Comprehensive Enzymatic Characterization of Glycosyltransferases with a β3GT or β4GT Motif

Glycosyltransferases: glycoengineers in human milk oligosaccharide synthesis and manufacturing

Human milk oligosaccharides (HMOs) are a diverse group of complex carbohydrates that play crucial roles in infant health, promoting a beneficial gut microbiota, modulating immune responses, and protecting against pathogens. Central to the synthesis of HMOs are glycosyltransferases, a specialized class of enzymes that catalyse the transfer of sugar moieties to form the complex glycan structures characteristic of HMOs. This review provides an in-depth analysis of glycosyltransferases, beginning with their classification based on structural and functional characteristics. The catalytic activity of these enzymes is explored, highlighting the mechanisms by which they facilitate the precise addition of monosaccharides in HMO biosynthesis. Structural insights into glycosyltransferases are also discussed, shedding light on how their conformational features enable specific glycosidic bond formations. This review maps out the key biosynthetic pathways involved in HMO production, including the synthesis of lactose, and subsequent fucosylation and sialylation processes, all of which are intricately regulated by glycosyltransferases. Industrial methods for HMO synthesis, including chemical, enzymatic, and microbial approaches, are examined, emphasizing the role of glycosyltransferases in these processes. Finally, the review discusses future directions in glycosyltransferase research, particularly in enhancing the efficiency of HMO synthesis and developing advanced analytical techniques to better understand the structural complexity and biological functions of HMOs.

The Uremic Toxins Inorganic Phosphate, Indoxylsulphate, p-Cresylsulphate, and TMAO Induce the Generation of Sulphated Glycosaminoglycans in Aortic Tissue and Vascular Cells via pAKT Signaling: A Missing Link in the "Gut-Matrix Axis"

Gut-derived uremic toxins (UTs) contribute to cardiovascular disorders like atherosclerosis and cardiomyopathy in patients with chronic kidney disease (CKD), causing increased cardiovascular morbidity and mortality. The intermediate steps between higher concentrations of gut-derived UTs and organ damage caused by UTs are still insufficiently understood. Glycosaminoglycans (GAGs) as components of the extracellular matrix are known to interact with various ligands such as growth factors or receptors, thereby influencing (patho)physiological processes. We previously found that the UT inorganic phosphate (Pi) induces the synthesis and sulphation of the GAGs heparan sulphate and chondroitin sulphate in the rat vascular smooth muscle cell (VSMC) line A7r5 and in the human endothelial cell (EC) line EA.Hy926. The aim of this study was to investigate if other organic UTs modulate GAGs in vascular cells as well. We treated ex vivo cultures of rat aortic rings as well as primary rat VSMCs and human ECs with the UTs Pi, indoxylsulphate (IS), p-cresylsulphate (pCS), trimethylamine N-oxide (TMAO), and urea, and analyzed the samples by histological staining, qPCR, western blot, HPLC, and colorimetric assays. The UT treatment of aortic rings and cells increased contents of sulphated GAGs and hyaluronic acid. UT-treated cells contained higher amounts of 4S- and 6S-sulphated GAGs compared to controls. This was accompanied by altered expressions of genes and proteins relevant for GAG metabolism. Mechanistically, the effects of the UTs on GAGs involve the activation of the PI3K/Akt pathway and of the transcription factor NF-κB. In conclusion, the UT-induced remodeling of the cardiovascular matrix by upregulation of sulphated GAGs and hyaluronic acid in aortic tissue and vascular cells might be a missing link between gut-derived UT and pathophysiological alterations in the cardiovascular system in the sense of a gut-matrix axis.

Impacts of β-1, 3-N-acetylglucosaminyltransferases (B3GNTs) in human diseases

Glycosylation modification of proteins is a common post-translational modification that exists in various organisms and has rich biological functions. It is usually catalyzed by multiple glycosyltransferases located in the Golgi apparatus. β-1,3-N-acetylglucosaminyltransferases (B3GNTs) are members of the glycosyltransferases and have been found to be involved in the occurrence and development of a variety of diseases including autoimmunity diseases, cancers, neurodevelopment, musculoskeletal system, and metabolic diseases. The functions of B3GNTs represent the glycosylation of proteins is a crucial and frequently life-threatening step in progression of most diseases. In this review, we give an overview about the roles of B3GNTs in tumor, nervous system, musculoskeletal and metabolic diseases, describing the recent results about B3GNTs, in order to provide a research direction and exploration value for the prevention, diagnosis and treatment of these diseases.

Uremic Toxin-Induced Exosome-like Extracellular Vesicles Contain Enhanced Levels of Sulfated Glycosaminoglycans which Facilitate the Interaction with Very Small Superparamagnetic Iron Oxide Particles

Uremic toxins exert pathophysiological effects on cells and tissues, such as the generation of a pro-calcifying subtype of exosome-like extracellular vesicles (EVs) in vascular cells. Little is known about the effects of the toxins on the surface structure of EVs. Thus, we studied the effects of uremic toxins on the abundance of sulfated glycosaminoglycans (GAGs) in EVs, and the implications for binding of ligands such as very small superparamagnetic iron oxide particles (VSOPs) which could be of relevance for radiological EV-imaging. Vascular cells were treated with the uremic toxins NaH2PO4 and a mixture of urea and indoxyl sulfate. Uremia in rats was induced by adenine feeding. EVs were isolated from culture supernatants and plasma of rats. By proton T1-relaxometry, magnetic particle spectroscopy, and analysis of genes, proteins, and GAG-contents, we analyzed the roles of GAGs in the ligand binding of EVs. By influencing GAG-associated genes in host cells, uremic toxins induced higher GAG contents in EVs, particularly of sulfated chondroitin sulfate and heparan sulfate chains. EVs with high GAG content interacted stronger with VSOPs compared to control ones. This was confirmed by experiments with GAG-depleted EVs from genetically modified CHO cells and with uremic rat-derived EVs. Mechanistically, uremic toxin-induced PI3K/AKT-signaling and expression of the sulfate transporter SLC26A2 in host cells contributed to high GAG contents in EVs. In conclusion, uremic conditions induce enhanced GAG contents in EVs, which entails a stronger interaction with VSOPs. VSOPs might be suitable for radiological imaging of EVs rich in GAGs.

