Lipid scrambling via TMEM16F mediates the formation and release of apoptotic vesicles
The ubiquitous and highly conserved programmed cell death pathways that are essential for tissue development and homeostasis are accompanied by distinct morphological alterations. Apoptotic cells undergo fragmentation that is concomitant with the exposure of phosphatidylserine (PS) on the membrane surface. Large fragments, called apoptotic bodies, as well as much smaller and more numerous vesicles are released. While the molecular mechanisms underlying apoptotic body formation have been explored, much less is known about vesicle biogenesis. We used an inducible, active form of TMEM16F to determine the role of lipid scrambling in vesiculation, separately from other apoptotic signaling events. Plasmalemmal lipid scrambling sufficed to release apoptotic-like vesicles without causing changes in cytosolic calcium or the submembrane cytoskeleton. The scrambled bilayer showed pronounced segregation of exofacial lipids and redistribution of apparent cholesterol to the inner leaflet. The clustering of raft-associated components with bulky headgroups -typified by glycophosphatidylinositol-linked proteins- formed domains of outward (convex) curvature, while regions of accumulation of phosphatidylethanolamine (PE) generated inward (concave) curvature that facilitated the scission of vesicles. Thus, scrambling of plasma membrane lipids suffices to induce regions of acute membrane curvature and facilitates detachment of vesicles analogous to those released from the surface of apoptotic cells.