Lipid polymorphism as observed by cryo-electron microscopy

Preparation of nanoliposome loaded with peanut peptide fraction: stability and bioavailability

Nanoliposome loaded with peanut peptide fraction (PPF) prepared by high pressure microfluidization (HPM) treatment was investigated as well as its stability and bioavailability.

Spatial distribution of nanocrystals imaged at the liquid-air interface

The 3D distribution of nanocrystals at the liquid-air interface is imaged for the first time on a single-particle level by cryogenic electron tomography, revealing the equilibrium concentration profile from the interface to the bulk of the liquid. When the surface tension of the liquid is decreased, the interaction of the nanocrystals with the liquid-air interface shifts from adsorption to desorption. Macroscopic surface tension measurements do not detect this transition, due to the presence of surface-active molecular species.

[FeFe]-hydrogenase models assembled into vesicular structures

Compartmentalization is a major prerequisite for the origin of life on earth according to Wächtershäuser "Iron-Sulfur-World". The hypothesis is mainly based on an autocatalytic inorganic energy reproducing redox system consisting of iron and sulfur as requirement for the subsequent synthesis of complex organic structures. Here, we modified [FeFe]-hydrogenase models by means of covalent coupling to either oleic acid or the amphiphilic block copolymer polybutadiene-polyethyleneoxide (PB-PEO) and incorporated those into the membranes of vesicles composed of phospholipids (liposomes) or the unmodified amphiphilic polymer (polymersomes). We employed a [2Fe-2S] cluster as a hydrogenase model, since these structures are known to be suitable catalysts for the generation of H2 in the presence of weak acids. Successful incorporation was confirmed by spectrophotometric iron quantification and the vesicles formed were characterized by size determination (photon correlation spectroscopy (PCS)), and zeta potential as well as by cryo-transmission electron microscopy (Cryo-TEM). The modified models could be incorporated into liposomes or polymersomes up to molar proportions of 3.15% and 28%, respectively. Due to the immobilization in vesicular bilayers the [FeFe]-hydrogenase models can even exhibit catalytic action under the particular conditions of the intravesicular microenvironment. Our results suggest that the vesicular systems described may be applied as a nanoreactor for the reduction of encapsulated substances by generating hydrogen and thus as a minimal cell model.

Polymersomes containing iron sulfide (FeS) as primordial cell model : for the investigation of energy providing redox reactions

According to Wächtershäuser's "Iron-Sulfur-World" one major requirement for the development of life on the prebiotic Earth is compartmentalization. Vesicles spontaneously formed from amphiphilic components containing a specific set of molecules including sulfide minerals may have lead to the first autotrophic prebiotic units. The iron sulfide minerals may have been formed by geological conversions in the environment of deep-sea volcanos (black smokers), which can be observed even today. Wächtershäuser postulated the evolution of chemical pathways as fundamentals of the origin of life on earth. In contrast to the classical Miller-Urey experiment, depending on external energy sources, the "Iron-Sulfur-World" is based on the catalytic and energy reproducing redox system FeS+H2S-->FeS2+H2. The energy release out of this redox reaction (∆RG°=-38 kJ/mol, pH 0) could be the cause for the subsequent synthesis of complex organic molecules and the precondition for the development of more complex units similar to cells known today. Here we show the possibility for precipitating iron sulfide inside vesicles composed of amphiphilic block-copolymers as a model system for a first prebiotic unit. Our findings could be an indication for a chemoautotrophic FeS based origin of life.

Liposome characterization by quartz crystal microbalance measurements and atomic force microscopy

This chapter reviews liposome characterization by quartz crystal microbalance (QCM) measurements and atomic force microscopy (AFM). In many studies, AFM imaging is simply used to image liposomes with resolution often that does not allow morphological analysis. Although liposome size can be obtained by processing AFM images, it is found that liposomes flatten upon surface adsorption or immobilization. Liposome stability and stiffness have been characterized by using AFM imaging or AFM force measurements, although the latter method, using a microsphere attached on the AFM cantilever, seems more appropriate to limit liposome damage and to obtain more quantitative analysis, such as the Young's modulus. Investigation of liposome layers by QCM revealed that liposomes can be detected from a combined analysis of frequency and bandwidth shifts. However, QCM by itself provides only limited information on liposomes. QCM can be used to assess the presence of a layer and also to discriminate between rigid and viscoelastic ones. Liposome properties have been derived from QCM curves, but often this requires making hypotheses that are difficult to assess. AFM and QCM analyses need to be combined with other techniques to provide complementary information.

Arsonoliposomes: Effect of Lipid Composition on Their Stability and Morphology

The influence of the lipid composition of arsonoliposomes on their membrane integrity was investigated to evaluate whether it is possible to combine their action with drugs that can be encapsulated in their aqueous interior. This was investigated by measuring the retention of vesicle-encapsulated calcein (100 mM) during incubation, in the absence and presence of serum proteins. Liposomes containing various concentrations of arsonolipid (with the palmitoyl side chain) as well as egg-lecithin (phosphatidylcholine, PC) and cholesterol (lipid/chol 2:1 mol:mol) were prepared. In some experiments, PC was replaced by the synthetic phospholipid DSPC. All PC/arsonoliposomes tested are stable after 24 h of incubation in buffer at 37 degrees C. After incubation in the presence of serum proteins, arsonoliposomes that contain low amounts of arsonolipid (up to 5 mol% of the lipid content without cholesterol) are stable, whereas increased release of calcein is observed when vesicle arsonolipid concentration is raised (from 5 to 15 mol%). Further increase of arsonolipid content results in immediate decrease of calcein latency while the remaining calcein is rapidly released during incubation. DSPC/arsonoliposomes are comparably more stable, and membrane integrity is independent of the vesicle arsonolipid content, in the range investigated (15-40 mol% of the lipid content without cholesterol). Thereby, we conclude that more stable arsonoliposomes that incorporate high arsonolipid concentrations may be produced when PC is replaced by DSPC. The latter arsonoliposomes provide a system that may be used for combining arsonolipid activity with the activity of other drugs.

