Regulated expression of claudin-4 decreases paracellular conductance through a selective decrease in sodium permeability

Claudin-4 knockout by TALEN-mediated gene targeting in MDCK cells: Claudin-4 is dispensable for the permeability properties of tight junctions in wild-type MDCK cells

Epithelia act as a barrier between the internal and external environments, and the movement of substances via the paracellular pathway is regulated by tight junctions (TJs). Claudins are major determinants of TJ permeability. Claudin-4 was the first claudin whose involvement in the TJ permeability in cultured cells was directly demonstrated, but the permeability properties of individual claudins including claudin-4 are still incompletely clarified. In this study, we established claudin-4 knockout cells using transcription activator-like effector nucleases (TALENs), a recently developed method for genome editing, and investigated the permeability property of claudin-4 in MDCK II cells. We found that claudin-4 knockout has no apparent effect on the localization of other claudins and electrophysiological properties in MDCK II cells. Therefore we further established claudin-2 and claudin-4 double knockout clones and investigated the effects on TJs. Claudin-4 knockout in addition to claudin-2 knockout slightly increased the localization of other claudins at TJs but showed no obvious effects on the electrophysiological properties in MDCK II cells. These results indicate that claudin-4 is dispensable for the barrier property of TJs in wild-type as well as claudin-2 knockout MDCK II cells. Our results suggest the need for further knockout analysis to reveal the permeability properties of individual claudins.

Lactoferrin protects against intestinal inflammation and bacteria-induced barrier dysfunction in vitro

The iron-binding glycoprotein lactoferrin (LF) is naturally present in human breast milk. Several studies suggest that LF contributes to infant health and development owing to a variety of protective effects, including antimicrobial and anti-inflammatory features. Therefore, we aimed to elucidate its protective properties on intestinal epithelial barrier dysfunction induced by infection or inflammation using the human epithelial cell culture models HT-29/B6 and T84. During barrier perturbation induced by the proinflammatory cytokine tumor necrosis factor α (TNF-α), bovine LF restored tight junction (TJ) morphometry and inhibited TNF-α-induced epithelial apoptosis. This resulted in an attenuation of the TNF-α-induced decrease in transepithelial resistance (TER) and increases in permeability of fluorescein and FITC-dextran (4 kDa) and was as effective as the apoptosis inhibitor Q-VD-Oph. The enteropathogenic bacterium Yersinia enterocolitica is a frequent cause of diarrhea in early childhood. This involves focal changes in TJ protein expression and localization. LF diminished the Y. enterocolitica-induced drop in TER in the present in vitro model, which was paralleled by an inhibition of the Yersinia-induced reduction of claudin-8 expression via c-Jun kinase signaling. In conclusion, LF exerts protective effects against inflammation- or infection-induced barrier dysfunction in human intestinal cell lines, supporting its relevance for healthy infant development.

Zonula occludens-2 regulates Rho proteins activity and the development of epithelial cytoarchitecture and barrier function

Silencing Zonula occludens 2 (ZO-2), a tight junctions (TJ) scaffold protein, in epithelial cells (MDCK ZO-2 KD) triggers: 1) Decreased cell to substratum attachment, accompanied by reduced expression of claudin-7 and integrin β1, and increased vinculin recruitment to focal adhesions and stress fibers formation; 2) Lowered cell-cell aggregation and appearance of wider intercellular spaces; 3) Increased RhoA/ROCK activity, mediated by GEF-HI recruitment to cell borders by cingulin; 4) Increased Cdc42 activity, mitotic spindle disorientation and the appearance of cysts with multiple lumens; 5) Increased Rac and cofilin activity, multiple lamellipodia formation and random cell migration but increased wound closure; 6) Diminished cingulin phosphorylation and disappearance of planar network of microtubules at the TJ region; and 7) Increased transepithelial electrical resistance at steady state, coupled to an increased expression of ZO-1 and claudin-4 and a decreased expression of claudin-2 and paracingulin. Hence, ZO-2 is a crucial regulator of Rho proteins activity and the development of epithelial cytoarchitecture and barrier function.

