Multiphoton excitation fluorescence microscopy in planar membrane systems

Molecular mapping and functional validation of GLP-1R cholesterol binding sites in pancreatic beta cells

G protein-coupled receptors (GPCRs) are integral membrane proteins which closely interact with their plasma membrane lipid microenvironment. Cholesterol is a lipid enriched at the plasma membrane with pivotal roles in the control of membrane fluidity and maintenance of membrane microarchitecture, directly impacting on GPCR stability, dynamics, and function. Cholesterol extraction from pancreatic beta cells has previously been shown to disrupt the internalisation, clustering, and cAMP responses of the glucagon-like peptide-1 receptor (GLP-1R), a class B1 GPCR with key roles in the control of blood glucose levels via the potentiation of insulin secretion in beta cells and weight reduction via the modulation of brain appetite control centres. Here, we unveil the detrimental effect of a high cholesterol diet on GLP-1R-dependent glucoregulation in vivo, and the improvement in GLP-1R function that a reduction in cholesterol synthesis using simvastatin exerts in pancreatic islets. We next identify and map sites of cholesterol high occupancy and residence time on active vs inactive GLP-1Rs using coarse-grained molecular dynamics (cgMD) simulations, followed by a screen of key residues selected from these sites and detailed analyses of the effects of mutating one of these, Val229, to alanine on GLP-1R-cholesterol interactions, plasma membrane behaviours, clustering, trafficking and signalling in INS-1 832/3 rat pancreatic beta cells and primary mouse islets, unveiling an improved insulin secretion profile for the V229A mutant receptor. This study (1) highlights the role of cholesterol in regulating GLP-1R responses in vivo; (2) provides a detailed map of GLP-1R - cholesterol binding sites in model membranes; (3) validates their functional relevance in beta cells; and (4) highlights their potential as locations for the rational design of novel allosteric modulators with the capacity to fine-tune GLP-1R responses.

Significance of in situ quantitative membrane property-morphology relation (QmPMR) analysis

Deformation of the cell membrane is well understood from the viewpoint of protein interactions and free energy balance. However, the various dynamic properties of the membrane, such as lipid packing and hydrophobicity, and their relationship with cell membrane deformation are unknown. Therefore, the deformation of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and oleic acid (OA) giant unilamellar vesicles (GUVs) was induced by heating and cooling cycles, and time-lapse analysis was conducted based on the membrane hydrophobicity and physical parameters of "single-parent" and "daughter" vesicles. Fluorescence ratiometric analysis by simultaneous dual-wavelength detection revealed the variation of different hydrophilic GUVs and enabled inferences of the "daughter" vesicle composition and the "parent" membrane's local composition during deformation; the "daughter" vesicle composition of OA was lower than that of the "parents", and lateral movement of OA was the primary contributor to the formation of the "daughter" vesicles. Thus, our findings and the newly developed methodology, named in situ quantitative membrane property-morphology relation (QmPMR) analysis, would provide new insights into cell deformation and accelerate research on both deformation and its related events, such as budding and birthing.

Perspectives on the yogurt rheology

The oral processing of yogurt is a dynamic process involving a series of deformation processes. Rheological knowledge is essential to understand the structure and flow properties of yogurt in the mouth and to explore its relationship with sensory perception. Yogurt is rheologically characterized as a non-Newtonian viscoelastic material. The rheological properties of yogurt are affected by many factors, from production to consumption. Therefore, rheological measurements are widely used to predict and control the final quality and structure of yogurts. Recent studies focus on the elucidation of the effects of cultures and processes used in production, as well as the design of different formulations to improve the rheological properties of yogurts. Moreover, the science of tribology, which dominates the surface properties of interacting substances in relative motion to evaluate the structural sensation in the later stages of eating in addition to the rheological properties that give the feeling of structure in the early stages of eating, has also become the focus of recent studies. For a detailed comprehension of the rheological properties of yogurt, this review deals with the factors affecting the rheology of yogurt, analytical methods used to determine rheological properties, microstructural and rheological characterization of yogurt, and tribological evaluations.

Exposure to Aldehyde Cherry e-Liquid Flavoring and Its Vaping Byproduct Disrupt Pulmonary Surfactant Biophysical Function

Over the past decade, there has been a significant rise in the use of vaping devices, particularly among adolescents, raising concerns for effects on respiratory health. Pressingly, many recent vaping-related lung injuries are unexplained by current knowledge, and the overall implications of vaping for respiratory health are poorly understood. This study investigates the effect of hydrophobic vaping liquid chemicals on the pulmonary surfactant biophysical function. We focus on the commonly used flavoring benzaldehyde and its vaping byproduct, benzaldehyde propylene glycol acetal. The study involves rigorous testing of the surfactant biophysical function in Langmuir trough and constrained sessile drop surfactometer experiments with both protein-free synthetic surfactant and hydrophobic protein-containing clinical surfactant models. The study reveals that exposure to these vaping chemicals significantly interferes with the synthetic and clinical surfactant biophysical function. Further atomistic simulations reveal preferential interactions with SP-B and SP-C surfactant proteins. Additionally, data show surfactant lipid-vaping chemical interactions and suggest significant transfer of vaping chemicals to the experimental subphase, indicating a toxicological mechanism for the alveolar epithelium. Our study, therefore, reveals novel mechanisms for the inhalational toxicity of vaping. This highlights the need to reassess the safety of vaping liquids for respiratory health, particularly the use of aldehyde chemicals as vaping flavorings.

Advantages of the classical thermodynamic analysis of single-and multi-component Langmuir monolayers from molecules of biomedical importance-theory and applications

The Langmuir monolayer technique has been successfully used for decades to model biological membranes and processes occurring at their interfaces. Classically, this method involves surface pressure measurements to study interactions within membrane components as well as between external bioactive molecules (e.g. drugs) and the membrane. In recent years, surface-sensitive techniques were developed to investigate monolayers in situ; however, the obtained results are in many cases insufficient for a full characterization of biomolecule-membrane interactions. As result, description of systems using parameters such as mixing or excess thermodynamic functions is still relevant, valuable and irreplaceable in biophysical research. This review article summarizes the theory of thermodynamics of single- and multi-component Langmuir monolayers. In addition, recent applications of this approach to characterize surface behaviour and interactions (e.g. orientation of bipolar molecules, drug-membrane affinity, lateral membrane heterogeneity) are presented.

