Possible involvement of phosphorylation of occludin in tight junction formation

Shear stress regulates occludin content and phosphorylation

Previous studies determined that shear stress imposed on bovine aortic endothelial cell (BAEC) monolayers increased the hydraulic conductivity (L(P)); however, the mechanism by which shear stress increases L(P) remains unknown. This study tested the hypothesis that shear stress regulates paracellular transport by altering the expression and phosphorylation state of the tight junction protein occludin. The effect of shear stress on occludin content was examined by Western blot analysis. Ten dyn/cm(2) significantly reduced occludin content in a time-dependent manner such that after a 3 h exposure to shear, occludin content decreased to 44% of control. Twenty dyn/cm(2) decreased occludin content to 50% of control and increased L(P) by 4.7-fold after 3 h. Occludin expression and L(P) depend on tyrosine kinase activity because erbstatin A (10 microM) attenuated both the shear-induced decrease in occludin content and increase in L(P). Shear stress increased occludin phosphorylation after 5 min, 15 min, and 3 h exposures. The shear-induced increase in occludin phosphorylation was attenuated with dibutyryl (DB) cAMP (1 mM), a reagent previously shown to reverse the shear-induced increase in L(P). We conclude that shear stress rapidly (< or = 5 min) increases occludin phosphorylation and significantly decreases the expression of occludin over 1-4 h. Alterations in the occludin phosphorylation state and occludin total content are potential mechanisms by which shear stress increases L(P).

Reassembly of the tight junction after oxidative stress depends on tyrosine kinase activity

Oxidative stress compromises the tight junction, but the mechanisms underlying its recovery remain unclear. We developed a model in which oxidative stress reversibly disrupts the tight junction. Exposure of Madin-Darby canine kidney cells to hydrogen peroxide markedly reduced transepithelial resistance and disrupted the staining patterns of the tight junction proteins ZO-1 and occludin. These changes were reversed by catalase. The short-term reassembly of tight junctions was not dependent on new protein synthesis, suggesting that recovery occurs through re-utilization of existing proteins. Although ATP levels were reduced, the reduction was insufficient to explain the observed changes, since a comparable reduction of ATP levels (with 2-deoxy-D-glucose) did not induce these changes. The intracellular hydrogen peroxide scavenger pyruvate protected Madin-Darby canine kidney cells from loss of transepithelial resistance as did the heavy metal scavenger N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine. Of a wide variety of agents examined, only tyrosine kinase inhibitors and protein kinase C inhibitors markedly inhibited tight junction reassembly. During reassembly, tyrosine phosphorylation in or near the lateral membrane, was detected by immunofluorescence. The tyrosine kinase inhibitors genistein and PP-2 inhibited the recovery of transepithelial resistance and perturbed the relocalization of ZO-1 and occludin to the tight junction, indicating that tyrosine kinases, possibly members of the Src family, are critical for reassembly after oxidative stress.

Etk/Bmx activation modulates barrier function in epithelial cells

Etk/Bmx is a member of the Tec family of cytoplasmic non-receptor tyrosine kinases known to express in epithelial cells. We demonstrate herein that Etk activation in stably Etk-transfected epithelial Pa-4 cells resulted in a consistently increased transepithelial resistance (TER). After 24 h of hypoxic (1% O2) exposure, the TER and equivalent active ion transport rate ( I eq) were reduced to <5% of the normoxia control in Pa-4 cells, whereas both TER and I eqwere maintained at comparable and 60% levels, respectively, relative to their normoxic controls in cells with Etk activation. Moreover, Pa-4 cells exhibited an abundant actin stress fiber network with a diffuse distribution of β-catenin at the cell periphery. By contrast, Etk-activated cells displayed a redistribution of actin to an exclusively peripheral network, with a discrete band of β-catenin also concentrated at the cell periphery, and an altered occludin distribution profile. On the basis of these findings, we propose that Etk may be a novel regulator of epithelial junctions during physiological and pathophysiological conditions.

Fatty acids and epithelial permeability: effect of conjugated linoleic acid in Caco-2 cells

Conjugated linoleic acid (CLA) is a collective term referring to the positional and geometric isomers of linoleic acid. This novel fatty acid has been shown to have a number of beneficial actions, including immunomodulatory, anticarcinogenic, and antiatherogenic effects. Tight junctions of epithelial cells determine epithelial membrane integrity and selective paracellular permeability to ions and macromolecules. Occludin and ZO-1 are integral structural components of the tight junction, which are involved in the biogenesis and functional integrity of the epithelial monolayer. This study investigated the effects of two isomers of CLA (cis-9 and trans-10 isomers) on Caco-2 cell transepithelial resistance (TER) development, paracellular epithelial permeability, and occludin and ZO-1 expression. Caco-2 cells were grown in media supplemented with 0.05 mM linoleic acid, cis-9 CLA, or trans-10 CLA for 21 days. The trans-10 CLA isomer delayed Caco-2 cell TER development, which is an in vitro measure of epithelial cell integrity, and increased paracellular epithelial permeability. Immunofluorescent staining of Caco-2 cell epithelial monolayers grown in media supplemented trans-10 CLA showed that the trans-10 CLA isomer altered distribution of occludin and ZO-1. The trans-10 CLA isomer delayed the acquisition of transepithelial resistance and altered the cellular distribution of occludin, which have important implications in relation to epithelial permeability.

The effects of the Na(+)/Ca(++) exchange blocker on osmotic blood-brain barrier disruption

Osmotic disruption of the blood-brain barrier (BBB) by mannitol is currently being used to enhance drug delivery in human brains. Despite clinical and experimental interest, to date the time course in the early phase of disruption has not been accurately identified. The mechanism in barrier closure also remains elusive. We first studied the rapid change in cerebrovascular permeability after BBB disruption in rats, and then demonstrated that the Na(+)/Ca(++) exchange blocker (KB-R7943) prolongs osmotic disruption. Osmotic BBB disruption was attained by using intra-arterial infusion of hypertonic mannitol in Sprague-Dawley (SD) rats. To measure the changes in cerebrovascular permeability, perfusate containing [14C]-sucrose was infused intra-arterially at different time points following osmotic stress. Cerebrovascular permeability was then measured with the in situ brain perfusion technique. This is the first in vivo study demonstrating that osmotic disruption is prolonged by the Na(+)/Ca(++) exchange blocker, which did not affect the peak level of BBB disruption. The exact time course of cerebrovascular reversibility was studied and the earliest BBB disruption was seen to occur 5 min after osmotic stress. Histopathological examination after osmotic disruption with the Na(+)/Ca(++) exchange blocker showed no neuronal damage in rat brains. Our findings represent important experimental information regarding pharmacological manipulation of BBB disruption. The possibility of prolonging the transient opening of the BBB has major clinical implications.

Multifunctional strands in tight junctions

Tight junctions are one mode of cell-cell adhesion in epithelial and endothelial cellular sheets. They act as a primary barrier to the diffusion of solutes through the intercellular space, create a boundary between the apical and the basolateral plasma membrane domains, and recruit various cytoskeletal as well as signalling molecules at their cytoplasmic surface. New insights into the molecular architecture of tight junctions allow us to now discuss the structure and functions of this unique cell-cell adhesion apparatus in molecular terms.

Regulation of tight junction permeability and occludin phosphorylation by Rhoa-p160ROCK-dependent and -independent mechanisms

In epithelial and endothelial cells, tight junctions regulate the paracellular permeability of ions and proteins. Disruption of tight junctions by inflammation is often associated with tissue edema, but regulatory mechanisms are not fully understood. Using ECV304 cells as a model system, lysophosphatidic acid and histamine were found to increase the paracellular permeability of the tracer horseradish peroxidase. Cytoskeletal changes induced by these agents included stimulation of stress fiber formation and myosin light chain phosphorylation. Additionally, occludin, a tight junction protein, was a target for signaling events triggered by lysophosphatidic acid and histamine, events that resulted in its phosphorylation. A dominant-negative mutant of RhoA, RhoA T19N, or a specific inhibitor of Rho-activated kinases, Y-27632, prevented stress fiber formation, myosin light chain phosphorylation, occludin phosphorylation, and the increase in tracer flux in response to lysophosphatidic acid. In contrast, although RhoA T19N and Y-27632 blocked the cytoskeletal events induced by histamine, they had no effect on the stimulation of occludin phosphorylation or increased tracer flux, indicating that occludin phosphorylation may regulate tight junction permeability independently of cytoskeletal events. Thus, occludin is a target for receptor-initiated signaling events regulating its phosphorylation, and this phosphorylation may be a key regulator of tight junction permeability.

