An intercrypt subpopulation of goblet cells is essential for colonic mucus barrier function

Research progress on the relationship between intestinal microecology and intestinal bowel disease

Intestinal microecology is the main component of human microecology. Intestinal microecology consists of intestinal microbiota, intestinal epithelial cells, and intestinal mucosal immune system. These components are interdependent and establish a complex interaction network that restricts each other. According to the impact on the human body, there are three categories of symbiotic bacteria, opportunistic pathogens, and pathogenic bacteria. The intestinal microecology participates in digestion and absorption, and material metabolism, and inhibits the growth of pathogenic microorganisms. It also acts as the body's natural immune barrier, regulates the innate immunity of the intestine, controls the mucosal barrier function, and also participates in the intestinal epithelial cells' physiological activities such as hyperplasia or apoptosis. When the steady‐state balance of the intestinal microecology is disturbed, the existing core intestinal microbiota network changes and leads to obesity, diabetes, and many other diseases, especially irritable bowel syndrome, inflammatory bowel disease (IBD), and colorectal malignancy. Intestinal diseases, including tumors, are particularly closely related to intestinal microecology. This article systematically discusses the research progress on the relationship between IBD and intestinal microecology from the pathogenesis, treatment methods of IBD, and the changes in intestinal microbiota.

Activation of 5-Hydroxytryptamine 4 Receptor Improves Colonic Barrier Function by Triggering Mucin 2 Production in a Mouse Model of Type 1 Diabetes

Diabetes leads to intestinal barrier dysfunction. 5-Hydroxytryptamine 4 receptor (5-HT₄R) is distributed in the colonic mucosa, but little is known about the role of its activation in diabetes-evoked colonic barrier dysfunction. This study investigates whether activation of 5-HT₄Rs on goblet cells (GCs) protects the colon from commensal bacterial translocation in diabetic mice. Expression of 5-HT₄R detected inside the colonic epithelium by RNAscope in situ hybridization was further observed within the mucin 2 (MUC2)-immunoreactive GCs. In diabetic mice, neither 5-HT₄R transcription nor protein levels were altered compared with those in nondiabetic mice. Bacterial translocation was characterized by 16S rRNA RNAscope in situ hybridization and manifested in both crypts and lamina propria of the colon in diabetic mice. Mucin production and MUC2 expression were significantly decreased in diabetic mice. Furthermore, the loss of mitochondrial cristae of GCs and the down-regulation of mitofilin, the core protein maintaining mitochondrial homeostasis, were observed in diabetic mice. Long-term treatment with 5-HT₄R agonist in diabetic mice not only prevented bacterial penetration of the whole colonic mucosa but also promoted mucin production and MUC2 expression. Markedly, 5-HT₄R agonist also restored the mitochondrial cristae of GCs and up-regulated mitofilin. However, co-administration of 5-HT₄R antagonist abolished the effects of 5-HT₄R agonist on diabetic mice. These findings indicate that 5-HT₄R in colonic mucosa is an effective target for the treatment of diabetes-induced colonic mucous barrier dysfunction.

Multi-Omics Strategies for Investigating the Microbiome in Toxicology Research

Microbial communities on and within the host contact environmental pollutants, toxic compounds, and other xenobiotic compounds. These communities of bacteria, fungi, viruses, and archaea possess diverse metabolic potential to catabolize compounds and produce new metabolites. Microbes alter chemical disposition thus making the microbiome a natural subject of interest for toxicology. Sequencing and metabolomics technologies permit the study of microbiomes altered by acute or long-term exposure to xenobiotics. These investigations have already contributed to and are helping to re-interpret traditional understandings of toxicology. The purpose of this review is to provide a survey of the current methods used to characterize microbes within the context of toxicology. This will include discussion of commonly used techniques for conducting omic-based experiments, their respective strengths and deficiencies, and how forward-looking techniques may address present shortcomings. Finally, a perspective will be provided regarding common assumptions that currently impede microbiome studies from producing causal explanations of toxicologic mechanisms.

