Resveratrol Prevents Campylobacter jejuni-Induced Leaky gut by Restoring Occludin and Claudin-5 in the Paracellular Leak Pathway

Campylobacter jejuni is a bacterial human pathogen causing gastroenteritis and sequelae like irritable bowel syndrome. Epidemiologists count the human campylobacteriosis by C. jejuni as the most common foodborne zoonosis and bacterial diarrheal disease worldwide. Based on bioinformatics predictions for potential protective compounds in campylobacteriosis, the question was raised whether the plant-based polyphenol resveratrol is sufficient to attenuate intestinal epithelial damage induced by C. jejuni. We investigated this by performing experimental infection studies in an epithelial cell culture and the secondary abiotic IL-10-/- mouse model. In C. jejuni-infected human colonic HT-29/B6 cell monolayers, transepithelial electrical resistance (TER) was decreased and the paracellular marker flux of fluorescein (332 Da) increased. Concomitantly, the tight junction (TJ) proteins occludin and claudin-5 were re-distributed off the tight junction domain. This was accompanied by an increased induction of epithelial apoptosis, both changes contributing to compromised barrier function and the opening of the leak pathway induced by C. jejuni. In parallel, the recovery experiments with the application of resveratrol revealed a functional improvement of the disturbed epithelial barrier in both models in vitro and in vivo. During treatment with resveratrol, TJ localization of occludin and claudin-5 was fully restored in the paracellular domain of HT-29/B6 cells. Moreover, resveratrol decreased the rate of epithelial apoptosis. These resveratrol-induced molecular and cellular effects would therefore be expected to improve epithelial barrier function, thereby minimizing the so-called leaky gut phenomenon. In conclusion, the induction of the leak pathway by C. jejuni and the restoration of barrier function by resveratrol demonstrates its effectiveness as a potential preventive or therapeutic method of mitigating the leaky gut associated with campylobacteriosis.

Campylobacter jejuni impairs sodium transport and epithelial barrier function via cytokine release in human colon

This corrects the article DOI: 10.1038/mi.2017.66.

Architectural and functional alterations of the small intestinal mucosa in classical Whipple's disease

Classical Whipple's disease (CWD) affects the gastrointestinal tract and rather elicits regulatory than inflammatory immune reactions. Mechanisms of malabsorption, diarrhea, and systemic immune activation are unknown. We here analyzed mucosal architecture, barrier function, and immune activation as potential diarrheal trigger in specimens from 52 CWD patients. Our data demonstrate villus atrophy and crypt hyperplasia associated with epithelial apoptosis and reduced alkaline phosphatase expression in the duodenum of CWD patients. Electrophysiological and flux experiments revealed increased duodenal permeability to small solutes and macromolecules. Duodenal architecture and permeability ameliorated upon antibiotic treatment. Structural correlates for these alterations were concordant changes of membranous claudin-1, claudin-2, claudin-3, and tricellulin expression. Tumor necrosis factor-α and interleukin-13 were identified as probable mediators of epithelial apoptosis, and altered tight junction expression. Increased serum markers of microbial translocation and their decline following treatment corroborated the biological significance of the mucosal barrier defect. Hence, mucosal immune responses in CWD elicit barrier dysfunction. Diarrhea is caused by loss of absorptive capacity and leak flux of ions and water. Downregulation of tricellulin causes increased permeability to macromolecules and subsequent microbial translocation contributes to systemic inflammation. Thus, therapeutic strategies to reconstitute the mucosal barrier and control inflammation could assist symptomatic control of CWD.

Claudin-2 as a mediator of leaky gut barrier during intestinal inflammation

The epithelial tight junction determines the paracellular water and ion movement in the intestine and also prevents uptake of larger molecules, including antigens, in an uncontrolled manner. Claudin-2, one of the 27 mammalian claudins regulating that barrier function, forms a paracellular channel for small cations and water. It is typically expressed in leaky epithelia like proximal nephron and small intestine and provides a major pathway for the paracellular transport of sodium, potassium, and fluid. In intestinal inflammation (Crohn's disease, ulcerative colitis), immune-mediated diseases (celiac disease), and infections (HIV enteropathy), claudin-2 is upregulated in small and large intestine and contributes to diarrhea via a leak flux mechanism. In parallel to that upregulation, other epithelial and tight junctional features are altered and the luminal uptake of antigenic macromolecules is enhanced, for which claudin-2 may be partially responsible through induction of tight junction strand discontinuities.

TcpC protein from E. coli Nissle improves epithelial barrier function involving PKCζ and ERK1/2 signaling in HT-29/B6 cells

The probiotic Escherichia coli Nissle 1917 (EcN) is widely used to maintain remission in ulcerative colitis. This is thought to be mediated by various immunomodulatory and barrier-stabilizing effects in the intestine. In this study, the mechanisms of barrier modulation by EcN were studied in the human epithelial HT-29/B6 cell culture model.EcN supernatant increased transepithelial resistance (TER) and reduced permeability to mannitol because of sealing of the paracellular passage pathway as revealed by two-path impedance spectroscopy. This increase in TER was attributed to the TcpC protein of EcN. TcpC induced protein kinase C-ζ (PKCζ) and extracellular-signal-regulated kinase 1/2 (ERK1/2) phosphorylation, which in turn resulted in upregulation of the barrier-forming tight junction protein claudin-14. By specific silencing of protein expression by small interfering RNA (siRNA), the sealing function of claudin-14 was confirmed. In conclusion, the TcpC protein of EcN affects innate immunity by improving intestinal barrier function through upregulation of claudin-14 via PKCζ and ERK1/2 signaling.

Prostaglandin I(2) sensory input into the enteric nervous system during distension-induced colonic chloride secretion in rat colon

AIM

Intestinal pressure differences or experimental distension induce ion secretion via the enteric nervous system, the sensorial origin of which is only poorly understood. This study aimed to investigate sensorial inputs and the role of afferent and interneurones in mechanically activated submucosal secretory reflex circuits.

METHODS

Distension-induced rheogenic chloride secretion was measured as increase in short-circuit current 10 min after distension (DeltaI(SC)(10); distension parameters +/- 100 microL, 2 Hz, 20 s) in partially stripped rat distal colon in the Ussing-chamber in vitro. PGE(2) and PGI(2) were measured by radioimmunoassay.

RESULTS

DeltaI(SC)(10) was 2.0 +/- 0.2 micromol h(-1) cm(-2) and could be attenuated by lobeline, mecamylamine and dimethylphenylpiperazine, indicating an influence of nicotinergic interneurones. Additionally, a contribution of afferent neurones was indicated from the short-term potentiation of DeltaI(SC)(10) by capsaicin (1 microm). As evidence for its initial event, indomethacin (1 microm) inhibited distension-induced secretion and the release of PGI(2) was directly detected after distension. Furthermore, serotoninergic mediation was confirmed by granisetron (100 microm) which was functionally localized distally to PGI(2) in this reflex circuit, as granisetron inhibited an iloprost-induced I(SC), while indomethacin did not affect serotonin-activated ion secretion.

