Templating membrane assembly, structure, and dynamics using engineered interfaces

Self-Assembly of Graphene Oxide Nanoflakes in a Lipid Monolayer at the Air-Water Interface

The graphene family, especially graphene oxide (GO), has captured increasing prospects in the biomedical field due to its excellent physicochemical properties. Understanding the health and environmental impact of GO is of great importance for guiding future applications. Although their interactions with living organisms are omnipresent, the exact molecular mechanism is yet to be established. The cellular membrane is the first barrier for a foreign molecule to interact before entering into the cell. In the present study, a model system consisting of a lipid monolayer at the air-water interface represents one of the leaflets of this membrane. Surface pressure-area isotherms and advanced synchrotron X-ray scattering techniques have been employed to comprehend the interaction by varying the electrostatics of the membrane. The results depict a strong GO interaction with positively charged phospholipids, weak interaction with zwitterionic lipids, and interestingly negligible interaction with negatively charged lipids. GO flakes induce significant changes in the out-of-plane organization of a positively charged lipid monolayer with a minor influence on in-plane assembly of lipid chains. This interaction is packing-specific, and the influence of GO is much stronger at lower surface pressure. Even though for zwitterionic phospholipids, the GO flakes may partly insert into the lipid chains, the X-ray scattering results indicate that the flakes preferentially lie horizontally underneath the positively charged lipid monolayer. This in-depth structural description may pave new perspectives for the scientific community for the development of GO-based biosensors and biomedical materials.

Model Lipid Membranes Assembled from Natural Plant Thylakoids into 2D Microarray Patterns as a Platform to Assess the Organization and Photophysics of Light-Harvesting Proteins

Natural photosynthetic "thylakoid" membranes found in green plants contain a large network of light-harvesting (LH) protein complexes. Rearrangement of this photosynthetic machinery, laterally within stacked membranes called "grana", alters protein-protein interactions leading to changes in the energy balance within the system. Preparation of an experimentally accessible model system that allows the detailed investigation of these complex interactions can be achieved by interfacing thylakoid membranes and synthetic lipids into a template comprised of polymerized lipids in a 2D microarray pattern on glass surfaces. This paper uses this system to interrogate the behavior of LH proteins at the micro- and nanoscale and assesses the efficacy of this model. A combination of fluorescence lifetime imaging and atomic force microscopy reveals the differences in photophysical state and lateral organization between native thylakoid and hybrid membranes, the mechanism of LH protein incorporation into the developing hybrid membranes, and the nanoscale structure of the system. The resulting model system within each corral is a high-quality supported lipid bilayer that incorporates laterally mobile LH proteins. Photosynthetic activity is assessed in the hybrid membranes versus proteoliposomes, revealing that commonly used photochemical assays to test the electron transfer activity of photosystem II may actually produce false-positive results.

Exploring Molecular-Biomembrane Interactions with Surface Plasmon Resonance and Dual Polarization Interferometry Technology: Expanding the Spotlight onto Biomembrane Structure

The molecular analysis of biomolecular-membrane interactions is central to understanding most cellular systems but has emerged as a complex technical challenge given the complexities of membrane structure and composition across all living cells. We present a review of the application of surface plasmon resonance and dual polarization interferometry-based biosensors to the study of biomembrane-based systems using both planar mono- or bilayers or liposomes. We first describe the optical principals and instrumentation of surface plasmon resonance, including both linear and extraordinary transmission modes and dual polarization interferometry. We then describe the wide range of model membrane systems that have been developed for deposition on the chips surfaces that include planar, polymer cushioned, tethered bilayers, and liposomes. This is followed by a description of the different chemical immobilization or physisorption techniques. The application of this broad range of engineered membrane surfaces to biomolecular-membrane interactions is then overviewed and how the information obtained using these techniques enhance our molecular understanding of membrane-mediated peptide and protein function. We first discuss experiments where SPR alone has been used to characterize membrane binding and describe how these studies yielded novel insight into the molecular events associated with membrane interactions and how they provided a significant impetus to more recent studies that focus on coincident membrane structure changes during binding of peptides and proteins. We then discuss the emerging limitations of not monitoring the effects on membrane structure and how SPR data can be combined with DPI to provide significant new information on how a membrane responds to the binding of peptides and proteins.

