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

The B7 family member B7-H6 is a tumor cell ligand for the activating natural killer cell receptor NKp30 in humans

Cancer development is often associated with the lack of specific and efficient recognition of tumor cells by the immune system. Natural killer (NK) cells are lymphocytes of the innate immune system that participate in the elimination of tumors. We report the identification of a tumor cell surface molecule that binds NKp30, a human receptor which triggers antitumor NK cell cytotoxicity and cytokine secretion. This previously unannotated gene belongs to the B7 family and, hence, was designated B7-H6. B7-H6 triggers NKp30-mediated activation of human NK cells. B7-H6 was not detected in normal human tissues but was expressed on human tumor cells, emphasizing that the expression of stress-induced self-molecules associated with cell transformation serves as a mode of cell recognition in innate immunity.

Structure determination of a sulfated N-glycans, candidate for a precursor of the selectin ligand in bovine lung

To clarify the structure of non-sialic acid anionic residue on N-glycans in the mammalian tissues, we have isolated sialidase-resistant anionic residue on N-glycans from bovine lung. Analyses by partial acid hydrolysis and glycosidase digestions combined with a two-dimensional HPLC mapping method revealed that the major sialidase-resistant anionic N-glycan had a fucosylbianntenary core structure. The anionic residue was identified as a sulfate ester by methanolysis, anion-exchange chromatography, and mass spectrometry. The linkage position of the sulfate ester was the 6-position of the GlcNAc residue on the Manalpha1-6 branch. This conclusion was based on the results of glycosidase digestions followed by two-dimensional HPLC mapping. Furthermore, the disialylated form of this sulfated glycan was dominant, and no asialo form was detected. The structure of the major anionic N-glycan prepared from bovine lung and having a sulfate was proposed to be the pyridylamino derivative of Siaalpha2-3Galphalbeta1-4(HSO(3)-6)GlcNAcbeta1-2Manalpha1-6(Siaalpha2-3Galbeta1-4GlcNAcbeta1-2Manalpha1-3)Manbeta1-4GlcNAcbeta1-4(Fucalpha1-6)GlcNAc.

The amyloid-β precursor-like protein APLP2 and its relative APP are differentially regulated during neuroendocrine cell activation

The amyloid-beta precursor-like protein APLP2 is structurally and functionally related to the amyloid-beta precursor protein APP, the protein generally accepted to be involved in Alzheimer's disease. Since we previously observed that the levels of APP mRNA and protein were up-regulated threefold in the active intermediate pituitary melanotrope cells of black-adapted Xenopus laevis, we now decided to study the regulation of APLP2 in these physiologically inducible neuroendocrine cells. Interestingly, both the mRNA and protein levels of Xenopus APLP2 were similar in the melanotrope cells of black and white frogs. Newly synthesized APLP2 became glycosylated and sulfated, chondroitin sulfate glycosaminoglycan chains were added, and eventually the protein was proteolytically cleaved. Unlike for APP, no phosphorylated APLP2 was observed. Our results show that, although APP and APLP2 are thought to be functionally related, their responses to neuroendocrine cell activation differ, suggesting distinct roles for these proteins.

Determination of the glycosaminoglycan-protein linkage region oligosaccharide structures of proteoglycans from Drosophila melanogaster and Caenorhabditis elegans

