Molecular cloning and characterization of UDP-GlcNAc:lactosylceramide beta 1,3-N-acetylglucosaminyltransferase (beta 3Gn-T5), an essential enzyme for the expression of HNK-1 and Lewis X epitopes on glycolipids

Helicobacter pylori induces beta3GnT5 in human gastric cell lines, modulating expression of the SabA ligand sialyl-Lewis x

Chronic Helicobacter pylori infection is recognized as a cause of gastric cancer. H. pylori adhesion to gastric cells is mediated by bacterial adhesins such as sialic acid-binding adhesin (SabA), which binds the carbohydrate structure sialyl-Lewis x. Sialyl-Lewis x expression in the gastric epithelium is induced during persistent H. pylori infection, suggesting that H. pylori modulates host cell glycosylation patterns for enhanced adhesion. Here, we evaluate changes in the glycosylation-related gene expression profile of a human gastric carcinoma cell line following H. pylori infection. We observed that H. pylori significantly altered expression of 168 of the 1,031 human genes tested by microarray, and the extent of these alterations was associated with the pathogenicity of the H. pylori strain. A highly pathogenic strain altered expression of several genes involved in glycan biosynthesis, in particular that encoding beta3 GlcNAc T5 (beta3GnT5), a GlcNAc transferase essential for the biosynthesis of Lewis antigens. beta3GnT5 induction was specific to infection with highly pathogenic strains of H. pylori carrying a cluster of genes known as the cag pathogenicity island, and was dependent on CagA and CagE. Further, beta3GnT5 overexpression in human gastric carcinoma cell lines led to increased sialyl-Lewis x expression and H. pylori adhesion. This study identifies what we believe to be a novel mechanism by which H. pylori modulates the biosynthesis of the SabA ligand in gastric cells, thereby strengthening the epithelial attachment necessary to achieve successful colonization.

Polylactosamine on glycoproteins influences basal levels of lymphocyte and macrophage activation

beta1,3-N-acetylglucosaminyltransferase 2 (beta3GnT2) is a polylactosamine synthase that synthesizes a backbone structure of carbohydrate structures onto glycoproteins. Here we generated beta3GnT2-deficient (beta3GnT2(-/-)) mice and showed that polylactosamine on N-glycans was markedly reduced in their immunological tissues. In WT mice, polylactosamine was present on CD28 and CD19, both known immune costimulatory molecules. However, polylactosamine levels on these molecules were reduced in beta3GnT2(-/-) mice. beta3GnT2(-/-) T cells lacking polylactosamine were more sensitive to the induction of intracellular calcium flux on stimulation with anti-CD3epsilon/CD28 and proliferated more strongly than T cells from WT mice. beta3GnT2(-/-) B cells also showed hyperproliferation on BCR stimulation. Macrophages from beta3GnT2(-/-) mice had higher cell surface CD14 levels and enhanced responses to endotoxin. These results indicate that polylactosamine on N-glycans is a putative immune regulatory factor presumably suppressing excessive responses during immune reactions.

Long-term evolution of the CAZY glycosyltransferase 6 (ABO) gene family from fishes to mammals—a birth-and-death evolution model

Functional glycosyltransferase 6 (GT6) family members catalyze the transfer of galactose or N-acetylgalactosamine in alpha1,3 linkage to various substrates and synthesize structures related to the A and B histo-blood group antigens, the Forssman antigen, alphaGal epitope, and iGb3 glycolipid. In rat, mouse, dog, and cow genomes, we have identified three new mammalian genes (GT6m5, GT6m6, and GT6m7) encoding putative proteins belonging to the GT6 family. Among these, GT6m6 protein does not display major alterations of the GT6 motifs involved in binding of the divalent cation and the substrate. Based on protein sequence comparison, gene structure, and synteny, GT6 homologous sequences were also identified in bird, fish, and amphibian genomes. Strikingly, the number and type of GT6 genes varied widely from species to species, even within phylogenetically related groups. In human, except ABO functional alleles, all other GT6 genes are either absent or nonfunctional. Human, mouse, and cow have only one ABO gene, whereas rat and dog have several. In the chicken, the Forssman synthase-like is the single GT6 family member. Five Forssman synthase-like genes were found in zebrafish, but are absent from three other fishes (fugu, puffer fish, and medaka). Two iGb3 synthase-like genes were found in medaka, which are absent from zebrafish. Fugu, puffer fish, and medaka have an additional GT6 gene that we termed GT6m8, which is absent from all other species analyzed here. These observations indicate that individual GT6 genes have expanded and contracted by recurrent duplications and deletions during vertebrate evolution, following a birth-and-death evolution type.

Patterning the developing and regenerating olfactory system

The olfactory system is a remarkable model for investigating the factors that influence the guidance of sensory axon populations to specific targets in the CNS. Since the initial discovery of the vast odorant receptor (ORs) gene family in rodents and the subsequent finding that these molecules directly influence targeting, several additional olfactory axon guidance cues have been identified. Two of these, ephrins and semaphorins, have well-established functions in patterning axon connections in other systems. In addition, lactosamine-containing glycans are also required for proper targeting and maintenance of olfactory axons, and may also function in other sensory regions. It is now apparent that these and likely other additional molecules are required along with ORs to orchestrate the complex pattern of convergence and divergence that is unique to the olfactory system.

Mice lacking alpha1,3-fucosyltransferase IX demonstrate disappearance of Lewis x structure in brain and increased anxiety-like behaviors

The 3-fucosyl-N-acetyllactosamine [Lewis x (Le(x)), CD15, SSEA-1] carbohydrate structure is expressed on several glycolipids, glycoproteins, and proteoglycans of the nervous system and has been implicated in cell-cell recognition, neurite outgrowth, and neuronal migration during development. To characterize the functional role of Le(x) carbohydrate structure in vivo, we have generated mutant mice that lack alpha1,3-fucosyltransferase IX (Fut9(-/-)). Fut9(-/-) mice were unable to synthesize the Le(x) structure carried on glycoproteins and glycolipids in embryonic and adult brain. However, no obvious pathological differences between wild-type and Fut9(-/-) mice were found in brain. In behavioral tests, Fut9(-/-) mice exhibited increased anxiety-like responses in dark-light preference and in elevated plus maze tests. Immunohistochemical analysis showed that the number of calbindin-positive neurons was decreased in the basolateral amygdala in Fut9(-/-) mice. These observations indicated that the carbohydrates synthesized by Fut9 play critical roles in functional regulations of interneurons in the amygdalar subdivisions and suggested a role for the Le(x) structure in some aspects of emotional behavior in mice.

