Objective: 

Recent molecular identification of tight junction constituents has paved the way for analyses of structure, function and regulation of the barrier. Whereas perturbation and internalisation of tight junction proteins away from tight junction strands have been reported, so far information concerning induction of tight junction proteins are relatively scarce. In our studies, we have analysed effects of induction of sodium transport and of inflammatory processes on tight junction regulation, in vitro.

Methods: 

Preparations of human colon (inflamed or non-inflamed) and monolayers of the epithelial colon cell line HT-29/B6 grown on permeable supports were mounted in Ussing chambers and corticosteroids and proinflammatory cytokines were added, respectively. After incubation, molecular analyses including confocal laser-scanning microscopy, real time PCR and immunoblotting were performed on selfsame cells and tissues.

Results: 

After induction and subsequent inhibition of the epithelial sodium channel employing corticosteroids and the specific blocker amiloride, an induction of occludin and claudin-8 was observed. Claudin-8 was induced on transcriptional level, and Na⁺-entry into the cell was a prerequisite for this effect in both, colon preparations and epithelial cells. In a separate approach, tight junction proteins were monitored in inflamed colon epithelia of patients with Crohn's disease and ulcerative colitis. These tissues show an increased expression of claudin-2, which could be evaluated by incubation of the epithelial colon cell line with proinflammatory cytokines.

Conclusions: 

In these studies, novel mechanisms of barrier regulation are reported: a synergistic effect of transport and barrier function during physiological Na⁺ absorption, and the perturbation of barrier properties in IBD, which can be attributed to direct effects of proinflammatory cytokines on claudin-2.