Dynamics and size of cross-linking-induced lipid nanodomains in model membranes

A comparison of lipid diffusive dynamics in monolayers and bilayers in the context of interleaflet coupling

Cellular membranes are composed of lipids typically organized in a double-leaflet structure. Interactions between these two leaflets - often referred to as interleaflet coupling - play a crucial role in various cellular processes. Despite extensive study, the mechanisms governing such interactions remain incompletely understood. Here, we investigate the effects of interleaflet coupling from a specific point of view, i.e. by comparing diffusive dynamics in bilayers and monolayers, focusing on potential lipid-specific interactions between opposing leaflets. Through quantitative fluorescence microscopy techniques, we characterize lipid diffusion and mean molecular area in monolayers and bilayers composed of different lipids. Our results suggest that the observed decrease in bilayer lipid diffusion compared to monolayers depends on lipid identity. Furthermore, our analysis suggests that lipid acyl chain structure and spatial configuration at the bilayer may strongly influence interleaflet interactions and dynamics in bilayers. These findings provide insights into the role of lipid structure in mediating interleaflet coupling and underscore the need for further experimental investigations to elucidate the underlying mechanisms.

Which Moiety Drives Gangliosides to Form Nanodomains?

Gangliosides are important glycosphingolipids involved in a multitude of physiological functions. From a physicochemical standpoint, this is related to their ability to self-organize into nanoscopic domains, even at molar concentrations of one per 1000 lipid molecules. Despite recent experimental and theoretical efforts suggesting that a hydrogen bonding network is crucial for nanodomain stability, the specific ganglioside moiety decisive for the development of these nanodomains has not yet been identified. Here, we combine an experimental technique achieving nanometer resolution (Förster resonance energy transfer analyzed by Monte Carlo simulations) with atomistic molecular dynamic simulations to demonstrate that the sialic acid (Sia) residue(s) at the oligosaccharide headgroup dominates the hydrogen bonding network between gangliosides, driving the formation of nanodomains even in the absence of cholesterol or sphingomyelin. Consequently, the clustering pattern of asialoGM₁, a Sia-depleted glycosphingolipid bearing three glyco moieties, is more similar to that of structurally distant sphingomyelin than that of the closely related gangliosides GM₁ and GD_1a with one and two Sia groups, respectively.

The ganglioside GM1a functions as a coreceptor/attachment factor for dengue virus during infection

Dengue virus (DENV) is a flavivirus causing an estimated 390 million infections per year around the world. Despite the immense global health and economic impact of this virus, its true receptor(s) for internalization into live cells has not yet been identified, and no successful antivirals or treatments have been isolated to this date. This study aims to improve our understanding of virus entry routes by exploring the sialic acid-based cell surface molecule GM1a and its role in DENV infection. We studied the interaction of the virus with GM1a using fluorescence correlation spectroscopy, fluorescence crosscorrelation spectroscopy, imaging fluorescence correlation spectroscopy, amide hydrogen/deuterium exchange mass spectrometry, and isothermal titration calorimetry. Additionally, we explored the effect of this interaction on infectivity and movement of the virus during infection was explored using plaque assay and fluorescence-based imaging and single particle tracking. GM1a was deemed to interact with DENV at domain I (DI) and domain II (DII) of the E protein of the protein coat at quaternary contacts of a fully assembled virus, leading to a 10-fold and 7-fold increase in infectivity for DENV1 and DENV2 in mammalian cell systems, respectively. We determined that the interaction of the virus with GM1a triggers a speeding up of virus movement on live cell surfaces, possibly resulting from a reduction in rigidity of cellular rafts during infection. Collectively, our results suggest that GM1a functions as a coreceptor/attachment factor for DENV during infection in mammalian systems.

Exploring the binding kinetics and behaviors of self-aggregated beta-amyloid oligomers to phase-separated lipid rafts with or without ganglioside-clusters

Lipid binding kinetics and energetics of self-aggregated and disordered beta-amyloid oligomers of various sizes, from solution to lipid raft surfaces, were investigated using MD simulations. Our systems include small (monomers to tetramers) and larger (octamers and dodecamers) oligomers. Our lipid rafts contain saturated and unsaturated phosphatidylcholine (PC), cholesterol, and with or without asymmetrically distributed monosialotetrahexosylganglioside (GM1). All rafts exhibited dynamic and structurally diversified domains including liquid-ordered (Lo), liquid-disordered (Ld), and interfacial Lod domains. For rafts without GM1, all oligomers bound to the Lod domain. For GM1-containing rafts, all small oligomers and most larger oligomers bound specifically to the GM1-clusters embedded in the Lo domain. Lipid-protein binding energies followed an order of GM1 >> unsaturated PC > saturated PC > cholesterol for all rafts. In addition, protein-induced membrane structural disruption increased progressively with the size of the oligomer for the annular lipids surrounding the membrane-bound protein in non-GM1-containing rafts. We propose that the tight binding of beta-amyloid oligomers to the GM1-clusters and the structural perturbation of lipids surrounding the membrane-bound proteins at the Lod domain are early molecular events of the beta-amyloid aggregation process on neuronal membrane surfaces that trigger the onset of Alzheimer's.

