Sphingomyelinase activity causes transbilayer lipid translocation in model and cell membranes

(1-Deoxy)ceramides in bilayers containing sphingomyelin and cholesterol

The discovery of a novel sphingolipid subclass, the (1-deoxy)sphingolipids, which lack the 1-hydroxy group, attracted considerable attention in the last decade, mainly due to their involvement in disease. They differed in their physico-chemical properties from the canonical (or 1-hydroxy) sphingolipids and they were more toxic when accumulated in cells, inducing neurodegeneration and other dysfunctions. (1-Deoxy)ceramides, (1-deoxy)dihydroceramides, and (1- deoxymethyl)dihydroceramides, the latter two containing a saturated sphingoid chain, have been studied in this work using differential scanning calorimetry, confocal fluorescence and atomic force microscopy, to evaluate their behavior in bilayers composed of mixtures of three or four lipids. When compared to canonical ceramides (Cer), a C16:0 (1-deoxy)Cer shows a lower miscibility in mixtures of the kind C16:0 sphingomyelin/cholesterol/XCer, where XCer is any (1-deoxy)ceramide, giving rise to the coexistence of a liquid-ordered phase and a gel phase. The latter resembles, in terms of thermotropic behavior and nanomechanical resistance, the gel phase of the C16:0 sphingomyelin/cholesterol/C16:0 Cer mixture [Busto et al., Biophys. J. 2014, 106, 621-630]. Differences are seen between the various C16:0 XCer under study in terms of nanomechanical resistance, bilayer thickness and bilayer topography. When examined in a more fluid environment (bilayers based on C24:1 SM), segregated gel phases are still present. Probably related to such lateral separation, XCer preserve the capacity for membrane permeation, but their effects are significantly lower than those of canonical ceramides. Moreover, C24:1 XCer show significantly lower membrane permeation capacity than their C16:0 counterparts. The above data may be relevant in the pathogenesis of certain sphingolipid-related diseases, including certain neuropathies, diabetes, and glycogen storage diseases.

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.

Phospholipid Scramblase 1 Localizes Proximal to Sphingomyelin Synthase Isoforms but Is Not Involved in Sphingomyelin Synthesis

Ceramide (Cer) is synthesized de novo in the bilayer of the endoplasmic reticulum and transported to the cytosolic leaflet of the trans-Golgi apparatus for sphingomyelin (SM) synthesis. As the active site of SM synthase (SMS) is located on the luminal side of the Golgi membrane, Cer translocates to the lumen via transbilayer movement for SM synthesis. However, the mechanism of transbilayer movement is not fully understood. As the Cer-related translocases seem to localize near the SMS, the protein was identified using proximity-dependent biotin identification proteomics. Phospholipid scramblase 1 (PLSCR1), which is thought to act as a scramblase for phosphatidylserine and phosphatidylethanolamine, was identified as a protein proximal to the SMS isoforms SMS1 and SMS2. Although five isoforms of PLSCR have been reported in humans, only PLSCR1, PLSCR3, and PLSCR4 are expressed in HEK293T cells. Confocal microscopic analysis showed that PLSCR1 and PLSCR4 partially co-localized with p230, a trans-Golgi network marker, where SMS isoforms are localized. We established CRISPR/Cas9-mediated PLSCR1, PLSCR3, and PLSCR4 single-knockout cells and PLSCR1, 3, 4 triple knockout HEK293T cells. Liquid chromatography-tandem mass spectrometry revealed that the levels of species with distinct acyl chains in Cer and SM were not significantly different in single knockout cells or in the triple knockout cells compared to the wild-type cells. Our findings suggest that PLSCR1 is localized in the vicinity of SMS isoforms, however is not involved in the transbilayer movement of Cer for SM synthesis.

The pathophysiological role of dihydroceramide desaturase in the nervous system

Dihydroceramide desaturase 1 (DEGS1) converts dihydroceramide (dhCer) to ceramide (Cer) by inserting a C4-C5 trans (∆4E) double bond into the sphingoid backbone. Low DEGS activity causes accumulation of dhCer and other dihydrosphingolipid species. Although dhCer and Cer are structurally very similar, their imbalances can have major consequences both in vitro and in vivo. Mutations in the human DEGS1 gene are known to cause severe neurological defects, such as hypomyelinating leukodystrophy. Likewise, inhibition of DEGS1 activity in fly and zebrafish models causes dhCer accumulation and subsequent neuronal dysfunction, suggesting that DEGS1 activity plays a conserved and critical role in the nervous system. Dihydrosphingolipids and their desaturated counterparts are known to control various essential processes, including autophagy, exosome biogenesis, ER stress, cell proliferation, and cell death. Furthermore, model membranes with either dihydrosphingolipids or sphingolipids exhibit different biophysical properties, including membrane permeability and packing, thermal stability, and lipid diffusion. However, the links between molecular properties, in vivo functional data, and clinical manifestations that underlie impaired DEGS1 function remain largely unresolved. In this review, we summarize the known biological and pathophysiological roles of dhCer and its derivative dihydrosphingolipid species in the nervous system, and we highlight several possible disease mechanisms that warrant further investigation.

