Correlated fluorescence-atomic force microscopy of membrane domains: structure of fluorescence probes determines lipid localization

Kinetic Mechanism of Surfactant-Based Molecular Recognition: Selective Permeability across an Oil-Water Interface Regulated by Supramolecular Aggregates

Lipids are known to play a vital role in the molecular organization of all cellular life. Molecular recognition is another fundamental biological process that is generally attributed to biological polymers, such as proteins and nucleic acids. However, there is evidence that aggregates of lipids and lipid-like molecules are also capable of selectively binding to or regulating the partitioning of other molecules. We previously demonstrated that a model two-phase octanol/water system can selectively partition Red 40 and Blue 1 dyes added to an aqueous phase, with the selectivity depending on the surfactant (e.g., cetyltrimethylammonium bromide) dissolved in the organic phase. Here, we elucidate the mechanism of molecular recognition in this system by using quantitative partitioning experiments and molecular dynamics (MD) simulations. Our results indicate that the selectivity for the red dye is thermodynamically favored at all surfactant concentrations, while selectivity for the blue dye is kinetically favored at high surfactant concentrations. The kinetic selectivity for the blue dye correlates with the presence of molecular aggregation at the oil-water interface. Coarse-grained MD simulations elucidate nanoscale supramolecular structures that can preferentially bind one small molecule rather than another at an interface, providing a selectively permeable barrier in the absence of proteins. The results suggest a new supramolecular mechanism for molecular recognition with potential applications in drug delivery, drug discovery, and biosensing.

Visualization of accessible cholesterol using a GRAM domain-based biosensor

Cholesterol is important for membrane integrity and cell signaling, and dysregulation of the distribution of cellular cholesterol is associated with numerous diseases, including neurodegenerative disorders. While regulated transport of a specific pool of cholesterol, known as "accessible cholesterol", contributes to the maintenance of cellular cholesterol distribution and homeostasis, tools to monitor accessible cholesterol in live cells remain limited. Here, we engineer a highly sensitive accessible cholesterol biosensor by taking advantage of the cholesterol-sensing element (the GRAM domain) of an evolutionarily conserved lipid transfer protein, GRAMD1b. Using this cholesterol biosensor, which we call GRAM-W, we successfully visualize in real time the distribution of accessible cholesterol in many different cell types, including human keratinocytes and iPSC-derived neurons, and show differential dependencies on cholesterol biosynthesis and uptake for maintaining levels of accessible cholesterol. Furthermore, we combine GRAM-W with a dimerization-dependent fluorescent protein (ddFP) and establish a strategy for the ultrasensitive detection of accessible plasma membrane cholesterol. These tools will allow us to obtain important insights into the molecular mechanisms by which the distribution of cellular cholesterol is regulated.

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.

Refinement of Singer-Nicolson fluid-mosaic model by microscopy imaging: Lipid rafts and actin-induced membrane compartmentalization

This year celebrates the 50th anniversary of the Singer-Nicolson fluid mosaic model for biological membranes. The next level of sophistication we have achieved for understanding plasma membrane (PM) structures, dynamics, and functions during these 50 years includes the PM interactions with cortical actin filaments and the partial demixing of membrane constituent molecules in the PM, particularly raft domains. Here, first, we summarize our current knowledge of these two structures and emphasize that they are interrelated. Second, we review the structure, molecular dynamics, and function of raft domains, with main focuses on raftophilic glycosylphosphatidylinositol-anchored proteins (GPI-APs) and their signal transduction mechanisms. We pay special attention to the results obtained by single-molecule imaging techniques and other advanced microscopy methods. We also clarify the limitations of present optical microscopy methods for visualizing raft domains, but emphasize that single-molecule imaging techniques can "detect" raft domains associated with molecules of interest in the PM.

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.

Cellular cholesterol and how to find it

Cholesterol is an essential component of eukaryotic cellular membranes. Information about its subcellular localization and transport pathways inside cells are key for the understanding and treatment of cholesterol-related diseases. In this review we give an overview over the most commonly used methods that contributed to our current understanding of subcellular cholesterol localization and transport routes. First, we discuss methods that provide insights into cholesterol metabolism based on readouts of downstream effects such as esterification. Subsequently, we focus on the use of cholesterol-binding molecules as probes that facilitate visualization and quantification of sterols inside of cells. Finally, we explore different analogues of cholesterol which, when taken up by living cells, are integrated and transported in a similar fashion as endogenous sterols. Taken together, we highlight the challenges and advantages of each method such that researchers studying aspects of cholesterol transport may choose the most pertinent approach for their problem.

