Fusion of dipalmitoylphosphatidylcholine vesicles

Azide-Modified Membrane Lipids: Miscibility with Saturated Phosphatidylcholines

In this study, we describe the miscibility of four azide-modified membrane phospholipids (azidolipids) with conventional phospholipids. The azidolipids bear an azide group at different positions of the sn-1 or sn-2 alkyl chain and they further differ in the type of linkage (ester vs ether) of the sn-2 alkyl chain. Investigations regarding the miscibility of the azidolipids with bilayer-forming phosphatidylcholines will evaluate lipid mixtures that are suitable for the production of stable azidolipid-doped liposomes. These vesicles then serve as model membranes for the incorporation of model peptides or proteins in the future. The miscibility of both types of phospholipids was studied by calorimetric assays, electron microscopy, small-angle X-ray scattering, infrared spectroscopy, and dynamic light scattering to provide a complete biophysical characterization of the mixed systems.

Highly Stable, Ultrasmall Polymer-Grafted Nanobins (usPGNs) with Stimuli-Responsive Capability

Highly stable and stimuli/pH-responsive ultrasmall polymer-grafted nanobins (usPGNs) have been developed by grafting a small amount (10 mol %) of short (4.3 kDa) cholesterol-terminated poly(acrylic acid) (Chol-PAA) into an ultrasmall unilamellar vesicle (uSUV). The usPGNs are stable against fusion and aggregation over several weeks, exhibiting over 10-fold enhanced cargo retention in biologically relevant media at pH 7.4 in comparison with the parent uSUV template. Coarse-grained molecular dynamics (CGMD) simulations confirm that the presence of the cholesterol moiety can greatly stabilize the lipid bilayer. They also show extended PAA chain conformations that can be interpreted as causing repulsion between colloidal particles, thus stabilizing them against fusion. Notably, CGMD predicted a clustering of the Chol-PAA chains on the lipid bilayer under acidic conditions due to intra- and interchain hydrogen bonding, leading to the destabilization of local membrane areas. This explains the experimental observation that usPGNs can be triggered to release a significant amount of cargo upon acidification to pH 5. These developments put the lipid-bilayer-embedded Chol-PAA in stark contrast with traditional poly(acrylic acid) systems where the molar mass (Mn) of the polymer chains must exceed 16.5 kDa to achieve stimuli-responsive changes in conformation. They also distinguish the small usPGNs from the much-larger polymer-caged nanobin platform where the Chol-PAA chains must be covalently cross-linked to engender stimuli-responsive behaviors.

Azide-Modified Membrane Lipids: Synthesis, Properties, and Reactivity

In the present work, we describe the synthesis and the temperature-dependent behavior of photoreactive membrane lipids as well as their capability to study peptide/lipid interactions. The modified phospholipids contain an azide group either in the middle part or at the end of an alkyl chain and also differ in the linkage (ester vs ether) of the second alkyl chain. The temperature-dependent aggregation behavior of the azidolipids was studied using differential scanning calorimetry (DSC), Fourier-transform infrared (FTIR) spectroscopy, and small-angle X-ray scattering (SAXS). Aggregate structures were visualized by stain and cryo transmission electron microscopy (TEM) and were further characterized by dynamic light scattering (DLS). We show that the position of the azide group and the type of linkage of the alkyl chain at the sn-2 position of the glycerol influences the type of aggregates formed as well as their long-term stability: P10AzSPC and r12AzSHPC show the formation of extrudable liposomes, which are stable in size during storage. In contrast, azidolipids that carry a terminal azido moiety either form extrudable liposomes, which show time-dependent vesicle fusion (P15AzPdPC), or self-assemble in large sheet-like, nonextrudable aggregates (r15AzPdHPC) where the lipid molecules are arranged in an interdigitated orientation at temperatures below T_m (L_βI phase). Finally, a P10AzSPC:DMPC mixture was used for photochemically induced cross-linking experiments with a transmembrane peptide (WAL-peptide) to demonstrate the applicability of the azidolipids for the analysis of peptide/lipid interactions. The efficiency of photo-cross-linking was monitored by attenuated total reflection infrared (ATR-IR) spectroscopy and mass spectrometry (MS).

Lipid nanoparticle delivery systems for siRNA-based therapeutics

Therapeutics based on small interfering RNA (siRNA) have a huge potential for the treatment of disease but requires sophisticated delivery systems for in vivo applications. Lipid nanoparticles (LNP) are proven delivery systems for conventional small molecule drugs with over eight approved LNP drugs. Experience gained in the clinical development of LNP for the delivery of small molecules, combined with an understanding of the physical properties of lipids, can be applied to design LNP systems for in vivo delivery of siRNA. In particular, cationic lipids are required to achieve efficient encapsulation of oligonucleotides; however, the presence of a charge on LNP systems can result in toxic side effects and rapid clearance from the circulation. To address these problems, we have developed ionizable cationic lipids with pKa values below 7 that allow oligonucleotide encapsulation at low pH (e.g., pH 4) and a relatively neutral surface at physiological pH. Further optimization of cationic lipids to achieve maximized endosomal destabilization following uptake has resulted in LNP siRNA systems that can silence genes in hepatocytes at doses as low as 0.005 mg siRNA/kg body weight in mouse models. These systems have been shown to be highly effective clinically, with promising results for the treatment of hypercholesterolemia and transthyretin-induced amyloidosis among others. More LNP siRNA therapeutics, targeting different tissues and diseases, are expected to become available in the near future.

Lactadherin binds to phosphatidylserine-containing vesicles in a two-step mechanism sensitive to vesicle size and composition

Lactadherin binds to phosphatidylserine (PS) in a stereospecific and calcium independent manner that is promoted by vesicle curvature. Because membrane binding of lactadherin is supported by a PS content of as little as 0.5%, lactadherin is a useful marker for cell stress where limited PS is exposed, as well as for apoptosis where PS freely traverses the plasma membrane. To gain further insight into the membrane-binding mechanism, we have utilized intrinsic lactadherin fluorescence. Our results indicate that intrinsic fluorescence increases and is blue-shifted upon membrane binding. Stopped-flow kinetic experiments confirm the specificity for PS and that the C2 domain contains a PS recognition motif. The stopped-flow kinetic data are consistent with a two-step binding mechanism, in which initial binding is followed by a slower step that involves either a conformational change or an altered degree of membrane insertion. Binding is detected at concentrations down to 0.03% PS and the capacity of binding reaches saturation around 1% PS (midpoint 0.15% PS). Higher concentrations of PS (and also to some extent PE) increase the association kinetics and the affinity. Increasing vesicle curvature promotes association. Remarkably, replacement of vesicles with micelles destroys the specificity for PS lipids. We conclude that the vesicular environment provides optimal conditions for presentation and recognition of PS by lactadherin in a simple binding mechanism. This article is part of a Special Issue entitled: Protein Folding in Membranes.