Mapping trasmembrane distribution of sphingomyelin

Our knowledge on the asymmetric distribution of sphingomyelin (SM) in the plasma membrane is largely based on the biochemical analysis of erythrocytes using sphingomyelinase (SMase). However, recent studies showed that the product of SMase, ceramide, disturbs transmembrane lipid distribution. This led to the development of the complimentary histochemical method, which combines electron microscopy and SM-binding proteins. This review discusses the advantages and caveats of published methods of measuring transbilayer distribution of SM. Recent finding of the proteins involved in the transbilayer movement of SM will also be summarized.

Lipid Self-Assemblies under the Atomic Force Microscope

Lipid model membranes are important tools in the study of biophysical processes such as lipid self-assembly and lipid–lipid interactions in cell membranes. The use of model systems to adequate and modulate complexity helps in the understanding of many events that occur in cellular membranes, that exhibit a wide variety of components, including lipids of different subfamilies (e.g., phospholipids, sphingolipids, sterols…), in addition to proteins and sugars. The capacity of lipids to segregate by themselves into different phases at the nanoscale (nanodomains) is an intriguing feature that is yet to be fully characterized in vivo due to the proposed transient nature of these domains in living systems. Model lipid membranes, instead, have the advantage of (usually) greater phase stability, together with the possibility of fully controlling the system lipid composition. Atomic force microscopy (AFM) is a powerful tool to detect the presence of meso- and nanodomains in a lipid membrane. It also allows the direct quantification of nanomechanical resistance in each phase present. In this review, we explore the main kinds of lipid assemblies used as model membranes and describe AFM experiments on model membranes. In addition, we discuss how these assemblies have extended our knowledge of membrane biophysics over the last two decades, particularly in issues related to the variability of different model membranes and the impact of supports/cytoskeleton on lipid behavior, such as segregated domain size or bilayer leaflet uncoupling.

Acid Sphingomyelinase, a Lysosomal and Secretory Phospholipase C, Is Key for Cellular Phospholipid Catabolism

Here, we present the main features of human acid sphingomyelinase (ASM), its biosynthesis, processing and intracellular trafficking, its structure, its broad substrate specificity, and the proposed mode of action at the surface of the phospholipid substrate carrying intraendolysosomal luminal vesicles. In addition, we discuss the complex regulation of its phospholipid cleaving activity by membrane lipids and lipid-binding proteins. The majority of the literature implies that ASM hydrolyses solely sphingomyelin to generate ceramide and ignores its ability to degrade further substrates. Indeed, more than twenty different phospholipids are cleaved by ASM in vitro, including some minor but functionally important phospholipids such as the growth factor ceramide-1-phosphate and the unique lysosomal lysolipid bis(monoacylglycero)phosphate. The inherited ASM deficiency, Niemann-Pick disease type A and B, impairs mainly, but not only, cellular sphingomyelin catabolism, causing a progressive sphingomyelin accumulation, which furthermore triggers a secondary accumulation of lipids (cholesterol, glucosylceramide, GM2) by inhibiting their turnover in late endosomes and lysosomes. However, ASM appears to be involved in a variety of major cellular functions with a regulatory significance for an increasing number of metabolic disorders. The biochemical characteristics of ASM, their potential effect on cellular lipid turnover, as well as a potential impact on physiological processes will be discussed.

C24:0 and C24:1 sphingolipids in cholesterol-containing, five- and six-component lipid membranes

The biophysical properties of sphingolipids containing lignoceric (C24:0) or nervonic (C24:1) fatty acyl residues have been studied in multicomponent lipid bilayers containing cholesterol (Chol), by means of confocal microscopy, differential scanning calorimetry and atomic force microscopy. Lipid membranes composed of dioleoyl phosphatidylcholine and cholesterol were prepared, with the addition of different combinations of ceramides (C24:0 and/or C24:1) and sphingomyelins (C24:0 and/or C24:1). Results point to C24:0 sphingolipids, namely lignoceroyl sphingomyelin (lSM) and lignoceroyl ceramide (lCer), having higher membrane rigidifying properties than their C24:1 homologues (nervonoyl SM, nSM, or nervonoyl Cer, nCer), although with a similar strong capacity to induce segregated gel phases. In the case of the lSM-lCer multicomponent system, the segregated phases have a peculiar fibrillar or fern-like morphology. Moreover, the combination of C24:0 and C24:1 sphingolipids generates interesting events, such as a generalized bilayer dynamism/instability of supported planar bilayers. In some cases, these sphingolipids give rise to exothermic curves in thermograms. These peculiar features were not present in previous studies of C24:1 combined with C16:0 sphingolipids. Conclusions of our study point to nSM as a key factor governing the relative distribution of ceramides when both lCer and nCer are present. The data indicate that lCer could be easier to accommodate in multicomponent bilayers than its C16:0 counterpart. These results are relevant for events of membrane platform formation, in the context of sphingolipid-based signaling cascades.

Lipid ratios as a marker for red blood cell storage quality and as a possible explanation for donor gender differences in storage quality

BACKGROUND AND OBJECTIVES

Red blood cells that are stored for transfusions as red cell concentrates (RCCs) undergo changes during the storage period, culminating in the lysis of the cells. The goal of this work is to find markers that are linked to high haemolysis, in order to explain the inter-donor variability that is known to occur in storage quality, and also the known differences between RCCs from male and female donors.

