The polymorphic phase behaviour and miscibility properties of synthetic phosphatidylethanolamines

Membrane Fusion Biophysical Analysis of Fusogenic Liposomes

Liposomes represent important drug carrier vehicles in biological systems. A fusogenic liposomal system composed of equimolar mixtures of the cationic lipid DOTAP and the phospholipid DOPE showed high fusion and delivery efficiencies with cells and lipid vesicles. However, aspects of the thermodynamics involving the interaction of these fusogenic liposomes and biomimetic systems remain unclear. Here, we investigate the fusion of this system with large unilamellar vesicles (LUVs) composed of the zwitterionic lipid POPC and increasing fractions of the anionic lipid POPG and up to 30 mol % cholesterol. The focus here is to concomitantly follow changes in size, zeta-potential, and enthalpy binding upon membrane interaction and fusion. Isothermal titration calorimetry (ITC) data showed that membrane fusion in our system is an exothermic process in the absence of cholesterol, suggesting that electrostatic attraction is the driving force for fusion. An endothermic component appeared and eventually dominated the titration at 30 mol % cholesterol, which we propose is caused by membrane fluidification when cholesterol is diluted upon fusion. The inflection points of the ITC data occurred around 0.5-0.7 POPG/DOTAP for all systems, the same stoichiometry for which zeta-potential and dynamic light scattering measurements showed an increase in size coupled with charge neutralization of the system, which is consistent with the fact that fusion in our system is charge-mediated. Microscopy observations of the final mixtures revealed the presence of giant vesicles, which is a clear indication of fusion, coexisting with intermediate-sized objects that could be the result of both fusion and/or aggregation. The results show that the fusion efficiency of the DOTAP:DOPE fusogenic system is modulated by the charge and membrane packing of the acceptor membrane and explain why the system fuses very efficiently with cells.

The conical shape of DIM lipids promotes Mycobacterium tuberculosis infection of macrophages

Phthiocerol dimycocerosate (DIM) is a major virulence factor of the pathogen Mycobacterium tuberculosis (Mtb). While this lipid promotes the entry of Mtb into macrophages, which occurs via phagocytosis, its molecular mechanism of action is unknown. Here, we combined biophysical, cell biology, and modeling approaches to reveal the molecular mechanism of DIM action on macrophage membranes leading to the first step of Mtb infection. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry showed that DIM molecules are transferred from the Mtb envelope to macrophage membranes during infection. Multiscale molecular modeling and ³¹P-NMR experiments revealed that DIM adopts a conical shape in membranes and aggregates in the stalks formed between 2 opposing lipid bilayers. Infection of macrophages pretreated with lipids of various shapes uncovered a general role for conical lipids in promoting phagocytosis. Taken together, these results reveal how the molecular shape of a mycobacterial lipid can modulate the biological response of macrophages.

An NMR database for simulations of membrane dynamics

Computational methods are powerful in capturing the results of experimental studies in terms of force fields that both explain and predict biological structures. Validation of molecular simulations requires comparison with experimental data to test and confirm computational predictions. Here we report a comprehensive database of NMR results for membrane phospholipids with interpretations intended to be accessible by non-NMR specialists. Experimental ¹³C-¹H and ²H NMR segmental order parameters (S(CH) or S(CD)) and spin-lattice (Zeeman) relaxation times (T(1Z)) are summarized in convenient tabular form for various saturated, unsaturated, and biological membrane phospholipids. Segmental order parameters give direct information about bilayer structural properties, including the area per lipid and volumetric hydrocarbon thickness. In addition, relaxation rates provide complementary information about molecular dynamics. Particular attention is paid to the magnetic field dependence (frequency dispersion) of the NMR relaxation rates in terms of various simplified power laws. Model-free reduction of the T(1Z) studies in terms of a power-law formalism shows that the relaxation rates for saturated phosphatidylcholines follow a single frequency-dispersive trend within the MHz regime. We show how analytical models can guide the continued development of atomistic and coarse-grained force fields. Our interpretation suggests that lipid diffusion and collective order fluctuations are implicitly governed by the viscoelastic nature of the liquid-crystalline ensemble. Collective bilayer excitations are emergent over mesoscopic length scales that fall between the molecular and bilayer dimensions, and are important for lipid organization and lipid-protein interactions. Future conceptual advances and theoretical reductions will foster understanding of biomembrane structural dynamics through a synergy of NMR measurements and molecular simulations.

Intracellular distribution of fluorescent probes delivered by vesicles of different lipidic composition

In order to study mechanisms involved in liposome-cell interaction, this work attempted to assess the influence of vesicle composition on the delivery of liposomal content to Hela cells. In particular, to evaluate pH-sensitive properties and cell interaction of the prepared liposomes, the lipid formulations contained cholesterol (Chol) and they were varied by using phosphatidylcholines with different purity degree: soy lecithin (SL; 80% phosphatidylcholine), a commercial mixture of soy phosphatidylcholine (P90; 90% phosphatidylcholine) or dipalmitoylphosphatidylcholine (DPPC; 99% of purity). A second series of liposomes also contained stearylamine (SA). Dehydration-rehydration vesicles (DRV) were prepared and then sonicated to decrease vesicle size. Vesicle-cell interactions and liposomal uptake were examined by fluorescence microscopy using carboxyfluorescein (CF) and phosphatidylethanolamine-dioleoyl-sulforhodamine B (Rho-PE) as fluorescent markers. Fluorescence dequenching assay was used to study the influence of pH on CF release from the liposomal formulations. Liposome adhesion on the cell surface and internalization were strongly dependent on vesicle bilayer composition. SA vesicles were not endocytosed. DPPC/Chol liposomes were endocytosed but did not release their fluorescent content into the cytosol. SL/Chol and P90/Chol formulations displayed a diffuse cytoplasmic fluorescence of liposomal marker.

