Formation and characterization of supported lipid bilayers containing phosphatidylinositol-4,5-bisphosphate and cholesterol as functional surfaces

Formation of Highly Negatively Charged Supported Lipid Bilayers on a Silica Surface: Effects of Ionic Strength and Osmotic Stress

Solid supported lipid bilayers (SLBs) serve as an excellent platform for biophysical studies. However, the formation of highly negatively charged SLBs on negatively charged surfaces remains a challenge due to electrostatic repulsion. Here, we study the effects of ionic strength and osmotic stress on the formation of highly negatively charged SLBs on the silica surface. We used quartz crystal microbalance-dissipation to study the adsorption and rupture of highly negatively charged small unilamellar vesicles on the silica surface in different concentrations of NaCl and under different osmotic stresses. It was demonstrated that an increase in the ionic strength of the solution enhances SLB formation. Both hypertonic and moderate hypotonic osmotic stress can promote the formation of SLBs. However, the SLB cannot be formed under high hypotonic osmotic stress. Importantly, osmotic stress alone without a change in ionic strength is insufficient to promote SLB formation. Moreover, the topographical images obtained by atomic force microscopy showed that complete bilayers were formed under hypertonic osmotic stress and high ionic strength, whereas defects were noticed in the bilayers formed under hypotonic osmotic stress. Furthermore, the fluidity of the lipid bilayers was studied by fluorescence recovery after photobleaching. A higher membrane fluidity was observed for the complete lipid bilayers compared to that of the lipid bilayers with defects. Our findings further the understanding of how ionic strength and osmotic stress affect the formation of highly negatively charged SLBs on negatively charged surfaces, providing insights for preparing model biological membranes.

Advancements in Engineering Planar Model Cell Membranes: Current Techniques, Applications, and Future Perspectives

Cell membranes are crucial elements in living organisms, serving as protective barriers and providing structural support for cells. They regulate numerous exchange and communication processes between cells and their environment, including interactions with other cells, tissues, ions, xenobiotics, and drugs. However, the complexity and heterogeneity of cell membranes-comprising two asymmetric layers with varying compositions across different cell types and states (e.g., healthy vs. diseased)-along with the challenges of manipulating real cell membranes represent significant obstacles for in vivo studies. To address these challenges, researchers have developed various methodologies to create model cell membranes or membrane fragments, including mono- or bilayers organized in planar systems. These models facilitate fundamental studies on membrane component interactions as well as the interactions of membrane components with external agents, such as drugs, nanoparticles (NPs), or biomarkers. The applications of model cell membranes have extended beyond basic research, encompassing areas such as biosensing and nanoparticle camouflage to evade immune detection. In this review, we highlight advancements in the engineering of planar model cell membranes, focusing on the nanoarchitectonic tools used for their fabrication. We also discuss approaches for incorporating challenging materials, such as proteins and enzymes, into these models. Finally, we present our view on future perspectives in the field of planar model cell membranes.

Fluorescence imaging of lamellipodin-mediated biomolecular condensates on solid supported lipid bilayer membranes

Biomolecular condensates play a major role in numerous cellular processes, including several that occur on the surface of lipid bilayer membranes. There is increasing evidence that cellular membrane trafficking phenomena, including the internalization of the plasma membrane through endocytosis, are mediated by multivalent protein-protein interactions that can lead to phase separation. We have recently found that proteins involved in the clathrin-independent endocytic pathway named Fast Endophilin Mediated Endocytosis can undergo liquid-liquid phase separation (LLPS) in solution and on lipid bilayer membranes. Here, the protein solution concentrations required for phase separation to be observed are significantly smaller compared to those required for phase separation in solution. LLPS is challenging to systematically characterize in cellular systems in general, and on biological membranes in particular. Model membrane approaches are more suitable for this purpose as they allow for precise control over the nature and amount of the components present in a mixture. Here we describe a method that enables the imaging of LLPS domain formation on solid supported lipid bilayers. These allow for facile imaging, provide long-term stability, and avoid clustering of vesicles and vesicle-attached features (such as buds and tethers) in the presence of multi-valent membrane interacting proteins.

