Tissue barriers that restrict passage of liquids, ions, and larger solutes are essential for the development of multicellular organisms. In simple organisms this allows distinct cell types to interface with the external environment. In more complex species, the diversity of cell types capable of forming barriers increases dramatically. Although the plasma membranes of these barrier-forming cells prevent flux of most hydrophilic solutes, the paracellular, or shunt, pathway between cells must also be sealed. This function is accomplished in vertebrates by the zonula occludens, or tight junction. The tight junction barrier is not absolute but is selectively permeable and is able to discriminate between solutes on the basis of size and charge. Many tight junction components have been identified over the past 20 years, and recent progress has provided new insights into the proteins and interactions that regulate structure and function. This review presents these data in a historical context and proposes an integrated model in which dynamic regulation of tight junction protein interactions determines barrier function.

Epithelial paracellular barrier function, determined primarily by tight junction permeability, is frequently disrupted in disease. In the intestine, barrier loss can be mediated by tumor necrosis factor (α) (TNF) signaling and epithelial myosin light chain kinase (MLCK) activation. However, TNF induces only limited alteration of tight junction morphology, and the events that couple structural reorganization to barrier regulation have not been defined. We have used in vivo imaging and transgenic mice expressing fluorescent-tagged occludin and ZO-1 fusion proteins to link occludin endocytosis to TNF-induced tight junction regulation. This endocytosis requires caveolin-1 and is essential for structural and functional tight junction regulation. These data demonstrate that MLCK activation triggers caveolin-1–dependent endocytosis of occludin to effect structural and functional tight junction regulation.

Depressed contractility is a central feature of the failing human heart and has been attributed to altered [Ca ²⁺ ] _i . This study examined the respective roles of the L-type Ca ²⁺ current ( I _Ca ), SR Ca ²⁺ uptake, storage and release, Ca ²⁺ transport via the Na ⁺ -Ca ²⁺ exchanger (NCX), and Ca ²⁺ buffering in the altered Ca ²⁺ transients of failing human ventricular myocytes. Electrophysiological techniques were used to measure and control V _m and measure I _m , respectively, and Fluo-3 was used to measure [Ca ²⁺ ] _i in myocytes from nonfailing (NF) and failing (F) human hearts. Ca ²⁺ transients from F myocytes were significantly smaller and decayed more slowly than those from NF hearts. Ca ²⁺ uptake rates by the SR and the amount of Ca ²⁺ stored in the SR were significantly reduced in F myocytes. There were no significant changes in the rate of Ca ²⁺ removal from F myocytes by the NCX, in the density of NCX current as a function of [Ca ²⁺ ] _i , I _Ca density, or cellular Ca ²⁺ buffering. However, Ca ²⁺ influx during the late portions of the action potential seems able to elevate [Ca ²⁺ ] _i in F but not in NF myocytes. A reduction in the rate of net Ca ²⁺ uptake by the SR slows the decay of the Ca ²⁺ transient and reduces SR Ca ²⁺ stores. This leads to reduced SR Ca ²⁺ release, which induces additional Ca ²⁺ influx during the plateau phase of the action potential, further slowing the decay of the Ca ²⁺ transient. These changes can explain the defective Ca ²⁺ transients of the failing human ventricular myocyte.

Somatic mutations in tet methylcytosine dioxygenase 2 (TET2), which encodes an epigenetic modifier enzyme, drive the development of haematopoietic malignancies^{1,2,3,4,5,6,7}. In both humans and mice, TET2 deficiency leads to increased self-renewal of haematopoietic stem cells with a net developmental bias towards the myeloid lineage^1,4,8,9. However, pre-leukaemic myeloproliferation (PMP) occurs in only a fraction of Tet2^−/− mice^8,9 and humans with TET2 mutations^1,3,5,6,7, suggesting that extrinsic non-cell-autonomous factors are required for disease onset. Here we show that bacterial translocation and increased interleukin-6 production, resulting from dysfunction of the small-intestinal barrier, are critical for the development of PMP in mice that lack Tet2 expression in haematopoietic cells. Furthermore, in symptom-free Tet2^−/− mice, PMP can be induced by disrupting intestinal barrier integrity, or in response to systemic bacterial stimuli such as the toll-like receptor 2 agonist. PMP was reversed by antibiotic treatment and failed to develop in germ-free Tet2^−/− mice, which illustrates the importance of microbial signals in the development of this condition. Our findings demonstrate the requirement for microbial-dependent inflammation in the development of PMP and provide a mechanistic basis for the variation in PMP penetrance observed in Tet2^−/− mice. This study will prompt new lines of investigation that may profoundly affect the prevention and management of haematopoietic malignancies.

Patients with inflammatory bowel disease (IBD) are at increased risk of developing colorectal adenocarcinoma. The factors that result in IBD-associated carcinogenesis are not understood. We hypothesized that altered expression of intestinal epithelial tight junction proteins might contribute to neoplastic progression. Semiquantitative immunohistochemical staining of human biopsies was used to assess expression of the tight junction proteins claudin-1, claudin-2, claudin-4, and occludin in IBD, IBD-associated dysplasia, acute, self-limited colitis (ASLC), and sporadic adenomas. Claudin-1 and claudin-2 expression was elevated in active IBD, adenomas, and IBD-associated dysplasia, but not ASLC. In contrast, claudin-4 expression was elevated in both active IBD and ASLC. Occludin expression was similar to control in all cases. Importantly, in IBD, claudin-1 and claudin-2 expression correlated positively with inflammatory activity. To investigate mechanisms underlying altered claudin expression, β-catenin activation was assessed as nuclear localization. Like claudin-1 and claudin-2, β-catenin was markedly activated in IBD, sporadic dysplasia, and IBD-associated dysplasia, but was only slightly activated in ASLC. Taken together, these data suggest that β-catenin transcriptional activity is elevated in chronic injury and that this may contribute to increased claudin-1 and claudin-2 expression. We speculate that increased claudin-1 and claudin-2 expression may be involved at early stages of transformation in IBD-associated neoplasia.