Nanoscale analysis of supported lipid bilayers using atomic force microscopy

Spontaneous nanotube formation of an asymmetric glycolipid

Over the past decades, advances in lipid nanotechnology have shown that self-assembled lipid structures providing ease of preparation, chemical stability, and biocompatibility represent a landmark on the development of multidisciplinary technologies. Lipid nanotubes (LNTs) are a unique class of lipid self-assembled structures, bearing unique properties such as high-aspect ratio, tunable diameter size, and precise molecular recognition. They can be obtained either by the action of external factors to already formed vesicles or spontaneously, the latter depending strongly on subtle molecular features. Here, we report on the spontaneous formation of supported lipid nanotubes of a particular type of glycolipid, ohmline, whose hydrophobic core displays remarkable asymmetry. The combination of bulk and surface-sensitive techniques indicates that below its main transition, ohmline displays an interdigitated gel phase, likely driven by the unique asymmetry in its hydrophobic core. Enhanced order packing by interdigitation favors the formation of ohmline nanotubes in agreement with chiral-based models of nanotube formation. The findings presented in this work call for additional studies to link lipid molecular structure-assembly relationships, whose understanding is relevant for the controlled design of lipid nanotubes networks in particular and controlled design of soft-matter nanomaterials in general.

Localized surface plasmon resonance and atomic force microscopy study of model lipid membranes and their interactions with amyloid and melatonin

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by the accumulation of amyloid plaques in the brain. The toxicity of amyloid to neuronal cell surfaces arises from interactions between small intermediate aggregates, namely amyloid oligomers, and the cell membrane. The nature of these interactions changes with age and disease progression. In our previous work, we demonstrated that both membrane composition and nanoscale structure play crucial roles in amyloid toxicity, and that membrane models mimicking healthy neuron were less affected by amyloid than model membranes mimicking AD neuronal membranes. This understanding introduces the possibility of modifying membrane properties with membrane-active molecules, such as melatonin, to protect them from amyloid-induced damage. In this study, we employed atomic force microscopy and localized surface plasmon resonance to investigate the protective effects of melatonin. We utilized synthetic lipid membranes that mimic the neuronal cellular membrane at various stages of AD and explored their interactions with amyloid-β(1-42) in the presence of melatonin. Our findings reveal that the early diseased membrane model is particularly vulnerable to amyloid binding and subsequent damage. However, melatonin exerts its most potent protective effect on this early-stage membrane. These results suggest that melatonin could act at the membrane level to alleviate amyloid toxicity, offering the most protection during the initial stages of AD.

The intricate link between membrane lipid structure and composition and membrane structural properties in bacterial membranes

It is now evident that the cell manipulates lipid composition to regulate different processes such as membrane protein insertion, assembly and function. Moreover, changes in membrane structure and properties, lipid homeostasis during growth and differentiation with associated changes in cell size and shape, and responses to external stress have been related to drug resistance across mammalian species and a range of microorganisms. While it is well known that the biomembrane is a fluid self-assembled nanostructure, the link between the lipid components and the structural properties of the lipid bilayer are not well understood. This perspective aims to address this topic with a view to a more detailed understanding of the factors that regulate bilayer structure and flexibility. We describe a selection of recent studies that address the dynamic nature of bacterial lipid diversity and membrane properties in response to stress conditions. This emerging area has important implications for a broad range of cellular processes and may open new avenues of drug design for selective cell targeting.

Elucidating the Influence of Lipid Composition on Bilayer Perturbations Induced by the N-terminal Region of the Huntingtin Protein

Understanding the membrane interactions of the N-terminal 17 residues of the huntingtin protein (HttN) is essential for unraveling its role in cellular processes and its impact on huntingtin misfolding. In this study, we used atomic force microscopy (AFM) to examine the effects of lipid specificity in mediating bilayer perturbations induced by HttN. Across various lipid environments, the peptide consistently induced bilayer disruptions in the form of holes. Notably, our results unveiled that cholesterol enhanced bilayer perturbation induced by HttN, while phosphatidylethanolamine (PE) lipids suppressed hole formation. Furthermore, anionic phosphatidylglycerol (PG) and cardiolipin lipids, along with cholesterol at high concentrations, promoted the formation of double-bilayer patches. This unique structure suggests that the synergy among HttN, anionic lipids, and cholesterol can enhance bilayer fusion, potentially by facilitating lipid intermixing between adjacent bilayers. Additionally, our AFM-based force spectroscopy revealed that HttN enhanced the mechanical stability of lipid bilayers, as evidenced by an elevated bilayer puncture force. These findings illuminate the complex interplay between HttN and lipid membranes and provide useful insights into the role of lipid composition in modulating membrane interactions with the huntingtin protein.

Imaging Amyloid-β Membrane Interactions: Ion-Channel Pores and Lipid-Bilayer Permeability in Alzheimer's Disease

The accumulation of the amyloid-β peptides (Aβ) is central to the development of Alzheimer's disease. The mechanism by which Aβ triggers a cascade of events that leads to dementia is a topic of intense investigation. Aβ self-associates into a series of complex assemblies with different structural and biophysical properties. It is the interaction of these oligomeric, protofibril and fibrillar assemblies with lipid membranes, or with membrane receptors, that results in membrane permeability and loss of cellular homeostasis, a key event in Alzheimer's disease pathology. Aβ can have an array of impacts on lipid membranes, reports have included: a carpeting effect; a detergent effect; and Aβ ion-channel pore formation. Recent advances imaging these interactions are providing a clearer picture of Aβ induced membrane disruption. Understanding the relationship between different Aβ structures and membrane permeability will inform therapeutics targeting Aβ cytotoxicity.