3'-sulfated LewisA/C: An oncofetal epitope associated with metaplastic and oncogenic plasticity of the gastrointestinal foregut

Metaplasia, dysplasia, and cancer arise from normal epithelia via a plastic cellular transformation, typically in the setting of chronic inflammation. Such transformations are the focus of numerous studies that strive to identify the changes in RNA/Protein expression that drive such plasticity along with the contributions from the mesenchyme and immune cells. However, despite being widely utilized clinically as biomarkers for such transitions, the role of glycosylation epitopes is understudied in this context. Here, we explore 3'-Sulfo-Lewis A/C, a clinically validated biomarker for high-risk metaplasia and cancer throughout the gastrointestinal foregut: esophagus, stomach, and pancreas. We discuss the clinical correlation of sulfomucin expression with metaplastic and oncogenic transformation, as well as its synthesis, intracellular and extracellular receptors and suggest potential roles for 3'-Sulfo-Lewis A/C in contributing to and maintaining these malignant cellular transformations.

Elevated transcription and glycosylation of B3GNT5 promotes breast cancer aggressiveness

BACKGROUND

Basal-like breast cancer (BLBC) is the most aggressive subtype of breast cancer because of its aggressive biological characteristics and no effective targeted agents. However, the mechanism underlying its aggressive behavior remain poorly understood. β1,3-N-acetylglucosaminyltransferase V (B3GNT5) overexpression occurs specifically in BLBC. Here, we studied the possible molecular mechanisms of B3GBT5 promoting the aggressiveness of BLBC.

METHODS

The potential effects of B3GNT5 on breast cancer cells were tested by colony formation, mammosphere formation, cell proliferation assay, flow cytometry and Western blotting. The glycosylation patterns of B3GNT5 and associated functions were determined by Western blotting, quantitative real-time PCR and flow cytometry. The effect of B3GNT5 expression on BLBC was assessed by in vitro and in vivo tumorigenesis model.

RESULTS

In this study, we showed that B3GNT5 copy number amplification and hypomethylation of B3GNT5 promoter contributed to the overexpression of B3GNT5 in BLBC. Knockout of B3GNT5 strongly reduced surface expression of SSEA-1 and impeded cancer stem cell (CSC)-like properties of BLBC cells. Our results also showed that B3GNT5 protein was heavily N-glycosylated, which is critical for its protein stabilization. Clinically, elevated expression of B3GNT5 was correlated with high grade, large tumor size and poor survival, indicating poor prognosis of breast cancer patients.

CONCLUSIONS

Our work uncovers the critical association of B3GNT5 overexpression and glycosylation with enhanced CSCs properties in BLBC. These findings suggest that B3GNT5 has the potential to become a prognostic marker and therapeutic target for BLBC.

CRISPR activation screen identifies BCL-2 proteins and B3GNT2 as drivers of cancer resistance to T cell-mediated cytotoxicity

The cellular processes that govern tumor resistance to immunotherapy remain poorly understood. To gain insight into these processes, here we perform a genome-scale CRISPR activation screen for genes that enable human melanoma cells to evade cytotoxic T cell killing. Overexpression of four top candidate genes (CD274 (PD-L1), MCL1, JUNB, and B3GNT2) conferred resistance in diverse cancer cell types and mouse xenografts. By investigating the resistance mechanisms, we find that MCL1 and JUNB modulate the mitochondrial apoptosis pathway. JUNB encodes a transcription factor that downregulates FasL and TRAIL receptors, upregulates the MCL1 relative BCL2A1, and activates the NF-κB pathway. B3GNT2 encodes a poly-N-acetyllactosamine synthase that targets >10 ligands and receptors to disrupt interactions between tumor and T cells and reduce T cell activation. Inhibition of candidate genes sensitized tumor models to T cell cytotoxicity. Our results demonstrate that systematic gain-of-function screening can elucidate resistance pathways and identify potential targets for cancer immunotherapy.

