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

Ceramide regulation of autophagy: A biophysical approach

Specific membrane lipids play unique roles in (macro)autophagy. Those include phosphatidylethanolamine, to which LC3/GABARAP autophagy proteins become covalently bound in the process, or cardiolipin, an important effector in mitochondrial autophagy (or mitophagy). Ceramide (Cer), or N-acyl sphingosine, is one of the simplest sphingolipids, known as a stress signal in the apoptotic pathway. Moreover, Cer is increasingly being recognized as an autophagy activator, although its mechanism of action is unclear. In the present review, the proposed Cer roles in autophagy are summarized, together with some biophysical properties of Cer in membranes. Possible pathways for Cer activation of autophagy are discussed, including specific protein binding of the lipid, and Cer-dependent perturbation of bilayer properties. Cer generation of lateral inhomogeneities (domain formation) is given special attention. Recent biophysical results, including fluorescence and atomic force microscopy data, show Cer-promoted enhanced binding of LC3/GABARAP to lipid bilayers. These observations could be interpreted in terms of the putative formation of Cer-rich nanodomains.

Lipids in Mitochondrial Macroautophagy: Phase Behavior of Bilayers Containing Cardiolipin and Ceramide

Cardiolipin (CL) is a key lipid for damaged mitochondrial recognition by the LC3/GABARAP human autophagy proteins. The role of ceramide (Cer) in this process is unclear, but CL and Cer have been proposed to coexist in mitochondria under certain conditions. Varela et al. showed that in model membranes composed of egg sphingomyelin (eSM), dioleoyl phosphatidylethanolamine (DOPE), and CL, the addition of Cer enhanced the binding of LC3/GABARAP proteins to bilayers. Cer gave rise to lateral phase separation of Cer-rich rigid domains but protein binding took place mainly in the fluid continuous phase. In the present study, a biophysical analysis of bilayers composed of eSM, DOPE, CL, and/or Cer was attempted to understand the relevance of this lipid coexistence. Bilayers were studied by differential scanning calorimetry, confocal fluorescence microscopy, and atomic force microscopy. Upon the addition of CL and Cer, one continuous phase and two segregated ones were formed. In bilayers with egg phosphatidylcholine instead of eSM, in which the binding of LC3/GABARAP proteins hardly increased with Cer in the former study, a single segregated phase was formed. Assuming that phase separation at the nanoscale is ruled by the same principles acting at the micrometer scale, it is proposed that Cer-enriched rigid nanodomains, stabilized by eSM:Cer interactions formed within the DOPE- and CL-enriched fluid phase, result in structural defects at the rigid/fluid nanointerfaces, thus hypothetically facilitatingLC3/GABARAP protein interaction.

Ceramide enhances binding of LC3/GABARAP autophagy proteins to cardiolipin-containing membranes

Macroautophagy, or autophagy, is a process in which cell macromolecules, or even organelles, are engulfed in a double-membrane vesicle, the autophagosome, and directed to a lysosome. Among autophagy-related proteins, LC3/GABARAP constitute a protein family derived from yeast Atg8. They play important roles in autophagosome formation, binding future cargo organelles and promoting autophagosome growth. The involvement of specific lipids in this process is poorly understood. The present study explores the interaction of LC3/GABARAP proteins with phospholipid monolayers and bilayers based on phosphatidylcholine or on sphingomyelin. Cardiolipin is found to be essential for the protein interaction with such bilayers, as measured through gradient centrifugation experiments, while ceramide markedly increases binding. Giant unilamellar vesicles examined under confocal fluorescence microscopy reveal that ceramide segregates laterally into very rigid domains, while GABARAP binds only the more fluid regions, suggesting that the enhancing role of ceramide is exerted by the minority of ceramide molecules dispersed in the fluid phase. Although in further autophagy steps the LC3/GABARAP proteins are covalently bound to a phospholipid, this is not the case in our system, thus it is proposed that the observed ceramide effects would correspond to very early stages in the process, such as cargo recognition.

Preparation of Nitrogen Analogues of Ceramide and Studies of Their Aggregation in Sphingomyelin Bilayers

Ceramides can regulate biological processes probably through the formation of laterally segregated and highly packed ceramide-rich domains in lipid bilayers. In the course of preparation of its analogues, we found that a hydrogen-bond-competent functional group in the C1 position is necessary to form ceramide-rich domains in lipid bilayers [Matsufuji; Langmuir 2018]. Hence, in the present study, we newly synthesized three ceramide analogues: CerN₃, CerNH₂, and CerNHAc, in which the 1-OH group of ceramide is substituted with a nitrogen functionality. CerNH₂ and CerNHAc are capable of forming hydrogen bonds in their headgroups, whereas CerN₃ is not. Fluorescent microscopy observation and differential scanning calorimetry analysis disclosed that these ceramide analogues formed ceramide-rich phases in sphingomyelin bilayers, although their thermal stability was slightly inferior to that of normal ceramides. Moreover, wide-angle X-ray diffraction analysis showed that the chain packing structure of ceramide-rich phases of CerNHAc and CerN₃ was similar to that of normal ceramide, while the CerNH₂-rich phase showed a slightly looser chain packing due to the formation of CerNH₃⁺. Although the domain formation of CerN₃ was unexpected because of the lack of hydrogen-bond capability in the headgroup, it may become a promising tool for investigating the mechanistic link between the ceramide-rich phase and the ceramide-related biological functions owing to its Raman activity and applicability to click chemistry.

