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

Untargeted metabolomics reveals homogeneity and heterogeneity between physiological and pathological ovarian aging

BACKGROUND

Ovarian aging is the main cause of reduced reproductive life span, yet its metabolic profiles remain poorly understood. This study aimed to reveal the metabolic homogeneity and heterogeneity between physiological and pathological ovarian aging.

METHODS

Seventy serum samples from physiological ovarian aging participants, pathological ovarian aging participants (including diminished ovarian reserve (DOR), subclinical premature ovarian insufficiency (scPOI) and premature ovarian insufficiency (POI)), as well as healthy participants were collected and analyzed by untargeted metabolomics.

RESULTS

Five homogeneous differential metabolites (neopterin, menaquinone, sphingomyelin (SM) (d14:1/24:2), SM (d14:0/21:1) and SM (d17:0/25:1)) were found in both physiological and pathological ovarian aging. While five distinct metabolites, including phosphoglyceride (PC) (17:0/18:2), PC (18:2e/17:2), SM (d22:1/14:1), SM (d14:1/20:1) and 4-hydroxyretinoic acid were specific to pathological ovarian aging. Functional annotation of differential metabolites suggested that folate biosynthesis, ubiquinone and other terpenoid-quinone biosynthesis pathways, were mainly involved in the ovarian aging process. Meanwhile, dopaminergic synapses pathway was strongly associated with scPOI, vitamin digestion and absorption and retinol metabolism were associated with POI. Furthermore, testosterone sulfate, SM (d14:0/28:1), PC (18:0e/4:0) and 4-hydroxyretinoic acid, were identified as potential biomarkers for diagnosing physiological ovarian aging, DOR, scPOI, and POI, respectively. Additionally, SM (d14:1/24:2) strongly correlated with both physiological and pathological ovarian aging. 4-hydroxyretinoic acid was strongly correlated with pathological ovarian aging.

CONCLUSIONS

Metabolic homogeneity of physiological and pathological ovarian aging was related to disorders of lipid, folate, ubiquinone metabolism, while metabolic heterogeneity between them was related to disorders of lipid, vitamin and retinol metabolism.

CLINICAL TRIAL NUMBER

Not applicable.

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

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

Shaping intercellular channels of plasmodesmata: the structure-to-function missing link

Plasmodesmata (PD) are a hallmark of the plant kingdom and a cornerstone of plant biology and physiology, forming the conduits for the cell-to-cell transfer of proteins, RNA and various metabolites, including hormones. They connect the cytosols and endomembranes of cells, which allows enhanced cell-to-cell communication and synchronization. Because of their unique position as intercellular gateways, they are at the frontline of plant defence and signalling and constitute the battleground for virus replication and spreading. The membranous organization of PD is remarkable, where a tightly furled strand of endoplasmic reticulum comes into close apposition with the plasma membrane, the two connected by spoke-like elements. The role of these structural features is, to date, still not completely understood. Recent data on PD seem to point in an unexpected direction, establishing a close parallel between PD and membrane contact sites and defining plasmodesmal membranes as microdomains. However, the implications of this new viewpoint are not fully understood. Aided by available phylogenetic data, this review attempts to reassess the function of the different elements comprising the PD and the relevance of membrane lipid composition and biophysics in defining specialized microdomains of PD, critical for their function.

Molecular Dynamics Simulations of Ceramide and Ceramide-Phosphatidylcholine Bilayers

Recent studies in lipid raft formation and stratum corneum permeability have focused on the role of ceramides (CER). In this study, we use the all-atom CHARMM36 (C36) force field to simulate bilayers using N-palmitoylsphingosine (CER16) or α-hydroxy-N-stearoyl phytosphingosine (CER[AP]) in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), which serve as general membrane models. Conditions are replicated from experimental studies for comparison purposes, and concentration (X_CER) is varied to probe the effect of CER on these systems. Comparisons with experiment based on deuterium order parameters and bilayer thickness demonstrate good agreement, thus supporting further use of the C36 force field. CER concentration is shown to have a profound effect on nearly all membrane properties including surface area per lipid, chain order and tilt, area compressibility moduli, bilayer thickness, hydrogen bonding, and lipid clustering. Hydrogen bonding in particular can significantly affect other membrane properties and can even encourage transition to a gel phase. Despite CER's tendency to condense the membrane, an expansion of CER lipids with increasing X_CER is possible depending on how the balance between various hydrogen-bond pairs and lipid clustering is perturbed. Based on gel phase transitions, support is given for phytosphingosine's role as a hydrogen-bond bridge between sphingosine ordered domains in the stratum corneum.

