Phase and surface properties of lipid bilayers containing neoglycolipids

The physical properties conferred to DPPC bilayers by including neoglycolipids composed by two different trisaccharides: mannose-mannose-mannose (3M) and glucose-mannose-glucose (GMG) attached to a cholesterol (cho) and a distearylglycerol (diC18) lipid moiety by a spacer were evaluated by means of the measurement of the electrokinetic potential and interfacial fluorescent probes. The phase properties measured with diphenylhexatriene (DPH) were correlated with the surface properties measured with merocyanine 540, dansyl, and Laurdan probes. The results show that the surface properties of large unilamellar vesicles depend on the sugar exposure to the water phase and also on the hydrocarbon moiety by which it is anchored to the bilayer. The combination of the cholesterol moiety with the saccharide attenuates the cooperativity decrease induced by the cholesterol moiety without the sugar portion. The neoglycolipid GMG-diC18 promotes opposite effects affecting slightly the cooperativity at the hydrocarbon core of DPPC and displacing the phase transition temperature to higher values. The presence of neoglycolipid with diC18 introduces defects in the packing at the interface of the membrane in the gel state. It is concluded that a relatively low proportion of neoglycolipids affects significantly the interfacial properties of DPPC bilayers in large unilamellar vesicles in the absence of changes at the membrane bulk at 25 degrees C.     

Structure and dynamics of sphingomyelin bilayer: insight gained through systematic comparison to phosphatidylcholine

Sphingomyelin, one of the main lipid components of biological membranes, is actively involved in various cellular processes such as protein trafficking and signal transduction. In particular, specific lateral domains enriched in sphingomyelin and cholesterol have been proposed to play an important functional role in biomembranes, although their precise characteristics have remained unclear. A thorough understanding of the functional role of membranes requires detailed knowledge of their individual lipid components. Here, we employ molecular dynamics simulations to conduct a systematic comparison of a palmitoylsphingomyelin (PSM, 16:0-SM) bilayer with a membrane that comprises dipalmitoylphosphatidylcholine (DPPC) above the main phase transition temperature. We clarify atomic-scale properties that are specific to sphingomyelin due to its sphingosine moiety, and further discuss their implications for SM-rich membranes. We find that PSM bilayers, and in particular the dynamics of PSM systems, are distinctly different from those of a DPPC bilayer. When compared with DPPC, the strong hydrogen bonding properties characteristic to PSM are observed to lead to considerable structural changes in the polar headgroup and interface regions. The strong ordering of PSM acyl chains and specific ordering effects in the vicinity of a PSM-water interface reflect this issue clearly. The sphingosine moiety and related hydrogen bonding further play a crucial role in the dynamics of PSM bilayers, as most dynamic properties, such as lateral and rotational diffusion, are strongly suppressed. This is most evident in the rotational motion characterized by spin-lattice relaxation times and the decay of hydrogen bond autocorrelation functions that are expected to be important in complexation of SM with other lipids in many-component bilayers. A thorough understanding of SM bilayers would greatly benefit from nuclear magnetic resonance experiments for acyl chain ordering and dynamics, allowing full comparison of these simulations to experiments.     

Membrane properties of and cholesterol's interactions with a biologically relevant three-chain sphingomyelin: 3O-palmitoyl-N-palmitoyl-D-erythro-sphingomyelin

Sphingomyelins (SMs) are order-imposing phospholipids in cell membranes which interact favorably with cholesterol. The hydrophobic part of SM constitutes a long-chain base with an amide-linked acyl chain, whereas the polar head group is phosphocholine. The long-chain base has a free hydroxyl group in position 3, which is an important donor/acceptor in hydrogen bonding. In newborn mammals, a SM in which a palmitic acid is esterified to the 3-OH has been reported. We have synthesized this SM analog (3O-P-PSM) and studied its properties in bilayer membranes, and also determined its interactions with cholesterol. Fully hydrated 3O-P-PSM bilayers underwent a gel-to-liquid crystalline phase transition at 55.5 °C (ΔH 8 kcal/mol), which is about 15 °C higher than the phase transition temperature of PSM. The 3O-P-PSM displayed rather poor miscibility with PSM in mixed bilayers, suggesting that the third acyl chain interfered significantly with lateral interactions. Bilayers made from 3O-P-PSM were much more resistant to detergent-induced solubilization than bilayers made from PSM. In binary bilayers, cholesterol was able to destabilize the gel phase, and order the fluid phase of 3O-P-PSM, in a concentration-dependent manner. Cholesterol was also able to form sterol-enriched ordered domains with 3O-P-PSM in fluid POPC bilayers. The interaction between cholesterol and 3O-P-PSM was not, however, as favorable as the interaction between cholesterol and PSM. It is unclear what physiological role 3O-P-PSM could play in newborn mammalian membranes. However, it is clear that 3O-P-PSM will form more highly ordered domains than PSM while still having a limited ability to interact with cholesterol.     

