Adult stem cells are capable of self renewal and differentiation. Although they possess self-renewal properties, this potential is not limitless. This begs the following question: how are stem cells maintained throughout life? The answer lies in the reversible state of quiescence. Adult stem cells are predominantly in a quiescent state interspersed with rare traverses into the cell cycle unlike their downstream descendents that spend more time in cycle. Using cell labeling techniques to monitor cell division history, stem cells that divide less frequently are enriched for self-renewal potential. In muscular dystrophies, muscle stem cells (satellite cells, SCs) undergo continuous bouts of proliferation, which leads to a depletion of the stem cell pool over time. In aged muscle the SC pool is diminished and displays impaired renewal and differentiation. Therefore understanding how 'proliferation-restricted' stem cells may hold the key to optimizing stem cell function during aging and disease. In this grant proposal we will interrogate 1) the functional heterogeneity between slow and faster dividing SCs. 2) How the slow dividing SC population is specified and maintained throughout life through Spry, an inhibitor of growth factor signaling. 3) The consequences of losing slow dividing subset of SCs SC function and muscle homeostasis during aging. Slow dividing SCs will be identified in vivo based on label retaining character (LRC) using a transgenic H2B-GFP approach; cells that undergo fewer divisions retain more label. Satellite cells will be characterized for LRC and non-LRC; both subpopulations will be tested for their functional differences based on in vivo and in vitro assays. We have generated preliminary data that demonstrates Spry1 expression is enriched in LRCs and genetic disruption of Spry1 leads to a loss of LRC in growing muscle and an accelerated decline in the number of SCs during aging. We will extend these studies to delete and overexpress Spry1 specifically in SCs in a temporally inducible manner. We will disrupt Spry1 to alter LRC establishment and maintenance and study the effects on SC function and muscle phenotype. The specific aims of this proposal are: 1) To study the slow division dynamics of satellite cells during developmental and postnatal myogenesis, 2) to identify whether Spry1 is required and sufficient to regulate LRC SCs, and 3) to understand the consequences of losing SC LRC throughout life.