward increased endurance was observed in regular chow-fed mice .
Interestingly, decreased NAD+ levels and Nampt expression were also observed in Duchenne’s muscular dystrophy (DMD), a condition histologically and transcriptionally similar to progressive wasting syndrome [112,113]. DMD is also characterized by increased PARP activity, fibrosis, and muscular degeneration. NR was found to improve muscular function and heart pathology in mdx mice models of DMD and decrease PARylation, inflammation, and fibrosis .
Moreover, NR treatment has been shown to improve stem cell function and thereby ameliorate the muscle wasting phenotype in mdx mice, supporting the use of NR for the human condition [38,112]. Improvement in stem cell function appears to be a general phenomenon during NR treatment and has been suggested to underlie a small, but significant extension of lifespan in mice . Altogether, these findings support that NR could be effective in managing the progression of muscular dystrophy and degeneration, by improving muscular strength, rejuvenating senescent muscle stem cells, and reducing levels of inflammation and fibrosis.
Effects on Longevity
Calorie restriction (CR) is considered the most effective approach to extend lifespan in eukaryotes since the first report of lifespan extension in wild-type yeast cells via regulation of Sir2 and NAD+ . CR life-prolonging effects may partly be mediated via increased sirtuin function, while the requirement of NAD+ for their activity suggests a possible connection between aging and metabolism. However, the nutritional approach for increasing Sir2 activity and longevity has been accomplished by engineered gene overexpression in yeast , while NA failed to extend lifespan, and NAM shortened it [116,117].
On the other hand, as with CR, NR can increase NAD+ levels and Sir2 function, while exogenous NR promotes Sir2-dependent repression of recombination, improves gene silencing, and extends lifespan without calorie restriction . Moreover, the mechanism of action of NR is completely dependent on increased net NAD+ synthesis through the Nrk1 and the Urh1/Pnp1/4 pathways.
The latter is Nrk1 independent and represents a newly discovered NR salvage pathway . Furthermore, a study in mouse models documented that a one-day fast increases NAD+ in the liver  whereas CR elevates NAD+ and reduces NAM in the brain . This implies that the increased levels of NAD+ appear to mediate several beneficial effects of CR, supporting the life-prolonging effects of NR supplementation.
These effects are mediated via improvement of metabolism and decrease in chronic inflammation, a hallmark of aging . Preclinical studies have reported that NR reduces macrophage infiltration in damaged muscles [38,112] and attenuates plasma TNF-α in models of fatty liver disease .
Nevertheless, a recent clinical study confirmed NR availability in muscular tissue in aged human subjects  and its anti-inflammatory effects. Namely, a 21-day supplementation of NR decreased numerous circulating inflammatory cytokines , implying additional mechanisms through which NR can potentially modulate the aging process and thereby exhibit life-prolonging effects. While the exact mechanisms through which NR exerts these effects remain unclear, the apparent health benefits described indicate positive effects of NR on longevity.
Infection Treatment and Immunomodulatory Effects
NAD+ intermediates have been recognized for their beneficial health effects during infection with several pathogens. Studies have confirmed antimycobacterial effect of NAM in patients infected with Mycobacterium tuberculosis [121,122], while immune-mediated elimination was reported for Staphylococcus aureus, including MRSA and other major human pathogens such as Klebsiella pneumoniae and Pseudomonas aeruginosa .
In addition, NAM and its analogues exhibited antiviral effect in patients with HIV  and hepatitis B . The potential treatment with NAD+ intermediates has been recognized recently for combating COVID-19 infection that currently lacks efficient therapeutic or preventive agents and represents a global concern for public health.
SARS-CoV-2 infection triggers a maladaptive immune response. Notably an exaggerated proinflammatory response leading to a “cytokine storm” in the lung tissue, and lymphopenia with a drastic decline of CD4+ and CD8+ T cells . On the molecular level, as the innate immune response activates to fight the infection, the activation of PARPs increases due to extensive DNA damage and IFN-induced MARylation (mono-ADP-ribosylation) of the target SARS-CoV-2 proteins [126,127].
