The TNF-family cytokine RANKL regulates bone resorption and formation

RANKL is a pleiotropic TNF-family cytokine that governs the differentiation and function of bone cells, lymphoid tissue, and mammary epithelium. In bone, RANKL regulates the ontogeny of the osteoclast, the principal resorptive cell of the skeleton. RANKL, via its receptor RANK, is requisite for the inflammatory cytokine TNF to induce bone destruction. TNF potentiates the ability of RANKL to induce osteoclastogenesis, exhibiting synergism at the level of NFκB and SAPK/JNK, essential signaling pathways for osteoclastogenesis. In vivo administration of TNF prompts robust osteoclast formation in marrow-transplanted hosts in which β-galactosidase positive, TNF-responsive osteoclast progenitors develop within a TNF-nonresponsive stroma. While TNF alone does not induce osteoclastogenesis, it does so in vitro and in vivo by directly targeting osteoclast precursors within a stroma that expresses permissive levels of RANKL. The ability of RANKL, but not TNF, to induce osteoclastogenesis lies within specific structural components that enable it to activate RANK. The crystal structure of RANKL at 2.6 Å reveals that it self-associates as a homotrimer with unique surface loops that distinguish it from other TNF-family cytokines. Mutagenesis of its unique loops significantly alters the ability of RANKL to activate RANK, thereby identifying elements that are indispensable for receptor engagement. A polypeptide corresponding to the most unique surface loop of RANKL acts as an antagonist in vitro, successfully competing against the biological activity of RANKL. Such structural determinants of receptor-ligand specificity enables the design of compounds to block osteoclastic bone resorption. Systemic administration of RANKL to mice, initially performed to assess synergy with TNFα, induces an exuberant proliferation of osteoblasts, cells that synthesize bone matrix. Paradoxically, RANKL, the key osteoclastogenic cytokine, profoundly stimulates anabolic bone formation in vitro, ex vivo, and in vivo. The anabolic effect occurs by a mechanism involving RANK signaling in the osteoblast to the transcription factor Cbfa-1 and the ERKs, thereby inducing the osteoblast differentiation and function. RANK-deficient mice exhibit a significantly impairment in bone formation, thus RANKL:RANK signaling plays a physiological role in bone anabolism. When administered exogenously, RANKL exerts an anabolic effect on bone that may be of use in treating diseases of bone loss.