Simulations of lipid vesicle adsorption for different lipid mixtures

Monitoring the electroactive cargo of extracellular vesicles can differentiate various cancer cell lines

Extracellular vesicles (EVs) are pivotal in cell-to-cell communication due to the array of cargo contained within these vesicles. EVs are considered important biomarkers for identification of disease, however most measurement approaches have focused on monitoring specific surface macromolecular targets. Our study focuses on exploring the electroactive component present within cargo from EVs obtained from various cancer and non-cancer cell lines using a disk carbon fiber microelectrode. Variations in the presence of oxidizable components were observed when the total cargo from EVs were measured, with the highest current detected in EVs from MCF7 cells. There were differences observed in the types of oxidizable species present within EVs from MCF7 and A549 cells. Single entity measurements showed clear spikes due to the detection of oxidizable cargo within EVs from MCF7 and A549 cells. These studies highlight the promise of monitoring EVs through the presence of varying electroactive components within the cargo and can drive a wave of new strategies towards specific detection of EVs for diagnosis and prognosis of various diseases.

Electrostatic interactions control the adsorption of extracellular vesicles onto supported lipid bilayers

Communication between cells located in different parts of an organism is often mediated by membrane-enveloped nanoparticles, such as extracellular vesicles (EVs). EV binding and cell uptake mechanisms depend on the heterogeneous composition of the EV membrane. From a colloidal perspective, the EV membrane interacts with other biological interfaces via both specific and non-specific interactions, where the latter include long-ranged electrostatic and van der Waals forces, and short-ranged repulsive "steric-hydration" forces. While electrostatic forces are generally exploited in most EV immobilization protocols, the roles played by various colloidal forces in controlling EV adsorption on surfaces have not yet been thoroughly addressed. In the present work, we study the adsorption of EVs onto supported lipid bilayers (SLBs) carrying different surface charge densities using a combination of quartz crystal microbalance with dissipation monitoring (QCM-D) and confocal laser scanning microscopy (CLSM). We demonstrate that EV adsorption onto lipid membranes can be controlled by varying the strength of electrostatic forces and we theoretically describe the observed phenomena within the framework of nonlinear Poisson-Boltzmann theory. Our modelling results confirm the experimental observations and highlight the crucial role played by attractive electrostatics in EV adsorption onto lipid membranes. They furthermore show that simplified theories developed for model lipid systems can be successfully applied to the study of their biological analogues and provide new fundamental insights into EV-membrane interactions with potential use in developing novel EV separation and immobilization strategies.

Modelling the adsorption of phospholipid vesicles to a silicon dioxide surface using Langmuir kinetics

Supported Lipid Bilayers (SLBs) are model biological membranes that have been developed to study the interactions between biomolecules in a cell membrane. Though forming SLBs is relatively easy, their formation mechanism remains a topic of debate. When buffered solutions containing phosphatidylcholine vesicles are flowed over a silicon dioxide (SiO₂) surface they adsorb intact to the surface to form a Supported Vesicle Layer (SVL) if the pH of the buffer is above 9. We have run experiments with buffers with a pH at or above 9 to study the kinetics of the adsorption of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) vesicles to an SiO₂ surface, which is the first step in the formation of an SLB. We used a quartz crystal microbalance (QCM) to monitor the real-time changes in the mass of the SVL as it formed from solutions with different lipid concentrations. Increases in the maximum frequency change with increasing lipid concentration indicated that both adsorption and desorption of DOPC vesicles were occurring, and that an equilibrium was established between the DOPC vesicles in the SVL and in the bulk solution. From the data acquired we were able to determine that the equilibrium constant for the adsorption and desorption of DOPC vesicles was 18 ± 1. The data was fitted to a Langmuir adsorption model from which the rate constants for the adsorption and desorption of DOPC vesicles were determined to be k_a = (0.0107 ± 0.0004) mL mg^-1 s^-1 and k_d = (5.8 ± 0.3) × 10^-4 s^-1. The best fit to the experimental data was achieved if a parameter (α = (0.035 ± 0.003) s^-1) was used to account for the time taken for the lipid concentration to reach its steady state value in the flow cell used in the experiments.

Formation of Supported Lipid Bilayers (SLBs) from Buffers Containing Low Concentrations of Group I Chloride Salts

Supported lipid bilayers (SLBs) are a useful tool for studying the interactions between lipids and other biomolecules that make up a cell membrane. SLBs are typically formed by the adsorption and rupture of vesicles from solution. Although it is known that many experimental factors can affect whether SLB formation is successful, there is no comprehensive understanding of the mechanism. In this work, we have used a quartz crystal microbalance (QCM) to investigate the role of the salt in the buffer on the formation of phosphatidylcholine SLBs on a silicon dioxide (SiO₂) surface. We varied the concentration of sodium chloride in the buffer, from 5 to 150 mM, to find the minimum concentration of NaCl that was required for the successful formation of an SLB. We then repeated the experiments with other group I chloride salts (LiCl, KCl, and CsCl) and found that at higher salt concentrations (150 mM) SLB formation was successful for all of the salts used, and the degree of deformation of the adsorbed vesicles at the critical vesicle coverage was cation-dependent. The results showed that at an intermediate salt concentration (50 mM) the critical vesicle coverage was cation-dependent and at low salt concentrations (12.5 mM) the cation used determined whether SLB formation was successful. We found that the successful formation of SLBs could occur at lower electrolyte concentrations for KCl and CsCl than it did for NaCl. To understand these results, we calculated the magnitude of the vesicle-surface interaction energy using the Derjaguin-Landau-Verwey-Overbeek (DLVO) and extended-DLVO theory. We managed to explain the results obtained at higher salt concentrations by including cation-dependent surface potentials in the calculations and at lower salt concentrations by the addition of a cation-dependent hydration force. These results showed that the way that different cations in solution affect the 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)-SiO₂ surface interaction energy depends on the ionic strength of the solution.

