Structural diversity and specific distribution of O-glycans in normal human mucins along the intestinal tract

A complex, but uniform O-glycosylation of the human MUC2 mucin from colonic biopsies analyzed by nanoLC/MSn

High-sensitivity glycan profiling providing detailed structural information is very important in the search for glycan disease markers. By combining a straight-forward and fast preparation protocol of mucins with high-throughput nanoLC/MS, we have been able to study the O-glycosylation of the colon MUC2 mucin from one single biopsy (approximately 5 mg wet tissue as starting material) collected from the sigmoid colon during routine colonoscopy of 25 normal control patients. This large mucin glycoprotein was recovered from the guanidinium chloride-extracted insoluble pellet, reduced and alkylated, separated by SDS-agarose polyacrylamide composite gel electrophoresis, and transferred to a PVDF membrane. The O-linked oligosaccharides of the major MUC2 monomer band were released by reductive beta-elimination and analyzed by nanoLC/mass spectrometry and MS(n). The aim was to identify the MUC2 O-glycans of the sigmoid colon and provide a comprehensive catalog of the O-glycan repertoire. More than 100 complex O-linked oligosaccharides were identified, of which some had not been described before. Most of the oligosaccharides were based on the core 3 structure with sialic acid at the 6-position of the GalNAc and the substructure Gal beta 1-3/4-GlcNAc beta 1-3(NeuAc-6)GalNAcol was found in most glycans. The most abundant components were -Gal-(Fuc)GlcNAc-3(NeuAc-6)GalNAcol, GalNAc-(NeuAc-)Gal-4/3GlcNAc-3(NeuAc-6)GalNAcol, GalNAc-3(NeuAc-6) GalNAcol, and GlcNAc-3(NeuAc-6)GalNAcol. In contrast to the O-glycans of other mucins, the sigmoid MUC2 O-glycan repertoire and relative amounts in normal individuals were relatively constant.

Sialic acid (N-acetyl neuraminic acid) utilization by Bacteroides fragilis requires a novel N-acetyl mannosamine epimerase

We characterized the nanLET operon in Bacteroides fragilis, whose products are required for the utilization of the sialic acid N-acetyl neuraminic acid (NANA) as a carbon and energy source. The first gene of the operon is nanL, which codes for an aldolase that cleaves NANA into N-acetyl mannosamine (manNAc) and pyruvate. The next gene, nanE, codes for a manNAc/N-acetylglucosamine (NAG) epimerase, which, intriguingly, possesses more similarity to eukaryotic renin binding proteins than to other bacterial NanE epimerase proteins. Unphosphorylated manNAc is the substrate of NanE, while ATP is a cofactor in the epimerase reaction. The third gene of the operon is nanT, which shows similarity to the major transporter facilitator superfamily and is most likely to be a NANA transporter. Deletion of any of these genes eliminates the ability of B. fragilis to grow on NANA. Although B. fragilis does not normally grow with manNAc as the sole carbon source, we isolated a B. fragilis mutant strain that can grow on this substrate, likely due to a mutation in a NAG transporter; both manNAc transport and NAG transport are affected in this strain. Deletion of the nanE epimerase gene or the rokA hexokinase gene, whose product phosphorylates NAG, in the manNAc-enabled strain abolishes growth on manNAc. Thus, B. fragilis possesses a new pathway of NANA utilization, which we show is also found in other Bacteroides species.

Differential carbohydrate recognition by Campylobacter jejuni strain 11168: influences of temperature and growth conditions

The pathogenic clinical strain NCTC11168 was the first Campylobacter jejuni strain to be sequenced and has been a widely used laboratory model for studying C. jejuni pathogenesis. However, continuous passaging of C. jejuni NCTC11168 has been shown to dramatically affect its colonisation potential. Glycan array analysis was performed on C. jejuni NCTC11168 using the frequently passaged, non-colonising, genome sequenced (11168-GS) and the infrequently passaged, original, virulent (11168-O) isolates grown or maintained under various conditions. Glycan structures recognised and bound by C. jejuni included terminal mannose, N-acetylneuraminic acid, galactose and fucose. Significantly, it was found that only when challenged with normal oxygen at room temperature did 11168-O consistently bind to sialic acid or terminal mannose structures, while 11168-GS bound these structures regardless of growth/maintenance conditions. Further, binding of un-capped galactose and fucosylated structures was significantly reduced when C. jejuni was maintained at 25°C under atmospheric oxygen conditions. These binding differences identified through glycan array analysis were confirmed by the ability of specific lectins to competitively inhibit the adherence of C. jejuni to a Caco-2 intestinal cell line. Our data suggests that the binding of mannose and/or N-acetylneuraminic acid may provide the initial interactions important for colonisation following environmental exposure.

