Ganglioside partitioning and aggregation in phase-separated monolayers characterized by bodipy GM1 monomer/dimer emission

Rafting on the Plasma Membrane: Lipid Rafts in Signaling and Disease

The plasma membrane is not a uniform phospholipid bilayer; it has specialized membrane nano- or microdomains called lipid rafts. Lipid rafts are small cholesterol and sphingolipid-rich plasma membrane islands. Although their existence was long debated, their presence in the plasma membrane of living cells is now well accepted with the advent of super-resolution imaging techniques. It is interesting to note that lipid rafts function to compartmentalize receptors and their regulators and substantially modulate cellular signaling. In this review, we will examine the role of lipid rafts and caveolae-lipid raft-like microdomains with a distinct 3D morphology-in cellular signaling. Moreover, we will investigate how raft compartmentalized signaling regulates diverse physiological processes such as proliferation, apoptosis, immune signaling, and development. Also, the deregulation of lipid raft-mediated signaling during tumorigenesis and metastasis will be explored.

Imaging Sphingomyelin- and Cholesterol-Enriched Domains in the Plasma Membrane Using a Novel Probe and Super-Resolution Microscopy

In this chapter, we show the visualization of lipid domains using a specific lipid-binding protein and super-resolution microscopy. Lipid rafts are plasma membrane domains enriched in both sphingolipids and sterols that play key roles in various physiological events. We identified a novel protein that specifically binds to a complex of sphingomyelin (SM) and cholesterol (Chol). The isolated protein, nakanori, labels the SM/Chol complex at the outer leaflet of the plasma membrane in mammalian cells. Structured illumination microscopic images suggested that the influenza virus buds from the edges of the SM/Chol domains in MDCK cells. Furthermore, a photoactivated localization microscopy analysis indicated that the SM/Chol complex forms domains in the outer leaflet, just above the phosphatidylinositol 4,5-bisphosphate domains in the inner leaflet. These observations provide significant insight into the structure and function of lipid rafts.

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.

Super-resolution optical microscopy for studying membrane structure and dynamics

Investigation of cell membrane structure and dynamics requires high spatial and temporal resolution. The spatial resolution of conventional light microscopy is limited due to the diffraction of light. However, recent developments in microscopy enabled us to access the nano-scale regime spatially, thus to elucidate the nanoscopic structures in the cellular membranes. In this review, we will explain the resolution limit, address the working principles of the most commonly used super-resolution microscopy techniques and summarise their recent applications in the biomembrane field.

GM1 Softens POPC Membranes and Induces the Formation of Micron-Sized Domains

The influence of the glycolipid GM1 on the physical properties of POPC membranes was studied systematically by using different methods applied to giant and large unilamellar vesicles. The charge per GM1 molecule in the membrane was estimated from electrophoretic mobility measurements. Optical microscopy and differential scanning calorimetry were employed to construct a partial phase diagram of the GM1/POPC system. At room temperature, phase separation in the membrane was detected for GM1 fractions at and above ∼5 mol %, whereby GM1-rich gel-like domains were observed by fluorescent microscopy. Fluctuation analysis, vesicle electrodeformation, and micropipette aspiration were used to assess the bending rigidity of the membrane as a function of GM1 content. In the fluid phase, GM1 was shown to strongly soften the bilayer. In the region of coexistence of fluid and gel-like domains, the micropipette aspiration technique allowed measurements of the bending rigidity of the fluid phase only, whereas electrodeformation and fluctuation analysis were affected by the presence of the gel-phase domains. The observation that GM1 decreased the bilayer bending rigidity is important for understanding the role of this ganglioside in the flexibility of neuronal membranes.

