Expression cloning of a human sulfotransferase that directs the synthesis of the HNK-1 glycan on the neural cell adhesion molecule and glycolipids

Installation of O-glycan sulfation capacities in human HEK293 cells for display of sulfated mucins

The human genome contains at least 35 genes that encode Golgi sulfotransferases that function in the secretory pathway, where they are involved in decorating glycosaminoglycans, glycolipids, and glycoproteins with sulfate groups. Although a number of important interactions by proteins such as selectins, galectins, and sialic acid–binding immunoglobulin-like lectins are thought to mainly rely on sulfated O-glycans, our insight into the sulfotransferases that modify these glycoproteins, and in particular GalNAc-type O-glycoproteins, is limited. Moreover, sulfated mucins appear to accumulate in respiratory diseases, arthritis, and cancer. To explore further the genetic and biosynthetic regulation of sulfated O-glycans, here we expanded a cell-based glycan array in the human embryonic kidney 293 (HEK293) cell line with sulfation capacities. We stably engineered O-glycan sulfation capacities in HEK293 cells by site-directed knockin of sulfotransferase genes in combination with knockout of genes to eliminate endogenous O-glycan branching (core2 synthase gene GCNT1) and/or sialylation capacities in order to provide simplified substrates (core1 Galβ1–3GalNAcα1–O-Ser/Thr) for the introduced sulfotransferases. Expression of the galactose 3-O-sulfotransferase 2 in HEK293 cells resulted in sulfation of core1 and core2 O-glycans, whereas expression of galactose 3-O-sulfotransferase 4 resulted in sulfation of core1 only. We used the engineered cell library to dissect the binding specificity of galectin-4 and confirmed binding to the 3-O-sulfo-core1 O-glycan. This is a first step toward expanding the emerging cell-based glycan arrays with the important sulfation modification for display and production of glycoconjugates with sulfated O-glycans.

High degree of conservation of the enzymes synthesizing the laminin-binding glycoepitope of α-dystroglycan

The dystroglycan (DG) complex plays a pivotal role for the stabilization of muscles in Metazoa. It is formed by two subunits, extracellular α-DG and transmembrane β-DG, originating from a unique precursor via a complex post-translational maturation process. The α-DG subunit is extensively glycosylated in sequential steps by several specific enzymes and employs such glycan scaffold to tightly bind basement membrane molecules. Mutations of several of these enzymes cause an alteration of the carbohydrate structure of α-DG, resulting in severe neuromuscular disorders collectively named dystroglycanopathies. Given the fundamental role played by DG in muscle stability, it is biochemically and clinically relevant to investigate these post-translational modifying enzymes from an evolutionary perspective. A first phylogenetic history of the thirteen enzymes involved in the fabrication of the so-called 'M3 core' laminin-binding epitope has been traced by an overall sequence comparison approach, and interesting details on the primordial enzyme set have emerged, as well as substantial conservation in Metazoa. The optimization along with the evolution of a well-conserved enzymatic set responsible for the glycosylation of α-DG indicate the importance of the glycosylation shell in modulating the connection between sarcolemma and surrounding basement membranes to increase skeletal muscle stability, and eventually support movement and locomotion.

Neural glycomics: the sweet side of nervous system functions

The success of investigations on the structure and function of the genome (genomics) has been paralleled by an equally awesome progress in the analysis of protein structure and function (proteomics). We propose that the investigation of carbohydrate structures that go beyond a cell's metabolism is a rapidly developing frontier in our expanding knowledge on the structure and function of carbohydrates (glycomics). No other functional system appears to be suited as well as the nervous system to study the functions of glycans, which had been originally characterized outside the nervous system. In this review, we describe the multiple studies on the functions of LewisX, the human natural killer cell antigen-1 (HNK-1), as well as oligomannosidic and sialic (neuraminic) acids. We attempt to show the sophistication of these structures in ontogenetic development, synaptic function and plasticity, and recovery from trauma, with a view on neurodegeneration and possibilities to ameliorate deterioration. In view of clinical applications, we emphasize the need for glycomimetic small organic compounds which surpass the usefulness of natural glycans in that they are metabolically more stable, more parsimonious to synthesize or isolate, and more advantageous for therapy, since many of them pass the blood brain barrier and are drug-approved for treatments other than those in the nervous system, thus allowing a more ready access for application in neurological diseases. We describe the isolation of such mimetic compounds using not only Western NIH, but also traditional Chinese medical libraries. With this review, we hope to deepen the interests in this exciting field.

Knockdown of chondroitin-4-sulfotransferase-1, but not of dermatan-4-sulfotransferase-1, accelerates regeneration of zebrafish after spinal cord injury

Glycosaminoglycans such as chondroitin sulfate (CS) and dermatan sulfate (DS) are long chains of repeating disaccharide units, covalently linked to core proteins to form proteoglycans. Proteoglycans can be cell membrane-bound or are part of the extracellular matrix. They are important in a wide range of biologic processes, including development, synaptic plasticity, and regeneration after injury, as well as modulation of growth factor signaling, cell migration, survival, and proliferation. Synthesis of CS and DS in the Golgi apparatus is mediated by sulfotransferases that modify sugar chains through transfer of sulfate groups to specific positions on the sugar moieties. To clarify the functions of CS and DS during nervous system regeneration, we studied the effect of chondroitin 4- O-sulfotransferase-1/carbohydrate sulfotransferase-11 (C4ST-1/Chst-11) and dermatan 4- O-sulfotransferase-1/Chst-14 (D4ST-1/Chst-14) down-regulation on spinal cord regeneration in larval and adult zebrafish. In our study, knockdown of C4ST1/Chst-11 accelerated regeneration after spinal cord injury in larval and adult zebrafish and knockdown of D4ST1/Chst-14 did not alter regenerative capacity. From these and previous observations, we drew the conclusion that different CS and DS expression patterns can be growth permitting, growth inhibiting, or neutral for regrowing or sprouting axons, depending on the tissue environment of a particular animal species.-Sahu, S., Li, R., Loers, G., Schachner, M. Knockdown of chondroitin-4-sulfotransferase-1, but not of dermatan-4-sulfotransferase-1, accelerates regeneration of zebrafish after spinal cord injury.

The role of human natural killer-1 (HNK-1) carbohydrate in neuronal plasticity and disease

BACKGROUND

The human natural killer-1 (HNK-1) carbohydrate, a unique trisaccharide possessing sulfated glucuronic acid in a non-reducing terminus (HSO₃-3GlcAß1-3Galß1-4GlcNAc-), is highly expressed in the nervous system and its spatiotemporal expression is strictly regulated. Mice deficient in the gene encoding a key enzyme, GlcAT-P, of the HNK-1 biosynthetic pathway exhibit almost complete disappearance of the HNK-1 epitope in the brain, significant reduction of long-term potentiation, and aberration of spatial learning and memory formation. In addition to its physiological roles in higher brain function, the HNK-1 carbohydrate has attracted considerable attention as an autoantigen associated with peripheral demyelinative neuropathy, which relates to IgM paraproteinemia, because of high immunogenicity. It has been suggested, however, that serum autoantibodies in IgM anti-myelin-associated glycoprotein (MAG) antibody-associated neuropathy patients show heterogeneous reactivity to the HNK-1 epitope.

SCOPE OF REVIEW

We have found that structurally distinct HNK-1 epitopes are expressed in specific proteins in the nervous system. Here, we overview the current knowledge of the involvement of these HNK-1 epitopes in the regulation of neural plasticity and discuss the impact of different HNK-1 antigens of anti-MAG neuropathy patients.

MAJOR CONCLUSIONS

We identified the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type glutamate receptor subunit GluA2 and aggrecan as HNK-1 carrier proteins. The HNK-1 epitope on GluA2 and aggrecan regulates neural plasticity in different ways. Furthermore, we found the clinical relationship between reactivity of autoantibodies to the different HNK-1 epitopes and progression of anti-MAG neuropathy.

GENERAL SIGNIFICANCE

The HNK-1 epitope is indispensable for the acquisition of normal neuronal function and can be a good target for the establishment of diagnostic criteria for anti-MAG neuropathy.

