Do the long fatty acid chains of sphingolipids interdigitate across the center of a bilayer of shorter chain symmetric phospholipids?

Is cholesterol both the lock and key to abnormal transmembrane signals in Autism Spectrum Disorder?

Disturbances in cholesterol homeostasis have been associated with ASD. Lipid rafts are central in many transmembrane signaling pathways (including mTOR) and changes in raft cholesterol content affect their order function. Cholesterol levels are controlled by several mechanisms, including endoplasmic reticulum associated degradation (ERAD) of the rate limiting HMGCoA reductase. A new approach to increase cholesterol via temporary ERAD blockade using a benign bacterial toxin-derived competitor for the ERAD translocon is suggested.A new lock and key model for cholesterol/lipid raft dependent signaling is proposed in which the rafts provide both the afferent and efferent 'tumblers' across the membrane to allow 'lock and key' receptor transmembrane signals.

Fumonisin B Series Mycotoxins' Dose Dependent Effects on the Porcine Hepatic and Pulmonary Phospholipidome

Male weaned piglets n = 6/group were fed Fumonisin B₁₊₂₊₃ (FBs) mycotoxins at 0, 15, or 30 mg/kg diet for 3 weeks to assess the fatty acid (FA) composition of membrane lipid classes, lipid peroxidation, and histomorphological changes in the liver and lung. Growth performance and lipid peroxidation were unaltered, but histomorphological lesion scores increased in the liver. Linear dose-response was detected in liver phosphatidylcholines for C16:1n7, C18:1n9, and total monounsaturation and in lungs for C22:6n3, total n-3 and n-3:n-6, in pulmonary phosphatidylserines C20:0 and C24:0. Alterations associated with the highest FBs dose were detected in sphingomyelins (liver: total saturation ↓, total monounsaturation ↑), phosphatidylcholines (liver: total n-6 ↓, n-6:n-3 ↑; in lungs: total monounsaturation ↑, total polyunsaturation ↑), phosphatidylethanolamines (liver: total n-3 ↓; in lungs: total monounsaturation ↑ and n-6:n-3 ↑), phosphatidylserines (liver: n-6:n-3 ↑; in lungs: total saturation ↓, total polyunsatuartion ↑, and total n-6 and its ratio to n-3 ↑), and phosphatidylinositol (n-6:n-3 ↑; lungs: C22:1n9 ↑, C22:6n3 ↓, total saturation ↓, total monounsaturaion ↑). In conclusion, FBs exposures neither impaired growth nor induced substantial lipid peroxidation, but hepatotoxicity was proven with histopathological alterations at the applied exposure period and doses. FA results imply an enzymatic disturbance in FA metabolism, agreeing with earlier findings in rats.

The Gb3-enriched CD59/flotillin plasma membrane domain regulates host cell invasion by Pseudomonas aeruginosa

The opportunistic pathogen Pseudomonas aeruginosa has gained precedence over the years due to its ability to develop resistance to existing antibiotics, thereby necessitating alternative strategies to understand and combat the bacterium. Our previous work identified the interaction between the bacterial lectin LecA and its host cell glycosphingolipid receptor globotriaosylceramide (Gb3) as a crucial step for the engulfment of P. aeruginosa via the lipid zipper mechanism. In this study, we define the LecA-associated host cell membrane domain by pull-down and mass spectrometry analysis. We unraveled a predilection of LecA for binding to saturated, long fatty acyl chain-containing Gb3 species in the extracellular membrane leaflet and an induction of dynamic phosphatidylinositol (3,4,5)-trisphosphate (PIP₃) clusters at the intracellular leaflet co-localizing with sites of LecA binding. We found flotillins and the GPI-anchored protein CD59 not only to be an integral part of the LecA-interacting membrane domain, but also majorly influencing bacterial invasion as depletion of either of these host cell proteins resulted in about 50% reduced invasiveness of the P. aeruginosa strain PAO1. In summary, we report that the LecA-Gb3 interaction at the extracellular leaflet induces the formation of a plasma membrane domain enriched in saturated Gb3 species, CD59, PIP₃ and flotillin thereby facilitating efficient uptake of PAO1.

Complex Phase Behavior of GUVs Containing Different Sphingomyelins

Understanding the lateral organization of biological membranes plays a key role on the road to fully appreciate the physiological functions of this fundamental barrier between the inside and outside regions of a cell. Ternary lipid bilayers composed of a high and a low melting temperature lipid and cholesterol represent a model system that mimics some of the important thermodynamical features of much more complex lipid mixtures such as those found in mammal membranes. The phase diagram of these ternary mixtures can be studied exploiting fluorescence microscopy in giant unilamellar vesicles, and it is typically expected to give rise, for specific combinations of composition and temperature, to regions of two-phase coexistence and a region with three-phase coexistence, namely, the liquid-ordered, liquid-disordered, and solid phases. Whereas the observation of two-phase coexistence is routinely possible using fluorescence microscopy, the three-phase region is more elusive to study. In this article, we show that particular lipid mixtures containing diphytanoyl-phosphatidylcholine and cholesterol plus different types of sphingomyelin (SM) are prone to produce bilayer regions with more than two levels of fluorescence intensity. We found that these intensity levels occur at low temperature and are linked to the copresence of long and asymmetric chains in SMs and diphytanoyl-phosphatidylcholine in the lipid mixtures. We discuss the possible interpretations for this observation in terms of bilayer phase organization in the presence of sphingolipids. Additionally, we also show that in some cases, liposomes in the three-phase coexistence state exhibit extreme sensitivity to lateral tension. We hypothesize that the appearance of the different phases is related to the asymmetric structure of SMs and to interdigitation effects.