Inhibition of mitochondrial fusion by α-synuclein is rescued by PINK1, Parkin and DJ-1

Aggregation of α-synuclein (αS) is involved in the pathogenesis of Parkinson's disease (PD) and a variety of related neurodegenerative disorders. The physiological function of αS is largely unknown. We demonstrate with in vitro vesicle fusion experiments that αS has an inhibitory function on membrane fusion. Upon increased expression in cultured cells and in Caenorhabditis elegans, αS binds to mitochondria and leads to mitochondrial fragmentation. In C. elegans age-dependent fragmentation of mitochondria is enhanced and shifted to an earlier time point upon expression of exogenous αS. In contrast, siRNA-mediated downregulation of αS results in elongated mitochondria in cell culture. αS can act independently of mitochondrial fusion and fission proteins in shifting the dynamic morphologic equilibrium of mitochondria towards reduced fusion. Upon cellular fusion, αS prevents fusion of differently labelled mitochondrial populations. Thus, αS inhibits fusion due to its unique membrane interaction. Finally, mitochondrial fragmentation induced by expression of αS is rescued by coexpression of PINK1, parkin or DJ-1 but not the PD-associated mutations PINK1 G309D and parkin Δ1-79 or by DJ-1 C106A.

Lipid vesicle aggregation induced by cooling

Lipid bilayer fusion is a complex process requiring several intermediate steps. Initially, the two bilayers are brought into close contact following removal of intervening water layers and overcoming electrostatic repulsions between opposing bilayer head groups. In this study we monitor by light scattering the reversible aggregation of phosphatidylcholine single shell vesicles during which adhesion occurs but stops prior to a fusion process. Light scattering measurements of dimyristoyl-sn-glycero-3-phosphocholine (DMPC), dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) in water show that lowering the temperature of about 0.14 micron single shell vesicles of DPPC (from 20 °C to 5 °C) and about 2 micron vesicles of DSPC (from 20 °C to 15 °C), but not of 1 micron vesicles of DMPC, results in extensive aggregation within 24 hours that is reversible by an increase in temperature. Aggregation of DSPC vesicles was confirmed by direct visual observation. Orientation of lipid head groups parallel to the plane of the bilayer and consequent reduction of the negative surface charge can account for the ability of DPPC and DSPC vesicles to aggregate. Retention of negatively charged phosphates on the surface and the burial of positively charged cholines within the bilayer offer an explanation for the failure of DMPC vesicles to aggregate. Lowering the temperature of 1,2-dipalmitoyl-sn-glycero-3-phosphoserine (DPPS) vesicles from 20 °C to 5 °C failed to increase aggregation within 24 hours at Mg⁺⁺/DPPS ratios that begin to initiate aggregation and fusion.

The influence of vesicle size and composition on alpha-synuclein structure and stability

Monomeric alpha-synuclein (alphaSN), which has no persistent structure in aqueous solution, is known to bind to anionic lipids with a resulting increase in alpha-helix structure. Here we show that at physiological pH and ionic strength, alphaSN incubated with different anionic lipid vesicles undergoes a marked increase in alpha-helical content at a temperature dictated either by the temperature of the lipid phase transition, or (in 1,2-DilauroylSN-Glycero-3-[Phospho-rac-(1-glycerol)] (DLPG), which is fluid down to 0 degrees C) by an intrinsic cold denaturation that occurs around 10-20 degrees C. This structure is subsequently lost in a thermal transition around 60 degrees C. Remarkably, this phenomenon is only observed for vesicles >100 nm in diameter and is sensitive to lipid chain length, longer chain lengths, and larger vesicles giving more cooperative unfolding transitions and a greater degree of structure. For both vesicle size and chain length, a higher degree of compressibility or permeability in the lipid thermal transition region is associated with a higher degree of alphaSN folding. Furthermore, the degree of structural change is strongly reduced by an increase in ionic strength or a decrease in the amount of anionic lipid. A simple binding-and-folding model that includes the lipid phase transition, exclusive binding of alphaSN to the liquid disordered phase, the thermodynamics of unfolding, and the electrostatics of binding of alphaSN to lipids is able to reproduce the two thermal transitions as well as the effect of ionic strength and anionic lipid. Thus the nature of alphaSN's binding to phospholipid membranes is intimately tied to the lipids' physico-chemical properties.

Effect of the Nature of the 3β-Substitution in Manoyl Oxides on the Thermotropic Behavior of DPPC Lipid Bilayer and on DPPC Liposomes

Functionalized manoyl oxide derivatives have been proved over the years to evoke several biological responses. Among them, 3beta-hydroxy-manoyl oxide (1) and 3beta-acetoxy-manoyl oxide (2) have been shown to exhibit in vitro antimicrobial and cytotoxic activity, while N-imidazole-3 beta-thiocarbonyl ester of manoyl oxide (3) was found to exhibit potent cytotoxic effect. Their partitioning into phospholipid bilayers may lead to membrane structure modifications that are crucial in liposome development as they may influence their maintenance and integrity. DSC was used to study the modifications induced in DPPC bilayers by incorporating increasing concentrations of the three manoyl oxide derivatives. All derivatives were found to strongly affect the bilayer structural organization in terms of a decrease of the cooperativity, the fluidization and partially destabilization of the gel phase and the induction of a lateral phase separation in clustering domains. Derivatives 1 and 3 were incorporated into DPPC liposomes and their physicochemical stability was monitored at 4 degrees C. The stability of liposomes was strongly influenced by the presence of 1 and 3 at any molar ratio studied. DPPC/1 liposomes were found to retain its stability for 48 h at low concentration of 10% mol, while at higher concentrations up to 30% mol they collapsed into aggregated material. In all cases DPPC/3 liposomes were found unstable and sticky aggregated structures precipitated from the bulk suspension.

