Ceramides in phospholipid membranes: effects on bilayer stability and transition to nonlamellar phases

Undulating tubular liposomes through incorporation of a synthetic skin ceramide into phospholipid bilayers

Nonspherical liposomes were prepared by doping L-alpha-phosphatidylcholine (PC) with ceramide VI (a skin lipid). Cryo-transmission electron microscopy shows the liposome shape changing from spherical to an undulating tubular morphology, when the amount of ceramide VI is increased. The formation of tubular liposomes is energetically favorable and is attributed to the association of ceramide VI with PC creating regions of lower curvature. Since ceramides are the major component of skin lipids in the stratum corneum, tubular liposomes containing ceramide may potentially serve as self-enhanced nanocarriers for transdermal delivery.

Ceramide-1-phosphate, in contrast to ceramide, is not segregated into lateral lipid domains in phosphatidylcholine bilayers

Sphingolipids are key lipid regulators of cell viability: ceramide is one of the key molecules in inducing programmed cell death (apoptosis), whereas other sphingolipids, such as ceramide 1-phosphate, are mitogenic. The thermotropic and structural behavior of binary systems of N-hexadecanoyl-D-erythro-ceramide (C(16)-ceramide) or N-hexadecanoyl-D-erythro-ceramide-1-phosphate (C(16)-ceramide-1-phosphate; C(16)-C1P) with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was studied with DSC and deuterium nuclear magnetic resonance ((2)H-NMR). Partial-phase diagrams (up to a mole fraction of sphingolipids X = 0.40) for both mixtures were constructed based on DSC and (2)H-NMR observations. For C(16)-ceramide-containing bilayers DSC heating scans showed already at X(cer) = 0.025 a complex structure of the main-phase transition peak suggestive of lateral-phase separation. The transition width increased significantly upon increasing X(cer), and the upper-phase boundary temperature of the mixture shifted to approximately 65 degrees C at X(cer) = 0.40. The temperature range over which (2)H-NMR spectra of C(16)-ceramide/DPPC-d(62) mixtures displayed coexistence of gel and liquid crystalline domains increased from approximately 10 degrees for X(cer) = 0.1 to approximately 21 degrees for X(cer) = 0.4. For C16-C1P/DPPC mixtures, DSC and (2)H-NMR observations indicated that two-phase coexistence was limited to significantly narrower temperature ranges for corresponding C1P concentrations. To complement these findings, C(16)-ceramide/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and C16-C1P/POPC mixtures were also studied by (2)H-NMR and fluorescence techniques. These observations indicate that DPPC and POPC bilayers are significantly less perturbed by C(16)-C1P than by C(16)-ceramide and that C(16)-C1P is miscible within DPPC bilayers at least up to X(C1P) = 0.30.

Phase behavior of aqueous dispersions of mixtures of N-palmitoyl ceramide and cholesterol: a lipid system with ceramide-cholesterol crystalline lamellar phases

Ceramides are particularly abundant in the stratum corneum lipid matrix, where they determine its unusual mesostructure, are involved in the lateral segregation of lipid domains in biological cell membranes, and are also known to act as signaling agents in cells. The importance attributed to ceramides in several biological processes has heightened in recent years, demanding a better understanding of their interaction with other membrane components, namely, cholesterol. Structural data concerning pure ceramides in water are relatively scarce, and this is even more the case for mixtures of ceramides with other lipids commonly associated with them in biological systems. We have derived the thermotropic binary phase diagram of mixtures of N-palmitoyl- D-erythro-sphingosine, C16:0-ceramide, and cholesterol in excess water, using differential scanning calorimetry and small- and wide-angle X-ray diffraction. These mixtures are self-organized in lamellar mesostructures that, between other particularities, show two ceramide to cholesterol crystalline phases with molar proportions that approach 2:3 and 1:3. The 2:3 phase crystallizes in a tetragonal arrangement with a lamellar repeat distance of 3.50 nm, which indicates an unusual lipid stacking, probably unilamellar. The uncommon mesostructures formed by ceramides with cholesterol should be considered in the rationalization of their singular structural role in biological systems.

Ceramide-induced transbilayer (flip-flop) lipid movement in membranes

Lipids in biological membranes are asymmetrically distributed across the bilayer. The choline-containing lipids, phosphatidylcholine (PtdCho) and sphingomyelin (SM), are more abundant in the external leaflet. In contrast, the amino-containing glycerophospholipids, phosphatidylserine (PtdSer) and phosphatidylethanolamine (PtdEth), are located preferentially on the cytoplasmic leaflet. The maintenance of transbilayer lipid asymmetry is essential for normal membrane function, and disruption of this asymmetry is associated with cell activation or pathological condition. The physiological role of ceramide formation in response to cell stimulation remains controversial. Ceramide formation serves many different functions at various locations in the cell. Despite the limited capacity for spontaneous intracellular diffusion or membrane flip-flop of lipids in membranes, we have found that ceramide production, via sphingomyelinase action or addition of external ceramide, induces the transbilayer lipid motion of the lipids within the cellular membrane. This chapter outlines various commonly used assays for measuring lipid flip-flop induced by ceramide in cell and model membranes.

Ceramide in bacterial infections and cystic fibrosis

Ceramide is formed by the activity of sphingomyelinases, by degradation of complex sphingolipids, reverse ceramidase activity or de novo synthesized. The formation of ceramide within biological membranes results in the formation of large ceramide-enriched membrane domains. These domains serve the spatial and temporal organization of receptors and signaling molecules. The acid sphingomyelinase-ceramide system plays an important role in the infection of mammalian host cells with bacterial pathogens such as Neisseria gonorrhoeae, Escherichia coli, Staphylococcus aureus, Listeria monocytogenes, Salmonella typhimurium and Pseudomonas aeruginosa. Ceramide and ceramide-enriched membrane platforms are also involved in the induction of apoptosis in infected cells, such as in epithelial and endothelial cells after infection with Pseudomonas aeruginosa and Staphylococcus aureus, respectively. Finally, ceramide-enriched membrane platforms are critical regulators of the release of pro-inflammatory cytokines upon infection. The diverse functions of ceramide in bacterial infections suggest that ceramide and ceramide-enriched membrane domains are key players in host responses to many pathogens and thus are potential novel targets to treat infections.