Supported lipid bilayer platforms to probe cell mechanobiology

Mammalian and bacterial cells sense and exert mechanical forces through the process of mechanotransduction, which interconverts biochemical and physical signals. This is especially important in contact-dependent signaling, where ligand-receptor binding occurs at cell-cell or cell-ECM junctions. By virtue of occurring within these specialized junctions, receptors engaged in contact-dependent signaling undergo oligomerization and coupling with the cytoskeleton as part of their signaling mechanisms. While our ability to measure and map biochemical signaling within cell junctions has advanced over the past decades, physical cues remain difficult to map in space and time. Recently, supported lipid bilayer (SLB) technologies have emerged as a flexible platform to mimic and perturb cell-cell and cell-ECM junctions, allowing one to study membrane receptor mechanotransduction. Changing the lipid composition and underlying substrate tunes bilayer fluidity, and lipid and ligand micro- and nano-patterning spatially control positioning and clustering of receptors. Patterning metal gridlines within SLBs confines lipid mobility and introduces mechanical resistance. Here we review fundamental SLB mechanics and how SLBs can be engineered as tunable cell substrates for mechanotransduction studies. Finally, we highlight the impact of this work in understanding the biophysical mechanisms of cell adhesion. This article is part of a Special Issue entitled: Interactions between membrane receptors in cellular membranes edited by Kalina Hristova.

Reversible Monolayer-Bilayer Transition in Supported Phospholipid LB Films under the Presence of Water: Morphological and Nanomechanical Behavior

Mixed monolayer Langmuir-Blodgett (LB) films of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and cholesterol (Chol) in the 1:1 ratio have been prepared onto solid mica substrates. Upon immersion in water or in an aqueous HEPES solution (pH 7.4) the monolayer LB films were spontaneously converted into well-organized bilayers leaving free mica areas. The process has been demonstrated to be reversible upon removal of the aqueous solution, resulting in remarkably free of defects monolayers that are homogeneously distributed onto the mica. In addition, the nanomechanical properties exhibited by the as-formed bilayers have been determined by means of AFM breakthrough force studies. The bilayers formed by immersion of the monolayer in an aqueous media exhibit nanomechanical properties and stability under compression analogous to those of DPPC:Chol supported bilayers obtained by other methods previously described in the literature. Consequently, the hydration of a monolayer LB film has been revealed as an easy method to produce well-ordered bilayers that mimic the cell membrane and that could be used as model cell membranes.

pH-dependent lipid vesicle interactions with plasma polymerized thin films

Model lipid vesicle and supported lipid bilayer (SLB) systems are used in a variety of applications including biosensing, cell membrane mimics, and drug delivery. Exposure of a surface to a vesicle solution provides a straightforward method for creating such systems via vesicle adsorption and collapse. However, this process is complex and the relationship between the surface physicochemical properties and vesicle collapse is poorly understood. Plasma polymers are thin conformal films that can be applied to a variety of materials to modify surface properties. This paper uses quartz crystal microbalance with dissipation and fluorescence recovery after photobleaching (FRAP) to explore lipid vesicle interactions with plasma polymerized acrylic acid (ppAAc), allylamine (ppAAm), and ppAAc/ppAAm micropatterns. Vesicle interactions were dependent on plasma polymer chemistry and pH of the buffer solution. Vesicles readily and stably adsorbed to ppAAm over a wide pH range. ppAAc demonstrated limited interactions at pH 7 and vesicle adsorption at pH 4. Vesicle collapse and SLB formation could be induced using a pH change. FRAP was used to explore the fluidity of the lipid structures on both the patterned and unpatterned plasma polymer films. On ppAAm/ppAAc micropatterns, pH transitions combined with the presence of chemically distinct regions on the same substrate enabled immobile lipid islands on ppAAc to be surrounded by fluid lipid regions on ppAAm. This work demonstrates that plasma polymer films could enable spatially controlled vesicle adsorption and SLB formation on a wide variety of different substrates.

Adsorption of Nanoceria by Phosphocholine Liposomes

Nanoceria (CeO₂ nanoparticle) possesses a number of enzyme-like activities. In particular, it scavenges reactive oxygen species based on in vitro and in vivo antioxidation studies. An important aspect of fundamental physical understanding is its interaction with lipid membranes that are the main components of the cell membrane. In this work, adsorption of nanoceria onto phosphocholine (PC) liposomes was performed. PC lipids are the main constituents of the cell outer membrane. Using a fluorescence quenching assay, a nanoceria adsorption isotherm was determined at various pH values and ionic strengths. A non-Langmuir isotherm occurred at pH 4 because of lateral electrostatic repulsion among the adsorbed cationic nanoceria. The phosphate group in the PC lipid is mainly responsible for the interaction, and the adsorbed nanoceria can be displaced by free inorganic phosphate. The tendency of the system to form large aggregates is a function of pH and the concentration of nanoceria, attributable to nanoceria being positively charged at pH 4 and neutral at physiological pH. Calcein leakage tests indicate that nanoceria induces liposome leakage because of transient lipid phase transition, and cryo-transmission electron microscopy indicates that the overall shape of the liposome is retained although deformation is still observed. This study provides fundamental biointerfacial information at a molecular level regarding the interaction of nanoceria and model cell membranes.