Effect of the polymer nature on the structural organization of lipid/polymer particle assemblies

The nano-organized LipoParticle assemblies, consisting of polymer particles coated with lipid layers, are investigated with the aim of evidencing the impact of the particle chemical nature on their physicochemical behavior. To this end, these colloidal systems are elaborated from anionic submicrometer poly(styrene) (P(St)) or poly(lactic acid) (PLA) particles, and lipid mixtures composed of zwitterionic 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) and cationic 1,2-dipalmitoyl-3-trimethylammonium-propane (DPTAP). As revealed by various experimental techniques, such as quasielastic light scattering, zeta potential measurements, transmission electron microscopy, and 1H NMR spectroscopy, the features of both LipoParticle systems are similar when cationic lipid formulations (DPPC/DPTAP mixtures) are used. This result emphasizes the major role of electrostatic interactions as driving forces in the assembly elaboration process. Conversely, the assemblies prepared only with the zwitterionic DPPC lipid are strongly dependent on the particle chemical nature. The structural characteristics of the assemblies based on PLA particles are not controlled and correspond to aggregates, contrary to P(St) particles. To understand this specific phenomenon, and to consequently improve the final organization of these assemblies which are potentially of great interest in biotechnology and biomedicine, numerous investigations are carried out such as the studies of the impact of the ionic strength and the pH of the preparation media, as well as the presence of ethanol (involved in the PLA particle synthesis) or the mean size of the lipid vesicles. From the resulting data and according to the nature of spherical solid support, hydrophobic effects, hydrogen bonds, or dipole-dipole interactions would also appear to influence the LipoParticle elaboration in the case of zwitterionic lipid formulation.

High-density encapsulation of Fe3O4 nanoparticles in lipid vesicles

We report a morphological study of the encapsulation of 12-nm Fe3O4 nanoparticles (NPs) in large unilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC). Preparation was done by reverse-phase evaporation. Phase behavior of the NP-lipid system was studied so that the loading of NPs in vesicles could be maximized. Increasing NP concentration significantly affects the resulting lipid morphology in a manner similar to increasing lipid concentration. Optimal production of high-density NP-loaded vesicles (HNLVs) requires temperatures of 50 degrees C, higher than the main phase transition (Tm) of DPPC. The formation of fully enclosed HNLVs requires incubation times of at least hours.

NMR of molecules interacting with lipids in small unilamellar vesicles

Detailed characterization of protein, peptide or drug interactions with natural membrane is still a challenge. This review focuses on the use of nuclear magnetic resonance (NMR) for the analysis of interaction of molecules with small unilamellar vesicles (SUV). These phospholipid vesicles are often used as model membranes for fluorescence or circular dichroism experiments. The various NMR approaches for studying molecule-lipid association are reviewed. After a brief survey of the SUV characterization, the use of heteronuclear NMR (phosphorous, carbon, fluorine) is described. Applications of proton NMR through transferred nuclear Overhauser effect to perform structural determination of peptide are presented. Special care is finally given to the influence of the kinetic of the interactions for the proton NMR of bound molecules in SUV, which can constitute a good model for the study of dynamical processes at the membrane surface.

Calorimetric study on the induction of interdigitated phase in hydrated DPPC bilayers by bioactive labdanes and correlation to their liposome stability

Labd-7,13-dien-15-ol (1), labd-13-ene-8alpha,15-diol (2), and labd-14-ene-8,13-diol (sclareol) have been found to exhibit cytotoxic and cytostatic effects. Their partitioning into phospholipid bilayers may induce membrane structure modifications, crucial in the development of liposomes. DSC was used to elucidate the profile of modifications induced in DPPC bilayers by incorporating increasing concentrations of the labdanes. Labdanes 1, 2 and sclareol were incorporated into SUV liposomes composed of DPPC their physicochemical stability was monitored (4 degrees C) and was compared to liposomes incorporating cholesterol. All labdanes strongly affect the bilayer organization in a concentration dependent manner in terms of a decrease of the cooperativity, the fluidization and partially destabilization of the gel phase, the induction of a lateral phase separation and the possible existence of interdigitated domains in the bilayer. The physicochemical stability of liposomes was strongly influenced by the chemical features of the labdanes. The liposomal preparations were found to retain their stability at low labdane concentration (10 mol%), while at higher concentrations up to 30 mol% a profound decrease in intact liposomes occurred, and a possible existence of interdigitated sheets was concluded.

New insights into self-organization of a model lipid mixture and quantification of its adsorption on spherical polymer particles

The adsorption of lipids onto spherical polymer colloids led to original assemblies presenting structural characteristics adjustable with the lipid formulation. The model system selected for this work involved sulfate-charged poly(styrene) submicrometer particles and zwitterionic/cationic lipid mixtures composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-3-trimethylammonium-propane (DPTAP). According to the theoretical packing parameter calculations and whatever the DPPC/DPTAP ratio, the two lipids self-assembled in aqueous media to spontaneously form vesicles. A phase transition investigation of these DPPC/DPTAP vesicles using differential scanning calorimetry revealed particular thermotropic behaviors, especially for the equimolar formulation where very strong interactions occurred between DPPC and DPTAP. Furthermore, the coating of the lipids around particles was monitored versus DPPC/DPTAP ratio by means of numerous appropriate techniques. First, a thermogravimetric analysis, providing decomposition profiles of lipid/polymer particle assemblies with temperature, was atypically carried out for such nanostructures. Then, 1H NMR spectroscopy enabled the exact DPPC/DPTAP molar ratios adsorbed on particles to be determined by differentiating both lipids. Subsequently, it also pointed out the major role of electrostatic interactions as driving forces in the assembly elaboration process. In addition to these findings, quantitative information has been collected and correlated with chemical lipid assays and permitted the statement of a lipid bilayer coverage for the assemblies prepared in water, in agreement with quasi-elastic light scattering data.

