No evidence of the presence of dystrophin protein in rodent muscle stem cells

The polarized distribution of dystrophin in muscle stem cells (MuSCs) has been found to regulate asymmetric cell division and maintain the balance between stem cell self-renewal and myogenic commitment. In Duchenne muscular dystrophy, which is associated with dystrophin deficiency, MuSC dysfunction is believed to contribute to fatal muscle wasting. This study investigated the dynamics of the polarized dystrophin distribution during MuSC activation. To this end, we used live fluorescence imaging to visualize EGFP-labeled dystrophin in muscle fibers and MuSCs in the Dmd ^EGFP reporter mouse model. We also investigated this phenomenon in rats to assess the existence of interspecies consistency. Dystrophin was nearly absent on the apical side of quiescent MuSCs, whereas it rapidly accumulated near a subset of activated mouse MuSCs and 100% of rat MuSCs following enzymatic myofiber isolation and culture. Surprisingly, dystrophin that accumulated near MuSCs accumulated in membranes, and it was always found in continuity with the sarcolemma. Live imaging revealed that MuSCs could move from the condensed dystrophin, which remained attached to the myofiber. Additionally, we detected no dystrophin protein in MuSCs using different techniques, including immunocytochemistry capillary electrophoresis and fluorescence-activated cell sorting, whereas it was present in primary myoblasts and myotubes. Our findings indicate that dystrophin attached to activated MuSCs represents condensed sarcolemmal membranes. Our data suggest that dystrophin protein in MuSCs is not required for normal muscle regeneration.