Glycosphingolipids with extended sugar chain have specialized functions in development and behavior of Drosophila

Bombyx mori nuclear polyhedrosis virus infection regulated by host glycosphingolipids

Glycosylation is an important post-translational modification commonly found in eukaryotes, and plays crucial roles in many biological activities. The silkworm Bombyx mori (B. mori), an important economic insect and a model organism in biology, has recently been found to be abundantly glycosylated. In this study, we established the role of silkworm glycosphingolipids (GSLs), the glycoconjugates formed by covalent attachment of a glycan to the lipid class of ceramide, during B. mori nuclear polyhedrosis virus (BmNPV) infection. The levels of cellular glycosphingolipids (GSLs), particularly the glucosylceramide (Glc-Cer) series, were modulated by targeting uridine diphosphate-glucose ceramide glycosyltransferase (UGCG), the enzyme responsible for Glc-Cer synthesis. Inhibiting UGCG activity by Genz-123346 (Genz), an inhibitor and substrate analogue of UGCG, reduced BmNPV binding, internalization, and viral protein expression in BmN cells. A general reduction in the cellular GSL contents was observed following Genz treatment. Overexpression of UGCG increased cellular GSL levels overall while still caused suppression in viral infection. It is postulated that GSLs are highly regulated membrane components that are crucial for viral entry, and disturbing the balance, either by increasing or decreasing cellular GSL components, alters membrane traffic and transport, which is unfavorable for viral infection. Therefore, highly regulated cellular GSLs are required for effective BmNPV infection. This study provides direct evidence linking GSL levels to BmNPV infection, offering new insights into the role of GSLs in viral infection.

Dual roles of α1,4-galactosyltransferase 1 in spermatogenesis of Drosophila melanogaster

Spermatogenesis is critical for insect reproduction and the process is regulated by multiple genes. Glycosyltransferases have been shown to participate in the development of Drosophila melanogaster; however, their role in spermatogenesis is still unclear. In this study, we found that α1,4-galactosyltransferase 1 (α4GT1) was expressed at a significantly higher level in the testis than in the ovary of Drosophila. Importantly, the hatching rate was significantly decreased when α4GT1 RNA interference (RNAi) males were crossed with w1118 females, with only a few mature sperm being present in the seminal vesicle of α4GT1 RNAi flies. Immunofluorescence staining further revealed that the individualization complex (IC) in the testes from α4GT1 RNAi flies was scattered and did not move synchronically, compared with the clustered IC observed in the control flies. Terminal deoxyribonucleotide transferase (TdT)-mediated dUTP nick end labeling (TUNEL) assay showed that apoptosis signals in the sperm bundles of α4GT1 RNAi flies were significantly increased. Moreover, the expression of several individualization-related genes, such as Shrub, Obp44a and Hanabi, was significantly decreased, whereas the expression of several apoptosis-related genes, including Dronc and Drice, was significantly increased in the testes of α4GT1 RNAi flies. Together, these results suggest that α4GT1 may play dual roles in Drosophila spermatogenesis by regulating the sperm individualization process and maintaining the survival of sperm bundles.

Glial control of sphingolipid levels sculpts diurnal remodeling in a circadian circuit

Structural plasticity in the brain often necessitates dramatic remodeling of neuronal processes, with attendant reorganization of the cytoskeleton and membranes. Although cytoskeletal restructuring has been studied extensively, how lipids might orchestrate structural plasticity remains unclear. We show that specific glial cells in Drosophila produce glucocerebrosidase (GBA) to locally catabolize sphingolipids. Sphingolipid accumulation drives lysosomal dysfunction, causing gba1b mutants to harbor protein aggregates that cycle across circadian time and are regulated by neural activity, the circadian clock, and sleep. Although the vast majority of membrane lipids are stable across the day, a specific subset that is highly enriched in sphingolipids cycles daily in a gba1b-dependent fashion. Remarkably, both sphingolipid biosynthesis and degradation are required for the diurnal remodeling of circadian clock neurites, which grow and shrink across the day. Thus, dynamic sphingolipid regulation by glia enables diurnal circuit remodeling and proper circadian behavior.

Glycosyltransferases: the multifaceted enzymatic regulator in insects

Glycosyltransferases (GTs) catalyse the reaction of glyco-conjugation of various biomolecules by transferring the saccharide moieties from an activated nucleotide sugar to nucleophilic glycosyl acceptor. In insects, GTs show diverse temporal and site-specific expression patterns and thus play significant roles in forming the complex biomolecular structures that are necessary for insect survival, growth and development. Several insects exhibit GT-mediated detoxification as a key defence strategy against plant allelochemicals and xenobiotic compounds, as well as a mechanism for pesticide cross-resistance. Also, these enzymes act as crucial effectors and modulators in various developmental processes of insects such as eye development, UV shielding, cuticle formation, epithelial development and other specialized functions. Furthermore, many of the known insect GTs have been shown to play a fundamental role in other physiological processes like body pigmentation, cuticular tanning, chemosensation and stress response. This review provides a detailed overview of the multifaceted functionality of insect GTs and summarizes numerous case studies associated with it.

