Bilayer polarity and its thermal dependency in the l(o) and l(d) phases of binary phosphatidylcholine/cholesterol mixtures

Diverse variations in membrane properties are observed in binary phosphatidylcholine/cholesterol mixtures. These mixtures are nonideal, displaying single or phase coexistence, depending on chemical composition and other thermodynamic parameters. When compared with pure phospholipid bilayers, there are changes in water permeability, bilayer thickness and thermomechanical properties, molecular packing and conformational freedom of phospholipid acyl chains, in internal dipolar potential and in lipid lateral diffusion. Based on the phase diagrams for DMPC/cholesterol and DPPC/cholesterol, we compare the equivalent polarity of pure bilayers with specific compositions of these mixtures, by using the Py empirical scale of polarity. Besides the contrast between pure and mixed lipid bilayers, we find that liquid-ordered (l(o)) and liquid-disordered (l(d)) phases display significantly different polarities. Moreover, in the l(o) phase, the polarities of bilayers and their thermal dependences vary with the chemical composition, showing noteworthy differences for cholesterol proportions at 35, 40, and 45 mol%. At 20 degrees C, for DMPC/cholesterol at 35 and 45 mol%, the equivalent dielectric constants are 21.8 and 23.8, respectively. Additionally, we illustrate potential implications of polarity in various membrane-based processes and reactions, proposing that for cholesterol containing bilayers, it may also go along with the occurrence of lateral heterogeneity in biological membranes.     

Role of cholesterol in the formation and nature of lipid rafts in planar and spherical model membranes

Sterols play a crucial regulatory and structural role in the lateral organization of eukaryotic cell membranes. Cholesterol has been connected to the possible formation of ordered lipid domains (rafts) in mammalian cell membranes. Lipid rafts are composed of lipids in the liquid-ordered (l(o)) phase and are surrounded with lipids in the liquid-disordered (l(d)) phase. Cholesterol and sphingomyelin are thought to be the principal components of lipid rafts in cell and model membranes. We have used fluorescence microscopy and fluorescence recovery after photobleaching in planar supported lipid bilayers composed of porcine brain phosphatidylcholine (bPC), porcine brain sphingomyelin (bSM), and cholesterol to map the composition-dependence of l(d)/l(o) phase coexistence. Cholesterol decreases the fluidity of bPC bilayers, but disrupts the highly ordered gel phase of bSM, leading to a more fluid membrane. When mixed with bPC/bSM (1:1) or bPC/bSM (2:1), cholesterol induces the formation of l(o) phase domains. The fraction of the membrane in the l(o) phase was found to be directly proportional to the cholesterol concentration in both phospholipid mixtures, which implies that a significant fraction of bPC cosegregates into l(o) phase domains. Images reveal a percolation threshold, i.e., the point where rafts become connected and fluid domains disconnected, when 45-50% of the total membrane is converted to the l(o) phase. This happens between 20 and 25 mol % cholesterol in 1:1 bPC/bSM bilayers and between 25 and 30 mol % cholesterol in 2:1 bPC/bSM bilayers at room temperature, and at approximately 35 mol % cholesterol in 1:1 bPC/bSM bilayers at 37 degrees C. Area fractions of l(o) phase lipids obtained in multilamellar liposomes by a fluorescence resonance energy transfer method confirm and support the results obtained in planar lipid bilayers.     

