Three-Phase Coexistence in Lipid Membranes

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.

PDMS as a Substrate for Lipid Bilayers

PDMS (polydimethylsiloxane) is a cheap, optically clear polymer that is elastic and can be easily and quickly fabricated into a wide array of microscale and nanoscale architectures, making it a versatile substrate for biophysical experiments on cell membranes. It is easy to imagine many new experiments will be devised that require a bilayer to be placed upon a substrate that is flexible or easily cast into a desired geometry, such as in lab-on-a-chip, organ-on-chip, and microfluidic applications, or for building accurate membrane models that replicate the surface structure and elasticity of the cytoskeleton. However, PDMS has its limitations, and the extent to which the behavior of membranes is affected on PDMS has not been fully explored. We use AFM and fluorescence optical microscopy to investigate the use of PDMS as a substrate for the formation and study of supported lipid bilayers (SLBs). Lipid bilayers form on plasma-treated PDMS and show free diffusion and normal phase transitions, confirming its suitability as a model bilayer substrate. However, lipid-phase separation on PDMS is severely restricted due to the pinning of domains to surface roughness, resulting in the cessation of lateral hydrodynamic flow. We show the high-resolution porous structure of PDMS and the extreme smoothing effect of oxygen plasma treatment used to hydrophilize the surface, but this is not flat enough to allow domain formation. We also observe bilayer degradation over hour timescales, which correlates with the known hydrophobic recovery of PDMS, and establish a critical water contact angle of 30°, above which bilayers degrade or not form at all. Care must be taken as incomplete surface oxidation and hydrophobic recovery result in optically invisible membrane disruption, which will also be transparent to fluorescence microscopy and lipid diffusion measurements in the early stages.

Activity modulation of the Escherichia coli F1FO ATP synthase by a designed antimicrobial peptide via cardiolipin sequestering

Most antimicrobial peptides (AMPs) exert their microbicidal activity through membrane permeabilization. The designed AMP EcDBS1R4 has a cryptic mechanism of action involving the membrane hyperpolarization of Escherichia coli, suggesting that EcDBS1R4 may hinder processes involved in membrane potential dissipation. We show that EcDBS1R4 can sequester cardiolipin, a phospholipid that interacts with several respiratory complexes of E. coli. Among these, F₁F_O ATP synthase uses membrane potential to fuel ATP synthesis. We found that EcDBS1R4 can modulate the activity of ATP synthase upon partition to membranes containing cardiolipin. Molecular dynamics simulations suggest that EcDBS1R4 alters the membrane environment of the transmembrane F_O motor, impairing cardiolipin interactions with the cytoplasmic face of the peripheral stalk that binds the catalytic F₁ domain to the F_O domain. The proposed mechanism of action, targeting membrane protein function through lipid reorganization may open new venues of research on the mode of action and design of other AMPs.

The potential of AFM in studying the role of the nanoscale amphipathic nature of (lipo)-peptides interacting with lipid bilayers

Antimicrobial peptides (AMPs) and lipopeptides (LPs) represent very promising molecules to fight resistant bacterial infections due to their broad-spectrum of activity, their first target, i.e. the bacterial membrane, and the rapid bactericidal action. For both types of molecules, the action mechanism starts from the membrane of the pathogen agents, producing a disorganization of their phase structure or the formation of pores of different size altering their permeability. This mechanism of action is based on physical interactions more than on a lock-and-key recognition event and it is difficult for the pathogens to rapidly develop an effective resistance. Very small differences in the sequence of both AMPs and LPs might lead to very different effects on the target membrane. Therefore, a correct understanding of their mechanism of action is required with the aim of developing new synthetic peptides, analogues of the natural ones, with specific and more powerful bactericidal activity. Atomic force microscopy (AFM), with its high resolution and the associated force spectroscopy resource, provides a valuable technique to investigate the reorganization of lipid bilayers exposed to antimicrobial or lipopeptides. Here, we present AFM results obtained by ours and other groups on the action of AMPs and LPs on supported lipid bilayers (SLBs) of different composition. We also consider data obtained by fluorescence microscopy to compare the AFM data with another technique which can be used on different lipid bilayer model systems such as SLBs and giant unilamellar vesicles. The outcomes here presented highlight the powerful of AFM-based techniques in detecting nanoscale peptide-membrane interactions and strengthen their use as an exceptional complementary tool toin vivoinvestigations. Indeed, the combination of these approaches can help decipher the mechanisms of action of different antimicrobials and lipopeptides at both the micro and nanoscale levels, and to design new and more efficient antimicrobial compounds.

