Interleaflet coupling mechanisms in bilayers of lipids and cholesterol

Measuring the Transmembrane Registration of Lipid Domains in Droplet Interface Bilayers through Tensiometry

Diverse collections of lipids self-assemble into domains within biological membranes, and these domains are typically organized in both the transverse and lateral directions of the membrane. The ability of the membrane to link these domains across the membrane's interior grants cells control over features on the external cellular surface. Numerous hypothesized factors drive the cross-membrane (or transverse) coupling of lipid domains. In this work we seek to isolate these transverse lipid-lipid influences in a simple model system using droplet interface bilayers (DIBs) to better understand the associated mechanics. DIBs enable symmetric and asymmetric combinations of domain-forming lipid mixtures within a model bilayer, and the evolving energetics of the membrane may be tracked using drop-shape analysis. We find that symmetric distributions of domain-forming lipids produce long-lasting, gradual shifts in the DIB membrane energetics that are not observed in asymmetric distributions of the lipids where the domain-forming lipids are only within one leaflet. The approach selected for this work provides experimental measurement of the mismatch penalty associated with antiregistered lipid domains as well as measurements of the influence of rafts on DIB behaviors with suggestions for their future use as a model platform.

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.

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.

Cholesterol, Eukaryotic Lipid Domains, and an Evolutionary Perspective of Transmembrane Signaling

Transmembrane signaling is essential for complex life forms. Communication across a bilayer lipid barrier is elaborately organized to convey precision and to fine-tune strength. Looking back, the steps that it has taken to enable this seemingly mundane errand are breathtaking, and with our survivorship bias, Darwinian. While this review is to discuss eukaryotic membranes in biological functions for coherence and theoretical footing, we are obliged to follow the evolution of the biological membrane through time. Such a visit is necessary for our hypothesis that constraints posited on cellular functions are mainly via the biomembrane, and relaxation thereof in favor of a coordinating membrane environment is the molecular basis for the development of highly specialized cellular activities, among them transmembrane signaling. We discuss the obligatory paths that have led to eukaryotic membrane formation, its intrinsic ability to signal, and how it set up the platform for later integration of protein-based receptor activation.

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.

Structure and Interdigitation of Chain-Asymmetric Phosphatidylcholines and Milk Sphingomyelin in the Fluid Phase

We addressed the frequent occurrence of mixed-chain lipids in biological membranes and their impact on membrane structure by studying several chain-asymmetric phosphatidylcholines and the highly asymmetric milk sphingomyelin. Specifically, we report trans-membrane structures of the corresponding fluid lamellar phases using small-angle X-ray and neutron scattering, which were jointly analyzed in terms of a membrane composition-specific model, including a headgroup hydration shell. Focusing on terminal methyl groups at the bilayer center, we found a linear relation between hydrocarbon chain length mismatch and the methyl-overlap for phosphatidylcholines, and a non-negligible impact of the glycerol backbone-tilting, letting the sn1-chain penetrate deeper into the opposing leaflet by half a CH2 group. That is, penetration-depth differences due to the ester-linked hydrocarbons at the glycerol backbone, previously reported for gel phase structures, also extend to the more relevant physiological fluid phase, but are significantly reduced. Moreover, milk sphingomyelin was found to follow the same linear relationship suggesting a similar tilt of the sphingosine backbone. Complementarily performed molecular dynamics simulations revealed that there is always a part of the lipid tails bending back, even if there is a high interdigitation with the opposing chains. The extent of this back-bending was similar to that in chain symmetric bilayers. For both cases of adaptation to chain length mismatch, chain-asymmetry has a large impact on hydrocarbon chain ordering, inducing disorder in the longer of the two hydrocarbons.

Investigation of the domain line tension in asymmetric vesicles prepared via hemifusion

The plasma membrane (PM) is asymmetric in lipid composition. The distinct and characteristic lipid compositions of the exoplasmic and cytoplasmic leaflets lead to different lipid-lipid interactions and physical-chemical properties in each leaflet. The exoplasmic leaflet possesses an intrinsic ability to form coexisting ordered and disordered fluid domains, whereas the cytoplasmic leaflet seems to form a single fluid phase. To better understand the interleaflet interactions that influence domains, we compared asymmetric model membranes that capture salient properties of the PM with simpler symmetric membranes. Using asymmetric giant unilamellar vesicles (aGUVs) prepared by hemifusion with a supported lipid bilayer, we investigate the domain line tension that characterizes the behavior of coexisting ordered + disordered domains. The line tension can be related to the contact perimeter of the different phases. Compared to macroscopic phase separation, the appearance of modulated phases was found to be a robust indicator of a decrease in domain line tension. Symmetric GUVs of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC)/1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC)/cholesterol (chol) were formed into aGUVs by replacing the GUV outer leaflet with DOPC/chol = 0.8/0.2 in order to create a cytoplasmic leaflet model. These aGUVs revealed lower line tension for the ordered + disordered domains of the exoplasmic model leaflet.

