Myostatin (Mstn) is a secreted TGF- β family member that controls skeletal muscle growth, and binds with high affinity to the activin type IIB receptor (ActRIIB). The soluble ligand-binding domain of ActRIIB fused to the Fc domain of IgG (ActRIIB-Fc) potently binds and inhibits TGF-β family members in muscle, leading to rapid and marked muscle growth. The present study was designed to assess the combinative effects of myostatin-targeting siRNA (Mstn-siRNA) and ActRIIB-Fc on murine myoblast in vitro and in vivo. C2C12 cells were treated by Mstn-siRNA with or without ActRIIB-Fc at 0 and 48 h after differentiation. Myotube size was measured, and gene expression of Mstn, MuRF-1, MyoD and myogenin were analyzed. Furthermore, 11-week-old, male C57BL/6 mice were injected with atelocollagen (ATCOL)-mediated Mstn-siRNA and Mstn-siRNA/ActRIIB-Fc locally into the masseter muscle twice a week. Histological and biochemical analyses were performed using the dissected muscles. Transfection of Mstn-siRNA and Mstn-siRNA/ActRIIB-Fc resulted in significant increases in the myotube diameter of the C2C12 cells compared with untreated control. Also, treatment with Mstn-siRNA and Mstn-siRNA/ActRIIB-Fc could lead to an upregulation of MyoD and myogenin gene expression and downregulation of Mstn and MuRF-1. In vivo, muscle fibril hypertrophy was observed in both Mstn-siRNA and Mstn-siRNA/ActRIIB-Fc treated groups. Moreover, western blotting analysis showed that the p-Smad2/3 expression level was decreased by treatment of Mstn-siRNA/ActRIIB-Fc. In contrast, MyoD and myogenin protein levels were increased by combined treatment, compared with the other groups. These suggest that double inhibition of myostain is potentially useful for myogenesis and muscle growth promotion. This may be a good as new treatment remedy for patients with various muscle atrophies, including muscular dystrophy.

Myostatin, a member of the TGF-beta superfamily, is a negative regulator of skeletal muscle cell growth and differentiation, and binds with high affinity to the activin type IIB receptor (ActRIIB). The soluble ligand-binding domain of ActRIIB fused to the Fc domain of IgG (ActRIIB-Fc) potently binds and inhibits TGF-beta family members in muscle, leading to rapid and marked muscle growth. The present study was designed to assess the effectiveness of the co-delivery of myostatin-targeting siRNA (Mstn-siRNA) and ActRIIB-Fc into skeletal muscle as a potential treatment of atrophic myopathies. Eleven-week-old, male C57BL/6 mice were injected with atelocollagen (ATCOL)-mediated Mstn-siRNA with/ without ActRIIB-Fc locally into the masseter muscle twice a week. Inhibition of myostatin function by the combination of Mstn-siRNA and ActRIIB-Fc increased muscle weight and myofibril size in murine masseter muscle. Real-time RT-PCR analysis revealed significant downregulation of myostatin mRNA expression in both the Mstn-siRNA-treated and the combination treatment group. Furthermore, myogenin mRNA expression was upregulated in the combination treatment group, while MuRF-1 and Atrogin-1 mRNA expression was downregulated compared to administration of each compound alone. These findings suggest that double inhibition of myostatin is a potentially useful treatment strategy to increase muscle mass and fiber size and could be a useful treatment of patients with various muscle atrophies, including muscular dystrophy.