Domain Size Regulation in Phospholipid Model Membranes Using Oil Molecules and Hybrid Lipids

The formation of domains in multicomponent lipid mixtures has been suggested to play a role in moderating signal transduction in cells. Understanding how domain size may be regulated by both hybrid lipid molecules and impurities is important for understanding real biological processes; at the same time, developing model systems where domain size can be regulated is crucial to enable systematic studies of domain formation kinetics and thermodynamics. Here, we perform a model study of the effects of oil molecules, which swell the bilayer, and line-active hybrid phospholipids using a thermally induced liquid-solid phase separation in planar, free-standing lipid bilayers consisting of DOPC and DPPC (1,2-dioleoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, respectively). The experiments show that the kinetics of domain growth are significantly affected by the type and molecular structure of the oil (squalene, hexadecane, or decane), with the main contributing factors being the degree of swelling of the bilayer and the changes in line tension induced by the different oils, with smaller domains resulting from systems with smaller values of the line tension. POPC (1-palmitoyl-sn-2-oleoyl-glycero-3-phosphocholine), on the other hand, acts as a line-active hybrid lipid, reducing the domain size when added in small amounts and slowing down domain coarsening. Finally, we show that despite the regulation of domain size by both methods, the phase transition temperature is influenced by the presence of oil molecules but not significantly by the presence of hybrid lipids. Overall, our results show how to regulate domain size in binary membrane model systems, over a wide range of length scales, by incorporating oil molecules and hybrid lipids.

Membrane Dynamics and Remodelling in Response to the Action of the Membrane-Damaging Pore-Forming Toxins

Pore-forming protein toxins (PFTs) represent a diverse class of membrane-damaging proteins that are produced by a wide variety of organisms. PFT-mediated membrane perforation is largely governed by the chemical composition and the physical properties of the plasma membranes. The interaction between the PFTs with the target membranes is critical for the initiation of the pore-formation process, and can lead to discrete membrane reorganization events that further aids in the process of pore-formation. Punching holes on the plasma membranes by the PFTs interferes with the cellular homeostasis by disrupting the ion-balance inside the cells that in turn can turn on multiple signalling cascades required to restore membrane integrity and cellular homeostasis. In this review, we discuss the physicochemical attributes of the plasma membranes associated with the pore-formation processes by the PFTs, and the subsequent membrane remodelling events that may start off the membrane-repair mechanisms.

Farnesylation and lipid unsaturation are critical for the membrane binding of the C-terminal segment of G-Protein Receptor Kinase 1

Many proteins are modified by the covalent addition of different types of lipids, such as myristoylation, palmitoylation and prenylation. Lipidation is expected to promote membrane association of proteins. Visual phototransduction involves many lipid-modified proteins. The G-Protein-coupled receptor of rod photoreceptors, rhodopsin, is inactivated by G-Protein-coupled Receptor Kinase 1 (GRK1). The C-terminus of GRK1 is farnesylated and its truncation has been shown to result in a very high decrease of its enzymatic activity, most likely because of the loss of its membrane localization. Little information is available on the membrane binding of GRK1 as well as of most prenylated proteins. Measurements of the membrane binding of the non-farnesylated and farnesylated C-terminal segment of GRK1 were thus performed using lipids typical of those found in rod outer segment disk membranes. Their random coil secondary structure was determined using circular dichroism and infrared spectroscopy. The non-farnesylated C-terminal segment of GRK1 has no surface activity. In contrast, the farnesylated C-terminal segment of GRK1 shows a particularly strong binding to lipid monolayers bearing at least one unsaturated fatty acyl chain. No binding is observed in the presence of monolayers of saturated phospholipids, in agreement with the low affinity of farnesylated Ras proteins for lipids in the liquid-ordered state. Altogether, these data demonstrate that the farnesyl group of the C-terminal segment of GRK1 is mandatory for its membrane binding, which is favored by particular lipids or lipid mixtures. This information will also be useful for the understanding of the membrane binding of other prenylated proteins.

