Axonal degeneration is an active program of self-destruction that is observed in many physiological and pathological settings. In Wallerian degeneration slow (wlds) mice, Wallerian degeneration in response to axonal injury is delayed because of a mutation that results in overexpression of a chimeric protein (Wlds) composed of the ubiquitin assembly protein Ufd2a and the nicotinamide adenine dinucleotide (NAD) biosynthetic enzyme Nmnat1. We demonstrate that increased Nmnat activity is responsible for the axon-sparing activity of the Wlds protein. Furthermore, we demonstrate that SIRT1, a mammalian ortholog of Sir2, is the downstream effector of increased Nmnat activity that leads to axonal protection. These findings suggest that novel therapeutic strategies directed at increasing the supply of NAD and/or Sir2 activation may be effective for treatment of diseases characterized by axonopathy and neurodegeneration.

Intracellular nicotinamide phosphoribosyltransferase (iNampt) is an essential enzyme in the NAD biosynthetic pathway. An extracellular form of this protein (eNampt) has been reported to act as a cytokine named PBEF or an insulin-mimetic hormone named visfatin, but its physiological relevance remains controversial. Here we show that eNampt does not exert insulin-mimetic effects in vitro or in vivo but rather exhibits robust NAD biosynthetic activity. Haplodeficiency and chemical inhibition of Nampt cause defects in NAD biosynthesis and glucose-stimulated insulin secretion in pancreatic islets in vivo and in vitro. These defects are corrected by administration of nicotinamide mononucleotide (NMN), a product of the Nampt reaction. A high concentration of NMN is present in mouse plasma, and plasma eNampt and NMN levels are reduced in Nampt heterozygous females. Our results demonstrate that Nampt-mediated systemic NAD biosynthesis is critical for beta cell function, suggesting a vital framework for the regulation of glucose homeostasis.

The Wnt signaling pathway is essential for development and organogenesis. Wnt signaling stabilizes beta-catenin, which accumulates in the cytoplasm, binds to 1-cell factor (TCF; also known as lymphocyte enhancer-binding factor, LEF) and then upregulates downstream genes. Mutations in CTNNB1 (encoding beta-catenin) or APC (adenomatous polyposis coli) have been reported in human neoplasms including colon cancers and hepatocellular carcinomas (HCCs). Because HCC5 tend to show accumulation of beta-catenin more often than mutations in CTNNB1, we looked for mutations in AXIN1, encoding a key factor for Wnt signaling, in 6 HCC cell lines and 100 primary HCC5. Among the 4 cell lines and 87 HCC5 in which we did not detect CTNNB1 mutations, we identified AXIN1 mutations in 3 cell lines and 6 mutations in 5 of the primary HCCs. In cell lines containing mutations in either gene, we observed increased DNA binding of TCF associated with beta-catenin in nuclei. Adenovirus mediated gene transfer of wild-type AXINI induced apoptosis in hepatocellular and colorectal cancer cells that had accumulated beta-catenin as a consequence of either APC, CTNNB1 or AXIN1 mutation, suggesting that axin may be an effective therapeutic molecule for suppressing growth of hepatocellular and colorectal cancers.

SUMMARY Axonal degeneration is an early and prominent feature of many neurological disorders. SARM1 is the central executioner of the axonal degeneration pathway that culminates in depletion of axonal NAD+; yet the identity of the underlying NAD+ depleting enzyme(s) is unknown. Here, in a series of experiments using purified proteins from mammalian cells, bacteria, and a cell-free protein translation system, we show that the SARM1-TIR domain itself has intrinsic NADase activity – cleaving NAD+ into ADP Ribose (ADPR), cyclic ADPR, and Nicotinamide, with Nicotinamide serving as a feedback inhibitor of the enzyme. Using traumatic and vincristine-induced injury models in neurons, we demonstrate that the NADase activity of full-length SARM1 is required in axons to promote axonal NAD+ depletion and axonal degeneration after injury. Hence, the SARM1 enzyme represents a novel therapeutic target for axonopathies. Moreover, the widely utilized TIR-domain is a protein motif that can possess enzymatic activity.

Summary NAD+ availability decreases with age and in certain disease conditions. Nicotinamide mononucleotide (NMN), a key NAD+ intermediate, has been shown to enhance NAD+ biosynthesis and ameliorate various pathologies in mouse disease models. In this study, we conducted a 12 month-long NMN administration to regular chow-fed wild-type C57BL/6N mice during their normal aging. Orally administered NMN was quickly utilized to synthesize NAD+ in tissues. Remarkably, NMN effectively mitigates age-associated physiological decline in mice. Without any obvious toxicity or deleterious effects, NMN suppressed age-associated body weight gain, enhanced energy metabolism, promoted physical activity, improved insulin sensitivity and plasma lipid profile, and ameliorated eye function and other pathophysiologies. Consistent with these phenotypes, NMN prevented age-associated gene expression changes in key metabolic organs and enhanced mitochondrial oxidative metabolism and mitonuclear protein imbalance in skeletal muscle. These effects of NMN highlight the preventive and therapeutic potential of NAD+ intermediates as effective anti-aging interventions in humans.