Insect glycolipids

Mass spectrometric and glycan microarray-based characterization of the filarial nematode Brugia malayi glycome reveals anionic and zwitterionic glycan antigens

Millions of people worldwide are infected with filarial nematodes, responsible for lymphatic filariasis (LF) and other diseases causing chronic disablement. Elimination programs have resulted in a substantial reduction of the rate of infection in certain areas creating a need for improved diagnostic tools to establish robust population surveillance and avoid LF resurgence. Glycans from parasitic helminths are emerging as potential antigens for use in diagnostic assays. However, despite its crucial role in host–parasite interactions, filarial glycosylation is still largely, structurally, and functionally uncharacterized. Therefore, we investigated the glycan repertoire of the filarial nematode Brugia malayi. Glycosphingolipid and N-linked glycans were extracted from several life-stages using enzymatic release and characterized using a combination of MALDI-TOF-MS and glycan sequencing techniques. Next, glycans were purified by HPLC and printed onto microarrays to assess the host anti-glycan antibody response. Comprehensive glycomic analysis of B. malayi revealed the presence of several putative antigenic motifs such as phosphorylcholine and terminal glucuronic acid. Glycan microarray screening showed a recognition of most B. malayi glycans by immunoglobulins from rhesus macaques at different time points after infection, which permitted the characterization of the dynamics of anti-glycan immunoglobulin G and M during the establishment of brugian filariasis. A significant level of IgG binding to the parasite glycans was also detected in infected human plasma, while IgG binding to glycans decreased after anthelmintic treatment. Altogether, our work identifies B. malayi glycan antigens and reveals antibody responses from the host that could be exploited as potential markers for LF.

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Antigenic B. malayi N-linked and GSL glycans were structurally defined.

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IgG/IgM is induced to a subset of B. malayi glycans upon infection of rhesus macaques.

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Preferential IgG response to B. malayi glycans observed in chronically infected humans.

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Marked drop of anti-glycan IgG following treatment of individuals with anthelminthic.

Zwitterionic Phosphodiester-Substituted Neoglycoconjugates as Ligands for Antibodies and Acute Phase Proteins

Zwitterionic modifications of glycans, such as phosphorylcholine and phosphoethanolamine, are known from a range of prokaryotic and eukaryotic species and are recognized by mammalian antibodies and pentraxins; however, defined saccharide ligands modified with these zwitterionic moieties for high-throughput studies are lacking. In this study, we prepared and tested example mono- and disaccharides 6-substituted with either phosphorylcholine or phosphoethanolamine as bovine serum albumin neoglycoconjugates or printed in a microarray format for subsequent assessment of their binding to lectins, pentraxins, and antibodies. C-Reactive protein and anti-phosphorylcholine antibodies bound specifically to ligands with phosphorylcholine, but recognition by concanavalin A was abolished or decreased as compared with that to the corresponding nonzwitterionic compounds. Furthermore, in array format, the phosphorylcholine-modified ligands were recognized by IgG and IgM in sera of either non-infected or nematode-infected dogs and pigs. Thereby, these new compounds are defined ligands which allow the assessment of glycan-bound phosphorylcholine as a target of both the innate and adaptive immune systems in mammals.

Anionic and zwitterionic moieties as widespread glycan modifications in non-vertebrates

Glycan structures in non-vertebrates are highly variable; it can be assumed that this is a product of evolution and speciation, not that it is just a random event. However, in animals and protists, there is a relatively limited repertoire of around ten monosaccharide building blocks, most of which are neutral in terms of charge. While two monosaccharide types in eukaryotes (hexuronic and sialic acids) are anionic, there are a number of organic or inorganic modifications of glycans such as sulphate, pyruvate, phosphate, phosphorylcholine, phosphoethanolamine and aminoethylphosphonate that also confer a 'charged' nature (either anionic or zwitterionic) to glycoconjugate structures. These alter the physicochemical properties of the glycans to which they are attached, change their ionisation when analysing them by mass spectrometry and result in different interactions with protein receptors. Here, we focus on N-glycans carrying anionic and zwitterionic modifications in protists and invertebrates, but make some reference to O-glycans, glycolipids and glycosaminoglycans which also contain such moieties. The conclusion is that 'charged' glycoconjugates are a widespread, but easily overlooked, feature of 'lower' organisms.

Magi Is Associated with the Par Complex and Functions Antagonistically with Bazooka to Regulate the Apical Polarity Complex

The mammalian MAGI proteins play important roles in the maintenance of adherens and tight junctions. The MAGI family of proteins contains modular domains such as WW and PDZ domains necessary for scaffolding of membrane receptors and intracellular signaling components. Loss of MAGI leads to reduced junction stability while overexpression of MAGI can lead to increased adhesion and stabilization of epithelial morphology. However, how Magi regulates junction assembly in epithelia is largely unknown. We investigated the single Drosophila homologue of Magi to study the in vivo role of Magi in epithelial development. Magi is localized at the adherens junction and forms a complex with the polarity proteins, Par3/Bazooka and aPKC. We generated a Magi null mutant and found that Magi null mutants were viable with no detectable morphological defects even though the Magi protein is highly conserved with vertebrate Magi homologues. However, overexpression of Magi resulted in the displacement of Baz/Par3 and aPKC and lead to an increase in the level of PIP3. Interestingly, we found that Magi and Baz functioned in an antagonistic manner to regulate the localization of the apical polarity complex. Maintaining the balance between the level of Magi and Baz is an important determinant of the levels and localization of apical polarity complex.

