DR3 Regulates Intestinal Epithelial Homeostasis and Regeneration After Intestinal Barrier Injury

BACKGROUND & AIMS

Tumor necrosis factor (TNF) superfamily member tumor necrosis factor-like protein 1A (TL1A) has been associated with the susceptibility and severity of inflammatory bowel diseases. However, the function of the tumor necrosis factor-like protein 1A and its receptor death receptor 3 (DR3) in the development of intestinal inflammation is incompletely understood. We investigated the role of DR3 expressed by intestinal epithelial cells (IECs) during intestinal homeostasis, tissue injury, and regeneration.

METHODS

Clinical phenotype and histologic inflammation were assessed in C57BL/6 (wild-type), Tl1a-/- and Dr3-/- mice in dextran sulfate sodium (DSS)-induced colitis. We generated mice with an IEC-specific deletion of DR3 (Dr3ΔIEC) and assessed intestinal inflammation and epithelial barrier repair. In vivo intestinal permeability was assessed by fluorescein isothiocyanate dextran uptake. Proliferation of IECs was analyzed by bromodeoxyuridine incorporation. Expression of DR3 messenger RNA was assessed by fluorescent in situ hybridization. Small intestinal organoids were used to determine ex vivo regenerative potential.

RESULTS

Dr3-/- mice developed more severe colonic inflammation than wild-type mice in DSS-induced colitis with significantly impaired IEC regeneration. Homeostatic proliferation of IECs was increased in Dr3-/- mice, but blunted during regeneration. Cellular localization and expression of the tight junction proteins Claudin-1 and zonula occludens-1 were altered, leading to increased homeostatic intestinal permeability. Dr3ΔIEC mice recapitulated the phenotype observed in Dr3-/- mice with increased intestinal permeability and IEC proliferation under homeostatic conditions and impaired tissue repair and increased bacterial translocation during DSS-induced colitis. Impaired regenerative potential and altered zonula occludens-1 localization also were observed in Dr3ΔIEC enteroids.

CONCLUSIONS

Our findings establish a novel function of DR3 in IEC homeostasis and postinjury regeneration independent of its established role in innate lymphoid cells and T-helper cells.

Gut Epithelial Inositol Polyphosphate Multikinase Alleviates Experimental Colitis via Governing Tuft Cell Homeostasis

BACKGROUND & AIMS

Inositol polyphosphate multikinase (IPMK), an essential enzyme for inositol phosphate metabolism, has been known to mediate major biological events such as growth. Recent studies have identified single-nucleotide polymorphisms in the IPMK gene associated with inflammatory bowel disease predisposition. Therefore, we aimed to investigate the functional significance of IPMK in gut epithelium.

METHODS

We generated intestinal epithelial cell (IEC)-specific Ipmk knockout (IPMK^ΔIEC) mice, and assessed their vulnerability against dextran sulfate sodium-induced experimental colitis. Both bulk and single-cell RNA sequencing were performed to analyze IPMK-deficient colonic epithelial cells and colonic tuft cells.

RESULTS

Although IPMK^ΔIEC mice developed normally and showed no intestinal abnormalities during homeostasis, Ipmk deletion aggravated dextran sulfate sodium-induced colitis, with higher clinical colitis scores, and increased epithelial barrier permeability. Surprisingly, Ipmk deletion led to a significant decrease in the number of tuft cells without influencing other IECs. Single-cell RNA sequencing of mouse colonic tuft cells showed 3 distinct populations of tuft cells, and further showed that a transcriptionally inactive population was expanded markedly in IPMK^ΔIEC mice, while neuronal-related cells were relatively decreased.

CONCLUSIONS

Cholinergic output from tuft cells is known to be critical for the restoration of intestinal architecture upon damage, supporting that tuft cell-defective IPMK^ΔIEC mice are more prone to colitis. Thus, intestinal epithelial IPMK is a critical regulator of colonic integrity and tissue regeneration by determining tuft cell homeostasis and affecting cholinergic output.

The Glucocorticoid Receptor in Intestinal Epithelial Cells Alleviates Colitis and Associated Colorectal Cancer in Mice

BACKGROUND & AIMS

Inflammatory bowel disease is commonly treated by administration of glucocorticoids. While the importance of intestinal epithelial cells for the pathogenesis of this disorder is widely accepted, their role as target cells for glucocorticoids has not been explored. To address this issue, we induced colonic inflammation in GR^villin mice, which carry an inducible deletion of the glucocorticoid receptor in intestinal epithelial cells.