Bioinformatics for comprehensive finding and analysis of glycosyltransferases

Bioinformatics is a very powerful tool in the field of glycoproteomics as well as genomics and proteomics. As a part of the Glycogene Project (GG project), we have developed a novel bioinformatics system for the comprehensive identification and in silico cloning of human glycogenes. Using our system, a total of 105 candidate human glycogenes were identified and then engineered for heterologous expression. Of these candidates, 38 recombinant proteins were successfully identified for their enzyme activity and substrate specificity. We also classified 47 out of 60 carbohydrate-active enzyme glycosyltransferase families into 4 superfamilies using the profile Hidden Markov Model method. On the basis of our classification and the relationship between glycosylation pathways and superfamilies, we propose the evolution of glycosyltransferases.

Comprehensive Enzymatic Characterization of Glycosyltransferases with a β3GT or β4GT Motif

Bioinformatics is a very powerful tool in the field of glycoproteomics, as well as genomics and proteomics. The bioinformatics technique accelerates the comprehensive identification and in silico cloning of human glycogenes containing glycosyltransferases, glycolytic enzymes, sugar-nucleotide synthetases, sugar-nucleotide transporters, and so forth. Glycosyltransferase genes play central roles in carbohydrate chain biosynthesis and have been analyzed for their biological functions. At present, over 180 human glycosyltransferases were identified, cloned, and expressed in various expression systems to detect the activity for carbohydrate synthesis. The recombinant proteins for glycosyltransferase were successfully identified for their enzyme activities and substrate specificities. Their substrate specificities were determined using various donor substrates and acceptors. This section reviews the functions, substrate specificities, and enzymatic reactions of glycosyltransferases such as beta1,3-glycosyltransferase family and beta1,4-glycosyltransferase family.

Characterization of a novel galactose beta1,3-N-acetylglucosaminyltransferase (beta3Gn-T8): the complex formation of beta3Gn-T2 and beta3Gn-T8 enhances enzymatic activity

We characterized a novel member of the beta1,3-N-acetylglucosaminyltransferase (beta3Gn-T) gene family, beta3Gn-T8. A recombinant soluble form of beta3Gn-T8 was expressed in Pichia pastoris (P. pastoris), and its substrate specificity was compared with that of beta3Gn-T2. The two enzymes had similar substrate specificities and recognized tetraantennary N-glycans and 2,6-branched triantennary glycans in preference to 2,4-branched triantennary glycans, biantennary glycans, and lacto-N-neotetraose (LNnT), indicating their specificity for 2,6-branched structures such as [Galbeta1-->4GlcNAcbeta1-->2(Galbeta1-->4GlcNAcbeta1-->6)Manalpha1--> 6Man]. Interestingly, when soluble recombinant beta3Gn-T2 and beta3Gn-T8 were mixed, the Vmax/Km value of the mixture was 9.3- and 160-fold higher than those of individual beta3Gn-T2 and -T8, respectively. Sephacryl S-300 gel filtration of the enzymes revealed that apparent molecular weights of each beta3Gn-T2, beta3Gn-T8, and the mixture were 90-160, 45-65, and 110-210 kDa, respectively, suggesting that beta3Gn-T2 and -T8 can form a complex with enhanced enzymatic activity. This is the first report demonstrating that in vitro mixed glycosyltransferases show enhanced enzymatic activity through the formation of a heterocomplex. These results suggested that beta3Gn-T8 and beta3Gn-T2 are cooperatively involved in the elongation of specific branch structures of multiantennary N-glycans.

Characterization of a cDNA encoding a protein with limited similarity to beta1, 3-N-acetylglucosaminyltransferase

Glycosyltransferases constitute a large group of enzymes that are involved in a wide range of functions in all living organisms. By large-scale sequencing analysis of a human fetal brain cDNA library, we isolated a novel human putative glycosyltransferase gene named beta3GnTL1. Its cDNA is 1372 base pair in length, encoding a predicted protein with 361 amino acid residues. The human beta3GnTL1 is located to chromosome 17q25.3 by comparison of its cDNA with human gemome database. RT-PCR result shows the beta3GnTL1 is expressed at various levels in most of tissues examined.

The effects of N-glycosylation sites and the N-terminal region on the biological function of β1,3-N-acetylglucosaminyltransferase 2 and its secretion

Human beta1,3-N-acetylglucosaminyltransferase 2 (beta3GnT2) is thought to be an enzyme that extends the polylactosamine acceptor chains, but its function and structure analysis are unknown. To obtain insight into the structure of beta3GnT2, the effects of N-glycosylation on its biological function were evaluated using the addition of inhibitors, site-directed mutagenesis of potential N-glycosylation sites, and deletion of its N-terminal region using a fusion protein with GFP(uv) in a baculovirus expression system. Four of five potential N-glycosylation sites were found to be occupied, and their biological function and secretion were inhibited with the treatment of N-glycosylation inhibitor, tunicamycin. The N-glycosylation at Asn219 was necessary for the beta3GnT activity; moreover, N-glycosylation at Asn127 and Asn219 was critical for efficient protein secretion. When Ser221 was replaced with Thr, fusion protein was expressed as a single band, indicating that the double band of the expressed fusion protein was due to the heterogeneity of the glycosylation at Asn219. The truncated protein consisting of amino acids 82-397 (GFP(uv)-beta3GnT2Delta83), which lacked both one N-glycosylation site at Asn79 and the stem region of glycosyltransferase, was expressed as only a small form and showed no beta3GnT activity. These results suggest that the N-glycosylation site at Asn219, which is conserved throughout the beta1,3-glycosyltransferase family, is indispensable not only with regard to its biological function, but also to its secretion. The N-terminal region, which belongs to a stem region of glycosyltransferase, might also be important to the active protein structure.