Konjac oligosaccharides attenuate DSS-induced ulcerative colitis in mice: mechanistic insights

This study aims to explore the protective effect of konjac oligosaccharides (KOS) on inflammatory bowel disease in colitis mice. During the experimental period, mice were administered 200 mg kg^-1 or 600 mg kg^-1 KOS, 200 mg kg^-1 sulfasalazine and a combination of KOS and sulfasalazine for 14 days. The mice were then treated with drinking water containing 2.5% DSS for 9 days, while the intervention of KOS and sulfasalazine continued. At the end of the experiment, the phenotype, pathological lesion of the colon, parameters of cytokines and gut microbiota were evaluated. The results showed that mice treated with KOS exhibited alleviated pathological lesion of the colon tissue and significantly increased expression of tight junction proteins (p < 0.05). The level of inflammatory cytokines in the colon tissue of the colitis mice tended to be normal. Moreover, the analysis of the gut microbiota revealed that the structures and composition of the intestinal microorganisms were also regulated by KOS treatment. The possible internal mechanism is that KOS down-regulates the abundance of pro-inflammatory bacteria (Proteobacteria, Campilobacterota and Clostridiaceae) and up-regulates the abundance of anti-inflammatory bacteria (Bifidobacteriaceae and Akkermansiaceae). These findings provide new insights into dietary management for patients with inflammatory bowel disease.

MTG16 (CBFA2T3) regulates colonic epithelial differentiation, colitis, and tumorigenesis by repressing E protein transcription factors

Aberrant epithelial differentiation and regeneration contribute to colon pathologies including inflammatory bowel disease (IBD) and colitis-associated cancer (CAC). MTG16 (CBFA2T3) is a transcriptional corepressor expressed in the colonic epithelium. MTG16 deficiency in mice exacerbates colitis and increases tumor burden in CAC, though the underlying mechanisms remain unclear. Here, we identified MTG16 as a central mediator of epithelial differentiation, promoting goblet and restraining enteroendocrine cell development in homeostasis and enabling regeneration following dextran sulfate sodium (DSS)-induced colitis. Transcriptomic analyses implicated increased E box-binding transcription factor (E protein) activity in MTG16-deficient colon crypts. Using a novel mouse model with a point mutation that attenuates MTG16:E protein interactions (Mtg16P209T), we established that MTG16 exerts control over colonic epithelial differentiation and regeneration by repressing E protein-mediated transcription. Mimicking murine colitis, MTG16 expression was increased in biopsies from patients with active IBD compared to unaffected controls. Finally, uncoupling MTG16:E protein interactions partially phenocopied the enhanced tumorigenicity of Mtg16-/- colon in the azoxymethane(AOM)/DSS-induced model of CAC, indicating that MTG16 protects from tumorigenesis through additional mechanisms. Collectively, our results demonstrate that MTG16, via its repression of E protein targets, is a key regulator of cell fate decisions during colon homeostasis, colitis, and cancer.

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.

EPA and DHA differentially coordinate the crosstalk between host and gut microbiota and block DSS-induced colitis in mice by a reinforced colonic mucus barrier

Background: Ulcerative colitis (UC) is a chronic inflammatory disorder of the colon with a continuously remitting and relapsing course. Its etiology is closely related to abnormal interactions between host and gut microbiota. The mucus barrier lining the gastrointestinal tract is necessary to coordinate host and gut microbiota interaction by nourishing and modulating the microbiota. Differential effects of the anti-inflammatory fatty acids docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) on UC progression in mice were firstly addressed by our previous work; here, the mechanism for their respective effects were further uncovered from host-microbiome crosstalk based on mucus barrier modulation to pave the way for UC therapy. Methods: Assessment of the disease activity index and histopathology score was conducted in mice with dextran sodium sulfate (DSS)-induced colitis pre-treated with different doses of EPA and DHA. Mucin generation, glycosylation and secretion were evaluated by a combination of electron microscopy, specific mucous staining, and qPCR. Western blotting was used to analyze the underlying molecular events. Fecal short chain fatty acids were detected using gas chromatography, and the gut microbial composition was analyzed using 16S rRNA sequencing. Results: Compared with DHA, the more potent inhibitory effect of high dose EPA on DSS-induced colitis was reconfirmed, which was underlain by a reinforced mucus layer as indicated by increased mucin granule release, mucus layer stratification and markedly upregulated expression of the key modulators involved in goblet cell differentiation. In turn a remarkably enhanced mucus barrier in the EPA group functioned to modulate the gut microbiome, as demonstrated by the enriched abundance of the phylum Bacteroidetes and mucin-degrading bacterium Akkermansia muciniphila producing acetic and propionic acids. Conclusions: EPA and DHA differentially coordinate the interaction between the host and the gut microbiota and relieve mucus barrier disruption in DSS-induced colitis. EPA may develop into a promising adjunctive therapy for UC.

