Structural diversity of sphingomyelin microdomains

Studying lipid flip-flop in asymmetric liposomes using 1H NMR and TR-SANS

The specific spatial and temporal distribution of lipids in membranes play a crucial role in determining the biochemical and biophysical properties of the system. In nature, the asymmetric distribution of lipids is a dynamic process with ATP-dependent lipid transporters maintaining asymmetry, and passive transbilayer diffusion, that is, flip-flop, counteracting it. In this chapter, two probe-free techniques, 1H NMR and time-resolved small angle neutron scattering, are described in detail as methods of investigating lipid flip-flop rates in synthetic liposomes that have been generated with an asymmetric bilayer composition.

Negatively-charged supported lipid bilayers regulate neuronal adhesion and outgrowth

Primary cortical neurons were cultured on negatively charged supported lipid bilayers (SLBs). Such membranes demonstrated the potential effect of negative charges on neuronal growth and neurite branching and elongation.

Placental polar lipid composition is associated with placental gene expression and neonatal body composition

The polar-lipid composition of the placenta reflects its cellular heterogeneity and metabolism. This study explored relationships between placental polar-lipid composition, gene expression and neonatal body composition. Placental tissue and maternal and offspring data were collected in the Southampton Women's Survey. Lipid and RNA were extracted from placental tissue and polar lipids measured by mass spectrometry, while gene expression was assessed using the nCounter analysis platform. Principal component analysis was used to identify patterns within placental lipid composition and these were correlated with neonatal body composition and placental gene expression. In the analysis of placental lipids, the first three principal components explained 19.1%, 12.7% and 8.0% of variation in placental lipid composition, respectively. Principal component 2 was characterised by high principal component scores for acyl-alkyl-glycerophosphatidylcholines and lipid species containing DHA. Principal component 2 was associated with placental weight and neonatal lean mass; this component was associated with gene expression of APOE, PLIN2, FATP2, FABP4, LEP, G0S2, PNPLA2 and SRB1. Principal components 1 and 3 were not related to birth outcomes but they were associated with the gene expression of lipid related genes. Principal component 1 was associated with expression of LEP, APOE, FATP2 and ACAT2. Principal component 3 was associated with expression of PLIN2, PLIN3 and PNPLA2. This study demonstrates that placentas of different sizes have specific differences in polar-lipid composition and related gene expression. These differences in lipid composition were associated with birth weight and neonatal lean mass, suggesting that placental lipid composition may influence prenatal lean mass accretion.

Asymmetric bilayers mimicking membrane rafts prepared by lipid exchange: Nanoscale characterization using AFM-Force spectroscopy

Sphingolipids-enriched rafts domains are proposed to occur in plasma membranes and to mediate important cellular functions. Notwithstanding, the asymmetric transbilayer distribution of phospholipids that exists in the membrane confers the two leaflets different potentials to form lateral domains as next to no sphingolipids are present in the inner leaflet. How the physical properties of one leaflet can influence the properties of the other and its importance on signal transduction across the membrane are questions still unresolved. In this work, we combined AFM imaging and Force spectroscopy measurements to assess domain formation and to study the nanomechanical properties of asymmetric supported lipid bilayers (SLBs) mimicking membrane rafts. Asymmetric SLBs were formed by incorporating N-palmitoyl-sphingomyelin (16:0SM) into the outer leaflet of preformed 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC)/Cholesterol SLBs through methyl-β-cyclodextrin-mediated lipid exchange. Lipid domains were detected after incorporation of 16:0SM though their phase state varied from gel to liquid ordered (Lo) phase if the procedure was performed at 24 or 37 °C, respectively. When comparing symmetric and asymmetric Lo domains, differences in size and morphology were observed, with asymmetric domains being smaller and more interconnected. Both types of Lo domains showed similar mechanical stability in terms of rupture forces and Young's moduli. Notably, force curves in asymmetric domains presented two rupture events that could be attributed to the sequential rupture of a liquid disordered (Ld) and a Lo phase. Interleaflet coupling in asymmetric Lo domains could also be inferred from those measurements. The experimental approach outlined here would significantly enhance the applicability of membrane models.

