Structural aspects of blood group glycosphingolipids in the gastrointestinal tract

Characterization of moose intestinal glycosphingolipids

As a part of a systematic investigation of the species-specific expression of glycosphingolipids, acid and non-acid glycosphingolipids were isolated from three small intestines and one large intestine of the moose (Alces alces). The glycosphingolipids were characterized by binding of monoclonal antibodies, lectins and bacteria in chromatogram binding assays, and by mass spectrometry. The non-acid fractions were complex mixtures, and all had glycosphingolipids belonging to the lacto- and neolactoseries (lactotriaosylceramide, lactotetraosylceramide, neolactotetraosylceramide, Galα3-Le^x hexaosylceramide, and lacto-neolactohexaosylceramide), globo-series (globotriaosylceramide and globotetraosylceramide), and isogloboseries (isoglobotriaosylceramide). Penta- and heptaglycosylceramides with terminal Galili determinants were also characterized. Furthermore, glycosphingolipids with terminal blood group O determinants (H triaosylceramide, H type 2 pentaosylceramide, H type 1 penta- and heptaosylceramide) were characterized in two of the moose small intestines, and in the one large intestine, while the third small intestine had glycosphingolipids with terminal blood group A determinants (A tetraosylceramide, A type 1 hexa- and octaosylceramide, A dodecaosylceramide). The acid glycosphingolipid fractions of moose small and large intestine contained sulfatide, and the gangliosides GM3, GD3, GD1a, GD1b, and also NeuGc and NeuAc variants of the Sd^a ganglioside and the sialyl-globopenta/SSEA-4 ganglioside. In humans, the NeuAc-globopenta/SSEA-4 ganglioside is a marker of embryonic and adult stem cells, and is also expressed in several human cancers. This is the first time sialyl-globopentaosylceramide/SSEA-4 has been characterized in a fully differentiated normal tissue, and also the first time NeuGc-globopentaosylceramide has been characterized.

Altered O-glycosylation profile of MUC2 mucin occurs in active ulcerative colitis and is associated with increased inflammation

BACKGROUND

The MUC2 mucin organizes the two mucus layers in the colon. This mucin carries a large number of O-glycans that are assumed to be attachment sites for the commensal flora found in the outer mucus layer.

METHODS

Single biopsies from the sigmoid colon of controls (25) and patients with inactive (13) or active (15) ulcerative colitis (UC) were collected during routine colonoscopy. The insoluble MUC2 mucin was prepared and separated by gel electrophoresis, its relative amount estimated, its O-glycans released, and glycans analyzed by novel sensitive glycomics chromatography / mass spectrometry providing information on glycan structures and relative abundances. The glycosylation pattern was related to the degree of mucosal inflammation and clinical severity of the disease.

RESULTS

The relative abundance of MUC2 showed high individual variability. Two major glycan profiles were found; a normal pattern in the control and inactive UC patients and an aberrant profile in patients with active colitis with an increase in a subset of the smaller glycans and a decrease of several complex glycans. The magnitude of this phenomenon was significantly related to both the degree of inflammation in the biopsies and also to some extent the severity of disease course. The aberrant profile was further shown to be reversible upon remission.

CONCLUSIONS

In the majority of the active UC patients MUC2 mucin has an altered glycan profile as compared to inactive UC and control patients. Patients with strong alterations in the glycan pattern tended to have a more severe disease course.

Composition and functional role of the mucus layers in the intestine

In discussions on intestinal protection, the protective capacity of mucus has not been very much considered. The progress in the last years in understanding the molecular nature of mucins, the main building blocks of mucus, has, however, changed this. The intestinal enterocytes have their apical surfaces covered by transmembrane mucins and the whole intestinal surface is further covered by mucus, built around the gel-forming mucin MUC2. The mucus of the small intestine has only one layer, whereas the large intestine has a two-layered mucus where the inner, attached layer has a protective function for the intestine, as it is impermeable to the luminal bacteria.