A biophysical perspective of the regulatory mechanisms of ezrin/radixin/moesin proteins

Many signal transductions resulting from ligand-receptor interactions occur at the cell surface. These signaling pathways play essential roles in cell polarization, membrane morphogenesis, and the modulation of membrane tension at the cell surface. However, due to the large number of membrane-binding proteins, including actin-membrane linkers, and transmembrane proteins present at the cell surface, the molecular mechanisms underlying the regulation at the cell surface are yet unclear. Here, we describe the molecular functions of one of the key players at the cell surface, ezrin/radixin/moesin (ERM) proteins from a biophysical point of view. We focus our discussion on biophysical properties of ERM proteins revealed by using biophysical tools in live cells and in vitro reconstitution systems. We first describe the structural properties of ERM proteins and then discuss the interactions of ERM proteins with PI(4,5)P₂ and the actin cytoskeleton. These properties of ERM proteins revealed by using biophysical approaches have led to a better understanding of their physiological functions in cells and tissues.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12551-021-00928-0.

Direct-Write Patterning of Biomimetic Lipid Membranes In Situ with FluidFM

The creation of biologically inspired artificial membranes on substrates with custom size and in close proximity to each other not only provides a platform to study biological processes in a simplified manner, but they also constitute building blocks for chemical or biological sensors integrated in microfluidic devices. Scanning probe lithography tools such as dip-pen nanolithography (DPN) have opened a new paradigm in this regard, although they possess some inherent drawbacks like the need to operate in air environment or the limited choice of lipids that can be patterned. In this work, we propose the use of the fluid force microscopy (FluidFM) technology to fabricate biomimetic membranes without losing the multiplexing capability of DPN but gaining flexibility in lipid inks and patterning environment. We shed light on the driving mechanisms of the FluidFM-mediated lithography processes in air and liquid. The obtained results should prompt the creation of more realistic biomimetic membranes with arbitrary complex phospholipid mixtures, cholesterol, and potential functional membrane proteins directly patterned in physiological environment.

Mechanical properties of the high cholesterol-containing membrane: An AFM study

Cholesterol (Chol) content in most cellular membranes does not exceed 50 mol%, only in the eye lens's fiber cell plasma membrane, its content surpasses 50 mol%. At this high concentration, Chol induces the formation of pure cholesterol bilayer domains (CBDs), which coexist with the surrounding phospholipid-cholesterol domain (PCD). Here, we applied atomic force microscopy to study the mechanical properties of Chol/phosphatidylcholine membranes where the Chol content was increased from 0 to 75 mol%, relevant to eye lens membranes. The surface roughness of the membrane decreases with an increase of Chol content until it reaches 60 mol%, and roughness increases with a further increment in Chol content. We propose that the increased roughness at higher Chol content results from the formation of CBDs. Force spectroscopy on the membrane with Chol content of 50 mol% or lesser exhibited single breakthrough events, whereas two distinct puncture events were observed for membranes with the Chol content greater than 50 mol%. We propose that the first puncture force corresponds to the membranes containing coexisting PCD and CBDs. In contrast, the second puncture force corresponds to the "CBD water pocket" formed due to coexisting CBDs and PCD. Membrane area compressibility modulus (K_A) increases with an increase in Chol content until it reaches 60 mol%, and with further increment in Chol content, CBDs are formed, and K_A starts to decrease. Our results report the increase in membrane roughness and decrease K_A at very high Chol content (>60 mol%) relevant to the eye lens membrane.

Leaflet-Dependent Distribution of PtdIns[4,5]P2 in Supported Model Membranes

Supported planar lipid bilayers (SLBs) prepared by spreading of unilamellar vesicles on hydrophilic substrates such as silicon dioxide are frequently used to investigate lipid-protein interactions by means of surface-sensitive methods. In recent years, the receptor lipid phosphatidylinositol-4,5-bisphosphate (PtdIns[4,5]P₂) became particularly important as a significant number of proteins bind to this lipid at the inner leaflet of the plasma membrane. Here, we investigated how the lipid PtdIns[4,5]P₂ distributes between the two leaflets of an SLB on SiO₂ surfaces. We prepared SLBs on SiO₂ by spreading small unilamellar vesicles and quantified the adsorption of PtdIns[4,5]P₂ binding proteins providing information about the accessibility of PtdIns[4,5]P₂. We compared protein binding to PtdIns[4,5]P₂ in SLBs with that in lipid monolayers on a 1,1,1-trimethyl-N-(trimethylsilyl)silanamine-functionalized SiO₂ surface using reflectometric interference spectroscopy and atomic force microscopy. Our results clearly demonstrate that the accessibility of PtdIns[4,5]P₂ for protein binding is reduced in SLBs compared to that in supported hybrid membranes, which is discussed in terms of PtdIns[4,5]P₂ distribution in the two leaflets of SLBs.