Effect of Different Pig Fecal Microbiota Transplantation on Mice Intestinal Function and Microbiota Changes During Cold Exposure

Cold stress influences intestinal processes, causing physiological and immunological responses in animals. Intestinal microbiota participates in maintaining the stability of the intestinal environment. However, phenotypic characteristics and the effects of porcine microbiota changes under cold conditions remain poorly understood. Here, the fecal microbiota of cold tolerant breed (Mashen) and cold sensitive breed (Duroc-Landrace-Yorkshire) was transferred to germ-free mice, respectively. After a cold exposure (4°C) for 21 days, intestinal function and microbe changes of mice were explored. The results showed that Mashen pigs microbiota transplantation made the body temperature of the mice stable, in which the fat weight and expression of uncoupling protein 1 (UCP1), carnitine palmitoyltransferase 1B (Cpt1b), and Peroxisome proliferator-activated receptor-gamma coactivator (PGC-1α) were significantly higher (P &lt; 0.05) than those of the control group. The results of intestinal structure and expression of serum inflammatory factors showed that fecal microbiota transplantation (FMT) mice have more intact intestinal structure and high expression of proinflammatory factor such as interleukin-4 (IL-4). The study of mice fecal microbiome characterized via 16S rRNA sequencing found that pig microbiota transplantation changed the abundance of Firmicutes. In addition, it identified discriminative features of Firmicutes in the microbiota between two breeds of pig, in which Clostridiaceae were enriched in the microbiota community of Mashen pig and Coriobacteriales were significantly (P &lt; 0.05) enriched in the Duroc-Landrace-Yorkshire pig microbiota transplantation group based on linear discriminant analysis effect size (LEfSe) analysis. Finally, we found that the content of propionic acid and butyric acid in rectal contents significantly changed and the abundances of Clostridium and Lachnospira showed significant correlations with changes in short-chain fatty acids. The results suggest that pig fecal microbiota transplantation can alleviate the changes in physiological and biochemical indicators in mice caused by cold exposure. Mice have gut microbes altered and improved gut barrier function via fecal microbiota transplantation in pigs.

The barrier and beyond: Roles of intestinal mucus and mucin-type O-glycosylation in resistance and tolerance defense strategies guiding host-microbe symbiosis

Over the past two decades, our appreciation of the gut mucus has moved from a static lubricant to a dynamic and essential component of the gut ecosystem that not only mediates the interface between host tissues and vast microbiota, but regulates how this ecosystem functions to promote mutualistic symbioses and protect from microbe-driven diseases. By delving into the complex chemistry and biology of the mucus, combined with innovative in vivo and ex vivo approaches, recent studies have revealed novel insights into the formation and function of the mucus system, the O-glycans that make up this system, and how they mediate two major host-defense strategies - resistance and tolerance - to reduce damage caused by indigenous microbes and opportunistic pathogens. This current review summarizes these findings by highlighting the emerging roles of mucus and mucin-type O-glycans in influencing host and microbial physiology with an emphasis on host defense strategies against bacteria in the gastrointestinal tract.

The Protective Effects of Sour Orange (Citrus aurantium L.) Polymethoxyflavones on Mice Irradiation-Induced Intestinal Injury

Sour orange (Citrus aurantium L.) is one of the biological sources of polymethoxyflavones (PMFs), which are often used to deal with gastrointestinal diseases. The intestine is highly sensitive to irradiation damage. However, limited certain cures have been released for irradiation-induced gastrointestinal injury, and the potentials of sour orange PMFs as radio-resistance agents have not been fully discussed yet. The present study aims to (1) investigate the PMF components in 12 sour orange cultivars, (2) determine the protective effects of PMFs on irradiation-induced intestinal injury by treating mice that received 12 Gy abdominal irradiation with different doses of PMFs and observing the changes in organ indexes and pathological sections and (3) test cytotoxicity of PMFs by CCK-8 method. The results showed that sour orange PMFs appeared to have high intraspecies similarity. Besides, PMFs protected mice from irradiation-induced injury by alleviating body weight loss, reliving organ index changing and maintaining the intestinal structure. Finally, IC₅₀ concentrations to cell line CCD 841 CoN of PMFs and nobiletin were calculated as 42.23 μg/mL and 51.58 μg/mL, respectively. Our study uncovered PMF contents in 12 sour orange materials and determined the protective effects on irradiation-induced intestinal injuries, providing guidance for the utilization of sour orange resources.

