Glucocorticoid receptor is indispensable for physiological responses to aldosterone in epithelial Na+ channel induction via the mineralocorticoid receptor in a human colonic cell line

The epithelial Na+ channel (ENaC) plays a crucial role in electrogenic Na(+) absorption in the distal colon. ENaC induction via the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR) is differentially regulated by modulatory components. As most existing epithelial cell lines including colonic epithelial cell lines miss the co-expression of functional GR and MR, signaling on ENaC is only poorly characterized regarding the interplay of glucocorticoids and mineralocorticoids. In the present study, we show that GR expression and activity are indispensable for MR-dependent induction of ENaC-mediated Na(+) transport. Cooperativity of the two receptors has been studied in the highly differentiated, epithelial colonic cell line HT-29/B6-GR/MR which is equipped with the complete receptor repertoire of both GR and MR due to stable transfection. In contrast to HT-29/B6 cells solely expressing the MR, this cell line displays a physiological response to aldosterone regarding ENaC induction. To achieve this, a pre-incubation step with the GR agonist dexamethasone was required to allow for the subsequent stimulation of ENaC by aldosterone. As a result of cooperative effects between the activated GR and the MR, MR protein levels were elevated and MR-dependent transcription of ENaC subunits β and γ was increased. As an additional mechanism involved, transcription of SGK-1 (serum- and glucocorticoid-induced kinase 1) and GILZ (glucocorticoid-induced leucin zipper)--both essential for the insertion of ENaC into the apical enterocyte membrane--were also augmented by the activated MR.

Inflamed pouch mucosa possesses altered tight junctions indicating recurrence of inflammatory bowel disease

BACKGROUND AND AIMS

The etiology of pouchitis after coloproctomucosectomy with ileal pouch-anal anastomosis in patients with ulcerative colitis is still unknown. Beside changes in luminal antigens, the immunological predisposition is assumed to be responsible. In previous electrophysiological studies, we showed that mucosal barrier and transport function in pouchitis is markedly reduced. Thus, the aim of the present study was to analyze barrier function on the molecular level.

MATERIAL AND METHODS

Pouch biopsies of 36 ulcerative colitis patients were analyzed. Time points were (1) intraoperative immediately prior to ileal pouch-anal anastomosis (n = 13), (2) >1 year after ileostomy closure (pouch, n = 12), and (3) during pouchitis (n = 11). Control terminal ileum biopsies were obtained from eight patients undergoing hemicolectomy due to carcinoma. Expression of tight junction proteins was analyzed by Western blotting and confocal laser-scanning microscopy. To elucidate effects on epithelial barrier properties, impedance spectroscopy was performed in miniaturized Ussing chambers.

RESULTS

In pouchitis, epithelial resistance was markedly reduced compared to non-inflamed pouch and control ileum. Expression of tight junction proteins claudin-1, 3, 4, 5, and 7 and occludin revealed differential expression regulation with the tightening tight junction protein claudin-1 being decreased and an increase of the pore-forming claudin-2, whereas other claudins remained constant. Morphometry indicated the mucosal surface to be unchanged.

CONCLUSION

Pouchitis is characterized by a selective change of tight junction proteins in favor of opening the epithelial tight junction and, thus, the paracellular pathway, which contributes to the inflammatory process. This resembles changes in inflammatory bowel disease (IBD) and indicates IBD recurrence in pouchitis.

Tricellulin forms a barrier to macromolecules in tricellular tight junctions without affecting ion permeability

Tricellulin is a tight junction protein localized in tricellular tight junctions (tTJs), the meeting points of three cells, but also in bicellular tight junctions (bTJs). To investigate its specific barrier functions in bTJs and tTJs, TRIC-a was expressed in low-level tricellulin-expressing cells, and MDCK II, either in all TJs or only in tTJs. When expressed in all TJs, tricellulin increased paracellular electrical resistance and decreased permeability to ions and larger solutes, which are associated with enhanced ultrastructural integrity of bTJs toward enhanced strand linearity. In tTJs in contrast, ultrastructure was unchanged and tricellulin minimized permeability to macromolecules but not to ions. This paradox is explained by properties of the tTJ central tube which is wide enough for passage of macromolecules, but too rare to contribute significantly to ion permeability. In conclusion, at low tricellulin expression the tTJ central tube forms a pathway for macromolecules. At higher expression, tricellulin forms a barrier in tTJs effective only for macromolecules and in bTJs for solutes of all sizes.

