Functional UDP-xylose transport across the endoplasmic reticulum/Golgi membrane in a Chinese hamster ovary cell mutant defective in UDP-xylose Synthase

Xylosyltransferase Bump-and-hole Engineering to Chemically Manipulate Proteoglycans in Mammalian Cells

Mammalian cells orchestrate signalling through interaction events on their surfaces. Proteoglycans are an intricate part of these interactions, carrying large glycosaminoglycan polysaccharides that recruit signalling molecules. Despite their importance in development, cancer and neurobiology, a relatively small number of proteoglycans have been identified. In addition to the complexity of glycan extension, biosynthetic redundancy in the first protein glycosylation step by two xylosyltransferase isoenzymes XT1 and XT2 complicates annotation of proteoglycans. Here, we develop a chemical genetic strategy that manipulates the glycan attachment site of cellular proteoglycans. By employing a tactic termed bump- and-hole engineering, we engineer the two isoenzymes XT1 and XT2 to specifically transfer a chemically modified xylose analogue to target proteins. The chemical modification contains a bioorthogonal tag, allowing the ability to visualise and profile target proteins modified by both transferases in mammalian cells. The versatility of our approach allows pinpointing glycosylation sites by tandem mass spectrometry, and exploiting the chemical handle to manufacture proteoglycans with defined GAG chains for cellular applications. Engineered XT enzymes permit a view into proteoglycan biology that is orthogonal to conventional techniques in biochemistry.

Metabolism-driven glycosylation represents therapeutic opportunities in interstitial lung diseases

Metabolic changes are coupled with alteration in protein glycosylation. In this review, we will focus on macrophages that are pivotal in the pathogenesis of pulmonary fibrosis and sarcoidosis and thanks to their adaptable metabolism are an attractive therapeutic target. Examples presented in this review demonstrate that protein glycosylation regulates metabolism-driven immune responses in macrophages, with implications for fibrotic processes and granuloma formation. Targeting proteins that regulate glycosylation, such as fucosyltransferases, neuraminidase 1 and chitinase 1 could effectively block immunometabolic changes driving inflammation and fibrosis, providing novel avenues for therapeutic interventions.

Proteomic Investigation of Differential Interactomes of Glypican 1 and a Putative Disease-Modifying Variant of Ataxia

In a currently 13-year-old girl of consanguineous Turkish parents, who developed unsteady gait and polyneuropathy at the ages of 3 and 6 years, respectively, we performed whole genome sequencing and identified a biallelic missense variant c.424C>T, p.R142W in glypican 1 (GPC1) as a putative disease-associated variant. Up to date, GPC1 has not been associated with a neuromuscular disorder, and we hypothesized that this variant, predicted as deleterious, may be causative for the disease. Using mass spectrometry-based proteomics, we investigated the interactome of GPC1 WT and the missense variant. We identified 198 proteins interacting with GPC1, of which 16 were altered for the missense variant. This included CANX as well as vacuolar ATPase (V-ATPase) and the mammalian target of rapamycin complex 1 (mTORC1) complex members, whose dysregulation could have a potential impact on disease severity in the patient. Importantly, these proteins are novel interaction partners of GPC1. At 10.5 years, the patient developed dilated cardiomyopathy and kyphoscoliosis, and Friedreich's ataxia (FRDA) was suspected. Given the unusually severe phenotype in a patient with FRDA carrying only 104 biallelic GAA repeat expansions in FXN, we currently speculate that disturbed GPC1 function may have exacerbated the disease phenotype. LC-MS/MS data are accessible in the ProteomeXchange Consortium (PXD040023).

Two nucleotide sugar transporters are important for cell wall integrity and full virulence of Magnaporthe oryzae

Cell wall polysaccharides play key roles in fungal development, virulence, and resistance to the plant immune system, and are synthesized from many nucleotide sugars in the endoplasmic reticulum (ER)-Golgi secretory system. Nucleotide sugar transporters (NSTs) are responsible for transporting cytosolic-derived nucleotide sugars to the ER lumen for processing, but their roles in plant-pathogenic fungi remain to be revealed. Here, we identified two important NSTs, NST1 and NST2, in the rice blast fungus Magnaporthe oryzae. Both NSTs were localized in the ER, which was consistent with a function in transporting nucleotide sugar for processing in the ER. Sugar transport property analysis suggested that NST1 is involved in transportation of mannose and glucose, while NST2 is only responsible for mannose transportation. Accordingly, deletion of NSTs resulted in a significant decrease in corresponding soluble saccharides abundance and defect in sugar utilization. Moreover, both NSTs played important roles in cell wall integrity, were involved in asexual development, and were required for full virulence. The NST mutants exhibited decreasing external glycoproteins and exposure of inner chitin, which resulted in activation of the host defence response. Altogether, our results revealed that two sugar transporters are required for fungal cell wall polysaccharides accumulation and full virulence of M. oryzae.

Collision-induced dissociation of xylose and its applications in linkage and anomericity identification

Different dehydration barrier heights result in different branching ratio, a simple and fast anomeric configuration identification for xylose.

The carboxyl-terminal di-lysine motif is essential for catalytic activity of UDP-glucuronosyltransferase 1A9

UDP-Glucuronosyltransferase (UGT) is a type I membrane protein localized to the endoplasmic reticulum (ER). UGT has a di-lysine motif (KKXX/KXKXX) in its cytoplasmic domain, which is defined as an ER retention signal. However, our previous study has revealed that UGT2B7, one of the major UGT isoform in human, localizes to the ER in a manner that is independent of this motif. In this study, we focused on another UGT isoform, UGT1A9, and investigated the role of the di-lysine motif in its ER localization, glucuronidation activity, and homo-oligomer formation. Immunofluorescence microscopy indicated that the cytoplasmic domain of UGT1A9 functioned as an ER retention signal in a chimeric protein with CD4, but UGT1A9 itself could localize to the ER in a di-lysine motif-independent manner. In addition, UGT1A9 formed homo-oligomers in the absence of the motif. However, deletion of the di-lysine motif or substitution of lysines in the motif for alanines, severely impaired glucuronidation activity of UGT1A9. This is the first study that re-defines the cytoplasmic di-lysine motif of UGT as an essential peptide for retaining glucuronidation capacity.