METHODS

Colitis and colitis-associated colorectal cancer were induced by administration of dextran sulfate sodium and azoxymethane in mice. Clinical parameters, epithelial permeability and tumor development were monitored during disease progression. Colon tissue, lamina propria cells and intestinal epithelial cells were examined by gene expression analyses, flow cytometry, histopathology, and immunohistochemistry.

RESULTS

The absence of the intestinal epithelial glucocorticoid receptor aggravated clinical symptoms and tissue damage, and compromised epithelial barrier integrity during colitis. Gene expression of chemokines, pattern recognition receptors and molecules controlling epithelial permeability was dysregulated in intestinal epithelial cells of GR^villin mice, leading to a reduced recruitment and a hyperactivation of leukocytes in the lamina propria of the colon. Importantly, the exaggerated inflammatory response in GR^villin mice also enhanced associated tumorigenesis, resulting in a higher number and larger size of tumors in the colon.

CONCLUSIONS

Our results reveal an important role of intestinal epithelial cells as targets of glucocorticoid action in inflammatory bowel disease and suggest that the efficacy with which colitis is kept at bay directly affects the progression of colorectal cancer.

Deficiency of Stomach-Type Claudin-18 in Mice Induces Gastric Tumor Formation Independent of H pylori Infection

BACKGROUND & AIMS

Epithelial cells are joined by tight junctions (TJs) to form a cell sheet. In the stomach, epithelial cell sheet forms an essential barrier against gastric material, including gastric acid. Although the decreased expression of stomach-type claudin-18 (stCldn18), a TJ protein, is generally observed in human gastritis and gastric cancer, its pathological roles are not fully understood. We previously reported that mice lacking stCldn18 (stCldn18-/-) exhibit gastric acid leakage through TJs, which induces active gastritis at a young age. Here, we examined the gastric pathologies in mice after long-term stCldn18 deficiency.

METHODS

The gastric pathologies in stCldn18-/- mice were sequentially analyzed from youth to old age, and compared to those in humans. To examine the relationship between stCldn18 deficiency-induced gastric pathologies and Wnt-dependent tumorigenesis, we generated Wnt1-overexpressing stCldn18-/- mice.

RESULTS

StCldn18-/- mice developed chronic active gastritis at middle age, with expression of the chemoattractant CCL28. At old age, 20-30% of these mice developed gastric tumors with CXCL5 expression, indicative of EMT. In this process, spasmolytic polypeptide-expressing metaplasia (SPEM) cells appeared. Increased expressions of CD44-variants, TLR2, and CXCL5 indicated age-dependent changes in cell characteristics. Some features of the stCldn18-/- mouse gastric tumorigenesis resembled H pylori-infection-related human carcinogenesis. The gastric tumorigenesis was accelerated in Wnt1-overexpressing stCldn18-/- mice, indicating that Wnt is involved in the stCldn18-/- mouse gastric tumorigenesis.

CONCLUSIONS

StCldn18 deficiency induced gastric tumorigenesis in mice without H pylori infection. Our findings revealed that several signaling networks, including the cytokine-, stemness-, and Wnt-signaling pathways, may be activated under the stCldn18-deficiency-induced chronic active gastritis to accelerate the gastric tumorigenesis.

Claudin-3 regulates bile canalicular paracellular barrier and cholesterol gallstone core formation in mice

BACKGROUND & AIMS

Most cholesterol gallstones have a core consisting of inorganic and/or organic calcium salts, although the mechanisms of core formation are poorly understood. We examined whether the paracellular permeability of ions at hepatic tight junctions is involved in the core formation of cholesterol gallstones, with particular interest in the role of phosphate ion, a common food additive and preservative.

METHODS

We focused on claudin-3 (Cldn3), a paracellular barrier-forming tight junction protein whose expression in mouse liver decreases with age. Since Cldn3-knockout mice exhibited gallstone diseases, we used them to assess the causal relationship between paracellular phosphate ion permeability and the core formation of cholesterol gallstones.

RESULTS

In the liver of Cldn3-knockout mice, the paracellular phosphate ion permeability through hepatic tight junctions was significantly increased, resulting in calcium phosphate core formation. Cholesterol overdose caused cholesterol gallstone disease in these mice.

