Interactions of cholesterol with lipid membranes and cyclodextrin characterized by calorimetry

Reversible tuning of membrane sterol levels by cyclodextrin in a dialysis setting

Large unilamellar vesicles are popular membrane models for studying the impact of lipids and bilayer properties on the structure and function of transmembrane proteins. However, the functional reconstitution of transmembrane proteins in liposomes can be challenging, especially if the hydrophobic thickness of the protein does not match the thickness of the lipid bilayer. Such hydrophobic mismatch causes protein aggregation and low yields during the reconstitution procedure, which are exacerbated in sterol-rich membranes featuring low membrane compressibility. Here, we explore new approaches to reversibly tune the sterol content of (proteo)liposomes with methyl-β-cyclodextrin (mβCD) in a dialysis setting. Maintaining (proteo)liposomes in a confined compartment minimizes loss of material during cholesterol transfer and facilitates efficient removal of mβCD. We monitor the sterol concentration in the membrane with help of the solvatochromic probe C-Laurdan, which reports on lipid packing. Using Förster resonance energy transfer, we show that cholesterol delivery to proteoliposomes induces the oligomerization of a membrane property sensor, whereas a subsequent removal of cholesterol demonstrates full reversibility. We propose that tuning membrane compressibility by mβCD-meditated cholesterol delivery and removal in a dialysis setup provides a new handle to study the impact of sterols and membrane compressibility on membrane protein structure, function, and dynamics.

Asymmetric phase transitions in lipid bilayers: coupling or bending?

Biomembranes show asymmetric lipid composition of their two leaflets. The phenomenon that ordered domains in one leaflet may affect the order of the other has been referred to as interleaflet coupling and discussed in terms of transmembrane signaling. Many coupling mechanisms have been proposed; one potential mechanism should arise from the fact that the isolated melting of an ordered, e.g., gel phase gives rise to a significant expansion of this leaflet, resulting in a mismatch between the intrinsic areas of the leaflets. This asymmetry stress can be accommodated in a number of ways. One is interleaflet coupling - individually higher- and lower-melting leaflets melt together at intermediate melting temperature. Alternatively, the membrane may bend towards the larger-intrinsic-area leaflet, bud and release very small daughter vesicles (DVs). Here, we prepared lipid-asymmetric large unilamellar vesicles (aLUVs) with low-melting stearyl-oleyl-phosphatidylcholine (SOPC) in the inner and SOPC with ∼20 mol% of high-melting dipalmitoyl phosphatidylglycerol (DPPG) in the outer leaflet. Phase transitions in aLUVs versus LUVs were recorded with pressure perturbation calorimetry; vesicle budding was monitored by asymmetric flow field-flow fractionation (AF4) and visualized by cryo-transmission electron microscopy. An HPLC protocol was established to quantify the total DPPG content; zeta potential was used to detect outer-leaflet DPPG. It turned out to be possible to prepare aLUVs at 5 and 15 °C, with the outer leaflet partially in the gel phase. The properties of the final aLUVs depended on the preparation temperature. aLUVs prepared at 5 and 15 °C caused the budding of exovesicles upon heating and only weak coupling of the phase transitions of the leaflets. aLUVs prepared at 30 °C with both leaflets in the fluid state showed stronger coupling upon asymmetric freezing/melting at lower temperature. This is in line with the hypotheses that (i) the exchange of lipid between close-to lipid-saturated cyclodextrin and acceptor vesicles at a given temperature results in largely stress-free bilayers and (ii) that outside budding and coupling are, to some extent, alternative responses of the bilayer to asymmetric expansion. These hypotheses help explaining our and some literature data; the overall understanding and prediction of coupling for any given aLUV system remains an urgent, open question.

Monitoring the conformational ensemble and lipid environment of a mechanosensitive channel under cyclodextrin-induced membrane tension

Membrane forces shift the equilibria of mechanosensitive channels enabling them to convert mechanical cues into electrical signals. Molecular tools to stabilize and methods to capture their highly dynamic states are lacking. Cyclodextrins can mimic tension through the sequestering of lipids from membranes. Here we probe the conformational ensemble of MscS by EPR spectroscopy, the lipid environment with NMR, and function with electrophysiology under cyclodextrin-induced tension. We show the extent of MscS activation depends on the cyclodextrin-to-lipid ratio, and that lipids are depleted slower when MscS is present. This has implications in MscS' activation kinetics when distinct membrane scaffolds such as nanodiscs or liposomes are used. We find MscS transits from closed to sub-conducting state(s) before it desensitizes, due to the lack of lipid availability in its vicinity required for closure. Our approach allows for monitoring tension-sensitive states in membrane proteins and screening molecules capable of inducing molecular tension in bilayers.

Ex Vivo Drug Screening Assay with Artificial Membranes: Characterizing Cholesterol Desorbing Competencies of Beta-Cyclodextrins

Despite advancements in contemporary therapies, cardiovascular disease from atherosclerosis remains a leading cause of mortality worldwide. Supported lipid bilayers (SLBs) are membrane interfaces that can be constructed with varying lipid compositions. Herein, we use a solvent-assisted lipid bilayer (SALB) construction method to build SLB membranes with varying cholesterol compositions to create a lipid-sterol interface atop a piezoelectric sensor. These cholesterol-laden SLBs were utilized to investigate the mechanisms of various cholesterol-lowering drug molecules. Within a flow-cell, membranes with varying cholesterol content were exposed to cyclodextrins 2-hydroxypropyl-beta-cyclodextrin (HPβCD) and methyl-beta-cyclodextrin (MβCD). Quartz-crystal microgravimetry with dissipation monitoring (QCM-D) enabled the collection of in vitro, real-time changes in relative areal mass and dissipation. We define the cholesterol desorbing competency of a cyclodextrin species via measures of the rate of cholesterol removal, the rate of the transfer of membrane-bound cholesterol to drug-complexed cholesterol, and the binding strength of the drug to the cholesterol-ladened membrane. Desorption data revealed distinct cholesterol removal kinetics for each cyclodextrin while also supporting a model for the lipid-cholesterol-drug interface. We report that MβCD removes a quantity of cholesterol 1.61 times greater, with a speed 2.12 times greater, binding affinity to DOPC lipid interfaces 1.97 times greater, and rate of internal cholesterol transfer 3.41 times greater than HPβCD.

