The second loop of occludin is required for suppression of Raf1-induced tumor growth

Paracellular permeability and tight junction regulation in gut health and disease

Epithelial tight junctions define the paracellular permeability of the intestinal barrier. Molecules can cross the tight junctions via two distinct size-selective and charge-selective paracellular pathways: the pore pathway and the leak pathway. These can be distinguished by their selectivities and differential regulation by immune cells. However, permeability increases measured in most studies are secondary to epithelial damage, which allows non-selective flux via the unrestricted pathway. Restoration of increased unrestricted pathway permeability requires mucosal healing. By contrast, tight junction barrier loss can be reversed by targeted interventions. Specific approaches are needed to restore pore pathway or leak pathway permeability increases. Recent studies have used preclinical disease models to demonstrate the potential of pore pathway or leak pathway barrier restoration in disease. In this Review, we focus on the two paracellular flux pathways that are dependent on the tight junction. We discuss the latest evidence that highlights tight junction components, structures and regulatory mechanisms, their impact on gut health and disease, and opportunities for therapeutic intervention.

Role of Cell-Cell Junctions in Oesophageal Squamous Cell Carcinoma

Cell-cell junctions comprise various structures, including adherens junctions, tight junctions, desmosomes, and gap junctions. They link cells to each other in tissues and regulate tissue homeostasis in critical cellular processes. Recent advances in cell-cell junction research have led to critical discoveries. Cell-cell adhesion components are important for the invasion and metastasis of tumour cells, which are not only related to cell-cell adhesion changes, but they are also involved in critical molecular signal pathways. They are of great significance, especially given that relevant molecular mechanisms are being discovered, there are an increasing number of emerging biomarkers, targeted therapies are becoming a future therapeutic concern, and there is an increased number of therapeutic agents undergoing clinical trials. Oesophageal squamous cell carcinoma (ESCC), the most common histological subtype of oesophageal cancer, is one of the most common cancers to affect epithelial tissue. ESCC progression is accompanied by the abnormal expression or localisation of components at cell-cell junctions. This review will discuss the recent scientific developments related to the molecules at cell-cell junctions and their role in ESCC to offer valuable insights for readers, provide a global view of the relationships between position, construction, and function, and give a reference for future mechanistic studies, diagnoses, and therapeutic developments.

Tight junction proteins occludin and ZO-1 as regulators of epithelial proliferation and survival

Epithelial cells are the first line of mucosal defense. In the intestine, a single layer of epithelial cells must establish a selectively permeable barrier that supports nutrient absorption and waste secretion while preventing the leakage of potentially harmful luminal materials. Key to this is the tight junction, which seals the paracellular space and prevents unrestricted leakage. The tight junction is a protein complex established by interactions between members of the claudin, zonula occludens, and tight junction-associated MARVEL protein (TAMP) families. Claudins form the characteristic tight junction strands seen by freeze-fracture microscopy and create paracellular channels, but the functions of ZO-1 and occludin, founding members of the zonula occludens and TAMP families, respectively, are less well defined. Recent studies have revealed that these proteins have essential noncanonical (nonbarrier) functions that allow them to regulate epithelial apoptosis and proliferation, facilitate viral entry, and organize specialized epithelial structures. Surprisingly, neither is required for intestinal barrier function or overall health in the absence of exogenous stressors. Here, we provide a brief overview of ZO-1 and occludin canonical (barrier-related) functions, and a more detailed examination of their noncanonical functions.

Modulated Start-Up Mode of Cancer Cell Migration Through Spinophilin-Tubular Networks

Spinophilin (SPL) is a multifunctional actin-binding scaffolding protein. Although increased research on SPL in cancer biology has revealed a tumor suppressive role, its modulation in cancer biology, and oncological relevance remains elusive. Thus, we determined the role of SPL in the modulation of the junctional network and cellular migration in A549 lung cancer cell line. Knockdown of SPL promoted cancer cell invasion in agarose spot and scratch wound assays. Attenuation of SPL expression also enhanced invadopodia, as revealed by enhanced vinculin spots, and enhanced sodium bicarbonate cotransporter NBC activity without enhancing membranous expression of NBCn1. Disruption of the tubular structure with nocodazole treatment revealed enhanced SPL expression and reduced NBC activity and A549 migration. SPL-mediated junctional modulation and tubular stability affected bicarbonate transporter activity in A549 cells. The junctional modulatory function of SPL in start-up migration, such as remodeling of tight junctions, enhanced invadopodia, and increased NBC activity, revealed here would support fundamental research and the development of an initial target against lung cancer cell migration.

Role of tight junctions in the epithelial-to-mesenchymal transition of cancer cells

The epithelial-mesenchymal transition (EMT) is an essential step in cancer progression. Epithelial cells possess several types of cell-cell junctions, and tight junctions are known to play important roles in maintaining the epithelial program. EMT is characterized by a loss of epithelial markers, including E-cadherin and tight junction proteins. Somewhat surprisingly, the evidence is accumulating that upregulated expression of tight junction proteins plays an important role in the EMT of cancer cells. Tight junctions have distinct tissue-specific and cancer-specific regulatory mechanisms, enabling them to play different roles in EMT. Tight junctions and related signaling pathways are attractive targets for cancer treatments; signal transduction inhibitors and monoclonal antibodies for tight junction proteins may be used to suppress EMT, invasion, and metastasis. Here we review the role of bicellular and tricellular tight junction proteins during EMT. Further investigation of regulatory mechanisms of tight junctions during EMT in cancer cells will inform the development of biomarkers for predicting prognosis as well as novel therapies.

Dietary Puerarin Supplementation Alleviates Oxidative Stress in the Small Intestines of Diquat-Challenged Piglets

Simple Summary

The oxidant stress which piglets suffer from during the weaning period has caused huge losses to the pig farm industry. It is important for scientists to find an effective way to alleviate the oxidant stress in weaned piglets. This study was designed to test the hypothesis that dietary puerarin supplementation alleviates oxidative stress in the small intestine of diquat-challenged piglets. Interestingly, dietary puerarin supplementation improved intestinal morphology, cell proliferation, barrier function, and increased Nrf2 and its downstream enzymes in diquat-challenged piglets, which shows that puerarin has potent protective effects against diquat-induced oxidative stress. These findings will be very beneficial to the pig industry, especially to the development of antibiotic-free diets, new anti-inflammatory drugs and the application of puerarin in piglets.