Molecular Genetic Contributions to Social Deprivation and Household Income in UK Biobank

Individuals with lower socio-economic status (SES) are at increased risk of physical and mental illnesses and tend to die at an earlier age [1, 2, 3]. Explanations for the association between SES and health typically focus on factors that are environmental in origin [4]. However, common SNPs have been found collectively to explain around 18% of the phenotypic variance of an area-based social deprivation measure of SES [5]. Molecular genetic studies have also shown that common physical and psychiatric diseases are partly heritable [6]. It is possible that phenotypic associations between SES and health arise partly due to a shared genetic etiology. We conducted a genome-wide association study (GWAS) on social deprivation and on household income using 112,151 participants of UK Biobank. We find that common SNPs explain 21% of the variation in social deprivation and 11% of household income. Two independent loci attained genome-wide significance for household income, with the most significant SNP in each of these loci being rs187848990 on chromosome 2 and rs8100891 on chromosome 19. Genes in the regions of these SNPs have been associated with intellectual disabilities, schizophrenia, and synaptic plasticity. Extensive genetic correlations were found between both measures of SES and illnesses, anthropometric variables, psychiatric disorders, and cognitive ability. These findings suggest that some SNPs associated with SES are involved in the brain and central nervous system. The genetic associations with SES obviously do not reflect direct causal effects and are probably mediated via other partly heritable variables, including cognitive ability, personality, and health.

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Common SNPs explain 21% of social deprivation and 11% of household income

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Two loci attained genome-wide significance for household income

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Genes in these loci have been linked to synaptic plasticity

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Genetic correlations were found between both measures of SES and many other traits

HNK-1 sulfotransferase-dependent sulfation regulating laminin-binding glycans occurs in the post-phosphoryl moiety on α-dystroglycan

Dystroglycan (DG) is a cell surface glycoprotein that connects extracellular matrix molecules to the intracellular cytoskeleton, functioning as mechanical and signaling axes in various physiological events. Since the ligand-binding activity of DG strictly depends on O-mannosyl glycans attached to its extracellular α-DG subunit, aberrant glycosylation causes dystroglycanopathy, a subclass of congenital muscular dystrophy. Accumulating evidence shows that like-acetylglucosaminyltransferase (LARGE), a glycosyltransferase involved in the biosynthesis of a phosphodiester-linked modification on O-mannose, is essential for α-DG to gain the ligand-binding activity. We previously reported that human natural killer-1 sulfotransferase (HNK-1ST), which was originally reported as one of the enzymes responsible for HNK-1 glycoepitope, had an ability to suppress the glycosylation and the function of α-DG. In this study, we investigated how HNK-1ST regulates the glycosylation of α-DG using deletion and mutation analyses. We generated an α-DG mutant which has only one threonine residue capable of being modified by LARGE. Focusing on the single post-phosphoryl modification site, we found that HNK-1ST showed an almost complete inhibition of the LARGE-dependent modification and transferred a sulfate group to the phosphodiester-linked moiety on O-mannose. Furthermore, using an in vitro enzymatic assay system, we demonstrated that the sulfated α-DG by HNK-1ST is no longer glycosylated by LARGE. These results illustrate one possible glycosylation pathway where α-DG function is regulated by opposing actions of HNK-1ST and LARGE.

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.

Human natural killer-1 sulfotransferase (HNK-1ST)-induced sulfate transfer regulates laminin-binding glycans on α-dystroglycan

Retinoic acid (RA) is a well established anti-tumor agent inducing differentiation in various cancer cells. Recently, a robust up-regulation of human natural killer-1 sulfotransferase (HNK-1ST) was found in several subsets of melanoma cells during RA-mediated differentiation. However, the molecular mechanism underlying the tumor suppression mediated by HNK-1ST remains unclear. Here, we show that HNK-1ST changed the glycosylation state and reduced the ligand binding activity of α-dystroglycan (α-DG) in RA-treated S91 melanoma cells, which contributed to an attenuation of cell migration. Knockdown of HNK-1ST restored the glycosylation of α-DG and the migration of RA-treated S91 cells, indicating that HNK-1ST functions through glycans on α-DG. Using CHO-K1 cells, we provide direct evidence that HNK-1ST but not other homologous sulfotransferases (C4ST1 and GalNAc4ST1) suppresses the glycosylation of α-DG. The activity-abolished mutant of HNK-1ST did not show the α-DG-modulating function, indicating that the sulfotransferase activity of HNK-1ST is essential. Finally, the HNK-1ST-dependent incorporation of [(35)S]sulfate groups was detected on α-DG. These findings suggest a novel role for HNK-1ST as a tumor suppressor controlling the functional glycans on α-DG and the importance of sulfate transfer in the glycosylation of α-DG.

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.

Dermatan sulfotransferase Chst14/D4st1, but not chondroitin sulfotransferase Chst11/C4st1, regulates proliferation and neurogenesis of neural progenitor cells

Chondroitin sulfates (CSs) and dermatan sulfates (DSs) are enriched in the microenvironment of neural stem cells (NSCs) during development and in the adult neurogenic niche, and have been implicated in mechanisms governing neural precursor migration, proliferation and differentiation. In contrast to previous studies, in which a chondroitinaseABC-dependent unselective deglycosylation of both CSs and DSs was performed, we used chondroitin 4-O-sulfotransferase-1 (Chst11/C4st1)- and dermatan 4-O-sulfotransferase-1 (Chst14/D4st1)-deficient NSCs specific for CSs and DSs, respectively, to investigate the involvement of specific sulfation profiles of CS and DS chains, and thus the potentially distinct roles of CSs and DSs in NSC biology. In comparison to wild-type controls, deficiency for Chst14 resulted in decreased neurogenesis and diminished proliferation of NSCs accompanied by increased expression of GLAST and decreased expression of Mash-1, and an upregulation of the expression of the receptors for fibroblast growth factor-2 (FGF-2) and epidermal growth factor (EGF). By contrast, deficiency in Chst11 did not influence NSC proliferation, migration or differentiation. These observations indicate for the first time that CSs and DSs play distinct roles in the self-renewal and differentiation of NSCs.

Involvement of human natural killer-1 (HNK-1) sulfotransferase in the biosynthesis of the GlcUA(3-O-sulfate)-Gal-Gal-Xyl tetrasaccharide found in α-thrombomodulin from human urine

Thrombomodulin (TM) is an integral membrane glycoprotein, which occurs as both a chondroitin sulfate (CS) proteoglycan (PG) form (β-TM) and a non-PG form without a CS chain (α-TM) and hence is a part-time PG. An α-TM preparation isolated from human urine contained the glycosaminoglycan linkage region tetrasaccharide GlcUAβ1-3Galβ1-3Galβ1-4xylose, and the nonreducing terminal GlcUA residue is 3-O-sulfated. Because the human natural killer-1 sulfotransferase (HNK-1ST) transfers a sulfate group from 3'-phosphoadenosine 5'-phosphosulfate to the C-3 position of the nonreducing terminal GlcUA residue in the HNK-1 antigen precursor trisaccharide, GlcUAβ1-3Galβ1-4GlcNAc, the sulfotransferase activity toward the linkage region was investigated. In fact, the activity of HNK-1ST toward the linkage region was much higher than that toward the glucuronylneolactotetraosylceramide, the precursor of the HNK-1 epitope. HNK-1ST may be responsible for regulating the sorting of α- and β-TM. Furthermore, HNK-1ST also transferred a sulfate group from 3'-phosphoadenosine 5'-phosphosulfate to the C-3 position of the nonreducing terminal GlcUA residue of a chondroitin chain. Intriguingly, the HNK-1 antibody recognized CS chains and the linkage region if they contained GlcUA(3-O-sulfate), suggesting that HNK-1ST not only synthesizes the HNK-1 epitope but may also be involved in the generation of part-time PGs.

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.