Structures and functions of invertebrate glycosylation

Glycosylation refers to the covalent attachment of sugar residues to a protein or lipid, and the biological importance of this modification has been widely recognized. While glycosylation in mammals is being extensively investigated, lower level animals such as invertebrates have not been adequately interrogated for their glycosylation. The rich diversity of invertebrate species, the increased database of sequenced invertebrate genomes and the time and cost efficiency of raising and experimenting on these species have enabled a handful of the species to become excellent model organisms, which have been successfully used as tools for probing various biologically interesting problems. Investigation on invertebrate glycosylation, especially on model organisms, not only expands the structural and functional knowledgebase, but also can facilitate deeper understanding on the biological functions of glycosylation in higher organisms. Here, we reviewed the research advances in invertebrate glycosylation, including N- and O-glycosylation, glycosphingolipids and glycosaminoglycans. The aspects of glycan biosynthesis, structures and functions are discussed, with a focus on the model organisms Drosophila and Caenorhabditis. Analytical strategies for the glycans and glycoconjugates are also summarized.

The glycosphingolipid MacCer promotes synaptic bouton formation in Drosophila by interacting with Wnt

Lipids are structural components of cellular membranes and signaling molecules that are widely involved in development and diseases, but the underlying molecular mechanisms are poorly understood, partly because of the vast variety of lipid species and complexity of synthetic and turnover pathways. From a genetic screen, we identify that mannosyl glucosylceramide (MacCer), a species of glycosphingolipid (GSL), promotes synaptic bouton formation at the Drosophila neuromuscular junction (NMJ). Pharmacological and genetic analysis shows that the NMJ growth-promoting effect of MacCer depends on normal lipid rafts, which are known to be composed of sphingolipids, sterols and select proteins. MacCer positively regulates the synaptic level of Wnt1/Wingless (Wg) and facilitates presynaptic Wg signaling, whose activity is raft-dependent. Furthermore, a functional GSL-binding motif in Wg exhibiting a high affinity for MacCer is required for normal NMJ growth. These findings reveal a novel mechanism whereby the GSL MacCer promotes synaptic bouton formation via Wg signaling.

Structural analysis of neutral glycosphingolipids from the silkworm Bombyx mori and the difference in ceramide composition between larvae and pupae

Glycosphingolipids (GSLs) from the silkworm Bombyx mori were identified and GSL expression patterns between larvae and pupae were compared. The structural analysis of neutral GSLs from dried pupae revealed the following predominant species: Glcβ1Cer, Manβ4Glcβ1Cer, GlcNAcβ3Manβ4Glcβ1Cer, Galβ3Manβ4Glcβ1Cer, GalNAcα4Galβ3Manβ4Glcβ1Cer, GlcNAcβ3Galβ3Manβ4Glcβ1Cer, Galα4Galβ3Manβ4Glcβ1Cer and (GalNAcα4)1-4 GalNAcα4Galβ3Manβ4Glcβ1Cer. Lin-ear elongation of α4-GalNAc was observed at the non-reducing end of Galβ3Manβ4Glcβ1Cer with up to five GalNAc repeats. The arthro-series GSL GlcNAcβ3Manβ4Glcβ1Cer, a characteristic GSL-glycan sequence of other Arthropoda, was detected in silkworms. The main ceramide species in each purified GSL fraction were h20:0-d14:1 and h22:0-d14:1. GSL expression patterns in larvae and pupae were compared using thin-layer chromatography, which demonstrated differences among acidic, polar and neutral GSL fractions, while the zwitterionic fraction showed no difference. Neutral GSLs such as ceramides di-, tri- and tetrasaccharides in larvae showed less abundant than those in pupae. MALDI-TOF MS analysis revealed that larval GSLs contained four types of ceramide species, whereas pupal GSLs contained only two types. The structural analysis of neutral GSLs from silkworms revealed a novel series of GSLs. The comparison of GSL expression patterns between larvae and pupae demonstrated differences in several fractions. Alterations in GSL ceramide composition between larvae and pupae were observed by MALDI-TOF MS analysis.

