Rat gastric mucin is synthesized and secreted exclusively as filamentous oligomers

Hyaluronic Acid in the Intestinal Tract: Influence of Structure, Rheology, and Mucoadhesion on the Intestinal Uptake in Rats

Oral hyaluronic acid (HA) is a ubiquitous biopolymer that has gained attention as a treatment for local or systemic diseases. Here, we prepared and characterized structures of free HA (f-HA) with a high (>105 Da), intermediate (≤105 Da), and low (≤104 Da) average molar mass (MM); nanoparticles crosslinked with adipic dihydrazide (n-HA); and mixed formulations (mixed-HA) containing f-HA and n-HA. MM distribution determined the structure, hydrodynamic diameter, and zeta potential of the f-HAs. Crosslinking changed the physicochemical properties in n-HA. In vitro tack adhesion assays, using mucin tablets or a viable rat intestinal mucosa, showed better mucoadhesion with f-HA (intermediate MM) and mixed-HA (25% n-HA), especially in the jejunum segment. High MM f-HA presented negligible mucoadhesion. n-HA showed the deepest diffusion into the porous of the membranes. In vivo results showed that, except for high MM f-HA, there is an inverse relationship between rheological changes in the intestinal membrane macerates resulting from mucoadhesion and the effective intestinal permeability that led to blood clearance of the structures. We conclude that the n-HA formulations are promising for targeting other tissues, while formulations of f-HA (intermediate MM) and mixed-HA are better for treating dysbiosis.

Mucin-bacterial interactions in the human oral cavity and digestive tract

Mucins are a family of heavily glycosylated proteins that are the major organic components of the mucus layer, the protective layer covering the epithelial cells in many human and animal organs, including the entire gastro-intestinal tract. Microbes that can associate with mucins benefit from this interaction since they can get available nutrients, experience physico-chemical protection and adhere, resulting in increased residence time. Mucin-degrading microorganisms, which often are found in consortia, have not been extensively characterized as mucins are high molecular weight glycoproteins that are hard to study because of their size, complexity and heterogeneity. The purpose of this review is to discuss how advances in mucus and mucin research, and insight in the microbial ecology promoted our understanding of mucin degradation. Recent insight is presented in mucin structure and organization, the microorganisms known to use mucin as growth substrate, with a specific attention on Akkermansia muciniphila, and the molecular basis of microbial mucin degradation owing to availability of genome sequences.

Barrier properties of mucus

Mucus is tenacious. It sticks to most particles, preventing their penetration to the epithelial surface. Multiple low-affinity hydrophobic interactions play a major role in these adhesive interactions. Mucus gel is also shear-thinning, making it an excellent lubricant that ensures an unstirred layer of mucus remains adherent to the epithelial surface. Thus nanoparticles (NP) must diffuse readily through the unstirred adherent layer if they are to contact epithelial cells efficiently. This article reviews some of the physiological and biochemical properties that form the mucus barrier. Capsid viruses can diffuse through mucus as rapidly as through water and thereby penetrate to the epithelium even though they have to diffuse 'upstream' through mucus that is being continuously secreted. These viruses are smaller than the mucus mesh spacing, and have surfaces that do not stick to mucus. They form a useful model for developing NP for mucosal drug delivery.

The thrombospondin type 1 repeat (TSR) and neuronal differentiation: roles of SCO-spondin oligopeptides on neuronal cell types and cell lines

SCO-spondin is a large glycoprotein secreted by ependymal cells of the subcommissural organ. It shares functional domains called thrombospondin type 1 repeats (TSRs) with a number of developmental proteins expressed in the central nervous system, and involved in axonal pathfinding. Also, SCO-spondin is highly conserved in the chordate phylum and its multiple domain organization is probably a chordate innovation. The putative involvement of SCO-spondin in neuron/glia interaction in the course of development is assessed in various cell culture systems. SCO-spondin interferes with several developmental processes, including neuronal survival, neurite extension, neuronal aggregation, and fasciculation. The TSR motifs, and especially the WSGWSSCSVSCG sequence, are most important in these neuronal responses. Integrins and growth factor receptors may cooperate as integrative signals. We discuss the putative involvement of the subcommissural organ/Reissner's fiber complex in developmental events, as a particular extracellular signaling system.

