Exchange and interactions between lipid layers at the surface of a liposome solution

Particle Metrology Approach to Understanding How Storage Conditions Affect Long-Term Liposome Stability

Lipid nanoparticles are a generic type of nanomaterial with broad applicability in medicine as drug delivery vehicles. Liposomes are a subtype of lipid nanoparticles and, as a therapeutic platform, can be loaded with a genetic material or pharmaceutical agents for use as drug treatments. An open question for these types of lipid nanoparticles is what factor(s) affect the long-term stability of the particles. The stability of the particle is of great interest to understand and predict the effective shelf-life and storage requirements. In this report, we detail a one-year study of liposome stability as a function of lipid composition, buffer composition/pH, and storage temperature. This was done in aqueous solution without freezing. The effect of lipid composition is shown to be a critical factor when evaluating stability of the measured particle size and number concentration. Other factors (i.e., storage temperature and buffer pH/composition) were shown to be less critical but still have some effect. The stability of these particles informs formulation and optimal storage requirements and assists with future developmental planning of a NIST liposome-based reference material. This work also highlights the complex nature of long-term soft particle storage in biopharmaceutical applications.

Interfacial behavior of phospholipid monolayers revealed by mesoscopic simulation

A mesoscopic model with molecular resolution is presented for dipalmitoyl phosphatidylcholine (DPPC) and palmitoyl oleoyl phosphatidylcholine (POPC) monolayer simulations at the air-water interface using many-body dissipative particle dynamics (MDPD). The parameterization scheme is rigorously based on reproducing the physical properties of water and alkane and the interfacial property of the phospholipid monolayer by comparison with experimental results. Using much less computing cost, these MDPD simulations yield a similar surface pressure-area isotherm as well as similar pressure-related morphologies as all-atom simulations and experiments. Moreover, the compressibility modulus, order parameter of lipid tails, and thickness of the phospholipid monolayer are quantitatively in line with the all-atom simulations and experiments. This model also captures the sensitive changes in the pressure-area isotherms of mixed DPPC/POPC monolayers with altered mixing ratios, indicating that the model is promising for applications with complex natural phospholipid monolayers. These results demonstrate a significant improvement of quantitative phospholipid monolayer simulations over previous coarse-grained models.

Mechanism of Time-Dependent Adsorption for Phosphatidylcholine onto a Clean Air-Water Interface from a Dispersion of Vesicles: Effect of Temperature and Acyl Chain Length

Dynamic surface tension measurements were used to track adsorption kinetics for dilauroylphosphatidylcholine (DLPC) or dimyristoylphosphatidylcholine (DMPC) from monodisperse vesicle dispersions to an air-water interface at elevated temperatures ≥30 °C. Effects of vesicle concentration, aqueous solubility of the lipids, and temperature T on the adsorption kinetics were determined, and the controlling transport pathway was identified. Adsorption dynamics were tracked for 0.1-10 mM DLPC at 30 and 38 °C and for 1-10 mM DMPC at 30, 50, and 58 °C. Experimental results were compared to theoretical predictions for a reaction-enhanced, molecular transport mechanism, which was previously shown to effectively predict DLPC adsorption kinetics at 22 °C. At higher temperatures, for DLPC concentrations ≥0.25 mM or DMPC concentrations ≥1 mM, a weak dependence of adsorption time on concentration was observed, again consistent with the reaction-enhanced molecular pathway. Molecular release rates from vesicles increased with increasing temperature or decreasing acyl chain length. At equivalent ratios T/T_m of the dispersion temperature to the lipid chain phase transition temperature T_m, measured adsorption times for DLPC were approximately 10-fold shorter than those for DMPC, suggesting that the fluidity of the acyl tails is not the only lipid property determining adsorption rates. Despite the significant difference in aqueous solubility and chain phase transition temperature between DLPC and DMPC, the results provide further evidence for an adsorption mechanism that is well described by diffusion of molecular lipid, with rates of molecular diffusion near the interface enhanced by release from nearby vesicles.

Adsorption of Phospholipids at the Air-Water Surface

Phospholipids are ubiquitous components of biomembranes and common biomaterials used in many bioengineering applications. Understanding adsorption of phospholipids at the air-water surface plays an important role in the study of pulmonary surfactants and cell membranes. To date, however, the biophysical mechanisms of phospholipid adsorption are still unknown. It is challenging to reveal the molecular structure of adsorbed phospholipid films. Using combined experiments with constrained drop surfactometry and molecular dynamics simulations, here, we studied the biophysical mechanisms of dipalmitoylphosphatidylcholine (DPPC) adsorption at the air-water surface. It was found that the DPPC film adsorbed from vesicles showed distinct equilibrium surface tensions from the DPPC monolayer spread via organic solvents. Our simulations revealed that only the outer leaflet of the DPPC vesicle is capable of unzipping and spreading at the air-water surface, whereas the inner leaflet remains intact and forms an inverted micelle to the interfacial monolayer. This inverted micelle increases the local curvature of the monolayer, thus leading to a loosely packed monolayer at the air-water surface and hence a higher equilibrium surface tension. These findings provide novel insights, to our knowledge, into the mechanism of the phospholipid and pulmonary surfactant adsorption and may help understand the structure-function correlation in biomembranes.