Construction of a human glycogene library and comprehensive functional analysis

Eighteen years have passed after the first mammalian glycosyltransferase was cloned. At the beginning of April, 2001, 110 genes for human glycosyltransferases, including modifying enzymes for carbohydrate chains such as sulfotransferases, had been cloned and analyzed. We started the Glycogene Project (GG project) in April 2001, a comprehensive study on human glycogenes with the aid of bioinformatic technology. The term glycogene includes the genes for glycosyltransferases, sulfotransferases adding sulfate to carbohydrates and sugar-nucleotide transporters, etc. Firstly, as many novel genes, which are the candidates for glycogenes, as possible were searched using bioinformatic technology in databases. They were then cloned and expressed in various expression systems to detect the activity for carbohydrate synthesis. Their substrate specificity was determined using various acceptors.

A novel beta1,3-N-acetylglucosaminyltransferase (beta3Gn-T8), which synthesizes poly-N-acetyllactosamine, is dramatically upregulated in colon cancer

A new member of the UDP-N-acetylglucosamine: beta-galactose beta1,3-N-acetylglucosaminyltransferase (beta3Gn-T) family having the beta3-glycosyltransferase motifs was identified using an in silico method. This novel beta3Gn-T was cloned from a human colon cancer cell line and named beta3Gn-T8 based on its position in a phylogenetic tree and enzymatic activity. Beta3Gn-T8 transfers GlcNAc to the non-reducing terminus of the Galbeta1-4GlcNAc of tetraantennary N-glycan in vitro. HCT15 cells transfected with beta3Gn-T8 cDNA showed an increase in reactivity to both LEA and PHA-L4 in a flow cytometric analysis. These results indicated that beta3Gn-T8 is involved in the biosynthesis of poly-N-acetyllactosamine chains on tetraantennary (beta1,6-branched) N-glycan. In most of the colorectal cancer tissues examined, the level of beta3Gn-T8 transcript was significantly higher than in normal tissue. Beta3Gn-T8 could be an enzyme involved in the synthesis of poly-N-acetyllactosamine on beta1-6 branched N-glycans in colon cancer.

Egghead and brainiac are essential for glycosphingolipid biosynthesis in vivo

The Drosophila genes, brainiac and egghead, encode glycosyltransferases predicted to act sequentially in early steps of glycosphingolipid biosynthesis, and both genes are required for development in Drosophila. egghead encodes a beta4-mannosyltransferase, and brainiac encodes a beta3-N-acetylglucosaminyltransferase predicted by in vitro analysis to control synthesis of the glycosphingolipid core structure, GlcNAcbeta1-3Manbeta1-4Glcbeta1-Cer, found widely in invertebrates but not vertebrates. In this report we present direct in vivo evidence for this hypothesis. egghead and brainiac mutants lack elongated glycosphingolipids and exhibit accumulation of the truncated precursor glycosphingolipids. Furthermore, we demonstrate that despite fundamental differences in the core structure of mammalian and Drosophila glycosphingolipids, the Drosophila egghead mutant can be rescued by introduction of the mammalian lactosylceramide glycosphingolipid biosynthetic pathway (Galbeta1-4Glcbeta1-Cer) using a human beta4-galactosyltransferase (beta4Gal-T6) transgene. Conversely, introduction of egghead in vertebrate cells (Chinese hamster ovary) resulted in near complete blockage of biosynthesis of glycosphingolipids and accumulation of Manbeta1-4Glcbeta1-Cer. The study demonstrates that glycosphingolipids are essential for development of complex organisms and suggests that the function of the Drosophila glycosphingolipids in development does not depend on the core structure.

Improvement of GFPuv-beta 3GnT2 fusion protein production by suppressing protease in baculovirus expression system

The effects of protease inhibitors on the production of recombinant protein were investigated using a recombinant baculovirus containing GFPuv-human beta 1,3-N-acetylglucosaminyltransferase 2 (beta 3GnT2) connected to the prepromelittin signal sequence. The addition of leupeptin as a cysteine protease inhibitor at 2.5 microg/ml improved intra- and extracellular beta 3GnT activities 5- and 3-fold, respectively, compared to those without addition, which was due to a suppression of protease activity. With the leupeptin addition only four degraded molecular bands lower than 32 kDa appeared, but 9 degraded molecular bands between 29 kDa and 41 kDa existed without addition. In contrast, pepstatin A as a carboxyl protease inhibitor had no influence on the improvement of beta 3GnT production, judging from SDS-PAGE. Moreover, when 50 microM carbobenzoxy-L-leucyl-L-leucyl-L-leucinal (MG-132), known as a proteasome inhibitor, was used in combination with the leupeptin, a ladder of low molecular mass bands of fusion protein was diminished. The intracellular beta 3GnT activity increased 9-fold, to as high as that without addition of two kinds of protease, but the extracellular activity was not different from that with the addition of only leupeptin. These findings indicate that the decrease in cell viability causes the decrease in the secretion rate of intracellular fusion protein, resulting the accumulation of the full-length of fusion protein.

Comparative analysis of GFPUV-β1,3-N-acetylglucosaminyltransferase 2 production in two insect-cell-based expression systems

Active beta1,3-N-acetylglucosaminyltransferase 2 (beta3GnT2) was produced in the baculovirus expression system (BES) and in stably transformed insect Tn-5B1-4 cells. beta3GnT2 was expressed as a secreted fusion protein with GFP(UV) with three different types of signal sequence to enhance the secretion of the fusion protein. In the stably transformed cells, the maximal beta3GnT2 activity differed between isolates, but their secretion efficiencies were similar. The difference between the maximal beta3GnT activities of the isolates studied was considered to be due to the presence of a copy number of the fusion gene, as determined on the basis of the results of Southern blot analysis. The beta3GnT activities of the culture supernatant in BES (Tn-5B1-4 cells) without or with the addition of the protease inhibitor, leupeptin, were 0.68 and 2.01 mU/ml, respectively. The stably transformed Tn-5B1-4 cells (Tn-pXme11) exhibited a beta3GnT activity of 6.83 mU/ml, which was 3.4-fold higher than that observed for BES with the leupeptin addition. The purity of fusion protein purified from the culture supernatant of the Tn-pXme11 was higher than 95% on SDS-PAGE, in contrast with that purified from the culture supernatant of the baculovirus-infected cells which contained low-molecular-weight fragments of the fusion protein. The stably transformed cell line is more suitable than BES for the efficient production of the secretory protein, beta3GnT2.

