Analysis of the role of bone morphogenetic protein signaling in skeletal development

Bone morphogenetic proteins (BMPs) are important regulators of vertebrate limb development and have been suggested to control patterning formation, cartilage condensation, chondrocyte maturation, apoptosis, and joint morphogenesis. BMPs exert their effects via binding to two types of transmembrane receptors, and activate intracellular molecules in the Smad and MAPK pathways. To further analyze the role of BMP signaling in skeletal development, three genes in the BMP signaling pathway, Chordin, ALK2, and Smad1, had been mis-expressed in the developing chick limb. Chordin is a BMP antagonist and was expressed in developing joints. Over-expression of Chordin protein delayed chondrocyte maturation in vivo as well as in the cultured chondrocytes, which strongly supports a role for Chordin as a negative regulator of endochondral ossification. ALK2 is a TGF-β type I receptor. ALK2 expression was detected in developing cartilage and it functioned as a BMP type I receptor in chondrocytes. Constitutively active (CA) ALK2 viral infection in embryonic limbs induced Ihh and PTHrP, which indicates that the BMP signaling is upstream of Ihh and integrated into the Ihh/PTHrP signaling pathway. Smad1 is a transcriptional factor that is specifically activated by BMP signaling. Mis-expression of Smad1 protein in vivo caused truncation, delayed cartilage maturation, joint fusion, and ectopic ossification of the skeletal elements, suggesting fine regulation of Smad1 is critical for limb outgrowth and joint development. In summary, my in vivo gain-of-function studies provide new insights into the roles of BMPs in skeletal development: BMP signaling promotes cartilage differentiation in vivo, and it acts upstream of Ihh/PTHrP feedback loop; BMP signaling is involved in the process of joint formation as well.