The many faces (and phases) of ceramide and sphingomyelin I - single lipids

Ceramides, the simplest kind of two-chained sphingolipids, contain a single hydroxyl group in position 1 of the sphingoid base. Sphingomyelins further contain a phosphocholine group at the OH of position 1 of ceramide. Ceramides and sphingomyelins show a variety of species depending on the fatty acyl chain length, hydroxylation, and unsaturation. Because of the relatively high transition temperature of sphingomyelin compared to lecithin and, particularly, of ceramides with 16:0-18:0 saturated chains, a widespread idea on their functional importance refers to formation of rather solid domains enriched in sphingomyelin and ceramide. Frequently, and especially in the cell biology field, these are generally (and erroneously) assumed to occur irrespective on the type of N-acyl chain in these lipids. This is because most studies indicating such condensed ordered domains employed sphingolipids with acyl chains with 16 carbons while scarce attention has been focused on the influence of the N-acyl chain on their surface properties. However, abundant evidence has shown that variations of the N-acyl chain length in ceramides and sphingomyelins markedly affect their phase state, interfacial elasticity, surface topography, electrostatics and miscibility and that, even the usually conceived "condensed" sphingolipids and many of their mixtures, may exhibit liquid-like expanded states. This review is a summarized overview of our work and of related others on some facts regarding membranes composed of single molecular species of ceramide and sphingomyelin. A second part is dedicated to discuss the miscibility properties between species of sphingolipids that differ in N-acyl and oligosaccharide chains.

The Pseudomonas aeruginosa lectin LecA triggers host cell signalling by glycosphingolipid-dependent phosphorylation of the adaptor protein CrkII

The human pathogen Pseudomonas aeruginosa induces phosphorylation of the adaptor protein CrkII by activating the non-receptor tyrosine kinase Abl to promote its uptake into host cells. So far, specific factors of P. aeruginosa, which induce Abl/CrkII signalling, are entirely unknown. In this research, we employed human lung epithelial cells H1299, Chinese hamster ovary cells and P. aeruginosa wild type strain PAO1 to study the invasion process of P. aeruginosa into host cells by using microbiological, biochemical and cell biological approaches such as Western Blot, immunofluorescence microscopy and flow cytometry. Here, we demonstrate that the host glycosphingolipid globotriaosylceramide, also termed Gb3, represents a signalling receptor for the P. aeruginosa lectin LecA to induce CrkII phosphorylation at tyrosine 221. Alterations in Gb3 expression and LecA function correlate with CrkII phosphorylation. Interestingly, phosphorylation of CrkII^Y221 occurs independently of Abl kinase. We further show that Src family kinases transduce the signal induced by LecA binding to Gb3, leading to Crk^Y221 phosphorylation. In summary, we identified LecA as a bacterial factor, which utilizes a so far unrecognized mechanism for phospho-CrkII^Y221 induction by binding to the host glycosphingolipid receptor Gb3. The LecA/Gb3 interaction highlights the potential of glycolipids to mediate signalling processes across the plasma membrane and should be further elucidated to gain deeper insights into this non-canonical mechanism of activating host cell processes.

Interleaflet Coupling, Pinning, and Leaflet Asymmetry-Major Players in Plasma Membrane Nanodomain Formation

The plasma membrane has a highly asymmetric distribution of lipids and contains dynamic nanodomains many of which are liquid entities surrounded by a second, slightly different, liquid environment. Contributing to the dynamics is a continuous repartitioning of components between the two types of liquids and transient links between lipids and proteins, both to extracellular matrix and cytoplasmic components, that temporarily pin membrane constituents. This make plasma membrane nanodomains exceptionally challenging to study and much of what is known about membrane domains has been deduced from studies on model membranes at equilibrium. However, living cells are by definition not at equilibrium and lipids are distributed asymmetrically with inositol phospholipids, phosphatidylethanolamines and phosphatidylserines confined mostly to the inner leaflet and glyco- and sphingolipids to the outer leaflet. Moreover, each phospholipid group encompasses a wealth of species with different acyl chain combinations whose lateral distribution is heterogeneous. It is becoming increasingly clear that asymmetry and pinning play important roles in plasma membrane nanodomain formation and coupling between the two lipid monolayers. How asymmetry, pinning, and interdigitation contribute to the plasma membrane organization is only beginning to be unraveled and here we discuss their roles and interdependence.