Structural foundations of sticholysin functionality

Actinoporins constitute a family of α pore-forming toxins produced by sea anemones. The soluble fold of these proteins consists of a β-sandwich flanked by two α-helices. Actinoporins exert their activity by specifically recognizing sphingomyelin at their target membranes. Once there, they penetrate the membrane with their N-terminal α-helices, a process that leads to the formation of cation-selective pores. These pores kill the target cells by provoking an osmotic shock on them. In this review, we examine the role and relevance of the structural features of actinoporins, down to the residue level. We look at the specific amino acids that play significant roles in the function of actinoporins and their fold. Particular emphasis is given to those residues that display a high degree of conservation across the actinoporin sequences known to date. In light of the latest findings in the field, the membrane requirements for pore formation, the effect of lipid composition, and the process of pore formation are also discussed.

Exploring polar headgroup interactions between sphingomyelin and ceramide with infrared spectroscopy

Ceramide is a major actor in the sphingolipid signaling pathway elicited by various kinds of cell stress. Under those conditions ceramide (Cer) is produced in the plasma membrane as a product of sphingomyelin (SM) hydrolysis, and this may lead to apoptosis. Thus, SM and Cer coexist in the membrane for some time, and they are known to separate laterally from the (more abundant) glycerolipids, giving rise to highly rigid domains or platforms. The properties of these domains/platforms are rather well understood, but the underlying SM:Cer molecular interactions have not been explored in detail. Infrared (IR) spectroscopy is a powerful analytical technique that provides information on all the chemical groupings in a molecule, and that can be applied to membranes and lipid bilayers in aqueous media. IR spectra can be conveniently retrieved as a function of temperature, thus revealing the thermotropic transitions of SM and its mixtures with Cer. Four regions of the IR spectrum of these sphingolipids have been examined, two of them dominated by the hydrophobic regions in the molecules, namely the C–H stretching vibrations (2800–3000 cm⁻¹), and the CH₂ scissoring vibrations (1455–1485 cm⁻¹), and two others arising from chemical groups at the lipid-water interface, the sphingolipid amide I band (1600–1680 cm⁻¹), and the phosphate vibrations in the 1000–1110 cm⁻¹ region. The latter two regions have been rarely studied in the past. The IR data from the hydrophobic components show a gel (or ripple)-fluid transition of SM at 40 °C, that is shifted up to about 70 °C when Cer is added to the bilayers, in agreement with previous studies using a variety of techniques. IR information concerning the polar parts is more interesting. The amide I (carbonyl) band of pure SM exhibits a maximum at 1638 cm⁻¹ at room temperature, and its position is shifted by about 10 cm⁻¹ in the presence of Cer. Cer causes also a change in the overall band shape, but no signs of band splitting are seen, suggesting that SM and Cer carbonyl groups are interacting tightly, presumably through H-bonds. The 1086 cm⁻¹ band, corresponding to PO₂⁻ vibrations, appears more stable in SM than in DPPC, and it is further stabilized by Cer, again suggesting an important role of H-bonds in the formation of SM:Cer clusters. Thus, SM and Cer can interact through their polar headgroups, in a way that is not accessible to other lipid classes.

Lamellar Phases Composed of Phospholipid, Cholesterol, and Ceramide, as Studied by 2H NMR

Sphingolipids constitute a significant fraction of cellular plasma membrane lipid content. Among sphingolipids, ceramide levels are usually very low. However, in some cell processes like apoptosis, cell membrane ceramide levels increase markedly because of the activation of enzymes like acid sphingomyelinase. This increase can change the physical state of the membrane by promoting molecular order and inducing solid-ordered (So) phase domains. This effect has been observed in a previous 2H NMR study on membranes consisting of palmitoyl sphingomyelin (PSM) and palmitoyl ceramide (PCer). Cholesterol (Chol), too, is present at high concentrations in mammalian plasma membranes and has a favorable interaction with sphingomyelin (SM), together forming domains in the liquid-ordered phase in model membranes. There are reports that Chol is able to displace ceramide (Cer) in SM bilayers and abolish the So phase domains formed by SM:Cer. This ability of Chol appears to be concentration dependent; in membranes with low Chol and high Cer contents, So phase domains rich in Cer coexist with the continuous fluid phase of the membrane. Here, we studied the effect of increasing PCer concentration in PSM:Chol bilayers, using 2H NMR. Chol:PCer mole ratios were 3:1, 3:2, and 3:3, at a fixed 7:3 phospholipid:cholesterol mol ratio. Both PSM and PCer were monitored in separate samples for changes in their physical state by introducing a perdeuterated palmitoyl chain in either molecule. Moreover, the effect of replacing PSM with DPPC was investigated to test the impact on membrane phase behavior of replacing the sphingosine with a palmitoylated glycerol backbone. We found that PCer can increase acyl chain order in both PSM:Chol and DPPC:Chol bilayers. Especially in bilayers with Chol:PCer 1:1 molar ratios, PCer induces highly stable So phase domains in both PSM and DPPC bilayers near 37°C. However, PCer has a more pronounced ordering effect on PSM compared to DPPC bilayers.