Importance of the phosphocholine linkage on sphingomyelin molecular properties and interactions with cholesterol; a study with phosphate oxygen modified sphingomyelin-analogues

We have characterized the molecular properties and membrane behavior of synthetically modified sphingomyelin analogues, modified on the oxygen connecting the phosphocholine group to the ceramide backbone. The oxygen was replaced with an S-atom (S-PSM), an NH-group (NH-PSM) or a CH(2)-group (CH(2)-PSM). Diphenylhexatriene and Laurdan anisotropy experiments showed that an S-linkage increased and NH- and CH(2)-linkages decreased the stability of PSM-analogue bilayer membranes as compared to PSM. When the polarity of the interface was probed using Laurdan, S-PSM appeared to have a lower polarity as compared to PSM whereas NH-PSM and CH(2)-PSM had higher polarities of their respective interfaces. Fluorescence quenching-studies with cholestatrienol showed that all compounds formed SM/cholesterol-rich domains. The S-PSM/cholesterol and PSM/cholesterol domains displayed a similar thermostability, whereas NH-PSM/cholesterol and CH(2)-PSM/cholesterol domains were less thermostable. DSC on vesicles containing the PSM-analogues showed a more complex melting behavior as compared to PSM, whereas equimolar mixtures of the PSM-analogues and PSM showed almost ideal mixing with PSM for NH- and S-PSM. Our data show that the properties of the bond linking the phosphocholine head group to the 1-hydroxyl on the ceramide molecule is important for the stability of SM/SM and SM/cholesterol interactions.     

Examining the effects of cholesterol on model membranes at high temperatures: Laurdan and Patman see it differently

At high temperature, the presence of cholesterol in phospholipid membranes alters the influence of membrane dipoles, including water molecules, on naphthalene-based fluorescent probes such as Laurdan and Patman (solvatochromism). Although both of these probes report identical changes to their emission spectra as a function of temperature in pure phosphatidylcholine bilayers, they differ in their response to cholesterol. Computer simulations of the spectra based on a simple model of solvatochromism indicated that the presence of cholesterol reduces the probability of bilayer dipole relaxation and also blunts the tendency of heat to enhance that probability. While the overall effect of cholesterol on membrane dipoles was detected identically by the two probes, Laurdan was influenced much more by the additional effect on temperature sensitivity than was Patman. A comparison of the fluorescence data with simulations using a coarse-grained bilayer model (de Meyer et al., 2010) suggested that these probes may be differentially sensitive to two closely related properties distinguishable in the presence of cholesterol. Specifically, Patman fluorescence correlated best with the average phospholipid acyl chain order. On the other hand, Laurdan fluorescence tracked more closely with the area per lipid molecule which, although affected generally by chain order, is also impacted by additional membrane-condensing effects of cholesterol. We postulate that this difference between Laurdan and Patman may be attributed to the bulkier charged headgroup of Patman which may cause the probe to preferentially locate in juxtaposition to the diminutive headgroup of cholesterol as the membrane condenses.     

Structure and cohesive properties of sphingomyelin/cholesterol bilayers.

