ZO-2 silencing in epithelial cells perturbs the gate and fence function of tight junctions and leads to an atypical monolayer architecture

The Role of ZO-2 in Modulating JAM-A and γ-Actin Junctional Recruitment, Apical Membrane and Tight Junction Tension, and Cell Response to Substrate Stiffness and Topography

This work analyzes the role of the tight junction (TJ) protein ZO-2 on mechanosensation. We found that the lack of ZO-2 reduced apical membrane rigidity measured with atomic force microscopy, inhibited the association of γ-actin and JAM-A to the cell border, and instead facilitated p114RhoGEF and afadin accumulation at the junction, leading to an enhanced mechanical tension at the TJ measured by FRET, with a ZO-1 tension probe, and increased tricellular TJ tension. Simultaneously, adherens junction tension measured with an E-cadherin probe was unaltered. The stability of JAM-A and ZO-2 binding was assessed by a collaborative in silico study. The absence of ZO-2 also impacted the cell response to the substrate, as monolayers plated in 20 kPa hydrogels developed holes not seen in parental cultures and displayed a retarded elongation and formation of cell aggregates. The absence of ZO-2 was sufficient to induce YAP and Snail nuclear accumulation in cells cultured over glass, but when ZO-2 KD cells were plated in nanostructured ridge arrays, they displayed an increased abundance of nuclear Snail and conspicuous internalization of claudin-4. These results indicate that the absence of ZO-2 also impairs the response of cells to substrate stiffness and exacerbates transformation triggered by substrate topography.

Dietary supplementation with Dendrobium officinale leaves improves growth, antioxidant status, immune function, and gut health in broilers

Background

The Dendrobium officinale leaves (DOL) is an underutilized by-product with a large biomass, which have been shown to exhibit immunomodulatory and antioxidant functions. The purpose of this research was to investigate the effects of DOL on broiler growth performance, antioxidant status, immune function, and gut health.

Methods

One hundred and ninety-two 1-day-old chicks were selected and divided into 4 groups at random, 6 replicates for each group and 8 in each. Chicks were given a basal diet supplemented with different amounts of DOL: 0% (control group, NC), 1% (LD), 5% (MD), or 10% (HD). During the feeding trial (70 days), broiler body weight, feed intake, and residual feeding were recorded. On d 70, 12 broilers from each group were sampled for serum antioxidant and immune indexes measurement, intestinal morphological analysis, as well as 16S rRNA sequencing of cecal contents and short-chain fatty acid (SCFA) determination.

Results

In comparison to the NC group, the LD group had greater final body weight and average daily gain, and a lower feed conversion ratio (p < 0.05, d 1 to 70). However, in MD group, no significant change of growth performance occurred (p > 0.05). Furthermore, DOL supplementation significantly improved the levels of serum total antioxidant capacity, glutathione peroxidase, superoxide dismutase, and catalase, but reduced the level of malondialdehyde (p < 0.05). Higher serum immunoglobulin A (IgA) content and lower cytokine interleukin-2 (IL-2) and IL-6 contents were observed in DOL-fed broilers than in control chickens (p <0.05). Compared to the NC group, duodenal villus height (VH) and villus height-to-crypt depth (VH:CD) ratio were considerably higher in three DOL supplementation groups (p < 0.05). Further, 16S rRNA sequencing analysis revealed that DOL increased the diversity and the relative abundance of cecal bacteria, particularly helpful microbes like Faecalibacterium, Lactobacillus, and Oscillospira, which improved the production of SCFA in cecal content. According to Spearman correlation analysis, the increased butyric acid and acetic acid concentrations were positively related to serum antioxidant enzyme activities (T-AOC and GSH-Px) and immunoglobulin M (IgM) level (p < 0.05).

Conclusion

Overall, the current study demonstrated that supplementing the dies with DOL in appropriate doses could enhance growth performance, antioxidant capacity, and immune response, as well as gut health by promoting intestinal integrity and modulating the cecal microbiota in broilers. Our research may serve as a preliminary foundation for the future development and application of DOL as feed additive in broiler chicken diets.

Tight junction proteins related to blood-brain barrier and their regulatory signaling pathways in ischemic stroke

Tight junctions (TJs) are crucial for intercellular connections. The abnormal expression of proteins related to TJs can result in TJ destruction, structural damage, and endothelial and epithelial cell dysfunction. These factors are associated with the occurrence and progression of several diseases. Studies have shown that blood-brain barrier (BBB) damage and dysfunction are the prominent pathological features of stroke. TJs are directly associated with the BBB integrity. In this article, we first discuss the structure and function of BBB TJ-related proteins before focusing on the crucial events that cause TJ dysfunction and BBB damage, as well as the regulatory mechanisms that affect the qualitative and quantitative expression of TJ proteins during ischemic stroke. Multiple regulatory mechanisms, including phosphorylation, matrix metalloproteinases (MMPs), and microRNAs, regulate TJ-related proteins and affect BBB permeability. Some signaling pathways and mechanisms have been demonstrated to have dual functions. Hopefully, our understanding of the regulation of BBB TJs in ischemic stroke will be applied to the development of targeted medications and therapeutic therapies.

Genome-wide identification, evolution and expression analysis of tight junction gene family and the immune roles of claudin5 gene in turbot (Scophthalmus maximus L.)

Tight junction proteins (TJs) are important component proteins that maintaining the structure and function of TJs, connecting to each other to form a TJ complex between cells, maintaining the biological homeostasis of the internal environment. In this study, a total of 103 TJ genes were identified in turbot according to our whole-transcriptome database. Transmembrane TJs were divided into seven subfamilies, including claudin (CLDN), occludin (OCLD), tricellulin (MARVELD2), MARVEL domain containing 3 (MARVELD3), junctional adhesion molecules (JAM), immunoglobulin superfamily member 5 (IGSF5/JAM4), blood vessel epicardial substance (BVEs). Moreover, the majority of homologous pairs of TJ genes showed highly conserved alongside length, exon/intron number and motifs. As for phylogenetic analysis for 103 TJ genes, eight of them have undergone a positive selection and JAMB-like has undergone the most neutral evolution. The expression patterns of several TJ genes showed the lowest expression levels in blood, while the highest expression levels were detected in intestine, gill and skin, which all belong to mucosal tissues. Meanwhile, most examined TJ genes showed down-regulated expression patterns during bacterial infection, while several TJ genes exhibited up-regulated expression patterns at a later stage (24 h). At the same time, several potential candidate genes (such as CLDN-15, CLDN-3, CLDN-12, CLDN-5 and OCLD) were significantly down-regulated, which may indicate their important functions that involved in the regulation of bacterial infection. Currently, there is little research on CLDN5 in the intestine, but it is highly expressed in the intestine and has significant changes in intestinal expression after bacterial infection. Thus, we knocked down CLDN5 by the method of lentiviral infection. The result showed CLDN5 was related to cell migration (wound healing) and apoptosis, and the method of dualluciferasereporterassay showed that the functions of CLDN5 could be regulated by miR-24. The study of TJs may lead to a better understanding of the function of TJs in teleost.

How Nanoparticles Open the Paracellular Route of Biological Barriers: Mechanisms, Applications, and Prospects

Biological barriers are essential physiological protective systems and obstacles to drug delivery. Nanoparticles (NPs) can access the paracellular route of biological barriers, either causing adverse health impacts on humans or producing therapeutic opportunities. This Review introduces the structural and functional influences of NPs on the key components that govern the paracellular route, mainly tight junctions, adherens junctions, and cytoskeletons. Furthermore, we evaluate their interaction mechanisms and address the influencing factors that determine the ability of NPs to open the paracellular route, which provides a better knowledge of how NPs can open the paracellular route in a safer and more controllable way. Finally, we summarize limitations in the research models and methodologies of the existing research in the field and provide future research direction. This Review demonstrates the in-depth causes for the reversible opening or destruction of the integrity of barriers generated by NPs; more importantly, it contributes insights into the design of NP-based medications to boost paracellular drug delivery efficiency.