Aldosterone signaling regulates the over-expression of claudin-4 and -8 at the distal nephron from type 1 diabetic rats

Hyperglycemia in diabetes alters tight junction (TJ) proteins in the kidney. We evaluated the participation of aldosterone (ALD), and the effect of spironolactone (SPL), a mineralocorticoid receptor antagonist, on the expressions of claudin-2, -4, -5 and -8, and occludin in glomeruli, proximal and distal tubules isolated from diabetic rats. Type 1 diabetes was induced in female Wistar rats by a single tail vein injection of streptozotocin (STZ), and SPL was administrated daily by gavage, from days 3–21. Twenty-one days after STZ injection the rats were sacrificed. In diabetic rats, the serum ALD levels were increased, and SPL-treatment did not have effect on these levels or in hyperglycemia, however, proteinuria decreased in SPL-treated diabetic rats. Glomerular damage, evaluated by nephrin and Wilm’s tumor 1 (WT1) protein expressions, and proximal tubular damage, evaluated by kidney injury molecule 1 (Kim-1) and heat shock protein 72 kDa (Hsp72) expressions, were ameliorated by SPL. Also, SPL prevented decrement in claudin-5 in glomeruli, and claudin-2 and occludin in proximal tubules by decreasing oxidative stress, evaluated by superoxide anion (O₂^●―) production, and oxidative stress markers. In distal tubules, SPL ameliorated increase in mRNA, protein expression, and phosphorylation in threonine residues of claudin-4 and -8, through a serum and glucocorticoid-induced kinase 1 (SGK1), and with-no-lysine kinase 4 (WNK4) signaling pathway. In conclusion, this is the first study that demonstrates that ALD modulates the expression of renal TJ proteins in diabetes, and that the blockade of its actions with SPL, may be a promising therapeutic strategy to prevent alterations of TJ proteins in diabetic nephropathy.

Insulin signaling via the PI3-kinase/Akt pathway regulates airway glucose uptake and barrier function in a CFTR-dependent manner

Cystic fibrosis-related diabetes is the most common comorbidity associated with cystic fibrosis (CF) and correlates with increased rates of lung function decline. Because glucose is a nutrient present in the airways of patients with bacterial airway infections and because insulin controls glucose metabolism, the effect of insulin on CF airway epithelia was investigated to determine the role of insulin receptors and glucose transport in regulating glucose availability in the airway. The response to insulin by human airway epithelial cells was characterized by quantitative PCR, immunoblot, immunofluorescence, and glucose uptake assays. Phosphatidylinositol 3-kinase/protein kinase B (Akt) signaling and cystic fibrosis transmembrane conductance regulator (CFTR) activity were analyzed by pharmacological and immunoblot assays. We found that normal human primary airway epithelial cells expressed glucose transporter 4 and that application of insulin stimulated cytochalasin B-inhibitable glucose uptake, consistent with a requirement for glucose transporter translocation. Application of insulin to normal primary human airway epithelial cells promoted airway barrier function as demonstrated by increased transepithelial electrical resistance and decreased paracellular flux of small molecules. This provides the first demonstration that airway cells express insulin-regulated glucose transporters that act in concert with tight junctions to form an airway glucose barrier. However, insulin failed to increase glucose uptake or decrease paracellular flux of small molecules in human airway epithelia expressing F508del-CFTR. Insulin stimulation of Akt1 and Akt2 signaling in CF airway cells was diminished compared with that observed in airway cells expressing wild-type CFTR. These results indicate that the airway glucose barrier is regulated by insulin and is dysfunctional in CF.

Porcine milk induces a strengthening of barrier function in porcine jejunal epithelium in vitro

Milk contains a variety of components that have been shown to affect the expression and localization of epithelial tight junction proteins and therefore the intestinal barrier. Thus, we hypothesized that milk would have an effect on intestinal barrier properties, owing to effects on the tight junction in an intraspecies porcine intestinal in vitro model. Jejunal samples of piglets derived from different age groups were analyzed. Transepithelial electrical resistance was recorded employing the Ussing chamber technique. Porcine milk or predigested milk in buffer solution was added to the apical side, and effects were compared to untreated controls. Unidirectional paracellular flux measurements were performed using sodium fluorescein. Tight junction protein expression and localization were analyzed by immunoblotting and immunofluorescence microscopy. Incubation with milk or predigested milk led to an increase in transepithelial electrical resistance, while paracellular permeability for sodium fluorescein did not result in significant changes. Densitometric analysis of immunoblot signals did not show significant alterations in claudin expression, but a reduction of claudin signals in apicolateral membrane compartments in both approaches became apparent via immunohistology. The functional effect might reflect a physiological protective mechanism, when offspring exclusively rely on their mother's milk and are exposed to a plethora of potentially barrier-perturbing factors.

Claudin-1, -2, -4, and -5: comparison of expression levels and distribution in equine tissues

Claudins, which are known as transmembrane proteins play an essential role in tight junctions (TJs) to form physical barriers and regulate paracellular transportation. To understand equine diseases, it is helpful to measure the tissue-specific expression of TJs in horses. Major equine diseases such as colic and West Nile cause damage to TJs. In this study, the expression level and distribution of claudin-1, -2, -4, and -5 in eight tissues were assessed by Western blotting and immunohistochemistry methods. Claudin-1 was primarily identified in the lung, duodenum, and uterus, claudin-2 was evenly observed in equine tissues, claudin-4 was abundantly detected in the liver, kidney and uterus, and claudin-5 was strongly expressed in the lung, duodenum, ovary, and uterus, as determined by Western blotting method. The localization of equine claudins was observed by immunohistochemistry methods. These findings provide knowledge regarding the expression patterns and localization of equine claudins, as well as valuable information to understand tight junction-related diseases according to tissue specificity and function of claudins in horses.