Association of junctional adhesion molecule with calcium/calmodulin-dependent serine protein kinase (CASK/LIN-2) in human epithelial caco-2 cells

We report here that junctional adhesion molecule (JAM) interacts with calcium/calmodulin-dependent serine protein kinase (CASK), a protein related to membrane-associated guanylate kinases. In Caco-2 cells, JAM and CASK were coprecipitated and found to colocalize at intercellular contacts along the lateral surface of the plasma membrane. Association of JAM with CASK requires the PSD95/dlg/ZO-1 (PDZ) domain of CASK and the putative PDZ-binding motif Phe-Leu-Val(COOH) in the cytoplasmic tail of JAM. Temporal dissociation in the junctional localization of the two proteins suggests that the association with CASK is not required for recruiting JAM to intercellular junctions. Compared with mature intercellular contacts, junction assembly was characterized by both enhanced solubility of CASK in Triton X-100 and reduced amounts of Triton-insoluble JAM-CASK complexes. We propose that JAM association with CASK is modulated during junction assembly, when CASK is partially released from its cytoskeletal associations.

Inflammatory pain alters blood-brain barrier permeability and tight junctional protein expression

Effects of inflammatory pain states on functional and molecular properties of the rat blood-brain barrier (BBB) were investigated. Inflammation was produced by subcutaneous injection of formalin, lambda-carrageenan, or complete Freund's adjuvant (CFA) into the right hind paw. In situ perfusion and Western blot analyses were performed to assess BBB integrity after inflammatory insult. In situ brain perfusion determined that peripheral inflammation significantly increased the uptake of sucrose into the cerebral hemispheres. Capillary depletion and cerebral blood flow analyses indicated the perturbations were due to increased paracellular permeability rather than vascular volume changes. Western blot analyses showed altered tight junctional protein expression during peripheral inflammation. Occludin significantly decreased in the lambda-carrageenan- and CFA-treated groups. Zonula occluden-1 expression was significantly increased in all pain models. Claudin-1 protein expression was present at the BBB and remained unchanged during inflammation. Actin expression was significantly increased in the lambda-carrageenan- and CFA-treated groups. We have shown that inflammatory-mediated pain alters both the functional and molecular properties of the BBB. Inflammatory-induced changes may significantly alter delivery of therapeutic agents to the brain, thus affecting dosing regimens during chronic pain.

The transmembrane protein occludin of epithelial tight junctions is a functional target for serine peptidases from faecal pellets of Dermatophagoides pteronyssinus

There have been only a few studies of how allergens cross the airway epithelium to cause allergic sensitization. House dust mite fecal pellets (HDMFP) contain several proteolytic enzymes. Group 1 allergens are cysteine peptidases, whilst those of groups 3, 6 and 9 have catalytic sites indicative of enzymes that mechanistically behave as serine peptidases. We have previously shown that the group 1 allergen Der p 1 leads to cleavage of tight junctions (TJs), allowing allergen delivery to antigen presenting cells. In this study we determined whether HDMFP serine peptidases similarly compromise the airway epithelium by attacking TJs, desmosomes and adherens junctions. Experiments were performed in monolayers of MDCK, Calu-3 or 16HBE14o-epithelial cells. Cell junction morphology was examined by 2-photon molecular excitation microscopy and digital image analysis. Barrier function was measured as mannitol permeability. Cleavage of cell adhesion proteins was studied by immunoblotting and mass spectrometry. HDMFP serine peptidases led to a progressive cleavage of TJs and increased epithelial permeability. Desmosomal puncta became more concentrated. Cleavage of TJs involved proteolysis of the TJ proteins, occludin and ZO-1. This was associated with activation of intracellular proteolysis of ZO-1. In contrast to occludin, E-cadherin of adherens junctions was cleaved less extensively. Although Calu-3 and 16HBE14o-cells expressed tethered ligand receptors for serine peptidases, these were not responsible for transducing the changes in TJs. HDMFP serine peptidases cause cleavage of TJs. This study identifies a second general class of HDM peptidase capable of increasing epithelial permeability and thereby creating conditions that would favour transepithelial delivery of allergens.

The roles of claudin superfamily proteins in paracellular transport

The claudin superfamily consists of at least 18 homologous proteins in humans. These proteins are important structural and functional components of tight junctions in paracellular transport. Complexed with two other integral transmembrane proteins, occludin and junctional adhesion molecule, claudins are located in both epithelial and endothelial cells in all tight junction-bearing tissues. Claudins interact directly with tight junction-specific, membrane-associated guanylate kinase homologues, ZO-1, ZO-2, and ZO-3, and indirectly with AF-6 and the myosin-binding molecule cingulin. These protein-protein interactions promote scaffolding of the tight junction transmembrane proteins and provide a link to the actin cytoskeleton for transducing regulatory signals to and from tight junctions. The distinct permeability properties observed in different epithelia and endothelia seemingly result from the restricted tissue expression, variability of the homopolymer and heteropolymer assembly, regulated transcription and translation, and the subcellular localization of claudin family proteins. Defects in claudins are causatively associated with a variety of human diseases, demonstrating that claudins play important roles in human physiology. In conditions where the cell adhesion function contributed by tight junctions is essential, such as in altered paracellular transport, in proliferative diseases, and during morphogenesis, the claudin superfamily of homologous proteins provides the molecular basis for the uniqueness of tight junctions and emerges as a new target for intervention.

VEGF increases BMEC monolayer permeability by affecting occludin expression and tight junction assembly

Tight junctions between brain microvessel endothelial cells (BMECs) maintain the blood-brain barrier. Barrier breakdown is associated with brain tumors and central nervous system diseases. Tumor cell-secreted vascular endothelial growth factor (VEGF) increases microvasculature permeability in vivo and is correlated with the induction of clinically severe brain tumor edema. Here we investigated the permeability-increasing effect and tight junction formation of VEGF. By measuring [(14)C]sucrose flux and transendothelial electrical resistance (TER) across BMEC monolayer cultures, we found that VEGF increased sucrose permeability and decreased TER. VEGF also caused a loss of occludin and ZO-1 from the endothelial cell junctions and changed the staining pattern of the cell boundary. Western blot analysis of BMEC lysates revealed that the level of occludin but not of ZO-1 was lowered by VEGF treatment. These results suggest that VEGF increases BMEC monolayer permeability by reducing occludin expression and disrupting ZO-1 and occludin organization, which leads to tight junction disassembly. Occludin and ZO-1 appear to be downstream effectors of the VEGF signaling pathway.

Complex phenotype of mice lacking occludin, a component of tight junction strands

Occludin is an integral membrane protein with four transmembrane domains that is exclusively localized at tight junction (TJ) strands. Here, we describe the generation and analysis of mice carrying a null mutation in the occludin gene. Occludin -/- mice were born with no gross phenotype in the expected Mendelian ratios, but they showed significant postnatal growth retardation. Occludin -/- males produced no litters with wild-type females, whereas occludin -/- females produced litters normally when mated with wild-type males but did not suckle them. In occludin -/- mice, TJs themselves did not appear to be affected morphologically, and the barrier function of intestinal epithelium was normal as far as examined electrophysiologically. However, histological abnormalities were found in several tissues, i.e., chronic inflammation and hyperplasia of the gastric epithelium, calcification in the brain, testicular atrophy, loss of cytoplasmic granules in striated duct cells of the salivary gland, and thinning of the compact bone. These phenotypes suggested that the functions of TJs as well as occludin are more complex than previously supposed.