Structural features of the acyl chain determine self-phospholipid antigen recognition by a CD1d-restricted invariant NKT (iNKT) cell

Little is known about the antigen specificity of CD1d-restricted T cells, except that they frequently recognize CD1d-expressing antigen-presenting cells in the absence of exogenous antigen. We previously demonstrated that the 24.8.A iNKT cell hybridoma was broadly reactive with CD1d-transfected cell lines and recognized the polar lipid fraction of a tumor cell extract. In the present study, the antigen recognized by the 24.8.A iNKT cell hybridoma was purified to homogeneity and identified as palmitoyl-oleoyl-sn-glycero-3-phosphoethanolamine (16:0-18:1 PE). The 24.8.A iNKT cell hybridoma recognized synthetic 16:0-18:1[cis] PE, confirming that this phospholipid is antigenic. Recognition correlated with the degree of unsaturation of the acyl chains. Using a panel of synthetic PEs, the 24.8.A iNKT cell hybridoma was shown to be activated by PEs that contained at least one unsaturated acyl chain. The configuration of the double bonds was important, as the 24.8.A iNKT cell hybridoma recognized unsaturated acyl chains in the cis, but not the trans, configuration. PEs with multiple double bonds were recognized better than those with a single double bond, and increasing acyl chain unsaturation correlated with increased binding of PE to CD1d. These data illustrate the potential importance of the acyl chain structure for phospholipid antigen binding to CD1d.

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.

Phospholipid-assisted refolding of an integral membrane protein. Minimum structural features for phosphatidylethanolamine to act as a molecular chaperone

Escherichia coli-derived phosphatidylethanolamine (PE) or PE with fully saturated fatty acids was able to correct in vitro a defect in folding in the lipid-dependent epitope 4B1 of lactose permease (LacY) resulting from in vivo assembly in the absence of PE. PE plasmalogen, PE with two unsaturated fatty acids, and lyso-PE, which all do not favor bilayer organization, did not support proper refolding. Proper refolding occurred when these latter lipids were mixed with a bilayer-forming lipid (phosphatidylglycerol), which alone could not support refolding. L-Phosphatidylserine (PS; natural diastereomer) did support proper refolding. PE derivatives of increasing degrees of methylation were progressively less effective in supporting refolding, with phosphatidylcholine being completely ineffective. Therefore, the properties of nonmethylated aminophospholipids capable of organization into a bilayer configuration are essential for the recovery of the native state of epitope 4B1 after misassembly in vivo in the absence of PE. Neither D-PS (sn-glycero-1-phosphate backbone) nor P-D-S (D-serine in the head group) is competent in supporting proper refolding unless used in binary mixtures with phosphatidylglycerol. The detailed characterization of phospholipid-assisted refolding reported here further supports a specific rather than nonspecific role for PE in structural maturation of lactose permease in vivo (Bogdanov, M., and Dowhan, W. (1998) EMBO J. 17, 5255-5264).

Nisin promotes the formation of non-lamellar inverted phases in unsaturated phosphatidylethanolamines

Nisin, a peptide used as a food preservative, is shown, by 31P-nuclear magnetic resonance and infrared spectroscopy, to perturb the structure of membranes formed of unsaturated phosphatidylethanolamine (PE) and to induce the formation of inverted non-lamellar phases. In the case of dioleoyl-sn-glycero-3-phosphatidylethanolamine (DOPE), nisin promotes the formation of inverted hexagonal phase. Similarly, the peptide induces the formation of an isotropic phase, most likely a cubic phase, with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylethanolamine (POPE). It is proposed that the insertion of the peptide in the bilayer shifts the amphiphilic balance by increasing the hydrophobic contribution and is at the origin of the changes in the polymorphic propensities of PE. This is supported by the fact that the presence of cholesterol in the PE bilayer inhibits the power of nisin to perturb the membrane structure, most likely because the peptide insertion is difficult in the fluid ordered phase. This finding provides insight into possible antibacterial mechanisms of nisin.

Tracking phospholipid populations in polymorphism by sideband analyses of 31P magic angle spinning NMR

A method was developed to track the distributional preferences of phospholipids in polymorphism based on sideband analyses of the 31P magic angle spinning nuclear magnetic resonance spectra. The method was applied to lipid mixtures containing phosphatidylcholine (PtdCho), phosphatidylethanolamine (PtdEtn) and either cholesterol (Chol) or tetradecane, as well as mixtures containing the anionic phosphatidylmethanol, phosphatidylethanolamine, and diolein. The phospholipid composition of coexisting lamellar (Lalpha) and inverted hexagonal (HII) phases remained constant throughout the Lalpha --> HII transition in all mixtures, except those that contained saturated PtdCho and unsaturated PtdEtn in the presence of cholesterol-mixtures that are known to be microimmiscible because of favored associations between Chol and saturated acyl chains. In the latter mixture, saturated PtdCho was enriched in the planar bilayer structure, and unsaturated PtdEtn was enriched in the highly curved HII structure. This enrichment was coincident with an increase in the transition width. When compositional heterogeneity among coexisting phases was observed, it appeared that preexisting lateral microheterogeneities led to compositionally distinct transitional clusters, such that the distributional preferences that resulted were not those of the individual phospholipids.

