Lateral organization and domain formation in a two-component lipid membrane system

Spin label EPR-based characterization of biosystem complexity

Following the widely spread EPR spin-label applications for biosystem characterization, a novel approach is proposed for EPR-based characterization of biosystem complexity. Hereto a computational method based on a hybrid evolutionary optimization (HEO) is introduced. The enormous volume of information obtained from multiple HEO runs is reduced with a novel so-called GHOST condensation method for automatic detection of the degree of system complexity through the construction of two-dimensional solution distributions. The GHOST method shows the ability of automatic quantitative characterization of groups of solutions, e.g. the determination of average spectral parameters and group contributions. The application of the GHOST condensation algorithm is demonstrated on four synthetic examples of different complexity and applied to two physiologically relevant examples--the determination of domains in biomembranes (lateral heterogeneity) and the study of the low-resolution structure of membrane proteins.

Fluorescence-quenching and resonance energy transfer studies of lipid microdomains in model and biological membranes

Measurements of contact-dependent fluorescence quenching and of fluorescence resonance energy transfer (FRET) within bilayers provide information concerning the spatial relationships between molecules on distance scales of a few nm or up a few tens of nm, respectively, and are therefore well suited to detect the presence and composition of membrane microdomains. As described in this review, techniques based on fluorescence quenching and FRET have been used to demonstrate the formation of nanoscale liquid-ordered domains in cholesterol-containing model membranes under physiological conditions, and to investigate the structural features of lipids and proteins that influence their partitioning between liquid-ordered and liquid-disordered domains. FRET-based methods have also been used to test for the presence of 'raft' microdomains in the plasma membranes of mammalian cells. We discuss the sometimes divergent findings of these studies, possible modifications to the 'raft hypothesis' suggested by studies using FRET and other techniques, and the further potential of FRET-based methods to test and to refine current models of the nature and organization of membrane microdomains.

Absence of fluid-ordered/fluid-disordered phase coexistence in ceramide/POPC mixtures containing cholesterol

The effect of temperature on the lateral structure of lipid bilayers composed of porcine brain ceramide and 1-palmitoyl 2-oleoyl-phosphatidylcholine (POPC), with and without addition of cholesterol, were studied using differential scanning calorimetry, Fourier transformed infrared spectroscopy, atomic force microscopy, and confocal/two-photon excitation fluorescence microscopy (which included LAURDAN generalized polarization function images). A broad gel/fluid phase coexistence temperature regime, characterized by the presence of micrometer-sized gel-phase domains with stripe and flowerlike shapes, was observed for different POPC/ceramide mixtures (up to approximately 25 mol % ceramide). This observed phase coexistence scenario is in contrast to that reported previously for this mixture, where absence of gel/fluid phase coexistence was claimed using bulk LAURDAN generalized polarization (GP) measurements. We demonstrate that this apparent discrepancy (based on the direct comparison between the LAURDAN GP data obtained in the microscope and the fluorometer) disappears when the additive property of the LAURDAN GP function is taken into account to examine the data obtained using bulk fluorescence measurements. Addition of cholesterol to the POPC/ceramide mixtures shows a gradual transition from a gel/fluid to gel/liquid-ordered phase coexistence scenario as indicated by the different experimental techniques used in our experiments. This last result suggests the absence of fluid-ordered/fluid-disordered phase coexistence in the ternary mixtures studied in contrast to that observed at similar molar concentrations with other ceramide-base-containing lipid mixtures (such as POPC/sphingomyelin/cholesterol, which is used as a canonical raft model membrane). Additionally, we observe a critical cholesterol concentration in the ternary mixtures that generates a peculiar lateral pattern characterized by the observation of three distinct regions in the membrane.

Domain-induced activation of human phospholipase A2 type IIA: local versus global lipid composition

Secretory human phospholipase A2 type IIA (PLA2-IIA) catalyzes the hydrolysis of the sn-2 ester bond in glycerolipids to produce fatty acids and lysolipids. The enzyme is coupled to the inflammatory response, and its specificity toward anionic membrane interfaces suggests a role as a bactericidal agent. PLA2-IIA may also target perturbed native cell membranes that expose anionic lipids to the extracellular face. However, anionic lipid contents in native cells appear lower than the threshold levels necessary for activation. By using phosphatidylcholine/phosphatidylglycerol model systems, we show that local enrichment of anionic lipids into fluid domains triggers PLA2-IIA activity. In addition, the compositional range of enzyme activity is shown to be related to the underlying lipid phase diagram. A comparison is done between PLA2-IIA and snake venom PLA2, which in contrast to PLA2-IIA hydrolyzes both anionic and zwitterionic membranes. In general, this work shows that PLA2-IIA activation can be accomplished through local enrichment of anionic lipids into domains, indicating a mechanism for PLA2-IIA to target perturbed native membranes with low global anionic lipid contents. The results also show that the underlying lipid phase diagram, which determines the lipid composition at a local level, can be used to predict PLA2-IIA activity.

