Bilayer edges catalyze supported lipid bilayer formation

Microelectrode Arrays Measure Blocking of Voltage-Gated Calcium Ion Channels on Supported Lipid Bilayers Derived from Primary Neurons

Drug studies targeting neuronal ion channels are crucial to understand neuronal function and develop therapies for neurological diseases. The traditional method to study neuronal ion-channel activities heavily relies on the whole-cell patch clamp as the industry standard. However, this technique is both technically challenging and labour-intensive, while involving the complexity of keeping cells alive with low throughput. Therefore, the shortcomings are limiting the efficiency of ion-channel-related neuroscience research and drug testing. Here, this work reports a new system of integrating neuron membranes with organic microelectrode arrays (OMEAs) for ion-channel-related drug studies. This work demonstrates that the supported lipid bilayers (SLBs) derived from both neuron-like (neuroblastoma) cells and primary neurons are integrated with OMEAs for the first time. The increased expression of voltage-gated calcium (CaV) ion channels on differentiated SH-SY5Y SLBs  compared to non-differentiated ones is sensed electrically. Also, dose-response of the CaV ion-channel blocking effect on primary cortical neuronal SLBs from rats is monitored. The dose range causing ion channel blocking is comparable to literature. This system overcomes the major challenges from traditional methods (e.g., patch clamp) and showcases an easy-to-test, rapid, ultra-sensitive, cell-free, and high-throughput platform to monitor dose-dependent ion-channel blocking effects on native neuronal membranes.

Direct detection of deformation modes on varying length scales in active biopolymer networks

Correlated flows and forces that emerge from active matter orchestrate complex processes such as shape regulation and deformations in biological cells and tissues. The active materials central to cellular mechanics are cytoskeletal networks, where molecular motor activity drives deformations and remodeling. Here, we investigate deformation modes in actin networks driven by the molecular motor myosin II through quantitative fluorescence microscopy. We examine the deformation anisotropy at different length scales in networks of entangled, cross-linked, and bundled actin. In sparsely cross-linked networks, we find myosin-dependent biaxial buckling modes across length scales. In cross-linked bundled networks, uniaxial contraction is predominate on long length scales, while the uniaxial or biaxial nature of the deformation depends on bundle microstructure at shorter length scales. The anisotropy of deformations may provide insight to regulation of collective behavior in a variety of active materials.

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.

Observation of Liquid-Liquid-Phase Separation and Vesicle Spreading during Supported Bilayer Formation via Liquid-Phase Transmission Electron Microscopy

Liquid-phase transmission electron microscopy (LP-TEM) enables the real-time visualization of nanoscale dynamics in solution. This technique has been used to study the formation and transformation mechanisms of organic and inorganic nanomaterials. Here, we study the formation of block-copolymer-supported bilayers using LP-TEM. We observe two formation pathways that involve either liquid droplets or vesicles as intermediates toward supported bilayers. Quantitative image analysis methods are used to characterize vesicle spread rates and show the origin of defect formation in supported bilayers. Our results suggest that bilayer assembly methods that proceed via liquid droplet intermediates should be beneficial for forming pristine supported bilayers. Furthermore, supported bilayers inside the liquid cells may be used to image membrane interactions with proteins and nanoparticles in the future.

Bioelectrochemical platforms to study and detect emerging pathogens

The ongoing SARS-CoV-2 pandemic has emphasized the importance of technologies to rapidly detect emerging pathogens and understand their interactions with hosts. Platforms based on the combination of biological recognition and electrochemical signal transduction, generally termed bioelectrochemical platforms, offer unique opportunities to both sense and study pathogens. Improved bio-based materials have enabled enhanced control over the biotic-abiotic interface in these systems. These improvements have generated platforms with the capability to elucidate biological function rather than simply detect targets. This advantage is a key feature of recent bioelectrochemical platforms applied to infectious disease. Here, we describe developments in materials for bioelectrochemical platforms to study and detect emerging pathogens. The incorporation of host membrane material into electrochemical devices has provided unparalleled insights into the interaction between viruses and host cells, and new capture methods have enabled the specific detection of bacterial pathogens, such as those that cause secondary infections with SARS-CoV-2. As these devices continue to improve through the merging of hi-tech materials and biomaterials, the scalability and commercial viability of these devices will similarly improve.

Interactions of different lipoproteins with supported phospholipid raft membrane (SPRM) patterns to understand similar in-vivo processes

To better understand how lipoproteins interact and enter endothelium and participate in cellular processes, we investigated preferential lipid partitioning of triglyceride rich lipoproteins (TGRL), chylomicrons (CM), low density lipoproteins (LDL), very low density lipoproteins (VLDL) and their lipolysis products using supported phospholipid raft membrane (SPRM) patterns. We prepared SPRM patterns with Texas red labeled phospholipid patterns and Marina blue labeled raft patterns and added Atto-520 labeled lipoproteins (TGRL, CM, VLDL, LDL) and their lipolysis products in separate experiments and characterized these interactions using fluorescence microscopy. We observed that VLDL and LDL preferentially interacted with raft patterns. In contrast the TGRL and lipolysed products of TGRL interacted with both the patterns, slightly elevated preference for raft patterns and CM and its lipolysis products showed greater affinity to phospholipid patterns. The clear preference of VLDL and LDL for raft patterns suggests that these lipoproteins associate with cholesterol and sphingomyelin rich lipid micro-domains during their early interactions with endothelial cells, leading to atherosclerosis.

