Tumor necrosis factor-α induces a biphasic change in claudin-2 expression in tubular epithelial cells: role in barrier functions

Deciphering the complex interplay of obesity, epithelial barrier dysfunction, and tight junction remodeling: Unraveling potential therapeutic avenues

Obesity stands as a formidable global health challenge, predisposing individuals to a plethora of chronic illnesses such as cardiovascular disease, diabetes, and cancer. A confluence of genetic polymorphisms, suboptimal dietary choices, and sedentary lifestyles significantly contribute to the elevated incidence of obesity. This multifaceted health issue profoundly disrupts homeostatic equilibrium at both organismal and cellular levels, with marked alterations in gut permeability as a salient consequence. The intricate mechanisms underlying these alterations have yet to be fully elucidated. Still, evidence suggests that heightened inflammatory cytokine levels and the remodeling of tight junction (TJ) proteins, particularly claudins, play a pivotal role in the manifestation of epithelial barrier dysfunction in obesity. Strategic targeting of proteins implicated in these pathways and metabolites such as short-chain fatty acids presents a promising intervention for restoring barrier functionality among individuals with obesity. Nonetheless, recognizing the heterogeneity among affected individuals is paramount; personalized medical interventions or dietary regimens tailored to specific genetic backgrounds and allergy profiles may prove indispensable. This comprehensive review delves into the nexus of obesity, tight junction remodeling, and barrier dysfunction, offering a critical appraisal of potential therapeutic interventions.

Claudin-2 Mediates the Proximal Tubular Epithelial Cell-Fibroblast Crosstalk via Paracrine CTGF

Purpose

Proximal tubular epithelial cell (PTEC) is vulnerable to injury in diabetic kidney disease (DKD) due to high energy expenditure. The injured PTECs-derived profibrotic factors are thought to be driving forces in tubulointerstitial fibrosis (TIF) as they activate surrounding fibroblasts. However, the mechanisms remain unclear.

Methods

The diabetes with uninephrectomy (DKD) rats were used to evaluated renal histological changes and the expression of Claudin-2 by immunofluorescence staining. Then, Claudin-2 expression in PTECs were modulated and subsequently determined the proliferation and activation of fibroblasts by building a transwell co-culture system in normal glucose (NG)or high glucose (HG) condition.

Results

Decreased expression of Claudin-2 in PTECs accompanied by tight junction disruption and increased interstitial fibrosis, were detected in DKD rats. In vitro, downregulated Claudin-2 in PTECs promoted proliferation and activation of fibroblasts, which coincided with elevated expression of profibrotic connective tissue growth factor (CTGF) in PTECs. Silenced CTGF inhibited the profibrotic of PTECs via Claudin-2 inhibition. Fibroblasts co-cultured with PTECs transitioned more to myofibroblasts and generated extracellular matrix (ECM) significantly in response to high glucose (HG) stimulation whereas overexpression of Claudin-2 in PTECs reversed the above results. Upregulating CTGF disrupted the beneficial anti-fibrosis effects obtained by overexpression of Claudin-2 in HG condition.

Conclusion

Our study suggested that Claudin-2 in PTECs, a key mediator of paracellular cation and water transport, promotes the activation and proliferation of surrounding fibroblasts significantly via CTGF in a paracrine manner.

Fructo-oligosaccharide-mediated alteration in claudin expression in small intestinal absorptive Caco-2 cells is positively associated with the induction of inflammatory genes and the glucan receptor gene CLEC7A

OBJECTIVES

Indigestible carbohydrates may strengthen tight junctions (TJs) independently of intestinal bacteria. This study investigated whether indigestible carbohydrates (i.e., fructo-oligosaccharides [FOS]) promote TJs directly to intestinal absorptive Caco-2 cells and examined the association between the expression of genes constructing TJs and other genes using mRNA microarray analysis.

METHODS

Caco-2 cells at 1.0 × 105/mL were seeded in a type I collagen plate and cultured in high-glucose Dulbecco's modified Eagle medium (DMEM) supplemented with 10% fetal calf serum (FCS); the cells reached confluence at 7 d after seeding. Ten days after the cells reached confluency, they were cultured for 24 h in 10% FCS-containing DMEM medium supplemented with 0%, 5%, or 10% FOS. We performed mRNA microarray to identify the genes whose expression was altered by FOS. Subsequently, quantitative reverse transcription polymerase chain reaction was performed for these altered genes, including CLEC7A encoding the glucan receptor, and for the claudin (CLDN) family genes. The expression of CLDN2, CLDN4, and CLEC7A proteins was assessed using western blot analysis.

RESULTS

FOS decreased the mRNA and protein expression of CLDN2, which weakens TJs, and increased those of CLDN4, which strengthens TJs, in Caco-2 cells. FOS treatment (10%) reduced the mRNA expression of antioxidative genes and induced the expression of immune response-related genes, including CLEC7A, CCL2, and ITGA2. Furthermore, the expression of CLEC7A protein was enhanced by FOS.

CONCLUSIONS

Induction of TJ-strengthening CLDN4 and reduction of TJ-weakening CLDN2 by FOS treatment in small intestinal absorptive Caco-2 cells is positively associated with the induction of inflammatory genes, including CLEC7A.

The role of claudins in homeostasis

Sequential expression of claudins, a family of tight junction proteins, along the nephron mirrors the sequential expression of ion channels and transporters. Only by the interplay of transcellular and paracellular transport can the kidney efficiently maintain electrolyte and water homeostasis in an organism. Although channel and transporter defects have long been known to perturb homeostasis, the contribution of individual tight junction proteins has been less clear. Over the past two decades, the regulation and dysregulation of claudins have been intensively studied in the gastrointestinal tract. Claudin expression patterns have, for instance, been found to be affected in infection and inflammation, or in cancer. In the kidney, a deeper understanding of the causes as well as the effects of claudin expression alterations is only just emerging. Little is known about hormonal control of the paracellular pathway along the nephron, effects of cytokines on renal claudin expression or relevance of changes in paracellular permeability to the outcome in any of the major kidney diseases. By summarizing current findings on the role of specific claudins in maintaining electrolyte and water homeostasis, this Review aims to stimulate investigations on claudins as prognostic markers or as druggable targets in kidney disease.

Claudin-2 in hyperproliferative migrating keratinocytes and migration inhibition via siRNA knockdown

Claudin-2 is a tight junction protein found in various tissues including the epidermis of the skin. Intracellular signalling via claudin-2 may have an effect on cell proliferation and migration. While the role of claudin-2 in the epidermis has not been established, here we show an increase in claudin-2 expression in hyperproliferative archival skin samples. To further examine the role of claudin-2 in cell migration we examined its expression in cultured keratinocytes and found it was increased in wound margins in an in vitro scratch test assay. We then used a claudin-2 knockdown assay using small interfering ribonucleic acid (siRNA) with a 77% transfection efficiency and decrease in claudin-2 protein via Western blot analysis to examine cell migration, which was inhibited following claudin-2 knockdown over a 5-day period. Cells transfected with claudin-2 siRNA also showed a decreased size compared to controls and a more diffuse staining pattern. Lastly we examined claudin-2 expression in migrating keratinocytes by Western blot analysis and found a significant decrease in protein staining in scratch-test assay cultures after 4 h, followed by a significant increase in claudin-2 protein after 24 h. Taken together these results indicate a role for claudin-2 signalling in proliferation and cell migration in the epidermis of the skin.

