A tetravalent peptide that binds to the RANK-binding region of TRAF6 via a multivalent interaction efficiently inhibits osteoclast differentiation

Inhibition of osteoclast differentiation is a promising approach for the treatment of osteoporosis and rheumatoid arthritis. Receptor activator of nuclear factor kappa B (NF-κB) (RANK), which is an essential molecule for osteoclast differentiation, interacts with tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6) to transduce downstream signals. Both RANK and TRAF6 have homo-trimeric structures, forming a multivalent interaction between the Pro-X-Glu-X-X-(aromatic/acidic) motif of RANK and the C-terminal domain of TRAF6 (TRAF-C), that markedly increases the binding affinity. Here, we designed a tetravalent peptide, RANK-tet, containing the TRAF-C-binding motif of RANK and found that RANK-tet binds to TRAF-C with high affinity. In contrast, a monomeric form of RANK-tet (RANK-mono) with the same TRAF-C-binding motif did not bind to TRAF-C, clearly indicating the multivalent interaction is strictly required for the high-affinity binding to TRAF-C. RANK-tet did not bind to a series of TRAF-C-mutants with an amino acid substitution in the RANK-binding region, indicating that RANK-tet specifically targets the RANK-binding region of TRAF-C. A cell-permeable form of RANK-tet that has poly-Arg residues at each C-terminal of the TRAF-C-binding motif efficiently inhibited the RANK ligand (RANKL)-induced differentiation of bone marrow cells to osteoclasts. Thus, this compound can be an effective anti-osteoclastogenic agent.