Immunological characterization of deglycosylated human and swine trachea and Cowper's gland mucin glycoproteins

Transdifferentiation of outgrowth cells and cultured epithelial cells from swine trachea

The morphologic and functional properties of explant out-growth cells and epithelial cells isolated from swine trachea epithelium by proteolysis were examined. A mixed population of ciliated, serous, and basal cells, obtained from out-growths, from proteolysis of trachea epithelium, and from unattached explants in organ culture, all yielded cell cultures that werecomposed almost entirely of mucus-secreting cells. When the cells were grown in primary or secondary culture on a modified collagen matrix in supplemented HAM:DMEM (1:1) medium they expressed a mucus-secreting phenotype with numerous mucus granules at various stages of maturation and incorporated [3H]GlcN and 35SO4 into secreied mucin glycoproteins. Results obtained in these studies suggest that extensive transdifferentiation of ciliated and serous cells to mucus-secreting-cells occurs after the release and during subsequent attachment and culture. Ciliated cells containing mucus granules were seen in various stages of cilia resorption. Basal cells containing mucus granules were also frequently observed. The number of mucus-secreting cells and the synthesis of mucin glycoproteins increased dramatically with time of attachment and culture, whereas cell proliferation, population doubling time of 72 h, and incorporation of [3H]-thymidine into DNA increased much more slowly. The number of mucus-secreting cells correlated closely with the level of secretion of mucin glycoproteins. Taken collectively, these studies help to elucidate the transdifferentiation process, which dramatically increases the number of mucus-secreting cells after disruption and release of epithelial cells from swine tracheobronchial epithelium. A similar mechanism involving disruption of the extracellular matrix may be involved in the stimulation of hypersecretion of mucus and mucin glycoproteins by chemical and infections irritants.

Cloning, expression and properties of porcine trachea UDP-galnac: polypeptide N-acetylgalactosaminyl transferase

A UDP-GalNAc:polypeptide N-acetyl-galactosaminyl transferase which catalyses the transfer of GalNAc from UDP-GalNAc to serine and threonine residues in mucin polypeptide chains was purified to homogeneity from swine trachea epithelium (Mendicino J, Sangadala S: Mol Cell Biochem 185: 135-145, 1998). Peptides obtained by proteolysis of the purified enzyme were isolated, sequenced and used to prepare degenerate oligonucleotide primers. Amplified segments of a gene encoding GalNAc transferase were synthesised using the primers and a swine trachea epithelial cDNA library. Selected cDNA fragments were then used to screen the cDNA library, and a clone containing an open reading frame encoding 559 amino acids was isolated. The predicted amino acid sequence contains type II transmembrane region, three potential N-glycosylation sites as well as all of the isolated peptide sequences. The nucleotide sequence and predicted primary protein structure of the transferase were very similar to those of type T-1 GalNAc transferases. The isolated clone was transiently expressed in COS 7 cells and the recombinant enzyme, which contained an N-terminal hexa-histidine tag, was purified to homogeneity and its enzymatic properties were examined. The Vmax of the recombinant enzyme, 2.08 micromol/(min mg), was nearly the same as the native enzyme, 2.12 micromol/(min mg), when assayed with partially deglycosylated mucins as glycosyl acceptors. Both enzymes showed much higher activities when assayed with peptides prepared by limited acid hydrolysis of incompletely deglycosylated Cowper's gland, swine, and human respiratory mucins and tryptic peptides isolated from deglycosylated mucin polypeptide chains. However, as noted earlier (Mendicino J, Sangadala S: Mol Cell Biochem 185: 135-145, 1998), these enzymes showed very little activity with completely deglycosylated mucin polypeptide chains. When completely deglycosylated polypeptide chains were partially glycosylated by incubation with microsome preparations they were again good glycosyl acceptors for the T1-GalNAc transferases isolated from swine trachea. These results show for the first time that multiple isoforms of GalNAc transferases acting in sequence may be required for the complete O-glycosylation of mucin polypeptide chains, and those acting on the nacent polypeptide chain synthesize intermediates which can serve as glycosyl acceptors for other isoforms of the enzyme.

Synthesis of sulfated oligosaccharides by cystic fibrosis trachea epithelial cells