Rat testicular myotubularin, a protein tyrosine phosphatase expressed by Sertoli and germ cells, is a potential marker for studying cell-cell interactions in the rat testis

The full-length cDNA encoding the entire open reading frame (ORF) of rat myotubularin (rMTM) was isolated from a rat testis expression library by PCR. Among the three approximately 2.9-kb cDNAs that were sequenced, one clone was different from the other two clones. It contained seven extra amino acids of FVVLNLQ; this short stretch of extra sequence was found between Gln(421) and Phe(422) within the SET (Suvar3-9, Enhancer-of-zeste, Trithorax) interacting domain (SID) of rMTM. The rMTM ORF had 1,713 bp encoding for a 571 amino acid polypeptide and a calculated molecular weight of 65.8 kDa. A comparison between its deduced amino acid sequence and the GenBank database using BLAST revealed a 53.1% identity with human myotubularin protein (hMTM1), which is a member of the protein tyrosine phosphatase (PTP) family associated with X-linked myotubular myopathy. A 22 amino acid peptide NH(2)-TKVNERYELCDTYPALLAVPAN was synthesized based on the deduced amino acid sequence of rMTM and used for antibody production. By using immunoblot analysis, a 66-kDa protein was indeed detected in both Sertoli and germ-cell cytosols. rMTM mRNA was found in various tissues but was predominantly expressed in the testis, ovary, and skeletal muscle. Sertoli cell rMTM expression was stimulated by germ cells and enhanced when inter-Sertoli junctions were being assembled in vitro. A drastic reduction in testicular rMTM steady-state mRNA level correlated with the depletion of germ cells from the testis in vivo following either glycerol or lonidamine treatment. These results indicate that rMTM is a rat homologue of hMTM1 that may be a useful marker in monitoring the events of cell-cell interactions in the testis.

Exogenous expression of the amino-terminal half of the tight junction protein ZO-3 perturbs junctional complex assembly

The functional characteristics of the tight junction protein ZO-3 were explored through exogenous expression of mutant protein constructs in MDCK cells. Expression of the amino-terminal, PSD95/dlg/ZO-1 domain-containing half of the molecule (NZO-3) delayed the assembly of both tight and adherens junctions induced by calcium switch treatment or brief exposure to the actin-disrupting drug cytochalasin D. Junction formation was monitored by transepithelial resistance measurements and localization of junction-specific proteins by immunofluorescence. The tight junction components ZO-1, ZO-2, endogenous ZO-3, and occludin were mislocalized during the early stages of tight junction assembly. Similarly, the adherens junction proteins E-cadherin and beta-catenin were also delayed in their recruitment to the cell membrane, and NZO-3 expression had striking effects on actin cytoskeleton dynamics. NZO-3 expression did not alter expression levels of ZO-1, ZO-2, endogenous ZO-3, occludin, or E-cadherin; however, the amount of Triton X-100-soluble, signaling-active beta-catenin was increased in NZO-3-expressing cells during junction assembly. In vitro binding experiments showed that ZO-1 and actin preferentially bind to NZO-3, whereas both NZO-3 and the carboxy-terminal half of the molecule (CZO-3) contain binding sites for occludin and cingulin. We hypothesize that NZO-3 exerts its dominant-negative effects via a mechanism involving the actin cytoskeleton, ZO-1, and/or beta-catenin.

Correlation of tight junction morphology with the expression of tight junction proteins in blood-brain barrier endothelial cells

Endothelial cells of the blood-brain barrier form complex tight junctions, which are more frequently associated with the protoplasmic (P-face) than with the exocytoplasmic (E-face) membrane leaflet. The association of tight junctional particles with either membrane leaflet is a result of the expression of various claudins, which are transmembrane constituents of tight junction strands. Mammalian brain endothelial tight junctions exhibit an almost balanced distribution of particles and lose this morphology and barrier function in vitro. Since it was shown that the brain endothelial tight junctions of submammalian species form P-face-associated tight junctions of the epithelial type, the question of which molecular composition underlies the morphological differences and how do these brain endothelial cells behave in vitro arose. Therefore, rat and chicken brain endothelial cells were investigated for the expression of junctional proteins in vivo and in vitro and for the morphology of the tight junctions. In order to visualize morphological differences, the complexity and the P-face association of tight junctions were quantified. Rat and chicken brain endothelial cells form tight junctions which are positive for claudin-1, claudin-5, occludin and ZO-1. In agreement with the higher P-face association of tight junctions in vivo, chicken brain endothelia exhibited a slightly stronger labeling for claudin-1 at membrane contacts. Brain endothelial cells of both species showed a significant alteration of tight junctions in vitro, indicating a loss of barrier function. Rat endothelial cells showed a characteristic switch of tight junction particles from the P-face to the E-face, accompanied by the loss of claudin-1 in immunofluorescence labeling. In contrast, chicken brain endothelial cells did not show such a switch of particles, although they also lost claudin-1 in culture. These results demonstrate that the maintenance of rat and chicken endothelial barrier function depends on the brain microenvironment. Interestingly, the alteration of tight junctions is different in rat and chicken. This implies that the rat and chicken brain endothelial tight junctions are regulated differently.

Enhancing paracellular permeability by modulating epithelial tight junctions

The intestinal epithelium is a major barrier to the absorption of hydrophilic drugs. The presence of intercellular junctional complexes, particularly the tight junctions (zona occludens), renders the epithelium impervious to hydrophilic drugs, which cannot diffuse across the cells through the lipid bilayer of the cell membranes. There have been significant advances in understanding the structure and cellular regulation of tight junctions over the past decade. This article reviews current knowledge regarding the physiological regulation of tight junctions and paracellular permeability, and recent progress towards the rational design of agents that can effectively and safely increase paracellular permeability via modulation of tight junctions.

The coiled-coil domain of occludin can act to organize structural and functional elements of the epithelial tight junction

Occludin is an integral membrane protein that has been suggested to play a role in the organization and dynamic function of the epithelial tight junction (TJ). A number of other proteins have also been described to localize to the TJ. We have used a novel bait peptide method to investigate potential protein-protein interactions of the putative coiled-coil domain of occludin with some of these other TJ proteins. A 27-amino acid peptide of the human occludin sequence was synthesized, biotinylated at the N terminus, and modified to contain a photoactive moiety at either its hydrophobic or hydrophilic surface. These bait peptides were alpha-helical in solution, characteristic of coiled-coil structures. Photoactivation studies in the presence and absence of control peptides were used to assess the potential interactions in polarized sheets of a human intestinal cell line T84. Although a large number of proteins associated with the TJ or that are known to be involved in regulatory events of epithelial cells failed to be specifically labeled, occludin itself, ZO-1, protein kinase C-zeta, c-Yes, the regulatory subunit of phosphatidylinositol 3-kinase, and the gap junction component connexin 26 were specifically labeled. Our data demonstrate the potential of one specific domain of occludin, contained within 27 amino acids, to coordinate the binding of proteins that have been previously suggested to modulate TJ structure and function.

Protein kinase C activation leads to dephosphorylation of occludin and tight junction permeability increase in LLC-PK1 epithelial cell sheets

Activation of protein kinase C by exposure of LLC-PK1 renal epithelial cells to 10(−7) M TPA, a tumor promoting phorbol ester, results in a rapid and sustained increase in paracellular permeability as evidenced by a decrease in transepithelial electrical resistance. Occludin, the first identified transmembrane protein to be localized to the tight junction of both epithelial and endothelial cells is thought play an important role in tight junction barriers. Although transepithelial electrical resistance fell to less than 20% of initial values within 1 hour of TPA exposure, transmission electron microscopy showed no change in the gross morphology of the tight junction of cells treated with 10(−7) M TPA for up to 2 hours. Immunofluorescence microscopy revealed a more rapid change in the membrane distribution of ZO-1 compared to occludin in the TPA-treated cells. Immunoblot analysis indicated that occludin levels in total cell lysates as well as cytosolic, membrane (Triton-X soluble) and cytoskeletal (Triton-X insoluble) fractions remained unchanged for at least 2 hours in cells treated with 10(−7) M TPA compared to their corresponding control cells. As the phosphorylation state of occludin is thought to be important in both tight junction assembly and regulation, the effect of phorbol ester treatment on the phosphorylation of occludin was investigated. Surprisingly, activation of protein kinase C with 10(−7) M TPA resulted in a time-dependent decrease in threonine phosphorylation of occludin which correlated closely with the rapid decrease in transepithelial electrical resistance. This dephosphorylation of occludin, occurring after activation of a serine/threonine kinase by TPA, suggested that protein kinase C was not acting directly on this tight junction target protein. If occludin dephosphorylation is involved in increasing tight junction permeability, then protein kinase C is apparently further upstream in the signaling pathway regulating epithelial barrier function, with a downstream serine/threonine phosphatase acting upon occludin.