Chemical potential measurements constrain models of cholesterol-phosphatidylcholine interactions

Bilayer membranes composed of cholesterol and phospholipids exhibit diverse forms of nonideal mixing. In particular, many previous studies document macroscopic liquid-liquid phase separation as well as nanometer-scale heterogeneity in membranes of phosphatidylcholine (PC) lipids and cholesterol. Here, we present experimental measurements of cholesterol chemical potential (μc) in binary membranes containing dioleoyl PC (DOPC), 1-palmitoyl-2-oleoyl PC (POPC), or dipalmitoyl PC (DPPC), and in ternary membranes of DOPC and DPPC, referenced to crystalline cholesterol. μc is the thermodynamic quantity that dictates the availability of cholesterol to bind other factors, and notably must be equal between coexisting phases of a phase separated mixture. It is simply related to concentration under conditions of ideal mixing, but is far from ideal for the majority of lipid mixtures investigated here. Measurements of μc can vary with phospholipid composition by 1.5 kBT at constant cholesterol mole fraction implying a more than fivefold change in its availability for binding receptors and other reactions. Experimental measurements are fit to thermodynamic models including cholesterol-DPPC complexes or pairwise interactions between lipid species to provide intuition about the magnitude of interactions. These findings reinforce that μc depends on membrane composition overall, suggesting avenues for cells to alter the availability of cholesterol without varying cholesterol concentration.

A Guide to Your Desired Lipid-Asymmetric Vesicles

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

Enzyme-Free Liposome Active Motion via Asymmetrical Lipid Efflux

As a class of biocompatible, water-dispersed colloids, liposomes have found widespread applications ranging from food to drug delivery. Adding mobility to these colloids, i.e., liposome micromotors, represents an attractive approach to next-generation liposome carriers with enhanced functionality and effectiveness. Currently, it remains unclear as to the scope of material features useful for building liposome micromotors or how they may differ functionally from their inorganic/polymer counterparts. In this work, we demonstrate liposome active motion taking advantage of mainly a pair of intrinsic material properties associated with these assemblies: lipid phase separation and extraction. We show that global phase separation of ternary lipid systems (such as DPPC/DOPC/cholesterol) within individual liposomes yields stable Janus particles with two distinctive liquid domains. While these anisotropic liposomes undergo pure Brownian diffusion in water, similar to their homogeneous analogues, adding extracting agents, cyclodextrins, to the system triggers asymmetrical cholesterol efflux about the liposomes, setting the latter into active motion. We present detailed analyses of liposome movement and cholesterol extraction kinetics to establish their correlation. We explore various experimental parameters as well as mechanistic details to account for such motion. Our results highlight the rich possibility to hierarchically design lipid-based artificial motors, from individual lipids, to their organization, surface chemistry, and interfacial mechanics.

Synergistic Effect of Acetazolamide-(2-hydroxy)propyl β-Cyclodextrin in Timolol Liposomes for Decreasing and Prolonging Intraocular Pressure Levels

The purpose of this study was to design, for the first time, a co-loaded liposomal formulation (CLL) for treatment of glaucoma including timolol maleate (TM) in the lipid bilayer and acetazolamide (Acz)-(2-hydroxy)propyl β-cyclodextrin (HPβCD) complexes (AczHP) solubilized in the aqueous core of liposomes. Formulations with TM (TM-L) and AczHP (AczHP-L), separately, were also prepared and characterized. A preliminary study comprising the Acz/HPβCD complexes and their interaction with cholesterol (a component of the lipid bilayer) was realized. Then, a screening study on formulation factors affecting the quality of the product was carried out following the design of the experiment methodology. In addition, in vitro release and permeation studies and in vivo lowering intraocular pressure (IOP) studies were performed. The results of the inclusion complexation behavior, characterization, and binding ability of Acz with HPβCD showed that HPβCD could enhance the water solubility of Acz despite the weak binding ability of the complex. Ch disturbed the stability and solubility parameters of Acz due to the fact of its competence by CD; thus, Chems (steroid derivative) was selected for further liposome formulation studies. The optimization of the lipid bilayer composition (DDAB, 0.0173 mmol and no double loading) and the extrusion as methods to reduce vesicle size were crucial for improving the physico-chemical properties and encapsulation efficiency of both drugs. In vitro release and permeation studies demonstrated that the CLL formulation showed improvement in in vitro drug release and permeation compared to the liposomal formulations with a single drug (TM-L and AczHP-L) and the standard solutions (TM-S and AczHP-S). CLL showed high efficacy in reducing and prolonging IOP, suggesting that the synergistic effect of TM and Acz on aqueous humor retention and the presence of this cyclodextrin and liposomes as permeation enhancers are responsible for the success of this strategy of co-loading for glaucoma therapy.

Cyclodextrin dimers: A versatile approach to optimizing encapsulation and their application to therapeutic extraction of toxic oxysterols

We have developed a novel class of specifically engineered, dimerized cyclodextrin (CD) nanostructures for the encapsulation of toxic biomolecules such as 7-ketocholesterol (7KC). 7KC accumulates over time and causes dysfunction in many cell types, linking it to several age-related diseases including atherosclerosis and age-related macular degeneration (AMD). Presently, treatments for these diseases are invasive, expensive, and show limited benefits. CDs are cyclic glucose oligomers utilized to capture small, hydrophobic molecules. Here, a combination of in silico, in vitro, and ex vivo methods is used to implement a synergistic rational drug design strategy for developing CDs to remove atherogenic 7KC from cells and tissues. Mechanisms by which CDs encapsulate sterols are discussed, and we conclude that covalently linked head-to-head dimers of βCDs have substantially improved affinity for 7KC compared to monomers. We find that inclusion complexes can be stabilized or destabilized in ways that allow the design of CD dimers with increased 7KC selectivity while maintaining an excellent safety profile. These CD dimers are being developed as therapeutics to treat atherosclerosis and other debilitating diseases of aging.

