Lipid bilayers: clusters, domains and phases

Dysregulation of cellular membrane homeostasis as a crucial modulator of cancer risk

Cellular membranes serve as an epicentre combining extracellular and cytosolic components with membranous effectors, which together support numerous fundamental cellular signalling pathways that mediate biological responses. To execute their functions, membrane proteins, lipids and carbohydrates arrange, in a highly coordinated manner, into well-defined assemblies displaying diverse biological and biophysical characteristics that modulate several signalling events. The loss of membrane homeostasis can trigger oncogenic signalling. More recently, it has been documented that select membrane active dietaries (MADs) can reshape biological membranes and subsequently decrease cancer risk. In this review, we emphasize the significance of membrane domain structure, organization and their signalling functionalities as well as how loss of membrane homeostasis can steer aberrant signalling. Moreover, we describe in detail the complexities associated with the examination of these membrane domains and their association with cancer. Finally, we summarize the current literature on MADs and their effects on cellular membranes, including various mechanisms of dietary chemoprevention/interception and the functional links between nutritional bioactives, membrane homeostasis and cancer biology.

Anomalous lateral diffusion of lipids during the fluid/gel phase transition of a lipid membrane

A lipid membrane undergoes a phase transition from fluid to gel phase upon changing external thermodynamic conditions, such as decreasing temperature and increasing pressure. Extremophilic organisms face the challenge of preventing this deleterious phase transition. The main focus of their adaptive strategy is to facilitate effective temperature sensing through sensor proteins, relying on the drastic changes in packing density and membrane fluidity during the phase transition. Although the changes in packing density parameters due to the fluid/gel phase transition are studied in detail, the impact on membrane fluidity is less explored in the literature. Understanding the lateral diffusive dynamics of lipids in response to temperature, particularly during the fluid/gel phase transition, is albeit crucial. Here we have simulated the phase transition of a single component lipid membrane composed of dipalmitoylphosphatidylcholine (DPPC) lipids using a coarse-grained (CG) model and studied the changes of the structural and dynamical properties. It is observed that near the phase transition point, both fluid and gel phase domains coexist together. The dynamics remains highly non-Gaussian for a long time even when the mean square displacement reaches the Fickian regime at a much earlier time. This Fickian yet non-Gaussian diffusion (FnGD) is a characteristic of a highly heterogeneous system, previously observed for the lateral diffusion of lipids in raft mimetic membranes having liquid-ordered and liquid-disordered phases co-existing together. We have analyzed the molecular trajectories and calculated the jump-diffusion of the lipids, stemming from sudden jump translations, using a translational jump-diffusion (TJD) approach. An overwhelming contribution of the jump-diffusion of the lipids is observed suggesting anomalous diffusion of lipids during fluid/gel phase transition of the membrane. These results are important in unravelling the intricate nature of lipid diffusion during the phase transition of the membrane and open up a new possibility of investigating the most significant change of membrane properties during phase transition, which can be effectively sensed by proteins.

Kinetics of Ganglioside-Rich Supported Lipid Bilayer Formation with Tracer Vesicle Fluorescence Imaging

Gangliosides, forming a class of lipids complemented by sugar chains, influence the lateral distribution of membrane proteins or membrane-binding proteins, act as receptors for viruses and bacterial toxins, and mediate several types of cellular signaling. Gangliosides incorporated into supported lipid bilayers (SLBs) have been widely applied as a model system to examine these biological processes. In this work, we explored how ganglioside composition affects the kinetics of SLB formation using the vesicle rupturing method on a solid surface. We imaged the attachment of vesicles and the subsequent SLB formation using the time-lapse total internal reflection fluorescence microscopy technique. In the early phase, the ganglioside type and concentration influence the adsorption kinetics of vesicles and their residence/lifetime on the surface before rupturing. Our data confirm that a simultaneous rupturing of neighboring surface-adsorbed vesicles forms microscopic lipid patches on the surface and it is triggered by a critical coverage of the vesicles independent of their composition. In the SLB growth phase, lipid patches merge, forming a continuous SLB. The propagation of patch edges catalyzes the process and depends on the ganglioside type. Our pH-dependent experiments confirm that the polar/charged head groups of the gangliosides have a critical role in these steps and phases of SLB formation kinetics.