Intestinal Tuft-2 cells exert antimicrobial immunity via sensing bacterial metabolite N-undecanoylglycine

Tuft cells are a type of intestinal epithelial cells that exist in epithelial barriers and play a critical role in immunity against parasite infection. It remains insufficiently clear whether Tuft cells participate in bacterial eradication. Here, we identified Sh2d6 as a signature marker for CD45⁺ Tuft-2 cells. Depletion of Tuft-2 cells resulted in susceptibility to bacterial infection. Tuft-2 cells quickly expanded in response to bacterial infection and sensed the bacterial metabolite N-undecanoylglycine through vomeronasal receptor Vmn2r26. Mechanistically, Vmn2r26 engaged with N-undecanoylglycine activated G-protein-coupled receptor-phospholipase C gamma2 (GPCR-PLCγ2)-Ca²⁺ signaling axis, which initiated prostaglandin D2 (PGD2) production. PGD2 enhanced the mucus secretion of goblet cells and induced antibacterial immunity. Moreover, Vmn2r26 signaling also promoted SpiB transcription factor expression, which is responsible for Tuft-2 cell development and expansion in response to bacterial challenge. Our findings reveal an additional function of Tuft-2 cells in immunity against bacterial infection through Vmn2r26-mediated recognition of bacterial metabolites.

Bacillus velezensis A2 Inhibited the Cecal Inflammation Induced by Zearalenone by Regulating Intestinal Flora and Short-Chain Fatty Acids

Zearalenone (ZEA) as an estrogen-like mycotoxin can cause the inflammatory injury of the cecum. How to reduce the harm that ZEA causes to humans and animals is a current concern for researchers. In this study, we aimed to ascertain whether Bacillus velezensis A2 (A2) could alleviate injury caused by ZEA by regulating the intestinal flora and the content of short chain fatty acids in the cecum among mice. Our results showed that Bacillus velezensis A2 improved the fold height, myometrial thickness, and crypt depth of the cecum induced by ZEA. Enzyme-linked immunosorbent assay and Western blotting results showed that A2 could decrease the ZEA-induced increase in expression levels of IL-2, IL-6, IFN-γ, TNF-α, and FC. Studies also showed that A2 increased the content of SCFA in the cecum which was decreased by ZEA. The microbial communities in the cecum were changed when given ZEA or A2. A2 was found to greatly reduce the ZEN-induced increase in the relative abundance of p_Actinobacteria, p_Protebacteria, o_Coriobacteriales, g_Anaerotruncus, g_Pseudoflavonifractor, g_Lachnoclostridium, g_Enterorhabdus, and f_Oscillospiraceae, and increase the ZEN-induced decrease in the relative abundance of f_Coriobacteriales. Results indicated that Bacillus velezensis A2 can largely ameliorate the intestinal inflammatory injury induced by ZEA in mice by regulating the microflora and short chain fatty acids content.

The LRRC family of BK channel regulatory subunits: potential roles in health and disease

Large conductance K⁺ channels, termed BK channels, regulate a variety of cellular and physiological functions. Although universally activated by changes in voltage or [Ca²⁺ ]_i , the threshold for BK channel activation varies among loci of expression, often arising from cell-specific regulatory subunits including a family of leucine rich repeat-containing (LRRC) γ subunits (LRRC26, LRRC52, LRRC55 and LRRC38). The 'founding' member of this family, LRRC26, was originally identified as a tumour suppressor in various cancers. An LRRC26 knockout (KO) mouse model recently revealed that LRRC26 is also highly expressed in secretory epithelial cells and partners with BK channels in the salivary gland and colonic goblet cells to promote sustained K⁺ fluxes likely essential for normal secretory function. To accomplish this, LRRC26 negatively shifts the range of BK channel activation such that channels contribute to K⁺ flux near typical epithelial cell resting conditions. In colon, the absence of LRRC26 increases vulnerability to colitis. LRRC26-containing BK channels are also likely important regulators of epithelial function in other loci, including airways, female reproductive tract and mammary gland. Based on an LRRC52 KO mouse model, LRRC52 regulation of large conductance K⁺ channels plays a role both in sperm function and in cochlear inner hair cells. Although our understanding of LRRC-containing BK channels remains rudimentary, KO mouse models may help define other organs in which LRRC-containing channels support normal function. A key topic for future work concerns identification of endogenous mechanisms, whether post-translational or via gene regulation, that may impact LRRC-dependent pathologies.