CONCLUSIONS

Distension-induced active electrogenic chloride secretion in rat colon is mediated by a neuronal reflex circuit which includes afferent neurones and nicotinergic interneurones. It is initiated by distension-induced PGI(2) release from subepithelial cells triggering this reflex via serotoninergic 5-HT(3) receptor transmission. Functionally, this mechanism may help to protect against intestinal stasis but could also contribute to luminal fluid loss, e.g. during intestinal obstruction.

TNFalpha up-regulates claudin-2 expression in epithelial HT-29/B6 cells via phosphatidylinositol-3-kinase signaling

Our aim has been to characterize the molecular mechanisms regulating the expression of the channel-forming tight-junctional protein claudin-2 in response to the pro-inflammatory cytokine tumor necrosis factor-alpha (TNFalpha), which is elevated, for example, in active Crohn's disease. TNFalpha caused an 89% decrease of the paracellular resistance in colonic HT-29/B6 cells, whereas transcellular resistance was unaltered. The claudin-2 protein level was increased by TNFalpha without changes in subcellular tight-junctional protein localization as revealed by confocal laser scanning microscopy. Enhanced gene expression was identified as the source of this increase, since claudin-2-specific mRNA and promoter activity was elevated, whereas mRNA stability remained unaltered. Specific inhibitors and phospho-specific antibodies revealed that the increased gene expression of claudin-2 after TNFalpha treatment was mediated by the phosphatidylinositol-3-kinase pathway. Thus, the up-regulation of claudin-2 by TNFalpha is attributable to the regulation of the expression of the gene, as a result of which epithelial barrier function is disturbed, for example, during chronic intestinal inflammation.

Mechanisms of epithelial translocation of the alpha(2)-gliadin-33mer in coeliac sprue

BACKGROUND AND AIMS

The alpha(2)-gliadin-33mer has been shown to be important in the pathogenesis of coeliac disease. We aimed to study mechanisms of its epithelial translocation and processing in respect to transcytotic and paracellular pathways.

METHODS

Transepithelial passage of a fluorescence-labelled alpha(2)-gliadin-33mer was studied in Caco-2 cells by using reverse-phase high-performance liquid chromatography, mass spectrometry, confocal laser scanning microscopy (LSM) and fluorescence activated cell sorting (FACS). Endocytosis mechanisms were characterised with rab-GFP constructs transiently transfected into Caco-2 cells and in human duodenal biopsy specimens.

RESULTS

The alpha(2)-gliadin-33mer dose-dependently crossed the epithelial barrier in the apical-to-basal direction. Degradation analysis revealed translocation of the 33mer polypeptide in the uncleaved as well as in the degraded form. Transcellular passage was identified by confocal LSM, inhibitor experiments and FACS. Rab5 but not rab4 or rab7 vesicles were shown to be part of the transcytotic pathway. After pre-incubation with interferon-gamma, translocation of the 33mer was increased by 40%. In mucosal biopsies of the duodenum, epithelial 33mer uptake was significantly higher in untreated coeliac disease patients than in healthy controls or coeliac disease patients on a gluten-free diet.

CONCLUSION

Epithelial translocation of the alpha(2)-gliadin-33mer occurs by transcytosis after partial degradation through a rab5 endocytosis compartment and is regulated by interferon-gamma. Uptake of the 33mer is higher in untreated coeliac disease than in controls and coeliac disease patients on a gluten-free diet.

Effects of endotoxaemia on markers of permeability, metabolism and inflammation in the large bowel of healthy subjects

BACKGROUND

Increased permeability and increased luminal concentrations of L-lactate have previously been shown in the large bowel in septic patients. To advance these observations, a human model of colorectal barrier failure in sepsis is desirable. Therefore, we assessed the effects of endotoxaemia on markers of permeability, metabolism and inflammation in the large bowel in healthy subjects.

METHODS

Twelve healthy male subjects received intravenous endotoxin (2 ng/kg body weight) or saline in a paired cross-over design. Colorectal permeability was assessed after 3, 6, 9 and 12 h by the systemic recovery of luminally instilled (99m)Tc-diethylenetriaminepentaacetate. Luminal concentrations of L-lactate were assessed by equilibrium dialysis. Mucosal biopsies from the large bowel were sampled after 6 and 12 h, and the apoptotic ratio of the epithelium was assessed by terminal deoxynucleotidyl transferase-mediated desoxyuridinetriphosphate nick end-labelling (TUNEL) assay and the expression of inducible nitric oxide synthase (iNOS) mRNA by reverse transcriptase-polymerase chain reaction.

RESULTS

Systemic effects of endotoxaemia were observed, including fever, tachycardia and strongly increased plasma values of tumour necrosis factor-alpha. By contrast, the colorectal permeability, luminal lactate concentrations, mucosal infiltration of inflammatory cells, epithelial apoptotic ratio and expression of iNOS were all unaffected by endotoxin.

CONCLUSIONS

No effect of a single intravenous dose of endotoxin was observed on markers of large bowel permeability, metabolism and inflammation in healthy subjects. This suggests that this part of the gut is relatively resistant to the systemic inflammation induced by experimental endotoxaemia in humans.

Restitution of single-cell defects in the mouse colon epithelium differs from that of cultured cells

Integrity of colon epithelium is of crucial importance and, as small defects occur constantly, rapid repair (restitution) is essential. To investigate the mechanism of restitution, single-cell lesions were induced in mouse colonic surface epithelia by iontophoretic injection of Ca2+. Closure of the resulting defects was monitored using confocal laser scanning microscopy (CLSM), and functional sealing by electrophysiological techniques. Restitution was evaluated as the time constant tau of the exponential decrease in conductance of an induced leak and amounted to 0.28 min under control conditions. After 4 min, the leak was completely sealed. Repair was thus considerably faster than in previously investigated HT-29/B6 cells (tau=5.73 min). As in cultured cells, cytochalasin D delayed restitution in native colon epithelia (tau=0.69 min), indicating the involvement of actin in the healing process; however, no accumulation of actin surrounding the lesion was detected. Long-term incubation of epithelia with IFN-gamma alone or in combination with TNF-alpha increased tau to 0.49 and 0.59 min, respectively. In contrast to cultured cells, TNF-alpha alone did not affect restitution. A brief (<10 min) exposure to the sterile filtered supernatant of hemolytic E. coli O4 cultures did not affect the morphology of the epithelium, but delayed restitution. In CLSM studies, defects were still clearly visible 4 min after the onset of lesion induction. The supernatant of a nonhemolytic E. coli O4 mutant did not exhibit this effect. In conclusion, single-cell defects in native colon cause functional leaks that seal faster than in cell cultures. Proinflammatory cytokines and pathogenic bacteria delay restitution. This suggests a key role of very small lesions at the onset of pathogenic processes in the intestine.

