Brain contains HNK-1 immunoreactive O-glycans of the sulfoglucuronyl lactosamine series that terminate in 2-linked or 2,6-linked hexose (mannose)

Golgi Stress Response: New Insights into the Pathogenesis and Therapeutic Targets of Human Diseases

The Golgi apparatus modifies and transports secretory and membrane proteins. In some instances, the production of secretory and membrane proteins exceeds the capacity of the Golgi apparatus, including vesicle trafficking and the post-translational modification of macromolecules. These proteins are not modified or delivered appropriately due to insufficiency in the Golgi function. These conditions disturb Golgi homeostasis and induce a cellular condition known as Golgi stress, causing cells to activate the 'Golgi stress response,' which is a homeostatic process to increase the capacity of the Golgi based on cellular requirements. Since the Golgi functions are diverse, several response pathways involving TFE3, HSP47, CREB3, proteoglycan, mucin, MAPK/ETS, and PERK regulate the capacity of each Golgi function separately. Understanding the Golgi stress response is crucial for revealing the mechanisms underlying Golgi dynamics and its effect on human health because many signaling molecules are related to diseases, ranging from viral infections to fatal neurodegenerative diseases. Therefore, it is valuable to summarize and investigate the mechanisms underlying Golgi stress response in disease pathogenesis, as they may contribute to developing novel therapeutic strategies. In this review, we investigate the perturbations and stress signaling of the Golgi, as well as the therapeutic potentials of new strategies for treating Golgi stress-associated diseases.

Determination of terminal glycan and total monosaccharide profiles of reelin glycoprotein in SH-SY5Y neuroblastoma cell line by lectin blotting and capillary liquid chromatography electrospray ionization-ion trap tandem mass spectrometry system

Reelin (400 kDa) is an extracellular matrix glycoprotein that is a key regulator of the many significant biological processes including the brain formation, cell aggregation, and dendrite formation. The glycosylation contributes to the nature of the protein through folding, localization and trafficking, solubility, antigenicity, biological activity, and half-life. Although reelin is to be known as a glycoprotein, the knowledge of its glycosylation is very limited. In this study, we aimed to characterize the terminal glycan profile of reelin by lectin blotting and monosaccharide analysis of glycan chains by capillary liquid chromatography electrospray ionization ion trap tandem mass spectrometry (CapLC-ESI-MS/MS) in SH-SY5Y neuroblastoma cell line. According to our results, reelin was detected in different protein fragments (310, 250, and 85 kDa) in addition to full-length form (400 kDa) in the cell line. The reelin glycoprotein was found to carry the β-N-Acetylglucosamine, α-Mannose, β-Galactose, and α-2,3 and α2,6 linked sialic acids by lectin blotting. Nevertheless, these terminal monosaccharides were found in different intensity according to reelin fragments. Besides, we purified a reelin fragment (250 kDa), and we analyzed it for their monosaccharide by CapLC-ESI-MS/MS. We found that reelin contained five types of monosaccharides, which were consisted of N-Acetylgalactosamine, N-Acetylglucosamine, Galactose, Glucose, Mannose and Sialic acid, from high to low abundance respectively. The present results provide a valuable guide for biochemical, genetic, and glycobiology based further experiments about reelin glycosylation in cancer perspective.

HNK-1 sulfotransferase modulates α-dystroglycan glycosylation by 3-O-sulfation of glucuronic acid on matriglycan

Mutations in multiple genes required for proper O-mannosylation of α-dystroglycan are causal for congenital/limb-girdle muscular dystrophies and abnormal brain development in mammals. Previously, we and others further elucidated the functional O-mannose glycan structure that is terminated by matriglycan, [(-GlcA-β3-Xyl-α3-)n]. This repeating disaccharide serves as a receptor for proteins in the extracellular matrix. Here, we demonstrate in vitro that HNK-1 sulfotransferase (HNK-1ST/carbohydrate sulfotransferase) sulfates terminal glucuronyl residues of matriglycan at the 3-hydroxyl and prevents further matriglycan polymerization by the LARGE1 glycosyltransferase. While α-dystroglycan isolated from mouse heart and kidney is susceptible to exoglycosidase digestion of matriglycan, the functional, lower molecular weight α-dystroglycan detected in brain, where HNK-1ST expression is elevated, is resistant. Removal of the sulfate cap by a sulfatase facilitated dual-glycosidase digestion. Our data strongly support a tissue specific mechanism in which HNK-1ST regulates polymer length by competing with LARGE for the 3-position on the nonreducing GlcA of matriglycan.

Chemoenzymatic Synthesis of O-Mannose Glycans Containing Sulfated or Nonsulfated HNK-1 Epitope

The human natural killer-1 (HNK-1) epitope is a unique sulfated trisaccharide sequence presented on O- and N-glycans of various glycoproteins and on glycolipids. It is overexpressed in the nervous system and plays crucial roles in nerve regeneration, synaptic plasticity, and neuronal diseases. However, the investigation of functional roles of HNK-1 in a more complex glycan context at the molecular level remains a big challenge due to lack of access to related structurally well-defined complex glycans. Herein, we describe a highly efficient chemoenzymatic approach for the first collective synthesis of HNK-1-bearing O-mannose glycans with different branching patterns, and for their nonsulfated counterparts. The successful strategy relies on both chemical glycosylation of a trisaccharide lactone donor for the introduction of sulfated HNK-1 branch and substrate promiscuities of bacterial glycosyltransferases that can tolerate sulfated substrates for enzymatic diversification. Glycan microarray analysis with the resulting complex synthetic glycans demonstrated their recognition by two HNK-1-specific antibodies including anti-HNK-1/N-CAM (CD57) and Cat-315, which provided further evidence for the recognition epitopes of these antibodies and the essential roles of the sulfate group for HNK-1 glycan-antibody recognition.

Nanolithography of biointerfaces

This article is based on the Concluding remarks made at the Faraday Discussion meeting on Nanolithography of Biointerfaces, held in London, UK, 3-5th July 2019.

Multiple distinct O-Mannosylation pathways in eukaryotes

Protein O-mannosylation (O-Man), originally discovered in yeast five decades ago, is an important post-translational modification (PTM) conserved from bacteria to humans, but not found in plants or nematodes. Until recently, the homologous family of ER-located protein O-mannosyl transferases (PMT1-7 in yeast; POMT1/POMT2 in humans), were the only known enzymes involved in directing O-Man biosynthesis in eukaryotes. However, recent studies demonstrate the existence of multiple distinct O-Man glycosylation pathways indicating that the genetic and biosynthetic regulation of O-Man in eukaryotes is more complex than previously envisioned. Introduction of sensitive glycoproteomics strategies provided an expansion of O-Man glycoproteomes in eukaryotes (yeast and mammalian cell lines) leading to the discovery of O-Man glycosylation on important mammalian cell adhesion (cadherin superfamily) and signaling (plexin family) macromolecules, and to the discovery of unique nucleocytoplasmic O-Man glycosylation in yeast. It is now evident that eukaryotes have multiple distinct O-Man glycosylation pathways including: i) the classical PMT1-7 and POMT1/POMT2 pathway conserved in all eukaryotes apart from plants; ii) a yet uncharacterized nucleocytoplasmic pathway only found in yeast; iii) an ER-located pathway directed by the TMTC1-4 genes found in metazoans and protists and primarily dedicated to the cadherin superfamily; and iv) a yet uncharacterized pathway found in metazoans primarily dedicated to plexins. O-Man glycosylation is thus emerging as a much more widespread and evolutionary diverse PTM with complex genetic and biosynthetic regulation. While deficiencies in the POMT1/POMT2 O-Man pathway underlie muscular dystrophies, the TMTC1-4 pathway appear to be involved in distinct congenital disorders with neurodevelopmental phenotypes. Here, we review and discuss the recent discoveries of the new non-classical O-Man glycosylation pathways, their substrates, functions and roles in disease.

