A RANK/TRAF6-dependent signal transduction pathway is essential for osteoclast cytoskeletal organization and resorptive function.

Signaling through receptor activator of nuclear factor-kappaB (RANK) is essential for the differentiation and activation of osteoclasts, the cell principally responsible for bone resorption. Animals genetically deficient in RANK or the cognate RANK ligand are profoundly osteopetrotic because of the lack of bone resorption and remodeling. RANK provokes biochemical signaling via the recruitment of intracellular tumor necrosis factor receptor-associated factors (TRAFs) after ligand binding and receptor oligomerization. To understand the RANK-mediated signal transduction mechanism in osteoclastogenesis, we have designed a system to recapitulate osteoclast differentiation and activation in vitro by transfer of the RANK cDNA into hematopoietic precursors genetically deficient in RANK. Gene transfer of RANK constructs that are selectively incapable of binding different TRAF proteins revealed that TRAF pathways downstream of RANK that affect osteoclast differentiation are functionally redundant. In contrast, the interaction of RANK with TRAF6 is absolutely required for the proper formation of cytoskeletal structures and functional resorptive activity of osteoclasts. Moreover, signaling via the interleukin-1 receptor, which also utilizes TRAF6, rescues the osteoclast activation defects observed in the absence of RANK/TRAF6 interactions. These studies are the first to define the functional domains of the RANK cytoplasmic tail that control specific differentiation and activation pathways in osteoclasts.