Co-Encapsulation and Co-Delivery of Peptide Drugs via Polymeric Nanoparticles

Combination therapy is a promising form of treatment. In particular, co-treatment of P3 and QBP1 has been shown to enhance therapeutic effect in vivo in treating polyglutamine diseases. These peptide drugs, however, face challenges in clinical administration due to poor stability, inability to reach intracellular targets, and lack of method to co-deliver both drugs. Here we demonstrate two methods of co-encapsulating the peptide drugs via polymer poly(ethylene glycol)-block-polycaprolactone (PEG-b-PCL) based nanoparticles. Nanoparticles made by double emulsion were 100⁻200 nm in diameter, with drug encapsulation efficiency of around 30%. Nanoparticles made by nanoprecipitation with lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG) were around 250⁻300 nm in diameter, with encapsulation efficiency of 85⁻100%. Particles made with both formulations showed cellular uptake when decorated with a mixture of peptide ligands that facilitate endocytosis. In vitro assay showed that nanoparticles could deliver bioactive peptides and encapsulation by double emulsion were found to be more effective in rescuing cells from polyglutamine-induced toxicity.

Mechanism of Time-Dependent Adsorption for Dilauroyl Phosphatidylcholine onto a Clean Air-Water Interface from a Dispersion of Vesicles

This study focused on mechanisms of adsorption for dilauroyl phosphatidylcholine (DLPC) from a dispersion of large, unilamellar vesicles (LUVs) onto a clean air-water interface. The adsorption kinetics were tracked using dynamic surface tension measurements for 0.01-10 mM concentrations of DLPC, contained within monodisperse LUVs with mean diameters between 100 and 300 nm. Any lipid in excess of the solubility limit, determined to be 1.1(±0.7) × 10^-5 mM (1.1 × 10^-8 M), was assumed to be in vesicle form. The adsorption rate was found to increase with increasing lipid concentration and decreasing vesicle diameter, indicating a clear mechanistic role for the vesicles. An induction regime was observed, during which lipid adsorption occurred without significantly changing the surface tension. Pressure-area isotherm data suggested that the surface concentration at the end of this induction period was ∼50% of the concentration at saturation, with the latter estimated as 4.2(±0.7) × 10^-6 mol/m². Convection was also introduced into these experiments to probe the importance of bulk transport mechanisms to the overall kinetics. Theoretical expressions for possible contributing mechanisms and pathways, via molecular and/or vesicle transport, were developed and used to predict associated transport time scales for different scenarios. These theoretical time scales were compared to experimentally measured characteristic times for a variety of DLPC concentrations, vesicle diameters, and convection rates. For DLPC concentrations ≥0.25 mM, our results were consistent with the monolayer formation arising from a molecular transport mechanism that is enhanced by vesicle-to-monomer exchange beneath the interface. At lower concentrations, experimental rates of adsorption increased with increasing convection, and a strong effect of lipid concentration was also observed. For DLPC ≤0.25 mM, transport controlled by direct interfacial vesicle adsorption reasonably captured the observed effect of lipid concentration; however, neither monomer nor vesicle pathway mechanisms captured the influence of convection. Understanding the adsorption kinetics for such nearly insoluble surfactant systems is important in several areas, including food emulsification, foam or microbubble formulation, spray drying techniques, and therapeutics.

Interactions of Lipidic Cubic Phase Nanoparticles with Lipid Membranes

The interactions of liquid-crystalline monoolein (GMO) cubic phase nanoparticles with various model lipid membranes spread at the air-solution interface by the Langmuir technique were investigated. Cubosomes have attracted attention as potential biocompatible drug delivery systems, and thus understanding their mode of interaction with membranes is of special interest. Cubosomes spreading at the air-water interface as well as interactions with a monolayer of 1, 2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) compressed to different surface pressures were studied by monitoring surface pressure-time dependencies at constant area. Progressive incorporation of the nanoparticles was shown to lead to mixed monolayer formation. The concentration of cubosomes influenced the mechanism of incorporation, as well as the fluidity and permeability of the resulting lipid membranes. Brewster angle microscopy images reflected the dependence of the monolayer structure on the cubosomes presence in the subphase. A parameter Csat was introduced to indicate the point of saturation of the lipid membrane with the cubosomal material. This parameter was found to depend on the surface pressure showing that the cubosomes disintegrate in prolonged contact with the membrane, filling available voids in the lipid membrane. At highest surface pressures when the layer is most compact, the penetration of cubosomal material is not possible and only some exchange with the membrane lipid becomes the route of including GMO into the layer. Finally, comparative studies of the interactions between lipids with various headgroup charges with cubosomes suggest that at high surface pressure an exchange of lipid component between the monolayer and the cubosome in its intact form may occur.