Phospholipid Topography of Whole-Body Sections of the Anopheles stephensi Mosquito, Characterized by High-Resolution Atmospheric-Pressure Scanning Microprobe Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Imaging

High-resolution atmospheric-pressure scanning microprobe matrix-assisted laser desorption/ionization mass spectrometry imaging (AP-SMALDI MSI) has been employed to study the molecular anatomical structure of rodent malaria vector Anopheles stephensi mosquitoes. A dedicated sample preparation method was developed which suits both, the special tissue properties of the sample and the requirements of high-resolution MALDI imaging. Embedding in 5% carboxymethylcellulose (CMC) was used to maintain the tissue integrity of the whole mosquitoes, being very soft, fragile, and difficult to handle. Individual lipid compounds, specifically representing certain cell types, tissue areas, or organs, were detected and imaged in 20 μm-thick whole-body tissue sections at a spatial resolution of 12 μm per image pixel. Mass spectrometric data and information quality were based on a mass resolution of 70,000 (at m/z 200) and a mass accuracy of better than 2 ppm in positive-ion mode on an orbital trapping mass spectrometer. A total of 67 imaged lipids were assigned by database search and, in a number of cases, identified via additional MS/MS fragmentation studies directly from tissue. This is the first MSI study at 12 μm spatial resolution of the malaria vector Anopheles. The study provides insights into the molecular anatomy of Anopheles stephensi and the distribution and localization of major classes of glycerophospholipids and sphingolipids. These data can be a basis for future experiments, investigating, e.g., the metabolism of Plasmodium-infected and -uninfected Anopheles mosquitoes.

Co-evolution of sphingomyelin and the ceramide transport protein CERT

Life creates many varieties of lipids. The choline-containing sphingophospholipid sphingomyelin (SM) exists ubiquitously or widely in vertebrates and lower animals, but is absent or rare in bacteria, fungi, protists, and plants. In the biosynthesis of SM, ceramide, which is synthesized in the endoplasmic reticulum, is transported to the Golgi region by the ceramide transport protein CERT, probably in a non-vesicular manner, and is then converted to SM by SM synthase, which catalyzes the reaction of phosphocholine transfer from phosphatidylcholine (PtdCho) to ceramide. Recent advances in genomics and lipidomics indicate that the phylogenetic occurrence of CERT and its orthologs is nearly parallel to that of SM. Based on the chemistry of lipids together with evolutionary aspects of SM and CERT, several concepts are here proposed. SM may serve as a chemically inert and robust, but non-covalently interactive lipid class at the outer leaflet of the plasma membrane. The functional domains and peptidic motifs of CERT are separated by exon units, suggesting an exon-shuffling mechanism for the generation of an ancestral CERT gene. CERT may have co-evolved with SM to bypass a competing metabolic reaction at the bifurcated point in the anabolism of ceramide. Human CERT is identical to the splicing variant of human Goodpasture antigen-binding protein (GPBP) annotated as an extracellular non-canonical serine/threonine protein kinase. The relationship between CERT and GPBP has also been discussed from an evolutionary aspect. Moreover, using an analogy of "compatible (or osmoprotective) solutes" that can accumulate to very high concentrations in the cytosol without denaturing proteins, choline phospholipids such as PtdCho and SM may act as compatible phospholipids in biomembranes. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

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.

N-terminal aromatic residues closely impact the cytolytic activity of cupiennin 1a, a major spider venom peptide

Cupiennins are small cationic α-helical peptides from the venom of the ctenid spider Cupiennius salei which are characterized by high bactericidal as well as hemolytic activities. To gain insight into the determinants responsible for the broad cytolytic activities, two analogues of cupiennin 1a with different N-terminal hydrophobicities were designed. The insecticidal, bactericidal and hemolytic activities of these analogues were assayed and compared to the native peptide. Specifically, substitution of two N-terminal Phe residues by Ala results in less pronounced insecticidal and cytolytic activity, whereas a substitution by Lys reduces strongly its bactericidal activity and completely diminishes its hemolytic activity up to very high tested concentrations. Biophysical analyses of peptide/bilayer membrane interactions point to distinct interactions of the analogues with lipid bilayers, and dependence upon membrane surface charge. Indeed, we find that lower hemolytic activity was correlated with less surface association of the analogues. In contrast, our data indicate that the reduced bactericidal activity of the two cupiennin 1a analogues likely correspond to greater bilayer-surface localization of the peptides. Overall, ultimate insertion and destruction of the host cell membrane is highly dependent on the presence of Phe-2 and Phe-6 (Cu 1a) or Leu-6 (Cu 2a) in the N-terminal sequences of native cupiennins.

Cell-specific in vivo functions of glycosphingolipids: lessons from genetic deletions of enzymes involved in glycosphingolipid synthesis

Glycosphingolipids (GSLs) are believed to be involved in many cellular events including trafficking, signaling and cellular interactions. Over the past decade considerable progress was made elucidating the function of GSLs by generating and exploring animal models with GSL-deficiency. Initial studies focused on exploring the role of complex sialic acid containing GSLs (gangliosides) in neuronal tissue. Although complex gangliosides were absent, surprisingly, the phenotype observed was rather mild. In subsequent studies, several mouse models with combinations of gene-deletions encoding GSL-synthesizing enzymes were developed. The results indicated that reduction of GSL-complexity correlated with severity of phenotypes. However, in these mice, accumulation of precursor GSLs or neobiosynthesized GSL-series seemed to partly compensate the loss of GSLs. Thus, UDP-glucose:ceramide glucosyltransferase (Ugcg), catalyzing the basic step of the glucosylceramide-based GSL-biosynthesis, was genetically disrupted. A total systemic deletion of Ugcg caused early embryonic lethality. Therefore, Ugcg was eliminated in a cell-specific manner using the cre/loxP-system. New insights into the cellular function of GSLs were gained. It was demonstrated that neurons require GSLs for differentiation and maintenance. In keratinocytes, preservation of the skin barrier depends on GSL synthesis and in enterocytes of the small intestine GSLs are involved in endocytosis and vesicular transport.

Structure and glycolipid binding properties of the nematicidal protein Cry5B

Crystal (Cry) proteins are globally used in agriculture as proteinaceous insecticides. They have also been recently recognized to have great potential as anthelmintic agents in targeting parasitic roundworms (e.g., hookworms). The most extensively characterized of the anthelmintic Cry proteins is Cry5B. We report here the 2.3 Å resolution structure of the proteolytically activated form of Cry5B. This structure, which is the first for a nematicidal Cry protein, shows the familiar three-domain arrangement seen in insecticidal Cry proteins. However, domain II is unusual in that it more closely resembles a banana lectin than it does other Cry proteins. This result is consistent with the fact that the receptor for Cry5B consists of a set of invertebrate-specific glycans (attached to lipids) and also suggests that domain II is important for receptor binding. We found that not only galactose but also N-acetylgalactosamine (GalNAc) is an efficient competitor for binding between Cry5B and glycolipids. GalNAc is one of the core arthroseries tetrasaccharides of the Cry5B receptor and galactose an antennary sugar that emanates from this core. These and prior data suggest that the minimal binding determinant for Cry5B consists of a core GalNAc and two antennary galactoses. Lastly, the protoxin form of Cry5B was found to bind nematode glycolipids with a specificity equal to that of activated Cry5B, but with lower affinity. This suggests that the initial binding of Cry5B protoxin to glycolipids can be stabilized at the nematode cell surface by proteolysis. These results lay the groundwork for the design of effective Cry5B-based anthelmintics.