Thermal, structural, and cohesive measurements have been obtained for both bovine brain sphingomyelin (BSM) and N-tetracosanoylsphingomyelin (C24-SM) in the presence and absence of cholesterol. A goal of these experiments has been to clarify the mechanisms responsible for the strong interaction between sphingomyelin and cholesterol. Differential scanning calorimetry shows that fully hydrated bilayers of BSM and C24-SM have main endothermic phase transitions at 39 and 46 degrees C, respectively, that reflect the melting of the acyl chains from a gel to a liquid-crystalline phase. For each lipid, the addition of cholesterol monotonically reduces the enthalpy of this transition, so that at equimolar cholesterol the transition enthalpy is zero. The addition of equimolar cholesterol to either BSM or C24-SM coverts the wide-angle X-ray diffraction reflection at 4.15 A to a broad band centered at 4.5 A. Electron density profiles of gel-phase C24-SM bilayers contain two terminal methyl dips in the center of the bilayer, indicating that the lipid hydrocarbon chains partially interdigitate so that the long saturated 24-carbon acyl chains in one monolayer cross the bilayer center and appose the shorter sphingosine chains from the other monolayer. The incorporation of cholesterol adds electron density to the hydrocarbon chain region near the head group and removes the double terminal methyl dip. These wide- and low-angle X-ray data indicate that cholesterol packs into the hydrocarbon chain region near the sphingomyelin head group, fluidizes the methylene chains near the center of the bilayer compared to the gel phase, and reduces the extent of methylene chain interdigitation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Importance of the phosphocholine linkage on sphingomyelin molecular properties and interactions with cholesterol; a study with phosphate oxygen modified sphingomyelin-analogues

We have characterized the molecular properties and membrane behavior of synthetically modified sphingomyelin analogues, modified on the oxygen connecting the phosphocholine group to the ceramide backbone. The oxygen was replaced with an S-atom (S-PSM), an NH-group (NH-PSM) or a CH₂-group (CH₂-PSM). Diphenylhexatriene and Laurdan anisotropy experiments showed that an S-linkage increased and NH- and CH₂-linkages decreased the stability of PSM-analogue bilayer membranes as compared to PSM. When the polarity of the interface was probed using Laurdan, S-PSM appeared to have a lower polarity as compared to PSM whereas NH-PSM and CH₂-PSM had higher polarities of their respective interfaces. Fluorescence quenching-studies with cholestatrienol showed that all compounds formed SM/cholesterol-rich domains. The S-PSM/cholesterol and PSM/cholesterol domains displayed a similar thermostability, whereas NH-PSM/cholesterol and CH₂-PSM/cholesterol domains were less thermostable. DSC on vesicles containing the PSM-analogues showed a more complex melting behavior as compared to PSM, whereas equimolar mixtures of the PSM-analogues and PSM showed almost ideal mixing with PSM for NH- and S-PSM. Our data show that the properties of the bond linking the phosphocholine head group to the 1-hydroxyl on the ceramide molecule is important for the stability of SM/SM and SM/cholesterol interactions.     

Phase separation is induced by phenothiazine derivatives in phospholipid/sphingomyelin/cholesterol mixtures containing low levels of cholesterol and sphingomyelin

Lipid rafts are membrane structures enriched in cholesterol, sphingomyelin and glycolipids. In majority raft-mimicking model systems high contents of cholesterol and sphingomyelin (approximately 30 mol%) are used. Existence of raft-like structures was, however, reported also in model and natural membranes containing low levels of cholesterol and sphingomyelin. In the present work differential scanning calorimetry and fluorescence spectroscopy with the use of Laurdan probe was employed to demonstrate the existence of phase separation in model systems containing DPPC with addition of 5 mol% or 10 mol% of both cholesterol and sphingomyelin. Additionally, the influence of three phenothiazine derivatives on phase separation in mixed DPPC/cholesterol/sphingomyelin bilayers was investigated. Chlorpromazine, thioridazine and trifluoperazine were able to induce phase separation in DPPC and DPPC/cholesterol/sphingomyelin bilayers in temperatures below lipid main phase transition. However, only trifluoperazine induced phase separation in temperatures close to or above main phase transition. Trifluoperazine also induced phase separation in bilayers composed of egg yolk PC or DOPC mixed with cholesterol and sphingomyelin. We concluded that presence of lipid domains can be observed in model membranes containing low levels of cholesterol and sphingomyelin. Among three phenothiazine derivatives studied, only trifluoperazine was able to induce a permanent phase separation in phosphatidylcholine/cholesterol/sphingomyelin systems.     

