Protein-mediated membrane remodeling

Eunice Kennedy Shriver National Institute Of Child Health & Human Development

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Abstract

Cell-cell fusion plays a critical role in fertilization, the lifelong remodeling of bones and skeletal muscles, the formation of the placental syncytiotrophoblast, and other physiological processes. Some mechanistic motifs appear to be strikingly conserved across these diverse cell-cell fusion processes. All listed cell-cell fusions depend on lipid scramblases, proteins that mediate the nonselective, bidirectional movement of lipids between the inner (cytofacial) and outer leaflets of the plasma membrane (PM); and on scramblase-mediated non-apoptotic cell surface exposure of an anionic lipid phosphatidylserine (PS), normally almost exclusively retained in the inner leaflet of the PM. These fusions also depend on Annexin A5 (Anx A5). Membrane fusion depends on membrane deformations and initiation of PM deformations depends on the local and transient detachment of a patch of membrane from the actin cortex (AC), membrane-linked actin-rich protein network. AC is attached to the inner leaflet of the PM via membrane-anchoring proteins of the ezrin/radixin/moesin (ERM) family. ERM binding to the PM and organization of the actomyosin network depend on the lipid composition of the inner (cytosolic) leaflet of PM and are promoted by PS and another anionic lipid phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2). Like PS, PI(4,5)P2 is highly enriched in the inner leaflet of PM, but in some physiological processes it is also found at the cell surface. Local weakening of the AC-PM attachment, via mechanisms such as reduction of the negative surface charge of the inner leaflet, allows intracellular pressure to locally bulge the PM. Why do diverse cell fusion processes depend on lipid scramblase activity and on the appearance of PS- and Anx A5 at the cell surface? In our most recent study we address this question for osteoclast fusion, a critical stage in the formation of bone-resorbing multinucleated osteoclasts. Each fusion event increases the bone-resorbing activity of osteoclasts, which balances the bone-forming activity of osteoblasts in the tightly regulated, lifelong remodeling of the skeleton. We found that activation of lipid scrambling reduces PS and PI(4,5)P₂ levels in the inner leaflet of the PM, leading to local AC–PM detachment. Binding of extracellular Anx A5 to cell-surface PS enhances PS depletion from the inner leaflet and thus further weakens the AC-PM connection. Based on these findings and theoretical analysis, we propose that PS exposure-Anx A5 binding-AC detachment pathway facilitates osteoclast fusion and other cell-cell fusions by promoting membrane deformations required for formation of prefusion membrane contacts. Additionally, our model suggests that a local increase in tension in the AC detachment region of the membrane promotes fusion pore expansion. Dissection of the signaling pathways that control fusion and bone-resorbing activity can help in identifying potential therapeutic targets for bone diseases. Indeed, some of the proteins and pathways involved in the early stages of osteoclastogenic differentiation have already been tested in animal and clinical studies as potential therapeutic targets. Moreover, we suggest that weakening the AC-PM attachment via lipid scrambling and Anx A5-PS interactions at the surface of the cells may, in addition to fusion, play an important role in other physiological contexts, including the immune response; neuronal regeneration and degeneration, and in cancer cells in metastases. A better understanding of the cascade of events within this emerging inside-out/outside-in signaling pathway will likely help in uncovering novel mechanisms that coordinate diverse aspects of cellular responses in various biological contexts.