Structural and functional consequences of reversible lipid asymmetry in living membranes

Abstract

Maintenance of lipid asymmetry across the two leaflets of the plasma membrane (PM) bilayer is a ubiquitous feature of eukaryotic cells. Loss of this asymmetry has been widely associated with cell death. However, increasing evidence points to the physiological importance of non-apoptotic, transient changes in PM asymmetry. Such transient scrambling events are associated with a range of biological functions, including intercellular communication and intracellular signaling. Thus, regulation of interleaflet lipid distribution in the PM is a broadly important but underappreciated cellular process with key physiological and structural consequences. Here, we compile the mounting evidence revealing multifaceted, functional roles of PM asymmetry and transient loss thereof. We discuss the consequences of reversible asymmetry on PM structure, biophysical properties and interleaflet coupling. We argue that despite widespread recognition of broad aspects of membrane asymmetry, its importance in cell biology demands more in-depth investigation of its features, regulation, and physiological and pathological implications.

Figure 1.. The interleaflet asymmetry of the PM bilayer is complex and dynamic.

(A) Data-driven schematic illustration of the phospholipid asymmetry of the RBC PM. Lipid asymmetry can be released by lipid scrambling (i.e. interleaflet lipid mixing, blue regions) that occurs either (B) globally or (C) at specialized local sites. Legend on the bottom left applies to all panels; SFA denotes saturated, MFA monounsaturated, and PUFA polyunsaturated fatty acids.

Figure 2.. Functional roles of reversible PM lipid asymmetry.

Transient, non-apoptotic changes in PM lipid asymmetry have been implicated in a variety of biological processes including (A-C) cell-cell communication and contact and (D-E) intracellular signaling. (A) Surface exposed PS facilitates intercellular contact and fusion by interacting either directly (with PS receptors) or indirectly (through bridging molecules) with PM components on neighboring cells. (B) Small molecule signals can lead to changes in PM asymmetry, e.g. ATP released at sites of cell damage, peroxynitrate secreted by microglia to facilitate engulfment of viable neurons, bicarbonate that induces sperm capacitation, and immune antigens. These signals either open calcium channels on the PM or lead to release of ER calcium stores. Increased cytosolic calcium inhibits flippases and activates scramblases, leading to loss of asymmetry. (C) Microvesicles can bud out of regions of altered PM asymmetry. These vesicles can be sensed by their exposed PS via bridging molecules or lipid-binding proteins on other cells. (D) Loss of PM asymmetry can affect the function of transmembrane proteins. The P-glycoprotein responsible for export of cytotoxic drugs has reduced efflux activity upon PM scrambling. (E) Relatedly, proteases of the ADAM family (10 and 17) contain polybasic motifs whose conformational changes upon PS exposure may putatively promote their sheddase function^,.

Figure 3.. Effects of lipid scrambling on PM physical properties.

(A) Asymmetric PMs can exhibit unique structural and mechanical characteristics, including higher lipid packing and lower fluidity in the exoplasmic leaflet. Also, due to the asymmetric phospholipid distribution, such membranes likely have non-zero spontaneous curvatures. In contrast, fully scrambled membranes have identical packing, thickness and fluidity in the two leaflets, and no spontaneous curvature. (B) Depending on the interleaflet abundances of phospholipids, an asymmetric membrane may be subject to intrinsic leaflet stresses arising from suboptimal packing densities of the two leaflets. Symmetrizing the lipid compositions of the leaflets would restore optimal lipid packing and eliminate differential stress. (C) Communication between membrane leaflets can be achieved through interleaflet coupling. Clustering of glycolipids or GPI-anchored proteins in the exoplasmic leaflet, or charged lipids interacting with the cytoskeleton in the cytosolic leaflet, can transmit signals across the membrane and promote lateral reorganization of the opposing leaflet. Changes in PM asymmetry leading to surface exposure of PS may result in local detachment of the cytoskeleton and/or promote pinning from the outside via PS-binding molecules.

Figure 4.. Possible configurations for cholesterol’s interleaflet distribution in the PM.

The same data-driven representation of PL asymmetry as in Fig 1, but with cholesterol (green) enriched either in the (A) top, (B) neither or (C) bottom leaflet, as indicated by the exo/endo percentages (left). In all bilayers the phospholipid compositions of the two leaflets, as well as the total cholesterol mole fraction in the membrane (40%), correspond to those measured in RBC PMs^–, but the abundance of phospholipids between leaflets varies, resulting in the leaflet cholesterol mol% indicated on the right. An imbalance in total lipid abundance between leaflets may give rise to leaflet stresses (indicated by packing of headgroups) that remain stable as long as the phospholipid asymmetry is maintained. Potential contributors to cholesterol interleaflet distribution include: (D) recruitment of cholesterol to the exoplasmic leaflet by strong preference for saturated sphingomyelin, (E) active pumping of cholesterol to the exoplasmic leaflet by transporter proteins, and (F) exclusion of cholesterol from the exoplasmic leaflet via long-chain sphingomyelin species.