Pivotal Role of Interdigitation in Interleaflet Interactions: Implications from Molecular Dynamics Simulations

The asymmetric lipid composition in plasma membranes within the inner leaflet is not typically suitable for domain formation. Thus elucidation of the likelihood of the formation or stability of a raft-like domain in the inner leaflet is necessary. Herein we investigated the phase behavior of asymmetric membranes using coarse-grained molecular dynamics simulations. The lipid leaflet comprising dioleoylphosphatidylcholine (DOPC) and cholesterol (Chol) does not typically show well-developed domains in symmetric bilayer membranes; however, it does separate into liquid ordered (L_o) and liquid disordered (L_d) phases when the opposing leaflet containing sphingomyelin (SM), DOPC, and Chol demonstrates domain formation. We determine that interdigitated acyl chains modulated the partitioning of Chol in the opposing leaflet, resulting in phase separation. Similarly, the acyl chain length of SM within the opposing leaflet affected the phase behavior of the leaflet. Our results reveal the crucial role of interdigitation in determining the phase status in asymmetric membranes.

Interleaflet Coupling of Lipid Nanodomains - Insights From in vitro Systems

The plasma membrane is a complex system, consisting of two layers of lipids and proteins compartmentalized into small structures called nanodomains. Despite the asymmetric composition of both leaflets, coupling between the layers is surprisingly strong. This can be evidenced, for example, by recent experimental studies performed on phospholipid giant unilamellar vesicles showing that nanodomains formed in the outer layer are perfectly registered with those in the inner leaflet. Similarly, microscopic phase separation in one leaflet can induce phase separation in the opposing leaflet that would otherwise be homogeneous. In this review, we summarize the current theoretical and experimental knowledge that led to the current view that domains are – irrespective of their size – commonly registered across the bilayer. Mechanisms inducing registration of nanodomains suggested by theory and calculations are discussed. Furthermore, domain coupling is evidenced by experimental studies based on the sparse number of methods that can resolve registered from independent nanodomains. Finally, implications that those findings using model membrane studies might have for cellular membranes are discussed.

Photopolymerized microdomains in both lipid leaflets establish diffusive transport pathways across biomimetic membranes

Controlled transport within a network of aqueous subcompartments provides a foundation for the construction of biologically-inspired materials. These materials are commonly assembled using the droplet interface bilayer (DIB) technique, adhering droplets together into a network of lipid membranes. DIB structures may be functionalized to generate conductive pathways by enhancing the permeability of pre-selected membranes, a strategy inspired by nature. Traditionally these pathways are generated by dissolving pore-forming toxins (PFTs) in the aqueous phase. A downside of this approach when working with larger DIB networks is that transport is enabled in all membranes bordering the droplets containing the PFT, instead of occurring exclusively between selected droplets. To rectify this limitation, photopolymerizable phospholipids (23:2 DiynePC) are incorporated within the aqueous phase of the DIB platform, forming conductive pathways in the lipid membranes post-exposure to UV-C light. Notably these pathways are only formed in the membrane if both adhered droplets contain the photo-responsive lipids. Patterned DIB networks can then be generated by controlling the lipid composition within select droplets which creates conductive routes one droplet thick. We propose that the incorporation of photo-polymerizable phospholipids within the aqueous phase of DIB networks will improve the resolution of the patterned conductive pathways and reduce diffusive loss within the synthetic biological network.

HIV-1 Gag specifically restricts PI(4,5)P2 and cholesterol mobility in living cells creating a nanodomain platform for virus assembly

HIV-1 Gag protein assembles at the plasma membrane of infected cells for viral particle formation. Gag targets lipids, mainly PI(4,5)P2, at the inner leaflet of this membrane. Here, we address the question whether Gag is able to trap specifically PI(4,5)P2 or other lipids during HIV-1 assembly in the host CD4+ T lymphocytes. Lipid dynamics within and away from HIV-1 assembly sites were determined using super-resolution microscopy coupled with scanning fluorescence correlation spectroscopy in living cells. Analysis of HIV-1-infected cells revealed that, upon assembly, HIV-1 is able to specifically trap PI(4,5)P2 and cholesterol, but not phosphatidylethanolamine or sphingomyelin. Furthermore, our data showed that Gag is the main driving force to restrict the mobility of PI(4,5)P2 and cholesterol at the cell plasma membrane. This is the first direct evidence highlighting that HIV-1 creates its own specific lipid environment by selectively recruiting PI(4,5)P2 and cholesterol as a membrane nanoplatform for virus assembly.