The role of claudin-2 in kidney function and dysfunction

Claudin-2 is a tight junction protein expressed in leaky epithelia where it forms paracellular pores permeable to cations and water. The paracellular pore formed by claudin-2 is important in energy-efficient cation and water transport in the proximal tubules of the kidneys. Mounting evidence now suggests that claudin-2 may modulate cellular processes often altered in disease, including cellular proliferation. Also, dysregulation of claudin-2 expression has been linked to various diseases, including kidney stone disease and renal cell carcinoma. However, the mechanisms linking altered claudin-2 expression and function to disease are poorly understood and require further investigation. The aim of this review is to discuss the current understanding of the role of claudin-2 in kidney function and dysfunction. We provide a general overview of the claudins and their organization in the tight junction, the expression, and function of claudin-2 in the kidney, and the evolving evidence for its role in kidney disease.

Digested Cinnamon (Cinnamomum verum J. Presl) Bark Extract Modulates Claudin-2 Gene Expression and Protein Levels under TNFα/IL-1β Inflammatory Stimulus

Epigenetic changes, host-gut microbiota interactions, and environmental factors contribute to inflammatory bowel disease (IBD) onset and progression. A healthy lifestyle may help to slow down the chronic or remitting/relapsing intestinal tract inflammation characteristic of IBD. In this scenario, the employment of a nutritional strategy to prevent the onset or supplement disease therapies included functional food consumption. Its formulation consists of the addition of a phytoextract enriched in bioactive molecules. A good candidate as an ingredient is the Cinnamon verum aqueous extract. Indeed, this extract, subjected to a process of gastrointestinal digestion simulation (INFOGEST), exhibits beneficial antioxidant and anti-inflammatory properties in an in vitro model of the inflamed intestinal barrier. Here, we deepen the study of the mechanisms related to the effect of digested cinnamon extract pre-treatment, showing a correlation between transepithelial electrical resistance (TEER) decrement and alterations in claudin-2 expression under Tumor necrosis factor-α/Interleukin-1β (TNF-α/IL-1) β cytokine administration. Our results show that pre-treatment with cinnamon extract prevents TEER loss by claudin-2 protein level regulation, influencing both gene transcription and autophagy-mediated degradation. Hence, cinnamon polyphenols and their metabolites probably work as mediators in gene regulation and receptor/pathway activation, leading to an adaptive response against renewed insults.

The multiphasic TNF-α-induced compromise of Calu-3 airway epithelial barrier function

Purpose: Airway epithelial barrier leak and the involvement of proinflammatory cytokines play a key role in a variety of diseases. This study evaluates barrier compromise by the inflammatory mediator Tumor Necrosis Factor-α (TNF-α) in the human airway epithelial Calu-3 model. Methods: We examined the effects of TNF-α on barrier function in Calu-3 cell layers using Transepithelial Electrical Resistance (TER) and transepithelial diffusion of radiolabeled probe molecules. Western immunoblot analyses of tight junctional (TJ) proteins in detergent soluble fractions were performed. Results: TNF-α dramatically reduced TER and increased paracellular permeability of both 14C-D-mannitol and the larger 5 kDa probe, 14C-inulin. A time course of the effects shows two separate actions on barrier function. An initial compromise of barrier function occurs 2-4 hours after TNF-α exposure, followed by complete recovery of barrier function by 24 hrs. Beginning 48 hrs. post-exposure, a second more sustained barrier compromise ensues, in which leakiness persists through 144 hrs. There were no changes in TJ proteins observed at 3 hrs. post exposure, but significant increases in claudins-2, -3, -4, and -5, as well as a decrease in occludin were seen at 72 hrs. post TNF-α exposure. Both the 2-4 hr. and the 72 hr. TNF-α induced leaks are shown to be mediated by the ERK signaling pathway. Conclusion: TNF-α induced a multiphasic transepithelial leak in Calu-3 cell layers that was shown to be ERK mediated, as well as involve changes in the TJ complex. The micronutrients, retinoic acid and calcitriol, were effective at reducing this barrier compromise caused by TNF-α. The significance of these results for airway disease and for COVID-19 specifically are discussed.

PI3Kγ is a novel regulator of TNFα signaling in the human colon cell line HT29/B6

Phosphoinositide 3-kinases (PI3Ks) are a family of enzymes phosphorylating phospholipids in the membrane, thereby, promoting the PI3K/AKT signaling cascade. PI3Ks are involved in a variety of fundamental cellular functions, including tumor necrosis factor α (TNFα)-induced tight junction (TJ) impairment-a hallmark of inflammatory bowel diseases. Most of the studies analyzing the role of class I PI3K signaling in epithelial barrier maintenance did not decipher which of the isoforms are responsible for the observed effects. By using wild-type and PI3Kγ-deficient HT-29/B6 cells, we characterized the functional role of PI3Kγ in these cells under inflammatory conditions. Measurement of the transepithelial electrical resistance and the paracellular flux of macromolecules revealed that monolayers of PI3Kγ-deficient cells, compared with wild-type cells, were protected against TNFα-induced barrier dysfunction. This effect was independent of any PI3K activity because treatment with a pan-PI3K inhibitor did not alter this observation. By immunostaining, we found correlative changes in the distribution of the TJ marker ZO-1. Furthermore, the absence of PI3Kγ reduced the basal level of the pore-forming TJ protein claudin-2. Our study suggests a novel noncanonical, kinase-independent scaffolding function of PI3Kγ in TNFα-induced barrier dysfunction.

Chronic psychological stress alters gene expression in rat colon epithelial cells promoting chromatin remodeling, barrier dysfunction and inflammation

Chronic stress is commonly associated with enhanced abdominal pain (visceral hypersensitivity), but the cellular mechanisms underlying how chronic stress induces visceral hypersensitivity are poorly understood. In this study, we examined changes in gene expression in colon epithelial cells from a rat model using RNA-sequencing to examine stress-induced changes to the transcriptome. Following chronic stress, the most significantly up-regulated genes included Atg16l1, Coq10b, Dcaf13, Nat2, Ptbp2, Rras2, Spink4 and down-regulated genes including Abat, Cited2, Cnnm2, Dab2ip, Plekhm1, Scd2, and Tab2. The primary altered biological processes revealed by network enrichment analysis were inflammation/immune response, tissue morphogenesis and development, and nucleosome/chromatin assembly. The most significantly down-regulated process was the digestive system development/function, whereas the most significantly up-regulated processes were inflammatory response, organismal injury, and chromatin remodeling mediated by H3K9 methylation. Furthermore, a subpopulation of stressed rats demonstrated very significantly altered gene expression and transcript isoforms, enriched for the differential expression of genes involved in the inflammatory response, including upregulation of cytokine and chemokine receptor gene expression coupled with downregulation of epithelial adherens and tight junction mRNAs. In summary, these findings support that chronic stress is associated with increased levels of cytokines and chemokines, their downstream signaling pathways coupled to dysregulation of intestinal cell development and function. Epigenetic regulation of chromatin remodeling likely plays a prominent role in this process. Results also suggest that super enhancers play a primary role in chronic stress-associated intestinal barrier dysfunction.

Dissection of Barrier Dysfunction in Organoid-Derived Human Intestinal Epithelia Induced by Giardia duodenalis

BACKGROUND & AIMS

The protozoa Giardia duodenalis is a major cause of gastrointestinal illness worldwide, but underlying pathophysiological mechanisms remain obscure, partly due to the absence of adequate cellular models. We aimed at overcoming these limitations and recapitulating the authentic series of pathogenic events in the primary human duodenal tissue by using the human organoid system.