The Lord of the NanoRings: Cyclodextrins and the battle against SARS-CoV-2

A handful of singular structures and laws can be observed in nature. They are not always evident but, once discovered, it seems obvious how to take advantage of them. In chemistry, the discovery of reproducible patterns stimulates the imagination to develop new functional materials and technological or medical applications. Two clear examples are helical structures at different levels in biological polymers as well as ring and spherical structures of different size and composition. Rings are intuitively observed as holes able to thread elongated structures. A large number of real and fictional stories have rings as inanimate protagonists. The design, development or just discovering of a special ring has often been taken as a symbol of power or success. Several examples are the Piscatory Ring wore by the Pope of the Catholic Church, the NBA Championship ring and the One Ring created by the Dark Lord Sauron in the epic story The Lord of the Rings. In this work, we reveal the power of another extremely powerful kind of rings to fight against the pandemic which is currently affecting the whole world. These rings are as small as ~1 nm of diameter and so versatile that they are able to participate in the attack of viruses, and specifically SARS-CoV-2, in a large range of different ways. This includes the encapsulation and transport of specific drugs, as adjuvants to stabilize proteins, vaccines or other molecules involved in the infection, as cholesterol trappers to destabilize the virus envelope, as carriers for RNA therapies, as direct antiviral drugs and even to rescue blood coagulation upon heparin treatment.

“One ring to rule them all. One ring to find them. One ring to bring them all and in the darkness bind them.” J. R. R. Tolkien.

Stairway to Asymmetry: Five Steps to Lipid-Asymmetric Proteoliposomes

Membrane proteins are embedded in a complex lipid environment that influences their structure and function. One key feature of nearly all biological membranes is a distinct lipid asymmetry. However, the influence of membrane asymmetry on proteins is poorly understood, and novel asymmetric proteoliposome systems are beneficial. To our knowledge, we present the first study on a multispanning protein incorporated in large unilamellar liposomes showing a stable lipid asymmetry. These asymmetric proteoliposomes contain the Na+/H+ antiporter NhaA from Salmonella Typhimurium. Asymmetry was introduced by partial, outside-only exchange of anionic phosphatidylglycerol (PG), mimicking this key asymmetry of bacterial membranes. Outer-leaflet and total fractions of PG were determined via ζ-potential (ζ) measurements after lipid exchange and after scrambling of asymmetry. ζ-Values were in good agreement with exclusive outside localization of PG. The electrogenic Na+/H+ antiporter was active in asymmetric liposomes, and it can be concluded that reconstitution and generation of asymmetry were successful. Lipid asymmetry was stable for more than 7 days at 23°C and thus enabled characterization of the Na+/H+ antiporter in an asymmetric lipid environment. We present and validate a simple five-step protocol that addresses key steps to be taken and pitfalls to be avoided for the preparation of asymmetric proteoliposomes: 1) optimization of desired lipid composition, 2) detergent-mediated protein reconstitution with subsequent detergent removal, 3) generation of lipid asymmetry by partial exchange of outer-leaflet lipid, 4) verification of lipid asymmetry and stability, and 5) determination of protein activity in the asymmetric lipid environment. This work offers guidance in designing asymmetric proteoliposomes that will enable researchers to compare functional and structural properties of membrane proteins in symmetric and asymmetric lipid environments.

Cholesterol-Induced Conformational Change in the Sphingomyelin Headgroup

Sphingomyelin (SM) and cholesterol (Cho) are the important lipids for the formation of biologically functional membrane domains, lipid rafts. However, the interaction between Cho and the headgroup of SM remains unclear. In this study, we performed solid-state NMR experiments to reveal the Cho effects on the headgroup conformation using ²H-labeled stearoyl-SM (SSM). Deuterated SSMs at the Cα, Cβ, and Cγ positions of a choline moiety were separately prepared and subjected to NMR measurements to determine the quadrupolar splitting of ²H signals in hydrated SSM unitary and SSM/Cho (1:1) bilayers. Using ²H NMR and ¹³C-³¹P REDOR data, the conformation and orientation of the choline moiety were deduced and compared with those derived from molecular dynamics simulations. In SSM unitary bilayers, three torsional angles in the phosphocholine moiety, P-O-Cα-Cβ, were found to be consecutive +gauche(g)/+g/+g or -g/-g/-g. The orientation and conformation of the SSM headgroup were consistent with the results of our molecular dynamics simulations and the previous results on phosphatidylcholines. The quadrupolar coupling at the α methylene group slightly increased in the presence of Cho, and those at the Cβ and Cγ decreased more significantly, thus suggesting that Cho reduced the gauche conformation at the Cα-Cβ torsion. The conformational ensemble in the presence of Cho may enhance the so-called umbrella effect of the SSM headgroup, resulting in the stabilization of Cho near the SM molecules by concealing the hydrophobic Cho core from interfacial water. We also examined the effect of the chiral centers at the sphingosine chain to the headgroup conformation by determining the enantiomeric excess between the diastereomeric +g/+g/+g and -g/-g/-g conformers using (S)-Cα-deuterated and (R)-Cα-deuterated SSMs. Their ²H NMR measurements showed that the chiral centers induced the slight diastereomeric excess in the SM headgroup conformation.

Mechanism of local anesthetic-induced disruption of raft-like ordered membrane domains

BACKGROUND

Because ordered membrane domains, called lipid rafts, regulate activation of ion channels related to the nerve pulse, lipids rafts are thought to be a possible target for anesthetic molecules. To understand the mechanism of anesthetic action, we examined influence of representative local anesthetics (LAs); dibucaine, tetracaine, and lidocaine, on raft-like liquid-ordered (L_o)/non-raft-like liquid-disordered (L_d) phase separation.

