Phase structures of binary lipid bilayers as revealed by permeability of small molecules

Divergent Effects of Cholesterol on the Structure and Fluidity of Liposome and Catanionic Vesicle Membranes

Lipid-like ion-pair amphiphile vesicles, or catanionic vesicles, have emerged as potential drug carriers. The effects of cholesterol on the properties of catanionic vesicles have not yet been systematically studied. Here, we compared the effects of cholesterol on the structures and fluidities of dipalmitoylphosphatidylcholine liposomes and catanionic vesicles with similar main transition temperatures (T_m ). For liposomes, fluorescence anisotropy (FA) thermograms reveal typical condensing and disordering effects of cholesterol above and below T_m , respectively. In contrast, FA and molecular simulation data reveal that catanionic bilayers below T_m are more fluidic due to shorter alkyl chains. This leads to only condensing effects of cholesterol for catanionic vesicles at all temperatures. Our results provide important insights into the fabrication of catanionic vesicles as novel drug carriers.

How Do Urea and Trimethylamine N-Oxide Influence the Dehydration-Induced Phase Transition of a Lipid Membrane?

Living organisms are often exposed to extreme dehydration, which is detrimental to the structure and function of the cell membrane. The lipid membrane undergoes fluid-to-gel phase transition due to dehydration and thus loses fluidity and functionality. To protect the fluid phase of the bilayer these organisms adopt several strategies. Enhanced production of small polar organic solutes (also called osmolytes) is one such strategy. Urea and trimethylamine N-oxide (TMAO) are two osmolytes found in different organisms combating osmotic stress. Previous experiments have found that both these osmolytes have strong effects on lipid membrane under different hydration conditions. Urea prevents the dehydration-induced phase transition of the lipid membrane by directly interacting with the lipids, while TMAO does not inhibit the phase transition. To provide atomistic insights, we have carried out all-atom molecular dynamics (MD) simulation of a lipid membrane under varying hydration levels and studied the effect of these osmolytes on different structural and dynamic properties of the membrane. This study suggests that urea significantly inhibits the dehydration-induced fluid-to-gel phase transition by strongly interacting with the lipid membrane via hydrogen bonds, which balances the reduced lipid hydration due to the decreasing water content. In contrast, TMAO is excluded from the membrane surface due to unfavorable interaction with the lipids. This induces further dehydration of the lipids which reinforces the fluid-to-gel phase transition. We have also studied the counteractive role of TMAO on the effect of urea on lipid membrane when both the osmolytes are present. TMAO draws some urea molecules out of the membrane and thereby reduces the effect of urea on the lipid membrane at lower hydration levels. This is similar to the counteraction of urea's deleterious effects on protein by TMAO. All these observations are consistent with the experimental results and thus provide deep molecular insights into the role of these osmolytes in protecting the fluid phase of the membrane, the key survival strategy against osmotic-stress-induced dehydration.

Skin hydration as a tool to control the distribution and molecular effects of intermediate polarity compounds in intact stratum corneum

The barrier function of the skin is mainly assured by its outermost layer, stratum corneum (SC), which consists of dead keratin-filled cells embedded in a lipid matrix. The skin is daily exposed to an environment with changing conditions in terms of hydration and different chemicals. Here we investigate how a molecule that has reasonable solubility in both hydrophobic and hydrophilic environments can be directed to certain regions in SC by changing the skin hydration. We use 1,2,3-trimethoxy propane (TMP) as a model substance and solid-state NMR on natural abundance ¹³C to obtain atomically resolved information on the molecular dynamics of TMP as well as SC lipid and protein components at varying hydration conditions. Upon dehydration, TMP redistributes from the hydrophilic corneocytes to the hydrophobic SC lipid regions. In this way, TMP can act to prevent the fluid-solid lipid transition in drying conditions and be present in the corneocytes in more humid conditions. Hydration can thereby be used as a switch to control the location and action of TMP or similar compounds in complex materials like SC. The general principles described here can also have impact on other applications including lipid-based formulations in food, drug delivery and cosmetics.

Quantification of the amount of mobile components in intact stratum corneum with natural-abundance 13C solid-state NMR

The outermost layer of the skin is the stratum corneum (SC), which is mainly comprised of solid proteins and lipids. Minor amounts of mobile proteins and lipids are crucial for the macroscopic properties of the SC, including softness, elasticity and barrier function. Still this minor number of mobile components are not well characterized in terms of structure or amount. Conventional quantitative direct polarization (Q-DP) 13C solid-state NMR gives signal amplitudes proportional to concentrations, but fails to quantify the SC mobile components because of spectral overlap with the overwhelming signals from the solids. Spectral editing with the INEPT scheme suppresses the signals from solids, but also modulates the amplitudes of the mobile components depending on their values of the transverse relaxation times T2, scalar couplings JCH, and number of covalently bound hydrogens nH. This study describes a quantitative INEPT (Q-INEPT) method relying on systematic variation of the INEPT timing variables to estimate T2, JCH, nH, and amplitude for each of the resolved resonances from the mobile components. Q-INEPT is validated with a series of model systems containing molecules with different hydrophobicity and dynamics. For selected systems where Q-DP is applicable, the results of Q-INEPT and Q-DP are similar with respect to the linearity and uncertainty of the obtained molar ratios. Utilizing a reference compound with known concentration, we quantify the concentrations of mobile lipids and proteins within the mainly solid SC. By melting all lipids at high temperature, we obtain the total lipid concentration. These Q-INEPT results are the first steps towards a quantitative understanding of the relations between mobile component concentrations and SC macroscopic properties.

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.