Heterogeneous Ganglioside-Enriched Nanoclusters with Different Densities in Membrane Rafts Detected by a Peptidyl Molecular Probe

The specific features of the lateral distribution of gangliosides play key roles in cell-cell communications and the onset of various diseases related to the plasma membrane. We herein demonstrated that an artificial peptide identified from a phage-displayed library is available as a molecular probe for specific ganglioside nanoclustering sites in caveolae/membrane rafts on the cell surface. Atomic force microscopy studies indicated that the peptide specifically binds to the highly enriched monosialoganglioside GM1 nanodomains of reconstituted lipid bilayers composed of GM1, sphingomyelin, cholesterol, and unsaturated phospholipids. The ganglioside-containing area recognized by the peptide on the surface of PC12 cells was part of the area recognized by the cholera toxin B subunit, which has high affinity for GM1. Furthermore, the peptide bound to the cell surface after a treatment with methyl-β-cyclodextrin (MβCD), which disrupts membrane rafts by removing cholesterol. The present results indicate that there are heterogeneous ganglioside clusters with different ganglioside densities in caveolae/membrane rafts, and the peptidyl probe selectively recognizes the high-density ganglioside nanodomain that resists the MβCD treatment. This peptidyl probe will be useful for obtaining information on the lipid organization of the cell membrane and will help clarify the mechanisms by which the lateral distribution of gangliosides affects biological functions and the onset of diseases.

Single-lipid dynamics in phase-separated supported lipid bilayers

Phase separation is a fundamental organizing mechanism on cellular membranes. Lipid phases have complex dependencies on the membrane composition, curvature, tension, and temperature. Lipid diffusion rates vary by up to ten-fold between liquid-disordered (L_d) and liquid-ordered (L_o) phases depending on the membrane composition, measurement technique, and the surrounding environment. This manuscript reports the lipid diffusion on phase-separated supported lipid bilayers (SLBs) with varying temperature, composition, and lipid phase. Lipid diffusion is measured by single-particle tracking (SPT) and fluorescence correlation spectroscopy (FCS) via custom data acquisition and analysis protocols that apply to diverse membranes systems. Traditionally, SPT is sensitive to diffuser aggregation, whereas the diffusion rates reported by FCS are unaffected by the presence of immobile aggregates. Within this manuscript, we report (1) improved single-particle tracking analysis of lipid diffusion, (2) comparison and consistency between diffusion measurement methods for non-Brownian diffusers, and (3) the application of these methods to measure the phase, temperature, and composition dependencies in lipid diffusion. We demonstrate improved SPT analysis methods that yield consistent FCS and SPT diffusion results even when most fluorescent lipids are frequently confined within aggregates within the membrane. With varying membrane composition and temperature, we demonstrate differences in diffusion between the L_d and L_o phases of SLBs.

FRET from phase-separated vesicles: An analytical solution for a spherical geometry

Förster resonance energy transfer (FRET) is a powerful tool for investigating heterogeneity in lipid bilayers. In model membrane studies, samples are frequently unilamellar vesicles with diameters of 20-200 nm. It is well-known that FRET efficiency is insensitive to vesicle curvature in uniformly mixed lipid bilayers, and consequently theoretical models for FRET typically assume a planar geometry. Here, we use a spherical harmonic expansion of the acceptor surface density to derive an analytical solution for FRET between donor and acceptor molecules distributed on the surface of a sphere. We find excellent agreement between FRET predicted from the model and FRET calculated from corresponding Monte Carlo simulations, thus validating the model. An extension of the model to the case of a non-uniform acceptor surface density (i.e., a phase-separated vesicle) reveals that FRET efficiency depends on vesicle size when acceptors partition between the coexisting phases, and approaches the efficiency of a uniformly mixed bilayer as the vesicle size decreases. We show that this is an indirect effect of constrained domain size, rather than an intrinsic effect of vesicle curvature. Surprisingly, the theoretical predictions were not borne out in experiments: we did not observe a statistically significant change in FRET efficiency in phase-separated vesicles as a function of vesicle size. We discuss factors that likely mask the vesicle size effect in extruded samples.