MATERIALS AND METHODS

The relative amounts of lipids at the end of the storage period were compared for one group of low haemolysis samples (24 units, all ≤0·15% haemolysis), and one group of high haemolysis samples (26 units, all ≥0·5% haemolysis). Representative lipids were analysed from different lipid classes, including cholesterol, phosphatidylcholine, phosphatidylethanolamine, sphingomyelin and ceramide. Whole membrane preparations were analysed with one mass spectrometry technique, and lipid extracts were analysed with a second mass spectrometry technique.

RESULTS

The ratio of palmitoyl-oleoyl phosphatidylcholine (POPC) to sphingomyelin was different for the high and low haemolysis groups (P = 0·0001) and for the RCCs from male and female donors (P = 0·0009). The ratio of cholesterol to phospholipids showed only minimal links to haemolysis. Higher relative amounts of sphingomyelin were associated with lower haemolysis, and higher relative amounts of ceramides were associated with increased haemolysis.

CONCLUSION

The level of sphingomyelinase activity and the resulting ratio of sphingomyelin to POPC is proposed as a possible marker for RCC storage quality.

Changes in Membrane Ceramide Pools in Rat Soleus Muscle in Response to Short-Term Disuse

Lipid raft disruption is an early event during skeletal muscle unloading. Ceramide (Cer) serves as a signaling lipid that can contribute to lipid raft disturbance and muscle atrophy. Using biochemical and fluorescent approaches, the distribution of Cer and related molecules in the rat soleus muscle subjected to 12 h of hindlimb suspension (HS) was studied. HS led to upregulation of TNFα receptor 1 (TNFR1), Cer-producing enzymes, and acid and neutral sphingomyelinase (SMase) in detergent-resistant membranes (lipid rafts), which was accompanied by an increase in Cer and a decrease in sphingomyelin in this membrane fraction. Fluorescent labeling indicated increased Cer in the sarcoplasm as well as the junctional (synaptic) and extrajunctional compartments of the suspended muscles. Also, a loss of membrane asymmetry (a hallmark of membrane disturbance) was induced by HS. Pretreatment with clomipramine, a functional inhibitor of acid SMase, counteracted HS-mediated changes in the Cer/sphingomyelin ratio and acid SMase abundance as well as suppressed Cer accumulation in the intracellular membranes of junctional and extrajunctional regions. However, the elevation of plasma membrane Cer and disturbance of the membrane asymmetry were suppressed only in the junctional compartment. We suggest that acute HS leads to TNFR1 and SMase upregulation in the lipid raft fraction and deposition of Cer throughout the sarcolemma and intracellularly. Clomipramine-mediated downregulation of acid SMase can suppress Cer accumulation in all compartments, excluding the extrajunctional plasma membrane.

Mixing brain cerebrosides with brain ceramides, cholesterol and phospholipids

The properties of bilayers composed of pure brain cerebroside (bCrb) or of binary mixtures of bCrb with brain ceramide, cholesterol, egg phosphatidylcholine or brain sphingomyelin have been studied using a combination of physical techniques. Pure bCrb exhibits a rather narrow gel-fluid transition centred at ≈65 °C, with a half-width at half-height T1/2 ≈ 3 °C. bCrb mixes well with both fluid and gel phospholipids and ceramide, and it rigidifies bilayers of egg phosphatidylcholine or brain sphingomyelin when the latter are in the fluid state. Cholesterol markedly widens the bCrb gel-fluid transition, while decreasing the associated transition enthalpy, in the manner of cholesterol mixtures with saturated phosphatidylcholines, or sphingomyelins. Laurdan and DPH fluorescence indicate the formation of fluid ordered phases in the bCrb:cholesterol mixtures. Macroscopic phase separation of more and less fluid domains is observed in giant unilamellar vesicles consisting of bCrb:egg phosphatidylcholine or bCrb:sphingomyelin. Crb capacity to induce bilayer permeabilization or transbilayer (flip-flop) lipid motion is much lower than those of ceramides. The mixtures explored here contained mostly bCrb concentrations >50 mol%, mimicking the situation of cell membranes in Gaucher's disease, or of the Crb-enriched microdomains proposed to exist in healthy cell plasma membranes.

Characterization of Lipid-Protein Interactions and Lipid-Mediated Modulation of Membrane Protein Function through Molecular Simulation

The cellular membrane constitutes one of the most fundamental compartments of a living cell, where key processes such as selective transport of material and exchange of information between the cell and its environment are mediated by proteins that are closely associated with the membrane. The heterogeneity of lipid composition of biological membranes and the effect of lipid molecules on the structure, dynamics, and function of membrane proteins are now widely recognized. Characterization of these functionally important lipid-protein interactions with experimental techniques is however still prohibitively challenging. Molecular dynamics (MD) simulations offer a powerful complementary approach with sufficient temporal and spatial resolutions to gain atomic-level structural information and energetics on lipid-protein interactions. In this review, we aim to provide a broad survey of MD simulations focusing on exploring lipid-protein interactions and characterizing lipid-modulated protein structure and dynamics that have been successful in providing novel insight into the mechanism of membrane protein function.

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.