Interfacial Aβ fibril formation is modulated by the disorder-order state of the lipids: The concept of the physical environment as amyloid inductor in biomembranes

The behavior of amphiphilic molecules such as lipids, peptides and their mixtures at the air/water interface allow us to evaluate and visualize the arrangement formed in a confined and controlled surface area. We have studied the surface properties of the zwitterionic DPPC lipid and Aβ(1-40) amyloid peptide in mixed films at different temperatures (from 15 to 40 °C). In this range of temperature the surface properties of pure Aβ(1-40) peptide remained unchanged, whereas DPPC undergoes its characteristic liquid-expanded → liquid-condensed bidimensional phase transition that depends on the temperature and lateral pressure. This particular property of DPPC makes it possible to dynamically study the influence of the lipid phase state on amyloid structure formation at the interface in a continuous, isothermal and abrupt change on the environmental condition. As the mixed film is compressed the fibril-like structure of Aβ(1-40) is triggered specifically in the liquid-expanded region, independently of temperature, and it is selectively excluded from the well-visible liquid condensed domains of DPPC. The Aβ amyloid fibers were visualized by using BAM and AFM and they were Thio T positive. In mixed DPPC/Aβ(1-40) films the condensed domains (in between 11 mN/m to 20 mN/m) become irregular probably due to the fibril-like structures is imposing additional lateral stress sequestering lipid molecules in the surrounding liquid-expanded phase to self-organize into amyloids.

pH-Dependence Cytotoxicity Evaluation of Artepillin C against Tumor Cells

Brazilian green propolis is a well-known product that is consumed globally. Its major component, Artepillin C, showed potential as an antitumor product. This study explored the impact of Artepillin C on fibroblast and glioblastoma cell lines, used as healthy and very aggressive tumor cell lines, respectively. The focus of the study was to evaluate the pH-dependence of Artepillin C cytotoxicity, since tumor cells are known to have a more acidic extracellular microenvironment compared to healthy cells, and Artepillin C was shown to become more lipophilic at lower pH values. Investigations into the pH-dependency of Artepillin C (6.0-7.4), through viability assays and live cell imaging, revealed compelling insights. At pH 6.0, MTT assays showed the pronounced cytotoxic effects of Artepillin C, yielding a notable reduction in cell viability to less than 12% among glioblastoma cells following a 24 h exposure to 100 µM of Artepillin C. Concurrently, LDH assays indicated significant membrane damage, affecting approximately 50% of the total cells under the same conditions. Our Laurdan GP analysis suggests that Artepillin C induces autophagy, and notably, provokes a lipid membrane packing effect, contributing to cell death. These combined results affirm the selective cytotoxicity of Artepillin C within the acidic tumor microenvironment, emphasizing its potential as an effective antitumor agent. Furthermore, our findings suggest that Artepillin C holds promise for potential applications in the realm of anticancer therapies given its pH-dependence cytotoxicity.

Eukaryotic Cell Membranes: Structure, Composition, Research Methods and Computational Modelling

This paper deals with the problems encountered in the study of eukaryotic cell membranes. A discussion on the structure and composition of membranes, lateral heterogeneity of membranes, lipid raft formation, and involvement of actin and cytoskeleton networks in the maintenance of membrane structure is included. Modern methods for the study of membranes and their constituent domains are discussed. Various simplified models of biomembranes and lipid rafts are presented. Computer modelling is considered as one of the most important methods. This is stated that from the study of the plasma membrane structure, it is desirable to proceed to the diverse membranes of all organelles of the cell. The qualitative composition and molar content of individual classes of polar lipids, free sterols and proteins in each of these membranes must be considered. A program to create an open access electronic database including results obtained from the membrane modelling of individual cell organelles and the key sites of the membranes, as well as models of individual molecules composing the membranes, has been proposed.

Melittin-solid phospholipid mixed films trigger amyloid-like nano-fibril arrangements at air-water interface

We used the Langmuir monolayers technique to study the surface properties of melittin toxin mixed with either liquid-condensed DSPC or liquid-expanded POPC phospholipids. Pure melittin peptide forms stable insoluble monolayers at the air-water interface without interacting with Thioflavin T (Th-T), a sensitive probe to detect protein amyloid formation. When melittin peptide is mixed with DSPC lipid at 50 % of peptide area proportion at the surface, we observed the formation of fibril-like structures detected by Brewster angle microscopy (BAM), but they were not observable with POPC. The nano-structures in the melittin-DSPC mixtures became Th-T positive labeling when the arrangement was observed with fluorescence microscopy. In this condition, Th-T undergoes an unexpected shift in the typical emission wavelength of this amyloid marker when a 2D fluorescence analysis is conducted. Even when reflectivity analysis of BAM imaging evidenced that these structures would correspond to the DSPC lipid component of the mixture, the interpretation of ATR-FTIR and Th-T data suggested that both components were involved in a new lipid-peptide rearrangement. These nano-fibril arrangements were also evidenced by scanning electron and atomic force microscopy when the films were transferred to a mica support. The fibril formation was not detected when melittin was mixed with the liquid-expanded POPC lipid. We postulated that DSPC lipids can dynamically trigger the process of amyloid-like nano-arrangement formation at the interface. This process is favored by the relative peptide content, the quality of the interfacial environment, and the physical state of the lipid at the surface.

The innards of the cell: studies of water dipolar relaxation using the ACDAN fluorescent probe

This article reviews the use of the 6-acetyl-2-(dimethylamino)naphthalene (ACDAN) fluorophore to study dipolar relaxation in cells, tissues, and biomimetic systems. As the most hydrophilic member of the 6-acyl-2-(dimethylamino)naphthalene series, ACDAN markedly partitions to aqueous environments. In contrast to 6-lauroyl-2-(dimethylamino)naphthalene (LAURDAN), the hydrophobic and best-known member of the series used to explore relaxation phenomena in biological (or biomimetic) membranes, ACDAN allows mapping of spatial and temporal water dipolar relaxation in cytosolic and intra-organelle environments of the cell. This is also true for the 6-propionyl-2-(dimethylamino)naphthalene (PRODAN) derivative which, unlike LAURDAN, partitions to both hydrophobic and aqueous environments. We will i) summarize the mechanism which underlies the solvatochromic properties of the DAN probes, ii) expound on the importance of water relaxation to understand the intracellular environment, iii) discuss technical aspects of the use of ACDAN in eukaryotic cells and some specialized structures, including liquid condensates arising from processes leading to liquid immiscibility and, iv) present some novel studies in plant cells and tissues which demonstrate the kinds of information that can be uncovered using this approach to study dipolar relaxation in living systems.