Effects of ceramide and other simple sphingolipids on membrane lateral structure

The available data concerning the ability of ceramide and other simple sphingolipids to segregate laterally into rigid, gel-like domains in a fluid bilayer has been reviewed. Ceramides give rise to rigid ceramide-enriched domains when their N-acyl chain is longer than C12. The high melting temperature of hydrated ceramides, revealing a tight intermolecular interaction, is probably responsible for their lateral segregation. Ceramides compete with cholesterol for the formation of domains with lipids such as sphingomyelin or saturated phosphatidylcholines; under these conditions displacement of cholesterol by ceramide involves a transition from a liquid-ordered to a gel-like phase in the domains involved. When ceramide is generated in situ by a sphingomyelinase, instead of being premixed with the other lipids, gel-like domain formation occurs as well, although the topology of the domains may not be the same, the enzyme causing clustering of domains that is not detected with premixed ceramide. Ceramide-1-phosphate is not likely to form domains in fluid bilayers, and the same is true of sphingosine and of sphingosine-1-phosphate. However, sphingosine does rigidify pre-existing gel domains in mixed bilayers.

Cholesterol displacement by ceramide in sphingomyelin-containing liquid-ordered domains, and generation of gel regions in giant lipidic vesicles

Fluorescence confocal microscopy and differential scanning calorimetry are used in combination to study the phase behaviour of bilayers composed of PC:PE:SM:Chol equimolecular mixtures, in the presence or absence of 10 mol% egg ceramide. In the absence of ceramide, separate liquid-ordered and liquid-disordered domains are observed in giant unilamellar vesicles. In the presence of ceramide, gel-like domains appear within the liquid-ordered regions. The melting properties of these gel-like domains resemble those of SM:ceramide binary mixtures, suggesting Chol displacement by ceramide from SM:Chol-rich liquid-ordered regions. Thus three kinds of domains coexist within a single vesicle in the presence of ceramide: gel, liquid-ordered, and liquid-disordered. In contrast, when 10 mol% egg diacylglycerol is added instead of ceramide, homogeneous vesicles, consisting only of liquid-disordered bilayers, are observed.

Ceramide-enriched membrane domains--structure and function

Membrane lipids seem to be organized and not randomly distributed in the cell membrane. In particular, sphingolipids seem to interact with cholesterol in the outer leaflet of the cell membrane resulting in the formation of distinct membrane domains, i.e. rafts. The generation of ceramide within rafts alters their biophysical properties and results in the formation of large ceramide-enriched membrane platforms. These platforms serve to cluster receptor molecules and to organize intracellular signalling molecules to facilitate signal transduction via a receptor upon stimulation. Thus, ceramide-enriched membrane domains amplify not only receptor-, but also stress-mediated signalling events. Although many receptors cluster, the molecular mechanisms mediating this important and general event in signal transduction need to be identified.

Fluorescent annexin A1 reveals dynamics of ceramide platforms in living cells

Upon its genesis during apoptosis, ceramide promotes gross reorganization of the plasma membrane structure involving clustering of signalling molecules and an amplification of vesicle formation, fusion and trafficking. The annexins are a family of proteins, which in the presence of Ca(2+), bind to membranes containing negatively charged phospholipids. Here, we show that ceramide increases affinity of annexin A1-membrane interaction. In the physiologically relevant range of Ca(2+) concentrations, this leads to an increase in the Ca(2+)sensitivity of annexin A1-membrane interaction. In fixed cells, using a ceramide-specific antibody, we establish a direct interaction of annexin A1 with areas of the plasma membrane enriched in ceramide (ceramide platforms). In living cells, the intracellular dynamics of annexin A1 match those of plasmalemmal ceramide. Among proteins of the annexin family, the interaction with ceramide platforms is restricted to annexin A1 and is conveyed by its unique N-terminal domain. We demonstrate that intracellular Ca(2+)overload occurring at the conditions of cellular stress induces ceramide production. Using fluorescently tagged annexin A1 as a reporter for ceramide platforms and annexin A6 as a non-selective membrane marker, we visualize ceramide platforms for the first time in living cells and provide evidence for a ceramide-driven segregation and internalization of membrane-associated proteins.

The role of sphingomyelin in regulating phase coexistence in complex lipid model membranes: competition between ceramide and cholesterol

The structure, thermotropic phase behavior, dynamic motion and order parameters of bilayer dispersions of egg phosphatidylcholine, egg sphingomyelin, egg ceramide and cholesterol have been determined. The coexistence of gel, liquid-ordered and liquid-disordered structure has been determined by peak fitting analysis of synchrotron X-ray powder patterns. Order parameters and extent of distribution of 16-doxyl-stearic acid spin probe between ordered and disordered environments has been estimated by ESR spectral simulation methods. The presence of ceramide in proportions up to 20 mol% in phosphatidylcholine is characterized by gel-fluid phase coexistence at temperatures up to 46 degrees C depending on the amount of ceramide. Cholesterol tends to destabilize the ceramide-rich domains formed in phosphatidylcholine while sphingomyelin, by formation of stable complexes with ceramide, tends to stabilize these domains. The stability of sphingomyelin-ceramide complexes is evident from the persistence of highly ordered structure probed by ESR spectroscopy and appearance of a sharp wide-angle X-ray reflection at temperatures higher than the gel-fluid transition of ceramide alone in egg phosphatidylcholine bilayers. The competition between ceramide and cholesterol for interaction with sphingomyelin is discussed in terms of control of lipid-mediated signaling pathways by sphingomyelinase and phospholipase A2.

Identification of Mg2+ -dependent neutral sphingomyelinase 1 as a mediator of heat stress-induced ceramide generation and apoptosis

Neutral sphingomyelinases (SMases) are involved in the induction of ceramide-mediated proapoptotic signaling under heat stress conditions. Although ceramide is an important mediator of apoptosis, the neutral SMase that is activated under heat stress has not been identified. In this study, we cloned an Mg(2+)-dependent neutral SMase from a zebrafish embryonic cell cDNA library using an Escherichia coli expression-cloning vector. Screening of the clones using an SMase activity assay with C(6)-7-nitro-2-1,3-benzoxadiazol-4-yl-sphingomyelin as the substrate resulted in the isolation of one neutral SMase cDNA clone. This cDNA encoded a polypeptide of 420 amino acids (putative molecular weight: 46,900) containing two predicted transmembrane domains in its C-terminal region. The cloned neutral SMase 1 acted as a mediator of stress-induced apoptosis. Bacterially expressed recombinant neutral SMase 1 hydrolyzed [choline-methyl-(14)C]sphingomyelin optimally at pH 7.5 in the presence of an Mg(2+) ion. In zebrafish embryonic cells, the endogenous SMase enzyme was localized in the microsomal fraction. In FLAG-tagged SMase-overexpressing cells, neutral SMase 1 colocalized with a Golgi marker in a cytochemical analysis. Inactivation of the enzyme by an antisense phosphorothioate oligonucleotide repressed the induction of ceramide generation, caspase-3 activation, and apoptotic cell death by heat stress. Thus, neutral SMase 1 participates in an inducible ceramide-mediating, proapoptotic signaling pathway that operates in heat-induced apoptosis in zebrafish embryonic cells.