Functional link between plasma membrane spatiotemporal dynamics, cancer biology, and dietary membrane-altering agents

The cell plasma membrane serves as a nexus integrating extra- and intracellular components, which together enable many of the fundamental cellular signaling processes that sustain life. In order to perform this key function, plasma membrane components assemble into well-defined domains exhibiting distinct biochemical and biophysical properties that modulate various signaling events. Dysregulation of these highly dynamic membrane domains can promote oncogenic signaling. Recently, it has been demonstrated that select membrane-targeted dietary bioactives (MTDBs) have the ability to remodel plasma membrane domains and subsequently reduce cancer risk. In this review, we focus on the importance of plasma membrane domain structural and signaling functionalities as well as how loss of membrane homeostasis can drive aberrant signaling. Additionally, we discuss the intricacies associated with the investigation of these membrane domain features and their associations with cancer biology. Lastly, we describe the current literature focusing on MTDBs, including mechanisms of chemoprevention and therapeutics in order to establish a functional link between these membrane-altering biomolecules, tuning of plasma membrane hierarchal organization, and their implications in cancer prevention.

Complex formation of sphingomyelin synthase 1 with glucosylceramide synthase increases sphingomyelin and decreases glucosylceramide levels

Sphingolipids, including sphingomyelin (SM) and glucosylceramide (GlcCer), are generated by the addition of a polar head group to ceramide (Cer). Sphingomyelin synthase 1 (SMS1) and glucosylceramide synthase (GCS) are key enzymes that catalyze the conversion of Cer to SM and GlcCer, respectively. GlcCer synthesis has been postulated to occur mainly in cis-Golgi, and SM synthesis is thought to occur in medial/trans-Golgi; however, SMS1 and GCS are known to partially co-localize in cisternae, especially in medial/trans-Golgi. Here, we report that SMS1 and GCS can form a heteromeric complex, in which the N terminus of SMS1 and the C terminus of GCS are in close proximity. Deletion of the N-terminal sterile α-motif of SMS1 reduced the stability of the SMS1-GCS complex, resulting in a significant reduction in SM synthesis in vivo In contrast, chemical-induced heterodimerization augmented SMS1 activity, depending on an increase in the amount and stability of the complex. Fusion of the SMS1 N terminus to the GCS C terminus via linkers of different lengths increased SM synthesis and decreased GlcCer synthesis in vivo These results suggest that formation of the SMS1-GCS heteromeric complex increases SM synthesis and decreases GlcCer synthesis. Importantly, this regulation of relative Cer levels by the SMS1-GCS complex was confirmed by CRISPR/Cas9-mediated knockout of SMS1 or GCS combined with pharmacological inhibition of Cer transport protein in HEK293T cells. Our findings suggest that complex formation between SMS1 and GCS is part of a critical mechanism controlling the metabolic fate of Cer in the Golgi.

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.

Pb(II) Induces Scramblase Activation and Ceramide-Domain Generation in Red Blood Cells

The mechanisms of Pb(II) toxicity have been studied in human red blood cells using confocal microscopy, immunolabeling, fluorescence-activated cell sorting and atomic force microscopy. The process follows a sequence of events, starting with calcium entry, followed by potassium release, morphological change, generation of ceramide, lipid flip-flop and finally cell lysis. Clotrimazole blocks potassium channels and the whole process is inhibited. Immunolabeling reveals the generation of ceramide-enriched domains linked to a cell morphological change, while the use of a neutral sphingomyelinase inhibitor greatly delays the process after the morphological change, and lipid flip-flop is significantly reduced. These facts point to three major checkpoints in the process: first the upstream exchange of calcium and potassium, then ceramide domain formation, and finally the downstream scramblase activation necessary for cell lysis. In addition, partial non-cytotoxic cholesterol depletion of red blood cells accelerates the process as the morphological change occurs faster. Cholesterol could have a role in modulating the properties of the ceramide-enriched domains. This work is relevant in the context of cell death, heavy metal toxicity and sphingolipid signaling.

Does Ceramide Form Channels? The Ceramide-Induced Membrane Permeabilization Mechanism

Ceramide is a sphingolipid involved in several cellular processes, including apoptosis. It has been proposed that ceramide forms large and stable channels in the mitochondrial outer membrane that induce cell death through direct release of cytochrome c. However, this mechanism is still debated because the membrane permeabilizing activity of ceramide remains poorly understood. To determine whether the mechanism of ceramide-induced membrane leakage is consistent with the hypothesis of an apoptotic ceramide channel, we have used here assays of calcein release from liposomes. When assaying liposomes containing sphingomyelin and cholesterol, we observed an overall gradual phenomenon of contents release, together with some all-or-none leakage (at low ceramide concentrations or short times). The presence of channels in the bilayer should cause only an all-or-none leakage. When liposomes poor in sphingomyelin/cholesterol or mimicking the lipid composition of the mitochondrial outer membrane were tested, we did not detect any leakage. In consequence, the hypothesis of formation of large ceramide channels in the membrane is not consistent with our results. Instead we propose that the presence of ceramide in one of the membrane monolayers causes a surface area mismatch between both monolayers, which leads to vesicle collapse. The gradual phenomenon of calcein release would be due to a competition between two ceramide effects; namely, lateral segregation that facilitates permeabilization, and at longer times, trans-bilayer flip-flop that opposes asymmetric lateral segregation and causes a mismatch.

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.

