Clones of ectopic stem cells in the regeneration of muscle defects in vivo

Little is known about whether clones of ectopic, non-muscle stem cells contribute to muscle regeneration. Stem/progenitor cells that are isolated for experimental research or therapeutics are typically heterogeneous. Non-myogenic lineages in a heterogeneous population conceptually may compromise tissue repair. In this study, we discovered that clones of mononucleated stem cells of human tooth pulp fused into multinucleated myotubes that robustly expressed myosin heavy chain in vitro with or without co-culture with mouse skeletal myoblasts (C2C12 cells). Cloned cells were sustainably Oct4+, Nanog+ and Stro1+. The fusion indices of myogenic clones were approximately 16-17 folds greater than their parent, heterogeneous stem cells. Upon infusion into cardio-toxin induced tibialis anterior muscle defects, undifferentiated clonal progenies not only engrafted and colonized host muscle, but also expressed human dystrophin and myosin heavy chain more efficaciously than their parent heterogeneous stem cell populations. Strikingly, clonal progenies yielded ～9 times more human myosin heavy chain mRNA in regenerating muscles than those infused with their parent, heterogeneous stem cells. The number of human dystrophin positive cells in regenerating muscles infused with clonal progenies was more than ～3 times greater than muscles infused with heterogeneous stem cells from which clonal progenies were derived. These findings suggest the therapeutic potential of ectopic myogenic clones in muscle regeneration.