The many faces (and phases) of ceramide and sphingomyelin II - binary mixtures

A rather widespread idea on the functional importance of sphingolipids in cell membranes refers to the occurrence of ordered domains enriched in sphingomyelin and ceramide that are largely assumed to exist irrespective of the type of N-acyl chain in the sphingolipid. Ceramides and sphingomyelins are the simplest kind of two-chained sphingolipids and show a variety of species, depending on the fatty acyl chain length, hydroxylation, and unsaturation. Abundant evidences have shown that variations of the N-acyl chain length in ceramides and sphingomyelins markedly affect their phase state, interfacial elasticity, surface topography, electrostatics, and miscibility, and that even the usually conceived "condensed" sphingolipids and many of their mixtures may exhibit liquid-like expanded states. Their lateral miscibility properties are subtlety regulated by those chemical differences. Even between ceramides with different acyl chain length, their partial miscibility is responsible for a rich two-dimensional structural variety that impacts on the membrane properties at the mesoscale level. In this review, we will discuss the miscibility properties of ceramide, sphingomyelin, and glycosphingolipids that differ in their N-acyl or oligosaccharide chains. This work is a second part that accompanies a previous overview of the properties of membranes formed by pure ceramides or sphingomyelins, which is also included in this Special Issue.

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

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

Computational Investigation of the Effect of Lipid Membranes on Ion Permeation in Gramicidin A

Membrane proteins are embedded in a lipid bilayer and interact with the lipid molecules in subtle ways. This can be studied experimentally by examining the effect of different lipid bilayers on the function of membrane proteins. Understanding the causes of the functional effects of lipids is difficult to dissect experimentally but more amenable to a computational approach. Here we perform molecular dynamics simulations and free energy calculations to study the effect of two lipid types (POPC and NODS) on the conductance of the gramicidin A (gA) channel. A larger energy barrier is found for the K⁺ potential of mean force in gA embedded in POPC compared to that in NODS, which is consistent with the enhanced experimental conductance of cations in gA embedded in NODS. Further analysis of the contributions to the potential energy of K⁺ reveals that gA and water molecules in gA make similar contributions in both bilayers but there are significant differences between the two bilayers when the lipid molecules and interfacial waters are considered. It is shown that the stronger dipole moments of the POPC head groups create a thicker layer of interfacial waters with better orientation, which ultimately is responsible for the larger energy barrier in the K⁺ PMF in POPC.

Tackling the biophysical properties of sphingolipids to decipher their biological roles

From the most simple sphingoid bases to their complex glycosylated derivatives, several sphingolipid species were shown to have a role in fundamental cellular events and/or disease. Increasing evidence places lipid-lipid interactions and membrane structural alterations as central mechanisms underlying the action of these lipids. Understanding how these molecules exert their biological roles by studying their impact in the physical properties and organization of membranes is currently one of the main challenges in sphingolipid research. Herein, we review the progress in the state-of-the-art on the biophysical properties of sphingolipid-containing membranes, focusing on sphingosine, ceramides, and glycosphingolipids.

N-Acyl Chain in Ceramide and Sphingomyelin Determines Their Mixing Behavior, Phase State, and Surface Topography in Langmuir Films

Sphingolipids are membrane lipids composed by a long chain aminediol base, usually sphingosine, with a N-linked fatty acyl chain whose quality depends on the membrane type. The effect of length and unsaturation of the N-acyl chain on the mixing behavior of different sphingolipids has scarcely been studied, and in this work this issue is addressed employing Langmuir monolayers at the air-water interface, in order to assess the surface mixing in binary mixtures of different species of sphingomyelins and ceramides. The dependence on the monolayer composition of the mean molecular area, perpendicular dipole moment, domain segregation, and surface topography, as well as the film elasticity and optical thickness were studied. The results indicate that composition-dependent favorable interactions among sphingomyelin and ceramide occur as a consequence of complementary lateral packing and increased acyl chain ordering; the phase state of the components appears as a major factor determining miscibility among sphingomyelins and ceramides even in cases where the lipids have a considerable hydrocarbon chain length mismatch.