Fluorescence sensors for imaging membrane lipid domains and cholesterol

Lipid membrane domains are supramolecular lateral heterogeneities of biological membranes. Of nanoscopic dimensions, they constitute specialized hubs used by the cell as transient signaling platforms for a great variety of biologically important mechanisms. Their property to form and dissolve in the bulk lipid bilayer endow them with the ability to engage in highly dynamic processes, and temporarily recruit subpopulations of membrane proteins in reduced nanometric compartments that can coalesce to form larger mesoscale assemblies. Cholesterol is an essential component of these lipid domains; its unique molecular structure is suitable for interacting intricately with crevices and cavities of transmembrane protein surfaces through its rough β face while "talking" to fatty acid acyl chains of glycerophospholipids and sphingolipids via its smooth α face. Progress in the field of membrane domains has been closely associated with innovative improvements in fluorescence microscopy and new fluorescence sensors. These advances enabled the exploration of the biophysical properties of lipids and their supramolecular platforms. Here I review the rationale behind the use of biosensors over the last few decades and their contributions towards elucidation of the in-plane and transbilayer topography of cholesterol-enriched lipid domains and their molecular constituents. The challenges introduced by super-resolution optical microscopy are discussed, as well as possible scenarios for future developments in the field, including virtual ("no staining") staining.

Adaptive Lipid Immiscibility and Membrane Remodeling Are Active Functional Determinants of Primary Ciliogenesis

Lipid liquid-liquid immiscibility and its consequent lateral heterogeneity have been observed under thermodynamic equilibrium in model and native membranes. However, cholesterol-rich membrane domains, sometimes referred to as lipid rafts, are difficult to observe spatiotemporally in live cells. Despite their importance in many biological processes, robust evidence for their existence remains elusive. This is mainly due to the difficulty in simultaneously determining their chemical composition and physicochemical nature, whilst spatiotemporally resolving their nanodomain lifetime and molecular dynamics. In this study, a bespoke method based on super-resolution stimulated emission depletion (STED) microscopy and raster imaging correlation spectroscopy (RICS) is used to overcome this issue. This methodology, laser interleaved confocal RICS and STED-RICS (LICSR), enables simultaneous tracking of lipid lateral packing and dynamics at the nanoscale. Previous work indicated that, in polarized epithelial cells, the midbody remnant licenses primary cilium formation through an unidentified mechanism. LICSR shows that lipid immiscibility and its adaptive collective nanoscale self-assembly are crucial for the midbody remnant to supply condensed membranes to the centrosome for the biogenesis of the ciliary membrane. Hence, this work poses a breakthrough in the field of lipid biology by providing compelling evidence of a functional role for liquid ordered-like membranes in primary ciliogenesis.

Understanding the Functional Properties of Lipid Heterogeneity in Pulmonary Surfactant Monolayers at the Atomistic Level

Pulmonary surfactant is a complex mixture of lipids and proteins lining the interior of the alveoli, and constitutes the first barrier to both oxygen and pathogens as they progress toward blood circulation. Despite decades of study, the behavior of the pulmonary surfactant at the molecular scale is poorly understood, which hinders the development of effective surfactant replacement therapies, useful in the treatment of several lung-related diseases. In this work, we combined all-atom molecular dynamics simulations, Langmuir trough measurements, and AFM imaging to study synthetic four-component lipid monolayers designed to model protein-free pulmonary surfactant. We characterized the structural and dynamic properties of the monolayers with a special focus on lateral heterogeneity. Remarkably, simulations reproduce almost quantitatively the experimental data on pressure-area isotherms and the presence of lateral heterogeneities highlighted by AFM. Quite surprisingly, the pressure-area isotherms do not show a plateau region, despite the presence of liquid-condensed nanometer-sized domains at surface pressures larger than 20 mN/m. In the simulations, the liquid-condensed domains were small and transient, but they did not coalesce to yield a separate phase. They were only slightly enriched in DPPC and cholesterol, and their chemical composition remained very similar to the overall composition of the monolayer membrane. Instead, they differed from liquid-expanded regions in terms of membrane thickness (in agreement with AFM data), diffusion rates, as well as acyl chain packing and orientation. We hypothesize that such lateral heterogeneities are crucial for lung surfactant function, as they allow both efficient packing, to achieve low surface tension, and sufficient fluidity, critical for rapid adsorption to the air–liquid interface during the breathing cycle.

Breakdown of classical paradigms in relation to membrane structure and functions

Updates of the mosaic fluid membrane model implicitly sustain the paradigms that bilayers are closed systems conserving a state of fluidity and behaving as a dielectric slab. All of them are a consequence of disregarding water as part of the membrane structure and its essential role in the thermodynamics and kinetics of membrane response to bioeffectors. A correlation of the thermodynamic properties with the structural features of water makes possible to introduce the lipid membrane as a responsive structure due to the relaxation of water rearrangements in the kinetics of bioeffectors' interactions. This analysis concludes that the lipid membranes are open systems and, according to thermodynamic of irreversible formalism, bilayers and monolayers can be reasonable compared under controlled conditions. The inclusion of water in the complex structure makes feasible to reconsider the concept of dielectric slab and fluidity.

Addressing Differentiation in Live Human Keratinocytes by Assessment of Membrane Packing Order

Differentiation of keratinocytes is critical for epidermal stratification and formation of a protective stratum corneum. It involves a series of complex processes leading through gradual changes in characteristics and functions of keratinocytes up to their programmed cell death via cornification. The stratum corneum is a relatively impermeable barrier, comprised of dead cell remnants (corneocytes) embedded in lipid matrix. Corneocyte membranes are comprised of specialized lipids linked to late differentiation proteins, contributing to the formation of a stiff and mechanically strengthened layer. To date, the assessment of the progression of keratinocyte differentiation is only possible through determination of specific differentiation markers, e.g., by using proteomics-based approaches. Unfortunately, this requires fixation or cell lysis, and currently there is no robust methodology available to study keratinocyte differentiation in living cells in real-time. Here, we explore new live-cell based approaches for screening differentiation advancement in keratinocytes, in a "calcium switch" model. We employ a polarity-sensitive dye, Laurdan, and Laurdan general polarization function (GP) as a reporter of the degree of membrane lateral packing order or condensation, as an adequate marker of differentiation. We show that the assay is straightforward and can be conducted either on a single cell level using confocal spectral imaging or on the ensemble level using a fluorescence plate reader. Such systematic quantification may become useful for understanding mechanisms of keratinocyte differentiation, such as the role of membrane in homogeneities in stiffness, and for future therapeutic development.