A Proximal-to-Distal Survey of Healthy Adult Human Small Intestine and Colon Epithelium by Single-Cell Transcriptomics

BACKGROUND & AIMS

Single-cell transcriptomics offer unprecedented resolution of tissue function at the cellular level, yet studies analyzing healthy adult human small intestine and colon are sparse. Here, we present single-cell transcriptomics covering the duodenum, jejunum, ileum, and ascending, transverse, and descending colon from 3 human beings.

METHODS

A total of 12,590 single epithelial cells from 3 independently processed organ donors were evaluated for organ-specific lineage biomarkers, differentially regulated genes, receptors, and drug targets. Analyses focused on intrinsic cell properties and their capacity for response to extrinsic signals along the gut axis across different human beings.

RESULTS

Cells were assigned to 25 epithelial lineage clusters. Multiple accepted intestinal stem cell markers do not specifically mark all human intestinal stem cells. Lysozyme expression is not unique to human Paneth cells, and Paneth cells lack expression of expected niche factors. Bestrophin 4 (BEST4)⁺ cells express Neuropeptide Y (NPY) and show maturational differences between the small intestine and colon. Tuft cells possess a broad ability to interact with the innate and adaptive immune systems through previously unreported receptors. Some classes of mucins, hormones, cell junctions, and nutrient absorption genes show unappreciated regional expression differences across lineages. The differential expression of receptors and drug targets across lineages show biological variation and the potential for variegated responses.

CONCLUSIONS

Our study identifies novel lineage marker genes, covers regional differences, shows important differences between mouse and human gut epithelium, and reveals insight into how the epithelium responds to the environment and drugs. This comprehensive cell atlas of the healthy adult human intestinal epithelium resolves likely functional differences across anatomic regions along the gastrointestinal tract and advances our understanding of human intestinal physiology.

Profiling of the Bacterial Microbiota along the Murine Alimentary Tract

Here, the spatial distribution of the bacterial flora along the murine alimentary tract was evaluated using high throughput sequencing in wild-type and Tff3-deficient (Tff3^KO) animals. Loss of Tff3 was linked to increased dextran sodium sulfate-induced colitis. This systematic study shows the results of 13 different regions from the esophagus to the rectum. The number of bacterial species (richness) increased from the esophagus to the rectum, from 50 to 200, respectively. Additionally, the bacterial community structure changed continuously; the highest changes were between the upper/middle and lower gastrointestinal compartments when comparing adjacent regions. Lactobacillus was the major colonizer in the upper/middle gastrointestinal tract, especially in the esophagus and stomach. From the caecum, a drastic diminution of Lactobacillus occurred, while members of Lachnospiraceae significantly increased. A significant change occurred in the bacterial community between the ascending and the transverse colon with Bacteroidetes being the major colonizers with relative constant abundance until the rectum. Interestingly, wild-type and Tff3^KO animals did not show significant differences in their bacterial communities, suggesting that Tff3 is not involved in alterations of intraluminal or adhesive microbiota but is obviously important for mucosal protection, e.g., of the sensitive stem cells in the colonic crypts probably by a mucus plume.

Hyaluronan-induced alterations of the gut microbiome protects mice against Citrobacter rodentium infection and intestinal inflammation