Complex Effects of 24:1 Sphingolipids in Membranes Containing Dioleoylphosphatidylcholine and Cholesterol

The effects of C24:1 sphingolipids have been tested in phospholipid bilayers containing cholesterol. Confocal microscopy, differential scanning calorimetry, and atomic force microscopy imaging and force curves have been used. More precisely, the effects of C24:1 ceramide (nervonoyl ceramide, nCer) were evaluated and compared to those of C16:0 ceramide (palmitoyl ceramide, pCer) in bilayers composed basically of dioleoylphosphatidylcholine, sphingomyelin (either C24:1, nSM or C16:0, pSM) and cholesterol. Combination of equimolecular amounts of C24:1 and C16:0 sphingolipids were also studied under the same conditions. Results show that both pCer and nCer are capable of forming segregated gel domains. Force spectroscopy data point to nCer having a lower stiffening effect than pCer, while the presence of nSM reduces the stiffness. DSC reveals T_m reduction by nSM in every case. Furthermore, pSM seems to better accommodate both ceramides in a single phase of intermediate properties, while nSM partial accommodation of ceramides generates different gel phases with higher stiffnesses caused by interceramide cooperation. If both pSM and nSM are present, a clear preference of both ceramides toward pSM is observed. These findings show the sharp increase in complexity when membranes exhibit different sphingolipids of varying N-acyl chains, which should be a common issue in an actual cell membrane environment.

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.

C8-glycosphingolipids preferentially insert into tumor cell membranes and promote chemotherapeutic drug uptake

Insufficient drug delivery into tumor cells limits the therapeutic efficacy of chemotherapy. Co-delivery of liposome-encapsulated drug and synthetic short-chain glycosphingolipids (SC-GSLs) significantly improved drug bioavailability by enhancing intracellular drug uptake. Investigating the mechanisms underlying this SC-GSL-mediated drug uptake enhancement is the aim of this study. Fluorescence microscopy was used to visualize the cell membrane lipid transfer intracellular fate of fluorescently labeled C6-NBD-GalCer incorporated in liposomes in tumor and non-tumor cells. Additionally click chemistry was applied to image and quantify native SC-GSLs in tumor and non-tumor cell membranes. SC-GSL-mediated flip-flop was investigated in model membranes to confirm membrane-incorporation of SC-GSL and its effect on membrane remodeling. SC-GSL enriched liposomes containing doxorubicin (Dox) were incubated at 4°C and 37°C and intracellular drug uptake was studied in comparison to standard liposomes and free Dox. SC-GSL transfer to the cell membrane was independent of liposomal uptake and the majority of the transferred lipid remained in the plasma membrane. The transfer of SC-GSL was tumor cell-specific and induced membrane rearrangement as evidenced by a transbilayer flip-flop of pyrene-SM. However, pore formation was measured, as leakage of hydrophilic fluorescent probes was not observed. Moreover, drug uptake appeared to be mediated by SC-GSLs. SC-GSLs enhanced the interaction of doxorubicin (Dox) with the outer leaflet of the plasma membrane of tumor cells at 4°C. Our results demonstrate that SC-GSLs preferentially insert into tumor cell plasma membranes enhancing cell intrinsic capacity to translocate amphiphilic drugs such as Dox across the membrane via a biophysical process.

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.

Lung injury and lung cancer caused by cigarette smoke-induced oxidative stress: Molecular mechanisms and therapeutic opportunities involving the ceramide-generating machinery and epidermal growth factor receptor

Chronic obstructive pulmonary disease (COPD) and lung cancer are frequently caused by tobacco smoking. However, these diseases present opposite phenotypes involving redox signaling at the cellular level. While COPD is characterized by excessive airway epithelial cell death and lung injury, lung cancer is caused by uncontrolled epithelial cell proliferation. Notably, epidemiological studies have demonstrated that lung cancer incidence is significantly higher in patients who have preexisting emphysema/lung injury. However, the molecular link and common cell signaling events underlying lung injury diseases and lung cancer are poorly understood. This review focuses on studies of molecular mechanism(s) underlying smoking-related lung injury (COPD) and lung cancer. Specifically, the role of the ceramide-generating machinery during cigarette smoke-induced oxidative stress leading to both apoptosis and proliferation of lung epithelial cells is emphasized. Over recent years, it has been established that ceramide is a sphingolipid playing a major role in lung epithelia structure/function leading to lung injury in chronic pulmonary diseases. However, new and unexpected findings draw attention to its potential role in lung development, cell proliferation, and tumorigenesis. To address this dichotomy in detail, evidence is presented regarding several protein targets, including Src, p38 mitogen-activated protein kinase, and neutral sphingomyelinase 2, the major sphingomyelinase that controls ceramide generation during oxidative stress. Furthermore, their roles are presented not only in apoptosis and lung injury but also in enhancing cell proliferation, lung cancer development, and resistance to epidermal growth factor receptor-targeted therapy for treating lung cancer.