CONCLUSION

We revealed that in the hepatobiliary system, Cldn3 functions as a paracellular barrier for phosphate ions, to help maintain biliary ion homeostasis. We provide in vivo evidence that elevated phosphate ion concentrations play a major role in the lifestyle- and age-related risks of developing cholesterol gallstone disease under cholesterol overdose.

LAY SUMMARY

Herein, we reveal a new mechanism for cholesterol gallstone formation, in which increased paracellular phosphate ion permeability across hepatobiliary epithelia causes calcium phosphate core formation and cholesterol gallstones. Thus, altered phosphate ion metabolism under cholesterol overdose plays a major role in the lifestyle- and age-related risks of developing cholesterol gallstone disease.

Increased Pancreatic Protease Activity in Response to Antibiotics Impairs Gut Barrier and Triggers Colitis

Background & Aims

Antibiotic (ABx) therapy is associated with increased risk for Crohn's disease but underlying mechanisms are unknown. We observed high fecal serine protease activity (PA) to be a frequent side effect of ABx therapy. The aim of the present study was to unravel whether this rise in large intestinal PA may promote colitis development via detrimental effects on the large intestinal barrier.

Methods

Transwell experiments were used to assess the impact of high PA in ABx-treated patients or vancomycin/metronidazole-treated mice on the epithelial barrier. Serine protease profiling was performed using liquid chromatography-mass spectrometry/mass spectrometry analysis. The impact of high large intestinal PA on the intestinal barrier in wild-type and interleukin (IL)10-/- mice and on colitis development in IL10-/- mice was investigated using vancomycin/metronidazole with or without oral serine protease inhibitor (AEBSF) treatment.

Results

The ABx-induced, high large intestinal PA was caused by significantly increased levels of pancreatic proteases and impaired epithelial barrier integrity. In wild-type mice, the rise in PA caused a transient increase in intestinal permeability but did not affect susceptibility to chemically induced acute colitis. In IL10-/- mice, increased PA caused a consistent impairment of the intestinal barrier associated with inflammatory activation in the large intestinal tissue. In the long term, the vancomycin/metronidazole-induced lasting increase in PA aggravated colitis development in IL10-/- mice.

Conclusions

High large intestinal PA is a frequent adverse effect of ABx therapy, which is detrimental to the large intestinal barrier and may contribute to the development of chronic intestinal inflammation in susceptible individuals.

Enteric Glia Regulate Gastrointestinal Motility but Are Not Required for Maintenance of the Epithelium in Mice

BACKGROUND & AIMS

When the glial fibrillary acidic protein (GFAP) promoter is used to express cellular toxins that eliminate glia in mice, intestinal epithelial permeability and proliferation increase; this led to the concept that glia are required for maintenance of the gastrointestinal epithelium. Many enteric glia, however, particularly in the mucosa, do not express GFAP. In contrast, virtually all enteric glia express proteolipid protein 1 (PLP1). We investigated whether elimination of PLP1-expressing cells compromises epithelial maintenance or gastrointestinal motility.

METHODS

We generated mice that express tamoxifen-inducible Cre recombinase under control of the Plp1 promoter and carry the diptheria toxin subunit A (DTA) transgene in the Rosa26 locus (Plp1^CreER;Rosa26^DTA mice). In these mice, PLP1-expressing glia are selectively eliminated without affecting neighboring cells. We measured epithelial barrier function and gastrointestinal motility in these mice and littermate controls, and analyzed epithelial cell proliferation and ultrastructure from their intestinal tissues. To compare our findings with those from previous studies, we also eliminated glia with ganciclovir in Gfap^HSV-TK mice.

RESULTS

Expression of DTA in PLP1-expressing cells selectively eliminated enteric glia from the small and large intestines, but caused no defects in epithelial proliferation, barrier integrity, or ultrastructure. In contrast, administration of ganciclovir to Gfap^HSV-TK mice eliminated fewer glia but caused considerable non-glial toxicity and epithelial cell death. Elimination of PLP1-expressing cells did not reduce survival of neurons in the intestine, but altered gastrointestinal motility in female, but not male, mice.

CONCLUSIONS

Using the Plp1 promoter to selectively eliminate glia in mice, we found that enteric glia are not required for maintenance of the intestinal epithelium, but are required for regulation of intestinal motility in females. Previous observations supporting the concept that maintenance of the intestinal epithelium requires enteric glia can be attributed to non-glial toxicity in Gfap^HSV-TK mice and epithelial-cell expression of GFAP. Contrary to widespread notions, enteric glia are therefore not required for epithelial homeostasis. However, they regulate intestinal motility in a sex-dependent manner.

