Controlling two-dimensional tethered vesicle motion using an electric field: interplay of electrophoresis and electro-osmosis

Rapid Enrichment of a Native Multipass Transmembrane Protein via Cell Membrane Electrophoresis through Buffer pH and Ionic Strength Adjustment

Supported membrane electrophoresis is a promising technique for collecting membrane proteins in native bilayer environments. However, the slow mobility of typical transmembrane proteins has impeded the technique's advancement. Here, we successfully applied cell membrane electrophoresis to rapidly enrich a 12-transmembrane helix protein, glucose transporter 1 with antibodies (GLUT1 complex), by tuning the buffer pH and ionic strength. The identified conditions allowed the separation of the GLUT1 complex and a lipid probe, Fast-DiO, within a native-like environment in a few minutes. A force model was developed to account for distinct electric and drag forces acting on the transmembrane and aqueous-exposed portion of a transmembrane protein as well as the electroosmotic force. This model not only elucidates the impact of size and charge properties of transmembrane proteins but also highlights the influence of pH and ionic strength on the driving forces and, consequently, electrophoretic mobility. Model predictions align well with experimentally measured electrophoretic mobilities of the GLUT1 complex and Fast-DiO at various pH and ionic strengths as well as with several lipid probes, lipid-anchored proteins, and reconstituted membrane proteins from previous studies. Force analyses revealed the substantial membrane drag of the GLUT1 complex, significantly slowing down electrophoretic mobility. Besides, the counterbalance of similar magnitudes of electroosmotic and electric forces results in a small net driving force and, consequently, reduced mobility under typical neutral pH conditions. Our results further highlight how the size and charge properties of transmembrane proteins influence the suitable range of operating conditions for effective movement, providing potential applications for concentrating and isolating membrane proteins within this platform.

Implications of different membrane compartmentalization models in particle-based in silico studies

Studying membrane dynamics is important to understand the cellular response to environmental stimuli. A decisive spatial characteristic of the plasma membrane is its compartmental structure created by the actin-based membrane-skeleton (fences) and anchored transmembrane proteins (pickets). Particle-based reaction-diffusion simulation of the membrane offers a suitable temporal and spatial resolution to analyse its spatially heterogeneous and stochastic dynamics. Fences have been modelled via hop probabilities, potentials or explicit picket fences. Our study analyses the different approaches' constraints and their impact on simulation results and performance. Each of the methods comes with its own constraints; the picket fences require small timesteps, potential fences might induce a bias in diffusion in crowded systems, and probabilistic fences, in addition to carefully scaling the probability with the timesteps, induce higher computational costs for each propagation step.

Development of Cell Membrane Electrophoresis to Measure the Diffusivity of a Native Transmembrane Protein

The lateral diffusion of transmembrane proteins in cell membranes is an important process that controls the dynamics and functions of the cell membrane. Several fluorescence-based techniques have been developed to study the diffusivities of transmembrane proteins. However, it is challenging to measure the diffusivity of a transmembrane protein with slow diffusion because of the photobleaching effect caused by long exposure times or multiple exposures to light. In this study, we developed a cell membrane electrophoresis platform to measure diffusivity. We deposited cell membrane vesicles derived from HeLa cells to form supported cell membrane patches. We demonstrated that the electrophoresis platform can be used to drive the movement of not only a lipid probe but also a native transmembrane protein, GLUT1. The movements were halted by the boundaries of the membrane patches and the concentration profiles reached steady states when the diffusion mass flux was balanced with the electrical mass flux. We used the Nernst-Planck equation as the mass balance equation to describe the steady concentration profiles and fitted these equations to our data to obtain the diffusivities. The obtained diffusivities were comparable to those obtained by fluorescence recovery after photobleaching, suggesting the validity of this new method of diffusivity measurement. Only a single snapshot is required for the diffusivity measurement, addressing the problems associated with photobleaching and allowing researchers to measure the diffusivity of transmembrane proteins with slow diffusion.

Integrated magneto-electrophoresis microfluidic chip purification on library preparation device for preimplantation genetic testing for aneuploidy detection

Next generation sequencing (NGS) technology has revolutionized the field of personalized medicine through providing patient specific diagnostic information on a nucleic acid level. A key bottleneck in the NGS workflow is the preparation of nucleic acids for sequencing, or library preparation. One approach to overcoming this bottleneck on time and resources is through automating library preparation as much as possible from the stage of DNA extraction to a sequence-ready sample. Here, we have integrated microscale purification and macroscale PCR amplification to create an automated platform to replace manual DNA library preparation and magnetic bead-based cleanup steps. This microfluidic chip integrates magnetic bead transport and electrokinetic flow to remove unbound adapter dimers and other impurities from samples. We incorporate this method to develop an automated NGS DNA library preparation device that also includes macro- and microfluidic reagent movement and mixing and a thermoelectric cooler for controlled capillary heating and cooling. We greatly reduce the hands-on time, amount of pipetting required, and volumes of reagents needed as we test the feasibility of the platform on the clinically important diagnostic field of preimplantation genetic testing for aneuploidy (PGT-A). We prepared euploid and aneuploid five cell samples for sequencing and found our results were accurate for the cell samples with a sequencing quality equivalent to the standard of the DNA libraries prepared manually. Our device platform utilizes concepts such as: magneto-electrophoresis, integrated capillary PCR, and automated sample loading and unloading onto a microfluidic chip.

