Inhibition of myogenic differentiation in myoblasts expressing a truncated type II TGF-β receptor

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Abstract
Transforming growth factor-β (TGF-β? is thought to play a role in mesenchymal cell development and, specifically, in muscle differentiation, yet its precise role in the latter process remains unclear. TGF-β has been shown to both inhibit and induce myoblast maturation in vitro, depending on the culture conditions. Whether the type I or type II TGF-β receptor mediates the various TGF-β effects on myogenesis is not known. In the present study, C2C12 myoblasts were transfected with an expression vector for a truncated type II TGF-β receptor, which has been shown to act as a dominant negative inhibitor of type II receptor signaling. In contrast to the parental cells, the transfected clones did not efficiently form myotubes or induce expression of MyoD, myogenin and several other differentiation markers following incubation in low serum media. However, some muscle differentiation markers continued to be expressed in the transfected cells suggesting that at least two pathways are involved in muscle cell differentiation. These cells could still growth arrest in low serum media, showing that decreased proliferation can be dissociated from differentiation. Unlike several oncogenes known to block myogenic differentiation, expression of the truncated TGF-β receptor did not result in myoblast transformation. Injection of the parental or the transfected C2C12 cells into the limb muscle of nude mice revealed quantitative and qualitative differences in their behavior, and suggested that myoblasts expressing the truncated TGF-β receptor cannot fuse in vivo. Finally, retrovirusmediated expression of MyoD in the transfected cells restored their ability to form myotubes in vitro, indicating that inhibition of myoblast differentiation by the truncated TGF-β receptor may depend on decreased MyoD expression. We propose that TGF-β signaling through the type II receptor is required for several distinct aspects of myogenic differentiation and that TGF-β acts as a competence factor in this multistep process.