Proteomic analyses of the two mucus layers of the colon barrier reveal that their main component, the Muc2 mucin, is strongly bound to the Fcgbp protein

Mucin dynamics and enteric pathogens

The extracellular secreted mucus and the cell surface glycocalyx prevent infection by the vast numbers of microorganisms that live in the healthy gut. Mucin glycoproteins are the major component of these barriers. In this Review, we describe the components of the secreted and cell surface mucosal barriers and the evidence that they form an effective barricade against potential pathogens. However, successful enteric pathogens have evolved strategies to circumvent these barriers. We discuss the interactions between enteric pathogens and mucins, and the mechanisms that these pathogens use to disrupt and avoid mucosal barriers. In addition, we describe dynamic alterations in the mucin barrier that are driven by host innate and adaptive immune responses to infection.

Toward quantitative proteomics of organ substructures: implications for renal physiology

Organs are complex structures that consist of multiple tissues with different levels of gene expression. To achieve comprehensive coverage and accurate quantitation data, organs ideally should be separated into morphologic and/or functional substructures before gene or protein expression analysis. However, because of complex morphology and elaborate isolation protocols, to date this often has been difficult to achieve. Kidneys are organs in which functional and morphologic subdivision is especially important. Each subunit of the kidney, the nephron, consists of more than 10 subsegments with distinct morphologic and functional characteristics. For a full understanding of kidney physiology, global gene and protein expression analyses have to be performed at the level of the nephron subsegments; however, such studies have been extremely rare to date. Here we describe the latest approaches in quantitative high-accuracy mass spectrometry-based proteomics and their application to quantitative proteomics studies of the whole kidney and nephron subsegments, both in human beings and in animal models. We compare these studies with similar studies performed on other organ substructures. We argue that the newest technologies used for preparation, processing, and measurement of small amounts of starting material are finally enabling global and subsegment-specific quantitative measurement of protein levels in the kidney and other organs. These new technologies and approaches are making a decisive impact on our understanding of the (patho)physiological processes at the molecular level.

Antibiotic treatment alters the colonic mucus layer and predisposes the host to exacerbated Citrobacter rodentium-induced colitis

Antibiotics are often used in the clinic to treat bacterial infections, but the effects of these drugs on microbiota composition and on intestinal immunity are poorly understood. Citrobacter rodentium was used as a model enteric pathogen to investigate the effect of microbial perturbation on intestinal barriers and susceptibility to colitis. Streptomycin and metronidazole were used to induce alterations in the composition of the microbiota prior to infection with C. rodentium. Metronidazole pretreatment increased susceptibility to C. rodentium-induced colitis over that of untreated and streptomycin-pretreated mice, 6 days postinfection. Both antibiotic treatments altered microbial composition, without affecting total numbers, but metronidazole treatment resulted in a more dramatic change, including a reduced population of Porphyromonadaceae and increased numbers of lactobacilli. Disruption of the microbiota with metronidazole, but not streptomycin treatment, resulted in an increased inflammatory tone of the intestine characterized by increased bacterial stimulation of the epithelium, altered goblet cell function, and thinning of the inner mucus layer, suggesting a weakened mucosal barrier. This reduction in mucus thickness correlates with increased attachment of C. rodentium to the intestinal epithelium, contributing to the exacerbated severity of C. rodentium-induced colitis in metronidazole-pretreated mice. These results suggest that antibiotic perturbation of the microbiota can disrupt intestinal homeostasis and the integrity of intestinal defenses, which protect against invading pathogens and intestinal inflammation.

Intestinal goblet cells and mucins in health and disease: recent insights and progress

The mucus layer coating the gastrointestinal tract is the front line of innate host defense, largely because of the secretory products of intestinal goblet cells. Goblet cells synthesize secretory mucin glycoproteins (MUC2) and bioactive molecules such as epithelial membrane-bound mucins (MUC1, MUC3, MUC17), trefoil factor peptides (TFF), resistin-like molecule beta (RELMbeta), and Fc-gamma binding protein (Fcgbp). The MUC2 mucin protein forms trimers by disulfide bonding in cysteine-rich amino terminal von Willebrand factor (vWF) domains, coupled with crosslinking provided by TFF and Fcgbp proteins with MUC2 vWF domains, resulting in a highly viscous extracellular layer. Colonization by commensal intestinal microbiota is limited to an outer "loose" mucus layer, and interacts with the diverse oligosaccharides of mucin glycoproteins, whereas an "inner" adherent mucus layer is largely devoid of bacteria. Defective mucus layers resulting from lack of MUC2 mucin, mutated Muc2 mucin vWF domains, or from deletion of core mucin glycosyltransferase enzymes in mice result in increased bacterial adhesion to the surface epithelium, increased intestinal permeability, and enhanced susceptibility to colitis caused by dextran sodium sulfate. Changes in mucin gene expression and mucin glycan structures occur in cancers of the intestine, contributing to diverse biologic properties involved in the development and progression of cancer. Further research is needed on identification and functional significance of various components of mucus layers and the complex interactions among mucus layers, microbiota, epithelial cells, and the underlying innate and adaptive immunity. Further elucidation of the regulatory mechanisms involved in mucin changes in cancer and inflammation may lead to the development of novel therapeutic approaches.