METHODS

We established a compartmentalized cellular transwell system with electrophysiological and barrier properties akin to duodenal mucosa and dissected the events leading to G. duodenalis-induced barrier breakdown by functional analysis of transcriptional, electrophysiological, and tight junction components.

RESULTS

Organoid-derived cell layers of different donors showed a time- and parasite load-dependent leak flux indicated by collapse of the epithelial barrier upon G. duodenalis infection. Gene set enrichment analysis suggested major expression changes, including gene sets contributing to ion transport and tight junction structure. Solute carrier family 12 member 2 and cystic fibrosis transmembrane conductance regulator-dependent chloride secretion was reduced early after infection, while changes in the tight junction composition, localization, and structural organization occurred later as revealed by immunofluorescence analysis and freeze fracture electron microscopy. Functionally, barrier loss was linked to the adenosine 3',5'-cyclic monophosphate (cAMP)/protein kinase A-cAMP response element-binding protein signaling pathway.

CONCLUSIONS

Data suggest a previously unknown sequence of events culminating in intestinal barrier dysfunction upon G. duodenalis infection during which alterations of cellular ion transport were followed by breakdown of the tight junctional complex and loss of epithelial integrity, events involving a cAMP/protein kinase A-cAMP response element-binding protein mechanism. These findings and the newly established organoid-derived model to study G. duodenalis infection may help to explore new options for intervening with disease and infection, in particular relevant for chronic cases of giardiasis.

Cow Milk Extracellular Vesicle Effects on an In Vitro Model of Intestinal Inflammation

Extracellular vesicles (EVs) are lipid bilayer nano-dimensional spherical structures and act mainly as signaling mediators between cells, in particular modulating immunity and inflammation. Milk-derived EVs (mEVs) can have immunomodulatory and anti-inflammatory effects, and milk is one of the most promising food sources of EVs. In this context, this study aimed to evaluate bovine mEVs anti-inflammatory and immunomodulating effects on an in vitro co-culture (Caco-2 and THP-1) model of intestinal inflammation through gene expression evaluation with RT-qPCR and cytokine release through ELISA. After establishing a pro-inflammatory environment due to IFN-γ and LPS stimuli, CXCL8, IL1B, TNFA, IL12A, IL23A, TGFB1, NOS2, and MMP9 were significantly up-regulated in inflamed Caco-2 compared to the basal co-culture. Moreover, IL-17, IL-1β, IL-6, TNF-α release was increased in supernatants of THP-1. The mEV administration partially restored initial conditions with an effective anti-inflammatory activity. Indeed, a decrease in gene expression and protein production of most of the tested cytokines was detected, together with a significant gene expression decrease in MMP9 and the up-regulation of MUC2 and TJP1. These results showed a fundamental capability of mEVs to modulate inflammation and their potential beneficial effect on the intestinal mucosa.

IL-1β is a key inflammatory cytokine that weakens lactation-specific tight junctions of mammary epithelial cells

In lactating mammary glands, alveolar mammary epithelial cells (MECs) produce milk and form less-permeable tight junctions (TJs). However, alveolar TJs are weakened with a reduction in milk production in mammary glands due to mastitis or weaning in the presence of high levels of IL-1β, IL-6, or TNF-α. In this study, using in vitro cultured model of MECs with milk-producing ability and lactation-specific TJs, we investigated whether the aforementioned cytokines affect MEC TJs. The results showed that TNF-α, IL-1β, and IL-6 affected lactation-specific TJs in different ways. In particular, upon activation of p38 and JNK signalling, IL-1β caused rapid disruption of TJs at tricellular contact points. IL-1β treatment led to decreased CLDN3, CLDN4, and OCLN levels and a weakened TJ barrier. The adverse effects of IL-1β on TJs were mimicked by anisomycin, which is an activator of p38 and JNK signalling, and were blocked by MEC pretreatment with a p38 inhibitor but not a JNK inhibitor. The mislocalization of tricellulin at tricellular contact areas was confirmed in MECs treated with IL-1β or anisomycin. These results indicate that IL-1β is a key cytokine that adversely affects the TJs between MECs by activating p38.

Tumor Necrosis Factor Alpha Effects on the Porcine Intestinal Epithelial Barrier Include Enhanced Expression of TNF Receptor 1

Tumor necrosis factor alpha (TNFα) has been shown to impair the intestinal barrier, inducing and maintaining inflammatory states of the intestine. The aim of the current study was to analyze functional, molecular and regulatory effects of TNFα in a newly established non-transformed jejunal enterocyte model, namely IPEC-J2 monolayers. Incubation with 1000 U/mL TNFα induced a marked decrease in transepithelial electrical resistance (TEER), and an increase in permeability for the paracellular flux marker [³H]-D-mannitol compared to controls. Immunoblots revealed a significant decrease in tight junction (TJ) proteins occludin, claudin-1 and claudin-3. Moreover, a dose-dependent increase in the TNF receptor (TNFR)-1 was detected, explaining the exponential nature of pro-inflammatory effects, while TNFR-2 remained unchanged. Recovery experiments revealed reversible effects after the removal of the cytokine, excluding apoptosis as a reason for the observed changes. Furthermore, TNFα signaling could be inhibited by the specific myosin light chain kinase (MLCK) blocker ML-7. Results of confocal laser scanning immunofluorescence microscopy were in accordance with all quantitative changes. This study explains the self-enhancing effects of TNFα mediated by MLCK, leading to a differential regulation of TJ proteins resulting in barrier impairment in the intestinal epithelium.

Impaired airway epithelial barrier integrity was mediated by PI3Kδ in a mouse model of lipopolysaccharide-induced acute lung injury

Cell-cell junctions are critical for the maintenance of cellular as well as tissue polarity and integrity. Dysfunction of airway epithelial barrier has been shown to be involved in the pathogenesis of acute lung injury (ALI). Yet the role of phosphatidylinositol 3-kinase delta (PI3Kδ) in dysregulation of airway epithelial barrier integrity in ALI has not been addressed. Mice were subjected to intratracheal instillation of lipopolysaccharide (LPS) to generate a ALI model. Two pharmacological inhibitors of PI3Kδ, IC87114 and AMG319, were respectively given to the mice. Expression of p110δ and its downstream substrate phospho-AKT (Ser473) was increased in LPS-exposed lungs. These increases were inhibited by IC87114 or AMG319. LPS led to pronounced lung injury that was accompanied by significant airway neutrophil recruitment and bronchial epithelial morphological alterations 72 h after exposure. We also found compromised expression of adherens junction protein E-cadherin and tight junction protein claudin-2 in the airway epithelial cells. Treatment with either IC87114 or AMG319 not only attenuated LPS-induced edema, lung injury and neutrophilc inflammation, reduced total protein concentration and IL-6, TNF-α secretion in BALF, but also restored epithelial E-cadherin and claudin-2 expression. In summary, our results showed that LPS can induce a delayed effect on airway epithelial barrier integrity that is mediated by PI3Kδ in a mouse model of ALI.