Bobbing of Oxysterols: Molecular Mechanism for Translocation of Tail-Oxidized Sterols through Biological Membranes

Translocation of sterols between cellular membrane leaflets is of key importance in membrane organization, dynamics, and signaling. We present a novel translocation mechanism that differs in a unique manner from the established ones. The bobbing mechanism identified here is demonstrated for tail-oxidized sterols, but is expected to be viable for any molecule containing two polar centers at the opposite sides of the molecule. The mechanism renders translocation across a lipid membrane possible without a change in molecular orientation. For tail-oxidized sterols, the bobbing mechanism provides an exceptionally facile means to translocate these signaling molecules across membrane structures and may thus represent an important pathway in the course of their biological action.

A Universal Correlation Predicts Permeability Coefficients of Fluid- and Gel-Phase Phospholipid and Phospholipid-Cholesterol Bilayers for Arbitrary Solutes

The permeability of gel-phase phospholipids is typically about an order of magnitude lower than that of the same compositions in the fluid phase, yet a quantitative description of the ordering factors leading to this difference has been elusive. The present analysis examines these factors with particular focus on the area per phospholipid chain, Ac, and its relationship to the minimum area per molecule in the crystalline state, A0. It is shown that fluid- and gel-phase phospholipid permeabilities can be reconciled by postulating a minimum area per chain Ac,0 = 17.1 Å(2), substantially less than one would estimate by dividing the accepted value A0 = 40.8 Å(2) by 2. An extended data set of phospholipid and phospholipid-cholesterol bilayer permeability data extending over 9 orders of magnitude is analyzed and correlated according to the developed relationship (N = 85, s = 0.3024, r(2) = 0.9332). Individual permeability values are consequently predicted to within an average deviation of 10(0.3024) or about a factor of 2. The analysis is broadly applicable in the fluid phase but is restricted to gel-phase phospholipid compositions that do not contain cholesterol. Guidance for the latter scenario is provided.

Tracking solvents in the skin through atomically resolved measurements of molecular mobility in intact stratum corneum

Solvents are commonly used in pharmaceutical and cosmetic formulations and sanitary products and cleansers. The uptake of solvent into the skin may change the molecular organization of skin lipids and proteins, which may in turn alter the protective skin barrier function. We herein examine the molecular effects of 10 different solvents on the outermost layer of skin, the stratum corneum (SC), using polarization transfer solid-state NMR on natural abundance ¹³C in intact SC. With this approach it is possible to characterize the molecular dynamics of solvent molecules when present inside intact SC and to simultaneously monitor the effects caused by the added solvent on SC lipids and protein components. All solvents investigated cause an increased fluidity of SC lipids, with the most prominent effects shown for the apolar hydrocarbon solvents and 2-propanol. However, no solvent other than water shows the ability to fluidize amino acids in the keratin filaments. The solvent molecules themselves show reduced molecular mobility when incorporated in the SC matrix. Changes in the molecular properties of the SC, and in particular alternation in the balance between solid and fluid SC components, may have significant influences on the macroscopic SC barrier properties as well as mechanical properties of the skin. Deepened understanding of molecular effects of foreign compounds in SC fluidity can therefore have strong impact on the development of skin products in pharmaceutical, cosmetic, and sanitary applications.

DPPC-cholesterol phase diagram using coarse-grained Molecular Dynamics simulations

Cholesterol-phospholipid bilayers continue to be the current state of the art in membrane models and serve as representative systems for studying the effect of cholesterol on the cell membrane. As the mixing of different lipid species requires long spatio-temporal scales, coarse-grained models have gained increasing popularity in modeling such membrane systems. In this paper, a systematic study of the MARTINI coarse-grained model for the DPPC-cholesterol binary system has been performed. We construct the phase diagram of DPPC lipid bilayers in the presence of different cholesterol concentrations and at different temperatures using coarse-grained Molecular Dynamics (MD) simulations with the MARTINI force field. The phase diagram based on the condensation effect is directly comparable to available experimental data and demonstrates qualitative agreement over all cholesterol concentrations. Self-assembled bilayers quantitatively reproduce experimental observables, such as lateral diffusion of lipids, electron density, area per lipid and lipid order parameters. The phase diagram of the DPPC-cholesterol binary system also reveals the profound effect of cholesterol on the physical properties of phospholipid bilayers such lipid order, diffusion, and fluidity. Cholesterol induces the liquid-ordered phase, which increases the fluidity of the phospholipid hydrocarbon chains above the gel to liquid-crystalline phase transition temperature and decreases it below the phase transition. The present study suggests that the MARTINI force field can be successfully used to obtain molecular level insights into cholesterol-DPPC model membranes.

Chemical penetration enhancers in stratum corneum - Relation between molecular effects and barrier function

Skin is attractive for drug therapy because it offers an easily accessible route without first-pass metabolism. Transdermal drug delivery is also associated with high patient compliance and through the site of application, the drug delivery can be locally directed. However, to succeed with transdermal drug delivery it is often required to overcome the low permeability of the upper layer of the skin, the stratum corneum (SC). One common strategy is to employ so-called penetration enhancers that supposedly act to increase the drug passage across SC. Still, there is a lack of understanding of the molecular effects of so-called penetration enhancers on the skin barrier membrane, the SC. In this study, we provide a molecular characterization of how different classes of compounds, suggested as penetration enhancers, influence lipid and protein components in SC. The compounds investigated include monoterpenes, fatty acids, osmolytes, surfactant, and Azone. We employ natural abundance (13)C polarization transfer solid-state nuclear magnetic resonance (NMR) on intact porcine SC. With this method it is possible to detect small changes in the mobility of the minor fluid lipid and protein SC components, and simultaneously obtain information on the major fraction of solid SC components. The balance between fluid and solid components in the SC is essential to determine macroscopic material properties of the SC, including barrier and mechanical properties. We study SC at different hydration levels corresponding to SC in ambient air and under occlusion. The NMR studies are complemented with diffusion cell experiments that provide quantitative data on skin permeability when treated with different compounds. By correlating the effects on SC molecular components and SC barrier function, we aim at deepened understanding of diffusional transport in SC, and how this can be controlled, which can be utilized for optimal design of transdermal drug delivery formulations.

