Macrophages seed all tissues in which they have the ability, in specific and rare instances, to fuse with themselves and to differentiate into osteoclasts in bone or into giant cells in chronic inflammatory reactions. Although these cells play a central role in osteoporosis and in foreign body rejection, respectively, the molecular mechanism used by macrophages to fuse remains poorly understood. Macrophages might also fuse with somatic and tumor cells to promote tissue repair and metastasis, respectively. We reported that CD44 expression is highly induced in macrophages at the onset of fusion in which it plays a role. We report now that the intracellular domain of CD44 (CD44ICD) is cleaved in macrophages undergoing fusion and that presenilin inhibitors prevent the release of CD44ICD and fusion. We also show that CD44ICD promotes the fusion of tissue macrophages and bone marrow-derived macrophages. IntroductionMacrophages seed all tissues and have the capacity, in specific and rare instances, to undergo homotypic fusion and to differentiate into multinucleate osteoclasts or giant cells, in bone or in chronic inflammatory reactions, respectively. Multinucleation endows macrophages with the ability to resorb components located extracellularly such as bone and foreign bodies, respectively. 1,2 In as much as osteoclasts are essential for the remodeling of bone, they are chiefly responsible for the loss of bone that leads to osteoporosis. Osteoclasts are also responsible for the local loss of bone that results from inflammation, such as in rheumatoid arthritis and in periodontal disease. Giant cells differentiate around foreign bodies such as pathogens, implants, and transplants that they resorb.Increasing evidence suggests that macrophages possess the ability to fuse also with somatic cells to repair tissues and organs and with tumor cells to trigger the metastatic process. 2 So macrophages, which are ubiquitously present in tissues where they perform a wide array of functions, might be endowed with a larger repertoire of biologic activities than originally anticipated. 3 The molecular mechanisms used by macrophages to adhere to, and to fuse with each other, and possibly with other cells, is an essential step that remains to be characterized. Indeed, cell-to-cell fusion itself, whether it concerns that of sperm cells with oocytes or myoblasts with myoblasts, leading to fertilization and muscle development, respectively, remains unclear. 4 To gain insight into the fusion mechanism of macrophages, we had subjected fusing rat alveolar macrophages to genome-wide oligonucleotide microarray, an approach that revealed the transiently induced expression of presenilin 2 (PS2), a protein associated with Alzheimer disease (for a review, see Hutton and Hardy 5 ). The fact that the deletion of the ps1 gene in mice leads to severe skeletal defects, 6 and that the deletion of the ps2 gene enhances the embryonic lethal phenotype of ps1 deletion, 7 triggered our interest in PS.Presenilins (PS1 and PS2) are transmembrane proteins ide...

Fusion of macrophages is an essential step in the differentiation of osteoclasts, which play a central role in the development and remodeling of bone. Osteoclasts are important mediators of bone loss, which leads, for example, to osteoporosis. Macrophage fusion receptor/signal regulatory protein ␣ (MFR/SIRP␣) and its ligand CD47, which are members of the Ig superfamily (IgSF), have been implicated in the fusion of macrophages. We show that CD200, which is not expressed in cells that belong to the myeloid lineage, is strongly expressed in macrophages at the onset of fusion. By contrast, the CD200 receptor (CD200R), which, like CD200, belongs to the IgSF, is expressed only in cells that belong to the myeloid lineage, including osteoclasts, and in CD4 ؉ T cells. Osteoclasts from CD200 ؊/؊ mice differentiated at a reduced rate. Activation of the NF-B and MAP kinase signaling pathways downstream of RANK, a receptor that plays a central role in the differentiation of osteoclasts, was depressed in these cells. A soluble recombinant protein that included the extracellular domain of CD200 rescued the fusion of CD200 ؊/؊ macrophages and their activation downstream of RANK. Conversely, addition of a soluble recombinant protein that included the extracellular domain of CD200R or short-hairpin RNA-mediated silencing of the expression of CD200R prevented fusion. Thus CD200 engagement of the CD200R at the initiation of macrophage fusion regulated further differentiation to osteoclasts. Consistent with in vitro observations, CD200 ؊/؊ mice contained fewer osteoclasts and accumulated more bone than CD200 ؉/؉ mice. The CD200-CD200R axis is therefore a putative regulator of bone mass, via the formation of osteoclasts.fusion ͉ macrophage ͉ RANK ͉ MAPK