Indirect evidence of submicroscopic pores in giant unilamellar [correction of unilamelar] vesicles

Formation of pore-like structures in cell membranes could participate in exchange of matter between cell compartments and modify the lipid distribution between the leaflets of a bilayer. We present experiments on two model systems in which major lipid redistribution is attributed to few submicroscopic transient pores. The first kind of experiments consists in destabilizing the membrane of a giant unilamellar vesicle by inserting conic-shaped fluorescent lipids from the outer medium. The inserted lipids (10% of the vesicle lipids) should lead to membrane rupture if segregated on the outer leaflet. We show that a 5-nm diameter pore is sufficient to ease the stress on the membrane by redistributing the lipids. The second kind of experiments consists in forcing giant vesicles containing functionalized lipids to adhere. This adhesion leads to hemifusion (merging of the outer leaflets). In certain cases, the formation of pores in one of the vesicles is attested by contrast loss on this vesicle and redistribution of fluorescent labels between the leaflets. The kinetics of these phenomena is compatible with transient submicroscopic pores and long-lived membrane defects.     

Loss of plasma membrane lipid asymmetry can induce ordered domain (raft) formation

In some cases, lipids in one leaflet of an asymmetric artificial lipid vesicle suppress the formation of ordered lipid domains (rafts) in the opposing leaflet. Whether this occurs in natural membranes is unknown. Here, we investigated this issue using plasma membrane vesicles (PMVs) from rat leukemia RBL-2H3 cells. Membrane domain formation and order was assessed by fluorescence resonance energy transfer and fluorescence anisotropy. We found that ordered domains in PMVs prepared from cells by N-ethyl maleimide (NEM) treatment formed up to ～37°C, whereas ordered domains in symmetric vesicles formed from the extracted PMV lipids were stable up to 55°C, indicating the stability of ordered domains was substantially decreased in intact PMVs. This behavior paralleled lesser ordered domain stability in artificial asymmetric lipid vesicles relative to the corresponding symmetric vesicles, suggesting intact PMVs exhibit some degree of lipid asymmetry. This was supported by phosphatidylserine mislocalization on PMV outer leaflets as judged by annexin binding, which indicated NEM-induced PMVs are much more asymmetric than PMVs formed by dithiothreitol/paraformaldehyde treatment. Destroying asymmetry by reconstitution of PMVs using detergent dilution also showed stabilization of domain formation, even though membrane proteins remained associated with reconstituted vesicles. Similar domain stabilization was observed in artificial asymmetric lipid vesicles after destroying asymmetry via detergent reconstitution. Proteinase K digestion of proteins had little effect on domain stability in NEM PMVs. We conclude that loss of PMV lipid asymmetry can induce ordered domain formation. The dynamic control of lipid asymmetry in cells may regulate domain formation in plasma membranes.     

Lipid Unsaturation Properties Govern the Sensitivity of Membranes to Photoinduced Oxidative Stress

Unsaturated lipid oxidation is a fundamental process involved in different aspects of cellular bioenergetics; dysregulation of lipid oxidation is often associated with cell aging and death. To study how lipid oxidation affects membrane biophysics, we used a chlorin photosensitizer to oxidize vesicles of various lipid compositions and degrees of unsaturation in a controlled manner. We observed different shape transitions that can be interpreted as an increase in the area of the targeted membrane followed by a decrease. These area modifications induced by the chemical modification of the membrane upon oxidation were followed in situ by Raman tweezers microspectroscopy. We found that the membrane area increase corresponds to the lipids’ peroxidation and is initiated by the delocalization of the targeted double bonds in the tails of the lipids. The subsequent decrease of membrane area can be explained by the formation of cleaved secondary products. As a result of these area changes, we observe vesicle permeabilization after a time lag that is characterized in relation with the level of unsaturation. The evolution of photosensitized vesicle radius was measured and yields an estimation of the mechanical changes of the membrane over oxidation time. The membrane is both weakened and permeabilized by the oxidation. Interestingly, the effect of unsaturation level on the dynamics of vesicles undergoing photooxidation is not trivial and thus carefully discussed. Our findings shed light on the fundamental dynamic mechanisms underlying the oxidation of lipid membranes and highlight the role of unsaturations on their physical and chemical properties.     