A Novel Human β1,3-N-Acetylgalactosaminyltransferase That Synthesizes a Unique Carbohydrate Structure, GalNAcβ1-3GlcNAc

We found, using a BLAST search, a novel human gene (GenBank trade mark accession number BC029564) that possesses beta3-glycosyltransferase motifs. The full-length open reading frame consists of 500 amino acids and encodes a typical type II membrane protein. This enzyme has a domain containing beta1,3-glycosyltransferase motifs, which are widely conserved in the beta1,3-galactosyltransferase and beta1,3-N-acetylglucosaminyltransferase families. The putative catalytic domain was expressed in human embryonic kidney 293T cells as a soluble protein. Its N-acetylgalactosaminyltransferase activity was observed when N-acetylglucosamine (GlcNAc) beta1-O-benzyl was used as an acceptor substrate. The enzyme product was determined to have a beta1,3-linkage by NMR spectroscopic analysis, and was therefore named beta1,3-N-acetylgalactosaminyltransferase-II (beta3GalNAc-T2). The acceptor substrate specificity of beta3GalNAc-T2 was examined using various oligosaccharide substrates. Galbeta1-3(GlcNAcbeta1-6)GalNAcalpha1-O-para-nitrophenyl (core 2-pNP) was the best acceptor substrate for beta3GalNAc-T2, followed by GlcNAcbeta1-4GlcNAcbeta1-O-benzyl, and GlcNAcbeta1-6GalNAcalpha1-O-para-nitrophenyl (core 6-pNP), among the tested oligosaccharide substrates. Quantitative real time PCR analysis revealed that the beta3Gal-NAc-T2 transcripts was restricted in its distribution mainly to the testis, adipose tissue, skeletal muscle, and ovary. Its putative orthologous gene, mbeta3GalNAc-T2, was also found in a data base of mouse expressed sequence tags. In situ hybridization analysis with mouse testis showed that the transcripts are expressed in germ line cells. beta3GalNAc-T2 efficiently transferred GalNAc to N-glycans of fetal calf fetuin, which was treated with neuraminidase and beta-galactosidase. However, it showed no activity toward any glycolipid examined. Although the GalNAcbeta1-3GlcNAcbeta1-R structure has not been reported in humans or other mammals, we have discovered a novel human glycosyltransferase producing this structure on N- and O-glycans.

β1,3-N-Acetylglucosaminyltransferase-7 (β3Gn-T7) acts efficiently on keratan sulfate-related glycans

beta1,3-N-Acetylglucosaminyltransferase-7 (beta3Gn-T7) has been identified as an anti-migration factor for a lung cancer cell line but its enzymatic activity has not yet been characterized. Here we show that beta3Gn-T7 efficiently acts on keratan sulfate-related glycans including Galbeta1-->4(SO(3)(-)-->6)GlcNAcbeta1-->3Galbeta1-->4(SO(3)(-)-->6)GlcNAc (L2L2), while lacto-N-tetraose and lacto-N-neo-tetraose were poor substrates. Moreover, we found that among the other five beta3Gn-Ts and i antigen-producing beta3Gn-T (iGn-T), beta3Gn-T2 and iGn-T act well on L2L2, although these specific activities were lower than those for a tetraantennary N-glycan. These results indicate that beta3Gn-T7 is the one that most efficiently elongates L2L2 and may be involved in the biosynthesis of keratan sulfate.

Expression of zebrafish Lc3 synthase gene in embryonic lens requires hedgehog signaling

Glycosyltransferases are involved in synthesis of various glycolipids and glycoproteins that play important roles in many biological processes. We have identified a zebrafish gene encoding a member of beta1,3-N-acetylglucosaminyltransferase family, the Lc3 synthase/bGn-T5. Whole-mount in situ hybridization reveals that the Lc3 synthase gene is expressed in two distinct phases during the zebrafish embryogenesis. The early phase extends from late blastulation to the completion of epiboly, during which the expression occurs in the superficial layer of the embryos. The second phase of expression starts during mid-segmentation and persists until day 3, during which the expression occurs prominently in the developing lens. The expression of the Lc3 synthase gene in the lens is inhibited in you-too (yot) mutant embryos that are defective in Hedgehog signaling. The expression in the lens also decreases in cyclops (cyc) and one-eyed-pinhead (oep) mutant embryos and lefty1-injected embryos, which are deficient in Nodal signaling and lack Hedgehog activity in the ventral brain. These results suggest that Hedgehog signaling is required for the Lc3 synthase expression in embryonic lens.

A novel I-branching beta-1,6-N-acetylglucosaminyltransferase involved in human blood group I antigen expression

The human blood group i and I antigens are determined by linear and branched poly-N-acetyllactosamine structures, respectively. In erythrocytes, the fetal i antigen is converted to the adult I antigen by I-branching beta-1,6-N-acetylglucosaminyltransferase (IGnT) during development. Dysfunction of the I-branching enzyme may result in the adult i phenotype in erythrocytes. However, the I gene responsible for blood group I antigen has not been fully confirmed. We report here a novel human I-branching enzyme, designated IGnT3. The genes for IGnT1 (reported in 1993), IGnT2 (also presented in this study), and IGnT3 consist of 3 exons and share the second and third exons. Bone marrow cells preferentially expressed IGnT3 transcript. During erythroid differentiation using CD34(+) cells, IGnT3 was markedly up-regulated with concomitant decrease in IGnT1/2. Moreover, reticulocytes expressed the IGnT3 transcript, but IGnT1/2 was below detectable levels. By molecular genetic analyses of an adult i pedigree, individuals with the adult i phenotype were revealed to have heterozygous alleles with mutations in exon 2 (1006G>A; Gly336Arg) and exon 3 (1049G>A; Gly350Glu), respectively, of the IGnT3 gene. Chinese hamster ovary (CHO) cells transfected with each mutated IGnT3 cDNA failed to express I antigen. These findings indicate that the expression of the blood group I antigen in erythrocytes is determined by a novel IGnT3, not by IGnT1 or IGnT2.

