Assembly and sealing of tight junctions: possible participation of G-proteins, phospholipase C, protein kinase C and calmodulin

Tight junction proteins in glial tumors development and progression

Tight junctions form a paracellular barrier in epithelial and endothelial cells, and they regulate the diffusion of fluids, molecules, and the penetration of cells across tissue compartments. Tight junctions are composed of a group of integral membrane proteins, which include the claudin family, tight junction-associated Marvel protein family, junctional adhesion molecule family, and proteins that anchor the cytoskeleton, such as zonula occludens proteins and the cingulin family. Several factors, such as neurotransmitters or cytokines, and processes like ischemia/hypoxia, inflammation, tumorigenesis, phosphorylation/dephosphorylation, ubiquitination, and palmitoylation, regulate tight junction proteins. Claudins are involved in tumorigenesis processes that lead to glioma formation. In gliomas, there is a noticeable dysregulation of claudins, occludin, and zonula occludens-1 abundance, and their dislocation has been observed. The weakening of intercellular adhesion and cell detachment is responsible for glioma infiltration into surrounding tissues. Furthermore, the paracellular permeability of the blood-brain barrier, formed with the involvement of tight junction proteins, influences the development of peritumoral edema - and, simultaneously, the rate of drug delivery to the glial tumor. Understanding the junctional and paracellular environments in brain tumors is crucial to predicting glial tumor progression and the feasibility of chemotherapeutic drug delivery. This knowledge may also illuminate differences between high and low-grade gliomas.

Junctions at the crossroads: the impact of mechanical cues on endothelial cell-cell junction conformations and vascular permeability

Cells depend on precisely regulating barrier function within the vasculature to maintain physiological stability and facilitate essential substance transport. Endothelial cells achieve this through specialized adherens and tight junction protein complexes, which govern paracellular permeability across vascular beds. Adherens junctions, anchored by vascular endothelial (VE)-cadherin and associated catenins to the actin cytoskeleton, mediate homophilic adhesion crucial for barrier integrity. In contrast, tight junctions composed of occludin, claudin, and junctional adhesion molecule A interact with Zonula Occludens proteins, reinforcing intercellular connections essential for barrier selectivity. Endothelial cell-cell junctions exhibit dynamic conformations during development, maturation, and remodeling, regulated by local biochemical and mechanical cues. These structural adaptations play pivotal roles in disease contexts such as chronic inflammation, where junctional remodeling contributes to increased vascular permeability observed in conditions from cancer to cardiovascular diseases. Conversely, the brain microvasculature's specialized junctional arrangements pose challenges for therapeutic drug delivery due to their unique molecular compositions and tight organization. This commentary explores the molecular mechanisms underlying endothelial cell-cell junction conformations and their implications for vascular permeability. By highlighting recent advances in quantifying junctional changes and understanding mechanotransduction pathways, we elucidate how physical forces from cellular contacts and hemodynamic flow influence junctional dynamics.

Stroke-induced damage on the blood-brain barrier

The blood-brain barrier (BBB) is a functional phenotype exhibited by the neurovascular unit (NVU). It is maintained and regulated by the interaction between cellular and non-cellular matrix components of the NVU. The BBB plays a vital role in maintaining the dynamic stability of the intracerebral microenvironment as a barrier layer at the critical interface between the blood and neural tissues. The large contact area (approximately 20 m2/1.3 kg brain) and short diffusion distance between neurons and capillaries allow endothelial cells to dominate the regulatory role. The NVU is a structural component of the BBB. Individual cells and components of the NVU work together to maintain BBB stability. One of the hallmarks of acute ischemic stroke is the disruption of the BBB, including impaired function of the tight junction and other molecules, as well as increased BBB permeability, leading to brain edema and a range of clinical symptoms. This review summarizes the cellular composition of the BBB and describes the protein composition of the barrier functional junction complex and the mechanisms regulating acute ischemic stroke-induced BBB disruption.

Storming the gate: New approaches for targeting the dynamic tight junction for improved drug delivery

As biologics used in the clinic outpace the number of new small molecule drugs, an important challenge for their efficacy and widespread use has emerged, namely tissue penetrance. Macromolecular drugs - bulky, high-molecular weight, hydrophilic agents - exhibit low permeability across biological barriers. Epithelial and endothelial layers, for example within the gastrointestinal tract or at the blood-brain barrier, present the most significant obstacle to drug transport. Within epithelium, two subcellular structures are responsible for limiting absorption: cell membranes and intercellular tight junctions. Previously considered impenetrable to macromolecular drugs, tight junctions control paracellular flux and dictate drug transport between cells. Recent work, however, has shown tight junctions to be dynamic, anisotropic structures that can be targeted for delivery. This review aims to summarize new approaches for targeting tight junctions, both directly and indirectly, and to highlight how manipulation of tight junction interactions may help usher in a new era of precision drug delivery.

Fluoride disrupts intestinal epithelial tight junction integrity through intracellular calcium-mediated RhoA/ROCK signaling and myosin light chain kinase

Fluoride is a common contaminant of groundwater and agricultural commodity, which poses challenges to animal and human health. A wealth of research has demonstrated its detrimental effects on intestinal mucosal integrity; however, the underlying mechanisms remain obscure. This study aimed to investigate the role of the cytoskeleton in fluoride-induced barrier dysfunction. After sodium fluoride (NaF) treatment of the cultured Caco-2 cells, both cytotoxicity and cytomorphological changes (internal vacuoles or massive ablation) were observed. NaF lowered transepithelial electrical resistance (TEER) and enhanced paracellular permeation of fluorescein isothiocyanate dextran 4 (FD-4), indicating Caco-2 monolayers hyperpermeability. In the meantime, NaF treatment altered both the expression and distribution of the tight junction protein ZO-1. Fluoride exposure increased myosin light chain II (MLC2) phosphorylation and triggered actin filament (F-actin) remodeling. While inhibition of myosin II by Blebbistatin blocked NaF-induced barrier failure and ZO-1 discontinuity, the corresponding agonist Ionomycin had effects comparable to those of fluoride, suggesting that MLC2 serves as an effector. Given the mechanisms upstream of p-MLC2 regulation, further studies demonstrated that NaF activated RhoA/ROCK signaling pathway and myosin light chain kinase (MLCK), strikingly increasing the expression of both. Pharmacological inhibitors (Rhosin, Y-27632 and ML-7) reversed NaF-induced barrier breakdown and stress fiber formation. The role of intracellular calcium ions ([Ca²⁺]_i) in NaF effects on Rho/ROCK pathway and MLCK was investigated. We found that NaF elevated [Ca²⁺]_i, whereas chelator BAPTA-AM attenuated increased RhoA and MLCK expression as well as ZO-1 rupture, thus, restoring barrier function. Collectively, abovementioned results suggest that NaF induces barrier impairment via Ca²⁺-dependent RhoA/ROCK pathway and MLCK, which in turn triggers MLC2 phosphorylation and rearrangement of ZO-1 and F-actin. These results provide potential therapeutic targets for fluoride-induced intestinal injury.