Simple graphical approach to investigate differences in transepithelial paracellular leak pathway permeability

Although many studies have reported differences in epithelial paracellular Leak Pathway permeability following genetic manipulations and treatment with various agents, the basis for these differences remains mostly unclear. Two primary mechanisms which could underlie differences in Leak Pathway permeability are differences in the density of Leak Pathway openings and differences in the opening size. Using a computational approach, we demonstrate that these two possibilities can be readily distinguished graphically by comparing the apparent paracellular permeabilities of a size panel of solutes measured across different cell layers. Using this approach, we demonstrated that depletion of ZO-1 protein in MDCK Type II renal epithelial cells decreased Leak Pathway opening size and increased opening density. Depletion of ZO-2 protein either had no effect or minimally decreased opening size and did not markedly change opening density. Comparison of MDCK Type II cells with MDCK Type I cells revealed that Type I cells exhibited a substantially smaller Leak Pathway permeability than did Type II cells. This lower permeability was due to a decrease in opening density with little or no change in opening size. These results demonstrate the utility of this approach to provide insights into the basis for observed differences in epithelial Leak Pathway permeability. This approach has wide applications including analysis of the molecular basis for Leak Pathway permeability, the effects of specific manipulations on Leak Pathway permeability properties, and the effects of permeation enhancers on Leak Pathway permeability properties.

Epithelial integrity, junctional complexes, and biomarkers associated with intestinal functions

An intact intestinal barrier is crucial for immune homeostasis and its impairment activates the immune system and may result in chronic inflammation. The epithelial cells of the intestinal barrier are connected by tight junctions, which form an anastomosing network sealing adjacent epithelial cells. Tight junctions are composed of transmembrane and cytoplasmic scaffolding proteins. Transmembrane tight junction proteins at the apical-lateral membrane of the cell consist of occludin, claudins, junctional adhesion molecules, and tricellulin. Cytoplasmic scaffolding proteins, including zonula occludens, cingulin and afadin, provide a direct link between transmembrane tight junction proteins and the intracellular cytoskeleton. Each individual component of the tight junction network closely interacts with each other to form an efficient intestinal barrier. This review aims to describe the molecular structure of intestinal epithelial tight junction proteins and to characterize their organization and interaction. Moreover, clinically important biomarkers associated with impairment of gastrointestinal integrity are discussed.

ZO-2 favors Hippo signaling, and its re-expression in the steatotic liver by AMPK restores junctional sealing

ZO-2 is a peripheral tight junction (TJ) protein whose silencing in renal epithelia induces cell hypertrophy. Here, we found that in ZO-2 KD MDCK cells, in compensatory renal hypertrophy triggered in rats by a unilateral nephrectomy and in liver steatosis of obese Zucker (OZ) rats, ZO-2 silencing is accompanied by the diminished activity of LATS, a kinase of the Hippo pathway, and the nuclear concentration of YAP, the final effector of this signaling route. ZO-2 appears to function as a scaffold for the Hippo pathway as it associates to LATS1. ZO-2 silencing in hypertrophic tissue is due to a diminished abundance of ZO-2 mRNA, and the Sp1 transcription factor is critical for ZO-2 transcription in renal cells. Treatment of OZ rats with metformin, an activator of AMPK that blocks JNK activity, augments ZO-2 and claudin-1 expression in the liver, reduces the paracellular permeability of hepatocytes, and serum bile acid content. Our results suggest that ZO-2 silencing is a common feature of hypertrophy, and that ZO-2 is a positive regulator of the Hippo pathway that regulates cell size. Moreover, our observations highlight the importance of AMPK, JNK, and ZO-2 as therapeutic targets for blood-bile barrier dysfunction.

Zonula occludens 2 and Cell-Cell Contacts Are Required for Normal Nuclear Shape in Epithelia

MAGUK protein ZO-2 is present at tight junctions (TJs) and nuclei. In MDCK ZO-2 knockdown (KD) cells, nuclei exhibit an irregular shape with lobules and indentations. This condition correlates with an increase in DNA double strand breaks, however cells are not senescent and instead become resistant to UV-induced senescence. The irregular nuclear shape is also observed in isolated cells and in those without TJs, due to the lack of extracellular calcium. The aberrant nuclear shape of ZO-2 KD cells is not accompanied by a reduced expression of lamins A/C and B and lamin B receptors. Instead, it involves a decrease in constitutive and facultative heterochromatin, and microtubule instability that is restored with docetaxel. ZO-2 KD cells over-express SUN-1 that crosses the inner nuclear membrane and connects the nucleoskeleton of lamin A to nesprins, which traverse the outer nuclear membrane. Nesprins-3 and -4 that indirectly bind on their cytoplasmic face to vimentin and microtubules, respectively, are also over-expressed in ZO-2 KD cells, whereas vimentin is depleted. SUN-1 and lamin B1 co-immunoprecipitate with ZO-2, and SUN-1 associates to ZO-2 in a pull-down assay. Our results suggest that ZO-2 forms a complex with SUN-1 and lamin B1 at the inner nuclear membrane, and that ZO-2 and cell–cell contacts are required for a normal nuclear shape.

The Epithelial Cell Leak Pathway

The epithelial cell tight junction structure is the site of the transepithelial movement of solutes and water between epithelial cells (paracellular permeability). Paracellular permeability can be divided into two distinct pathways, the Pore Pathway mediating the movement of small ions and solutes and the Leak Pathway mediating the movement of large solutes. Claudin proteins form the basic paracellular permeability barrier and mediate the movement of small ions and solutes via the Pore Pathway. The Leak Pathway remains less understood. Several proteins have been implicated in mediating the Leak Pathway, including occludin, ZO proteins, tricellulin, and actin filaments, but the proteins comprising the Leak Pathway remain unresolved. Many aspects of the Leak Pathway, such as its molecular mechanism, its properties, and its regulation, remain controversial. In this review, we provide a historical background to the evolution of the Leak Pathway concept from the initial examinations of paracellular permeability. We then discuss current information about the properties of the Leak Pathway and present current theories for the Leak Pathway. Finally, we discuss some recent research suggesting a possible molecular basis for the Leak Pathway.

E7 oncoprotein from human papillomavirus 16 alters claudins expression and the sealing of epithelial tight junctions

Tight junctions (TJs) are cell-cell adhesion structures frequently altered by oncogenic transformation. In the present study the role of human papillomavirus (HPV) 16 E7 oncoprotein on the sealing of TJs was investigated and also the expression level of claudins in mouse cervix and in epithelial Madin-Darby Canine Kidney (MDCK) cells. It was found that there was reduced expression of claudins -1 and -10 in the cervix of 7-month-old transgenic K14E7 mice treated with 17β-estradiol (E₂), with invasive cancer. In addition, there was also a transient increase in claudin-1 expression in the cervix of 2-month-old K14E7 mice, and claudin-10 accumulated at the border of cells in the upper layer of the cervix in FvB mice treated with E₂, and in K14E7 mice treated with or without E₂. These changes were accompanied by an augmented paracellular permeability of the cervix in 2- and 7-monthold FvB mice treated with E₂, which became more pronounced in K14E7 mice treated with or without E₂. In MDCK cells the stable expression of E7 increased the space between adjacent cells and altered the architecture of the monolayers, induced the development of an acute peak of transepithelial electrical resistance accompanied by a reduced expression of claudins -1, -2 and -10, and an increase in claudin-4. Moreover, E7 enhances the ability of MDCK cells to migrate through a 3D matrix and induces cell stiffening and stress fiber formation. These observations revealed that cell transformation induced by HPV16 E7 oncoprotein was accompanied by changes in the pattern of expression of claudins and the degree of sealing of epithelial TJs.

