Evidence for phase boundary lipid. Permeability of Tempo-choline into dimyristoylphosphatidylcholine vesicles at the phase transition

Conformations of influenza A M2 protein in DOPC/DOPS and E. coli native lipids and proteins

We compared the conformations of the transmembrane domain (TMD) of influenza A M2 (IM2) protein reconstituted in 1,2-dioleoyl-sn-glycero-3-phosphocholine/1,2-dioleoyl-sn-glycero-3-phospho-L-serine (DOPC/DOPS) bilayers to those in isolated Escherichia coli (E. coli) membranes, having preserved its native proteins and lipids. IM2 is a single-pass transmembrane protein known to assemble into a homo-tetrameric proton channel. To represent this channel, we made a construct containing the IM2's TMD region flanked by the juxtamembrane residues. The single cysteine substitution, L43C, of leucine located in the bilayer polar region was paramagnetically tagged with a methanethiosulfonate nitroxide label for the electron spin resonance (ESR) study. For this particular residue, we probed the conformations of the spin-labeled IM2 reconstituted in DOPC/DOPS and isolated E. coli membranes using continuous-wave ESR and double electron-electron resonance (DEER) spectroscopy. The total protein-to-lipid molar ratio spanned the range from 1:230 to 1:10,400. The continuous-wave ESR spectra corresponded to very slow spin-label motion in both environments. In all cases, the DEER data were reconstructed into distance distributions with well-resolved peaks at 1.68 and 2.37 nm in distance and amplitude ratios of 1.41 ± 0.2 and 2:1, respectively. This suggests four nitroxide spin labels located at the corners of a square, indicative of an axially symmetric tetramer. The distance modeling of DEER data with molecular modeling software applied to the NMR molecular structures (PDB: 2L0J) confirmed the symmetry and closed state of the C-terminal exit pore of the IM2 TMD tetramer in agreement with the model. Thus, we can conclude that, under conditions of pH 7.4 used in this study, IM2 TMD has similar conformations in model lipid bilayers and membranes made of native E. coli lipids and proteins of comparable thickness and fluidity, notwithstanding the complexity of the E. coli membranes caused by their lipid diversity and the abundance of integral and peripheral membrane proteins.

A comparative study of influenza A M2 protein conformations in DOPC/DOPS liposomes and in native E. coli membranes

We compared the conformations of the transmembrane domain (TMD) of influenza A M2 (IAM2) protein reconstituted at pH 7.4 in DOPC/DOPS bilayers to those in isolated E. coli membranes, having preserved its native proteins and lipids. IAM2 is a single-pass transmembrane protein known to assemble into homo-tetrameric proton channel. To represent this channel, we made a construct containing the IAM2's TMD region flanked by the juxtamembrane residues. The single cysteine substitute, L43C, of leucine located in the bilayer polar region was paramagnetically tagged with a methanethiosulfonate nitroxide label for the ESR (electron spin resonance) study. We compared the conformations of the spin-labeled IAM2 residing in DOPC/DOPS and native E. coli membranes using continuous-wave (CW) ESR and double electron-electron resonance (DEER) spectroscopy. The total protein-to-lipid molar ratio spanned the range from 1:230 to 1:10,400⩦ The CW ESR spectra corresponded to a nearly rigid limit spin label dynamics in both environments. In all cases, the DEER data were reconstructed into the distance distributions showing well-resolved peaks at 1.68 nm and 2.37 nm. The peak distance ratio was 1.41±0.2 and the amplitude ratio was 2:1. This is what one expects from four nitroxide spin-labels located at the corners of a square, indicative of an axially symmetric tetramer. Distance modeling of DEER data with molecular modeling software applied to the NMR molecular structures (PDB: 2L0J) confirmed the symmetry and closed state of the C-terminal exit pore of the IAM2 tetramer in agreement with the NMR model. Thus, we can conclude that IAM2 TMD has similar conformations in model and native E. coli membranes of comparable thickness and fluidity, notwithstanding the complexity of the E. coli membranes caused by their lipid diversity and the abundance of integral and peripheral membrane proteins.

Location of the TEMPO moiety of TEMPO-PC in phosphatidylcholine bilayers is membrane phase-dependent

The (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) moiety tethered to the headgroup of phosphatidylcholine (PC) lipid is employed in spin labeling electron paramagnetic resonance (EPR) spectroscopy to probe the water dynamics near lipid bilayer interfaces. Due to its amphiphilic character, however, TEMPO spin label could partition between aqueous and lipid phases, and may even be stabilized in the lipid phase. Accurate assessment of the TEMPO-PC configuration in bilayer membranes is essential for correctly interpreting the data from measurements. Here, we carry out all-atom molecular dynamics (MD) simulations of TEMPO-PC probe in single-component lipid bilayers at varying temperatures, using two standard MD force fields. We find that for DPPC membrane whose gel-to-fluid lipid phase transition occurs at 314 K, while the TEMPO spin label is stabilized above the bilayer interface in the gel phase, there is a preferential location of TEMPO below the membrane interface in the fluid phase. For bilayers made of unsaturated lipids, DOPC and POPC, which adopt the fluid phase at ambient temperature, TEMPO is unequivocally stabilized inside the bilayers. Our finding of membrane phase-dependent positioning of TEMPO moiety highlights the importance of assessing the packing order and fluidity of lipids under a given measurement condition.

The antioxidant vitamin E as a membrane raft modulator: Tocopherols do not abolish lipid domains

The antioxidant vitamin E is a commonly used vitamin supplement. Although the multi-billion dollar vitamin and nutritional supplement industry encourages the use of vitamin E, there is very little evidence supporting its actual health benefits. Moreover, vitamin E is now marketed as a lipid raft destabilizing anti-cancer agent, in addition to its antioxidant behaviour. Here, we studied the influence of vitamin E and some of its vitamers on membrane raft stability using phase separating unilamellar lipid vesicles in conjunction with small-angle scattering techniques and fluorescence microscopy. We find that lipid phase behaviour remains unperturbed well beyond physiological concentrations of vitamin E (up to a mole fraction of 0.10). Our results are consistent with a proposed line active role of vitamin E at the domain boundary. We discuss the implications of these findings as they pertain to lipid raft modification in native membranes, and propose a new hypothesis for the antioxidant mechanism of vitamin E.

