Mechanically Driven Counter-Regulation of Cortical Bone Formation in Response to Sclerostin-Neutralizing Antibodies

Sclerostin (Scl) antibodies (Scl-Ab) potently stimulate bone formation, but these effects are transient. Whether the rapid inhibition of Scl-Ab anabolic effects is due to a loss of bone cells' capacity to form new bone or to a mechanostatic downregulation of Wnt signaling once bone strength exceeds stress remains unclear. We hypothesized that bone formation under Scl-Ab could be reactivated by increasing the dose of Scl-Ab and/or by adding mechanical stimuli, and investigated the molecular mechanisms involved in this response, in particular the role of periostin (Postn), a co-activator of the Wnt pathway in bone. For this purpose, C57Bl/6, Postn^-/- and Postn^+/+ mice were treated with vehicle or Scl-Ab (50 to 100 mg/kg/wk) for various durations and subsequently subjected to tibia axial compressive loading. In wild-type (WT) mice, Scl-Ab anabolic effects peaked between 2 and 4 weeks and declined thereafter, with no further increase in bone volume and strength between 7 and 10 weeks. Doubling the dose of Scl-Ab did not rescue the decline in bone formation. In contrast, mechanical stimulation was able to restore cortical bone formation concomitantly to Scl-Ab treatment at both doses. Several Wnt inhibitors, including Dkk1, Sost, and Twist1, were upregulated, whereas Postn was markedly downregulated by 2 to 4 weeks of Scl-Ab. Mechanical loading specifically upregulated Postn gene expression. In turn, Scl-Ab effects on cortical bone were more rapidly downregulated in Postn^-/- mice. These results indicate that bone formation is not exhausted by Scl-Ab but inhibited by a mechanically driven downregulation of Wnt signaling. Hence, increasing mechanical loads restores bone formation on cortical surfaces, in parallel with Postn upregulation. © 2020 American Society for Bone and Mineral Research (ASBMR).