The neoglycolipid (NGL) technology-based microarrays and future prospects

The neoglycolipid (NGL) technology is the basis of a state-of-the-art oligosaccharide microarray system, which we offer for screening analyses to the broad scientific community. We review here the sequential development of the technology and its power in pinpointing and isolating naturally occurring ligands for glycan-binding proteins (GBPs) within glycan populations. We highlight our Designer Array approach and Beam Search Array approach for generating natural glycome arrays to identify novel ligands of biological relevance. These two microarray approaches have been applied for assignments of ligands or antigens on glucan polysaccharides for effector proteins of the immune system (Dectin-1, DC-SIGN and DC-SIGNR) and carbohydrate-binding modules (CBMs) on bacterial hydrolases. We also discuss here the more recent applications to elucidate the structure of a prostate cancer- associated antigen F77 and identify ligands for adhesins of two rotaviruses, P[10] and P[19], expressed on an epithelial mucin glycoprotein.

O-Glycome Beam Search Arrays for Carbohydrate Ligand Discovery

O-glycosylation is a post-translational modification of proteins crucial to molecular mechanisms in health and disease. O-glycans are typically highly heterogeneous. The involvement of specific O-glycan sequences in many bio-recognition systems is yet to be determined because of a lack of efficient methodologies. We describe here a targeted microarray approach: O-glycome beam search that is both robust and efficient for O-glycan ligand-discovery. Substantial simplification of the complex O-glycome profile and facile chromatographic resolution is achieved by arraying O-glycans as branches, monitoring by mass spectrometry, focusing on promising fractions, and on-array immuno-sequencing. This is orders of magnitude more sensitive than traditional methods. We have applied beam search approach to porcine stomach mucin and identified extremely minor components previously undetected within the O-glycome of this mucin that are ligands for the adhesive proteins of two rotaviruses. The approach is applicable to O-glycome recognition studies in a wide range of biological settings to give insights into glycan recognition structures in natural microenvironments.

Glycosylation with ribitol-phosphate in mammals: New insights into the O-mannosyl glycan

BACKGROUND

O-mannosyl glycans have been found in a limited number of glycoproteins of the brain, nerves, and skeletal muscles, particularly in α-dystroglycan (α-DG). Defects in O-mannosyl glycan on α-DG are the primary cause of a group of congenital muscular dystrophies, which are collectively termed α-dystroglycanopathy. Recent studies have revealed various O-mannosyl glycan structures, which can be classified as core M1, core M2, and core M3 glycans. Although many dystroglycanopathy genes are involved in core M3 processing, the structure and biosynthesis of core M3 glycan remains only partially understood.

SCOPE OF REVIEW

This review presents recent findings about the structure, biosynthesis, and pathology of O-mannosyl glycans.

MAJOR CONCLUSIONS

Recent studies have revealed that the entire structure of core M3 glycan, including ribitol-5-phosphate, is a novel structure in mammals; its unique biosynthetic pathway has been elucidated by the identification of new causative genes for α-dystroglycanopathies and their functions.

GENERAL SIGNIFICANCE

O-mannosyl glycan has a novel, unique structure that is important for the maintenance of brain and muscle functions. These findings have opened up a new field in glycoscience. These studies will further contribute to the understanding of the pathomechanism of α-dystroglycanopathy and the development of glycotherapeutics. This article is part of a Special Issue entitled Neuro-glycoscience, edited by Kenji Kadomatsu and Hiroshi Kitagawa.

Induction of Antibodies Directed Against Branched Core O-Mannosyl Glycopeptides-Selectivity Complimentary to the ConA Lectin

Mammalian protein O-mannosylation, initiated by attachment of α-mannopyranose to Ser or Thr residues, comprise a group of post-translational modifications (PTMs) involved in muscle and brain development. Recent advances in glycoproteomics methodology and the "SimpleCell" strategy have enabled rapid identification of glycoproteins and specific glycosylation sites. Despite the enormous progress made, the biological impact of the mammalian O-mannosyl glycoproteome remains largely unknown to date. Tools are still needed to investigate the structure, role, and abundance of O-mannosyl glycans. Although O-mannosyl branching has been shown to be of relevance in integrin-dependent cell migration, and also plays a role in demyelinating diseases, such as multiple sclerosis, a broader understanding of the biological roles of branched O-mannosyl glycans is lacking in part due to the paucity of detection tools. In this work, a glycopeptide vaccine construct was synthesized and used to generate antibodies against branched O-mannosyl glycans. Glycopeptide microarray screening revealed high selectivity of the induced antibodies for branched glycan core structures presented on different peptide backbones, with no cross-reactivity observed with related linear glycans. For comparison, microarray screening of the mannose-binding lectin concanavalin A (ConA), which is commonly used in glycoproteomics workflows to enrich tryptic O-mannosyl peptides, showed that the ConA lectin did not recognize branched O-mannosyl glycans. The binding preference of ConA for short linear O-mannosyl glycans was rationalized in terms of molecular structure using crystallographic data augmented by molecular modeling. The contrast between the ConA binding specificity and that of the new antibodies indicates a novel role for the antibodies in studies of protein O-mannosylation.

Protein O-Mannosylation in the Murine Brain: Occurrence of Mono-O-Mannosyl Glycans and Identification of New Substrates

Protein O-mannosylation is a post-translational modification essential for correct development of mammals. In humans, deficient O-mannosylation results in severe congenital muscular dystrophies often associated with impaired brain and eye development. Although various O-mannosylated proteins have been identified in the recent years, the distribution of O-mannosyl glycans in the mammalian brain and target proteins are still not well defined. In the present study, rabbit monoclonal antibodies directed against the O-mannosylated peptide YAT(α1-Man)AV were generated. Detailed characterization of clone RKU-1-3-5 revealed that this monoclonal antibody recognizes O-linked mannose also in different peptide and protein contexts. Using this tool, we observed that mono-O-mannosyl glycans occur ubiquitously throughout the murine brain but are especially enriched at inhibitory GABAergic neurons and at the perineural nets. Using a mass spectrometry-based approach, we further identified glycoproteins from the murine brain that bear single O-mannose residues. Among the candidates identified are members of the cadherin and plexin superfamilies and the perineural net protein neurocan. In addition, we identified neurexin 3, a cell adhesion protein involved in synaptic plasticity, and inter-alpha-trypsin inhibitor 5, a protease inhibitor important in stabilizing the extracellular matrix, as new O-mannosylated glycoproteins.

