Role of claudin species-specific dynamics in reconstitution and remodeling of the zonula occludens

A loss of function mutation in CLDN25 causing Pelizaeus-Merzbacher-like leukodystrophy

Claudin-25 (CLDN-25), also known as Claudin containing domain 1, is an uncharacterized claudin family member. It has less conserved amino acid sequences when compared to other claudins. It also has a very broad tissue expression profile and there is currently a lack of functional information from murine knockout models. Here, we report a de novo missense heterozygous variant in CLDN25 (c. 745G>C, p. A249P) found in a patient diagnosed with Pelizaeus-Merzbacher-like leukodystrophy and presenting with symptoms such as delayed motor development, several episodes of tonic absent seizures and generalized dystonia. The variant protein does not localize to the cell-cell borders where it would normally be expected to be expressed. Amino acid position 249 is located 4 amino acids from the C-terminal end of the protein where most claudin family members have a conserved binding motif for the key scaffolding protein ZO-1. However, CLDN-25 does not contain this motif. Here, we show that the C-terminal end of CLDN-25 is required for its junctional localization in a ZO-1 independent manner. The A249P mutant protein as well as a deletion mutant lacking its last 5 C-terminal amino acids also failed to localize to the cell-cell border in vitro. Intriguingly, cellular knockout of CLDN25, in vitro, appeared to increase the integrity of the tight junction between 2 contacting cells, while driving highly unusual increased movement of solutes between cells. We propose that the barrier function of CLDN-25 is akin to a decoy claudin, whereby decreasing its expression in "leaky" epithelial cells and endothelial cells will drive dynamic changes in the adhesion and interaction capacity of cell-cell contact points. While it remains unclear how this de novo CLDN-25 mutant induces leukodystrophy, our findings strongly suggest that this mutation induces haploinsufficiency of CLDN-25. Elucidating the function of this uncharacterized claudin protein will lead to a better understanding of the role of claudin proteins in health and disease.

Claudin-23 reshapes epithelial tight junction architecture to regulate barrier function

Claudin family tight junction proteins form charge- and size-selective paracellular channels that regulate epithelial barrier function. In the gastrointestinal tract, barrier heterogeneity is attributed to differential claudin expression. Here, we show that claudin-23 (CLDN23) is enriched in luminal intestinal epithelial cells where it strengthens the epithelial barrier. Complementary approaches reveal that CLDN23 regulates paracellular ion and macromolecule permeability by associating with CLDN3 and CLDN4 and regulating their distribution in tight junctions. Computational modeling suggests that CLDN23 forms heteromeric and heterotypic complexes with CLDN3 and CLDN4 that have unique pore architecture and overall net charge. These computational simulation analyses further suggest that pore properties are interaction-dependent, since differently organized complexes with the same claudin stoichiometry form pores with unique architecture. Our findings provide insight into tight junction organization and propose a model whereby different claudins combine to form multiple distinct complexes that modify epithelial barrier function by altering tight junction structure.

The role of claudin-2 in kidney function and dysfunction

Claudin-2 is a tight junction protein expressed in leaky epithelia where it forms paracellular pores permeable to cations and water. The paracellular pore formed by claudin-2 is important in energy-efficient cation and water transport in the proximal tubules of the kidneys. Mounting evidence now suggests that claudin-2 may modulate cellular processes often altered in disease, including cellular proliferation. Also, dysregulation of claudin-2 expression has been linked to various diseases, including kidney stone disease and renal cell carcinoma. However, the mechanisms linking altered claudin-2 expression and function to disease are poorly understood and require further investigation. The aim of this review is to discuss the current understanding of the role of claudin-2 in kidney function and dysfunction. We provide a general overview of the claudins and their organization in the tight junction, the expression, and function of claudin-2 in the kidney, and the evolving evidence for its role in kidney disease.

Claudin Barriers on the Brink: How Conflicting Tissue and Cellular Priorities Drive IBD Pathogenesis

Inflammatory bowel diseases (IBDs) are characterized by acute or chronic recurring inflammation of the intestinal mucosa, often with increasing severity over time. Life-long morbidities and diminishing quality of life for IBD patients compel a search for a better understanding of the molecular contributors to disease progression. One unifying feature of IBDs is the failure of the gut to form an effective barrier, a core role for intercellular complexes called tight junctions. In this review, the claudin family of tight junction proteins are discussed as they are a fundamental component of intestinal barriers. Importantly, claudin expression and/or protein localization is altered in IBD, leading to the supposition that intestinal barrier dysfunction exacerbates immune hyperactivity and disease. Claudins are a large family of transmembrane structural proteins that constrain the passage of ions, water, or substances between cells. However, growing evidence suggests non-canonical claudin functions during mucosal homeostasis and healing after injury. Therefore, whether claudins participate in adaptive or pathological IBD responses remains an open question. By reviewing current studies, the possibility is assessed that with claudins, a jack-of-all-trades is master of none. Potentially, a robust claudin barrier and wound restitution involve conflicting biophysical phenomena, exposing barrier vulnerabilities and a tissue-wide frailty during healing in IBD.

Cholesterol-rich domain formation mediated by ZO proteins is essential for tight junction formation

Tight junctions (TJs) are cell-adhesion structures responsible for the epithelial barrier. We reported that accumulation of cholesterol at the apical junctions is required for TJ formation [K. Shigetomi, Y. Ono, T. Inai, J. Ikenouchi, J. Cell Biol. 217, 2373-2381 (2018)]. However, it is unclear how cholesterol accumulates and informs TJ formation-and whether cholesterol enrichment precedes or follows the assembly of claudins in the first place. Here, we established an epithelial cell line (claudin-null cells) that lacks TJs by knocking out claudins. Despite the lack of TJs, cholesterol normally accumulated in the vicinity of the apical junctions. Assembly of claudins at TJs is thought to require binding to zonula occludens (ZO) proteins; however, a claudin mutant that cannot bind to ZO proteins still formed TJ strands. ZO proteins were however necessary for cholesterol accumulation at the apical junctions through their effect on the junctional actomyosin cytoskeleton. We propose that ZO proteins not only function as scaffolds for claudins but also promote TJ formation of cholesterol-rich membrane domains at apical junctions.