Characterization of lipid insertion into monomolecular layers mediated by lung surfactant proteins SP-B and SP-C

Pulmonary surfactant proteins, SP-B and SP-C, if present in preformed monolayers can induce lipid insertion from lipid vesicles into the monolayer after the addition of (divalent) cations [Oosterlaken-Dijksterhuis, M. A., Haagsman, H. P., van Golde, L. M. G., & Demel, R. A. (1991) Biochemistry 30, 8276-8287]. This model system was used to study the mechanisms by which SP-B and SP-C induce monolayer formation from vesicles. Lipid insertion proceeds irrespectively of the molecular class, and PG is not required for this process. In addition to lipids that are immediately inserted from vesicles into the monolayer, large amounts of vesicles are bound to the monolayer and their lipids eventually inserted when the surface area is expanded. SP-B and SP-C are directly responsible for the binding of vesicles to the monolayer. By weight, the vesicle binding capacity of SP-B is approximately 4 times that of SP-C. For vesicle binding and insertion, the formation of close contacts between monolayer and vesicles is essential. SP-B and SP-C show very similar surface properties. Both proteins form extremely stable monolayers (collapse pressures 36-37 mN/m) of alpha-helical structures oriented parallel to the interface. In monolayers consisting of DPPC and SP-B or SP-C, an increase in mean molecular area is observed, which is mainly attributed to the phospholipid. This will greatly enhance the insertion of new lipid material into the monolayer. The results of this study suggest that the surface properties and the hydrophobic nature of SP-B and SP-C are important for the protein-mediated monolayer formation.     

Divalent cation induced fusion and lipid lateral segregation in phosphatidylcholine-phosphatidic acid vesicles

The interactions of unilamellar vesicles containing phosphatidylcholine (PC) and phosphatidic acid (PA) in the presence of calcium and magnesium were examined by fluorometric assays of vesicle lipid mixing, contents mixing, and contents leakage and by spray-freezing freeze-fracture electron microscopy. These results were correlated with calorimetric and fluorometric measurements of divalent cation induced lateral segregation of lipids in these vesicles under comparable conditions. PA-PC vesicles in the presence of calcium show a rapid but limited intermixing of vesicle lipids and contents, the extent of which increases as the vesicle size decreases or the PA content increases. Calcium produces massive aggregation and efficient mixing of the contents of vesicles containing high proportions of dioleoyl-PA or egg PA, but vesicle coalescence in the latter case is followed rapidly by vesicle collapse and massive leakage of contents. The effects of magnesium are similar for vesicles of very high PA content. However, in the presence of magnesium, vesicles containing lower amounts of PA exhibit "hemifusion", a mode of interaction in which vesicles aggregate and mix approximately 50% of their lipids, apparently representing the lipids of the outer monolayer of each vesicle, without significant mixing of vesicle contents or collapse of the vesicles. Fluorometric measurements of lipid lateral segregation demonstrate that lateral redistribution of lipids in PA-PC vesicles begins at submillimolar concentrations of divalent cations and shows no abrupt change at the "threshold" divalent cation concentration, above which coalescence of vesicles is observed. By correlating calorimetric and fluorometric measurements of lipid lateral segregation and mixing of vesicle components, we can demonstrate that lipid segregation is at least strongly correlated with calcium-promoted coalescence of PA-PC vesicles and is essential to the magnesium-promoted interactions of vesicles of low PA contents.     

Effect of alkali cations on freeze-thaw-dependent reconstitution of amino acid transport from Ehrlich ascites cell plasma membrane

Na+-dependent amino acid transport can be reconstituted by gel filtration of disaggregated plasma membrane and asolectin vesicles coupled to a freeze-thaw cycle. The resultant transport activity is markedly affected by the nature of the reconstitution medium. Reconstitution in K+ permits the formation of active liposomes, whereas reconstitution in Na+, Li+, or choline does not. Electron micrographs of K+ liposomes show a wide variation in liposome sizes. Ficoll density gradient fractionation of K+ liposomes shows that the largest vesicles are lipid rich, have the lowest density, and have the highest level of Na+-dependent amino acid transport. Liposomes formed in Na+ have a 34% smaller trapped volume than K+ liposomes and lack a population of large vesicles. A second freeze-thaw in K+ restores activity to Na+ liposomes which now contain large low density active vesicles. Fluorescence measurements of freeze-thaw-induced mixing of vesicle lipids indicates that the absence of large vesicles in Na+ liposomes is due to inhibition by Na+ of lipid vesicle fusion events during freezing and thawing. The large vesicle fraction is enriched in a 125-kDa peptide. It has not yet been established whether this peptide is part of the transport system for neutral amino acids.     