Gastrointestinal microflora and mucins may play a critical role in the development of 5-Fluorouracil-induced gastrointestinal mucositis

5-Fluorouracil (5-FU) is a commonly used chemotherapy agent in clinical oncology practice. Two of its major side effects are mucositis and diarrhoea. The structure of mucins offers mucosal protection, and allows maintenance of intestinal flora by providing attachment sites and preventing bacterial overgrowth and/or penetration. The aim of this study was to investigate changes in mucin secretion and microflora following treatment with 5-FU. Female DA rats were given a single 150 mg/ kg i.p. dose of 5-FU. Rats were killed at various time points after treatment. Control rats received no treatment. Jejunum, colon and faecal samples were collected. Standard microbiological culture techniques were used to identify bacteria, and real-time PCR was used to quantify bacteria in faecal samples. Goblet cells and cavitated goblet cells (having undergone mucus exocytosis) were also counted. Statistical analysis was carried out using Kruskal-Wallis test, a non-parametric method of testing equality of group medians. Following treatment with 5-FU, we showed decreases in Clostridium spp., Lactobacillus spp. and Streptococcus spp., and an increase in Escherichia spp. in the jejunum. In the colon, 5-FU caused decreases in Enterococcus spp., Lactobacillus spp. and Streptococcus spp. Real-time PCR of faecal samples showed decreasing trends in Lactobacillus spp. and Bacteroides spp., and an increasing trend in E. coli. Significant increases (P < 0.05) were seen in Clostridium spp. and Staphylococcus spp. at 24 h. Goblet cell numbers decreased significantly in the jejunum from 24-72 h, with a significant increase in the percentage of cavitated goblet cells. In conclusion, 5-FU treatment causes significant changes in intestinal flora and mucin secretion in rats. These changes could result in systemic effects and, in particular, may contribute to the development of chemotherapy-induced mucositis.

Irinotecan-induced mucositis is associated with changes in intestinal mucins

PURPOSE

Mucositis is a major oncological problem, caused by the cytotoxic effects of cancer chemotherapy and radiotherapy. Irinotecan is used to treat a variety of solid tumours, through the inhibition of DNA topoisomerase I and is linked with severe mucositis and diarrhoea. Mucus production appears to be increased, which may contribute to the development of diarrhoea.

METHODS

Dark agouti rats were treated with irinotecan, and tissues collected at several time points up to 72 h. Goblet cells and mucin secretion were investigated, as well as mucin expression (Muc2 and Muc4) and kruppel-like factor (Klf) 4 using immunohistochemistry in the gastrointestinal tract. Both goblet cells and cells positive for Muc expression were counted, and analysed statistically using the Mann-Whitney U test with Bonferroni correction.

RESULTS

Goblet cells decreased significantly after irinotecan treatment. However, mucin secretion increased. Mucin expression changed significantly after treatment. Muc2 and Muc4 decreased significantly in the villi of the jejunum after treatment, Muc2 and Muc4 decreased significantly in the crypts. Muc2 decreased significantly in the colon.

CONCLUSIONS

Irinotecan causes an increase in mucin secretion and a net decrease in mucin-producing goblet cells, and the expression of Muc2 and Muc4 in the gastrointestinal tract is altered following treatment. Increased mucin secretion is likely to be related to altered mucin expression, and may contribute to chemotherapy-induced diarrhoea.