Adsorption Kinetics Dictate Monolayer Self-Assembly for Both Lipid-In and Lipid-Out Approaches to Droplet Interface Bilayer Formation

The droplet interface bilayer (DIB)--a method to assemble planar lipid bilayer membranes between lipid-coated aqueous droplets--has gained popularity among researchers in many fields. Well-packed lipid monolayer on aqueous droplet-oil interfaces is a prerequisite for successfully assembling DIBs. Such monolayers can be achieved by two different techniques: "lipid-in", in which phospholipids in the form of liposomes are placed in water, and "lipid-out", in which phospholipids are placed in oil as inverse micelles. While both approaches are capable of monolayer assembly needed for bilayer formation, droplet pairs assembled with these two techniques require significantly different incubation periods and exhibit different success rates for bilayer formation. In this study, we combine experimental interfacial tension measurements with molecular dynamics simulations of phospholipids (DPhPC and DOPC) assembled from water and oil origins to understand the differences in kinetics of monolayer formation. With the results from simulations and by using a simplified model to analyze dynamic interfacial tensions, we conclude that, at high lipid concentrations common to DIBs, monolayer formation is simple adsorption controlled for lipid-in technique, whereas it is predominantly adsorption-barrier controlled for the lipid-out technique due to the interaction of interface-bound lipids with lipid structures in the subsurface. The adsorption barrier established in lipid-out technique leads to a prolonged incubation time and lower bilayer formation success rate, proving a good correlation between interfacial tension measurements and bilayer formation. We also clarify that advective flow expedites monolayer formation and improves bilayer formation success rate by disrupting lipid structures, rather than enhancing diffusion, in the subsurface and at the interface for lipid-out technique. Additionally, electrical properties of DIBs formed with varying lipid placement and type are characterized.

Maintenance of ischemic β cell viability through delivery of lipids and ATP by targeted liposomes

Islet transplantation is a promising treatment for type 1 diabetes, but despite the successes, existing challenges prevent widespread application. Ischemia, occurring during pancreas preservation and isolation, as well as after islet transplantation, decreases islet viability and function. We hypothesized that the liposomal delivery of adenosine triphosphate (ATP) could prevent the loss of cell viability during an ischemic insult. In this work we use a model β cell line, INS-1 to probe the liposome/cell interactions and examined the ability of liposomes functionalized with the fibronectin-mimetic peptide PR_b to facilitate the delivery of ATP to ischemic β cells. We demonstrate that PR_b increases the binding and internalization of liposomes to the β cells. Unexpectedly, when comparing the ability of PR_b liposomes with and without ATP to protect INS-1 cells from ischemia we found that both formulations increased cell survival. By probing the functional activity of ischemic cells treated with PR_b functionalized liposomes with and without ATP we find that both lipids and ATP play a role in maintaining cell metabolic activity after an ischemic insult and preventing cell necrosis. This approach may be beneficial for preventing ischemia related damage to islet cells, especially in the organ preservation stage.

Adsorption, organization, and rheology of catanionic layers at the air/water interface

We have investigated the adsorption and organization at the air/water interface of catanionic molecules released from a dispersion of solid-like catanionic vesicles composed of myristic acid and cetyl trimethylammonium chloride at the 2:1 ratio. These vesicles were shown recently to be promising foam stabilizers. Using Brewster angle microscopy, we observed the formation of a catanionic monolayer at the air/water interface composed of liquid-condensed domains in a liquid-expanded matrix. Further adsorption of catanionic molecules forced them to pack, thereby forming a very dense monolayer that prevented further vesicle rupture by avoiding contact of the vesicles with air. Moreover, confocal fluorescence microscopy revealed the presence of layers of intact vesicles that were progressively creaming toward this catanionic monolayer; the surface tension of the vesicle dispersion remained constant upon creaming. The catanionic monolayer behaved as a soft glassy material, an amorphous solid with time- and temperature-dependent properties. Using interfacial oscillatory rheology, we found that the monolayer relaxed mechanical stresses in seconds and melted at a temperature very close to the melting transition temperature of the vesicle bilayers. These results have potential application in the design of smart foams that have temperature-tunable stability.

