Mouse myoblast fusion - Separating myoblast differentiation from muscle cell fusion using IGF-I and the p38 MAP kinase inhibitor SB202190

The p38 MAP kinases play critical roles in skeletal muscle biology, but the specific processes that they regulate remain poorly defined. Here we find that activity of p38α/β is important not only for early phases of myoblast differentiation, but also in later stages of myocyte fusion and myofibrillogenesis. By treating C2 myoblasts with the pro-myogenic growth factor, IGF-I, we were able to overcome the early block in differentiation imposed by co-incubation with the p38 chemical inhibitor, SB202190. Yet, IGF-I could not overcome the later impairment of muscle cell fusion, as marked by the nearly complete absence of multinucleated myofibers. Removal of SB202190 from the medium of differentiating myoblasts showed that the fusion block was reversible, as multinucleated myofibers were detected several hours later, and reached ~90% of the culture within 30 hours. Analysis by quantitative mass spectroscopy of proteins that changed in abundance following removal of the inhibitor revealed a cohort of up-regulated muscle-enriched molecules that may be important for both myofibrillogenesis and fusion. We have thus developed a model system that allows separation of myoblast differentiation from muscle cell fusion, which should be useful in identifying specific steps regulated by p38 MAP kinase-mediated signaling in myogenesis.