Intra- and intermembrane pairwise molecular recognition between synthetic hydrogen-bonding phospholipids

Multivalency and preorganization are fundamental aspects of molecular recognition at the lipid membrane-water interface and can render weak monomeric binding interactions selective and robust; this concept is important throughout biology, biotechnology, and materials science. Though hydrogen bonding is typically weakened in water, intramembrane hydrogen bonding between native lipids has been well-studied and is thought to contribute to lipid bioactivity and membrane function. We hypothesized that avidity and preorganization effects at the lipid-water interface could overcome solvent competition and allow for selective hydrogen-bond recognition between small, unstructured components. We have found that electrostatically identical vesicular membranes composed of cyanuric acid and melamine functionalized phospholipids 1 and 2 undergo selective apposition, fusion and adhesion in suspension and on solid support, indicating that their well-known low-dielectric hydrogen bonding properties translate effectively to the lipid-water interface. This work is notable and of general interest given the few detailed studies of aqueous phase hydrogen-bonding systems; we have extensively characterized this system, gaining structural, functional, and thermodynamic data. Furthermore, we have found that the designed lipid-lipid headgroup interactions result in dramatic alteration of the lipid phase morphology, providing insight into the coupling of molecular interactions with assembly state. As such, this work contributes to our understanding of fundamental phenomena such as molecular recognition at the lipid-water interface membrane chemistry and further illustrates the general possibility of designing selective hydrogen-bonding adhesive interactions from simple starting materials at other polar-apolar interfaces; this could have numerous materials and biotechnological applications.

Crystallography of dispersed liquid crystalline phases studied by cryo-transmission electron microscopy

Low molecular weight surfactants, for example monoglycerides and phospholipids, form a multitude of self-assembled structures, such as inverted cubic or hexagonal mesophases, if brought into contact with water/oil. These mesophases can be dispersed in water using adequate surface-active materials such as low molecular weight surfactants or surface active polymers. In order to use such mesophase particles for incorporating drugs and aromas, it is essential to determine their internal crystallographic structure and to understand their mechanism of stabilization. Cryo-transmission electron microscopy was used to investigate the internal structure of different dispersed particles at various temperatures and oil contents. It is shown here that cryo-transmission electron microscopy, in combination with fast Fourier transform and tilting experiments, is effective in obtaining information on crystallographic structure, space group and morphology of particles with reversed bicontinuous cubic and hexagonal structures. In particular, using the presence or the absence of the {111} reflections and viewing the same particle under different axes of observation allows one to discriminate between the Im3m and Pn3m space groups. A major advantage of cryo-transmission electron microscopy is the ability to analyse single particles. This allows the identification of particles present at very low concentrations and the coexistence of particles with different internal self-assembly structures. With this technique we have obtained strong evidence for the presence of two cubic internal self-assembly structures with different space groups within the same dispersion. In addition, we found that cryo-transmission electron microscopy combined with tilting experiments enables the analysis of internal particle morphology, allowing the discussion of mechanisms for hexosome stabilization.

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

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

Cryo-TEM investigation of phase behaviour and aggregate structure in dilute dispersions of monoolein and oleic acid

Cryo-transmission electron microscopy (cryo-TEM) was used to image the microstructure in dilute sonicated dispersions of monoolein and oleic acid. The aim of the study was to explore how different experimental parameters, such as sample composition, total lipid concentration, pH, and ageing affect the phase behaviour and aggregate structure. Our investigations show that a rich variety of lamellar and non-lamellar structures, including liposomes and particles of cubic and inverted hexagonal phase, may form depending on the experimental conditions. The results are analyzed and discussed in relation to existing phase diagrams and earlier investigations concerning phase- and structural behaviour in monoolein/oleic acid/water systems.

Incorporation of PEG-lipids in arsonoliposomes results in formation of highly stable arsenic-containing vesicles

