Muc1 mucin limits both Helicobacter pylori colonization of the murine gastric mucosa and associated gastritis

Glycan-mediated adhesion mechanisms in antibiotic-resistant bacteria

Bacterial adhesins play a central role in host-pathogen interactions, with many specifically targeting glycans to mediate bacterial colonization, influence infection dynamics, and evade host immune responses. In this review, we focus on bacterial pathogens identified by the World Health Organization as critical threats to public health and in urgent need of new treatments. We summarize glycoconjugate targets identified in the literature across 19 bacterial genera and species. This comprehensive review provides a foundation for the development of innovative therapeutic strategies to effectively combat these pathogens.

Human milk as a complex natural product

Covering: up to the end of 2024Breastfeeding is one of the most effective ways to promote child health. However, characterizing the chemistry that fortifies the benefits of breastfeeding remains a grand challenge. Current efforts in the community are focused on characterizing the roles of the different carbohydrates, proteins, and fats in milk. The goal of this review is to highlight and describe current knowledge about the major classes of macromolecules in human milk and their potential role in infant health and wellness.

Bacillus coagulans alleviates intestinal barrier injury induced by Klebsiella pneumoniae in rabbits by regulating the TLR4/MyD88/NF-κB signalling pathway

Probiotics effectively alleviate host diarrhoea, but the specific mechanism is not clear. Therefore, we explored the protective mechanism of Bacillus coagulans (BC) on intestinal barrier injury induced by Klebsiella pneumoniae (K. pneumoniae) in rabbits by HE, immunofluorescence and 16S rRNA. The results showed that BC pretreatment alleviated the changes in average daily gain, average daily feed intake and FCR caused by K. pneumoniae in rabbits. Moreover, BC alleviated the inflammatory cell infiltration, intestinal villus reduction, crypt deepening and goblet cell reduction caused by K. pneumoniae in rabbits. Further research revealed that BC improved the intestinal barrier by improving the mechanical barrier, chemical barrier, immune barrier and microbial barrier. Specifically, BC improved the intestinal mechanical barrier by improving the intestinal structure, increasing the protein expression of PCNA, increasing the number of goblet cells, and altering the expression of occludin, claudin-1 and ZO-1. BC improved the intestinal chemical barrier by regulating the expression of MUC1 and MUC2 and inhibited the TLR4/MyD88/NF-κB signalling pathway by altering the expression levels of the inflammatory factors IL-1β, IL-6 and TNF-α, thus optimizing the intestinal immune barrier. In addition, adding BC to the diet improved the intestinal microbial barrier of rabbits by reducing the abundance of harmful bacteria and increasing the abundance of beneficial bacteria. In summary, BC protects against K. pneumoniae-induced intestinal barrier damage by improving intestinal morphology, mitigating the inflammatory response and regulating the microbial composition. Among the pretreatments, the pretreatment effect of 1 × 106 CFU/g was the best. This study provides a theoretical basis for the use of BC to prevent and treat diarrhoea caused by K. pneumoniae in rabbits.

Expression and distribution of MUC1 in the developing and adult kidney

Mucin 1 (or MUC1) is a heterodimeric transmembrane glycoprotein expressed on the apical surface of polarized epithelial cells in several tissues including the kidney. Recent studies have revealed several novel roles of MUC1 in the kidney, potentially including bacterial infection, mineral balance, and genetic interstitial kidney disease, even though MUC1 levels are reduced not only in the kidney but also in all tissues due to MUC1 mutations. A careful localization of MUC1 in discrete segments of the nephron is the first step in understanding the multiple functional roles of MUC1 in the kidney. Most published reports of MUC1 expression to date have been largely confined to cultured cells, tumor tissues, and selective nephron segments of experimental rodents, and very few studies have been performed using human kidney tissues. Given the rising attention to the role of MUC1 in differing components of renal physiology, we carefully examined the kidney distribution of MUC1 in both human and mouse kidney sections using well-defined markers for different nephron segments or cell types. We further extended our investigation to include sections of early stages of mouse kidney development and upon injury in humans. We included staining for MUC1 in urothelial cells, the highly specialized epithelial cells lining the renal pelvis and bladder. These data implicate a role for MUC1 in antimicrobial defense. Our study provides the groundwork to test the physiological relevance of MUC1 in the urinary tract.NEW & NOTEWORTHY MUC1 is a transmembrane glycoprotein expressed on the apical surface of polarized epithelial tissues and most carcinomas. MUC1 may play novel roles in the kidney including defense against infections. Here, we examine the expression of MUC1 in mouse and human kidneys. We show that the distal nephron and the urinary system are the predominant sites of expression of both message and protein, implicating segment-specific roles including distal nephron defense against ascending bacteria.

MUC17 is an essential small intestinal glycocalyx component that is disrupted in Crohn's disease

Crohn's disease (CD) is the chronic inflammation of the terminal ileum and colon triggered by a dysregulated immune response to bacteria, but insights into specific molecular perturbations at the critical bacteria-epithelium interface are limited. Here, we report that the membrane mucin MUC17 protected small intestinal enterocytes against commensal and pathogenic bacteria. In noninflamed CD ileum, reduced MUC17 levels and a compromised glycocalyx barrier allowed recurrent bacterial contact with enterocytes. Muc17 deletion in mice rendered the small intestine particularly prone to atypical bacterial infection while maintaining resistance to colitis. The loss of Muc17 resulted in spontaneous deterioration of epithelial homeostasis and in the extraintestinal translocation of bacteria. Finally, Muc17-deficient mice harbored specific small intestinal bacterial taxa observed in patients with CD. Our findings highlight MUC17 as an essential region-specific line of defense in the small intestine with relevance for early epithelial defects in CD.

FGF20 modulates gut microbiota to mitigate dextran sodium sulfate-induced ulcerative colitis in mouse models

Ulcerative colitis (UC), a prevalent form of inflammatory bowel disease (IBD), presents a significant clinical challenge due to the lack of optimal therapeutic strategies. Emerging evidence suggests that fibroblast growth factor 20 (FGF20) may play a crucial role in mitigating UC symptoms, though the mechanistic underpinnings remain elusive. In this study, a mouse model of UC was established using dextran sodium sulfate (DSS) to investigate the potential role of FGF20. Our findings revealed a marked reduction in FGF20 expression in the serum and colonic tissues of DSS-treated mice. Furthermore, FGF20 knockout did not exacerbate colonic damage in these mice. Conversely, overexpression of FGF20 via adeno-associated virus (AAV) significantly alleviated UC-associated symptoms. This alleviation was evidenced by attenuated intestinal shortening, mitigated weight loss, increased colonic goblet cell density and crypt formation, reduced inflammation severity and inflammatory cell infiltration, and enhanced expression of tight junction and mucin proteins. Moreover, FGF20 significantly ameliorated the dysbiosis of gut microbiota in DSS-treated mice by increasing the abundance of beneficial bacteria and decreasing the abundance of harmful bacteria. The beneficial effects of FGF20 were notably attenuated following gut microbiota depletion with an antibiotic regimen. Fecal microbiota transplantation experiments further supported the critical role of gut microbiota in mediating the effects of FGF20 on DSS-treated mice. In conclusion, these findings highlight the potential involvement of gut microbiota in the therapeutic effects of FGF20 in UC.

The role of the mucosal barrier system in maintaining gut symbiosis to prevent intestinal inflammation

In the intestinal tract, where numerous intestinal bacteria reside, intestinal epithelial cells produce and release various antimicrobial molecules that form a complex barrier on the mucosal surface. These barrier molecules can be classified into two groups based on their functions: those that exhibit bactericidal activity through chemical reactions, such as antimicrobial peptides, and those that physically hinder bacterial invasion, like mucins, which lack bactericidal properties. In the small intestine, where Paneth cells specialize in producing antimicrobial peptides, the chemical barrier molecules primarily inhibit bacterial growth. In contrast, in the large intestine, where Paneth cells are absent, allowing bacterial growth, the primary defense mechanism is the physical barrier, mainly composed of mucus, which controls bacterial movement and prevents their invasion of intestinal tissues. The expression of these barrier molecules is regulated by metabolites produced by bacteria in the intestinal lumen and cytokines produced by immune cells in the lamina propria. This regulation establishes a defense mechanism that adapts to changes in the intestinal environment, such as alterations in gut microbial composition and the presence of pathogenic bacterial infections. Consequently, when the integrity of the gut mucosal barrier is compromised, commensal bacteria and pathogenic microorganisms from outside the body can invade intestinal tissues, leading to conditions such as intestinal inflammation, as observed in cases of inflammatory bowel disease.