Membrane deformation and layer-by-layer peeling of giant vesicles induced by the pore-forming toxin pneumolysin

Membranes under attack by the pore-forming toxin pneumolysin reveal a hitherto unknown layer-by-layer peeling mechanism and disclose the multilamellar structure.

Bridging of membrane surfaces by annexin A2

The protein-mediated formation of membrane contacts is a crucial event in many cellular processes ranging from the establishment of organelle contacts to the docking of vesicles to a target membrane. Annexins are Ca²⁺ regulated membrane-binding proteins implicated in providing such membrane contacts; however, the molecular basis of membrane bridging by annexins is not fully understood. We addressed this central question using annexin A2 (AnxA2) that functions in secretory vesicle exocytosis possibly by providing membrane bridges. By quantitatively analyzing membrane contact formation using a novel assay based on quartz crystal microbalance recordings, we show that monomeric AnxA2 can bridge membrane surfaces Ca²⁺ dependently. However, this activity depends on an oxidative crosslink involving a cysteine residue in the N-terminal domain and thus formation of disulfide-linked dimers. Alkylated AnxA2 in which this cysteine residue has been modified and AnxA2 mutants lacking the N-terminal domain are not capable of bridging membrane surfaces. In contrast, a heterotetrameric complex comprising two membrane binding AnxA2 subunits linked by a S100A10 dimer can provide membrane contacts irrespective of oxidation status. Thus, monomeric AnxA2 only contains one lipid binding site and AnxA2-mediated linking of membrane surfaces under non-oxidative intracellular conditions most likely requires AnxA2-S100 complex formation.

Interfacial Forces Dictate the Pathway of Phospholipid Vesicle Adsorption onto Silicon Dioxide Surfaces

The pathway of vesicle adsorption onto a solid support depends on the material composition of the underlying support, and there is significant interest in developing material-independent strategies to modulate the spectrum of vesicle-substrate interactions on a particular surface. Herein, using the quartz crystal microbalance-dissipation (QCM-D) technique, we systematically investigated how solution pH and membrane surface charge affect vesicle adsorption onto a silicon dioxide surface. While vesicle adsorption and spontaneous rupture to form complete supported lipid bilayer (SLBs) occurred in acidic conditions, it was discovered that a wide range of adsorption pathways occurred in alkaline conditions, including (i) vesicle adsorption and spontaneous rupture to form complete SLBs, (ii) vesicle adsorption and spontaneous rupture to form incomplete SLBs, (iii) irreversible adsorption of intact vesicles, (iv) reversible adsorption of intact vesicles, and (v) negligible adsorption. In general, SLB formation became more favorable with increasingly positive membrane surface charge although there were certain conditions at which attractive electrostatic forces were insufficient to promote vesicle rupture. To rationalize these findings, we discuss how solution pH and membrane surface charge affect interfacial forces involved in vesicle-substrate interactions. Taken together, our findings present a comprehensive picture of how interfacial forces dictate the pathway of phospholipid vesicle adsorption onto silicon dioxide surfaces and offer a broadly applicable framework to characterize the interactions between phospholipid vesicles and inorganic material surfaces.

Role of Phosphorylation in Moesin Interactions with PIP2-Containing Biomimetic Membranes

Moesin, a protein of the ezrin, radixin, and moesin family, which links the plasma membrane to the cytoskeleton, is involved in multiple physiological and pathological processes, including viral budding and infection. Its interaction with the plasma membrane occurs via a key phosphoinositide, the phosphatidyl(4,5)inositol-bisphosphate (PIP2), and phosphorylation of residue T558, which has been shown to contribute, in cellulo, to a conformationally open protein. We study the impact of a double phosphomimetic mutation of moesin (T235D, T558D), which mimics the phosphorylation state of the protein, on protein/PIP2/microtubule interactions. Analytical ultracentrifugation in the micromolar range showed moesin in the monomer and dimer forms, with wild-type (WT) moesin containing a slightly larger fraction (∼30%) of dimers than DD moesin (10-20%). Only DD moesin was responsive to PIP2 in its micellar form. Quantitative cosedimentation assays using large unilamellar vesicles and quartz crystal microbalance on supported lipid bilayers containing PIP2 reveal a specific cooperative interaction for DD moesin with an ability to bind two PIP2 molecules simultaneously, whereas WT moesin was able to bind only one. In addition, DD moesin could subsequently interact with microtubules, whereas WT moesin was unable to do so. Altogether, our results point to an important role of these two phosphorylation sites in the opening of moesin: since DD moesin is intrinsically in a more open conformation than WT moesin, this intermolecular interaction is reinforced by its binding to PIP2. We also highlight important differences between moesin and ezrin, which appear to be finely regulated and to exhibit distinct molecular behaviors.