Selective Deuteration Improves the Affinity of Adenosine A2A Receptor Ligands: A Computational Case Study with Istradefylline and Caffeine

We used a range of computational techniques to assess the effect of selective C-H deuteration on the antagonist istradefylline affinity for the adenosine A_2A receptor, which was discussed relative to its structural analogue caffeine, a well-known and likely the most widely used stimulant. The obtained results revealed that smaller caffeine shows high receptor flexibility and exchanges between two distinct poses, which agrees with crystallographic data. In contrast, the additional C8-trans-styryl fragment in istradefylline locks the ligand within a uniform binding pose, while contributing to the affinity through the C-H···π and π···π contacts with surface residues, which, together with its much lower hydration prior to binding, enhances the affinity over caffeine. In addition, the aromatic C8-unit shows a higher deuteration sensitivity over the xanthine part, so when both of its methoxy groups are d₆-deuterated, the affinity improvement is -0.4 kcal mol^-1, which surpasses the overall affinity gain of -0.3 kcal mol^-1 in the perdeuterated d₉-caffeine. Yet, the latter predicts around 1.7-fold potency increase, being relevant for its pharmaceutical implementations, and also those within the coffee and energy drink production industries. Still, the full potential of our strategy is achieved in polydeuterated d₁₉-istradefylline, whose A_2A affinity improves by -0.6 kcal mol^-1, signifying a 2.8-fold potency increase that strongly promotes it as a potential synthetic target. This knowledge supports deuterium application in drug design, and while the literature already reports about over 20 deuterated drugs currently in the clinical development, it is easily foreseen that more examples will hit the market in the years to come. With this in mind, we propose that the devised computational methodology, involving the ONIOM division of the QM region for the ligand and the MM region for its environment, with an implicit quantization of nuclear motions relevant for the H/D exchange, allows fast and efficient estimates of the binding isotope effects in any biological system.

Impact of sphingomyelin acyl chain heterogeneity upon properties of raft-like membranes

Sphingomyelin (SM) is a main component of lipid rafts and characteristic of abundance of long and saturated acyl chains. Recently, we reported that fluorescence-labeled lipids including C16:0 and C18:0SMs retained membrane behaviors of inherent lipids. Here, we newly prepared fluorescent SMs with longer acyl chains, C22:0 and C24:1, for observing their partition and diffusion in SM/cholesterol (chol)/dioleoylphosphatidylcholine (DOPC) bilayers. Although fluorescent C24:1SM underwent a uniform distribution between ordered (Lo) and disordered (Ld) phases, other fluorescent SMs with saturated acyl chains were preferentially distributed in the Lo phase. Interestingly, when the acyl chains of fluorescent and membrane SMs are different, distribution of fluorescent SM to the Lo phase was reduced compared to when the acyl chains are the same. This tendency was also observed for C16:0SM/C22:0SM/chol/DOPC quaternary bilayers, where the minor SM was more excluded out of the Lo phase than the major SM. We also found that the coexistence of SMs induces SM efflux out of the Lo phase and simultaneous DOPC influx to the Lo phase, consequently reducing the difference in fluidity between the two phases. These results suggest that physicochemical properties of lipid rafts are regulated by the acyl chain heterogeneity of SMs.

Serotonergic drugs modulate the phase behavior of complex lipid bilayers

Serotonin is an endogenous neurotransmitter involved in both physiological and pathophysiological processes. Traditionally, serotonin acts as a ligand for G protein-coupled receptors (GPCRs) leading to subsequent cell signaling. However, serotonin can also bind to lipid membranes with high affinity and modulate the phase behavior in 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC)/N-palmitoyl-D-erythro-sphingosylphosphorylcholine (PSM)/cholesterol model membranes mimicking the outer leaflet of the plasma membrane. Here, we investigated if serotonergic drugs containing the pharmacophore from serotonin would also modulate phase behavior in lipid membranes in a similar fashion. We used ²H NMR spectroscopy to explore the phase behavior of POPC/PSM/cholesterol (4/4/2 molar ratio) mixtures in the presence of the serotonergic drugs aripiprazole, BRL-54443, BW-723C86, and CP-135807 at a lipid to drug molar ratio of 10:1. POPC and PSM were perdeuterated in the palmitoyl chain, respectively, and prepared in individual samples. Numerical lineshape simulations of the ²H NMR spectra were used to calculate the order parameter profiles and projected lengths of the saturated acyl chains. All serotonergic drugs induce two components in ²H NMR spectra, indicating that they increased the hydrophobic mismatch between the thickness of the coexisting lipid phases leading to larger domain sizes, relatively similarly to serotonin. AFM force indentation and Raman spectral studies, which interrogate membrane mechanical properties, also indicate changes in membrane order in the presence of these drugs. These findings highlight how serotonergic drugs alter membrane phase behavior and could modulate both target and other membrane proteins, possibly explaining the side effects observed for serotonergic and other clinically relevant drugs.