Distribution of Glycan Motifs at the Surface of Midgut Cells in the Cotton Leafworm (Spodoptera littoralis) Demonstrated by Lectin Binding

Glycans are involved in many biological phenomena, including signal transduction, cell adhesion, immune response or differentiation. Although a few papers have reported on the role of glycans in the development and proper functioning of the insect midgut, no data are available regarding the localization of the glycan structures on the surface of the cells in the gut of insects. In this paper, we analyzed the spatial distribution of glycans present on the surface of the midgut cells in larvae of the cotton leafworm Spodoptera littoralis, an important agricultural pest insect worldwide. For this purpose, we established primary midgut cell cultures, probed these individual cells that are freely suspended in liquid medium with a selection of seven fluorescently labeled lectins covering a range of different carbohydrate binding specificities [mannose oligomers (GNA and HHA), GalNAc/Gal (RSA and SSA), GlcNAc (WGA and Nictaba) and Neu5Ac(α-2,6)Gal/GalNAc (SNA-I)], and visualized the interaction of these lectins with the different zones of the midgut cells using confocal microscopy. Our analysis focused on the typical differentiated columnar cells with a microvillar brush border at their apical side, which are dominantly present in the Lepidopteran midgut and function in food digestion and absorption, and as well as on the undifferentiated stem cells that are important for midgut development and repair. Confocal microscopy analyses showed that the GalNAc/Gal-binding lectins SSA and RSA and the terminal GlcNAc-recognizing WGA bound preferentially to the apical microvillar zone of the differentiated columnar cells as compared to the basolateral pole. The reverse result was observed for the mannose-binding lectins GNA and HHA, as well as Nictaba that binds preferentially to GlcNAc oligomers. Furthermore, differences in lectin binding to the basal and lateral zones of the cell membranes of the columnar cells were apparent. In the midgut stem cells, GNA and Nictaba bound more strongly to the membrane of these undifferentiated cells compared to the microvillar pole of the columnar cells, while SSA, HHA, WGA, and SNA-I showed stronger binding to the microvilli. Our results indicated that polarization of the midgut cells is also reflected by a specific distribution of glycans, especially between the basal and microvillar pole. The data are discussed in relation to the functioning and development of the insect midgut.

Diversity and functions of protein glycosylation in insects

The majority of proteins is modified with carbohydrate structures. This modification, called glycosylation, was shown to be crucial for protein folding, stability and subcellular location, as well as protein-protein interactions, recognition and signaling. Protein glycosylation is involved in multiple physiological processes, including embryonic development, growth, circadian rhythms, cell attachment as well as maintenance of organ structure, immunity and fertility. Although the general principles of glycosylation are similar among eukaryotic organisms, insects synthesize a distinct repertoire of glycan structures compared to plants and vertebrates. Consequently, a number of unique insect glycans mediate functions specific to this class of invertebrates. For instance, the core α1,3-fucosylation of N-glycans is absent in vertebrates, while in insects this modification is crucial for the development of wings and the nervous system. At present, most of the data on insect glycobiology comes from research in Drosophila. Yet, progressively more information on the glycan structures and the importance of glycosylation in other insects like beetles, caterpillars, aphids and bees is becoming available. This review gives a summary of the current knowledge and recent progress related to glycan diversity and function(s) of protein glycosylation in insects. We focus on N- and O-glycosylation, their synthesis, physiological role(s), as well as the molecular and biochemical basis of these processes.

Acsl, the Drosophila ortholog of intellectual-disability-related ACSL4, inhibits synaptic growth by altered lipids

Nervous system development and function are tightly regulated by metabolic processes, including the metabolism of lipids such as fatty acids. Mutations in long-chain acyl-CoA synthetase 4 (ACSL4) are associated with non-syndromic intellectual disabilities. We previously reported that Acsl, the Drosophila ortholog of mammalian ACSL3 and ACSL4, inhibits neuromuscular synapse growth by suppressing bone morphogenetic protein (BMP) signaling. Here, we report that Acsl regulates the composition of fatty acids and membrane lipids, which in turn affects neuromuscular junction (NMJ) synapse development. Acsl mutant brains had a decreased abundance of C16:1 fatty acyls; restoration of Acsl expression abrogated NMJ overgrowth and the increase in BMP signaling. A lipidomic analysis revealed that Acsl suppressed the levels of three lipid raft components in the brain, including mannosyl glucosylceramide (MacCer), phosphoethanolamine ceramide and ergosterol. The MacCer level was elevated in Acsl mutant NMJs and, along with sterol, promoted NMJ overgrowth, but was not associated with the increase in BMP signaling in the mutants. These findings suggest that Acsl inhibits NMJ growth by stimulating C16:1 fatty acyl production and concomitantly suppressing raft-associated lipid levels.

