Sterol structure determines the separation of phases and the curvature of the liquid-ordered phase in model membranes

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.

Influenza viral membrane fusion is sensitive to sterol concentration but surprisingly robust to sterol chemical identity

Influenza virions are enriched in cholesterol relative to the plasma membrane from which they bud. Previous work has shown that fusion between influenza virus and synthetic liposomes is sensitive to the amount of cholesterol in either the virus or the target membrane. Here, we test the chemical properties of cholesterol required to promote influenza fusion by replacing cholesterol with other sterols and assaying viral fusion kinetics. We find that influenza fusion with liposomes is surprisingly robust to sterol chemical identity, showing no significant dependence on sterol identity in target membranes for any of the sterols tested. In the viral membrane, lanosterol slowed fusion somewhat, while polar sterols produced a more pronounced slowing and inhibition of fusion. No other sterols tested showed a significant perturbation in fusion rates, including ones previously shown to alter membrane bending moduli or phase behavior. Although fusion rates depend on viral cholesterol, they thus do not require cholesterol’s ability to support liquid-liquid phase coexistence. Using electron cryo-microscopy, we further find that sterol-dependent changes to hemagglutinin spatial patterning in the viral membrane do not require liquid-liquid phase coexistence. We therefore speculate that local sterol-hemagglutinin interactions in the viral envelope may control the rate-limiting step of fusion.

Inhibition of T cell receptor signaling by cholesterol sulfate, a naturally occurring derivative of membrane cholesterol

Most adaptive immune responses require the activation of specific T cells through the T cell antigen receptor (TCR)-CD3 complex. Here we show that cholesterol sulfate (CS), a naturally occurring analog of cholesterol, inhibits CD3 ITAM phosphorylation, a crucial first step in T cell activation. In biochemical studies, CS disrupted TCR multimers, apparently by displacing cholesterol, which is known to bind TCRβ. Moreover, CS-deficient mice showed heightened sensitivity to a self-antigen, whereas increasing CS content by intrathymic injection inhibited thymic selection, indicating that this molecule is an intrinsic regulator of thymocyte development. These results reveal a regulatory role for CS in TCR signaling and thymic selection, highlighting the importance of the membrane microenvironment in modulating cell surface receptor activation.

Solution Asymmetry and Salt Expand Fluid-Fluid Coexistence Regions of Charged Membranes

Liquid-liquid phase separation in giant unilamellar vesicles (GUVs) leads to the formation of intramembrane domains. To mimic charged biological membranes, we studied phase separation and domain formation in GUVs of ternary lipid mixtures composed of egg sphingomyelin, cholesterol, and the negatively charged lipid dioleoylphosphatidylglycerol. The GUVs were exposed to solutions of sucrose and high-saline buffer. The phase diagram was determined using epifluorescence microscopy for vesicle populations with symmetric and asymmetric solution compositions across the membranes. Trans-membrane solution asymmetry was found to affect the membrane phase state. Furthermore, compared to the case of salt-free conditions, the phase diagram in the presence of high-saline buffer (both symmetrically or asymmetrically present across the membrane) was found to exhibit a significantly extended region of liquid-ordered and liquid-disordered coexistence. These observations were confirmed on single GUVs using microfluidics and confocal microscopy. Moreover, we found that the miscibility temperatures markedly increased for vesicles in the presence of symmetric and asymmetric salt solutions. Our results demonstrate a substantial effect of salt and solution asymmetry on the phase behavior of charged membranes, which has direct implications for protein adsorption onto these membranes and for the repartitioning of proteins within the membrane domains.

Posing for a picture: vesicle immobilization in agarose gel

Taking a photo typically requires the object of interest to stand still. In science, imaging is potentiated by optical and electron microscopy. However, living and soft matter are not still. Thus, biological preparations for microscopy usually include a fixation step. Similarly, immobilization strategies are required for or substantially facilitate imaging of cells or lipid vesicles, and even more so for acquiring high-quality data via fluorescence-based techniques. Here, we describe a simple yet efficient method to immobilize objects such as lipid vesicles with sizes between 0.1 and 100 μm using agarose gel. We show that while large and giant unilamellar vesicles (LUVs and GUVs) can be caged in the pockets of the gel meshwork, small molecules, proteins and micelles remain free to diffuse through the gel and interact with membranes as in agarose-free solutions, and complex biochemical reactions involving several proteins can proceed in the gel. At the same time, immobilization in agarose has no adverse effect on the GUV size and stability. By applying techniques such as FRAP and FCS, we show that the lateral diffusion of lipids is not affected by the gel. Finally, our immobilization strategy allows capturing high-resolution 3D images of GUVs.

