Interleaflet coupling and domain registry in phase-separated lipid bilayers

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.

Antivesiculation and Complete Unbinding of Tail-Tethered Lipids

We report the effect of tail-tethering on vesiculation and complete unbinding of bilayered membranes. Amphiphilic molecules of a bolalipid, resembling the tail-tethered molecular structure of archaeal lipids, with two identical zwitterionic phosphatidylcholine headgroups self-assemble into a large flat lamellar membrane, in contrast to the multilamellar vesicles (MLVs) observed in its counterpart, monopolar nontethered zwitterionic lipids. The antivesiculation is confirmed by small-angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (cyro-TEM). With the net charge of zero and higher bending rigidity of the membrane (confirmed by neutron spin echo (NSE) spectroscopy), the current membrane theory would predict that membranes should stack with each other (aka "bind") due to dominant van der Waals attraction, while the outcome of the nonstacking ("unbinding") membrane suggests that the theory needs to include entropic contribution for the nonvesicular structures. This report pioneers an understanding of how the tail-tethering of amphiphiles affects the structure, enabling better control over the final nanoscale morphology.

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.

Systematic measurements of interleaflet friction in supported bilayers

When lipid membranes curve or are subjected to strong shear forces, the two apposed leaflets of the bilayer slide past each other. The drag that one leaflet creates on the other is quantified by the coefficient of interleaflet friction, b. Existing measurements of this coefficient range over several orders of magnitude, so we used a recently developed microfluidic technique to measure it systematically in supported lipid membranes. Fluid shear stress was used to force the top leaflet of a supported membrane to slide over the stationary lower leaflet. Here, we show that this technique yields a reproducible measurement of the friction coefficient and is sensitive enough to detect differences in friction between membranes made from saturated and unsaturated lipids. Adding cholesterol to saturated and unsaturated membranes increased interleaflet friction significantly. We also discovered that fluid shear stress can reversibly induce gel phase in supported lipid bilayers that are close to the gel-transition temperature.

Enthalpic and entropic contributions to interleaflet coupling drive domain registration and antiregistration in biological membrane

Biological membrane is a complex self-assembly of lipids, sterols, and proteins organized as a fluid bilayer of two closely stacked lipid leaflets. Differential molecular interactions among its diverse constituents give rise to heterogeneities in the membrane lateral organization. Under certain conditions, heterogeneities in the two leaflets can be spatially synchronized and exist as registered domains across the bilayer. Several contrasting theories behind mechanisms that induce registration of nanoscale domains have been suggested. Following a recent study showing the effect of position of lipid tail unsaturation on domain registration behavior, we decided to develop an analytical theory to elucidate the driving forces that create and maintain domain registry across leaflets. Towards this, we formulated a Hamiltonian for a stacked lattice system where site variables capture the lipid molecular properties such as the position of unsaturation and various other interactions that could drive phase separation and interleaflet coupling. We solve the Hamiltonian using Monte Carlo simulations and create a complete phase diagram that reports the presence or absence of registered domains as a function of various Hamiltonian parameters. We find that the interleaflet coupling should be described as a competing enthalpic contribution due to interaction of lipid tail termini, primarily due to saturated-saturated interactions, and an interleaflet entropic contribution from overlap of unsaturated tail termini. A higher position of unsaturation is seen to provide weaker interleaflet coupling. Thermodynamically stable nanodomains could also be observed for certain points in the parameter space in our bilayer model, which were further verified by carrying out extended Monte Carlo simulations. These persistent noncoalescing registered nanodomains close to the lower end of the accepted nanodomain size range also point towards a possible "nanoscale" emulsion description of lateral heterogeneities in biological membrane leaflets.

A Theoretical Basis for Nanodomains

We review the current theories of nanodomain, or "raft," formation. We emphasize that the idea that they are co-exisiting Lo and Ld phases is fraught with difficulties, as is the closely related idea that they are due to critical fluctuations. We then review an alternate theory that the plasma membrane is a two-dimensional microemulsion, and that the mechanism that drives to zero the line tension between Lo and Ld phases is the coupling of height and composition fluctuations. The theory yields rafts of SM and cholesterol in the outer leaf and POPS and POPC in the inner leaf. The "sea" between rafts consists of POPC in the outer leaf and POPE and cholesterol in the inner leaf. The characteristic size of the domain structures is tens of nanometers.