We investigated the effect of pegylation on the physical stability, morphology and membrane integrity of arsonoliposomes. Arsonoliposomes composed of distearoylglycerophosphocholine (DSPC), cholesterol (Chol) and the palmitoyl side chain arsonolipid (with concentrations ranging from 0 mol% [DSPC/Chol vesicles] to 53 mol% of total lipid) containing either 4 or 8 mol% DPPE-PEG2000 or DSPE-PEG2000, were prepared by sonication. Arsonoliposome membrane integrity was evaluated by measuring the retention of encapsulated calcein in vesicles (during incubation in buffer or fetal calf serum [FCS]) while physical stability was evaluated by measuring vesicle dispersion turbidity (during incubation in water or CaCl(2)). Vesicle morphology was studied by cryo-electron microscopy. Experimental results show that: (i) PEG-lipids are incorporated in arsonoliposomes (as confirmed by the vesicle zeta potential modulation), (ii) pegylation of arsonoliposomes prevents their aggregation and fusion in the presence of calcium ions and (iii) when 8 mol% of PEG-DSPE is incorporated in arsonoliposomes based on their arsonolipid content, two groups of pegylated vesicles are formed: low content arsonoliposomes (<20 mol% arsonolipid) which are highly leaky and high content arsonoliposomes (>27 mol% arsonolipid) which are highly stable (70% calcein retention after 24h incubation in fetal calf serum [FCS]). In addition to high membrane integrity, the high content pegylated arsonoliposomes are morphologically perfect round-shaped vesicles without the sharp edges typically observed with non-pegylated DSPC-containing arsonoliposomes.

Transmembrane peptides stabilize inverted cubic phases in a biphasic length-dependent manner: implications for protein-induced membrane fusion

WALP peptides consist of repeating alanine-leucine sequences of different lengths, flanked with tryptophan "anchors" at each end. They form membrane-spanning alpha-helices in lipid membranes, and mimic protein transmembrane domains. WALP peptides of increasing length, from 19 to 31 amino acids, were incorporated into N-monomethylated dioleoylphosphatidylethanolamine (DOPE-Me) at concentrations up to 0.5 mol % peptide. When pure DOPE-Me is heated slowly, the lamellar liquid crystalline (L(alpha)) phase first forms an inverted cubic (Q(II)) phase, and the inverted hexagonal (H(II)) phase at higher temperatures. Using time-resolved x-ray diffraction and slow temperature scans (1.5 degrees C/h), WALP peptides were shown to decrease the temperatures of Q(II) and H(II) phase formation (T(Q) and T(H), respectively) as a function of peptide concentration. The shortest and longest peptides reduced T(Q) the most, whereas intermediate lengths had weaker effects. These findings are relevant to membrane fusion because the first step in the L(alpha)/Q(II) phase transition is believed to be the formation of fusion pores between pure lipid membranes. These results imply that physiologically relevant concentrations of these peptides could increase the susceptibility of biomembrane lipids to fusion through an effect on lipid phase behavior, and may explain one role of the membrane-spanning domains in the proteins that mediate membrane fusion.

Phosphatidylcholine structure determines cholesterol solubility and lipid polymorphism

In the present work, we demonstrate that phosphatidylcholine with (16:1)9 acyl chains undergoes polymorphic rearrangements in mixtures with 0.6-0.8 mol fraction cholesterol. Studies were performed using differential scanning calorimetry, X-ray diffraction, cryo-electron microscopy, 31P NMR static powder patterns and 13C MAS/NMR. Mixtures of phosphatidylcholine with (16:1)9 acyl chains and 0.6 mol fraction cholesterol, after being heated to 100 degrees C, can form an ordered array with periodicity 14 nm which may be indicative of a cubic phase. Our results indicate that the formation of highly curved bilayer structures, such as those required for membrane fusion, can occur in mixtures of cholesterol with certain phosphatidylcholines that do not form non-lamellar structures in the absence of cholesterol. We also determine the polymorphic behavior of mixtures of symmetric phosphatidylcholines with cholesterol. Species of phosphatidylcholine with (20:1)11, (22:1)13 or (24:1)15 acyl chains in mixtures with 0.6-0.8 mol fraction cholesterol undergo a transition to the hexagonal phase at temperatures 70-80 degrees C. This is not the case for phosphatidylcholine with (18:1)6 acyl chains which remains in the lamellar phase up to 100 degrees C when mixed with as much as 0.8 mol fraction cholesterol. Thus, the polymorphic behavior of mixtures of phosphatidylcholine and cholesterol is not uncommon and is dependent on the intrinsic curvature of the phospholipid. Crystals of cholesterol can be detected in mixtures of all of these phosphatidylcholines at sufficiently high cholesterol mole fraction. What is unusual about the formation of these crystals in several cases is that cholesterol crystals are present in the monohydrate form in preference to the anhydrous form. Furthermore, after heating to 100 degrees C and recooling, the cholesterol crystals are again observed to be in the monohydrate form, although pure cholesterol crystals require many hours to rehydrate after being heated to 100 degrees C. Both the nature of the acyl chain as well as the mole fraction cholesterol determine whether cholesterol crystals in mixtures with the phospholipids will be in the monohydrate or in the anhydrous form.

Particulate and Bulk Bicontinuous Cubic Phases Obtained from Mixtures of Glyceryl Monooleate and Copolymers Bearing Blocks of Lipid−Mimetic Anchors in Water