Liposome-Mediated Chemotherapeutic Delivery Is Synergistically Enhanced by Ternary Lipid Compositions and Cationic Lipids

Most small molecule chemotherapeutics must cross one or more cellular membrane barriers to reach their biochemical targets. Owing to the relatively low solubility of chemotherapeutics in the lipid membrane environment, high doses are often required to achieve a therapeutic effect. The resulting systemic toxicity has motivated efforts to improve the efficiency of chemotherapeutic delivery to the cellular interior. Toward this end, liposomes containing lipids with cationic head groups have been shown to permeabilize cellular membranes, resulting in the more efficient release of encapsulated drugs into the cytoplasm. However, the high concentrations of cationic lipids required to achieve efficient delivery remain a key limitation, frequently resulting in toxicity. Toward overcoming this limitation, here, we investigate the ability of ternary lipid mixtures to enhance liposomal delivery. Specifically, we investigate the delivery of the chemotherapeutic, doxorubicin, using ternary liposomes that are homogeneous at physiological temperature but have the potential to undergo membrane phase separation upon contact with the cell surface. This approach, which relies upon the ability of membrane phase boundaries to promote drug release, provides a novel method for reducing the overall concentration of cationic lipids required for efficient delivery. Our results show that this approach improves the performance of doxorubicin by up to 5-fold in comparison to the delivery of the same drug by conventional liposomes. These data demonstrate that ternary lipid compositions and cationic lipids can be combined synergistically to substantially improve the efficiency of chemotherapeutic delivery in vitro.

Magnetic liposome design for drug release systems responsive to super-low frequency alternating current magnetic field (AC MF)

HYPOTHESIS

Magnetic liposomes are shown to release the entrapped dye once modulated by low frequency AC MF. The mechanism and effectiveness of MF application should depend on lipid composition, magnetic nanoparticles (MNPs) properties, temperature and field parameters.

EXPERIMENTS

The study was performed using liposomes of various lipid composition and embedded hydrophobic MNPs. The liposomes structural changes were studied by the transmission electron microscopy (TEM) and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and the leakage was monitored by the fluorescent dye release.

FINDINGS

Magnetic liposomes exposure to the AC MF resulted in the clustering of the MNPs in the membranes and disruption of the lipid packaging. Addition of cholesterol diminished the dye release from the saturated lipid-based liposomes. Replacement of the saturated lipid for unsaturated one also decreased the dye release. The dye release depended on the strength, but not the frequency of the field. Thus, the oscillating motion of MNPs in AC MF ruptures the gel phase membranes of saturated lipids. As the temperature increases the disruption also increases. In the liquid crystalline membranes formed by unsaturated lipids the deformations and defects created by mechanical motion of the MNPs are more likely to heal and results in decreased release.

Coupling of Membrane Nanodomain Formation and Enhanced Electroporation near Phase Transition

Biological cells are enveloped by a heterogeneous lipid bilayer that prevents the uncontrolled exchange of substances between the cell interior and its environment. In particular, membranes act as a continuous barrier for salt and macromolecules to ensure proper physiological functions within the cell. However, it has been shown that membrane permeability strongly depends on temperature and, for phospholipid bilayers, displays a maximum at the transition between the gel and fluid phase. Here, extensive molecular dynamics simulations of dipalmitoylphosphatidylcholine bilayers were employed to characterize the membrane structure and dynamics close to phase transition, as well as its stability with respect to an external electric field. Atomistic simulations revealed the dynamic appearance and disappearance of spatially related nanometer-sized thick ordered and thin interdigitating domains in a fluid-like bilayer close to the phase transition temperature (Tm). These structures likely represent metastable precursors of the ripple phase that vanished at increased temperatures. Similarly, a two-phase bilayer with coexisting gel and fluid domains featured a thickness minimum at the interface because of splaying and interdigitating lipids. For all systems, application of an external electric field revealed a reduced bilayer stability with respect to pore formation for temperatures close to Tm. Pore formation occurred exclusively in thin interdigitating membrane nanodomains. These findings provide a link between the increased membrane permeability and the structural heterogeneity close to phase transition.

Daptomycin-Phosphatidylglycerol Domains in Lipid Membranes

Daptomycin is an acidic, 13-amino acid, cyclic polypeptide that contains a number of nonproteinogenic residues and is modified at its N-terminus with a decanoyl chain. It has been in clinical use since 2003 against selected drug-resistant Staphylococcus aureus and Enterococcus spp infections. In vitro, daptomycin is active against Gram-positive pathogens at low concentrations but its antibiotic activity depends critically on the presence of calcium ions. This dependence has been thought to arise from binding of one or two Ca²⁺ ions to daptomycin as a required step in its interaction with the bacterial membrane. Here, we investigated the interaction of daptomycin with giant unilamellar vesicles (GUVs) composed 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) and 1-palmitoyl-2-oleoylphosphatidylglycerol (POPG). We used fluorescence confocal microscopy to monitor binding of the peptide to GUVs and follow its effect on the membrane of the vesicle. We found that in the absence of POPG or Ca²⁺ daptomycin does not bind measurably to the lipid membrane. In the presence of 20-30% PG in the membrane and 2 mM Ca²⁺, daptomycin induces the formation of membrane domains rich in acidic lipids. This effect is not induced by Ca²⁺ alone. In addition, daptomycin causes GUV collapse, but it does not translocate across the membrane to the inside of intact POPC/POPG vesicles. We conclude that pore formation is probably not the mechanism by which the peptide functions. On the other hand, we found that daptomycin coclusters with the anionic phospholipid POPG and the fluorescent probes used, leading to extensive formation of daptomycin-POPG domains in the membrane.