Phase separation of an actin nucleator by junctional microtubules regulates epithelial function

Liquid-liquid phase separation (LLPS) is involved in various dynamic biological phenomena. In epithelial cells, dynamic regulation of junctional actin filaments tethered to the apical junctional complex (AJC) is critical for maintaining internal homeostasis against external perturbations; however, the role of LLPS in this process remains unknown. Here, after identifying a multifunctional actin nucleator, cordon bleu (Cobl), as an AJC-enriched microtubule-associated protein, we conducted comprehensive in vitro and in vivo analyses. We found that apical microtubules promoted LLPS of Cobl at the AJC, and Cobl actin assembly activity increased upon LLPS. Thus, microtubules spatiotemporally regulated junctional actin assembly for epithelial morphogenesis and paracellular barriers. Collectively, these findings established that LLPS of the actin nucleator Cobl mediated dynamic microtubule-actin cross-talk in junctions, which fine-tuned the epithelial barrier.

EpCAM proteolysis and release of complexed claudin-7 repair and maintain the tight junction barrier

TJs maintain the epithelial barrier by regulating paracellular permeability. Since TJs are under dynamically fluctuating intercellular tension, cells must continuously survey and repair any damage. However, the underlying mechanisms allowing cells to sense TJ damage and repair the barrier are not yet fully understood. Here, we showed that proteinases play an important role in the maintenance of the epithelial barrier. At TJ break sites, EpCAM-claudin-7 complexes on the basolateral membrane become accessible to apical membrane-anchored serine proteinases (MASPs) and the MASPs cleave EpCAM. Biochemical data and imaging analysis suggest that claudin-7 released from EpCAM contributes to the rapid repair of damaged TJs. Knockout (KO) of MASPs drastically reduced barrier function and live-imaging of TJ permeability showed that MASPs-KO cells exhibited increased size, duration, and frequency of leaks. Together, our results reveal a novel mechanism of TJ maintenance through the localized proteolysis of EpCAM at TJ leaks, and provide a better understanding of the dynamic regulation of epithelial permeability.

Role of TM3 in claudin-15 strand flexibility: A molecular dynamics study

Claudins are cell-cell adhesion proteins within tight junctions that connect epithelial cells together. Claudins polymerize into a network of strand-like structures within the membrane of adjoining cells and create ion channels that control paracellular permeability to water and small molecules. Tight junction morphology and barrier function is tissue specific and regulated by claudin subtypes. Here, we present a molecular dynamics study of claudin-15 strands within lipid membranes and the role of a single-point mutation (A134P) on the third transmembrane helix (TM3) of claudin-15 in determining the morphology of the strand. Our results indicate that the A134P mutation significantly affects the lateral flexibility of the strands, increasing the persistence length of claudin-15 strands by a factor of three. Analyses of claudin-claudin contact in our μsecond-long trajectories show that the mutation does not alter the intermolecular contacts (interfaces) between claudins. However, the dynamics and frequency of interfacial contacts are significantly affected. The A134P mutation introduces a kink in TM3 of claudin-15 similar to the one observed in claudin-3 crystal structure. The kink on TM3 skews the rotational flexibility of the claudins in the strands and limits their fluctuation in one direction. This asymmetric movement in the context of the double rows reduces the lateral flexibility of the strand and leads to higher persistence lengths of the mutant.

Role of Cell-Cell Junctions in Oesophageal Squamous Cell Carcinoma

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

Effects of TAMP family on the tight junction strand network and barrier function in epithelial cells

Occludin, tricellulin, and marvelD3 belong to the tight junction (TJ)-associated MARVEL protein family. Occludin and tricellulin jointly contribute to TJ strand branching point formation and epithelial barrier maintenance. However, whether marvelD3 has the same function remains unclear. Furthermore, the roles of the carboxy-terminal cytoplasmic tail, which is conserved in occludin and tricellulin, on the regulation of TJ strand morphology have not yet been explored in epithelial cells. We established tricellulin/occludin/marveld3 triple-gene knockout (tKO) MDCK II cells and evaluated the roles of marvelD3 in the TJ strand structure and barrier function using MDCK II cells and a mathematical model. The complexity of TJ strand networks and paracellular barrier did not change in tKO cells compared to that in tricellulin/occludin double-gene knockout (dKO) cells. Exogenous marvelD3 expression in dKO cells did not increase the complexity of TJ strand networks and epithelial barrier tightness. The expression of the carboxy-terminal truncation mutant of tricellulin restored the barrier function in the dKO cells, whereas occludin lacking the carboxy-terminal cytoplasmic tail was not expressed on the plasma membrane. These data suggest that marvelD3 does not affect the morphology of TJ strands and barrier function in MDCK II cells and that the carboxy-terminal cytoplasmic tail of tricellulin is dispensable for barrier improvement.

Tight junction channel regulation by interclaudin interference

Tight junctions form selectively permeable seals across the paracellular space. Both barrier function and selective permeability have been attributed to members of the claudin protein family, which can be categorized as pore-forming or barrier-forming. Here, we show that claudin-4, a prototypic barrier-forming claudin, reduces paracellular permeability by a previously unrecognized mechanism. Claudin-4 knockout or overexpression has minimal effects on tight junction permeability in the absence of pore-forming claudins. However, claudin-4 selectively inhibits flux across cation channels formed by claudins 2 or 15. Claudin-4-induced loss of claudin channel function is accompanied by reduced anchoring and subsequent endocytosis of pore-forming claudins. Analyses in nonepithelial cells show that claudin-4, which is incapable of independent polymerization, disrupts polymeric strands and higher order meshworks formed by claudins 2, 7, 15, and 19. This process of interclaudin interference, in which one claudin disrupts higher order structures and channels formed by a different claudin, represents a previously unrecognized mechanism of barrier regulation.

Claudin18.2 is a novel molecular biomarker for tumor-targeted immunotherapy

The claudin18.2 (CLDN18.2) protein, an isoform of claudin18, a member of the tight junction protein family, is a highly selective biomarker with limited expression in normal tissues and often abnormal expression during the occurrence and development of various primary malignant tumors, such as gastric cancer/gastroesophageal junction (GC/GEJ) cancer, breast cancer, colon cancer, liver cancer, head and neck cancer, bronchial cancer and non-small-cell lung cancer. CLDN18.2 participates in the proliferation, differentiation and migration of tumor cells. Recent studies have identified CLDN18.2 expression as a potential specific marker for the diagnosis and treatment of these tumors. With its specific expression pattern, CLDN18.2 has become a unique molecule for targeted therapy in different cancers, especially in GC; for example, agents such as zolbetuximab (claudiximab, IMAB362), a monoclonal antibody (mAb) against CLDN18.2, have been developed. In this review, we outline recent advances in the development of immunotherapy strategies targeting CLDN18.2, including monoclonal antibodies (mAbs), bispecific antibodies (BsAbs), chimeric antigen receptor T (CAR-T) cells redirected to target CLDN18.2, and antibody–drug conjugates (ADCs).

