Targeting mitochondrial bioenergetics and NAD+ metabolism during sarcopenia and regeneration of skeletal muscle

The progressive decline of muscle mass and function called sarcopenia is a multi-factorial process associated to frailty, disability, and low quality of life in the elderly. The co-factor nicotinamide adenine dinucleotide (NAD+) becomes a limiting factor in the course of aging and other pathological conditions. It has emerged as a major regulator of cellular metabolism, which is modulated by dietary precursors of the vitamin B3 family. So far, no direct link between sarcopenia and alterations in NAD+ metabolism has been reported. In this thesis, I have taken a multi-disciplinary approach combining molecular profiling in humans and dietary and genetic manipulations in rodents to characterize the skeletal muscle NAD+ metabolism in the context of muscle plasticity and aging. We have performed a multi-center study to characterize the molecular signature of human sarcopenia compared to age-matched controls. Sarcopenic participants of three cohorts in Singapore, the United Kingdom and Jamaica presented a prominent transcriptional signature of mitochondrial dysfunction, which translated into functional bioenergetic deficiency with lower mitochondrial protein expression and activity. Furthermore, we established a concurrent decrease in NAD+ content of sarcopenic muscle, which correlated to lower muscle strength and function in these patients. Supplementation with the dietary NAD+ precursor nicotinamide riboside (NR) was shown to improve pathological muscle conditions and revert age-related mitochondrial dysfunction and stem cell senescence in mice. To test whether boosting NAD+ through NR can be beneficial in the context of sarcopenia, we investigated the acute response to NR supplementation in young adult and old sarcopenic rats. Acute NR treatment efficiently increased NAD+ levels and normalized specific age-related gene expression signatures in old rats, in particular related to fibrosis. Interestingly, our transcriptional profiling also revealed that the molecular response to NR is partially blunted in aging and suggests that aged muscle could be in a state of partial NAD+ resistance. To investigate the role of endogenous NR as NAD+ precursor in the muscle, we studied mice deficient for NR kinases 1 and 2 (NRK1/2 dKO), the rate-limiting enzymes for NR salvage. NRK1/2 dKO mice develop normally, but exhibit elevated salvage of the NAD+ precursor nicotinamide (NAM) to maintain tissue NAD+. Thus, our results demonstrate for the first time that an endogenous NRK-dependent flux actively converts NR into NAD+ in healthy skeletal muscle. Moreover, we demonstrated that NRKs are required for muscle regeneration, as dKO mice failed to efficiently regenerate the NAD+ pool during the active phases of myofiber remodeling, which coincided with a transient delay of myofiber maturation. Conversely, dietary NR improved the activation of muscle stem cells and accelerated recovery of NAD+ levels in regenerating myofibers of wild-type mice. Altogether, this work demonstrates that both in humans and preclinical models, sarcopenia is tightly linked to mitochondrial bioenergetics and NAD+ metabolism. Dietary manipulation of NAD+ levels is a promising therapeutic strategy for the management of age-related muscle dysfunction for which we uncovered important mechanistic insights linking metabolic fluxes through the NAD+ pathway to physiological adaptations.