Claudins of intestine and nephron - a correlation of molecular tight junction structure and barrier function

Altered expression of tight junction proteins in mammary epithelium after discontinued suckling in mice

Milk production is modulated by the paracellular barrier function of tight junction (TJ) proteins located in the mammary epithelium. The aim of our study was the molecular analysis of TJs in native lactating murine mammary gland epithelium as this process may strongly challenge epithelial barrier properties and regulation. Mammary gland tissue specimens from lactating control mice and animals after a 20-h interruption of suckling were prepared; histological analyses were performed by light and electron microscopy; and expression of TJ proteins was detected by PCR, Western blotting, immunofluorescent staining, and confocal laser scanning microscopy. Discontinuation of suckling resulted in a substantial accumulation of milk in mammary glands, an increase of alveolar size, and a flattening of epithelial cells without effects on inflammatory indicators. In control tissues, PCR and Western blots showed signals for occludin, and claudin-1, -2, -3, -4, -5, -7, -8, -15, and -16. After a 20-h accumulation of milk, expression of two sealing TJ proteins, claudin-1 and -3, was markedly increased, whereas two TJ proteins involved in cation transport, claudin-2 and -16, were reduced. Real-time PCR validated increased transcripts of claudin-1 and claudin-3. During extension of mammary glands in the process of lactation, claudin-1 and -3 are markedly induced and claudin-2 and -16 are decreased. Volume and composition of milk might be strongly dependent on this counter-regulation of sealing claudins with permeability-mediating claudins, indicating a physiological process of a tightening of TJs against a back-leak of solutes and ions from the alveolar lumen.

Intestinal epithelial CD98 synthesis specifically modulates expression of colonic microRNAs during colitis

The transmembrane glycoprotein CD98 is known to be involved in intestinal inflammation. In the present study, we found that CD98 overexpression in intestinal epithelial cells does not normally affect the expression of colonic (epithelial and immune cell) microRNAs (miRNAs), small noncoding RNAs that posttranscriptionally regulate a wide variety of biological processes. However, upon dextran sulfate sodium (DSS) treatment, the expression of several colonic miRNAs, but not miRNAs from other tissues such as liver and spleen, were differentially regulated in mice overexpressing CD98 in epithelial cells compared with wild-type (WT) animals. For example, the level of colonic miRNA 132 was not affected by DSS treatment in WT animals but was upregulated in mice overexpressing CD98 in intestinal epithelial cells. Other colonic miRNAs, including colonic miRNA 23a and 23b, were downregulated in WT animals after DSS treatment but not in colonic epithelial cell CD98-overexpressing mice. Interestingly, the expression of potential miRNA target genes affected intestinal epithelial cells that overexpress CD98 and cell types that did not overexpress CD98 but were in close proximity to CD98-overexpressing intestinal epithelial cells. Taken together, these observations show that the combination of an inflammatory context and intestinal epithelial cell expression of CD98 affects the regulation of miRNA expression in colonic epithelial and immune cells. This is new evidence that protein expression modulates miRNA expression and suggests the existence of regulatory crosstalk between proteins and miRNAs in diseases such as colitis.

Drug-induced alterations in Mg2+ homoeostasis

Magnesium (Mg2+) balance is tightly regulated by the concerted actions of the intestine, bone and kidneys. This balance can be disturbed by a broad variety of drugs. Diuretics, modulators of the EGFR (epidermal growth factor receptor), proton pump inhibitors, antimicrobials, calcineurin inhibitors and cytostatics may all cause hypomagnesaemia, potentially leading to tetany, seizures and cardiac arrhythmias. Conversely, high doses of Mg2+ salts, frequently administered as an antacid or a laxative, may lead to hypermagnesaemia causing various cardiovascular and neuromuscular abnormalities. A better understanding of the molecular mechanisms underlying the adverse effects of these medications on Mg2+ balance will indicate ways of prevention and treatment of these adverse effects and could potentially provide more insight into Mg2+ homoeostasis.

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis: phenotype-genotype correlation and outcome in 32 patients with CLDN16 or CLDN19 mutations

BACKGROUND AND OBJECTIVES

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis is a rare autosomal recessive renal tubular disease. It is caused by mutations in CLDN16 and CLDN19, encoding claudin-16 and -19, respectively. Familial hypomagnesemia with hypercalciuria and nephrocalcinosis is usually complicated by progressive CKD. The objectives of this study were to describe the clinical and genetic features of familial hypomagnesemia with hypercalciuria and nephrocalcinosis and analyze phenotype-genotype associations in patients with CLDN16 or CLDN19 mutations.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Data from 32 genetically confirmed patients (9 patients with CLDN16 and 23 patients with CLDN19 mutations) from 26 unrelated families were retrospectively reviewed.

