Cell Fusion Stage in Osteoclast Formation

Little is known about cell fusion that generates osteoclasts, multinucleated cells that resorb our bones in their continuous remodeling. Here we isolated fusion stage from preceding and subsequent stages of osteoclast formation and identified some proteins involved in this fusion process. To trigger osteoclastogenesis, we applied RANKL to RAW macrophage-like cells or M-CSF and RANKL to human monocytes. We blocked fusion by applying a reversible hemifusion inhibitor lysophosphatidylcholine, accumulated the ready-to-fuse macrophages for 16h and then removed our inhibitor. This fusion-synchronization approach has allowed us to concentrate within an hour cell fusion events that would normally develop within 16h. At the time of fusion, macrophages exposed at their surface phosphatidylserine (PS), lipid that is normally found only in the inner leaflet of the plasma membrane. CaCCinh-A01, an inhibitor of TMEM16 scramblases that mediate non-apoptotic PS externalization, suppressed both PS exposure at the surface of fusion-committed cells and synchronized fusion indicating the functional importance of PS externalization. We also found that fusion was suppressed by peptide inhibitors and antibodies to PS-binding proteins annexins A1 and A5 (AnxA1&A5) for RAW cells and AnxA5 for human monocytes. The importance of Anxs in osteoclast formation was further confirmed by finding that bone marrow cells from mice deficient in both AnxA1 and A5 have a lowered ability to generate osteoclasts. In addition to PS and Anxs, synchronized fusion involves endogenous retroviral human syncytin-1 protein that along with Anx A5 is present at the surface of fusion-committed human osteoclast precursors. We hypothesize that Anxs accumulated at TMEM16-externalized PS regulate activity of syncytin1-dependent fusion machinery.