Combination of antitumor ether lipid with lipids of complementary molecular shape reduces its hemolytic activity

Because the therapeutic use of the antitumor ether lipid 1-O-octadecyl-2-O-methyl-sn-glycero-3-phosphorylcholine (ET-18-OCH3) is restricted by its hemolytic activity we explored the use of lipid packing parameters to reduce this toxicity by creating structurally optimized ET-18-OCH3 liposomes. We postulated that combination of ET-18-OCH3, which is similar in structure to lysophosphatidylcholine, with lipid molecules of complementary molecular shape (opposite headgroup/chain volume) would likely yield a stable lamellar phase from which ET-18-OCH3 exchange to red blood cell membranes would be curtailed. To quantitate the degree of shape complementarity, we used a Langmuir trough and measured the mean molecular area per molecule (MMAM) for monolayers comprised of ET-18-OCH3, the host lipids, and binary mixtures of varying mole percentage ET-18-OCH3. The degree of complementarity was taken as the reduction in MMAM from the value expected based on simple additivity of the individual components. The greatest degree of shape complementarity was observed with cholesterol: the order of complementarity for the ET-18-OCH3-lipid mixtures examined was cholesterol >> DOPE > POPC approximately DOPC. Phosphorus NMR and TLC analysis of aqueous suspensions of ET-18-OCH3 (40 mol%) with the host lipids revealed them to all be lamellar phase. For ET-18-OCH3 at 40 mol% in liposomes, the hemolytic activity followed the trend of the reduction in MMAM and was least for the ET-18-OCH3/cholesterol system (H50 = 661 microM ET-18-OCH3) followed by ET-18-OCH3/DOPE (H50 = 91 microM) and mixtures with POPC and DOPC which were comparable at H50 = 26 microM and 38 microM, respectively: the H50 concentration for free ET-18-OCH3 was 16 microM. This experimental strategy for designing optimized liposomes with a reduction in exchange, and hence toxicity, may be useful for other amphipathic/lipophilic drugs that are dimensionally compatible with lipid bilayers.

Novel cationic liposomes for DNA-transfection with high efficiency and low toxicity

Liposomes containing the natural cationic amphiphile, sphingosine and some of its derivatives were used for transfection of DNA in vitro. Multilamellar liposomes comprised of dioleoylphosphatidylethanolamine (DOPE), different sphingosine derivatives, and diacylglycerols with varying fatty acid chains, preincubated with DNA, transfected efficiently the KK-1 murine granulosa cells. Most efficient transfection on this cell line was achieved with liposomes composed of phytosphingosine, DOPE, and dioctanoylglycerol (DC8G) (64:31:4.8, molar stoichiometry), which gave expression of the transfected gene 2-10-fold higher than the commercial reagent Lipofectin. At higher doses the new liposomes also caused markedly less cell death of KK-1 cells. On COS-7 cells these liposomes showed slightly, but significantly lower transfection, of approximately 70%, of that gained with Lipofectin. The murine Sertoli cells, MSC-1, selectively resisted transfection by the sphingosine derivative based liposomes tested, giving only 11-14% of the expression detected in Lipofectin transfected cells of the same line. In conclusion, the novel liposomes formulated offer an effective, technically easy and economical method of transfection for a variety of cultured cell lines.

X-ray diffraction study of bilayer to non-bilayer phase transitions in aqueous dispersions of di-polyenoic phosphatidylethanolamines

The low temperature phase properties of aqueous dispersions of di-18:2 and di-18:3 phosphatidylethanolamine are strongly influenced by the presence of ice. In the presence of cryoprotectants to inhibit ice formation, these lipids persist in the H(II) phase down to at least -50 degrees C. Ice formation, however, leads to a drastic reduction in the amount of available free water and a rapid reduction in the diameter of the inverted cylindrical micelles of the H(II) phase. The resulting increase in surface curvature of the micelles induces an imbalance in the forces acting in the lipid surface and the hydrophobic core which is relieved by formation of the L(alpha) phase. On reheating the lipid samples undergo an abrupt L(alpha) --> H(II) phase transition at about -20 degrees C. The radius of the water core of the inverted micelles at their point of formation is estimated to be 0.9 nm. This increases with temperature as more unfrozen water becomes available until the normal equilibrium radius of about 2.3 nm is reached at 0 degrees C when the bulk water in the sample finally melts. A small proportion of the H(II) phase lipid enters an as yet unidentified cubic phase on freezing. The spacings of the (10) planes of the H(II) phase, the (111) planes of the cubic phase and the d-spacing of the L(alpha) phase were found to be almost identical at the phase transition temperature. The cubic phase appears to disappear at low temperature but to reform on heating. Freeze-fracture studies revealed no unequivocal evidence for cubic phase lipid but the presence of residual non-bilayer lipid structures was observed even at temperatures as low as -80 degrees C. The presence of intersecting stacks of lamellar sheets in the replicas strongly suggest the existence of an epitaxial relationship between the L(alpha) and H(II) phases in these systems.

Is the high propensity of ethanolamine plasmalogens to form non-lamellar lipid structures manifested in the properties of biomembranes?

