The bone morphogenetic protein 2 signaling mediator Smad1 participates predominantly in osteogenic and not in chondrogenic differentiation in mesenchymal progenitors C3H1OT1/2

The role of the bone morphogenetic protein (BMP)-signaling mediator Smad1 i n osteogenic or chondrogenic differentiation was investigated in murine par ental mesenchymal progenitors C3H10T1/2 and its derivatives constitutively expressing BMP-2 (C3H10T1/2-BMP-2) and, therefore, undergo BMP-mediated ost eogenic/chondrogenic development. The functions of the three Smad1 domains, that is, the N-terminal (MH1) domain, the C-terminal (MH2) domain, and the midregional proline-rich linker domain, were documented and compared with full-length Smad1, We showed that expression of the MH2 domain in parental C3H10T1/2 cells was sufficient to initiate osteogenic differentiation. Inte restingly, MH1 was sufficient to initiate transcription of osteogenic marke r genes like the osteocalcin or parathyroid hormone/parathyroid hormone-rel ated protein (PTH/PTHrP) receptor. However, MH1 interfered with the histolo gically distinct formation of osteoblast-like cells. A dominant-negative ef fect on MH2-mediated osteogenic development in C3H10T1/2 cells was observed by the dose-dependent trans-expression of the midregional linker domain. I mportantly, in contrast to osteogenic differentiation, Smad1 and its domain s do not mimic or interfere with BMP-2-dependent chondrogenic development a s monitored by the inability of MH2 to give rise to histologically distinct chondrocytes in parental C3H10T 1/2 cells and by the inefficiency of the M H1 or linker domain to interfere with BMP-2 mediated chondrogenic different iation.