Ceramide: a simple sphingolipid with unique biophysical properties

Ceramides are involved in a variety of cellular processes and in disease. Their biological functions are thought to depend on ceramides' unique biophysical properties, which promote strong alterations of cell membrane properties and consequent triggering of signaling events. Over the last decades, efforts were made to understand the impact of ceramide on membrane biophysical features. Several studies, performed in a multitude of membrane models, address ceramides' specific interactions, the effect of their acyl chain structure and the influence of membrane lipid composition and properties on ceramide biophysical outcome. In this review, a rationale for the multiple and complex changes promoted by ceramide is provided, highlighting, on a comprehensive and critical manner, the interactions between ceramides and specific lipids and/or lipid phases. Focus is also given to the interplay between ceramide and cholesterol, particularly in lipid raft-mimicking mixtures, an issue of intense debate due to the urgent need to understand the biophysical impact of ceramide formation in models resembling the cell membrane. The implications of ceramide-induced biophysical changes on lipid-protein interactions and cell signaling are also discussed, together with the emerging evidence for the existence of ceramide-gel like domains in cellular membranes.

Importance of the sphingoid base length for the membrane properties of ceramides

The sphingoid bases of sphingolipids, including ceramides, can vary in length from 12 to >20 carbons. To study how such length variation affects the bilayer properties of ceramides, we synthesized ceramides consisting of a C12-, C14-, C16-, C18-, or C20-sphing-4-enin derivative coupled to palmitic acid. The ceramides were studied in mixtures with palmitoyloleoylphosphocholine (POPC) and/or palmitoylsphingomyelin (PSM), and in more complex bilayers also containing cholesterol. The trans-parinaric acid lifetimes showed that 12:1- and 14:1-PCer failed to increase the order of POPC bilayers, whereas 16:1-, 18:1-, and 20:1-PCer induced ordered- or gel-phase formation. Nevertheless, all of the analogs were able to thermally stabilize PSM, and a chain-length-dependent increase in the main phase transition temperature of equimolar PSM/Cer bilayers was revealed by differential scanning calorimetry. Similar thermal stabilization of PSM-rich domains by the ceramides was observed in POPC bilayers with a trans-parinaric acid-quenching assay. A cholestatrienol-quenching assay and sterol partitioning experiments showed that 18:1- and 20:1-PCer formed sterol-excluding gel phases with PSM, reducing the overall bilayer affinity of sterol. The effect of 16:1-PCer on sterol distribution was less dramatic, and no displacement of sterol from the PSM environment was observed with 12:1- and 14:1-PCer. The results are discussed in relation to other structural features that affect the bilayer properties of ceramides.

Palmitate diet-induced loss of cardiac caveolin-3: a novel mechanism for lipid-induced contractile dysfunction

Obesity is associated with an increased risk of cardiomyopathy, and mechanisms linking the underlying risk and dietary factors are not well understood. We tested the hypothesis that dietary intake of saturated fat increases the levels of sphingolipids, namely ceramide and sphingomyelin in cardiac cell membranes that disrupt caveolae, specialized membrane micro-domains and important for cellular signaling. C57BL/6 mice were fed two high-fat diets: palmitate diet (21% total fat, 47% is palmitate), and MCT diet (21% medium-chain triglycerides, no palmitate). We established that high-palmitate feeding for 12 weeks leads to 40% and 50% increases in ceramide and sphingomyelin, respectively, in cellular membranes. Concomitant with sphingolipid accumulation, we observed a 40% reduction in systolic contractile performance. To explore the relationship of increased sphingolipids with caveolins, we analyzed caveolin protein levels and intracellular localization in isolated cardiomyocytes. In normal cardiomyocytes, caveolin-1 and caveolin-3 co-localize at the plasma membrane and the T-tubule system. However, mice maintained on palmitate lost 80% of caveolin-3, mainly from the T-tubule system. Mice maintained on MCT diet had a 90% reduction in caveolin-1. These data show that caveolin isoforms are sensitive to the lipid environment. These data are further supported by similar findings in human cardiac tissue samples from non-obese, obese, non-obese cardiomyopathic, and obese cardiomyopathic patients. To further elucidate the contractile dysfunction associated with the loss of caveolin-3, we determined the localization of the ryanodine receptor and found lower expression and loss of the striated appearance of this protein. We suggest that palmitate-induced loss of caveolin-3 results in cardiac contractile dysfunction via a defect in calcium-induced calcium release.