Hypoxia Inducible Factor (HIF) Hydroxylases as Regulators of Intestinal Epithelial Barrier Function

Human health is dependent on the ability of the body to extract nutrients, fluids, and oxygen from the external environment while at the same time maintaining a state of internal sterility. Therefore, the cell layers that cover the surface areas of the body such as the lung, skin, and gastrointestinal mucosa provide vital semipermeable barriers that allow the transport of essential nutrients, fluid, and waste products, while at the same time keeping the internal compartments free of microbial organisms. These epithelial surfaces are highly specialized and differ in their anatomic structure depending on their location to provide appropriate and effective site-specific barrier function. Given this important role, it is not surprising that significant disease often is associated with alterations in epithelial barrier function. Examples of such diseases include inflammatory bowel disease, chronic obstructive pulmonary disease, and atopic dermatitis. These chronic inflammatory disorders often are characterized by diminished tissue oxygen levels (hypoxia). Hypoxia triggers an adaptive transcriptional response governed by hypoxia-inducible factors (HIFs), which are repressed by a family of oxygen-sensing HIF hydroxylases. Here, we review recent evidence suggesting that pharmacologic hydroxylase inhibition may be of therapeutic benefit in inflammatory bowel disease through the promotion of intestinal epithelial barrier function through both HIF-dependent and HIF-independent mechanisms.

α-Haemolysin of Escherichia coli in IBD: a potentiator of inflammatory activity in the colon

OBJECTIVE

α-Haemolysin (HlyA) influences host cell ionic homeostasis and causes concentration-dependent cell lysis. As a consequence, HlyA-producing Escherichia coli is capable of inducing 'focal leaks' in colon epithelia, through which bacteria and antigens translocate. This study addressed the role of HlyA as a virulence factor in the pathogenesis of colitis according to the 'leaky gut' concept.

DESIGN

To study the action of HlyA in the colon, we performed oral administration of HlyA-expressing E coli-536 and its isogenic α-haemolysin-deficient mutant (HDM) in three mouse models: wild type, interleukin-10 knockout mice (IL-10(-/-)) and monoassociated mice. Electrophysiological properties of the colonised colon were characterised in Ussing experiments. Inflammation scores were evaluated and focal leaks in the colon were assessed by confocal laser-scanning microscopy. HlyA quantity in human colon biopsies was measured by quantitative PCR.

RESULTS

All three experimental mouse models infected with HlyA-producing E coli-536 showed an increase in focal leak area compared with HDM. This was associated with a decrease in transepithelial electrical resistance and an increase in macromolecule uptake. As a consequence, inflammatory activity index was increased to a higher degree in inflammation-prone mice. Mucosal samples from human colon were E coli HlyA-positive in 19 of 22 patients with ulcerative colitis, 9 of 9 patients with Crohn's disease and 9 of 12 healthy controls. Moreover, focal leaks were found together with 10-fold increased levels of HlyA in active ulcerative colitis.

CONCLUSIONS

E coli HlyA impairs intestinal barrier function via focal leak induction in the epithelium, thereby intensifying antigen uptake and triggering intestinal inflammation in vulnerable mouse models. Therefore, HlyA-expressing E coli strains should be considered as potential cofactors in the pathogenesis of intestinal inflammation.

Proton pump inhibitors partially restore mucosal integrity in patients with proton pump inhibitor-responsive esophageal eosinophilia but not eosinophilic esophagitis

BACKGROUND & AIMS

Histologic analysis is used to distinguish patients with proton pump inhibitor-responsive eosinophilia (PPI-REE) from those with eosinophilic esophagitis (EoE). It is not clear whether these entities have different etiologies. Exposure to acid reflux can impair the integrity of the esophageal mucosal. We proposed that patients with EoE and PPI-REE might have reflux-induced esophageal mucosal damage that promotes transepithelial flux of allergens. We therefore assessed the integrity of the esophageal mucosal in these patients at baseline and after PPI.

METHODS

We performed a prospective study of 16 patients with suspected EoE and 11 controls. Patients had dysphagia, endoscopic signs of EoE, and esophageal eosinophilia (>15 eosinophils/high-power field [eos/hpf]). All subjects underwent endoscopy at baseline; endoscopy was performed again on patients after 8 weeks of treatment with high-dose esomeprazole. After PPI treatment, patients were diagnosed with EoE (>10 eos/hpf; n = 8) or PPI-REE (≤10 eos/hpf; n = 8). We evaluated the structure (intercellular spaces) and function (electrical tissue impedance, transepithelial electrical resistance, transepithelial molecule flux) of the esophageal mucosal barrier.