Glycosylation of human fetal mucins: a similar repertoire of O-glycans along the intestinal tract

Intestinal mucins are very high molecular weight glycoproteins secreted by goblet cells lining the crypt and the surface of the colonic mucosa. Profound alterations of mucin O-glycans are observed in diseases such as cancer and inflammation, modifying the function of the cell and its antigenic and adhesive properties. Based on immunohistochemical studies, certain cancer- and inflammation- associated glycans have been defined as oncofetal antigens. However, little or no chemical analysis has allowed the structural elucidation of O-glycans expressed on human fetal mucins. In this paper, mucins were isolated from different regions of the normal human intestine (ileum, right, transverse and left colon) of eight fetuses with A, B or O blood group. After alkaline borohydride treatment, the released oligosaccharides were investigated by nanoESI Q-TOF MS/MS (electrospray ionization quadrupole time-of-flight tandem mass spectrometry). More than 117 different glycans were identified, mainly based on core 2 structures. Some core 1, 3 and 4 oligosaccharides were also found. Most of the structures were acidic with NeuAc residues mainly alpha2-6 linked to the N-acetylgalactosaminitol and sulphate residues 3-linked to galactose or 6-linked to GlcNAc. In contrast to adult human intestinal mucins, Sda/Cad determinants were not expressed on fetal mucin O-glycans and the presence of an acidic gradient along the intestinal tract was not observed. Similar patterns of glycosylation were found in each part of the intestine and the level of expression of the major oligosaccharides was in the same order of magnitude. This study could help determining new oncofetal antigens, which can be exploited for the diagnosis or the treatment of intestinal diseases.

Campylobacter jejuni response to human mucin MUC2: modulation of colonization and pathogenicity determinants

Campylobacter jejuni is the main cause of bacterial acute gastroenteritis worldwide. In its colonization of the host intestinal tract, it encounters secreted mucins in the mucus layer and surface mucins in the epithelial cells. Mucins are complex glycoproteins that comprise the major component of mucus and give mucus its viscous consistency. MUC2 is the most abundant secreted mucin in the human intestine; it is a major chemoattractant for C. jejuni, and the bacterium binds to it. There are no studies on the transcriptional response of the bacterium to this mucin. Here, cell-culture techniques and quantitative RT-PCR were used to characterize in vitro the effects of MUC2 on C. jejuni growth and the changes in expression of 20 C. jejuni genes related to various functions. The genes encoding cytolethal distending toxin protein (cdtABC), vacuolating cytotoxin (vacB), C. jejuni lipoprotein (jlpA), Campylobacter invasion antigen (ciaB), the multidrug efflux system (cmeAB), putative mucin-degrading enzymes (cj1344c, cj0843c, cj0256 and cj1055c), flagellin A (flaA) and putative rod-shape-determining proteins (mreB and mreC) were upregulated, whereas those encoding Campylobacter adhesion fibronectin-binding protein (cadF) and sialic acid synthase (neuB1) were downregulated. These results showed that C. jejuni utilizes MUC2 as an environmental cue for the modulation of expression of genes with various functions including colonization and pathogenicity.

Characterization of two different endo-alpha-N-acetylgalactosaminidases from probiotic and pathogenic enterobacteria, Bifidobacterium longum and Clostridium perfringens

Endo-alpha-N-acetylgalactosaminidase (endo-alpha-GalNAc-ase) catalyzes the hydrolysis of the O-glycosidic bond between alpha-GalNAc at the reducing end of mucin-type sugar chains and serine/threonine of proteins to release oligosaccharides. Previously, we identified the gene engBF encoding endo-alpha-GalNAc-ase from Bifidobacterium longum, which specifically released the disaccharide Gal beta 1-3GalNAc (Fujita K, Oura F, Nagamine N, Katayama T, Hiratake J, Sakata K, Kumagai H, Yamamoto K. 2005. Identification and molecular cloning of a novel glycoside hydrolase family of core 1 type O-glycan-specific endo-alpha-N-acetylgalactosaminidase from Bifidobacterium longum. J Biol Chem. 280:37415-37422). Here we cloned a similar gene named engCP from Clostridium perfringens, a pathogenic enterobacterium, and characterized the gene product EngCP. Detailed analyses on substrate specificities of EngCP and EngBF using a series of p-nitrophenyl-alpha-glycosides chemically synthesized by the di-tert-butylsilylene-directed method revealed that both enzymes released Hex/HexNAc beta 1-3GalNAc (Hex = Gal or Glc). EngCP could also release the core 2 trisaccharide Gal beta 1-3(GlcNAc beta 1-6)GalNAc, core 8 disaccharide Gal alpha 1-3GalNAc, and monosaccharide GalNAc. Our results suggest that EngCP possesses broader substrate specificity than EngBF. Actions of the two enzymes on native glycoproteins and cell surface glycoproteins were also investigated.

