Osteoclasts are multinucleated cells responsible for bone resorption and are formed through the cell-cell fusion of mononuclear macrophage-lineage precursors. Cell-cell fusion is a fundamental process essential not only for osteoclast differentiation but also for various physiological and pathological events, such as myogenesis, fertilization, viral infection, and cancer progression. Despite its broad importance, the universal mechanical conditions that enable membrane fusion remain poorly understood. Recent studies have suggested that in cell fusion processes, including osteoclast differentiation, the formation of membrane protrusions driven by actin cytoskeleton remodeling at the fusion interface plays a critical role. In this study, we reveal that decreased plasma membrane (PM) tension, mediated by reduced membrane-cortex attachment (MCA), is a key mechanical requirement for the formation of actin-based protrusive structures known as invadosomes and for the progression of cell-cell fusion during osteoclastogenesis. Using live-cell imaging, we observed that cells involved in cell-cell fusion show a transition from filopodia-rich morphology to rounded shape immediately before the fusion event. This morphological change coincides with the downregulation of ezrin, one of the ezrin-radixin-moesin (ERM) families of membrane-actin linker proteins, leading to weakened MCA and a reduction in PM tension. Artificial enhancement of MCA led to increased PM tension, and cell fusion was significantly impaired. Importantly, reduced PM tension promoted invadosome formation in a manner dependent on BAR domain proteins, which are known to generate membrane curvature. Knockdown of these BAR proteins suppressed invadosome formation and inhibited cell-cell fusion. Together, these findings demonstrate that reducing PM tension facilitates the formation of curvature-generating membrane structures, enabling cytoskeletal rearrangements required for cell-cell fusion. Our study uncovers a biophysical requirement for osteoclast fusion and highlights membrane mechanics as a key factor in cell-cell fusion processes.
Nemoto et al. (Sun,) studied this question.