Fluorescence strategies for mapping cell membrane dynamics and structures

Fluorescence spectroscopy has been a cornerstone of research in membrane dynamics and organization. Technological advances in fluorescence spectroscopy went hand in hand with discovery of various physicochemical properties of membranes at nanometric spatial and microsecond timescales. In this perspective, we discuss the various challenges associated with quantification of physicochemical properties of membranes and how various modes of fluorescence spectroscopy have overcome these challenges to shed light on the structure and organization of membranes. Finally, we discuss newer measurement strategies and data analysis tools to investigate the structure, dynamics, and organization of membranes.

Lipid vesicle composition influences the incorporation and fluorescence properties of the lipophilic sulphonated carbocyanine dye SP-DiO

Lipid membrane composition influences insertion efficiency and photophysical properties of lipophilic membrane-inserting dyes.

The Effect of Charge on Melittin-Induced Changes in Membrane Structure and Morphology

The binding of melittin to a range of phospholipid bilayers was studied using dual polarisation interferometry and atomic force microscopy. The phospholipid model membranes included zwitterionic dimyristylphosphatidylcholine (DMPC), together with mixtures of DMPC/dimyristylphosphatidylglycerol (DMPG) and DMPC/DMPG/cholesterol. Melittin caused significant disruption on all bilayers, but differences in morphological changes during binding were different on each membrane. Overall, the results demonstrate that the process of membrane disruption follows distinct structural changes for different lipid mixtures irrespective of the strength of binding to the membrane surface.

Raman Spectroscopy Study of Curvature-Mediated Lipid Packing and Sorting in Single Lipid Vesicles

Cellular plasma membrane deformability and stability is important in a range of biological processes. Changes in local curvature of the membrane affect the lateral movement of lipids, affecting the biophysical properties of the membrane. An integrated holographic optical tweezers and Raman microscope was used to investigate the effect of curvature gradients induced by optically stretching individual giant unilamellar vesicles (GUVs) on lipid packing and lateral segregation of cholesterol in the bilayer. The spatially resolved Raman analysis enabled detection of induced phase separation and changes in lipid ordering in individual GUVs. Using deuterated cholesterol, the changes in lipid ordering and phase separation were linked to lateral sorting of cholesterol in the stretched GUVs. Stretching the GUVs in the range of elongation factors 1-1.3 led to an overall decrease in cholesterol concentration at the edges compared to the center of stretched GUVs. The Raman spectroscopy results were consistent with a model of the bilayer accounting for cholesterol sorting in both bilayer leaflets, with a compositional asymmetry of 0.63 ± 0.04 in favor of the outer leaflet. The results demonstrate the potential of the integrated holographic optical tweezers-Raman technique to induce deformations to individual lipid vesicles and to simultaneously provide quantitative and spatially resolved molecular information. Future studies can extend to include more realistic models of cell membranes and potentially live cells.

Computational Characterization of a Cholesterol-Based Molecular Rotor in Lipid Membranes

Biophysical properties of cellular membranes critically depend on their content of cholesterol and its interaction with various other lipid species. Cholesterol-dependent friction at the nanoscale can be studied with molecular rotors, whose quantum yield depends on rotational dynamics of functional groups during their excited state lifetime. Here, we present a detailed computational analysis of a phenyl-BODIPY-linked cholesterol based molecular rotor in direct comparison with the well-known TopFluor-cholesterol. We describe a new parametrization strategy of force field parameters for the BODIPY moiety and carry out extensive molecular dynamics simulations of the probe in membranes in the absence or presence of cholesterol. Our study quantifies the extent of membrane perturbation by these probes, analyzes their tilting resistance in the bilayer and derives dynamic properties directly related to the rotor propensity. We show that phenyl-BODIPY-cholesterol bears potential as a cholesterol-dependent molecular rotor to report about microviscosity of sterol-containing model and cell membranes.

Mechanism of local anesthetic-induced disruption of raft-like ordered membrane domains

BACKGROUND

Because ordered membrane domains, called lipid rafts, regulate activation of ion channels related to the nerve pulse, lipids rafts are thought to be a possible target for anesthetic molecules. To understand the mechanism of anesthetic action, we examined influence of representative local anesthetics (LAs); dibucaine, tetracaine, and lidocaine, on raft-like liquid-ordered (L_o)/non-raft-like liquid-disordered (L_d) phase separation.

METHODS

Impact of LAs on the phase separation was observed by fluorescent microscopy. LA-induced perturbation of the L_o and L_d membranes was examined by DPH anisotropy measurements. Incorporation of LAs to the membranes was examined by fluorescent anisotropy of LAs. The biding location of the LAs was indicated by small angle x-ray diffraction (SAXD).

RESULTS

Fluorescent experiments showed that dibucaine eliminated the phase separation the most effectively, followed by tetracaine and lidocaine. The disruption of the phase separation can be explained by their disordering effects on the L_o membrane. SAXD and other experiments further suggested that dibucaine's most potent perturbation of the L_o membrane is attributable to its deeper immersion and bulky molecular structure. Tetracaine, albeit immersed in the L_o membrane as deeply as dibucaine, less perturbs the L_o membrane probably because of its smaller bulkiness. Lidocaine hardly reaches the hydrophobic region, resulting in the weakest L_o membrane perturbation.

