Sulfated N-linked Oligosaccharides Research Articles

Hallmarks of the relationship between host and Trypanosoma cruzi sulfated glycoconjugates along the course of Chagas disease.

American Trypanosomiasis or Chagas disease (ChD), a major problem that is still endemic in large areas of Latin America, is caused by Trypanosoma cruzi. This agent holds a major antigen, cruzipain (Cz). Its C-terminal domain (C-T) is retained in the glycoprotein mature form and bears several post-translational modifications. Glycoproteins containing sulfated N-linked oligosaccharides have been mostly implicated in numerous specific procedures of molecular recognition. The presence of sulfated oligosaccharides was demonstrated in Cz, also in a minor abundant antigen with serine-carboxypeptidase (SCP) activity, as well as in parasite sulfatides. Sulfate-bearing glycoproteins in Trypanosomatids are targets of specific immune responses. T. cruzi chronically infected subjects mount specific humoral immune responses to sulfated Cz. Unexpectedly, in the absence of infection, mice immunized with C-T, but not with sulfate-depleted C-T, showed ultrastructural heart anomalous pathological effects. Moreover, the synthetic anionic sugar conjugate GlcNAc6SO3-BSA showed to mimic the N-glycan-linked sulfated epitope (sulfotope) humoral responses that natural Cz elicits. Furthermore, it has been reported that sulfotopes participate via the binding of sialic acid Ig-like-specific lectins (Siglecs) to sulfosialylated glycoproteins in the immunomodulation by host-parasite interaction as well as in the parasite infection process. Strikingly, recent evidence involved Cz-sulfotope-specific antibodies in the immunopathogenesis and infection processes during the experimental ChD. Remarkably, sera from chronically T. cruzi-infected individuals with mild disease displayed higher levels of IgG2 antibodies specific for sulfated glycoproteins and sulfatides than those with more severe forms of the disease, evidencing that T. cruzi sulfotopes are antigenic independently of the sulfated glycoconjugate type. Ongoing assays indicate that antibodies specific for sulfotopes might be considered biomarkers of human cardiac ChD progression, playing a role as predictors of stability from the early mild stages of chronic ChD.

Sulfation of LH does not affect intracellular trafficking

LH and FSH are produced by the same gonadotrope cells of the anterior pituitary but differ in their mode of secretion. LH secretion is primarily episodic, or regulated, while FSH secretion is primarily basal, or constitutive. The asparagine (N)-linked oligosaccharides of LH and FSH terminate with sulfate and sialic acid, respectively. TSH also contains sulfated N-linked oligosaccharides and is secreted through the regulated pathway. It has been hypothesized that sulfate plays a role in segregating LH to the regulated pathway. Using a mouse pituitary model, we tested this hypothesis by examining the secretory fate of LH from pituitaries treated with sodium chlorate, a known inhibitor of sulfation. Here we show that mouse LH is sulfated and secreted through the regulated pathway, while FSH is secreted constitutively. LH secretion from chlorate-treated pituitaries, which showed complete inhibition of sulfation, was similar to untreated pituitaries. These data suggest that the metabolic role for sulfated N-linked oligosaccharides is not for intracellular trafficking but for the extracellular bioactivity of LH.

Structural analysis of sulfated N-linked oligosaccharides in erythropoietin.

We previously demonstrated that high-performance liquid chromatography with electrospray ionization mass spectrometry (LC/MS) equipped with a graphitized carbon column (GCC) is useful for the structural analysis of carbohydrates in glycoproteins. Using LC/MS with GCC, sulfated N-linked oligosaccharides were found in erythropoietin (EPO) expressed in baby hamster kidney cells. Sulfation occurs in a part of the N-linked oligosaccharides in the EPO. Sulfated monosaccharide residue in the sulfated N-linked oligosaccharide was determined by exoglycosidase digestion followed by sugar mapping by LC/MS. The linkage position and branch-location of the sulfate group in the tetraantennary oligosaccharide were analyzed by (1)H-nuclear magnetic resonance. It was suggested that sulfation occurs on the C-6 position of GlcNAc located in the GlcNAcbeta1-4Manalpha1-3 branch.

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.

