Expression and glycosylation of the filamentous brush border glycocalyx (FBBG) during rabbit enterocyte differentiation along the crypt-villus axis

Designing the new generation of intelligent biocompatible carriers for protein and peptide delivery

Therapeutic proteins and peptides have revolutionized treatment for a number of diseases, and the expected increase in macromolecule-based therapies brings a new set of challenges for the pharmaceutics field. Due to their poor stability, large molecular weight, and poor transport properties, therapeutic proteins and peptides are predominantly limited to parenteral administration. The short serum half-lives typically require frequent injections to maintain an effective dose, and patient compliance is a growing issue as therapeutic protein treatments become more widely available. A number of studies have underscored the relationship of subcutaneous injections with patient non-adherence, estimating that over half of insulin-dependent adults intentionally skip injections. The development of oral formulations has the potential to address some issues associated with non-adherence including the interference with daily activities, embarrassment, and injection pain. Oral delivery can also help to eliminate the adverse effects and scar tissue buildup associated with repeated injections. However, there are several major challenges associated with oral delivery of proteins and peptides, such as the instability in the gastrointestinal (GI) tract, low permeability, and a narrow absorption window in the intestine. This review provides a detailed overview of the oral delivery route and associated challenges. Recent advances in formulation and drug delivery technologies to enhance bioavailability are discussed, including the co-administration of compounds to alter conditions in the GI tract, the modification of the macromolecule physicochemical properties, and the use of improved targeted and controlled release carriers.

The mucus and mucins of the goblet cells and enterocytes provide the first defense line of the gastrointestinal tract and interact with the immune system

The gastrointestinal tract is covered by mucus that has different properties in the stomach, small intestine, and colon. The large highly glycosylated gel-forming mucins MUC2 and MUC5AC are the major components of the mucus in the intestine and stomach, respectively. In the small intestine, mucus limits the number of bacteria that can reach the epithelium and the Peyer's patches. In the large intestine, the inner mucus layer separates the commensal bacteria from the host epithelium. The outer colonic mucus layer is the natural habitat for the commensal bacteria. The intestinal goblet cells secrete not only the MUC2 mucin but also a number of typical mucus components: CLCA1, FCGBP, AGR2, ZG16, and TFF3. The goblet cells have recently been shown to have a novel gate-keeping role for the presentation of oral antigens to the immune system. Goblet cells deliver small intestinal luminal material to the lamina propria dendritic cells of the tolerogenic CD103(+) type. In addition to the gel-forming mucins, the transmembrane mucins MUC3, MUC12, and MUC17 form the enterocyte glycocalyx that can reach about a micrometer out from the brush border. The MUC17 mucin can shuttle from a surface to an intracellular vesicle localization, suggesting that enterocytes might control and report epithelial microbial challenge. There is communication not only from the epithelial cells to the immune system but also in the opposite direction. One example of this is IL10 that can affect and improve the properties of the inner colonic mucus layer. The mucus and epithelial cells of the gastrointestinal tract are the primary gate keepers and controllers of bacterial interactions with the host immune system, but our understanding of this relationship is still in its infancy.

Identification of fitness determinants in Enterococcus faecalis by differential proteomics

Enterococcus (E.) faecalis is found as commensal in healthy humans, in a variety of fermented foods. It can serve as probiotic but also as pathogen causing endocarditis, bacteremia and urinary tract infections. We have employed a proteomic study with E. faecalis strain OG1RF under different growth conditions and in contact to mouse intestinal cells to identify novel latent and adaptive fitness determinants. These relate to changes in catabolic pathways (BudA), protein biosynthesis (AsnS), cellular surface biosynthesis (RmlA) and regulatory mechanisms (OmpR). This knowledge can be used to derive novel evidence-based targets, which can be used to further elucidate gene expression changes enhancing pathogenicity or fitness in a commensal strain and possibly delineate this species into groups of higher and lower risk for applications in a food or a medical context versus improved treatment strategies of the so far hard to cure diseases.