Loss-of-function CRISPR-based screens have identified several genes associated with cancer resistance to T cell-induced cytotoxicity. Here the authors perform a genome-scale, gain-of-function CRISPR screen and identify candidate genes, including the poly-N-acetyllactosamine synthase B3GNT2, whose overexpression confers tumor cell resistance to T cell cytotoxicity

Bombyx mori β1,4-N-acetylgalactosaminyltransferase possesses relaxed donor substrate specificity in N-glycan synthesis

N-Glycosylation is one of the most important post-translational protein modifications in eukaryotic cells. Although more than 200 N-glycogenes contributing to N-glycan biosynthesis have been identified and characterized, the information on insect N-glycosylation is still limited. Here, focusing on insect N-glycosylation, we characterized Bombyx mori N-acetylgalactosaminyltransferase (BmGalNAcT) participating in complex N-glycan biosynthesis in mammals. BmGalNAcT localized at the Golgi and was ubiquitously expressed in every organ and in the developmental stage of the middle silk gland of fifth instar larvae. Analysis of recombinant BmGalNAcT expressed in Sf9 cells showed that BmGalNAcT transferred GalNAc to non-reducing terminals of GlcNAcβ1,2-R with β1,4-linkage. In addition, BmGalNAcT mediated transfer of galactose and N-acetylglucosamine residues but not transfer of either glucose or glucuronic acid from the UDP-sugar donor substrate to the N-glycan. Despite this tri-functional sugar transfer activity, however, most of the endogenous glycoproteins of insect cells were present without GalNAc, Gal, or GlcNAc residues at the non-reducing terminal of β1,2-GlcNAc residue(s). Moreover, overexpression of BmGalNAcT in insect cells had no effect on N-acetylgalactosaminylation, galactosylation, or N-acetylglucosaminylation of the major N-glycan during biosynthesis. These results suggested that B. mori has a novel multifunctional glycosyltransferase, but the N-glycosylation is highly and strictly regulated by the endogenous N-glycosylation machineries.

Genetic glycoengineering in mammalian cells

Advances in nuclease-based gene-editing technologies have enabled precise, stable, and systematic genetic engineering of glycosylation capacities in mammalian cells, opening up a plethora of opportunities for studying the glycome and exploiting glycans in biomedicine. Glycoengineering using chemical, enzymatic, and genetic approaches has a long history, and precise gene editing provides a nearly unlimited playground for stable engineering of glycosylation in mammalian cells to explore and dissect the glycome and its many biological functions. Genetic engineering of glycosylation in cells also brings studies of the glycome to the single cell level and opens up wider use and integration of data in traditional omics workflows in cell biology. The last few years have seen new applications of glycoengineering in mammalian cells with perspectives for wider use in basic and applied glycosciences, and these have already led to discoveries of functions of glycans and improved designs of glycoprotein therapeutics. Here, we review the current state of the art of genetic glycoengineering in mammalian cells and highlight emerging opportunities.

Comparison of human poly-N-acetyl-lactosamine synthase structure with GT-A fold glycosyltransferases supports a modular assembly of catalytic subsites

Poly-N-acetyl-lactosamine (poly-LacNAc) structures are composed of repeating [-Galβ(1,4)-GlcNAcβ(1,3)-]n glycan extensions. They are found on both N- and O-glycoproteins and glycolipids and play an important role in development, immune function, and human disease. The majority of mammalian poly-LacNAc is synthesized by the alternating iterative action of β1,3-N-acetylglucosaminyltransferase 2 (B3GNT2) and β1,4-galactosyltransferases. B3GNT2 is in the largest mammalian glycosyltransferase family, GT31, but little is known about the structure, substrate recognition, or catalysis by family members. Here we report the structures of human B3GNT2 in complex with UDP:Mg2+ and in complex with both UDP:Mg2+ and a glycan acceptor, lacto-N-neotetraose. The B3GNT2 structure conserves the GT-A fold and the DxD motif that coordinates a Mg2+ ion for binding the UDP-GlcNAc sugar donor. The acceptor complex shows interactions with only the terminal Galβ(1,4)-GlcNAcβ(1,3)- disaccharide unit, which likely explains the specificity for both N- and O-glycan acceptors. Modeling of the UDP-GlcNAc donor supports a direct displacement inverting catalytic mechanism. Comparative structural analysis indicates that nucleotide sugar donors for GT-A fold glycosyltransferases bind in similar positions and conformations without conserving interacting residues, even for enzymes that use the same donor substrate. In contrast, the B3GNT2 acceptor binding site is consistent with prior models suggesting that the evolution of acceptor specificity involves loops inserted into the stable GT-A fold. These observations support the hypothesis that GT-A fold glycosyltransferases employ coevolving donor, acceptor, and catalytic subsite modules as templates to achieve the complex diversity of glycan linkages in biological systems.

Structures and mechanism of human glycosyltransferase β1,3-N-acetylglucosaminyltransferase 2 (B3GNT2), an important player in immune homeostasis

β1,3-N-acetylglucosaminyltransferases (B3GNTs) are Golgi-resident glycosyltransferases involved in the biosynthesis of poly-N-acetyl-lactosamine chains. They catalyze the addition of the N-acetylglucosamine to the N-acetyl-lactosamine repeat as a key step of the chain elongation process. Poly-N-acetyl-lactosamine is involved in the immune system in many ways. Particularly, its long chain has been demonstrated to suppress excessive immune responses. Among the characterized B3GNTs, B3GNT2 is the major poly-N-acetyl-lactosamine synthase, and deletion of its coding gene dramatically reduced the cell surface poly-N-acetyl-lactosamine and led to hypersensitive and hyperresponsive immunocytes. Despite the extensive functional studies, no structural information is available to understand the molecular mechanism of B3GNT2, as well as other B3GNTs. Here we present the structural and kinetic studies of the human B3GNT2. Five crystal structures of B3GNT2 have been determined in the unliganded, donor substrate-bound, acceptor substrate-bound, and product(s)-bound states at resolutions ranging from 1.85 to 2.35 Å. Kinetic study shows that the transglycosylation reaction follows a sequential mechanism. Critical residues involved in recognition of both donor and acceptor substrates as well as catalysis are identified. Mutations of these invariant residues impair B3GNT2 activity in cell assays. Structural comparison with other glycosyltransferases such as mouse Fringe reveals a novel N-terminal helical domain of B3GNTs that may stabilize the catalytic domain and distinguish among different acceptor substrates.