Thermodynamics of lipid interactions with cell-penetrating peptides

Cationic peptides are efficiently taken up by biological cells through different pathways, which can be exploited for delivery of intracellular drugs. For example, their endocytosis is known since 1967, and this typically produces entrapment of the peptides in endocytotic vesicles. The resulting peptide (and cargo) degradation in lysosomes is of little therapeutic interest. Beside endocytosis (and various subtypes thereof), cationic cell-penetrating peptides (CPPs) may also gain access to cytosol and nucleus of livings cells. This process is known since 1988, but it is poorly understood whether the cytosolic CPP appearance requires an active cellular machinery with membrane proteins and signaling molecules, or whether this translocation occurs by passive diffusion and thus can be mimicked with model membranes devoid of proteins or glycans. In the present chapter, protocols are presented that allow for testing the membrane binding and disturbance of CPPs on model membranes with special focus on particular CPP properties. Protocols include vesicle preparation, lipid quantification, and analysis of membrane leakage, lipid polymorphism ((31)P NMR), and membrane binding (isothermal titration calorimetry). Using these protocols, a major difference among CPPs is observed: At low micromolar concentration, nonamphipathic CPPs, such as nona-arginine (WR(9)) and penetratin, have only a poor affinity for model membranes with a lipid composition typical of eukaryotic membranes. No membrane leakage is induced by these compounds at low micromolar concentration. In contrast, their amphipathic derivatives, such as acylated WR(9) (C(14), C(16), C(18)) or amphipathic penetratin mutant p2AL (Drin et al., Biochemistry 40:1824-1834, 2001), bind and disturb lipid model membranes already at low micromolar peptide concentration. This suggests that the mechanism for cytosolic CPP delivery (and potential toxicity) differs among CPPs despite their common name.

Test systems to study the structure and function of uncoupling protein 1: a critical overview

The discovery of active brown adipose tissue (BAT) in healthy adult humans has renewed interest in the biology of this organ. BAT is capable of distributing nutrient energy in the form of heat allowing small mammals to efficiently defend their body temperature when acutely exposed to the cold. On the other hand BAT might be a target for the treatment of obesity and related diseases, as its pharmacological activation could allow release of excess energy stored in white adipose tissue depots. Energy dissipation in BAT depends on the activity of uncoupling protein 1 (UCP1), therefore a BAT-based obesity therapy requires a detailed understanding of structure and function of UCP1. Although UCP1 has been in the focus of research since its discovery, central questions concerning its mechanistic function and regulation are not yet resolved. They have been addressed in native mitochondria but also in several test systems, which are generally used to lower inter-experimental variability and to simplify analysis conditions. Different test systems have contributed to our current knowledge about UCP1 but of course all of them have certain limitations. We here provide an overview about research on UCP1 structure and function in test systems. So far, these have nearly exclusively been employed to study rodent and not human UCP1. Considering that the amino acid sequence of mouse and human UCP1 is only 79% identical, it will be essential to test whether the human version has a similarly high catalytic activity, allowing a relevant amount of energy dissipation in human BAT. Besides the issue of comparable mechanistic function a sufficiently high expression level of human UCP1 is a further prerequisite for anti-obesity therapeutic potential. Treatments which induce BAT hyperplasia and UCP1 expression in humans might therefore be equally important to discover as mere activators of the thermogenic process.

Exploring the interactions of gliadins with model membranes: effect of confined geometry and interfaces

Mechanisms leading to the assembly of wheat storage proteins into proteins bodies within the endoplasmic reticulum (ER) of endosperm cells are unresolved today. In this work, physical chemistry parameters which could be involved in these processes were explored. To model the confined environment of proteins within the ER, the dynamic behavior of gamma-gliadins inserted inside lyotropic lamellar phases was studied using FRAP experiments. The evolution of the diffusion coefficient as a function of the lamellar periodicity enabled to propose the hypothesis of an interaction between gamma-gliadins and membranes. This interaction was further studied with the help of phospholipid Langmuir monolayers. gamma- and omega-gliadins were injected under DMPC and DMPG monolayers and the two-dimensional (2D) systems were studied by Brewster angle microscopy (BAM), polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS), and surface tension measurements. Results showed that both gliadins adsorbed under phospholipid monolayers, considered as biological membrane models, and formed micrometer-sized domains at equilibrium. However, their thicknesses, probed by reflectance measurements, were different: omega-gliadins aggregates displayed a constant thickness, consistent with a monolayer, while the thickness of gamma-gliadins aggregates increased with the quantity of protein injected. These different behaviors could find some explanations in the difference of aminoacid sequence distribution: an alternate repeated - unrepeated domain within gamma-gliadin sequence, while one unique repeated domain was present within omega-gliadin sequence. All these findings enabled to propose a model of gliadins self-assembly via a membrane interface and to highlight the predominant role of wheat prolamin repeated domain in the membrane interaction. In the biological context, these results would mean that the repeated domain could be considered as an anchor for the interaction with the ER membrane and a nucleus point for the formation and growth of protein bodies within endosperm cells. (c) 2009 Wiley Periodicals, Inc. Biopolymers 91: 610-622, 2009.This article was originally published online as an accepted preprint. The "Published Online" date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com.

