Finally, NAD+ is used in cells to generate other important bioactive derivatives, such as cyclic ADP ribose (cADPR) and 1-methylnicotinamide (Figures 1d and and2).2). cADPR is generated (and can be hydrolyzed) by CD38 and its relative CD157, and mutations in CD38 not only lower production of cADPR but also substantially raise NAD+ levels in mice 40, 41. cADPR can play an important role in signaling by stimulating intracellular calcium release, and the range of its biological functions are just beginning to be uncovered 42. 1-methylnicotinamide is made by nicotinamide-N-methyltransferase from the NAD+ cleavage product nicotinamide (Figure 2). A recent study has shown that 1-methylnicotinamide plays an important role in the extension of worm life span by the sirtuin SIR-2.1, the ortholog of mammalian SIRT1 21.

NAD+ declines with aging and can be restored by supplementation with NAD+ precursors

Several studies have reported that the activity of sirtuins decays with aging 34, 43, 44. The mammalian Sir2 ortholog SIRT1 can be regulated by many mechanisms, including transcriptionally, and post-translationally by changes in stability, phosphorylation, SIRT1-binding proteins, and by changes in NAD+ levels 14. Of these mechanisms regulating SIRT1, a systemic decline in NAD+ has emerged as a likely explanation for why aging affects sirtuins. The decline in NAD+ was first noticed in transgenic mice overexpressing SIRT1 in pancreatic β cells (BESTO mice) 44. BESTO mice showed enhanced glucose-stimulated insulin secretion when they were young, but lost this phenotype when they became old. Importantly, administration of a key NAD+ intermediate, nicotinamide mononucleotide (NMN), restored the metabolic phenotype in old BESTO mice and enhanced insulin secretion in old wild-type control mice. Note NMN can be converted into NAD+ by NMN adenylyltransferases (NMNATs) in one step (Figure 2). This finding suggests that a decrease in NAD+ with aging was responsible for the loss of the phenotype in pancreatic β cells of BESTO mice. Consistent with this surmise, NAD+ levels have been shown to decline approximately 2-fold in old worms and in multiple tissues, including liver and skeletal muscle, in aged mice 18, 43, 45.
Another supplementation study with NMN has been shown to restore NAD+ levels and prevent diet- and age-induced type 2 diabetes in wild-type mice 45. In a recent study, NMN was reported to dramatically reverse the effects of aging at the cellular and organismal levels 43. Another NAD+ intermediate, nicotinamide riboside (NR), can also be converted to NAD+, after conversion to NMN via NR kinase (Nrk) 46, 47 (Figure 2). Like NMN, NR boosts NAD+ levels in worms and mice and can counter effects of aging 18, 48. NR supplementation also increases mitochondrial NAD+ levels and stimulates SIRT3-mediated deacetylation of mitochondrial proteins 48.
Importantly, NAD+ intermediate supplementation appears to restore NAD+ levels in both nuclear and mitochondrial compartments of cells. In one study, aging was shown to trigger SIRT1 inactivation, which was reversed by NMN, demonstrating supplementation of an NAD+ deficiency in the nuclear/cytosolic pool 43. In another study, a mitochondrial deficiency in complex I of the electron transport chain led to depletion of mitochondrial NAD+ due to accumulation of NADH, inactivation of the mitochondrial SIRT3, and severe cardiac damage 49. These effects could also be corrected by supplementation with NMN 49. Thus, the benefits of NAD+ intermediate supplementation appear to be due to reactivation of sirtuins. Alternatively, reactivation of other NAD+-dependent enzymes may be critical in improving health by this supplementation.

Possible mechanisms for how NAD+ levels decline in aging

Why do NAD+ levels decline with aging? One possibility is that one or more of the NAD+ biosynthetic pathways decline. Indeed, there is some evidence that levels of NAMPT decline during aging 45, whereas exercise training has the opposite effect, at least in skeletal muscle 50. Moreover, as discussed above, NAMPT is a major output of the circadian transcription factors BMAL and CLOCK. If the activity of the circadian machinery systemically declined with aging, as appears to be the case in the SCN 34, a deficit in NAMPT and NAD+ would result (Figure 3). Under such conditions, the use of NAD+ intermediates, such as NMN and NR, rather than earlier NAD+ precursors like nicotinamide, would be critical to enhance NAD+ biosynthesis efficiently in aged individuals.