Design and use of model membranes to study biomolecular interactions using complementary surface-sensitive techniques

Cellular membranes are complex structures and simplified analogues in the form of model membranes or biomembranes are used as platforms to understand fundamental properties of the membrane itself as well as interactions with various biomolecules such as drugs, peptides and proteins. Model membranes at the air-liquid and solid-liquid interfaces can be studied using a range of complementary surface-sensitive techniques to give a detailed picture of both the structure and physicochemical properties of the membrane and its resulting interactions. In this review, we will present the main planar model membranes used in the field to date with a focus on monolayers at the air-liquid interface, supported lipid bilayers at the solid-liquid interface and advanced membrane models such as tethered and floating membranes. We will then briefly present the principles as well as the main type of information on molecular interactions at model membranes accessible using a Langmuir trough, quartz crystal microbalance with dissipation monitoring, ellipsometry, atomic force microscopy, Brewster angle microscopy, Infrared spectroscopy, and neutron and X-ray reflectometry. A consistent example for following biomolecular interactions at model membranes is used across many of the techniques in terms of the well-studied antimicrobial peptide Melittin. The overall objective is to establish an understanding of the information accessible from each technique, their respective advantages and limitations, and their complementarity.

Relevance of charges and polymer mechanical stiffness in the mechanism and kinetics of formation of liponanoparticles probed by the supported bilayer model approach

Parameters controlling the mechanism and kinetics of formation of liponanoparticles are determined using supported lipid bilayer models.

Adsorption and fusion of hybrid lipid/polymer vesicles onto 2D and 3D surfaces

We investigated the formation of hybrid lipid/polymer (1,2-dioleoyl-sn-glycero-3-phosphocholine and poly(ethylene oxide-b-butadiene); DOPC/EO22Bd37) films onto planar silica surfaces. Using laser scanning confocal microscopy, atomic force microscopy, and quartz crystal microbalance analysis, we monitored the adsorption and fusion of hybrid lipid/polymer vesicles onto planar borosilicate glass cleaned via chemical etching or RF/air plasma treatment. In addition we used cryo-electron microscopy to characterize film formation on mesoporous silica nanoparticles. As the polymer content in the vesicles increased, the resulting hybrid lipid/polymer films on borosilicate glass - cleaned by chemical etching or plasma treatment - were more heterogeneous, indicating a large number of adsorbed vesicles rather than continuous bilayer films at higher polymer loadings. The observed lateral fluidity of both DOPC and hybrid lipid/polymer films also decreased substantially with increasing polymer fraction and was found to be relatively insensitive to changes in pH. Films prepared from vesicles with higher polymer loadings were completely immobile. We also found that polymer vesicles did not interact with clean plasma-treated glass surfaces, which may be due to elevated OH and Si-OH on plasma-treated surfaces. Conformal hybrid lipid/polymer coatings consistent with bilayers could be formed on mesoporous silica nanoparticles and imaged via cryo-electron microscopy. These results expand the library of biocompatible materials that can be used for coating silica-based materials and nanoparticles.

On the mechanism of electrochemical vesicle cytometry: chromaffin cell vesicles and liposomes

The mechanism of mammalian vesicle rupture onto the surface of a polarized carbon fiber microelectrode during electrochemical vesicle cytometry is investigated. It appears that following adsorption to the surface of the polarized electrode, electroporation leads to the formation of a pore at the interface between a vesicle and the electrode and this is shown to be potential dependent. The chemical cargo is then released through this pore to be oxidized at the electrode surface. This makes it possible to quantify the contents as it restricts diffusion away from the electrode and coulometric oxidation takes place. Using a bottom up approach, lipid-only transmitter-loaded liposomes were used to mimic native vesicles and the rupture events occurred much faster in comparison with native vesicles. Liposomes with added peptide in the membrane result in rupture events with a lower duration than that of liposomes and faster in comparison to native vesicles. Diffusional models have been developed and suggest that the trend in pore size is dependent on soft nanoparticle size and diffusion of the content in the nanometer vesicle. In addition, it appears that proteins form a barrier for the membrane to reach the electrode and need to move out of the way to allow close contact and electroporation. The protein dense core in vesicles matrixes is also important in the dynamics of the events in that it significantly slows diffusion through the vesicle.

Effects of mono- and di-valent metal cations on the morphology of lipid vesicles

Lipid vesicles are an attractive model membrane experimental platform that is widely used in a biological context. The stability of vesicles can affect their performance and depends on various experimental conditions. How bio-related ions affect vesicle morphology is poorly understood in some cases. Herein, we investigated changes in vesicle morphology influenced by cation in the static and flowing environments. The effects of different mono- and di-valent metal cations on the morphology of lipid vesicles were systematically studied using the various techniques. The results showed that divalent cations caused significant aggregation or fusion of lipid vesicles, but monovalent cations had little effect on the vesicle morphology. Cation binding increased the net surface potential of vesicles, leading to changes in the zeta potential. The same qualitative kinetics were observed for cations that had the same valence at the same ionic strength. However, different types of cations gave different quantitative effects. The order of the ability to destroy the vesicle morphology was Cu²⁺ > Mg²⁺ > Ca²⁺ > Na⁺ > K⁺. These results are of practical value in the use of lipid vesicles as a bionic model, and help to shed light on the role of ions at membrane surfaces and interfaces.

