Immune cells surveil aberrantly sialylated O-glycans on megakaryocytes to regulate platelet count

Immune thrombocytopenia (ITP) is a platelet disorder. Pediatric and adult ITP have been associated with sialic acid alterations, but the pathophysiology of ITP remains elusive, and ITP is often a diagnosis of exclusion. Our analysis of pediatric ITP plasma samples showed increased anti-Thomsen-Friedenreich antigen (TF antigen) antibody representation, suggesting increased exposure of the typically sialylated and cryptic TF antigen in these patients. The O-glycan sialyltransferase St3gal1 adds sialic acid specifically on the TF antigen. To understand if TF antigen exposure associates with thrombocytopenia, we generated a mouse model with targeted deletion of St3gal1 in megakaryocytes (MK) (St3gal1MK-/-). TF antigen exposure was restricted to MKs and resulted in thrombocytopenia. Deletion of Jak3 in St3gal1MK-/- mice normalized platelet counts implicating involvement of immune cells. Interferon-producing Siglec H-positive bone marrow (BM) immune cells engaged with O-glycan sialic acid moieties to regulate type I interferon secretion and platelet release (thrombopoiesis), as evidenced by partially normalized platelet count following inhibition of interferon and Siglec H receptors. Single-cell RNA-sequencing determined that TF antigen exposure by MKs primed St3gal1MK-/- BM immune cells to release type I interferon. Single-cell RNA-sequencing further revealed a new population of immune cells with a plasmacytoid dendritic cell-like signature and concomitant upregulation of the immunoglobulin rearrangement gene transcripts Igkc and Ighm, suggesting additional immune regulatory mechanisms. Thus, aberrant TF antigen moieties, often found in pathological conditions, regulate immune cells and thrombopoiesis in the BM, leading to reduced platelet count.

Transfer of Functional Cargo in Exomeres

Exomeres are a recently discovered type of extracellular nanoparticle with no known biological function. Herein, we describe a simple ultracentrifugation-based method for separation of exomeres from exosomes. Exomeres are enriched in Argonaute 1-3 and amyloid precursor protein. We identify distinct functions of exomeres mediated by two of their cargo, the β-galactoside α2,6-sialyltransferase 1 (ST6Gal-I) that α2,6- sialylates N-glycans, and the EGFR ligand, amphiregulin (AREG). Functional ST6Gal-I in exomeres can be transferred to cells, resulting in hypersialylation of recipient cell-surface proteins including β1-integrin. AREG-containing exomeres elicit prolonged EGFR and downstream signaling in recipient cells, modulate EGFR trafficking in normal intestinal organoids, and dramatically enhance the growth of colonic tumor organoids. This study provides a simplified method of exomere isolation and demonstrates that exomeres contain and can transfer functional cargo. These findings underscore the heterogeneity of nanoparticles and should accelerate advances in determining the composition and biological functions of exomeres.

Prognostic value of Tumoral Sialyltransferase Expression and Circulating E-Selectin Concentrations in Node-Negative Breast Cancer Patients

Aims and Background

A crucial step in the metastatic process is the interaction between the endothelial molecule E-selectin and its tumoral ligands sialyl-Lewisx and sialyl-Lewisa. Sialyltranferases are involved in the biosynthesis of these ligands. The aim of this study was to assess the prognostic value of tumoral sialyltransferase expression and of circulating soluble E-selectin (sE-selectin) in node-negative breast cancer patients.

Methods

Using a multiplex RT-PCR method, we measured the expression of five sialyltransferases (ST3Gal III, ST6Gal I, ST3Gal IV, ST3Gal I and ST3Gal II) in tumors of 135 surgically treated node-negative breast cancer patients. Circulating sE-selectin concentrations were measured by an ELISA method prior to surgery. We also analyzed tumor size, histoprognostic grading and steroid hormone receptor status.

Results

The median follow-up was 7.5 years. Expression of estrogen receptors was associated with a good prognosis for relapse-free survival in univariate analysis. A high ST3Gal III/ST6Gal I ratio and a high sE-selectin concentration were associated with a bad prognosis for relapse-free survival and overall survival in univariate and multivariate analysis.

Conclusion

In the present study, tumoral sialyltransferase expression and circulating sE-selectin concentrations had prognostic value in patients with node-negative breast cancer. This result provides further evidence for the important role of these agents in the metastatic process.

A new assay for determining ganglioside sialyltransferase activities lactosylceramide-2,3-sialyltransferase (SAT I) and monosialylganglioside-2,3-sialyltransferase (SAT IV)

A new assay for the determination of lactosylceramide-2,3-sialyltransferase (SAT I, EC 2.4.99.9) and monosialoganglioside sialyltransferase (SAT IV, EC 2.4.99.2) is described. The assay utilised the commercially available fluorophore labelled sphingolipids, boron dipyrromethene difluoride (BODIPY) lactosylceramide (LacCer), and BODIPY-monosialotetrahexosylganglioside (GM1) as the acceptor substrates, for SAT I and SAT IV, respectively. HPLC coupled with fluorescence detection was used to analyse product formation. The analysis was performed in a quick and automated fashion. The assay showed good linearity for both BODIPY sphingolipids with a quantitative detection limit of 0.05 pmol. The high sensitivity enabled the detection of SAT I and SAT IV activities as low as 0.001 μU, at least 200 fold lower than that of most radiometric assays. This new assay was applied to the screening of SAT I and SAT IV activities in ovine and bovine organs (liver, heart, kidney, and spleen). The results provided evidence that young animals, such as calves, start to produce ganglioside sialyltransferases as early as 7 days after parturition and that levels change during maturation. Among the organs tested from a bovine source, spleen had the highest specific ganglioside sialyltransferase activity. Due to the organ size, the greatest total ganglioside sialyltransferase activities (SAT I and SAT IV) were detected in the liver of both bovine and ovine origin.

A beta-galactoside alpha2,6-sialyltransferase produced by a marine bacterium, Photobacterium leiognathi JT-SHIZ-145, is active at pH 8

A gene encoding a sialyltransferase produced by Photobacterium leiognathi JT-SHIZ-145 was cloned, sequenced, and expressed in Escherichia coli. The sialyltransferase gene contained an open reading frame of 1494 base pairs (bp) encoding a predicted protein of 497 amino acid residues. The deduced amino acid sequence of the sialyltransferase had no significant similarity to mammalian sialyltransferases and did not contain sialyl motifs, but did show high homology to another marine bacterial sialyltransferase, a beta-galactoside alpha2,6-sialyltransferase produced by P. damselae JT0160. The acceptor substrate specificity of the new enzyme was similar to that of the alpha2,6-sialyltransferase from P. damselae JT0160, but its activity was maximal at pH 8. This property is quite different from the properties of all mammalian and bacterial sialyltransferases reported previously, which have maximal activity at acidic pH. In general, both sialosides and cytidine-5'-monophospho-N-acetylneuraminic acid, the common donor substrate of sialyltransferases, are more stable under basic conditions. Therefore, a sialyltransferase with an optimum pH in the basic range should be useful for the preparation of sialosides and the modification of glycoconjugates, such as asialo-glycoproteins and asialo-glycolipids. Thus, the sialyltransferase obtained from P. leiognathi JT-SHIZ-145 is a promising tool for the efficient production of sialosides.

Beta-galactoside alpha2,6-sialyltransferase and the sialyl alpha2,6-galactosyl-linkage in tissues and cell lines

beta-Galactoside alpha2,6-sialyltransferase (ST6Gal.I) is the principal sialyltransferase responsible for the biosynthesis of the sialyl alpha2,6-galactosyl linkage. This enzyme and its cognate glycosidic structure are overexpressed in several malignancies and are related to cancer progression. The expression of the enzyme is regulated primarily through the expression of three principal mRNA species differing in the 5'-untranslated exons. The form known as YZ is considered associated with the basal expression of the gene, while forms H and X are specific to the liver and B-lymphocytes, respectively. The authors have studied the expression of ST6Gal.I activity by two different methods using a panel of human cancer cell lines: the expression of alpha2,6-sialylated sugar chains by the lectin from Sambucus nigra (SNA), and the expression of the different mRNA species by RT-PCR using oligonucleotide primers complementary to the isoform-specific regions. Very high levels of ST6Gal.I activity result in high levels of SNA reactivity and are associated with the expression of the H transcript in colon and liver cell lines, and of the X transcript in B cells.