ICAM-1-coupled cytoskeletal rearrangements and transendothelial lymphocyte migration involve intracellular calcium signaling in brain endothelial cell lines

Endothelium of the cerebral blood vessels, which constitutes the blood-brain barrier, controls adhesion and trafficking of leukocytes into the brain. Investigating signaling pathways triggered by the engagement of adhesion molecules expressed on brain endothelial cells using two rat brain endothelial cell lines (RBE4 and GP8), we report in this paper that ICAM-1 cross-linking induces a sustained tyrosine phosphorylation of the phosphatidylinositol-phospholipase C (PLC)gamma1, with a concomitant increase in both inositol phosphate production and intracellular calcium concentration. Our results suggest that PLC are responsible, via a calcium- and protein kinase C (PKC)-dependent pathway, for p60Src activation and tyrosine phosphorylation of the p60Src substrate, cortactin. PKCs are also required for tyrosine phosphorylation of the cytoskeleton-associated proteins, focal adhesion kinase and paxillin, but not for ICAM-1-coupled p130Cas phosphorylation. PKC's activation is also necessary for stress fiber formation induced by ICAM-1 cross-linking. Finally, cell pretreatment with intracellular calcium chelator or PKC inhibitors significantly diminishes transmonolayer migration of activated T lymphocytes, without affecting their adhesion to brain endothelial cells. In summary, our data demonstrate that ICAM-1 cross-linking induces calcium signaling which, via PKCs, mediates phosphorylation of actin-associated proteins and cytoskeletal rearrangement in brain endothelial cell lines. Our results also indicate that these calcium-mediated intracellular events are essential for lymphocyte migration through the blood-brain barrier.

Enteropathogenic Escherichia coli dephosphorylates and dissociates occludin from intestinal epithelial tight junctions

Enteropathogenic Escherichia coli (EPEC) increases tight junction permeability in part by phosphorylating the 20 kDa myosin light chain (MLC20) that induces cytoskeletal contraction. The impact of this enteric pathogen on specific tight junction (TJ) proteins has not been investigated. We examined the effect of EPEC infection on occludin localization and phosphorylation in intestinal epithelial cells. After infection by EPEC, a progressive shift of occludin from a primarily TJ-associated domain to an intracellular compartment occurred, as demonstrated by immunofluorescent staining. A reverse in the ratio of phosphorylated to dephosphorylated occludin accompanied this morphological change. Eradication of EPEC with gentamicin resulted in the normalization of occludin localization and phosphorylation. The serine/threonine phosphatase inhibitor, calyculin A, prevented these events. The EPEC-associated decrease in transepithelial electrical resistance, a measure of TJ barrier function, returned to baseline after gentamicin treatment. Non-pathogenic E. coli, K-12, did not induce these changes. Transformation of K-12 with the pathogenicity island of EPEC, however, conferred the phenotype of wild-type EPEC. Deletion of specific EPEC genes encoding proteins involved in EPEC type III secretion markedly attenuated these effects. These findings suggest that EPEC-induced alterations in occludin contribute to the pathophysiology associated with this infection.

Transmembrane proteins of tight junctions

Tight junctions from a morphological and functional boundary between the apical and basolateral cell surface domains of epithelia and endothelia, and regulate selective diffusion along the paracellular space. Two types of four-span transmembrane proteins, occludin and claudins, as well as the single-span protein JAM are associated with tight junctions. The functional analysis of these proteins starts to reveal how they are involved in the functions of tight junctions, which of their domains are important for these functions, and how they interact with each other to form the junctional diffusion barriers.

H(2)O(2)-mediated permeability: role of MAPK and occludin

H(2)O(2)-mediated elevation in endothelial solute permeability is associated with pathological events such as ischemia-reperfusion and inflammation. To understand how H(2)O(2) mediates increased permeability, we investigated the effects of H(2)O(2) administration on vascular endothelial barrier properties and tight junction organization and function. We report that H(2)O(2) exposure caused an increase in endothelial solute permeability in a time-dependent manner through extracellularly regulated kinase 1 and 2 (ERK1/ERK2) signal pathways. H(2)O(2) exposure caused the tight junctional protein occludin to be rearranged from endothelial cell-cell junctions. Occludin rearrangement involved redistribution of occludin on the cell surface and dissociation of occludin from ZO-1. Occludin also was heavily phosphorylated on serine residues upon H(2)O(2) administration. H(2)O(2) mediates changes in ERK1/ERK2 phosphorylation, increases endothelial solute permeability, and alters occludin localization and phosphorylation were all blocked by PD-98059, a specific mitogen-activated protein (MAP) or ERK kinase 1 inhibitor. These data strongly suggest that H(2)O(2)-mediated increased endothelial solute permeability involves the loss of endothelial tight junction integrity through increased ERK1/ERK2 activation.

Entamoeba histolytica disturbs the tight junction complex in human enteric T84 cell layers

Entamoeba (E.) histolytica trophozoites initiate amebiasis through invasion into the enteric mucosa. It was our aim to understand the molecular interactions between amebic trophozoites and enterocytes during the early steps of invasion. Trophozoites of E. histolytica strain HM1:IMSS were seeded on the apical side of enteric T84 cell layers, which were established on filters in two-compartment culture chambers. Cocultures were analyzed for paracellular permeability by measurement of transepithelial electrical resistance (TER) and for the tight junction proteins ZO-1, ZO-2, occludin, and cingulin by immunocytochemistry and immunoprecipitation. On direct contact with the apical side of the enteric cells, trophozoites caused an increase in paracellular permeability as evidenced by a decrease of TER associated with an increase in [(3)H]mannitol flux. Immunoprecipitation of cocultures revealed dephosphorylation of ZO-2, loss of ZO-1 from ZO-2, and degradation of ZO-1 but less so of ZO-2 and none of occludin or E-cadherin. In conclusion, trophozoite-associated increase in paracellular permeability of enteric cell layers is ascribed to disturbance of the molecular organization of tight junction proteins.

Expression from the human occludin promoter is affected by tumor necrosis factor alpha and interferon gamma

The 65 kDa protein occludin is a membrane-spanning part of the epithelial tight junction, which is the main barrier of the paracellular pathway. The function of occludin as part of tight junctions is still poorly understood and even less is known about the regulatory mechanisms that influence occludin gene expression. This study aimed to identify the sequences essential in cis for genomic regulation of tight junction formation and to investigate their funcional role in cytokine-dependent tight junction regulation. Using genome walking cloning of occludin-specific human genomic DNA sequences, a 1853 bp DNA fragment containing the transcription start point of occludin cDNA sequences was amplified and sequenced. Subcloning of this fragment in front of the luciferase reporter gene revealed strong expression of enzymatic activity after transfection of the human intestinal cell line HT-29/B6. With subsequent deletions of parts of the promoter fragment, its size was reduced to 280 bp that are necessary and sufficient to mediate promoter activity. Tumor necrosis factor alpha and another cytokine involved in inflammation, interferon gamma, reduced transepithelial resistance in HT-29/B6 cells, which was preceded by a decrease in occludin mRNA expression as revealed by northern blot analysis. Tumor necrosis factor alpha and interferon gamma diminished occludin promoter activity alone and even synergistically, suggesting a genomic regulation of alterations of the paracellular barrier. In conclusion, proinflammatory cytokines such as tumor necrosis factor alpha and interferon gamma can downregulate the expression of the transmembrane tight junction strand protein occludin, paralleling the barrier disturbance detected electrophysiologically. This could be an important mechanism in gastrointestinal diseases accompanied by barrier defects, for example inflammatory bowel diseases.

