Osteoclasts and giant cells: macrophage-macrophage fusion mechanism

Membrane fusion is a ubiquitous event that occurs in a wide range of biolog ical processes. While intracellular membrane fusion mediating organelle tra fficking is well understood, much less is known about cell-cell fusion medi ating sperm cell-oocyte, myoblast-myoblast and macrophage-macrophage fusion . In the case of mononuclear phagocytes, their fusion is not only associate d with the differentiation of osteoclasts, cells which play a key role in t he pathogenesis of osteoporosis, but also of giant cells that are present i n chronic inflammatory reactions and in tumours. Despite the biological and pathophysiological importance of intercellular fusion events, the actual m olecular mechanism of macrophage fusion is still unclear. One of the main r esearch themes in my laboratory has been to investigate the molecular mecha nism of mononuclear phagocyte fusion. Our hypothesis has been that macropha ge-macrophage fusion, similar to virus-cell fusion, is mediated by specific cell surface proteins. But, in contrast with myoblasts and sperm cells, ma crophage fusion is a rare event that occurs in specific instances. To test our hypothesis, we established an in vitro cell-cell fusion assay as a mode l system which uses alveolar macrophages. Upon multinucleation, these macro phages acquire the osteoclast phenotype. This indicates that multinucleatio n of macrophages leads to a specific and novel functional phenotype in macr ophages. To identify the components of the fusion machinery, we generated f our monoclonal antibodies (mAbs) which block the fusion of alveolar macroph ages and purified the unique antigen recognized by these mAbs. This led us to the cloning of MFR (Macrophage Fusion Receptor). MFR was cloned simultan eously as P84/ SHPS-1/SIRP alpha /BIT by other laboratories. We subsequentl y showed that the recombinant extracellular domain of MFR blocks fusion. Mo st recently, we identified a lower molecular weight form of MFR that is mis sing two extracellular immunoglobulin (Ig) C domains. Shortly after we clon ed MFR, CD47 was reported to be a ligand for P84/SIRP alpha. We have since generated preliminary results which suggest that CD47 interacts with MFR du ring adhesion/fusion and is a member of the fusion machinery. We also ident ified CD44 as a plasma membrane protein which, like MFR, is highly expresse d at the onset of fusion. The recombinant soluble extracellular domain of C D44 blocks fusion by interacting with a cell-surface binding site. We now p ropose a model in which both forms of MFR, CD44, and CD47 mediate macrophag e adhesion/fusion and therefore the differentiation of osteoclasts and gian t cells.