Determining the conductance of the SecY protein translocation channel for small molecules

The channel formed by the SecY complex must maintain the membrane barrier for ions and other small molecules during the translocation of membrane or secretory proteins. We have tested the permeability of the channel by using planar bilayers containing reconstituted purified E. coli SecY complex. Wild-type SecY complex did not show any conductance for ions or water. Deletion of the "plug," a short helix normally located in the center of the SecY complex, or modification of a cysteine introduced into the plug resulted in transient channel openings; a similar effect was seen with a mutation in the pore ring, a constriction in the center of the channel. Permanent channel opening occurred when the plug was moved out of the way by disulfide-bridge formation. These data show that the resting channel on its own forms a barrier for small molecules, with both the pore ring and the plug required for the seal; channel opening requires movement of the plug.

Kinetic studies of spreading DMPC vesicles at the air-solution interface using film pressure measurements

DMPC vesicles were injected into a 50 mM NaF solution in water, and the kinetics of a monolayer formation at the air-solution interface was investigated by measuring changes of the film pressure, pi, as a function of time. The studies were carried out in temperature range from 5 to 35 degrees C. The solutions were stirred in order to eliminate the mass transport limitations. Under these conditions, the monolayer formation was controlled by the surface processes only. At temperatures above the critical temperature Tc, compression isotherms were measured and used to convert the pi-t curves into the gamma-t plots, with gamma being the surface concentration of DMPC. These kinetic data indicate that the monolayer formation involves fast rupture of vesicles and formation of a constant number of disk-shaped monolayer islands at the air-solution interface that grow with a constant radial rate. At higher coverages, the growth is restricted by the availability of the monolayer-free surface area. At temperatures below Tc, the pi-t curves cannot be converted into the gamma-t plots. Here the kinetics can be discussed only qualitatively. The data indicate that the kinetics of the monolayer formation involves two steps. Initially, an expanded film is formed. At higher film pressures, the expanded film is slowly transformed into a liquid condensed state.

Formation and characterization of phospholipid monolayers spontaneously assembled at interfaces between aqueous phases and thermotropic liquid crystals

This paper reports an experimental investigation of the self-assembly of phospholipids (l-alpha-phosphatidylcholine-beta-oleoyl-gamma-palmitoyl (l-POPC), dipalmitoyl phosphatidylcholine (DPPC), and l-alpha-dilauroyl phosphatidylcholine (l-DLPC)) at interfaces between aqueous phases and the nematic liquid crystal (LC) 4'-pentyl-4-cyanobiphenyl. Stable planar interfaces between the aqueous phases and LCs were created by hosting the LCs within gold grids (square pores with widths of 283 microm and depths of 20 microm). At these interfaces, the presence and lateral organization of the phospholipids leads to interface-driven orientational transitions within the LC. By doping the phospholipids with a fluorescently labeled lipid (Texas Red-1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (TR-DPPE)), quantitative epifluorescence microscopy revealed the saturation coverage of phospholipid at the interface to be that of a monolayer with an areal density of approximately 49 +/- 8% relative to hydrated lipid bilayers. By adsorbing phospholipids to the aqueous-LC interface from either vesicles or mixed micelles of dodecyltrimethylammonium and phospholipid, control of the areal density of phospholipid from 42 +/- 10 to 102 +/-18% of saturation monolayer coverage was demonstrated. Fluorescence recovery after photobleaching (FRAP) experiments performed by using laser scanning confocal microscopy (LSCM) revealed the lateral mobility of fluorescently labeled DPPE in l-DLPC assembled at the interface with the liquid crystal to be (6 +/- 1) x 10(-12) m(2)/s for densely packed monolayers. Variation of the surface coverage and composition of phospholipid led to changes in lateral diffusivity between (0.2 +/- 0.1) x 10(-12) and (15 +/- 2) x 10(-12) m(2)/s. We also observed the phospholipid-laden interface to be compartmentalized by the gold grid, thus allowing for the creation of patterned arrays of phospholipids at the LC-aqueous interface.