Integration of Sugar Metabolism and Proteoglycan Synthesis by UDP-glucose Dehydrogenase

Regulation of proteoglycan and glycosaminoglycan synthesis is critical throughout development, and to maintain normal adult functions in wound healing and the immune system, among others. It has become increasingly clear that these processes are also under tight metabolic control and that availability of carbohydrate and amino acid metabolite precursors has a role in the control of proteoglycan and glycosaminoglycan turnover. The enzyme uridine diphosphate (UDP)-glucose dehydrogenase (UGDH) produces UDP-glucuronate, an essential precursor for new glycosaminoglycan synthesis that is tightly controlled at multiple levels. Here, we review the cellular mechanisms that regulate UGDH expression, discuss the structural features of the enzyme, and use the structures to provide a context for recent studies that link post-translational modifications and allosteric modulators of UGDH to its function in downstream pathways.

Proteoglycan-Dependent Endo-Lysosomal Fusion Affects Intracellular Survival of Salmonella Typhimurium in Epithelial Cells

Proteoglycans (PGs) are glycoconjugates which are predominately expressed on cell surfaces and consist of glycosaminoglycans (GAGs) linked to a core protein. An initial step of GAGs assembly is governed by the β-D-xylosyltransferase enzymes encoded in mammals by the XylT1/XylT2 genes. PGs are essential for the interaction of a cell with other cells as well as with the extracellular matrix. A number of studies highlighted a role of PGs in bacterial adhesion, invasion, and immune response. In this work, we investigated a role of PGs in Salmonella enterica serovar Typhimurium (S. Typhimurium) infection of epithelial cells. Gentamicin protection and chloroquine resistance assays were applied to assess invasion and replication of S. Typhimurium in wild-type and xylosyltransferase-deficient (ΔXylT2) Chinese hamster ovary (CHO) cells lacking PGs. We found that S. Typhimurium adheres to and invades CHO WT and CHO ΔXylT2 cells at comparable levels. However, 24 h after infection, proteoglycan-deficient CHO ΔXylT2 cells are significantly less colonized by S. Typhimurium compared to CHO WT cells. This proteoglycan-dependent phenotype could be rescued by addition of PGs to the cell culture medium, as well as by complementation of the XylT2 gene. Chloroquine resistance assay and immunostaining revealed that in the absence of PGs, significantly less bacteria are associated with Salmonella-containing vacuoles (SCVs) due to a re-distribution of endocytosed gentamicin. Inhibition of endo-lysosomal fusion by a specific inhibitor of phosphatidylinositol phosphate kinase PIKfyve significantly increased S. Typhimurium burden in CHO ΔXylT2 cells demonstrating an important role of PGs for PIKfyve dependent vesicle fusion which is modulated by Salmonella to establish infection. Overall, our results demonstrate that PGs influence survival of intracellular Salmonella in epithelial cells via modulation of PIKfyve-dependent endo-lysosomal fusion.

Making sense out of missense mutations: Mechanistic dissection of Notch receptors through structure-function studies in Drosophila

Notch signaling is involved in the development of almost all organ systems and is required post-developmentally to modulate tissue homeostasis. Rare variants in Notch signaling pathway genes are found in patients with rare Mendelian disorders, while unique or recurrent somatic mutations in a similar set of genes are identified in cancer. The human genome contains four genes that encode Notch receptors, NOTCH1-4, all of which are linked to genetic diseases and cancer. Although some mutations have been classified as clear loss- or gain-of-function alleles based on cellular or rodent based assay systems, the functional consequence of many variants/mutations in human Notch receptors remain unknown. In this review, I will first provide an overview of the domain structure of Notch receptors and discuss how each module is known to regulate Notch signaling activity in vivo using the Drosophila Notch receptor as an example. Next, I will introduce some interesting mutant alleles that have been isolated in the fly Notch gene over the past > 100 years of research and discuss how studies of these mutations have facilitated the understanding of Notch biology. By identifying unique alleles of the fly Notch gene through forward genetic screens, mapping their molecular lesions and characterizing their phenotypes in depth, one can begin to unravel new mechanistic insights into how different domains of Notch fine-tune signaling output. Such information can be useful in deciphering the functional consequences of rare variants/mutations in human Notch receptors, which in turn can influence disease management and therapy.

Hysteresis and Allostery in Human UDP-Glucose Dehydrogenase Require a Flexible Protein Core

Human UDP-glucose dehydrogenase (hUGDH) oxidizes UDP-glucose to UDP-glucuronic acid, an essential substrate in the phase II metabolism of drugs. The activity of hUGDH is regulated by the conformation of a buried allosteric switch (T131 loop/α6 helix). Substrate binding induces the allosteric switch to slowly isomerize from an inactive E* conformation to the active E state, which can be observed as enzyme hysteresis. When the feedback inhibitor UDP-xylose binds, the allosteric switch and surrounding residues in the protein core repack, converting the hexamer into an inactive, horseshoe-shaped complex (EΩ). This allosteric transition is facilitated by large cavities and declivities in the protein core that provide the space required to accommodate the alternate packing arrangements. Here, we have used the A104L substitution to fill a cavity in the E state and sterically prevent repacking of the core into the EΩ state. Steady state analysis shows that hUGDHA104L binds UDP-xylose with lower affinity and that the inhibition is no longer cooperative. This means that the allosteric transition to the high-UDP-xylose affinity EΩ state is blocked by the substitution. The crystal structures of hUGDHA104L show that the allosteric switch still adopts the E and E* states, albeit with a more rigid protein core. However, the progress curves of hUGDHA104L do not show hysteresis, which suggests that the E* and E states are now in rapid equilibrium. Our data suggest that hysteresis in native hUGDH originates from the conformational entropy of the E* state protein core.

Structural basis of Notch O-glucosylation and O-xylosylation by mammalian protein-O-glucosyltransferase 1 (POGLUT1)

Protein O-glucosyltransferase 1/Rumi-mediated glucosylation of Notch epidermal growth factor-like (EGF-like) domains plays an important role in Notch signaling. Protein O-glucosyltransferase 1 shows specificity for folded EGF-like domains, it can only glycosylate serine residues in the C¹XSXPC² motif, and it possesses an uncommon dual donor substrate specificity. Using several EGF-like domains and donor substrate analogs, we have determined the structures of human Protein O-glucosyltransferase 1 substrate/product complexes that provide mechanistic insight into the basis for these properties. Notably, we show that Protein O-glucosyltransferase 1’s requirement for folded EGF-like domains also leads to its serine specificity and that two distinct local conformational states are likely responsible for its ability to transfer both glucose and xylose. We also show that Protein O-glucosyltransferase 1 possesses the potential to xylosylate a much broader range of EGF-like domain substrates than was previously thought. Finally, we show that Protein O-glucosyltransferase 1 has co-evolved with EGF-like domains of the type found in Notch.