Phenotype-based clustering of glycosylation-related genes by RNAi-mediated gene silencing

Glycan structures are synthesized by a series of reactions conducted by glycosylation-related (GR) proteins such as glycosyltransferases, glycan-modifying enzymes, and nucleotide-sugar transporters. For example, the common core region of glycosaminoglycans (GAGs) is sequentially synthesized by peptide-O-xylosyltransferase, β1,4-galactosyltransferase I, β1,3-galactosyltransferase II, and β1,3-glucuronyltransferase. This raises the possibility that functional impairment of GR proteins involved in synthesis of the same glycan might result in the same phenotypic abnormality. To examine this possibility, comprehensive silencing of genes encoding GR and proteoglycan core proteins was conducted in Drosophila. Drosophila GR candidate genes (125) were classified into five functional groups for synthesis of GAGs, N-linked, O-linked, Notch-related, and unknown glycans. Spatiotemporally regulated silencing caused a range of malformed phenotypes that fell into three types: extra veins, thick veins, and depigmentation. The clustered phenotypes reflected the biosynthetic pathways of GAGs, Fringe-dependent glycan on Notch, and glycans placed at or near nonreducing ends (herein termed terminal domains of glycans). Based on the phenotypic clustering, CG33145 was predicted to be involved in formation of terminal domains. Our further analysis showed that CG33145 exhibited galactosyltransferase activity in synthesis of terminal N-linked glycans. Phenotypic clustering, therefore, has potential for the functional prediction of novel GR genes.

Biochemical membrane lipidomics during Drosophila development

Lipids play critical roles in energy homeostasis, membrane structure, and signaling. Using liquid chromatography and mass spectrometry, we provide a comprehensive semiquantification of lipids during the life cycle of Drosophila melanogaster (230 glycerophospholipids, 210 sphingolipids, 6 sterols and sterol esters, and 60 glycerolipids) and obtain biological insights through this biochemical resource. First, we find a high and constant triacylglycerol-to-membrane lipid ratio during pupal stage, which is nonobvious in the absence of nutrient uptake and tissue remodeling. Second, sphingolipids undergo specific changes in headgroup (glycosylation) and tail configurations (unsaturation and hydroxylation on sphingoid base and fatty acyls, respectively), which correlate with gene expression of known (GlcT/CG6437; FA2H/ CG30502) and putative (Cyt-b5-r/CG13279) enzymes. Third, we identify a gender bias in phosphoethanolamine-ceramides as a lead for future investigation into sexual maturation. Finally, we partially characterize ghiberti, required for male meiotic cytokinesis, as a homolog of mammalian serine palmitoyltransferase.

The N's and O's of Drosophila glycoprotein glycobiology

The past 25 years have seen significant advances in understanding the diversity and functions of glycoprotein glycans in Drosophila melanogaster. Genetic screens have captured mutations that reveal important biological activities modulated by glycans, including protein folding and trafficking, as well as cell signaling, tissue morphogenesis, fertility, and viability. Many of these glycan functions have parallels in vertebrate development and disease, providing increasing opportunities to dissect pathologic mechanisms using Drosophila genetics. Advances in the sensitivity of structural analytic techniques have allowed the glycan profiles of wild-type and mutant tissues to be assessed, revealing novel glycan structures that may be functionally analogous to vertebrate glycans. This review describes a selected set of recent advances in understanding the functions of N-linked and O-linked (non-glycosaminoglycan) glycoprotein glycans in Drosophila with emphasis on their relatedness to vertebrate organisms.

Biochemical studies on sphingolipids of Artemia franciscana: complex neutral glycosphingolipids

