Sterol affinity for bilayer membranes is affected by their ceramide content and the ceramide chain length

It is known that ceramides can influence the lateral organization in biological membranes. In particular ceramides have been shown to alter the composition of cholesterol and sphingolipid enriched nanoscopic domains, by displacing cholesterol, and forming gel phase domains with sphingomyelin. Here we have investigated how the bilayer content of ceramides and their chain length influence sterol partitioning into the membranes. The effect of ceramides with saturated chains ranging from 4 to 24 carbons in length was investigated. In addition, unsaturated 18:1- and 24:1-ceramides were also examined. The sterol partitioning into bilayer membranes was studied by measuring the distribution of cholestatrienol, a fluorescent cholesterol analogue, between methyl-β-cyclodextrin and large unilamellar vesicle with defined lipid composition. Up to 15 mol% ceramide was added to bilayers composed of DOPC:PSM:cholesterol (3:1:1), and the effect on sterol partitioning was measured. Both at 23 and 37 °C addition of ceramide affected the sterol partitioning in a chain length dependent manner, so that the ceramides with intermediate chain lengths were the most effective in reducing sterol partitioning into the membranes. At 23 °C the 18:1-ceramide was not as effective at inhibiting sterol partitioning into the vesicles as its saturated equivalent, but at 37 °C the additional double bond had no effect. The longer 24:1-ceramide behaved as 24:0-ceramide at both temperatures. In conclusion, this work shows how the distribution of sterols within sphingomyelin-containing membranes is affected by the acyl chain composition in ceramides. The overall membrane partitioning measured in this study reflects the differential partitioning of sterol into ordered domains where ceramides compete with the sterol for association with sphingomyelin.     

Ablation of ceramide synthase 2 strongly affects biophysical properties of membranes

Little is known about the effects of altering sphingolipid (SL) acyl chain structure and composition on the biophysical properties of biological membranes. We explored the biophysical consequences of depleting very long acyl chain (VLC) SLs in membranes prepared from lipid fractions isolated from a ceramide synthase 2 (CerS2)-null mouse, which is unable to synthesize C22-C24 ceramides. We demonstrate that ablation of CerS2 has different effects on liver and brain, causing a significant alteration in the fluidity of the membrane and affecting the type and/or extent of the phases present in the membrane. These changes are a consequence of the depletion of VLC and unsaturated SLs, which occurs to a different extent in liver and brain. In addition, ablation of CerS2 causes changes in intrinsic membrane curvature, leading to strong morphological alterations that promote vesicle adhesion, membrane fusion, and tubule formation. Together, these results show that depletion of VLC-SLs strongly affects membrane biophysical properties, which may compromise cellular processes that critically depend on membrane structure, such as trafficking and sorting.     

The hydrophobic mismatch determines the miscibility of ceramides in lipid monolayers

The organization of lipids within membranes strongly depends on the interaction with other lipid and protein molecules. Sphingolipids comprise a structurally diverse family, the ceramides being some of the simplest members. Although small chemical modifications of ceramide structure, such as varying the N-acyl chain length, lead to a complex polymorphism of this lipid, only long acyl chain ceramides have usually been studied and their properties became a putative hallmark for all ceramides. In this work, we studied the mixing behavior of C10:0 Cer, which has the N-acyl chain shorter than that of the sphingosine acyl chain and displays an expanded to condensed phase transition at 25mNm(-1) at 24°C, with ceramides N-acylated with longer fatty acyl chains C12:0, C14:0 and C18:0. The N-acyl chain length determined the miscibility of ceramides in Langmuir monolayers, as it was ascertained by the dependence of the mean molecular area, perpendicular dipole moment, surface topography and film thickness with the mixture composition. We found that, as the hydrophobic mismatch in ceramides increased complete miscibility, partial or complete immiscibility can occur.     