BMAL1 Regulates the Daily Timing of Colitis

Many physiological functions exhibit circadian rhythms: oscillations in biological processes that occur in a 24-hour period. These daily rhythms are maintained through a highly conserved molecular pacemaker known as the circadian clock. Circadian disruption has been proposed to cause increased risk of Inflammatory Bowel Disease (IBD) but the underlying mechanisms remain unclear. Patients with IBD experience chronic inflammation and impaired regeneration of intestinal epithelial cells. Previous animal-based studies have revealed that colitis models of IBD are more severe in mice without a circadian clock but the timing of colitis, and whether its inflammatory and regenerative processes have daily rhythms, remains poorly characterized. We tested circadian disruption using Bmal1-/- mutant mice that have a non-functional circadian clock and thus no circadian rhythms. Dextran Sulfate Sodium (DSS) was used to induce colitis. The disease activity of colitis was found to exhibit time-dependent variation in Bmal1+/+ control mice but is constant and elevated in Bmal1-/- mutants, who exhibit poor recovery. Histological analyses indicate worsened colitis severity in Bmal1-/- mutant colon, and colon infiltration of immune system cells shows a daily rhythm that is lost in the Bmal1-/- mutant. Similarly, epithelial proliferation in the colon has a daily rhythm in Bmal1+/+ controls but not in Bmal1-/- mutants. Our results support a critical role of a functional circadian clock in the colon which drives 24-hour rhythms in inflammation and healing, and whose disruption impairs colitis recovery. This indicates that weakening circadian rhythms not only worsens colitis, but delays healing and should be taken into account in the management of IBD. Recognition of this is important in the management of IBD patients required to do shift work.

An Experimental Workflow for Studying Barrier Integrity, Permeability, and Tight Junction Composition and Localization in a Single Endothelial Cell Monolayer: Proof of Concept

Endothelial and epithelial barrier function is crucial for the maintenance of physiological processes. The barrier paracellular permeability depends on the composition and spatial distribution of the cell-to-cell tight junctions (TJ). Here, we provide an experimental workflow that yields several layers of physiological data in the setting of a single endothelial cell monolayer. Human umbilical vein endothelial cells were grown on Transwell filters. Transendothelial electrical resistance (TER) and 10 kDa FITC dextran flux were measured using Alanyl-Glutamine (AlaGln) as a paracellular barrier modulator. Single monolayers were immunolabelled for Zonula Occludens-1 (ZO-1) and Claudin-5 (CLDN5) and used for automated immunofluorescence imaging. Finally, the same monolayers were used for single molecule localization microscopy (SMLM) of ZO-1 and CLDN5 at the nanoscale for spatial clustering analysis. The TER increased and the paracellular dextran flux decreased after the application of AlaGln and these functional changes of the monolayer were mediated by an increase in the ZO-1 and CLDN5 abundance in the cell–cell interface. At the nanoscale level, the functional and protein abundance data were accompanied by non-random increased clustering of CLDN5. Our experimental workflow provides multiple data from a single monolayer and has wide applicability in the setting of paracellular studies in endothelia and epithelia.

Increased integrity of cell-cell junctions accompanied by increased expression of claudin 4 in keratinocytes stimulated with vitamin D3

The stratified squamous epithelium has a multilayer structure formed by the differentiation of the keratinized epithelium, which covers the skin and oral mucosa. The epithelium plays a central role in regulating the interactions between the immune system and pathogens. The tight junction (TJ) barrier, which is composed of adhesion molecules called claudins (CLDN), is critical for the homeostasis of the skin and oral mucosa. Furthermore, the crucial roles of vitamin D3 (VD3) in the pathogeneses of skin and oral mucosal disease have been suggested. The aim of this in vitro study was to observe the correlations between the integrity of the keratinocyte population and the expression levels of CLDN1 and CLDN4 in gingival epithelial cells, stimulated with VD3. CLDN 1 and 4 expression levels were down and upregulated, respectively, in the cells stimulated with VD3. Additionally, transepithelial electrical resistance (TEER) levels were increased in the stimulated cells when compared to the controls. These findings indicate that CLDN 4 may play a more important role in the TJ barrier than CLDN 1. Hence, the therapeutic effect of VD3 in skin and oral diseases may be regulated by the increase in the expression of CLDN 4.

Context-Dependent Roles of Claudins in Tumorigenesis

The barrier and fence functions of the claudin protein family are fundamental to tissue integrity and human health. Increasing evidence has linked claudins to signal transduction and tumorigenesis. The expression of claudins is frequently dysregulated in the context of neoplastic transformation. Studies have uncovered that claudins engage in nearly all aspects of tumor biology and steps of tumor development, suggesting their promise as targets for treatment or biomarkers for diagnosis and prognosis. However, claudins can be either tumor promoters or tumor suppressors depending on the context, which emphasizes the importance of taking various factors, including organ type, environmental context and genetic confounders, into account when studying the biological functions and targeting of claudins in cancer. This review discusses the complicated roles and intrinsic and extrinsic determinants of the context-specific effects of claudins in cancer.

Brain endothelial tricellular junctions as novel sites for T cell diapedesis across the blood-brain barrier

The migration of activated T cells across the blood–brain barrier (BBB) is a critical step in central nervous system (CNS) immune surveillance and inflammation. Whereas T cell diapedesis across the intact BBB seems to occur preferentially through the BBB cellular junctions, impaired BBB integrity during neuroinflammation is accompanied by increased transcellular T cell diapedesis. The underlying mechanisms directing T cells to paracellular versus transcellular sites of diapedesis across the BBB remain to be explored. By combining in vitro live-cell imaging of T cell migration across primary mouse brain microvascular endothelial cells (pMBMECs) under physiological flow with serial block-face scanning electron microscopy (SBF-SEM), we have identified BBB tricellular junctions as novel sites for T cell diapedesis across the BBB. Downregulated expression of tricellular junctional proteins or protein-based targeting of their interactions in pMBMEC monolayers correlated with enhanced transcellular T cell diapedesis, and abluminal presence of chemokines increased T cell diapedesis through tricellular junctions. Our observations assign an entirely novel role to BBB tricellular junctions in regulating T cell entry into the CNS.

This article has an associated First Person interview with the first author of the paper.

Highlighted Article: Ultrastructural analysis of T cell migration across the blood–brain barrier (BBB) under physiological flow identifies BBB tricellular junctions as sites of T cell diapedesis.