Dynamics imaging of lipid phases and lipid-marker interactions in model biomembranes

Biomembranes are complex systems that regulate numerous biological processes. Lipid phases that constitute these membranes influence their properties and transport characteristics. Here, we demonstrate the potential of short-range dynamics imaging (excited-state lifetime, rotational diffusion, and order parameter) as a sensitive probe of lipid phases in giant unilamellar vesicles (GUVs). Liquid-disordered and gel phases were labeled with Bodipy-PC at room temperature. Two-photon fluorescence lifetime imaging microscopy of single-phase GUVs reveals more heterogeneity in fluorescence lifetimes of Bodipy in the gel phase (DPPC: 3.8+/-0.6 ns) as compared with the fluid phase (DOPC: 5.2+/-0.2 ns). The phase-specificity of excited-state lifetime of Bodipy-PC is attributed to the stacking of ordered lipid molecules that possibly enhances homo-FRET. Fluorescence polarization anisotropy imaging also reveals distinctive molecular order that is phase specific. The results are compared with DiI-C12-labeled fluid GUVs to investigate the sensitivity of our fluorescence dynamics assay to different lipid-marker interactions. Our results provide a molecular perspective of lipid phase dynamics and the nature of their microenvironments that will ultimately help our understanding of the structure-function relationship of biomembranes in vivo. Furthermore, these ultrafast excited-state dynamics will be used for molecular dynamics simulation of lipid-lipid, lipid-marker and lipid-protein interactions.

Seeing spots: Complex phase behavior in simple membranes

Liquid domains in model lipid bilayers are frequently studied as models of raft domains in cell plasma membranes. Micron-scale liquid domains are easily produced in vesicles composed of ternary mixtures of a high melting temperature lipid, a low melting temperature lipid, and cholesterol. Here, we describe the rich phase behavior observed in binary and ternary systems. We then discuss experimental challenges inherent in mapping phase diagrams of even simple lipid systems. For example, miscibility behavior varies with lipid type, lipid ratio, lipid oxidation, and level of impurity. Liquid domains are often circular, but can become noncircular when membranes are near critical points. Finally, we reflect on applications of phase diagrams in model systems to rafts in cell membranes.

Detection of submicron-sized raft-like domains in membranes by small-angle neutron scattering

Using coarse grained models of heterogeneous vesicles we demonstrate the potential for small-angle neutron scattering (SANS) to detect and distinguish between two different categories of lateral segregation: 1) unilamellar vesicles (ULV) containing a single domain and 2) the formation of several small domains or "clusters" (approximately 10 nm in radius) on a ULV. Exploiting the unique sensitivity of neutron scattering to differences between hydrogen and deuterium, we show that the liquid ordered (lo) DPPC-rich phase can be selectively labeled using chain deuterated dipalymitoyl phosphatidylcholine (dDPPC), which greatly facilitates the use of SANS to detect membrane domains. SANS experiments are then performed in order to detect and characterize, on nanometer length scales, lateral heterogeneities, or so-called "rafts", in approximately 30 nm radius low polydispersity ULV made up of ternary mixtures of phospholipids and cholesterol. For 1:1:1 DOPC:DPPC:cholesterol (DDC) ULV we find evidence for the formation of lateral heterogeneities on cooling below 30 degrees C. These heterogeneities do not appear when DOPC is replaced by SOPC. Fits to the experimental data using coarse grained models show that, at room temperature, DDC ULV each exhibit approximately 30 domains with average radii of approximately 10 nm.

alpha-Lactalbumin binding and membrane integrity--effect of charge and degree of unsaturation of glycerophospholipids

Several studies have shown that the physical state of the phospholipid membrane has an important role in protein-membrane interactions, involving both electrostatic and hydrophobic forces. We have investigated the influence of the interaction of the calcium-depleted, (apo)-conformation of bovine alpha-lactalbumin (BLA) on the integrity of anionic glycerophospholipid vesicles by leakage experiments using fluorescence spectroscopy. The stability of the membranes was also studied by measuring surface tension/molecular area relationships with phospholipid monolayers. We show that the degree of unsaturation of the acyl chains and the proportion of charged phospholipid species in the membranes made of neutral and acidic glycerophospholipids are determinants for the association of BLA with liposomes and for the impermeability of the bilayer. Particularly, tighter packing counteracted interaction with BLA, while unsaturation-leading to looser packing-promoted interaction and leakage of contents. Equimolar mixtures of neutral and acidic glycerophospholipids were more permeable upon protein binding than pure acidic lipids. The effect of lipid structure on BLA-membrane interaction and bilayer integrity may throw new light on the membrane disrupting mechanism of a conformer of human alpha-lactalbumin (HAMLET) that induces death of tumour cells but not of normal cells.