Caveolin- and clathrin-independent entry of BKPyV into primary human proximal tubule epithelial cells

BK polyomavirus (BKPyV) is a human pathogen that causes polyomavirus-associated nephropathy and hemorrhagic cystitis in transplant patients. Gangliosides and caveolin proteins have previously been reported to be required for BKPyV infection in animal cell models. Recent studies from our lab and others, however, have indicated that the identity of the cells used for infection studies can greatly influence the behavior of the virus. We therefore wished to re-examine BKPyV entry in a physiologically relevant primary cell culture model, human renal proximal tubule epithelial cells. Using siRNA knockdowns, we interfered with expression of UDP-glucose ceramide glucosyltransferase (UGCG), and the endocytic vesicle coat proteins caveolin 1, caveolin 2, and clathrin heavy chain. The results demonstrate that while BKPyV does require gangliosides for efficient infection, it can enter its natural host cells via a caveolin- and clathrin-independent pathway. The results emphasize the importance of studying viruses in a relevant cell culture model.

ABCA1-dependent sterol release: sterol molecule specificity and potential membrane domain for HDL biogenesis

Mammalian cells synthesize various sterol molecules, including the C30 sterol, lanosterol, as cholesterol precursors in the endoplasmic reticulum. The build-up of precursor sterols, including lanosterol, displays cellular toxicity. Precursor sterols are found in plasma HDL. How these structurally different sterols are released from cells is poorly understood. Here, we show that newly synthesized precursor sterols arriving at the plasma membrane (PM) are removed by extracellular apoA-I in a manner dependent on ABCA1, a key macromolecule for HDL biogenesis. Analysis of sterol molecules by GC-MS and tracing the fate of radiolabeled acetate-derived sterols in normal and mutant Niemann-Pick type C cells reveal that ABCA1 prefers newly synthesized sterols, especially lanosterol, as the substrates before they are internalized from the PM. We also show that ABCA1 resides in a cholesterol-rich membrane domain resistant to the mild detergent, Brij 98. Blocking ACAT activity increases the cholesterol contents of this domain. Newly synthesized C29/C30 sterols are transiently enriched within this domain, but rapidly disappear from this domain with a half-life of less than 1 h. Our work shows that substantial amounts of precursor sterols are transported to a certain PM domain and are removed by the ABCA1-dependent pathway.

Quantitative analysis of endosome tubulation and microdomain organization mediated by the WASH complex

Sorting of cargoes in endosomes occurs through their concentration into sorting platforms, called microdomains, from which transport intermediates are formed. The WASH complex localizes to such endosomal microdomains and triggers localized branched actin nucleation by activating the Arp2/3 complex. These branched actin networks are required for both the lateral compartmentalization of endosome membranes into distinct microdomains and for the fission of transport intermediates from these sorting platforms. In this chapter, we provide experimental protocols to study these two aspects of WASH physiology. We first describe how to image the dynamic membrane tubules resulting from the defects of WASH-mediated fission. We then describe how to study quantitatively the microdomain localization of WASH in live and fixed cells. Since microdomains are below the resolution limit of conventional light-microscopy techniques, this required the development of specific image analysis pipelines, which are detailed. The guidelines presented in this chapter can apply to other endomembrane microdomains beyond WASH in order to increase our understanding of trafficking in molecular and quantitative terms.

Dendritic domains with hexagonal symmetry formed by x-shaped bolapolyphiles in lipid membranes

A novel class of bolapolyphile (BP) molecules are shown to integrate into phospholipid bilayers and self-assemble into unique sixfold symmetric domains of snowflake-like dendritic shapes. The BPs comprise three philicities: a lipophilic, rigid, π-π stacking core; two flexible lipophilic side chains; and two hydrophilic, hydrogen-bonding head groups. Confocal microscopy, differential scanning calorimetry, XRD, and solid-state NMR spectroscopy confirm BP-rich domains with transmembrane-oriented BPs and three to four lipid molecules per BP. Both species remain well organized even above the main 1,2-dipalmitoyl-sn-glycero-3-phosphocholine transition. The BP molecules only dissolve in the fluid membrane above 70 °C. Structural variations of the BP demonstrate that head-group hydrogen bonding is a prerequisite for domain formation. Independent of the head group, the BPs reduce membrane corrugation. In conclusion, the BPs form nanofilaments by π stacking of aromatic cores, which reduce membrane corrugation and possibly fuse into a hexagonal network in the dendritic domains.