Imaging Amyloid-β Membrane Interactions: Ion-Channel Pores and Lipid-Bilayer Permeability in Alzheimer's Disease

The accumulation of the amyloid-β peptides (Aβ) is central to the development of Alzheimer's disease. The mechanism by which Aβ triggers a cascade of events that leads to dementia is a topic of intense investigation. Aβ self-associates into a series of complex assemblies with different structural and biophysical properties. It is the interaction of these oligomeric, protofibril and fibrillar assemblies with lipid membranes, or with membrane receptors, that results in membrane permeability and loss of cellular homeostasis, a key event in Alzheimer's disease pathology. Aβ can have an array of impacts on lipid membranes, reports have included: a carpeting effect; a detergent effect; and Aβ ion-channel pore formation. Recent advances imaging these interactions are providing a clearer picture of Aβ induced membrane disruption. Understanding the relationship between different Aβ structures and membrane permeability will inform therapeutics targeting Aβ cytotoxicity.

Visualisation of microalgal-viral interactions by high-speed atomic force microscopy

Visualization of viruses and their hosts has been paramount to their study and understanding. The direct observation of the morphological dynamics of infection is a highly desired capability and the focus of instrument development across a variety of microscopy technologies. This study demonstrates progress that has been made in exploiting the capabilities offered by HS-AFM to characterise the interactions between coccolithoviruses and their globally important coccolithophore hosts. We observe whole Emiliania huxleyi Virus capsids, transient binding to Emiliania huxleyi derived supported lipid bilayers, and host-virus binding in real-time in an environmentally relevant, aqueous environment.

A Versatile Suspended Lipid Membrane System for Probing Membrane Remodeling and Disruption

Artificial membrane systems can serve as models to investigate molecular mechanisms of different cellular processes, including transport, pore formation, and viral fusion. However, the current, such as SUVs, GUVs, and the supported lipid bilayers suffer from issues, namely high curvature, heterogeneity, and surface artefacts, respectively. Freestanding membranes provide a facile solution to these issues, but current systems developed by various groups use silicon or aluminum oxide wafers for fabrication that involves access to a dedicated nanolithography facility and high cost while conferring poor membrane stability. Here, we report the development, characterization and applications of an easy-to-fabricate suspended lipid bilayer (SULB) membrane platform leveraging commercial track-etched porous filters (PCTE) with defined microwell size. Our SULB system offers a platform to study the lipid composition-dependent structural and functional properties of membranes with exceptional stability. With dye entrapped in PCTE microwells by SULB, we show that sphingomyelin significantly augments the activity of pore-forming toxin, Cytolysin A (ClyA) and the pore formation induces lipid exchange between the bilayer leaflets. Further, we demonstrate high efficiency and rapid kinetics of membrane fusion by dengue virus in our SULB platform. Our suspended bilayer membrane mimetic offers a novel platform to investigate a large class of biomembrane interactions and processes.

Hybrid bilayer membranes as platforms for biomimicry and catalysis

Hybrid bilayer membrane (HBM) platforms represent an emerging nanoscale bio-inspired interface that has broad implications in energy catalysis and smart molecular devices. An HBM contains multiple modular components that include an underlying inorganic surface with a biological layer appended on top. The inorganic interface serves as a support with robust mechanical properties that can also be decorated with functional moieties, sensing units and catalytic active sites. The biological layer contains lipids and membrane-bound entities that facilitate or alter the activity and selectivity of the embedded functional motifs. With their structural complexity and functional flexibility, HBMs have been demonstrated to enhance catalytic turnover frequency and regulate product selectivity of the O₂ and CO₂ reduction reactions, which have applications in fuel cells and electrolysers. HBMs can also steer the mechanistic pathways of proton-coupled electron transfer (PCET) reactions of quinones and metal complexes by tuning electron and proton delivery rates. Beyond energy catalysis, HBMs have been equipped with enzyme mimics and membrane-bound redox agents to recapitulate natural energy transport chains. With channels and carriers incorporated, HBM sensors can quantify transmembrane events. This Review serves to summarize the major accomplishments achieved using HBMs in the past decade.

Quartz crystal microbalance and atomic force microscopy to characterize mimetic systems based on supported lipids bilayer

Quartz Crystal Microbalance (QCM) with dissipation and Atomic Force Microscopy (AFM) are two characterization techniques that allow describing processes taking place at solid-liquid interfaces. Both are label-free and, when used in combination, provide kinetic, thermodynamic and structural information at the nanometer scale of events taking place at surfaces. Here we describe the basic operation principles of both techniques, addressing a non-specialized audience, and provide some examples of their use for describing biological events taking place at supported lipid bilayers (SLBs). The aim is to illustrate current strengths and limitations of the techniques and to show their potential as biophysical characterization techniques.