Tuning molecular motor transport through cytoskeletal filament network organization

Within cells, crosslinking proteins organize cytoskeletal filaments both temporally and spatially to create dynamic and structurally diverse networks. Molecular motors move on these networks for both force generation and transport processes. How the transport statistics depend on the network architecture remains poorly characterized. Using cross-linking proteins (α-actinin, fimbrin, fascin, or filamin) and purified actin, we create cytoskeletal networks with diverse microscopic architectures. We track the motion of myosin II motor proteins moving on these networks and calculate transport statistics. We observe that motor dynamics change predictably based on the bundling of filaments within the underlying networks and discuss implications for network function.

Surface-Enhanced Raman Spectroscopy of Fluid-Supported Lipid Bilayers

Supported lipid bilayers are essential model systems for studying biological membranes and for membrane-based sensor development. Surface-enhanced Raman spectroscopy (SERS) stands to add considerably to our understanding of the dynamics and interactions of these systems through direct chemical information. Despite this potential, SERS of lipid bilayers is not routinely achieved. Here, we carried out the first measurements of a solid-supported lipid bilayer on a SERS-active substrate and characterized the bilayer using SERS, atomic force microscopy, surface plasmon resonance spectroscopy, ellipsometry, and fluorescence recovery after photobleaching (FRAP). The creation of a fluid, SERS-active supported lipid bilayer was accomplished through use of a novel silica-coated silver film-over-nanosphere substrate. These substrates offer a powerful new platform to couple common surface techniques that are challenging on the nanoscale, for example, ellipsometry and FRAP, with SERS for studying biological membranes and their dynamics.

Spatiotemporal Kinetics of Supported Lipid Bilayer Formation on Glass via Vesicle Adsorption and Rupture

Supported lipid bilayers (SLBs) represent one of the most popular mimics of the cell membrane. Herein, we have used total internal reflection fluorescence microscopy for in-depth characterization of the vesicle-mediated SLB formation mechanism on a common silica-rich substrate, borosilicate glass. Fluorescently labeling a subset of vesicles allowed us to monitor the adsorption of individual labeled vesicles, resolve the onset of SLB formation from small seeds of SLB patches, and track their growth via SLB-edge-induced autocatalytic rupture of adsorbed vesicles. This made it possible to perform the first quantitative measurement of the SLB front velocity, which is shown to increase up to 1 order of magnitude with time. This effect can be classified as dramatic because in many other physical, chemical, or biological kinetic processes the front velocity is either constant or decreasing with time. The observation was successfully described with a theoretical model and Monte Carlo simulations implying rapid local diffusion of lipids upon vesicle rupture.

Surface Plasmon Resonance Study of the Binding of PEO-PPO-PEO Triblock Copolymer and PEO Homopolymer to Supported Lipid Bilayers

Poloxamer 188 (P188), a poly(ethylene oxide)- b-poly(propylene oxide)- b-poly(ethylene oxide) triblock copolymer, protects cell membranes against various external stresses, whereas poly(ethylene oxide) (PEO; 8600 g/mol) homopolymer lacks protection efficacy. As part of a comprehensive effort to elucidate the protection mechanism, we used surface plasmon resonance (SPR) to obtain direct evidence of binding of the polymers onto supported lipid bilayers. Binding kinetics and coverage of P188 and PEO were examined and compared. Most notably, PEO exhibited membrane association comparable to that of P188, evidenced by comparable association rate constants and coverage. This result highlights the need for additional mechanistic understanding beyond simple membrane association to explain the differential efficacy of P188 in therapeutic applications.

Lipid Pore-Filled Silica Thin-Film Membranes for Biomimetic Recovery of Dilute Carbohydrates

Selectively permeable biological membranes containing lipophilic barriers inspire the design of biomimetic carrier-mediated membranes for aqueous solute separation. The recovery of glucose, which can reversibly bind to boronic acid (BA) carriers, is examined in lipid pore-filled silica thin-film composite membranes with accessible mesopores. The successful incorporation of lipids (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC) and BA carriers (4-((N-Boc-amino)methyl)phenylboronic acid, BAMP-BA) in the pores of mesoporous silica (∼10 nm pore diameter) through evaporation deposition is verified by confocal microscopy and differential scanning calorimetry. In the absence of BA carriers, lipids confined inside the pores of silica thin films (∼200 nm thick) provide a factor of 14 increase in diffusive transport resistance to glucose, relative to traditional supported lipid bilayers formed by vesicle fusion on the porous surface. The addition of lipid-immobilized BAMP-BA (59 mol % in DPPC) facilitates the transport of glucose through the membrane; glucose flux increases from 45 × 10^-8 to 225 × 10^-8 mol/m²/s in the presence of BAMP-BA. Furthermore, the transport can be improved by environmental factors including pH gradient (to control the binding and release of glucose) and temperature (to adjust lipid bilayer fluidity). The successful development of biomimetic nanocomposite membranes demonstrated here is an important step toward the efficient dilute aqueous solute upgrading or separations, such as the processing of carbohydrates from lignocellulose hydrolysates, using engineered carrier/catalyst/support systems.

Filament rigidity and connectivity tune the deformation modes of active biopolymer networks

Molecular motors embedded within collections of actin and microtubule filaments underlie the dynamics of cytoskeletal assemblies. Understanding the physics of such motor-filament materials is critical to developing a physical model of the cytoskeleton and designing biomimetic active materials. Here, we demonstrate through experiments and simulations that the rigidity and connectivity of filaments in active biopolymer networks regulates the anisotropy and the length scale of the underlying deformations, yielding materials with variable contractility. We find that semiflexible filaments can be compressed and bent by motor stresses, yielding materials that undergo predominantly biaxial deformations. By contrast, rigid filament bundles slide without bending under motor stress, yielding materials that undergo predominantly uniaxial deformations. Networks dominated by biaxial deformations are robustly contractile over a wide range of connectivities, while networks dominated by uniaxial deformations can be tuned from extensile to contractile through cross-linking. These results identify physical parameters that control the forces generated within motor-filament arrays and provide insight into the self-organization and mechanics of cytoskeletal assemblies.