Establishment of Intestinal Epithelial Cell Monolayers and Their Use in Calcium Switch Assay for Assessment of Intestinal Tight Junction Assembly

Intestinal barrier function relies primarily on the assembly and integrity of tight junctions, which forms a size-selective barrier. This barrier restricts paracellular movement of solutes in various types of epithelia. Of note, extracellular Ca²⁺ concentration affects tight junction assembly. Therefore, the removal and re-addition of Ca²⁺ into cell culture medium of cultured intestinal epithelial cells causes destabilization and reassembly of tight junction to membrane periphery near apical surface, respectively. Based on this principle, the Ca²⁺-switch assay was established to investigate tight junction assembly in fully differentiated intestinal epithelial cells. This chapter provides a stepwise protocol for culture of intestinal epithelial cell monolayers using T84 cell line as an in vitro model and the Ca²⁺-switch assay for evaluating tight junction assembly.

Tight Junctions as a Key for Pathogens Invasion in Intestinal Epithelial Cells

Tight junctions play a major role in maintaining the integrity and impermeability of the intestinal barrier. As such, they act as an ideal target for pathogens to promote their translocation through the intestinal mucosa and invade their host. Different strategies are used by pathogens, aimed at directly destabilizing the junctional network or modulating the different signaling pathways involved in the modulation of these junctions. After a brief presentation of the organization and modulation of tight junctions, we provide the state of the art of the molecular mechanisms leading to permeability breakdown of the gut barrier as a consequence of tight junctions’ attack by pathogens, including bacteria, viruses, fungi, and parasites.

Ca2+ homeostasis in brain microvascular endothelial cells

Blood brain barrier (BBB) is formed by the brain microvascular endothelial cells (BMVECs) lining the wall of brain capillaries. Its integrity is regulated by multiple mechanisms, including up/downregulation of tight junction proteins or adhesion molecules, altered Ca²⁺ homeostasis, remodeling of cytoskeleton, that are confined at the level of BMVECs. Beside the contribution of BMVECs to BBB permeability changes, other cells, such as pericytes, astrocytes, microglia, leukocytes or neurons, etc. are also exerting direct or indirect modulatory effects on BBB. Alterations in BBB integrity play a key role in multiple brain pathologies, including neurological (e.g. epilepsy) and neurodegenerative disorders (e.g. Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis etc.). In this review, the principal Ca²⁺ signaling pathways in brain microvascular endothelial cells are discussed and their contribution to BBB integrity is emphasized. Improving the knowledge of Ca²⁺ homeostasis alterations in BMVECa is fundamental to identify new possible drug targets that diminish/prevent BBB permeabilization in neurological and neurodegenerative disorders.

A blood-brain barrier overview on structure, function, impairment, and biomarkers of integrity

The blood–brain barrier is playing a critical role in controlling the influx and efflux of biological substances essential for the brain’s metabolic activity as well as neuronal function. Thus, the functional and structural integrity of the BBB is pivotal to maintain the homeostasis of the brain microenvironment. The different cells and structures contributing to developing this barrier are summarized along with the different functions that BBB plays at the brain–blood interface. We also explained the role of shear stress in maintaining BBB integrity. Furthermore, we elaborated on the clinical aspects that correlate between BBB disruption and different neurological and pathological conditions. Finally, we discussed several biomarkers that can help to assess the BBB permeability and integrity in-vitro or in-vivo and briefly explain their advantages and disadvantages.

High dose of baicalin or baicalein can reduce tight junction integrity by partly targeting the first PDZ domain of zonula occludens-1 (ZO-1)

The tight junction (TJ) is the apical-most intercellular junction complex, serving as a biological barrier of intercellular spaces between epithelial cells. The TJ's integrity is maintained by a key protein-protein interaction between C-terminal motifs of claudins (CLDs) and the postsynaptic density 95 (PSD-95)/discs large/zonula occludens 1 (ZO-1; PDZ) domains of ZO-1. Weak but direct interaction of baicalin and its aglycon, baicalein-which are pharmacologically active components of Chinese skullcap (Radix scutellariae)-with ZO-1(PDZ1) have been observed in NMR experiments. Next, we observed TJ-mitigating activity of these flavonoids against Madin-Darby canine kidney (MDCK) II cells with the downregulation of subcellular localization of CLD-2 at TJs. Meanwhile, baicalein-but not baicalin-induced a slender morphological change of MDCK cells' shape from their normal cobblestone-like shapes. Since baicalin and baicalein did not induce a localization change of occludin (OCLN), a "partial" epithelial-mesenchymal transition (EMT) induced by these flavonoids was considered. SB431542, an ALK-5 inhibitor, reversed the CLD-2 downregulation of both baicalin and baicalein, while SB431542 did not reverse the slender morphology. In contrast, the MEK/ERK inhibitor U0126 reversed the slender shape change. Thus, in addition to inhibition of the ZO-1-CLD interaction, activation of both transforming growth factor-β (TGF-β) and MEK/ERK signaling pathways have been suggested to be involved in TJ reduction by these flavonoids. Finally, we demonstrated that baicalin enhanced the permeability of fluorescence-labeled insulin via the paracellular pathway of the Caco-2 cell layer. We propose that baicalin, baicalein, and Radix scutellariae extract are useful as drug absorption enhancers.

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

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

The multifarious regulation of the apical junctional complex

Epithelial cells form highly organized polarized sheets with characteristic cell morphologies and tissue architecture. Cell–cell adhesion and intercellular communication are prerequisites of such cohesive sheets of cells, and cell connectivity is mediated through several junctional assemblies, namely desmosomes, adherens, tight and gap junctions. These cell–cell junctions form signalling hubs that not only mediate cell–cell adhesion but impact on multiple aspects of cell behaviour, helping to coordinate epithelial cell shape, polarity and function. This review will focus on the tight and adherens junctions, constituents of the apical junctional complex, and aims to provide a comprehensive overview of the complex signalling that underlies junction assembly, integrity and plasticity.