Target specific tight junction modulators

Intercellular tight junctions represent a formidable barrier against paracellular drug absorption at epithelia (e.g., nasal, intestinal) and the endothelium (e.g., blood-brain barrier). In order to enhance paracellular transport of drugs and increase their bioavailability and organ deposition, active excipients modulating tight junctions have been applied. First-generation of permeation enhancers (PEs) acted by unspecific interactions, while recently developed PEs address specific physiological mechanisms. Such target specific tight junction modulators (TJMs) have the advantage of a defined specific mechanism of action. To date, merely a few of these novel active excipients has entered into clinical trials, as their lack in safety and efficiency in vivo often impedes their commercialisation. A stronger focus on the development of such active excipients would result in an economic and therapeutic improvement of current and future drugs.

Molecular architecture and assembly of the tight junction backbone

The functional and structural concept of tight junctions has developed after discovery of claudin and TAMP proteins. Many of these proteins contribute to epi- and endothelial barrier but some, in contrast, form paracellular channels. Claudins form the backbone of tight junction (TJ) strands whereas other proteins regulate TJ dynamics. The current joined double-row model of TJ strands and channels is crucially based on the linear alignment of claudin-15 in the crystal. Molecular dynamics simulations, protein docking, mutagenesis, cellular TJ reconstitution, and electron microscopy studies largely support stability and functionality of the model. Here, we summarize in silico and in vitro data about TJ strand assembly including comparison of claudin crystal structures and alternative models. Sequence comparisons, experimental and structural data substantiate differentiation of classic and non-classic claudins differing in motifs related to strand assembly. Classic claudins seem to share a similar mechanism of strand formation. Interface variations likely contribute to TJ strand flexibility. Combined in vitro/in silico studies are expected to elucidate mechanistic keys determining TJ regulation.

Contributions of Myosin Light Chain Kinase to Regulation of Epithelial Paracellular Permeability and Mucosal Homeostasis

Intestinal barrier function is required for the maintenance of mucosal homeostasis. Barrier dysfunction is thought to promote progression of both intestinal and systemic diseases. In many cases, this barrier loss reflects increased permeability of the paracellular tight junction as a consequence of myosin light chain kinase (MLCK) activation and myosin II regulatory light chain (MLC) phosphorylation. Although some details about MLCK activation remain to be defined, it is clear that this triggers perijunctional actomyosin ring (PAMR) contraction that leads to molecular reorganization of tight junction structure and composition, including occludin endocytosis. In disease states, this process can be triggered by pro-inflammatory cytokines including tumor necrosis factor-α (TNF), interleukin-1β (IL-1β), and several related molecules. Of these, TNF has been studied in the greatest detail and is known to activate long MLCK transcription, expression, enzymatic activity, and recruitment to the PAMR. Unfortunately, toxicities associated with inhibition of MLCK expression or enzymatic activity make these unsuitable as therapeutic targets. Recent work has, however, identified a small molecule that prevents MLCK1 recruitment to the PAMR without inhibiting enzymatic function. This small molecule, termed Divertin, restores barrier function after TNF-induced barrier loss and prevents disease progression in experimental chronic inflammatory bowel disease.

Role of Claudin Proteins in Regulating Cancer Stem Cells and Chemoresistance-Potential Implication in Disease Prognosis and Therapy

Claudins are cell–cell adhesion proteins, which are expressed in tight junctions (TJs), the most common apical cell-cell adhesion. Claudin proteins help to regulate defense and barrier functions, as well as differentiation and polarity in epithelial and endothelial cells. A series of studies have now reported dysregulation of claudin proteins in cancers. However, the precise mechanisms are still not well understood. Nonetheless, studies have clearly demonstrated a causal role of multiple claudins in the regulation of epithelial to mesenchymal transition (EMT), a key feature in the acquisition of a cancer stem cell phenotype in cancer cells. In addition, claudin proteins are known to modulate therapy resistance in cancer cells, a feature associated with cancer stem cells. In this review, we have focused primarily on highlighting the causal link between claudins, cancer stem cells, and therapy resistance. We have also contemplated the significance of claudins as novel targets in improving the efficacy of cancer therapy. Overall, this review provides a much-needed understanding of the emerging role of claudin proteins in cancer malignancy and therapeutic management.

Multiscale dynamics of tight junction remodeling

Epithelial cells form tissues that generate biological barriers in the body. Tight junctions (TJs) are responsible for maintaining a selectively permeable seal between epithelial cells, but little is known about how TJs dynamically remodel in response to physiological forces that challenge epithelial barrier function, such as cell shape changes (e.g. during cell division) or tissue stretching (e.g. during developmental morphogenesis). In this Review, we first introduce a framework to think about TJ remodeling across multiple scales: from molecular dynamics, to strand dynamics, to cell- and tissue-scale dynamics. We then relate knowledge gained from global perturbations of TJs to emerging information about local TJ remodeling events, where transient localized Rho activation and actomyosin-mediated contraction promote TJ remodeling to repair local leaks in barrier function. We conclude by identifying emerging areas in the field and propose ideas for future studies that address unanswered questions about the mechanisms that drive TJ remodeling.

Potential for Tight Junction Protein-Directed Drug Development Using Claudin Binders and Angubindin-1

The tight junction (TJ) is an intercellular sealing component found in epithelial and endothelial tissues that regulates the passage of solutes across the paracellular space. Research examining the biology of TJs has revealed that they are complex biochemical structures constructed from a range of proteins including claudins, occludin, tricellulin, angulins and junctional adhesion molecules. The transient disruption of the barrier function of TJs to open the paracellular space is one means of enhancing mucosal and transdermal drug absorption and to deliver drugs across the blood–brain barrier. However, the disruption of TJs can also open the paracellular space to harmful xenobiotics and pathogens. To address this issue, the strategies targeting TJ proteins have been developed to loosen TJs in a size- or tissue-dependent manner rather than to disrupt them. As several TJ proteins are overexpressed in malignant tumors and in the inflamed intestinal tract, and are present in cells and epithelia conjoined with the mucosa-associated lymphoid immune tissue, these TJ-protein-targeted strategies may also provide platforms for the development of novel therapies and vaccines. Here, this paper reviews two TJ-protein-targeted technologies, claudin binders and an angulin binder, and their applications in drug development.