Time-Resolved Neutron Reflectivity during Supported Membrane Formation by Vesicle Fusion

The formation of supported lipid bilayers (SLB) on hydrophilic substrates through the method of unilamelar vesicle fusion is used routinely in a wide range of biophysical studies. In an effort to control and better understand the fusion process on the substrate, many experimental studies employing different techniques have been devoted to the elucidation of the fusion mechanism. In the present work, we follow the kinetics of membrane formation using time-resolved (TR) neutron reflectivity, focusing on the structural changes near the solid/liquid interface. A clear indication of stacked bilayer structure is observed during the intermediate phase of SLB formation. Adsorbed lipid mass decrease is also measured in the final stage of the process. We have found that it is essential for the analysis of the experimental results to treat the shape of adsorbed lipid vesicles on an attractive substrate theoretically. The overall findings are discussed in relation to proposed fusion mechanisms from the literature, and we argue that our observations favor a model involving enhanced adhesion of incoming vesicles on the edges of already-formed bilayer patches.

Optimizing the Formation of Supported Lipid Bilayers from Bicellar Mixtures

Supported lipid bilayers (SLBs) are widely studied model membrane platforms that are compatible with various surface-sensitive measurement techniques. SLBs are typically formed on silica-based materials, and there are numerous possible fabrication routes involving either bottom-up molecular self-assembly or vesicle adsorption and rupture. In between these two classes of fabrication strategies lies an emerging approach based on depositing quasi-two-dimensional lamellar, bicellar disks composed of a mixture of long-chain and short-chain phospholipids to promote the formation of SLBs. This approach takes advantage of the thermodynamic preference of long-chain phospholipids to form planar SLBs, whereas short-chain phospholipids have brief residence times. Although a few studies have shown that SLBs can be formed on silica-based materials from bicellar mixtures, outstanding questions remain about the self-assembly mechanism as well as the influence of the total phospholipid concentration, ratio of the two phospholipids (termed the "q-ratio"), and process of sample preparation. Herein, we address these questions through comprehensive quartz crystal microbalance-dissipation, fluorescence microscopy, and fluorescence recovery after photobleaching experiments. Our findings identify that optimal SLB formation occurs at lower total concentrations of phospholipids than previously used as short-chain phospholipids behave like membrane-destabilizing detergents at higher concentrations. Using lower phospholipid concentrations, we also discovered that the formation of SLBs proceeds through a two-step mechanism involving a critical coverage of bicellar disks akin to vesicle fusion. In addition, the results indicate that at least one cycle of freeze-thaw-vortexing is useful during the sample preparation process to produce SLBs. Taken together, the findings in this work identify optimal routes for fabricating SLBs from bicellar mixtures and reveal mechanistic details about the bicelle-mediated SLB formation process, which will aid further exploration of bicellar mixtures as tools for model membrane fabrication.

Liquid behavior of cross-linked actin bundles

The actin cytoskeleton is a critical regulator of cytoplasmic architecture and mechanics, essential in a myriad of physiological processes. Here we demonstrate a liquid phase of actin filaments in the presence of the physiological cross-linker, filamin. Filamin condenses short actin filaments into spindle-shaped droplets, or tactoids, with shape dynamics consistent with a continuum model of anisotropic liquids. We find that cross-linker density controls the droplet shape and deformation timescales, consistent with a variable interfacial tension and viscosity. Near the liquid-solid transition, cross-linked actin bundles show behaviors reminiscent of fluid threads, including capillary instabilities and contraction. These data reveal a liquid droplet phase of actin, demixed from the surrounding solution and dominated by interfacial tension. These results suggest a mechanism to control organization, morphology, and dynamics of the actin cytoskeleton.

A Stable Lipid/TiO2 Interface with Headgroup-Inversed Phosphocholine and a Comparison with SiO2

Zwitterionic phosphocholine (PC) lipids are highly biocompatible, representing a major component of the cell membrane. A simple mixing of PC liposomes and silica (SiO2) surface results in liposome fusion with the surface and formation of supported lipid bilayers. However, the stability of this bilayer is relatively low because adsorption is based mainly on weak van der Waals force. PC lipids strongly adsorb by TiO2 via chemical bonding with the lipid phosphate. The lack of fusion on TiO2 is attributable to the steric effect from the choline group in PC. In this study, inverse phosphocholine lipids (CP) are used, directly exposing the phosphate. Using a calcein leakage assay and cryo-TEM, fusion of CP liposome with TiO2 is demonstrated. The stability of this supported bilayer is significantly higher than that of the PC/SiO2 system, as indicated by washing the membrane under harsh conditions. Adsorption of CP liposomes by TiO2 is inhibited at high pH. Interestingly, the CP liposome cannot fuse with silica surface because of a strong charge repulsion. This study demonstrates an interesting interplay between a soft matter surface and metal oxides. By tuning the lipid structure, it is possible to rationally control the interaction force. This study provides an alternative system for forming stable supported bilayers on TiO2, and represents the first example of interfacing inverse lipids with inorganic surfaces.