Changes in the expression and distribution of junction and polarity proteins in the porcine endometrium during early pregnancy period

The maternal endometrium undergoes transformations during early pregnancy period to regulate the paracellular permeability across the epithelium and to enable adhesion between the trophoblast and endometrial epithelial cells. These transformations, under the influence of ovarian hormones, are associated with a partial loss in polarity of epithelial cell that is regulated by tight junctions (TJ), adherens junctions (AJ) and associated polarity protein complexes. This study examined the change in expression and distribution of proteins associated with TJs, AJs and apical partition defective (PAR) complex in porcine endometrium on Days 10, 13 and 16 of estrous cycle and pregnancy. Moreover, effect of hormones, progesterone (P4) and 17-β estradiol (E2) on polar phenotype of endometrial epithelial cells was also investigated in vitro. There was pregnancy induced increase in gene and protein expression of TJ associated claudin-1 (CLDN1) on Day 13 of pregnancy as compared to corresponding day of estrous cycle and a decrease in TJ protein, zona occludens-1 (ZO-1) and PAR complex associated PAR6 expression levels on Day 16 of pregnancy (P < 0.05). Immunofluorescence studies revealed that on Days 10 and 13, TJ proteins occludin (OCLN) and ZO-1were primarily present in the apical region of lateral epithelial membrane. On Day 16 of pregnancy, whereas, OCLN redistributed into cytoplasm, ZO-1 decreased apically but was found to localize in the basal epithelium. The AJ proteins cadherin and β-catenin were located at the apical epithelium on Day 10 of estrous cycle and pregnancy and Day 13 of estrous cycle. On Days 13 and 16 of pregnancy both proteins were expressed in the lateral membrane and co-localization between these proteins was observed on Day 16. On Day 10, PAR complex proteins PAR3, cell division control protein 42 (CDC42) and atypical protein kinase C (aPKC) ζ were observed in apical epithelium and in lateral membrane and CDC42 was also present in the cytoplasm of epithelium. Pregnancy induced redistribution of aPKCζ to cytoplasm and CDC42 to apical surface of luminal epithelium was observed on Days 13 and 16. The in vitro P4 and E2 treatment of epithelial cells mimicked in vivo results. These results indicate that P4 and E2 regulate alterations in epithelium that may facilitate embryo implantation and given the role of cadherin, catenin and CDC42 in embryo invasion, change in distribution of these proteins may limit the invasiveness of porcine conceptuses into the stroma.

Disruption of the Blood-Brain Barrier During Neuroinflammatory and Neuroinfectious Diseases

As the organ of highest metabolic demand, utilizing over 25% of total body glucose utilization via an enormous vasculature with one capillary every 73 μm, the brain evolves a barrier at the capillary and postcapillary venules to prevent toxicity during serum fluctuations in metabolites and hormones, to limit brain swelling during inflammation, and to prevent pathogen invasion. Understanding of neuroprotective barriers has since evolved to incorporate the neurovascular unit (NVU), the blood-cerebrospinal fluid (CSF) barrier, and the presence of CNS lymphatics that allow leukocyte egress. Identification of the cellular and molecular participants in BBB function at the NVU has allowed detailed analyses of mechanisms that contribute to BBB dysfunction in various disease states, which include both autoimmune and infectious etiologies. This chapter will introduce some of the cellular and molecular components that promote barrier function but may be manipulated by inflammatory mediators or pathogens during neuroinflammation or neuroinfectious diseases.

Early Porcine Sapovirus Infection Disrupts Tight Junctions and Uses Occludin as a Coreceptor

The genus Sapovirus belongs to the family Caliciviridae, and its members are common causative agents of severe acute gastroenteritis in both humans and animals. Some caliciviruses are known to use either terminal sialic acids or histo-blood group antigens as attachment factors and/or cell surface proteins, such as CD300lf, CD300ld, and junctional adhesion molecule 1 of tight junctions (TJs), as receptors. However, the roles of TJs and their proteins in sapovirus entry have not been examined. In this study, we found that porcine sapovirus (PSaV) significantly decreased transepithelial electrical resistance and increased paracellular permeability early in infection of LLC-PK cells, suggesting that PSaV dissociates TJs of cells. This led to the interaction between PSaV particles and occludin, which traveled in a complex into late endosomes via Rab5- and Rab7-dependent trafficking. Inhibition of occludin using small interfering RNA (siRNA), a specific antibody, or a dominant-negative mutant significantly blocked the entry of PSaV. Transient expression of occludin in nonpermissive Chinese hamster ovary (CHO) cells conferred susceptibility to PSaV, but only for a limited time. Although claudin-1, another TJ protein, neither directly interacted nor was internalized with PSaV particles, it facilitated PSaV entry and replication in the LLC-PK cells. We conclude that PSaV particles enter LLC-PK cells by binding to occludin as a coreceptor in PSaV-dissociated TJs. PSaV and occludin then form a complex that moves to late endosomes via Rab5- and Rab7-dependent trafficking. In addition, claudin-1 in the TJs opened by PSaV infection facilitates PSaV entry and infection as an entry factor.IMPORTANCE Sapoviruses (SaVs) cause severe acute gastroenteritis in humans and animals. Although they replicate in intestinal epithelial cells, which are tightly sealed by apical-junctional complexes, such as tight junctions (TJs), the mechanisms by which SaVs hijack TJs and their proteins for successful entry and infection remain largely unknown. Here, we demonstrate that porcine SaVs (PSaVs) induce early dissociation of TJs, allowing them to bind to the TJ protein occludin as a functional coreceptor. PSaVs then travel in a complex with occludin into late endosomes through Rab5- and Rab7-dependent trafficking. Claudin-1, another TJ protein, does not directly interact with PSaV but facilitates the entry of PSaV into cells as an entry factor. This work contributes to our understanding of the entry of SaV and other caliciviruses into cells and may aid in the development of efficient and affordable drugs to treat SaV infections.

Spatial Overlap of Claudin- and Phosphatidylinositol Phosphate-Binding Sites on the First PDZ Domain of Zonula Occludens 1 Studied by NMR

Background: The tight junction is an intercellular adhesion complex composed of claudins (CLDs), occludin, and the scaffolding proteins zonula occludens 1 (ZO-1) and its two paralogs ZO-2 and ZO-3. ZO-1 is a multifunctional protein that contains three PSD95/Discs large/ZO-1(PDZ) domains. A key functional domain of ZO-1 is the first PDZ domain (ZO-1(PDZ1)) that recognizes the conserved C-termini of CLDs. Methods: In this study, we confirmed that phosphoinositides bound directly to ZO-1(PDZ1) by biochemical and solution NMR experiments. We further determined the solution structure of mouse ZO-1(PDZ1) by NMR and mapped the phosphoinositide binding site onto its molecular surface. Results: The phosphoinositide binding site was spatially overlapped with the CLD-binding site of ZO-1(PDZ1). Accordingly, inositol-hexaphosphate (phytic acid), an analog of the phosphoinositide head group, competed with ZO-1(PDZ)-CLD interaction. Conclusions: The results suggested that the PDZ domain–phosphoinositide interaction plays a regulatory role in biogenesis and homeostasis of the tight junction.

