GRAF1 promotes ferlin-dependent myoblast fusion

• GRAF1 deficient mice exhibit a reduction in skeletal myofiber size.
• GRAF1 plays an important role in muscle fusion in vivo.
• GRAF1 regulates vesicle trafficking to promote myoblast fusion in vitro.
• GRAF1 regulates trafficking of EHD1, myoferlin and Fer1L5 to pre-fusion complexes.
• GRAF2 also plays a role in myoblast fusion and EHD1 membrane recruitment.

Myoblast fusion (a critical process by which muscles grow) occurs in a multi-step fashion that requires actin and membrane remodeling; but important questions remain regarding the spatial/temporal regulation of and interrelationship between these processes. We recently reported that the Rho-GAP, GRAF1, was particularly abundant in muscles undergoing fusion to form multinucleated fibers and that enforced expression of GRAF1 in cultured myoblasts induced robust fusion by a process that required GAP-dependent actin remodeling and BAR domain-dependent membrane sculpting. Herein we developed a novel line of GRAF1-deficient mice to explore a role for this protein in the formation/maturation of myotubes in vivo. Post-natal muscles from GRAF1-depleted mice exhibited a significant and persistent reduction in cross-sectional area, impaired regenerative capacity and a significant decrease in force production indicative of lack of efficient myoblast fusion. A significant fusion defect was recapitulated in isolated myoblasts depleted of GRAF1 or its closely related family member GRAF2. Mechanistically, we show that GRAF1 and 2 facilitate myoblast fusion, at least in part, by promoting vesicle-mediated translocation of fusogenic ferlin proteins to the plasma membrane.

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