Development of an all-in-one technology for glycan profiling targeting formalin-embedded tissue sections

An ultra-sensitive method for glycan analysis targeting small tissue sections (1.5mm in diameter) is described as an application of a recently-established lectin microarray technology. The developed system achieved a high level of detection of a tissue section consisting of approximately 500 cells for differential profiling, where both N- and O-glycans attached to a pool of glycoproteins are subjected to multiplex analysis with 43 lectins. By using an optimized protocol for differential glycan analysis, sections of adenocarcinoma (n=28) and normal epithelia (n=12) of the colon were analyzed in an all-in-one manner. As a result, Wisteria floribunda agglutinin (WFA) was found to clearly differentiate cancerous from normal epithelia with P<0.0001. The obtained results correlated well with the subsequent histochemical study using biotinylated WFA. Thus, the developed technology proved to be valid for expanding the lectin microarray applications to tissue-based glycomics, and hence, should accelerate a discovery phase of glycan-related biomarkers.

Glycosylation of the two O-glycosylated domains of human MUC2 mucin in patients transposed with artificial urinary bladders constructed from proximal colonic tissue

Transposition of intestinal segments is frequently used for bladder reconstruction. Following transposition, bowel segments continue to produce mucus and a correlation between excessive mucus production and complications such as urinary tract infection or catheter blockage has been observed for a long time. However, no information is currently available on the change of mucin expression and glycosylation under these abnormal conditions. In this study, the variable number tandem repeat region and the irregular repeat domain of human MUC2 were isolated as two glycopeptide populations after reduction and trypsin digestion followed by gel chromatography from urine of patients transposed with urinary bladders. After alkaline borohydride treatment, the oligosaccharides released from the whole MUC2 mucin and the two glycosylated domains were investigated by nanoESI Q-TOF MS/MS (electrospray ionization quadrupole time-of-flight tandem mass spectrometry). More than 60 different glycans were identified, mainly based on sialylated core 3 structures. Some core 1, 2 and 4 oligosaccharides were also found. Most of the structures were acidic with NeuAc residues mainly alpha2-6 linked to the N-acetylgalactosaminitol and sulphate residues exclusively 3-linked to galactose. No expression of blood group A and B or Sda/Cad determinants was observed. Similar patterns of glycosylation were found in the tandem repeat region and the irregular repeat domain and the level of expression of the major oligosaccharides were in the same order of magnitude. The most interesting feature of this study was that sialyl-Tn antigen, which is considered as a tumour antigen, was the oligosaccharide most highly expressed. This result suggests that mucins from intestinal transposed segments are abnormally glycosylated.

Differential Salmonella survival against communities of intestinal amoebae

Predation from intestinal amoebae may provide selective pressure for the maintenance of high genetic diversity at the Salmonella enterica rfb locus, whereby serovars better escape predators in particular environments depending on the O-antigens they express. Here, the hypothesis that amoebae from a particular intestinal environment collectively prefer one serovar over another is tested. Collections of Acanthamoeba, Tetramitus, Naegleria and Hartmannella were isolated from the intestinal tracts of several vertebrate hosts, including bullfrog tadpoles, goldfish, turtles and bearded dragons, and their feeding preferences were determined. Congeneric amoebae from the same environment had significantly similar feeding preferences. Strikingly, even unrelated amoebae - such as Naegleria and Tetramitus from goldfish - also had significantly similar feeding preferences. Yet amoebae isolated from different environments showed no similarity in prey choice. Thus, feeding preferences of amoebae appear to reflect their environment, not their taxonomic relationships. A mechanism mediating this phenotypic convergence is discussed.

Glycomic mapping of O- and N-linked glycans from major rat sublingual mucin

Carbohydrate moieties of salivary mucins play various roles in life processes, especially as a microbial trapping agent. While structural details of the salivary O-glycans from several mammalian sources are well studied, very little information is currently available for the corresponding N-glycans. The existence of N-glycans alongside O-glycans on mucin isolated from rat sublingual gland has previously been implicated by total glycosyl compositional analysis but the respective structural data are both lacking. The advent of facile glycomic mapping and sequencing methods by mass spectrometry (MS) has enabled a structural reinvestigation into many previously unsolved issues. For the first time, high energy collision induced dissociation (CID) MALDI-MS/MS as implemented on a TOF/TOF instrument was applied to permethyl derivatives of mucin type O-glycans and N-glycans, from which the linkage specific fragmentation pattern could be established. The predominant O-glycans carried on the rat sublingual mucin were defined as sialylated core 3 and 4 types whereas the N-glycans were determined to be non-bisected hybrid types similarly carrying a sialylated type II chain. The masking effect of terminal sialylation on the tight binding of rat sublingual mucin to Galbeta1-->4GlcNAc specific lectins and three oligomannose specific lectins were clearly demonstrated in this study.