Non-linear van't Hoff behavior in pulmonary surfactant model membranes

Pulmonary surfactant exhibits phase coexistence over a wide range of surface pressure and temperature. Less is known about the effect of temperature on pulmonary surfactant models. Given the lack of studies on this issue, we used electron paramagnetic resonance (EPR) and nonlinear least-squares (NLLS) simulations to investigate the thermotropic phase behavior of the matrix that mimics the pulmonary surfactant lipid complex, i.e., the lipid mixture composed of dipalmitoyl phosphatidylcholine (DPPC), palmitoyl-oleoyl phosphatidylcholine (POPC) and palmitoyl-oleoyl phosphatidylglycerol (POPG). Irrespective of pH, the EPR spectra recorded from 5°C to 25°C in the DPPC/POPC/POPG (4:3:1) model membrane contain two spectral components corresponding to lipids in gel-like and fluid-like phases, indicating a coexistence of two domains in that range. The temperature dependence of the distribution of spin labels between the domains yielded nonlinear van't Hoff plots. The thermodynamic parameters evaluated were markedly different for DPPC and for the ternary DPPC/POPC/POPG (4:3:1) membranes and exhibited a dependence on chemical environment. While enthalpy and entropy changes for DPPC were always positive and presented a quadratic behavior with temperature, those of the ternary mixture were linearly dependent on temperature and changed from negative to positive values. Despite that, enthalpy-entropy compensation takes place in the two systems. The thermotropic process associated with the coexistence of the two domains is entropically-driven in DPPC and either entropically- or enthalpically-driven in the pulmonary surfactant membrane depending on the pH, ionic strength and temperature. The significance of these results to the structure and function of the pulmonary surfactant lipid matrix is discussed.

Line Tension Assists Membrane Permeation at the Transition Temperature in Mixed-Phase Lipid Bilayers

The umbrella sampling method has been used to evaluate the free energy profile for a large permeant moving through a lipid bilayer, represented using a coarse-grained simulation model, at and below its gel-fluid transition temperature. At the lipid transition temperature, determined to be 302 K for the MARTINI 2.0 model of DPPC, the permeation barrier for passage through an enclosed fluid domain embedded in a patch of gel was significantly lower than that for passage through a fluid stripe domain. In contrast, permeation through a fluid domain in a stripe geometry produced a free energy profile nearly identical to that of a gel-free fluid bilayer. In both cases, insertion of the permeant into a fluid domain coexisting with the gel phase led to a shift in phase composition, as lipids transitioned from fluid to gel to accommodate the area occupied by the permeant. In the case of the enclosed fluid domain, this transition produced a decrease in the length of the fluid-gel interface as the approximately circular fluid domain shrank. The observed decrease in the apparent permeation barrier, combined with an approximation for the change in interfacial length, enabled estimation of the interfacial line tension to be between 10 and 13 pN for this model. The permeation barrier was shown to drop even further in simulations performed at temperatures below the transition temperature. The results suggest a mechanism to explain the experimentally observed anomalous peak in the temperature-dependent permeability of lipid bilayers near their transition temperatures. The contribution of this mechanism toward the permeability of a gel phase containing a thermal distribution of fluid-phase domains is estimated using a simple statistical thermodynamic model.

Adsorption Behaviors of Mixed Monolayers of n-Alkanes at the Liquid-Solid Interface

To understand the self-assembly of monolayers at the liquid-solid interface, a thermodynamic model, which describes the contributions of the molecular interactions, is essential. We present an adapted Zimm-Bragg model of the cooperativity transitions for determining the Gibbs free energy for self-assembly at the liquid-solid interface. Scanning tunneling microscopy was used to observe the monolayers formed on graphite from phenyloctane solutions of binary mixtures of n-hexacosane (C26H54) and n-tetratriacontane (C34H70). This revealed that the sharp transition in the monolayers from the full surface coverage of the long-chain alkane, which is adsorbed preferentially, to the full coverage of the short-chain alkane is a function of the mixture composition. The model allows for the estimation of the free-energy changes associated with the difference in the alkyl chain length and the interface between the two different alkane regions in the monolayers. It is also suitable for understanding more complex systems that exhibit intermolecular interactions.

Simulation study of the permeability of a model lipid membrane at the fluid-solid phase transition

When a range of lipid bilayers are melted to the disordered fluid phase from the (much less permeable) ordered gel phase, their permeability to a variety of permeants shows a peak at the transition temperature and drops off with increasing temperature, rather than just rising as melting proceeds. To explore this anomalous behavior, a simulated coarse-grained lipid membrane model that exhibits a phase transition upon expansion or compression was studied to determine how the permeation rate of a simple particle depends on the phase composition in the two-phase region and on particle size. The permeation rate and each phase's area fraction and area density could be directly calculated, along with the probability that the permeant would cross in either phase or in interfacial regions. For large permeants and system sizes, conditions could be found where permeability increases upon compression of the bilayer. Permeation was negligible in the gel phase and, in contrast to the predictions of the "leaky interface" hypothesis, was not enriched in interfacial regions. The anomalous effect could instead be attributed to an increase in the area per lipid of fluid-phase domains. This result motivated a model for the decrease in effective permeability barrier through fluid-phase domains arising from a decrease in the length of the gel/fluid interface at the midpoint of a permeation event.

Biocompatible cationic lipids for the formulation of liposomal DNA vectors

Ethylphosphocholine lipids are highly biocompatible cationic amphiphiles that can be used for the formulation of liposomal DNA vectors, with negligible toxic effects on cells and organisms. Here we report the characterization of EDPPC (1,2-dipalmitoyl-sn-glycero-O-ethyl-3-phosphocholine chloride) liposomes, containing two different zwitterionic helper lipids, POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) and DOPE (1,2-dioleoyl-sn-glycero-3-phosphoethanolamine). Depending on the nature of the helper lipid, a phase separation in the bilayer is found at room temperature, where domains enriched in the cationic component coexist in a relatively large temperature range with regions where the zwitterionic lipids are predominant. We studied DNA complexation, the internal structure of lipoplexes and their docking and fusogenic ability with model target bilayers. The structural and functional modifications caused by DNA binding were studied using Dynamic Light Scattering (DLS), zeta potential, and small and wide angle X-ray scattering (SAXS-WAXS) measurements, while the interaction with membranes was assessed by using Giant Unilamellar Vesicles (GUVs) as model target bilayers. The results presented establish a connection between the physicochemical properties of lipid bilayers, and in particular of lipid demixing, with the phase state of the complexes and their ability to interact with model membranes.