Hyaluronan is a glycosaminoglycan polymer that has been shown to play an important role in homeostasis of the gastrointestinal tract. However, its mechanistic significance in gastrointestinal epithelial barrier elements remain unexplored. Here, our results revealed that hyaluronan treatment resulted in significant changes in the gut microbiota in mice. To demonstrate the functional consequences of hyaluronan-treatment and hyaluronan-induced microbiota alterations, Citrobacter rodentium- and DSS-induced colitis models and microbiota transplantation approaches were utilized. We showed that hyaluronan alleviated intestinal inflammation in both pathogen and chemically induced intestinal mucosal damage. The protection in bacterial colitis was associated with enhanced C. rodentium clearance and alleviation of pathogen-induced gut dysbiosis. Microbiota transplantation experiments showed that the hyaluronan-altered microbiota is sufficient to confer protection against C. rodentium infection. Colonization with Akkermansia muciniphila, a commensal bacterium that is greatly enriched by hyaluronan treatment, alleviated C. rodentium-induced bacterial colitis in mice. Additionally, Akkermansia-induced protection was found to be associated with the induction of goblet cells and the production of mucins and epithelial antimicrobial peptides. Collectively, these results provide novel insights into the regulatory role of hyaluronan in modulating the gut microbiota and immunity in enteric infection and inflammation, with therapeutic potential for gut microbiome-targeted immunotherapy.

Transglutaminase 3 crosslinks the secreted gel-forming mucus component Mucin-2 and stabilizes the colonic mucus layer

The colonic mucus layer is organized as a two-layered system providing a physical barrier against pathogens and simultaneously harboring the commensal flora. The factors contributing to the organization of this gel network are not well understood. In this study, the impact of transglutaminase activity on this architecture was analyzed. Here, we show that transglutaminase TGM3 is the major transglutaminase-isoform expressed and synthesized in the colon. Furthermore, intrinsic extracellular transglutaminase activity in the secreted mucus was demonstrated in vitro and ex vivo. Absence of this acyl-transferase activity resulted in faster degradation of the major mucus component the MUC2 mucin and changed the biochemical properties of mucus. Finally, TGM3-deficient mice showed an early increased susceptibility to Dextran Sodium Sulfate-induced colitis. Here, we report that natural isopeptide cross-linking by TGM3 is important for mucus homeostasis and protection of the colon from inflammation, reducing the risk of colitis.

The colonic mucus layer is an organized system providing a physical barrier against pathogens and simultaneously harbouring the commensal flora. Here the authors report that transglutaminase 3 activity contributes to homeostasis of the colonic mucus layer and the lack of this enzymatic activity leads to increased susceptibility against DSS-induced colitis in mice.

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.

Lactobacillus acidophilus LA85 ameliorates cyclophosphamide-induced immunosuppression by modulating Notch and TLR4/NF-κB signal pathways and remodeling the gut microbiota

With the prevalence of coronavirus disease 2019 (COVID-19), we found that probiotics may be effective in organism immune recovery and remodeling of gut microbiota in their patients and recovered individuals....

Epithelial GPR35 protects from Citrobacter rodentium infection by preserving goblet cells and mucosal barrier integrity

Goblet cells secrete mucin to create a protective mucus layer against invasive bacterial infection and are therefore essential for maintaining intestinal health. However, the molecular pathways that regulate goblet cell function remain largely unknown. Although GPR35 is highly expressed in colonic epithelial cells, its importance in promoting the epithelial barrier is unclear. In this study, we show that epithelial Gpr35 plays a critical role in goblet cell function. In mice, cell-type-specific deletion of Gpr35 in epithelial cells but not in macrophages results in goblet cell depletion and dysbiosis, rendering these animals more susceptible to Citrobacter rodentium infection. Mechanistically, scRNA-seq analysis indicates that signaling of epithelial Gpr35 is essential to maintain normal pyroptosis levels in goblet cells. Our work shows that the epithelial presence of Gpr35 is a critical element for the function of goblet cell-mediated symbiosis between host and microbiota.

Finding the sweet spot: glycosylation mediated regulation of intestinal inflammation

Glycans are essential cellular components that facilitate a range of critical functions important for tissue development and mucosal homeostasis. Furthermore, specific alterations in glycosylation represent important diagnostic hallmarks of cancer that contribute to tumor cell dissociation, invasion, and metastasis. However, much less is known about how glycosylation contributes to the pathobiology of inflammatory mucosal diseases. Here we will review how epithelial and immune cell glycosylation regulates gut homeostasis and how inflammation-driven changes in glycosylation contribute to intestinal pathobiology.

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.