POGLUT1 is a protein-O-glucosyltransferase that transfers glucose and xylose to the EGF-like domains of Notch and other signaling receptors. Here the authors report the structure of human POGLUT1 in complexes with 3 different EGF-like domains and donor substrates and shed light on the enzyme’s substrate specificity and catalytic mechanism

Engineering of CHO Cells for the Production of Recombinant Glycoprotein Vaccines with Xylosylated N-glycans

Xylose is a general component of O-glycans in mammals. Core-xylosylation of N-glycans is only found in plants and helminth. Consequently, xylosylated N-glycans cause immunological response in humans. We have used the F-protein of the human respiratory syncytial virus (RSV), one of the main causes of respiratory tract infection in infants and elderly, as a model protein for vaccination. The RSV-F protein was expressed in CHO-DG44 cells, which were further modified by co-expression of β1,2-xylosyltransferase from Nicotiana tabacum. Xylosylation of RSV-F N-glycans was shown by monosaccharide analysis and MALDI-TOF mass spectrometry. In immunogenic studies with a human artificial lymph node model, the engineered RSV-F protein revealed improved vaccination efficacy.

Dual Roles of O-Glucose Glycans Redundant with Monosaccharide O-Fucose on Notch in Notch Trafficking

Notch is a transmembrane receptor that mediates cell-cell interactions and controls various cell-fate specifications in metazoans. The extracellular domain of Notch contains multiple epidermal growth factor (EGF)-like repeats. At least five different glycans are found in distinct sites within these EGF-like repeats. The function of these individual glycans in Notch signaling has been investigated, primarily by disrupting their individual glycosyltransferases. However, we are just beginning to understand the potential functional interactions between these glycans. Monosaccharide O-fucose and O-glucose trisaccharide (O-glucose-xylose-xylose) are added to many of the Notch EGF-like repeats. In Drosophila, Shams adds a xylose specifically to the monosaccharide O-glucose. We found that loss of the terminal dixylose of O-glucose-linked saccharides had little effect on Notch signaling. However, our analyses of double mutants of shams and other genes required for glycan modifications revealed that both the monosaccharide O-glucose and the terminal dixylose of O-glucose-linked saccharides function redundantly with the monosaccharide O-fucose in Notch activation and trafficking. The terminal dixylose of O-glucose-linked saccharides and the monosaccharide O-glucose were required in distinct Notch trafficking processes: Notch transport from the apical plasma membrane to adherens junctions, and Notch export from the endoplasmic reticulum, respectively. Therefore, the monosaccharide O-glucose and terminal dixylose of O-glucose-linked saccharides have distinct activities in Notch trafficking, although a loss of these activities is compensated for by the presence of monosaccharide O-fucose. Given that various glycans attached to a protein motif may have redundant functions, our results suggest that these potential redundancies may lead to a serious underestimation of glycan functions.

Emerging tools to study proteoglycan function during skeletal development

In the past 20years, appreciation for the varied roles of proteoglycans (PGs), which are specific types of sugar-coated proteins, has increased dramatically. PGs in the extracellular matrix were long known to impart structural functions to many tissues, especially articular cartilage, which cushions bones and allows mobility at skeletal joints. Indeed, osteoarthritis is a debilitating disease associated with loss of PGs in articular cartilage. Today, however, PGs have a demonstrated role in cell biological processes, such as growth factor signalling, prompting new perspectives on the etiology of PG-associated diseases. Here, we review diseases associated with defects in PG synthesis and sulfation, also highlighting current understanding of the underlying genetics, biochemistry, and cell biology. Since most research has analyzed a class of PGs called heparan sulfate PGs, more attention is paid here to studies of chondroitin sulfate PGs (CSPGs), which are abundant in cartilage. Interestingly, CSPG synthesis is tightly linked to the cell biological processes of secretion and lysosomal degradation, suggesting that these systems may be linked genetically. Animal models of loss of CSPG function have revealed CSPGs to impact skeletal development. Specifically, our work from a mutagenesis screen in zebrafish led to the hypothesis that cartilage PGs normally delay the timing of endochondral ossification. Finally, we outline emerging approaches in zebrafish that may revolutionize the study of cartilage PG function, including transgenic methods and novel imaging techniques. Our recent work with X-ray fluorescent imaging, for example, enables direct correlation of PG function with PG-dependent biological processes.

Small molecules enhance functional O-mannosylation of Alpha-dystroglycan

Alpha-dystroglycan (α-DG), a highly glycosylated receptor for extracellular matrix proteins, plays a critical role in many biological processes. Hypoglycosylation of α-DG results in various types of muscular dystrophies and is also highly associated with progression of majority of cancers. Currently, there are no effective treatments for those devastating diseases. Enhancing functional O-mannosyl glycans (FOG) of α-DG on the cell surfaces is a potential approach to address this unmet challenge. Based on the hypothesis that the cells can up-regulate FOG of α-DG in response to certain chemical stimuli, we developed a cell-based high-throughput screening (HTS) platform for searching chemical enhancers of FOG of α-DG from a large chemical library with 364,168 compounds. Sequential validation of the hits from a primary screening campaign and chemical works led to identification of a cluster of compounds that positively modulate FOG of α-DG on various cell surfaces including patient-derived myoblasts. These compounds enhance FOG of α-DG by almost ten folds, which provide us powerful tools for O-mannosylation studies and potential starting points for the development of drug to treat dystroglycanopathy.

Chemistry of xylopyranosides

Xylose is one of the few monosaccharidic building blocks that are used by mammalian cells. In comparison with other monosaccharides, xylose is rather unusual and, so far, only found in two different mammalian structures, i.e. in the Notch receptor and as the linker between protein and glycosaminoglycan (GAG) chains in proteoglycans. Interestingly, simple soluble xylopyranosides can not only initiate the biosynthesis of soluble GAG chains but also function as inhibitors of important enzymes in the biosynthesis of proteoglycans. Furthermore, xylose is a major constituent of hemicellulosic xylans and thus one of the most abundant carbohydrates on Earth. Altogether, this has spurred a strong interest in xylose chemistry. The scope of this review is to describe synthesis of xylopyranosyl donors, as well as protective group chemistry, modifications, and conformational analysis of xylose.