Different Usage of the Glycosaminoglycan Attachment Sites of Biglycan

Biglycan is a member of the small leucine-rich proteoglycan family. Its core protein comprises two chondroitin/dermatan sulfate attachment sites on serine 42 and serine 47, respectively, which are the fifth and tenth amino acid residues, respectively, after removal of the prepro peptide. Because the regulation of glycosaminoglycan chain assembly is not fully understood and because of the in vivo existence of monoglycanated biglycan, mutant core proteins were stably expressed in human 293 and Chinese hamster ovary cells in which i) either one or both serine residues were converted into alanine or threonine residues, ii) the number of acidic amino acids N-terminal of the respective serine residues was altered, and iii) a hexapeptide was inserted between the mutated site 1 and the unaltered site 2. Labeling experiments with [(35)S]sulfate and [(35)S]methionine indicated that serine 42 was almost fully used as the glycosaminoglycan attachment site regardless of whether site 2 was available or not for chain assembly. In contrast, substitution of site 2 was greatly influenced by the presence or absence of serine 42, although additional mutations demonstrated a direct influence of the amino acid sequence between the two sites. When site 2 was not substituted with a glycosaminoglycan chain, there was also no assembly of the linkage region. These results indicate that xylosyltransferase is the rate-limiting enzyme in glycosaminoglycan chain assembly and implicate a cooperative effect on the xylosyl transfer to site 2 by xylosylation of site 1, which probably becomes manifest before the removal of the propeptide. It is shown additionally that biglycan expressed in 293 cells may still contain the propeptide sequence and may carry heparan sulfate chains as well as sulfated N-linked oligosaccharides.

Sulfated N-linked oligosaccharides affect secretion but are not essential for the transport, proteolytic processing, and sorting of lysosomal enzymes in Dictyostelium discoideum.

Although previous studies have indicated that N-linked oligosaccharides on lysosomal enzymes in Dictyostelium discoideum are extensively phosphorylated and sulfated, the role of these modifications in the sorting and function of these enzymes remains to be determined. We have used radiolabel pulse-chase, subcellular fractionation, and immunofluorescence microscopy to analyze the transport, processing, secretion, and sorting of two lysosomal enzymes in a mutant, HL244, which is almost completely defective in sulfation. [3H]Mannose-labeled N-linked oligosaccharides were released from immunoprecipitated alpha-mannosidase and beta-glucosidase of HL244 by digestion with peptide: N-glycosidase. The size, Man9-10GlcNAc2, and processing of the neutral species were similar to that found in the wild type, but the anionic oligosaccharides were less charged than those from the wild-type enzymes. All of the negative charges on the oligosaccharides for HL244 were due to the presence of 1, 2, or 3 phosphodiesters and not to sulfate esters. The rate of proteolytic processing of precursor forms of alpha-mannosidase and beta-glucosidase to mature forms in HL244 was identical to wild type. The precursor polypeptides in the mutant and the wild type were membrane associated until being processed to mature forms; therefore, sulfated sugars are not essential for this association. Furthermore, the rate of transport of alpha-mannosidase and beta-glucosidase from the endoplasmic reticulum to the Golgi complex was normal in the mutant as determined by the rate at which the newly synthesized proteins became resistant to the enzyme, endo-beta-N-acetylglucosaminidase H. There was no increase in the percentage of newly synthesized mutant precursors which escaped sorting and were secreted, and the intracellularly retained lysosomal enzymes were properly localized to lysosomes as determined by fractionation of cell organelles on Percoll gradients and immunofluorescence microscopy. However, the mutant secreted lysosomally localized mature forms of the enzymes at 2-fold lower rates than wild-type cells during both growth and during starvation conditions that stimulate secretion. Furthermore, the mutant was more resistant to the effects of chloroquine treatment which results in the missorting and oversecretion of lysosomal enzymes. Together, these results suggest that sulfation of N-linked oligosaccharides is not essential for the transport, processing, or sorting of lysosomal enzymes in D. discoideum, but these modified oligosaccharides may function in the secretion of mature forms of the enzymes from lysosomes.