Relationship between apical junction proteins, gene expression and cancer

The apical junctional complex (AJC) is a cell-cell adhesion system present at the upper portion of the lateral membrane of epithelial cells integrated by the tight junction (TJ) and the adherens junction (AJ). This complex is crucial to initiate and stabilize cell-cell adhesion, to regulate the paracellular transit of ions and molecules and to maintain cell polarity. Moreover, we now consider the AJC as a hub of signal transduction that regulates cell-cell adhesion, gene transcription and cell proliferation and differentiation. The molecular components of the AJC are multiple and diverse and depending on the cellular context some of the proteins in this complex act as tumor suppressors or as promoters of cell transformation, migration and metastasis outgrowth. Here, we describe these new roles played by TJ and AJ proteins and their potential use in cancer diagnostics and as targets for therapeutic intervention.

Simvastatin improves lysosome function via enhancing lysosome biogenesis in endothelial cells

Nlrp3 inflammasomes were shown to play a critical role in triggering obesity-associated early onsets of cardiovascular complications such as endothelial barrier dysfunction with endothelial hyperpermeability. Statins prevent endothelial dysfunction and decrease cardiovascular risk in patients with obesity and diabetes. However, it remains unclear whether statin treatment for obesity-induced endothelial barrier dysfunction is in part due to the blockade of Nlrp3 inflammasome signaling axis. The results showed that simvastatin, a clinically and widely used statin, prevented free fatty acid-induced endothelial hyperpermeability and disruption of ZO-1 and VE-cadherin junctions in mouse microvascular endothelial cells (MVECs). This protective effect of simvastatin was largely due to improved lysosome function that attenuated lysosome injury-mediated Nlrp3 inflammasome activation and subsequent release of high mobility group box protein-1 (HMGB1). Mechanistically, simvastatin induces autophagy that promotes removal of damaged lysosomes and also promotes lysosome regeneration that preserves lysosome function. Collectively, simvastatin treatment improves lysosome function via enhancing lysosome biogenesis and its autophagic turnover, which may be an important mechanism to suppress Nlrp3 inflammasome activation and prevents endothelial hyperpermeability in obesity.

Contribution of cathepsin B-dependent Nlrp3 inflammasome activation to nicotine-induced endothelial barrier dysfunction

Recent studies indicate that endothelial Nlrp3 inflammasome is critically involved in the development of cardiovascular complications. However, it remains unknown whether endothelial inflammasome is involved in endothelial barrier dysfunction associated with smoking. This study aims to investigate the role of endothelial Nlrp3 inflammasome in nicotine-induced disruption of inter-endothelial tight junctions and consequent endothelial barrier dysfunction. The confocal microscopic analysis demonstrated that mice treated with nicotine exhibited disrupted inter-endothelial tight junctions as shown by decreased ZO-1 and ZO-2 expression in the coronary arterial endothelium, whereas the decreases in ZO-1/2 were prevented by Nlrp3 gene deficiency. In cultured endothelial cells, nicotine caused Nlrp3 inflammasome complex formation and enhances the inflammasome activity as shown by increased cleavage of pro-caspase-1, and interleukin-1β (IL-1β) production. Further, nicotine disrupted tight junction and increased permeability in an endothelial cell monolayer, and this nicotine-induced effect was prevented by silencing of Nlrp3 gene, inhibition of caspase-1, or blockade of high mobility group box 1 (HMGB1). Nicotine increased endothelial cell lysosomal membrane permeability and triggered the lysosomal release of cathepsin B, whereas these events were prevented by pretreating cells with a lysosome stabilizing agent, dexamethasone. Collectively, our data suggest that nicotine enhances cathepsin B-dependent Nlrp3 inflammasome activation and the consequent production of a novel permeability factor HMGB1, which causes disruption of inter-endothelial tight junctions leading to endothelial hyperpermeability. Instigation of endothelial inflammasomes may serve as an important pathogenic mechanism contributing to the early onset of vasculopathy associated with smoking.

Endothelial tight junctions and their regulatory signaling pathways in vascular homeostasis and disease

Endothelial tight junctions (TJs) regulate the transport of water, ions, and molecules through the paracellular pathway, serving as an important barrier in blood vessels and maintaining vascular homeostasis. In endothelial cells (ECs), TJs are highly dynamic structures that respond to multiple external stimuli and pathological conditions. Alterations in the expression, distribution, and structure of endothelial TJs may lead to many related vascular diseases and pathologies. In this review, we provide an overview of the assessment methods used to evaluate endothelial TJ barrier function both in vitro and in vivo and describe the composition of endothelial TJs in diverse vascular systems and ECs. More importantly, the direct phosphorylation and dephosphorylation of TJ proteins by intracellular kinases and phosphatases, as well as the signaling pathways involved in the regulation of TJs, including and the protein kinase C (PKC), PKA, PKG, Ras homolog gene family member A (RhoA), mitogen-activated protein kinase (MAPK), phosphatidylinositol 3-kinase (PI3K)/Akt, and Wnt/β-catenin pathways, are discussed. With great advances in this area, targeting endothelial TJs may provide novel treatment for TJ-related vascular pathologies.

Intestinal epithelial barrier functions in ageing

The intestinal epithelial barrier protects the mucosa of the gastrointestinal (GI)-tract and plays a key role in maintaining the host homeostasis. It encompasses several elements that include the intestinal epithelium and biochemical and immunological products, such as the mucus layer, antimicrobial peptides (AMPs) and secretory immunologlobulin A (sIgA). These components are interlinked with the large microbial community inhabiting the gut to form a highly sophisticated biological system that plays an important role on many aspects of human health both locally and systemically. Like any other organ and tissue, the intestinal epithelial barrier is affected by the ageing process. New insights have surfaced showing that critical functions, including intestinal stem cell regeneration and regulation of the intestinal crypt homeostasis, barrier integrity, production of regulatory cytokines, and epithelial innate immunity to pathogenic antigens change across life. Here we review the age-associated changes of the various components of the intestinal epithelial barrier and we highlight the necessity to elucidate further the mechanisms underlying these changes. Expanding our knowledge in this area is a goal of high medical relevance and it will help to define intervention strategies to ameliorate the quality of life of the ever-expanding elderly population.

ZO-2 Is a Master Regulator of Gene Expression, Cell Proliferation, Cytoarchitecture, and Cell Size

ZO-2 is a cytoplasmic protein of tight junctions (TJs). Here, we describe ZO-2 involvement in the formation of the apical junctional complex during early development and in TJ biogenesis in epithelial cultured cells. ZO-2 acts as a scaffold for the polymerization of claudins at TJs and plays a unique role in the blood–testis barrier, as well as at TJs of the human liver and the inner ear. ZO-2 movement between the cytoplasm and nucleus is regulated by nuclear localization and exportation signals and post-translation modifications, while ZO-2 arrival at the cell border is triggered by activation of calcium sensing receptors and corresponding downstream signaling. Depending on its location, ZO-2 associates with junctional proteins and the actomyosin cytoskeleton or a variety of nuclear proteins, playing a role as a transcriptional repressor that leads to inhibition of cell proliferation and transformation. ZO-2 regulates cell architecture through modulation of Rho proteins and its absence induces hypertrophy due to inactivation of the Hippo pathway and activation of mTOR and S6K. The interaction of ZO-2 with viral oncoproteins and kinases and its silencing in diverse carcinomas reinforce the view of ZO-2 as a tumor regulator protein.