Roles of sphingolipids in Drosophila development and disease

The last 10 years have seen a rebirth of interest in lipid biology in the fields of Drosophila development and neurobiology, and sphingolipids have emerged as controlling many processes that have not previously been studied from the viewpoint of lipid biochemistry. Mutations in sphingolipid regulatory enzymes have been pinpointed as affecting cell survival and growth in tissues ranging from muscle to retina. Specification of cell types are also influenced by sphingolipid regulatory pathways, as genetic interactions of glycosphingolipid biosynthetic enzymes with many well-known signaling receptors such as Notch and epidermal growth factor receptor reveal. Furthermore, studies in flies are now uncovering unexpected roles of sphingolipids in controlling lipid storage and response to nutrient availability. The sophisticated genetics of Drosophila is particularly well suited to uncover the roles of sphingolipid regulatory enzymes in development and metabolism, especially in light of conserved pathways that are present in both flies and mammals. The challenges that remain in the field of sphingolipid biology in Drosophila are to combine traditional developmental genetics with more analytical biochemical and biophysical methods, to quantify and localize the responses of these lipids to genetic and metabolic perturbations.

Glycolipid trafficking in Drosophila undergoes pathway switching in response to aberrant cholesterol levels

In lipid storage diseases, the intracellular trafficking of sphingolipids is altered by conditions of aberrant cholesterol accumulation. Drosophila has been used recently to model lipid storage diseases, but the effects of sterol accumulation on sphingolipid trafficking are not known in the fly, and the trafficking of sphingolipids in general has not been studied in this model organism. Here, we examined the uptake and intracellular distribution of a fluorescent glycolipid analog, BODIPY-lactosyl-ceramide, in Drosophila neurons. The uptake mechanism and intracellular trafficking route of this simple glycolipid are largely conserved. Our principle finding is that cholesterol steers trafficking of the glycolipid between Golgi, lysosome, and recycling compartments. Our analyses support the idea that cholesterol storage in Drosophila triggers a switch in glycolipid trafficking from the biosynthetic to the degradative endolysosomal pathway, whereas cholesterol depletion eliminates recycling of the glycolipid. Unexpectedly, we observe a novel phenomenon we term "hijacking," whereby lactosyl-ceramide diverts the trafficking pathway of an endocytic cargo, dextran, completely away from its lysosomal target. This work establishes that glycolipid trafficking in Drosophila undergoes changes similar to those seen in mammalian cells under conditions of cholesterol storage and therefore validates Drosophila as a suitable model organism in which to study lipid storage diseases.

The glycomics of glycan glucuronylation in Drosophila melanogaster

As glycan characterization methods increase in sensitivity, new opportunities arise to undertake glycomic analyses on limiting amounts of material. Developing systems present special challenges since the amount of available tissue can restrict deep glycan characterization. We have optimized mass spectrometric methods with the goal of obtaining full glycan profiles from small amounts of tissue derived from organisms of particular interest. A major target of our efforts has been the Drosophila embryo, allowing us to leverage the tools already developed in this organism to meld glycomics, genomics, and molecular genetics. Our analysis of the N-linked, O-linked (non-GAG), and glycosphingolipid (GSL) glycans of the Drosophila embryo have identified expected and unexpected glycan structures. We have verified previous findings regarding the predominance of high-Man and pauci-Man N-linked glycans, but have also detected minor families of sialylated and glucuronylated N-linked structures. Glucuronic acid (GlcA) also presents itself as an abundant modification of O-linked and GSL glycans. We describe critical advancements in our methodology and present the broad range of contexts in which GlcA is found in the Drosophila embryo.

The diversity of O-linked glycans expressed during Drosophila melanogaster development reflects stage- and tissue-specific requirements for cell signaling

Appropriate glycoprotein O-glycosylation is essential for normal development and tissue function in multicellular organisms. To comprehensively assess the developmental and functional impact of altered O-glycosylation, we have extensively analyzed the non-glycosaminoglycan, O-linked glycans expressed in Drosophila embryos. Through multidimensional mass spectrometric analysis of glycans released from glycoproteins by beta-elimination, we detected novel as well as previously reported O-glycans that exhibit developmentally modulated expression. The core 1 mucin-type disaccharide (Galbeta1-3GalNAc) is the predominant glycan in the total profile. HexNAcitol, hexitol, xylosylated hexitol, and branching extension of core 1 with HexNAc (to generate core 2 glycans) were also evident following release and reduction. After Galbeta1-3GalNAc, the next most prevalent glycans were a mixture of novel, isobaric, linear, and branched forms of a glucuronyl core 1 disaccharide. Other less prevalent structures were also extended with HexA, including an O-fucose glycan. Although the expected disaccharide product of the Fringe glycosyltransferase, (GlcNAcbeta1-3)fucitol, was not detectable in whole embryos, mass spectrometry fragmentation and exoglycosidase sensitivity defined a novel glucuronyl trisaccharide as GlcNAcbeta1-3(GlcAbeta1-4)fucitol. Consistent with the spatial distribution of the Fringe function, the GlcA-extended form of the Fringe product was enriched in the dorsal portion of the wing imaginal disc. Furthermore, loss of Fringe activity reduced the prevalence of the O-Fuc trisaccharide. Therefore, O-Fuc glycans necessary for the modulation of important signaling events in Drosophila are, as in vertebrates, substrates for extension beyond the addition of a single HexNAc.