Introduction to section I: overview of approaches to study cystic fibrosis pathophysiology

Mutation of the CFTR chloride channel was identified as the genetic basis of cystic fibrosis over 20 years ago; however, correlation of the pathophysiological changes occurring in CF lung disease with the mutation of a chloride channel is ongoing. The failure of innate lung defense in CF, and the subsequent cyclical microbial colonization of airways, explains the gross anatomical changes that occur in CF pathophysiology. However, ongoing research is focused on how the lack of the CFTR channel explains the failure of innate lung defense. Hydration status of the mucus blanket is key to understanding this link, and this series of chapters details the recent progress that has been made in understanding the interplay between ion transport activity and innate lung defense, and the initiation of CF lung pathophysiology.

Changes in the mucosal barrier during acute and chronic Trichuris muris infection

The intestinal mucosal barrier, part of the innate immune defence, is responsive to the external environment and changes in response to infection. There is disparate evidence for the epithelial and goblet cell products within the intrinsic barrier being part of a response to resolve infection. We comprehensively analysed the changes of mucosal glycoconjugates during acute and chronic infection by utilising the Trichuris muris (T. muris) model. Transcription factors, atonal homolog 1 (Math-1) and SAM pointed domain containing ETS transcription factor (Spdef) were activated during acute infection, which promoted stem cell fate towards a secretory cell phenotype. The thickness of the intermediate barrier, the carbohydrate-rich glycocalyx, composed of cell surface mucins increased with exposure to T. muris, with an increase in Muc4, Muc13 and Muc17. Overall, hypersecretion of glycoproteins into the extrinsic barrier (mediated by IL-13) via the gamma amino-butyric acid-α3 receptor (GABA-α3), was observed during acute infection. Furthermore, altered glycosylation was observed during acute and chronic infection; mucins were more highly charged during acute infection than during chronic infection. This study readdresses the changes within the mucosal barrier, in particular in the cell surface and secreted mucins during acute and chronic nematode infection.

Human intestinal TFF3 forms disulfide-linked heteromers with the mucus-associated FCGBP protein and is released by hydrogen sulfide

TFF3 is a secretory peptide belonging to the trefoil factor family with a predicted size of 59 amino acid residues containing seven cysteine residues. It is predominantly expressed in intestinal goblet cells where it plays a key role in mucosal regeneration and repair processes. In the course of these studies, human colonic TFF3 was shown to exist mainly as a high molecular weight heteromer. Purification of this heteromer and characterization by LC-ESI-MS/MS analysis identified the IgG Fc binding protein (FCGBP) as the disulfide-linked partner protein of TFF3. FCGBP is a constituent of intestinal mucus secreted by goblet cells. Furthermore, low amounts of TFF3/monomer and only little TFF3/dimer were detected in human colonic extracts. Here, we show that these TFF3 forms can be released from the purified TFF3-FCGBP heteromer complex in vitro by reduction with hydrogen sulfide (H(2)S). Such a mechanism would be in line with the high H(2)S concentrations reported to occur in the lumen of the colon. Of special note, this points to intestinal mucus as a reservoir for a biologically active peptide. Also proteolytic processing of FCGBP was observed which is in line with multiple autocatalytic cleavages as proposed earlier by Johansson et al. (J. Proteome Res. 2009 , 8 , 3549 - 3557).

The interaction of large bowel microflora with the colonic mucus barrier

The colonic mucus barrier is the first line of defence that the underlying mucosa has against the wide range of potentially damaging agents of microbial, endogenous, and dietary origin that occur within the colonic lumen. The functional component of mucus is the secreted, polymeric glycoprotein mucin. The mucus barrier can either act as an energy source or a support medium for growth to the intestinal microflora. The mucus barrier appears to effectively partition the vast number of microbial cells from the underlying epithelium. The normal functionality and biochemistry of this mucus barrier appears to be lost in diseases of the colorectal mucosa. Germ-free animal studies have highlighted the necessity of the presence of the colonic microflora to drive the maturation of the colonic mucosa and normal mucus production. A number of by-products of the microflora have been suggested to be key luminal drivers of colonic mucus secretion.