A Sulfated Glycosaminoglycan Linkage Region is a Novel Type of Human Natural Killer-1 (HNK-1) Epitope Expressed on Aggrecan in Perineuronal Nets

Human natural killer-1 (HNK-1) carbohydrate (HSO₃-3GlcAβ1-3Galβ1-4GlcNAc-R) is highly expressed in the brain and required for learning and neural plasticity. We previously demonstrated that expression of the HNK-1 epitope is mostly abolished in knockout mice for GlcAT-P (B3gat1), a major glucuronyltransferase required for HNK-1 biosynthesis, but remained in specific regions such as perineuronal nets (PNNs) in these mutant mice. Considering PNNs are mainly composed of chondroitin sulfate proteoglycans (CSPGs) and regulate neural plasticity, GlcAT-P-independent expression of HNK-1 in PNNs is suggested to play a role in neural plasticity. However, the function, structure, carrier glycoprotein and biosynthetic pathway for GlcAT-P-irrelevant HNK-1 epitope remain unclear. In this study, we identified a unique HNK-1 structure on aggrecan in PNNs. To determine the biosynthetic pathway for the novel HNK-1, we generated knockout mice for GlcAT-S (B3gat2), the other glucuronyltransferase required for HNK-1 biosynthesis. However, GlcAT-P and GlcAT-S double-knockout mice did not exhibit reduced HNK-1 expression compared with single GlcAT-P-knockout mice, indicating an unusual biosynthetic pathway for the HNK-1 epitope in PNNs. Aggrecan was purified from cultured cells in which GlcAT-P and -S are not expressed and we determined the structure of the novel HNK-1 epitope using liquid chromatography/mass spectrometry (LC/MS) as a sulfated linkage region of glycosaminoglycans (GAGs), HSO₃-GlcA-Gal-Gal-Xyl-R. Taken together, we propose a hypothetical model where GlcAT-I, the sole glucuronyltransferase required for synthesis of the GAG linkage, is also responsible for biosynthesis of the novel HNK-1 on aggrecan. These results could lead to discovery of new roles of the HNK-1 epitope in neural plasticity.

Matriglycan: a novel polysaccharide that links dystroglycan to the basement membrane

Associations between cells and the basement membrane are critical for a variety of biological events including cell proliferation, cell migration, cell differentiation and the maintenance of tissue integrity. Dystroglycan is a highly glycosylated basement membrane receptor, and is involved in physiological processes that maintain integrity of the skeletal muscle, as well as development and function of the central nervous system. Aberrant O-glycosylation of the α subunit of this protein, and a concomitant loss of dystroglycan's ability to function as a receptor for extracellular matrix (ECM) ligands that bear laminin globular (LG) domains, occurs in several congenital/limb-girdle muscular dystrophies (also referred to as dystroglycanopathies). Recent genetic studies revealed that mutations in DAG1 (which encodes dystroglycan) and at least 17 other genes disrupt the ECM receptor function of dystroglycan and cause disease. Here, we summarize recent advances in our understanding of the enzymatic functions of two of these disease genes: the like-glycosyltransferase (LARGE) and protein O-mannose kinase (POMK, previously referred to as SGK196). In addition, we discuss the structure of the glycan that directly binds the ECM ligands and the mechanisms by which this functional motif is linked to dystroglycan. In light of the fact that dystroglycan functions as a matrix receptor and the polysaccharide synthesized by LARGE is the binding motif for matrix proteins, we propose to name this novel polysaccharide structure matriglycan.

Glycobiology of α-dystroglycan and muscular dystrophy

Most proteins are modified by glycans, which can modulate the biological properties and functions of glycoproteins. The major glycans can be classified into N-glycans and O-glycans according to their glycan-peptide linkage. This review will provide an overview of the O-mannosyl glycans, one subtype of O-glycans. Originally, O-mannosyl glycan was only known to be present on a limited number of glycoproteins, especially α-dystroglycan (α-DG). However, once a clear relationship was established between O-mannosyl glycan and the pathological mechanisms of some congenital muscular dystrophies in humans, research on the biochemistry and pathology of O-mannosyl glycans has been expanding. Because α-DG glycosylation is defective in congenital muscular dystrophies, which also feature abnormal neuronal migration, these disorders are collectively called α-dystroglycanopathies. In this article, I will describe the structure, biosynthesis and pathology of O-mannosyl glycans.

Mammalian O-mannosylation pathway: glycan structures, enzymes, and protein substrates

The mammalian O-mannosylation pathway for protein post-translational modification is intricately involved in modulating cell–matrix interactions in the musculature and nervous system. Defects in enzymes of this biosynthetic pathway are causative for multiple forms of congenital muscular dystophy. The application of advanced genetic and biochemical technologies has resulted in remarkable progress in this field over the past few years, culminating with the publication of three landmark papers in 2013 alone. In this review, we will highlight recent progress focusing on the dramatic expansion of the set of genes known to be involved in O-mannosylation and disease processes, the concurrent acceleration of the rate of O-mannosylation pathway protein functional assignments, the tremendous increase in the number of proteins now known to be modified by O-mannosylation, and the recent progress in protein O-mannose glycan quantification and site assignment. Also, we attempt to highlight key outstanding questions raised by this abundance of new information.

Synthesis of a glycopeptide vaccine conjugate for induction of antibodies recognizing O-mannosyl glycopeptides

In spite of the clear importance of protein O-mannosylation in brain glycobiology, tools are lacking for specific detection, enrichment, and identification of proteins containing these modifycations. We envisioned inducing antibodies that specifically recognize O-mannose glycans on proteins and peptides. With this in mind, we prepared a glycopeptide vaccine construct containing the N-acetyllactosamine-extended mannose motif Galβ1-4GlcNAcβ1-2ManαThr, found as a common core structure on almost all mammalian O-mannosyl glycoproteins identified. O-mannose glycosylated amino acid building blocks and the corresponding glycopeptides were prepared by chemical synthesis and then conjugated to an immune carrier protein. After administration of the synthetic vaccine into rabbits, strong immune responses were obtained. Further evaluation by ELISA neutralization experiments and glycopeptide microarrays showed that the induced antibodies were highly specific to the glycopeptide antigen.

