Microbial-induced meprin β cleavage in MUC2 mucin and a functional CFTR channel are required to release anchored small intestinal mucus. Proc Natl Acad Sci U S A. 2014;111:12396-12401. [PMID: 25114233 DOI: 10.1073/pnas.1407597111]

Clca1 deficiency exacerbates colitis susceptibility via impairment of mucus barrier integrity and gut microbiota homeostasis

The intestinal mucus barrier has emerged as a promising therapeutic target for inflammatory bowel disease. Understanding its regulatory mechanisms is critical for elucidating ulcerative colitis (UC) pathogenesis, improving diagnostics, guiding treatments, and preventing relapse. Chloride Channel Accessory 1 (Clca1), a constituent of the mucus layer, remains understudied in colitis. Here, we investigated Clca1's role in mucosal immunity and intestinal homeostasis using experimental colitis models. Clca1-deficient (Clca1-/-) mice displayed compromised mucus layer integrity, reduced neutrophil infiltration, and gut microbiota dysbiosis. Notably, Clca1-/- mice exhibited exacerbated colitis severity following dextran sulfate sodium (DSS) challenge, accompanied by a diminished goblet cell populations. Fecal microbiota transplantation (FMT) studies revealed that gut microbiota critically modulates divergent phenotypic outcomes between genotypes. Our findings establish Clca1 as a multifunctional regulator of mucus barrier integrity through mechanisms involving goblet cell maintenance, neutrophil-mediated immunity, and host-microbiota crosstalk. These results advance the understanding of UC pathogenesis and identify Clca1-associated pathways as potential targets for barrier restoration therapies.

Meprin β Modulates Brevican Proteolysis Impairing Neural Plasticity and Memory Formation

The metalloprotease meprin β is known for its multifunctional involvement in various physiological processes throughout the body including the brain. However, its broader functions within the brain besides amyloid β generation remain largely unexplored. To investigate this, we utilized a mouse model overexpressing meprin β in neurons within the cortex and hippocampus, regions crucial for learning and memory. Behavioral assessments, employing the Morris' Water Maze paradigm test, revealed impaired cognitive functions in animals overexpressing meprin β. Furthermore, electrophysiological recordings in hippocampal slices using multielectrode arrays showed an impaired long-term potentiation (LTP) in meprin β-overexpressing mice compared to wild-type counterparts. Intriguingly, concomitant with the LTP impairment, we observed an increased neuronal excitability. These findings underline the complicated interplay between meprin β abundance and behavioral manifestations, suggesting a broader impact on neural circuit dynamics. To elucidate the molecular mechanisms underlying these observed deficits, western blotting analyses were conducted to address the expression of glutamatergic receptors. Neither the expression of the N-methyl-D-aspartate (NMDA) nor the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor showed variation relative to each other. The application of N-terminomics identified brevican as a proteolytic substrate of meprin β and thus a potential key mediator linking meprin β overexpression to the observed effects. Previous studies have reported that brevican knockout in animal models influences learning and memory. Our data demonstrate that meprin β modulates brevican expression, likely contributing to the effects we have observed in our mouse model. These results shed light on the broader functional significance of meprin β in neurological processes.

Mucins and Their Roles in Asthma

Mucus is a crucial component of airway host defense. For optimal protection, its chief components-the mucins MUC5AC and MUC5B-need to be tightly regulated. Their expression localizes to specific secretory epithelial cell types capable of producing and secreting massive glycopolymers. In asthma, abnormal mucus is an important clinical problem that is effectively treated with therapies that directly target mucins. This review summarizes what is known about how mucin gene regulation, protein synthesis, and secretion are regulated in healthy and asthmatic lungs. Ultimately, a better understanding of these processes could help identify novel ways of preventing or reversing airway mucus dysfunction.

HYTANE-Identified Latrophilin-3 Cleavage by Meprin β Leads to Loss of the Interaction Domains

The metalloprotease meprin β is upregulated in neurons and astrocytes of Alzheimer's disease patients' brains. While the role of meprin β as the β-secretase of amyloid precursor protein (APP) has been characterized, its broader substrate profile within the brain remains largely unexplored. Hence, to identify additional substrates, we conducted N-terminomics of brain lysates from mice overexpressing meprin β in astrocytes employing the Hydrophobic Tagging-Assisted N-terminal Enrichment (HYTANE) strategy. We observed 3906 (82.2%) N-terminal peptides and identified seven new substrates that match meprin β in terms of localization and cleavage specificity. Of note, the meprin β overexpressing mice show mild cognitive impairments caused by amyloidogenic APP processing alongside hyperactivity and altered exploratory behavior seemingly independent of APP cleavage. Hence, latrophilin-3 was of particular interest, as latrophilin-3 defects are associated with hyperactivity in mice and human. In brain lysates from mice overexpressing meprin β in astrocytes as well as in cellulo, we validated the cleavage of latrophilin-3, resulting in the release of two N-terminal domains. These domains promote interactions with neuronal proteins such as fibronectin leucine-rich repeat transmembrane proteins, promoting adequate synapse formation. Thus, meprin β might affect synaptic integrity by cleaving interaction domains of latrophilin-3, potentially exacerbating the observed hyperactivity phenotype.

Influence of Alcohol on the Intestinal Immune System

PURPOSE

Alcohol misuse is associated with disruption of the microbial homeostasis (dysbiosis) and microbial overgrowth in the gut, gut barrier disruption, and translocation of microbes into the systemic circulation. It also induces changes in regulatory mechanisms of the gut, which is the largest peripheral immune organ. The gut-liver axis is important for health and disease, and alterations in the intestinal immune system contribute to alcohol-associated liver disease (ALD). Understanding these changes might help discover new targets for drugs and therapeutic approaches.

SEARCH METHODS

A systematic literature search was conducted in PubMed, Medline, and Embase of manuscripts published between January 2000 and November 2023 using the terms ("alcohol" or "ethanol") AND ("immune" or "immunol") AND ("intestine," "colon," or "gut"). Eligible manuscripts included studies and reviews that discussed the effects of ethanol on immune cells in the intestine.

SEARCH RESULTS

A total of 506 publications were found in the databases on November 20, 2023. After excluding duplicates and research not covering ALD (415 articles), 91 studies were reviewed. Also included were manuscripts covering specific immune cells in the context of ALD.

DISCUSSION AND CONCLUSIONS

Balancing immune tolerance vs. initiating an immune response challenges the intestinal immune system. Alcohol induces disruption of the intestinal barrier, which is accompanied by a thicker mucus layer and reduced anti-microbial peptides. This leads to longer attachment of bacteria to epithelial cells and consequently greater translocation into the circulation. Bacterial translocation activates the immune system, reducing the activity of regulatory T cells and inducing T helper 17 response via a variety of pathways. The role of innate immune cells, especially Type 3 innate lymphoid cells, and of specific B- and T-cell subsets in ALD remains elusive. Gut dysbiosis, translocation of viable bacteria and bacterial products into the circulation, and changes in the intestinal barrier have been linked to immune deficiency and infections in patients with cirrhosis. Modifying the intestinal immune system could reduce intestinal inflammation and alcohol-induced liver injury. Understanding the underlying pathophysiology can help to detect new targets for drugs and design therapeutic strategies.

