Phase diagram of a polyunsaturated lipid mixture: Brain sphingomyelin/1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine/cholesterol

Cholesterol and Lipid Rafts in the Biogenesis of Amyloid-β Protein and Alzheimer's Disease

Cholesterol has been conjectured to be a modulator of the amyloid cascade, the mechanism that produces the amyloid-β (Aβ) peptides implicated in the onset of Alzheimer's disease. We propose that cholesterol impacts the genesis of Aβ not through direct interaction with proteins in the bilayer, but indirectly by inducing the liquid-ordered phase and accompanying liquid-liquid phase separations, which partition proteins in the amyloid cascade to different lipid domains and ultimately to different endocytotic pathways. We explore the full process of Aβ genesis in the context of liquid-ordered phases induced by cholesterol, including protein partitioning into lipid domains, mechanisms of endocytosis experienced by lipid domains and secretases, and pH-controlled activation of amyloid precursor protein secretases in specific endocytotic environments. Outstanding questions on the essential role of cholesterol in the amyloid cascade are identified for future studies.

Secondary Ion Mass Spectrometry of Single Giant Unilamellar Vesicles Reveals Compositional Variability

Giant unilamellar vesicles (GUVs) are a widely used model system to interrogate lipid phase behavior, study biomembrane mechanics, reconstitute membrane proteins, and provide a chassis for synthetic cells. It is generally assumed that the composition of individual GUVs is the same as the nominal stock composition; however, there may be significant compositional variability between individual GUVs. Although this compositional heterogeneity likely impacts phase behavior, the function and incorporation of membrane proteins, and the encapsulation of biochemical reactions, it has yet to be directly quantified. To assess heterogeneity, we use secondary ion mass spectrometry (SIMS) to probe the composition of individual GUVs using non-perturbing isotopic labels. Both 13C- and 2H-labeled lipids are incorporated into a ternary mixture, which is then used to produce GUVs via gentle hydration or electroformation. Simultaneous detection of seven different ion species via SIMS allows for the concentration of 13C- and 2H-labeled lipids in single GUVs to be quantified using calibration curves, which correlate ion intensity to composition. Additionally, the relative concentration of 13C- and 2H-labeled lipids is assessed for each GUV via the ion ratio 2H-/13C-, which is highly sensitive to compositional differences between individual GUVs and circumvents the need for calibration by using standards. Both quantification methods suggest that gentle hydration produces GUVs with greater compositional variability than those formed by electroformation. However, both gentle hydration and electroformation display standard deviations in composition (n = 30 GUVs) on the order of 1-4 mol %, consistent with variability seen in previous indirect measurements.

How Neuromembrane Lipids Modulate Membrane Proteins: Insights from G-Protein-Coupled Receptors (GPCRs) and Receptor Tyrosine Kinases (RTKs)

Lipids play a diverse and critical role in cellular processes in all tissues. The unique lipid composition of nerve membranes is particularly interesting because it contains, among other things, polyunsaturated lipids, such as docosahexaenoic acid, which the body only gets through the diet. The crucial role of lipids in neurological processes, especially in receptor-mediated cell signaling, is emphasized by the fact that in many neuropathological diseases there are significant deviations in the lipid composition of nerve membranes compared to healthy individuals. The lipid composition of neuromembranes can significantly affect the function of receptors by regulating the physical properties of the membrane or by affecting specific interactions between receptors and lipids. In addition, it is worth noting that the ligand-binding pocket of many receptors is located inside the cell membrane, due to which lipids can even modulate the binding of ligands to their receptors. These mechanisms highlight the importance of lipids in the regulation of membrane receptor activation and function. In this article, we focus on two major protein families: G-protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs) and discuss how lipids affect their function in neuronal membranes, elucidating the basic mechanisms underlying neuronal function and dysfunction.

Mimicking effects of cholesterol in lipid bilayer membranes by self-assembled amphiphilic block copolymers

The effect of cholesterol on biological membranes is important in biochemistry. In this study, a polymer system is used to simulate the consequences of varying cholesterol content in membranes. The system consists of an AB-diblock copolymer, a hydrophilic homopolymer hA, and a hydrophobic rigid homopolymer C, corresponding to phospholipid, water, and cholesterol, respectively. The effect of the C-polymer content on the membrane is studied within the framework of a self-consistent field model. The results show that the liquid-crystal behavior of B and C has a great influence on the chemical potential of cholesterol in bilayer membranes. The effects of the interaction strength between components, characterized by the Flory-Huggins parameters and the Maier-Saupe parameter, were studied. Some consequences of adding a coil headgroup to the C-rod are presented. Results of our model are compared to experimental findings for cholesterol-containing lipid bilayer membranes.

Vitamin E Does Not Disturb Polyunsaturated Fatty Acid Lipid Domains

The function of vitamin E in biomembranes remains a prominent topic of discussion. As its limitations as an antioxidant persist and novel functions are discovered, our understanding of the role of vitamin E becomes increasingly enigmatic. As a group of lipophilic molecules (tocopherols and tocotrienols), vitamin E has been shown to influence the properties of its host membrane, and a wealth of research has connected vitamin E to polyunsaturated fatty acid (PUFA) lipids. Here, we use contrast-matched small-angle neutron scattering and differential scanning calorimetry to integrate these fields by examining the influence of vitamin E on lipid domain stability in PUFA-based lipid mixtures. The influence of α-tocopherol, γ-tocopherol, and α-tocopherylquinone on the lateral organization of a 1:1 lipid mixture of saturated distearoylphosphatidylcholine (DSPC) and polyunsaturated palmitoyl-linoleoylphosphatidylcholine (PLiPC) with cholesterol provides a complement to our growing understanding of the influence of tocopherol on lipid phases. Characterization of domain melting suggests a slight depression in the transition temperature and a decrease in transition cooperativity. Tocopherol concentrations that are an order of magnitude higher than anticipated physiological concentrations (2 mol percent) do not significantly perturb lipid domains; however, addition of 10 mol percent is able to destabilize domains and promote lipid mixing. In contrast to this behavior, increasing concentrations of the oxidized product of α-tocopherol (α-tocopherylquinone) induces a proportional increase in domain stabilization. We speculate how the contrasting effect of the oxidized product may supplement the antioxidant response of vitamin E.