Histo-blood group antigens act as attachment factors of rabbit hemorrhagic disease virus infection in a virus strain-dependent manner

Rabbit Hemorrhagic disease virus (RHDV), a calicivirus of the Lagovirus genus, and responsible for rabbit hemorrhagic disease (RHD), kills rabbits between 48 to 72 hours post infection with mortality rates as high as 50–90%. Caliciviruses, including noroviruses and RHDV, have been shown to bind histo-blood group antigens (HBGA) and human non-secretor individuals lacking ABH antigens in epithelia have been found to be resistant to norovirus infection. RHDV virus-like particles have previously been shown to bind the H type 2 and A antigens. In this study we present a comprehensive assessment of the strain-specific binding patterns of different RHDV isolates to HBGAs. We characterized the HBGA expression in the duodenum of wild and domestic rabbits by mass spectrometry and relative quantification of A, B and H type 2 expression. A detailed binding analysis of a range of RHDV strains, to synthetic sugars and human red blood cells, as well as to rabbit duodenum, a likely gastrointestinal site for viral entrance was performed. Enzymatic cleavage of HBGA epitopes confirmed binding specificity. Binding was observed to blood group B, A and H type 2 epitopes in a strain-dependent manner with slight differences in specificity for A, B or H epitopes allowing RHDV strains to preferentially recognize different subgroups of animals. Strains related to the earliest described RHDV outbreak were not able to bind A, whereas all other genotypes have acquired A binding. In an experimental infection study, rabbits lacking the correct HBGA ligands were resistant to lethal RHDV infection at low challenge doses. Similarly, survivors of outbreaks in wild populations showed increased frequency of weak binding phenotypes, indicating selection for host resistance depending on the strain circulating in the population. HBGAs thus act as attachment factors facilitating infection, while their polymorphism of expression could contribute to generate genetic resistance to RHDV at the population level.

Rabbit hemorrhagic disease virus (RHDV), detected as late as 1984, has spread to large parts of the world, threatening rabbit populations and other species dependent on rabbits in many European countries. Mortality has been shown to be as high as 90% and rabbits are killed 48 to 72 hours after infection. Related viruses called noroviruses, infect humans in a manner dependent on the expression of histo-blood group antigens (HBGAs), which are not only expressed on red blood cells, but also on epithelial cells, in saliva and on mucins of the intestinal tract. RHDV also binds to HBGA and in this report we characterize binding of strains of all genetic groups of RHDV to different HBGAs. We also demonstrate HBGAs to function as attachment factors in a challenge experiment. As polymorphisms of genes involved in HBGA synthesis divide the rabbit population into different subgroups, we find selection of low-binding subgroups of wild rabbits in populations recovering from devastating outbreaks of RHDV. This is the first demonstration of differential HBGA specificities of RHDV strains, description of function in infection and demonstration of host selection due to RHDV infection based on HBGA phenotype.

Initial adherence of EPEC, EHEC and VTEC to host cells

Initial adherence to host cells is the first step of the infection of enteropathogenic Escherichia coli (EPEC), enterohaemorrhagic Escherichia coli (EHEC) and verotoxigenic Escherichia coli (VTEC) strains. The importance of this step in the infection resides in the fact that (1) adherence is the first contact between bacteria and intestinal cells without which the other steps cannot occur and (2) adherence is the basis of host specificity for a lot of pathogens. This review describes the initial adhesins of the EPEC, EHEC and VTEC strains. During the last few years, several new adhesins and putative colonisation factors have been described, especially in EHEC strains. Only a few adhesins (BfpA, AF/R1, AF/R2, Ral, F18 adhesins) appear to be host and pathotype specific. The others are found in more than one species and/or pathotype (EPEC, EHEC, VTEC). Initial adherence of EPEC, EHEC and VTEC strains to host cells is probably mediated by multiple mechanisms.