Bicellular Tight Junctions and Wound Healing

Bicellular tight junctions (TJs) are intercellular junctions comprised of a variety of transmembrane proteins including occludin, claudins, and junctional adhesion molecules (JAMs) as well as intracellular scaffold proteins such as zonula occludens (ZOs). TJs are functional, intercellular structures that form a barrier between adjacent cells, which constantly seals and unseals to control the paracellular passage of molecules. They are primarily present in the epithelial and endothelial cells of all tissues and organs. In addition to their well-recognized roles in maintaining cell polarity and barrier functions, TJs are important regulators of signal transduction, which modulates cell proliferation, migration, and differentiation, as well as some components of the immune response and homeostasis. A vast breadth of research data is available on TJs, but little has been done to decipher their specific roles in wound healing, despite their primary distribution in epithelial and endothelial cells, which are essential contributors to the wound healing process. Some data exists to indicate that a better understanding of the functions and significance of TJs in healing wounds may prove crucial for future improvements in wound healing research and therapy. Specifically, recent studies demonstrate that occludin and claudin-1, which are two TJ component proteins, are present in migrating epithelial cells at the wound edge but are absent in chronic wounds. This indicates that functional TJs may be critical for effective wound healing. A tremendous amount of work is needed to investigate their roles in barrier function, re-epithelialization, angiogenesis, scar formation, and in the interactions between epithelial cells, endothelial cells, and immune cells both in the acute wound healing process and in non-healing wounds. A more thorough understanding of TJs in wound healing may shed new light on potential research targets and reveal novel strategies to enhance tissue regeneration and improve wound repair.

Rhubarb Monomers Protect Intestinal Mucosal Barrier in Sepsis via Junction Proteins

Background:

Leakage of the intestinal mucosal barrier may cause translocation of bacteria, then leading to multiorgan failure. This study hypothesized that rhubarb monomers might protect the gut mucosal barrier in sepsis through junction proteins.

Methods:

Healthy male Sprague-Dawley rats (weighing 230–250 g) under anesthesia and sedation were subjected to cecal ligation and perforation (CLP). After surgical preparation, rats were randomly assigned to eight groups (n = 6 or 8 each group): sham group (Group A: normal saline gavage); sepsis group (Group B: normal saline gavage); Group C (intraperitoneally, dexamethasone 0.5 mg/kg) immediately after CLP surgery; and rhubarb monomer (100 mg/kg in normal saline)-treated groups (Group D: rhein; Group E: emodin; Group F: 3,8-dihydroxy-1-methyl-anthraquinone-2-carboxylic acid; Group G: 1-O-caffeoyl-2-(4-hydroxy-O-cinnamoyl)-D-glucose; and Group H: daucosterol linoleate). Animals were sacrificed after 24 h. Intestinal histology, lactulose, mannitol concentrations were measured, and zonula occludens (ZO)-1, occludin and claudin-5 transcription (polymerase chain reaction), translation (by Western blot analysis), and expression (by immunohistochemistry) were also measured.

Results:

Intestinal histology revealed injury to intestinal mucosal villi induced by sepsis in Group B, compared with Group A. Compared with Group A (0.17 ± 0.41), the pathological scores in Groups B (2.83 ± 0.41, P < 0.001), C (1.83 ± 0.41, P < 0.001), D (2.00 ± 0.63, P < 0.001), E (1.83 ± 0.41, P < 0.001), F (1.83 ± 0.75, P < 0.001), G (2.17 ± 0.41, P < 0.001), and H (1.83 ± 0.41, P < 0.001) were significantly increased. Lactulose/mannitol (L/M) ratio in Group B (0.046 ± 0.003) was significantly higher than in Group A (0.013 ± 0.001, P < 0.001) while L/M ratios in Groups C (0.028 ± 0.002, P < 0.001), D (0.029 ± 0.003, P < 0.001), E (0.026 ± 0.003, P < 0.001), F (0.027 ± 0.003, P < 0.001), G (0.030 ± 0.005, P < 0.001), and H (0.026 ± 0.002, P < 0.001) were significantly lower than that in Group B. ZO-1, occludin and claudin-5 transcription, translation, and expression in Group B were significantly lower than that in Group A (P < 0.001), but they were significantly higher in Groups C, D, E, F, G, and H than those in Group B (P < 0.05).

Conclusion:

Rhubarb monomer treatment ameliorated mucosal damage in sepsis via enhanced transcription, translation, and expression of junction proteins.

Tension-Dependent Stretching Activates ZO-1 to Control the Junctional Localization of Its Interactors

Tensile forces regulate epithelial homeostasis, but the molecular mechanisms behind this regulation are poorly understood. Using structured illumination microscopy and proximity ligation assays, we show that the tight junction protein ZO-1 exists in stretched and folded conformations within epithelial cells, depending on actomyosin-generated force. We also show that ZO-1 and ZO-2 regulate the localization of the transcription factor DbpA and the tight junction membrane protein occludin in a manner that depends on the organization of the actin cytoskeleton, myosin-II activity, and substrate stiffness, resulting in modulation of gene expression, cell proliferation, barrier function, and cyst morphogenesis. Pull-down experiments show that interactions between N-terminal (ZPSG) and C-terminal domains of ZO-1 prevent binding of DbpA to the ZPSG, suggesting that force-dependent intra-molecular interactions regulate ZPSG binding to ligands within cells. In vivo and in vitro experiments also suggest that ZO-1 heterodimerization with ZO-2 promotes the stretched conformation and ZPSG interaction with ligands. Magnetic tweezers single-molecule experiments suggest that pN-scale tensions (∼2-4 pN) are sufficient to maintain the stretched conformation of ZO-1, while keeping its structured domains intact, and that 5-20 pN force is required to disrupt the interaction between the extreme C-terminal and the ZPSG domains of ZO-1. We propose that tensile forces regulate epithelial homeostasis by activating ZO proteins through stretching, to control the junctional recruitment and downstream signaling of their interactors.

Numerical investigation of the role of intercellular interactions on collective epithelial cell migration

During collective cell migration, the intercellular forces will significantly affect the collective migratory behaviors. However, the measurement of mechanical stresses exerted at cell-cell junctions is very challenging. A recent experimental observation indicated that the intercellular adhesion sites within a migrating monolayer are subjected to both normal stress exerted perpendicular to cell-cell junction surface and shear stress exerted tangent to cell-cell junction surface. In this study, an interfacial interaction model was proposed to model the intercellular interactions for the first time. The intercellular interaction model-based study of collective epithelial migration revealed that the direction of cell migration velocity has better alignment with the orientation of local principal stress at higher maximum shear stress locations in an epithelial monolayer sheet. Parametric study of the effects of adhesion strength indicated that normal adhesion strength at the cell-cell junction surface has dominated effect on local alignment between the direction of cell velocity vector and the principal stress orientation, while the shear adhesion strength has little effect, which provides compelling evidence to help explain the force transmission via cell-cell junctions between adjacent cells in collective cell motion and provides new insights into "adhesive belt" effects at cell-cell junction.

PARP‑1 may be involved in hydroquinone‑induced apoptosis by poly ADP‑ribosylation of ZO‑2

Hydroquinone (HQ), a major reactive metabolite of benzene, contributes to benzene-induced leukemia. The molecular mechanisms that underlie this activity remain to be elucidated. Poly ADP-ribosylation (PARylation) is a type of reversible posttranslational modification that is performed by enzymes in the PAR polymerase (PARP) family and mediates different biological processes, including apoptosis. Zona occludens 2 (ZO-2) is a tight junction scaffold protein, which is involved in cell proliferation and apoptosis. The present study investigated the activity and mechanisms regulated by PARP-1 during HQ-induced apoptosis using TK6 lymphoblastoid cells and PARP-1-silenced TK6 cells. The results revealed that exposure to 10 µM HQ for 72 h induced apoptosis in TK6 cells and that apoptosis was attenuated in PARP-1-silenced TK6 cells. In cells treated with HQ, inhibition of PARP-1 increased the expression of B cell leukemia/lymphoma 2 (Bcl-2), increased ATP production and reduced reactive oxygen species (ROS) production relative to the levels observed in cells treated with HQ alone. Co-localization of ZO-2 and PAR (or PARP-1 protein) was determined using immunofluorescence confocal microscopy. The findings of the present study revealed that ZO-2 was PARylated via an interaction with PARP-1, which was consistent with an analysis of protein expression that was performed using western blot analysis, which determined that ZO-2 protein expression was upregulated in HQ-treated control cells and downregulated in HQ-treated PARP-1-silenced TK6 cells. These findings indicated that prolonged exposure to a low dose of HQ induced TK6 cells to undergo apoptosis, whereas inhibiting PARP-1 attenuates cellular apoptosis by activating Bcl-2 and energy-saving processes and reducing ROS. The present study determined that PARP-1 was involved in HQ-induced apoptosis by PARylation of ZO-2.