Aβ-Amyloid Fibrils Are Self-Triggered by the Interfacial Lipid Environment and Low Peptide Content

We studied the surface properties of Aβ(1-40) amyloid peptides mixed with 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) (liquid state) or 1,2-disteraoyl-phosphatidylcholine (DSPC) (solid state) phospholipids by using nanostructured lipid/peptide films (Langmuir monolayers). Pure Aβ(1-40) amyloid peptides form insoluble monolayers without forming fibril-like structures. In a lipid environment [phospholipid/Aβ(1-40) peptide mixtures], we observed that both miscibility and stability of the films depend on the peptide content. At low Aβ(1-40) amyloid peptide proportion (from 2.5 to 10% of peptide area proportion), we observed the formation of a fibril-like structure when mixed only with POPC lipids. The stability acquired by these mixed films is within 20-35 mN·m^-1 compatible with the equivalent surface pressure postulated for natural biomembranes. Fibrils are clearly evidenced directly from the monolayers by using Brewster angle microscopy. The so-called nanostructured fibrils are thioflavin T positive when observed by fluorescence microscopy. The amyloid fibril network at the surface was also evidenced by atomic force microscopy when the films are transferred onto a mica support. Aβ(1-40) amyloid mixed with the solid DSPC lipid showed an immiscible behavior in all peptide proportions without fibril formation. We postulated that the amyloid fibrillogenesis at the membrane can be dynamically nano-self-triggered at the surface by the quality of the interfacial environment, that is, the physical state of the water-lipid interface and the relative content of amyloid protein present at the interface.

Effects of artepillin C on model membranes displaying liquid immiscibility

It has been hypothesized that the therapeutic effects of artepillin C, a natural compound derived from Brazilian green propolis, are likely related to its partition in the lipid bilayer component of biological membranes. To test this hypothesis, we investigated the effects of the major compound of green propolis, artepillin C, on model membranes (small and giant unilamelar vesicles) composed of ternary lipid mixtures containing cholesterol, which display liquid-ordered (l_o) and liquid-disordered (l_d) phase coexistence. Specifically, we explored potential changes in relevant membrane parameters upon addition of artepillin C presenting both neutral and deprotonated states by means of small angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and confocal and multiphoton excitation fluorescence microscopy. Thermotropic analysis obtained from DSC experiments indicated a loss in the lipid cooperativity of l_o phase at equilibrium conditions, while at similar conditions spontaneous formation of unilamellar vesicles from SAXS experiments showed that deprotonated artepillin C preferentially located at the surface of the membrane. Time-resolved experiments using fluorescence microscopy showed that at doses above 100 µM, artepillin C in its neutral state interacted with both liquid-ordered and liquid-disordered phases, inducing curvature stress and promoting dehydration at the membrane interface.

Direct visualization of the lateral structure of giant vesicles composed of pseudo-binary mixtures of sulfatide, asialo-GM1 and GM1 with POPC

We compared the lateral structure of giant unilamellar vesicles (GUVs) composed of three pseudo binary mixtures of different glycosphingolipid (GSL), i.e. sulfatide, asialo-GM1 or GM1, with POPC. These sphingolipids possess similar hydrophobic residues but differ in the size and charge of their polar head group. Fluorescence microscopy experiments using LAURDAN and DiIC₁₈ show coexistence of micron sized domains in a molar fraction range that depends on the nature of the GSLs. In all cases, experiments with LAURDAN show that the membrane lateral structure resembles the coexistence of solid ordered and liquid disordered phases. Notably, the overall extent of hydration measured by LAURDAN between the solid ordered and liquid disordered membrane regions show marked similarities and are independent of the size of the GSL polar head group. In addition, the maximum amount of GSL incorporated in the POPC bilayer exhibits a strong dependence on the size of the GSL polar head group following the order sulfatide>asialo-GM1>GM1. This observation is in full harmony with previous experiments and theoretical predictions for mixtures of these GSL with glycerophospholipids. Finally, compared with previous results reported in GUVs composed of mixtures of POPC with the sphingolipids cerebroside and ceramide, we observed distinctive curvature effects at particular molar fraction regimes in the different mixtures. This suggests a pronounced effect of these GSL on the spontaneous curvature of the bilayer. This observation may be relevant in a biological context, particularly in connection with the highly curved structures found in neural cells.

Spectral imaging toolbox: segmentation, hyperstack reconstruction, and batch processing of spectral images for the determination of cell and model membrane lipid order

BACKGROUND

Spectral imaging with polarity-sensitive fluorescent probes enables the quantification of cell and model membrane physical properties, including local hydration, fluidity, and lateral lipid packing, usually characterized by the generalized polarization (GP) parameter. With the development of commercial microscopes equipped with spectral detectors, spectral imaging has become a convenient and powerful technique for measuring GP and other membrane properties. The existing tools for spectral image processing, however, are insufficient for processing the large data sets afforded by this technological advancement, and are unsuitable for processing images acquired with rapidly internalized fluorescent probes.

RESULTS

Here we present a MATLAB spectral imaging toolbox with the aim of overcoming these limitations. In addition to common operations, such as the calculation of distributions of GP values, generation of pseudo-colored GP maps, and spectral analysis, a key highlight of this tool is reliable membrane segmentation for probes that are rapidly internalized. Furthermore, handling for hyperstacks, 3D reconstruction and batch processing facilitates analysis of data sets generated by time series, z-stack, and area scan microscope operations. Finally, the object size distribution is determined, which can provide insight into the mechanisms underlying changes in membrane properties and is desirable for e.g. studies involving model membranes and surfactant coated particles. Analysis is demonstrated for cell membranes, cell-derived vesicles, model membranes, and microbubbles with environmentally-sensitive probes Laurdan, carboxyl-modified Laurdan (C-Laurdan), Di-4-ANEPPDHQ, and Di-4-AN(F)EPPTEA (FE), for quantification of the local lateral density of lipids or lipid packing.