Role of Transmembrane Proteins for Phase Separation and Domain Registration in Asymmetric Lipid Bilayers

It is well known that the formation and spatial correlation of lipid domains in the two apposed leaflets of a bilayer are influenced by weak lipid–lipid interactions across the bilayer’s midplane. Transmembrane proteins span through both leaflets and thus offer an alternative domain coupling mechanism. Using a mean-field approximation of a simple bilayer-type lattice model, with two two-dimensional lattices stacked one on top of the other, we explore the role of this “structural” inter-leaflet coupling for the ability of a lipid membrane to phase separate and form spatially correlated domains. We present calculated phase diagrams for various effective lipid–lipid and lipid–protein interaction strengths in membranes that contain a binary lipid mixture in each leaflet plus a small amount of added transmembrane proteins. The influence of the transmembrane nature of the proteins is assessed by a comparison with “peripheral” proteins, which result from the separation of one single integral protein into two independent units that are no longer structurally connected across the bilayer. We demonstrate that the ability of membrane-spanning proteins to facilitate domain formation requires sufficiently strong lipid–protein interactions. Weak lipid–protein interactions generally tend to inhibit phase separation in a similar manner for transmembrane as for peripheral proteins.

Intrinsic Curvature-Mediated Transbilayer Coupling in Asymmetric Lipid Vesicles

We measured the effect of intrinsic lipid curvature, J0, on structural properties of asymmetric vesicles made of palmitoyl-oleoyl-phosphatidylethanolamine (POPE; J0<0) and palmitoyl-oleoyl-phosphatidylcholine (POPC; J0∼0). Electron microscopy and dynamic light scattering were used to determine vesicle size and morphology, and x-ray and neutron scattering, combined with calorimetric experiments and solution NMR, yielded insights into leaflet-specific lipid packing and melting processes. Below the lipid melting temperature we observed strong interleaflet coupling in asymmetric vesicles with POPE inner bilayer leaflets and outer leaflets enriched in POPC. This lipid arrangement manifested itself by lipids melting cooperatively in both leaflets, and a rearrangement of lipid packing in both monolayers. On the other hand, no coupling was observed in vesicles with POPC inner bilayer leaflets and outer leaflets enriched in POPE. In this case, the leaflets melted independently and did not affect each other’s acyl chain packing. Furthermore, we found no evidence for transbilayer structural coupling above the melting temperature of either sample preparation. Our results are consistent with the energetically preferred location of POPE residing in the inner leaflet, where it also resides in natural membranes, most likely causing the coupling of both leaflets. The loss of this coupling in the fluid bilayers is most likely the result of entropic contributions.

How to Determine Lipid Interactions in Membranes from Experiment Through the Ising Model

The determination and the meaning of interactions in lipid bilayers are discussed and interpreted through the Ising model. Originally developed to understand phase transitions in ferromagnetic systems, the Ising model applies equally well to lipid bilayers. In the case of a membrane, the essence of the Ising model is that each lipid is represented by a site on a lattice and that the interaction of each site with its nearest neighbors is represented by an energy parameter ω. To calculate the thermodynamic properties of the system, such as the enthalpy, the Gibbs energy, and the heat capacity, the partition function is derived. The calculation of the configurational entropy factor in the partition function, however, requires approximations or the use of Monte Carlo (MC) simulations. Those approximations are described. Ultimately, MC simulations are used in combination with experiment to determine the interaction parameters ω in lipid bilayers. Several experimental approaches are described, which can be used to obtain interaction parameters. They include nearest-neighbor recognition, differential scanning calorimetry, and Förster resonance energy transfer. Those approaches are most powerful when used in combination of MC simulations of Ising models. Lipid membranes of different compositions are discussed, which have been studied with these approaches. They include mixtures of cholesterol, saturated (ordered) phospholipids, and unsaturated (disordered) phospholipids. The interactions between those lipid species are examined as a function of molecular properties such as the degree of unsaturation and the acyl chain length. The general rule that emerges is that interactions between different lipids are usually unfavorable. The exception is that cholesterol interacts favorably with saturated (ordered) phospholipids. However, the interaction of cholesterol with unsaturated phospholipids becomes extremely unfavorable as the degree of unsaturation increases.

Effects of Passive Phospholipid Flip-Flop and Asymmetric External Fields on Bilayer Phase Equilibria

Compositional asymmetry between the leaflets of bilayer membranes modifies their phase behavior and is thought to influence other important features such as mechanical properties and protein activity. We address here how phase behavior is affected by passive phospholipid flip-flop, such that the compositional asymmetry is not fixed. We predict transitions from "pre-flip-flop" behavior to a restricted set of phase equilibria that can persist in the presence of passive flip-flop. Surprisingly, such states are not necessarily symmetric. We further account for external symmetry breaking, such as a preferential substrate interaction, and show how this can stabilize strongly asymmetric equilibrium states. Our theory explains several experimental observations of flip-flop-mediated changes in phase behavior and shows how domain formation and compositional asymmetry can be controlled in concert, by manipulating passive flip-flop rates and applying external fields.