Copolymers based on poly(ethylene glycol) bearing one or more lipid-mimetic anchors were mixed with glycerylmonooleate (GMO)-a lipid with nonlamellar propensity-to form bulk and particulate bicontinuous cubic phases in water. The particulate phase was obtained via a liquid precursor method. Three forms of copolymer/GMO mixtures were investigated-precursor dispersions in glycerol and bulk and particulate phases in water-by visual observations, dynamic light scattering (DLS), and cryogenic transmission electron microscopy (cryo-TEM). The bulk phases were found to very slowly develop a macroscopic appearance that can be associated with the bicontinuous cubic phase. They were prepared in a slight excess of water, which became opalescent in some of the preparations. Cryo-TEM investigation of the excess showed that vesicles and particles with a dense interior coexisted. The precursors were prepared as solutions in glycerol. The viscous liquid material was investigated by DLS. Diffusion coefficients and the corresponding hydrodynamic radii, ranging from about 10 to 30 nm, were calculated. The particles are presumably of a structure similar to that of conventional emulsion droplets with GMO in the interior and copolymer molecules in the outer regions. The particulate phase in water was obtained upon hydration of the liquid precursors. The dispersions were investigated by DLS and cryo-TEM. DLS revealed the formation of nanosized particles. The size was found to increase with increasing copolymer content for copolymers with only one lipid-mimetic anchor, whereas the opposite trend was observed for the formulations with copolymers bearing more than one lipid-mimetic anchor. The shape and interior of the particles were studied by cryo-TEM. It was found that most particles were globular. For some of the compositions, particles with a dense internal structure dominated. The texture of the internal structures was assigned to dispersed bicontinuous cubic or L3 phases. In other compositions, the interior seemingly consists of arrays of interlamellar attachments.

Physical Stability of Sonicated Arsonoliposomes: Effect of Calcium Ions

The physical stability of sonicated arsonoliposomes in the absence and presence of Ca(2+) ions is evaluated. Cholesterol-containing arsonoliposomes composed of arsonolipids [having different acyl chains (C(12)-C(18))], or mixtures of arsonolipids with phospholipids (phosphatidylcholine or distearoyl-phosphatidylcholine) were prepared, and physical stability was evaluated in the absence and presence of CaCl(2), by vesicle dispersions turbidity measurements and cryo-electron microscopy morphological assessment. In some cases, vesicle zeta-potential was measured, under identical conditions. Results demonstrate that self-aggregation of the vesicles studied is low and influenced by the acyl chain length of the arsonolipid used, whereas calcium-induced aggregation is higher, correlating well with the decreased values of vesicle zeta-potential in the presence of Ca(2+) ions (weaker electrostatic repulsion). Acyl chain length of arsonolipids used has a significant quantitative effect on Ca(2+)-induced vesicle aggregation mainly for arsonoliposomes that contain phospholipids (mixed), compared with the vesicles that consist of plain arsonolipids (significant effect only for initial aggregation at time 0). Another difference between plain and mixed arsonoliposomes is that for mixed arsonoliposomes Ca(2+)-induced increases in turbidity are irreversible by ethylenediaminotetraacetic acid, suggesting that vesicle fusion is taking place. This was confirmed by cryo-electron microscopy observations. Finally, when phosphatidylcholine is replaced by distearoyl-phosphatidylcholine, arsonoliposomes are more stable in terms of self-aggregation, but in the presence of calcium, the turbidity and morphology results are similar.

Mechanism of formation of multilayered 2D crystals of the Enzyme IIC-mannitol transporter

We have recently reported the crystallization by reconstitution into lipid bilayer structures of Enzyme IIC(mtl), the transmembrane C-domain of the mannitol transporter from E. coli. The projected structure was determined to a resolution of 0.5 nm [J. Mol. Biol. 287 5 (1999) 845]. However, further investigation proved that these crystals were multilamellar stacks instead of 2D crystals, and therefore were unsuitable for three-dimensional structural analysis by electron crystallography. Understanding the crystallogenesis of these crystals could reveal the mechanism of formation of multilayers. In the present study, cryo-electron microscopy (cryo-EM) and turbidimetry are used to study the successive steps of reconstitution of Enzyme IIC(mtl) into phospholipid-containing structures and its crystallization under different conditions. Our experimental approach enabled us to distinguish the separate steps of reconstitution and crystallization. The salt concentration especially influenced the nature of the vesicles, either half open unilamellar or aggregated multilamellar, formed during reconstitution of Enzyme IIC(mtl). The presence of DOPE and DOPC and the temperature influenced the type of lipid structures that were formed during the crystallization phase of Enzyme IIC(mtl). Cryo-EM showed that protein crystallization is closely associated with the formation of isotropic lipid (cubic) phases. We believe that DOPE is responsible for the formation of these lipid cubic phases, and that crystallization is driven by exclusion of protein from these phases and its concentration into the lamellar phases. This mechanism is inextricably associated with the formation of multilayers.

Vesicle formation and other structures in aqueous dispersions of monoolein and sodium oleate

The macroscopic appearance and microstructure in the dilute corner of the ternary monoolein (MO)-sodium oleate (NaO)-water (2H2O) system have been investigated by visual inspection and by using direct structural imaging with light microscopy and cryogenic transmission electron microscopy. The microstructural transformations that take place between the micellar phase (binary NaO2H2O axis) and the dispersed cubic phase (binary MO2H2O axis) upon increasing the ratio of MO to NaO are micelles, ruptured multilamellar vesicles together with flexible threads, various vesicle structures, vesicles in equilibrium with densely packed layers that either represent bilayers or domains of H(II) tubes visible from the side, and finally a pattern that may be either a cubic phase or domains of H(II) tubes visible in a cross section. Spontaneously formed uni- and multilamellar vesicles that show long-term stability are found to be the dominant structure for mixed dispersions over almost the entire concentration range. The addition of NaOH to the non-bilayer-forming system, the ternary MOOA (oleic acid)-2H2O system, leads to the formation of vesicles. Vesicles were also observed in other ternary MO-aqueous-based systems with potassium oleate, cetyltrimethylammonium bromide, sodium taurodeoxycholate, or dipalmitoylphosphatidylcholine instead of NaO.