Uncovering cryptic glycan markers in multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE)

Using an integrated antigen microarray approach, we observed epitope-spreading of autoantibody responses to a variety of antigenic structures in the cerebrospinal fluid (CSF) of patients with multiple sclerosis (MS) and in the serum of mice with experimental autoimmune encephalomyelitis (EAE). These included previously described protein- and lipid-based antigenic targets and newly discovered autoimmunogenic sugar moieties, notably, autoantibodies specific for the oligomannoses in both MS patient CSF and the sera of mice with EAE. These glycans are often masked by other sugar moieties and belong to a class of cryptic autoantigens. We further determined that these targets are highly expressed on multiple cell types in MS and EAE lesions. Co-immunization of SJL/J mice with a Man9-KLH conjugate at the time of EAE induction elicited highly significant levels of anti-Man9-cluster autoantibodies. Nevertheless, this anti-glycan autoantibody response was associated with a significantly reduced clinical severity of EAE. The potential of these cryptic glycan markers and targeting antibodies for diagnostic and therapeutic interventions of neurological disorders has yet to be explored.

Protein O-mannosylation in metazoan organisms

Protein O-mannosylation is a special type of glycosylation that plays prominent roles in metazoans, affecting development and physiology of the nervous system and muscles. A major biological effect of O-mannosylation involves the regulation of α-dystroglycan, a membrane glycoprotein mediating cell-extracellular matrix interactions. Genetic defects of O-mannosylation result in the loss of ligand-binding activity of α-dystroglycan and cause congenital muscular dystrophies termed dystroglycanopathies. Recent progress in mass spectrometry and in vitro analyses has shed new light on the mechanism of α-dystroglycan glycosylation; however, this mechanism is underlain by complex genetic and molecular elements that remain poorly understood. Protein O-mannosylation is evolutionarily conserved in metazoans, yet the pathway is simplified and more amenable to genetic analyses in invertebrate organisms, indicating that genetically tractable in vivo models could facilitate research in this area. This unit describes recent methodological strategies for studying protein O-mannosylation using in vitro and in vivo approaches.

Loss of branched O-mannosyl glycans in astrocytes accelerates remyelination

In demyelinating diseases such as multiple sclerosis, a critical problem is failure of remyelination, which is important for protecting axons against degeneration and restoring conduction deficits. However, the underlying mechanism of demyelination/remyelination remains unclear. N-acetylglucosaminyltransferase-IX (GnT-IX; also known as GnT-Vb) is a brain-specific glycosyltransferase that catalyzes the branched formation of O-mannosyl glycan structures. O-Mannosylation of α-dystroglycan is critical for its function as an extracellular matrix receptor, but the biological significance of its branched structures, which are exclusively found in the brain, is unclear. In this study, we found that GnT-IX formed branched O-mannosyl glycans on receptor protein tyrosine phosphatase β (RPTPβ) in vivo. Since RPTPβ is thought to play a regulatory role in demyelinating diseases, GnT-IX-deficient mice were subjected to cuprizone-induced demyelination. Cuprizone feeding for 8 weeks gradually promoted demyelination in wild-type mice. In GnT-IX-deficient mice, the myelin content in the corpus callosum was reduced after 4 weeks of treatment, but markedly increased at 8 weeks, suggesting enhanced remyelination under GnT-IX deficiency. Furthermore, astrocyte activation in the corpus callosum of GnT-IX-deficient mice was significantly attenuated, and an oligodendrocyte cell lineage analysis indicated that more oligodendrocyte precursor cells differentiated into mature oligodendrocytes. Together, branched O-mannosyl glycans in the corpus callosum in the brain are a necessary component of remyelination inhibition in the cuprizone-induced demyelination model, suggesting that modulation of O-mannosyl glycans is a likely candidate for therapeutic strategies.

Carbohydrate recognition in the immune system: contributions of neoglycolipid-based microarrays to carbohydrate ligand discovery

Oligosaccharide sequences in glycomes of eukaryotes and prokaryotes are enormously diverse. The reasons are not fully understood, but there is an increasing number of examples of the involvement of specific oligosaccharide sequences as ligands in protein-carbohydrate interactions in health and, directly or indirectly, in every major disease, be it infectious or noninfectious. The pinpointing and characterizing of oligosaccharide ligands within glycomes has been one of the most challenging aspects of molecular cell biology, as oligosaccharides cannot be cloned and are generally available in limited amounts. This overview recounts the background to the development of a microarray system that is poised for surveying proteomes for carbohydrate-binding activities and glycomes for assigning the oligosaccharide ligands. Examples are selected by way of illustrating the potential of "designer" microarrays for ligand discovery at the interface of infection, immunity, and glycobiology. Particularly highlighted are sulfo-oligosaccharide and gluco-oligosaccharide recognition systems elucidated using microarrays.

In vivo regulation of steroid hormones by the Chst10 sulfotransferase in mouse

Chst10 adds sulfate to glucuronic acid to form a carbohydrate antigen, HNK-1, in glycoproteins and glycolipids. To determine the role of Chst10 in vivo, we generated systemic Chst10-deficient mutant mice. Although Chst10^−/− mice were born and grew to adulthood with no gross defects, they were subfertile. Uteri from Chst10^−/− females at the pro-estrus stage were larger than those from wild-type females and exhibited a thick uterine endometrium. Serum estrogen levels in Chst10^−/− females were higher than those from wild-type females, suggesting impaired down-regulation of estrogen. Because steroid hormones are often conjugated to glucuronic acid, we hypothesized that Chst10 sulfates glucuronidated steroid hormone to regulate steroid hormone in vivo. Enzymatic activity assays and structural analysis of Chst10 products by HPLC and mass spectrometry revealed that Chst10 indeed sulfates glucuronidated estrogen, testosterone, and other steroid hormones. We also identified an HPLC peak corresponding to sulfated and glucuronidated estradiol in serum from wild-type but not from Chst10 null female mice. Estrogen-response element reporter assays revealed that Chst10-modified estrogen likely did not bind to its receptor. These results suggest that subfertility exhibited by female mice following Chst10 loss results from dysregulation of estrogen. Given that Chst10 transfers sulfates to several steroid hormones, Chst10 likely functions in widespread regulation of steroid hormones in vivo.

Regulated expression and neural functions of human natural killer-1 (HNK-1) carbohydrate

Human natural killer-1 (HNK-1) carbohydrate, comprising a unique trisaccharide HSO(3)-3GlcAβ1-3Galβ1-4GlcNAc, shows well-regulated expression and unique functions in the nervous system. Recent studies have revealed sophisticated and complicated expression mechanisms for HNK-1 glycan. Activities of biosynthetic enzymes are controlled through the formation of enzyme-complexes and regulation of subcellular localization. Functional aspects of HNK-1 carbohydrate were examined by overexpression, knockdown, and knockout studies of these enzymes. HNK-1 is involved in several neural functions such as synaptic plasticity, learning and memory, and the underlying molecular mechanisms have been illustrated upon identification of the target carrier glycoproteins of HNK-1 such as the glutamate receptor subunit GluA2 or tenascin-R. In this review, we describe recent findings about HNK-1 carbohydrate that provide further insights into the mechanism of its expression and function in the nervous system.