Host Cell Prediction of Exosomes Using Morphological Features on Solid Surfaces Analyzed by Machine Learning

Exosomes are extracellular nanovesicles released from any cells and found in any body fluid. Because exosomes exhibit information of their host cells (secreting cells), their analysis is expected to be a powerful tool for early diagnosis of cancers. To predict the host cells, we extracted multidimensional feature data about size, shape, and deformation of exosomes immobilized on solid surfaces by atomic force microscopy (AFM). The key idea is combination of support vector machine (SVM) learning for individual exosome particles and their interpretation by principal component analysis (PCA). We observed exosomes derived from three different cancer cells on SiO₂/Si, 3-aminopropyltriethoxysilane-modified-SiO₂/Si, and TiO₂ substrates by AFM. Then, 14-dimensional feature vectors were extracted from AFM particle data, and classifiers were trained in 14-dimensional space. The prediction accuracy for host cells of test AFM particles was examined by the cross-validation test. As a result, we obtained prediction of exosome host cells with the best accuracy of 85.2% for two-class SVM learning and 82.6% for three-class one. By PCA of the particle classifiers, we concluded that the main factors for prediction accuracy and its strong dependence on substrates are incremental decrease in the PCA-defined aspect ratio of the particles with their volume.

Excited Fluorophores Enhance the Opening of Vesicles at Electrode Surfaces in Vesicle Electrochemical Cytometry

Electrochemical cytometry is a method developed recently to determine the content of an individual cell vesicle. The mechanism of vesicle rupture at the electrode surface involves the formation of a pore at the interface between a vesicle and the electrode through electroporation, which leads to the release and oxidation of the vesicle's chemical cargo. We have manipulated the membrane properties using excited fluorophores conjugated to lipids, which appears to make the membrane more susceptible to electroporation. We propose that by having excited fluorophores in close contact with the membrane, membrane lipids (and perhaps proteins) are oxidized upon production of reactive oxygen species, which then leads to changes in membrane properties and the formation of water defects. This is supported by experiments in which the fluorophores were placed on the lipid tail instead of the headgroup, which leads to a more rapid onset of vesicle opening. Additionally, application of DMSO to the vesicles, which increases the membrane area per lipid, and decreasing the membrane thickness result in the same enhancement in vesicle opening, which confirms the mechanism of vesicle opening with excited fluorophores in the membrane. Light-induced manipulation of membrane vesicle pore opening might be an attractive means of controlling cell activity and exocytosis. Additionally, our data confirm that in experiments in which cells or vesicle membranes are labeled for fluorescence monitoring, the properties of the excited membrane change substantially.

Solid-supported polymer bilayers formed by coil-coil block copolymers

The formation and physical properties of solid-supported polymer bilayers (SPBs) on an adhesive substrate have been explored by dissipative particle dynamics simulations. A SPB is developed by the adsorption of vesicles formed by diblock copolymers in a selective solvent. The adsorbed vesicle can remain intact or become ruptured into a SPB, depending on the interaction between solvophobic blocks and solvent and the interaction between solvophilic blocks and the substrate. The morphological phase diagram of adsorbed vesicles is acquired. The influence of polymer adhesion strength and solvophobicity on the geometrical and mechanical properties of a SPB is systematically studied as well. It is found that vesicular disruption is easily triggered for strong adhesion strength. Moreover, for strong adhesion strength and weak solvophobicity, the fluctuation of membrane height is impeded while the area of fluctuation is enhanced.

Influence of Divalent Cations on Deformation and Rupture of Adsorbed Lipid Vesicles

The fate of adsorbed lipid vesicles on solid supports depends on numerous experimental parameters and typically results in the formation of a supported lipid bilayer (SLB) or an adsorbed vesicle layer. One of the poorly understood questions relates to how divalent cations appear to promote SLB formation in some cases. The complexity arises from the multiple ways in which divalent cations affect vesicle-substrate and vesicle-vesicle interactions as well as vesicle properties. These interactions are reflected, e.g., in the degree of deformation of adsorbed vesicles (if they do not rupture). It is, however, experimentally challenging to measure the extent of vesicle deformation in real-time. Herein, we investigated the effect of divalent cations (Mg(2+), Ca(2+), Sr(2+)) on the adsorption of zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) lipid vesicles onto silicon oxide- and titanium oxide-coated substrates. The vesicle adsorption process was tracked using the quartz crystal microbalance-dissipation (QCM-D) and localized surface plasmon resonance (LSPR) measurement techniques. On silicon oxide, vesicle adsorption led to SLB formation in all cases, while vesicles adsorbed but did not rupture on titanium oxide. It was identified that divalent cations promote increased deformation of adsorbed vesicles on both substrates and enhanced rupture on silicon oxide in the order Ca(2+) > Mg(2+) > Sr(2+). The influence of divalent cations on different factors in these systems is discussed, clarifying experimental observations on both substrates. Taken together, the findings in this work offer insight into how divalent cations modulate the interfacial science of supported membrane systems.