CONCLUSION

Dibcaine perturbs the L_o membrane the most effectively, followed by tetracaine and lidocaine. This ranking correlates with their anesthetic potency.

GENERAL SIGNIFICANCE

This study suggests a possible mechanistic link between anesthetic action and perturbation of lipid rafts.

Toxic HypF-N Oligomers Selectively Bind the Plasma Membrane to Impair Cell Adhesion Capability

The deposition of fibrillar protein aggregates in human organs is the hallmark of several pathological states, including highly debilitating neurodegenerative disorders and systemic amyloidoses. It is widely accepted that small oligomers arising as intermediates in the aggregation process, released by fibrils, or growing in secondary nucleation steps are the cytotoxic entities in protein-misfolding diseases, notably neurodegenerative conditions. Increasing evidence indicates that cytotoxicity is triggered by the interaction between nanosized protein aggregates and cell membranes, even though little information on the molecular details of such interaction is presently available. In this work, we propose what is, to our knowledge, a new approach, based on the use of single-cell force spectroscopy applied to multifunctional substrates, to study the interaction between protein oligomers, cell membranes, and/or the extracellular matrix. We compared the interaction of single Chinese hamster ovary cells with two types of oligomers (toxic and nontoxic) grown from the N-terminal domain of the Escherichia coli protein HypF. We were able to quantify the affinity between both oligomer type and the cell membrane by measuring the mechanical work needed to detach the cells from the aggregates, and we could discriminate the contributions of the membrane lipid and protein fractions to such affinity. The fundamental role of the ganglioside GM1 in the membrane-oligomers interaction was also highlighted. Finally, we observed that the binding of toxic oligomers to the cell membrane significantly affects the functionality of adhesion molecules such as Arg-Gly-Asp binding integrins, and that this effect requires the presence of the negatively charged sialic acid moiety of GM1.

Sphingolipids and lipid rafts: Novel concepts and methods of analysis

About twenty years ago, the functional lipid raft model of the plasma membrane was published. It took into account decades of research showing that cellular membranes are not just homogenous mixtures of lipids and proteins. Lateral anisotropy leads to assembly of membrane domains with specific lipid and protein composition regulating vesicular traffic, cell polarity, and cell signaling pathways in a plethora of biological processes. However, what appeared to be a clearly defined entity of clustered raft lipids and proteins became increasingly fluid over the years, and many of the fundamental questions about biogenesis and structure of lipid rafts remained unanswered. Experimental obstacles in visualizing lipids and their interactions hampered progress in understanding just how big rafts are, where and when they are formed, and with which proteins raft lipids interact. In recent years, we have begun to answer some of these questions and sphingolipids may take center stage in re-defining the meaning and functional significance of lipid rafts. In addition to the archetypical cholesterol-sphingomyelin raft with liquid ordered (Lo) phase and the liquid-disordered (Ld) non-raft regions of cellular membranes, a third type of microdomains termed ceramide-rich platforms (CRPs) with gel-like structure has been identified. CRPs are "ceramide rafts" that may offer some fresh view on the membrane mesostructure and answer several critical questions for our understanding of lipid rafts.

Three-Dimensional Heterogeneous Structure Formation on a Supported Lipid Bilayer Disclosed by Single-Particle Tracking

Three-dimensional (3D) single-particle tracking was employed to study the lipid membrane morphology change at different pHs on glass supported lipid bilayers (SLBs) [1,2-dioleoyl- sn-glycero-3-phosphoethanolamine/1,2-dioleoyl- sn-glycero-3-phospho-l-serine (sodium salt)/1,2-dioleoyl- sn-glycero-3-phosphocholine = 5:3:2]. Fluorescently tagged, carboxylated polystyrene nanoparticles (of 100 nm) were used as the probes. At neutral pHs, the particles' diffusion was close to two-dimensional Brownian motion, indicating a mainly planar structure of the SLBs. When the environmental pH was tuned to be basic at 10.0, transiently confined diffusions within small areas were frequently observed. These confinements had a lateral dimension of 100-200 nm. Most interestingly, they showed 3D bulged structures protruding from the planar lipid bilayer. The particles were trapped by these 3D structures for a short period of time (∼0.75 s), with an estimated escape activation energy of ∼4.2 k_B T. Nonuniform distribution of pH-sensitive lipids in the membrane was proposed to explain the formation of these 3D heterogeneous structures. This work suggests that the geometry of the 3D lipid structures can play a role in tuning the particle-lipid surface interactions. It sheds new light on the origin of lateral heterogeneity on the lipid membrane.