Teleost Muc2 and Muc5ac: Key guardians of mucosal immunity in flounder (Paralichthys olivaceus)

Mucins secreted by mucous cells constitute a core part of the defense line against the invasion of pathogens. However, mucins' structure and immunological functions remain largely unknown in teleost fish. In this study, two typical mucins, Muc2 and Muc5ac of flounder (Paralichthys olivaceus), were cloned and their physicochemical properties, structure and conservation were analyzed. Notably, specific antibodies against flounder Muc2 and Muc5ac were developed. It was verified at the gene and protein level that Muc2 was expressed in the hindgut and gills but not in the skin, while Muc5ac was expressed in the skin and gills but not in the hindgut. After flounders were immunized by immersion with inactivated Edwardsiella tarda, Muc2 and Muc5ac were significantly upregulated at both the gene expression and protein levels, and Muc2+/Muc5ac+ mucous cells proliferated and increased secretion of Muc2 and Muc5ac. Moreover, Muc2 and Muc5ac exerted retention and clearance effects on E. tarda in a short period (within 1 dpi). These results revealed the characterization of fish mucins Muc2 and Muc5ac at the protein level and clarified the role of mucins as key guardians to maintain the mucus barrier, which advanced our understanding of teleost mucosal barrier.

Nurturing gut health: role of m6A RNA methylation in upholding the intestinal barrier

The intestinal lumen acts as a critical interface connecting the external environment with the body's internal state. It's essential to prevent the passage of harmful antigens and bacteria while facilitating nutrient and water absorption. The intestinal barriers encompass microbial, mechanical, immunological, and chemical elements, working together to maintain intestinal balance. Numerous studies have associated m6A modification with intestinal homeostasis. This review comprehensively outlines potential mechanisms through which m6A modification could initiate, exacerbate, or sustain barrier damage from an intestinal perspective. The pivotal role of m6A modification in preserving intestinal equilibrium provides new insights, guiding the exploration of m6A modification as a target for optimizing preventive and therapeutic strategies for intestinal homeostasis.

Regional Conjunctival Differences in Glycocalyx Mucin Expression in Dry Eye and Normal Subjects

Purpose

To compare regional conjunctival expression of membrane-associated mucins (MAMs) MUC1, MUC4, and MUC16 in normal and dry eye (DE) subjects.

Methods

Adults with and without signs and symptoms of DE were recruited. Impression cytology was performed to collect MAMs from four bulbar and upper eyelid palpebral conjunctival regions of both eyes. After protein extraction, samples from both eyes of a single subject were pooled by region, and expression was analyzed using a capillary electrophoresis nano-immunoassay system. The chemiluminescence intensity of each antigen binding signal was calculated after normalization to the total protein amount. Statistical analyses were conducted using GraphPad Prime 9.

Results

Samples from thirteen to sixteen DE and seven to eleven normal subjects were analyzed. In normal samples, MUC1 expression from the nasal bulbar conjunctiva was significantly greater than superior (P = 0.004) and inferior (P = 0.005). In DE samples, MUC1 expression was highest superiorly. Significant differences in MUC4 and MUC16 expression were not seen in normal samples. MUC4 and MUC16 expression was upregulated superiorly (P < 0.0001) and inferiorly (P < 0.0001) in DE compared with those regions in normal samples.

Conclusions

Although MAMs form a hydrophilic barrier called the glycocalyx, each mucin may have unique functions that are currently unexplored. All MAMs were expressed in the upper palpebral conjunctiva. Increased MUC1 expression nasally in healthy subjects suggests a functional need for increased protection. When comparing DE with normal eyes, upregulation of MUC1 superiorly, and in both MUC4 and MUC16 both superiorly and inferiorly, may indicate a need to decrease eyelid friction during blinking, especially in DE.

BLV-miR-B1-5p Promotes Staphylococcus aureus Adhesion to Mammary Epithelial Cells by Targeting MUC1

Bovine leukemia virus (BLV) is widely prevalent worldwide and can persistently infect mammary epithelial cells in dairy cows, leading to reduced cellular antimicrobial capacity. BLV-encoded microRNAs (BLV-miRNAs) can modify host genes and promote BLV replication. We previously showed that BLV-miR-B1-5p significantly promoted Staphylococcus aureus (S. aureus) adhesion to bovine mammary epithelial (MAC-T) cells; however, the pathway responsible for this effect remained unclear. This study aims to examine how BLV-miR-B1-5p promotes S. aureus adhesion to MAC-T cells via miRNA target gene prediction and validation. Target site prediction showed that BLV-miR-B1-5p could target the mucin family gene mucin 1 (MUC1). Real-time polymerase chain reaction, immunofluorescence, and dual luciferase reporter assay further confirmed that BLV-miR-B1-5p could target and inhibit the expression of MUC1 in bovine MAC-T cells while interfering with the expression of MUC1 promoted S. aureus adhesion to MAC-T cells. These results indicate that BLV-miR-B1-5p promotes S. aureus adhesion to mammary epithelial cells by targeting MUC1.

MUC13 Cell Surface Mucin Limits Salmonella Typhimurium Infection by Protecting the Mucosal Epithelial Barrier

BACKGROUND & AIMS

MUC13 cell surface mucin is highly expressed on the mucosal surface throughout the intestine, yet its role against bacterial infection is unknown. We investigated how MUC13 impacts Salmonella typhimurium (S Tm) infection and elucidated its mechanisms of action.

METHODS

Muc13-/- and wild-type littermate mice were gavaged with 2 isogenic strains of S Tm after pre-conditioning with streptomycin. We assessed clinical parameters, cecal histology, local and systemic bacterial load, and proinflammatory cytokines after infection. Cecal enteroids and epithelial cell lines were used to evaluate the mechanism of MUC13 activity after infection. The interaction between bacterial SiiE and MUC13 was assessed by using siiE-deficient Salmonella.

RESULTS

S Tm-infected Muc13-/- mice had increased disease activity, histologic damage, and higher local and systemic bacterial loads. Mechanistically, we found that S Tm binds to MUC13 through its giant SiiE adhesin and that MUC13 acts as a pathogen-binding decoy shed from the epithelial cell surface after pathogen engagement, limiting bacterial invasion. In addition, MUC13 reduces epithelial cell death and intestinal barrier breakdown by enhancing nuclear factor kappa B signaling during infection, independent of its decoy function.

CONCLUSIONS

We show for the first time that MUC13 plays a critical role in antimicrobial defense against pathogenic S Tm at the intestinal mucosal surface by both acting as a releasable decoy limiting bacterial invasion and reducing pathogen-induced cell death. This further implicates the cell surface mucin family in mucosal defense from bacterial infection.

The Regulated Secretion and Models of Intracellular Transport: The Goblet Cell as an Example

Transport models are extremely important to map thousands of proteins and their interactions inside a cell. The transport pathways of luminal and at least initially soluble secretory proteins synthesized in the endoplasmic reticulum can be divided into two groups: the so-called constitutive secretory pathway and regulated secretion (RS) pathway, in which the RS proteins pass through the Golgi complex and are accumulated into storage/secretion granules (SGs). Their contents are released when stimuli trigger the fusion of SGs with the plasma membrane (PM). In specialized exocrine, endocrine, and nerve cells, the RS proteins pass through the baso-lateral plasmalemma. In polarized cells, the RS proteins secrete through the apical PM. This exocytosis of the RS proteins increases in response to external stimuli. Here, we analyze RS in goblet cells to try to understand the transport model that can be used for the explanation of the literature data related to the intracellular transport of their mucins.