Membrane interactions of ionic liquids and imidazolium salts

Room-temperature ionic liquids (RTILs) have attracted considerable attention in recent years due to their versatile properties such as negligible volatility, inflammability, high extractive selectivity and thermal stability. In general, RTILs are organic salts with a melting point below ~100 °C determined by the asymmetry of at least one of their ions. Due to their amphiphilic character, strong interactions with biological materials can be expected. However, rising attention has appeared towards their similarity and interaction with biomolecules. By employing structural modifications, the biochemical properties of RTILs can be designed to mimic lipid structures and to tune their hydrophobicity towards a lipophilic behavior. This is evident for the interaction with lipid-membranes where some of these compounds present membrane-disturbing effects or cellular toxicity. Moreover, they can form micelles or lipid-like bilayer structures by themselves. Both aspects, cellular effects and membrane-forming capacities, of a novel class of lipophilic imidazolium salts will be discussed.

Microfluidic device as a facile in vitro tool to generate and investigate lipid gradients

This work describes a method that utilizes a microfluidic gradient generator to develop lateral lipid gradients in supported lipid bilayers (SLB). The new methodology provides freedom of choice with respect to the lipid composition of the SLB. In addition, the device has the ability to create a protein or bivalent cation gradient in the aqueous phase above the lipid bilayer which can elicit a gradient specific response in the SLB. To highlight these features we demonstrate that we can create a phosphoinositide gradient on various length scales, ranging from 2mm to 50μm. We further show that a Ca²⁺ gradient in the aqueous phase above the SLB causes anionic lipid clustering mirroring the cation gradient. We demonstrate this effect for mixed phosphatidylcholine/phosphatidylinositol-4,5-bisphosphate bilayers and fora mixed phosphatidylcholine/phosphatidylserine bilayers. The biomimetic platform can be combined with a Total Internal Reflection Fluorescence (TIRF) microscopy setup, which allows for the convenient observation of the time evolution of the gradient and the interaction of ligands with the lipid bilayer. The method provides unprecedented access to study the dynamics and mechanics of protein-lipid interactions on membranes with micron level gradients, mimicking plasma membrane gradients observed in organisms such as Dictyostelium discodeum and neutrophils.

Imidazolium Salts Mimicking the Structure of Natural Lipids Exploit Remarkable Properties Forming Lamellar Phases and Giant Vesicles

Tailor-made ionic liquids based on imidazolium salts have recently attracted a large amount of attention because of their extraordinary properties and versatile functionality. An intriguing ability to interact with and stabilize membranes has already been reported for 1,3-dialkylimidazolium compounds. We now reveal further insights into the field by investigating 1,3-dimethyl-4,5-dialkylimidazolium (C_n-IMe·HI, n = 7, 11, 15) and 1,3-dibenzyl-4,5-dialkylimidazolium (C_n-IBn·HBr, n = 7, 11, 15) salts. Diverse alkyl chain lengths and headgroups differing in their steric demand were employed for the membrane interface interaction with bilayer membranes imitating the cellular plasma membrane. Membrane hydration properties and domain fluidization were analyzed by fluorescent bilayer probes in direct comparison to established model membranes in a buffered aqueous environment, which resembles the salt content and pH of the cytosol of living cells. Membrane binding and insertion was analyzed via a quartz crystal microbalance and confocal laser scanning microscopy. We show that short-chain 4,5-dialkylimidazolium salts with a bulky headgroup were able to disintegrate membranes. Long-chain imidazolium salts form bilayer membrane vesicles spontaneously and autonomously without the addition of other lipids. These 4,5-dialkylimidazolium salts are highly eligible for further biochemical engineering and drug delivery.