We have shown that distinct proteostatic activities, macroautophagy (i.e. autophagy) and chaperone- mediated autophagy (CMA) are active in satellite cells although exerting distinct roles. While autophagy is required for quiescence, CAM is needed for satellite cell activation in response to injury. Genetic loss of either autophagy or CMA results in defective muscle regeneration. These findings uncover specialization of distinct proteostatic activities in distinct stem cell states along the muscle regeneration process. We have also found that 1) the DNA/RNA helicase Dhx36 controls satellite cell activation from quiescence, and that its loss provokes defective muscle regeneration; 2) satellite cell activation through NurD complex component Chd4 and p38gamma MAPK in collaboration with the groups of Drs T. Braun and M. Rudnicki); 3) A new mechanism for repair of myofibers after a localized damage, such as during exercise.
We have shown that quiescence not only allows stem cells to survive for prolonged periods, but also instructs heterogeneity. This finding is based on our identification and molecular and functional characterization of “genuine” and “primed” quiescent stem-cell states. We have demonstrated that the genuine state is spared into late life. Nonetheless, in geriatric age, this state undergoes a steep functional decline. This indicates that aging is not a uniform period of functional decline and that loss of stem-cell heterogeneity and capacity for efficient repair become prominent only very late in life. We also showed that genetic loss of the mitochondria fission regulator DRP1 in muscle stem cells (or during aging) blunts their proliferation and regenerative capacity, whereas its reestablishment rescues these defects. In fact, normalizing mitochondrial dynamics (or increasing OXPHOS and mitophagy) in aged muscle stem cells restored tissue regeneration. This opens the way to improve the health of elderly people who are debilitated by the loss of muscle regenerative capacity.
Clear isolation and characterization of senescent cells from mammalian tissues is still pending, likely due to their low number and the lack of in vivo isolation procedures. We have been able to isolate distinct types of senescent cells from regenerating muscle tissue and characterized them molecular and functionally. Unexpectedly, we found that very few genes are shared among the distinct senescent cell types in aging muscle. This indicates that cellular senescence is heterogeneous in vivo. We also demonstrated that elimination of senescent cells improves muscle regeneration in geriatric mice, which opens possibilities for rejuvenation in regenerative medicine. Lastly, we found that senescent cells secrete proinflammatory and profibrotic factors which, in turn, impact the nearby stem cells and hampers their regenerative capacity, thus impairing muscle regeneration. This has implications for regenerative medicine and aging.
We have observed that aged mice remain behaviorally circadian, and that their muscle stem cells retain a robustly rhythmic core circadian machinery. However, the oscillating transcriptome was found to be extensively reprogrammed in aged stem cells, switching from genes involved in homeostasis to those involved in tissue-specific stresses, such as inefficient autophagy. Furthermore, while age-associated rewiring of the oscillatory diurnal transcriptome is not recapitulated by a high-fat diet in young mice, it is significantly prevented by caloric restriction in aged mice. Thus, stem cells rewire their diurnal timed functions to adapt to metabolic cues and to tissue-specific age-related traits. We also found a muscle regeneration defect in Bmal1-deficient mice, indicating that circadian rhythms are needed for efficient tissue regeneration. Finally, we have found communication among skeletal muscle and both central and peripheral clocks, which is necessary for daily tissue physiology.
