Confocal imaging to quantify passive transport across biomimetic lipid membranes

Assessing the mechanism of facilitated proton transport across GUVs trapped in a microfluidic device

Proton transport across lipid membranes is one of the most fundamental reactions that make up living organisms. In vitro, however, the study of proton transport reactions can be very challenging due to limitations imposed by proton concentrations, compartment size, and unstirred layers as well as buffer exchange and buffer capacity. In this study, we have developed a proton permeation assay based on the microfluidic trapping of giant vesicles enclosing the pH-sensitive dye pyranine to address some of these challenges. Time-resolved fluorescence imaging upon a rapid pH shift enabled us to investigate the facilitated H+ permeation mediated by either a channel or a carrier. Specifically, we compared the proton transport rates as a function of different proton gradients of the channel gramicidin D and the proton carrier carbonyl cyanide-m-chlorophenyl hydrazone. Our results demonstrate the efficacy of the assay in monitoring proton transport reactions and distinguishing between a channel-like and a carrier-like mechanism. This groundbreaking result enabled us to elucidate the enigmatic mode of the proton permeation mechanism of the recently discovered natural fibupeptide lugdunin.

Advances in giant unilamellar vesicle preparation techniques and applications

Giant unilamellar vesicles (GUVs) are versatile and promising cell-sized bio-membrane mimetic platforms. Their applications range from understanding and quantifying membrane biophysical processes to acting as elementary blocks in the bottom-up assembly of synthetic cells. Definite properties and requisite goals in GUVs are dictated by the preparation techniques critical to the success of their applications. Here, we review key advances in giant unilamellar vesicle preparation techniques and discuss their formation mechanisms. Developments in lipid hydration and emulsion techniques for GUV preparation are described. Novel microfluidic-based techniques involving lipid or surfactant-stabilized emulsions are outlined. GUV immobilization strategies are summarized, including gravity-based settling, covalent linking, and immobilization by microfluidic, electric, and magnetic barriers. Moreover, some of the key applications of GUVs as biomimetic and synthetic cell platforms during the last decade have been identified. Membrane interface processes like phase separation, membrane protein reconstitution, and membrane bending have been deciphered using GUVs. In addition, vesicles are also employed as building blocks to construct synthetic cells with defined cell-like functions comprising compartments, metabolic reactors, and abilities to grow and divide. We critically discuss the pros and cons of preparation technologies and the properties they confer to the GUVs and identify potential techniques for dedicated applications.

A study on attempt for determination of permeation kinetics of coumarin at lipid bilayer of liposomes by using capillary electrophoresis with moment analysis theory

It was tried to develop a moment analysis method for the determination of lipid membrane permeability. The first absolute and second central moments of elution peaks measured by liposome electrokinetic chromatography (LEKC) are analyzed by using moment equations. As a concrete example, elution peak profiles of coumarin in a LEKC system, in which liposomes consisting of 1-palmitoyl-2-oleoyl-sn‑glycero-3-phosphocholine (POPC) and phosphatidylserine (PS) are used as a pseudo-stationary phase, were analyzed. It seems that lipid membrane permeability of coumarin across the lipid bilayer of POPC/PS liposomes was measured by the moment analysis method because previous permeability measurements using parallel artificial membrane permeability assay (PAMPA) and Caco-2 cells indicated that coumarin is permeable across lipid bilayer. However, it was also pointed out that the moment analysis method with LEKC is not effective for the determination of lipid membrane permeability and that it provides information about adsorption/desorption kinetics at lipid bilayer of liposomes. Therefore, different moment equations were also developed for the determination of adsorption/desorption rate constants of coumarin from the LEKC data. It was demonstrated that permeation rate constants at lipid bilayer or adsorption/desorption rate constants can be determined from the LEKC data on the basis of moment analysis theory for the mass transfer phenomena of coumarin at the lipid bilayer of POPC/PS liposomes. Mass transfer kinetics of solutes at lipid bilayer should be determined under the conditions that liposomes originally be because they are self-assembling and dynamic systems formed through weak interactions between phospholipid monomers. The moment analysis method using LEKC is effective for the experimental determination of the mass transfer rate constants at the lipid bilayer of liposomes because neither immobilization nor chemical modification of liposomes is necessary when LEKC data are measured. It is expected that the results of this study contribute to the dissemination of an opportunity for the determination of permeation rate constants or adsorption/desorption rate constants at the lipid bilayer of liposomes to many researchers because capillary electrophoresis is widespread.