Characteristics of the sulfation of N-linked oligosaccharides in vesicles from Dictyostelium discoideum: In vitro sulfation of lysosomal enzymes

Lysosomal enzymes from Dictyostelium discoideum contain unusual sulfated N-linked oligosaccharides, whose synthesis has been well studied in vivo. However, little is known about the properties of the pertinent sulfotransferases. To study these transferases, we have prepared a cell-free system which transfers 35SO 4 from 3′-phosphoadenosine 5′-phosphosulfate to either endogenous or exogenous acceptors. We found that the 35SO 4 was released from macromolecules by protein N-glycanase F to yield a mixture of anionic oligosaccharides with 1–6 negative charges. Some of the labeled molecules contained acid-stable methyl phosphodiesters but none contained phosphomoesters or acid-labile diesters. The sulfate was found in molecules with the acid stability characteristic of esters of primary alcohols. In all these ways, the products resembled those generated in vivo. We also demonstrated that a membrane-associated form of β-hexosaminidase and the precursor of α-mannosidase were among the products. In addition, glycoproteins prepared from a sulfation-deficient mutant strain could act as exogenous acceptors in permeabilized vesicles.

Sulfated N-linked oligosaccharides in mammalian cells. III. Characterization of a pancreatic carcinoma cell surface glycoprotein with N- and O-sulfate esters on asparagine-linked glycans.

In the preceding two papers, we described two new classes of sulfated N-linked oligosaccharides isolated from total cellular 35SO4-labeled macromolecules of different mammalian cell lines. The first class carries various combinations of sialic acids and 6-O-sulfate esters on typical complex-type chains, while the second carries heparin and heparan-like sequences. In this study, we have characterized a sulfophosphoglycoprotein of 140 kDa from FG-Met-2 pancreatic cancer cells whose oligosaccharides share some properties of both these classes. The molecule was localized to the cell surface by electron microscopy using a monoclonal antibody (S3-53) and by cell surface 125I-labeling. Metabolic labeling of the cells with radioactive glucosamine, methionine, inorganic sulfate, or phosphate all demonstrated a single 140-kDa molecule. Pulse-chase analysis and tunicamycin treatment indicated the glycosylation of a putative primary translation product of 110 kDa via an intermediate (120 kDa) to the mature form (140 kDa). Digestion with peptide:N-glycosidase F (PNGaseF) indicated a minimum of four N-linked glycosylation sites. PNGaseF released more than 90% of the [6-3H]GlcNH2 label and 40-70% of 35SO4 label from the immunoprecipitated 140-kDa molecule. The isolated oligosaccharides were characterized as described in the preceding two papers. The majority of [6-3H]GlcNH2-labeled molecules were susceptible to neuraminidase. More than 50% of the 35SO4 label was associated with only 5-10% of the 3H-labeled chains. Some of the sulfated chains were partly sialylated molecules with four to five negative charges. Treatment with nitrous acid released about 25% of the 35SO4 label as free sulfate, together with 6% of the [6-3H]GlcNH2 label, indicating the presence of N-sulfated glucosamine residues. Some of these oligosaccharides were degraded by heparinase and heparitinase. Therefore, while they are not as highly charged as typical heparin or heparan chains, they must share structural features that permit recognition by the enzymes. Thus, this 140-kDa glycoprotein contains at least four asparagine-linked chains substituted with a heterogeneous mixture of sulfated sequences. The heterogeneity of these molecules is as extensive as that described for whole-cell sulfated N-linked oligosaccharides in the preceding two papers.

Sulfated N-linked oligosaccharides in mammalian cells. I. Complex-type chains with sialic acids and O-sulfate esters.