The effects of <Emphasis Type="Italic">N</Emphasis>-acyl chain methylations on ceramide molecular properties in bilayer membranes

Long-chain saturated ceramides possess the ability to form gel domains in fluid bilayer membranes. Such domains may also contain sphingomyelin, but generally exclude cholesterol. We studied the effect of N-acyl chain methylations on the ability of ceramide to form ceramide- and sphingomyelin-containing gel domains that displace sterol. Fluorescence quenching of probes displaying different lateral partitioning in heterogeneous lipid bilayers showed that the methyl branches induced position-dependent changes in the lateral distribution of the ceramides. Distally monomethylated ceramides interacted with sphingomyelin and displaced sterol, whereas proximally monomethylated and polymethylated ceramides appeared to be located outside of sterol/sphingomyelin-enriched domains. The branched ceramides also markedly reduced the bilayer affinity for sterol as determined from the equilibrium partitioning of sterol between lipid vesicles and cyclodextrin. Altogether, alterations in intermolecular interactions induced by the methyl branches markedly affected the molecular properties of ceramide in artificial bilayers.     

Ceramide acyl chain length markedly influences miscibility with palmitoyl sphingomyelin in bilayer membranes

Ceramides are precursors of major sphingolipids and can be important cellular effectors. The biological effects of ceramides have been suggested to stem from their biophysical effects on membrane structure affecting the lateral and transbilayer organization of other membrane components. In this study we investigated the effect of acyl chain composition in ceramides (C4-C24:1) on their miscibility with N-palmitoyl-sphingomyelin (PSM) using differential scanning calorimetry. We found that short-chain (C4 and C8) ceramides induced phase separation and lowered the T _m and enthalpy of the PSM endotherm. We conclude that short-chain ceramides were more miscible in the fluid-phase than in the gel-phase PSM bilayers. Long-chain ceramides induced apparent heterogeneity in the bilayers. The main PSM endotherm decreased in cooperativity and enthalpy with increasing ceramide concentration. New ceramide-enriched components could be seen in the thermograms at all ceramide concentrations above X _Cer = 0.05. These broad components had higher T _m values than pure PSM. C24:1 ceramide exhibited complex behavior in the PSM bilayers. The miscibility of C24:1 ceramide with PSM at low (X _Cer = 0.05–0.10) concentrations was exceptionally good according to the cooperativity of the transition. At higher concentrations, multiple components were detected, which might have arisen from interdigitated gel-phases formed by this very asymmetric ceramide. The results of this study indicate that short-chain and long-chain ceramides have very different effects on the sphingomyelin bilayers. There also seems to be a correlation between their miscibility in binary systems and the effect of ceramides of different hydrophobic length on sphingomyelin-rich domains in multicomponent membranes.     

Effect of hydrophobic mismatch and interdigitation on sterol/sphingomyelin interaction in ternary bilayer membranes

Sphingomyelin (SM) is a major phospholipid in most cell membranes. SMs are composed of a long-chain base (often sphingosine, 18:1(Δ4t)), and N-linked acyl chains (often 16:0, 18:0 or 24:1(Δ15c)). Cholesterol interacts with SM in cell membranes, but the acyl chain preference of this interaction is not fully elucidated. In this study we have examined the effects of hydrophobic mismatch and interdigitation on cholesterol/sphingomyelin interaction in complex bilayer membranes. We measured the capacity of cholestatrienol (CTL) and cholesterol to form sterol-enriched ordered domains with saturated SM species having different chain lengths (14 to 24 carbons) in ternary bilayer membranes. We also determined the equilibrium bilayer partitioning coefficient of CTL with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membranes containing 20mol% of saturated SM analogs. Ours results show that while CTL and cholesterol formed sterol-enriched domains with both short and long-chain SM species, the sterols preferred interaction with 16:0-SM over any other saturated chain length SM analog. When CTL membrane partitioning was determined with fluid POPC bilayers containing 20mol% of a saturated chain length SM analog, the highest affinity was seen with 16:0-SM (both at 23 and 37°C). These results indicate that hydrophobic mismatch and/or interdigitation attenuate sterol/SM association and thus affect lateral distribution of sterols in the bilayer membrane.     