Direct and potent regulation of gamma-secretase by its lipid microenvironment

gamma-Secretase is an unusual and ubiquitous aspartyl protease with an intramembrane catalytic site that cleaves many type-I integral membrane proteins, most notably APP and Notch. Several reports suggest that cleavage of APP to produce the Abeta peptide is regulated in part by lipids. As gamma-secretase is a multipass protein complex with 19 transmembrane domains, it is likely that the local lipid composition of the membrane can regulate gamma-activity. To determine the direct contribution of the lipid microenvironment to gamma-secretase activity, we purified the human protease from overexpressing mammalian cells, reconstituted it in vesicles of varying lipid composition, and examined the effects of individual phospholipids, sphingolipids, cholesterol, and complex lipid mixtures on substrate cleavage. A conventional gamma-activity assay was modified to include a detergent-removal step to facilitate proteoliposome formation, and this increased baseline activity over 2-fold. Proteoliposomes containing sphingolipids significantly increased gamma-secretase activity over a phosphatidylcholine-only baseline, whereas the addition of phosphatidylinositol significantly decreased activity. Addition of soluble cholesterol in the presence of phospholipids and sphingolipids robustly increased the cleavage of APP- and Notch-like substrates in a dose-dependent manner. Reconstitution of gamma-secretase in complex lipid mixtures revealed that a lipid raft-like composition supported the highest level of activity compared with other membrane compositions. Taken together, these results demonstrate that membrane lipid composition is a direct and potent modulator of gamma-secretase and that cholesterol, in particular, plays a major regulatory role.     

The effects of temperature, pressure and peptide incorporation on ternary model raft mixtures--a Laurdan fluorescence spectroscopy study

Recently, an increasing evidence accumulated for the existence of lipid microdomains, called lipid rafts, in cell membranes, which may play an important role in many important membrane-associated biological processes. Suitable model systems for studying biophysical properties of lipid rafts are lipid vesicles composed of three-component lipid mixtures, such as POPC/SM/cholesterol, which exhibit a rich phase diagram, including raft-like liquid-ordered/liquid-disordered phase coexistence regions. We explored the temperature, pressure and concentration-dependent phase behavior of such canonical model raft mixtures using the Laurdan fluorescence spectroscopic technique. Hydrostatic pressure has not only been used as a physical parameter for studying the stability and energetics of these systems, but also because high pressure is an important feature of certain natural membrane environments. We show that the liquid-disordered/liquid-ordered phase coexistence regions of POPC/SM/cholesterol model raft mixtures extends over a very wide temperature range of about 50 degrees C. Upon pressurization, an overall ordered membrane state is reached at pressures of approximately 1,000 bar at 20 degrees C, and of approximately 2,000 bar at 40 degrees C. Incorporation of 5 mol% gramicidin as a model ion channel slightly increases the overall order parameter profile in the l(o)+l(d) two-phase coexistence region, probably by selectively partitioning into l(d) domains, does not change the overall phase behavior, however. This behavior is in contrast to the effect of the peptide incorporation into simple, one-component phospholipid bilayer systems.     

Direct measurement of phase coexistence in DPPC/cholesterol vesicles using Raman spectroscopy

The phase behavior of bilayers of binary mixtures of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and cholesterol has been studied using Raman spectroscopy. It is observed that the shape of the cholesterol vibrational spectrum in lipid-cholesterol binary mixtures does not vary significantly with either the cholesterol concentration or the temperature. This permits determination of the lipid vibrational signatures of the liquid-disordered (l(d)), solid-ordered (s(o)) and liquid-ordered (l(o)) phases. Within the phase coexistence region, the measured spectra are described very well by a linear combination of the different spectral components, which permits a quantitative analysis of the phase diagram. In contrast to earlier findings, our experiments provide no indication of a phase boundary at low cholesterol concentration. The upper boundary of the phase coexistence region is found at approximately 27 and approximately 22 mol% for l(d)-l(o) and s(o)-l(o) coexistence region, respectively. Within these phase coexistence regions, the partitioning of cholesterol between the cholesterol-poor and the cholesterol-rich phases is in close agreement with the lever rule.     

Lateral diffusion coefficients of separate lipid species in a ternary raft-forming bilayer: a Pfg-NMR multinuclear study