Plasmalogens are glycerophospholipids characterized by an alk-1'-enylether bond in position sn-1 and an acyl bond in position sn-2. These ubiquitous etherlipids exhibit a different molecular structure as compared to diacyl phospholipids. The most peculiar change is a perpendicular orientation of the sn-2 acyl chain at all segments to the membrane surface. This extended conformation results in an effectively longer aliphatic chain in plasmalogen than in the diacyl analog. Moreover, the lack of the carbonyl oxygen in position sn-1 affects the hydrophilicity of the headgroup and allows stronger intermolecular hydrogen-bonding between the headgroups of the lipid. These properties favour the formation of non-lamellar structures which are expressed in the high affinity of ethanolamine plasmalogen to adopt the inverse hexagonal phase. Such structures may be involved in membrane processes, either temporarily, like in membrane fusion or locally, e.g. to affect the activity of membrane-bound proteins. The predominant distribution of ethanolamine plasmalogens in some cellular membranes like nerve tissues or plasma membranes and their distinctly different properties in model membranes as compared to diacyl phospholipids impose the question, whether these differences are also manifested in the heterogeneous environment of biological membranes. The integration of biophysical studies and biochemical findings clearly indicated that the high propensity of ethanolamine plasmalogen to form non-lamellar structures is reflected in several physiological functions. So far it seems to be evident that ethanolamine plasmalogens play an important role in maintaining the balance between bilayer and non-lamellar phases which is crucial for proper cell function. Furthermore, they are the major phospholipid component of inverse hexagonal phase inclusions in native retina and are able to mediate membrane fusion as demonstrated between neurotransmitter vesicles and presynaptic membranes.

Capsaicin affects the structure and phase organization of phospholipid membranes

Capsaicin is a natural compound with pharmacological and toxicological effects, which given its hydrophobicity, can influence the structure of membranes. The interaction of capsaicin with model membranes of dipalmitoylphosphatidylcholine and dielaidoylphosphatidylethanolamine has been studied by using differential scanning calorimetry, fluorescent probe spectroscopy and 31P-nuclear magnetic resonance. Capsaicin remarkably affects the phase transition of dipalmitoylphosphatidylcholine, shifting the transition temperature to lower values, and giving rise, at relatively high capsaicin concentrations, to the appearance of two peaks in the thermogram. These peaks may correspond to separated phases as indicated by the partial phase diagram. Whereas capsaicin did not affect the fluorescence polarization of the probes diphenylhexatriene and trimethylammonium-diphenylhexatriene, it clearly affected that of the probe 2-anthroyloxystearic acid, indicating that the perturbation produced by capsaicin on the membrane would be mainly at the position where this fluorophore is located. On the other hand, capsaicin, at relatively low concentrations, gives rise to immiscible phases in the presence of dielaidoylphosphatidylethanolamine and decrease the temperature of the lamellar to hexagonal HII phase transition. At concentrations of capsaicin higher than 0.3 mol fraction, isotropic phases were detected. The possible implications of the effects of capsaicin on biological membranes are discussed.

Membrane lipid composition and cell size of Acholeplasma laidlawii strain A are strongly influenced by lipid acyl chain length

The small, cell-wall-less prokaryote Acholeplasma laidlawii strain A-EF22 could grow with membrane lipids having an average acyl chain length Cn varying over 14.5- almost 20 carbons by exogenous supplementation with selected fatty acids. For 16 < Cn < 18, the cells grew with lipids containing 100% (mol/100 mol) monounsaturated acyl chains, whereas for Cn < 16 and Cn > 18, cell growth only occurred with gradually lower fractions of unsaturated chains. Cn was actively increased and decreased by chain elongation or de novo fatty acid synthesis upon incorporation of short-chain and long-chain fatty acids, respectively. The membrane lipid composition was strongly affected by the acyl chain length and unsaturation, and the metabolic responses are readily explained as a regulation mechanism based on the established phase equilibria of the individual lipids in the A. laidlawii membrane. Monoglucosyldiacylglycerol (Glc-acyl2-Gro) was the dominating lipid with short chains but the fraction of this lipid decreased with increasing Cn, correlating with the decreasing lamellar to nonlamellar phase transition temperatures for this lipid. The fractions of diglucosyldiacylglycerol (Glc2-acyl2Gro) and phosphatidylglycerol (PtdGro), forming lamellar phases only, increased with increasing Cn over the entire chain-length interval. A weaker correlation was usually observed between the relative amount of a lipid and the extent of chain unsaturation; however, the fractions of Glc2-acyl2Gro and PtdGro increased clearly with an increasing degree of unsaturation. Moreover, the synthesis of the nonbilayer-forming lipids acyl2Gro and monoacyl-Glc-acyl2Gro was strongly stimulated by a high degree of chain saturation. Concomitantly, the phase equilibria of Glc-acyl2Gro are shifted towards lamellar phases at the growth temperature. The fraction of the three potentially nonbilayer-forming lipids varied over 10-80% (mol/100 mol) total lipids as a function of the acyl chain composition. The combined molar fractions of the three phospholipids increased strongly with chain unsaturation. However, the fraction of phosphate moieties in the different lipids was constant over the entire chain-length interval. It is concluded that the regulation of the membrane lipid composition aims at maintaining similar phase equilibria and surface charge densities of the lipid bilayer. The size of A. laidlawii cells was changed in a systematic manner and correlated qualitatively with the packing properties of the lipids. Cell diameters were increased by an increase in acyl chain length and saturation, and was affected by additives such an n-dodecane and acyl2Gro.