Transdermal delivery of pergolide from surfactant-based elastic and rigid vesicles: characterization and in vitro transport studies

PURPOSE

The aim of this study was to investigate the effect of elastic and rigid vesicles on the penetration of pergolide across human skin.

METHODS

Vesicles used consisted of the bilayer-forming surfactant L-595 (sucrose laurate ester) and the micelle-forming surfactant PEG-8-L (octaoxyethylene laurate ester), together with the stabilizer sulfosuccinate. A series of L-595/PEG-8-L/sulfosuccinate vesicles were investigated, ranging from very rigid to very elastic. Pergolide-loaded elastic and rigid vesicles were visualized using Cryo-TEM and characterized for size and stability. Transdermal penetration of pergolide from different vesicle compositions was studied in vitro using flow-through Franz diffusion cells. A saturated buffer solution served as the control.

RESULTS

Vesicle composition had a major effect on the physicochemical characteristics, morphology and drug solubility of the vesicular system. L-595/PEG-8-L/sulfosuccinate (70/30/5) elastic vesicles gave the best balance between vesicle stability and elasticity, as well as the highest drug solubility. Transport studies clearly showed that elastic vesicles were superior to rigid vesicles. Elastic vesicles enhanced the drug transport compared to the buffer control, although rigid vesicles decreased the drug transport. The best drug transport was achieved from L-595/PEG-8-L/sulfosuccinate (70/30/5) elastic vesicles, resulting in a steady-state flux of 13.6 +/- 2.3 ng/ (h*cm2). This was a 6.2-fold increase compared to the most rigid vesicles.

CONCLUSIONS

This study supports the hypothesis that elastic vesicles are superior to rigid vesicles as vehicles for transdermal drug delivery.

Electron microscopic investigation of the morphology and calcium-induced fusion of lipid vesicles with an oligomerised inner leaflet

The lipid head groups in the inner leaflet of unilamellar bilayer vesicles of the synthetic lipids DHPBNS and DDPBNS can be selectively oligomerised. Earlier studies have established that these vesicles fuse much slower and less extensively upon oligomerisation of the lipid head groups. The morphology and calcium-induced fusion of vesicles of DHPBNS and DDPBNS were investigated using cryo-electron microscopy. DHPBNS vesicles are not spherical but flattened, ellipsoidal structures. Upon addition of CaCl(2), DHPBNS vesicles with an oligomerised inner leaflet were occasionally observed in an arrested hemifused state. However, the evidence for hemifusion is not equivocal due to potential artefacts of sample preparation. DDPBNS vesicles show the expected spherical morphology. Upon addition of excess CaCl(2), DDPBNS vesicles fuse into dense aggregates that show a regular spacing corresponding to the bilayer width. Upon addition of EDTA, the aggregates readily disperse into large unilamellar vesicles. At low concentration of calcium ion, DDPBNS vesicles with an oligomerised inner leaflet form small multilamellar aggregates, in which a spacing corresponding to the bilayer width appears. Addition of excess EDTA results in slow dispersal of the Ca2+-lipid aggregates into a heterogeneous mixture of bilamellar, spherical vesicles and networks of thread-like vesicles. These lipid bilayer rearrangements are discussed within the context of shape transformations and fusion of lipid membranes.

Interactions of the HIV-1 fusion peptide with large unilamellar vesicles and monolayers. A cryo-TEM and spectroscopic study

We have examined the interaction of the human immunodeficiency virustype 1 fusion peptide (23 amino acid residues) and of a Trp-containing analog with vesicles composed of dioleoylphosphatidylcholine, dioleoylphosphatidylethanolamine and cholesterol (molar ratio, 1:1:1). Both the native and the Trp-substituted peptides bound the vesicles to the same extent and induced intervesicular lipid mixing with comparable efficiency. Infrared reflection-absorption spectroscopy data are compatible with the adoption by the peptide of a main beta-sheet structure in a cospread lipid/peptide monolayer. Cryo-transmission electron microscopy observations of peptide-treated vesicles reveal the existence of a peculiar morphology consisting of membrane tubular elongations protruding from single vesicles. Tryptophan fluorescence quenching by brominated phospholipids and by water-soluble acrylamide further indicated that the peptide penetrated into the acyl chain region closer to the interface rather than into the bilayer core. We conclude that the differential partition and shallow penetration of the fusion peptide into the outer monolayer of a surface-constrained bilayer may account for the detected morphological effects. Such single monolayer-restricted interaction and its structural consequences are compatible with specific predictions of current theories on viral fusion.

X-ray diffraction study of feline leukemia virus fusion peptide and lipid polymorphism

The structural effects of the fusion peptide of feline leukemia virus (FeLV) on the lipid polymorphism of N-methylated dioleoylphosphatidylethanolamine were studied using a temperature ramp with sequential X-ray diffraction. This peptide, the hydrophobic amino-terminus of p15E, has been proven to be fusogenic and to promote the formation of highly curved, intermediate structures on the lamellar liquid-crystal to inverse hexagonal phase transition pathway. The FeLV peptide produces marked effects on the thermotropic mesomorphic behaviour of MeDOPE, a phospholipid with an intermediate spontaneous radius of curvature. The peptide is shown to reduce the lamellar repeat distance of the membrane prior to the onset of an inverted cubic phase. This suggests that membrane thinning may play a role in peptide-induced membrane fusion and strengthens the link between the fusion pathway and inverted cubic phase formation. The results of this study are interpreted in relation to models of the membrane fusion mechanism.