POPC Bilayers Supported on Nanoporous Substrates: Specific Effects of Silica-Type Surface Hydroxylation and Charge Density

Recent advances in nanotechnology bring to the forefront a new class of extrinsic constraints for remodeling lipid bilayers. In this next-generation technology, membranes are supported over nanoporous substrates. The nanometer-sized pores in the substrate are too small for bilayers to follow the substrate topology; consequently, the bilayers hang over the pores. Experiments demonstrate that nanoporous substrates remodel lipid bilayers differently from continuous substrates. The underlying molecular mechanisms, however, remain largely undetermined. Here we use molecular dynamics (MD) simulations to probe the effects of silica-type hydroxylation and charge densities on adsorbed palmitoyl-oleoylphosphatidylcholine (POPC) bilayers. We find that a 50% porous substrate decorated with a surface density of 4.6 hydroxyls/nm(2) adsorbs a POPC bilayer at a distance of 4.5 Å, a result consistent with neutron reflectivity experiments conducted on topologically similar silica constructs under highly acidic conditions. Although such an adsorption distance suggests that the interaction between the bilayer and the substrate will be buffered by water molecules, we find that the substrate does interact directly with the bilayer. The substrate modifies several properties of the bilayer-it dampens transverse lipid fluctuations, reduces lipid diffusion rates, and modifies transverse charge densities significantly. Additionally, it affects lipid properties differently in the two leaflets. Compared to substrates functionalized with sparser surface hydroxylation densities, this substrate adheres to bilayers at smaller distances and also remodels POPC more extensively, suggesting a direct correspondence between substrate hydrophilicity and membrane properties. A partial deprotonation of surface hydroxyls, as expected of a silica substrate under mildly acidic conditions, however, produces an inverse effect: it increases the substrate-bilayer distance, which we attribute to the formation of an electric double layer over the negatively charged substrate, and restores, at least partially, leaflet asymmetry and headgroup orientations. Overall, this study highlights the intrinsic complexity of lipid-substrate interactions and suggests the prospect of making two surface attributes-dipole densities and charge densities-work antagonistically toward remodeling lipid bilayer properties.

Spontaneous vesiculation: a mechanistic insight from the study of hybrid peptide molecules

We report a series of hybrid peptide molecules that display spontaneous vesiculation. A closer look at their vesicle formation revealed a toroidal intermediate en route to the final vesicular form.

Photosynthetic Proteins in Supported Lipid Bilayers: Towards a Biokleptic Approach for Energy Capture

In nature, plants and some bacteria have evolved an ability to convert solar energy into chemical energy usable by the organism. This process involves several proteins and the creation of a chemical gradient across the cell membrane. To transfer this process to a laboratory environment, several conditions have to be met: i) proteins need to be reconstituted into a lipid membrane, ii) the proteins need to be correctly oriented and functional and, finally, iii) the lipid membrane should be capable of maintaining chemical and electrical gradients. Investigating the processes of photosynthesis and energy generation in vivo is a difficult task due to the complexity of the membrane and its associated proteins. Solid, supported lipid bilayers provide a good model system for the systematic investigation of the different components involved in the photosynthetic pathway. In this review, the progress made to date in the development of supported lipid bilayer systems suitable for the investigation of membrane proteins is described; in particular, there is a focus on those used for the reconstitution of proteins involved in light capture.

Fluorescence imaging of MACPF/CDC proteins: new techniques and their application

Structural and biochemical investigations have helped illuminate many of the important details of MACPF/CDC pore formation. However, conventional techniques are limited in their ability to tackle many of the remaining key questions, and new biophysical techniques might provide the means to improve our understanding. Here we attempt to identify the properties of MACPF/CDC proteins that warrant further study, and explore how new developments in fluorescence imaging are able to probe these properties.

Validity and applicability of membrane model systems for studying interactions of peripheral membrane proteins with lipids

The cell membrane serves, at the same time, both as a barrier that segregates as well as a functional layer that facilitates selective communication. It is characterized as much by the complexity of its components as by the myriad of signaling process that it supports. And, herein lays the problems in its study and understanding of its behavior - it has a complex and dynamic nature that is further entangled by the fact that many events are both temporal and transient in their nature. Model membrane systems that bypass cellular complexity and compositional diversity have tremendously accelerated our understanding of the mechanisms and biological consequences of lipid-lipid and protein-lipid interactions. Concurrently, in some cases, the validity and applicability of model membrane systems are tarnished by inherent methodical limitations as well as undefined quality criteria. In this review we introduce membrane model systems widely used to study protein-lipid interactions in the context of key parameters of the membrane that govern lipid availability for peripheral membrane proteins. This article is part of a Special Issue entitled Tools to study lipid functions.