Circulating tight junction proteins mirror blood-brain barrier integrity in leukaemia central nervous system metastasis

The aim of this study was to evaluate the clinical significance of circulating tight junction (TJ) proteins as biomarkers reflecting of leukaemia central nervous system (CNS) metastasis. TJs [claudin5 (CLDN5), occludin (OCLN) and ZO-1] concentrations were measured in serum and cerebrospinal fluid (CSF) samples obtained from 45 leukaemia patients. Serum ZO-1 was significantly higher (p < 0.05), but CSF ZO-1 levels were not significantly higher in the CNS leukaemia (CNSL) compared to the non-CNSL. The CNSL patients also had a lower CLDN5/ZO1 ratio in both serum and CSF than in non-CNSL patients (p < 0.05). The TJ index was negatively associated with WBC_CSF , ALB_CSF and BBB values in leukaemia patients. Among all of the parameters studied, CLDN5_CSF had the highest specificity in discriminating between CNSL and non-CNSL patients. Therefore, analysing serum and CSF levels of CLDN5, OCLN and the CLDN5/ZO1 ratio is valuable in evaluating the potential of leukaemia CNS metastasis. Copyright © 2016 John Wiley & Sons, Ltd.

Cell-specific diversity in the expression and organization of cytoplasmic plaque proteins of apical junctions

Tight and adherens junctions play critical roles in the barrier, adhesion, and signaling functions of epithelial and endothelial cells. How the molecular organization of these junctions is tuned to the widely diverse physiological requirements of each tissue type is not well understood. Here, we address this question by examining the expression, localization, and interactions of major cytoplasmic plaque proteins of tight and adherens junctions in different cultured epithelial and endothelial cell lines. Immunoblotting and immunofluorescence analyses show that the expression profiles of cingulin, paracingulin, ZO-1, ZO-2, ZO-3, PLEKHA7, afadin, PDZD11, p120-catenin, and α-catenin, as well as the transmembrane junctional proteins occludin, E-cadherin, and VE-cadherin, are significantly diverse when comparing kidney cells (MDCK, mCCD), keratinocytes (HaCaT), lung carcinoma (A427, A549), and endothelium-derived cells (bEnd.3, meEC, H5V). Proximity ligation and co-immunoprecipitation assays show that PLEKHA7 and PDZD11 are significantly more associated with the tight junction proteins cingulin and ZO-1 in aortic endothelium-derived (meEC) cells but not kidney collecting duct epithelial (mCCD) cells. These results provide evidence that the cytoplasmic plaques of tight and adherens junctions are diverse in their composition and molecular architecture and establish a conceptual framework by which we can rationally address the mechanisms of tissue-dependent junction physiology and signaling by cytoplasmic junctional proteins.

Zonula occludens-2 regulates Rho proteins activity and the development of epithelial cytoarchitecture and barrier function

Silencing Zonula occludens 2 (ZO-2), a tight junctions (TJ) scaffold protein, in epithelial cells (MDCK ZO-2 KD) triggers: 1) Decreased cell to substratum attachment, accompanied by reduced expression of claudin-7 and integrin β1, and increased vinculin recruitment to focal adhesions and stress fibers formation; 2) Lowered cell-cell aggregation and appearance of wider intercellular spaces; 3) Increased RhoA/ROCK activity, mediated by GEF-HI recruitment to cell borders by cingulin; 4) Increased Cdc42 activity, mitotic spindle disorientation and the appearance of cysts with multiple lumens; 5) Increased Rac and cofilin activity, multiple lamellipodia formation and random cell migration but increased wound closure; 6) Diminished cingulin phosphorylation and disappearance of planar network of microtubules at the TJ region; and 7) Increased transepithelial electrical resistance at steady state, coupled to an increased expression of ZO-1 and claudin-4 and a decreased expression of claudin-2 and paracingulin. Hence, ZO-2 is a crucial regulator of Rho proteins activity and the development of epithelial cytoarchitecture and barrier function.

ZO-2, a tight junction protein involved in gene expression, proliferation, apoptosis, and cell size regulation

ZO-2 is a peripheral tight junction protein that belongs to the membrane-associated guanylate kinase protein family. Here, we explain the modular and supramodular organization of ZO-2 that allows it to interact with a wide variety of molecules, including cell-cell adhesion proteins, cytoskeletal components, and nuclear factors. We also describe how ZO proteins evolved through metazoan evolution and analyze the intracellular traffic of ZO-2, as well as the roles played by ZO-2 at the plasma membrane and nucleus that translate into the regulation of proliferation, cell size, and apoptosis. In addition, we focus on the impact of ZO-2 expression on male fertility and on maladies like cancer, cholestasis, and hearing loss.

The role of apical cell-cell junctions and associated cytoskeleton in mechanotransduction

Tissues of multicellular organisms are characterised by several types of specialised cell-cell junctions. In vertebrate epithelia and endothelia, tight and adherens junctions (AJ) play critical roles in barrier and adhesion functions, and are connected to the actin and microtubule cytoskeletons. The interaction between junctions and the cytoskeleton is crucial for tissue development and physiology, and is involved in the molecular mechanisms governing cell shape, motility, growth and signalling. The machineries which functionally connect tight and AJ to the cytoskeleton comprise proteins which either bind directly to cytoskeletal filaments, or function as adaptors for regulators of the assembly and function of the cytoskeleton. In the last two decades, specific cytoskeleton-associated junctional molecules have been implicated in mechanotransduction, revealing the existence of multimolecular complexes that can sense mechanical cues and translate them into adaptation to tensile forces and biochemical signals. Here, we summarise the current knowledge about the machineries that link tight and AJ to actin filaments and microtubules, and the molecular basis for mechanotransduction at epithelial and endothelial AJ.

Endothelial Dysfunction in Obesity

Chronic inflammatory state in obesity causes dysregulation of the endocrine and paracrine actions of adipocyte-derived factors, which disrupt vascular homeostasis and contribute to endothelial vasodilator dysfunction and subsequent hypertension. While normal healthy perivascular adipose tissue (PVAT) ensures the dilation of blood vessels, obesity-associated PVAT leads to a change in profile of the released adipo-cytokines, resulting in a decreased vasorelaxing effect. Adipose tissue inflammation, nitric oxide (NO)-bioavailability, insulin resistance and oxidized low-density lipoprotein (oxLDL) are main participating factors in endothelial dysfunction of obesity. In this chapter, disruption of inter-endothelial junctions between endothelial cells, significant increase in the production of reactive oxygen species (ROS), inflammation mediators, which are originated from inflamed endothelial cells, the balance between NO synthesis and ROS , insulin signaling and NO production, and decrease in L-arginine/endogenous asymmetric dimethyl-L-arginine (ADMA) ratio are discussed in connection with endothelial dysfunction in obesity.