IL-22-Activated MUC13 Impacts on Colonic Barrier Function through JAK1/STAT3, SNAI1/ZEB1 and ROCK2/MAPK Signaling

Overexpression of the transmembrane mucin MUC13, as seen in inflammatory bowel diseases (IBD), could potentially impact barrier function. This study aimed to explore how inflammation-induced MUC13 disrupts epithelial barrier integrity by affecting junctional protein expression in IBD, thereby also considering the involvement of MUC1. RNA sequencing and permeability assays were performed using LS513 cells transfected with MUC1 and MUC13 siRNA and subsequently stimulated with IL-22. In vivo intestinal permeability and MUC13-related signaling pathways affecting barrier function were investigated in acute and chronic DSS-induced colitis wildtype and Muc13-/- mice. Finally, the expression of MUC13, its regulators and other barrier mediators were studied in IBD and control patients. Mucin knockdown in intestinal epithelial cells affected gene expression of several barrier mediators in the presence/absence of inflammation. IL-22-induced MUC13 expression impacted barrier function by modulating the JAK1/STAT3, SNAI1/ZEB1 and ROCK2/MAPK signaling pathways, with a cooperating role for MUC1. In response to DSS, MUC13 was protective during the acute phase whereas it caused more harm upon chronic colitis. The pathways accounting for the MUC13-mediated barrier dysfunction were also altered upon inflammation in IBD patients. These novel findings indicate an active role for aberrant MUC13 signaling inducing intestinal barrier dysfunction upon inflammation with MUC1 as collaborating partner.

Targeting Host Sulphonyl Urea Receptor 2 Can Reduce Severity of Helicobacter pylori Associated Gastritis

Background and Aims

While most Helicobacter pylori-infected individuals remain asymptomatic throughout their lifetime, in a significant proportion, the resulting severe chronic gastritis drives the development of gastric cancer. In this study, we examine a new therapeutic target, a host potassium channel regulatory subunit, SUR2 (encoded by ABCC9), with potential to protect against H pylori-associated diseases.

Methods

SUR2 gene (ABCC9) expression in human gastric biopsies was analyzed by quantitative polymerase chain reactions. Helicobacter-infected mice were administered the SUR2-channel agonists, pinacidil and nicorandil, then gastric tissues analyzed by histology, immunohistochemistry and quantitative polymerase chain reaction, and splenic tissues by enzyme-linked immunosorbent assays. In vitro studies were performed on human and mouse macrophages, human gastric epithelial cells and mouse splenocytes.

Results

ABCC9 expression in human and mouse stomachs is downregulated with H pylori infection. Treatment of Helicobacter-infected mice with SUR2 channel modulators, pinacidil or nicorandil, significantly reduced gastritis severity. In gastric epithelial cells, nicorandil-induced opening of the SUR2 channel increased intracellular K+ and prevented H pylori-mediated Ca2+ influx and downstream pro-inflammatory signaling.

Conclusion

SUR2 is a novel host factor that regulates Helicobacter pathogenesis. Pharmacological targeting of SUR2 provides a potential approach for reducing the severity of H pylori-associated gastritis, without eradicating infection.

A comprehensive evaluation of an animal model for Helicobacter pylori-associated stomach cancer: Fact and controversy

Even though Helicobacter pylori infection was the most causative factor of gastric cancer, numerous in vivo studies failed to induce gastric cancer using H. pylori infection only. The utilization of established animal studies in cancer research is crucial as they aim to investigate the coincidental association between suspected oncogenes and pathogenesis as well as generate models for the development and testing of potential treatments. The methods to establish gastric cancer using infected animal models remain limited, diverse in methods, and showed different results. This study investigates the differences in animal models, which highlight different pathological results in gaster by literature research. Electronic databases searched were performed in PubMed, Science Direct, and Cochrane, without a period filter. A total of 135 articles were used in this study after a full-text assessment was conducted. The most frequent animal models used for gastric cancer were Mice, while Mongolian gerbils and Transgenic mice were the most susceptible model for gastric cancer associated with H. pylori infection. Additionally, transgenic mice showed that the susceptibility to gastric cancer progression was due to genetic and epigenetic factors. These studies showed that in Mongolian gerbil models, H. pylori could function as a single agent to trigger stomach cancer. However, most gastric cancer susceptibilities were not solely relying on H. pylori infection, and numerous factors are involved in cancer progression. Further study using Mongolian gerbils and Transgenic mice is crucial to conduct and establish the best models for gastric cancer associated H. pylori.

The ulcerative colitis-associated gene FUT8 regulates the quantity and quality of secreted mucins

Mucins are the main macrocomponents of the mucus layer that protects the digestive tract from pathogens. Fucosylation of mucins increases mucus viscoelasticity and its resistance to shear stress. These properties are altered in patients with ulcerative colitis (UC), which is marked by a chronic inflammation of the distal part of the colon. Here, we show that levels of Fucosyltransferase 8 (FUT8) and specific mucins are increased in the distal inflamed colon of UC patients. Recapitulating this FUT8 overexpression in mucin-producing HT29-18N2 colonic cell line increases delivery of MUC1 to the plasma membrane and extracellular release of MUC2 and MUC5AC. Mucins secreted by FUT8 overexpressing cells are more resistant to removal from the cell surface than mucins secreted by FUT8-depleted cells (FUT8 KD). FUT8 KD causes intracellular accumulation of MUC1 and alters the ratio of secreted MUC2 to MUC5AC. These data fit well with the Fut8^-/- mice phenotype, which are protected from UC. Fut8^-/- mice exhibit a thinner proximal colon mucus layer with an altered ratio of neutral to acidic mucins. Together, our data reveal that FUT8 modifies the biophysical properties of mucus by controlling levels of cell surface MUC1 and quantity and quality of secreted MUC2 and MUC5AC. We suggest that these changes in mucus viscoelasticity likely facilitate bacterial-epithelial interactions leading to inflammation and UC progression.

6'-O-Galloylpaeoniflorin attenuates Helicobacter pylori-associated gastritis via modulating Nrf2 pathway

As a common disease of the digestive system, chronic gastritis is inflammation of the gastric mucosa caused by various factors. Helicobacter pylori (H. pylori) is one of the main causes of chronic gastritis, which can lead to gastric mucosal damage and gland atrophy, thereby promoting gastrocarcinogenesis. Oxidative stress and the inflammatory response are important mechanisms of H. pylori-induced gastritis. 6'-O-Galloylpaeoniflorin (GPF) is a substance isolated from peony root with antioxidant and anti-inflammatory activities. However, its role and mechanism in the pathogenesis of H. pylori-induced chronic gastritis remain unclear. This study explored the effects of GPF on H. pylori-induced gastric mucosal oxidative stress and inflammation using flow cytometry, western blotting, real-time quantitative PCR, and immunohistochemistry. We found that H. pylori infection increased oxidative stress and expression of inflammatory cytokines in vitro and in vivo and that these outcomes were inhibited by GPF. Furthermore, GPF activated nuclear factor erythroid-related factor-2 (Nrf2) and its downstream target genes in H. pylori-infected GES-1 cells and mice. The anti-inflammatory and antioxidant effects of GPF on H. pylori-infected cells were attenuated by an Nrf2 inhibitor. Taken together, these data suggest that GPF reduces H. pylori-induced gastric mucosa injury by activating Nrf2 signaling and that GPF is a potential candidate for the treatment of H. pylori-associated gastritis.

A Complex Connection Between the Diversity of Human Gastric Mucin O-Glycans, Helicobacter pylori Binding, Helicobacter Infection and Fucosylation

Helicobacter pylori colonizes the stomach of half of the human population. Most H. pylori are located in the mucus layer, which is mainly comprised by glycosylated mucins. Using mass spectrometry, we identified 631 glycans (whereof 145 were fully characterized and the remainder assigned as compositions) on mucins isolated from 14 Helicobacter spp.-infected and 14 Helicobacter spp.-noninfected stomachs. Only six identified glycans were common to all individuals, from a total of 60 to 189 glycans in each individual. An increased number of unique glycan structures together with an increased intraindividual diversity and larger interindividual variation were identified among O-glycans from Helicobacter spp.-infected stomachs compared with noninfected stomachs. H. pylori strain J99, which carries the blood group antigen-binding adhesin (BabA), the sialic acid-binding adhesin (SabA), and the LacdiNAc-binding adhesin, bound both to Lewis b (Leb)-positive and Leb-negative mucins. Among Leb-positive mucins, H. pylori J99 binding was higher to mucins from Helicobacter spp.-infected individuals than noninfected individuals. Statistical correlation analysis, binding experiments with J99 wt, and J99ΔbabAΔsabA and inhibition experiments using synthetic glycoconjugates demonstrated that the differences in H. pylori-binding ability among these four groups were governed by BabA-dependent binding to fucosylated structures. LacdiNAc levels were lower in mucins that bound to J99 lacking BabA and SabA than in mucins that did not, suggesting that LacdiNAc did not significantly contribute to the binding. We identified 24 O-glycans from Leb-negative mucins that correlated well with H. pylori binding whereof 23 contained α1,2-linked fucosylation. The large and diverse gastric glycan library identified, including structures that correlated with H. pylori binding, could be used to select glycodeterminants to experimentally investigate further for their importance in host-pathogen interactions and as candidates to develop glycan-based therapies.