Domain Size Regulation in Phospholipid Model Membranes Using Oil Molecules and Hybrid Lipids

The formation of domains in multicomponent lipid mixtures has been suggested to play a role in moderating signal transduction in cells. Understanding how domain size may be regulated by both hybrid lipid molecules and impurities is important for understanding real biological processes; at the same time, developing model systems where domain size can be regulated is crucial to enable systematic studies of domain formation kinetics and thermodynamics. Here, we perform a model study of the effects of oil molecules, which swell the bilayer, and line-active hybrid phospholipids using a thermally induced liquid-solid phase separation in planar, free-standing lipid bilayers consisting of DOPC and DPPC (1,2-dioleoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, respectively). The experiments show that the kinetics of domain growth are significantly affected by the type and molecular structure of the oil (squalene, hexadecane, or decane), with the main contributing factors being the degree of swelling of the bilayer and the changes in line tension induced by the different oils, with smaller domains resulting from systems with smaller values of the line tension. POPC (1-palmitoyl-sn-2-oleoyl-glycero-3-phosphocholine), on the other hand, acts as a line-active hybrid lipid, reducing the domain size when added in small amounts and slowing down domain coarsening. Finally, we show that despite the regulation of domain size by both methods, the phase transition temperature is influenced by the presence of oil molecules but not significantly by the presence of hybrid lipids. Overall, our results show how to regulate domain size in binary membrane model systems, over a wide range of length scales, by incorporating oil molecules and hybrid lipids.

Particle-based Ising model

We characterize equilibrium properties and relaxation dynamics of a two-dimensional lattice containing, at each site, two particles connected by a double-well potential (dumbbell). Dumbbells are oriented in the orthogonal direction with respect to the lattice plane and interact with each other through a Lennard-Jones potential truncated at the nearest neighbor distance. We show that the system's equilibrium properties are accurately described by a two-dimensional Ising model with an appropriate coupling constant. Moreover, we characterize the coarsening kinetics by calculating the cluster size as a function of time and compare the results with Monte Carlo simulations based on Glauber or reactive dynamics rate constants.

Inhibition of Amyloid β-Induced Lipid Membrane Permeation and Amyloid β Aggregation by K162

Alzheimer's disease (AD) is characterized by progressive neurodegeneration associated with amyloid β (Aβ) peptide aggregation. The aggregation of Aβ monomers (AβMs) leads to the formation of Aβ oligomers (AβOs), the neurotoxic Aβ form, capable of permeating the cell membrane. Here, we investigated the effect of a fluorene-based active drug candidate, named K162, on both Aβ aggregation and AβO toxicity toward the bilayer lipid membrane (BLM). Electrochemical impedance spectroscopy (EIS), atomic force microscopy (AFM), and molecular dynamics (MD) were employed to show that K162 inhibits AβOs-induced BLM permeation, thus preserving BLM integrity. In the presence of K162, only shallow defects on the BLM surface were formed. Apparently, K162 modifies Aβ aggregation by bypassing the formation of toxic AβOs, and only nontoxic AβMs, dimers (AβDs), and fibrils (AβFs) are produced. Unlike other Aβ toxicity inhibitors, K162 preserves neurologically beneficial AβMs. This unique K162 inhibition mechanism provides an alternative AD therapeutic strategy that could be explored in the future.

Increasing complexity in small-angle X-ray and neutron scattering experiments: from biological membrane mimics to live cells

Small-angle X-ray and neutron scattering are well-established, non-invasive experimental techniques to interrogate global structural properties of biological membrane mimicking systems under physiologically relevant conditions. Recent developments, both in bottom-up sample preparation techniques for increasingly complex model systems, and in data analysis techniques have opened the path toward addressing long standing issues of biological membrane remodelling processes. These efforts also include emerging quantitative scattering studies on live cells, thus enabling a bridging of molecular to cellular length scales. Here, we review recent progress in devising compositional models for joint small-angle X-ray and neutron scattering studies on diverse membrane mimics - with a specific focus on membrane structural coupling to amphiphatic peptides and integral proteins - and live Escherichia coli. In particular, we outline the present state-of-the-art in small-angle scattering methods applied to complex membrane systems, highlighting how increasing system complexity must be followed by an advance in compositional modelling and data-analysis tools.

Single-lipid dynamics in phase-separated supported lipid bilayers

Phase separation is a fundamental organizing mechanism on cellular membranes. Lipid phases have complex dependencies on the membrane composition, curvature, tension, and temperature. Lipid diffusion rates vary by up to ten-fold between liquid-disordered (L_d) and liquid-ordered (L_o) phases depending on the membrane composition, measurement technique, and the surrounding environment. This manuscript reports the lipid diffusion on phase-separated supported lipid bilayers (SLBs) with varying temperature, composition, and lipid phase. Lipid diffusion is measured by single-particle tracking (SPT) and fluorescence correlation spectroscopy (FCS) via custom data acquisition and analysis protocols that apply to diverse membranes systems. Traditionally, SPT is sensitive to diffuser aggregation, whereas the diffusion rates reported by FCS are unaffected by the presence of immobile aggregates. Within this manuscript, we report (1) improved single-particle tracking analysis of lipid diffusion, (2) comparison and consistency between diffusion measurement methods for non-Brownian diffusers, and (3) the application of these methods to measure the phase, temperature, and composition dependencies in lipid diffusion. We demonstrate improved SPT analysis methods that yield consistent FCS and SPT diffusion results even when most fluorescent lipids are frequently confined within aggregates within the membrane. With varying membrane composition and temperature, we demonstrate differences in diffusion between the L_d and L_o phases of SLBs.