Claudin expression in the rat endolymphatic duct and sac - first insights into regulation of the paracellular barrier by vasopressin

Hearing and balance functions of the inner ear rely on the homeostasis of the endolymphatic fluid. When disturbed, pathologic endolymphatic hydrops evolves as observed in Menière’s disease. The molecular basis of inner ear fluid regulation across the endolymphatic epithelium is largely unknown. In this study we identified the specific expression of the tight junction (TJ) molecules Claudin 3, 4, 6, 7, 8, 10, and 16 in epithelial preparations of the rat inner ear endolymphatic duct (ED) and endolymphatic sac (ES) by high-throughput qPCR and immunofluorescence confocal microscopy. Further we showed that Claudin 4 in the ES is a target of arginine-vasopressin (AVP), a hormone elevated in Menière’s disease. Moreover, our transmission-electron microscopy (TEM) analysis revealed that the TJs of the ED were shallow and shorter compared to the TJ of the ES indicating facilitation of a paracellular fluid transport across the ED epithelium. The significant differences in the subcellular localization of the barrier-forming protein Claudin 3 between the ED and ES epithelium further support the TEM observations. Our results indicate a high relevance of Claudin 3 and Claudin 4 as important paracellular barrier molecules in the ED and ES epithelium with potential involvement in the pathophysiology of Menière’s disease.

The epithelial barrier and beyond: Claudins as amplifiers of physiological organ functions

Epithelial cell layers are interconnected by a meshwork of tight junction (TJ) protein strands, which are localized within apicolateral membranes. The proteins that form TJs are regarded to provide a static barrier, determining epithelial properties. However, recent findings in the field of barriology suggest that TJs contribute to more physiological aspects than indicated by the sum of the qualities of the single TJ proteins. Generally, TJs exhibit four major functions: (i) a "gate function," defining transepithelial permeability (i.e., barrier) properties, (ii) a "fence function" determining epithelial cell polarity, (iii) a "signaling function," affecting regulatory pathways, and (iv) a "stabilizing function," maintaining the integrity of the epithelium. This review presents a critical view on how the efficacy of physiological processes in epithelia and thus organ function might be improved by changes in the expression of claudins, the latter representing the largest and most variable family of TJ proteins. Major focus is set on (i) the coordinated regulation of transport and barrier in the intestine, (ii) the role of TJs in defining the route for antigen uptake and presentation in intestinal Peyer's patches, and (iii) the TJ function in mammary glands in response to milk accumulation, which represent impressive examples to highlight the amplification of epithelial functions by TJ proteins. © 2017 IUBMB Life, 69(5):290-296, 2017.

Perinatal administration of bisphenol A alters the expression of tight junction proteins in the uterus and reduces the implantation rate

We studied the effect of bisphenol-A (BPA) administration to rats, during the perinatal period, on the fertility of F1 generation and on the expression of tight junction (TJ) proteins in the uterus during early pregnancy. Pregnant Wistar dams (F0) received: BPA-L (0.05mg/kg/day), BPA-H (20mg/kg/day) or vehicle, from gestational day (GD) 6 to lactation day 21. F1 female pups were mated at 3 months of age and sacrificed at GD 1, 3, 6, and 7. Serum hormonal levels, ovulation rate, number of implantation sites and expression of TJ proteins in the uterus of F1 females were evaluated. BPA treatment induced no change in ovulation rate, but induced alterations in progesterone (P₄) and estradiol (E₂) serum levels, and in implantation rate. With regards to TJ proteins, BPA-H increased claudin-1 during all GDs; eliminated the peaks of claudins -3 and -4 at GD 3 and 6, respectively; and decreased claudin-7 at GD 6, ZO-1 from GD 1-6, and claudin-3 at GD 7 in stromal cells. BPA-L instead, eliminated claudin-3 peak at GD 3, increased claudin-4 and decreased claudin-7 from GD 1-6, decreased claudin-1 at GD 3 and 7 and claudin-4 at GD 7 in stromal cells. BPA-L also decreased ZO-1 at GDs 1 and 3 and increased ZO-1 at GD 6. Thus, BPA treatment during perinatal period perturbed, when the animals reached adulthood and became pregnant, the particular expression of TJ proteins in the uterine epithelium and reduced in consequence the number of implantation sites.