Preferential orientation of an immunoglobulin in a glycolipid monolayer controlled by the disintegration kinetics of proteo-lipidic vesicles spread at an air–buffer interface

The insertion of immunoglobulin (IgG) in a glycolipid monolayer was achieved by using the ability of new proteo-glycolipid vesicles to disintegrate into a mixed IgG-glycolipid interfacial film after spreading at an air-buffer interface. The interfacial disintegration kinetics was shown to be directly dependent on the initial vesicle surface density and on the buffer ionic strength. The presence of the immunoglobulin in the glycolipid film was displayed by an increase of the lateral compressibility (Cs) during monolayer compression. Cs magnitude modifications, due to the antibody effect on the monolayer packing, decreases as the spread vesicle density increases. At interfacial saturation, the lateral compressibility profile becomes similar to that of a control monolayer without antibody. However, the careful analysis of the mixed monolayer after transfer by Langmuir-Blodgett technique (ATR-FTIR characterisation, enzyme immunoassociation) clearly demonstrated that the antibody was still present in such conditions and was not completely squeezed out from the interface as compressibility changes could have meant. At nonsaturating vesicle surface density, IgG molecules initially lying in the lipid matrix with the Y-shape plane parallel to the interface move to a standing-up position during the compression, leading to lateral compressibility modifications. For a saturating vesicle surface density, the glycolipid molecules force the IgG molecules to directly adopt a more vertical position in the interfacial film and, consequently, no lateral compressibility modification was recorded during the compression.

Interaction of synapsin I with membranes

The synapsins (I, II, and III) comprise a family of peripheral membrane proteins that are involved in both regulation of neurotransmitter release and synaptogenesis. Synapsins are concentrated at presynaptic nerve terminals and are associated with the cytoplasmic surface of synaptic vesicles. Membrane-binding of synapsins involves interaction with both protein and lipid components of synaptic vesicles. Synapsin I binds rapidly and with high affinity to liposomes containing anionic lipids. The binding of bovine synapsin I to liposomes was studied using fluoresceinphosphatidyl-ethanolamine (FPE) to measure membrane electrostatic potential. Synapsin binding to liposomes caused a rapid increase in FPE fluorescence, indicating an increase in positive charge at the membrane surface. Synapsin I binding to monolayers resulted in a substantial increase in monolayer surface pressure. At higher initial surface pressures, the synapsin-induced increase in monolayer surface pressure is dependent on the presence of anionic lipids in the monolayer. Synapsin I also induced rapid aggregation of liposomes, but did not induce leakage of entrapped carboxyfluorescein, while other aggregation-inducing agents promoted extensive leakage. These results are in agreement with the presence of amphipathic stretches of amino acids in synapsin I that exhibit both electrostatic and hydrophobic interactions with membranes, and offer a molecular explanation for the high affinity binding of synapsin I to liposomes and for stabilization of membranes by synapsin I.

Comparative membrane interaction study of viscotoxins A3, A2 and B from mistletoe (Viscum album) and connections with their structures

Viscotoxins A2 (VA2) and B (VB) are, together with viscotoxin A3 (VA3), among the most abundant viscotoxin isoforms that occur in mistletoe-derived medicines used in anti-cancer therapy. Although these isoforms have a high degree of amino-acid-sequence similarity, they are strikingly different from each other in their in vitro cytotoxic potency towards tumour cells. First, as VA3 is the only viscotoxin whose three-dimensional (3D) structure has been solved to date, we report the NMR determination of the 3D structures of VA2 and VB. Secondly, to account for the in vitro cytotoxicity discrepancy, we carried out a comparative study of the interaction of the three viscotoxins with model membranes. Although the overall 3D structure is highly conserved among the three isoforms, some discrete structural features and associated surface properties readily account for the different affinity and perturbation of model membranes. VA3 and VA2 interact in a similar way, but the weaker hydrophobic character of VA2 is thought to be mainly responsible for the apparent different affinity towards membranes. VB is much less active than the other two viscotoxins and does not insert into model membranes. This could be related to the occurrence of a single residue (Arg25) protruding outside the hydrophobic plane formed by the two amphipathic alpha-helices, through which viscotoxins are supposed to interact with plasma membranes.

Lipid compositional analysis of pulmonary surfactant monolayers and monolayer-associated reservoirs

Pulmonary surfactant is a lipid:protein complex containing dipalmitoyl-phosphatidylcholine (DPPC) as the major component. Recent studies indicate adsorbed surfactant films consist of a surface monolayer and a monolayer-associated reservoir. It has been hypothesized that the monolayer and its functionally contiguous reservoir may be enriched in DPPC relative to bulk phase surfactant. We investigated the compositional relationship between the monolayer and its reservoir using paper-supported wet bridges to transfer films from adsorbing dishes to clean surfaces on spreading dishes. Spreading films appear to form monolayers in the spreading dishes. We employed bovine lipid extract surfactant [BLES(chol)] containing [3H]DPPC and either [14C]palmitoyl, oleoyl-phosphatidylcholine (POPC), [14C]dipalmitoyl-phosphatidylglycerol (DPPG), [14C]palmitoyl, oleoyl-phosphatidylglycerol (POPG), or [14C]cholesterol. Radiolabeled phosphatidylglycerols were prepared using phospholipase D. The studies demonstrated that the [3H]DPPC-[14C] POPC ratios were the same in the prepared BLES dispersions as in Langmuir-Blodgett films, indicating a lack of DPPC selectivity during film formation. Furthermore, identical 3H-14C isotopic ratios were observed with DPPC and either 14C-labeled POPC, DPPG, POPG, or cholesterol in the original dispersions, the bulk phases in adsorption dish D1, and monolayers recovered from spreading dish D2. These relationships remained unperturbed with 2-fold increases in bulk concentrations in D1 and 10-fold variations in D1-D2 surface area. These results indicate adsorbed surfactant monolayers and their associated reservoirs possess similar lipid compositions and argue against selective adsorption of DPPC.