Brine shrimp are primitive crustacean arthropodal model organisms, second to daphnia, which can survive in high-salinity environments. Their oviposited cysts, cuticle-covered diapausing eggs, are highly resistant to dryness. To elucidate specialties of brine shrimp, this study characterized glycosphingolipids, which are signal transduction-associated material. A group of novel and complex fucosyl glycosphingolipids were separated and identified from cysts of the brine shrimp Artemia franciscana by repeated lipid extraction, alkaline methanolysis, acid treatment, successive column chromatography, and post-source decay measurements by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Structures of the glycosphingolipids were elucidated by conventional structural characterization and mass spectrometry, and the compounds were identified as GlcNAcβ1-3GalNAcβ1-4(GlcNAcα1-2Fucα1-3)GlcNAcβ1-3Manβ1-4Glcβ1-Cer, GalNAcβ1-4(Fucα1-3)GlcNAcβ1-3GalNAcβ1-4(GlcNAcα1-2Fucα1-3)GlcNAcβ1-3Manβ1-4Glcβ1-Cer, and GalNAcβ1-4(GlcNAcα1-2Fucα1-3)GlcNAcβ1-3GalNAcβ1-4(GlcNAcα1-2Fucα1-3)GlcNAcβ1-3Manβ1-4Glcβ1-Cer. These compounds also contained a branching, non-arthro-series disaccharide with an α-GlcNAc terminus, similar to that found in a previously reported ceramide hexasaccharide (III³(GlcNAcα2Fucα)-At₄Cer). The glycans within these complex GSLs are longer than reported glycans of the animal kingdom containing α-GlcNAc terminus. These complex GSLs as well as the longest GSL with ten sugar residues, ceramide decasaccharide (CDeS), contain the fucosylated LacdiNAc sequence reported to associate with parasitism/immunosuppression and the α-GlcNAc terminus reported to show a certain antibacterial effect in other reports. CDeS, the longest GSL of this species, was found in the highest amount, which indicates that CDeS may be functionally important.

Genetic Interactions Between Drosophila sialyltransferase and β1,4-N-acetylgalactosaminyltransferase-A Genes Indicate Their Involvement in the Same Pathway

Sialylated glycans play a prominent role in the Drosophila nervous system where they are involved in the regulation of neural transmission. However, the functional pathway of sialylation in invertebrates, including Drosophila, remains largely unknown. Here we used a combination of genetic and behavioral approaches to shed light on the Drosophila sialylation pathway. We examined genetic interactions between Drosophila sialyltransferase (DSiaT) and β1,4-N-acetylgalactosaminyltransferase (β4GalNAcT) genes. Our results indicated that β4GalNAcTA and DSiaT cooperate within the same functional pathway that regulates neural transmission. We found that β4GalNAcTA is epistatic to DSiaT. Our data suggest an intriguing possibility that β4GalNAcTA may participate in the biosynthesis of sialylated glycans.

Glycosylated synaptomatrix regulation of trans-synaptic signaling

Synapse formation is driven by precisely orchestrated intercellular communication between the presynaptic and the postsynaptic cell, involving a cascade of anterograde and retrograde signals. At the neuromuscular junction (NMJ), both neuron and muscle secrete signals into the heavily glycosylated synaptic cleft matrix sandwiched between the two synapsing cells. These signals must necessarily traverse and interact with the extracellular environment, for the ligand-receptor interactions mediating communication to occur. This complex synaptomatrix, rich in glycoproteins and proteoglycans, comprises heterogeneous, compartmentalized domains where specialized glycans modulate trans-synaptic signaling during synaptogenesis and subsequent synapse modulation. The general importance of glycans during development, homeostasis and disease is well established, but this important molecular class has received less study in the nervous system. Glycan modifications are now understood to play functional and modulatory roles as ligands and co-receptors in numerous tissues; however, roles at the synapse are relatively unexplored. We highlight here properties of synaptomatrix glycans and glycan-interacting proteins with key roles in synaptogenesis, with a particular focus on recent advances made in the Drosophila NMJ genetic system. We discuss open questions and interesting new findings driving this investigation of complex, diverse, and largely understudied glycan mechanisms at the synapse.

Biochemical studies on sphingolipids of Artemia franciscana: novel neutral glycosphingolipids

Neutral glycosphingolipids containing one to six sugars in their oligosaccharide chains have been isolated from cysts of the brine shrimp Artemia franciscana. The structures of these glycolipids were identified by methylation analysis, partial acid hydrolysis, gas-liquid chromatography, combined gas-liquid chromatography-mass spectrometry, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, and proton nuclear magnetic resonance spectroscopy to be Glcβ1-Cer, Manβ1-4Glcβ1-Cer, Fucα1-3Manβ1-4Glcβ1-Cer, GlcNAcβ1-3Manβ1-4Glcβ1-Cer, GlcNAcα1-2Fucα1-3Manβ1-4Glcβ1-Cer, GalNAcβ1-4GlcNAcβ1-3Manβ1-4Glcβ1-Cer, GalNAcβ1-4(Fucα1-3)GlcNAcβ1-3Manβ1-4Glcβ1-Cer (CPS), and GalNAcβ1-4(GlcNAcα1-2Fucα1-3)GlcNAcβ1-3Manβ1-4Glcβ1-Cer (CHS). Two glycosphingolipids, CPS and CHS, were characterized as novel structures. Because Artemia contains a certain series of glycosphingolipids (-Fucα3Manβ4GlcβCer), which differ from the core sugar sequences reported thus far, we tentatively designated the glycosphingolipids characterized as nonarthro-series ones. Furthermore, CHS exhibited a hybrid structure of arthro-series and nonarthro-series sugar chain. Two novel glycosphingolipids were characterized from the brine shrimp Artemia franciscana; one was composed of arthrotetraose and a branching fucose attached to N-acetylglucosamine residue, and the other was composed of CPS with an additional N-acetylglucosamine residue attached to the branching fucose.

