The role of nicotinamide riboside kinases in mammalian NAD⁺ metabolism

NAD+ has emerged as a central metabolic node and an important co-substrate for the activity of various enzymes, including the protein deacetylase SIRT1. Different strategies to increase NAD+ bioavailability have been shown to boost SIRT1 activity, which results in protection against metabolic disease, neurodegenerative disorders and certain types of cancer. Nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN) have recently been discovered as new NAD+ precursors. Recent studies have shown that dietary supplementation with NR or NMN prevented against high-fat diet-induced body weight gain. However, the physiological role of NR in mammalian metabolism remains largely unexplored. In this project, we focused on the first step of NR metabolism – the phosphorylation of NR by nicotinamide riboside kinases (NRK1 and NRK2). We have investigated the role of NRKs in the metabolism of NAD+ precursors and mammalian physiology in vivo. The results obtained indicate that NRKs are the rate-limiting and essential step for NR- and NMN-driven NAD+ synthesis in hepatocytes and that NMN has to be extracellularly converted to NR for its use as an NAD+ precursor. Accordingly, animals lacking NRK1 have defects in NR/NMN utilization. We demonstrated that effective NR utilization is key for the maintenance of metabolic homeostasis as NRK1 whole-body KO mice display numerous alterations in glucose and lipid management. Using NRK1 liver-specific KO mice, we demonstrate that NRK1 is required to maintain hepatic NAD+ levels and mitochondrial function upon high-fat feeding. Consequently, NRK1LKO mice are sensitized to diet-induced glucose intolerance and insulin resistance. Finally, we show that both isoforms of NRK are redundant in mediating NR/NMN effects in muscle. Hence, this work identifies NRKs as a novel valuable target in the fight against metabolic disease and indicates that the regulation of NRK will enable us to find novel ways of enhancing NAD+ availability. Moreover, it has unveiled a critical role of NRK1 for the metabolism of diverse forms of Vitamin B3 and highlights how circulating NR is key to preserve metabolic health upon dietary challenges.