Surface behavior, microheterogeneity and adsorption equilibrium of myelin at the air-water interface

Interfacial films of whole myelin membrane adsorb at the air-water interface from myelin vesicles. The films show a liquid state and their equilibrium spreading pressure is equal to the collapse pressure (about 47 mN/m). The films appear microheterogeneous as seen by epifluorescence microscopy, consisting in two liquid phases over all the adsorption isotherm, starting with rounded liquid expanded domains (low surface pressure) immersed in a cholesterol enriched phase and reaching a fractal pattern at high surface pressure similar to those previously observed by compressing the film. Vesicles adsorb to the interfacial film mainly at the lateral interfaces. The high surface pressure at equilibrium (almost equal to the collapse pressure) indicates the formation of surface multilayers, also shown by fluorescence microscopy.

Compositional domain immiscibility in whole myelin monolayers at the air-water interface and Langmuir-Blodgett films

Monomolecular layers of whole myelin membrane can be formed at the air-water interface from vesicles or from solvent solution of myelin. The films appear microheterogeneous as seen by epifluorescence and Brewster angle microscopy. The pattern consists mainly of two coexisting liquid phases over the whole compression isotherm. The liquid nature of the phases is apparent from the fluorescent probe behavior, domain mobility, deformability and boundary relaxation due to the line tension of the surface domains. The monolayers were transferred to alkylated glass and fluorescently labeled against myelin components. The immunolabeling of two major proteins of myelin (myelin basic protein, proteolipid-DM20) and of 2',3'-cyclic nucleotide 3'-phosphodiesterase shows colocalization with probes partitioning preferentially in liquid-expanded lipid domains also containing ganglioside G(M1). A different phase showing an enrichment in cholesterol, galactocerebroside and phosphatidylserine markers is also found. The distribution of components is qualitatively independent of the lateral surface pressure and is generally constituted by one phase enriched in charged components in an expanded state coexisting with another phase enriched in non-charged constituents of lower compressibility. The domain immiscibility provides a physical basis for the microheterogeneity found in this membrane model system.

Modes of membrane interaction of a natural cysteine-rich peptide: viscotoxin A3

Among the very homologous family of alpha- and beta-thionins, known for their antimicrobial activity, the viscotoxin subfamily differs from other members because it is cytotoxic against tumoral cells but weakly hemolytic. We studied the interactions between the most active of these toxins, viscotoxin A3 (VA3), and model membranes made of phosphatidylcholine and phosphatidylserine (PS), the major zwitterionic and acidic phospholipids found in eukaryotic cells. Monolayer studies showed that electrostatic forces are essential for the interaction and are mainly involved in modulating the embedding of the toxin in the PS head group region. This in turn induces membrane stiffening, as shown by fluorescence polarization assays with 1,6-diphenyl-1,3,5-hexatriene and its derivatives. Moreover, vesicle permeabilization analyses showed that there are two modes of interaction, which are directly related to the stiffening effect and depend on the amount of VA3 bound to the surface of the vesicles. We propose an interaction model in which the embedding of VA3 in the membrane induces membrane defects leading to the gradual release of encapsulated dye. When the surfaces of the vesicles are saturated with the viscotoxin, complete vesicle destabilization is induced which leads to bilayer disruption, all-or-none encapsulated dye release and rearrangement of the vesicles.

Water and ion permeation of aquaporin-1 in planar lipid bilayers. Major differences in structural determinants and stoichiometry

The aquaporin-1 (AQP1) water channel protein is known to facilitate the rapid movement of water across cell membranes, but a proposed secondary role as an ion channel is still unsettled. Here we describe a method to simultaneously measure water permeability and ion conductance of purified human AQP1 after reconstitution into planar lipid bilayers. Water permeability was determined by measuring Na(+) concentrations adjacent to the membrane. Comparisons with the known single channel water permeability of AQP1 indicate that the planar lipid bilayers contain from 10(6) to 10(7) water channels. Addition of cGMP induced ion conductance in planar bilayers containing AQP1, whereas cAMP was without effect. The number of water channels exceeded the number of active ion channels by approximately 1 million-fold, yet p-chloromethylbenzenesulfonate inhibited the water permeability but not ion conductance. Identical ion channel parameters were achieved with AQP1 purified from human red blood cells or AQP1 heterologously expressed in Saccharomyces cerevisae and affinity purified with either N- or C-terminal poly-histidine tags. Rp-8-Br-cGMP inhibited all of the observed conductance levels of the cation selective channel (2, 6, and 10 pS in 100 mm Na(+) or K(+)). Deletion of the putative cGMP binding motif at the C terminus by introduction of a stop codon at position 237 yielded a truncated AQP1 protein that was still permeated by water but not by ions. Our studies demonstrate a method for simultaneously measuring water permeability and ion conductance of AQP1 reconstituted into planar lipid bilayers. The ion conductance occurs (i) through a pathway distinct from the aqueous pathway, (ii) when stimulated directly by cGMP, and (iii) in only an exceedingly small fraction of AQP1 molecules.