Structural and morphological changes of breast cancer cells induced by iron(II) complexes

Metal-based complexes are well-established cancer chemotherapeutic drug candidates. Although our knowledge regarding their exact activity vs. toxicity profile is incomplete, changes in cell membrane biophysical properties and cytoskeletal structures have been implicated as part of the mechanism of action. Thus, in this work, we characterised the effects of iron(II)-based complexes on the structural and morphological properties of epithelial non-tumorigenic (MCF 10A) and tumorigenic (MDA-MB-231) breast cell lines using atomic force microscopy (AFM), flow cytometry and immunofluorescence microscopy. At 24 h of exposure, both the MCF 10A and MDA-MB-231 cells experienced a cell softening, and an increase in size followed by a re-stiffening at 96 h. In addition, the triple negative breast cancer cell line, MDA-MB-231, sustained a notable cytoskeletal and mitochondrial reorganization with increased actin stress fibers and cell-to-cell communication structures. An extensive all-atom molecular dynamic simulation suggests a possible direct and unassisted internalization of the metallodrug candidate, and confirmed that the cellular effects could not be ascribed to the simple physical interaction of the iron-based complexes with the biological membrane. These observations provide an insight into a link between the mechanisms of action of such iron-based complexes as anti-cancer treatment and cytoskeletal architecture.

Atomic force microscopy for quantitative understanding of peptide-induced lipid bilayer remodeling

A number of peptides are known to bind lipid bilayer membranes and cause these natural barriers to leak in an uncontrolled manner. Though membrane permeabilizing peptides play critical roles in cellular activity and may have promising future applications in the therapeutic arena, significant questions remain about their mechanisms of action. The atomic force microscope (AFM) is a single molecule imaging tool capable of addressing lipid bilayers in near-native fluid conditions. The apparatus complements traditional assays by providing local topographic maps of bilayer remodeling induced by membrane permeabilizing peptides. The information garnered from the AFM includes direct visualization and statistical analyses of distinct bilayer remodeling modes such as highly localized pore-like voids in the bilayer and dispersed thinned membrane regions. Colocalization of distinct remodeling modes can be studied. Here we examine recent work in the field and outline methods used to achieve precise AFM image data. Experimental challenges and common pitfalls are discussed as well as techniques for unbiased analysis including the Hessian blob detection algorithm, bootstrapping, and the Bayesian information criterion. When coupled with robust statistical analyses, high precision AFM data is poised to advance understanding of an important family of peptides that cause poration of membrane bilayers.

Coexistence of lipid phases in multilayer phospholipid films probed by Raman mapping

Raman spectroscopy of stacked phospholipid bilayers reveals the chemical and conformational composition of coexisting domains.

Controllable membrane remodeling by a modified fragment of the apoptotic protein Bax

Intrinsic apoptosis is orchestrated by a group of proteins that mediate the coordinated disruption of mitochondrial membranes. Bax is a multi-domain protein that, upon activation, disrupts the integrity of the mitochondrial outer membrane by forming pores. We strategically introduced glutamic acids into a short sequence of the Bax protein that constitutively creates membrane pores. The resulting BaxE5 peptide efficiently permeabilizes membranes at acidic pH, showing low permeabilization at neutral pH. Atomic force microscopy (AFM) imaging showed that at acidic pH BaxE5 established several membrane remodeling modalities that progressively disturbed the integrity of the lipid bilayer. The AFM data offers vistas on the membrane disruption process, which starts with pore formation and progresses through localized exposure of membrane monolayers leading to stable and small (height ∼ 16 Å) lipid-peptide complexes. The different types of membrane morphology observed in the presence of BaxE5 suggest that the peptide can establish different types of membrane interactions. BaxE5 adopts a rare unstructured conformation when bound to membranes, which might facilitate the dynamic transition between those different states, and then promote membrane digestion.

Acyl-chain saturation regulates the order of phosphatidylinositol 4,5-bisphosphate nanodomains

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) plays a critical role in the regulation of various plasma membrane processes and signaling pathways in eukaryotes. A significant amount of cellular resources are spent on maintaining the dominant 1-stearoyl-2-arachidonyl PI(4,5)P₂ acyl-chain composition, while less abundant and more saturated species become more prevalent in response to specific stimuli, stress or aging. Here, we report the impact of acyl-chain structure on the biophysical properties of cation-induced PI(4,5)P₂ nanodomains. PI(4,5)P₂ species with increasing levels of acyl-chain saturation cluster in progressively more ordered nanodomains, culminating in the formation of gel-like nanodomains for fully saturated species. The formation of these gel-like domains was largely abrogated in the presence of 1-stearoyl-2-arachidonyl PI(4,5)P_2. This is, to the best of our knowledge, the first report of the impact of PI(4,5)P₂ acyl-chain composition on cation-dependent nanodomain ordering, and provides important clues to the motives behind the enrichment of PI(4,5)P₂ with polyunsaturated acyl-chains. We also show how Ca²⁺-induced PI(4,5)P₂ nanodomains are able to generate local negative curvature, a phenomenon likely to play a role in membrane remodeling events.

Lipid phase transitions in cat oocytes supplemented with deuterated fatty acids

Cryopreservation of oocytes has already been used to preserve genetic resources, but this technology faces limitations when applied to the species whose oocytes contain large amounts of cytoplasmic lipid droplets. Although cryoinjuries in such oocytes are usually associated with the lipid phase transition in lipid droplets, this phenomenon is still poorly understood. We applied Raman spectroscopy of deuterium-labeled lipids to investigate the freezing of lipid droplets inside cat oocytes. Lipid phase separation was detected in oocytes cryopreserved by slow-freezing protocol. For oocytes supplemented with stearic acid, we found that saturated lipids form the ordered phase being distributed at the periphery of lipid droplets. When an oocyte is warmed to physiological temperatures after cooling, a fraction of saturated lipids may remain in the ordered conformational state. The fractions of monounsaturated and polyunsaturated lipids redistribute to the core of lipid droplets. Monounsaturated lipids undergo the transition to the ordered conformational state below -10°C. Using deuterated fatty acids with a different number of double bonds, we reveal how different lipid fractions are involved in the lipid phase transition of a cytoplasmic lipid droplet and how they can affect cell survival. Raman spectroscopy of deuterated lipids has proven to be a promising tool for studying the lipid phase transitions and lipid redistributions inside single organelles within living cells.