Bacteria penetrate the inner mucus layer before inflammation in the dextran sulfate colitis model

Background

Protection of the large intestine with its enormous amount of commensal bacteria is a challenge that became easier to understand when we recently could describe that colon has an inner attached mucus layer devoid of bacteria (Johansson et al. (2008) Proc. Natl. Acad. Sci. USA 105, 15064–15069). The bacteria are thus kept at a distance from the epithelial cells and lack of this layer, as in Muc2-null mice, allow bacteria to contact the epithelium. This causes colitis and later on colon cancer, similar to the human disease Ulcerative Colitis, a disease that still lacks a pathogenetic explanation. Dextran Sulfate (DSS) in the drinking water is the most widely used animal model for experimental colitis. In this model, the inflammation is observed after 3–5 days, but early events explaining why DSS causes this has not been described.

Principal Findings

When mucus formed on top of colon explant cultures were exposed to 3% DSS, the thickness of the inner mucus layer decreased and became permeable to 2 µm fluorescent beads after 15 min. Both DSS and Dextran readily penetrated the mucus, but Dextran had no effect on thickness or permeability. When DSS was given in the drinking water to mice and the colon was stained for bacteria and the Muc2 mucin, bacteria were shown to penetrate the inner mucus layer and reach the epithelial cells already within 12 hours, long before any infiltration of inflammatory cells.

Conclusion

DSS thus causes quick alterations in the inner colon mucus layer that makes it permeable to bacteria. The bacteria that reach the epithelial cells probably trigger an inflammatory reaction. These observations suggest that altered properties or lack of the inner colon mucus layer may be an initial event in the development of colitis.

The two mucus layers of colon are organized by the MUC2 mucin, whereas the outer layer is a legislator of host-microbial interactions

The normal intestinal microbiota inhabits the colon mucus without triggering an inflammatory response. The reason for this and how the intestinal mucus of the colon is organized have begun to be unraveled. The mucus is organized in two layers: an inner, stratified mucus layer that is firmly adherent to the epithelial cells and approximately 50 μm thick; and an outer, nonattached layer that is usually approximately 100 μm thick as measured in mouse. These mucus layers are organized around the highly glycosylated MUC2 mucin, forming a large, net-like polymer that is secreted by the goblet cells. The inner mucus layer is dense and does not allow bacteria to penetrate, thus keeping the epithelial cell surface free from bacteria. The inner mucus layer is converted into the outer layer, which is the habitat of the commensal flora. The outer mucus layer has an expanded volume due to proteolytic activities provided by the host but probably also caused by commensal bacterial proteases and glycosidases. The numerous O-glycans on the MUC2 mucin not only serve as nutrients for the bacteria but also as attachment sites and, as such, probably contribute to the selection of the species-specific colon flora. This observation that normal human individuals carry a uniform MUC2 mucin glycan array in colon may indicate such a specific selection.

Phosphatidylcholine as a constituent in the colonic mucosal barrier--physiological and clinical relevance

Phosphatidylcholine (PC) is an important constituent of the gastrointestinal tract. PC molecules are not only important in intestinal cell membranes but also receiving increasing attention as protective agents in the gastrointestinal barrier. They are largely responsible for establishing the hydrophobic surface of the colon. Decreased phospholipids in colonic mucus could be linked to the pathogenesis of ulcerative colitis, a chronic inflammatory bowel disease. Clinical studies revealed that therapeutic addition of PC to the colonic mucus of these patients alleviated the inflammatory activity. This positive role is still elusive, however, we hypothesized that luminal PC has two possible functions: first, it is essential for surface hydrophobicity, and second, it is integrated into the plasma membrane of enterocytes and it modulates the signaling state of the mucosa. The membrane structure and lipid composition of cells is a regulatory component of the inflammatory signaling pathways. In this perspective, we will shortly summarize what is known about the localization and protective properties of PC in the colonic mucosa before turning to its evident medical importance. We will discuss how PC contributes to our understanding of the pathogenesis of ulcerative colitis and how reinforcing the luminal phospholipid monolayer can be used as a therapeutic concept in humans.

ER stress and the unfolded protein response in intestinal inflammation

Endoplasmic reticulum (ER) stress is a phenomenon that occurs when excessive protein misfolding occurs during biosynthesis. ER stress triggers a series of signaling and transcriptional events known as the unfolded protein response (UPR). The UPR attempts to restore homeostasis in the ER but if unsuccessful can trigger apoptosis in the stressed cells and local inflammation. Intestinal secretory cells are susceptible to ER stress because they produce large amounts of complex proteins for secretion, most of which are involved in mucosal defense. This review focuses on ER stress in intestinal secretory cells and describes how increased protein misfolding could occur in these cells, the process of degradation of misfolded proteins, the major molecular elements of the UPR pathway, and links between the UPR and inflammation. Evidence is reviewed from mouse models and human inflammatory bowel diseases that ties ER stress and activation of the UPR with intestinal inflammation, and possible therapeutic approaches to ameliorate ER stress are discussed.