Colon epithelial cell TGFβ signaling modulates the expression of tight junction proteins and barrier function in mice

Defective barrier function is a predisposing factor in inflammatory bowel disease (IBD) and colitis-associated cancer (CAC). Although TGFβ signaling defects have been associated with IBD and CAC, few studies have examined the relationship between TGFβ and intestinal barrier function. Here, we examine the role of TGFβ signaling via SMAD4 in modulation of colon barrier function. The Smad4 gene was conditionally deleted in the intestines of adult mice and intestinal permeability assessed using an in vivo 4 kDa FITC-Dextran (FD4) permeability assay. Mouse colon was isolated for gene expression (RNA-sequencing), Western blot, and immunofluorescence analysis. In vitro colon organoid culture was utilized to assess junction-related gene expression by qPCR and transepithelial resistance (TER). In silico analyses of human IBD and colon cancer databases were performed. Mice lacking intestinal expression of Smad4 demonstrate increased colonic permeability to FD4 without gross mucosal damage. mRNA/protein expression analyses demonstrate significant increases in Cldn2/Claudin 2 and Cldn8/Claudin 8, and decreases in Cldn3, Cldn4, and Cldn7/Claudin 7 with intestinal SMAD4 loss in vivo without changes in Claudin protein localization. TGFβ1/BMP2 treatment of polarized SMAD4+ colonoids increases TER. Cldn2, Cldn4, Cldn7, and Cldn8 are regulated by canonical TGFβ signaling, and TGFβ-dependent regulation of these genes is dependent on nascent RNA transcription (Cldn2, Cldn4, Cldn8) but not nascent protein translation (Cldn4, Cldn8). Human IBD/colon cancer specimens demonstrate decreased SMAD4, CLDN4, CLDN7, and CLDN8 and increased CLDN2 compared with healthy controls. Canonical TGFβ signaling modulates the expression of tight junction proteins and barrier function in mouse colon.NEW & NOTEWORTHY We demonstrate that canonical TGFβ family signaling modulates the expression of critical tight junction proteins in colon epithelial cells, and that expression of these tight junction proteins is associated with maintenance of colon epithelial barrier function in mice.

Electric cell-substrate impedance sensing in kidney research

Electric cell-substrate impedance sensing (ECIS) is a quantitative, label-free, non-invasive analytical method allowing continuous monitoring of the behaviour of adherent cells by online recording of transcellular impedance. ECIS offers a wide range of practical applications to study cell proliferation, migration, differentiation, toxicity and monolayer barrier integrity. All of these applications are relevant for basic kidney research, e.g. on endothelial cells, tubular and glomerular epithelial cells. This review gives an overview on the fundamental principles of the ECIS technology. We name strengths and remaining hurdles for practical applications, present an ECIS array reuse protocol, and review its past, present and potential future contributions to preclinical kidney research.

[Immunity and tubular dysfunction in case of systemic disease]

The immune renal tubular diseases are known since five decades, but their prevalence remains to be defined. They are caused by humoral and cellular effectors of innate and adaptative immunities on several targets of the renal tubule: protein channels, co or counter transporters, luminal or cytosolic enzymes, tight junctions. Genetic or epigenetic variations are also involved. Clinical manifestations are various and make the diagnosis difficult. They can precede the causal affection and they worsen the prognosis. The classical model consists in hypokalemic tubular distal acidosis observed in Sjögren's syndrome which illustrates the auto-immune epithelitis concept. Cellular immunity can act through other ways, like tertiary lymphoid neogenesis in systemic lupus. Humoral immunity through autoantibodies targets several membrane, cytosolic or nuclear proteins, causing specific tubular dysfonctions. It is also implied in the epithelial-mesenchymal transition of tubular cells. Innate immunity through cytokines may be involved. Treatment consists in electrolytic disorders correction and immunosupppressive medication: the choice should be guided at best by physiopathology.

Corticosteroid enhances epithelial barrier function in intestinal organoids derived from patients with Crohn's disease

Abstract

Corticosteroids (CS), first-line therapeutics for Crohn’s disease (CD) with moderate or severe disease activity, were found to restore intestinal permeability in CD patients, whereas the underlying molecular events are still largely unknown. This study aimed to investigate the effect and mechanisms of CS prednisolone on epithelial barrier using CD patient-derived intestinal organoids. 3D intestinal organoids were generated from colon biopsies of inactive CD patients. To mimic the inflammatory microenvironment, a mixture of cytokines containing TNF-α, IFN-γ, and IL-1β were added to the organoid culture with or without pre-incubation of prednisolone or mifepristone. Epithelial permeability of the organoids was assessed by FITC-D4 flux from the basal to luminal compartment using confocal microscopy. Expression of junctional components were analyzed by qRT-PCR, immunofluorescence staining, and western blot. Activity of signaling pathways were analyzed using western blot. Exposure of the cytokines significantly disrupted epithelial barrier of the intestinal organoids, which was partially restored by prednisolone. On the molecular level, the cytokine mixture resulted in a significant reduction in E-cadherin and ILDR-1, an increase in CLDN-2, MLCK, and STAT1 phosphorylation, whereas prednisolone ameliorated the abovementioned effects induced by the cytokine mixture. This study demonstrates that prednisolone confers a direct effect in tightening the epithelial barrier, identifies novel junctional targets regulated by prednisolone, and underscores intestinal barrier restoration as a potential mechanism that contributes to the clinical efficacy of prednisolone in CD patients.

Key messages

Prednisolone confers a direct preventive effect against cytokine-induced barrier dysfunction.

Prednisolone regulates the expression of CLDN-2, E-cadherin, and ILDR-1.

The effect of prednisolone is GR-, MLCK-, and STAT1-dependent.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00109-021-02045-7.

Protective Role of Natural and Semi-Synthetic Tocopherols on TNFα-Induced ROS Production and ICAM-1 and Cl-2 Expression in HT29 Intestinal Epithelial Cells

Vitamin E, a fat-soluble compound, possesses both antioxidant and non-antioxidant properties. In this study we evaluated, in intestinal HT29 cells, the role of natural tocopherols, α-Toc and δ-Toc, and two semi-synthetic derivatives, namely bis-δ-Toc sulfide (δ-Toc)₂S and bis-δ-Toc disulfide (δ-Toc)₂S₂, on TNFα-induced oxidative stress, and intercellular adhesion molecule-1 (ICAM-1) and claudin-2 (Cl-2) expression. The role of tocopherols was compared to that of N-acetylcysteine (NAC), an antioxidant precursor of glutathione synthesis. The results show that all tocopherol containing derivatives used, prevented TNFα-induced oxidative stress and the increase of ICAM-1 and Cl-2 expression, and that (δ-Toc)₂S and (δ-Toc)₂S₂ are more effective than δ-Toc and α-Toc. The beneficial effects demonstrated were due to tocopherol antioxidant properties, but suppression of TNFα-induced Cl-2 expression seems not only to be related with antioxidant ability. Indeed, while ICAM-1 expression is strongly related to the intracellular redox state, Cl-2 expression is TNFα-up-regulated by both redox and non-redox dependent mechanisms. Since ICAM-1 and Cl-2 increase intestinal bowel diseases, and cause excessive recruitment of immune cells and alteration of the intestinal barrier, natural and, above all, semi-synthetic tocopherols may have a potential role as a therapeutic support against intestinal chronic inflammation, in which TNFα represents an important proinflammatory mediator.