N- and O-methylation of sphingomyelin markedly affects its membrane properties and interactions with cholesterol

We have prepared palmitoyl sphingomyelin (PSM) analogs in which either the 2-NH was methylated to NMe, the 3-OH was methylated to OMe, or both were methylated simultaneously. The aim of the study was to determine how such modifications in the membrane interfacial region of the molecules affected interlipid interactions in bilayer membranes. Measuring DPH anisotropy in vesicle membranes prepared from the SM analogs, we observed that methylation decreased gel-phase stability and increased fluid phase disorder, when compared to PSM. Methylation of the 2-NH had the largest effect on gel-phase instability (T(m) was lowered by ~7°C). Atomistic molecular dynamics simulations showed that fluid phase bilayers with methylated SM analogs were more expanded but thinner compared to PSM bilayers. It was further revealed that 3-OH methylation dramatically attenuated hydrogen bonding also via the amide nitrogen, whereas 2-NH methylation did not similarly affect hydrogen bonding via the 3-OH. The interactions of sterols with the methylated SM analogs were markedly affected. 3-OH methylation almost completely eliminated the capacity of the SM analog to form sterol-enriched ordered domains, whereas the 2-NH methylated SM analog formed sterol-enriched domains but these were less thermostable (and thus less ordered) than the domains formed by PSM. Cholestatrienol affinity to bilayers containing methylated SM analogs was also markedly reduced as compared to its affinity for bilayers containing PSM. Molecular dynamics simulations revealed further that cholesterol's bilayer location was deeper in PSM bilayers as compared to the location in bilayers made from methylated SM analogs. This study shows that the interfacial properties of SMs are very important for interlipid interactions and the formation of laterally ordered domains in complex bilayers.     

Effect of sphingomyelin headgroup size on molecular properties and interactions with cholesterol

Sphingomyelins (SMs) and sterols are important constituents of the plasma membrane and have also been identified as major lipid components in membrane rafts. Using SM analogs with decreasing headgroup methylation, we systemically analyzed the effect of headgroup size on membrane properties and interactions with cholesterol. An increase in headgroup size resulted in a decrease in the main phase transition. Atom-scale molecular-dynamics simulations were in agreement with the fluorescence anisotropy experiments, showing that molecular areas increased and acyl chain order decreased with increasing headgroup size. Furthermore, the transition temperatures were constantly higher for SM headgroup analogs compared to corresponding phosphatidylcholine headgroup analogs. The sterol affinity for phospholipid bilayers was assessed using a sterol-partitioning assay and an increased headgroup size increased sterol affinity for the bilayer, with a higher sterol affinity for SM analogs as compared to phosphatidylcholine analogs. Moreover, the size of the headgroup affected the formation and composition of cholesterol-containing ordered domains. Palmitoyl-SM (the largest headgroup) seemed to attract more cholesterol into ordered domains than the other SM analogs with smaller headgroups. The ordering and condensing effect of cholesterol on membrane lipids was also largest for palmitoyl-SM as compared to the smaller SM analogs. The results show that the size of the SM headgroup is crucially important for SM-SM and SM-sterol interactions. Our results further emphasize that interfacial electrostatic interactions are important for stabilizing cholesterol interactions with SMs.     

N-cholesteryl sphingomyelin-A synthetic sphingolipid with unique membrane properties

A sphingomyelin chimera in which the amide-linked acyl chain was replaced with cholesterol carbamate was prepared and its properties examined. The sphingomyelin/cholesterol chimera (N-cholesterol-D-erythro-sphingomyelin) was able to form unilamellar vesicles of defined size when extruded through 200nm pore size membranes. These N-cholesteryl sphingomyelin bilayers were resistant to solubilization by Triton X-100. When N-cholesteryl sphingomyelin was added to N-palmitoyl sphingomyelin (N-palmitoyl-d-erythro-sphingomyelin) bilayers, it increased acyl chain order as determined by 1,6-diphenyl-1,3,5-hexatriene fluorescence anisotropy. N-cholesteryl sphingomyelin was, however, not as good an inducer of membrane order compared to cholesterol on a molar basis. Differential scanning calorimetry studies further showed that the miscibility of N-cholesteryl sphingomyelin with N-palmitoyl-d-erythro-sphingomyelin bilayers was non-ideal, and the effect of N-cholesteryl sphingomyelin on the N-palmitoyl-d-erythro-sphingomyelin gel-fluid transition enthalpy differed from that seen with cholesterol. Together with N-palmitoyl-d-erythro-sphingomyelin, the N-cholesteryl sphingomyelin chimera was able to form sterol-enriched ordered domains in a fluid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayer. N-cholesteryl sphingomyelin in the absence of N-palmitoyl-d-erythro-sphingomyelin was unable to form such sterol-enriched ordered domains in a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayer. However, N-cholesteryl sphingomyelin markedly increased the affinity of cholestatrienol for N-cholesteryl sphingomyelin containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayers, suggesting that N-cholesteryl sphingomyelin was able to somehow stabilize sterol interaction in fluid bilayers. Based on our results, we conclude that N-cholesteryl sphingomyelin behaved more like a cholesterol than a sphingolipid in fluid bilayer membranes. Because N-cholesteryl sphingomyelin increased bilayer order, conferred resistance against detergent solubilization, and is not degradable by phospholipases A(2), it could constitute a good lipocomplex matrix for drug delivery vehicles.     