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

Changes in order parameters associated with ceramide-mediated membrane reorganization measured using pTIRFM

The enzymatic generation of ceramide has significant effects on the biophysical properties of lipid bilayers and can lead to the extensive reorganization of cell membranes. We have synthesized and characterized a headgroup-labeled fluorescent lipid probe (NBD-ceramide, NBD-Cer) and demonstrated that it can be used for polarized total internal reflection fluorescence microscopy experiments to probe changes in membrane order that result from ceramide incorporation. NBD-Cer measures significantly higher order parameters for the liquid-ordered (Lo) domains ([P2] = 0.40 ± 0.03) than for the liquid-disordered phase (Ld, fluid, [P2] = 0.22 ± 0.02) of phase-separated bilayers prepared from egg sphingomyelin, dioleolyphosphatidylcholine, and cholesterol mixtures. The probe also responds to changes in packing induced by the direct incorporation of ceramide or the variation in the ionic strength of the aqueous medium. Order parameter maps obtained after enzyme treatment of bilayers with coexisting Lo and Ld phases show two distinct types of behavior. In regions of high enzyme activity, the initial Lo/Ld domains are replaced by large, dark features that have high membrane order corroborating previous hypotheses that these are ceramide-enriched regions of the membrane. In areas of low enzyme activity, the size and shape of the Lo domains are conserved, but there is an increase in the order parameter for the initial Ld phase ([P2] = 0.30 ± 0.01). This is attributed to the incorporation of ceramide in the Lo domains with the concomitant expulsion of cholesterol into the surrounding fluid phase, increasing its order parameter.

Partitioning, diffusion, and ligand binding of raft lipid analogs in model and cellular plasma membranes

Several simplified membrane models featuring coexisting liquid disordered (Ld) and ordered (Lo) lipid phases have been developed to mimic the heterogeneous organization of cellular membranes, and thus, aid our understanding of the nature and functional role of ordered lipid-protein nanodomains, termed "rafts". In spite of their greatly reduced complexity, quantitative characterization of local lipid environments using model membranes is not trivial, and the parallels that can be drawn to cellular membranes are not always evident. Similarly, various fluorescently labeled lipid analogs have been used to study membrane organization and function in vitro, although the biological activity of these probes in relation to their native counterparts often remains uncharacterized. This is particularly true for raft-preferring lipids ("raft lipids", e.g. sphingolipids and sterols), whose domain preference is a strict function of their molecular architecture, and is thus susceptible to disruption by fluorescence labeling. Here, we analyze the phase partitioning of a multitude of fluorescent raft lipid analogs in synthetic Giant Unilamellar Vesicles (GUVs) and cell-derived Giant Plasma Membrane Vesicles (GPMVs). We observe complex partitioning behavior dependent on label size, polarity, charge and position, lipid headgroup, and membrane composition. Several of the raft lipid analogs partitioned into the ordered phase in GPMVs, in contrast to fully synthetic GUVs, in which most raft lipid analogs mis-partitioned to the disordered phase. This behavior correlates with the greatly enhanced order difference between coexisting phases in the synthetic system. In addition, not only partitioning, but also ligand binding of the lipids is perturbed upon labeling: while cholera toxin B binds unlabeled GM1 in the Lo phase, it binds fluorescently labeled GMI exclusively in the Ld phase. Fluorescence correlation spectroscopy (FCS) by stimulated emission depletion (STED) nanoscopy on intact cellular plasma membranes consistently reveals a constant level of confined diffusion for raft lipid analogs that vary greatly in their partitioning behavior, suggesting different physicochemical bases for these phenomena.

Ganglioside influence on phospholipid films investigated with single molecule fluorescence measurements

Single molecule fluorescence measurements are used to probe the effects of GM1 in DPPC monolayers. Langmuir-Blodgett films of GM1 and DPPC were doped with ~10(-8) mol % of the fluorescent lipid probe, BODIPY-PC, and transferred onto glass substrates at 23 mN/m. As shown previously, the individual orientation of each BODIPY-PC probe in the membrane can be measured using defocused polarized total internal reflection fluorescence microscopy, revealing changes in film properties at the molecular level. Here, BODIPY-PC tilt angle histograms are used to characterize the effects of GM1 in DPPC films from 0.05 to 100 mol % GM1. At high GM1 levels (>5 mol % GM1), trends in the single molecule measurements agree with previous bulk measurements showing the turnover from condensing to expanding influence of GM1 at 15-20 mol %, thus validating the single molecule approach. At biologically relevant, low concentrations of GM1 (<5 mol % GM1), where bulk fluorescence measurements are less informative, the single molecule measurements reveal a marked influence of GM1 on film properties. The addition of trace amounts of GM1 to DPPC films leads to an expansion of the film which continues to 0.10 mol % GM1, above which the trend reverses and the condensing effect previously noted is observed.

