Assembly of tight junction is regulated by the antagonism of conventional and novel protein kinase C isoforms

Apparently conflicting observations indicated that protein kinase C both may block and support the assembly of tight junctions. We therefore tested the hypothesis that different isoenzymes antagonistically affect tight junction proteins and function. Thus, by using specific inhibitors we investigated the involvement of conventional and novel protein kinase C of kidney tubule cells in tight junction assembly. In low Ca2+ medium, the application of pan-protein kinase C inhibitor GF-109203X blocked the formation of tight junctions induced by protein kinase C agonist diacyglycerol. G?6976, inhibitor of conventional protein kinase C, promoted the formation of tight junctions and occludin phosphorylation in cells cultivated in low Ca2+ medium and attenuated the disruption of tight junction complex induced by the switch to low Ca2+ medium. In addition, G?6976 accelerated the occludin phosphorylation and the formation of tight junction barrier during assembly of tight junctions induced by Ca2+ re-addition. This phosphorylation was accompanied by accelerated occludin incorporation into newly forming tight junctions and by reducing the paracellular permeability. In contrast, inhibitor of novel protein kinase C rottlerin blocked the occludin phosphorylation and the formation of tight junction barrier, both caused by re-addition of normal Ca2+ medium. It is concluded that the conventional protein kinase C alpha participates in tight junction disassembly while the novel protein kinase C epsilon plays a role in tight junction formation of kidney epithelial cells. The discovered antagonism contributes to a better understanding of the regulation of the structure and function of tight junctions and hence to that of the epithelial barrier.     

Connexin 26 expression prevents down-regulation of barrier and fence functions of tight junctions by Na+/K+-ATPase inhibitor ouabain in human airway epithelial cell line Calu-3

Gap junctions are considered to play a crucial role in differentiation of epithelial cells and to be associated with tight junction proteins. In this study, to investigate the role of gap junctions in regulation of the barrier function and fence function on the tight junctions, we introduced the Cx26 gene into human airway epithelial cell line Clau-3 and used a disruption model of tight junctions employing the Na(+)/K(+)-ATPase inhibitor ouabain. In parental Calu-3 cells, gap junction proteins Cx32 and Cx43, but not Cx26, and tight junction proteins occludin, JAM-1, ZO-1, claudin-1, -2, -3, -4, -5, -6, -7, -8, -9, and -14 were detected by RT-PCR. The barrier function and fence function of tight junctions were well maintained, whereas the GJIC was low level. Treatment with ouabain caused disruption of the barrier function and fence function of tight junctions together with down-regulation of occludin, JAM-1, claudin-2, and -4 and up-regulation of ZO-1 and claudin-14. In Cx26 transfectants, Cx26 protein was detected by Western blotting and immunocytochemistry, and many gap junction plaques were observed with well-developed tight junction strands. Expression of claudin-14 was significantly increased in Cx26 transfectants compared to parental cells, and in some cells, Cx26 was co-localized with claudin-14. Interestingly, transfection with Cx26 prevented disruption of both functions of tight junctions by treatment with ouabain without changes in the tight junction proteins. Pretreatment with the GJIC blockers 18beta-glycyrrhetinic acid and oleamide did not affect the changes induced by Cx26 transfection. These results suggest that Cx26 expression, but not the mediated intercellular communication, may regulate tight junction barrier and fence functions in human airway epithelial cell line Calu-3.     

Stimulus-induced reorganization of tight junction structure: The role of membrane traffic

The tight junction forms a barrier that limits paracellular movement of water, ions, and macromolecules. The permeability properties of this barrier are regulated in response to both physiological and pathophysiological stimuli, and this regulation has been modeled by pharmacological agents. Although it is now known that vesicular traffic plays important roles in tight junction assembly, the molecular mechanisms by which vesicular traffic contributes to tight junction regulation remain to be defined. This review summarizes recent progress in understanding mechanisms and pathways of tight junction protein internalization and the relevance of these to tight junction regulation.     