Epithelial transport and barrier function in occludin-deficient mice

BACKGROUND AND AIMS

This study aimed at functional characterization of the tight junction protein occludin using the occludin-deficient mouse model.

METHODS

Epithelial transport and barrier functions were characterized in Ussing chambers. Impedance analysis revealed the ionic permeability of the epithelium (Re, epithelial resistance). Conductance scanning differentiated transcellular (Gc) and tight junctional conductance (Gtj). The pH-stat technique quantified gastric acid secretion.

RESULTS

In occludin+/+ mice, Re was 23+/-5 Omega cm2 in jejunum, 66+/-5 Omega cm2 in distal colon and 33+/-6 Omega cm2 in gastric corpus and was not altered in heterozygotic occludin+/- or homozygotic occludin-/- mice. Additionally, [3H]mannitol fluxes were unaltered. In the control colon, Gc and Gtj were 7.6+/-1.0 and 0.3+/-0.1 mS/cm2 and not different in occludin deficiency. Epithelial resistance after mechanical perturbation or EGTA exposition (low calcium switch) was not more affected in occludin-/- mice than in control. Barrier function was measured in the urinary bladder, a tight epithelium, and in the stomach. Control Rt was 5.8+/-0.8 kOmega cm2 in urinary bladder and 33+/-6 Omega cm2 in stomach and not altered in occludin-/- mice. In gastric corpus mucosa, the glandular structure exhibited a complete loss of parietal cells and mucus cell hyperplasia, as a result of which acid secretion was virtually abolished in occludin-/- mice.

CONCLUSION

Epithelial barrier characterization in occludin-deficiency points against an essential barrier function of occludin within the tight junction strands or to a substitutional redundancy of single tight junction molecules like occludin. A dramatic change in gastric morphology and secretory function indicates that occludin is involved in gastric epithelial differentiation.

Downregulation of epithelial apoptosis and barrier repair in active Crohn's disease by tumour necrosis factor alpha antibody treatment

BACKGROUND AND AIMS

Barrier dysfunction is an important feature contributing to inflammation and diarrhoea in Crohn's disease (CD). Recently, tumour necrosis factor alpha (TNF-alpha) antibodies were recognised as effective in steroid refractory CD. The aim of this study was to characterise the effects of this therapy on the epithelial barrier.

PATIENTS AND METHODS

Forceps biopsies were obtained from the sigmoid colon before and 14 days after TNF-alpha antibody therapy in 11 patients treated for chronic active CD (Crohn's disease activity index >150). Epithelial apoptoses were measured after terminal deoxynucleotidyl transferase mediated deoxyuridine triphosphate nick end labelling (TUNEL) and 4',6-diamidino-2-phenylindole staining. Epithelial resistance was determined by alternating current impedance analysis in miniaturised Ussing chambers. Occludin, claudin 1, and claudin 4 expression was quantified in immunoblots.

RESULTS

The epithelial apoptotic ratio was 2.1 (0.2)% in controls and increased to 5.3 (1.0)% in CD. TNF-alpha antibody therapy decreased the apoptotic ratio to 2.9 (1.0)% (normalised in 10 of 11 patients). In parallel, epithelial resistance was lower in CD than in controls (24 (3) v 42 (3) Omegaxcm(2)) and improved to 34 (3) Omegaxcm(2) after therapy. Occludin, claudin 1, and claudin 4 were not affected by TNF-alpha antibody therapy. In support of a functional role of epithelial apoptoses in CD, a similar decrease in resistance of -40% was observed when the apoptotic rate was selectively upregulated from 2.6% to 5.4% with camptothecin in HT-29/B6 cells.

CONCLUSIONS

Epithelial apoptoses were upregulated in the colon in CD and restored to normal in 10 of 11 patients by TNF-alpha antibody therapy. This is the structural correlate of epithelial barrier dysfunction measured as epithelial resistance while expression of tight junction proteins did not contribute to this therapeutic effect.

Aeromonas hydrophila beta-hemolysin induces active chloride secretion in colon epithelial cells (HT-29/B6)

The diarrheal mechanisms in Aeromonas enteritis are not completely understood. In this study we investigated the effect of aeromonads and of their secretory products on ion secretion and barrier function of monolayers of human intestinal cells (HT-29/B6). Ion secretion was determined as a short-circuit current (I(SC)) of HT-29/B6 monolayers mounted in Ussing-type chambers. Transepithelial resistance (R(t)) served as a measure of permeability. A diarrheal strain of Aeromonas hydrophila (strain Sb) added to the mucosal side of HT-29/B6 monolayers induced a significant I(SC) (39 +/- 3 microA/cm(2)) and decreased the R(t) to approximately 10% of the initial value. A qualitatively identical response was obtained with sterile supernatant of strain Sb, and Aeromonas supernatant also induced a significant I(SC) in totally stripped human colon. Tracer flux and ion replacement studies revealed the I(SC) to be mainly accounted for by electrogenic Cl(-) secretion. Supernatant applied serosally completely abolished basal I(SC). The supernatant-induced I(SC) was inhibited by the protein kinase C inhibitor chelerythrine, whereas a protein kinase A inhibitor (H8) and a Ca(2+) chelator (BAPTA-AM) had no effect. Physicochemical properties indicated that the supernatant's active compound was an aerolysin-related Aeromonas beta-hemolysin. Accordingly, identical I(SC) and R(t) responses were obtained with Escherichia coli lysates harboring the cloned beta-hemolysin gene from strain SB or the aerA gene encoding for aerolysin. Sequence comparison revealed a 64% homology between aerolysin and the beta-hemolysin cloned from Aeromonas sp. strain Sb. In conclusion, beta-hemolysin secreted by pathogenic aeromonads induces active Cl(-) secretion in the intestinal epithelium, possibly by channel insertion into the apical membrane and by activation of protein kinase C.

Mechanisms of diarrhea in the interleukin-2-deficient mouse model of colonic inflammation

Colitis in interleukin-2-deficient (IL-2(-/-)) mice resembles ulcerative colitis in humans. We studied epithelial transport and barrier function in IL-2(-/-) mice and used this model to characterize mechanisms of diarrhea during intestinal inflammation. (22)Na(+) and (36)Cl(-) fluxes were measured in proximal colon. Net Na(+) flux was reduced from 4.0 +/- 0.5 to 0.8 +/- 0.5 micromol.h(-1).cm(-2), which was paralleled by diminished mRNA and protein expression of the Na(+)/H(+) exchanger NHE3. Net Cl(-) flux was also decreased from 2.2 +/- 1.6 to -2.7 +/- 0.6 micromol.h(-1).cm(-2), indicating impaired Na(+)-Cl(-) absorption. In distal colon, aldosterone-induced electrogenic Na(+) absorption was 6.1 +/- 0.9 micromol.h(-1).cm(-2) in controls and was abolished in IL-2(-/-) mice. Concomitantly, mRNA expression of beta- and gamma-subunits of the epithelial sodium channel (ENaC) was reduced. Epithelial barrier was studied in proximal colon by impedance technique and mannitol fluxes. In contrast to ulcerative colitis, epithelial resistance was increased and mannitol fluxes were decreased in IL-2(-/-) mice. This was in accord with the findings of reduced ion transport as well as increased expression of tight junction proteins occludin and claudin-1, -2, -3, and -5. In conclusion, the IL-2(-/-) mucosa exhibits impaired electroneutral Na(+)-Cl(-) absorption and electrogenic Na(+) transport due to reduced mRNA and protein expression of NHE3 and ENaC beta- and gamma-subunit mRNA. This represents a model of early intestinal inflammation with absorptive dysfunction due to impaired transport protein expression/function while epithelial barrier is still intact. Therefore, this model is ideal to study regulation of transporter expression independent of barrier defects.