Membrane domain formation, interdigitation, and morphological alterations induced by the very long chain asymmetric C24:1 ceramide

Ceramide (Cer) is involved in the regulation of several biological processes, such as apoptosis and cell signaling. The alterations induced by Cer in the biophysical properties of membranes are thought to be one of the major routes of Cer action. To gain further knowledge about the alterations induced by Cer, membrane reorganization by the very long chain asymmetric nervonoylceramide (NCer) was studied. The application of an established fluorescence multiprobe approach, together with x-ray diffraction, differential scanning calorimetry, and confocal fluorescence microscopy, allowed the characterization of NCer and the determination of the phase diagram of palmitoyloleoylphosphatidylcholine (POPC)/NCer binary mixtures. Nervonoylceramide undergoes a transition from a mixed interdigitated gel phase to a partially interdigitated gel phase at approximately 20 degrees C, and a broad main transition to the fluid phase at approximately 52 degrees C. The solubility of NCer in the fluid POPC is low, driving gel-fluid phase separation, and the binary-phase diagram is characterized by multiple and large coexistence regions between the interdigitated gel phases and the fluid phase. At 37 degrees C, the relevant phases are the fluid and the partially interdigitated gel. Moreover, the formation of NCer interdigitated gel phases leads to strong morphological alterations in the lipid vesicles, driving the formation of cochleate-type tubular structures.

Physicochemical and biological characterization of ceramide-containing liposomes: paving the way to ceramide therapeutic application

Ceramides mediate antiproliferative responses, and it has been proposed that increasing the level of ceramides in cancer cells may have a therapeutic antitumor effect. However, ceramides, because of their high "packing parameter" (PP), do not form lipid assemblies that can be dispersed in a form suitable for intravenous administration. We found that nanoliposomes containing short- or medium-chain ceramides are unstable because of their very high (>1.3) PP. To overcome this major obstacle, we included the lipopolymer 2kPEG-DSPE, which reduces the additive PP. The presence of PEG-DSPE allows the formation of highly stable (>1 year) ceramide (Cer)-containing nanoliposomes suitable for systemic administration. Using tumor cell lines, we found that the ceramide cytotoxicity was not impaired by their inclusion in nanoliposomes. The use of 14C-labeled ceramides shows that the C6Cer, but not C16Cer, was transferred from the nanoliposomes to the cells and metabolized efficiently. The difference between the two ceramides is related to the large difference between their critical aggregation concentration and was correlated with the much higher cytotoxity of liposomal C6Cer. The activity of 2kPEG-DSPE as a steric stabilizer (as previously shown for Doxil) was also confirmed for C6Cer-containing nanoliposomes. The 2kPEG-DSPE lipopolymer significantly reduced the desorption rate of the ceramide from the liposome bilayer, thereby allowing liposomes containing C6Cer to reach the tumor site and to demonstrate therapeutic efficacy.

Sphingolipids and cell death

Sphingolipids (SLs) have been considered for many years as predominant building blocks of biological membranes with key structural functions and little relevance in cellular signaling. However, this view has changed dramatically in recent years with the recognition that certain SLs such as ceramide, sphingosine 1-phosphate and gangliosides, participate actively in signal transduction pathways, regulating many different cell functions such as proliferation, differentiation, adhesion and cell death. In particular, ceramide has attracted considerable attention in cell biology and biophysics due to its key role in the modulation of membrane physical properties, signaling and cell death regulation. This latter function is largely exerted by the ability of ceramide to activate the major pathways governing cell death such as the endoplasmic reticulum and mitochondria. Overall, the evidence so far indicates a key function of SLs in disease pathogenesis and hence their regulation may be of potential therapeutic relevance in different pathologies including liver diseases, neurodegeneration and cancer biology and therapy.

Ceramide-enriched membrane domains in infectious biology and development

Ceramide has been shown to be critically involved in multiple biological processes, for instance induction of apoptosis after ligation of death receptors or application of gamma-irradiation or UV-A light, respectively, regulation of cell differentiation, control of tumor cell growth, infection of mammalian cells with pathogenic bacteria and viruses or the control of embryo and organ development to name a few examples. Ceramide molecules form distinct large domains in the cell membrane, which may serve to re-organize cellular receptors and signalling molecules. Thus, in many conditions, ceramide may be involved in the spatial and temporal organisation of specific signalling pathways explaining the pleiotrophic effects of this lipid. Here, we focus on the role of ceramide and ceramide-enriched membrane domains, respectively, in bacterial infections, in particular of the lung, and sepsis. We describe the role of ceramide for infections with Neisseriae gonorhoeae, Staphylococcus aureus and Pseudomonas aeruginosa. Finally, we discuss newly emerging aspects of the cellular function of ceramide, i.e. its role in germ line and embryo development.

Visualisation of liposomes prepared from skin and stratum corneum lipids by transmission electron microscopy

Transmission electron microscopy was used to visualize the liposomes prepared from total lipids extracted from mouse, human and porcine skin and stratum corneum. The total lipid composition was monitored by high precision thin layer chromatography coupled with a flame identification detector (HPTLC/FID, Iatroscan) and the fatty acid content of the samples was monitored by gas chromatography. The liposomes were prepared by the thin lipid film hydration method and they were visualized by transmission electron microscopy after negative staining using uranyl acetate. The structure of the vesicular structures present in the formulations largely depended on the lipid composition of the samples. The liposomes with high ceramide content were drop like vesicles with sharp tips, whereas the presence of excessive phospholipid content lead to bag like liposomes with two hemispheres divided by a membrane. Finally, the tendency of triacylglycerides to accumulate in the lipophylic region of the lipid bilayer, forms membranes with uneven thickness, resulting in structures with undulated membranes. A degree of fusion depending on the phospholipid content was also observed.

