Structure of an unusual complex-type oligosaccharide isolated from Chinese hamster ovary cells

Cloning, Expression, and Functional Characterization of FUT1, a Key Gene for Histo-Blood Group Antigens Synthesis in Crassostrea gigas

Histo-blood group antigens (HBGAs) comprise a family of cell-surface carbohydrates that are considered norovirus-specific binding receptors or ligands. HBGA-like molecules have also been detected in oysters as common norovirus carriers, although the pathway involved in the synthesis of these molecules in oysters has yet to be elucidated. We isolated and identified a key gene involved in the synthesis of HBGA-like molecules, FUT1, from Crassostrea gigas, named CgFUT1. Real-time quantitative polymerase chain reaction analysis showed that CgFUT1 mRNA was expressed in the mantle, gill, muscle, labellum, and hepatopancreatic tissues of C. gigas, with the hepatopancreas exhibiting the highest expression level. A recombinant CgFUT1 protein with a molecular mass of 38.0 kDa was expressed in Escherichia coli using a prokaryotic expression vector. A eukaryotic expression plasmid was constructed and transfected into Chinese hamster ovary (CHO) cells. The expression of CgFUT1 and membrane localization of type H-2 HBGA-like molecules in CHO cells were detected using Western blotting and cellular immunofluorescence, respectively. This study indicated that CgFUT1, expressed in C. gigas tissues, can synthesize type H-2 HBGA-like molecules. This finding provides a new perspective for analyzing the source and synthetic pathway of HBGA-like molecules in oysters.

Complex N-glycans are the major ligands for galectin-1, -3, and -8 on Chinese hamster ovary cells

Galectins are implicated in a large variety of biological functions, many of which depend on their carbohydrate-binding ability. Fifteen members of the family have been identified in vertebrates based on binding to galactose (Gal) that is mediated by one or two, evolutionarily conserved, carbohydrate-recognition domains (CRDs). Variations in glycan structures expressed on glycoconjugates at the cell surface may, therefore, affect galectin binding and functions. To identify roles for different glycans in the binding of the three types of mammalian galectins to cells, we performed fluorescence cytometry at 4 degrees C with recombinant rat galectin-1, human galectin-3, and three forms of human galectin-8, to Chinese hamster ovary (CHO) cells and 12 different CHO glycosylation mutants. All galectin species bound to parent CHO cells and binding was inhibited >90% by 0.2 M lactose. Galectin-8 isoforms with either a long or a short inter-CRD linker bound similarly to CHO cells. However, a truncated form of galectin-8 containing only the N-terminal CRD bound only weakly to CHO cells and the C-terminal galectin-8 CRD exhibited extremely low binding. Binding of the galectins to the different CHO glycosylation mutants revealed that complex N-glycans are the major ligands for each galectin except the N-terminal CRD of galectins-8, and also identified some fine differences in glycan recognition. Interestingly, increased binding of galectin-1 at 4 degrees C correlated with increased propidium iodide (PI) uptake, whereas galectin-3 or -8 binding did not induce permeability to PI. The CHO glycosylation mutants with various repertoires of cell surface glycans are a useful tool for investigating galectin-cell interactions as they present complex and simple glycans in a natural mixture of multivalent protein and lipid glycoconjugates anchored in a cell membrane.

Modification of a recombinant GPI-anchored metalloproteinase for secretion alters the protein glycosylation

The N-linked glycans of recombinant leishmanolysin (GP63) expressed as a glycosylphosphatidylinositol (GPI)-anchored membrane protein or modified for secretion in Chinese hamster ovary (CHO) cells were analyzed by fast atom bombardment-mass spectrometry (FAB-MS). The glycans isolated from both membrane and secreted protein were predominantly complex biantennary structures. However other aspects of the glycan profiles showed striking differences. The degree of sialylation of the membrane form was greatly reduced and the core fucosylation of biantennary structures was increased compared to the secreted form. Glycans isolated from membrane expressed protein also contained a higher proportion of lactosamine repeats. Residence times in the secretory pathway were similar for both secreted and membrane protein. Glycosylation differences may therefore be due to differences in protein conformation and accessibility to glycosyltransferases or glycosidases. These differences in glycosylation represent an important factor when considering modifying membrane expressed proteins for secreted production.

alpha(1,2)-fucosylation prevents sialyl Lewis x expression and E-selectin-mediated adhesion of fucosyltransferase VII-transfected cells

E-selectin is a cytokine-inducible, calcium-dependent endothelial cell adhesion molecule that plays a critical role in the leucocyte-endothelium interaction during inflammation and is thought to contribute to the metastatic dissemination of tumour cells. Like the other selectins, E-selectin binds to ligands carrying the tetrasaccharide sialyl-Lewis x (NeuAcalpha2,3Galbeta1,4[Fucalpha1, 3]GlcNAc)1 or its isomer sialyl-Lewis a (NeuAcalpha2, 3Galbeta1, 3[Fucalpha1,4]GlcNAc). We examined the effect of expressing the H-type alpha(1,2)-fucosyltransferase or the alpha(2, 6)-sialyltransferase on the synthesis of sialyl-Lewis x by alpha(1, 3)fucosyltransferase. We found that H-type alpha(1, 2)-fucosyltransferase but not alpha(2,6)-sialyltransferase, strongly inhibited sialyl-Lewis x expression and E-selectin adhesion. We assume that H-type alpha(1,2)-fucosyltransferase competes with the endogenous alpha(2,3)-sialyltransferase for the N-acetyllactosamine structures assigned to further serve as acceptors for alpha(1, 3)fucosyltransferase.