SCO-spondin and RF-GlyI: two designations for the same glycoprotein secreted by the subcommissural organ

SCO-spondin and RF-GlyI are two designations for cDNAs strongly expressed in the bovine subcommissural organ (SCO), characterized, respectively, in 1996 and 1998 by two different research groups. Because both cDNAs were partial sequences and exhibited close similarities in their nucleotide and deduced amino acid sequences, it was thought that they might be part of the same encoding sequence. To find out, we performed 3'RACE using a SCO-spondin-specific upstream primer. From the RT-PCR product generated and by nested PCR techniques, we amplified both SCO-spondin and RF-GlyI specific products with the expected length. Also, probes generated from both PCR products hybridized to the same major 14 kb transcript in Northern blot analyses, clearly showing that SCO-spondin and RF-GlyI cDNAs do belong to the same encoding sequence. In addition, we amplified, cloned, and sequenced a PCR product of 3 kb spanning both the known SCO-spondin and RF-GlyI sequences. The deduced amino acid sequence contains nine thrombospondin type 1 repeats that alternate with sequences sharing similarities with the D-domain of von Willebrand factor. Taken together, these findings show that SCO-spondin and RF-GlyI are two designations of the same gene encoding proteins secreted by the bovine SCO and forming Reissner's fiber. In addition, compared to the sequence provided by Nualart et al. (1998), we extended the reading frame and identified new conserved domains in the 3' end of SCO-spondin. The putative function of SCO-spondin on axonal pathfinding is discussed regarding the presence of a great number of thrombospondin type 1 repeats.

Ecabet sodium, a novel locally-acting anti-ulcer agent, protects the integrity of the gastric mucosal gel layer from pepsin-induced disruption in the rat

BACKGROUND

Ecabet sodium, a novel non-systemic anti-ulcer agent, possesses high affinity to gastric adherent mucus, which plays an important role in the protection of the gastric epithelium against acid and pepsin.

AIM

To assess the effect of ecabet on pepsin-induced degradation of the structure of the mucus gel layer.

METHODS

Everted sacs of rat stomach were incubated in HCl solution containing pepsin with or without ecabet. Pepsin-induced release of the cleaved peptides and hexosamine from the sacs was determined. Changes in the molecular size of glycoproteins in the adherent mucus (using gel filtration methods) and in the morphology of the epithelium (using both light and scanning electron microscopy) were also examined.

RESULTS

Ecabet reduced the pepsin-induced release of peptides and hexosamine, depending on its content in the adherent mucus. Pepsin treatment partially lowered the molecular weight of native glycoproteins in the adherent mucus, caused exfoliation of the epithelial cells, and degraded the network-like ultrastructure of the mucus layer, giving it a lumpy, globular appearance. Ecabet prevented both the pepsin-induced molecular size shift in mucus glycoproteins, and morphological alteration of the epithelium, including ultrastructural derangement of the mucus gel layer.

CONCLUSION

Ecabet protects the polymeric structure of mucus glycoproteins from proteolytic degradation by pepsin, and thus maintains integrity of the gastric mucus gel 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.

The oligomerization of a family of four genetically clustered human gastrointestinal mucins

Mucins are synthesized and secreted by many epithelia. They are complex glycoproteins that offer cytoprotection. In their functional configuration, mucins form oligomers by a biosynthetic process that is poorly understood. A family of four human gastrointestinal mucin genes (MUC2, MUC5AC, MUC5B, and MUC6) is clustered to chromosome 11p15.5. To study oligomerization of these related mucins, we performed metabolic labeling experiments with [35S]amino acids in LS174T cells, and isolated mucin precursors by specific immunoprecipitations that were analyzed on SDS-PAGE. Each of the precursors of MUC2, MUC5AC, MUC5B, and MUC6 formed a single species of disulfide-linked homo-oligomer within 1 h after pulse labeling. Based on apparent molecular masses, these oligomeric precursors were most likely dimers. Inhibition of vesicular RER-to-Golgi transport, with brefeldin A and CCCP, did not affect the dimerization of MUC2 precursors, localizing dimerization to the RER. O-Glycosylation of MUC2 followed dimerization. Inhibition of N-glycosylation by tunicamycin retarded, but did not inhibit, dimerization, indicating that N-glycans play a role in efficient dimerization of MUC2 precursors. Based on sequence homology, the ability of MUC2, MUC5AC, MUC5B and MUC6 to dimerize most likely resides in their C-terminal domains. Thus, the RER-localized dimerization of secretory mucins likely proceeds by similar mechanisms, which is an essential step in the formation of the human gastrointestinal mucus-gels.

