Edited by Xiao-Fan Wang Bone-resorbing multinucleated osteoclasts that play a central role in the maintenance and repair of our bones are formed from bone marrow myeloid progenitor cells by a complex differentiation process that culminates in fusion of mononuclear osteoclast precursors. In this study, we uncoupled the cell fusion step from both pre-fusion stages of osteoclastogenic differentiation and the post-fusion expansion of the nascent fusion connections. We accumulated ready-to-fuse

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... in the presence of the fusion inhibitor lysophosphatidylcholine and then removed the inhibitor to study synchronized cell fusion. We found that osteoclast fusion required the dendrocyte-expressed seven transmembrane protein (DC-STAMP)-dependent non-apoptotic exposure of phosphatidylserine at the surface of fusion-committed cells. Fusion also depended on extracellular annexins, phosphatidylserine-binding proteins, which, along with annexin-binding protein S100A4, regulated fusogenic activity of syncytin 1. Thus, in contrast to fusion processes mediated by a single protein, such as epithelial cell fusion in Caenorhabditis elegans, the cell fusion step in osteoclastogenesis is controlled by phosphatidylserine-regulated activity of several proteins. 2 The abbreviations used are: LPC, lysophosphatidylcholine; Anx, annexin; BMC, bone marrow cell; HM, human monocyte; M-CSF, macrophage colony-stimulating factor; PS, phosphatidylserine; RANK, receptor activator of nuclear factor-B; RANKL, receptor activator of nuclear factor-B ligand; OCP, osteoclast precursor cell; TRAP, tartrate-resistant acidic phosphatase; MEM-␣, minimal essential medium-␣; CDF, cumulative distribution function; DiI, 1,1Јdioctadecyl-3,3,3Ј,3Ј-tetramethylindocarbocyanine perchlorate. cro ARTICLE 254