Identification and Characterization of a Golgi-Localized UDP-Xylose Transporter Family from Arabidopsis

Most glycosylation reactions require activated glycosyl donors in the form of nucleotide sugars to drive processes such as posttranslational modifications and polysaccharide biosynthesis. Most plant cell wall polysaccharides are biosynthesized in the Golgi apparatus from cytosolic-derived nucleotide sugars, which are actively transferred into the Golgi lumen by nucleotide sugar transporters (NSTs). An exception is UDP-xylose, which is biosynthesized in both the cytosol and the Golgi lumen by a family of UDP-xylose synthases. The NST-based transport of UDP-xylose into the Golgi lumen would appear to be redundant. However, employing a recently developed approach, we identified three UDP-xylose transporters in the Arabidopsis thaliana NST family and designated them UDP-XYLOSE TRANSPORTER1 (UXT1) to UXT3. All three transporters localize to the Golgi apparatus, and UXT1 also localizes to the endoplasmic reticulum. Mutants in UXT1 exhibit ∼30% reduction in xylose in stem cell walls. These findings support the importance of the cytosolic UDP-xylose pool and UDP-xylose transporters in cell wall biosynthesis.

Transporter-mediated uptake of UDP-glucuronic acid by human liver microsomes: assay conditions, kinetics, and inhibition

This study characterized the kinetics, variability, and factors that affect UDP-glucuronic acid (UDP-GlcUA) uptake by human liver microsomes (HLM). Biphasic kinetics were observed for UDP-GlcUA uptake by HLM. Uptake affinities (assessed as Kd) of the high- and low-affinity components differed by more than an order of magnitude (13 ± 6 vs. 374 ± 175 µM), but were comparable in terms of the maximal rate of uptake, with mean Vmax values differing less than 2.3-fold (56 ± 26 vs. 131 ± 35 pmol/min per mg). Variability in total intrinsic transporter activity (Uint) for microsomal UDP-GlcUA uptake across 12 livers was less than 4-fold. Experiments performed to optimize the conditions for microsomal UDP-GlcUA uptake demonstrated that both components were trans-stimulated by preloading (luminal addition) with an alternate UDP-sugar, and essentially abolished by the thiol-alkylating agent N-ethylmaleimide. Furthermore, interaction studies undertaken with a panel of drugs, alternate UDP-sugars, and glucuronide conjugates, at low (2.5 μM) and high (1000 μM) UDP-GlcUA concentrations, demonstrated that both components were inhibited to varying extents. Notably, the nucleoside analogs zidovudine, stavudine, lamivudine, and acyclovir inhibited both the high- and low- affinity components of microsomal UDP-GlcUA uptake by >45% at an inhibitor concentration of 100 μM. Taken together, these data demonstrate that human liver microsomal UDP-GlcUA uptake involves multiple protein-mediated components, and raises the possibility of impaired in vivo glucuronidation activity resulting from inhibition of UDP-GlcUA uptake into the endoplasmic reticulum membrane by drugs and other compounds.

The glucuronyltransferase B4GAT1 is required for initiation of LARGE-mediated α-dystroglycan functional glycosylation

Dystroglycan is a cell membrane receptor that organizes the basement membrane by binding ligands in the extracellular matrix. Proper glycosylation of the α-dystroglycan (α-DG) subunit is essential for these activities, and lack thereof results in neuromuscular disease. Currently, neither the glycan synthesis pathway nor the roles of many known or putative glycosyltransferases that are essential for this process are well understood. Here we show that FKRP, FKTN, TMEM5 and B4GAT1 (formerly known as B3GNT1) localize to the Golgi and contribute to the O-mannosyl post-phosphorylation modification of α-DG. Moreover, we assigned B4GAT1 a function as a xylose β1,4-glucuronyltransferase. Nuclear magnetic resonance studies confirmed that a glucuronic acid β1,4-xylose disaccharide synthesized by B4GAT1 acts as an acceptor primer that can be elongated by LARGE with the ligand-binding heteropolysaccharide. Our findings greatly broaden the understanding of α-DG glycosylation and provide mechanistic insight into why mutations in B4GAT1 disrupt dystroglycan function and cause disease.

DOI:http://dx.doi.org/10.7554/eLife.03941.001

Dystroglycan is a protein that is critical for the proper function of many tissues, especially muscles and brain. Dystroglycan helps to connect the structural network inside the cell with the matrix outside of the cell. The extracellular matrix fills the space between the cells to serve as a scaffold and hold cells together within a tissue. It is well established that the interaction of cells with their extracellular environments is important for structuring tissues, as well as for helping cells to specialize and migrate. These interactions also play a role in the progression of cancer.

As is the case for many proteins, dystroglycan must be modified with particular sugar molecules in order to work correctly. Enzymes called glycosyltransferases are responsible for sequentially assembling a complex array of sugar molecules on dystroglycan. This modification is essential for making dystroglycan ‘sticky’, so it can bind to the components of the extracellular matrix. If sugar molecules are added incorrectly, dystroglycan loses its ability to bind to these components. This causes congenital muscular dystrophies, a group of diseases that are characterized by a progressive loss of muscle function.

Willer et al. use a wide range of experimental techniques to investigate the types of sugar molecules added to dystroglycan, the overall structure of the resulting ‘sticky’ complex and the mechanism whereby it is built. This reveals that a glycosyltransferase known as B3GNT1 is one of the enzymes responsible for adding a sugar molecule to the complex. This enzyme was first described in the literature over a decade ago, and the name B3GNT1 was assigned, according to a code, to reflect the sugar molecule it was thought to transfer to proteins. However, Willer et al. (and independently, Praissman et al.) find that this enzyme actually attaches a different sugar modification to dystroglycan, and so should therefore be called B4GAT1 instead.

Willer et al. find that the sugar molecule added by the B4GAT1 enzyme acts as a platform for the assembly of a much larger sugar polymer that cells use to anchor themselves within a tissue. Some viruses–including Lassa virus, which causes severe fever and bleeding–also use the ‘sticky’ sugar modification of dystroglycan to bind to and invade cells, causing disease in humans. Understanding the structure of this complex, and how these sugar modifications are added to dystroglycan, could therefore help to develop treatments for a wide range of diseases like progressive muscle weakening and viral infections.

DOI:http://dx.doi.org/10.7554/eLife.03941.002

Significance of glycosylation in Notch signaling

Notch signaling is essential for cell-fate specification in metazoans, and dysregulation of the pathway leads to a variety of human diseases including heart and vascular defects as well as cancer. Glycosylation of the Notch extracellular domain has emerged as an elegant means for regulating Notch activity, especially since the discovery that Fringe is a glycosyltransferase that modifies O-fucose in 2000. Since then, several other O-glycans on the extracellular domain have been demonstrated to modulate Notch activity. Here we will describe recent results on the molecular mechanisms by which Fringe modulates Notch activity, summarize recent work on how O-glucose, O-GlcNAc, and O-GalNAc glycans affect Notch, and discuss several human genetic disorders resulting from defects in Notch glycosylation.