Highly selective water channel activity measured by voltage clamp: analysis of planar lipid bilayers reconstituted with purified AqpZ

Aquaporins are membrane channels selectively permeated by water or water plus glycerol. Conflicting reports have described ion conductance associated with some water channels, raising the question of whether ion conductance is a general property of the aquaporin family. To clarify this question, a defined system was developed to simultaneously measure water permeability and ion conductance. The Escherichia coli water channel aquaporin-Z (AqpZ) was studied, because it is a highly stable tetramer. Planar lipid bilayers were formed from unilamellar vesicles containing purified AqpZ. The hydraulic conductivity of bilayers made from the total extract of E. coli lipids increased 3-fold if reconstituted with AqpZ, but electric conductance was unchanged. No channel activity was detected under voltage-clamp conditions, indicating that less than one in 10(9) transport events is electrogenic. Microelectrode measurements were simultaneously undertaken adjacent to the membrane. Changes in sodium concentration profiles accompanying transmembrane water flow permitted calculation of the activation energies: 14 kcal/mol for protein-free lipid bilayers and 4 kcal/mol for lipid bilayers containing AqpZ. Neither the water permeability nor the electric conductivity exhibited voltage dependence. This sensitive system demonstrated that AqpZ is permeated by water but not charged ions and should permit direct analyses of putative electrogenic properties of other aquaporins.

The role of the gel <=> liquid-crystalline phase transition in the lung surfactant cycle

Lipid polymorphism plays an important role in the lung surfactant cycle. A better understanding of the influence of phase transitions on the formation of a lipid film from dispersions of vesicles will help to describe the mechanism of action of lung surfactant. The surface pressure (or tension) of dispersions of DPPC, DMPC, and DPPE unilamellar vesicles was studied as a function of temperature. These aggregates rapidly fuse with a clean air-water interface when the system is at their phase transition temperature (Tm), showing a direct correlation between phase transition and film formation. Based on these results, an explanation on how fluid aggregates in the alveolar subphase can form a rigid monolayer at the alveolar interface is proposed.

Fusion of vesicles with the air-water interface: the influence of polar head group, salt concentration, and vesicle size

Fusion of vesicles with the air-water interface and consequent monolayer formation has been studied as a function of temperature. Unilamellar vesicles of DMPC, DPPC, and DODAX (X=Cl(-), Br(-)) were injected into a subphase containing NaCl, and the surface pressure (tension) was recorded on a Langmuir Balance (Tensiometer) using the Wilhelmy plate (Ring) method. For the zwitterionic vesicles, plots of the initial surface pressure increase rate (surface tension decrease rate) as a function of temperature show a peak at the phase transition temperature (T(m)) of the vesicles, whereas for ionic ones they show a sharp rise. At high concentrations of NaCl, ionic DODA(Cl) vesicles seem to behave like zwitterionic ones, and the rate of fusion is higher at the T(m). The influence of size was studied comparing large DODA(Cl) vesicles with small sonicated ones, and no significant changes were found regarding the rate of fusion with the air-water interface.

On the role of sterol in the formation of the amphotericin B channel

Amphotericin B is an antimycotic agent that has been studied for a long time, both because of its pharmacological action and the interest in understanding how this ionic channel works. It has been proposed that the channel is formed by a barrel of monomers, and that the presence of sterol is needed for the formation of such a barrel. As a matter of fact this need of a sterol has been used as a guiding idea in attempts to design derivatives more efficient in the discrimination of the cholesterol containing membranes, as compared to the ergosterol containing ones, henceforth diminishing the unwanted side effects in its pharmacological use. In this work we show that unitary channels that appear in a cholesterol containing membrane also appear when this membrane is free of cholesterol. We prove this to be the case for two membranes, a biological one, asolectin, and a synthetic one, DMPC. We then advance the idea that the role of sterols in the formation of the amphotericin B channel is related to the effects they have on the structure of the membrane itself, rather than to a direct involvement in the channel formation. We further look into the effect that different cholesterol concentrations in the membrane produce on the single channel properties.