Polyene Antibiotics Physical Chemistry and Their Effect on Lipid Membranes; Impacting Biological Processes and Medical Applications

This review examined a collection of studies regarding the molecular properties of some polyene antibiotic molecules as well as their properties in solution and in particular environmental conditions. We also looked into the proposed mechanism of action of polyenes, where membrane properties play a crucial role. Given the interest in polyene antibiotics as therapeutic agents, we looked into alternative ways of reducing their collateral toxicity, including semi-synthesis of derivatives and new formulations. We follow with studies on the role of membrane structure and, finally, recent developments regarding the most important clinical applications of these compounds.

Order vs. Disorder: Cholesterol and Omega-3 Phospholipids Determine Biomembrane Organization

Lipid structural diversity strongly affects biomembrane chemico-physical and structural properties in addition to membrane-associated events. At high concentrations, cholesterol increases membrane order and rigidity, while polyunsaturated lipids are reported to increase disorder and flexibility. How these different tendencies balance in composite bilayers is still controversial. In this study, electron paramagnetic resonance spectroscopy, small angle neutron scattering, and neutron reflectivity were used to investigate the structural properties of cholesterol-containing lipid bilayers in the fluid state with increasing amounts of polyunsaturated omega-3 lipids. Either the hybrid 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine or the symmetric 1,2-docosahexaenoyl-sn-glycero-3-phosphocholine were added to the mixture of the naturally abundant 1-palmitoyl-2-oleyl-sn-glycero-3-phosphocholine and cholesterol. Our results indicate that the hybrid and the symmetric omega-3 phospholipids affect the microscopic organization of lipid bilayers differently. Cholesterol does not segregate from polyunsaturated phospholipids and, through interactions with them, is able to suppress the formation of non-lamellar structures induced by the symmetric polyunsaturated lipid. However, this order/disorder balance leads to a bilayer whose structural organization cannot be ascribed to either a liquid ordered or to a canonical liquid disordered phase, in that it displays a very loose packing of the intermediate segments of lipid chains.

Identifying Membrane Lateral Organization by Contrast-Matched Small Angle Neutron Scattering

Lipid domains in model membranes are routinely studied to provide insight into the physical interactions that drive raft formation in cellular membranes. Using small angle neutron scattering, contrast-matching techniques enable the detection of lipid domains ranging from tens to hundreds of nanometers which are not accessible to other techniques without the use of extrinsic probes. Here, we describe a probe-free experimental approach and model-free analysis to identify lipid domains in freely floating vesicles of ternary phase separating lipid mixtures.

Critical Phenomena in Plasma Membrane Organization and Function

Lateral organization in the plane of the plasma membrane is an important driver of biological processes. The past dozen years have seen increasing experimental support for the notion that lipid organization plays an important role in modulating this heterogeneity. Various biophysical mechanisms rooted in the concept of liquid-liquid phase separation have been proposed to explain diverse experimental observations of heterogeneity in model and cell membranes with distinct but overlapping applicability. In this review, we focus on the evidence for and the consequences of the hypothesis that the plasma membrane is poised near an equilibrium miscibility critical point. Critical phenomena explain certain features of the heterogeneity observed in cells and model systems but also go beyond heterogeneity to predict other interesting phenomena, including responses to perturbations in membrane composition.

Spicule movement on RBCs during echinocyte formation and possible segregation in the RBC membrane

We use phase contrast microscopy of red blood cells to observe the transition between the initial discocyte shape and a spiculated echinocyte form. During the early stages of this change, spicules can move across the surface of the cell; individual spicules can also split apart into pairs. One possible explanation of this behaviour is that the membrane forms large scale domains in association with the spicules. The spicules are formed initially at the rim of the cell and then move at speeds of up to 3 μm/min towards the centre of the disc. Spicule formation that was reversed and then allowed to proceed a second time resulted in spicules at reproducible places, a shape memory effect that implies that the cytoskeleton contributes towards stopping the spicule movement. The splitting of the spicules produces a well-defined shape change with an increase in membrane curvature associated with formation of the daughter pair of spicules; the total boundary length around the spicules also increases. Following the model in which the spicules are associated with lipid domains, these observations suggest an experimental procedure that could potentially be applied to the calculation of the line tension of lipid domains in living cells.

A machine learning approach to estimation of phase diagrams for three-component lipid mixtures

The plasma membrane of eukaryotic cells is commonly believed to contain ordered lipid domains. The interest in understanding the origin of such domains has led to extensive studies on the phase behavior of mixed lipid systems. Three-component phase diagrams, composed of a high melting temperature (Tm) lipid, cholesterol, and a low Tm lipid have been valuable in studying lipid phase behavior. However, developing phase diagrams over the entire composition space and with precise tie-lines requires significant experimental effort. In this study, a machine learning approach was used to predict the Tm of lipids and generate phase diagrams from lipid mixtures. First, artificial neural network (ANN) was used for the prediction of Tm. The network was trained using available Tm data and was able to generate Tm values that closely matched literature results for its testing dataset. This model was then used to predict the Tm for lipids that have not yet been experimentally tested. Then, random forests (RF) and support vector machines (SVM) were trained and tested for their ability to predict a test three-component phase diagram. The model from the RF algorithm was able to generate a diagram that closely matched published results. This model was then used to generate phase diagrams for lipid mixtures at various temperatures and various degrees of unsaturation. This machine learning approach to the generation of lipid phase diagrams has the potential to save significant time and resources in studies of lipid phase behavior.