Caenorhabditis elegans and Drosophila melanogaster are relevant models for studying the roles of glycosaminoglycans (GAG) during the development of multicellular organisms. The genome projects of these organisms have revealed the existence of multiple genes related to GAG-synthesizing enzymes. Although the putative genes encoding the enzymes that synthesize the GAG-protein linkage region have also been identified, there is no direct evidence that the GAG chains bind covalently to core proteins. This study aimed to clarify whether GAG chains in these organisms are linked to core proteins through the conventional linkage region tetrasaccharide sequence found in vertebrates and whether modifications by phosphorylation and sulfation reported for vertebrates are present also in invertebrates. The linkage region oligosaccharides were isolated from C. elegans chondroitin in addition to D. melanogaster heparan and chondroitin sulfate after digestion with the respective bacterial eliminases and were then derivatized with a fluorophore 2-aminobenzamide. Their structures were characterized by gel filtration and anion-exchange high performance liquid chromatography in conjunction with enzymatic digestion and matrix-assisted laser desorption ionization time-of-flight spectrometry, which demonstrated a uniform linkage tetrasaccharide structure of -GlcUA-Gal-Gal-Xyl- or -GlcUA-Gal-Gal-Xyl(2-O-phosphate)- for C. elegans chondroitin and D. melanogaster CS, respectively. In contrast, the unmodified and phosphorylated counterparts were demonstrated in heparan sulfate of adult flies at a molar ratio of 73:27, and in that of the immortalized D. melanogaster S2 cell line at a molar ratio of 7:93, which suggests that the linkage region in the fruit fly first becomes phosphorylated uniformly on the Xyl residue and then dephosphorylated. It has been established here that GAG chains in both C. elegans and D. melanogaster are synthesized on the core protein through the ubiquitous linkage region tetrasaccharide sequence, suggesting that indispensable functions of the linkage region in the GAG synthesis have been well conserved during evolution.

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.

Occurrence and structural analysis of highly sulfated multiantennary N-linked glycan chains derived from a fertilization-associated carbohydrate-rich glycoprotein in unfertilized eggs of Tribolodon hakonensis

This study represents the first detailed investigation of the nature of highly sulfated (keratan-sulfate-like) complex-type asparagine-linked glycans having a tetraantennary core structure and shows the effectiveness of fast-atom-bombardment mass spectrometric (FAB-MS) methods incorporating derivatization and mild methanolysis for analyzing such complex types of sulfated glycans. The structure of the N-glycan chains was unambiguously established by a combination of compositional analysis, methylation analysis, mild methanolysis for desulfation, hydrazinolysis/nitrous acid deamination, enzymatic (endo-beta-galactosidase and peptide:N-glycosidase F) digestions, and instrumental analyses (1H-NMR spectroscopy and FAB-MS) which revealed the novel repeating sulfated carbohydrate sequences, +/- Gal beta 1-->4Gal beta 1[-->(HSO3-->6)GlcNAc beta 1-->3(+/- Gal beta 1-->4)Gal beta 1]n--> (see Structure I; p + q + r + s approximately 14). This sequence is unique in: (a) the skeletal structure is similar to that of keratan sulfate but is completely devoid of 6-O-sulfated Gal residues and (b) the presence of branched Gal residues in the sequence -->4GlcNAc beta 1-->3(Gal beta 1-->4)Gal beta 1-->. [formula: see text]

Unusual anionic N-linked oligosaccharides from bovine lung

We previously described a diverse family of sulfated anionic N-linked oligosaccharides released by peptide: N-glycosidase F (PNGaseF) from calf pulmonary artery endothelial (CPAE) cells (Roux, L., Holoyda, S., Sundblad, G., Freeze, H.H., and Varki, A. (1988) J. Biol. Chem. 263, 8879-8889). Since a major fraction of the intact lung consists of endothelial cells, we reasoned that bovine lung might be a rich source of similar molecules. Total N-linked oligosaccharides from bovine lung acetone powder were released by PNGaseF, labeled by [3H]NaBH4 reduction, and the anionic fractions were studied with a variety of techniques. The sugar chains with lesser negative charge (designated Class I) share several properties of conventional multiantennary complex-type chains. However, unlike the case with CPAE cells, sialic acids account only for a minority of the anionic properties and only a small proportion carry sulfate esters. A variety of different treatments indicate that most of the unexplained negative charge is due to multiple carboxylic acid groups. Resistance to beta-glucuronidase and alpha-iduronidase suggests that these may be previously undescribed modifications of mammalian oligosaccharides. The most highly charged N-linked chains (designated Class II) are more similar in general structure to the corresponding ones from CPAE cells, although relatively more abundant. Their high charge is primarily due to chondroitin sulfate, heparin/heparan sulfate, or keratan sulfate glycosaminoglycan chains. Sequential digestion studies suggest that a significant proportion of these molecules have more than one type of glycosaminoglycan chain associated with them. Compositional analysis indicates the presence of xylose residues in Class II, but not Class I molecules. However, unlike the case with conventional glycosaminoglycans, these residues are not at the reducing terminus. Most previously reported structures of complex-type N-linked oligosaccharides are derived from the glycoproteins of blood cells, plasma, or the secretions of cultured mammalian cells. This library of N-linked oligosaccharides from an intact mammalian organ (lung) contains a high proportion of novel anionic sugar chains whose structures are different from conventional complex-type sialylated chains and only partially related to those from CPAE cells. Further exploration of the N-linked chains of intact mammalian tissues seems warranted.