Cyclic and Linear Monoterpenes in Phospholipid Membranes: Phase Behavior, Bilayer Structure, and Molecular Dynamics

Monoterpenes are abundant in essential oils extracted from plants. These relatively small and hydrophobic molecules have shown important biological functions, including antimicrobial activity and membrane penetration enhancement. The interaction between the monoterpenes and lipid bilayers is considered important to the understanding of the biological functions of monoterpenes. In this study, we investigated the effect of cyclic and linear monoterpenes on the structure and dynamics of lipids in model membranes. We have studied the ternary system 1,2-dimyristoyl-sn-glycero-3-phosphocholine-monoterpene-water as a model with a focus on dehydrated conditions. By combining complementary techniques, including differential scanning calorimetry, solid-state nuclear magnetic resonance, and small- and wide-angle X-ray scattering, bilayer structure, phase transitions, and lipid molecular dynamics were investigated at different water contents. Monoterpenes cause pronounced melting point depression and phase segregation in lipid bilayers, and the extent of these effects depends on the hydration conditions. The addition of a small amount of thymol to the fluid bilayer (volume fraction of 0.03 in the bilayer) leads to an increased order in the acyl chain close to the bilayer interface. The findings are discussed in relation to biological systems and lipid formulations.

Relationship between the mobility of phosphocholine headgroup and the protein-liposome interaction: a dielectric spectroscopic study

Proteins could affect the headgroup mobility of phospholipid within liposome membranes through the protein-liposome interaction. The variation of headgroup mobility of phospholipid was then investigated by using the dielectric dispersion analysis. The eight proteins (Mw = 4.2-28.7 kDa) were used to investigate the protein-liposome interaction. It has been revealed that the strength of the protein-liposome interaction at 25 °C was linearly correlated with the stability of intramolecular hydrogen bondings of proteins, better than with their hydrophobicity and the surface charge density. Overall, liposomes composed of binary lipid system, appeared to strongly interact with proteins, in contrast to liposomes composed of single, ternary, and quaternary lipid systems. This is probably because liposomes composed of binary lipid system favored to form the microscopic environment where proteins could interact. The present result suggested the heterogeneous phase state of lipid membranes was one of dominant factors for the interaction between proteins and lipid membranes.

Permeability of fluid-phase phospholipid bilayers: assessment and useful correlations for permeability screening and other applications

Permeability data (P(lip/w) ) for liquid crystalline phospholipid bilayers composed of egg lecithin and dimyristoylphosphatidylcholine (DMPC) are analyzed in terms of a mathematical model that accounts for free surface area and chain-ordering effects in the bilayer as well as size and lipophilicity of the permeating species. Free surface area and chain ordering are largely determined by temperature and cholesterol content of the membrane, molecular size is represented by molecular weight, and lipophilicity of the barrier region is represented by the 1,9-decadiene/water partition coefficient, following earlier work by Xiang, Anderson, and coworkers. A correlating variable χ = MW(n) σ/(1 -σ) is used to link the results from different membrane systems, where different values of n are tried, and σ denotes a reduced phospholipid density. The group (1 -σ)/σ is a measure of free surface area, but can also be interpreted in terms of free volume. A single exponential function of χ is developed that is able to correlate 39 observations of P(lip/w) for different compounds in egg lecithin at low density, and 22 observations for acetic acid in DMPC at higher densities, spanning nine orders of magnitude to within an rms error for log 10 P(lip/w) of 0.20. The best fit found for n = 0.87 ultimately makes χ much closer to the ratio of molecular to free volumes than surface areas. The results serve as a starting point for estimating passive permeability of cell membranes to nonionized solutes as a function of temperature and cholesterol content of the membrane.

NMR-NOE and MD simulation study on phospholipid membranes: dependence on membrane diameter and multiple time scale dynamics

Motional correlation times between the hydrophilic and hydrophobic terminal groups in lipid membranes are studied over a wide range of curvatures using the solution-state (1)H NMR-nuclear Overhauser effect (NOE) and molecular dynamics (MD) simulation. To enable (1)H NMR-NOE measurements for large vesicles, the transient NOE method is combined with the spin-echo method, and is successfully applied to a micelle of 1-palmitoyl-lysophosphatidylcholine (PaLPC) with diameter of 5 nm and to vesicles of dipalmitoylphosphatidylcholine (DPPC) with diameters ranging from 30 to 800 nm. It is found that the NOE intensity increases with the diameter up to ∼100 nm, and the model membrane is considered planar on the molecular level beyond ∼100 nm. While the NOE between the hydrophilic terminal and hydrophobic terminal methyl groups is absent for the micelle, its intensity is comparable to that for the neighboring group for vesicles with larger diameters. The origin of NOE signals between distant sites is analyzed by MD simulations of PaLPC micelles and DPPC planar bilayers. The slow relaxation is shown to yield an observable NOE signal even for the hydrophilic and hydrophobic terminal sites. Since the information on distance and dynamics cannot be separated in the experimental NOE alone, the correlation time in large vesicles is determined by combining the experimental NOE intensity and MD-based distance distribution. For large vesicles, the correlation time is found to vary by 2 orders of magnitude over the proton sites. This study shows that NOE provides dynamic information on large vesicles when combined with MD, which provides structural information.