Cholesterol is enriched in the sphingolipid patches on the substrate near nonpolarized MDCK cells, but not in the sphingolipid domains in their plasma membranes

Information about the distributions of cholesterol and sphingolipids within the plasma membranes of mammalian cells provides insight into the roles of these molecules in membrane function. In this report, high-resolution secondary ion mass spectrometry was used to image the distributions of metabolically incorporated rare isotope-labeled sphingolipids and cholesterol on the surfaces of nonpolarized epithelial cells. Sphingolipid domains that were not enriched with cholesterol were detected in the plasma membranes of subconfluent Madin-Darby canine kidney cells. Surprisingly, cholesterol-enriched sphingolipid patches were observed on the substrate adjacent to these cells. Based on the shapes of these cholesterol-enriched sphingolipid patches on the substrate and their proximity to cellular projections, we hypothesize that they are deposits of membranous particles released by the cell.

High-resolution mapping and recognition of lipid domains using AFM with toxin-derivatized probes

Specific mapping using AFM tips derivatized with toxin fragments targeting specific lipids as a novel approach to evidence lateral lipid heterogeneities at high-resolution.

Continuous Pore-Spanning Lipid Bilayers on Silicon Oxide-Coated Porous Substrates

A number of techniques has been developed and analyzed in recent years to generate pore-spanning membranes (PSMs). While quite a number of methods rely on nanoporous substrates, only a few use micrometer-sized pores to be able to individually resolve suspending membranes by means of fluorescence microscopy. To be able to produce PSMs on pores that are micrometer in size, an orthogonal functionalization strategy resulting in a hydrophilic surface is highly desirable. Here, we report on a method to prepare PSMs based on the evaporation of a thin layer of silicon monoxide on top of the porous substrate. PM-IRRAS experiments demonstrate that the final surface is composed of SiO_x with 1 < x < 2. The hydrophilic surface turned out to be well suited to spread giant unilamellar vesicles forming PSMs. As the method does not rely on a gold coating as frequently used for orthogonal functionalization, fluorescence micrographs provide information not only from the freestanding membrane areas but also from the supported ones. The observation of the entire PSM area enabled us to observe phase-separation in these membranes on the freestanding and supported parts as well as protein binding and possible lipid reorganization of the membranes induced by binding of the protein Shiga toxin.

NanoSIMS for biological applications: Current practices and analyses

Secondary ion mass spectrometry (SIMS) has become an increasingly utilized tool in biologically relevant studies. Of these, high lateral resolution methodologies using the NanoSIMS 50/50L have been especially powerful within many biological fields over the past decade. Here, the authors provide a review of this technology, sample preparation and analysis considerations, examples of recent biological studies, data analyses, and current outlooks. Specifically, the authors offer an overview of SIMS and development of the NanoSIMS. The authors describe the major experimental factors that should be considered prior to NanoSIMS analysis and then provide information on best practices for data analysis and image generation, which includes an in-depth discussion of appropriate colormaps. Additionally, the authors provide an open-source method for data representation that allows simultaneous visualization of secondary electron and ion information within a single image. Finally, the authors present a perspective on the future of this technology and where they think it will have the greatest impact in near future.

Structural templating of J-aggregates: Visualizing bis(monoacylglycero)phosphate domains in live cells

Identifying the key structural and dynamical determinants that drive the association of biomolecules, whether in solution, or perhaps more importantly in a membrane environment, has critical implications for our understanding of cellular dynamics, processes, and signaling. With recent advances in high-resolution imaging techniques, from the development of new molecular labels to technical advances in imaging methodologies and platforms, researchers are now reaping the benefits of being able to directly characterize and quantify local dynamics, structures, and conformations in live cells and tissues. These capabilities are providing unique insights into association stoichiometries, interactions, and structures on sub-micron length scales. We previously examined the role of lipid headgroup chemistry and phase state in guiding the formation of pseudoisocyanine (PIC) dye J-aggregates on supported planar bilayers [Langmuir, 25, 10719]. We describe here how these same J-aggregates can report on the in situ formation of organellar membrane domains in live cells. Live cell hyperspectral confocal microscopy using GFP-conjugated GTPase markers of early (Rab5) and late (Rab7) endosomes revealed that the PIC J-aggregates were confined to domains on either the limiting membrane or intralumenal vesicles (ILV) of late endosomes, known to be enriched in the anionic lipid bis(monoacylglycero)phosphate (BMP). Correlated confocal fluorescence - atomic force microscopy performed on endosomal membrane-mimetic supported planar lipid bilayers confirmed BMP-specific templating of the PIC J-aggregates. These data provide strong evidence for the formation of BMP-rich lipid domains during multivesicular body formation and portend the application of structured dye aggregates as markers of cellular membrane domain structure, size, and formation.