Sphingomyelinases D From Loxosceles Spider Venoms and Cell Membranes: Action on Lipid Rafts and Activation of Endogenous Metalloproteinases

Loxosceles spider venom contains Sphingomyelinase D (SMase D), the key toxin causing pathology. SMase D hydrolyzes the main component of lipid rafts, sphingomyelin, which changes the membrane microenvironment resulting in the activation of endogenous metalloproteinase from the ADAMs family. Alterations in membrane microenvironment of lipid rafts contribute to the activation of several cell surface molecules. Serine proteinases convertases acting on the pro-domain of membrane metalloproteinases, such as ADAMs, increase the cleavage and the release of proteins ectodomains and receptors located at the cell surface areas containing lipid rafts. We, therefore, investigated the interaction of SMases D with these membrane microdomains (lipid rafts) in human keratinocytes, to better understand the molecular mechanism of SMases D action, and identify the ADAM(s) responsible for the cleavage of cell surface molecules. Using specific inhibitors, we observed that ADAMs 10 and 17 are activated in the cell membrane after SMase D action. Furthermore, proproteins convertases, such as furin, are involved in the SMase D induced ADAMs activation. One of the signaling pathways that may be involved in the activation of these proteases is the MAPK pathway, since phosphorylation of ERK1/2 was observed in cells treated with SMase D. Confocal analysis showed a strong colocalization between SMase D and GM1 ganglioside present in rafts. Analysis of structural components of rafts, such as caveolin-1 and flotillin-1, showed that the action of SMase D on cell membranes leads to a reduction in caveolin-1, which is possibly degraded by toxin-induced superoxide production in cells. The action of the toxin also results in flotilin-1 increased detection in the cell membrane. These results indicate that SMases D from Loxosceles venoms alter membrane rafts structure, leading to the activation of membrane bound proteases, which may explain why the lipase action of this toxin can result in proteolytic cleavage of cell surface proteins, ultimately leading to pathology.

Biophysical approaches for exploring lipopeptide-lipid interactions

In recent years, lipopeptides (LPs) have attracted a lot of attention in the pharmaceutical industry due to their broad-spectrum of antimicrobial activity against a variety of pathogens and their unique mode of action. This class of compounds has enormous potential for application as an alternative to conventional antibiotics and for pest control. Understanding how LPs work from a structural and biophysical standpoint through investigating their interaction with cell membranes is crucial for the rational design of these biomolecules. Various analytical techniques have been developed for studying intramolecular interactions with high resolution. However, these tools have been barely exploited in lipopeptide-lipid interactions studies. These biophysical approaches would give precise insight on these interactions. Here, we reviewed these state-of-the-art analytical techniques. Knowledge at this level is indispensable for understanding LPs activity and particularly their potential specificity, which is relevant information for safe application. Additionally, the principle of each analytical technique is presented and the information acquired is discussed. The key challenges, such as the selection of the membrane model are also been briefly reviewed.

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A brief overview of topics to understand the generalities of lipopeptide (LP) science.

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Main analytical techniques used to reveal the interaction and the distorting effect of LP on artificial membranes.

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Guidelines for selecting of the most adequate membrane models for the given analytical technique.

1H NMR Shows Slow Phospholipid Flip-Flop in Gel and Fluid Bilayers

We measured the transbilayer diffusion of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in large unilamellar vesicles, in both the gel (L_β') and fluid (L_α) phases. The choline resonance of headgroup-protiated DPPC exchanged into the outer leaflet of headgroup-deuterated DPPC-d13 vesicles was monitored using ¹H NMR spectroscopy, coupled with the addition of a paramagnetic shift reagent. This allowed us to distinguish between the inner and outer bilayer leaflet of DPPC, to determine the flip-flop rate as a function of temperature. Flip-flop of fluid-phase DPPC exhibited Arrhenius kinetics, from which we determined an activation energy of 122 kJ mol^-1. In gel-phase DPPC vesicles, flip-flop was not observed over the course of 250 h. Our findings are in contrast to previous studies of solid-supported bilayers, where the reported DPPC translocation rates are at least several orders of magnitude faster than those in vesicles at corresponding temperatures. We reconcile these differences by proposing a defect-mediated acceleration of lipid translocation in supported bilayers, where long-lived, submicron-sized holes resulting from incomplete surface coverage are the sites of rapid transbilayer movement.