Morphologic determinant of tight junctions revealed by claudin-3 structures

Tight junction is a cell adhesion apparatus functioning as barrier and/or channel in the paracellular spaces of epithelia. Claudin is the major component of tight junction and polymerizes to form tight junction strands with various morphologies that may correlate with their functions. Here we present the crystal structure of mammalian claudin-3 at 3.6 Å resolution. The third transmembrane helix of claudin-3 is clearly bent compared with that of other subtypes. Structural analysis of additional two mutants with a single mutation representing other subtypes in the third helix indicates that this helix takes a bent or straight structure depending on the residue. The presence or absence of the helix bending changes the positions of residues related to claudin-claudin interactions and affects the morphology and adhesiveness of the tight junction strands. These results evoke a model for tight junction strand formation with different morphologies – straight or curvy strands – observed in native epithelia.

The main components of tight junctions (TJ) are claudins that polymerize and form meshwork architectures called TJ strands. Here the authors present the 3.6 Å crystal structure of murine claudin-3 and show that residue P134 causes a bending of the third transmembrane helix which affects the morphology and adhesiveness of the TJ strands.

Dynamic modeling of the tight junction pore pathway

Claudins define paracellular permeability to small molecules by forming ion-selective pores within the tight junction. We recently demonstrated that claudin-2 channels are gated and open and close on a submillisecond timescale. To determine if and how the ensemble behavior of this unique class of entirely extracellular gated ion channels could define global epithelial barrier function, we have developed an in silico model of local claudin-2 behavior. This model considers the complex anastomosing ultrastructure of tight junction strands and can be scaled to show that local behavior defines global epithelial barrier function of epithelial monolayers expressing different levels of claudin-2. This is the first mathematical model to describe global epithelial barrier function in terms of the dynamic behavior of single tight junction channels and establishes a framework to consider gating kinetics as a means to regulate barrier function.

The mucosal barrier at a glance

Mucosal barriers separate self from non-self and are essential for life. These barriers, which are the first line of defense against external pathogens, are formed by epithelial cells and the substances they secrete. Rather than an absolute barrier, epithelia at mucosal surfaces must allow selective paracellular flux that discriminates between solutes and water while preventing the passage of bacteria and toxins. In vertebrates, tight junctions seal the paracellular space; flux across the tight junction can occur through two distinct routes that differ in selectivity, capacity, molecular composition and regulation. Dysregulation of either pathway can accompany disease. A third, tight-junction-independent route that reflects epithelial damage can also contribute to barrier loss during disease. In this Cell Science at a Glance article and accompanying poster, we present current knowledge on the molecular components and pathways that establish this selectively permeable barrier and the interactions that lead to barrier dysfunction during disease.

Dose-dependent role of claudin-1 in vivo in orchestrating features of atopic dermatitis

Atopic dermatitis (AD) is a chronic inflammatory skin disease in humans. It was recently noted that the characteristics of epidermal barrier functions critically influence the pathological features of AD. Evidence suggests that claudin-1 (CLDN1), a major component of tight junctions (TJs) in the epidermis, plays a key role in human AD, but the mechanism underlying this role is poorly understood. One of the main challenges in studying CLDN1's effects is that Cldn1 knock-out mice cannot survive beyond 1 d after birth, due to lethal dehydration. Here, we established a series of mouse lines that express Cldn1 at various levels and used these mice to study Cldn1's effects in vivo. Notably, we discovered a dose-dependent effect of Cldn1's expression in orchestrating features of AD. In our experimental model, epithelial barrier functions and morphological changes in the skin varied exponentially with the decrease in Cldn1 expression level. At low Cldn1 expression levels, mice exhibited morphological features of AD and an innate immune response that included neutrophil and macrophage recruitment to the skin. These phenotypes were especially apparent in the infant stages and lessened as the mice became adults, depending on the expression level of Cldn1 Still, these adult mice with improved phenotypes showed an enhanced hapten-induced contact hypersensitivity response compared with WT mice. Furthermore, we revealed a relationship between macrophage recruitment and CLDN1 levels in human AD patients. Our findings collectively suggest that CLDN1 regulates the pathogenesis, severity, and natural course of human AD.

Claudin-21 Has a Paracellular Channel Role at Tight Junctions

Claudin protein family members, of which there are at least 27 in humans and mice, polymerize to form tight junctions (TJs) between epithelial cells, in a tissue- and developmental stage-specific manner. Claudins have a paracellular barrier function. In addition, certain claudins function as paracellular channels for small ions and/or solutes by forming selective pores at the TJs, although the specific claudins involved and their functional mechanisms are still in question. Here we show for the first time that claudin-21, which is more highly expressed in the embryonic than the postnatal stages, acts as a paracellular channel for small cations, such as Na(+), similar to the typical channel-type claudins claudin-2 and -15. Claudin-21 also allows the paracellular passage of larger solutes. Our findings suggest that claudin-21-based TJs allow the passage of small and larger solutes by both paracellular channel-based and some additional mechanisms.

Intestinal deletion of Claudin-7 enhances paracellular organic solute flux and initiates colonic inflammation in mice

OBJECTIVE

To design novel anti-inflammation treatments, it is important to recognise two distinct steps of inflammation: initiation and acceleration. In IBDs, intestinal inflammation is reported to be accelerated by dysfunction in the epithelial paracellular barrier formed by tight junctions (TJs). However, it is unclear whether changes in paracellular barrier function initiate inflammation. Some of the intestinal claudin-family proteins, which form the paracellular barrier, show aberrant expression levels and localisations in IBDs. We aimed to elucidate the role of paracellular-barrier change in initiating colonic inflammation.

DESIGN

We generated intestine-specific conditional knockout mice of claudin-7 (Cldn7), one of the predominant intestinal claudins.

RESULTS

The intestine-specific Cldn7 deficiency caused colonic inflammation, even though TJ structures were still present due to other claudins. The paracellular flux (pFlux), determined by measuring the paracellular permeability across the colon epithelium, was enhanced by the Cldn7 deficiency for the small organic solute Lucifer Yellow (457 Da), but not for the larger organic solute FITC-Dextran (4400 Da). Consistent with these results, the intestine-specific claudin-7 deficiency enhanced the pFlux for N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP) (438 Da), a major bacterial product, to initiate colonic inflammation.

CONCLUSIONS

These findings suggest that specific enhancement of the pFlux for small organic solutes across the claudin-based TJs initiates colonic inflammation.

The importance of claudin-7 palmitoylation on membrane subdomain localization and metastasis-promoting activities

BACKGROUND

Claudin-7 (cld7), a tight junction (TJ) component, is also found basolaterally and in the cytoplasm. Basolaterally located cld7 is enriched in glycolipid-enriched membrane domains (GEM), where it associates with EpCAM (EpC). The conditions driving cld7 out of TJ into GEM, which is associated with a striking change in function, were not defined. Thus, we asked whether cld7 serines or palmitoylation affect cld7 location and protein, particularly EpCAM, associations.