The interleukin-2-deficient mouse model

Interleukin-2-deficient (IL-2(-/-)) mice develop colitis with striking clinical and morphological similarities to ulcerative colitis. Since transport and barrier properties are impaired in ulcerative colitis, we studied transport and barrier functions in IL-2(-/-) mice in order to gain insight for the first time into the general pathomechanisms of disturbed transport and barrier function of the intestine during inflammation. Alternating current impedance analysis was used to determine tissue conductance in the inflamed proximal colon of IL-2(-/-) mice and to discriminate between pure epithelial and subepithelial conductance. Surprisingly, epithelial conductance was not increased but diminished in IL-2(-/-) mice compared to controls (20.2 +/- 1.3 versus 28.8 +/- 2.8 mS/cm(2)). Concomitantly, conductance of the subepithelial tissue layers was decreased in IL-2(-/-) mice as a result of edema and infiltration with inflammatory cells. In the distal colon, electrogenic Na(+) transport (J(Na)) mediated by the epithelial Na(+) channel (ENaC) was measured 8 h after stimulation with 3.10(-9) M aldosterone in vitro as the drop in I(SC) (short circuit current) after addition of 10(-4) M amiloride. In controls, J(Na) was 6.9 +/- 0.9 micromol x h(-1) x cm(-2), whereas it was abolished in IL-2(-/-) mice. In conclusion, the inflamed colon of IL-2(-/-) mice exhibits a severe disturbance in Na(+) uptake via the ENaC in the absence of a barrier defect. Thus, reduced expression of active absorptive transport and not a barrier defect is responsible for the diarrhea in this model of intestinal inflammation. This makes this model suitable for studying the general pathomechanisms of the inflammatory downregulation of intestinal transport proteins.

Single-cell epithelial defects close rapidly by an actinomyosin purse string mechanism with functional tight junctions

Restitution of single-cell defects, a frequent event in epithelia with high turnover, is poorly understood. Morphological and functional changes were recorded, using intravital time-lapse video microscopy, confocal fluorescence microscopy, and conductance scanning techniques. After artificial single-cell loss from an HT-29/B6 colonic cell monolayer, the basal ends of adjacent cells extended. Concurrently, the local conductive leak associated with the defect sealed with an exponential time course (from 0.48 +/- 0.05 microS 2 min post lesion to 0.17 +/- 0.02 microS 8 min post lesion, n = 17). Between 3 and 10 min post lesion, a band of actin arose around the gap, which colocalized with a ring of ZO-1 and occludin. Hence, tight junction proteins bound to the actin band facing the gap, and competent tight junctions assembled in the adjoining cell membranes. Closure and sealing were inhibited when actin polymerization was blocked by cytochalasin D, delayed following decrease of myosin-ATPase activity by butanedione monoxime, and blocked after myosin light chain kinase inhibition by ML-7. The Rho-associated protein kinase inhibitor Y-27632 did not affect restitution. After loosening of intercellular contacts in low Ca(2+) Ringer solution, the time course of restitution was not significantly altered. Albeit epithelial conductivity was 12-fold higher in low Ca(2+) Ringer solution than in controls, under both conditions the repaired epithelium assumed the same conductivity as distant intact epithelium. In conclusion, epithelial restitution of single-cell defects comprises rapid closure by an actinomyosin 'purse-string' mechanism and simultaneous formation of a functional barrier from tight junction proteins also associated with the purse string.

[Appendectomy reduces the risk of development of ulcerative colitis]

The etiology and pathogenesis of ulcerative colitis are still unclear. There is, however, a growing consensus that the disease is characterized by a complex interaction between genetic and environmental influences. Many studies in the last few years have suggested that appendectomy reduces the risk of development and improves the course of ulcerative colitis. The underlying mechanism of this effect is unknown. The appendix seems to play a more important role in the development and modulation of the mucosal immune system than believed so far.

Impairment of epithelial transport but not of barrier function in idiopathic pouchitis after ulcerative colitis

BACKGROUND AND AIMS

Little is known of the permeability of ileoanal pouches. Hence the aim of the present study was to determine changes in permeability and mucosal function after ileo-pouchanal anastomosis (IPAA) in patients with ulcerative colitis.

MATERIALS AND METHODS

Biopsies were taken from 43 patients (male:female ratio 28:15; mean age 35.2 (12.5) years) prior to colectomy (ileum prior to pouch), prior to closure of ileostomy (deviation), and after closure of ileostomy (intact pouch) in the case of pouchitis, and from 14 healthy controls. Tissues were mounted in a miniaturised Ussing chamber. Epithelial and subepithelial resistance was determined by transmural impedance analysis. Active Na(+)-glucose cotransport was measured as change in short circuit current after stepwise addition of glucose, and active Cl(-) secretion was measured after stimulation with theophylline and prostaglandin E(2).

RESULTS

Neither epithelial resistance nor mannitol fluxes were significantly altered compared with intact controls, indicating no barrier defect in pouchitis. Subepithelial resistances of intact pouches and pouchitis were increased compared with deviation (18.2 (1.6) and 24.3 (1.5) v. 13.6 (1.0) Omegaxcm(2)) consistent with an adaptive thickening of the subepithelial layer. In contrast, active Cl(-) secretion of pouchitis tissues was reduced versus intact pouch and controls (1.4 (0.3) v. 4.3 (0.7) and 4.6 (0.7) micromol/h/cm(2)), and Na(+)-glucose cotransport of pouchitis was reduced compared with intact pouch and controls (1.8 (0.5) v. 4.2 (0.8) and 8.8 (1.3) micromol/h/cm(2)).

CONCLUSIONS

Ileal mucosa in pouchitis and terminal ileum prior to IPAA exhibit impaired secretory and absorptive transport functions whereas the epithelial barrier function remains unchanged. This differs from findings in ulcerative colitis. Thus the hypothesis that pouchitis represents a remanifestation of ulcerative colitis has to be questioned.

Epithelial barrier defects in ulcerative colitis: characterization and quantification by electrophysiological imaging

BACKGROUND & AIMS

In ulcerative colitis (UC), the epithelial barrier is impaired by erosion/ulcer-type lesions and epithelial apoptosis causing local leaks, and generalized tight junction alterations increasing the basal permeability. We quantified the contribution of these mechanisms to the increased colonic ion permeability.