Homogeneous and Heterogeneous Bilayers of Ternary Lipid Compositions Containing Equimolar Ceramide and Cholesterol

Cell membranes have been proposed to be laterally inhomogeneous, particularly in the case of mammalian cells, due to the presence of "domains" enriched in sphingolipids and cholesterol (Chol). Among membrane sphingolipids, sphingomyelin (SM) in the cell plasma membrane is known to be degraded to ceramide (Cer) by acid sphingomyelinases under stress conditions. Since cholesterol (Chol) is abundant in the plasma membrane, the study of ternary mixtures SM:Chol:Cer is interesting from the point of view of membrane biophysics, and it might be physiologically relevant. In previous studies, we have described the homogeneous gel phase formed by phospholipid:Chol:Cer at 54:23:23 mol ratios, where phospholipid was either SM or dipalmitoylphosphatidylcholine (DPPC). We now provide new data, based on trans-parinaric acid and diphenylhexatriene fluorescence, supporting that the gel phase includes all three components in a single bilayer. The main question addressed in this paper is the stability of the ternary gel phase when bilayer composition is changed, specifically when the SM proportion is varied. To this aim, we have prepared bilayers of composition phospholipid:Chol:Cer at X:Y:Y ratios, in which phospholipid increased between 54 and 70 mol %. The N-palmitoyl derivatives of SM (pSM) and Cer (pCer) have been used. We observe that for X = 54 or 60 mol %, a gel phase is clearly predominant. However, when the proportion of phospholipid increases beyond 60 mol %, i.e., in 66:17:17 or 70:15:15 mixtures, a lateral phase separation occurs at the micrometer scale. These data can be interpreted in terms of a pCer:Chol interaction, that would predominate at the lower phospholipid concentrations. The putative pCer:Chol complexes (or nanodomains) would mix well with the phospholipid. At the higher SM concentrations pSM:pCer and pSM:Chol interactions would become more important, giving rise to the coexisting gel and liquid-ordered phases respectively. Heterogeneity, or lateral phase separation, occurs more easily with pSM than with DPPC, indicating a higher affinity of SM over DPPC for Chol or Cer. The observation that heterogeneity, or lateral phase separation, occurs more easily with pSM than with DPPC, indicates a higher affinity of SM over DPPC for Chol or Cer, and can be related to cell regulation through the sphingolipid signaling pathway.

Preparation and Membrane Distribution of Fluorescent Derivatives of Ceramide

Ceramide is a bioactive lipid with significant roles in several biological processes including cell proliferation, apoptosis, and raft formation. Although fluorescent derivatives of ceramide are required to probe the behavior of ceramide in cells and cell membranes, commercial fluorescent ceramide derivatives do not reproduce the membrane behavior of native ceramide because of the introduction of bulky fluorophores in the acyl chain. Recently, we developed novel fluorescent analogs of sphingomyelin in which the hydrophilic fluorophores, ATTO488 and ATTO594, are attached to the polar head of sphingomyelin via a nonaethylene glycol linker and demonstrated that their partition and dynamic behaviors in bilayer membranes are similar to native sphingomyelin. In this report, by extending the concept used for the development of fluorescent analogs of sphingomyelin, we prepared novel fluorescent ceramides that exhibit membrane behaviors similar to native ceramide and succeeded in visualizing ceramide-rich membrane domains segregated from ceramide-poor domains.

Clearly Detectable, Kinetically Restricted Solid-Solid Phase Transition in cis-Ceramide Monolayers

Sphingosine [(2 S,3 R,4 E)-2-amino-4-octadecene-1,3-diol] is the most common sphingoid base in mammals. Ceramides are N-acyl sphingosines. Numerous small variations on this canonical structure are known, including the 1-deoxy, the 4,5-dihydro, and many others. However, whenever there is a Δ4 double bond, it adopts the trans (or E) configuration. We synthesized a ceramide containing 4 Z-sphingosine and palmitic acid ( cis-pCer) and studied its behavior in the form of monolayers extended on an air-water interface. cis-pCer acted very differently from the trans isomer in that, upon lateral compression of the monolayer, a solid-solid transition was clearly observed at a mean molecular area ≤44 Ų·molecule^-1, whose characteristics depended on the rate of compression. The solid-solid transition, as well as states of domain coexistence, could be imaged by atomic force microscopy and by Brewster-angle microscopy. Atomistic molecular dynamics simulations provided results compatible with the experimentally observed differences between the cis and trans isomers. The data can help in the exploration of other solid-solid transitions in lipids, both in vitro and in vivo, that have gone up to now undetected because of their less obvious change in surface properties along the transition, as compared to cis-pCer.

The Physical Properties of Ceramides in Membranes

Ceramides are sphingolipids containing a sphingosine or a related base, to which a fatty acid is linked through an amide bond. When incorporated into a lipid bilayer, ceramides exhibit a number of properties not shared by almost any other membrane lipid: Ceramides ( a) are extremely hydrophobic and thus cannot exist in suspension in aqueous media; ( b) increase the molecular order (rigidity) of phospholipids in membranes; ( c) give rise to lateral phase separation and domain formation in phospholipid bilayers; ( d) possess a marked intrinsic negative curvature that facilitates formation of inverted hexagonal phases; ( e) make bilayers and cell membranes permeable to small and large (i.e., protein-size) solutes; and ( f) promote transmembrane (flip-flop) lipid motion. Unfortunately, there is hardly any link between the physical studies reviewed here and the mass of biological and clinical studies on the effects of ceramides in health and disease.

