Improving our picture of the plasma membrane: Rafts induce ordered domains in a simplified model cytoplasmic leaflet

Engineering Phosphatidylserine Containing Asymmetric Giant Unilamellar Vesicles

The plasma membrane lipid distribution is asymmetric, with several anionic lipid species located in its inner leaflet. Among these, phosphatidylserine (PS) plays a crucial role in various important physiological functions. Over the last decade several methods have been developed that allow for the fabrication of large or giant unilamellar vesicles (GUVs) with an asymmetric lipid composition. Investigating the physicochemical properties of PS in such asymmetric lipid bilayers and studying its interactions with proteins necessitates the reliable fabrication of asymmetric GUVs (aGUVs) with a high degree of asymmetry that exhibit PS in the outer leaflet so that the interaction with peptides and proteins can be studied. Despite progress, achieving aGUVs with well-defined PS asymmetry remains challenging. Recently, a Ca2+-initiated hemifusion method has been introduced, utilizing the fusion of symmetric GUVs (sGUVs) with a supported lipid bilayer (SLB) for the fabrication of aGUVs. We extend this approach to create aGUVs with PS in the outer bilayer leaflet. Comparing the degree of asymmetry between aGUVs obtained via Ca2+ or Mg2+ initiated hemifusion of a phosphatidylcholine (PC) sGUVwith a PC/PS-supported lipid bilayer, we observe for both bivalent cations a significant number of aGUVs with near-complete asymmetry. The degree of asymmetry distribution is narrower for physiological salt conditions than at lower ionic strengths. While Ca2+ clusters PS in the SLB, macroscopic domain formation is absent in the presence of Mg2+. However, the clustering of PS upon the addition of Ca2+ is apparently too slow to have a negative effect on the quality of the obtained aGUVs. We introduce a data filtering method to select aGUVs that are best suited for further investigation.

Lipid-driven interleaflet coupling of plasma membrane order regulates FcεRI signaling in mast cells

Interleaflet coupling-the influence of one leaflet on the properties of the opposing leaflet-is a fundamental plasma membrane organizational principle. This coupling is proposed to participate in maintaining steady-state biophysical properties of the plasma membrane, which in turn regulates some transmembrane signaling processes. A prominent example is antigen (Ag) stimulation of signaling by clustering transmembrane receptors for immunoglobulin E (IgE), FcεRI. This transmembrane signaling depends on the stabilization of ordered regions in the inner leaflet for sorting of intracellular signaling components. The resting inner leaflet has a lipid composition that is generally less ordered than the outer leaflet and that does not spontaneously phase separate in model membranes. We propose that interleaflet coupling can mediate ordering and disordering of the inner leaflet, which is poised in resting cells to reorganize upon stimulation. To test this in live cells, we first established a straightforward approach to evaluate induced changes in membrane order by measuring inner leaflet diffusion of lipid probes by imaging fluorescence correlation spectroscopy, by imaging fluorescence correlation spectroscopy (ImFCS), before and after methyl-α-cyclodexrin (mαCD)-catalyzed exchange of outer leaflet lipids (LEX) with exogenous order- or disorder-promoting phospholipids. We examined the functional impact of LEX by monitoring two Ag-stimulated responses: recruitment of cytoplasmic Syk kinase to the inner leaflet and exocytosis of secretory granules (degranulation). Based on the ImFCS data in resting cells, we observed global increase or decrease of inner leaflet order when outer leaflet is exchanged with order- or disorder-promoting lipids, respectively. We find that the degree of both stimulated Syk recruitment and degranulation correlates positively with LEX-mediated changes of inner leaflet order in resting cells. Overall, our results show that resting-state lipid ordering of the outer leaflet influences the ordering of the inner leaflet, likely via interleaflet coupling. This imposed lipid reorganization modulates transmembrane signaling stimulated by Ag clustering of IgE-FcεRI.