Tight junctions are membrane microdomains

Tight junctions (TJ) of polarized epithelial cells regulate barrier function at mucosal surfaces. Structural proteins of TJs include hyperphosphorylated occludin (HO) and the peripheral membrane protein, ZO-1. Since TJs are dynamically regulated, and lipid-modified signal transduction proteins localize to TJs, we considered the possibility that the TJ itself is composed of microdomains with unique structure. Differential detergent extraction and isopycnic sucrose density gradients were utilized to isolate TJ-enriched membranes from a polarized intestinal epithelial cell line, T84. Here we report that major pools of hyperphosphorylated occludin (HO) and ZO-1 are found in raft-like membrane microdomains with characteristics of the previously described detergent-insoluble glycolipid rafts (DIGs). Properties of such gradient fractions included Triton X-100 (TX-100) insolubility, light scattering at 600 nm, buoyant density of approximately 1.08 g/cm(3) and increased cholesterol content compared to high density fractions. Similar results were obtained using natural epithelium. Unlike the TJ proteins HO and ZO-1, other basolateral transmembrane proteins including E-cadherin, c-met and β 1 integrin were not increased in DIG-like fractions. Immunoprecipitation studies revealed coprecipitation of a pool of occludin with caveolin-1, a scaffolding protein abundant in DIGs. Coprecipitation results were supported by immunofluorescence and immunogold labeling studies demonstrating caveolin-1 localization in the apical membrane and focal colocalization with occludin in TJs. TJ disassembly by calcium chelation resulted in displacement of TJ proteins from the ‘raft-like’ compartment. Our findings suggest that raft-like compartments play an important role in the spatial organization of TJs and probably in regulation of paracellular permeability in epithelial cells.

Inhibiting cadherin function by dominant mutant E-cadherin expression increases the extent of tight junction assembly

Previous studies have shown that induction of cadherin-mediated cell-cell adhesion leads to tight junction formation, and that blocking cadherin-mediated cell-cell adhesion inhibits tight junction assembly. Here we report analysis of tight junction assembly in MDCK cells overexpressing a mutant E-cadherin protein that lacks an adhesive extracellular domain (T151 cells). Mutant E-cadherin overexpression caused a dramatic reduction in endogenous cadherin levels. Despite this, tight junction assembly was extensive. The number of tight junction strands observed by freeze-fracture electron microscopy significantly increased in T151 cells compared to that in control cells. Our data indicate that the hierarchical regulation of junctional complex assembly is not absolute, and that inhibition of cadherin function has both positive and negative effects on tight junction assembly.

Chat, a Cas/HEF1-associated adaptor protein that integrates multiple signaling pathways

Cas (Crk-associated substrate) and HEF1 (human enhancer of filamentation) are related adaptor proteins that function in integrin-mediated cell adhesion and antigen receptor signaling pathways. We report here a molecular cloning of Chat (Cas/HEF1-associated signal transducer) that associates with Cas and HEF1. Chat is a 78-kDa signaling molecule with an N-terminal SH2 domain and is expressed in a wide range of tissues. In hematopoietic cells, a 115-kDa isoform of Chat (Chat-H) was specifically expressed. Chat is associated with Cas in brain, and Chat-H is associated with HEF1 in splenocytes. Deletion analyses revealed that Chat and Cas are associated with each other by their C-terminal domains. Treatment of PC12 cells with epidermal growth factor or nerve growth factor increased the phosphorylation level of Chat. This increase was suppressed by an inhibitor of mitogen-activated protein (MAP) kinase kinase, PD98059, suggesting the phosphorylation of Chat by MAP kinase. In Chat-overexpressed COS7 cells, the activity of c-Jun N-terminal kinase was up-regulated. After the epidermal growth factor stimulation, Chat and Cas were colocalized with actin filaments at ruffling membranes. These findings suggest that Chat transduces signals of tyrosine kinases and MAP kinase to Cas signaling pathway.

Restoration of tight junction structure and barrier function by down-regulation of the mitogen-activated protein kinase pathway in ras-transformed Madin-Darby canine kidney cells

In the Madin-Darby canine kidney epithelial cell line, the proteins occludin and ZO-1 are structural components of the tight junctions that seal the paracellular spaces between the cells and contribute to the epithelial barrier function. In Ras-transformed Madin-Darby canine kidney cells, occludin, claudin-1, and ZO-1 were absent from cell-cell contacts but were present in the cytoplasm, and the adherens junction protein E-cadherin was weakly expressed. After treatment of the Ras-transformed cells with the mitogen-activated protein kinase kinase (MEK1) inhibitor PD98059, which blocks the activation of mitogen-activated protein kinase (MAPK), occludin, claudin-1, and ZO-1 were recruited to the cell membrane, tight junctions were assembled, and E-cadherin protein expression was induced. Although it is generally believed that E-cadherin-mediated cell-cell adhesion is required for tight junction assembly, the recruitment of occludin to the cell-cell contact area and the restoration of epithelial cell morphology preceded the appearance of E-cadherin at cell-cell contacts. Both electron microscopy and a fourfold increase in the transepithelial electrical resistance indicated the formation of functional tight junctions after MEK1 inhibition. Moreover, inhibition of MAPK activity stabilized occludin and ZO-1 by differentially increasing their half-lives. We also found that during the process of tight junction assembly after MEK1 inhibition, tyrosine phosphorylation of occludin and ZO-1, but not claudin-1, increased significantly. Our study demonstrates that down-regulation of the MAPK signaling pathway causes the restoration of epithelial cell morphology and the assembly of tight junctions in Ras-transformed epithelial cells and that tyrosine phosphorylation of occludin and ZO-1 may play a role in some aspects of tight junction formation.

Post-translational control of occludin membrane assembly in mouse trophectoderm: a mechanism to regulate timing of tight junction biogenesis and blastocyst formation

The mouse blastocyst forms during the 32-cell stage with the emergence of the blastocoelic cavity. This developmental transition is dependent upon the differentiation and transport function of the trophectoderm epithelium which forms the wall of the blastocyst and exhibits functional intercellular tight junctions (TJs) to maintain epithelial integrity during blastocoele expansion. To investigate mechanisms regulating the timing of blastocyst formation, we have examined the dynamics of expression of occludin, an integral membrane protein of the TJ. Confocal microscopy of intact embryos and synchronised cell clusters revealed that occludin first assembles at the apicolateral membrane contact site between nascent trophectoderm cells usually during the early 32-cell stage, just prior to the time of blastocoele cavitation. This is a late event in the assembly of TJ-associated proteins within trophectoderm which, from our previous data, spans from 8- to 32-cell stages. Occludin membrane assembly is dependent upon prior E-cadherin-mediated cell-cell adhesion and is sensitive to brefeldin A, an inhibitor of Golgi-to-membrane transport. Occludin is delivered to the TJ site in association with the TJ plaque protein, ZO-1(α)+, which we have shown previously is newly transcribed and translated during late cleavage. Immediately after assembly and before cavitation, occludin localised at the TJ site switches from a Triton X-100-soluble to -insoluble form indicative of actin cytoskeletal and/or membrane anchorage. Occludin mRNA and protein are detectable throughout cleavage by RT-PCR and immunoblotting, respectively, indicating that timing of membrane assembly is not controlled by expression alone. Rather, we have identified changes in the pattern of different occludin forms expressed during cleavage which, using phosphatase treatment of embryo lysates, include post-translational modifications. We propose that the phosphorylation of one form of occludin (band 2, 65–67 kDa) during late cleavage, which leads to its exclusive conversion from a Triton X-100-soluble to -insoluble pool, may regulate occludin association with ZO-1(α)+ and membrane assembly, and thereby act to control completion of TJ biogenesis and the timing of blastocyst formation.