Alpha-synuclein has a high affinity for packing defects in a bilayer membrane: a thermodynamics study

A number of neurodegenerative disorders, including Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy, are characterized by the intracellular deposition of fibrillar aggregates that contain a high proportion of alpha-synuclein (alphaS). The interaction with the membrane-water interface strongly modulates folding and aggregation of the protein. The present study investigates the lipid binding and the coil-helix transition of alphaS, using titration calorimetry, differential scanning calorimetry, and circular dichroism spectroscopy. Titration of the protein with small unilamellar vesicles composed of zwitterionic phospholipids below the chain melting temperature of the lipids yielded exceptionally large exothermic heat values. The sigmoidal titration curves were evaluated in terms of a simple model that assumes saturable binding sites at the vesicle surface. The cumulative heat release and the ellipticity were linearly correlated as a result of simultaneous binding and helix folding. There was no heat release and folding of alphaS in the presence of large unilamellar vesicles, indicating that a small radius of curvature is necessary for the alphaS-membrane interaction. The heat release and the negative heat capacity of the protein-vesicle interaction could not be attributed to the coil-helix transition of the protein alone. We speculate that binding and helix folding of alphaS depends on the presence of defect structures in the membrane-water interface, which in turn results in lipid ordering in the highly curved vesicular membranes. This will be discussed with regard to a possible role of the protein for the stabilization of synaptic vesicle membranes.

Preparation and evaluation of proliposomes containing salmon calcitonin

Salmon calcitonin (sCT)-containing proliposomes were prepared by penetrating a methanol-chloroformic solution of sCT and phosphatidylcholine (PC) into microporous sorbitol particles, followed by vacuum evaporation of the solvent. As a result, sCT proliposomes with free-flowing flowability were obtained. On contact with water, the proliposomes were rapidly converted into a liposomal dispersion, in which a certain amount of sCT was entrapped by the liposomes. The apparent permeability of sCT across Caco-2 cell monolayers was increased as the result of incorporating sCT into the proliposomes, suggesting that the pharmacokinetics of sCT would be modified through the administration of proliposomes. This is the first study that reports the successful loading of sCT, a protein drug, in proliposomes. The development of various dosage forms of sCT, especially solid dosage forms, appears be feasible using proliposomes.

Ultradeformable lipid vesicles can penetrate the skin and other semi-permeable barriers unfragmented. Evidence from double label CLSM experiments and direct size measurements

The stability of various aggregates in the form of lipid bilayer vesicles was tested by three different methods before and after crossing different semi-permeable barriers. First, polymer membranes with pores significantly smaller than the average aggregate diameter were used as the skin barrier model; dynamic light scattering was employed to monitor vesicle size changes after barrier passage for several lipid mixtures with different bilayer elasticities. This revealed that vesicles must adapt their size and/or shape, dependent on bilayer stability and elasto-mechanics, to overcome an otherwise confining pore. For the mixed lipid aggregates with highly flexible bilayers (Transfersomes), the change is transient and only involves vesicle shape and volume adaptation. The constancy of ultradeformable vesicle size before and after pores penetration proves this. This is remarkable in light of the very strong aggregate deformation during an enforced barrier passage. Simple phosphatidylcholine vesicles, with less flexible bilayers, lack such capability and stability. Conventional liposomes are therefore fractured during transport through a semi-permeable barrier; as reported by other researchers, liposomes are fragmented to the size of a narrow pore if sufficient pressure is applied across the barrier; otherwise, liposomes clog the pores. The precise outcome depends on trans-barrier flux and/or on relative vesicle vs. pore size. Lipid vesicles applied on the skin behave accordingly. Mixed lipid vesicles penetrate the skin if they are sufficiently deformable. If this is the case, they cross inter-cellular constrictions in the organ without significant composition or size modification. To prove this, we labelled vesicles with two different fluorescent markers and applied the suspension on intact murine skin without occlusion. The confocal laser scanning microscopy (CLSM) of the skin then revealed a practically indistinguishable distribution of both labels in the stratum corneum, corroborating the first assumption. To confirm the second postulate, we compared vesicle size in the starting suspension and in the blood after non-invasive transcutaneous aggregate delivery. Size exclusion chromatograms of sera from the mice that received ultradeformable vesicles on the skin were undistinguishable from the results measured with the original vesicle suspension. Taken together, the results support our previous postulate that ultradeformable vesicles penetrate the skin intact, that is, without permanent disintegration.

The contrasting kinetics of peroxidation of vitamin E-containing phospholipid unilamellar vesicles and human low-density lipoprotein

It is well established that alpha-tocopherol, TocH, is an outstanding lipid-soluble, peroxyl radical trapping antioxidant in homogeneous systems. It is also well established that TocH functions as a prooxidant in human low-density lipoprotein, LDL, subjected to attack by peroxyl radicals generated in the aqueous phase by, for example, thermal decomposition of the azo compound, ABAP. This tocopherol-mediated peroxidation, TMP, of LDL involves a three-step chain reaction, one step being hydrogen atom abstraction from the LDL lipids by the tocopheroxyl radical, Toc*. The occurrence of TMP has been attributed to three factors, (i) translocation by TocH of radical character from the aqueous phase into LDL lipid, (ii) isolation of the water-insoluble Toc* in the LDL particle in which it is formed for times sufficient to permit it to react with the lipid, and (iii) the small lipid volume of LDL which ensures that no particle can contain more than a single radical for a significant length of time. This consensus view of TMP implies that it should occur in any TocH-containing dispersion of small lipid particles. However, the present examination of the kinetics of the ABAP-initiated peroxidation of small unilamellar vesicles, SUVs, made from palmitoyllinoleoylphosphatidylcholine and cholesterol with a composition designed to mimic the surface coat of LDL, has shown that TocH functions as an antioxidant in such systems and that TMP does not occur under conditions where it would have occurred if the particles had been LDL. Several possible reasons for the kinetic differences between SUVs and LDL have been considered and ruled out by experiment. It is concluded that TMP can occur in LDL because these particles contain a lipid core in which the Toc* radical "hides" for much of its lifetime well away from the peroxyl radicals in the aqueous phase. In contrast, because SUVs have no lipid core, the Toc(*) radical is always "exposed" and available to aqueous peroxyl radicals with which it reacts rapidly and is destroyed before it can abstract a hydrogen atom from the lipid.