Regulation of phase boundaries and phase-segregated patterns in model membranes

Demixing of components has long been described in model membranes. It is a consequence of non-ideal lateral interactions between membrane components, and it causes the presence of segregated phases, forming patches (domains) of different properties, thus introducing heterogeneity into the membrane. In the present review we first describe the processes through which domains are generated, how they grow, and why they are rounded, striped or fractal-like, as well as why they get distributed forming defined patterns. Next, we focus on the effect of an additive on a lipid mixture, which usually induces shifts in demixing points, thus stabilizing or destabilizing the phase-segregated state. Results found for different model membranes are summarized, detailing the ways in which phase segregation and the generated patterns may be modulated. We focus on which are, from our viewpoint, the most relevant regulating factors affecting the surface texture observed in model membranes. This article is part of a Special Issue entitled: Emergence of Complex Behavior in Biomembranes edited by Marjorie Longo.

Preparation and Membrane Properties of Oxidized Ceramide Derivatives

Ceramide is a bioactive lipid with important roles in several biological processes including cell proliferation and apoptosis. Although 3-ketoceramides that contain a keto group in place of the 3-OH group of ceramide occur naturally, ceramide derivatives oxidized at the primary 1-OH group have not been identified to date. To evaluate how the oxidative state of the 1-OH group affects the physical properties of membranes, we prepared novel ceramide derivatives in which the 1-OH group was oxidized to a carboxylic acid (PCerCOOH) or methylester (PCerCOOMe) and examined the rigidity of their monolayers and the formation of gel domains in palmitoyloleoylphosphatidylcholine (POPC) or sphingomyelin (SM) bilayers. As a result, PCerCOOH and PCerCOOMe exhibited membrane properties similar to those of native ceramide, although the deprotonated form of PCerCOOH, PCerCOO^-, exhibited markedly lower rigidity and higher miscibility with POPC and SM. This was attributed to the electrostatic repulsion of the negative charge, which hampered the formation of the ceramide-enriched gel domain. The similarities in the properties of PCerCOOMe and ceramide revealed the potential to introduce various functional groups onto PCerCOOH via ester or amide linkages; therefore, these derivatives will also provide a new strategy for developing molecular probes, such as fluorescent ceramides, and inhibitors of ceramide-related enzymes.

The enzymatic sphingomyelin to ceramide conversion increases the shear membrane viscosity at the air-water interface

Whereas most of lipids have viscous properties and they do not have significant elastic features, ceramides behave as very rigid solid assemblies, displaying viscoelastic behaviour at physiological temperatures. The present review addresses the surface rheology of lipid binary mixtures made of sphingomyelin and ceramide. However, ceramide is formed by the enzymatic cleavage of sphingomyelin in cell plasma membranes. The consequences of the enzymatically-driven ceramide formation involve mechanical alterations of the embedding membrane. Here, an increase on surface shear viscosity was evidenced upon enzymatic incubation of sphingomyelin monolayers. The overall rheological data are discussed in terms of the current knowledge of the thermotropic behaviour of ceramide-containing model membranes.

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.

Ceramide-C16 Is a Versatile Modulator of Phosphatidylethanolamine Polymorphism

Ceramide-C16 (CerC16) is a sphingolipid associated with several diseases like diabetes, obesity, Parkinson disease, and certain types of cancers. As a consequence, research efforts are devoted to identify the impact of CerC16 on the behavior of membranes, and to understand how it is involved in these diseases. In this work, we investigated the impacts of CerC16 (up to 20 mol %) on the lipid polymorphism of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), using differential scanning calorimetry, and sequential ²H and ³¹P solid-state nuclear magnetic resonance spectroscopy. A partial phase diagram is proposed. The results indicate that the presence of CerC16 leads to an upshift of the temperature of the gel-to-liquid crystalline (L_β - L_α) phase transition, leading to a large L_β/L_α phase coexistence region where gel-phase domains contain ∼35 mol % CerC16. It also leads to a downshift of the temperature of the lamellar-to-inverted hexagonal (L - H_II) phase transition of POPE. The opposite influence on the two-phase transitions of POPE brings a three-phase coexistence line when the two transitions overlap. The resulting H_II phase can be ceramide enriched, coexisting with a L_α phase, or ceramide depleted, coexisting with a L_β phase, depending on the CerC16 proportions. The uncommon capability of CerC16 to modulate the membrane fluidity, its curvature propensity, and the membrane interface properties highlights its potential as a versatile messenger in cell membrane events.