Enteric Pathogens and Their Toxin-Induced Disruption of the Intestinal Barrier through Alteration of Tight Junctions in Chickens

Maintaining a healthy gut environment is a prerequisite for sustainable animal production. The gut plays a key role in the digestion and absorption of nutrients and constitutes an initial organ exposed to external factors influencing bird’s health. The intestinal epithelial barrier serves as the first line of defense between the host and the luminal environment. It consists of a continuous monolayer of intestinal epithelial cells connected by intercellular junctional complexes which shrink the space between adjacent cells. Consequently, free passing of solutes and water via the paracellular pathway is prevented. Tight junctions (TJs) are multi-protein complexes which are crucial for the integrity and function of the epithelial barrier as they not only link cells but also form channels allowing permeation between cells, resulting in epithelial surfaces of different tightness. Tight junction’s molecular composition, ultrastructure, and function are regulated differently with regard to physiological and pathological stimuli. Both in vivo and in vitro studies suggest that reduced tight junction integrity greatly results in a condition commonly known as “leaky gut”. A loss of barrier integrity allows the translocation of luminal antigens (microbes, toxins) via the mucosa to access the whole body which are normally excluded and subsequently destroys the gut mucosal homeostasis, coinciding with an increased susceptibility to systemic infection, chronic inflammation and malabsorption. There is considerable evidence that the intestinal barrier dysfunction is an important factor contributing to the pathogenicity of some enteric bacteria. It has been shown that some enteric pathogens can induce permeability defects in gut epithelia by altering tight junction proteins, mediated by their toxins. Resolving the strategies that microorganisms use to hijack the functions of tight junctions is important for our understanding of microbial pathogenesis, because some pathogens can utilize tight junction proteins as receptors for attachment and subsequent internalization, while others modify or destroy the tight junction proteins by different pathways and thereby provide a gateway to the underlying tissue. This review aims to deliver an overview of the tight junction structures and function, and its role in enteric bacterial pathogenesis with a special focus on chickens. A main conclusion will be that the molecular mechanisms used by enteric pathogens to disrupt epithelial barrier function in chickens needs a much better understanding, explicitly highlighted for Campylobacter jejuni, Salmonella enterica and Clostridium perfringens. This is a requirement in order to assist in discovering new strategies to avoid damages of the intestinal barrier or to minimize consequences from infections.

Enhancement of Outflow Facility in the Murine Eye by Targeting Selected Tight-Junctions of Schlemm's Canal Endothelia

The juxtacanalicular connective tissue of the trabecular meshwork together with inner wall endothelium of Schlemm’s canal (SC) provide the bulk of resistance to aqueous outflow from the anterior chamber. Endothelial cells lining SC elaborate tight junctions (TJs), down-regulation of which may widen paracellular spaces between cells, allowing greater fluid outflow. We observed significant increase in paracellular permeability following siRNA-mediated suppression of TJ transcripts, claudin-11, zonula-occludens-1 (ZO-1) and tricellulin in human SC endothelial monolayers. In mice claudin-11 was not detected, but intracameral injection of siRNAs targeting ZO-1 and tricellulin increased outflow facility significantly. Structural qualitative and quantitative analysis of SC inner wall by transmission electron microscopy revealed significantly more open clefts between endothelial cells treated with targeting, as opposed to non-targeting siRNA. These data substantiate the concept that the continuity of SC endothelium is an important determinant of outflow resistance, and suggest that SC endothelial TJs represent a specific target for enhancement of aqueous movement through the conventional outflow system.

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.

Physiological roles of claudins in kidney tubule paracellular transport

The paracellular pathways in renal tubular epithelia such as the proximal tubules, which reabsorb the largest fraction of filtered solutes and water and are leaky epithelia, are important routes for transepithelial transport of solutes and water. Movement occurs passively via an extracellular route through the tight junction between cells. The characteristics of paracellular transport vary among different nephron segments with leaky or tighter epithelia. Claudins expressed at tight junctions form pores and barriers for paracellular transport. Claudins are from a multigene family, comprising at least 27 members in mammals. Multiple claudins are expressed at tight junctions of individual nephron segments in a nephron segment-specific manner. Over the last decade, there have been advances in our understanding of the structure and functions of claudins. This paper is a review of our current knowledge of claudins, with special emphasis on their physiological roles in proximal tubule paracellular solute and water transport.