Characterization of Plasma Membrane Ceramides by Super-Resolution Microscopy

The sphingolipid ceramide regulates cellular processes such as differentiation, proliferation, growth arrest, and apoptosis. Ceramide-rich membrane areas promote structural changes within the plasma membrane that segregate membrane receptors and affect membrane curvature and vesicle formation, fusion, and trafficking. Ceramides were labeled by immunocytochemistry to visualize their distribution on the plasma membrane of different cells with virtually molecular resolution by direct stochastic optical reconstruction microscopy (dSTORM). Super-resolution images show that independent of labeling conditions and cell type 50-60 % of all membrane ceramides are located in ceramide-rich platforms (CRPs) with a size of about 75 nm that are composed of at least about 20 ceramides. Treatment of cells with Bacillus cereus sphingomyelinase (bSMase) increases the overall ceramide concentration in the plasma membrane, the quantity of CRPs, and their size. Simultaneously, the ceramide concentration in CRPs increases approximately twofold.

Sphingolipid Organization in the Plasma Membrane and the Mechanisms That Influence It

Sphingolipids are structural components in the plasma membranes of eukaryotic cells. Their metabolism produces bioactive signaling molecules that modulate fundamental cellular processes. The segregation of sphingolipids into distinct membrane domains is likely essential for cellular function. This review presents the early studies of sphingolipid distribution in the plasma membranes of mammalian cells that shaped the most popular current model of plasma membrane organization. The results of traditional imaging studies of sphingolipid distribution in stimulated and resting cells are described. These data are compared with recent results obtained with advanced imaging techniques, including super-resolution fluorescence detection and high-resolution secondary ion mass spectrometry (SIMS). Emphasis is placed on the new insight into the sphingolipid organization within the plasma membrane that has resulted from the direct imaging of stable isotope-labeled lipids in actual cell membranes with high-resolution SIMS. Super-resolution fluorescence techniques have recently revealed the biophysical behaviors of sphingolipids and the unhindered diffusion of cholesterol analogs in the membranes of living cells are ultimately in contrast to the prevailing hypothetical model of plasma membrane organization. High-resolution SIMS studies also conflicted with the prevailing hypothesis, showing sphingolipids are concentrated in micrometer-scale membrane domains, but cholesterol is evenly distributed within the plasma membrane. Reductions in cellular cholesterol decreased the number of sphingolipid domains in the plasma membrane, whereas disruption of the cytoskeleton eliminated them. In addition, hemagglutinin, a transmembrane protein that is thought to be a putative raft marker, did not cluster within sphingolipid-enriched regions in the plasma membrane. Thus, sphingolipid distribution in the plasma membrane is dependent on the cytoskeleton, but not on favorable interactions with cholesterol or hemagglutinin. The alternate views of plasma membrane organization suggested by these findings are discussed.

Fluorescent Sterols and Cholesteryl Esters as Probes for Intracellular Cholesterol Transport

Cholesterol transport between cellular organelles comprised vesicular trafficking and nonvesicular exchange; these processes are often studied by quantitative fluorescence microscopy. A major challenge for using this approach is producing analogs of cholesterol with suitable brightness and structural and chemical properties comparable with those of cholesterol. This review surveys currently used fluorescent sterols with respect to their behavior in model membranes, their photophysical properties, as well as their transport and metabolism in cells. In the first part, several intrinsically fluorescent sterols, such as dehydroergosterol or cholestatrienol, are discussed. These polyene sterols (P-sterols) contain three conjugated double bonds in the steroid ring system, giving them slight fluorescence in ultraviolet light. We discuss the properties of P-sterols relative to cholesterol, outline their chemical synthesis, and explain how to image them in living cells and organisms. In particular, we show that P-sterol esters inserted into low-density lipoprotein can be tracked in the fibroblasts of Niemann–Pick disease using high-resolution deconvolution microscopy. We also describe fluorophore-tagged cholesterol probes, such as BODIPY-, NBD-, Dansyl-, or Pyrene-tagged cholesterol, and eventual esters of these analogs. Finally, we survey the latest developments in the synthesis and use of alkyne cholesterol analogs to be labeled with fluorophores by click chemistry and discuss the potential of all approaches for future applications.

BODIPY–steroid conjugates: Syntheses and biological applications

BODIPY and aza-BODIPY fluorophores linked to steroids are being developed as multimodal-imaging agents to monitor the mechanism of action of biologically active components in living systems.