A combined ligand-based and structure-based in silico molecular modeling approach to pinpoint the key structural attributes of hydroxamate derivatives as promising meprin β inhibitors

Human meprin β is a Zn2+-containing multidomain metalloprotease enzyme that belongs to the astacin family of the metzincin endopeptidase superfamily. Meprin β, with its diverse tissue expression pattern and wide substrate specificity, plays a significant role in various biological processes, including regulation of IL-6R pathways, lung fibrosis, collagen deposition, cellular migration, neurotoxic amyloid β levels, and inflammation. Again, meprin β is involved in Alzheimer's disease, hyperkeratosis, glomerulonephritis, diabetic kidney injury, inflammatory bowel disease, and cancer. Despite a crucial role in diverse disease processes, no such promising inhibitors of meprin β are marketed to date. Thus, it is an unmet requirement to find novel promising meprin β inhibitors that hold promise as potential therapeutics. In this study, a series of arylsulfonamide and tertiary amine-based hydroxamate derivatives as meprin β inhibitors has been analyzed through ligand-based and structure-based in silico approaches to pinpoint their structural and physiochemical requirements crucial for exerting higher inhibitory potential. This study identified different crucial structural features such as arylcarboxylic acid, sulfonamide, and arylsulfonamide moieties, as well as hydrogen bond donor and hydrophobicity, inevitable for exerting higher meprin β inhibition, providing valuable insight for their further future development.

Structure of the MUC5AC VWD3 assembly responsible for the formation of net-like mucin polymers

Gel-forming mucins MUC5AC and MUC5B constitute the main structural component of the mucus in the respiratory system. Secreted mucins interact specifically with each other and other molecules giving mucus specific properties. We determined the cryoEM structures of the wild type D3 assembly of the human MUC5AC mucin and the structural single nucleotide polymorphisms (SNP) variants Arg996Gln and Arg1201Trp that affect intermolecular interactions. Our structures explain the MUC5AC N-terminal non-covalent oligomerization after secretion. The D3 assembly forms covalent dimers that can appear in two alternative conformations, open and closed, where the closed conformation dimers interact through an arginine-rich loop in the TIL3 domain to form tetramers. Our study provides a model to explain MUC5AC net-like structures and how the two SNPs will affect mucus organization, something that might affect lung and other diseases.

Targeted approach to determine the impact of cancer-associated protease variants

Several steps of cancer progression, from tumor onset to metastasis, critically involve proteolytic activity. To elucidate the role of proteases in cancer, it is particularly important to consider single-nucleotide variants (SNVs) that affect the active site of proteases, thereby influencing cleavage specificity, substrate processing, and thus cancer cell behavior. To facilitate systematic studies, we here present a targeted approach to determine the impact of cancer-associated protease variants (TACAP). Starting with the semiautomated identification of potential specificity-modulating SNVs, our workflow comprises mass spectrometry-based cleavage specificity profiling and substrate identification, localization, and inhibitor studies, followed by functional analyses investigating cancer cell properties. To demonstrate the feasibility of TACAP, we analyzed the meprin β R238Q variant. This amino acid exchange R238Q leads to a loss of meprin β's characteristic cleavage preference for acidic amino acids at P1' position, accompanied with changes in substrate pool and inhibitor affinity compared to meprin β wild type.

The Role of Gut Microbiome in Irritable Bowel Syndrome: Implications for Clinical Therapeutics

Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder (FGID) characterized by chronic or recurrent gastrointestinal symptoms without organic changes, and it is also a common disorder of gut-brain interaction (DGBIs).. The symptoms of IBS not only affect the quality of life for individual patients but also place a significant burden on global healthcare systems. The lack of established and universally applicable biomarkers for IBS, along with the substantial variability in symptoms and progression, presents challenges in developing effective clinical treatments. In recent years, preclinical and clinical studies have linked the pathogenesis of IBS to alterations in the composition and function of the intestinal microbiota. Within the complex microbial community of the gut, intricate metabolic and spatial interactions occur among its members and between microbes and their hosts. Amid the multifaceted pathophysiology of IBS, the role of intestinal microenvironment factors in symptom development has become more apparent. This review aims to delve into the changes in the composition and structure of the gut microbiome in individuals with IBS. It explores how diet-mediated alterations in intestinal microbes and their byproducts play a role in regulating the pathogenesis of IBS by influencing the "brain-gut" axis, intestinal barrier function, immune responses, and more. By doing so, this review seeks to lay a theoretical foundation for advancing the development of clinical therapeutics for IBS.

Goblet cells: guardians of gut immunity and their role in gastrointestinal diseases

Goblet cells (GCs) are specialised guardians lining the intestine. They play a critical role in gut defence and immune regulation. GCs continuously secrete mucus creating a physical barrier to protect from pathogens while harbouring symbiotic gut bacteria adapted to live within the mucus. GCs also form specialised GC-associated passages in a dynamic and regulated manner to deliver luminal antigens to immune cells, promoting gut tolerance and preventing inflammation. The composition of gut bacteria directly influences GC function, highlighting the intricate interplay between these components of a healthy gut. Indeed, imbalances in the gut microbiome can disrupt GC function, contributing to various gastrointestinal diseases like colorectal cancer, inflammatory bowel disease, cystic fibrosis, pathogen infections and liver diseases. This review explores the interplay between GCs and the immune system. We delve into the underlying mechanisms by which GC dysfunction contributes to the development and progression of gastrointestinal diseases. Finally, we examine current and potential treatments that target GCs and represent promising avenues for further investigation.

Cy3-Based Nanoviscosity Determination of Mucus: Effect of Mucus Collection Methods and Antibiotics Treatment

The integrity of the protective mucus layer as a primary defense against pathogen invasion and microbial leakage into the intestinal epithelium can be compromised by the effects of antibiotics on the commensal microbiome. Changes in mucus integrity directly affect the solvent viscosity in the immediate vicinity of the mucin network, that is, the nanoviscosity, which in turn affects both biochemical reactions and selective transport. To assess mucus nanoviscosity, a reliable readout via the viscosity-dependent fluorescence lifetime of the molecular rotor dye cyanine 3 is established and nanoviscosities from porcine and murine ex vivo mucus are determined. To account for different mucin concentrations due to the removal of digestive residues during mucus collection, the power law dependence of mucin concentration on viscosity is used. The impact of antibiotics combinations (meropenem/vancomycin, gentamycin/ampicillin) on ex vivo intestinal mucus nanoviscosity is presented. The significant increase in viscosity of murine intestinal mucus after treatment suggests an effect of antibiotics on the microbiota that affects mucus integrity. This method will be a useful tool to assess how drugs, directly or indirectly, affect mucus integrity. Additionally, the method can be utilized to analyze the role of mucus nanoviscosity in health and disease, as well as in drug development.

The anion exchanger slc26a3 regulates colonic mucus expansion during steady state and in response to prostaglandin E2, while Cftr regulates de novo mucus release in response to carbamylcholine

The intestinal epithelium is covered by mucus that protects the tissue from the luminal content. Studies have shown that anion secretion via the cystic fibrosis conductance regulator (Cftr) regulates mucus formation in the small intestine. However, mechanisms regulating mucus formation in the colon are less understood. The aim of this study was to explore the role of anion transport in the regulation of mucus formation during steady state and in response to carbamylcholine (CCh) and prostaglandin E2 (PGE2). The broad-spectrum anion transport inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulfonate (DIDS), CftrdF508 (CF) mice, and the slc26a3 inhibitor SLC26A3-IN-2 were used to inhibit anion transport. In the distal colon, steady-state mucus expansion was reduced by SLC26A3-IN-2 and normal in CF mice. PGE2 stimulated mucus expansion without de novo mucus release in wild type (WT) and CF colon via slc26a3 sensitive mechanisms, while CCh induced de novo mucus secretion in WT but not in CF colon. However, when added simultaneously, CCh and PGE2 stimulated de novo mucus secretion in the CF colon via DIDS-sensitive pathways. A similar response was observed in CF ileum that responded to CCh and PGE2 with DIDS-sensitive de novo mucus secretion. In conclusion, this study suggests that slc26a3 regulates colonic mucus expansion, while Cftr regulates CCh-induced de novo mucus secretion from ileal and distal colon crypts. Furthermore, these findings demonstrate that in the absence of a functional Cftr channel, parallel stimulation with CCh and PGE2 activates additional anion transport processes that help release mucus from intestinal goblet cells.