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.

Non-raft submicron domain formation in cholesterol-containing lipid bilayers induced by polyunsaturated phosphatidylethanolamine

Domain formation in "HLC" ternary lipid bilayers, comprising a high transition temperature (High-T_m) lipid, a Low-T_m lipid, and cholesterol (Chol), has been extensively studied as raft-resembling systems. Recently, we reported the formation of submicron domains in an "LLC" lipid bilayer, encompassing Low-T_m phosphatidylethanolamine (PE), Low-T_m phosphatidylcholine (PC), and Chol. We hypothesized that the formation of this unique domain is driven by polyunsaturated PE. In this study, we explored the effects of the degree of PE unsaturation and the double bond distribution at the sn-position on the mechanism of formation and the composition of submicron domains. Supported lipid bilayers (SLBs), comprising PE with various degrees of unsaturation, monounsaturated PC (POPC), and Chol, were investigated using fluorescence microscopy, atomic force microscopy, and the force-distance curve measurement. The area fraction of submicron domains in PE+POPC+Chol-SLB increased with the PE concentration and degree of unsaturation of the PE acyl chain. The results indicated that the submicron domains were enriched with polyunsaturated PE and were in the liquid-disordered-like state, whereas their surrounding regions were in the liquid-ordered-like state. Segregation of polyunsaturated PE from the Chol-containing region generated submicron domains in the LLC lipid bilayer. We propose a mechanism for the formation of these submicron domains based on molecular interactions involving the hydrophobic and hydrophilic parts of the bilayer membrane.

Ceramide-mediation of diffusion in supported lipid bilayers

The fluidity and compositional heterogeneity of the mammalian plasma membrane play deterministic roles in a variety of membrane functions. Designing model bilayer systems allows for compositional control over these properties. Ceramide is a phospholipid capable of extensive headgroup-region hydrogen bonding, and we report here on the role of ceramide in planar model bilayers. We use fluorescence recovery after photobleaching (FRAP) to obtain translational diffusion constants of two chromophores in supported model bilayers composed of cholesterol, 1,2-dioleoyl-sn-phosphatidylcholine (DOPC), sphingomyelin, and ceramide. FRAP data for perylene report on the acyl chain region of the model bilayer and FRAP data for 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(lissamine rhodamine B sulfonyl) sense diffusional dynamics in the bilayer headgroup region. Dynamics in the headgroup region exhibit anomalous diffusion behavior that is characteristic of spatially heterogeneous media.

How Does Liquid-Liquid Phase Separation in Model Membranes Reflect Cell Membrane Heterogeneity?

Although liquid-liquid phase separation of cytoplasmic or nuclear components in cells has been a major focus in cell biology, it is only recently that the principle of phase separation has been a long-standing concept and extensively studied in biomembranes. Membrane phase separation has been reconstituted in simplified model systems, and its detailed physicochemical principles, including essential phase diagrams, have been extensively explored. These model membrane systems have proven very useful to study the heterogeneity in cellular membranes, however, concerns have been raised about how reliably they can represent native membranes. In this review, we will discuss how phase-separated membrane systems can mimic cellular membranes and where they fail to reflect the native cell membrane heterogeneity. We also include a few humble suggestions on which phase-separated systems should be used for certain applications, and which interpretations should be avoided to prevent unreliable conclusions.