Interactions of the intestinal epithelium with the pathogen and the indigenous microbiota: a three-way crosstalk

The mucosal surfaces of the gastrointestinal tract harbor a vast number of commensal microbiota that have coevolved with the host, and in addition display one of the most complex relationships with the host. This relationship affects several important aspects of the biology of the host including the synthesis of nutrients, protection against infection, and the development of the immune system. On the other hand, despite the existence of several lines of mucosal defense mechanisms, pathogenic organisms such as Shigella and Salmonella have evolved sophisticated virulence strategies for breaching these barriers. The constant challenge from these pathogens and the attempts by the host to counter them set up a dynamic equilibrium of cellular and molecular crosstalk. Even slight perturbations in this equilibrium may be detrimental to the host leading to severe bacterial infection or even autoimmune diseases like inflammatory bowel disease. Several experimental model systems, including germ-free mice and antibiotic-treated mice, have been used by various researchers to study this complex relationship. Although it is only the beginning, it promises to be an exciting era in the study of these host-microbe relationships.

Three-dimensional ultrastructure of the brush border glycocalyx in the mouse small intestine: a high resolution scanning electron microscopic study

The three-dimensional ultrastructure of the filamentous glycocalyx of the brush border in the mouse small intestine was successfully demonstrated by high resolution scanning electron microscopy (SEM). The specimens were fixed with 2% glutaraldehyde in a 0.1M phosphate buffer (pH 7.4), and rinsed with buffered solutions with differently adjusted pH values (pH 3.0, 7.0 or 11.0). They were then osmicated, dried, spatter-coated with gold (1.0-1.5 nm), and observed under a high resolution SEM. The glycocalyx on the luminal surface of the intestinal villi covered the top of the microvilli of the epithelial cells and were well preserved in the specimens treated with an alkaline buffer (pH 11.0). The glycocalyx was observed as filamentous structures, 7 to 15 nm thick in diameter. These filaments repeatedly branched and anastomosed with neighboring ones to form an actual network or plexus as a whole, in contrast with superimposed images in transmission electron microscopy (TEM) which suggested that such anastomoses were pseudo-networks. The filaments thickened globularly at the sites of the filament bifurcation or branching. On the other hand, specimens rinsed with an acid or neutral buffer showed no glycocalyx on their microvilli, whose naked top had knob-like structures. Thus, the pH values of the washing buffer solutions were considered to affect the preservation of the surface coat due to molecular characteristics.

Expression of apical membrane L-glutamate transporters in neonatal porcine epithelial cells along the small intestinal crypt-villus axis

Enteral l-glutamate is extensively utilized as an oxidative fuel by the gut mucosa in the neonate. To identify major uptake pathways and to understand uptake regulation, we examined transport kinetics and molecular identities of apical membrane l-glutamate transporters in epithelial cells sequentially isolated along the small intestinal crypt-villus axis from milk protein-fed, 16-day-old pigs. The distended intestinal sac method was used to isolate 12 sequential cell fractions from the tip villus to the bottom crypt. Initial rates and kinetics of l-glutamate uptake were measured with l-[G-(3)H]glutamate by fast filtration in apical membrane vesicles prepared by Mg(2+) precipitation and differential centrifugation, with membrane potential clamped by SCN(-). Initial l-glutamate uptake results suggested the presence of B(o) and X(AG)(-) transport systems, but the X(AG)(-) system was predominant for uptake across the apical membrane. Kinetic data suggested that l-glutamate uptake through the X(AG)(-) system was associated with higher maximal transport activity but lower transporter affinity in crypt than in villus cells. Molecular identity of the X(AG)(-) glutamate transporter, based on immunoblot and RT-PCR analysis, was primarily the defined excitatory amino acid carrier (EAAC)-1. EAAC-1 expression was increased with cell differentiation and regulated at transcription and translation levels from crypt to upper villus cells. In conclusion, efficiency and capacity of luminal l-glutamate uptake across the apical membrane are regulated by changing expression of the X(AG)(-) system transporter gene EAAC-1 at transcription and translation levels as well as maximal uptake activity and transporter affinity along the intestinal crypt-villus axis in the neonate.