A dry membrane protection technique to allow surface acoustic wave biosensor measurements of biological model membrane approaches

Model membrane approaches have attracted much attention in biomedical sciences to investigate and simulate biological processes. The application of model membrane systems for biosensor measurements is partly restricted by the fact that the integrity of membranes critically depends on the maintenance of an aqueous surrounding, while various biosensors require a preconditioning of dry sensors. This is for example true for the well-established surface acoustic wave (SAW) biosensor SAM®5 blue. Here, a simple drying procedure of sensor-supported model membranes is introduced using the protective disaccharide trehalose. Highly reproducible model membranes were prepared by the Langmuir-Blodgett technique, transferred to SAW sensors and supplemented with a trehalose solution. Membrane rehydration after dry incorporation into the SAW device becomes immediately evident by phase changes. Reconstituted model membranes maintain their full functionality, as indicated by biotin/avidin binding experiments. Atomic force microscopy confirmed the morphological invariability of dried and rehydrated membranes. Approximating to more physiological recognition phenomena, the site-directed immobilization of the integrin VLA-4 into the reconstituted model membrane and subsequent VCAM-1 ligand binding with nanomolar affinity were illustrated. This simple drying procedure is a novel way to combine the model membrane generation by Langmuir-Blodgett technique with SAW biosensor measurements, which extends the applicability of SAM®5 blue in biomedical sciences.

Lipid membrane domains for the selective adsorption and surface patterning of conjugated polyelectrolytes

Conjugated polyelectrolytes (CPEs) are promising materials for generating optoelectronics devices under environmentally friendly processing conditions, but challenges remain to develop methods to define lateral features for improved junction interfaces and direct optoelectronic pathways. We describe here the potential to use a bottom-up approach that employs self-assembly in lipid membranes to form structures to template the selective adsorption of CPEs. Phase separation of gel phase anionic lipids and fluid phase phosphocholine lipids allowed the formation of negatively charged domain assemblies that selectively adsorb a cationic conjugated polyelectrolyte (P2). Spectroscopic studies found the adsorption of P2 to negatively charged membranes resulted in minimal structural change of the solution phase polymer but yielded an enhancement in fluorescence intensity (~50%) due to loss of quenching pathways. Fluorescence microscopy, dynamic light scattering, and AFM imaging were used to characterize the polymer-membrane interaction and the polymer-bound domain structures of the biphasic membranes. In addition to randomly formed circular gel phase domains, we also show that predefined features, such as straight lines, can be directed to form upon etched patterns on the substrate, thus providing potential routes toward the self-organization of optoelectronic architectures.

Diffusion and interaction dynamics of individual membrane protein complexes confined in micropatterned polymer-supported membranes

Micropatterned polymer-supported membranes (PSM) are established as a tool for confining the diffusion of transmembrane proteins for single molecule studies. To this end, a photochemical surface modification with hydrophobic tethers on a PEG polymer brush is implemented for capturing of lipid vesicles and subsequent fusion. Formation of contiguous membranes within micropatterns is confirmed by scanning force microscopy, fluorescence recovery after photobleaching (FRAP), and super-resolved single-molecule tracking and localization microscopy. Free diffusion of transmembrane proteins reconstituted into micropatterned PSM is demonstrated by FRAP and by single-molecule tracking. By exploiting the confinement of diffusion within micropatterned PSM, the diffusion and interaction dynamics of individual transmembrane receptors are quantitatively resolved.

Nonintercalating nanosubstrates create asymmetry between bilayer leaflets

The physical properties of lipid bilayers can be remodeled by a variety of environmental factors. Here we investigate using molecular dynamics simulations the specific effects of nanoscopic substrates or external contact points on lipid membranes. We expose palmitoyl-oleoyl phosphatidylcholine bilayers unilaterally and separately to various model nanosized substrates differing in surface hydroxyl densities. We find that a surface hydroxyl density as low as 10% is sufficient to keep the bilayer juxtaposed to the substrate. The bilayer interacts with the substrate indirectly through multiple layers of water molecules; however, despite such buffered interaction, the bilayers exhibit certain properties different from unsupported bilayers. The substrates modify transverse lipid fluctuations, charge density profiles, and lipid diffusion rates, although differently in the two leaflets, which creates an asymmetry between bilayer leaflets. Other properties that include lipid cross-sectional areas, component volumes, and order parameters are minimally affected. The extent of asymmetry that we observe between bilayer leaflets is well beyond what has been reported for bilayers adsorbed on infinite solid supports. This is perhaps because the bilayers are much closer to our nanosized finite supports than to infinite solid supports, resulting in a stronger support-bilayer electrostatic coupling. The exposure of membranes to nanoscopic contact points, therefore, cannot be considered as a simple linear interpolation between unsupported membranes and membranes supported on infinite supports. In the biological context, this suggests that the exposure of membranes to nonintercalating proteins, such as those belonging to the cytoskeleton, should not always be considered as passive nonconsequential interactions.

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.