Sprouty1 Regulates Reversible Quiescence of a Self-Renewing Adult Muscle Stem Cell Pool during Regeneration

Abstract

SummarySatellite cells are a heterogeneous population of skeletal muscle specific stem cells capable of self-renewal and differentiation after transplantation. Whether quiescent satellite cells can self-renew and contribute to muscle fiber repair in their endogenous environment in normal regenerating muscle has remained unknown. The transcription factor Pax7 is expressed in satellite cells and is critical for establishing the adult satellite cell pool. Using a temporally-inducible genetic lineage tracing approach (Pax7-CreERtm; R26R-lacZ) to fate-map adult satellite cells, we show that in response to injury quiescent adult Pax7+ cells enter the cell cycle; a subpopulation return to quiescence to fully replenish the satellite cell compartment and the others contribute to de novo muscle fiber formation. We demonstrate that Sprouty1 (Spry1), an inhibitor of receptor tyrosine kinase signaling, is robustly expressed in quiescent Pax7+ satellite cells in uninjured adult muscle, down-regulated in proliferating myogenic cells in injured muscles, and re-induced as Pax7+ cells return to quiescence in regenerated muscles. We show through deletion of Spry1 specifically in cycling adult Pax7+ satellite cells, that Spry1 is required for the return to quiescence and homeostasis of the self-renewing Pax7+ satellite cell pool during repair. Satellite cells unable to return to quiescence succumb to apoptosis leading to a diminished self-renewing Pax7-derived satellite cell pool. Our results define a novel role for Spry1 in adult stem cell biology and tissue repair.

Muscle stem cells and reversible quiescence: The role of sprouty

“…49,50 However, satellite cells are capable of self-renewal and differentiation during repair. 20,21 This confirms that adult Pax7 + satellite cells have stem cell properties in their endogenous environment.…”

“…However overexpression studies have demonstrated that Pax7 is sufficient to promote a return to quiescence through repression of MyoD and myogenin 35,37 suggesting downregulation of MyoD is a critical component of reversible quiescence. During the proliferative phase of regeneration in vivo, the majority of the satellite cells that repopulated the renewed Pax7 + stem cell pool progressed through a MyoD + state, 21 therefore expressing MyoD during developmental history does not exclude a satellite cell from returning to quiescence in vivo.…”

“…21,23,25,27,29 Whether RCs are identical to satellite cells is still a matter of debate; however they do retain some of their characteristics, i.e., their ability to return to quiescence.…”

“…After muscle injury satellite cells that replenished the quiescent satellite cell pool, had derived from a Pax7 + /BrdU + population. 21 It should be emphasized that to fully label the self-renewing pool, relatively long periods of BrdU administration during the regeneration bout was required. In contrast, satellite cells that contributed to fiber repair could be labeled with ease by a short duration BrdU pulse (Shea and Brack AS, unpublished observations).…”

p38α MAPK Regulates Adult Muscle Stem Cell Fate by Restricting Progenitor Proliferation During Postnatal Growth and Repair

“…6A). During regeneration, the satellite cell pool expands in response to the injury, before differentiating to repair the damaged area and returning to a baseline satellite cell number, maintaining the stem cell population [39,50]. While the p38a fl muscle responded to the muscle injury as expected by displaying a 10-fold satellite cell number increase and returning to baseline levels by 25 DPI, the p38a KO muscle displayed a heightened satellite cell expansion ( Fig.…”