Glycoproteins in culture medium: a comparison from cystic fibrosis and control skin fibroblasts

Terminal glycosylation in cystic fibrosis (CF): a review emphasizing the airway epithelial cell

Altered terminal glycosylation, with increased fucosylation and decreased sialylation is a hallmark of the cystic fibrosis (CF) glycosylation phenotype. Oligosaccharides purified from the surface membrane glycoconjugates of CF airway epithelial cells have the Lewis x, selectin ligand in terminal positions. This review is focused on the investigations of the glycoconjugates of the CF airway epithelial cell surface. Two of the major bacterial pathogens in CF, Pseudomonas aeruginosa and Haemophilus influenzae, have binding proteins which recognize fucose in alpha-1,3 linkage and asialoglycoconjugates. Therefore, consideration has been given to the possibility that the altered terminal glycosylation of airway epithelial glycoproteins in CF contributes to both the chronic infection and the robust, but ineffective, inflammatory response in the CF lung. Since the glycosylation phenotype of CF airway epithelial cells have been modulated by the expression of wtCFTR, the hypotheses which have been proposed to relate altered function of CFTR to the regulation of the glycosyltransferases are discussed. Understanding the effects of mutant CFTR on glycosylation may provide further insight into the regulation of glycoconjugate processing as well as new approaches to the therapy of CF.

Terminal glycosylation in cystic fibrosis

Cystic fibrosis (CF) is a common genetic disease for which the gene was identified within the last decade. Pulmonary disease predominates in this ultimately fatal disease and current therapy only slows the progression. CF transmembrane regulator (CFTR), the gene product, is an integral membrane glycoprotein that normally functions as a chloride channel in epithelial cells. The most common mutation, deltaF508, results in mislocalization and altered glycosylation of CFTR. Altered fucosylation and sialylation are hallmarks of both membrane and secreted glycoproteins in CF and the focus here is on these investigations. Oligosaccharides from CF membrane glycoproteins have the Lewis x, selectin ligand in terminal positions. In addition, two major bacterial pathogens in CF, Pseudomonas aeruginosa and Haemophilus influenzae, have binding proteins, which recognize fucose in alpha1,3 linkage and asialoglycoconjugates. We speculate that the altered terminal glycosylation of airway epithelial glycoproteins in CF contributes to the chronic infection and robust inflammatory response in the CF lung. Understanding the effects of mutant CFTR on glycosylation may provide further insight into the regulation of glycoconjugate processing as well as therapy for CF.

Increased monocyte oxidase activity in cystic fibrosis heterozygotes and homozygotes

Freshly isolated monocytes from cystic fibrosis (CF) heterozygotes and homozygotes had significantly increased oxygen uptake and superoxide formation after surface glycoprotein stimulation than did monocytes from age- and sex-matched controls. Lack of differences among the genotypes in inhibition by simple sugars of the concanavalin A-stimulated superoxide production and lack of differences in concanavalin A-binding surface proteins suggested that different regulation of the oxidase pathway produced the increased oxygen uptake and superoxide formation in CF patients and carriers. This regulatory role is consistent with the predicted structure of the CF gene product. The results support the hypothesis that the mononuclear phagocytes of CF heterozygotes have a significantly increased ability to kill intracellular microbes and may confer a selective advantage to the host.

Additional fucosyl residues on membrane glycoproteins but not a secreted glycoprotein from cystic fibrosis fibroblasts