Abstract

This study was conducted to demonstrate that dietary puerarin supplementation alleviates oxidative stress in the small intestine of diquat-challenged piglets. The results showed that puerarin administration markedly alleviated diquat-induced intestinal injury, which was indicated by the improvement of intestinal morphology, cell proliferation and barrier function. One of the potential mechanisms responsible for this was the decrease in oxidative stress, as evidenced by the increase in activities of superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), and total antioxidant capacity (T-AOC) in the small intestine. Puerarin increased the protein expression levels of NF-E2-related factor 2 (Nrf2) and its downstream enzymes, including heme oxygenase 1 (HO-1), glutamate–cysteine ligase catalytic and its modifier subunit (GCLc and GCLm) in the jejunal mucosa of diquat-induced piglets. Puerarin administration improved intestinal morphology, cell proliferation, and barrier function, and increased Nrf2 and its downstream enzymes. These findings indicate that the dietary supplementation of puerarin attenuates the oxidative stress involving Nrf2 signaling pathways in diquat-challenged piglets.

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.

Tight Junctions in Cell Proliferation

Tight junction (TJ) proteins form a continuous intercellular network creating a barrier with selective regulation of water, ion, and solutes across endothelial, epithelial, and glial tissues. TJ proteins include the claudin family that confers barrier properties, members of the MARVEL family that contribute to barrier regulation, and JAM molecules, which regulate junction organization and diapedesis. In addition, the membrane-associated proteins such as MAGUK family members, i.e., zonula occludens, form the scaffold linking the transmembrane proteins to both cell signaling molecules and the cytoskeleton. Most studies of TJ have focused on the contribution to cell-cell adhesion and tissue barrier properties. However, recent studies reveal that, similar to adherens junction proteins, TJ proteins contribute to the control of cell proliferation. In this review, we will summarize and discuss the specific role of TJ proteins in the control of epithelial and endothelial cell proliferation. In some cases, the TJ proteins act as a reservoir of critical cell cycle modulators, by binding and regulating their nuclear access, while in other cases, junctional proteins are located at cellular organelles, regulating transcription and proliferation. Collectively, these studies reveal that TJ proteins contribute to the control of cell proliferation and differentiation required for forming and maintaining a tissue barrier.

Inflammation-induced Occludin Downregulation Limits Epithelial Apoptosis by Suppressing Caspase-3 Expression

BACKGROUND & AIMS

Epithelial tight junctions are compromised in gastrointestinal disease. Processes that contribute to the resulting barrier loss include endocytic occludin removal from the tight junction and reduced occludin expression. Nevertheless, the relatively-normal basal phenotype of occludin knockout (KO) mice has been taken as evidence that occludin does not contribute to gastrointestinal barrier function. We asked whether stress could unmask occludin functions within intestinal epithelia.

METHODS

Wildtype (WT), universal and intestinal epithelial-specific occludin KO, and villin-EGFP-occludin transgenic mice as well as WT and occludin knockdown (KD) Caco-2_BBe cell monolayers were challenged with DSS, TNBS, staurosporine, 5-FU, or TNF. Occludin and caspase-3 expression were assessed in patient biopsies.

RESULTS

Intestinal epithelial occludin loss limited severity of DSS- and TNBS-induced colitis due to epithelial resistance to apoptosis; activation of both intrinsic and extrinsic apoptotic pathways was blocked in occludin KO epithelia. Promoter analysis revealed that occludin enhances CASP3 transcription and, conversely, that occludin downregulation reduces caspase-3 expression. Analysis of biopsies from Crohn's disease and ulcerative colitis patients and normal controls demonstrated that disease-associated occludin downregulation was accompanied by and correlated with reduced caspase-3 expression. In vitro, cytokine-induced occludin downregulation resulted in reduced caspase-3 expression and resistance to intrinsic and extrinsic pathway apoptosis, demonstrating an overall protective effect of inflammation-induced occludin loss.

CONCLUSIONS

The tight junction protein occludin regulates apoptosis by enhancing caspase-3 transcription. These data suggest that reduced epithelial caspase-3 expression downstream of occludin downregulation is a previously-unappreciated anti-apoptotic process that contributes to mucosal homeostasis in inflammatory conditions.

Bergamottin Suppresses Metastasis of Lung Cancer Cells through Abrogation of Diverse Oncogenic Signaling Cascades and Epithelial-to-Mesenchymal Transition

Bergamottin (BGM) is a naturally occurring furanocoumarin and is known to inhibit the growth of tumor cells. However, there is no available evidence that BGM has an inhibitory effect on cancer metastasis, specifically on the epithelial-to-mesenchymal transition (EMT) process in the malignant cells. Here we aimed to evaluate the antimetastatic potential of BGM in human lung adenocarcinoma cells. Our results demonstrate that BGM can block EMT, and observed inhibition was accompanied by downregulation of fibronectin, vimentin, N-cadherin, twist and snail expression, and upregulation of occludin and E-cadherin. Interestingly, transforming growth factor-β (TGF-β)-induced upregulation of fibronectin, vimentin, N-cadherin, twist and snail, and downregulation of occludin and E-cadherin, were abrogated by BGM treatment. Moreover, the treatment of BGM repressed TGF-β-induced cell invasive potential. BGM treatment also inhibited multiple oncogenic cascades such as PI3K/Akt/mTOR. Overall, the results demonstrate the potential antimetastatic activity of BGM against lung cancer cells.

Intestinal epithelial claudins: expression and regulation in homeostasis and inflammation

The intestinal epithelium forms a highly dynamic and selective barrier that controls absorption of fluid and solutes while restricting pathogen access to underlying tissues. Barrier properties are achieved by intercellular junctions that include an apical tight junction (TJ) and subjacent adherens junctions and desmosomes. The TJ tetraspan claudin proteins form pores between epithelial cells to control paracellular fluid and ion movement. In addition to regulation of barrier function, claudin family members control epithelial homeostasis and are expressed in a spatiotemporal manner in the intestinal crypt-luminal axis. This delicate balance of physiologic differential claudin protein expression is altered during mucosal inflammation. Inflammatory mediators influence transcriptional regulation, as well as endocytic trafficking, targeting, and retention of claudins in the TJ. Increased expression of intestinal epithelial claudin-1, -2, and -18 with downregulation of claudin-3, -4, -5, -7, -8, and -12 has been observed in intestinal inflammatory disorders. Such changes in claudin proteins modify the epithelial barrier function in addition to influencing epithelial and mucosal homeostasis. An improved understanding of the regulatory mechanisms that control epithelial claudin proteins will provide strategies to strengthen the epithelial barrier function and restore mucosal homeostasis in inflammatory disorders.