MUC2 and related bacterial factors: Therapeutic targets for ulcerative colitis

The mucin2 (MUC2) mucus barrier acts as the first barrier that prevents direct contact between intestinal bacteria and colonic epithelial cells. Bacterial factors related to the MUC2 mucus barrier play important roles in the response to changes in dietary patterns, MUC2 mucus barrier dysfunction, contact stimulation with colonic epithelial cells, and mucosal and submucosal inflammation during the occurrence and development of ulcerative colitis (UC). In this review, these underlying mechanisms are summarized and updated, and related interventions for treating UC, such as dietary adjustment, exogenous repair of the mucus barrier, microbiota transplantation and targeted elimination of pathogenic bacteria, are suggested. Such interventions are likely to induce and maintain a long and stable remission period and reduce or even avoid the recurrence of UC. A better mechanistic understanding of the MUC2 mucus barrier and its related bacterial factors may help researchers and clinicians to develop novel approaches for treating UC.

Functional and structural characterization of a GH3 β-N-acetylhexosaminidase from Akkermansia muciniphila involved in mucin degradation

Akkermansia muciniphila is a probiotic that colonizes the outer layer of intestinal mucus and is negatively associated with metabolic disorders. Amuc_2109 protein, a β-N-acetylhexosaminidase from A. muciniphila, may be involved in the degradation of mucins and is associated with intestinal health. Here, we reported the crystal structure of Amuc_2109, which belongs to the GH family 3 enzymes and fell into the canonical (α/β)₈ TIM barrel structure with GlcNAc bound to the active center. Biochemical assay characterization of Amuc_2109 revealed that Amuc_2109 is a GlcNAc-specific glycosidase active over a wide temperature and pH range, reflecting the survival advantage of Amuc_2109 in the intestinal environment. Our structural and biochemical results will contribute to the understanding of the catalytic mechanism of the GH3 β-N-acetylhexosaminidase and help to gain insight into the molecular mechanism of complex carbohydrate utilization and restoration of the intestinal barrier in A. muciniphila.

Role of microRNAs in the Pathophysiology of Ulcerative Colitis

Ulcerative colitis (UC) is an intractable disorder characterized by a chronic inflammation of the colon. Studies have identified UC as a multifactorial disorder affected by both genetic and environmental factors; however, the precise mechanism remains unclear. Recent advances in the field of microRNA (miRNA) research have identified an association between this small non-coding RNA in the pathophysiology of UC and altered miRNA expression profiles in patients with UC. Nevertheless, the roles of individual miRNAs are uncertain due to heterogeneity in both research samples and clinical backgrounds. In this review, we focus on miRNA expression in colonic mucosa where inflammation occurs in UC and discuss the potential roles of individual miRNAs in disease development, outlining the pathophysiology of UC.

Intestinal mucus barrier: a missing piece of the puzzle in food allergy

The prevalence of food allergies has reached epidemic levels but the cause remains largely unknown. We discuss the clinical relevance of the gut mucosal barrier as a site for allergic sensitization to food. In this context, we focus on an important but overlooked part of the mucosal barrier in pathogenesis, the glycoprotein-rich mucus layer, and call attention to both beneficial and detrimental aspects of mucus-gut microbiome interactions. Studying the intricate links between the mucus barrier, the associated bacteria, and the mucosal immune system may advance our understanding of the mechanisms and inform prevention and treatment strategies in food allergy.

Type 2 immunity in intestinal homeostasis and inflammatory bowel disease

Type 2 immune responses commonly emerge during allergic reactions or infections with helminth parasites. Most of the cytokines associated with type 2 immune responses are IL-4, IL-5, and IL13, which are mainly produced by T helper 2 cells (T_H2), eosinophils, basophils, mast cells, and group 2 innate lymphoid cells (ILC2s). Over the course of evolution, humans have developed type 2 immune responses to fight infections and to protect tissues from the potential collateral damage caused by inflammation. For example, worm parasites induce potent type 2 immune responses, which are needed to simultaneously clear the pathogen and to promote tissue repair following injury. Due to the strong type 2 immune responses induced by helminths, which can promote tissue repair in the damaged epithelium, their use has been suggested as a possible treatment for inflammatory bowel disease (IBD); however, the role of type 2 immune responses in the initiation and progression of IBD is not fully understood. In this review, we discuss the molecular and cellular mechanisms that regulate type 2 immune responses during intestinal homeostasis, and we briefly discuss the scarce evidence linking type 2 immune responses with the aetiology of IBD.