A fluorescent glycolipid-binding peptide probe traces cholesterol dependent microdomain-derived trafficking pathways

BACKGROUND

The uptake and intracellular trafficking of sphingolipids, which self-associate into plasma membrane microdomains, is associated with many pathological conditions, including viral and toxin infection, lipid storage disease, and neurodegenerative disease. However, the means available to label the trafficking pathways of sphingolipids in live cells are extremely limited. In order to address this problem, we have developed an exogenous, non-toxic probe consisting of a 25-amino acid sphingolipid binding domain, the SBD, derived from the amyloid peptide Abeta, and conjugated by a neutral linker with an organic fluorophore. The current work presents the characterization of the sphingolipid binding and live cell trafficking of this novel probe, the SBD peptide. SBD was the name given to a motif originally recognized by Fantini et al in a number of glycolipid-associated proteins, and was proposed to interact with sphingolipids in membrane microdomains.

METHODOLOGY/PRINCIPAL FINDINGS

In accordance with Fantini's model, optimal SBD binding to membranes depends on the presence of sphingolipids and cholesterol. In synthetic membrane binding assays, SBD interacts preferentially with raft-like lipid mixtures containing sphingomyelin, cholesterol, and complex gangliosides in a pH-dependent manner, but is less glycolipid-specific than Cholera toxin B (CtxB). Using quantitative time-course colocalization in live cells, we show that the uptake and intracellular trafficking route of SBD is unlike that of either the non-raft marker Transferrin or the raft markers CtxB and Flotillin2-GFP. However, SBD traverses an endolysosomal route that partially intersects with raft-associated pathways, with a major portion being diverted at a late time point to rab11-positive recycling endosomes. Trafficking of SBD to acidified compartments is strongly disrupted by cholesterol perturbations, consistent with the regulation of sphingolipid trafficking by cholesterol.

CONCLUSIONS/SIGNIFICANCE

The current work presents the characterization and trafficking behavior of a novel sphingolipid-binding fluorescent probe, the SBD peptide. We show that SBD binding to membranes is dependent on the presence of cholesterol, sphingomyelin, and complex glycolipids. In addition, SBD targeting through the endolysosomal pathway in neurons is highly sensitive to cholesterol perturbations, making it a potentially useful tool for the analysis of sphingolipid trafficking in disease models that involve changes in cholesterol metabolism and storage.

Biodiversity of sphingoid bases ("sphingosines") and related amino alcohols

"Sphingosin" was first described by J. L. W. Thudichum in 1884 and structurally characterized as 2S,3R,4E-2-aminooctadec-4-ene-1,3-diol in 1947 by Herb Carter, who also proposed the designation of "lipides derived from sphingosine as sphingolipides." This category of amino alcohols is now known to encompass hundreds of compounds that are referred to as sphingoid bases and sphingoid base-like compounds, which vary in chain length, number, position, and stereochemistry of double bonds, hydroxyl groups, and other functionalities. Some have especially intriguing features, such as the tail-to-tail combination of two sphingoid bases in the alpha,omega-sphingoids produced by sponges. Most of these compounds participate in cell structure and regulation, and some (such as the fumonisins) disrupt normal sphingolipid metabolism and cause plant and animal disease. Many of the naturally occurring and synthetic sphingoid bases are cytotoxic for cancer cells and pathogenic microorganisms or have other potentially useful bioactivities; hence, they offer promise as pharmaceutical leads. This thematic review gives an overview of the biodiversity of the backbones of sphingolipids and the broader field of naturally occurring and synthetic sphingoid base-like compounds.

Structural studies on the neutral glycosphingolipids of Manduca sexta

Glycosphingolipids (GSLs) have been implicated as playing major roles in cellular interactions and control of cell proliferation in muticellular organisms. Moreover GSLs and other sphingolipids such as sphingomyelins, ceramides and sphingosines serve a variety of roles in signal transduction. Hence, identification of structures of GSLs in different biota will shed light in understanding their physiological role. During this study, the major glycosphingolipid component present in the extracts of stage-12 and stage-17/18 metamorphosing adults of Manduca sexta was identified as mactosyl ceramide. We report the isolation of several ceramide disaccharides, a ceramide trisaccharide and a ceramide tetrasaccharide. The GSL structures were confirmed by high-resolution mass spectrometry and tandem mass spectrometry. The identity of the monosaccharides was proved using exoglycosidases. The predominant sphingosine chain-length varied from C-14 (tetradecasphing-4-enine) to C-16 (hexadecasphing-4-enine) in these GSLs. Sphingosines of both chain lengths were accompanied by their doubly unsaturated counterparts tetradecasphinga-4,6-diene and hexadecasphinga-4,6-diene. It is also interesting to note the presence of tetradecasphinganine and hexadecasphinganine in minute amounts in the form of a GSL in the extracts of M. sexta. The varying degrees of unsaturation in the sphingosine moiety of GSLs in M. sexta may be biologically significant in insect metamorphosis. The ceramide trisaccharides and ceramide tetrasaccharide belong to the arthro-series, The observation of fucose in the M. sexta GSLs is the first report of the presence of fucose in an arthroseries GSL.

Distinct contributions of β4GalNAcTA and β4GalNAcTB to Drosophila glycosphingolipid biosynthesis

Drosophila melanogaster has two beta4-N-acetylgalactosaminyltransferases, beta4GalNAcTA and beta4GalNAcTB, that are able to catalyse the formation of lacdiNAc (GalNAcbeta,4GlcNAc). LacdiNAc is found as a structural element of Drosophila glycosphingolipids (GSLs) suggesting that beta4GalNAcTs contribute to the generation of GSL structures in vivo. Mutations in Egghead and Brainaic, enzymes that generate the beta4GalNAcT trisaccharide acceptor structure GlcNAcbeta,3Manbeta,4GlcbetaCer, are lethal. In contrast, flies doubly mutant for the beta4GalNAcTs are viable and fertile. Here, we describe the structural analysis of the GSLs in beta4GalNAcT mutants and find that in double mutant flies no lacdiNAc structure is generated and the trisaccharide GlcNAcbeta,3Manbeta,4GlcbetaCer accumulates. We also find that phosphoethanolamine transfer to GlcNAc in the trisaccharide does not occur, demonstrating that this step is dependent on prior or simultaneous transfer of GalNAc. By comparing GSL structures generated in the beta4GalNAcT single mutants we show that beta4GalNAcTB is the major enzyme for the overall GSL biosynthesis in adult flies. In beta4GalNAcTA mutants, composition of GSL structures is indistinguishable from wild-type animals. However, in beta4GalNAcTB mutants precursor structures are accumulating in different steps of GSL biosynthesis, without the complete loss of lacdiNAc, indicating that beta4GalNAcTA plays a minor role in generating GSL structures. Together our results demonstrate that both beta4GalNAcTs are able to generate lacdiNAc structures in Drosophila GSL, although with different contributions in vivo, and that the trisaccharide GlcNAcbeta,3Manbeta,4GlcbetaCer is sufficient to avoid the major phenotypic consequences associated with the GSL biosynthetic defects in Brainiac or Egghead.

