Purification and immunofluorescent localization of rat submandibular mucin

Purified mucins in drug delivery research

One of the main challenges in the field of drug delivery remains the development of strategies to efficiently transport pharmaceuticals across mucus barriers, which regulate the passage and retention of molecules and particles in all luminal spaces of the body. A thorough understanding of the molecular mechanisms, which govern such selective permeability, is key for achieving efficient translocation of drugs and drug carriers. For this purpose, model systems based on purified mucins can contribute valuable information. In this review, we summarize advances that were made in the field of drug delivery research with such mucin-based model systems: First, we give an overview of mucin purification procedures and discuss the suitability of model systems reconstituted from purified mucins to mimic native mucus. Then, we summarize techniques to study mucin binding. Finally, we highlight approaches that made use of mucins as building blocks for drug delivery platforms or employ mucins as active compounds.

Lectin histochemistry of glycoconjugates in horse salivary glands

The glycoconjugate content of major horse salivary glands was investigated by means of horseradish peroxidase-conjugated lectins. Qualitative differences were observed in the terminal sugar residues of secretory glycoproteins and glycoconjugates linked to the apical surface of excretory duct epithelial cells. Mucous acinar cells in mandibular and sublingual glands contained oligosaccharides with D-galactose, alpha- and beta-N-acetylgalactosamine, N-acetylglucosamine and fucose residues, whereas mandibular, sublingual and parotid serous cells contained only oligosaccharides with terminal alpha- and beta-N-acetylgalactosamine residues. The apical portion of striated and interlobular duct lining cells of mandibular and sublingual glands stained for alpha- and beta-N-acetylgalactosamine and for N-acetylglucosamine. In parotid gland the cytoplasm of intercalated duct cells and the apical surface of striated duct epithelial cells stained for alpha-N-acetylgalactosamine.

Determination in situ of neutral and acidic fucose-containing oligosaccharides in the bovine submandibular gland

Lectins conjugated to horseradish peroxidase in combination with selected exoglycosidase digestion procedures were used to localize fucoglycoconjugates in the bovine submandibular gland. In particular, sequential treatments were employed to determine the distribution of neutral and acidic fucose-containing oligosaccharides that were previously shown to be present by biochemical techniques. Information was obtained on the distribution of the acidic oligosaccharide A-1a, alpha-Fuc(1-->2)-beta-Gal-(1-->4)-beta-GlcNAc-(1-->3)-[alpha-NeuAc-(2--> 6)]-GalNAc-ol, which was sequenced in situ and localized in acinar cells.

Biochemical and immunological properties of the human carcinoma antigen CAR-5 defined by the monoclonal antibody BD-5

The monoclonal antibody (MAb) BD-5 reacts with an epitope (CAR-5) expressed in 83% of the gastric carcinomas and in 51% of the ductal pancreatic carcinomas. This MAb reacts also with epithelial cells of colorectal mucosa, but does not react at all with normal adult gastric mucosa or normal adult pancreas. We report the biochemical and immunochemical characterization of CAR-5-bearing molecule. The epitope was found to be carried on a mucin of more than 400 kDa with a density of 1.45 g/ml, metabolically labelled with 35S-sulfate, 3H-glucosamine, 3H-mannose and 35S-methionine. Antigenicity survived metaperiodate oxidation and alkalinization, while it was fully destroyed by pronase or papain. Trypsin, although cleaving the molecule, did not affect its antigenic activity. CAR-5 epitope is thus carried on the protein moiety of a sulfo-mucin. On the basis of its biochemical properties, the antigen was purified by a 3-step procedure, consisting of perchloric acid extraction, molecular sieving on Sepharose CL-4B and affinity chromatography on wheat-germ agglutinin coupled to Sepharose 4B. Cross-competition experiments, together with the chemical properties displayed by the different epitopes, clearly indicate that CAR-5 is different from all previously characterized carcinoma-associated determinants. Cross-DDIRMA experiments performed with different "catcher" and "tracer" antibody combinations showed that CAR-5 epitope may be expressed on the same mucin bearing CA 19-9, MOv2, DU-PAN-2, Lewisa and Lewisb epitopes.

Biochemistry and lectin binding properties of mammalian salivary mucous glycoproteins

The molecules responsible for the highly viscous properties of mucus are secretory glycoproteins referred to as mucins. Salivary mucins are characterized by a high sugar to protein ratio and are of a broad range of molecular weight from 7 x 10(4) to millions. With a few exceptions, they contain up to 30% of hexosamine (galactosamine and glucosamine), 8-33% of sialic acid, trace to 15% of galactose or fucose and little or no mannose. The size of carbohydrate side chains of these glycoproteins ranges from one to about fifteen units of sugar. These carbohydrate side chains are usually O-glycosidically linked through N-acetylgalactosamine to a peptidyl serine or threonine. In some instances, ester sulfate groups, mainly on N-acetylglucosamine, are also a structural feature. In many of these glycoproteins, the saccharide sequence is the same as that which determines the specificity of blood groups. Carbohydrate sequence analysis shows that salivary mucins exhibit considerable polydispersity, great diversity and remarkable structural flexibility not only among animal species but also within the same mucin molecule. Based on their lectin-binding ability, they can be used for purification of lectins, and lectins coupled to resin may be useful for the isolation of mucin-type glycoproteins. The epithelial mucous secretions modulate oral microbial flora; many secretory components serve as lectin-receptors for the attachment of microbes. The judicious use of lectins with widely differing binding characteristics has already been valuable in the in situ localization of salivary glycoproteins, in elucidating structural details, recording sugar density within a given tissue section, and defining host-parasite interactions. It is hoped that their use, together with monoclonal antibody (158) and tissue culture techniques (159, 160) will further clarify the roles of individual secretory mucous glycoproteins in health and disease.