Detectors for evaluating the cellular landscape of sphingomyelin- and cholesterol-rich membrane domains

Although sphingomyelin and cholesterol are major lipids of mammalian cells, the detailed distribution of these lipids in cellular membranes remains still obscure. However, the recent development of protein probes that specifically bind sphingomyelin and/or cholesterol provides new information about the landscape of the lipid domains that are enriched with sphingomyelin or cholesterol or both. Here, we critically summarize the tools to study distribution and dynamics of sphingomyelin and cholesterol. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

The structure and function of cell membranes examined by atomic force microscopy and single-molecule force spectroscopy

The cell membrane is one of the most complicated biological complexes, and long-term fierce debates regarding the cell membrane persist because of technical hurdles. With the rapid development of nanotechnology and single-molecule techniques, our understanding of cell membranes has substantially increased. Atomic force microscopy (AFM) has provided several unprecedented advances (e.g., high resolution, three-dimensional and in situ measurements) in the study of cell membranes and has been used to systematically dissect the membrane structure in situ from both sides of membranes; as a result, novel models of cell membranes have recently been proposed. This review summarizes the new progress regarding membrane structure using in situ AFM and single-molecule force spectroscopy (SMFS), which may shed light on the study of the structure and functions of cell membranes.

Molecular modeling of lipid probes and their influence on the membrane

In this review a number of Molecular Dynamics simulation studies are discussed which focus on the understanding of the behavior of lipid probes in biomembranes. Experiments often use specialized probe moieties or molecules to report on the behavior of a membrane and try to gain information on the membrane as a whole from the probe lipids as these probes are the only things an experiment sees. Probes can be used to make NMR, EPR and fluorescence accessible to the membrane and use fluorescent or spin-active moieties for this purpose. Clearly membranes with and without probes are not identical which makes it worthwhile to elucidate the differences between them with detailed atomistic simulations. In almost all cases these differences are confined to the local neighborhood of the probe molecules which are sparsely used and generally present as single molecules. In general, the behavior of the bulk membrane lipids can be qualitatively understood from the probes but in most cases their properties cannot be directly quantitatively deduced from the probe behavior. This article is part of a Special Issue entitled: Biosimulations edited by Ilpo Vattulainen and Tomasz Róg.

Bright and photostable push-pull pyrene dye visualizes lipid order variation between plasma and intracellular membranes

Imaging lipid organization in cell membranes requires advanced fluorescent probes. Here, we show that a recently synthesized push-pull pyrene (PA), similarly to popular probe Laurdan, changes the emission maximum as a function of lipid order, but outperforms it by spectroscopic properties. In addition to red-shifted absorption compatible with common 405 nm diode laser, PA shows higher brightness and much higher photostability than Laurdan in apolar membrane environments. Moreover, PA is compatible with two-photon excitation at wavelengths >800 nm, which was successfully used for ratiometric imaging of coexisting liquid ordered and disordered phases in giant unilamellar vesicles. Fluorescence confocal microscopy in Hela cells revealed that PA efficiently stains the plasma membrane and the intracellular membranes at >20-fold lower concentrations, as compared to Laurdan. Finally, ratiometric imaging using PA reveals variation of lipid order within different cellular compartments: plasma membranes are close to liquid ordered phase of model membranes composed of sphingomyelin and cholesterol, while intracellular membranes are much less ordered, matching well membranes composed of unsaturated phospholipids without cholesterol. These differences in the lipid order were confirmed by fluorescence lifetime imaging (FLIM) at the blue edge of PA emission band. PA probe constitutes thus a new powerful tool for biomembrane research.

Hemagglutinin clusters in the plasma membrane are not enriched with cholesterol and sphingolipids

The clusters of the influenza envelope protein, hemagglutinin, within the plasma membrane are hypothesized to be enriched with cholesterol and sphingolipids. Here, we directly tested this hypothesis by using high-resolution secondary ion mass spectrometry to image the distributions of antibody-labeled hemagglutinin and isotope-labeled cholesterol and sphingolipids in the plasma membranes of fibroblast cells that stably express hemagglutinin. We found that the hemagglutinin clusters were neither enriched with cholesterol nor colocalized with sphingolipid domains. Thus, hemagglutinin clustering and localization in the plasma membrane is not controlled by cohesive interactions between hemagglutinin and liquid-ordered domains enriched with cholesterol and sphingolipids, or from specific binding interactions between hemagglutinin, cholesterol, and/or the majority of sphingolipid species in the plasma membrane.

Assemblies of pore-forming toxins visualized by atomic force microscopy

A number of pore-forming toxins (PFTs) can assemble on lipid membranes through their specific interactions with lipids. The oligomeric assemblies of some PFTs have been successfully revealed either by electron microscopy (EM) and/or atomic force microscopy (AFM). Unlike EM, AFM imaging can be performed under physiological conditions, enabling the real-time visualization of PFT assembly and the transition from the prepore state, in which the toxin does not span the membrane, to the pore state. In addition to characterizing PFT oligomers, AFM has also been used to examine toxin-induced alterations in membrane organization. In this review, we summarize the contributions of AFM to the understanding of both PFT assembly and PFT-induced membrane reorganization. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

The Interaction of Alpha-synuclein with Membranes and its Implication in Parkinson's Disease: A Literature Review

Alpha-synuclein belongs to the class of intrinsically disordered proteins lacking a well-folded structure under physiological conditions. The conversion of alpha-synuclein from a soluble monomer to an insoluble fibril may underlie the neurodegeneration associated with Parkinson's disease (PD). Although the exact mechanism of alpha-synuclein toxicity is still unknown, it has been proposed that alpha-synuclein disturbs membrane structure, leading to increased membrane permeability and eventual cell death. This review highlights the significant role played by fluorescence techniques in unraveling the nature of interactions between alpha-synuclein and membranes and its implications in PD.