Effects of fluorescent probe NBD-PC on the structure, dynamics and phase transition of DPPC. A molecular dynamics and differential scanning calorimetry study

We present a combined theoretical (molecular dynamics, MD) and experimental (differential scanning calorimetry, DSC) study of the effect of 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) acyl chain-labeled fluorescent phospholipid analogs (C6-NBD-PC and C12-NBD-PC) on 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers. DSC measurements reveal that <1 mol% of NBD-PC causes elimination of the pre-transition and a large loss of cooperativity of the main transition of DPPC. Labeling with C6-NBD-PC or C12-NBD-PC shifts the main transition temperature to lower or higher values, respectively. Following our recent report on the location and dynamics of these probes (BBA 1768 (2007) 467-478) in fluid phase DPPC, we present a detailed analysis of 100-ns MD simulations of systems containing either C6-NBD-PC or C12-NBD-PC, focused on their influence on several properties of the host bilayer. Whereas most monitored parameters are not severely affected for 1.6 mol% of probe, for the higher concentration studied (6.2 mol%) important differences are evident. In agreement with published reports, we observed that the average area per phospholipid molecule increases, whereas DPPC acyl chain order parameters decrease. Moreover, we predict that incorporation of NBD-PC should increase the electrostatic potential across the bilayer and, especially for C12-NBD-PC, slow lateral diffusion of DPPC molecules and rotational mobility of DPPC acyl chains.

Raft domain reorganization driven by short- and long-chain ceramide: a combined AFM and FCS study

Naturally occurring long-chain ceramides (Cer) are known to alter the lateral organization of biological membranes. In particular, they produce alterations of microdomains that are involved in several cellular processes, ranging from apoptosis to immune response. In order to induce similar biological effects, short-chain Cer are extensively used in in vivo experiments to replace their long-chain analogues. In this work, we used the combined approach of atomic force microscopy (AFM) and fluorescence correlation spectroscopy (FCS) to investigate the effect of Cer chain length in lipid bilayers composed of sphingomyelin, dioleoyl-phosphatidylcholine, and cholesterol. Our results show that only long-chain Cer, like C18 and C16, are able to segregate from the liquid-ordered phase, forming separate Cer-enriched domains. Conversely, short-chain Cer do not form a separate phase but alter the physical properties of the liquid-ordered domains, decreasing their stability and viscosity and perturbing the lipid packing. These differences may contribute to the explanation of the different physiological effects that are often observed for the long- and short-chain Cer.

Biological aspects of ceramide-enriched membrane domains

Ceramide has been shown to be critically involved in many aspects of cellular responses to receptor-dependent and -independent stimuli. For instance, ceramide was demonstrated to be a central component of the signaling cascades mediating apoptosis after death receptor stimulation, treatment with chemotherapy or exposure to gamma-irradiation or UV-A light. Further studies indicated the importance of ceramide for the infection of mammalian cells with bacterial, viral and parasitic pathogens. Ceramide is released by the activity of acid, neutral or alkaline sphingomyelinases or de novo synthesized. A concept unifying the diverse biological functions of ceramide indicates that ceramide forms distinct membrane domains, named ceramide-enriched membrane domains or platforms. These domains serve the clustering of receptor molecules, the re-organization of signaling proteins, the exclusion of inhibitory signals and, thus, initiate and greatly amplify a primary signal. In addition, ceramide directly interacts with and stimulates intracellular enzymes that may act together with signals initiated in ceramide-enriched membrane domains to transmit signals into a cell.

Surface tension induced by sphingomyelin to ceramide conversion in lipid membranes

We have investigated the effect of sphingomyelin (SM) to ceramide enzymatic conversion on lipid bilayers using Giant Unilamellar Vesicles (GUVs). Sphingomyelinase was added externally to GUVs containing various proportions of SM. In situ asymmetrical SM conversion to ceramide reduced the area of one leaflet. In the absence of equilibration of all the lipids between the two leaflets, a mismatch between the two monolayers was generated. The tension generated by this mismatch was sufficient to trigger the formation of membrane defects and total vesicle collapse at relatively low percentage of SM ( approximately 5% mol). The formation of nanometric size defects was visualised by AFM in supported bilayers. Vesicle rupture was prevented in two circumstances: (a) in GUVs containing a mixture of l(d) and l(o) domains and (b) in GUVs containing 5% lyso-phosphatidylcholine. In both cases, the accumulation of enough ceramide (at initial SM concentration of 10%) allowed the formation of ceramide-rich domains. The coupling between the two asymmetrical monolayers and the condensing effect produced by the newly formed ceramide generated a tension that could underlie the mechanism through which ceramide formation induces membrane modifications observed during the late stages of apoptosis.

Sphingolipids and the formation of sterol-enriched ordered membrane domains

This review is focused on the formation of lateral domains in model bilayer membranes, with an emphasis on sphingolipids and their interaction with cholesterol. Sphingolipids in general show a preference for partitioning into ordered domains. One of the roles of cholesterol is apparently to modulate the fluidity of the sphingolipid domains and also to help segregate the domains for functional purposes. Cholesterol shows a preference for sphingomyelin over phosphatidylcholine with corresponding acyl chains. The interaction of cholesterol with different sphingolipids is largely dependent on the molecular properties of the particular sphingolipid in question. Small head group size clearly has a destabilizing effect on sphingolipid/cholesterol interaction, as exemplified by studies with ceramide and ceramide phosphoethanolamine. Ceramides actually displace sterol from ordered domains formed with saturated phosphatidylcholine or sphingomyelin. The N-linked acyl chain is known to be an important stabilizer of the sphingolipid/cholesterol interaction. However, N-acyl phosphatidylethanolamines failed to interact favorably with cholesterol and to form cholesterol-enriched lateral domains in bilayer membranes. Glycosphingolipids also form ordered domains in membranes but do not show a strong preference for interacting with cholesterol. It is clear from the studies reviewed here that small changes in the structure of sphingolipids alter their partitioning between lateral domains substantially.