Active and passive involvement of claudins in the pathophysiology of intestinal inflammatory diseases

Intestinal inflammatory diseases, four of which are discussed here, are associated with alterations of claudins. In ulcerative colitis, diarrhea and antigen entry into the mucosa occurs. Claudin-2 is upregulated but data on other claudins are still limited or vary (e.g., claudin-1 and -4). Apart from that, tight junction changes contribute to diarrhea via a leak flux mechanism, while protection against antigen entry disappears behind epithelial gross lesions (erosions) and apoptotic foci. Crohn's disease is additionally characterized by a claudin-5 and claudin-8 reduction which plays an active role in antigen uptake already before gross lesions appear. In microscopic colitis (MC), upregulation of claudin-2 expression is weak and a reduction in claudin-4 may be only passively involved, while sodium malabsorption represents the main diarrheal mechanism. However, claudin-5 is removed from MC tight junctions which may be an active trigger for inflammation through antigen uptake along the so-called leaky gut concept. In celiac disease, primary barrier defects are discussed in the context of candidate genes as PARD3 which regulate cell polarity and tight junctions. The loss of claudin-5 allows small antigens to invade, while the reductions in others like claudin-3 are rather passive events. Taken together, the specific role of single tight junction proteins for the onset and perpetuation of inflammation and the recovery from these diseases is far from being fully understood and is clearly dependent on the stage of the disease, the background of the other tight junction components, the transport activity of the mucosa, and the presence of other barrier features like gross lesions, an orchestral interplay which is discussed in this article.

Claudins in barrier and transport function-the kidney

Claudins are discovered to be key players in renal epithelial physiology. They are involved in developmental, physiological, and pathophysiological differentiation. In the glomerular podocytes, claudin-1 is an important determinant of cell junction fate. In the proximal tubule, claudin-2 plays important roles in paracellular salt reabsorption. In the thick ascending limb, claudin-14, -16, and -19 regulate the paracellular reabsorption of calcium and magnesium. Recessive mutations in claudin-16 or -19 cause an inherited calcium and magnesium losing disease. Synonymous variants in claudin-14 have been associated with hypercalciuric nephrolithiasis by genome-wide association studies (GWASs). More importantly, claudin-14 gene expression can be regulated by extracellular calcium levels via the calcium sensing receptor. In the distal tubules, claudin-4 and -8 form paracellular chloride pathway to facilitate electrogenic sodium reabsorption. Aldosterone, WNK4, Cap1, and KLHL3 are powerful regulators of claudin and the paracellular chloride permeability. The lessons learned on claudins from the kidney will have a broader impact on tight junction biology in other epithelia and endothelia.

An emerging role for IQGAP1 in tight junction control

IQGAP1 is a scaffold protein involved in the assembly of adherens junctions. Our work has recently revealed a novel role for IQGAP1 in the regulation of tight junctions (TJ) through differential recruitment of claudins to the nascent TJ. Here, we discuss the potential mechanisms of this regulation, including IQGAP1 effects on CDC42, and IQGAP1 interactions with sorting/trafficking molecules (e.g. Exo70). Given the many roles of IQGAP1 and the large number of interacting partners, we focus our discussion of these functions in the context of junction formation, trafficking, growth factor signaling and cancer. We also propose a potential role for IQGAP1 in regulating epithelial integrity and compartmentalized signaling in epithelia.

Claudin-4 activity in ovarian tumor cell apoptosis resistance and migration

Background

Claudin-4 is a transmembrane protein expressed at high levels in the majority of epithelial ovarian tumors, irrespective of subtype, and has been associated with tumor cells that are both chemoresistant and highly mobile. The objective of this study was to determine the functional role that claudin-4 plays in apoptosis resistance and migration as well as the therapeutic utility of targeting claudin-4 activity with a small mimic peptide.

Methods

We examined claudin-4 activity in human ovarian tumor cell lines (SKOV3, OVCAR3, PEO4) using in vitro caspase and scratch assays as well as an in vivo mouse model of ovarian cancer. Claudin-4 activity was disrupted by treating cells with a small peptide that mimics the DFYNP sequence in the second extracellular loop of claudin-4. Claudin-4 expression was also altered using shRNA-mediated gene silencing.

Results

Both the disruption of claudin-4 activity and the loss of claudin-4 expression significantly increased tumor cell caspase-3 activation (4 to 10-fold, respectively) in response to the apoptotic inducer staurosporine and reduced tumor cell migration by 50 %. The mimic peptide had no effect on cells that lacked claudin-4 expression. Female athymic nude mice bearing ZsGreen-PEO4 ovarian tumors showed a significant decrease in ovarian tumor burden, due to increased apoptosis, after treatment with intraperitoneal injections of 4 mg/kg mimic peptide every 48 h for three weeks, compared to control peptide treated mice.

Conclusion

Claudin-4 functionally contributes to both ovarian tumor cell apoptosis resistance and migration and targeting extracellular loop interactions of claudin-4 may have therapeutic implications for reducing ovarian tumor burden.