Mucosal barrier and immune mediators

The intestinal mucosa functions is an immunologic organ that plays a major role in the development of oral tolerance and host-defense mechanisms. Antigens must cross the intestinal epithelium in a controlled manner to interact with dendritic antigen-presenting cells, because bacteria or their products are a primary risk factor for the development of intestinal inflammation. Therefore, the regulation of the intestinal epithelial cell barrier is central to the development of intestinal immunity and inflammation, but the involved mechanisms are largely unknown. Intestinal barrier function relies on the formation of tight junctions at the apical contact areas of intestinal epithelial cells. Tight junctions have a highly dynamic structure whose permeability, assembly, or disassembly can be regulated by a variety of cellular and metabolic mediators, including cytokines, which have major functions in the immune system. Immune modulators control tight junction dependent intestinal barrier function during development, wound healing, and pathologic processes such as cancer, infection, and chronic inflammation.

Glycocalyx on rabbit intestinal M cells displays carbohydrate epitopes from Muc2

It is essential to investigate the apical surface properties of both M cells and dome enterocytes to understand the mechanisms involved in the binding of pathogens to M cells. In rabbit appendix tissue, monoclonal antibodies (MAbs) highlight differences between M cells (MAb 58) and dome enterocytes (MAb 214). Such antibodies ultimately recognized intestinal mucin-related epitopes. To further characterize these differences, the labeling patterns obtained with these MAbs were compared to those obtained with other antibodies to intestinal mucins on dissected domes from all gut-associated lymphoid tissues. A glycoprotein recognized by MAb 58 was purified on a CsCl isopycnic density gradient and microsequenced, and its mRNA expression was localized by in situ hybridization. It was identified as the rabbit homologue of human Muc2, i.e., the major mucin secreted in intestine tissue. Two other Muc2 carbohydrate epitopes were also expressed on M cells, although Muc2 mRNA was not detected. All results indicated that M cells express, on their apical membrane, glycoconjugates bearing at least three glycosidic epitopes from Muc2. MAb 214 and MAb 6G2, which recognized a partially characterized mucin expressed on dome enterocytes, were negative markers for M cells in rabbit gut-associated lymphoid tissues. We propose that the presence, on the surface of M cells, of carbohydrates also expressed on Muc2, together with the absence of an enterocyte-associated mucin, could favor pathogen attachment and accessibility to the M-cell luminal membrane.

Accessibility of glycolipid and oligosaccharide epitopes on rabbit villus and follicle-associated epithelium

The initial step in many mucosal infections is pathogen attachment to glycoconjugates on the apical surfaces of intestinal epithelial cells. We examined the ability of virus-sized (120-nm) and bacterium-sized (1-microm) particles to adhere to specific glycolipids and protein-linked oligosaccharides on the apical surfaces of rabbit Peyer's patch villus enterocytes, follicle-associated enterocytes, and M cells. Particles coated with the B subunit of cholera toxin, which binds the ubiquitous glycolipid GM1, were unable to adhere to enterocytes or M cells. This confirms that both the filamentous brush border glycocalyx on enterocytes and the thin glycoprotein coat on M cells can function as size-selective barriers. Oligosaccharides containing terminal beta(1,4)-linked galactose were accessible to soluble lectin Ricinus communis type I on all epithelial cells but were not accessible to lectin immobilized on beads. Oligosaccharides containing alpha(2, 3)-linked sialic acid were recognized on all epithelial cells by soluble Maackia amurensis lectin II (Mal II). Mal II coated 120-nm (but not 1-microm) particles adhered to follicle-associated enterocytes and M cells but not to villus enterocytes. The differences in receptor availability observed may explain in part the selective attachment of viruses and bacteria to specific cell types in the intestinal mucosa.