Expression and characterization of a lactosaminoglycan-carrying glycoprotein of Zajdela hepatoma cell surface--structural analysis of the carbohydrate moiety

In poorly differentiated hepatoma cells, a glycoprotein carrying lactosaminoglycans is identified, and the structure of its glycan moiety is proposed. After membrane solubilization, protein fractionation by gel filtration, and electroelution, this glycoprotein (GPIII) was identified by its affinity for Datura stramonium lectin and its content in large glycopeptides. As shown by PAGE, GPIII has an apparent molecular mass of 100 kDa and is highly glycosylated (36%). It appears as an integral membrane glycoprotein. It is absent from normal hepatocytes, in that no heavy glycopeptides could be detected that bound to Datura lectin or to specific antiserum. The glycan moiety of GPIII has been analyzed according to carbohydrate composition, glycosidase treatment, affinity chromatography on immobilized pokeweed, Datura and Griffonia lectins, and by NMR and methylation analyses. The glycan is a N-linked tetraantennary lactosaminoglycan of 6.6 kDa, containing Gal, GlcNAc, Man, and NeuNAc in a 16:14:3:4 molar ratio, with an average of three repeating units/branch. Its beta-Gal residues are in the penultimate position and are linked in beta1-4 at least in four structural elements (three peripheral and one internal). It contains a very branched structure with Gal alpha1-3Gal beta1-4GlcNAc side chains linked in the C6 position to an inner Gal residue in a main branch. Alpha-Gal and NeuNAc residues [mainly NeuNAc alpha(2-3) linkage] are expressed as the nonreducing terminal groups. A possible structural model is proposed for this heterogeneous lactosaminoglycan, although no definitive structure can be established. That this lactosaminoglycan-carrying glycoprotein GPIII is not expressed in hepatocytes suggests its expression to be linked to the undifferentiated and/or malignant state of this hepatoma.

Expression of human H-type alpha1,2-fucosyltransferase encoding for blood group H(O) antigen in Chinese hamster ovary cells. Evidence for preferential fucosylation and truncation of polylactosamine sequences

The human H(O) blood group is specified by the structure Fucalpha1-2Galbeta1-R, but the factors regulating expression of this determinant on cell surface glycoconjugates are not well understood. To learn more about the regulation of H blood group expression, cDNA encoding the human H-type GDPFuc:beta-D-galactoside alpha1, 2-fucosyltransferase (alpha1,2FT) was stably transfected into Chinese hamster ovary (CHO) cells. The new cell line, designated CHO(alpha1,2)FT, expressed surface neoglycans containing the H antigen. The structures of the fucosylated neoglycans in CHO(alpha1, 2)FT cells and the distribution of these glycans on glycoproteins were characterized. Seventeen percent of the [3H]Gal-labeled glycopeptides from CHO(alpha1,2)FT cells bound to the immobilized H blood group-specific lectin Ulex europaeus agglutinin-I (UEA-I), whereas none from parental CHO cells bound to the lectin. The glycopeptides from CHO(alpha1,2)FT cells binding to UEA-I contained polylactosamine [3Galbeta1-4GlcNAcbeta1-]n with the terminal sequence Fucalpha1-2Galbeta1- 4GlcNAc-R. Fucosylation of the polylactosamine sequences on complex-type N-glycans in CHO(alpha1, 2)FT cells caused a decrease in both sialylation and length of polylactosamine. Unexpectedly, only small amounts of terminal fucosylation was found in diantennary complex-type N-glycans. The O-glycans and glycolipids were not fucosylated by the H-type alpha1, 2FT. Two major high molecular weight glycoproteins, one of which was shown to be the lysosome-associated membrane glycoprotein LAMP-1, preferentially contained the H-type structure and were bound by immobilized UEA-I. These results demonstrate that in CHO cells the expressed H-type alpha1,2FT does not indiscriminately fucosylate terminal galactosyl residues in complex-type N-glycans, but it favors glycans containing polylactosamine and dramatically alters their length and sialylation.

N-glycans modulate in vivo and in vitro thyroid hormone synthesis. Study at the N-terminal domain of thyroglobulin

Thyroglobulin (Tg) is the substrate for thyroid hormone biosynthesis, which requires tyrosine iodination and iodotyrosine coupling and occurs at the apical membrane of the thyrocytes. Tg glycoconjugates have been shown to play a major role in Tg routing through cellular compartments and recycling after endocytosis. Here we show that glycoconjugates also play a direct role in hormonosynthesis. The N-terminal domain (NTD; Asn1-Met171) of human Tg, which bears the preferential hormonogenic site, brings two N-glycans (Asn57 and Asn91). NTD preparations were purified from Tg with low and mild iodine content in vivo and from poorly iodinated Tg after in vitro iodination and coupling. NTD separated from poorly iodinated Tg was also submitted to iodination and coupling after desialylation and deglycosylation. The various NTD isoforms were analyzed for their N-glycan structures and hormone contents. Our results show that 1) in vivo as well as in vitro unglycosylated isoforms did not synthesize hormones, whereas fully or partially (at Asn91) glycosylated isoforms did; 2) high mannose type structures enhanced the hormone content; and 3) desialylation did not affect in vitro hormone synthesis. Evidence of a direct involvement in hormonosynthesis adds to the role of N-glycans in Tg function and opens the way to new mechanisms for regulation (e.g. TSH modulation of N-glycan) or alteration (e.g. Asn91 mutation) of thyroid hormone synthesis.

Structural differences between complex-type Asn-linked glycan chains of glycoproteins in rat hepatocytes and Zajdela hepatoma cells

Using serial lectin-affinity chromatography, glycosidase digestion, and NMR and methylation analysis, the structures of complex N-linked glycan chains (M(r) range 2000-3500) of rat hepatocytes and poorly differentiated chemically transformed Zajdela ascites hepatoma cells were determined and compared. The results revealed considerable differences between the two cell types: (i) hepatoma cells only expressed tri- and/or tetra-antennary complex N-linked glycan chains, whereas hepatocytes displayed large amounts of bi-antennary N-linked structures and smaller amounts of tri-/tetra-antennary structures; (ii) 20% of the glycan chains in hepatoma cells contained a bisecting GlcNAc residue which was beta (1,4)-linked to the beta-mannosyl residue of the core and was not detected in the hepatocytes; (iii) hepatoma cells expressed a high proportion of the fucosylated or not GlNAc beta (1,6) Man alpha 1-->branch, whereas hepatocytes only contained a little of this branch; (iv) hepatoma cells, but not hepatocytes, exhibited a repeating (Gal beta(1,4) GlcNAc beta (1,3)) sequence characteristic of poly-N-acetyllactosaminoglycans. These glycans were capped by both alpha-galactosyl and sialyl residues; (v) The alpha (2,3)/alpha (2,6)-linkage ratio of sialic acid was significantly higher in hepatoma cells (4/1 vs. 2/1 in hepatocytes); (vi) Only hepatocytes expressed an unusual structure in which a sialyl residue was alpha (2,6)-linked to a GlcNAc residue located within a NeuAc alpha (2,3) Gal beta (1,3) GlcNAc branch which was beta (1,4)-linked to Man alpha 1,3-->. The differences between these complex N-linked glycan chains in hepatocytes and hepatoma cells seem to be both quantitative and qualitative, since some glycan structures were only present in one cell type.