Mucus and Mucins: The Underappreciated Host Defence System

The mucosal surfaces that form the boundary between the external environment and the underlying tissue are protected by a mucus barrier. Mucin glycoproteins, both secreted and cell surface mucins, are the major components of the barrier. They can exclude pathogens and toxins while hosting the commensal bacteria. In this review, we highlight the dynamic function of the mucins and mucus during infection, how this mucosal barrier is regulated, and how pathogens have evolved mechanisms to evade this defence system.

Mammalian Neuraminidases in Immune-Mediated Diseases: Mucins and Beyond

Mammalian neuraminidases (NEUs), also known as sialidases, are enzymes that cleave off the terminal neuraminic, or sialic, acid resides from the carbohydrate moieties of glycolipids and glycoproteins. A rapidly growing body of literature indicates that in addition to their metabolic functions, NEUs also regulate the activity of their glycoprotein targets. The simple post-translational modification of NEU protein targets-removal of the highly electronegative sialic acid-affects protein folding, alters protein interactions with their ligands, and exposes or covers proteolytic sites. Through such effects, NEUs regulate the downstream processes in which their glycoprotein targets participate. A major target of desialylation by NEUs are mucins (MUCs), and such post-translational modification contributes to regulation of disease processes. In this review, we focus on the regulatory roles of NEU-modified MUCs as coordinators of disease pathogenesis in fibrotic, inflammatory, infectious, and autoimmune diseases. Special attention is placed on the most abundant and best studied NEU1, and its recently discovered important target, mucin-1 (MUC1). The role of the NEU1 - MUC1 axis in disease pathogenesis is discussed, along with regulatory contributions from other MUCs and other pathophysiologically important NEU targets.

Vitamin A supplementation ameliorates ulcerative colitis in gut microbiota-dependent manner

Ulcerative colitis (UC), is a chronic relapsing inflammatory condition of the gastrointestinal track. The purpose of this study is to explore whether Vitamin A (VA) can treat UC and its mechanisms. A mouse model of UC was established using 3.0% (w/v) dextran sodium sulfate (DSS). VA was used to treat UC by intragastric administration of 5000 international unit (IU) retinyl acetate. Fecal microbiota transplantation (FMT) was also used to treat the UC model mice to verify the effect of influenced gut microbiota. The content of short-chain fatty acids (SCFAs) in cecal contents was quantitatively detected by gas chromatography and mass spectrometry. VA supplementation significantly ameliorated UC. 16S rRNA sequencing indicated that VA-treated mice exhibited much more abundant gut microbial diversity and flora composition. Targeted metabolomics analysis manifested the increased production of SCFAs in VA-treated mice. Gut microbiota depletion and FMT results confirmed the gut microbiota-dependent mechanism as that VA relieved UC via regulating gut microbiota: increase in SCFA-producing genera and decrease in UC-related genera. The restore of intestinal barrier and the inhibition of inflammation were also found to contribute to the amelioration of UC by VA. It was concluded that a VA supplement was enough to cause a significant change in gut microbiota and amelioration of UC.

Elucidating bacterial adhesion to mucosal surface by an original AFM approach

Background

Fish skin represents an ancient vertebrate mucosal surface, sharing characteristics with other mucosal surfaces including those of the intestine. The skin mucosa is continuously exposed to microbes in the surrounding water and is therefore important in the first line defense against environmental pathogens by preventing bacteria from accessing the underlying surfaces. Understanding the microbe-host interactions at the fish skin mucosa is highly relevant in order to understand and control infection, commensalism, colonization, persistence, infection, and disease. Here we investigate the interactions between the pathogenic bacteria Aeromonas salmonicida (A. salmonicida) and Yersinia ruckeri (Y. ruckeri), respectively, and the skin mucosal surface of Atlantic salmon fry using AFM force spectroscopy.

Results

The results obtained revealed that when retracting probes functionalized with bacteria from surfaces coated with immobilized mucins, isolated from salmon mucosal surfaces, rupture events reflecting the disruption of adhesive interactions were observed, with rupture strengths centered around 200 pN. However, when retracting probes functionalized with bacteria from the intact mucosal surface of salmon fish fry no adhesive interactions could be detected. Furthermore, rheological measurements revealed a near fluid-like behavior for the fish fry skin mucus. Taken together, the experimental data indicate that the adhesion between the mucin molecules within the mucous layer may be significantly weaker than the interaction between the bacteria and the mucin molecules. The bacteria, immobilized on the AFM probe, do bind to individual mucins in the mucosal layer, but are released from the near fluid mucus with little resistance upon retraction of the AFM probe, to which they are immobilized.

Conclusion

The data provided in the current paper reveal that A. salmonicida and Y. ruckeri do bind to the immobilized mucins. However, when retracting the bacteria from intact mucosal surfaces, no adhesive interactions are detected. These observations suggest a mechanism underlying the protective function of the mucosal surface based on the clearing of potential threats by adhering them to loosely attached mucus that is subsequently released from the fish skin.

Interaction Between the Microbiota, Epithelia, and Immune Cells in the Intestine

The gastrointestinal tract harbors numerous commensal bacteria, referred to as the microbiota, that benefit host health by digesting dietary components and eliminating pathogens. The intestinal microbiota maintains epithelial barrier integrity and shapes the mucosal immune system, balancing host defense and oral tolerance with microbial metabolites, components, and attachment to host cells. To avoid aberrant immune responses, epithelial cells segregate the intestinal microbiota from immune cells by constructing chemical and physical barriers, leading to the establishment of host-commensal mutualism. Furthermore, intestinal immune cells participate in the maintenance of a healthy microbiota community and reinforce epithelial barrier functions. Perturbations of the microbiota composition are commonly observed in patients with autoimmune diseases and chronic inflammatory disorders. An understanding of the intimate interactions between the intestinal microbiota, epithelial cells, and immune cells that are crucial for the maintenance of intestinal homeostasis might promote advances in diagnostic and therapeutic approaches for various diseases.

Pathogen Challenge and Dietary Shift Alter Microbiota Composition and Activity in a Mucin-Associated in vitro Model of the Piglet Colon (MPigut-IVM) Simulating Weaning Transition

Enterotoxigenic Escherichia coli (ETEC) is the principal pathogen responsible for post-weaning diarrhea in newly weaned piglets. Expansion of ETEC at weaning is thought to be the consequence of various stress factors such as transient anorexia, dietary change or increase in intestinal inflammation and permeability, but the exact mechanisms remain to be elucidated. As the use of animal experiments raise more and more ethical concerns, we used a recently developed in vitro model of piglet colonic microbiome and mucobiome, the MPigut-IVM, to evaluate the effects of a simulated weaning transition and pathogen challenge at weaning. Our data suggested that the tested factors impacted the composition and functionality of the MPigut-IVM microbiota. The simulation of weaning transition led to an increase in relative abundance of the Prevotellaceae family which was further promoted by the presence of the ETEC strain. In contrast, several beneficial families such as Bacteroidiaceae or Ruminococcaceae and gut health related short chain fatty acids like butyrate or acetate were reduced upon simulated weaning. Moreover, the incubation of MPigut-IVM filtrated effluents with porcine intestinal cell cultures showed that ETEC challenge in the in vitro model led to an increased expression of pro-inflammatory genes by the porcine cells. This study provides insights about the etiology of a dysbiotic microbiota in post-weaning piglets.