Cholesterol Flip-Flop Impacts Domain Registration in Plasma Membrane Models

The plasma membrane is a highly complex multicomponent system that is central to the functioning of cells. Cholesterol, a key lipid component of the plasma membrane, promotes the formation of nanodomains. These nanodomains are often correlated across the two membrane leaflets, but the underlying physical mechanism remains unclear. Using coarse-grained molecular dynamics simulations, we investigate the influence of cholesterol flip-flop on membrane properties, in particular, the interleaflet coupling of cholesterol-enriched domains. We show that the cholesterol density correlation between the leaflets of an average mammalian plasma membrane is significantly reduced by suppressing interleaflet cholesterol population. Our results suggest an amplifying role of cholesterol in signal transduction across the leaflets.

Cholesterol induced asymmetry in DOPC bilayers probed by AFM force spectroscopy

Cholesterol induced mechanical effects on artificial lipid bilayers are well known and have been thoroughly investigated by AFM force spectroscopy. However, dynamics of cholesterol impingement into bilayers at various cholesterol concentrations and their effects have not been clearly understood. In this paper we present, the effect of cholesterol as a function of its concentration in a simple single component dioleoylphosphatidylcholine (DOPC) bilayer. The nature of measured breakthrough forces on a bilayer with the addition of cholesterol, suggested that it is not just responsible to increase the mechanical stability but also introduces irregularities across the leaflets of the bilayer. This cholesterol induced asymmetry across the (in the inner and outer leaflets) bilayer is related to the phenomena of interleaflet coupling and is a function of cholesterol concentration probed by AFM can provide an unprecedented direction on mechanical properties of lipid membrane as it can be directly correlated to biophysical properties of a cell membrane.

Dissipative dynamics of fluid lipid membranes enriched in cholesterol

Cholesterol is an intriguing component of fluid lipid membranes: It makes them stiffer but also more fluid. Despite the enormous biological significance of this complex dynamical behavior, which blends aspects of membrane elasticity with viscous friction, their mechanical bases remain however poorly understood. Here, we show that the incorporation of physiologically relevant contents of cholesterol in model fluid membranes produces a fourfold increase in the membrane bending modulus. However, the increase in the compression rigidity that we measure is only twofold; this indicates that cholesterol increases coupling between the two membrane leaflets. In addition, we show that although cholesterol makes each membrane leaflet more fluid, it increases the friction between the membrane leaflets. This dissipative dynamics causes opposite but advantageous effects over different membrane motions: It allows the membrane to rearrange quickly in the lateral dimension, and to simultaneously dissipate out-of-plane stresses through friction between the two membrane leaflets. Moreover, our results provide a clear correlation between coupling and friction of membrane leaflets. Furthermore, we show that these rigid membranes are optimal to resist slow deformations with minimum energy dissipation; their optimized stability might be exploited to design soft technological microsystems with an encoded mechanics, vesicles or capsules for instance, useful beyond classical applications as model biophysical systems.

Lipid Domain Co-localization Induced by Membrane Undulations

Multicomponent lipid bilayer membranes display rich phase transition and associated compositional lipid domain formation behavior. When both leaflets of the bilayer contain domains, they are often found co-localized across the leaflets, implying the presence of a thermodynamic interleaflet coupling. In this work, it is demonstrated that fluctuation-induced interactions between domains embedded within opposing membrane leaflets provide a robust means to co-localize the domains. In particular, it is shown via a combination of a mode-counting argument, a perturbative calculation, and a non-perturbative treatment of a special case, that spatial variations in membrane bending rigidity associated with lipid domains embedded within the background phase always lead to an attractive interleaflet coupling with a magnitude of ∼0.01k_BT/nm² in simple model membrane systems. Finally, it is demonstrated that the fluctuation-induced coupling is very robust against membrane tension and substrate interactions.

Interleaflet Coupling, Pinning, and Leaflet Asymmetry-Major Players in Plasma Membrane Nanodomain Formation

The plasma membrane has a highly asymmetric distribution of lipids and contains dynamic nanodomains many of which are liquid entities surrounded by a second, slightly different, liquid environment. Contributing to the dynamics is a continuous repartitioning of components between the two types of liquids and transient links between lipids and proteins, both to extracellular matrix and cytoplasmic components, that temporarily pin membrane constituents. This make plasma membrane nanodomains exceptionally challenging to study and much of what is known about membrane domains has been deduced from studies on model membranes at equilibrium. However, living cells are by definition not at equilibrium and lipids are distributed asymmetrically with inositol phospholipids, phosphatidylethanolamines and phosphatidylserines confined mostly to the inner leaflet and glyco- and sphingolipids to the outer leaflet. Moreover, each phospholipid group encompasses a wealth of species with different acyl chain combinations whose lateral distribution is heterogeneous. It is becoming increasingly clear that asymmetry and pinning play important roles in plasma membrane nanodomain formation and coupling between the two lipid monolayers. How asymmetry, pinning, and interdigitation contribute to the plasma membrane organization is only beginning to be unraveled and here we discuss their roles and interdependence.