Lipid vesicles and membrane fusion

Membrane fusion is essential for cell survival and has attracted a great deal of both theoretical and experimental interest. Fluorescence (de)quenching measurements were designed to distinguish between bilayermerging and vesicle-mixing. Theoretical studies and various microscopic and diffraction methods have elucidated the mechanism of membrane fusion. These have revealed that membrane proximity and high defect density in the adjacent bilayers are the only prerequisites for fusion. Intermediates, such as stalk or inverse micellar structures can, but need not, be involved in vesicle fusion. Nonlamellar phase creation is accompanied by massive membrane fusion although it is not a requirement for bilayer merging. Propensity for membrane fusion is increased by increasing the local membrane disorder as well by performing manipulations that bring bilayers closer together. Membrane rigidification and enlarged bilayer separation opposes this trend. Membrane fusion is promoted by defects created in the bilayer due to the vicinity of lipid phase transition, lateral phase separation or domain generation, high local membrane curvature, osmotic or electric stress in or on the membrane; the addition of amphiphats or macromolecules which insert themselves into the membrane, freezing or other mechanical membrane perturbation have similar effects. Lowering the water activity by the addition of water soluble polymers or by partial system dehydration invokes membrane aggregation and hence facilitates fusion; as does the membrane charge neutralization after proton or other ion binding to the lipids and intermembrane scaffolding by proteins or other macromolecules. The alignment of defect rich domains and polypeptides or protein binding is pluripotent: not only does it increase the number of proximal defects in the bilayers, it triggers the vesicle aggregation and is fusogenic. Exceptions are the bound molecules that create steric or electrical barriers between the membranes which prevent fusion. Membrane fusion can be non-leaky but it is very common to lose material from the vesicle interior during the later stages of membrane unification, that is, after a few hundred microseconds following the induction of fusion.

Physicochemical characterization and purification of cationic lipoplexes

Cationic lipid-nucleic acid complexes (lipoplexes) consisting of dioleoyltrimethylammoniumpropane (DOTAP) liposomes and plasmid DNA were prepared at various charge ratios (cationic group to nucleotide phosphate), and the excess component was separated from the lipoplex. We measured the stoichiometry of the lipoplex, noted its colloidal properties, and observed its morphology and structure by electron microscopy. The colloidal properties of the lipoplexes were principally determined by the cationic lipid/DNA charge ratio and were independent of the lipid composition. In lipoplexes, the lipid membranes as observed in freeze-fracture electron microscopy were deformed into high-radius-of-curvature features whose characteristics depended on the lipid composition. Lipoplexes prepared at a threefold or greater excess of either DOTAP or DNA could be resolved into complexes with a defined stoichiometry and the excess component by sedimentation to equilibrium on sucrose gradients. The separated, positively charged complex retained high transfection activity and had reduced toxicity. The negatively charged lipoplex showed increased transfection activity compared to the starting mixture. In cryoelectron micrographs the positively charged complex was spherical and contained a condensed but indistinct interior structure. In contrast, the separated negatively charged lipoplexes had a prominent internal 5.9 +/- 0.1-nm periodic feature with material projecting as spikes from the spherical structure into the solution. It is likely that these two lipoplexes represent structures with different lipid and DNA packing.

The modified stalk mechanism of lamellar/inverted phase transitions and its implications for membrane fusion

A model of the energetics of lipid assemblies (Siegel. 1993. Biophys. J. 65:2124-2140) is used to predict the relative free energy of intermediates in the transitions between lamellar (Lalpha) inverted hexagonal (HII), and inverted cubic (QII) phases. The model was previously used to generate the modified stalk theory of membrane fusion. The modified stalk theory proposes that the lowest energy structures to form between apposed membranes are the stalk and the transmonolayer contact (TMC), respectively. The first steps in the Lalpha/HII and Lalpha/QII phase transitions are also intermembrane events: bilayers of the Lalpha phase must interact to form new topologies during these transitions. Hence the intermediates in these phase transitions should be similar to the intermediates in the modified stalk mechanism of fusion. The calculations here show that stalks and TMCs can mediate transitions between the Lalpha, QII, and HII phases. These predictions are supported by studies of the mechanism of these transitions via time-resolved cryoelectron microscopy (. Biophys. J. 66:402-414; Siegel and Epand. 1997. Biophys. J. 73:3089-3111), whereas the predictions of previously proposed transition mechanisms are not. The model also predicts that QII phases should be thermodynamically stable in all thermotropic lipid systems. The profound hysteresis in Lalpha/QII transitions in some phospholipid systems may be due to lipid composition-dependent effects other than differences in lipid spontaneous curvature. The relevant composition-dependent properties are the Gaussian curvature modulus and the membrane rupture tension, which could change the stability of TMCs. TMC stability also influences the rate of membrane fusion of apposed bilayers, so these two properties may also affect the fusion rate in model membrane and biomembrane systems. One way proteins catalyze membrane fusion may be by making local changes in these lipid properties. Finally, although the model identifies stalks and TMCs as the lowest energy intermembrane intermediates in fusion and lamellar/inverted phase transitions, the stalk and TMC energies calculated by the present model are still large. This suggests that there are deficiencies in the current model for intermediates or intermediate energies. The possible nature of these deficiencies is discussed.