The structural and functional effects of Hg(II) and Cd(II) on lipid model systems and human erythrocytes: A review

The anthropogenic mobilization of mercury and cadmium into the biosphere has led to an increased and ineludible entry of these metals into biological systems. Here we discuss the impact of Hg(II) and Cd(II) on lipid model systems and human erythrocytes from a biophysical perspective. After a brief introduction to their implications on human health, studies that have investigated the effects of Hg(II) and Cd(II) on lipid model systems and human erythrocytes are discussed. In terms of lipids as toxicological target sites, predominantly variations in lipid head groups have been the source of investigation. However, as research in this field progresses, the effects of Hg(II) and Cd(II) on other structural features, such as acyl chain length and unsaturation, and other important lipid components and complex biomimetic lipid mixtures, will require further examinations. This review provides an analysis of what has been learned collectively from the diverse methodologies and experimental conditions used thus far. Consequently, there is a need for more comprehensive and thorough investigations into the effects of Hg(II) and Cd(II) on lipid membranes under consistent experimental conditions such as pH, ionic strength, temperature, and choice of lipid model system.

Scanning tunneling microscope observation of the phosphatidylserine domains in the phosphatidylcholine monolayer

A mixed monolayer of 1,2-dihexanoyl-sn-glycero-3-phospho-l-serine (DHPS) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) on an 1-octanethiol-modified gold substrate was visualized on the nanometer scale using in situ scanning tunneling microscopy (STM) in aqueous solution. DHPS clusters were evident as spotty domains. STM enabled us to distinguish DHPS molecules from DHPC molecules depending on their electronic structures. The signal of the DHPS domains was abolished by neutralization with Ca(2+). The addition of the PS + Ca(2+)-binding protein of annexin V to the Ca(2+)-treated monolayer gave a number of spots corresponding to a single annexin V molecule.

Milk sphingomyelin domains in biomimetic membranes and the role of cholesterol: morphology and nanomechanical properties investigated using AFM and force spectroscopy

Milk sphingomyelin (MSM) and cholesterol segregate into domains in the outer bilayer membrane surrounding milk fat globules. To elucidate the morphology and mechanical properties of theses domains, supported lipid bilayers with controlled molar proportions of MSM, dioleoylphosphatidylcholine (DOPC) and cholesterol were produced in buffer mimicking conditions of the milk aqueous phase. Atomic force microscopy imaging showed that (i) for T < 35 °C MSM segregated in gel phase domains protruding above the fluid phase, (ii) the addition of 20 mol % cholesterol resulted in smaller and more elongated l(o) phase domains than in equimolar MSM/DOPC membranes, (iii) the MSM/cholesterol-enriched l(o) phase domains were less salient than the MSM gel phase domains. Force spectroscopy measurements furthermore showed that cholesterol reduced the resistance of MSM/DOPC membrane to perforation. The results are discussed with respect to the effect of cholesterol on the biophysical properties of lipid membranes. The combination of AFM imaging and force mapping provides unprecedented insight into the structural and mechanical properties of milk lipid membranes, and opens perspectives for investigation of the functional properties of MSM domains during milk fat processing or digestion.

Biological functions of sphingomyelins

Sphingomyelin (SM) is a dominant sphingolipid in membranes of mammalian cells and this lipid class is specifically enriched in the plasma membrane, the endocytic recycling compartment, and the trans Golgi network. The distribution of SM and cholesterol among cellular compartments correlate. Sphingolipids have extensive hydrogen-bonding capabilities which together with their saturated nature facilitate the formation of sphingolipid and SM-enriched lateral domains in membranes. Cholesterol prefers to interact with SMs and this interaction has many important functional consequences. In this review, the synthesis, regulation, and intracellular distribution of SMs are discussed. The many direct roles played by membrane SM in various cellular functions and processes will also be discussed. These include involvement in the regulation of endocytosis and receptor-mediated ligand uptake, in ion channel and G-protein coupled receptor function, in protein sorting, and functioning as receptor molecules for various bacterial toxins, and for non-bacterial pore-forming toxins. SM is also an important constituent of the eye lens membrane, and is believed to participate in the regulation of various nuclear functions. SM is an independent risk factor in the development of cardiovascular disease, and new studies have shed light on possible mechanism behind its role in atherogenesis.