METHODS

Impact of LAs on the phase separation was observed by fluorescent microscopy. LA-induced perturbation of the L_o and L_d membranes was examined by DPH anisotropy measurements. Incorporation of LAs to the membranes was examined by fluorescent anisotropy of LAs. The biding location of the LAs was indicated by small angle x-ray diffraction (SAXD).

RESULTS

Fluorescent experiments showed that dibucaine eliminated the phase separation the most effectively, followed by tetracaine and lidocaine. The disruption of the phase separation can be explained by their disordering effects on the L_o membrane. SAXD and other experiments further suggested that dibucaine's most potent perturbation of the L_o membrane is attributable to its deeper immersion and bulky molecular structure. Tetracaine, albeit immersed in the L_o membrane as deeply as dibucaine, less perturbs the L_o membrane probably because of its smaller bulkiness. Lidocaine hardly reaches the hydrophobic region, resulting in the weakest L_o membrane perturbation.

CONCLUSION

Dibcaine perturbs the L_o membrane the most effectively, followed by tetracaine and lidocaine. This ranking correlates with their anesthetic potency.

GENERAL SIGNIFICANCE

This study suggests a possible mechanistic link between anesthetic action and perturbation of lipid rafts.

The Chemical Potential of Plasma Membrane Cholesterol: Implications for Cell Biology

Cholesterol is abundant in plasma membranes and exhibits a variety of interactions throughout the membrane. Chemical potential accounts for thermodynamic consequences of molecular interactions, and quantifies the effective concentration (i.e., activity) of any substance participating in a process. We have developed, to our knowledge, the first method to measure cholesterol chemical potential in plasma membranes. This was accomplished by complexing methyl-β-cyclodextrin with cholesterol in an aqueous solution and equilibrating it with an organic solvent containing dissolved cholesterol. The chemical potential of cholesterol was thereby equalized in the two phases. Because cholesterol is dilute in the organic phase, here activity and concentration were equivalent. This equivalence allowed the amount of cholesterol bound to methyl-β-cyclodextrin to be converted to cholesterol chemical potential. Our method was used to determine the chemical potential of cholesterol in erythrocytes and in plasma membranes of nucleated cells in culture. For erythrocytes, the chemical potential did not vary when the concentration was below a critical value. Above this value, the chemical potential progressively increased with concentration. We used standard cancer lines to characterize cholesterol chemical potential in plasma membranes of nucleated cells. This chemical potential was significantly greater for highly metastatic breast cancer cells than for nonmetastatic breast cancer cells. Chemical potential depended on density of the cancer cells. A method to alter and fix the cholesterol chemical potential to any value (i.e., a cholesterol chemical potential clamp) was also developed. Cholesterol content did not change when cells were clamped for 24-48 h. It was found that the level of activation of the transcription factor STAT3 increased with increasing cholesterol chemical potential. The cholesterol chemical potential may regulate signaling pathways.

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.

Preparation of Asymmetric Liposomes Using a Phosphatidylserine Decarboxylase

Lipid asymmetries between the outer and inner leaflet of the lipid bilayer exist in nearly all biological membranes. Although living cells spend great effort to adjust and maintain these asymmetries, little is known about the biophysical phenomena within asymmetric membranes and their role in cellular function. One reason for this lack of insight into such a fundamental membrane property is the fact that the majority of model-membrane studies have been performed on symmetric membranes. Our aim is to overcome this problem by employing a targeted, enzymatic reaction to prepare asymmetric liposomes with phosphatidylserine (PS) primarily in the inner leaflet. To achieve this goal, we use a recombinant version of a water soluble PS decarboxylase from Plasmodium knowlesi, which selectively decarboxylates PS in the outer leaflet, converting it to phosphatidylethanolamine. The extent of decarboxylation is quantified using high-performance thin-layer chromatography, and the local concentration of anionic PS in the outer leaflet is monitored in terms of the ζ potential. Starting, for example, with 21 mol % 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine sodium salt, the assay leads to liposomes with 21 mol % in the inner and 6 mol % PS in the outer leaflet. This asymmetry persists virtually unchanged for at least 4 days at 20°C and at least 2 days at 40°C. The use of a highly specific enzyme carries the advantage that a minor component such as PS can be adjusted without affecting or being affected by the other lipid species present in the model membrane. The phenomena governing the residual outside PS content are addressed but warrant further study.

Effect of Lung Surfactant Protein SP-C and SP-C-Promoted Membrane Fragmentation on Cholesterol Dynamics

To allow breathing and prevent alveolar collapse, lung surfactant (LS) develops a complex membranous system at the respiratory surface. LS is defined by a specific protein and lipid composition, including saturated and unsaturated phospholipid species and cholesterol. Surfactant protein C (SP-C) has been suggested to be an essential element for sustaining the presence of cholesterol in surfactant without functional impairment. In this work, we used a fluorescent sterol-partitioning assay to assess the effect of the surfactant proteins SP-B and SP-C on cholesterol distribution in membranes. Our results suggest that in the LS context, the combined action of SP-B and SP-C appears to facilitate cholesterol dynamics, whereas SP-C does not seem to establish a direct interaction with cholesterol that could increase the partition of free cholesterol into membranes. Interestingly, SP-C exhibits a membrane-fragmentation behavior, leading to the conversion of large unilamellar vesicles into highly curved vesicles ∼25 nm in diameter. Sterol partition was observed to be sensitive to the bending of bilayers, indicating that the effect of SP-C to mobilize cholesterol could be indirectly associated with SP-C-mediated membrane remodeling. Our results suggest a potential role for SP-C in generating small surfactant structures that may participate in cholesterol mobilization and pulmonary surfactant homeostasis at the alveolar interfaces.

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.