The aliphatic chain of cholesterol modulates bilayer interleaflet coupling and domain registration

Cholesterol is a necessary component and critical regulator of liquid-ordered membrane domains. However, the structural features that determine its unique physicochemical behaviors are not fully understood. In particular, very little is known about the specific functions of the terminal aliphatic chain of cholesterol, as previous studies have focused mainly on the rigid sterol ring structure and its hydroxyl head. In the current work, we used coarse-grained molecular dynamics simulations to investigate the effect of cholesterol aliphatic chain length on the dynamics and structure of coexisting lipid domains. We found that the aliphatic chain has no appreciable effect on phase separation per se, but it significantly affects the rate of cholesterol flip-flop and intermonolayer interaction. These effects are accompanied by changes in domain dynamics, lateral pressure, and interleaflet coupling. Our study provides useful insight into how biological sterols modulate communication between the outer and inner surfaces of the plasma membrane and, therefore, cellular signaling.

Transbilayer Colocalization of Lipid Domains Explained via Measurement of Strong Coupling Parameters

When micron-scale compositional heterogeneity develops in membranes, the distribution of lipids on one face of the membrane strongly affects the distribution on the other. Specifically, when lipid membranes phase separate into coexisting liquid phases, domains in each monolayer leaflet of the membrane are colocalized with domains in the opposite leaflet. Colocalized domains have never been observed to spontaneously move out of registry. This result indicates that the lipid compositions in one leaflet are strongly coupled to compositions in the opposing leaflet. Predictions of the interleaflet coupling parameter, Λ, vary by a factor of 50. We measure the value of Λ by applying high shear forces to supported lipid bilayers. This causes the upper leaflet to slide over the lower leaflet, moving domains out of registry. We find that the threshold shear stress required to deregister domains in the upper and lower leaflets increases with the inverse length of domains. We derive a simple, closed-form expression relating the threshold shear to Λ, and find Λ = 0.016 ± 0.004 kBT/nm2.

Continuity of Monolayer-Bilayer Junctions for Localization of Lipid Raft Microdomains in Model Membranes

We show that the selective localization of cholesterol-rich domains and associated ganglioside receptors prefer to occur in the monolayer across continuous monolayer-bilayer junctions (MBJs) in supported lipid membranes. For the MBJs, glass substrates were patterned with poly(dimethylsiloxane) (PDMS) oligomers by thermally-assisted contact printing, leaving behind 3 nm-thick PDMS patterns. The hydrophobicity of the transferred PDMS patterns was precisely tuned by the stamping temperature. Lipid monolayers were formed on the PDMS patterned surface while lipid bilayers were on the bare glass surface. Due to the continuity of the lipid membranes over the MBJs, essentially free diffusion of lipids was allowed between the monolayer on the PDMS surface and the upper leaflet of the bilayer on the glass substrate. The preferential localization of sphingomyelin, ganglioside GM1 and cholesterol in the monolayer region enabled to develop raft microdomains through coarsening of nanorafts. Our methodology provides a simple and effective scheme of non-disruptive manipulation of the chemical landscape associated with lipid phase separations, which leads to more sophisticated applications in biosensors and as cell culture substrates.

Relaxation dynamics of two-component fluid bilayer membranes

We theoretically investigate the relaxation dynamics of a nearly flat binary lipid bilayer membrane by taking into account the membrane tension, hydrodynamics of the surrounding fluid, inter-monolayer friction and mutual diffusion. Mutual diffusion is the collective irreversible process that leads to homogenization of the density difference between the two lipid species. We find that two relaxation modes associated with the mutual diffusion appear in addition to the three previously discussed relaxation modes reflecting the bending and compression of the membrane. Because of the symmetry, only one of the two diffusive modes is coupled to the bending mode. The two diffusive modes are much slower than the bending and compression modes in the entire realistic wave number range. This means that the long time relaxation behavior is dominated by the mutual diffusion in binary membranes. The two diffusive modes become even slower in the vicinity of the unstable region towards phase separation, while the other modes are almost unchanged. In short time scales, on the other hand, the lipid composition heterogeneity induces in-plane compression and bending of the bilayer.

Effects of Transmembrane α-Helix Length and Concentration on Phase Behavior in Four-Component Lipid Mixtures: A Molecular Dynamics Study

We used coarse-grained molecular dynamics simulations to examine the effects of transmembrane α-helical WALP peptides on the behavior of four-component lipid mixtures. These mixtures contain a high-melting temperature (high-Tm) lipid, a nanodomain-inducing low-Tm lipid, a macrodomain-inducing low-Tm lipid and cholesterol to model the outer leaflet of cell plasma membranes. In a series of simulations, we incrementally replace the nanodomain-inducing low-Tm lipid by the macrodomain-inducing low-Tm lipid and measure how lipid and phase properties are altered by the addition of WALPs of different length. Regardless of the ratio of the two low-Tm lipids, shorter WALPs increase domain size and all WALPs increase domain alignment between the two leaflets. These effects are smallest for the longest WALP tested, and increase with increasing WALP concentration. Thus, our simulations explain the experimental observation that WALPs induce macroscopic domains in otherwise nanodomain-forming lipid-only mixtures (unpublished). Since the cell plasma membrane contains a large fraction of transmembrane proteins, these findings link the behavior of lipid-only model membranes in vitro to phase behavior in vivo.