Molecular basis for the presence of glycosylated onco-foetal fibronectin in oral carcinomas: the production of glycosylated onco-foetal fibronectin by carcinoma cells

Glycosylated onco-foetal fibronectin (GOF) deposited in the stroma of oral squamous cell carcinomas correlates with survival. One of the two polypeptide GalNAc-transferases, GalNAc-T3 or GalNAc-T6, is required for the biosynthesis of GOF by the initiation of a unique O-glycan in the alternative spliced IIICS region. Using cell culture experiments, immunohistochemical staining of primary tissue, and RT-PCR of tumour cells isolated by laser capture techniques we have examined the molecular basis for the production of GOF in oral carcinomas. Immuno-histochemical investigation confirmed the stromal deposition of GOF in oral carcinomas. However, neither GalNAc-T3 nor GalNAc-T6 could be detected in stromal fibroblasts. In contrast both transferases were present in the oral squamous carcinoma cells, suggesting that GOF is produced by the oral cancer cells and not only the stromal cells. RT-PCR analysis of RNA isolated from carcinoma cells provided further support for this conclusion by demonstrating in-splicing of the alternative spliced IIICS domain in GOF.

Sialyltransferase STX (ST8SiaII): A novel molecular marker of metastatic neuroblastoma

Polysialic acid (PSA) is highly expressed in many human cancers, including neuroblastoma (NB), and is critical for cellular adhesion, neuronal migration and tumor metastasis. The key enzyme responsible for PSA synthesis is sialyltransferase STX (ST8SiaII). Using quantitative RT-PCR we (i) studied STX expression in 39 NB tumors and 8 cell lines and (ii) examined its potential clinical utility as an early response marker in the bone marrows of the entire cohort of 136 high-risk NB patients treated with an immunotherapy protocol utilizing anti-GD2 antibody 3F8 and GM-CSF. Based on the quantitation of 24 normal marrow and peripheral blood samples, a normalized STX transcript value below the mean + 2SD was defined as negative. Sensitivity of this assay was 1 NB cell in 10(6) normal mononuclear cells. STX expression was high among NB tumors of all stages, as well as NB cell lines of different phenotypes. Evaluation for early (2.5 months from protocol entry) marrow response by univariate Cox model indicated that STX marker status (positive versus negative) was strongly associated with both progression-free and overall survival (p < 0.0005 for both). Similarly, the STX transcript level of posttreatment marrows was also highly prognostic of outcome (PFS, p = 0.001; OS, p < 0.0005). We conclude that STX mRNA has potential clinical utility as a molecular marker of metastatic NB.

Correlation analysis between tumor-associated antigen sialyl-Tn expression and ST6GalNAc I activity in human colon adenocarcinoma

OBJECTIVES

Sialyl-Tn (sTn) is a mucin carbohydrate-associated antigen that is strongly expressed in a large number of colorectal carcinomas. In this study, we combined immunohistochemical and enzymatic techniques in order to find the correlation between sTn tissue expression and the sialyltransferase activity (ST6GalNAc I) responsible for its synthesis in colorectal cancer (CRC) patients.

METHODS

We compared sTn expression in healthy (n = 46), tumorous (n = 60) and transitional tissue (n = 46) from CRC patients, and correlated sTn altered expression with clinicopathologic variables of the patient. Furthermore, we determined ST6GalNAc I tissue activity employing asialo-ovine submaxillary mucin (asialo-OSM) as glycoprotein acceptor (n = 27).

RESULTS

The rates of sTn positive expression obtained for healthy, tumorous and transitional tissues were 15, 67 and 63%, respectively. These rates led to statistically significant differences between healthy and tumorous or transitional tissue (p = 0.001); sTn expression was related to the first stages of the tumor invasion in transitional tissue. As regards ST6GalNAc I activity, we found an enhancement in transitional tissue. Statistical correlation analysis did not reveal association between sTn expression and ST6GalNAc I activity.

CONCLUSIONS

Our findings indicated that sTn antigen tissue expression and ST6GalNAc I activity levels were not correlated in CRC, in spite of the overexpression of the antigen in tumorous and transitional tissue.

Sialylated core 1 based O-linked glycans enhance the growth rate of mammary carcinoma cells in MUC1 transgenic mice

The MUC1 mucin, found on the luminal surface of simple epithelial cells is upregulated and aberrantly glycosylated in many carcinomas particularly breast and ovarian. MUC1 expressed by normal mammary epithelial cells, carries core 2 glycans but in breast carcinomas the simple core 1 based glycans are added. The binding of the monoclonal antibody SM3 to its peptide epitope in the tandem repeat of MUC1 is blocked by the branched core 2 glycans found on MUC1 expressed by normal cells. Thus SM3 does not bind to MUC1 expressed by normal mammary epithelial cells but reacts with more than 90% of breast carcinomas, suggesting that the loss of at least some core 2 glycans is a very common event in breast carcinogenesis. To determine if the change in glycosylation observed in breast carcinomas confers an advantage to cancer cells, murine mammary carcinoma cell lines were developed that express MUC1 carrying core 2 or core 1 linked glycans. The in vivo growth rate in wild-type and nude mice were identical regardless of the O-glycosylation patterns. However, the tumors that grew out of wild-type mice lost most of their MUC1 expression. In contrast, in MUC1 transgenic mice, where expression of MUC1 was retained by the tumor, a striking difference in growth rate was observed. In these mice, cells expressing core 1 glycans grew significantly faster than cells expressing core 2 glycans. These data suggest that MUC1 transgenic mice are more tolerant to core 1 expressing tumors than to tumors expressing core 2.

First Synthetic Probe for the Detection and Quantification of a Protein with a Potential α,(2→8)Sialyltransferase Activity

There is considerable interest in monitoring alpha,(2-->8)sialyltransferase (ST8) levels; however, there are few specific and sensitive methods to directly detect and quantitate the protein. This paper reports the development of a synthetic probe composed of oxidized colominic acid coupled to biotinyl-L-lysine hydrazide to detect and quantify ST8 with putative "initiase" activity and its use in three solid-phase applications. The detection limit observed for ST8 purified from K562 cells was approximately 2 pg by dot-blot analysis. In Western blots the probe bound and specifically recognized a protein band corresponding to ST8. In ELISA a linear dose response was obtained for pure protein in the range of 50-200 pg. Analysis of 3'-azido-3'deoxythymidine-treated cells by all three methods showed a reduction in ST8 compared to control cells; treated cells had 73% of control levels by ELISA. This probe will be useful for studies on the expression ST8 and its role in glycoconjugate biosynthesis.