Depletion with Cyclodextrin Reveals Two Populations of Cholesterol in Model Lipid Membranes

Recent results provide evidence that cholesterol is highly accessible for removal from both cell and model membranes above a threshold concentration that varies with membrane composition. Here we measured the rate at which methyl-β-cyclodextrin depletes cholesterol from a supported lipid bilayer as a function of cholesterol mole fraction. We formed supported bilayers from two-component mixtures of cholesterol and a PC (phosphatidylcholine) lipid, and we directly visualized the rate of decrease in area of the bilayers with fluorescence microscopy. Our technique yields the accessibility of cholesterol over a wide range of concentrations (30-66 mol %) for many individual bilayers, enabling fast acquisition of replicate data. We found that the bilayers contain two populations of cholesterol, one with low surface accessibility and the other with high accessibility. A larger fraction of the total membrane cholesterol appears in the more accessible population when the acyl chains of the PC-lipid tails are more unsaturated. Our findings are most consistent with the predictions of the condensed-complex and cholesterol bilayer domain models of cholesterol-phospholipid interactions in lipid membranes.

Structural Stringency of Cholesterol for Membrane Protein Function Utilizing Stereoisomers as Novel Tools: A Review

Cholesterol is an important lipid in the context of membrane protein function. The function of a number of membrane proteins, including G protein-coupled receptors (GPCRs) and ion channels, has been shown to be dependent on membrane cholesterol. However, the molecular mechanism underlying such regulation is still being explored. In some cases, specific interaction between cholesterol and the protein has been implicated. In other cases, the effect of cholesterol on the membrane properties has been attributed for the regulation of protein function. In this article, we have provided an overview of experimental approaches that are useful for determining the degree of structural stringency of cholesterol for membrane protein function. In the process, we have highlighted the role of immediate precursors in cholesterol biosynthetic pathway in the function of membrane proteins. Special emphasis has been given to the application of stereoisomers of cholesterol in deciphering the structural stringency required for regulation of membrane protein function. A comprehensive examination of these processes would help in understanding the molecular basis of cholesterol regulation of membrane proteins in subtle details.

Bioactive peptide functionalized aligned cyclodextrin nanofibers for neurite outgrowth

Guidance of neurite extension and establishment of neural connectivity hold great importance for neural tissue regeneration and neural conduit implants. Although bioactive-epitope functionalized synthetic or natural polymeric materials have been proposed for the induction of neural regeneration, chemical modifications of these materials for neural differentiation still remain a challenge due to the harsh conditions of chemical reactions, along with non-homogeneous surface modifications. In this study, a facile noncovalent functionalization method is proposed by exploiting host-guest interactions between an adamantane-conjugated laminin derived bioactive IKVAV epitope and electrospun cyclodextrin nanofibers (CDNFs) to fabricate implantable scaffolds for peripheral nerve regeneration. While electrospun CDNFs introduce a three-dimensional biocompatible microenvironment to promote cellular viability and adhesion, the bioactive epitopes presented on the surface of electrospun CDNFs guide the cellular differentiation of PC-12 cells. In addition to materials synthesis and smart functionalization, physical alignment of the electrospun nanofibers guides the cells for enhanced differentiation. Cells cultured on aligned and IKVAV functionalized electrospun CDNFs had significantly higher expression of neuron-specific βIII-tubulin and synaptophysin. The neurite extension is also higher on the bioactive aligned scaffolds compared to random and non-functionalized electrospun CDNFs. Both chemical and physical cues were utilized for an effective neuronal differentiation process.

Evaluation of the potential toxicity of unmodified and modified cyclodextrins on murine blood-brain barrier endothelial cells

In this study, we investigated the cytotoxic effects of unmodified α-cyclodextrin (α-CD) and modified cyclodextrins, including trimethyl-β-cyclodextrin (TRIMEB) and hydroxypropyl-β-cyclodextrin (HPβCD), on immortalized murine microvascular endothelial (cEND) cells of the blood-brain barrier (BBB). A CellTiter-Glo viability test, performed on the cEND cells showed significant differences among the different cyclodextrins. After 24 hr of incubation, TRIMEB was the most cytotoxic, and HPβCD was non-toxic. α-CD and TRIMEB exhibited greater cytotoxicity in the Dulbecco's modified Eagle's medium than in heat-inactivated human serum indicating protective properties of the human serum. The predicted dynamic toxicity profiles (Td) for α-CD and TRIMEB indicated higher cytotoxicity for these cyclodextrins compared to the reference compound (dimethylsulfoxide). Molecular dynamics simulation of cholesterol binding to the CDs suggested that not just cholesterol but phospholipids extraction might be involved in the cytotoxicity. Overall, the results demonstrate that HPβCD has the potential to be used as a candidate for drug delivery vector development and signify a correlation between the in vitro cytotoxic effect and cholesterol binding of cyclodextrins.

Cholesterol Depletion from a Ceramide/Cholesterol Mixed Monolayer: A Brewster Angle Microscope Study

Cholesterol is crucial to the mechanical properties of cell membranes that are important to cells' behavior. Its depletion from the cell membranes could be dramatic. Among cyclodextrins (CDs), methyl beta cyclodextrin (MβCD) is the most efficient to deplete cholesterol (Chol) from biomembranes. Here, we focus on the depletion of cholesterol from a C16 ceramide/cholesterol (C16-Cer/Chol) mixed monolayer using MβCD. While the removal of cholesterol by MβCD depends on the cholesterol concentration in most mixed lipid monolayers, it does not depend very much on the concentration of cholesterol in C16-Cer/Chol monolayers. The surface pressure decay during depletion were described by a stretched exponential that suggested that the cholesterol molecules are unable to diffuse laterally and behave like static traps for the MβCD molecules. Cholesterol depletion causes morphology changes of domains but these disrupted monolayers domains seem to reform even when cholesterol level was low.

