Nonequilibrium phenomena in the phase separation of a two-component lipid bilayer

Ceramide-platform formation and -induced biophysical changes in a fluid phospholipid membrane

To understand the formation and properties of ceramide-enriched domains in cell membranes, the biophysical properties of the binary system palmitoyloleoylphosphatidylcholine (POPC)/palmitoylceramide were thoroughly characterized. Diverse fluorescent probes and parameters were necessary to unravel the complexity of this apparently simple system. For the first time, a complete phase diagram is reported, characterizing the lamellar phases of these mixtures, and providing a quantitative framework integrating biophysical and biological studies. The diagram suggests that in resting cells no ceramide domains exist, but upon apoptotic stimuli, platforms may form. Moreover, our data show that 2 mol% of Cer strongly affects the POPC fluid matrix, suggesting that a small increase in Cer levels can significantly affect cell membrane properties. In this work, we also show that Cer domains, formed in conditions similar to physiological, are extremely ordered and rigid. The domains composition is estimated from the phase diagram and their large size was concluded from fluorescence resonance energy transfer. Dynamic light scattering and electron microscopy were used to characterize the system morphology, which is highly dependent on ceramide content and includes vesiculation and tubular structure formation.

Structural Effects of a Basic Peptide on the Organization of Dipalmitoylphosphatidylcholine/Dipalmitoylphosphatidylserine Membranes: A Fluorescent Resonance Energy Transfer Study

We studied the effect of a model basic peptide, hexalysiltryptophan, on the organization of dipalmitoylphosphatidylcholine/dipalmitoylphosphatidylserine unilamellar vesicles by means of fluorescent resonance energy transfer (FRET) between fluorescently labeled phospholipids. Several FRET theoretical models assuming different bilayer geometries and probe distributions were fitted to the time-resolved data. The experiments were carried out at two temperatures in different regions of the lipid mixture phase diagram. At 45 degrees C, the expected gel/fluid phase separation was verified by model fitting in peptide-free vesicles, which from the FRET approach means that domains are larger than approximately 200 A. No noticeable alteration of membrane organization was detected upon increasing the peptide concentration. At variance, for the single fluid phase at 60 degrees C, there was a large increase in FRET efficiency upon peptide addition to the lipid vesicles, mainly caused by peptide-induced vesicle aggregation. The system gradually changed from unilamellar lipid vesicles to a multibilayer geometry, and a limit lamellar repeat distance of approximately 57 A was recovered. Furthermore, no evidence for lateral domain formation on the FRET length scale was found at this temperature, the cationic peptide being only able to induce local lipid demixing, causing a short-range sequestration of 2-3 acidic lipids around each surface-adsorbed peptide.

Galactosylceramide domain microstructure: impact of cholesterol and nucleation/growth conditions

Galactosylceramide (GalCer), a glycosphingolipid, is believed to exist in the extracellular leaflet of cell membranes in nanometer-sized domains or rafts. The local clustering of GalCer within rafts is thought to facilitate the initial adhesion of certain viruses, including HIV-1, and bacteria to cells through multivalent interactions between receptor proteins (gp120 for HIV-1) and GalCer. Here we use atomic force microscopy (AFM) to study the effects of cholesterol on solid-phase GalCer domain microstructure and miscibility with a fluid lipid 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC) in supported lipid bilayers. Using "slow-cooled vesicle fusion" to prepare the supported lipid bilayers, we were able to overcome the nonequilibrium effects of the substrate (verified by comparison to results for giant unilamellar vesicles) and accurately quantify the dramatic effect of cholesterol on the GalCer domain surface area/perimeter ratio (A(D)/P) and DLPC-GalCer miscibility. We compare these results to a supported lipid bilayer system in which the bilayer is rapidly cooled (nonequilibrium conditions), "quenched vesicle fusion", and find that the microstructures are remarkably similar above a cholesterol mol fraction of approximately 0.06. We determined that GalCer domains were contained in one leaflet distal to the mica substrate through qualitative binding experiments with Trichosanthes kirilowii agglutinin (TKA), a galactose-specific lectin, and AFM of Langmuir-Blodgett deposited GalCer/DLPC supported lipid bilayers. In addition, GalCer domains in bilayers containing cholesterol rearranged upon tip-sample contact. Our results further serve to clarify why discrepancies exist between different model membrane systems and between model membranes and cell membranes. In addition, these results offer new insight into the effect of cholesterol and surrounding lipid on domain microstructure and behavior. Finally, our observations may be pertinent to cell membrane structure, dynamics, and HIV infection.