Electrochemical Detection of Single Phospholipid Vesicle Collisions at a Pt Ultramicroelectrode

We report the collision behavior of single unilamellar vesicles, composed of a bilayer lipid membrane (BLM), on a platinum (Pt) ultramicroelectrode (UME) by two electrochemical detection methods. In the first method, the blocking of a solution redox reaction, induced by the single vesicle adsorption on the Pt UME, can be observed in the amperometric i-t response as current steps during the electrochemical oxidation of ferrocyanide. In the second technique, the ferrocyanide redox probe is directly encapsulated inside vesicles and can be oxidized during the vesicle collision on the UME if the potential is poised positive enough for ferrocyanide oxidation to occur. In the amperometric i-t response for the latter experiment, a current spike is observed. Here, we report the vesicle blocking (VB) method as a relevant technique for determining the vesicle solution concentration from the collisional frequency and also for observing the vesicle adhesion on the Pt surface. In addition, vesicle reactor (VR) experiments show clear evidence that the lipid bilayer membrane does not collapse or break open at the Pt UME during the vesicle collision. Because the bilayer is too thick for electron tunneling to occur readily, an appropriate concentration of a surfactant, such as Triton X-100 (TX100), was added in the VR solution to induce loosening of the bilayer (transfection conditions), allowing the electrode to oxidize the contents of the vesicle. With this technique, the TX100 effect on the vesicle lipid bilayer permeability can be evaluated through the current spike charge and frequency corresponding to redox vesicle collisions.

Quantitative measurement of transmitters in individual vesicles in the cytoplasm of single cells with nanotip electrodes

The quantification of vesicular transmitter content is important for studying the mechanisms of neurotransmission and malfunction in disease, and yet it is incredibly difficult to measure the tiny amounts of neurotransmitters in the attoliter volume of a single vesicle, especially in the cell environment. We introduce a novel method, intracellular vesicle electrochemical cytometry. A nanotip conical carbon-fiber microelectrode was used to electrochemically measure the total content of electroactive neurotransmitters in individual nanoscale vesicles in single PC12 cells as these vesicles lysed on the electrode inside the living cell. The results demonstrate that only a fraction of the quantal neurotransmitter content is released during exocytosis. These data support the intriguing hypothesis that the vesicle does not open all the way during the normal exocytosis process, thus resulting in incomplete expulsion of the vesicular contents.

New poly(amino acid methacrylate) brush supports the formation of well-defined lipid membranes

A novel poly(amino acid methacrylate) brush comprising zwitterionic cysteine groups (PCysMA) was utilized as a support for lipid bilayers. The polymer brush provides a 12-nm-thick cushion between the underlying hard support and the aqueous phase. At neutral pH, the zeta potential of the PCysMA brush was ∼-10 mV. Cationic vesicles containing >25% DOTAP were found to form a homogeneous lipid bilayer, as determined by a combination of surface analytical techniques. The lipid mobility as measured by FRAP (fluorescence recovery after photobleaching) gave diffusion coefficients of ∼1.5 μm(2) s(-1), which are comparable to those observed for lipid bilayers on glass substrates.

Characterizing the catecholamine content of single mammalian vesicles by collision-adsorption events at an electrode

We present the electrochemical response to single adrenal chromaffin vesicles filled with catecholamine hormones as they are adsorbed and rupture on a 33 μm diameter disk-shaped carbon electrode. The vesicles adsorb onto the electrode surface and sequentially spread out over the electrode surface, trapping their contents against the electrode. These contents are then oxidized, and a current (or amperometric) peak results from each vesicle that bursts. A large number of current transients associated with rupture of single vesicles (86%) are observed under the experimental conditions used, allowing us to quantify the vesicular catecholamine content.

New poly(amino acid methacrylate) brush supports the formation of well-defined lipid membranes

A novel poly(amino acid methacrylate) brush comprising zwitterionic cysteine groups (PCysMA) was utilized as a support for lipid bilayers. The polymer brush provides a 12-nm-thick cushion between the underlying hard support and the aqueous phase. At neutral pH, the zeta potential of the PCysMA brush was ∼-10 mV. Cationic vesicles containing >25% DOTAP were found to form a homogeneous lipid bilayer, as determined by a combination of surface analytical techniques. The lipid mobility as measured by FRAP (fluorescence recovery after photobleaching) gave diffusion coefficients of ∼1.5 μm(2) s(-1), which are comparable to those observed for lipid bilayers on glass substrates.

Induced rupture of vesicles adsorbed on glass by pore formation at the surface-bilayer interface

Supported lipid bilayers (SLBs) are often formed by spontaneous vesicle rupture and fusion on a solid surface. A well-characterized rupture mechanism for isolated vesicles is pore nucleation and expansion in the solution-exposed nonadsorbed area. In contrast, pore formation in the adsorbed bilayer region has not been investigated to date. In this work, we studied the detailed mechanisms of asymmetric rupture of giant unilamellar vesicles (GUVs) adsorbed on glass using fluorescence microscopy. Asymmetric rupture is the pathway where a rupture pore forms in a GUV near the edge of the glass-bilayer interface with high curvature and then expansion of the pore yields a planar bilayer patch. We show that asymmetric rupture occasionally resulted in SLB patches bearing a defect pore. The defect formation probability depended on lipid composition, salt concentration, and pH. Approximately 40% of negatively charged GUVs under physiological conditions formed pore-containing SLB patches, while negatively charged GUVs at low salt concentration or pH 4.0 and positively charged GUVs exhibited a low probability of defect inclusion. The edge of the defect pore was either in contact with (on-edge) or away from (off-edge) the edge of the planar bilayer. On-edge pores were predominantly formed over off-edge defects. Pores initially formed in the glass-adsorbed region before rupture, most frequently in close contact with the edge of the adsorbed region. When a pore formed near the edge of the adsorbed area or when the edge of a pore reached that of the adsorbed area by pore expansion, asymmetric rupture was induced from the defect site. These induced rupture mechanisms yielded SLB patches with an on-edge pore. In contrast, off-edge pores were produced when defect pore generation and subsequent vesicle rupture were uncoupled. The current results demonstrate that pore formation in the surface-adsorbed region of GUVs is not a negligible event.