RESULTS

HEK cells were transfected with EpCAM and wild type cld7 or cld7, where serine phopsphorylation or the palmitoylation sites (AA184, AA186) (cld7(mPalm)) were mutated. Exchange of individual serine phosphorylation sites did not significantly affect the GEM localization and the EpCAM association. Instead, cld7(mPalm) was poorly recruited into GEM. This has consequences on migration and invasiveness as palmitoylated cld7 facilitates integrin and EpCAM recruitment, associates with cytoskeletal linker proteins and cooperates with MMP14, CD147 and TACE, which support motility, matrix degradation and EpCAM cleavage. On the other hand, only cld7(mPalm) associates with TJ proteins.

CONCLUSION

Cld7 palmitoylation prohibits TJ integration and fosters GEM recruitment. Via associated molecules, palmitoylated cld7 supports motility and invasion.

Claudin switching: Physiological plasticity of the Tight Junction

Tight Junctions (TJs) are multi-molecular complexes in epithelial tissues that regulate paracellular permeability. Within the TJ complex, claudins proteins span the paracellular space to form a seal between adjacent cells. This seal allows regulated passage of ions, fluids, and solutes, contingent upon the complement of claudins expressed. With as many as 27 claudins in the human genome, the TJ seal is complex indeed. This review focuses on changes in claudin expression within the epithelial cells of the gastrointestinal tract, where claudin differentiation results in several physiologically distinct TJs within the lifetime of the cell. We also review mechanistic studies revealing that TJs are highly dynamic, with the potential to undergo molecular remodeling while structurally intact. Therefore, physiologic Tight Junction plasticity involves both the adaptability of claudin expression and gene specific retention in the TJ; a process we term claudin switching.

Conceptual barriers to understanding physical barriers

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

Claudin-2 knockout by TALEN-mediated gene targeting in MDCK cells: claudin-2 independently determines the leaky property of tight junctions in MDCK cells

Tight junctions (TJs) regulate the movements of substances through the paracellular pathway, and claudins are major determinants of TJ permeability. Claudin-2 forms high conductive cation pores in TJs. The suppression of claudin-2 expression by RNA interference in Madin-Darby canine kidney (MDCK) II cells (a low-resistance strain of MDCK cells) was shown to induce a three-fold increase in transepithelial electrical resistance (TER), which, however, was still lower than in high-resistance strains of MDCK cells. Because RNA interference-mediated knockdown is not complete and only reduces gene function, we considered the possibility that the remaining claudin-2 expression in the knockdown study caused the lower TER in claudin-2 knockdown cells. Therefore, we investigated the effects of claudin-2 knockout in MDCK II cells by establishing claudin-2 knockout clones using transcription activator-like effector nucleases (TALENs), a recently developed genome editing method for gene knockout. Surprisingly, claudin-2 knockout increased TER by more than 50-fold in MDCK II cells, and TER values in these cells (3000–4000 Ω·cm²) were comparable to those in the high-resistance strains of MDCK cells. Claudin-2 re-expression restored the TER of claudin-2 knockout cells dependent upon claudin-2 protein levels. In addition, we investigated the localization of claudin-1, -2, -3, -4, and -7 at TJs between control MDCK cells and their respective knockout cells using their TALENs. Claudin-2 and -7 were less efficiently localized at TJs between control and their knockout cells. Our results indicate that claudin-2 independently determines the ‘leaky’ property of TJs in MDCK II cells and suggest the importance of knockout analysis in cultured cells.

Tight junctions. Structural insight into tight junction disassembly by Clostridium perfringens enterotoxin

The C-terminal region of Clostridium perfringens enterotoxin (C-CPE) can bind to specific claudins, resulting in the disintegration of tight junctions (TJs) and an increase in the paracellular permeability across epithelial cell sheets. Here we present the structure of mammalian claudin-19 in complex with C-CPE at 3.7 Å resolution. The structure shows that C-CPE forms extensive hydrophobic and hydrophilic interactions with the two extracellular segments of claudin-19. The claudin-19/C-CPE complex shows no density of a short extracellular helix that is critical for claudins to assemble into TJ strands. The helix displacement may thus underlie C-CPE-mediated disassembly of TJs.

Differential expression of claudin family members during osteoblast and osteoclast differentiation: Cldn-1 is a novel positive regulator of osteoblastogenesis

Claudins (Cldns), a family of 27 transmembrane proteins, represent major components of tight junctions. Aside from functioning as tight junctions, Cldns have emerging roles as regulators of cell proliferation and differentiation. While Cldns are known to be expressed and have important functions in various tissues, their expression and function in bone cells is ill-defined. In this study, the expression of Cldns was examined during osteoblast and osteoclast differentiation. The expression of Cldn-1, -7, -11, and -15 was downregulated during early stages of osteoclast differentiation, whereas Cldn-6 was upregulated. Moreover, the expression of several Cldns increased 3–7 fold in fully differentiated osteoclasts. As for osteoblasts, the expression of several Cldns was found to increase more than 10-fold during differentiation, with some peaking at early, and others at late stages. By contrast, only expression of Cldn-12, and -15 decreased during osteoblast differentiation. In subsequent studies, we focused on the role of Cldn-1 in osteoblasts as its expression was increased by more than 10 fold during osteoblast differentiation and was found to be regulated by multiple osteoregulatory agents including IGF-1 and Wnt3a. We evaluated the consequence of lentiviral shRNA-mediated knockdown of Cldn-1 on osteoblast proliferation and differentiation using MC3T3-E1 mouse osteoblasts. Cldn-1 knockdown caused a significant reduction in MC3T3-E1 cell proliferation and ALP activity. Accordingly, expression levels of cyclinD1 and ALP mRNA levels were reduced in Cldn-1 shRNA knockdown cells. We next determined if Cldn-1 regulates the expression of Runx-2 and osterix, master transcription factors of osteoblast differentiation, and found that their levels were reduced significantly as a consequence of Cldn-1 knockdown. Moreover, knocking down Cldn-1 reduced β-catenin level. In conclusion, the expression of Cldn family members during bone cell differentiation is complex and involves cell type and differentiation stage-dependent regulation. In addition, Cldn-1 is a positive regulator of osteoblast proliferation and differentiation.