Occludin S471 Phosphorylation Contributes to Epithelial Monolayer Maturation

Multiple organ systems require epithelial barriers for normal function, and barrier loss is a hallmark of diseases ranging from inflammation to epithelial cancers. However, the molecular processes regulating epithelial barrier maturation are not fully elucidated. After contact, epithelial cells undergo size-reductive proliferation and differentiate, creating a dense, highly ordered monolayer with high resistance barriers. We provide evidence that the tight junction protein occludin contributes to the regulation of epithelial cell maturation upon phosphorylation of S471 in its coiled-coil domain. Overexpression of a phosphoinhibitory occludin S471A mutant prevents size-reductive proliferation and subsequent tight junction maturation in a dominant manner. Inhibition of cell proliferation in cell-contacted but immature monolayers recapitulated this phenotype. A kinase screen identified G-protein-coupled receptor kinases (GRKs) targeting S471, and GRK inhibitors delayed epithelial packing and junction maturation. We conclude that occludin contributes to the regulation of size-reductive proliferation and epithelial cell maturation in a phosphorylation-dependent manner.

Occludin S490 Phosphorylation Regulates Vascular Endothelial Growth Factor-Induced Retinal Neovascularization

Occludin is a transmembrane tight junction protein that contributes to diverse cellular functions, including control of barrier properties, cell migration, and proliferation. Vascular endothelial growth factor (VEGF) induces phosphorylation of occludin at S490, which is required for VEGF-induced endothelial permeability. Herein, we demonstrate that occludin S490 phosphorylation also regulates VEGF-induced retinal endothelial cell proliferation and neovascularization. Using a specific antibody, phospho-occludin was located in centrosomes in endothelial cell cultures, animal models, and human surgical samples of retinal neovessels. Occludin S490 phosphorylation was found to increase with endothelial tube formation in vitro and in vivo during retinal neovascularization after induction of VEGF expression. More important, expression of occludin mutated at S490 to Ala, completely inhibited angiogenesis in cell culture models and in vivo. Collectively, these data suggest a novel role for occludin in regulation of endothelial proliferation and angiogenesis in a phosphorylation-dependent manner. These findings may lead to methods of regulating pathological neovascularization by specifically targeting endothelial cell proliferation.

Tight Junctions and the Tumor Microenvironment

PURPOSE OF REVIEW

Tight junctions (TJs) are specialized differentiations of epithelial and endothelial cell membranes. TJs play an important role in the adhesion of cells and their interaction with each other. Most cancers originate from epithelial cells. Thus, it is of significance to examine the role of TJs in the tumor microenvironment (TME) and how they affect cancer metastasis.

RECENT FINDINGS

In epithelium-derived cancers, intactness of the primary tumor mass is influenced by intercellular structures as well as cell-to-cell adhesion. Irregularities of these factors may lead to tumor dissociation and subsequent metastasis. Low expression of TJs is observed among highly metastatic cancer cells.

SUMMARY

In this review, we summarized findings from current literature in consideration of the role of TJs in relation to the TME and cancer. Deeper understanding of the mechanisms leading to TJ dysregulation is needed to facilitate the design and conceptualization of new and better therapeutic strategies for cancer.

Signalling at tight junctions during epithelial differentiation and microbial pathogenesis

Tight junctions are a component of the epithelial junctional complex, and they form the paracellular diffusion barrier that enables epithelial cells to create cellular sheets that separate compartments with different compositions. The assembly and function of tight junctions are intimately linked to the actomyosin cytoskeleton and, hence, are under the control of signalling mechanisms that regulate cytoskeletal dynamics. Tight junctions not only receive signals that guide their assembly and function, but transmit information to the cell interior to regulate cell proliferation, migration and survival. As a crucial component of the epithelial barrier, they are often targeted by pathogenic viruses and bacteria, aiding infection and the development of disease. In this Commentary, we review recent progress in the understanding of the molecular signalling mechanisms that drive junction assembly and function, and the signalling processes by which tight junctions regulate cell behaviour and survival. We also discuss the way in which junctional components are exploited by pathogenic viruses and bacteria, and how this might affect junctional signalling mechanisms.

JAM-related proteins in mucosal homeostasis and inflammation

Mucosal surfaces are lined by epithelial cells that form a physical barrier protecting the body against external noxious substances and pathogens. At a molecular level, the mucosal barrier is regulated by tight junctions (TJs) that seal the paracellular space between adjacent epithelial cells. Transmembrane proteins within TJs include junctional adhesion molecules (JAMs) that belong to the cortical thymocyte marker for Xenopus family of proteins. JAM family encompasses three classical members (JAM-A, JAM-B, and JAM-C) and related molecules including JAM4, JAM-like protein, Coxsackie and adenovirus receptor (CAR), CAR-like membrane protein and endothelial cell-selective adhesion molecule. JAMs have multiple functions that include regulation of endothelial and epithelial paracellular permeability, leukocyte recruitment during inflammation, angiogenesis, cell migration, and proliferation. In this review, we summarize the current knowledge regarding the roles of the JAM family members in the regulation of mucosal homeostasis and leukocyte trafficking with a particular emphasis on barrier function and its perturbation during pathological inflammation.

MarvelD3 couples tight junctions to the MEKK1-JNK pathway to regulate cell behavior and survival

MarvelD3 is a transmembrane component of tight junctions, but there is little evidence for a direct involvement in the junctional permeability barrier. Tight junctions also regulate signaling mechanisms that guide cell proliferation; however, the transmembrane components that link the junction to such signaling pathways are not well understood. In this paper, we show that MarvelD3 is a dynamic junctional regulator of the MEKK1-c-Jun NH2-terminal kinase (JNK) pathway. Loss of MarvelD3 expression in differentiating Caco-2 cells resulted in increased cell migration and proliferation, whereas reexpression in a metastatic tumor cell line inhibited migration, proliferation, and in vivo tumor formation. Expression levels of MarvelD3 inversely correlated with JNK activity, as MarvelD3 recruited MEKK1 to junctions, leading to down-regulation of JNK phosphorylation and inhibition of JNK-regulated transcriptional mechanisms. Interplay between MarvelD3 internalization and JNK activation tuned activation of MEKK1 during osmotic stress, leading to junction dissociation and cell death in MarvelD3-depleted cells. MarvelD3 thus couples tight junctions to the MEKK1-JNK pathway to regulate cell behavior and survival.