Rupture of Stochastically Occurring Vesicle Clusters Limits Bilayer Formation on Alkane-PEG-Type Supports: Uncoupling Clustering from Surface Coverage

Polymer-supported bilayers (PSBs) are a recognized tool for drug discovery through function-interaction analysis of membrane proteins. While silica-supported bilayers (SSBs) spontaneously form from surface-adsorbed vesicles, successful PSB formation via a similar method has thus far been limited by an insufficient understanding of the underlying vesicle-remodelling processes. Here, we generated a polymer support through the incubation of poly-L-lysine conjugated to alkyl-chain-terminated poly(ethylene)glycol on silica. This polymer-coated silica substrate yielded efficient vesicle adsorption and spontaneous bilayer formation, thereby providing a rare opportunity to address the mechanism of PSB formation and compare it to that of SSB. The combined use of super-resolution imaging, kinetics, and simulations indicates that the rupture of stochastically formed vesicle clusters is the rate-limiting step, which is an order of magnitude higher for silica than for polymer-coated silica. This was confirmed by directly demonstrating increased rupture rates for surface adsorbed multivesicle assemblies formed by vesicle cross-linking in solution. On the basis of this key insight we surmised that a low propensity of cluster rupture can be compensated for by an increase in the number density of clusters: the deposition of a mixture of oppositely charged vesicles resulted in bilayer formation on another alkane-PEG type of interface, which despite efficient vesicle adsorption otherwise fails to support spontaneous bilayer formation. This potentially provides a universal strategy for promoting bilayer formation on resistant surfaces without resorting to modifying the surface itself. Therefore, multivesicle assemblies with tailored geometries not only could facilitate bilayer formation on polymers with interesting functional properties but also could instigate the exploration of vesicle architecture for other processes involving vesicle remodelling such as drug delivery.

Lipid exchange and transfer on nanoparticle supported lipid bilayers: effect of defects, ionic strength, and size

Lipid exchange/transfer has been compared for zwitterionic 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dimyristoyl-d54-sn-glycero-3-phosphocholine (DMPC) small unilamellar vesicles (SUVs) and for the same lipids on silica (SiO2) nanoparticle supported lipid bilayers (NP-SLBs) as a function of ionic strength, temperature, temperature cycling, and NP size, above the main gel-to-liquid crystal phase transition temperature, Tm, using d- and h-DMPC and DPPC. Increasing ionic strength decreases the exchange kinetics for the SUVs, but more so for the NP-SLBs, due to better packing of the lipids and increased attraction between the lipid and support. When the NP-SLBs (or SUVs) are cycled above and below Tm, the exchange rate increases compared with exchange at the same temperature without cycling, for similar total times, suggesting that defects provide sites for more facile removal and thus exchange of lipids. Defects can occur: (i) at the phase boundaries between coexisting gel and fluid phases at Tm; (ii) in bare regions of exposed SiO2 that form during NP-SLB formation due to mismatched surface areas of lipid and NPs; and (iii) during cycling as the result of changes in area of the lipids at Tm and mismatched thermal expansion coefficient between the lipids and SiO2 support. Exchange rates are faster for NP-SLBs prepared with the nominal amount of lipid required to form a NP-SLB compared with NP-SLBs that have been prepared with excess lipids to minimize SiO2 patches. Nanosystems prepared with equimolar mixtures of NP-SLBs composed of d-DMPC (d(DMPC)-NP-SLB) and SUVs composed of h-DMPC (h(DMPC)-SUV) show that the calorimetric transition of the "donor" h(DMPC)-SUV decreases in intensity without an initial shift in Tm, indicating that the "acceptor" d(DMPC)-NP-SLB can accommodate more lipids, through either further fusion or insertion of lipids into the distal monolayer. Exchange for d/h(DMPC)-NP-SLB is in the order 100 nm SiO2 > 45 nm SiO2 > 5 nm SiO2.

Development of hat-shaped liposomes on solid supports

We introduce a simple dynamical model which can explain the development of hat-shaped surface attached liposomes by taking membrane self-adhesion into account. The model reveals that hat formation is a general phenomenon, although it is difficult to observe experimentally. We show under what conditions hat-shaped vesicles can become observable. One such scenario, in consistency with AFM measurements, is that the dynamics is slowed down by the low rate of the outflow of the internal fluid of the vesicle through the narrow space between the two bilayers of the brim.

Rupture of lipid vesicles near solid surfaces

The behavior of lipid vesicles near solid surfaces, despite its scientific and technological significance, is poorly understood. By simultaneously taking into account (i) the dynamics of spontaneous pore opening and closing in surface bound vesicles; (ii) their volume loss via leakage through the pores; (iii) and the propagation of their contact line, we have developed a simple model that can fully describe the detailed mechanism of and provide the necessary conditions for the rupture of vesicles and the subsequent formation of supported lipid bilayers. The predictions of the model are in qualitative agreement with many of the experimental observations.

Solvent-assisted lipid bilayer formation on silicon dioxide and gold

Planar lipid bilayers on solid supports mimic the fundamental structure of biological membranes and can be investigated using a wide range of surface-sensitive techniques. Despite these advantages, planar bilayer fabrication is challenging, and there are no simple universal methods to form such bilayers on diverse material substrates. One of the novel methods recently proposed and proven to form a planar bilayer on silicon dioxide involves lipid deposition in organic solvent and solvent exchange to influence the phase of adsorbed lipids. To scrutinize the specifics of this solvent-assisted lipid bilayer (SALB) formation method and clarify the limits of its applicability, we have developed a simplified, continuous solvent-exchange version to form planar bilayers on silicon dioxide, gold, and alkanethiol-coated gold (in the latter case, a lipid monolayer is formed to yield a hybrid bilayer) and varied the type of organic solvent and rate of solvent exchange. By tracking the SALB formation process with simultaneous quartz crystal microbalance-dissipation (QCM-D) and ellipsometry, it was determined that the acoustic, optical, and hydration masses along with the acoustic and optical thicknesses, measured at the end of the process, are comparable to those observed by employing conventional fabrication methods (e.g., vesicle fusion). As shown by QCM-D measurements, the obtained planar bilayers are highly resistant to protein adsorption, and several, but not all, water-miscible organic solvents could be successfully used in the SALB procedure, with isopropanol yielding particularly high-quality bilayers. In addition, fluorescence recovery after photobleaching (FRAP) measurements demonstrated that the coefficient of lateral lipid diffusion in the fabricated bilayers corresponds to that measured earlier in the planar bilayers formed by vesicle fusion. With increasing rate of solvent exchange, it was also observed that the bilayer became incomplete and a phenomenological model was developed in order to explain this feature. The results obtained allowed us to clarify and discriminate likely steps of the SALB formation process as well as determine the corresponding influence of organic solvent type and flow conditions on these steps. Taken together, the findings demonstrate that the SALB formation method can be adapted to a continuous solvent-exchange procedure that is technically minimal, quick, and efficient to form planar bilayers on solid supports.