Hydrophobic glycosides of N-acetylglucosamine can act as primers for polylactosamine synthesis and can affect glycolipid synthesis in vivo

Several hydrophobic glycosides of N-acetylglucosamine (GlcNAc) served as primers for polylactosamine synthesis when added to Chinese hamster ovary (CHO) cells. The modified glycosides, containing one to six lactosamine repeats in linear array, were sialylated and secreted into the culture medium. The relative efficiencies of the glycosides to serve as primers were dependent on the nature of the aglycone and on the anomeric configuration of the GlcNAc residue. The same compounds were tested for their effects on glycolipid synthesis in CHO cells. All of the beta-glycosides significantly inhibited the synthesis of the lactoseries glycolipid GM3 whereas the alpha-glycoside was inactive. The compound GlcNAc alpha 1-O-benzyl- was the most efficient primer of polylactosamine synthesis and had no effect on glycolipid synthesis. This compound may have potential for the assay of the polylactosamine synthetic capacity of living cells.

Mediation of Trypanosoma cruzi invasion by sialic acid on the host cell and trans-sialidase on the trypanosome

Trypanosoma cruzi attaches and invades a large variety of mammalian cells. The nature of the cell receptors and of the corresponding parasite counter-receptors that mediate T. cruzi-host cell interaction are not known. Three sialic acid-deficient mutants of Chinese hamster ovary (CHO) cells were used to probe the role of host sialyl residues in T. cruzi infection. All three mutants supported adhesion and infection to a much lower extent than the parental CHO cells. One of the mutants, Lec2, contains sugar chains terminating in non-reducing beta Gal residues, which are acceptors for sialylation by the T. cruzi trans-sialidase. Re-sialylation of Lec2 cells restored T. cruzi adhesion and invasion to about the same extent as wild-type cells. Digestion of wild-type cells with bacterial sialidase reduced T. cruzi interaction but after re-sialylation, the cells were almost as good as control, naturally sialylated parental cells. These results suggest that T. cruzi recognizes sialyl residues on the surface of host cells during invasion. On the other hand, affinity-purified trans-sialidase blocked T. cruzi adherence and invasion of sialylated cells, and had no effect on parasite interaction with sialic acid-deficient Lec2 mutant. Furthermore, 2,3-sialyllactose, a substrate for the trans-sialidase, competitively inhibited T. cruzi invasion of sialylated parental K1 cells, but 2,6-sialyllactose, which does not react with the trans-sialidase, was without effect, as were other sugars that do not contain alpha 2,3 sialyl residues. These results suggest that the trans-sialidase functions as a counter-receptor for trypomastigote binding to alpha 2,3-sialyl receptors on host cells as a prelude to T. cruzi invasion.

Polylactosaminoglycan modification of the respiratory syncytial virus small hydrophobic (SH) protein: a conserved feature among human and bovine respiratory syncytial viruses

We investigated the nature of the oligosaccharide modification of the glycosylated forms of the small hydrophobic integral membrane protein, SH (previously designated 1A), of respiratory syncytial (RS) virus. Analysis of SH protein expressed in cells infected with RS virus or with a recombinant vaccinia virus revealed two glycosylated SH protein species, SHg and SHp, which contained N-linked carbohydrate residues. SHp migrated diffusely on polyacrylamide gels, which suggested modification by polylactosaminoglycan oligosaccharides. Polylactosaminoglycan modification of SHp was established from three lines of investigation: (1) the synthesis of SHp in a cell line (IdID) conditionally defective in the ability to add specific carbohydrate residues to N- or O-linked oligosaccharide chains required the addition of galactose, which is a component of the N-acetyllactosamine repeating unit; (2) SHp was sensitive to digestion with endo-beta-galactosidase, which cleaves the beta 1-4 linkage between galactose and N-acetylglucosamine of the repeated N-acetyllactosamine subunit; and (3) SHp was selected by Datura stramonium lectin (Dsl), which has specificity for polylactosaminoglycans. The presence of SHp as a component of purified human subgroups A and B and bovine RS virus particles was demonstrated by Dsl affinity selection. In addition to SHp, nonglycosylated SHo was selected by Dsl affinity, indicating that SHp and SHo may associate to form complexes within infected cells and virus particles. To identify conserved amino acid residues among the human and bovine SH glycoproteins that may function as signals for polylactosaminoglycan modification, the nucleotide sequences of the SH protein genes of a human subgroup B virus (8/60) and a bovine virus (391-2) were determined and compared to those of a human subgroup A virus (A2), a subgroup B virus (18537), and a bovine virus (A51908). A comparison of the deduced amino acid sequences of the human and bovine RS virus SH proteins indicated that a central hydrophobic region and the presence of potential N-linked glycosylation sites on either side of the central hydrophobic region were conserved features that may be required for the polylactosaminoglycan modification of SH.

The hamster transferrin receptor contains Ser/Thr-linked oligosaccharides: use of a lectin-resistant CHO cell line to identify glycoproteins containing these linkages

We recently reported that the human transferrin receptor (TfR) contains O-linked GalNAc residues [1]. To investigate whether this modification is shared by transferrin receptors in other mammals, we investigated the glycosylation of TfR in hamster cells. To facilitate our analysis the lectin-resistant Chinese hamster ovary (CHO) cell line Lec8 was used. These cells are unable to galactosylate glycoproteins, resulting in truncation of the Ser/Thr-linked oligosaccharides to a single residue of terminal alpha-linked GalNAc. This structure is bound with high affinity by the lectin Helix pomatia agglutinin (HPA). The TfR was affinity purified from Lec8 cells metabolically radiolabeled with [3H]glucosamine and the receptor was found to bind tightly to HPA-Sepharose. Treatment of the purified TfR with mild alkaline/borohydride released [3H]GalNAcitol, demonstrating the presence of O-linked GalNAc. We also found that many other unidentified [3H]glucosamine-labeled glycoproteins from Lec8 cells were bound by HPA-Sepharose. The bound and unbound glycoproteins were separated by SDS/PAGE and individual species were selected for treatment with mild base/borohydride. Treatment of glycoproteins bound by HPA, but not those unbound, resulted in the release of [3H]GalNAcitol. These studies demonstrate both that the hamster TfR contains O-linked oligosaccharides and that this approach may have general utility for identifying the presence of these oligosaccharides in other glycoproteins.

