Sulphation causes heterogeneity of gastric mucins

Gastrointestinal expression and partial cDNA cloning of murine Muc2

To help us investigate the role of mucin in the protection of the colonic epithelium in the mouse, we aimed to identify the murine colonic mucin (MCM) and its encoding gene. We isolated MCM, raised an anti-MCM antiserum, and studied the biosynthesis of MCM in the gastrointestinal tract. Isolated MCM resembled other mucins in physicochemical properties. Anti-MCM recognized MCM as well as rat and human MUC2 on Western blots, interacting primarily with peptide epitopes, indicating that MCM was identical to murine Muc2. Using anti-MCM and previously characterized anti-human and anti-rat MUC2 antibodies, we identified a murine Muc2 precursor in the colon of approximately 600 kDa, which appeared similar in size to rat and human MUC2 precursors. Western blotting, immunoprecipitation of metabolically labeled mucins, and immunohistochemistry showed that murine Muc2 was expressed in the colon and the small intestine but was absent in the stomach. To independently identify murine Muc2, we cloned a cDNA fragment from murine colonic mRNA, encoding the 302 NH2-terminal amino acids of murine Muc2. The NH2 terminus of murine Muc2 showed 86 and 75% identity to the corresponding rat and human MUC2 peptide sequences, respectively. Northern blotting with a murine Muc2 cDNA probe showed hybridization to a very large mRNA, which was expressed highly in the colon and to some extend in the small intestine but was absent in the stomach. In situ hybridization showed that the murine Muc2 mRNA was confined to intestinal goblet cells. In conclusion, by two independent sets of experiments we identified murine Muc2, which appears homologous to rat and human MUC2. Because Muc2 is prominently expressed in the colon, it is most likely to be the predominant mucin in the colonic mucus layer.

Strategic biochemical analysis of mucins

MUC-type mucins comprise a family of structurally related molecules, which are expressed in epithelia of the body that are in close contact with the milieu. Because of their large sizes and very complex structures, containing very extensive O-glycosylation, MUC-type mucins are difficult to study by conventional techniques. Many see MUC-type mucins as protective molecules; however, functional studies on the individual MUC-type mucins are very scarce. At present, essential steps in MUC research are to characterize the specific expression patterns of each MUC-type mucin in the body and to find methods to reliably quantify these MUC-type mucins. These aims can only be met at the level of the primary sequences of the MUC-type mucins, as the O-glycosylation even within one species of MUC-type mucin is not only very complex, but may also vary among individuals, organs, and cell types. We will discuss some recent advances in mucin research, particularly the identification of MUC precursor molecules in metabolic labeling experiments. We will try to define some strategic considerations in the study of the expression patterns of MUC-type mucins, which circumvent the complications caused by the very complex and heterogeneous O-glycosylation of the molecules.

Biosynthesis of human colonic mucin: Muc2 is the prominent secretory mucin

BACKGROUND/AIMS

Human colonic epithelium produces large amounts of mucin. The aim of this study was to examine mucin biosynthesis in the human colon.

METHODS

Human colonic mucin was isolated using CsCl density gradients, and polyclonal antiserum was raised. Biosynthesis of colonic mucins was studied by labeling colonic explants with 35S-labeled amino acids or [35S]sulfate and subsequent immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE).

RESULTS

The polyclonal antiserum specifically recognized colonic mucin, primarily reacting with peptide epitopes. Biosynthetic pulse/chase experiments showed a 35S-amino acid-labeled mucin precursor of about 600 kilodaltons, which was converted into a mature, glycosylated, and sulfated mucin and subsequently secreted into the medium. This mature mucin comigrated with isolated colonic mucin with an apparent molecular weight of 550 kilodaltons on SDS-PAGE, whereas gel filtration indicated that the molecular weight is actually much larger. Independent immunoprecipitation with an anti-Muc2 antiserum showed cross-reactivity with the 600-kilodalton precursor.

CONCLUSIONS

These results show the biosynthesis of a secretory colonic mucin for the first time. This mucin is synthesized as a precursor protein of approximately 600 kilodaltons, which, after glycosylation, is secreted as a glycoprotein with an apparent molecular weight of 550 kilodaltons on SDS-PAGE. It is very likely that this mucin is Muc2.