The mucin glycoproteins in tracheal mucus of patients with cystic fibrosis is more highly sulfated than the corresponding secretions from healthy individuals [16]. In order to further characterize these differences in sulfation and possibly also glycosylation patterns, we compared the structures of sulfated mucin oligosaccharides synthesized by continuously cultured human tracheal cells transformed by simian virus 40. The synthesis of highly sulfated oligosaccharide chains in mucins secreted by normal human epithelial and submucosal cell lines were compared with mucins formed by cystic fibrosis tracheal epithelial and submucosal cell lines. The epithelial cell lines from cystic fibrosis trachea showed a higher rate of sulfate uptake and a significantly higher rate of synthesis and sulfation of high molecular weight chains. Mucins synthesized by each cell line in the presence of 35SO4 were isolated and oligosaccharide chains were released by beta-elimination and separated by ion exchange chromatography and gel filtration. The sulfated high molecular weight chains synthesized by the cystic fibrosis cell lines were characterized by methylation analysis and sequential glycosidase digestion before and after desulfation. Carbohydrate analysis yielded Fuc, Gal and GlcNAc in a ratio of 1:2:2.2 and only one galactosaminitol residue for about every 150-200 sugar residues present. The average molecular size of oligosaccharide chains in these fractions was between 30,000-40,000 daltons. These studies show that increased sulfation of oligosaccharides in mucins synthesized by cells from cystic fibrosis trachea is accompanied by a significant increase in the extension of a basic branched structure present in many of the lower molecular weight oligosaccharides.

Purification and characterization of UDP-GalNAc:polypeptide N-acetylgalactosaminyl transferase from swine trachea epithelium

UDP-GalNac: polypeptide N-acetylgalactosaminyltransferase from swine trachea epithelium was purified to homogeneity by procedures which included affinity chromatography on Sepharose 4B columns containing bound deglycosylated Cowper's gland mucin. The enzyme, purified 12,000-fold from microsomes with a yield of 40%, showed only a single band on dodecyl sulfate polyacrylamide gel electrophoresis. The homogenous enzyme has an apparent molecular mass of 70,000 Da, as determined by gel electrophoresis or gel filtration. The transferase has a broad pH optimum between 6.7-7.8 with maximal activity at pH 7.2, and required Mn2+ for activity with maximal activity at 5-7.5 mM. Higher concentrations of Mn2+, inhibited the enzyme. The purified transferase was specific for UDPGalNAc and glycosylated both threonine and serine residues in tryptic peptides prepared from deglycosylated Cowper's gland and swine and human trachea mucins. The apparent Km of the transferase for UDPGalNAc was 6.3 microM, and the Km values for deglycosylated Cowper's gland and human and swine trachea mucins were 0.83, 1.12 and 0.94 mg/ml, respectively. The Vmax of the purified enzyme was 2.1 micromol/min/mg with deglycosylated Cowper's gland mucin, as the glycosyl acceptor. However, the activities with peptides prepared from deglycosylated mucins by limited acid hydrolysis were 20-fold greater than the intact glycoprotein under identical conditions. The deglycosylated mucins and larger peptides aggregated with time of storage and precipitated from solution. Aggregation was accompanied by a corresponding loss of enzymatic activity even after dispersion of the aggregate by sonication. The deglycosylated mucins which were prepared by chemical treatment and periodate oxidation still contained about 20% of the N-acetylgalactosamine present in the intact mucin. When this residual amino sugar was removed by periodate oxidation the completely deglycosylated mucins became very poor substrates for the purified transferase. Data obtained in the current study indicate that the accessibility of serine and threonine in the polypeptide chains of mucin glycoproteins significantly influences the rate of glycosylation of these amino acids. The best substrates and affinity ligand for the enzyme were fragments of incompletely deglycosylated mucin polypeptide chains.

Characterization of mucin glycoprotein-specific translation products from swine and human trachea, pancreas and colon

RNA was isolated from cultured swine trachea epithelial cells and mucus-secreting tumor cell lines from human pancreas, lung and colon by extraction with guanidine isothiocyanate. Poly(A)+mRNA rich fractions were purified by repeated chromatography on oligo (dT)-cellulose columns and they were translated in a cell-free rabbit reticulocyte system. Translation products labelled with 35S-methionine were isolated by immunoprecipitation with specific antibodies to the polypeptide chains of mucin glycoproteins and they were analyzed by SDS-PAGE and fluorography. A single principal polypeptide band of 67 kDa was found in all cases when the immunoprecipitates were washed with buffer containing bovine serum albumin and unlabeled deglycosylated mucin glycoprotein. The intensity of the 67 kDa band decreased when unlabeled deglycosylated mucin glycoprotein was added to the translation mixture before immunoprecipitation. Affinity purified monospecific antibodies elicited against chemically deglycosylated polypeptide chains of purified mucin glycoproteins from human and swine trachea and Cowper's gland were all equally effective in immunoprecipitating the 67 kDa translation product. Monospecific antibodies directed against the glycosylated and unglycosylated regions of the polypeptide chain yielded single bands with a molecular size of 67 kDa in each case. Peptide profiles obtained by digestion of the 67 kDa translation product with S. aureus V-8 protease were identical to those obtained with deglycosylated human and swine trachea mucin glycoproteins. These studies clearly demonstrate that the translation product of swine trachea and human lung, colon and pancreatic mucin glycoprotein gene is a single polypeptide chain of 67 kDa. The relative size and properties of the translation products synthesized with poly (A)+RNA isolated from mucus-secreting cells derived from three different tissues are similar to those of mucin glycoproteins purified directly from mucus secretions of human and swine trachea epithelium.