Load Regulates Bone Formation and Sclerostin Expression through a TGFβ-Dependent Mechanism

Bone continually adapts to meet changing physical and biological demands. Osteoblasts, osteoclasts, and osteocytes cooperate to integrate these physical and biochemical cues to maintain bone homeostasis. Although TGF beta acts on all three of these cell types to maintain bone homeostasis, the extent to which it participates in the adaptation of bone to mechanical load is unknown. Here, we investigated the role of the TGF beta pathway in load-induced bone formation and the regulation of Sclerostin, a mechanosensitive antagonist of bone anabolism. We found that mechanical load rapidly represses the net activity of the TGF beta pathway in osteocytes, resulting in reduced phosphorylation and activity of key downstream effectors, Smad2 and Smad3. Loss of TGF beta sensitivity compromises the anabolic response of bone to mechanical load, demonstrating that the mechanosensitive regulation of TGF beta signaling is essential for load-induced bone formation. Furthermore, sensitivity to TGF beta is required for the mechanosensitive regulation of Sclerostin, which is induced by TGF beta in a Smad3-dependent manner. Together, our results show that physical cues maintain bone homeostasis through the TGF beta pathway to regulate Sclerostin expression and the deposition of new bone.