The structures of sulfated N-linked oligosaccharides have been reported for a few specific proteins. We recently demonstrated that such oligosaccharides occur in many different types of tissue culture cell lines (Freeze, H. H., and Varki, A. (1986) Biochem. Biophys. Res. Commun. 140, 967-973). Here we report improved methods to metabolically label cell lines with 35SO4 and to release sulfated N-linked oligosaccharides with peptide:N-glycosidase F as well as the partial structure of some of these novel oligosaccharides. The released 35SO4-labeled chains from Chinese hamster ovary (CHO) cells and bovine pulmonary artery endothelial cells (CPAE) were characterized by gel filtration, anion exchange and lectin affinity chromatography, and various enzymatic and chemical treatments. Each cell line contains a class of sulfated oligosaccharide chains bearing from two to six negative charges in varying combinations of O-sulfate esters and sialic acids. These molecules represent a significant proportion of both the total 35SO4 label and the total anionic N-linked oligosaccharides. They are also relatively enriched in a CHO mutant that is deficient in glycosaminoglycan chain synthesis. Lectin affinity chromatography of such molecules from CPAE cells indicates that the majority are sialylated multiantennary complex-type chains. The sulfate esters are exclusively of the primary type. Sequential exoglycosidase digestions, including beta-hexosaminidase A treatment at low pH, demonstrate that at least one-third of these sulfate esters are found in the following structure, (formula; see text) where R is the remainder of the underlying oligosaccharide, and SA is sialic acid. In addition to these molecules, a more highly charged group of sulfated N-linked oligosaccharides sharing structural features with glycosaminoglycans was found in CPAE cells, but not in CHO cells. These are described in the following paper (Sundblad, G., Holojda, S., Roux, L., Varki, A., and Freeze, H. H. (1988) J. Biol. Chem. 263, 8890-8896).

Sulfated N-linked oligosaccharides in mammalian cells. II. Identification of glycosaminoglycan-like chains attached to complex-type glycans.

In the preceding paper (Roux, L., Holojda, S., Sundblad, G., Freeze, H. H., and Varki, A. (1988) J. Biol. Chem. 263, 8879-8889) we described the metabolic labeling and isolation of sulfated N-linked oligosaccharides from mammalian cell lines. All cell lines studied contained a class of sulfated sialylated complex-type chains with 2-6 negative charges. In this paper, we show that bovine pulmonary arterial endothelial (CPAE) and human erythroleukemia (K562) cell lines also contain a class of more highly charged sulfated but less sialylated oligosaccharides. These molecules were further characterized by ion exchange chromatography and various enzymatic and chemical treatments. In both cell lines they contained greater than 6 negative charges, but those from K562 were even more highly charged than those from CPAE. Nitrous acid, heparinase, and heparitinase degradation of K562 oligosaccharides released 88, 64, and 78%, respectively, of 35S label. Combined digestion with the two enzymes resulted in 87% release. The corresponding values for CPAE were 48, 25, and 50% (60% for the two enzymes together). Chondroitinase ABC (or AC) digestion of K562 and CPAE oligosaccharides released 10 and 5%, respectively. About 30% of the 35S-labeled oligosaccharides from CPAE were sensitive to endo-beta-galactosidase, indicating that poly-N-acetyl-lactosamine structures were present on some chains. Highly charged [3H]mannose-labeled sulfated oligosaccharides from CPAE cells became neutral after treatment with heparinase/heparitinase but were resistant to Pronase, further proving that glycosaminoglycan (GAG)-like chains were directly attached to N-linked oligosaccharides. Such neutralized oligosaccharides did not bind to concanavalin A-Sepharose, but some interacted with phytohemagglutinin L4, indicating that they were bi-, tri-, or tetra-antennary complex-type chains. Thus, K562 and CPAE cells contain different types of GAG chains directly attached to asparagine-linked oligosaccharides. Such molecules were not found in many other cell lines that synthesize the more typical O-linked GAG chains. This suggests that the occurrence of these novel N-linked chains is not a random event resulting from accidental initiation of GAG chain synthesis on N-linked intermediates in the Golgi apparatus.

Endo-glycosidase F and peptide N-glycosidase F release the great majority of total cellular N-linked oligosaccharides: Use in demonstrating that sulfated N-linked oligosaccharides are frequently found in cultured cells

Subtle modifications of N-linked oligosaccharides are known to mediate important biological functions. However, many of these modifications are destroyed by chemical treatment used to release oligosaccharides from the proteins. Therefore, enzymatic release of the intact, modified oligosaccharides is preferred. We have used a mixture of endoglycosidaseF and Peptide: N-glycosidaseF to release most, if not all, of the [2- 3H]Man-labelled N-linked oligosaccharides from a wide variety of mammalian cell lines. This approach combined with biosynthetic radiolabelling could, therefore, be used to identify novel, rare or transient modifications of N-linked oligosaccharides. This treatment indeed released metabolically incorporated [ 35SO 4] from each of 9 different cell lines demonstrating that sulfated N-linked oligosaccharides are much more common than previously believed.

