Periodic Reporting for period 4 - STEM-AGING (Tissue regeneration and aging: the decisive quiescent stem-cell state)
Período documentado: 2022-05-01 hasta 2022-10-31
The possibility of slowing down or reversing stem cell functional decline with aging is one of the most fascinating challenges in regenerative medicine. Hallmarks of aging have been described and they converge in the exhaustion of stem cells, which provokes tissue regenerative decline. Skeletal muscle provides a stark example of this decline. Skeletal muscle stem cells (satellite cells) spend most of their life in quiescence (reversible G0 arrest), and only activate and proliferate in response to muscle trauma to either self-renew or differentiate and form new myofibers (myogenesis). How these long-lived stem cells are maintained in quiescence is basically unknown. With aging, satellite cell regenerative functions decline, through poorly understood mechanisms.
The underlying hypothesis of this project is that life-long persistence of stem cells in quiescence makes them susceptible to the accumulation of cellular damage (by loss of autophagic activity), which may cause senescence (irreversible G0 arrest) and loss of regenerative function (see Image Summary). We also hypothesize that muscle stem cells are a heterogeneous population, subjected to daily circadian oscillations, which may change with aging and influence regeneration. This project therefore aimed to explore the possible connections between muscle regenerative decline with aging and stem-cell quiescence loss and acquisition of senescence. We also aimed to understand whether changes in stem cell heterogeneity and circadian regulation contribute to regenerative loss with aging.
The underlying hypothesis of this project is that life-long persistence of stem cells in quiescence makes them susceptible to the accumulation of cellular damage (by loss of autophagic activity), which may cause senescence (irreversible G0 arrest) and loss of regenerative function (see Image Summary). We also hypothesize that muscle stem cells are a heterogeneous population, subjected to daily circadian oscillations, which may change with aging and influence regeneration. This project therefore aimed to explore the possible connections between muscle regenerative decline with aging and stem-cell quiescence loss and acquisition of senescence. We also aimed to understand whether changes in stem cell heterogeneity and circadian regulation contribute to regenerative loss with aging.
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.
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.
Three achievements obtained from this ERC grant have been unexpected findings and constitute breakthroughs in the stem cell/regeneration/aging fields, which advance these fields and extend beyond them:
1. By generating the first transcriptomic atlas of senescent cells in vivo, we found that senescent cells are widely heterogeneous, yet they display common traits, including the secretion of proinflammatory and profibrotic factors. This proinflammatory and profibrotic secretome, in turn, impacts the nearby stem cells and hampers their regenerative capacity, thus impairing muscle regeneration. We have also identified new senolytic and senomorphic targets. This has implications for regenerative medicine and aging. Moiseeva V et al. Senescence atlas reveals an aged-like inflamed niche that blunts muscle regeneration. Nature 2022
2. We have found that muscle can be repaired by a rapid and autonomous mechanism, independent of stem cells. In fact, we discovered a new mechanism to repair muscle fibers after physiological damage (i.e. exercise) based on myonucleus reorganization, which does not require stem cells. These findings are relevant for the fields of physical activity and physiotherapy, as well as for muscle aging.
Roman W et al. . Muscle repair after physiological damage relies on nuclear migration for cellular reconstruction. Science 2021
3. We identified two distinct quiescent satellite cell (QSC) states that we termed genuine (QSCgenuine) and primed (QSCprimed). We showed that the genuine stem-cell state is molecularly and functionally preserved up to old age. However, it is compromised in extreme old age, accounting for the sharp regenerative failure in this last stage of life. Our results therefore indicate that ageing is not a uniform life-period of tissue decline.
García-Prat L et al. FoxO maintains a genuine muscle stem-cell quiescent state until geriatric age. Nature Cell Biol. 2020
1. By generating the first transcriptomic atlas of senescent cells in vivo, we found that senescent cells are widely heterogeneous, yet they display common traits, including the secretion of proinflammatory and profibrotic factors. This proinflammatory and profibrotic secretome, in turn, impacts the nearby stem cells and hampers their regenerative capacity, thus impairing muscle regeneration. We have also identified new senolytic and senomorphic targets. This has implications for regenerative medicine and aging. Moiseeva V et al. Senescence atlas reveals an aged-like inflamed niche that blunts muscle regeneration. Nature 2022
2. We have found that muscle can be repaired by a rapid and autonomous mechanism, independent of stem cells. In fact, we discovered a new mechanism to repair muscle fibers after physiological damage (i.e. exercise) based on myonucleus reorganization, which does not require stem cells. These findings are relevant for the fields of physical activity and physiotherapy, as well as for muscle aging.
Roman W et al. . Muscle repair after physiological damage relies on nuclear migration for cellular reconstruction. Science 2021
3. We identified two distinct quiescent satellite cell (QSC) states that we termed genuine (QSCgenuine) and primed (QSCprimed). We showed that the genuine stem-cell state is molecularly and functionally preserved up to old age. However, it is compromised in extreme old age, accounting for the sharp regenerative failure in this last stage of life. Our results therefore indicate that ageing is not a uniform life-period of tissue decline.
García-Prat L et al. FoxO maintains a genuine muscle stem-cell quiescent state until geriatric age. Nature Cell Biol. 2020