Analysis and refinement of 2D single-particle tracking experiments

In recent decades, single particle tracking (SPT) has been developed into a sophisticated analytical approach involving complex instruments and data analysis schemes to extract information from time-resolved particle trajectories. Very often, mobility-related properties are extracted from these particle trajectories, as they often contain information about local interactions experienced by the particles while moving through the sample. This tutorial aims to provide a comprehensive overview about the accuracies that can be achieved when extracting mobility-related properties from 2D particle trajectories and how these accuracies depend on experimental parameters. Proper interpretation of SPT data requires an assessment of whether the obtained accuracies are sufficient to resolve the effect under investigation. This is demonstrated by calculating mean square displacement curves that show an apparent super- or subdiffusive behavior due to poor measurement statistics instead of the presence of true anomalous diffusion. Furthermore, the refinement of parameters involved in the design or analysis of SPT experiments is discussed and an approach is proposed in which square displacement distributions are inspected to evaluate the quality of SPT data and to extract information about the maximum distance over which particles should be tracked during the linking process.

Single-Vesicle Assays Using Liposomes and Cell-Derived Vesicles: From Modeling Complex Membrane Processes to Synthetic Biology and Biomedical Applications

The plasma membrane is of central importance for defining the closed volume of cells in contradistinction to the extracellular environment. The plasma membrane not only serves as a boundary, but it also mediates the exchange of physical and chemical information between the cell and its environment in order to maintain intra- and intercellular functions. Artificial lipid- and cell-derived membrane vesicles have been used as closed-volume containers, representing the simplest cell model systems to study transmembrane processes and intracellular biochemistry. Classical examples are studies of membrane translocation processes in plasma membrane vesicles and proteoliposomes mediated by transport proteins and ion channels. Liposomes and native membrane vesicles are widely used as model membranes for investigating the binding and bilayer insertion of proteins, the structure and function of membrane proteins, the intramembrane composition and distribution of lipids and proteins, and the intermembrane interactions during exo- and endocytosis. In addition, natural cell-released microvesicles have gained importance for early detection of diseases and for their use as nanoreactors and minimal protocells. Yet, in most studies, ensembles of vesicles have been employed. More recently, new micro- and nanotechnological tools as well as novel developments in both optical and electron microscopy have allowed the isolation and investigation of individual (sub)micrometer-sized vesicles. Such single-vesicle experiments have revealed large heterogeneities in the structure and function of membrane components of single vesicles, which were hidden in ensemble studies. These results have opened enormous possibilities for bioanalysis and biotechnological applications involving unprecedented miniaturization at the nanometer and attoliter range. This review will cover important developments toward single-vesicle analysis and the central discoveries made in this exciting field of research.

Brownian Motion at Lipid Membranes: A Comparison of Hydrodynamic Models Describing and Experiments Quantifying Diffusion within Lipid Bilayers

The capability of lipid bilayers to exhibit fluid-phase behavior is a fascinating property, which enables, for example, membrane-associated components, such as lipids (domains) and transmembrane proteins, to diffuse within the membrane. These diffusion processes are of paramount importance for cells, as they are for example involved in cell signaling processes or the recycling of membrane components, but also for recently developed analytical approaches, which use differences in the mobility for certain analytical purposes, such as in-membrane purification of membrane proteins or the analysis of multivalent interactions. Here, models describing the Brownian motion of membrane inclusions (lipids, peptides, proteins, and complexes thereof) in model bilayers (giant unilamellar vesicles, black lipid membranes, supported lipid bilayers) are summarized and model predictions are compared with the available experimental data, thereby allowing for evaluating the validity of the introduced models. It will be shown that models describing the diffusion in freestanding (Saffman-Delbrück and Hughes-Pailthorpe-White model) and supported bilayers (the Evans-Sackmann model) are well supported by experiments, though only few experimental studies have been published so far for the latter case, calling for additional tests to reach the same level of experimental confirmation that is currently available for the case of freestanding bilayers.

Fabrication of microstructured binary polymer brush "corrals" with integral pH sensing for studies of proton transport in model membrane systems

Binary brush structures consisting of poly(cysteine methacrylate) (PCysMA) "corrals" enclosed within poly(oligoethylene glycol methyl ether methacrylate) (POEGMA) "walls" are fabricated simply and efficiently using a two-step photochemical process. First, the C-Cl bonds of 4-(chloromethyl)phenylsilane monolayers are selectively converted into carboxylic acid groups by patterned exposure to UV light through a mask and POEGMA is grown from unmodified chlorinated regions by surface-initiated atom-transfer radical polymerisation (ATRP). Incorporation of a ratiometric fluorescent pH indicator, Nile Blue 2-(methacryloyloxy)ethyl carbamate (NBC), into the polymer brushes facilitates assessment of local changes in pH using a confocal laser scanning microscope with spectral resolution capability. Moreover, the dye label acts as a radical spin trap, enabling removal of halogen end-groups from the brushes via in situ dye addition during the polymerisation process. Second, an initiator is attached to the carboxylic acid-functionalised regions formed by UV photolysis in the patterning step, enabling growth of PCysMA brushes by ATRP. Transfer of the system to THF, a poor solvent for PCysMA, causes collapse of the PCysMA brushes. At the interface between the collapsed brush and solvent, selective derivatisation of amine groups is achieved by reaction with excess glutaraldehyde, facilitating attachment of aminobutyl(nitrile triacetic acid) (NTA). The PCysMA brush collapse is reversed on transfer to water, leaving it fully expanded but only functionalized at the brush-water interface. Following complexation of NTA with Ni2+, attachment of histidine-tagged proteorhodopsin and lipid deposition, light-activated transport of protons into the brush structure is demonstrated by measuring the ratiometric response of NBC in the POEGMA walls.