Listeriolysin O affects barrier function and induces chloride secretion in HT-29/B6 colon epithelial cells

Listeria monocytogenes is a food-borne pathogen, which is able to induce diarrhea when residing in the intestine. We studied the effect of listeriolysin O (LLO), an extracellular virulence factor of L. monocytogenes, on intestinal transport and barrier function in monolayers of HT-29/B6 human colon cells using the Ussing technique to understand the pathomechanisms involved. Mucosal addition of LLO, but not a LLO mutant, induced a dose- and pH-dependent increase in short-circuit current (I(SC)). Sodium and chloride tracer flux and DIDS sensitivity studies revealed that I(SC) was mainly due to electrogenic chloride secretion. Barrier function was impaired by LLO, as assessed by transepithelial resistance (R(t)) and mannitol flux measurements. Intracellular signal transduction occurred through Ca(2+) release from intracellular stores and PKC activation. In conclusion, listeriolysin induces chloride secretion and perturbs epithelial barrier function, thus potentially contributing to Listeria-induced diarrhea.

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.

Quercetin enhances epithelial barrier function and increases claudin-4 expression in Caco-2 cells

Quercetin is the most abundant flavonoid and is assumed to have positive effects on the gastrointestinal mucosa after dietary intake. The aim of the study was to analyze the influence of quercetin on intestinal barrier function using the human colonic epithelial cell line Caco-2. Transepithelial resistance (R(t)), tracer fluxes of [(3)H]-mannitol, 22Na+, and 36Cl- as well as electrogenic ion transport were determined in Ussing chambers. Expression of tight junction (TJ) proteins and mRNA was analyzed in Western blots and quantitative RT-PCR, respectively. Regulation of transcription was analyzed by reporter gene assay. Cellular distribution of TJ proteins was examined by confocal laser scanning microscopy (LSM). Apoptotic rate was determined by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling staining. Quercetin induced a dose-dependent increase of R(t) persisting for >2 d. Daily addition of quercetin was able to perpetuate the effect, which was seen whether quercetin was added apically or to the basolateral compartment. Parallel to the R(t) increase, quercetin induced a strong increase of the TJ protein claudin-4 but not of other claudins. Confocal LSM showed a localization of claudin-4 in TJ. Apoptotic rate was not affected by quercetin. Consistent with these changes, fluxes of Na+ and Cl-, but not of mannitol, were reduced. Reporter gene assays revealed a stimulatory effect of quercetin on claudin-4 transcription. The flavonoid quercetin enhances barrier function via transcriptional expression regulation of the TJ protein claudin-4, which represents an important protective effect of this food component against barrier disturbance in intestinal inflammation.

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.

Restoration of ENaC expression by glucocorticoid receptor transfection in human HT-29/B6 colon cells

The epithelial sodium channel (ENaC) controls colonic sodium absorption. So far, investigation of ENaC was limited by an unexplained lack of steroid-dependent ENaC expression in cultured intestinal cells, which we aimed to resolve. HT-29/B6 cells constitutively expressed the alpha-ENaC subunit, while beta- and gamma-ENaC subunits could not be detected due to deficient basal as well as corticosteroid-induced transcription. This was due to a lack of expression of both activating and inhibiting isoforms of glucocorticoid receptor (GR-alpha, -beta) and mineralocorticoid receptor. Stable transfection of GR-alpha restored intestine-specific glucocorticoid upregulation of beta- and gamma-ENaC in HT-29/B6 cells, which was followed by intact targeting of ENaC channels to the apical cell membrane and dose-dependent induction of electrogenic sodium absorption. In conclusion, ENaC deficiency is due to a lack of steroid receptors and not the consequence of a crypt-like phenotype of cultured intestinal cells. By stable GR transfection we obtained a model, in which ENaC regulation can be studied.

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.

Interleukin-13 is the key effector Th2 cytokine in ulcerative colitis that affects epithelial tight junctions, apoptosis, and cell restitution

BACKGROUND & AIMS

Ulcerative colitis (UC) is characterized by a Th2 immune response with inflammation and epithelial barrier dysfunction. So far, Th2 cytokines have not been shown to directly influence epithelial barrier function.

METHODS

Lamina propria mononuclear cells (LPMCs) were stimulated and interleukin (IL)-13 was measured by enzyme-linked immunosorbent assay. Functional IL-13 and IL-4 effects were studied on HT-29/B6 colonic epithelial cells in Ussing chambers and by conductance scanning. Apoptosis was detected by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling assays. IL-13/IL-4 receptors were analyzed by reverse-transcription polymerase chain reaction and immunofluorescence. Western blotting combined with immunofluorescence was used to detect tight junction proteins. Furthermore, restitution velocity was measured. Finally, mucosal biopsy specimens from patients with UC were compared with cultured cells for these features.