Structural analysis of the N-linked oligosaccharides from murine glycophorin

Glycophorins, isolated from BALB/c mouse erythrocytes, were degraded under mild and strong reductive alkaline conditions and the N-linked oligosaccharides were isolated as alditols. The oligosaccharide alditols were fractionated and purified using gel filtration, concanavalin A-Sepharose affinity chromatography, and high-performance ion-exchange chromatography. Structural analysis was carried out by chemical analyses, periodate oxidation in combination with fast atom bombardment mass spectrometry, and 500-MHz 1H NMR spectroscopy. The results revealed the presence of sialylated biantennary, triantennary, and tetraantennary complex type oligosaccharides, all fucosylated at the innermost N-acetylglucosamine residue. The tri- and tetraantennary oligosaccharide-containing fractions also contained species elongated by one and/or two N-acetyllactosamine (-3Gal beta 1-4GlcNAc beta 1-) sequences. The N-linked oligosaccharides were shown to be combined only with one (the low molecular weight) of the two mouse glycophorins.

The selectin GMP-140 binds to sialylated, fucosylated lactosaminoglycans on both myeloid and nonmyeloid cells

Granule membrane protein-140 (GMP-140) is an inducible receptor for myeloid leukocytes on activated platelets and endothelium. Like other selectins, GMP-140 recognizes specific oligosaccharide ligands. However, prior data on the nature of these ligands are contradictory. We investigated the structural features required for ligand interaction with GMP-140 using purified GMP-140, cells naturally expressing specific oligosaccharides, and cells expressing cloned glycosyltransferases. Like the related selectin endothelial leukocyte adhesion molecule-1 (ELAM-1), GMP-140 recognizes alpha(2-3)sialylated, alpha(1-3)fucosylated lactosaminoglycans on both myeloid and nonmyeloid cells, including the sequence Neu5Ac alpha 2-3Gal beta 1-4(Fuc alpha 1-3)GlcNac beta-R (sialyl Lewis x). Recognition requires sialic acid, because cells expressing large amounts of Lewis x, but not sialyl Lewis x, do not interact with GMP-140. Although sialyl Lewis x is expressed by both myeloid HL-60 cells and CHO cells transfected with an alpha 1-3/4 fucosyltransferase, GMP-140 binds with significantly higher affinity to HL-60 cells. Thus, the sialyl Lewis x tetrasaccharide may require additional structural modifications or specific presentations in order for leukocytes in flowing blood to interact rapidly and with high affinity to GMP-140 on activated platelets or endothelium.

The dysfunctional LDL receptor in a monensin-resistant mutant of Chinese hamster ovary cells lacks selected O-linked oligosaccharides

The Chinese hamster ovary (CHO) cell line Monr31, which is resistant to the cytotoxic ionophore monensin, produces a receptor for the low density lipoprotein (LDL) that has a lowered binding affinity for LDL and is approximately 5 kDa smaller in size than the receptor from parental CHO cells. It has been proposed that the reduced size and affinity for LDL are associated with a reduced level of O-glycosylation of Ser/Thr residues in the receptor. To examine this possibility in more detail, both parental CHO and Monr31 cells were metabolically radiolabeled with [3H]glucosamine, and the labeled LDL receptors were purified by immunoprecipitation and identified by SDS-PAGE-fluorography. The Ser/Thr-linked oligosaccharides in the receptors from both parental CHO and Monr31 cells are mono- and desialylated species having the common core structure Gal beta 1-3GalNAc. The receptor from Monr31 cells, however, contains about one-third fewer Ser/Thr-linked oligosaccharides than the receptor from parental CHO cells. Analysis of the glycopeptides derived from the Monr31 cell LDL receptors indicates that they contain Ser/Thr-linked oligosaccharides only in the clustered domain and are missing Ser/Thr-linked oligosaccharides in the unclustered regions of the protein. Additionally, analysis of a human LDL receptor lacking the domain for attachment of the clustered Ser/Thr-linked oligosaccharides and expressed in both parental CHO and Monr31 cells indicated that the truncated human receptor from Monr31 cells is devoid of Ser/Thr-linked oligosaccharides. In contrast, the truncated human receptor produced by parental CHO cells contains Ser/Thr-linked oligosaccharides contributing approximately 5 kDa to its apparent size. Collectively, these results demonstrate that the LDL receptor produced by the Monr31 cells contains Ser/Thr-linked oligosaccharides in the clustered domain but is missing Ser/Thr-linked oligosaccharides in the unclustered, NH2-terminal domains of the receptor.

LAMP-1 in CHO cells is a primary carrier of poly-N-acetyllactosamine chains and is bound preferentially by a mammalian S-type lectin

Recent studies indicate that some mammalian S-type lectins bind preferentially to oligosaccharides containing the repeating disaccharide [3Gal beta 1,4GlcNAc beta 1]n or poly-N-acetyllactosamine (PL) sequence. We report here our investigation on the distribution of these sequences in glycoproteins in Chinese hamster ovary (CHO) cells and the interaction of glycoproteins containing PL chains with an immobilized S-type lectin (L14) from calf heart tissue. Our results demonstrate that PL chains are carried by a few high molecular weight glycoproteins which are bound by tomato-lectin Sepharose and one of these was precipitated by antibody to LAMP-1 (a lysosomal-associated membrane glycoprotein). More importantly, these high molecular weight glycoproteins, including LAMP-1, were bound with high affinity by L14. These results indicate that mammalian S-type lectins are highly specific in their interactions with glycoproteins and that LAMPs carry important recognition sequences for these lectins.