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.

Model Plasma Membrane Exhibits a Microemulsion in Both Leaves Providing a Foundation for "Rafts"

We consider a model lipid plasma membrane, one that describes the outer leaf as consisting of sphingomyelin, phosphatidylcholine, and cholesterol and the inner leaf of phosphatidylethanolamine, phosphatidylserine, phosphatidylcholine, and cholesterol. Their relative compositions are taken from experiment; the cholesterol freely interchanges between leaves. Fluctuations in local composition are coupled to fluctuations in the local membrane curvature, as in the Leibler-Andelman mechanism. Structure factors of components in both leaves display a peak at nonzero wavevector. This indicates that the disordered fluid membrane is characterized by structure of the corresponding wavelength. The scale is given by membrane properties: its bending modulus and its surface tension, which arises from the membrane's connections to the cytoskeleton. From measurements on the plasma membrane, this scale is on the order of 100 nm. We find that the membrane can be divided into two different kinds of domains that differ not only in their composition but also in their curvature. The first domain in the outer, exoplasmic leaf is rich in cholesterol and sphingomyelin, whereas the inner, cytoplasmic leaf is rich in phosphatidylserine and phosphatidylcholine. The second kind of domain is rich in phosphatidylcholine in the outer leaf and in cholesterol and phosphatidylethanolamine in the inner leaf. The theory provides a tenable basis for the origin of structure in the plasma membrane and an illuminating picture of the organization of lipids therein.

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.

Cholesterol Flip-Flop in Heterogeneous Membranes

Cholesterol is the most abundant molecule in the plasma membrane of mammals. Its distribution across the two membrane leaflets is critical for understanding how cells work. Cholesterol trans-bilayer motion (flip-flop) is a key process influencing its distribution in membranes. Despite extensive investigations, the rate of cholesterol flip-flop and its dependence on the lateral heterogeneity of membranes remain uncertain. In this work, we used atomistic molecular dynamics simulations to sample spontaneous cholesterol flip-flop events in a DPPC:DOPC:cholesterol mixture with heterogeneous lateral distribution of lipids. In addition to an overall flip-flop rate at the time scale of sub-milliseconds, we identified a significant impact of local environment on flip-flop rate. We discuss the atomistic details of the flip-flop events observed in our simulations.

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.

Accurate In Silico Modeling of Asymmetric Bilayers Based on Biophysical Principles

Technological advances in the last decade have enabled the study of ever more complex and physiologically relevant model membranes to help dispel the mystery surrounding the role of plasma membrane asymmetry in various cellular processes. The slowly accumulating body of experimental data is fueling renewed interest in and the need for computational methods to support interpretations and address a wide range of problems that are still not amenable to direct experimental study. The specific appeal of molecular dynamics simulations for this purpose lies in their ability to access information at atomic resolution, which is useful for the formulation of testable mechanistic hypotheses. But, the range of questions that can be addressed reliably with such simulations is determined by the appropriate construction and simulation of asymmetric bilayer models. One essential way to achieve this goal is to follow rigorous biophysical criteria and principles. In this context, we show that the requirement for a robust comparison between the properties of simulated asymmetric and symmetric model membranes is for the tension in each bilayer leaflet to be zero. Commonly used methods for constructing asymmetric bilayers, including matching the average areas of the leaflets from the corresponding symmetric systems, do not ensure zero leaflet tension, thus precluding physically realistic changes in the areas of the two leaflets. We present, to our knowledge, a new method for identifying the ideal lipid packing in bilayers with different leaflet compositions that achieves the zero-tension goal, and discuss the basic principles underlying the biophysically correct computational study of asymmetric membranes.