Characterization of mucin-type core-1 beta1-3 galactosyltransferase homologous enzymes in Drosophila melanogaster

Mucin type O-glycosylation is a widespread modification of eukaryotic proteins. The transfer of N-acetylgalactosamine to selected serine or threonine residues is catalyzed by a family of polypeptide N-acetylgalactosaminyltransferases localized in the Golgi apparatus. The most abundant elongation of O-glycans is the addition of a beta1-3 linked galactose by the core-1 beta1-3 galactosyltransferase (core-1 beta3GalT), thereby building the T-antigen or core-1 structure Gal(beta1-3)GalNAc(alpha1-O). We have isolated four Drosophila melanogaster cDNAs encoding proteins structurally similar to the human core-1 beta3GalT enzyme and expressed them as FLAG-tagged proteins in Sf9 insect cells. The identity of these D. melanogasterbeta3GalT enzymes with a core-1 beta3GalT activity was confirmed by utilization of MUC5AC mucin derived O-glycopeptide acceptors. In addition to the core-1 beta3GalT activity toward O-glycoprotein substrates, one member of this enzyme family showed a strong activity towards glycolipid acceptors, thereby building the core-1 terminated Nz6 glycosphingolipid. Transcripts of the embryonically expressed core-1 beta3GalTs were found in the maternally deposited mRNA, in salivary glands and in the amnioserosa. The presence of multiple core-1 beta3GalT genes in D. melanogaster suggests an increased complexity of core-1 O-glycan expression, which is possibly related to multiple developmental and physiological functions attributable to this class of glycans.

Presence of unsaturated sphingomyelins and changes in their composition during the life cycle of the moth Manduca sexta

NMR and electrospray ionization tandem mass spectrometry were used to show for the first time the presence of sphingomyelins in extracts of the tobacco hornworm Manduca sexta (Lepidoptera). The sphingosine in the ceramide was identified as tetradecasphing-4-enine, and the fatty acids were C18:0, C20:0, C22:0, and C24:0 (compound 1). Heterogeneity in the ceramide was observed in sphingomyelins from M. sexta. All of the sphingomyelins were associated with their doubly unsaturated sphingosine, tetradecasphing-4,6-dienine (compound 2), which contained the same set of fatty acids as compound 1 and represents a novel set of sphingomyelins not previously reported in Lepidoptera. Lipid rafts were isolated from brains of M. sexta, and the association of these novel sphingomyelins with rafts was confirmed. The existence of the additional double bond was also observed in ceramide and ceramide phosphoethanolamine isolated from M. sexta. The levels of the doubly unsaturated ceramide showed modest changes during metamorphosis of M. sexta. These results suggest that Manduca sphingomyelins may participate in the formation of lipid rafts, in keeping with their function in vertebrates.

Ceramides and skin function

Ceramides are the major lipid constituent of lamellar sheets present in the intercellular spaces of the stratum corneum. These lamellar sheets are thought to provide the barrier property of the epidermis. It is generally accepted that the intercellular lipid domain is composed of approximately equimolar concentrations of free fatty acids, cholesterol, and ceramides. Ceramides are a structurally heterogeneous and complex group of sphingolipids containing derivatives of sphingosine bases in amide linkage with a variety of fatty acids. Differences in chain length, type and extent of hydroxylation, saturation etc. are responsible for the heterogeneity of the epidermal sphingolipids. It is well known that ceramides play an essential role in structuring and maintaining the water permeability barrier function of the skin. In conjunction with the other stratum corneum lipids, they form ordered structures. An essential factor is the physical state of the lipid chains in the nonpolar regions of the bilayers. The stratum corneum intercellular lipid lamellae, the aliphatic chains in the ceramides and the fatty acids are mostly straight long-chain saturated compounds with a high melting point and a small polar head group. This means that at physiological temperatures, the lipid chains are mostly in a solid crystalline or gel state, which exhibits low lateral diffusional properties and is less permeable than the state of liquid crystalline membranes, which are present at higher temperatures. The link between skin disorders and changes in barrier lipid composition, especially in ceramides, is difficult to prove because of the many variables involved. However, most skin disorders that have a diminished barrier function present a decrease in total ceramide content with some differences in the ceramide pattern. Formulations containing lipids identical to those in skin and, in particular, some ceramide supplementation could improve disturbed skin conditions. Incomplete lipid mixtures yield abnormal lamellar body contents, and disorder intercellular lamellae, whereas complete lipid mixtures result in normal lamellar bodies and intercellular bilayers. The utilization of physiological lipids according to these parameters have potential as new forms of topical therapy for dermatoses. An alternative strategy to improving barrier function by topical application of the various mature lipid species is to enhance the natural lipid-synthetic capability of the epidermis through the topical delivery of lipid precursors.