Carbohydrate conformation and lipid condensation in monolayers containing glycosphingolipid Gb3: influence of acyl chain structure

Globotriaosylceramide (Gb3), a glycosphingolipid found in the plasma membrane of animal cells, is the endocytic receptor of the bacterial Shiga toxin. Using x-ray reflectivity (XR) and grazing incidence x-ray diffraction (GIXD), lipid monolayers containing Gb3 were investigated at the air-water interface. XR probed Gb3 carbohydrate conformation normal to the interface, whereas GIXD precisely characterized Gb3's influence on acyl chain in-plane packing and area per molecule (APM). Two phospholipids, 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE), were used to study Gb3 packing in different lipid environments. Furthermore, the impact on monolayer structure of a naturally extracted Gb3 mixture was compared to synthetic Gb3 species with uniquely defined acyl chain structures. XR results showed that lipid environment and Gb3 acyl chain structure impact carbohydrate conformation with greater solvent accessibility observed for smaller phospholipid headgroups and long Gb3 acyl chains. In general, GIXD showed that Gb3 condensed phospholipid packing resulting in smaller APM than predicted by ideal mixing. Gb3's capacity to condense APM was larger for DSPC monolayers and exhibited different dependencies on acyl chain structure depending on the lipid environment. The interplay between Gb3-induced changes in lipid packing and the lipid environment's impact on carbohydrate conformation has broad implications for glycosphingolipid macromolecule recognition and ligand binding.

Glycosphingolipid functions

The combination of carbohydrate and lipid generates unusual molecules in which the two distinctive halves of the glycoconjugate influence the function of each other. Membrane glycolipids can act as primary receptors for carbohydrate binding proteins to mediate transmembrane signaling despite restriction to the outer bilayer leaflet. The extensive heterogeneity of the lipid moiety plays a significant, but still largely unknown, role in glycosphingolipid function. Potential interplay between glycolipids and their fatty acid isoforms, together with their preferential interaction with cholesterol, generates a complex mechanism for the regulation of their function in cellular physiology.

Ceramide N-acyl chain length: a determinant of bidimensional transitions, condensed domain morphology, and interfacial thickness

Several lipids of biological interest are able to form monomolecular surfaces with a rich variety of thickness and lateral topography that can be precisely controlled by defined variations of the film composition. Ceramide is one of the simplest sphingolipids, consisting of a sphingosine base N-linked to a fatty acid, and is a membrane mediator for cell-signaling events. In this work, films of ceramides N-acylated with the saturated fatty acids C10, C12, C14, and C16 were studied at the air-aqueous interface. The dipole moment contribution (from surface potential measurements) and the surface topography and thickness (as revealed by Brewster angle microscopy) were measured simultaneously with the surface pressure at different molecular areas. Several surface features were observed depending on the asymmetry between the sphingosine and the N-linked acyl chains. At 21 °C, the C16:0 and C14:0 ceramides showed condensed isotherms and the film topography revealed solid film patches (17.3-15.7 Å thick) that coalesced into a homogeneous surface by further compression. On the other hand, in the more asymmetric C12:0 and C10:0 ceramides, liquid expanded states and liquid expanded-condensed transitions occurred. In the phase coexistence region, the condensed state of these compounds formed flowerlike domains (11.1-13.3 Å thick). C12:0 ceramide domains were larger and more densely branched than those of C10:0 ceramide. Both the film thickness and the surface dipole moment of the condensed state increased with ceramide N-acyl chain length. Bending of the sphingosine chain over the N-linked acyl chain in the more asymmetric ceramides can account for the variation of the surface electrostatics, topography, and thickness of the films with the acyl chain mismatch.

Lateral diffusion in sphingomyelin bilayers

Sphingomyelin (SM) is an important lipid of eukaryotic cellular membranes and neuronal tissues. We studied lateral diffusion in macroscopically oriented bilayers of synthetic palmitoylsphingomyelin (PSM) and natural sphingomyelins of egg yolk (eSM), bovine brain (bSM) and bovine milk (mSM) by pulsed field gradient NMR (PFG NMR) in the temperature range 45-60 °C. We found that the mean values of lateral diffusion coefficients (LDCs) of SMs are 1.9-fold lower compared with those of dipalmitoylphosphatidylcholine (DPPC), which is similar in molecular structure. This discrepancy could be explained by the characteristics of intermolecular SM interactions. The LDCs of different SMs differ: egg SM is most similar to PSM; both of them have a 10% higher LDC value compared with the other two natural SMs. Besides, all natural SMs show a complicated form of the spin-echo diffusion decay (DD), which is an indicator of a distribution of LDC values in bilayers. This peculiarity is explained by the broad distributions of hydrocarbon chain lengths of the natural SMs studied here, especially mSM and bSM. We confirmed the relationship between chain length and LDC in the bilayers by computer analysis of a set of (1)H NMR spectra obtained by scanning the value of the pulsed field gradient. There is a correlation between lower LDC values and SM molecules with longer acyl chains. The most probable mechanisms by which long-chain SM molecules decrease their lateral diffusion relative to the average value are protrusion into the other side of the bilayer or lateral separation into areas that diverge with their LDCs.