Neurofascin 186 is O-mannosylated within and outside of the mucin domain

Protein O-mannosylation is an important modification in mammals, and deficiencies thereof lead to a variety of severe phenotypes. Although it has already been shown that the amount of O-mannosyl glycans in brain is very high, only very few proteins have been identified as O-mannosylated. Additionally, the functions of the O-mannose-based glycans are still speculative and only investigated for α-dystroglycan. In a previous study a cis-located peptide was identified, which controls O-mannosylation in mammals. A BLAST search on the basis of this peptidic determinant identified other potential O-mannosylated proteins. Among these neurofascin was chosen for further analysis as a recombinant probe (mucin domain) and as an endogenous protein from mouse brain. Mass spectrometric data for both proteins confirmed that neurofascin186 is indeed O-mannosylated. Glycopeptide analysis by liquid chromatography-tandem mass spectrometry allowed for the identification of some of the O-mannosylation sites, which are not restricted to the mucin domain but were found also within N-terminal IgG and Fibronectin domains of the protein.

RPTPζ/phosphacan is abnormally glycosylated in a model of muscle-eye-brain disease lacking functional POMGnT1

Congenital muscular dystrophies (CMDs) with associated brain abnormalities are a group of disorders characterized by muscular dystrophy and brain and eye abnormalities that are frequently caused by mutations in known or putative glycotransferases involved in protein O-mannosyl glycosylation. Previous work identified α-dystroglycan as the major substrate for O-mannosylation and its altered glycosylation the major cause of these disorders. However, work from several labs indicated that other proteins in the brain are also O-mannosylated and therefore could contribute to CMD pathology in patients with mutations in the protein O-mannosylation pathway, however few of these proteins have been identified and fully characterized in CMDs. In this study we identify receptor protein tyrosine phosphatase ζ (RPTPζ) and its secreted variant, phosphacan, as another potentially important substrate for protein O-mannosylation in the brain. Using a mouse model of muscle-eye-brain disease lacking functional protein O-mannose β-1,2-N-acetylglucosaminyltransferase (POMGnT1), we show that RPTPζ/phosphacan is shifted to a lower molecular weight and distinct carbohydrate epitopes normally detected on the protein are either absent or substantially reduced, including Human Natural Killer-1 (HNK-1) reactivity. The spatial and temporal expression patterns of these O-mannosylated forms of RPTPζ/phosphacan and its hypoglycosylation and loss of HNK-1 glycan epitopes in POMGnT1 knockouts are suggestive of a role in the neural phenotypes observed in patients and animal models of CMDs.

Neoglycolipid-based oligosaccharide microarray system: preparation of NGLs and their noncovalent immobilization on nitrocellulose-coated glass slides for microarray analyses

Carbohydrate microarrays, since their advent in 2002, are revolutionizing studies of the molecular basis of protein-carbohydrate interactions both in endogenous recognition systems and pathogen-host interactions. We have developed a unique carbohydrate microarray system based on the neoglycolipid (NGL) technology, a well-validated microscale approach for generating lipid-tagged oligosaccharide probes for use in carbohydrate recognition studies. This chapter provides an overview of the principles and key features of the NGL-based oligosaccharide microarrays, and describes in detail the basic techniques - from the preparation of NGL probes to the generation of microarrays using robotic arraying hardware, as well as a general protocol for probing the microarrays with carbohydrate-binding proteins.

Mammalian O-mannosylation: unsolved questions of structure/function

Post-translational modification of polypeptides with glycans increases the diversity of the structures of proteins and imparts increased functional diversity. Here, we review the current literature on a relatively new O-glycosylation pathway, the mammalian O-mannosylation pathway. The importance of O-mannosylation is illustrated by the fact that O-mannose glycan structures play roles in a variety of processes including viral entry into cells, metastasis, cell adhesion, and neuronal development. Furthermore, mutations in the enzymes of this pathway are causal for a variety of congenital muscular dystrophies. Here we highlight the protein substrates, glycan structures, and enzymes involved in O-mannosylation as well as our gaps in understanding structure/function relationships in this biosynthetic pathway.

Golgi glycosylation and human inherited diseases

The Golgi factory receives custom glycosylates and dispatches its cargo to the correct cellular locations. The process requires importing donor substrates, moving the cargo, and recycling machinery. Correctly glycosylated cargo reflects the Golgi's quality and efficiency. Genetic disorders in the specific equipment (enzymes), donors (nucleotide sugar transporters), or equipment recycling/reorganization components (COG, SEC, golgins) can all affect glycosylation. Dozens of human glycosylation disorders fit these categories. Many other genes, with or without familiar names, well-annotated pedigrees, or likely homologies will join the ranks of glycosylation disorders. Their broad and unpredictable case-by-case phenotypes cross the traditional medical specialty boundaries. The gene functions in patients may be elusive, but their common feature may include altered glycosylation that provide clues to Golgi function. This article focuses on a group of human disorders that affect protein or lipid glycosylation. Readers may find it useful to generalize some of these patient-based, translational observations to their own research.

HNK-1 glycan functions as a tumor suppressor for astrocytic tumor

Astrocytic tumor is the most prevalent primary brain tumor. However, the role of cell surface carbohydrates in astrocytic tumor invasion is not known. In a previous study, we showed that polysialic acid facilitates astrocytic tumor invasion and thereby tumor progression. Here, we examined the role of HNK-1 glycan in astrocytic tumor invasion. A Kaplan-Meier analysis of 45 patients revealed that higher HNK-1 expression levels were positively associated with increased survival of patients. To determine the role of HNK-1 glycan, we transfected C6 glioma cells, which lack HNK-1 glycan expression, with β1,3-glucuronyltransferase-P cDNA, generating HNK-1-positive cells. When these cells were injected into the mouse brain, the resultant tumors were 60% smaller than tumors emerging from injection of the mock-transfected HNK-1-negative C6 cells. HNK-1-positive C6 cells also grew more slowly than mock-transfected C6 cells in anchorage-dependent and anchorage-independent assays. C6-HNK-1 cells migrated well after treatment of anti-β1 integrin antibody, whereas the same treatment inhibited cell migration of mock-transfected C6 cells. Similarly, α-dystroglycan containing HNK-1 glycan is different from those containing the laminin-binding glycans, supporting the above conclusion that C6-HNK-1 cells migrate independently from β1-integrin-mediated signaling. Moreover, HNK-1-positive cells exhibited attenuated activation of ERK 1/2 compared with mock-transfected C6 cells, whereas focal adhesion kinase activation was equivalent in both cell types. Overall, these results indicate that HNK-1 glycan functions as a tumor suppressor.

Cell surface N-glycans-mediated isolation of mouse neural stem cells

The isolation of neural stem cells (NSCs) has been hampered by the lack of valid cell-surface antigens on NSCs, and novel valuable markers have been proposed. Glycan (oligosaccharide chain) is a potential candidate as a marker to isolate NSCs, because the species and the combination order of saccharides in glycan generate remarkable structural diversity and specificity. At present, the expression of hundreds of glycoconjugates with glycans have been found in the NSCs; however, just a few glycan-epitopes have been identified as valuable cell-surface markers. This review focused on the isolation of NSC using glycoprotein, especially complex type N-glycans. The cell-surface N-glycan-mediated isolation of NSCs is therefore expected to provide a comprehensive understanding of the biologic characteristics of NSCs in the brain, and thereby help to develop novel strategies in the field of regenerative medicine.