A phase behavior study of mixtures of sphingosine with zwitterionic phospholipids

The interactions of sphingosine (SPH) with dipalmitoylphosphatidylcholine (DPPC) and dielaidoylphosphatidylethanolamine (DEPE) have been studied by means of differential scanning calorimetry (DSC) and 31P-nuclear magnetic resonance (31P-NMR). Experiments were carried out with the fully protonated form of SPH, at pH 6.0. DSC studies showed that the main Tc transition temperature of DPPC was perturbed by the presence of SPH so that Tc of the mixture was higher than those of pure components at concentrations of SPH up to 50 mol%, with an azeotropic point at 30 mol% of SPH. At higher concentrations solid phase separations were observed from 70 to 95 mol% of SPH with an eutectic point at 90 mol% of SPH. 31P-NMR showed lamellar phases in DPPC/SPH mixtures, at all the range of concentrations. The behavior of DEPE/SPH mixtures was somewhat different since no azeotropic point was detected, the gel to liquid-crystalline transition being depressed by the presence of SPH, and an eutectic point was detected at 60 mol%. Solid phase immiscibilities were present between 50 mol% and 85 mol% of SPH. It is also remarkable that the liquid-crystalline to hexagonal HII phase transition of DEPE was only slightly shifted to lower temperatures at concentrations of SPH lower than 33 mol% of SPH but, this transition disappeared at concentrations of SPH higher than 33 mol% of SPH, so that isotropic phases were formed instead, as seen through 31P-NMR. The present results show the importance of taking into account the effects appearing in mixtures of SPH with zwitterionic phospholipids when considering their influence on the organization of biomembranes.

Phases and phase transitions of the hydrated phosphatidylethanolamines

LIPIDAT is a computerized database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior. Here, a review of the LIPIDAT data subset referring to hydrated phosphatidylethanolamines (PE) is presented together with an analysis of these data. The PE subset represents 14% of all LIPIDAT records. It includes data collected over a 38-year period and consists of 1511 records obtained from 203 articles in 35 different journals. An analysis of the data in the subset has allowed us to identify trends in synthetic PE phase behavior reflecting changes in lipid chain length, chain unsaturation (number, isomeric type and position of double bonds), chain asymmetry and branching, type of chain-glycerol linkage (ether vs. ester) and headgroup modification. Also included is a summary of the data concerning the effect of pH, stereochemical purity, and different additives such as salts, saccharides, alcohols, amino adds and alkanes on PE phase behavior. Information on the phase behavior of biologically derived PE is also presented. This review includes 236 references.

Molecular basis for low temperature compartment formation by transitional endoplasmic reticulum of rat liver

The molecular basis for temperature compartment formation was investigated using a cell-free system from rat liver. The donor was from liver slices prelabeled with [3H]acetate. Unlabeled Golgi apparatus membranes were immobilized on nitrocellulose as the acceptor. When transfer was determined as a function of temperature, a transition in transfer activity was observed at low temperatures (less than or equal to 20 degrees C) similar to that seen in vivo. The decrease in transfer efficiency correlated with a decrease in phosphatidylethanolamine and phosphatidylserine content of the transition vesicles formed. By adding lipid mixtures enriched in these lipids to the vesicles, their ability to fuse with the cis Golgi apparatus was reconstituted. These findings provide evidence for a role for lipids in low temperature compartment formation.

Nonlamellar phases formed by membrane lipids

A brief review of membrane lipids forming cubic and reversed hexagonal phases is presented. An emphasis is made on anionic lipids and particular microbial lipids.

Chemical exchange between lamellar and non-lamellar lipid phases. A one- and two-dimensional 31P-NMR study

One- and two-dimensional 31P-exchange NMR has been used to investigate chemical exchange between coexisting lamellar (L alpha) and non-lamellar (hexagonal HII and cubic I2) lipid phases. Samples of DOPE, DOPE/DOPC (9:1 and 7:3), DOPE/cholesterol sulfate (9:1), DOPC/monoolein (MO) (3:7 and 1:1), and DOPC/DOPE/cholesterol (1:1:2) were macroscopically oriented on glass plates and studied at the 0 degree orientation (angle between the bilayer normal and the external magnetic field), where the L alpha, HII, and I2 resonances are resolved. A reversible L alpha to HII transition was observed for all of the samples except for the DOPC/MO mixtures, which displayed a reversible L alpha to I2 transition. Near-equilibrium mixtures of L alpha and either HII or I2 were obtained after prolonged incubation at a given temperature. Two-dimensional exchange experiments were performed on DOPE at 9-14 degrees C for mixing times ranging from 500 ms to 2 s. For all samples, one-dimensional exchange experiments were performed for mixing times ranging from 100 ms to 4 s, at temperatures ranging from 3 degrees C to 73 degrees C. No evidence of lipid exchange between lamellar and non-lamellar phases was observed, indicating that if such a process occurs it is either very slow on the seconds' timescale, or involves an undetectable quantity of lipid. The results place constraints on the stability or kinetic behaviour of proposed transition intermediates (Siegel, D.P. (1986) Biophys. J. 49, 1155-1170).

Effect of head-group structure and counterion condensation on phase equilibria in anionic phospholipid-water systems studied by 2H, 23Na, and 31P NMR and X-ray diffraction