The Influence of Lactobacillus Acidophilus on MUC1, GAL-3, IL-1β and IL-17 Gene Expression in BALB/c Mice Stomach

Background and Objective:

Lactobacillus acidophilus has been widely used for the management of gastrointestinal carcinoma owing to its immunomodulation effect; however, the role of L. acidophilus and its specific mechanism of action in the stomach is not fully comprehended. The present study evaluated the expression profile of MUC-1, GAL-3, IL -1β, and IL-17 in the L. acidophilus treated mice stomach.

Methods:

The study was conducted utilizing three groups of mice, 6 mice for each group, administered with different doses of L. acidophilus and a control group treated with normal saline. The results were analyzed with the Mann-Whitney Test.

Results:

The results demonstrated that L. acidophilus elevated IL-1β insignificantly and inhibited the expression of IL-17. The MUC-1 expression is influenced by L. acidophilus and inversely proportional to GAL-3 expression.

Conclusion:

Lactobacillus acidophilus plays a prominent role against inflammatory responses and has a potential in the treatment of gastrointestinal cancer.

The Transmembrane Mucin MUC1 Facilitates β1-Integrin-Mediated Bacterial Invasion

At the intestinal host-microbe interface, the transmembrane mucin MUC1 can function as a physical barrier as well as a receptor for bacteria. MUC1 also influences epithelial cell morphology and receptor function. Various bacterial pathogens can exploit integrins to infect eukaryotic cells. It is yet unclear whether MUC1 influences the interaction of bacteria with integrins. We used Escherichia coli expressing the invasin (inv) protein of Yersinia pseudotuberculosis (E. coli inv) to assess the effects of MUC1 on β1 integrin (ITGB1)-mediated bacterial invasion. Our results show that expression of full-length MUC1 does not yield a physical barrier but slightly enhances E. coli inv uptake. Enzymatic removal of the MUC1 extracellular domain (ED) using a secreted protease of C1 esterase inhibitor (StcE) of pathogenic Escherichia coli had no additional effect on E. coli inv invasion. In contrast, expression of a truncated MUC1 that lacks the cytoplasmic tail (CT) reduced bacterial entry substantially. Substitution of tyrosine residues in the MUC1 CT also reduced bacterial uptake, while deletion of the C-terminal half of the cytoplasmic tail only had a minor effect, pointing to a regulatory role of tyrosine phosphorylation and the N-terminal region of the MUC1 CT in integrin-mediated uptake process. Unexpectedly, StcE removal of the ED in MUC1-ΔCT cells reversed the block in bacterial invasion. Together, these findings indicate that MUC1 can facilitate β1-integrin-mediated bacterial invasion by a concerted action of the large glycosylated extracellular domain and the membrane-juxtaposed cytoplasmic tail region.IMPORTANCE Bacteria can exploit membrane receptor integrins for cellular invasion, either by direct binding of bacterial adhesins or utilizing extracellular matrix components. MUC1 is a large transmembrane glycoprotein expressed by most epithelial cells that can have direct defensive or receptor functions at the host-microbe interface and is involved in facilitating integrin clustering. We investigated the role of epithelial MUC1 on β1 integrin-mediated bacterial invasion. We discovered that MUC1 does not act as a barrier but facilitates bacterial entry through β1 integrins. This process involves a concerted action of the MUC1 O-glycosylated extracellular domain and cytoplasmic tail. Our findings add a new dimension to the complexity of bacterial invasion mechanisms and provide novel insights into the distinct functions of MUC1 domains at the host-microbe interface.

GWAS of peptic ulcer disease implicates Helicobacter pylori infection, other gastrointestinal disorders and depression

Genetic factors are recognized to contribute to peptic ulcer disease (PUD) and other gastrointestinal diseases, such as gastro-oesophageal reflux disease (GORD), irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD). Here, genome-wide association study (GWAS) analyses based on 456,327 UK Biobank (UKB) individuals identify 8 independent and significant loci for PUD at, or near, genes MUC1, MUC6, FUT2, PSCA, ABO, CDX2, GAST and CCKBR. There are previously established roles in susceptibility to Helicobacter pylori infection, response to counteract infection-related damage, gastric acid secretion or gastrointestinal motility for these genes. Only two associations have been previously reported for duodenal ulcer, here replicated trans-ancestrally. The results highlight the role of host genetic susceptibility to infection. Post-GWAS analyses for PUD, GORD, IBS and IBD add insights into relationships between these gastrointestinal diseases and their relationships with depression, a commonly comorbid disorder.

The role of mucin 1 in respiratory diseases

Recent evidence has demonstrated that mucin 1 (MUC1) is involved in many pathological processes that occur in the lung. MUC1 is a transmembrane protein mainly expressed by epithelial and hematopoietic cells. It has a receptor-like structure, which can sense the external environment and activate intracellular signal transduction pathways through its cytoplasmic domain. The extracellular domain of MUC1 can be released to the external environment, thus acting as a decoy barrier to mucosal pathogens, as well as serving as a serum biomarker for the diagnosis and prognosis of several respiratory diseases such as lung cancer and interstitial lung diseases. Furthermore, bioactivated MUC1-cytoplasmic tail (CT) has been shown to act as an anti-inflammatory molecule in several airway infections and mediates the expression of anti-inflammatory genes in lung diseases such as chronic rhinosinusitis, chronic obstructive pulmonary disease and severe asthma. Bioactivated MUC1-CT has also been reported to interact with several effectors linked to cellular transformation, contributing to the progression of respiratory diseases such as lung cancer and pulmonary fibrosis. In this review, we summarise the current knowledge of MUC1 as a promising biomarker and drug target for lung disease.

Breastfeeding importance and its therapeutic potential against SARS-CoV-2

During postnatal development, colostrum and breastmilk are sequentially the first sources of nutrition with protein components and bioactive molecules that confer protection and immunostimulatory function to the gut. Caseins, whey proteins, secretory immunoglobulin A (sIgA), mucins, tryptophan, and growth factors are among milk-borne elements that are directly important in the control of mucosa development and protection. Consequently, breastfeeding is associated with the low incidence of gastrointestinal inflammation and with the decrease in respiratory diseases during postnatal period. The novel coronavirus (SARS-CoV-2) binds to angiotensin II-converting enzyme (ACE2) on the cell membrane, allowing virus entrance, replication, and host commitment. ACE2 is expressed by different cell types, which include ciliated cells in the lungs and enterocytes in the intestine. Such cells are highly active in metabolism, as they internalize molecules to be processed and used by the organism. The disruption of ACE2 impairs leads to intestinal inflammation and decreased synthesis of serotonin, affecting motility. By reviewing the effects of SARS-CoV-2 in the gastrointestinal and respiratory tracts in infants, and gut responses to breastfeeding interruption, we suggest that it is important to maintain breastfeeding during SARS-CoV-2 infection, as it might be essential to protect newborns from gastrointestinal-associated disorders and relieve disease symptoms.

Early detection of tumor cells in bone marrow and peripheral blood in a fast‑progressing gastric cancer model

Helicobacter pylori (H. pylori) infection is a major risk factor for the development of gastric cancer. The authors previously demonstrated that in mice deficient in myeloid differentiation primary response 88 (Myd88^−/−), infection with Helicobacter felis (H. felis) a close relative of H. pylori, subsequently rapidly progressed to neoplasia. The present study examined circulating tumor cells (CTCs) by measuring the expression of cytokeratins, epithelial-to-mesenchymal transition (EMT)-related markers and cancer stem cell (CSC) markers in bone marrow and peripheral blood from Myd88^−/− and wild-type (WT) mice. Cytokeratins CK8/18 were detected as early as 4 months post-infection in Myd88^−/− mice. By contrast, cytokeratins were not detected in WT mice even after 7 months post-infection. The expression of Mucin-1 (MUC1) was observed in both bone marrow and peripheral blood at different time points, suggesting its role in gastric cancer metastasis. Snail, Twist and ZEB were expressed at different levels in bone marrow and peripheral blood. The expression of these EMT-related markers suggests the manifestation of cancer metastasis in the early stages of disease development. LGR5, CD44 and CD133 were the most prominent CSC markers detected. The detection of CSC and EMT markers along with cytokeratins does reinforce their use as biomarkers for gastric cancer metastasis. This early detection of markers suggests that CTCs leave primary site even before cancer is well established. Thus, cytokeratins, EMT, and CSCs could be used as biomarkers to detect aggressive forms of gastric cancers. This information may prove to be of significance in stratifying patients for treatment prior to the onset of severe disease-related characteristics.