Origin of laurdan sensitivity to the vesicle-to-micelle transition of phospholipid-octylglucoside system: a time-resolved fluorescence study

The fluorescent probe laurdan has been shown to be sensitive to the vesicle-to-micelle transition of phosphatidylcholine/octylglucoside (M. Paternostre, O. Meyer, C. Grabielle-Madelmont, S. Lesieur, and, Biophys. J. 69:2476-2488). On the other hand, a study on the photophysics of laurdan in organic solvents has shown that the complex de-excitation pathway of the probe can be described by two successive processes, i.e., an intramolecular charge transfer followed by dielectric relaxation of the solvent if polar. These two excited-state reactions lead to three emitting states, i.e., a locally excited state, a charge transfer state, and a solvent relaxed state (M. Viard, J. Gallay, M. Vincent, B. Robert and, Biophys. J. 73:2221-2234). Experiments have been performed using time-resolved fluorescence on the probe inserted in amphiphile aggregates (mixed liposomes, mixed micelles) different in detergent-to-lipid ratios. The results have been compared with those obtained for laurdan inserted in dipalmitoyl phosphatidylcholine liposomes in the gel and in the fluid lamellar phase. Except for laurdan in dipalmitoyl phosphatidylcholine liposomes in the gel lamellar phase, the red part of the emission spectra originates from the de-excitation of the relaxed excited state of laurdan, indicating that indeed the dielectric relaxation process is an important phenomena in the ground-state return pathway of this probe. On the other hand, the maximization entropy method (MEM) analysis of the fluorescence decay recorded in the blue part of the emission spectra indicates that the dielectric relaxation is not the only reaction occurring to the excited state of laurdan. Moreover, the analysis of the fluorescence decays of laurdan inserted in gel lamellar dipalmitoylphosphatidylcholine (DPPC) liposomes indicates excited-state reactions, although dielectric relaxation is impossible. These results are in agreement with the de-excitation pathway determined from laurdan behavior in organic solvent even if, in most of the aggregates studied in this work, the major phenomenon is the dielectric relaxation of the solvent. All along the vesicle-to-micelle transition, we have observed that the lifetime of the relaxed excited state of laurdan continuously decreases probably due to a dynamic quenching process by water molecules. On the other hand, the time constant of the dielectric relaxation process remains almost unchanged in the lamellar part of the transition but abruptly decreases as soon as the first mixed micelle is formed. This decrease is continuous all over the rest of the transition even if it is more pronounced in the mixed liposomes' and mixed micelles' coexistence. The increase of the octylglucoside-to-lipid ratio of the mixed micelles via the change of the size and the shape of the aggregates may facilitate the penetration and the mobility of water molecules. Therefore, during the vesicle-to-micelle transition, laurdan probes the evolution of both the amphiphile packing in the aggregates and the increase of the interface polarity. This study finally shows that the detergent-to-lipid ratio of the mixed micelles is an important parameter to control to limit the penetration and the mobility of water within the amphiphile aggregates and that laurdan is a nice tool to monitor this phenomenon.

A high-throughput assay for rat liver golgi and Saccharomyces cerevisiae-expressed murine CMP-N-acetylneuraminic acid transport proteins

Rat liver Golgi and Saccharomyces cerevisiae-expressed CMP-Neu5Ac transport protein were reconstituted in phosphatidylcholine liposomes and transport of CMP-Neu5Ac into these proteoliposomes was determined. The separation of transported substrate from free substrate was performed using Multiscreen minicolumns loaded with Sephadex G-50 resin (fine). The CMP-Neu5Ac transport characteristics of the rat liver Golgi and S. cerevisiae-expressed transporters, determined using this separation system, were very similar to those previously reported. Inhibition studies, utilizing the above procedure, revealed that the main structural features required for recognition of glycosyl nucleosides by the rat liver Golgi CMP-Neu5Ac transport protein were the nature of the nucleoside base and the anomeric configuration of the associated carbohydrate. In general, pyrimidine-based glycosyl nucleosides were found to inhibit transport to a far greater extent than purine-based glycosyl nucleosides, an observation that is in good agreement with previous reports. These results indicate that the reconstitution procedure, in conjunction with Multiscreen minicolumns, is an effective high-throughput method for the determination of CMP-Neu5Ac transport.

Effects of complexation between liposome and poly(malic acid) on aggregation and leakage behaviour

The design and development of novel pH-sensitive liposomes were investigated to improve the release of liposome-encapsulated chemicals. Stable liposomes comprising of L-alpha-dipalmitoylphosphatidylcholine (DPPC) and poly(carboxylic acid) were prepared and characterized. Poly(malic acid) (PMLA) was chosen as a fusogen, because of its excellent biodegradability in physiological regions. Octyl groups introduced in the poly(malic acid) worked as anchors at the surface of the liposomes and made a remarkable contribution to complexing. The interaction between the liposomes and the polyacids was studied in terms of the change in size of the liposomes. The influences of molecular weight and amounts of polymer upon their characteristics, especially fusion, were discussed. The influences of pH change with respect to the association behavior of the liposomes such as aggregation and fusion were estimated by the particle size of the liposomes, turbidimetry of the solution and resonance energy transfer assay. From the results of these studies, it was shown that more tightly complexed liposomes aggregated and fused more positively with increasing acidity of the solution. The leakage of calcein entrapped in the inner aqueous phase of the liposomes increased with decreasing pH. The effect of pH on the liposome aggregation in a solution qualitatively paralleled that found in the leakage behavior.

A differential scanning calorimetry study of the interaction of lasalocid antibiotic with phospholipid bilayers

Interaction of lasalocid sodium salt (Las-Na) with dipalmitoylphosphatidylcholine (DPPC) as a membrane model was investigated by highly-sensitive differential scanning calorimetry (DSC). The insertion properties of the antibiotic were studied both in multilamellar suspensions and unilamellar vesicles, for Las-Na/DPPC molar ratios (r) ranging from 0.005 to 0.1. The effect of the antibiotic on the lipid thermotropic behavior is concentration dependent and drastically changes at a critical r of 0.04 in both model membranes. Below this ratio, Las-Na molecules interact with DPPC bilayers without disrupting the global organization of the membrane. In the multilamellar systems only the transition cooperativity is affected whereas for the mixed vesicles, a decrease in the enthalpy change suggests a different mode of insertion. Above this ratio, implantation of the antibiotic give rise to lateral phase separation in multilamellar systems. These structural modifications have repercussions on the formation of mixed LAS-Na/DPPC vesicles which seems limited to an r value of 0.04.