Tumor necrosis factor-α induces claudin-3 upregulation in kidney tubular epithelial cells through NF-κB and CREB1

Claudins are essential for tight junction formation and paracellular transport, and they affect key cellular events including proliferation and migration. The properties of tight junctions are dynamically modulated by a variety of inputs. We previously showed that the inflammatory cytokine tumor necrosis factor-α (TNFα), a major pathogenic factor in kidney disease, alters epithelial permeability by affecting the expression of claudin-1, -2, and -4 in kidney tubular cells. Here, we explored the effect of TNFα on claudin-3 (Cldn-3), a ubiquitous barrier-forming protein. We found that TNFα elevated Cldn-3 protein expression in tubular epithelial cells (LLC-PK1 and IMCD3) as early as 3 h post treatment. Bafilomycin A and bortezomib, inhibitors of lysosomal and proteasomes, respectively, reduced Cldn-3 degradation. However, TNFα caused a strong upregulation of Cldn-3 in the presence of bafilomycin, suggesting an effect independent from lysosomes. Blocking protein synthesis using cycloheximide prevented Cldn-3 upregulation by TNFα, verifying the contribution of de novo Cldn-3 synthesis. Indeed, TNFα elevated Cldn-3 mRNA levels at early time points. Using pharmacological inhibitors and siRNA-mediated silencing, we determined that the effect of TNFα on Cldn-3 was mediated by extracellular signal regulated kinase (ERK)-dependent activation of NF-κB and PKA-induced activation of CREB1. These two pathways were turned on by TNFα in parallel and both were required for the upregulation of Cldn-3. Because Cldn-3 was suggested to modulate cell migration and epithelial-mesenchymal transition (EMT), and TNFα was shown to affect these processes, Cldn-3 upregulation may modulate regeneration of the tubules following injury.

Histone H3K9 methylation regulates chronic stress and IL-6-induced colon epithelial permeability and visceral pain

BACKGROUND

Chronic stress is associated with activation of the HPA axis, elevation in pro-inflammatory cytokines, decrease in intestinal epithelial cell tight junction (TJ) proteins, and enhanced visceral pain. It is unknown whether epigenetic regulatory pathways play a role in chronic stress-induced intestinal barrier dysfunction and visceral hyperalgesia.

METHODS

Young adult male rats were subjected to water avoidance stress ± H3K9 methylation inhibitors or siRNAs. Visceral pain response was assessed. Differentiated Caco-2/BBE cells and human colonoids were treated with cortisol or IL-6 ± antagonists. Expression of TJ, IL-6, and H3K9 methylation status at gene promoters was measured. Transepithelial electrical resistance and FITC-dextran permeability were evaluated.

KEY RESULTS

Chronic stress induced IL-6 up-regulation prior to a decrease in TJ proteins in the rat colon. The IL-6 level inversely correlated with occludin expression. Treatment with IL-6 decreased occludin and induced visceral hyperalgesia. Chronic stress and IL-6 increased H3K9 methylation and decreased transcriptional GR binding to the occludin gene promoter, leading to down-regulation of protein expression and increase in paracellular permeability. Intrarectal administration of a H3K9 methylation antagonist prevented chronic stress-induced visceral hyperalgesia in the rat. In a human colonoid model, cortisol decreased occludin expression, which was prevented by the GR antagonist RU486, and IL-6 increased H3K9 methylation and decreased TJ protein levels, which were prevented by inhibitors of H3K9 methylation.

CONCLUSIONS & INFERENCES

Our findings support a novel role for methylation of the repressive histone H3K9 to regulate chronic stress, pro-inflammatory cytokine-mediated reduction in colon TJ protein levels, and increase in paracellular permeability and visceral hyperalgesia.

Anti-Obesity Effect of Dictyophora indusiata Mushroom Polysaccharide (DIP) in High Fat Diet-Induced Obesity via Regulating Inflammatory Cascades and Intestinal Microbiome

Obesity is a multifactorial metabolic disorder characterized by low-grade chronic inflammation, hyper-permeability of the gut epithelium, and perturbation of the intestinal microbiome. Despite the numerous therapeutic efficacies of Dictyophora indusiata mushroom, its biological activity in alleviating obesity through regulation of the gut microbiota and inflammatory cascades remain obscure. Henceforth, we determined the modulatory impact of D. indusiata polysaccharide (DIP) in the high-fat diet (HFD)-induced obesity mice model. The experimental subjects (BALB/C mice) were supplemented with chow diet (Control group), high-fat diet (HFD group), or HFD along with DIP at a low dose [HFD + DIP(L)] and high dose [HFD + DIP(H)]. Obesity-related parameters, including body weight gain, epididymal adipocyte size, fat accumulation, adipogenic markers, lipogenic markers, inflammatory associated markers, intestinal integrity, and intestinal microbiome, were elucidated. Our findings demonstrated that the oral administration of DIP at low dose partially and at high dose significantly reversed HFD-induced obesity parameters. Furthermore, the body weight, fat accumulation, adipocyte size, adipogenic and liver associated markers, glucose levels, inflammatory cytokines, and endotoxin (Lipopolysaccharide, LPS) levels were reduced considerably. Moreover, the study revealed that DIP treatment reversed the dynamic alterations of the gut microbiome community by decreasing the Firmicutes to Bacteroidetes ratio. These findings led us to infer the therapeutic potential of DIP in alleviating HFD-induced obesity via regulating inflammatory cascades, modulating intestinal integrity and intestinal microbiome community.

Measuring Cell Growth and Junction Development in Epithelial Cells Using Electric Cell-Substrate Impedance Sensing (ECIS)

Electric Cell-substrate Impedance Sensing (ECIS) is an automated method that can be used to quantify processes such as cell attachment, growth, migration and barrier functions (i.e., the properties of tight junctions). The method provides simultaneous information on cell number and tight junction function by detecting electric parameters of cells grown on electrodes. Samples are probed with small alternating current (AC) over a range of frequencies, and changes in capacitance and impedance are measured over time. Capacitance reflects the degree of electrode coverage by cells, that correlates with cell number, and can be used to assess cell proliferation or migration. Impedance values inform about barrier function. Obtaining real-time simultaneous information on these parameters is unique to this system and is of great value for addressing fundamental questions such as the role of tight junction proteins in cell growth and migration. This protocol describes the use of ECIS to follow cell growth and tight junction-dependent barrier generation in tubular epithelial cells. We used this method to explore how depleting claudin-2, a tight junction protein affects tubular cell growth and barrier function. During the process, cells are transfected with control or claudin-2-specific siRNA, and 24h later plated on electrodes. ECIS automatically collects information on cell growth and barrier as the monolayer develops. The data are initially analyzed using the ECIS software and exported into a graph software for further processing.

Immunofluorescent Staining of Claudin-2 in Cultured Kidney Tubular Cells

Members of the claudin family of tight junction proteins regulate paracellular permeability and modulate cell signaling. During junction remodeling, these proteins are selectively inserted into or retrieved from the tight junctions, but the control and coordination of these processes remain incompletely understood. Visualization of claudins allows the assessment of changes in their localization and abundance. We use the described protocol to stain claudin-2, but it can also be adapted to stain any tight junction protein. We found that using methanol for fixing allows the best preservation of claudin-2 both at the membrane and in cytoplasmic vesicles. Staining is done using a claudin-2 specific primary and a fluorescently labelled secondary antibody, along with DAPI to label nuclei. The samples are then imaged using confocal microscopy, and a z-stack is obtained allowing visualization of both junctional and intracellular claudin-2. Total claudin-2 signal can be quantified after 3D reconstruction of the images using the Imaris software.