Ceramide drives cholesterol out of the ordered lipid bilayer phase into the crystal phase in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine/cholesterol/ceramide ternary mixtures

The effect of brain ceramide on the maximum solubility of cholesterol in ternary mixtures of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), cholesterol, and ceramide was investigated at 37 degrees C by a cholesterol oxidase (COD) reaction rate assay and by optical microscopy. The COD reaction rate assay showed a sharp increase in cholesterol chemical potential as the cholesterol mole fraction approaches the solubility limit. A decline in the COD reaction rate was found after the formation of cholesterol crystals. The maximum solubility of brain ceramide in POPC bilayers was determined to be 68 +/- 2 mol % by microscopy. We found that ceramide has a much higher affinity for the ordered bilayers than cholesterol, and the maximum solubility of cholesterol decreases with the increase in ceramide content. More significantly, the displacement of cholesterol by ceramide follows a 1:1 relation. At the cholesterol solubility limit, adding one more ceramide molecule to the lipid bilayer drives one cholesterol out of the bilayer into the cholesterol crystal phase, and cholesterol is incapable of displacing ceramide from the bilayer phase. On the basis of these findings, a ternary phase diagram of the POPC/cholesterol/ceramide mixture was constructed. The behaviors of ceramide and cholesterol can be explained by the umbrella model. Both ceramide and cholesterol have small polar headgroups and relatively large nonpolar bodies. In a PC bilayer, ceramide and cholesterol compete for the coverage of the headgroups of neighboring PC to prevent the exposure of their nonpolar bodies to water. This competition results in the 1:1 displacement as well as the displacement of cholesterol by ceramide from lipid raft domains.

Biophysics of sphingolipids I. Membrane properties of sphingosine, ceramides and other simple sphingolipids

Some of the simplest sphingolipids, namely sphingosine, ceramide, some closely related molecules (eicosasphingosine, phytosphingosine), and their phosphorylated compounds (sphingosine-1-phosphate, ceramide-1-phosphate), are potent metabolic regulators. Each of these lipids modifies in marked and specific ways the physical properties of the cell membranes, in what can be the basis for some of their physiological actions. This paper reviews the mechanisms by which these sphingolipid signals, sphingosine and ceramide in particular, are able to modify the properties of cell membranes.

Membrane rafts in host-pathogen interactions

Central elements in the infection of mammalian cells with viral, bacterial and parasitic pathogens include the adhesion of the pathogen to surface receptors of the cell, recruitment of additional receptor proteins to the infection-site, a re-organization of the membrane and, in particular, the intracellular signalosome. Internalization of the pathogen results in the formation of a phagosome that is supposed to fuse with lysosomes to form phagolysosomes, which serve the degradation of the pathogen, an event actively prevented by some pathogens. In summary, these changes in the infected cell permit pathogens to trigger apoptosis (for instance of macrophages paralysing the initial immune response), to invade the cell and/or to survive in the cell, but they also serve the mammalian cell to defeat the infection, for instance by activation of transcription factors and the release of cytokines. Distinct membrane domains in the plasma membrane and intracellular vesicles that are mainly composed of sphingolipids and cholesterol or enriched with the sphingolipid ceramide, are critically involved in all of these events occurring during the infection. These membrane structures are therefore very attractive targets for novel drugs to interfere with bacterial, viral and parasitic infections.

Sphingolipid signaling and redox regulation

Sphingolipids including ceramide and its derivatives such as ceramide-1-phosphate, glycosyl-ceramide, and sphinogosine (-1-phosphate) are now recognized as novel intracellular signal mediators for regulation of inflammation, apoptosis, proliferation, and differentiation. One of the important and regulated steps in these events is the generation of these sphingolipids via hydrolysis of sphingomyelin through the action of sphingomyelinases (SMase). Several lines of evidence suggest that reactive oxygen species (ROS; O2-, H2O2, and OH-,) and reactive nitrogen species (RNS; NO, and ONOO-) and cellular redox potential, which is mainly regulated by cellular glutathione (GSH), are tightly linked to the regulation of SMase activation. On the other hand, sphingolipids are also known to play an important role in maintaining cellular redox homeostasis through regulation of NADPH oxidase, mitochondrial integrity, and antioxidant enzymes. Therefore, this paper reviews the relationship between cellular redox and sphingolipid metabolism and its biological significance.

Regulation of the activity and fatty acid specificity of lecithin-cholesterol acyltransferase by sphingomyelin and its metabolites, ceramide and ceramide phosphate

Sphingomyelin (SM), the second most abundant phospholipid in plasma lipoproteins, was previously shown to be a physiological inhibitor of the lecithin-cholesterol acyltransferase (LCAT) reaction. In this study, we investigated the effects of its metabolites, ceramide and ceramide phosphate, on the activity and fatty acid specificity of LCAT in vitro. Treatment of SM-containing substrate with SMase C, which hydrolyzes SM to ceramide, abolished the inhibitory effect of SM, whereas treatment with SMase D, which hydrolyzes it to ceramide phosphate, increased the level of inhibition. Although incorporation of ceramide into the substrate in the absence of SM activated the LCAT reaction only modestly, its co-incorporation with SM neutralized the inhibitory effect of SM. Ceramide phosphate, on the other hand, inhibited the LCAT reaction more strongly than SM. The effects of the sphingolipids on the phospholipase A and cholesterol esterification reactions of the enzyme were similar, indicating that they regulate the binding of phosphatidylcholine (PC) to the active site, rather than the esterification step. Incorporation of ceramide into the substrate stimulated the synthesis of unsaturated cholesteryl esters at the expense of saturated esters. However, these effects on fatty acid specificity disappeared when the PC substrates were incorporated into an inert diether PC matrix, suggesting that ceramide increases the availability of polyunsaturated PCs to the enzyme by altering the macromolecular structure of the substrate particle. Since the plasma ceramide levels are increased during inflammation, these results indicate that the activity and fatty acid specificity of LCAT may be altered during the inflammatory response.

Effects of ceramide on liquid-ordered domains investigated by simultaneous AFM and FCS

The sphingolipid ceramides are known to influence lipid lateral organization in biological membranes. In particular, ceramide-induced alterations of microdomains can be involved in several cell functions, ranging from apoptosis to immune response. We used a combined approach of atomic force microscopy, fluorescence correlation spectroscopy, and confocal fluorescence imaging to investigate the effects of ceramides in model membranes of biological relevance. Our results show that physiological quantities of ceramide in sphingomyelin/dioleoylphosphatidylcholine/cholesterol supported bilayers lead to a significant rearrangement of lipid lateral organization. Our experimental setup allowed a simultaneous characterization of both structural and dynamic modification of membrane microdomains, induced by the presence of ceramide. Formation of similar ceramide-enriched domains and, more general, alterations of lipid-lipid interactions can be of crucial importance for the biological function of cell membranes.