Stimulus-induced reorganization of tight junction structure: the role of membrane traffic

The tight junction forms a barrier that limits paracellular movement of water, ions, and macromolecules. The permeability properties of this barrier are regulated in response to both physiological and pathophysiological stimuli, and this regulation has been modeled by pharmacological agents. Although it is now known that vesicular traffic plays important roles in tight junction assembly, the molecular mechanisms by which vesicular traffic contributes to tight junction regulation remain to be defined. This review summarizes recent progress in understanding mechanisms and pathways of tight junction protein internalization and the relevance of these to tight junction regulation.     

Tight Junctions of the Blood–Brain Barrier

1. The blood–brain barrier is essential for the maintainance 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. 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.     

Role of epithelial cells in initiation and propagation of intestinal inflammation. Eliminating the static: tight junction dynamics exposed

Like all mucosal surfaces, the intestine forms a barrier that separates the external environment, i.e., the gut lumen, from the protected internal milieu. The intestinal barrier is formed by the epithelial cells that line the luminal surface. Plasma membranes of these cells prevent free passage of hydrophilic molecules across this barrier but do not seal the space between cells. This function is provided by the tight junction. Each cell is encircled at the apicolateral boundary by the tight junction, which seals the paracellular space. The tight junction does not form a completely impermeant seal, however, because that would prevent paracellular absorption of essential nutrients and ions; intestinal tight junctions are "leaky" and allow solutes to be transported paracellularly according to size and charge. Abundant data are available to demonstrate that barrier properties of tight junctions can be modulated in response to physiological, pharmacological, and pathophysiological stimuli, but the structural modifications responsible for these responses are poorly defined. Recent advances in understanding the role of tight junction dynamics in response to such stimuli are the focus of this review.     

Structure, regulation, and pathophysiology of tight junctions in the gastrointestinal tract.

The tight junction, or zonula occludens, forms an intercellular barrier between epithelial cells within the gastrointestinal tract and liver and, by limiting the movement of water and solutes through the intercellular space, maintains the physicochemical separation of tissue compartments. The paracellular barrier properties of junctions are regulated and quite different among epithelia. The junction also forms an intramembrane barrier between the apical and basolateral membrane domains, contributing to segregation of biochemically distinct components of these plasma membrane surfaces. Here we briefly review three rapidly developing areas of medically relevant basic knowledge about the tight junction. First, we describe the presently incomplete knowledge of the molecular structure of the tight junction as a framework for understanding its functional properties. Second, we consider experimental evidence defining how the barrier properties of junctions are physiologically regulated and, third, how barrier properties are specifically altered in, and contribute to, pathologic processes affecting epithelia.     

Afadin controls p120-catenin-ZO-1 interactions leading to endothelial barrier enhancement by oxidized phospholipids

Afadin is a novel regulator of epithelial cell junctions assembly. However, its role in the formation of endothelial cell junctions and the regulation of vascular permeability remains obscure. We previously described protective effects of oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (OxPAPC) in the in vitro and in vivo models of lung endothelial barrier dysfunction and acute lung injury, which were mediated by Rac GTPase. This study examined a role of afadin in the OxPAPC-induced enhancement of interactions between adherens junctions and tight junctions as a novel mechanism of endothelial cell (EC) barrier preservation. OxPAPC induced Rap1-dependent afadin accumulation at the cell periphery and Rap1-dependent afadin interaction with adherens junction and tight junction proteins p120-catenin and ZO-1, respectively. Afadin knockdown using siRNA or ectopic expression of afadin mutant lacking Rap1 GTPase binding domain suppressed OxPAPC-induced EC barrier enhancement and abolished barrier protective effects of OxPAPC against thrombin-induced EC permeability. Afadin knockdown also abolished protective effects of OxPAPC against ventilator-induced lung injury in vivo. These results demonstrate for the first time a critical role of afadin in the regulation of vascular barrier function in vitro and in vivo via coordination of adherens junction-tight junction interactions.     