Claudin-2: Roles beyond Permeability Functions

Claudin-2 is expressed in the tight junctions of leaky epithelia, where it forms cation-selective and water permeable paracellular channels. Its abundance is under fine control by a complex signaling network that affects both its synthesis and turnover in response to various environmental inputs. Claudin-2 expression is dysregulated in many pathologies including cancer, inflammation, and fibrosis. Claudin-2 has a key role in energy-efficient ion and water transport in the proximal tubules of the kidneys and in the gut. Importantly, strong evidence now also supports a role for this protein as a modulator of vital cellular events relevant to diseases. Signaling pathways that are overactivated in diseases can alter claudin-2 expression, and a good correlation exists between disease stage and claudin-2 abundance. Further, loss- and gain-of-function studies showed that primary changes in claudin-2 expression impact vital cellular processes such as proliferation, migration, and cell fate determination. These effects appear to be mediated by alterations in key signaling pathways. The specific mechanisms linking claudin-2 to these changes remain poorly understood, but adapters binding to the intracellular portion of claudin-2 may play a key role. Thus, dysregulation of claudin-2 may contribute to the generation, maintenance, and/or progression of diseases through both permeability-dependent and -independent mechanisms. The aim of this review is to provide an overview of the properties, regulation, and functions of claudin-2, with a special emphasis on its signal-modulating effects and possible role in diseases.

Changes in Radixin Expression and Interaction with Efflux Transporters in the Liver of Adjuvant-Induced Arthritic Rats

Scaffold proteins such as radixin help to modulate the plasma membrane localization and transport activity of the multidrug resistance-associated protein 2 (MRP2/ABCC2) and P-glycoprotein (P-gp/ABCB1) efflux transporters in the liver. We examined changes in radixin expression and interaction with efflux transporters in adjuvant-induced arthritic (AA) rats, an animal model of rheumatoid arthritis, as well as in human liver cancer (HepG2) cells because inflammation affects drug pharmacokinetics via the efflux transporters. The expression levels of radixin and phosphorylated radixin (p-radixin) were measured 24 h after treatment with inflammatory cytokines comprising tumor necrosis factor (TNF)-α, interleukin (IL)-1β, IL-6 or sodium nitroprusside (SNP; a nitric oxide donor). The protein levels of radixin, MRP2, and P-gp in the rat liver were next examined. We also investigated whether inflammation affected the formation of complexes between radixin and MRP2 or P-gp. The mRNA and protein levels of radixin in HepG2 cells were significantly decreased by TNF-α treatment, while minimal changes were observed after treatment with IL-1β, IL-6 or SNP. TNF-α also significantly decreased the protein levels of p-radixin, suggesting that TNF-α inhibited the activation of radixin and thereby reduced the activity of the efflux transporters. Complex formation of radixin with MRP2 and P-gp was significantly decreased in AA rats but this was reversed by prednisolone and dexamethasone treatment, indicating that decreased interactions of radixin with MRP2 and P-gp likely occur during liver inflammation. These data suggest that liver inflammation reduces radixin function by decreasing its interactions with MRP2 and P-gp.

CLDN1 induces autophagy to promote proliferation and metastasis of esophageal squamous carcinoma through AMPK/STAT1/ULK1 signaling

Tight junction is a structural constitution in cell-cell adhesion and play an important role in the maintenance of permeability and integrity of normal epithelial cell barrier. The protein encoded by Claudin 1 (CLDN1), a member of the claudin family, is an integral membrane protein and a component of tight junction strands. CLDN1 has been proved to regulate the proliferation and metastasis of multiple tumors, but little is known about its role in esophageal squamous cell carcinoma (ESCC). Here, we found that CLDN1 was aberrantly increased in ESCC tissues and cell lines, and mainly distributed in the nucleus of tumor cells. Furthermore, we confirmed that CLDN1 promoted the proliferation and metastasis of ESCC by triggering autophagy both in vitro and in vivo. Mechanically, we validated that CLDN1-induced autophagy via increasing Unc-51 like autophagy activating kinase 1 (ULK1) expression through AMP-activated protein kinase (AMPK)/signal transducer and activator of transcription 1 (STAT1) signaling pathway in ESCC cells. Taken together, our findings demonstrated that aberrant expression and distribution of CLDN1 promoted the proliferation and metastasis of esophageal squamous carcinoma by triggering autophagy through AMPK/STAT1/ULK1 signaling pathway.

Claudin-2 suppresses GEF-H1, RHOA, and MRTF, thereby impacting proliferation and profibrotic phenotype of tubular cells

The tight junctional pore-forming protein claudin-2 (CLDN-2) mediates paracellular Na⁺ and water transport in leaky epithelia and alters cancer cell proliferation. Previously, we reported that tumor necrosis factor-α time-dependently alters CLDN-2 expression in tubular epithelial cells. Here, we found a similar expression pattern in a mouse kidney injury model (unilateral ureteral obstruction), consisting of an initial increase followed by a drop in CLDN-2 protein expression. CLDN-2 silencing in LLC-PK¹ tubular cells induced activation and phosphorylation of guanine nucleotide exchange factor H1 (GEF-H1), leading to Ras homolog family member A (RHOA) activation. Silencing of other claudins had no such effects, and re-expression of an siRNA-resistant CLDN-2 prevented RHOA activation, indicating specific effects of CLDN-2 on RHOA. Moreover, kidneys from CLDN-2 knockout mice had elevated levels of active RHOA. Of note, CLDN-2 silencing reduced LLC-PK1 cell proliferation and elevated expression of cyclin-dependent kinase inhibitor P27 (P27KIP1) in a GEF-H1/RHOA-dependent manner. P27KIP1 silencing abrogated the effects of CLDN-2 depletion on proliferation. CLDN-2 loss also activated myocardin-related transcription factor (MRTF), a fibrogenic RHOA effector, and elevated expression of connective tissue growth factor and smooth muscle actin. Finally, CLDN-2 down-regulation contributed to RHOA activation and smooth muscle actin expression induced by prolonged tumor necrosis factor-α treatment, because they were mitigated by re-expression of CLDN-2. Our results indicate that CLDN-2 suppresses GEF-H1/RHOA. CLDN-2 down-regulation, for example, by inflammation, can reduce proliferation and promote MRTF activation through RHOA. These findings suggest that the initial CLDN-2 elevation might aid epithelial regeneration, and CLDN-2 loss could contribute to fibrotic reprogramming.

Integrated analysis of circRNAs and mRNAs expression profile revealed the involvement of hsa_circ_0007919 in the pathogenesis of ulcerative colitis

BACKGROUND

Ulcerative colitis (UC) is characterized by chronic inflammation in the colon and epigenetic factors underlying the occurrence. Circular RNAs (circRNAs) have been under intensive focus due to the circular construct and gene-regulating functions. However, the changes and roles of circRNAs in UC remain unknown.

METHODS

Microarrays were used to detect the differentially expressed genes, and quantitative real-time PCR was used to identify the changes in UC. In silico analyses were performed to predict the functions of circRNAs and mRNAs. In vitro, epithelial cell lines were stimulated by pro-inflammation effectors to test the alterations in circRNAs. CircRNAs-microRNAs-mRNAs network clarified the potential mechanisms underlying circRNAs in UC. The binding site between hsa_circ_0007919 and miR-138 or let-7a was verified using dual-luciferase assay.

RESULTS

A total of 264 significantly dysregulated circRNAs and 1869 differentially expressed mRNAs in inflamed mucosa were compared with the non-inflamed mucosa in UC. Hsa_circ_0004662 and hsa_circ_0007919 were altered largely in UC tissues. Hsa_circ_0007919 was reduced persistently after inflammatory treatments, and it was relevant to Mayo endoscopic subscores and the expression of tight junction molecules. Finally, hsa_circ_0007919 could harbor miR-138, and let-7a to regulate the targeted mRNAs EPC1 and VIPR1.

CONCLUSIONS

Several circRNAs were differentially expressed in UC. Hsa_circ_0007919 is related to clinical characteristics and epithelial integrity by binding to hsa-let-7a, hsa-miR-138 to regulate the target genes. CircRNAs, especially hsa_circ_0007919, are associated with the pathogenesis and development of UC, with potential diagnostic and therapeutic implications.