Lipid bilayer dynamics in plasma and coated vesicle membranes from bovine adrenal cortex. Evidence of two types of coated vesicle involved in the LDL receptor traffic

Pure coated vesicles have been prepared from the bovine adrenal cortex and two homogeneous populations have been separated, one of large diameter (100 nm) and one of small diameter (70 nm). The chemical composition in lipids and proteins of coated vesicles has been compared with that of partially purified plasma membranes and evidences a higher protein/lipid ratio and a higher concentration in phosphatidylethanolamine and unsaturated fatty acids. Evaluation of the lateral diffusion of pyrene in the lipid bilayer of coated vesicles as compared to uncoated vesicles evidences a slowing-down effect of clathrin. Measurements of lipids' rotational diffusion by time-resolved fluorescence indicate a decrease in the order parameter of the lipids in the coated vesicles due to clathrin. A hypothesis is proposed for a possible role of the clathrin coat in the concerted motion of lipids and proteins toward coated pits and in the mechanism of formation of coated vesicles. Separation of the large from the small coated vesicles made it possible to reveal different protein components in the two types of vesicle by electrophoresis and autoradiograms of the [γ-³²P]adenosine triphosphate- (ATP-) treated vesicles. Visualisation of the low-density lipoprotein receptor by ligand blotting and enzyme-linked immunosorbent assay (ELISA) techniques indicates an increased low-density lipoprotein receptor binding capacity in small coated vesicles as compared to large ones and plasma membranes.     

Real-time Observation of Lipoplex Formation and Interaction with Anionic Bilayer Vesicles

A novel development has allowed for the direct observation of single, pairwise interactions of linear DNA with cationic vesicles and of DNA-cationic lipid complexes with anionic vesicles. A new cationic phospholipid derivative, l,2-dioleoyl-sn-glycero-3-ethylphosphocholine, was used to prepare giant bilayer vesicles and to form DNA-cationic lipid complexes (lipoplexes). The cationic vesicles were electrophoretically maneuvered into contact with DNA, and similarly, complexes were brought into contact with anionic phospholipid vesicles composed of dioleoylphosphatidylglycerol (DOPG; 100%), DOPG/dioleoylphosphatidylethanolamine (DOPE; 1:1) or DOPG/dioleoylphosphatidylcholine (DOPC; 1:1). Video fluorescence microscopy revealed that upon contact with phospholipid anionic vesicles, lipoplexes exhibited four different types of behavior: adhesion, vesicle rupture, membrane perforation (manifested as vesicle shrinkage and/or content loss), and expansion of DNA (which was always concomitant with membrane perforation.) In one instance, the lipoplex was injected into the target vesicle just prior to DNA expansion. In all other instances, the DNA expanded over the outer surface of the vesicle, and expansion was faster, the larger the area of vesicle over which it expanded. Given the likelihood of incorporation of cellular anionic lipids into lipoplexes, the expansion of the DNA could be important in DNA release during cell transfection. Upon contact with naked DNA, giant cationic vesicles usually ruptured and condensed the DNA into a small particle. Contact of cationic vesicles that were partially coated with DNA usually caused the DNA to wrap around the vesicle, leading to vesicle rupture, vesicle fusion (with other attached vesicles or lipid aggregates), or simply cessation of movement. These behaviors clearly indicated that both DNA and vesicles could be partly or fully covered by the other, thus modifying surface charges, which, among others, allowed adhesion of DNA-coated vesicles with uncoated vesicles and of lipid-coated DNA with uncoated DNA.     