High-sensitivity O-glycomic analysis of mice deficient in core 2 {beta}1,6-N-acetylglucosaminyltransferases

Core 2 β1,6-N-acetylglucosaminyltransferase (C2GnT), which exists in three isoforms, C2GnT1, C2GnT2 and C2GnT3, is one of the key enzymes in the O-glycan biosynthetic pathway. These isoenzymes produce core 2 O-glycans and have been correlated with the biosynthesis of core 4 O-glycans and I-branches. Previously, we have reported mice with single and multiple deficiencies of C2GnT isoenzyme(s) and have evaluated the biological and structural consequences of the loss of core 2 function. We now present more comprehensive O-glycomic analyses of neutral and sialylated glycans expressed in the colon, small intestine, stomach, kidney, thyroid/trachea and thymus of wild-type, C2GnT2 and C2GnT3 single knockouts and the C2GnT1-3 triple knockout mice. Very high-quality data have emerged from our mass spectrometry techniques with the capability of detecting O-glycans up to at least 3500 Da. We were able to unambiguously elucidate the types of O-glycan core, branching location and residue linkages, which allowed us to exhaustively characterize structural changes in the knockout tissues. The C2GnT2 knockout mice suffered a major loss of core 2 O-glycans as well as glycans with I-branches on core 1 antennae especially in the stomach and the colon. In contrast, core 2 O-glycans still dominated the O-glycomic profile of most tissues in the C2GnT3 knockout mice. Analysis of the C2GnT triple knockout mice revealed a complete loss of both core 2 O-glycans and branched core 1 antennae, confirming that the three known isoenzymes are entirely responsible for producing these structures. Unexpectedly, O-linked mannosyl glycans are upregulated in the triple deficient stomach. In addition, our studies have revealed an interesting terminal structure detected on O-glycans of the colon tissues that is similar to the RM2 antigen from glycolipids.

Comparison of the substrate specificities and catalytic properties of the sister N-acetylglucosaminyltransferases, GnT-V and GnT-Vb (IX)

N-Acetylglucosaminlyltransferase-V (GnT-V) synthesizes GlcNAcbeta1,6Man branched N-glycans both in vitro and in vivo. A paralog, GnT-Vb (or GnT-IX), has also been shown to synthesize both GlcNAcbeta1,6Man branched N- and O-glycans. GnT-V is expressed in most human and rodent tissues while GnT-Vb expression is limited mainly to neural tissue and testes. It is of interest, therefore, to compare the catalytic properties and reaction kinetics of these sister enzymes. The results demonstrate that while GnT-V was fully active without exogenous cation and in the presence of EDTA, the activity of GnT-Vb was stimulated over 4-fold in the presence of 10 mM Mn(++). The pH optimum for GnT-V was in the range of 6.5-7.0, while that of GnT-Vb was 8.0. common for glycosyltransferases active in brain. Both enzymes transferred GlcNAcbeta1,6 to the Man residue of the GlcNAcbeta1,2Man moiety of glycan substrates, and both enzymes acted effectively on a synthetic GlcNAcbeta1,2Manalpha1,2Glc-O-octyl trisaccharide acceptor. Moreover, although both enzymes utilized an N-linked asialo-agalacto-biantennary glycan as an acceptor, GnT-Vb displayed an almost 2.5-fold higher apparent K(m) value compared to GnT-V. Conversely, GnT-Vb very efficiently glycosylated a synthetic glycopeptide, Ac-H(2)N-Val-Glu-Pro-(GlcNAcbeta1,2-Man-O-)Thr-Ala-Val-CO-Ac, while GnT-V showed relatively poor activity toward this O-Man-linked glycopeptide acceptor, with a K(m) value of 20-fold higher than that of GnT-Vb. When the N-linked asialo-agalacto-biantennary glycan acceptor was utilized with GnT-Vb, the expected triantennary beta1,6-branched product was observed up to 8 h incubation. An additional product with two beta1,6-linked GlcNAc resides, however, was observed after prolonged (>8 h) incubation, consistent with an earlier report. This unusual tetraantennary product was observed with GnT-Vb only after substantial accumulation of the first triantennary product and not during the early stages of incubation.

Carbohydrate microarrays: key developments in glycobiology

Carbohydrate chains of glycoproteins, glycolipids, proteoglycans, and polysaccharides mediate processes of biological and medical importance through their interactions with complementary proteins. The unraveling of these interactions is therefore a priority in biomedical sciences. Carbohydrate microarray technology is a new development at the frontier of glycomics that is revolutionizing the study of carbohydrate-protein interactions and the elucidation of their specificities in endogenous biological processes, microbe-host interactions, and immune defense mechanisms. In this review, we briefly refer to the principles of numerous platforms since the introduction of carbohydrate microarrays in 2002, and we highlight platforms that are beyond proof-of-concept and have provided new biological information.

O-glycosylation pattern of CD24 from mouse brain

The cell adhesion molecule CD24 is a highly glycosylated glycoprotein that plays important roles in the central nervous system, the immune system and in tumor biology. Since CD24 comprises only a short protein core of approximately 30 amino acids and low conservation among species, it has been proposed that the functions of CD24 are mediated by its glycosylation pattern. Our present study provides evidence that interaction of CD24 with the cell adhesion molecule L1 is mediated by O-linked glycans carrying alpha2,3-linked sialic acid. Furthermore, de-N-glycosylated CD24 was shown to promote or inhibit neurite outgrowth of cerebellar neurons or dorsal root ganglion neurons, respectively, to the same extent as untreated CD24. Therefore, this study is focused on the structural elucidation of the chemically released, permethylated CD24 O-glycans by electrospray ionization ion trap mass spectrometry. Our analyses revealed the occurrence of a diverse mixture of mucin-type and O-mannosyl glycans carrying, in part, functionally relevant epitopes, such as 3-linked sialic acid, disialyl motifs, Le(X), sialyl-Le(X) or HNK-1 units. Hence, our data provide the basis for further studies on the contribution of carbohydrate determinants to CD24-mediated biological activities.

The repertoire of glycan determinants in the human glycome

The number of glycan determinants that comprise the human glycome is not known. This uncertainty arises from limited knowledge of the total number of distinct glycans and glycan structures in the human glycome, as well as limited information about the glycan determinants recognized by glycan-binding proteins (GBPs), which include lectins, receptors, toxins, microbial adhesins, antibodies, and enzymes. Available evidence indicates that GBP binding sites may accommodate glycan determinants made up of 2 to 6 linear monosaccharides, together with their potential side chains containing other sugars and modifications, such as sulfation, phosphorylation, and acetylation. Glycosaminoglycans, including heparin and heparan sulfate, comprise repeating disaccharide motifs, where a linear sequence of 5 to 6 monosaccharides may be required for recognition. Based on our current knowledge of the composition of the glycome and the size of GBP binding sites, glycoproteins and glycolipids may contain approximately 3000 glycan determinants with an additional approximately 4000 theoretical pentasaccharide sequences in glycosaminoglycans. These numbers provide an achievable target for new chemical and/or enzymatic syntheses, and raise new challenges for defining the total glycome and the determinants recognized by GBPs.