Acyl chain length affects ceramide action on sterol/sphingomyelin-rich domains

The effects of ceramides with varying saturated N-linked acyl chains (C2-C14) on cholesterol displacement from sphingomyelin-rich domains and on the stability of ordered domains were studied. The bilayers examined were made from 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), D-erythro-N-palmitoyl-sphingomyelin (PSM), D-erythro-N-acyl-sphingosine, and cholesterol (60:15:15:10 mol%, respectively). Cholestatrienol (CTL) or D-erythro-N-trans-parinoyl-sphingomyelin (tParSM) were used as reporter molecules (at 1 mol%) for the ordered domains, and 1-palmitoyl-2-stearoyl-(7-doxyl)-sn-glycero-3-phosphocholine (7SLPC) as a fluorescence quencher (30 mol%, replacing POPC) in the liquid-disordered phase. The results indicate that the ceramide had to have an N-linked acyl chain with at least 8 methylene units in order for it to displace cholesterol from the sphingomyelin-rich domains at the concentration used. The melting of the sphingomyelin-rich domain shifted to higher temperatures (compared to the ceramide-free control) with C2, C12 and longer chain ceramides, whereas C4-C10 ceramides led to domain melting at lower temperatures than control. This study shows that short-chain ceramides do not have the same effects on sterol- and sphingomyelin-rich domains as naturally occurring longer-chain ceramides have.     

Effect of fatty acyl domain of phospholipids on the membrane-channel formation of Staphylococcus aureus alpha-toxin in liposome membrane.

By use of carboxyfluorescein-loaded multilamellar liposomes prepared from synthetic phosphatidylcholine (PC) or sphingomyelin and cholesterol in a molar ratio of 1:1, we studied whether or not fatty acyl domain of the phospholipids affects the membrane-damaging action (or channel formation) of Staphylococcus aureus alpha-toxin on the phospholipid-cholesterol membranes. Our data indicated: (1) that toxin-induced carboxyfluorescein-leakage from the liposomes composed of saturated fatty acyl residue-carrying PC and cholesterol was decreased with increasing chain length of the acyl residues between 12 and 18 carbon atoms, although toxin-binding to the liposomes was not significantly affected by the length of fatty acyl residue; (2) that unsaturated fatty acyl residue in PC or sphingomyelin molecule conferred higher sensitivity to alpha-toxin on the phospholipid-cholesterol liposomes, compared with saturated fatty acyl residues; and (3) that hexamerization of alpha-toxin, estimated by SDS-polyacrylamide gel electrophoresis, occurred more efficiently on the liposomes composed of PC with shorter fatty acyl chain or unsaturated fatty acyl chain. Thus, hydrophobic domain of the phospholipids influences membrane-channel formation of alpha-toxin in the phospholipid-cholesterol membrane, perhaps by modulating packing of phospholipid, cholesterol and the toxin in membrane.     

Stage-associated expression of ceramide structures in glycosphingolipids from the human trematode parasite Schistosoma mansoni

Glycosphingolipids of Schistosoma mansoni adults, cercariae and eggs comprise ceramide monohexosides (CMH) with glucose or galactose and ceramide dihexosides (CDH) with the schistosome-specific structure GalNAc(beta1-4)Glc(1-1)ceramide. Ceramide analysis revealed C18- and C20-phytosphingosines in egg CMH, C18-sphinganine as well as C18-, C19- and C20-phytosphingosines in cercarial CMH, and C18- and C20-phytosphingosines as well as C18-sphingosine and C18-sphinganine in adult CMH. For all three life cycle stages, the predominant fatty acid was C16h:0. As a characteristic feature, a range of saturated, unsaturated and hydroxylated long-chain fatty acids with 24-28 carbon atoms were additionally found in minor cercarial CMH species. The corresponding ceramides represented major constituents in cercarial CDH, while adult and egg CDH were dominated by ceramides with short fatty acid chains. The resultant ceramide patterns could be correlated with the differential expression of carbohydrate antigens on schistosomal glycolipids at various stages. A possible impact of ceramide structure on the biosynthesis of the carbohydrate moieties is discussed.     

Interplay of unsaturated phospholipids and cholesterol in membranes: effect of the double-bond position

The structural and dynamical properties of lipid membranes rich in phospholipids and cholesterol are known to be strongly affected by the unsaturation of lipid acyl chains. We show that not only unsaturation but also the position of a double bond has a pronounced effect on membrane properties. We consider how cholesterol interacts with phosphatidylcholines comprising two 18-carbon long monounsaturated acyl chains, where the position of the double bond is varied systematically along the acyl chains. Atomistic molecular dynamics simulations indicate that when the double bond is not in contact with the cholesterol ring, and especially with the C18 group on its rough β-side, the membrane properties are closest to those of the saturated bilayer. However, any interaction between the double bond and the ring promotes membrane disorder and fluidity. Maximal disorder is found when the double bond is located in the middle of a lipid acyl chain, the case most commonly found in monounsaturated acyl chains of phospholipids. The results suggest a cholesterol-mediated lipid selection mechanism in eukaryotic cell membranes. With saturated lipids, cholesterol promotes the formation of highly ordered raft-like membrane domains, whereas domains rich in unsaturated lipids with a double bond in the middle remain highly fluid despite the presence of cholesterol.     