Identification of glycosphingolipid receptors for pierisin-1, a guanine-specific ADP-ribosylating toxin from the cabbage butterfly

Pierisin-1, a cytotoxic protein found naturally in the cabbage butterfly, induces apoptosis of mammalian cells. Our recent studies suggest that pierisin-1 consists of an N-terminal ADP-ribosyltransferase domain, and a C-terminal region that binds to receptors on the surfaces of target cells and incorporates the protein into cells. The present study was undertaken to identify receptors for pierisin-1. The cross-linking and cloning experiments suggested that the proteins on cell membrane had no binding ability to pierisin-1. Inhibitory assays of fractionated lipids from human cervical carcinoma HeLa cells, which are highly sensitive to pierisin-1, indicated neutral glycosphingolipids on the cell surface to show receptor activity. Inhibitory assays and TLC immunostaining using anti-pierisin-1 antibodies demonstrated two neutral glycosphingolipids as active components. Analysis of their structures with glycosphingolipid-specific antibodies and negative secondary ion mass spectrometry identified them as globotriaosylceramide (Gb3) and globotetraosylceramide (Gb4). The receptor activities of Gb3 and Gb4 for pierisin-1 were also confirmed with these authentic compounds. Pierisin-1-insensitive mouse melanoma MEB4 cells were found to lack pierisin-1 receptors, including Gb3 and Gb4, but pretreatment of the cells with glycosphingolipid Gb3 or Gb4 enhanced their sensitivity to pierisin-1. Thus, Gb3 and Gb4 were proven to serve as pierisin-1 receptors. The C-terminal region of pierisin-1 consists of possible lectin domains of a ricin B-chain, containing QXW sequences, which are essential for its structural organization. Alteration of QXW by site-directed mutagenesis caused marked reduction of pierisin-1 cytotoxicity. Thus, our results suggest that pierisin-1 binds to Gb3 and Gb4 receptors at the C-terminal region, in a manner similar to ricin, and then exhibits cytotoxicity after incorporation into the cell.

Effects of membrane potential and sphingolipid structures on fusion of Semliki Forest virus

Cells expressing the E1 and E2 envelope proteins of Semliki Forest virus (SFV) were fused to voltage-clamped planar lipid bilayer membranes at low pH. Formation and evolution of fusion pores were electrically monitored by capacitance measurements, and membrane continuity was tracked by video fluorescence microscopy by including rhodamine-phosphatidylethanolamine in the bilayer. Fusion occurred without leakage for a negative potential applied to the trans side of the planar membrane. When a positive potential was applied, leakage was severe, obscuring the observation of any fusion. E1-mediated cell-cell fusion occurred without leakage for negative intracellular potentials but with substantial leakage for zero membrane potential. Thus, negative membrane potentials are generally required for nonleaky fusion. With planar bilayers as the target, the first fusion pore that formed almost always enlarged; pore flickering was a rare event. Similar to other target membranes, fusion required cholesterol and sphingolipids in the planar membrane. Sphingosine did not support fusion, but both ceramide, with even a minimal acyl chain (C(2)-ceramide), and lysosphingomyelin (lyso-SM) promoted fusion with the same kinetics. Thus, unrelated modifications to different parts of sphingosine yielded sphingolipids that supported fusion to the same degree. Fusion studies of pyrene-labeled SFV with cholesterol-containing liposomes showed that C(2)-ceramide supported fusion while lyso-SM did not, apparently due to its positive curvature effects. A model is proposed in which the hydroxyls of C-1 and C-3 as well as N of C-2 of the sphingosine backbone must orient so as to form multiple hydrogen bonds to amino acids of SFV E1 for fusion to proceed.

The Drosophila melanogaster brainiac protein is a glycolipid-specific beta 1,3N-acetylglucosaminyltransferase

Mutations at the Drosophila melanogaster brainiac locus lead to defective formation of the follicular epithelium during oogenesis and to neural hyperplasia. The brainiac gene encodes a type II transmembrane protein structurally similar to mammalian beta1,3-glycosyltransferases. We have cloned the brainiac gene from D. melanogaster genomic DNA and expressed it as a FLAG-tagged recombinant protein in Sf9 insect cells. Glycosyltransferase assays showed that brainiac is capable of transferring N-acetylglucosamine (GlcNAc) to beta-linked mannose (Man), with a marked preference for the disaccharide Man(beta1,4)Glc, the core of arthro-series glycolipids. The activity of brainiac toward arthro-series glycolipids was confirmed by showing that the enzyme efficiently utilized glycolipids from insects as acceptors whereas it did not with glycolipids from mammalian cells. Methylation analysis of the brainiac reaction product revealed a beta1,3 linkage between GlcNAc and Man, proving that brainiac is a beta1,3GlcNAc-transferase. Human beta1,3GlcNAc-transferases structurally related to brainiac were unable to transfer GlcNAc to Man(beta1,4)Glc-based acceptor substrates and failed to rescue a homozygous lethal brainiac allele, indicating that these proteins are paralogous and not orthologous to brainiac.

The organizing potential of sphingolipids in intracellular membrane transport

Eukaryotes are characterized by endomembranes that are connected by vesicular transport along secretory and endocytic pathways. The compositional differences between the various cellular membranes are maintained by sorting events, and it has long been believed that sorting is based solely on protein-protein interactions. However, the central sorting station along the secretory pathway is the Golgi apparatus, and this is the site of synthesis of the sphingolipids. Sphingolipids are essential for eukaryotic life, and this review ascribes the sorting power of the Golgi to its capability to act as a distillation apparatus for sphingolipids and cholesterol. As Golgi cisternae mature, ongoing sphingolipid synthesis attracts endoplasmic reticulum-derived cholesterol and drives a fluid-fluid lipid phase separation that segregates sphingolipids and sterols from unsaturated glycerolipids into lateral domains. While sphingolipid domains move forward, unsaturated glycerolipids are retrieved by recycling vesicles budding from the sphingolipid-poor environment. We hypothesize that by this mechanism, the composition of the sphingolipid domains, and the surrounding membrane changes along the cis-trans axis. At the same time the membrane thickens. These features are recognized by a number of membrane proteins that as a consequence of partitioning between domain and environment follow the domains but can enter recycling vesicles at any stage of the pathway. The interplay between protein- and lipid-mediated sorting is discussed.