Extended core 1 and core 2 branched O-glycans differentially modulate sialyl Lewis X-type L-selectin ligand activity

It has been established that sialyl Lewis x in core 2 branched O-glycans serves as an E- and P-selectin ligand. Recently, it was discovered that 6-sulfosialyl Lewis x in extended core 1 O-glycans, NeuNAcalpha2-->3Galbeta1-->4(Fucalpha1-->3(sulfo-->6))GlcNAcbeta1--> 3Galbeta1-->3GalNAcalpha1-->Ser/Thr, functions as an L-selectin ligand in high endothelial venules. Extended core 1 O-glycans can be synthesized when a core 1 extension enzyme is present. In this study, we first show that beta1,3-N-acetylglucosaminyltransferase-3 (beta3GlcNAcT-3) is almost exclusively responsible for core 1 extension among seven different beta3GlcNAcTs and thus acts on core 1 O-glycans attached to PSGL-1. We found that transcripts encoding beta3GlcNAcT-3 were expressed in human neutrophils and lymphocytes but that their levels were lower than those of transcripts encoding core 2 beta1,6-N-acetylglucosaminyltransferase I (Core2GlcNAcT-I). Neutrophils also expressed transcripts encoding fucosyltransferase VII (FucT-VII) and Core2GlcNAcT-I, whereas lymphocytes expressed only small amounts of transcripts encoding FucT-VII. To determine the roles of sialyl Lewis x in extended core 1 O-glycans, Chinese hamster ovary (CHO) cells were stably transfected to express PSGL-1, FucT-VII, and either beta3GlcNAcT-3 or Core2GlcNAcT-I. Glycan structural analyses disclosed that PSGL-1 expressed in these transfected cells carried comparable amounts of sialyl Lewis x in extended core 1 and core 2 branched O-glycans. In a rolling assay, CHO cells expressing sialyl Lewis x in extended core 1 O-glycans supported a significant degree of shear-dependent tethering and rolling of neutrophils and lymphocytes, although less than CHO cells expressing sialyl Lewis x in core 2 branched O-glycans. These results indicate that sialyl Lewis x in extended core 1 O-glycans can function as an L-selectin ligand and is potentially involved in neutrophil adhesion on neutrophils bound to activated endothelial cells.

Cloning and characterization of a novel UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase, pp-GalNAc-T14

A novel member of the human UDP-N-acetyl-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase (pp-GalNAc-T) gene family was cloned and designated pp-GalNAc-T14. This type II membrane protein contains all motifs that are conserved in the pp-GalNAc-T family proteins and forms a subfamily with pp-GalNAc-T2 on the phylogenetic tree. Quantitative real time PCR analysis revealed significantly high expression of the pp-GalNAc-T14 transcript in kidney, although the transcripts were ubiquitously expressed in all tissues examined. Furthermore, the recombinant pp-GalNAc-T14 transferred GalNAc to a panel of mucin-derived peptide substrates such as Muc2, Muc5AC, Muc7, and Muc13 (-58). Our results provide evidence that pp-GalNAc-T14 is a new member of the pp-GalNAc-T family and suggest that pp-GalNAc-T14 may be involved in the O-glycosylation in kidney.

Drosophila egghead encodes a beta 1,4-mannosyltransferase predicted to form the immediate precursor glycosphingolipid substrate for brainiac

The neurogenic Drosophila genes brainiac and egghead are essential for epithelial development in the embryo and in oogenesis. Analysis of egghead and brainiac mutants has led to the suggestion that the two genes function in a common signaling pathway. Recently, brainiac was shown to encode a UDP-N-acetylglucosamine:beta Man beta 1,3-N-acetylglucosaminyltransferase (beta 3GlcNAc-transferase) tentatively assigned a key role in biosynthesis of arthroseries glycosphingolipids and forming the trihexosylceramide, GlcNAc beta 1-3Man beta 1-4Glc beta 1-1Cer. In the present study we demonstrate that egghead encodes a Golgi-located GDP-mannose:beta Glc beta 1,4-mannosyltransferase tentatively assigned a biosynthetic role to form the precursor arthroseries glycosphingolipid substrate for Brainiac, Man beta 1-4Glc beta 1-1Cer. Egghead is unique among eukaryotic glycosyltransferase genes in that homologous genes are limited to invertebrates, which correlates with the exclusive existence of arthroseries glycolipids in invertebrates. We propose that brainiac and egghead function in a common biosynthetic pathway and that inactivating mutations in either lead to sufficiently early termination of glycolipid biosynthesis to inactivate essential functions mediated by glycosphingolipids.

Cloning and characterization of a new human UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase, designated pp-GalNAc-T13, that is specifically expressed in neurons and synthesizes GalNAc alpha-serine/threonine antigen

To date, 10 members of the UDP-N-acetyl-alpha-d-galactosamine:polypeptide N-acetylgalactosaminyltransferase (pp-GalNAc-T) family have been cloned and analyzed in human. In this study, we cloned and analyzed a novel human pp-GalNAc-T from an NT2 cell cDNA library, and we named it pp-GalNAc-T13. In amino acid sequences, pp-GalNAc-T13 was highly homologous, showing 84.3% identity, to pp-GalNAc-T1. Real time PCR analysis revealed pp-GalNAc-T13 to be highly and restrictively expressed in the brain and present at very low or undetectable levels in other tissues, in contrast to the ubiquitous expression of pp-GalNAc-T1. pp-GalNAc-T13 was abundantly expressed in all neuroblastoma cells examined and primary cultured neurons but not in glioblastoma cells and primary cultured astrocytes. pp-GalNAc-T13 exhibited much stronger activity to transfer GalNAc to mucin peptides, such as Muc5Ac and MUC7, than did pp-GalNAc-T1. In addition, pp-GalNAc-T13 differed in substrate specificity to pp-GalNAc-T1. pp-GalNAc-T13 was able to form a triplet Tn epitope, three consecutive GalNAc-Ser/Thr structures, on peptides encoded in syndecan-3, a proteoglycan expressed in neurons. pp-GalNAc-T13-deficient mice have been established in a previous work. Immunohistochemical study showed a remarkable decrease in Tn antigen expression in the cerebellum of the pp-GalNAc-T13 knockout mouse. pp-GalNAc-T13 would be a major enzyme responsible for the synthesis of O-glycan and specifically the Tn antigen epitope in neurons.