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.

Notch ligand activity is modulated by glycosphingolipid membrane composition in Drosophila melanogaster

Endocytosis of the transmembrane ligands Delta (Dl) and Serrate (Ser) is required for the proper activation of Notch receptors. The E3 ubiquitin ligases Mindbomb1 (Mib1) and Neuralized (Neur) regulate the ubiquitination of Dl and Ser and thereby promote both ligand endocytosis and Notch receptor activation. In this study, we identify the alpha1,4-N-acetylgalactosaminyltransferase-1 (alpha4GT1) gene as a gain of function suppressor of Mib1 inhibition. Expression of alpha4GT1 suppressed the signaling and endocytosis defects of Dl and Ser resulting from the inhibition of mib1 and/or neur activity. Genetic and biochemical evidence indicate that alpha4GT1 plays a regulatory but nonessential function in Notch signaling via the synthesis of a specific glycosphingolipid (GSL), N5, produced by alpha4GT1. Furthermore, we show that the extracellular domain of Ser interacts with GSLs in vitro via a conserved GSL-binding motif, raising the possibility that direct GSL-protein interactions modulate the endocytosis of Notch ligands. Together, our data indicate that specific GSLs modulate the signaling activity of Notch ligands.

Roles of glycosylation in Notch signaling

Notch and the DSL Notch ligands Delta and Serrate/Jagged are glycoproteins with a single transmembrane domain. The extracellular domain (ECD) of both Notch receptors and Notch ligands contains numerous epidermal growth factor (EGF)-like repeats which are post-translationally modified by a variety of glycans. Inactivation of a subset of genes that encode glycosyltransferases which initiate and elongate these glycans inhibits Notch signaling. In the formation of developmental boundaries in Drosophila and mammals, in mouse T-cell and marginal zone B-cell development, and in co-culture Notch signaling assays, the regulation of Notch signaling by glycans is to date a cell-autonomous effect of the Notch-expressing cell. The regulation of Notch signaling by glycans represents a new paradigm of signal transduction. O-fucose glycans modulate the strength of Notch binding to DSL Notch ligands, while O-glucose glycans facilitate juxta-membrane cleavage of Notch, generating the substrate for intramembrane cleavage and Notch activation. Identifying precisely how the addition of particular sugars at specific locations on Notch modifies Notch signaling is a challenge for the future.

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.

Pathophysiological actions of neuropathy-related anti-ganglioside antibodies at the neuromuscular junction

The outer leaflet of neuronal membranes is highly enriched in gangliosides. Therefore, specific neuronal roles have been attributed to this family of sialylated glycosphingolipids, e.g. in modulation of ion channels and transporters, neuronal interaction and recognition, temperature adaptation, Ca(2+) homeostasis, axonal growth, (para)node of Ranvier stability and synaptic transmission. Recent developmental, ageing and injury studies on transgenic mice lacking subsets of gangliosides indicate that gangliosides are involved in maintenance rather than development of the nervous system and that ganglioside family members are able to act in a mutually compensatory manner. Besides having physiological functions, gangliosides are the likely antigenic targets of autoantibodies present in Guillain-Barré syndrome (GBS), a group of neuropathies with clinical symptoms of motor- and/or sensory peripheral nerve dysfunction. Antibody binding to peripheral nerves is thought to either interfere with ganglioside function or activate complement, causing axonal damage and thereby disturbed action potential conduction. The presynaptic motor nerve terminal at the neuromuscular junction (NMJ) may be a prominent target because it is highly enriched in gangliosides and lies outside the blood-nerve barrier, allowing antibody access. The ensuing neuromuscular synaptopathy might contribute to the muscle weakness in GBS patients. Several groups, including our own, have studied the effects of anti-ganglioside antibodies in ex vivo and in vivo experimental settings at mouse NMJs. Here, after providing a background overview on ganglioside synthesis, localization and physiology, we will review those studies, which clearly show that anti-ganglioside antibodies are capable of binding to NMJs and thereby can exert a variety of pathophysiological effects. Furthermore, we will discuss the human clinical electrophysiological and histological evidence produced so far of the existence of a neuromuscular synaptopathy contributing to muscle weakness in GBS patients.