Lipid rafts sense and direct electric field-induced migration

Endogenous electric fields (EFs) are involved in developmental regulation and wound healing. Although the phenomenon is known for more than a century, it is not clear how cells perceive the external EF. Membrane proteins, responding to electrophoretic and electroosmotic forces, have long been proposed as the sensing molecules. However, specific charge modification of surface proteins did not change cell migration motility nor directionality in EFs. Moreover, symmetric alternating current (AC) EF directs cell migration in a frequency-dependent manner. Due to their charge and ability to coalesce, glycolipids are therefore the likely primary EF sensor driving polarization of membrane proteins and intracellular signaling. We demonstrate that detergent-resistant membrane nanodomains, also known as lipid rafts, are the primary response element in EF sensing. The clustering and activation of caveolin and signaling proteins further stabilize raft structure and feed-forward downstream signaling events, such as rho and PI3K activation. Theoretical modeling supports the experimental results and predicts AC frequency-dependent cell and raft migration. Our results establish a fundamental mechanism for cell electrosensing and provide a role in lipid raft mechanotransduction.

Modulating Vesicle Adhesion by Electric Fields

We introduce an experimental setup for modulating adhesion of giant unilamellar vesicles to a planar substrate. Adhesion is induced by the application of an external potential to a transparent indium tin oxide-coated electrode (the substrate), which enables single-vesicle studies. We demonstrate tunable and reversible adhesion of negatively charged vesicles. The adhesion energy at different potentials is calculated from the vesicle shape assessed with confocal microscopy. Two approaches for these estimates are employed: one based on the whole contour of the vesicle and a second based on the contact curvature of the membrane in the vicinity of the substrate. Both approaches agree well with each other and show that the adhering vesicles are in the weak adhesion regime for the range of explored external potentials. Using fluorescence quenching assays, we detect that, in the adhering membrane segment, only the outer bilayer leaflet of the vesicle is depleted of negatively charged fluorescent lipids, while the inner leaflet remains unaffected. We show that depletion of negatively charged lipids is consistent Poisson-Boltzmann theory, taking into account charge regulation from lipid mobility. Finally, we also show that lipid diffusion is not significantly affected in the adhering membrane segment. We believe that the approaches introduced here for modulating and assessing vesicle adhesion have many potential applications in the field of single-vesicle studies and research on membrane adhesion.

Micrometre and nanometre scale patterning of binary polymer brushes, supported lipid bilayers and proteins

Binary polymer brush patterns were fabricated via photodeprotection of an aminosilane with a photo-cleavable nitrophenyl protecting group. UV exposure of the silane film through a mask yields micrometre-scale amine-terminated regions that can be derivatised to incorporate a bromine initiator to facilitate polymer brush growth via atom transfer radical polymerisation (ATRP). Atomic force microscopy (AFM) and imaging secondary ion mass spectrometry (SIMS) confirm that relatively thick brushes can be grown with high spatial confinement. Nanometre-scale patterns were formed by using a Lloyd's mirror interferometer to expose the nitrophenyl-protected aminosilane film. In exposed regions, protein-resistant poly(oligo(ethylene glycol)methyl ether methacrylate) (POEGMEMA) brushes were grown by ATRP and used to define channels as narrow as 141 nm into which proteins could be adsorbed. The contrast in the pattern can be inverted by (i) a simple blocking reaction after UV exposure, (ii) a second deprotection step to expose previously intact protecting groups, and (iii) subsequent brush growth via surface ATRP. Alternatively, two-component brush patterns can be formed. Exposure of a nitrophenyl-protected aminosilane layer either through a mask or to an interferogram, enables growth of an initial POEGMEMA brush. Subsequent UV exposure of the previously intact regions allows attachment of ATRP initiator sites and growth of a second poly(cysteine methacrylate) (PCysMA) brush within photolithographically-defined micrometre or nanometre scale regions. POEGMEMA brushes resist deposition of liposomes, but fluorescence recovery after photobleaching (FRAP) studies confirm that liposomes readily rupture on PCysMA "corrals" defined within POEGMEMA "walls". This leads to the formation of highly mobile supported lipid bilayers that exhibit similar diffusion coefficients to lipid bilayers formed on surfaces such as glass.