Monitoring membranes: The exploration of biological bilayers with second harmonic generation

Nature's seemingly controlled chaos in heterogeneous two-dimensional cell membranes stands in stark contrast to the precise, often homogeneous, environment in an experimentalist's flask or carefully designed material system. Yet cell membranes can play a direct role, or serve as inspiration, in all fields of biology, chemistry, physics, and engineering. Our understanding of these ubiquitous structures continues to evolve despite over a century of study largely driven by the application of new technologies. Here, we review the insight afforded by second harmonic generation (SHG), a nonlinear optical technique. From potential measurements to adsorption and diffusion on both model and living systems, SHG complements existing techniques while presenting a large exploratory space for new discoveries.

Role of Negatively Charged Lipids Achieving Rapid Accumulation of Water-Soluble Molecules and Macromolecules into Cell-Sized Liposomes against a Concentration Gradient

Liposomes, molecular self-assemblies resembling biological membranes, are a promising scaffold to investigate the physicochemical logic behind the complexity of living cells. Despite elaborate synthetic studies constructing cell-like chemical systems using liposomes, less attention has been paid to the proactive role of the membrane emerging as dynamics of the molecular self-assembly. This study investigated the liposomes containing anionic phospholipids by exposing them to steady flow conditions using a newly constructed automatic microfluidic observation platform. We demonstrated that the liposomes accumulated even macromolecules under the microfluidic condition without pore formation. By investigating the effect of composition of liposomes and visualizing negatively charged phospholipids upon the flow, we presumed that the external flow caused a compositional asymmetry of anionic phospholipids between the inner/outer leaflets, and the asymmetry enabled a rapid accumulation of those molecules against the concentration gradient. The current study opens new research interests regarding the nature of biological membranes under steady flow conditions.

Recent Experimental Developments in Studying Passive Membrane Transport of Drug Molecules

The ability to measure the passive membrane permeation of drug-like molecules is of fundamental biological and pharmaceutical importance. Of significance, passive diffusion across the cellular membranes plays an effective role in the delivery of many pharmaceutical agents to intracellular targets. Hence, approaches for quantitative measurement of membrane permeability have been the topics of research for decades, resulting in sophisticated biomimetic systems coupled with advanced techniques. In this review, recent developments in experimental approaches along with theoretical models for quantitative and real-time analysis of membrane transport of drug-like molecules through mimetic and living cell membranes are discussed. The focus is on time-resolved fluorescence-based, surface plasmon resonance, and second-harmonic light scattering approaches. The current understanding of how properties of the membrane and permeant affect the permeation process is discussed.

Permeation Studies across Symmetric and Asymmetric Membranes in Microdroplet Arrays

We investigated the permeation of molecules across lipid membranes on an open microfluidic platform. An array of droplet pairs was created by spotting aqueous droplets, dispersed in a lipid oil solution, onto a plate with cavities surrounded by a hydrophobic substrate. Droplets in two adjacent cavities come in contact and form an artificial lipid bilayer, called a droplet interface bilayer (DIB). The method allows for monitoring permeation of fluorescently tagged compounds from a donor droplet to an acceptor droplet. A mathematical model was applied to describe the kinetics and determine the permeation coefficient. We also demonstrate that permeation kinetics can be followed over a series of droplets, all connected via DIBs. Moreover, by changing the lipid oil composition after spotting donor droplets, we were able to create asymmetric membranes that we used to mimic the asymmetry of the cellular plasma membrane. Finally, we developed a protocol to separate and extract the droplets for label-free analysis of permeating compounds by liquid chromatography-mass spectrometry. Our versatile platform has the potential to become a new tool for the screening of drug membrane permeability in the future.