Contribution of the hydration force to vesicle adhesion on titanium oxide

Titanium oxide is a biocompatible material that supports vesicle adhesion. Depending on experimental parameters, adsorbed vesicles remain intact or rupture spontaneously. Vesicle rupture has been attributed to electrostatic attraction between vesicles and titanium oxide, although the relative contribution of various interfacial forces remains to be clarified. Herein, we investigated the influence of vesicle surface charge on vesicle adsorption onto titanium oxide and observed that electrostatic attraction is insufficient for vesicle rupture. Following this line of evidence, a continuum model based on the DLVO forces and a non-DLVO hydration force was applied to investigate the role of different interfacial forces in modulating the lipid-substrate interaction. Within an experimentally significant range of conditions, the model shows that the magnitude of the repulsive hydration force strongly influences the behavior of adsorbed vesicles, thereby supporting that the hydration force makes a strong contribution to the fate of adsorbed vesicles on titanium oxide. The findings are consistent with literature reports concerning phospholipid assemblies on solid supports and nanoparticles and underscore the importance of the hydration force in influencing the behavior of phospholipid films on hydrophilic surfaces.

Formation of supported lipid bilayers on silica: relation to lipid phase transition temperature and liposome size

DPPC liposomes ranging from 90 nm to 160 nm in diameter were prepared and used for studies of the formation of supported lipid membranes on silica (SiO2) at temperatures below and above the gel to liquid-crystalline phase transition temperature (Tm = 41 °C), and by applying temperature gradients through Tm. The main method was the quartz crystal microbalance with dissipation (QCM-D) technique. It was found that liposomes smaller than 100 nm spontaneously rupture on the silica surface when deposited at a temperature above Tm and at a critical surface coverage, following a well-established pathway. In contrast, DPPC liposomes larger than 160 nm do not rupture on the surface when adsorbed at 22 °C or at 50 °C. However, when liposomes of this size are first adsorbed at 22 °C and at a high enough surface coverage, after which they are subject to a constant temperature gradient up to 50 °C, they rupture and fuse to a bilayer, a process that is initiated around Tm. The results are discussed and interpreted considering a combination of effects derived from liposome-surface and liposome-liposome interactions, different softness/stiffness and shape of liposomes below and above Tm, the dynamics and thermal activation of the bilayers occurring around Tm and (for liposomes containing 33% of NaCl) osmotic pressure. These findings are valuable both for preparation of supported lipid bilayer cell membrane mimics and for designing temperature-responsive material coatings.

Effect of osmotic stress on membrane fusion on solid substrate

There is currently a lack of comprehensive understanding of osmotic effect on lipid vesicle fusion on solid oxide surface. The question has both biological and biomedical implications. We studied the effect by quartz crystal microbalance with dissipation monitoring using NaCl, sucrose as osmolytes, and two different osmotic stress imposition methods, which allowed us to separate the osmotic effects from the solute impacts. Osmotic stress was found to have limited influence on the fusion kinetics, independently of the direction of the gradient. Further atomic force microscopy experiments and energy consideration implied that osmotic stress spends the majority of chemical potential energy associated in directed transport of water across membrane. Its contribution to vesicle deformation and fusion on substrate is therefore small compared to that of adhesion.

Mechanisms of vesicle spreading on surfaces: coarse-grained simulations

Exposition of unilamellar vesicles to attractive surfaces is a frequently used way to create supported lipid bilayers. Although this approach is known to produce continuous supported bilayer coatings, the mechanism of their formation and its dependence on factors like surface interaction and roughness or membrane tension as well as the interplay between neighboring vesicles or the involvement of preadsorbed bilayer patches are not well understood. Using dissipative particle dynamics simulations, we assess different mechanisms of vesicle spreading on attractive surfaces, placing special emphasis on the orientation of the resulting bilayer. Making use of the universality of collective phenomena in lipid membranes, we employed a solvent-free coarse-grained model, enabling us to cover the relatively large system sizes and time scales required. Our results indicate that one can control the mechanism of vesicle spreading by tuning the strength and range of the interactions with the substrate as well as the surface's roughness, resulting in a switch from a predominant inside-up to an outside-up orientation of the created supported bilayer.

Assembly of lipid bilayers on silica and modified silica colloids by reconstitution of dried lipid films

A method is presented for the assembly of lipid bilayers on silica colloids via reconstitution of dried lipid films solvent-cast from chloroform within packed beds of colloids ranging from 100 nm to 10 μm in diameter. Rapid solvent evaporation from the packed bed void volume results in uniform distribution of dried lipid throughout the colloidal bed. Fluorescence measurements indicate that significant, if not quantitative, retention of DOPC or DPPC films cast between sub-bilayer and multilayer quantities occurs when the colloids are redispersed in aqueous solution. Phospholipid bilayers assembled in this manner are shown to effectively passivate the surface of 250 nm colloids to nonspecific adsorption of bovine serum albumin. The method is shown to be capable of preparing supported bilayers on colloid surfaces that do not generally support vesicle fusion such as poly(ethylene glycol) (PEG) modified silica colloids. Bilayers of lipids that have not been reported to self-assemble by vesicle fusion, including gel-phase lipids and single-chain diacetylene amphiphiles, can also be formed by this method. The utility of the solid-core support is demonstrated by the facile assembly of supported lipid bilayers within fused silica capillaries to generate materials that are potentially suitable for the analysis of membrane interactions in a microchannel format.