A phylogenetic view and functional annotation of the animal β1,3-glycosyltransferases of the GT31 CAZy family

The formation of β1,3-linkages on animal glycoconjugates is catalyzed by a subset of β1,3-glycosyltransferases grouped in the Carbohydrate-Active enZYmes family glycosyltransferase-31 (GT31). This family represents an extremely diverse set of β1,3-N-acetylglucosaminyltransferases [B3GNTs and Fringe β1,3-N-acetylglucosaminyltransferases], β1,3-N-acetylgalactosaminyltransferases (B3GALNTs), β1,3-galactosyltransferases [B3GALTs and core 1 β1,3-galactosyltransferases (C1GALTs)], β1,3-glucosyltransferase (B3GLCT) and β1,3-glucuronyl acid transferases (B3GLCATs or CHs). The mammalian enzymes were particularly well studied and shown to use a large variety of sugar donors and acceptor substrates leading to the formation of β1,3-linkages in various glycosylation pathways. In contrast, there are only a few studies related to other metazoan and lower vertebrates GT31 enzymes and the evolutionary relationships of these divergent sequences remain obscure. In this study, we used bioinformatics approaches to identify more than 920 of putative GT31 sequences in Metazoa, Fungi and Choanoflagellata revealing their deep ancestry. Sequence-based analysis shed light on conserved motifs and structural features that are signatures of all the GT31. We leverage pieces of evidence from gene structure, phylogenetic and sequence-based analyses to identify two major subgroups of GT31 named Fringe-related and B3GALT-related and demonstrate the existence of 10 orthologue groups in the Urmetazoa, the hypothetical last common ancestor of all animals. Finally, synteny and paralogy analysis unveiled the existence of 30 subfamilies in vertebrates, among which 5 are new and were named C1GALT2, C1GALT3, B3GALT8, B3GNT10 and B3GNT11. Altogether, these various approaches enabled us to propose the first comprehensive analysis of the metazoan GT31 disentangling their evolutionary relationships.

Glycosyltransferase genes that cause monogenic congenital disorders of glycosylation are distinct from glycosyltransferase genes associated with complex diseases

Glycosylation of proteins, lipids and proteoglycans in human cells involves at least 167 identified glycosyltransferases (GTfs), and these orchestrate the biosynthesis of diverse types of glycoconjugates and glycan structures. Mutations in this part of the genome-the GTf-genome-cause more than 58 rare, monogenic congenital disorders of glycosylation (CDGs). They are also statistically associated with a large number of complex phenotypes, diseases or predispositions to complex diseases based on Genome-Wide Association Studies (GWAS). CDGs are extremely rare and often with severe medical consequences. In contrast, GWAS are likely to identify more common genetic variations and generally involve less severe and distinct traits. We recently confirmed that structural defects in GTf genes are extremely rare, which seemed at odds with the large number of GWAS pointing to GTf-genes. To resolve this issue, we surveyed the GTf-genome for reported CDGs and GWAS candidates; we found little overlap between the two groups of genes. Moreover, GTf-genes implicated by CDG or GWAS appear to constitute different classes with respect to their: (i) predicted roles in glycosylation pathways; (ii) potential for partial redundancy by closely homologous genes; and (iii) transcriptional regulation as evaluated by RNAseq data. Our analysis suggest that more complex traits are caused by dysregulation rather than structural deficiency of GTfs, which suggests that some glycosylation reactions may be predicted to be under tight regulation for fine-tuning of important biological functions.

Identification of Poly-N-Acetyllactosamine-Carrying Glycoproteins from HL-60 Human Promyelocytic Leukemia Cells Using a Site-Specific Glycome Analysis Method, Glyco-RIDGE

To elucidate the relationship between the protein function and the diversity and heterogeneity of glycans conjugated to the protein, glycosylation sites, glycan variation, and glycan proportions at each site of the glycoprotein must be analyzed. Glycopeptide-based structural analysis technology using mass spectrometry has been developed; however, complicated analyses of complex spectra obtained by multistage fragmentation are necessary, and sensitivity and throughput of the analyses are low. Therefore, we developed a liquid chromatography/mass spectrometry (MS)-based glycopeptide analysis method to reveal the site-specific glycome (Glycan heterogeneity-based Relational IDentification of Glycopeptide signals on Elution profile, Glyco-RIDGE). This method used accurate masses and retention times of glycopeptides, without requiring MS2, and could be applied to complex mixtures. To increase the number of identified peptide, fractionation of sample glycopeptides for reduction of sample complexity is required. Therefore, in this study, glycopeptides were fractionated into four fractions by hydrophilic interaction chromatography, and each fraction was analyzed using the Glyco-RIDGE method. As a result, many glycopeptides having long glycans were enriched in the highest hydrophilic fraction. Based on the monosaccharide composition, these glycans were thought to be poly-N-acetyllactosamine (polylactosamine [pLN]), and 31 pLN-carrier proteins were identified in HL-60 cells. Gene ontology enrichment analysis revealed that pLN carriers included many molecules related to signal transduction, receptors, and cell adhesion. Thus, these findings provided important insights into the analysis of the glycoproteome using our novel Glyco-RIDGE method. Graphical Abstract ᅟ.

