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Although heterogeneity is recognized within the murine satellite cell pool, a comprehensive understanding of distinct subpopulations and their functional relevance in human satellite cells is lacking. We used a combination of single cell RNA sequencing and flow cytometry to identify, distinguish, and physically separate novel subpopulations of human PAX7+ satellite cells (Hu-MuSCs) from normal muscles. We found that, although relatively homogeneous compared to activated satellite cells and committed progenitors, the Hu-MuSC pool contains clusters of transcriptionally distinct cells with consistency across human individuals. New surface marker combinations were enriched in transcriptional subclusters, including a subpopulation of Hu-MuSCs marked by CXCR4/CD29/CD56/CAV1 (CAV1+). In vitro, CAV1+ Hu-MuSCs are morphologically distinct, and characterized by resistance to activation compared to CAV1- Hu-MuSCs. In vivo, CAV1+ Hu-MuSCs demonstrated increased engraftment after transplantation. Our findings provide a comprehensive transcriptional view of normal Hu-MuSCs and describe new heterogeneity, enabling separation of functionally distinct human satellite cell subpopulations.

Satellite cells (SCs) reside in a dormant state during tissue homeostasis. The specific paracrine agents and niche cells that maintain SC quiescence remain unknown. We find that Wnt4 produced by the muscle fiber maintains SC quiescence through RhoA. Using cell-specific inducible genetics, we find that a Wnt4-Rho signaling axis constrains SC numbers and activation during tissue homeostasis in adult mice. Wnt4 activates Rho in quiescent SCs to maintain mechanical strain, restrict movement in the niche, and repress YAP. The induction of YAP upon disruption of RhoA is essential for SC activation under homeostasis. In the context of injury, the loss of Wnt4 from the niche accelerates SC activation and muscle repair, whereas overexpression of Wnt4 transitions SCs into a deeper state of quiescence and delays muscle repair. In conclusion, the SC pool undergoes dynamic transitions during early activation with changes in mechano-properties and cytoskeleton signaling preceding cell-cycle entry.

Stem cell heterogeneity is recognized as functionally relevant for tissue homeostasis and repair. The identity, context dependence, and regulation of skeletal muscle satellite cell (SC) subsets remains poorly understood. We identify a minor subset of Pax7+ SCs that is indelibly marked by an inducible Mx1-Cre transgene in vivo, is enriched for Pax3 expression, and has reduced ROS (reactive oxygen species) levels. Mx1+ SCs possess potent stem cell activity upon transplantation but minimally contribute to endogenous muscle repair, due to their relative low abundance. In contrast, a dramatic clonal expansion of Mx1+ SCs allows extensive contribution to muscle repair and niche repopulation upon selective pressure of radiation stress, consistent with reserve stem cell (RSC) properties. Loss of Pax3 in RSCs increased ROS content and diminished survival and stress tolerance. These observations demonstrate that the Pax7+ SC pool contains a discrete population of radiotolerant RSCs that undergo clonal expansion under severe stress.

Cell populations display heterogeneous and dynamic phenotypic states at multiple scales. Similar to molecular features commonly used to explore cell heterogeneity, cell behavior is a rich phenotypic space that may allow for identification of relevant cell states. Inference of cell state from cell behavior across a time course may enable the investigation of dynamics of transitions between heterogeneous cell states, a task difficult to perform with destructive molecular observations. Cell motility is one such easily observed cell behavior with known biomedical relevance. To investigate heterogenous cell states and their dynamics through the lens of cell behavior, we developed Heteromotility, a software tool to extract quantitative motility features from timelapse cell images. In mouse embryonic fibroblasts (MEFs), myoblasts, and muscle stem cells (MuSCs), Heteromotility analysis identifies multiple motility phenotypes within the population. In all three systems, the motility state identity of individual cells is dynamic. Quantification of state transitions reveals that MuSCs undergoing activation transition through progressive motility states toward the myoblast phenotype. Transition rates during MuSC activation suggest non-linear kinetics. By probability flux analysis, we find that this MuSC motility state system breaks detailed balance, while the MEF and myoblast systems do not. Balanced behavior state transitions can be captured by equilibrium formalisms, while unbalanced switching between states violates equilibrium conditions and would require an external driving force. Our data indicate that the system regulating cell behavior can be decomposed into a set of attractor states which depend on the identity of the cell, together with a set of transitions between states. These results support a conceptual view of cell populations as dynamical systems, responding to inputs from signaling pathways and generating outputs in the form of state transitions and observable motile behaviors.