Characterization of supported lipid bilayers incorporating the phosphoinositides phosphatidylinositol 4,5-biphosphate and phosphoinositol-3,4,5-triphosphate by complementary techniques

Phosphoinositides are involved in a large number of processes in cells and it is very demanding to study individual protein-lipid interactions in vivo due to their rapid turnover and involvement in simultaneous events. Supported lipid bilayers (SLBs) containing controlled amounts of phosphoinositides provide a defined model system where important specific recognition events involving phosphoinositides can be systematically investigated using surface sensitive analytical techniques. The authors have demonstrated the formation and characterized the assembly kinetics of SLBs incorporating phosphatidylinositol 4,5-biphosphate (PIP(2); 1, 5, and 10 wt %) and phosphoinositol-3,4,5-triphosphate (1 wt %) using the quartz crystal microbalance with dissipation monitoring and fluorescence recovery after photobleaching. An increased fraction of phosphoinositides led to a higher barrier to liposome fusion, but full fluidity for the phosphatidylcholine lipids in the formed SLB. Significantly, the majority of phosphoinositides were shown to be immobile. X-ray photoelectron spectroscopy was used for the first time to verify that the PIP(2) fraction of lipids in the SLB scales linearly with the amount mixed in from stock solutions.

Weakly coupled lipid bilayer membranes on multistimuli-responsive poly(N-isopropylacrylamide) copolymer cushions

Polymer-cushioned lipid bilayers are frequently used to mimic the native environment of cellular membranes in respect to the extracellular matrix and intracellular structures. With the aim to actively tune lipid membrane characteristics, we pursue the approach to use temperature and pH responsive polymer thin films of poly(N-isopropylacrylamide-co-carboxyacrylamide) (PNIPAAm-co-carboxyAAM) as cushions for supported lipid bilayers. A cationic lipid bilayer composed of dioleoylphosphatidylcholine (DOPC) and dioleoyltrimethylammoniumpropane (DOTAP) (9:1) was formed on top of the polymer thin film in a drying/rehydration process. Fluorescence recovery after photobleaching (FRAP) yielded higher lipid diffusion coefficients (6.3-9.6 μm(2) s(-1)) on polymer cushions in comparison to solid glass supports (3.0-5.9 μm(2) s(-1)). No correlation of the lipid mobility was found with the swelling state of (PNIPAAm-co-carboxyAAM), which is ascribed to restrained interfacial electrostatic interactions and dispersion forces. The results revealed a minimal coupling of the lipid bilayer with the polymer cushions, and thus, bilayers supported by (PNIPAAm-co-carboxyAAM) provide interesting opportunities for unperturbed lipid diffusion combined with control of transmembrane protein mobility due to the impact of a tunable frictional drag.

Building biomimetic membrane at a gold electrode surface

This article describes efforts to build a model biological membrane at a surface of a gold electrode. In this architecture, the membrane may be exposed to static electric fields on the order of 10(7) to 10(8) V m(-1). These fields are comparable in magnitude to the static electric field acting on a natural biological membrane. The field may be conveniently used to manipulate organic molecules within the membrane. By turning a knob on the control instrument one can deposit or lift the membrane from the gold surface. Electrochemical techniques can be used to control the physical state of the film while the infrared reflection absorption spectroscopy (IRRAS), surface imaging by STM and AFM and neutron scattering techniques can be employed to study conformational changes of organic molecules and their ordering within the membrane. This is shown on examples of membranes built of a simple zwitterionic phospholipid such as 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) and a mixed membrane composed of DMPC and cholesterol. The results illustrate the tremendous effect of cholesterol on the membrane structure. Two methods of membrane deposition at the electrode surface, namely by unilamellar vesicles fusion and using the Langmuir-Blodgett technique, are compared. Applications of these model systems to study interactions of small antibiotic peptides with lipids are discussed.

Influence of lipid-bilayer-associated molecules on lipid-vesicle adsorption

Supported lipid bilayers (SLBs) containing different types of bilayer-associated molecules (membrane-bound molecules) where one part of the molecule resides inside the lipid bilayer and another part of the molecule sticks out of the bilayer (e.g., membrane proteins) are important biophysical model systems. SLBs are commonly formed via lipid vesicle adsorption on certain surfaces (e.g., SiO(2)). However, vesicles doped with different types of (bio)molecules often do not form an SLB on the surface, and the reasons for this are not clear. Using a newly developed model of a lipid bilayer, simulations were performed to clarify the influence of the bilayer-associated molecules on vesicle adsorption and rupture. It is shown that by increasing the concentration of membrane-bound molecules in the vesicles the tendency for vesicle rupture decreases markedly and for a certain concentration rupture does not happen. The reason for this is that vesicles containing significant concentrations of such molecules tend to deform less on the surface (lower vesicle strain), especially for a significantly corrugated bilayer-surface potential. After vesicle rupture, membrane-bound molecules face either the surface or the solution in the resulting bilayer patch on the surface, depending on whether the molecules point outward or inward in the original vesicle, respectively. Vesicle surface diffusion is also studied for weak and strong surface corrugation, where vesicles are found to be almost immobile in the latter case.