Live attenuated Salmonella: a paradigm of mucosal vaccines

Two key steps control immune responses in mucosal tissues: the sampling and transepithelial transport of antigens, and their targeting into professional antigen-presenting cells in mucosa-associated lymphoid tissue. Live Salmonella bacteria use strategies that allow them to cross the epithelial barrier of the gut, to survive in antigen-presenting cells where bacterial antigens are processed and presented to the immune cells, and to express adjuvant activity that prevents induction of oral tolerance. Two Salmonella serovars have been used as vaccines or vectors, S. typhimurium in mice and S. typhi in humans. S. typhimurium causes gastroenteritis in a broad host range, including humans, while S. typhi infection is restricted to humans. Attenuated S. typhimurium has been used successfully in mice to induce systemic and mucosal responses against more than 60 heterologous antigens. This review aims to revisit S. typhimurium-based vaccination, as an alternative to S. typhi, with special emphasis on the molecular pathogenesis of S. typhimurium and the host response. We then discuss how such knowledge constitutes the basis for the rational design of novel live mucosal vaccines.

Mucin-related epitopes distinguish M cells and enterocytes in rabbit appendix and Peyer's patches

The biochemical composition of the apical membranes of epithelial M cells overlying the gut-associated lymphoid tissues (GALT) is still largely unknown. We have prepared monoclonal antibodies (MAbs) directed against carbonate-washed plasma membranes from epithelial cells detached with EDTA from rabbit appendix, a tissue particularly rich in GALT. As determined by immunofluorescence microscopy, several MAbs specifically recognized either M cells or enterocyte-like cells of the domes from rabbit appendix, sacculus rotundus, and Peyer's patches. M cells were identified by their large ventral pocket containing lymphoid cells and by specific labeling with antivimentin. Among various characterized MAbs, MAb 104 recognized rabbit immunoglobulins and was used as an apical marker for M cells in the rabbit appendix, MAb 58 selectively stained an integral membrane glycoprotein of greater than 205 kDa located at the apex of M cells, and MAb 214 stained a smaller soluble glycoprotein associated with the apical surfaces from neighboring enterocytes. In addition, both MAbs 58 and 214 also labeled luminal mucus and secretory granules in goblet cells. The selective association of mucin-related molecules at the surfaces of either M cells or enterocyte-like cells of the follicle-associated epithelium suggests that specific carbohydrate antigens are differentially expressed by epithelial cells and could account for the differential binding properties of pathogens.

Glycoconjugate expression defines the origin and differentiation pathway of intestinal M-cells

Intestinal M-cells are specialized epithelial cells located in the domes of the gut-associated lymphoid tissues, which transport antigens from the lumen to the underlying lymphoid tissue, thereby initiating immune reactions. It is assumed that M-cells arise from stem cells in the crypts, from which they migrate to the top of the domes. To study the differentiation pathway of M-cells, we used the rabbit cecal lymphoid patch in which the M-cells express high levels of alpha 1-2-linked fucose and N-acetyl-galactosamine residues in their apical membrane. Dome areas were labeled with fluorescein- and rhodamine-conjugated lectins specific for alpha 1-2-linked fucose and N-acetyl-galactosamine in vivo and in vitro, and were observed with confocal laser scanning microscopy. Ultrathin sections were double labeled with lectin-gold conjugates and the labeling density was quantified by computer-based image analysis. All cecal patch M-cells expressed alpha 1-2-linked fucose and N-acetyl-galactosamine, but the amount of the two saccharides varied considerably depending on the position of the M-cells at the base, flank, or top of the dome. In eight of 18 rabbits studied, radial strips of M-cells with common glycosylation patterns were observed, each strip associated with an individual crypt. Confocal microscopy revealed that lectin-labeled M-cells were not restricted to the dome epithelium but were also detected in the upper third of crypts surrounding the domes. The results show that M-cells are heterogeneous concerning the glycosylation pattern of membrane glycoconjugates. This pattern is modified as the M-cells differentiate and migrate from the base to the top of the dome. Radial strips of M-cells with a common proclivity of glycoconjugate expression suggest that those M-cells that derive from the same crypt have a clonal origin. The presence of (pre-) M-cells in the crypts surrounding the domes indicates that M-cells derive directly from undifferentiated crypt cells and do not develop from differentiated enterocytes.