Anesthetics interacting with lipid rafts

The exact mechanism by which anesthetics induce cell membrane-mediated modifications is still an open question. Although the fluidization effect of the anesthetic molecules on the cellular membrane is widely recognized, it is not known if anesthetics show any preference for specific membrane domains, namely the lipid rafts. The importance of these membrane micro-domains derives from the fact that they have been associated with cell signaling pathways, as well as with specific drug interactions. The objective of this work is to contribute for the elucidation of this question through the comparison of the anesthetic interactions with membranes of various lipid compositions. Liposomes prepared with an equimolar mixture of POPC, sphingomyelin and cholesterol, were chosen as models for lipid rafts. The interactions of these liposomes with two local anesthetics, tetracaine and lidocaine, and one general anesthetic, propofol, were studied. The effect of cholesterol was investigated by comparing anesthetic interactions with POPC/SM liposomes and POPC/SM/CHOL liposomes. The following experimental techniques were used: quartz crystal microbalance with dissipation, differential scanning calorimetry and phosphorus nuclear magnetic resonance. Although the liposomes investigated by the different techniques are not in the same conditions, it is possible to assemble the information obtained from all experimental techniques employed to reach a general conclusion. Tetracaine interacts more with raftlike domains, lidocaine induces stronger modifications on POPC/SM liposomes and the results for propofol are not fully conclusive but it seems to be the least prone to lipid interactions. The results were compared with those obtained with DMPC-containing liposomes, reported in a previous work.

Effects of lipid composition and phase on the membrane interaction of the prion peptide 106-126 amide

Lipid rafts are specialized liquid-ordered (L(o)) phases of the cell membrane that are enriched in sphingolipids and cholesterol (Chl), and surrounded by a liquid-disordered (L(d)) phase enriched in glycerophospholipids. Lipid rafts are involved in the generation of pathological forms of proteins that are associated with neurodegenerative diseases. To investigate the effects of lipid composition and phase on the generation of pathological forms of proteins, we constructed an L(d)-gel phase-separated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/sphingomyelin (from bovine brain (BSM))-supported lipid bilayer (SLB) and an L(d)-L(o) phase-separated POPC/BSM/Chl SLB. We used in situ time-lapse atomic force microscopy to study the interactions between these SLBs and the prion peptide K(106)TNMKHMAGAAAAGAVVGGLG(126) (PrP106-126) amide, numbered according to the human prion-peptide sequence. Our results show that: 1), with the presence of BSM in the L(d) phase, the PrP106-126 amide induces fully penetrated porations in the L(d) phase of POPC/BSM SLB and POPC/BSM/Chl SLB; 2), with the presence of both BSM and Chl in the L(d) phase, the PrP106-126 amide induces the disintegration of the L(d) phase of POPC/BSM/Chl SLB; and 3), with the presence of both BSM and Chl in the L(o) phase, PrP106-126 amide induces membrane thinning in the L(o) phase of POPC/BSM/Chl SLB. These results provide comprehensive insight into the process by which the PrP106-126 amide interacts with lipid membranes.

Raftlike mixtures of sphingomyelin and cholesterol investigated by solid-state 2H NMR spectroscopy

Sphingomyelin is a lipid that is abundant in the nervous systems of mammals, where it is associated with putative microdomains in cellular membranes and undergoes alterations due to aging or neurodegeneration. We investigated the effect of varying the concentration of cholesterol in binary and ternary mixtures with N-palmitoylsphingomyelin (PSM) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) using deuterium nuclear magnetic resonance ((2)H NMR) spectroscopy in both macroscopically aligned and unoriented multilamellar dispersions. In our experiments, we used PSM and POPC perdeuterated on the N-acyl and sn-1 acyl chains, respectively. By measuring solid-state (2)H NMR spectra of the two lipids separately in mixtures with the same compositions as a function of cholesterol mole fraction and temperature, we obtained clear evidence for the coexistence of two liquid-crystalline domains in distinct regions of the phase diagram. According to our analysis of the first moments M1 and the observed (2)H NMR spectra, one of the domains appears to be a liquid-ordered phase. We applied a mean-torque potential model as an additional tool to calculate the average hydrocarbon thickness, the area per lipid, and structural parameters such as chain extension and thermal expansion coefficient in order to further define the two coexisting phases. Our data imply that phase separation takes place in raftlike ternary PSM/POPC/cholesterol mixtures over a broad temperature range but vanishes at cholesterol concentrations equal to or greater than a mole fraction of 0.33. Cholesterol interacts preferentially with sphingomyelin only at smaller mole fractions, above which a homogeneous liquid-ordered phase is present. The reasons for these phase separation phenomena seem to be differences in the effects of cholesterol on the configurational order of the palmitoyl chains in PSM-d31 and POPC-d31 and a difference in the affinity of cholesterol for sphingomyelin observed at low temperatures. Hydrophobic matching explains the occurrence of raftlike domains in cellular membranes at intermediate cholesterol concentrations but not saturating amounts of cholesterol.