Distinct effects of Vibrio cholerae haemagglutinin/protease on the structure and localization of the tight junction-associated proteins occludin and ZO-1

Vibrio cholerae produces a little-studied cytotoxin, haemagglutinin/protease (HA/P), in addition to several better-characterized enterotoxins, i.e. cholera toxin (CT), zonula occludens toxin (ZOT) and accessory cholera enterotoxin (Ace). We have found recently that HA/P perturbs the barrier function of Mardin-Darby canine kidney epithelial cell line I (MDCK-I) by affecting the intercellular tight junctions (TJs) and the F-actin cytoskeleton. In the present study we have assessed more specifically how TJs are affected by HA/P by investigating the cellular localization and biochemical integrity of two well-characterized TJ-associated proteins, occludin and ZO-1. Western blot analysis showed that occludin bands of 66-85 kDa were digested by HA/P to two predominant bands of around 50 kDa and 35 kDa, and that this degradation was greatly attenuated when the specific bacterial metalloproteinase inhibitor Zincov was co-administered. Trypsin, on the other hand, did not degrade occludin when it was applied in the same way, suggesting that the degradation of occludin by HA/P is an early and specific event. The other TJ-associated protein ZO-1 was not degraded by HA/P in parallel experiments, suggesting the selectivity of HA/P-associated protein degradation. Moreover, immunofluorescence labelling and confocal microscopy showed that ZO-1, but not occludin, around cell-cell boundaries was rearranged by HA/P treatment. Since ZO-1 is located on the inside of the plasma membrane and is directly associated with occludin, the results indicate that breakdown of occludin may send signals to ZO-1 that affect its organization and the structure of the F-actin cytoskeleton. Our finding that the zinc-containing metalloprotease of V. cholerae specifically degraded occludin suggests that specific degradation of important host proteins by bacterial zinc-containing metalloproteases may be an important mechanism in microbial pathogenesis.

Tight junctions of the blood-brain barrier

1. The blood-brain barrier is essential for the maintenance and regulation of the neural microenvironment. The blood-brain barrier endothelial cells comprise an extremely low rate of transcytotic vesicles and a restrictive paracellular diffusion barrier. The latter is realized by the tight junctions between the endothelial cells of the brain microvasculature, which are subject of this review. Morphologically, blood-brain barrier-tight junctions are more similar to epithelial tight junctions than to endothelial tight junctions in peripheral blood vessels. 2. Although blood-brain barrier-tight junctions share many characteristics with epithelial tight junctions, there are also essential differences. However, in contrast to tight junctions in epithelial systems, structural and functional characteristics of tight junctions in endothelial cells are highly sensitive to ambient factors. 3. Many ubiquitous molecular constituents of tight junctions have been identified and characterized including claudins, occludin, ZO-1, ZO-2, ZO-3, cingulin, and 7H6. Signaling pathways involved in tight junction regulation comprise, among others, G-proteins, serine, threonine, and tyrosine kinases, extra- and intracellular calcium levels, cAMP levels, proteases, and TNF alpha. Common to most of these pathways is the modulation of cytoskeletal elements which may define blood-brain barrier characteristics. Additionally, cross-talk between components of the tight junction- and the cadherin-catenin system suggests a close functional interdependence of the two cell-cell contact systems. 4. Recent studies were able to elucidate crucial aspects of the molecular basis of tight junction regulation. An integration of new results into previous morphological work is the central intention of this review.

Occludin 1B, a variant of the tight junction protein occludin

Occludin and claudin are the major integral membrane components of the mammalian tight junction. Although more than 11 distinct claudins have been identified, only 1 occludin transcript has been reported thus far. Therefore, we searched by reverse transcription-PCR for occludin-related sequences in Madin-Darby canine kidney (MDCK) mRNA and identified a transcript encoding an alternatively spliced form of occludin, designated occludin 1B. The occludin 1B transcript contained a 193-base pair insertion encoding a longer form of occludin with a unique N-terminal sequence of 56 amino acids. Analysis of the MDCK occludin gene revealed an exon containing the 193-base pair sequence between the exons encoding the original N terminus and the distal sequence, suggesting that occludin and occludin 1B arise from alternative splicing of one transcript. To assess the expression and distribution of occludin 1B, an antibody was raised against its unique N-terminal domain. Immunolabeling of occludin 1B in MDCK cells revealed a distribution indistinguishable from that of occludin. Furthermore, occludin 1B staining at cell-to-cell contacts was also found in cultured T84 human colon carcinoma cells and in frozen sections of mouse intestine. Immunoblots of various mouse tissues revealed broad coexpression of occludin 1B with occludin. The wide epithelial distribution and the conservation across species suggests a potentially important role for occludin 1B in the structure and function of the tight junction.

Direct binding of three tight junction-associated MAGUKs, ZO-1, ZO-2, and ZO-3, with the COOH termini of claudins

ZO-1, ZO-2, and ZO-3, which contain three PDZ domains (PDZ1 to -3), are concentrated at tight junctions (TJs) in epithelial cells. TJ strands are mainly composed of two distinct types of four-transmembrane proteins, occludin, and claudins, between which occludin was reported to directly bind to ZO-1/ZO-2/ZO-3. However, in occludin-deficient intestinal epithelial cells, ZO-1/ZO-2/ZO-3 were still recruited to TJs. We then examined the possible interactions between ZO-1/ZO-2/ZO-3 and claudins. ZO-1, ZO-2, and ZO-3 bound to the COOH-terminal YV sequence of claudin-1 to -8 through their PDZ1 domains in vitro. Then, claudin-1 or -2 was transfected into L fibroblasts, which express ZO-1 but not ZO-2 or ZO-3. Claudin-1 and -2 were concentrated at cell-cell borders in an elaborate network pattern, to which endogenous ZO-1 was recruited. When ZO-2 or ZO-3 were further transfected, both were recruited to the claudin-based networks together with endogenous ZO-1. Detailed analyses showed that ZO-2 and ZO-3 are recruited to the claudin-based networks through PDZ2 (ZO-2 or ZO-3)/PDZ2 (endogenous ZO-1) and PDZ1 (ZO-2 or ZO-3)/COOH-terminal YV (claudins) interactions. In good agreement, PDZ1 and PDZ2 domains of ZO-1/ZO-2/ZO-3 were also recruited to claudin-based TJs, when introduced into cultured epithelial cells. The possible molecular architecture of TJ plaque structures is discussed.

In vitro transepithelial drug transport by on-line measurement: cellular control of paracellular and transcellular transport

Studies on transcellular transport across epithelial cell layers are performed mostly by discontinuous sampling of the transported compound. This has several drawbacks, e.g., it gives disturbances in volume, it limits the time-resolution, and is often laborious. In this report we introduce a method to measure transepithelial transport of fluorescent compounds continuously. The time-resolution is at the (sub)minute scale, allowing the measurement of the change in transport rate before and after transport modulation. We will describe how we used the method to measure transcellular and paracellular transport. For highly membrane-impermeable compounds, the paracellular transport and the regulation of the tight junctions was studied in wild-type and MDR1 cDNA transfected epithelial canine kidney cells (MDCKII). The effect of the multidrug transporter P-glycoprotein (Pgp) on the transepithelial transport was studied. Addition of the Pgp inhibitor SDZ PSC 833 showed a modulation of the idarubicin (IDA) and daunorubicin (DNR) transport, which was larger during transport from the basolateral to the apical side than in the reverse direction. By modeling the transepithelial transport, we found that in these cells Pgp had more effect on the basolateral to apical transport than vice versa, which can be attributed to a relatively large passive permeation coefficient for the cellular basolateral plasma membrane.