Doxorubicin loaded magnetic polymersomes: theranostic nanocarriers for MR imaging and magneto-chemotherapy

Hydrophobically modified maghemite (γ-Fe(2)O(3)) nanoparticles were encapsulated within the membrane of poly(trimethylene carbonate)-b-poly(l-glutamic acid) (PTMC-b-PGA) block copolymer vesicles using a nanoprecipitation process. This formation method gives simple access to highly magnetic nanoparticles (MNPs) (loaded up to 70 wt %) together with good control over the vesicles size (100-400 nm). The simultaneous loading of maghemite nanoparticles and doxorubicin was also achieved by nanoprecipitation. The deformation of the vesicle membrane under an applied magnetic field has been evidenced by small angle neutron scattering. These superparamagnetic hybrid self-assemblies display enhanced contrast properties that open potential applications for magnetic resonance imaging. They can also be guided in a magnetic field gradient. The feasibility of controlled drug release by radio frequency magnetic hyperthermia was demonstrated in the case of encapsulated doxorubicin molecules, showing the viability of the concept of magneto-chemotherapy. These magnetic polymersomes can be used as efficient multifunctional nanocarriers for combined therapy and imaging.

Dynamic organization of lymphocyte plasma membrane: lessons from advanced imaging methods

Lipids and lipid domains are suggested to play an essential role in the heterogeneous organization of the plasma membrane in eukaryotic cells, including cells of the immune system. We summarize the results of advanced imaging and physical studies of membrane organization with special focus on the plasma membrane of lymphocytes. We provide a comprehensive up-to-date view on the existence of membrane lipid and protein clusters such as lipid rafts and suggest research directions to better understand these highly dynamic entities on the surface of immune cells.

Liquid-ordered phases induced by cholesterol: a compendium of binary phase diagrams

Mixtures of phospholipids with cholesterol are able to form liquid-ordered phases that are characterised by short-range orientational order and long-range translational disorder. These L(o)-phases are distinct from the liquid-disordered, fluid L(alpha)-phases and the solid-ordered, gel L(beta)-phases that are assumed by the phospholipids alone. The liquid-ordered phase can produce spatially separated in-plane fluid domains, which, in the form of lipid rafts, are thought to act as platforms for signalling and membrane sorting in cells. The areas of domain formation are defined by the regions of phase coexistence in the phase diagrams for the binary mixtures of lipid with cholesterol. In this paper, the available binary phase diagrams of lipid-cholesterol mixtures are all collected together. It is found that there is not complete agreement between different determinations of the phase diagrams for the same binary mixture. This can be attributed to the indirect methods largely used to establish the phase boundaries. Intercomparison of the various data sets allows critical assessment of which phase boundaries are rigorously established from direct evidence for phase coexistence.

CYTOTOXICITY AND PHOSPHOLIPID-LIPOSOME PHASE-TRANSITION PROPERTIES OF 2-HYDROXYETHYL METHACRYLATE (HEMA)

To elucidate the cytotoxic induction mechanisms of the hydrophilic HEMA, the comparative cytotoxic activities of HEMA and the hydrophobic monomers TEGDMA and MMA were studied, using erythrocytes, gingival fibroblasts and a salivary gland carcinoma cell line. Also, the gel-to-fluid phase transition properties (i.e. temperature, Tm; cooperativity, H/HHW; enthalpy, deltaH) of dipalmitoylphosphatidylcholine (DPPC) and DPPC/cholesterol (CS) liposomes (as a model for biological membranes) induced by methacrylates were investigated, using differential scanning calorimetry (DSC). In addition, the methacrylate-chemical-shifts in DPPC liposomes were assayed using NMR spectroscopy. Both the hemo lytic and cytotoxic activity declined in the order: TEGDMA> HEMA>MMA. The changes in Tm increased in the order: HEMA <MMA<TEGDMA. The H/HHW declined in the order of MMA >>TEGDMA, while in contrast, that of HEMA was slightly increased without changes in the deltaH. The DSC changes in DPPC/CS liposomes with HEMA were the largest of those recorded. The cytotoxicity of HEMA may be induced by the hydrophobic interaction derived from the molecular association of OH groups of HEMA and, in addition, by the preferential interaction with CS.

Liposome-templated supramolecular assembly of responsive alginate nanogels

Nanosized gel particles (nanogels) are of interest for a variety of applications, including drug delivery and single-molecule encapsulation. Here, we employ the cores of nanoscale liposomes as reaction vessels to template the assembly of calcium alginate nanogels. For our experiments, a liposome formulation with a high bilayer melting temperature (Tm) is selected, and sodium alginate is encapsulated in the liposomal core. The liposomes are then placed in an aqueous buffer containing calcium chloride, and the temperature is raised up to Tm. This allows permeation of Ca2+ ions through the bilayer and into the core, whereupon these ions gel the encapsulated alginate. Subsequently, the lipid bilayer covering the gelled core is removed by the addition of a detergent. The resulting alginate nanogels have a size distribution consistent with that of the template liposomes (ca. 120-200 nm), as confirmed by transmission electron microscopy and light scattering. Nanogels of different average sizes can be synthesized by varying the template dimensions, and the gel size can be further tuned after synthesis by the addition of monovalent salt to the solution.