Innate immune lectins kill bacteria expressing blood group antigen

The expression of ABO(H) blood group antigens causes deletion of cells that generate self anti-blood group antibodies, but this deletion limits adaptive immunity toward pathogens bearing cognate blood group antigens. To explore potential defense mechanisms against these pathogens, given such limitations in adaptive immunity, we screened for innate proteins that could recognize human blood group antigens. Here we report that two innate immune lectins, galectins-4 and -8, which are expressed in the intestinal tract, recognize and kill human blood group antigen-expressing E. coli, while failing to alter viability of other E. coli strains or other gram-negative or gram-positive organisms both in vitro and in vivo. Killing by both galectins-4 and -8 resides within their C-terminal domains, occurs rapidly and independently of complement, and is accompanied by disruption of membrane integrity. These results demonstrate that innate defense lectins can provide immunity against pathogens that display blood group self-antigens on their surface.

The Multiple Carbohydrate Binding Specificities of Helicobacter pylori

Persistent colonization of the human stomach by Helicobacter pylori is a risk factor for the development of peptic ulcer disease and gastric cancer. Adhesion of microbes to the target tissue is an important determinant for successful initiation, establishment and maintenance of infection, and a variety of different candidate carbohydrate receptors for H. pylori have been identified. Here the different the binding specifities, and their potential role in adhesion to human gastric epithelium are described. Finally, recent findings on the roles of sialic acid binding SabA adhesin in interactions with human neutrophils and erythrocytes are discussed.

Blood group A glycosyltransferase occurring as alleles with high sequence difference is transiently induced during a Nippostrongylus brasiliensis parasite infection

Neutral mucin oligosaccharides from the small intestine of control rats and rats infected with the parasite Nippostrongylus brasiliensis were released and analyzed by gas chromatography-mass spectrometry. Infected animals expressed seven blood group A-like structures that were all absent in the control animals. The blood group A nature of these epitopes was confirmed by blood group A reactivity of the prepared mucins, of which Muc2 was one. Transferase assays and Northern blotting on small intestines from infected animals showed that an alpha-N-acetylgalactosaminyltransferase similar to the human blood group A glycosyltransferase had been induced. The expression was a transient event, with a maximum at day 6 of the 13-day-long infection. The rat blood group A glycosyltransferase was cloned, revealing two forms with an amino acid similarity of 95%. Both types had blood group A transferase activity and were probably allelic because none of 12 analyzed inbred strains carried both types. The second type was found in outbred rats and in one inbred strain. First generation offspring of inbred rats of each type were heterozygous, further supporting the allelic hypothesis. The transient induction and the large allelic variation could suggest that glycosyltransferases are part of a dynamic system altering mucins and other glycoconjugates as a protecting mechanism against microbial challenges.

Two glycosylation alterations of mouse intestinal mucins due to infection caused by the parasite Nippostrongylus brasiliensis

The glycosylation alterations of mouse small intestinal mucins during a 12-day infectious cycle caused by the parasite Nippostrongylus brasiliensis have been studied. The guanidinium chloride insoluble mucins were isolated at day 0 to 12 from the small intestine of infected and non-infected C57BL/6 mice. The O-linked oligosaccharides were released by reductive beta-elimination from the mucins and separated into neutral, sialylated and sulfated fractions. All fractions were analyzed by monosaccharide composition analysis and the neutral oligosaccharides were structurally characterized by gas chromatography/mass spectrometry. Two oligosaccharides containing blood group H-type epitopes (Fucalpha1-2Gal-) were transiently expressed with a maximum at day 6. Additional oligosaccharides with the common structure HexNAc-Gal-3GalNAcol were transiently induced with a maximum at day 10. Northern blot analysis on total RNA showed a transient expression at day 4-6 of the Fut2 gene encoding a Fucalpha1-2 fucosyltransferase, probably responsible for the detected blood group H-type epitopes. Comparisons with the corresponding infection in rat studied previously, revealed structurally different alterations, although occurring as transient events in both species. Both showed an induced blood group-type transferase halfway through the infection (a blood group A transferase in rat) and an induced transferase adding a terminal GalNAc (to a sialic acid- containing epitope in rat) towards the end of the infection. These differences between closely related species suggest rapid evolutionary alterations in glycosyltransferase expression.