Simple, Direct Routes to Polymer Brush Traps and Nanostructures for Studies of Diffusional Transport in Supported Lipid Bilayers

Patterned poly(oligo ethylene glycol) methyl ether methacrylate (POEGMEMA) brush structures may be formed by using a combination of atom-transfer radical polymerization (ATRP) and UV photopatterning. UV photolysis is used to selectively dechlorinate films of 4-(chloromethyl)phenyltrichlorosilane (CMPTS) adsorbed on silica surfaces, by exposure either through a mask or using a two-beam interferometer. Exposure through a mask yields patterns of carboxylic acid-terminated adsorbates. POEGMEMA may be grown from intact Cl initiators that were masked during exposure. Corrals, traps, and other structures formed in this way enable the patterning of proteins, vesicles, and, following vesicle rupture, supported lipid bilayers (SLBs). Bilayers adsorbed on the carboxylic acid-terminated surfaces formed by C-Cl bond photolysis in CMPTS exhibit high mobility. SLBs do not form on POEGMEMA. Using traps consisting of carboxylic acid-functionalized regions enclosed by POEGMEMA structures, electrophoresis may be observed in lipid bilayers containing a small amount of a fluorescent dye. Segregation of dye at one end of the traps was measured by fluorescence microscopy. The increase in the fluorescence intensity was found to be proportional to the trap length, while the time taken to reach the maximum value was inversely proportional to the trap length, indicating uniform, rapid diffusion in all of the traps. Nanostructured materials were formed using interferometric lithography. Channels were defined by exposure of CMPTS films to maxima in the interferogram, and POEGMEMA walls were formed by ATRP. As for the micrometer-scale patterns, bilayers did not form on the POEGMEMA structures, and high lipid mobilities were measured in the polymer-free regions of the channels.

Hydrodynamic Propulsion of Liposomes Electrostatically Attracted to a Lipid Membrane Reveals Size-Dependent Conformational Changes

The efficiency of lipid nanoparticle uptake across cellular membranes is strongly dependent on the very first interaction step. Detailed understanding of this step is in part hampered by the large heterogeneity in the physicochemical properties of lipid nanoparticles, such as liposomes, making conventional ensemble-averaging methods too blunt to address details of this complex process. Here, we contribute a means to explore whether individual liposomes become deformed upon binding to fluid cell-membrane mimics. This was accomplished by using hydrodynamic forces to control the propulsion of nanoscale liposomes electrostatically attracted to a supported lipid bilayer. In this way, the size of individual liposomes could be determined by simultaneously measuring both their individual drift velocity and diffusivity, revealing that for a radius of ∼45 nm, a close agreement with dynamic light scattering data was observed, while larger liposomes (radius ∼75 nm) displayed a significant deformation unless composed of a gel-phase lipid. The relevance of being able to extract this type of information is discussed in the context of membrane fusion and cellular uptake.

Two-dimensional flow nanometry of biological nanoparticles for accurate determination of their size and emission intensity

Biological nanoparticles (BNPs) are of high interest due to their key role in various biological processes and use as biomarkers. BNP size and composition are decisive for their functions, but simultaneous determination of both properties with high accuracy remains challenging. Optical microscopy allows precise determination of fluorescence/scattering intensity, but not the size of individual BNPs. The latter is better determined by tracking their random motion in bulk, but the limited illumination volume for tracking this motion impedes reliable intensity determination. Here, we show that by attaching BNPs to a supported lipid bilayer, subjecting them to hydrodynamic flows and tracking their motion via surface-sensitive optical imaging enable determination of their diffusion coefficients and flow-induced drifts, from which accurate quantification of both BNP size and emission intensity can be made. For vesicles, the accuracy of this approach is demonstrated by resolving the expected radius-squared dependence of their fluorescence intensity for radii down to 15 nm.

Design of surface modifications for nanoscale sensor applications

Nanoscale biosensors provide the possibility to miniaturize optic, acoustic and electric sensors to the dimensions of biomolecules. This enables approaching single-molecule detection and new sensing modalities that probe molecular conformation. Nanoscale sensors are predominantly surface-based and label-free to exploit inherent advantages of physical phenomena allowing high sensitivity without distortive labeling. There are three main criteria to be optimized in the design of surface-based and label-free biosensors: (i) the biomolecules of interest must bind with high affinity and selectively to the sensitive area; (ii) the biomolecules must be efficiently transported from the bulk solution to the sensor; and (iii) the transducer concept must be sufficiently sensitive to detect low coverage of captured biomolecules within reasonable time scales. The majority of literature on nanoscale biosensors deals with the third criterion while implicitly assuming that solutions developed for macroscale biosensors to the first two, equally important, criteria are applicable also to nanoscale sensors. We focus on providing an introduction to and perspectives on the advanced concepts for surface functionalization of biosensors with nanosized sensor elements that have been developed over the past decades (criterion (iii)). We review in detail how patterning of molecular films designed to control interactions of biomolecules with nanoscale biosensor surfaces creates new possibilities as well as new challenges.

Supported lipid bilayers as dynamic platforms for tethered particles

Nanoparticle tethering to lipid bilayers enables the observation of hundreds of diffusing particles that are confined within a single field of view. A wide variety of materials ranging from plasmonic metals to soft matter can be stably tethered to the surface of a fluid bilayer by covalent or non-covalent means. The controlled environment of this experimental platform allows direct control over surface compositions and accurate tracking of nanoparticle interactions. This minireview will cover studies that use bilayer-tethered nanoparticles to investigate physical properties related to lipid mobility, biomolecule sensing, and surface interactions, as well as experiments to reversibly manipulate tethered nanoparticles by electric fields.