Drosophila alpha-1,4-glycosyltransferase (alpha-4GT1) inhibits reaper-mediated apoptosis in the eye

In a genetic screen, alpha-4GT1 has been identified as a potential enhancer of Hairless-mediated cell death in the eye of Drosophila. alpha-4GT1 encodes an alpha-1,4-glycosyltransferase, known to catalyze the fifth step in a series of ceramide glycosylation events. As reported for other enzymes involved in the glycosylation of ceramide, alpha-4GT1 is strongly expressed during oogenesis and is deposited maternally in the egg. Moreover, the protein is enriched at cell membranes. Unexpectedly, overexpression of alpha-4GT1 does not enhance Hairless-mediated cell death; instead, Hairless enhancement is caused by an allele of Scutoid present on the alpha-4GT1 chromosome. Interestingly, the downregulation of alpha-4GT1 during eye development amplifies cell death induction by the pro-apoptotic gene reaper. Accordingly, overexpression of alpha-4GT1 represses reaper-induced cell death. Thus, alpha-4GT1 appears to be an inhibitor of apoptosis, as has previously been observed for other ceramide glycosylating enzymes, suggesting that it likewise contributes to ceramide anchoring in the membrane.

Expression of ceramide glucosyltransferases, which are essential for glycosphingolipid synthesis, is only required in a small subset of C. elegans cells

Glycosphingolipids (GSLs) are glycosylated derivatives of ceramide in the lipid bilayer. Their ubiquitous distribution and complexity suggest that they have important functions, but what these are in vivo is still poorly understood. Here, we characterize the phenotype of Caenorhabditis elegans mutants with essentially no GSLs. The C. elegans genome encodes three ceramide glucosyltransferase (CGT) genes, which encode enzymes required for GSL biosynthesis. Animals lacking CGT do not synthesize GSLs, arrest growth at the first larval stage, and display defects in a subset of cells in their digestive tract; these defects impair larval feeding, resulting in a starvation-induced growth arrest. Restoring CGT function in these digestive tract cells - but not in a variety of other tissues - is sufficient to rescue the phenotypes associated with loss of CGT function. These unexpected findings suggest that GSLs are dispensable in most C. elegans cells, including those of the nervous system.

Glycosphingolipids control the extracellular gradient of the Drosophila EGFR ligand Gurken

Glycosphingolipids (GSLs) are present in all eukaryotic membranes and are implicated in neuropathologies and tumor progression in humans. Nevertheless, their in vivo functions remain poorly understood in vertebrates, partly owing to redundancy in the enzymes elongating their sugar chains. In Drosophila, a single GSL biosynthetic pathway is present that relies on the activity of the Egghead and Brainiac glycosyltransferases. Mutations in these two enzymes abolish GSL elongation and yield oogenesis defects, providing a unique model system in which to study GSL roles in signaling in vivo. Here, we use egghead and brainiac mutants to show that GSLs are necessary for full activation of the EGFR pathway during oogenesis in a time-dependent manner. In contrast to results from in vitro studies, we find that GSLs are required in cells producing the TGFalpha-like ligand Gurken, but not in EGFR-expressing cells. Strikingly, we find that GSLs are not essential for Gurken trafficking and secretion. However, we characterize for the first time the extracellular Gurken gradient and show that GSLs affect its formation by controlling Gurken planar transport in the extracellular space. This work presents the first in vivo evidence that GSLs act in trans to regulate the EGFR pathway and shows that extracellular EGFR ligand distribution is tightly controlled by GSLs. Our study assigns a novel role for GSLs in morphogen diffusion, possibly through regulation of their conformation.

Optic lobe development

The optic lobes comprise approximately half of the fly's brain. In four major synaptic ganglia, or neuropils, the visual input from the compound eyes is received and processed for higher order visual functions like motion detection and color vision. A common characteristic of vertebrate and invertebrate visual systems is the point-to-point mapping of the visual world to synaptic layers in the brain, referred to as visuotopy. Vision requires the parallel extraction of numerous parameters in a visuotopic manner. Consequently, the optic neuropils are arranged in columns and perpendicularly oriented synaptic layers that allow for the selective establishment of synapses between columnar neurons. How this exquisite synaptic specificity is established during approximately 100 hours of brain development is still poorly understood. However, the optic lobe contains one of the best characterized brain structures in any organism-both anatomically and developmentally. Moreover, numerous molecules and their function illuminate some of the basic mechanisms involved in brain wiring. The emerging picture is that the development of the visual system of Drosophila is (epi-)genetically hard-wired; it supplies the emerging fly with vision without requiring neuronal activity for fine tuning of neuronal connectivity. Elucidating the genetic and cellular principles by which gene activity directs the assembly of the optic lobe is therefore a fascinating task and the focus of this chapter.