Atomic force microscopy of lipid domains in supported model membranes

Atomic force microscopy (AFM) has been a significant tool in the characterization of lipid domains in model membranes. With AFM, one can image the structure of membranes in a natural fluid environment with a lateral resolution that approaches 1 nm and vertical resolution of 0.1 nm. The AFM technique is discussed, with a special emphasis on imaging soft, compliant memranes that are supported on solid substrates such as glass or mica. In typical model membranes, lipid domains are formed by phase separation in multicomponent lipid mixtures and are observed by nm-level height differences owing to lipid packing. A general procedure for creating supported lipid bilayers through vesicle fusion is discussed.

Remodeling of ordered membrane domains by GPI-anchored intestinal alkaline phosphatase

Glycosylphosphatidyl-inositol (GPI)-anchored proteins preferentially localize in the most ordered regions of the cell plasma membrane. Acyl and alkyl chain composition of GPI anchors influence the association with the ordered domains. This suggests that, conversely, changes in the fluid and in the ordered domains lipid composition affect the interaction of GPI-anchored proteins with membrane microdomains. Validity of this hypothesis was examined by investigating the spontaneous insertion of the GPI-anchored intestinal alkaline phophatase (BIAP) into the solid (gel) phase domains of preformed supported membranes made of dioleoylphosphatidylcholine/dipalmitoylphosphatidylcholine (DOPC/DPPC), DOPC/sphingomyelin (DOPC/SM), and palmitoyloleoylphosphatidylcholine/SM (POPC/SM). Atomic force microscopy (AFM) showed that BIAP inserted in the gel phases of the three mixtures. However, changes in the lipid composition of membranes had a marked effect on the protein containing bilayer topography. Moreover, BIAP insertion was associated with a net transfer of phospholipids from the fluid to the gel (DOPC/DPPC) or from the gel to the fluid (POPC/SM) phases. For DOPC/SM bilayers, transfer of lipids was dependent on the homogeneity of the gel SM phase. The data strongly suggest that BIAP interacts with the most ordered lipid species present in the gel phases of phase-separated membranes. They also suggest that GPI-anchored proteins might contribute to the selection of their own microdomain environment.

A Mycobacterium tuberculosis-derived lipid inhibits membrane fusion by modulating lipid membrane domains

Tuberculosis is an infectious and potentially fatal disease caused by the acid-fast bacillus Mycobacterium tuberculosis (MTB). One hallmark of a tuberculosis infection is the ability of the bacterium to subvert the normal macrophage defense mechanism of the host immune response. Lipoarabinomannan (LAM), an integral component of the MTB cell wall, is released when MTBs are taken into phagosomes and has been reported to be involved in the inhibition of phago-lysosomal (P-L) fusion. However, the physical chemistry of the effects of LAM on lipid membrane structure relative to P-L fusion has not been studied. We produced membranes in vitro composed of dioleoylphosphatidylcholine, sphingomyelin, and cholesterol to simulate phagosomal lipid membranes and quantified the effects of the addition of LAM to these membranes, using fluorescence resonance energy transfer assays and atomic force microscopy. We found that LAM inhibits vesicle fusion and markedly alters lipid membrane domain morphology and sphingomyelin-chollesterol/dioleoylphosphatidylcholine ratios. These data demonstrate that LAM induces a dramatic reorganization of lipid membranes in vitro and clarifies the role of LAM in the inhibition of P-L fusion and the survival of the MTB within the macrophage.