Docosahexaenoic acid promotes micron scale liquid-ordered domains. A comparison study of docosahexaenoic versus oleic acid containing phosphatidylcholine in raft-like mixtures

The understanding of the functional role of the lipid diversity in biological membranes is a major challenge. Lipid models have been developed to address this issue by using lipid mixtures generating liquid-ordered (Lo)/liquid-disordered (Ld) immiscibility. The present study examined mixtures comprising Egg sphingomyelin (SM), cholesterol (chol) and phosphatidylcholine (PC) either containing docosahexaenoic (PDPC) or oleic acid (POPC). The mixtures were examined in terms of their capability to induce phase separation at the micron- and nano-scales. Fluorescence microscopy, electron spin resonance (ESR), X-ray diffraction (XRD) and calorimetry methods were used to analyze the lateral organization of the mixtures. Fluorescence microscopy of giant vesicles could show that the temperature of the micron-scale Lo/Ld miscibility is higher for PDPC than for POPC ternary mixtures. At 37°C, no micron-scale Lo/Ld phase separation could be identified in the POPC containing mixtures while it was evident for PDPC. In contrast, a phase separation was distinguished for both PC mixtures by ESR and XRD, indicative that PDPC and POPC mixtures differed in micron vs nano domain organization. Compared to POPC, the higher line tension of the Lo domains observed in PDPC mixtures is assumed to result from the higher difference in Lo/Ld order parameter rather than hydrophobic mismatch.

A single herpesvirus protein can mediate vesicle formation in the nuclear envelope

Herpesviruses assemble capsids in the nucleus and egress by unconventional vesicle-mediated trafficking through the nuclear envelope. Capsids bud at the inner nuclear membrane into the nuclear envelope lumen. The resulting intralumenal vesicles fuse with the outer nuclear membrane, delivering the capsids to the cytoplasm. Two viral proteins are required for vesicle formation, the tail-anchored pUL34 and its soluble interactor, pUL31. Whether cellular proteins are involved is unclear. Using giant unilamellar vesicles, we show that pUL31 and pUL34 are sufficient for membrane budding and scission. pUL34 function can be bypassed by membrane tethering of pUL31, demonstrating that pUL34 is required for pUL31 membrane recruitment but not for membrane remodeling. pUL31 can inwardly deform membranes by oligomerizing on their inner surface to form buds that constrict to vesicles. Therefore, a single viral protein can mediate all events necessary for membrane budding and abscission.

Photo-triggered transformation from vesicles to branched nanotubes fabricated by a cholesterol-appended cyanostilbene

Vesicles constructed from cholesteryl cyanostilbene could transform into nanotubes upon UV light irradiation, in which the photoluminescence could be fine-tuned.

Dynamic shaping of cellular membranes by phospholipids and membrane-deforming proteins

All cellular compartments are separated from the external environment by a membrane, which consists of a lipid bilayer. Subcellular structures, including clathrin-coated pits, caveolae, filopodia, lamellipodia, podosomes, and other intracellular membrane systems, are molded into their specific submicron-scale shapes through various mechanisms. Cells construct their micro-structures on plasma membrane and execute vital functions for life, such as cell migration, cell division, endocytosis, exocytosis, and cytoskeletal regulation. The plasma membrane, rich in anionic phospholipids, utilizes the electrostatic nature of the lipids, specifically the phosphoinositides, to form interactions with cytosolic proteins. These cytosolic proteins have three modes of interaction: 1) electrostatic interaction through unstructured polycationic regions, 2) through structured phosphoinositide-specific binding domains, and 3) through structured domains that bind the membrane without specificity for particular phospholipid. Among the structured domains, there are several that have membrane-deforming activity, which is essential for the formation of concave or convex membrane curvature. These domains include the amphipathic helix, which deforms the membrane by hemi-insertion of the helix with both hydrophobic and electrostatic interactions, and/or the BAR domain superfamily, known to use their positively charged, curved structural surface to deform membranes. Below the membrane, actin filaments support the micro-structures through interactions with several BAR proteins as well as other scaffold proteins, resulting in outward and inward membrane micro-structure formation. Here, we describe the characteristics of phospholipids, and the mechanisms utilized by phosphoinositides to regulate cellular events. We then summarize the precise mechanisms underlying the construction of membrane micro-structures and their involvements in physiological and pathological processes.