Computational Lipidomics of the Neuronal Plasma Membrane

Membrane lipid composition varies greatly within submembrane compartments, different organelle membranes, and also between cells of different cell stage, cell and tissue types, and organisms. Environmental factors (such as diet) also influence membrane composition. The membrane lipid composition is tightly regulated by the cell, maintaining a homeostasis that, if disrupted, can impair cell function and lead to disease. This is especially pronounced in the brain, where defects in lipid regulation are linked to various neurological diseases. The tightly regulated diversity raises questions on how complex changes in composition affect overall bilayer properties, dynamics, and lipid organization of cellular membranes. Here, we utilize recent advances in computational power and molecular dynamics force fields to develop and test a realistically complex human brain plasma membrane (PM) lipid model and extend previous work on an idealized, "average" mammalian PM. The PMs showed both striking similarities, despite significantly different lipid composition, and interesting differences. The main differences in composition (higher cholesterol concentration and increased tail unsaturation in brain PM) appear to have opposite, yet complementary, influences on many bilayer properties. Both mixtures exhibit a range of dynamic lipid lateral inhomogeneities ("domains"). The domains can be small and transient or larger and more persistent and can correlate between the leaflets depending on lipid mixture, Brain or Average, as well as on the extent of bilayer undulations.

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.

Membrane stress profiles from self-consistent field theory

Using self-consistent field theory (SCFT), we develop an accurate, local expression for the stress profiles in membranes and soft matter interfaces, in general. The bond stresses are expressed in terms of pre-computed chain propagators, which are used to describe the statistical weight of the molecules and therefore require minimal additional calculations. In addition, we overcome the resolution limit of the molecular bond length by including the Irving and Kirkwood bond assignment and recover a constant normal stress profile across an interface. Using this theory, we find that the membrane lateral stress profile contains repulsive (positive) stresses in the regions of the head and tail groups, and attractive (negative) stresses near the hydrophobic/hydrophilic interface. We also verify that the zeroth and first moments of the stress profile correspond to the thermodynamic tension and product of the bending modulus and the spontaneous curvature, respectively.

Molecular Insight into the Line Tension of Bilayer Membranes Containing Hybrid Polyunsaturated Lipids

Line tension (γ) is a key parameter for the structure and dynamics of membrane domains. It was proposed that hybrid lipids, with mixed saturated and unsaturated acyl chains, participate in the relaxation of γ through different mechanisms. In this work, we used molecular dynamics simulations of the coarse-grained MARTINI model to measure γ in liquid-ordered-liquid-disordered (Lo-Ld) membranes, with increasingly larger relative proportion of the hybrid polyunsaturated lipid PAPC (4:0-5:4PC) to DAPC (di5:4PC) (i.e., X_H). We also calculated an elastic contribution to γ by the Lo-Ld thickness mismatch, tilt moduli, and bending moduli, as predicted by theory. We found that an increase in X_H decreased the overall γ value and the elastic contribution to line tension. The effect on the elastic line tension is driven by a reduced hydrophobic mismatch. Changes in the elastic constants of the phases due to an increase in X_H produced a slightly larger elastic γ term. In addition to this elastic energy, other major contributions to γ are found in these model membranes. Increasing X_H decreases both elastic and nonelastic contributions to γ. Finally, PAPC also behaves as a linactant, relaxing γ through an interfacial effect, as predicted by theoretical results. This study gives insight into the actual contribution of distinct energy terms to γ in bilayers containing polyunsaturated hybrid lipids.

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.

Daptomycin inhibits cell envelope synthesis by interfering with fluid membrane microdomains

Daptomycin is a highly efficient last-resort antibiotic that targets the bacterial cell membrane. Despite its clinical importance, the exact mechanism by which daptomycin kills bacteria is not fully understood. Different experiments have led to different models, including (i) blockage of cell wall synthesis, (ii) membrane pore formation, and (iii) the generation of altered membrane curvature leading to aberrant recruitment of proteins. To determine which model is correct, we carried out a comprehensive mode-of-action study using the model organism Bacillus subtilis and different assays, including proteomics, ionomics, and fluorescence light microscopy. We found that daptomycin causes a gradual decrease in membrane potential but does not form discrete membrane pores. Although we found no evidence for altered membrane curvature, we confirmed that daptomycin inhibits cell wall synthesis. Interestingly, using different fluorescent lipid probes, we showed that binding of daptomycin led to a drastic rearrangement of fluid lipid domains, affecting overall membrane fluidity. Importantly, these changes resulted in the rapid detachment of the membrane-associated lipid II synthase MurG and the phospholipid synthase PlsX. Both proteins preferentially colocalize with fluid membrane microdomains. Delocalization of these proteins presumably is a key reason why daptomycin blocks cell wall synthesis. Finally, clustering of fluid lipids by daptomycin likely causes hydrophobic mismatches between fluid and more rigid membrane areas. This mismatch can facilitate proton leakage and may explain the gradual membrane depolarization observed with daptomycin. Targeting of fluid lipid domains has not been described before for antibiotics and adds another dimension to our understanding of membrane-active antibiotics.