Impacts of PFOS, PFOA and their alternatives on the gut, intestinal barriers and gut-organ axis

With the restricted use of perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA), a number of alternatives to PFOS and PFOA have attracted great interest. Most of the alternatives are still characterized by persistence, bioaccumulation, and a variety of toxicity. Due to the production and use of these substances, they can be detected in the atmosphere, soil and water body. They affect human health through several exposure pathways and especially enter the gut by drinking water and eating food, which results in gut toxicity. In this review, we summarized the effects of PFOS, PFOA and 9 alternatives on pathological changes in the gut, the disruption of physical, chemical, biological and immune barriers of the intestine, and the gut-organ axis. This review provides a valuable understanding of the gut toxicity of PFOS, PFOA and their alternatives as well as the human health risks of emerging contaminants.

The MUC2 Gene Product: Polymerisation and Post-Secretory Organisation-Current Models

MUC2 mucin, the primary gel-forming component of intestinal mucus, is well researched and a model of polymerisation and post-secretory organisation has been published previously. Recently, several significant developments have been made which either introduce new ideas or challenge previous theories. New ideas include an overhaul of the MUC2 C-terminal globular structure which is proposed to harbour several previously unobserved domains, and include a site for an extra intermolecular disulphide bridge dimer between the cysteine 4379 of adjacent MUC2 C-termini. MUC2 polymers are also now thought to be secreted attached to the epithelial surface of goblet cells in the small intestine and removed following secretion via a metalloprotease meprin β-mediated cleavage of the von Willebrand D2 domain of the N-terminus. It remains unclear whether MUC2 forms intermolecular dimers, trimers, or both, at the N-termini during polymerisation, with several articles supporting either trimer or dimer formation. The presence of a firm inner mucus layer in the small intestine is similarly unclear. Considering this recent research, this review proposes an update to the previous model of MUC2 polymerisation and secretion, considers conflicting theories and data, and highlights the importance of this research to the understanding of MUC2 mucus layers in health and disease.

The structure of the second CysD domain of MUC2 and role in mucin organization by transglutaminase-based cross-linking

The MUC2 mucin protects the colonic epithelium by a two-layered mucus with an inner attached bacteria-free layer and an outer layer harboring commensal bacteria. CysD domains are 100 amino-acid-long sequences containing 10 cysteines that separate highly O-glycosylated proline, threonine, serine (PTS) regions in mucins. The structure of the second CysD, CysD2, of MUC2 is now solved by nuclear magnetic resonance. CysD2 shows a stable stalk region predicted to be partly covered by adjacent O-glycans attached to neighboring PTS sequences, whereas the CysD2 tip with three flexible loops is suggested to be well exposed. It shows transient dimer interactions at acidic pH, weakened at physiological pH. This transient interaction can be stabilized in vitro and in vivo by transglutaminase 3-catalyzed isopeptide bonds, preferring a specific glutamine residue on one flexible loop. This covalent dimer is modeled suggesting that CysD domains act as connecting hubs for covalent stabilization of mucins to form a protective mucus.

In Vitro Models for Investigating Intestinal Host-Pathogen Interactions

Infectious diseases are increasingly recognized as a major threat worldwide due to the rise of antimicrobial resistance and the emergence of novel pathogens. In vitro models that can adequately mimic in vivo gastrointestinal physiology are in high demand to elucidate mechanisms behind pathogen infectivity, and to aid the design of effective preventive and therapeutic interventions. There exists a trade-off between simple and high throughput models and those that are more complex and physiologically relevant. The complexity of the model used shall be guided by the biological question to be addressed. This review provides an overview of the structure and function of the intestine and the models that are developed to emulate this. Conventional models are discussed in addition to emerging models which employ engineering principles to equip them with necessary advanced monitoring capabilities for intestinal host-pathogen interrogation. Limitations of current models and future perspectives on the field are presented.

Orally-administered nanomedicine systems targeting colon inflammation for the treatment of inflammatory bowel disease: latest advances

Inflammatory bowel disease (IBD) is a chronic and idiopathic condition that results in inflammation of the gastrointestinal tract, leading to conditions such as ulcerative colitis and Crohn's disease. Commonly used treatments for IBD include anti-inflammatory drugs, immunosuppressants, and antibiotics. Fecal microbiota transplantation is also being explored as a potential treatment method; however, these drugs may lead to systemic side effects. Oral administration is preferred for IBD treatment, but accurately locating the inflamed area in the colon is challenging due to multiple physiological barriers. Nanoparticle drug delivery systems possess unique physicochemical properties that enable precise delivery to the target site for IBD treatment, exploiting the increased permeability and retention effect of inflamed intestines. The first part of this review comprehensively introduces the pathophysiological environment of IBD, covering the gastrointestinal pH, various enzymes in the pathway, transport time, intestinal mucus, intestinal epithelium, intestinal immune cells, and intestinal microbiota. The second part focuses on the latest advances in the mechanism and strategies of targeted delivery using oral nanoparticle drug delivery systems for colitis-related fields. Finally, we present challenges and potential directions for future IBD treatment with the assistance of nanotechnology.

Discovery of dual-target natural inhibitors of meprins α and β metalloproteases for inflammation regulation: pharmacophore modelling, molecular docking, ADME prediction, and molecular dynamics studies

Meprins, zinc-dependent metalloproteinases belonging to the metzincin family, have been associated with various inflammatory diseases due to their abnormal expression and activity. In this study, we utilized pharmacophore modelling to identify crucial features for discovering potential dual inhibitors targeting meprins α and β. We screened four pharmacophoric features against a library of 270,540 natural compounds from the Zinc database, resulting in 84,092 matching compounds. Molecular docking was then performed on these compounds, targeting the active sites of meprins α and β. Docking results revealed six compounds capable of interacting with both isoforms, with binding affinities ranging from -10.0 to -10.5 kcal/mol and -6.9 to -9.9 kcal/mol for meprin α and β, respectively. Among these compounds, ZINC000008790788 and ZINC000095099469 displayed superior docking scores and MM-GBSA binding free energy compared to reference ligands. Furthermore, these two compounds exhibited acceptable predicted pharmacokinetic properties and stable interactions with meprins α and β during molecular dynamics simulations. This study presents a comprehensive approach for identifying potential dual inhibitors of meprin α and β, offering insights into the development of therapeutic interventions for inflammatory diseases associated with meprin dysregulation.

Goblet cell interactions reorient bundled mucus strands for efficient airway clearance

The respiratory tract of larger animals is cleared by sweeping bundled strands along the airway surface. These bundled strands can be millimetric in length and consist of MUC5B mucin. They are produced by submucosal glands, and upon emerging from these glands, the long axis of the bundled strands is oriented along the cilia-mediated flow toward the oral cavity. However, after release, the bundled strands are found to have turned orthogonal to the flow, which maximizes their clearance potential. How this unexpected reorientation is accomplished is presently not well understood. Recent experiments suggest that the reorientation process involves bundled strands sticking to MUC5AC mucus threads, which are tethered to the goblet cells. Such goblet cells are present in small numbers throughout the airway epithelium. Here, we develop a minimal model for reorientation of bundled mucus strands through adhesive interactions with surface goblet cells. Our simulations reveal that goblet cell interactions can reorient the bundled strands within 10 mm of release-making reorientation on the length scale of the tracheal tube feasible-and can stabilize the orthogonal orientation. Our model also reproduces other experimental observations such as strong velocity fluctuations and significant slow-down of the bundled strand with respect to the cilia-mediated flow. We further provide insight into the strand turning mechanism by examining the effect of strand shape on the impulse exerted by a single goblet cell. We conclude that goblet cell-mediated reorientation is a viable route for bundled strand reorientation, which should be further validated in future experiment.