Mucus glycoproteins and their role in colorectal disease

In this review, the nature and impact of progress in the study of mucins is outlined, emphasizing the current understanding of the structure and physiological function of these molecules in the colorectum. The use of new methods for preparation and separation has led to improvements in the analysis of mucins; these are detailed, as are their difficulties and pitfalls. Results obtained with these methods are correlated with long-established histochemical techniques and the use of chemical, lectin, and antibody reagents for general and specific detection of mucins in all procedures is described. Improvements in the detection and analysis of mucins in biopsy-size tissue samples and in larger numbers of individual clinical cases have now permitted a much wider approach to the pathological evaluation of mucin biology and progress with these techniques is outlined. The significance of the discovery of a family of mucin genes is presented and new concepts of mucin structure resulting from these studies are described. Bacterial degradation of the mucus layer at the surface of the colorectal mucosa is considered in line with the homeostatic relationship with mucosal mucin synthesis. Finally, the implications of abnormal mucins in colorectal disease are considered in the light of recent methodological advances.

Biosynthesis of rat MUC2 in colon and its analogy with human MUC2

In order to identify the mucins synthesized and secreted in the rat colon, we studied their biochemical characteristics and biosynthesis and evaluated their analogy to human colonic mucins. Purified mucin from both species appeared similar with respect to composition, buoyant density and mobility on SDS/PAGE. Isolated rat colonic mucin (RCM) was used to elicit a polyclonal antiserum, which was used in metabolic labelling studies to identify mucins and mucin precursors. RCM is synthesized as a 600 kDa precursor protein, which oligomerizes before O-glycosylation. The mature, high-molecular mass mucin is secreted and displays an anomalous molecular mass on SDS/PAGE of approximately 650 kDa. Polymorphism in precursor size was found among different rats, suggesting genetic heterogeneity. Molecular mass, biosynthesis and secretion of RCM appeared similar to human MUC2. Moreover, RCM precursor could be immunoprecipitated using specific anti-(human MUC2) antisera, indicating that the RCM can be designated rat MUC2. This study describes the biosynthesis of two homologous mucins in two different species. The high degree of similarity suggests functional analogy.

Cloning and analysis of human gastric mucin cDNA reveals two types of conserved cysteine-rich domains

Human gastric mucin was isolated by successive CsCl-gradient ultracentrifugation in the presence of guanidinium hydrochloride to prevent degradation of the polypeptide moieties of the molecules. The amino acid sequence of a tryptic fragment of this molecule was identical to that of a tryptic fragment of tracheobronchial mucin. An oligonucleotide based on this sequence hybridized specifically to human stomach mRNA and was subsequently used to screen a human stomach lambda ZAPII cDNA library. The largest of 10 positive clones encoded 850 amino acid residues, including the tryptic fragment, with high amounts of threonine, serine and proline residues. Interestingly, cysteine accounted for almost 8% of the amino acid residues. The 3' part of the sequence was very similar but not identical to the 3' region of human tracheobronchial cDNA. No tandem repeated sequences were present and the deduced polypeptide sequence contained two potential N-linked glycosylation sites. Four cysteine-rich clusters were detected, one of which was apparently homologous to the D-domains present in other mucins and in von Willebrand factor. The arrangement of the cysteines in three other cysteine-rich clusters was conserved in the human gastric mucin cDNA in a similar fashion as in two domains in the MUC2 gene product. The cysteine-rich domains were separated by short stretches of non-repetitive amino acid residues with a very high content of threonine and serine residues. These data suggest that the encoded polypeptide of this clone may be involved in disulphide-bond-mediated oligomerization of the mucin, and provide new insights into the molecular organization of mammalian apomucins.