Chloroform alters interleaflet coupling in lipid bilayers: an entropic mechanism

The interaction of the two leaflets of the plasmatic cell membrane is conjectured to play an important role in many cell processes. Experimental and computational studies have investigated the mechanisms that modulate the interaction between the two membrane leaflets. Here, by means of coarse-grained molecular dynamics simulations, we show that the addition of a small and polar compound such as chloroform alters interleaflet coupling by promoting domain registration. This is interpreted in terms of an entropic gain that would favour frequent chloroform commuting between the two leaflets. The implication of this effect is discussed in relation to the general anaesthetic action.

Ordered raft domains induced by outer leaflet sphingomyelin in cholesterol-rich asymmetric vesicles

Sphingolipid- and cholesterol-rich liquid-ordered (Lo) lipid domains (rafts) are thought to be important organizing elements in eukaryotic plasma membranes. How they form in the sphingolipid-poor cytosolic (inner) membrane leaflet is unclear. Here, we characterize how outer-leaflet Lo domains induce inner-leaflet-ordered domains, i.e., interleaflet coupling. Asymmetric vesicles studied contained physiologically relevant cholesterol levels (∼ 37 mol %), a mixture of SM (sphingomyelin) and DOPC (dioleoylphosphatidylcholine) in their outer leaflets, and DOPC in their inner leaflets. Lo domains were observed in both leaflets, and were in register, indicative of coupling between SM-rich outer-leaflet-ordered domains and inner-leaflet-ordered domains. For asymmetric vesicles with outer-leaflet egg SM or milk SM, a fluorescent lipid with unsaturated acyl chains (NBD-DOPE) was depleted in both the outer- and inner-leaflet-ordered domains. This suggests the inner-leaflet-ordered domains were depleted in unsaturated lipid (i.e., DOPC) and thus rich in cholesterol. For asymmetric vesicles containing egg SM, outer-leaflet Lo domains were also depleted in a saturated fluorescent lipid (NBD-DPPE), while inner-leaflet Lo domains were not. This indicates that inner- and outer-leaflet Lo domains can have significantly different physical properties. In contrast, in asymmetric vesicles containing outer-leaflet milk SM, which has long acyl chains capable of interdigitating into the inner leaflet, both outer- and inner-leaflet Lo domains were depleted, to a similar extent, in NBD-DPPE. This is indicative of interdigitation-enhanced coupling resulting in inner- and outer-leaflet Lo domains with similar physical properties.

Interleaflet Coupling, Pinning, and Leaflet Asymmetry-Major Players in Plasma Membrane Nanodomain Formation

The plasma membrane has a highly asymmetric distribution of lipids and contains dynamic nanodomains many of which are liquid entities surrounded by a second, slightly different, liquid environment. Contributing to the dynamics is a continuous repartitioning of components between the two types of liquids and transient links between lipids and proteins, both to extracellular matrix and cytoplasmic components, that temporarily pin membrane constituents. This make plasma membrane nanodomains exceptionally challenging to study and much of what is known about membrane domains has been deduced from studies on model membranes at equilibrium. However, living cells are by definition not at equilibrium and lipids are distributed asymmetrically with inositol phospholipids, phosphatidylethanolamines and phosphatidylserines confined mostly to the inner leaflet and glyco- and sphingolipids to the outer leaflet. Moreover, each phospholipid group encompasses a wealth of species with different acyl chain combinations whose lateral distribution is heterogeneous. It is becoming increasingly clear that asymmetry and pinning play important roles in plasma membrane nanodomain formation and coupling between the two lipid monolayers. How asymmetry, pinning, and interdigitation contribute to the plasma membrane organization is only beginning to be unraveled and here we discuss their roles and interdependence.

Modeling Lipid-Lipid Correlations across a Bilayer Membrane Using the Quasi-chemical Approximation

Mixed fluid-like lipid membranes exhibit interactions not only among the lipids within a given leaflet but also across the bilayer. The ensuing collective interleaflet coupling of entire membrane domains has been modeled previously using various mean-field approaches. Yet, also on the level of individual lipids have correlations across the bilayer been observed experimentally for binary mixtures of charged/uncharged lipids with mismatching combinations of short and long acyl chain lengths. The present study proposes a lattice gas model to quantify these correlations. To this end, we represent a macroscopically homogeneous lipid bilayer by two coupled two-dimensional lattice gases that we study using the quasi-chemical approximation. We demonstrate that the rationalization of previous experimental results is only possible if besides two-body lipid-lipid interactions within and across the bilayer our model also accounts for an additional multibody interaction mechanism, namely the local hydrophobic height mismatch created by pairing short and long chain lipids together. The robustness of the quasi-chemical approximation is verified by comparison with Monte Carlo simulations.

