Blocking interaction of sclerostin loop3 with osteoblastic LRP4 counteracts bone loss without increasing arterial stiffness during mechanical unloading

Background: Mechanical unloading leads to bone loss and cardiovascular deconditioning, accompanied by elevated sclerostin expression. Genetic Sost knockout or pharmacologic sclerostin antibody treatment was reported to counteract bone loss during mechanical unloading in mice. However, severe cardiovascular events were reported in postmenopausal osteoporotic patients treated with commercially available sclerostin antibody targeting loop2. It is desirable to develop a precise sclerostin inhibition strategy to counteract unloading-induced bone loss, without increasing cardiovascular risk.

Methods and results: In a previously published rodent studies under normal loading condition, it was found that sclerostin loop3 participated in the inhibitory effect of sclerostin on bone formation, while the preventive action of sclerostin against cardiovascular events was independent of sclerostin loop3. Nevertheless, whether and how sclerostin loop3 contributes to bone formation reduction and bone loss under mechanical unloading condition remains unclear. In this study under mechanical unloading condition, either sclerostin loop3-specific deficiency in Sost^loop3−/− mice or sclerostin loop3-specific inhibition by our tailor-made aptamer Apc001 counteracted unloading-induced bone loss without increasing arterial stiffness, whereas either Sost knockout or romosozumab treatment significantly increased unloading-induced arterial stiffness in mice. These findings indicated sclerostin loop3 as a therapeutic target with cardiovascular safety against unloading-induced bone loss. Mechanistically, we identified that sclerostin loop3 bound to LRP4 in osteoblasts under mechanical unloading condition. Osteoblast-specific Lrp4 knockout counteracted unloading-induced bone formation reduction and bone loss in OB. Lrp4−/− mice. Further, blocking the interaction of sclerostin loop3 with LRP4 via mutation of the interaction residues (Lrp4m) or pharmacologic inhibition with LRP4 peptide tool (LRP4-Pep) dramatically attenuated binding of sclerostin to LRP6, counteracted decrease of Wnt/β-catenin signaling activity and osteogenic potential in osteoblasts under mechanical unloading condition in vitro. Consistently, Lrp4m counteracted unloading-induced bone formation reduction and bone loss in mice in vivo. In Lrp4m/OB-Lrp4 mice, osteoblast-conditional correction of Lrp4m to wild-type Lrp4 attenuated the counteractive effect of Lrp4m on unloading-induced bone loss. Pharmacologically, osteoblasts-targeted LRP4-Pep counteracted bone formation reduction and bone loss during mechanical unloading in wild-type mice.

Conclusion: Sclerostin loop3-mediated anchoring of sclerostin to LRP4 facilitated its binding to LRP6 in osteoblasts, contributing to bone formation reduction and bone loss under mechanical unloading condition.

The translational potential of this article: Specifically blocking the interaction of sclerostin loop3 with LRP4 in osteoblasts would offer a precise strategy with cardiovascular safety for treatment of unloading-induced bone loss.