RESULTS

Diagnosis was based on clinical criteria at a median age of 9.5 years and confirmed by genetic testing at a median age of 15.5 years. In total, 13 CLDN16 or CLDN19 mutations were identified, including 8 novel mutations. A founder effect was detected for the recurrent CLDN16 p.Ala139Val mutation in North African families and the CLDN19 p.Gly20Asp mutation in Spanish and French families. CKD was more frequently observed in patients with CLDN19 mutations: survival without CKD or ESRD was 56% at 20 years of age in CLDN19 versus 100% in CLDN16 mutations (log rank P<0.01). Ocular abnormalities were observed in 91% of patients with CLDN19 mutations and none of the patients with CLDN16 mutations (P<0.01). Treatments seem to have no effect on hypercalciuria and CKD progression.

CONCLUSIONS

Patients with CLDN19 mutations may display more severe renal impairment than patients with CLDN16 mutations. Ocular abnormalities were observed only in patients with CLDN19 mutations.

Mucosal injuries due to ribosome-inactivating stress and the compensatory responses of the intestinal epithelial barrier

Ribosome-inactivating (ribotoxic) xenobiotics are capable of using cleavage and modification to damage 28S ribosomal RNA, which leads to translational arrest. The blockage of global protein synthesis predisposes rapidly dividing tissues, including gut epithelia, to damage from various pathogenic processes, including epithelial inflammation and carcinogenesis. In particular, mucosal exposure to ribotoxic stress triggers integrated processes that are important for barrier regulation and re-constitution to maintain gut homeostasis. In the present study, various experimental models of the mucosal barrier were evaluated for their response to acute and chronic exposure to ribotoxic agents. Specifically, this review focuses on the regulation of epithelial junctions, epithelial transporting systems, epithelial cytotoxicity, and compensatory responses to mucosal insults. The primary aim is to characterize the mechanisms associated with the intestinal epithelial responses induced by ribotoxic stress and to discuss the implications of ribotoxic stressors as chemical modulators of mucosa-associated diseases such as ulcerative colitis and epithelial cancers.

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.

Paracellular permeability of bronchial epithelium is controlled by CFTR

In normal airway epithelium, the cystic fibrosis transmembrane conductance regulator (CFTR) transports Cl(-) ions to the apical surface of the epithelium paralleled by the flow of water through transcellular and paracellular pathways. The hypothesis was tested whether CFTR not only regulates the transcellular but also the paracellular shunt pathway. Therefore, we performed measurements of transepithelial electrical resistance (TER) and paracellular (14)C-mannitol permeability in wtCFTR (16HBE14o(-)) and delF508-CFTR (CFBE41o(-)) expressing human bronchial epithelial cells. Under resting conditions, CFBE41o(-) cell monolayers exhibit a higher paracellular permeability and lower TER as compared to 16HBE14o(-) monolayers. Stimulation of CFTR by cAMP induces opposite effects in the two cell lines. 16HBE14o(-) monolayers show a sharp decrease of TER, in parallel with a concomitant increase of paracellular permeability. The change in paracellular permeability is mediated by a myosin II dependent mechanism because it can be blocked by the myosin light chain kinase inhibitor ML-7. In contrast, CFBE41o(-) cells respond to cAMP stimulation with a decrease of paracellular permeability, paralleled by slight increase of TER. We conclude that stimulation of wtCFTR increases vectorial transcellular salt transport and, simultaneously, the paracellular permeability allowing water to follow through the paracellular pathway. In contrast, in CF epithelium cAMP stimulation increases neither vectorial salt transport nor paracellular permeability which is likely to contribute to the CF pulmonary phenotype. Taken together, our results link CFTR dysfunction to an improper regulation of the paracellular transport route.

Advances in research of intestinal epithelial tight junctions and intestinal permeability

Intestinal epithelial tight junctions are a structural basis for the intestinal barrier and play an important role in the regulation of intestinal permeability. Increased intestinal permeability caused by the destruction of tight junctions may result in bacterial translocation, systemic inflammatory response, and multiple organ dysfunction syndrome. In this paper, we review the structure and function of tight junctions, factors affecting intestinal permeability, and measures for improving the dysfunction in intestinal permeability.

Tight junction proteins contribute to barrier properties in human pleura

The permeability of pleural mesothelium helps to control the volume and composition of the liquid lubricating pleural surfaces. Information on pleural barrier function in health and disease, however, is scarce. Tissue specimens of human pleura were mounted in Ussing chambers for measurement of transmesothelial resistance. Expression of tight junction (TJ) proteins was studied by Western blots and immune fluorescence confocal microscopy. Both visceral and parietal pleura showed barrier properties represented by transmesothelial resistance. Occludin, claudin-1, -3, -5, and -7, were detected in visceral pleura. In parietal pleura, the same TJ proteins were detected, except claudin-7. In tissues from patients with pleural inflammation these tightening claudins were decreased and in visceral pleura claudin-2, a paracellular channel former, became apparent. We report that barrier function in human pleura coincides with expression of claudins known to be key determinants of epithelial barrier properties. In inflamed tissue, claudin expression indicates a reduced barrier function.