Human serotonin receptor 5-HT(1A) preferentially segregates to the liquid disordered phase in synthetic lipid bilayers

We demonstrate successful incorporation of the G protein coupled receptor 5-HT_1A into giant unilamellar vesicles using an agarose rehydration method. With direct observation using fluorescence techniques, we report preferential segregation of 5-HT_1A into the cholesterol-poor liquid disordered phase of the membrane, contradicting previous reports of lipid raft segregation. Furthermore, altering the concentration of cholesterol and sphingomyelin in ternary mixtures does not alter 5-HT_1A segregation into the liquid disordered phase.

Fluorescence correlation spectroscopy: principles and applications

Fluorescence correlation spectroscopy (FCS) is used to study the movements and the interactions of biomolecules at extremely dilute concentrations, yielding results with good spatial and temporal resolutions. Using a number of technical developments, FCS has become a versatile technique that can be used to study a variety of sample types and can be advantageously combined with other methods. Unlike other fluorescence-based techniques, the analysis of FCS data is not based on the average intensity of the fluorescence emission but examines the minute intensity fluctuations caused by spontaneous deviations from the mean at thermal equilibrium. These fluctuations can result from variations in local concentrations owing to molecular mobility or from characteristic intermolecular or intramolecular reactions of fluorescently labeled biomolecules present at low concentrations. Here, we provide a basic introduction to FCS, including its technical development and theoretical basis, experimental setup of an FCS system, adjustment of a setup, data acquisition, and analysis of FCS measurements. Finally, the application of FCS to the study of lipid bilayer membranes and to living cells is discussed.

Phospholipid Architecture of the Bovine Milk Fat Globule Membrane Using Giant Unilamellar Vesicles as a Model

Giant unilamellar vesicles (GUVs) were constructed using an electroformation technique to mimic the morphology of the native milk fat globule membrane (MFGM) for the purpose of structural investigation. Bovine milk derived phospholipids were selected to manufacture GUVs which were characterized by confocal laser scanning microscopy after fluorescent staining. Circular nonfluorescent dark regions were observed in a 3/7 (mol/mol) surface mixture of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dioleoyl-sn-glycero-3 phosphoethanolamine. Linear shaped dark lipid domains were found in GUVs containing sphingomyelin (SM) in the absence of cholesterol. The dark regions were interpreted as a gel phase formed by a high gel-liquid phase transition temperature (T_m) of DPPC and SM. This study provides a strategy for investigating the lipid structural organization within the native MFGM using a model lipid bilayer system and reveals that a SM and cholesterol association network is not the only requirement for nonfluorescent lipid domain formation and that PE is preferably located in the inner leaflet of the phospholipid bilayer.

Function and regulation of the endosomal fusion and fission machineries

Organelles within the endomembrane system are connected via vesicle flux. Along the endocytic pathway, endosomes are among the most versatile organelles. They sort cargo through tubular protrusions for recycling or through intraluminal vesicles for degradation. Sorting involves numerous machineries, which mediate fission of endosomal transport intermediates and fusion with other endosomes or eventually with lysosomes. Here we review the recent advances in our understanding of these processes with a particular focus on the Rab GTPases, tethering factors, and retromer. The cytoskeleton has also been recently recognized as a central player in membrane dynamics of endosomes, and this review covers the regulation of the machineries that govern the formation of branched actin networks through the WASH and Arp2/3 complexes in relation with cargo recycling and endosomal fission.

Lateral surface engineering of hybrid lipid-BCP vesicles and selective nanoparticle embedding

Bio-inspired recognition between macromolecules and antibodies can be used to reveal the location of amphiphilic block copolymers (BCPs) in model biomembranes and their subsequent scaffolding with nanoparticles (NPs). Potential applications of this novel class of lipid-BCP membranes require an understanding of their compositional heterogeneities with a variety of different molecules including natural proteins or synthetic NPs, whose selective incorporation into a specific part of phase separated membranes can serve as a model system for the targeted delivery of therapeutics. We demonstrate the selective incorporation of polymer-functionalized CdSe NPs into the polymer-rich domains in vesicular hybrid membranes using 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC, Tm = 41 °C) or 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC, Tm = -20 °C) as the lipid component. Furthermore, we demonstrate a method to detect PIB-PEO based amphiphilic BCPs on liposomal surfaces by a PEO binding antibody (anti-PEO). As a result, hybrid membrane morphologies, which depend on the lipid/BCP composition, are selectively monitored and engineered.

