Epithelial barrier and ion transport in coeliac sprue: electrical measurements on intestinal aspiration biopsy specimens

A physiologically based model of bile acid metabolism in mice

Bile acid (BA) metabolism is a complex system that includes a wide variety of primary and secondary, as well as conjugated and unconjugated BAs that undergo continuous enterohepatic circulation (EHC). Alterations in both composition and dynamics of BAs have been associated with various diseases. However, a mechanistic understanding of the relationship between altered BA metabolism and related diseases is lacking. Computational modeling may support functional analyses of the physiological processes involved in the EHC of BAs along the gut-liver axis. In this study, we developed a physiologically based model of murine BA metabolism describing synthesis, hepatic and microbial transformations, systemic distribution, excretion, and EHC of BAs at the whole-body level. For model development, BA metabolism of specific pathogen-free (SPF) mice was characterized in vivo by measuring BA levels and composition in various organs, expression of transporters along the gut, and cecal microbiota composition. We found significantly different BA levels between male and female mice that could only be explained by adjusted expression of the hepatic enzymes and transporters in the model. Of note, this finding was in agreement with experimental observations. The model for SPF mice could also describe equivalent experimental data in germ-free mice by specifically switching off microbial activity in the intestine. The here presented model can therefore facilitate and guide functional analyses of BA metabolism in mice, e.g., the effect of pathophysiological alterations on BA metabolism and translation of results from mouse studies to a clinically relevant context through cross-species extrapolation.

A case for improved assessment of gut permeability: a meta-analysis quantifying the lactulose:mannitol ratio in coeliac and Crohn's disease

BACKGROUND

A widely used method in assessing small bowel permeability is the lactulose:mannitol test, where the lactulose:mannitol ratio (LMR) is measured. However, there is discrepancy in how the test is conducted and in the values of LMR obtained across studies. This meta-analysis aims to determine LMR in healthy subjects, coeliac and Crohn's disease.

METHODS

A literature search was performed using PRISMA guidance to identify studies assessing LMR in coeliac or Crohn's disease. 19 studies included in the meta-analysis measured gut permeability in coeliac disease, 17 studies in Crohn's disease. Outcomes of interest were LMR values and comparisons of standard mean difference (SMD) and weighted mean difference (WMD) in healthy controls, inactive Crohn's, active Crohn's, treated coeliac and untreated coeliac. Pooled estimates of differences in LMR were calculated using the random effects model.

RESULTS

Pooled LMR in healthy controls was 0.014 (95% CI: 0.006-0.022) while pooled LMRs in untreated and treated coeliac were 0.133 (95% CI: 0.089-0.178) and 0.037 (95% CI: 0.019-0.055). In active and inactive Crohn's disease, pooled LMRs were 0.093 (95% CI: 0.031-0.156) and 0.028 (95% CI: 0.015-0.041). Significant differences were observed in LMR between: (1) healthy controls and treated coeliacs (SMD = 0.409 95% CI 0.034 to 0.783, p = 0.032), (2) healthy controls and untreated coeliacs (SMD = 1.362 95% CI: 0.740 to 1.984, p < 0.001), (3) treated coeliacs and untreated coeliacs (SMD = 0.722 95% CI: 0.286 to 1.157, p = 0.001), (4) healthy controls and inactive Crohn's (SMD = 1.265 95% CI: 0.845 to 1.686, p < 0.001), (5) healthy controls and active Crohn's (SMD = 2.868 95% CI: 2.112 to 3.623, p < 0.001), and (6) active Crohn's and inactive Crohn's (SMD = 1.429 (95% CI: 0.580 to 2.278, p = 0.001). High heterogeneity was observed, which was attributed to variability in protocols used across different studies.

CONCLUSION

The use of gut permeability measurements in screening and monitoring of coeliac and Crohn's disease is promising. LMR is useful in performing this function with significant limitations. More robust alternative tests with higher degrees of clinical evidence are needed if measurements of gut permeability are to find widespread clinical use.

The Role of Intestinal Permeability in Gastrointestinal Disorders and Current Methods of Evaluation

An increased intestinal permeability has been described in various gastrointestinal and non-gastrointestinal disorders. Nevertheless, the concept and definition of intestinal permeability is relatively broad and includes not only an altered paracellular route, regulated by tight junction proteins, but also the transcellular route involving membrane transporters and channels, and endocytic mechanisms. Paracellular intestinal permeability can be assessed in vivo by using different molecules (e.g., sugars, polyethylene glycols, ⁵¹Cr-EDTA) and ex vivo in Ussing chambers combining electrophysiology and probes of different molecular sizes. The latter is still the gold standard technique for assessing the epithelial barrier function, whereas in vivo techniques, including putative blood biomarkers such as intestinal fatty acid-binding protein and zonulin, are broadly used despite limitations. In the second part of the review, the current evidence of the role of impaired barrier function in the pathophysiology of selected gastrointestinal and liver diseases is discussed. Celiac disease is one of the conditions with the best evidence for impaired barrier function playing a crucial role with zonulin as its proposed regulator. Increased permeability is clearly present in inflammatory bowel disease, but the question of whether this is a primary event or a consequence of inflammation remains unsolved. The gut-liver axis with a crucial role in impaired intestinal barrier function is increasingly recognized in chronic alcoholic and metabolic liver disease. Finally, the current evidence does not support an important role for increased permeability in bile acid diarrhea.

The tight junction and the epithelial barrier in coeliac disease

Epithelial barriers are essential to maintain multicellular organisms well compartmentalized and protected from external environment. In the intestine, the epithelial layer orchestrates a dynamic balance between nutrient absorption and prevention of microorganisms, and antigen intrusion. Intestinal barrier function has been shown to be altered in coeliac disease but whether it contributes to the pathogenesis development or if it is merely a phenomenon secondary to the aberrant immune response is still unknown. The tight junction complexes are multiprotein cell-cell adhesions that seal the epithelial intercellular space and regulate the paracellular permeability of ions and solutes. These structures have a fundamental role in epithelial barrier integrity as well as in signaling mechanisms that control epithelial-cell polarization, the formation of apical domains and cellular processes such as cell proliferation, migration, differentiation, and survival. In coeliac disease, the molecular structures and function of tight junctions appear disrupted and are not completely recovered after treatment with gluten-free diet. Moreover, zonulin, the only known physiological regulator of the tight junction permeability, appears augmented in autoimmune conditions associated with TJ dysfunction, including coeliac disease. This chapter will examine recent discoveries about the molecular architecture of tight junctions and their functions. We will discuss how different factors contribute to tight junction disruption and intestinal barrier impairment in coeliac disease. To conclude, new insights into zonulin-driven disruption of tight junction structures and barrier integrity in coeliac disease are presented together with the advancements in novel therapy to treat the barrier defect seen in pathogenesis.