Protection of the neonate by the innate immune system of developing gut and of human milk

The neonatal adaptive immune system, relatively naïve to foreign antigens, requires synergy with the innate immune system to protect the intestine. Goblet cells provide mucins, Paneth cells produce antimicrobial peptides, and dendritic cells (DCs) present luminal antigens. Intracellular signaling by Toll-like receptors (TLRs) elicits chemokines and cytokines that modulate inflammation. Enteric neurons and lymphocytes provide paracrine and endocrine signaling. However, full protection requires human milk. Breast-feeding reduces enteric infection and may reduce chronic disease in later life. Although human milk contains significant secretory immunoglobulin A (sIgA), most of its protective factors are constitutively expressed. Multifunctional milk components are nutrients whose partial digestion products inhibit pathogens. Cytokines, cytokine receptors, TLR agonists and antagonists, hormones, anti-inflammatory agents, and nucleotides in milk modulate inflammation. Human milk is rich in glycans (complex carbohydrates): As prebiotics, indigestible glycans stimulate colonization by probiotic organisms, modulating mucosal immunity and protecting against pathogens. Through structural homology to intestinal cell surface receptors, glycans inhibit pathogen binding, the essential first step of pathogenesis. Bioactive milk components comprise an innate immune system of human milk whereby the mother protects her nursing infant. Interactions between human milk glycans, intestinal microflora, and intestinal mucosa surface glycans underlie ontogeny of innate mucosal immunity, pathobiology of enteric infection, and inflammatory bowel diseases.

The Interaction of a Carbohydrate-binding Module from a Clostridium perfringens N-Acetyl-β-hexosaminidase with Its Carbohydrate Receptor

Clostridium perfringens is a notable colonizer of the human gastrointestinal tract. This bacterium is quite remarkable for a human pathogen by the number of glycoside hydrolases found in its genome. The modularity of these enzymes is striking as is the frequent occurrence of modules having amino acid sequence identity with family 32 carbohydrate-binding modules (CBMs), often referred to as F5/8 domains. Here we report the properties of family 32 CBMs from a C. perfringens N-acetyl-beta-hexosaminidase. Macroarray, UV difference, and isothermal titration calorimetry binding studies indicate a preference for the disaccharide LacNAc (beta-d-galactosyl-1,4-beta-d-N-acetylglucosamine). The molecular details of the interaction of this CBM with galactose, LacNAc, and the type II blood group H-trisaccharide are revealed by x-ray crystallographic studies at resolutions of 1.49, 2.4, and 2.3 A, respectively.

Rôles physiologiques des mucines dans la barrière colique

Colonic mucus is a key element of colonic barrier as it is located at the frontier between luminal microflora and colonic mucosa itself. Colonic mucus is mainly composed of high molecular weight glycoproteins called mucins that can be either secreted or membrane-linked. The expression of various colonic mucins is altered in colorectal cancers or inflammations. The aim of this review is to highlight the crucial role played by colonic mucins in the maintenance of colonic barrier integrity, both because they are part of the protective mucus layer, and because they individually exert specific functions involved in epithelial barrier, like cell growth and differentiation, immunomodulation, signal transduction or cell adhesion.

Multiple hepatic receptors cooperate to eliminate secretory mucins aberrantly entering the bloodstream: are circulating cancer mucins the "tip of the iceberg"?