Epithelial TNF controls cell differentiation and CFTR activity to maintain intestinal mucin homeostasis

The gastrointestinal tract relies on the production, maturation, and transit of mucin to protect against pathogens and to lubricate the epithelial lining. Although the molecular and cellular mechanisms that regulate mucin production and movement are beginning to be understood, the upstream epithelial signals that contribute to mucin regulation remain unclear. Here, we report that the inflammatory cytokine tumor necrosis factor (TNF), generated by the epithelium, contributes to mucin homeostasis by regulating both cell differentiation and cystic fibrosis transmembrane conductance regulator (CFTR) activity. We used genetic mouse models and noninflamed samples from patients with inflammatory bowel disease (IBD) undergoing anti-TNF therapy to assess the effect of in vivo perturbation of TNF. We found that inhibition of epithelial TNF promotes the differentiation of secretory progenitor cells into mucus-producing goblet cells. Furthermore, TNF treatment and CFTR inhibition in intestinal organoids demonstrated that TNF promotes ion transport and luminal flow via CFTR. The absence of TNF led to slower gut transit times, which we propose results from increased mucus accumulation coupled with decreased luminal fluid pumping. These findings point to a TNF/CFTR signaling axis in the adult intestine and identify epithelial cell-derived TNF as an upstream regulator of mucin homeostasis.

The putative pleiotropic functions of meprin β in gastric cancer

BACKGROUND

The gastric microbiome and inflammation play a key role in gastric cancer (GC) by regulating the immune response in a complex manner and by inflammatory events supporting carcinogenesis. Meprin β is a zinc endopeptidase and participates in tissue homeostasis, intestinal barrier function and immunological processes. It influences local inflammatory processes, dysbiosis and the microbiome. Here, we tested the hypothesis that meprin β is expressed in GC and of tumor biological significance.

PATIENTS AND METHODS

Four hundred forty whole mount tissue sections of patients with therapy-naive GC were stained with an anti-meprin β antibody. The histoscore and staining pattern were analyzed for each case. Following dichotomization at the median histoscore into a "low" and "high" group, the expression was correlated with numerous clinicopathological patient characteristics.

RESULTS

Meprin β was found intracellularly and at the cell membrane of GC. Cytoplasmic expression correlated with the phenotype according to Lauren, microsatellite instability and PD-L1 status. Membranous expression correlated with intestinal phenotype, mucin-1-, E-cadherin-, β-catenin status, mucin typus, microsatellite instability, KRAS mutation and PD-L1-positivity. Patients with cytoplasmic expression of meprin β showed a better overall and tumor-specific survival.

CONCLUSIONS

Meprin β is differentially expressed in GC and has potential tumor biological relevance. It might function as a tumor suppressor or promotor depending on histoanatomical site and context.

The intestinal MUC2 mucin C-terminus is stabilized by an extra disulfide bond in comparison to von Willebrand factor and other gel-forming mucins

The MUC2 mucin polymer is the main building unit of the intestinal mucus layers separating intestinal microbiota from the host epithelium. The MUC2 mucin is a large glycoprotein with a C-terminal domain similar to the MUC5AC and MUC5B mucins and the von Willebrand factor (VWF). A structural model of the C-terminal part of MUC2, MUC2-C, was generated by combining Cryo-electron microscopy, AlphaFold prediction, information of its glycosylation, and small angle X-ray scattering information. The globular VWD4 assembly in the N-terminal of MUC2-C is followed by 3.5 linear VWC domains that form an extended flexible structure before the C-terminal cystine-knot. All gel-forming mucins and VWF form tail-tail disulfide-bonded dimers in their C-terminal cystine-knot domain, but interestingly the MUC2 mucin has an extra stabilizing disulfide bond on the N-terminal side of the VWD4 domain, likely essential for a stable intestinal mucus barrier.

Proteolytic processing of galectin-3 by meprin metalloproteases is crucial for host-microbiome homeostasis

The metalloproteases meprin α and meprin β are highly expressed in the healthy gut but significantly decreased in inflammatory bowel disease, implicating a protective role in mucosal homeostasis. In the colon, meprin α and meprin β form covalently linked heterodimers tethering meprin α to the plasma membrane, therefore presenting dual proteolytic activity in a unique enzyme complex. To unravel its function, we applied N-terminomics and identified galectin-3 as the major intestinal substrate for meprin α/β heterodimers. Galectin-3-deficient and meprin α/β double knockout mice show similar alterations in their microbiome in comparison to wild-type mice. We further demonstrate that meprin α/β heterodimers differentially process galectin-3 upon bacterial infection, in germ-free, conventionally housed (specific pathogen-free), or wildling mice, which in turn regulates the bacterial agglutination properties of galectin-3. Thus, the constitutive cleavage of galectin-3 by meprin α/β heterodimers may play a key role in colon host-microbiome homeostasis.

Orchestration of MUC2 - The key regulatory target of gut barrier and homeostasis: A review

The gut mucosa of human is covered by mucus, functioning as a crucial defense line for the intestine against external stimuli and pathogens. Mucin2 (MUC2) is a subtype of secretory mucins generated by goblet cells and is the major macromolecular component of mucus. Currently, there is an increasing interest on the investigations of MUC2, noting that its function is far beyond a maintainer of the mucus barrier. Moreover, numerous gut diseases are associated with dysregulated MUC2 production. Appropriate production level of MUC2 and mucus contributes to gut barrier function and homeostasis. The production of MUC2 is regulated by a series of physiological processes, which are orchestrated by various bioactive molecules, signaling pathways and gut microbiota, etc., forming a complex regulatory network. Incorporating the latest findings, this review provided a comprehensive summary of MUC2, including its structure, significance and secretory process. Furthermore, we also summarized the molecular mechanisms of the regulation of MUC2 production aiming to provide developmental directions for future researches on MUC2, which can act as a potential prognostic indicator and targeted therapeutic manipulation for diseases. Collectively, we elucidated the micro-level mechanisms underlying MUC2-related phenotypes, hoping to offer some constructive guidance for intestinal and overall health of mankind.

Muc2-dependent microbial colonization of the jejunal mucus layer is diet sensitive and confers local resistance to enteric pathogen infection

Intestinal mucus barriers normally prevent microbial infections but are sensitive to diet-dependent changes in the luminal environment. Here we demonstrate that mice fed a Western-style diet (WSD) suffer regiospecific failure of the mucus barrier in the small intestinal jejunum caused by diet-induced mucus aggregation. Mucus barrier disruption due to either WSD exposure or chromosomal Muc2 deletion results in collapse of the commensal jejunal microbiota, which in turn sensitizes mice to atypical jejunal colonization by the enteric pathogen Citrobacter rodentium. We illustrate the jejunal mucus layer as a microbial habitat, and link the regiospecific mucus dependency of the microbiota to distinctive properties of the jejunal niche. Together, our data demonstrate a symbiotic mucus-microbiota relationship that normally prevents jejunal pathogen colonization, but is highly sensitive to disruption by exposure to a WSD.

Anthocyanin actions at the gastrointestinal tract: Relevance to their health benefits

Anthocyanins (AC) are flavonoids abundant in the human diet, which consumption has been associated to several health benefits, including the mitigation of cardiovascular disease, type 2 diabetes, non-alcoholic fatty liver disease, and neurological disorders. It is widely recognized that the gastrointestinal (GI) tract is not only central for food digestion but actively participates in the regulation of whole body physiology. Given that AC, and their metabolites reach high concentrations in the intestinal lumen after food consumption, their biological actions at the GI tract can in part explain their proposed local and systemic health benefits. In terms of mechanisms of action, AC have been found to: i) inhibit GI luminal enzymes that participate in the absorption of lipids and carbohydrates; ii) preserve intestinal barrier integrity and prevent endotoxemia, inflammation and oxidative stress; iii) sustain goblet cell number, immunological functions, and mucus production; iv) promote a healthy microbiota; v) be metabolized by the microbiota to AC metabolites which will be absorbed and have systemic effects; and vi) modulate the metabolism of GI-generated hormones. This review will summarize and discuss the latest information on AC actions at the GI tract and their relationship to overall health benefits.