A visualizable chain-terminating inhibitor of glycosaminoglycan biosynthesis in developing zebrafish

Heparan sulfate (HS) and chondroitin sulfate (CS) glycosaminoglycans (GAG) are proteoglycan-associated polysaccharides with essential functions in animals. They have been studied extensively by genetic manipulation of biosynthetic enzymes, but chemical tools for probing GAG function are limited. HS and CS possess a conserved xylose residue that links the polysaccharide chain to a protein backbone. Here we report that, in zebrafish embryos, the peptide-proximal xylose residue can be metabolically replaced with a chain-terminating 4-azido-4-deoxyxylose (4-XylAz) residue by administration of UDP-4-azido-4-deoxyxylose (UDP-4-XylAz). UDP-4-XylAz disrupted both HS and CS biosynthesis and caused developmental abnormalities reminiscent of GAG biosynthesis and laminin mutants. The azide substituent of protein-bound 4-XylAz allowed for rapid visualization of the organismal sites of chain termination in vivo through bioorthogonal reaction with fluorescent cyclooctyne probes. UDP-4-XylAz therefore complements genetic tools for studies of GAG function in zebrafish embryogenesis.

Roles of the nucleotide sugar transporters (SLC35 family) in health and disease

Nucleotide sugars and adenosine 3'-phospho 5'-phosphosulfate (PAPS) are transported from the cytosol to the endoplasmic reticulum (ER) and the Golgi apparatus where they serve as substrates for the glycosylation and sulfation of proteins, lipids and proteoglycans. The translocation is accomplished by the nucleotide sugar transporters (NSTs), a family of highly conserved hydrophobic proteins with multiple transmembrane domains that are part of the solute carrier family 35 (SLC35). NSTs are antiporters responsible not only for transporting nucleotide sugars and PAPS into the Golgi, but also for the transport of the reaction products back to the cytosol. The initial reaction products - the nucleoside diphosphates - must be first converted to nucleoside monophosphates by a group of enzymes called ectonucleoside triphosphate diphosphohydrolases (ENTPDs) before they can exit the Golgi. The transport role of NSTs is essential to glycosylation and development. Mutations in two NST genes, SLC35A1 and SLC35C1, have been related to congenital disorder of glycosylation II (CDG II).

Negative regulation of notch signaling by xylose

The Notch signaling pathway controls a large number of processes during animal development and adult homeostasis. One of the conserved post-translational modifications of the Notch receptors is the addition of an O-linked glucose to epidermal growth factor-like (EGF) repeats with a C-X-S-X-(P/A)-C motif by Protein O-glucosyltransferase 1 (POGLUT1; Rumi in Drosophila). Genetic experiments in flies and mice, and in vivo structure-function analysis in flies indicate that O-glucose residues promote Notch signaling. The O-glucose residues on mammalian Notch1 and Notch2 proteins are efficiently extended by the addition of one or two xylose residues through the function of specific mammalian xylosyltransferases. However, the contribution of xylosylation to Notch signaling is not known. Here, we identify the Drosophila enzyme Shams responsible for the addition of xylose to O-glucose on EGF repeats. Surprisingly, loss- and gain-of-function experiments strongly suggest that xylose negatively regulates Notch signaling, opposite to the role played by glucose residues. Mass spectrometric analysis of Drosophila Notch indicates that addition of xylose to O-glucosylated Notch EGF repeats is limited to EGF14-20. A Notch transgene with mutations in the O-glucosylation sites of Notch EGF16-20 recapitulates the shams loss-of-function phenotypes, and suppresses the phenotypes caused by the overexpression of human xylosyltransferases. Antibody staining in animals with decreased Notch xylosylation indicates that xylose residues on EGF16-20 negatively regulate the surface expression of the Notch receptor. Our studies uncover a specific role for xylose in the regulation of the Drosophila Notch signaling, and suggest a previously unrecognized regulatory role for EGF16-20 of Notch.

LARGE2 generates the same xylose- and glucuronic acid-containing glycan structures as LARGE

LARGE (like-glycosyltransferase) and LARGE2 (glycosyltransferase-like 1B (GYLTL1B)) are homologous Golgi glycosyltransferases possessing two catalytic domains with homology to members of glycosyltransferase families GT8 and GT49. Mutations in human and mouse Large result in muscular dystrophy due to underglycosylation of dystroglycan. The systemic function of LARGE2 is unknown, but at a cellular level the enzyme can substitute for LARGE in glycosylating dystroglycan. Here, we show that LARGE2 catalyzes the same glycosylation reaction as LARGE. It is a bifunctional glycosyltransferase using uridine diphosphate (UDP)-xylose (Xyl) and UDP-glucuronic acid (GlcA) as donor sugars to produce a xyloglucuronan with alternating Xyl and GlcA residues.

Xylosyl- and glucuronyltransferase functions of LARGE in α-dystroglycan modification are conserved in LARGE2

LARGE-dependent modification enables α-dystroglycan (α-DG) to bind to its extracellular matrix ligands. Mutations in the LARGE gene and several others involved in O-mannosyl glycan synthesis have been identified in congenital and limb-girdle muscular dystrophies that are characterized by perturbed glycosylation and reduced ligand-binding affinity of α-DG. LARGE is a bifunctional glycosyltransferase that alternately transfers xylose and glucuronic acid, thereby generating the heteropolysaccharides on α-DG that confer its ligand binding. Although the LARGE paralog LARGE2 (also referred to as GYLTL1B) has likewise been shown to enhance the functional modification of α-DG in cultured cells, its enzymatic activities have not been identified. Here, we report that LARGE2 is also a bifunctional glycosyltransferase and compare its properties with those of LARGE. By means of a high-performance liquid chromatography-based enzymatic assay, we demonstrate that like LARGE, LARGE2 has xylosyltransferase (Xyl-T) and glucuronyltransferase (GlcA-T) activities, as well as polymerizing activity. Notably, however, the pH optima of the Xyl-T and GlcA-T of LARGE2 are distinct from one another and also from those of LARGE. Our results suggest that LARGE and LARGE2 catalyze the same glycosylation reactions for the functional modification of α-DG, but that they have different biochemical properties.

