Belt-like tight junctions (TJs), referred to as zonula occludens, have long been regarded as a specialized differentiation of epithelial cell membranes. They are required for cell adhesion and paracellular barrier functions, and are now thought to be partly involved in fence functions and in cell polarization. Recently, the molecular bases of TJs have gradually been unveiled. TJs are constructed by TJ strands, whose basic frameworks are composed of integral membrane proteins with four transmembrane domains, designated claudins. The claudin family is supposedly composed of at least 24 members in mice and humans. Other types of integral membrane proteins with four transmembrane domains, namely occludin and tricellulin, as well as the single transmembrane proteins, JAMs (junctional adhesion molecules) and CAR (coxsackie and adenovirus receptor), are associated with TJ strands, and the high-level organization of TJ strands is likely to be established by membrane-anchored scaffolding proteins, such as ZO-1/2. Recent functional analyses of claudins in cell cultures and in mice have suggested that claudin-based TJs may have pivotal functions in the regulation of the epithelial microenvironment, which is critical for various biological functions such as control of cell proliferation. These represent the dawn of ‘Barriology’ (defined by Shoichiro Tsukita as the science of barriers in multicellular organisms). Taken together with recent reports regarding changes in claudin expression levels, understanding the regulation of the TJ-based microenvironment system will provide new insights into the regulation of polarization in the respect of epithelial microenvironment system and new viewpoints for developing anticancer strategies.

Zonula occludens (ZO)-1/2/3 are the members of the TJ-MAGUK family of membrane-associated guanylate kinases associated with tight junctions. To investigate the role of ZO-1 (encoded by Tjp1) in vivo, ZO-1 knockout (Tjp1^−/−) mice were generated by gene targeting. Although heterozygous mice showed normal development and fertility, delayed growth and development were evident from E8.5 onward in Tjp1^−/−embryos, and no viable Tjp1^−/−embryos were observed beyond E11.5. Tjp1^−/−embryos exhibited massive apoptosis in the notochord, neural tube area, and allantois at embryonic day (E)9.5. In the yolk sac, the ZO-1 deficiency induced defects in vascular development, with impaired formation of vascular trees, along with defective chorioallantoic fusion. Immunostaining of wild-type embryos at E8.5 for ZO-1/2/3 revealed that ZO-1/2 were expressed in almost all embryonic cells, showing tight junction-localizing patterns, with or without ZO-3, which was confined to the epithelial cells. ZO-1 deficiency depleted ZO-1-expression without influence on ZO-2/3 expression. In Tjp1^+/+yolk sac extraembryonic mesoderm, ZO-1 was dominant without ZO-2/3 expression. Thus, ZO-1 deficiency resulted in mesoderms with no ZO-1/2/3, associated with mislocalization of endothelial junctional adhesion molecules. As a result, angiogenesis was defected in Tjp1^−/−yolk sac, although differentiation of endothelial cells seemed to be normal. In conclusion, ZO-1 may be functionally important for cell remodeling and tissue organization in both the embryonic and extraembryonic regions, thus playing an essential role in embryonic development.

Background & Aims In the small intestine, the paracellular transport of Na⁺ is thought to be critical for luminal Na⁺-homeostasis and the transcellular absorption of nutrients by Na⁺-driven transporters. Na⁺ is supplied to the intestinal lumen from the submucosa and serum through tight junctions, which form a paracellular barrier between the cells of epithelial sheets. However, the molecular basis for this paracellular transport of Na⁺ is not well understood. Here, we examined this mechanism by performing loss-of-function studies of claudin-2 and claudin-15, two tight-junctional membrane proteins that are specifically and age-dependently expressed in the villi and/or crypts of small intestinal epithelia. Methods Knockout mice for claudin-2 or claudin-15 were subjected to histologic, cell biologic, electrophysiologic, and physiologic analyses. Results Examination of the knockout mice revealed that both claudin-2 and claudin-15 play crucial roles in the transepithelial paracellular channel-like permselectivity for extracellular monovalent cations, particularly Na⁺, in infants and adults. Especially in Cldn15^−/− adults, the luminal Na⁺ concentration in the small intestine measured directly in vivo was abnormally low, and glucose absorption was impaired, as assessed by the oral glucose tolerance test and estimation of unabsorbed glucose. Conclusions We propose that the “Na⁺-leaky” claudin-15 is indispensable in vivo for the paracellular Na⁺ permeability, luminal Na⁺-homeostasis, and efficient glucose absorption in the small intestine, but claudin-2 is indispensable for only the first of these functions. Claudin-15 knockout leads to Na⁺ deficiency and glucose malabsorption in the mouse adult small intestine.

Objective To design novel anti-inflammation treatments, it is important to recognise two distinct steps of inflammation: initiation and acceleration. In IBDs, intestinal inflammation is reported to be accelerated by dysfunction in the epithelial paracellular barrier formed by tight junctions (TJs). However, it is unclear whether changes in paracellular barrier function initiate inflammation. Some of the intestinal claudin-family proteins, which form the paracellular barrier, show aberrant expression levels and localisations in IBDs. We aimed to elucidate the role of paracellular-barrier change in initiating colonic inflammation. Design We generated intestine-specific conditional knockout mice of claudin-7 (Cldn7), one of the predominant intestinal claudins. Results The intestine-specific Cldn7 deficiency caused colonic inflammation, even though TJ structures were still present due to other claudins. The paracellular flux (pFlux), determined by measuring the paracellular permeability across the colon epithelium, was enhanced by the Cldn7 deficiency for the small organic solute Lucifer Yellow (457 Da), but not for the larger organic solute FITC-Dextran (4400 Da). Consistent with these results, the intestine-specific claudin-7 deficiency enhanced the pFlux for N-formyl-l-methionyl-l-leucyl-l-phenylalanine (fMLP) (438 Da), a major bacterial product, to initiate colonic inflammation. Conclusions These findings suggest that specific enhancement of the pFlux for small organic solutes across the claudin-based TJs initiates colonic inflammation.