The Whole Is Bigger than the Sum of Its Parts: Drug Transport in the Context of Two Membranes with Active Efflux

Cell envelope plays a dual role in the life of bacteria by simultaneously protecting it from a hostile environment and facilitating access to beneficial molecules. At the heart of this ability lie the restrictive properties of the cellular membrane augmented by efflux transporters, which preclude intracellular penetration of most molecules except with the help of specialized uptake mediators. Recently, kinetic properties of the cell envelope came into focus driven on one hand by the urgent need in new antibiotics and, on the other hand, by experimental and theoretical advances in studies of transmembrane transport. A notable result from these studies is the development of a kinetic formalism that integrates the Michaelis-Menten behavior of individual transporters with transmembrane diffusion and offers a quantitative basis for the analysis of intracellular penetration of bioactive compounds. This review surveys key experimental and computational approaches to the investigation of transport by individual translocators and in whole cells, summarizes key findings from these studies and outlines implications for antibiotic discovery. Special emphasis is placed on Gram-negative bacteria, whose envelope contains two separate membranes. This feature sets these organisms apart from Gram-positive bacteria and eukaryotic cells by providing them with full benefits of the synergy between slow transmembrane diffusion and active efflux.

Physical Properties and Reactivity of Microdomains in Phosphatidylinositol-Containing Supported Lipid Bilayer

We characterized the size, distribution, and fluidity of microdomains in a lipid bilayer containing phosphatidylinositol (PI) and revealed their roles during the two-dimensional assembly of a membrane deformation protein (FBP17). The morphology of the supported lipid bilayer (SLB) consisting of PI and phosphatidylcholine (PC) on a mica substrate was observed with atomic force microscope (AFM). Single particle tracking (SPT) was performed for the PI+PC-SLB on the mica substrate by using the diagonal illumination setup. The AFM topography showed that PI-derived submicron domains existed in the PI+PC-SLB. The spatiotemporal dependence of the lateral lipid diffusion obtained by SPT showed that the microdomain had lower fluidity than the surrounding region and worked as the obstacles for the lipid diffusion. We observed the two-dimensional assembly of FBP17, which is one of F-BAR family proteins included in endocytosis processes and has the function generating lipid bilayer tubules in vitro. At the initial stage of the FBP17 assembly, the PI-derived microdomain worked as a scaffold for the FBP17 adsorption, and the fluid surrounding region supplied FBP17 to grow the FBP17 domain via the lateral molecular diffusion. This study demonstrated an example clearly revealing the roles of two lipid microregions during the protein reaction on a lipid bilayer.

Compressibility of Multicomponent, Charged Model Biomembranes Tunes Permeation of Cationic Nanoparticles

Cells respond to external stress by altering their membrane lipid composition to maintain fluidity, integrity and net charge. However, in interactions with charged nanoparticles (NPs), altering membrane charge could adversely affect its ability to transport ions across the cell membrane. Hence, it is important to understand possible pathways by which cells could alter zwitterionic lipid composition to respond to NPs without compromising membrane integrity and charge. Here, we report in situ synchrotron X-ray reflectivity (XR) measurements to monitor the interaction of cationic NPs in the form of quantum dots, with phase-separated supported lipid bilayers of different compositions containing an anionic lipid and zwitterionic lipids having variable degrees of stiffness. We observe that the extent of NP penetration into the respective membranes, as estimated from XR data analysis, is inversely related to membrane compression moduli, which was tuned by altering the stiffness of the zwitterionic lipid component. For a particular membrane composition with a discernible height difference between ordered and disordered phases, we were able to observe subtle correlations between the extent of charge on the NPs and the specificity to bind to the charged and ordered phase, contrary to that observed earlier for phase-separated model biomembranes containing no charged lipids. Our results provide microscopic insight into the role of membrane rigidity and electrostatics in determining membrane permeation. This can lead to great potential benefits in rational designing of NPs for bioimaging and drug delivery applications as well as in assessing and alleviating cytotoxicity of NPs.

Impact of the number of rhamnose moieties of rhamnolipids on the structure, lateral organization and morphology of model biomembranes

Various studies have described remarkable biological activities and surface-active properties of rhamnolipids, leading to their proposed use in a wide range of industrial applications. Here, we report on a study of the effects of monorhamnolipid RhaC₁₀C₁₀ and dirhamnolipid RhaRhaC₁₀C₁₀ incorporation into model membranes of varying complexity, including bacterial and heterogeneous model biomembranes. For comparison, we studied the effect of HAA (C₁₀C₁₀, lacking a sugar headgroup) partitioning into these membrane systems. AFM, confocal fluorescence microscopy, DSC, and Laurdan fluorescence spectroscopy were employed to yield insights into the rhamnolipid-induced morphological changes of lipid vesicles as well as modifications of the lipid order and lateral membrane organization of the model biomembranes upon partitioning of the different rhamnolipids. The partitioning of the three rhamnolipids into phospholipid bilayers changes the phase behavior, fluidity, lateral lipid organization and morphology of the phospholipid membranes dramatically, to what extent, depends on the headgroup structure of the rhamnolipid, which affects its packing and hydrogen bonding capacity. The incorporation into giant unilamellar vesicles (GUVs) of a heterogeneous anionic raft membrane system revealed budding of domains and fission of daughter vesicles and small aggregates for all three rhamnolipids, with major destabilization of the lipid vesicles upon insertion of RhaC₁₀C₁₀, and also formation of huge GUVs upon the incorporation of RhaRhaC₁₀C₁₀. Finally, we discuss the results with regard to the role these biosurfactants play in biology and their possible impact on applications, ranging from agricultural to pharmaceutical industries.