Molecular cloning and characterization of a novel UDP-Gal:GalNAc(alpha) peptide beta 1,3-galactosyltransferase (C1Gal-T2), an enzyme synthesizing a core 1 structure of O-glycan

Recently, a UDP-Gal:GalNAc(alpha) peptide beta1,3-galactosyltransferase (core 1 synthase 1; C1Gal-T1) has been purified from rat liver and its complementary DNA cloned from several species. We isolated a second candidate for core 1 synthase from a Colo205 cDNA library and named it C1Gal-T2. The deduced amino acid sequence of C1Gal-T2, having 26% homology to C1Gal-T1, showed a topology typical of a type II membrane protein. Real time PCR analysis revealed that the expression of C1Gal-T2 transcripts was widespread in many tissues and of relatively high level in salivary gland, stomach, small intestine, kidney, testis, thymus, and spleen. LSC cells, having no core 1 synthase activity, were transfected stably with the C1Gal-T2 gene. Their microsome fraction showed beta1,3-galactosyltransferase activity toward GalNAc-alpha-para-nitrophenyl and GalNAc(alpha)1 peptides resulting in the synthesis of the core 1 structure. The core 1 synthesizing activity of C1Gal-T2 was also determined by flow cytometry and lectin blotting using the LSC cells stably expressing C1Gal-T2. Finally, LSC cells, and Jurkat cells that also lack the core 1 synthase activity, were found to have null alleles of C1Gal-T2. These results indicated that C1Gal-T2 is the second candidate for core 1 synthase that plays an important role in synthesizing O-glycans in digestive organs.

Enzymatic synthesis of chondroitin with a novel chondroitin sulfate N-acetylgalactosaminyltransferase that transfers N-acetylgalactosamine to glucuronic acid in initiation and elongation of chondroitin sulfate synthesis

We found a novel glycosyltransferase gene having a hypothetical beta 1,4-galactosyltransferase motif (GenBank accession number ) by a BLAST search and cloned its full-length open reading frame using the 5'-rapid amplification of cDNA ends method. The truncated form was expressed in insect cells as a soluble enzyme. It transferred N-acetylgalactosamine, not galactose, to para-nitrophenyl-beta-glucuronic acid. The N-acetylgalactosamine-glucuronic acid linkage has been identified only in chondroitin sulfate; therefore, we examined its chondroitin elongation and initiation activities. N-Acetylgalactosaminyltransferase activity was observed toward chondroitin poly- and oligosaccharides, chondroitin sulfate oligosaccharides, and linkage tetrasaccharide (GlcA-Gal-Gal-Xyl-O-methoxyphenyl), and the chondroitin polysaccharide and linkage tetrasaccharide were better acceptor substrates than the others. Northern blot analysis and quantitative real-time PCR analysis revealed that its 4-kb transcripts were highly expressed in thyroid and placenta, although they were ubiquitously expressed in various tissues and cells. These results suggest that this enzyme has N-acetylgalactosaminyltransferase activity in both the elongation and initiation of chondroitin sulfate synthesis. Furthermore, we performed enzymatic synthesis of chondroitin pentasaccharide in vitro. In one tube reaction with four enzymes, beta 1,4-galactosyltransferase-VII, beta 1,3-galactosyltransferase-VI, glucuronyltransferase-I, and this enzyme, and a synthetic xylose-peptide acceptor, the structure GalNAc-GlcA-Gal-Gal-Xyl-peptide was constructed. This is the first report of a chondroitin pentasaccharide constructed with recombinant glycosyltransferases in vitro.

Molecular cloning and characterization of a novel chondroitin sulfate glucuronyltransferase that transfers glucuronic acid to N-acetylgalactosamine

We found a novel human gene (GenBank accession number, Kazusa DNA Research Institute KIAA1402) that possesses homology with chondroitin synthase. The full-length open reading frame consists of 772 amino acids and encodes a typical type II membrane protein. This enzyme had a domain containing beta 3-glycosyltransferase motifs, which might be a beta3-glucuronyltransferase domain, but no domain with beta 4-glycosyltransferase motifs, although both are found in chondroitin synthase. The putative catalytic domain was expressed in COS-7 cells as a soluble enzyme. Its glucuronyltransferase activity was observed when chondroitin and chondroitin sulfate polysaccharides and oligosaccharides were used as acceptor substrates. However, it was not detected when dermatan sulfate, hyaluronan, heparan sulfate, heparin, N-acetylheparosan, lactosamine tetrasaccharide, and linkage tri- and tetrasaccharide acceptors were employed. The reaction product, which was speculated to exhibit a GlcA beta 1-3GalNAc linkage structure at its non-reducing terminus, showed the following characteristics. 1) It was catabolized by beta-glucuronidase. 2) It was an acceptor for Escherichia coli K4 chondroitin polymerase (K4 chondroitin polymerase). 3) The product of K4 chondroitin polymerase was cleaved by chondroitinase ACII. On the other hand, no N-acetylgalactosaminyltransferase activity was detected toward any acceptors. Quantitative real time PCR analysis revealed that its transcripts were highly expressed in the placenta, small intestine, and pancreas, although they were ubiquitously expressed in various tissues and cell lines. This enzyme could play a role in the synthesis of chondroitin sulfate as a glucuronyltransferase.