Purification and characterization of N-acetylglucosamine-6-sulfate sulfatase from bovine kidney: evidence for the presence of a novel endosulfatase activity

N-Acetylglucosamine-6-sulfate sulfatase (NG6SS) is an enzyme that catalyzes the hydrolysis of sulfate esters from the C-6 hydroxyl of N-acetylglucosamine. We report our purification and characterization of the enzyme and the discovery that it can remove sulfate from internally sulfated GlcNAc on glycopeptides and glycoproteins. The enzyme was purified from bovine kidney over 200,000-fold using a combination of ion-exchange and size-exclusion chromatography. NG6SS is soluble and occurs as a single subunit with apparent solution molecular weight of 60.2 kDa on gel filtration chromatography and approximately 52.5 and 57.8 kDa on reducing and nonreducing SDS/PAGE, respectively. The enzyme is highly basic and exhibits a broad pH range with an optimum at pH 6.5 and a temperature optimum of 37 degrees C. Among the mono- and disaccharide sulfates tested, only GlcNAc-6-SO4 is an effective substrate with a Km of 4.7 mM, and either free sulfate or phosphate inhibits the activity. Unexpectedly, we found that the enzyme displays endosulfatase activity and quantitatively releases 35SO4 from 35SO4-labeled glycopeptides and intact glycoproteins isolated from human Molt-3 cells, which we have previously shown to synthesize glycoproteins containing GlcNAc-6-SO4 residues within the sequence Gal beta 1-4[SO-3-6]-GlcNAc beta 1-R of complex-type N-linked oligosaccharides. The N-terminal sequence of the bovine NG6SS was homologous to a human-liver-derived N-acetylglucosamine-6-sulfatase. The endosulfatase activity of bovine kidney NG6SS may be important in its potential role in the degradation of sulfated glycans and may make this enzyme a valuable reagent to study the biological functions of sulfated glycoproteins.

Use of large-scale hydrazinolysis in the preparation of N-linked oligosaccharide libraries: application to brain tissue

In this report, we describe the preparation of a library of N-linked glycans from whole murine brain obtained by the large-scale hydrazinolysis of an acetone powder of the tissue followed by chromatographic procedures. 84% of the characterized oligosaccharides were found to be anionic, the remainder neutral. The anionic species were successively neutralized by neuraminidase (29%), aq. hydrofluoric acid (30%), and methanolysis (26%), indicating that approximately equal portions were sensitive to desialylation, dephosphorylation and desulfation, respectively. The presence of the sulfated fraction was confirmed by direct 35SO4 metabolic labelling. A residual partially characterized fraction was found to be anionic through possession of carboxylic acid groups, unrelated to sialic acid. The purified oligosaccharides, in the absence of their original protein conjugates, were shown to retain those immunological characteristics essential for recognition by a specific monoclonal antibody, LS (412), that is known to recognize a carbohydrate epitope present on a number of neural adhesion molecules and functional in neural cell adhesion. These properties confirm the viability of scaling up the size of the hydrazinolysis procedure and adapting it to whole tissue for the production of glycan libraries and for the probing of structures of interest.