Environmental conditions that influence the ability of humic acids to induce permeability in model biomembranes

The interaction of humic acids (HAs) with 1-palmitoyl-2-oleoyl-Sn-glycero-3-phosphocholine (POPC) large unilamellar vesicle (LUV) model biomembrane system was studied by fluorescence spectroscopy. HAs from aquatic and terrestrial (including coal) sources were studied. The effects of HA concentration and temperature over environmentally relevant ranges of 0 to 20 mg C/L and 10 to 30 °C, respectively, were investigated. The dosage studies revealed that the aquatic Suwannee River humic acid (SRHA) causes an increased biomembrane perturbation (percent leakage of the fluorescent dye, Sulforhodamine B) over the entire studied concentration range. The two terrestrial HAs, namely Leonardite humic acid (LAHA) and Florida peat humic acid (FPHA), at concentrations above 5 mg C/L, show a decrease or a plateau effect attributable to the competition within the HA mixture and/or the formation of "partial aggregates". The temperature studies revealed that biomembrane perturbation increases with decreasing temperature for all three HAs. Kinetic studies showed that the membrane perturbation process is complex with both fast and slow absorption (sorption into the bilayer) components and that the slow component could be fitted by first order kinetics. A mechanism based on "lattice errors" within the POPC LUVs is put forward to explain the fast and slow components. A rationale behind the concentration and temperature findings is provided, and the environmental implications are discussed.

Quantitative analysis of molecular transport across liposomal bilayer by J-mediated 13C Overhauser dynamic nuclear polarization

We introduce a new NMR technique to dramatically enhance the solution-state (13)C NMR sensitivity and contrast at 0.35 T and at room temperature by actively transferring the spin polarization from Overhauser dynamic nuclear polarization (ODNP)-enhanced (1)H to (13)C nuclei through scalar (J) coupling, a method that we term J-mediated (13)C ODNP. We demonstrate the capability of this technique by quantifying the permeability of glycine across negatively charged liposomal bilayers composed of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG). The permeability coefficient of glycine across this DPPC/DPPG bilayer is measured to be (1.8 ± 0.1) × 10(-11)m/s, in agreement with the literature value. We further observed that the presence of 20 mol % cholesterol within the DPPC/DPPG lipid membrane significantly retards the permeability of glycine by a factor of 4. These findings demonstrate that the high sensitivity and contrast of J-mediated (13)C ODNP affords the measurement of the permeation kinetics of small hydrophilic molecules across lipid bilayers, a quantity that is difficult to accurately measure with existing techniques.

Confocal Raman microscopy probing of temperature-controlled release from individual, optically-trapped phospholipid vesicles

Control of permeability of phospholipid vesicle (liposome) membranes is critical to their applications in analytical sensing, in fundamental studies of chemistry in small volumes, and in encapsulation and release of payloads for site-directed drug delivery. Applications of liposome formulations in drug delivery often take advantage of the enhanced permeability of phospholipid membranes at their gel-to-fluid phase transition, where the release of encapsulated molecules can be initiated by an increase in temperature. Despite numerous successful liposome formulations for encapsulation and release methods to study the kinetics, this process has been limited to investigations of bulk vesicle dispersions, which provide little or no information about the vesicle membrane structure and its relationship to the kinetics of trans-membrane transport. In this work, confocal Raman microscopy is adapted to study temperature-dependent release of a model compound, 3-nitrobenzene sulfonate (3-NBS), from individual optically trapped phospholipid vesicles, while simultaneously monitoring structural changes in the vesicle membrane reported by vibrational modes of phospholipid acyl chains and the local environment of the encapsulated compound. The confocal geometry allows efficient excitation and collection of Raman scattering from a single vesicle, while optical trapping allows more than hour-long observations of the same vesicle. With window factor analysis to resolve component spectra, temperature-controlled release of 3-NBS through vesicle membranes composed of pure 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was measured and compared to transport through a lysolipid-containing membrane specifically formulated for efficient drug delivery.

Nanoscale Drug Delivery and Hyperthermia: The Materials Design and Preclinical and Clinical Testing of Low Temperature-Sensitive Liposomes Used in Combination with Mild Hyperthermia in the Treatment of Local Cancer

The overall objective of liposomal drug delivery is to selectively target drug delivery to diseased tissue, while minimizing drug delivery to critical normal tissues. The purpose of this review is to provide an overview of temperature-sensitive liposomes in general and the Low Temperature-Sensitive Liposome (LTSL) in particular. We give a brief description of the material design of LTSL and highlight the likely mechanism behind temperature-triggered drug release. A complete review of the progress and results of the latest preclinical and clinical studies that demonstrate enhanced drug delivery with the combined treatment of hyperthermia and liposomes is provided as well as a clinical perspective on cancers that would benefit from hyperthermia as an adjuvant treatment for temperature-triggered chemotherapeutics. This review discusses the ideas, goals, and processes behind temperature-sensitive liposome development in the laboratory to the current use in preclinical and clinical settings.

Relationship between membrane permeability and specificity of human secretory phospholipase A(2) isoforms during cell death

During apoptosis, a number of physical changes occur in the cell membrane including a gradual increase in permeability to vital stains such as propidium iodide. This study explored the possibility that one consequence of membrane changes concurrent with early modest permeability is vulnerability to degradation by secretory phospholipase A(2). The activity of this hydrolytic enzyme toward mammalian cells depends on the health of the cell; healthy cells are resistant, but they become susceptible early during programmed death. Populations of S49 lymphoma cells during programmed death were classified by flow cytometry based on permeability to propidium iodide and susceptibility to secretory phospholipase A(2). The apoptotic inducers thapsigargin and dexamethasone caused modest permeability to propidium iodide and increased staining by merocyanine 540, a dye sensitive to membrane perturbations. Various secretory phospholipase A(2) isozymes (human groups IIa, V, X, and snake venom) preferentially hydrolyzed the membranes of cells that displayed enhanced permeability. In contrast, cells exposed briefly to a calcium ionophore showed the increase in cell staining intensity by merocyanine 540 without accompanying uptake of propidium iodide. Under that condition, only the snake venom and human group X enzymes hydrolyzed cells that were dying. These results suggested that cells showing modest permeability to propidium iodide during the early phase of apoptosis are substrates for secretory phospholipase A(2) and that specificity among isoforms of the enzyme depends on the degree to which the membrane has been perturbed during the death process. This susceptibility to hydrolysis may be important as part of the signal to attract macrophages toward apoptotic cells.