Photouncaging of ceramides promotes reorganization of liquid-ordered domains in supported lipid bilayers

6-Bromo-7-hydroxycoumarin (Bhc)-caged ceramide (Cer) analogs were incorporated into supported lipid bilayers containing a mixture of coexisting liquid-ordered (Lo) and liquid-disordered (Ld) phases. The release of N-palmitoyl and N-butanoyl-D-erythro-sphingosine (C16- and C4-Cer) by the photolysis of caged Cers using long-wavelength UV light was studied using a combination of atomic force microscopy and fluorescence microscopy. This approach demonstrated the ability to generate Cer with spatial and temporal control, providing an alternative method to the enzymatic generation of Cer. The generation of C16-Cer from Bhc-C16-Cer disrupted the Lo domains, with the incorporation of small fluid-phase regions and the disappearance of some smaller domains. Cer-rich gel-phase domains were not observed, in contrast to results reported by either direct Cer incorporation or enzymatic Cer generation. The photorelease of C4-Cer from Bhc-C4-Cer resulted in qualitatively similar changes in bilayer morphology, with the disappearance of some Lo domains and no evidence of Cer-rich gel domains but with a smaller height difference between the ordered and disordered phases.

In situ synthesis of fluorescent membrane lipids (ceramides) using click chemistry

Ceramide analogues containing azide groups either in the polar head or in the hydrocarbon chains are non-fluorescent. When incorporated into phospholipid bilayers, they can react in situ with a non-fluorescent 1,8-naphthalimide using click chemistry giving rise to fluorescent ceramide derivatives emitting at ≈440 nm. When incorporated into giant unilamellar vesicles, two-photon excitation at 760 nm allows visualization of the ceramide-containing bilayers. This kind of method may be of general applicability in the study of model and cell membranes.

Effect of ceramide structure on membrane biophysical properties: the role of acyl chain length and unsaturation

Ceramide is an important bioactive sphingolipid involved in a variety of biological processes. The mechanisms by which ceramide regulates biological events are not fully understood, but may involve alterations in the biophysical properties of membranes. We now examine the properties of ceramide with different acyl chains including long chain (C16- and C18-), very long chain (C24-) and unsaturated (C18:1- and C24:1-) ceramides, in phosphatidylcholine model membranes. Our results show that i) saturated ceramides have a stronger impact on the fluid membrane, increasing its order and promoting gel/fluid phase separation, while their unsaturated counterparts have a lower (C24:1-) or no (C18:1-) ability to form gel domains at 37°C; ii) differences between saturated species are smaller and are mainly related to the morphology and size of the gel domains, and iii) very long chain ceramides form tubular structures likely due to their ability to form interdigitated phases. These results suggest that generation of different ceramide species in cell membranes has a distinct biophysical impact with acyl chain saturation dictating membrane lateral organization, and chain asymmetry governing interdigitation and membrane morphology.

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

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

Cholesterol displaces palmitoylceramide from its tight packing with palmitoylsphingomyelin in the absence of a liquid-disordered phase

A set of different biophysical approaches has been used to explore the phase behavior of palmitoylsphingomyelin (pSM)/cholesterol (Chol) model membranes in the presence and absence of palmitoylceramide (pCer). Fluorescence spectroscopy of di-4-ANEPPDHQ-stained pSM/Chol vesicles and atomic force microscopy of supported planar bilayers show gel L(beta)/liquid-ordered (L(o)) phase coexistence within the range X(Chol) = 0-0.25 at 22 degrees C. At the latter compositional point and beyond, a single L(o) pSM/Chol phase is detected. In ternary pSM/Chol/pCer mixtures, differential scanning calorimetry of multilamellar vesicles and confocal fluorescence microscopy of giant unilamellar vesicles concur in showing immiscibility, but no displacement, between L(o) cholesterol-enriched (pSM/Chol) and gel-like ceramide-enriched (pSM/pCer) phases at high pSM/(Chol + pCer) ratios. At higher cholesterol content, pCer is unable to displace cholesterol at any extent, even at X(Chol) < 0.25. It is interesting that an opposite strong cholesterol-mediated pCer displacement from its tight packing with pSM is clearly detected, completely abolishing the pCer ability to generate large microdomains and giving rise instead to a single ternary phase. These observations in model membranes in the absence of the lipids commonly used to form a liquid-disordered phase support the role of cholesterol as the key determinant in controlling its own displacement from L(o) domains by ceramide upon sphingomyelinase activity.

