Complex roles for Ste20-like kinase in muscle development and regeneration. (A) After muscle injury (left side), SLK is required for proliferation of activated satellite cells. Upon terminal differentiation, SLK activity is downregulated (down arrow), leading to cell-cycle exit and growth arrest . SLK activity is then upregulated upon myoblast fusion and myofiber maturation (up arrow). Similarly, in cultured myoblasts (right side) SLK is downregulated for growth arrest and upregulated during fusion and maturation. Expression of kinase-dead SLK (K63R) as myoblasts enter the differentiation pathway enhances cell-cycle arrest and differentiation. (B) In the developing embryos, expression of kinase-dead SLK from the skeletal actin promoter delays terminal differentiation and maturation, suggesting a distinct role for SLK in embryonic myogenesis.