PARPs response is required for inhibition of viral replication ; however, this antiviral effect is reversed by the ADP-ribosylhydrolase macrodomain of the viral nonstructural protein, nsp3, whose activity is required for virulence [126,127,129]. In addition, nsp10 of SARS-CoV was found to inhibit electron transport at the NADH site of complex I in the mitochondrial electron transport chain , suggesting that key events in the innate immune response to viral infections are occurring within the infected cell’s NAD+ metabolome .
Recent study investigated PARPs expression and NAD+ metabolome dysregulation due to coronavirus infection. The examined SARS-CoV-2 infected cell lines of a ferret and deceased patients’ lungs showed disturbed NAD+ metabolism and gene expression with respect to synthesis and utilization of NAD+ .
Furthermore, the expression of the NMRK1 pathway was upregulated together with the expression of the concentrative nucleoside transporter CNT3, indicating a higher capacity for NR conversion to NAD+ and NADP+ during the infection . An upregulation of the NMRK gene was previously associated with therapeutic efficacy of NR [66,105].
In addition, the expression of NNMT (nicotinamide N-methyltransferase) was decreased  due to lower NAM methylation, suggesting a promotion of the NAM salvage pathway  and increased efficiency of NR treatment to replenish NAD+ . These data indicate that boosting NAD+ content through NAM and NR kinase pathways may restore antiviral PARPs functions to support innate immunity to SARS-CoV-2 .
Upon activation of the adaptive immune response, the overexpression of CD38 in both CD4+ and CD8+ lymphocytes further exaggerates NAD+ depletion [133,134] leading to increased production and release of proinflammatory cytokines, reactive oxygen species, and macrophage infiltration [135,136].
Furthermore, drastic NAD+ depletion impairs the function of sirtuins, the regulators of cell death and viability . Specifically, SIRT1 regulates the expression of genes including tumor suppressors, cytokines and proto-oncogenes and ultimately modulates inflammation, cell survival, and apoptosis mechanisms .
Loss of sirtuin function along with increased oxidative damage and an overall energy decrease finally culminates in cell death. Replenishment of NAD+ body content could restore energy levels and impaired sirtuin function and possibly rebalance the maladaptive immune response to SARS-CoV-2 infection. Namely, both SARS-CoV and SARS-CoV-2 induce maladaptive hyperinflammation followed by increased leucocyte infiltration into the lungs resulting in extensive tissue damage and subsequent organ failure with reduced lung capacity [138,139,140].
Emerging evidence has demonstrated that NAD+ is released during the early phase of inflammation and has an immunoregulatory role in vivo [141,142].
Moreover, niacin was previously suggested as an anti-inflammatory therapy in one preclinical study as a potent agent to decrease proinflammatory cytokines, including IL-1, IL-6, and TNFα . NR can similarly decrease IL-2, IL-5, IL-6, and TNFα .
Targeting IL-6 has been recently proposed as a promising treatment to block the inflammatory storm, especially in severe COVID-19 patients . Furthermore, niacin might also decrease neutrophil infiltration, while exhibiting a prolonged anti-inflammatory effect during ventilator-induced lung injury.
However, niacin exacerbated hypoxemia regardless of the neutrophil infiltration decrease, suggesting a different cause of hypoxemia, independent of neutrophil decline [144,145], which requires further investigation. In addition, high efficiency of vitamin B3 (niacin or nicotinamide) in preventing lung tissue damage was confirmed in several animal models with bleomycin- and LPS-induced lung injury [146,147,148].
Considering vitamin B3′s strong lung-protective effects, it has been proposed as an early treatment-supporting agent against COVID-19 . This implies that NR should be considered as a potential therapeutic or supporting agent to reduce hyperinflammation and regenerate damaged lung tissue.
reference link : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7352172/
More information: Mario Romani et al, NAD+ boosting reduces age-associated amyloidosis and restores mitochondrial homeostasis in muscle, Cell Reports (2021). DOI: 10.1016/j.celrep.2020.108660