Influence of silver and titanium dioxide nanoparticles on in vitro blood-brain barrier permeability

An in vitro blood-brain barrier (BBB) model being composed of co-culture with endothelial (bEnd.3) and astrocyte-like (ALT) cells was established to evaluate the toxicity and permeability of Ag nanoparticles (AgNPs; 8nm) and TiO₂ nanoparticles (TiO₂NPs; 6nm and 35nm) in normal and inflammatory central nervous system. Lipopolysaccharide (LPS) was pre-treated to simulate the inflammatory responses. Both AgNPs and Ag ions can decrease transendothelial electrical resistance (TEER) value, and cause discontinuous tight junction proteins (claudin-5 and zonula occludens-1) of BBB. However, only the Ag ions induced inflammatory cytokines to release, and had less cell-to-cell permeability than AgNPs, which indicated that the toxicity of AgNPs was distinct from Ag ions. LPS itself disrupted BBB, while co-treatment with AgNPs and LPS dramatically enhanced the disruption and permeability coefficient. On the other hand, TiO₂NPs exposure increased BBB penetration by size, and disrupted tight junction proteins without size dependence, and many of TiO₂NPs accumulated in the endothelial cells were observed. This study provided the new insight of toxic potency of AgNPs and TiO₂NPs in BBB.

Protective effect of bone marrow derived mesenchymal stem cells in lipopolysaccharide-induced acute lung injury mediated by claudin-4 in a rat model

Our study aims to investigate the effects of bone marrow derived mesenchymal stem cells (BM-MSCs) in lipopolysaccharide (LPS)-induced acute lung injury (ALI) as well as the underlying mechanism. In our study, Wistar rats were randomly divided into four groups: control group; ALI group; ALI+MSCs group and ALI+MSCs claudin-4 siRNA group. MRC-5 and BEAS-2B cell lines were used for in vitro assay. Flow cytometry, western blot, hematoxylin and eosin (H&E) staining, CCK-8 assay, enzyme-linked immunosorbent assay (ELISA) were involved to measure the pathological changes in lung tissues. Results showed that in vivo MSCs administration significantly attenuated pulmonary edema (wet/dry ratio), inflammation cytokines levels (TGF-α), pathological alternations and cell apoptosis which were mediated by claudin-4 in LPS-induced acute lung injury in rats. In vitro experiment showed that hypoxia could induce the expression of claudin-4 in MSCs, and MSCs treatment showed significantly enhanced cell viability (by CCK-8 assay) and reduced cell apoptosis. In conclusion, the present study demonstrated that BM-MSCs can protect against LPS-induced ALI in vivo and in vitro, at least partly mediated by claudin-4.

Claudins in cancer: bench to bedside

The claudin family, in mammals, encoded by at least 27 members of a single ancestral gene, CLDN, is the main constituent as integral membrane proteins of tight junctions. It has been shown that the expression levels of claudins are often decreased or that their expressions are absent in human neoplasias. These findings are consistent with the well-accepted concept that carcinogenesis is accompanied by the disruption or loss of functional tight junctions. In contrast, accumulating data have showed elevated or aberrant expression of claudins in various cancers, indicating specific roles of claudins in tumorigenesis. Importantly, dysregulated claudins play an oncogenic role or conversely have a tumor-suppressive effect depending on target tissues or cell types, and thus, they contribute to tumor development and progression. Although tight junctions are intercellular structures in epithelial cells, specific roles of claudins in cancer are supported by the evidence that TJs are not simple static constituents for establishing cell adhesion structures but are also cell signaling components that have functions in receiving environmental cues and transmitting signals inside cells. Since the expression profile of claudins is associated with patients' outcome and prognosis in several cancer types, an understanding of the expression pattern and subcellular localization of claudins in various pathologies will lead to the establishment of claudins as useful biomarkers for the detection and diagnosis of cancers.

Expression and Function of Tight Junctions in the Crypt Epithelium of Human Palatine Tonsils

The human palatine tonsils have surface and crypt stratified epithelium and may be initiated via the epithelium to mount immune responses to various presenting antigens. Here we investigated the expression and function of tight junctions in the epithelium of human palatine tonsils from patients with tonsillar hypertrophy or recurrent tonsillitis. Occludin, ZO-1, JAM-1, and claudin-1, −3, −4, −7, −8, and −14 mRNAs were detected in tonsillar hypertrophy. Occludin and claudin-14 were expressed in the uppermost layer of the tonsil surface epithelium, whereas ZO-1, JAM-1, and claudin-1, −4, and −7 were found throughout the epithelium. In the crypt epithelium, claudin-4 was preferentially expressed in the upper layers. In freeze-fracture replicas, short fragments of continuous tight junction strands were observed but never formed networks. In the crypt epithelium of recurrent tonsillitis, the tracer was leaked from the surface regions where occludin and claudin-4 disappeared. Occludin, ZO-1, JAM-1, and claudin-1, −3, −4, and −14, but not claudin-7, mRNAs were decreased in recurrent tonsillitis compared with those of tonsillar hypertrophy. These studies suggest unique expression of tight junctions in human palatine tonsillar epithelium, and the crypt epithelium may possess an epithelial barrier different from that of the surface epithelium.