Meprin A impairs epithelial barrier function, enhances monocyte migration, and cleaves the tight junction protein occludin

Meprin metalloproteases are highly expressed at the luminal interface of the intestine and kidney and in certain leukocytes. Meprins cleave a variety of substrates in vitro, including extracellular matrix proteins, adherens junction proteins, and cytokines, and have been implicated in a number of inflammatory diseases. The linkage between results in vitro and pathogenesis, however, has not been elucidated. The present study aimed to determine whether meprins are determinative factors in disrupting the barrier function of the epithelium. Active meprin A or meprin B applied to Madin-Darby canine kidney (MDCK) cell monolayers increased permeability to fluorescein isothiocyanate-dextran and disrupted immunostaining of the tight junction protein occludin but not claudin-4. Meprin A, but not meprin B, cleaved occludin in MDCK monolayers. Experiments with recombinant occludin demonstrated that meprin A cleaves the protein between Gly(100) and Ser(101) on the first extracellular loop. In vivo experiments demonstrated that meprin A infused into the mouse bladder increased the epithelium permeability to sodium fluorescein. Furthermore, monocytes from meprin knockout mice on a C57BL/6 background were less able to migrate through an MDCK monolayer than monocytes from their wild-type counterparts. These results demonstrate the capability of meprin A to disrupt epithelial barriers and implicate occludin as one of the important targets of meprin A that may modulate inflammation.

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.

Triggers and effectors of oxidative stress at blood-brain barrier level: relevance for brain ageing and neurodegeneration

As fundamental research advances, it is becoming increasingly clear that a clinically expressed disease implies a mixture of intertwining molecular disturbances. Oxidative stress is one of such pathogenic pathways involved in virtually all central nervous system pathologies, infectious, inflammatory, or degenerative in nature. Since brain homeostasis largely depends on integrity of blood-brain barrier (BBB), many studies focused lately on BBB alteration in a wide spectrum of brain diseases. The proper two-way molecular transfer through BBB depends on several factors, including the functional status of its tight junction (TJ) complexes of proteins sealing neighbour endothelial cells. Although there is abundant experimental work showing that oxidative stress associates BBB permeability alteration, less is known about its implications, at molecular level, in TJ protein expression or TJ-related cell signalling. In this paper, oxidative stress is presented as a common pathway for different brain pathogenic mechanisms which lead to BBB dysregulation. We revise here oxidative-induced molecular mechanisms of BBB disruption and TJ protein expression alteration, in relation to ageing and neurodegeneration.

Tight junctions in skin: new perspectives

Tight junctions (TJs) are intercellular contacts that seal the space between the individual cells of an epithelial sheet or stratifying epithelia, such as the epidermis, so that they can collectively separate tissue compartments. Intercellular junctions, such as adherens and TJs, play a crucial role in the formation and maintenance of epithelial and endothelial barriers. A variety of components including claudins, occludin, tricellulin, zonula occluden proteins and junctional adhesion molecules have been identified in complex localization patterns in mammalian epidermis. In several skin diseases that are characterized by impaired skin barrier function, altered proliferation/differentiation of the epidermis and/or infiltration of inflammatory cells, altered expression patterns of TJ proteins have been observed. This review is aimed at providing an insight into the molecular composition, tools for identification and understanding the role of TJs in skin diseases and barrier function regulation.

Regulation of epithelial proliferation by tight junction proteins

The epithelial tight junction (TJ) is the apical-most intercellular junction and serves as a gatekeeper for the paracellular pathway by permitting regulated passage of fluid and ions while restricting movement of large molecules. In addition to these vital barrier functions, TJ proteins are emerging as major signaling molecules that mediate crosstalk between the extracellular environment, the cell surface, and the nucleus. Biochemical studies have recently determined that epithelial TJs contain over a hundred proteins that encompass transmembrane proteins, scaffolding molecules, cytoskeletal components, regulatory elements, and signaling molecules. Indeed, many of these proteins have defined roles in regulating epithelial polarity, differentiation, and proliferation. This review will focus on recent findings that highlight a role for TJ proteins in controlling cell proliferation during epithelial homeostasis, wound healing, and carcinogenesis.

Occludin protein family: oxidative stress and reducing conditions

The occludin-like proteins belong to a family of tetraspan transmembrane proteins carrying a marvel domain. The intrinsic function of the occludin family is not yet clear. Occludin is a unique marker of any tight junction and is found in polarized endothelial and epithelial tissue barriers, at least in the adult vertebrate organism. Occludin is able to oligomerize and to form tight junction strands by homologous and heterologous interactions, but has no direct tightening function. Its oligomerization is affected by pro- and antioxidative agents or processes. Phosphorylation of occludin has been described at multiple sites and is proposed to play a regulatory role in tight junction assembly and maintenance and, hence, to influence tissue barrier characteristics. Redox-dependent signal transduction mechanisms are among the pathways modulating occludin phosphorylation and function. This review discusses the novel concept that occludin plays a key role in the redox regulation of tight junctions, which has a major impact in pathologies related to oxidative stress and corresponding pharmacologic interventions.

Occludin localizes to centrosomes and modifies mitotic entry

Proper control of cell cycle progression and barrier function are essential processes to the maintenance of epithelial cell homeostasis. The contribution of tight junction proteins to barrier function is well established, whereas their contribution to cell cycle control is only beginning to be understood. Centrosomes are the principal microtubule organizing centers in eukaryotic cells and centrosome duplication and separation are linked to the cell cycle and mitotic entry. Here we demonstrate that occludin localizes with centrosomes in Madin-Darby canine kidney cells. Immunocytochemistry and biochemical fractionation studies reveal occludin localizes with centrosomes during interphase and occludin Ser-490 phosphorylation at centrosomes increases with mitotic entry. Stable expression of aspartic acid phosphomimetic (S490D) results in centrosomal localization of occludin and increases cell numbers. Furthermore, we provide evidence that occludin regulates centrosome separation and mitotic entry as the nonphosphorylatable alanine mutation (S490A) impedes centrosome separation, delays mitotic entry, and reduces proliferation. Collectively, these studies demonstrate a novel location and function for occludin in centrosome separation and mitosis.