Membrane fusion in vesicles of oligomerizable lipids

Membrane fusion has been examined in a model system of small unilamellar vesicles of synthetic lipids that can be oligomerized through the lipid headgroups. The oligomerization can be induced either in both bilayer leaflets or in the inner leaflet exclusively. Oligomerization leads to denser lipid headgroup packing, with concomitant reduction of lipid lateral diffusion and membrane permeability. As evidenced by lipid mixing assays, electron microscopy, and light scattering, calcium-induced fusion of the bilayer vesicles is strongly retarded and inhibited by oligomerization. Remarkably, oligomerization of only the inner leaflet of the bilayer is already sufficient to affect fusion. The efficiency of inhibition and retardation of fusion critically depend on the relative amount of oligomeric lipid present, on the concentration of calcium ions, and on temperature. Implications for the mechanism of bilayer membrane fusion are discussed in terms of lipid lateral diffusion and membrane curvature effects.

The mechanism of lamellar-to-inverted hexagonal phase transitions in phosphatidylethanolamine: implications for membrane fusion mechanisms

We studied the mechanism of the lamellar-to-inverted hexagonal (L alpha/H[II]) phase transition, using time-resolved cryotransmission electron microscopy (TRC-TEM), 31P-NMR, and differential scanning calorimetry. The transition was initiated in dispersions of large unilamellar vesicles of dipalmitoleoyl phosphatidylethanolamine (DiPoPE). We present evidence that the transition proceeds in three steps. First, many small connections form between apposed membranes. Second, the connections aggregate within the planes of the bilayers, forming arrays with hexagonal order in some projections. Third, these quasihexagonal structures elongate into small domains of H(II) phase, acquiring lipid molecules by diffusion from contiguous bilayers. A previously proposed membrane fusion mechanism rationalizes these results. The modified stalk theory predicts that the L alpha/H(II) phase transition involves some of the same intermediate structures as membrane fusion. The small interbilayer connections observed via TRC-TEM are compatible with the structure of a critical intermediate in the modified stalk mechanism: the trans monolayer contact (TMC). The theory predicts that 1) TMCs should form starting at tens of degrees below TH; 2) when TMCs become sufficiently numerous, they should aggregate into transient arrays like the quasihexagonal arrays observed here by TRC-TEM; and 3) these quasihexagonal arrays can then elongate directly into H(II) phase domains. These predictions rationalize the principal features of our data, which are incompatible with the other transition mechanisms proposed to date. Thus these results support the modified stalk mechanism for both membrane fusion and the L alpha/H(II) phase transition. We also discuss some implications of the modified stalk theory for fusion in protein-containing systems. Specifically, we point out that recent data on the effects of hydrophobic peptides and viral fusion peptides on lipid phase behavior are consistent with an effect of the peptides on TMC stability.

Morphological changes induced by phospholipase C and by sphingomyelinase on large unilamellar vesicles: a cryo-transmission electron microscopy study of liposome fusion

Cryo-transmission electron microscopy has been applied to the study of the changes induced by phospholipase C on large unilamellar vesicles containing phosphatidylcholine, as well as to the action of sphingomyelinase on vesicles containing sphingomyelin. In both cases vesicle aggregation occurs as the earliest detectable phenomenon; later, each system behaves differently. Phospholipase C induces vesicle fusion through an intermediate consisting of aggregated and closely packed vesicles (the "honeycomb structure") that finally transforms into large spherical vesicles. The same honeycomb structure is also observed in the absence of enzyme when diacylglycerols are mixed with the other lipids in organic solution, before hydration. In this case the sample then evolves toward a cubic phase. The fact that the same honeycomb intermediate can lead to vesicle fusion (with enzyme-generated diacylglycerol) or to a cubic phase (when diacylglycerol is premixed with the lipids) is taken in support of the hypothesis according to which a highly curved lipid structure ("stalk") would act as a structural intermediate in membrane fusion. Sphingomyelinase produces complete leakage of vesicle aqueous contents and an increase in size of about one-third of the vesicles. A mechanism of vesicle opening and reassembling is proposed in this case.

Acid-catalyzed plasmenylcholine hydrolysis and its effect on bilayer permeability: a quantitative study

