Dysregulation of 5-hydroxytryptamine 6 receptor accelerates maturation of bone-resorbing osteoclasts and induces bone loss

Rationale: Characterizing the regulation of bone-resorbing osteoclasts is central to the understanding of the pathogenesis and treatment of bone diseases, such as osteoporosis and periodontitis. 5-hydroxytryptamine (5-HT) has drawn considerable attention for its role in bone; however, it remains unknown whether the intracellular signaling of 5-HT receptors (5-HTRs) is linked to any of the regulatory mechanisms in osteoclasts. Herein, we report 5-HT(6)R to be a key regulatory receptor for osteoclastogenesis. Methods: In order to explore the critical role of 5-HT(6)R in bone-resorbing osteoclasts, in vitro experiments were performed using mouse whole bone marrow cells isolated from femora and tibiae and In vivo animal experiments were performed using 5-HT(6)R-deficient (5-HT(6)R(KO-/-)) mice, bone resorption mice model, and osteoporosis mice model. Results: Compared to other 5HTRs, activation of 5-HT(6)R relatively increased TRAP (tartrate-resistant acid phosphatase) activity during osteoclastogenesis. 5-HT(6)R(KO(-/-)) mice and 5-HT(6)R(KO(-/-)) osteoclast lineages presented with an abnormal phenotype and impaired osteoclastogenesis and impaired osteoclastogenesis. Activation of 5-HT(6)R increased the number of TRAP-positive multinuclear osteoclasts, actin ring formation, and expression of early osteoclast markers with osteoclast lineage commitment. Intracellular 5-HT(6)R signaling was found to be linked to RhoA GTPase activation and was involved in the maturation of osteoclasts. This signaling pathway also showed enhanced bone destruction after lipopolysaccharide (LPS) administration in mice. Furthermore, inhibition of 5-HT(6)R-mediated RhoA GTPase signaling protected against ovariectomy(OVX)-induced bone loss in mice. Conclusion: Taken together, our findings place the 5-HT(6)R system in a new context of osteoclast lineages in both an in vitro and in vivo system, and also it may offer a novel molecular target for the treatment of bone diseases.