Phosphatidylserine exposure and annexin A5 weaken the actin cortex in osteoclast fusion

Diverse cell–cell fusions involve Ca²⁺ signaling, exposure of phosphatidylserine (PS) at the cell surface and binding of extracellular annexin A5 (Anx A5). Here we report that in the fusion stage of osteoclast formation, each of these shared hallmarks of cell fusion represents a step in a novel signaling pathway. A rise in intracellular Ca²⁺ activates a lipid scramblase that translocates PS from the inner to the outer leaflet of the plasma membrane. This redistribution is enhanced by binding of extracellular Anx A5 to PS. Depletion of PS in the inner leaflet weakens actin cortex-plasma membrane attachment, as evidenced by the preferential localization of the cortex detachment areas within PS-enriched regions at the cell surface. Weakening of the cortex attachment promotes osteoclast fusion. Based on these findings and theoretical analysis, we propose that PS exposure-to-cortex detachment pathway facilitates pre-fusion membrane contacts and fusion pore expansion in osteoclast fusion and other cell–cell fusions by promoting outward membrane deformations with locally elevated tension.