Microbial influences on gut development and gut-brain communication

The developmental programs that build and sustain animal forms also encode the capacity to sense and adapt to the microbial world within which they evolved. This is abundantly apparent in the development of the digestive tract, which typically harbors the densest microbial communities of the body. Here, we review studies in human, mouse, zebrafish and Drosophila that are revealing how the microbiota impacts the development of the gut and its communication with the nervous system, highlighting important implications for human and animal health.

Host-Viral Interactions in the Pathogenesis of Ulcerative Colitis

Ulcerative colitis is characterized by relapsing and remitting colonic mucosal inflammation. During the early stages of viral infection, innate immune defenses are activated, leading to the rapid release of cytokines and the subsequent initiation of downstream responses including inflammation. Previously, intestinal viruses were thought to be either detrimental or neutral to the host. However, persisting viruses may have a role as resident commensals and confer protective immunity during inflammation. On the other hand, the dysregulation of gut mucosal immune responses to viruses can trigger excessive, pathogenic inflammation. The purpose of this review is to discuss virus-induced innate immune responses that are at play in ulcerative colitis.

Chronic Inflammation in Ulcerative Colitis Causes Long-Term Changes in Goblet Cell Function

BACKGROUND & AIMS

One of the features of ulcerative colitis (UC) is a defect in the protective mucus layer. This has been attributed to a reduced number of goblet cells (GCs). However, it is not known whether abnormal GC mucus secretion also contributes to the reduced mucus layer. Our aims were to investigate whether GC secretion was abnormal in UC and exists as a long-term effect of chronic inflammation.

METHODS

Colonoids were established from intestinal stem cells of healthy subjects (HS) and patients with UC. Colonoids were maintained as undifferentiated (UD) or induced to differentiate (DF) and studied as three-dimensional or monolayers on Transwell filters. Total RNA was extracted for quantitative real-time polymerase chain reaction analysis. Carbachol and prostaglandin E2 mediated mucin stimulation was examined by MUC2 IF/confocal microscopy and transmission electron microscopy.

RESULTS

Colonoids from UC patients can be propagated over many passages; however, they exhibit a reduced rate of growth and transepithelial electrical resistance compared with HS. Differentiated UC colonoid monolayers form a thin and non-continuous mucus layer. UC colonoids have increased expression of secretory lineage markers ATOH1 and SPDEF, along with MUC2 positive GCs, but failed to secrete mucin in response to the cholinergic agonist carbachol and prostaglandin E2, which caused increased secretion in HS. Exposure to tumor necrosis factor α (5 days) reduced the number of GCs, with a greater percentage decrease in UC colonoids compared with HS.

CONCLUSIONS

Chronic inflammation in UC causes long-term changes in GCs, leading to abnormal mucus secretion. This continued defect in GC mucus secretion may contribute to the recurrence in UC.

Pain in Inflammatory Bowel Disease: Optogenetic Strategies for Study of Neural-Epithelial Signaling

Abdominal pain is common in patients with active inflammation of the colon but can persist even in its absence, suggesting other mechanisms of pain signaling. Recent findings suggest colon epithelial cells are direct regulators of pain-sensing neurons. Optogenetic activation of epithelial cells evoked nerve firing and pain-like behaviors. Inhibition of epithelial cells caused the opposite effect, reducing responses to colon distension and inflammatory hypersensitivity. Thus, epithelial cells alone can regulate the activation of pain circuits. Future goals are to define the anatomical and cellular mechanisms that underlie epithelial-neural pain signaling and how it is altered in response to colon inflammation.

Evolution and function of the epithelial cell-specific ER stress sensor IRE1β

Barrier epithelial cells lining the mucosal surfaces of the gastrointestinal and respiratory tracts interface directly with the environment. As such, these tissues are continuously challenged to maintain a healthy equilibrium between immunity and tolerance against environmental toxins, food components, and microbes. An extracellular mucus barrier, produced and secreted by the underlying epithelium plays a central role in this host defense response. Several dedicated molecules with a unique tissue-specific expression in mucosal epithelia govern mucosal homeostasis. Here, we review the biology of Inositol-requiring enzyme 1β (IRE1β), an ER-resident endonuclease and paralogue of the most evolutionarily conserved ER stress sensor IRE1α. IRE1β arose through gene duplication in early vertebrates and adopted functions unique from IRE1α which appear to underlie the basic development and physiology of mucosal tissues.