Liposome transport of hydrophobic drugs: gel phase lipid bilayer permeability and partitioning of the lactone form of a hydrophobic camptothecin, DB-67

The design of liposomal delivery systems for hydrophobic drug molecules having improved encapsulation efficiency and enhanced drug retention would be highly desirable. Unfortunately, the poor aqueous solubility and high membrane binding affinity of hydrophobic drugs necessitates extensive validation of experimental methods to determine both liposome loading and permeability and thus the development of a quantitative understanding of the factors governing the encapsulation and retention/release of such compounds has been slow. This report describes an efflux transport method using dynamic dialysis to study the liposomal membrane permeability of hydrophobic compounds. A mathematical model has been developed to calculate liposomal membrane permeability coefficients of hydrophobic compounds from dynamic dialysis experiments and partitioning experiments using equilibrium dialysis. Also reported is a simple method to study the release kinetics of liposome encapsulated camptothecin lactone in plasma by comparing the hydrolysis kinetics of liposome entrapped versus free drug. DB-67, a novel hydrophobic camptothecin analogue has been used as a model permeant to validate these methods. Theoretical estimates of DB-67 permeability obtained from the bulk solubility diffusion model and the "barrier-domain" solubility diffusion model are compared to the experimentally observed value. The use of dynamic dialysis in drug release studies of liposome and other nanoparticle formulations is further discussed and experimental artifacts that can arise without adequate validation are illustrated through simulations.

Liposomal delivery of hydrophobic weak acids: enhancement of drug retention using a high intraliposomal pH

Clinical development of highly potent lipophilic neutral camptothecins has been impeded by the poor solubility, stability, and nonspecific toxicity of these compounds. Liposomal encapsulation offers a promising formulation route for tumor site-specific delivery of these novel drug candidates. However, the development of formulation strategies for liposomal loading and retention of hydrophobic drugs such as the neutral camptothecins has been lacking. In the studies presented here, we explored the potential of a trans-bilayer pH gradient strategy for prolonging the liposome retention of DB-67, a novel lipophilic camptothecin that can undergo lactone ring-opening to form a hydrophobic weak acid. The liposome membrane permeability of DB-67 was obtained as a function of pH in aqueous buffers. A permeability model was developed and liposome membrane permeability was shown to be controlled by the fraction of unbound neutral lactone entrapped in the vesicles. Liposome membrane permeability of DB-67 was also studied under physiological conditions. The high membrane partitioning of DB-67 in the intraliposomal microenvironment was found to shift the equilibrium between lactone and carboxylate towards the lactone species resulting in a faster than desired drug release under physiological conditions. The effectiveness of the pH gradient strategy was further reduced under physiological conditions by the rapid loss of trans-membrane pH gradients due to CO(2) uptake. Simulations were conducted to explore the role of membrane binding, intravesicular pH, and carbonate buffer concentration in successful utilization of the pH gradient strategy for hydrophobic weak acids.

Lateral organization of mixed, two-phosphatidylcholine liposomes as investigated by GPS, the slope of Laurdan generalized polarization spectra

The effect of the excitation or emission wavelengths on Laurdan generalized polarization (GP) can be evaluated by GPS, a quantitative, simplified determination of the GP spectrum slope, the thermotropic dependence of which allows the assessment of phospholipid lamellar membrane phase, as shown in a recent publication of our laboratory [J.B. Velázquez, M.S. Fernández, Arch. Biochem. Biophys. 455 (2006) 163-174]. In the present work, we applied Laurdan GPS to phase transition studies of mixed, two-phosphatidylcholine liposomes prepared from variable proportions of dimyristoyl- and dipalmitoylphosphatidylcholine (DMPC and DPPC, respectively). We have found that the GPS function reports a clear limit between the gel/liquid-crystalline phase coexistence region and the liquid-crystalline state, not only at a certain temperature T(c) for liposomes of constant composition submitted to temperature scans, but also at a defined mole fraction X(c), for two-component liposomes of variable composition at constant temperature. The T(c) or the X(c) values obtained from GPS vs. temperature or GPS vs. composition plots, respectively, allow the construction of a partial phase diagram for the DMPC-DPPC mixtures, showing the boundary between the two-phase coexisting region and the liquid-crystalline state. Likewise, at the onset of the transition region, i.e., the two-phase coexisting region as detected by GPS, it is possible to determine, although with less precision, a temperature T(o) or a mole fraction X(o) defining a boundary located below but near the limit between the gel and ripple phase, reported in the literature. These GPS results are consistent with the proposal by several authors that a fraction of L(alpha) phospholipids coexists with gel phospholipids in the rippled phase.

Dynamic molecular structure and phase diagram of DPPC-cholesterol binary mixtures: a 2D-ELDOR study

This paper is an application of 2D electron-electron double resonance (2D-ELDOR) with the "full Sc- method" to study model membranes. We obtain and confirm the phase diagram of 1,2-dipalmitoyl-sn-glycerophosphatidylcholine (DPPC)-cholesterol binary mixtures versus temperature and provide quantitative descriptions for its dynamic molecular structure using 2D-ELDOR at the Ku band. The spectra from the end-chain 16-PC spin label in multilamellar phospholipid vesicles are obtained for cholesterol molar concentrations ranging from 0 to 50% and from 25 to 60 degrees C. This phase diagram consists of liquid-ordered, liquid-disordered, and gel phases and phase coexistence regions. The phase diagram is carefully examined according to the spectroscopic evidence, and the rigorous interpretation for the line shape changes. We show that the 2D-ELDOR spectra differ markedly with variation in the composition. The extensive line shape changes in the 2D-plus-mixing-time representation provide useful information to define and characterize the membrane phases with respect to their dynamic molecular structures and to determine the phase boundaries. The homogeneous T2's are extracted from the pure absorption spectra and are used to further distinguish the membrane phases. These results show 2D-ELDOR to be naturally suitable for probing and reporting the dynamic structures of microdomains in model membrane systems and, moreover, providing a very detailed picture of their molecular dynamic structure, especially with the aid of the "full Sc- method".