The Drosophila gene brainiac encodes a glycosyltransferase putatively involved in glycosphingolipid synthesis

The Drosophila genes fringe and brainiac exhibit sequence similarities to glycosyltransferases. Drosophila and mammalian fringe homologs encode UDP-N-acetylglucosamine:fucose-O-Ser beta1,3-N-acetylglucosaminyltransferases that modulate the function of Notch family receptors. The biological function of brainiac is less well understood. brainiac is a member of a large homologous mammalian beta3-glycosyltransferase family with diverse functions. Eleven distinct mammalian homologs have been demonstrated to encode functional enzymes forming beta1-3 glycosidic linkages with different UDP donor sugars and acceptor sugars. The putative mammalian homologs with highest sequence similarity to brainiac encode UDP-N-acetylglucosamine:beta1,3-N-acetylglucosaminyltransferases (beta3GlcNAc-transferases), and in the present study we show that brainiac also encodes a beta3GlcNAc-transferase that uses beta-linked mannose as well as beta-linked galactose as acceptor sugars. The inner disaccharide core structures of glycosphingolipids in mammals (Galbeta1-4Glcbeta1-Cer) and insects (Manbeta1-4Glcbeta1-Cer) are different. Both disaccharide glycolipids served as substrates for brainiac, but glycolipids of insect cells have so far only been found to be based on the GlcNAcbeta1-3Manbeta1-4Glcbeta1-Cer core structure. Infection of High Five(TM) cells with baculovirus containing full coding brainiac cDNA markedly increased the ratio of GlcNAcbeta1-3Manbeta1-4Glcbeta1-Cer glycolipids compared with Galbeta1-4Manbeta1-4Glcbeta1-Cer found in wild type cells. We suggest that brainiac exerts its biological functions by regulating biosynthesis of glycosphingolipids.

Molecular cloning and characterization of a novel member of the UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase family, pp-GalNAc-T12

We cloned in silico a novel human UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase (pp-GalNAc-T), pp-GalNAc-T12. The deduced amino acid sequence of pp-GalNAc-T12 contains all conserved motifs in pp-GalNAc-T family proteins. Quantitative real time polymerase chain reaction analysis revealed that the pp-GalNAc-T12 transcript was expressed mainly in digestive organs such as stomach, small intestine and colon. The recombinant pp-GalNAc-T12 transferred GalNAc to the mucin-derived peptides such as the Muc1a, Muc5AC, EA2 peptides and the GalNAc-Muc5AC glycopeptide. Since mucins are glycoproteins mainly produced in the digestive organs, our results suggest that pp-GalNAc-T12 plays an important role in the initial step of mucin-type oligosaccharide biosynthesis in digestive organs.

Molecular cloning and characterization of a novel UDP-GlcNAc:GalNAc-peptide beta1,3-N-acetylglucosaminyltransferase (beta 3Gn-T6), an enzyme synthesizing the core 3 structure of O-glycans

The core 3 structure of the O-glycan, GlcNAcbeta1-3GalNAcalpha1-serine/threonine, an important precursor in the biosynthesis of mucin-type glycoproteins, is synthesized by UDP-N-acetylglucosamine:GalNAc-peptide beta1,3-N- acetylglucosaminyltransferase (beta3Gn-T; core 3 synthase). The core 3 structure is restricted in its occurrence to mucins from specific tissues such as the stomach, small intestine, and colon. A partial sequence encoding a novel member of the human beta3Gn-T family was found in one of the data bases. We cloned a complementary DNA of this gene and named it beta3Gn-T6. The putative amino acid sequence of beta3Gn-T6 retains the beta3Gn-T motifs and is predicted to comprise a typical type II membrane protein. The soluble form of beta3Gn-T6 expressed in insect cells showed beta3Gn-T activity toward GalNAcalpha-p-nitrophenyl and GalNAcalpha1-serine/threonine. The beta1,3-linkage between GlcNAc and GalNAc of the enzyme reaction product was confirmed by high performance liquid chromatography and NMR analyses. beta3Gn-T6 effectively transferred a GlcNAc to the GalNAc residue on MUC1 mucin, resulting in the synthesis of a core 3 structure. Real time PCR analysis revealed that the beta3Gn-T6 transcript was restricted in its distribution, mainly to the stomach, colon, and small intestine. We concluded that beta3Gn-T6 is the most logical candidate for the core 3 synthase, which plays an important role in the synthesis of mucin-type O-glycans in digestive organs.

Functional assignment of motifs conserved in beta 1,3-glycosyltransferases

The beta 1,3-glycosyltransferase enzymes identified to date share several conserved regions and conserved cysteine residues, all being located in the putative catalytic domain. To investigate the importance of these motifs and cysteines for the enzymatic activity, 14 mutants of the murine beta 1,3-galactosyltransferase-I gene were constructed and expressed in Sf9 insect cells. Seven mutations abolished the galactosyltransferase activity. Kinetic analysis of the other seven active mutants revealed that three of them showed a threefold to 21-fold higher apparent K(m) with regard to the donor substrate UDP-galactose relative to the wild-type enzyme, while two mutants had a sixfold to 7.5-fold increase of the apparent K(m) value for the acceptor substrate N-acetylglucosamine-beta-p-nitrophenol. Taken together, our results indicate that the conserved residues W101 and W162 are involved in the binding of the UDP-galactose donor, the residue W315 in the binding of the N-acetylglucosamine-beta-p-nitrophenol acceptor, and the domain including E264 appears to participate in the binding of both substrates.

Biosynthesis of the linkage region of glycosaminoglycans: cloning and activity of galactosyltransferase II, the sixth member of the beta 1,3-galactosyltransferase family (beta 3GalT6)

A family of five beta1,3-galactosyltransferases has been characterized that catalyze the formation of Galbeta1,3GlcNAcbeta and Galbeta1,3GalNAcbeta linkages present in glycoproteins and glycolipids (beta3GalT1, -2, -3, -4, and -5). We now report a new member of the family (beta3GalT6), involved in glycosaminoglycan biosynthesis. The human and mouse genes were located on chromosomes 1p36.3 and 4E2, respectively, and homologs are found in Drosophila melanogaster and Caenorhabditis elegans. Unlike other members of the family, beta3GalT6 showed a broad mRNA expression pattern by Northern blot analysis. Although a high degree of homology across several subdomains exists among other members of the beta3-galactosyltransferase family, recombinant enzyme did not utilize glucosamine- or galactosamine-containing acceptors. Instead, the enzyme transferred galactose from UDP-galactose to acceptors containing a terminal beta-linked galactose residue. This product, Galbeta1,3Galbeta is found in the linkage region of heparan sulfate and chondroitin sulfate (GlcAbeta1,3Galbeta1,3Galbeta1,4Xylbeta-O-Ser), indicating that beta3GalT6 is the so-called galactosyltransferase II involved in glycosaminoglycan biosynthesis. Its identity was confirmed in vivo by siRNA-mediated inhibition of glycosaminoglycan synthesis in HeLa S3 cells. Its localization in the medial Golgi indicates that this is the major site for assembly of the linkage region.