Differential expression of beta-galactoside alpha2,6 sialyltransferase and sialoglycans in normal and cirrhotic liver and hepatocellular carcinoma

SUMMARY

Sialyltransferases sialylate plasma glycoproteins in hepatocytes and may (as hepatic key enzymes) constitute markers for liver diseases. We examined expression of the prevalent alpha2,6 sialyltransferase (ST6Gal I) and sialoglycans in normal liver, cirrhotic liver, and hepatocellular carcinoma (HCC) using a new ST6Gal I-specific mAb and recombinant fusion proteins of CD22 and sialoadhesin recognizing alpha2,6- or alpha2,3-sialylated glycans in immunohistology and flow cytometry. In normal and cirrhotic liver, ST6Gal I and sialoglycans were localized in the Golgi region of hepatocytes surrounding the bile canaliculi and along the bile canaliculi, respectively. Sialoglycans were additionally recognized in Kupffer cells, bile ducts, endothelial cells, and oval cells. Well-differentiated and moderately differentiated HCC showed Golgi and diffuse cytoplasmic staining of ST6Gal I and sialoglycans, whereas the cytoplasmic staining for ST6Gal I and sialoglycans was decreased or even absent in poorly differentiated HCC. Detection of sialoglycans by the recombinant fusion proteins in Western blots of cell lysates derived from cell lines revealed two major double bands of sialoglycoproteins at 65 and 120 kDa for hepatocytes, three major bands at 54, 49, and 44 kDa for colonic epithelial cells, and one band at 60 kDa for endothelial cells. Our results describe the expression patterns of ST6Gal I and sialoglycans in various liver tissues and demonstrate an altered expression of these structures between benign and malignant hepatocellular lesions.

Detection of the lst gene in different serogroups and LOS immunotypes of Neisseria meningitidis

The sialylation of the lipooligosaccharide (LOS) of Neisseria meningitidis is mediated by the LOS sialyltransferase enzyme encoded by the lst gene. PCR using four sets of primers that targeted to different regions of the lst gene was used to survey the distribution of lst in different serogroups and LOS immunotypes of N. meningitidis as well as other Neisseria species. While the lst gene was found in N. meningitidis strains regardless of their capsular serogroup and LOS structures, the gene is also found in N. gonorrhoeae, N. lactamica, N. polysaccharea, and N. subflava biovar subflava. The presence of the lst gene in these organisms and the sialylation of their LOS antigens were discussed.

A new fluorescent assay for sialyltransferase

A new fluorescent assay for the sialyltransferase reaction was established. After incubation of the sialyltransferase reaction, the sialyloligosaccharide obtained was treated by acid hydrolysis, and then the NeuAc that was released was labeled with 1,2-diamino-4,5-methylenedioxibenzene. The fluorescent-labeled NeuAc could be estimated by HPLC (excitation: 373 nm; emission: 448 nm) and a Lineweaver-Burk plot could be plotted with the data from this analysis.

Sialyltransferase activity in normal and atherosclerotic human aorta intima

Sialyltransferase activity has been determined in Golgi membrane fractions isolated from atherosclerotic and normal intima of human aorta by measuring the transfer of N-acetylneuraminic acid (NeuAc) from CMP-NeuAc to asialofetuin. The asialofetuin-sialyltransferase activity was found to be twofold higher in the atherosclerotic intima than in the normal intima. The mean value of the apparent Michaelis constant (Km) for the sialylating enzyme in both tissues did not differ and was 57 microM. In contrast, the maximal velocity (Vmax) was 2-fold higher for the atherosclerotic intima than for the normal intima. These results suggest that expression of asialofetuin-sialyltransferases of the aortal intima may be increased in atherosclerosis.

Thin-layer chromatography and polyacrylamide gel electrophoresis-based assays for sialyltransferases using tetramethylrhodamine-labeled acceptors

Two novel assay systems for the determination of sialyltransferase activity using a tetramethylrhodamine-labeled disaccharide Galbeta1-4GlcNAc (2) as the acceptor are described. The TMR-labeled disaccharide 2 was synthesized by directly coupling Galbeta1-4GlcNAc-O-(CH(2))(6)NH(2) (1) with 5-tetramethylrhodamine N-hydroxysuccinimide ester. The K(m) value for compound 2 obtained with alpha-2,6-sialyltransferase from rat liver (EC 2.4.99.1) was 160 +/- 20 microM. After incubation of compound 2 with sialyltransferase the product and the unreacted acceptor substrate were separated either by thin-layer chromatography (TLC) on C-18 silica gel plates or by polyacrylamide gel electrophoresis (PAGE). The density of the spots on the TLC plates and the fluorescence of the bands on the gel were quantified. The assay conditions were optimized using crude bovine colostrum extract and also alpha-2, 6-sialyltransferase from rat liver. The detection limits for the TLC and PAGE assays were 1 and 0.4 microU of the rat liver enzyme, respectively. Either assay allows the parallel investigation of several samples at a time and is useful for the testing of fractions during enzyme purification.

Terminal glycosylation of bovine uroplakin III, one of the major integral-membrane glycoproteins of mammalian bladder

Uroplakin III (UPIII) is one of the major transmembrane glycoproteins exposed at the luminal face of mammalian bladder. We investigated the terminal glycosylation of bovine UPIII in order to ascertain whether it contains the alpha 2,3-sialylated sequence thus potentially serving as a receptor for uropathogenic Escherichia coli expressing type S adhesins. We report the occurrence of sialic acid in alpha 2,3- and alpha 2,6-linkage to galactose in bovine UPIII glycans as evidenced by the sensitivity of UPIII to both Vibrio cholera and Newcastle disease virus neuraminidase and by the colocalization of UPIII antigen and material detected by lectins of Sambucus nigra and Maackia amurensis on the luminal face of the bladder. We also present evidence that UPIII glycans are capped by Gal-alpha 1,3-Gal epitope. Consistently, alpha 2,3- and alpha 2, 6-sialyltransferase, as well as alpha 1,3-galactosyltransferase were found to be present in the cells detached from the luminal side of bovine bladder, which are responsible for the UPIII biosynthesis. The putative role of UPIII sialylated glycans in enhancing the uropathogenicity of E. coli expressing type S adhesins is discussed.

Design of fluorogenic substrates for continuous assay of sialyltransferase by resonance energy transfer

Glycosyltransferases are important synthetic enzymes for the construction of naturally occurring glycoconjugates as well as for the design of neoglycoconjugates. The assay methods currently available for these enzymes require tedious and time-consuming procedures for separation of products and do not permit continual assay of enzyme activities. As a set of convenient fluorogenic substrates for continuous monitoring of sialyltransferase activities, we designed and synthesized a novel CMP-Neu5Ac derivative with a naphthylmethyl group at the C-9 position and N-acetyllactosamine derivative containing a dansyl group at the terminal position of aglycon. In such substrates, the emission peak of the naphthylmethyl group (lambdaem = 340 nm) of the glycosyl donor is successfully overlapped with the excitation peak due to the dansyl group (lambdaex = 335 nm) of the glycosyl acceptor. A coupling reaction of these two substrates catalyzed by rat liver 2,6-sialyltransferase caused an increase of dansyl fluorescence (lambdaem = 525 nm) and a decrease of naphthylmethyl fluorescence on the basis of resonance energy transfer between two fluorescence probes. The substrates presented here permit continuous fluorescent monitoring of enzymatic sugar combining reactions. Actually, using this time course of enzymatic reactions, kinetic constants of rat liver 2,6-sialyltransferase against glycosyl donor substrates were estimated to be Km = 4.85 microM and Vmax. = 0.119 micromol/min, respectively. This strategy allows precise and efficient analyses of enzyme kinetics not possible with the conventional assay methods for the glycosyltransferases that usually require separation of products from the reaction mixture.