Claudin-1 contributes to the epithelial barrier function in MDCK cells

Tight junctions (TJs) create a paracellular permeability barrier and also act as a fence preventing intermixing of proteins and lipids between the apical and basolateral plasma membranes. Recently, claudin-1 has been identified as an integral membrane protein localizing at TJs, and introduced claudin-1 can form TJ-like networks in fibroblasts. To investigate the function of claudin-1, MDCK cells were transfected with a mammalian expression vector containing myc-tagged mouse claudin-1, and four stable clones were obtained. The myc-tagged claudin-1 precisely colocalized with both occludin and ZO-1 at cell-cell contact sites, indicating that exogenous claudin-1 was properly targeted to the TJs. Immunoblot analysis revealed that overexpression of claudin-1 increased expression of ZO-1 but not of occludin or ZO-2. The barrier functions of these cells were evaluated by transepithelial electrical resistance (TER) and paracellular flux. Claudin-1-expressing cells exhibited about four times higher TER than wild-type MDCK cells. Consistent with the increase of TER, the cells overexpressing claudin-1 showed reduced paracellular flux, estimated at 4 and 40 kD FITC-dextrans. These results suggest that claudin-1 is involved in the barrier function at TJs.

Relationship between Na(+),K(+)-ATPase and cell attachment

A prolonged ouabain blockade of the Na(+),K(+)-ATPase detaches cells from each other and from the substrate. This suggests the existence of a link between pump (P) and attachment (A). In the present work, we report that MDCK-W cells treated with ouabain increase tyrosine phosphorylation and content of active MAP kinase, redistribute molecules involved in cell attachment (occludin, ZO-1, desmoplakin, cytokeratin, alpha-actinin, vinculin and actin), and detach. Genistein and UO126, inhibitors of protein tyrosine kinase and of MAP kinase kinase, respectively, block this detachment. The content of P190(Rho-GAP), a GTPase activating protein of the Rho small G-protein subfamily, is increased by ouabain, suggesting that both the Rho/Rac and MAPK pathways are involved. Another clone of MDCK cells whose Na(+),K(+)-ATPase has a negligible affinity for the drug, show none of the effects described for MDCK-W and remain attached. Ma104 cells, a line that has a high affinity for ouabain and stops pumping, fail to modify phosphorylation, as well as the pattern of distribution of attaching molecules, and remain in the monolayer. Taken together, these results suggest that there is a mechanism (P--&gt;A) that transduces a blockade of the pump in a detachment of the cell from neighbors and substrate, in which Ma104 cells are faulty.

Occludin proteolysis and increased permeability in endothelial cells through tyrosine phosphatase inhibition

Regulation of epithelial and endothelial permeability is essential for proper function of compartmentalized organisms, and tyrosine phosphorylation plays an important role in this process. We analyzed the impact of protein tyrosine phosphatase (PTP) inhibition on the structure of endothelial junctional proteins. In human umbilical vein endothelial cells (HUVECs) the PTP inhibitors phenylarsine oxide (PAO) and pervanadate induced proteolysis of the tight junction protein occludin. Occludin proteolysis was inhibited by the metalloproteinase inhibitor 1,10-phenanthroline (PHEN), but not by inhibitors against other types of proteases. The junctional proteins ZO-1, cadherin and beta-catenin were not cleaved. Under conditions of occludin proteolysis, PAO treatment elevated permeability for FITC-dextran. Simultaneous incubation of HUVECs with PAO and PHEN inhibited the rise in permeability by more than 60%. PAO treatment lead to progressive disappearance of occludin from the cell periphery. In contrast, ZO-1, cadherin and beta-catenin retained their positions at the sites of intercellular contact. Simultaneous administration of PAO and PHEN greatly prevented the redistribution of occludin. These results demonstrate a selective cleavage of occludin by a metalloproteinase and suggest that this process can contribute to the control of paracellular permeability in endothelial cells.

Requirement for Ras and phosphatidylinositol 3-kinase signaling uncouples the glucocorticoid-induced junctional organization and transepithelial electrical resistance in mammary tumor cells

In Con8 rat mammary epithelial tumor cells, the synthetic glucocorticoid dexamethasone stimulates the remodeling of the apical junction (tight and adherens junctions) and the transepithelial electrical resistance (TER), which reflects tight junction sealing. Indirect immunofluorescence revealed that dexamethasone induced the recruitment of endogenous Ras and the p85 regulatory subunit of phosphatidylinositol (PI) 3-kinase to regions of cell-cell contact, concurrently with the stimulation of TER. Expression of dominant-negative RasN17 abolished the dexamethasone stimulation in TER, whereas, dexamethasone induced the reorganization of tight junction and adherens junction proteins, ZO-1 and beta-catenin, as well as F-actin, to precise regions of cell-cell contact in a Ras-independent manner. Confocal microscopy revealed that RasN17 and the p85 regulatory subunit of PI 3-kinase co-localized with ZO-1 and F-actin at the tight junction and adherens junction, respectively. Treatment with either of the PI 3-kinase inhibitors, wortmannin or LY294002, or the MEK inhibitor PD 098059, which prevents MAPK signaling, attenuated the dexamethasone stimulation of TER without affecting apical junction remodeling. Similar to dominant-negative RasN17, disruption of both Ras effector pathways using a combination of inhibitors abolished the glucocorticoid stimulation of TER. Thus, the glucocorticoiddependent remodeling of the apical junction and tight junction sealing can be uncoupled by their dependence on Ras and/or PI 3-kinase-dependent pathways, implicating a new role for Ras and PI 3-kinase cell signaling events in the steroid control of cell-cell interactions.

Independent regulation of adherens and tight junctions by tyrosine phosphorylation in Caco-2 cells

To study the role of tyrosine phosphorylation in the control of intercellular adhesion of intestinal cells, we have generated several clones of Caco-2 cells that express high levels of pp60v-src only after addition of butyrate. Expression of this oncogene in cells 5 days after confluence induced beta-catenin and p120-ctn tyrosine phosphorylation, redistribution of E-cadherin to the cytosol and disassembly of adherens junctions. However, tight junctions of Caco-2 cells at 5 days after confluence were not altered by expression of pp60v-src. Similar results were obtained when Caco-2 cells were incubated with phosphotyrosine phosphatase inhibitor orthovanadate. Although addition of this compound to postconfluent cells disrupt adherens junctions, tight junctions remain unaltered, as determined measuring monolayer permeability to mannitol or hyperphosphorylation of Triton-insoluble occludin. Modifications in tight junction permeability of Caco-2 were only observed at high concentrations of orthovanadate (1 mM). Interestingly, this tyrosine phosphorylation-refractory state was achieved after confluence since early postconfluent cells (day 2) showed a limited but significant response to low doses of orthovanadate. These results suggest that tight junctions of differentiated Caco-2 cells are uncoupled from adherens junctions and are insensitive to regulation by tyrosine phosphorylation.

Hepatocyte growth factor/scatter factor decreases the expression of occludin and transendothelial resistance (TER) and increases paracellular permeability in human vascular endothelial cells

Hepatocyte growth factor/scatter factor (HGF/SF), a multi function cytokine has been shown to regulate the expression of cell adhesion molecules in endothelial cells. In the current study, we examined the effects of HGF/SF on the function of tight junctions and the expression of occludin in these cells. Stimulation of human vascular endothelial cells with HGF/SF resulted in a concentration dependent increase in the paracellular permeability as measured using fluorescence labelled dextran but a decrease in the transendothelial cell resistance (TER) of the endothelial cells. Western blotting revealed that HGF/SF decreased the level of occludin in the cells, a primary tight junction forming protein. Immunofluorescence study indicated that shortly after treatment with HGF/SF there was a disturbance of the distribution of occludin and then a reduction in the staining of the molecule. It is concluded that HGF/SF decreases the expression of occludin, resulting in the functional change of tight junction.