Cholesterol-induced microdomain formation in lipid bilayer membranes consisting of completely miscible lipids

Recently, we reported that a ternary lipid bilayer comprising phosphatidylethanolamine (PE), phosphatidylcholine (PC), which were both derived from chicken egg, and cholesterol (Chol) generates microdomains that function as specific fusion sites for proteoliposomes. Chol-induced microdomain formation in a completely miscible lipid bilayer is an exceptional phenomenon. Numerous studies have elucidated the formation of domains in liquid ordered (L_o) and liquid disordered (L_d) phases of ternary bilayers, which comprise two partially miscible lipids and Chol. Herein, we investigated the composition and mechanism of formation of these unique microdomains in supported lipid bilayers (SLBs) using a fluorescence microscope and an atomic force microscope (AFM). We prepared ternary SLBs using egg-derived PC (eggPC), Chol and three different types of PE: egg-derived PE, 1-palmitoyl-2-oleoyl-PE, and 1,2-didocosahexaenoyl-PE (diDHPE). Fluorescence microscopy observations revealed that fluid and continuous SLBs were formed at PE concentrations (C_PE) of ≥6 mol%. Fluorescence recovery after photobleaching measurement revealed that the microdomain was more fluid than the surrounding region that showed typical diffusion coefficient of the L_o phase. The microdomains were observed as depressions in the AFM topographies. Their area fraction (θ) increased with C_PE, and diDHPE produced a significantly large θ among the three PEs. The microdomains in the PE+eggPC+Chol-SLBs were rich in polyunsaturated PE and were in the L_d-like phase. Associating eggPC and Chol caused polyunsaturated PE to segregate, resulting in a microdomain formation by conferring the umbrella effect on Chol, entropic effect of disordered acyl chains, and π-π interactions in the hydrophobic core.

Critical Phenomena in Plasma Membrane Organization and Function

Lateral organization in the plane of the plasma membrane is an important driver of biological processes. The past dozen years have seen increasing experimental support for the notion that lipid organization plays an important role in modulating this heterogeneity. Various biophysical mechanisms rooted in the concept of liquid-liquid phase separation have been proposed to explain diverse experimental observations of heterogeneity in model and cell membranes with distinct but overlapping applicability. In this review, we focus on the evidence for and the consequences of the hypothesis that the plasma membrane is poised near an equilibrium miscibility critical point. Critical phenomena explain certain features of the heterogeneity observed in cells and model systems but also go beyond heterogeneity to predict other interesting phenomena, including responses to perturbations in membrane composition.

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.

Impact of Glycans on Lipid Membrane Dynamics at the Nanoscale Unveiled by Planar Plasmonic Nanogap Antennas and Atomic Force Spectroscopy

Lateral compartmentalization of the plasma membrane is a prominent feature present at multiple spatiotemporal scales that regulates key cellular functions. The extracellular glycocalyx matrix has recently emerged as an important player that modulates the organization of specific receptors and patterns the lipid bilayer itself. However, experimental limitations in investigating its impact on the membrane nanoscale dynamics have hampered detailed studies. Here, we used photonic nanoantenna arrays combined with fluorescence correlation spectroscopy to investigate the influence of hyaluronic acid (HA), a prominent glycosaminoglycan, on the nanoscale organization of mimetic lipid bilayers. Using atomic force microscopy and force spectroscopy, we further correlated our dynamic measurements with the morphology and mechanical properties of bilayers at the nanoscale. Overall, we find that HA has a profound effect on the dynamics, nanoscale organization, and mechanical properties of lipid bilayers that are enriched in sphingolipids and/or cholesterol, such as those present in living cells.

The impact of antibacterial peptides on bacterial lipid membranes depends on stage of growth

Impact of maculatin 1.1 on supported lipid bilayers (SLBs) derived from early growth phase (EGP) or stationary growth phase (SGP) E. coli lipid extracts, monitored by atomic force microscopy which images bilayer morphology in real time.