The proprotein convertase PC7: unique zymogen activation and trafficking pathways

The zymogen activation mechanism and physiological functions of the most ancient and highly conserved basic amino acid-specific proprotein convertase 7 (PC7) are not known. Herein, we characterized the biosynthesis, subcellular localization, and trafficking of the membrane-bound full-length rat and human PC7. The prosegment of PC7 is primarily secreted alone as a non-inhibitory protein via the conventional, Golgi-dependent, secretory pathway. Mature PC7 is partially sulfated and thus reaches the cell surface via the conventional route. However, a fraction of PC7 reaches the cell surface through a brefeldin A- and COPII-independent unconventional secretory pathway. The latter trafficking may explain the rapid (<10 min) transit of a fraction of PC7 from the ER to the cell surface. Electron microscopy further confirmed the localization of PC7 to the cell surface of HEK293 cells. Within the cytosolic tail, only two cysteines (Cys(699) and Cys(704)) are palmitoylated, but this modification does not affect the choice of trafficking pathway. Swapping the transmembrane-cytosolic tail (TMCT) sequences of the convertases Furin and PC7 revealed that PC7(TMCT-Furin) is much more sulfated and hence traffics more efficiently through the conventional secretory pathway. In contrast, the Furin(TMCT-PC7) is no longer sulfated and thus reaches the cell surface by the unconventional pathway. Because trafficking of PC7(CT-Furin) and Furin(CT-PC7) resemble their wild type counterparts, we deduce that the transmembrane domain of PC7 regulates the sorting of PC7 toward the unconventional secretory pathway. In conclusion, PC7 is distinct from other proprotein convertases in its zymogen activation, subcellular localization, and trafficking.

Candidate gene analysis of the human natural killer-1 carbohydrate pathway and perineuronal nets in schizophrenia: B3GAT2 is associated with disease risk and cortical surface area

BACKGROUND

The Human Natural Killer-1 carbohydrate (HNK-1) is involved in neurodevelopment and synaptic plasticity. Extracellular matrix structures called perineuronal nets, condensed around subsets of neurons and proximal dendrites during brain maturation, regulate synaptic transmission and plasticity.

METHODS

Ten genes of importance for HNK-1 biosynthesis (B3GAT1, B3GAT2, and CHST10) or for the formation of perineuronal nets (TNR, BCAN, NCAN, HAPLN1, HAPLN2, HAPLN3, and HAPLN4) were investigated for potential involvement in schizophrenia (SCZ) susceptibility, by genotyping 104 tagSNPs in the Scandinavian Collaboration on Psychiatric Etiology sample (849 cases; 1602 control subjects). Genome-wide association study imputation data from the European SGENE-plus sample (2663 cases; 13,498 control subjects) were used for comparison. The effect of SCZ risk alleles on brain structure was investigated in a Norwegian subset (98 cases; 177 control subjects) with structural magnetic resonance imaging data.

RESULTS

Five single nucleotide polymorphisms (SNPs), located in two adjacent estimated linkage disequilibrium blocks in the first intron of β-1,3-glucuronyltransferase 2 (B3GAT2), were nominally associated with SCZ (.004 ≤ P(empirical) ≤ .05). The rs2460691 was significantly associated in the comparison sample and in the meta-analysis after correction for all 121 SNP/haplotype tests (P(raw) = 1 × 10(-4); P(corrected) = .018). Increased dosage of the rs2460691 SCZ risk allele was associated with decreased cortical area (p = .002) but not thickness or hippocampal volume. A second SNP (r(2) = .24 with rs10945275), which conferred the highest SCZ risk effect in the Norwegian subset, was also associated with cortical area.

CONCLUSIONS

The present results suggest that effects on biosynthesis of the neuronal epitope HNK-1, through common B3GAT2 variation, could increase the risk of SCZ, possibly by decreasing cortical area.

N-linked oligosaccharides are required to produce and stabilize the active form of chondroitin 4-sulphotransferase-1

C4ST-1 (chondroitin 4-sulphotransferase-1) transfers sulphate to position 4 of N-acetylgalactosamine in chondroitin. We showed previously that purified C4ST-1 from the culture medium of rat chondrosarcoma cells was a glycoprotein containing approx. 35% N-linked oligosaccharides. In the present paper, we investigated the functional role of the N-linked oligosaccharides attached to C4ST-1. We found that (i) treatment of recombinant C4ST-1 with peptide N-glycosidase F caused a marked decrease in activity, (ii) production of the active form of C4ST-1 by COS-7 cells transfected with cDNA of C4ST-1 was inhibited by tunicamycin, (iii) deletion of the N-glycosylation site located at the C-terminal region of C4ST-1 abolished activity, (iv) attachment of a single N-glycan at the C-terminal region supported production of the active form of C4ST-1, but the resulting recombinant enzyme was much more unstable at 37 degrees C than the control recombinant protein, and (v) truncation of C-terminal region up to the N-glycosylation site at the C-terminal region resulted in total loss of activity. These observations strongly suggest that N-linked oligosaccharides attached to C4ST-1 contribute to the production and stability of the active form of C4ST-1. In addition, the N-linked oligosaccharide at the C-terminal region appears to affect the glycosylation pattern of recombinant C4ST; a broad protein band of the wildtype protein resulting from microheterogeneity of N-linked oligosaccharides disappeared and four discrete protein bands with different numbers of N-linked oligosaccharides appeared when the N-linked oligosaccharide at the C-terminal region was deleted.

Mechanism of regulation and suppression of melanoma invasiveness by novel retinoic acid receptor-gamma target gene carbohydrate sulfotransferase 10

Retinoic acid (RA) induces growth arrest and differentiation of S91 murine melanoma cells and serves as a valuable model for this disease. RA acts through activation of RA receptors (RAR), which are members of the nuclear receptor superfamily of ligand-inducible transcription factors. Interestingly, differentiation is mediated by RARgamma, but not by RARalpha or RARbeta, suggesting that RARgamma possesses unique and uncharacterized molecular properties. To address this question, DNA microarrays in combination with RAR isoform-specific agonists were employed to identify novel RARgamma target genes that may play a role in this process. Here, we identified and validated carbohydrate sulfotransferase 10 (CHST10) as a novel RARgamma target gene in S91 cells. The RARgamma-inducible CHST10 promoter was obtained, and two atypical, independently functioning RA response elements were identified in a 425 bp region. Surprisingly, this fragment is bound by RARgamma, but not by RARalpha or RARbeta, thus providing a mechanism for the observed RARgamma-specific regulation. CHST10 is a sulfotransferase that forms HNK-1 glycan on neural cell adhesion proteins and glycolipids, and HNK-1 is thought to modulate cell adhesion and possibly metastasis. We show that CHST10 is also regulated by RARgamma in a significant subset of human melanoma cells, and three-dimensional cell culture migration assays suggest that CHST10 functions as a suppressor of invasiveness, but not proliferation, in these cells. Induction of CHST10 by RARgamma-activating retinoids may present a novel therapeutic strategy to inhibit invasiveness in a subset of melanoma patients.

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.

Tissue-specific mRNA expression profiles of human carbohydrate sulfotransferase and tyrosylprotein sulfotransferase

Pairs of forward and reverse primers and TaqMan probes specific to each of 15 human sulfotransferases were prepared. The mRNA expression level of each target enzyme was analyzed in total RNA from single and pooled specimens of various human tissues (adrenal gland, bone marrow, brain, colon, heart, kidney, liver, lung, pancreas, peripheral leukocytes, placenta, prostate, salivary gland, skeletal muscle, small intestine, spinal cord, spleen, stomach, testis, thymus, thyroid gland, trachea, and uterus) by real-time reverse transcription PCR using an ABI PRISM 7700 Sequence Detection System. The mRNA expression profiles of the sulfotransferases in these 23 different human tissues were used to identify the tissues exhibiting high transcriptional activity for these enzymes. These results provide valuable information for studies concerning the human carbohydrate sulfotransferase and tyrosylprotein sulfotransferase genes in various tissues.

Expression of specific carbohydrates by transfection with carbohydrate modifying enzymes

The identification of cDNAs encoding glycosyltransferases and carbohydrate-modifying enzymes such as sulfotransferases has allowed expression of a given enzyme in cells that lack the enzyme or express it at very low levels. By comparing the function and/or structure of carbohydrates expressed in cells before and after transfection, we can determine the function of the ectopically expressed enzyme. This assay is less time consuming than assaying function by obtaining cells deficient in a given enzyme. Moreover, it is a more definitive method for establishing the function of the enzyme because the result is derived from an enzyme introduced by transfection. Using this method, an enormous amount of knowledge relevant to the structure and function of glycoenzymes has been derived from such studies. In this chapter, we describe methods used to obtain mammalian cells that have acquired new carbohydrate structures and function following transfection of mammalian expression vectors harboring glycoenzymes.