Untangling membrane rearrangement in the nidovirales

All known positive sense single-stranded RNA viruses induce host cell membrane rearrangement for purposes of aiding viral genome replication and transcription. Members of the Nidovirales order are no exception, inducing intricate regions of double membrane vesicles and convoluted membranes crucial for the production of viral progeny. Although these structures have been well studied for some members of this order, much remains unclear regarding the biogenesis of these rearranged membranes. Here, we discuss what is known about these structures and their formation, compare some of the driving viral proteins behind this process across the nidovirus order, and examine possible routes of mechanism by which membrane rearrangement may occur.

Probing the subcellular localization of hopanoid lipids in bacteria using NanoSIMS

The organization of lipids within biological membranes is poorly understood. Some studies have suggested lipids group into microdomains within cells, but the evidence remains controversial due to non-native imaging techniques. A recently developed NanoSIMS technique indicated that sphingolipids group into microdomains within membranes of human fibroblast cells. We extended this NanoSIMS approach to study the localization of hopanoid lipids in bacterial cells by developing a stable isotope labeling method to directly detect subcellular localization of specific lipids in bacteria with ca. 60 nm resolution. Because of the relatively small size of bacterial cells and the relative abundance of hopanoid lipids in membranes, we employed a primary ²H-label to maximize our limit of detection. This approach permitted the analysis of multiple stable isotope labels within the same sample, enabling visualization of subcellular lipid microdomains within different cell types using a secondary label to mark the growing end of the cell. Using this technique, we demonstrate subcellular localization of hopanoid lipids within alpha-proteobacterial and cyanobacterial cells. Further, we provide evidence of hopanoid lipid domains in between cells of the filamentous cyanobacterium Nostoc punctiforme. More broadly, our method provides a means to image lipid microdomains in a wide range of cell types and test hypotheses for their functions in membranes.

Influence of different membrane environments on the behavior of cholesterol

Our results show the distribution of cholesterol between stress-free and stressed membranes or between the inner leaflet and the outer leaflet of curved membrane.

Toward atomic force microscopy and mass spectrometry to visualize and identify lipid rafts in plasmodesmata

Plant cell-to-cell communication is mediated by nanopores called plasmodesmata (PDs) which are complex structures comprising plasma membrane (PM), highly packed endoplasmic reticulum and numerous membrane proteins. Although recent advances on proteomics have led to insights into mechanisms of transport, there is still an inadequate characterization of the lipidic composition of the PM where membrane proteins are inserted. It has been postulated that PDs could be formed by lipid rafts, however no structural evidence has shown to visualize and analyse their lipid components. In this perspective article, we discuss proposed experiments to characterize lipid rafts and proteins in the PDs. By using atomic force microscopy (AFM) and mass spectrometry (MS) of purified PD vesicles it is possible to determine the presence of lipid rafts, specific bound proteins and the lipidomic profile of the PD under physiological conditions and after changing transport permeability. In addition, MS can determine the stoichiometry of intact membrane proteins inserted in lipid rafts. This will give novel insights into the role of membrane proteins and lipid rafts on the PD structure.

Bending "on the rocks"--a cocktail of biophysical modules to build endocytic pathways

Numerous biological processes rely on endocytosis. The construction of endocytic pits is achieved by a bewildering complexity of biochemical factors that function in clathrin-dependent and -independent pathways. In this review, we argue that this complexity can be conceptualized by a deceptively small number of physical principles that fall into two broad categories: passive mechanisms, such as asymmetric transbilayer stress, scaffolding, line tension, and crowding, and active mechanisms driven by mechanochemical enzymes and/or cytoskeleton. We illustrate how the functional identity of biochemical modules depends on system parameters such as local protein density on membranes, thus explaining some of the controversy in the field. Different modules frequently operate in parallel in the same step and often are shared by apparently divergent uptake processes. The emergence of a novel endocytic classification system may thus be envisioned in which functional modules are the elementary bricks.