Characterization of cell surface carbohydrate receptors for Entamoeba histolytica adherence lectin

Binding and cytolysis of Chinese hamster ovary (CHO) cells by Entamoeba histolytica trophozoites is inhibitable by galactose (Gal) or N-acetyl-D-galactosamine (GalNAc). To better define the carbohydrate receptor for E. histolytica, we compared the binding and cytolytic target properties of 10 CHO glycosylation mutants. Each mutant expresses a uniquely altered array of N- and/or O-linked cell surface carbohydrates. Amebic adherence was reduced when lactosamine-containing N-linked carbohydrates were essentially absent (Lec1 mutant), almost undetectable when Gal and GalNAc residues were absent on both N- and O-linked carbohydrates (ldlD.Lec1 mutant), and enhanced for mutants with increased terminal Gal residues (Lec2 and Lec3). Parental CHO cells treated with neuraminidase to expose Gal residues behaved like Lec2 mutants. Binding of purified Gal or GalNAc lectin to parental, Lec1, ldlD.Lec1, and Lec2 mutant CHO cells corroborated the adherence results. The suitability of CHO cell mutants as targets for amebic cytolysis correlated with their glycosylation phenotype: the Lec1 mutants were less susceptible than parental CHO cells, the ldlD.Lec1 mutants were highly resistant, and the Lec2 mutants required higher concentrations of Gal for inhibition. The E. histolytica Gal or GalNAc adherence lectin bound preferentially to beta 1-6-branched, N-linked carbohydrates lacking terminal sialic acid or fucose residues. However, amebic lectin binding to either N- or O-linked cell surface carbohydrates was sufficient to initiate parasite cytolytic activity.

Control of carbohydrate processing: increased beta-1,6 branching in N-linked carbohydrates of Lec9 CHO mutants appears to arise from a defect in oligosaccharide-dolichol biosynthesis

A correlation between increased beta-1,6 branching of N-linked carbohydrates and the ability of a cell to metastasize or to form a tumor has been observed in several experimental models. Lec9 Chinese hamster ovary (CHO) mutants exhibit a drastic reduction in tumorigenicity in nude mice, and this phenotype directly correlates with their ability to attach an increased proportion of beta-1,6-branched carbohydrates to the G glycoprotein of vesicular stomatitis virus (J. Ripka, S. Shin, and P. Stanley, Mol. Cell. Biol. 6:1268-1275, 1986). In this paper we provide evidence that cellular carbohydrates from Lec9 cells also contain an increased proportion of beta-1,6-branched carbohydrates, although they do not possess significantly increased activity of the beta-1,6 branching enzyme (GlcNAc-transferase V). Biosynthetic labeling experiments show that a substantial degree of underglycosylation occurs in Lec9 cells and that this affects several classes of glycoproteins. Lec9 cells synthesize ca. 40-fold less Glc3Man9GlcNAc2-P-P-lipid and ca. 2-fold less Man5GlcNAc2-P-P-lipid than parental cells do. In addition, Lec9 cells possess ca. fivefold less protein-bound oligosaccharide intermediates, and one major species is resistant to release by endo-beta-N-acetylglucosaminidase H (endo H). Membranes of Lec9 cells exhibit normal mannosylphosphoryldolichol synthase, glucosylphosphoryldolichol synthase, and N-acetylglucosaminylphosphate transferase activities in the presence of exogenous dolichyl phosphate. However, in the absence of exogenous dolichyl phosphate, mannosylphosphoryldolichol synthase and glucosylphosphoryldolichol synthase activities are reduced in membranes of Lec9 cells, indicating that membranes of Lec9 cells are deficient in lipid phosphate. This was confirmed by analysis of lipids labeled by [3H]mevalonate, which showed that Lec9 cells have less lipid phosphate than parental CHO cells. Mechanisms by which a defect in the synthesis of dolichol-oligosaccharides might alter the degree of beta-1,6 branching in N-linked carbohydrates are discussed.

Chinese hamster ovary cell mutants with multiple glycosylation defects for production of glycoproteins with minimal carbohydrate heterogeneity

The production of glycoproteins with carbohydrates of defined structure and minimal heterogeneity is important for functional studies of mammalian carbohydrates. To facilitate such studies, several Chinese hamster ovary mutants that carry between two and four glycosylation mutations were developed. All of the lines grew readily in culture despite the drastic simplification of their surface carbohydrates. Therefore, both endogenous glycoproteins and those introduced by transfection can be obtained with specifically tailored carbohydrates. The lectin resistance properties of the mutants showed that each line expresses a novel array of cell surface carbohydrates useful for identifying specific roles for carbohydrates in cellular interactions. In addition, they showed that the epistatic relationships among different glycosylation mutations are not entirely predictable, providing insight into the complexity of the carbohydrate structures at the Chinese hamster ovary cell surface.

Biosynthesis of high molecular weight polylactosamine-type glycopeptides in rat Zajdela hepatoma ascites cells

The first steps of the biosynthetic pathway of high molecular weight polylactosamine-type glycopeptides from rat Zajdela hepatoma cells were studied by pulse-chase experiments, biochemical analysis and by inhibition of N-glycosylation. It is clear that this process involves firstly the transfer of a lipid-linked high-mannose oligosaccharide precursor to a protein moiety in a similar way to that of N-linked glycopeptides of a more common size range according to the classical 'cycle of dolichol'. In the presence of enzymes which are inhibitors of the processing of high-mannose oligosaccharide chains, this class of oligosaccharides was considerably increased, whereas polylactosamine chains and lower complex N-linked glycopeptides were concomitantly decreased in the same kinetics and the same ratio. As expected in the presence of N-methyldeoxynojirimycin, which is an alpha-glucosidase inhibitor, high-mannose oligosaccharides remained glycosylated and are mostly of the Glc1-3Man9GlcNAc type. In the presence of swainsonine, which is an alpha-mannosidase (EC 3.2.1.24) inhibitor, these chains were devoid of glucose residues. In addition, some chains displayed hybrid structures. It appears, therefore, that the first steps of the biosynthesis of polylactosamine-type and N-linked oligosaccharides of a more common size range proceed similarly and that differences between their biosynthetic pathways occur during the elongation phase, which leads to their final respective structures. Glycopeptides prepared from the cell surface by mild trypsin treatment as well as from entire cells, previously treated or not by processing inhibitors, display the same gel filtration patterns indicating that modifications in protein glycosylation do not prevent glycoprotein insertion into the cell membrane.