Differential expression and enzymatic properties of GalNAc-4-sulfotransferase-1 and GalNAc-4-sulfotransferase-2

We have cloned two GalNAc-4-sulfotransferases, GalNAc-4-ST1 and GalNAc-4-ST2, that transfer sulfate to terminal beta1,4-linked GalNAc. In conjunction with the action of protein-specific beta1,4GalNAc-transferases, GalNAc-4-ST1 and GalNAc-4-ST2 account for the presence of terminal beta1,4-linked GalNAc-4-SO(4) on glycoproteins such as lutropin, thyrotropin (TSH), proopiomelanocortin (POMC), carbonic anhydratase-VI (CA-VI), and tenascin-R. GalNAc-4-ST1 and GalNAc-4-ST2 can be distinguished by their differing specificity for oligosaccharide acceptors and temperature lability. The differences in properties have been used to show that the levels of GalNAc-4-ST1 and GalNAc-4-ST2 activity are proportionate to the levels of their respective transcripts. Furthermore, we have found that both transcript and activity levels of GalNAc-4-ST1 and GalNAc-4-ST2 vary widely among different tissues indicating that the regulation of their expression differs. Differences in specificity and the regulation of expression may account for existence of two GalNAc-4-sulfotransferases in vivo. The highest levels of both GalNAc-4-ST1 and GalNAc-4-ST2 transcripts are present in the pituitary of the mouse with multiple cell types that produce glycoproteins terminating with GalNAc-4-SO(4). Genetic ablation of both GalNAc-4-ST1 and GalNAc-4-ST2 may be necessary to alter the pattern and/or extent of sulfate addition to terminal beta1,4GalNAc in tissues such as pituitary.

A non-sulfated form of the HNK-1 carbohydrate is expressed in mouse kidney

The HNK-1 carbohydrate, which is recognized by anti-HNK-1 antibody, is well known to be expressed predominantly in the nervous system. The characteristic structural feature of the HNK-1 carbohydrate is 3-sulfo-glucuronyl residues attached to lactosamine structures (Gal beta1-4GlcNAc) on glycoproteins and glycolipids. The biosynthesis of the HNK-1 carbohydrate is regulated mainly by two glucuronyltransferases (GlcAT-P and GlcAT-S) and a sulfotransferase. In this study, we found that GlcAT-S mRNA was expressed at higher levels in the kidney than in the brain, but that both GlcAT-P and HNK-1 sulfotransferase mRNAs, which were expressed at high levels in the brain, were not detected in the kidney. These results suggested that the HNK-1 carbohydrate without sulfate (non-sulfated HNK-1 carbohydrate) is expressed in the kidney. We substantiated this hypothesis using two different monoclonal antibodies: one (anti-HNK-1 antibody) requires sulfate on glucuronyl residues for its binding, and the other (antibody M6749) does not. Western blot analyses of mouse kidney revealed that two major bands (80 and 140 kDa) were detected with antibody M6749, but not with anti-HNK-1 antibody. The 80- and 140-kDa band materials were identified as meprin alpha and CD13/aminopeptidase N, respectively. We also confirmed the presence of the non-sulfated HNK-1 carbohydrate on N-linked oligosaccharides by multistage tandem mass spectrometry. Immunofluorescence staining with antibody M6749 revealed that the non-sulfated HNK-1 carbohydrate was expressed predominantly on the apical membranes of the proximal tubules in the cortex and was also detected in the thin ascending limb in the inner medulla. This is the first study indicating the presence of the non-sulfated HNK-1 carbohydrate being synthesized by GlcAT-S in the kidney. The results presented here constitute novel knowledge concerning the function of the HNK-1 carbohydrate.

Glycosylation-related Gene Expression in Prion Diseases

Several lines of evidence indicate that some glycoconjugates are efficient effectors of the cellular prion protein (PrP(C)) conversion into its pathogenic (PrP(Sc)) isoform. To assess how glycoconjugate glycan moieties participate in the biogenesis of PrP(Sc), an exhaustive comparative analysis of the expression of about 200 glycosylation-related genes was performed on prion-infected or not, hypothalamus-derived GT1 cells by hybridization of DNA microarrays, semiquantitative RT-PCR, and biochemical assays. A significant up- (30-fold) and down- (17-fold) regulation of the expression of the ChGn1 and Chst8 genes, respectively, was observed in prion-infected cells. ChGn1 and Chst8 are involved in the initiation of the synthesis of chondroitin sulfate and in the 4-O-sulfation of non-reducing N-acetylgalactosamine residues, respectively. A possible role for a hyposulfated chondroitin in PrP(Sc) accumulation was evidenced at the protein level and by determination of chondroitin and heparan sulfate amounts. Treatment of Sc-GT1 cells with a heparan mimetic (HM2602) induced an important reduction of the amount of PrP(Sc), associated with a total reversion of the transcription pattern of the N-acetylgalactosamine-4-O-sulfotransferase 8. It suggests a link between the genetic control of 4-O-sulfation and PrP(Sc) accumulation.

Strategies for drug discovery by targeting sulfation pathways

Posttranslational modifications of proteins such as phosphorylation have been recognized as pivotal modulators of biological activity in healthy and diseased tissues. Sulfation is a key posttranslational modification the role of which in physiology and pathology is only now becoming appreciated. Whereas phosphorylation is central to intracellular signal transduction, sulfation modulates cell-cell and cell-matrix communication. Sulfation involves a class of enzymes known as sulfotransferases, which transfer sulfate from the ATP-like sulfate donor 3'phosphoadenosine-5'phosphosulate to glycoproteins, glycolipids or metabolites. This review focuses on Golgi-localized sulfotransferases, their molecular biology and biochemistry, and strategies towards discovery of sulfotransferase inhibitors that could have potential as therapeutics in inflammation, cancer and infectious diseases.

Deregulation of the carbohydrate (chondroitin 4) sulfotransferase 11 (CHST11) gene in a B-cell chronic lymphocytic leukemia with a t(12;14)(q23;q32)

The t(12;14)(q23;q32) breakpoints in a case of B-cell chronic lymphocytic leukemia (B-CLL) were mapped by fluorescence in situ hybridization (FISH) and Southern blot analysis and cloned using an IGH switch-gamma probe. The translocation affected a productively rearranged IGH allele and the carbohydrate (chondroitin 4) sulfotransferase 11 (CHST11) locus at 12q23, with a reciprocal break in intron 2 of the CHST11 gene. CHST11 belongs to the HNK1 family of Golgi-associated sulfotransferases, a group of glycosaminoglycan-modifying enzymes, and is expressed mainly in the hematopoietic lineage. Northern Blot analysis of tumor RNA using CHST11-specific probes showed expression of two CHST11 forms of abnormal size. 5'- and 3'-Rapid Amplification of cDNA Ends (RACE) revealed IGH/CHST11 as well as CHST11/IGH fusion RNAs expressed from the der(14) and der(12) chromosomes. Both fusion species contained open reading frames making possible the translation of two truncated forms of CHST11 protein. The biological consequence of t(12;14)(q23;q32) in this case presumably is a disturbance of the cellular distribution of CHST11 leading to deregulation of a chondroitin-sulfate-dependent pathway specific to the hematopoietic lineage.