RESULTS

LPMCs from patients with UC produced large amounts of IL-13 (985 +/- 73 pg/mL), much more than from controls or patients with Crohn's disease. IL-13Ralpha1 and IL-4Ralpha receptors were present in HT-29/B6 cells and colonic epithelial cells of control patients and patients with UC. IL-13 had a dose-dependent effect on transepithelial resistance of HT-29/B6 monolayers (reduction to 60% +/- 4%), whereas IL-4 had no effect. This was due to an increased number of apoptotic cells (5.6-fold +/- 0.9-fold) and an increased expression of the pore-forming tight junction protein claudin-2 to 295% +/- 37%, both of which contributed equally. Finally, epithelial restitution velocity decreased from 15.1 +/- 0.6 to 10.6 +/- 0.5 microm/h after treatment with IL-13. Parallel changes were observed in human samples, with an increase in claudin-2 expression to 956% +/- 252%.

CONCLUSIONS

IL-13 was identified as an important effector cytokine in UC that impairs epithelial barrier function by affecting epithelial apoptosis, tight junctions, and restitution velocity.

Interleukin-13 is the key effector Th2 cytokine in ulcerative colitis that affects epithelial tight junctions, apoptosis, and cell restitution

BACKGROUND & AIMS

Ulcerative colitis (UC) is characterized by a Th2 immune response with inflammation and epithelial barrier dysfunction. So far, Th2 cytokines have not been shown to directly influence epithelial barrier function.

METHODS

Lamina propria mononuclear cells (LPMCs) were stimulated and interleukin (IL)-13 was measured by enzyme-linked immunosorbent assay. Functional IL-13 and IL-4 effects were studied on HT-29/B6 colonic epithelial cells in Ussing chambers and by conductance scanning. Apoptosis was detected by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling assays. IL-13/IL-4 receptors were analyzed by reverse-transcription polymerase chain reaction and immunofluorescence. Western blotting combined with immunofluorescence was used to detect tight junction proteins. Furthermore, restitution velocity was measured. Finally, mucosal biopsy specimens from patients with UC were compared with cultured cells for these features.

RESULTS

LPMCs from patients with UC produced large amounts of IL-13 (985 +/- 73 pg/mL), much more than from controls or patients with Crohn's disease. IL-13Ralpha1 and IL-4Ralpha receptors were present in HT-29/B6 cells and colonic epithelial cells of control patients and patients with UC. IL-13 had a dose-dependent effect on transepithelial resistance of HT-29/B6 monolayers (reduction to 60% +/- 4%), whereas IL-4 had no effect. This was due to an increased number of apoptotic cells (5.6-fold +/- 0.9-fold) and an increased expression of the pore-forming tight junction protein claudin-2 to 295% +/- 37%, both of which contributed equally. Finally, epithelial restitution velocity decreased from 15.1 +/- 0.6 to 10.6 +/- 0.5 microm/h after treatment with IL-13. Parallel changes were observed in human samples, with an increase in claudin-2 expression to 956% +/- 252%.

CONCLUSIONS

IL-13 was identified as an important effector cytokine in UC that impairs epithelial barrier function by affecting epithelial apoptosis, tight junctions, and restitution velocity.

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.

Claudin-2 expression induces cation-selective channels in tight junctions of epithelial cells

Tight junctions seal the paracellular pathway of epithelia but, in leaky tissues, also exhibit specific permeability. In order to characterize the contribution of claudin-2 to barrier and permeability properties of the tight junction in detail, we studied two strains of Madin-Darby canine kidney cells (MDCK-C7 and MDCK-C11) with different tight junctional permeabilities. Monolayers of C7 cells exhibited a high transepithelial resistance (>1 kOhms cm(2)), compared with C11 cells (<100 Ohms cm(2)). Genuine expression of claudin-1 and claudin-2, but not of occludin or claudin-3, was reciprocal to transepithelial resistance. However, confocal microscopy revealed a marked subjunctional localization of claudin-1 in C11 cells, indicating that claudin-1 is not functionally related to the low tight junctional resistance of C11 cells. Strain MDCK-C7, which endogenously does not express junctional claudin-2, was transfected with claudin-2 cDNA. In transfected cells, but not in vector controls, the protein was detected in colocalization with junctional occludin by means of immunohistochemical analyses. Overexpression of claudin-2 in the originally tight epithelium with claudin-2 cDNA resulted in a 5.6-fold higher paracellular conductivity and relative ion permeabilities of Na(+) identical with 1, K(+)=1.02, NMDG(+)=0.79, choline(+)=0.71, Cl(-)=0.12, Br(-)=0.10 (vector control, 1:1.04:0.95:0.94:0.85:0.83). By contrast, fluxes of (radioactively labeled) mannitol and lactulose and (fluorescence labeled) 4 kDa dextran were not changed. Hence, with regular Ringer's, Na(+) conductivity was 0.2 mS cm(-2) in vector controls and 1.7 mS cm(-2) in claudin-2-transfected cells, while Cl(-) conductivity was 0.2 mS cm(-2) in both cells. Thus, presence of junctional claudin-2 causes the formation of cation-selective channels sufficient to transform a 'tight' tight junction into a leaky one.

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.