Deformation of adsorbed lipid vesicles as a function of vesicle size

Experimental indications that adsorbed lipid vesicles are deformed on the surface (e.g., on SiO(2)) and that the deformation seems to be more pronounced for larger vesicles have been reported. In general, it has been assumed that larger vesicles should show a stronger tendency for spontaneous rupture, which is also backed up by thermodynamic considerations (Seifert, U.; Lipowsky, R. Phys. Rev. A 1990, 42, 4768; Seifert, U. Adv. Phys. 1997, 46, 13). However, using a newly developed model of a lipid bilayer, simulations were performed to study the shape of adsorbed lipid vesicles for different vesicle sizes, with the observation that larger vesicles indeed are more deformed on the surface, but that there is no additional tendency for larger vesicles to rupture spontaneously. It is shown here that the radius of curvature, on the portions of the vesicle membrane that are most strained, is practically independent of the vesicle size. A kinetic barrier for vesicle rupture is proposed to be the reason for the observed disagreement with thermodynamic theory.

Simulations of lipid transfer between a supported lipid bilayer and adsorbing vesicles

Recent experiments demonstrate transfer of lipid molecules between a charged, supported lipid membrane (SLB) and vesicles of opposite charge when the latter adsorb on the SLB. A simple phenomenological bead model has been developed to simulate this process. Beads were defined to be of three types, 'n', 'p', and '0', representing POPS (negatively charged), POEPC (positively charged), and POPC (neutral but zwitterionic) lipids, respectively. Phenomenological bead-bead interaction potentials and lipid transfer rate constants were used to account for the overall interaction and transfer kinetics. Using different bead mixtures in both the adsorbing vesicle and in the SLB (representing differently composed/charged vesicles and SLBs as in the reported experiments), we clarify under which circumstances a vesicle adsorbs to the SLB, and whether it, after lipid transfer and changed composition of the SLB and vesicle, desorbs back to the bulk again or not. With this model we can reproduce and provide a conceptual picture for the experimental findings.

Influence of lipid vesicle composition and surface charge density on vesicle adsorption events: a kinetic phase diagram

Lipid vesicle adsorption on a solid surface, from a bulk liquid solution, results in different final situations on the surface depending on the vesicles' composition, properties of the solution (pH, ion types, and concentration), and surface properties. The main alternative outcomes of the adsorption event are (i) a lipid bilayer with vesicle rupture immediately upon the adsorption event or (ii) bilayer formation only at and after a critical vesicle coverage, (iii) adsorption of intact vesicles, or (iv) repulsion (no adsorption). We have simulated these different events for the case of vesicles consisting of pure neutral (zwitterionic) lipids and mixtures of neutral and positive or negative lipids, keeping the bulk conditions fixed, and have compiled the different resulting lipid structures on the surface as a function of vesicle composition and surface charge density, in a kinetic phase diagram.

Influence of surface pinning points on diffusion of adsorbed lipid vesicles

Using a simple model of a vesicle and a substrate, we have studied the surface diffusion of an adsorbed vesicle. We show that the experimentally observed but unexplained fact, that a neutral (POPC) vesicle adsorbed to a SiO(2) or mica surface does not diffuse but can be moved laterally by an atomic force microscope (AFM) tip, without rupture, can be explained by transient (i.e., temporary) pinning of lipid head groups to surface charges. We studied the surface diffusion for different vesicle adsorption strengths (without any pinning taking place), with the observation that a stronger vesicle-surface attraction leads to slower surface diffusion. However, the surface diffusion was still significant and too high to explain the experimentally observed immobility. When allowing transient lipid pinning between the vesicle and the surface, a 1-2 orders of magnitude decrease in the surface diffusion coefficient was observed. For a lipid adsorption potential of around 20 k(B)T and a lipid pinning potential of about 25 k(B)T, the vesicle is found to be practically immobile on the surface.

Influence of mono- and divalent ions on the formation of supported phospholipid bilayers via vesicle adsorption

We have used the quartz crystal microbalance with dissipation monitoring (QCM-D) technique to investigate how mono- and divalent cations influence the formation of supported (phospho)lipid bilayers (SPB, SLB), occurring via deposition of nanosized palmitoyloleoyl phosphatidylcholine (POPC) vesicles on a SiO2 support. This process is known to proceed via initial adsorption of intact vesicles until a critical surface coverage is reached, where the combination of vesicle-surface and vesicle-vesicle interaction causes the vesicles to rupture. New vesicles then rupture and the lipid fragments fuse until a final continuous bilayer is formed. We have explored how this process and the critical coverage are influenced by different mono- and divalent ions and ion concentrations, keeping the anions the same throughout the experiments. The same qualitative kinetics is observed for all cations. However, different ions cause quite different quantitative kinetics. When compared with monovalent ions, even very small added concentrations of divalent cations cause a strong reduction of the critical coverage, where conversion of intact, adsorbed vesicles to bilayer occurs. This bilayer promoting effect increases in the order Sr2+<Ca2+<Mg2+. Monovalent cations exhibit a much weaker but similar effect in the order Li+>Na+>K+. The results are of practical value for preparation of lipid bilayers and help shed light on the role of ions and on electrostatic effects at membrane surfaces/interfaces.

Dynamics of vesicle formation from lipid droplets: mechanism and controllability

A coarse-grained model developed by Marrink et al. [J. Phys. Chem. B 111, 7812 (2007)] is applied to investigate vesiculation of lipid [dipalmitoylphosphatidylcholine (DPPC)] droplets in water. Three kinds of morphologies of micelles are found with increasing lipid droplet size. When the initial lipid droplet is smaller, the equilibrium structure of the droplet is a spherical micelle. When the initial lipid droplet is larger, the lipid ball starts to transform into a disk micelle or vesicle. The mechanism of vesicle formation from a lipid ball is analyzed from the self-assembly of DPPC on the molecular level, and the morphological transition from disk to vesicle with increasing droplet size is demonstrated. Importantly, we discover that the transition point is not very sharp, and for a fixed-size lipid ball, the disk and vesicle appear with certain probabilities. The splitting phenomenon, i.e., the formation of a disk/vesicle structure from a lipid droplet, is explained by applying a hybrid model of the Helfrich membrane theory. The elastic module of the DPPC bilayer and the smallest size of a lipid droplet for certain formation of a vesicle are successfully predicted.