The human MUC2 mucin apoprotein appears to dimerize before O-glycosylation and shares epitopes with the 'insoluble' mucin of rat small intestine

Rabbit antiserum against a synthetic peptide corresponding to a tandemly repeated amino acid sequence in the human intestinal mucin apoprotein MUC2 was used in immunoprecipitation to study the biosynthesis of MUC2 in the colon-carcinoma cell line LS 174T. Under non-reducing conditions, two bands were precipitated, the smaller with an apparent size of about 700 kDa on SDS/PAGE. When analysed by two-dimensional electrophoresis after reduction, the larger band migrated to the same position as the smaller band and was interpreted as a putative disulphide-bond-stabilized dimer. Pulse-chase experiments showed only the monomer after 5 min and the appearance of the putative dimer after 30 min. The MUC2 apoprotein was also precipitated by antisera against the HF-deglycosylated peptides of the two highly glycosylated domains of the 'insoluble' mucin complex of rat small intestine [Carlstedt, Herrmann, Karlsson, Sheehan, Fransson and Hansson (1993) J. Biol. Chem. 268, [18771-18781]. Endoprotease Lys-C cleavage of the immunopurified apoprotein gave a large fragment of about 250 kDa that was detected by both the antiserum against the MUC2 tandem repeat and one of the glycopeptide antisera. This supports the view that the 'insoluble' mucin of rat small intestine is encoded by the Muc2 gene, as recently indicated by a partial cDNA sequence [Hansson, Baeckström, Carlstedt and Klinga-Levan (1994) Biochem. Biophys. Res. Commun. 198, 181-190] and that parts of the apoprotein are conserved between the species. A lectin from the snail Helix pomatia that detects terminal alpha-GalNAc residues did not bind to the monomer or putative dimer, suggesting that O-glycosylation starts after dimerization. The results indicate that the biosynthetic pathway of the MUC2 mucin may be similar to that of the von Willebrand factor with which MUC2 shares sequence similarities at its C- and N-termini.

Identification of a human gastric mucin precursor: N-linked glycosylation and oligomerization

Gastric mucin plays an important role in the protection of the stomach wall from chemical, microbiological and mechanical damage. We have previously isolated human gastric mucus glycoproteins and raised a polyclonal antiserum against these macromolecules. This antiserum specifically reacted with gastric mucins in immunoblotting experiments and stained mucous granules at the apical side of gastric surface epithelial cells. A similar staining pattern was obtained after incubation with an antiserum against rat gastric mucin. Next we used the antiserum in pulse-chase experiments of human stomach tissue explants. After short labelling periods with [35S]methionine and [35S]cysteine, the antiserum reacted with a polypeptide with an apparent molecular mass of approx. 500 kDa as determined by SDS/PAGE, which was converted after 90 min into a heterogeneous high-molecular-mass glycoprotein. This high-molecular-mass form, but not the 500 kDa polypeptide, was detectable in the culture medium after 2 h. This strongly suggests that the 500 kDa polypeptide is the precursor of the purified gastric mucin. Analysis of pulse-chase experiments by non-reducing SDS/PAGE revealed that the precursors form disulphide-linked oligomers early in biosynthesis, before the addition of O-linked sugars. After preincubation with the N-glycosylation inhibitor, tunicamycin, the apparent molecular mass of the precursor decreased marginally but consistently, indicating that N-linked glycan chains are present on the mucin precursor.

Biosynthesis of mucin derived from a 60-kDa precursor protein in the human stomach

We studied the biosynthesis of mucin in the human stomach using an anti-mucin core peptide monoclonal antibody, 3G12. Human stomach mucosa was labeled with [35S]methionine, and chased for 3 h. An approximately 60-kDa subunit of human gastric mucin precursor protein was detected in the intracellular product. Under nonreducing conditions, dimer, trimer, and tetramer mucin precursor protein (120, 180, 240 kDa) were detected. Treatment with tunicamycin or endo-beta-N-acetylglucosaminidase H had no effect on the 60-kDa subunit and its oligomers. Extracellular products contained only the high molecular weight mucin, and the secretion was not affected by tunicamycin. By treatment with monensin or brefeldin A, the mature mucin was not secreted extracellularly. These findings suggested that a 60-kDa subunit of the mucin precursor protein was biosynthesized into mature mucin after oligomerization to tetramers, and that neither the oligomerization nor the intracellular transport of the mucin in the human stomach was associated with N-glycosylation.