Oral Exposure to Polystyrene Microplastics of Mice on a Normal or High-Fat Diet and Intestinal and Metabolic Outcomes

BACKGROUND

Microplastics (MPs) are small particles of plastic (≤5mm in diameter). In recent years, oral exposure to MPs in living organisms has been a cause of concern. Leaky gut syndrome (LGS), associated with a high-fat diet (HFD) in mice, can increase the entry of foreign substances into the body through the intestinal mucosa.

OBJECTIVES

We aimed to evaluate the pathophysiology of intestinal outcomes associated with consuming a high-fat diet and simultaneous intake of MPs, focusing on endocrine and metabolic systems.

METHODS

C57BL6/J mice were fed a normal diet (ND) or HFD with or without polystyrene MP for 4 wk to investigate differences in glucose tolerance, intestinal permeability, gut microbiota, as well as metabolites in serum, feces, and liver.

RESULTS

In comparison with HFD mice, mice fed the HFD with MPs had higher blood glucose, serum lipid concentrations, and nonalcoholic fatty liver disease (NAFLD) activity scores. Permeability and goblet cell count of the small intestine (SI) in HFD-fed mice were higher and lower, respectively, than in ND-fed mice. There was no obvious difference in the number of inflammatory cells in the SI lamina propria between mice fed the ND and mice fed the ND with MP, but there were more inflammatory cells and fewer anti-inflammatory cells in mice fed the HFD with MPs in comparison with mice fed the HFD without MPs. The expression of genes related to inflammation, long-chain fatty acid transporter, and Na+/glucose cotransporter was significantly higher in mice fed the HFD with MPs than in mice fed the HFD without MPs. Furthermore, the genus Desulfovibrio was significantly more abundant in the intestines of mice fed the HFD with MPs in comparison with mice fed the HFD without MPs. Muc2 gene expression was decreased when palmitic acid and microplastics were added to the murine intestinal epithelial cell line MODE-K cells, and Muc2 gene expression was increased when IL-22 was added.

DISCUSSION

Our findings suggest that in this study, MP induced metabolic disturbances, such as diabetes and NAFLD, only in mice fed a high-fat diet. These findings suggest that LGS might have been triggered by HFD, causing MPs to be deposited in the intestinal mucosa, resulting in inflammation of the intestinal mucosal intrinsic layer and thereby altering nutrient absorption. These results highlight the need for reducing oral exposure to MPs through remedial environmental measures to improve metabolic disturbance under high-fat diet conditions. https://doi.org/10.1289/EHP11072.

Hericium caput-medusae (Bull.:Fr.) Pers. fermentation concentrate polysaccharides improves intestinal bacteria by activating chloride channels and mucus secretion

ETHNOPHARMACOLOGICAL RELEVANCE

As a traditional edible fungus in China and many other Asian countries, Hericium caput-medusae (Bull. Fr.) Pers. is widely used to improve the health of the gastrointestinal tract. For example, the drug "Weilexin Granules" is mainly composed of H. caput-medusae (Bull. Fr.) Pers. fermentation concentrate. However, the mechanism of action remains to be elucidated.

AIMS OF THE STUDY

The purpose of this study was to assess whether polysaccharides from H. caput-medusae (Bull. Fr.) Pers. fermentation concentrate (HFP) exerts a gut protective effect and a regulatory effect on the intestinal microbiota through the chloride channels and mucus secretion.

MATERIALS AND METHODS

HFP was extracted, characterized and different concentrations of HFP (100, 200, 400 mg/kg) were administered to mice for 14 days. The changes in gut microbiota were observed via 16S high throughput sequencing. Short-chain fatty acids (SCFAs) was detected by GC-MS. AB-PAS staining was used to observe the secretion of mucus. The chloride channel activity and protein expression were verified by short-circuit current measurement and Western blot.

RESULTS

HFP regulated the abundance of gut microbiota in mice, with increased levels of Ruminococcaceae and Lachnospiraceae and reduced proportions of Staphylococcus and Enterobacter. HFP enhanced mucus volume as well as increased intestinal fluid secretion by activating the chloride channels. In addition, short-circuit current experiments also proved that HFP activates Cl⁻ currents targeting cystic fibrosis transmembrane conductance regulator (CFTR) and Anoamin1 (ANO1).

CONCLUSION

In conclusion, HFP might increase intestinal fluid secretion by promoting Cl⁻ secretion, which in turn advanced mucus hydration as well as regulated gut microbiota to improve intestinal health. Therefore, H. caput-medusae (Bull. Fr.) Pers. could be potentially used in the regulation of intestinal secretion and microbes.

Modeling mucus physiology and pathophysiology in human organs-on-chips

The surfaces of human internal organs are lined by a mucus layer that ensures symbiotic relationships with commensal microbiome while protecting against potentially injurious environmental chemicals, toxins, and pathogens, and disruption of this layer can contribute to disease development. Studying mucus biology has been challenging due to the lack of physiologically relevant human in vitro models. Here we review recent progress that has been made in the development of human organ-on-a-chip microfluidic culture models that reconstitute epithelial tissue barriers and physiologically relevant mucus layers with a focus on lung, colon, small intestine, cervix and vagina. These organ-on-a-chip models that incorporate dynamic fluid flow, air-liquid interfaces, and physiologically relevant mechanical cues can be used to study mucus composition, mechanics, and structure, as well as investigate its contributions to human health and disease with a level of biomimicry not possible in the past.

The role of goblet cells and mucus in intestinal homeostasis

The intestinal tract faces numerous challenges that require several layers of defence. The tight epithelium forms a physical barrier that is further protected by a mucus layer, which provides various site-specific protective functions. Mucus is produced by goblet cells, and as a result of single-cell RNA sequencing identifying novel goblet cell subpopulations, our understanding of their various contributions to intestinal homeostasis has improved. Goblet cells not only produce mucus but also are intimately linked to the immune system. Mucus and goblet cell development is tightly regulated during early life and synchronized with microbial colonization. Dysregulation of the developing mucus systems and goblet cells has been associated with infectious and inflammatory conditions and predisposition to chronic disease later in life. Dysfunctional mucus and altered goblet cell profiles are associated with inflammatory conditions in which some mucus system impairments precede inflammation, indicating a role in pathogenesis. In this Review, we present an overview of the current understanding of the role of goblet cells and the mucus layer in maintaining intestinal health during steady-state and how alterations to these systems contribute to inflammatory and infectious disease.

Intestinal mucin is a chaperone of multivalent copper

Mucus protects the epithelial cells of the digestive and respiratory tracts from pathogens and other hazards. Progress in determining the molecular mechanisms of mucus barrier function has been limited by the lack of high-resolution structural information on mucins, the giant, secreted, gel-forming glycoproteins that are the major constituents of mucus. Here, we report how mucin structures we determined enabled the discovery of an unanticipated protective role of mucus: managing the toxic transition metal copper. Using two juxtaposed copper binding sites, one for Cu²⁺ and the other for Cu¹⁺, the intestinal mucin, MUC2, prevents copper toxicity by blocking futile redox cycling and the squandering of dietary antioxidants, while nevertheless permitting uptake of this important trace metal into cells. These findings emphasize the value of molecular structure in advancing mucosal biology, while introducing mucins, produced in massive quantities to guard extensive mucosal surfaces, as extracellular copper chaperones.