Cholesterol-induced fluid membrane domains: a compendium of lipid-raft ternary phase diagrams

The biophysical underpinning of the lipid-raft concept in cellular membranes is the liquid-ordered phase that is induced by moderately high concentrations of cholesterol. Although the crucial feature is the coexistence of phase-separated fluid domains, direct evidence for this in mixtures of cholesterol with a single lipid is extremely sparse. More extensive evidence comes from ternary mixtures of a high chain-melting lipid and a low chain-melting lipid with cholesterol, including those containing sphingomyelin that are taken to be a raft paradigm. There is, however, not complete agreement between the various phase diagrams and their interpretation. In this review, the different ternary phase diagrams of cholesterol-containing systems are presented in a uniform way, using simple x,y-coordinates to increase accessibility for the non-specialist. It is then possible to appreciate the common features and examine critically the discrepancies and hence what direct biophysical evidence there is that supports the raft concept.

A raft-associated species of phosphatidylethanolamine interacts with cholesterol comparably to sphingomyelin. A Langmuir-Blodgett monolayer study

Background

Specific interactions between sphingomyelin (SM) and cholesterol (Ch) are commonly believed to play a key role in the formation of rafts in the biological membranes. A weakness of this model is the implication that these microdomains are confined to the outer bilayer leaflet. The cytoplasmic leaflet, which contains the bulk of phosphatidylethanolamine (PE), phosphatidylserine (PS) and phosphatidylinositol (PI), is thought also to harbour half of the membrane cholesterol. Moreover, SLPE (1-stearoyl-2-linoleoyl-sn-glycero-3-phosphatidyl-ethanolamine) has recently been shown to be enriched in isolated detergent-resistant membranes (DRM), and this enrichment was independent of the method of isolation of DRM.

Methodology/Principal Findings

Here we present quantitative evidence coming from Langmuir-Blodgett monolayer experiments that SLPE forms complex with Ch similar to that between SM and Ch. The energies of these interactions as calculated form the monolayer studies are highly negative. FRAP analysis showed that NBD-Ch recovery was similar in liposomes composed of DOPC/Ch SM or SLPE but not DPPE, providing further evidence that SLPE may form an l_o phase in the presence of high Ch concentration. Experiments on the solubility of DOPC liposomes containing DPPE/Ch (1∶1), SM/Ch (1∶1) or SLPE/Ch (1∶1) showed the presence of Triton X-100 insoluble floating fraction (TIFF) in the case of SM/Ch or SLPE/Ch but not in DPPE/Ch containing liposomes. Quantitative determination of particular lipid species in the TIFF fraction confirms the conclusion that SLPE (or similar PE species) could be an important constituent of the inner leaflet raft.

Conclusion

Such interactions suggest a possible existence of inner-leaflet nanoscale assemblies composed of cholesterol complexes with SLPE or similar unsaturated PE species.

Controlled release mechanisms of spontaneously forming unilamellar vesicles

Spontaneously forming small unilamellar vesicles (SULVs) are easy to prepare and show great promise for use in delivering therapeutic payloads. We report of SULVs made up of the ternary phospholipid mixture, dimyristoyl-phosphatidylcholine (DMPC), dihexanoyl-phosphatidylcholine (DHPC) and dimyristoyl-phosphatidylglycerol (DMPG), which have been characterized by small angle neutron scattering (SANS). These low-polydispersity (0.14-0.19) SULVs range in size (i.e., radius) from 110 to 215 A and are capable of entrapping, and subsequently releasing, hydrophilic molecules (e.g., fluorescent dyes and quenchers) in a controlled fashion over two different temperature ranges. The low-temperature release mechanism involves the SULVs transforming into discoidal micelles, with an onset temperature (T(o)) of ~32 degrees C, while the high-temperature release mechanism is more gradual, presumably the result of defects formed through the continuous dissolution of DHPC into solution. Both of these mechanisms differ from other, previously reported thermosensitive liposomes.

Phase transition of individually addressable microstructured membranes visualized by imaging ellipsometry

The phase transition of individually addressable microstructured lipid bilayers was investigated by means of imaging ellipsometry. Microstructured bilayers were created on silicon substrates by micromolding in capillaries, and the thermotropic behavior of various saturated diacyl phosphatidylcholine (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, 1,2-dipentadecoyl-sn-glycero-3-phosphocholine, and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)) bilayers as well as DMPC/cholesterol membranes was determined by measuring the area expansion and thickness of the bilayer as a function of temperature. We found an increase in the main phase transition temperature T(M) of 2-6 degrees C and a substantially reduced cooperativity compared to multilamellar vesicles. Measurements of lateral diffusion constants D employing fluorescence recovery after photobleaching revealed, however, only a marginal decrease in D compared to those found for vesicles and multibilayers. The known dependencies of T(M) both on the chain length of diacyl PC membranes and on the cholesterol content were reproduced on a solid support. Microstructured bilayers offer the unique advantage of integrating an internal standard of known thermotropic properties, which turned out to be important for reducing the measurement error and for ruling out the slightly changing impact of the surface on the phase transition behavior due to the surface pretreatment.