Visualization and analysis of apolipoprotein A-I interaction with binary phospholipid bilayers

Apolipoprotein A-I (apoA-I) interaction with specific cell lipid domains was suggested to trigger cholesterol and phospholipid efflux. We analyzed here apoA-I interaction with dimyristoylphosphatidylcholine/distearoylphosphatidylcholine (DMPC/DSPC) bilayers at a temperature showing phase coexistence. Solid and liquid-crystalline domains were visualized by two-photon fluorescence microscopy on giant unilamellar vesicles (GUVs) labeled with 6-dodecanoyl-2-dimethyl-amino-naphthalene (Laurdan). A decrease of vesicle size was detected as long as they were incubated with lipid-free apoA-I, together with a shape deformation and a relative enrichment in DSPC. Selective lipid removal mediated by apoA-I from different domains was followed in real time by changes in the Laurdan generalized polarization. The data show a selective interaction of apoA-I with liquid-crystalline domains, from which it removes lipids, at a molar ratio similar to the domain compositions. Next, apoA-I was incubated with DMPC/DSPC small unilamellar vesicles, and products were isolated and quantified. Protein solubilized both lipids but formed complexes relatively enriched in the liquid component. We also show changes in the GUV morphology when cooling down. Our results suggest that the most efficient reaction between apoA-I and DMPC/DSPC occurs in particular bilayer conditions, probably when small fluid domains are nucleated within a continuous gel phase and interfacial packing defects are maximal.

pH-dependent domain formation in phosphatidylinositol polyphosphate/phosphatidylcholine mixed vesicles

Phosphatidylinositol polyphosphates (PI-PPs) have been shown to mediate a large variety of physiological processes by attracting proteins to specific cellular sites. Such site-specific signaling requires local accumulation of PI-PPs, and in light of the rich headgroup functionality, it is conceivable that hydrogen bond formation between adjacent headgroups is a contributing factor to the formation of PI-PP-enriched domains. To explore the significance of hydrogen bond formation for the mutual interaction of PI-PPs, this study aims to characterize the pH-dependent phase behavior of phosphatidylcholine/phosphatidylinositol bisphosphate and trisphosphate mixed vesicles by differential scanning calorimetry, infrared transmission spectroscopy, and fluorescence resonance energy transfer measurements. For pH values >7-7.5, the experiments yielded results consistent with dipalmitoylphosphatidylcholine/dipalmitoylphosphatidylinositol polyphosphate gel phase demixing, whereas for moderately acidic conditions, an enhanced mixing was observed. Similarly, this pH-dependent formation of PI-PP-enriched domains was also found for the physiologically important fluid phase. The stability of PI-PP-enriched domains and to some extent the pH dependence of the domain formation was governed by the number as well as the position of the phosphomonoester groups at the inositol ring.

Domain formation in phosphatidylinositol monophosphate/phosphatidylcholine mixed vesicles

Phosphoinositides have been shown to control membrane trafficking events by targeting proteins to specific cellular sites, which requires a tight regulation of phosphoinositide generation and turnover as well as a high degree of compartmentalization. To shed light on the processes that lead to the formation of phosphoinositide-enriched microdomains, phosphatidylcholine/phosphatidylinositol monophosphate (phosphatidylinositol-3-phosphate (PI-3P), -4-phosphate (PI-4P), or -5-phosphate (PI-5P)) mixed vesicles were investigated by calorimetric (DSC) Fourier transform infrared spectroscopic (FTIR), and fluorescence resonance energy transfer (FRET) measurements. The experiments furnished results consistent with a pH-dependent formation of phosphatidylinositol monophosphate-enriched microdomains. The domain formation was most pronounced between pH approximately 7 and approximately 9.5, whereas slightly acidic pH values (pH 4) resulted in the disintegration of the domains. This pH-dependent phosphatidylcholine/phosphatidylinositol monophosphate demixing was observed for the gel phase (FTIR experiments) as well as for the fluid lipid phase (FRET measurements). The observed microdomains are presumably stabilized by hydroxyl/hydroxyl as well as hydroxyl/phosphomonoester and phosphodiester interactions. While the pH dependence of the mutual phosphatidylinositol monophosphate interaction was largely the same for all investigated phosphatidylinositol monophosphates, it turned out that the relative stability of phosphatidylinositol monophosphate-enriched microdomains (pH 7-9.5) was governed by the position of the phosphomonoester group at the inositol ring (PI-4P > PI-5P > PI-3P). Demixing was also observed for phosphatidylcholine/phosphatidylinositol mixed vesicles; however, in this case the microdomain formation was only slightly affected by pH changes.