HIV induces production of IL-18 from intestinal epithelial cells that increases intestinal permeability and microbial translocation

Interleukin-18 (IL-18) is a pleiotropic cytokine of the IL-1 family with multiple context dependent functions. We and others have shown that HIV infection is accompanied by increased circulating levels of IL-18 along with decreased levels of its antagonist, Interleukin-18 Binding Protein (IL-18BP). The infection is also accompanied by intestinal inflammation and decreased intestinal integrity as measured by intestinal permeability, regeneration and repair. However, little is known concerning the relation between high level of IL-18 associated with the viral infection and intestinal permeability. Here we demonstrate that HIV treatment increases production of IL-18 and decreases that of IL-18BP production in human intestinal epithelial cell (IEC) lines. IL-18 causes apoptosis of the IEC by activating caspase-1 and caspase-3. It induces epithelial barrier hyperpermeability by decreasing and disrupting both tight and adherens junction proteins, occludin, claudin 2 and beta-catenin. Disorganization of F-actin was also observed in the IEC that were exposed to the cytokine. Moreover IL-18 decreases transepithelial electrical resistance (TEER) in Caco-2 and increases permeability in HT29 monolayers. The cells' treatment with IL-18 causes an increase in the expression of phosphorylated myosin II regulatory light-chain (p-MLC) and myosin light-chain kinase (MLCK), and a decrease in phosphorylated Signal Transducer and Activator of Transcription (p-STAT)-5. This increase in p-MLC is suppressed by a Rho-kinase (ROCK)-specific inhibitor. Interestingly, the levels of the cytokine correlate with those of LPS in the circulation in three different categories of HIV infected patients (HAART-naïve and HAART-treated HIV-infected individuals, and Elite controls) as well as in healthy controls. Collectively, these results suggest that the HIV-induced IL-18 plays a role in increased intestinal permeability and microbial translocation observed in HIV-infected individuals.

Temporal and regional intestinal changes in permeability, tight junction, and cytokine gene expression following ovariectomy-induced estrogen deficiency

Estrogen deficiency that occurs during menopause is associated with wide‐ranging consequences, including effects on the gastrointestinal system. Although previous studies have implicated a role for estrogen in modulating colonic permeability and inflammatory gene expression, the kinetics of these changes following loss of estrogen and whether they are intestinal region specific are unknown. To test this, we performed sham or ovariectomy (OVX) surgery in BALB/c mice and examined permeability (in vivo and ex vivo) and gene expression changes in the duodenum, jejunum, ileum, and colon at 1, 4, and 8 weeks postsurgery. In vivo permeability, assessed by FITC‐dextran gavage and subsequent measures of serum levels, indicated that OVX significantly increased whole intestinal permeability 1 week postsurgery before returning to sham levels at 4 and 8 weeks. Permeability of individual intestinal sections, measured ex vivo by Ussing chambers, revealed specific regional and temporal responses to OVX, with the most dynamic changes exhibited by the ileum. Analysis of gene expression, by qPCR and by mathematical modeling, revealed an OVX‐specific effect with tight junction and inflammatory gene expression elevated and suppressed with both temporal and regional specificity. Furthermore, ileal and colonic expression of the tight junction protein occludin was found to be significantly correlated with expression of TNFα and IL‐1β. Together, our studies reveal previously unappreciated effects of estrogen deficiency in specific intestinal segments and further demonstrate temporal links between estrogen deficiency, inflammatory genes, and intestinal permeability.

Cell confluence regulates claudin-2 expression: possible role for ZO-1 and Rac

Claudin-2 (Cldn-2) is a channel-forming tight junction (TJ) protein in the proximal tubules that mediates paracellular Na+ transport and has also emerged as a regulator of proliferation and migration. Expression of Cldn-2 is altered by numerous stimuli, but the underlying mechanisms remain incompletely understood. Here we show that Cldn-2 protein and mRNA expression were low in subconfluent tubular cells and increased during junction maturation. Cldn-1 or occludin did not exhibit similar confluence-dependence. Conversely, disruption of TJs by Ca2+ removal or silencing of zonula occludens-1 (ZO-1) or ZO-2 induced a large drop in Cldn-2 abundance. Immunofluorescent staining revealed a more uneven Cldn-2 staining in nascent, Cldn-1-positive TJs. Subconfluence and ZO-1 silencing augmented Cldn-2 degradation and reduced Cldn-2 promoter activity, suggesting that insertion into the TJs slows Cldn-2 turnover. Indeed, blocking endocytosis or lysosomal degradation increased Cldn-2 abundance. Cell confluence increased expression of the junctional adapters ZO-1 and -2, and the small GTPase Rac, and elevated Rac activity and p21-activated kinase (Pak) phosphorylation, suggesting that they might mediate confluence-dependent Cldn-2 regulation. Indeed, Rac silencing or Pak inhibition strongly reduced Cldn-2 protein abundance, which was likely the combined effect on turnover, as these interventions reduced Cldn-2 promoter activity and augmented Cldn-2 degradation. Taken together, our data suggest that TJ integrity and maturity, ZO-1 expression/TJ localization, and Rac/Pak control Cldn-2 degradation and synthesis. A feedback mechanism connecting Cldn-2 expression with junction remodeling, e.g., during wound healing, epithelial-mesenchymal transition, or tumor metastasis formation, may have important downstream effects on permeability, proliferation, and migration.

IL-22 Increases Permeability of Intestinal Epithelial Tight Junctions by Enhancing Claudin-2 Expression

Dysfunction of the epithelial barrier is a hallmark of inflammatory intestinal diseases. The intestinal epithelial barrier is maintained by expression of tight junctions that connect adjacent epithelial cells and seal the paracellular space. IL-22 is critical for the maintenance of intestinal barrier function through promoting antipathogen responses and regeneration of epithelial tissues in the gut. However, little is known about the effects of IL-22 on the regulation of tight junctions in the intestinal epithelium. In this study we report that IL-22 signals exclusively through the basolateral side of polarized Caco-2 cell monolayers. IL-22 treatment does not affect the flux of uncharged macromolecules across cell monolayers but significantly reduces transepithelial electrical resistance (TEER), indicating an increase of paracellular permeability for ions. IL-22 treatment on Caco-2 monolayers and on primary human intestinal epithelium markedly induces the expression of Claudin-2, a cation-channel-forming tight junction protein. Furthermore, treatment of IL-22 in mice upregulates Claudin-2 protein in colonic epithelial cells. Knocking down Claudin-2 expression with small interfering RNA reverses the reduction of TEER in IL-22-treated cells. Moreover, IL-22-mediated upregulation of Claudin-2 and loss of TEER can be suppressed with the treatment of JAK inhibitors. In summary, our results reveal that IL-22 increases intestinal epithelial permeability by upregulating Claudin-2 expression through the JAK/STAT pathway. These results provide novel mechanistic insights into the role of IL-22 in the regulation and maintenance of the intestinal epithelial barrier.