A solute gradient in the tear meniscus. I. A hypothesis to explain Marx's line

Marx's line is a line of mucosal staining behind the mucocutaneous junction. It can be demonstrated throughout life in all normal lids by staining with lissamine green and related dyes. Of all the body orifices, only the mucosae of the eye and mouth are directly exposed to the atmosphere. In this paper, we suggest that for the eye, this exposure leads to the formation of Marx's line. The tear meniscus thins progressively toward its apex, where it is pinned at the mucocutaneous junction of the lid. It also thins toward the black line, which segregates the meniscus from the tear film after the blink. We predict that, because of the geometry of the tear meniscus, evaporation generates a solute gradient across the meniscus profile in the anteroposterior plane, which peaks at the meniscus apices at the end of the interblink. One outcome would be to amplify the level of tear molarity at these sites so that they reach hyperosmolar proportions. Preliminary mathematical modeling suggests that dilution of this effect by advection and diffusion of solute away from the meniscus apex at the mucocutaneous junction will be restricted by spatial constraints, the presence of tear and surface mucins at this site, and limited fluid flow. We conclude that evaporative water loss from the tear meniscus may result in a physiological zone of hyperosmolar and related stresses to the occlusal conjunctiva, directly behind the mucocutaneous junction. We hypothesize that this stimulates a high epithelial cell turnover at this site, incomplete epithelial maturation, and a failure to express key molecules such as MUC 16 and galectin-3, which, with the tight junctions between surface epithelial cells, are necessary to seal the ocular surface and prevent penetration of dyes and other molecules into the epithelium. This is proposed as the basis for Marx's line. In Part II of this paper (also published in this issue of The Ocular Surface), we address additional pathophysiological consequences of this mechanism, affecting lid margins.

Review: molecular pathogenesis of blood-brain barrier breakdown in acute brain injury

Historically, the blood-brain barrier (BBB) was considered to be at the level of cerebral endothelium. Currently, the interaction of endothelium with other components of the vessel wall and with neurones and glial cells is considered to constitute a functional unit, termed the neurovascular unit that maintains cerebral homeostasis in steady states and brain injury. The emphasis of this review is on cerebral endothelium, the best-studied component of the neurovascular unit, and its permeability mechanisms in health and acute brain injury. Major advances have been made in unravelling the molecular structure of caveolae and tight junctions, both of which are components of the structural barrier to the entry of plasma proteins into brain. Time course studies suggest that caveolar changes precede junctional changes in acute brain injury. Additional factors modulating BBB permeability in acute brain injury are matrix metalloproteinases-2 and 9 and angiogenic factors, the most notable being vascular endothelial growth factor-A and angiopoietins (Ang) 1 and 2. Vascular endothelial growth factor-A and Ang2 have emerged as potent inducers of BBB breakdown while Ang1 is a potent anti-leakage factor. These factors have the potential to modulate permeability in acute brain injury and this is an area of ongoing research. Overall, a combination of haemodynamic, structural and molecular alterations affecting brain endothelium results in BBB breakdown in acute brain injury.

Morphology and properties of brain endothelial cells

The molecular advances in various aspects of brain endothelial cell function in steady states are considerable and difficult to summarize in one chapter. Therefore, this chapter focuses on endothelial permeability mechanisms in steady states and disease namely vasogenic edema. The morphology and properties of caveolae and tight junctions that are involved in endothelial permeability to macromolecules are reviewed. Endothelial transport functions are briefly reviewed. Diseases with alterations of endothelial permeability are mentioned and details are provided of the molecular alterations in caveolae and tight junctions in vasogenic edema. Other factors involved in increased endothelial permeability such as the matrix metalloproteinases are briefly discussed. Of the modulators of endothelial permeability, angioneurins such as the vascular endothelial growth factors and angiopoietins are discussed. The chapter concludes with a brief discussion on delivery of therapeutic substances across endothelium.

The second extracellular loop dictates Occludin-mediated HCV entry

Recent findings have implicated tight junction (TJ) protein Occludin (OCLN) as an essential factor for hepatitis C virus (HCV) to enter human hepatocytes. To gain insights into OCLN-mediated HCV entry, we created a panel of OCLN deletion mutants and found that without impairing OCLN's cell surface localization, removal of the extracellular loop 2 (EL2) from OCLN abolished both its ability to mediate HIV-HCV pseudotypes' (HCVpp) entry as well as its ability to coprecipitate HCV glycoprotein E2. Recombinant OCLN EL2, however, failed to robustly bind soluble E2 (sE2) in pull-down assays. Subsequent studies revealed that OCLN formed complex with Dynamin II, an important GTPase for endocytosis, in an EL2-dependent fashion. HCVpp, as well as cell culture grown HCV (HCVcc), was sensitive to Dynamin knockdown or inhibition. We conclude that OCLN EL2 dictates the Dynamin-dependent HCV entry. Furthermore, OCLN could function to bridge virions to Dynamin-dependent endocytic machineries.

Physiology and function of the tight junction

Understanding of tight junctions has evolved from their historical perception as inert solute barriers to recognition of their physiological and biochemical complexity. Many proteins are specifically localized to tight junctions, including cytoplasmic actin-binding proteins and adhesive transmembrane proteins. Among the latter are claudins, which are critical barrier proteins. Current information suggests that the paracellular barrier is most usefully modeled as having two physiologic components: a system of charge-selective small pores, 4 A in radius, and a second pathway created by larger discontinuities in the barrier, lacking charge or size discrimination. The first pathway is influenced by claudin expression patterns and the second is likely controlled by different proteins and signals. Recent information on claudin function and disease-causing mutations have led to a more complete understanding of their role in barrier formation, but progress is impeded by lack of high resolution structural information.

Glycogen Synthase Kinase 3 (GSK-3) influences epithelial barrier function by regulating occludin, claudin-1 and E-cadherin expression

The Apical Junctional Complex (AJC) encompassing the tight junction (TJ) and adherens junction (AJ) plays a pivotal role in regulating epithelial barrier function and epithelial cell proliferative processes through signaling events that remain poorly characterized. A potential regulator of AJC protein expression is Glycogen Synthase Kinase-3 (GSK-3). GSK-3 is a constitutively active kinase that is repressed during epithelial-mesenchymal transition (EMT). In the present study, we report that GSK-3 activity regulates the structure and function of the AJC in polarized model intestinal (SK-CO15) and kidney (Madin-Darby Canine Kidney (MDCK)) epithelial cells. Reduction of GSK-3 activity, either by small molecule inhibitors or siRNA targeting GSK-3 alpha and beta mRNA, resulted in increased permeability to both ions and bulk solutes. Immunofluorescence labeling and immunoblot analyses revealed that the barrier defects correlated with decreased protein expression of AJC transmembrane proteins Occludin, Claudin-1 and E-cadherin without influencing other TJ proteins, Zonula Occludens-1 (ZO-1) and Junctional Adhesion Molecule A (JAM-A). The decrease in Occludin and E-cadherin protein expression correlated with downregulation of the corresponding mRNA levels for these respective proteins following GSK-3 inhibition. These observations implicate an important role of GSK-3 in the regulation of the structure and function of the AJC that is mediated by differential modulation of mRNA transcription of key AJC proteins, Occludin, Claudin-1 and E-cadherin.

