DC-STAMP and OC-STAMP cooperatively regulate osteoclast and foreign body giant cell cell–cell fusion

INTRODUCTION: Osteoclasts and foreign body giant cells (FBGCs) are multi-nuclear cells established by fusion of their mono-nuclear forms. Multi-nucleation via cell-cell fusion is a common characteristic of osteoclasts and FBGCs, and Dendritic Cell-Specific Transmembrane Protein (DC-STAMP) and Osteoclast Stimulatory Transmembrane Protein (OC-STAMP) are both required for the process. Thus, it is thought that DC-STAMP and OC-STAMP interaction likely induces this fusion, but details of these mechanisms are not clear. MATERIALS AND METHODS: We crossed DC-STAMP knockout (KO) with OC-STAMP KO mice to obtain DC-STAMP/OC-STAMP doubly deficient (DKO) mice. Osteoclasts and FBGC common progenitors were isolated from wild-type (WT), DC-STAMP KO, OC-STAMP KO or DKO mice. We then set up 4 co-culture systems: (1) WT with DC-STAMP KO cells, (2) WT with OC-STAMP KO cells, (3) WT with DKO cells, and (4) DC-STAMP KO with OC-STAMP KO cells to induce osteoclast or FBGC formation. We evaluated osteoclast and FBGC formation by TRAP and May-Gruenwald Giemsa staining, respectively. Finally, we performed bone morphometric analysis of WT, DC-STAMP KO, OC-STAMP KO, and DKO mice. RESULTS: Cell-cell fusion occurs normally in co-cultures of DC KO with WT cells, OC KO with WT cells, and DKO with WT cells in both osteoclast and FBGC-inducing conditions. By contrast, co-cultures of DC-STAMP KO with OC-STAMP KO cells did not show cell-cell fusion. Bone morphometric analysis revealed enhanced bone formation in DKO mice. CONCLUSION: DC-STAMP and OC-STAMP cooperate to promote osteoclast and FBGC cell-cell fusion. DC-STAMP and OC-STAMP doubly deficient mice exhibit increased osteogenesis.