A lipid matrix model of membrane raft structure

Domains in cell membranes are created by lipid-lipid interactions and are referred to as membrane rafts. Reliable isolation methods have been developed which have shown that rafts from the same membranes have different proteins and can be sub-fractionated by immunoaffinity methods. Analysis of these raft subfractions shows that they are also comprised of different molecular species of lipids. The major lipid classes present are phospholipids, glycosphingolipids and cholesterol. Model studies show that mixtures of phospholipids, particularly sphingomyelin, and cholesterol form liquid-ordered phase with properties intermediate between a gel and fluid phase. This type of liquid-ordered phase dominates theories of domain formation and raft structure in biological membranes. Recently it has been shown that sphingolipids with long (22-26C) N-acyl fatty acids form quasi-crystalline bilayer structures with diacylphospholipids that have well-defined stoichiometries. A two tier heuristic model of membrane raft structure is proposed in which liquid-ordered phase created by a molecular complex between sphingolipids with hydrocarbon chains of approximately equal length and cholesterol acts as a primary staging area for selecting raft proteins. Tailoring of the lipid anchors of raft proteins takes place at this site. Assembly of lipid-anchored proteins on a scaffold of sphingolipids with asymmetric hydrocarbon chains and phospholipids arranged in a quasi-crystalline bilayer structure serves to concentrate and orient the proteins in a manner that couples them functionally within the membrane. Specificity is inherent in the quasi-crystalline lipid structure of liquid-ordered matrices formed by both types of complex into which protein lipid anchors are interpolated. An interaction between the sugar residues of the glycolipids and the raft proteins provides an additional level of specificity that distinguishes one raft from another.

Myelin, DIGs, and membrane rafts in the central nervous system

Over the past 40 years our understanding of the organization of cell membranes has changed dramatically. Membranes are no longer viewed as a homogenous sea of phospholipids studded with randomly positioned islands of proteins. Our current view of the membrane involves the formation of small lipid clusters, comprised mainly of cholesterol and sphingolipids, known as membrane rafts. These lipid clusters apparently include and exclude specific proteins leading to the hypothesis that these domains (1) regulate cellular polarity and compartmentalization through trafficking and sorting, (2) provide platforms for cellular signaling and adhesion, and (3) function as cellular gate keepers. Tremendous controversy surrounds the concept of membrane rafts primarily because these small, highly dynamic entities are too small to be observed with traditional microscopic methods and the most utilized approach for raft analysis relies on poorly quantified, inconsistent biochemical extractions. New analytical approaches are being developed and applied to the study of membrane rafts and these techniques provide great promise for furthering our understanding of these enigmatic domains. In this review we will provide a brief summary of the current understanding of membrane rafts, utilizing the CNS myelin literature for illustrative purposes, and present caveats that should be considered when studying these domains.

Thermotropic behavior and lateral distribution of very long chain sphingolipids

Sphingolipids containing very long acyl chains are abundant in certain specialized tissues and minor components of plasma membranes in most mammalian cells. There are cellular processes in which these sphingolipids are required, and the function seems to be mediated through sphingolipid-rich membrane domains. This study was conducted to explore how very long acyl chains of sphingolipids influence their lateral distribution in membranes. Differential scanning calorimetry showed that 24:0- and 24:1-sphingomyelins, galactosylceramides and glucosylceramides exhibited complex thermotropic behavior and partial miscibility with palmitoyl sphingomyelin. The T(m) was decreased by about 20 degrees C for all 24:1-sphingolipids compared to the corresponding 24:0-sphingolipids. The ability to pack tightly with ordered and extended acyl chains is a necessity for membrane lipids to partition into ordered domains in membranes and thus the 24:1-sphingolipids appeared less likely to do so. Fluorescence quenching measurements showed that the 24:0-sphingolipids formed ordered domains in multicomponent membranes, both as the only sphingolipid and mixed with palmitoyl sphingomyelin. These domains had a high packing density which appeared to hinder the partitioning of sterols into them, as reported by the fluorescent cholesterol analog cholestatrienol. 24:0-SM was, however, better able to accommodate sterol than the glycosphingolipids. The 24:1-sphingolipids could, depending on head group structure, either stabilize or disrupt ordered sphingolipid/cholesterol domains. We conclude that very long chain sphingolipids, when present in biological membranes, may affect the physical properties of or the distribution of sterols between lateral domains. It was also evident that not only the very long acyl chain but also the specific molecular structure of the sphingolipids was of importance for their membrane properties.

Composition-driven surface domain structuring mediated by sphingolipids and membrane-active proteins. Above the nano- but under the micro-scale: mesoscopic biochemical/structural cross-talk in biomembranes

Biomembranes contain a wide variety of lipids and proteins within an essentially two-dimensional structure. The coexistence of such a large number of molecular species causes local tensions that frequently relax into a phase or compositional immiscibility along the lateral and transverse planes of the interface. As a consequence, a substantial microheterogeneity of the surface topography develops and that depends not only on the lipid-protein composition, but also on the lateral and transverse tensions generated as a consequence of molecular interactions. The presence of proteins, and immiscibility among lipids, constitute major perturbing factors for the membrane sculpturing both in terms of its surface topography and dynamics. In this work, we will summarize some recent evidences for the involvement of membrane-associated, both extrinsic and amphitropic, proteins as well as membrane-active phosphohydrolytic enzymes and sphingolipids in driving lateral segregation of phase domains thus determining long-range surface topography.

Biophysics of sphingolipids II. Glycosphingolipids: An assortment of multiple structural information transducers at the membrane surface

Glycosphingolipids are ubiquitous components of animal cell membranes. They are constituted by the basic structure of ceramide with its hydroxyl group linked to single carbohydrates or oligosaccharide chains of different complexity. The combination of the properties of their hydrocarbon moiety with those derived from the variety and complexity of their hydrophilic polar head groups confers to these lipids an extraordinary capacity for molecular-to-supramolecular transduction across the lateral/transverse planes in biomembranes and beyond. In our opinion, most of the advances made over the last decade on the biophysical behavior of glycosphingolipids can be organized into three related aspects of increasing structural complexity: (1) intrinsic codes: local molecular interactions of glycosphingolipids translated into structural self-organization. (2) Surface topography: projection of molecular shape and miscibility of glycosphingolipids into formation of coexisting membrane domains. (3) Beyond the membrane interface: glycosphingolipid as modulators of structural topology, bilayer recombination and surface biocatalysis.