Effect of Cyclodextrins on Morphology and Barrier Characteristics of Isolated Rabbit Corneas

The objective of the present study is to investigate the confounding effects, if any, of beta-cyclodextrins (βCDs) on corneal permeability coefficients obtained from in vitro transmembrane diffusion studies. Transcorneal permeability studies were carried out with 2-hydroxypropyl-beta-cyclodextrin (HPβCD) and randomly methylated-beta-cyclodextrin (RMβCD) at 5 and 2.5%w/v in isotonic phosphate-buffered solution (IPBS) (pH 7.4). Rabbit corneas received from Pel-Freez Biologicals® were used for these studies. Propranolol hydrochloride (PHCl) (1 mg/mL) was used as the paracellular permeability marker. A series of permeation studies were carried out with IPBS as the control, with CDs on the donor side only, CDs on the receiver side only, and CDs on both the donor and receiver sides. At the end of 1 or 3 h, corneas were collected and fixed using a solution containing 2%v/v glutaraldehyde + 2%w/v paraformaldehyde + IPBS and histological examinations were performed (Excalibur Pathology, Inc). The order of transcorneal permeability of PHCl was found to be CDs on the receiver side > control (no CDs) ≈ CDs on both the receiver and donor sides > CDs on the donor side. Histology studies revealed that the corneal epithelial and endothelial layers remained intact in the control sets. Damage to the cornea was observed in the order of CDs on the receiver side > CDs on the donor side > CDs on both sides > control. The use of CDs in solutions for in vitro permeation experiments with rabbit corneas needs to be carefully considered to avoid confounding effects in the data obtained.

Unfolding and refolding of a protein by cholesterol and cyclodextrin: a single molecule study

Cholesterol induced unfolding of a globular protein, human serum albumin (HSA), and β-cyclodextrin induced refolding of the unfolded protein is demonstrated in this study.

Preparation of artificial plasma membrane mimicking vesicles with lipid asymmetry

Lipid asymmetry, the difference in lipid distribution across the lipid bilayer, is one of the most important features of eukaryotic cellular membranes. However, commonly used model membrane vesicles cannot provide control of lipid distribution between inner and outer leaflets. We recently developed methods to prepare asymmetric model membrane vesicles, but facile incorporation of a highly controlled level of cholesterol was not possible. In this study, using hydroxypropyl-α-cyclodextrin based lipid exchange, a simple method was devised to prepare large unilamellar model membrane vesicles that closely resemble mammalian plasma membranes in terms of their lipid composition and asymmetry (sphingomyelin (SM) and/or phosphatidylcholine (PC) outside/phosphatidylethanolamine (PE) and phosphatidylserine (PS) inside), and in which cholesterol content can be readily varied between 0 and 50 mol%. We call these model membranes “artificial plasma membrane mimicking” (“PMm”) vesicles. Asymmetry was confirmed by both chemical labeling and measurement of the amount of externally-exposed anionic lipid. These vesicles should be superior and more realistic model membranes for studies of lipid-lipid and lipid-protein interaction in a lipid environment that resembles that of mammalian plasma membranes.

Computational microscopy of cyclodextrin mediated cholesterol extraction from lipid model membranes

Beta-cyclodextrins (β-CDs) can form inclusion complexes with cholesterol, and are commonly used to manipulate cholesterol levels of biomembranes. In this work, we have used multiscale molecular dynamics simulations to provide a detailed view on the interaction between β-CDs and lipid model membranes. We show that cholesterol can be extracted efficiently upon adsorption of β-CD dimers at the membrane/water interface. However, extraction is only observed to occur spontaneously in membranes with high cholesterol levels. Free energy calculations reveal the presence of a kinetic barrier for cholesterol extraction in the case of low cholesterol content. Cholesterol uptake is facilitated in case of (poly)unsaturated lipid membranes, which increases the free energy of the membrane bound state of cholesterol. Comparing lipid/cholesterol compositions typical of liquid-disordered (L(d)) and liquid-order (L(o)) domains, we furthermore show that cholesterol is preferentially extracted from the disordered regions, in line with recent experimental data.

Preparation of semisynthetic lipoproteins with fluorescent cholesterol anchor and their introduction to the cell membrane with minimal disruption of the membrane

The exogenous introduction of fluorescent lipoproteins into cell membranes is a method for visualizing the cellular traffic of membrane associated proteins, and also for altering the cell surface in a controlled manner. In order to achieve the cell membrane anchoring of proteins and their subsequent fluorescence based detection, a cholesterol derivative was designed. The headgroup of the novel cholesterol anchor contains a fluorescent reporter and a thiol reactive maleimide for protein conjugation. Protein conjugation was demonstrated by the addition of a green fluorescent maleimido anchor to the C-terminus of a Cys extended red fluorescent protein, mCherry. The resulting dual fluorescent cholesteryl lipoprotein was successfully separated from the micellar associates of the surplus fluorescent lipid anchor without denaturing the protein, and the lipoprotein containing only the covalently linked, stoichiometric fluorescent lipid was efficiently delivered to the plasma membrane of live cells. It was demonstrated that the membrane fluorescence could be directly assigned to the protein-anchor conjugate, because no excess of fluorescent lipid species were present during the imaging experiment and the protein and anchor fluorescence colocalized in the cell membrane. Molecular dynamics simulations and subsequent trajectory analysis suggest also the spontaneous and stable membrane association of the cholesterol anchor. Thus, the method could be beneficially applied for studying membrane associated proteins and for preparing mimetics of glycosylphosphatidylinositol (GPI)-anchored proteins to target cholesterol-rich membrane microdomains.