Function and structure of Drosophila glycans

Through the application of classic organismal genetic strategies, such as mutagenesis and interaction screens, Drosophila melanogaster provides opportunities to understand glycan function. For instance, screens for Drosophila genes that establish dorsal-ventral polarity in the embryo or that influence cellular differentiation through signal modulation have identified putative glycan modifying enzymes. Other genetic and molecular approaches have demonstrated the existence of phylogenetically conserved and novel oligosaccharide processing activities and carbohydrate binding proteins. While the structural characterization of Drosophila oligosaccharide diversity has lagged behind the elucidation of glycan function, landmarks are becoming apparent in the carbohydrate terrain. For instance, O-linked GlcNAc and mucins, spatially and temporally regulated N-linked oligosaccharide expression, glycosphingolipids, heparan sulfate, chondroitin sulfate and polysialic acid have all been described. A major challenge for Drosophila glycobiology is to expand the oligosaccharide structural database while endeavoring to link glycan characterization to functional analysis. The completion of the Drosophila genome sequencing project will yield a broad portfolio of glycosyltransferases, glycan modifying enzymes and lectins requiring characterization. To this end, the great range of genetic tools that allow the controlled spatial and temporal expression of transgenes in Drosophila will permit unprecedented manipulation of glycosylation in a whole organism.

Zwitterionic and acidic glycosphingolipids of the Drosophila melanogaster embryo

Defining glycosphingolipid structures in species amenable to genetic manipulation, such as Drosophila melanogaster, provides a foundation for investigating mechanisms that regulate glycolipid expression. Therefore, eight of the 12 major glycosphingolipids, accounting for 64% of lipid-linked carbohydrate in Drosophila embryos, were purified after separation into acidic and zwitterionic pools. The zwitterionic lipids possess phosphoethanolamine (PEtn) linked to one or more GlcNAc residues and comprise a family of serially related structures. The longest characterized glycolipid, an octaosylceramide, designated Nz28, has the structure: GalNAcbeta, 4(PEtn-6)GlcNAcbeta,3Galbeta,3GalNAcalpha,4Ga lNAcbeta, 4(PEtn-6)GlcNAcbeta,3Manbeta,4GlcbetaCer. Heptaosyl (Nz7), hexaosyl (Nz6), pentaosyl (Nz5) and tetraosyl (Nz4) forms of Nz28, sequentially truncated from the nonreducing terminus, possess only one PEtn moiety. The major acidic lipid, designated Az29, possesses two PEtn moieties and a glucuronic acid linked to a Gal-extended Nz28. Two other acidic glycolipids, Az9 and Az6, exhibit one PEtn moiety and the same hexose and N-acetylhexosamine composition as Az29 and Nz6, respectively. The fully extended Drosophila core oligosaccharide differs from that of other dipterans in the linkage at a single glycosidic bond, a distinction with significant structural and biosynthetic consequences. Furthermore, acidic species account for a larger proportion of total glycosphingolipid, and PEtn substitution of GlcNAc is more complete in the Drosophila embryo. Divergent characteristics may reflect interspecies variation or stage-specific glycosphingolipid expression in dipterans.

Association of sterol- and glycosylphosphatidylinositol-linked proteins with Drosophila raft lipid microdomains

In vertebrates, the formation of raft lipid microdomains plays an important part in both polarized protein sorting and signal transduction. To establish a system in which raft-dependent processes could be studied genetically, we have analyzed the protein and lipid composition of these microdomains in Drosophila melanogaster. Using mass spectrometry, we identified the phospholipids, sphingolipids, and sterols present in Drosophila membranes. Despite chemical differences between Drosophila and mammalian lipids, their structure suggests that the biophysical properties that allow raft formation have been preserved. Consistent with this, we have identified a detergent-insoluble fraction of Drosophila membranes that, like mammalian rafts, is rich in sterol, sphingolipids, and glycosylphosphatidylinositol-linked proteins. We show that the sterol-linked Hedgehog N-terminal fragment associates specifically with this detergent-insoluble membrane fraction. Our findings demonstrate that raft formation is preserved across widely separated phyla in organisms with different lipid structures. They further suggest sterol modification as a novel mechanism for targeting proteins to raft membranes and raise the possibility that signaling and polarized intracellular transport of Hedgehog are based on raft association.

Mutational analysis of caveolin-induced vesicle formation. Expression of caveolin-1 recruits caveolin-2 to caveolae membranes

Caveolae are vesicular organelles with a characteristic uniform diameter in the range of 50-100 nm. Although recombinant expression of caveolin-1 is sufficient to drive caveolae formation, it remains unknown what controls the uniform diameter of these organelles. One hypothesis is that specific caveolin-caveolin interactions regulate the size of caveolae, as caveolin-1 undergoes two stages of self-oligomerization. To test this hypothesis directly, we have created two caveolin-1 deletion mutants that lack regions of caveolin-1 that are involved in directing the self-assembly of caveolin-1 oligomers. More specifically, Cav-1 delta61-100 lacks a region of the N-terminal domain that directs the formation of high molecular mass caveolin-1 homo-oligomers, while Cav-1 deltaC lacks a complete C-terminal domain that is required to allow caveolin homo-oligomers to interact with each other, forming a caveolin network. It is important to note that these two mutants retain an intact transmembrane domain. Our current results show that although Cav-1 delta61-100 and Cav-1 deltaC are competent to drive vesicle formation, these vesicles vary widely in their size and shape with diameters up to 500-1000 nm. In addition, caveolin-induced vesicle formation appears to be isoform-specific. Recombinant expression of caveolin-2 under the same conditions failed to drive the formation of vesicles, while caveolin-3 expression yielded caveolae-sized vesicles. These results are consistent with the previous observation that in transformed NIH 3T3 cells that lack caveolin-1 expression, but continue to express caveolin-2, no morphologically distinguishable caveolae are observed. In addition, as caveolin-2 alone exists mainly as a monomer or homo-dimer, while caveolins 1 and 3 exist as high molecular mass homo-oligomers, our results are consistent with the idea that the formation of high molecular mass oligomers of caveolin are required to regulate the formation of uniform caveolae-sized vesicles. In direct support of this notion, regulated induction of caveolin-1 expression in transformed NIH 3T3 cells was sufficient to recruit caveolin-2 to caveolae membranes. The ability of caveolin-1 to recruit caveolin-2 most likely occurs through a direct interaction between caveolins 1 and 2, as caveolins 1 and 2 are normally co-expressed and interact with each other to form high molecular mass hetero-oligomers containing both caveolins 1 and 2.