The antioxidant vitamin E as a membrane raft modulator: Tocopherols do not abolish lipid domains

The antioxidant vitamin E is a commonly used vitamin supplement. Although the multi-billion dollar vitamin and nutritional supplement industry encourages the use of vitamin E, there is very little evidence supporting its actual health benefits. Moreover, vitamin E is now marketed as a lipid raft destabilizing anti-cancer agent, in addition to its antioxidant behaviour. Here, we studied the influence of vitamin E and some of its vitamers on membrane raft stability using phase separating unilamellar lipid vesicles in conjunction with small-angle scattering techniques and fluorescence microscopy. We find that lipid phase behaviour remains unperturbed well beyond physiological concentrations of vitamin E (up to a mole fraction of 0.10). Our results are consistent with a proposed line active role of vitamin E at the domain boundary. We discuss the implications of these findings as they pertain to lipid raft modification in native membranes, and propose a new hypothesis for the antioxidant mechanism of vitamin E.

Principles and Applications of Biological Membrane Organization

Many critical biological events, including biochemical signaling, membrane traffic, and cell motility, originate at membrane surfaces. Each such event requires that members of a specific group of proteins and lipids rapidly assemble together at a specific site on the membrane surface. Understanding the biophysical mechanisms that stabilize these assemblies is critical to decoding and controlling cellular functions. In this article, we review progress toward a quantitative biophysical understanding of the mechanisms that drive membrane heterogeneity and organization. We begin from a physical perspective, reviewing the fundamental principles and key experimental evidence behind each proposed mechanism. We then shift to a biological perspective, presenting key examples of the role of heterogeneity in biology and asking which physical mechanisms may be responsible. We close with an applied perspective, noting that membrane heterogeneity provides a novel therapeutic target that is being exploited by a growing number of studies at the interface of biology, physics, and engineering.

Phase separation in pore-spanning membranes induced by differences in surface adhesion

A porous scaffold providing different adhesion energies alters the behaviour of coexisting phases in lipid membranes considerably.

Synthesis of Gb3 Glycosphingolipids with Labeled Head Groups: Distribution in Phase-Separated Giant Unilamellar Vesicles

The receptor lipid Gb3 is responsible for the specific internalization of Shiga toxin (STx) into cells. The head group of Gb3 defines the specificity of STx binding, and the backbone with different fatty acids is expected to influence its localization within membranes impacting membrane organization and protein internalization. To investigate this influence, a set of Gb3 glycosphingolipids labeled with a BODIPY fluorophore attached to the head group was synthesized. C24 fatty acids, saturated, unsaturated, α-hydroxylated derivatives, and a combination thereof, were attached to the sphingosine backbone. The synthetic Gb3 glycosphingolipids were reconstituted into coexisting liquid-ordered (lo )/liquid-disordered (ld ) giant unilamellar vesicles (GUVs), and STx binding was verified by fluorescence microscopy. Gb3 with the C24:0 fatty acid partitioned mostly in the lo phase, while the unsaturated C24:1 fatty acid distributes more into the ld phase. The α-hydroxylation does not influence its partitioning.

Impact of Acyl Chain Mismatch on the Formation and Properties of Sphingomyelin-Cholesterol Domains

Lateral segregation and the formation of lateral domains are well-known phenomena in ternary lipid bilayers composed of an unsaturated (low gel-to-liquid phase transition temperature (T_m)) phospholipid, a saturated (high-T_m) phospholipid, and cholesterol. The formation of lateral domains has been shown to be influenced by differences in phospholipid acyl chain unsaturation and length. Recently, we also showed that differential interactions of cholesterol with low- and high-T_m phospholipids in the bilayer can facilitate phospholipid segregation. Now, we have investigated phospholipid-cholesterol interactions and their role in lateral segregation in ternary bilayers composed of different unsaturated phosphatidylcholines (PCs) with varying acyl chain lengths, N-palmitoyl-D-erythro-sphingomyelin (PSM), and cholesterol. Using deuterium NMR spectroscopy, we determined how PSM was influenced by the acyl chain composition in surrounding PC environments and correlated this with the affinity of cholestatrienol (a fluorescent cholesterol analog) for PSM in the different PC environments. Results from a combination of time-resolved fluorescence measurements of trans-parinaric acid and Förster resonance energy transfer experiments showed that the relative affinity of cholesterol for phospholipids determined the degree to which the sterol promoted domain formation. From Förster resonance energy transfer, deuterium NMR, and differential scanning calorimetry results, it was clear that cholesterol also influenced both the thermostability of the domains and the degree of order in and outside the PSM-rich domains. The results of this study have shown that the affinity of cholesterol for both low-T_m and high-T_m phospholipids and the effects of low- and high-T_m phospholipids on each other influence both lateral structure and domain properties in complex bilayers. We envision that similar effects also contribute to lateral heterogeneity in even more complex biological membranes.