Relationship between G proteins coupled receptors and tight junctions

Tight junctions (TJs) are sites of cell-cell adhesion, constituted by a cytoplasmic plaque of molecules linked to integral proteins that form a network of strands around epithelial and endothelial cells at the uppermost portion of the lateral membrane. TJs maintain plasma membrane polarity and form channels and barriers that regulate the transit of ions and molecules through the paracellular pathway. This structure that regulates traffic between the external milieu and the organism is affected in numerous pathological conditions and constitutes an important target for therapeutic intervention. Here, we describe how a wide array of G protein-coupled receptors that are activated by diverse stimuli including light, ions, hormones, peptides, lipids, nucleotides and proteases, signal through heterotrimeric G proteins, arrestins and kinases to regulate TJs present in the blood-brain barrier, the blood-retinal barrier, renal tubular cells, keratinocytes, lung and colon, and the slit diaphragm of the glomerulus.

New insights into the mechanisms of pulmonary edema in acute lung injury

Appearance of alveolar protein-rich edema is an early event in the development of acute respiratory distress syndrome (ARDS). Alveolar edema in ARDS results from a significant increase in the permeability of the alveolar epithelial barrier, and represents one of the main factors that contribute to the hypoxemia in these patients. Damage of the alveolar epithelium is considered a major mechanism responsible for the increased pulmonary permeability, which results in edema fluid containing high concentrations of extravasated macromolecules in the alveoli. The breakdown of the alveolar-epithelial barrier is a consequence of multiple factors that include dysregulated inflammation, intense leukocyte infiltration, activation of pro-coagulant processes, cell death and mechanical stretch. The disruption of tight junction (TJ) complexes at the lateral contact of epithelial cells, the loss of contact between epithelial cells and extracellular matrix (ECM), and relevant changes in the communication between epithelial and immune cells, are deleterious alterations that mediate the disruption of the alveolar epithelial barrier and thereby the formation of lung edema in ARDS.

TRPV4 Regulates Tight Junctions and Affects Differentiation in a Cell Culture Model of the Corneal Epithelium

TRPV4 (transient receptor potential vanilloid 4) is a cation channel activated by hypotonicity, moderate heat, or shear stress. We describe the expression of TRPV4 during the differentiation of a corneal epithelial cell model, RCE1(5T5) cells. TRPV4 is a late differentiation feature that is concentrated in the apical membrane of the outmost cell layer of the stratified epithelia. Ca²⁺ imaging experiments showed that TRPV4 activation with GSK1016790A produced an influx of calcium that was blunted by the specific TRPV4 blocker RN-1734. We analyzed the involvement of TRPV4 in RCE1(5T5) epithelial differentiation by measuring the development of transepithelial electrical resistance (TER) as an indicator of the tight junction (TJ) assembly. We showed that TRPV4 activity was necessary to establish the TJ. In differentiated epithelia, activation of TRPV4 increases the TER and the accumulation of claudin-4 in cell-cell contacts. Epidermal Growth Factor (EGF) up-regulates the TER of corneal epithelial cultures, and we show here that TRPV4 activation mimicked this EGF effect. Conversely, TRPV4 inhibition or knock down by specific shRNA prevented the increase in TER. Moreover, TRPP2, an EGF-activated channel that forms heteromeric complexes with TRPV4, is also concentrated in the outmost cell layer of differentiated RCE1(5T5) sheets. This suggests that the EGF regulation of the TJ may involve a heterotetrameric TRPV4-TRPP2 channel. These results demonstrated TRPV4 activity was necessary for the correct establishment of TJ in corneal epithelia and as well as the regulation of both the barrier function of TJ and its ability to respond to EGF. J. Cell. Physiol. 232: 1794-1807, 2017. © 2016 Wiley Periodicals, Inc.

Tight junctions: from simple barriers to multifunctional molecular gates

Epithelia and endothelia separate different tissue compartments and protect multicellular organisms from the outside world. This requires the formation of tight junctions, selective gates that control paracellular diffusion of ions and solutes. Tight junctions also form the border between the apical and basolateral plasma-membrane domains and are linked to the machinery that controls apicobasal polarization. Additionally, signalling networks that guide diverse cell behaviours and functions are connected to tight junctions, transmitting information to and from the cytoskeleton, nucleus and different cell adhesion complexes. Recent advances have broadened our understanding of the molecular architecture and cellular functions of tight junctions.

Signalling at tight junctions during epithelial differentiation and microbial pathogenesis

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

Claudin-4 is required for modulation of paracellular permeability by muscarinic acetylcholine receptor in epithelial cells

The epithelial cholinergic system plays an important role in water, ion and solute transport. Previous studies have shown that activation of muscarinic acetylcholine receptors (mAChRs) regulates paracellular transport of epithelial cells; however, the underlying mechanism is still largely unknown. Here, we found that mAChR activation by carbachol and cevimeline reduced the transepithelial electrical resistance (TER) and increased the permeability of paracellular tracers in rat salivary epithelial SMG-C6 cells. Carbachol induced downregulation and redistribution of claudin-4, but not occludin or ZO-1 (also known as TJP1). Small hairpin RNA (shRNA)-mediated claudin-4 knockdown suppressed, whereas claudin-4 overexpression retained, the TER response to carbachol. Mechanistically, the mAChR-modulated claudin-4 properties and paracellular permeability were triggered by claudin-4 phosphorylation through ERK1/2 (also known as MAPK3 and MAPK1, respectively). Mutagenesis assay demonstrated that S195, but not S199, S203 or S207, of claudin-4, was the target for carbachol. Subsequently, the phosphorylated claudin-4 interacted with β-arrestin2 and triggered claudin-4 internalization through the clathrin-dependent pathway. The internalized claudin-4 was further degraded by ubiquitylation. Taken together, these findings suggested that claudin-4 is required for mAChR-modulated paracellular permeability of epithelial cells through an ERK1/2, β-arrestin2, clathrin and ubiquitin-dependent signaling pathway.