By isotopical labeling lipid lateral diffusion coefficients for each of the membrane constituents, including cholesterol, have been measured by 1H, 2H, and 19F pulsed field gradient NMR spectroscopy in macroscopically oriented lipid bilayers. This provides a means of obtaining detailed dynamic and compositional information in raft-forming lipid bilayers without introducing foreign molecules into the systems. The raft systems studied contained dioleoylphosphatidylcholine/dipalmitoylphosphatidylcholine (DPPC)/cholesterol at the molar ratios of 42.5:42.5:15 and 35:35:30 in excess water. At temperatures below 30 degrees C the raft system forms large (>1 microm) domains of a liquid ordered (l(o)) phase, in which the lipid lateral diffusion was approximately 5 times slower than for the lipids in the surrounding liquid disordered (l(d)) phase. Within each domain all lipid species showed the same diffusion coefficient, despite the very different structures of cholesterol and phospholipids. DPPC partitions exclusively into the l(o) domains, whereas cholesterol and dioleoylphosphatidylcholine were distributed in both l(o) and l(d) phases. The cholesterol concentration was found to be 10-20 mol % in the l(d) domain and 30-40 mol % in the l(o) domain. Comparison of these results with data from sphingomyelin-containing systems suggests that DPPC interacts more weakly with cholesterol than does sphingomyelin.     

The polar nature of 7-ketocholesterol determines its location within membrane domains and the kinetics of membrane microsolubilization by apolipoprotein A-I

7-Ketocholesterol is an oxidized derivative of cholesterol with numerous physiological effects. In model membranes, 7-ketocholesterol and cholesterol were compared by physical measures of bilayer order and polarity, formation of detergent resistant domains (DRM), phase separation, and membrane microsolubilization by apolipoprotein A-I. In binary mixtures of a saturated phosphatidylcholine (PC), dipalmitoyl-PC (DPPC), and cholesterol or 7-ketocholesterol, the sterols modulate bilayer order and polarity and induce DRM formation to a similar extent. Cholesterol induces formation of ordered lipid domains (rafts) in tertiary mixtures with dioleoyl-PC (DOPC) and DPPC, or DOPC and sphingomyelin (SM). In tertiary mixtures, cholesterol increased lipid order and reduces bilayer polarity more than 7-ketocholesterol. This effect was more pronounced when the mixtures were in a miscible liquid-disordered (L(d)) phase. Substitution of 7-ketocholesterol for cholesterol dramatically reduced the extent of DRM formation in DOPC/DPPC and DOPC/SM bilayers and ordered lipid phase separation in mixtures of a spin-labeled PC with DPPC and with SM. Compared to cholesterol, 7-ketocholesterol decreased the rate for the microsolubilization of dimyristoyl-PC multilamellar vesicles by apolipoprotein A-I. The membrane effects of 7-ketocholesterol were dependent on the phospholipid matrix. In L(d) phase phospholipids, a model for 7-ketocholesterol indicates that the proximity of the 7-keto and 3beta-OH groups puts both polar moieties at the lipid-water interface to tilt the sterol nucleus to the plane of the bilayer. 7-Ketocholesterol was less effective in forming ordered lipid domains, in decreasing the level of bilayer hydration, and in forming phase boundary bilayer defects. Compared to cholesterol, 7-ketocholesterol can differentially modulate membrane properties involved in protein-membrane association and function.     

Transmembrane helices can induce domain formation in crowded model membranes

We studied compositionally heterogeneous multi-component model membranes comprised of saturated lipids, unsaturated lipids, cholesterol, and α-helical TM protein models using coarse-grained molecular dynamics simulations. Reducing the mismatch between the length of the saturated and unsaturated lipid tails reduced the driving force for segregation into liquid-ordered (l(o)) and liquid-disordered (l(d)) lipid domains. Cholesterol depletion had a similar effect, and binary lipid mixtures without cholesterol did not undergo large-scale phase separation under the simulation conditions. The phase-separating ternary dipalmitoyl-phosphatidylcholine (DPPC)/dilinoleoyl-PC (DLiPC)/cholesterol bilayer was found to segregate into l(o) and l(d) domains also in the presence of a high concentration of ΤΜ helices. The l(d) domain was highly crowded with TM helices (protein-to-lipid ratio ~1:5), slowing down lateral diffusion by a factor of 5-10 as compared to the dilute case, with anomalous (sub)-diffusion on the μs time scale. The membrane with the less strongly unsaturated palmitoyl-linoleoyl-PC instead of DLiPC, which in the absence of TM α-helices less strongly deviated from ideal mixing, could be brought closer to a miscibility critical point by introducing a high concentration of TM helices. Finally, the 7-TM protein bacteriorhodopsin was found to partition into the l(d) domains irrespective of hydrophobic matching. These results show that it is possible to directly study the lateral reorganization of lipids and proteins in compositionally heterogeneous and crowded model biomembranes with coarse-grained molecular dynamics simulations, a step toward simulations of realistic, compositionally complex cellular membranes. This article is part of a Special Issue entitled: Protein Folding in Membranes.     