CONCLUSIONS

The Spectral Imaging Toolbox is a powerful tool for the segmentation and processing of large spectral imaging datasets with a reliable method for membrane segmentation and no ability in programming required. The Spectral Imaging Toolbox can be downloaded from https://uk.mathworks.com/matlabcentral/fileexchange/62617-spectral-imaging-toolbox .

Effects of seaweed sterols fucosterol and desmosterol on lipid membranes

Higher sterols are universally present in large amounts (20-30%) in the plasma membranes of all eukaryotes whereas they are universally absent in prokaryotes. It is remarkable that each kingdom of the eukaryotes has chosen, during the course of evolution, its preferred sterol: cholesterol in animals, ergosterol in fungi and yeast, phytosterols in higher plants, and e.g., fucosterol and desmosterol in algae. The question arises as to which specific properties do sterols impart to membranes and to which extent do these properties differ among the different sterols. Using a range of biophysical techniques, including calorimetry, fluorescence microscopy, vesicle-fluctuation analysis, and atomic force microscopy, we have found that fucosterol and desmosterol, found in red and brown macroalgae (seaweeds), similar to cholesterol support liquid-ordered membrane phases and induce coexistence between liquid-ordered and liquid-disordered domains in lipid bilayers. Fucosterol and desmosterol induce acyl-chain order in liquid membranes, but less effectively than cholesterol and ergosterol in the order: cholesterol>ergosterol>desmosterol>fucosterol, possibly reflecting the different molecular structure of the sterols at the hydrocarbon tail.

The diffusion dynamics of PEGylated liposomes in the intact vitreous of the ex vivo porcine eye: A fluorescence correlation spectroscopy and biodistribution study

The diffusion dynamics of nanocarriers in the vitreous and the influence of nanocarrier physicochemical properties on these dynamics is an important aspect of the efficacy of intravitreal administered nanomedicines for the treatment of posterior segment eye diseases. Here we use fluorescence correlation spectroscopy (FCS) to determine liposome diffusion coefficients in the intact vitreous (D_Vit) of ex vivo porcine eyes using a modified Miyake-Apple technique to minimize the disruption of the vitreous fine structure. We chose to investigate whether the zeta potential of polyethylene glycol functionalized (i.e. PEGylated) liposomes altered liposome in situ diffusion dynamics in the vitreous. Non-PEGylated cationic nanocarriers have previously shown little to no diffusion in the vitreous, whilst neutral and anionic have shown diffusion. The liposomes investigated had diameters below 150nm and zeta potentials ranging from -20 to +12mV. We observed that PEGylated cationic liposomes had significantly lower D_Vit values (1.14μm²s^-1) than PEGylated neutral and anionic liposomes (2.78 and 2.87μm²s^-1). However, PEGylated cationic liposomes had a similar biodistribution profile across the vitreous to the other systems. These results show that PEGylated cationic liposomes with limited cationic charge can diffuse across the vitreous and indicate that the vitreous as a barrier to nanocarriers (Ø<500nm) is more complicated than simply an electrostatic barrier as previously suggested.

Evidence of proteolipid domain formation in an inner mitochondrial membrane mimicking model

BACKGROUND

Mitochondrial creatine kinase (mtCK) is highly abundant in mitochondria; its quantity is equimolecular to the Adenylic Nucleotide Translocator and represents 1% of the mitochondrial proteins. It is a multitask protein localized in the mitochondria intermembrane space where it binds to the specific cardiolipin (CL) phospholipid. If mtCK was initially thought to be exclusively implicated in energy transfer between mitochondria and cytosol through a mechanism referred to as the phosphocreatine shuttle, several recent studies suggested an additional role in maintaining mitochondria membrane structure.

METHODS

To further characterized mtCK binding process we used multiphoton excitation fluorescence microscopy coupled with Giant Unilamellar Vesicles (GUV) and laurdan as fluorescence probe.

RESULTS

We gathered structural and dynamical information on the molecular events occurring during the binding of mtCK to the mitochondria inner membrane. We present the first visualization of mtCK-induced CL segregation on a bilayer model forming micrometer-size proteolipid domains at the surface of the GUV. Those microdomains, which only occurred when CL is included in the lipid mixture, were accompanied by the formation of protein multimolecular assembly, vesicle clamping, and changes in both vesicle curvature and membrane fluidity CONCLUSION: Those results highlighted the importance of the highly abundant mtCK in the lateral organization of the mitochondrial inner membrane.

GENERAL SIGNIFICANCE

Microdomains were induced in mitochondria-mimicking membranes composed of natural phospholipids without cholesterol and/or sphingolipids differing from the proposed cytoplasmic membrane rafts. Those findings as well as membrane curvature modification were discussed in relation with protein-membrane interaction and protein cluster involvement in membrane morphology.

Properties of Langmuir and solid supported lipid films with sphingomyelin

Biological cell membranes play a crucial role in various biological processes and their functionality to some extent is determined by the hydrophilic/hydrophobic balance. A significant progress in understanding the membrane structure was the discovery of laterally segregated lipid domains, called the lipid rafts. These raft domains are of ordered lamellar liquid-crystalline phase, while rest of the membrane exists in a relatively disordered lamellar liquid-crystalline phase. Moreover, the chemical constitution of the lipid rafts consists of a higher content (up to 50%) of cholesterol (Chol) and sphingomyelin (SM). Sphingomyelin also plays a significant role in the red cells of blood and nerves, in some diseases, as a precursor to ceramides, and other sphingolipid metabolites. In this paper properties of Langmuir and solid supported mixed lipid films of DPPC/SM, DOPC/SM, and Chol/SM are described. Special attention has been paid to wetting properties (hydrophobic/hydrophilic balance) of these films transferred onto a hydrophilic glass surface. To our knowledge such results have not yet been published in the literature. The properties were determined via contact angle measurements and then calculation of the films' apparent surface free energy. The films' wettability and their apparent surface free energy strongly depend on their composition. The energy is affected by both the structure of hydrocarbon chains of glycerophospholipids (DPPC and DOPC) and their interactions with SM. Properties of mixed Chol/SM monolayer depend also on the film stoichiometry. At a low Chol content (XChol=0.25) the interactions between SM and Chol are strong and hence the formation of binary complex is possible. This is accompanied by a decrease in the film surface free energy in comparison to that of pure SM monolayer, contrary to a higher Chol content where the monolayer energy increases. This suggests that cholesterol is excluded from the membrane thus increasing the film hydrophilicity. These results are consistent with the literature data and somehow confirm the hypothesis of lipid raft formation. The roughness of the investigated monolayer surfaces was also determined using optical profilometry. The roughness parameters of the DPPC, SM, and mixed DPPC/SM generally correlate with the changes of their apparent surface free energy, i.e. with the decreasing roughness the apparent surface free energy also decreases. However, this is not the case for mixed DOPC/SM monolayers. Although the roughness increases with SM content the apparent surface free energy decreases. Therefore some other factors, like the presence of unsaturated bonds in the DOPC molecule, influence the film phase state and the energy too. More experiments are needed to explain this hypothesis.