Gut microbiota: a new avenue to reveal pathological mechanisms of constipation

Constipation is very pervasive all over the world. It is a common multifactorial gastrointestinal disease, and its etiology and pathomechanism are not completely clear. Now, increasing evidence shows that intestinal flora is closely related to constipation. Intestinal flora is the largest microbiota in the human body and has powerful metabolic functions. Intestinal flora can produce a variety of metabolites, such as bile acids, short-chain fatty acids, tryptophan metabolites, and methane, which have important effects on intestinal motility and secretion. The host can also monitor the intestinal flora and regulate gut dysbacteriosis in constipation. To explore the relationship between intestinal flora and host, the combination of multiomics technology has become the powerful and effective method. Furthermore, the homeostasis restoration of intestinal flora also provides a new strategy for the treatment of constipation. This review aims to explore the interaction between intestinal flora and host in constipation, which contributes to disclose the pathogenesis of constipation and the development of novel drugs for the treatment of constipation from the perspective of intestinal flora. KEY POINTS: • This review highlights the regulation of gut microbiota on the intestinal motility and secretion of host. • The current review gives an insight into the role of the host on the recognition and regulation of intestinal ecology under constipation. • The article also introduces some novel methods of current gut microbiota research and gut microbiota-based constipation therapies.

Physiology and pathophysiology of human airway mucus

The mucus clearance system is the dominant mechanical host defense system of the human lung. Mucus is cleared from the lung by cilia and airflow, including both two-phase gas-liquid pumping and cough-dependent mechanisms, and mucus transport rates are heavily dependent on mucus concentration. Importantly, mucus transport rates are accurately predicted by the gel-on-brush model of the mucociliary apparatus from the relative osmotic moduli of the mucus and periciliary-glycocalyceal (PCL-G) layers. The fluid available to hydrate mucus is generated by transepithelial fluid transport. Feedback interactions between mucus concentrations and cilia beating, via purinergic signaling, coordinate Na+ absorptive vs Cl- secretory rates to maintain mucus hydration in health. In disease, mucus becomes hyperconcentrated (dehydrated). Multiple mechanisms derange the ion transport pathways that normally hydrate mucus in muco-obstructive lung diseases, e.g., cystic fibrosis (CF), chronic obstructive pulmonary disease (COPD), non-CF bronchiectasis (NCFB), and primary ciliary dyskinesia (PCD). A key step in muco-obstructive disease pathogenesis is the osmotic compression of the mucus layer onto the airway surface with the formation of adherent mucus plaques and plugs, particularly in distal airways. Mucus plaques create locally hypoxic conditions and produce airflow obstruction, inflammation, infection, and, ultimately, airway wall damage. Therapies to clear adherent mucus with hydrating and mucolytic agents are rational, and strategies to develop these agents are reviewed.

Clearance of small intestinal crypts involves goblet cell mucus secretion by intracellular granule rupture and enterocyte ion transport

Goblet cells in the small intestinal crypts contain large numbers of mucin granules that are rapidly discharged to clean bacteria from the crypt. Because acetylcholine released by neuronal and nonneuronal cells controls many aspects of intestinal epithelial function, we used tissue explants and organoids to investigate the response of the small intestinal crypt to cholinergic stimulation. The activation of muscarinic acetylcholine receptors initiated a coordinated and rapid emptying of crypt goblet cells that flushed the crypt contents into the intestinal lumen. Cholinergic stimulation induced an expansion of the granule contents followed by intracellular rupture of the mucin granules. The mucus expanded intracellularly before the rupture of the goblet cell apical membrane and continued to expand after its release into the crypt lumen. The goblet cells recovered from membrane rupture and replenished their stores of mucin granules. Mucus secretion from the goblet cells depended on Ca2+ signaling and the expansion of the mucus in the crypt depended on gap junctions and on ion and water transport by enterocytes adjacent to the goblet cells. This distinctive mode of mucus secretion, which we refer to as "expanding secretion," efficiently cleans the small intestine crypt through coordinated mucus, ion, and fluid secretion by goblet cells and enterocytes.

Diet, microbiota, and the mucus layer: The guardians of our health

The intestinal tract is an ecosystem in which the resident microbiota lives in symbiosis with its host. This symbiotic relationship is key to maintaining overall health, with dietary habits of the host representing one of the main external factors shaping the microbiome-host relationship. Diets high in fiber and low in fat and sugars, as opposed to Western and high-fat diets, have been shown to have a beneficial effect on intestinal health by promoting the growth of beneficial bacteria, improve mucus barrier function and immune tolerance, while inhibiting pro-inflammatory responses and their downstream effects. On the contrary, diets low in fiber and high in fat and sugars have been associated with alterations in microbiota composition/functionality and the subsequent development of chronic diseases such as food allergies, inflammatory bowel disease, and metabolic disease. In this review, we provided an updated overview of the current understanding of the connection between diet, microbiota, and health, with a special focus on the role of Western and high-fat diets in shaping intestinal homeostasis by modulating the gut microbiota.

Intestinal Microbiota - An Unmissable Bridge to Severe Acute Pancreatitis-Associated Acute Lung Injury

Severe acute pancreatitis (SAP), one of the most serious abdominal emergencies in general surgery, is characterized by acute and rapid onset as well as high mortality, which often leads to multiple organ failure (MOF). Acute lung injury (ALI), the earliest accompanied organ dysfunction, is the most common cause of death in patients following the SAP onset. The exact pathogenesis of ALI during SAP, however, remains unclear. In recent years, advances in the microbiota-gut-lung axis have led to a better understanding of SAP-associated lung injury (PALI). In addition, the bidirectional communications between intestinal microbes and the lung are becoming more apparent. This paper aims to review the mechanisms of an imbalanced intestinal microbiota contributing to the development of PALI, which is mediated by the disruption of physical, chemical, and immune barriers in the intestine, promotes bacterial translocation, and results in the activation of abnormal immune responses in severe pancreatitis. The pathogen-associated molecular patterns (PAMPs) mediated immunol mechanisms in the occurrence of PALI via binding with pattern recognition receptors (PRRs) through the microbiota-gut-lung axis are focused in this study. Moreover, the potential therapeutic strategies for alleviating PALI by regulating the composition or the function of the intestinal microbiota are discussed in this review. The aim of this study is to provide new ideas and therapeutic tools for PALI patients.

Disease-associated dysbiosis and potential therapeutic role of Akkermansia muciniphila, a mucus degrading bacteria of gut microbiome

The unique functionality of Akkermansia muciniphila in gut microbiota indicates it to be an indispensable microbe for human welfare. The importance of A. muciniphila lies in its potential to convert mucin into beneficial by-products, regulate intestinal homeostasis and maintain gut barrier integrity. It is also known to competitively inhibit other mucin-degrading bacteria and improve metabolic functions and immunity responses in the host. It finds a pivotal perspective in various diseases and their treatment. It has future as a promising probiotic, disease biomarker and therapeutic agent for chronic diseases. Disease-associated dysbiosis of A. muciniphila in the gut microbiome makes it a potential candidate as a biomarker for some diseases and can provide future theranostics by suggesting ways of diagnosis for the patients and best treatment method based on the screening results. Manipulation of A. muciniphila in gut microbiome may help in developing a novel personalized therapeutic action and can be a suitable next generation medicine. However, the actual pathway governing A. muciniphila interaction with hosts remains to be investigated. Also, due to the limited availability of products containing A. muciniphila, it is not exploited to its full potential. The present review aims at highlighting the potential of A. muciniphila in mucin degradation, contribution towards the gut health and host immunity and management of metabolic diseases such as obesity and type 2 diabetes, and respiratory diseases such as cystic fibrosis and COVID-19.

In vivo models of mucin biosynthesis and function

The secreted mucus layer that lines and protects epithelial cells is conserved across diverse species. While the exact composition of this protective layer varies between organisms, certain elements are conserved, including proteins that are heavily decorated with N-acetylgalactosamine-based sugars linked to serines or threonines (O-linked glycosylation). These heavily O-glycosylated proteins, known as mucins, exist in many forms and are able to form hydrated gel-like structures that coat epithelial surfaces. In vivo studies in diverse organisms have highlighted the importance of both the mucin proteins as well as their constituent O-glycans in the protection and health of internal epithelia. Here, we summarize in vivo approaches that have shed light on the synthesis and function of these essential components of mucus.