Structured clustering of the glycosphingolipid GM1 is required for membrane curvature induced by cholera toxin

AB₅ bacterial toxins and polyomaviruses induce membrane curvature as a mechanism to facilitate their entry into host cells. How membrane bending is accomplished is not yet fully understood but has been linked to the simultaneous binding of the pentameric B subunit to multiple copies of glycosphingolipid receptors. Here, we probe the toxin membrane binding and internalization mechanisms by using a combination of superresolution and polarized localization microscopy. We show that cholera toxin subunit B (CTxB) can induce membrane curvature only when bound to multiple copies of its glycosphingolipid receptor, GM1, and the ceramide structure of GM1 is likely not a determinant of this activity as assessed in model membranes. A mutant CTxB capable of binding only a single GM1 fails to generate curvature either in model membranes or in cells, and clustering the mutant CTxB-single-GM1 complexes by antibody cross-linking does not rescue the membrane curvature phenotype. We conclude that both the multiplicity and specific geometry of GM1 binding sites are necessary for the induction of membrane curvature. We expect this to be a general rule of membrane behavior for all AB₅ toxins and polyomaviruses that bind glycosphingolipids to invade host cells.

Interleaflet Coupling of Lipid Nanodomains - Insights From in vitro Systems

The plasma membrane is a complex system, consisting of two layers of lipids and proteins compartmentalized into small structures called nanodomains. Despite the asymmetric composition of both leaflets, coupling between the layers is surprisingly strong. This can be evidenced, for example, by recent experimental studies performed on phospholipid giant unilamellar vesicles showing that nanodomains formed in the outer layer are perfectly registered with those in the inner leaflet. Similarly, microscopic phase separation in one leaflet can induce phase separation in the opposing leaflet that would otherwise be homogeneous. In this review, we summarize the current theoretical and experimental knowledge that led to the current view that domains are – irrespective of their size – commonly registered across the bilayer. Mechanisms inducing registration of nanodomains suggested by theory and calculations are discussed. Furthermore, domain coupling is evidenced by experimental studies based on the sparse number of methods that can resolve registered from independent nanodomains. Finally, implications that those findings using model membrane studies might have for cellular membranes are discussed.

Membrane Protein Dimerization in Cell-Derived Lipid Membranes Measured by FRET with MC Simulations

Many membrane proteins are thought to function as dimers or higher oligomers, but measuring membrane protein oligomerization in lipid membranes is particularly challenging. Förster resonance energy transfer (FRET) and fluorescence cross-correlation spectroscopy are noninvasive, optical methods of choice that have been applied to the analysis of dimerization of single-spanning membrane proteins. However, the effects inherent to such two-dimensional systems, such as the excluded volume of polytopic transmembrane proteins, proximity FRET, and rotational diffusion of fluorophore dipoles, complicate interpretation of FRET data and have not been typically accounted for. Here, using FRET and fluorescence cross-correlation spectroscopy, we introduce a method to measure surface protein density and to estimate the apparent Förster radius, and we use Monte Carlo simulations of the FRET data to account for the proximity FRET effect occurring in confined two-dimensional environments. We then use FRET to analyze the dimerization of human rhomboid protease RHBDL2 in giant plasma membrane vesicles. We find no evidence for stable oligomers of RHBDL2 in giant plasma membrane vesicles of human cells even at concentrations that highly exceed endogenous expression levels. This indicates that the rhomboid transmembrane core is intrinsically monomeric. Our findings will find use in the application of FRET and fluorescence correlation spectroscopy for the analysis of oligomerization of transmembrane proteins in cell-derived lipid membranes.

Roughness of Transmembrane Helices Reduces Lipid Membrane Dynamics

The dynamics of cellular membranes is primarily determined by lipid species forming a bilayer. Proteins are considered mainly as effector molecules of diverse cellular processes. In addition to large assemblies of proteins, which were found to influence properties of fluid membranes, biological membranes are densely populated by small, highly mobile proteins. However, little is known about the effect of such proteins on the dynamics of membranes. Using synthetic peptides, we demonstrate that transmembrane helices interfere with the mobility of membrane components by trapping lipid acyl chains on their rough surfaces. The effect is more pronounced in the presence of cholesterol, which segregates from the rough surface of helical peptides. This may contribute to the formation or stabilization of membrane heterogeneities. Since roughness is a general property of helical transmembrane segments, our results suggest that, independent of their size or cytoskeleton linkage, integral membrane proteins affect local membrane dynamics and organization.