Human stratum corneum lipid organization as observed by atomic force microscopy on Langmuir–Blodgett films

The barrier function of skin ultimately depends on the physical state and structural organisation of the stratum corneum extracellular lipid matrix. Ceramides, cholesterol and a broad distribution of saturated long-chain free fatty acids dominate the stratum corneum lipid composition. Additionally, smaller amounts of cholesterol sulfate and cholesteryl oleate may be present. A key feature determining skin barrier capacity is thought to be whether or not different lipid domains coexist laterally in the stratum corneum extracellular lipid matrix. In this study, the overall tendency for lipid domain formation in different mixtures of extracted human stratum corneum ceramides, cholesterol, free fatty acids, cholesterol sulfate and cholesteryl oleate were studied using atomic force microscopy (AFM) on Langmuir-Blodgett (LB) films on mica. It is shown that the saturated long-chain free fatty acid distribution of human stratum corneum prevents hydrocarbon chain segregation. Further, LB-films of human stratum corneum ceramides express a pattern of connected elongated domains with a granular domain interface. The dominating effect of both cholesterol and cholesterol sulfate is that of increased ceramide domain dispersion. This effect is counteracted by the presence of free fatty acids, which preferentially mix with ceramides and not with cholesterol. Cholesteryl oleate does not mix with other skin lipid components, supporting the hypothesis of an extra-endogenous origin. In the system composed of endogenous human ceramides and cholesterol plus 15 wt% stratum corneum distributed free fatty acids, i.e., the system mimicking most closely the lipid composition of the stratum corneum extracellular space, LB-films on mica express lateral domain formation.

Infrared spectroscopic study of stratum corneum model membranes prepared from human ceramides, cholesterol, and fatty acids

The outermost layer of the skin, the stratum corneum, consists of corneocytes surrounded by lipid domains. The main lipid classes in stratum corneum are cholesterol, ceramides (CER), and free fatty acids forming two crystalline lamellar phases. However, only limited information is available on whether the various lipid classes participate in the same crystalline lattices or if separate domains are formed within the lipid lamellae. In this article infrared spectroscopic studies are reported of hydrated mixtures prepared from cholesterol, human CER, and free fatty acids. Evaluation of the methylene stretching vibrations revealed a conformational disordering starting at approximately 60 degrees C for all mixtures. Examination of the rotational ordering (scissoring and rocking vibrations) of mixtures prepared from equimolar cholesterol and CER with a variation in the level of free fatty acids showed that at lower free fatty acid content orthorhombic and hexagonal domains coexist in the lipid lamellae. Increasing the fatty acid level to an equimolar cholesterol/CER/fatty acid mixture reveals the dominant presence of an orthorhombic lattice, confirming x-ray diffraction studies. Replacing the protonated free fatty acid chains by their perdeuterated counterparts demonstrates that free fatty acids and CER participate in the same orthorhombic lattice up to a level of slightly less than 1:1:0.75 cholesterol/CER/free fatty acids molar ratio but that free fatty acids also form separate domains within the lipid lamellae at equimolar ratios at room temperature. However, no evidence for this has been observed at 32 degrees C. Extrapolating these findings to the situation in stratum corneum led us conclude that in stratum corneum, fatty acids and CER participate in the orthorhombic lattice at 32 degrees C, the skin temperature.

Liposomal drug transport: a molecular perspective from molecular dynamics simulations in lipid bilayers

Computational methods to predict drug permeability across biomembranes prior to synthesis are increasingly desirable to minimize the investment in drug design and development. Significant progress in molecular dynamics (MD) simulation methodologies applied to lipid bilayer membranes, for example, is making it possible to move beyond characterization of the membranes themselves to explore various thermodynamic and kinetic processes governing membrane binding and transport. Such methods are also likely to be directly applicable to the design and optimization of liposomal delivery systems. MD simulations are particularly valuable in addressing issues that are difficult to explore in laboratory experiments due to the heterogeneity of lipid bilayer membranes at the molecular level. Insights emerging from MD simulations are contributing to an understanding of which regions within bilayers are most and least favored by solutes at equilibrium as the solute structure is varied, local diffusivities of permeants, and the origin of the amplified selectivity to permeant size imposed by lipid bilayer membranes, particularly as changes in composition increase acyl chain ordering.

Computer simulation of small molecule permeation across a lipid bilayer: dependence on bilayer properties and solute volume, size, and cross-sectional area

Cell membrane permeation is required for most drugs to reach their biological target, and understanding this process is therefore crucial for rational drug design. Recent molecular dynamics simulations have studied the permeation of eight small molecules through a phospholipid bilayer. Unlike experiments, atomistic simulations allow the direct calculation of diffusion and partition coefficients of solutes at different depths inside a lipid membrane. Further analyses of the simulations suggest that solute diffusion is less size-dependent and solute partitioning more size-dependent than was commonly thought.

Lipid rafts: elusive or illusive?

There has been considerable recent interest in the possibility that the plasma membrane contains lipid "rafts," microdomains enriched in cholesterol and sphingolipids. It has been suggested that such rafts could play an important role in many cellular processes including signal transduction, membrane trafficking, cytoskeletal organization, and pathogen entry. However, rafts have proven difficult to visualize in living cells. Most of the evidence for their existence and function relies on indirect methods such as detergent extraction, and a number of recent studies have revealed possible problems with these methods. Direct studies of the distribution of raft components in living cells have not yet reached a consensus on the size or even the presence of these microdomains, and hence it seems that a definitive proof of raft existence has yet to be obtained.

Hypothesis: could the signalling function of membrane microdomains involve a localized transition of lipids from liquid to solid state?