Helical self-assembly of a mucin segment suggests an evolutionary origin for von Willebrand factor tubules

Extracellular proteins with mechanical functions often require specialized assembly processes to form covalent oligomers. Progress in tissue bioengineering and repair will benefit from an understanding of how to harness and manipulate these processes. Here, we show that a particular supramolecular assembly mode was pre-encoded in the ancient domain organization common to gel-forming mucins and von Willebrand factor, glycoproteins that are deceptively different due to their divergence for distinct mechanical tasks. This finding highlights symmetry principles and building blocks retooled in nature to construct polymers with wide-ranging properties. These building blocks and knowledge of their self-assembly can be used to design new polymeric structures.

The glycoprotein von Willebrand factor (VWF) contributes to hemostasis by stanching injuries in blood vessel walls. A distinctive feature of VWF is its assembly into long, helical tubules in endothelial cells prior to secretion. When VWF is released into the bloodstream, these tubules unfurl to release linear polymers that bind subendothelial collagen at wound sites, recruit platelets, and initiate the clotting cascade. VWF evolved from gel-forming mucins, the polymeric glycoproteins that coat and protect exposed epithelia. Despite the divergent function of VWF in blood vessel repair, sequence conservation and shared domain organization imply that VWF retained key aspects of the mucin bioassembly mechanism. Here, we show using cryo-electron microscopy that the ability to form tubules, a property hitherto thought to have arisen as a VWF adaptation to the vasculature, is a feature of the amino-terminal region of mucin. This segment of the human intestinal gel-forming mucin (MUC2) was found to self-assemble into tubules with a striking resemblance to those of VWF itself. To facilitate a comparison, we determined the residue-resolution structure of tubules formed by the homologous segment of VWF. The structures of the MUC2 and VWF tubules revealed the flexible joints and the intermolecular interactions required for tubule formation. Steric constraints in full-length MUC2 suggest that linear filaments, a previously observed supramolecular assembly form, are more likely than tubules to be the physiological mucin storage intermediate. Nevertheless, MUC2 tubules indicate a possible evolutionary origin for VWF tubules and elucidate design principles present in mucins and VWF.

Integrity of the Intestinal Barrier: The Involvement of Epithelial Cells and Microbiota-A Mutual Relationship

Simple Summary

The gastrointestinal tract is a complex organization of various types of epithelial cells forming a single layer of the mucosal barrier, the host mucosal immune system, and microorganisms termed as gut microbiota inhabiting this area. The mucosal barrier, including physical and chemical factors, spatially segregates gut microbiota and the host immune system preventing the development of immune response directed towards non-pathogenic commensals and dietary antigens. However, for the maintenance of the integrity of the mucosal surfaces, cross-talk between epithelial cells and microbiota is required. The microbiome and the intestinal epithelium developed a complex dependence necessary for sustaining intestinal homeostasis. In this review, we highlight the role of specific epithelial cell subtypes and their role in barrier arrangement, the mechanisms employed by them to control intestinal microbiota as well as the mechanisms utilized by the microbiome to regulate intestinal epithelial function. This review will provide information regarding the development of inflammatory disorders dependent on the loss of intestinal barrier function and composition of the intestinal microbiota.

Abstract

The gastrointestinal tract, which is constantly exposed to a multitude of stimuli, is considered responsible for maintaining the homeostasis of the host. It is inhabited by billions of microorganisms, the gut microbiota, which form a mutualistic relationship with the host. Although the microbiota is generally recognized as beneficial, at the same time, together with pathogens, they are a permanent threat to the host. Various populations of epithelial cells provide the first line of chemical and physical defense against external factors acting as the interface between luminal microorganisms and immunocompetent cells in lamina propria. In this review, we focus on some essential, innate mechanisms protecting mucosal integrity, thus responsible for maintaining intestine homeostasis. The characteristics of decisive cell populations involved in maintaining the barrier arrangement, based on mucus secretion, formation of intercellular junctions as well as production of antimicrobial peptides, responsible for shaping the gut microbiota, are presented. We emphasize the importance of cross-talk between gut microbiota and epithelial cells as a factor vital for the maintenance of the homeostasis of the GI tract. Finally, we discuss how the imbalance of these regulations leads to the compromised barrier integrity and dysbiosis considered to contribute to inflammatory disorders and metabolic diseases.

Mucins Dynamics in Physiological and Pathological Conditions

Maintaining intestinal health requires clear segregation between epithelial cells and luminal microbes. The intestinal mucus layer, produced by goblet cells (GCs), is a key element in maintaining the functional protection of the epithelium. The importance of the gut mucus barrier is highlighted in mice lacking Muc2, the major form of secreted mucins. These mice show closer bacterial residence to epithelial cells, develop spontaneous colitis and became moribund when infected with the attaching and effacing pathogen, Citrobacter rodentium. Furthermore, numerous observations have associated GCs and mucus layer dysfunction to the pathogenesis of inflammatory bowel disease (IBD). However, the molecular mechanisms that regulate the physiology of GCs and the mucus layer remain obscured. In this review, we consider novel findings describing divergent functionality and expression profiles of GCs subtypes within intestinal crypts. We also discuss internal (host) and external (diets and bacteria) factors that modulate different aspects of the mucus layer as well as the contribution of an altered mucus barrier to the onset of IBD.

A preliminary study of possible fibrotic role of meprin metalloproteases in scleroderma patients

OBJECTIVES

This study aims to investigate the possible fibrotic role of meprin metalloproteases and possible fibrotic effects of activator protein-1 (AP-1) in scleroderma patients.

PATIENTS AND METHODS

Between April 2018 and April 2019, a total of 85 scleroderma patients (9 males, 76 females; mean age: 54.9 years; range, 22 to 80 years) who met the 2013 American College of Rheumatology/European League Against Rheumatism criteria and 80 healthy control individuals (10 males, 70 females; mean age 42.9 years; range, 19 to 65 years) were included. Patients' data and blood samples were collected. Messenger ribonucleic acid expressions of interleukin (IL)-6, AP-1 subunits, and tumor necrosis factor-alpha (TNF-α) were analyzed by quantitative real-time polymerase chain reaction. Serum meprin alpha and beta protein levels were analyzed using the enzyme-linked immunosorbent assay.

RESULTS

Meprin alpha and meprin beta protein levels increased in scleroderma patients. The AP-1 subunits (c-Fos, c-Jun), IL-6, and TNF-α increased in scleroderma patients, compared to controls.

CONCLUSION

Our results provide evidence showing that increased meprins levels may be related to AP-1 levels and increased meprins levels may responsible for increased inflammatory TNF-α and IL-6 levels. All these data suggest meprins as promising therapeutic targets to restore the balance between inflammation and extracellular matrix deposition in scleroderma.

The Impact of MicroRNAs during Inflammatory Bowel Disease: Effects on the Mucus Layer and Intercellular Junctions for Gut Permeability

Research on inflammatory bowel disease (IBD) has produced mounting evidence for the modulation of microRNAs (miRNAs) during pathogenesis. MiRNAs are small, non-coding RNAs that interfere with the translation of mRNAs. Their high stability in free circulation at various regions of the body allows researchers to utilise miRNAs as biomarkers and as a focus for potential treatments of IBD. Yet, their distinct regulatory roles at the gut epithelial barrier remain elusive due to the fact that there are several external and cellular factors contributing to gut permeability. This review focuses on how miRNAs may compromise two components of the gut epithelium that together form the initial physical barrier: the mucus layer and the intercellular epithelial junctions. Here, we summarise the impact of miRNAs on goblet cell secretion and mucin structure, along with the proper function of various junctional proteins involved in paracellular transport, cell adhesion and communication. Knowledge of how this elaborate network of cells at the gut epithelial barrier becomes compromised as a result of dysregulated miRNA expression, thereby contributing to the development of IBD, will support the generation of miRNA-associated biomarker panels and therapeutic strategies that detect and ameliorate gut permeability.