The phase equilibria, hydration, and sodium counterion association for the systems DOPA-2H2O, DOPS-2H2O, DOPG-2H2O, and DPG-2H2O were investigated with 2H, 23Na, and 31P NMR and X-ray diffraction. The following one-phase regions were found in the DOPA-water system: a reversed hexagonal liquid-crystalline (HII) phase up to about 35 wt % water and a lamellar liquid-crystalline (L alpha) phase between about 55 and 98 wt % water. The area per DOPA molecule was 36-65 A2 in the HII phase (10-40 wt % water) and 69 A2 in the L alpha phase (60 wt % water). DOPS and DOPG with 10-98 wt % water, and DPG with 20-95 wt % water formed an L alpha phase at temperatures between 25 and 55 degrees C. At temperatures above 55 degrees C, DPG with 20 and 30 wt % water formed a mixture of L alpha, HII, and cubic liquid-crystalline phases, the mole percent of lipid forming nonlamellar phases being smaller at 30 wt % water than at 20 wt % water. DPG with 10 wt % water probably formed a mixture of an L alpha phase and at least one nonlamellar liquid-crystalline phase at 25 and 35 degrees C, and a pure HII phase at 45 degrees C and higher temperatures. At water concentrations above about 50 wt % the 23Na quadrupole splitting was constant for all four lipid-water systems studied, implying that the counterion association to the charged lipid aggregates did not change upon dilution. These experimental observations can be described with an ion condensation model but not with a simple equilibrium model. The fraction of counterions located close to the lipid-water interface was calculated to be greater than 95%. The 2H and 23Na NMR quadrupole splittings of 2H2O and sodium counterions, respectively, indicate that the molecular order in the polar head-group region decreases for the L alpha phase in the order DOPA approximately DPG greater than DOPS greater than DOPG.

Influence of vitamin E on phosphatidylethanolamine lipid polymorphism

The effect of vitamin E, in its major form alpha-tocopherol and its synthetic analog alpha-tocopheryl acetate, on phosphatidylethanolamine lipid polymorphism has been studied by mean of differential scanning calorimetry and 31P-nuclear magnetic resonance techniques. From the interaction of these tocopherols with dielaidoylphosphatidylethanolamine it is concluded that both molecules promote the formation of the hexagonal HII phase at temperatures lower than those of the pure phospholipid. When the tocopherols were incorporated in the saturated dimiristoylphosphatidylethanolamine, which has been shown not to undergo bilayer to hexagonal HII phase transition, up to 90 degrees C, they induce the phospholipid to partially organize in hexagonal HII phase. From our experiments it is shown that alpha-tocopherol is more effective than its analog in promoting HII phase in these systems. It is also shown that, while alpha-tocopheryl acetate does not significantly perturb the gel to liquid-crystalline phase transition of dimirystoylphosphatidylethanolamine, alpha-tocopherol does so and more than one peak appears in the calorimetric profile, indicating that lateral phase separations are taking place.

The polymorphic phase behavior of dielaidoylphosphatidylethanolamine. Effect of n-alkanols

The polymorphic phase behavior of dielaidoylphosphatidylethanolamine (DEPE) has been investigated using spectrophotometry and 31P nuclear magnetic resonance (NMR). It has been demonstrated that the bilayer to inverted hexagonal phase transition can be observed by spectrophotometry. The effects of the methanol, ethanol, and propanol on both the gel to liquid crystal transition and the bilayer to inverted hexagonal transition were investigated by spectrophotometry. It was shown that these alcohols shift the gel to liquid-crystalline phase transition to lower temperature, whereas the bilayer to inverted hexagonal phase transition is shifted to higher temperatures by these alcohols. The structural transition between the bilayer and inverted hexagonal phase of pure DEPE was also investigated by 31P-NMR.

Freeze/thaw-induced destabilization of the plasma membrane and the effects of cold acclimation

Disruption of the plasma membrane is a primary cause of freezing injury. In this review, the mechanisms of injury resulting from freeze-induced cell dehydration are presented, including destabilization of the plasma membrane resulting from (a) freeze/thaw-induced osmotic excursions and (b) lyotropic phase transitions in the plasma membrane lipids. Cold acclimation dramatically alters the behavior of the plasma membrane during a freeze/thaw cycle--increasing the tolerance to osmotic excursions and decreasing the propensity for dehydration-induced lamellar to hexagonal-II phase transitions. Evidence for a casual relationship between the increased cryostability of the plasma membrane and alterations in the lipid composition is reviewed.

Adaptive modifications in membranes of halotolerant and halophilic microorganisms

Halotolerant and halophilic microorganisms can grow in (hyper)saline environments, but only halophiles specifically require salt. Genotypic and phenotypic adaptations are displayed by halophiles; the halotolerants adapt phenotypically, but it is not established whether they show genotypic adaptation. This paper reviews the various strategies of haloadaptation of membrane proteins and lipids by halotolerant and halophilic microorganisms. Moderate halophiles and halotolerants adapt their membrane lipid composition by increasing the proportion of anionic lipids, often phosphatidylglycerol and/or glycolipids, which in the moderately halophilic bacterium Vibrio costicola appears to be part of an osmoregulatory response to minimize membrane stress at high salinities. Extreme halophiles possess typical archaebacterial ether lipids, which are genotypically adapted by having additional substitutions with negatively-charged residues such as sulfate. In contrast to the lipids, it is less clear whether membrane proteins are haloadapted, although they may be more acidic; very few depend on salt for their activity.

Influence of membrane-lipid composition on translocation of nascent proteins in heated Escherichia coli

In studies using Escherichia coli we have shown that new protein species appear in the outer membrane fraction with concomitant losses of nascent proteins from the soluble and inner membrane fractions following heat exposure. Of the various explanations for this phenomenon, temperature-induced membrane disorganization appeared the most likely. It is suggested that heat mimics the action of the signal sequence of a protein on the lipid bilayer allowing non-signal-sequence-containing proteins to be translocated. To test this hypothesis we grew E. coli K1060 cells, an unsaturated fatty acid requiring auxotroph, supplemented during growth with fatty acids of varying chain length in an attempt to determine whether biological membranes of varying ability to maintain their bilayer configuration could be constructed. The rationale being that such membranes would allow us to determine whether differences in translocation would occur in cells grown at the same temperature supplemented with either 16:1 or 20:1 unsaturated fatty acids when the cells were subjected to a series of thermal insults. Protein translocation occurred to a greater extent and at lower temperatures in cells supplemented with the longer chain fatty acid. Treatment of outer membranes with either 1 M salt, 6 M urea or high pH and studies determining fluorescent polarization values by scanning up and down through a series of temperatures ranging from 15 to 49 degrees C indicate that the proteins translocated by heat to the outer membrane are integral. Protein translocation may represent an adaptive response to an altered environment enabling the cell to respond to stress by stabilizing its outer membrane.