Lymphocyte function-associated antigen 1 (LFA-1) contains sulfated N-linked oligosaccharides.

The murine lymphocyte function-associated antigen 1 (LFA-1) is a glycoprotein heterodimer consisting of an Mr 180,000 alpha-chain and an Mr 95,000 beta-chain. Although LFA-1 has been studied extensively in the past few years due to its involvement in various antigen-specific T lymphocyte responses, virtually nothing is known about its glycosylation. In this report, we have analyzed the oligosaccharide moieties of the murine LFA-1 molecule. Utilizing a T lymphoma cell line, EL-4, it was found that [35S] sulfate, [3H]glucosamine, [3H]mannose, and [3H]fucose were incorporated into both the alpha- and beta-chains of LFA-1. Isolated alpha- and beta-chains from anti-LFA-1 immunoprecipitates of [3H]glucosamine-labeled NP-40 lysates were subjected to tryptic-chymotryptic digestion, and the resulting glycopeptides were fractionated by reverse-phase high performance liquid chromatography. Five major [3H]glucosamine-labeled glycopeptides were generated by this procedure from each of the two polypeptide chains. Treatment of the individual glycopeptides with almond emulsin peptide:N-glycosidase or Endo F demonstrated that the [3H]glucosamine label existed almost entirely in N-linked oligosaccharide structures (Mr 5000 to 10,000). By using similar techniques, the majority of the [35S]sulfate moieties were also found covalently bound to N-linked oligosaccharides. In addition, both [35S]sulfate-labeled alpha- and beta-chains were susceptible to Keratanase and endo-beta-galactosidase digestions, indicating the presence of sulfated N-acetyllactosamine sequences. The expression of [35S]sulfate-labeled LFA-1 on various cell types was also examined. LFA-1 was found to be sulfated only on thymocytes and splenic T cells, but not on macrophages, splenic B, or bone marrow cells.

Developmental changes in the modification of lysosomal enzymes in Dictyostelium discoideum

Evidence has been found for a generalized change in the post-translational modification of lysosomal enzymes during development of Dictyostelium discoideum. The physical and antigenic properties of four developmentally regulated lysosomal enzymes. N-acetylglucosaminidase, β-glucosidase, α-mannosidase, and acid phosphatase, have been examined throughout the life cycle. In vegetative cells, a single major isoelectric species is detected for each enzymatic activity on native nonequilibrium isoelectric focusing gels. Between 6 and 10 hr of development, all activities, including the preformed enzyme, become less negatively charged, resulting in a modest but reproducible shift in the isoelectric focusing pattern. This alteration is not detected by native gel electrophoresis at constant pH. As development continues, the specific activity of β-glucosidase, α-mannosidase, and acid phosphatase continues to increase and coincidentally, new, less acidic isozymic bands of activity can be observed on both gel systems. Some of these new isozymes accumulate preferentially in anterior cells, while others accumulate preferentially in posterior cells of migrating slugs. N-Acetylglucosaminidase does not increase in specific activity late in development and no new isozymic species appear. Using a monoclonal antibody that reacts with sulfated N-linked oligosaccharides shared by vegetative lysosomal enzymes in D. discoideum, the antigenicity of the developmental isozymes has been characterized. All of the enzymatic activity present during vegetative growth and early development is immunoprecipitable. However, the less negatively charged isozymes that accumulate after aggregation are not recognized by the antibody. Nonantigenic acid phosphatase and α-mannosidase are found in both anterior and posterior cells from migrating pseudoplasmodia. Since each enzyme is coded by a single structural gene, these results suggest that the isozymes present late in development arise from the synthesis of the same polypeptides with altered post-translational modifications. The appearance of anterior and posterior specific isozymes is likely to be the result of cell type specific changes in the glycoprotein modification pathway for newly synthesized proteins.