Enzyme-catalyzed oxidation of cholesterol in pure monolayers at the air/water interface.

Mean molecular area vs. lateral surface pressure isotherms were determined for monolayers containing cholesterol, 4-cholesten-3-one (cholestenone), or binary mixtures of the two. At all lateral surface pressures examined, cholestenone had a larger mean molecular area requirement than cholesterol. Results with the binary mixtures of cholesterol and cholestenone suggested that the sterols did not mix ideally (non additive mean molecular area) with each other in the monolayer; the observed mean molecular area for mixtures was less than would be expected based on ideal mixing. The mixed sterol monolayers also displayed a reduction in the lateral collapse pressure which appeared to be a linear function of the mole fraction of cholestenone in the monolayer, suggesting that cholesterol and cholestenone were completely miscible in the mixed monolayer. The pure cholesterol monolayer was next used to examine the cholesterol oxidase-catalyzed (Brevibacterium sp.) oxidation of cholesterol to cholestenone at different lateral surface pressures at 22 degrees C. The difference in mean molecular area requirements of cholesterol and cholestenone was directly used to convert monolayer area changes (at constant lateral surface pressure) into average reaction rates. It was observed that the average catalytic activity of cholesterol oxidase increased linearly with increased lateral surface pressure in the range of 1 to 20 mN/m. In addition, the enzyme was capable to oxidize cholesterol in monolayers with a lateral surface pressure close to the collapse pressure of cholesterol monolayers (collapse pressure 45 mN/m; oxidation was observed at 40 mN/m). The adsorption of cholesterol oxidase to an inert sterol monolayer film at low surface pressures (around 9 mN/m) was marginal, although clearly detectable at very low (0.5-4 mN/m) lateral surface pressures, suggesting that the enzyme did not penetrate deeply into the monolayer in order to reach the 3 beta-hydroxy group of cholesterol. This interpretation is further supported by the finding that a maximally compressed cholesterol monolayer (40 mN/m) was readily susceptible to enzyme-catalyzed oxidation. It is concluded that cholesterol oxidase is capable of oxidizing cholesterol in laterally expanded monolayers as well as in tightly packed monolayers, where the lateral surface pressure is close to the collapse pressure. The kinetic results suggested that the rate-limiting step in the overall process was the substrate availability per surface area (or surface concentration) at the water/lipid interface.     

Fluid-fluid membrane microheterogeneity: a fluorescence resonance energy transfer study

Large unilamellar vesicles of dimyristoylphosphatidylcholine/cholesterol mixtures were studied using fluorescence techniques (steady-state fluorescence intensity and anisotropy, fluorescence lifetime, and fluorescence resonance energy transfer (FRET)). Three compositions (cholesterol mole fraction 0.15, 0.20, and 0.25) and two temperatures (30 and 40 degrees C) inside the coexistence range of liquid-ordered (l(o)) and liquid-disordered (l(d)) phases were investigated. Two common membrane probes, N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-dimyristoylphosphatidylethanolamine (NBD-DMPE) and N-(lissamine(TM)-rhodamine B)-dimyristoylphosphatidylethanolamine (Rh-DMPE), which form a FRET pair, were used. The l(o)/l(d) partition coefficients of the probes were determined by individual photophysical measurements and global analysis of time-resolved FRET decays. Although the acceptor, Rh-DMPE, prefers the l(d) phase, the opposite is observed for the donor, NBD-DMPE. Accordingly, FRET efficiency decreases as a consequence of phase separation. Comparing the independent measurements of partition coefficient, it was possible to detect very small domains (<20 nm) of l(o) in the cholesterol-poor end of the phase coexistence range. In contrast, domains of l(d) in the cholesterol-rich end of the coexistence range have comparatively large size. These observations are probably related to different processes of phase separation, nucleation being preferred in formation of l(o) phase from initially pure l(d), and domain growth being faster in formation of l(d) phase from initially pure l(o).     