The Intestinal Barrier and Current Techniques for the Assessment of Gut Permeability

The intestinal barrier is essential in human health and constitutes the interface between the outside and the internal milieu of the body. A functional intestinal barrier allows absorption of nutrients and fluids but simultaneously prevents harmful substances like toxins and bacteria from crossing the intestinal epithelium and reaching the body. An altered intestinal permeability, a sign of a perturbed barrier function, has during the last decade been associated with several chronic conditions, including diseases originating in the gastrointestinal tract but also diseases such as Alzheimer and Parkinson disease. This has led to an intensified interest from researchers with diverse backgrounds to perform functional studies of the intestinal barrier in different conditions. Intestinal permeability is defined as the passage of a solute through a simple membrane and can be measured by recording the passage of permeability markers over the epithelium via the paracellular or the transcellular route. The methodological tools to investigate the gut barrier function are rapidly expanding and new methodological approaches are being developed. Here we outline and discuss, in vivo, in vitro and ex vivo techniques and how these methods can be utilized for thorough investigation of the intestinal barrier.

Enteric parasitic infection disturbs bacterial structure in Mexican children with autoantibodies for type 1 diabetes and/or celiac disease

Background

Intestinal bacterial dysbiosis and increased gut permeability are associated with higher risk of developing type 1 diabetes (T1D) or celiac disease (CD). There is a lack of information on parasitism involved in gut disturbance of predisposed children. We evaluated the effect of enteropathogenic parasites (Cryptosporidium spp., Cyclospora spp. G. lamblia, and Blastocystis spp.) on the bacterial structure of feces from children with autoantibodies for T1D or CD. Participants included 37 children under 18 years of age, from whom stools were analyzed for enteric parasites by qPCR and 22/37 for bacterial profile by sequencing the V3–V4 region of the 16s rRNA gene. Dietary, clinical, and socioeconomic data was recorded.

Results

Pathogens parasitized 28/37 participants, Cryptosporidium spp. was the most prevalent (62.2%), followed by both Cyclospora cayetanensis and Blastocystis spp (37.8%). There were no dietary differences (p > 0.05) attributable to parasitism. Co-infected participants with Cryptosporidium and Cyclospora did not differ (p = 0.064) from non-infected participants in bacterial alpha phylogenetic diversity. The same parasites’ co-infection was associated with a decreased abundance of the Ruminococaceae (p = 0.04) and Verrucomicrobioceae families, of the Akkermansia genus (p = 0.009). There was a lower Firmicutes/Bacteroidetes ratio (p = 0.02) in infected than in uninfected participants.

Conclusions

Cryptosporidium and Cyclospora affected the bacterial structure at family and genus levels, decreasing the ratio between Firmicutes and Bacteroidetes in children with auto-antibodies for T1D or CD, which could increase the risk of illness onset.

Intestinal Anti-tissue Transglutaminase2 Autoantibodies: Pathogenic and Clinical Implications for Celiac Disease

Celiac disease (CD) is a systemic disease that primarily affects the small intestine. The presence of anti-tissue transglutaminase 2 (anti-TG2) antibodies in the serum, as well as the presence of autoimmune phenomena, account for the inclusion of CD among autoimmune diseases. Anti-TG2 autoantibodies are produced at intestinal level, where they are deposited even before they appear in circulation. The pathogenic events that lead to their production are still not completely defined, but a central role seems to be played by gliadin-specific T cells. Interestingly, limited somatic mutations have been observed in VH and VL genes in TG2-specific plasma cells, another important aspect being the biased use of a heavy chain encoded by the VH5 gene. Conflicting data have been produced over the years on the effect of anti-TG2 antibodies on TG2 function. Although the presence of anti-TG2 antibodies in serum is considered a hallmark of CD and relevant from a clinical viewpoint, the role of these autoantibodies in the development of the celiac lesion remains to be defined. In the years, different technical approaches have been implemented to detect and measure intestinal CD-associated autoantibody production. Two aspects can make intestinal anti-TG2 antibodies relevant: from a clinical viewpoint: the first is their proposed ability in potential coeliac patients to predict the development of a full-blown enteropathy; the second is their possible role in revealing a condition of reactivity to gluten in patients with no circulating CD-associated autoantibodies. In fact, the detection of CD-specific autoantibodies production in the intestine, in the absence of serum positivity for the same antibodies, could be suggestive of a very early condition of gluten reactivity; alternatively, it could be not specific for CD and merely attributable to intestinal inflammation. In conclusion, the role of mucosal anti-TG2 antibodies in pathogenesis of CD is unknown. Their presence, the modalities of their production, their gluten dependence render them a unique model to study autoimmunity.

Intestinal Barrier Function in Gluten-Related Disorders

Gluten-related disorders include distinct disease entities, namely celiac disease, wheat-associated allergy and non-celiac gluten/wheat sensitivity. Despite having in common the contact of the gastrointestinal mucosa with components of wheat and other cereals as a causative factor, these clinical entities have distinct pathophysiological pathways. In celiac disease, a T-cell mediate immune reaction triggered by gluten ingestion is central in the pathogenesis of the enteropathy, while wheat allergy develops as a rapid immunoglobulin E- or non-immunoglobulin E-mediated immune response. In non-celiac wheat sensitivity, classical adaptive immune responses are not involved. Instead, recent research has revealed that an innate immune response to a yet-to-be-defined antigen, as well as the gut microbiota, are pivotal in the development in this disorder. Although impairment of the epithelial barrier has been described in all three clinical conditions, its role as a potential pathogenetic co-factor, specifically in celiac disease and non-celiac wheat sensitivity, is still a matter of investigation. This article gives a short overview of the mucosal barrier of the small intestine, summarizes the aspects of barrier dysfunction observed in all three gluten-related disorders and reviews literature data in favor of a primary involvement of the epithelial barrier in the development of celiac disease and non-celiac wheat sensitivity.

Soluble Syndecan-1: A Novel Biomarker of Small Bowel Mucosal Damage in Children with Celiac Disease

BACKGROUND

Syndecan-1 (SDC1) is essential for maintaining normal epithelial barrier. Shedding of SDC1 ectodomain, reflected by serum soluble syndecan-1 (SSDC1) levels, is regulated by inflammation. Increased intestinal permeability plays a central role in celiac disease (CD). The association between SSDC1 levels and mucosal damage in CD has not been evaluated.

AIMS

To evaluate serum SSDC1 levels in children with CD and to determine its relationship with histological grading classified by modified Marsh criteria.

METHODS

This is a cross-sectional, pilot study, in which serum SSDC1 was analyzed by ELISA in a cohort of 49 untreated children with CD and 15 children with nonspecific abdominal pain (AP). CD was diagnosed based on positive celiac serology and small intestinal biopsy. SSDC1 levels at the time of biopsy were correlated with Marsh grading. Controls were defined by AP, negative celiac serology, normal upper endoscopy, and small intestinal biopsies.

RESULTS

SSDC1 levels were significantly higher in CD patients compared to AP controls (116.2 ± 161 vs. 41.3 ± 17.5 ng/ml, respectively, p < 0.01). SSDC1 levels were significantly higher in patients with Marsh 3c lesion compared to AP controls (170.6 ± 201 vs. 41.3 ± 17.5 ng/ml, respectively, p < 0.05). SSDC1 concentrations displayed a significant correlation with mucosal damage defined by Marsh (r = 0.39, p < 0.05).

CONCLUSION

This is the first study demonstrating elevated levels of serum SSDC1 in children with CD. Our results suggest that SSDC1 is a potentially novel marker of intestinal mucosal damage in patients with CD. Its applicability as a surrogate biomarker in CD remains to be determined.