Background

Over the past decade, it has become apparent that specialised membrane microdomains, commonly called rafts, where lipids like sphingolipids and cholesterol are arranged compactly in a liquid ordered phase are involved in cell signalling.

Hypothesis

The core of the hypothesis presented here is that resting cells may actively maintain their plasma membrane in liquid phase, corresponding to a metastable thermodynamic state. Following a physiological stimulus such as ligands binding to their membrane receptors, the tendency of membrane components to undergo a localised transition towards a gel state would increase, resulting in initial minute solid structures. These few membrane components having undergone a liquid to solid state transition, would then act as seeds for the specific recruitment of additional membrane components whose properties are compatible with the crystalline growth of these initial docks. Cells could therefore be using the propensity of lipids to assemble selectively to generate stable platforms of particular cellular components either for intra-cellular transport or for signal transduction.

Testing the hypothesis

could presumably be done via biophysical approaches such as EPR spin labelling, X-ray diffraction or FRET coupled to direct microscopic observation of cells to which very localized stimuli would be delivered.

Implications

Such a model of selective growth of membrane docks would provide an explanation for the existence of different types of microdomains, and for the fact that, depending on the state of the cells and on the procedures used to isolate them, membrane microdomains can vary greatly in their properties and composition. Ultimately, a thorough understanding of how and why lipid domains are assembled in biological membranes will be essential for many aspects of cell biology and medicine.

Differential effects induced by alpha- and beta-endosulfan in lipid bilayer organization are reflected in proton permeability

The effects of two insecticides isomers, alpha- and beta-endosulfan, on the passive proton permeability of large unilamellar vesicles (LUV) reconstituted with dipalmitoylphosphatidylcholine (DPPC) or mitochondrial lipids were reported. In DPPC (LUV) gel phase, at 30 degrees C, the global kinetic constant (K) of proton permeability (proportional to the proton permeability) initially increased slightly with the increase of alpha-endosulfan/lipid molar ratio up to 0.143. In the range from 0.143 to 0.286, a discontinuity in the increment occurred and, above this range, the proton permeability increased substantially. In DPPC fluid phase, at 48 degrees C, the proton permeability showed a behavior identical to that observed in gel DPPC, with a sharp increase for alpha-endosulfan/lipid molar ratios ranging from 0.143 to 0.286. At these and higher concentrations, alpha-endosulfan induced phase separation in the plane of DPPC membranes, as revealed by differential scanning calorimetry (DSC). Conversely to alpha-endosulfan, beta-endosulfan induced only a slight increase in the proton permeability, either in the fluid or the gel phase of DPPC, for all beta-endosulfan/lipid molar ratios tested. Additionally, the effects of the endosulfan isomers on the proton permeability of mitochondrial fluid lipid dispersions, at 37 degrees C, are similar to those described for DPPC. The beta-isomer induced a very small effect, and alpha-endosulfan, at low concentrations, increased slightly the proton permeability, but for insecticide/lipid molar ratios above 0.143 the permeability increased substantially. Consequently, the membrane physical state of synthetic and native lipid dispersions, as affected by the structural features of alpha- and beta-endosulfan, influenced the proton permeability. The effects here observed in vitro suggest that the formation of lateral membrane domains may underlay the biological activity of alpha-endosulfan in vivo, contributing to its higher degree of toxicity as compared with beta-endosulfan.

Different interactions of egg-yolk phosphatidylcholine and sphingomyelin with detergent bile salts

To examine physical-chemical aspects of bile salt-phospholipid interactions that could contribute to preferential phosphatidylcholine (PC) secretion into bile, we have compared transitions between vesicles and micelles in model systems containing taurocholate (TC) and either egg-yolk PC (EYPC), egg-yolk sphingomyelin (EYSM), buttermilk SM (BMSM) or dipalmitoyl PC (DPPC). Phase transitions from micelles to vesicles were observed at 4-fold dilution of serially diluted EYPC/TC systems, but not earlier than at 16-fold dilution of SM/TC or DPPC/TC systems, indicating lower concentrations of the detergent required for micellization in the case of SM or DPPC. Cryo-transmission electron microscopy of phase transitions initiated by addition of TC to phospholipid vesicles revealed extremely long SM-containing intermediate structures, but shorter EYPC-containing intermediate structures. Again, larger amounts of bile salt were required to induce phase transitions in the case of EYPC compared to SM. Sizes of TC-phospholipid micelles increased progressively upon increasing phospholipid contents in the rank order: DPPC-TC<EYSM-TC<BMSM-TC<EYPC-TC, consistent with higher micellization concentrations in the case of EYPC. Micelles were also separated from vesicular phases in two-phase model systems composed with TC, both EYPC and EYSM and 0, 10, 20 or 30 mol% cholesterol, by ultracentrifugation and ultrafiltration of the supernatant. At increasing cholesterol contents, EYPC preferentially distributed into the micellar phase. In contrast, no preferential micellar EYPC distribution occurred in the absence of the sterol. These results indicate different structural arrangements of EYPC-TC micelles compared to SM-TC micelles and lower detergent concentrations required for micellization in the case of SM-containing vesicles.