25-Hydroxycholesterol Effect on Membrane Structure and Mechanical Properties

Cholesterol is responsible for the plasticity of plasma membranes and is involved in physiological and pathophysiological responses. Cholesterol homeostasis is regulated by oxysterols, such as 25-hydroxycholesterol. The presence of 25-hydroxycholesterol at the membrane level has been shown to interfere with several viruses’ entry into their target cells. We used atomic force microscopy to assess the effect of 25-hydroxycholesterol on different properties of supported lipid bilayers with controlled lipid compositions. In particular, we showed that 25-hydroxycholesterol inhibits the lipid-condensing effects of cholesterol, rendering the bilayers less rigid. This study indicates that the inclusion of 25-hydroxycholesterol in plasma membranes or the conversion of part of their cholesterol content into 25-hydroxycholesterol leads to morphological alterations of the sphingomyelin (SM)-enriched domains and promotes lipid packing inhomogeneities. These changes culminate in membrane stiffness variations.

Estimation of pore dimensions in lipid membranes induced by peptides and other biomolecules: A review

The cytoplasmic membrane is one of the most frequent cell targets of antimicrobial peptides (AMPs) and other biomolecules. Understanding the mechanism of action of AMPs at the molecular level is of utmost importance for designing of new membrane-specific molecules. In particular, the formation of pores, the structure and size of these pores are of great interest and require nanoscale resolution approaches, therefore, biophysical strategies are essential to achieve an understanding of these processes at this scale. In the case of membrane active peptides, pore formation or general membrane disruption is usually the last step before cell death, and so, pore size is generally directly associated to pore structure and stability and loss of cellular homeostasis, implicated in overall peptide activity. Up to date, there has not been a critical review discussing the methods that can be used specifically for estimating the pore dimensions induced by membrane active peptides. In this review we discuss the scope, relevance and popularity of the different biophysical techniques such as liposome leakage experiments, advanced microscopy, neutron or X-ray scattering, electrophysiological techniques and molecular dynamics studies, all of them useful for determining pore structure and dimension.

Lipid Composition of Liposomal Membrane Largely Affects Its Transport and Uptake through Small Intestinal Epithelial Cell Models

Lipid composition of liposomal bilayer should alter the cell response for permeability, transport, and uptake in small intestine. This work was done to investigate the transport and uptake of liposomes composed of docosahexaenoic acid-enriched phosphatidylcholine (PtdCho), phosphatidylserine (PtdSer), and sulfoquinovosyl diacylglycerol (SQDG) derived from marine products on multilamellar vesicles (MLV) in small intestinal epithelial cell models. The results showed that addition of PtdSer and SQDG as liposomal bilayer could improve the efficiency entrapment of liposomes. The liposomes containing PtdSer showed higher transport and uptake through both Caco-2 cell and M cell monolayers as compared to PtdCho-MLV. SQDG-containing liposomes exhibited only higher transport through M cell monolayer, while its uptake effect was higher both in Caco-2 cell and M cell monolayers. The results of experiments done with endocytosis inhibitors indicated that PtdCho-MLV must be transported via macropinocytosis and uptaken by phagocytosis in M cell monolayer model. PtdCho/PtdSer-MLV and PtdCho/SQDG-MLV might be transported and uptaken through M cell monolayer by phagocytosis. The result also indicated that PtdCho/SQDG-MLV could open the tight junction of small intestinal epithelial cell monolayers. Furthermore, our findings demonstrated that the surface status of cholesterol-containing liposomes were smooth, but they did not affect their transport and uptake through Caco-2 cell and M cell monolayers.

Lipid bilayers: Phase behavior and nanomechanics

Lipid membranes are involved in many physiological processes like recognition, signaling, fusion or remodeling of the cell membrane or some of its internal compartments. Within the cell, they are the ultimate barrier, while maintaining the fluidity or flexibility required for a myriad of processes, including membrane protein assembly. The physical properties of in vitro model membranes as model cell membranes have been extensively studied with a variety of techniques, from classical thermodynamics to advanced modern microscopies. Here we review the nanomechanics of solid-supported lipid membranes with a focus in their phase behavior. Relevant information obtained by quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM) as complementary techniques in the nano/mesoscale interface is presented. Membrane morphological and mechanical characterization will be discussed in the framework of its phase behavior, phase transitions and coexistence, in simple and complex models, and upon the presence of cholesterol.

Lipid membranes supported by planar porous substrates

Biological membranes play key roles in cell life, but their intrinsic complexity motivated the study and development of artificial lipid membranes with the primary aim to reconstitute and understand the natural functions in vitro. Porous-supported lipid membrane (pSLM) has emerged as a flexible platform for studying the surface chemistry of the cell due to their high stability and fluidity, and their ability to study the transmembrane process of the molecules. In this review, the pSLM, for the first time, to our knowledge, was divided into three types according to the way of the porous materials support the lipid membrane, containing the lipid membrane on the pores of the porous materials, the lipid membrane on both sides of the porous materials, the lipid membrane in the pores of the porous materials. All of these pSLMs were systematically elaborated from several aspects, including the substrates, formation, and characterization. Meanwhile, the advantages and disadvantages of each model membranes were summarized. Finally, suggestions for selecting appropriate pSLM and future directions in this area are discussed.