Membrane Lipid Nanodomains

Lipid membranes can spontaneously organize their components into domains of different sizes and properties. The organization of membrane lipids into nanodomains might potentially play a role in vital functions of cells and organisms. Model membranes represent attractive systems to study lipid nanodomains, which cannot be directly addressed in living cells with the currently available methods. This review summarizes the knowledge on lipid nanodomains in model membranes and exposes how their specific character contrasts with large-scale phase separation. The overview on lipid nanodomains in membranes composed of diverse lipids (e.g., zwitterionic and anionic glycerophospholipids, ceramides, glycosphingolipids) and cholesterol aims to evidence the impact of chemical, electrostatic, and geometric properties of lipids on nanodomain formation. Furthermore, the effects of curvature, asymmetry, and ions on membrane nanodomains are shown to be highly relevant aspects that may also modulate lipid nanodomains in cellular membranes. Potential mechanisms responsible for the formation and dynamics of nanodomains are discussed with support from available theories and computational studies. A brief description of current fluorescence techniques and analytical tools that enabled progress in lipid nanodomain studies is also included. Further directions are proposed to successfully extend this research to cells.

Evidence from ITIR-FCS Diffusion Studies that the Amyloid-Beta (Aβ) Peptide Does Not Perturb Plasma Membrane Fluidity in Neuronal Cells

The amyloid-beta (Aβ) peptide, commonly found in elevated levels in the brains of patients with Alzheimer's disease (AD) and in the cerebrospinal fluid of individuals presenting mild cognitive impairment, is thought to be one of the major factors resulting in the onset of AD. Although observed and studied at the molecular level for several decades, the exact disease pathology of AD is still not totally clear. One way in which Aβ is thought to affect neurons is by influencing cell membrane fluidity, which could result in abnormal synaptic or signaling function. The effects of Aβ on the fluidity of biological membranes have been studied using numerous membrane models such as artificial lipid bilayers and vesicles, living cells and membranes extracted from animal models of AD, yet there is still no consensus as to what effects Aβ has, if any, on membrane fluidity. As one of the most precise and accurate means of assaying membrane dynamics, we have thus chosen fluorescence correlation spectroscopy to investigate the issue, using fluorescent membrane-targeted probes on living cells treated with Aβ(1-42) oligomers and observing possible changes in membrane diffusion. Effects of Aβ on viability in different cell types varied from no detectable effect to extensive cell death by 72 h post-exposure. However, there was no change in the fluidity of either ordered membrane domains or the bulk membrane in any of these cells within this period. Our conclusion from these results is that perturbation of membrane fluidity is not likely to be a factor in acute Aβ-induced cytotoxicity.

Nanoscale Membrane Budding Induced by CTxB and Detected via Polarized Localization Microscopy

For endocytosis and exocytosis, membranes transition among planar, budding, and vesicular topographies through nanoscale reorganization of lipids, proteins, and carbohydrates. However, prior attempts to understand the initial stages of nanoscale bending have been limited by experimental resolution. Through the implementation of polarized localization microscopy, this article reports the inherent membrane bending capability of cholera toxin subunit B (CTxB) in quasi-one-component-supported lipid bilayers. Membrane buds were first detected with <50 nm radius, grew to >200 nm radius, and extended into longer tubules with dependence on the membrane tension and CTxB concentration. Compared to the concentration of the planar-supported lipid bilayers, CTxB was (12 ± 4)× more concentrated on the positive curvature top and (26 ± 11)× more concentrated on the negative Gaussian curvature neck of the nanoscale membrane buds. CTxB is frequently used as a marker for liquid-ordered lipid phases; however, the coupling between CTxB and membrane bending provides an alternate understanding of CTxB-induced membrane reorganization. These findings allow for the reinterpretation of prior observations by correlating CTxB clustering and diffusion to CTxB-induced membrane bending. Single-particle tracking was performed on single lipids and CTxB to reveal the correlations among single-molecule diffusion, CTxB accumulation, and membrane topography. Slowed lipid and CTxB diffusion was observed at the nanoscale bud locations, suggesting a local increase in the effective membrane viscosity or molecular crowding upon membrane bending. These results suggest inherent CTxB-induced membrane bending as a mechanism for initiating CTxB internalization in cells that could be independent of clathrin, caveolin, actin, and lipid phase separation.