Ceramide-Induced Lamellar Gel Phases in Fluid Cell Lipid Extracts

The effects of increasing amounts of palmitoylceramide (pCer) on human red blood cell lipid membranes have been studied using atomic force microscopy of supported lipid bilayers, in both imaging (bilayer thickness) and force-spectroscopy (nanomechanical resistance) modes. Membranes appeared homogeneous with pCer concentrations up to 10 mol % because of the high concentration of cholesterol (Chol) present in the membrane (∼45 mol %). However, the presence of pCer at 30 mol % gave rise to a clearly distinguishable segregated phase with a nanomechanical resistance 7-fold higher than the continuous phase. These experiments were validated using differential scanning calorimetry. Furthermore, Chol depletion of the bilayers caused lipid domain generation in the originally homogeneous samples, and Chol-depleted domain stiffness significantly increased with higher amounts of pCer. These results point to the possibility of different kinds of transient and noncompositionally constant, complex gel-like phases present in RBC lipid membranes rich in both pCer and Chol, in contrast to the widespread opinion about the displacements between pCer-enriched "gel-like" domains and liquid-ordered "raft-like" Chol-enriched phases. Changes in the biophysical properties of these complex gel-like phases governed by local modulation of pCer:Chol ratios could be a cell mechanism for fine-tuning the properties of membranes as required.

Influence of Hydroxylation, Chain Length, and Chain Unsaturation on Bilayer Properties of Ceramides

Mammalian ceramides constitute a family of at least a few hundred closely related molecules distinguished by small structural differences, giving rise to individual molecular species that are expressed in distinct cellular compartments, or tissue types, in which they are believed to execute distinct functions. We have examined how specific structural details influence the bilayer properties of a selection of biologically relevant ceramides in mixed bilayers together with sphingomyelin, phosphatidylcholine, and cholesterol. The ceramide structure varied with regard to interfacial hydroxylation, the identity of the headgroup, the length of the N-acyl chain, and the position of cis-double bonds in the acyl chains. The interactions of the ceramides with sphingomyelin, their lateral segregation into ceramide-rich domains in phosphatidylcholine bilayers, and the effect of cholesterol on such domains were studied with DSC and various fluorescence-based approaches. The largest differences arose from the presence and relative position of cis-double bonds, causing destabilization of the ceramide's interactions and lateral packing relative to common saturated and hydroxylated species. Less variation was observed as a consequence of interfacial hydroxylation and the N-acyl chain length, although an additional hydroxyl in the sphingoid long-chain base slightly destabilized the ceramide's interactions and packing relative to a nonhydroxyceramide, whereas an additional hydroxyl in the N-acyl chain had the opposite effect. In conclusion, small structural details conferred variance in the bilayer behavior of ceramides, some causing more dramatic changes in the bilayer properties, whereas others imposed only fine adjustments in the interactions of ceramides with other membrane lipids, reflecting possible functional implications in distinct cell or tissue types.

Biophysical properties of novel 1-deoxy-(dihydro)ceramides occurring in mammalian cells

Ceramides and dihydroceramides are N-acyl derivatives of sphingosine and sphinganine, respectively, which are the major sphingoid-base backbones of mammals. Recent studies have found that mammals, like certain other organisms, also produce 1-deoxy-(dihydro)ceramides (1-deoxyDHCers) that contain sphingoid bases lacking the 1-hydroxyl- or 1-hydroxymethyl- groups. The amounts of these compounds can be substantial-indeed, we have found comparable levels of 1-deoxyDHCers and ceramides in RAW 264.7 cells maintained in culture. The biophysical properties of 1-deoxyDHCers have not yet been reported, although these lipids might play important roles in normal cell regulation and in the pathology of diseases in which they are elevated, such as hereditary sensory autonomic neuropathies or diabetes. This study uses several approaches, including surface-pressure measurements, differential scanning calorimetry, and confocal microscopy, to study the behavior of 1-deoxyDHCers of different N-acyl-chain lengths and their interaction with sphingomyelin (SM). The thermotropic behaviors of 1-deoxyDHCers alone and in mixtures with SM are described, together with their interactions in monolayers and giant unilamellar vesicles. The gel-fluid transition temperatures of the pure compounds increase in the order 1-deoxyceramide < ceramide ≈ 1-deoxyDHCer < 1-(deoxymethyl)DHCer. In general, canonical ceramides are more miscible with SM in bilayers than are 1-deoxyceramides, and 1-(deoxymethyl)DHCers are the most hydrophobic among them, not even capable of forming monolayers at the air-water interface. Thus, these properties suggest that 1-deoxyDHCer can influence the properties of cellular membranes in ways that might affect biological function/malfunction.

Cholesterol interactions with ceramide and sphingomyelin

Sphingolipids contain in their polar heads chemical groups allowing them to establish a complex network of H-bonds (through different OH and NHgroups) with other lipids in the bilayer. In the recent years the specific interaction of sphingomyelin (SM) with cholesterol (Chol) has been examined, largely in the context of the "lipid raft" hypothesis. Formation of SM-Ceramide (Cer) complexes, proposed to exist in cell membranes in response to stress, has also been described. More recently, a delicate balance of phase formation and transformation in ternary mixtures of SM, Chol and Cer, with mutual displacement of Chol and Cer from their interaction with SM is considered to exist. In addition, data demonstrating direct Chol-Cer interaction are becoming available.