Biochemical and functional characterization of glycosylation-associated mutational landscapes in colon cancer

The molecular basis of aberrant protein glycosylation, a pathological alteration widespread in colorectal cancers (CRC), and the mechanisms by which it contributes to tumor progression remain largely unknown. We performed targeted re-sequencing of 430 glycosylation-associated genes in a series of patient-derived CRC cell lines (N = 31) and matched primary tumor tissues, identifying 12 new significantly mutated glycosylation-associated genes in colon cancer. In particular, we observed an enrichment of mutations in genes (B3GNT2, B4GALT2, ST6GALNAC2) involved in the biosynthesis of N- and Cores 1–3 O-linked glycans in the colon, accounting for ~16% of the CRCs tested. Analysis of independent large-scale tumor tissue datasets confirmed recurrent mutations within these genes in colon and other gastrointestinal cancers. Systematic biochemical and phenotypic characterization of the candidate wild-type and mutant glycosyltransferases demonstrated these mutations as either markedly altering protein localization, post-translational modification, encoded enzymatic activities and/or the migratory potential of colon carcinoma cells. These findings suggest that functionally deleterious mutations in glycosyltransferase genes in part underlie aberrant glycosylation, and contribute to the pathogenesis of molecular subsets of colon and other gastrointestinal malignancies.

Investigation of glycan evolution based on a comprehensive analysis of glycosyltransferases using phylogenetic profiling

Glycans play important roles in such cell-cell interactions as signaling and adhesion, including processes involved in pathogenic infections, cancers, and neurological diseases. Glycans are biosynthesized by multiple glycosyltransferases (GTs), which function sequentially. Excluding mucin-type O-glycosylation, the non-reducing terminus of glycans is biosynthesized in the Golgi apparatus after the reducing terminus is biosynthesized in the ER. In the present study, we performed genome-wide analyses of human GTs by investigating the degree of conservation of homologues in other organisms, as well as by elucidating the phylogenetic relationship between cephalochordates and urochordates, which has long been controversial in deuterostome phylogeny. We analyzed 173 human GTs and functionally linked glycan synthesis enzymes by phylogenetic profiling and clustering, compiled orthologous genes from the genomes of other organisms, and converted them into a binary sequence based on the presence (1) or absence (0) of orthologous genes in the genomes. Our results suggest that the non-reducing terminus of glycans is biosynthesized by newly evolved GTs. According to our analysis, the phylogenetic profiles of GTs resemble the phylogenetic tree of life, where deuterostomes, metazoans, and eukaryotes are resolved into separate branches. Lineage-specific GTs appear to play essential roles in the divergence of these particular lineages. We suggest that urochordates lose several genes that are conserved among metazoans, such as those expressing sialyltransferases, and that the Golgi apparatus acquires the ability to synthesize glycans after the ER acquires this function.

Assembly, organization and regulation of cell-surface receptors by lectin–glycan complexes

Galectins are a family of β-galactoside-binding lectins carrying at least one consensus sequence in the carbohydrate-recognition domain. Properties of glycosylated ligands, such as N- and O-glycan branching, LacNAc (N-acetyl-lactosamine) content and the balance of α2,3- and α2,6-linked sialic acid dramatically influence galectin binding to a preferential set of counter-receptors. The presentation of specific glycans in galectin-binding partners is also critical, as proper orientation and clustering of oligosaccharide ligands on multiple carbohydrate side chains increase the binding avidity of galectins for particular glycosylated receptors. When galectins are released from the cells, they typically concentrate on the cell surface and the local matrix, raising their local concentration. Thus galectins can form their own multimers in the extracellular milieu, which in turn cross-link glycoconjugates on the cell surface generating galectin–glycan complexes that modulate intracellular signalling pathways, thus regulating cellular processes such as apoptosis, proliferation, migration and angiogenesis. Subtle changes in receptor expression, rates of protein synthesis, activities of Golgi enzymes, metabolite concentrations supporting glycan biosynthesis, density of glycans, strength of protein–protein interactions at the plasma membrane and stoichiometry may modify galectin–glycan complexes. Although galectins are key contributors to the formation of these extended glycan complexes leading to promotion of receptor segregation/clustering, and inhibition of receptor internalization by surface retention, when these complexes are disrupted, some galectins, particularly galectin-3 and -4, showed the ability to drive clathrin-independent mechanisms of endocytosis. In the present review, we summarize the data available on the assembly, hierarchical organization and regulation of conspicuous galectin–glycan complexes, and their implications in health and disease.

RNAi screening of human glycogene orthologs in the nematode Caenorhabditis elegans and the construction of the C. elegans glycogene database

In this study, we selected 181 nematode glycogenes that are orthologous to human glycogenes and examined their RNAi phenotypes. The results are deposited in the Caenorhabditis elegans Glycogene Database (CGGDB) at AIST, Tsukuba, Japan. The most prominent RNAi phenotypes observed are disruptions of cell cycle progression in germline mitosis/meiosis and in early embryonic cell mitosis. Along with the previously reported roles of chondroitin proteoglycans, glycosphingolipids and GPI-anchored proteins in cell cycle progression, we show for the first time that the inhibition of the functions of N-glycan synthesis genes (cytoplasmic alg genes) resulted in abnormal germline formation, ER stress and small body size phenotypes. The results provide additional information on the roles of glycoconjugates in the cell cycle progression mechanisms of germline and embryonic cells.