Absence of fluid-ordered/fluid-disordered phase coexistence in ceramide/POPC mixtures containing cholesterol

The effect of temperature on the lateral structure of lipid bilayers composed of porcine brain ceramide and 1-palmitoyl 2-oleoyl-phosphatidylcholine (POPC), with and without addition of cholesterol, were studied using differential scanning calorimetry, Fourier transformed infrared spectroscopy, atomic force microscopy, and confocal/two-photon excitation fluorescence microscopy (which included LAURDAN generalized polarization function images). A broad gel/fluid phase coexistence temperature regime, characterized by the presence of micrometer-sized gel-phase domains with stripe and flowerlike shapes, was observed for different POPC/ceramide mixtures (up to approximately 25 mol % ceramide). This observed phase coexistence scenario is in contrast to that reported previously for this mixture, where absence of gel/fluid phase coexistence was claimed using bulk LAURDAN generalized polarization (GP) measurements. We demonstrate that this apparent discrepancy (based on the direct comparison between the LAURDAN GP data obtained in the microscope and the fluorometer) disappears when the additive property of the LAURDAN GP function is taken into account to examine the data obtained using bulk fluorescence measurements. Addition of cholesterol to the POPC/ceramide mixtures shows a gradual transition from a gel/fluid to gel/liquid-ordered phase coexistence scenario as indicated by the different experimental techniques used in our experiments. This last result suggests the absence of fluid-ordered/fluid-disordered phase coexistence in the ternary mixtures studied in contrast to that observed at similar molar concentrations with other ceramide-base-containing lipid mixtures (such as POPC/sphingomyelin/cholesterol, which is used as a canonical raft model membrane). Additionally, we observe a critical cholesterol concentration in the ternary mixtures that generates a peculiar lateral pattern characterized by the observation of three distinct regions in the membrane.

Sphingosine increases the permeability of model and cell membranes

Sphingosine, at 5-15 mol % total lipids, remarkably increases the permeability to aqueous solutes of liposomal and erythrocyte ghost membranes. The increased permeability cannot be interpreted in terms of leakage occurring at the early stages of a putative membrane solubilization by sphingosine, nor is it due to a sphingosine-induced generation of nonlamellar structures, or flip-flop lipid movement. Instead, sphingosine stabilizes (rigidifies) gel domains in membranes, raising their melting temperatures and increasing the transition cooperativity. Structural defects originating during the lateral phase separation of the "more rigid" and "less rigid" domains are likely sites for the leakage of aqueous solutes to the extravesicular medium. The presence of coexisting domains in the plasma membrane makes it a target for sphingosine permeabilization. The sphingosine-induced increase in rigidity and breakdown of the plasma membrane permeability barrier could be responsible for some of the physiological effects of sphingosine.

Rapid phase change of lipid microdomains in giant vesicles induced by conversion of sphingomyelin to ceramide

To understand the role of sphingomyelinase (SMase) in the function of biological membranes, we have investigated the effect of conversion of sphingomyelin (SM) to ceramide (Cer) on the assembly of domains in giant unilamellar vesicles (GUVs). The GUVs were prepared from mixture of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), N-palmitoly-D-erythro-sphingosine (C16Cer), N-palmitoyl-D-erythro-sphingosylphosphorylcholine (C16SM) and cholesterol. The amounts of DOPC, sum of C16Cer and C16SM, and cholesterol were kept constant (the ratio of these four lipids is shown as 1:X:1-X:1 (molar ratio), i.e., X is C16Cer/(C16Cer+C16SM)). Shape and distribution of domains formed in the GUVs were monitored by a fluorescent lipid, Texas Red 1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (0.1 mol%). In GUVs containing low C16Cer (X=0 and 0.25), round-shaped domains labeled by the fluorescent lipid were present, suggesting coexistence of liquid-ordered and disordered domains. In GUVs containing intermediate Cer concentration (X=0.5), the fluorescent domain covered most of GUV surface, which was surrounded by gel-like domains. Differential scanning calorimetry of multilamellar vesicles prepared in the presence of higher Cer concentration (X>or=0.5) suggested existence of a Cer-enriched gel phase. Video microscopy showed that the enzymatic conversion of SM to Cer caused rapid change in the domain structure: several minutes after the SMase addition, the fluorescent region spread over the GUV surface, within which regions with darker contrast existed. Image-based measurement of generalized polarization (GP) of 6-dodecanoyl-2-dimethylaminonaphthalene (Laurdan), which is related to the acyl chain ordering of the lipids, was performed. Before the SMase treatment domains with high (0.65) and low (below 0.4) GP values coexisted, presumably reflecting the liquid-ordered and disordered domains; after the SMase treatment regions with intermediate GP values (0.5) and smaller regions with higher GP values (0.65) were present. Generation of Cer thus caused a phase transition from liquid-ordered and disordered phases to a gel and liquid phase.

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 phytosphingosine-type ceramides on the structure of DMPC membrane

The present paper describes the influence of the ceramides with phytosphingosine base, N-stearoylphytosphingosine (Cer[NP]) and alpha-hydroxy-N-stearoylphytosphingosine (Cer[AP]), on the structure and properties of multilamellar (MLVs) and unilamellar vesicles (ULVs) of dimyristoylphosphatidylcholine (DMPC). The lamellar repeat distance, D, has been measured at various temperatures using small angle X-ray diffraction. The incorporation of ceramides into the DMPC membrane causes larger D compared to pure DMPC membrane. For both ceramide types, at 32 degrees C, there is a linear relationship between the D value and the ceramide concentration. However, there is no such dependence at 13 or 60 degrees C. Unlike Cer[AP], Cer[NP] induces a new phase with a repeat distance of 38.5A. The membrane thickness and the vesicle radius of ULVs in water and in sucrose solution were calculated from small angle neutron scattering curves. Phytosphingosine ceramides increase both the membrane thickness and the radius in comparison to pure DMPC ULVs. The stability of ULVs in time was studied by dynamic light scattering. Both ceramides induce an aggregation of the ULVs into micrometer sized non-multilamellar structures in pure water. Presence of sucrose in the environment averts the vesicle aggregation.