Membrane permeability to water measured by microfluidic trapping of giant vesicles

We use a microfluidic method to estimate the water permeability coefficient (p) of membranes. As model lipid membranes we employ giant unilamellar vesicles (GUVs) composed of palmitoyloleoyl phosphatidylcholine and cholesterol (10 mol%). We have developed a microfluidic device with multiple chambers to trap GUVs and allow controlled osmotic exchange. Each chamber has a ring-shaped pressure-controlled valve which upon closure allows isolation of the GUVs in a defined volume. Opening the valves leads to a rapid fluid exchange between the trapping region and the microchannel network outside, thus allowing precise control over solution concentration around the GUVs contrary to other experimental approaches for permeability measurements reported in the literature. The area and volume changes of individual vesicles are monitored with confocal microscopy. The solute concentration in the immediate vicinity of the GUVs, and thus the concentration gradient across the membrane, is independently assessed. The data are well fitted by a simple model for water permeability which assumes that the rate of change in volume of a GUV per unit area is linearly proportional to concentration difference with permeability as the proportionality constant. Experiments of GUV osmotic deflation with hypertonic solutions yield the permeability of POPC/cholesterol 9/1 membranes to be p = 15.7 ± 5.5 μm s-1. For comparison, we also show results using two other approaches, which either do not take into account local concentration changes and/or do not resolve the precise vesicle shape. We point out the errors associated with these limitations. Finally, we also demonstrate the applicability of the microfluidic device for studying the dynamics of vesicles under flow.

Hydrodynamic accumulation of small molecules and ions into cell-sized liposomes against a concentration gradient

In investigations of the emergence of protocells at the origin of life, repeatable and continuous supply of molecules and ions into the closed lipid bilayer membrane (liposome) is one of the fundamental challenges. Demonstrating an abiotic process to accumulate substances into preformed liposomes against the concentration gradient can provide a clue. Here we show that, without proteins, cell-sized liposomes under hydrodynamic environment repeatedly permeate small molecules and ions, including an analogue of adenosine triphosphate, even against the concentration gradient. The mechanism underlying this accumulation of the molecules and ions is shown to involve their unique partitioning at the liposomal membrane under forced external flow in a constrained space. This abiotic mechanism to accumulate substances inside of the liposomal compartment without light could provide an energetically up-hill process for protocells as a critical step toward the contemporary cells.

Intrinsic Membrane Permeability to Small Molecules

Spontaneous solute and solvent permeation through membranes is of vital importance to human life, be it gas exchange in red blood cells, metabolite excretion, drug/toxin uptake, or water homeostasis. Knowledge of the underlying molecular mechanisms is the sine qua non of every functional assignment to membrane transporters. The basis of our current solubility diffusion model was laid by Meyer and Overton. It correlates the solubility of a substance in an organic phase with its membrane permeability. Since then, a wide range of studies challenging this rule have appeared. Commonly, the discrepancies have their origin in ill-used measurement approaches, as we demonstrate on the example of membrane CO₂ transport. On the basis of the insight that scanning electrochemical microscopy offered into solute concentration distributions in immediate membrane vicinity of planar membranes, we analyzed the interplay between chemical reactions and diffusion for solvent transport, weak acid permeation, and enzymatic reactions adjacent to membranes. We conclude that buffer reactions must also be considered in spectroscopic investigations of weak acid transport in vesicular suspensions. The evaluation of energetic contributions to membrane translocation of charged species demonstrates the compatibility of the resulting membrane current with the solubility diffusion model. A local partition coefficient that depends on membrane penetration depth governs spontaneous membrane translocation of both charged and uncharged molecules. It is determined not only by the solubility in an organic phase but also by other factors like cholesterol concentration and intrinsic electric membrane potentials.

Direct Observation of Bacterial Growth in Giant Unilamellar Vesicles: A Novel Tool for Bacterial Cultures

Bacterial cultivation techniques are classic, basic, and common processes used to characterize the physiological activity of bacteria in their environment. Owing to recent advances in bacterial cultivation techniques, the physiological activity of bacteria can be elucidated at the single-cell culture level. Here, we report a novel method to monitor the real-time activity of bacterial growth at the single-cell level inside giant unilamellar vesicles (GUVs). This method consists of two steps: 1) encapsulation of single bacteria in 1-33 pL scale GUVs and 2) immobilization of the GUVs on a planar lipid bilayer membrane on a glass surface. We directly observed single E. coli cells actively growing to a great number of cells inside GUVs. GUVs also protected the bacteria from external antibiotic compounds during prolonged cultivation for more than 24 h. This approach can be applied widely in the fields of biochemistry, biotechnology, microbiology, and synthetic biology.