Membrane Protein Modified Electrodes in Bioelectrocatalysis

Transmembrane proteins involved in metabolic redox reactions and photosynthesis catalyse a plethora of key energy-conversion processes and are thus of great interest for bioelectrocatalysis-based applications. The development of membrane protein modified electrodes has made it possible to efficiently exchange electrons between proteins and electrodes, allowing mechanistic studies and potentially applications in biofuels generation and energy conversion. Here, we summarise the most common electrode modification and their characterisation techniques for membrane proteins involved in biofuels conversion and semi-artificial photosynthesis. We discuss the challenges of applications of membrane protein modified electrodes for bioelectrocatalysis and comment on emerging methods and future directions, including recent advances in membrane protein reconstitution strategies and the development of microbial electrosynthesis and whole-cell semi-artificial photosynthesis.

Melatonin Alters Fluid Phase Coexistence in POPC/DPPC/Cholesterol Membranes

The structure and biophysical properties of lipid membranes are important for cellular functions in health and disease. In Alzheimer's disease, the neuronal membrane is a target for toxic amyloid-β (Aβ). Melatonin is an important pineal gland hormone that has been shown to protect against Aβ toxicity in cellular and animal studies, but the molecular mechanism of this protection is not fully understood. Melatonin is a small membrane-active molecule that has been shown to interact with model lipid membranes and alter the membrane biophysical properties, such as membrane molecular order and dynamics. This effect of melatonin has been previously studied in simple model bilayers with one or two lipid components. To make it more relevant to neuronal membranes, we used a more complex ternary lipid mixture as our membrane model. In this study, we used ²H-NMR to investigate the effect of melatonin on the phase behavior of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), and cholesterol lipid membranes. We used deuterium-labeled POPC-d₃₁ and DPPC-d₆₂,separately to probe the changes in hydrocarbon chain order as a function of temperature and melatonin concentration. We find that POPC/DPPC/cholesterol at molar proportions of 3:3:2 is close to liquid-disordered/liquid-ordered phase separation and that melatonin can induce phase separation in these ternary mixtures by preferentially incorporating into the disordered phase and increasing its level of disorder. At 5 mol% melatonin, we observed phase separation in samples with POPC-d₃₁, but not with DPPC-d₆₂, whereas at 10 mol% melatonin, phase separation was observed in both samples with either POPC-d₃₁ or DPPC-d₆₂. These results indicate that melatonin can have a strong effect on membrane structure and physical properties, which may provide some clues to understanding how melatonin protects against Aβ, and that choice of chain perdeuteration is an important consideration from a technical point of view.

Asymmetric bilayers mimicking membrane rafts prepared by lipid exchange: Nanoscale characterization using AFM-Force spectroscopy

Sphingolipids-enriched rafts domains are proposed to occur in plasma membranes and to mediate important cellular functions. Notwithstanding, the asymmetric transbilayer distribution of phospholipids that exists in the membrane confers the two leaflets different potentials to form lateral domains as next to no sphingolipids are present in the inner leaflet. How the physical properties of one leaflet can influence the properties of the other and its importance on signal transduction across the membrane are questions still unresolved. In this work, we combined AFM imaging and Force spectroscopy measurements to assess domain formation and to study the nanomechanical properties of asymmetric supported lipid bilayers (SLBs) mimicking membrane rafts. Asymmetric SLBs were formed by incorporating N-palmitoyl-sphingomyelin (16:0SM) into the outer leaflet of preformed 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC)/Cholesterol SLBs through methyl-β-cyclodextrin-mediated lipid exchange. Lipid domains were detected after incorporation of 16:0SM though their phase state varied from gel to liquid ordered (Lo) phase if the procedure was performed at 24 or 37 °C, respectively. When comparing symmetric and asymmetric Lo domains, differences in size and morphology were observed, with asymmetric domains being smaller and more interconnected. Both types of Lo domains showed similar mechanical stability in terms of rupture forces and Young's moduli. Notably, force curves in asymmetric domains presented two rupture events that could be attributed to the sequential rupture of a liquid disordered (Ld) and a Lo phase. Interleaflet coupling in asymmetric Lo domains could also be inferred from those measurements. The experimental approach outlined here would significantly enhance the applicability of membrane models.