Effect of alcohol on the phase separation in model membranes

The discovery of coexisting liquid-ordered and liquid-disordered phases in multicomponent lipid bilayers has received widespread attention due to its potential relevance for biological systems. One of the many open questions is how the presence of additional components affects the nature of the coexisting phases. Of particular interest is the addition of alcohols because their anesthetic properties may arise from modulating bilayer behavior. We use coarse-grained Molecular Dynamics simulations to gain insight into the partitioning preferences of linear n-alcohols into ordered and disordered bilayers alongside their effects on local membrane structure. We find that alcohols cause only small changes to membrane composition alongside a lack of significant effects on membrane thickness and lipid tail order. Cholesterol and n-alcohol trans-bilayer motion is measured and found to be near or within the range of previous atomistic results. The cholesterol flip-flop rates increase with both n-alcohol length and concentration for octanol, dodecanol, and hexadecanol, indicating a decrease in lipid order. Umbrella sampling simulations of removing cholesterol from tertiary membranes find no significant difference with or without n-alcohols at various concentrations. Simulations of a phase-separated bilayer show that octanol preferentially partitions into the liquid-disordered phase in a ratio of approximately 3:1 over the liquid-ordered phase. Furthermore, partition coefficients of alcohol in single-phase membranes show a preference for longer alcohols (dodecanol and hexadecanol) to partition preferentially into the liquid-ordered phase, while decreasing the length of the alcohol reverses this trend. Our work tests experimental results while also investigating the ability for coarse-grained MARTINI simulations to capture minute differences in model membrane spatial arrangements on the nanoscale level.

C24:0 and C24:1 sphingolipids in cholesterol-containing, five- and six-component lipid membranes

The biophysical properties of sphingolipids containing lignoceric (C24:0) or nervonic (C24:1) fatty acyl residues have been studied in multicomponent lipid bilayers containing cholesterol (Chol), by means of confocal microscopy, differential scanning calorimetry and atomic force microscopy. Lipid membranes composed of dioleoyl phosphatidylcholine and cholesterol were prepared, with the addition of different combinations of ceramides (C24:0 and/or C24:1) and sphingomyelins (C24:0 and/or C24:1). Results point to C24:0 sphingolipids, namely lignoceroyl sphingomyelin (lSM) and lignoceroyl ceramide (lCer), having higher membrane rigidifying properties than their C24:1 homologues (nervonoyl SM, nSM, or nervonoyl Cer, nCer), although with a similar strong capacity to induce segregated gel phases. In the case of the lSM-lCer multicomponent system, the segregated phases have a peculiar fibrillar or fern-like morphology. Moreover, the combination of C24:0 and C24:1 sphingolipids generates interesting events, such as a generalized bilayer dynamism/instability of supported planar bilayers. In some cases, these sphingolipids give rise to exothermic curves in thermograms. These peculiar features were not present in previous studies of C24:1 combined with C16:0 sphingolipids. Conclusions of our study point to nSM as a key factor governing the relative distribution of ceramides when both lCer and nCer are present. The data indicate that lCer could be easier to accommodate in multicomponent bilayers than its C16:0 counterpart. These results are relevant for events of membrane platform formation, in the context of sphingolipid-based signaling cascades.

Peripheral myelin protein 22 preferentially partitions into ordered phase membrane domains

The preferential partitioning of single-span membrane proteins for ordered phase domains in phase-separated membranes is now reasonably well understood, but little is known about this phase preference for multispan helical membrane proteins. Here, it is shown that the disease-linked tetraspan membrane protein, PMP22, displays a pronounced preference to partition into the ordered phase, a preference that is reversed by disease mutations. This phase preference may be related to the role of PMP22 in cholesterol homeostasis in myelinating Schwann cells, a role that is also known to be disrupted under conditions of Charcot–Marie–Tooth disease (CMTD) peripheral neuropathy caused by pmp22 mutations.

The ordered environment of cholesterol-rich membrane nanodomains is thought to exclude many transmembrane (TM) proteins. Nevertheless, some multispan helical transmembrane proteins have been proposed to partition into these environments. Here, giant plasma membrane vesicles (GPMVs) were employed to quantitatively show that the helical tetraspan peripheral myelin protein 22 (PMP22) exhibits a pronounced preference for, promotes the formation of, and stabilizes ordered membrane domains. Neither S-palmitoylation of PMP22 nor its putative cholesterol binding motifs are required for this preference. In contrast, Charcot–Marie–Tooth disease-causing mutations that disrupt the stability of PMP22 tertiary structure reduce or eliminate this preference in favor of the disordered phase. These studies demonstrate that the ordered phase preference of PMP22 derives from global structural features associated with the folded form of this protein, providing a glimpse at the structural factors that promote raft partitioning for multispan helical membrane proteins.