Cardiomyocyte specific deficiency of serine palmitoyltransferase subunit 2 reduces ceramide but leads to cardiac dysfunction

The role of serine palmitoyltransferase (SPT) and de novo ceramide biosynthesis in cardiac ceramide and sphingomyelin metabolism is unclear. To determine whether the de novo synthetic pathways, rather than ceramide uptake from circulating lipoproteins, is important for heart ceramide levels, we created cardiomyocyte-specific deficiency of Sptlc2, a subunit of SPT. Heart-specific Sptlc2-deficient (hSptlc2 KO) mice had a >35% reduction in ceramide, which was limited to C18:0 and very long chain ceramides. Sphingomyelinase expression, and levels of sphingomyelin and diacylglycerol were unchanged. But surprisingly phospholipids and acyl CoAs contained increased saturated long chain fatty acids. hSptlc2 KO mice had decreased fractional shortening and thinning of the cardiac wall. While the genes regulating glucose and fatty acid metabolism were not changed, expression of cardiac failure markers and the genes involved in the formation of extracellular matrices were up-regulated in hSptlc2 KO hearts. In addition, ER-stress markers were up-regulated leading to increased apoptosis. These results suggest that Sptlc2-mediated de novo ceramide synthesis is an essential source of C18:0 and very long chain, but not of shorter chain, ceramides in the heart. Changes in heart lipids other than ceramide levels lead to cardiac toxicity.     

Effect of ceramide acyl chain length on skin permeability and thermotropic phase behavior of model stratum corneum lipid membranes

Stratum corneum ceramides play an essential role in the barrier properties of skin. However, their structure-activity relationships are poorly understood. We investigated the effects of acyl chain length in the non-hydroxy acyl sphingosine type (NS) ceramides on the skin permeability and their thermotropic phase behavior. Neither the long- to medium-chain ceramides (8-24 C) nor free sphingosine produced any changes of the skin barrier function. In contrast, the short-chain ceramides decreased skin electrical impedance and increased skin permeability for two marker drugs, theophylline and indomethacin, with maxima in the 4-6C acyl ceramides. The thermotropic phase behavior of pure ceramides and model stratum corneum lipid membranes composed of ceramide/lignoceric acid/cholesterol/cholesterol sulfate was studied by differential scanning calorimetry and infrared spectroscopy. Differences in thermotropic phase behavior of these lipids were found: those ceramides that had the greatest impact on the skin barrier properties displayed the lowest phase transitions and formed the least dense model stratum corneum lipid membranes at 32°C. In conclusion, the long hydrophobic chains in the NS-type ceramides are essential for maintaining the skin barrier function. However, this ability is not shared by their short-chain counterparts despite their having the same polar head structure and hydrogen bonding ability.     

Very long chain sphingolipids: Tissue expression, function and synthesis

Forecort membranes of all mammalian cells contain, in addition to phosphoglycerolipids and cholesterol, substantial amounts of sphingolipids. In most cells the acyl moieties of these sphingolipids are of long chain type (C16-24) and often saturated. However, epidermal keratinocytes and male germ cells largely express sphingolipids with very long chain-acyl moieties (C26-C36) during their differentiation and maturation. This expression seems to depend on CerS3, one of six ceramide synthases. However, the complex biosynthetic sequence for both epidermal and testicular sphingolipids has not been elucidated completely. Whereas it is established that omega-hydroxylated very long chain-sphingolipids are essential for proper skin barrier function, the role of polyunsaturated very long chain-sphingolipids (ceramides, sphingomyelins and glycosphingolipids) in differentiating spermatocytes/spermatids is just beginning to be revealed.     