Celiac Disease: Role of the Epithelial Barrier

In celiac disease (CD) a T-cell-mediated response to gluten is mounted in genetically predisposed individuals, resulting in a malabsorptive enteropathy histologically highlighted by villous atrophy and crypt hyperplasia. Recent data point to the epithelial layer as an under-rated hot spot in celiac pathophysiology to date. This overview summarizes current functional and genetic evidence on the role of the epithelial barrier in CD, consisting of the cell membranes and the apical junctional complex comprising sealing as well as ion and water channel-forming tight junction proteins and the adherens junction. Moreover, the underlying mechanisms are discussed, including apoptosis of intestinal epithelial cells, biology of intestinal stem cells, alterations in the apical junctional complex, transcytotic uptake of gluten peptides, and possible implications of a defective epithelial polarity. Current research is directed toward new treatment options for CD that are alternatives or complementary therapeutics to a gluten-free diet. Thus, strategies to target an altered epithelial barrier therapeutically also are discussed.

Active and passive involvement of claudins in the pathophysiology of intestinal inflammatory diseases

Intestinal inflammatory diseases, four of which are discussed here, are associated with alterations of claudins. In ulcerative colitis, diarrhea and antigen entry into the mucosa occurs. Claudin-2 is upregulated but data on other claudins are still limited or vary (e.g., claudin-1 and -4). Apart from that, tight junction changes contribute to diarrhea via a leak flux mechanism, while protection against antigen entry disappears behind epithelial gross lesions (erosions) and apoptotic foci. Crohn's disease is additionally characterized by a claudin-5 and claudin-8 reduction which plays an active role in antigen uptake already before gross lesions appear. In microscopic colitis (MC), upregulation of claudin-2 expression is weak and a reduction in claudin-4 may be only passively involved, while sodium malabsorption represents the main diarrheal mechanism. However, claudin-5 is removed from MC tight junctions which may be an active trigger for inflammation through antigen uptake along the so-called leaky gut concept. In celiac disease, primary barrier defects are discussed in the context of candidate genes as PARD3 which regulate cell polarity and tight junctions. The loss of claudin-5 allows small antigens to invade, while the reductions in others like claudin-3 are rather passive events. Taken together, the specific role of single tight junction proteins for the onset and perpetuation of inflammation and the recovery from these diseases is far from being fully understood and is clearly dependent on the stage of the disease, the background of the other tight junction components, the transport activity of the mucosa, and the presence of other barrier features like gross lesions, an orchestral interplay which is discussed in this article.

Claudin-related intestinal diseases

With up to 200 m(2) the human intestine is the organ with the largest absorptive surface of the body. It is lined by a single layer of epithelial cells that separates the host from the environment. The intestinal epithelium provides both, selective absorption of nutrients, ions, and water but also a highly effective barrier function which includes the first line of defense against environmental antigens. The paracellular part of this barrier function is provided by tight junction (TJ) proteins, especially the large family of claudins. Changes in abundance or molecular structure of claudins can generally result in three typical effects, (i) decreased absorptive passage, (ii) increased secretory passage of small solutes and water causing leak flux diarrhea and (iii) increased absorptive passage of macromolecules which may induce inflammatory processes. Several intestinal diseases are associated with such changes that can result in intestinal inflammation and symptoms like weight loss, abdominal pain or diarrhea. This review summarizes our current knowledge on barrier dysfunction and claudin dysregulation in several intestinal diseases gastroenterologists are often faced with, like inflammatory bowel disease, microscopic colitis, celiac disease, irritable bowel syndrome, gallstones and infectious diseases like HIV enteropathy, Campylobacter jejuni and Clostridium perfringens infection.

Abnormal apical-to-basal transport of dietary ovalbumin by secretory IgA stimulates a mucosal Th1 response

In celiac disease, enhanced permeability to gliadin peptides can result from their apico-basal transport by secretory immunoglobulin A1 (SIgA1) binding to the CD71 receptor ectopically expressed at the gut epithelial surface. Herein, we have established a mouse model in which there is apico-basal transport of the model antigen ovalbumin (OVA) by specific SIgA1 and have analyzed local T-cell activation. Transgenic DO11.10 mice were grafted with a hybridoma-secreting OVA-specific humanized IgA1, which could bind mouse CD71 and which were released in the intestinal lumen as SIgA. CD71 expression was induced at the gut apical surface by treating the mice with tyrphostin A8. Following gavage of the mice with OVA, OVA-specific CD4⁺ T cells isolated from the mesenteric lymph nodes displayed higher expression of the activation marker CD69 and produced more interferon gamma in mice bearing the hybridoma-secreting OVA-specific IgA1, than in ungrafted mice or in mice grafted with an irrelevant hybridoma. These results indicate that the protective role of SIgA1 might be jeopardized in human pathological conditions associated with ectopic expression of CD71 at the gut surface.

A subset of anti-rotavirus antibodies directed against the viral protein VP7 predicts the onset of celiac disease and induces typical features of the disease in the intestinal epithelial cell line T84

Celiac disease (CD) is an autoimmune disorder of the small intestine triggered by environmental factors in genetically predisposed individuals. A strong association between type 1 diabetes (T1DM) and CD has been reported. We have previously shown that rotavirus infection may be involved in the pathogenesis of CD through a mechanism of molecular mimicry. Indeed, we identified a subset of anti-transglutaminase IgA antibodies that recognize the rotavirus viral protein VP7. In this study, we aimed at evaluating whether such antibodies may predict the onset of CD in children affected by T1DM. Moreover, to further analyze the link between rotavirus infection and pathogenesis of CD, we analyzed the effect of anti-rotavirus VP7 antibodies on T84 intestinal epithelial cells using the gene-array technique, complemented by the analysis of molecules secreted in the supernatant of stimulated cells. We found that anti-rotavirus VP7 antibodies are present in the vast majority (81%) of T1DM-CD tested sera, but are detectable also in a fraction (27%) of T1DM children without CD. Moreover, we found that anti-rotavirus VP7 antibodies are present before the CD onset, preceding the detection of anti-tTG and anti-endomysium antibodies. The gene-array analysis showed that purified anti-rotavirus VP7 antibodies modulate genes that are involved in apoptosis, inflammation, and alteration of the epithelial barrier integrity in intestinal epithelial cells, all typical features of CD. Taken together, these new data further support the involvement of rotavirus infection in the pathogenesis of CD and suggest a predictive role of anti-rotavirus VP7 antibodies.

Discerning apical and basolateral properties of HT-29/B6 and IPEC-J2 cell layers by impedance spectroscopy, mathematical modeling and machine learning

Quantifying changes in partial resistances of epithelial barriers in vitro is a challenging and time-consuming task in physiology and pathophysiology. Here, we demonstrate that electrical properties of epithelial barriers can be estimated reliably by combining impedance spectroscopy measurements, mathematical modeling and machine learning algorithms. Conventional impedance spectroscopy is often used to estimate epithelial capacitance as well as epithelial and subepithelial resistance. Based on this, the more refined two-path impedance spectroscopy makes it possible to further distinguish transcellular and paracellular resistances. In a next step, transcellular properties may be further divided into their apical and basolateral components. The accuracy of these derived values, however, strongly depends on the accuracy of the initial estimates. To obtain adequate accuracy in estimating subepithelial and epithelial resistance, artificial neural networks were trained to estimate these parameters from model impedance spectra. Spectra that reflect behavior of either HT-29/B6 or IPEC-J2 cells as well as the data scatter intrinsic to the used experimental setup were created computationally. To prove the proposed approach, reliability of the estimations was assessed with both modeled and measured impedance spectra. Transcellular and paracellular resistances obtained by such neural network-enhanced two-path impedance spectroscopy are shown to be sufficiently reliable to derive the underlying apical and basolateral resistances and capacitances. As an exemplary perturbation of pathophysiological importance, the effect of forskolin on the apical resistance of HT-29/B6 cells was quantified.