Isolation and partial characterization of serine- and threonine-rich porcine gastric mucus glycopeptides

1. Two subfractions from purified porcine gastric mucus glycopeptide were found to separate from each other by cesium chloride equilibrium centrifugation. The highest density fraction and two lower density fractions separated were designated VHD, HD and LD, respectively. A comparative study of these components was made. 2. The high and low density fractions, HD and LD, appeared almost the same or identical, while VHD differed completely from either of them in the following respects: (1) VHD exhibited strong alcian blue binding activity. (2) 57% of VHD bound to the DEAE-Toyopearl column equilibrated with 0.2 M NaCl. (3) VHD eluted from the Sephacryl S-400 column as a lower molecular subunit. (4) One third of the sialic acid as a minor component in VHD was constituted by N-glycolylneuraminic acid. (5) Carbohydrate composition showed typical mucus glycoprotein with slightly higher fucose content. (6) Amino acid compositions of the anionic components prepared from VHD showed the highest Ser/Thr ratio, 1.92 compared to 0.46 for LD and 0.62 for HD. (7) Oligosaccharide released from VHD by alkaline-sodium borohydride treatment was larger than that from HD or LD. 3. The above results indicate the minor component, VHD, separated from the major components, to be a quite similar but not identical component to the so-called sulfated mucus glycoprotein reported previously [Slomiany et al. (1972) J. biol. Chem. 247, 5062-5070].

Role of sulfation in post-translational processing of gastric mucins

1. Gastric mucosal segments were incubated in MEM supplemented with various sulfate concentrations in the presence of [3H]glucosamine, [3H]proline and [35S]Na2SO4, with and without chlorate, an inhibitor of 3'-phosphoadenosine-5'-phosphosulfate formation. 2. Incorporation of glucosamine and sulfate depended upon the sulfate content of the medium and reached a maximum at 300 microM sulfate. Introduction of chlorate into the medium, while having no effect on protein synthesis as evidenced by proline incorporation, caused, at its optimal concentration of 2 mM, a 90% decrease in mucin sulfation and a 40% drop in glycosylation. 3. At low sulfate content in the medium and in the presence of chlorate, the incorporation of sulfate and glucosamine was mainly into the low molecular-weight form of mucin. An increase in sulfate in the medium caused an increase in the high molecular-weight form of mucin and in the extent of sulfation in its carbohydrate chain. 4. The results suggest that the sulfation process is an early event taking place at the stage of mucin subunit assembly and that sulfate availability is essential for the formation of the high molecular-weight mucin polymer.

Isolation of different high-Mr mucin species from human whole saliva

By using CsCl-density-gradient ultracentrifugation, two high-Mr mucin species were isolated from human whole saliva, having buoyant densities in 0.2 M-guanidinium chloride of approx. 1.56 g/ml (pool IA) and 1.48 g/ml (pool IIA). Analytical density-gradient centrifugation of submandibular, sublingual, labial and palatal saliva, followed by immunochemical analysis with anti-mucin monoclonal antibodies, indicated immunochemical and physicochemical similarities between the high-density mucins of pool IA and mucins from palatal salivary glands. Chemical analysis indicated that the putative palatal mucin was rich in sulphate, but poor in sialic acid. The lower-density mucins of pool IIA equated with the high-Mr mucins of submandibular-sublingual saliva, both immunochemically and physicochemically (buoyant density).

Mucin-type glycoproteins

Considerable advances have been made in recent years in our understanding of the biochemistry of mucin-type glycoproteins. This class of compounds is characterized mainly by a high level of O-linked oligosaccharides. Initially, the glycoproteins were solely known as the major constituents of mucus. Recent studies have shown that mucins from the gastrointestinal tract, lungs, salivary glands, sweat glands, breast, and tumor cells are structurally related to high-molecular-weight glycoproteins, which are produced by epithelial cells as membrane proteins. During mucin synthesis, an orchestrated sequence of events results in giant molecules of Mr 4 to 6 x 10(6), which are stored in mucous granules until secretion. Once secreted, mucin forms a barrier, not only to protect the delicate epithelial cells against the extracellular environment, but also to select substances for binding and uptake by these epithelia. This review is designed to critically examine relations between structure and function of the different compounds categorized as mucin glycoproteins.

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.