A synthetic enzyme built from DNA flips 107 lipids per second in biological membranes

Mimicking enzyme function and increasing performance of naturally evolved proteins is one of the most challenging and intriguing aims of nanoscience. Here, we employ DNA nanotechnology to design a synthetic enzyme that substantially outperforms its biological archetypes. Consisting of only eight strands, our DNA nanostructure spontaneously inserts into biological membranes by forming a toroidal pore that connects the membrane’s inner and outer leaflets. The membrane insertion catalyzes spontaneous transport of lipid molecules between the bilayer leaflets, rapidly equilibrating the lipid composition. Through a combination of microscopic simulations and fluorescence microscopy we find the lipid transport rate catalyzed by the DNA nanostructure exceeds 10⁷ molecules per second, which is three orders of magnitude higher than the rate of lipid transport catalyzed by biological enzymes. Furthermore, we show that our DNA-based enzyme can control the composition of human cell membranes, which opens new avenues for applications of membrane-interacting DNA systems in medicine.

Mimicking enzyme function and improving upon it is a challenge facing nanotechnology. Here the authors design a DNA nanostructure that catalyzes the transport of lipids between bilayers at a rate three orders of magnitude higher than biological enzymes.

Reconstitution and functional studies of hamster P-glycoprotein in giant liposomes

This paper describes the preparation of giant unilamellar vesicles with reconstituted hamster P-glycoprotein (Pgp, ABCB1) for studying the transport activity of this efflux pump in individual liposomes using optical microscopy. Pgp, a member of ABC (ATP-binding cassette) transporter family, is known to contribute to the cellular multidrug resistance (MDR) against variety of drugs. The efficacy of many therapeutics is, thus, hampered by this efflux pump, leading to a high demand for simple and effective strategies to monitor the interactions of candidate drugs with this protein. Here, we applied small Pgp proteoliposomes to prepare giant Pgp-bearing liposomes via modified electroformation techniques. The presence of Pgp in the membrane of giant proteoliposomes was confirmed using immunohistochemistry. Assessment of Pgp ATPase activity suggested that this transporter retained its activity upon reconstitution into giant liposomes, with an ATPase specific activity of 439 ± 103 nmol/mg protein/min. For further confirmation, we assessed the transport activity of Pgp in these proteoliposomes by monitoring the translocation of rhodamine 123 (Rho123) across the membrane using confocal microscopy at various ATP concentrations (0-2 mM) and in the presence of Pgp inhibitors. Rate of change in Rho123 concentration inside the liposomal lumen was used to estimate the Rho123 transport rates (1/s) for various ATP concentrations, which were then applied to retrieve the Michaelis-Menten constant (Km) of ATP in Rho123 transport (0.42 ± 0.75 mM). Similarly, inhibitory effects of verapamil, colchicine, and cyclosporin A on Pgp were studied in this system and the IC50 values for these Pgp inhibitors were found 26.6 ± 6.1 μM, 94.6 ± 47.6 μM, and 0.21 ± 0.07 μM, respectively. We further analyzed the transport data using a kinetic model that enabled dissecting the passive diffusion of Rho123 from its Pgp-mediated transport across the membrane. Based on this model, the permeability coefficient of Rho123 across the liposomal membrane was approximately 1.25×10-7 cm/s. Comparing the membrane permeability in liposomes with and without Pgp revealed that the presence of this protein did not have a significant impact on membrane integrity and permeability. Furthermore, we used this model to obtain transport rate constants for the Pgp-mediated transport of Rho123 (m3/mol/s) at various ATP and inhibitor concentrations, which were then applied to estimate values of 0.53 ± 0.66 mM for Km of ATP and 25.2 ± 5.0 μM for verapamil IC50, 61.8 ± 34.8 μM for colchicine IC50, and 0.23 ± 0.09 μM for cyclosporin A IC50. The kinetic parameters obtained from the two analyses were comparable, suggesting a minimal contribution from the passive Rho123 diffusion across the membrane. This approach may, therefore, be applied for screening the transport activity of Pgp against potential drug candidates.