Simultaneous measurements of K+ and calcein release from liposomes and the determination of pore size formed in a membrane

The changes induced by biologically active substances in the permeability to K+ and calcein of liposomes composed of egg phosphatidylcholine and cholesterol were measured simultaneously in order to rapidly screen the sizes of pores formed in a membrane, using different sized markers. The substances examined in the present study were classified into three types based on differences in the rates at which K+ and calcein were released. The first type released only K+, and included gramicidin A. The second type predominantly released K+, preceding the release of calcein, and included amphotericin B and nystatin. The third type, including antimicrobial peptides, such as gramicidin S, alamethicin, and melittin, and several membrane-active drugs, like celecoxib (non-steroidal anti-inflammatory drug), 1-dodecylazacycloheptan-2-one (named azone; skin permeation enhancer), and chlorpromazine (tranquilizer), caused the release of K+ and calcein simultaneously. Thus, the sizes of pores formed in a liposomal membrane increased in the following order: types one, two, and three. We determined the size more precisely by conducting an osmotic protection experiment, measuring the release of calcein in the presence of osmotic protectants of different sizes. The radii of pores formed by the second type, amphotericin B and nystatin, were 0.36 - 0.46 nm, while the radii of pores formed by the third type were much larger, 0.63 - 0.67 nm or more. The permeability changes induced by substances of the third type are discussed in connection with a transient pore formed in a lipid packing mismatch taking place during the phase transition of dipalmitoylphosphatidylcholine liposomes.

Synergy in lipofection by cationic lipid mixtures: superior activity at the gel-liquid crystalline phase transition

Some mixtures of two cationic lipids including phospholipid compounds (O-ethylphosphatidylcholines) as well as common, commercially available cationic lipids, such as dimethylammonium bromides and trimethylammonium propanes, deliver therapeutic DNA considerably more efficiently than do the separate molecules. In an effort to rationalize this widespread "mixture synergism", we examined the phase behavior of the cationic lipid mixtures and constructed their binary phase diagrams. Among a group of more than 50 formulations, the compositions with maximum delivery activity resided unambiguously in the solid-liquid crystalline two-phase region at physiological temperature. Thus, the transfection efficacy of formulations exhibiting solid-liquid crystalline phase coexistence is more than 5 times higher than that of formulations in the gel (solid) phase and over twice that of liquid crystalline formulations; phase coexistence occurring at physiological temperature thus appears to contribute significantly to mixture synergism. This relationship between delivery activity and physical property can be rationalized on the basis of the known consequences of lipid-phase transitions, namely, the accumulation of defects and increased disorder at solid-liquid crystalline phase boundaries. Packing defects at the borders of coexisting solid and liquid crystalline domains, as well as large local density fluctuations, could be responsible for the enhanced fusogenicity of mixtures. This study leads to the important conclusion that manipulating the composition of the lipid carriers so that their phase transition takes place at physiological temperature can enhance their delivery efficacy.

An intracellular lamellar-nonlamellar phase transition rationalizes the superior performance of some cationic lipid transfection agents

Two cationic phospholipid derivatives with asymmetric hydrocarbon chains were synthesized: ethyl esters of oleoyldecanoyl-ethylphosphatidylcholine (C18:1/C10-EPC) and stearoyldecanoyl-ethylphosphatidylcholine (C18:0/C10-EPC). The former was 50 times more effective as a DNA transfection agent (human umbilical artery endothelial cells) than the latter, despite their similar chemical structure and virtually identical lipoplex organization. A likely reason for the superior effectiveness of C18:1/C10-EPC relative to C18:0/C10-EPC (and to many other cationic lipoids) was suggested by the phases that evolved when these lipoids were mixed with negatively charged membrane lipid formulations. The saturated C18:0/C10-EPC remained lamellar in mixtures with biomembrane-mimicking lipid formulations [e.g., dioleoyl-phosphatidylcholine/dioleoyl-phosphatidylethanolamine/dioleoyl-phosphatidylserine/cholesterol at 45:20:20:15 (wt/wt)]; in contrast, the unsaturated C18:1/C10-EPC exhibited a lamellar-nonlamellar phase transition in such mixtures, which took place at physiological temperatures, approximately 37 degrees C. As is well known, lipid vehicles exhibit maximum leakiness and contents release in the vicinity of phase transitions, especially those involving nonlamellar phase formation. Moreover, nonlamellar phase-forming compositions are frequently highly fusogenic. Indeed, FRET experiments showed that C18:1/C10-EPC exhibits lipid mixing with negatively charged membranes that is several times more extensive than that of C18:0/C10-EPC. Thus, C18:1/C10-EPC lipoplexes are likely to easily fuse with membranes, and, as a result of lipid mixing, the resultant aggregates should exhibit extensive phase coexistence and heterogeneity, thereby facilitating DNA release and leading to superior transfection efficiency. These results highlight the phase properties of the carrier lipid/cellular lipid mixtures as a decisive factor for transfection success and suggest a strategy for the rational design of superior cationic lipid carriers.

Transfection activity of binary mixtures of cationic o-substituted phosphatidylcholine derivatives: the hydrophobic core strongly modulates physical properties and DNA delivery efficacy

A combination of two cationic lipid derivatives having the same headgroup but tails of different chain lengths has been shown to have considerably different transfection activity than do the separate molecules. Such findings point to the importance of investigating the hydrophobic portions of cationic amphiphiles. Hence, we have synthesized a variety of cationic phosphatidylcholines with unusual hydrophobic moieties and have evaluated their transfection activity and that of their mixtures with the original molecule of this class, dioleoyl-O-ethylphosphatidylcholine (EDOPC). Four distinct relationships between transfection activity and composition of the mixture (plotted as percent of the new compound added to EDOPC) were found, namely: with a maximum or minimum; with a proportional change; or with essentially no change. Relevant physical properties of the lipoplexes were also examined; specifically, membrane fusion (by fluorescence resonance energy transfer between cationic and anionic lipids) and DNA unbinding (measured as accessibility of DNA to ethidium bromide by electrophoresis and by fluorescence resonance energy transfer between DNA and cationic lipid), both after the addition of negatively charged membrane lipids. Fusibility increased with increasing content of second cationic lipid, regardless of the transfection pattern. However, the extent of DNA unbinding after addition of negatively charged membrane lipids did correlate with extent of transfection. The phase behavior of cationic lipids per se as well as that of their mixtures with membrane lipids revealed structural differences that may account for and support the hypothesis that a membrane lipid-triggered, lamellar-->nonlamellar phase transition that facilitates DNA release is critical to efficient transfection by cationic lipids.