Disruption of liquid/liquid phase separation in asymmetric GUVs prepared by hemifusion

Model asymmetric bilayers are useful for studying the coupling between lateral and transverse lipid organization. Here, we used calcium-induced hemifusion to create asymmetric giant unilamellar vesicles (aGUVs) for exploring the phase behavior of 16:0-PC/16:1-PC/Cholesterol, a simplified model for the mammalian plasma membrane. Symmetric GUVs (sGUVs) were first prepared using a composition that produced coexisting liquid-disordered and liquid-ordered phases visible by confocal fluorescence microscopy. The sGUVs were then hemifused to a supported lipid bilayer (SLB) composed of uniformly mixed 16:1-PC/Cholesterol. The extent of outer leaflet exchange was quantified in aGUVs in two ways: (1) from the reduction in fluorescence intensity of a lipid probe initially in the sGUV ("probe exit"); or (2) from the gain in intensity of a probe initially in the SLB ("probe entry"). These measurements revealed a large variability in the extent of outer leaflet exchange in aGUVs within a given preparation, and two populations with respect to their phase behavior: a subset of vesicles that remained phase separated, and a second subset that appeared uniformly mixed. Moreover, a correlation between phase behavior and extent of asymmetry was observed, with more strongly asymmetric vesicles having a greater probability of being uniformly mixed. We also observed substantial overlap between these populations, an indication that the uncertainty in measured exchange fraction is high. We developed models to determine the position of the phase boundary (i.e., the fraction of outer leaflet exchange above which domain formation is suppressed) and found that the phase boundaries determined separately from probe-entry and probe-exit data are in good agreement. Our models also provide improved estimates of the compositional uncertainty of individual aGUVs. We discuss several potential sources of uncertainty in the determination of lipid exchange from fluorescence measurements.

Polymerization mechanism of the Candida albicans virulence factor candidalysin

Candida albicans is a commensal fungus that can cause epithelial infections and life-threatening invasive candidiasis. The fungus secretes candidalysin (CL), a peptide that causes cell damage and immune activation by permeation of epithelial membranes. The mechanism of CL action involves strong peptide assembly into polymers in solution. The free ends of linear CL polymers can join, forming loops that become pores upon binding to membranes. CL polymers constitute a therapeutic target for candidiasis, but little is known about CL self-assembly in solution. Here, we examine the assembly mechanism of CL in the absence of membranes using complementary biophysical tools, including a new fluorescence polymerization assay, mass photometry, and atomic force microscopy. We observed that CL assembly is slow, as tracked with the fluorescent marker C-laurdan. Single-molecule methods showed that CL polymerization involves a convolution of four processes. Self-assembly begins with the formation of a basic subunit, thought to be a CL octamer that is the polymer seed. Polymerization proceeds via the addition of octamers, and as polymers grow they can curve and form loops. Alternatively, secondary polymerization can occur and cause branching. Interplay between the different rates determines the distribution of CL particle types, indicating a kinetic control mechanism. This work elucidates key physical attributes underlying CL self-assembly which may eventually evoke pharmaceutical development.

The use of hemifusion to create asymmetric giant unilamellar vesicles: Insights on induced order domains

The natural asymmetry of the lipid bilayer in biological membranes is, in part, a testament to the complexity of the structure and function of this barrier limiting and protecting cells (or organelles). These lipid bilayers consist of two lipid leaflets with different lipid compositions, resulting in unique interactions within each leaflet. These interactions, combined with interactions between the two leaflets, determine the overall behavior of the membrane. Model membranes provide the most suitable option for investigating the fundamental interactions of lipids. This report describes a comprehensive method to make asymmetric giant unilamellar vesicles (aGUVs) using the technique of hemifusion. In this method, calcium ions induce the hemifusion of giant unilamellar vesicles (GUVs) with a supported lipid bilayer (SLB), both having different lipid compositions. During hemifusion, a stalk, or a more commonly seen hemifusion diaphragm, connects the outer leaflets of GUVs and the SLB. The lateral diffusion of lipids naturally promotes the lipid exchange between the connected outer leaflets. After calcium chelation to prevent further fusion, a mechanical shear detaches aGUVs from the SLB. A fluorescence quench assay is employed to test the extent of bilayer asymmetry. A fluorescence quenching assay tests bilayer asymmetry and verifies dye and lipid migration to a GUV's outer leaflet.