B4GAT1 is the priming enzyme for the LARGE-dependent functional glycosylation of α-dystroglycan

Recent studies demonstrated that mutations in B3GNT1, an enzyme proposed to be involved in poly-N-acetyllactosamine synthesis, were causal for congenital muscular dystrophy with hypoglycosylation of α-dystroglycan (secondary dystroglycanopathies). Since defects in the O-mannosylation protein glycosylation pathway are primarily responsible for dystroglycanopathies and with no established O-mannose initiated structures containing a β3 linked GlcNAc known, we biochemically interrogated this human enzyme. Here we report this enzyme is not a β-1,3-N-acetylglucosaminyltransferase with catalytic activity towards β-galactose but rather a β-1,4-glucuronyltransferase, designated B4GAT1, towards both α- and β-anomers of xylose. The dual-activity LARGE enzyme is capable of extending products of B4GAT1 and we provide experimental evidence that B4GAT1 is the priming enzyme for LARGE. Our results further define the functional O-mannosylated glycan structure and indicate that B4GAT1 is involved in the initiation of the LARGE-dependent repeating disaccharide that is necessary for extracellular matrix protein binding to O-mannosylated α-dystroglycan that is lacking in secondary dystroglycanopathies.

Differential expression patterns of N-acetylglucosaminyl transferases and polylactosamines in uterine lesions

Polylactosamine (polyLacNAc) is a fundamental structure in glycoconjugates and it is expressed in specific cells/tissues associated with the development and carcinogenesis. β1,3-N-acetylglucosaminyl transferases ((β3GnTs) play an important role in polyLacNAc synthesis, however the roles of these glycosyltransferases and their products in cancer progression are still unclear. In this sense, this work aimed to evaluate differential expression pattern of the N-acetylglucosaminyl transferases and polylactosamines in invasive and premalignant lesions of the uterus cervix. The expression of β3GnT2 and β3GnT3 were evaluated in normal (n=10) and uterine cervix lesions (n=120), both malignant [squamous carcinoma (SC)] and premalignant [cervical intraepithelial neoplasia (CIN), grades 1, 2 and 3] using immunohistochemistry. Besides, lectin histochemistry with Phytolacca americana lectin (PWM) and Wheat germ agglutinin (WGA) was also carried out to observe the presence of polyLacNAc chains and N-acetylglucosamine (GlcNAc), respectively. The β3GnT3 was expressed in almost all samples (99%) and β3GnT2 was higher expressed in disease samples mainly in CIN 3, when compared with normal (P=0.002), CIN 1 (P=0.009) and CIN 2 (P=0.03). The expression of polyLacNAc was higher is SC samples, when compared with normal (P=0.03), CIN 1 (P=0.02) and CIN 3 (P=0.004), and was observed only nuclear expression in nearly 50% of the SC samples, showing a statistically significant when compared with normal (P=0.01), CIN 1 (P=0.002), CIN 2 (P=0.007) and CIN 3 (P=0.04). Deferring from transferases and polyLacNAc chains, GlcNAc (WGA ligand) reveals a gradual staining pattern decrease with the increase of the lesion degree, being more expressed in CIN 1 lesions when compared with normal (P<0.0001), CIN 2 (P<0.0001), SC (P<0.0001) and CIN 3 (P=0.0003). Our data reveal that β3GnT2 and polyLacNAc may be involved in the progression of the pre-malignant lesions of the human uterine cervix. In addition, polyLacNAc expression only in the nucleus can be associated a poor prognostic in uterine lesions.

Molecular deciphering of the ABO system as a basis for novel diagnostics and therapeutics in ABO incompatible transplantation

In recent years ABO incompatible kidney transplantation (KTx) has become a more or less clinical routine procedure with graft and patient survival similar to those of ABO compatible transplants. Antigen-specific immunoadsorption (IA) for anti-A and anti-B antibody removal constitutes in many centers an important part of the treatment protocol. ABO antibody titration by hemagglutination is guiding the treatment; both if the recipient can be transplanted as well as in cases of suspected rejections if antibody removal should be performed. Despite the overall success of ABO incompatible KTx, there is still room for improvements and an extension of the technology to include other solid organs. Based on an increased understanding of the structural complexity and tissue distribution of ABH antigens and the fine epitope specificity of the ABO antibody repertoire, improved IA matrices and ABO antibody diagnostics should be developed. Furthermore, understanding the molecular mechanisms behind accommodation of ABO incompatible renal allografts could make it possible to induce long-term allograft acceptance also in human leukocyte antigen (HLA) sensitized recipients and, perhaps, also make clinical xenotransplantation possible.

Mutations in B3GALT6, which encodes a glycosaminoglycan linker region enzyme, cause a spectrum of skeletal and connective tissue disorders

Proteoglycans (PGs) are a major component of the extracellular matrix in many tissues and function as structural and regulatory molecules. PGs are composed of core proteins and glycosaminoglycan (GAG) side chains. The biosynthesis of GAGs starts with the linker region that consists of four sugar residues and is followed by repeating disaccharide units. By exome sequencing, we found that B3GALT6 encoding an enzyme involved in the biosynthesis of the GAG linker region is responsible for a severe skeletal dysplasia, spondyloepimetaphyseal dysplasia with joint laxity type 1 (SEMD-JL1). B3GALT6 loss-of-function mutations were found in individuals with SEMD-JL1 from seven families. In a subsequent candidate gene study based on the phenotypic similarity, we found that B3GALT6 is also responsible for a connective tissue disease, Ehlers-Danlos syndrome (progeroid form). Recessive loss-of-function mutations in B3GALT6 result in a spectrum of disorders affecting a broad range of skeletal and connective tissues characterized by lax skin, muscle hypotonia, joint dislocation, and spinal deformity. The pleiotropic phenotypes of the disorders indicate that B3GALT6 plays a critical role in a wide range of biological processes in various tissues, including skin, bone, cartilage, tendon, and ligament.