Lipid asymmetry in DLPC/DSPC-supported lipid bilayers: a combined AFM and fluorescence microscopy study

A fundamental attribute of cell membranes is transmembrane asymmetry, specifically the formation of ordered phase domains in one leaflet that are compositionally different from the opposing leaflet of the bilayer. Using model membrane systems, many previous studies have demonstrated the formation of ordered phase domains that display complete transmembrane symmetry; but there have been few reports on the more biologically relevant asymmetric membrane structures. Here we report on a combined atomic force microscopy and fluorescence microscopy study whereby we observe three different states of transmembrane symmetry in phase-separated supported lipid bilayers formed by vesicle fusion. We find that if the leaflets differ in gel-phase area fraction, then the smaller domains in one leaflet are in registry with the larger domains in the other leaflet and the system is dynamic. In a presumed lipid flip-flop process similar to Ostwald ripening, the smaller domains in one leaflet erode away whereas the large domains in the other leaflet grow until complete compositional asymmetry is reached and remains stable. We have quantified this evolution and determined that the lipid flip-flop event happens most frequently at the interface between symmetric and asymmetric DSPC domains. If both leaflets have identical area fraction of gel-phase, gel-phase domains are in registry and are static in comparison to the first state. The stability of these three DSPC domain distributions, the degree of registry observed, and the domain immobility have biological significance with regards to maintenance of lipid asymmetry in living cell membranes, communication between inner leaflet and outer leaflet, membrane adhesion, and raft mobility.

Observation of the main phase transition of dinervonoylphosphocholine giant liposomes by fluorescence microscopy

The phase heterogeneity of giant unilamellar dinervonoylphosphocholine (DNPC) vesicles in the course of the main phase transition was investigated by confocal fluorescence microscopy observing the fluorescence from the membrane incorporated lipid analog, 1-palmitoyl-2-(N-4-nitrobenz-2-oxa-1,3-diazol)aminocaproyl-sn-glycero-3-phosphocholine (NBDPC). These data were supplemented by differential scanning calorimetry (DSC) of DNPC large unilamellar vesicles (LUV, diameter approximately 0.1 and 0.2 microm) and multilamellar vesicles (MLV). The present data collected upon cooling reveal a lack of micron-scale gel and fluid phase coexistence in DNPC GUVs above the temperature of 20.5 degrees C, this temperature corresponding closely to the heat capacity maxima (T(em)) of DNPC MLVs and LUVs (T(em) approximately 21 degrees C), measured upon DSC cooling scans. This is in keeping with the model for phospholipid main transition inferred from our previous fluorescence spectroscopy data for DMPC, DPPC, and DNPC LUVs. More specifically, the current experiments provide further support for the phospholipid main transition involving a first-order process, with the characteristic two-phase coexistence converting into an intermediate phase in the proximity of T(em). This at least macroscopically homogenous intermediate phase would then transform into the liquid crystalline state by a second-order process, with further increase in acyl chain trans-->gauche isomerization.

Local mobility in lipid domains of supported bilayers characterized by atomic force microscopy and fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) is used to examine mobility of labeled probes at specific sites in supported bilayers consisting of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid domains in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). Those sites are mapped beforehand with simultaneous atomic force microscopy and submicron confocal fluorescence imaging, allowing characterization of probe partitioning between gel DPPC and disordered liquid DOPC domains with corresponding topography of domain structure. We thus examine the relative partitioning and mobility in gel and disordered liquid phases for headgroup- and tailgroup-labeled GM1 ganglioside probes and for headgroup- and tailgroup-labeled phospholipid probes. For the GM1 probes, large differences in mobility between fluid and gel domains are observed; whereas unexpected mobility is observed in submicron gel domains for the phospholipid probes. We attribute the latter to domain heterogeneities that could be induced by the probe. Furthermore, fits to the FCS data for the phospholipid probes in the DOPC fluid phase require two components (fast and slow). Although proximity to the glass substrate may be a factor, local distortion of the probe by the fluorophore could also be important. Overall, we observe nonideal aspects of phospholipid probe mobility and partitioning that may not be restricted to supported bilayers.