Vasoactive intestinal peptide prevents PKCε-induced intestinal epithelial barrier disruption during EPEC infection

We previously showed that vasoactive intestinal peptide (VIP) protects against bacterial pathogen-induced epithelial barrier disruption and colitis, although the mechanisms remain poorly defined. The aim of the current study was to identify cellular pathways of VIP-mediated protection with use of pharmacological inhibitors during enteropathogenic Escherichia coli (EPEC) infection of Caco-2 cell monolayers and during Citrobacter rodentium-induced colitis. EPEC-induced epithelial barrier disruption involved the PKC pathway but was independent of functional cAMP, Rho, and NF-κB pathways. VIP mediated its protective effects by inhibiting EPEC-induced PKC activity and increasing expression of the junctional protein claudin-4. Short-term treatment with TPA, which is known to activate PKC, was inhibited by VIP pretreatment, while PKC degradation via long-term treatment with TPA mimicked the protective actions of VIP. Immunostaining for specific PKC isotypes showed upregulated expression of PKCθ and PKCε during EPEC infection. Treatment with specific inhibitors revealed a critical role for PKCε in EPEC-induced barrier disruption. Furthermore, activation of PKCε and loss of barrier integrity correlated with claudin-4 degradation. In contrast, inhibition of PKCε by VIP pretreatment or the PKCε inhibitor maintained membrane-bound claudin-4 levels, along with barrier function. Finally, in vivo treatment with the PKCε inhibitor protected mice from C. rodentium-induced colitis. In conclusion, EPEC infection increases intracellular PKCε levels, leading to decreased claudin-4 levels and compromising epithelial barrier integrity. VIP inhibits PKCε activation, thereby attenuating EPEC-induced barrier disruption.

Sensing of EGTA Mediated Barrier Tissue Disruption with an Organic Transistor

Barrier tissue protects the body against external factors by restricting the passage of molecules. The gastrointestinal epithelium is an example of barrier tissue with the primary purpose of allowing the passage of ions and nutrients, while restricting the passage of pathogens and toxins. It is well known that the loss of barrier function can be instigated by a decrease in extracellular calcium levels, leading to changes in protein conformation and an increase in paracellular transport. In this study, ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetra acetic acid (EGTA), a calcium chelator, was used to disrupt the gastrointestinal epithelial barrier. The effect of EGTA on barrier tissue was monitored by a novel label-free method based on an organic electrochemical transistor (OECT) integrated with living cells and validated against conventional methods for measuring barrier tissue integrity. We demonstrate that the OECT can detect breaches in barrier tissue upon exposure to EGTA with the same sensitivity as existing methods but with increased temporal resolution. Due to the potential of low cost processing techniques and the flexibility in design associated with organic electronics, the OECT has great potential for high-throughput, disposable sensing and diagnostics.

Glutamine enhances tight junction protein expression and modulates corticotropin-releasing factor signaling in the jejunum of weanling piglets

BACKGROUND

Dysfunction of tight junction integrity is associated with decreased nutrient absorption and numerous gastrointestinal diseases in humans and piglets. Although l-glutamine has been reported to enhance intestinal-mucosal mass and barrier function under stressful conditions, in vivo data to support a functional role for l-glutamine on intestinal tight junction protein (TJP) expression in weanling mammals are limited.

OBJECTIVE

This study tested the hypothesis that glutamine regulates expression of TJPs and stress-related corticotropin-releasing factor (CRF) signaling in the jejunum of weanling piglets.

METHODS

Piglets were reared by sows or weaned at 21 d of age to a corn and soybean meal-based diet that was or was not supplemented with 1% l-glutamine for 7 d. Growth performance, intestinal permeability, TJP abundance, and CRF expression were examined.

RESULTS

Weaning caused increases (P < 0.05) in intestinal permeability by 40% and in CRF concentrations by 4.7 times in association with villus atrophy (P < 0.05). Western blot analysis showed reductions (P < 0.05) in jejunal expression of occludin, claudin-1, zonula occludens (ZO) 2, and ZO-3, but no changes in the abundance of claudin-3, claudin-4, or ZO-1 in weanling piglets compared with age-matched suckling controls. Glutamine supplementation improved (P < 0.05) intestinal permeability and villus height, while reducing (P < 0.05) jejunal mRNA and protein levels for CRF and attenuating (P < 0.05) weanling-induced decreases in occludin, claudin-1, ZO-2, and ZO-3 protein abundances.

CONCLUSION

Collectively, our results support an important role for l-glutamine in regulating expression of TJPs and CRF in the jejunum of weanling piglets.

Monitoring of cell layer coverage and differentiation with the organic electrochemical transistor

High resolution monitoring of cell layer integrity with the OECT was up until now, limited to high resistance, barrier tissue type cells. In this work, the sensitivity and versatility of the device is expanded to monitor all adherent cell types.

Combined optical and electronic sensing of epithelial cells using planar organic transistors

A planar, conducting-polymer-based transistor for combined optical and electronic monitoring of live cells provides a unique platform for monitoring the health of cells in vitro. Monitoring of MDCK-I epithelial cells over several days is shown, along with a demonstration of the device for toxicology studies, of use in future drug discovery or diagnostics applications.

IBD candidate genes and intestinal barrier regulation

Technological advances in the large scale analysis of human genetics have generated profound insights into possible genetic contributions to chronic diseases including the inflammatory bowel diseases (IBDs), Crohn's disease and ulcerative colitis. To date, 163 distinct genetic risk loci have been associated with either Crohn's disease or ulcerative colitis, with a substantial degree of genetic overlap between these 2 conditions. Although many risk variants show a reproducible correlation with disease, individual gene associations only affect a subset of patients, and the functional contribution(s) of these risk variants to the onset of IBD is largely undetermined. Although studies in twins have demonstrated that the development of IBD is not mediated solely by genetic risk, it is nevertheless important to elucidate the functional consequences of risk variants for gene function in relevant cell types known to regulate key physiological processes that are compromised in IBD. This article will discuss IBD candidate genes that are known to be, or are suspected of being, involved in regulating the intestinal epithelial barrier and several of the physiological processes presided over by this dynamic and versatile layer of cells. This will include assembly and regulation of tight junctions, cell adhesion and polarity, mucus and glycoprotein regulation, bacterial sensing, membrane transport, epithelial differentiation, and restitution.

The emergence of the concept of tight junctions and physiological regulation by ouabain

The exchange of substances between metazoan and the environment takes place across transporting epithelia that have two fundamental differentiated features: tight junctions (TJ) and apical/basolateral polarity. Usually, reviews of the structure and function of transporting epithelia follow a historical description of major biological findings, but seldom refer to the fact that it also required fundamental theoretical changes in the physics and chemistry involved. We make a brief description of the concatenation of both types of achievements, in which it becomes clear that the major source of conflicts was the enzyme Na(+),K(+)-ATPase (also referred to as "the pump"), because of its intrinsic mechanisms and its asymmetric expression on one side of epithelial cells only (polarity). This enzyme is also the receptor of the newly recognized hormone ouabain, whose chief function is to modulate cell contacts, such as TJs, several types of cell-cell contacts participating in polarization (as gauged through ciliogenesis).