Super-resolved FT-IR spectroscopy: Strategies, challenges, and opportunities for membrane biophysics

Direct correlation of molecular conformation with local structure is critical to studies of protein- and peptide-membrane interactions, particularly in the context of membrane-facilitated aggregation, and disruption or disordering. Infrared spectroscopy has long been a mainstay for determining molecular conformation, following folding dynamics, and characterizing reactions. While tremendous advances have been made in improving the spectral and temporal resolution of infrared spectroscopy, it has only been with the introduction of scanned-probe techniques that exploit the raster-scanning tip as either a source, scattering tool, or measurement probe that researchers have been able to obtain sub-diffraction limit IR spectra. This review will examine the history of correlated scanned-probe IR spectroscopies, from their inception to their use in studies of molecular aggregates, membrane domains, and cellular structures. The challenges and opportunities that these platforms present for examining dynamic phenomena will be discussed. This article is part of a Special Issue entitled: FTIR in membrane proteins and peptide studies.

Correlative optical and scanning probe microscopies for mapping interactions at membranes

Innovative approaches for real-time imaging on molecular-length scales are providing researchers with powerful strategies for characterizing molecular and cellular structures and dynamics. Combinatorial techniques that integrate two or more distinct imaging modalities are particularly compelling as they provide a means for overcoming the limitations of the individual modalities and, when applied simultaneously, enable the collection of rich multi-modal datasets. Almost since its inception, scanning probe microscopy has closely associated with optical microscopy. This is particularly evident in the fields of cellular and molecular biophysics where researchers are taking full advantage of these real-time, in situ, tools to acquire three-dimensional molecular-scale topographical images with nanometer resolution, while simultaneously characterizing their structure and interactions though conventional optical microscopy. The ability to apply mechanical or optical stimuli provides an additional experimental dimension that has shown tremendous promise for examining dynamic events on sub-cellular length scales. In this chapter, we describe recent efforts in developing these integrated platforms, the methodology for, and inherent challenges in, performing coupled imaging experiments, and the potential and future opportunities of these research tools for the fields of molecular and cellular biophysics with a specific emphasis on the application of these coupled approaches for the characterization of interactions occurring at membrane interfaces.

Atomic force microscopy: a versatile tool to probe the physical and chemical properties of supported membranes at the nanoscale

Atomic force microscopy (AFM) was developed in the 1980s following the invention of its precursor, scanning tunneling microscopy (STM), earlier in the decade. Several modes of operation have evolved, demonstrating the extreme versatility of this method for measuring the physicochemical properties of samples at the nanoscopic scale. AFM has proved an invaluable technique for visualizing the topographic characteristics of phospholipid monolayers and bilayers, such as roughness, height or laterally segregated domains. Implemented modes such as phase imaging have also provided criteria for discriminating the viscoelastic properties of different supported lipid bilayer (SLB) regions. In this review, we focus on the AFM force spectroscopy (FS) mode, which enables determination of the nanomechanical properties of membrane models. The interpretation of force curves is presented, together with newly emerging techniques that provide complementary information on physicochemical properties that may contribute to our understanding of the structure and function of biomembranes. Since AFM is an imaging technique, some basic indications on how real-time AFM imaging is evolving are also presented at the end of this paper.

