TGF-β signaling is essential for joint morphogenesis

Despite its clinical significance, joint morphogenesis is still an obscure process. In this study, we determine the role of transforming growth factor β (TGF-β) signaling in mice lacking the TGF-β type II receptor gene (Tgfbr2) in their limbs (Tgfbr2PRX⁻¹KO). In Tgfbr2PRX⁻¹KO mice, the loss of TGF-β responsiveness resulted in the absence of interphalangeal joints. The Tgfbr2Prx¹KO joint phenotype is similar to that in patients with symphalangism (SYM1-OMIM185800). By generating a Tgfbr2-green fluorescent protein-β-GEO-bacterial artificial chromosome β-galactosidase reporter transgenic mouse and by in situ hybridization and immunofluorescence, we determined that Tgfbr2 is highly and specifically expressed in developing joints. We demonstrated that in Tgfbr2PRX⁻¹KO mice, the failure of joint interzone development resulted from an aberrant persistence of differentiated chondrocytes and failure of Jagged-1 expression. We found that TGF-β receptor II signaling regulates Noggin, Wnt9a, and growth and differentiation factor-5 joint morphogenic gene expressions. In Tgfbr2PRX⁻¹KO growth plates adjacent to interphalangeal joints, Indian hedgehog expression is increased, whereas Collagen 10 expression decreased. We propose a model for joint development in which TGF-β signaling represents a means of entry to initiate the process.