Static and Dynamic Permeability Assay for Hydrophilic Small Molecules Using a Planar Droplet Interface Bilayer

Because numerous drugs are administered through an oral route and primarily absorbed at the intestine, the prediction of drug permeability across an intestinal epithelial cell membrane has been a crucial issue in drug discovery. Thus, various in vitro permeability assays have been developed such as the Caco-2 assay, the parallel artificial membrane permeability assay (PAMPA), the phospholipid vesicle-based permeation assays (PVPA) and Permeapad. However, because of the time-consuming and quite expensive process for culturing cells in the Caco-2 assay and the unknown microscopic membrane structures of the other assays, a simpler yet more accurate and versatile technique is still required. Accordingly, we developed a new platform to measure the permeability of small molecules across a planar freestanding lipid bilayer with a well-defined area and structure. The lipid bilayer was constructed within a conventional UV spectrometer cell, and the transport of drug molecules across the bilayer was recorded by UV absorbance over time. We then computed the permeability from the time-dependent diffusion equation. We tested this assay for five exemplary hydrophilic drugs and compared their values with previously reported ones. We found that our assay has a much higher permeability compared to the other techniques, and this higher permeability is related to the thickness of the lipid bilayer. Also we were able to measure the dynamic permeability upon the addition of a membrane-disrupting surfactant demonstrating that our assay has the capability to detect real-time changes in permeability across the lipid bilayer.

Imaging and lipidomics methods for lipid analysis in metabolic and cardiovascular disease

Cardiometabolic diseases exhibit changes in lipid biology, which is important as lipids have critical roles in membrane architecture, signalling, hormone synthesis, homoeostasis and metabolism. However, Developmental Origins of Health and Disease studies of cardiometabolic disease rarely include analysis of lipids. This short review highlights some examples of lipid pathology and then explores the technology available for analysing lipids, focussing on the need to develop imaging modalities for intracellular lipids. Analytical methods for studying interactions between the complex endocrine and intracellular signalling pathways that regulate lipid metabolism have been critical in expanding our understanding of how cardiometabolic diseases develop in association with obesity and dietary factors. Biochemical methods can be used to generate detailed lipid profiles to establish links between lifestyle factors and metabolic signalling pathways and determine how changes in specific lipid subtypes in plasma and homogenized tissue are associated with disease progression. New imaging modalities enable the specific visualization of intracellular lipid traffic and distribution in situ. These techniques provide a dynamic picture of the interactions between lipid storage, mobilization and signalling, which operate during normal cell function and are altered in many important diseases. The development of methods for imaging intracellular lipids can provide a dynamic real-time picture of how lipids are involved in complex signalling and other cell biology pathways; and how they ultimately regulate metabolic function/homoeostasis during early development. Some imaging modalities have the potential to be adapted for in vivo applications, and may enable the direct visualization of progression of pathogenesis of cardiometabolic disease after poor growth in early life.

Addition of Cleaved Tail Fragments during Lipid Oxidation Stabilizes Membrane Permeability Behavior

Lipid oxidation has been linked to plasma membrane damage leading to cell death. In previous work, we examined the effect of oxidation on bilayer permeability by replacing defined amounts of an unsaturated lipid species with the corresponding phospholipid product that would result from oxidative tail scission of that species. This study adds the cleaved tail fragment, better mimicking the chemical results of oxidation. Permeability of PEG12-NBD, a small, uncharged molecule, was measured for vesicles with oxidation concentration corresponding to between 0 and 18 mol % of total lipid content. Permeability was measured using a microfluidic trap to capture the vesicles and spinning disk confocal microscopy (SDCM) to measure the transport of fluorescent PEG12-NBD at the equatorial plane. The thicknesses of lipid bilayers containing oxidized species were estimated by measuring capacitance of a black lipid membrane while simultaneously measuring bilayer area. We found that relative to chemically modeled oxidized bilayers without tail fragments, bilayers containing cleaved tail groups were less permeable for the same degree of oxidation. Curiously, membrane capacitance measurements indicated that the addition of tail fragments to chemically modeled oxidized bilayers also thinned these bilayers relative to samples with no tail fragments; in other words, the more permeable membranes were thicker. Above 12.5% chemically modeled oxidation, compositions both with and without the cleaved tail groups showed pore formation. This work highlights the complexity of the relationship between chemically modeled lipid bilayer oxidation and cell membrane properties.