Subcellular localization of the carbohydrate Lewis(x) adhesion structure in hippocampus cell cultures

The Lewis(x) (Le(x)) epitope (Gal(beta1-4)[Fuc(alpha1-3)]GlcNAc-R) has been associated with the development of the central nervous system of diverse species including human and rodents. In this work, Le(x) has been found in the tetanus neurotoxin insensitive vesicle-associated membrane protein (TI-VAMP) compartment of rat hippocampus neurons in culture, at 7 days in vitro (DIV), when neurite extension is abundant. The TI-VAMP compartment is known to be associated with neurite outgrowth. Le(x) was found predominantly in neurites but also in somata and in growth cones. Abundant Le(x)-carrier glycoproteins specific to neurons have been identified at this stage of differentiation. At a later stage of differentiation, at 14 DIV, Le(x) appeared in extrasynaptic sites of GABAergic neurons, and in synaptic sites of glutamatergic neurons.

Protein O-mannosylation: conserved from bacteria to humans

Protein O-mannosylation is an essential modification in fungi and animals. Different from most other types of O-glycosylation, protein O-mannosylation is initiated in the endoplasmic reticulum by the transfer of mannose from dolichol monophosphate-activated mannose to serine and threonine residues of secretory proteins. In recent years, it has emerged that even bacteria are capable of O-mannosylation and that the biosynthetic pathway of O-mannosyl glycans is conserved between pro- and eukaryotes. In this review, we summarize the observations that have opened up the field and highlight characteristics of O-mannosylation in the different domains/kingdoms of life.

Receptor tyrosine phosphatase beta (RPTPbeta) activity and signaling are attenuated by glycosylation and subsequent cell surface galectin-1 binding

O-Mannosyl-linked glycosylation is abundant within the central nervous system, yet very few glycoproteins with this glycan modification have been identified. Congenital diseases with significant neurological defects arise from inactivating mutations found within the glycosyltransferases that act early in the O-mannosyl glycosylation pathway. The N-acetylglucosaminyltransferase known as GnT-Vb or -IX is highly expressed in brain and branches O-mannosyl-linked glycans. Our results using SH-SY5Y neuroblastoma cells indicate that GnT-Vb activity promotes the addition of the O-mannosyl-linked HNK-1 modification found on the developmentally regulated and neuron-specific receptor protein-tyrosine phosphatase beta (RPTPbeta). These changes in glycosylation accompany decreased cell-cell adhesion and increased rates of migration on laminin. In addition, we show that expression of GnT-Vb promotes its dimerization and inhibits RPTPbeta intrinsic phosphatase activity, resulting in higher levels of phosphorylated beta-catenin, suggesting a mechanism by which GnT-Vb glycosylation couples to changes in cell adhesion. GnT-Vb-mediated glycosylation of RPTPbeta promotes galectin-1 binding and RPTPbeta levels of retention on the cell surface. N-Acetyllactosamine, but not sucrose, treatment of cells results in decreased RPTP retention, showing that galectin-1 binding contributes to the increased retention after GnT-Vb expression. These results place GnT-Vb as a regulator of RPTPbeta signaling that influences cell-cell and cell-matrix interactions in the developing nervous system.

Neoglycolipid probes prepared via oxime ligation for microarray analysis of oligosaccharide-protein interactions

Neoglycolipid technology is the basis of a microarray platform for assigning oligosaccharide ligands for carbohydrate-binding proteins. The strategy for generating the neoglycolipid probes by reductive amination results in ring opening of the core monosaccharides. This often limits applicability to short-chain saccharides, although the majority of recognition motifs are satisfactorily presented with neoglycolipids of longer oligosaccharides. Here, we describe neoglycolipids prepared by oxime ligation. We provide evidence from NMR studies that a significant proportion of the oxime-linked core monosaccharide is in the ring-closed form, and this form selectively interacts with a carbohydrate-binding protein. By microarray analyses we demonstrate the effective presentation with oxime-linked neoglycolipids of (1) Lewis(x) trisaccharide to antibodies to Lewis(x), (2) sialyllactose analogs to the sialic acid-binding receptors, siglecs, and (3) N-glycans to a plant lectin that requires an intact N-acetylglucosamine core.

Protein Glycosylation, Conserved from Yeast to Man: A Model Organism Helps Elucidate Congenital Human Diseases

Proteins can be modified by a large variety of covalently linked saccharides. The present review concentrates on two types, protein N-glycosylation and protein O-mannosylation, which, with only a few exceptions, are evolutionary conserved from yeast to man. They are also distinguished by some special features: The corresponding glycosylation processes start in the endoplasmatic reticulum, are continued in the Golgi apparatus, and require dolichol-activated precursors for the initial biosynthetic steps. With respect to the molecular biology of both types of protein glycosylation, the pathways and the genetic background of the reactions have most successfully been studied with the genetically easy-to-handle baker's yeast, Saccharomyces cerevisae. Many of the severe developmental disturbances in children are related to protein glycosylation, for example, the CDG syndrome (congenital disorders of glycosylation) as well as congenital muscular dystrophies with neuronal-cell-migration defects have been elucidated with the help of yeast.

Monoclonal antibody Cat-315 detects a glycoform of receptor protein tyrosine phosphatase beta/phosphacan early in CNS development that localizes to extrasynaptic sites prior to synapse formation

Perineuronal nets (PNs) are lattice-like condensations of the extracellular matrix (ECM) that envelop synapses and decorate the surface of subsets of neurons in the CNS. Previous work has suggested that, despite the fact that PNs themselves are not visualized until later in development, some PN component molecules are expressed in the rodent CNS even before synaptogenesis. In the adult mammalian brain, monoclonal antibody Cat-315 recognizes a glycoform of aggrecan, a major component of PNs. In primary cortical cultures, a Cat-315-reactive chondroitin sulfate proteoglycan (CSPG) is also expressed on neuronal surfaces and is secreted into culture media as early as 24 h after plating. In this study, we show that in primary cortical cultures, the Cat-315 CSPG detected in early neural development is expressed in extrasynaptic sites prior to synapse formation. This suggests that ECM components in the CNS, as in the neuromuscular junction (NMJ), may prepattern neuronal surfaces prior to innervation. We further show that while the Cat-315-reactive carbohydrate decorates aggrecan in the adult, it decorates a different CSPG in the developing CNS. Using receptor protein tyrosine phosphatase beta (RPTPbeta/protein tyrosine phosphatase zeta) knock-out mice and immunoprecipitation techniques, we demonstrate here that in the developing rodent brain Cat-315 recognizes RPTPbeta isoforms. Our further examination of the Cat-315 epitope suggests that it is an O-mannose linked epitope in the HNK-1 family. The presence of the Cat-315 reactive carbohydrate on different PN components--RPTPbeta and aggrecan--at different stages of synapse development suggests a potential role for this neuron-specific carbohydrate motif in synaptogenesis.