ZO-2 silencing induces renal hypertrophy through a cell cycle mechanism and the activation of YAP and the mTOR pathway

Renal compensatory hypertrophy (RCH) restores normal kidney function after disease or loss of kidney tissue and is characterized by an increase in organ size due to cell enlargement and not to cell proliferation. In MDCK renal epithelial cells, silencing of the tight junction protein zona occludens 2 (ZO-2 KD) induces cell hypertrophy by two mechanisms: prolonging the time that cells spend at the G1 phase of the cell cycle due to an increase in cyclin D1 level, and augmenting the rate of protein synthesis. The latter is triggered by the nuclear accumulation and increased transcriptional activity of Yes-associated protein (YAP), the main target of the Hippo pathway, which results in decreased expression of phosphatase and tensin homologue. This in turn increased the level of phosphatidylinositol (3,4,5)-triphosphate, which transactivates the Akt/mammalian target of rapamycin pathway, leading to activation of the kinase S6K1 and increased synthesis of proteins and cell size. In agreement, in a rat model of uninephrectomy, RCH is accompanied by decreased expression of ZO-2 and nuclear expression of YAP. Our results reveal a novel role of ZO-2 as a modulator of cell size.

Chronic hypoxia down-regulates tight junction protein ZO-2 expression in children with cyanotic congenital heart defect

Aims

Tight junction protein zonula occludens protein 2 (ZO‐2) is a member of the membrane‐associated guanylate kinases protein family known to be expressed at tight junctions of epithelial and endothelial cells and at adherens junctions (AJs) in cardiomyocytes. Little is known about ZO‐2 expression and function in the human heart. Here, we examined the hypothesis that chronic hypoxia down‐regulates ZO‐2 expression in human myocardium and cultured rat cardiomyocytes.

Methods and results

Patients with a diagnosis of cyanotic (n = 10) or acyanotic (n = 10) Tetralogy of Fallot undergoing surgical repair were used to examine ZO‐2 messenger RNA and protein expression by real time‐PCR, immunohistochemistry, and western blotting. A model of cultured rat cardiomyocytes was used to measure ZO‐2 and AJ proteins levels in response to hypoxia and to investigate ZO‐2 cellular localization. We showed that ZO‐2 is expressed in myocardial tissue in acyanotic and cyanotic children with congenital heart defects. ZO‐2 was specifically down‐regulated in cyanotic myocardium at both the messenger RNA and protein levels when compared with acyanotic patients. This specific down‐regulation can be mimicked in cultured rat cardiomyocytes by treating them with hypoxic conditions confirming that ZO‐2 gene down‐regulation is specifically due to cyanosis. Furthermore, in addition to its cytoplasmic expression, ZO‐2 showed nuclear expression in cultured rat cardiomyocytes suggesting potential role in transcription regulation.

Conclusions

Hypoxia down‐regulates ZO‐2 expression in both cyanotic patient's myocardium and cultured rat cardiomyocytes. This down‐regulation suggest an involvement of ZO‐2 in cardiac remodelling of AJs in cyanotic children and may explain the greater susceptibility of cyanotic patients to corrective heart surgery.

Instigation of endothelial Nlrp3 inflammasome by adipokine visfatin promotes inter-endothelial junction disruption: role of HMGB1

Recent studies have indicated that the inflammasome plays a critical role in the pathogenesis of vascular diseases. However, the pathological relevance of this inflammasome activation, particularly in vascular cells, remains largely unknown. Here, we investigated the role of endothelial (Nucleotide‐binding Oligomerization Domain) NOD‐like receptor family pyrin domain containing three (Nlrp3) inflammasomes in modulating inter‐endothelial junction proteins, which are associated with endothelial barrier dysfunction, an early onset of obesity‐associated endothelial injury. Our findings demonstrate that the activation of Nlrp3 inflammasome by visfatin markedly decreased the expression of inter‐endothelial junction proteins including tight junction proteins ZO‐1, ZO‐2 and occludin, and adherens junction protein VE‐cadherin in cultured mouse vascular endothelial (VE) cell monolayers. Such visfatin‐induced down‐regulation of junction proteins in endothelial cells was attributed to high mobility group box protein 1 (HMGB1) release derived from endothelial inflammasome‐dependent caspase‐1 activity. Similarly, in the coronary arteries of wild‐type mice, high‐fat diet (HFD) treatment caused a down‐regulation of inter‐endothelial junction proteins ZO‐1, ZO‐2, occludin and VE‐cadherin, which was accompanied with enhanced inflammasome activation and HMGB1 expression in the endothelium as well as transmigration of CD43⁺ T cells into the coronary arterial wall. In contrast, all these HFD‐induced alterations in coronary arteries were prevented in mice with Nlrp3 gene deletion. Taken together, these data strongly suggest that the activation of endothelial Nlrp3 inflammasomes as a result of the increased actions of injurious adipokines such as visfatin produces HMGB1, which act in paracrine or autocrine fashion to disrupt inter‐endothelial junctions and increase paracellular permeability of the endothelium contributing to the early onset of endothelial injury during metabolic disorders such as obesity or high‐fat/cholesterol diet.

Conceptual barriers to understanding physical barriers

The members of the large family of claudin proteins regulate ion and water flux across the tight junction. Many claudins, e.g. claudins 2 and 15, accomplish this by forming size- and charge-selective paracellular channels. Claudins also appear to be essential for genesis of tight junction strands and recruitment of other proteins to these sites. What is less clear is whether claudins form the paracellular seal. While this seal is defective when claudins are disrupted, some results, including ultrastructural and biochemical data, suggest that lipid structures are an important component of tight junction strands and may be responsible for the paracellular seal. This review highlights current understanding of claudin contributions to barrier function and tight junction structure and suggests a model by which claudins and other tight junction proteins can drive assembly and stabilization of a lipid-based strand structure.

Beyond cell-cell adhesion: Emerging roles of the tight junction scaffold ZO-2

Zonula occludens proteins (ZO-1, ZO-2, ZO-3), which belong to the family of membrane-associated guanylate kinase (MAGUK) homologs, serve as molecular hubs for the assembly of multi-protein networks at the cytoplasmic surface of intercellular contacts in epithelial and endothelial cells. These multi-PDZ proteins exert crucial functions in the structural organization of intercellular contacts and in transducing intracellular signals from the plasma membrane to the nucleus. The junctional MAGUK protein ZO-2 not only associates with the C-terminal PDZ-binding motif of various transmembrane junctional proteins but also transiently targets to the nucleus and interacts with a number of nuclear proteins, thereby modulating gene expression and cell proliferation. Recent evidence suggests that ZO-2 is also involved in stress response and cytoprotective mechanisms, which further highlights the multi-faceted nature of this PDZ domain-containing protein.

This review focuses on ZO-2 acting as a molecular scaffold at the cytoplasmic aspect of tight junctions and within the nucleus and discusses additional aspects of its cellular activities. The multitude of proteins interacting with ZO-2 and the heterogeneity of proteins either influencing or being influenced by ZO-2 suggests an exceptional functional capacity of this protein far beyond merely serving as a structural component of cellular junctions.

ZO-1 and ZO-2 are required for extra-embryonic endoderm integrity, primitive ectoderm survival and normal cavitation in embryoid bodies derived from mouse embryonic stem cells

The Zonula Occludens proteins ZO-1 and ZO-2 are cell-cell junction-associated adaptor proteins that are essential for the structural and regulatory functions of tight junctions in epithelial cells and their absence leads to early embryonic lethality in mouse models. Here, we use the embryoid body, an in vitro peri-implantation mouse embryogenesis model, to elucidate and dissect the roles ZO-1 and ZO-2 play in epithelial morphogenesis and de novo tight junction assembly. Through the generation of individual or combined ZO-1 and ZO-2 null embryoid bodies, we show that their dual deletion prevents tight junction formation, resulting in the disorganization and compromised barrier function of embryoid body epithelial layers. The disorganization is associated with poor microvilli development, fragmented basement membrane deposition and impaired cavity formation, all of which are key epithelial tissue morphogenetic processes. Expression of Podocalyxin, which positively regulates the formation of microvilli and the apical membrane, is repressed in embryoid bodies lacking both ZO-1 and ZO-2 and this correlates with an aberrant submembranous localization of Ezrin. The null embryoid bodies thus give an insight into how the two ZO proteins influence early mouse embryogenesis and possible mechanisms underlying the embryonic lethal phenotype.