Effect of N-glycosylation on turnover and subcellular distribution of N-acetylgalactosaminyltransferase I and sialyltransferase II in neuroblastoma cells

Gangliosides are sialylated glycosphingolipids whose biosynthesis is catalyzed by a series of endoplasmic reticulum (ER)- and Golgi-resident glycosyltransferases. Protein expression, processing, and subcellular localization of the key regulatory enzymes for ganglioside biosynthesis, sialyltransferase II (ST-II) and N-acetylgalactosaminyltransferase I (GalNAcT), were analyzed upon transient expression of the two enzymes in the neuroblastoma cell lines NG108-15 and F-11. The enzymes were endowed with a C-terminal epitope tag peptide (FLAG) for immunostaining and immunoaffinity purification using a FLAG-specific antibody. Mature ST-II-FLAG and GalNAcT-FLAG were expressed as N-glycoproteins with noncomplex oligosaccharides. ST-II-FLAG was distributed to the Golgi apparatus, whereas GalNAcT-FLAG was found in the ER and Golgi. Inhibition of early N-glycoprotein processing with castanospermine resulted in a distribution of ST-II-FLAG to the ER, whereas that of GalNAcT-FLAG remained unaltered. In contrast to GalNAcT, the activity of ST-II and the amount of immunostained enzyme were reduced concomitantly by 75% upon incubation with castanospermine. This was due to a fourfold increased turnover of ST-II-FLAG, which was not found with GalNAcT-FLAG. The ER retention and increased turnover of ST-II-FLAG were most likely due to its inability to bind to calnexin upon inhibition of early N-glycoprotein processing. Calnexin binding was not observed for GalNAcT-FLAG, indicating a differential effect of N-glycosylation on the turnover and subcellular localization of the two glycosyltransferases.

GM3 alpha2,8-sialyltransferase (GD3 synthase): protein characterization and sub-golgi location in CHO-K1 cells

GD3 synthase (Sial-T2) is a key enzyme of ganglioside synthesis that, in concert with GM2 synthase (GalNAc-T), regulates the ratio of a- and b-pathway gangliosides. In this work, we study the sub-Golgi location of an epitope-tagged version of chicken Sial-T2 transfected to CHO-K1 cells. The expressed protein was enzymatically active both in vitro and in vivo and showed a molecular mass of approximately 47 or approximately 95 kDa on sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the presence or absence of, respectively, beta-mercaptoethanol. The 95-kDa form of Sial-T2 was also detected if the protein was retained in the endoplasmic reticulum (ER) due to impaired glycosylation, indicating that it was formed in the ER. Confocal immunofluorescence microscopy showed Sial-T2 localized to the Golgi complex and, within the organelle, partially co-localizing with the mannose-6-phosphate receptor, a marker of the trans-Golgi network (TGN). In cells treated with brefeldin A, a major fraction of Sial-T2 redistributed to the ER, even under controlled expression to control for mislocalization due to protein overloading. In experiments of incorporation of sugars into endogenous acceptors of Golgi membranes in vitro, GD3 molecules formed by incubation with CMP-NeuAc were converted to GD2 upon incubation with UDP-GalNAc. These results indicate that Sial-T2 localizes mainly to the proximal Golgi, although a fraction is located in the TGN functionally coupled to GalNAc-T. Consistent with this, most of the enzyme was in an endoglycosidase H (Endo-H)-sensitive, neuraminidase (NANase)-insensitive form. A minor secreted form lacking approximately 40 amino acids was Endo-H-resistant and NANase-sensitive, indicating that the cells were able to process N-glycans to Endo-H-resistant forms. Taken together, the results of these biochemical and immunocytochemical experiments indicate that in CHO-K1 cells, most Sial-T2 localizes in the proximal Golgi and that a functional fraction is also present in the TGN.

Molecular identification, tissue distribution and subcellular localization of mST3GalV/GM3 synthase

A molecular screen for a mouse homologue of a Drosophila carbohydrate binding protein, called Gliolectin, yielded a cDNA encoding mST3GalV/GM3 synthase (CMP-NeuAc: lactosylceramide alpha2, 3-sialyltransferase). By in situ hybridization and immunohistochemistry, mST3GalV exhibits differential expression in neural and non-neural tissues. Although expressed by all neurons in the central nervous system, neuronal populations that contribute their axons to myelinated efferent projections, such as cerebellar Purkinje cells and spinal motorneurons, demonstrate the highest ST3GalV expression. When stained with anti-mST3GalV antiserum (designated CS2), subpopulations of neurons display an elaborate Golgi apparatus, frequently extending into one or more dendritic processes. The extended spatial distribution of the neuronal Golgi apparatus, particularly in spinal motorneurons, allowed the confocal immunohistochemical colocalization of mST3GalV with markers for medial/trans-Golgi but not the cis-Golgi or trans-Golgi network, consistent with previous observations suggesting that ganglioside glycosyltransferases are enriched in late Golgi compartments. Among non-neural tissues, liver and testes demonstrate cell-type specific CS2 staining. In liver, endothelial cells lining a ring of sinusoids, concentric with the central vein, express mST3GalV. Kupffer cells are also stained with CS2 antiserum but hepatocyte expression is undetectable. In the seminiferous tubules of the testes, ST3GalV is found in somatic (Leydig, Sertoli) and early germline cells (spermatogonia and primary spermatocytes); the epididymal epithelium exhibits intense ST3GalV expression. Since GM3 is a precursor for the synthesis of a- and b-series gangliosides, the range of mST3GalV/GM3 synthase expression among various cell populations indicates that certain cell types possess greater reliance on ganglioside function than others.

Quantitative analysis of expression of mouse sialyltransferase genes by competitive PCR

The present paper describes a rapid and systematic method for semi-quantitative analysis of the expression of sialyltransferase genes. So far, fifteen sialyltransferase cDNAs have been cloned from mice. Most of these genes are expressed in developmental stage-dependent and/or tissue-specific manners, and the expression levels of some of them are too low to detect on Northern blot analysis. To resolve how each sialyltransferase contribute to synthesize sialylglycoconjugates, it is necessary to establish the method for quantification of gene expression levels of these fifteen sialyltransferases. Therefore, we developed a competitive PCR-based method for analyzing the quantitative relationship of the gene expression of fifteen sialyltransferases. Using this method, we can investigate the levels of gene expression of sialyltransferases in various cell lines and various tissues of mice, and can accurately determine their expression levels.

GD3 and GM2 synthase activities in rat testes during the period of sexual development

Activities of two key enzymes of gangliosides biosynthesis were determined in rat testes during development. GD3 synthase activity was low and showed small variations with age. GM2 synthase activity increased 10-fold in testes from 10- to 30-d-old animals, showing a maximum activity at 30 d, followed by a small decrease until 45 d and then a constant activity up to adulthood. These developmental changes in the activity of both glycosyltransferases were related to the increasing complexity in the ganglioside pattern observed in rats testes during the period of sexual development.

Multiplex reverse transcription polymerase chain reaction assessment of sialyltransferase expression in human breast cancer

Increased sialylation, especially involving the Sialyl-Lewisa and Sialyl-Lewisx determinants, has been reported in breast cancer. A multiplex reverse transcription-PCR method was used here to determine the expression of five sialyltransferases (ST3Gal III, ST6Gal I, ST3Gal IV, ST3Gal I, and ST3Gal II) in 49 patients surgically treated for locoregional breast cancer. We assessed the relationship between these expressions and clinical, pathological, and biological features. The most expressed sialyltransferase was ST3Gal 1II, which is involved in Sialyl-Lewisa synthesis. ST3Gal III expression was positively correlated to ST6Gal I and ST3Gal IV expressions, to tumor size, and to the number of involved axillary nodes. Patients with high ST3Gal III expression had a shorter overall survival. High ST6Gal I expression was associated with histoprognostic grade III. ST6Gal I expression was negatively correlated to expression of progesterone receptor. In conclusion, high ST3Gal III and ST6Gal I expressions in human breast tumors are associated with poor prognosis markers.