RhoGTPases, actomyosin signaling and regulation of the epithelial Apical Junctional Complex

Epithelial cells form regulated and selective barriers between distinct tissue compartments. The Apical Junctional Complex (AJC) consisting of the tight junction (TJ) and adherens junction (AJ) control epithelial homeostasis, paracellular permeability and barrier properties. The AJC is composed of mutliprotein complexes consisting of transmembrane proteins that affiliate with an underlying perijunctional F-actin myosin ring through cytoplasmic scaffold proteins. AJC protein associations with the apical actin-myosin cytoskeleton are tightly controlled by a number of signaling proteins including the Rho family of GTPases that orchestrate junctional biology, epithelial homeostasis and barrier function. This review highlights the vital relationship of Rho GTPases and AJCs in controlling the epithelial barrier. The pathophysiologic relationship of Rho GTPases, AJC, apical actomyosin cytoskeleton and epithelial barrier function is discussed.

Functions of the CB1 and CB 2 receptors in neuroprotection at the level of the blood-brain barrier

The cannabinoid (CB) receptors are the main targets of the cannabinoids, which include plant cannabinoids, endocannabinoids and synthetic cannabinoids. Over the last few years, accumulated evidence has suggested a role of the CB receptors in neuroprotection. The blood-brain barrier (BBB) is an important brain structure that is essential for neuroprotection. A link between the CB receptors and the BBB is thus likely, but this possible connection has only recently gained attention. Cannabinoids and the BBB share the same mechanisms of neuroprotection and both protect against excitotoxicity (CB1), cell death (CB1), inflammation (CB2) and oxidative stress (possibly CB independent)-all processes that also damage the BBB. Several examples of CB-mediated protection of the BBB have been found, such as inhibition of leukocyte influx and induction of amyloid beta efflux across the BBB. Moreover, the CB receptors were shown to improve BBB integrity, particularly by restoring the tightness of the tight junctions. This review demonstrated that both CB receptors are able to restore the BBB and neuroprotection, but much uncertainty about the underlying signaling cascades still exists and further investigation is needed.

Recent progress in tight junction modulation for improving bioavailability

INTRODUCTION

Currently, there are many novel drugs that belong to class III or IV of the Biopharmaceutics Classification System, showing low bioavailability. Tight junction (TJ) modulation offers an approach to increase bioavailability of pharmaceutical compounds. Furthermore, some diseases are accompanied by disturbed barrier function or TJ dysregulation and thus represent a second application for TJ modulators.

AREAS COVERED

This review contains a summary of three different TJ modulators: AT1002, PN159 and labradimil. Within this summary, the authors provide a description of their effects on TJs, their adverse effects and their success in clinical trials. Furthermore, the authors present the current understanding of TJ regulation and highlight opportunities to develop new TJ modulators; they also review the problems that might occur.

EXPERT OPINION

The development of new mechanism-based (MB) TJ modulators is a very promising field of research. MB approaches are expected to have the best future prospects. Further elucidation of signaling pathways and TJ regulation will be necessary for advancing MB TJ modulator research.

Activation of the Ca²+-sensing receptor induces deposition of tight junction components to the epithelial cell plasma membrane

The Ca(2+)-sensing receptor (CaSR) belongs to the G-protein-coupled receptor superfamily and plays essential roles in divalent ion homeostasis and cell differentiation. Because extracellular Ca(2+) is essential for the development of stable epithelial tight junctions (TJs), we hypothesized that the CaSR participates in regulating TJ assembly. We first assessed the expression of the CaSR in Madin-Darby canine kidney (MDCK) cells at steady state and following manipulations that modulate TJ assembly. Next, we examined the effects of CaSR agonists and antagonists on TJ assembly. Immunofluorescence studies indicate that endogenous CaSR is located at the basolateral pole of MDCK cells. Stable transfection of human CaSR in MDCK cells further reveals that this protein co-distributes with β-catenin on the basolateral membrane. Switching MDCK cells from low-Ca(2+) medium to medium containing a normal Ca(2+) concentration significantly increases CaSR expression at both the mRNA and protein levels. Exposure of MDCK cells maintained in low-Ca(2+) conditions to the CaSR agonists neomycin, Gd(3+) or R-568 causes the transient relocation of the tight junction components ZO-1 and occludin to sites of cell-cell contact, while inducing no significant changes in the expression of mRNAs encoding junction-associated proteins. Stimulation of CaSR also increases the interaction between ZO-1 and the F-actin-binding protein I-afadin. This effect does not involve activation of the AMP-activated protein kinase. By contrast, CaSR inhibition by NPS-2143 significantly decreases interaction of ZO-1 with I-afadin and reduces deposition of ZO-1 at the cell surface following a Ca(2+) switch from 5 µM to 200 µM [Ca(2+)]e. Pre-exposure of MDCK cells to the cell-permeant Ca(2+) chelator BAPTA-AM, similarly prevents TJ assembly caused by CaSR activation. Finally, stable transfection of MDCK cells with a cDNA encoding a human disease-associated gain-of-function mutant form of the CaSR increases the transepithelial electrical resistance of these cells in comparison to expression of the wild-type human CaSR. These observations suggest that the CaSR participates in regulating TJ assembly.

Measurement of barrier tissue integrity with an organic electrochemical transistor

The integration of an organic electrochemical transistor with human barrier tissue cells provides a novel method for assessing toxicology of compounds in vitro. Minute variations in paracellular ionic flux induced by toxic compounds are measured in real time, with unprecedented temporal resolution and extreme sensitivity.