N-Acetylglucosaminyltransferase IX Acts on the GlcNAcβ1,2-Manα1-Ser/Thr Moiety, Forming a 2,6-Branched Structure in Brain O-Mannosyl Glycan

Mammals contain O-linked mannose residues with 2-mono- and 2,6-di-substitutions by GlcNAc in brain glycoproteins. It has been demonstrated that the transfer of GlcNAc to the 2-OH position of the mannose residue is catalyzed by the enzyme, protein O-mannose beta1,2-N-acetylglucosaminyltransferase (POMGnT1), but the enzymatic basis of the transfer to the 6-OH position is unknown. We recently reported on a brain-specific beta1,6-N-acetylglucosaminyltransferase, GnT-IX, that catalyzes the transfer of GlcNAc to the 6-OH position of the mannose residue of GlcNAcbeta1,2-Manalpha on both the alpha1,3- and alpha1,6-linked mannose arms in the core structure of N-glycan (Inamori, K., Endo, T., Ide, Y., Fujii, S., Gu, J., Honke, K., and Taniguchi, N. (2003) J. Biol. Chem. 278, 43102-43109). Here we examined the issue of whether GnT-IX is able to act on the same sequence of the GlcNAcbeta1,2-Manalpha in O-mannosyl glycan. Using three synthetic Ser-linked mannose-containing saccharides, Manalpha1-Ser, GlcNAcbeta1,2-Manalpha1-Ser, and Galbeta1,4-GlcNAcbeta1,2-Manalpha1-Ser as acceptor substrates, the findings show that (14)C-labeled GlcNAc was incorporated only into GlcNAcbeta1,2-Manalpha1-Ser after separation by thin layer chromatography. To simplify the assay, high performance liquid chromatography was employed, using a fluorescence-labeled acceptor substrate GlcNAcbeta1,2-Manalpha1-Ser-pyridylaminoethylsuccinamyl (PAES). Consistent with the above data, GnT-IX generated a new product which was identified as GlcNAcbeta1,2-(GlcNAcbeta1,6-)Manalpha1-Ser-PAES by mass spectrometry and (1)H NMR. Furthermore, incorporation of an additional GlcNAc residue into a synthetic mannosyl peptide Ac-Ala-Ala-Pro-Thr(Man)-Pro-Val-Ala-Ala-Pro-NH(2) by GnT-IX was only observed in the presence of POMGnT1. Collectively, these results strongly suggest that GnT-IX may be a novel beta1,6-N-acetylglucosaminyltransferase that is responsible for the formation of the 2,6-branched structure in the brain O-mannosyl glycan.

Specificities of three distinct human chondroitin/dermatan N-acetylgalactosamine 4-O-sulfotransferases demonstrated using partially desulfated dermatan sulfate as an acceptor: implication of differential roles in dermatan sulfate biosynthesis

4-O-Sulfation of GalNAc is a high frequency modification of chondroitin sulfate and dermatan sulfate (DS), and three major GalNAc 4-O-sulfotransferases including dermatan 4-O-sulfotransferase-1 (D4ST-1) and chondroitin 4-O-sulfotransferases-1 and -2 (C4ST-1 and -2) have been identified. 4-O-Sulfation of GalNAc during DS biosynthesis had long been postulated to be a prerequisite for iduronic acid (IdoUA) formation by C5-epimerization of GlcUA. This hypothesis has recently been argued based on enzymological studies using microsomes that C5-epimerization precedes 4-O-sulfation, which was further supported by the specificity of the cloned D4ST-1 with predominant preference for IdoUA-GalNAc flanked by GlcUA-GalNAc over IdoUA-GalNAc flanked by IdoUA-GalNAc in exhaustively desulfated dermatan. Whereas the counterproposal explains the initial reactions, apparently it cannot rationalize the synthetic mechanism of IdoUA-GalNAc(4-O-sulfate)-rich clusters typical of mature DS chains. In this study, we examined detailed specificities of the three recombinant human 4-O-sulfotransferases using partially desulfated DS as an acceptor. Enzymatic analysis of the transferase reaction products showed that D4ST-1 far more efficiently transferred sulfate to GalNAc residues in -IdoUA-Gal-NAc-IdoUA-than in -GlcUA-GalNAc-GlcUA-sequences. In contrast, C4ST-1 showed the opposite preference, and C4ST-2 used GalNAc residues in both sequences to comparable degrees, being consistent with its phylogenetic relations to D4ST-1 and C4ST-1. Structural analysis of the oligosaccharides, which were isolated after chondroitinase AC-I digestion of the 35S-labeled transferase reaction products, revealed for the first time that D4ST-1, as compared with C4ST-1 and C4ST-2, most efficiently utilized GalNAc residues located not only in the sequence -IdoUA-GalNAc-IdoUA- but also in -GlcUA-Gal-NAc-IdoUA- and -IdoUA-GalNAc-GlcUA-. The isolated oligosaccharide structures also suggest that 4-O-sulfation promotes subsequent 4-O-sulfation of GalNAc in the neighboring disaccharide unit.

Identification and characterization of three Drosophila melanogaster glucuronyltransferases responsible for the synthesis of the conserved glycosaminoglycan-protein linkage region of proteoglycans. Two novel homologs exhibit broad specificity toward oligosaccharides from proteoglycans, glycoproteins, and glycosphingolipids

The Drosophila melanogaster genome contains three putative glucuronyltransferases homologous to human GlcAT-I and GlcAT-P. These enzymes are predicted to be beta1,3-glucuronyltransferases involved in the synthesis of the glycosaminoglycan (GAG)-protein linkage region of proteoglycans and the HNK-1 carbohydrate epitope of glycoproteins, respectively. The genes encode active enzymes, which we have designated DmGlcAT-I, DmGlcAT-BSI, and DmGlcAT-BSII (where BS stands for "broad specificity"). Protein A-tagged truncated soluble forms of all three enzymes efficiently transfer GlcUA from UDP-GlcUA to the linkage region trisaccharide Galbeta1-3Galbeta1-4Xyl. Strikingly, DmGlcAT-I has specificity for Galbeta1-3Galbeta1-4Xyl, whereas DmGlcAT-BSI and DmGlcAT-BSII act on a wide array of substrates with non-reducing terminal beta1,3- and beta1,4-linked Gal residues. Their highest activities are obtained with asialoorosomucoid with a terminal Galbeta1-4GlcNAc sequence, indicating their possible involvement in the synthesis of the HNK-1 epitope in addition to the GAG-protein linkage region. Galbeta1-3GlcNAc and Galbeta1-3GalNAc, disaccharide structures widely found in N- and O-glycans of glycoproteins and glycolipids, also serve as acceptors for DmGlcAT-BSI and -BSII. Transcripts of all three enzymes are ubiquitously expressed throughout the developmental stages and in adult tissues of Drosophila. Thus, all three glucuronyltransferases are likely involved in the synthesis of the GAG-protein linkage region in Drosophila, and DmGlcAT-BSI and -BSII appear to be involved in various GlcUA transfer reactions for the synthesis of proteoglycans, glycoproteins, and glycolipids. This activity distinguishes these glucuronyltransferases from their mammalian homologs GlcAT-P and GlcAT-D (or -S). Sequence alignment of the Drosophila glucuronyltransferases with homologs in human, rat, and Caenorhabditis elegans demonstrates the conservation of a majority of the critical amino acid residues in the active sites of the three Drosophila enzymes.

Developmental expression of the HNK-1 carbohydrate epitope on aggrecan during chondrogenesis

Previously, we showed that the HNK-1 carbohydrate epitope is expressed on aggrecan synthesized in the notochord but not in mature cartilage. In the present study, we demonstrate that in immature cartilage (embryonic day 6) the HNK-1 epitope is also expressed predominantly on aggrecan proteoglycan molecules. This finding was verified by using an aggrecan-deficient mutant, the nanomelic chick, which lacks HNK-1 immunostaining in the extracellular matrix of dividing and hypertrophic chondrocytes as late as embryonic day 12. By using both biochemical and immunologic approaches, the initially prominent expression of the HNK-1 epitope is down-regulated as development of limb and vertebral cartilage proceeds, so that by embryonic day 14 no HNK-1 is detectable. Localization changes with development and the HNK-1-aggrecan matrix becomes restricted to dividing and hypertrophic chondrocytes and is particularly concentrated in the intraterritorial matrix. Concomitant with the temporal and spatial decreases in HNK-1, there is a significant increase in keratan-sulfate content and the aggrecan-borne HNK-1 epitope is closely associated with proteolytic peptides that contain keratan sulfate chains, rather than chondroitin sulfate chains or carbohydrate-free domains. Lastly, the diminution in HNK-1 expression is consistent with a reduction in mRNA transcripts specific for at least one of the key enzymes in HNK-1 oligosaccharide biosynthesis, the HNK-1 sulfotransferase. These findings indicate that the HNK-1 carbohydrate may be a common modifier of several proteoglycans (such as aggrecan) that are usually expressed early in development, and that HNK-1 addition to these molecules may be regulated by tissue- and temporal-specific expression of requisite sulfotransferases and glycosyltransferases.