Exploring membrane asymmetry and its effects on membrane proteins

Plasma membranes utilize free energy to maintain highly asymmetric, non-equilibrium distributions of lipids and proteins between their two leaflets. In this review we discuss recent progress in quantitative research enabled by using compositionally controlled asymmetric model membranes. Both experimental and computational studies have shed light on the nuanced mechanisms that govern the structural and dynamic coupling between compositionally distinct bilayer leaflets. This coupling can increase the membrane bending rigidity and induce order - or lipid domains - across the membrane. Furthermore, emerging evidence indicates that integral membrane proteins not only respond to asymmetric lipid distributions but also exhibit intriguing asymmetric properties themselves. We propose strategies to advance experimental research, aiming for a deeper, quantitative understanding of membrane asymmetry, which carries profound implications for cellular physiology.

Curcumin Modulates the Membrane Raft Integrity via Phase Separation and Induces CD44 Shedding in Tumor Cells

CD44 is a transmembrane cell adhesion molecule that is cleaved by the membrane proteinase, a disintegrin and metalloproteinase 10 (ADAM10), on the cell surface via ectodomain shedding after cholesterol depletion. Lipid raft-mediated CD44 shedding is essential for cancer cell invasion. As cell-cell and cell-matrix adhesions are critical for cancer progression, lipid raft-targeting agents may be effective for cancer therapy. Here, we found that curcumin and its derivatives induced the ADAM10-mediated shedding of CD44 in tumor cells. We also found that curcumin and the derivatives are membrane-active compounds whose effect depends on its planar backbone and the spatial arrangement of methoxy groups substituted on the two aromatic rings using giant unilamellar and plasma membrane vesicles. Curcumin and its derivatives with rigid backbones and hydroxy groups exerted membrane-domain-modulating activity, which may account for their pleiotropic effects via multiple signaling pathways involving membrane receptors. This study provides a basis for the use of membrane-active compounds, such as curcuminoids, to elucidate the roles of lipid rafts in cellular signaling, regulation of membrane-bound ADAM metalloproteinases, and the development of novel membrane lipid-based therapies.

Seeing the Membrane from Both Sides Now: Lipid Asymmetry and Its Strange Consequences

Almost all biomembranes are constructed as lipid bilayers and, in almost all of these, the two opposing monolayers (leaflets) have distinct lipid compositions. This lipid asymmetry arises through the concerted action of a suite of energy-dependent enzymes that maintain living bilayers in a far-from-equilibrium steady-state. Recent discoveries reveal that lipid compositional asymmetry imparts biophysical asymmetries and that this dualistic organization may have major consequences for cellular physiology. Importantly, while transbilayer asymmetry appears to be an essential, near-ubiquitous characteristic of biological membranes, it has been challenging to reproduce in reconstituted or synthetic systems. Although recent methodological developments have overcome some critical challenges, it remains difficult to extrapolate results from available models to biological systems. Concurrently, there are few experimental approaches for targeted, controlled manipulation of lipid asymmetry in living cells. Thus, the biophysical and functional consequences of membrane asymmetry remain almost wholly unexplored. This perspective summarizes the current state of knowledge and highlights emerging themes that are beginning to make inroads into the fundamental question of why life tends toward asymmetry in its bilayers.