Glycopeptides derived from peripheral membrane glycoproteins of skin fibroblasts of seven patients with cystic fibrosis (CF) had an increase in fucosyl residues when compared with those of seven age, race and sex matched controls (Pediatr Res 1985;19:368-374). To further define these results, the membrane glycopeptides which bound to immobilized lentil lectin and thereby enriched in fucosyl residues linked alpha 1----6 to N-acetylglucosamine attached to asparagine, were Pronase digested, partially purified and examined by 500-MHz 1H-NMR spectroscopy. The CF derived glycopeptides had two different features when compared to those from Controls (1) an increased number of fucosyl residues linked alpha 1----6 to the N-acetylglucosamine attached to asparagine and (2) fucosyl residues linked alpha 1----3 to a branch N-acetylglucosamine. The glycopeptides from both sources were of the di and triantennary type containing sialic acid linked alpha 2----3 and alpha 2----6 to galactose in an approximate molar ratio of 3:2 and 2:1, from CF and Control, respectively. Glycopeptides derived from a glycoprotein, fibronectin, secreted from CF fibroblasts were also examined by 1H-NMR spectroscopy and showed no evidence of fucosyl residues linked alpha 1----3 to branch N-acetylglucosamine and a lesser percentage of core fucose than found in the peripheral membrane glycopeptides. These results define further the altered fucosylation of the CF peripheral membrane glycoproteins.

Colorimetric assay for free and bound L-fucose

A novel, rapid, and reliable colorimetric method for measuring L-fucose has been developed. This method utilizes NADH formed from the interaction of L-fucose with fucose dehydrogenase and NAD to generate color in a reaction involving CuSO4 and neocuproine. NADH reduces Cu2+ to Cu1+ and the latter interacts with neocuproine to yield a complex with a maximal absorption at 455 nm. The reaction of NADH with copper-neocuproine is immediate and under the conditions of the assay the color formed remains stable for at least 2 h. When the assay is used to determine levels of L-fucose, the absorbance is found to be linearly proportional to exogenously added fucose concentrations from 16 to 179 nmol with resulting molar extinction coefficient of 13,660. Using this procedure, L-fucose released by acid hydrolysis from porcine submaxillary mucin, and by alpha-L-fucosidase from p-nitrophenyl-alpha-L-fucopyranoside, was quantitated.

Increased sulfate uptake in skin fibroblasts isolated from cystic fibrosis patients

Sulfate uptake into skin fibroblasts from patients with cystic fibrosis is increased. Sulfate transport studies were carried out in skin fibroblasts isolated from age/sex matched cystic fibrosis and normal subjects. Sulfate transport occurred mainly via a carrier-mediated proton-stimulated S04(2)-/Cl-exchange. The capacity (Vmax) of the uptake system operating at physiological concentrations of sulfate was stimulated in cystic fibrosis, but the affinity of the carrier for sulfate was not altered.

A quantitative method for GDP-L-Fuc:N-acetyl-beta-D-glucosaminide alpha 1----6fucosyltransferase activity with lectin affinity chromatography

A quantitative method for the activity of GDP-L-Fuc:N-acetyl-beta-D-glucosaminide alpha 1----6fucosyltransferase has been developed using a well-characterized substrate to which other fucosyltransferases fail to transfer and lentil lectin-Sepharose, which will bind this substrate only after fucosylation of the asparagine-linked N-acetylglucosamine. The enzyme was extracted from human skin fibroblasts and incubated with GDP-[14C]fucose and a specific substrate, asialo-agalactotransferrin glycopeptide. The product of the enzyme reaction, [14C]fucose alpha 1----6 to the asparagine-linked N-acetylglucosamine of the substrate, bound to lentil lectin-Sepharose and was eluted with 0.4 M methyl alpha-D mannopyranoside. The method was shown to be specific after characterization of the lentil lectin-bound glycopeptides by enzyme degradation and affinity chromatography. Quantitation of the method was shown by several parameters, including the linearity of product formed with respect to time, GDP-[14C]fucose concentration and enzyme concentration.