Identification of MarvelD3 as a tight junction-associated transmembrane protein of the occludin family

Background

Tight junctions are an intercellular adhesion complex of epithelial and endothelial cells, and form a paracellular barrier that restricts the diffusion of solutes on the basis of size and charge. Tight junctions are formed by multiprotein complexes containing cytosolic and transmembrane proteins. How these components work together to form functional tight junctions is still not well understood and will require a complete understanding of the molecular composition of the junction.

Results

Here we identify a new transmembrane component of tight junctions: MarvelD3, a four-span transmembrane protein. Its predicted transmembrane helices form a Marvel (MAL and related proteins for vesicle traffic and membrane link) domain, a structural motif originally discovered in proteins involved in membrane apposition and fusion events, such as the tight junction proteins occludin and tricellulin. In mammals, MarvelD3 is expressed as two alternatively spliced isoforms. Both isoforms exhibit a broad tissue distribution and are expressed by different types of epithelial as well as endothelial cells. MarvelD3 co-localises with occludin at tight junctions in intestinal and corneal epithelial cells. RNA interference experiments in Caco-2 cells indicate that normal MarvelD3 expression is not required for the formation of functional tight junctions but depletion results in monolayers with increased transepithelial electrical resistance.

Conclusions

Our data indicate that MarvelD3 is a third member of the tight junction-associated occludin family of transmembrane proteins. Similar to occludin, normal expression of MarvelD3 is not essential for the formation of functional tight junctions. However, MarvelD3 functions as a determinant of epithelial paracellular permeability properties.

Tight junctions: a barrier to the initiation and progression of breast cancer?

Breast cancer is a complex and heterogeneous disease that arises from epithelial cells lining the breast ducts and lobules. Correct adhesion between adjacent epithelial cells is important in determining the normal structure and function of epithelial tissues, and there is accumulating evidence that dysregulated cell-cell adhesion is associated with many cancers. This review will focus on one cell-cell adhesion complex, the tight junction (TJ), and summarize recent evidence that TJs may participate in breast cancer development or progression. We will first outline the protein composition of TJs and discuss the functions of the TJ complex. Secondly we will examine how alterations in these functions might facilitate breast cancer initiation or progression; by focussing on the regulatory influence of TJs on cell polarity, cell fate and cell migration. Finally we will outline how pharmacological targeting of TJ proteins may be useful in limiting breast cancer progression. Overall we hope to illustrate that the relationship between TJ alterations and breast cancer is a complex one; but that this area offers promise in uncovering fundamental mechanisms linked to breast cancer progression.

Identification and analysis of occludin phosphosites: a combined mass spectrometry and bioinformatics approach

The molecular function of occludin, an integral membrane component of tight junctions, remains unclear. VEGF-induced phosphorylation sites were mapped on occludin by combining MS data analysis with bioinformatics. In vivo phosphorylation of Ser490 was validated and protein interaction studies combined with crystal structure analysis suggest that Ser490 phosphorylation attenuates the interaction between occludin and ZO-1. This study demonstrates that combining MS data and bioinformatics can successfully identify novel phosphorylation sites from limiting samples.

Regulation of renal epithelial tight junctions by the von Hippel-Lindau tumor suppressor gene involves occludin and claudin 1 and is independent of E-cadherin

Epithelial-to-mesenchymal transitions (EMT) are important in renal development, fibrosis, and cancer. Loss of function of the tumor suppressor VHL leads to many features of EMT, and it has been hypothesized that the pivotal mediator is down-regulation of the adherens junction (AJ) protein E-cadherin. Here we show that VHL loss-of-function also has striking effects on the expression of the tight junction (TJ) components occludin and claudin 1 in vitro in VHL-defective clear cell renal cell carcinoma (CCRCC) cells and in vivo in VHL-defective sporadic CCRCCs (compared with normal kidney). Occludin is also down-regulated in premalignant foci in kidneys from patients with germline VHL mutations, consistent with a contribution to CCRCC initiation. Reexpression of E-cadherin was sufficient to restore AJ but not TJ assembly, indicating that the TJ defect is independent of E-cadherin down-regulation. Additional experiments show that activation of hypoxia inducible factor (HIF) contributes to both TJ and AJ abnormalities, thus the VHL/HIF pathway contributes to multiple aspects of the EMT phenotype that are not interdependent. Despite the independent nature of the defects, we show that treatment with the histone deacetylase inhibitor sodium butyrate, which suppresses HIF activation, provides a method for reversing EMT in the context of VHL inactivation.

Tumor necrosis factor receptor-associated factor 4 is a dynamic tight junction-related shuttle protein involved in epithelium homeostasis

Background

Despite numerous in vivo evidences that Tumor Necrosis Factor Receptor-Associated Factor 4 (TRAF4) plays a key biological function, how it works at the cellular and molecular level remains elusive.

Methodology/Principal Findings

In the present study, we show using immunofluorescence and immuohistochemistry that TRAF4 is a novel player at the tight junctions (TJs). TRAF4 is connected to assembled TJs in confluent epithelial cells, but accumulates in the cytoplasm and/or nucleus when TJs are open in isolated cells or EGTA-treated confluent cells. In vivo, TRAF4 is consistently found at TJs in normal human mammary epithelia as well as in well-differentiated in situ carcinomas. In contrast, TRAF4 is never localized at the plasma membrane of poorly-differentiated invasive carcinomas devoid of correct TJs, but is observed in the cytoplasm and/or nucleus of the cancer cells. Moreover, TRAF4 TJ subcellular localization is remarkably dynamic. Fluorescence recovery after photobleaching (FRAP) experiments show that TRAF4 is highly mobile and shuttles between TJs and the cytoplasm. Finally, we show that intracellular TRAF4 potentiates ERK1/2 phosphorylation in proliferating HeLa cells, an epithelial cell line known to be devoid of TJs.