In vitro assessment of epithelial electrical resistance in human esophageal and jejunal mucosae and in Caco-2 cell layers

OBJECTIVE

There is a need for a technique allowing studies of human mucosal specimens collected during different clinical conditions. This study elucidates if square wave pulse analysis discriminates between epithelial and transmural electrical resistance and if there is an association with transepithelial permeability of molecular probes.

METHODS

Mucosae from esophagus (surgical resections: n = 14; endoscopic biopsies: n = 15) and jejunum (n = 12) and Caco-2 cell monolayers were investigated in Ussing chambers. Transmural and epithelial electrical resistance were recorded by the use of standardized current pulses. Permeability was assessed using two fluorescein-labeled probes (weight 376 and 4000 Da).

RESULTS

Baseline epithelial electrical resistance was higher in esophageal mucosa (~280 Ω*cm(2)), than in jejunal (~10 Ω*cm(2)) and Caco-2 cells (~140 Ω*cm(2)). The subepithelial contribution to the transmural resistance was higher in jejunal preparations (+88%) and Caco-2 cells (+75%), than in esophageal (+30%). During hypoxia the subepithelial resistance was unchanged, whereas the epithelial resistance decreased significantly in jejunal mucosa and Caco-2 cells. These findings coincided with increased transepithelial probe permeability and signs of disturbed morphology. Esophageal epithelia were resistant to hypoxia. However, exposure to deoxycholic acid and trypsin abolished the esophageal epithelial resistance and increased probe permeability. Endoscopic esophageal biopsies from patients with erosive reflux disease exhibited significantly lower epithelial resistance and higher current than healthy subjects.

CONCLUSION

Square wave pulse analysis in Ussing chambers is suitable for assessment of epithelial electrical resistance that can reflect transepithelial permeability of molecular probes with known size. Moreover, the technique discriminated between healthy and reflux-diseased esophageal mucosal biopsies.

Interactions among secretory immunoglobulin A, CD71, and transglutaminase-2 affect permeability of intestinal epithelial cells to gliadin peptides

BACKGROUND & AIMS

The transferrin receptor (CD71) is up-regulated in duodenal biopsy samples from patients with active celiac disease and promotes retrotransport of secretory immunoglobulin A (SIgA)-gliadin complexes. We studied intestinal epithelial cell lines that overexpress CD71 to determine how interactions between SIgA and CD71 promote transepithelial transport of gliadin peptides.

METHODS

We analyzed duodenal biopsy specimens from 8 adults and 1 child with active celiac disease. Caco-2 and HT29-19A epithelial cell lines were transfected with fluorescence-labeled small interfering RNAs against CD71. Interactions among IgA, CD71, and transglutaminase 2 (Tgase2) were analyzed by flow cytometry, immunoprecipitation, and confocal microscopy. Transcytosis of SIgA-CD71 complexes and intestinal permeability to the gliadin 3H-p31-49 peptide were analyzed in polarized monolayers of Caco-2 cells.

RESULTS

Using fluorescence resonance energy transfer and in situ proximity ligation assays, we observed physical interactions between SIgA and CD71 or CD71 and Tgase2 at the apical surface of enterocytes in biopsy samples and monolayers of Caco-2 cells. CD71 and Tgase2 were co-precipitated with SIgA, bound to the surface of Caco-2 cells. SIgA-CD71 complexes were internalized and localized in early endosomes and recycling compartments but not in lysosomes. In the presence of celiac IgA or SIgA against p31-49, transport of intact 3H-p31-49 increased significantly across Caco-2 monolayers; this transport was inhibited by soluble CD71 or Tgase2 inhibitors.

CONCLUSIONS

Upon binding to apical CD71, SIgA (with or without gliadin peptides) enters a recycling pathway and avoids lysosomal degradation; this process allows apical-basal transcytosis of bound peptides. This mechanism is facilitated by Tgase2 and might be involved in the pathogenesis of celiac disease.

In vitro models for gluten toxicity: relevance for celiac disease pathogenesis and development of novel treatment options

In genetically predisposed individuals, dietary gluten in wheat, rye and barley triggers celiac disease, a systemic autoimmune disorder hallmarked by an extensive small-bowel mucosal immune response. The current conception of celiac disease pathogenesis is that it involves components of both innate and adaptive immunity whose activation typically leads to small-bowel villous atrophy with crypt hyperplasia. Currently, the only effective treatment for celiac disease is a strict lifelong gluten-free diet excluding all wheat-, rye- and barley-containing food products. During the diet, the clinical symptoms improve and the small-bowel mucosal damage recovers, while re-introduction of gluten into the diet leads to re-appearance of the symptoms and deterioration of the small-bowel mucosal architecture. In view of the restricted nature of the diet, alternative treatment is warranted. Improved understanding of the molecular basis of celiac disease has enabled researchers to suggest other therapeutic approaches. Although there is no animal model reproducing all features of celiac disease, the use of in vitro approaches including a variety of cell lines and the celiac patient small-bowel mucosal biopsy organ culture has generated knowledge about pathogenesis of celiac disease. In these culture systems, gluten induces different effects that can be quantified, thus also enabling studies concerning the efficacy of candidate therapeutic compounds for celiac disease. This review describes the intestinal epithelial cell models, celiac patient T-cell lines and clones, as well as the small-bowel mucosal organ culture methods widely used in studies of celiac disease, and summarizes the major findings obtained with these systems.

Immunogenicity of two oat varieties, in relation to their safety for celiac patients

OBJECTIVE

Most of the recent studies suggest that oats are well tolerated by celiac disease (CD) patients. However, it is still possible that different oat cultivars may display different biological properties relevant for CD pathogenesis. We aimed to investigate biological and immunological properties of two oat varieties, Avena genziana and Avena potenza, in relation to their safety for CD patients.

MATERIAL AND METHODS

Phosphorylation of extracellular signal-regulated kinase (ERK) and trans-epithelial electrical resistance (TEER) were evaluated in CaCo-2 cells treated with peptic-tryptic (PT) digests from the two oats and from gliadin (PTG). With the same PT-digests, duodenal biopsies from 22 CD patients were treated in vitro for 24 h and density of CD25+ cells in lamina propria and of intraepithelial CD3+ T cells was measured, as well as crypt cell proliferation and epithelial expression of interleukin 15. Finally, interferon γ (IFN-γ) production was measured as evidence of gliadin-specific T-cell activation by PT-digests.

