Dysfunction of Caveolae-Mediated Endocytic TRI Degradation Results in Hypersensitivity of TGF-/Smad Signaling in Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a genetic disorder caused by mutations of type I collagen-related genes, and excessive transforming growth factor-beta (TGF-beta) signaling is a common mechanism. TGF-beta/Smad signaling has inhibitory effects on osteoblast differentiation and maturation and is mainly transduced and regulated by the internalization of a tetrameric receptor complex comprising types I and II TGF-beta receptors (T beta RI and T beta RII). During internalization, clathrin-mediated endocytosis enhances TGF-beta/Smad signaling via Smad2/3 phosphorylation and receptors recycling, while caveolae-mediated endocytosis turns off TGF-beta/Smad signaling by promoting receptor ubiquitination and degradation. In this study, using an animal model of OI (Colla2oim, osteogenesis imperfecta murine [oim]/oim mouse), we found that osteoblastic cells of oim/oim mice were more sensitive to the inhibitory effects of TGF-beta on osteoblast differentiation and maturation and had much higher cell membrane protein levels of TGF-beta receptors than those of wild-type (wt)/wt mice. Further results showed that clathrin-mediated endocytosis of T beta RI was enhanced, whereas caveolae-mediated T beta RI endocytic degradation was reduced in oim/oim mice, combined with reduced caveolin-1 (Cav-1) phosphorylation. In addition, type I collagen downregulated T beta RI via focal adhesion kinase (FAK) and Src activation-dependent Cav-1 phosphorylation. To further examine this mechanism, 4-week-old oim/oim and wt/wt mice were treated with either T beta RI kinase inhibitor (SD-208) or vehicle for 8 weeks. SD-208 treatment significantly reduced the fracture incidence in oim/oim mice. Micro-computed tomography and biomechanical testing showed that femoral bone mass and strength were significantly improved with SD-208 treatment in both genotypes. Additionally, SD-208 significantly promoted osteoblast differentiation and bone formation and inhibited bone resorption. In conclusion, dysfunction of caveolae-mediated endocytic T beta RI degradation is a possible mechanism for the enhanced TGF-beta/Smad signaling in OI. Targeting this mechanism using a T beta RI kinase inhibitor effectively reduced fractures and improved bone mass and strength in OI model and, thus, may offer a new strategy for the treatment of OI. (C) 2022 American Society for Bone and Mineral Research (ASBMR).