Poly(ethylene glycol) (PEG)-mediated fusion between pure lipid bilayers: a mechanism in common with viral fusion and secretory vesicle release?

Membrane fusion is fundamental to the life of eukaryotic cells. Cellular trafficking and compartmentalization, import of food stuffs and export of waste, inter-cellular communication, sexual reproduction, and cell division are all dependent on this basic process. Yet, little is known about the molecular mechanism(s) by which fusion occurs. It is known that fusing membranes must somehow be docked and brought into close contact. Specific proteins, many of which have been identified within the past decade, accomplish this. An electrical connection or 'fusion pore' is established between compartments surrounded by the fusing membranes. Three primary views of the mechanism of pore formation during secretory and viral fusion have been proposed within the past decade. In one view, a protein ring forms an initial transient connection that expands slowly by recruiting lipid so as to form a lipidic junction. In another view, the initial fusion pore consists of a protein-lipid complex that transforms slowly until the fusion proteins dissociate from the complex to form an irreversible lipidic pore. In a third view, the initial pore is a transient lipid pore that fluctuates between open and closed states before either expanding irreversibly or closing. Recent work has helped define the mechanism by which poly(ethylene glycol) (PEG) mediates fusion of highly curved model membranes composed only of synthetic phospholipids. PEG is a highly hydrated polymer that can bring vesicle membranes to near molecular contact by making water between them thermodynamically unfavourable. Disrupted packing in the contacting monolayers of these vesicle membranes is necessary to induce fusion. The time course and sequence of molecular events of the ensuing fusion process have also been defined. This sequence of events involves the formation of an initial, transient intermediate in which outer leaflet lipids have mixed and small transient pores join fusing compartments ('stalk'). The transient intermediate transforms in 1-3 min to a fusion-committed, second intermediate ('septum') that then 'pops' to form the fusion pore. Inner leaflet mixing, which is shown to be distinct from outer leaflet mixing, accompanies contents mixing that marks formation of the fusion pore. Both the sequence of events and the activation energies of these events correspond well to those observed in viral membrane fusion and secretory granule fusion. These results strongly support the contention that both viral and secretory fusion events occur by lipid molecule rearrangements that can be studied and defined through the use of PEG-mediated vesicle fusion as a model system. A possible mechanism by which fusion proteins might mediate this lipidic process is described.

Rapid diffusion of the lipid phosphorus of phosphatidylglycerol liposomes through polycarbonate membranes is caused by the oxidation of the unsaturated fatty acids

The lipid phosphorus of phosphatidylglycerol liposomes was found to diffuse extensively, after a lag time of 1 to 2 days, through a 0.1 micron pore size polycarbonate membrane in a two compartment system. Diffusion occurred when either multilamellar or large unilamellar vesicles were studied, even if they were sedimented to eliminate any smaller particles. The lipid of liposomes prepared under sterile conditions also diffused extensively. Diffusion appeared to be related to the age of the vesicles, and could be eliminated by incorporating antioxidants into the liposomes, or by using liposomes prepared from saturated phospholipids (C14 or larger). This indicated that diffusion was caused by phospholipid oxidation, which was confirmed by HPLC analysis. Phospholipid phosphorus that diffused through a membrane appeared more polar, as indicated by its capacity to distribute into the upper phase of a two phase extraction. Phospholipid phosphorus diffusion was preceded by the complete loss of liposomes contents, indicated by the complete diffusion of encapsulated carboxyfluorescein through the membrane. Oxidation of the lipid could be prevented by inclusion of either butylated hydroxytoluene or alpha-tocopherol in the membrane. The best retention of liposomal contents was achieved when both antioxidants and cholesterol were included in the liposome preparation. The antioxidant incorporated in the liposomes remained effective in protecting the phospholipids upon storage at 4 degrees C for 2 months. The inclusion of EDTA in the suspension medium retarded the rapid oxidation, suggesting that the presence of trace amounts of heavy metal ions in the buffer catalyzed the oxidation. Phospholipid oxidation was most effectively inhibited by the presence of serum or chemically defined medium, suggesting that oxidation of liposomal lipids in a biological environment may be minimized if appropriate steps are taken.

Liposome fusion and lipid exchange on ultraviolet irradiation of liposomes containing a photochromic phospholipid

A photochromic phospholipid, 1,2-bis[4-4(4-n-butylphenylazo) phenylbutyroyl] phosphatidylcholine (Bis-Azo PC) has been incorporated into liposomes of gel- and liquid-crystalline- phase phospholipids. Liposomes of gel-phase phospholipid are stable in the presence of the trans photostationary state Bis-Azo PC and can encapsulate fluorescent marker dye. On photoisomerization to the cis photostationary state, trapped marker is rapidly released. Liposomes containing Bis-Azo PC can rapidly fuse together after UV isomerization, this process continuing in the dark. Exposure to white light causes reversion of Bis-Azo Pc to the trans form and halts dye leakage and vesicle fusion. Both unilamellar and multilamellar liposomes are able to fuse together on UV exposure. On UV photolysis, liposomes containing Bis-Azo PC do not fuse with a large excess of unlabeled liposomes, but transfer of Bis-Azo PC can be demonstrated spectrophotometrically. Vesicles of pure gel-phase lipid containing trapped marker dye but initially no Bis-Azo PC become leaky as a result of this lipid transfer. Liposomes composed of liquid-crystalline-phase phosphatidylcholine- containing Bis-Azo PC neither leak trapped marker no fuse together on photolysis, nor do liquid-crystalline-phase liposomes fuse with gel-phase liposomes under these conditions. These results are discussed together with some possible applications of liposome photodestabilization.