Insights into adenosine A2A receptor activation through cooperative modulation of agonist and allosteric lipid interactions

The activation process of G protein-coupled receptors (GPCRs) has been extensively studied, both experimentally and computationally. In particular, Molecular Dynamics (MD) simulations have proven useful in exploring GPCR conformational space. The typical behaviour of class A GPCRs, when subjected to unbiased MD simulations from their crystallized inactive state, is to fluctuate between inactive and intermediate(s) conformations, even with bound agonist. Fully active conformation(s) are rarely stabilized unless a G protein is also bound. Despite several crystal structures of the adenosine A2a receptor (A2aR) having been resolved in complex with co-crystallized agonists and Gs protein, its agonist-mediated activation process is still not completely understood. In order to thoroughly examine the conformational landscape of A2aR activation, we performed unbiased microsecond-length MD simulations in quadruplicate, starting from the inactive conformation either in apo or with bound agonists: endogenous adenosine or synthetic NECA, embedded in two homogeneous phospholipid membranes: 1,2-dioleoyl-sn-glycerol-3-phosphoglycerol (DOPG) or 1,2-dioleoyl-sn-glycerol-3-phosphocholine (DOPC). In DOPC with bound adenosine or NECA, we observe transition to an intermediate receptor conformation consistent with the known adenosine-bound crystal state. In apo state in DOPG, two different intermediate conformations are obtained. One is similar to that observed with bound adenosine in DOPC, while the other is closer to the active state but not yet fully active. Exclusively, in DOPG with bound adenosine or NECA, we reproducibly identify receptor conformations with fully active features, which are able to dock Gs protein. These different receptor conformations can be attributed to the action/absence of agonist and phospholipid-mediated allosteric effects on the intracellular side of the receptor.

Liposome-Mediated Chemotherapeutic Delivery Is Synergistically Enhanced by Ternary Lipid Compositions and Cationic Lipids

Most small molecule chemotherapeutics must cross one or more cellular membrane barriers to reach their biochemical targets. Owing to the relatively low solubility of chemotherapeutics in the lipid membrane environment, high doses are often required to achieve a therapeutic effect. The resulting systemic toxicity has motivated efforts to improve the efficiency of chemotherapeutic delivery to the cellular interior. Toward this end, liposomes containing lipids with cationic head groups have been shown to permeabilize cellular membranes, resulting in the more efficient release of encapsulated drugs into the cytoplasm. However, the high concentrations of cationic lipids required to achieve efficient delivery remain a key limitation, frequently resulting in toxicity. Toward overcoming this limitation, here, we investigate the ability of ternary lipid mixtures to enhance liposomal delivery. Specifically, we investigate the delivery of the chemotherapeutic, doxorubicin, using ternary liposomes that are homogeneous at physiological temperature but have the potential to undergo membrane phase separation upon contact with the cell surface. This approach, which relies upon the ability of membrane phase boundaries to promote drug release, provides a novel method for reducing the overall concentration of cationic lipids required for efficient delivery. Our results show that this approach improves the performance of doxorubicin by up to 5-fold in comparison to the delivery of the same drug by conventional liposomes. These data demonstrate that ternary lipid compositions and cationic lipids can be combined synergistically to substantially improve the efficiency of chemotherapeutic delivery in vitro.

Recent Lipid Membrane-Based Biosensing Platforms

The investigation of lipid films for the construction of biosensors has recently given the opportunity to manufacture devices to selectively detect a wide range of food toxicants, environmental pollutants, and compounds of clinical interest. Biosensor miniaturization using nanotechnological tools has provided novel routes to immobilize various “receptors” within the lipid film. This chapter reviews and exploits platforms in biosensors based on lipid membrane technology that are used in food, environmental, and clinical chemistry to detect various toxicants. Examples of applications are described with an emphasis on novel systems, new sensing techniques, and nanotechnology-based transduction schemes. The compounds that can be monitored are insecticides, pesticides, herbicides, metals, toxins, antibiotics, microorganisms, hormones, dioxins, etc.

DHA Modifies the Size and Composition of Raftlike Domains: A Solid-State 2H NMR Study

