ADAM8 enhances osteoclast precursor fusion and osteoclast formation in vitro and in vivo.

ADAM8 expression is increased in the interface tissue around a loosened hip prosthesis and in the pannus and synovium of patients with rheumatoid arthritis, but its potential role in these processes is unclear. ADAM8 stimulates osteoclast (OCL) formation, but the effects of overexpression or loss of expression of ADAM8 in vivo and the mechanisms responsible for the effects of ADAM8 on osteoclastogenesis are unknown. Therefore, to determine the effects of modulating ADAM expression, we generated tartrate-resistant acid phosphatase (TRAP)-ADAM8 transgenic mice that overexpress ADAM8 in the OCL lineage and ADAM8 knockout (ADAM8 KO) mice. TRAP-ADAM8 mice developed osteopenia and had increased numbers of OCL precursors that formed hypermultinucleated OCLs with an increased bone-resorbing capacity per OCL. They also had an enhanced differentiation capacity, increased TRAF6 expression, and increased NF-κB, Erk, and Akt signaling compared with wild-type (WT) littermates. This increased bone-resorbing capacity per OCL was associated with increased levels of p-Pyk2 and p-Src activation. In contrast, ADAM8 KO mice did not display a bone phenotype in vivo, but unlike WT littermates, they did not increase RANKL production, OCL formation, or calvarial fibrosis in response to tumor necrosis factor α (TNF-α) in vivo. Since loss of ADAM8 does not inhibit basal bone remodeling but only blocks the enhanced OCL formation in response to TNF-α, these results suggest that ADAM8 may be an attractive therapeutic target for preventing bone destruction associated with inflammatory disease.

A SQSTM1/p62 mutation linked to Paget's disease increases the osteoclastogenic potential of the bone microenvironment.

Paget's disease of bone (PDB) is the second most common bone disease and is characterized by focal bone lesions which contain large numbers of abnormal osteoclasts (OCLs) and very active normal osteoblasts in a highly osteoclastogenic marrow microenvironment. The etiology of PDB is not well understood and both environmental and genetic causes have been implicated in its pathogenesis. Mutations in the SQSTM1/p62 gene have been identified in up to 30% of Paget's patients. To determine if p62 mutation is sufficient to induce PDB, we generated mice harboring a mutation causing a P-to-L (proline-to-leucine) substitution at residue 394 (the murine equivalent of human p62(P392L), the most common PDB-associated mutation). Bone marrow cultures from p62(P394L) mice formed increased numbers of OCLs in response to receptor activator of NF-kappaB ligand (RANKL), tumor necrosis factor alpha (TNF-alpha) or 1alpha,25-(OH)(2)D(3), similar to PDB patients. However, purified p62(P394L) OCL precursors depleted of stromal cells were no longer hyper-responsive to 1alpha,25-(OH)(2)D(3), suggesting effects of the p62(P394L) mutation on the marrow microenvironment in addition to direct effects on OCLs. Co-cultures of purified p62(P394L) stromal cells with either wild-type (WT) or p62(P394L) OCL precursors formed more OCLs than co-cultures containing WT stromal cells due to increased RANKL production by the mutant stromal cells. However, despite the enhanced osteoclastogenic potential of both OCL precursors and marrow stromal cells, the p62(P394L) mice had histologically normal bones. These results indicate that this PDB-associated p62 mutation is not sufficient to induce PDB and suggest that additional factors acting together with p62 mutation are necessary for the development of PDB in vivo.