Protein kinase Cζ phosphorylates occludin and promotes assembly of epithelial tight junctions

Protein kinases play an important role in the regulation of epithelial tight junctions. In the present study, we investigated the role of PKCζ (protein kinase Cζ) in tight junction regulation in Caco-2 and MDCK (Madin-Darby canine kidney) cell monolayers. Inhibition of PKCζ by a specific PKCζ pseudosubstrate peptide results in redistribution of occludin and ZO-1 (zona occludens 1) from the intercellular junctions and disruption of barrier function without affecting cell viability. Reduced expression of PKCζ by antisense oligonucleotide or shRNA (short hairpin RNA) also results in compromised tight junction integrity. Inhibition or knockdown of PKCζ delays calcium-induced assembly of tight junctions. Tight junction disruption by PKCζ pseudosubstrate is associated with the dephosphorylation of occludin and ZO-1 on serine and threonine residues. PKCζ directly binds to the C-terminal domain of occludin and phosphorylates it on threonine residues. Thr403, Thr404, Thr424 and Thr438 in the occludin C-terminal domain are the predominant sites of PKCζ-dependent phosphorylation. A T424A or T438A mutation in full-length occludin delays its assembly into the tight junctions. Inhibition of PKCζ also induces redistribution of occludin and ZO-1 from the tight junctions and dissociates these proteins from the detergent-insoluble fractions in mouse ileum. The present study demonstrates that PKCζ phosphorylates occludin on specific threonine residues and promotes assembly of epithelial tight junctions.     

Claudins--key pieces in the tight junction puzzle

Tight junctions restrict the flow of ions and aqueous molecules between cells by forming a selective barrier to the paracellular pathway. Permeability of the tight junction barrier is determined by a class of transmembrane proteins known as claudins. The relationship between claudins and paracellular permeability is complex and determined not only by the profile of claudin expression but also by the arrangement of claudins and other proteins into tight junction strands. This review summarizes progress in understanding how claudins are assembled into tight junctions and how they interact with other tight junction proteins.     

ZO-1 controls endothelial adherens junctions,cell–cell tension,angiogenesis, and barrier formation

Intercellular junctions are crucial for mechanotransduction, but whether tight junctions contribute to the regulation of cell–cell tension and adherens junctions is unknown. Here, we demonstrate that the tight junction protein ZO-1 regulates tension acting on VE-cadherin–based adherens junctions, cell migration, and barrier formation of primary endothelial cells, as well as angiogenesis in vitro and in vivo. ZO-1 depletion led to tight junction disruption, redistribution of active myosin II from junctions to stress fibers, reduced tension on VE-cadherin and loss of junctional mechanotransducers such as vinculin and PAK2, and induced vinculin dissociation from the α-catenin–VE-cadherin complex. Claudin-5 depletion only mimicked ZO-1 effects on barrier formation, whereas the effects on mechanotransducers were rescued by inhibition of ROCK and phenocopied by JAM-A, JACOP, or p114RhoGEF down-regulation. ZO-1 was required for junctional recruitment of JACOP, which, in turn, recruited p114RhoGEF. ZO-1 is thus a central regulator of VE-cadherin–dependent endothelial junctions that orchestrates the spatial actomyosin organization, tuning cell–cell tension, migration, angiogenesis, and barrier formation.     

c‐Jun N‐terminal kinase is largely involved in the regulation of tricellular tight junctions via tricellulin in human pancreatic duct epithelial cells