Binding of α-Synuclein Affects the Lipid Packing in Bilayers of Small Vesicles

The intracellular deposition of fibrillar aggregates of alpha-synuclein is a characteristic feature of Parkinson disease. Alternatively, as a result of its unusual conformational plasticity, alpha-synuclein may fold into an amphipathic helix upon contact with a lipid-water interface. Using spin label ESR and fluorescence spectroscopy, we show here that alpha-synuclein affects the lipid packing in small unilamellar vesicles. The ESR hyperfine splittings of spin-labeled phospholipid probes revealed that alpha-synuclein induces chain ordering at carbon 14 of the acyl chains below the chain melting phase transition temperature but not in the liquid crystalline state of electroneutral vesicle membranes. Binding of alpha-synuclein leads to an increase in the temperature and cooperativity of the phase transition according to the fluorescence anisotropy of the hydrophobic polyene 1,6-diphenylhexatriene and of the fluorescence emission maxima of the amphiphilic probe 6-dodecanoyl-2-dimethylaminonaphthalene. Binding parameters were obtained from the fluorescence anisotropy measurements in combination with our previous determinations by titration calorimetry (Nuscher, B., Kamp, F., Mehnert, T., Odoy, S., Haass, C., Kahle, P. J., and Beyer, K. (2004) J. Biol. Chem. 279, 21966-21975). We also show that alpha-synuclein interacts with vesicle membranes containing sphingomyelin and cholesterol. We propose that the protein is capable of annealing defects in curved vesicle membranes, which may prevent synaptic vesicles from premature fusion.

Lysolipid incorporation in dipalmitoylphosphatidylcholine bilayer membranes enhances the ion permeability and drug release rates at the membrane phase transition

The enhanced permeability of lipid bilayer membranes at their gel-to-liquid phase transition has been explained using a "bilayer lipid heterogeneity" model, postulating leaky interfacial regions between still solid and melting liquid phases. The addition of lysolipid to dipalmitoylphosphatidylcholine bilayers dramatically enhances the amount of, and speed at which, encapsulated markers or drugs are released at this, already leaky, phase transition through these interfacial regions. To characterize and attempt to determine the mechanism behind lysolipid-generated permeability enhancement, dithionite permeability and doxorubicin release were measured for lysolipid and non-lysolipid, containing membranes. Rapid release of contents from lysolipid-containing membranes appears to occur through lysolipid-stabilized pores rather than a simple enhancement due to increased drug solubility in the bilayer. A dramatic enhancement in the permeability rate constant begins about two degrees below the calorimetric peak of the thermal transition, and extends several degrees past it. The maximum permeability rate constant coincides exactly with this calorimetric peak. Although some lysolipid desorption from liquid state membranes cannot be dismissed, dialyzation above T(m) and mass spectrometry analysis indicate lysolipid must, and can, remain in the membrane for the permeability enhancement, presumably as lysolipid stabilized pores in the grain boundary regions of the partially melted solid phase.

Fluorescence correlation spectroscopy relates rafts in model and native membranes

The lipid raft model has evoked a new perspective on membrane biology. Understanding the structure and dynamics of lipid domains could be a key to many crucial membrane-associated processes in cells. However, one shortcoming in the field is the lack of routinely applicable techniques to measure raft association without perturbation by detergents. We show that both in cell and in domain-exhibiting model membranes, fluorescence correlation spectroscopy (FCS) can easily distinguish a raft marker (cholera toxin B subunit bound to ganglioside (GM1) and a nonraft marker (dialkylcarbocyanine dye diI)) by their decidedly different diffusional mobilities. In contrast, these markers exhibit only slightly different mobilities in a homogeneous artificial membrane. Performing cholesterol depletion with methyl-beta-cyclodextrin, which disrupts raft organization, we find an analogous effect of reduced mobility for the nonraft marker in domain-exhibiting artificial membranes and in cell membranes. In contrast, cholesterol depletion has differential effects on the raft marker, cholera toxin B subunit-GM1, rendering it more mobile in artificial domain-exhibiting membranes but leaving it immobile in cell membranes, where cytoskeleton disruption is required to achieve higher mobility. Thus, fluorescence correlation spectroscopy promises to be a valuable tool to elucidate lipid raft associations in native cells and to gain deeper insight into the correspondence between model and natural membranes.

Factors affecting responsivity of unilamellar liposomes to 20 kHz ultrasound

Ultrasound is commonly used in the preparation of unilamellar liposome dispersions and is often considered for cell membrane disruption for drug delivery or DNA transfection applications. To better understand the physical and chemical properties of lipid membranes that render them susceptible to ultrasonic permeabilization, the roles of temperature, lipid composition (cholesterol and PEG-lipid content), and liposome size have been studied. The results of these studies suggest that lipid packing is very important to ultrasound responsiveness; surprisingly, cohesive energy and tensile strength are not. Taken together, the experimental results implicate a defect-mediated permeabilization mechanism, rather than pore formation or membrane tearing. The implications of this work for drug release from liposomes and ultrasound-mediated DNA transfection are discussed.

Studies on molecular interactions between nalidixic acid and liposomes

The interaction between nalidixic acid sodium salt (NANa) and liposomes prepared from alpha-L-dipalmitoyl-phosphatidylcholine (DPPC) or from its binary mixture with dioleoyl-phosphatidylcholine (DOPC) was studied with differential scanning calorimetry (DSC) and electron paramagnetic resonance (EPR) spectroscopy. We evaluated the role of broadband ultraviolet-B (UV-B) irradiation on the molecular interactions between the lipids and the NANa, and determined the decay-kinetics of the incorporated spin labeled fatty-acid free radicals. Multilamellar and unilamellar vesicles were prepared by sonication and extrusion. The entrapment efficiencies were determined spectrophotometrically. The size-distribution of the liposomes and its change in time was checked by dynamic light scattering (DLS). Our results indicate that NANa mainly interacts with lipid head groups. However, its effect and presumably the formation of the free radicals, induced by broadband ultraviolet-B, is not localized only to the head group region of the lipid molecules. Depending on DOPC content, interaction between the NANa and the lipids modifies the phase-transition parameters of the liposome dispersions.