The oligomeric structure of rat and human gastric mucins

Intact oligomeric gastric mucins were isolated from the fundic part of rat and human stomach. Physicochemical properties of the oligomeric mucins from both species, such as buoyant density, molecular mass, proteinase-resistance, amino acid composition and monosaccharide composition were similar. Biochemical analysis showed that the oligomeric mucins from both species consist of disulphide-linked mucin monomers exclusively: no other covalently attached proteins were detected in purified monomeric mucin. Four major differences were found between the monomeric mucins of these species: (1) the human monomer is larger, (2) the proteolytic-digest peptides derived from proteinase-sensitive portions of the polypeptide backbone displayed no sequence similarity, (3) the human mucin was less sulphated compared with rat mucin, and (4) the proteinase-sensitive part of the human mucin was relatively larger. However, analyses of [3H]galactose-labelled mucin from both species on gel filtration revealed that both gastric mucins were exclusively synthesized as oligomers. The results indicate that the oligomeric structures of human and rat gastric mucin are similar and their biosyntheses are not affected by the differences in the subunits.

Sulfated O-glycoproteins secreted by guinea pig trachea in organ culture

Organ culture of guinea pig trachea was performed in the presence of [35S]sulfate in order to characterize the sulfated glycoproteins released from the respiratory epithelium and mucosa. The sulfated macromolecules that were synthesized during a 6-h incorporation were separated by CsBr density-gradient centrifugation and gel-filtration chromatography successively. Most of the sulfated secreted macromolecules corresponded to a population of glycoproteins sensitive to reductive beta-elimination but resistant to both chondroitinase ABC and heparinase. These glycoproteins had different buoyant densities (ranging from 1.48 g/ml to 1.16 g/ml) and could be subfractionated according to molecular mass. A major part of the radioactivity was incorporated into high-molecular-mass mucins that were excluded from a Sepharose CL-2B column and did not penetrate into polyacrylamide gel in PAGE. However, a mixture of sulfated O-glycoproteins of much lower molecular mass was also characterized in addition to low amounts of chondroitin sulfate. Epithelial goblet cells are the predominant mucin-containing cells of the respiratory guinea pig trachea. Our results suggest that a wide range of sulfated O-glycoproteins are secreted by the guinea pig tracheal mucosa.

Role of sulphation in post-translational processing of rat salivary mucins

Segments of rat submandibular salivary gland were incubated in MEM supplemented with 10-800 microM sulphate in the presence of [3H]-glucosamine, [3H]-proline and [35S]-Na2SO4, with 0-8 mM chlorate, an inhibitor of 3'-phosphoadenosine-5'-phosphosulphate formation. Incorporation of glucosamine and sulphate depended upon the sulphate content of the medium and reached a maximum at 400 microM sulphate. The introduction of chlorate into the medium, while having no effect on the protein synthesis as shown by [3H]-proline incorporation, caused, at its optimal concentration of 4 mM, a 90% decrease in mucin sulphation and a 29% drop in mucin glycosylation. At low sulphate content in the medium and in the presence of chlorate the incorporation of sulphate and glucosamine was mainly into the low molecular-weight form of mucin. An increase in sulphate in the medium caused an increase in the high molecular-weight form of mucin and in the extent of sulphation in its carbohydrate chain. This effect of sulphate was, however, inhibited by chlorate. The results suggest that sulphation takes place at an early stage of mucin assembly and that sulphate availability is essential for the formation of the high molecular-weight mucin.

Analysis of mucin-derived oligosaccharides by medium-pressure gel-permeation and amino-plate thin-layer chromatography conducted in conjunction

Oligosaccharides present in mucin were labeled by reduction with NaB3H4 and separated by gel-permeation chromatography with a Toyopearl HW-40S column using 0.1 M pyridine acetate, pH 5.0, as the solvent. Each fraction was further analyzed by thin-layer chromatography (TLC) on a Funagel AMP plate, a glass plate precoated with 3-aminopropyl-bonded silica. Acetonitrile/10 mM triethylamine acetate (3/2, by volume) served as the solvent. The sites of oligosaccharides on the TLC plate could be determined according to size, anionic charge, and sugar composition. They could thus be "mapped" on the plate. In this manner, the distribution of oligosaccharides on bovine submaxillary mucin and rat gastric mucin was determined. Each radiolabeled oligosaccharide in newly synthesized rat gastric mucin, metabolically labeled with [14C]glucosamine or [35S]sulfate, was also identified by this method.