Conclusions/Significance

Collectively, our data strongly support the new concept of TJs as a dynamic structure. Moreover, our results implicate TRAF4 in one of the emerging TJ-dependent signaling pathways that responds to cell polarity by regulating the cell proliferation/differentiation balance, and subsequently epithelium homeostasis. Drastic phenotypes or lethality in TRAF4-deficient mice and drosophila strongly argue in favor of such a function.

A novel splice variant of occludin deleted in exon 9 and its role in cell apoptosis and invasion

The tight junction protein occludin participates in cell adhesion and migration and has been shown to possess antitumorigenic properties; however, the exact mechanism underlying these effects is poorly understood. In liver cell lines, we identified an occludin splice variant deleted in exon 9 (Occ(DeltaE9)). Furthermore, comparison analysis of wild-type occludin (Occ(WT)) and Occ(DeltaE9) revealed that exon 9 played important roles in the induction of mitochondria-mediated apoptosis and the inhibition of invasion, along with the downregulation of matrix metalloproteinase expression. In addition, by using the calcium indicator X-rhod-1, and the inositol trisphosphate receptor inhibitor 2-aminoethoxydiphenyl borate, we found that Occ(WT) but not Occ(DeltaE9) increased calcium release from the endoplasmic reticulum. In conclusion, our results showed that occludin mediates apoptosis and invasion by elevating the cytoplasmic calcium concentration and that exon 9 of occludin is an important region that mediates these effects.

Tight junctions and the modulation of barrier function in disease

Tight junctions create a paracellular barrier in epithelial and endothelial cells protecting them from the external environment. Two different classes of integral membrane proteins constitute the tight junction strands in epithelial cells and endothelial cells, occludin and members of the claudin protein family. In addition, cytoplasmic scaffolding molecules associated with these junctions regulate diverse physiological processes like proliferation, cell polarity and regulated diffusion. In many diseases, disruption of this regulated barrier occurs. This review will briefly describe the molecular composition of the tight junctions and then present evidence of the link between tight junction dysfunction and disease.

A key claudin extracellular loop domain is critical for epithelial barrier integrity

Intercellular tight junctions (TJs) regulate epithelial barrier properties. Claudins are major structural constituents of TJs and belong to a large family of tetra-spanning membrane proteins that have two predicted extracellular loops (ELs). Given that claudin-1 is widely expressed in epithelia, we further defined the role of its EL domains in determining TJ function. The effects of several claudin-1 EL mimetic peptides on epithelial barrier structure and function were examined. Incubation of model human intestinal epithelial cells with a 27-amino acid peptide corresponding to a portion of the first EL domain (Cldn-1(53-80)) reversibly interfered with epithelial barrier function by inducing the rearrangement of key TJ proteins: occludin, claudin-1, junctional adhesion molecule-A, and zonula occludens-1. Cldn-1(53-80) associated with both claudin-1 and occludin, suggesting both the direct interference with the ability of these proteins to assemble into functional TJs and their close interaction under physiological conditions. These effects were specific for Cldn-1(53-80), because peptides corresponding to other claudin-1 EL domains failed to influence TJ function. Furthermore, the oral administration of Cldn-1(53-80) to rats increased paracellular gastric permeability. Thus, the identification of a critical claudin-1 EL motif, Cldn-1(53-80), capable of regulating TJ structure and function, offers a useful adjunct to treatments that require drug delivery across an epithelial barrier.

Investigation of allelic imbalances on chromosome 3p in nasopharyngeal carcinoma in Tunisia: high frequency of microsatellite instability in patients with early-onset of the disease

Tunisia is one of the world's intermediate risk areas for nasopharyngeal carcinoma (NPC). Loss of heterozygosity (LOH) on the short arm of chromosome 3 (3p) is the most frequent genetic change reported in NPC from endemic areas. In the present study, we investigate the incidence of LOH and microsatellite instability (MSI) on chromosome 3p in 49 microdissected primary NPC specimens and corresponding non-cancerous tissues from Tunisian patients using six microsatellite polymorphic markers. LOH at one or more markers was observed in 40 out of 48 informative cases (83.3%). The markers D3S1038 at 3p25.2-26.1 and D3S1076 at 3p21.1-21.2 have showed the highest frequency of LOH (51.3%), followed by D3S1067 at 3p14.3-21.1 (48.7%), D3S1568 at 3p21.3 (47.4%), D3S659 at 3p13 (15.3%), and D3S1228 at 3p14.1-14.2 (11%). Interestingly, MSI at one or more microsatellite markers was observed in 15 cases (31.2%). The highest frequency of MSI was presented by D3S1568 (18.4%), D3S1067 (17.9%), and D3S1038 (12.8%). With regard to clinicopathological features, LOH was found to be less common in young patients (under 25 years) than in adults (p=0.04), whereas MSI was found to be more frequent in patients under 45 years than in older patients (p=0.006). No significant correlation was found between LOH or MSI and the other clinicopathological features investigated including, gender, tumor size, lymph node metastasis, UICC clinical stage, and histological subtype. This study revealed different patterns of allelic imbalance on chromosome 3P in NPC between age groups in Tunisia, and suggests an alteration in the DNA mismatch repair machinery that may be, in part, responsible of the early age onset form of this disease in North African populations. More attention should be given to the mismatch repair system in the juvenile form of this disease in future studies.

Occludin independently regulates permeability under hydrostatic pressure and cell division in retinal pigment epithelial cells

PURPOSE

The aim of this study was to determine the function of the tight junction protein occludin in the control of permeability, under diffusive and hydrostatic pressures, and its contribution to the control of cell division in retinal pigment epithelium.

METHODS

Occludin expression was inhibited in the human retinal pigment epithelial cell line ARPE-19 by siRNA. Depletion of occludin was confirmed by Western blot, confocal microscopy, and RT-PCR. Paracellular permeability of cell monolayers to fluorescently labeled 70 kDa dextran, 10 kDa dextran, and 467 Da tetramethylrhodamine (TAMRA) was examined under diffusive conditions or after the application of 10 cm H2O transmural pressure. Cell division rates were determined by tritiated thymidine incorporation and Ki67 immunoreactivity. Cell cycle inhibitors were used to determine whether changes in cell division affected permeability.

RESULTS

Occludin depletion increased diffusive paracellular permeability to 467 Da TAMRA by 15%, and permeability under hydrostatic pressure was increased 50% compared with control. Conversely, depletion of occludin protein with siRNA did not alter diffusive permeability to 70 kDa and 10 kDa RITC-dextran, and permeability to 70 kDa dextran was twofold lower in occludin-depleted cells under hydrostatic pressure conditions. Occludin depletion also increased thymidine incorporation by 90% and Ki67-positive cells by 50%. Finally, cell cycle inhibitors did not alter the effect of occludin siRNA on paracellular permeability.