RESULTS

In contrast to PTG, oats PT-digests were not able to induce significant increase in ERK phosphorylation and decrease in TEER in CaCo-2 cells. In the organ culture system, oats PT-digests, unlike PTG, did not induce significant increase in crypt enterocyte proliferation, increase in interleukin 15 expression or in lamina propria CD25+ cells. Nevertheless Avena potenza increased intraepithelial T-cell density, while Avena genziana-induced IFN-γ production in 3/8 CD intestinal T cell lines.

CONCLUSIONS

Our data show that Avena genziana and Avena potenza do not display in vitro activities related to CD pathogenesis. Some T-cell reactivity could be below the threshold for clinical relevance.

Epithelial barriers in intestinal inflammation

The gastrointestinal epithelium transports solutes and water between lumen and blood and at the same time forms a barrier between these compartments. This highly selective and regulated barrier permits ions, water, and nutrients to be absorbed, but normally restricts the passage of harmful molecules, bacteria, viruses and other pathogens. During inflammation, the intestinal barrier can be disrupted, indicated by a decrease in transcellular electrical resistance and an increase in paracellular permeability for tracers of different size. Such inflammatory processes are accompanied by increased oxidative stress, which in turn can impair the epithelial barrier. In this review, we discuss the role of inflammatory oxidative stress on barrier function with special attention on the epithelial tight junctions. Diseases discussed causing barrier changes include the inflammatory bowel diseases Crohn's disease, ulcerative colitis, and microscopic colitis, the autoimmune disorder celiac disease, and gastrointestinal infections. In addition, the main cytokines responsible for these effects and their role during oxidative stress and intestinal inflammation will be discussed, as well as therapeutic approaches and their mode of action.

Acute HIV infection induces mucosal infiltration with CD4+ and CD8+ T cells, epithelial apoptosis, and a mucosal barrier defect

BACKGROUND & AIMS

A barrier defect of the intestinal mucosa is thought to affect the progression of human immunodeficiency virus (HIV) infection. It is not clear whether the mucosal barrier impairment already is present in acute infection and what mechanisms cause this defect. We analyzed T-cell subsets, epithelial apoptosis, and barrier function of the duodenal mucosa in patients with acute HIV infection.

METHODS

Mucosal T-cell subsets, epithelial apoptosis, and barrier function were assessed by immunohistochemistry, immunofluorescence, flow cytometry, and impedance spectroscopy in duodenal samples from 8 patients with early acute infection, 8 patients with chronic infection, and 9 HIV-negative individuals (controls). One patient was analyzed serially, before and during acute infection.

RESULTS

Compared with controls, densities of mucosal CD8+ and, surprisingly, of mucosal CD4+ T cells too, increased in patients with acute infection. Most mucosal CD4+ T cells had an activated effector memory phenotype (CD45RA-CD45RO+CD62L-CD40L+CD38+) and did not proliferate. Perforin-expressing mucosal CD8+ T cells also were increased in acutely infected patients; their frequency correlated with epithelial apoptosis. The epithelial barrier was impaired significantly in patients with acute HIV infection. The patient analyzed serially developed increased densities of mucosal CD4+ and CD8+ T cells, increased apoptosis of epithelial cells, and mucosal barrier impairment during acute infection.

CONCLUSIONS

Before depleting CD4+ T cells, acute HIV infection induces infiltration of the mucosa with activated effector memory CD4+ and CD8+ T cells. The HIV-induced barrier defect of the intestinal mucosa is evident already in acute infection; it might arise from increased epithelial apoptosis, induced by perforin-positive mucosal cytotoxic T cells.

Degradation of coeliac disease-inducing rye secalin by germinating cereal enzymes: diminishing toxic effects in intestinal epithelial cells

Currently the only treatment for coeliac disease is a lifelong gluten-free diet excluding food products containing wheat, rye and barley. There is, however, only scarce evidence as to harmful effects of rye in coeliac disease. To confirm the assumption that rye should be excluded from the coeliac patient's diet, we now sought to establish whether rye secalin activates toxic reactions in vitro in intestinal epithelial cell models as extensively as wheat gliadin. Further, we investigated the efficacy of germinating cereal enzymes from oat, wheat and barley to hydrolyse secalin into short fragments and whether secalin-induced harmful effects can be reduced by such pretreatment. In the current study, secalin elicited toxic reactions in intestinal Caco-2 epithelial cells similarly to gliadin: it induced epithelial cell layer permeability, tight junctional protein occludin and ZO-1 distortion and actin reorganization. In high-performance liquid chromatography and mass spectroscopy (HPLC-MS), germinating barley enzymes provided the most efficient degradation of secalin and gliadin peptides and was thus selected for further in vitro analysis. After germinating barley enzyme pretreatment, all toxic reactions induced by secalin were ameliorated. We conclude that germinating enzymes from barley are particularly efficient in the degradation of rye secalin. In future, these enzymes might be utilized as a novel medical treatment for coeliac disease or in food processing in order to develop high-quality coeliac-safe food products.

Solute transporters and aquaporins are impaired in celiac disease

BACKGROUND INFORMATION

Celiac disease is a chronic inflammatory disorder of the small bowel induced in genetically susceptible subjects by gluten ingestion. Diarrhoea, weight loss and malabsorption represent the major clinical presentation of the disease. Here we examined the possible alteration in the expression and localization of water channels [AQPs (aquaporins)] and some solute transporters in duodenal mucosa of celiac disease patients. Duodenal biopsies from untreated celiacs, treated celiacs, healthy controls and disease controls were considered in the present study. The expressions of some AQPs and transporter mRNAs in human duodenal biopsies were determined by semi-quantitative RT-PCR (reverse transcription PCR) and real-time RT-PCR. The localization of AQPs 3, 7 and 10 and of SGLT1 (Na+/glucose co-transporter 1), PEPT1 (H+/oligopeptide transporter 1) and NHE3 (Na+/H+ exchanger 3) was evaluated by immunohistochemistry.

RESULTS

AQPs 3, 7, 10 and 11, SGLT, PEPT and NHE, CFTR (cystic fibrosis transmembrane conductance regulator) and NKCC (Na-K-2Cl co-transporter) mRNAs were expressed in duodenal biopsies of healthy controls, treated celiac patients and disease controls. The expression of transcripts was virtually absent in duodenal biopsies of untreated celiac disease patients except for CFTR and NKCC. In healthy controls, immunohistochemistry revealed a labelling in the apical membrane of surface epithelial cells of the duodenum. The immunolabelling was heavily reduced or absent in untreated celiac patients, while it was normal in patients consuming a gluten-free diet for at least 12 months.

CONCLUSIONS

Our results indicate that the main routes for water and solute absorption are deficient in celiac disease and may play a role in the onset of malabsorption symptoms.

Inhibition of epithelial growth factor receptor signalling does not preserve epithelial barrier function after in vitro gliadin insult

OBJECTIVE

In coeliac disease, small-bowel mucosal permeability is increased in response to gluten consumption. However, the signalling routes leading to such a barrier defect remain obscure. As the epidermal growth factor receptor (EGFR) pathway is up-regulated in untreated coeliac disease, and since this cascade has been related to epithelial hyperpermeability, the aim of this study was to establish whether blocking the EGFR route would restore the barrier after gliadin insult in vitro.