Cholesterol modulation of membrane resistance to Triton X-100 explored by atomic force microscopy

Biomembranes are not homogeneous, they present a lateral segregation of lipids and proteins which leads to the formation of detergent-resistant domains, also called "rafts". These rafts are particularly enriched in sphingolipids and cholesterol. Despite the huge body of literature on raft insolubility in non-ionic detergents, the mechanisms governing their resistance at the nanometer scale still remain poorly documented. Herein, we report a real-time atomic force microscopy (AFM) study of model lipid bilayers exposed to Triton X-100 (TX-100) at different concentrations. Different kinds of supported bilayers were prepared with dioleoylphosphatidylcholine (DOPC), sphingomyelin (SM) and cholesterol (Chol). The DOPC/SM 1:1 (mol/mol) membrane served as the non-resistant control, and DOPC/SM/Chol 2:1:1 (mol/mol/mol) corresponded to the raft-mimicking composition. For all the lipid compositions tested, AFM imaging revealed that TX-100 immediately solubilized the DOPC fluid phase leaving resistant patches of membrane. For the DOPC/SM bilayers, the remaining SM-enriched patches were slowly perforated leaving crumbled features reminiscent of the initial domains. For the raft model mixture, no holes appeared in the remaining SM/Chol patches and some erosion occurred. This work provides new, nanoscale information on the biomembranes' resistance to the TX-100-mediated solubilization, and especially about the influence of Chol.

Gating and conductance changes in BK(Ca) channels in bilayers are reciprocal

The energy associated with a mismatch between the hydrocarbon portions of a lipid bilayer and the hydrophobic regions of a transmembrane protein requires that one or both components deform in an attempt to minimize the energy difference. Transmembrane potassium channel subunits are composed of different structural motifs, each responsible for ion-selectivity, conductance and gating capabilities. Each has an inherent degree of flexibility commensurate with its amino acid composition. It is not clear, however, how each structural motif will respond to a fixed amount of distortion applied to the whole structure. We examined the single-channel conductance (G(c)) and gating (open probability, P (o)) of single BK(Ca) channels (hslo alpha-subunits) inserted into planar lipid bilayers containing 1,2-dioleoyl-3-phosphatidylethanolamine (DOPE) or DOPE with either 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) or sphingomyelin (SPM) and 1-palmitoyl-2-oleoyl-3-phosphatidylethanolamine (POPE) with SPM. These latter three binary mixtures formed stable membranes with different distributions of thickness domains as determined by atomic force microscopy. Channels placed in each composition should be exposed to different amounts of distortion. BK(Ca) channels forced into the DOPE/SPM bilayer containing lipid domains with two different thicknesses showed two distinct levels of G(c) and P(o). The alterations in G(c) and P(o) were reciprocal. A larger conductance was accompanied by a smaller value for gating and vice versa. Channels forced into the POPE/SPM bilayer containing lipid domains with different thicknesses showed more than two distinct levels of G(c) and P(o). Channels placed in a uniform bilayer (DOPE/DOPC) showed a uniform distribution of conductance and activation. We conclude that both the inner and outer domains of the channel where these two channel functions are localized respond to deformation and that a fixed amount of distortion results in reciprocal changes in protein function.

Visualizing the localization of sulfoglycolipids in lipid raft domains in model membranes and sperm membrane extracts

Sulfogalactosylglycerolipid (SGG) is found in detergent-resistant lipid raft fractions isolated from sperm plasma membranes and has been shown to be important in sperm-egg adhesion. In order to provide more direct evidence for the association of sulfoglycolipids with lipid raft domains, we have examined the distribution of two sulfoglycolipids in supported membranes prepared from artificial lipid mixtures and cellular lipid extracts. Atomic force microscopy has been used to visualize the localization of SGG and sulfogalactosylceramide (SGC) in liquid-ordered domains in supported bilayers of ternary lipid mixtures comprised of dipalmitoylphosphatidylcholine, cholesterol and palmitoyldocosahexaenoylphosphatidylcholine. The localization of SGC/SGG in the liquid-ordered raft domains is demonstrated by changes in bilayer morphology in the presence of sulfoglycolipid, by selective antibody labeling of the domains with anti-SGC/SGG and by the effects of the cholesterol-sequestering agent, methyl-beta-cyclodextrin, on the supported membranes. In addition, we use a combination of atomic force microscopy and immunofluorescence to show that supported bilayers made from lipids extracted from sperm anterior head plasma membranes (APM) and isolated APM vesicles exhibit small SGG-rich domains that are similar to those observed in bilayers of artificial lipid mixtures. The possible implications of these results for the involvement of SGG-rich lipid rafts in modulating sperm-egg interactions in vivo and the utility of model membranes for studying the behavior of lipid rafts are discussed.