METHODS

Sigmoid colon was stripped, and the spatial distribution of current clamped across the viable epithelium was recorded by a microelectrode probe, using the conductance scanning method. Local leaks (circumscribed conductive peaks) were marked, and structural changes were studied in H&E-stained series sections.

RESULTS

Overall conductivity increased from 8.4 +/- 0.7 mS/cm(2) (mean +/- SEM) in controls to 11.7 +/- 0.6 in specimens with mild inflammation (i.e., with intact epithelium) and 34.4 +/- 6.2 mS/cm(2) in moderate-to-severe inflammation (i.e., with visible epithelial lesions). Only in part this was caused by a generalized increase in basal conductivity (12.2 +/- 1.5 mS/cm(2) in moderate-to-severe UC vs. 8.3 +/- 0.7 in controls). More importantly, the spatial distribution of conductivity, which was even in controls, showed dramatic leaks in UC. Leaks found in mild inflammation without epithelial lesion turned out to be foci of epithelial apoptosis. In moderate-to-severe inflammation, leaks correlated with epithelial erosion/ulcer-type lesions or crypt abscesses.

CONCLUSIONS

In early UC, but not in controls, seemingly intact epithelium comprises leaks at apoptotic foci. With more intensive inflammation, erosion/ulcer-type lesions are highly conductive, even if covered with fibrin. Local leaks contribute 19% to the overall epithelial conductivity in mild and 65% in moderate-to-severe inflammation.

Celiac disease-like abnormalities in a subgroup of patients with irritable bowel syndrome

BACKGROUND & AIMS

Abdominal symptoms in the absence of mucosal abnormalities are features of both the irritable bowel syndrome (IBS) and latent/potential celiac disease (cd). To identify a possible subgroup of IBS patients with latent/potential cd, surrogate markers of cd were investigated in IBS patients.

METHODS

IBS patients suffering from diarrhea (n = 102), and patients with active cd (n = 10), treated cd (n = 26), and latent cd (n = 5) were included in the study. We measured serum immunoglobulin (Ig) A against gliadin and tissue-transglutaminase, and IgA and IgM against gliadin, tissue-transglutaminase (intestinal cd-associated antibodies), and the dietary proteins beta-lactoglobulin and ovalbumin in duodenal aspirate by enzyme-linked immunosorbent assay. Intraepithelial lymphocytes (IELs) were counted in histology sections, and the expression of HLA-DQ2 (A1*0501/B1*0201) was investigated by polymerase chain reaction. In 26 IBS patients, the effect of 6 months of gluten withdrawal was examined.

RESULTS

Most cd patients expressed HLA-DQ2 and had increased intestinal cd-associated antibodies, whereas cd-associated serum IgA and IEL counts were increased in active cd in contrast to treated or latent cd. In IBS patients, 35% were HLA-DQ2-positive, 23% had increased IEL counts, and 0% and 30% had increased cd-associated antibodies in serum and duodenal aspirate, respectively. Furthermore, stool frequency and intestinal IgA decreased significantly under a gluten-free diet in the subgroups of HLA-DQ2-positive and intestinal antibody-positive IBS patients when compared with IBS patients without these markers.

CONCLUSIONS

HLA-DQ2 expression and increased intestinal cd-associated antibodies are markers that can identify latent/potential cd in a subgroup of IBS patients who consequently appear to profit from a gluten-free diet.

Permeability of human HT-29/B6 colonic epithelium as a function of apoptosis

1. The barrier function of colonic epithelia is challenged by apoptotic loss of enterocytes. In monolayers of human colonic HT-29/B6 cells, apoptosis induced by camptothecin was assessed by poly-(ADP-ribose)-polymerase (PARP) cleavage, histone ELISA and DNA-specific fluorochrome staining (with 4',6'-diamidino-2'-phenylindoladihydrochloride (DAPI)). Epithelial barrier function was studied in Ussing chambers by measuring transepithelial conductivity and unidirectional tracer fluxes. The ion permeability associated with single cell apoptoses was investigated with the conductance scanning technique. 2. The spontaneous rate of apoptotic cells was 3.5 +/- 0.3 % with an overall epithelial conductivity of 3.2 +/- 0.1 mS cm(-2). Camptothecin induced a time- and dose-dependent increase of apoptosis and permeability. With 20 microg ml(-1) of camptothecin for 48 h, apoptosis increased 4.1-fold to 14.3 +/- 1.5 % and the conductivity doubled to 6.4 +/- 1.0 mS cm(-2). 3. While 3H-mannitol flux increased 3.8-fold and 3H-lactulose flux increased 2.6-fold, the flux of 3H-polyethylene glycol 4000 remained unchanged. Hence, the higher permeability was limited to molecules < 4000 Da. 4. The local epithelial conductivity was higher at the sites of apoptosis than in non-apoptotic areas. With camptothecin the leaks associated with apoptosis became more numerous and more conductive, while in non-apoptotic areas the conductivity remained at control level. Hence, the camptothecin-induced increase in epithelial conductivity reflected the opening of apoptotic leaks and thus the results described, for the first time, epithelial permeability as a function of apoptosis only. 5. The conductivity of apoptotic leaks contributed 5.5 % to the epithelial conductivity of controls and 60 % to the conductivity of monolayers treated with 20 microg ml(-1) of camptothecin. Thus apoptosis increased the contribution of paracellular pathways to the overall epithelial permeability. Under control conditions the paracellular conductivity (G(para)) was smaller than the transcellular (G(trans)), but with 12 % apoptosis, G(para) exceeded G(trans). By definition, the epithelium became 'leaky'.

Antisecretory effect of loperamide in colon epithelial cells by inhibition of basolateral K+ conductance

BACKGROUND

The mechanism of the antisecretory effect of loperamide was investigated in cultured highly differentiated colon epithelial cells (HT-29/B6).

METHODS

Chloride secretion was stimulated via cAMP by forskolin (FSK, 10(-5) M), via Ca2+ by the muscarinic agonist carbachol (CCh, 10(-4) M), and via protein kinase C by the phorbol ester PMA (5 x 10(-9) M). Stimulated Cl- secretion was quantified as short circuit current (I(SC)) of HT-29/B6 monolayers mounted in Ussing-type chambers.

RESULTS

Loperamide (5 x 10(-5) M) inhibited I(SC) stimulated by FSK, CCh and PMA. The antisecretory action of loperamide was unaffected by preincubation with naloxone (10(-5) M). Furthermore, loperamide strongly inhibited basolateral 86Rb efflux. Like loperamide, the calmodulin antagonist trifluoperazine (10(-4) M) inhibited I(SC) induced by FSK, CCh or PMA. The Ca2+ channel blocker verapamil (5 x 10(-5) M), on the other hand, inhibited only PMA-stimulated I(SC),but had no effect on FSK or CCh-induced I(SC) CONCLUSIONS: Loperamide exerts a direct antisecretory action on chloride secretion of colon epithelial cells independently of the respective stimulatory signal transduction pathway. This antisecretory effect is not mediated by opiate receptors and reflects inhibition of basolateral K+ conductance.