The asymmetric plasma membrane-A composite material combining different functionalities?: Balancing Barrier Function and Fluidity for Effective Signaling

One persistent puzzle in the life sciences is the asymmetric lipid composition of the cellular plasma membrane: while the exoplasmic leaflet is enriched in lipids carrying predominantly saturated fatty acids, the cytoplasmic leaflet hosts preferentially lipids with (poly-)unsaturated fatty acids. Given the high energy requirements necessary for cells to maintain this asymmetry, the question naturally arises regarding its inherent benefits. In this paper, we propose asymmetry to represent a potential solution for harmonizing two conflicting requirements for the plasma membrane: first, the need to build a barrier for the uncontrolled influx or efflux of substances; and second, the need to form a fluid and dynamic two-dimensional substrate for signaling processes. We hence view here the plasma membrane as a composite material, where the exoplasmic leaflet is mainly responsible for the functional integrity of the barrier and the cytoplasmic leaflet for fluidity. We reinforce the validity of the proposed mechanism by presenting quantitative data from the literature, along with multiple examples that bolster our model.

Experimentally determined leaflet-leaflet phase diagram of an asymmetric lipid bilayer

We have determined the partial leaflet-leaflet phase diagram of an asymmetric lipid bilayer at ambient temperature using asymmetric giant unilamellar vesicles (aGUVs). Symmetric GUVs with varying amounts of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine) were hemifused to a supported lipid bilayer (SLB) composed of DOPC, resulting in lipid exchange between their outer leaflets. The GUVs and SLB contained a red and green lipid fluorophore, respectively, thus enabling the use of confocal fluorescence imaging to determine both the extent of lipid exchange (quantified for individual vesicles by the loss of red intensity and gain of green intensity) and the presence or absence of phase separation in aGUVs. Consistent with previous reports, we found that hemifusion results in large variation in outer leaflet exchange for individual GUVs, which allowed us to interrogate the phase behavior at multiple points within the asymmetric composition space of the binary mixture. When initially symmetric GUVs showed coexisting gel and fluid domains, aGUVs with less than ~50% outer leaflet exchange were also phase-separated. In contrast, aGUVs with greater than 50% outer leaflet exchange were uniform and fluid. In some cases, we also observed three coexisting bilayer-spanning phases: two registered phases and an anti-registered phase. These results suggest that a relatively large unfavorable midplane interaction between ordered and disordered phases in opposing leaflets (i.e., a midplane surface tension) can overwhelm the driving force for lateral phase separation within one of the leaflets, resulting in an asymmetric bilayer with two uniformly mixed leaflets that is poised to phase-separate upon leaflet scrambling.

An Unexpected Driving Force for Lipid Order Appears in Asymmetric Lipid Bilayers

An ordered phase in one leaflet of an asymmetric bilayer can induce a precisely superimposed induced order domain in the apposed leaflet. Order is induced in such simple lipid compositions as dioleoylphosphatidylcholine/cholesterol (DOPC)/chol) which is expected to be a uniform and disordered lipid mixture. Dye partitioning can be used to label and identify coexisting liquid-disordered (Ld), liquid-ordered (Lo), or gel-ordered (Lβ) molecules in a phase-separated leaflet. In the other leaflet of an asymmetric bilayer, dye partitioning also labels and identifies any induced order domains created by an Lo or gel phase domain in the apposed leaflet as well as the state of disorder of the lipid surrounding the induced ordered region. We explore a molecular level mechanism by which a disorder-prone uniform mixture of DOPC/chol = 0.8/0.2 would spontaneously separate into ordered regions coexisting with disordered regions. A redistribution of cholesterol seems to take place in the regions apposed to the ordered phase. The precision of the superposition of Lo or gel domains with their induced order domains implies a strong energy penalty that would be incurred if order/disorder interfaces were to form at the bilayer midplane. We conclude that the energy penalty for Lo/Ld or gel/Ld contact in the bilayer midplane is sufficient to drive disorderly DOPC/chol into an ordered state that reduces unfavorable order-disorder contacts at the bilayer midplane interface.