Imaging and shape analysis of GUVs as model plasma membranes: effect of trans DOPC on membrane properties

Unsaturated trans fatty acids have been linked to a higher incidence of coronary artery disease, but not enough is known about the effect of trans lipids on membrane properties. Liquid-ordered (l(o)) and liquid-disordered (l(d)) membrane domains are implicated in various biological processes, such as endocytosis, adhesion, signaling, protein transport, apoptosis, and disease pathogenesis. The physical forces that induce domain formation and thus orchestrate cell function need to be further addressed and quantified. Here, we test the effect of trans DOPC (dielaidoyl phosphatidylcholine or DEPC) on the morphology of giant unilamellar vesicles (GUVs, used as a biomembrane model) made by electroformation with varying compositions of egg sphingomyelin, trans DOPC, cis DOPC, and cholesterol. GUVs were imaged by confocal fluorescence microscopy and then analyzed for changes in membrane morphology and properties such as l(o)/l(d) phase coexistence and area fractions, distribution of meridional curvature, and fluorescent-probe intensity distribution. BODIPY-FL-C(12)-sphingomyelin, Lissamine rhodamine B dioleoylphosphatidylethanolamine and BODIPY-TR-C(12)-sphingomyelin were used as fluorescent probes to differentially label the l(o) and l(d) phases. Trans DOPC induces some vesicles to form multidomain, invaginated morphologies that differ from the typical two-domain circular and truncated spherical shapes observed in its absence. Trans DOPC also alters the membrane curvature distribution; this is more pronounced in the l(o) phase near the phase boundary, where significantly negative curvatures (<-0.5 microm(-1)) are observed. A narrower distribution of meridional curvatures in GUVs with trans DOPC is suggestive of higher membrane bending rigidity. The ratio of average fluorescent intensities in the l(d)/l(o) phases indicates a greater concentration or brightness of the probes BODIPY-FL-C(12)-sphingomyelin and BODIPY-TR-C(12)-sphingomyelin in the l(o) phase in the presence of trans DOPC. Addition of trans DOPC does not alter the l(o)/l(d) area fractions, indicating that it does not act like egg sphingomyelin, a saturated lipid. These changes in membrane properties seen in the presence of trans lipids could significantly impact cell function.     

The characterization of plasma membrane Ca2+-ATPase in rich sphingomyelin-cholesterol domains

According to the raft hypothesis, sphingolipid-cholesterol (CHOL) microdomains are involved in numerous cellular functions. Here, we have prepared liposomes to simulate the lipid composition of rafts/caveolae using phosphatidylchone, sphingomyelin (SPM)-CHOL in vitro. Experiments of both 1,6-diphenyl-1,3,5-hexatriene and merocyanine-540 fluorescence showed that a phase transition from l(d) to l(o) can be observed clearly. In particular, we investigated the behavior of a membrane protein, plasma membrane Ca(2+)-ATPase (PMCA), in lipid rafts (l(o) phase). Three complementary approaches to characterize the physical appearance of PMCA were employed in the present study. Tryptophan intrinsic fluorescence increase, fluorescence quenching by both acrylamid and hypocrellin B decrease, and MIANS fluorescence decrease, indicate that the conformation of PMCA embedded in lipid l(o) phase is more compact than in lipid l(d) phase. Also, our results showed that PMCA activity decreased with the increase of SPM-CHOL content, in other words, with the increase of l(o) phase. This suggests that the specific domains containing high SPM-CHOL concentration are not a favorable place for PMCA activity. Finally, a possible explanation about PMCA molecules concentrated in caveolae/rafts was discussed.     