Imaging of the domain organization in sphingomyelin and phosphatidylcholine monolayers

The lateral organization of biomembranes has gained significant interest when the fluid mosaic model was challenged by the model of "lipid rafts". Several lipid classes like cholesterol and sphingolipids are considered to be essential for their formation. Here we investigate the lateral domain formation in binary mixtures of sphingomyelin and phosphatidylcholine. Both are major lipid components of lipoproteins and mammalian cell membranes at various molar ratios. Surface pressure-area isotherms and surface potential-area isotherms of monolayers composed of these lipids clearly indicated non-ideal mixing. In addition, Brewster angle microscopy provided a well-suited approach to image the formation of lateral domains. These images demonstrated that pure sphingomyelin forms very stable finger-like domains that exhibit a distinct internal organization suggesting an anisotropic orientation of the acyl side chains. Similar behavior was found for mixtures containing more than 60 mol% sphingomyelin. With increasing content of phosphatidylcholine the domain size decreased and the surface pressure, where domain formation occurred, increased. At lower sphingomyelin content (30-60 mol%) rather round-shaped, smaller domains were observed. Thus, the potential of sphingomyelin domains as potentially important building blocks for actual domains that could be building blocks for raft formation is suggested, even without the presence of cholesterol. In addition, these observations may suggest a role for the distinct molar ratio of these key lipids frequently found in physiologically relevant particles such as low and high density lipoproteins or the outer leaflet of the human erythrocyte membrane.

Transition from nanodomains to microdomains induced by exposure of lipid monolayers to air

The morphology of monolayers prepared from ternary lipid mixtures that have coexisting fluid phases has been examined by atomic force microscopy for samples transferred to mica before and after exposure to air. Mixtures of 1,2-dioleoyl-sn-glycero-3-phosphocholine and cholesterol with either egg sphingomyelin or 1,2-dipalmitoyl-sn-glycero-3-phosphocholine were studied at several surface pressures. Both lipid mixtures have a combination of small islands and large microdomains at low surface pressure (5-10 mN/m) for monolayers deposited in either air or nitrogen. By contrast, monolayers have small interconnected nanodomains when deposited under nitrogen at 30 mN/m but mixtures of large microdomains and small nanodomains when transferred after exposure to air. These results are consistent with an earlier report that concluded that the formation of large domains at high surface pressures (>30 mN/m) for monolayers exposed to air is caused by lipid oxidation. However, the higher spatial resolution available with atomic force microscopy indicates that exposure of the monolayers to air leads to an increase in the size of preexisting nanodomains, rather than a change in the miscibility pressure. Examination of changes in surface morphology as a function of surface pressure demonstrate a gradual evolution in size and surface coverage for both nano- and microdomains, before formation of a network of interconnected nanodomains. Similar studies for binary mixtures in the absence of cholesterol indicate that lipid oxidation results in analogous changes in domain size for monolayers with coexisting gel and fluid phases. These results illustrate the importance of using techniques capable of probing the nanoscale organization of membranes.

Temperature-dependent localization of GPI-anchored intestinal alkaline phosphatase in model rafts

In plasma membranes, most of glycosylphosphatidylinositol (GPI)-anchored proteins would be associated with rafts, a category of ordered microdomains enriched in sphingolipids and cholesterol (Ch). They would be also concentrated in the detergent resistant membranes (DRMs), a plasma membrane fraction extracted at low temperature. Preferential localization of GPI-anchored proteins in these membrane domains is essentially governed by their high lipid order, as compared to their environment. Changes in the temperature are expected to modify the membrane lipid order, suggesting that they could affect the distribution of GPI-anchored proteins between membrane domains. Validity of this hypothesis was examined by investigating the temperature-dependent localization of the GPI-anchored bovine intestinal alkaline phophatase (BIAP) into model raft made of palmitoyloleoylphosphatidylcholine/sphingomyelin/cholesterol (POPC/SM/Chl) supported membranes. Atomic force microscopy (AFM) shows that the inserted BIAP is localized in the SM/Chl enriched ordered domains at low temperature. Above 30 degrees C, BIAP redistributes and is present in both the 'fluid' POPC enriched and the ordered SM/Chl domains. These data strongly suggest that in cells the composition of plasma membrane domains at low temperature differs from that at physiological temperature.