Interaction of the chemopreventive agent resveratrol and its metabolite,piceatannol, with model membranes

Resveratrol and piceatannol are plant-derived polyphenols possessing extremely wide range of biological activities such as cancer chemopreventive, cardio- and neuroprotective, antioxidant, anti-inflammatory, anticancer and lifespan extending properties. Despite great interest in these stilbenes, their interactions with lipid bilayers have not been extensively studied. In the present work, the interaction of both resveratrol and piceatannol with model membranes composed of phosphatidylcholine (DMPC and DPPC) was investigated by means of fluorescence spectroscopy, differential scanning calorimetry (DSC) and electron spin resonance spectroscopy (ESR). Generalized polarization of two fluorescent probes Laurdan and Prodan measured in pure lipid and lipid:stilbene mixtures revealed that resveratrol and piceatannol changed bilayer properties in both gel-like and liquid crystalline phase and interacted with lipid headgroup region of the membrane. These findings were corroborated by DSC experiments in which the stilbene-induced decrease of lipid melting temperature and transition cooperativity were recorded. Resveratrol and piceatannol restricted also the ESR-measured mobility of spin probes GluSIN18, 5DSA and 16DSA with nitroxide group localized at different depths. Since the most pronounced effect was exerted on the spin probe located near membrane surface, we concluded that also ESR results pointed to the preferential interaction of resveratrol and piceatannol with headgroup region of lipid bilayer.     

Effects of Natural and Enantiomeric Cholesterol on the Thermotropic Phase Behavior and Structure of Egg Sphingomyelin Bilayer Membranes

Phospholipids, sphingolipids, and sterols are the major lipid components of the plasma membranes of eukaryotic cells. Because these three lipid classes occur naturally as enantiomerically pure compounds, enantiospecific lipid-lipid and lipid-sterol interactions could in principle occur in the lipid bilayers of eukaryotic plasma membranes. Although previous biophysical studies of phospholipid and phospholipid-sterol model membrane systems have consistently failed to observe such enantiomerically selective interactions, a recent monolayer study of the interactions of natural and enantiomeric cholesterol with egg sphingomyelin has apparently revealed the existence of enantiospecific sterol-sphingolipid interactions. To determine whether enantiospecific sterol-sphingolipid interactions also occur in more biologically relevant lipid-bilayer systems, differential scanning calorimetric, x-ray diffraction, and neutral buoyant-density measurements were utilized to study the effects of natural and enantiomeric cholesterol on the thermotropic phase behavior and structure of egg sphingomyelin bilayers. The calorimetry experiments show that the natural and enantiomeric cholesterol have essentially identical effects on the temperature, enthalpy, and cooperativity of the gel/liquid-crystalline phase transition of egg sphingomyelin bilayers within the limits of experimental error. As well, the x-ray diffraction and neutral buoyancy experiments indicate that bilayers formed from mixtures of natural or enantiomeric cholesterol and egg sphingomyelin have, within experimental uncertainty, the same structure and mass density. We thus conclude that significant enantioselective cholesterol-sphingolipid interactions do not occur in this lipid-bilayer model membrane system.     