Docosahexaenoic acid is an omega-3 polyunsaturated fatty acid that relieves the symptoms of a wide variety of chronic inflammatory disorders. The structural mechanism is not yet completely understood. Our focus here is on the plasma membrane as a site of action. We examined the molecular organization of [²H₃₁]-N-palmitoylsphingomyelin (PSM-d₃₁) mixed with 1-palmitoyl-2-docosahexaenoylphosphatylcholine (PDPC) or 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), as a monounsaturated control, and cholesterol (chol) (1:1:1 mol) in a model membrane by solid-state ²H NMR. The spectra were analyzed in terms of segregation into ordered SM-rich/chol-rich (raftlike) and disordered PC-rich/chol-poor (nonraft) domains that are nanoscale in size. An increase in the size of domains is revealed when POPC was replaced by PDPC. Spectra that are single-component, attributed to fast exchange between domains (<45 nm), for PSM-d₃₁ mixed with POPC and chol become two-component, attributed to slow exchange between domains (r > 30 nm), for PSM-d₃₁ mixed with PDPC and chol. The resolution of separate signals from PSM-d₃₁, and correspondingly from [3α-²H₁]cholesterol (chol-d₁) and 1-[²H₃₁]palmitoyl-2-docosahexaenoylphosphatidylcholine (PDPC-d₃₁), in raftlike and nonraft domains enabled us to determine the composition of the domains in the PDPC-containing membrane. Most of the lipid (28% SM, 29% chol, and 23% PDPC with respect to total lipid at 30°C) was found in the raftlike domain. Despite substantial infiltration of PDPC into raftlike domains, there appears to be minimal effect on the order of SM, implying the existence of internal structure that limits contact between SM and PDPC. Our results suggest a significant refinement to the model by which DHA regulates the architecture of ordered, sphingolipid-chol-enriched domains (rafts) in membranes.

Hydration-mediated stiffening of collective membrane dynamics by cholesterol

Hydration water governs the cholesterol-induced changes in collective headgroup dynamics in lipid bilayers.

Sphingolipids and lipid rafts: Novel concepts and methods of analysis

About twenty years ago, the functional lipid raft model of the plasma membrane was published. It took into account decades of research showing that cellular membranes are not just homogenous mixtures of lipids and proteins. Lateral anisotropy leads to assembly of membrane domains with specific lipid and protein composition regulating vesicular traffic, cell polarity, and cell signaling pathways in a plethora of biological processes. However, what appeared to be a clearly defined entity of clustered raft lipids and proteins became increasingly fluid over the years, and many of the fundamental questions about biogenesis and structure of lipid rafts remained unanswered. Experimental obstacles in visualizing lipids and their interactions hampered progress in understanding just how big rafts are, where and when they are formed, and with which proteins raft lipids interact. In recent years, we have begun to answer some of these questions and sphingolipids may take center stage in re-defining the meaning and functional significance of lipid rafts. In addition to the archetypical cholesterol-sphingomyelin raft with liquid ordered (Lo) phase and the liquid-disordered (Ld) non-raft regions of cellular membranes, a third type of microdomains termed ceramide-rich platforms (CRPs) with gel-like structure has been identified. CRPs are "ceramide rafts" that may offer some fresh view on the membrane mesostructure and answer several critical questions for our understanding of lipid rafts.

Muscle membrane integrity in Duchenne muscular dystrophy: recent advances in copolymer-based muscle membrane stabilizers

The scientific premise, design, and structure-function analysis of chemical-based muscle membrane stabilizing block copolymers are reviewed here for applications in striated muscle membrane injury. Synthetic block copolymers have a rich history and wide array of applications from industry to biology. Potential for discovery is enabled by a large chemical space for block copolymers, including modifications in block copolymer mass, composition, and molecular architecture. Collectively, this presents an impressive chemical landscape to leverage distinct structure-function outcomes. Of particular relevance to biology and medicine, stabilization of damaged phospholipid membranes using amphiphilic block copolymers, classified as poloxamers or pluronics, has been the subject of increasing scientific inquiry. This review focuses on implementing block copolymers to protect fragile muscle membranes against mechanical stress. The review highlights interventions in Duchenne muscular dystrophy, a fatal disease of progressive muscle deterioration owing to marked instability of the striated muscle membrane. Biophysical and chemical engineering advances are presented that delineate and expand upon current understanding of copolymer-lipid membrane interactions and the mechanism of stabilization. The studies presented here serve to underscore the utility of copolymer discovery leading toward the therapeutic application of block copolymers in Duchenne muscular dystrophy and potentially other biomedical applications in which membrane integrity is compromised.

Phospholipid Chain Interactions with Cholesterol Drive Domain Formation in Lipid Membranes

Cholesterol is a key component of eukaryotic membranes, but its role in cellular biology in general and in lipid rafts in particular remains controversial. Model membranes are used extensively to determine the phase behavior of ternary mixtures of cholesterol, a saturated lipid, and an unsaturated lipid with liquid-ordered and liquid-disordered phase coexistence. Despite many different experiments that determine lipid-phase diagrams, we lack an understanding of the molecular-level driving forces for liquid phase coexistence in bilayers with cholesterol. Here, we use atomistic molecular dynamics computer simulations to address the driving forces for phase coexistence in ternary lipid mixtures. Domain formation is directly observed in a long-timescale simulation of a mixture of 1,2-distearoyl-sn-glycero-3-phosphocholine, unsaturated 1,2-dilinoleoyl-sn-glycero-3-phosphocholine, and cholesterol. Free-energy calculations for the exchange of the saturated and unsaturated lipids between the ordered and disordered phases give insight into the mixing behavior. We show that a large energetic contribution to domain formation is favorable enthalpic interactions of the saturated lipid in the ordered phase. This favorable energy for forming an ordered, cholesterol-rich phase is opposed by a large unfavorable entropy. Martini coarse-grained simulations capture the unfavorable free energy of mixing but do not reproduce the entropic contribution because of the reduced representation of the phospholipid tails. Phospholipid tails and their degree of unsaturation are key energetic contributors to lipid phase separation.