Effects of cholesterol and temperature on the permeability of dimyristoylphosphatidylcholine bilayers near the chain melting phase transition

The passive leakage of glucose across bilayers of dimyristoylphosphatidylcholine (DMPC), cholesterol (variable), and dicetyl phosphate (constant 5.9 mol%) has been measured as efflux over 30 min from multilamellar vesicles. Bilayer cholesterol was varied from 20 mol% to 40 mol%. Glucose permeation rates were measured from 10 degrees C to 36 degrees C, and showed a maximum in permeability at 24 degrees C, the DMPC phase transition temperature. Increasing the bilayer cholesterol content above 20 mol% reduced that permeability peak. These results are quite consistent with a large number of similar bilayer permeability studies over the past 25 years. However, they are not consistent with a previous study of these same systems, which reported increased glucose permeability with temperature, without any maximum at or near the lipid chain melting temperature (K. Inoue, Biochim. Biophys. Acta 339 (1974) 390-402).

Miscibility gap in fluid dimyristoylphosphatidylcholine:cholesterol as "seen" by x rays

A binary mixture of dimyristoylphosphatidylcholine (DMPC) and cholesterol displays a fluid miscibility gap under excess water conditions. Effects due to the imperfect miscibility of the two amphiphiles are studied near to and far from thermodynamic equilibrium by time-resolved small angle x-ray diffraction. The experiment discloses that this mixture phase separates when leaving the miscibility gap upon heating, a transition that is not included in current phase diagrams. This transition appears to be reversible and shows a temperature hysteresis of only a few degrees. We suggest a model in which the transition is driven with increasing temperature by a movement of the cholesterol away from the hydrophilic-hydrophobic interface toward the hydrophobic core of the bilayer.

Association of a fluorescent amphiphile with lipid bilayer vesicles in regions of solid-liquid-disordered phase coexistence

The effects of solid-fluid phase separations on the kinetics of association of a single-chain fluorescent amphiphile were investigated in two different systems: pure DMPC (dimyristoylphosphatidylcholine) and a 1:1 mixture of DMPC and DSPC (distearoylphosphatidylcholine). In pure DMPC vesicles, solid (s) and fluid (l(d)) phases coexist at the phase transition temperature, T(m), whereas a 1:1 mixture of DMPC and DSPC shows a stable s-l(d) phase separation over a large temperature interval. We found that in single-component bilayers, within the main phase transition, the experimental kinetics of association are clearly not single-exponential, the deviation from that function becoming maximal at the T(m). This observation can be accounted for by a rate of desorption that is slower than desorption from either fluid or solid phases, leaving the rates of insertion unchanged, but a treatment in terms of stable fluid and solid domains may not be adequate for the analysis of the association of an amphiphile with pure DMPC vesicles at the T(m). In DMPC/DSPC mixtures with solid-fluid phase coexistence, association occurs overall faster than expected based on phase composition. The observed kinetics can be described by an increase in the rate of insertion, leaving the desorption rates unchanged. The fast kinetics of insertion of the amphiphile into two-phase bilayers in two-component vesicles is attributed to a more rapid insertion into defect-rich regions, which are most likely phase boundaries between solid and fluid domains. A two-component mixture of lipids that shows a stable phase separation between l(d)-s phases over a large temperature interval thus behaves very differently from a single-component bilayer at the T(m), with respect to insertion of amphiphiles.

Molecular diffusion into ferritin: pathways, temperature dependence, incubation time, and concentration effects

The detailed kinetics of permeation and effusion of small nitroxide spin probe radicals with the protein shells of horse spleen ferritin (HoSF) and human H-chain ferritin (HuHF) and a 3-fold channel variant D131H+E134H of HuHF were studied by electron paramagnetic resonance spectroscopy and gel permeation chromatography under a variety of experimental conditions. The results confirm that the permeation of molecular species of 7-9-A diameter into ferritin is a charge selective process and that the threefold channels are the likely pathways for entry into the protein. Studies with holoHoSF show that increased temperature increases the rates of penetration and effusion and also increases the concentration of positively charged spin probe accumulated within the protein in excess of that in the external solution. The interior of HoSF is much more accessible to small molecules at physiological temperature of approximately 40 degrees C than at room temperature. The large activation energy of 63-67 kJ/mol measured for the effusion/penetration and the small diffusion coefficient, D approximately 5 x 10(-22) m(2)/s at 20 degrees C, corresponding to a time of approximately 60 min for traversing the protein shell, is consistent with the kinetics of diffusion being largely controlled by the restrictive porosity of the protein itself. An inverse dependence of the first-order rate constant for effusion out of the protein channel on the incubation time used for radical penetration into the protein is attributed to increased binding of the radical within the funnel-shaped channel.

Effect of free fatty acids on the permeability of 1,2-dimyristoyl-sn-glycero-3-phosphocholine bilayer at the main phase transition

We measured the influence of saturated and unsaturated free fatty acids on the permeability and partition of ions into 1, 2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayers. The bilayer permeability was measured using the depletion of N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-1, 2-dihexadecanoyl-sn-glycero-3-phosphatidylethanolamine (N-NBD-PE) fluorescence as a result of its reduction by dithionite. We observed a distinct increase of dithionite permeability at the main gel-fluid phase transition of DMPC. When vesicles were formed from a mixture of DMPC and oleic acid, the membrane permeability at the phase transition was reduced drastically. Stearic acid and methyl ester of oleic acid have little effect. Similar results in the quenching of pyrene-PC in DMPC vesicles by iodide were obtained. Again, the increase of iodide partition into the lipid phase at the main phase transition of DMPC was abolished by the addition of unsaturated free fatty acids. Free fatty acids, in concentrations up to 5 mol%, do not abolish DMPC phase transition when measured by differential scanning calorimetry. It seems that unsaturated, but not saturated, free fatty acids reduce the lipid bilayer permeability to dithionite and iodide ions at the main phase transition of DMPC, without altering the thermodynamic properties of the bilayer.

Kinetics of amphiphile association with two-phase lipid bilayer vesicles

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