Creating Supported Plasma Membrane Bilayers Using Acoustic Pressure

Model membrane systems are essential tools for the study of biological processes in a simplified setting to reveal the underlying physicochemical principles. As cell-derived membrane systems, giant plasma membrane vesicles (GPMVs) constitute an intermediate model between live cells and fully artificial structures. Certain applications, however, require planar membrane surfaces. Here, we report a new approach for creating supported plasma membrane bilayers (SPMBs) by bursting cell-derived GPMVs using ultrasound within a microfluidic device. We show that the mobility of outer leaflet molecules is preserved in SPMBs, suggesting that they are accessible on the surface of the bilayers. Such model membrane systems are potentially useful in many applications requiring detailed characterization of plasma membrane dynamics.

Visualization and measurement of the local absorption coefficients of single bilayer phospholipid membranes using scanning near-field optical microscopy

Here we report the results of shear-mode thicknesses and absorption coefficient measurements made on neat membranes using scanning near-field optical microscopy (SNOM). Biomimic neat membranes composed of two different types of phoshpholipid molecules: 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2- dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) were found to exhibit different absorption coefficients under the SNOM. The localization of the lipids could be identified and correlated to the morphology of the membrane domains indicating that SNOM can be an effective and accurate approach for the label-free characterization of the structure-function relationships in cell membranes.

Amyloid-β oligomers have a profound detergent-like effect on lipid membrane bilayers, imaged by atomic force and electron microscopy

The ability of amyloid-β peptide (Aβ) to disrupt membrane integrity and cellular homeostasis is believed to be central to Alzheimer's disease pathology. Aβ is reported to have various impacts on the lipid bilayer, but a clearer picture of Aβ influence on membranes is required. Here, we use atomic force and transmission electron microscopies to image the impact of different isolated Aβ assembly types on lipid bilayers. We show that only oligomeric Aβ can profoundly disrupt the bilayer, visualized as widespread lipid extraction and subsequent deposition, which can be likened to an effect expected from the action of a detergent. We further show that Aβ oligomers cause widespread curvature and discontinuities within lipid vesicle membranes. In contrast, this detergent-like effect was not observed for Aβ monomers and fibers, although Aβ fibers did laterally associate and embed into the upper leaflet of the bilayer. The marked impact of Aβ oligomers on membrane integrity identified here reveals a mechanism by which these oligomers may be cytotoxic.

Imaging Artificial Membranes Using High-Speed Atomic Force Microscopy

Supported lipid bilayers represent a very attractive way to mimic biological membranes, especially to investigate molecular mechanisms associated with the lateral segregation of membrane components. Observation of these model membranes with high-speed atomic force microscopy (HS-AFM) allows the capture of both topography and dynamics of membrane components, with a spatial resolution in the nanometer range and image capture time of less than 1 s. In this context, we have developed new protocols adapted for HS-AFM to form supported lipid bilayers on small mica disks using the vesicle fusion or Langmuir-Blodgett methods. In this chapter we describe in detail the protocols to fabricate supported artificial bilayers as well as the main guidelines for HS-AFM imaging of such samples.

Preparation and Characterization of Solid-Supported Lipid Bilayers Formed by Langmuir-Blodgett Deposition: A Tutorial

The structure, phase behavior, and properties of cellular membranes are derived from their composition, which includes phospholipids, sphingolipids, sterols, and proteins with various levels of glycosylation. Because of the intricate nature of cellular membranes, a plethora of in vitro studies have been carried out with model membrane systems that capture particular properties such as fluidity, permeability, and protein binding but vastly simplify the membrane composition in order to focus in detail on a specialized property or function. Supported lipid bilayers (SLB) are widely used as archetypes for cellular membranes, and this instructional review primarily focuses on the preparation and characterization of SLB systems formed by Langmuir deposition methods. Typical characterization methods, which take advantage of the planar orientation of SLBs, are illustrated, and references that go into more depth are included. This invited instructional review is written so that nonexperts can quickly gain in-depth knowledge regarding the preparation and characterization of SLBs. In addition, this work goes beyond traditional instructional reviews to provide expert readers with new results that cover a wider range of SLB systems than those previously reported in the literature. The quality of an SLB is frequently not well described, and details such as topological defects can influence the results and conclusions of an individual study. This article quantifies and compares the quality of SLBs fabricated from a variety of gel and fluid compositions, in correlation with preparation techniques and parameters, to generate general rules of thumb to guide the construction of designed SLB systems.

Interdigitation in spin-coated lipid layers in air

In this study, we show that dry saturated phospholipid layers prepared by the spin-coating technique could present thinner regions associated to interdigitated phases under some conditions. The morphological characteristics of lipid layers of saturated phosphocholines, such as dilauroylphosphatidylcholine (DLPC), dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine (DPPC) and distearoylphosphatidylcholine (DSPC), have been measured by Atomic Force Microscopy and revealed that the presence of interdigitated regions is not induced by the same parameters that induce them in hydrated samples. To achieve these results the effect of the lipid hidrocabonated chain length, the presence of alcohol in the coating solution, the spinning velocity and the presence of cholesterol were tested. We showed that DPPC and DSPC bilayers, on the one side, can show structures with similar height than interdigitated regions observed in hydrated samples, while, on the other side, DLPC and DMPC tend to show no evidence of interdigitation. Results indicate that the presence of interdigitated areas is due to the presence of lateral tensions and, hence, that they can be eliminated by releasing these tensions by, for instance, the addition of cholesterol. These results demonstrate that interdigitation in lipid layers is a rather general phenomena and can be observed in lipid bilayers in dry conditions.

