A Unique Twist-Dependent Progenitor Cell Contributes to Adult Skeletal Muscle [Tw2 overexpression]. Mus musculus

Skeletal muscle possesses remarkable regenerative potential due to satellite cells, a stem cell population located beneath the muscle basal lamina. By lineage tracing of progenitor cells expressing the Twist2 (Tw2) transcription factor in mice, we discovered a unique myogenic lineage that resides outside the basal lamina of adult muscle and contributes specifically to type IIb/x myofibers during adulthood and muscle regeneration. Tw2+ progenitors are molecularly and anatomically distinct from satellite cells, are highly myogenic in vitro and can fuse with satellite cells. Transplantation of Tw2+ progenitors into adult mice is sufficient to reconstitute new myofibers, and genetic ablation of endogenous Tw2+ progenitors causes wasting of type IIb myofibers. We show that Tw2 expression maintains progenitor cells in an undifferentiated state that is poised to initiate myogenesis in response to appropriate cues that suppress Tw2 expression. Tw2-expressing progenitors represent a previously unrecognized, fiber-type specific progenitor cell involved in muscle growth and regeneration. Overall design: Gene expression profile was generated by comparing Twist2- or GFP-overexpressing Tw2 myoblasts before and after differentiation.