NAD(+) repletion improves muscle function in muscular dystrophy and counters global PARylation

Neuromuscular diseases are often caused by inherited mutations that lead to progressive skeletal muscle weakness and degeneration. In diverse populations of normal healthy mice, we observed correlations between the abundance of mRNA transcripts related to mitochondrial biogenesis, the dystrophin-sarcoglycan complex, and nicotinamide adenine dinucleotide (NAD(+)) synthesis, consistent with a potential role for the essential cofactor NAD(+) in protecting muscle from metabolic and structural degeneration. Furthermore, the skeletal muscle transcriptomes of patients with Duchene's muscular dystrophy (DMD) and other muscle diseases were enriched for various poly[adenosine 5'-diphosphate (ADP)-ribose] polymerases (PARPs) and for nicotinamide N-methyltransferase (NNMT), enzymes that are major consumers of NAD(+) and are involved in pleiotropic events, including inflammation. In the mdx mouse model of DMD, we observed significant reductions in muscle NAD(+) levels, concurrent increases in PARP activity, and reduced expression of nicotinamide phosphoribosyltransferase (NAMPT), the rate-limiting enzyme for NAD+ biosynthesis. Replenishing NAD(+) stores with dietary nicotinamide riboside supplementation improved muscle function and heart pathology in mdx and mdx/Utr(-/-)mice and reversed pathology in Caenorhabditis elegans models of DMD. The effects of NAD(+) repletion in mdx mice relied on the improvement in mitochondrial function and structural protein expression (alpha-dystrobrevin and delta-sarcoglycan) and on the reductions in general poly(ADP)-ribosylation, inflammation, and fibrosis. In combination, these studies suggest that the replenishment of NAD(+) may benefit patients with muscular dystrophies or other neuromuscular degenerative conditions characterized by the PARP/NNMT gene expression signatures.

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.

EPFL2016

Effective treatment of mitochondrial myopathy by nicotinamide riboside, a vitamin B3

Johan Auwerx, Eija Riitta Pirinen

Nutrient availability is the major regulator of life and reproduction, and a complex cellular signaling network has evolved to adapt organisms to fasting. These sensor pathways monitor cellular energy metabolism, especially mitochondrial ATP production and NAD+/NADH ratio, as major signals for nutritional state. We hypothesized that these signals would be modified by mitochondrial respiratory chain disease, because of inefficient NADH utilization and ATP production. Oral administration of nicotinamide riboside (NR), a vitamin B3 and NAD+ precursor, was previously shown to boost NAD+ levels in mice and to induce mitochondrial biogenesis. Here, we treated mitochondrial myopathy mice with NR. This vitamin effectively delayed early- and late-stage disease progression, by robustly inducing mitochondrial biogenesis in skeletal muscle and brown adipose tissue, preventing mitochondrial ultrastructure abnormalities and mtDNA deletion formation. NR further stimulated mitochondrial unfolded protein response, suggesting its protective role in mitochondrial disease. These results indicate that NR and strategies boosting NAD+ levels are a promising treatment strategy for mitochondrial myopathy.

Wiley Open Access2014

Mitochondrial stress signalling and its impacts on physiology

Adrienne Joëlle Laurence Mottis

Mitochondria are responsible for respiration and energy harvesting across the eukaryotic kingdom. They also take part in other cellular processes like intermediary metabolism. As a result, mitochondrial function directly impacts organismal features such as metabolic homeostasis, fitness and aging. Moreover, mitochondrial dysfunction is involved in numerous pathologies, such as neurodegenerative disease, obesity, diabetes and cancer. Several surveillance pathways constantly monitor mitochondria to ensure their proper function. Among the pathways triggered by stress, the mitochondrial unfolded protein response (UPRmt) aims to restore proteostasis within this organelle. A better understanding of the cellular response to mitochondrial stress would expand our fundamental knowledge of physiological and pathological processes involving mitochondria, leading to potential new therapeutics that target these organelles. This thesis focuses on the mechanistic and physiological characterization of the mitochondrial stress response in several organisms. I used the model organism Caenorhabtis elegans to screen for novel players of the UPRmt and found the poly(A)-binding protein pab-1. Using transcript profiling, we showed that the induction of immune genes is a common consequence of several mitochondrial stressors. We demonstrated that pab-1 regulates the activation of the mitochondrial stress response and innate immunity as well. On top, pab-1 is required for the survival of C. elegans upon bacterial infection. Transcriptomic data from multiple human tissues suggest that the human pab-1 orthologue, PABPC1, has a conserved role in immunity. In mice, I explored the regulation of the mitochondrial stress response using a combination of bioinformatics and in vivo experimental approaches. Combined transcriptomic and proteomic analyses in the BXD mouse genetic reference population revealed a tight co-regulation of the orthologues of the UPRmt under normal physiological conditions. As a complementary approach, we triggered mitochondrial stress pharmacologically in newly born and adult mice. Due to the bacterial ancestry of mitochondria, the effects of the antibiotic doxycycline (dox) are also deleterious to this organelle. We found that dox impairs mitochondrial proteostasis and oxygen consumption in adult mice. However, mitochondrial stress induced by post-natal dox treatment did not cause long-lasting effects on mouse physiology and longevity, which is very distinct from observations in C. elegans. The mammalian gut flora plays an important role in organismal homeostasis. The use of an antibiotic causes perturbations of the microbiota, with repercussions on metabolism, inflammation, and the nervous system. To eliminate these confounding effects, we also treated germ-free mice with dox to characterize the response of the mouse to a so-called âpureâ mitochondrial stress. Using multi-omics profiling, we found that organs highly dependent on mitochondria display specific transcriptomic and proteomic signatures following dox treatment. In the kidney, we observed an inhibition of translation and of the mTOR pathway, accompanied by an activation of the ATF4 integrated stress response (ISR). In the liver, dox led to a remodelling of lipid metabolism. In both organs, target transcripts of type I interferon anti-viral response were induced. This thesis demonstrates that the response to mitochondrial stress is a multi-faceted process with conserved aspects acr