Maintenance and repair of skeletal muscle throughout adulthood relies on the essential contributions of resident muscle stem cells. Our lab has pioneered the prospective isolation of mouse skeletal muscle stem cells by FACS and has shown that these cells contribute extensively to the regeneration of damaged muscles and also replenish the endogenous stem cell pool. Although much insight has been gained in mice, in humans there is a scarcity of knowledge of the molecular identities of the resident stem and progenitor cells and the fundamental cellular and molecular mechanisms regulating their contributions to muscle repair. This unmet need is particularly underscored in dystrophic and aged individuals in which muscle function is compromised. Our project aims to elucidate the stem cell populations in young and aged human muscles to better understand the poor ability to repair damage to aged muscles. Furthermore, we are developing in vitro strategies to propagate human muscle stem cells with the goal of enhancing their numbers and improving their therapeutic function in clinical settings.
During the first year of this grant period, we have made substantial progress on Milestones 1, 2 and 3. Human muscle stem cells have been isolated by flow cytometry in our laboratory based on the expression of positive and negative cell surface selection markers. We have shown that the purified cells can be transplanted in vivo into damaged muscles of immunodeficient recipient mice and generate muscle fibers, demonstrating the potential of isolated human cells to make muscle in vivo. We observed using non-invasive bioluminescence imaging that the purified human muscle stem cells expand in vivo more than 100-fold during the repair process, in contrast to human myoblast progenitors, which do not expand in vivo. Notably, we now have data demonstrating that using our culture conditions, these cells can be successfully maintained as stem cells as shown by their ability to engraft and generate muscle in vivo. We are actively investigating factors that expand skeletal muscle stem cells (MuSCs) purified from human muscle tissues obtained from aged individuals ex vivo. Thus, we have been able to translate our prior findings with murine MuSCs to human MuSCs.