Architecture of tight junctions and principles of molecular composition

The tight junction creates an intercellular barrier limiting paracellular movement of solutes and material across epithelia. Currently many proteins have been identified as components of the tight junction and understanding their architectural organization and interactions is critical to understanding the biology of the barrier. In general the architecture can be conceptualized into compartments with the transmembrane barrier proteins (claudins, occludin, JAM-A, etc.), linked to peripheral scaffolding proteins (such as ZO-1, afadin, MAGI1, etc.) which are in turned linked to actin and microtubules through numerous linkers (cingulin, myosins, protein 4.1, etc.). Within this complex network are associated many signaling proteins that affect the barrier and broader cell functions. The PDZ domain is a commonly used motif to specifically link individual junction protein pairs. Here we review some of the key proteins defining the tight junction and general themes of their organization with the perspective that much will be learned about function by characterizing the detailed architecture and subcompartments within the junction.

Proinflammatory cytokine-induced tight junction remodeling through dynamic self-assembly of claudins

Epithelial barriers are vital components of the innate immune system. This barrier is provided by tight junctions and compromised by proinflammatory cytokine signaling. Study of claudin 4 live-cell protein dynamics shows that tight junctions are self-assembling systems that undergo remodeling through heterotypic claudin incompatibility.

Tight junctions (TJs) are dynamic, multiprotein intercellular adhesive contacts that provide a vital barrier function in epithelial tissues. TJs are remodeled during physiological development and pathological mucosal inflammation, and differential expression of the claudin family of TJ proteins determines epithelial barrier properties. However, the molecular mechanisms involved in TJ remodeling are incompletely understood. Using acGFP-claudin 4 as a biosensor of TJ remodeling, we observed increased claudin 4 fluorescence recovery after photobleaching (FRAP) dynamics in response to inflammatory cytokines. Interferon γ and tumor necrosis factor α increased the proportion of mobile claudin 4 in the TJ. Up-regulation of claudin 4 protein rescued these mobility defects and cytokine-induced barrier compromise. Furthermore, claudins 2 and 4 have reciprocal effects on epithelial barrier function, exhibit differential FRAP dynamics, and compete for residency within the TJ. These findings establish a model of TJs as self-assembling systems that undergo remodeling in response to proinflammatory cytokines through a mechanism of heterotypic claudin-binding incompatibility.

Oxidative stress induces claudin-2 nitration in experimental type 1 diabetic nephropathy

Renal complications in diabetes are severe and may lead to renal insufficiency. Early alterations in tight junction (TJ) proteins in diabetic nephropathy (DN) have not been explored and the role of oxidative stress in their disassembly has been poorly characterized. We investigated the expression and distribution of TJ proteins: claudin-5 in glomeruli (GL), occludin and claudin-2 in proximal tubules (PTs), and ZO-1 and claudin-1, -4, and -8 in distal tubules (DTs) of rats 21 days after streptozotocin injection. Redox status along the nephron segments was evaluated. Diabetes increased kidney injury molecule-1 expression. Expression of sodium glucose cotransporters (SGLT1 and SGLT2) and facilitative glucose transporter (GLUT2) was induced. Increased oxidative stress was present in GL and PTs and to a lesser extent in DTs (measured by superoxide production and PKCβ2 expression), owing to NADPH oxidase activation and uncoupling of the endothelial nitric oxide synthase-dependent pathway. Claudin-5, occludin, and claudin-2 expression was decreased, whereas claudin-4 and -8 expression increased. ZO-1 was redistributed from membrane to cytosol. Increased nitration of tyrosine residues in claudin-2 was found, which might contribute to decrement of this protein in proximal tubule. In contrast, occludin was not nitrated. We suggest that loss of claudin-2 is associated with increased natriuresis and that loss of glomerular claudin-5 might explain early presence of proteinuria. These findings suggest that oxidative stress is related to alterations in TJ proteins in the kidney that are relevant to the pathogenesis and progression of DN and for altered sodium regulation in diabetes.

Ligand-dependent stabilization of androgen receptor in a novel mouse ST38c Sertoli cell line

Mature Sertoli cells (SC) are critical mediators of androgen regulation of spermatogenesis, via the androgen receptor (AR) signaling. Available immortalized SC lines loose AR expression or androgen responsiveness, hampering the study of endogenous AR regulation in SC. We have established and characterized a novel clonal mouse immortalized SC line, ST38c. These cells express some SC specific genes (sox9, wt1, tjp1, clu, abp, inhbb), but not fshr, yet more importantly, maintain substantial expression of endogenous AR as determined by PCR, immunocytochemistry, testosterone binding assays and Western blots. Microarrays allowed identification of some (146) but not all (rhox5, spinlw1), androgen-dependent, SC expressed target genes. Quantitative Real-Time PCR validated regulation of five up-regulated and two down-regulated genes. We show that AR undergoes androgen-dependent transcriptional activation as well as agonist-dependent posttranslational stabilization in ST38c cells. This cell line constitutes a useful experimental tool for future investigations on the molecular and cellular mechanisms of androgen receptor signaling in SC function.

HGF signaling regulates Claudin-3 dynamics through its C-terminal tyrosine residues

The hormone HGF regulates morphogenesis and regeneration of multiple organs and increased HGF signaling is strongly associated with metastatic cancer. At the cellular level, one of the distinct effects of HGF is the de-stabilization of cell-cell junctions. Several molecular mechanisms have been shown to be involved that mostly culminate at the E-cadherin adhesion complex. One of the key determinants in HGF-driven morphological changes is the actomyosin cytoskeleton whose organization and physical parameters changes upon stimulation. Here we have investigated how HGF affects the different actomyosin-associated cell-cell junction complexes, Nectin Junctions, Adherens Junctions and Tight Junctions in MDCK cells. We find that components of all complexes stay present at cell-cell contacts until their physical dissociation. We find that at cell-cell junctions, the mobility of Claudin-3, but not that of other cell-cell adhesion receptors, is affected by HGF. This depends on tyrosine residues that likely affect PDZ-domain interactions at the C-terminal tail of Claudin-3, although their phosphorylation is not directly regulated by HGF. Thus we uncovered Claudins as novel targets of HGF signaling at cell-cell junctions.