Phase separation in pore-spanning membranes induced by differences in surface adhesion

A porous scaffold providing different adhesion energies alters the behaviour of coexisting phases in lipid membranes considerably.

Synthesis of Gb3 Glycosphingolipids with Labeled Head Groups: Distribution in Phase-Separated Giant Unilamellar Vesicles

The receptor lipid Gb3 is responsible for the specific internalization of Shiga toxin (STx) into cells. The head group of Gb3 defines the specificity of STx binding, and the backbone with different fatty acids is expected to influence its localization within membranes impacting membrane organization and protein internalization. To investigate this influence, a set of Gb3 glycosphingolipids labeled with a BODIPY fluorophore attached to the head group was synthesized. C24 fatty acids, saturated, unsaturated, α-hydroxylated derivatives, and a combination thereof, were attached to the sphingosine backbone. The synthetic Gb3 glycosphingolipids were reconstituted into coexisting liquid-ordered (lo )/liquid-disordered (ld ) giant unilamellar vesicles (GUVs), and STx binding was verified by fluorescence microscopy. Gb3 with the C24:0 fatty acid partitioned mostly in the lo phase, while the unsaturated C24:1 fatty acid distributes more into the ld phase. The α-hydroxylation does not influence its partitioning.

Multicomponent flow on curved surfaces: A vielbein lattice Boltzmann approach

We develop and implement a finite difference lattice Boltzmann scheme to study multicomponent flows on curved surfaces, coupling the continuity and Navier-Stokes equations with the Cahn-Hilliard equation to track the evolution of the binary fluid interfaces. The standard lattice Boltzmann method relies on regular Cartesian grids, which makes it generally unsuitable to study flow problems on curved surfaces. To alleviate this limitation, we use a vielbein formalism to write the Boltzmann equation on an arbitrary geometry, and solve the evolution of the fluid distribution functions using a finite difference method. Focusing on the torus geometry as an example of a curved surface, we demonstrate drift motions of fluid droplets and stripes embedded on the surface of the torus. Interestingly, they migrate in opposite directions: fluid droplets to the outer side while fluid stripes to the inner side of the torus. For the latter we demonstrate that the global minimum configuration is unique for small stripe widths, but it becomes bistable for large stripe widths. Our simulations are also in agreement with analytical predictions for the Laplace pressure of the fluid stripes, and their damped oscillatory motion as they approach equilibrium configurations, capturing the corresponding decay timescale and oscillation frequency. Finally, we simulate the coarsening dynamics of phase separating binary fluids in the hydrodynamics and diffusive regimes for tori of various shapes, and compare the results against those for a flat two-dimensional surface. Our finite difference lattice Boltzmann scheme can be extended to other surfaces and coupled to other dynamical equations, opening up a vast range of applications involving complex flows on curved geometries.

Nanoscale Substrate Roughness Hinders Domain Formation in Supported Lipid Bilayers

Supported lipid bilayers are model membranes formed at solid substrate surfaces. This architecture renders the membrane experimentally accessible to surface-sensitive techniques used to study their properties, including atomic force microscopy, optical fluorescence microscopy, quartz crystal microbalance, and X-ray/neutron reflectometry, and allows integration with technology for potential biotechnological applications such as drug screening devices. The experimental technique often dictates substrate choice or treatment, and it is anecdotally recognized that certain substrates are suitable for a particular experiment, but the exact influence of the substrate has not been comprehensively investigated. Here, we study the behavior of a simple model bilayer, phase-separating on a variety of commonly used substrates, including glass, mica, silicon, and quartz, with drastically different results. The distinct micron-scale domains observed on mica, identical to those seen in free-floating giant unilamellar vesicles, are reduced to nanometer-scale domains on glass and quartz. The mechanism for the arrest of domain formation is investigated, and the most likely candidate is nanoscale surface roughness, acting as a drag on the hydrodynamic motion of small domains during phase separation. Evidence was found that the physicochemical properties of the surface have a mediating effect, most likely because of the changes in the lubricating interstitial water layer between the surface and bilayer.