Detergent-resistant, ceramide-enriched domains in sphingomyelin/ceramide bilayers

When cell membranes are treated with Triton X-100 or other detergents at 4 degrees C, a nonsolubilized fraction can often be recovered, the "detergent-resistant membranes", that is not found when detergent treatment takes place at 37 degrees C. Detergent-resistant membranes may be related in some cases to membrane "rafts". However, several basic aspects of the formation of detergent-resistant membranes are poorly understood. To answer some of the relevant questions, a simple bilayer composition that would mimic detergent-resistant membranes was required. The screening of multiple lipid compositions has shown that the binary mixture egg sphingomyelin/egg ceramide (SM/Cer) exhibits the required detergent resistance. In detergent-free membranes composed of different mixtures of SM and Cer (5-30 mol % of Cer) differential scanning calorimetry, fluorescence spectroscopy, and fluorescence microscopy experiments reveal the presence of discrete, Cer-enriched gel domains in a broad temperature range. In particular, at temperatures below SM phase transition ( approximately 40 degrees C) two gel (respectively Cer-rich and SM-rich) phases are directly observed using fluorescence microscopy. Although pure SM membranes are fully solubilized by Triton X-100 at room temperature, 5 mol % Cer is also enough to induce detergent resistance, even with a large detergent excess and lengthy equilibration times. Short-chain Cers do not give rise to detergent resistance. SM/Cer mixtures containing up to 30 mol % Cer become fully soluble at approximately 50 degrees C, i.e., well above the gel-fluid transition temperature of SM. The combined results of temperature-dependent solubilization and differential scanning calorimetry reveal that SM-rich domains are preferentially solubilized over the Cer-rich ones as soon as the former melt (i.e., at approximately 40 degrees C). As a consequence, at temperatures allowing only partial solubilization, the nonsolubilized residue is enriched in Cer with respect to the original bilayer composition. Fluorescence microscopy of giant unilamellar vesicles at room temperature clearly shows that SM-rich domains are preferentially solubilized over the Cer-rich ones and that the latter become more rigid and extensive as a consequence of the detergent effects. These observations may be relevant to the phenomena of sphingomyelinase-dependent signaling, generation of "raft platforms", and detergent-resistant cell membranes.

Ceramide-enriched membrane domains

Cellular activation involves the re-organization of receptor molecules and the intracellular signalosom in the cell membrane. Recent studies indicate that specialized domains of the cell membrane, termed rafts, are central for the spatial organization of receptors and signaling molecules. Rafts are converted into larger membrane platforms by activity of the acid sphingomyelinase, which hydrolyses raft-sphingomyelin to ceramide. Ceramide molecules spontaneously associate to form ceramide-enriched microdomains, which fuse to large ceramide-enriched membrane platforms. The acid sphingomyelinase is activated by multiple stimuli including CD95, CD40, DR5/TRAIL, CD20, FcgammaRII, CD5, LFA-1, CD28, TNF, the Interleukin-1 receptor, the PAF-receptor, CD14, infection with P. aeruginosa, S. aureus, N. gonorrhoeae, Sindbis-Virus, Rhinovirus, treatment with gamma-irradiation, UV-light, doxorubicin, cisplatin, disruption of integrin-signaling and under some conditions of developmental death. Ceramide-enriched membrane platforms serve the clustering of receptors, the recruitment of intracellular signaling molecules and the exclusion of inhibitory signaling factors and, thus, facilitate signal transduction initiated by the specific stimulus.

Role of membrane sphingomyelin and ceramide in platform formation for Fas-mediated apoptosis

Engagement of the Fas receptor (CD95) initiates multiple signaling pathways that lead to apoptosis, such as the formation of death-inducing signaling complex (DISC), activation of caspase cascades, and the generation of the lipid messenger, ceramide. Sphingomyelin (SM) is a major component of lipid rafts, which are specialized structures that enhance the efficiency of membrane receptor signaling and are a main source of ceramide. However, the functions of SM in Fas-mediated apoptosis have yet to be clearly defined, as the responsible genes have not been identified. After cloning a gene responsible for SM synthesis, SMS1, we established SM synthase–defective WR19L cells transfected with the human Fas gene (WR/Fas-SM(−)), and cells that have been functionally restored by transfection with SMS1 (WR/Fas-SMS1). We show that expression of membrane SM enhances Fas-mediated apoptosis through increasing DISC formation, activation of caspases, efficient translocation of Fas into lipid rafts, and subsequent Fas clustering. Furthermore, WR/Fas-SMS1 cells, but not WR/Fas-SM(−) cells, showed a considerable increase in ceramide generation within lipid rafts upon Fas stimulation. These data suggest that a membrane SM is important for Fas clustering through aggregation of lipid rafts, leading to Fas-mediated apoptosis.

Caspase-dependent and -independent Activation of Acid Sphingomyelinase Signaling

Recent evidence suggests clustering of plasma membrane rafts into ceramide-enriched platforms serves as a transmembrane signaling mechanism for a subset of cell surface receptors and environmental stresses (Grassme, H., Jekle, A., Riehle, A., Schwarz, H., Berger, J., Sandhoff, K., Kolesnick, R., and Gulbins, E. (2001) J. Biol. Chem. 276, 20589-20596; Cremesti, A., Paris, F., Grassme, H., Holler, N., Tschopp, J., Fuks, Z., Gulbins, E., and Kolesnick, R. (2001) J. Biol. Chem. 276, 23954-23961). Translocation of the secretory form of acid sphingomyelinase (ASMase) into microscopic rafts generates therein the ceramide that drives raft coalescence. This process serves to feed forward Fas activation, with approximately 2% of full caspase 8 activation sufficient for maximal ASMase translocation, leading to death-inducing signaling complex formation within ceramide-rich platforms, and apoptosis. Here we report that treatment of Jurkat T cells with UV-C also induces ASMase translocation into rafts within 1 min, catalyzing sphingomyelin hydrolysis to ceramide and raft clustering. In contrast to Fas, UV-induced ASMase translocation and activation were caspase-independent. Nonetheless, ceramide-rich platforms promoted UV-C-induced death signaling, because ASMase inhibition or raft disruption inhibited apoptosis, improving clonogenic cell survival. These studies thus define two distinct mechanisms for biologically relevant ASMase activation within rafts; a Fas-mediated mechanism dependent upon caspase 8 and FADD, and a UV-induced mechanism independent of caspase activation. Consistent with this notion, genetic depletion or pharmacologic inhibition of caspase 8 or FADD, which render Jurkat cells incapable of sphingolipid signaling and apoptosis upon Fas ligation, did not impair these events upon UV-C stimulation.