FRET Detects the Size of Nanodomains for Coexisting Liquid-Disordered and Liquid-Ordered Phases

Biomembranes with as few as three lipid components can form coexisting liquid-disordered (Ld) and liquid-ordered (Lo) phases. In the coexistence region of Ld and Lo phases, the lipid mixtures 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC)/1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/chol or brain sphingomyelin (bSM)/DOPC/chol form micron-scale domains that are easily visualized with light microscopy. Although large domains are not observed in the mixtures DSPC/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/chol and bSM/POPC/chol, lateral heterogeneity is nevertheless detected using techniques with nanometer-scale spatial resolution. We propose a simple and accessible method to measure domain sizes below optical resolution (∼200 nm). We measured nanodomain size for the latter two mixtures by combining experimental Förster resonance energy transfer data with a Monte-Carlo-based analysis. We found a domain radius of 7.5-10 nm for DSPC/POPC/chol, similar to values obtained previously by neutron scattering, and ∼5 nm for bSM/POPC/chol, slightly smaller than measurable by neutron scattering. These analyses also detect the domain-size transition that is observed by fluorescence microscopy in the four-component lipid mixture bSM/DOPC/POPC/chol. Accurate measurements of fluorescent-probe partition coefficients are especially important for the analysis; therefore, we exploit three different methods to measure the partition coefficient of fluorescent molecules between Ld and Lo phases.

Reduced level of docosahexaenoic acid shifts GPCR neuroreceptors to less ordered membrane regions

G protein-coupled receptors (GPCRs) control cellular signaling and responses. Many of these GPCRs are modulated by cholesterol and polyunsaturated fatty acids (PUFAs) which have been shown to co-exist with saturated lipids in ordered membrane domains. However, the lipid compositions of such domains extracted from the brain cortex tissue of individuals suffering from GPCR-associated neurological disorders show drastically lowered levels of PUFAs. Here, using free energy techniques and multiscale simulations of numerous membrane proteins, we show that the presence of the PUFA DHA helps helical multi-pass proteins such as GPCRs partition into ordered membrane domains. The mechanism is based on hybrid lipids, whose PUFA chains coat the rough protein surface, while the saturated chains face the raft environment, thus minimizing perturbations therein. Our findings suggest that the reduction of GPCR partitioning to their native ordered environments due to PUFA depletion might affect the function of these receptors in numerous neurodegenerative diseases, where the membrane PUFA levels in the brain are decreased. We hope that this work inspires experimental studies on the connection between membrane PUFA levels and GPCR signaling.

Our current picture of cellular membranes depicts them as laterally heterogeneous sheets of lipids crowded with membrane proteins. These proteins often require a specific lipid environment to efficiently perform their functions. Certain neuroreceptor proteins are regulated by membrane cholesterol that is considered to be enriched in ordered membrane domains. In the brain, these very same domains also contain a fair amount of polyunsaturated fatty acids (PUFAs) that have also been discovered to interact favorably with many receptor proteins. However, certain neurological diseases—associated with the inadequate functioning of the neuroreceptors—seem to result in the decrease of brain PUFA levels. We hypothesized that this decrease in PUFA levels somehow inhibits receptor partitioning to cholesterol-rich domains, which could further compromise their function. We verified our hypothesis by an extensive set of computer simulations. They demonstrated that the PUFA–receptor interaction indeed leads to favorable partitioning of the receptors in the cholesterol-rich ordered domains. Moreover, the underlying mechanism based on the shielding of the rough protein surface by the PUFAs seems to be exclusive for multi-helical protein structures, of which neuroreceptors are a prime example.

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.

Complex Phase Behavior of GUVs Containing Different Sphingomyelins

Understanding the lateral organization of biological membranes plays a key role on the road to fully appreciate the physiological functions of this fundamental barrier between the inside and outside regions of a cell. Ternary lipid bilayers composed of a high and a low melting temperature lipid and cholesterol represent a model system that mimics some of the important thermodynamical features of much more complex lipid mixtures such as those found in mammal membranes. The phase diagram of these ternary mixtures can be studied exploiting fluorescence microscopy in giant unilamellar vesicles, and it is typically expected to give rise, for specific combinations of composition and temperature, to regions of two-phase coexistence and a region with three-phase coexistence, namely, the liquid-ordered, liquid-disordered, and solid phases. Whereas the observation of two-phase coexistence is routinely possible using fluorescence microscopy, the three-phase region is more elusive to study. In this article, we show that particular lipid mixtures containing diphytanoyl-phosphatidylcholine and cholesterol plus different types of sphingomyelin (SM) are prone to produce bilayer regions with more than two levels of fluorescence intensity. We found that these intensity levels occur at low temperature and are linked to the copresence of long and asymmetric chains in SMs and diphytanoyl-phosphatidylcholine in the lipid mixtures. We discuss the possible interpretations for this observation in terms of bilayer phase organization in the presence of sphingolipids. Additionally, we also show that in some cases, liposomes in the three-phase coexistence state exhibit extreme sensitivity to lateral tension. We hypothesize that the appearance of the different phases is related to the asymmetric structure of SMs and to interdigitation effects.

Flexible lipid nanomaterials studied by NMR spectroscopy

Advances in solid-state nuclear magnetic resonance spectroscopy inform the emergence of material properties from atomistic-level interactions in membrane lipid nanostructures.

Rapid diffusion of cholesterol along polyunsaturated membranes via deep dives

Cholesterol regulates the function of membrane proteins either via direct or membrane-mediated effects. Therefore, its ready availability is crucial for many protein-governed cellular processes. Recent studies suggest that cholesterol can partition to the core of polyunsaturated membranes, where cholesterol binding sites of many membrane proteins are also located. This core region is characterized by a lower viscosity. Therefore, we hypothesized that cholesterol partitioning into the membrane interior increases the rate of its diffusion in polyunsaturated membrane environments. We studied the behavior of cholesterol in membranes with increasing level of lipid chain unsaturation using a combination of atomistic and coarse-grained molecular dynamics simulations. Our simulations suggest a strong correlation between entropy-driven enhanced cholesterol partitioning to the membrane core and its faster lateral diffusion, which indicates that the less viscous membrane core indeed provides an efficient means for cholesterol movement in polyunsaturated membrane environments.