Droplet-interface-bilayer assays in microfluidic passive networks

Basic biophysical studies and pharmacological processes can be investigated by mimicking the intracellular and extracellular environments across an artificial cell membrane construct. The ability to reproduce in vitro simplified scenarios found in live cell membranes in an automated manner has great potential for a variety of synthetic biology and compound screening applications. Here, we present a fully integrated microfluidic system for the production of artificial lipid bilayers based on the miniaturisation of droplet-interface-bilayer (DIB) techniques. The platform uses a microfluidic design that enables the controlled positioning and storage of phospholipid-stabilized water-in-oil droplets, leading successfully to the scalable and automated formation of arrays of DIBs to mimic cell membrane processes. To ensure robustness of operation, we have investigated how lipid concentration, immiscible phase flow velocities and the device geometrical parameters affect the system performance. Finally, we produced proof-of-concept data showing that diffusive transport of molecules and ions across on-chip DIBs can be studied and quantified using fluorescence-based assays.

Low levels of lipid oxidation radically increase the passive permeability of lipid bilayers

Oxidation of unsaturated lipids in cellular membranes has been shown to cause severe membrane damage and potentially cell death. The presence of oxidized lipid species in the membrane is known to cause changes in membrane properties, such as decreased fluidity. This study uses giant unilamellar vesicles (GUVs) to measure passive transport across membranes containing defined concentrations of oxidized lipid species. GUVs consisting of a saturated phospholipid, an unsaturated phospholipid, and cholesterol were used as model membranes. By replacing defined amounts of the unsaturated lipid with a corresponding oxidized product, the oxidation process could be mimicked, yielding vesicles of varying oxidized lipid concentration. Oxidized lipid concentration was varied from 0 mol% to 18 mol% of the total lipid concentration. Passive transport of PEG12-NBD, an uncharged fluorescent molecule, was measured using a microfluidic trap to capture the GUVs and spinning disk confocal microscopy (SDCM) to track the transport of a fluorescent species in the equatorial plane of each GUV. Membrane permeability was determined by fitting the resulting concentration profiles to a finite difference model of diffusion and permeation around and through the membrane. Experiments showed three permeability regimes. Without oxidation, transport was slow, with a measured permeability on the order of 1.5 × 10(-6) cm s(-1). At 2.5-10% oxidized species permeation was fast (1.5 × 10(-5) cm s(-1)). Above 12.5% oxidized species, the bilayer was disrupted by the formation of pore defects. As passive transport is an important mechanism for drug delivery, understanding the relationship between oxidation and permeation could provide insight into the pharmaceutical characteristics of tissues with oxidative damage.

Visualization and quantification of transmembrane ion transport into giant unilamellar vesicles

Transmembrane ion transporters (ionophores) are widely investigated as supramolecular agents with potential for biological activity. Tests are usually performed in synthetic membranes that are assembled into large unilamellar vesicles (LUVs). However transport must be followed through bulk properties of the vesicle suspension, because LUVs are too small for individual study. An alternative approach is described whereby ion transport can be revealed and quantified through direct observation. The method employs giant unilamellar vesicles (GUVs), which are 20–60 μm in diameter and readily imaged by light microscopy. This allows characterization of individual GUVs containing transporter molecules, followed by studies of transport through fluorescence emission from encapsulated indicators. The method provides new levels of certainty and relevance, given that the GUVs are similar in size to living cells. It has been demonstrated using a highly active anion carrier, and should aid the development of compounds for treating channelopathies such as cystic fibrosis.

A new approach for the fabrication of microscale lipid bilayers at glass pipets: application to quantitative passive permeation visualization

A new method of planar bilayer lipid membrane (BLM) formation is presented that allows stable, solvent-free lipid bilayers exhibiting high seal resistances to be formed rapidly, easily and reproducibly. Using these bilayers the passive permeation of a series of carboxylic acids is investigated, to determine quantitatively the trend in permeability with lipophilicity of the acid. BLMs are formed at the tip openings of pulled theta pipets, and the rate of permeation of each carboxylic acid across the bilayer, from within the pipet into the bulk solution is determined. This is achieved through spatially-resolved measurements of the pH change that occurs upon the permeation of the weak acid, visualized using a pH-sensitive fluorophore with a confocal laser scanning microscope. The extracted fluorescence profiles are matched to finite element method (FEM) simulations, to allow the associated permeation coefficient for each weak acid to be determined with high accuracy, since this is the only adjustable parameter used to fit the experimental data. For bilayers formed in this way, the weak acids show increasing permeability with lipophilicity. Furthermore, the arrangement allows the effect of a trans-membrane electric field on permeation to be explored. For both propanoic and hexanoic acid it is found that an applied electric field enhances molecular transport, which is attributed to the formation of pores within the membrane.