Biosynthesis of two distinct types of mucin in HM3 human colon cancer cells

Mucins, high-M(r) glycoproteins with a large amount of O-glycosidically linked carbohydrate, protect the colonic epithelial surface and are altered in ulcerative colitis and colon cancer. At least two mucin genes, MUC2 and MUC3, are expressed at high levels in the human intestine. As an experimental model for studying the biosynthesis of human intestinal mucins, we used HM3 colon cancer cells. When mature mucins labelled with [3H]glucosamine or [3H]threonine were analysed by gel filtration, it was found that secreted mucins (M(r) > 10(8) were larger than soluble cellular mucins (M(r) approx. 5 x 10(6)). Only secreted mucin was sensitive to reduction. Both MUC2 and MUC3 proteins, identified by labelling with [3H]threonine or [35S]cysteine and immunoprecipitation with antibodies to synthetic mucin peptides, were already of large size (M(r) > 180,000) by the earliest labelling time (5 min). The MUC3 precursor was completely degraded by trypsin, but the MUC2 precursor had a trypsin-resistant fragment of M(r) approx. 240,000 containing threonine and cysteine. The trypsin-resistant MUC2 fragment contained N-linked carbohydrate, as indicated by a decrease in size as a result of peptidyl N-glycosidase digestion or tunicamycin treatment of HM3 cells. These results show that HM3 colon cancer cells produce at least two distinct human intestinal mucins. They also indicate that the mechanisms of biosynthesis of intestinal mucins differ from those of other mucin-like glycoproteins that have been studied.

Biosynthesis of frog skin mucins: cysteine-rich shuffled modules, polydispersities and genetic polymorphism

1. Frog integumentary mucins (FIM-A.1, FIM-B.1 and FIM-C.1) consist of typical threonine-rich highly O-glycosylated (semi)repetitive domains, and cysteine-rich modules, i.e. the P-domain, the short consensus repeat and a region with high similarity to the C-terminal end of von Willebrand factor (designated here CC29-motif). 2. These modules are thought to be involved in protein-protein interactions and they have been observed in a variety of extracellular proteins. In FIMs, these modules may be involved in oligomerization processes leading to an entangled mucin network. 3. Polydispersities have been detected in FIM-B.1 and FIM-C.1 within single individuals. Multiple transcripts are probably generated by alternative splicing of a huge array of different (semi)repetitive cassettes encoding the threonine-rich domains. 4. Furthermore, genetic polymorphism is observed between different individuals, probably due to allelic variations in the number of (semi)repetitive cassettes.

Mucin-like glycoproteins in the equine embryonic capsule

The equine embryonic capsule replaces the zona pellucida and envelopes the conceptus during the second and third weeks of pregnancy. Although this capsule was described more than 100 years ago, its molecular structure has not been characterized. Here we present evidence that the glycoprotein(s) of the equine capsule resembles those of the mucin glycoprotein family. The resistance of the capsule to chemical and enzymatic solubilization was confirmed, and, as in mucins, protein constituted only 35-40% of its total dry mass. Determination of the sugar composition of the capsule using colorimetric assays and high-performance anion-exchange chromatography also showed it to have mucin-like characteristics. Gal, GalNAc, sulfated sugars, and sialic acid make up a high proportion of the capsular carbohydrate, while GlcNAc, Glc, and Man are minor components. These findings were verified using lectin histochemical staining of frozen sections of conceptuses. The results of amino acid analysis were also consistent with the proposal that the capsular glycoproteins belong to the mucin family. Removal of the covalently bound carbohydrate by beta-elimination under reducing conditions demonstrated that the capsule is O-glycosylated mainly on threonine residues. Affinity chromatography on jacalin-agarose confirmed that, like mucins, the capsular glycoproteins are heavily O-glycosylated. SDS-PAGE analysis revealed a prominent 21-kDa band, specific to the capsule, in preparations solubilized by trypsin but not by other proteases. Characterization of its constituent glycoprotein(s) should be helpful in elucidating the role of the capsule (and analogous blastocyst coverings in other species) during early pregnancy.