Lipid transfer between charged supported lipid bilayers and oppositely charged vesicles

The bidirectional transfer of phospholipids between a charged, supported lipid bilayer (SLB) on SiO(2) and oppositely charged, unilamellar vesicles was studied by means of quartz crystal microbalance with dissipation (QCM-D) and optical reflectometry techniques. SLBs and vesicles were prepared from binary mixtures of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) mixed with different fractions of either 1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-l-serine] (POPS) (negatively charged) or 1-palmitoyl-2-oleoyl-sn-glycero-3-ethylphosphocholine (POEPC) (positively charged). The interaction process consists of an attachment-transfer-detachment (ATD) sequence, where added vesicles first attach to and interact with the SLB, after which they detach, leaving behind a compositionally modified SLB and ditto vesicles. When the process is complete, there is no net addition or reduction of total lipid mass in the SLB, but lipid exchange has occurred. The time scale of the process varies from a few to many tens of minutes depending on the type of charged lipid molecule and the relative concentration of charged lipids in the two membranes. Electrostatically symmetric cases, where only the charge sign (but not the fraction of charged lipid) was reversed between the SLB and the vesicles, produce qualitatively similar but quantitatively different kinetics. The time scale of the interaction varies significantly between the two cases, which is attributed to a combination of the differences in the molecular structure of the lipid headgroup for the positively and the negatively charged lipids used, and to nonsymmetric distribution of charged lipids in the lipid membranes. The maximum amounts of attached vesicles during the ATD process were estimated to be 25-40% of a full monolayer of vesicles, with the precise amount depending on the actual charge fractions in the vesicles and the SLB. Interrupted vesicle exposure experiments, and experiments where the bulk concentration of vesicles was varied, show that vesicles in some cases may be trapped irreversibly on the SLB, when only partial transfer of lipid molecules has occurred. Additional supply of vesicles and further transfer induces detachment, when a sufficient amount of oppositely charged lipids has been transferred to the SLB, so that the latter becomes repulsive to the attached vesicles. Possible mechanistic scenarios, including monomer insertion and hemifusion models, are discussed. The observed phenomena and the actual SLB preparation process form a platform both for studies of various intermembrane molecular transfer processes and for modifying the composition of SLBs in a controlled way, for example, for biosensor and cell culture applications.

Rupture pathway of phosphatidylcholine liposomes on silicon dioxide

We have investigated the pathway by which unilamellar POPC liposomes upon adsorption undergo rupture and form a supported lipid bilayer (SLB) on a SiO(2) surface. Biotinylated lipids were selectively incorporated in the outer monolayer of POPC liposomes to create liposomes with asymmetric lipid compositions in the outer and inner leaflets. The specific binding of neutravidin and anti-biotin to SLBs formed by liposome fusion, prior to and after equilibrated flip-flop between the upper and lower monolayers in the SLB, were then investigated. It was concluded that the lipids in the outer monolayer of the vesicle predominantly end up on the SLB side facing the SiO(2) substrate, as demonstrated by having maximum 30-40% of lipids in the liposome outer monolayer orienting towards the bulk after forming the SLB.

AFM studies of the effect of temperature and electric field on the structure of a DMPC-cholesterol bilayer supported on a Au(111) electrode surface

Atomic force microscopy (AFM) was used to characterize a phospholipid bilayer composed of 70 mol % 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 30 mol % cholesterol, at a Au(111) electrode surface. Results indicate that addition of cholesterol relaxes membrane elastic stress, increases membrane thickness, and reduces defect density. The thickness and thermotropic properties of the mixed DMPC-cholesterol bilayer supported at the gold electrode surface are quite similar to the properties of the mixed membrane in unilamellar vesicles. The stability of the supported membrane at potentials negative to the potential of zero charge E(pzc) was investigated. This study demonstrates that the bilayer supported at the gold electrode surface is stable provided the applied potential (E - E(pzc)) is less than -0.3 V. At larger polarizations, swelling of the membrane is observed. Polarizations larger than -1 V cause electrodewetting of the bilayer from the gold surface. At these negative potentials, the bilayer remains in close proximity to the metal surface, separated from it by a approximately 2 nm thick layer of electrolyte.

AFM studies of solid-supported lipid bilayers formed at a Au(111) electrode surface using vesicle fusion and a combination of Langmuir-Blodgett and Langmuir-Schaefer techniques

Atomic force microscopy (AFM) has been used to characterize the formation of a phospholipid bilayer composed of 1,2-dimyristyl-sn-glycero-3-phosphocholine (DMPC) at a Au(111) electrode surface. The bilayer was formed by one of two methods: fusion of lamellar vesicles or by the combination of Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) deposition. Results indicate that phospholipid vesicles rapidly adsorb and fuse to form a film at the electrode surface. The resulting film undergoes a very slow structural transformation until a characteristic corrugated phase is formed. Force-distance curve measurements reveal that the thickness of the corrugated phase is consistent with the thickness of a bilayer lipid membrane. The formation of the corrugated phase may be explained by considering the elastic properties of the film and taking into account spontaneous curvature induced by the asymmetric environment of the bilayer, in which one side faces the gold substrate and the other side faces the solution. The effect of temperature and electrode potential on the stability of the corrugated phase has also been described.