Quantitative immunoelectron microscopy reveals alpha2,6 sialyltransferase is concentrated in the central cisternae of rat hepatocyte Golgi apparatus

The Golgi apparatus is a membrane bound organelle involved in synthesis of N-linked oligosaccharides which are trimmed and then lengthened by a series of sugar transferases adding N-acetylglucosamine, galactose and sialic acid in sequence. We previously published qualitative work which localized Galbeta1,4GlcNAc alpha2,6 sialyltransferase of rat hepatocytes to the trans cisternae and the trans Golgi network. We now report the use of combined stereological and immunoelectron microscopical techniques for mapping the Golgi stack composition and distribution of sialyltransferase protein in rat hepatocytes. The Golgi stack showed substantial variation in composition consisting of 1, 2, 3, 4, or 5 cisternae with an average of 2.5 cisternae. Sialyltransferase labeling was mainly located in the central cisternae of the Golgi stacks irrespective of whether the stacks were oriented in a cis/trans direction using morphological criteria. Only 20% of the total sialyltransferase labeling was present in the transmost cisterna and 2% in the trans Golgi Network. The low labeling in the transmost cisterna was essentially due to the presence of a sialyltransferase negative cisterna. These data emphasize the importance of quantitation in obtaining a representative picture of Golgi enzyme distribution in three dimensions. They indicate that central cisternae, rather than the transmost cisterna and TGN, function in sialylation along the secretory pathway of rat hepatocytes.

Differential expression of alpha2,6-sialyltransferase in colon tumors recognized by a monoclonal antibody

It has been reported that alpha2,6-sialyltransferase (alpha2,6-ST; E.C. 2.4.99.1) activity is associated with cellular differentiation. To define its role in colon carcinoma differentiation, we have generated murine monoclonal antibodies (MAb) against alpha2,6-sialyltransferase. The MAb, designated 6B9 of IgM isotype, showed strong reactivity with the purified and crude alpha2,6-ST by ELISA and dot blot assays. Western blotting with MAb 6B9 identified purified alpha2,6-ST of MW 47 kDa and the same MW protein from rat and human liver extracts. The MAb also reacted with two other liver proteins of approximate MW 65 and 100 kDa. Immunoperoxidase studies with formalin-fixed paraffin-embedded tissues showed that MAb 6B9 reacts with liver tissues, the staining of hepatocytes was granular and cytoplasmic. There was a distinct pattern of zonal distribution of this enzyme in hepatocytes located particularly in the portal areas of the liver corresponding to zone 1. Normal colon (100%) and hyperplastic polyps (100%) showed very weak to no reactivity. Adenomas (100%) demonstrated moderate reactivity, while the poor (33%), moderate (100%) and well-differentiated (80%) colon adenocarcinomas showed strong reactivity. Results suggest that alpha2,6-ST is associated with the differentiation state of colon tumors.

New sialyltransferase inhibitors based on CMP-quinic acid: development of a new sialyltransferase assay

Quinic acid (4) was transformed into phosphitamides 6, 14, and 15, which could be readily linked to 5'-O-unprotected cytidine derivative 7; ensuing oxidation of the obtained phosphite triesters with tert-butylhydroperoxide furnished the corresponding phosphate triesters 8, 16, and 17, respectively. Hydrogenolytic debenzylation of the phosphate moiety, base catalysed removal of acetyl protective groups, and basic hydrolysis of the methylester of the quinic acid moiety furnished CMP-Neu5Ac analogues 1-3. In order to measure their inhibition of sialyltransferases, a nonradioactive sialyltransferase assay [employed for alpha(2-6)-sialyltransferase from rat liver (EC 2.4.99.1)] based on reversed-phase HPLC separation of UV-labelled acceptor 20 (p-nitrophenyl glycoside of N-acetyllactosamine) from the UV-labelled product 21 (p-nitrophenyl glycoside of sialyl alpha(2-6')-N-acetyllactosamine) and p-nitrophenylalanine as internal standard was developed. The assay reproduced the reported K(M) values for CMP-Neu5Ac and N-acetyllactosamine and the Ki values for CDP. 1 and 2 turned out to be potent sialyltransferase inhibitors.

Immunocytochemical localization of alpha2,3(N)-sialyltransferase (ST3Gal III) in cell lines and rat kidney tissue sections: evidence for golgi and post-golgi localization

Sialylation is a biosynthetic process occurring in the trans compartments of the Golgi apparatus. Corresponding evidence is based on localization and biochemical studies of alpha2, 6(N)-sialyltransferase (ST6Gal I) as previously reported. Here we describe generation and characterization of polyclonal antibodies to recombinant rat alpha2,3(N)-sialyltransferase (ST3Gal III) expressed as a soluble enzyme in Sf9 cells or as a beta-galactosidase-human-ST3Gal III fusion-protein from E.coli , respectively. These antibodies were used to localize ST3Gal III by immunofluorescence in various cell lines and rat kidney tissue sections. In transiently transfected COS cells the antibodies directed to soluble sialyltransferase or the sialyltransferase portion of the fusion-protein only recognized the recombinant antigen retained in the endoplasmic reticulum. However, an antibody fraction crossreactive with beta-galactosidase recognized natively expressed ST3Gal III which was found to be colocalized with beta1, 4-galactosyltransferase in the Golgi apparatus of several cultured cell lines. Antibodies affinity purified on the beta-galactosidase-ST3Gal III fusion-protein column derived from both antisera have then been used to localize the enzyme in perfusion-fixed rat kidney sections. We found strong staining of the Golgi apparatus of tubular epithelia and a brush-border-associated staining which colocalized with cytochemical staining of the H+ATPase. This subcellular localization was not observed for ST6Gal I which localized to the Golgi apparatus. These data show colocalization in the Golgi apparatus and different post-Golgi distributions of the two sialyltransferases.

Absorbance- and light-based solid-phase assays for CMPNeuAc:Galbeta1-4GlcNAc-R alpha-2,3-sialyltransferase

We now report a solid-phase assay for CMPNeuAc: Galbeta1-3/4GlcNAc-R alpha-2,3-sialyltransferase (alpha2,3ST) that is nonradioactive and allows specific identification of the sialylated product. An acceptor glycoprotein, desialylated fetuin, is immobilized on a microtiter plate. The transfer of sialic acid from CMPNeuAc generates the product NeuAc alpha2-3Gal beta1-4GlcNAc-R that is specifically bound by biotinylated Maackia amurensis leukoagglutinin (MAL). The binding of biotinylated MAL is measured with either an absorbance-based reagent (streptavidin conjugated to alkaline phosphatase) or a light-based reagent (streptavidin conjugated to the bioluminescent protein aequorin, Aqualite). The rat liver alpha2,3ST was used to optimize the assay. The formation of product is linear with respect to time and dependence on the amounts of CMPNeuAc, enzyme, and acceptor coated on the plates. As little as 0.2 microU of enzyme can be measured using the streptavidin-aequorin reagent. The assay is useful with crude tissue extracts, as demonstrated by the determination of the alpha2,3ST activity in human serum and in microsomes of HL-60 and Chinese hamster ovary cells.

CMP-NeuAc:(NeuAc alpha 2-->8)n (colominic acid) sialyltransferase activity in rat brain and in tumour cells that express polysialic acid on neural cell adhesion molecules

A method for the assay of CMP-NeuAc:(NeuAc alpha 2-->8)n (colominic acid) sialyltransferase activity was developed. Using a 1-day-old rat brain membrane fraction as an enzyme preparation optimal activity was obtained at pH 6.5, 0.3% Triton X-100, and 5 mM MnCl2. However, no absolute cation requirement was found as EDTA only partially inhibited the activity. Within a concentration range of 0.3-3 mg colominic acid (which consists of a mixture of oligomers of alpha 2-->8-linked sialic acid) per 50 microliters a V of 0.61 nmol per mg protein h-1 was estimated while a half-maximal reaction velocity was obtained at a concentration of 1.75 mg per 50 microliters. High performance anion-exchange chromatography of the radioactive products formed in the reaction showed that sialic acid oligomers ranging in size from a degree of polymerization (DP) of 2 up to at least DP 9 could serve as acceptor substrates. Comparison of the acceptor properties of DP 3 and DP 6 showed that the larger oligomer was acted upon with a 10-fold higher efficiency. Periodate oxidation of the products followed by reduction and hydrolysis yielded the C7 analogue of NeuAc as the only radioactive product, indicating that under the conditions of the assay only a single sialic acid residue was introduced into the acceptor molecules. Using the assay it appeared that in rat brain the activity of this sialyltransferase decreased six-fold during postnatal development to the adult stage. The assay method was also applied to lysates of several neuroblastoma and small cell lung tumour cell lines, which differ in the expression of polysialic acid as well as of the neural cell adhesion molecule NCAM, a major carrier of this polymer. Activity of the sialyltransferase appeared to be correlated with the expression of polysialic acid present on NCAM. These results indicate that this sialyltransferase might function in the process of poly-sialylation.