Peripheral and integral membrane binding of peptides characterized by time-dependent fluorescence shifts: focus on antimicrobial peptide LAH₄

Positioning of peptides with respect to membranes is an important parameter for biological and biophysical studies using model systems. Our experiments using five different membrane peptides suggest that the time-dependent fluorescence shift (TDFS) of Laurdan can help when distinguishing between peripheral and integral membrane binding and can be a useful, novel tool for studying the impact of transmembrane peptides (TMP) on membrane organization under near-physiological conditions. This article focuses on LAH4, a model α-helical peptide with high antimicrobial and nucleic acid transfection efficiencies. The predominantly helical peptide has been shown to orient in supported model membranes parallel to the membrane surface at acidic and, in a transmembrane manner, at basic pH. Here we investigate its interaction with fully hydrated large unilamellar vesicles (LUVs) by TDFS and fluorescence correlation spectroscopy (FCS). TDFS shows that at acidic pH LAH4 does not influence the glycerol region while at basic pH it makes acyl groups at the glycerol level of the membrane less mobile. TDFS experiments with antimicrobial peptides alamethicin and magainin 2, which are known to assume transmembrane and peripheral orientations, respectively, prove that changes in acyl group mobility at the glycerol level correlate with the orientation of membrane-associated peptide molecules. Analogous experiments with the TMPs LW21 and LAT show similar effects on the mobility of those acyl groups as alamethicin and LAH4 at basic pH. FCS, on the same neutral lipid bilayer vesicles, shows that the peripheral binding mode of LAH4 is more efficient in bilayer permeation than the transmembrane mode. In both cases, the addition of LAH4 does not lead to vesicle disintegration. The influence of negatively charged lipids on the bilayer permeation is also addressed.

Direct observation and control of supported lipid bilayer formation with interferometric scattering microscopy

Supported lipid bilayers (SLB) are frequently used to study processes associated with or mediated by lipid membranes. The mechanism by which SLBs form is a matter of debate, largely due to the experimental difficulty associated with observing the adsorption and rupture of individual vesicles. Here, we used interferometric scattering microscopy (iSCAT) to directly visualize membrane formation from nanoscopic vesicles in real time. We observed a number of previously proposed phenomena such as vesicle adsorption, rupture, movement, and a wave-like bilayer spreading. By varying the vesicle size and the lipid-surface interaction strength, we rationalized and tuned the relative contributions of these phenomena to bilayer formation. Our results support a model where the interplay between bilayer edge tension and the overall interaction energy with the surface determine the mechanism of SLB formation. The unique combination of sensitivity, speed, and label-free imaging capability of iSCAT provides exciting prospects not only for investigations of SLB formation, but also for studies of assembly and disassembly processes on the nanoscale with previously unattainable accuracy and sensitivity.

Nanoscale departures: excess lipid leaving the surface during supported lipid bilayer formation

The behavior of small liposomes on surfaces of inorganic oxides remains enigmatic. Under appropriate conditions it results in the formation of supported lipid bilayers (SLBs). During this process, some lipids leave the surface (desorb). We were able to visualize this by a combination of time-resolved fluorescence microscopy and fluorescence recovery after photobleaching studies. Our observations also allowed us to analyze the kinetics of bilayer patch growth during the late stages of SLB formation. We found that it entails a balance between desorption of excess lipids and further adsorption of liposomes from solution. These studies were performed with liposomes containing zwitterionic phospholipids (dioleoylphosphatidylcholine alone or a mixture of dioleoylphosphatidylcholine, dipalmitoylphosphatidylcholine, and cholesterol) on TiO2 in the presence of Ca(2+) but in the absence of other salts.

Characterization of PAH/DPPG layer-by-layer films by VUV spectroscopy

The spectroscopic characterization of layer-by-layer (LbL) films containing liposomes is essential not only for determining the precise film architecture but also to guide the design of drug delivery systems. In this study we provide the first report of vacuum ultraviolet spectroscopy (VUV) characterization of LbL films made with liposomes from 1.2-dipalmitoyl-sn-Glycero-3-[Phospho-rac-(1-glycerol)] (Sodium Salt) (DPPG) alternated with poly(allylamine hydrochloride) (PAH). Measurements in the 6.0-9.5eV range allowed us to identify the electronic transitions responsible for the spectra, which were assigned to carboxyl, hydroxyl and phosphate groups in DPPG while the PAH spectra were governed by electronic transitions in the amino groups. The surface mass density of the LbL films could be determined, from which the formation of a DPPG bilayer was inferred. This rupture of the liposomes into bilayers was confirmed with atomic force microscopy measurements. In subsidiary experiments we ensured that the UV irradiation in vacuum had negligible damage in the DPPG liposomes during the course of the VUV measurements. In addition to demonstrating the usefulness of VUV spectroscopy, the results presented here may be exploited in biological applications of liposome-containing films.