Incorporation of cholesterol in sphingomyelin- phosphatidylcholine vesicles has profound effects on detergent-induced phase transitions

Vesicle <--> micelle transitions are important phenomena during bile formation and intestinal lipid processing. The hepatocyte canalicular membrane outer leaflet contains appreciable amounts of phosphatidylcholine (PC) and sphingomyelin (SM), and both phospholipids are found in the human diet. Dietary SM enrichment inhibits intestinal cholesterol absorption. We therefore studied detergent-induced vesicle --> micelle transitions in SM-PC vesicles. Phase transitions were evaluated by spectrophotometry and cryotransmission electron microscopy (cryo-TEM) after addition of taurocholate (3-7 mM) to SM-PC vesicles (4 mM phospholipid, SM/PC 40%/60%, without or with 1.6 mM cholesterol). After addition of excess (5-7 mM) taurocholate, SM-PC vesicles were more sensitive to micellization than PC vesicles. As shown by sequential cryo-TEM, addition of equimolar (4 mM) taurocholate to SM-PC vesicles induced formation of open vesicles, then (at the absorbance peak) fusion of bilayer fragments into large open structures (around 200 nm diameter) coexisting with some multilamellar or fused vesicles and thread-like micelles and, finally, transformation into an uniform picture with long thread-like micelles. Incorporation of cholesterol in the SM/PC bilayer changed initial vesicular shape from spherical into ellipsoid and profoundly increased detergent resistance. Disk-like micelles and multilamellar vesicles, and then extremely large vesicular structures, were observed by sequential cryo-TEM under these circumstances, with persistently increased absorbance values by spectrophotometry. These findings may be relevant for bile formation and intestinal lipid processing. Inhibition of intestinal cholesterol absorption by dietary SM enrichment may relate to high resistance against bile salt-induced micellization of intestinal lipids in presence of the sphingolipid.

An index of lipid phase diagrams

There is a growing awareness of the utility of lipid phase behavior data in studies of membrane-related phenomena. Such miscibility information is commonly reported in the form of temperature-composition (T-C) phase diagrams. The current index is a conduit to the relevant literature. It lists lipid phase diagrams, their components and conditions of measurement, and complete bibliographic information. The main focus of the index is on lipids of membrane origin where water is the dispersing medium. However, it also includes records on acylglycerols, fatty acids, cationic lipids, and detergent-containing systems. The miscibility of synthetic and natural lipids with other lipids, with water, and with biomolecules (proteins, nucleic acids, carbohydrates, etc.) and non-biological materials (drugs, anesthetics, organic solvents, etc.) is within the purview of the index. There are 2188 phase diagram records in the index, the bulk (81%) of which refers to binary (two-component) T-C phase diagrams. The remainder is made up of more complex (ternary, quaternary) systems, pressure-T phase diagrams, and other more exotic miscibility studies. The index covers the period from 1965 through to July, 2001.

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).

Ethylazinphos interaction with membrane lipid organization induces increase of proton permeability and impairment of mitochondrial bioenergetic functions

Ethylazinphos increases the passive proton permeability of lipid bilayers reconstituted with dipalmitoylphosphatidylcholine (DPPC) and mitochondrial lipids. A sharp increase of proton permeability is detected at insecticide/lipid molar ratios identical to those inducing phase separation in the plane of DPPC bilayers, as revealed by differential scanning calorimetry (DSC). Ethylazinphos progressively depresses the transmembrane potential (DeltaPsi) of mitochondria supported by piruvate/malate, succinate, or ascorbate/TMPD. Additionally, a decreased depolarization induced by ADP depends on ethylazinphos concentration, reflecting a phosphorylation depression. This loss of phosphorylation is a consequence of a decreased DeltaPsi. A decreased respiratory control ratio is also observed, since ethylazinphos stimulates state 4 respiration and inhibits ADP-stimulated respiration (state 3). Ethylazinphos concentrations up to 100 nmol/mg mitochondrial protein increase the rate of state 4 together with a decrease in DeltaPsi, without significant perturbation of state 3 and carbonyl cyanide-p-trifluoromethoxyphenylhydrazone (FCCP)-uncoupled respiration. For increased insecticide concentrations, the state 3 and FCCP-uncoupled respiration are inhibited to approximately the same extent. The perturbations are more pronounced when the energization is supported by pyruvate/malate and less effective when succinate is used as substrate. The present data, in association with previous DSC studies, indicate that ethylazinphos, at concentrations up to 100 nmol/mg mitochondrial protein, interacts with the lipid bilayer of mitochondrial membrane, changing the lipid organization and increasing the proton permeability of the inner membrane. The increased proton permeability explains the decreased oxidative phosphorylation coupling. Resulting disturbed ATP synthesis may significantly underlie the mechanisms of ethylazinphos toxicity, since most of cell energy in eukaryotes is provided by mitochondria.

Responding phospholipid membranes--interplay between hydration and permeability

Osmotic forces are important in regulating a number of physiological membrane processes. The effect of osmotic pressure on lipid phase behavior is of utmost importance for the extracellular lipids in stratum corneum (the outer part of human skin), due to the large gradient in water chemical potential between the water-rich tissue on the inside, and the relative dry environment on the outside of the body. We present a theoretical model for molecular diffusional transport over an oriented stack of two-component lipid bilayers in the presence of a gradient in osmotic pressure. This gradient serves as the driving force for diffusional motion of water. It also causes a gradient in swelling and phase transformations, which profoundly affect the molecular environment and thus the local diffusion properties. This feedback mechanism generates a nonlinear transport behavior, which we illustrate by calculations of the flux of water and solute (nicotine) through the bilayer stack. The calculated water flux shows qualitative agreement with experimental findings for water flux through stratum corneum. We also present a physical basis for the occlusion effect. Phase behavior of binary phospholipid mixtures at varying osmotic pressures is modeled from the known interlamellar forces and the regular solution theory. A first-order phase transformation from a gel to a liquid--crystalline phase can be induced by an increase in the osmotic pressure. In the bilayer stack, a transition can be induced along the gradient. The boundary conditions in water chemical potential can thus act as a switch for the membrane permeability.