Fusion of Sindbis virus with model membranes containing phosphatidylethanolamine: implications for protein-induced membrane fusion

The pH-induced fusion of Sindbis virus with model lipid membranes containing phosphatidylethanolamine has been studied using a quantitative fluorescence technique. The headgroup and acyl chain domains of the lipids have been altered systematically to determine their effect on fusion. Unsaturated phosphatidylethanolamines (PE) have been found to promote fusion, either by themselves, or in combination with phosphatidylcholines (PC). Cholesterol added to a mixture of unsaturated PE and PC was also shown to increase the extent of viral fusion. The results of these studies have been interpreted in terms of a tentative model for the molecular aspects of the target membrane which are necessary for viral fusion. In this model, the target membrane must have a sufficiently-sized domain containing poorly hydrated lipids which are capable of existing in a non-bilayer arrangement.

Phase behavior of synthetic N-acylethanolamine phospholipids

Both saturated and unsaturated N-acylethanolamine phospholipids form lamellar structures when dispersed in buffer. The addition of excess Ca2+ (Ca2+/N-acylphosphatidylethanolamine greater than 0.5) results in precipitation. Freeze-fracture replicas indicate that the addition of Ca2+ to the unsaturated lipid results in a non-bilayer structure while the Ca2+-complex of the saturated lipid is lamellar. Since unsaturated phosphatidylethanolamine (PE) is a non-bilayer lipid, its N-acylation with a saturated fatty acid converts a non-bilayer lipid into an acidic bilayer lipid capable of interacting with Ca2+ to return to a non-bilayer structure. Ca2+ may thereby exert an influence on membrane phenomena by regulating phase behavior within certain membrane domains. Differential scanning calorimetry (DSC) indicates that N-acylation of unsaturated PE with a saturated fatty acid also results in changes in thermotropic phase behavior. Therefore, N-acylation may affect fluidity within certain membrane domains.

Inverted micellar intermediates and the transitions between lamellar, cubic, and inverted hexagonal amphiphile phases. III. Isotropic and inverted cubic state formation via intermediates in transitions between L alpha and HII phases

Inverted cubic and isotropic phases have been observed in phospholipid and glycolipid systems. These phases exhibit characteristic morphologies in freeze-fracture electron micrographs, isotropic 31P-NMR resonances and (in some cases) cubic X-ray diffraction patterns. It is proposed here that these phases may form from the same intermediates that are involved in lamellar/inverted hexagonal (L alpha/HII) phase transitions, and that it is possible that these cubic and isotropic phases are metastable. According to a kinetic theory of L alpha/HII phase transitions, intermediates in such transitions can form structures known as interlamellar attachments (ILAs). It is shown that ILAs should form in large numbers during L alpha/HII transitions in systems like those reported to form inverted cubic or isotropic structures. ILAs cannot readily assemble into either the HII phase or well-ordered arrays of L alpha phase bilayers, and represent a kinetic trap for intermediates in L alpha/HII transitions (although it is possible that they are marginally more stable in a thermodynamic sense than the L alpha phase in a small temperature range below TH). It is also shown that arrays of ILAs should form metastable arrays with the same morphology and isotropic 31P-NMR resonances that are observed in isotropic and inverted cubic states. In particular, under some circumstances ILAs will assemble into a structure identical to the bicontinuous inverted cubic phase previously described in monoglycerides and very similar in morphology to structures observed in phospholipid systems. Finally, since isotropic and cubic states form from ILAs, which also can mediate fusion of unilamellar vesicles, unilamellar vesicles should fuse to at least some extent under the same conditions in which multilamellar samples of the same lipid form isotropic or inverted cubic states. This correlation has been observed.

Lipid polymorphism

In this review the polymorphic phase behaviour of several of the major classes of lipids found in biological membranes, both in isolation and also in mixtures, is briefly described. Emphasis is given to the ability of many membrane lipids to adopt non-lamellar phases in response to a variety of factors such as temperature, the presence of divalent cations or changes in pH. The phase behaviour of mixed lipid systems and factors which can modulate the phase preferences of such systems are considered in some detail particularly with regard to the effect of cholesterol upon lipid polymorphism.

Thermotropic properties of bilayers containing branched-chain phospholipids. Calorimetric, Raman, and 31P NMR studies

Diisopalmitoylphosphatidylcholine (DIPPC), -phosphatidylethanolamine (DIPPE), and -phosphatidylglycerol (DIPPG) have been synthesized, and the structures of aqueous dispersions of these lipids have been examined by high-sensitivity differential scanning calorimetry, 31P nuclear magnetic resonance, and Raman spectroscopy. DIPPC at temperatures below 23.1 degrees C readily forms a gel phase with the acyl chains packed in an orthorhombic subcell. Above this temperature, this "orthorhombic" phase converts directly to the liquid-crystalline phase. The phase diagram for the system DIPPC--dipalmitoyl-PC (DIPPC-DPPC) shows that the gel phases formed by either lipid can accommodate only limited amounts of the other species and suggests that the low-temperature orthorhombic phase of DIPPC is distinct in its structure from the "subgel" phase of DPPC. DIPPE forms a well-ordered gel phase only in samples that are equilibrated at low temperatures for long times (approximately days to weeks) or at very high lipid concentrations. However, this lipid readily forms an "intermediate" phase with a very disordered acyl chain packing upon cooling from the liquid-crystalline state. Mixtures of DIPPE with DIPPG exhibit similar thermotropic properties. Hydrated DIPPE appears to be stable in the lamellar phase up to at least 98 degrees C, while di-cis- and di-trans-9-hexadecenoyl-PE convert to the hexagonal II phase at 43.5 and 92.5 degrees C, respectively. We discuss the relevance of these results to the structure and stability of bacterial membranes containing branched-chain acyl lipids.