Expression of Gal beta 1,4GlcNAc alpha 2,6-sialyltransferase and alpha 2,6-linked sialoglycoconjugates in normal human and rat tissues

We performed histochemical studies on normal human and rat tissues using anti-Gal beta 1,4GlcNAc alpha 2,6-sialyltransferase (alpha 2,6-ST) antibody and Sambucus nigra agglutinin (SNA). alpha 2,6-ST and its products were detected in almost all tissues examined. However, the staining intensities varied significantly with different cell types. Some secretory epithelial cells, such as hepatocytes and choroid plexus cells, were vividly stained with either anti-alpha 2,6-ST or SNA. In several cell types the intensity of alpha 2,6-ST staining did not always correlate with SNA stainability. Neurons and gastrointestinal epithelia were rarely stained with SNA, even though they were positive for alpha 2,6-ST. In contrast, the endothelial cells of blood vessels strongly reacted with SNA despite their weak alpha 2,6-ST expression. The precise physiological roles played by alpha 2,6-linked sialylated glycoconjugates have been unclear. However, the findings described here lend further support to their important role in cell growth and differentiation, since immature blood cells, including megakaryocytes in bone marrow, were intensely stained with anti-alpha 2,6-ST and SNA, and SNA reaction products were primarily observed in the basal and suprabasal layers of the stratified epithelia rather than in the more differentiated upper layers. In view of the vivid reactivity of anti-alpha 2,6-ST in the decidual cells of the placenta, it seems likely that alpha 2,6-ST expression is under hormonal control.

Immunocytochemical localization of beta-COP to the ER-Golgi boundary and the TGN

Recent data strongly suggest that the coatomer (COP) complex is involved in membrane transport between the ER and Golgi complex. This vesicular coat has been implicated in ER to Golgi, in intra Golgi as well as in Golgi to ER traffic. In this study we present a detailed immunocytochemical analysis of the distribution of beta-COP in different tissue culture cells. Our results extend previous studies by showing, using electron microscopy, that beta-COP accumulates on vesicular profiles and buds in the intermediate compartment (IC) under conditions that block ER to Golgi transport (15 degrees C). Importantly, under these conditions beta-COP co-localizes on these structures with a passenger protein, the membrane glycoprotein of vesicular stomatis virus (ts-O45-G). Furthermore, quantitative immunofluorescence microscopy of cells with ts-045-G accumulated in the ER, IC and trans-Golgi network, shifted briefly to the permissive temperature, showed that beta-COP was associated with many of the putative transport intermediates containing the viral glycoprotein which is in transit between the ER/IC and the cis-Golgi. The simplest interpretation of these data is that COP-coated vesicles are involved in anterograde transport of ts-045-G from the IC to the Golgi complex. Since many putative COP vesicle lacked the G protein following release of the 15 degrees C block this pool could be involved in retrograde transport. We also show that beta-COP is present on the membranes of the trans-Golgi network. However, in contrast to the ER-Golgi boundary, we could find no convincing evidence that this pool of beta-COP is associated with buds or trans-Golgi network-derived transport vesicles.

Comparative localization of mannose-6-phosphate receptor with 2,6sialyltransferase in HepG2 cells: an analysis by confocal double immunofluorescence microscopy

Recent advances in confocal immunofluorescent microscopy have led to significant improvements in delineating membrane-bounded organelles. In this study using HepG2 cells we focused on two functionally distinct but closely apposed organelles that have been difficult to distinguish by conventional immunofluorescent microscopy, namely the Golgi apparatus, the trans Golgi network (TGN) and late endosomes. The following markers were used: for the Golgi apparatus beta 1,4galactosyltransferase (gal-T), for the TGN, 2, 6(N)sialytransferase (sia-T) and for late endosomes/TGN, the mannose-6-phosphate/insulin growth factor II receptor (CIMPR). In addition, that part of the TGN previously shown to contain CIMPR was also identified using antibodies to the gamma-chain of the HA-1 adaptor (Klumperman et al. J. Cell Biol. 121, 997-1010 (1993)). True colocalization of intracellular antigens was ascertained by double staining of gal-T using both monoclonal and polyclonal antibodies. As previously reported, our results revealed essentially complete colocalization of gal-T and sia-T in this cell line. While the compartments containing CIMPR appeared to overlap with those containing sia-T by conventional immunofluorescence, both compartments were clearly distinct by double-label confocal microscopy. Differences between these organelles became more evident following treatment with brefeldin A. Finally, HA-1 gamma-chain was also localized to structures that were close to but clearly different from the sia-T-containing compartment. Absence of colocalization of CIMPR or HA-1 gamma-chain with sia-T indicates that these markers are enriched in distinct domains of the trans Golgi network.

Mapping the distribution of Golgi enzymes involved in the construction of complex oligosaccharides

The distribution of beta 1,2 N-acetylglucosaminyltransferase I (NAGT I), alpha 1,3-1,6 mannosidase II (Mann II), beta 1,4 galactosyltransferase (GalT), alpha 2,6 sialyltransferase (SialylT) was determined by immuno-labelling of cryo-sections from HeLa cell lines. Antibody labelling in the HeLa cell line was made possible by stable expression of epitope-tagged forms of these proteins or forms from species to which specific antibodies were available. NAGT I and Mann II had the same distribution occupying the medial and trans cisternae of the stack. GalT and SialylT also had the same distribution but they occupied the trans cisterna and the trans-Golgi network (TGN). These results generalise our earlier observations on the overlapping distribution of Golgi enzymes and show that each of the trans compartments of the Golgi apparatus in HeLa cells contains unique mixtures of those Golgi enzymes involved in the construction of complex, N-linked oligosaccharides.

CD22-mediated cell adhesion to cytokine-activated human endothelial cells. Positive and negative regulation by alpha 2-6-sialylation of cellular glycoproteins

We previously showed that cultured human umbilical vein endothelial cells (HEC) exposed to the inflammatory cytokines tumor necrosis factor-alpha or interleukin-1 display increased activity of beta-galactoside alpha 2,6-sialyltransferase. This is associated with enhanced expression of ligands for the B cell receptor CD22 beta, which recognizes alpha 2-6-linked sialic acids (Hanasaki, K., Varki, A., Stamenkovic, I., and Bevilacqua, M. P. (1994) J. Biol. Chem. 269, 10637-10643). Here we report that increased expression of CD22 ligands is a feature of dermal microvascular endothelial cells as well, and is also observed in response to the cytokine interleukin-4. Tumor necrosis factor-alpha stimulation of HEC causes no change in the profile of endothelial glycoproteins recognized by CD22, but doubles the proportion of total cellular N-linked oligosaccharides capable of binding tightly to CD22. This modest change is sufficient to cause a marked increase in alpha 2-6-linked sialic acid-dependent binding of Chinese hamster ovary (CHO) cells expressing recombinant human CD22. In contrast, B lymphoma cell lines expressing higher levels of cell surface CD22 do not show such sialic acid-dependent binding to activated HEC. Since B lymphoma cells themselves also express high levels of alpha 2-6-linked sialic acids, their CD22 molecules might be rendered nonfunctional by endogenous ligands. In support of this, the lectin function of CD22 can be directly detected on transfected CHO cells, but not on B lymphoma cells. Furthermore, coexpression of beta-galactoside alpha 2,6-sialyltransferase with CD22 in the CHO cells abrogates sialic acid-dependent binding to cytokine-activated HEC. However, such co-transfected cells can bind to B lymphoma cells in a manner apparently less dependent upon alpha 2-6-linked sialic acid, suggesting CD22-mediated interactions that may not be directly dependent on its lectin function. Thus, CD22-mediated interactions between B cells and activated vascular endothelium may be positively regulated by induction of alpha 2-6-linked sialic acid-bearing endothelial cell ligands, but negatively regulated by such ligands on the B cells expressing CD22. Since expression of both CD22 and beta-galactoside alpha 2,6-sialyltransferase are regulated during B cell ontogeny, these findings could be of importance in B cell function and/or trafficking.

