Cell fusion dynamics: mechanisms of multinucleation in osteoclasts and macrophages

Abstract

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.

Keywords: Cell fusion; Macrophages; Multinucleated giant cells; Multinucleation; Osteoclasts.

Fig. 1

Types of cell fusion and the process of cell membrane fusion. a Three types of syncytia: Cells that undergo fusion are collectively referred to as syncytia and include three types—homokaryon, heterokaryon, and synkaryon. Homokaryon formation: Fusion of cells with the same type of nuclei results in a multinucleated cell. Heterokaryon formation: Fusion of cells with distinct types of nuclei forms a multinucleated cell. Synkaryon formation: A single-nucleus cell is formed through the fusion of cells with either the same or different types of nuclei. b Membrane fusion process: Cells approach and interact with each other (outer coat interaction). This interaction leads to the formation of a stalk-like structure. The stalk then expands into a diaphragm. Finally, the fusion pore forms, completing the membrane fusion. c Syncytin-1 mediated fusion in trophoblasts: The receptor-binding domain (RBD) of syncytin-1 binds to the Na-dependent neutral amino acid transporter type 2 (ASCT2). The surface unit domain (SU) is removed from the transmembrane domain (TM) by cleaving disulfide (SS) bonds. The fusion peptide (FP) is inserted into the target plasma membrane, initiating host plasma membrane bending

Fig. 2

Types and characteristics of multinucleated giant cells. a Types of multinucleated giant cells: Langhans giant cells, found in granulomatous conditions such as tuberculosis and sarcoidosis, have nuclei arranged in a horseshoe pattern. Foreign body giant cells, formed in response to foreign materials, have randomly distributed nuclei within the cytoplasm. Osteoclasts, involved in bone resorption, are characterized by their large size and multiple nuclei. b In vitro differentiation of multinucleated giant cells: Macrophages can differentiate into different types of giant cells when stimulated with specific cytokines. IFN-γ stimulation leads to the formation of Langhans giant cells. IL-4 stimulation results in the differentiation of macrophages into foreign body giant cells. RANKL induces the formation of osteoclasts, which are involved in bone resorption

Fig. 3

Multinucleated cells share transcription products within a single cell. A parabiosis experiment was conducted with a mouse expressing Cre under the control of the Ctsk gene and a tdTomato reporter mouse. When their circulations are shared, cells differentiate into tdTomato-expressing osteoclasts through cell-cell fusion. This occurs because the Cre protein induces recombination between loxP sites, resulting in the elimination of the STOP sequence. This demonstrates that multinucleated cells share transcription products within a single cell. Scale bar, 10μm

Fig. 4

Molecular mechanisms of cell-cell fusion in osteoclastogenesis. a Acquisition of fusion competence: Stimulation of TREM2 leads to DAP12 phosphorylation, activating SYK and ZAP70. This triggers pathways that, along with RANKL/RANK signaling, enhance the induction of NFATC1, which is a master regulator of osteoclast differentiation. b, c Chemotaxis and adhesion: CCL2/CCR2 signaling mediates chemotaxis and migration, facilitating fusion competency. Cell-cell adhesion is mediated by E-cadherins and SIRPα/CD47 interactions, which are crucial for initiating and maintaining the cell contacts necessary for fusion. d Cytoskeletal reorganization: GTPases, including RAC1, CDC42, and RHOA, which are regulated by MT1-MMP, orchestrate the dynamic rearrangement of the actin cytoskeleton, resulting in membrane protrusions. e Alterations of lipid bilayers and membrane fusion: RANKL/RANK signaling activates caspase-8 and caspase-3, leading to Xkr8-mediated PS exposure. The fusion partner recognizes PS, which is essential for multinucleation. f Bone resorption: Osteoclasts secrete H+, CTSK, TRAP, and MMPs from the ruffled border onto the bone surface, dissolving the bone