Adsorption kinetics of DPPG liposome layers: a quantitative analysis of surface roughness

Roughness of a positively charged poly(allylamine hydrochloride) (PAH) polyelectrolyte surface was shown to strongly influence the adsorption of 1.2-dipalmitoyl-sn-3-glycero-[phosphorrac-(1-glycerol)] (DPPG) liposomes on it. The adsorption kinetic curves of DPPG liposomes onto a low-roughness PAH layer reveal an adsorbed amount of 5 mg/m², pointing to liposome rupture, whereas a high-roughness surface leads to adsorbed amounts of 51 mg/m², signifying adsorption of intact liposomes. The adsorption kinetic parameters calculated from adsorption kinetic curves allow us to conclude that the adsorption process is due to electrostatic interactions and also depends on processes such as diffusion and reorganization of lipids on the surface. Analysis of the roughness kinetics enabled us to calculate a growth exponent of 0.19 ± 0.07 and a roughness exponent of around 0.84, revealing that DPPG liposomes adsorbed onto rough surfaces follow the Villain self-affine model. By relating self-affine surfaces with hydrophobicity, the liposome integrity was explained by the reduction in the number of water molecules on the PAH surface, contributing to counterion anchorage near PAH ionic groups, reducing the liposome/PAH layer electrostatic forces and, consequently, avoiding liposome rupture.

Sialylneolacto-N-tetraose c (LSTc)-bearing liposomal decoys capture influenza A virus

Influenza is a severe disease in humans and animals with few effective therapies available. All strains of influenza virus are prone to developing drug resistance due to the high mutation rate in the viral genome. A therapeutic agent that targets a highly conserved region of the virus could bypass resistance and also be effective against multiple strains of influenza. Influenza uses many individually weak ligand binding interactions for a high avidity multivalent attachment to sialic acid-bearing cells. Polymerized sialic acid analogs can form multivalent interactions with influenza but are not ideal therapeutics due to solubility and toxicity issues. We used liposomes as a novel means for delivery of the glycan sialylneolacto-N-tetraose c (LSTc). LSTc-bearing decoy liposomes form multivalent, polymer-like interactions with influenza virus. Decoy liposomes competitively bind influenza virus in hemagglutination inhibition assays and inhibit infection of target cells in a dose-dependent manner. Inhibition is specific for influenza virus, as inhibition of Sendai virus and respiratory syncytial virus is not observed. In contrast, monovalent LSTc does not bind influenza virus or inhibit infectivity. LSTc decoy liposomes prevent the spread of influenza virus during multiple rounds of replication in vitro and extend survival of mice challenged with a lethal dose of virus. LSTc decoy liposomes co-localize with fluorescently tagged influenza virus, whereas control liposomes do not. Considering the conservation of the hemagglutinin binding pocket and the ability of decoy liposomes to form high avidity interactions with influenza hemagglutinin, our decoy liposomes have potential as a new therapeutic agent against emerging influenza strains.

Few-atom fluorescent silver clusters assemble at programmed sites on DNA nanotubes

We show that DNA hairpins template the site-specific assembly of fluorescent few-atom Ag clusters on DNA nanotubes. Fluorescent clusters form only at hairpin sites and not on the double-stranded DNA scaffold, allowing for spatially programmed self-assembly. Ag clusters synthesized on hairpins protruding from DNA nanotubes can have nearly identical fluorescence spectra to those synthesized on free hairpins of identical sequence. Analysis of the stepwise photobleaching of individual clusters suggests a chemical yield of ~45%. Given the well-established sequence-specific optical properties of DNA stabilized Ag clusters, these results point the way toward high yield assembly of metal cluster fluorophores with control over spectra as well as spatial arrangement.

Mechanistic insight into patterned supported lipid bilayer self-assembly

Patterned supported lipid bilayers (SLBs) provide a model system for studying fluid lipid bilayers and transmembrane proteins in an array format. SLB arrays self-assemble on patterned self-assembled monolayers (SAMs) consisting of hexadecanethiol and glycol-terminated regions. While the mechanism of SLB formation on glass has been studied extensively, the formation of SLBs on other substrates is not necessarily well understood. Moreover, SLB arrays on patterned SAMs represent an intriguing system, since lipid vesicles do not adhere to glycol-terminated monolayers. Here, we utilize surface plasmon resonance imaging (SPRi) and kinetic analysis to examine the mechanism of SLB formation on the glycol-terminated regions of patterned SAMs and supported lipid monolayer (SLM) formation on alkyl-terminated regions of patterned SAMs. We determine that vesicles rupture to form a patterned SLB through a two-step mechanism that is dependent upon vesicle attachment at the interface of the two regions of the patterned monolayer.

The role of lateral tension in calcium-induced DPPS vesicle rupture

We assess the role of lateral tension in rupturing anionic dipalmitoylphosphatidyserine (DPPS), neutral dipalmitoylphosphatidylcholine (DPPC), and mixed DPPS-DPPC vesicles. Binding of Ca(2+) is known to have a significant impact on the effective size of DPPS lipids and little effect on the size of DPPC lipids in bilayer structures. In the present work we utilized laser transmission spectroscopy (LTS) to assess the effect of Ca(2+)-induced stress on the stability of the DPPS and DPPC vesicles. The high sensitivity and resolution of LTS has permitted the determination of the size and shape of liposomes in solution. The results indicate a critical size after which DPPS single shell vesicles are no longer stable. Our measurements indicate Ca(2+) promotes bilayer fusion up to a maximum diameter of ca. 320 nm. These observations are consistent with a straightforward free-energy-based model of vesicle rupture involving lateral tension between lipids regulated by the binding of Ca(2+). Our results support a critical role of lateral interactions within lipid bilayers for controlling such processes as the formation of supported bilayer membranes and pore formation in vesicle fusion. Using this free energy model we are able to infer a lower bound for the area dilation modulus for DPPS (252 pN/nm) and demonstrate a substantial free energy increase associated with vesicle rupture.