VAP-33 localizes to both an intracellular vesicle population and with occludin at the tight junction

Tight junctions create a regulated intercellular seal between epithelial and endothelial cells and also establish polarity between plasma membrane domains within the cell. Tight junctions have also been implicated in many other cellular functions, including cell signaling and growth regulation, but they have yet to be directly implicated in vesicle movement. Occludin is a transmembrane protein located at tight junctions and is known to interact with other tight junction proteins, including ZO-1. To investigate occludin's role in other cellular functions we performed a yeast two-hybrid screen using the cytoplasmic C terminus of occludin and a human liver cDNA library. From this screen we identified VAP-33 which was initially cloned from Aplysia by its ability to interact with VAMP/synaptobrevin and thus was implicated in vesicle docking/fusion. Extraction characteristics indicated that VAP-33 was an integral membrane protein. Antibodies to the human VAP-33 co-localized with occludin at the tight junction in many tissues and tissue culture cell lines. Subcellular fractionation of liver demonstrated that 83% of VAP-33 co-isolated with occludin and DPPIV in a plasma membrane fraction and 14% fractionated in a vesicular pool. Thus, both immunofluorescence and fractionation data suggest that VAP-33 is present in two distinct pools in the cells. In further support of this conclusion, a GFP-VAP-33 chimera also distributed to two sites within MDCK cells and interestingly shifted occludin's localization basally. Since VAP-33 has previously been implicated in vesicle docking/fusion, our results suggest that tight junctions may participate in vesicle targeting at the plasma membrane or alternatively VAP-33 may regulate the localization of occludin.

Clostridium perfringens enterotoxin fragment removes specific claudins from tight junction strands: Evidence for direct involvement of claudins in tight junction barrier

Claudins, comprising a multigene family, constitute tight junction (TJ) strands. Clostridium perfringens enterotoxin (CPE), a single approximately 35-kD polypeptide, was reported to specifically bind to claudin-3/RVP1 and claudin-4/CPE-R at its COOH-terminal half. We examined the effects of the COOH-terminal half fragment of CPE (C-CPE) on TJs in L transfectants expressing claudin-1 to -4 (C1L to C4L, respectively), and in MDCK I cells expressing claudin-1 and -4. C-CPE bound to claudin-3 and -4 with high affinity, but not to claudin-1 or -2. In the presence of C-CPE, reconstituted TJ strands in C3L cells gradually disintegrated and disappeared from their cell surface. In MDCK I cells incubated with C-CPE, claudin-4 was selectively removed from TJs with its concomitant degradation. At 4 h after incubation with C-CPE, TJ strands were disintegrated, and the number of TJ strands and the complexity of their network were markedly decreased. In good agreement with the time course of these morphological changes, the TJ barrier (TER and paracellular flux) of MDCK I cells was downregulated by C-CPE in a dose-dependent manner. These findings provided evidence for the direct involvement of claudins in the barrier functions of TJs.

A role for intracellular calcium in tight junction reassembly after ATP depletion-repletion

The integrity of the tight junction (TJ), which is responsible for the permeability barrier of the polarized epithelium, is disrupted during ischemic injury and must be reestablished for recovery. Recently, with the use of an ATP depletion-repletion model for ischemia and reperfusion injury in Madin-Darby canine kidney cells, TJ proteins such as zonula occludens-1 (ZO-1) were shown to reversibly form large complexes and associate with cytoskeletal proteins (T. Tsukamoto and S. K. Nigam, J. Biol. Chem. 272: 16133-16139, 1997). In this study, we examined the role of intracellular calcium in TJ reassembly after ATP depletion-repletion by employing the cell-permeant calcium chelator 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM (BAPTA-AM). Lowering intracellular calcium during ATP depletion is associated with significant inhibition of the reestablishment of the permeability barrier following ATP repletion as measured by transepithelial electrical resistance and mannitol flux, marked alterations in the subcellular localization of occludin by immunofluorescent analysis, and decreased solubility of ZO-1 and other TJ proteins by Triton X-100 extraction assay, suggesting that lowering intracellular calcium potentiates the interaction of TJ proteins with the cytoskeleton. Coimmunoprecipitation studies indicated that decreased solubility may partly result from the stabilization of large TJ protein-containing complexes with fodrin. Although ionic detergents (SDS and deoxycholate) appeared to cause a dissociation of ZO-1-containing complexes from the cytoskeleton, sucrose gradient analyses of the solubilized proteins suggested that calcium chelation leads to self-association of these complexes. Together, these results raise the possibility that intracellular calcium plays an important facilitatory role in the reassembly of the TJ damaged by ischemic insults. Calcium appears to be necessary for the dissociation of TJ-cytoskeletal complexes, thus permitting functional TJ reassembly and paracellular permeability barrier recovery.

Structural and signalling molecules come together at tight junctions

Tight junctions (TJs) have been suggested to act both as barriers and fences, but lack of information on the constituents of TJ strands has hampered the direct assessment of these functions. Over the past year, our understanding of the molecular architecture of TJ strands has increased markedly and we are ready to experimentally examine how TJs are involved in their dual functions.

PKC-dependent regulation of transepithelial resistance: roles of MLC and MLC kinase

The mechanisms by which protein kinase C (PKC) activation results in increased transepithelial resistance (TER) are unknown [G. Hecht, B. Robinson, and A. Koutsouris. Am. J. Physiol. 266 (Gastrointest. Liver Physiol. 29): G214-G221, 1994]. We have previously shown that phosphorylation of the regulatory light chain of myosin II (MLC) is associated with decreases in TER and have suggested that contraction of the perijunctional actomyosin ring (PAMR) increases tight junction (TJ) permeability [J. R. Turner, B. K. Rill, S. L. Carlson, D. Carnes, R. Kerner, R. J. Mrsny, and J. L. Madara. Am. J. Physiol. 273 (Cell Physiol. 42): C1378-C1385, 1997]. We therefore hypothesized that PKC activation alters TER via relaxation of the PAMR. Activation of PKC by the phorbol ester phorbol 12-myristate 13-acetate (PMA) resulted in a progressive dose-dependent increase in TER that was apparent within 15 min (111% of controls) and maximal within 2 h (142% of controls). Similar increases were induced by a diacylglycerol analog, and the effects of both PMA and the diacylglycerol analog were prevented by the PKC inhibitor bisindolylmaleimide I. PMA treatment caused progressive decreases in MLC phosphorylation, by 12% at 15 min and 41% at 2 h. Phosphorylation of MLC kinase (MLCK) increased by 64% within 15 min of PMA treatment and was stable over 2 h (51% greater than that of controls). Thus increases in MLCK phosphorylation preceded decreases in MLC phosphorylation. These data suggest that PKC regulates TER via decreased phosphorylation of MLC, possibly due to inhibitory phosphorylation of MLCK. The decreased phosphorylation of MLC likely reduces PAMR tension, leading to decreased TJ permeability.

Xenopus laevis occludin. Identification of in vitro phosphorylation sites by protein kinase CK2 and association with cingulin

Occludin is a protein component of the membrane domain of tight junctions, and has been shown to be phosphorylated in vivo in cultured cells and Xenopus laevis embryos. However, nothing is known about the identity of specific occludin kinase(s) and occludin phosphorylation site(s). Furthermore, nothing is known about the interaction of occludin with cingulin, a cytoplasmic plaque component of tight junctions. Here we report the isolation and sequencing of a complete X. laevis occludin cDNA, and experiments aimed at mapping X. laevis occludin in vitro phosphorylation site(s) and characterizing occludin interaction with cingulin. The sequence of Xenopus occludin is homologous to that of occludins from other species, with identities ranging from 41% to 58%. Bacterially expressed domain E of Xenopus occludin (amino acids 247-493) was a good substrate for protein kinase CK2 (stoichiometry 10.8%, Km 8.4 microM) but not for CK1 kinase, protein kinase A, cdc2 kinase, MAP kinase or syk kinase. Residues Thr375 and Ser379 were identified as potential CK2 phosphorylation sites in this region based on sequence analysis. Mutation of Ser379 to aspartic acid or alanine reduced phosphorylation by CK2 by approximately 50%, and double mutation of Ser379 into aspartic acid and Thr375 into aspartic acid essentially abolished phosphorylation. Glutathione S-transferase (GST) pull-down experiments using extracts of Xenopus A6 epithelial cells showed that constructs of GST fused to wild-type and mutant forms of the C-terminal region of X. laevis occludin associate with several polypeptides, and immunoblot analysis showed that one of these polypeptides is cingulin. GST pull-down experiments using in vitro translated, full-length Xenopus cingulin indicated that cingulin interacts directly with the C-terminal region of occludin.