Complex dynamics at the nanoscale in simple biomembranes

Nature is known to engineer complex compositional and dynamical platforms in biological membranes. Understanding this complex landscape requires techniques to simultaneously detect membrane re-organization and dynamics at the nanoscale. Using super-resolution stimulated emission depletion (STED) microscopy coupled with fluorescence correlation spectroscopy (FCS), we reveal direct experimental evidence of dynamic heterogeneity at the nanoscale in binary phospholipid-cholesterol bilayers. Domain formation on the length scale of ~200–600 nm due to local cholesterol compositional heterogeneity is found to be more prominent at high cholesterol content giving rise to distinct intra-domain lipid dynamics. STED-FCS reveals unique dynamical crossover phenomena at length scales of ~100–150 nm within each of these macroscopic regions. The extent of dynamic heterogeneity due to intra-domain hindered lipid diffusion as reflected from the crossover length scale, is driven by cholesterol packing and organization, uniquely influenced by phospholipid type. These results on simple binary model bilayer systems provide novel insights into pathways leading to the emergence of complex nanodomain substructures with implications for a wide variety of membrane mediated cellular events.

Development of new ganglioside probes and unraveling of raft domain structure by single-molecule imaging

Gangliosides are involved in a variety of biological roles and are a component of lipid rafts found in cell plasma membranes (PMs). Gangliosides are especially abundant in neuronal PMs and are essential to their physiological functions. However, the dynamic behaviors of gangliosides have not been investigated in living cells due to a lack of fluorescent probes that behave like their parental molecules. We have recently developed, using an entirely chemical method, four new ganglioside probes (GM1, GM2, GM3, and GD1b) that act similarly to their parental molecules in terms of raft partitioning and binding affinity. Using single fluorescent-molecule imaging, we have found that ganglioside probes dynamically enter and leave rafts featuring CD59, a GPI-anchored protein. This occurs both before and after stimulation. The residency time of our ganglioside probes in rafts with CD59 oligomers was 48ms, after stimulation. The residency times in CD59 homodimer and monomer rafts were 40ms and 12ms, respectively. In this review, we introduce an entirely chemical-based ganglioside analog synthesis method and describe its application in single-molecule imaging and for the study of the dynamic behavior of gangliosides in cell PMs. Finally, we discuss how raft domains are formed, both before and after receptor engagement. This article is part of a Special Issue entitled Neuro-glycoscience, edited by Kenji Kadomatsu and Hiroshi Kitagawa.

Lipid Driven Nanodomains in Giant Lipid Vesicles are Fluid and Disordered

It is a fundamental question in cell biology and biophysics whether sphingomyelin (SM)- and cholesterol (Chol)- driven nanodomains exist in living cells and in model membranes. Biophysical studies on model membranes revealed SM and Chol driven micrometer-sized liquid-ordered domains. Although the existence of such microdomains has not been proven for the plasma membrane, such lipid mixtures have been often used as a model system for ‘rafts’. On the other hand, recent super resolution and single molecule results indicate that the plasma membrane might organize into nanocompartments. However, due to the limited resolution of those techniques their unambiguous characterization is still missing. In this work, a novel combination of Förster resonance energy transfer and Monte Carlo simulations (MC-FRET) identifies directly 10 nm large nanodomains in liquid-disordered model membranes composed of lipid mixtures containing SM and Chol. Combining MC-FRET with solid-state wide-line and high resolution magic angle spinning NMR as well as with fluorescence correlation spectroscopy we demonstrate that these nanodomains containing hundreds of lipid molecules are fluid and disordered. In terms of their size, fluidity, order and lifetime these nanodomains may represent a relevant model system for cellular membranes and are closely related to nanocompartments suggested to exist in cellular membranes.

Investigation of Nanoscopic Phase Separations in Lipid Membranes Using Inverse FCS

Measurement of the sizes of nanoscopic particles is a difficult challenge, especially in two-dimensional systems such as cell membranes. We have extended inverse fluorescence correlation spectroscopy (iFCS) to endow it with unique advantages for measuring particle size from the nano- to the microscale. We have augmented iFCS with an analysis of moments of fluorescence fluctuations and used it to measure stages of phase separation in model lipid bilayer membranes. We observed two different pathways for the growth of phase domains. In one, nanoscopic gel domains appeared first and then gradually grew to micrometer size. In the other, the domains reached micrometer size quickly, and their number gradually increased. These measurements demonstrate the value of iFCS measurements through their ability, to our knowledge, to provide new information about the mechanism of lipid phase separation and potentially about the physical basis of naturally occurring nanodomains such as lipid rafts.

Impact of GM1 on Membrane-Mediated Aggregation/Oligomerization of β-Amyloid: Unifying View

In this perspective we summarize current knowledge of the effect of monosialoganglioside GM₁ on the membrane-mediated aggregation of the β-amyloid (Aβ) peptide. GM₁ has been suggested to be actively involved in the development of Alzheimer's disease due to its ability to seed the aggregation of Aβ. However, GM₁ is known to be neuroprotective against Aβ-induced toxicity. Here we suggest that the two scenarios are not mutually exclusive but rather complementary, and might depend on the organization of GM₁ in membranes. Improving our understanding of the molecular details behind the role of gangliosides in neurodegenerative amyloidoses might help in developing disease-modifying treatments.