Microbe-Driven Genotoxicity in Gastrointestinal Carcinogenesis

The intestinal epithelium serves as a barrier to discriminate the outside from the inside and is in constant exchange with the luminal contents, including nutrients and the microbiota. Pathogens have evolved mechanisms to overcome the multiple ways of defense in the mucosa, while several members of the microbiota can exhibit pathogenic features once the healthy barrier integrity of the epithelium is disrupted. This not only leads to symptoms accompanying the acute infection but may also contribute to long-term injuries such as genomic instability, which is linked to mutations and cancer. While for Helicobacter pylori a link between infection and cancer is well established, many other bacteria and their virulence factors have only recently been linked to gastrointestinal malignancies through epidemiological as well as mechanistic studies. This review will focus on those pathogens and members of the microbiota that have been linked to genotoxicity in the context of gastric or colorectal cancer. We will address the mechanisms by which such bacteria establish contact with the gastrointestinal epithelium-either via an existing breach in the barrier or via their own virulence factors as well as the mechanisms by which they interfere with host genomic integrity.

Differential Expression of Mucins in Murine Olfactory Versus Respiratory Epithelium

Mucins are a key component of the surface mucus overlying airway epithelium. Given the different functions of the olfactory and respiratory epithelia, we hypothesized that mucins would be differentially expressed between these 2 areas. Secondarily, we evaluated for potential changes in mucin expression with radiation exposure, given the clinical observations of nasal dryness, altered mucus rheology, and smell loss in radiated patients. Immunofluorescence staining was performed to evaluate expression of mucins 1, 2, 5AC, and 5B in nasal respiratory and olfactory epithelia of control mice and 1 week after exposure to 8 Gy of radiation. Mucins 1, 5AC, and 5B exhibited differential expression patterns between olfactory and respiratory epithelium (RE) while mucin 2 showed no difference. In the olfactory epithelium (OE), mucin 1 was located in a lattice-like pattern around gaps corresponding to dendritic knobs of olfactory sensory neurons, whereas in RE it was intermittently expressed by surface goblet cells. Mucin 5AC was expressed by subepithelial glands in both epithelial types but to a higher degree in the OE. Mucin 5B was expressed by submucosal glands in OE and by surface epithelial cells in RE. At 1-week after exposure to single-dose 8 Gy of radiation, no qualitative effects were seen on mucin expression. Our findings demonstrate that murine OE and RE express mucins differently, and characteristic patterns of mucins 1, 5AC, and 5B can be used to define the underlying epithelium. Radiation (8 Gy) does not appear to affect mucin expression at 1 week.

LEVEL OF EVIDENCE

N/A (Basic Science Research).IACUC-approved study [Protocol 200065].

Unraveling mucin domains in cancer and metastasis: when protectors become predators

A dynamic mucosal layer shields the epithelial cells lining the body cavities and is made up of high molecular weight, heavily glycosylated, multidomain proteins called mucins. Mucins, broadly grouped into transmembrane and secreted mucins, are the first responders to any mechanical or chemical insult to the epithelia and help maintain tissue homeostasis. However, their intrinsic properties to protect and repair the epithelia are exploited during oncogenic processes, where mucins are metamorphosed to aid the tumor cells in their malignant journey. Diverse domains, like the variable number tandem repeats (VNTR), sea urchin sperm protein enterokinase and agrin (SEA), adhesion-associated domain (AMOP), nidogen-like domain (NIDO), epidermal growth factor-like domain (EGF), and von Willebrand factor type D domain (vWD) on mucins, including MUC1, MUC4, MUC5AC, MUC5B, and MUC16, have been shown to facilitate cell-to-cell and cell-to-matrix interactions, and cell-autonomous signaling to promote tumorigenesis and distant dissemination of tumor cells. Several obstacles have limited the study of mucins, including technical difficulties in working with these huge glycoproteins, the dearth of scientific tools, and lack of animal models; thus, the tissue-dependent and domain-specific roles of mucins during mucosal protection, chronic inflammation, tumorigenesis, and hematological dissemination of malignant cells are still unclear. Future studies should try to integrate information on the rheological, molecular, and biological characteristics of mucins to comprehensively delineate their pathophysiological role and evaluate their suitability as targets in future diagnostic and therapeutic strategies.

Influence of the MUC1 Cell Surface Mucin on Gastric Mucosal Gene Expression Profiles in Response to Helicobacter pylori Infection in Mice

The cell surface mucin MUC1 is an important host factor limiting Helicobacter pylori (H. pylori) pathogenesis in both humans and mice by providing a protective barrier and modulating mucosal epithelial and leukocyte responses. The aim of this study was to establish the time-course of molecular events in MUC1-modulated gene expression profiles in response to H. pylori infection in wild type (WT) and MUC1-deficient mice using microarray-determined mRNA expression, gene network analysis and Ingenuity Pathway Analysis (IPA). A time-course over the first 72 h of infection showed significantly higher mucosal loads of bacteria at 8 h of infection in Muc1^−/− mice compared with WT, confirming its importance in the early stages of infection (P = 0.0003). Microarray analysis revealed 266 differentially expressed genes at one or more time-points over 72 h in the gastric mucosa of Muc1^−/− mice compared with WT control using a threshold of 2-fold change. The SPINK1 pancreatic cancer canonical pathway was strongly inhibited in Muc1^−/− mice compared with WT at sham and 8 h infection (P = 6.08E-14 and P = 2.25 E-19, respectively) but potently activated at 24 and 72 h post-infection (P = 1.38E-22 and P = 5.87E-13, respectively). The changes in this pathway are reflective of higher expression of genes mediating digestion and absorption of lipids, carbohydrates, and proteins at sham and 8 h infection in the absence of MUC1, but that this transcriptional signature is highly down regulated as infection progresses in the absence of MUC1. Uninfected Muc1^−/− gastric tissue was highly enriched for expression of factors involved in lipid metabolism and 8 h infection further activated this network compared with WT. As infection progressed, a network of antimicrobial and anti-inflammatory response genes was more highly activated in Muc1^−/− than WT mice. Key target genes identified by time-course microarrays were independently validated using RT-qPCR. These results highlight the dynamic interplay between the host and H. pylori, and the role of MUC1 in host defense, and provide a general picture of changes in cellular gene expression modulated by MUC1 in a time-dependent manner in response to H. pylori infection.

Gastric Mucosa-Associated Microbial Signatures of Early Gastric Cancer

Alterations in the microbiome are associated with the development of gastric cancer. Our study aimed to identify dysbiotic features in early gastric cancer (EC). The gastric microbiome was assessed in EC (n = 30), advanced gastric cancer (AC) (n = 30), and chronic gastritis (CG) (n = 60). The results demonstrated significant differences in the microbial profile and composition between EC and AC, suggesting alterations associated with gastric cancer progression. Linear discriminant analysis (LDA) effect size (LEfSe) analyses identified 32 bacterial genera that were associated with EC. Functional analyses of the gastric microbiome showed that the production of urease and synthesis of bacterial flagella were weakened in EC, while the glycolysis of fructose and hydrolysis of glycosides were enhanced. A classifier based on a random forest (RF) machine learning algorithm identified a microbial signature that distinguished EC from CG or AC with high accuracy. The correct identification of the signature was further validated in independent cohorts. This signature enriched of bacteria with varied abundance, high degree of bacterial interactions and carcinogenic potentials. Constrained principal coordinate analyses revealed that the presence of Helicobacter pylori and the cagA and vacA virulence genotypes influenced the structure of the gastric microbiome. To determine the impacts of host genetic variations on the gastric microbiome, six previously reported single nucleotide polymorphisms (SNPs) were examined. The minor allele of MUC1 rs4072037 was associated with an increased abundance of Ochrobactrum. The gastric microbiome altered in EC, which might be attributed in part to host genetic variations, H. pylori infection, bacterial virulence and environmental adaptations. The identified microbial signature could serve as biomarkers for clinical assessment of gastric cancer risk in high-risk patients.

