MT1-MMP: A novel component of the macrophage cell fusion machinery

Cell fusion dynamics: mechanisms of multinucleation in osteoclasts and macrophages

Cell-cell fusion is a vital biological process where the membranes of two or more cells merge to form a syncytium. This phenomenon is critical in various physiological and pathological contexts, including embryonic development, tissue repair, immune responses, and the progression of several diseases. Osteoclasts, which are cells from the monocyte/macrophage lineage responsible for bone resorption, have enhanced functionality due to cell fusion. Additionally, other multinucleated giant cells (MGCs) also arise from the fusion of monocytes and macrophages, typically during chronic inflammation and reactions to foreign materials such as prostheses or medical devices. Foreign body giant cells (FBGCs) and Langhans giant cells (LGCs) emerge only under pathological conditions and are involved in phagocytosis, antigen presentation, and the secretion of inflammatory mediators. This review provides a comprehensive overview of the mechanisms underlying the formation of multinucleated cells, with a particular emphasis on macrophages and osteoclasts. Elucidating the intracellular structures, signaling cascades, and fusion-mediating proteins involved in cell-cell fusion enhances our understanding of this fundamental biological process and helps identify potential therapeutic targets for disorders mediated by cell fusion.

Multinucleated Giant Cells: Current Insights in Phenotype, Biological Activities, and Mechanism of Formation

Monocytes and macrophages are innate immune cells with diverse functions ranging from phagocytosis of microorganisms to forming a bridge with the adaptive immune system. A lesser-known attribute of macrophages is their ability to fuse with each other to form multinucleated giant cells. Based on their morphology and functional characteristics, there are in general three types of multinucleated giant cells including osteoclasts, foreign body giant cells and Langhans giant cells. Osteoclasts are bone resorbing cells and under physiological conditions they participate in bone remodeling. However, under pathological conditions such as rheumatoid arthritis and osteoporosis, osteoclasts are responsible for bone destruction and bone loss. Foreign body giant cells and Langhans giant cells appear only under pathological conditions. While foreign body giant cells are found in immune reactions against foreign material, including implants, Langhans giant cells are associated with granulomas in infectious and non-infectious diseases. The functionality and fusion mechanism of osteoclasts are being elucidated, however, our knowledge on the functions of foreign body giant cells and Langhans giant cells is limited. In this review, we describe and compare the phenotypic aspects, biological and functional activities of the three types of multinucleated giant cells. Furthermore, we provide an overview of the multinucleation process and highlight key molecules in the different phases of macrophage fusion.

The Macrophage-Osteoclast Axis in Osteoimmunity and Osteo-Related Diseases

Osteoimmunity is involved in regulating the balance of bone remodeling and resorption, and is essential for maintaining normal bone morphology. The interaction between immune cells and osteoclasts in the bone marrow or joint cavity is the basis of osteoimmunity, in which the macrophage-osteoclast axis plays a vital role. Monocytes or tissue-specific macrophages (macrophages resident in tissues) are an important origin of osteoclasts in inflammatory and immune environment. Although there are many reports on macrophages and osteoclasts, there is still a lack of systematic reviews on the macrophage-osteoclast axis in osteoimmunity. Elucidating the role of the macrophage-osteoclast axis in osteoimmunity is of great significance for the research or treatment of bone damage caused by inflammation and immune diseases. In this article, we introduced in detail the concept of osteoimmunity and the mechanism and regulators of the differentiation of macrophages into osteoclasts. Furthermore, we described the role of the macrophage-osteoclast axis in typical bone damage caused by inflammation and immune diseases. These provide a clear knowledge framework for studying macrophages and osteoclasts in inflammatory and immune environments. And targeting the macrophage-osteoclast axis may be an effective strategy to treat bone damage caused by inflammation and immune diseases.