The Rheological Properties of Lipid Monolayers Modulate the Incorporation of l-Ascorbic Acid Alkyl Esters

In this work, we tested the hypothesis that the incorporation of amphiphilic drugs into lipid membranes may be regulated by their rheological properties. For this purpose, two members of the l-ascorbic acid alkyl esters family (ASCn) were selected, ASC16 and ASC14, which have different rheological properties when organized at the air/water interface. They are lipophilic forms of vitamin C used in topical pharmacological preparations. The effect of the phase state of the host lipid membranes on ASCn incorporation was explored using Langmuir monolayers. Films of pure lipids with known phase states have been selected, showing liquid-expanded, liquid-condensed, and solid phases as well as pure cholesterol films in liquid-ordered state. We also tested ternary and quaternary mixed films that mimic the properties of cholesterol containing membranes and of the stratum corneum. The compressibility and shear properties of those monolayers were assessed in order to define its phase character. We found that the length of the acyl chain of the ASCn compounds induces differential changes in the rheological properties of the host membrane and subtly regulates the kinetics and extent of the penetration process. The capacity for ASCn uptake was found to depend on the phase state of the host film. The increase in surface pressure resultant after amphiphile incorporation appears to be a function of the capacity of the host membrane to incorporate such amphiphile as well as the rheological response of the film. Hence, monolayers that show a solid phase state responded with a larger surface pressure increase to the incorporation of a comparable amount of amphiphile than liquid-expanded ones. The cholesterol-containing films, including the mixture that mimics stratum corneum, allowed a very scarce ASCn uptake independently of the membrane diffusional properties. This suggests an important contribution of Cho on the maintenance of the barrier function of stratum corneum.

Thermomechanical transitions of egg-ceramide monolayers

Ceramides have unique biophysical properties. Their high melting temperature and their ability to form lateral domains have converted ceramides into the paradigm of rigid lipids. Here, using shear surface rheology of egg-ceramide Langmuir monolayers, a solid to fluid transition was evidenced as a vanishing shear rigidity at lower temperatures than the lipid melting temperature. Such a mechanical transition, which depends on the lipid lateral pressure, was found in a broad range temperature (40-50 °C). The solid to fluid transition was correlated to a LC to LC+LE phase transition, as confirmed by BAM experiments. Interestingly, together with the softening transition, a supercooling process compatible with a glassy behavior was found upon freezing. A new phase scenario is then depicted that broadens the mechanical behavior of natural ceramides. The phase diversity of ceramides might have important implications in their physiological roles.

Pressure-temperature phase behavior of mixtures of natural sphingomyelin and ceramide extracts

Ceramides are a group of sphingolipids that act as highly important signaling molecules in a variety of cellular processes including differentiation and apoptosis. The predominant in vivo synthetic pathway for ceramide formation is via sphingomyelinase catalyzed hydrolysis of sphingomyelin. The biochemistry of this essential pathway has been studied in detail; however, there is currently a lack of information on the structural behavior of sphingomyelin- and ceramide-rich model membrane systems, which is essential for developing a bottom-up understanding of ceramide signaling and platform formation. We have studied the lyotropic phase behavior of sphingomyelin-ceramide mixtures in excess water as a function of temperature (30-70 °C) and pressure (1-200 MPa) by small- and wide-angle X-ray scattering. At low ceramide concentrations the mixtures form the ripple gel phase (P(β)') below the gel transition temperature for sphingomyelin, and this observation has been confirmed by atomic force microscopy. Formation of the ripple gel phase can also be induced at higher temperatures via the application of hydrostatic pressure. At high ceramide concentration an inverse hexagonal phase (HII) is formed coexisting with a cubic phase.

Atomic force microscopy characterization of palmitoylceramide and cholesterol effects on phospholipid bilayers: a topographic and nanomechanical study

Supported planar bilayers (SPBs) on mica substrates have been studied at 23 °C under atomic force microscopy (AFM)-based surface topography and force spectroscopy with two main objectives: (i) to characterize palmitoylceramide (pCer)-induced gel (Lβ) domains in binary mixtures with either its sphingolipid relative palmitoylsphingomyelin (pSM) or the glycerophospholipid dipalmitoylphosphorylcholine (DPPC) and (ii) to evaluate effects of incorporating cholesterol (Chol) into the previous mixtures in terms of Cer and Chol cooperation for the generation of lamellar gel (Lβ) phases of ternary composition. Binary phospholipid/pCer mixtures at XpCer < 0.33 promote the generation of laterally segregated micron-sized pCer-rich domains. Their analysis at different phospholipid/pCer ratios, by means of domain thickness, roughness, and mechanical resistance to tip piercing, reveals unvarying AFM-derived features over increasing pCer concentrations. These results suggest that the domains grow in size with increasing pCer concentrations while keeping a constant phospholipid/pCer stoichiometry. Moreover, the data show important differences between pCer interactions with pSM or DPPC. Gel domains generated in pSM/pCer bilayers are thinner than the pSM-rich surrounding phase, while the opposite is observed in DPPC/pCer mixtures. Furthermore, a higher breakthrough force is observed for pSM/pCer as compared to DPPC/pCer domains, which can be associated with the preferential pCer interaction with its sphingolipid relative pSM. Cholesterol incorporation into both binary mixtures at a high Chol and pCer ratio abolishes any phospholipid/pCer binary domains. Bilayers with properties different from any of the pure or binary samples are observed instead. The data support no displacement of Chol by pCer or vice versa under these conditions, but rather a preferential interaction between the two hydrophobic lipids.

Ceramide increases free volume voids in DPPC membranes

We have measured by positron annihilation lifetime spectroscopy (PALS) that ceramide increases the size of the free volume holes in DPPC lipid membranes.