Effect of PEGylated lipid and Lecinol S-10 on physico-chemical properties and encapsulation efficiency of palmitoleate–palmitoleic acid vesicles

A vesicle is a microscopic particle composed of a lipid bilayer membrane that separates the inner aqueous compartment from the outer aqueous environment. Palmitoleate–palmitoleic acid vesicles were prepared and their physico-chemical properties were investigated. Moreover, mixed vesicles composed of palmitoleic acid and PEGylated lipid and/or a mixture of phospholipids were also prepared. The stabilizing effects of these double-chain lipids on the formation of palmitoleate–palmitoleic acid vesicles were studied. Stability of the vesicle suspension was examined using particle size and zeta potential at 30?°C. The magnitude of the zeta potential was relatively lower in the vesicle suspension with the presence of phospholipid. Although some of the mixed vesicles that were formed were not very stable, they displayed potential for encapsulating the active ingredient calcein and the encapsulation efficiencies of calcein were encouraging. The palmitoleate–palmitoleic acid–DPPE-PEG2000 vesicle showed the most promising stability and encapsulation efficiency.     

Interactions between human defensins and lipid bilayers: evidence for formation of multimeric pores.

Defensins comprise a family of broad-spectrum antimicrobial peptides that are stored in the cytoplasmic granules of mammalian neutrophils and Paneth cells of the small intestine. Neutrophil defensins are known to permeabilize cell membranes of susceptible microorganisms, but the mechanism of permeabilization is uncertain. We report here the results of an investigation of the mechanism by which HNP-2, one of 4 human neutrophil defensins, permeabilizes large unilamellar vesicles formed from the anionic lipid palmitoyloleoylphosphatidylglycerol (POPG). As observed by others, we find that HNP-2 (net charge = +3) cannot bind to vesicles formed from neutral lipids. The binding of HNP-2 to vesicles containing varying amounts of POPG and neutral (zwitterionic) palmitoyloleoylphosphatidylcholine (POPC) demonstrates that binding is initiated through electrostatic interactions. Because vesicle aggregation and fusion can confound studies of the interaction of HNP-2 with vesicles, those processes were explored systematically by varying the concentrations of vesicles and HNP-2, and the POPG:POPC ratio. Vesicles (300 microM POPG) readily aggregated at HNP-2 concentrations above 1 microM, but no mixing of vesicle contents could be detected for concentrations as high as 2 microM despite the fact that intervesicular lipid mixing could be demonstrated. This indicates that if fusion of vesicles occurs, it is hemi-fusion, in which only the outer monolayers mix at bilayer contact sites. Under conditions of limited aggregation and intervesicular lipid mixing, the fractional leakage of small solutes is a sigmoidal function of peptide concentration. For 300 microM POPG vesicles, 50% of entrapped solute is released by 0.7 microM HNP-2. We introduce a simple method for determining whether leakage from vesicles is graded or all-or-none. We show by means of this fluorescence "requenching" method that native HNP-2 induces vesicle leakage in an all-or-none manner, whereas reduced HNP-2 induces partial, or graded, leakage of vesicle contents. At HNP-2 concentrations that release 100% of small (approximately 400 Da) markers, a fluorescent dextran of 4,400 Da is partially retained in the vesicles, and a 18,900-Da dextran is mostly retained. These results suggest that HNP-2 can form pores that have a maximum diameter of approximately 25 A. A speculative multimeric model of the pore is presented based on these results and on the crystal structure of a human defensin.     

Early steps of supported bilayer formation probed by single vesicle fluorescence assays

We have developed a single vesicle assay to study the mechanisms of supported bilayer formation. Fluorescently labeled, unilamellar vesicles (30-100 nm diameter) were first adsorbed to a quartz surface at low enough surface concentrations to visualize single vesicles. Fusion and rupture events during the bilayer formation, induced by the subsequent addition of unlabeled vesicles, were detected by measuring two-color fluorescence signals simultaneously. Lipid-conjugated dyes monitored the membrane fusion while encapsulated dyes reported on the vesicle rupture. Four dominant pathways were observed, each exhibiting characteristic two-color fluorescence signatures: 1) primary fusion, in which an unlabeled vesicle fuses with a labeled vesicle on the surface, is signified by the dequenching of the lipid-conjugated dyes followed by rupture and final merging into the bilayer; 2) simultaneous fusion and rupture, in which a labeled vesicle on the surface ruptures simultaneously upon fusion with an unlabeled vesicle; 3) no dequenching, in which loss of fluorescence signal from both dyes occur simultaneously with the final merger into the bilayer; and 4) isolated rupture (pre-ruptured vesicles), in which a labeled vesicle on the surface spontaneously undergoes content loss, a process that occurs with high efficiency in the presence of a high concentration of Texas Red-labeled lipids. Vesicles that have undergone content loss appear to be more fusogenic than intact vesicles.     