Tight Junction Ultrastructure Alterations in a Mouse Model of Enteral Nutrient Deprivation

BACKGROUND

Total parenteral nutrition (TPN), a necessary treatment for patients who cannot receive enteral nutrition, is associated with infectious complications due in part to a loss of intestinal epithelial barrier function (EBF). Using a mouse model of TPN, with enteral nutrient deprivation, we previously demonstrated an increase in mucosal interferon-γ and tumor necrosis factor-α; these cytokine changes are a major mediator driving a reduction in epithelial tight junction (TJ) protein expression. However, the exact ultrastructural changes to the intestinal epithelial barrier have not been previously described.

AIM

We hypothesized that TPN dependence results in ultrastructural changes in the intestinal epithelial TJ meshwork.

METHODS

C57BL/6 mice underwent internal jugular venous cannulation and were given enteral nutrition or TPN with enteral nutrient deprivation for 7 days. Freeze-fracture electron microscopy was performed on ileal tissue to characterize changes in TJ ultrastructure. EBF was measured using transepithelial resistance and tracer permeability, while TJ expression was measured via Western immunoblotting and immunofluorescence staining.

RESULTS

While strand density, linearity, and appearance were unchanged, TPN dependence led to a mean reduction in one horizontal strand out of the TJ compact meshwork to a more basal region, resulting in a reduction in meshwork depth. These findings were correlated with the loss of TJ localization of claudin-4 and tricellulin, reduced expression of claudin-5 and claudin-8, and reduced ex vivo EBF.

CONCLUSION

Tight junction ultrastructural changes may contribute to reduced EBF in the setting of TPN dependence.

Chloride secretion by renal collecting ducts

PURPOSE OF REVIEW

Renal collecting ducts maintain NaCl homeostasis by fine-tuning urinary excretion to balance dietary salt intake. This review focuses on recent studies on transcellular Cl secretion by collecting ducts, its regulation and its role in cyst growth in autosomal dominant polycystic kidney disease (ADPKD).

RECENT FINDINGS

Lumens of nonperfused rat medullary collecting ducts collapse in control media but expand with fluid following treatment with cAMP, demonstrating the capacity for both salt absorption and secretion. Recently, inhibition of apical epithelial Na channels (ENaC) unmasked Cl secretion in perfused mouse cortical collecting ducts (CCDs), involving Cl uptake by basolateral NKCC1 and efflux through apical Cl channels. AVP, the key hormone for osmoregulation, promotes cystic fibrosis transmembrane conductance regulator (CFTR)-mediated Cl secretion. In addition, prostaglandin E2 stimulates Cl secretion through both CFTR and Ca-activated Cl channels.

SUMMARY

Renal Cl secretion has been commonly overlooked because of the overwhelming capacity for the nephron to reabsorb NaCl from the glomerular filtrate. In ADPKD, Cl secretion plays a central role in the accumulation of cyst fluid and the remarkable size of the cystic kidneys. Investigation of renal Cl secretion may provide a better understanding of NaCl homeostasis and identify new approaches to reduce cyst growth in PKD.

Mycotoxins modify the barrier function of Caco-2 cells through differential gene expression of specific claudin isoforms: Protective effect of illite mineral clay

Aflatoxin B1 (AFB1), fumonisin B1 (FB1), ochratoxin A (OTA) and T-2 toxin (T2) are mycotoxins that commonly contaminate the food chain and cause various toxicological effects. Their global occurrence is regarded as an important risk factor for human and animal health. In this study, the results demonstrate that, in human Caco-2 cells, AFB1, FB1, OTA and T2 origin cytotoxic effects, determining cell viability through MTT assay and LDH leakage, and decrease trans-epithelial electrical resistance (TEER). The decrease in barrier properties is concomitant with a reduction in the expression levels of the tight junction constituents claudin-3, claudin-4 and occludin. The protective effect of mineral clays (diosmectite, montmorillonite and illite) on alterations in cell viability and epithelial barrier function induced by the mycotoxins was also evaluated. Illite was the best clay to prevent the mycotoxin effects. Illite plus mycotoxin co-treatment completely abolished AFB1 and FB1-induced cytotoxicity. Also, the decreases in the gene expression of claudins and the reduction of TEER induced by mycotoxins were reversed by the illite plus mycotoxin co-treatment. In conclusion, these results demonstrated that mycotoxins AFB1, FB1, T2 and OTA disrupt the intestinal barrier permeability by a mechanism involving reduction of claudin isoform expressions, and illite counteracts this disruption.