A lack of role for antibodies in regulating Helicobacter pylori colonization and associated gastritis

BACKGROUND

Helicobacter pylori occupy a unique niche, located within the mucus layer lining the stomach, and attached to the apical surface of the gastric epithelium. As such, antibodies would be expected to play a major role in regulating infection and/or pathogenesis. However, experiments using antibody-deficient mice to study gastric helicobacter infection have yielded inconsistent results, although some pointed toward antibodies increasing colonization levels and decreasing gastritis severity. The variability in these studies is possibly due to their use of nonmatched wild-type controls. This current study presents the first evaluation of the role of antibodies in H pylori infection by comparing antibody-deficient mice with matched wild-type siblings.

METHODS

Matched wild-type and antibody-deficient μMT mice were generated by heterozygous crossings. In two separate experiments, appropriately genotyped sibling littermates were infected with H pylori for 4 months and then sera and stomachs were collected.

RESULTS

There was no difference in H pylori colonization levels between infected μMT mice and sibling wild-type controls. Similarly, there was no significant difference in the severity of gastritis between these groups of mice, although there was a trend toward less severe gastritis in μMT mice which was supported by a significantly lower IFNγ (Th1) gastric cytokine response.

CONCLUSIONS

Comparing matched antibody-deficient and antibody-competent mice indicates that an antibody response does not influence H pylori colonization levels. Contrary to previous studies, these results suggest antibodies might have a minor pro-inflammatory effect by promoting gastric Th1 cytokines, although this did not translate to a significant effect on gastritis severity.

Membrane mucins of the intestine at a glance

Membrane mucins cover most mucosal surfaces throughout the human body. The intestine harbors complex population of microorganisms (the microbiota) and numerous exogenous molecules that can harm the epithelium. In the colon, where the microbial burden is high, a mucus barrier forms the first line of defense by keeping bacteria away from the epithelial cells. In the small intestine where the mucus layer is less organized, microbes are kept at bay by peristalsis and antimicrobial peptides. Additionally, a dense glycocalyx consisting of extended and heavily glycosylated membrane mucins covers the surface of enterocytes. Whereas many aspects of mucosal barriers are being discovered, the function of membrane mucins remains a largely overlooked topic, mainly because we lack the necessary reagents and experimental animal models to investigate these large glycoproteins. In this Cell Science at a Glance article and accompanying poster, we highlight central concepts of membrane mucin biology and the role of membrane mucins as integral components of intestinal mucosal barriers. We also present the current consensus concerning the role of membrane mucins in host-microbe interactions. Moreover, we discuss how regulatory circuits that govern membrane mucins in the healthy gut display strong overlap with pathways that are perturbed during chronic inflammation. Finally, we review how dysregulation of intestinal membrane mucins may contribute to human diseases, such as inflammation and cancer.

Formyl peptide receptor 2 orchestrates mucosal protection against Citrobacter rodentium infection

Citrobacter rodentium is an attaching and effacing intestinal murine pathogen which shares similar virulence strategies with the human pathogens enteropathogenic- and enterohemorrhagic Escherichia coli to infect their host. C. rodentium is spontaneously cleared by healthy wild-type (WT) mice whereas mice lacking Muc2 or specific immune regulatory genes demonstrate an impaired ability to combat the pathogen. Here we demonstrate that apical formyl peptide receptor 2 (Fpr2) expression increases in colonic epithelial cells during C. rodentium infection. Using a conventional inoculum dose of C. rodentium, both WT and Fpr2^−/− mice were infected and displayed similar signs of disease, although Fpr2^−/− mice recovered more slowly than WT mice. However, Fpr2^−/− mice exhibited increased susceptibility to C. rodentium colonization in response to low dose infection: 100% of the Fpr2^−/− and 30% of the WT mice became colonized and Fpr2^−/− mice developed more severe colitis and more C. rodentium were in contact with the colonic epithelial cells. In line with the larger amount of C. rodentium detected in the spleen in Fpr2^−/− mice, more C. rodentium and enteropathogenic Escherichia coli translocated across an in vitro mucosal surface to the basolateral compartment following FPR2 inhibitor treatment. Fpr2^−/− mice also lacked the striated inner mucus layer that was present in WT mice. Fpr2^−/− mice had decreased mucus production and different mucin O-glycosylation in the colon compared to WT mice, which may contribute to their defect inner mucus layer. Thus, Fpr2 contributes to protection against infection and influence mucus production, secretion and organization.

MUC1 is associated with TFF2 methylation in gastric cancer

Background

Emerging evidence has shown that MUC1 and TFF2 play crucial roles in the H. pylori-infected pathogenesis of gastric cancer (GC). A recent study revealed that H. pylori infection induced obviously increased Tff2 methylation levels in Muc1^−/− mice compared with controls. However, little is known of the molecular mechanism on MUC1 regulating the expression of TFF2.

Methods

We conducted a correlation analysis of MUC1 and TFF2 in public databases and our adjacent GC tissues. Besides, MUC1 overexpression vector or small interfering RNA (siRNA) was transfected into GC cells to assess the change in TFF2 expression. Furthermore, the methylation status of TFF2 was measured by bisulfite sequencing PCR (BSP).

Results

The expression of MUC1 was significantly lower in non-cardia and cardia tumor tissues than that in normal tissues. Downregulation of TFF2 expression was also observed in GC tissues. In addition, we found that MUC1 expression was positively associated with TFF2 expression in GC tissues, especially among GC patients with H. pylori infection. Overexpression of MUC1 in BGC-823 and SGC-7901 cell lines substantially increased the TFF2 expression, whereas knockdown of MUC1 reverted this effect. Moreover, MUC1 was negatively related to the methylation of TFF2 in the co-expression analysis. The results of BSP experiments showed that compared with negative vector group, the methylation level of TFF2 was decreased in GC cells transfected with MUC1 overexpression vector. Additionally, survival analysis indicated that GC patients with lower level of MUC1 or TFF2 had a worse outcome.

Conclusion

Our results indicated that MUC1 was associated with the methylation of TFF2, which may have implications for TFF2 expression in GC. These findings warrant further research toward the underlying mechanism of MUC1 influenced the TFF2 methylation.

Microbe-MUC1 Crosstalk in Cancer-Associated Infections

Infection-associated cancers account for ∼20% of all malignancies. Understanding the molecular mechanisms underlying infection-associated malignancies may help in developing diagnostic biomarkers and preventative vaccines against malignancy. During infection, invading microbes interact with host mucins lining the glandular epithelial cells and trigger inflammation. MUC1 is a transmembrane mucin glycoprotein that is present on the surface of almost all epithelial cells, and is known to interact with invading microbes. This interaction can trigger pro- or anti-inflammatory responses depending on the microbe and the cell type. In this review we summarize the mechanisms of microbe and MUC1 interactions, and highlight how MUC1 plays contrasting roles in different cells. We also share perspectives on future research that may support clinical advances in infection-associated cancers.

Core 1-derived mucin-type O-glycosylation protects against spontaneous gastritis and gastric cancer

Core 1-derived mucin-type O-glycans (O-glycans) are a major component of gastric mucus with an unclear role. To address this, we generated mice lacking gastric epithelial O-glycans (GEC C1galt1-/-). GEC C1galt1-/- mice exhibited spontaneous gastritis that progressed to adenocarcinoma with ∼80% penetrance by 1 yr. GEC C1galt1-/- gastric epithelium exhibited defective expression of a major mucus forming O-glycoprotein Muc5AC relative to WT controls, which was associated with impaired gastric acid homeostasis. Inflammation and tumorigenesis in GEC C1galt1-/- stomach were concurrent with activation of caspases 1 and 11 (Casp1/11)-dependent inflammasome. GEC C1galt1-/- mice genetically lacking Casp1/11 had reduced gastritis and gastric cancer progression. Notably, expression of Tn antigen, a truncated form of O-glycan, and CASP1 activation was associated with tumor progression in gastric cancer patients. These results reveal a critical role of O-glycosylation in gastric homeostasis and the protection of the gastric mucosa from Casp1-mediated gastric inflammation and cancer.

