Flippases and Scramblases at Plasma Membranes that Regulate Phosphatidylserine Exposure
Authors: Nagata Shigekazu 
Published: October 2019

Abstract
One of the hallmarks of apoptosis is the caspase-dependent exposure of phosphatidylserine (PtdSer) on cell surface, which is recognized by macrophages for engulfment of dead cells (1). How PtdSer is exposed to the cell surface had been elusive for a long time. We recently identified two membrane proteins (TMEM16F and Xkr8) that are involved in scrambling of phospholipids in plasma membrane (2, 3). TMEM16F carries 8 transmembrane regions, and requires Ca2+ to mediate phospholipid scrambling. It plays a role in the PtdSer-exposure in activated platelets for blood clotting, and patients of Scott Syndrome who suffer bleeding disorder carry a mutation in TMEM16F gene. Xkr8 is a protein carrying 6 transmembrane regions. Caspase 3 and 7 cleave off the C-terminal tail of Xkr8, and the cleaved Xkr8 promotes the PtdSer-exposure. In addition to the activation of scramblase, the flippase that translocates PtdSer from outer to inner leaflets was thought to be inactivated during apoptosis. In fact, we recently found that a pair of molecules, ATP11C of a P4-type ATPase and its chaperon CDC50A work as a flippase at plasma membrane (4, 5). ATP11C carries three caspase recognition sites in the middle of the molecule, and is cleaved during apoptosis. When ATP11C gene is mutated, the cells lose most of the flippase activity, but the asymmetrical distribution of PtdSer was still maintained at plasma membrane. Whereas, the cells lacking CDC50A completely lost the flippase activity and constitutively exposed PtdSer. The PtdSer-exposing living CDC50A-null cells were engulfed by thioglycollate-elicited macrophages, indicating that PtdSer exposed on the cell surface is necessary and sufficient to be recognized by macrophages for engulfment. Several molecules such as MFG-E8, Tim-4, Gas6, and Protein S specifically bind to PtdSer with high affinity, and promote the engulfment of PtdSer-exposing cells. However, how they work for the engulfment of apoptotic cells in certain macrophages has not been clear. We recently found that that resident peritoneal macrophages require both Tim4 and Protein S for engulfment, and Tim4, PtdSer-receptor, was involved in tethering of apoptotic cells, while Protein S promoted the engulfment of apoptotic cells by binding to MerTK, a tyrosine kinase receptor (6, 7). Here, I discuss how PdtSer is exposed during apoptotic cell death, and how dead cells are engulfed by macrophages. 1. Nagata S, Hanayama R, Kawane K. Autoimmunity and the clearance of dead cells. Cell. 2010;140:619-630. 2. Suzuki J, Umeda M, Sims PJ, Nagata S. Calcium-dependent phospholipid scrambling by TMEM16F. Nature. 2010;468:834-838. 3. Suzuki J, Denning DP, Imanishi E, Horvitz HR, Nagata S. Xk-related protein 8 and CED-8 promote phosphatidylserine exposure in apoptotic cells. Science. 2013;341:403-406. 4. Segawa K, Suzuki J, Nagata S. Flippases and scramblases in the plasma membrane. Cell Cycle. 2014;13:2990-2991. 5. Segawa K, Kurata S, Yanagihashi Y, Brummelkamp T, Matsuda F, Nagata S. Caspase-mediated cleavage of phospholipid flippase for apoptotic phosphatidylserine exposure. Science. 2014;344:1164-1168. 6. Nishi C, Toda S, Segawa K, Nagata S. Tim4- and MerTK-mediated engulfment of apoptotic cells by mouse resident peritoneal macrophages. Mol Cell Biol. 2014;34:1512-1520. 7. Toda S, Segawa K, Nagata S. MerTK-mediated engulfment of pyrenocytes by central macrophages in erythroblastic islands. Blood. 2014;123:3963-3971.

Disclosures
No relevant conflicts of interest to declare.