Temperature and pressure dependent growth and morphology of DMPC/DSPC domains studied by Brewster angle microscopy

In this work, the temperature and pressure dependent growth of domains in DMPC/DSPC monolayers at various molar ratios was studied by Brewster angle microscopy. Upon compression, roughly discoidal domains with some branching are formed. Further compression leads to an increase in both the number and the average size of the domains, which range between ca. 5 and 20 microm. The isobaric heating of the monolayers results in a gradual decrease of the domain size until their disappearance. The size and morphology of the domains depend not only on equilibrium parameters such as temperature, pressure and composition, but appear to be also strongly dependent on non-equilibrium parameters such as the rate of perturbation. The comparison between our results and those previously published for bilayers allows us to infer that the growth behaviour in monolayers can be qualitatively but not quantitatively extrapolated to bilayers.

Temperature dependence of the surface topography in dimyristoylphosphatidylcholine/distearoylphosphatidylcholine multibilayers

Simple lipid binary systems are intensively used to understand the formation of domains in biological membranes. The size of individual domains present in the gel/fluid coexistence region of single supported bilayers, determined by atomic force microscopy (AFM), generally exceeds by two to three orders of magnitude that estimated from multibilayers membranes by indirect spectroscopic methods. In this article, the topography of equimolar dimyristoylphosphatidylcholine/distearoylphosphatidylcholine (DMPC/DSPC) multibilayers, made of two superimposed bilayers supported on mica surface, was studied by AFM in a temperature range from room temperature to 45 degrees C. In the gel/fluid phase coexistence region the size of domains, between approximately 100 nm and a few micrometers, was of the same order for the first bilayer facing the mica and the top bilayer facing the buffer. The gel to fluid phase separation temperature of the first bilayer, however, could be increased by up to 8 degrees C, most likely as a function of the buffer layer thickness that separated it from the mica. Topography of the top bilayer revealed the presence of lipids in ripple phase up to 38-40 degrees C. Above this temperature, a pattern characteristic of the coexistence of fluid and gel domains was observed. These data show that difference in the size of lipid domains given by AFM and spectroscopy can hardly be attributed to the use of multibilayers models in spectroscopy experiments. They also provide a direct evidence for metastable ripple phase transformation into a gel/fluid phase separated structure upon heating.

Optical-trapping Raman microscopy detection of single unilamellar lipid vesicles

Raman spectra of individual unilamellar phospholipid vesicles ( approximately 0.6 microm in size) have been acquired by optical-trapping confocal Raman microscopy over the 900-3200-cm(-)(1) region. Raman scattering from the phospholipid bilayer of a single, trapped liposome could be detected, along with molecular species trapped within the vesicle. The Raman spectra of vesicles prepared from four different phosphatidylcholine lipids, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), could be readily distinguished by evaluating differences in the skeletal C-C and C-H stretching modes of the acyl hydrocarbon tails. These differences correlate with changes in lipid organization for different gel to liquid-crystal transition temperatures (T(m)): 41, 24, 7, and -20 degrees C for DPPC, DMPC, DLPC, and DOPC, respectively. The spectra could be acquired on the same trapped vesicle for several hours, which allowed the permeability of the bilayer to be investigated by monitoring the leakage of perchlorate anions from the vesicle. Vesicles prepared from pure DPPC or DOPC, with gel to liquid-crystal transition temperatures well above and well below room temperature, exhibited no detectable anion transfer. DLPC and DMPC vesicles permitted rapid ion transfer across the bilayer. The lengths of hydrocarbon tails were shorter in these two lipids, which could indicate that shorter chains lower the hydrophobic barrier of a membrane to ion transport. While the DMPC chains were longer than DLPC with a correspondingly higher T(m), the temperature of the experiment corresponds to the T(m) of DMPC, and domain boundaries between gel and liquid-crystal phases could contribute to high membrane permeability.

Partitioning of dual-lipidated peptides into membrane microdomains: lipid sorting vs peptide aggregation