The atomic force microscope as a tool for studying phase separation in lipid membranes

Atomic force microscopy has developed into a powerful tool in the study of phase separation in lipid bilayers. Its ability to image a semi-fluid surface under buffer at nanometre lateral resolution and Angstrom resolution vertically allows us to distinguish phase separated lipid domains, models of the elusive rafts postulated to exist as functional platforms in the cellular membrane, which may only rise 0.3 nm above the surrounding membrane. This review charts the history of this development, and includes a description of sample preparation techniques, factors affecting image contrast mechanisms, its use in the investigation of the pre-transition ripple phase, and in the localization of cell surface proteins.

Domain Formation in Lipid Bilayers Probed by Two-Dimensional Infrared Spectroscopy

Two-dimensional infrared (2D-IR) spectroscopy has been used to probe structure and dynamics in binary sphingomyelin/phospholipid liposomes. The liposomes consist of 1-palmitoyl-2-linoleyl phosphatidylcholine (PLPC) and sphingomyelin (SPM) in the ratio 1:1. The diagonal part of the 2D-IR spectra shows two bands which are due to amide I of SPM and to the carbonyl moieties of PLPC. The diagonal components of the 2D-IR spectra reveal a difference in the molecular dynamics. The presence of off-diagonal cross-peaks indicates the occurrence of intermolecular structural correlation. The intensity of the cross-peaks is consistent with segregation of two lipid components into PLPC and SPM molecular domains.

Plasmon-waveguide Resonance Studies of Lateral Segregation of Lipids and Proteins into Microdomains (Rafts) in Solid-supported Bilayers

Plasmon-waveguide resonance (PWR) spectroscopy has been used to examine solid-supported lipid bilayers consisting of dioleoylphosphatidylcholine (DOPC), palmitoyloleoylphosphatidylcholine (POPC), sphingomyelin (SM), and phosphatidylcholine/SM binary mixtures. Spectral simulation of the resonance curves demonstrated an increase in bilayer thickness, long-range order, and molecular packing density in going from DOPC to POPC to SM single component bilayers, as expected based on the decreasing level of unsaturation in the fatty acyl chains. DOPC/SM and POPC/SM binary mixtures yielded PWR spectra that can be ascribed to a superposition of two resonances corresponding to microdomains (rafts) consisting of phosphatidylcholine- and SM-rich phases coexisting within a single bilayer. These were formed spontaneously over time as a consequence of lateral phase separation. Each microdomain contained a small proportion (<20%) of the other lipid component, which increased their kinetic and thermodynamic stability. Incorporation of a glycosylphosphatidylinositol-linked protein (placental alkaline phosphatase) occurred within each of the single component bilayers, although the insertion was less efficient into the DOPC bilayer. Incorporation of placental alkaline phosphatase into a DOPC/SM binary bilayer occurred with preferential insertion into the SM-rich phase, although the protein incorporated into both phases at higher concentrations. These results demonstrate the utility of PWR spectroscopy to provide insights into raft formation and protein sorting in model lipid membranes.

Use of cyclodextrin for AFM monitoring of model raft formation

The lipid rafts membrane microdomains, enriched in sphingolipids and cholesterol, are implicated in numerous functions of biological membranes. Using atomic force microscopy, we have examined the effects of cholesterol-loaded methyl-beta-cyclodextrin (MbetaCD-Chl) addition to liquid disordered (l(d))-gel phase separated dioleoylphosphatidylcholine (DOPC)/sphingomyelin (SM) and 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC)/SM supported bilayers. We observed that incubation with MbetaCD-Chl led to the disappearance of domains with the formation of a homogeneously flat bilayer, most likely in the liquid-ordered (l(o)) state. However, intermediate stages differed with the passage through the coexistence of l(o)-l(d) phases for DOPC/SM samples and of l(o)-gel phases for POPC/SM bilayers. Thus, gel phase SM domains surrounded by a l(o) matrix rich in cholesterol and POPC could be observed just before reaching the uniform l(o) state. This suggests that raft formation in biological membranes could occur not only via liquid-liquid but also via gel-liquid immiscibility. The data also demonstrate that MbetaCD-Chl as well as the unloaded cyclodextrin MbetaCD make holes and preferentially extract SM in supported bilayers. This strongly suggests that interpretation of MbetaCD and MbetaCD-Chl effects on cell membranes only in terms of cholesterol movements have to be treated with caution.