Structural insights into positive and negative allosteric regulation of a G protein-coupled receptor through protein-lipid interactions

Lipids are becoming known as essential allosteric modulators of G protein-coupled receptor (GPCRs). However, how they exert their effects on GPCR conformation at the atomic level is still unclear. In light of recent experimental data, we have performed several long-timescale molecular dynamics (MD) simulations, totalling 24 μs, to rigorously map allosteric modulation and conformational changes in the β₂ adrenergic receptor (β2AR) that occur as a result of interactions with three different phospholipids. In particular, we identify different sequential mechanisms behind receptor activation and deactivation, respectively, mediated by specific lipid interactions with key receptor regions. We show that net negatively charged lipids stabilize an active-like state of β2AR that is able to dock G_sα protein. Clustering of anionic lipids around the receptor with local distortion of membrane thickness is also apparent. On the other hand, net-neutral zwitterionic lipids inactivate the receptor, generating either fully inactive or intermediate states, with kinetics depending on lipid headgroup charge distribution and hydrophobicity. These chemical differences alter membrane thickness and density, which differentially destabilize the β2AR active state through lateral compression effects.

Capacitive Detection of Low-Enthalpy, Higher-Order Phase Transitions in Synthetic and Natural Composition Lipid Membranes

In-plane lipid organization and phase separation in natural membranes play key roles in regulating many cellular processes. Highly cooperative, first-order phase transitions in model membranes consisting of few lipid components are well understood and readily detectable via calorimetry, densitometry, and fluorescence. However, far less is known about natural membranes containing numerous lipid species and high concentrations of cholesterol, for which thermotropic transitions are undetectable by the above-mentioned techniques. We demonstrate that membrane capacitance is highly sensitive to low-enthalpy thermotropic transitions taking place in complex lipid membranes. Specifically, we measured the electrical capacitance as a function of temperature for droplet interface bilayer model membranes of increasing compositional complexity, namely, (a) a single lipid species, (b) domain-forming ternary mixtures, and (c) natural brain total lipid extract (bTLE). We observed that, for single-species lipid bilayers and some ternary compositions, capacitance exhibited an abrupt, temperature-dependent change that coincided with the transition detected by other techniques. In addition, capacitance measurements revealed transitions in mixed-lipid membranes that were not detected by the other techniques. Most notably, capacitance measurements of bTLE bilayers indicated a transition at ∼38 °C not seen with any other method. Likewise, capacitance measurements detected transitions in some well-studied ternary mixtures that, while known to yield coexisting lipid phases, are not detected with calorimetry or densitometry. These results indicate that capacitance is exquisitely sensitive to low-enthalpy membrane transitions because of its sensitivity to changes in bilayer thickness that occur when lipids and excess solvent undergo subtle rearrangements near a phase transition. Our findings also suggest that heterogeneity confers stability to natural membranes that function near transition temperatures by preventing unwanted defects and macroscopic demixing associated with high-enthalpy transitions commonly found in simpler mixtures.

Complex dynamics at the nanoscale in simple biomembranes

Nature is known to engineer complex compositional and dynamical platforms in biological membranes. Understanding this complex landscape requires techniques to simultaneously detect membrane re-organization and dynamics at the nanoscale. Using super-resolution stimulated emission depletion (STED) microscopy coupled with fluorescence correlation spectroscopy (FCS), we reveal direct experimental evidence of dynamic heterogeneity at the nanoscale in binary phospholipid-cholesterol bilayers. Domain formation on the length scale of ~200–600 nm due to local cholesterol compositional heterogeneity is found to be more prominent at high cholesterol content giving rise to distinct intra-domain lipid dynamics. STED-FCS reveals unique dynamical crossover phenomena at length scales of ~100–150 nm within each of these macroscopic regions. The extent of dynamic heterogeneity due to intra-domain hindered lipid diffusion as reflected from the crossover length scale, is driven by cholesterol packing and organization, uniquely influenced by phospholipid type. These results on simple binary model bilayer systems provide novel insights into pathways leading to the emergence of complex nanodomain substructures with implications for a wide variety of membrane mediated cellular events.