Complex Phase Behavior of GUVs Containing Different Sphingomyelins

Understanding the lateral organization of biological membranes plays a key role on the road to fully appreciate the physiological functions of this fundamental barrier between the inside and outside regions of a cell. Ternary lipid bilayers composed of a high and a low melting temperature lipid and cholesterol represent a model system that mimics some of the important thermodynamical features of much more complex lipid mixtures such as those found in mammal membranes. The phase diagram of these ternary mixtures can be studied exploiting fluorescence microscopy in giant unilamellar vesicles, and it is typically expected to give rise, for specific combinations of composition and temperature, to regions of two-phase coexistence and a region with three-phase coexistence, namely, the liquid-ordered, liquid-disordered, and solid phases. Whereas the observation of two-phase coexistence is routinely possible using fluorescence microscopy, the three-phase region is more elusive to study. In this article, we show that particular lipid mixtures containing diphytanoyl-phosphatidylcholine and cholesterol plus different types of sphingomyelin (SM) are prone to produce bilayer regions with more than two levels of fluorescence intensity. We found that these intensity levels occur at low temperature and are linked to the copresence of long and asymmetric chains in SMs and diphytanoyl-phosphatidylcholine in the lipid mixtures. We discuss the possible interpretations for this observation in terms of bilayer phase organization in the presence of sphingolipids. Additionally, we also show that in some cases, liposomes in the three-phase coexistence state exhibit extreme sensitivity to lateral tension. We hypothesize that the appearance of the different phases is related to the asymmetric structure of SMs and to interdigitation effects.

Correlated fluorescence quenching and topographic mapping of Light-Harvesting Complex II within surface-assembled aggregates and lipid bilayers

Light-Harvesting Complex II (LHCII) is a chlorophyll-protein antenna complex that efficiently absorbs solar energy and transfers electronic excited states to photosystems I and II. Under excess light intensity LHCII can adopt a photoprotective state in which excitation energy is safely dissipated as heat, a process known as Non-Photochemical Quenching (NPQ). In vivo NPQ is triggered by combinatorial factors including transmembrane ΔpH, PsbS protein and LHCII-bound zeaxanthin, leading to dramatically shortened LHCII fluorescence lifetimes. In vitro, LHCII in detergent solution or in proteoliposomes can reversibly adopt an NPQ-like state, via manipulation of detergent/protein ratio, lipid/protein ratio, pH or pressure. Previous spectroscopic investigations revealed changes in exciton dynamics and protein conformation that accompany quenching, however, LHCII-LHCII interactions have not been extensively studied. Here, we correlated fluorescence lifetime imaging microscopy (FLIM) and atomic force microscopy (AFM) of trimeric LHCII adsorbed to mica substrates and manipulated the environment to cause varying degrees of quenching. AFM showed that LHCII self-assembled onto mica forming 2D-aggregates (25-150 nm width). FLIM determined that LHCII in these aggregates were in a quenched state, with much lower fluorescence lifetimes (~0.25 ns) compared to free LHCII in solution (2.2-3.9 ns). LHCII-LHCII interactions were disrupted by thylakoid lipids or phospholipids, leading to intermediate fluorescent lifetimes (0.6-0.9 ns). To our knowledge, this is the first in vitro correlation of nanoscale membrane imaging with LHCII quenching. Our findings suggest that lipids could play a key role in modulating the extent of LHCII-LHCII interactions within the thylakoid membrane and so the propensity for NPQ activation.