Identification of new binding partners of the chemosensory signaling protein Gγ13 expressed in taste and olfactory sensory cells

Tastant detection in the oral cavity involves selective receptors localized at the apical extremity of a subset of specialized taste bud cells called taste receptor cells (TRCs). The identification of the genes coding for the taste receptors involved in this process have greatly improved our understanding of the molecular mechanisms underlying detection. However, how these receptors signal in TRCs, and whether the components of the signaling cascades interact with each other or are organized in complexes is mostly unexplored. Here we report on the identification of three new binding partners for the mouse G protein gamma 13 subunit (Gγ13), a component of the bitter taste receptors signaling cascade. For two of these Gγ13 associated proteins, namely GOPC and MPDZ, we describe the expression in taste bud cells for the first time. Furthermore, we demonstrate by means of a yeast two-hybrid interaction assay that the C terminal PDZ binding motif of Gγ13 interacts with selected PDZ domains in these proteins. In the case of the PDZ domain-containing protein zona occludens-1 (ZO-1), a major component of the tight junction defining the boundary between the apical and baso-lateral region of TRCs, we identified the first PDZ domain as the site of strong interaction with Gγ13. This association was further confirmed by co-immunoprecipitation experiments in HEK 293 cells. In addition, we present immunohistological data supporting partial co-localization of GOPC, MPDZ, or ZO-1, and Gγ13 in taste buds cells. Finally, we extend this observation to olfactory sensory neurons (OSNs), another type of chemosensory cells known to express both ZO-1 and Gγ13. Taken together our results implicate these new interaction partners in the sub-cellular distribution of Gγ13 in olfactory and gustatory primary sensory cells.

Larazotide acetate promotes tight junction assembly in epithelial cells

Tight junctions (TJ) control paracellular permeability and apical-basolateral polarity of epithelial cells. Dysregulated permeability is associated with pathological conditions, such as celiac disease and inflammatory bowel disease. TJ formation is dependent on E-cadherin-mediated cell-cell adhesion and actin rearrangement, and is regulated by the Rho family GTPase and aPKC signaling pathways. Larazotide acetate, an 8-mer peptide and TJ modulator, inhibits TJ disassembly and dysfunction caused by endogenous and exogenous stimuli in intestinal epithelial cells. Here, we examined the effect of larazotide acetate on de novo TJ assembly using 2 different model systems. In MDCK cells, larazotide acetate promoted TJ assembly in a calcium switch assay. Larazotide acetate also promoted actin rearrangement, and junctional distribution of zonula occludens-1 (ZO-1), occludin, claudins, and E-cadherin. Larazotide acetate promoted TJ maturation and decreased paracellular permeability in "leaky" Caco-2 cells. Taken together, our data indicate that larazotide acetate enhances TJ assembly and barrier function by promoting actin rearrangement and redistribution of TJ and AJ proteins.

Inhibition of protein kinase C attenuates Pseudomonas aeruginosa elastase-induced epithelial barrier disruption

Pseudomonas aeruginosa pulmonary infection compromises the human airway epithelium, and can be especially devastating to immunocompromised or debilitated individuals. We have reported earlier that P. aeruginosa elastase (PE) increases paracellular permeability in epithelial cell monolayers by mechanisms involving tight junction (TJ) disruption and cytoskeletal reorganization, leading to destruction of epithelial barrier function. The aim of this study was to investigate putative TJ targets and potential mechanisms by which PE induces barrier disruption. We found that PE decreased localization of TJ proteins, occludin and zonula occludens (ZO)-1, in membrane fractions, and induced reorganization of F-actin within 1 hour. Although inhibition of protein kinase (PK) C α/β signaling modestly altered the extent of cytoskeletal disruption and ZO-1 translocation, we found PKC signaling to play a significant role in decreased occludin functionality during PE exposure. Furthermore, elevated PKC levels correlated with decreased levels of TJ proteins in membrane fractions, and increased paracellular permeability in a time-dependent manner. Therefore, we conclude that PKC signaling is involved during PE-induced epithelial barrier disruption via TJ translocation and cytoskeletal reorganization. Specifically, occludin, as well as associated ZO-1 and F-actin, may be early targets of PE pathogenesis occurring via a PKC-dependent pathway.

CaV1.3 channels and intracellular calcium mediate osmotic stress-induced N-terminal c-Jun kinase activation and disruption of tight junctions in Caco-2 CELL MONOLAYERS

We investigated the role of a Ca(2+) channel and intracellular calcium concentration ([Ca(2+)](i)) in osmotic stress-induced JNK activation and tight junction disruption in Caco-2 cell monolayers. Osmotic stress-induced tight junction disruption was attenuated by 1,2-bis(2-aminophenoxyl)ethane-N,N,N',N'-tetraacetic acid (BAPTA)-mediated intracellular Ca(2+) depletion. Depletion of extracellular Ca(2+) at the apical surface, but not basolateral surface, also prevented tight junction disruption. Similarly, thapsigargin-mediated endoplasmic reticulum (ER) Ca(2+) depletion attenuated tight junction disruption. Thapsigargin or extracellular Ca(2+) depletion partially reduced osmotic stress-induced rise in [Ca(2+)](i), whereas thapsigargin and extracellular Ca(2+) depletion together resulted in almost complete loss of rise in [Ca(2+)](i). L-type Ca(2+) channel blockers (isradipine and diltiazem) or knockdown of the Ca(V)1.3 channel abrogated [Ca(2+)](i) rise and disruption of tight junction. Osmotic stress-induced JNK2 activation was abolished by BAPTA and isradipine, and partially reduced by extracellular Ca(2+) depletion, thapsigargin, or Ca(V)1.3 knockdown. Osmotic stress rapidly induced c-Src activation, which was significantly attenuated by BAPTA, isradipine, or extracellular Ca(2+) depletion. Tight junction disruption by osmotic stress was blocked by tyrosine kinase inhibitors (genistein and PP2) or siRNA-mediated knockdown of c-Src. Osmotic stress induced a robust increase in tyrosine phosphorylation of occludin, which was attenuated by BAPTA, SP600125 (JNK inhibitor), or PP2. These results demonstrate that Ca(V)1.3 and rise in [Ca(2+)](i) play a role in the mechanism of osmotic stress-induced tight junction disruption in an intestinal epithelial monolayer. [Ca(2+)](i) mediate osmotic stress-induced JNK activation and subsequent c-Src activation and tyrosine phosphorylation of tight junction proteins. Additionally, inositol 1,4,5-trisphosphate receptor-mediated release of ER Ca(2+) also contributes to osmotic stress-induced tight junction disruption.

