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

Transforming growth factor beta (TGF-beta) 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-beta receptor type I under the control of Mx1-Cre (Mx1;T beta RICA mice). mu CT 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 T beta RI 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;T beta RICA femurs. Interestingly, Gli1, Wnt3a, Sp7, Alpl, Ptch1, Ptch2, and Shh mRNA expression were reduced whereas Tnfsf11/Tnfrsf11b ratio was increased in Mx1;T beta RICA mandibles. Similarly, osteoclast-related genes were increased in Mx1;TbRICA osteoclasts whereas osteoblast-related genes were reduced in Mx1;T beta RICA osteoblasts. Western blot analysis indicated that SMAD2 and SMAD3 phosphorylation was increased in Mx1;TbRICA osteoblasts, and SMAD3 phosphorylation was increased in Mx1;T beta RICA osteoclasts. CTSK was increased while RUNX2 and PTCH1 was decreased in Mx1;T beta RICA mice. Microindentation analysis indicated decreased hardness in Mx1;T beta RICA mice. Our study indicated that Mx1;T beta RICA mice were osteopenic by increasing osteoclast number and decreasing osteoblast number, possibly by suppressing Hedgehog signaling pathways.