JAM-A associates with ZO-2, afadin, and PDZ-GEF1 to activate Rap2c and regulate epithelial barrier function

Intestinal barrier function is regulated by epithelial tight junctions, structures that control paracellular permeability. JAM-A regulates epithelial permeability through association with ZO-2, afadin, and PDZ-GEF1 to activate Rap2c and control contraction of the apical cytoskeleton.

Intestinal barrier function is regulated by epithelial tight junctions (TJs), structures that control paracellular permeability. Junctional adhesion molecule-A (JAM-A) is a TJ-associated protein that regulates barrier; however, mechanisms linking JAM-A to epithelial permeability are poorly understood. Here we report that JAM-A associates directly with ZO-2 and indirectly with afadin, and this complex, along with PDZ-GEF1, activates the small GTPase Rap2c. Supporting a functional link, small interfering RNA–mediated down-regulation of the foregoing regulatory proteins results in enhanced permeability similar to that observed after JAM-A loss. JAM-A–deficient mice and cultured epithelial cells demonstrate enhanced paracellular permeability to large molecules, revealing a potential role of JAM-A in controlling perijunctional actin cytoskeleton in addition to its previously reported role in regulating claudin proteins and small-molecule permeability. Further experiments suggest that JAM-A does not regulate actin turnover but modulates activity of RhoA and phosphorylation of nonmuscle myosin, both implicated in actomyosin contraction. These results suggest that JAM-A regulates epithelial permeability via association with ZO-2, afadin, and PDZ-GEF1 to activate Rap2c and control contraction of the apical cytoskeleton.

The intracellular fate of zonula occludens 2 is regulated by the phosphorylation of SR repeats and the phosphorylation/O-GlcNAcylation of S257

ZO-2 nuclear import and accumulation in speckles is regulated by phosphorylation of its SR repeats by SRPK1 in a process initiated by EGF activation of AKT. ZO-2 nuclear exportation is favored by O-GlcNAc of S257 at the nucleus, whereas maturation of tight junctions is accompanied by ZO-2 phosphorylation at S257 by PKCζ.

Zona occludens 2 (ZO-2) has a dual localization. In confluent epithelia, ZO-2 is present at tight junctions (TJs), whereas in sparse proliferating cells it is also found at the nucleus. Previously we demonstrated that in sparse cultures, newly synthesized ZO-2 travels to the nucleus before reaching the plasma membrane. Now we find that in confluent cultures newly synthesized ZO-2 goes directly to the plasma membrane. Epidermal growth factor induces through AKT activation the phosphorylation of the kinase for SR repeats, serine arginine protein kinase 1, which in turn phosphorylates ZO-2, which contains 16 SR repeats. This phosphorylation induces ZO-2 entry into the nucleus and accumulation in speckles. ZO-2 departure from the nucleus requires intact S257, and stabilizing the β-O-linked N-acetylglucosylation (O-GlcNAc) of S257 with O-(2-acetamido-2-deoxy-d-glucopyranosylidene)amino-N-phenylcarbamate, an inhibitor of O-GlcNAcase, triggers nuclear exportation and proteosomal degradation of ZO-2. At the plasma membrane ZO-2 is not O-GlcNAc, and instead, as TJs mature, it becomes phosphorylated at S257 by protein kinase Cζ. This late phosphorylation of S257 is required for the correct cytoarchitecture to develop, as cells transfected with ZO-2 mutant S257A or S257E form aberrant cysts with multiple lumens. These results reveal novel posttranslational modifications of ZO-2 that regulate the intracellular fate of this protein.

Build them up and break them down: Tight junctions of cell lines expressing typical hepatocyte polarity with a varied repertoire of claudins

Tight junctions (TJs) of cells expressing simple epithelial polarity have been extensively studied, but less is known about TJs of cells expressing complex polarity. In this paper we analyzed, TJs of four different lines, that form bile canaliculi (BC) and express typical hepatocyte polarity; WIF-B9, 11–3, Can 3–1, Can 10. Striking differences were observed in claudin expression. None of the cell lines produced claudin-1. WIF-B9 and 11–3 expressed only claudin-2 while Can 3–1 and Can 10 expressed claudin-2,-3,-4,-5. TJs of these two classes of lines differed in their ultra-stucture, paracellular permeability, and robustness. Lines expressing a large claudin repertoire, especially Can 10, had complex and efficient TJs, that were maintained when cells were depleted in calcium. Inversely, TJs of WIF-B9 and 11–3 were leaky, permissive and dismantled by calcium depletion. Interestingly, we found that during the polarization process, TJ proteins expressed by all lines were sequentially settled in a specific order: first occludin, ZO-1 and cingulin, then JAM-A and ZO-2, finally claudin-2. Claudins expressed only in Can lines were also sequentially settled: claudin-3 was the first settled. Inhibition of claudin-3 expression delayed BC formation in Can10 and induced the expression of simple epithelial polarity. These results highlight the role of claudins in the settlement and the efficiency of TJs in lines expressing typical hepatocyte polarity. Can 10 seems to be the most promising of these lines because of its claudin repertoire near that of hepatocytes and its capacity to form extended tubular BC sealed by efficient TJs.

Tight junction proteins: from barrier to tumorigenesis

The tight junction is a multi-protein complex and is the apical most junctional complex in certain epithelial and endothelial cells. A great deal of attention has been devoted to the understanding of these proteins in contributing to the barrier function - that is, regulating the paracellular flux or permeability between adjacent cells. However, tight junction proteins are now recognized as having functions beyond the barrier. The focus of this review is to discuss the barrier function of the tight junction and to summarize the literature with a focus on the role of tight junction proteins in proliferation, transformation, and metastasis.

Claudins and the modulation of tight junction permeability

Claudins are tight junction membrane proteins that are expressed in epithelia and endothelia and form paracellular barriers and pores that determine tight junction permeability. This review summarizes our current knowledge of this large protein family and discusses recent advances in our understanding of their structure and physiological functions.

Epithelial barrier assembly requires coordinated activity of multiple domains of the tight junction protein ZO-1

Tight junctions (TJs) regulate the paracellular movement of ions, macromolecules and immune cells across epithelia. Zonula occludens (ZO)-1 is a multi-domain polypeptide required for the assembly of TJs. MDCK II cells lacking ZO-1, and its homolog ZO-2, have three distinct phenotypes: reduced localization of occludin and some claudins to the TJs, increased epithelial permeability, and expansion of the apical actomyosin contractile array found at the apical junction complex (AJC). However, it is unclear exactly which ZO-1 binding domains are required to coordinate these activities. We addressed this question by examining the ability of ZO-1 domain-deletion transgenes to reverse the effects of ZO depletion. We found that the SH3 domain and the U5 motif are required to recruit ZO-1 to the AJC and that localization is a prerequisite for normal TJ and cytoskeletal organization. The PDZ2 domain is not required for localization of ZO-1 to the AJC, but is necessary to establish the characteristic continuous circumferential band of ZO-1, occludin and claudin-2. PDZ2 is also required to establish normal permeability, but is not required for normal cytoskeletal organization. Finally, our results demonstrate that PDZ1 is crucial for the normal organization of both the TJ and the AJC cytoskeleton. Our results establish that ZO-1 acts as a true scaffolding protein and that the coordinated activity of multiple domains is required for normal TJ structure and function.