Mixing Block Copolymers with Phospholipids at the Nanoscale: From Hybrid Polymer/Lipid Wormlike Micelles to Vesicles Presenting Lipid Nanodomains

Hybrids, i.e., intimately mixed polymer/phospholipid vesicles, can potentially marry in a single membrane the best characteristics of the two separate components. The ability of amphiphilic copolymers and phospholipids to self-assemble into hybrid membranes has been studied until now on the submicrometer scale using optical microscopy on giant hybrid unilamellar vesicles (GHUVs), but limited information is available on large hybrid unilamellar vesicles (LHUVs). In this work, copolymers based on poly(dimethylsiloxane) and poly(ethylene oxide) with different molar masses and architectures (graft, triblock) were associated with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). Classical protocols of LUV formation were used to obtain nanosized self-assembled structures. Using small-angle neutron scattering (SANS), time-resolved Förster resonance energy transfer (TR-FRET), and cryo-transmission electron microscopy (cryo-TEM), we show that copolymer architecture and molar mass have direct influences on the formation of hybrid nanostructures that can range from wormlike hybrid micelles to hybrid vesicles presenting small lipid nanodomains.

Glycolipid Crosslinking Is Required for Cholera Toxin to Partition Into and Stabilize Ordered Domains

Current models of lipid rafts propose that lipid domains exist as nanoscale compositional fluctuations and these fluctuations can potentially be stabilized into larger domains, consequently better compartmentalizing cellular functions. However, the mechanisms governing stabilized raft assembly and function remain unclear. Here, we test the role of glycolipid crosslinking as a raft targeting and ordering mechanism using the well-studied raft marker cholera toxin B pentamer (CTxB) that binds up to five GM1 glycosphingolipids to enter host cells. We show that when applied to cell-derived giant plasma membrane vesicles, a variant of CTxB containing only a single functional GM1 binding site exhibits significantly reduced partitioning to the ordered phase compared to wild-type CTxB with five binding sites. Moreover, monovalent CTxB does not stabilize membrane domains, unlike wild-type CTxB. These results support the long-held hypothesis that CTxB stabilizes raft domains via a lipid crosslinking mechanism and establish a role for crosslinking in the partitioning of CTxB to ordered domains.

Time-resolved fluorescence in lipid bilayers: selected applications and advantages over steady state

Fluorescence methods are versatile tools for obtaining dynamic and topological information about biomembranes because the molecular interactions taking place in lipid membranes frequently occur on the same timescale as fluorescence emission. The fluorescence intensity decay, in particular, is a powerful reporter of the molecular environment of a fluorophore. The fluorescence lifetime can be sensitive to the local polarity, hydration, viscosity, and/or presence of fluorescence quenchers/energy acceptors within several nanometers of the vicinity of a fluorophore. Illustrative examples of how time-resolved fluorescence measurements can provide more valuable and detailed information about a system than the time-integrated (steady-state) approach will be presented in this review: 1), determination of membrane polarity and mobility using time-dependent spectral shifts; 2), identification of submicroscopic domains by fluorescence lifetime imaging microscopy; 3), elucidation of membrane leakage mechanisms from dye self-quenching assays; and 4), evaluation of nanodomain sizes by time-resolved Förster resonance energy transfer measurements.

A protocol for the systematic and quantitative measurement of protein-lipid interactions using the liposome-microarray-based assay

Lipids organize the activity of the cell's proteome through a complex network of interactions. The assembly of comprehensive atlases embracing all protein-lipid interactions is an important challenge that requires innovative methods. We recently developed a liposome-microarray-based assay (LiMA) that integrates liposomes, microfluidics and fluorescence microscopy and which is capable of measuring protein recruitment to membranes in a quantitative and high-throughput manner. Compared with previous assays that are labor-intensive and difficult to scale up, LiMA improves the protein-lipid interaction assay throughput by at least three orders of magnitude. Here we provide a step-by-step LiMA protocol that includes the following: (i) the serial and generic production of the liposome microarray; (ii) its integration into a microfluidic format; (iii) the measurement of fluorescently labeled protein (either purified proteins or from cell lysate) recruitment to liposomal membranes using high-throughput microscopy; (iv) automated image analysis pipelines to quantify protein-lipid interactions; and (v) data quality analysis. In addition, we discuss the experimental design, including the relevant quality controls. Overall, the protocol-including device preparation, assay and data analysis-takes 6-8 d. This protocol paves the way for protein-lipid interaction screens to be performed on the proteome and lipidome scales.