CONCLUSIONS

The data suggest that occludin regulates tight junction permeability in response to changes in hydrostatic pressure. Furthermore, these data suggest that occludin also contributes to the control of cell division, demonstrating a novel function for this tight junction protein.

Role of the intestinal barrier in inflammatory bowel disease

A critical function of the intestinal mucosa is to form a barrier that separates luminal contents from the interstitium. The single layer of intestinal epithelial cells (IECs) serves as a dynamic interface between the host and its environment. Cell polarity and structural properties of the epithelium is complex and is important in the development of epithelial barrier function. Epithelial cells associate with each other via a series of intercellular junctions. The apical most intercellular junctional complex referred to as the Apical Junction Complex (AJC) is important in not only cell-cell recognition, but also in the regulation of paracellular movement of fluid and solutes. Defects in the intestinal epithelial barrier function have been observed in a number of intestinal disorders such as inflammatory bowel disease (IBD). It is now becoming evident that an aberrant epithelial barrier function plays a central role in the pathophysiology of IBD. Thus, a better understanding of the intestinal epithelial barrier structure and function in healthy and disease states such as IBD will foster new ideas for the development of therapies for such chronic disorders.

Glucocorticoids induce transactivation of tight junction genes occludin and claudin-5 in retinal endothelial cells via a novel cis-element

Tight junctions between vascular endothelial cells help to create the blood-brain and blood-retinal barriers. Breakdown of the retinal tight junction complex is problematic in several disease states including diabetic retinopathy. Glucocorticoids can restore and/or preserve the endothelial barrier to paracellular permeability, although the mechanism remains unclear. We show that glucocorticoid treatment of primary retinal endothelial cells increases content of the tight junction proteins occludin and claudin-5, co-incident with an increase in barrier properties of endothelial monolayers. The glucocorticoid receptor antagonist RU486 reverses both the glucocorticoid-stimulated increase in occludin content and the increase in barrier properties. Transcriptional activity from the human occludin and claudin-5 promoters increases in retinal endothelial cells upon glucocorticoid treatment, and is dependent on the glucocorticoid receptor (GR) as demonstrated by siRNA. Deletion analysis of the occludin promoter reveals a 205bp sequence responsible for the glucocorticoid response. However, this region does not possess a canonical glucocorticoid response element and does not bind to the GR in a chromatin immunoprecipitation (ChIP) assay. Mutational analysis of this region revealed a novel 40bp occludin enhancer element (OEE), containing two highly conserved regions of 10 and 13 base pairs, that is both necessary and sufficient for glucocorticoid-induced gene expression in retinal endothelial cells. These data suggest a novel mechanism for glucocorticoid induction of vascular endothelial barrier properties through increased occludin and claudin-5 gene expression.

Transmembrane proteins of tight junctions

Tight junctions contribute to the paracellular barrier, the fence dividing plasma membranes, and signal transduction, acting as a multifunctional complex in vertebrate epithelial and endothelial cells. The identification and characterization of the transmembrane proteins of tight junctions, claudins, junctional adhesion molecules (JAMs), occludin and tricellulin, have led to insights into the molecular nature of tight junctions. We provide an overview of recent progress in studies on these proteins and highlight their roles and regulation, as well as their functional significance in human diseases.

Occludin-mediated premature senescence is a fail-safe mechanism against tumorigenesis in breast carcinoma cells

We have previously demonstrated that epigenetic silencing of occludin, a tight junction-associated membrane protein, results in the acquisition of apoptotic resistance to various apoptogenic stimuli, causally contributing to the enhanced tumorigenicity of cancer cells. However, it remains to be examined whether occludin expression in transformed cells has an alternative impact that is important for cancer progression. Here we show that forced expression of occludin induces anoikis and promotes oxidative stress-induced premature senescence in breast carcinoma cells, which is accompanied by upregulation of negative cell cycle regulators such as p16(INK4A), p21(Waf1/Cip1) and p27(Kip1) but not p53. The senescent phenotype is reversed by specific inhibition of mitogen-activated protein kinase. Endogenous reexpression of occludin mediated by a synergistic effect with a demethylator and histone deacetylase inhibitor or retinoids that stimulate retinoic acid receptor alpha is also sufficient for provoking the senescent phenotype. In addition, tumors that developed from occludin-expressing cells in mice showed a feature of cellular senescence that has not been described as a consequence of occludin signaling. These findings suggest that the loss of occludin expression is at least partially involved in the senescence-escape program during mammary tumorigenesis.

Vascular permeability in ocular disease and the role of tight junctions

Vascular permeability is closely linked with angiogenesis in a number of pathologies. In the retina, the normally well-developed blood-retinal barrier is altered in a host of eye diseases preceding or commensurate with angiogenesis. This review examines the literature regarding the tight junction complex that establishes the blood-retinal barrier focusing on the transmembrane proteins occludin and the claudin family and the membrane associated protein zonula occludens. The changes observed in these proteins associated with vascular and epithelial permeability is discussed. Finally, novel literature addressing the link between the tight junction complex and angiogenesis is considered.

Role of tight junctions in cell proliferation and cancer

The acquisition of a cancerous phenotype by epithelial cells involves the disruption of intercellular adhesions. The reorganization of the E-cadherin/beta-catenin complex in adherens junctions during cell transformation is widely recognized. Instead the implication of tight junctions (TJs) in this process is starting to be unraveled. The aim of this article is to review the role of TJ proteins in cell proliferation and cancer.

Tight junctions and cell polarity

The tight junction is an intracellular junctional structure that mediates adhesion between epithelial cells and is required for epithelial cell function. Tight junctions control paracellular permeability across epithelial cell sheets and also serve as a barrier to intramembrane diffusion of components between a cell's apical and basolateral membrane domains. Recent genetic and biochemical studies in invertebrates and vertebrates indicate that tight junction proteins play an important role in the establishment and maintenance of apico-basal polarity. Proteins involved in epithelial cell polarization form evolutionarily conserved multiprotein complexes at the tight junction, and these protein complexes regulate the architecture of epithelia throughout the polarization process. Accumulating information regarding the regulation of these polarity proteins will lead to a better understanding of the molecular mechanisms whereby cell polarity is established.