Effects of a short-chain ceramide on bilayer domain formation, thickness, and chain mobililty: DMPC and asymmetric ceramide mixtures

An important part of natural ceramides contain asymmetric hydrocarbon chains. We have used calorimetry, atomic force microscopy, and electron paramagnetic resonance to study the effect of ceramide chain asymmetry in mixtures of C8Cer with DMPC as a model system of hydrocarbon chain disparity. A phase diagram is provided along with information on the thickness of the membrane and the mobility of the chains at different temperatures both below and above the phase transition temperature of the mixtures. The results indicate a partial interdigitation of C8Cer chains in the gel phase, producing a correlation between the organization of both hemilayers. Our data suggest that the effects of ceramides on biomembranes may be bimodal and similar to those of cholesterol.

Influence of chain length and unsaturation on sphingomyelin bilayers

Sphingomyelins (SMs) are among the most common phospholipid components of plasma membranes, usually constituting a mixture of several molecular species with various fatty acyl chain moieties. In this work, we utilize atomistic molecular dynamics simulations to study the differences in structural and dynamical properties of bilayers comprised of the most common natural SM species. Keeping the sphingosine moiety unchanged, we vary the amide bonded acyl chain from 16 to 24 carbons in length and examine the effect of unsaturation by comparing lipids with saturated and monounsaturated chains. As for structural properties, we find a slight decrease in average area per lipid and a clear linear increase in bilayer thickness with increasing acyl chain length both in saturated and unsaturated systems. Increasing the acyl chain length is found to further the interdigitation across the bilayer center. This is related to the dynamics of SM molecules, as the lateral diffusion rates decrease slightly for an increasing acyl chain length. Interdigitation also plays a role in interleaflet friction, which is stronger for unsaturated chains. The effect of the cis double bond is most significant on the local order parameters and rotation rates of the chains, though unsaturation shows global effects on overall lipid packing and dynamics as well. Regarding hydrogen bonding or properties related to the lipid/water interface region, no significant effects were observed due to varying chain length or unsaturation. The significance of the findings presented is discussed.

A glycosynapse in myelin?

Myelin, the multilayered membrane which surrounds nerve axons, is the only example of a membranous structure where contact between extracellular surfaces of membrane from the same cell occurs. The two major glycosphingolipids (GSLs) of myelin, galactosylceramide (GalC) and its sulfated form, galactosylceramide I(3)-sulfate (SGC), can interact with each other by trans carbohydrate-carbohydrate interactions across apposed membranes. They occur in detergent-insoluble lipid rafts containing kinases and thus may be located in membrane signaling domains. These signaling domains may contact each other across apposed extracellular membranes, thus forming glycosynapses in myelin. Multivalent forms of these carbohydrates, GalC/SGC-containing liposomes, or galactose conjugated to albumin, have been added to cultured oligodendrocytes (OLs) to mimic interactions which might occur between these signaling domains when OL membranes or the extracellular surfaces of myelin come into contact. These interactions between multivalent carbohydrate and the OL membrane cause co-clustering or redistribution of myelin GSLs, GPI-linked proteins, several transmembrane proteins, and signaling proteins to the same membrane domains. They also cause depolymerization of the cytoskeleton, indicating that they cause transmission of a signal across the membrane. Their effects have similarities to those of anti-GSL antibodies on OLs, shown by others, suggesting that the multivalent carbohydrate interacts with GalC/SGC in the OL membrane. Communication between the myelin sheath and the axon regulates both axonal and myelin function and is necessary to prevent neurodegeneration. Participation of transient GalC and SGC interactions in glycosynapses between the apposed extracellular surfaces of mature compact internodal myelin might allow transmission of signals throughout the myelin sheath and thus facilitate myelin-axonal communication.

Building up of the liquid-ordered phase formed by sphingomyelin and cholesterol

The long-range and molecular orders and dynamics in codispersions of egg sphingomyelin-cholesterol have been investigated by synchrotron x-ray diffraction and electron spin resonance using phosphatidylcholine spin-labeled at several positions on the sn-2 chain. Mixtures containing 0, 17, 33, 41, 50 mol% cholesterol exhibited a single phase by x-ray diffraction methods. The temperature dependence of the d-spacing between 20 and 50 degrees C is attenuated with increasing proportions of cholesterol, becoming invariant for cholesterol contents of 41 and 50 mol% on completion of the liquid-ordered phase. Electron spin resonance revealed two sites for 17 and 33 mol% cholesterol. One site is highly ordered and the other is less ordered than the fluid phase of pure sphingomyelin as shown by the molecular and the intramolecular order parameters reflecting the segmental motions of the probe. The two-sites exchange rate indicates a mean lifetime of the sites of approximately 0.1 micros during which the lipid displacement is approximately 1 nm. The short lifetime of the sites probed by ESR and the single phase detected by x-ray diffraction support in this binary mixture, the building up of the Lo phase by a progressive accumulation of randomly distributed sphingomyelin-cholesterol condensed complexes rather than by diffusional exchange between extended domains.