O-Mannosylation and human disease

Glycosylation of proteins is arguably the most prevalent co- and post-translational modification. It is responsible for increased heterogeneity and functional diversity of proteins. Here we discuss the importance of one type of glycosylation, specifically O-mannosylation and its relationship to a number of human diseases. The most widely studied O-mannose modified protein is alpha-dystroglycan (α-DG). Recent studies have focused intensely on α-DG due to the severity of diseases associated with its improper glycosylation. O-mannosylation of α-DG is involved in cancer metastasis, arenavirus entry, and multiple forms of congenital muscular dystrophy [1, 2]. In this review, we discuss the structural and functional characteristics of O-mannose-initiated glycan structures on α-DG, enzymes involved in the O-mannosylation pathway, and the diseases that are a direct result of disruptions within this pathway.

Enhancing a Pathway-Genome Database (PGDB) to capture subcellular localization of metabolites and enzymes: the nucleotide-sugar biosynthetic pathways of Populus trichocarpa

Understanding how cellular metabolism works and is regulated requires that the underlying biochemical pathways be adequately represented and integrated with large metabolomic data sets to establish a robust network model. Genetically engineering energy crops to be less recalcitrant to saccharification requires detailed knowledge of plant polysaccharide structures and a thorough understanding of the metabolic pathways involved in forming and regulating cell-wall synthesis. Nucleotide-sugars are building blocks for synthesis of cell wall polysaccharides. The biosynthesis of nucleotide-sugars is catalyzed by a multitude of enzymes that reside in different subcellular organelles, and precise representation of these pathways requires accurate capture of this biological compartmentalization. The lack of simple localization cues in genomic sequence data and annotations however leads to missing compartmentalization information for eukaryotes in automatically generated databases, such as the Pathway-Genome Databases (PGDBs) of the SRI Pathway Tools software that drives much biochemical knowledge representation on the internet. In this report, we provide an informal mechanism using the existing Pathway Tools framework to integrate protein and metabolite sub-cellular localization data with the existing representation of the nucleotide-sugar metabolic pathways in a prototype PGDB for Populus trichocarpa. The enhanced pathway representations have been successfully used to map SNP abundance data to individual nucleotide-sugar biosynthetic genes in the PGDB. The manually curated pathway representations are more conducive to the construction of a computational platform that will allow the simulation of natural and engineered nucleotide-sugar precursor fluxes into specific recalcitrant polysaccharide(s).

Database URL: The curated Populus PGDB is available in the BESC public portal at http://cricket.ornl.gov/cgi-bin/beocyc_home.cgi and the nucleotide-sugar biosynthetic pathways can be directly accessed at http://cricket.ornl.gov:1555/PTR/new-image?object=SUGAR-NUCLEOTIDES.

Dystroglycan function requires xylosyl- and glucuronyltransferase activities of LARGE

Posttranslational modification of alpha-dystroglycan (α-DG) by the like-acetylglucosaminyltransferase (LARGE) is required for it to function as an extracellular matrix (ECM) receptor. Mutations in the LARGE gene have been identified in congenital muscular dystrophy patients with brain abnormalities. However, the precise function of LARGE remains unclear. Here we found that LARGE could act as a bifunctional glycosyltransferase, with both xylosyltransferase and glucuronyltransferase activities, which produced repeating units of [-3-xylose-α1,3-glucuronic acid-β1-]. This modification allowed α-DG to bind laminin-G domain-containing ECM ligands.

Heparan sulfate proteoglycans as multifunctional cell regulators: cell surface receptors

Proteoglycans are macromolecules expressed on the cell surfaces and in the extracellular matrix of most animal tissues (Annu Rev Biochem 68:729-777, 1999; Int Rev Cell Mol Biol 276:105-159, 2009). Heparan sulfate proteoglycans (HSPGs) are essential for animal development and homeostasis, and are involved in various pathological processes. The functions of HSPGs are largely exerted through interaction of the heparan sulfate (HS) side chains with different types of ligands, including diverse molecules such as cytokines, enzymes, and pathogens. One of the important roles of cell surface HSPGs is to mediate cytokine-induced cell signaling through interaction with growth factors (GFs) and their cognate receptors. A selective dependence of GFs for different structural features of HS has been demonstrated by applying cell models that are mutated variously in HS structure due to deficiency in enzymes involved in the biosynthesis of HS chains.

Rumi functions as both a protein O-glucosyltransferase and a protein O-xylosyltransferase

Mutations in rumi result in a temperature-sensitive loss of Notch signaling in Drosophila. Drosophila Rumi is a soluble, endoplasmic reticulum-retained protein with a CAP10 domain that functions as a protein O-glucosyltransferase. In human and mouse genomes, three potential Rumi homologues exist: one with a high degree of identity to Drosophila Rumi (52%), and two others with lower degrees of identity but including a CAP10 domain (KDELC1 and KDELC2). Here we show that both mouse and human Rumi, but not KDELC1 or KDELC2, catalyze transfer of glucose from UDP-glucose to an EGF repeat from human factor VII. Similarly, human Rumi, but not KDELC1 or KDELC2, rescues the Notch phenotypes in Drosophila rumi clones. During characterization of the Rumi enzymes, we noted that, in addition to protein O-glucosyltransferase activity, both mammalian and Drosophila Rumi also showed significant protein O-xylosyltransferase activity. Rumi transfers Xyl or glucose to serine 52 in the O-glucose consensus sequence ( ) of factor VII EGF repeat. Surprisingly, the second serine (S53) facilitates transfer of Xyl, but not glucose, to the EGF repeat by Rumi. EGF16 of mouse Notch2, which has a diserine motif in the consensus sequence ( ), is also modified with either O-Xyl or O-glucose glycans in cells. Mutation of the second serine (S590A) causes a loss of O-Xyl but not O-glucose at this site. Altogether, our data establish dual substrate specificity for the glycosyltransferase Rumi and provide evidence that amino acid sequences of the recipient EGF repeat significantly influence which donor substrate (UDP-glucose or UDP-Xyl) is used.

Fringe benefits: functional and structural impacts of O-glycosylation on the extracellular domain of Notch receptors

The Notch family of receptors plays essential roles in many phases of development, and dysregulation of Notch activity is increasingly recognized as a player in many diseases. O-Glycosylation of the Notch extracellular domain is essential for Notch activity, and tissue-specific alterations in the glycan structures are known to regulate activity. Here we review recent advances in identification and characterization of the enzymes responsible for glycosylating Notch and molecular mechanisms for how these O-glycans affect Notch activity.