RAS GTPases and Interleaflet Coupling in the Plasma Membrane

RAS genes are frequently mutated in cancer. The primary signaling compartment of wild-type and constitutively active oncogenic mutant RAS proteins is the inner leaflet of the plasma membrane (PM). Thus, a better understanding of the unique environment of the PM inner leaflet is important to shed further light on RAS function. Over the past few decades, an integrated approach of superresolution imaging, molecular dynamic simulations, and biophysical assays has yielded new insights into the capacity of RAS proteins to sort lipids with specific headgroups and acyl chains, to assemble signaling nanoclusters on the inner PM. RAS proteins also sense and respond to changes in components of the outer PM leaflet, including glycophosphatidylinositol-anchored proteins, sphingophospholipids, glycosphingolipids, and galectins, as well as cholesterol that translocates between the two leaflets. Such communication between the inner and outer leaflets of the PM, called interleaflet coupling, allows RAS to potentially integrate extracellular mechanical and electrostatic information with intracellular biochemical signaling events, and reciprocally allows mutant RAS-transformed tumor cells to modify tumor microenvironments. Here, we review RAS-lipid interactions and speculate on potential mechanisms that allow communication between the opposing leaflets of the PM.

Nano-scale domains in the plasma membrane are like macroscopic domains in asymmetric bilayers

Unfavorable lipid-lipid pairwise interactions between HiTm and LowTm lipids drive liquid-disordered (Ld) + liquid-ordered (Lo) phase separation. Large size of phase domains is opposed by lipid dipole repulsions, which are more significant compared with the pairwise interactions for naturally abundant LowTm lipids such as palmitoyl oleoyl phosphatidylcholine. During the nano-to-macro domain size transition, no lipid phase transition occurs, and measured properties of Ld + Lo nanodomains are found to be essentially the same as those of macrodomains. Use of macrodomains in mixtures to model cell plasma membranes (PM) is helpful, enabling study by optical microscopy. Use of asymmetric giant unilamellar vesicles to model a PM reveals that ordered phase domains in one leaflet induce ordered domains in an otherwise uniform phase in the apposing leaflet that models a cytoplasmic leaflet. Because macro and nano phase properties are so similar, we conclude that a cell PM that has nano-scale Ld + Lo phase domains in the exoplasmic leaflet is likely to induce nano-scale ordered domains in the cytoplasmic leaflet.

A Guide to Your Desired Lipid-Asymmetric Vesicles

Liposomes are prevalent model systems for studies on biological membranes. Recently, increasing attention has been paid to models also representing the lipid asymmetry of biological membranes. Here, we review in-vitro methods that have been established to prepare free-floating vesicles containing different compositions of the classic two-chain glycero- or sphingolipids in their outer and inner leaflet. In total, 72 reports are listed and assigned to four general strategies that are (A) enzymatic conversion of outer leaflet lipids, (B) re-sorting of lipids between leaflets, (C) assembly from different monolayers and (D) exchange of outer leaflet lipids. To guide the reader through this broad field of available techniques, we attempt to draw a road map that leads to the lipid-asymmetric vesicles that suit a given purpose. Of each method, we discuss advantages and limitations. In addition, various verification strategies of asymmetry as well as the role of cholesterol are briefly discussed. The ability to specifically induce lipid asymmetry in model membranes offers insights into the biological functions of asymmetry and may also benefit the technical applications of liposomes.

Ordered Domain (Raft) Formation in Asymmetric Vesicles and Its Induction upon Loss of Lipid Asymmetry in Artificial and Natural Membranes

Lipid asymmetry, the difference in the lipid composition in the inner and outer lipid monolayers (leaflets) of a membrane, is an important feature of eukaryotic plasma membranes. Investigation of the biophysical consequences of lipid asymmetry has been aided by advances in the ability to prepare artificial asymmetric membranes, especially by use of cyclodextrin-catalyzed lipid exchange. This review summarizes recent studies with artificial asymmetric membranes which have identified conditions in which asymmetry can induce or suppress the ability of membranes to form ordered domains (rafts). A consequence of the latter effect is that, under some conditions, a loss of asymmetry can induce ordered domain formation. An analogous study in plasma membrane vesicles has demonstrated that asymmetry can also suppress domain formation in natural membranes. Thus, it is possible that a loss of asymmetry can induce domain formation in vivo.