A KINASE DOMAIN-TRUNCATED TYPE-I RECEPTOR BLOCKS BONE MORPHOGENETIC PROTEIN-2-INDUCED SIGNAL-TRANSDUCTION IN C2C12 MYOBLASTS

Members of the transforming growth factor (TGF)-beta superfamily bind the transmembrane serine/threonine kinase complex consisting of type I and type II receptors. Their intracellular signals are propagated via respective type I receptors. Bone morphogenetic protein (BMP)-2, a me mber of the TGF-beta superfamily, induces ectopic bone formation when implanted into muscular tissues. Two type I receptors (BMPR-IA and BMP R-IB) have been identified for BMP-2. We have reported that BMP-2 inhi bits the terminal differentiation of C2C12 myoblasts and converts thei r differentiation pathway into that of osteoblast lineage cells (Katag iri, T., Yamaguchi, A., Komaki, M., Abe, E., Takahashi, N., Ikeda, T., Rosen, V., Wozney, J. M., Fujisawa-Sehara, A. and Suda, T. (1994) J. Cell Biol. 127, 1755-1766). In the present study, we examined the invo lvement of functional BMP-2 type I receptors in signal transduction in C2C12 cells, which expressed mRNA for BMPR-IA, but not for BMPR-IB in Northern blotting. TGF-beta type I receptor (T beta R-I) mRNA was als o expressed in C2C12 cells. Subclonal cell lines of C2C12 that stably expressed a kinase domain-truncated BMPR-IA (Delta BMPR-IA) differenti ated into myosin heavy chain-expressing myotubes but not into alkaline phosphatase (ALP)-positive cells, even in the presence of BMP-2. In c ontrast, the differentiation of the Delta BMPR-IA-transfected C2C12 ce lls into myotubes was suppressed by TGF-beta 1, as in the parental C2C 12 cells. BMP-2 did not efficiently suppress the mRNA expression of mu scle-specific genes such as muscle creatine kinase, MyoD, and myogenin , nor did it induce the expression of ALP mRNA in the Delta BMPR-IA-tr ansfected C2C12 cells. In contrast, TGF-beta 1 inhibited mRNA expressi on of the muscle-specific genes in those cells. When wild-type BMPR-IA was transiently transfected into the Delta BMPR-IA-transfected C2C12 cells, a number of ALP-positive cells appeared in the presence of BMP- 2. Transfection of wild-type BMPR-IB or T beta R-I failed to increase the number of ALP-positive cells. These results suggest that the BMP-2 -induced signals, which inhibit myogenic differentiation and induce os teoblast differentiation, are transduced via BMPR-IA in C2C12 myoblast s.