Interactions of polymerizable phosphatidylcholine vesicles with blood components: relevance to biocompatibility

We have studied the biocompatibility properties of polymerizable phosphatidylcholine bilayer membranes, in the form of liposomes, with a view toward the eventual utilization of such polymerized lipid assemblies in drug carrier systems or as surface coatings for biomaterials. The SH-based polymerizable lipid 1,2-bis[1,2-(lipoyl)dodecanoyl]-sn-glycero-3-phosphocholine (dilipoyl lipid, DLL) and the methacryl-based lipid 1,2-bis[(methacryloyloxy)dodecanoyl]-sn-glycero-3-phosphocholine (dipolymerizable lipid, DPL) were studied in comparison to ‘conventional’ zwitterionic or charged phospholipids. We examined binding of serum proteins to liposomes and effects of liposomes on fibrin clot formation and on platelet aggregation. All types of liposomes tested bound complex mixtures of serum proteins with IgG being the most abundant bound component. DPL vesicles and anionic vesicles bound substantially more protein than other vesicle types. Polymerized DPL vesicles uniquely bound a protein of about 53 kDa which was not bound to other types of phosphatidylcholine liposomes. Likewise polymerized DPL vesicles, but not other types of phosphatidylcholine vesicles, caused a marked alteration in coagulation as measured by activated partial thromboplastin time (APTT) and prothrombin time (PT) tests; this effect was shown to be due to binding and depletion of clothing factor V by the DPL polymerized vesicles. Polymerized DPL liposomes and DLL liposomes in polymerized or nonpolymerized form, were without substantial effect on platelet aggregation. However, DPL nonpolymerized vesicles, while not causing aggregation, did impair ADP-induced aggregation of platelets. These studies suggest that SH based polymerizable lipids of the DLL type may be very suitable for in vivo use in the contexts of drug delivery systems or biomaterials development. Methacryloyl-based lipids of the DPL type seem to display interactions with the hemostatic process which militate against their in vivo utilization.     

The influence of lipid composition on the barrier properties of band 3-containing lipid vesicles

Band 3 protein has been incorporated into lipid vesicles consisting of 94:6 (molar ratio) egg phosphatidylcholine-bovine heart phosphatidylserine or total erythrocyte lipids by means of a Triton X-100 Bio-Beads method, with an additional sonication step prior to the removal of the detergent. This methods results, for both types of band 3 lipid vesicles, in rather homogeneous vesicles with comparable protein content and vesicle trap. Freeze-fracture electron microscopy revealed that band 3-egg phosphatidylcholine-bovine heart phosphatidylserine vesicles have considerably more intramembrane particles as compared to the band 3-erythrocyte lipid vesicles. The dimensions of the nonspecific permeation pathways present in the band 3-lipid vesicles were measured using an influx assay procedure for nonelectrolytes of different size, in which the vesicles were sampled and subsequently freed from nonenclosed labeled permeant by means of gel-filtration. The band 3-egg phosphatidylcholine-bovine heart phosphatidylserine vesicles have nonspecific permeation pathways (pores), with diameters of up to 60 A. In contrast, the band 3-total erythrocyte lipid vesicles are more homogeneous and show much smaller nonspecific permeation pathways, having a diameter of about 12 A. These results suggest that the nonspecific permeability of the band 3-lipid vesicles is strongly lipid-dependent. Increase in specific anion permeability expected as a consequence of the presence of band 3 in the erythrocyte lipid vesicles was found to be very limited. However, stereospecific, phloretin-inhibitable D-glucose permeability could clearly be demonstrated in these vesicles. The difference of the nonspecific permeability of the band 3-egg phosphatidylcholine-bovine heart phosphatidylserine vesicles and band 3-erythrocyte lipid vesicles, is discussed in the light of the presence of defects at the lipid/protein interface and protein aggregation, which may induce formation of pores.     