Application of nucleic acid-lipid conjugates for the programmable organisation of liposomal modules

We present a critical review of recent work related to the assembly of multicompartment liposome clusters using nucleic acids as a specific recognition unit to link liposomal modules. The asymmetry in nucleic acid binding to its non-self complementary strand allows the controlled association of different compartmental modules into composite systems. These biomimetic multicompartment architectures could have future applications in chemical process control, drug delivery and synthetic biology. We assess the different methods of anchoring DNA to lipid membrane surfaces and discuss how lipid and DNA properties can be tuned to control the morphology and properties of liposome superstructures. We consider different methods for chemical communication between the contents of liposomal compartments within these clusters and assess the progress towards making this chemical mixing efficient, switchable and chemically specific. Finally, given the current state of the art, we assess the outlook for future developments towards functional modular networks of liposomes.

Electrophoretic measurements of lipid charges in supported bilayers

While electrophoresis in lipid bilayers has been performed since the 1970s, the technique has until now been unable to accurately measure the charge on lipids and proteins within the membrane based on drift velocity measurements. Part of the problem is caused by the use of the Einstein-Smoluchowski equation to estimate the electrophoretic mobility of such species. The source of the error arises from the fact that a lipid headgroup is typically smaller than the Debye length of the adjacent aqueous solution in most electrophoresis experiments. Instead, the Henry equation can more accurately predict the electrophoretic mobility at sufficient ionic strength. This was done for three dye-labeled lipids with different sized head groups and a charge on each lipid of -1. Also, the charge was measured as a function of pH for two titratable lipids that were fluorescently labeled. Finally, it was shown that the Henry equation also has difficulties measuring the correct lipid charge at salt concentrations below 5 mM, where electroosmotic forces are more significant.

Forming lipid bilayer membrane arrays on micropatterned polyelectrolyte film surfaces

A novel method of forming lipid bilayer membrane arrays on micropatterned polyelectrolyte film surfaces is introduced. Polyelectrolyte films were fabricated by the layer-by-layer technique on a silicon oxide surface modified with a 3-aminopropyltriethoxysilane (APTES) monolayer. The surface pK(a) value of the APTES monolayer was determined by cyclic voltammetry to be approximately 5.61, on the basis of which a pH value of 2.0 was chosen for layer-by-layer assembly. Micropatterned polyelectrolyte films were obtained by deep-UV (254 nm) photolysis though a mask. Absorbed fluorescent latex beads were used to visualize the patterned surfaces. Lipid bilayer arrays were fabricated on the micropatterned surfaces by immersing the patterned substrates into a solution containing egg phosphatidylcholine vesicles. Fluorescence recovery after photobleaching studies yielded a lateral diffusion coefficient for probe molecules of 1.31±0.17 μm(2) s(-1) in the bilayer region, and migration of the lipid NBD PE in bilayer lipid membrane arrays was observed in an electric field.

Electrically induced lipid migration in non-lamellar phase

Inverted hexagonal blocks of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) lipid adsorbed on a polyethyleneimine (PEI)-coated surface in deionized water transformed its shape upon the application of an electric field, forming lipid objects in a variety of shapes (e.g. lines with a width of 10-50 μm). The phenomenon was driven by the electrophoresis, because the zwitterionic lipid, DOPE turned out to be highly negatively charged in deionized water. The interaction between DOPE and the PEI surface stabilized the system, assuring a lifetime over several weeks for the formed structures after the electric field was switched off. The free-drawing of microscopic objects (lines, crosses, and jelly fish) was also achieved by controlling the direction of the lipid movement with the field direction.

Tethered fibronectin liposomes on supported lipid bilayers as a prepackaged controlled-release platform for cell-based assays

A biomimetic construct containing an extracellular matrix protein-liposome composite tethered on supported lipid bilayers (SLBs) was formed with fibronectin (FN), and polyethylene glycol (PEG) and cholesterol-containing liposomes. The construct can serve as a multifunctional platform for cell attachment and drug release. The successful fabrication of the FN-liposome-SLB model platform was analyzed in situ with a quartz crystal microbalance with dissipation. The long-term stability of the surface tethered liposomes was measured via an encapsulated fluorescent probe. Less than 20% of the fluorescent probe content was released in 8 days, which compared favorably to the release of 90% of the probe content in one day from a similar construct made without PEG and cholesterol. HeLa cells were used to study the cellular interactions with the model platform. The extracellular matrix composition, FN, was found to be essential to promote HeLa cell adhesion on the liposome-SLB surfaces. Upon cell adhesion, the liposomes were spatially reorganized and absorbed by the cells. The rate of HeLa cell apoptosis was correlated with the surface density of doxorubicin-loaded liposomes, confirming the effective drug delivery through liposomes. The multifunctional model platform could be useful as preadministered, controlled-release platforms for cell- and tissue-based assays.