Single plasmonic nanoparticle tracking studies of solid supported bilayers with ganglioside lipids

Single-particle tracking experiments were carried out with gold nanoparticle-labeled solid supported lipid bilayers (SLBs) containing increasing concentrations of ganglioside (GM(1)). The negatively charged nanoparticles electrostatically associate with a small percentage of positively charged lipids (ethyl phosphatidylcholine) in the bilayers. The samples containing no GM(1) show random diffusion in 92% of the particles examined with a diffusion constant of 4.3(±4.5) × 10(-9) cm(2)/s. In contrast, samples containing 14% GM(1) showed a mixture of particles displaying both random and confined diffusion, with the majority of particles, 62%, showing confined diffusion. Control experiments support the notion that the nanoparticles are not associating with the GM(1) moieties but instead most likely confined to regions in between the GM(1) clusters. Analysis of the root-mean-squared displacement plots for all of the data reveals decreasing trends in the confined diffusion constant and diameter of the confining region versus increasing GM(1) concentration. In addition, a linearly decreasing trend is observed for the percentage of randomly diffusing particles versus GM(1) concentration, which offers a simple, direct way to measure the percolation threshold for this system, which has not previously been measured. The percolation threshold is found to be 22% GM(1) and the confining diameter at the percolation threshold only ∼50 nm.

Fluorescence techniques to study lipid dynamics

Biological research has always tremendously benefited from the development of key methodology. In fact, it was the advent of microscopy that shaped our understanding of cells as the fundamental units of life. Microscopic techniques are still central to the elucidation of biological units and processes, but equally important are methods that allow access to the dimension of time, to investigate the dynamics of molecular functions and interactions. Here, fluorescence spectroscopy with its sensitivity to access the single-molecule level, and its large temporal resolution, has been opening up fully new perspectives for cell biology. Here we summarize the key fluorescent techniques used to study cellular dynamics, with the focus on lipid and membrane systems.

Atomic force microscopy study of ganglioside GM1 concentration effect on lateral phase separation of sphingomyelin/dioleoylphosphatidylcholine/cholesterol bilayers

The effect of monosialoganglioside GM1 (GM1) concentration on the lateral phase separation in the sphingomyelin/1,2-dioleoyl-sn-glycero-3-phosphocholine/cholesterol (SM/DOPC/Chol) bilayers was studied by using atomic force microscopy. The results show that, with the increase of GM1 mol fraction (x), the dominant composition of liquid-ordered (L(o)) domains changes from SM to SM/GM1 and finally to GM1. Meanwhile, the decrease of domain area (A) of the L(o) phase with the increase of x follows a scaling law of A ∼ x(-3/2), for x > 0.005, indicating that the domain growth is pinned with high GM1 concentration. Results of in situ experiments of GM1 insertion into SM/DOPC/cholesterol bilayers further supported our observations.

The European nanometrology landscape

This review paper summarizes the European nanometrology landscape from a technical perspective. Dimensional and chemical nanometrology are discussed first as they underpin many of the developments in other areas of nanometrology. Applications for the measurement of thin film parameters are followed by two of the most widely relevant families of functional properties: measurement of mechanical and electrical properties at the nanoscale. Nanostructured materials and surfaces, which are seen as key materials areas having specific metrology challenges, are covered next. The final section describes biological nanometrology, which is perhaps the most interdisciplinary applications area, and presents unique challenges. Within each area, a review is provided of current status, the capabilities and limitations of current techniques and instruments, and future directions being driven by emerging industrial measurement requirements. Issues of traceability, standardization, national and international programmes, regulation and skills development will be discussed in a future paper.

Near-field scanning optical microscopy: a tool for nanometric exploration of biological membranes

Near-field scanning optical microscopy (NSOM) is an emerging optical technique that enables simultaneous high-resolution fluorescence and topography measurements. Here we discuss selected applications of NSOM to biological systems that help illustrate the utility of its high spatial resolution and simultaneous collection of both fluorescence and topography. For the biological sciences, these attributes seem particularly well suited for addressing ongoing issues in membrane organization, such as those regarding lipid rafts, and protein-protein interactions. Here we highlight a few NSOM measurements on model membranes, isolated biological membranes, and cultured cells that help illustrate some of these capabilities. We finish by highlighting nontraditional applications of NSOM that take advantage of the small probe to create nanometric sensors or new modes of imaging.