Tricellulin (TRIC) is a tight junction protein at tricellular contacts where three epithelial cells meet, and it is required for the maintenance of the epithelial barrier. To investigate whether TRIC is regulated via a c‐Jun N‐terminal kinase (JNK) pathway, human pancreatic HPAC cells, highly expressed at tricellular contacts, were exposed to various stimuli such as the JNK activators anisomycin and 12‐O‐tetradecanoylphorbol 13‐acetate (TPA), and the proinflammatory cytokines IL‐1β, TNFα, and IL‐1α. TRIC expression and the barrier function were moderated by treatment with the JNK activator anisomycin, and suppressed not only by inhibitors of JNK and PKC but also by siRNAs of TRIC. TRIC expression was induced by treatment with the PKC activator TPA and proinflammatory cytokines IL‐1β, TNFα, and IL‐1α, whereas the changes were inhibited by a JNK inhibitor. Furthermore, in normal human pancreatic duct epithelial cells using hTERT‐transfected primary cultured cells, the responses of TRIC expression to the various stimuli were similar to those in HPAC cells. TRIC expression in tricellular tight junctions is strongly regulated together with the barrier function via the JNK transduction pathway. These findings suggest that JNK may be involved in the regulation of tricellular tight junctions including TRIC expression and the barrier function during normal remodeling of epithelial cells, and prevent disruption of the epithelial barrier in inflammation and other disorders in pancreatic duct epithelial cells. J. Cell. Physiol. 225: 720–733, 2010. © 2010 Wiley‐Liss, Inc.     

Structural organization of the tight junctions

Tight junctions are the most apical organelle of the apical junctional complex and are primarily involved in the regulation of paracellular permeability and membrane polarity. Extensive research in the past two decades has identified not only the individual molecules of the tight junctions but also their mutual interactions, which are the focus of the present review article. While a complete map of the interactions among the tight junction molecules is probably far from being complete, the available evidence already allows outlining the general molecular architecture of the tight junctions. Here, with the aim of gaining deeper mechanistic understanding of tight junction assembly, regulation and function, we have subdivided the known molecular interactions into four major clusters that are centered on cell surface, polarity, cytoskeletal and signaling molecules.     

Structural organization of the tight junctions

Tight junctions are the most apical organelle of the apical junctional complex and are primarily involved in the regulation of paracellular permeability and membrane polarity. Extensive research in the past two decades has identified not only the individual molecules of the tight junctions but also their mutual interactions, which are the focus of the present review article. While a complete map of the interactions among the tight junction molecules is probably far from being complete, the available evidence already allows outlining the general molecular architecture of the tight junctions. Here, with the aim of gaining deeper mechanistic understanding of tight junction assembly, regulation and function, we have subdivided the known molecular interactions into four major clusters that are centered on cell surface, polarity, cytoskeletal and signaling molecules.     

FRAP Analysis Reveals Stabilization of Adhesion Structures in the Epidermis Compared to Cultured Keratinocytes

Proper development and tissue maintenance requires cell-cell adhesion structures, which serve diverse and crucial roles in tissue morphogenesis. Epithelial tissues have three main types of cell-cell junctions: tight junctions, which play a major role in barrier formation, and adherens junctions and desmosomes, which provide mechanical stability and organize the underlying cytoskeleton. Our current understanding of adhesion function is hindered by a lack of tools and methods to image junctions in mammals. To better understand the dynamics of adhesion in tissues we have created a knock-in ZO-1-GFP mouse and a BAC-transgenic mouse expressing desmoplakin I-GFP. We performed fluorescence recovery after photobleaching (FRAP) experiments to quantify the turnover rates of the tight junction protein ZO-1, the adherens junction protein E-cadherin, and the desmosomal protein desmoplakin in the epidermis. Proteins at each type of junction are remarkably stable in the epidermis, in contrast to the high observed mobility of E-cadherin and ZO-1 at adherens junctions and tight junctions, respectively, in cultured cells. Our data demonstrate that there are additional mechanisms for stabilizing junctions in tissues that are not modeled by cell culture.     