Cholesterol Effects on the Physical Properties of Lipid Membranes Viewed by Solid-state NMR Spectroscopy

In this chapter, we review the physical properties of lipid/cholesterol mixtures involving studies of model membranes using solid-state NMR spectroscopy. The approach allows one to quantify the average membrane structure, fluctuations, and elastic deformation upon cholesterol interaction. Emphasis is placed on understanding the membrane structural deformation and emergent fluctuations at an atomistic level. Lineshape measurements using solid-state NMR spectroscopy give equilibrium structural properties, while relaxation time measurements study the molecular dynamics over a wide timescale range. The equilibrium properties of glycerophospholipids, sphingolipids, and their binary and tertiary mixtures with cholesterol are accessible. Nonideal mixing of cholesterol with other lipids explains the occurrence of liquid-ordered domains. The entropic loss upon addition of cholesterol to sphingolipids is less than for glycerophospholipids, and may drive formation of lipid rafts. The functional dependence of ²H NMR spin-lattice relaxation (R _1Z) rates on segmental order parameters (S _CD) for lipid membranes is indicative of emergent viscoelastic properties. Addition of cholesterol shows stiffening of the bilayer relative to the pure lipids and this effect is diminished for lanosterol. Opposite influences of cholesterol and detergents on collective dynamics and elasticity at an atomistic scale can potentially affect lipid raft formation in cellular membranes.

Cholesterol in phospholipid bilayers: positions and orientations inside membranes with different unsaturation degrees

Cholesterol is an essential component of all animal cell membranes and plays an important role in maintaining the membrane structure and physical-chemical properties necessary for correct cell functioning. The presence of cholesterol is believed to be responsible for domain formation (lipid rafts) due to different interactions of cholesterol with saturated and unsaturated lipids. In order to get detailed atomistic insight into the behaviour of cholesterol in bilayers composed of lipids with varying degrees of unsaturation, we have carried out a series of molecular dynamics simulations of saturated and polyunsaturated lipid bilayers with different contents of cholesterol, as well as well-tempered metadynamics simulations with a single cholesterol molecule in these bilayers. From these simulations we have determined distributions of cholesterol across the bilayer, its orientational properties, free energy profiles, and specific interactions of molecular groups able to form hydrogen bonds. Both molecular dynamics and metadynamics simulations showed that the most unsaturated bilayer with 22:6 fatty acid chains shows behaviour which is most different from other lipids. In this bilayer, cholesterol is relatively often found in a "flipped" configuration with the hydroxyl group oriented towards the membrane middle plane. This bilayer has also the highest (least negative) binding free energy among liquid phase bilayers, and the lowest reorientation barrier. Furthermore, cholesterol molecules in this bilayer are often found to form head-to-tail contacts which may lead to specific clustering behaviour. Overall, our simulations support ideas that there can be a subtle interconnection between the contents of highly unsaturated fatty acids and cholesterol, deficiency or excess of each of them is related to many human afflictions and diseases.

The Affinity of Sterols for Different Phospholipid Classes and Its Impact on Lateral Segregation

Cholesterol is an essential molecule in the membranes of mammalian cells. It is known to be distributed heterogeneously within the cells, between the bilayer leaflets, as well as between lateral domains within the bilayer. However, we do not know exactly how cholesterol is distributed and what forces drive this sorting process because it extremely difficult to study using currently available methods. To further elucidate this distribution, we measured how cholesterol partitions between different phospholipid (PL) environments using different methods based on cholesterol, TopFluor-cholesterol, and cholesta-5,7,9(11)-triene-3-β-ol. Based on the obtained relative partition coefficients, we made predictions regarding how cholesterol would be distributed between lateral domains and between the inner and outer leaflets of the plasma membrane. In addition, using a trans-parinaric acid fluorescence-based method, we tested how cholesterol could influence lateral segregation through its interaction with unsaturated PLs with different headgroups. The results showed that the lower the affinity of cholesterol was for the different unsaturated PLs, the more cholesterol stimulated lateral segregation in a ternary bilayer of unsaturated PL/N-palmitoyl-D-erythro-sphingomyelin and cholesterol. Overall, the results indicate that both the distribution of cholesterol between different lipid environments and the impact of cholesterol on lateral segregation can be predicted relatively accurately from determined relative partition coefficients.

Membrane Lipid Nanodomains

Lipid membranes can spontaneously organize their components into domains of different sizes and properties. The organization of membrane lipids into nanodomains might potentially play a role in vital functions of cells and organisms. Model membranes represent attractive systems to study lipid nanodomains, which cannot be directly addressed in living cells with the currently available methods. This review summarizes the knowledge on lipid nanodomains in model membranes and exposes how their specific character contrasts with large-scale phase separation. The overview on lipid nanodomains in membranes composed of diverse lipids (e.g., zwitterionic and anionic glycerophospholipids, ceramides, glycosphingolipids) and cholesterol aims to evidence the impact of chemical, electrostatic, and geometric properties of lipids on nanodomain formation. Furthermore, the effects of curvature, asymmetry, and ions on membrane nanodomains are shown to be highly relevant aspects that may also modulate lipid nanodomains in cellular membranes. Potential mechanisms responsible for the formation and dynamics of nanodomains are discussed with support from available theories and computational studies. A brief description of current fluorescence techniques and analytical tools that enabled progress in lipid nanodomain studies is also included. Further directions are proposed to successfully extend this research to cells.