Coexistence of immiscible mixtures of palmitoylsphingomyelin and palmitoylceramide in monolayers and bilayers

A combination of lipid monolayer- and bilayer-based model systems has been applied to explore in detail the interactions between and organization of palmitoylsphingomyelin (pSM) and the related lipid palmitoylceramide (pCer). Langmuir balance measurements of the binary mixture reveal favorable interactions between the lipid molecules. A thermodynamically stable point is observed in the range approximately 30-40 mol % pCer. The pSM monolayer undergoes hyperpolarization and condensation with small concentrations of pCer, narrowing the liquid-expanded (LE) to liquid-condensed (LC) pSM main phase transition by inducing intermolecular interactions and chain ordering. Beyond this point, the phase diagram no longer reveals the presence of the pSM-enriched phase. Differential scanning calorimetry (DSC) of multilamellar vesicles reveals a widening of the pSM main gel-fluid phase transition (41 degrees C) upon pCer incorporation, with formation of a further endotherm at higher temperatures that can be deconvoluted into two components. DSC data reflect the presence of pCer-enriched domains coexisting, in different proportions, with a pSM-enriched phase. The pSM-enriched phase is no longer detected in DSC thermograms containing >30 mol % pCer. Direct domain visualization has been carried out by fluorescence techniques on both lipid model systems. Epifluorescence microscopy of mixed monolayers at low pCer content shows concentration-dependent, morphologically different pCer-enriched LC domain formation over a pSM-enriched LE phase, in which pCer content close to 5 and 30 mol % can be determined for the LE and LC phases, respectively. In addition, fluorescence confocal microscopy of giant vesicles further confirms the formation of segregated pCer-enriched lipid domains. Vesicles cannot form at >40 mol % pCer content. Altogether, the presence of at least two immiscible phase-segregated pSM-pCer mixtures of different compositions is proposed at high pSM content. A condensed phase (with domains segregated from the liquid-expanded phase) showing enhanced thermodynamic stability occurs near a compositional ratio of 2:1 (pSM/pCer). These observations become significant on the basis of the ceramide-induced microdomain aggregation and platform formation upon sphingomyelinase enzymatic activity on cellular membranes.

Basic nanostructure of stratum corneum lipid matrices based on ceramides [EOS] and [AP]: a neutron diffraction study

The goal of this study was to investigate the nanostructure of SC lipid model membranes comprising the most relevant SC lipids such as the unique-structured omega-acylceramide [EOS] in a near natural ratio with neutron diffraction. In models proposed recently the presence of ceramide [EOS] and FFA are necessary for the formation of one of the two existent crystalline lamellar phases of the SC lipids, the long-periodicity phase as well as for the normal barrier function of the SC. The focus of this study was placed on the influence of the FFA BA on the membrane structure and its localization within the membrane based on the ceramides [EOS] and [AP]. The internal nanostructure of such membranes was obtained by Fourier synthesis from the experimental diffraction patterns. The resulting neutron scattering length density profiles showed that the exceptionally long ceramide [EOS] is arranged in a short-periodicity phase created by ceramide [AP] by spanning through the whole bilayer and extending even further into the adjacent bilayer. Specifically deuterated BA allowed us to determine the exact position of this FFA inside this SC lipid model membrane. Furthermore, hydration experiments showed that the presented SC mimic system shows an extremely small intermembrane hydration of approximately 1 A, consequently the headgroups of the neighboring leaflets are positioned close to each other.

Lipid domain morphologies in phosphatidylcholine-ceramide monolayers

In cells, one of the main roles of ceramide-enriched membrane domains is to recruit or exclude intracellular signaling molecules and receptors, thereby facilitating signal transduction cascades. Accordingly, in model membranes, even low contents of ceramide segregate into lateral domains. The impact of the N-acyl chain on this segregation and on the morphology of the domains remains to be explored. Using Langmuir monolayers, we have systematically studied binary mixtures of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and ceramide (2:1, molar ratio) and varied the N-acyl chain length of ceramide from 2 to 24 carbon atoms (Cer2 to Cer24). Fluid Cer2, Cer6, and Cer8/DMPC mixtures were miscible at all surface pressures. Longer ceramides, however, formed surface pressure-dependent immiscible mixtures with DMPC. The domain morphology under fluorescence microscopy after including a trace amount of fluorescent NBD-phosphatidylcholine into DMPC/Cer mixtures was found to be very sensitive to the N-acyl chain length. Shorter ceramides (Cer10-Cer14) formed flower-like (seaweed) domains, whereas longer ceramides (N-acyl chain length>14 carbon atoms) formed round and regular domains. We attribute the formation of the flower patterns to diffusive morphological instabilities during domain growth.