Microsomal cholesterol ester hydrolase of rat brain: lipids as effector of enzymatic activity

Evidence is presented that lipid plays an important role in the function of the microsomal cholesterol ester hydrolase of rat brain. The catalytic activity was almost completely lost when most of cholesterol and up to 70% of phospholipids were removed from lyophilized microsomes by extraction with chloroform at ?20 °C. The activity was completely restored when the chloroform-extracted lipid was added back to the assay mixture in the amount equal to the original concentration. Cholesterol or individual phospholipid alone was not effective in reconstituting the lost enzymatic activity. Effective restoration of the activity required addition of cholesterol and a phospholipid. Among the phospholipids tested, phosphatidylserine was the most effective, followed by ethanolamine phospholipids and phosphatidylcholine. The apparent V was dependent on the amount of the lipid added, while the K_m for the substrate, cholesteryl oleate, remained relatively constant, indicating that the effect of the added lipid was primarily on the reaction rate and not on the affinity of the enzyme to the substrate. The similar lipid dependence was observed with the Triton X-100-solubilized enzyme preparation. When the lipid phase of the microsomal membrane was perturbed, the enzyme became unstable when heated at 50 °C and its activity showed a discontinuity in the Arrhenius plots. Therefore, not only the concentration of the added lipid but also the physical state of the lipid phase around the enzyme appeared to be important for the activity of the rat brain microsomal cholesterol ester hydrolase.     

Kinetics and thermodynamics of association of a fluorescent lysophospholipid derivative with lipid bilayers in liquid-ordered and liquid-disordered phases

We have measured the rates of insertion into, desorption from, and spontaneous interlayer translocation (flip-flop) of the fluorescent lysophospholipid derivative NBD-lyso-1-myristoylphosphatidylethanolamine in l(d) and l(o) phase lipid bilayer membranes. The lipid bilayers, studied as LUV, were prepared from pure 1-palmitoyl-2-oleoylphosphatidylcholine, in the l(d) phase; and from two Chol-containing binary lipid mixtures, 1-palmitoyl-2-oleoylphosphatidylcholine and Chol (molar ratio of 1:1) and SpM and Chol (molar ratio of 6:4), both in the l(o) phase. Insertion, desorption, and translocation rate constants and equilibrium constants for association of the amphiphile monomer with the lipid bilayers were measured between 15 degrees C and 35 degrees C, and the standard free energies, enthalpies, and entropies, as well as the activation energies for these processes were derived from these data. The equilibrium partition coefficients for partitioning of the amphiphile between the aqueous phase and the different membrane phases were also derived, and an estimation was made of hypothetical partition coefficients and the respective energetic parameters for partitioning between the different lipid phases if these were to coexist in the same membrane. We show that, contrary to general belief, the association of NBD-lysoMPE with lipid bilayers is not a diffusion-controlled process, the rate-limiting step in insertion being the formation of a free area in the membrane surface of an adequate size for insertion to occur.     

Kinetics of amphiphile association with two-phase lipid bilayer vesicles

We examined the consequences of membrane heterogeneity for the association of a simple amphiphilic molecule with phospholipid vesicles with solid-liquid and liquid-liquid phase coexistence. To address this problem we studied the association of a single-chain, fluorescent amphiphile with dimyristoylphosphatidylcholine (DMPC) vesicles containing varying amounts of cholesterol. DMPC bilayers containing 15 mol% cholesterol show a region of solid-liquid-ordered (s-l(o)) coexistence below the T(m) of pure DMPC (23.9 degrees C) and a region of liquid-disordered-liquid-ordered coexistence (l(d)-l(o)) above the T(m). We first examined equilibrium binding and kinetics of amphiphile insertion into single-phase vesicles (s, l(d), and l(o) phase). The data obtained were then used to predict the behavior of the equivalent process in a two-phase system, taking into account the fractions of phases present. Next, the predicted kinetics were compared to experimental kinetics obtained from a two-phase system. We found that association of the amphiphile with lipid vesicles is not influenced by the existence of l(d)-l(o) phase boundaries but occurs much more slowly in the s-l(o) phase coexistence region than expected on the basis of phase composition.     