The N-acetyl-binding pocket of N-acetylglucosaminyltransferases also accommodates a sugar analog with a chemical handle at C2

In recent years, sugars with a unique chemical handle have been used to detect and elucidate the function of glycoconjugates. Such chemical handles have generally been part of an N-acetyl moiety of a sugar. We have previously developed several applications using the single mutant Y289L-β1,4-galactosyltransferase I (Y289L-β4Gal-T1) and the wild-type polypeptide-α-GalNAc-T enzymes with UDP-C2-keto-Gal. Here, we describe for the first time that the GlcNAc-transferring enzymes-R228K-Y289L-β4Gal-T1 mutant enzyme, the wild-type human β1,3-N-acetylglucosaminyltransferase-2 and human Maniac Fringe-can also transfer the GlcNAc analog C2-keto-Glc molecule from UDP-C2-keto-Glc to their respective acceptor substrates. Although the R228K-Y289L-β4Gal-T1 mutant enzyme transfers the donor sugar substrate GlcNAc or its analog C2-keto-Glc only to its natural acceptor substrate, GlcNAc, it does not transfer to its analog C2-keto-Glc. Thus, these observations suggest that the GlcNAc-transferring glycosyltransferases can generally accommodate a chemical handle in the N-acetyl-binding cavity of the donor sugar substrate, but not in the N-acetyl-binding cavity of the acceptor sugar.

A novel glycosylation signal regulates transforming growth factor beta receptors as evidenced by endo-beta-galactosidase C expression in rodent cells

The αGal (Galα1-3Gal) epitope is a xenoantigen that is responsible for hyperacute rejection in xenotransplantation. This epitope is expressed on the cell surface in the cells of all mammals except humans and Old World monkeys. It can be digested by the enzyme endo-β-galactosidase C (EndoGalC), which is derived from Clostridium perfringens. Previously, we produced EndoGalC transgenic mice to identify the phenotypes that would be induced following EndoGalC overexpression. The mice lacked the αGal epitope in all tissues and exhibited abnormal phenotypes such as postnatal death, growth retardation, skin lesion and abnormal behavior. Interestingly, skin lesions caused by increased proliferation of keratinocytes suggest the role of a glycan structure [in which the αGal epitope has been removed or the N-acetylglucosamine (GlcNAc) residue is newly exposed] as a regulator of signal transduction. To verify this hypothesis, we introduced an EndoGalC expression vector into cultured mouse NIH3T3 cells and obtained several EndoGalC-expressing transfectants. These cells lacked αGal epitope expression and exhibited 1.8-fold higher proliferation than untransfected parental cells. We then used several cytokine receptor inhibitors to assess the signal transduction cascades that were affected. Only SB431542 and LY364947, both of which are transforming growth factor β (TGFβ) receptor type-I (TβR-I) inhibitors, were found to successfully reverse the enhanced cell proliferation rate of EndoGalC transfectants, indicating that the glycan structure is a regulator of TβRs. Biochemical analysis demonstrated that the glycan altered association between TβR-I and TβR-II in the absence of ligands.

Structure-based evolutionary relationship of glycosyltransferases: a case study of vertebrate β1,4-galactosyltransferase, invertebrate β1,4-N-acetylgalactosaminyltransferase and α-polypeptidyl-N-acetylgalactosaminyltransferase

Cell surface glycans play important cellular functions and are synthesized by glycosyltransferases. Structure and function studies show that the donor sugar specificity of the invertebrate β1,4-N-acetyl-glactosaminyltransferase (β4GalNAc-T) and the vertebrate β1,4-galactosyltransferase I (β4Gal-T1) are related by a single amino acid residue change. Comparison of the catalytic domain crystal structures of the β4Gal-T1 and the α-polypeptidyl-GalNAc-T (αppGalNAc-T) shows that their protein structure and sequences are similar. Therefore, it seems that the invertebrate β4GalNAc-T and the catalytic domain of αppGalNAc-T might have emerged from a common primordial gene. When vertebrates emerged from invertebrates, the amino acid that determines the donor sugar specificity of the invertebrate β4GalNAc-T might have mutated, thus converting the enzyme to a β4Gal-T1 in vertebrates.

Lack of lacto/neolacto-glycolipids enhances the formation of glycolipid-enriched microdomains, facilitating B cell activation