RESULTS

Compared with controls, electrical tissue impedance and transepithelial electrical resistance were reduced in patients with EoE (P < .001 and P < .001, respectively) and PPI-REE (P = .01 and P = .06, respectively), enabling transepithelial small-molecule flux. PPI therapy partially restored these changes in integrity and inflammation in patients with PPI-REE, but not in those with EoE.

CONCLUSIONS

The integrity of the esophageal mucosa is impaired in patients with EoE and PPI-REE, allowing transepithelial transport of small molecules. PPI therapy partially restores mucosal integrity in patients with PPI-REE, but not in those with EoE. Acid reflux might contribute to transepithelial allergen flux in patients with PPI-REE. Trialregister.nl number: NTR3480.

Psychological stress and corticotropin-releasing hormone increase intestinal permeability in humans by a mast cell-dependent mechanism

OBJECTIVE

Intestinal permeability and psychological stress have been implicated in the pathophysiology of IBD and IBS. Studies in animals suggest that stress increases permeability via corticotropin-releasing hormone (CRH)-mediated mast cell activation. Our aim was to investigate the effect of stress on intestinal permeability in humans and its underlying mechanisms.

DESIGN

Small intestinal permeability was quantified by a 2 h lactulose-mannitol urinary excretion test. In a first study, 23 healthy volunteers were subjected to four different conditions: control; indomethacin; public speech and anticipation of electroshocks. In a second study, five test conditions were investigated in 13 volunteers: control; after pretreatment with disodium cromoglycate (DSCG); administration of CRH; DSCG+CRH and DSCG+public speech.

RESULTS

Indomethacin, as a positive comparator (0.071±0.040 vs 0.030±0.022; p<0.0001), and public speech (0.059±0.040; p<0.01), but not the shock protocol increased intestinal permeability. Similarly, salivary cortisol was only increased after public speech. Subgroup analysis demonstrated that the effect of public speech on permeability was only present in subjects with a significant elevation of cortisol. CRH increased the lactulose-mannitol ratio (0.042±0.021 vs 0.028±0.009; p=0.02), which was inhibited by the mast cell stabiliser DSCG. Finally, intestinal permeability was unaltered by public speech with DSCG pretreatment.

CONCLUSIONS

Acute psychological stress increases small intestinal permeability in humans. Peripheral CRH reproduces the effect of stress and DSCG blocks the effect of both stress and CRH, suggesting the involvement of mast cells. These findings provide new insight into the complex interplay between the central nervous system and GI function in man.

Neutralisation of the interleukin-33/ST2 pathway ameliorates experimental colitis through enhancement of mucosal healing in mice

OBJECTIVE

Inflammatory bowel diseases (IBD) have been intrinsically linked to a deregulated cytokine network, but novel therapeutic principles are urgently needed. Here we identify the interleukin (IL)-33 and its receptor ST2 as key negative regulators of wound healing and permeability in the colon of mice.

DESIGN

Expression of IL-33 and ST2 was determined by qRT-PCR, ELISA, immunohistochemistry and western-blot analysis. Wild-type and St2(-/-) mice were used in wound healing experiments and in two experimental models of IBD triggered by 2,4,6-trinitrobenzene sulphonic acid or dextran sodium sulphate (DSS). Neutralisation of ST2 was performed by using a specific blocking antibody.

RESULTS

Nuclear localisation and enhanced expression of IL-33 in myofibroblasts and enterocytes was linked to disease involvement independently of inflammation, while the expression of ST2 was primarily restricted to the colonic epithelia. In two experimental models of IBD, genetic ablation of ST2 significantly improved signs of colitis, while a sustained epithelial expression of the cyto-protective factor connexin-43 was observed in DSS-treated St2-deficient mice. Unexpectedly, absence of ST2 in non-hematopoietic cells was sufficient to protect against colitis. Consistently, specific inhibition of endogenous ST2-mediated signalling by treatment with neutralising antibody improved DSS-induced colitis. In addition, IL-33 treatment impaired epithelial barrier permeability in vitro and in vivo, whereas absence of ST2 enhanced wound healing response upon acute mechanical injury in the colon.