Lipid interactions of an actinoporin pore-forming oligomer

The actinoporins are cytolytic toxins produced by sea anemones. Upon encountering a membrane, preferably containing sphingomyelin, they oligomerize and insert their N-terminal helix into the membrane, forming a pore. Whether sphingomyelin is specifically recognized by the protein or simply induces phase coexistence in the membrane has been debated. Here, we perform multi-microsecond molecular dynamics simulations of an octamer of fragaceatoxin C, a member of the actinoporin family, in lipid bilayers containing either pure 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) or a 1:1 mixture of DOPC and palmitoyl sphingomyelin (PSM). The complex is highly stable in both environments, with only slight fraying of the inserted helices near their N-termini. Analyzing the structural parameters of the mixed membrane in the course of the simulation, we see signs of a phase transition for PSM in the inner leaflet of the bilayer. In both leaflets, cross-interactions between lipids of different type decrease over time. Surprisingly, the aromatic loop thought to be responsible for sphingomyelin recognition interacts more with DOPC than PSM by the end of the simulation. These results support the notion that the key membrane property that actinoporins recognize is lipid phase coexistence.     

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.     

Interaction of ceramides with phosphatidylcholine, sphingomyelin and sphingomyelin/cholesterol bilayers

Ceramides (Cers) may exert their biological activity through changes in membrane structure and organization. To understand this mechanism, the effect of Cer on the biophysical properties of phosphatidylcholine, sphingomyelin (SM) and SM/cholesterol bilayers was determined using fluorescence probe techniques. The Cers were bovine brain Cer and synthetic Cers that contained a single acyl chain species. The phospholipids were 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1,2-dipalmitoyl-sn-glyero-3-phosphocholine (DPPC) and bovine brain, egg yolk and bovine erythrocyte SM. The addition of Cer to POPC and DPPC bilayers that were in the liquid-crystalline phase resulted in a linear increase in acyl chain order and decrease in membrane polarity. The addition of Cer to DPPC and SM bilayers also resulted in a linear increase in the gel to liquid-crystalline phase transition temperature (T(M)). The magnitude of the change was dependent upon Cer lipid composition and was much higher in SM bilayers than DPPC bilayers. The addition of 33 mol% cholesterol essentially eliminated the thermal transition of SM and SM/Cer bilayers. However, there is still a linear increase in acyl chain order induced by the addition of Cer. The results are interpreted as the formation of DPPC/Cer and SM/Cer lipid complexes. SM/Cer lipid complexes have higher T(M)s than the corresponding SM because the addition of Cer reduces the repulsion between the bulky headgroup and allows closer packing of the acyl chains. The biophysical properties of a SM/Cer-rich bilayer are dependent upon the amount of cholesterol present. In a cholesterol-poor membrane, a sphingomyelinase could catalyze the isothermal conversion of a liquid-crystalline SM bilayer to a gel phase SM/Cer complex at physiological temperature.     

Glycosylation induces shifts in the lateral distribution of cholesterol from ordered towards less ordered domains

Several studies have indicated the involvement of steryl glycosides in the cellular stress response. In this work, we have compared the effect of 1-O-cholesteryl-beta-d-glucoside, 1-O-cholesteryl-beta-d-galactoside and cholesterol on the properties of glycerophospholipid and sphingolipid bilayers. The studies were performed in order to gain insight into the change in membrane properties that would follow upon the glycosylation of cholesterol in cells subjected to stress. DPH anisotropy measurements indicated that the cholesteryl glycosides (10-40 mol%) increased the order of the hydrophobic region of a POPC bilayer almost as efficiently as cholesterol. In a PSM bilayer, the cholesteryl glycosides were however shown to be much less effective compared to cholesterol in ordering the hydrocarbon chain region at temperatures above the gel to liquid-crystalline phase transition. Fluorescence quenching analysis of multicomponent lipid bilayers demonstrated that the cholesteryl glycosides, in contrast to cholesterol, were unable to stabilize ordered domains rich in PSM against temperature-induced dissociation. When the sterols were incorporated into bilayers composed of both POPC and PSM, the cholesteryl glycosides showed a higher propensity, compared to cholesterol, to influence the endothermal component representing the melting of POPC-rich domains, as determined by differential scanning calorimetry. Taken together, the results indicate that the glycosylation of cholesterol diminishes the ability of the sterol to reside in lateral domains constituted by membrane lipids having highly ordered hydrocarbon chains.     