Nanofabrication for the analysis and manipulation of membranes

Recent advancements and applications of nanofabrication have enabled the characterization and control of biological membranes at submicron scales. This review focuses on the application of nanofabrication towards the nanoscale observing, patterning, sorting, and concentrating membrane components. Membranes on living cells are a necessary component of many fundamental cellular processes that naturally incorporate nanoscale rearrangement of the membrane lipids and proteins. Nanofabrication has advanced these understandings, for example, by providing 30 nm resolution of membrane proteins with metal-enhanced fluorescence at the tip of a scanning probe on fixed cells. Naturally diffusing single molecules at high concentrations on live cells have been observed at 60 nm resolution by confining the fluorescence excitation light through nanoscale metallic apertures. The lateral reorganization on the plasma membrane during membrane-mediated signaling processes has been examined in response to nanoscale variations in the patterning and mobility of the signal-triggering molecules. Further, membrane components have been separated, concentrated, and extracted through on-chip electrophoretic and microfluidic methods. Nanofabrication provides numerous methods for examining and manipulating membranes for both greater understandings of membrane processes as well as for the application of membranes to other biophysical methods.

Protein separation by electrophoretic-electroosmotic focusing on supported lipid bilayers

An electrophoretic-electroosmotic focusing (EEF) method was developed and used to separate membrane-bound proteins and charged lipids based on their charge-to-size ratio from an initially homogeneous mixture. EEF uses opposing electrophoretic and electroosmotic forces to focus and separate proteins and lipids into narrow bands on supported lipid bilayers (SLBs). Membrane-associated species were focused into specific positions within the SLB in a highly repeatable fashion. The steady-state focusing positions of the proteins could be predicted and controlled by tuning experimental conditions, such as buffer pH, ionic strength, electric field, and temperature. Careful tuning of the variables should enable one to separate mixtures of membrane proteins with only subtle differences. The EEF technique was found to be an effective way to separate protein mixtures with low initial concentrations, and it overcame diffusive peak broadening to allow four bands to be separated simultaneously within a 380 μm wide isolated supported membrane patch.

Dynamic and reversible control of 2D membrane protein concentration in a droplet interface bilayer

We form an artificial lipid bilayer between a nanolitre aqueous droplet and a supporting hydrogel immersed in an oil/lipid solution. Manipulation of the axial position of the droplet relative to the hydrogel controls the size of the bilayer formed at the interface; this enables the surface density of integral membrane proteins to be controlled. We are able to modulate the surface density of the β-barrel pore-forming toxin α-hemolysin over a range of 4 orders of magnitude within a time frame of a few seconds. The concentration changes are fully reversible. Membrane protein function and diffusion are unaltered, as measured by single molecule microscopy and single channel electrical recording.

Stability of DNA-tethered lipid membranes with mobile tethers

We recently introduced two approaches for tethering planar lipid bilayers as membrane patches to either a supported lipid bilayer or DNA-functionalized surface using DNA hybridization (Chung, M.; Lowe, R. D.; Chan, Y-H. M.; Ganesan, P. V.; Boxer, S. G. J. Struct. Biol.2009, 168, 190-9). When mobile DNA tethers are used, the tethered bilayer patches become unstable, while they are stable if the tethers are fixed on the surface. Because the mobile tethers between a patch and a supported lipid bilayer offer a particularly interesting architecture for studying the dynamics of membrane-membrane interactions, we have investigated the sources of instability, focusing on membrane composition. The most stable patches were made with a mixture of saturated lipids and cholesterol, suggesting an important role for membrane stiffness. Other factors such as the effect of tether length, lateral mobility, and patch membrane edge were also investigated. On the basis of these results, a model for the mechanism of patch destruction is developed.

Concentration dependence of lipopolymer self-diffusion in supported bilayer membranes

Self-diffusion coefficients of poly(ethylene glycol)2k-derivatized lipids (DSPE-PEG2k-CF) in glass-supported DOPC phospholipid bilayers are ascertained from quantitative fluorescence recovery after photobleaching (FRAP). We developed a first-order reaction-diffusion model to ascertain the bleaching constant, mobile fraction and lipopolymer self-diffusion coefficient D(s) at concentrations in the range c ≈ 0.5-5 mol%. In contrast to control experiments with 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(7-nitro-2-1,3-benzoxadiazol-4-yl) (ammonium salt) (DOPE-NBD) in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), the lipopolymer self-diffusion coefficient decreases monotonically with increasing concentration, without a distinguishing mushroom-to-brush transition. Our data yield a correlation D(s) = D(0)/(1 + αc), where D(0) ≈ 3.36 µm(2) s(-1) and α ≈ 0.56 (with c expressed as a mole percent). Interpreting the dilute limit with the Scalettar-Abney-Owicki statistical mechanical theory for transmembrane proteins yields an effective disc radius a(e) ≈ 2.41 nm. On the other hand, the Bussell-Koch-Hammer theory, which includes hydrodynamic interactions, yields a(e) ≈ 2.92 nm. As expected, both measures are smaller than the Flory radius of the 2 kDa poly(ethylene glycol) (PEG) chains, R(F) ≈ 3.83 nm, and significantly larger than the nominal radius of the phospholipid heads, a(l) ≈ 0.46 nm. The diffusion coefficient at infinite dilution D(0) was interpreted using the Evans-Sackmann theory, furnishing an inter-leaflet frictional drag coefficient b(s) ≈ 1.33 × 10(8) N s m(-3). Our results suggest that lipopolymer interactions are dominated by the excluded volume of the PEG-chain segments, with frictional drag dominated by the two-dimensional bilayer hydrodynamics.

