Membrane asymmetry is a carefully controlled and energetically costly differential in lipid composition between the inner and outer plasma membrane (PM) leaflets. Asymmetry is created and maintained by lipid translocases (flippases) that force anionic and amino-phospholipids into the cytosolic leaflet and released by lipid channels (scramblases) that shuttle phospholipids down their concentration gradient to disrupt PM asymmetry. The release of lipid asymmetry exposes the charged lipid phosphatidylserine (PS) to the external leaflet, where it is essential for apoptosis and blood clotting. Additionally, exposure of inner leaflet phospholipids has been observed during the activation of several immune cell types (mast cells, basophils, and leukocytes). We confirmed these observations, showing exposure of PS and other inner leaflet phospholipids on the PM outer leaflet following antigen-mediated stimulation of T cells and mast cells. Importantly, in contrast to wholesale release of PM asymmetry by apoptosis, PS exposure in activated immune cells is focal, transient, reversible, and not associated with cell death. This lipid scrambling appears to be involved in immune cell function, as inhibition of the scramblase TMEM16F attenuates antigen-activated mast cell degranulation. To investigate the mechanisms underlying these effects, we knocked out (KO) TMEM16F, the ubiquitous Ca 2+ -dependent scramblase / lipid channel, which is putatively responsible for PS and PE translocation during immune cell signaling. Indeed, we find that TMEM16F is essential for activation-associated lipid scrambling. Finally, we combine theory and cell experiments to explore the molecular nature and functional role of localized, transient membrane scrambling domains during mast cell activation and degranulation. Altogether, our findings suggest that localized scrambling is an essential component of signal transduction downstream of immune receptor activation.
Sputay et al. (Sun,) studied this question.