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.

Roles of Interleaflet Coupling and Hydrophobic Mismatch in Lipid Membrane Phase-Separation Kinetics

Characterizing the nanoscale dynamic organization within lipid bilayer membranes is central to our understanding of cell membranes at a molecular level. We investigate phase separation and communication across leaflets in ternary lipid bilayers, including saturated lipids with between 12 and 20 carbons per tail. Coarse-grained molecular dynamics simulations reveal a novel two-step kinetics due to hydrophobic mismatch, in which the initial response of the apposed leaflets upon quenching is to increase local asymmetry (antiregistration), followed by dominance of symmetry (registration) as the bilayer equilibrates. Antiregistration can become thermodynamically preferred if domain size is restricted below ∼20 nm, with implications for the symmetry of rafts and nanoclusters in cell membranes, which have similar reported sizes. We relate our findings to theory derived from a semimicroscopic model in which the leaflets experience a “direct” area-dependent coupling, and an “indirect” coupling that arises from hydrophobic mismatch and is most important at domain boundaries. Registered phases differ in composition from antiregistered phases, consistent with a direct coupling between the leaflets. Increased hydrophobic mismatch purifies the phases, suggesting that it contributes to the molecule-level lipid immiscibility. Our results demonstrate an interplay of competing interleaflet couplings that affect phase compositions and kinetics, and lead to a length scale that can influence lateral and transverse bilayer organization within cells.

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.

Registered and antiregistered phase separation of mixed amphiphilic bilayers

We derive a mean-field free energy for the phase behavior of coupled bilayer leaflets, which is implicated in cellular processes and important to the design of artificial membranes. Our model accounts for amphiphile-level structural features, particularly hydrophobic mismatch, which promotes antiregistration, in competition with the direct transmidplane coupling usually studied, which promotes registration. We show that the phase diagram of coupled leaflets allows multiple metastable coexistences, and we illustrate the kinetic implications of this with a detailed study of a bilayer of equimolar overall composition. For approximate parameters estimated to apply to phospholipids, equilibrium coexistence is typically registered, but metastable antiregistered phases can be kinetically favored by hydrophobic mismatch. Thus, a bilayer in the spinodal region can require nucleation to equilibrate, in a novel manifestation of Ostwald's rule of stages. Our results provide a framework for understanding disparate existing observations in the literature, elucidating a subtle competition of couplings and a key role for phase-transition kinetics in bilayer phase behavior.

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.

Nucleation of symmetric domains in the coupled leaflets of a bilayer

We study the kinetics governing the attainment of inter-leaflet domain symmetry in a phase-separating amphiphilic bilayer. "Indirect" inter-leaflet coupling via hydrophobic mismatch can induce an instability towards a metastable pattern of locally asymmetric domains upon quenching from high temperature. This necessitates a nucleation step to form the conventional symmetric domains, which are favoured by a "direct" inter-leaflet coupling. We model the energetics for a symmetric domain to nucleate from the metastable state, and find that an interplay between hydrophobic mismatch and thickness stretching/compression causes the effective hydrophobic mismatch, and thus line tension, to depend on domain size. This leads to strong departure from classical nucleation theory. We speculate on implications for cell membrane rafts or clusters, whose size may be of similar magnitude to estimated critical radii for domain symmetry.