Mucin-lectin interactions assessed by flow cytometry

The O-glycosylated domains of mucins and mucin-type glycoproteins contain 50-80% of carbohydrate and possess expanded conformations. Herein, we describe a flow cytometry (FCM) method for determining the carbohydrate-binding specificities of lectins to mucin. Biotinylated mucin was immobilized on streptavidin-coated beads, and the binding specificities of the major mucin sugar chains, as determined by GC-MS and MALDI-ToF, were monitored using fluorescein-labeled lectins. The specificities of lectins toward specific biotinylated glycans were determined as controls. The advantage of flexibility, multiparametric data acquisition, speed, sensitivity, and high-throughput capability makes flow cytometry a valuable tool to study diverse interactions between glycans and proteins.

Muc2 protects against lethal infectious colitis by disassociating pathogenic and commensal bacteria from the colonic mucosa

Despite recent advances in our understanding of the pathogenesis of attaching and effacing (A/E) Escherichia coli infections, the mechanisms by which the host defends against these microbes are unclear. The goal of this study was to determine the role of goblet cell-derived Muc2, the major intestinal secretory mucin and primary component of the mucus layer, in host protection against A/E pathogens. To assess the role of Muc2 during A/E bacterial infections, we inoculated Muc2 deficient (Muc2^−/−) mice with Citrobacter rodentium, a murine A/E pathogen related to diarrheagenic A/E E. coli. Unlike wildtype (WT) mice, infected Muc2^−/− mice exhibited rapid weight loss and suffered up to 90% mortality. Stool plating demonstrated 10–100 fold greater C. rodentium burdens in Muc2^−/− vs. WT mice, most of which were found to be loosely adherent to the colonic mucosa. Histology of Muc2^−/− mice revealed ulceration in the colon amid focal bacterial microcolonies. Metabolic labeling of secreted mucins in the large intestine demonstrated that mucin secretion was markedly increased in WT mice during infection compared to uninfected controls, suggesting that the host uses increased mucin release to flush pathogens from the mucosal surface. Muc2 also impacted host-commensal interactions during infection, as FISH analysis revealed C. rodentium microcolonies contained numerous commensal microbes, which was not observed in WT mice. Orally administered FITC-Dextran and FISH staining showed significantly worsened intestinal barrier disruption in Muc2^−/− vs. WT mice, with overt pathogen and commensal translocation into the Muc2^−/− colonic mucosa. Interestingly, commensal depletion enhanced C. rodentium colonization of Muc2^−/− mice, although colonic pathology was not significantly altered. In conclusion, Muc2 production is critical for host protection during A/E bacterial infections, by limiting overall pathogen and commensal numbers associated with the colonic mucosal surface. Such actions limit tissue damage and translocation of pathogenic and commensal bacteria across the epithelium.

Enteropathogenic E. coli (EPEC) and Enterohemorrhagic E. coli (EHEC) are important causes of diarrheal disease and other serious complications worldwide. Despite many studies addressing the pathogenic strategies used by these microbes, how the host protects itself from these pathogens is poorly understood. A critical question we address here is whether the thick mucus layer that overlies the intestinal surface plays a role in host protection. Since EPEC and EHEC do not infect mice efficiently, we used a related mouse pathogen called Citrobacter rodentium to infect and compare responses between wildtype mice and Muc2-deficient mice, which are defective in mucus production. We show that Muc2-deficient mice are extremely susceptible to C. rodentium infection-induced mortality and disease. Muc2-deficient mice were also colonized faster and had higher pathogen burdens throughout the experiment. Resident (non-pathogenic) bacteria were found to interact with C. rodentium and host tissues in Muc2-deficient mice, indicating Muc2 regulates all forms of intestinal microbiota at the gut surface. Deficiency in mucus production also contributed to increased leakiness of the gut, which allowed microbes to enter mucosal tissues. Our study shows that Muc2-dependent mucus production is critical for effective management of both pathogenic and non-pathogenic bacteria during infection by an EPEC/EHEC-like pathogen.

The inner of the two Muc2 mucin-dependent mucus layers in colon is devoid of bacteria

We have recently shown that the colon is protected by an inner mucus layer that efficiently separates the bacteria in the outer mucus from the epithelial cells. The inner mucus is impervious for bacteria and built by a network formed by the MUC2 mucin. Lack or defects in this inner mucus layer allow bacteria to reach the epithelia, something that triggers colon inflammation.