Regimes of Complex Lipid Bilayer Phases Induced by Cholesterol Concentration in MD Simulation

Cholesterol is essential to the formation of phase-separated lipid domains in membranes. Lipid domains can exist in different thermodynamic phases depending on the molecular composition and play significant roles in determining structure and function of membrane proteins. We investigate the role of cholesterol in the structure and dynamics of ternary lipid mixtures displaying phase separation using molecular dynamics simulations, employing a physiologically relevant span of cholesterol concentration. We find that cholesterol can induce formation of three regimes of phase behavior: 1) miscible liquid-disordered bulk, 2) phase-separated, domain-registered coexistence of liquid-disordered and liquid-ordered domains, and 3) phase-separated, domain-antiregistered coexistence of liquid-disordered and newly identified nanoscopic gel domains composed of cholesterol threads we name "cholesterolic gel" domains. These findings are validated and discussed in the context of current experimental knowledge, models of cholesterol spatial distributions, and models of ternary lipid-mixture phase separation.

Docosahexaenoic acid regulates the formation of lipid rafts: A unified view from experiment and simulation

Docosahexaenoic acid (DHA, 22:6) is an n-3 polyunsaturated fatty acid (n-3 PUFA) that influences immunological, metabolic, and neurological responses through complex mechanisms. One structural mechanism by which DHA exerts its biological effects is through its ability to modify the physical organization of plasma membrane signaling assemblies known as sphingomyelin/cholesterol (SM/chol)-enriched lipid rafts. Here we studied how DHA acyl chains esterified in the sn-2 position of phosphatidylcholine (PC) regulate the formation of raft and non-raft domains in mixtures with SM and chol on differing size scales. Coarse grained molecular dynamics simulations showed that 1-palmitoyl-2-docosahexaenoylphosphatylcholine (PDPC) enhances segregation into domains more than the monounsaturated control, 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC). Solid state ²H NMR and neutron scattering experiments provided direct experimental evidence that substituting PDPC for POPC increases the size of raft-like domains on the nanoscale. Confocal imaging of giant unilamellar vesicles with a non-raft fluorescent probe revealed that POPC had no influence on phase separation in the presence of SM/chol whereas PDPC drove strong domain segregation. Finally, monolayer compression studies suggest that PDPC increases lipid-lipid immiscibility in the presence of SM/chol compared to POPC. Collectively, the data across model systems provide compelling support for the emerging model that DHA acyl chains of PC lipids tune the size of lipid rafts, which has potential implications for signaling networks that rely on the compartmentalization of proteins within and outside of rafts.

Computation of a Theoretical Membrane Phase Diagram and the Role of Phase in Lipid-Raft-Mediated Protein Organization

Lipid phase heterogeneity in the plasma membrane is thought to be crucial for many aspects of cell signaling, but the physical basis of participating membrane domains such as "lipid rafts" remains controversial. Here we consider a lattice model yielding a phase diagram that includes several states proposed to be relevant for the cell membrane, including microemulsion-which can be related to membrane curvature-and Ising critical behavior. Using a neural-network-based machine learning approach, we compute the full phase diagram of this lattice model. We analyze selected regions of this phase diagram in the context of a signaling initiation event in mast cells: recruitment of the membrane-anchored tyrosine kinase Lyn to a cluster of transmembrane IgE-FcεRI receptors. We find that model membrane systems in microemulsion and Ising critical states can mediate roughly equal levels of kinase recruitment (binding energy ∼ -0.6 k_B T), whereas a membrane near a tricritical point can mediate a much stronger kinase recruitment (-1.7 k_B T). By comparing several models for lipid heterogeneity within a single theoretical framework, this work points to testable differences between existing models. We also suggest the tricritical point as a new possibility for the basis of membrane domains that facilitate preferential partitioning of signaling components.

Concepts and Methods of Solid-State NMR Spectroscopy Applied to Biomembranes

Concepts of solid-state NMR spectroscopy and applications to fluid membranes are reviewed in this paper. Membrane lipids with ²H-labeled acyl chains or polar head groups are studied using ²H NMR to yield knowledge of their atomistic structures in relation to equilibrium properties. This review demonstrates the principles and applications of solid-state NMR by unifying dipolar and quadrupolar interactions and highlights the unique features offered by solid-state ²H NMR with experimental illustrations. For randomly oriented multilamellar lipids or aligned membranes, solid-state ²H NMR enables direct measurement of residual quadrupolar couplings (RQCs) due to individual C-²H-labeled segments. The distribution of RQC values gives nearly complete profiles of the segmental order parameters S_CD⁽ⁱ⁾ as a function of acyl segment position (i). Alternatively, one can measure residual dipolar couplings (RDCs) for natural abundance lipid samples to obtain segmental S_CH order parameters. A theoretical mean-torque model provides acyl-packing profiles representing the cumulative chain extension along the normal to the aqueous interface. Equilibrium structural properties of fluid bilayers and various thermodynamic quantities can then be calculated, which describe the interactions with cholesterol, detergents, peptides, and integral membrane proteins and formation of lipid rafts. One can also obtain direct information for membrane-bound peptides or proteins by measuring RDCs using magic-angle spinning (MAS) in combination with dipolar recoupling methods. Solid-state NMR methods have been extensively applied to characterize model membranes and membrane-bound peptides and proteins, giving unique information on their conformations, orientations, and interactions in the natural liquid-crystalline state.

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.