Lipid Rafts have Different Sizes Depending on Membrane Composition: A Time-resolved Fluorescence Resonance Energy Transfer Study

The ternary lipid system palmitoylsphingomyelin (PSM)/palmitoyloleoylphosphatidylcholine (POPC)/cholesterol is a model for lipid rafts. Previously the phase diagram for that mixture was obtained, establishing the composition and boundaries for lipid rafts. In the present work, this system is further studied in order to characterize the size of the rafts. For this purpose, a time-resolved fluorescence resonance energy transfer (FRET) methodology, previously applied with success to a well-characterized phosphatidylcholine/cholesterol binary system, is used. It is concluded that: (1) the rafts on the low raft fraction of the raft region are small (below 20 nm), whereas on the other side the domains are larger; (2) on the large domain region, the domains reach larger sizes in the ternary system (> approximately 75-100 nm) than in binary systems phosphatidylcholine/cholesterol (between approximately 20 and approximately 75-100 nm); (3) the raft marker ganglioside G(M1) in small amounts (and excess cholera toxin subunit B) does not affect the general phase behaviour of the lipid system, but can increase the size of the rafts on the small to intermediate domain region. In summary, lipid-lipid interactions alone can originate lipid rafts on very different length scales. The conclusions presented here are consistent with the literature concerning both model systems and cell membrane studies.

Cholesterol modulates the organization of the gammaM4 transmembrane domain of the muscle nicotinic acetylcholine receptor

A 28-mer gammaM4 peptide, obtained by solid-state synthesis and corresponding to the fourth transmembrane segment of the nicotinic acetylcholine receptor gamma-subunit, possesses a single tryptophan residue (Trp453), making it an excellent model for studying peptide-lipid interactions in membranes by fluorescence spectroscopy. The gammaM4 peptide was reconstituted with synthetic lipids (vesicles of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, i.e., POPC) rich and poor in cholesterol and analyzed using steady-state and time-resolved fluorescence techniques. The decrease in gammaM4 intrinsic fluorescence lifetime observed upon incorporation into a cholesterol-rich lo phase could be rationalized on the basis of a dynamic self-quenching owing to the formation of peptide-rich patches in the membrane. This agrees with the low Förster type resonance energy transfer efficiency from the Trp453 residue to the fluorescent cholesterol analog, dehydroergosterol, in the lo phase. In the absence of cholesterol the gammaM4 nicotinic acetylcholine receptor peptide is randomly distributed in the POPC bilayer with its hydrophobic moiety matching the membrane thickness, whereas in the presence of cholesterol the increase in the membrane thickness and variation of the material properties favor the formation of peptide-enriched patches, i.e., interhelix interaction energy is essential for obtaining a stabilized structure. Thus, the presence of a cholesterol-rich, ordered POPC phase drives the organization of peptide-enriched patches, in which the gammaM4 peptide occupies approximately 30% of the patch area.

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.

Model Systems, Lipid Rafts, and Cell Membranes

Views of how cell membranes are organized are presently changing. The lipid bilayer that constitutes these membranes is no longer understood to be a homogeneous fluid. Instead, lipid assemblies, termed rafts, have been introduced to provide fluid platforms that segregate membrane components and dynamically compartmentalize membranes. These assemblies are thought to be composed mainly of sphingolipids and cholesterol in the outer leaflet, somehow connected to domains of unknown composition in the inner leaflet. Specific classes of proteins are associated with the rafts. This review critically analyzes what is known of phase behavior and liquid-liquid immiscibility in model systems and compares these data with what is known of domain formation in cell membranes.

Sphingomyelin/phosphatidylcholine/cholesterol phase diagram: boundaries and composition of lipid rafts

The ternary system palmitoylsphingomyelin (PSM)/palmitoyloleoylphosphatidylcholine (POPC)/cholesterol is used to model lipid rafts. The phase behavior of the three binary systems PSM/POPC, PSM/cholesterol, and POPC/cholesterol is first experimentally determined. Phase coexistence boundaries are then determined for ternary mixtures at room temperature (23 degrees C) and the ternary phase diagram at that temperature is obtained. From the diagram at 23 degrees C and the binary phase diagrams, a reasonable expectation is drawn for the ternary phase diagram at 37 degrees C. Several photophysical methodologies are employed that do not involve detergent extraction, in addition to literature data (e.g., differential scanning calorimetry) and thermodynamic rules. For the ternary phase diagrams, some tie-lines are calculated, including the one that contains the PSM/POPC/ cholesterol 1:1:1 mixture, which is often used in model raft studies. The diagrams here described are used to rationalize literature results, some of them apparently discrepant, and to discuss lipid rafts within the framework of liquid-ordered/liquid-disordered phase coexistence.