In a previous study, we demonstrated that beta1,3-N-acetylglucosaminyltransferase 5 (B3gnt5) is a lactotriaosylceramide (Lc(3)Cer) synthase that synthesizes a precursor structure for lacto/neolacto-series glycosphingolipids (GSLs) in in vitro experiments. Here, we generated B3gnt5-deficient (B3gnt5(-/-)) mice to investigate the in vivo biological functions of lacto/neolacto-series GSLs. In biochemical analyses, lacto/neolacto-series GSLs were confirmed to be absent and no Lc(3)Cer synthase activity was detected in the tissues of these mice. These results demonstrate that beta3GnT5 is the sole enzyme synthesizing Lc(3)Cer in vivo. Ganglioside GM1, known as a glycosphingolipid-enriched microdomain (GEM) marker, was found to be up-regulated in B3gnt5(-/-) B cells by flow cytometry and fluorescence microscopy. However, no difference in the amount of GM1 was observed by TLC-immunoblotting analysis. The GEM-stained puncta on the surface of B3gnt5(-/-) resting B cells were brighter and larger than those of WT cells. These results suggest that structural alteration of GEM occurs in B3gnt5(-/-) B cells. We next examined whether BCR signaling-related proteins, such as BCR, CD19, and the signaling molecule Lyn, had moved into or out of the GEM fraction. In B3gnt5(-/-) B cells, these molecules were enriched in the GEM fraction or adjacent fraction. Moreover, B3gnt5(-/-) B cells were more sensitive to the induction of intracellular phosphorylation signals on BCR stimulation and proliferated more vigorously than WT B cells. Together, these results suggest that lacto/neolacto-series GSLs play an important role in clustering of GEMs and tether-specific proteins, such as BCR, CD19, and related signaling molecules to the GEMs.

Beta3GnT2 (B3GNT2), a major polylactosamine synthase: analysis of B3GNT2-deficient mice

The polylactosamine structure is a fundamental structure of carbohydrate chains and carries a lot of biofunctional carbohydrate epitopes. To investigate the biological function of polylactosamine chains, here we generated and analyzed knockout mice lacking the gene B3gnt2, which encodes a major polylactosamine synthase. In beta1,3-N-acetylglucosaminyltransferase (B3gnt2) B3gnt2-deficient (B3gnt2-/-) mice, the number of polylactosamine structures was markedly lower than in wild-type mice. Flow cytometry, LEL lectin-blotting, and glycan analysis by metabolic labeling demonstrated that the amount of polylactosamine chains on N-glycans was greatly reduced in the tissues of B3gnt2-/- mice. We examined whether immunological abnormalities were present in B3gnt2-/- mice. We screened polylactosamine-carrying molecules of wild-type mice by lectin microarray analysis and found that polylactosamine was present on CD28 and CD19, two established immune co-stimulatory molecules. Polylactosamine levels on these molecules were lower in B3gnt2-/- mice than in wild-type mice. B3gnt2-/- T cells were more sensitive to the induction of intracellular Ca2+ flux on stimulation with anti-CD3epsilon/CD28 antibodies and proliferated more strongly than wild-type T cells. B3gnt2-/- B cells also showed hyperproliferation on BCR stimulation. These results showed that hyperactivation of lymphocytes occurred due to a lack of polylactosamine on receptor molecules in B3gnt2-/- mice. This finding indicates that polylactosamine has an important role in immunological biofunctions. We can therefore attempt to identify the in vivo biological function of glycans using glycogene-deficient mice.

N-Glycans in cancer progression

N-Glycan branching in the medial-Golgi generates ligands for lattice-forming lectins (e.g., galectins) that regulate surface levels of glycoproteins including epidermal growth factor (EGF) and transforming growth factor-beta (TGF-beta) receptors. Moreover, functional classes of glycoproteins differ in N-glycan multiplicities (number of N-glycans/peptide), a genetically encoded feature of glycoproteins that interacts with metabolic flux (UDP-GlcNAc) and N-glycan branching to differentially regulate surface levels. Oncogenesis increases beta1,6-N-acetylglucosaminyltransferase V (encoded by Mgat5) expression, and its high-affinity galectin ligands promote surface retention of growth receptors with a reduced dependence on UDP-GlcNAc. Mgat5(-/-) tumor cells are less metastatic in vivo and less responsive to cytokines in vitro, but undergo secondary changes that support tumor cell proliferation. These include loss of Caveolin-1, a negative regulator of EGF signaling, and increased reactive oxygen species, an inhibitor of phosphotyrosine phosphatases. These studies suggest a systems approach to cancer treatment where the surface distribution of receptors is targeted through metabolism and N-glycan branching to induce growth arrest.

Regulation of glycan structures in animal tissues: transcript profiling of glycan-related genes

Glycan structures covalently attached to proteins and lipids play numerous roles in mammalian cells, including protein folding, targeting, recognition, and adhesion at the molecular or cellular level. Regulating the abundance of glycan structures on cellular glycoproteins and glycolipids is a complex process that depends on numerous factors. Most models for glycan regulation hypothesize that transcriptional control of the enzymes involved in glycan synthesis, modification, and catabolism determines glycan abundance and diversity. However, few broad-based studies have examined correlations between glycan structures and transcripts encoding the relevant biosynthetic and catabolic enzymes. Low transcript abundance for many glycan-related genes has hampered broad-based transcript profiling for comparison with glycan structural data. In an effort to facilitate comparison with glycan structural data and to identify the molecular basis of alterations in glycan structures, we have developed a medium-throughput quantitative real time reverse transcriptase-PCR platform for the analysis of transcripts encoding glycan-related enzymes and proteins in mouse tissues and cells. The method employs a comprehensive list of >700 genes, including enzymes involved in sugar-nucleotide biosynthesis, transporters, glycan extension, modification, recognition, catabolism, and numerous glycosylated core proteins. Comparison with parallel microarray analyses indicates a significantly greater sensitivity and dynamic range for our quantitative real time reverse transcriptase-PCR approach, particularly for the numerous low abundance glycan-related enzymes. Mapping of the genes and transcript levels to their respective biosynthetic pathway steps allowed a comparison with glycan structural data and provides support for a model where many, but not all, changes in glycan abundance result from alterations in transcript expression of corresponding biosynthetic enzymes.

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.