Lamellar gel (lβ) phases of ternary lipid composition containing ceramide and cholesterol

Lipid lateral segregation into specific domains in cellular membranes is associated with cell signaling and metabolic regulation. This phenomenon partially arises as a consequence of the very distinct bilayer-associated lipid physico-chemical properties that give rise to defined phase states at a given temperature. Until now lamellar gel (Lβ) phases have been described in detail in single or two-lipid systems. Using x-ray scattering, differential scanning calorimetry, confocal fluorescence microscopy, and atomic force microscopy, we have characterized phases of ternary lipid compositions in the presence of saturated phospholipids, cholesterol, and palmitoyl ceramide mixtures. These phases stabilized by direct cholesterol-ceramide interaction can exist either with palmitoyl sphingomyelin or with dipalmitoyl phosphatidylcholine and present intermediate properties between raft-associated phospholipid-cholesterol liquid-ordered and phospholipid-ceramide Lβ phases. The present data provide novel, to our knowledge, evidence of a chemically defined, multicomponent lipid system that could cooperate in building heterogeneous segregated platforms in cell membranes.

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.

Surface behavior of sphingomyelins with very long chain polyunsaturated fatty acids and effects of their conversion to ceramides

Molecular species of sphingomyelin (SM) with nonhydroxy (n) and 2-hydroxy (h) very long chain polyunsaturated fatty acids (n- and h-28:4, 30:5, and 32:5) abound in rat spermatogenic cells and spermatozoa. These SMs are located on the sperm head, where they are converted to the corresponding ceramides (Cer) after the completion of the acrosomal reaction, as induced in vitro. The aim of this study was to look into the surface properties of these unique SM species and how these properties change by the SM → Cer conversion. After isolation by HPLC, these SMs were organized in Langmuir films and studied alone, in combination with different proportions of Cer, and during their conversion to Cer by sphingomyelinase. Compression isotherms for all six SMs under study were compatible with a liquid-expanded (LE) state and showed large molecular areas. Only the longest SMs (n-32:5 and h-32:5 SM) underwent a phase transition upon cooling. Interestingly, the abundant h-28:4 Cer exhibited a highly compressible liquid-condensed (LC) phase compatible with a high conformational freedom of Cer molecules but with the characteristic low diffusional properties of the LC phase. In mixed films of h-28:4 SM/h-28:4 Cer, the components showed favorable mixing in the LE phase. The monolayer exhibited h-28:4 Cer-rich domains both in premixed films and when formed by the action of sphingomyelinase on pure h-28:4 SM films. Whereas the SMs from sperm behaved in a way similar to that of shorter acylated SMs, the corresponding Cers showed atypical rheological properties that may be relevant to the membrane structural rearrangements that take place on the sperm head after the completion of the acrosomal reaction.

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.

Changes in membrane biophysical properties induced by sphingomyelinase depend on the sphingolipid N-acyl chain

Ceramide (Cer) is involved in the regulation of several cellular processes by mechanisms that depend on Cer-induced changes on membrane biophysical properties. Accumulating evidence shows that Cers with different N-acyl chain composition differentially impact cell physiology, which may in part be due to specific alterations in membrane biophysical properties. We now address how the sphingolipid (SL) N-acyl chain affects membrane properties in cultured human embryonic kidney cells by overexpressing different Cer synthases (CerSs). Our results show an increase in the order of cellular membranes in CerS2-transfected cells caused by the enrichment in very long acyl chain SLs. Formation of Cer upon treatment of cells with bacterial sphingomyelinase promoted sequential changes in the properties of the membranes: after an initial increase in the order of the fluid plasma membrane, reorganization into domains with gel-like properties whose characteristics are dependent on the acyl chain structure of the Cer was observed. Moreover, the extent of alterations of membrane properties correlates with the amount of Cer formed. These data reinforce the significance of Cer-induced changes on membrane biophysical properties as a likely molecular mechanism by which different acyl chain Cers exert their specific biological actions.

Stiffness of lipid monolayers with phase coexistence

The surface dilational modulus--or compressibility modulus--has been previously studied for monolayers composed of pure materials, where a jump in this modulus was related with the onset of percolation as a result of the establishment of a connected structure at the molecular level. In this work, we focused on monolayers composed of two components of low lateral miscibility. Our aim was to investigate the compressibility of mixed monolayers at pressures and compositions in the two-phase region of the phase diagram, in order to analyze the effect of the mechanical properties of each phase on the stiffness of the composite. In nine different systems with distinct molecular dipoles and charges, the stiffness of each phase and the texture at the plane of the monolayer were studied. In this way, we were able to analyze the general compressibility of two-phase lipid monolayers, regardless of the properties of their constituent parts. The results are discussed in the light of the following two hypotheses: first, the stiffness of the composite could be dominated by the stiffness of each phase as a weighted sum according to the percentage of each phase area, regardless of the distribution of the phases in the plane of the monolayer. Alternatively, the stiffness of the composite could be dominated by the mechanical properties of the continuous phase. Our results were better explained by this latter proposal, as in all the analyzed mixtures it was found that the mechanical properties of the percolating phase were the determining factors. The value of the compression modulus was closer to the value of the connected phase than to that of the dispersed phase, indicating that the bidimensional composites displayed mechanical properties that were related to the properties of each phases in a rather complex manner.