Monitoring Membrane Hydration with 2-(Dimethylamino)-6-Acylnaphtalenes Fluorescent Probes

A family of polarity sensitive fluorescent probes (2-(dimethylamino)-6-acylnaphtalenes, i.e. LAURDAN, PRODAN, ACDAN) was introduced by Gregorio Weber in 1979, with the aim to monitor solvent relaxation phenomena on protein matrices. In the following years, however, PRODAN and particularly LAURDAN, were used to study membrane lateral structure and associated dynamics. Once incorporated into membranes, the (nanosecond) fluorescent decay of these probes is strongly affected by changes in the local polarity and relaxation dynamics of restricted water molecules existing at the membrane/water interface. For instance, when glycerophospholipid containing membranes undertake a solid ordered (gel) to liquid disordered phase transition the fluorescence emission maximum of these probes shift ~ 50 nm with a significant change in their fluorescence lifetime. Furthermore, the fluorescence parameters of LAURDAN and PRODAN are exquisitely sensitive to cholesterol effects, allowing interpretations that correlate changes in membrane packing with membrane hydration. Different membrane model systems as well as innate biological membranes have been studied with this family of probes allowing interesting comparative studies. This chapter presents a short historical overview about these fluorescent reporters, discusses on different models proposed to explain their sensitivity to membrane hydration, and includes relevant examples from experiments performed in artificial and biological membranes.

Nanoscale packing differences in sphingomyelin and phosphatidylcholine revealed by BODIPY fluorescence in monolayers: physiological implications

Phosphatidycholines (PC) with two saturated acyl chains (e.g., dipalmitoyl) mimic natural sphingomyelin (SM) by promoting raft formation in model membranes. However, sphingoid-based lipids, such as SM, rather than saturated-chain PCs have been implicated as key components of lipid rafts in biomembranes. These observations raise questions about the physical packing properties of the phase states that can be formed by these two major plasma membrane lipids with identical phosphocholine headgroups. To investigate, we developed a monolayer platform capable of monitoring changes in surface fluorescence by acquiring multiple spectra during measurement of a lipid force-area isotherm. We relied on the concentration-dependent emission changes of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-labeled PC to detect nanoscale alterations in lipid packing and phase state induced by monolayer lateral compression. The BODIPY-PC probe contained an indacene ring with four symmetrically located methyl (Me) substituents to enhance localization to the lipid hydrocarbon region. Surface fluorescence spectra indicated changes in miscibility even when force-area isotherms showed no deviation from ideal mixing behavior in the surface pressure versus cross-sectional molecular area response. We detected slightly better mixing of Me4-BODIPY-8-PC with the fluid-like, liquid expanded phase of 1-palmitoyl-2-oleoyl-PC compared to N-oleoyl-SM. Remarkably, in the gel-like, liquid condensed phase, Me4-BODIPY-8-PC mixed better with N-palmitoyl-SM than dipalmitoyl-PC, suggesting naturally abundant SMs with saturated acyl chains form gel-like lipid phase(s) with enhanced ability to accommodate deeply embedded components compared to dipalmitoyl-PC gel phase. The findings reveal a fundamental difference in the lateral packing properties of SM and PC that occurs even when their acyl chains match.

Quantitative optical microscopy and micromanipulation studies on the lipid bilayer membranes of giant unilamellar vesicles

This manuscript discusses basic methodological aspects of optical microscopy and micromanipulation methods to study membranes and reviews methods to generate giant unilamellar vesicles (GUVs). In particular, we focus on the use of fluorescence microscopy and micropipet manipulation techniques to study composition-structure-property materials relationships of free-standing lipid bilayer membranes. Because their size (∼5-100 μm diameter) that is well above the resolution limit of regular light microscopes, GUVs are suitable membrane models for optical microscopy and micromanipulation experimentation. For instance, using different fluorescent reporters, fluorescence microscopy allows strategies to study membrane lateral structure/dynamics at the level of single vesicles of diverse compositions. The micropipet manipulation technique on the other hand, uses Hoffman modulation contrast microscopy and allows studies on the mechanical, thermal, molecular exchange and adhesive-interactive properties of compositionally different membranes under controlled environmental conditions. The goal of this review is to (i) provide a historical perspective for both techniques; (ii) present and discuss some of their most important contributions to our understanding of lipid bilayer membranes; and (iii) outline studies that would utilize both techniques simultaneously on the same vesicle thus bringing the ability to characterize structure and strain responses together with the direct application of well-defined stresses to a single membrane or observe the effects of adhesive spreading. Knowledge gained by these studies has informed several applications of lipid membranes including their use as lung surfactants and drug delivery systems for cancer.

Effect of detergents on the physicochemical properties of skin stratum corneum: a two-photon excitation fluorescence microscopy study

OBJECTIVE

Understanding the structural and dynamical features of skin is critical for advancing innovation in personal care and drug discovery. Synthetic detergent mixtures used in commercially available body wash products are thought to be less aggressive towards the skin barrier when compared to conventional detergents. The aim of this work is to comparatively characterize the effect of a mild synthetic cleanser mixture (SCM) and sodium dodecyl sulphate (SDS) on the hydration state of the intercellular lipid matrix and on proton activity of excised skin stratum corneum (SC).

METHOD

Experiments were performed using two-photon excitation fluorescence microscopy. Fluorescent images of fluorescence reporters sensitive to proton activity and hydration of SC were obtained in excised skin and examined in the presence and absence of SCM and SDS detergents.