Multiple O-glucosylation sites on Notch function as a buffer against temperature-dependent loss of signaling

Mutations in Drosophila rumi result in a temperature-sensitive loss of Notch signaling. Rumi is a protein O-glucosyltransferase that adds glucose to EGF repeats with a C-X-S-X-P-C consensus sequence. Eighteen of the 36 EGF repeats in the Drosophila Notch receptor contain the consensus O-glucosylation motif. However, the contribution of individual O-glucose residues on Notch to the regulation of Notch signaling is not known. To address this issue, we carried out a mutational analysis of these glucosylation sites and determined their effects on Notch activity in vivo. Our results indicate that even though no single O-glucose mutation causes a significant decrease in Notch activity, all of the glucose residues on Notch contribute in additive and/or redundant fashions to maintain robust signaling, especially at higher temperatures. O-glucose motifs in and around the ligand-binding EGF repeats play a more important role than those in other EGF repeats of Notch. However, a single O-glucose mutation in EGF12 can be compensated by other O-glucose residues in neighboring EGF repeats. Moreover, timecourse cell aggregation experiments using a rumi null cell line indicate that a complete lack of Rumi does not affect Notch-Delta binding at high temperature. In addition, rumi fully suppresses the gain-of-function phenotype of a ligand-independent mutant form of Notch. Our data suggest that, at physiological levels of Notch, the combined effects of multiple O-glucose residues on this receptor allow productive S2 cleavage at high temperatures and thereby serve as a buffer against temperature-dependent loss of Notch signaling.

Role of packing defects in the evolution of allostery and induced fit in human UDP-glucose dehydrogenase

Allosteric feedback inhibition is the mechanism by which metabolic end products regulate their own biosynthesis by binding to an upstream enzyme. Despite its importance in controlling metabolism, there are relatively few allosteric mechanisms understood in detail. This is because allostery does not have an identifiable structural motif, making the discovery of new allosteric enzymes a difficult process. The lack of a conserved motif implies that the evolution of each allosteric mechanism is unique. Here we describe an atypical allosteric mechanism in human UDP-α-d-glucose 6-dehydrogenase (hUGDH) based on an easily acquired and identifiable structural attribute: packing defects in the protein core. In contrast to classic allostery, the active and allosteric sites in hUGDH are present as a single, bifunctional site. Using two new crystal structures, we show that binding of the feedback inhibitor, UDP-α-d-xylose, elicits a distinct induced-fit response; a buried loop translates ∼4 Å along and rotates ∼180° about the main chain axis, requiring surrounding side chains to repack. This allosteric transition is facilitated by packing defects, which negate the steric conformational restraints normally imposed by the protein core. Sedimentation velocity studies show that this repacking favors the formation of an inactive hexameric complex with unusual symmetry. We present evidence that hUGDH and the unrelated enzyme dCTP deaminase have converged to very similar atypical allosteric mechanisms using the same adaptive strategy, the selection for packing defects. Thus, the selection for packing defects is a robust mechanism for the evolution of allostery and induced fit.

Regulation of mammalian Notch signaling and embryonic development by the protein O-glucosyltransferase Rumi

Protein O-glucosylation is a conserved post-translational modification that occurs on epidermal growth factor-like (EGF) repeats harboring the C(1)-X-S-X-P-C(2) consensus sequence. The Drosophila protein O-glucosyltransferase (Poglut) Rumi regulates Notch signaling, but the contribution of protein O-glucosylation to mammalian Notch signaling and embryonic development is not known. Here, we show that mouse Rumi encodes a Poglut, and that Rumi(-/-) mouse embryos die before embryonic day 9.5 with posterior axis truncation and severe defects in neural tube development, somitogenesis, cardiogenesis and vascular remodeling. Rumi knockdown in mouse cell lines results in cellular and molecular phenotypes of loss of Notch signaling without affecting Notch ligand binding. Biochemical, cell culture and cross-species transgenic experiments indicate that a decrease in Rumi levels results in reduced O-glucosylation of Notch EGF repeats, and that the enzymatic activity of Rumi is key to its regulatory role in the Notch pathway. Genetic interaction studies show that removing one copy of Rumi in a Jag1(+/-) (jagged 1) background results in severe bile duct morphogenesis defects. Altogether, our data indicate that addition of O-glucose to EGF repeats is essential for mouse embryonic development and Notch signaling, and that Jag1-induced signaling is sensitive to the gene dosage of the protein O-glucosyltransferase Rumi. Given that Rumi(-/-) embryos show more severe phenotypes compared to those displayed by other global regulators of canonical Notch signaling, Rumi is likely to have additional important targets during mammalian development.

Down-regulation of UDP-glucose dehydrogenase affects glycosaminoglycans synthesis and motility in HCT-8 colorectal carcinoma cells

UDP-glucose dehydrogenase (UGDH) catalyzes oxidation of UDP-glucose to yield UDP-glucuronic acid, a precursor of hyaluronic acid (HA) and other glycosaminoglycans (GAGs) in extracellular matrix. Although association of extracellular matrix with cell proliferation and migration has been well documented, the importance of UGDH in these behaviors is not clear. Using UGDH-specific small interference RNA to treat HCT-8 colorectal carcinoma cells, a decrease in both mRNA and protein levels of UGDH, as well as the cellular UDP-glucuronic acid and GAG production was observed. Treatment of HCT-8 cells with either UGDH-specific siRNA or HA synthesis inhibitor 4-methylumbelliferone effectively delayed cell aggregation into multicellular spheroids and impaired cell motility in both three-dimensional collagen gel and transwell migration assays. The reduction in cell aggregation and migration rates could be restored by addition of exogenous HA. These results indicate that UGDH can regulate cell motility through the production of GAG. The enzyme may be a potential target for therapeutic intervention of colorectal cancers.

Role of glycans and glycosyltransferases in the regulation of Notch signaling

The evolutionarily conserved Notch signaling pathway plays broad and important roles during embryonic development and in adult tissue homeostasis. Unlike most other pathways used during animal development, Notch signaling does not rely on second messengers and intracellular signaling cascades. Instead, pathway activation results in the cleavage of the Notch intracellular domain and its translocation into the nucleus, where it functions as a transcriptional co-activator of the Notch target genes. To ensure tight spatial and temporal regulation of a pathway with such an unusually direct signaling transduction, animal cells have devised a variety of specialized modulatory mechanisms. One such mechanism takes advantage of decorating the Notch extracellular domain with rare types of O-linked glycans. In this review, we will discuss the genetic and biochemical data supporting the notion that carbohydrate modification is essential for Notch signaling and attempt to provide a brief historical overview of how we have learned what we know about the glycobiology of Notch. We will also summarize what is known about the contribution of specific nucleotide-sugar transporters to Notch biology and the roles-enzymatic and non-enzymatic-played by specific glycosyltransferases in the regulation of this pathway. Mutations in the Notch pathway components cause a variety of human diseases, and manipulation of Notch signaling is emerging as a powerful tool in regenerative medicine. Therefore, studying how sugar modification modulates Notch signaling provides a framework for better understanding the role of glycosylation in animal development and might offer new tools to manipulate Notch signaling for therapeutic purposes.