Lipoprotein-mediated delivery of BODIPY-labeled sterol and sphingolipid analogs reveals lipid transport mechanisms in mammalian cells

Lipids are often introduced into cell membranes directly from solvent or from lipophilic artificial carriers, such as cyclodextrins. A physiological lipid entry route into mammalian cells is via lipoprotein mediated uptake. In this review, we discuss the introduction of BODIPY-labeled sterol and sphingolipid analogs into mammalian cells via high- or low-density lipoproteins, and the novel findings made by using this strategy. Lipoprotein mediated delivery favors endocytic uptake and initial incorporation of the lipid into membranes of the endosomal compartments. This routing can therefore highlight physiological mechanisms of lipid entry into and exit from the endo-lysosomal membrane system. The underlying principles are of key importance for instance in controlling plasma cholesterol levels and in the development and regression of lysosomal lipid storage diseases. A common denominator for the BODIPY-labeled lipid analogs discussed in this review is that they were synthesized by late Robert Bittman, whose scientific impact radiates far beyond his lifework in organic chemistry.

Synthesis and validation of novel cholesterol-based fluorescent lipids designed to observe the cellular trafficking of cationic liposomes

Cholesterol-based fluorescent lipids with ether linker were synthesized using NBD (Chol-E-NBD) or Rhodamine B (Chol-E-Rh), and the usefulnesses as fluorescent probes for tracing cholesterol-based liposomes were validated. The fluorescent intensities of liposomes containing these modified lipids were measured and observed under a microscope. Neither compound interfered with the expression of GFP plasmid, and live cell images were obtained without interferences. Changes in the fluorescent intensity of liposomes containing Chol-E-NBD were followed by flow cytometry for up to 24h. These fluorescent lipids could be useful probes for trafficking of cationic liposome-mediated gene delivery.

Site of fluorescent label modifies interaction of melittin with live cells and model membranes

The mechanism of membrane disruption by melittin (MLT) of giant unilamellar vesicles (GUVs) and live cells was studied using fluorescence microscopy and two fluorescent synthetic analogues of MLT. The N-terminus of one of these was acylated with thiopropionic acid to enable labeling with maleimido-AlexaFluor 430 to study the interaction of MLT with live cells. It was compared with a second analogue labeled at P14C. The results indicated that the fluorescent peptides adhered to the membrane bilayer of phosphatidylcholine GUVs and inserted into the plasma membrane of HeLa cells. Fluorescence and light microscopy revealed changes in cell morphology after exposure to MLT peptides and showed bleb formation in the plasma membrane of HeLa cells. However, the membrane disruptive effect was dependent upon the location of the fluorescent label on the peptide and was greater when MLT was labeled at the N-terminus. Proline at position 14 appeared to be important for antimicrobial activity, hemolysis and cytotoxicity, but not essential for cell membrane disruption.

Sphingomyelin distribution in lipid rafts of artificial monolayer membranes visualized by Raman microscopy

Sphingomyelin (SM) and cholesterol (chol)-rich domains in cell membranes, called lipid rafts, are thought to have important biological functions related to membrane signaling and protein trafficking. To visualize the distribution of SM in lipid rafts by means of Raman microscopy, we designed and synthesized an SM analog tagged with a Raman-active diyne moiety (diyne-SM). Diyne-SM showed a strong peak in a Raman silent region that is free of interference from intrinsic vibrational modes of lipids and did not appear to alter the properties of SM-containing monolayers. Therefore, we used Raman microscopy to directly visualize the distribution of diyne-SM in raft-mimicking domains formed in SM/dioleoylphosphatidylcholine/chol ternary monolayers. Raman images visualized a heterogeneous distribution of diyne-SM, which showed marked variation, even within a single ordered domain. Specifically, diyne-SM was enriched in the central area of raft domains compared with the peripheral area. These results seem incompatible with the generally accepted raft model, in which the raft and nonraft phases show a clear biphasic separation. One of the possible reasons is that gradual changes of SM concentration occur between SM-rich and -poor regions to minimize hydrophobic mismatch. We believe that our technique of hyperspectral Raman imaging of a single lipid monolayer opens the door to quantitative analysis of lipid membranes by providing both chemical information and spatial distribution with high (diffraction-limited) spatial resolution.

