The Sandwich Assay: A Method for Subcellular Visualization of Paracellular Macromolecule Passage in Epithelial Sheets

EpCAM proteolysis and release of complexed claudin-7 repair and maintain the tight junction barrier

TJs maintain the epithelial barrier by regulating paracellular permeability. Since TJs are under dynamically fluctuating intercellular tension, cells must continuously survey and repair any damage. However, the underlying mechanisms allowing cells to sense TJ damage and repair the barrier are not yet fully understood. Here, we showed that proteinases play an important role in the maintenance of the epithelial barrier. At TJ break sites, EpCAM-claudin-7 complexes on the basolateral membrane become accessible to apical membrane-anchored serine proteinases (MASPs) and the MASPs cleave EpCAM. Biochemical data and imaging analysis suggest that claudin-7 released from EpCAM contributes to the rapid repair of damaged TJs. Knockout (KO) of MASPs drastically reduced barrier function and live-imaging of TJ permeability showed that MASPs-KO cells exhibited increased size, duration, and frequency of leaks. Together, our results reveal a novel mechanism of TJ maintenance through the localized proteolysis of EpCAM at TJ leaks, and provide a better understanding of the dynamic regulation of epithelial permeability.

Probing the leak pathway: Live-cell imaging of macromolecule passage through epithelia

Epithelia compartmentalize multicellular organisms and provide interfacing between the inside and outside. Apart from regulating the exchange of solutes, uptake of nutrients, and excretion of waste products, their major function is to prevent uncontrolled access of foreign material to immune-competent compartments. Progress in understanding this barrier function toward larger solutes and its possible defects, as can be seen in a variety of diseases, is largely hampered by a lack of methods to spatiotemporally resolve transepithelial passage of macromolecules. Using different cell culture epithelia, we applied biotinylated dextran tracers carrying an acceptor fluorophore. These bind to cell-adherent avidin carrying donor fluorophore at the basolateral membranes of single-layered epithelial sheets. Confocal fluorescence microscopy was applied to living epithelia in order to image apical-to-basolateral tracer passage as a Förster resonance energy transfer signal of the fluorescent dextran-avidin pair over time. Stimulated macromolecule passage using barrier-perturbing agents proved its effectiveness for the leak imaging method presented herein. Over hours of imaging, spontaneous leaks were rare, occurring transiently on the scale of minutes and for the most part associated with rearranging cell junctions. The discussed approach to leak imaging is expected to promote the understanding of epithelial barriers, particularly, the nature and dynamics of the epithelial cell leak pathway.

Post-translational modifications of tight junction transmembrane proteins and their direct effect on barrier function

Post-translational modifications (PTMs) such as phosphorylation, ubiquitination or glycosylation are processes affecting the conformation, stability, localization and function of proteins. There is clear evidence that PTMs can act upon tight junction (TJ) proteins, thus modulating epithelial barrier function. Compared to transcriptional or translational regulation, PTMs are rapid and more dynamic processes so in the context of barrier maintenance they might be essential for coping with changing environmental or external impacts. The aim of this review is to extract literature deciphering PTMs in TJ proteins directly contributing to epithelial barrier changes in permeability to ions and macromolecules. It is not intended to cover the entire scope of PTMs in TJ proteins and should rather be understood as a digest of TJ protein modifications directly resulting in the tightening or opening of the epithelial barrier.

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.

Occludin knockdown is not sufficient to induce transepithelial macromolecule passage

Occludin, a tight junction protein, has been reported to regulate barrier function - particularly the leak pathway for larger solutes - in epithelia. Therefore, we aimed to precisely define its role in macromolecule passage at single cell-cell junctions. A combination of varying occludin expression by transient and stable knockdown including systematic seeding strategies was employed to achieve a broad and defined pattern of variance in occludin expression over epithelia. This variance model enabled us to examine occludin function in the leak pathway using global and local analysis, i.e. to analyze macromolecule flux across epithelia and macromolecule passage at single-cell level. Macromolecular flux was found not to correlate with occludin expression in intestinal epithelial cells. In fact, by spatially resolving macromolecular permeation sites using a recently developed method we uncovered leaky cell junctions at the edge of Transwells resulting in increased passage. This demonstrates that rare leaks can determine net flux of macromolecules across epithelia while the vast majority of cellular junctions do not contribute significantly. Hence, concomitant local analysis of macromolecule passage across epithelial barriers is indispensable for interpretation of global flux data. By combining this new approach with cell culture models of the leak pathway, we can present evidence that lack of occludin is not sufficient to stimulate the epithelial leak pathway.