Ordered Membrane Domain-Forming Properties of the Lipids of Borrelia burgdorferi

Co-existing disordered and ordered (raft) membrane domains exist in Borrelia burgdorferi, the causative agent of Lyme disease. However, although B. burgdorferi contains cholesterol lipids, it lacks sphingolipids-a crucial component of rafts in eukaryotes. To define the principles of ordered lipid domain formation in Borrelia, the domain forming properties of vesicles composed of its three major lipids, acylated cholesteryl galactoside (ACGal), monogalactosyl diacyglycerol (MGalD), and phosphatidylcholine (PC) and/or their mixtures were studied. Anisotropy and fluorescence resonance energy transfer measurements were used to assay membrane order and ordered-domain formation. ACGal had the highest potential to form ordered domains. Interestingly, mixtures of ACGal with B. burgdorferi PC formed ordered domains more readily than mixtures of ACGal with MGalD. This appears to reflect the relatively high level of saturation observed for B. burgdorferi PC, as vesicles containing ACGal and PC, but in which the unsaturated lipid dioleoyl PC was substituted for Borrelia PC, failed to form ordered domains. In addition, the properties of ACGal were compared to those of cholesterol. Depending on what other lipids were present, ordered-domain formation in the presence of ACGal was greater than or equal to that in the presence of cholesterol. Giant unilamellar vesicles formed from ACGal-containing mixtures showed rounded domain shapes similar to those in analogous vesicles containing cholesterol, indicative of liquid-ordered state rather than solid-like gel-state domain formation. Over all, principles of ordered-domain formation in B. burgdorferi appear to be very similar to those in eukaryotes, with saturated PC taking the place of sphingolipids, but with ACGal being the main lipid component inducing ordered-domain formation.

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.

Line Tension Controls Liquid-Disordered + Liquid-Ordered Domain Size Transition in Lipid Bilayers

To better understand animal cell plasma membranes, we studied simplified models, namely four-component lipid bilayer mixtures. Here we describe the domain size transition in the region of coexisting liquid-disordered (Ld) + liquid-ordered (Lo) phases. This transition occurs abruptly in composition space with domains increasing in size by two orders of magnitude, from tens of nanometers to microns. We measured the line tension between coexisting Ld and Lo domains close to the domain size transition for a variety of lipid mixtures, finding that in every case the transition occurs at a line tension of ∼0.3 pN. A computational model incorporating line tension and dipole repulsion indicated that even small changes in line tension can result in domains growing in size by several orders of magnitude, consistent with experimental observations. We find that other properties of the coexisting Ld and Lo phases do not change significantly in the vicinity of the abrupt domain size transition.

Polyunsaturated chains in asymmetric lipids disorder raft mixtures and preferentially associate with α-Synuclein

Using molecular dynamics simulations, we have explored the effect of asymmetric lipids-specifically those that contain one polyunsaturated (PUFA) and one saturated fatty acid chain-on phase separation in heterogeneous membranes. These lipids are prevalent in neuronal membranes, particularly in synaptic membranes, where the Parkinson's Disease protein α-Synuclein (αS) is found. We have therefore explored the relationship between asymmetric, PUFA-containing lipids, and αS. The simulations show that asymmetric lipids partition to the liquid disordered (Ld) phase of canonical raft mixtures because of the highly disordered PUFA chain. In the case of a membrane built to mimic the lipid composition of a synaptic vesicle, the PUFA-containing asymmetric lipids completely disrupt phase separation. Because αS is positively charged, we show that it partitions with negatively charged lipids, regardless of the saturation state of the chains. Additionally, αS preferentially associates with the polyunsaturated fatty acid tails of both charged and neutral lipids. This is a consequence of those chains' ability to accommodate the void beneath the amphipathic helix. This article is part of a Special Issue entitled: Lipid order/lipid defects and lipid-control of protein activity edited by Dirk Schneider.

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.

Nanoviscosity Measurements Revealing Domain Formation in Biomimetic Membranes

Partitioning of lipid molecules in biomimetic membranes is a model system for the study of naturally occurring domains, such as rafts, in biological membranes. The existence of nanometer scale membrane domains in binary lipid mixtures has been shown with microscopy methods; however, the nature of these domains has not been established unequivocally. A common notion is to ascribe domain separation to thermodynamic phase equilibria. However, characterizing thermodynamic phases of single bilayer membranes has not been possible due to their extreme dimensions: the size of the domains falls to the order of tens to hundreds of nanometers whereas the membrane thickness is only a few nanometers. Here, we present direct measurements of phase transitions in single bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)/1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) phospholipid mixtures using quartz crystal microbalance-based nanoviscosity measurements. Coexisting thermodynamic phases have been successfully identified, and a phase diagram was constructed for the single bilayer binary lipid system. It was demonstrated that domain separation only takes place in planar membranes, and thus, it is absent in liposomes and not detectable in calorimetric measurements on liposome suspensions. On the basis of energetic analysis, the main transition was identified as the breaking of van der Waals interactions between the acyl chains.

Analytical approaches to study domain formation in biomimetic membranes

Novel characterization methods open new horizons in the study of membrane mixtures.

Domain Stability in Biomimetic Membranes Driven by Lipid Polyunsaturation

Biological membranes contain a broad variety of lipid species whose individual physicochemical properties and collective interactions ultimately determine membrane organization. A key aspect of the organization of cellular membranes is their lateral subdivision into domains of distinct structure and composition. The most widely studied membrane domains are lipid rafts, which are the biological manifestations of liquid-ordered phases that form in sterol-containing membranes. Detailed studies of biomimetic membrane mixtures have yielded wide-ranging insights into the physical principles behind lipid rafts; however, these simplified models do not fully capture the diversity and complexity of the mammalian lipidome, most notably in their exclusion of polyunsaturated lipids. Here, we assess the role of lipid acyl chain unsaturation as a driving force for phase separation using coarse-grained molecular dynamics (CGMD) simulations validated by model membrane experiments. The clear trends in our observations and good qualitative agreements between simulations and experiments support the conclusions that highly unsaturated lipids promote liquid-liquid domain stability by enhancing the differences in cholesterol content and lipid chain order between the coexisting domains. These observations reveal the important role of noncanonical biological lipids in the physical properties of membranes, showing that lipid polyunsaturation is a driving force for liquid-liquid phase separation.