Long chain ceramides and very long chain ceramides have opposite effects on human breast and colon cancer cell growth

Ceramides are known to be key players in intracellular signaling and are involved in apoptosis, cell senescence, proliferation, cell growth and differentiation. They are synthesized by ceramide synthases (CerS). So far, six different mammalian CerS (CerS1-6) have been described. Recently, we demonstrated that human breast cancer tissue displays increased activity of CerS2, 4, and 6, together with enhanced generation of their products, ceramides C(16:0), C(24:0), and C(24:1). Moreover, these increases were significantly associated with tumor dignity. To clarify the impact of this observation, we manipulated cellular ceramide levels by overexpressing ceramide synthases 2, 4 or 6 in MCF-7 (breast cancer) and HCT-116 (colon cancer) cells, respectively. Overexpression of ceramide synthases 4 and 6 elevated generation of short chain ceramides C(16:0), C(18:0) and C(20:0), while overexpression of ceramide synthase 2 had no effect on ceramide production in vivo, presumably due to limited substrate availability, because external addition of very long chain acyl-CoAs resulted in a significant upregulation of very long chain ceramides. We also demonstrated that upregulation of CerS4 and 6 led to the inhibition of cell proliferation and induction of apoptosis, whereas upregulation of CerS2 increased cell proliferation. On the basis of our data, we propose that a disequilibrium between ceramides of various chain length is crucial for cancer progression, while normal cells require an equilibrium between very long and long chain ceramides for normal physiology.     

Structure of the ceramide moiety of GM1 ganglioside determines its occurrence in different detergent-resistant membrane domains in HL-60 cells

To investigate the effect of the ceramide moiety of GM1 ganglioside on its association with detergent resistant membrane domains (DRMs) in human leukemia HL-60 cells, [(3)H] labeled GM1 molecular species (GM1s) with ceramides consisting of C18 sphingosine acetylated or acylated with C(8), C(12), C(14), C(16), C(18), C(22), C(24), C(18:1), C(22:1), or C(24:1) fatty acids (FAs), or C20 sphingosine acetylated or acylated with C(8) or C(18) FA were prepared and added to culture media. GM1s uptake by HL-60 cells was affected by the structure of their ceramides. Resistance to removal with trypsin and the stoichiometry of [(125)I] cholera toxin (CT) binding indicated that the added GM1s were incorporated into the membranes of the cells used for the isolation of DRMs in a manner resembling endogenous gangliosides. The ceramide moieties of the GM1s determined their occurrence in DRMs and the dependence of their recovery in this membrane fraction on the amount of Triton X-100 (TX) used for extraction as well as on cholesterol depletion. The GM1s with sphingosine acylated with C(14), C(16), C(18) C(22), or C(24) FAs were similarly abundant in DRMs. GM1s acylated with C(18:1), C(22:1), or C(24:1) were less abundant than those acylated with saturated FA of the same length. GM1s acetylated or acylated with C(8) FA were detected in DRMs in the lowest proportion. Depletion of 73% of cell cholesterol with methyl-beta-cyclodextrin significantly affected the recovery in DRMs of GM1s acetylated or acylated with C(8) or unsaturated FAs but not of GM1 acylated with C(18), C(22), or C(24) FAs. After cross-linking with CT B subunit, all GM1s were recovered in DRMs in a similarly high proportion irrespective of their ceramide structure or cholesterol depletion. DRMs prepared with low TX concentration at the TX/cell protein ratio of 0.3:1 were separated by multistep sucrose density gradient centrifugation into two fractions. The GM1s with sphingosine acetylated or acylated with C(18) or C(18:1) FAs occurred in these fractions in different proportions.     

Ceramide selectively displaces cholesterol from ordered lipid domains (rafts): implications for lipid raft structure and function