Introduction to porcine red blood cells: implications for xenotransfusion

Advances in the field of xenotransplantation raise the intriguing possibility of using porcine red blood cells (pRBCs) as an alternative source for blood transfusion. The domestic pig is considered the most likely donor species for xenotransplantation. However, identification of xenoantigens on porcine erythrocytes and elucidation of their possible roles in antibody-mediated RBC destruction are necessary for developing clinical strategies to circumvent immunological incompatibility between humans and pigs. Although the alphaGal epitope (Galalpha1,3Galbeta1,4GIcNAc-R) is the major xenoantigen on porcine erythrocytes and is responsible for the binding of the majority of human natural antibodies, other non-alphaGal xenoantigens have been identified. The importance of these non-alphaGal xenoantigens in binding human natural antibodies and subsequently triggering immunological responses cannot be underestimated. Our data suggest that non-alphaGal xenoantigen(s) identified on the porcine erythrocyte membrane are not only recognized by xenoreactive human natural antibodies but are also involved in complement-mediated hemolysis.

Creating and maintaining the gastrointestinal ecosystem: what we know and need to know from gnotobiology

Studying the cross talk between nonpathogenic organisms and their mammalian hosts represents an experimental challenge because these interactions are typically subtle and the microbial societies that associate with mammalian hosts are very complex and dynamic. A large, functionally stable, climax community of microbes is maintained in the murine and human gastrointestinal tracts. This open ecosystem exhibits not only regional differences in the composition of its microbiota but also regional differences in the differentiation programs of its epithelial cells and in the spatial distribution of its component immune cells. A key experimental strategy for determining whether "nonpathogenic" microorganisms actively create their own regional habitats in this ecosystem is to define cellular function in germ-free animals and then evaluate the effects of adding single or several microbial species. This review focuses on how gnotobiotics-the study of germ-free animals-has been and needs to be used to examine how the gastrointestinal ecosystem is created and maintained. Areas discussed include the generation of simplified ecosystems by using genetically manipulatable microbes and hosts to determine whether components of the microbiota actively regulate epithelial differentiation to create niches for themselves and for other organisms; the ways in which gnotobiology can help reveal collaborative interactions among the microbiota, epithelium, and mucosal immune system; and the ways in which gnotobiology is and will be useful for identifying host and microbial factors that define the continuum between nonpathogenic and pathogenic. A series of tests of microbial contributions to several pathologic states, using germ-free and ex-germ-free mice, are proposed.

Distinct sub-populations of carcinoma-associated MUC1 mucins as detected by the monoclonal antibody 9H8 and antibodies against the sialyl-Lewis a and sialyl-Lewis x epitopes in the circulation of breast-cancer patients

The cancer-associated epitope defined by the monoclonal antibody (MAb) 9H8 was shown to be closely related to the T antigen (Thomsen-Friedenreich antigen) by its sensitivity to 0-glycanase treatment of a mucin glycopeptide known to express this epitope. The reactivity with this glycopeptide increased upon neuraminiclase treatment, and among several MAbs tested for ability to block binding of the 9H8 antibody, the one specific for the T antigen was the most efficient. Out of 41 serum samples from breast-cancer patients, 11 showed elevated levels of the 9H8 epitope, and several sera also showed elevated levels of the cancer-associated carbohydrate epitopes sialyl-Lewis a and sialyl-Lewis x. By the use of antibodies specific for the MUC1 apoprotein (Ma552 and HMFG-2) it could be shown that these epitopes were attached to the MUC1 apoprotein in at least 4 of the cases. By combining antibodies specific to 9H8, sialyl-Lewis a and sialyl-Lewis x in catcher and tracer positions in several types of immunofluorometric assays, it was shown that the 9H8 epitope was rarely co-expressed with sialyl-Lewis a or sialyl-Lewis x epitopes an the same molecule, though all were expressed on MUC1 mucins. In fact, they can be considered as mutually exclusive epitopes, suggesting that these sera contained different populations of MUC1 mucins distinguishable by different sets of oligosaccharides. The existence of mutually exclusive carbohydrate epitopes on different MUC1 mucins in one and the same patient should be taken into account when designing immunoassays exploiting MUC1-reactive antibodies.