Hydrogel-assisted functional reconstitution of human P-glycoprotein (ABCB1) in giant liposomes

This paper describes the formation of giant proteoliposomes containing P-glycoprotein (P-gp) from a solution of small proteoliposomes that had been deposited and partially dried on a film of agarose. This preparation method generated a significant fraction of giant proteoliposomes that were free of internalized vesicles, making it possible to determine the accessible liposome volume. Measuring the intensity of the fluorescent substrate rhodamine 123 (Rho123) inside and outside these giant proteoliposomes determined the concentration of transported substrates of P-gp. Fitting a kinetic model to the fluorescence data revealed the rate of passive diffusion as well as active transport by reconstituted P-gp in the membrane. This approach determined estimates for the membrane permeability coefficient (Ps) of passive diffusion and rate constants of active transport (kT) by P-gp as a result of different experimental conditions. The Ps value for Rho123 was larger in membranes containing P-gp under all assay conditions than in membranes without P-gp indicating increased leakiness in the presence of reconstituted transmembrane proteins. For P-gp liposomes, the kT value was significantly higher in the presence of ATP than in its absence or in the presence of ATP and the competitive inhibitor verapamil. This difference in kT values verified that P-gp was functionally active after reconstitution and quantified the rate of active transport. Lastly, patch clamp experiments on giant proteoliposomes showed ion channel activity consistent with a chloride ion channel protein that co-purified with P-gp. Together, these results demonstrate several advantages of using giant rather than small proteoliposomes to characterize transport properties of transport proteins and ion channels.

Lipid directed intrinsic membrane protein segregation

We demonstrate a new approach for direct reconstitution of membrane proteins during giant vesicle formation. We show that it is straightforward to create a tissue-like giant vesicle film swelled with membrane protein using aquaporin SoPIP2;1 as an illustration. These vesicles can also be easily harvested for individual study. By controlling the lipid composition we are able to direct the aquaporin into specific immiscible liquid domains in giant vesicles. The oligomeric α-helical protein cosegregates with the cholesterol-poor domains in phase separating ternary mixtures.

A liposomal fluorescence assay to study permeation kinetics of drug-like weak bases across the lipid bilayer

Lipid bilayer permeation is considered the major route for in vivo barrier passage of drugs. Despite this fact, no technique is currently available to measure the kinetics of permeation across a single lipid bilayer of structurally unrelated drug-like solutes. We developed a liposomal fluorescence assay capable to determine permeation kinetics of basic drug-like solutes across lipid bilayers. The assay is based on the hypothesis that permeation of a weak base along a concentration gradient results in net proton release at the cis-side and net proton capture at the trans-side of the bilayer. The resulting pH changes were monitored with pH-sensitive fluorophores: Test compounds were incubated with liposomes containing a pH-sensitive fluorophore at the bilayer surfaces or in the aqueous lumen and fluorescence changes were monitored with a stopped-flow apparatus in solution or by total internal reflection fluorescence microscopy with surface-captured liposomes on a microfluidic platform. Incubation with lipophilic basic drugs resulted in the expected fluorescence changes while incubation with compounds without basic functionality or high polarity did not affect fluorescence. Kinetics of fluorescence changes followed bi-exponential functions. Logarithmic permeation coefficients (logPermapp) determined in solution and by microfluidics technology showed a good correlation (r(2)=0.94, n=7) and logPermapp increased with increasing lipophilicity. Neither diffusion in the aqueous phase nor partitioning into the bilayer was rate-limiting. PEGylation of 2% of the liposomal lipids reduced Permapp by a factor ~300. In conclusion, the presented liposomal fluorescence assay is capable to determine permeation kinetics of weak basic drug-like solutes across lipid bilayers. The method is adaptable to microfluidics technology for high-throughput measurements and can potentially be modified to work for weak acid solutes.