Epithelial barrier defects in HT-29/B6 colonic cell monolayers induced by tumor necrosis factor-alpha

The barrier function of intestinal epithelia relies upon the continuity of the enterocyte monolayer and intact tight junctions. After incubation with tumor necrosis factor-alpha TNF-alpha, however, the number of strands that form the tight junctions decreases, and apoptosis is induced in intestinal epithelial cells. These morphological changes lead to a rise of transepithelial ion permeability, because the paracellular ion permeability increases and leaks associated with sites of apoptosis increase by number and magnitude. Thus apoptosis and degradation of tight junctions contribute to the increased permeability observed after exposure to TNF-alpha. These mechanisms explain clinical manifestations in the inflamed intestinal wall containing cytokine-secreting macrophages--for example, leak flux diarrhea and invasion of bacterial enterotoxins.

Enhanced expression of iNOS in inflamed colons of IL-2-deficient mice does not impair colonic epithelial barrier function

On the basis of recently observed high levels of iNOS expression that correlated with intestinal inflammation in interleukin-2-deficient [IL-2(-/-)] mice, it was postulated that nitric oxide may damage colonic epithelial cells or impair intestinal epithelial barrier function. This damage may result in an increased permeability of the colonic epithelium leading to high antigenic exposure of the intestinal immune system, which may perpetuate chronic inflammation. Our data demonstrate that high expression of iNOS in IL-2(-/-) mice is correlated with the length/weight ratio (L/W ratio), a widely accepted marker for intestinal inflammation. However, no reduction of epithelial resistance was observed, as would be expected in case of a damaged, leaky epithelium. Our results suggest that enhanced formation of NO in IL-2(-/-) mice does not cause impairment of epithelial barrier function.

Apoptosis and intestinal barrier function

The signal transduction pathways of the induction of apoptosis in the gastrointestinal tract have in part been discovered. However, almost nothing is known about the functional influence of apoptotic signals on intestinal barrier function. In this study the effect of camptothecin-induced apoptosis in HT-29/B6 monolayers and the influence of apoptosis on epithelial barrier function were characterized. We demonstrated that camptothecin causes a decrease of transepithelial resistance and an increase in fluxes of the paracellular marker [3H]mannitol. Camptothecin increased the apoptotic rate and the conductance of single-cell apoptosis as measured by the conductance scanning technique. We conclude that in our model of HT-29/B6 cells camptothecin is a potent inductor of apoptosis that causes significant barrier defects measured by the Ussing chamber technique and the conductance scanning technique. Based on these results we are able to investigate the effect of other cytokines--TGF-beta, for instance, and its role in apoptotic conditions.

Mechanisms of epithelial barrier impairment in HIV infection

Diarrhea and malabsorption due to intestinal dysfunction are common symptoms in HIV infection. The pathophysiologic mechanisms of these alterations are often not known, and the role of HIV per se is still controversially discussed. We measured the epithelial transport and barrier function by means of a miniaturized Ussing chamber system in the duodenum of HIV-infected patients in different disease stages, determined by the CD4 cell count in the serum as well as symptoms in patients with and without diarrhea. We could show that diarrhea induced by HIV per se is caused by a leak flux mechanism due to impaired epithelial barrier function. Antisecretory therapy does not seem to be useful in these patients, because we did not find increased active ion secretion. Along the course of the HIV infection, the epithelial transport and barrier function varies with HIV disease stage (expressed by CD4 cell status). In addition, an in vitro model was studied to characterize the effect of HIV-infected human immune cells on the epithelial barrier function using the human colonic epithelial cell line HT-29/B6. HIV infection of human immune cells induced an increase in cytokine release--for example, TNF-alpha, IL-1 beta, IFN-alpha, and IFN-gamma--downregulating the epithelial barrier function of the human colonic epithelial cell line HT-29/B6. Taken together we postulate a specific stage-dependent cytokine pattern released from HIV-infected immune cells in the mucosa, which, corresponding to the HIV disease stage, is responsible for the variation in epithelial function.

Complex phenotype of mice lacking occludin, a component of tight junction strands

Occludin is an integral membrane protein with four transmembrane domains that is exclusively localized at tight junction (TJ) strands. Here, we describe the generation and analysis of mice carrying a null mutation in the occludin gene. Occludin -/- mice were born with no gross phenotype in the expected Mendelian ratios, but they showed significant postnatal growth retardation. Occludin -/- males produced no litters with wild-type females, whereas occludin -/- females produced litters normally when mated with wild-type males but did not suckle them. In occludin -/- mice, TJs themselves did not appear to be affected morphologically, and the barrier function of intestinal epithelium was normal as far as examined electrophysiologically. However, histological abnormalities were found in several tissues, i.e., chronic inflammation and hyperplasia of the gastric epithelium, calcification in the brain, testicular atrophy, loss of cytoplasmic granules in striated duct cells of the salivary gland, and thinning of the compact bone. These phenotypes suggested that the functions of TJs as well as occludin are more complex than previously supposed.

Expression from the human occludin promoter is affected by tumor necrosis factor alpha and interferon gamma

The 65 kDa protein occludin is a membrane-spanning part of the epithelial tight junction, which is the main barrier of the paracellular pathway. The function of occludin as part of tight junctions is still poorly understood and even less is known about the regulatory mechanisms that influence occludin gene expression. This study aimed to identify the sequences essential in cis for genomic regulation of tight junction formation and to investigate their funcional role in cytokine-dependent tight junction regulation. Using genome walking cloning of occludin-specific human genomic DNA sequences, a 1853 bp DNA fragment containing the transcription start point of occludin cDNA sequences was amplified and sequenced. Subcloning of this fragment in front of the luciferase reporter gene revealed strong expression of enzymatic activity after transfection of the human intestinal cell line HT-29/B6. With subsequent deletions of parts of the promoter fragment, its size was reduced to 280 bp that are necessary and sufficient to mediate promoter activity. Tumor necrosis factor alpha and another cytokine involved in inflammation, interferon gamma, reduced transepithelial resistance in HT-29/B6 cells, which was preceded by a decrease in occludin mRNA expression as revealed by northern blot analysis. Tumor necrosis factor alpha and interferon gamma diminished occludin promoter activity alone and even synergistically, suggesting a genomic regulation of alterations of the paracellular barrier. In conclusion, proinflammatory cytokines such as tumor necrosis factor alpha and interferon gamma can downregulate the expression of the transmembrane tight junction strand protein occludin, paralleling the barrier disturbance detected electrophysiologically. This could be an important mechanism in gastrointestinal diseases accompanied by barrier defects, for example inflammatory bowel diseases.