Ceramides mediate cigarette smoke-induced metabolic disruption in mice

Cigarette smoke exposure increases lung ceramide biosynthesis and alters metabolic function. We hypothesized that ceramides are released from the lung during cigarette smoke exposure and result in elevated skeletal muscle ceramide levels, resulting in insulin resistance and altered mitochondrial respiration. Employing cell and animal models, we explored the effect of cigarette smoke on muscle cell insulin signaling and mitochondrial respiration. Muscle cells were treated with conditioned medium from cigarette smoke extract (CSE)-exposed lung cells, followed by analysis of ceramides and assessment of insulin signaling and mitochondrial function. Mice were exposed to daily cigarette smoke and a high-fat, high-sugar (HFHS) diet with myriocin injections to inhibit ceramide synthesis. Comparisons were conducted between these mice and control animals on standard diets in the absence of smoke exposure and myriocin injections. Muscle cells treated with CSE-exposed conditioned medium were completely unresponsive to insulin stimulation, and mitochondrial respiration was severely blunted. These effects were mitigated when lung cells were treated with the ceramide inhibitor myriocin prior to and during CSE exposure. In mice, daily cigarette smoke exposure and HFHS diet resulted in insulin resistance, which correlated with elevated ceramides. Although myriocin injection was protective against insulin resistance with either smoke or HFHS, it was insufficient to prevent insulin resistance with combined CS and HFHS. However, myriocin injection restored muscle mitochondrial respiration in all treatments. Ceramide inhibition prevents metabolic disruption in muscle cells with smoke exposure and may explain whole body insulin resistance and mitochondrial dysfunction in vivo.

Diaphragm dysfunction caused by sphingomyelinase requires the p47(phox) subunit of NADPH oxidase

Sphingomyelinase (SMase) activity is elevated in inflammatory states and may contribute to muscle weakness in these conditions. Exogenous SMase depresses muscle force in an oxidant-dependent manner. However, the pathway stimulated by SMase that leads to muscle weakness is unclear. In non-muscle cells, SMase activates the Nox2 isoform of NADPH oxidase, which requires the p47(phox) subunit for enzyme function. We targeted p47(phox) genetically and pharmacologically (apocynin) to examine the role of NADPH oxidase on SMase-induced increase in oxidants and diaphragm weakness. SMase increased cytosolic oxidants (arbitrary units: control 203±15, SMase 276±22; P<0.05) and depressed maximal force in wild type mice (N/cm(2): control 20±1, SMase 16±0.6; P<0.05). However, p47(phox) deficient mice were protected from increased oxidants (arbitrary units: control 217±27, SMase 224±17) and loss of force elicited by SMase (N/cm(2): control 20±1, SMase 19±1). Apocynin appeared to partially prevent the decrease in force caused by SMase (n=3 mice/group). Thus, our study suggests that NADPH oxidase plays an important role on oxidant-mediated diaphragm weakness triggered by SMase. These observations provide further evidence that NADPH oxidase modulates skeletal muscle function.

Biophysical properties of sphingosine, ceramides and other simple sphingolipids

Some of the simplest sphingolipids, namely sphingosine, ceramide and their phosphorylated compounds [sphingosine 1-phosphate (Sph-1-P) and ceramide 1-phosphate (Cer-1-P)], 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. The present paper is an overview of the mechanisms by which these sphingolipid signals, sphingosine and ceramide, in particular, are able to modify the properties of cell membranes.

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.

The basic structure and dynamics of cell membranes: an update of the Singer-Nicolson model

The fluid mosaic model of Singer and Nicolson (1972) is a commonly used representation of the cell membrane structure and dynamics. However a number of features, the result of four decades of research, must be incorporated to obtain a valid, contemporary version of the model. Among the novel aspects to be considered are: (i) the high density of proteins in the bilayer, that makes the bilayer a molecularly "crowded" space, with important physiological consequences; (ii) the proteins that bind the membranes on a temporary basis, thus establishing a continuum between the purely soluble proteins, never in contact with membranes, and those who cannot exist unless bilayer-bound; (iii) the progress in our knowledge of lipid phases, the putative presence of non-lamellar intermediates in membranes, and the role of membrane curvature and its relation to lipid geometry, (iv) the existence of lateral heterogeneity (domain formation) in cell membranes, including the transient microdomains known as rafts, and (v) the possibility of transient and localized transbilayer (flip-flop) lipid motion. This article is part of a Special Issue entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.

Membrane signaling induced by high doses of ionizing radiation in the endothelial compartment. Relevance in radiation toxicity

Tumor areas can now be very precisely delimited thanks to technical progress in imaging and ballistics. This has also led to the development of novel radiotherapy protocols, delivering higher doses of ionizing radiation directly to cancer cells. Despite this, radiation toxicity in healthy tissue remains a major issue, particularly with dose-escalation in these new protocols. Acute and late tissue damage following irradiation have both been linked to the endothelium irrigating normal tissues. The molecular mechanisms involved in the endothelial response to high doses of radiation are associated with signaling from the plasma membrane, mainly via the acid sphingomyelinase/ceramide pathway. This review describes this signaling pathway and discusses the relevance of targeting endothelial signaling to protect healthy tissues from the deleterious effects of high doses of radiation.

Biological functions of sphingomyelins

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

Molecular properties of various structurally defined sphingomyelins -- correlation of structure with function

Sphingomyelins are important phospholipids in plasma membranes of most cells. Because of their dominantly saturated nature, they affect the lateral structure of membranes, and contribute to the regulation of cholesterol distribution within membranes, and in cells. However, the abundance of molecular species present in cells also implies that sphingomyelins have other, more specific functions. Many of these functions are currently unknown, but are under extensive study. Mostly model membrane studies have shown that sphingomyelins (and other sphingolipids), in contrast to glycerophospholipids, have important hydrogen bonding properties which in several important ways confer specific functional properties to this abundant class of membrane phospholipids. The often very asymmetric nature of sphingomyelins, arising from mismatch in length between the long chain base and N-acyl chains, also impose specific properties (e.g., interdigitation) to sphingomyelins not seen with glycerophospholipids. In this review, the latest sphingomyelin literature will be scrutinized, and an effort will be made to correlate the molecular structure of sphingomyelin with functional properties. In particular, the effects of head group properties, interfacial hydrogen bonding, long chain base hydroxylation, N-acyl chain hydroxylation, and N-acyl chain methyl-branching will be discussed.