Multiple facets of cAMP signalling and physiological impact: cAMP compartmentalization in the lung

Therapies involving elevation of the endogenous suppressor cyclic AMP (cAMP) are currently used in the treatment of several chronic inflammatory disorders, including chronic obstructive pulmonary disease (COPD). Characteristics of COPD are airway obstruction, airway inflammation and airway remodelling, processes encompassed by increased airway smooth muscle mass, epithelial changes, goblet cell and submucosal gland hyperplasia. In addition to inflammatory cells, airway smooth muscle cells and (myo)fibroblasts, epithelial cells underpin a variety of key responses in the airways such as inflammatory cytokine release, airway remodelling, mucus hypersecretion and airway barrier function. Cigarette smoke, being next to environmental pollution the main cause of COPD, is believed to cause epithelial hyperpermeability by disrupting the barrier function. Here we will focus on the most recent progress on compartmentalized signalling by cAMP. In addition to G protein-coupled receptors, adenylyl cyclases, cAMP-specific phospho-diesterases (PDEs) maintain compartmentalized cAMP signalling. Intriguingly, spatially discrete cAMP-sensing signalling complexes seem also to involve distinct members of the A-kinase anchoring (AKAP) superfamily and IQ motif containing GTPase activating protein (IQGAPs). In this review, we will highlight the interaction between cAMP and the epithelial barrier to retain proper lung function and to alleviate COPD symptoms and focus on the possible molecular mechanisms involved in this process. Future studies should include the development of cAMP-sensing multiprotein complex specific disruptors and/or stabilizers to orchestrate cellular functions. Compartmentalized cAMP signalling regulates important cellular processes in the lung and may serve as a therapeutic target.

ZO-2, a tight junction scaffold protein involved in the regulation of cell proliferation and apoptosis

ZO-2 is a membrane-associated guanylate kinase homologue (MAGUK) tight protein associated with the cytoplasmic surface of tight junctions. Here, we describe how ZO-2 is a multidomain molecule that binds to a variety of cell signaling proteins, to the actin cytoskeleton, and to gap, tight, and adherens junction proteins. In sparse cultures, ZO-2 is present at the nucleus and associates with molecules active in gene transcription and pre-mRNA processing. ZO-2 inhibits the Wnt signaling pathway, reduces cell proliferation, and promotes apoptosis; its absence, mutation, or overexpression is present in various human diseases, including deafness and cancer.

Zona occludens-2 protects against podocyte dysfunction induced by ADR in mice

Zona occludens-2 (ZO-2) is a protein present at the tight junction and nucleus of epithelial cells. ZO-2 represses the transcription of genes regulated by the Wnt/β-catenin pathway. This pathway plays a critical role in podocyte injury and proteinuria. Here, we analyze whether the overexpression of ZO-2 in the glomerulus, by hydrodynamics transfection, prevents podocyte injury mediated by the Wnt/β-catenin pathway in the mouse model of adriamycin (ADR) nephrosis. By immunofluorescence and immunogold electron microscopy, we show that ZO-2 is present in mice glomerulus, not at the slit diaphragms where nephrin concentrates, but in the cytoplasm and at processes of podocytes. Our results indicate that in the glomeruli of mice treated with ADR, ZO-2 overexpression increases the amount of phosphorylated β-catenin, inhibits the expression of the transcription factor snail, prevents nephrin and podocalyxin loss, reduces podocyte effacement and massive fusions, restrains proteinuria, and supports urea and creatinine clearance. These results suggest that ZO-2 could be a new target for the regulation of hyperactive Wnt/β-catenin signaling in proteinuric kidney diseases.

Zonula occludens-1 and -2 regulate apical cell structure and the zonula adherens cytoskeleton in polarized epithelia

ETOC: Our study reveals that ZO proteins in fully polarized cells regulate the assembly and contractility of the perijunctional actomyosin ring associated with the adherens junction.

The structure and function of both adherens (AJ) and tight (TJ) junctions are dependent on the cortical actin cytoskeleton. The zonula occludens (ZO)-1 and -2 proteins have context-dependent interactions with both junction types and bind directly to F-actin and other cytoskeletal proteins, suggesting ZO-1 and -2 might regulate cytoskeletal activity at cell junctions. To address this hypothesis, we generated stable Madin-Darby canine kidney cell lines depleted of both ZO-1 and -2. Both paracellular permeability and the localization of TJ proteins are disrupted in ZO-1/-2–depleted cells. In addition, immunocytochemistry and electron microscopy revealed a significant expansion of the perijunctional actomyosin ring associated with the AJ. These structural changes are accompanied by a recruitment of 1-phosphomyosin light chain and Rho kinase 1, contraction of the actomyosin ring, and expansion of the apical domain. Despite these changes in the apical cytoskeleton, there are no detectable changes in cell polarity, localization of AJ proteins, or the organization of the basal and lateral actin cytoskeleton. We conclude that ZO proteins are required not only for TJ assembly but also for regulating the organization and functional activity of the apical cytoskeleton, particularly the perijunctional actomyosin ring, and we speculate that these activities are relevant both to cellular organization and epithelial morphogenesis.

Emerging theme: cellular PDZ proteins as common targets of pathogenic viruses

More than a decade ago, three viral oncoproteins, adenovirus type 9 E4-ORF1, human T-lymphotropic virus type 1 Tax, and high-risk human papillomavirus E6, were found to encode a related carboxyl-terminal PDZ domain-binding motif (PBM) that mediates interactions with a select group of cellular PDZ proteins. Recent studies have shown that many other viruses also encode PBM-containing proteins that bind to cellular PDZ proteins. Interestingly, these recently recognized viruses include not only some with oncogenic potential (hepatitis B virus, rhesus papillomavirus, cottontail rabbit papillomavirus) but also many without this potential (influenza virus, Dengue virus, tick-borne encephalitis virus, rabies virus, severe acute respiratory syndrome coronavirus, human immunodeficiency virus). Examination of the cellular PDZ proteins that are targets of viral PBMs reveals that the viral proteins often interact with the same or similar types of PDZ proteins, most notably Dlg1 and other members of the membrane-associated guanylate kinase protein family, as well as Scribble. In addition, cellular PDZ protein targets of viral PBMs commonly control tight junction formation, cell polarity establishment, and apoptosis. These findings reveal a new theme in virology wherein many different virus families encode proteins that bind and perturb the function of cellular PDZ proteins. The inhibition or perturbation of the function of cellular PDZ proteins appears to be a widely used strategy for viruses to enhance their replication, disseminate in the host, and transmit to new hosts.

Tight junction proteins and the epidermis

Epithelia are found at the interfaces between body compartments where they act as selective permeability barriers that maintain the unique composition of the compartments on either side. Epithelial barrier function is dependent on tight junctions (TJs), which seal the intercellular or paracellular spaces but may permit selective permeability. In simple epithelia (one cell thick), the function of TJs is relatively well understood. By contrast, our understanding of TJ structure and function in stratified epithelia (e.g. the epidermis) is limited. This article briefly discusses what is known about TJs and their components in simple epithelia and speculates about their roles in the epidermis.

Integration of tight junctions and claudins with the barrier functions of the retinal pigment epithelium

The retinal pigment epithelium (RPE) forms the outer blood-retinal barrier by regulating the movement of solutes between the fenestrated capillaries of the choroid and the photoreceptor layer of the retina. Blood-tissue barriers use various mechanisms to accomplish their tasks including membrane pumps, transporters, and channels, transcytosis, metabolic alteration of solutes in transit, and passive but selective diffusion. The last category includes tight junctions, which regulate transepithelial diffusion through the spaces between neighboring cells of the monolayer. Tight junctions are extraordinarily complex structures that are dynamically regulated. Claudins are a family of tight junctional proteins that lend tissue specificity and selectivity to tight junctions. This review discusses how the claudins and tight junctions of the RPE differ from other epithelia and how its functions are modulated by the neural retina. Studies of RPE-retinal interactions during development lend insight into this modulation. Notably, the characteristics of RPE junctions, such as claudin composition, vary among species, which suggests the physiology of the outer retina may also vary. Comparative studies of barrier functions among species should deepen our understanding of how homeostasis is maintained in the outer retina. Stem cells provide a way to extend these studies of RPE-retinal interactions to human RPE.