Regulation of mucus secretion by cells isolated from the rat gastric mucosa

1. A suspension of cells, containing about 30% mucous cells, was isolated from the rat fundic mucosa, and was pre-incubated with D-[6-3H]glucosamine. 3H-labelled material subsequently released into the medium was separated by Fast Protein Liquid Chromatography on a Superose 6 column. 2. A sharp peak of labelled high molecular weight material eluted from the column close to the void volume. This material was identified as mucous glycoprotein by its similar chromatographic behaviour to partially purified rat gastric mucous glycoprotein, by its resistance to complete degradation by papain and by its behaviour on treatment with dithiothreitol. On a caesium chloride density gradient the labelled material was virtually all located between densities of 1.35 and 1.53 g/ml. with the main peak at 1.40 g/ml. 3. A broad peak of lower molecular weight material was also eluted from the column. The release of this unidentified material did not seem to be closely associated with the release of mucous glycoprotein from the cells. 4. Release of mucous glycoprotein was stimulated by secretin (half-maximally effective concentration 2.3 nM, 84% stimulation above basal release at 100 nM), and by isoprenaline (half-maximally effective concentration 34 nM, 33% stimulation at 1 microM). Carbachol (0.5 nM) produced a significant (18-29%) stimulation of mucus secretion, but gastrin (100 nM), histamine (0.5 mM) and epidermal growth factor (200 nM) were without effect. 5. The preparation should prove useful in the identification of the agents which regulate gastric mucus secretion.

Types of oligosaccharide sulphation, depending on mucus glycoprotein source, corpus or antral, in rat stomach

Radiolabelled mucus glycoprotein was obtained from tissue and a culture medium each of the corpus and antrum of rat stomach incubated with [35S]sulphate in vitro. Gel-filtration analysis of oligosaccharides liberated by alkaline-borohydride treatment from glycoproteins indicated that 35S-labelled oligosaccharides from the corpus vary considerably with respect to chain length whereas those from antral mucus glycoprotein are composed of small oligosaccharides. Examination of the reduced radiolabelled products obtained by HNO2 cleavage of the hydrazine-treated oligosaccharides indicated sulphate esters of N-acetylglucosamine to be present at three locations on a carbohydrate unit: [35S]sulphated monosaccharide (2,5-anhydromannitol 6-sulphate), [35S]sulphated disaccharide [galactosyl(beta 1-4)-2,5-anhydromannitol 6-sulphate] and [35S]sulphated trisaccharide [fucosyl(alpha 1-2)-galactosyl(beta 1-4)-2,5-anhydromannitol 6-sulphate]. Sulphated disaccharide and trisaccharide, possibly originating from the N-acetyl-lactosamine and fucosyl-N-acetyl-lactosamine sequences respectively, were detected in the corpus, especially as large oligosaccharides, but were present in the antrum in only very small amounts. The sulphated monosaccharide, however, most probably originating from 6-sulphated N-acetylglucosamine residues at non-reducing termini, was present in all oligosaccharide fractions in both the corpus and antrum.

Isolation and structural analysis of rat gastric mucus glycoprotein suggests a homogeneous protein backbone

We isolated monomeric gastric mucus glycoprotein from the rat stomach by applying three successive CsCl-density-gradient steps in the continuous presence of guanidinium chloride. The rat gastric mucin was pure as compared with mucin isolated without the chaotropic reagent. In addition, the presence of guanidinium chloride resulted in a better preservation of the protein moiety. The purified mucin was fractionated according to buoyant density and chemically radiolabelled on tyrosine or cysteine residues and digested with specific proteinases. Analysis of mucin fractions of various densities gave identical peptide patterns, suggesting that the fractions contain a common protein backbone. Electron-microscopic images of the individual mucin molecules were recorded using rotary shadowing. They showed large filamentous molecules with a mean length of 208 nm that, after proteolytic digestion, yielded glycopeptides with a mean length of 149 nm. Heterogeneity in buoyant density and electrophoretic mobility is located in this large glycopeptide which remains after proteolytic digestion. Metabolic labelling of the mucin with [35 S]sulphate and [3H]galactose, followed by purification and proteolytic digestion, revealed that this glycopeptide accounts for most of the mass and contains relatively little protein, but probably all the oligosaccharides and sulphate. As this protein part is masked by the oligosaccharides, detailed study by the methods described was not possible. The results indicate that rat gastric mucin is homogeneous in a major part of the protein backbone and that the heterogeneity of the molecule originates most likely from differences in sulphate and/or sugar composition.