Accumulated bending energy elicits neutral sphingomyelinase activity in human red blood cells

We propose that accumulated membrane bending energy elicits a neutral sphingomyelinase (SMase) activity in human erythrocytes. Membrane bending was achieved by osmotic or chemical processes, and SMase activity was assessed by quantitative thin-layer chromatography, high-performance liquid chromatography, and electrospray ionization-mass spectrometry. The activity induced by hypotonic stress in erythrocyte membranes had the pH dependence, ion dependence, and inhibitor sensitivity of mammalian neutral SMases. The activity caused a decrease in SM contents, with a minimum at 6 min after onset of the hypotonic conditions, and then the SM contents were recovered. We also elicited SMase activity by adding lysophosphatidylcholine externally or by generating it with phospholipase A(2). The same effect was observed upon addition of chlorpromazine or sodium deoxycholate at concentrations below the critical micellar concentration, and even under hypertonic conditions. A unifying factor of the various agents that elicit this SMase activity is the accumulated membrane bending energy. Both hypo-and hypertonic conditions impose an increased curvature, whereas the addition of surfactants or phospholipase A(2) activation increases the outer monolayer area, thus leading to an increased bending energy. The fact that this latent SMase activity is tightly coupled to the membrane bending properties suggests that it may be related to the general phenomenon of stress-induced ceramide synthesis and apoptosis.

Adenylyl cyclase AC8 directly controls its micro-environment by recruiting the actin cytoskeleton in a cholesterol-rich milieu

The central and pervasive influence of cAMP on cellular functions underscores the value of stringent control of the organization of adenylyl cyclases (ACs) in the plasma membrane. Biochemical data suggest that ACs reside in membrane rafts and could compartmentalize intermediary scaffolding proteins and associated regulatory elements. However, little is known about the organization or regulation of the dynamic behaviour of ACs in a cellular context. The present study examines these issues, using confocal image analysis of various AC8 constructs, combined with fluorescence recovery after photobleaching and fluorescence correlation spectroscopy. These studies reveal that AC8, through its N-terminus, enhances the cortical actin signal at the plasma membrane; an interaction that was confirmed by GST pull-down and immunoprecipitation experiments. AC8 also associates dynamically with lipid rafts; the direct association of AC8 with sterols was confirmed in Förster resonance energy transfer experiments. Disruption of the actin cytoskeleton and lipid rafts indicates that AC8 tracks along the cytoskeleton in a cholesterol-enriched domain, and the cAMP that it produces contributes to sculpting the actin cytoskeleton. Thus, an adenylyl cyclase is shown not just to act as a scaffold, but also to actively orchestrate its own micro-environment, by associating with the cytoskeleton and controlling the association by producing cAMP, to yield a highly organized signalling hub.

EGF receptor exposed to oxidative stress acquires abnormal phosphorylation and aberrant activated conformation that impairs canonical dimerization

Crystallographic studies have offered understanding of how receptor tyrosine kinases from the ErbB family are regulated by their growth factor ligands. A conformational change of the EGFR (ErbB1) was shown to occur upon ligand binding, where a solely ligand-mediated mode of dimerization/activation was documented. However, this dogma of dimerization/activation was revolutionized by the discovery of constitutively active ligand-independent EGFR mutants. In addition, other ligand-independent activation mechanisms may occur. We have shown that oxidative stress (ox-stress), induced by hydrogen peroxide or cigarette smoke, activates EGFR differently than its ligand, EGF, thereby inducing aberrant phosphorylation and impaired trafficking and degradation of EGFR. Here we demonstrate that ox-stress activation of EGFR is ligand-independent, does not induce "classical" receptor dimerization and is not inhibited by the tyrosine kinase inhibitor AG1478. Thus, an unprecedented, apparently activated, state is found for EGFR under ox-stress. Furthermore, this activation mechanism is temperature-dependent, suggesting the simultaneous involvement of membrane structure. We propose that ceramide increase under ox-stress disrupts cholesterol-enriched rafts leading to EGFR re-localization into the rigid, ceramide-enriched rafts. This increase in ceramide also supports EGFR aberrant trafficking to a peri-nuclear region. Therefore, the EGFR unprecedented and activated conformation could be sustained by simultaneous alterations in membrane structure under ox-stress.

Dynamic transbilayer lipid asymmetry

Cells have thousands of different lipids. In the plasma membrane, and in membranes of the late secretory and endocytotic pathways, these lipids are not evenly distributed over the two leaflets of the lipid bilayer. The basis for this transmembrane lipid asymmetry lies in the fact that glycerolipids are primarily synthesized on the cytosolic and sphingolipids on the noncytosolic surface of cellular membranes, that cholesterol has a higher affinity for sphingolipids than for glycerolipids. In addition, P4-ATPases, "flippases," actively translocate the aminophospholipids phosphatidylserine and phosphatidylethanolamine to the cytosolic surface. ABC transporters translocate lipids in the opposite direction but they generally act as exporters rather than "floppases." The steady state asymmetry of the lipids can be disrupted within seconds by the activation of phospholipases and scramblases. The asymmetric lipid distribution has multiple implications for physiological events at the membrane surface. Moreover, the active translocation also contributes to the generation of curvature in the budding of transport vesicles.