Sulfotransferases and sulfated oligosaccharides

Structural diversity of the sugar chains attached to proteins and lipids that arises from the variety of combinations of different monosaccharides, different types of linkages, branch formation and secondary modifications, such as sulfation, possesses a large amount of biological information. A number of proteoglycans, glycoproteins, and glycolipids contain sulfated carbohydrates. Their sulfate groups provide a negative charge and play a role in a specific molecular recognition process. The sulfation of oligosaccharides is catalyzed by the Golgi-associated sulfotransferases. Recent success in molecular cloning of these sulfotransferases has brought a breakthrough in the understanding of biological function of sulfated oligosaccharides in a variety of contexts. Investigations on the relationship of sulfated oligosaccharides to human diseases including hereditary deficiency, cancer, inflammation, and infection may provide hints for curing disastrous diseases.

Molecular cloning and characterization of chondroitin-4-O-sulfotransferase-3. A novel member of the HNK-1 family of sulfotransferases

We have identified and characterized an N-acetylgalactosamine-4-O-sulfotransferase designated chondroitin-4-sulfotransferase-3 (C4ST-3) (GenBank accession number AY120869) based on its homology to HNK-1 sulfotransferase (HNK-1 ST). The cDNA predicts an open reading frame encoding a type II membrane protein of 341 amino acids with a 12-amino acid cytoplasmic domain and a 311-amino acid luminal domain containing a single potential N-linked glycosylation site. C4ST-3 has the greatest amino acid sequence identity when aligned with chondroitin-4-O-sulfotransferase 1 (C4ST-1) (45%) but also shows significant amino acid identity with chondroitin-4-O-sulfotransferase 2 (C4ST-2) (27%), dermatan-4-O-sulfotransferase 1 (29%), HNK-1 ST (26%), N-acetylgalactosamine-4-O-sulfotransferase 1 (26%), and N-acetylgalactosamine-4-O-sulfotransferase 2 (23%). C4ST-3 transfers sulfate to the C-4 hydroxyl of beta1,4-linked GalNAc that is substituted with a beta-linked glucuronic acid at the C-3 hydroxyl. The open reading frame of C4ST-3 is encoded by three exons located on human chromosome 3q21.3. Northern blot analysis reveals a single 2.1-kilobase transcript. C4ST-3 message is expressed in adult liver and at lower levels in adult kidney, lymph nodes, and fetal liver. Although C4ST-3 and C4ST-1 have similar specificities, the highly restricted pattern of expression seen for C4ST-3 suggests that it has a different role than C4ST-1.

HNK-1 sulfotransferase null mice express glucuronyl glycoconjugates and show normal cerebellar granule neuron migration in vivo and in vitro

Sulfoglucuronyl carbohydrate (SGC), reactive with antibody against human natural killer cell antigen, is expressed in several glycolipids, glycoproteins and proteoglycans of the nervous system and has been implicated in cell-cell recognition, neurite outgrowth and neuronal migration during development, through its interaction with SGC-binding protein (SBP) 1. However, sulfotransferase (ST) null mutant mice, which lack SGC, were shown to have normal development with usual gross anatomy of the nervous system and other organs. Failure to observe a severe phenotype in the ST null mice prompted us to determine the compensatory molecular replacement of SGC by analyzing the carbohydrate of glycolipids and glycoproteins of the ST mutant nervous system. In the ST null mice, SGC-containing molecules were absent; instead the precursor glucuronyl carbohydrate (GC)-containing molecules accumulated. Other relevant glycolipids and proteins were not affected. The GC molecules in the mutant were localized at the same anatomical sites in the nervous system as the SGC molecules in the wild type. In vitro binding studies showed that, similar to sulfoglucuronyl glycolipids, glucuronyl glycolipids interacted with SBP-1, but with a lower binding capacity. In vitro studies with explant cultures of cerebellum indicated that neurite outgrowth and cell migration were not significantly affected in the mutant, possibly owing to interaction of SBP-1 with GC molecules. The results suggested that in vivo SBP-1-GC interaction was sufficient to allow normal neurite outgrowth and cell migration in the mutant, giving rise to a wild-type phenotype. However, the role of other compensatory molecules involved in these processes cannot be completely ruled out.

Biosynthesis of HNK-1 glycans on O-linked oligosaccharides attached to the neural cell adhesion molecule (NCAM): the requirement for core 2 beta 1,6-N-acetylglucosaminyltransferase and the muscle-specific domain in NCAM

The HNK-1 glycan, sulfo-->3GlcAbeta1-->3Galbeta1-->4GlcNAcbeta1-->R, is highly expressed in neuronal cells and apparently plays critical roles in neuronal cell migration and axonal extension. The HNK-1 glycan synthesis is initiated by the addition of beta1,3-linked GlcA to N-acetyllactosamine followed by sulfation of the C-3 position of GlcA. The cDNAs encoding beta1,3-glucuronyltransferase (GlcAT-P) and HNK-1 sulfotransferase (HNK-1ST) have been recently cloned. Among various adhesion molecules, the neural cell adhesion molecule (NCAM) was shown to contain HNK-1 glycan on N-glycans. In the present study, we first demonstrated that NCAM also bears HNK-1 glycan attached to O-glycans when NCAM contains the O-glycan attachment scaffold, muscle-specific domain, and is synthesized in the presence of core 2 beta1,6-N-acetylglucosaminyltransferase, GlcAT-P, and HNK-1ST. Structural analysis of the HNK-1 glycan revealed that the HNK-1 glycan is attached on core 2 branched O-glycans, sulfo-->3GlcAbeta1-->3Galbeta1-->4GlcNAcbeta1-->6(Galbeta1-->3)GalNAc. Using synthetic oligosaccharides as acceptors, we found that GlcAT-P and HNK-1ST almost equally act on oligosaccharides, mimicking N- and O-glycans. By contrast, HNK-1 glycan was much more efficiently added to N-glycans than O-glycans when NCAM was used as an acceptor. These results are consistent with our results showing that HNK-1 glycan is minimally attached to O-glycans of NCAM in fetal brain, heart, and the myoblast cell line, C2C12. These results combined together indicate that HNK-1 glycan can be synthesized on core 2 branched O-glycans but that the HNK-1 glycan is preferentially added on N-glycans over O-glycans of NCAM, probably because N-glycans are extended further than O-glycans attached to NCAM containing the muscle-specific domain.

Core structures of polysialylated glycans present in neural cell adhesion molecule from newborn mouse brain

Polysialylation of the neural cell adhesion molecule (N-CAM) is known to destabilize cell-cell adhesion and to promote plasticity in cell-cell interactions. To gain more insights into the molecular mechanisms regulating the selective expression of polysialic acid on distinct glycan chains, the underlying core structures of polysialylated N-CAM glycans from newborn mouse brain were examined. Starting from low picomolar amounts of oligosaccharides, a multistep approach was used that was based on various mass spectrometric techniques with minimized sample consumption. Evidence could be provided that polysialylated murine N-CAM glycans comprise diantennary, triantennary and tetraantennary core structures carrying, in part, type-1 N-acetyllactosamine antennae, sulfate groups linked to terminal galactose or subterminal N-acetylglucosamine residues and, as a characteristic feature, a sulfated glucuronic acid unit which was bound exclusively to C3 of terminal galactose in Manalpha3-linked type-2 antennae. Hence, our results reveal that part of the murine N-CAM carbohydrates are modified within a single oligosaccharide by polysialic acid plus a HSO3-GlcA-moiety, which is likely to represent a HNK1-epitope. As HNK1-carbohydrates are also known to modulate cell-cell interactions, the simultaneous presence of both carbohydrate epitopes may reflect a new mechanism involved in the fine-tuning of N-CAM functions.