Modulation of the phase heterogeneity of aminoglycerophospholipid mixtures by sphingomyelin and monovalent cations: maintenance of the lamellar arrangement in the biological membranes

The phase behaviour of mixed molecular species of phosphatidylethanolamine, phosphatidylserine and sphingomyelin of biological origin were examined in aqueous co-dispersions using synchrotron X-ray diffraction. The co-dispersions of phospholipids studied were aimed to model the mixing of lipids populating the cytoplasmic and outer leaflets in the resting or "scrambled" activated cell membrane. Mixtures enriched with phosphatidylethanolamine and phosphatidylserine were characterized by a phase separation of non-lamellar phases (cubic and inverted hexagonal) with a lamellar gel phase comprising the most saturated molecular species. Inclusion of sphingomyelin in the mixture resulted in a suppression of the hexagonal-II phase in favour of lamellar phases at temperatures where a proportion of the phospholipid was fluid. The effect was also dependent on the total amount of sphingomyelin in ternary mixtures, and the lamellar phase dominated in mixtures containing more than 30 mol%, irrespective of the relative proportions of phosphatidylserine/sphingomyelin. A transition from gel to liquid-crystal phase was detected by wide-angle scattering during heating scans of ternary mixtures enriched in sphingomyelin and was shown by thermal cycling experiments to be coupled with a hexagonal-II phase to lamellar transition. In such samples there was evidence of a coexistence of non-lamellar phases with a lamellar gel phase. A transition of the gel phase to the fluid state on heating from 35 to 41 degrees C was evidenced by a progressive increase in the lamellar d-spacing. The presence of calcium enhanced the phase separation of a lamellar gel phase from a hexagonal-II phase in mixtures enriched in phosphatidylserine. This effect was counteracted by charge screening with 150 mM NaCl. The effect of sphingomyelin on stabilizing the lamellar phase is discussed in the context of an altered composition in the cytoplasmic/outer leaflets of the plasma membrane resulting from scrambling of the phospholipid distribution. The results suggest that a lamellar structure can be retained by the inward translocation of sphingomyelin in biological membranes. The presence of monovalent cations serves also to stabilize the bilayer in activated cells where a translocation of aminoglycerophospholipids and an influx of calcium occur simultaneously.

Effect of selected environmental and physico-chemical factors on bacterial cytoplasmic membranes

Membranes lipids are one of the most adaptable molecules in response to perturbations. Even subtle changes of the composition of acyl chains or head groups can alter the packing arrangements of lipids within the bilayer. This changes the balance between bilayer and nonbilayer lipids, serving to affect bilayer stability and fluidity, as well as altering lipid-protein interactions. External factors can also change membrane fluidity and lipid composition; including temperature, chemicals, ions, pressure, nutrients and the growth phase of the microbial culture. Various biophysical techniques have been used to monitor fluidity changes within the bacterial membrane. In this review, bacterial cytoplasmic membrane changes and related functional effects will be examined as well as the use of fluorescence polarization methods and examples of data obtained from research with bacteria.

Characterization of cholesterol-sphingomyelin domains and their dynamics in bilayer membranes

Lipids segregate with each other into small domains in biological membranes, which can facilitate the associations of particular proteins. The segregation of cholesterol and sphingomyelin (SPM) into domains known as rafts is thought to be especially important. The formation of rafts was studied by using planar bilayer membranes that contained rhodamine-phosphatidylethanolamine (rho-DOPE) as a fluorescent probe, and wide-field fluorescence microscopy was used to detect phase separation of the probe. A fluorescently labeled GM(1), known to preferentially partition into rafts, verified that rho-DOPE faithfully reported the rafts. SPM-cholesterol domains did not form at high temperatures but spontaneously formed when temperature was lowered to below the melting temperature of the SPM. Saturated acyl chains on SPMs therefore promote the formation of rafts. The domains were circular (resolution > or = 0.5 microm), quickly reassumed their circular shape after they were deformed, and merged with each other to create larger domains, all phenomena consistent with liquid-ordered (l(o)) rather than solid-ordered (s(o)) domains. A saturated phosphatidylcholine (PC), disteoryl-PC, could substitute for SPM to complex with cholesterol into a l(o)-domain. But in the presence of cholesterol, a saturated phosphatidylethanolamine or phosphatidylserine yielded s(o)-domains of irregular shape. Lipids with saturated acyl chains can therefore pack well among each other and with cholesterol to form l(o)-domains, but domain formation is dependent on the polar headgroup of the lipid. An individual raft always extended through both monolayers. Degrading cholesterol in one monolayer with cholesterol oxidase first caused the boundary of the raft to become irregular; then the raft gradually disappeared. The fluid nature of rafts, demonstrated in this study, may be important for permitting dynamic interactions between proteins localized within rafts.