An obligate role for membrane-associated neutral sphingomyelinase activity in orienting chemotactic migration of human neutrophils

For polymorphonuclear neutrophils (PMNs) to orient migration to chemotactic gradients, weak external asymmetries must be amplified into larger internal signaling gradients. Lipid mediators, associated with the plasma membrane and within the cell, participate in generating these gradients. This study examined the role in PMN chemotaxis of neutral sphingomyelinase (N-SMase), a plasma membrane-associated enzyme that converts sphingomyelin to ceramide. A noncompetitive N-SMase inhibitor, GW4869 (5 mM, 5 minutes), did not inhibit PMN motility (as percentage of motile cells, or mean cell velocity), but it abrogated any orientation of movement toward the source of the chemotaxin, formylmethionylleucylphenylanaline (FMLP) (net displacement along the gradient axis in micrometers, or as percentage of total migration distance). This defect could be completely reversed by treatment with lignoceric ceramide (5 μg/ml, 15 minutes). Immunolocalization studies demonstrated that N-SMase (1) distributes preferentially toward the leading edge of some elongated cells, (2) is associated with the plasma membrane, (3) is more than 99.5% localized to the cytofacial aspect of the plasma membrane, (4) is excluded from pseudopodial extensions, and (5) increases rapidly in response to FMLP. Morphologically, the inhibition of N-SMase limited cellular spreading and the extension of sheet-like pseudopods. Elongated PMNs demonstrated a polarized distribution of GTPases, with Rac 1/2 accumulated at, and RhoA excluded from, the front of the cell. This polarity was negated by N-SMase inhibition and restored by lignoceric ceramide. We conclude that N-SMase at the cytofacial plasma membrane is an essential element for the proper orientation of PMNs in FMLP gradients, at least in part by polarizing the distribution of Rac 1/2 and RhoA GTPases.

Cellular mechanisms underlying the formation of circulating microparticles

Microparticles (MPs) derived from platelets, monocytes, endothelial cells, red blood cells, and granulocytes may be detected in low concentrations in normal plasma and at increased levels in atherothrombotic cardiovascular diseases. The elucidation of the cellular mechanisms underlying the generation of circulating MPs is crucial for improving our understanding of their pathophysiological role in health and disease. The flopping of phosphatidylserine (PS) to the outer leaflet of the plasma membrane is the key event that will ultimately lead to the shedding of procoagulant MPs from activated or apoptotic cells. Research over the last few years has revealed important roles for calcium-, mitochondrial-, and caspase-dependent mechanisms leading to PS exposure. The study of Scott cells has unraveled different molecular mechanisms that may contribute to fine-tuning of PS exposure and MP release in response to a variety of specific stimuli. The pharmacological modulation of MP release may have a substantial therapeutic impact in the management of atherothrombotic vascular disorders. Because PS exposure is a key feature in pathological processes different from hemostasis and thrombosis, the most important obstacle in the field of MP-modulating drugs seems to be carefully targeting MP release to relevant cell types at an optimal level, so as to achieve a beneficial action and limit possible adverse effects.

Regulation of phosphatidic Acid metabolism by sphingolipids in the central nervous system

This paper explores the way ceramide, sphingosine, ceramide 1-phosphate, and sphingosine 1-phosphate modulate the generation of second lipid messengers from phosphatidic acid in two experimental models of the central nervous system: in vertebrate rod outer segments prepared from dark-adapted retinas as well as in rod outer segments prepared from light-adapted retinas and in rat cerebral cortex synaptosomes under physiological aging conditions. Particular attention is paid to lipid phosphate phosphatase, diacylglycerol lipase, and monoacylglycerol lipase. Based on the findings reported in this paper, it can be concluded that proteins related to phototransduction phenomena are involved in the effects derived from sphingosine 1-phosphate/sphingosine or ceramide 1-phosphate/ceramide and that age-related changes occur in the metabolism of phosphatidic acid from cerebral cortex synaptosomes in the presence of either sphingosine 1-phosphate/sphingosine or ceramide 1-phosphate/ceramide. The present paper demonstrates, in two different models of central nervous system, how sphingolipids influence phosphatidic acid metabolism under different physiological conditions such as light and aging.

Implication of sphingomyelin/ceramide molar ratio on the biological activity of sphingomyelinase

Sphingolipid signaling plays an important, yet not fully understood, role in diverse aspects of cellular life. Sphingomyelinase is a major enzyme in these signaling pathways, catalyzing hydrolysis of sphingomyelin to ceramide and phosphocholine. To address the related membrane dynamical structural changes and their feedback to enzyme activity, we have studied the effect of enzymatically generated ceramide in situ on the properties of a well-defined lipid model system. We found a gel-phase formation that was about four times faster than ceramide generation due to ceramide-sphingomyelin pairing. The gel-phase formation slowed down when the ceramide molar ratios exceeded those of sphingomyelin and stopped just at the solubility limit of ceramide, due to unfavorable pairwise interactions of ceramide with itself and with monounsaturated phosphatidylcholine. A remarkable correlation to in vitro experiments suggests a regulation of sphingomyelinase activity based on the sphingomyelin/ceramide molar ratio.