Accelerated Bone Loss in Transgenic Mice Expressing Constitutively Active TGF-β Receptor Type I

Transforming growth factor beta (TGF-β) is a key factor mediating the intercellular crosstalk between the hematopoietic stem cells and their microenvironment. Here, we investigated the skeletal phenotype of transgenic mice expressing constitutively active TGF-β receptor type I under the control of Mx1-Cre (Mx1;TβRI mice). μCT analysis showed decreased cortical thickness, and cancellous bone volume in both femurs and mandibles. Histomorphometric analysis confirmed a decrease in cancellous bone volume due to increased osteoclast number and decreased osteoblast number. Primary osteoblasts showed decreased ALP and mineralization. Constitutive TβRI activation increased osteoclast differentiation. qPCR analysis showed that Tnfsf11/Tnfrsf11b ratio, Ctsk, Sufu, and Csf1 were increased whereas Runx2, Ptch1, and Ptch2 were decreased in Mx1;TβRI femurs. Interestingly, Gli1, Wnt3a, Sp7, Alpl, Ptch1, Ptch2, and Shh mRNA expression were reduced whereas Tnfsf11/Tnfrsf11b ratio was increased in Mx1;TβRI mandibles. Similarly, osteoclast-related genes were increased in Mx1;TβRI osteoclasts whereas osteoblast-related genes were reduced in Mx1;TβRI osteoblasts. Western blot analysis indicated that SMAD2 and SMAD3 phosphorylation was increased in Mx1;TβRI osteoblasts, and SMAD3 phosphorylation was increased in Mx1;TβRI osteoclasts. CTSK was increased while RUNX2 and PTCH1 was decreased in Mx1;TβRI mice. Microindentation analysis indicated decreased hardness in Mx1;TβRI mice. Our study indicated that Mx1;TβRI mice were osteopenic by increasing osteoclast number and decreasing osteoblast number, possibly by suppressing Hedgehog signaling pathways.