Towards understanding the molecular mechanism of the endocytosis-like process in the bacterium Gemmata obscuriglobus

An endocytosis-like process of protein uptake in the planctomycete Gemmata obscuriglobus is a recently discovered process unprecedented in the bacterial world. The molecular mechanisms underlying this process are not yet characterized. A homolog of the MC (membrane-coating) proteins of eukaryotes has been proposed to be involved in the mechanism of this process, but its relationship to eukaryote proteins is controversial. However, a number of other proteins of G. obscuriglobus with domains homologous to those involved in endocytosis in eukaryotes can also be identified. Here we critically evaluate current bioinformatic knowledge, and suggest practical experimental steps to overcome the limits of bioinformatics in elucidating the molecular mechanism of endocytosis in bacteria. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Biogenesis and transport of membrane domains-potential implications in brain pathologies

Lipids in biological membranes show astonishing chemical diversity, but they also show some key conserved structures in different organisms. In addition, some of their biophysical properties have been related to specific functions. In this review, we aim to discuss the role of sphingolipids- and cholesterol-rich micro- and nano-membrane domains (MD) and highlight their pivotal role in lipid-protein clustering processes, vesicle biogenesis and membrane fusion. We further review potential connections between human pathologies and defects in MD biosynthesis, recycling and homeostasis. Brain, which is second only to the adipose tissues in term of lipid abundance, is particularly affected by MD defects which are linked to neurodegenerative disorders. Finally we propose a potential connection between MD and several nutrient-related processes and envision how diet and autophagy could bring insights towards understanding the impact of global lipid homeostasis on human health and disease.

Phase diagram and tie-line determination for the ternary mixture DOPC/eSM/cholesterol

We propose a novel, to our knowledge, method for the determination of tie lines in a phase diagram of ternary lipid mixtures. The method was applied to a system consisting of dioleoylphosphatidylcholine (DOPC), egg sphingomyelin (eSM), and cholesterol (Chol). The approach is based on electrofusion of single- or two-component homogeneous giant vesicles in the fluid phase and analyses of the domain areas of the fused vesicle. The electrofusion approach enables us to create three-component vesicles with precisely controlled composition, in contrast to conventional methods for giant vesicle formation. The tie lines determined in the two-liquid-phase coexistence region are found to be not parallel, suggesting that the dominant mechanism of lipid phase separation in this region changes with the membrane composition. We provide a phase diagram of the DOPC/eSM/Chol mixture and predict the location of the critical point. Finally, we evaluate the Gibbs free energy of transfer of individual lipid components from one phase to the other.

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.

Lipid nanotechnology

Nanotechnology is a multidisciplinary field that covers a vast and diverse array of devices and machines derived from engineering, physics, materials science, chemistry and biology. These devices have found applications in biomedical sciences, such as targeted drug delivery, bio-imaging, sensing and diagnosis of pathologies at early stages. In these applications, nano-devices typically interface with the plasma membrane of cells. On the other hand, naturally occurring nanostructures in biology have been a source of inspiration for new nanotechnological designs and hybrid nanostructures made of biological and non-biological, organic and inorganic building blocks. Lipids, with their amphiphilicity, diversity of head and tail chemistry, and antifouling properties that block nonspecific binding to lipid-coated surfaces, provide a powerful toolbox for nanotechnology. This review discusses the progress in the emerging field of lipid nanotechnology.

Adhesion-induced phase behavior of two-component membranes and vesicles

The interplay of adhesion and phase separation is studied theoretically for two-component membranes that can phase separate into two fluid phases such as liquid-ordered and liquid-disordered phases. Many adhesion geometries provide two different environments for these membranes and then partition the membranes into two segments that differ in their composition. Examples are provided by adhering vesicles, by hole- or pore-spanning membranes, and by membranes supported by chemically patterned surfaces. Generalizing a lattice model for binary mixtures to these adhesion geometries, we show that the phase behavior of the adhering membranes depends, apart from composition and temperature, on two additional parameters, the area fraction of one membrane segment and the affinity contrast between the two segments. For the generic case of non-vanishing affinity contrast, the adhering membranes undergo two distinct phase transitions and the phase diagrams in the composition/temperature plane have a generic topology that consists of two two-phase coexistence regions separated by an intermediate one-phase region. As a consequence, phase separation and domain formation is predicted to occur separately in each of the two membrane segments but not in both segments simultaneously. Furthermore, adhesion is also predicted to suppress the phase separation process for certain regions of the phase diagrams. These generic features of the adhesion-induced phase behavior are accessible to experiment.