Immunity and Vaccine Development Against Helicobacter pylori

Helicobacter pylori is a highly-adapted gastrointestinal pathogen of humans and the immunology of this chronic infection is extremely complex. Despite the availability of antibiotic therapy, the global incidence of H. pylori infection remains high, particularly in low to middle-income nations. Failure of therapy and the spread of antibiotic resistance among the bacteria are significant problems and provide impetus for the development of new therapies and vaccines to treat or prevent gastric ulcer, and gastric carcinoma. The expansion of knowledge on gastric conventional and regulatory T cell responses, and the role of T_H17 in chronic gastritis from studies in mouse models and patients have provided valuable insights into how gastritis is initiated and maintained. The development of human challenge models for testing candidate vaccines has meant a unique opportunity to study acute infection, but the field of vaccine development has not progressed as rapidly as anticipated. One clear lesson learned from previous studies is that we need a better understanding of the immune suppressive mechanisms in vivo to be able to design vaccine strategies. There is still an urgent need to identify practical surrogate markers of protection that could be deployed in future field vaccine trials. Important developments in our understanding of the chronic inflammatory response, progress and problems arising from human studies, and an outlook for the future of clinical vaccine trials will be discussed.

Membrane-associated mucins of the ocular surface: New genes, new protein functions and new biological roles in human and mouse

The mucosal glycocalyx of the ocular surface constitutes the point of interaction between the tear film and the apical epithelial cells. Membrane-associated mucins (MAMs) are the defining molecules of the glycocalyx in all mucosal epithelia. Long recognized for their biophysical properties of hydration, lubrication, anti-adhesion and repulsion, MAMs maintain the wet ocular surface, lubricate the blink, stabilize the tear film and create a physical barrier to the outside world. However, it is increasingly appreciated that MAMs also function as cell surface receptors that transduce information from the outside to the inside of the cell. A number of excellent review articles have provided perspective on the field as it has progressed since 1987, when molecular cloning of the first MAM was reported. The current article provides an update for the ocular surface, placing it into the broad context of findings made in other organ systems, and including new genes, new protein functions and new biological roles. We discuss the epithelial tissue-equivalent with mucosal differentiation, the key model system making these advances possible. In addition, we make the first systematic comparison of MAMs in human and mouse, establishing the basis for using knockout mice for investigations with the complexity of an in vivo system. Lastly, we discuss findings from human genetics/genomics, which are providing clues to new MAM roles previously unimagined. Taken together, this information allows us to generate hypotheses for the next stage of investigation to expand our knowledge of MAM function in intracellular signaling and roles unique to the ocular surface.

The human transmembrane mucin MUC17 responds to TNFα by increased presentation at the plasma membrane

Transmembrane mucin MUC17 is an integral part of the glycocalyx as it covers the brush border membrane of small intestinal enterocytes and presents an extended O-glycosylated mucin domain to the intestinal lumen. Here, we identified two unknown phosphorylated serine residues, S4428 and S4492, in the cytoplasmic tail of human MUC17. We have previously demonstrated that MUC17 is anchored to the apical membrane domain via an interaction with the scaffolding protein PDZK1. S4492, localized in the C-terminal PDZ binding motif of MUC17, was mutated to generate phosphomimetic and phosphodeficient variants of MUC17. Using Caco-2 cells as a model system, we found that induction of an inflammatory state by long-term stimulation with the proinflammatory cytokine TNFα resulted in an increase of MUC17 protein levels and enhanced insertion of MUC17 and its two phospho-variants into apical membranes. Up-regulation and apical insertion of MUC17 was followed by shedding of MUC17-containing vesicles. Transmembrane mucins have previously been shown to play a role in the prevention of bacterial colonization by acting as sheddable decoys for encroaching bacteria. Overexpression and increased presentation at the plasma membrane of wild-type MUC17 and its phosphodeficient variant MUC17 S-4492A protected Caco-2 cells against adhesion of enteropathogenic Escherichia coli, indicating that C-terminal phosphorylation of MUC17 may play a functional role in epithelial cell protection. We propose a new function for MUC17 in inflammation, where MUC17 acts as a second line of defense by preventing attachment of bacteria to the epithelial cell glycocalyx in the small intestine.

Attenuated fibrosis in specific pathogen-free microbiota in experimental cholestasis- and toxin-induced liver injury

In advanced chronic liver disease (CLD), the translocation of intestinal bacteria and the resultant increase of proinflammatory cytokines in the splanchnic and systemic circulation may contribute to the progression of fibrosis. We therefore speculated that fibrosis and portal hypertension (PHT) would be attenuated in a mouse model of limited intestinal colonization with altered Schaedler flora (ASF) compared to a more complex colonization with specific pathogen-free (SPF) flora. We induced liver fibrosis in ASF and SPF mice by common bile duct ligation (BDL) or by carbon tetrachloride (CCl₄) treatment. We then measured portal pressure (PP), portosystemic shunts (PSSs), and harvested tissues for further analyses. There were no differences in PP between sham-treated ASF or SPF mice. After BDL or CCl₄ treatment, PP, PSSs, and hepatic collagen deposition increased in both groups. However, the increase in PP and the degree of fibrosis was significantly higher in ASF than SPF mice. Expression of fibrotic markers α-smooth muscle actin, desmin, and platelet-derived growth factor receptor β were significantly higher in ASF than SPF mice. This was associated with higher activation of hepatic immune cells (macrophages, neutrophils) and decreased expression of the intestinal epithelial tight junction proteins (claudin-1, occludin-1). In 2 models of advanced CLD, SPF mice presented significantly attenuated liver injury, fibrosis, and PHT compared to ASF mice. In contrast to our hypothesis, these findings suggest that a complex intestinal microbiota may play a "hepato-protective" role.-Moghadamrad, S., Hassan, M., McCoy, K. D., Kirundi, J., Kellmann, P., De Gottardi, A. Attenuated fibrosis in specific pathogen-free microbiota in experimental cholestasis- and toxin-induced liver injury.

Role of Sialic Acid in Brachyspira hyodysenteriae Adhesion to Pig Colonic Mucins

Infection with Brachyspira hyodysenteriae results in mucoid hemorrhagic diarrhea. This pathogen is associated with the colonic mucus layer, mainly composed of mucins. Infection regulates mucin O-glycosylation in the colon and increases mucin secretion as well as B. hyodysenteriae binding sites on mucins. Here, we analyzed potential mucin epitopes for B. hyodysenteriae adhesion in the colon, as well as the effect of colonic mucins on bacterial growth. Associations between B. hyodysenteriae binding to pig colonic mucins and mucin glycan data showed that B. hyodysenteriae binding was associated with the presence of N-glycolylneuraminic acid (NeuGc) on mucins. The role of sialic acid in B. hyodysenteriae adhesion was analyzed after the removal of sialic acid residues on the mucins by enzymatic treatment with sialidase A, which decreased bacterial binding to the mucins. The effect of pig colonic mucins on B. hyodysenteriae growth was determined in carbohydrate-free medium. B. hyodysenteriae growth increased in the presence of mucins from two out of five infected pigs, suggesting utilization of mucins as a carbon source for growth. Additionally, bacterial growth was enhanced by free sialic acid and N-acetylglucosamine. The results highlight a role of sialic acid as an adhesion epitope for B. hyodysenteriae interaction with colonic mucins. Furthermore, the mucin response and glycosylation changes exerted in the colon during B. hyodysenteriae infection result in a potentially favorable environment for pathogen growth in the intestinal mucus layer.

Experimental models to study intestinal microbes–mucus interactions in health and disease

A close symbiotic relationship exists between the intestinal microbiota and its host. A critical component of gut homeostasis is the presence of a mucus layer covering the gastrointestinal tract. Mucus is a viscoelastic gel at the interface between the luminal content and the host tissue that provides a habitat to the gut microbiota and protects the intestinal epithelium. The review starts by setting up the biological context underpinning the need for experimental models to study gut bacteria-mucus interactions in the digestive environment. We provide an overview of the structure and function of intestinal mucus and mucins, their interactions with intestinal bacteria (including commensal, probiotics and pathogenic microorganisms) and their role in modulating health and disease states. We then describe the characteristics and potentials of experimental models currently available to study the mechanisms underpinning the interaction of mucus with gut microbes, including in vitro, ex vivo and in vivo models. We then discuss the limitations and challenges facing this field of research.