This laboratory has previously shown (Anderson, V.C. and Thompson, D.H. (1992) Biochim. Biophys. Acta 1109, 33-42; Thompson, D.H., Gerasimov, O.V., Wheeler, J.J., Rui, Y. and Anderson, V.C. (1996) Biochim. Biophys. Acta 1279, 25-34), that plasmenylcholine (1-alk-1'-enyl-2-palmitoyl-sn-glycero-3-phosphocholine; PlsPamCho) liposomes release hydrophilic contents upon photooxidation or acid-catalyzed hydrolysis. We now report the kinetics and chemical mechanism of the acid-catalyzed reaction and its effect on calcein leakage rates. Hydrolysis of the plasmenylcholine vinyl ether linkage generates fatty aldehydes and 1-hydroxy-2-palmitoyl-sn-glycero-3-phosphocholine (lysolipid); HPLC and 1H-NMR experiments establish that the former is readily air-oxidized to fatty acids, while the latter undergoes rapid acid-catalyzed rearrangement to 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine. Lysolipid formation obeys first order kinetics, yielding observed pseudo-first order rate constants that are pH-dependent. Bimolecular hydrolysis rate constants, k(bi), have also been determined. Calcein release rates from plasmenylcholine liposomes are strongly dependent on both the dihydrocholesterol (DHC) content and the extent of PlsPamCho hydrolysis within the bilayer. DHC-free plasmenylcholine liposomes (38 degrees C, pH 2.5) require < 5% PlsPamCho hydrolysis to effect > 50% calcein release within 10 min. The presence of > or = 25 mol% DHC, however, greatly reduces the observed calcein release rate; nearly 30% PlsPamCho hydrolysis is required to effect 50% calcein release over a 70-min period in 6:4 PlsPamCho/DHC liposomes. Bacteriochlorophyll a-sensitized photooxidation of plasmenylcholine liposomes also produces fatty aldehyde and another intermediate, tentatively described as 1-formyl-2-palmitoyl-sn-glycero-3-phosphocholine, that hydrolyzes to form the 1-hydroxy lysolipid. These results have important implications for the quantitative description of lysolipid effects on membrane permeability and on the design of triggerable liposomes for drug delivery.

Topological properties of two cubic phases of a phospholipid:cholesterol:diacylglycerol aqueous system and their possible implications in the phospholipase C-induced liposome fusion

Water dispersions of phospholipid:cholesterol:diacylglycerol may, under certain conditions, originate either the lipid- and water-permeable Q224 cubic phase, or the lipid-permeable but water-impermeable Q227 cubic phase. These results are discussed within the framework of the phospholipase C-induced fusion of liposomes [Nieva et al. (1993) Biochemistry 32, 1054]. It is suggested that the cubic phases Q224 and Q227 represent two classes of lipid organization, one promoting, the other hindering the mixing of aqueous contents that is characteristic of membrane fusion. In this context, inverted micelles appear to be the end point of the fusion process, rather than fusion intermediates.

Complexes between cationic liposomes and DNA visualized by cryo-TEM

The association structures formed by cationic liposomes and DNA-plasmids have been successfully employed as gene carriers in transfection assays. In the present study such complexes was studied by cryo-TEM (cryo-transmission electron microscopy). Cationic liposomes made up by DOPE (dioleoylphosphatidylethanolamine) and various amounts of three different cationic surfactants were investigated. The cryo-TEM analysis suggests that an excess of lipid in terms of charge, leads to entrapment of the DNA molecules between the lamellas in clusters of aggregated multilamellar structures. With increasing amounts of DNA free or loosely bound plasmids were found in the vicinity of the complexes. The importance of the choice of surfactant, as reported from many transfection assays, was not reflected in changes of the type of DNA-vesicle association. A tendency towards polymorphism of the lipid mixtures is reported and its possible implications are discussed.

Effect of vesicle composition and curvature on the dissociation of phosphatidic acid in small unilamellar vesicles--a 31P-NMR study

Sonicated small unilamellar vesicles (SUVs) containing phosphatidic acid (PA) give two PA 31P-NMR resonances corresponding to PA molecules in the inner and outer leaflets of the bilayer. This NMR differentiation between the two monolayers is not due to a pH gradient across the membrane but instead reflects differential packing in the inner and outer leaflets imposed by the highly curved SUV surface. The apparent pKa of the outer-leaflet PA increases with decreasing surface curvature and with increasing PA content. The estimated relationship between the apparent pKa of the outer-leaflet PA headgroup and vesicle curvature may provide a qualitative probe for effects related to surface curvature in these model-membrane systems.

The mechanism of lamellar-to-inverted hexagonal phase transitions: a study using temperature-jump cryo-electron microscopy

The lamellar/inverted hexagonal (L alpha/HII) phase transition can be very fast, despite the drastic change in the topology of the lipid/water interfaces. The first structures to form in this transition may be similar to those that mediate membrane fusion in many lipid systems. To study the transition mechanism and other dynamic phenomena in membrane dispersions, we constructed an apparatus to rapidly trigger the transition and then vitrify the specimens to preserve the structure of transient intermediates. The apparatus applies millisecond-long temperature jumps of variable size to aqueous dispersions of lipids on electron microscope grids at times 9-16 ms before specimen vitrification. The vitrified specimens are then examined by cryo-transmission electron microscopy. Dispersions of egg phosphatidylethanolamine completed the transition within 9 ms when superheated by 20 K. Similar transition times have been observed in dioleoylphosphatidylethanolamine via time-resolved x-ray diffraction. N-monomethylated dioleoylphosphatidylethanolamine dispersions superheated to lesser extent exhibited slower transitions and more complex morphology. The structure of the first intermediates to form in the transition process could not be determined, probably because the intermediates are labile on the time scale of sample cooling and vitrification (< 1 ms) and because of the poor contrast developed by some of these small structures. However, the results are more compatible with a transition mechanism based on "stalk" intermediates than a mechanism involving inverted micellar intermediates. Temperature-jump cryo-transmission electron microscopy should be useful in studying dynamic phenomena in biomembranes, large protein complexes, and other colloidal dispersions. It should be especially helpful in studying the mechanism of protein-induced membrane fusion.