Mechanotransduction via a TRPV4-Rac1 signaling axis plays a role in multinucleated giant cell formation

Multinucleated giant cells are formed by the fusion of macrophages and are a characteristic feature in numerous pathophysiological conditions including the foreign body response (FBR). Foreign body giant cells (FBGCs) are inflammatory and destructive multinucleated macrophages and may cause damage and/or rejection of implants. However, while these features of FBGCs are well established, the molecular mechanisms underlying their formation remain elusive. Improved understanding of the molecular mechanisms underlying the formation of FBGCs may permit the development of novel implants that eliminate or reduce the FBR. Our previous study showed that transient receptor potential vanilloid 4 (TRPV4), a mechanosensitive ion channel/receptor, is required for FBGC formation and FBR to biomaterials. Here, we have determined that (a) TRPV4 is directly involved in fusogenic cytokine (interleukin-4 plus granulocyte macrophage-colony stimulating factor)-induced activation of Rac1, in bone marrow-derived macrophages; (b) TRPV4 directly interacts with Rac1, and their interaction is further augmented in the presence of fusogenic cytokines; (c) TRPV4-dependent activation of Rac1 is essential for the augmentation of intracellular stiffness and regulation of cytoskeletal remodeling; and (d) TRPV4-Rac1 signaling axis is critical in fusogenic cytokine-induced FBGC formation. Together, these data suggest a novel mechanism whereby a functional interaction between TRPV4 and Rac1 leads to cytoskeletal remodeling and intracellular stiffness generation to modulate FBGC formation.

Common signalling pathways in macrophage and osteoclast multinucleation

Macrophage cell fusion and multinucleation are fundamental processes in the formation of multinucleated giant cells (MGCs) in chronic inflammatory disease and osteoclasts in the regulation of bone mass. However, this basic cell phenomenon is poorly understood despite its pathophysiological relevance. Granulomas containing multinucleated giant cells are seen in a wide variety of complex inflammatory disorders, as well as in infectious diseases. Dysregulation of osteoclastic bone resorption underlies the pathogenesis of osteoporosis and malignant osteolytic bone disease. Recent reports have shown that the formation of multinucleated giant cells and osteoclast fusion display a common molecular signature, suggesting shared genetic determinants. In this Review, we describe the background of cell-cell fusion and the similar origin of macrophages and osteoclasts. We specifically focus on the common pathways involved in osteoclast and MGC fusion. We also highlight potential approaches that could help to unravel the core mechanisms underlying bone and granulomatous disorders in humans.

Transcriptional targeting of sphingosine-1-phosphate receptor S1P2 by epigallocatechin-3-gallate prevents sphingosine-1-phosphate-mediated signaling in macrophage-differentiated HL-60 promyelomonocytic leukemia cells

BACKGROUND

Macrophage chemotaxis followed by blood-brain barrier transendothelial migration is believed to be associated with inflammation in the central nervous system. Antineuroinflammatory strategies have identified the dietary-derived epigallocatechin-3-gallate (EGCG) as an efficient agent to prevent neuroinflammation-associated neurodegenerative diseases by targeting proinflammatory mediator signaling.

METHODS

Given that high levels of sphingosine kinase and its product, sphingosine-1-phosphate (S1P), are present in brain tumors, we used quantitative reverse-transcription polymerase chain reaction (qRT-PCR) and immunoblotting to test whether EGCG may impact on S1P receptor gene expression and prevent S1P response in undifferentiated and in terminally differentiated macrophages.

RESULTS

Promyelomonocytic human leukemia (HL)-60 cells were differentiated into macrophages, and S1P triggered phosphorylation in extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and P38 mitogen-activated protein kinase (MAPK) intracellular signaling, as shown by Western blot analysis. Pretreatment of cells with EGCG prior to differentiation inhibited the response to S1P in all three pathways, while EGCG abrogated P38 MAPK phosphorylation when present only during differentiation. Terminally-differentiated macrophages were, however, insensitive to EGCG treatment. Using qRT-PCR, gene expression of the S1P receptors S1P1, S1P2, and S1P5 was predominantly induced in terminally-differentiated macrophages, while the S1P2 was decreased by EGCG treatment.

CONCLUSION

Our data suggest that diet-derived EGCG achieves efficient effects as a preventive agent, targeting signaling pathways prior to cell terminal differentiation. Such properties could impact on cell chemotaxis through the blood-brain barrier and prevent cancer-related neuroinflammation.