The lateral membrane organization and phase behavior of the lipid mixture DMPC(di-C(14))/DSPC(di-C(18))/cholesterol (0-33 mol %) with and without an incorporated fluorescence-labeled palmitoyl/farnesyl dual-lipidated peptide, BODIPY-Gly-Cys(Pal)-Met-Gly-Leu-Pro-Cys(Far)-OMe, which represents a membrane recognition model system for Ras proteins, was studied by two-photon excitation fluorescence microscopy. Measurements were performed on giant unilamellar vesicles (GUVs) over a large temperature range, ranging from 30 to 80 degrees C to cover different lipid phase states (all-gel, fluid/gel, liquid-ordered, all-fluid). At temperatures where the fluid-gel coexistence region of the pure binary phospholipid system occurs, large-scale concentration fluctuations appear. Incorporation of cholesterol levels up to 33 mol % leads to a significant increase of conformational order in the membrane system and a reduction of large domain structures. Adding the peptide leads to dramatic changes in the lateral organization of the membrane. With cholesterol present, a phase separation is induced by a lipid sorting mechanism owing to the high affinity of the lipidated peptide to a fluid, DMPC-rich environment. This phase separation leads to the formation of peptide-containing domains with high fluorescence intensity that become progressively smaller with decreasing temperature. As a result, the local concentration of the peptide increases steadily within the confines of the shrinking domains. At the lowest temperatures, where the acyl-chain order parameter of the membrane has already drastically increased and the membrane achieves a liquid-ordered character, an efficient lipid sorting mechanism is no longer supported and aggregation of the peptide into small clusters prevails. We can conclude that palmitoyl/farnesyl dual-lipidated peptides do not associate with liquid-ordered or gel-like domains in phase-separated bilayer membranes. In particular, the study shows the interesting ability of the peptide to induce formation of fluid microdomains at physiologically relevant cholesterol concentrations, and this effect very much depends on the concentration of fluid vs ordered lipid molecules.

Membrane properties of plant sterols in phospholipid bilayers as determined by differential scanning calorimetry, resonance energy transfer and detergent-induced solubilization

The increased use of plant sterols as cholesterol-lowering agents warrants further research on the possible effects of plant sterols in membranes. In this study, the effects of the incorporation of cholesterol, campesterol, beta-sitosterol and stigmasterol in phospholipid bilayers were investigated by differential scanning calorimetry (DSC), resonance energy transfer (RET) between trans parinaric acid (tPA) and 2-(6-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)hexanoyl-1-hexadecanoyl-sn-glycero-3-phosphocholine (NBD-PC), and Triton X-100-induced solubilization. The phospholipids used were 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), D-erythro-N-palmitoyl-sphingomyelin (PSM), and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). In DSC experiments, it was demonstrated that the sterols differed in their effect on the melting temperatures of both the sterol-poor and the sterol-rich domains in DPPC and PSM bilayers. The plant sterols gave rise to lower temperatures of both transitions, when compared with cholesterol. The plant sterols also resulted in lower transition temperatures, in comparison with cholesterol, when sterol-containing DPPC and PSM bilayers were investigated by RET. In the detergent solubilization experiments, the total molar ratio between Triton X-100 and POPC at the onset of solubilization (R(t,sat)) was higher for bilayers containing plant sterols, in comparison with membranes containing cholesterol. Taken together, the observations presented in this study indicate that campesterol, beta-sitosterol and stigmasterol interacted less favorably than cholesterol with the phospholipids, leading to measurable differences in their domain properties.

Fluorescence energy transfer reveals microdomain formation at physiological temperatures in lipid mixtures modeling the outer leaflet of the plasma membrane

An approach is described using fluorescence resonance energy transfer (FRET) to detect inhomogeneity in lipid organization, on distance scales of the order of tens of nanometers or greater, in lipid bilayers. This approach compares the efficiency of energy transfer between two matched fluorescent lipid donors, differing in their affinities for ordered versus disordered regions of the bilayer, and an acceptor lipid that distributes preferentially into disordered regions. Inhomogeneities in bilayer organization, on spatial scales of tens of nanometers or greater, are detected as a marked difference in the efficiencies of quenching of fluorescence of the two donor species by the acceptor. Using a novel pair of 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)-labeled tetraacyl lipids as donor species with a rhodaminyl-labeled acceptor, this strategy faithfully reports homo- versus inhomogeneous mixing in each of several lipid bilayer systems whose organization on the FRET distance scale can be predicted from previous findings. Interestingly, however, the present FRET method reports clear evidence of inhomogeneity in the organization of mixtures combining sphingomyelin or saturated phospholipids with unsaturated phospholipids and physiological proportions of cholesterol, even at physiological temperatures where these systems have been reported to appear homogeneous by fluorescence microscopy. These results indicate that under physiological conditions, lipid mixtures mimicking the lipid composition of the outer leaflet of the plasma membrane can form domains on a spatial scale comparable to that inferred for the dimensions of lipid rafts in biological membranes.