Glycobiology on the fly: developmental and mechanistic insights from Drosophila

Drosophila melanogaster offers many unique advantages for deciphering the complexities of glycan biosynthesis and function. The completion of the Drosophila genome sequencing project as well as the comprehensive catalogue of existing mutations and phenotypes have lead to a prolific database where many of the genes involved in glycan synthesis, assembly, modification, and recognition have been identified and characterized. Recent biochemical and molecular studies have elucidated the structure of the glycans present in Drosophila. Powerful genetic approaches have uncovered a number of critical biological roles for glycans during development that impact on our understanding of their function during mammalian development. Here, we summarize key recent findings and provide evidence for the usefulness of this model organism in unraveling the complexities of glycobiology across many species.

Neuromuscular synaptic function in mice lacking major subsets of gangliosides

Gangliosides are a family of sialylated glycosphingolipids enriched in the outer leaflet of neuronal membranes, in particular at synapses. Therefore, they have been hypothesized to play a functional role in synaptic transmission. We have measured in detail the electrophysiological parameters of synaptic transmission at the neuromuscular junction (NMJ) ex vivo of a GD3-synthase knockout mouse, expressing only the O- and a-series gangliosides, as well as of a GM2/GD2-synthase*GD3-synthase double-knockout (dKO) mouse, lacking all gangliosides except GM3. No major synaptic deficits were found in either null-mutant. However, some extra degree of rundown of acetylcholine release at high intensity use was present at the dKO NMJ and a temperature-specific increase in acetylcholine release at 35 degrees C was observed in GD3-synthase knockout NMJs, compared with wild-type. These results indicate that synaptic transmission at the NMJ is not crucially dependent on the particular presence of most ganglioside family members and remains largely intact in the sole presence of GM3 ganglioside. Rather, presynaptic gangliosides appear to play a modulating role in temperature- and use-dependent fine-tuning of transmitter output.

Identification and characterization by electrospray mass spectrometry of endogenous Drosophila sphingadienes

Sphingolipids comprise a complex group of lipids concentrated in membrane rafts and whose metabolites function as signaling molecules. Sphingolipids are conserved in Drosophila, in which their tight regulation is required for proper development and tissue integrity. In this study, we identified a new family of Drosophila sphingolipids containing two double bonds in the long chain base (LCB). The lipids were found at low levels in wild-type flies and accumulated markedly in Drosophila Sply mutants, which do not express sphingosine-1-phosphate lyase and are defective in sphingolipid catabolism. To determine the identity of the unknown lipids, purified whole fly lipid extracts were separated on a C18-HPLC column and analyzed using electrospray mass spectrometry. The lipids contain a LCB of either 14 or 16 carbons with conjugated double bonds at C4,6. The Delta(4,6)-sphingadienes were found as free LCBs, as phosphorylated LCBs, and as the sphingoid base in ceramides. The temporal and spatial accumulation of Delta(4,6)-sphingadienes in Sply mutants suggests that these lipids may contribute to the muscle degeneration observed in these flies.

Regulation of Notch signaling by glycosylation

Notch receptors are approximately 300 kDa cell surface glycoproteins whose activation by Notch ligands regulates cell fate decisions in the metazoa. The extracellular domain of Notch receptors has many epidermal growth factor like repeats that are glycosylated with O-fucose and O-glucose glycans as well as N-glycans. Disruption of O-fucose glycan synthesis leads to severe Notch signaling defects in Drosophila and mammals. Removal or addition of O-fucose glycan consensus sites on Notch receptors also leads to Notch signaling defects. Ligand binding and ligand-induced Notch signaling assays have provided insights into how changes in the O-fucose glycans of Notch receptors alter Notch signaling.

Sphingolipids and membrane biology as determined from genetic models

The importance of sphingolipids in membrane biology was appreciated early in the twentieth century when several human inborn errors of metabolism were linked to defects in sphingolipid degradation. The past two decades have seen an explosion of information linking sphingolipids with cellular processes. Studies have unraveled mechanistic details of the sphingolipid metabolic pathways, and these findings are being exploited in the development of novel therapies, some now in clinical trials. Pioneering work in yeast has laid the foundation for identifying genes encoding the enzymes of the pathways. The advent of the era of genomics and bioinformatics has led to the identification of homologous genes in other species and the subsequent creation of animal knock-out lines for these genes. Discoveries from these efforts have re-kindled interest in the role of sphingolipids in membrane biology. This review highlights some of the recent advances in understanding sphingolipids' roles in membrane biology as determined from genetic models.

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.