Theory of tunable pH-sensitive vesicles of anionic and cationic lipids or anionic and neutral lipids

The design of vesicles that become unstable at an easily tuned value of pH is of great interest for targeted drug delivery. We present a microscopic theory for two forms of such vesicles. A model of lipids introduced by us previously is applied to a system of ionizable anionic lipid and permanently charged cationic lipid. We calculate the pH at which the lamellar phase becomes unstable with respect to an inverted hexagonal one, a value that depends continuously on the system composition. Identifying this instability with that displayed by unilamellar vesicles undergoing fusion, we obtain very good agreement with the recent experimental data of Hafez, Ansell, and Cullis, (2000, Biophys. J. 79:1438-1446) on the pH at which fusion occurs versus vesicle composition. We explicate the mechanism in terms of the role of the counterions. This understanding suggests that a system of a neutral, nonlamellar-forming lipid stabilized by an anionic lipid would serve equally well for preparing tunable, pH-sensitive vesicles. Our calculations confirm this. Further, we show that both forms of vesicle have the desirable feature of exhibiting a regime in which the pH at instability is a rapidly varying function of the vesicle composition.     

Bilayer Edges Catalyze Supported Lipid Bilayer Formation

Supported lipid bilayers (SLB) are important for the study of membrane-based phenomena and as coatings for biosensors. Nevertheless, there is a fundamental lack of understanding of the process by which they form from vesicles in solution. We report insights into the mechanism of SLB formation by vesicle adsorption using temperature-controlled time-resolved fluorescence microscopy at low vesicle concentrations. First, lipid accumulates on the surface at a constant rate up to ∼0.8 of SLB coverage. Then, as patches of SLB nucleate and spread, the rate of accumulation increases. At a coverage of ∼1.5 × SLB, excess vesicles desorb as SLB patches rapidly coalesce into a continuous SLB. Variable surface fluorescence immediately before SLB patch formation argues against the existence of a critical vesicle density necessary for rupture. The accelerating rate of accumulation and the widespread, abrupt loss of vesicles coincide with the emergence and disappearance of patch edges. We conclude that SLB edges enhance vesicle adhesion to the surface and induce vesicle rupture, thus playing a key role in the formation of continuous SLB.     

Affinity purification of lipid vesicles

We present a novel column chromatography technique for recovery and purification of lipid vesicles, which can be extended to other macromolecular assemblies. This technique is based on reversible binding of biotinylated lipids to monomeric avidin. Unlike the very strong binding of biotin and biotin-functionalized molecules to streptavidin, the interaction between biotin-functionalized molecules and monomeric avidin can be disrupted effectively by ligand competition from free biotin. In this work, biotin-functionalized lipids (biotin-PEG-PE) were incorporated into synthetic lipid vesicles (DOPC), resulting in unilamellar biotinylated lipid vesicles. The vesicles were bound to immobilized monomeric avidin, washed extensively with buffer, and eluted with a buffer supplemented with free biotin. Increasing the biotinyl lipid molar ratio beyond 0.53% of all lipids did not increase the efficiency of vesicle recovery. A simple adsorption model suggests 1.1 x 10(13) active binding sites/mL of resin with an equilibrium binding constant of K = 1.0 x 10(8) M(-1). We also show that this method is very robust and reproducible and can accommodate vesicles of varying sizes with diverse contents. This method can be scaled up to larger columns and/or high throughput analysis, such as a 96-well plate format.     