Leaks in the epithelial barrier caused by spontaneous and TNF-alpha-induced single-cell apoptosis

Current opinion assumes epithelial integrity during spontaneous apoptotic cell death. We measured, for the first time, the local conductances associated with apoptoses and show leaks of up to 280 nS (mean 48 +/- 19 nS) in human intestinal epithelium. The results disprove the dogma that isolated cell apoptosis occurs without affecting the epithelial cell permeability barrier. After induction by tumor necrosis factor alpha (TNF-alpha) the apoptotic leaks were dramatically enhanced: not only was the frequency increased by threefold, but the mean conductance also increased by 12-fold (597+/-98 nS). Thus, apoptosis accounted for about half (56%) of the TNF-alpha-induced permeability increase whereas the other half was caused by degradation of tight junctions in nonapoptotic areas. Hence, spontaneous and induced apoptosis hollow out the intestinal barrier and may facilitate loss of solutes and uptake of noxious agents.

Expression from the human occludin promoter is affected by tumor necrosis factor alpha and interferon gamma

The 65 kDa protein occludin is a membrane-spanning part of the epithelial tight junction, which is the main barrier of the paracellular pathway. The function of occludin as part of tight junctions is still poorly understood and even less is known about the regulatory mechanisms that influence occludin gene expression. This study aimed to identify the sequences essential in cis for genomic regulation of tight junction formation and to investigate their funcional role in cytokine-dependent tight junction regulation. Using genome walking cloning of occludin-specific human genomic DNA sequences, a 1853 bp DNA fragment containing the transcription start point of occludin cDNA sequences was amplified and sequenced. Subcloning of this fragment in front of the luciferase reporter gene revealed strong expression of enzymatic activity after transfection of the human intestinal cell line HT-29/B6. With subsequent deletions of parts of the promoter fragment, its size was reduced to 280 bp that are necessary and sufficient to mediate promoter activity. Tumor necrosis factor alpha and another cytokine involved in inflammation, interferon gamma, reduced transepithelial resistance in HT-29/B6 cells, which was preceded by a decrease in occludin mRNA expression as revealed by northern blot analysis. Tumor necrosis factor alpha and interferon gamma diminished occludin promoter activity alone and even synergistically, suggesting a genomic regulation of alterations of the paracellular barrier. In conclusion, proinflammatory cytokines such as tumor necrosis factor alpha and interferon gamma can downregulate the expression of the transmembrane tight junction strand protein occludin, paralleling the barrier disturbance detected electrophysiologically. This could be an important mechanism in gastrointestinal diseases accompanied by barrier defects, for example inflammatory bowel diseases.

Early aldosterone effect in distal colon by transcriptional regulation of ENaC subunits

Aldosterone-induced sodium absorption is mediated by the epithelial Na(+) channel (ENaC). It is thought that the "early effect" is not based on genomic regulation of ENaC expression, because ENaC subunit transcription was reported to start later than Na(+) transport. We investigated electrogenic Na(+) absorption (J(Na)) and, in identical tissues, mRNA expression of ENaC subunits in early (EDC) and late (LDC) distal colon of the rat. In both segments, 8-h in vitro incubation with 3 nM aldosterone enhanced expression of beta- and gamma-ENaC mRNA and induced J(Na). J(Na) was 10 times higher in LDC than in EDC. alpha-ENaC mRNA was unchanged in EDC, whereas it decreased in LDC. In LDC, beta- and gamma-ENaC mRNA was induced 1 h after aldosterone addition, whereas J(Na) became apparent >1 h later. Downregulation of alpha-ENaC mRNA did not take part in acute regulation because it started after a lag time of 3 h. Time correlation of beta- and gamma-ENaC induction and J(Na) stimulation suggests that the early aldosterone effect on Na(+) absorption in distal colon is caused by transcriptional upregulation of beta- and gamma-ENaC expression.

Patients with dyspepsia benefit from eradication of Helicobacter pylori if other organic causes for dyspepsia were carefully ruled out

AIM

In order to investigate the potential of Helicobacter pylori (HP) to induce dyspepsia, we performed a randomized prospective study on the long-term effect of HP-eradication on symptoms of HP-positive dyspeptic patients in whom other organic causes for dyspepsia were carefully ruled out.

PATIENTS

201 patients referred to our endoscopy unit with dyspeptic symptoms for at least six months entered the study. Patients with previous peptic ulcer were excluded.

METHODS

After endoscopy of the upper alimentary tract and 13C-urea breath test, patients with active peptic ulcer, hiatal hernia, macroscopic evidence for esophagitis and negative HP-status were excluded. The remaining patients underwent abdominal sonography, H2-exhalation test with lactose, and 24-h pH monitoring in order to exclude other organic causes for dyspepsia. In 20 patients, dyspepsia was assumed to be due to HP-gastritis. Patients received eradication therapy and were controlled as assessed by the 13C-urea breath test six weeks and six months after completion of the therapy. Dyspeptic symptoms were monitored by means of a validated symptom score.

RESULTS

Out of 20 patients with HP-gastritis the first eradication treatment was successful in 13, while seven patients remained HP-positive after antibiotic treatment. Six months after completion of therapy the symptoms of HP-eradicated patients improved considerably (score values 17.4 +/- 1.5 and 10.2 +/- 0.8, respectively, p < 0.01) whereas symptoms of patients with persistent infection remained unchanged (21.1 +/- 1.7 and 20.4 +/- 1.5, n.s.) and only improved after successful retherapy (20.4 +/- 1.5 and 11.7 +/- 2.1, p < 0.05). In total, 17 of 20 patients (85%) improved after successful eradication. Also, neutrophil infiltration in the gastric mucosa correlated to both dyspeptic symptoms before therapy (r = 0.85) and the decrease in symptom score after HP-eradication (r = 0.61). In contrast, the symptoms of eight patients with gastroesophageal reflux disease were not improved after eradication (20.0 +/- 1.1 and 18.2 +/- 1.0, n.s.)

CONCLUSIONS

HP-infection per se contributes to dyspepsia. 17 of 20 (85%) HP-positive dyspeptic patients improved after HP-eradication, when other potential organic causes for dyspepsia had been ruled out. However, many patients did not completely recover but the symptoms only partly decreased which parallels the persistence of part of the inflammatory infiltration in the gastric mucosa. This emphasizes the importance of HP-gastritis as an organic disease causing dyspeptic symptoms.