Lipid lateral diffusion in bilayers with phosphatidylcholine, sphingomyelin and cholesterol. An NMR study of dynamics and lateral phase separation

Pulsed field gradient (pfg)-NMR measurements of the lipid lateral diffusion coefficients in several macroscopically aligned bilayer systems were summarized from previous and new studies. The aim was to carry out a comparison of the translational dynamics for bilayers with various mixtures of l,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), l,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and chicken egg yolk sphingomyelin (eSM), with or without cholesterol. New useful information was obtained on the dynamics in these lipid bilayers that has not been previously appreciated. Thus, we were able to propose that the driving force behind the phase separation into l(d)and l(o)phases evolves from the increasing difficulty to incorpotate DOPC into a highly ordered phase. Our results suggest that DOPC has a preference to be located in a disordered phase, while DPPC and eSM prefer the ordered phase. Quite unexpectedly, CHOL seems to partition into both phases to roughly the same extent, indicating that CHOL has no particular preference for any of the l(d)or l(o) phases, and there are no specific interactions between CHOL and saturated lipids.     

Ethanol effects on binary and ternary supported lipid bilayers with gel/fluid domains and lipid rafts

Ethanol-lipid bilayer interactions have been a recurrent theme in membrane biophysics, due to their contribution to the understanding of membrane structure and dynamics. The main purpose of this study was to assess the interplay between membrane lateral heterogeneity and ethanol effects. This was achieved by in situ atomic force microscopy, following the changes induced by sequential ethanol additions on supported lipid bilayers formed in the absence of alcohol. Binary phospholipid mixtures with a single gel phase, dipalmitoylphosphatidylcholine (DPPC)/cholesterol, gel/fluid phase coexistence DPPC/dioleoylphosphatidylcholine (DOPC), and ternary lipid mixtures containing cholesterol, mimicking lipid rafts (DOPC/DPPC/cholesterol and DOPC/sphingomyelin/cholesterol), i.e., with liquid ordered/liquid disordered (ld/lo) phase separation, were investigated. For all compositions studied, and in two different solid supports, mica and silicon, domain formation or rearrangement accompanied by lipid bilayer thinning and expansion was observed. In the case of gel/fluid coexistence, low ethanol concentrations lead to a marked thinning of the fluid but not of the gel domains. In the case of ld/lo all the bilayer thins simultaneously by a similar extent. In both cases, only the more disordered phase expanded significantly, indicating that ethanol increases the proportion of disordered domains. Water/bilayer interfacial tension variation and freezing point depression, inducing acyl chain disordering (including opening and looping), tilting, and interdigitation, are probably the main cause for the observed changes. The results presented herein demonstrate that ethanol influences the bilayer properties according to membrane lateral organization.     

Exposure of phosphatidylinositol transfer proteins to sphingomyelin-cholesterol membranes suggests transient but productive interactions with raft-like, liquid-ordered domains

Both isoforms of rat phosphatidylinositol transfer protein (PITP) mediate the intermembrane transfer of sphingomyelin (CerPCho). In the plasma membrane, CerPCho often segregates with cholesterol into microdomains such as lipid rafts and caveolae. To test the hypothesis that PITP exhibits a preference for CerPCho- and cholesterol-rich membranes, we prepared unilamellar vesicles containing variable amounts of these two lipids. We also used CerPCho species with different acyl composition and treated vesicles with agents known to sequester and remove cholesterol. We observed that the beta isoform of rat PITP was more sensitive to membrane cholesterol than was the alpha isoform, as shown by increases in specific activities of lipid transfer of 2-6-fold. A relatively high membrane content of cholesterol (mole fraction > 0.4) was required to elicit such enhancements. Treatment of cholesterol-rich membranes with a series of beta cyclodextrins demonstrated that, upon depletion of cholesterol from participating membranes, the PITPbeta activity changes were fully reversible. We finally noted that the mechanism by which cholesterol enhances the activity of PITPbeta appeared to involve a decreased affinity of the protein for the membrane surface, in a manner that was independent of vesicle size and membrane microviscosity. We conclude that PITPbeta interacts transiently but productively with the liquid-ordered phase formed by CerPCho and cholesterol and discuss the possibility of PITP interactions in vivo with sphingolipid- and cholesterol-rich membrane microdomains.     