Control of carbohydrate processing: increased beta-1,6 branching in N-linked carbohydrates of Lec9 CHO mutants appears to arise from a defect in oligosaccharide-dolichol biosynthesis

A correlation between increased beta-1,6 branching of N-linked carbohydrates and the ability of a cell to metastasize or to form a tumor has been observed in several experimental models. Lec9 Chinese hamster ovary (CHO) mutants exhibit a drastic reduction in tumorigenicity in nude mice, and this phenotype directly correlates with their ability to attach an increased proportion of beta-1,6-branched carbohydrates to the G glycoprotein of vesicular stomatitis virus (J. Ripka, S. Shin, and P. Stanley, Mol. Cell. Biol. 6:1268-1275, 1986). In this paper we provide evidence that cellular carbohydrates from Lec9 cells also contain an increased proportion of beta-1,6-branched carbohydrates, although they do not possess significantly increased activity of the beta-1,6 branching enzyme (GlcNAc-transferase V). Biosynthetic labeling experiments show that a substantial degree of underglycosylation occurs in Lec9 cells and that this affects several classes of glycoproteins. Lec9 cells synthesize ca. 40-fold less Glc3Man9GlcNAc2-P-P-lipid and ca. 2-fold less Man5GlcNAc2-P-P-lipid than parental cells do. In addition, Lec9 cells possess ca. fivefold less protein-bound oligosaccharide intermediates, and one major species is resistant to release by endo-beta-N-acetylglucosaminidase H (endo H). Membranes of Lec9 cells exhibit normal mannosylphosphoryldolichol synthase, glucosylphosphoryldolichol synthase, and N-acetylglucosaminylphosphate transferase activities in the presence of exogenous dolichyl phosphate. However, in the absence of exogenous dolichyl phosphate, mannosylphosphoryldolichol synthase and glucosylphosphoryldolichol synthase activities are reduced in membranes of Lec9 cells, indicating that membranes of Lec9 cells are deficient in lipid phosphate. This was confirmed by analysis of lipids labeled by [3H]mevalonate, which showed that Lec9 cells have less lipid phosphate than parental CHO cells. Mechanisms by which a defect in the synthesis of dolichol-oligosaccharides might alter the degree of beta-1,6 branching in N-linked carbohydrates are discussed.

Chinese hamster ovary cell mutants with multiple glycosylation defects for production of glycoproteins with minimal carbohydrate heterogeneity

The production of glycoproteins with carbohydrates of defined structure and minimal heterogeneity is important for functional studies of mammalian carbohydrates. To facilitate such studies, several Chinese hamster ovary mutants that carry between two and four glycosylation mutations were developed. All of the lines grew readily in culture despite the drastic simplification of their surface carbohydrates. Therefore, both endogenous glycoproteins and those introduced by transfection can be obtained with specifically tailored carbohydrates. The lectin resistance properties of the mutants showed that each line expresses a novel array of cell surface carbohydrates useful for identifying specific roles for carbohydrates in cellular interactions. In addition, they showed that the epistatic relationships among different glycosylation mutations are not entirely predictable, providing insight into the complexity of the carbohydrate structures at the Chinese hamster ovary cell surface.

Secretion of a lactosaminoglycan-containing glycoprotein by peri-implantation sheep conceptuses

Sheep conceptuses from day 16 of pregnancy were cultured in the presence of [3H]glucosamine and [14C]leucine and a high-molecular-weight glycoprotein (HMWG) secreted into the culture medium was purified by a combination of anion-exchange and gel filtration chromatography. The HMWG was found to have a molecular weight between 800,000 and 900,000 and to be highly resistant to digestion with pronase. Characteristics of the carbohydrate portion of the purified glycoprotein were examined by selective chemical and enzymatic digestions and lectin binding studies. Mild alkaline reduction was ineffective in disassociating carbohydrate chains from the protein core. Furthermore, the protein was resistant to both O-glycanase and peptide:N-glycanase F. Harsh alkaline reduction caused the release of carbohydrates, however. After pronase digestion of these products, three molecular weight classes of carbohydrates were resolved by Sephadex G-25 chromatography. Two lines of evidence indicate that the HMWG contains lactosaminoglycan components. The intact molecule and two of the molecular weight classes of carbohydrates resolved by harsh alkaline reduction bind Datura stramonium lectin. Binding of HMWG to lectin could be partially inhibited by N-acetyllactosamine and completely inhibited by a mixture of N,N'-diacetylchitobiose and N,N',N"-triacetylchitotriose. Secondly, digestion with endo-beta-galactosidase causes the release of 16% of the [3H]glucosamine from the intact molecule. Therefore, the HMWG of the sheep conceptus is the first reported example of secretion of lactosaminoglycan-containing glycoprotein by peri-implantation embryos.

Polylactosaminoglycan modification of a small integral membrane glycoprotein, influenza B virus NB

The structure of the carbohydrate components of NB, the small integral membrane glycoprotein of influenza B virus, was investigated. The carbohydrate chains of NB are processed from the high-mannose form (NB18) to a heterogeneous form of much higher molecular weight, designated NBp. Selection of this carbohydrate-containing form of NB with Datura stramonium lectin, its susceptibility to digestion by endo-beta-galactosidase, and determination of the size of NBp glycopeptides by gel filtration chromatography suggested that the increase in molecular weight is due to processing to polylactosaminoglycan. Investigation of the polypeptides produced by influenza B/Lee/40 virus infection of several cell types and another strain of influenza B virus suggested that the signal for modification to polylactosaminoglycan is contained in NB. Expression of mutants of NB lacking either one or both of the normal N-terminal sites of asparagine-linked glycosylation indicated that both carbohydrate chains are modified to contain polylactosaminoglycan. NBp and a small amount of unprocessed NB18 are expressed at the infected-cell surface, as determined by digestion of the surfaces of intact cells with various endoglycosidases. Unglycosylated NB, expressed either in influenza B virus-infected cells treated with tunicamycin or in cells expressing the NB mutant lacking both N-linked glycosylation sites, was expressed at the cell surface, indicating that NB does not require carbohydrate addition for transport.

Use of Chinese hamster ovary cells with altered glycosylation patterns to define the carbohydrate specificity of Entamoeba histolytica adhesion

We compared the adherence of E. histolytica trophozoites with a panel of lectin-resistant CHO mutants with altered glycosylation patterns. Our results coupled with data from saccharide inhibition studies indicate that terminal N-acetyllactosamine units on Asn-linked complex type oligosaccharides provide the major determinants on the cellular receptor for E. histolytica adhesion.