Interaction of cholesterol in ternary lipid mixtures investigated using single-molecule fluorescence

Fluorescence measurements of the sterol analog 23-(dipyrrometheneboron difluoride)-24-norcholesterol (BODIPY-cholesterol) are used to compare the effects of cholesterol (Chol) in monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC)/1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/Chol and chicken egg sphingomyelin (SM)/DOPC/Chol. Monolayers are formed using the Langmuir-Blodgett technique and compared at surface pressures of 8 and 30 mN/m. In particular, these ternary lipid mixtures are compared using both ensemble and single-molecule fluorescence measurements of BODIPY-cholesterol. In mixed monolayers incorporating 0.10 mol % BODIPY-cholesterol, fluorescence microscopy measurements as a function of cholesterol added reveal similar trends in monolayer phase structure for both DPPC/DOPC/Chol and SM/DOPC/Chol films. With a probe concentration reduced to ∼10(-8) mol % BODIPY-cholesterol, single-molecule fluorescence measurements using defocused polarized total internal reflection microscopy are used to characterize the orientations of BODIPY-cholesterol in the monolayers. Population histograms of the BODIPY emission dipole tilt angle away from the membrane normal reveal distinct insertion geometries with a preferred angle observed near 78°. The measured angles and populations are relatively insensitive to added cholesterol and changes in surface pressure for monolayers of SM/DOPC/Chol. For monolayers of DPPC/DOPC/Chol, however, the single-molecule measurements reveal significant changes in the BODIPY-cholesterol insertion geometry when the surface pressure is increased to 30 mN/m. These changes are discussed in terms of a squeeze-out mechanism for BODIPY-cholesterol in these monolayers and provide insight into the partitioning and arrangement of BODIPY-cholesterol in ternary lipid mixtures.

Effect of neurosteroids on a model lipid bilayer including cholesterol: An Atomic Force Microscopy study

Amphiphilic molecules which have a biological effect on specific membrane proteins, could also affect lipid bilayer properties possibly resulting in a modulation of the overall membrane behavior. In light of this consideration, it is important to study the possible effects of amphiphilic molecule of pharmacological interest on model systems which recapitulate some of the main properties of the biological plasma membranes. In this work we studied the effect of a neurosteroid, Allopregnanolone (3α,5α-tetrahydroprogesterone or Allo), on a model bilayer composed by the ternary lipid mixture DOPC/bSM/chol. We chose ternary mixtures which present, at room temperature, a phase coexistence of liquid ordered (Lo) and liquid disordered (Ld) domains and which reside near to a critical point. We found that Allo, which is able to strongly partition in the lipid bilayer, induces a marked increase in the bilayer area and modifies the relative proportion of the two phases favoring the Ld phase. We also found that the neurosteroid shifts the miscibility temperature to higher values in a way similarly to what happens when the cholesterol concentration is decreased. Interestingly, an isoform of Allo, isoAllopregnanolone (3β,5α-tetrahydroprogesterone or isoAllo), known to inhibit the effects of Allo on GABAA receptors, has an opposite effect on the bilayer properties.

Plasma membrane reorganization: A glycolipid gateway for microbes

Ligand-receptor interactions, which represent the core for cell signaling and internalization processes are largely affected by the spatial configuration of host cell receptors. There is a growing piece of evidence that receptors are not homogeneously distributed within the plasma membrane, but are rather pre-clustered in nanodomains, or clusters are formed upon ligand binding. Pathogens have evolved many strategies to evade the host immune system and to ensure their survival by hijacking plasma membrane receptors that are most often associated with lipid rafts. In this review, we discuss the early stage molecular and physiological events that occur following ligand binding to host cell glycolipids. The ability of various biological ligands (e.g. toxins, lectins, viruses or bacteria) that bind to glycolipids to induce their own uptake into mammalian cells by creating negative membrane curvature and membrane invaginations is explored. We highlight recent trends in understanding nanoscale plasma membrane (re-)organization and present the benefits of using synthetic membrane systems. This article is part of a Special Issue entitled: Nanoscale membrane organisation and signalling.

Phase transitions in supported lipid bilayers studied by AFM

We review the capabilities of Atomic Force Microscopy (AFM) in the study of phase transitions in Supported Lipid Bilayers (SLBs). AFM represents a powerful technique to cover the resolution range not available to fluorescence imaging techniques and where spectroscopic data suggest what the relevant lateral scale for domain formation might be. Phase transitions of lipid bilayers involve the formation of domains characterized by different heights with respect to the surrounding phase and are therefore easily identified by AFM in liquid solution once the bilayer is confined to a flat surface. Even if not endowed with high time resolution, AFM allows light to be shed on some aspects related to lipid phase transitions in the case of both a single lipid component and lipid mixtures containing sterols also. We discuss here the obtained results in light of the peculiarities of supported lipid bilayer model systems.