Delayed astrocytic contact with cerebral blood vessels in FGF‐2 deficient mice does not compromise permeability properties at the developing blood‐brain barrier

The brain functions within a specialized environment tightly controlled by brain barrier mechanisms. Understanding the regulation of barrier formation is important for understanding brain development and may also lead to finding new ways to deliver pharmacotherapies to the brain; access of many potentially promising drugs is severely hindered by these barrier mechanisms. The cellular composition of the neurovascular unit of the blood‐brain barrier proper and their effects on regulation of its function are beginning to be understood. One hallmark of the neurovascular unit in the adult is the astroglial foot processes that tightly surround cerebral blood vessels. However their role in barrier formation is still unclear. In this study we examined barrier function in newborn, juvenile and adult mice lacking fibroblast growth factor‐2 (FGF‐2), which has been shown to result in altered astroglial differentiation during development. We show that during development of FGF‐2 deficient mice the astroglial contacts with cerebral blood vessels are delayed compared with wild‐type animals. However, this delay did not result in changes to the permeability properties of the blood brain barrier as assessed by exclusion of either small or larger sized molecules at this interface. In addition cerebral vessels were positive for tight‐junction proteins and we observed no difference in the ultrastructure of the tight‐junctions. The results indicate that the direct contact of astroglia processes to cerebral blood vessels is not necessary for either the formation of the tight‐junctions or for basic permeability properties and function of the blood‐brain barrier. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1201–1212, 2016     

Distinct domains of paracingulin are involved in its targeting to the actin cytoskeleton and regulation of apical junction assembly

Paracingulin is an M(r) 150-160 kDa cytoplasmic protein of vertebrate epithelial tight and adherens junctions and comprises globular head, coiled-coil rod, and globular tail domains. Unlike its homologous tight junction protein cingulin, paracingulin has been implicated in the control of junction assembly and has been localized at extrajunctional sites in association with actin filaments. Here we analyze the role of paracingulin domains, and specific regions within the head and rod domains, in the function and localization of paracingulin by inducible overexpression of exogenous proteins in epithelial Madin Darby canine kidney (MDCK) cells and by expression of mutated and chimeric constructs in Rat1 fibroblasts and MDCK cells. The overexpression of the rod + tail domains of paracingulin perturbs the development of the tight junction barrier and Rac1 activation during junction assembly by the calcium switch, indicating that regulation of junction assembly by paracingulin is mediated by these domains. Conversely, only constructs containing the head domain target to junctions in MDCK cells and Rat1 fibroblasts. Furthermore, expression of chimeric cingulin and paracingulin constructs in Rat1 fibroblasts and MDCK cells identifies specific sequences within the head and rod domains of paracingulin as critical for targeting to actin filaments and regulation of junction assembly, respectively. In summary, we characterize the functionally important domains of paracingulin that distinguish it from cingulin.     

Expression of kinase-inactive c-Src delays oxidative stress-induced disassembly and accelerates calcium-mediated reassembly of tight junctions in the Caco-2 cell monolayer

The activity of Src kinases appears to play a role in both assembly and disassembly of tight junction. However, the role of a specific isoform of Src kinase in regulation of tight junction is not known. In the present study the role of c-Src in regulation of epithelial tight junction was investigated in Caco-2 cell monolayers. Oxidative stress (xanthine oxidase + xanthine) induced an activation and membrane translocation of c-Src. The oxidative stress-induced decrease in transepithelial electrical resistance, increase in inulin permeability, and redistribution of occludin and ZO-1 from the intercellular junctions were prevented by PP2. The rates of oxidative stress-induced activation of c-Src, tyrosine phosphorylation of ZO-1 and beta-catenin, decrease in resistance, increase in permeability to inulin, and redistribution of occludin and ZO-1 were significantly greater in cells transfected with wild type c-Src, whereas it was low in cells transfected with kinase-inactive c-SrcK297R mutant, when compared with those in empty vector-transfected cells. The rates of recovery of resistance, increase in barrier to inulin, and reorganization of occludin and ZO-1 into the intercellular junctions during the calcium-induced reassembly of tight junction were much greater in Caco-2 cells transfected with c-SrcK297R as compared with those in cells transfected with empty vector or wild type c-Src. These results show that the dominant-negative expression of kinase-inactive c-Src delays the oxidative stress-induced disruption of tight junction and accelerates calcium-induced assembly of tight junction in Caco-2 cells and demonstrate that oxidative stress-induced disruption of tight junction is mediated by the activation of c-Src.     

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.