Molecular cloning and characterization of a dermatan-specific N-acetylgalactosamine 4-O-sulfotransferase

We have identified and characterized an N-acetylgalactosamine-4-O-sulfotransferase designated dermatan-4-sulfotransferase-1 (D4ST-1) (GenBank(TM) accession number AF401222) based on its homology to HNK-1 sulfotransferase. The cDNA predicts an open reading frame encoding a type II membrane protein of 376 amino acids with a 43-amino acid cytoplasmic domain and a 316-amino acid luminal domain containing two potential N-linked glycosylation sites. D4ST-1 has significant amino acid identity with HNK-1 sulfotransferase (21.4%), N-acetylgalactosamine-4-O-sulfotransferase 1 (GalNAc-4-ST1) (24.7%), N-acetylgalactosamine-4-O-sulfotransferase 2 (GalNAc-4-ST2) (21.0%), chondroitin-4-O-sulfotransferase 1 (27.3%), and chondroitin-4-O-sulfotransferase 2 (22.8%). D4ST-1 transfers sulfate to the C-4 hydroxyl of beta1,4-linked GalNAc that is substituted with an alpha-linked iduronic acid (IdoUA) at the C-3 hydroxyl. D4ST-1 shows a strong preference in vitro for sulfate transfer to IdoUAalpha1,3GalNAcbeta1,4 that is flanked by GlcUAbeta1,3GalNAcbeta1,4 as compared with IdoUAalpha1,3GalNAcbeta1,4 flanked by IdoUAalpha1,3GalNAcbeta1,4. The specificity of D4ST-1 when assayed in vitro suggests that the addition of sulfate to GalNAc occurs immediately after epimerization of GlcUA to IdoUA. The open reading frame of D4ST-1 is encoded by a single exon located on human chromosome 15q14. Northern blot analysis reveals a single 2.4-kilobase transcript. D4ST-1 message is expressed in virtually all tissues at some level but is most highly expressed in pituitary, placenta, uterus, and thyroid. The properties of D4ST-1 indicate that sulfation of the GalNAc moieties in dermatan is mediated by a distinct GalNAc-4-O-sulfotransferase and occurs following epimerization of GlcUA to IdoUA.

Human glycosaminoglycan glucuronyltransferase I gene and a related processed pseudogene: genomic structure, chromosomal mapping and characterization

Here we describe the characterization of the human glycosaminoglycan glucuronyltransferase I gene (GlcAT-I) and a related pseudogene. The GlcAT-I gene was localized to human chromosome 11q12-q13 by in situ hybridization of metaphase chromosomes. GlcAT-I spanned 7 kb of human genomic DNA and was divided into five exons. Northern blot analysis showed that GlcAT-I exhibited ubiquitous but markedly different expressions in the human tissues examined. The GlcAT-I promoter was approx. 3-fold more active in a melanoma cell line than in a hepatoma cell line, providing evidence for the differential regulation of the gene's expression. Stepwise 5' deletions of the promoter identified a strong enhancer element between -303 and -153 bp that included binding motifs for Ets, CREB (cAMP-response-element-binding protein) and STAT (signal transducers and activators of transcription). Screening of a human genomic library identified one additional distinct genomic clone containing an approx. 1.4 kb sequence region that shared an overall 95.3% nucleotide identity with exons 1-5 of GlcAT-I. However, a lack of intron sequences, as well as the presence of several nucleotide mutations, insertions and deletions that disrupted the potential GlcAT-I reading frame, suggested that the clone contained a processed pseudogene. The pseudogene was localized to chromosome 3. The human genome therefore contains two related GlcAT-I genes that are located on separate chromosomes.

Genome-wide screening of genes showing altered expression in liver metastases of human colorectal cancers by cDNA microarray

In spite of intensive and increasingly successful attempts to determine the multiple steps involved in colorectal carcinogenesis, the mechanisms responsible for metastasis of colorectal tumors to the liver remain to be clarified. To identify genes that are candidates for involvement in the metastatic process, we analyzed genome-wide expression profiles of 10 primary colorectal cancers and their corresponding metastatic lesions by means of a cDNA microarray consisting of 9121 human genes. This analysis identified 40 genes whose expression was commonly upregulated in metastatic lesions, and 7 that were commonly downregulated. The upregulated genes encoded proteins involved in cell adhesion, or remodeling of the actin cytoskeleton. Investigation of the functions of more of the altered genes should improve our understanding of metastasis and may identify diagnostic markers and/or novel molecular targets for prevention or therapy of metastatic lesions.

Epitope diversity of N-glycans from bovine peripheral myelin glycoprotein P0 revealed by mass spectrometry and nano probe magic angle spinning 1H NMR spectroscopy

The carbohydrate structures present on the glycoproteins in the central and peripheral nerve systems are essential in many cell adhesion processes. The P0 glycoprotein, expressed by myelinating Schwann cells, plays an important role during the formation and maintenance of myelin, and it is the most abundant constituent of myelin. Using monoclonal antibodies, the homophilic binding of the P0 glycoprotein was shown to be mediated via the human natural keller cell (HNK)-1 epitope (3-O-SO(3)H-GlcUA(beta1-3)Gal(beta1-4)GlcNAc) present on the N-glycans. We recently described the structure of the N-glycan carrying the HNK-1 epitope, present on bovine peripheral myelin P0 (Voshol, H., van Zuylen, C. W. E. M., Orberger, G., Vliegenthart, J. F. G., and Schachner, M. (1996) J. Biol. Chem. 271, 22957-22960). In this study, we report on the structural characterization of the detectable glycoforms, present on the single N-glycosylation site, using state-of-the-art NMR and mass spectrometry techniques. Even though all structures belong to the hybrid- or biantennary complex-type structures, the variety of epitopes is remarkable. In addition to the 3-O-sulfate present on the HNK-1-carrying structures, most of the glycans contain a 6-O-sulfated N-acetylglucosamine residue. This indicates the activity of a 6-O-sulfo-GlcNAc-transferase, which has not been described before in peripheral nervous tissue. The presence of the disialo-, galactosyl-, and 6-O-sulfosialyl-Lewis X epitopes provides evidence for glycosyltransferase activities not detected until now. The finding of such an epitope diversity triggers questions related to their function and whether events, previously attributed merely to the HNK-1 epitope, could be mediated by the structures described here.

Enhanced 3-O-sulfation of galactose in Asn-linked glycans and Maackia amurensis lectin binding in a new Chinese hamster ovary cell line

We report the characterization of two Chinese hamster ovary cell lines that produce large amounts of sulfated N-linked oligosaccharides. Clones 26 and 489 were derived by stable transfection of the glycosaminoglycan-deficient cell mutant pgsA-745 with a cDNA library prepared from wild-type cells. Peptide:N-glycanase F released nearly all of the sulfate label, indicating that sulfation had occurred selectively on the Asn-linked glycans. Hydrazinolysis followed by nitrous acid treatment at pH 4 and borohydride reduction yielded reduced sulfated disaccharides that comigrated with standard Gal3SO4beta1-4anhydromannitol. The disaccharides were resistant to periodate oxidation but became sensitive after the sulfate group was removed by methanolysis, indicating that the sulfate was located at C3 of the galactose residues. Maackia amurensis lectin bound to the sulfated glycopeptides on the cell surface and in free form, even after sialidase treatment. This finding indicates that the lectin requires only a charged group at C3 of the galactose unit and not an intact sialic acid. Growth of cells with chlorate restored sialidase sensitivity to lectin binding, indicating that sulfation and sialylation occurred largely at the same sites. The enhanced sulfation was due to elevated sulfotransferase activity that catalyzed transfer of sulfate from phosphoadenosine-5'-phosphosulfate to Galbeta1-4(3)GlcNAcbeta-O-naphthalenemethanol.

A family of novel, acidic N-glycans in Bowes melanoma tissue plasminogen activator have L2/HNK-1-bearing antennae, many with sulfation of the fucosylated chitobiose core

A family of about 20 novel acidic bi- and tri-antennary N-glycans, amounting to almost half those expressed on Bowes melanoma tissue-plasminogen activator (t-PA) were found to possess Galbeta1-->4GlcNAcbeta1-->, sulfated and sialylated GalNAcbeta1-->4GlcNAcbeta1--> or sulfated GlcAbeta1--> 3Galbeta1-->4GlcNAcbeta1--> antennae, of which those containing sulfated GlcA, depicting the L2/HNK-1 carbohydrate epitope, were preferentially located on the 6 arm. A proportion of the glycans were highly charged, because of multiple and variously distributed sulfation, some of which was located on the fucosylated chitobiose core. Multiple expression of the L2/HNK-1 epitope on a single glycan was observed. The most abundant compound was a biantennary glycan carrying sulfated GlcA on the 6-branched antenna and an alpha2-->6 sialylated GalNAc on the other. The N-glycosylation sequon containing Asn448, which is known to express all of the sulfate-carrying N-glycans contains, unusually, an arginine residue. An electrostatic interaction between this cationic amino acid and the core-sulfate group of the N-glycan is proposed to reduce mobility of the carbohydrate in the region of the t-PA active site. Because of the 'brain-type' nature of the N-glycans described in this neuro-ectodermal cell line, the possibility of neural t-PA interacting with the L2/HNK-1-recognizing molecule, laminin, of the central nervous system extracellular matrix is discussed.