Time-resolved fluorescence and fourier transform infrared spectroscopic investigations of lateral packing defects and superlattice domains in compositionally uniform cholesterol/phosphatidylcholine bilayers

Time-resolved fluorescence and Fourier transform infrared spectroscopies were used to investigate the lateral organization of lipids in compositionally uniform and fully equilibrated 1-palmitoyl-2-oleoyl-phosphatidylcholine/cholesterol (POPC/CHOL) liposomes prepared by a recently devised low-temperature trapping method. Independent fluorescence decay lifetime and rotational dynamics parameters of diphenylhexatriene (DPH) chain-labeled phosphatidylcholine (DPH-PC) in these liposomes were recovered from the time-resolved fluorescence measurements as a function of cholesterol molar fraction (X(CHOL)) at 23 degrees C. The results indicate significantly greater lifetime heterogeneity, shorter average lifetime, rotational correlation time, and lower order parameter of the DPH moiety at X(CHOL) approximately 0.40 and 0.50 as compared to the adjacent cholesterol concentrations. Less prominent changes were also detected at, for example, X(CHOL) approximately 0.20 and 0.33. These X(CHOL)'s coincide with the "critical" X(CHOL)'s predicted by the previously proposed superlattice (SL) model, thus indicating that POPC and cholesterol molecules tend to form SL domains where the components tend to be regularly distributed. The data also support another prediction of the SL model, namely that lateral packing defects coexist with the ordered SL domains. It appears that unfavorable interaction of the DPH-moiety of DPH-PC with cholesterol results in a preferential partition of DPH-PC to the defect regions. Fourier transform infrared analysis of the native lipid O=P=O, C=O, and C-H vibrational bands of POPC/CHOL liposomes in the absence of DPH-PC revealed an increase in the conformational order of the acyl chains and a decrease in the conformational order (or increased hydration) of the interfacial and headgroup regions at or close to the predicted critical X(CHOL)'s. This provides additional but probe-independent evidence for SL domain formations in the POPC/CHOL bilayers. We propose that the defect regions surrounding the putative SL domains could play an important role in modulating the activity of various membrane-associated enzymes, e.g., those regulating the lipid compositions of cell membranes.

Trehalose maintains phase separation in an air-dried binary lipid mixture

Mixing and thermal behavior of hydrated and air-dried mixtures of 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC) and 1,2-distearoyl-d70-sn-glycero-3-phosphocholine (DSPCd-70) in the absence and presence of trehalose were investigated by Fourier transform infrared spectroscopy. Mixtures of DLPC:DSPCd-70 (1:1) that were air-dried at 25 degrees C show multiple phase transitions and mixed phases in the dry state. After annealing at high temperatures, however, only one transition is seen during cooling scans. When dried in the presence of trehalose, the DLPC component shows two phase transitions at -22 degrees C and 75 degrees C and is not fully solidified at -22 degrees C. The DSPCd-70 component, however, shows a single phase transition at 78 degrees C. The temperatures of these transitions are dramatically reduced after annealing at high temperatures with trehalose. The data suggest that the sugar has a fluidizing effect on the DLPC component during drying and that this effect becomes stronger for both components with heating. Examination of infrared bands arising from the lipid phosphate and sugar hydroxyl groups suggests that the strong effect of trehalose results from direct interactions between lipid headgroups and the sugar and that these interactions become stronger after heating. The findings are discussed in terms of the protective effect of trehalose on dry membranes.

Ripples and the formation of anisotropic lipid domains: imaging two-component supported double bilayers by atomic force microscopy

Direct visualization of the fluid-phase/ordered-phase domain structure in mica-supported bilayers composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine/1,2-distearoyl-sn-glycero-3-phosphocholine mixtures is performed with atomic force microscopy. The system studied is a double bilayer supported on a mica surface in which the top bilayer (which is not in direct contact with the mica) is visualized as a function of temperature. Because the top bilayer is not as restricted by the interactions with the surface as single supported bilayers, its behavior is more similar to a free-standing bilayer. Intriguing straight-edged anisotropic fluid-phase domains were observed in the fluid-phase/ordered-phase coexistence temperature range, which resemble the fluid-phase/ordered-phase domain patterns observed in giant unilamellar vesicles composed of such phospholipid mixtures. With the high resolution provided by atomic force microscopy, we investigated the origin of these anisotropic lipid domain patterns, and found that ripple phase formation is directly responsible for the anisotropic nature of these domains. The nucleation and growth of fluid-phase domains are found to be directed by the presence of ripples. In particular, the fluid-phase domains elongate parallel to the ripples. The results show that ripple phase formation may have implications for domain formation in biological systems.

Biophysical mechanisms of phospholipase A2 activation and their use in liposome-based drug delivery

Secretory phospholipase A2 (PLA2) is a ubiquitous water-soluble enzyme found in venom, pancreatic, and cancerous fluid. It is also known to play a role in membrane remodeling processes as well as in cellular signaling cascades. PLA2 is interfacially active and functions mainly on organized types of substrate, e.g. micelles and lipid bilayers. Hence the activity of the enzyme is modulated by the lateral organization and the physical properties of the substrate, in particular the structure in the nanometer range. The evidence for nano-scale structure and lipid domains in bilayers is briefly reviewed. Results obtained from a variety of experimental and theoretical studies of PLA2 activity on lipid-bilayer substrates are then presented which provide insight into the biophysical mechanisms of PLA2 activation on lipid bilayers and liposomes of different composition. The insight into these mechanisms has been used to propose a novel principle for liposomal drug targeting, release, and absorption triggered by secretory PLA2.