Adsorption of lipid liquid crystalline nanoparticles on cationic, hydrophilic, and hydrophobic surfaces

Investigation of nonlamellar nanoparticles formed by dispersion of self-assembled lipid liquid crystalline phases is stimulated by their many potential applications in science and technology; resulting from their unique solubilizing, encapsulating, and space-dividing nature. Understanding the interfacial behavior of lipid liquid crystalline nanoparticles (LCNPs) at surfaces can facilitate the exploitation of such systems for a number of potentially interesting uses, including preparation of functional surface coatings and uses as carriers of biologically active substances. We have studied the adsorption of LCNP, based on phosphatidylcholine/glycerol dioleate and Polysorbate 80 as stabilizers, at different model surfaces by use of in situ ellipsometry. The technique allows time-resolved monitoring of the layer thickness and the amount adsorbed, thereby providing insights into the restructuring of the lipid nanoparticle upon adsorption. The effects of solvent condition, electrolyte concentration, particle size, and surface chemistry on adsorbed layer properties were investigated. Furthermore, the internal structures of the particles were investigated by cryo-transmission electron microscopy and small angle X-ray diffraction on the corresponding liquid crystalline phases in excess water. LCNPs are shown to form well-defined layers at the solid-liquid interface with a structure and coverage that are determined by the interplay between the self-assembly properties of the lipids and lipid surface interactions, respectively. At the hydrophobic surface, hydrophobic interaction results in a structural transition from the original LCNP morphology to a monolayer structure at the interface. In contrast, at cationic and hydrophilic surfaces, relaxation is a relatively slow process, resulting in much thicker adsorbed layers, with thickness and adsorption behavior that to a greater extent reflect the original bulk LCNP properties.

Minimal systems to study membrane-cytoskeleton interactions

In the context of minimal systems design, there are two areas in which the reductionist approach has been particularly successful: studies of molecular motors on cytoskeletal filaments, and of protein-lipid interactions in model membranes. However, a minimal cortex, that is, the interface between membrane and cytoskeleton, has just begun to be functionally reconstituted. A key property of living cells is their ability to change their shape in response to extracellular and intracellular stimuli. Although studied in live cells since decades, the mutual dependence between cytoskeleton and membrane dynamics in these large-scale transformations is still poorly understood. Here we report on inspiring recent in vitro work in this direction, and the promises it holds for a better understanding of key cellular processes.

Hemifusion of giant unilamellar vesicles with planar hydrophobic surfaces: a fluorescence microscopy study

Vesicle adhesion and fusion to interfaces are frequently used for the construction of biomimetic surfaces in biosensors and drug delivery. Ubiquitous in cell biology, vesicle fusion involves the transformation of two separate membranes into one contiguous lipid bilayer. In distinction, the deposition of vesicle membranes to hydrophobic surfaces requires the transformation of a lipidic bilayer into a monomolecular layer - a topologically distinct process termed hemifusion. Here, we used hydrophobically terminated self-assembled monolayers (SAMs) on solid surfaces to track the hemifusion of fluorescently labeled giant unilamellar vesicles (GUVs) at the single vesicle level with video time resolution (≈53 ms). We observed that a dilute monolayer, consisting of lipid extracted from the outer GUV leaflet, spreads outward across the hydrophobic surface from the vesicle adhesion site. Subsequently, bilayer hemifusion occurs by vesicle rupture near the hydrophobic surface, followed by spreading of lipid in a dense monolayer. GUV lipids thus transfer to the SAM surface in two concentric zones: an outer hemifusion zone comprises lipids drawn from the outer GUV leaflet and an inner hemifusion zone comprises lipids from both the inner and outer GUV leaflets and grows at a rate of ≈1000 µm2 s-1 (dA/dt = 970 ± 430 µm2 s-1 in n = 22 independent experiments). This growth rate is quantitatively consistent with the assumption that the spreading of the monolayer is entirely driven by the difference in surface energies of the hydrophobic and the lipid-covered SAM surfaces, which is dissipated by friction of the spreading monolayer on the SAM. Lipid transfer between the inner and outer GUV leaflets occurs via a hemifusion pore that forms early in the process near the membrane contact site. This pore also permits expulsion of water from the GUV interior as the vesicle contracts onto the contact site.

Supported lipid bilayer microarrays created by non-contact printing

Arrays of supported lipid bilayers (SLBs) provide great potential for future drug development and multiplexed biological research, but are difficult to prepare due to the sensitivity of both the lipid and protein structural arrangement to air exposure. A novel way to produce arrays of SLBs is presented based on non-contact dispensing of vesicles to a substrate through a thin surface confined water film. The approach presents many degrees of freedom since it is not limited to a specific substrate, lipid composition, linker or controlled environment. The method allows adjustment of spot size (180-360 μm) by repeated dispensing as well as control over the composition of the spots and subsequent analytes. SLB formation by vesicle adsorption and rupture allows for incorporation of membrane proteins through pre-formed proteoliposomes. Dispensing through a dip-and-rinse water film avoids contamination, disruptive drying and the need for complex buffer compositions. Furthermore, no humidity control is necessary which simplifies the production step and prolongs the life-time of the spotting system. We characterize the method with respect to control over spot size, bilayer mobility and the formation process as well as demonstrate the possibility to fuse bilayer spots with subsequently added vesicles. Since complex lipid compositions and multiple spotting nozzles can be used, this novel technique is expected to be a promising platform for future applications, e.g. patterning to monitor peptide/protein-lipid interactions, for glycomics using glycolipids or lipopolysaccharides, and to study mixing of spatially confined lipid membranes.