MATERIAL AND METHODS

Epithelial barrier function was assessed by measuring transepithelial electrical resistance (TER) in Caco-2 epithelial monolayers treated with pepsin trypsin (PT)-digested gliadin with or without monoclonal antibodies against EGFR family members or by inhibitors of the EGFR pathway signalling molecules. Furthermore, tight-junctional integrity was determined by Western blotting and immunofluorescence staining of the tight-junctional protein occludin.

RESULTS

PT-gliadin significantly reduced TER and the expression of occludin protein. Blocking of the EGFR signalling pathway could not prevent gliadin-triggered damage. In fact, a function-blocking monoclonal antibody against EGFR (ErbB1) actually potentiated the harmful effects of gliadin on TER.

CONCLUSIONS

The epithelial barrier-disrupting properties of gliadin are independent of the EGFR signalling cascade. However, our results suggest that activation of the EGFR pathway might actually be protective against gliadin-triggered hyperpermeability. Further studies are needed to elucidate the specific gliadin-triggered signalling cascades which lead to increased epithelial permeability in coeliac disease.

Tight junctions in salivary epithelium

Epithelial cell tight junctions (TJs) consist of a narrow belt-like structure in the apical region of the lateral plasma membrane that circumferentially binds each cell to its neighbor. TJs are found in tissues that are involved in polarized secretions, absorption functions, and maintaining barriers between blood and interstitial fluids. The morphology, permeability, and ion selectivity of TJ vary among different types of tissues and species. TJs are very dynamic structures that assemble, grow, reorganize, and disassemble during physiological or pathological events. Several studies have indicated the active role of TJ in intestinal, renal, and airway epithelial function; however, the functional significance of TJ in salivary gland epithelium is poorly understood. Interactions between different combinations of the TJ family (each with their own unique regulatory proteins) define tissue specificity and functions during physiopathological processes; however, these interaction patterns have not been studied in salivary glands. The purpose of this review is to analyze some of the current data regarding the regulatory components of the TJ that could potentially affect cellular functions of the salivary epithelium.

Antigen presentation in celiac disease

Celiac disease is caused by an inappropriate immune response to ingested gluten proteins. As a dietary antigen, gluten undergoes extensive but incomplete proteolytic digestion in the intestinal lumen. The resultant peptide fragments of gluten require deamidation, but not necessarily further intracellular processing for presentation. Recent studies reveal why the disease associated HLA-DQ2 molecule is particularly suited for binding proline-rich gluten peptides. In comparison, DQ8 exhibits different binding characteristics, which may explain the lesser risk for disease in association with this molecule.

Proinflammatory cytokines tumor necrosis factor-alpha and interferon-gamma alter tight junction structure and function in the rat parotid gland Par-C10 cell line

Sjögren's syndrome (SS) is an autoimmune disorder characterized by inflammation and dysfunction of salivary glands, resulting in impaired secretory function. The production of the proinflammatory cytokines tumor necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) is elevated in exocrine glands of patients with SS, although little is known about the effects of these cytokines on salivary epithelial cell functions necessary for saliva secretion, including tight junction (TJ) integrity and the establishment of transepithelial ion gradients. The present study demonstrates that chronic exposure of polarized rat parotid gland (Par-C10) epithelial cell monolayers to TNF-alpha and IFN-gamma decreases transepithelial resistance (TER) and anion secretion, as measured by changes in short-circuit current (I(sc)) induced by carbachol, a muscarinic cholinergic receptor agonist, or UTP, a P2Y(2) nucleotide receptor agonist. In contrast, TNF-alpha and IFN-gamma had no effect on agonist-induced increases in the intracellular calcium concentration [Ca(2+)](i) in Par-C10 cells. Furthermore, treatment of Par-C10 cell monolayers with TNF-alpha and IFN-gamma increased paracellular permeability to normally impermeant proteins, altered cell and TJ morphology, and downregulated the expression of the TJ protein, claudin-1, but not other TJ proteins expressed in Par-C10 cells. The decreases in TER, agonist-induced transepithelial anion secretion, and claudin-1 expression caused by TNF-alpha, but not IFN-gamma, were reversible by incubation of Par-C10 cell monolayers with cytokine-free medium for 24 h, indicating that IFN-gamma causes irreversible inhibition of cellular activities associated with fluid secretion in salivary glands. Our results suggest that cytokine production is an important contributor to secretory dysfunction in SS by disrupting TJ integrity of salivary epithelium.

Altered gene expression in highly purified enterocytes from patients with active coeliac disease

Background

Coeliac disease is a multifactorial inflammatory disorder of the intestine caused by ingestion of gluten in genetically susceptible individuals. Genes within the HLA-DQ locus are considered to contribute some 40% of the genetic influence on this disease. However, information on other disease causing genes is sparse. Since enterocytes are considered to play a central role in coeliac pathology, the aim of this study was to examine gene expression in a highly purified isolate of these cells taken from patients with active disease. Epithelial cells were isolated from duodenal biopsies taken from five coeliac patients with active disease and five non-coeliac control subjects. Contaminating T cells were removed by magnetic sorting. The gene expression profile of the cells was examined using microarray analysis. Validation of significantly altered genes was performed by real-time RT-PCR and immunohistochemistry.

Results

Enterocyte suspensions of high purity (98–99%) were isolated from intestinal biopsies. Of the 3,800 genes investigated, 102 genes were found to have significantly altered expression between coeliac disease patients and controls (p < 0.05). Analysis of these altered genes revealed a number of biological processes that are potentially modified in active coeliac disease. These processes include events likely to contibute to coeliac pathology, such as altered cell proliferation, differentiation, survival, structure and transport.

Conclusion

This study provides a profile of the molecular changes that occur in the intestinal epithelium of coeliac patients with active disease. Novel candidate genes were revealed which highlight the contribution of the epithelial cell to the pathogenesis of coeliac disease.

Elevated motility-related transmucosal potential difference in the upper small intestine in the irritable bowel syndrome

The pathophysiology of irritable bowel syndrome (IBS) is complex and incompletely known. Very little has been studied regarding the role of submucous neuronal activity. We therefore measured small intestinal transmural potential difference (PD, reflecting mainly electrogenic chloride secretion), and its linkage with fasting motor activity [migrating motor complex (MMC)] in controls (n = 16) and patients with IBS [n = 23, 14 diarrhoea predominant (d-IBS) and nine constipation predominant (c-IBS)]. Transmural-PD and its relation to MMC phase III was measured by modified multilumen manometry for 3 h in the fasting state using one jejunal and one duodenal infusion line as flowing electrodes. The amplitude and duration of motor phase III was similar in controls and IBS patients, but the propagation speed of phase III was higher in IBS patients. In IBS patients, maximal PD during MMC phase III was significantly elevated in both the duodenum and jejunum (P < 0.05) and the PD decline after phase III was significantly prolonged in the jejunum (P < 0.01). The PD elevation was seen in both duodenum and jejunum in d-IBS patients, but only in the jejunum in the c-IBS patients. On the basis of previous modelling studies, we propose that the enhanced secretion may reflect disturbed enteric network behaviour in some patients with IBS.

Epithelial electrical resistance as a measure of permeability changes in pediatric duodenal biopsies

OBJECTIVES

Intestinal permeability measured with medium-sized oral probes is increased in cystic fibrosis (CF) and celiac disease (CD), probably reflecting reduced tight junction resistance. The aim of this study was to evaluate whether square-pulse analysis of duodenal biopsies from children can be used to determine electrical tight junction resistance.

