[The potential of RANKL reverse signaling as a novel pharmacological target]

RANKL is a bidirectional signaling molecule; activation of the RANKL reverse signaling is triggered by the cross-linking of multiple RANKL trimers to form a molecular cluster. In adults, RANKL is expressed primarily in bone tissue and the immune system. In bone tissue, the functions of RANKL forward and reverse signaling have been separately analyzed; the forward signaling is responsible for bone resorption by inducing the maturation of osteoclasts, while the reverse signaling is activated by vesicular RANK, one of the osteoclast-derived coupling factors, leading to the promotion of early osteoblast differentiation. In the immune system, RANKL is expressed on lymphocytes and the interaction with antigen-presenting cells such as RANK-expressing dendritic cells should result in the activation of both the forward and the reverse signaling, however, the discrimination of the function of each pathway has not been achieved yet. To activate RANKL reverse signaling, a multivalent protein construct of anti-RANKL single-chain Fv multimerized with peptide linkers would be effective, since this type of construct can induce cluster formation by cross-linking RANKL trimers. It was also found that the divalent construct with minimal molecular size can cross-link the RANKL trimer without affecting the forward signaling. On the other hand, the design of constructs to inhibit the activation of RANKL reverse signaling needs to be tested experimentally. Particularly, it may be necessary to obtain small molecules that act on the RANKL intracellular domain to achieve selective inhibition of the reverse signaling without affecting the forward signaling.