Stability and stoichiometry of bilayer phospholipid-cholesterol complexes: relationship to cellular sterol distribution and homeostasis

Is cholesterol distributed among intracellular compartments by passive equilibration down its chemical gradient? If so, its distribution should reflect the relative cholesterol affinity of the constituent membrane phospholipids as well as their capacity for association with the sterol. We examined this issue by analyzing the reactivity to cholesterol oxidase of large unilamellar vesicles (LUVs) containing phospholipids and varied levels of cholesterol. The rates of cholesterol oxidation differed among the various phospholipid environments by roughly 4 orders of magnitude. Furthermore, accessibility to the enzyme increased by orders of magnitude at cholesterol thresholds that suggested cholesterol:phospholipid association ratios of 1:1, 2:3, or 1:2 (moles:moles). The accessibility of cholesterol above these thresholds was still constrained by its particular phospholipid environment. One phospholipid, 1-stearoyl-2-oleoyl-sn-glycero-3-phosphatidylserine, exhibited no threshold. The analysis suggested values for the stoichiometries of the putative cholesterol-phospholipid complexes, their relative stabilities, and the fractions of bilayer cholesterol not in complexes at the threshold equivalence points. Predictably, the saturated phosphorylcholine species had the lowest apparent stoichiometric ratios and the strongest associations with cholesterol. These results are in general agreement with the equilibrium distribution of cholesterol between the various LUVs and methyl-β-cyclodextrin. In addition, the behavior of the cholesterol in intact human red blood cells matched predictions made from LUVs of the corresponding composition. These results support a passive mechanism for the intracellular distribution of cholesterol that can provide a signal for its homeostatic regulation.

Genotype-induced changes in biophysical properties of frontal cortex lipid raft from APP/PS1 transgenic mice

Alterations in the lipid composition of lipid rafts have been demonstrated both in human brain and transgenic mouse models, and it has been postulated that aberrant lipid composition in lipid rafts is partly responsible for neuronal degeneration. In order to assess the impact of lipid changes on lipid raft functional properties, we have aimed at determining relevant physicochemical modifications in lipid rafts purified from frontal cortex of wild type (WT) and APP/PS1 double transgenic mice. By means of steady-state fluorescence anisotropy analyses using two lipid soluble fluorescent probes, TMA-DPH (1-[(4-trimethyl-amino)phenyl]-6-phenyl-1,3,5-hexatriene) and DPH (1,6-diphenyl-1,3,5-hexatriene), we demonstrate that cortical lipid rafts from WT and APP/PS1 animals exhibit different biophysical behaviors, depending on genotype but also on age. Thus, aged APP/PS1 animals exhibited slightly more liquid-ordered lipid rafts than WT counterparts. Membrane microviscosity η_app analyses demonstrate that WT lipid rafts are more fluid than APP/PS1 animals of similar age, both at the aqueous interface and hydrophobic core of the membrane. η_app in APP/PS1 animals was higher for DPH than for TMA-DPH under similar experimental conditions, indicating that the internal core of the membrane is more viscous than the raft membrane at the aqueous interface. The most dramatic changes in biophysical properties of lipid rafts were observed when membrane cholesterol was depleted with methyl-β-cyclodextrin. Overall, our results indicate that APP/PS1 genotype strongly affects physicochemical properties of lipid raft. Such alterations appear not to be homogeneous across the raft membrane axis, but rather are more prominent at the membrane plane. These changes correlate with aberrant proportions of sphingomyelin, cholesterol, and saturated fatty acids, as well as polyunsaturated fatty acids, measured in lipid rafts from frontal cortex in this familial model of Alzheimer's Disease.

Effect of hydrophobic mismatch and interdigitation on sterol/sphingomyelin interaction in ternary bilayer membranes

Sphingomyelin (SM) is a major phospholipid in most cell membranes. SMs are composed of a long-chain base (often sphingosine, 18:1(Δ4t)), and N-linked acyl chains (often 16:0, 18:0 or 24:1(Δ15c)). Cholesterol interacts with SM in cell membranes, but the acyl chain preference of this interaction is not fully elucidated. In this study we have examined the effects of hydrophobic mismatch and interdigitation on cholesterol/sphingomyelin interaction in complex bilayer membranes. We measured the capacity of cholestatrienol (CTL) and cholesterol to form sterol-enriched ordered domains with saturated SM species having different chain lengths (14 to 24 carbons) in ternary bilayer membranes. We also determined the equilibrium bilayer partitioning coefficient of CTL with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membranes containing 20mol% of saturated SM analogs. Ours results show that while CTL and cholesterol formed sterol-enriched domains with both short and long-chain SM species, the sterols preferred interaction with 16:0-SM over any other saturated chain length SM analog. When CTL membrane partitioning was determined with fluid POPC bilayers containing 20mol% of a saturated chain length SM analog, the highest affinity was seen with 16:0-SM (both at 23 and 37°C). These results indicate that hydrophobic mismatch and/or interdigitation attenuate sterol/SM association and thus affect lateral distribution of sterols in the bilayer membrane.

Molecular mechanism of cyclodextrin mediated cholesterol extraction

The depletion of cholesterol from membranes, mediated by β-cyclodextrin (β-CD) is well known and documented, but the molecular details of this process are largely unknown. Using molecular dynamics simulations, we have been able to study the CD mediated extraction of cholesterol from model membranes, in particular from a pure cholesterol monolayer, at atomic resolution. Our results show that efficient cholesterol extraction depends on the structural distribution of the CDs on the surface of the monolayer. With a suitably oriented dimer, cholesterol is extracted spontaneously on a nanosecond time scale. Additional free energy calculations reveal that the CDs have a strong affinity to bind to the membrane surface, and, by doing so, destabilize the local packing of cholesterol molecules making their extraction favorable. Our results have implications for the interpretation of experimental measurements, and may help in the rational design of efficient CD based nano-carriers.