Structural analysis of leech galactocerebrosides using 1D and 2D NMR spectroscopy, gas chromatography-mass spectrometry, and FAB mass spectrometry

Cerebrosides were isolated from the leech species, Hirudo medicinalis, and purified to homogeneity by silicic acid chromatography, followed by preparative thin-layer chromatography. Their structure was determined by spectroscopic and chemical methods. 1D and 2D 1H NMR spectroscopy, DQF-COSY and HMQC indicated that the head group consists of a single galactose residue in the beta configuration. The galacto configuration was determined by the characteristic chemical shift, the spin-spin splitting and the multiplicity of the characteristic resonance of its equatorial H-4 proton, as well as by the splittings of the other ring protons. GC, GC-MS and fast-atom-bombardment mass spectrometry studies indicated that C24:0 and C22:0 are the major saturated fatty acid species. Unsaturated fatty acids present were C25:2, C27:2, C27:3, C28:3, C29:3, C30:3, C33:3. GC-MS indicated the presence of hydroxylated C27:2 and one other unidentified hydroxylated fatty acid. The cerebroside contained an unusual polyunsaturated sphingosine analogue, namely 2-amino-1,3-dihydroxydocsatriene.

Structural characterization of a novel glycoinositolphospholipid from the parasitic nematode, Ascaris suum

A novel glycosphingolipid containing inositol phosphate as an acidic group has been demonstrated in whole tissues of the porcine roundworm, Ascaris suum. The thin layer chromatographic pattern of the total acidic glycolipid revealed the presence of several components, of which a major component (named AGL) with positive reactions toward both orcinol-sulfuric acid (sugar) and molybdate (phosphate) spray reagents was isolated and purified by the use of successive column chromatography on DEAE-Sephadex and silicic acid (latrobeads). From structural studies including compositional sugar analysis, hydrogen fluoride degradation, methylation analysis, periodate oxidation, proton magnetic resonance spectroscopy and fast atom bombardment mass spectrometry, the structure of AGL was deduced to be Gal alpha 1-2Ins(1-->)-P-Cer. Aliphatic constituents were lignoceric acid and its 2-hydroxy homologue as the principal fatty acids, and octadecasphinganine and branched heptadecasphinganine as the major sphingoids.

Gliolectin is a novel carbohydrate-binding protein expressed by a subset of glia in the embryonic Drosophila nervous system

Interactions between embryonic neural cells generate the specific patterns of connectivity observed in nervous systems. Cell surface carbohydrates have been proposed to function in cellular recognition events guiding such interactions. Carbohydrate-binding proteins (lectins) that recognize specific oligosaccharide ligands in embryonic neural tissue provide a molecular mechanism for carbohydrate-mediated cell-cell interactions in neural development. Therefore, we have screened an embryonic Drosophila melanogaster cDNA library, expressed in COS1 cells, for carbohydrate-binding activity. COS1 cells expressing putative Drosophila lectins were identified and recovered based on their adhesion to immobilized preparations of neutral and zwitterionic glycolipids extracted from Drosophila embryos. We have identified an endogenous lectin expressed during Drosophila embryogenesis. The cloned lectin, designated ‘gliolectin’, possesses a novel protein sequence with a calculated molecular mass of 24,993. When expressed in Drosophila S2 cells, the lectin mediates heterophilic cellular aggregation. In embryos, gliolectin is expressed by a subset of glial cells found at the midline of the developing nervous system. Expression is highest during the formation of the Drosophila embryonic axonal commissures, a process requiring midline glial cell funcion. Immunoprecipitation with a monoclonal antibody against gliolectin yields a protein of Mr=46,600 from Drosophila embryonic membranes, suggesting that post-translational modification of gliolectin is extensive. Epitope- tagged chimericproteins composed of the amino terminal one-half of gliolectin and the Fc region of human IgG bind a small subset of the total glycolipids extracted from Drosophila embryos, demonstrating that the lectin activity of gliolectin can discriminate between oligosaccharide structures. The presence of gliolectin in the developing Drosophila embryonic nervous system further supports a role for cell surface carbohydrates in cell-cell recognition and indicates that the molecular diversity of animal lectins is not yet completely defined.

Modulation by GM1 ganglioside of beta 1-adrenergic receptor-induced cyclic AMP formation in Sf9 cells

The effect of ganglioside GM1 on isoproterenol-induced cAMP accumulation was studied in insect Sf9 cells expressing the human beta 1-adrenergic receptor by infection with recombinant baculovirus. When such Sf9 cells were treated with isoproterenol plus IBMX, intracellular cAMP formation increased approximately 10-fold over the basal level. Preincubation of the baculovirus-infected cells with GM1 for 1 h caused a concentration-dependent inhibition of the isoproterenol-induced cAMP accumulation. Phosphatidylserine, GM3, GT1b and a bovine brain ganglioside preparation lacking GM1 did not cause significant inhibition. Forskolin-induced cAMP accumulation was not affected by the GM1 treatment. Inhibition of isoproterenol-induced cAMP formation by GM1 was not observed in Sf9 cells expressing beta 2-adrenergic receptor instead of the beta 1-adrenergic receptor. Binding studies with (-)-[3H]CGP12177 showed that preincubation with GM1 significantly reduced the affinity of antagonist binding to the beta 1-adrenergic receptor. These results suggest that GM1 or related ganglioside structure(s) may function as natural modulator(s) of the beta 1-adrenergic receptor.

A novel fucosylated glycosphingolipid from the millipede, Parafontaria laminata armigera

A novel fucosylated glycosphingolipid (GL-3a) was isolated and purified from whole tissues of the millipede, Parafontaria laminata armigera. Its chemical structure was characterized as Man beta 1-4(Fuc alpha 1-3)Glc beta 1-ceramide (I3 alpha Fuc,MlOse2Cer) by gas-liquid chromatography, permethylation study, partial acid hydrolysis, exoglycosidase degradation, TLC/enzyme-immunostaining, negative fast atom bombardment-mass spectrometry and proton nuclear magnetic resonance spectroscopy. This compound was unique in containing a fucose branch linked to the glucose residue of the disaccharide, mannosylglucose. The ceramide moiety was mainly composed of d17:1 (64.3%) and d18:1 (20.0%) sphingoids, and 22:0 (41.8%), 23:0 (16.4%) and 24:0 (15.8%) fatty acids.