Direct activation of human phospholipase C by its well known inhibitor u73122

Phospholipase C (PLC) enzymes are an important family of regulatory proteins involved in numerous cellular functions, primarily through hydrolysis of the polar head group from inositol-containing membrane phospholipids. U73122 (1-(6-((17β-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole-2,5-dione), one of only a few small molecules reported to inhibit the activity of these enzymes, has been broadly applied as a pharmacological tool to implicate PLCs in diverse experimental phenotypes. The purpose of this study was to develop a better understanding of molecular interactions between U73122 and PLCs. Hence, the effects of U73122 on human PLCβ3 (hPLCβ3) were evaluated in a cell-free micellar system. Surprisingly, U73122 increased the activity of hPLCβ3 in a concentration- and time-dependent manner; up to an 8-fold increase in enzyme activity was observed with an EC50=13.6±5 μm. Activation of hPLCβ3 by U73122 required covalent modification of cysteines as evidenced by the observation that enzyme activation was attenuated by thiol-containing nucleophiles, l-cysteine and glutathione. Mass spectrometric analysis confirmed covalent reaction with U73122 at eight cysteines, although maximum activation was achieved without complete alkylation; the modified residues were identified by LC/MS/MS peptide sequencing. Interestingly, U73122 (10 μm) also activated hPLCγ1 (>10-fold) and hPLCβ2 (∼2-fold); PLCδ1 was neither activated nor inhibited. Therefore, in contrast to its reported inhibitory potential, U73122 failed to inhibit several purified PLCs. Most of these PLCs were directly activated by U73122, and a simple mechanism for the activation is proposed. These results strongly suggest a need to re-evaluate the use of U73122 as a general inhibitor of PLC isozymes.

The modulation of protein kinase A and heat shock protein 70 is involved in the reversible increase of blood-brain tumor barrier permeability induced by papaverine

Intra-arterial administration of papaverine has been revealed to cause an increase in the blood-brain tumor barrier (BTB) permeability. The exact mechanism of papaverine opening the BTB in chemotherapy of malignant cerebral tumors, however, has not been well described. We used a rat brain glioma (C6) model for studying how papaverine modulates the permeability of BTB by monitoring the activities of the tight junction (TJ)-associated protein occludin, claudin-5 and cytoskeletal protein filamentous actin (F-actin) and whether protein kinase A (PKA) and heat shock protein 70 (HSP70) were involved in the regulation of this biological process. The levels of occludin, claudin-5 and F-actin protein in the tumor tissues were down-regulated by papaverine via immunohistochemistry, immunofluorescence assays and Western blot, corresponding to the time-dependent change of the BTB permeability. The most obvious attenuation of occludin, claudin-5 and F-actin protein was observed at 1h after papaverine perfusion, companied by a significant decrease in expression levels of PKA protein. The expression level of HSP70 in the tumor tissues was also progressively increased after papaverine perfusion and reached the maximum at 3h. The results demonstrate that the reversible openning of BTB mediated by papaverine may be associated with the functional combination between PKA and HSP70. That is, BTB opening may be attributable to the down-regulation of occludin, claudin-5 and F-actin, and cAMP/PKA signaling pathway might be involved in this process. HSP70 is likely responsible for the BTB closing, which helping the repairment of injured TJ protein and the rebuilding of the BTB.

Rho signaling and tight junction functions

Tight junctions are heteromeric protein complexes that act as signaling centers by mediating the bidirectional transmission of information between the environment and the cell interior to control paracellular permeability and differentiation. Insight into tight junction-associated signaling mechanisms is of fundamental importance for our understanding of the physiology of epithelia and endothelia in health and disease.

Structure and function of the blood-brain barrier

Neural signalling within the central nervous system (CNS) requires a highly controlled microenvironment. Cells at three key interfaces form barriers between the blood and the CNS: the blood-brain barrier (BBB), blood-CSF barrier and the arachnoid barrier. The BBB at the level of brain microvessel endothelium is the major site of blood-CNS exchange. The structure and function of the BBB is summarised, the physical barrier formed by the endothelial tight junctions, and the transport barrier resulting from membrane transporters and vesicular mechanisms. The roles of associated cells are outlined, especially the endfeet of astrocytic glial cells, and pericytes and microglia. The embryonic development of the BBB, and changes in pathology are described. The BBB is subject to short and long-term regulation, which may be disturbed in pathology. Any programme for drug discovery or delivery, to target or avoid the CNS, needs to consider the special features of the BBB.

Control of tight junctional sealing: roles of epidermal growth factor and prostaglandin E2

Epithelia can adjust the permeability of the paracellular permeation route by regulating the degree of sealing of the tight junction. This is reflected by a transepithelial electrical resistance (TER) ranging from a few tenths to several thousand ohms times square centimeters, depending on the difference in composition between the fluid in the lumen and the interstitial fluid. Although teleologically sound, such correlation requires a physiological explanation. We have previously shown that urine extracts from different animal species increase the TER of Madin-Darby canine kidney (MDCK) monolayers and that these effects are mediated by epidermal growth factor (EGF) contained in the flowing intratubular fluid that eventually reaches the urine. This increase in TER is accompanied by an enhanced expression of claudin-4 (cln-4) and a decrement of cln-2. These changes are transient, peaking at approximately 16 h and returning to control values in approximately 24 h. In the present work we investigated how EGF provokes this transient response, and we found that the activation of extracellular-regulated kinases 1/2 (ERK1/2) by EGF is essential to increase TER and cln-4 content, but it does not appear to participate in cln-2 downregulation. On the other hand, prostaglandin synthesis, stimulated by EGF, functions as a negative feedback, turning off the signal initiated by EGF. Thus, PGE(2) blocks ERK1/2 by a mechanism that involves the G alpha(s) protein, adenylyl cyclase as well as protein kinase A in MDCK cells. In summary, the permeability of a given segment of the nephron depends on the expression of different claudin types, which may be modulated by EGF and prostaglandins.

Deciliation is associated with dramatic remodeling of epithelial cell junctions and surface domains

Stress-induced shedding of motile cilia (autotomy) has been documented in diverse organisms and likely represents a conserved cellular reaction. However, little is known about whether primary cilia are shed from mammalian epithelial cells and what impact deciliation has on polarized cellular organization. We show that several chemically distinct agents trigger autotomy in epithelial cells. Surprisingly, deciliation is associated with a significant, but reversible increase in transepithelial resistance. This reflects substantial reductions in tight junction proteins associated with "leaky" nephron segments (e.g., claudin-2). At the same time, apical trafficking of gp80/clusterin and gp114/CEACAM becomes randomized, basal-lateral delivery of Na,K-ATPase is reduced, and expression of the nonciliary apical protein gp135/podocalyxin is greatly decreased. However, ciliogenesis-impaired MDCK cells do not undergo continual junction remodeling, and mature cilia are not required for autotomy-associated remodeling events. Deciliation and epithelial remodeling may be mechanistically linked processes, because RNAi-mediated reduction of Exocyst subunit Sec6 inhibits ciliary shedding and specifically blocks deciliation-associated down-regulation of claudin-2 and gp135. We propose that ciliary autotomy represents a signaling pathway that impacts the organization and function of polarized epithelial cells.