Polylactosaminoglycan modification of a small integral membrane glycoprotein, influenza B virus NB

The structure of the carbohydrate components of NB, the small integral membrane glycoprotein of influenza B virus, was investigated. The carbohydrate chains of NB are processed from the high-mannose form (NB18) to a heterogeneous form of much higher molecular weight, designated NBp. Selection of this carbohydrate-containing form of NB with Datura stramonium lectin, its susceptibility to digestion by endo-beta-galactosidase, and determination of the size of NBp glycopeptides by gel filtration chromatography suggested that the increase in molecular weight is due to processing to polylactosaminoglycan. Investigation of the polypeptides produced by influenza B/Lee/40 virus infection of several cell types and another strain of influenza B virus suggested that the signal for modification to polylactosaminoglycan is contained in NB. Expression of mutants of NB lacking either one or both of the normal N-terminal sites of asparagine-linked glycosylation indicated that both carbohydrate chains are modified to contain polylactosaminoglycan. NBp and a small amount of unprocessed NB18 are expressed at the infected-cell surface, as determined by digestion of the surfaces of intact cells with various endoglycosidases. Unglycosylated NB, expressed either in influenza B virus-infected cells treated with tunicamycin or in cells expressing the NB mutant lacking both N-linked glycosylation sites, was expressed at the cell surface, indicating that NB does not require carbohydrate addition for transport.

Developmentally regulated expression of cell surface carbohydrates during mouse embryogenesis

Cell surface carbohydrates undergo marked alterations during mouse embryogenesis. In preimplantation embryos, many carbohydrate markers show stage-specific expression in diverse ways. In early postimplantation embryos, certain carbohydrate markers are localized in defined regions in the embryo. Important carriers of stage-specific carbohydrates are the lactoseries structure (Gal beta 1----4GlcNAc) and the globoseries structure (Gal alpha 1----4Gal). Notably, the glycoprotein-bound large carbohydrate of poly-N-acetyllactosamine-type ([Gal beta 1----4GlcNAc beta 1----3]n) carries a number of markers preferentially expressed in early embryonic cells. These markers are of practical value in analyzing embryogenesis and cell differentiation. For example, in order to monitor in vitro differentiation of multipotential embryonal carcinoma cells, stage-specific embryonic antigen-1 (SSEA-1) and the Lotus agglutinin receptor have been used as markers of the undifferentiated cells, and the Dolichos agglutinin receptor has been used as a marker of extraembryonic endoderm cells. Developmental control of cell surface carbohydrates is attained by controlled expression of activities of key glycosyltransferases; for example, the activity of N-acetylglucosaminide alpha 1----3 fucosyltransferase is lost during in vitro differentiation of embryonal carcinoma cells to parietal endoderm cells, in parallel to the disappearance of SSEA-1. Accumulating evidence suggests that poly-N-acetyllactosamine-type glycans that are abundant in early embryonic cells are involved in cell surface recognition of these cells.

Differentiation-associated decrease in the proportion of fucosylated polylactosaminoglycans of CaCo-2 human colonic adenocarcinoma cells

CaCo-2 cells are human colonic adenocarcinoma cells which can differentiate spontaneously into enterocytes when maintained confluent for extended periods of time. Cells kept in culture for 4 days (rapidly growing), 7-9 days (early confluence) and 19-22 days (late confluence) were incubated for 24 h with L-[5,6-3H]fucose or D-[6-3H]glucosamine in order to examine the changes in glycoprotein carbohydrate structure that occur during this differentiation. Labelled glycopeptides obtained by exhaustive Pronase digestion of the cell-surface and cell-pellet fractions were fractionated on Bio-Gel P-6. A high-Mr glycopeptide fraction which was excluded from Bio-Gel P-6 was present in all cases. These glycopeptides were then fractionated by affinity chromatography on Datura stramonium agglutinin-agarose. The glycopeptides which were specifically bound to the lectin column were largely degraded by endo-beta-galactosidase, thereby indicating that they consisted of fucosylated polylactosaminoglycans. The proportion of labelled polylactosaminoglycans decreased with increasing time in culture, whereas sucrase activity, which is characteristic of differentiated enterocytes, increased. These results demonstrate that a relatively large decrease in the proportion of fucosylated polylactosaminoglycans occurs with differentiation of CaCo-2 cells.

Biosynthesis of polylactosaminoglycans. Novikoff ascites tumor cells contain two UDP-GlcNAc:beta-galactoside beta 1----6-N-acetylglucosaminyltransferase activities

Novikoff ascites tumor cells contain a UDP-GlcNAc:beta-galactoside beta 1----6-N-acetylglucosaminyltransferase (beta 6-GlcNAc-transferase B) that acts on galactosides and N-acetylgalactosaminides in which the accepting sugar is beta 1----3 substituted by a Gal or GlcNAc residue. Characterization of enzyme products by 1H-NMR and methylation analysis indicates that an R beta 1----3(GlcNAc beta 1----6)Gal- branching point is formed such as occurs in blood-group-I-active substances. The enzyme does not show an absolute divalent cation requirement and 20 mM EDTA is not inhibitory. The activity is strongly inhibited by Triton X-100 at concentrations of greater than or equal to 0.2%. Competition studies suggest that a single enzyme acts on Gal beta 1----3Gal beta 1----4Glc, GlcNAc beta 1----3Gal beta 1----4GlcNAc and GlcNAc beta 1----3GalNAc alpha-O-benzyl (Km values 0.71, 0.83 and 0.53 mM, respectively). Gal beta----3Gal beta 1----4Glc as an acceptor substrate for beta 6-GlcNAc-transferase B does not inhibit the incorporation of GlcNAc in beta 1----6 linkage to the terminal Gal residues of asialo-alpha 1-acid glycoprotein catalyzed by a beta-galactoside beta 1----6-N-acetylglucosaminyltransferase (beta 6-GlcNAc-transferase A) previously described in Novikoff ascites tumor cells [D. H. Van den Eijnden, H. Winterwerp, P. Smeeman & W.E.C.M. Schiphorst (1983) J. Biol. Chem. 258, 3435-3437]. Neither is Triton X-100 at a concentration of 0.8% inhibitory for the activity of beta 6-GlcNAc-transferase A. This activity is absent from hog gastric mucosa microsomes, which has been described to contain high levels of beta 6-GlcNAc-transferase B. [F. Piller, J. P. Cartron, A. Maranduba, A. Veyrières, Y. Leroy & B. Fournet (1984) J. Biol. Chem. 259, 13,385-13,390]. Our results show that Novikoff tumor cells contain two beta-galactoside beta 6-GlcNAc-transferases, which differ in acceptor specificity and tolerance towards Triton X-100. A role for these enzymes in the synthesis of branched polylactosaminoglycans and of O-linked oligosaccharide core structures having blood-group I activity is proposed.