Lipid lateral diffusion and membrane heterogeneity

The pulsed field gradient (pfg)-NMR method for measurements of translational diffusion of molecules in macroscopically aligned lipid bilayers is described. This technique is proposed to have an appreciable potential for investigations in the field of lipid and membrane biology. Transport of molecules in the plane of the bilayer can be successfully studied, as well as lateral phase separation of lipids and their dynamics within the bilayer organizations. Lateral diffusion coefficients depend on lipid packing and acyl chain ordering and investigations of order parameters of perdeuterated acyl chains, using (2)H NMR quadrupole splittings, are useful complements. In this review we summarize some of our recent achievements obtained on lipid membranes. In particular, bilayers exhibiting two-phase coexistence of liquid disordered (l(d)) and liquid ordered (l(o)) phases are considered in detail. Methods for obtaining good oriented lipid bilayers, necessary for the pfg-NMR method to be efficiently used, are also briefly described. Among our major results, besides determinations of l(d) and l(o) phases, belongs the finding that the lateral diffusion is the same for all components, independent of the molecular structure (including cholesterol (CHOL)), if they reside in the same domain or phase in the membrane. Furthermore, quite unexpectedly CHOL seems to partition into the l(d)and l(o) phases to roughly the same extent, indicating that CHOL has no strong preference for any of these phases, i.e. CHOL seems to have similar interactions with all of the lipids. We propose that the lateral phase separation in bilayers containing one high-T(m) and one low-T(m) lipid together with CHOL is driven by the increasing difficulty of incorporating an unsaturated or prenyl lipid into the highly ordered bilayer formed by a saturated lipid and CHOL, i.e. the phase transition is entropy driven to keep the disorder of the hydrocarbon chains of the unsaturated lipid.     

The effect of cholesterol on the lateral diffusion of phospholipids in oriented bilayers

Pulsed field gradient NMR was utilized to directly determine the lipid lateral diffusion coefficient for the following macroscopically aligned bilayers: dimyristoylphosphatidylcholine (DMPC), sphingomyelin (SM), palmitoyloleoylphosphatidylcholine (POPC), and dioleoylphosphatidylcholine (DOPC) with addition of cholesterol (CHOL) up to approximately 40 mol %. The observed effect of cholesterol on the lipid lateral diffusion is interpreted in terms of the different diffusion coefficients obtained in the liquid ordered (l(o)) and the liquid disordered (l(d)) phases occurring in the phase diagrams. Generally, the lipid lateral diffusion coefficient decreases linearly with increasing CHOL concentration in the l(d) phase for the PC-systems, while it is almost independent of CHOL for the SM-system. In this region the temperature dependence of the diffusion was always of the Arrhenius type with apparent activation energies (E(A)) in the range of 28-40 kJ/mol. The l(o) phase was characterized by smaller diffusion coefficients and weak or no dependence on the CHOL content. The E(A) for this phase was significantly larger (55-65 kJ/mol) than for the l(d) phase. The diffusion coefficients in the two-phase regions were compatible with a fast exchange between the l(d) and l(o) regions in the bilayer on the timescale of the NMR experiment (100 ms). Thus, strong evidence has been obtained that fluid domains (with size of micro m or less) with high molecular ordering are formed within a single lipid bilayer. These domains may play an important role for proteins involved in membrane functioning frequently discussed in the recent literature. The phase diagrams obtained from the analysis of the diffusion data are in qualitative agreement with earlier published ones for the SM/CHOL and DMPC/CHOL systems. For the DOPC/CHOL and the POPC/CHOL systems no two-phase behavior were observed, and the obtained E(A):s indicate that these systems are in the l(d) phase at all CHOL contents for temperatures above 25 degrees C.