UDP xylose synthase 1 is required for morphogenesis and histogenesis of the craniofacial skeleton

UDP-xylose synthase (Uxs1) is strongly conserved from bacteria to humans, but because no mutation has been studied in any animal, we do not understand its roles in development. Furthermore, no crystal structure has been published. Uxs1 synthesizes UDP-xylose, which initiates glycosaminoglycan attachment to a protein core during proteoglycan formation. Crystal structure and biochemical analyses revealed that an R233H substitution mutation in zebrafish uxs1 alters an arginine buried in the dimer interface, thereby destabilizing and, as enzyme assays show, inactivating the enzyme. Homozygous uxs1 mutants lack Alcian blue-positive, proteoglycan-rich extracellular matrix in cartilages of the neurocranium, pharyngeal arches, and pectoral girdle. Transcripts for uxs1 localize to skeletal domains at hatching. GFP-labeled neural crest cells revealed defective organization and morphogenesis of chondrocytes, perichondrium, and bone in uxs1 mutants. Proteoglycans were dramatically reduced and defectively localized in uxs1 mutants. Although col2a1a transcripts over-accumulated in uxs1 mutants, diminished quantities of Col2a1 protein suggested a role for proteoglycans in collagen secretion or localization. Expression of col10a1, indian hedgehog, and patched was disrupted in mutants, reflecting improper chondrocyte/perichondrium signaling. Up-regulation of sox9a, sox9b, and runx2b in mutants suggested a molecular mechanism consistent with a role for proteoglycans in regulating skeletal cell fate. Together, our data reveal time-dependent changes to gene expression in uxs1 mutants that support a signaling role for proteoglycans during at least two distinct phases of skeletal development. These investigations are the first to examine the effect of mutation on the structure and function of Uxs1 protein in any vertebrate embryos, and reveal that Uxs1 activity is essential for the production and organization of skeletal extracellular matrix, with consequent effects on cartilage, perichondral, and bone morphogenesis.

Identification of a bifunctional UDP-4-keto-pentose/UDP-xylose synthase in the plant pathogenic bacterium Ralstonia solanacearum strain GMI1000, a distinct member of the 4,6-dehydratase and decarboxylase family

The UDP-sugar interconverting enzymes involved in UDP-GlcA metabolism are well described in eukaryotes but less is known in prokaryotes. Here we identify and characterize a gene (RsU4kpxs) from Ralstonia solanacearum str. GMI1000, which encodes a dual function enzyme not previously described. One activity is to decarboxylate UDP-glucuronic acid to UDP-beta-l-threo-pentopyranosyl-4''-ulose in the presence of NAD(+). The second activity converts UDP-beta-l-threo-pentopyranosyl-4''-ulose and NADH to UDP-xylose and NAD(+), albeit at a lower rate. Our data also suggest that following decarboxylation, there is stereospecific protonation at the C5 pro-R position. The identification of the R. solanacearum enzyme enables us to propose that the ancestral enzyme of UDP-xylose synthase and UDP-apiose/UDP-xylose synthase was diverged to two distinct enzymatic activities in early bacteria. This separation gave rise to the current UDP-xylose synthase in animal, fungus, and plant as well as to the plant Uaxs and bacterial ArnA and U4kpxs homologs.

Identification of glycosyltransferase 8 family members as xylosyltransferases acting on O-glucosylated notch epidermal growth factor repeats

The epidermal growth factor repeats of the Notch receptor are extensively glycosylated with three different O-glycans. O-Fucosylation and elongation by the glycosyltransferase Fringe have been well studied and shown to be essential for proper Notch signaling. In contrast, biosynthesis of O-glucose and O-N-acetylglucosamine is less well understood. Recently, the isolation of the Drosophila mutant rumi has shown that absence of O-glucose impairs Notch function. O-Glucose is further extended by two contiguous alpha1,3-linked xylose residues. We have identified two enzymes of the human glycosyltransferase 8 family, now named GXYLT1 and GXYLT2 (glucoside xylosyltransferase), as UDP-d-xylose:beta-d-glucoside alpha1,3-d-xylosyltransferases adding the first xylose. The enzymes are specific for beta-glucose-terminating acceptors and UDP-xylose as donor substrate. Generation of the alpha1,3-linkage was confirmed by nuclear magnetic resonance. Activity on a natural acceptor could be shown by in vitro xylosylation of a Notch fragment expressed in a UDP-xylose-deficient cell line and in vivo by co-expression of the enzymes and the Notch fragment in insect cells followed by mass spectrometric analysis of peptide fragments.

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.

Mammalian Notch is modified by D-Xyl-alpha1-3-D-Xyl-alpha1-3-D-Glc-beta1-O-Ser: implementation of a method to study O-glucosylation

Notch is a key cell surface protein receptor that is a vital component of intercellular signaling occurring during development. The O-glucosylation of the extracellular Notch epidermal growth factor-like (EGF) repeats has recently been found to play an important role in the proper functioning of Notch in Drosophila. Previous efforts to identify the fine structure of the O-glucose-containing glycan of mammalian Notch have been hindered by limitations associated with approaches used to date. Here, we report the development of an alternative strategy that can be used to study this modification from a range of different tissues. To implement this approach, we have generated standards of the D-Xyl-alpha1-3-D-Xyl-alpha1-3-D-Glc trisaccharide, isomers of this structure, as well as the d-Xyl-alpha1-3-d-Glc disaccharide found previously on secreted EGF-containing proteins of the blood coagulation cascade. Following derivatization with 8-aminopyrene-1,3,6-trisulfonate (APTS), we use these standards in capillary electrophoretic analyses of O-glycans released from Notch1 EGF repeats in conjunction with exo-alpha-xylosidase digestion. These studies collectively reveal that the O-glucose-containing glycan decorating mammalian Notch is the D-Xyl-alpha1-3-D-Xyl-alpha1-3-D-Glc trisaccharide; an assignment in accord with previous predictions. Given the demonstrated importance of this modification in the function of Notch in Drosophila, we expect that the identification of this glycan decorating mammalian Notch1 should aid studies into the functional role of O-glycosylation of mammalian Notch isoforms. Wider application of this approach should facilitate identification of other EGF-containing proteins bearing this O-glycan and aid in their study.