Elastic Membrane Deformations Govern Interleaflet Coupling of Lipid-Ordered Domains

The mechanism responsible for domain registration in two membrane leaflets has thus far remained enigmatic. Using continuum elasticity theory, we show that minimum line tension is achieved along the rim between thicker (ordered) and thinner (disordered) domains by shifting the rims in opposing leaflets by a few nanometers relative to each other. Increasing surface tension yields an increase in line tension, resulting in larger domains. Because domain registration is driven by lipid deformation energy, it does not require special lipid components or interactions at the membrane midplane.

Multiscale modeling of four-component lipid mixtures: domain composition, size, alignment, and properties of the phase interface

Simplified lipid mixtures are often used to model the complex behavior of the cell plasma membrane. Indeed, as few as four components-a high-melting lipid, a nandomain-inducing low-melting lipid, a macrodomain-inducing low-melting lipid, and cholesterol (chol)-can give rise to a wide range of domain sizes and patterns that are highly sensitive to lipid compositions. Although these systems are studied extensively with experiments, the molecular-level details governing their phase behavior are not yet known. We address this issue by using molecular dynamics simulations to analyze how phase separation evolves in a four-component system as it transitions from small domains to large domains. To do so, we fix concentrations of the high-melting lipid 16:0,16:0-phosphatidylcholine (DPPC) and chol, and incrementally replace the nanodomain-inducing low-melting lipid 16:0,18:2-PC (PUPC) by the macrodomain-inducing low-melting lipid 18:2,18:2-PC (DUPC). Coarse-grained simulations of this four-component system reveal that lipid demixing increases as the amount of DUPC increases. Additionally, we find that domain size and interleaflet alignment change sharply over a narrow range of replacement of PUPC by DUPC, indicating that intraleaflet and interleaflet behaviors are coupled. Corresponding united atom simulations show that only lipids within ∼2 nm of the phase interface are significantly perturbed regardless of domain composition or size. Thus, whereas the fraction of interface-perturbed lipids is negligible for large domains, it is significant for smaller ones. Together, these results reveal characteristic traits of bilayer thermodynamic behavior in four-component mixtures, and provide a baseline for investigation of the effects of proteins and other lipids on membrane phase properties.

Kinetics of domain registration in multicomponent lipid bilayer membranes

The kinetics of registration of lipid domains in the apposing leaflets of symmetric bilayer membranes is investigated via systematic dissipative particle dynamics simulations. The decay of the distance between the centres of mass of the domains in the apposing leaflets is almost linear during early stages, and then becomes exponential during late times. The time scales of both linear and exponential decays are found to increase with decreasing strength of interleaflet coupling. The ratio between the time scales of the exponential and linear regimes decreases with increasing domain size, implying that the decay of the distance between the domains' centres of mass is essentially linear for large domains. These numerical results are largely in agreement with the recent theoretical predictions of Han and Haataja [Soft Matter, 2013, 9, 2120-2124]. We also found that the domains become elongated during the registration process.

Phase transitions in supported lipid bilayers studied by AFM

We review the capabilities of Atomic Force Microscopy (AFM) in the study of phase transitions in Supported Lipid Bilayers (SLBs). AFM represents a powerful technique to cover the resolution range not available to fluorescence imaging techniques and where spectroscopic data suggest what the relevant lateral scale for domain formation might be. Phase transitions of lipid bilayers involve the formation of domains characterized by different heights with respect to the surrounding phase and are therefore easily identified by AFM in liquid solution once the bilayer is confined to a flat surface. Even if not endowed with high time resolution, AFM allows light to be shed on some aspects related to lipid phase transitions in the case of both a single lipid component and lipid mixtures containing sterols also. We discuss here the obtained results in light of the peculiarities of supported lipid bilayer model systems.

Physical aspects of heterogeneities in multi-component lipid membranes

Ever since the raft model for biomembranes has been proposed, the traditional view of biomembranes based on the fluid-mosaic model has been altered. In the raft model, dynamical heterogeneities in multi-component lipid bilayers play an essential role. Focusing on the lateral phase separation of biomembranes and vesicles, we review some of the most relevant research conducted over the last decade. We mainly refer to those experimental works that are based on physical chemistry approach, and to theoretical explanations given in terms of soft matter physics. In the first part, we describe the phase behavior and the conformation of multi-component lipid bilayers. After formulating the hydrodynamics of fluid membranes in the presence of the surrounding solvent, we discuss the domain growth-law and decay rate of concentration fluctuations. Finally, we review several attempts to describe membrane rafts as two-dimensional microemulsion.