Component and state separation in DMPC/DSPC lipid bilayers: a Monte Carlo simulation study

In this paper a two-state, two-component, Ising-type model is used to simulate the lateral distribution of the components and gel/fluid state acyl chains in dimyristoylphosphatidylcholine/distearoylphosphatidylcholine (DMPC/DSPC) lipid bilayers. The same model has been successful in calculating the excess heat capacity curves, the fluorescence recovery after photobleaching (FRAP) threshold temperatures, the most frequent center-to-center distances between DSPC clusters, and the fractal dimensions of gel clusters (Sugar, I. P., T. E. Thompson, and R. L. Biltonen, 1999. Biophys. J. 76:2099-2110). Depending on the temperature and mole fraction the population of the cluster size is either homogeneous or inhomogeneous. In the inhomogeneous population the size of the largest cluster scales with the size of the system, while the rest of the clusters remain small with increasing system size. In a homogeneous population, however, every cluster remains small with increasing system size. For both compositional and fluid/gel state clusters, threshold temperatures-the so-called percolation threshold temperatures-are determined where change in the type of the population takes place. At a given mole fraction, the number of percolation threshold temperatures can be 0, 1, 2, or 3. By plotting these percolation threshold temperatures on the temperature/mole fraction plane, the diagrams of component and state separation of DMPC/DSPC bilayers are constructed. In agreement with the small-angle neutron scattering measurements, the component separation diagram shows nonrandom lateral distribution of the components not only in the gel-fluid mixed phase region, but also in the pure gel and pure fluid regions. A combined diagram of component and state separation is constructed to characterize the lateral distribution of lipid components and gel/fluid state acyl chains in DMPC/DSPC mixtures. While theoretical phase diagrams of two component mixtures can be constructed only in the case of first-order transitions, state and component separation diagrams can be constructed whether or not the system is involved in first-order transition. The effects of interchain interactions on the component and state separation diagrams are demonstrated on three different models. The influences of state and component separation on the in-plane and off-plane membrane reactions are discussed.

Modulation of concentration fluctuations in phase-separated lipid membranes by polypeptide insertion

The lateral membrane organization and phase behavior of the binary lipid mixture DMPC (1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine) - DSPC (1,2-distearoyl-sn-glycero-3-phosphatidylcholine) without and with incorporated gramicidin D (GD) as a model biomembrane polypeptide was studied by small-angle neutron scattering, Fourier-transform infrared spectroscopy, and by two-photon excitation fluorescence microscopy on giant unilamellar vesicles. The small-angle neutron scattering method allows the detection of concentration fluctuations in the range from 1 to 200 nm. Fluorescence microscopy was used for direct visualization of the lateral lipid organization and domain shapes on a micrometer length scale including information of the lipid phase state. In the fluid-gel coexistence region of the pure binary lipid system, large-scale concentration fluctuations appear. Infrared spectral parameters were used to determine the peptide conformation adopted in the different lipid phases. The data show that the structure of the temperature-dependent lipid phases is significantly altered by the insertion of 2 to 5 mol% GD. At temperatures corresponding to the gel-fluid phase coexistence region the concentration fluctuations drastically decrease, and we observe domains in the giant unilamellar vesicles, which mainly disappear by the incorporation of 2 to 5 mol% GD. Further, the lipid matrix has the ability to modulate the conformation of the inserted polypeptide. The balance between double-helical and helical dimer structures of GD depends on the phospholipid chain length and phase state. A large hydrophobic mismatch, such as in gel phase one-component DSPC bilayers, leads to an increase in population of double-helical structures. Using an effective molecular sorting mechanism, a large hydrophobic mismatch can be avoided in the DMPC-DSPC lipid mixture, which leads to significant changes in the heterogeneous lipid structure and in polypeptide conformation.

Lipid domain formation and ligand-receptor distribution in lipid bilayer membranes investigated by atomic force microscopy

A novel experimental technique, based on atomic force microscopy (AFM), is proposed to visualize the lateral organization of membrane systems in the nanometer range. The technique involves the use of a ligand-receptor pair, biotin-avidin, which introduces a height variation on a solid-supported lipid bilayer membrane. This leads to a height amplification of the lateral membrane organization that is large enough to be clearly imaged by scanning AFM. The power of the technique is demonstrated for a binary dipalmitoylphosphocholine-diarachidoylphosphocholine lipid mixture which is shown to exhibit a distinct lateral lipid domain formation. The new and simple ligand-receptor-based AFM approach opens up new ways to investigate lipid membrane microstructure in the nanometer range as well as the lateral distribution of ligand-lipid and receptor-protein complexes in supported membrane systems.