Enzymatic generation of ceramide induces membrane restructuring: Correlated AFM and fluorescence imaging of supported bilayers

The effect of enzymatic generation of ceramide on phase separated bilayers with a mixture of co-existing fluid and liquid-ordered phases has been examined using a combination of atomic force microscopy (AFM) and fluorescence imaging. Supported lipid bilayers prepared from a DOPC/sphingomyelin/cholesterol mixture were imaged prior to, during and after incubation with sphingomyelinase by total internal reflection fluorescence (TIRF) microscopy. Enzyme treatment resulted in the growth of large dye-excluded regions. The growth kinetics for these patches are consistent with activity of a variable number of enzyme molecules in different regions of the bilayer. Correlated AFM and fluorescence imaging shows that some of the large dye-excluded patches form around the original liquid-ordered domains, which become heterogeneous in height with many raised ceramide-rich regions around their periphery. However, some of the dye-excluded patches correspond to areas of the bilayer where the initial domains have largely or partially disappeared. The dye-excluded patches observed by fluorescence are shown to be areas of increased adhesion in lateral deflection AFM images and are postulated to form by incorporation of both cholesterol and ceramide in the original fluid phase and to vary in composition throughout the bilayer. This is evident from the observation that the dye-excluded areas are all detected as areas of increased friction, but do not always show a distinct height difference in topographic images. These results highlight the utility of a multi-modal imaging approach for understanding the complex membrane restructuring that occurs upon enzymatic generation of ceramide.

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.

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.

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

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

Sphingomyelinase generation of ceramide promotes clustering of nanoscale domains in supported bilayer membranes

The effects of ceramide incorporation in supported bilayers prepared from ternary lipid mixtures which have small nanoscale domains have been examined using atomic force and fluorescence microscopy. Both direct ceramide incorporation in vesicles used to prepare the supported bilayers and enzymatic hydrolysis of SM by sphingomyelinase were compared for membranes prepared from 5:5:1 DOPC/sphingomyelin/cholesterol mixtures. Both methods of ceramide incorporation resulted in enlargement of the initial small ordered domains. However, enzymatic ceramide generation led to a much more pronounced restructuring of the bilayer to give large clusters of domains with adjacent areas of a lower phase. The individual domains were heterogeneous with two distinct heights, the highest of which is assigned to a ceramide-rich phase which is hypothesized to occur via ceramide flip-flop to the lower leaflet with formation of a raised domain due to negative membrane curvature. A combination of AFM and fluorescence showed that the bilayer restructuring starts rapidly after enzyme addition, with formation of large clusters of domains at sites of high enzyme activity. The clustering of domains is accompanied by redistribution of fluid phase to the periphery of the domain clusters and there is a continued slow evolution of the bilayer over a period of an hour or more after the enzyme is removed. The relevance of the observed clustering of small nanoscale domains to the postulated coalescence of raft domains to form large signaling platforms is discussed.

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.

The permeability enhancing mechanism of DMSO in ceramide bilayers simulated by molecular dynamics

The lipids of the topmost layer of the skin, the stratum corneum, represent the primary barrier to molecules penetrating the skin. One approach to overcoming this barrier for the purpose of delivery of active molecules into or via the skin is to employ chemical permeability enhancers, such as dimethylsulfoxide (DMSO). How these molecules exert their effect at the molecular level is not understood. We have investigated the interaction of DMSO with gel-phase bilayers of ceramide 2, the predominant lipid in the stratum corneum, by means of molecular dynamics simulations. The simulations satisfactorily reproduce the phase behavior and the known structural parameters of ceramide 2 bilayers in water. The effect of DMSO on the gel-phase bilayers was investigated at various concentrations over the range 0.0-0.6 mol fraction DMSO. The DMSO molecules accumulate in the headgroup region and weaken the lateral forces between the ceramides. At high concentrations of DMSO (> or =0.4 mol fraction), the ceramide bilayers undergo a phase transition from the gel phase to the liquid crystalline phase. The liquid-crystalline phase of ceramides is expected to be markedly more permeable to solutes than the gel phase. The results are consistent with the experimental evidence that high concentrations of DMSO fluidize the stratum corneum lipids and enhance permeability.

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.

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.