Development and Functional Properties of Intestinal Mucus Layer in Poultry

Intestinal mucus plays important roles in protecting the epithelial surfaces against pathogens, supporting the colonization with commensal bacteria, maintaining an appropriate environment for digestion, as well as facilitating nutrient transport from the lumen to the underlying epithelium. The mucus layer in the poultry gut is produced and preserved by mucin-secreting goblet cells that rapidly develop and mature after hatch as a response to external stimuli including environmental factors, intestinal microbiota as well as dietary factors. The ontogenetic development of goblet cells affects the mucin composition and secretion, causing an alteration in the physicochemical properties of the mucus layer. The intestinal mucus prevents the invasion of pathogens to the epithelium by its antibacterial properties (e.g. β-defensin, lysozyme, avidin and IgA) and creates a physical barrier with the ability to protect the epithelium from pathogens. Mucosal barrier is the first line of innate defense in the gastrointestinal tract. This barrier has a selective permeability that allows small particles and nutrients passing through. The structural components and functional properties of mucins have been reviewed extensively in humans and rodents, but it seems to be neglected in poultry. This review discusses the impact of age on development of goblet cells and their mucus production with relevance for the functional characteristics of mucus layer and its protective mechanism in the chicken’s intestine. Dietary factors directly and indirectly (through modification of the gut bacteria and their metabolic activities) affect goblet cell proliferation and differentiation and can be used to manipulate mucosal integrity and dynamic. However, the mode of action and mechanisms behind these effects need to be studied further. As mucins resist to digestion processes, the sloughed mucins can be utilized by bacteria in the lower part of the gut and are considered as endogenous loss of protein and energy to animal. Hydrothermal processing of poultry feed may reduce this loss by reduction in mucus shedding into the lumen. Given the significance of this loss and the lack of precise data, this matter needs to be carefully investigated in the future and the nutritional strategies reducing this loss have to be defined better.

Structural and functional properties of meprin β metalloproteinase with regard to cell signaling

The metalloproteinase meprin β plays an important role during collagen I deposition in the skin, mucus detachment in the small intestine and also regulates the abundance of different cell surface proteins such as the interleukin-6 receptor (IL-6R), the triggering receptor expressed on myeloid cells 2 (TREM2), the cluster of differentiation 99 (CD99), the amyloid precursor protein (APP) and the cluster of differentiation 109 (CD109). With that, regulatory mechanisms that control meprin β activity and regulate its release from the cell surface to enable access to distant substrates are increasingly important. Here, we will summarize factors that alternate meprin β activity and thereby regulate its proteolytic activity on the cell surface or in the supernatant. We will also discuss cleavage of the IL-6R and TREM2 on the cell surface and compare it to CD109. CD109, as a substrate of meprin β, is cleaved within the protein core, thereby releasing defined fragments from the cell surface. At last, we will also summarize the role of proteases in general and meprin β in particular in substrate release on extracellular vesicles.

Regulation of meprin metalloproteases in mucosal homeostasis

Mucus is covering the entire epithelium of the gastrointestinal tract (GIT), building the interface for the symbiosis between microorganisms and their host. Hence, a disrupted mucosal barrier or alterations of proper mucus composition, including the gut microbiota, can cause severe infection and inflammation. Meprin metalloproteases are well-known to cleave various pro-inflammatory molecules, contributing to the onset and progression of pathological conditions including sepsis, pulmonary hypertension or inflammatory bowel disease (IBD). Moreover, meprins have an impact on migration and infiltration of immune cells like monocytes or leukocytes during intestinal inflammation by cleaving tight junction proteins or cell adhesion molecules, thereby disrupting epithelial cell barrier and promoting transendothelial cell migration. Interestingly, both meprin α and meprin β are susceptibility genes for IBD. However, both genes are significantly downregulated in inflamed intestinal tissue in contrast to healthy donors. Therefore, a detailed understanding of the underlying molecular mechanisms is the basis for developing new and effective therapies against manifold pathologies like IBD. This review focuses on the regulation of meprin metalloproteases and its impact on physiological and pathological conditions related to mucosal homeostasis.

New generation ENaC inhibitors detach cystic fibrosis airway mucus bundles via sodium/hydrogen exchanger inhibition

Cystic fibrosis (CF) is a recessive inherited disease caused by mutations affecting anion transport by the epithelial ion channel cystic fibrosis transmembrane conductance regulator (CFTR). The disease is characterized by mucus accumulation in the airways and intestine, but the major cause of mortality in CF is airway mucus accumulation, leading to bacterial colonization, inflammation and respiratory failure. Several drug targets are under evaluation to alleviate airway mucus obstruction in CF and one of these targets is the epithelial sodium channel ENaC. To explore effects of ENaC inhibitors on mucus properties, we used two model systems to investigate mucus characteristics, mucus attachment in mouse ileum and mucus bundle transport in piglet airways. We quantified mucus attachment in explants from CFTR null (CF) mice and tracheobronchial explants from newborn CFTR null (CF) piglets to evaluate effects of ENaC or sodium/hydrogen exchanger (NHE) inhibitors on mucus attachment. ENaC inhibitors detached mucus in the CF mouse ileum, although the ileum lacks ENaC expression. This effect was mimicked by two NHE inhibitors. Airway mucus bundles were immobile in untreated newborn CF piglets but were detached by the therapeutic drug candidate AZD5634 (patent WO, 2015140527). These results suggest that the ENaC inhibitor AZD5634 causes detachment of CF mucus in the ileum and airway via NHE inhibition and that drug design should focus on NHE instead of ENaC inhibition.

Phosphorylation of meprin β controls its cell surface abundance and subsequently diminishes ectodomain shedding

Meprin β is a zinc-dependent metalloprotease exhibiting a unique cleavage specificity with strong preference for acidic amino acids at the cleavage site. Proteomic studies revealed a diverse substrate pool of meprin β including the interleukin-6 receptor (IL-6R) and the amyloid precursor protein (APP). Dysregulation of meprin β is often associated with pathological conditions such as chronic inflammation, fibrosis, or Alzheimer's disease (AD). The extracellular regulation of meprin β including interactors, sheddases, and activators has been intensively investigated while intracellular regulation has been barely addressed in the literature. This study aimed to analyze C-terminal phosphorylation of meprin β with regard to cell surface expression and proteolytic activity. By immunoprecipitation of endogenous meprin β from the colon cancer cell line Colo320 and subsequent LC-MS analysis, we identified several phosphorylation sites in its C-terminal region. Here, T694 in the C-terminus of meprin β was the most preferred residue after phorbol 12-myristate 13-acetate (PMA) stimulation. We further demonstrated the role of protein kinase C (PKC) isoforms for meprin β phosphorylation and identified the involvement of PKC-α and PKC-β. As a result of phosphorylation, the meprin β activity at the cell surface is reduced and, consequently, the extent of substrate cleavage is diminished. Our data indicate that this decrease of the surface activity is caused by the internalization and degradation of meprin β.

Human molecular genetics and the long road to treating cystic fibrosis

The causative gene in cystic fibrosis was identified in 1989, three years before the publication of the first issue of Human Molecular Genetics. CFTR was among the first genes underlying a common inherited disorder to be cloned, and hence its subsequent utilization towards a cure for CF provides a roadmap for other monogenic diseases. Over the past 30 years the advances that built upon knowledge of the gene and the CFTR protein to develop effective therapeutics have been remarkable, and yet the setbacks have also been challenging. Technological progress in other fields has often circumvented the barriers. This review focuses on key aspects of CF diagnostics and current approaches to develop new therapies for all CFTR mutations. It also highlights the major research advances that underpinned progress towards treatments, and considers the remaining obstacles.