Chemical mapping of ceramide distribution in sphingomyelin-rich domains in monolayers

The incorporation of ceramide in phase-separated monolayers of ternary lipid mixtures has been studied by a combination of atomic force microscopy (AFM), fluorescence, and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Replacement of a fraction of the sphingomyelin by ceramide in DOPC/SM/cholesterol monolayers leads to changes in the SM-cholesterol-rich liquid-ordered domains. AFM shows the formation of heterogeneous domains with small raised islands that are assigned to a ceramide-rich gel phase. ToF-SIMS provides conclusive evidence for the localization of SM and ceramide in ordered domains and shows that ceramide is heterogeneously distributed in small islands throughout the domains. The results indicate the utility of combining AFM and ToF-SIMS for understanding compositions of phase-separated membranes.

Structural aspects of ganglioside-containing membranes

The demand for understanding the physical role of gangliosides in membranes is pressing, due to the high number of diverse and crucial biological functions in which they are involved, needing a unifying thread. To this purpose, model systems including gangliosides have been subject of extensive structural studies. Although showing different levels of complication, all models share the need for simplicity, in order to allow for physico-chemical clarity, so they keep far from the extreme complexity of the true biological systems. Nonetheless, as widely agreed, they provide a basic hint on the structural contribution specific molecules can pay to the complex aggregate. This topic we address in the present review. Gangliosides are likely to play their physical role through metamorphism, cooperativity and demixing, that is, they tend to segregate and identify regions where they can dictate and modulate the geometry and the topology of the structure, and its mechanical properties. Strong three-dimensional organisation and cooperativity are exploited to scale up the local arrangement hierarchically from the nano- to the mesoscale, influencing the overall morphology of the structure.

Molecular level investigation of organization in ternary lipid bilayer: a computational approach

The differential organization of lipid components in a multicomponent membrane leads to formation of domains having diverse composition and size. Cholesterol and glycosphingolipids are known to be important components of such lateral assembly. We report here the ordering of cholesterol around ganglioside GM1 and the nature of the cluster from an all-atom simulation of a ternary lipid system. The results are compared with a binary bilayer and a pure phospholipid bilayer. The difference in molecular rearrangements in ternary and binary lipid mixture shows the role of GM1 in the rearrangement of cholesterol. Calculation of the radial distribution function, rotational reorientation, and residence time analysis of cholesterol shows that cholesterol is preferentially accumulating near gangliosides, while the lateral translational motion, rotational diffusion, and order parameter of phospholipids characterize the amount of rigidity imparted on the phospholipid bilayer.

Combined scanning probe and total internal reflection fluorescence microscopy

Combining scanning probe and optical microscopy represents a powerful approach for investigating structure-function relationships and dynamics of biomolecules and biomolecular assemblies, often in situ and in real-time. This platform technology allows us to obtain three-dimensional images of individual molecules with nanometer resolution, while simultaneously characterizing their structure and interactions though complementary techniques such as optical microscopy and spectroscopy. We describe herein the practical strategies for the coupling of scanning probe and total internal reflection fluorescence microscopy along with challenges and the potential applications of such platforms, with a particular focus on their application to the study of biomolecular interactions at membrane surfaces.

Synthesis and spectral properties of cholesterol- and FTY720-containing boron dipyrromethene dyes

Two analogues (1, 2) of free cholesterol and one analogue (3) of the immunosuppressive sphingolipid FTY720 containing a boron dipyrromethene chromophore (BODIPY) were synthesized. The synthetic routes involved preparation of boron dipyrromethene moieties (5, 11), bearing a phenylethynyl group at different positions of the chromophore, and lipids (13, 20) bearing an azido group. The dye was tethered to the lipid via a 1,2,3-triazole in the linker by the click reaction. Analogues derived from 11 [in which an (E)-styrylethynyl moiety is bonded to C-5 of BODIPY] exhibited a marked red shift (approximately 70-80 nm) compared with those derived from 5 (in which a phenylethynyl moiety is bonded to C-8 of BODIPY).