Characterization of sialyltransferase-IV activity and its involvement in the c-pathway of brain ganglioside metabolism

To characterize the sialyltransferase-IV activity in brain tissues, the activities of GM1b-, GD1a-, GT1b-, and GQ1c-synthases in adult cichlid fish and rat brains were examined using GA1, GM1, GD1b, or a cod brain ganglioside mixture as the substrate. The GD1a-synthase activity in the total membrane fraction from cichlid fish brain required divalent cations such as Mg2+ or Mn2+ and Triton CF-54 for its full activity. The Vmax value was 1,340 pmol/mg of protein/h at an optimal pH of 6.5, whereas the apparent Km values for CMP-sialic acid and GM1 were 172 and 78 microM, respectively. Cichlid fish and rat brains also contained GM1b-, GT1b-, and GQ1c-synthase activities. The ratio of GM1b-, GD1a-, and GT1b-synthase activities in fish brain was 1.00:0.89:1.13, respectively, and in rat brain 1.00:0.60:0.63. Incubation of fish brain membranes with a cod brain ganglioside mixture, which contains GT1c, and [3H]CMP-sialic acid produced radiolabeled GQ1c. It is interesting that the adult rat brain also contains an appreciable level of GQ1c-synthase activity despite its very low concentrations of c-series gangliosides. The GD1a- or GQ1c-synthase activity in fish and rat brain was inhibited specifically by coincubation with the glycolipids that serve as the substrates for other sialyltransferase-IV reactions. Thus, the GD1a-synthase activity was inhibited by GA1 and GD1b, but not by LacCer, GM3, or GD3. In a similar manner, the synthesis of GQ1c was suppressed by GA1, GM1, and GD1b, but not by LacCer, GM3, or GD3.(ABSTRACT TRUNCATED AT 250 WORDS)

Trans-sialidase and sialidase activities discriminate between morphologically indistinguishable trypanosomatids

The expression of trans-sialidase and sialidase activities in the kinetoplastid protozoa was explored as a potential marker to discriminate between the morphologically indistinguishable flagellates isolated from human, insects and vertebrate reservoir hosts. By virtue of the differences observed in the ratios of these enzyme activities, a collection of 52 species and strains comprising the major taxa of these parasites could be separated into four expression types. Type-I parasites express comparable levels of both trans-sialidase and sialidase activities (Endotrypanum species and Trypanosoma lewisi). Type-II parasites express predominantly trans-sialidase activity (Trypanosoma cruzi and Trypanosoma conorhini). Type-III parasites express sialidase activity exclusively (Trypanosoma rangeli and Trypanosoma leeuwenhoeki). Type-IV parasites do not express either activity (Leishmania species and Trypanoplasma borreli). The measurement of trans-sialidase and sialidase activities thus permits the differentiation of parasites frequently found in the same insect vectors that are difficult to distinguish, such as T. cruzi and T. rangeli, or in the same sylvatic vertebrate and invertebrate hosts, such as Leishmania and Endotrypanum.

Expression cloning of a CMP-NeuAc:NeuAc alpha 2-3Gal beta 1-4Glc beta 1-1'Cer alpha 2,8-sialyltransferase (GD3 synthase) from human melanoma cells

Using an expression cloning approach, we have isolated a cDNA encoding GD3 synthase (CMP-NeuAc:NeuAc alpha 2-3Gal beta 1-4Glc beta 1-1'Cer alpha 2,8-sialyltransferase, EC 2.4.99.8), which is a key regulatory enzyme determining the prominence of the ganglioside biosynthesis pathway. The cloned cDNA encodes a 341-amino acid protein containing a single transmembrane domain at its N-terminal region, suggesting that the protein has a type II transmembrane topology. The sequence of alpha 2,8-sialyltransferase showed a high level of similarity with other sialyltransferases at two conserved regions typical in the sialyltransferase family. Transfected cells containing the cloned cDNA expressed GD3 ganglioside on the cell surface, which was detectable with specific anti-GD3 antibody by immunofluorescence and immunostaining after separation of isolated glycolipids on thin-layer chromatography. The cDNA hybridized to a single mRNA species of 2.4 kb in melanoma cells. This sialyltransferase is distinctive in catalyzing the formation of the alpha 2-8 linkage of sialic acids.

Enhanced activity of CMP-neuAc:Gal beta 1-4GlcNAc:alpha 2,6-sialyltransferase in metastasizing human colorectal tumor tissue and serum of tumor patients

The activity of sialyltransferases with different linkage specificities, of a Gal beta 1-4GlcNAc:alpha 2,6-sialyltransferase and a Gal beta 1-4GlcNAc:alpha 2,3-sialyltransferase, was studied in human colorectal tumor tissue from surgical specimens, normal mucosa, liver and liver metastases, and serum of patients suffering from colorectal carcinomas. While alpha 2,3-specific activity was equally high in tumor and mucosa samples, the activity of the alpha 2,6-specific enzyme was increased in tumor tissue and particularly in metastasizing tumors. Also, compared to healthy individuals, serum of patients suffering from metastasizing tumors contained a significantly higher activity of the alpha 2,6-specific enzyme. These results demonstrate that specific sialyltransferase isoforms are expressed in metastasizing tumors and that determination of such isoforms may be a new means for tumor detection and monitoring.

Alpha 2,6-sialyltransferase predominates in cultured jejunum of suckling rats: it is up-regulated by dexamethasone and secreted during cultivation

The alpha 2,6-sialyltransferase (alpha 2,6-ST) acting on N-acetyllactosamine is the major sialyltransferase of suckling rat jejunum. Jejunal explants of 7-day-old rats maintained in serum-free or serum-containing organ culture secreted alpha 2,6-ST into the cultivation medium. Dexamethasone (80 nM) stimulates primarily the secreted pool of alpha 2,6-ST. Fetal calf serum promotes the stimulatory effect of dexamethasone also on the bound form of alpha 2,6-ST.

Double immunofluorescent staining of alpha 2,6 sialyltransferase and beta 1,4 galactosyltransferase in monensin-treated cells: evidence for different Golgi compartments?

Beta 1,4 galactosyl- and alpha 2,6 sialyltransferase (gal-T EC 2.4.1.22 and sialyl-T EC 2.4.99.1) sequentially elongate and terminate complex N-glycan chains of glycoproteins. Both enzymes reside in trans Golgi cisternae; their ultrastructural relationship, however, is unknown. To delineate their respective Golgi compartment(s) we conducted a double label immunofluorescent study by conventional and confocal laser scanning microscopy in HepG2, HeLa, and other cells in presence of Golgi-disturbing agents. Polyclonal, peptide-specific antibodies to human sialyl-T expressed as a beta-galactosidase-sialyl-T fusion protein in E. coli were developed and applied together with mABs to human milk gal-T. In untreated HepG2 and HeLa cells Golgi morphology identified by immunofluorescent labeling of sialyl-T and gal-T, respectively, was nearly identical. Treatment of cells with brefeldin A (BFA) led to rapid and coordinated disappearance of immunostaining of both enzymes; after BFA washout, vesicular structures reappeared which first stained for gal-T followed by sialyl-T; in the reassembled Golgi apparatus sialyl-T and gal-T were co-localized again. In contrast, monensin treatment produced a reversible swelling and scattering of gal-T positive Golgi elements while sialyl-T positive structures showed little change. Treatment with nocodazole led to dispersal of Golgi elements in which gal-T and sialyl-T remained co-localized. Treatment with chloroquine affected Golgi structures less than monensin and led to condensation of gal-T positive and to slight enlargement of sialyl-T positive structures. Sequential recovery from BFA of gal-T and sialyl-T and their segregation by monensin suggest that these enzymes are targeted to different Golgi subcompartments.