Primary structure of O- and N-glycosylic carbohydrate chains derived from murine submandibular mucin (MSM)

The carbohydrate moiety of mouse submandibular mucin (MSM) contains mainly D-mannose and 2-acetamido-2-deoxy-D-glucose together with sialic acid, D-galactose, and 2-acetamido-2-deoxy-D-galactose. O-Glycosylically bound saccharides, obtained by treatment of MSM with alkaline borohydride, were shown by methylation analysis to have the structure: alpha-NeuAc-(2----3)-beta-Gal-(1----3)-GalNAc-ol. N-Glycosylically bound saccharides obtained from MSM by hydrazinolysis, and analysed by 500-MHz 1H-n.m.r. spectroscopy, were shown to have the following comprehensive structures. (Formula: see text).

Genetic and sex-related differences in the structure of submandibular glycoconjugates

Structures of oligosaccharides in submandibular glycoproteins were evaluated in situ. Sections of fixed paraffin-embedded glands from rats, mice, hamsters, sheep, and man were stained with a battery of lectins conjugated to horseradish peroxidase in conjunction with other methods, such as digestion with sialidase with or without prior saponification and/or periodate oxidation. Secretory glycoproteins showed a characteristic lectin binding pattern for each genus. Sialoglycoconjugates were detected in acinar cell secretions in all genera except the rat but differed with respect to the linkage of sialic acid to penultimate beta-galactose or alpha-N-acetylgalactosamine. Species and strains of mice showed minor differences in the structure of secretory glycoproteins. Sexes differed similarly in some but not other mouse species. Individual differences were seen in human glands, where oligosaccharide structure varied in relation to ABO blood group. In some species, heterogeneity in glycoprotein structure was observed among morphologically similar cells within a gland. Differences in the structure of salivary secretions between genera and between humans of different ABO blood type and secretor status substantiate biochemical and histochemical findings. The results showing species, sex, and individual differences in mice and heterogeneity in acinar cells in several species suggest a greater degree of genetic and perhaps hormonal influence on the synthesis of salivary glycoproteins than has previously been recognized.

Effects of a phorbol ester and diacylglycerols on secretion of mucin and arginine esterase by rat submandibular gland cells

The effects of a phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and a diacylglyceride, 1-oleoyl-2-acetyl-glycerol (OAG) on the secretion of two major exocrine products by dispersed rat submandibular cells were investigated. TPA stimulated the release of acinar cell mucin and ductal cell protease (arginine esterase) in a dose- and time-dependent manner. Mucin secretion was also provoked by OAG, which, however, had no effect on arginine esterase release. The unsaturated diacylglycerol, 1,2-diolein, elicited a greater mucosecretory response than did OAG at the same concentration, while the saturated 1,2-distearin produced a smaller response. Mucin and enzyme secretion caused by TPA or OAG in the rat submandibular model was not inhibited by either of two putative antagonists, the antipsychotic drug, fluphenazine, and the antibiotic, polymyxin B. The involvement of extracellular Ca2+ in TPA-induced secretion was examined by comparing responses of cells maintained in normal or Ca2+-free medium, or in medium containing the ionophore A23187. Although extracellular Ca2+ was not an absolute requirement for a secretory response, the results indicate a synergistic relationship between TPA and Ca2+ in stimulating the release of both mucin and arginine esterase. These results suggest a role for the Ca2+-, phospholipid-dependent enzyme, protein kinase C in the secretory mechanism of mucous and serous cells in the submandibular gland. This is consistent with the proposal that receptor-mediated hydrolysis of membrane phosphoinositides is an initial event in stimulus-response coupling in exocrine cells.

Isolation and characterization of a mucin-glycoprotein from rat submandibular glands

A blood group A+ mucin-glycoprotein was purified from aqueous extracts of rat submandibular glands by sequential chromatography on columns of Sepharose CL-6B and Sephacryl S-300 in urea-containing buffers. Final purification was facilitated by reductive methylation which appeared to release contaminating (hydrophobic) peptides. Homogeneity of the purified mucin was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis at varying concentrations of acrylamide, lectin affinity chromatography, and Western blot analysis. In contrast to previously described preparations, the purified mucin contained only trace amounts of N-acetylglucosamine and aromatic amino acids. In addition, only low levels of basic amino acids were present.