Molecular associations and surface-active properties of short- and long-N-acyl chain ceramides

The behaviour of N-hexadecanoylsphingosine (Cer16), N-hexanoylsphingosine (Cer6) and N-acetylsphingosine (Cer2) in aqueous media and in lipid-water systems, monolayers and bilayers has been comparatively examined using Langmuir balance and fluorescence techniques. Cer16 behaves as an insoluble non-swelling amphiphile, not partitioning into the air-water interface, thus not modifying the surface pressure of the aqueous solutions into which it is included. By contrast both Cer6 and Cer2 behave as soluble amphiphiles, up to approx. 100 microM. At low concentrations, they become oriented at the air-water interface, increasing surface pressure in a dose-dependent way up to ca. 5 microM bulk concentration. At higher concentrations, the excess ceramide forms micelles, critical micellar concentrations of both Cer6 and Cer2 being in the 5-6 microM range. When the air-water interface is occupied by a phospholipid, 6Cer2 and Cer6 become inserted in the phospholipid monolayer, causing a further increase in surface pressure. This increase is dose dependent, and reaches a plateau at ca. 2 microM ceramide bulk concentration. Both Cer2 and Cer6 become inserted in phospholipid monolayers with initial surface pressures of up to 43 and 46 mN m(-1), respectively, which ensures their capacity to become inserted into cell membranes whose monolayers are estimated to support a surface pressure of about 30 mN m(-1). Both Cer2 and Cer6, but not Cer16, had detergent-like properties, such as giving rise to phospholipid-ceramide mixed micelles, when added to phospholipid monolayers or bilayers. The short-chain ceramides form large aggregates and precipitate at concentrations above approx. 100 microM. These results are relevant in cell physiology studies in which short- and long-chain ceramides are sometimes used as equivalent molecules, in spite of their different biophysical behaviour.

Revaluation of the role of cholesterol in stabilizing rafts implicated in T cell receptor signaling

T lymphocytes contain two kinetic pools of cholesterol extractable with methyl-beta-cyclodextrin (m-beta-CD): a fast pool (31.5%, t1/2=17 s) and a slow pool (68.5%, t1/2=15 min). Purification of detergent-resistant membranes (DRMs) shows that the fast pool corresponds to buoyant cholesterol. Cholesterol extraction of the fast pool (i.e. cholesterol from rafts) still allows the buoyancy of signaling proteins and their phosphorylation under CD3 stimulation. Cholesterol depletion of the slow pool (i.e. cholesterol from membranes other than rafts) is accompanied by the extraction of the whole raft followed by the inhibition of CD3-induced tyrosine-phosphorylations. Cholesterol oxidase (COase) allows a specific oxidation of raft cholesterol into cholestenone. Cholestenone leaves the DRMs and accumulates as Triton X-100-soluble material. Specific cholesterol-rich raft disruption by COase does not inhibit the activation of either Jurkat cells or T CD4+ lymphocytes. Our study challenges the real role of cholesterol-rich rafts in CD3/TCR signaling and suggests that a cholesterol-poor subtype of rafts is involved in signal transmission via the TCR.

Effect of L-menthol and 1,8-cineole on phase behavior and molecular organization of SC lipids and skin permeation of zidovudine

The aim of this investigation was to study the effect of 1,8-cineole and L-menthol on phase behavior and molecular organization of Stratum corneum (SC) lipids and permeation of zidovudine (AZT) across human cadaver skin (HCS). Permeation studies were conducted across HCS using Franz diffusion cells at 37 degrees C. Differential scanning calorimetry (DSC) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) were employed to understand the effect of terpenes on phase behavior and molecular organization of a model SC lipid system consisting of an equimolar mixture of ceramide, palmitic acid and cholesterol. Both 1,8-cineole and L-menthol applied at 5% w/v in 66.6% ethanol as a vehicle significantly enhanced the pseudosteady state flux of AZT across HCS. The vehicle reduced the number of endothermic transitions observed in the DSC thermogram of a hydrated model SC lipid system from three to two with a lowered midtransition temperature (Tm), while the inclusion of terpenes resulted in a single but very broad endothermic transition for the model SC lipid system. Correspondingly, ATR-FTIR studies revealed that both 1,8-cineole and L-menthol increased CH2 stretching frequencies on either side of lipid transition in addition to lowering the Tm of model SC lipid system by approximately 2-8 degrees C. The alterations observed in the amide-I frequencies of model SC lipid system after the inclusion of terpenes suggest that they disrupt the interlamellar hydrogen-bonding network at the polar head group region. Further, terpenes also increased the hydration levels of the lipid system probably by forming new aqueous channels. These results indicate that terpenes enhance transdermal permeation of AZT and other drugs by transforming SC lipids from a highly ordered orthorhombic perpendicular subcellular packing to a less ordered hexagonal subcell packing.

Nanoscale properties of mixed fengycin/ceramide monolayers explored using atomic force microscopy

To gain insight into the interactions between fengycin and skin membrane lipids, mixed fengycin/ceramide monolayers were investigated using atomic force microscopy (AFM) (monolayers supported on mica) and surface pressure-area isotherms (monolayers at the air-water interface). AFM topographic images revealed phase separation in mixed monolayers prepared at 20 degrees C/pH 2 and composed of 0.25 and 0.5 fengycin molar ratios, in the form of two-dimensional (2-D) hexagonal crystalline domains of ceramide surrounded by a fengycin-enriched fluid phase. Surface pressure-area isotherms as well as friction and adhesion AFM images confirmed that the two phases had different molecular orientations: while ceramide formed a highly ordered phase with crystalline chain packing, fengycin exhibited a disordered fluid phase with the peptide ring lying horizontally on the substrate. Increasing the temperature and pH to values corresponding to the skin parameters, i.e., 37 degrees C/pH 5, was found to dramatically affect the film organization. At low fengycin molar ratio (0.25), the hexagonal ceramide domains transformed into round domains, while at higher ratio (0.5) these were shown to melt into a continuous fengycin/ceramide fluid phase. These observations were directly supported by the thermodynamic analysis (deviation from the additivity rule, excess of free energy) of the monolayer properties at the air-water interface. Accordingly, this study demonstrates that both the environmental conditions (temperature, pH) and fengycin concentration influence the molecular organization of mixed fengycin/ceramide monolayers. We believe that the ability to modulate the formation of 2-D domains in the skin membrane may be an important biological function of fengycin, which should be increasingly investigated in future pharmacological research.

Formation of ceramide-enriched domains in lipid particles enhances the binding of apolipoprotein E

We investigated the interaction between apolipoprotein E (apoE) and ceramide (CER)-enriched domains on the particles, by using lipid emulsions containing sphingomyelin (SM) or CER as model particles of lipoproteins. The sphingomyelinase (SMase)-induced aggregation of emulsion particles was prevented by apoE. CER increased the amount of apoE bound to emulsion particles. The confocal images of CER-containing large emulsions with two fluorescent probes showed three-dimensional microdomains enriched in CER. SMase also induced the formation of CER-enriched domains. We propose apoE prefers to bind on CER-enriched domains exposed on particle surface, and thus inhibits the aggregation or fusion of the particles.