Nanoscale mechanics of microgel particles

Microgel particles are highly tuneable materials that are useful for a wide range of industrial applications, such as drug delivery, sensing, nanoactuation, emulsion stabilisation and use as cell substrates. Microgels have also been used as model systems investigating physical phenomena such as crystallization, glass-formation, jamming, ageing and complex flow behaviour. The responsiveness of microgel systems such as poly(N-isopropylacrylamide) (PNIPAm) to external stimuli has been established in fundamental investigations and in applications and recent work has begun to quantify the mechanics of individual particles. However little focus has been placed on determining their internal mechanical properties, which is likely to relate to their nonuniform internal structure. In this work we combine atomic force microscopy, force spectroscopy and dynamic light scattering to mechanically profile the internal structure of microgel particles in the size range of ∼100 nm, which is commonly used both in practical applications and in fundamental studies. Nanoindentation using thermally stable cantilevers allows us to determine the particle moduli and the deformation profiles during particle compression with increasing force, while peak force nanomechanical mapping (PF-QNM) AFM is used to capture high resolution images of the particles' mechanical response. Combining these approaches with dynamic light scattering allows a quantitative profile of the particles' internal elastic response to be determined. Our results provide clear evidence for a radial distribution in particle mechanical response with a softer outer "corona" and a stiffer particle core. We determine the particle moduli in the core and corona, using different force microscopy approaches, and find them to vary systematically both in the core (∼17-50 kPa) and at the outer periphery of the particles (∼3-40 kPa). Importantly, we find that highly crosslinked particles have equivalent moduli across their radial profile, reflecting their significantly lower radial heterogeneity. This ability to accurately and precisely probe microgel radial profiles has clear implications both for fundamental science and for industrial applications.

Lipid self-assembly and lectin-induced reorganization of the plasma membrane

The plasma membrane represents an outstanding example of self-organization in biology. It plays a vital role in protecting the integrity of the cell interior and regulates meticulously the import and export of diverse substances. Its major building blocks are proteins and lipids, which self-assemble to a fluid lipid bilayer driven mainly by hydrophobic forces. Even if the plasma membrane appears-globally speaking-homogeneous at physiological temperatures, the existence of specialized nano- to micrometre-sized domains of raft-type character within cellular and synthetic membrane systems has been reported. It is hypothesized that these domains are the origin of a plethora of cellular processes, such as signalling or vesicular trafficking. This review intends to highlight the driving forces of lipid self-assembly into a bilayer membrane and the formation of small, transient domains within the plasma membrane. The mechanisms of self-assembly depend on several factors, such as the lipid composition of the membrane and the geometry of lipids. Moreover, the dynamics and organization of glycosphingolipids into nanometre-sized clusters will be discussed, also in the context of multivalent lectins, which cluster several glycosphingolipid receptor molecules and thus create an asymmetric stress between the two membrane leaflets, leading to tubular plasma membrane invaginations.This article is part of the theme issue 'Self-organization in cell biology'.

Exploring the biophysical properties of phytosterols in the plasma membrane for novel cancer prevention strategies

Cancer is a global problem with no sign that incidences are reducing. The great costs associated with curing cancer, through developing novel treatments and applying patented therapies, is an increasing burden to developed and developing nations alike. These financial and societal problems will be alleviated by research efforts into prevention, or treatments that utilise off-patent or repurposed agents. Phytosterols are natural components of the diet found in an array of seeds, nuts and vegetables and have been added to several consumer food products for the management of cardio-vascular disease through their ability to lower LDL-cholesterol levels. In this review, we provide a connected view between the fields of structural biophysics and cellular and molecular biology to evaluate the growing evidence that phytosterols impair oncogenic pathways in a range of cancer types. The current state of understanding of how phytosterols alter the biophysical properties of plasma membrane is described, and the potential for phytosterols to be repurposed from cardio-vascular to oncology therapeutics. Through an overview of the types of biophysical and molecular biology experiments that have been performed to date, this review informs the reader of the molecular and biophysical mechanisms through which phytosterols could have anti-cancer properties via their interactions with the plasma cell membrane. We also outline emerging and under-explored areas such as computational modelling, improved biomimetic membranes and ex vivo tissue evaluation. Focus of future research in these areas should improve understanding, not just of phytosterols in cancer cell biology but also to give insights into the interaction between the plasma membrane and the genome. These fields are increasingly providing meaningful biological and clinical data but iterative experiments between molecular biology assays, biosynthetic membrane studies and computational membrane modelling improve and refine our understanding of the role of different sterol components of the plasma membrane.