METHODS

Intestinal biopsies from children with different stages of CD and from patients with CF were studied in a modified Ussing chamber. The epithelium was assumed to act as an electrical circuit consisting of a current generator parallel with a resistance and a capacitance. Subepithelial and epithelial resistances were determined by square-pulse analysis, and the generated current was calculated.

RESULTS

Confirming data using permeability probes, reduced epithelial electrical resistance was found both in patients with CF and CD. Only the CF patients had reduced resting current as well. The secretagogues prostaglandin E2, cyclic adenosine monophosphate and acetylcholine increased the current in both control biopsies and biopsies with villous atrophy but had no significant effect on epithelial resistance.

CONCLUSIONS

Measurement of electrical resistance in duodenal biopsies can be used as an alternative method of quantifying permeability in pediatric biopsies.

Understanding the molecular basis of celiac disease: what genetic studies reveal

Celiac disease (CD) is characterized by a chronic immune reaction in the small intestine to the gluten proteins that are present in a (Western) daily diet. Besides the well known involvement of the HLA class II histocompatibility antigen (HLA)-DQ2.5 and -DQ8 heterodimers (encoded by particular combinations of the HLA-DQA1 and -DQB1 gene) in CD and the minor contribution of the CTLA-4 gene, recently the myosin IXB (MYO9B) gene has also been found to be genetically associated. This review covers the general aspects of CD as well as current insight into important molecular aspects. We evaluate the role of susceptibility genes in CD by following gluten along its path from ingestion to uptake in the body, which leads us through the three aspects of CD's pathology. The first is the presence of gluten in the lumen of the intestine, where it is broken down by several enzymes. The second is the intestinal barrier through which gluten peptides pass. The third is the reaction of the immune system in response to gluten peptides, in which both the innate and the adaptive immune systems play a role. Our main conclusion, based on the current genetic and functional studies, is that we should look for causal genes in the barrier function as well as in the immune systems.

Myosin IXB variant increases the risk of celiac disease and points toward a primary intestinal barrier defect

Celiac disease is probably the best-understood immune-related disorder. The disease presents in the small intestine and results from the interplay between multiple genes and gluten, the triggering environmental factor. Although HLA class II genes explain 40% of the heritable risk, non-HLA genes accounting for most of the familial clustering have not yet been identified. Here we report significant and replicable association (P = 2.1 x 10(-6)) to a common variant located in intron 28 of the gene myosin IXB (MYO9B), which encodes an unconventional myosin molecule that has a role in actin remodeling of epithelial enterocytes. Individuals homozygous with respect to the at-risk allele have a 2.3-times higher risk of celiac disease (P = 1.55 x 10(-5)). This result is suggestive of a primary impairment of the intestinal barrier in the etiology of celiac disease, which may explain why immunogenic gluten peptides are able to pass through the epithelial barrier.

Transcriptional downregulation of tight junction protein ZO-1 in active coeliac disease is reversed after a gluten-free diet

Coeliac disease is an autoimmune enteropathy characterized by an enhanced permeability of the intestinal epithelial barrier. In epithelial cells paracellular permeability is regulated by intercellular tight junction. The cytoplasmic protein ZO-1 interacts directly with F-actin and plays a pivotal role in the structural and functional organization of tight junction.

AIM

The aim of this study was to investigate the expression and localization of ZO-1 in the intestinal mucosa of coeliac patients.

PATIENTS AND METHODS

Twenty patients with active coeliac disease, seven of whom underwent a repeat biopsy following a gluten-free diet and 27 control subjects, were studied. In all subjects, three biopsies were obtained from distal duodenum during upper gastrointestinal endoscopy. ZO-1 protein localization and levels were detected by immunofluorescence followed by confocal microscopy analysis and immunoblotting. ZO-1 mRNA expression was assessed by RT-PCR. F-actin distribution was also investigated.

RESULTS

In patients with active coeliac disease, both ZO-1 protein levels and mRNA were clearly reduced. Cytoskeletal organization was disrupted with F-actin staining concentrated at the subcortical and basal surface regions. Abnormalities in ZO-1 expression and actin organization were reversed after a gluten-free diet.

CONCLUSIONS

In active coeliac disease, ZO-1 protein expression is downregulated at the transcriptional level in association with F-actin redistribution. These changes are completely reversed after a gluten-free diet and could contribute to the increased intestinal paracellular permeability observed in this disorder.

Alterations of the intestinal transport and processing of gliadin peptides in celiac disease

BACKGROUND & AIMS

The hypothesis of a defect in the intestinal transport and processing of toxic (31-49) or immunostimulant (57-68 and the 33-mer 56-89) gliadin peptides was tested in patients with active celiac disease (ACD), patients with treated celiac disease (TCD), and controls.

METHODS

Using duodenal biopsy specimens mounted in Ussing chambers, we measured electrical resistance, mucosal-to-serosal radiolabeled-peptide fluxes, and peptide processing during transport using radio-reverse-phase high-performance liquid chromatography.

RESULTS

Peptide 31-49 fluxes (24.7 microg x 3 h(-1). cm(-2)) were increased in patients with ACD compared with controls and patients with TCD (12.7 and 12.3 microg x 3 h(-1). cm(-2); P < 0.01). In contrast, no increase was observed for peptide 57-68 or 56-89 (33-mer). Electrical resistance was decreased in patients with ACD versus controls (15.3 vs. 23.9 ohms. cm(2); P < 0.001). Peptide 57-68 was partially degraded by brush-border peptidases in controls but not in patients with celiac disease. However, it was totally degraded after intestinal transport both in controls and patients with celiac disease. Peptides 31-49 and 56-89 were resistant to brush-border peptidases in all groups of patients but were totally degraded during intestinal transport in controls and patients with TCD. In patients with ACD, however, 50% of peptide 31-49 was delivered intact into the serosal compartment and only partial degradation of the 33-mer was observed. These abnormalities were not related to a nonspecific paracellular leakage.

CONCLUSIONS

Our data indicate that gliadin peptides, although poorly or not digested by intraluminal enzymes, can be fully digested by enterocytes in controls and patients with TCD. In patients with ACD, incomplete degradation of the 33-mer and protected transport of the peptide 31-49 might favor their respective immunostimulatory and toxic effects.

Low frequency electrical bioimpedance for the detection of inflammation and dysplasia in Barrett's oesophagus

Biological tissues undergoing inflammation and dysplasia seem to exhibit changes in the intercellular space that can be sensed using low frequency electrical impedance methods. Basically, low frequency electric current flows through this space and its widening as well as the disruption of the tight junction decrease the resistance, facilitating current flow. The electrical changes accompanying structural changes from columnar tissue to adenocarcinoma in Barrett's metaplastic mucosa and gastric tissue are illustrated using resected tissue from 32 patients. Two hundred and fifty-eight biopsies were analysed, correlating their electrical resistivity (R) at 9.6 kHz and their histopathological interpretation. Compared to non-inflamed non-dysplastic columnar tissue (R = 4.9 ohms m), the results suggest a small but statistically significant decrease of electrical impedance in columnar tissue showing inflammation (R = 4.2 ohms m, p = 0.016) and a larger decrease when dysplasia is present (R = 3.4 ohms m, p = 0.040). If this method is validated further, this technique could be used to obtain guided biopsies from patients undergoing surveillance programmes for Barrett's oesophagus. We aim to refine this technique using a new system with lower frequencies and, possibly, in vitro (cultured cells) and in vivo (rats) models of Barrett's oesophagus.