Skeletal muscle has a remarkable capacity to regenerate by virtue of its resident stem cells (satellite cells). This capacity declines with aging, although whether this is due to extrinsic changes in the environment and/or to cell-intrinsic mechanisms associated to aging has been a matter of intense debate. Furthermore, while some groups support that satellite cell aging is reversible by a youthful environment, others support cell-autonomous irreversible changes, even in the presence of youthful factors. Indeed, whereas the parabiosis paradigm has unveiled the environment as responsible for the satellite cell functional decline, satellite cell transplantation studies support cell-intrinsic deficits with aging. In this review, we try to shed light on the potential causes underlying these discrepancies. We propose that the experimental paradigm used to interrogate intrinsic and extrinsic regulation of stem cell function may be a part of the problem. The assays deployed are not equivalent and may overburden specific cellular regulatory processes and thus probe different aspects of satellite cell properties. Finally, distinct subsets of satellite cells may be under different modes of molecular control and mobilized preferentially in one paradigm than in the other. A better understanding of how satellite cells molecularly adapt during aging and their context-dependent deployment during injury and transplantation will lead to the development of efficacious compensating strategies that maintain stem cell fitness and tissue homeostasis throughout life.

Skeletal muscle has a remarkable capacity to regenerate by virtue of its resident stem cells (satellite cells). This capacity declines with aging, although whether this is due to extrinsic changes in the environment and/or to cell-intrinsic mechanisms associated to aging has been a matter of intense debate. Furthermore, while some groups support that satellite cell aging is reversible by a youthful environment, others support cell-autonomous irreversible changes, even in the presence of youthful factors. Indeed, whereas the parabiosis paradigm has unveiled the environment as responsible for the satellite cell functional decline, satellite cell transplantation studies support cell-intrinsic deficits with aging. In this review, we try to shed light on the potential causes underlying these discrepancies. We propose that the experimental paradigm used to interrogate intrinsic and extrinsic regulation of stem cell function may be a part of the problem. The assays deployed are not equivalent and may overburden specific cellular regulatory processes and thus probe different aspects of satellite cell properties. Finally, distinct subsets of satellite cells may be under different modes of molecular control and mobilized preferentially in one paradigm than in the other. A better understanding of how satellite cells molecularly adapt during aging and their context-dependent deployment during injury and transplantation will lead to the development of efficacious compensating strategies that maintain stem cell fitness and tissue homeostasis throughout life.

Satellite cells are skeletal muscle stem cells capable of self-renewal and differentiation after transplantation, but whether they contribute to endogenous muscle fiber repair has been unclear. The transcription factor Pax7 marks satellite cells and is critical for establishing the adult satellite cell pool. By using a lineage tracing approach, we show that after injury, quiescent adult Pax7(+) cells enter the cell cycle; a subpopulation returns to quiescence to replenish the satellite cell compartment, while others contribute to muscle fiber formation. We demonstrate that Sprouty1 (Spry1), a receptor tyrosine kinase signaling inhibitor, is expressed in quiescent Pax7(+) satellite cells in uninjured muscle, downregulated in proliferating myogenic cells after injury, and reinduced as Pax7(+) cells re-enter quiescence. We show that Spry1 is required for the return to quiescence and homeostasis of the satellite cell pool during repair. Our results therefore define a role for Spry1 in adult muscle stem cell biology and tissue repair.

The niche is a conserved regulator of stem cell quiescence and function. During ageing, stem cell function declines. To what extent and by what means age-related changes within the niche contribute to this phenomenon are unknown. Here we demonstrate that the aged muscle stem cell niche, the muscle fibre, expresses Fgf2 under homeostatic conditions, driving a subset of satellite cells to break quiescence and lose their self-renewing capacity. We show in mice that relatively dormant aged satellite cells robustly express sprouty 1 (Spry1), an inhibitor of fibroblast growth factor (FGF) signalling. Increasing FGF signalling in aged satellite cells under homeostatic conditions by removing Spry1 results in the loss of quiescence, satellite cell depletion and diminished regenerative capacity. Conversely, reducing niche-derived FGF activity through inhibition of Fgfr1 signalling or overexpression of Spry1 in satellite cells prevents their depletion. These experiments identify an age-dependent change in the stem cell niche that directly influences stem cell quiescence and function., Pax7-expressing ([Pax7.sup.+]) satellite cells function as an essential stem cell population capable of extensive self-renewal and skeletal muscle repair (1-6). During ageing, the regenerative capability of muscle is hampered owing [...]

To investigate the cause of skeletal muscle weakening during aging we examined the sequence of cellular changes in murine muscles. Satellite cells isolated from single muscle fibers terminally differentiate progressively less well with increasing age of donor. This change is detected before decline in satellite cell numbers and all histological changes examined here. In MSVski transgenic mice, which show type IIb fiber hypertrophy, initial muscle weakness is followed by muscle degeneration in the first year of life. This degeneration is accompanied by a spectrum of changes typical of normal muscle aging and a more marked decline in satellite cell differentiation efficiency. On a myoD-null genetic background, in which satellite cell differentiation is defective, the MSVski muscle phenotype is aggravated. This suggests that, on a wild-type genetic background, satellite cells are capable of repairing MSVski fibers and preserving muscle integrity in early life. We propose that decline in myogenic cell differentiation efficiency is an early event in aging-related loss of muscle function, both in normal aging and in some late-onset muscle degenerative conditions. muscle regeneration; MyoD; myonuclear domain size

Author(s): Susan Eliazer [1]; Andrew S. Brack (corresponding author) [1] As muscle stem cells age, their ability to regenerate skeletal muscle following injury declines. One factor that might be responsible [...]