Composition and asymmetry in supported membranes formed by vesicle fusion

The structure and formation of supported membranes at silica surfaces by vesicle fusion was investigated by neutron reflectivity and quartz crystal microbalance (QCM-D) measurements. The structure of equimolar phospholipid mixtures of DLPC-DPPC, DMPC-DPPC, and DOPC-DPPC depends intricately on the vesicle deposition conditions. The supported bilayer membranes exhibit varying degrees of compositional asymmetry between the monolayer leaflets, which can be modified by the deposition temperature as well as the salt concentration of the vesicle solution. The total lipid composition of the supported bilayers differs from the composition of the vesicles in solution, and the monolayer proximal to the silica surface is always enriched in DPPC compared to the distal monolayer. The results, which show unambiguougsly that some exchange and rearrangement of lipids occur during vesicle deposition, can be rationalized by considering the effects of salt screening and temperature on the rates of lipid exchange, rearrangement, and vesicle adsorption, but there is also an intricate dependence on the lipid-lipid interactions. Thus, although both symmetric and asymmetric supported bilayers can be prepared from vesicles, the optimal conditions are sensitive to the lipid composition of the system.

Formation of supported lipid bilayers at surfaces with controlled curvatures: influence of lipid charge

We have developed and characterized novel biomimetic membranes, formed at nanostructured sensor substrates with controlled curvatures, motivated by the many biological processes that involve membrane curvature. Model systems with convex nanostructures, with radii of curvatures (ROCs) of 70, 75, and 95 nm, were fabricated utilizing colloidal assembly and used as substrates for supported lipid bilayers (SLBs). The SLBs were formed via vesicle adsorption and rupture, and the vesicle deposition pathway was studied by means of quartz crystal microbalance with dissipation (QCM-D) and fluorescence microscopy. SLBs conforming to the underlying nanostructured surfaces, which exhibit increased surface area with decreased ROC, were confirmed from excess mass, monitored by QCM-D, and excess total fluorescence intensities. The formation of SLBs at the nanostructured surfaces was possible, however, depending on the ROC of the structures and the lipid vesicle charge the quality varied. The presence of nanostructures was shown to impair vesicle rupture and SLB formation was progressively hindered at surfaces with structures of decreasing ROCs. The introduction of a fraction of the positively charged lipid POEPC in the lipid vesicle membrane allowed for good quality and conformal bilayers at all surfaces. Alternatively, for vesicles formed from lipid mixtures with a fraction of the negatively charged lipid POPS, SLB formation was not at all possible at surfaces with the lowest ROC. Interestingly, the vesicle adsorption rate and the SLB formation were faster at surfaces with nanostructures of progressively smaller ROCs at high ratios of POPS in the vesicles. Development of templated SLBs with controlled curvatures provides a new experimental platform, especially at the nanoscale, at which membrane events such as lipid sorting, phase separation, and protein binding can be studied.

Supported bilayers with excess membrane reservoir: a template for reconstituting membrane budding and fission

A complete mechanistic understanding of membrane-localized processes in vesicular transport, such as membrane budding and fission, requires their reconstitution with biochemically-defined components from a biochemically-defined substrate. Supported bilayers formed by vesicle fusion represent an attractive substrate for this purpose. However, conventional supported bilayers lack a sufficient membrane reservoir to recreate membrane budding and fission events. We describe the formation of supported bilayers with excess membrane reservoir (SUPER) templates from the fusion of liposomes containing negatively charged lipids on silica beads under high-ionic-strength conditions. Using a fluorescence microscopy-based assay to monitor early and late stages of supported bilayer formation, we show that an increase in ionic strength leads to an increase in the rates of liposome adsorption and subsequent fusion during formation of supported bilayers. The two rates, however, increase disproportionally, leading to accumulation of excess reservoir with an increase in ionic strength. SUPER templates allow the seamless application of microscopy-based assays to analyze membrane-localized processes together with sedimentation-based assays to isolate vesicular and nonvesicular products released from the membrane. The results presented here emphasize the general utility of these templates for analyzing vesicular and nonvesicular transport processes.

Cholesterol-dependent nanomechanical stability of phase-segregated multicomponent lipid bilayers

Cholesterol is involved in endocytosis, exocytosis, and the assembly of sphingolipid/cholesterol-enriched domains, as has been demonstrated in both model membranes and living cells. In this work, we explored the influence of different cholesterol levels (5-40 mol%) on the morphology and nanomechanical stability of phase-segregated lipid bilayers consisting of dioleoylphosphatidylcholine/sphingomyelin/cholesterol (DOPC/SM/Chol) by means of atomic force microscopy (AFM) imaging and force mapping. Breakthrough forces were consistently higher in the SM/Chol-enriched liquid-ordered domains (Lo) than in the DOPC-enriched fluid-disordered phase (Ld) at a series of loading rates. We also report the activation energies (DeltaEa) for the formation of an AFM-tip-induced fracture, calculated by a model for the rupture of molecular thin films. The obtained DeltaEa values agree remarkably well with reported values for fusion-related processes using other techniques. Furthermore, we observed that within the Chol range studied, the lateral organization of bilayers can be categorized into three distinct groups. The results are rationalized by fracture nanomechanics of a ternary phospholipid/sphingolipid/cholesterol mixture using correlated AFM-based imaging and force mapping, which demonstrates the influence of a wide range of cholesterol content on the morphology and nanomechanical stability of model bilayers. This provides fundamental insights into the role of cholesterol in the formation and stability of sphingolipid/cholesterol-enriched domains, as well as in membrane fusion.