CONCLUSIONS

Our study unveiled a novel non-hematopoietic function of IL-33 in epithelial barrier function and wound healing. Therefore, blocking the IL-33/ST2 axis may represent an efficient therapy in IBD.

Butyricicoccus pullicaecorum in inflammatory bowel disease

OBJECTIVE

Many species within the phylum Firmicutes are thought to exert anti-inflammatory effects. We quantified bacteria belonging to the genus Butyricicoccus in stools of patients with ulcerative colitis (UC) and Crohn's disease (CD). We evaluated the effect of Butyricicoccus pullicaecorum in a rat colitis model and analysed the ability to prevent cytokine-induced increases in epithelial permeability.

DESIGN

A genus-specific quantitative PCR was used for quantification of Butyricicoccus in stools from patients with UC or CD and healthy subjects. The effect of B pullicaecorum on trinitrobenzenesulfonic (TNBS)-induced colitis was assessed and the effect of B pullicaecorum culture supernatant on epithelial barrier function was investigated in vitro.

RESULTS

The average number of Butyricicoccus in stools from patients with UC and CD in active (UC: 8.61 log10/g stool; CD: 6.58 log10/g stool) and remission phase (UC: 8.69 log10/g stool; CD: 8.38 log10/g stool) was significantly lower compared with healthy subjects (9.32 log10/g stool) and correlated with disease activity in CD. Oral administration of B pullicaecorum resulted in a significant protective effect based on macroscopic and histological criteria and decreased intestinal myeloperoxidase (MPO), tumour necrosis factor α (TNFα) and interleukin (IL)-12 levels. Supernatant of B pullicaecorum prevented the loss of transepithelial resistance (TER) and the increase in IL-8 secretion induced by TNFα and interferon γ (IFN gamma) in a Caco-2 cell model.

CONCLUSIONS

Patients with inflammatory bowel disease have lower numbers of Butyricicoccus bacteria in their stools. Administration of B pullicaecorum attenuates TNBS-induced colitis in rats and supernatant of B pullicaecorum cultures strengthens the epithelial barrier function by increasing the TER.

TNFR2 activates MLCK-dependent tight junction dysregulation to cause apoptosis-mediated barrier loss and experimental colitis

BACKGROUND & AIMS

Tight junction dysregulation and epithelial damage contribute to barrier loss in patients with inflammatory bowel disease. However, the mechanisms that regulate these processes and their relative contributions to disease pathogenesis are not completely understood. We investigated these processes using colitis models in mice.

METHODS

We induced colitis by adoptive transfer of CD4(+)CD45RB(hi) cells or administration of dextran sulfate sodium to mice, including those deficient in tumor necrosis factor receptor (TNFR) 1, TNFR2, or the long isoform of myosin light chain kinase (MLCK). Intestinal tissues and isolated epithelial cells were analyzed by immunoblot, immunofluorescence, enzyme-linked immunosorbent assay, and real-time polymerase chain reaction assays.

RESULTS

Induction of immune-mediated colitis by CD4(+)CD45RB(hi) adoptive transfer increased intestinal permeability, epithelial expression of claudin-2, the long isoform of MLCK, and TNFR2 (but not TNFR1) and phosphorylation of the myosin II light chain. Long MLCK upregulation, myosin II light chain phosphorylation, barrier loss, and weight loss were attenuated in TNFR2(-/-) , but not TNFR1(-/-) , recipients of wild-type CD4(+)CD45RB(hi) cells. Similarly, long MLCK(-/-) mice had limited increases in myosin II light chain phosphorylation, claudin-2 expression, and intestinal permeability and delayed onset of adoptive transfer-induced colitis. However, coincident with onset of epithelial apoptosis, long MLCK(-/-) mice ultimately developed colitis. This indicates that disease progresses via apoptosis in the absence of MLCK-dependent tight junction regulation. In support of this conclusion, long MLCK(-/-) mice were not protected from epithelial apoptosis-mediated, damage-dependent dextran sulfate sodium colitis.

CONCLUSIONS

In immune-mediated inflammatory bowel disease models, TNFR2 signaling increases long MLCK expression, resulting in tight junction dysregulation, barrier loss, and induction of colitis. At advanced stages, colitis progresses by apoptosis and mucosal damage that result in tight junction- and MLCK-independent barrier loss. Therefore, barrier loss in immune-mediated colitis occurs via two temporally and morphologically distinct mechanisms. Differential targeting of these mechanisms can lead to improved inflammatory bowel disease therapies.