The ability of certain molecules to capture other molecules forming so-called inclusion complexes has a range of potential important applications in e.g. drug delivery and chemical sensing. Here we study the complexation of cholesterol by small oligosaccharide rings named cyclodextrins (CDs). Cholesterol is an essential lipid in the plasma cell membrane, and the ability of CDs to extract cholesterol is widely used in the biomedical field to control the level of cholesterol in the membrane. The molecular mechanism of this process, however, is still not resolved. Using a detailed computational model of cholesterol and CD, we have succeeded to simulate this extraction process. We observe that the CDs are rapidly binding to the membrane surface in a dimeric form, and, provided that the CD dimers are in a suitable orientation, cholesterol molecules are being extracted spontaneously. The cholesterol/CD inclusion complex remains adsorbed on the surface; our simulations predict that the rate limiting step for the actual transport of cholesterol is the desorption of the complex from the membrane. With a clearer understanding of the basic molecular mechanism of the CD mediated process of cholesterol extraction, we can begin to rationalize the design of more efficient CDs in numerous applications.

Volume and expansivity changes of micelle formation measured by pressure perturbation calorimetry

We present the application of pressure perturbation calorimetry (PPC) as a new method for the volumetric characterization of the micelle formation of surfactants. The evaluation is realized by a global fit of PPC curves at different surfactant concentration ranging, if possible, from below to far above the CMC. It is based on the knowledge of the temperature dependence of the CMC, which can for example be characterized by isothermal titration calorimetry. We demonstrate the new approach for decyl-β-maltopyranoside (DM). It shows a strong volume increase upon micelle formation of 16 ± 2.5 mL/mol (+4%) at 25 °C, and changes with temperature by -0.1 mL/(mol K). The apparent molar expansivity (E(S)) decreases upon micelle formation from 0.44 to 0.31 mL/(mol K) at 25 °C. Surprisingly, the temperature dependence of the expansivity of DM in solution (as compared with that of maltose) does not agree with the principal behavior described for polar (E(S)(T) decreasing) and hydrophobic (E(S)(T) increasing) solutes or moieties before. The results are discussed in terms of changes in hydration of the molecules and internal packing of the micelles and compared with the volumetric effects of transitions of proteins, DNA, lipids, and polymers.

Modulation of plant mitochondrial VDAC by phytosterols

We have investigated the effect of cholesterol and two abundant phytosterols (sitosterol and stigmasterol) on the voltage-dependent anion-selective channel (VDAC) purified from mitochondria of bean seeds (Phaseolus coccineus). These sterols differ by the degree of freedom of their lateral chain. We show that VDAC displays sensitivity to the lipid-sterol ratio and to the type of sterol found in the membrane. The main findings of this study are: 1), cholesterol and phytosterols modulate the selectivity but only stigmasterol alters the voltage-dependence of the plant VDAC in the range of sterol fraction found in the plant mitochondrial membrane; 2), VDAC unitary conductance is not affected by the addition of sterols; 3), the effect of sterols on the VDAC is reversible upon sterol depletion with 10 μM methyl-β-cyclodextrins; and 4), phytosterols are essential for the channel gating at salt concentration prevailing in vivo. A quantitative analysis of the voltage-dependence indicates that stigmasterol inhibits the transition of the VDAC in the lowest subconductance states.

Transmembrane peptides influence the affinity of sterols for phospholipid bilayers

Cholesterol is distributed unevenly between different cellular membrane compartments, and the cholesterol content increases from the inner bilayers toward the plasma membrane. It has been suggested that this cholesterol gradient is important in the sorting of transmembrane proteins. Cholesterol has also been to shown play an important role in lateral organization of eukaryotic cell membranes. In this study the aim was to determine how transmembrane proteins influence the lateral distribution of cholesterol in phospholipid bilayers. Insight into this can be obtained by studying how cholesterol interacts with bilayer membranes of different composition in the presence of designed peptides that mimic the transmembrane helices of proteins. For this purpose we developed an assay in which the partitioning of the fluorescent cholesterol analog CTL between LUVs and mbetaCD can be measured. Comparison of how cholesterol and CTL partitioning between mbetaCD and phospholipid bilayers with different composition suggests that CTL sensed changes in bilayer composition similarly as cholesterol. Therefore, the results obtained with CTL can be used to understand cholesterol distribution in lipid bilayers. The effect of WALP23 on CTL partitioning between DMPC bilayers and mbetaCD was measured. From the results it was clear that WALP23 increased both the order in the bilayers (as seen from CTL and DPH anisotropy) and the affinity of the sterol for the bilayer in a concentration dependent way. Although WALP23 also increased the order in DLPC and POPC bilayers the effects on CTL partitioning was much smaller with these lipids. This indicates that proteins have the largest effect on sterol interactions with phospholipids that have longer and saturated acyl chains. KALP23 did not significantly affect the acyl chain order in the phospholipid bilayers, and inclusion of KALP23 into DMPC bilayers slightly decreased CTL partitioning into the bilayer. This shows that transmembrane proteins can both decrease and increase the affinity of sterols for the lipid bilayers surrounding proteins. This is likely to affect the sterol distribution within the bilayer and thereby the lateral organization in biomembranes.

The location and behavior of alpha-tocopherol in membranes

Vitamin E (alpha-tocopherol) has long been recognized as the major antioxidant in biological membranes, and yet many structurally related questions persist of how the vitamin functions. For example, the very low levels of alpha-tocopherol reported for whole cell extracts question how this molecule can successfully protect the comparatively enormous quantities of PUFA-containing phospholipids found in membranes that are highly susceptible to oxidative attack. The contemporary realization that membranes laterally segregate into regions of distinct lipid composition (domains), we propose, provides the answer. We hypothesize alpha-tocopherol partitions into domains that are enriched in polyunsaturated phospholipids, amplifying the concentration of the vitamin in the place where it is most needed. These highly disordered domains depleted in cholesterol are analogous, but organizationally antithetical, to the well-studied lipid rafts. We review here the ideas that led to our hypothesis. Experimental evidence in support of the formation of PUFA-rich domains in model membranes is presented, focusing upon docosahexaenoic acid that is the most unsaturated fatty acid commonly found. Physical methodologies are then described to elucidate the nature of the interaction of alpha-tocopherol with PUFA and to establish that the vitamin and PUFA-containing phospholipids co-localize in non-raft domains.