Up-regulation of claudin-2 expression by aldosterone in colonic epithelial cells of mice fed with NaCl-depleted diets

Dietary NaCl depletion increases Na⁺ absorption and K⁺ secretion in the colon, but the mechanisms are not fully understood. In mice fed with NaCl-depleted diets, the expression of claudin-2 and -7 increased compared to those in control mice. Aldosterone (ALD) concentration was also increased. We examined the regulatory mechanism of claudin expression by ALD using the murine colonic epithelial MCE301 cells. ALD dose-dependently increased claudin-2 expression without affecting the expression of claudin-4, -7, -8, and -15. ALD increased nuclear distribution of mineralocorticoid receptor (MR), which was inhibited by spironolactone, an MR antagonist. The ALD-induced elevation of claudin-2 mRNA and protein expression was inhibited by spironolactone, but not by RU-486, a glucocorticoid receptor antagonist. Luciferase reporter assay showed that ALD interacts with the promoter region between -2,021 and -2,008 of human claudin-2. The binding of MR on the promoter region of claudin-2 was increased by ALD, which was inhibited by spironolactone in chromatin immunoprecipitation assay. Our data suggest that ALD acts on MR and increases paracellular permeability to ions mediated by the elevation of claudin-2 expression in the colon. NaCl depletion may increase ALD secretion from adrenal cortex, resulting in the elevation of paracellular permeability to cations in the colon.

Obesity-induces Organ and Tissue Specific Tight Junction Restructuring and Barrier Deregulation by Claudin Switching

Obesity increases susceptibility to multiple organ disorders, however, underlying mechanisms remain unclear. The subclinical inflammation assisted by obesity-induced gut permeability may underlie obesity-associated co-morbidities. Despite eminent clinical significance of the obesity led gut barrier abnormalities, its precise molecular regulation remains unclear. It is also unknown whether barrier deregulations, similar to the gut, characterize other vital organs in obese individuals. The claudin family of proteins is integral to the tight junction (TJ), the apical cell-cell adhesion and a key regulator of the epithelial barrier. Using comprehensive physiological and biochemical analysis of intestinal and renal tissues from high-fat diet fed mice, critical for maintaining metabolic homeostasis, this study demonstrates that profound TJ-restructuring by organ and tissue-specific claudin switching characterize obese organs. Protein expression and cellular distribution were examined. In-silico analysis further highlighted potential association of select claudins, modulated by the obesity, with signaling and metabolic pathways of pathological significance. In vitro studies using Leptin or DCA-treatment suggested causal significance of obesity-induced changes in tissue microenvironment in regulating barrier deregulations in tissue-specific manner. Overall, current findings advances our understanding of the molecular undertakings of obesity associated changes that help predispose to specific diseases and also identifies novel windows of preventive and/or therapeutic interventions.

Effects of Ex Vivo Infection with ETEC on Jejunal Barrier Properties and Cytokine Expression in Probiotic-Supplemented Pigs

BACKGROUND AND AIM

Enterotoxigenic Escherichia coli (ETEC) strains are involved in piglet post-weaning diarrhea. Prophylactic measures including probiotics have been examined in infection experiments with live piglets. In the present study, we have tested whether the early effects of ETEC infection can also be evoked and studied in a model in which ETEC is added to whole mucosal tissues ex vivo, and whether this response can be modulated by prior supplementation of the piglets with probiotics.

METHODS

Jejunal barrier and transport properties of Enterococcus faecium-supplemented or control piglets were assessed in Ussing chambers. Part of the epithelia was challenged with an ETEC strain at the mucosal side. Fluxes of fluorescein as a marker of paracellular permeability, and the expression of selected tight junction (TJ) proteins and of proinflammatory cytokines were measured.

RESULTS

The addition of ETEC ex vivo induced an increase in transepithelial resistance peaking in the first 2 h with a concomitant reduction in fluorescein fluxes, indicating tightening effects on barrier function. The response of short-circuit current after stimulation with PGE₂ or glucose was reduced in epithelia treated with ETEC. ETEC induced a decrease in the TJ protein claudin-4 in the control diet group after 280 min and an increase in the mRNA expression of the proinflammatory cytokines interleukin-8 and TNF-α in both groups after 180 min.

CONCLUSIONS

The addition of ETEC ex vivo affected barrier function and transport properties of the jejunal tissues and enhanced cytokine expression. The differences in claudin-4 expression in the jejunum might indicate a beneficial effect of E. faecium prefeeding.

Inhibition of Autophagic Degradation Process Contributes to Claudin-2 Expression Increase and Epithelial Tight Junction Dysfunction in TNF-α Treated Cell Monolayers

Tight junction dysfunction plays a vital role in some chronic inflammatory diseases. Pro-inflammatory cytokines, especially tumor necrosis factor alpha (TNF-α), act as important factors in intestinal epithelial tight junction dysfunction during inflammatory conditions. Autophagy has also been shown to be crucial in tight junction function and claudin-2 expression, but whether autophagy has an effect on the change of claudin-2 expression and tight junction function induced by TNF-α is still unknown. To answer this question, we examined the expression of claudin-2 protein, transepithelial electrical resistance (TER), and permeability of cell monolayers, autophagy flux change, and lysosomal pH after TNF-α with or without PP242 treatment. Our study showed that claudin-2 expression, intestinal permeability, microtubule-associated protein 1 light chain 3B II (LC3B-II) and sequestosome 1 (P62) expression largely increased while TER values decreased in TNF-α treated cell monolayers. Further research using 3-methyladenine (3-MA), bafilomycin A1, and ad-mCherry-GFP-LC3B adenovirus demonstrated that LC3B-II increase induced by TNF-α was attributed to the inhibition of autophagic degradation. Moreover, both qualitative and quantitative method confirmed the increase of lysosomal pH, and mammalian target of rapamycin (mTOR) inhibitor PP242 treatment relieved this elevation. Moreover, PP242 treatment also alleviated the change of autophagy flux, TER, and claudin-2 expression induced by TNF-α. Therefore, we conclude that increase of claudin-2 levels and intestinal epithelial tight junction dysfunction are partly caused by the inhibition of autophagic degradation in TNF-α treated cell monolayers.

Claudin-1 promotes TNF-α-induced epithelial-mesenchymal transition and migration in colorectal adenocarcinoma cells

Epithelial-mesenchymal transition (EMT) is an important mechanism in cancer progression and malignancy including colorectal cancer (CRC). Importantly, inflammatory mediators are critical constituents of the local tumor environment and an intimate link between CRC progression and inflammation is now validated. We and others have reported key role of the deregulated claudin-1 expression in colon carcinogenesis including colitis-associated colon cancer (CAC). However, the causal association between claudin-1 expression and inflammation-induced colon cancer progression remains unclear. Here we demonstrate, TNF-α, a pro-inflammatory cytokine, regulates claudin-1 to modulate epithelial to mesenchymal transition (EMT) and migration in colon adenocarcinoma cells. Importantly, colon cancer cells cultured in the presence of TNF-α (10ng/ml), demonstrated a sharp decrease in E-cadherin expression and an increase in vimentin expression (versus control cells). Interestingly, TNF-α treatment also upregulated (and delocalized) claudin-1 expression in a time-dependent manner accompanied by increase in proliferation and wound healing. Furthermore, similar to our previous observation that claudin-1 overexpression in CRC cells induces ERK1/2 and Src- activation, signaling associated with colon cancer cell survival and transformation, TNF-α-treatment induced upregulation of phospho-ERK1/2 and -Src expression. The shRNA-mediated inhibition of claudin-1 expression largely abrogated the TNF-α-induced changes in EMT, proliferation, migration, p-Erk and p-Src expression. Taken together, our data demonstrate TNF-α mediated regulation of claudin-1 and tumorigenic abilities of colon cancer cells and highlights a key role of deregulated claudin-1 expression in inflammation-induced colorectal cancer growth and progression, through the regulation of the ERK and Src-signaling.