Intrinsic tyrosine fluorescence as a tool to study the interaction of the shaker B "ball" peptide with anionic membranes

Steady-state and time-resolved fluorescence from the single tyrosine in the inactivating peptide of the Shaker B potassium channel (ShB peptide) and in a noninactivating peptide mutant, ShB-L7E, has been used to characterize their interaction with anionic phospholipid membranes, a model target mimicking features of the inactivation site on the channel protein. Partition coefficients derived from steady-state anisotropy indicate that both peptides show a high affinity for anionic vesicles, being higher in ShB than in ShB-L7E. Moreover, differential quenching by lipophilic spin-labeled probes and fluorescence energy transfer using trans-parinaric acid as the acceptor confirm that the ShB peptide inserts deep into the membrane, while the ShB-L7E peptide remains near the membrane surface. The rotational mobility of tyrosine in membrane-embedded ShB, examined from the decay of fluorescence anisotropy, can be described by two different rotational correlation times and a residual constant value. The short correlation time corresponds to fast rotation reporting on local tyrosine mobility. The long rotational correlation time and the high residual anisotropy suggest that the ShB peptide diffuses in a viscous and anisotropic medium compatible with the aliphatic region of a lipid bilayer and support the hypothesis that the peptide inserts into it as a monomer, to configure an intramolecular beta-hairpin structure. Assuming that this hairpin structure behaves like a rigid body, we have estimated its dimensions and rotational dynamics, and a model for the peptide inserted into the bilayer has been proposed.

Material studies of lipid vesicles in the L(alpha) and L(alpha)-gel coexistence regimes

In this work, we utilize micropipette aspiration and fluorescence imaging to examine the material properties of lipid vesicles made from mixtures of palmitoyloleoylphosphocholine (POPC) and dipalmitoylphosphatidylcholine (DPPC). At elevated temperatures/low DPPC fractions, these lipids are in a miscible liquid crystalline (L(alpha)) state, whereas at lower temperatures/higher DPPC fractions they phase-separate into L(alpha) and gel phases. We show that the elastic modulus, K, and critical tension, tau(c), of L(alpha) vesicles are independent of DPPC fraction. However, as the sample temperature is increased from 15 degrees C to 45 degrees C, we measure decreases in both K and tau(c) of 20% and 50%, respectively. The elasticity change is likely driven by a change in interfacial tension. We describe the reduction in critical tension using a simple model of thermally activated membrane pores. Vesicles with two-phase coexistence exhibit material properties that differ from L(alpha) vesicles including critical tensions that are 20-40% lower. Fluorescence imaging of phase coexistent POPC/DPPC vesicles shows that the DPPC-rich domains exist in an extended network structure that exhibits characteristics of a solid. This gel network explains many of the unusual material properties of two-phase membranes.

Obstructed diffusion in phase-separated supported lipid bilayers: a combined atomic force microscopy and fluorescence recovery after photobleaching approach

Proteins and other macromolecules are believed to hinder molecular lateral diffusion in cellular membranes. We have constructed a well-characterized model system to better understand how obstacles in lipid bilayers obstruct diffusion. Fluorescence recovery after photobleaching was used to measure the lateral diffusion coefficient in single supported bilayers composed of mixtures of 1,2-dilauroylphosphotidylcholine (DLPC) and 1,2-distearoylphosphotidylcholine (DSPC). Because these lipids are immiscible and phase separate at room temperature, a novel quenching technique allowed us to construct fluid DLPC bilayers containing small disk-shaped gel-phase DSPC domains that acted as obstacles to lateral diffusion. Our experimental setup enabled us to analyze the same samples with atomic force microscopy and exactly characterize the size, shape, and number of gel-phase domains before measuring the obstacle-dependent diffusion coefficient. Lateral obstructed diffusion was found to be dependent on obstacle area fraction, size, and geometry. Analysis of our results using a free area diffusion model shows the possibility of unexpected long-range ordering of fluid-phase lipids around the gel-phase obstacles. This lipid ordering has implications for lipid-mediated protein interactions in cellular membranes.