RESULTS

Hydration of the intercellular lipid matrix to a depth of 10 μm into the SC was increased upon treatment with SCM, whereas SDS shows this effect only at the very surface of SC. The proton activity of SC remained unaffected by treatment with either detergent.

CONCLUSION

While our study indicates that the SC is very resistant to external stimuli, it also shows that, in contrast to the response to SDS, SCM to some extent modulates the in-depth hydration properties of the intercellular lipid matrix within excised skin SC.

Compositional and structural characterization of monolayers and bilayers composed of native pulmonary surfactant from wild type mice

This work comprises a structural and dynamical study of monolayers and bilayers composed of native pulmonary surfactant from mice. Spatially resolved information was obtained using fluorescence (confocal, wide field and two photon excitation) and atomic force microscopy methods. Lipid mass spectrometry experiments were also performed in order to obtain relevant information on the lipid composition of this material. Bilayers composed of mice pulmonary surfactant showed coexistence of distinct domains at room temperature, with morphologies and lateral packing resembling the coexistence of liquid ordered (lo)/liquid disordered (ld)-like phases reported previously in porcine lung surfactant. Interestingly, the molar ratio of saturated (mostly DPPC)/non-saturated phospholipid species and cholesterol measured in the innate material corresponds with that of a DOPC/DPPC/cholesterol mixture showing lo/ld phase coexistence at a similar temperature. This suggests that at quasi-equilibrium conditions, key lipid classes in this complex biological material are still able to produce the same scaffold observed in relevant but simpler model lipid mixtures. Also, robust structural and dynamical similarities between mono- and bi-layers composed of mice pulmonary surfactant were observed when the monolayers reach a surface pressure of 30mN/m. This value is in line with theoretically predicted and recently measured surface pressures, where the monolayer-bilayer equivalence occurs in samples composed of single phospholipids. Finally, squeezed out material attached to pulmonary surfactant monolayers was observed at surface pressures near the beginning of the monolayer reversible exclusion plateau (~40mN/m). Under these conditions this material adopts elongated tubular shapes and displays ordered lateral packing as indicated by spatially resolved LAURDAN GP measurements.

Influence of charge density on bilayer bending rigidity in lipid vesicles: a combined dynamic light scattering and neutron spin-echo study

We report a combined dynamic light scattering and neutron spin-echo study on vesicles composed of the uncharged stabilizing lipid 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) and the cationic lipid 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP). Mechanical properties of a model membrane and thus the corresponding bilayer undulation dynamics can be specifically tuned by changing its composition through lipid headgroup or acyl chain properties. We compare the undulation dynamics in lipid vesicles composed of DMPC/DOTAP to vesicles composed of a mixture of the uncharged helper lipid DMPC with the also uncharged reference lipid 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). We have performed dynamic light scattering on the lipid mixtures to investigate changes in lipid vesicle size and the corresponding center-of-mass diffusion. We study lipid translational diffusion in the membrane plane and local bilayer undulations using neutron spin-echo spectroscopy, on two distinct time scales, namely around 25 ns and around 150 ns. Finally, we calculate the respective bilayer bending rigidities κ for both types of lipid vesicles. We find that on the local length scale inserting lipid headgroup charge into the membrane influences the bilayer undulation dynamics and bilayer bending rigidity κ less than inserting lipid acyl chain unsaturation: We observe a bilayer softening with increasing inhomogenity of the lipid mixture, which could be caused by a hydrophobic mismatch between the acyl chains of the respective lipid components, causing a lateral phase segregation (domain formation) in the membrane plane.

Structural and dynamical aspects of skin studied by multiphoton excitation fluorescence microscopy-based methods

This mini-review reports on applications of particular multiphoton excitation microscopy-based methodologies employed in our laboratory to study skin. These approaches allow in-depth optical sectioning of the tissue, providing spatially resolved information on specific fluorescence probes' parameters. Specifically, by applying these methods, spatially resolved maps of water dipolar relaxation (generalized polarization function using the 6-lauroyl-2-(N,N-dimethylamino)naphthale probe), activity of protons (fluorescence lifetime imaging using a proton sensitive fluorescence probe--2,7-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein) and diffusion coefficients of distinct fluorescence probes (raster imaging correlation spectroscopy) can be obtained from different regions of the tissue. Comparative studies of different tissue strata, but also between equivalent regions of normal and abnormal excised skin, including applications of fluctuation correlation spectroscopy on transdermal penetration of liposomes are presented and discussed. The data from the different studies reported reveal the intrinsic heterogeneity of skin and also prove these strategies to be powerful noninvasive tools to explore structural and dynamical aspects of the tissue.

Multi-color single particle tracking with quantum dots

Quantum dots (QDs) have long promised to revolutionize fluorescence detection to include even applications requiring simultaneous multi-species detection at single molecule sensitivity. Despite the early promise, the unique optical properties of QDs have not yet been fully exploited in e. g. multiplex single molecule sensitivity applications such as single particle tracking (SPT). In order to fully optimize single molecule multiplex application with QDs, we have in this work performed a comprehensive quantitative investigation of the fluorescence intensities, fluorescence intensity fluctuations, and hydrodynamic radii of eight types of commercially available water soluble QDs. In this study, we show that the fluorescence intensity of CdSe core QDs increases as the emission of the QDs shifts towards the red but that hybrid CdSe/CdTe core QDs are less bright than the furthest red-shifted CdSe QDs. We further show that there is only a small size advantage in using blue-shifted QDs in biological applications because of the additional size of the water-stabilizing surface coat. Extending previous work, we finally also show that parallel four color multicolor (MC)-SPT with QDs is possible at an image acquisition rate of at least 25 Hz. We demonstrate the technique by measuring the lateral dynamics of a lipid, biotin-cap-DPPE, in the cellular plasma membrane of live cells using four different colors of QDs; QD565, QD605, QD655, and QD705 as labels.