Colloids versus solution state adsorption of proteins: Interaction of Myoglobin with supported lipid bilayers

This study examines adsorption of Myoglobin (Mb) in solution and as colloid on supported lipid bilayers of neutral phospholipids and a mixture of neutral+cationic lipids formed on gold coated quartz in a Quartz crystal microbalance (QCM). Results indicate that thin adsorbed films of Mb in solution and as colloids, show atleast 3 steps in the interaction with the bilayers: i) An initial strain of a viscoelastic film ii) Entrained water that moves in and out of the adsorbed film and iii) The coupled load from the bulk liquid which increases the strain of the film. These three components constitute an effective viscoelastic film which is rigidly coupled to the QCM. Grazing incidence XRD (GIXD) shows that the bilayer head group remains nearly undisturbed for Mb solution with pure (neutral) and (neutral+cationic) mixtures, whereas for the colloids there is an increase in head group thickness with neutral and decrease in the case of mixture. Unsaturation in the alkyl tails in the neutral lipid resulting in flexible disordered bilayers and more entrained water in the cationic system results in these changes. The sensitivity of QCM-D, makes it useful to study real-time monitoring of bilayer structural robustness cytotoxicity, drug delivery and lipid self-assembly.

Morphology and Physical Properties of Hydrophilic-Polymer-Modified Lipids in Supported Lipid Bilayers

Lipid molecules such as glycolipids that are modified with hydrophilic biopolymers participate in the biochemical reactions occurring on cell membranes. Their functions and efficiency are determined by the formation of microdomains and their physical properties. We investigated the morphology and properties of domains induced by the hydrophilic-polymer-modified lipid applying a polyethylene glycol (PEG)-modified lipid as a model modified lipid. We formed supported lipid bilayers (SLBs) using a 0-10 mol % range of PEG-modified lipid concentration ( C_PEG). We studied their morphology and fluidity by fluorescence microscopy, the fluorescence recovery after photobleaching method, and atomic force microscopy (AFM). Fluorescence images showed that domains rich in the PEG-modified lipid appeared and SLB fluidity decreased when C_PEG ≥ 5%. AFM topographies showed that clusters of the PEG-modified lipid appeared prior to domain formation and the PEG-lipid-rich domains were observed as depressions. Frequency-modulation AFM revealed a force-dependent appearance of the PEG-lipid-rich domain.

Properties of Artificial Phospholipid Membranes Containing Lauryl Gallate or Cholesterol

Lauryl gallate (LG) is an antioxidant agent. However, it exhibits poor solubility in water. Its interactions with the membrane result in structure evolution thus affecting the membrane functionality. In this paper the Brewster angle microscope coupled with the Langmuir trough was applied to determine the morphology, phase behaviour, thickness and miscibility of ternary Langmuir monolayers with equal mole fractions of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC); 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and an increasing mole fraction of LG. The results were discussed as regards analogous systems where cholesterol (Chol) was the third component. Moreover, the phosphatidylcholine–lauryl gallate (PC–LG) interactions were monitored by the attenuated total reflectance Fourier transform infrared spectroscopy and time-of-flight secondary ion mass spectrometry. Besides lipid composition, the addition of LG was found to be a significant factor to modulate the model membrane properties. The LG molecules adjust themselves to the PC monolayer structure. The hydrophobic fragment is dipped into the membrane interior while the hydroxyl groups of phenolic gallate moiety associate with the polar groups of PC mainly through hydrogen bonding inducing the compacting effect. LG is found to be deeply submerged within DOPC, closer to the double bonds, and its insertion practically does not affect the DPPC/DOPC membrane fluidity. This is crucial for getting more profound insight into the role of LG in stabilizing the non-raft domains, mostly exposed to oxidation in which LG can co-localize and serve its antioxidant function.

The Role of Nanomechanics in Healthcare

Nanomechanics has played a vital role in pushing our capability to detect, probe, and manipulate the biological species, such as proteins, cells, and tissues, paving way to a deeper knowledge and superior strategies for healthcare. Nanomechanical characterization techniques, such as atomic force microscopy, nanoindentation, nanotribology, optical tweezers, and other hybrid techniques have been utilized to understand the mechanics and kinetics of biospecies. Investigation of the mechanics of cells and tissues has provided critical information about mechanical characteristics of host body environments. This information has been utilized for developing biomimetic materials and structures for tissue engineering and artificial implants. This review summarizes nanomechanical characterization techniques and their potential applications in healthcare research. The principles and examples of label-free detection of cancers and myocardial infarction by nanomechanical cantilevers are discussed. The vital importance of nanomechanics in regenerative medicine is highlighted from the perspective of material selection and design for developing biocompatible scaffolds. This review interconnects the advancements made in fundamental materials science research and biomedical technology, and therefore provides scientific insight that is of common interest to the researchers working in different disciplines of healthcare science and technology.

