Metabolic rescue of muscle and muscle stem cells in muscle disease

Duchenne muscular dystrophy (DMD), caused by the mutation of dystrophin gene, is an X-linked disorder that affects 1 in 3500 males, leading to progressive muscle degeneration and eventually resulting in premature death. Increasing evidence indicates that DMD and age-related sarcopenia share clinical hallmarks, such as decreased metabolic activity of both myofibers and muscle stem cells (MuSCs), which results in defective MuSC function and impaired muscle regeneration. In my Ph.D. thesis, I focused on manipulating mitochondrial function, such as mitophagy, and lipid metabolism, particularly sphingolipid metabolic pathways, to restore muscle and MuSC functions in DMD and age-associated sarcopenia. My thesis consists of three projects:

Rescue of mitophagy improves muscle function in DMD. In this project, we identified defective mitophagy as a hallmark of muscular dystrophy. Rescue of dysfunctional mitophagy by dietary supplementation of Urolithin A (UA), a metabolite present in fruit extracts and a potent activator of mitophagy, improved muscle function in mdx mice. The beneficial effects of mitophagy restoration observed in dystrophic animals were attributed to the improvement in mitochondrial function, structural integrity of muscles, restoration of MuSC activity, and reduction in general inflammation and fibrosis.

Sphingolipid depletion improves muscle regeneration during ageing. We first established the link between sphingolipid metabolism and ageing by demonstrating that sphingolipids accumulate in skeletal muscle upon aging. We reduced skeletal muscle sphingolipid accumulation by treating aged mice with myriocin, a selective inhibitor of serine pamitoyltransferase (SPT), the first and rate-limiting enzyme of sphingolipid de novo synthesis pathway. Sphingolipid depletion increased MuSC regenerative ability through enhancing the proliferation and differentiation capacities of MuSCs, ultimately leading to the prevention of age-related decline in muscle mass and function.

Inhibition of sphingolipid synthesis reverts muscular dystrophy. By studying skeletal muscle transcript profiles of human muscular dystrophies, we identified upregulation of sphingolipid biosynthetic pathway as a universal feature of human patients affected with muscular dystrophy. To rescue the aberrant sphingolipid metabolism, we treated mdx mice with myriocin and showed that inhibition of sphingolipid generation enhanced muscle function, leading to amelioration of DMD. Furthermore, we uncovered a novel role of sphingolipid depletion in macrophage polarization and in reconstruction of the mdx MuSC niche, resulting in improved regenerative capacity of MuSCs.

In summary, our work establishes the essential roles of mitophagy and sphingolipid metabolism in maintaining muscle and MuSC function. Restoration of mitophagy and blockade of sphingolipid generation could therefore be attractive treatment strategies to delay the progression of DMD and age-related sarcopenia.

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

Tanja Sonntag

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.

EPFL2019

Metabolic control of muscle and muscle stem cell function

Hongbo Zhang

Skeletal muscle composed of myofibers and a small amount of muscle stem cells (MuSCs). It plays important roles in energy metabolism. Some of the key metabolites, such as NAD+ and acetyl-CoA were recently found to regulate muscle and MuSCs function. Most of these functions are related to the cellular mitochondria. In my PhD thesis projects, we aimed at exploring the link between NAD+ levels, mitochondrial function, muscle structural homeostasis and MuSCs senescence. We also investigated the impact of acetyl-CoA on MuSCs activation and proliferation, through the control of protein acetylation. These projects mainly focus on three aspects: Rejuvenation of adult stem cells by NAD+ repletion. Aging is accompanied by SCs senescence. However, the initial signaling events mediating SCs senescence are still not well defined. We demonstrate the importance of mitochondrial activity as a pivotal modulator of MuSCs senescence. MuSCs from aged mice have reduced NAD+ levels, mitochondrial content and capacity for oxidative respiration. Importantly, the induction of the mitochondrial unfolded protein response (UPRmt), subsequent to increasing cellular NAD+ with the precursor nicotinamide riboside (NR), prevents MuSCs senescence and improves muscle function and regeneration in aged mice. NR also prevents MuSCs dysfunction in the mdx mouse, a known mouse model of accelerated MuSCs senescence. Extending these observations to other SC pools and on the organism as a whole, we demonstrate that NR delays neural and melanocyte SCs senescence, while also increasing mouse lifespan. Control of MuSCs function by acetyltransferase KAT2A. MuSCs are essential for skeletal muscle homeostasis and repair. The complete mechanism controlling MuSCs maintenance and differentiation, however, remains elusive. We found that the acetyltransferase KAT2A expression is essential for myogenic differentiation of MuSCs. KAT2A loss-of-function in cells blocks MuSCs differentiation, and in mice impairs muscle regeneration after damage. The regulation of KAT2A in MuSCs function might rely on its ability to enzymatically acetylate other proteins, and in particular the transcription factor PAX7. NAD+ repletion improves muscle function in muscular dystrophy. Duchenne muscular dystrophy (DMD) is caused by dystrophin gene mutations that lead to progressive muscle degeneration. Our bioinformatics analysis in mice and DMD patients suggests a role for NAD+ in protecting the muscle from metabolic and structural degeneration. Validating these findings, we reveal that the mdx mouse is characterized by reductions in muscle NAD+ levels, concurrent to increased PARP activity and reduced expression of nicotinamide phosphoribosyltransferase (NAMPT), the key enzymes for NAD+ consumption and biosynthesis. Replenishing the NAD+ pool by dietary NR supplementation benefits muscle function in mdx and mdx/Utr-/- mice, an effect replicated in C. elegans models of DMD. The beneficial effects of NAD+ repletion in muscular dystrophy are pleiotropic and rely on the improvement in mitochondrial function, structural protein expression and on reductions in general PARylation, inflammation and fibrosis. In summary, our work demonstrates the critical role of NAD+ and Acetyl-coA in maintaining muscle and MuSCs function. Repletion of NAD+ levels, by NR administration, and/or boosting the function of acetyltransferase KAT2A might be attractive strategies to maintain muscle homeostasis and MuSCs function in aging and muscular dystrophy.