DNA as Membrane-Bound Ligand-Receptor Pairs: Duplex Stability Is Tuned by Intermembrane Forces

We use membrane-anchored DNA as model adhesion receptors between lipid vesicles. By studying the thermal stability of DNA duplex formation, which tethers the vesicles into superstructures, we show that the melting temperature of a 10-base DNA sequence is dependent on the lipid composition of the tethered vesicles. We propose a simple model that describes how the intermembrane interactions tilt the free energy landscape for DNA binding. From our model, we estimate the area per DNA in the binding sites between vesicles and also the total area of the adhesion plaques. We find that vesicles containing a small proportion of cationic lipid that are modified with membrane-anchored DNA can be reversibly tethered by specific DNA interactions and that the DNA also induces a small attraction between these membranes, which stabilizes the DNA duplex. By increasing the equilibrium intermembrane distance on binding, we show that intermembrane interactions become negligible for the binding thermodynamics of the DNA and hence the thermal stability of vesicle aggregates becomes independent of lipid composition at large enough intervesicle separations. We discuss the implications of our findings with regards to cell adhesion and fusion receptors, and the programmable self-assembly of nano-structured materials by DNA hybridization.     

Pardaxin Permeabilizes Vesicles More Efficiently by Pore Formation than by Disruption

Pardaxin is a 33-amino-acid neurotoxin from the Red Sea Moses sole Pardachirus marmoratus, whose mode of action shows remarkable sensitivity to lipid chain length and charge, although the effect of pH is unclear. Here we combine optical spectroscopy and dye release experiments with laser scanning confocal microscopy and natural abundance ¹³C solid-state nuclear magnetic resonance to provide a more complete picture of how pardaxin interacts with lipids. The kinetics and efficiency of release of entrapped calcein is highly sensitive to pH. In vesicles containing zwitterionic lipids (PC), release occurs most rapidly at low pH, whereas in vesicles containing 20% anionic lipid (PG), release occurs most rapidly at high pH. Pardaxin forms stable or transient pores in PC vesicles that allow release of contents without loss of vesicle integrity, whereas the inclusion of PG promotes total vesicle collapse. In agreement with this, solid-state nuclear magnetic resonance reveals that pardaxin takes up a trans-membrane orientation in 14-O-PC/6-O-PC bicelles, whereas the inclusion of 14-0-PG restricts it to contacts with lipid headgroups, promoting membrane lysis. Pore formation in zwitterionic vesicles is more efficient than lysis of anionic vesicles, suggesting that electrostatic interactions may trap pardaxin in several suboptimal interconverting conformations on the membrane surface.     

Lipid miscibility and size increase of vesicles composed of two phosphatidylcholines

The size increase of small unilamellar vesicles composed of binary mixtures either of saturated fatty acid phosphatidylcholines with different chain lengths or of saturated and unsaturated phosphatidylcholines was found to depend on the miscibility properties of the lipid components. No size increase was detected in vesicles formed by two miscible phosphatidylcholines. In vesicles composed of two lipids which are partially immiscible in the gel state, a size increase was observed at temperatures which mainly overlapped the range of temperatures of the lipid phase transition. The rate of size increase of vesicles composed of two lipids which are immiscible in the gel state was faster than that of vesicles composed of two partially immiscible phosphatidylcholines, and the process occurred not only at the temperature ranges of the lipid phase transition, but also when both lipids were in the gel state. The vesicle size increase process occurred without the mixing of the internal content of the vesicles. A model is proposed in which the presence of 'fractures' between membrane regions of different fluidity and/or lipid composition controls the rate of this process.     

Minimal Effect of Lipid Charge on Membrane Miscibility Phase Behavior in Three Ternary Systems

Giant unilamellar vesicles composed of a ternary mixture of phospholipids and cholesterol exhibit coexisting liquid phases over a range of temperatures and compositions. A significant fraction of lipids in biological membranes are charged. Here, we present phase diagrams of vesicles composed of phosphatidylcholine (PC) lipids, which are zwitterionic; phosphatidylglycerol (PG) lipids, which are anionic; and cholesterol (Chol). Specifically, we use DiPhyPG-DPPC-Chol and DiPhyPC-DPPG-Chol. We show that miscibility in membranes containing charged PG lipids occurs over similarly high temperatures and broad lipid compositions as in corresponding membranes containing only uncharged lipids, and that the presence of salt has a minimal effect. We verified our results in two ways. First, we used mass spectrometry to ensure that charged PC/PG/Chol vesicles formed by gentle hydration have the same composition as the lipid stocks from which they are made. Second, we repeated the experiments by substituting phosphatidylserine for PG as the charged lipid and observed similar phenomena. Our results consistently support the view that monovalent charged lipids have only a minimal effect on lipid miscibility phase behavior in our system.