N-linked glycopeptides with blood group determinants lacking neuraminic acid from the epithelial cells of rat small and large intestine

The N-linked type of glycans were prepared as their glycopeptides after pronase digestion of the epithelial cells from the small and large intestine of two inbred strains of rat. These glycopeptides were analysed for sugar composition, for blood-group activity, by 1H-NMR spectroscopy, and after permethylation by electron-impact mass spectrometry. The glycopeptides were of the triantennary and tetraantennary types with intersected GlcNAc. The terminal parts were, in contrast to most N-linked glycans, devoid of neuraminic acid residues. Instead they contained blood-group determinants. Blood-group-H types 1 (Fuc alpha 1-2Gal beta 1-3GlcNAc) and 2(Fuc alpha 1-2Gal beta 1-4GlcNAc) were found in the small and large intestines of both strains, although type-1 predominated. One rat strain (GOT-W) did not express blood-group-A glycopeptides in the small intestine, but the large intestine from the same strain did. The other strain (GOT-BW) expressed blood-group-A determinants in the small intestine. The lack of neuraminic acid residues in the small and large intestine and of blood-group-B activity in the large intestine differed from that found in glycosphingolipids obtained from the same organs.

A sialoglycoprotein complex linked to the microvillus cytoskeleton acts as a receptor for pilus (AF/R1) mediated adhesion of enteropathogenic Escherichia coli (RDEC-1) in rabbit small intestine

Escherichia coli strain RDEC-1 is an enteroadherent, diarrheagenic pathogen in rabbits that utilizes AF/R1 pili for initial (stage 1) adherence, but the host receptors for this adhesion are unknown. Here we demonstrate that RDEC-1 binds, via AF/R1 pili, to a specific rabbit ileal microvillus membrane glycoprotein receptor complex of subunits 130 and 140 kD. The binding involves sialic acid present on oligosaccharide moieties of the glycoprotein receptor. Furthermore, the microvillus membrane glycoprotein receptor complex appears to be associated with cytoskeletal components via brush border myosin 1. This newly described link between AF/R1 receptor and cytoskeletal components suggests that, in addition to this function in mucosal adherence, the pili may facilitate subsequent (second stage) close effacing attachment of RDEC-1 to the host epithelium by influencing cytoskeletal function.

Effects of alcohol on lectin binding affinity in rat gastric mucosa

Fluoresceinated lectins were employed to qualitatively evaluate cell surface carbohydrates, with and without ethanol exposure, in rat stomach mucosae. Rats received 1 ml of saline, or 50% or 100% ethanol orally. After 30 min, tissue samples of the glandular stomach were retrieved, cryosectioned, and incubated with one of a panel of lectins. Another set of sections was preincubated with neuraminidase to remove sialic acid residues. Qualitative evaluation of lectin binding showed that although several different sites stained, concanavalin A was the only lectin to stain the extracellular matrix, and soybean agglutinin the only lectin to stain chief cells. Neuraminidase preincubation enhanced lectin binding to both stained and previously unstained sites. Ethanol, both 50% and 100%, produced changes in both neuraminidase-treated and untreated tissues, increasing the specific binding of concanavalin A, Ulex europaeus agglutinin I, and wheat germ agglutinin, while decreasing Helix pomatia agglutinin and soybean agglutinin. These results suggest that ethanol can, through unknown mechanisms, alter carbohydrate binding affinity.