Light microscopic detection of sugar residues in glycoconjugates of salivary glands and the pancreas with lectin-horseradish peroxidase conjugates. II. Rat

Salivary glands and pancreases from male rats were stained with a battery of ten different lectin-horseradish peroxidase conjugates. Qualitative and quantitative differences were observed in the content of terminal sugar residues in stored secretory glycoproteins in parenchymal cells of glands having a similar histological structure. Heterogeneity in the content of secretory glycoconjugates was also found between cells in the same exocrine glands, which were previously thought to be identical on the basis of classical morphological and histochemical staining studies. Similar differences were observed in the structure of glycoconjugates associated with the apical surface of epithelial cells lining glandular excretory ducts. Intercalated ducts presented a gland specific staining pattern different from that of the glandular secretory cell population, whereas striated duct and interlobular duct epithelial cells stained similarly in all major rat exocrine glands. A comparison of lectin binding patterns in identical histological sites in the mouse, reported in a companion paper, is provided, and the similarities and differences between these two rodent species are discussed. In addition to providing valuable information concerning the localization and structure of tissue complex carbohydrates, a comparison of staining in the same tissue sites with labelled lectins reported biochemically to have similar binding specificity has revealed interesting differences in the binding specificity of these macromolecules.

Specificity of submaxillary gland sialyltransferases

A method has been developed to determine the activities of specific sialyltransferases by analysis of the products of the reaction. This method, which utilizes high performance liquid chromatography, distinguishes addition of sialic acid to the N-acetylgalactosamine vs. galactose residues of the mucin disaccharide Gal beta(1 leads to 3)GalNac, and can be used to distinguish formation of the 3'- and 6'-isomers of sialyllactose. For the bovine, ovine, and porcine submaxillary extracts, more than 95% of the activity with asialo ovine submaxillary mucin is due to formation of NeuAc alpha(2 leads to 6)GalNAc. With lactose as the acceptor, more than 95% of the alpha(2 leads to 3) isomer is produced. Activity with asialofetuin is due solely to the O-linked chain, with relative activity toward the galactose vs. GalNAc residues of 0.32, 1.5, and 0.10 for bovine, ovine, and porcine, respectively. The rat submaxillary gland extract showed equal formation of 3'- and 6'-sialyllactose, and very low activity with asialo ovine submaxillary mucin. However, at least 40% of the activity toward the Gal beta(1 leads to 3)GalNAc disaccharide of asialofetuin was directed toward the GalNAc residue. The relative preference of the N-acetylgalactosaminide alpha(2 leads to 6) sialyltransferase for a monosaccharide vs. a substituted GalNAc may play a role in regulation of chain length during mucin synthesis.

Comparison in vitro of the incorporation of D-[2-3H(N)]-mannose and D-[1-14C]-glucosamine into glycoproteins of dispersed rat submandibular salivary gland cells

The incorporation of two different radiolabelled sugars, D-[14C]-glucosamine and D-[2-3H(N)]-mannose, into cellular and secretory glycoproteins was compared using dispersed rat submandibular cells. Most of the de-novo biosynthesis appeared to be directed toward the synthesis of secretory material as the molecular profile of the 3H-labelled material released following sympathomimetic stimulation and the percentage of total 3H-labelled acid--precipitable material secreted following cholinergic- or adrenergic-receptor stimulation coincided with the data obtained from similar studies using [14C]-glucosamine. The [3H]-mannose label was found in the neutral sugars mannose, galactose, glucose and fucose, with trace amounts of radiolabel in the amino sugars, whereas the [14C]-glucosamine label was present in three different amino sugars; glucosamine, galactosamine and sialic acid.

Radioimmunoassay of rat submandibular salivary gland mucin

A solid phase radioimmunoassay (RIA) for rat submandibular mucin (RSM) was developed and applied to studies of mucin concentration, antigenicity and secretion. The assay utilizes the affinity of 125I-Protein A for IgG to quantitate antigen-antibody immunocomplexes immobilized in the wells of plastic microtitre plates. The technique was highly reproducible and capable of detecting as little as 3 ng mucin protein. The submandibular glands of rats weighing 150-180 g contained 257 micrograms of mucin (dry wt) per mg protein, which is equivalent to about 3.6 mg of mucin per gland. The antiserum to RSM was cross-reactive with mucins from rat sublingual and pig submandibular glands, and rat, human and pig small intestine. No cross-reactivity was detected with mucins from mouse, canine or bovine submandibular glands, and there was no evidence that ABH blood-group sugars contributed to mucin antigenicity. The RIA was used to estimate secretion from dispersed rat submandibular gland cells and gave a more specific and accurate assay of mucin release than previous assays of precursor-labelled radioactive glycoproteins in the culture medium. The beta-adrenergic agonist, isoproterenol, stimulated immunoreactive mucin secretion from cultured cells to approximately twice the level of unstimulated or propranolol-inhibited controls. The RIA appears to offer promising new approaches for studies on mucin metabolism and secretion in health and disease.