Nicotinamide riboside kinases display redundancy in mediating nicotinamide mononucleotide and nicotinamide riboside metabolism in skeletal muscle cells.

Objective: Augmenting nicotinamide adenine dinucleotide (NAD ⁺ ) availability may protect skeletal muscle from age-related metabolic decline. Dietary supplementation of NAD ⁺ precursors nicotinamide mononucleotide (NMN) and nicotinamide riboside (NR) appear efficacious in elevating muscle NAD ⁺ . Here we sought to identify the pathways skeletal muscle cells utilize to synthesize NAD ⁺ from NMN and NR and provide insight into mechanisms of muscle metabolic homeostasis.
Methods: We exploited expression profiling of muscle NAD ⁺ biosynthetic pathways, single and double nicotinamide riboside kinase 1/2 (NRK1/2) loss-of-function mice, and pharmacological inhibition of muscle NAD ⁺ recycling to evaluate NMN and NR utilization.
Results: Skeletal muscle cells primarily rely on nicotinamide phosphoribosyltransferase (NAMPT), NRK1, and NRK2 for salvage biosynthesis of NAD ⁺ . NAMPT inhibition depletes muscle NAD ⁺ availability and can be rescued by NR and NMN as the preferred precursors for elevating muscle cell NAD ⁺ in a pathway that depends on NRK1 and NRK2. Nrk2 knockout mice develop normally and show subtle alterations to their NAD+ metabolome and expression of related genes. NRK1, NRK2, and double KO myotubes revealed redundancy in the NRK dependent metabolism of NR to NAD ⁺ . Significantly, these models revealed that NMN supplementation is also dependent upon NRK activity to enhance NAD ⁺ availability.
Conclusions: These results identify skeletal muscle cells as requiring NAMPT to maintain NAD ⁺ availability and reveal that NRK1 and 2 display overlapping function in salvage of exogenous NR and NMN to augment intracellular NAD ⁺ availability.