Functions of cholera toxin B-subunit as a raft cross-linker

Lipid rafts are putative complexes of lipids and proteins in cellular membranes that are proposed to function in trafficking and signalling events. CTxB (cholera toxin B-subunit) has emerged as one of the most studied examples of a raft-associated protein. Consisting of the membrane-binding domain of cholera toxin, CTxB binds up to five copies of its lipid receptor on the plasma membrane of the host cell. This multivalency of binding gives the toxin the ability to reorganize underlying membrane structure by cross-linking otherwise small and transient lipid rafts. CTxB thus serves as a useful model for understanding the properties and functions of protein-stabilized domains. In the present chapter, we summarize current evidence that CTxB associates with and cross-links lipid rafts, discuss how CTxB binding modulates the architecture and dynamics of membrane domains, and describe the functional consequences of this cross-linking behaviour on toxin uptake into cells via endocytosis.

Nanodomain Formation of Ganglioside GM1 in Lipid Membrane: Effects of Cholera Toxin-Mediated Cross-Linking

Cross-linking of specific lipid components by proteins mediates transmembrane signaling and material transport. In this work, we conducted coarse-grained simulation to investigate the interactions of binding units of chorela toxin (CTB) with mixed ganglioside GM1 and dipalmitoylphosphatidylcholine (DPPC) lipid bilayer membrane. We determine that the binding of CTB pentamers cross-links GM1 molecules into protein-sized nanodomains that have distinct lipid order compared with the bulk. The toxin in the nanodomain partially penetrates into the membrane. The local disordering can also transmit across the membrane via lipid coupling. Comparison simulations on CTB binding to a membrane that is composed of various lipid components demonstrate that several factors are responsible for the nanodomain formation: (a) the negatively charged headgroup of a GM1 receptor is responsible for the multivalent binding; (b) the head groups being full of hydrogen-bonding donors and receptors stabilize the GM1 cluster itself and ensure the toxin binding with high affinity; and

Effect of receptor dimerization on membrane lipid raft structure continuously quantified on single cells by camera based fluorescence correlation spectroscopy

Membrane bound cell signaling is modulated by the membrane ultra-structure, which itself may be affected by signaling. However, measuring the interaction of membrane proteins with membrane structures in intact cells in real-time poses considerable challenges. In this paper we present a non-destructive fluorescence method that quantifies these interactions in single cells, and is able to monitor the same cell continuously to observe small changes. This approach combines total internal fluorescence microscopy with fluorescence correlation spectroscopy to measure the protein's diffusion and molecular concentration in different sized areas simultaneously. It correctly differentiates proteins interacting with membrane fences from proteins interacting with cholesterol-stabilized domains, or lipid rafts. This method detects small perturbations of the membrane ultra-structure or of a protein's tendency to dimerize. Through continuous monitoring of single cells, we demonstrate how dimerization of GPI-anchored proteins increases their association with the structural domains. Using a dual-color approach we study the effect of dimerization of one GPI-anchored protein on another type of GPI-anchored protein expressed in the same cell. Scans over the cell surface reveal a correlation between cholesterol stabilized domains and membrane cytoskeleton.

Phase Separation and Nanodomain Formation in Hybrid Polymer/Lipid Vesicles

Hybrid polymer/lipid large unilamellar vesicles (LUVs) were studied by small angle neutron scattering (SANS), time-resolved Förster resonance energy transfer (TR-FRET), and cryo-transmission electron microscopy (cryo-TEM). For the first time in hybrid vesicles, evidence for phase separation at the nanoscale was obtained, leading to the formation of stable nanodomains enriched either in lipid or polymer. This stability was allowed by using vesicle-forming copolymer with a membrane thickness close to the lipid bilayer thickness, thereby minimizing the hydrophobic mismatch at the domain periphery. Hybrid giant unilamellar vesicles (GUVs) with the same composition have been previously shown to be unstable and susceptible to fission, suggesting a role of curvature in the stabilization of nanodomains in these structures.

On multivalent receptor activity of GM1 in cholesterol containing membranes

Gangliosides located at the outer leaflet of plasma membrane are molecules that either participate in recognizing of exogenous ligand molecules or exhibit their own receptor activity, which are both essential phenomena for cell communication and signaling as well as for virus and toxin entry. Regulatory mechanisms of lipid-mediated recognition are primarily subjected to the physical status of the membrane in close vicinity of the receptor. Concerning the multivalent receptor activity of the ganglioside GM1, several regulatory strategies dealing with GM1 clustering and cholesterol involvement have been proposed. So far however, merely the isolated issues were addressed and no interplay between them investigated. In this work, several advanced fluorescence techniques such as Z-scan fluorescence correlation spectroscopy, Förster resonance energy transfer combined with Monte Carlo simulations, and a newly developed fluorescence antibunching assay were employed to give a more complex portrait of clustering and cholesterol involvement in multivalent ligand recognition of GM1. Our results indicate that membrane properties have an impact on a fraction of GM1 molecules that is not available for the ligand binding. While at low GM1 densities (~1 %) it is the cholesterol that turns GM1 headgroups invisible, at higher GM1 level (~4 %) it is purely the local density of GM1 molecules that inhibits the recognition. At medium GM1 content, cooperation of the two phenomena occurs. This article is part of a Special Issue entitled: Nanoscale membrane organisation and signalling.

