Elucidation of Bidirectional RANK–RANKL Signaling and Its Therapeutic Implications for Osteoporosis

Bone remodeling is a strongly regulated process that involves the coupling of osteoclastic bone resorption and osteoblastic bone formation. Receptor activator of nuclear factor-κB (RANK) ligand (RANKL) is a key regulator of osteoclast differentiation through its interaction with RANK. Our understanding of the intricate bone regulatory mechanisms has increased since 2011, when the primary source of RANKL was suggested to be not osteoblasts, as thought previously, but osteocytes embedded in the bone matrix. By analyzing the mechanisms controlling the intracellular localization of RANKL, we discovered that contact with RANK also generates a stimulus on the RANKL side (reverse signaling), and we have been conducting research on the physiological role of this bidirectional signaling pathway in bone metabolism. In this study, we have proposed a novel mechanism by which osteoclast-derived small extracellular vesicles (SEVs) carrying RANK interact with RANKL on osteoblasts, thereby triggering reverse signaling. This signaling activates downstream pathways, leading to enhanced osteoblast differentiation and mineralization. Analysis using RANKL P29A mutant mice, in which RANKL reverse signaling is impaired, suggested that this pathway mediates bone coupling. Furthermore, pharmacological inhibition of the secretion and interaction of SEVs suppressed the coupling from bone resorption to bone formation, strongly supporting this notion. Based on this concept, we developed a bifunctional anti-RANKL antibody that not only inhibits osteoclastogenesis but also activates RANKL reverse signaling. This agent preserved bone formation while suppressing bone resorption in vivo, and the RANKL reverse-signaling pathway identified in this study is a promising therapeutic target for osteoporosis.