Characterization of cellular oligosaccharides from normal and cystic fibrotic fibroblasts using sequential endoglycosidase digestions

A method was developed for obtaining detailed oligosaccharide profiles from [2-3H]mannose- or [6-3H]fucose-labeled cellular glycoproteins. The oligosaccharides were segregated first according to class, using endo-beta-N-acetylglucosaminidase H (Endo H) to release the high mannose species, and then with peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase (PNGase F), which provided a complete array of complex oligosaccharide chains. The high mannose and complex oligosaccharides were fractionated subsequently according to net negative charge on QAE-Sephadex. High resolution gel filtration on TSK HW-40(S) resolved the neutral high mannose population into species of the type Man9-5 N-acetylglucosamine. Desialylation of the complex chains with neuraminidase allowed resolution of these oligosaccharides into their corresponding asialo bi-, tri-, and tetraantennary species. Fibroblasts from normal and cystic fibrosis cells were analyzed for differences in their glycosylation patterns using these techniques. Over 95% of the [2-3H]mannose-labeled glycoproteins were susceptible to the combined glycosidase digestions, but no difference in either the high mannose or complex oligosaccharides were observed. Nonetheless, the methodology developed in this study provides an important new approach for investigating oligosaccharides of different cell types and variants of the same type. Metabolic changes induced in cellular glycoproteins, as illustrated by use of the processing inhibitor swainsonine, demonstrated the versatility of this procedure for investigating questions relating to glycoprotein structure and enzyme specificity. Thus, by employing a variation of this method, it was possible to confirm the location of fucose in the core of PNGase F-released hybrid oligosaccharides by the subsequent release with Endo H of the disaccharide, fucosyl-N-acetylglucosamine.

Characterization of human fibronectin glycopeptides from cystic fibrosis and control skin fibroblasts

Fibronectins isolated from different species and tissue sources are glycosylated differently. We report here a characterization of the glycopeptides of fibronectin isolated from the culture medium of skin fibroblasts from patients with cystic fibrosis together with age-, race-, and sex-matched control subjects. The characterization of this fibronectin is of special interest because it is derived from: a non-fetal, cellular source; eight different individuals; and cystic fibrosis and control individuals. The fibronectin glycopeptides were purified by gel-permeation chromatography and Con A-Sepharose and were analyzed by anion-exchange chromatography and affinity columns of immobilized 5-hydroxytryptamine and lectins. One half of the glycopeptides of skin fibroblast fibronectin were shown to contain biantennary oligosaccharides which were core-fucosylated and partially sialylated. Although the remaining half was a complex mixture of glycopeptides, there was remarkably little inter-individual variation. No difference between cystic fibrosis and control subjects was discernible by the techniques employed here. Unlike the biantennary glycopeptides of human plasma fibronectin, those from skin fibroblast fibronectin were core-fucosylated and less highly sialylated. However, compared to human cellular fibronectin glycopeptides from fetal sources, those from skin fibroblast fibronectin were both more highly fucosylated and sialylated.

Glycosyl acceptors in intact and permeabilized normal and cystic fibrosis fibroblasts

Using cell permeabilization, a technique which allows addition of exogenously supplied radiolabeled sugar nucleotides to serve as direct glycosyl donors, oligosaccharide biosynthesis was examined in fibroblasts obtained from normal and cystic fibrosis (CF) subjects. Incubation of logarithmically growing cells with either radiolabeled leucine or xylose has indicated that there was a difference in the synthetic rate between the cell types. Protein synthesis in normal cells made permeable with 50 micrograms/ml lysolecithin (LL) was demonstrated to be absent, and could not be induced to take place by adding exogenous components, including energy sources and amino acids, normally required for protein synthesis. Thus radiolabeled sugars were being added to peptide acceptors which were already present at the time of LL addition. Both permeable and intact fibroblasts were exposed to labeled UDP-xylose, UDP-galactose, and UDP-glucuronic acid, all donors of mucopolysaccharide precursors. The uptake of xylose into protein was the same for both normal and CF cells, but permeable CF fibroblasts incorporated statistically greater amounts of sugar from UDP-galactose and UDP-glucuronic acid. Intact CF cells were also labeled using these two sugar nucleotides. Trypan blue exclusion indicated CF and normal fibroblasts were equally intact. This and the fact that preincubation of CF cells with the appropriate cold sugar nucleotide eliminated the differences in incorporation between the normal and CF cells suggested that CF fibroblasts had more cell surface acceptor than the normal cells.