Influence of phospholipid peroxidation on the phase behavior of phosphatidylcholine and phosphatidylethanolamine in aqueous dispersions

The influence of oxygen-induced phospholipid peroxidation on the phase behavior of aqueous dispersions of both egg phosphatidylcholine (egg-PC) and egg phosphatidylethanolamine (egg-PE) has been investigated. Phospholipid peroxidation was followed via malondialdehyde formation and analyses of acyl chain compositions. 13C nuclear magnetic resonance spectroscopy (NMR) and the amino-indicating probe trinitrobenzenesulfonic acid were used to study the effect of peroxidation on the chemical structure of hydrated egg-PE. The macroscopic organization of the phospholipids was monitored by 31P NMR and small-angle X-ray diffraction. Differential scanning calorimetry was employed to study the influence of peroxidation on the thermotropic behavior of egg-PE. The results show that egg-PE is more sensitive to the effects of peroxidation than egg-PC. In the latter, no changes in the macromolecular organization were observed. However, peroxidation strongly influenced the polymorphic phase behavior of PE. Initial peroxidation stabilized hydrated egg-PE in a lamellar system up to 70 degrees C, presumably by modification of the head group. Such modifications were confirmed by 13C NMR experiments, which indicated the formation of Schiff bases between PE head groups and aldehydes. Furthermore, quantitative analyses of trinitrobenzenesulfonic acid reactable egg-PE and the corresponding fatty acid compositions revealed the presence of cross-links between the ethanolamine head groups, likely involving the bifunctional malondialdehyde. Prolonged peroxidation of egg-PE resulted in a loss of order in the system, possibly by the formation of intermediate nonbilayer structures.

Cation-dependent segregation phenomena and phase behavior in model membrane systems containing phosphatidylserine: influence of cholesterol and acyl chain composition

The addition of Ca2+ to model membrane systems containing phosphatidylserine (PS) can have remarkable effects on the distribution of PS and the overall polymorphic phase [bilayer or hexagonal (HII)] assumed by the lipid mixture. In this study, we examine the influence of Ca2+ on lipid mixtures composed of well-defined (synthetic) species of PS, phosphatidylethanolamine (PE), and phosphatidylcholine (PC) in the presence and absence of cholesterol by employing 31P and 2H NMR, freeze-fracture, and X-ray techniques. It is shown that whereas Ca2+ can segregate PS into crystalline cochleate domains in equimolar mixtures of dioleoyl-PE and dioleoyl-PS (DOPS), such effects are not observed for mixtures containing more unsaturated (dilinoleoyl) species of PS. The addition of cholesterol to these PE-PS systems inhibits Ca2+-induced segregation of DOPS and facilitates Ca2+-triggered hexagonal (HII) phase formation for both the PE and the PS components. In contrast, in equimolar mixtures of DOPS with dioleoyl-PC, Ca2+-induced segregation of phospholipid is not affected by the presence of up to 33 mol % cholesterol. These and related effects suggest that, in multicomponent biomembrane systems containing both PE and cholesterol, phase segregation of PS by Ca2+ may not be readily achievable. These results are discussed with regard to the reliability of 31P NMR phase identifications of phospholipid structure in model and biological membranes and demonstrate that in mixed lipid systems the influence of divalent cations on lipid distribution and structure can be exquisitely sensitive to details of the local lipid composition.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis and polymorphic phase behaviour of polyunsaturated phosphatidylcholines and phosphatidylethanolamines

A series of phosphatidylcholines and phosphatidylethanolamines was synthesized containing two acyl chains of the following polyunsaturated fatty acids: linoleic acid (18:2), linolenic acid (18:3), arachidonic acid (20:4) and docosahexaenoic acid (22:6). In addition two phospholipids with mixed acid composition were synthesized: 16:0/18:1c phosphatidylcholine and 16:0/18:1c phosphatidylethanolamine. The structural properties of these lipids in aqueous dispersions in the absence and in the presence of equimolar cholesterol were studied using 31P-NMR, freeze fracturing and differential scanning calorimetry (DSC). The phosphatidylcholines adopt a bilayer configuration above 0 degrees C. Incorporation of 50 mol% of cholesterol in polyunsaturated species induces a transition at elevated temperatures into structures with 31P-NMR characteristics typical of non-bilayer organizations. When the acyl chains contain three or more double bonds, this non-bilayer organization is most likely the hexagonal HII phase. 16:0/18:1c phosphatidylethanolamine shows a bilayer to hexagonal transition temperature of 75 degrees C. The polyunsaturated phosphatidylethanolamines exhibit a bilayer to hexagonal transition temperature below 0 degrees C which decreases with increasing unsaturation and which is lowered by approximately 10 degrees C upon incorporation of 50 mol% of cholesterol. Finally, it was found that small amounts of polyunsaturated fatty acyl chains in a phosphatidylethanolamine disproportionally lower its bilayer to hexagonal transition temperature.