Model of a raft in both leaves of an asymmetric lipid bilayer

We present a theory of inhomogeneities in the plasma membrane, or rafts, that can exist in both leaves of the plasma membrane. We note that although neither of the major phospholipid components of the outer leaf, sphingomyelin (SM) nor phosphatidylcholine (PC), evinces a tendency to form phases characterized by nonzero curvature, one of the major components of the inner leaf, phosphatidylethanolamine (PE), displays a strong tendency to do so whereas the other, phosphatidylserine (PS), does not. Therefore, we posit that the concentration difference of PS and PE couples to height fluctuations of the plasma membrane bilayer. This brings about a microemulsion in the inner leaf. Coupling of the concentration difference between PS and PE in the inner leaf and SM and PC in the outer leaf propagates the microemulsion to that leaf as well. The characteristic size of the inhomogeneities is equal to the square-root of the ratio of the bending modulus of the bilayer to its surface tension, a size which is ~100 nm for the plasma membrane. If the coupling between leaves were to be provided by the interchange of cholesterol, then our model raft would consist of SM and cholesterol in the outer leaf and PS and cholesterol in the inner leaf floating in a sea of PC and PE in both leaves.

Cholesterol translocation in a phospholipid membrane

Cholesterol (CHOL) molecules play a key role in modulating the rigidity of cell membranes and controlling intracellular transport and signal transduction. Using an all-atom molecular dynamics approach, we study the process of CHOL interleaflet transport (flip-flop) in a dipalmitoylphosphatidycholine (DPPC)-CHOL bilayer over a time period of 15 μs. We investigate the effect of the flip-flop process on mechanical stress across the bilayer and the role of CHOL in inducing molecular order in bilayer leaflets. The simulations are carried out at physiologically relevant CHOL concentration (30%), temperature (323 K), and pressure (1 bar). CHOL flip-flop events are observed with a rate constant of 3 × 10⁴s⁻¹. Once a flip-flop event is triggered, a CHOL molecule takes an average of 73 nanoseconds to migrate from one bilayer leaflet to the other.

Acyl chain length and saturation modulate interleaflet coupling in asymmetric bilayers: effects on dynamics and structural order

A long-standing question about membrane structure and function is the degree to which the physical properties of the inner and outer leaflets of a bilayer are coupled to one another. Using our recently developed methods to prepare asymmetric vesicles, coupling was investigated for vesicles containing phosphatidylcholine (PC) in the inner leaflet and sphingomyelin (SM) in the outer leaflet. The coupling of both lateral diffusion and membrane order was monitored as a function of PC and SM acyl chain structure. The presence in the outer leaflet of brain SM, which decreased outer-leaflet lateral diffusion, had little effect upon lateral diffusion in inner leaflets composed of dioleoyl PC (i.e., diffusion was only weakly coupled in the two leaflets) but did greatly reduce lateral diffusion in inner leaflets composed of PC with one saturated and one oleoyl acyl chain (i.e., diffusion was strongly coupled in these cases). In addition, reduced outer-leaflet diffusion upon introduction of outer-leaflet milk SM or a synthetic C24:0 SM, both of which have long interdigitating acyl chains, also greatly reduce diffusion of inner leaflets composed of dioleoyl PC, indicative of strong coupling. Strikingly, several assays showed that the ordering of the outer leaflet induced by the presence of SM was not reflected in increased lipid order in the inner leaflet, i.e., there was no detectable coupling between inner and outer leaflet membrane order. We propose a model for how lateral diffusion can be coupled in opposite leaflets and discuss how this might impact membrane function.

Long and short lipid molecules experience the same interleaflet drag in lipid bilayers

Membrane interleaflet viscosity ηe affects tether formation, phase separation into domains, cell shape changes, and budding. Contrary to the expected contribution to interleaflet coupling from interdigitation, the slide of lipid patches in opposing monolayers conferred the same value ηe≈3×10(9)  J s m-4 for the friction experienced by the ends of both short and long chain fluorescent lipid analogues. Consistent with the weak dependence of the translational diffusion coefficient on lipid length, the in-layer viscosity was, albeit length dependent, much smaller than ηe.

Dynamic domains in polymersomes: mixtures of polyanionic and neutral diblocks respond more rapidly to changes in calcium than to pH

Chemical triggering of membrane domain dynamics is of broad relevance to cell signaling through lipid bilayers and might also be exploited in application of phase-separated vesicles. Here we describe the morphodynamics and remixing kinetics of spotted polymersomes made with mixtures of polyanionic and neutral amphiphiles plus calcium. Addition of the calcium chelator EDTA to vesicle dispersions produced a decrease in domain size within minutes, whereas increasing the pH with NaOH led to the viscous fingering of domains and decreased domain size over hours. Although the latter suggests that the charge of the polyanion contributes to domain formation, the remixing of more negative chains at high pH is surprising. Domain roughening at high pH is also accelerated by EDTA, which highlights the dominance of cross-bridging. Importantly, even though vesicles were perturbed only externally, the inner and outer leaflets remain coupled throughout, consistent with molecular dynamics simulations and suggestive of an order-disorder transition that underlies the remixing kinetics.