Hollow organs lined by columnar epithelial cells normally secrete mucins and their proteolytic fragments vectorially into the lumen. These heterogeneously O-glycosylated molecules are known to aberrantly enter the bloodstream in the setting of epithelial carcinomas and possibly during injury or inflammation. We have recently shown that carcinoma mucin fragments can trigger the rapid formation of platelet-rich microthrombi in vivo. Thus, mechanisms to clear such aberrantly secreted mucins must exist. Indeed, we found that i.v. injected carcinoma mucin fragments had an approximately 1 minute half-life in mice, which was primarily due to rapid clearance by hepatic reticuloendothelial cells. Inhibition of known glycan-recognizing hepatic clearance receptors showed involvement of multiple partially overlapping clearance systems. Studies of genetically deficient mice and incomplete competition between different mucins confirmed this result. Thus, multiple hepatic clearance receptors cooperate to eliminate secretory mucins entering the circulation, limiting potential pathology. This may also explain why mucin-type clustered O-glycosylation is rare on plasma proteins. Notably, small subsets of injected carcinoma mucins remained unrecognized by clearance systems, had a much longer half-life, and carried highly sialylated O-glycans. Similar circulating mucins were found in tumor-bearing mice despite lack of saturation of hepatic clearance mechanisms. Thus, circulating cancer mucins currently used as clinical diagnostic markers likely represent only the clearance-resistant "tip of the iceberg." Such aberrantly circulating mucins could play pathologic roles not only in cancer but also during injury or inflammation of hollow organs and in liver disease.

DMBT1 expression and glycosylation during the adenoma-carcinoma sequence in colorectal cancer

The gene DMBT1 (deleted in malignant brain tumour-1) has been proposed to play a role in brain and epithelial cancer, but shows unusual features for a classical tumour-suppressor gene. On the one hand, DMBT1 has been linked to mucosal protection, whereas, on the other, it potentially plays a role in epithelial differentiation. Thus its function in a particular tissue is of mechanistic importance for its role in cancer. Because the former function requires secretion to the lumen and the latter function may depend on its presence in the extracellular matrix, we decided to investigate DMBT1 expression, location and its mode of secretion during malignant transformation in colorectal cancer. Using human colorectal PC/AA cell lines and tissue sections from individual patients, we have examined the expression of DMBT1 and its glycosylation in the adenoma-carcinoma sequence leading to the adenocarcinoma phenotype.

Gastrointestinal mucins of Fut2-null mice lack terminal fucosylation without affecting colonization by Candida albicans

Posttranslational modification of apomucins by the sequential action of glycosyltransferases is required to produce mature mucins. The Secretor gene (FUT2) encodes an alpha(1,2)fucosyltransferase (EC 2.4.1.69) that catalyzes addition of terminal alpha(1,2)fucose residues on mucins and other molecules in mucosal epithelium. Mutant mice containing targeted replacement of Fut2 with the bacterial reporter gene lacZ were studied to determine the affect of the loss of Fut2 on glycosylation of mucins in the gastrointestinal tract. By whole organ X-gal staining, lacZ activity is prominently expressed in the foveolar pit and chief cells of the glandular stomach, Brunner's glands of the duodenum, and goblet cells in the large intestine of Fut2-LacZ-null mice. Staining with Aleuria aurantia agglutinin demonstrates loss of L-fucosylated epithelial glycans throughout the gastrointestinal tract of Fut2-LacZ-null mice, however, histologic appearance of the tissues appears normal. Analysis of oligosaccharides released from insoluble colonic mucins, largely Muc2, by mass spectrometry shows complete lack of terminal fucosylation of O-linked oligosaccharides in Fut2-LacZ-null mice. Precursor glycans accumulate with no evidence of compensation by other fucosyltransferases or sialyltransferases on mucin glycosylation. Because Candida albicans has been reported to adhere to intestinal mucins creating a potential reservoir associated with vaginitis, Fut2-LacZ-null and wild-type mice were inoculated by gastric lavage with C. albicans. We observe no difference in colonization between genotypes suggesting mucin terminal fucosylation does not significantly influence C. albicans-host interaction in the intestine, highlighting that infections caused by the same organism at different mucosal surfaces are not equal.

Intestinal candyfloss

In this issue of the Biochemical Journal, Robbe et al. have employed a state-of-the-art MS technique to re-examine the sweet and sticky sugar covering of human intestinal mucosa. The availability of highly sensitive MS methods has been instrumental in determining the complex structures of mucin oligosaccharides, and in demonstrating that there are variations in structure along the sections of the gut. In contrast to previous studies, these results described by Robbe et al. show some correlation with the activities of enzymes synthesizing mucin oligosaccharides in the human colon. Many questions still remain to be answered: for example, regarding the regulation of the relative amounts of sugar chains in individual mucins, and their roles in the homoeostasis of the intestinal mucosa. Structural studies are a solid basis for understanding the functions of sugar chains, and the mechanisms and significance of changes during the development of intestinal disease.