Interaction between sulfogalactosylceramide and dimyristoylphosphatidylcholine increases the orientational fluctuation of their lipid hydrocarbon chains

The objective of this study was to investigate the interaction between sulfogalactosylceramide (SGC) and dimyristoylphosphatidylcholine (DMPC) in a mixed model liposomal system (molar ratio SGC:DMPC, 2:3). Structural and dynamic changes of the liposome components were monitored by Fourier-transform infrared spectroscopy (FTIR). Thermotropic FTIR analysis of the mixed liposomes showed a single gel/liquid crystalline phase transition, centered at approximately 42 degrees C. Spectral changes of the amide and ester C = O bands arising from functional groups at the interfacial region indicated a reduced hydrogen bonding of these groups in the mixed liposomes. Pressure-tuning FTIR of mixed liposomes showed that the methylene chains of SGC and DMPC were more orientationally disordered than those of the individual lipid SGC liposomes or DMPC liposomes. These results suggest that the mixed liposomes (molar ratio SGC:DMPC, 2:3) consisted of a homogeneous mixture of SGC and DMPC molecules in which mutual shielding reduced hydrogen bonding in the interfacial region, with a concurrent increase in the orientational disorder of the hydrocarbon chains of both SGC and DMPC.

Cholesterol is required for surface transport of influenza virus hemagglutinin

Transport from the TGN to the basolateral surface involves a rab/N-ethylmaleimide-sensitive fusion protein (NSF)/soluble NSF attachment protein (SNAP)/SNAP receptor (SNARE) mechanism. Apical transport instead is thought to be mediated by detergent-insoluble sphingolipid-cholesterol rafts. By reducing the cholesterol level of living cells by 60-70% with lovastatin and methyl-beta-cyclodextrin, we show that the TGN-to-surface transport of the apical marker protein influenza virus hemagglutinin was slowed down, whereas the transport of the basolateral marker vesicular stomatitis virus glycoprotein as well as the ER-to-Golgi transport of both membrane proteins was not affected. Reduction of transport of hemagglutinin was accompanied by increased solubility in the detergent Triton X-100 and by significant missorting of hemagglutinin to the basolateral membrane. In addition, depletion of cellular cholesterol by lovastatin and methyl-beta-cyclodextrin led to missorting of the apical secretory glycoprotein gp-80, suggesting that gp-80 uses a raft-dependent mechanism for apical sorting. Our data provide for the first time direct evidence for the functional significance of cholesterol in the sorting of apical membrane proteins as well as of apically secreted glycoproteins.

Sphingolipid organization in biomembranes: what physical studies of model membranes reveal

Recent cell biological studies suggest that sphingolipids and cholesterol may cluster in biomembranes to form raft-like microdomains. Such lipid domains are postulated to function as platforms involved in the lateral sorting of certain proteins during their trafficking within cells as well as during signal transduction events. Here, the physical interactions that occur between cholesterol and sphingolipids in model membrane systems are discussed within the context of microdomain formation. A model is presented in which the role of cholesterol is refined compared to earlier models.

Caveolae, DIGs, and the dynamics of sphingolipid-cholesterol microdomains

There is accumulating evidence that lateral assemblies (rafts) of sphingolipids and cholesterol form platforms that serve to support numerous cellular events in membrane traffic and signal transduction. Raft membrane microdomains are thought to function by preferentially associating with specific proteins while excluding others. The basic forces driving raft formation are lipid interactions which are, per se, weak and transient. Sphingolipid rafts should therefore be considered to be dynamic structures in which cholesterol plays an important role as a linker. Caveolins influence these dynamics by forming stabilized raft domains in intracellular membranes as well as at the plasma membrane. Recent data suggest that clustering of raft components could regulate raft dynamics and therefore represents an important feature in the function of these membrane microdomains.

Functional rafts in cell membranes

A new aspect of cell membrane structure is presented, based on the dynamic clustering of sphingolipids and cholesterol to form rafts that move within the fluid bilayer. It is proposed that these rafts function as platforms for the attachment of proteins when membranes are moved around inside the cell and during signal transduction.

Influence of phospholipid chain length on verotoxin/globotriaosyl ceramide binding in model membranes: comparison of a supported bilayer film and liposomes

The importance of the surrounding lipid environment on the availability of glycolipid carbohydrate for ligand binding was demonstrated by studying the influence of phosphatidylcholine fatty acid chain length on binding of verotoxins (VT1 and VT2c) to their specific cell surface receptor, globotriaosylceramide (Gb3) in the presence of auxiliary lipids both in a microtitre plate surface bilayer film and in a liposome membrane model system. In the microtitre assay, both VT1 and VT2c binding to Gb3 was increased as a function of decreasing PC acyl chain length likely resulting in increased Gb3 exposure. In the liposome assay VT1 binding was similarly modulated, however the effect of VT2c binding was more complex and did not follow a simple function of increased carbohydrate exposure. Earlier work established that C22:1 and C18:1Gb3 fatty acid homologues were the preferred Gb3 receptor containing liposomes, but in C14PC liposomes, binding to C22:1Gb3 (but not C18:1Gb3) was elevated such that this Gb3 species now became the preferred receptor for both toxins. This change in verotoxin/Gb3 homologue binding selectivity in the presence of C14PC did not occur in the microtitre bilayer format. These results are consistent with our proposal that these toxins recognize different epitopes on the Gb3 oligosaccharide. We infer that relative availability of these epitopes for toxin binding in an artificial bilayer is influenced not only by the exposure due to the discrepancy between the fatty acyl chain lengths of Gb3 and PC, but by the physical mode of presentation of the bilayer structure. Such acyl chain length differences have a more marked effect in a supported bilayer film whereas only the largest discrepancies affect Gb3 receptor function in liposomes. The basis of phospholipid modulation of glycolipid carbohydrate accessibility for receptor function is likely complex and will involve phase separation, gel/liquid crystalline transition, packing and lateral mobility within the bilayer, suggesting that such parameters should be considered in the assessment of glycolipid receptor function in cells.