Covalent attachment of lipid vesicles to a fluid-supported bilayer allows observation of DNA-mediated vesicle interactions

Specific membrane interactions such as lipid vesicle docking and fusion can be mediated by synthetic DNA-lipid conjugates as a model for the protein-driven processes that are ubiquitous in biological systems. Here we present a method of tethering vesicles to a supported lipid bilayer that allows the simultaneous deposition of cognate vesicle partners displaying complementary DNA, resulting in well-mixed populations of tethered vesicles that are laterally mobile. Vesicles are covalently attached to a supporting lipid bilayer using a DNA-templated click reaction; then DNA-mediated interactions between tethered vesicles are triggered by spiking the salt concentration. These interactions, such as docking and fusion, can then be observed for individual vesicles as they collide on the surface. The architecture of this new system also permits control over the number of lipid anchors that tether the vesicle to the supporting bilayer. The diffusion coefficient of tethered vesicles anchored by two lipids is approximately 1.6-fold slower than that of vesicles anchored only with a single lipid, consistent with a simple physical model.

Constructing size distributions of liposomes from single-object fluorescence measurements

We describe in detail a simple technique to construct the size distribution of liposome formulations from single-object fluorescence measurements. Liposomes that are fluorescently labeled in their membrane are first immobilized on a surface at dilute densities and then imaged individually using epi-fluorescence microscopy. The integrated intensities of several thousand single liposomes are collected and evaluated within minutes by automated image processing, using the user-friendly freeware ImageJ. The mean intensity of the liposome population is then calculated and scaled in units of length (nm) by relating the intensity data to the mean diameter obtained from a reference measurement with dynamic light scattering. We explain the process of constructing the size distributions in a step-by-step manner, starting with the preparation of liposomes through the final acquisition of size histograms. Detailed advice is given concerning critical parameters of image acquisition and processing. Size histograms constructed from single-particle measurements provide detailed information on complex distributions that may be easily averaged out in ensemble measurements (e.g., light scattering). In addition, the technique allows accurate measurements of polydisperse samples (e.g., nonextruded liposome preparations).

Segregation of molecules in lipid bilayer spreading through metal nanogates

A new methodology for nanoscopic molecular filtering was developed using a substrate with a periodic array of metallic nanogates with various widths between 75 and 500 nm. A self-spreading lipid bilayer was employed as the molecular transport and filtering medium. Dye-labeled molecules doped in the self-spreading lipid bilayer were filtered after the spreading less than a few tens of micrometers on the nanogate array. Quantitative analysis of the spreading dynamics suggests that the filtering effect originates from the formation of the chemical potential barrier at the nanogate region, which is believed to be due to structural change such as compression imposed on the spreading lipid bilayer at the gate. A highly localized chemical potential barrier affects the ability of the doped dye-labeled molecules to penetrate the gate. The use of the self-spreading lipid bilayer allows molecular transportation without the use of any external field such as an electric field as is used in electrophoresis. The present system could be applied micro- and nanoscopic device technologies as it provides a completely nonbiased filtering methodology.

Effects of linker sequences on vesicle fusion mediated by lipid-anchored DNA oligonucleotides

Synthetic lipid-oligonucleotide conjugates inserted into lipid vesicles mediate fusion when one population of vesicles displays the 5'-coupled conjugate and the other the 3'-coupled conjugate, so that anti-parallel hybridization allows the membrane surfaces to come into close proximity. Improved assays show that lipid mixing proceeds more quickly and to a much greater extent than content mixing, suggesting the latter is rate limiting. To test the effect of membrane-membrane spacing on fusion, a series of conjugates was constructed by adding 2-24 noncomplementary bases at the membrane-proximal ends of two complementary sequences. Increasing linker lengths generally resulted in progressively reduced rates and extents of lipid and content mixing, in contrast to higher vesicle docking rates. The relatively flexible, single-stranded DNA linker facilitates docking but allows greater spacing between the vesicles after docking, thus making the transition into fusion less probable, but not preventing it altogether. These experiments demonstrate the utility of DNA as a model system for fusion proteins, where sequence can easily be modified to systematically probe the effect of distance between bilayers in the fusion reaction.