Isolation and characterization of a wheat germ agglutinin-binding glycoprotein from B16 mouse melanoma cells

Cells of B16 mouse melanoma grown in serum-free medium in the presence of [3H]glucosamine secreted or shed labeled glycoproteins. A wheat germ agglutinin-binding glycoprotein was isolated that accounted for 37% of the total [3H]glucosamine incorporated; it had a molecular weight of approximately 50,000 and was absent in less-tumorgenic wheat germ agglutinin (isolectin I)-resistant variants of the cells. The glycoprotein contained approximately 25% of serine and threonine plus equimolar amounts of glucosamine and galactosamine, indicating both N- and O-linked oligosaccharide chains. Neuraminidase treatment released approximately 60% of the glycoprotein's 3H radioactivity as N-acetylneuraminic acid. The sialoglycoprotein did not, but the desialylated species did, bind (97%) to ricin-Sepharose, suggesting the presence of terminal sialic acid and penultimate galactose residues in most of the saccharide units. Alkaline borohydride released 61% of the glycoprotein's radioactivity as oligosaccharide alcohols that were mainly tetrasaccharides (75%) with some branched trisaccharides (10%) from the O-linked structures. Hydrazinolysis and analysis of the alkaline borohydride-resistant portion of the glycoprotein indicated the presence of mainly triantennary, complex-type structures (N-linked) containing three sialic acids residues plus L-fucose. Serial lectin-affinity chromatography of the hydrazine-released saccharides with concanavalin A-agarose, pea lectin-agarose, L-PHA-agarose, and E-PHA-agarose, indicated the absence of high-mannose or hybrid-type structures and further confirmed the presence of triantennary, complex-type units.

Effects of tunicamycin, N-methyl-1-deoxynojirimycin, and manno-1-deoxynojirimycin on the biosynthesis of lactosaminoglycans in F9 teratocarcinoma cells

F9 teratocarcinoma cells were incubated with D-[2-3H]mannose or D-[6-3H]galactose, and the labeled glycopeptides obtained after exhaustive digestion by pronase were fractionated on Bio-Gel P-6 before and after treatment by endo-beta-N-acetylglucosaminidase H. Tunicamycin almost completely inhibited the synthesis of lactosaminoglycans found in excluded glycopeptides of large molecular weight. Manno-1-deoxynojirimycin greatly inhibited the incorporation of labeled mannose into both lactosaminoglycan and complex oligosaccharides, while it greatly increased that into Man8GlcNAc and Man9GlcNAc oligosaccharides. In contrast, N-methyl-1-deoxynojirimycin only partially inhibited the incorporation into lactosaminoglycan and complex oligosaccharides, and caused the accumulation of Glc3Man7-9GlcNAc oligosaccharides. These results demonstrate that, in these cells, lactosaminoglycans are N-linked, and suggest that there is transfer of both glucosylated and nonglucosylated oligosaccharides from lipid to protein.

Structures of the asparagine-linked sugar chains of laminin

This investigation describes the isolation and characterization of oligosaccharides of the basement membrane glycoprotein, laminin. Pronase-released glycopeptides of isolated laminin, from a mouse Engelbreth-Holm-Swarm tumor, were fractionated using a combination of gel permeation chromatography and Con A-Sepharose affinity chromatography. The glycopeptides were analyzed for sugar linkage patterns by methylation analysis. Glycopeptides and hydrazine-released oligosaccharides were further analyzed using endo-beta-galactosidase, endo-beta-N-acetylglucosaminidase H and specific exoglycosidases in conjunction with calibrated gel permeation chromatography. Based on these experiments, murine tumor laminin was shown to contain asparagine-linked oligosaccharides with the following structures: bi-, tri- and tetraantennary complex-type oligosaccharides; polylactosaminyl side chains containing Gal(beta 1----4)GlcNAc(beta 1----3) repeating units attached to the trimannose core portion of the bi-, tri- and tetraantennary complex-type oligosaccharides; unusual complex-type oligosaccharides terminated at the nonreducing end with sialic acid, alpha-galactose, beta-galactose and beta-N-acetylglucosamine; alpha-galactosyl residues linked to N-acetyllactosamine sequences; high-mannose-type oligosaccharides. These results, in conjunction with analytical data, indicate that most of the carbohydrate of this laminin is N-linked to asparagine and that there are about 43 such N-linked oligosaccharides per laminin molecule.

Poly-N-acetyllactosamine alterations of Thy-1 glycoprotein in lymphocyte differentiation

Developmental changes in murine lymphocyte Thy-1 include both quantitative and qualitative alterations involving N-linked oligosaccharides. Comparison of immature with mature T-cells has shown that the oligosaccharides of Thy-1 are characterized by an increase in the number of sialic acid residues responsible for the acidic pI of peripheral T-cell Thy-1, and a decrease in those oligosaccharides responsible for Mr heterogeneity of thymocyte Thy-1. The research reported here suggests that the basis of the large Mr heterogeneity in Thy-1 of immature T-cells is the presence of repeating N-acetyllactosamine (R'Gal beta 1,4GlcNAc beta 1,3R") units in the oligosaccharide portion of the molecule. Lymphocytes were surface iodinated and 125I-thy-1 was purified by immunoprecipitation and preparative nonequilibrium pH gradient electrophoresis. The minimal Mr of unglycosylated Thy-1 after endoglycosidase F digestion was 15,000-16,000. Digestion of Thy-1 with endo-beta-galactosidase suggested that the complex type N-linked glycans in thymocytes, but not in lymph node T-cells, contained increased levels of polylactosamine. The presence of polylactosamine was confirmed by binding to a Datura stramonium lectin column which retarded and bound approx. 50% of thymocyte Thy-1 and only about 18% of lymph node T-cell Thy-1. Affinity chromatography using anti-i antibody immobilized on agarose beads indicated that the polylactosamine is probably present in a predominantly linear form. Since alterations of polylactosamine structures have been implicated in development and transformation in several systems, the present results suggest an important role for these glycans in immune-cell differentiation.