Interleaflet mixing and coupling in liquid-disordered phospholipid bilayers

Organized as bilayers, phospholipids are the fundamental building blocks of cellular membranes and determine many of their biological functions. Interactions between the two leaflets of the bilayer (interleaflet coupling) have been implicated in the passage of information through membranes. However, physically, the meaning of interleaflet coupling is ill defined and lacks a structural basis. Using all-atom molecular dynamics simulations of fluid phospholipid bilayers of five different lipids with differing degrees of acyl-chain asymmetry, we have examined interleaflet mixing to gain insights into coupling. Reasoning that the transbilayer distribution of terminal methyl groups is an appropriate measure of interleaflet mixing, we calculated the transbilayer distributions of the acyl chain terminal methyl groups for five lipids: dioleoylphosphatidylcholine (DOPC), palmitoyloleoylphosphatidylcholine (POPC), stearoyloleoylphosphatidylcholine (SOPC), oleoylmyristoylphosphatidylcholine (OMPC), and dimyristoylphosphatidylcholine (DMPC). We observed in all cases very strong mixing across the bilayer midplane that diminished somewhat with increasing acyl-chain ordering defined by methylene order parameters. A hallmark of the interleaflet coupling idea is complementarity, which postulates that lipids with short alkyl chains in one leaflet will preferentially associate with lipids with long alkyl chains in the other leaflet. Our results suggest a much more complicated picture for thermally disordered bilayers that we call distributed complementarity, as measured by the difference in the peak positions of the sn-1 and sn-2 methyl distributions in the same leaflet.

Kinetics of symmetry and asymmetry in a phase-separating bilayer membrane

We simulate a phase-separating bilayer in which the leaflets experience a direct coupling favoring local compositional symmetry ("registered" bilayer phases), and an indirect coupling due to hydrophobic mismatch that favors strong local asymmetry ("antiregistered" bilayer phases). For wide ranges of overall leaflet compositions, multiple competing states are possible. For estimated physical parameters, a quenched bilayer may first evolve toward a metastable state more asymmetric than if the leaflets were uncorrelated; subsequently, it must nucleate to reach its equilibrium, more symmetric, state. These phase-transition kinetics exhibit characteristic signatures through which fundamental and opposing interleaflet interactions may be probed. We emphasize how bilayer phase diagrams with a separate axis for each leaflet can account for overall and local symmetry or asymmetry, and capture a range of observations in the experiment and simulation literature.

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.

Resolving Internal Motional Correlations to Complete the Conformational Entropy Meter

Conformational entropy (SΩ) has long been used to theoretically characterize the dynamics of proteins, DNA, and other polymers. Though recent advances enabled its calculation also from simulations and nuclear magnetic resonance (NMR) relaxation experiments, correlated molecular motion has hitherto greatly hindered both numerical and experimental determination, requiring demanding empirical and computational calibrations. Herein, we show that these motional correlations can be estimated directly from the temperature-dependent SΩ series that reveal effective persistence lengths of the polymers, which we demonstrate by measuring SΩ of amphiphilic molecules in model lipid systems by spin-labeling electron paramagnetic resonance (EPR) spectroscopy. We validate our correlation-corrected SΩ meter against the basic biophysical interactions underlying biomembrane formation and stability, against the changes in enthalpy and diffusion coefficients upon phase transitions, and against the energetics of fatty acid dissociation. As the method can be directly applied to conformational analysis of proteins and other polymers, as well as adapted to NMR or polarized fluorescence techniques, we believe that the approach can greatly enrich the scope of experimentally available statistical thermodynamics, offering new physical insights into the behavior of biomolecules.

Membrane lateral structure: the influence of immobilized particles on domain size

In experiments on model membranes, formation of large domains of different lipid composition is readily observed. However, no such phase separation is observed in the membranes of intact cells. Instead, small transient inhomogeneities called lipid rafts are expected in these systems. One of the numerous attempts to explain small domains refers to the coupling of the membrane to its surroundings, which leads to the immobilization of some of the membrane molecules. These immobilized molecules then act as static obstacles for the remaining mobile ones. We present detailed Molecular Dynamics simulations demonstrating that this can indeed account for small domains. This confirms previous Monte Carlo studies based on simplified models. Furthermore, by directly comparing domain structures obtained using Molecular Dynamics to Monte Carlo simulations of the Ising model, we demonstrate that domain formation in the presence of obstacles is remarkably insensitive to the details of the molecular interactions.

Biomimetic monolayer and bilayer membranes made from amphiphilic block copolymer micelles

The deposition of amphiphilic poly(ethylene oxide)-block-poly(butadiene) (PEO-b-PBD) copolymer micelles is demonstrated on solid substrates. Depending upon surface chemistry, micelle adsorption creates either monolayer or bilayer films. Lateral diffusion measurements reveal that strong coupling between hydrophilic surfaces and PEO blocks creates immobile bilayers, while monolayers retain the fluidity previously observed in vesicular assemblies.