Integrin-dependent neuroblastoma cell adhesion and migration on laminin is regulated by expression levels of two enzymes in the O-mannosyl-linked glycosylation pathway, PomGnT1 and GnT-Vb

O-mannosyl-linked glycans constitute a third of all brain O-linked glycoproteins, and yet very little is understood about their functions. Several congenital muscular dystrophies with central nervous system defects are caused by genetic disruptions in glycosyltransferases responsible for the synthesis of O-mannosyl glycans. The glycosyltransferase GnT-Vb, also known as GnT-IX, is expressed abundantly in the brain and testis and is proposed to be the enzyme that branches O-mannosyl-linked glycans. In this study, we show in a human neuronal model that GnT-Vb expression enhances neurite outgrowth on laminin. GnT-Vb has been shown to perform both N-linked and O-mannosyl-linked glycosylation. To determine if the effect on neurite outgrowth was due to N-linked or O-mannosyl-linked glycosylation by GnT-Vb we suppressed the expression of glycosyltransferases important for the elongation of both N-linked and O-mannosyl-linked glycans using RNA interference. Our results suggest that GnT-Vb and PomGnT1, enzymes involved in the O-mannosyl glycosylation pathway, play an active role in modulating integrin and laminin-dependent adhesion and migration of human neuronal cells.

Brain alpha-dystroglycan displays unique glycoepitopes and preferential binding to laminin-10/11

alpha-Dystroglycan was quantitatively enriched from mammalian brain based on its uniform reactivity with Vicia villosa agglutinin and resolved into sub-populations possessing or lacking the sulfated glucuronic acid epitope recognized by monoclonal antibody HNK-1. We generated a new monoclonal antibody specific for a glycoepitope on brain alpha-dystroglycan but absent from alpha-dystroglycan expressed in all other tissues examined. Finally, we found that laminin-10/11 preferentially bound to brain alpha-dystroglycan compared to skeletal muscle alpha-dystroglycan. Our results suggest that tissue-specific glycosylation modifies the laminin binding specificity of alpha-dystroglycan.

N-acetylglucosaminyltranferase VB expression enhances beta1 integrin- dependent PC12 neurite outgrowth on laminin and collagen

N-acetylglucosaminyltransferase VB (GnT-VB, -IX) is a newly discovered glycosyltransferase expressed exclusively in high levels in neuronal tissue during early development. Its homolog, GnT-V, is expressed in many tissues and modulates cell-cell and cell-matrix adhesion. The ability of GnT-VB to regulate cell-matrix interactions was initially investigated using the rat pheochromocytoma PC12 neurite outgrowth model. PC12 cells stably transfected with GnT-VB consistently showed an enhanced rate of nerve growth factor (NGF)-induced neurite outgrowth on collagen and laminin substrates. Levels of TrkA receptor phosphorylation and downstream ERK activation induced by NGF were not influenced by GnT-VB expression. No significant difference was observed in the rate of neurite outgrowth when cells were cultured on non-coated culture dishes, indicating that integrin-ECM interaction is required for the stimulatory effects. Neurite outgrowth induced by manganese-dependent activation of beta1 integrin on collagen and laminin substrates, however, showed a significant increase in neurite length for the PC12/GnT-VB cells, compared with control cells, suggesting that the enhancement is most likely mediated by alteration of beta1 integrin-ECM interaction by GnT-VB. These results demonstrate that GnT-VB expression can modulate the rate of neurite outgrowth by affecting beta1 integrin-ECM interaction.

Demonstration of the expression and the enzymatic activity of N-acetylglucosaminyltransferase IX in the mouse brain

We recently reported on a brain-specific beta1,6-N-acetylglucosaminyltransferase IX (GnT-IX, also referred to as GnT-VB), a GnT-V homologue, which acts on alpha-linked mannose of N-glycans and O-mannosyl glycans. To distinguish functions of GnT-IX with GnT-V, we examined the distribution of GnT-IX and GnT-V transcripts in mouse tissues by Northern blot analysis. The two enzymes were differentially expressed as has previously been observed in human tissues. GnT-IX transcripts were restricted to the cerebrum, cerebellum, thymus and testis, whereas GnT-V transcripts were expressed ubiquitously in mouse tissues. To investigate the localization of these enzymes in mouse tissues in more detail, a polyclonal antibody against GnT-IX was prepared. The antibody specifically recognized GnT-IX, but not GnT-V, in the Golgi apparatus, as confirmed by the use of GnT-IX and GnT-V transfectants. In agreement with the Northern blot analysis data, an immunohistochemical study showed substantial expression of GnT-IX in the brain, while no expression was observed in the liver. Moreover, to exclude GnT-V contamination, we performed an enzymatic assay for GnT-IX using a Mgat5 (GnT-V)-null mouse brain as an enzyme source and found the enzymatic activities do, in fact, exist in mouse brain. The reaction product was confirmed by high performance liquid chromatography and mass spectrometry. These results demonstrate, for the first time, that GnT-IX protein is actually expressed and may function as a glycosyltransferase in the brain.

Mechanisms in protein O-glycan biosynthesis and clinical and molecular aspects of protein O-glycan biosynthesis defects: a review

BACKGROUND

Genetic diseases that affect the biosynthesis of protein O-glycans are a rapidly growing group of disorders. Because this group of disorders does not have a collective name, it is difficult to get an overview of O-glycosylation in relation to human health and disease. Many patients with an unsolved defect in N-glycosylation are found to have an abnormal O-glycosylation as well. It is becoming increasingly evident that the primary defect of these disorders is not necessarily localized in one of the glycan-specific transferases, but can likewise be found in the biosynthesis of nucleotide sugars, their transport to the endoplasmic reticulum (ER)/Golgi, and in Golgi trafficking. Already, disorders in O-glycan biosynthesis form a substantial group of genetic diseases. In view of the number of genes involved in O-glycosylation processes and the increasing scientific interest in congenital disorders of glycosylation, it is expected that the number of identified diseases in this group will grow rapidly over the coming years.

CONTENT

We first discuss the biosynthesis of protein O-glycans from their building blocks to their secretion from the Golgi. Subsequently, we review 24 different genetic disorders in O-glycosylation and 10 different genetic disorders that affect both N- and O-glycosylation. The key clinical, metabolic, chemical, diagnostic, and genetic features are described. Additionally, we describe methods that can be used in clinical laboratory screening for protein O-glycosylation biosynthesis defects and their pitfalls. Finally, we introduce existing methods that might be useful for unraveling O-glycosylation defects in the future.