The four-transmembrane protein IP39 of Euglena forms strands by a trimeric unit repeat

Euglenoid flagellates have striped surface structures comprising pellicles, which allow the cell shape to vary from rigid to flexible during the characteristic movement of the flagellates. In Euglena gracilis, the pellicular strip membranes are covered with paracrystalline arrays of a major integral membrane protein, IP39, a putative four-membrane-spanning protein with the conserved sequence motif of the PMP-22/EMP/MP20/Claudin superfamily. Here we report the three-dimensional structure of Euglena IP39 determined by electron crystallography. Two-dimensional crystals of IP39 appear to form a striated pattern of antiparallel double-rows in which trimeric IP39 units are longitudinally polymerised, resulting in continuously extending zigzag-shaped lines. Structural analysis revealed an asymmetric molecular arrangement in the trimer, and suggested that at least four different interactions between neighbouring protomers are involved. A combination of such multiple interactions would be important for linear strand formation of membrane proteins in a lipid bilayer.

CK2-dependent phosphorylation of occludin regulates the interaction with ZO-proteins and tight junction integrity

Background

Casein kinase 2 (CK2) is a ubiquitously expressed Ser/Thr kinase with multiple functions in the regulation of cell proliferation and transformation. In targeting adherens and tight junctions (TJs), CK2 modulates the strength and dynamics of epithelial cell-cell contacts. Occludin previously was identified as a substrate of CK2, however the functional consequences of CK2-dependent occludin phosphorylation on TJ function were unknown.

Results

Here, we present evidence that phosphorylation of a Thr400-XXX-Thr404-XXX-Ser408 motif in the C-terminal cytoplasmic tail of human occludin regulates assembly/disassembly and barrier properties of TJs. In contrast to wildtype and T400A/T404A/S408A-mutated occludin, a phospho-mimetic Occ-T400E/T404E/S408E construct was impaired in binding to ZO-2. Interestingly, pre-phosphorylation of a GST-Occ C-terminal domain fusion protein attenuated binding to ZO-2, whereas, binding to ZO-1 was not affected. Moreover, Occ-T400E/T404E/S408E showed delayed reassembly into TJs in Ca²⁺-switch experiments. Stable expression of Occ-T400E/T404E/S408E in MDCK C11 cells augments barrier properties in enhancing paracellular resistance in two-path impedance spectroscopy, whereas expression of wildtype and Occ-T400A/T404A/S408A did not affect transepithelial resistance.

Conclusions

These results suggest an important role of CK2 in epithelial tight junction regulation. The occludin sequence motif at amino acids 400–408 apparently represents a hotspot for Ser/Thr-kinase phosphorylation and depending on the residue(s) which are phosphorylated it differentially modulates the functional properties of the TJ.

Claudin interactions in and out of the tight junction

Claudins form the paracellular tight junction seal in epithelial tissues. Although there is still limited information on how these proteins are organized at the junction, a number of recent studies have provided useful insights both into claudin-claudin interactions and into interactions between claudins and other proteins. The focus of this review is to summarize recent information about claudin interactions and to identify critical unanswered questions about claudin organization and tight junction structure which will be required to understand claudin function.

Rab3Gap1 mediates exocytosis of Claudin-1 and tight junction formation during epidermal barrier acquisition

Epidermal barrier acquisition during late murine gestation is accompanied by an increase in Akt kinase activity and cJun dephosphorlyation. The latter is directed by the Ppp2r2a regulatory subunit of the Pp2a phosphatase. This was accompanied by a change of Claudin-1 localisation to the cell surface and interaction between Occludin and Claudin-1 which are thought to be required for tight junction formation.

The aim of this study was to determine the nature of the barrier defect caused by the loss of AKT/Ppp2r2a function. There was a paracellular barrier defect in rat epidermal keratinocytes expressing a Ppp2r2a siRNA. In Ppp2r2a knockdown cells, Claudin-1 was located to the cytoplasm and its expression was increased. Inhibiting cJun phosphorylation restored barrier function and plasma membrane localisation of Claudin-1. Expression of the Rab3 GTPase activating protein, Rab3Gap1, was restored in Ppp2r2a siRNA cells when cJun phosphorylation was inhibited. During normal mouse epidermal development, Claudin-1 plasma membrane localisation and Rab3Gap1 cell surface expression were co-incident with Akt activation in mouse epidermis, strongly suggesting a role of Rab3Gap1 in epidermal barrier acquisition. Supporting this hypothesis, siRNA knockdown of Rab3Gap1 prevented plasma membrane Claudin-1 expression and the formation of a barrier competent epithelium. Replacing Rab3Gap1 in Ppp2r2a knockdown cells was sufficient to rescue Claudin-1 transport to the cell surface. Therefore these data suggest Rab3Gap1 mediated exocytosis of Claudin-1 is an important component of epidermal barrier acquisition during epidermal development.

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Barrier acquisition correlates with Ppp2r2a and cell surface Claudin-1 expression.

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Ppp2r2a knockdown results in a paracellular barrier defect.

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Ppp2r2a knockdown prevents cell-surface claudin-1 expression in a c-Jun dependent fashion.

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Barrier rescue by inhibition of c-Jun phosphorylation involves exocytosis and Rab3Gap1.

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Rab3Gap1 is induced during barrier acquisition and is necessary for cell surface claudin-1.

Epithelial cell adhesion molecule (EpCAM) regulates claudin dynamics and tight junctions

Epithelial cell adhesion molecule (EpCAM) (CD326) is a surface glycoprotein expressed by invasive carcinomas and some epithelia. Herein, we report that EpCAM regulates the composition and function of tight junctions (TJ). EpCAM accumulated on the lateral interfaces of human colon carcinoma and normal intestinal epithelial cells but did not co-localize with TJ. Knockdown of EpCAM in T84 and Caco-2 cells using shRNAs led to changes in morphology and adhesiveness. TJ formed readily after EpCAM knockdown; the acquisition of trans-epithelial electroresistance was enhanced, and TJ showed increased resistance to disruption by calcium chelation. Preparative immunoprecipitation demonstrated that EpCAM bound tightly to claudin-7. Co-immunoprecipitation documented associations of EpCAM with claudin-7 and claudin-1 but not claudin-2 or claudin-4. Claudin-1 associated with claudin-7 in co-transfection experiments, and claudin-7 was required for association of claudin-1 with EpCAM. EpCAM knockdown resulted in decreases in claudin-7 and claudin-1 proteins that were reversed with lysosome inhibitors. Immunofluorescence microscopy revealed that claudin-7 and claudin-1 continually trafficked into lysosomes. Although EpCAM knockdown decreased claudin-1 and claudin-7 protein levels overall, accumulations of claudin-1 and claudin-7 in TJ increased. Physical interactions between EpCAM and claudins were required for claudin stabilization. These findings suggest that EpCAM modulates adhesion and TJ function by regulating intracellular localization and degradation of selected claudins.