Manipulation and charge determination of proteins in photopatterned solid supported bilayers

This work demonstrates the use of deep UV micropatterned chlorotrimethylsilane (TMS) monolayers to support lipid membranes on SiO(2) surfaces. After immersing such a patterned surface into a solution containing small unilamellar vesicles of egg PC, supported bilayer lipid membranes were formed on the hydrophilic, photolyzed regions and lipid monolayer over the hydrophobic, non-photolyzed regions. A barrier between the lipid monolayer and bilayer regions served to stop charged lipids migrating between the two. This allows the system to be used to separate charged lipids or proteins by electrophoresis. Either oppositely charged fluorescence labeled lipids [Texas Red DHPE (negative charge) and D291 (positive charge)] or lipids with different charge numbers [Texas Red DHPE (one negative charge) and NBD PS (two negative charges)] can be separated. We have also studied the migration of streptavidin attached to a biotinylated lipid. Negatively charged streptavidin responds to the applied electric field by moving in the direction of electroosmotic flow, i.e. towards the negative electrode. However the direction of streptavidin movement can be controlled by altering the difference in zeta potential between that of the streptavidin (zeta(1)) and the lipid membrane (zeta(2)). If zeta(1) > zeta(2), streptavidin moves to the negative electrode, while if zeta(1) < zeta(2), streptavidin moves to the positive electrode. This balance was manipulated by adding positively charged lipid DOTAP to the membrane. After measuring the average drift velocity of streptavidin as a function of DOTAP concentration, the point where zeta(1) approximately zeta(2) was found. At this point zeta(1) was calculated to be -9.8 mV which is in good agreement with the value of -13 mV from force measurements and corresponds to a charge of -2e per streptavidin, thus demonstrating the applicability of this method for determining protein charge.

Streptavidin crystals as nanostructured supports and image-calibration references for cryo-EM data collection

For cryo-EM structural studies, we seek to image membrane proteins as single particles embedded in proteoliposomes. One technical difficulty has been the low density of liposomes that can be trapped in the approximately 100nm ice layer that spans holes in the perforated carbon support film of EM grids. Inspired by the use of two-dimensional (2D) streptavidin crystals as an affinity surface for biotinylated DNA (Crucifix et al., 2004), we propose to use the crystals to tether liposomes doped with biotinylated lipids. The 2D crystal image also serves as a calibration of the image formation process, providing an absolute conversion from electrostatic potentials in the specimen to the EM image intensity, and serving as a quality control of acquired cryo-EM images. We were able to grow streptavidin crystals covering more than 90% of the holes in an EM grid, and which remained stable even under negative stain. The liposome density in the resulting cryo-EM sample was uniform and high due to the high-affinity binding of biotin to streptavidin. Using computational methods, the 2D crystal background can be removed from images without noticeable effect on image properties.

In situ synthesis of lipopeptides as versatile receptors for the specific binding of nanoparticles and liposomes to solid-supported membranes

A detailed study of the in situ coupling of small peptides such as CGGH6 (H6) and CGWK8 (K8) to maleimide functionalized phospholipid bilayers is presented. Individually addressable microstructured membranes are employed to unequivocally probe the conjugation. The in situ coupling of peptides via a terminal cysteine moiety to maleimide functionalized phospholipids is shown to be a convenient and versatile way to selectively fabricate peptide-modified phospholipid bilayers serving as specific receptor platforms for functionalized vesicles and nanoparticles. Specific binding of functional vesicles to the peptide-modified bilayers is achieved by either histidine complexation with Ni-NTA-DOGS containing vesicles or electrostatic interaction between positively charged oligolysine bearing lipopeptides and negatively charged POPC/POPG vesicles. Peptide receptors are also found to be easily accessible from the aqueous phase and not buried within the membrane interior.

Electrical manipulation of supported lipid membranes by embedded electrodes

Alkanethiol modified gold electrodes patterned over a silica surface provided a dual hydrophobic/hydrophilic surface suitable for phospholipid monolayer and bilayer formation over the alkylated gold and glass surfaces, respectively. The phospholipid monolayer and bilayer were connected, allowing free diffusion of lipids within both leaflets of the glass-supported bilayer over the alkanethiol/gold-to-glass interface. Application of large alternating current fields to these electrodes irreversibly switched the gold electrodes to diffusion barriers. Enclosure of the electrode devices within protein barriers revealed a resting state surface potential driven reorganization of the charged fluorescent probes. Application of lower magnitude direct current fields resulted in electrophoretic redistribution of the membrane probes and electro-osmotic reorganization of membrane associated proteins.

Model membrane systems and their applications

The complexity of biological membranes has motivated the development of a wide variety of simpler model systems whose size, geometry, and composition can be tailored with great precision. Approaches highlighted in this review are illustrated in Figure 1 including vesicles, supported bilayers, and hybrid membrane systems. These have been used to study problems ranging from phase behavior to membrane fusion. Experimental membrane models continue to advance in complexity with respect to architecture, size, and composition, as do computer simulations of their properties and dynamics. Analytical techniques such as imaging secondary ion mass spectrometry have also been developed and refined to give increasing spatial resolution and information content on membrane composition and dynamics.

Surface-based lipid vesicle reactor systems: fabrication and applications

Over the last ten years there has been a strong (bio)technological drive for the development of miniaturised reaction systems, motivated mainly by the need to reduce sample consumption and parallelise. Self-assembled soft-matter containers have naturally evolved to handle small volumes and could provide viable fluidic solutions especially in niche areas where ultra-miniaturisation, biocompatibility or cost are of critical importance. Here we focus on nanocontainers that are made of lipids and are immobilised on surfaces. We will highlight the most prominent contributions to date on the fabrication and the applications of surface-based vesicle systems as miniaturised reactors. Emphasis will be put on single-vesicle experiments.