Miscibility of ternary mixtures of phospholipids and cholesterol in monolayers, and application to bilayer systems

We investigate miscibility transitions of two different ternary lipid mixtures, DOPC/DPPC/Chol and POPC/PSM/Chol. In vesicles, both of these mixtures of an unsaturated lipid, a saturated lipid, and cholesterol form micron-scale domains of immiscible liquid phases for only a limited range of compositions. In contrast, in monolayers, both of these mixtures produce two distinct regions of immiscible liquid phases that span all compositions studied, the alpha-region at low cholesterol and the beta-region at high cholesterol. In other words, we find only limited overlap in miscibility phase behavior of monolayers and bilayers for the lipids studied. For vesicles at 25 degrees C, the miscibility phase boundary spans portions of both the monolayer alpha-region and beta-region. Within the monolayer beta-region, domains persist to high pressures, yet within the alpha-region, miscibility phase transition pressures always fall below 15 mN/m, far below the bilayer equivalent pressure of 32 mN/m. Approximately equivalent phase behavior is observed for monolayers of DOPC/DPPC/Chol and for monolayers of POPC/PSM/Chol. As expected, pressure-area isotherms of our ternary lipid mixtures yield smaller molecular area and compressibility for monolayers containing more saturated acyl chains and cholesterol. All monolayer experiments were conducted under argon. We show that exposure of unsaturated lipids to air causes monolayer surface pressures to decrease rapidly and miscibility transition pressures to increase rapidly.     

A study of quercetin effects on phospholipid membranes containing cholesterol using Laurdan fluorescence

Quercetin (QCT) is an important bioactive natural compound found in numerous edible plants. Since the lipid bilayer represents an essential compound of the cell membrane, QCT's direct interaction with this structure is of great interest. Therefore, we proposed to study the effects of QCT on DMPC liposomes containing cholesterol (Chol), and for this purpose Laurdan fluorescence was used. As a fluorescent probe, Laurdan is able to detect changes in membrane phase properties. When incorporated in lipid bilayers, Laurdan emits from two different excited states, a non-relaxed one when the bilayer packing is tight and a relaxed state when the bilayer packing is loose. The main tool for quantifying QCT's effects on phospholipid membranes containing Chol has been the analysis, the decomposition of Laurdan emission spectra in sums of two Gaussian functions on energy. This kind of approach has allowed good analysis of the balance between the two emitting states of Laurdan. Our results show that both Laurdan emission states are present to different extents in a wide temperature range for DMPC liposomes with Chol. QCT is decreasing the phase transition temperature in pure DMPC liposomes as proved by generalized polarization (GP) values. QCT also quenches Laurdan fluorescence, depending on the temperature and the presence of Chol in the membrane. Stern-Volmer constants were calculated for different lipid membrane compositions, and the conclusion was that the relaxed state favors the nonradiative transitions of the fluorophore.     

The influence of saturated fatty acid modulation of bilayer physical state on cellular and membrane structure and function

Cultured chick fibroblasts supplemented with stearic acid in the absence of serum at 37 degrees C degenerate and die in contrast to cells grown at 41 degrees C which appear normal in comparison with controls. These degenerative effects at 37 degrees C are alleviated by addition to stearate-containing media of fatty acids known to fluidize bilayers. These observations suggest that cell degeneration at 37 degrees C may involve alterations in the physical state of the membrane. Fatty acid analysis of plasma membrane obtained from stearate-supplemented cells clearly demonstrates the enrichment of this fatty acid species into bilayer phospholipids. Moreover, the extent of enrichment is similar in cells grown at both 37 and 41 degrees C. Stearate enrichment at either temperature does not appear to alter significantly membrane cholesterol or polar lipid content. Fluorescence anisotropy measurements for perylene and diphenylhexatriene incorporated into stearate-enriched membranes reveals changes suggestive of decreased bilayer fluidity. Moreover, analysis of temperature dependence of probe anisotropy indicates that a similarity in bilayer fluidity exists between stearate-enriched membranes at 41 degrees C and control membranes at 37 degrees C. Calorimetric data from liposomes prepared from polar lipids isolated from these membranes show similar melting profiles, consistent with the above lipid and fluorescence analyses. Arrhenius plot of stearate-enriched membrane glucose transporter function reveals breaks which coincide with the main endotherm of the pure phospholipid phase transition, indicating the sensitivity of the transporter to this transition which is undetectable in these native bilayers. These data suggest the existence of regions of bilayer lipid microheterogeneity which affect integral enzyme function, cell homeostasis and viability.