A Dual Color Pax7 and Myf5 In Vivo Reporter to Investigate Muscle Stem Cell Heterogeneity in Regeneration and Aging

Summary Increasing evidence suggests heterogeneity in the muscle stem cell (MuSC) pool. In particular, a rare subset of Pax7 positive MuSCs that has never expressed the myogenic regulatory factor Myf5 has enhanced self-renewal and engraftment characteristics. However, the scarcity and limited availability of protein markers make the characterization of these cells challenging. We describe the generation of StemRep reporter mice allowing to monitor Pax7 and Myf5 protein based on equimolar levels of dual nuclear fluorescence. High levels of Pax7 protein and low levels of Myf5 delineate a deeply quiescent MuSC subpopulation with distinct molecular signatures and dynamics of activation, proliferation, and commitment. Aging decreases the number of these cells, and skews the MuSC pool towards Myf5- High cells with impaired quiescence. Altogether, we describe a novel deeply quiescent MuSC subpopulation whose maintenance is impaired in old muscles, and establish the StemRep line as a versatile tool to study quiescence and MuSC heterogeneity.