Trans/paracellular, surface/crypt, and epithelial/subepithelial resistances of mammalian colonic epithelia

The epithelial barrier function of the large intestine resides in the trans- and paracellular pathways of the surface epithelium and crypts. Conventional transmural resistance and permeability measurements, however, yield only the resistance of the whole tissue and not that of its individual components. Combining conductance scanning techniques and impedance analysis, we determined the resistance of epithelial and subepithelial tissues, crypts and surface epithelium, and trans- and paracellular pathways of the mouse distal colon. The subepithelial tissue contributed 15% to the transmural resistance of 118+/-9 omega x cm2. In the epithelium proper the resistance of crypts (429+/-86 omega x cm2) exceeded that of the surface epithelium (132+/-15 omega x cm2). The paracellular resistance (3.2+/-0.4 k omega x cm2) of the surface epithelium was 23-fold higher than the transcellular resistance (137+/-16 omega x cm2), and thus the epithelium was classified as "medium tight". In order to investigate the trans- and paracellular resistances of the crypt epithelium as well, flat monolayers of HT-29/B6 cultured colon crypt cells were studied, which had a transepithelial resistance of 349+/-32 omega x cm2. With transcellular resistance (377+/-41 omega x cm2) tenfold lower than the paracellular resistance (3.9+/-1.3 k omega x cm2), this cryptal monolayer was also classified as "medium tight". Hence, considering the 1.2 times larger area of the crypt epithelium, the surface epithelium has a 4 times larger ion permeability than the crypt epithelium. However, the paracellular resistances are not different. Thus the lower transcellular resistance of the surface compared to the crypt epithelium suggests a higher density of ion channels in the apical membrane of surface cells.

Low edge damage container insert that adjusts intestinal forceps biopsies into Ussing chamber systems

Ussing chamber experiments with human intestinal tissue are impeded by the small size of forceps biopsy specimens. Therefore, a miniaturized container insert featuring low edge damage was designed with an exposure area of only 0.05 cm2. It allows measurement of short-circuit current (ISC) and transmural resistance (Rt) on endoscopically obtained biopsy specimens, as well as alternating current impedance analysis and conductance scanning. Comparison with larger specimens mounted in a conventional Ussing chamber without the insert (exposure area 0.54 cm2) was made using rat jejunum and rectum. No differences in ISC, Rt, or secretory response were found, indicating proper sealing and prevention of edge damage, as well as tissue viability in the container system. If biopsy samples obtained from human rectum were mounted in the insert, the local resistance near the edge was almost the same as the overall resistance (52.3 Omega.cm2). Epithelial and subepithelial resistances of human rectum were 43+/-1 Omega.cm2 and 10+/-1 Omega.cm2, respectively. In conclusion, we present a tool that allows reliable Ussing-type, impedance, and conductance scanning measurements to be made from intestinal biopsy specimens.

Electrogenic Na+ transport in rat late distal colon by natural and synthetic glucocorticosteroids

The potency of in vitro-added corticosteroids to stimulate electrogenic Na+ absorption (JNa, the Na+ absorptive short-circuit current blockable by 10(-4) M amiloride) was determined in rat late distal colon. JNa was determined 8 h after steroid addition from the drop in short-circuit current caused by 10(-4) M amiloride. The concentration dependency of JNa was obtained for seven corticosteroids and compared with that established for aldosterone. Apparent mineralocorticoid potencies as determined from apparent Michaelis-Menten constant (Km) values were as follows: aldosterone 1. 2 nM >> RU-28362 20 nM = deoxycorticosterone 20 nM > deoxycortisol 36 nM >/= dexamethasone 37 nM >> corticosterone 170 nM > cortisol 210 nM. These steroids exhibited Vmax values of 9-13 micromol. h-1. cm-2 and similar concentration dependencies. Hill coefficients were between 1.6 and 2.1, suggesting cooperative effects between activated receptors. We conclude that corticosteroids exhibit graded mineralocorticoid potency instead of a sharp partition into exclusive groups of mineralocorticoid and nonmineralocorticoid hormones. The low apparent Km value of RU-28362 for mineralocorticoid action and the need for high concentrations of the mineralocorticoid antagonist mespirenone to block this response indicated that JNa in a native mammalian epithelium can be mediated by the glucocorticoid receptor. Glucocorticoid receptor-specific amounts of RU-28362 in combination with mineralocorticoid receptor-specific amounts of aldosterone or of the mineralocorticoid antagonist spironolactone showed cooperative action, suggesting a heterodimeric activation of JNa by the glucocorticoid receptor and mineralocorticoid receptor.

Altered tight junction structure contributes to the impaired epithelial barrier function in ulcerative colitis

BACKGROUND & AIMS

Mechanisms of diarrhea in ulcerative colitis (UC) are still unknown. Functional and structural characterization of epithelial barrier and transport properties in ulcerative colitis (UC) was performed.

METHODS

Inflamed sigmoid colon epithelium from UC patients was studied by alternating current impedance analysis to determine the pure epithelial resistance as a measure of intestinal barrier function. Tight junction (TJ) structure was investigated by freeze-fracture electron microscopy.

RESULTS

Although total wall resistance was reduced in UC by 50%, impedance analysis uncovered a much more pronounced barrier defect. Epithelial resistance decreased from 95 +/- 5 to 20 +/- 3 omega3. cm2, which in conventional analysis is masked by an increase in subepithelial resistance from 14 +/- 1 to 36 +/- 3 omega3. cm2 caused by inflammation. This was paralleled by a change in epithelial cell TJ structure in UC. Strand count decreased from 6.94 +/- 0.25 to 4.76 +/- 0.47 at the surface and from 7.26 +/- 0.31 to 5.46 +/- 0.37 in the crypts.

CONCLUSIONS

The inflamed colonic mucosa in UC has an impaired barrier function that is much more pronounced than previously assumed. An altered TJ structure contributes to this barrier defect which, because of increased back leak, can reduce net ion transport. Thus, a leak-flux mechanism contributes to the diarrhea in UC.

Tumor necrosis factor-alpha (TNFalpha) regulates the epithelial barrier in the human intestinal cell line HT-29/B6

Cytokines are supposed to be mediators in diarrhoeal diseases. The aim of this study is to characterize the effect of tumor necrosis factor-alpha (TNFalpha) on epithelial barrier function in the colonic epithelial cell line HT-29/B6. Active ion transport and barrier function were measured as short-circuit current and transepithelial electrical resistance (Rt), respectively. In parallel, freeze-fracture electron microscopy (EM) of tight junctions (TJ) and immunofluorescence microscopy of the zonula occludens protein-1 (ZO-1) were performed. Serosal addition of TNF(alpha) (100 ng/ml) decreased Rt by 81%. This effect was dose-dependent and could be mimicked by antibodies against the p55 form of the TNF receptor. Cytotoxic effects were excluded by a negative lactate dehydrogenase (LDH) assay. Immunofluorescence localization with anti-ZO-1 antibodies revealed no evidence for disruption of the monolayer after TNFalpha treatment. In freeze-fracture EM, TJ complexity was decreased by TNFalpha, as indicated by a decrease in the number of strands from 4.7 to 3.4. The tyrosine kinase blocker genistein and the protein kinase A inhibitor H-8 reduced the effect of TNFalpha. A combination of TNFalpha with interferon-gamma acted synergistically on the epithelial barrier. In conclusion, TNFalpha impairs epithelial barrier function by altering structure and function of the tight junction, which could be of pathogenic relevance in intestinal inflammation.