Cholesterol induced asymmetry in DOPC bilayers probed by AFM force spectroscopy

Cholesterol induced mechanical effects on artificial lipid bilayers are well known and have been thoroughly investigated by AFM force spectroscopy. However, dynamics of cholesterol impingement into bilayers at various cholesterol concentrations and their effects have not been clearly understood. In this paper we present, the effect of cholesterol as a function of its concentration in a simple single component dioleoylphosphatidylcholine (DOPC) bilayer. The nature of measured breakthrough forces on a bilayer with the addition of cholesterol, suggested that it is not just responsible to increase the mechanical stability but also introduces irregularities across the leaflets of the bilayer. This cholesterol induced asymmetry across the (in the inner and outer leaflets) bilayer is related to the phenomena of interleaflet coupling and is a function of cholesterol concentration probed by AFM can provide an unprecedented direction on mechanical properties of lipid membrane as it can be directly correlated to biophysical properties of a cell membrane.

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.

Time lapse AFM on vesicle formation from mixed lipid bilayers induced by the membrane-active peptide melittin

Melittin is a model system for the action of antimicrobial peptides which are potential candidates for novel antibiotics. We investigated the membrane lysis effect of melittin on phase-separated supported lipid bilayers (DOPC-DPPC) by atomic force microscopy. AFM images show that the peptide first forms defects at the interface between the two lipid phases and then degrades preferentially the liquid-phase DOPC-enriched domains. Vesicular structures of 10-20 nm radius were observed to form, suggesting a mixed carpet-toroidal model mechanism for the resolved action of melittin.

Topographic analysis by atomic force microscopy of proteoliposomes matrix vesicle mimetics harboring TNAP and AnxA5

Atomic force microscopy (AFM) is one of the most commonly used scanning probe microscopy techniques for nanoscale imaging and characterization of lipid-based particles. However, obtaining images of such particles using AFM is still a challenge. The present study extends the capabilities of AFM to the characterization of proteoliposomes, a special class of liposomes composed of lipids and proteins, mimicking matrix vesicles (MVs) involved in the biomineralization process. To this end, proteoliposomes were synthesized, composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-sn-glycero-3-phospho-l-serine (DPPS), with inserted tissue-nonspecific alkaline phosphatase (TNAP) and/or annexin V (AnxA5), both characteristic proteins of osteoblast-derived MVs. We then aimed to study how TNAP and AnxA5 insertion affects the proteoliposomes' membrane properties and, in turn, interactions with type II collagen, thus mimicking early MV activity during biomineralization. AFM images of these proteoliposomes, acquired in dynamic mode, revealed the presence of surface protrusions with distinct viscoelasticity, thus suggesting that the presence of the proteins induced local changes in membrane fluidity. Surface protrusions were measurable in TNAP-proteoliposomes but barely detectable in AnxA5-proteoliposomes. More complex surface structures were observed for proteoliposomes harboring both TNAP and AnxA5 concomitantly, resulting in a lower affinity for type II collagen fibers compared to proteoliposomes harboring AnxA5 alone. The present study achieved the topographic analysis of lipid vesicles by direct visualization of structural changes, resulting from protein incorporation, without the need for fluorescent probes.

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.

Lipid Bilayer Membrane in a Silicon Based Micron Sized Cavity Accessed by Atomic Force Microscopy and Electrochemical Impedance Spectroscopy

Supported lipid bilayers (SLBs) are widely used in biophysical research to probe the functionality of biological membranes and to provide diagnoses in high throughput drug screening. Formation of SLBs at below phase transition temperature (Tm) has applications in nano-medicine research where low temperature profiles are required. Herein, we report the successful production of SLBs at above-as well as below-the Tm of the lipids in an anisotropically etched, silicon-based micro-cavity. The Si-based cavity walls exhibit controlled temperature which assist in the quick and stable formation of lipid bilayer membranes. Fusion of large unilamellar vesicles was monitored in real time in an aqueous environment inside the Si cavity using atomic force microscopy (AFM), and the lateral organization of the lipid molecules was characterized until the formation of the SLBs. The stability of SLBs produced was also characterized by recording the electrical resistance and the capacitance using electrochemical impedance spectroscopy (EIS). Analysis was done in the frequency regime of 10-2-10⁵ Hz at a signal voltage of 100 mV and giga-ohm sealed impedance was obtained continuously over four days. Finally, the cantilever tip in AFM was utilized to estimate the bilayer thickness and to calculate the rupture force at the interface of the tip and the SLB. We anticipate that a silicon-based, micron-sized cavity has the potential to produce highly-stable SLBs below their Tm. The membranes inside the Si cavity could last for several days and allow robust characterization using AFM or EIS. This could be an excellent platform for nanomedicine experiments that require low operating temperatures.

Undulations Drive Domain Registration from the Two Membrane Leaflets

Phase separation in biological membranes plays an important role in protein targeting and transmembrane signaling. Its occurrence in both membrane leaflets commonly gives rise to matching liquid or liquid-ordered domains in the opposing monolayers. The underlying mechanism of such co-localization is not fully understood. The decrease of the line tension around the thicker ordered domain constitutes an important driving force. Yet, robust domain coupling requires an additional energy source, which we have now identified as thermal undulations. Our theoretical analysis of elastic deformations in a lipid bilayer shows that stiffer lipid domains tend to distribute into areas with lower fluctuations of monolayer curvature. These areas naturally align in the opposing monolayers. Thus, coupling requires both membrane leafs to display a heterogeneity in splay rigidities. The heterogeneity may either originate from intrinsic lipid properties or be acquired by adsorption of peripheral molecules. Undulations and line tension act synergistically: the gain in energy due a minimized line tension is proportional to domain radius and thus primarily fuels the registration of smaller domains; whereas the energetic contribution of undulations increases with membrane area and thus primarily acts to coalesce larger domains.