Ceramide is a membrane lipid involved in a number of crucial biological processes. Recent evidence suggests that ceramide is likely to reside and function within lipid rafts; ordered sphingolipid and cholesterol-rich lipid domains believed to exist within many eukaryotic cell membranes. Using lipid vesicles containing co-existing raft domains and disordered fluid domains, we find that natural and saturated synthetic ceramides displace sterols from rafts. Other raft lipids remain raft-associated in the presence of ceramide, showing displacement is relatively specific for sterols. Like cholesterol-containing rafts, ceramide-rich "rafts" remain in a highly ordered state. Comparison of the sterol-displacing abilities of natural ceramides with those of saturated diglycerides and an unsaturated ceramide demonstrates that tight lipid packing is critical for sterol displacement by ceramide. Based on these results, and the fact that cholesterol and ceramides both have small polar headgroups, we propose that ceramides and cholesterol compete for association with rafts because of a limited capacity of raft lipids with large headgroups to accommodate small headgroup lipids in a manner that prevents unfavorable contact between the hydrocarbon groups of the small headgroup lipids and the surrounding aqueous environment. Minimizing the exposure of cholesterol and ceramide to water may be a strong driving force for the association of other molecules with rafts. Furthermore, displacement of sterol from rafts by ceramide is very likely to have marked effects upon raft structure and function, altering liquid ordered properties as well as molecular composition. In this regard, certain previously observed physiological processes may be a result of displacement. In particular, a direct connection to the previously observed sphingomyelinase-induced displacement of cholesterol from plasma membranes in cells is proposed.     

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.     

Phase behavior of galactocerebrosides from bovine brain

Bovine brain cerebrosides (BOV-CER) were separated by high-performance liquid chromatography into cerebroside fractions with a single acyl chain type or with a relatively homogeneous acyl chain distribution. The thermal behavior of these isolated cerebroside fractions was studied by differential scanning calorimetry. Nonhydroxy (n-acyl) fatty acid cerebrosides (NFA-CER) possessing a saturated acyl chain (C16:0, C18:0, C24:0) exhibit their major order-disorder transition temperature TM at 83 degrees C, independent of chain length. NFA-CER possessing primarily unsaturated acyl chains (C24:1) exhibits TM at 70 degrees C. 2-Hydroxy fatty acid cerebrosides (HFA-CER), which possess a saturated hydroxyacyl chain (C18:0h, C24:0h), exhibit TM at 70-72 degrees C. Thus, naturally occurring cerebrosides exhibit high TM's that do not depend significantly on acyl chain length and that depend only to a small degree on unsaturation and the presence of a 2-hydroxy branch in the amide-linked chain. Isolated NFA-CER's each exhibit metastable polymorphism of the type previously described for unfractionated NFA-CER [Curatolo, W. (1982) Biochemistry 21, 1761]. Polymorphism in HFA-CER is complex, with a different type of thermal behavior observed for each isolated acyl chain fraction studied. On prolonged storage at low temperature, unfractionated HFA-CER and unfractionated BOV-CER reach a highly ordered gel state similar to that which is readily reached by NFA-CER's. These results indicate that all cerebrosides exhibit metastable polymorphism. However, the kinetic barriers to reaching the stable gel state are greater for HFA-CER and BOV-CER than for NFA-CER.     

CERT mediates intermembrane transfer of various molecular species of ceramides

Ceramide produced at the endoplasmic reticulum is transported to the Golgi apparatus for conversion to sphingomyelin. The main pathway of endoplasmic reticulum-to-Golgi transport of ceramide is mediated by CERT, a cytosolic 68-kDa protein, in a nonvesicular manner. CERT contains a domain that catalyzes the intermembrane transfer of natural C(16)-ceramide. In this study, we examined the ligand specificity of CERT in detail by using a cell-free assay system for intermembrane transfer of lipids. CERT did not mediate the transfer of sphingosine or sphingomyelin at all. The activity of CERT to transfer saturated and unsaturated diacylglycerols, which structurally resemble ceramide, was 5-10% of the activity toward C(16)-ceramide. Among four stereoisomers of C(16)-ceramide, CERT specifically recognized the natural d-erythro isomer. CERT efficiently transferred ceramides having C(14), C(16), C(18), and C(20) chains, but not longer acyl chains, and also mediated efficient transfer of C(16)-dihydroceramide and C(16)-phyto-ceramide. Binding assays showed that CERT also recognizes short chain fluorescent analogs of ceramide with a stoichiometry of 1:1. Moreover, (1R,3R)-N-(3-hydroxy-1-hydroxymethyl-3-phenylpropyl)dodecamide, which inhibited the CERT-dependent pathway of ceramide trafficking in intact cells, was found to be an antagonist of the CERT protein. These results indicate that CERT can mediate transfer of various types of ceramides that naturally exist and their close relatives.     

Effects of ceramide and other simple sphingolipids on membrane lateral structure

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