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.

Effects of motility on epithelial transport in the human descending duodenum

The aim of the study was to determine how motility affects the balance between absorptive and secretory ion transport in the proximal human small intestine. Thirty-two healthy subjects and 16 patients (eight with villus atrophy, eight with normal duodenal biopsies) were studied. The absorptive sodium flux was estimated by measurement of bicarbonate absorption with a double-lumen perfusion technique. The secretory chloride flux was calculated from the ratio between the continuously recorded transmural potential difference (PD) and the epithelial parallel resistance, which was measured in forceps biopsies by square-wave current analysis. Graded variations in contraction frequency were obtained by recording during defined time periods before, during and after phase III of the migrating motor complex (MMC). Bicarbonate was absorbed by a process that led to CO2 formation, and both bicarbonate absorption and luminal PCO2 increased with contraction frequency. The motility-related PCO2 rise was reduced in patients with villus atrophy and by removal of bicarbonate from the perfusate. A higher motor activity was also associated with a larger PD (more lumen negative). Both the absorptive and the secretory fluxes were thus enhanced by motility. The estimated absorptive flux was approximately twice as large as the secretory flux during periods of low motor activity, and four times as large during submaximal motor activity. We conclude that motor activity affects both absorptive and secretory mucosal function in a quantifiable manner. Information about the behaviour of the respective linkage functions may make it possible to model the intestinal absorption process in vivo.

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.

Clinical models of intestinal adaptation

Mucosal adaptation of the small intestine is morphologically restricted to only three different patterns, namely, atrophy, hyperplasia, and hyperregeneration. The hyperplastic mucosa in the experimental short bowel syndrome exhibits unchanged epithelial barrier properties and a differential functional adaptation with a 150% increase in Na-glucose cotransport but no change in electroneutral NaCl cotransport. In the hyperregeneratively transformed mucosa of the self-filling blind loop of rat jejunum, absorption is seriously impaired, as indicated by the 80% decrease in Na-glucose cotransport. To compensate for this, epithelial barrier function is upregulated by an increase in tight junction complexity to prevent leak flux of ions and substrates. In contrast, the hyperregeneratively transformed mucosa in celiac sprue shows reduced tight junction complexity. Possible candidates responsible for the heterogeneity of tight junction adaptation in these conditions could be cytokines, because tumor necrosis factor-alpha can specifically downregulate the tight junction, as indicated in the intestinal HT-29/B6 cell model.

Impedance analysis for the determination of epithelial and subepithelial resistance in intestinal tissues

The barrier function of the intestinal wall plays a key role in body homeostasis and defense against noxious agents. Conventional Ussing chamber techniques determine the overall transmural resistance but do not differentiate epithelial and subepithelial tissues. The barrier function, however, resides in the epithelial cell layer only. Transmural impedance analysis can solve this problem, if adequate models are applied. We show that: (i) epithelial and subepithelial impedances are additive, (ii) the epithelium proper can be represented by a very general electrical model, which demonstrates short-circuiting at high frequencies (due to cell membrane capacitances), and (iii) the reactance of subepithelial tissue can be described phenomenologically. Using an empirical expression for description of the subepithelial impedance, the present method allows the determination of the epithelial and the subepithelial resistance. This was exemplified in rat ileum, which defied adequate impedance analysis so far. Of the transmural DC resistance of 61 +/- 5 omega.cm2 (n = 8) the subepithelial contribution was 28 +/- 2 omega.cm2 and the epithelial resistance was 33 +/- 4 omega.cm2.

Epithelial tight junction structure in the jejunum of children with acute and treated celiac sprue

Tight junction morphology was analyzed in freeze fracture electron micrographs from biopsies at two locations along the surface-crypt axis in the jejunum of children with treated and untreated sprue and in control subjects. In control jejunum, strand number, meshwork depth, and total depth of the tight junction decreased from surface to crypt, consistent with the concept of the crypt being more permeable than the surface epithelium. In acute sprue, strand number was reduced in all regions along the surface-crypt axis, from 5.5+/-0.2 to 3.4+/-0.3 (surface) and from 4.7+/-0.2 to 3.6+/-0.1 (crypt). Meshwork depth was also reduced at all regions along the surface-crypt axis. Strand discontinuities were more frequent in acute sprue. Aberrant strands appeared below the main meshwork of crypt tight junctions in acute sprue. In asymptomatic children treated with the gluten-free diet, jejunal tight junctional structure only partially recovered. Strand number was restored to normal at the surface, but was still decreased in the crypts, from 4.7+/-0.2 to 3.9+/-0.3. We conclude that the epithelial barrier function of the small intestine is seriously disturbed by structural modifications of the tight junction in acute symptomatic celiac disease, thereby accounting for increased ionic permeability noted in a parallel study on identical specimens. This epithelial barrier defect may contribute to diarrhea in celiac disease by a "leak flux mechanism." In children with sprue treated with a gluten-free diet, barrier dysfunction was only partly recovered, suggesting a level of "minimal damage."

Ussing chamber for high-frequency transmural impedance analysis of epithelial tissues

An Ussing chamber was designed for impedance analysis of epithelial tissue and optimized for the use of high-frequency alternating current stimuli. Shielded voltage electrodes, located axially within the electric field of the Ussing chamber, minimized the reactive properties of the set-up. By vectorial subtraction, the small reactive contribution of the optimized Ussing chamber was completely compensated for. This method allowed for transmural impedance measurements in a frequency range of 1-65 kHz. For the first time, Nyquist plots of cultured intestinal cell monolayers (HT-29/B6) are presented. The epithelial monolayers in different stages of confluence showed the impedance locus as a semicircle, with the high frequency end close to the origin. These epithelial monolayers could be modeled by a simple RC-parallel circuit without a series resistance.

Electrogenic ion transport along the human duodenum in childhood

BACKGROUND

To find reliable sites to study the effects of different secretagogues on electrogenic ion secretion, we investigated the secretion pattern in different parts of duodenum.

METHODS

Histologically normal routine intestinal biopsy specimens from children were mounted in a modified Ussing chamber. The secretory responses to prostaglandin E2 (PGE2), aminophylline, dibutyryl cyclic adenosine 5'-monophosphate (cAMP), and acetylcholine (ACh) were studied with continuous measurements of the potential difference. Tissue resistance and generated current were calculated.

RESULTS

ACh induced secretion in the whole of duodenum, although the secretory response was augmented distally. PGE2 and cAMP induced significant secretion only in the distal duodenum.

CONCLUSIONS

The ACh-induced, calcium-mediated, electrogenic secretion was expressed along the whole duodenum, whereas the cAMP-mediated secretion was only seen in the distal part. The fully expressed electrogenic chloride secretion was only seen at or distal to the duodenojejunal flexure. Our study shows that it is important to carefully define the localization of physiologic studies performed in the duodenum.