Regulation of sulfoglucuronyl glycolipid synthesis in the developing rat sciatic nerve

Sulfoglucuronyl glycolipids (SGGLs) have been considered as target antigens in demyelinating peripheral neuropathies associated with IgM monoclonal gammopathy. The regulation of expression of SGGLs in the rat sciatic nerve during development was studied by assaying the levels of SGGLs and activities of four glycosyltransferases sequentially involved in their synthesis from lactosylceramide. The levels of SGGLs in the sciatic nerve increased with development and reached a maximum at sixty days after birth. The rate of increase in the level of SGGLs between day 5 to 20 was similar to rate of deposition of myelin in the nerve. Analysis of the activities of the glycosyltransferases showed that only lactotriosylceramide galactosyltransferase (LcOse3Cer-GalTr) increased in parallel with the levels of SGGLs during development. The other three enzymes were not co-relative with the synthesis of SGGLs. The product of LcOse3Cer-GalTr reaction, nLcOse4Cer is the key intermediate for all neolactoglycolipids, particularly NeuAc alpha2-3nLcOse4Cer or nLM1, which is the major ganglioside (60%) of myelin in rat sciatic nerve. The results suggest that in the sciatic nerve SGGLs are mostly associated with Schwann cell myelin and their synthesis is regulated by LcOse3Cer-GalTr, unlike in the cerebral cortex and cerebellum where SGGLs are associated with the neuronal membranes and their synthesis is regulated by lactosylceramide N-acetylglucosaminyltransferase (LcOse2Cer-GlcNAcTr).

Cloning of a mouse beta 1,3 N-acetylglucosaminyltransferase GlcNAc(beta 1,3)Gal(beta 1,4)Glc-ceramide synthase gene encoding the key regulator of lacto-series glycolipid biosynthesis

The distinction between the different classes of glycolipids is conditioned by the action of specific core transferases. The entry point for lacto-series glycolipids is catalyzed by the beta1,3 N-acetylglucosaminyltransferase GlcNAc(beta1,3)Gal(beta1,4)Glc-ceramide (Lc3) synthase enzyme. The Lc3 synthase activity has been shown to be regulated during development, especially during brain morphogenesis. Here, we report the molecular cloning of a mouse gene encoding an Lc3 synthase enzyme. The mouse cDNA included an open reading frame of 1131 base pairs encoding a protein of 376 amino acids. The Lc3 synthase protein shared several structural motifs previously identified in the members of the beta1,3 glycosyltransferase superfamily. The Lc3 synthase enzyme efficiently utilized the lactosyl ceramide glycolipid acceptor. The identity of the reaction products of Lc3 synthase-transfected CHOP2/1 cells was confirmed by thin-layer chromatography immunostaining using antibodies TE-8 and 1B2 that recognize Lc3 and Gal(beta1,4)GlcNAc(beta1,3)Gal(beta1,4)Glc-ceramide (nLc4) structures, respectively. In addition to the initiating activity for lacto-chain synthesis, the Lc3 synthase could extend the terminal N-acetyllactosamine unit of nLc4 and also had a broad specificity for gangliosides GA1, GM1, and GD1b to generate neolacto-ganglio hybrid structures. The mouse Lc3 synthase gene was mainly expressed during embryonic development. In situ hybridization analysis revealed that that the Lc3 synthase was expressed in most tissues at embryonic day 11 with elevated expression in the developing central nervous system. Postnatally, the expression was restricted to splenic B-cells, the placenta, and cerebellar Purkinje cells where it colocalized with HNK-1 reactivity. These data support a key role for the Lc3 synthase in regulating neolacto-series glycolipid synthesis during embryonic development.

Suppression of integrin expression and tumorigenicity by sulfation of lactosylceramide in 3LL Lewis lung carcinoma cells

To investigate the cellular functions of sulfated glycosphingolipids, we introduced the cerebroside sulfotransferase (CST) gene into J5 cells, a subclone of 3LL Lewis lung carcinoma cells. The J5 cells lack acidic glycosphingolipids but accumulate their common biosynthetic precursor, lactosylceramide. We established the stable CST transfectants, J5/CST-1 and J5/CST-2 clones, highly expressing sulfated lactosylceramide (SM3). Both clones exhibited more spherical morphology in comparison to mock transfectant, and their adhesiveness to fibronectin and laminin was significantly lower. The loss of cell-substratum interactions in these SM3-expressing cells could be attributed to decreased expression of integrins (alpha(5), alpha(6), and beta(1)) on the cell surface and their whole cellular levels. However, the levels of H-2K(b) and H-2D(b) antigens remained unchanged. Reverse transcriptase-polymerase chain reaction and Northern blot analyses for these integrins exhibited significant decrease of beta(1) gene expression in J5/CST-1 and 2, but there was no change in the levels of alpha(5) and alpha(6) transcripts. Deglycosylation by endoglycosidase H treatment clearly demonstrated that the precursor form of beta(1) integrin, possessing high mannose oligosaccharide chains, was preferentially decreased in the CST transfectants. These results demonstrate that endogenous SM3 negatively regulates beta(1) integrin expression at the transcriptional level, and the decrease of alpha integrin proteins in the CST transfectants was due to the post-transcriptional modification. We suggest the putative importance of the intracellular pre-beta(1) integrin pool for normal integrin maturation and subsequent function. Although the rates of cell proliferation in vitro for mock and CST transfectants were similar, tumorigenicity of J5/CST-1 and -2 cells inoculated into syngeneic C57/BL6 mice was greatly decreased or even absent. This was probably due to global loss of the efficient cell-matrix interactions, which are essential for the development of malignant tumors in vivo. Thus, we showed the evidence that cellular SM3 negatively regulates the cell-substratum interaction, resulting in the loss of tumorigenicity.