Preparation and evaluation of proliposomes containing propranolol hydrochloride

Proliposomes were prepared by the penetration of a methanol-chloroform solution of propranolol hydrochloride (PH) and lecithin into microporous sorbitol, with subsequent vacuum drying. They were characterized for surface morphology and flowability, and following the conversion to liposomes upon hydration, size distribution, drug content and in vitro drug release of the reconstituted liposomes were examined. The porous structure of sorbitol was maintained in the proliposomes, affording the proliposomes good flowability at lecithin-to-sorbitol ratios (w/w) of not more than 0.2. Multilamellar liposomes were reconstituted spontaneously from the proliposomes upon hydration. The mean diameter of the resultant liposomes was highly dependent on the PH-to-lecithin ratio, but less dependent on the lecithin-to-sorbitol ratio and sorbitol particle size (105-710 microns). Entrapment efficiency of PH in liposomes showed a maximum 10% at PH-to-lecithin ratio < 0.5 and a lecithin-to-sorbitol ratio > 0.1. Sustained release of propranolol from the proliposomes was achieved when the lecithin-to-PH ratio was > 2, and the lecithin-to-sorbitol ratio was in the range examined (0.067-0.2). In conclusion, granular proliposomes of PH with good flowability and sustained release characteristics could be prepared by controlling the drug/lecithin/sorbitol ratio and sorbitol particle size. PH proliposomes can be potential candidates for the sustained drug delivery of propranolol when applied directly onto the mucosal membranes.

Temperature- and pH-controlled fusion between complex lipid membranes. Examples with the diacylphosphatidylcholine/fatty acid mixed liposomes

The fusion capability of complex lipid bilayers and its pH as well as temperature sensitivity have been studied by optical and spectroscopic means. The aggregation and fusion efficiency of such lipid membranes can be optimized by controlling the phase characteristics of the individual membrane components. For a practically relevant illustration, the stoichiometric 1:2 (mol/mol) mixtures of phosphatidylcholines and fatty acids are used. Perhaps the most interesting liposomes of this kind, which are made of dipalmitoylphosphatidylcholine/elaidic acid (DPPC/ELA-COOH (1:2)), undergo a chain-melting phase transition between 42 degrees C and 48 degrees C, depending on the bulk pH value. The highest chain-melting phase transition temperatures are measured with the fully protonated fatty acids at pH < or = 5.5 and involve a change into the non-bilayer high-temperature state. Upon increasing pH, this transition reverts into an ordinary gel-to-fluid lamellar phase change and occurs at 42 degrees C, by and large. Simultaneously, the rate and the efficacy of fusion between the PC/FA and PC/FA- mixed vesicles decreases. The fusion efficacy of the PC/FA(-) mixed liposomes at pH > or = pK(FA) approximately 7.5 is practically negligible. This is largely due to the increased interbilayer repulsion and to the relatively high water-solubility of the deprotonated fatty acid molecules at high pH. While the pH-variability chiefly affects the efficacy of the intermembrane aggregation, the vesicle fusion itself is more sensitive to temperature variations. It is more likely that the temperature dependence of the intramembrane defect density is chiefly responsible for this. Optimal conditions for the fusion between DPPC/ELA-COOH (1:2) mixed vesicles are thus 3.5 < or = pH < or = 5.5 (6.3) (aggregation maximum) and T > or = 41.5 degrees C = Tm(DPPC) (defect density and fusion maximum). Under such conditions the average size of PC/FA (1:2) mixed vesicles in a 1 mM suspension increases by a factor of 10 over a period of 10 min. Interbilayer fusion can also be catalyzed by the mechanically induced local membrane defects. Freshly made liposomes thus always fuse more avidly than aged vesicles. This permits estimates of the kinetics of membrane defects annihilation based on the measured temporal dependence of the maximum fusion-rate. From such studies, a quasi-exponential decay on the time scale of 1.2 h is found for the thermolabile fusogenic DPPC/ELA-COOH liposomes.

The pharmacokinetics of methotrexate after intravenous administration of methotrexate-loaded proliposomes to rats

The pharmacokinetics and tissue distribution of methotrexate (MTX) were investigated after intravenous (i.v.) injection of free MTX (treatment I), MTX-loaded proliposomes (treatment II), and empty proliposomes mixed manually with free MTX (treatment III), 8 mg kg-1, to rats using an HPLC assay. After i.v. infusion in 1 min, the plasma concentration of MTX (Cp), the area under the plasma concentration-time curve (AUC, 639 versus 913 micrograms min mL-1), the terminal half-life (t1/2, 48.8 versus 397 min), the mean residence time (MRT, 8.40 versus 325 min), and the apparent volume of distribution at steady state (Vss, 98.1 versus 2800 mL kg-1) were significantly higher; however, the total body clearance (CL, 12.5 versus 8.76 mL min-1 kg-1), renal clearance (CLR, 4.49 versus 2.78 mL min-1 kg-1), non-renal clearance (CLNR, 7.50 versus 5.99 mL min-1 kg-1), and the amount of MTX excreted in urine (Xu, 808 versus 685 micrograms, p < 0.0948) were significantly lower from treatment II than from treatment I. This could be due to the fact that some of the MTX-loaded liposomes (formed immediately after hydration of MTX-loaded proliposomes) are entrapped in tissues and the rest are present in the plasma (higher MRT and Vss from treatment II), and MTX is slowly released from MTX-loaded liposomes (higher t1/2 from treatment II). In the present HPLC assay, the concentrations of MTX represent the sum of free MTX and MTX loaded in liposomes (higher Cp and AUC, slower CL from treatment II). After i.v. infusion in 1 min, some pharmacokinetic parameters, such as t1/2, MRT, and Vss, were significantly different between treatments I and III; however, the differences seemed to be smaller than those between treatments I and II. After 30 min from i.v. infusion, the tissue to plasma (T/P) ratios of MTX in kidney and stomach from treatment II were significantly lower than those from treatment I. This suggested that the i.v. administration of MTX-loaded proliposomes might have fewer side effects in the organs than that of free MTX. The mean amount of MTX loaded in MTX-loaded proliposomes was 2.54 mg/g proliposomes and the MTX was released slowly from hydrated MTX-loaded proliposomes when incubated with phosphate-buffered saline (PBS), rat plasma, or rat liver homogenate.