The acyl-CoA binding protein is required for normal epidermal barrier function in mice

The acyl-CoA binding protein (ACBP) is a 10 kDa intracellular protein expressed in all eukaryotic species. Mice with targeted disruption of Acbp (ACBP(-/-) mice) are viable and fertile but present a visible skin and fur phenotype characterized by greasy fur and development of alopecia and scaling with age. Morphology and development of skin and appendages are normal in ACBP(-/-) mice; however, the stratum corneum display altered biophysical properties with reduced proton activity and decreased water content. Mass spectrometry analyses of lipids from epidermis and stratum corneum of ACBP(+/+) and ACBP(-/-) mice showed very similar composition, except for a significant and specific decrease in the very long chain free fatty acids (VLC-FFA) in stratum corneum of ACBP(-/-) mice. This finding indicates that ACBP is critically involved in the processes that lead to production of stratum corneum VLC-FFAs via complex phospholipids in the lamellar bodies. Importantly, we show that ACBP(-/-) mice display a ∼50% increased transepidermal water loss compared with ACBP(+/+) mice. Furthermore, skin and fur sebum monoalkyl diacylglycerol (MADAG) levels are significantly increased, suggesting that ACBP limits MADAG synthesis in sebaceous glands. In summary, our study shows that ACBP is required for production of VLC-FFA for stratum corneum and for maintaining normal epidermal barrier function.

Sphingomyelinase D activity in model membranes: structural effects of in situ generation of ceramide-1-phosphate

The toxicity of Loxosceles spider venom has been attributed to a rare enzyme, sphingomyelinase D, which transforms sphingomyelin to ceramide-1-phosphate. The bases of its inflammatory and dermonecrotic activity, however, remain unclear. In this work the effects of ceramide-1-phosphate on model membranes were studied both by in situ generation of this lipid using a recombinant sphingomyelinase D from the spider Loxosceles laeta and by pre-mixing it with sphingomyelin and cholesterol. The systems of choice were large unilamellar vesicles for bulk studies (enzyme kinetics, fluorescence spectroscopy and dynamic light scattering) and giant unilamellar vesicles for fluorescence microscopy examination using a variety of fluorescent probes. The influence of membrane lateral structure on the kinetics of enzyme activity and the consequences of enzyme activity on the structure of target membranes containing sphingomyelin were examined. The findings indicate that: 1) ceramide-1-phosphate (particularly lauroyl ceramide-1-phosphate) can be incorporated into sphingomyelin bilayers in a concentration-dependent manner and generates coexistence of liquid disordered/solid ordered domains, 2) the activity of sphingomyelinase D is clearly influenced by the supramolecular organization of its substrate in membranes and, 3) in situ ceramide-1-phosphate generation by enzymatic activity profoundly alters the lateral structure and morphology of the target membranes.

Formation of giant protein vesicles by a lipid cosolvent method

This paper describes a method to create giant protein vesicles (GPVs) of ≥10 μm by solvent-driven fusion of large vesicles (0.1-0.2 μm) with reconstituted membrane proteins. We found that formation of GPVs proceeded from rotational mixing of protein-reconstituted large unilamellar vesicles (LUVs) with a lipid-containing solvent phase. We made GPVs by using n-decane and squalene as solvents, and applied generalized polarization (GP) imaging to monitor the polarity around the protein transmembrane region of aquaporins labeled with the polarity-sensitive probe Badan. Specifically, we created GPVs of spinach SoPIP2;1 and E. coli AqpZ aquaporins. Our findings show that hydrophobic interactions within the bilayer of formed GPVs are influenced not only by the solvent partitioning propensity, but also by lipid composition and membrane protein isoform.

An epifluorescence microscopy method for generalized polarization imaging

Generalized polarization (GP) microscopy represents an excellent tool to study lipid-lipid and lipid-protein interactions in situ and in vitro. Here, we present an efficient and cost effective method to perform GP microscopy using a standard light-emitting diode (LED) epifluorescence microscope equipped with a digital color camera.

Model answers to lipid membrane questions

Ever since it was discovered that biological membranes have a core of a bimolecular sheet of lipid molecules, lipid bilayers have been a model laboratory for investigating physicochemical and functional properties of biological membranes. Experimental and theoretical models help the experimental scientist to plan experiments and interpret data. Theoretical models are the theoretical scientist's preferred toys to make contact between membrane theory and experiments. Most importantly, models serve to shape our intuition about which membrane questions are the more fundamental and relevant ones to pursue. Here we review some membrane models for lipid self-assembly, monolayers, bilayers, liposomes, and lipid-protein interactions and illustrate how such models can help answering questions in modern lipid cell biology.

Lipid lateral organization on giant unilamellar vesicles containing lipopolysaccharides

We developed a new (to our knowledge) protocol to generate giant unilamellar vesicles (GUVs) composed of mixtures of single lipopolysaccharide (LPS) species and Escherichia coli polar lipid extracts. Four different LPSs that differed in the size of the polar headgroup (i.e., LPS smooth > LPS-Ra > LPS-Rc > LPS-Rd) were selected to generate GUVs composed of different LPS/E. coli polar lipid mixtures. Our procedure consists of two main steps: 1), generation and purification of oligolamellar liposomes containing LPSs; and 2), electroformation of GUVs using the LPS-containing oligolamellar vesicles at physiological salt and pH conditions. Analysis of LPS incorporation into the membrane models (both oligolamellar vesicles and GUVs) shows that the final concentration of LPS is lower than that expected from the initial E. coli lipids/LPS mixture. In particular, our protocol allows incorporation of no more than 15 mol % for LPS-smooth and LPS-Ra, and up to 25 mol % for LPS-Rc and LPS-Rd (with respect to total lipids). We used the GUVs to evaluate the impact of different LPS species on the lateral structure of the host membrane (i.e., E. coli polar lipid extract). Rhodamine-DPPE-labeled GUVs show the presence of elongated micrometer-sized lipid domains for GUVs containing either LPS-Rc or LPS-Rd above 10 mol %. Laurdan GP images confirm this finding and show that this particular lateral scenario corresponds to the coexistence of fluid disordered and gel (LPS-enriched)-like micron-sized domains, in similarity to what is observed when LPS is replaced with lipid A. For LPSs containing the more bulky polar headgroup (i.e., LPS-smooth and LPS-Ra), an absence of micrometer-sized domains is observed for all LPS concentrations explored in the GUVs (up to ∼15 mol %). However, fluorescence correlation spectroscopy (using fluorescently labeled LPS) and Laurdan GP experiments in these microscopically homogeneous membranes suggests the presence of LPS clusters with dimensions below our microscope's resolution (∼380 nm radial). Our results indicate that LPSs can cluster into gel-like domains in these bacterial model membranes, and that the size of these domains depends on the chemical structure and concentration of the LPSs.