Molecular cloning and expression of a novel human beta-Gal-3-O-sulfotransferase that acts preferentially on N-acetyllactosamine in N- and O-glycans

A novel cDNA-encoding galactose 3-O-sulfotransferase was cloned by screening the expressed sequence tag data base using the previously cloned cDNA encoding a galactosyl ceramide 3-O-sulfotransferase, which we term Gal3ST-1. The newly isolated cDNA encodes a novel 3-O-sulfotransferase, termed Gal3ST-3, that acts exclusively on N-acetyllactosamine present in N-glycans and core2-branched O-glycans. These conclusions were confirmed by analyzing CD43 chimeric proteins in Chinese hamster ovary cells expressing core2 beta1,6-N-acetylglucosaminyltransferase. The acceptor specificity of Gal3ST-3 contrasts with that of the recently cloned galactose 3-O-sulfotransferase (Honke, K., Tsuda, M., Koyota, S., Wada, Y., Iida-Tanaka, N., Ishizuka, I., Nakayama, J., and Taniguchi, N. (2001) J. Biol. Chem. 276, 267-274), which we term Gal3ST-2 in the present study because the latter enzyme can also act on core1 O-glycan and type 1 oligosaccharides, Galbeta1-->3GlcNAc. Moreover, Gal3ST-3 but not Gal3ST-2 can act on Galbeta1-->4(sulfo-->6)GlcNAc, indicating that disulfated sulfo-->3Galbeta1-->4(sulfo-->6) GlcNAc-->R may be formed by Gal3ST-3 in combination with GlcNAc 6-O-sulfotransferase. Although both Gal3ST-2 and Gal3ST-3 do not act on galactosyl ceramide, Gal3ST-3 is only moderately more homologous to Gal3ST-2 (40.1%) than to Gal3ST-1 (38.0%) at the amino acid level. Northern blot analysis demonstrated that transcripts for Gal3ST-3 are predominantly expressed in the brain, kidney, and thyroid where the presence of 3'-sulfation of N-acetyllactosamine has been reported. These results indicate that the newly cloned Gal3ST-3 plays a critical role in 3'-sulfation of N-acetyllactosamine in both O- and N-glycans.

Molecular cloning and expression of two distinct human N-acetylgalactosamine 4-O-sulfotransferases that transfer sulfate to GalNAc beta 1-->4GlcNAc beta 1-->R in both N- and O-glycans

Recently, cDNAs encoding human chondroitin 4-O-sulfotransferase-1 and -2 (C4ST-1 and C4ST-2) were cloned based on their similarity to HNK-1 sulfotransferase (HNK-1ST) (Hiraoka, N., Nakagawa, H., Ong, E., Akama, T.O., Fukuda, M.N., and Fukuda, M. [2000] Molecular cloning and expression of two distinct human chondroitin 4-O-sulfotransferases that belong to the HNK-1 sulfotransferase gene family. J. Biol. Chem., 275, 20188--20196). In the present study, we identified two additional novel sulfotransferases by searching the expression sequence tag and genomic DNA database for enzymes similar to C4ST-1 and C4ST-2. These newly cloned enzymes, termed GalNAc4ST-1 and GalNAc4ST-2, belong to the HNK-1ST gene family having 40--42% identity with C4ST-1. GalNAc4ST-1 and -2 do not add sulfate to HNK-1 precursor glycans, chondroitin, or desulfated dermatan sulfate. Instead, both enzymes can transfer sulfate to the 4-position of GalNAc in the context of GalNAc beta 1-->4GlcNAc beta 1-->R attached to both N-linked and core 2 branched O-linked oligosaccharides. GalNAc4ST-1 and -2 transcripts are highly expressed in the pituitary gland and trachea, respectively, and GalNAc4ST-1 and -2 transcripts are reciprocally expressed in other tissues as well. Moreover, both enzymes can transfer sulfate to lutropin, a pituitary glycoprotein hormone. These combined results indicate that GalNAc4ST-1 and -2 play critical roles in forming sulfo-->4GalNAc beta 1-->4GlcNAc beta 1-->R in both N-glycans and O-glycans in a tissue-specific manner.

Molecular cloning and expression of an N-acetylgalactosamine-4-O-sulfotransferase that transfers sulfate to terminal and non-terminal beta 1,4-linked N-acetylgalactosamine

We have identified and characterized an N-acetylgalactosamine-4-O-sulfotransferase designated GalNAc-4-ST2 (GenBank(TM) accession number ) based on its homology to HNK-1 sulfotransferase (HNK-1 ST). The cDNA predicts an open reading frame encoding a type II membrane protein of 443 amino acids with a 12-amino acid cytoplasmic domain, a 23-amino acid transmembrane domain, and a 408-amino acid luminal domain containing four potential N-linked glycosylation sites. GalNAc-4-ST2 displays a high degree of amino acid sequence identity with GalNAc-4-ST1 (46%), HNK-1 ST (23%), chondroitin 4-O-sulfotransferase-1 (C4ST-1) (27%), and chondroitin 4-O-sulfotransferase-2 (C4ST-2) (24%). GalNAc-4-ST2 transfers sulfate to the C-4 hydroxyl of terminal beta1,4-linked GalNAc in the sequence GalNAc-beta1,4GlcNAcbeta-R found on N-linked oligosaccharides and nonterminal beta1,4-linked GalNAc in chondroitin and dermatan. The translated region of GalNAc-4-ST2 is encoded by five exons located on human chromosome 18q11.2. Northern blot analysis reveals a 2.1-kilobase transcript. GalNAc-4-ST2 message is most highly expressed in trachea and to a lesser extent in heart, liver, pancreas, salivary gland, and testis. The I.M.A.G.E. cDNA clone 49547 contains a putative GalNAc-4-ST2 splice form with an open reading frame encoding a protein of 358 amino acids that lacks the transmembrane domain and the stem region. This form of GalNAc-4-ST2 is not retained by transfected cells and is active against chondroitin but not terminal beta1,4-linked GalNAc. Thus, as with GalNAc-4-ST1, sequences N-terminal to the catalytic domain contribute to the specificity of GalNAc-4-ST2 toward terminal beta1,4-linked GalNAc.

Polysialyltransferase ST8Sia II (STX) polysialylates all of the major isoforms of NCAM and facilitates neurite outgrowth

The neural cell adhesion molecule (NCAM) has different isoforms due to different sizes in its polypeptide and plays a significant role in neural development. In neural development, the function of NCAM is modified by polysialylation catalyzed by two polysialyltransferases, ST8Sia II and ST8Sia IV. Previously, it was reported by others that ST8Sia II polysialylates only transmembrane isoforms of the NCAM, such as NCAM-140 and NCAM-180, but not NCAM-120 and NCAM-125 anchored by a glycosylphosphotidylinositol. In the present study, we first discovered that ST8Sia II polysialylates all isoforms of the NCAM examined, and we demonstrated that polysialylation of NCAM expressed on 3T3 cells facilitates neurite outgrowth regardless of isoforms of NCAM, where polysialic acid is attached. We then show that neurite outgrowth is significantly facilitated only when polysialylated NCAM is present in cell membranes. Moreover, the soluble NCAM coated on plates did not have an effect on neurite outgrowth exerted by soluble L1 adhesion molecule coated on plates. These results, taken together, indicate that ST8Sia II plays critical roles in modulating the function of all major isoforms of NCAM. The results also support previous studies showing that a signal cascade initiated by NCAM differs from that initiated by L1 molecule.