Optical stretching as a tool to investigate the mechanical properties of lipid bilayers

Measurements of lipid bilayer bending modulus by various techniques produce widely divergent results. We attempt to resolve some of this ambiguity by measuring bending modulus in a system that can rapidly process large numbers of samples, yielding population statistics. This system is based on optical stretching of giant unilamellar vesicles (GUVs) in a microfluidic dual-beam optical trap (DBOT). The microfluidic DBOT system is used here to measure three populations of GUVs with distinct lipid compositions. We find that gel-phase membranes are significantly stiffer than liquid-phase membranes, consistent with previous reports. We also find that the addition of cholesterol does not alter the bending modulus of membranes composed of a monounsaturated phospholipid.

Microfluidic methods for forming liposomes

Liposome structures have a wide range of applications in biology, biochemistry, and biophysics. As a result, several methods for forming liposomes have been developed. This review provides a critical comparison of existing microfluidic technologies for forming liposomes and, when applicable, a comparison with their analogous macroscale counterparts. The properties of the generated liposomes, including size, size distribution, lamellarity, membrane composition, and encapsulation efficiency, form the basis for comparison. We hope that this critique will allow the reader to make an informed decision as to which method should be used for a given biological application.

Imaging molecular transport across lipid bilayers

Low-molecular-weight carboxylic acids have many properties common to small molecule drugs. The transport of these acids across cell membranes has been widely studied, but these studies have produced wildly varying permeability values. Recent reports have even claimed that the transport behavior of these drugs is contrary to the rule of thumb called Overton's rule, which holds that more lipophilic molecules transport across lipid membranes more quickly. We used confocal microscopy to image the transport of carboxylic acids with different lipophilicities into a giant unilamellar lipid vesicle (GUV). Fluorescein-dextran, which acts as a pH-sensitive dye, was encapsulated in the GUV to trace the transport of acid. The GUV was immobilized on the surface of a microfluidic channel by biotin-avidin binding. This microchannel allows the rapid and uniform exchange of the solution surrounding the GUV. Using a spinning-disk confocal microscope, the entire concentration field is captured in a short (<100 ms) exposure. Results show that more lipophilic acids cross the bilayer more quickly. A finite difference model was developed to simulate the experimental process and derive permeabilities. The permeabilities change with the same trend as literature oil-water partition coefficients, demonstrating that Overton's rule applies to this class of molecules.

Vapor deposition of cross-linked fluoropolymer barrier coatings onto pre-assembled microfluidic devices

The interior surfaces of pre-assembled poly(dimethylsiloxane) (PDMS) microfluidic devices were modified with a cross-linked fluoropolymer barrier coating that significantly increased the chemical compatibility of the devices.

Microfluidic fabrication of asymmetric giant lipid vesicles

We have developed a microfluidic technology for the fabrication of compositionally asymmetric giant unilamellar vesicles (GUVs). The vesicles are assembled in two independent steps. In each step, a lipid monolayer is formed at a water-oil interface. The first monolayer is formed inside of a microfluidic device with a multiphase droplet flow configuration consisting of a continuous oil stream in which water droplets are formed. These droplets are dispensed into a vessel containing a layer of oil over a layer of water. The second lipid monolayer is formed by transferring the droplets through this second oil-water interface by centrifugation. By dissolving different lipid compositions in the different oil phases, the composition of each leaflet of the resulting lipid bilayer can be controlled. We have demonstrated membrane asymmetry by showing differential fluorescence quenching of labeled lipids in each leaflet and by demonstrating that asymmetric GUVs will bind an avidin-coated surface only when biotinylated lipids are targeted to the outer leaflet. In addition, we have demonstrated the successful asymmetric targeting of phosphatidylserine lipids to each leaflet, producing membranes with a biomimetic and physiologically relevant compositional asymmetry.

Excitation of Cy5 in self-assembled lipid bilayers using optical microresonators

Due to their sensitivity and temporal response, optical microresonators are used extensively in the biosensor arena, particularly in the development of label-free diagnostics and measurement of protein kinetics. In the present letter, we investigate using microcavities to probe molecules within biomimetic membranes. Specifically, a method for self-assembling lipid bilayers on spherical microresonators is developed and the bilayer-nature is verified. Subsequently, the microcavity is used to excite a Cy5-conjugated lipid located within the bilayer while the optical performance of the microcavity is characterized. The emission wavelength of the dye and the optical behavior of the microcavity agree with theoretical predictions.