Effects of sphingomyelin headgroup size on interactions with ceramide

Sphingomyelins (SMs) and ceramides are known to interact favorably in bilayer membranes. Because ceramide lacks a headgroup that could shield its hydrophobic body from unfavorable interactions with water, accommodation of ceramide under the larger phosphocholine headgroup of SM could contribute to their favorable interactions. To elucidate the role of SM headgroup for SM/ceramide interactions, we explored the effects of reducing the size of the phosphocholine headgroup (removing one, two, or three methyls on the choline moiety, or the choline moiety itself). Using differential scanning calorimetry and fluorescence spectroscopy, we found that the size of the SM headgroup had no marked effect on the thermal stability of ordered domains formed by SM analog/palmitoyl ceramide (PCer) interactions. In more complex bilayers composed of a fluid glycerophospholipid, SM analog, and PCer, the thermal stability and molecular order of the laterally segregated gel domains were roughly identical despite variation in SM headgroup size. We suggest that that the association between PCer and SM analogs was stabilized by ceramide's aversion for disordered phospholipids, by interfacial hydrogen bonding between PCer and the SM analogs, and by attractive van der Waals' forces between saturated chains of PCer and SM analogs.

Shear and compression rheology of Langmuir monolayers of natural ceramides: solid character and plasticity

The present work addresses the fundamental question of membrane elasticity of ceramide layers with a special focus on the plastic regime. The compression and shear viscoelasticity of egg-ceramide Langmuir monolayers were investigated using oscillatory surface rheology in the linear regime and beyond. High compression and shear moduli were measured at room temperature-a clear signature for a solid behavior. At deformations larger than one per mill, egg-ceramide monolayers display plastic features characterized by a decrease of the storage modulus followed by a viscous regime typical of fluid lipids. This behavior is accompanied by a marked decrease of the loss modulus with increasing stress above a yield point. The results permit to univocally classify ceramide monolayers as 2D solids able to undergo plastic deformations, at the difference of typical fluid lipid monolayers. These unusual features are likely to have consequences in the mechanical behavior of ceramide-rich emplacements in biological 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.

Insights into sphingolipid miscibility: separate observation of sphingomyelin and ceramide N-acyl chain melting

Ceramide produced from sphingomyelin in the plasma membrane is purported to affect signaling through changes in the membrane's physical properties. Thermal behavior of N-palmitoyl sphingomyelin (PSM) and N-palmitoyl ceramide (PCer) mixtures in excess water has been monitored by ²H NMR spectroscopy and compared to differential scanning calorimetry (DSC) data. The alternate use of either perdeuterated or proton-based N-acyl chain PSM and PCer in our ²H NMR studies has allowed the separate observation of gel-fluid transitions in each lipid in the presence of the other one, and this in turn has provided direct information on the lipids' miscibility over a wide temperature range. The results provide further evidence of the stabilization of the PSM gel state by PCer. Moreover, overlapping NMR and DSC data reveal that the DSC-signals parallel the melting of the major component (PSM) except at intermediate (20 and 30 mol %) fractions of PCer. In such cases, the DSC endotherm reports on the presumably highly cooperative melting of PCer. Up to at least 50 mol % PCer, PSM and PCer mix ideally in the liquid crystalline phase; in the gel phase, PCer becomes incorporated into PSM:PCer membranes with no evidence of pure solid PCer.

Effects of propofol and other GABAergic phenols on membrane molecular organization

GABA(A) receptor is the main inhibitory receptor of the central nervous system. The phenols propofol and thymol have been shown to act on this receptor. GABA(A) is an intrinsic protein, the activity of which may be affected by physical changes in the membrane. Taking into account the lipophilicity of phenols, their interaction with the membrane and a consequent non-specific receptor modulation cannot be discarded. By using Langmuir films, we analyze the comparative effects on the molecular properties of the membrane exerted by propofol, thymol and other related compounds, the activities of which on the GABA(A) are under investigation in our laboratory. All the compounds were able to expand phospholipid films, by their incorporation into the monolayer being favored by less-packed structures. Nonetheless, they were able to be incorporated at lateral pressures above the equilibrium pressure estimated for a natural membrane. Epifluorescence images revealed their presence between phospholipid molecules, probably at the head-group region. Hence, all results indicated that the phenols studied were clearly able to interact with membranes, suggesting that their anesthetic activity could be the combined result of their interaction with specific receptor proteins and with their surrounding lipid molecules modulating the supramolecular organization of the receptor environment.

Solid character of membrane ceramides: a surface rheology study of their mixtures with sphingomyelin

The compression and shear viscoelasticities of egg-ceramide and its mixtures with sphingomyelin were investigated using oscillatory surface rheology performed on Langmuir monolayers. We found high values for the compression and shear moduli for ceramide, compatible with a solid-state membrane, and extremely high surface viscosities when compared to typical fluid lipids. A fluidlike rheological behavior was found for sphingomyelin. Lateral mobilities, measured from particle tracking experiments, were correlated with the monolayer viscosities through the usual hydrodynamic relationships. In conclusion, ceramide increases the solid character of sphingomyelin-based membranes and decreases their fluidity, thus drastically decreasing the lateral mobilities of embedded objects. This mechanical behavior may involve important physiological consequences in biological membranes containing ceramides.