Cell-cell fusion is a highly regulated and dramatic cellular event that is required for development and homeostasis. Fusion may also play a role in the development of cancer and in tissue repair by stem cells. While virus-cell fusion and the fusion of intracellular membranes have been the subject of intense investigation during the past decade, cell-cell fusion remains poorly understood. Given the importance of this cell-biological phenomenon, a number of investigators have begun analyses of the molecular mechanisms that mediate the specialized fusion events of a variety of cell types and species. We discuss recent genetic and biochemical studies that are beginning to yield exciting insights into the fusion mechanisms of Saccharomyces cerevisiae mating pairs, Caenorhabditis elegans epithelial cells and gametes, Drosophila melanogaster and mammalian myoblasts, and mammalian macrophages.

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The proinflammatory and catabolic cytokine IL-1β has been implicated in the pathogenesis of osteoarthritis (OA) by mediating synovial inflammation and cartilage degeneration. Although synovial macrophages are suggested to be the source of IL-1β, the mechanism remains unclear. Ectopic deposition of hydroxyapatite (HA) crystals in joints is closely associated with OA and other arthropathies, but the precise role of HA in arthritis pathogenesis has not been clearly demonstrated. Here we show that HA crystals of a particular size and shape can stimulate robust secretion of proinflammatory cytokines IL-1β and IL-18 from murine macrophages in a NLRP3 inflammasome-dependent manner. HA-induced inflammasome activation is dependent on potassium efflux, generation of reactive oxygen species (ROS), and lysosomal damage, but independent of cell death. Mice lacking the inflammasome components are protected against HA-induced neutrophilic inflammation in the airpouch model of synovitis, and they show decreased joint pathology accompanying spontaneous HA deposition in the ank-deficient mouse model of arthritis. Moreover, calcium crystal positive synovial fluids from some OA patients exhibited inflammasome-stimulatory activity in vitro. These results demonstrate that the NLRP3 inflammasome mediates the pathological effect of HA crystals in vitro and in vivo and suggest a critical role for the inflammasome in the pathogenesis of OA.caspase-1 | ASC | basic calcium phosphate crystals

The macrophage fusion receptor (MFR), also called P84/BIT/SIRP␣/SHPS-1, is a transmembrane glycoprotein that belongs to the superfamily of immunoglobulins. Previously, we showed that MFR expression is highly induced at the onset of fusion in macrophages, and that MFR appears to play a role in macrophagemacrophage adhesion/fusion leading to multinucleation. The recent finding that IAP/CD47 acts as a ligand for MFR led us to hypothesize that it interacts with CD47 at the onset of cell-cell fusion. CD47 is a transmembrane glycoprotein, which, like MFR, belongs to the superfamily of immunoglobulins. We show that macrophages express the hemopoietic form of CD47, the expression of which is induced at the onset of fusion, but to a lower level than MFR. A glutathione S-transferase CD47 fusion protein engineered to contain the extracellular domain of CD47, binds macrophages, associates with MFR, and prevents multinucleation. CD47 and MFR associate via their amino-terminal immunoglobulin variable domain. Of the nine monoclonal antibodies raised against the extracellular domain of CD47, three block fusion, as well as MFR-CD47 interaction, whereas the others have no effect. Together, these data suggest that CD47 is involved in macrophage multinucleation by virtue of interacting with MFR during adhesion/fusion.Osteoclasts and giant cells are characterized by multinucleation and a powerful ability to resorb the substrate onto which they adhere. Although osteoclasts and giant cells play an important role in bone remodeling and immune defense, respectively, they are also associated with osteoporosis, granulomatous diseases, and tumors.Multinucleation appears to endow macrophages with the capacity to digest and resorb extracellular infectious agents, foreign material, and other components that are too large to be internalized, such as bone. This resorption occurs in an "extracellular lysosomal compartment" sealed off between the multinucleated cell and its target substrate (reviewed in Ref.1). The plasma membrane that faces that extracellular domain is highly ruffled and specialized. Multinucleation gives macrophages added resorptive capacity, in part by making available a large excess of plasma membrane.Understanding the mechanism by which macrophages differentiate into osteoclasts and multinucleated giant cells is of extreme importance. One of the key steps in the differentiation of osteoclasts and giant cells is the fusion mechanism of their mononucleated precursor cells. It is assumed that both osteoclasts and giant cells originate from the fusion of mononuclear phagocytes. Despite the pathophysiological importance of these cells, the mechanism by which their mononucleated precursors fuse remains poorly understood. Indeed, cell-cell fusion itself, whether it concerns that of sperm cells with oocytes in fertilization or myoblasts with myoblasts in muscle development, has not been investigated thoroughly. It is proposed that cell-cell fusion involves a set of proteins similar to those used by viruses to fuse with host cells be...

Membrane fusion is a ubiquitous event that occurs in a wide range of biological processes. While intracellular membrane fusion mediating organelle trafficking is well understood, much less is known about cell-cell fusion mediating sperm cell-oocyte, myoblast-myoblast and macrophage-macrophage fusion. In the case of mononuclear phagocytes, their fusion is not only associated with the differentiation of osteoclasts, cells which play a key role in the pathogenesis of osteoporosis, but also of giant cells that are present in chronic inflammatory reactions and in tumours. Despite the biological and pathophysiological importance of intercellular fusion events, the actual molecular mechanism of macrophage fusion is still unclear. One of the main research themes in my laboratory has been to investigate the molecular mechanism of mononuclear phagocyte fusion. Our hypothesis has been that macrophage-macrophage fusion, similar to virus-cell fusion, is mediated by specific cell surface proteins. But, in contrast with myoblasts and sperm cells, macrophage 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 model system which uses alveolar macrophages. Upon multinucleation, these macrophages acquire the osteoclast phenotype. This indicates that multinucleation of macrophages leads to a specific and novel functional phenotype in macrophages. To identify the components of the fusion machinery, we generated four monoclonal antibodies (mAbs) which block the fusion of alveolar macrophages and purified the unique antigen recognized by these mAbs. This led us to the cloning of MFR (Macrophage Fusion Receptor). MFR was cloned simultaneously as P84/SHPS-1/SIRPalpha/BIT by other laboratories. We subsequently showed that the recombinant extracellular domain of MFR blocks fusion. Most recently, we identified a lower molecular weight form of MFR that is missing two extracellular immunoglobulin (Ig) C domains. Shortly after we cloned MFR, CD47 was reported to be a ligand for P84/SIRPalpha. We have since generated preliminary results which suggest that CD47 interacts with MFR during adhesion/fusion and is a member of the fusion machinery. We also identified CD44 as a plasma membrane protein which, like MFR, is highly expressed at the onset of fusion. The recombinant soluble extracellular domain of CD44 blocks fusion by interacting with a cell-surface binding site. We now propose a model in which both forms of MFR, CD44, and CD47 mediate macrophage adhesion/fusion and therefore the differentiation of osteoclasts and giant cells.