Size distribution of liposomes by flow field-flow fractionation

The applicability of field-low fractionation (FFF) to the characterization of liposomes is discussed and theoretically described. Because of fundamental differences in their driving forces, sedimentation FFF and flow FFF measure different vesicle properties. Sedimentation FFF, although used previously to measure vesicle sizes and size distributions, is fundamentally a technique that measures the effective mass and mass distribution of particles. It is sensitive to small changes in the effective mass of either the biomembrane or its encapsulated load and thus is likely to be useful in characterizing such properties as drug loading, biomembrane volumes and areas, and distributions of these properties. Size characterization by sedimentation FFF can only be done by deducing size from effective mass. Flow FFF, by contrast, provides a direct measurement of vesicle size and size distribution. After demonstrating the high resolution and relative accuracy of size measurement of flow FFF by the separation of polystyrene latex standards, flow FFF was applied to two preparations of DSPC-DSPA liposomes that were sonicated under different temperature conditions. Fractograms and size distributions are reported as a function of sonication time. The rapid elimination of a large diameter tail to the distribution is shown to constitute a major mechanism for distribution narrowing. Finally, results are provided bearing on the reproducibility of size distribution measurements by flow FFF.

Omega-3 fatty acid modification of membrane structure and function. II. Alteration by docosahexaenoic acid of tumor cell sensitivity to immune cytolysis

Docosahexaenoic acid (DHA, 22:6) is a long-chain omega-3 fatty acid abundant in cold water fish; it is the most unsaturated fatty acid found in biologic systems and is reported to alter membrane structure. To explore DHA's effect on membrane function, we have fused tumor cells with synthetic phosphatidylcholine (PC) containing stearic acid in the sn-1 position and DHA in the sn-2 position (18:0, 22:6 PC) and have found the lipid-modified tumor cells to be more sensitive to cytolysis by alloreactive cytotoxic T lymphocytes. Cold target competition experiments suggested that fusion of tumor plasma membranes with 18:0, 22:6 PC produced a qualitative change in expression of surface antigens recognized by cytotoxic T lymphocytes. We monitored the expression of various epitopes on tumor cells by complement-mediated lysis and radioimmunoassay with monoclonal antibodies against H-2 class I antigens. Our results suggest that membrane-bound DHA increases the expression of some epitopes while decreasing the expression of others and that different tumor lines vary in the magnitude of DHA's effect. Our findings are consistent with a model in which DHA-containing phospholipids segregate into membrane domains, in turn altering the expression of membrane proteins.

Ultrastructure of sarcoballs on the surface of skinned amphibian skeletal muscle fibres

The formation of sarcoballs on the surface of skinned fibres from semitendinosus muscles of Xenopus laevis, and the sarcoplasmic reticulum content of the structures, have been studied using conventional electron microscopic techniques and immunoelectron microscopy. Examination of the fibres showed many membrane-bound blebs projecting from the surface in areas where vesicles of internal membranes (including sarcoplasmic reticulum, T-tubules and mitochondria) were clustered in interfilament spaces. The blebs varied in size from 1 micron to 150 microns and those with diameters > 10 microns are referred to as sarcoballs. Small blebs were often seen in close association with each other and might have fused during sarcoball formation. The interior of the sarcoball was filled with foam-like material made up of vesicles with diameters of 100 nm to 1.0 microns. The sarcoplasmic reticulum membrane content of the sarcoballs was evaluated using two monoclonal antibodies, one to the Ca2+ ATPase of the sarcoplasmic reticulum and the second to ryanodine receptor calcium release channels in the junctional-face membrane. The antibodies bound to some components of the surface and interior of the sarcoball, but not to mitochondrial-like structures and tubular vesicles. The results show that a large component of the sarcoball and its surface is derived from sarcoplasmic reticulum and suggest that mitochondria and T-tubules might also contribute membranes to the structures. Our hypothesis is that (a) blebs bud out from the surface of the skinned fibre following fusion of internal vesicles that are extruded along interfilament channels during unrestrained contractures, (b) blebs grow into sarcoballs by additional fusion of internal membrane vesicles and fusion of adjacent blebs, and (c) the sarcoball is a foam-like structure composed of bathing medium and membrane lipid (containing membrane proteins).

Kinetics of melittin binding to phospholipid small unilamellar vesicles

We have used the decrease in the fluorescence intensity of the single tryptophan residue of bee venom melittin at long emission wavelengths that accompanies binding of the peptide to phospholipid small unilamellar vesicles to determine the rate of binding through the use of stopped-flow fluorometry in the millisecond range. We have found the rate to depend on the degree of saturation of the lipid acyl chains as well as on the physical state of the bilayer, the net electric charge of the polar headgroups, and the lipid-to-melittin molar ratio R. For zwitterionic lipids (i) the binding process is found to exhibit negative cooperativity, and (ii) the rate-limiting step appears to be penetration of the protein into the hydrophobic region of the bilayer. For negatively charged lipids the results show that binding is a very fast process that seems to be electrostatic in nature.

Temperature-induced fusion of small unilamellar vesicles formed from saturated long-chain lecithins and diheptanoylphosphatidylcholine

Small unilamellar vesicles which form when gel-state long-chain phosphatidylcholines are mixed with micellar short-chain lecithins undergo an increase in size as the long-chain species melts to its liquid-crystalline form. Analysis of the vesicle population with quasi-elastic light scattering shows that the particle size increases from 90-A radius to greater than 5000-A radius. Resonance energy transfer experiments show total mixing of lipid probes with unlabeled vesicles only when the Tm of the long-chain phosphatidylcholine is exceeded. This implies that the large size change represents a fusion process. Aqueous compartments are also mixed during this transition. 31P NMR analysis of the vesicle mixtures above the phase transition shows a great degree of heterogeneity with large unilamellar particles coexisting with oligo- and multilamellar structures. Upon cooling the vesicles below the Tm, the original size distribution (e.g., small unilamellar vesicles) is obtained, as monitored by both quasi-elastic light scattering and 31P NMR spectroscopy. This temperature-induced fusion of unilamellar vesicles is concentration dependent and can be abolished at lower total phospholipid concentrations. It occurs over a wide range of long-chain to short-chain ratios and occurs with 1-palmitoyl-2-stearoylphosphatidylcholine and dimyristoylphosphatidylcholine as well. Characterization of this fusion event is used to understand the anomalous kinetics of water-soluble phospholipases toward these unusual vesicles.