Expression of oleosin and perilipins in yeast promote formation of lipid droplets from the endoplasmatic reticulum

Most cells store neutral lipids in a dedicated compartment, the lipid droplet (LD). These LDs are structurally and functionally conserved across species. In higher eukaryotes, LDs are covered by abundant scaffolding proteins, such as the oleosins in plants and perilipins (PLINs) in animal cells. S. cerevisiae, however, has no homologues of these scaffolding proteins. To analyze a possible function of these proteins in the biogenesis of LDs, oleosin and perilipin family members (PLIN1, ADRP/PLIN2, and TIP47/PLIN3) were expressed in yeast cells and their targeting to LDs, membrane association and function in neutral lipid homeostasis and LD biogenesis were analyzed. When expressed in wild-type cells, these proteins were properly targeted to LDs. However, when expressed in cells lacking LDs, oleosin was localized to the ER bilayer and was rapidly degraded. PLINs, on the other hand, did not localize to the ER membrane in the absence of LDs and lost their membrane association. Photobleaching experiments revealed that PLIN2 and PLIN3 rapidly exchanged their LD association but PLINs did not move over the LD surface as quickly as did an integral membrane protein, such as oleosin. Interestingly, expression of these scaffolding LD proteins in mutant cells containing elevated levels of neutral lipids within the ER bilayer resulted in the formation of LDs. These results suggest that these LD scaffolding proteins promote the sequestration of neutral lipids from the ER bilayer and thereby induce LD formation. Consistent with this proposition, addition of a cell permeable diacylglycerol (DAG) was sufficient to promote LD formation in cells expressing the LD scaffolding proteins but lacking the capacity to synthesize storage lipids.

Functional convergence of hopanoids and sterols in membrane ordering

Liquid-ordered phases are one of two biochemically active membrane states, which until now were thought to be a unique consequence of the interactions between eukaryotic membrane lipids. The formation of a liquid-ordered phase depends crucially on the ordering properties of sterols. However, it is not known whether this capacity exists in organisms that lack sterols, such as bacteria. We show that diplopterol, the simplest bacterial hopanoid, has similar properties and that hopanoids are bacterial "sterol surrogates" with the ability to order saturated lipids and to form a liquid-ordered phase in model membranes. These observations suggest that the evolution of an ordered biochemically active liquid membrane could have evolved before the oxygenation of Earth's surface and the emergence of sterols.

Curvature-driven effective attraction in multicomponent membranes

We study closed liquid membranes that segregate into three phases due to differences in the chemical and physical properties of its components. The shape and in-plane membrane arrangement of the phases are coupled through phase-specific bending energies and line tensions. We use simulated annealing Monte Carlo simulations to find low-energy structures, allowing both phase arrangement and membrane shape to relax. The three-phase system is the simplest one in which there are multiple interface pairs, allowing us to analyze interfacial preferences and pairwise distinct line tensions. We observe the system's preference for interface pairs that maximize differences in spontaneous curvature. From a pattern selection perspective, this acts as an effective attraction between phases of most disparate spontaneous curvature. We show that this effective attraction is robust enough to persist even when the interface between these phases is the most penalized by line tension. This effect is driven by geometry and not by any explicit component-component interaction.

Actin polymerization controls the organization of WASH domains at the surface of endosomes

Sorting of cargoes in endosomes occurs through their selective enrichment into sorting platforms, where transport intermediates are generated. The WASH complex, which directly binds to lipids, activates the Arp2/3 complex and hence actin polymerization onto such sorting platforms. Here, we analyzed the role of actin polymerization in the physiology of endosomal domains containing WASH using quantitative image analysis. Actin depolymerization is known to enlarge endosomes. Using a novel colocalization method that is insensitive to the heterogeneity of size and shape of endosomes, we further show that preventing the generation of branched actin networks induces endosomal accumulation of the WASH complex. Moreover, we found that actin depolymerization induces a dramatic decrease in the recovery of endosomal WASH after photobleaching. This result suggests a built-in turnover, where the actin network, i.e. the product of the WASH complex, contributes to the dynamic exchange of the WASH complex by promoting its detachment from endosomes. Our experiments also provide evidence for a role of actin polymerization in the lateral compartmentalization of endosomes: several WASH domains exist at the surface of enlarged endosomes, however, the WASH domains coalesce upon actin depolymerization or Arp2/3 depletion. Branched actin networks are thus involved in the regulation of the size of WASH domains. The potential role of this regulation in membrane scission are discussed.