Effects of Membrane Charge and Order on Membrane Binding of the Retroviral Structural Protein Gag

The retroviral structural protein Gag binds to the inner leaflet of the plasma membrane (PM), and many cellular proteins do so as well. We used Rous sarcoma virus (RSV) Gag together with membrane sensors to study the principles governing peripheral protein membrane binding, including electrostatics, specific recognition of phospholipid headgroups, sensitivity to phospholipid acyl chain compositions, preference for membrane order, and protein multimerization. We used an in vitro liposome-pelleting assay to test protein membrane binding properties of Gag, the well-characterized MARCKS peptide, a series of fluorescent electrostatic sensor proteins (mNG-KRn), and the specific phosphatidylserine (PS) binding protein Evectin2. RSV Gag and mNG-KRn bound well to membranes with saturated and unsaturated acyl chains, whereas the MARCKS peptide and Evectin2 preferentially bound to membranes with unsaturated acyl chains. To further discriminate whether the primary driving force for Gag membrane binding is electrostatic interactions or preference for membrane order, we measured protein binding to giant unilamellar vesicles (GUVs) containing the same PS concentration in both disordered (Ld) and ordered (Lo) phases. RSV Gag and mNG-KRn membrane association followed membrane charge, independent of membrane order. Consistent with pelleting data, the MARCKS peptide showed preference for the Ld domain. Surprisingly, the PS sensor Evectin2 bound to the PS-rich Ld domain with 10-fold greater affinity than to the PS-rich Lo domain. In summary, we found that RSV Gag shows no preference for membrane order, while proteins with reported membrane-penetrating domains show preference for disordered membranes.

IMPORTANCE

Retroviral particles assemble on the PM and bud from infected cells. Our understanding of how Gag interacts with the PM and how different membrane properties contribute to overall Gag assembly is incomplete. This study examined how membrane charge and membrane order influence Gag membrane association. Consistent with previous work on RSV Gag, we report here that electrostatic interactions provide the primary driving force for RSV Gag membrane association. Using phase-separated GUVs with known lipid composition of the Ld and Lo phases, we demonstrate for the first time that RSV Gag is sensitive to membrane charge but not membrane order. In contrast, the cellular protein domain MARCKS and the PS sensor Evectin2 show preference for disordered membranes. We also demonstrate how to define GUV phase composition, which could serve as a tool in future studies of protein membrane interactions.

Cholesterol-induced suppression of membrane elastic fluctuations at the atomistic level

Applications of solid-state NMR spectroscopy for investigating the influences of lipid-cholesterol interactions on membrane fluctuations are reviewed in this paper. Emphasis is placed on understanding the energy landscapes and fluctuations at an emergent atomistic level. Solid-state (2)H NMR spectroscopy directly measures residual quadrupolar couplings (RQCs) due to individual C-(2)H labeled segments of the lipid molecules. Moreover, residual dipolar couplings (RDCs) of (13)C-(1)H bonds are obtained in separated local-field NMR spectroscopy. The distributions of RQC or RDC values give nearly complete profiles of the order parameters as a function of acyl segment position. Measured equilibrium properties of glycerophospholipids and sphingolipids including their binary and tertiary mixtures with cholesterol show unequal mixing associated with liquid-ordered domains. The entropic loss upon addition of cholesterol to sphingolipids is less than for glycerophospholipids and may drive the formation of lipid rafts. In addition relaxation time measurements enable one to study the molecular dynamics over a wide time-scale range. For (2)H NMR the experimental spin-lattice (R1Z) relaxation rates follow a theoretical square-law dependence on segmental order parameters (SCD) due to collective slow dynamics over mesoscopic length scales. The functional dependence for the liquid-crystalline lipid membranes is indicative of viscoelastic properties as they emerge from atomistic-level interactions. A striking decrease in square-law slope upon addition of cholesterol denotes stiffening relative to the pure lipid bilayers that is diminished in the case of lanosterol. Measured equilibrium properties and relaxation rates infer opposite influences of cholesterol and detergents on collective dynamics and elasticity at an atomistic scale that potentially affects lipid raft formation in cellular membranes.

Phospholipase A2-Induced Remodeling Processes on Liquid-Ordered/Liquid-Disordered Membranes Containing Docosahexaenoic or Oleic Acid: A Comparison Study

Vesicle cycling, which is an important biological event, involves the interplay between membrane lipids and proteins, among which the enzyme phospholipase A2 (PLA2) plays a critical role. The capacity of PLA2 to trigger the budding and fission of liquid-ordered (L(o)) domains has been examined in palmitoyl-docosahexaenoylphosphatidylcholine (PDPC) and palmitoyl-oleoylphosphatidylcholine (POPC)/sphingomyelin/cholesterol membranes. They both exhibited a L(o)/liquid-disordered (L(d)) phase separation. We demonstrated that PLA2 was able to trigger budding in PDPC-containing vesicles but not POPC ones. The enzymatic activity, line tension, and elasticity of the membrane surrounding the L(o) domains are critical for budding. The higher line tension of Lo domains in PDPC mixtures was assigned to the greater difference in order parameters of the coexisting phases. The higher amount of lysophosphatidylcholine generated by PLA2 in the PDPC-containing mixtures led to a less-rigid membrane, compared to POPC. The more elastic L(d) membranes in PDPC mixtures exert a lower counteracting force against the L(o) domain bending.