Agomelatine strongly interacts with zwitterionic DPPC and charged DPPG membranes

Depression is one of the most common psychiatric diseases in the population. Agomelatine is a novel antidepressant drug with melatonin receptor agonistic and serotonin 5-HT2C antagonistic properties. Furthermore, being a melatonergic drug, agomelatine has the potential of being used in therapeutic applications like melatonin as an antioxidant, anti-inflammatory and antiapoptotic drug. The action mechanism of agomelatine on the membrane structure has not been clarified yet. In the present study, we aimed to investigate the interaction of agomelatine with model membranes of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylgylcerol (DPPG) by Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC). We found that agomelatine interacts with the head group in such a manner that it destabilizes the membrane architecture to a large extent. Thus, agomelatine causes alterations in the order, packing and dynamics of the DPPC and DPPG model membranes. Our results suggest that agomelatine strongly interacts with zwitterionic and charged membrane phospholipids. Because lipid structure and dynamics may have influence on the structure of membrane bound proteins and affect the signal transduction systems of membranes, these effects of agomelatine may be important in its action mechanism.

Weak glycolipid binding of a microdomain-tracer peptide correlates with aggregation and slow diffusion on cell membranes

Organized assembly or aggregation of sphingolipid-binding ligands, such as certain toxins and pathogens, has been suggested to increase binding affinity of the ligand to the cell membrane and cause membrane reorganization or distortion. Here we show that the diffusion behavior of the fluorescently tagged sphingolipid-interacting peptide probe SBD (Sphingolipid Binding Domain) is altered by modifications in the construction of the peptide sequence that both result in a reduction in binding to ganglioside-containing supported lipid membranes, and at the same time increase aggregation on the cell plasma membrane, but that do not change relative amounts of secondary structural features. We tested the effects of modifying the overall charge and construction of the SBD probe on its binding and diffusion behavior, by Surface Plasmon Resonance (SPR; Biacore) analysis on lipid surfaces, and by Fluorescence Correlation Spectroscopy (FCS) on live cells, respectively. SBD binds preferentially to membranes containing the highly sialylated gangliosides GT1b and GD1a. However, simple charge interactions of the peptide with the negative ganglioside do not appear to be a critical determinant of binding. Rather, an aggregation-suppressing amino acid composition and linker between the fluorophore and the peptide are required for optimum binding of the SBD to ganglioside-containing supported lipid bilayer surfaces, as well as for interaction with the membrane. Interestingly, the strength of interactions with ganglioside-containing artificial membranes is mirrored in the diffusion behavior by FCS on cell membranes, with stronger binders displaying similar characteristic diffusion profiles. Our findings indicate that for aggregation-prone peptides, aggregation occurs upon contact with the cell membrane, and rather than giving a stronger interaction with the membrane, aggregation is accompanied by weaker binding and complex diffusion profiles indicative of heterogeneous diffusion behavior in the probe population.

Förster resonance energy transfer (FRET) between heterogeneously distributed probes: application to lipid nanodomains and pores

The formation of membrane heterogeneities, e.g., lipid domains and pores, leads to a redistribution of donor (D) and acceptor (A) molecules according to their affinity to the structures formed and the remaining bilayer. If such changes sufficiently influence the Förster resonance energy transfer (FRET) efficiency, these changes can be further analyzed in terms of nanodomain/pore size. This paper is a continuation of previous work on this theme. In particular, it is demonstrated how FRET experiments should be planned and how data should be analyzed in order to achieve the best possible resolution. The limiting resolution of domains and pores are discussed simultaneously, in order to enable direct comparison. It appears that choice of suitable donor/acceptor pairs is the most crucial step in the design of experiments. For instance, it is recommended to use DA pairs, which exhibit an increased affinity to pores (i.e., partition coefficients K(D,A) > 10) for the determination of pore sizes with radii comparable to the Förster radius R(0). On the other hand, donors and acceptors exhibiting a high affinity to different phases are better suited for the determination of domain sizes. The experimental setup where donors and acceptors are excluded from the domains/pores should be avoided.