Evidence for a physiological role for membrane rafts in human platelets

We have investigated raft formation in human platelets in response to cell activation. Lipid phase separation and domain formation were detected using the fluorescent dye 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (diI-C(18)) that preferentially partitions into gel-like lipid domains. We showed that when human platelets are activated by cold and physiological agonists, rafts coalesce into visible aggregates. These events were disrupted by depletion of membrane cholesterol. Using Fourier transform infrared spectroscopy (FTIR), we measured a thermal phase transition at around 30 degrees C in intact platelets, which we have assigned as the liquid-ordered to the liquid-disordered phase transition of rafts. Phase separation of the phospholipid and the sphingomyelin-enriched rafts could be observed as two phase transitions at around 15 and 30 degrees C, respectively. The higher transition, assigned to the rafts, was greatly enhanced with removal of membrane cholesterol. Detergent-resistant membranes (DRMs) were enriched in cholesterol (50%) and sphingomyelin (20%). The multi-functional platelet receptor CD36 selectively partitioned into DRMs, whereas the GPI-linked protein CD55 and the major platelet integrin alpha(IIb)beta(3a) did not, which suggests that the clustering of proteins within rafts is a regulated process dependent on specific lipid protein interactions. We suggest that raft aggregation is a dynamic, reversible physiological event triggered by cell activation.

Phospholipase A(2) activity towards vesicles of DPPC and DMPC-DSPC containing small amounts of SMPC

Phospholipase A(2) (PLA(2)) is an interfacially active enzyme whose hydrolytic activity is known to be enhanced in one-component phospholipid bilayer substrates exhibiting dynamic micro-heterogeneity. In this study the activity of PLA(2) towards large unilamellar vesicles composed of DPPC:SMPC and DMPC:DSPC:SMPC is investigated using fluorescence and HPLC techniques. Phase diagrams of the mixtures are established by differential scanning calorimetry and the PLA(2) activity, monitored by the lag time, is correlated with the phase behavior of the mixtures. In addition, the degree of lipid hydrolysis in the DMPC:DSPC:SMPC lipid mixtures is detected by HPLC. The PLA(2) activity is found to be significantly increased in the temperature range of the coexistence region where the lipid mixtures exhibit lateral gel-fluid phase separation. Furthermore, in the entire temperature range it is demonstrated that PLA(2) preferentially hydrolyzes the short chain DMPC lipid. This discriminative effect becomes less pronounced when the asymmetric lipid SMPC is present in the lipid substrate. Inclusion of SMPC into either DPPC or DMPC:DSPC vesicles prolongs the lag time. The results clearly show that the PLA(2) activity is significantly enhanced by lipid bilayer micro-heterogeneity in both one-component and multi-component lipid bilayer substrates. The PLA(2) activity measurements are discussed in terms of dynamic gel-fluid lipid domain formation due to density fluctuations and static lipid domain formation due to gel-fluid phase separation.

Geometrical properties of gel and fluid clusters in DMPC/DSPC bilayers: Monte Carlo simulation approach using a two-state model

In this paper the geometrical properties of gel and fluid clusters of equimolar dimyristoylphosphatidylcholine/distearoylphosphatidylcholine (DMPC/DSPC) lipid bilayers are calculated by using an Ising-type model (Sugar, I. P., T. E. Thompson, and R. L. Biltonen. 1999. Biophys. J. 76:2099-2110). The model is able to predict the following properties in agreement with the respective experimental data: the excess heat capacity curves, fluorescence recovery after photobleaching (FRAP) threshold temperatures at different mixing ratios, the most frequent center-to-center distance between DSPC clusters, and the fractal dimension of gel clusters. In agreement with the neutron diffraction and fluorescence microscopy data, the simulations show that below the percolation threshold temperature of gel clusters many nanometer-size gel clusters co-exist with one large gel cluster of size comparable with the membrane surface area. With increasing temperature the calculated effective fractal dimension and capacity dimension of gel and fluid clusters decrease and increase, respectively, within the (0, 2) interval. In the region of the gel-to-fluid transition the following geometrical properties are independent from the temperature and the state of the cluster: 1) the cluster perimeter linearly increases with the number of cluster arms at a rate of 8.2 nm/arm; 2) the average number of inner islands in a cluster increases with increasing cluster size, S, according to a power function of 0.00427 x S(1.3); 3) the following exponential function describes the average size of an inner island versus the size of the host cluster, S: 1 + 1.09(1 - e(-0.0072xS)). By means of the equations describing the average geometry of the clusters the process of the association of clusters is investigated.