Complex biomembrane mimetics on the sub-nanometer scale

Biomimetic lipid vesicles are indispensable tools for gaining insight into the biophysics of cell physiology on the molecular level. The level of complexity of these model systems has steadily increased, and now spans from domain-forming lipid mixtures to asymmetric lipid bilayers. Here, we review recent progress in the development and application of elastic neutron and X-ray scattering techniques for studying these systems in situ and under physiologically relevant conditions on the nanometer to sub-nanometer length scales. In particular, we focus on: (1) structural details of coexisting liquid-ordered and liquid-disordered domains, including their thickness and lipid packing mismatch as a function of a size transition from nanoscopic to macroscopic domains; (2) membrane-mediated protein partitioning into lipid domains; (3) the role of the aqueous medium in tuning interactions between membranes and domains; and (4) leaflet-specific structure in asymmetric bilayers and passive lipid flip-flop.

The Affinity of Cholesterol for Different Phospholipids Affects Lateral Segregation in Bilayers

Saturated and unsaturated phospholipids (PLs) can segregate into lateral domains. The preference of cholesterol for saturated acyl chains over monounsaturated, and especially polyunsaturated ones, may also affect lateral segregation. Here we have studied how cholesterol influenced the lateral segregation of saturated and unsaturated PLs, for which cholesterol had a varying degree of affinity. The fluorescence lifetime of trans-parinaric acid reported the formation of ordered domains (gel or liquid-ordered (lo)) in bilayers composed of different unsaturated phosphatidylcholines, and dipalmitoyl-phosphatidylcholine or n-palmitoyl-sphingomyelin, in the presence or absence of cholesterol. We observed that cholesterol facilitated lateral segregations and the degree of facilitation correlated with the relative affinity of cholesterol for the different PLs in the bilayers. Differential scanning calorimetry and (2)H nuclear magnetic resonance showed that cholesterol increased the thermostability of both the gel and lo-domains. Increased number of double bonds in the unsaturated PL increased the order in the lo-domains, likely by enriching the ordered domains in saturated lipids and cholesterol. This supported the conclusions from the trans-parinaric acid experiments, and offers insight into how cholesterol facilitated lateral segregation. In conclusion, the relative affinity of cholesterol for different PLs appears to be an important determinant for the formation of ordered domains. Our data suggests that knowledge of the affinity of cholesterol for the different PLs in a bilayer allows prediction of the degree to which the sterol promotes lo-domain formation.

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.

Lateral pressure-mediated protein partitioning into liquid-ordered/liquid-disordered domains

We have studied the contributions of stored elastic energies in liquid-ordered (Lo) and liquid-disordered (Ld) domains to transmembrane proteins using the lateral pressure concept. In particular we applied previously reported experimental data for the membrane thickness, intrinsic curvature and bending elasticities of coexisting Lo/Ld domains to calculate whether proteins of simple geometric shapes would preferentially diffuse into Lo or Ld domains and form oligomers of a certain size. For the studied lipid mixture we generally found that proteins with convex shapes prefer sorting to Ld phases and the formation of large clusters. Lo domains in turn would be enriched in monomers of concave shaped proteins. We further observed that proteins which are symmetric with respect to the bilayer center prefer symmetric Lo or Ld domains, while asymmetric proteins favor a location in domains with Lo/Ld asymmetry. In the latter case we additionally retrieved a strong dependence on protein directionality, thus providing a mechanism for transmembrane protein orientation.

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.

Sub-ten-nanometer heterogeneity of solid supported lipid membranes determined by solution atomic force microscopy

Visually detecting nanoscopic structures in lipid membranes is important for elucidating lipid-lipid interactions, which are suggested to play a role in mediating membrane rafts. We use solution atomic force microscopy (AFM) to study lateral and normal organization in multicomponent lipid membranes supported by mica substrate. Nanoscopic heterogeneity is observed in a three-component system composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/brain-sphingomyelin (bSM)/cholesterol (Chol). We find sub-ten-nanometer correlation lengths that are used to describe membrane lateral organization. In addition, we find that the correlation length is independent on cholesterol concentration, while the height fluctuation (variation) is not. To explore the mechanism that controls the size of membrane heterogeneity, we extend our study to a four-component system composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/POPC/bSM/Chol. By systematically adjusting the relative amount of DOPC and POPC, we obtain macroscopic-to-nanoscopic size transition of membrane heterogeneity. In contrast to the results from vesicle based fluorescence microscopy, we find that the structural transition is continuous both in the lateral and normal directions. We compare our nanoscopic structures to two theoretical models, and find that both the critical fluctuations and the nanodomain models are not sufficient to account for our solution AFM data. Finally, we propose a nanoheterogeneity model that could serve as the organization principle of the observed nanoscopic structures in multicomponent lipid membranes.

Microemulsions, modulated phases and macroscopic phase separation: a unified picture of rafts

We consider two mechanisms that can lead to an inhomogeneous distribution of components in a multicomponent lipid bilayer: macroscopic phase separation and the formation of modulated phases. A simple model that encompasses both mechanisms displays a phase diagram that also includes a structured fluid, a microemulsion. Identifying rafts with the inhomogeneities of this structured fluid, we see how rafts are related to the occurrence of macroscopic phase separation or the formation of modulated phases in other systems, and focus our attention on specific differences between them.