Distribution of a lipooligosaccharide-specific sialyltransferase in pathogenic and non-pathogenic Neisseria

Sialyltransferase activity has been detected in Triton X100 extracts of all examined strains of pathogenic Neisseria as well as in 17 out of 18 Neisseria lactamica isolates. The enzyme was detected both in strains able to synthesize the 4.5 kDa lipooligosaccharide (LOS) component known to be sialylated in vivo and in vitro by cytidine 5'-monophospho-N-acetylneuraminic acid, and in some strains which lack this component. Exogenous 4.5 kDa+ LOS was required to detect the sialyltransferase activity in strains which lacked the LOS component. Sialyltransferase activity in a serogroup A, L11 (4.5 kDa-) meningococcal strain sialylated exogenous purified LOS from gonococci. The meningococcal serogroup B and C strain sialyltransferases active with LOS acceptors appeared to be distinct from the sialyltransferase required for the synthesis of the meningococcal polysialic acid capsule.

Human Golgi beta-galactoside alpha-2,6-sialyltransferase generates a group of sialylated B lymphocyte differentiation antigens

The role of the human beta-galactoside alpha-2,6-sialyltransferase (hu alpha-2,6-ST) in the generation of B cell surface antigens was investigated by selecting subclones of COS cells (monkey kidney epithelial cells) constitutively expressing a transfected cDNA which encodes the hu alpha-2,6-ST (COS alpha-2,6-ST cells). Expression of hu alpha-2,6-ST in COS cells was sufficient to generate sialylated cell surface epitopes on different glycosylated antigens recognized by monoclonal antibodies to CDw75, CD76, and the unclustered monoclonal antibodies HB-4 and EBU-65. These epitopes were sensitive to sialidase treatment and are likely to contain terminal alpha-2,6-linked sialic acid residues. A novel antiserum raised against bacterially expressed hu alpha-2,6-ST fusion protein was used to localize the sialyltransferase in two cell lines with high expression of either endogenous (B cell line JOK-1) or recombinant (COS alpha-2,6-ST cells) hu alpha-2,6-ST. In both cell lines, the enzyme was detected only intracellularly in the juxtanuclear region and not on the cell surface. In contrast, CDw75, formerly proposed to be identical with an alpha-2,6-ectosialyltransferase, was strongly expressed on the cell surface. The different expression patterns show that neither the CDw75 antigen nor any of the other sialylated antigens analyzed is identical with the hu alpha-2,6-ST. Furthermore, the presence of a surface-expressed alpha-2,6-ST appears unlikely in these cell lines. We propose that CDw75, CD76, HB-4, and EBU-65 represent a unique group of B cell differentiation antigens the production of which requires the enzymatic activity of alpha-2,6-ST.

A solid-phase assay for the activity of CMPNeuAc:Gal beta 1-4GlcNAc-R alpha-2,6-sialyltransferase

A solid-phase assay for the activity of CMPNeuAc:Gal beta 1-4GlcNAc-R alpha-2,6-sialyltransferase (2,6ST) has been developed. In the assay an acceptor glycoprotein is immobilized onto microtiter plate wells. The two glycoprotein acceptors used were asialofetuin (ASF), which contains oligosaccharides terminating in the sequence Gal beta 1-4GlcNAc-R, and a neoglycoprotein of bovine serum albumin containing covalently attached Gal beta 1-4GlcNAc-R units. Samples containing the donor CMPNeuAc and the 2,6ST were incubated with the immobilized acceptor to generate the product NeuAc alpha 2-6Gal beta 1-4GlcNAc-R. The product was detected by a biotin-streptavidin system using the biotinylated plant lectin Sambucus nigra agglutinin (SNA), which binds to sialic acid in alpha-2,6, but not in alpha-2,3, linkage. The biotinylated SNA bound to the product was then detected with streptavidin and biotinylated forms of either alkaline phosphatase or the recombinant bioluminescent protein aequorin. The assay was optimized with respect to the commercially available 2,6ST and shown to be dependent on the concentration of acceptor and CMPNeuAc and proportional to the 2,6ST activity in the range of 20 to 400 microU in a 1-h assay. The solid-phase assay also allows for the selective detection of 2,6ST activity in human and fetal bovine serum, where the activity was proportional in the range of 0.1 to 2 microliters of serum.

Identification and characterization of the murine homologue of CD22, a B lymphocyte-restricted adhesion molecule

The human B lymphocyte-specific Ag, CD22, is a cell adhesion molecule expressed on the surface during a narrow window of B cell development, coincident with surface IgD. A ligand for CD22 has recently been identified on human T cells as the low molecular mass isoform of the leukocyte common Ag, CD45RO. CD22 has been reported to function in the regulation of both T and B cell activation in vitro. In this study, we report the isolation and expression of a molecular cDNA clone encoding the murine homologue of CD22, mCD22. Within their predicted protein sequences, murine and human sequences overall have 62% identity, which includes 18 of 20 extracellular cysteines and six of six cytoplasmic tyrosines. BHK cells transfected with mCD22 cDNA specifically adhere to resting and activated T lymphocytes and in addition bound activated, but not resting, B cells. Five Th clones were analyzed for their ability to adhere to mCD22; two Th0 clones and one Th1 clone bound CD22+ BHK transfectants, but not all T cell clones bound CD22+ cells: another Th1 clone and a Th2 clone did not. mCD22+ BHK transfectants were also specifically bound by the B cell-specific mAb, NIM-R6, demonstrating that this mAb is specific for murine CD22. Human cell lines expressing the counter-receptors for human CD22 were also examined for adhesion to the murine CD22 homologue; the epitope responsible for B cell adhesion to CD22 is conserved, whereas the T cell epitope binding to CD22 is not. The cDNA and mAb to murine CD22 will be useful for defining the in vivo function of CD22.

Ricin and lentil lectin-affinity chromatography reveals oligosaccharide heterogeneity of thyrotropin secreted by 12 human pituitary tumors

Some patients with thyrotropin (TSH)-producing pituitary tumors are more hyperthyroid than others despite similar TSH levels in serum, suggesting that qualitatively different TSH molecules with differing bioactivities may be secreted by different tumors. We used ricin and lentil lectin-affinity chromatography to test whether the TSH oligosaccharides varied among 12 patients with TSH-producing tumors. We found that each tumor secreted heterogeneous isoforms of TSH that differed in their extents of exposed galactose (Gal) residues, and their degrees of sialylation and core fucosylation. These biochemical parameters also varied markedly for TSH secreted by different tumors. Isoforms appeared to reflect poor sialyltransferase activity in two tumors and efficient sialyltransferase in the remainder. TSH secreted by tumors was more fucosylated than TSH secreted by control euthyroid persons. There was an inverse relationship between the sialylation and fucosylation of tumor TSH. No simple relationship between TSH oligosaccharide structures and bioactivity was evident, although mixtures of isoforms having the least and most sialylated TSH seemed to be the most bioactive clinically. In three patients from whom serum and medium TSH were both available, TSH in serum was more sialylated than TSH secreted by the tumor in vitro, perhaps reflecting slow clearance of sialylated isoforms from the circulation. Core fucosylation of serum TSH was less than that of medium TSH. These data prove that human tumors secrete TSH with heterogeneous oligosaccharide structures.