Lateral organization of pyrene-labeled lipids in bilayers as determined from the deviation from equilibrium between pyrene monomers and excimers

In lipid bilayers, pyrene and pyrene-labeled lipids form excimers in a concentration-dependent manner. The aromatic amine N, N-diethylaniline (DEA), which has a high membrane-to-medium partition coefficient, quenches the monomers only, and therefore it is expected that under conditions in which the monomers are in equilibrium with the excimers due to the mass law, the Stern-Volmer coefficient (Ksv) for monomers (M), defined as KM, should be identical to that of the excimer (E), defined as KE, and KE/KM = 1. 0. This is indeed the case for pyrene and pyrene valerate in egg phosphatidylcholine small unilamellar vesicles. However, for pyrene decanoate and pyrene dodecanoate in these vesicles, and for N-[12-(1-pyrenyl)dodecanoyl]sphingosylphosphocholine in a matrix of either N-stearoyl sphingosylphosphocholine or 1-palmitoyl-2-oleoyl phosphatidylcholine, KE < KM. This can be explained either by the existence of (a) two subpopulations of excimers, one in fast equilibrium with the monomers and the other, related to ground-state protoaggregates of pyrene lipids; (b) two monomer subpopulations where part of M cannot be quenched by DEA; or (c) two monomer subpopulations, both quenched by DEA, but only one of which produces excimers. The good agreement between the photophysical processes determined by steady state and time-resolved measurements supports the third explanation for the bilayers containing pyrene phospholipids. It also suggests that the main factors determining the immiscibility of pyrene lipids in phospholipid bilayers are the temperature, the difference in the gel-to-liquid-crystalline phase transition temperature (deltaTm) between the matrix and the pyrene lipid, and the structural differences between the matrix lipid and the pyrene-labeled lipid. These results indicate that the KE/KM ratio can serve as a very sensitive tool to quantify isothermal microscopic immiscibility in membranes. This novel approach has the following advantages: applicability to fluid phase immiscibility, requirement of a relatively low mol fraction of pyrene lipids, and conceivably, applicability to biological membranes.

Alteration of glycerolipid and sphingolipid-derived second messenger kinetics in ras transformed 3T3 cells

The effect of ras transformation (rasB fibroblasts) on basal and serum-stimulated diacylglycerol (DAG) composition and mass was examined over time with respect to changes in membrane phospholipid composition and ceramide mass. RasB cells vs. nontransformed control cells (rasD and NR6) had chronically elevated DAG levels (up to 240 min) following serum stimulation, indicating a defect in the recovery phase of the intracellular DAG pulse. Ras transformation also had a dramatic effect on DAG composition. Molecular species analysis revealed that DAG from unstimulated rasB cells was enriched in the delta 9 desaturase fatty acyl species (monoenoate 18:1(n - 7) and 18:1(n - 9)), and depleted in arachidonic acid (20:4(n - 6)). With the exception of glycerophosphoinositol (GPI), DAG remodeling paralleled the compositional alterations in individual phospholipid classes. Importantly, ras transformation altered the fatty acyl composition of sphingomyelin, a precursor to the ceramide second messenger. With the addition of serum, control cells (rasD) had a progressive increase in ceramide mass with levels approximately 5-fold higher by 240 min. In contrast, ceramide levels did not increase in rasB cells at either 4 or 240 min. These results demonstrate that ras-oncogene, in addition to its effects on DAG metabolism, can also abolish the cellular increase in ceramide mass in response to serum stimulation. Since DAG and ceramide may have opposing biological functions, the prolonged elevation of DAG and the suppression of ceramide levels would be consistent with an enhanced proliferative capacity.

A transmembrane potential does not affect the vertical location of charged lipid spin labels with respect to the surface of a phosphatidylcholine bilayer

The effect of a transmembrane potential on the vertical location of a charged lipid in a neutral phosphatidylcholine (PC) lipid bilayer has been investigated using negatively and positively charged spin-labeled lipids. A transmembrane potential was generated across extruded large unilamellar vesicles either by using a K+/Na+ ion gradient and a K+ ionophore or by using a pH gradient. Since a transmembrane potential could have opposing effects on lipids in the inner and outer monolayer, some of the acidic spin labels were asymmetrically located in the inner monolayer as a result of a pH gradient. No significant effect on their order parameters was observed upon applying a transmembrane potential. The internal dipole potential of the bilayer was modified by using dialkyl-PC or by incorporating 10 mol% phloretin, or 6-ketocholestanol in the PC, but a transmembrane potential still had no detectable effect on the spin labeled lipids. Therefore, it is concluded that the electrochemical potential across membranes probably does not cause a significant change in the vertical location of charged lipids with respect to the surface of a PC bilayer. This suggests that polar interactions and/or van der Waals interactions between the spin probe and the surrounding lipids stabilize the overall structure of the membranes and these interactions are not disrupted by a selective effect of the transmembrane potential on the charged lipids.