Tight junctions in inflammatory bowel diseases and inflammatory bowel disease associated colorectal cancer

Inflammatory bowel diseases are characterised by inflammation that compromises the integrity of the epithelial barrier. The intestinal epithelium is not only a static barrier but has evolved complex mechanisms to control and regulate bacterial interactions with the mucosal surface. Apical tight junction proteins are critical in the maintenance of epithelial barrier function and control of paracellular permeability. The characterisation of alterations in tight junction proteins as key players in epithelial barrier function in inflammatory bowel diseases is rapidly enhancing our understanding of critical mechanisms in disease pathogenesis as well as novel therapeutic opportunities. Here we give an overview of recent literature focusing on the role of tight junction proteins, in particular claudins, in inflammatory bowel diseases and inflammatory bowel disease associated colorectal cancer.

Could tight junctions regulate the barrier function of the aged skin?

The skin is known to be the largest organ in human organism creating interface with outer environment. The skin provides protective barrier against pathogens, physical and chemical insults, and against uncontrolled loss of water. The barrier function was primarily attributed to the stratum corneum (SC) but recent studies confirmed that epidermal tight junctions (TJs) also play important role in maintaining barrier properties of the skin. Independent observations indicate that barrier function and its recovery is impaired in aged skin. However, trans-epidermal water loss (TEWL) values remains rather unchanged in elderly population. UV radiation as major factor of photoageing impairs TJ proteins, but TJs have great self-regenerative potential. Since it may be possible that TJs can compensate TEWL in elderly due to its regenerative and compensatory capabilities, important question remains to be answered: how are TJs regulated during skin ageing? This review provides an insight into TJs functioning as epidermal barrier and summarizes current knowledge about the impact of ageing on the barrier function of the skin and epidermal TJs.

A proposed route to independent measurements of tight junction conductance at discrete cell junctions

Direct recording of tight junction permeability is of pivotal importance to many biologic fields. Previous approaches bear an intrinsic disadvantage due to the difficulty of separating tight junction conductance from nearby membrane conductance. Here, we propose the design of Double whole-cell Voltage Clamp - Ion Conductance Microscopy (DVC-ICM) based on previously demonstrated potentiometric scanning of local conductive pathways. As proposed, DVC-ICM utilizes two coordinated whole-cell patch-clamps to neutralize the apical membrane current during potentiometric scanning, which in models described here will profoundly enhance the specificity of tight junction recording. Several potential pitfalls are considered, evaluated and addressed with alternative countermeasures.

The Immunomodulatory and Therapeutic Effects of Mesenchymal Stromal Cells for Acute Lung Injury and Sepsis

It is increasingly recognized that immunomodulation represents an important mechanism underlying the benefits of many stem cell therapies, rather than the classical paradigm of transdifferentiation and cell replacement. In the former paradigm, the beneficial effects of cell therapy result from paracrine mechanism(s) and/or cell-cell interaction as opposed to direct engraftment and repair of diseased tissue and/or dysfunctional organs. Depending on the cell type used, components of the secretome, including microRNA (miRNA) and extracellular vesicles, may be able to either activate or suppress the immune system even without direct immune cell contact. Mesenchymal stromal cells (MSCs), also referred to as mesenchymal stem cells, are found not only in the bone marrow, but also in a wide variety of organs and tissues. In addition to any direct stem cell activities, MSCs were the first stem cells recognized to modulate immune response, and therefore they will be the focus of this review. Specifically, MSCs appear to be able to effectively attenuate acute and protracted inflammation via interactions with components of both innate and adaptive immune systems. To date, this capacity has been exploited in a large number of preclinical studies and MSC immunomodulatory therapy has been attempted with various degrees of success in a relatively large number of clinical trials. Here, we will explore the various mechanism employed by MSCs to effect immunosuppression as well as review the current status of its use to treat excessive inflammation in the context of acute lung injury (ALI) and sepsis in both preclinical and clinical settings.

Digestive adaptations of aerial lifestyles

Flying vertebrates (birds and bats) are under selective pressure to reduce the size of the gut and the mass of the digesta it carries. Compared with similar-sized nonflying mammals, birds and bats have smaller intestines and shorter retention times. We review evidence that birds and bats have lower spare digestive capacity and partially compensate for smaller intestines with increased paracellular nutrient absorption.

Lactobacillus fermentum AGR1487 cell surface structures and supernatant increase paracellular permeability through different pathways

Lactobacillus fermentum is commonly found in food products, and some strains are known to have beneficial effects on human health. However, our previous research indicated that L. fermentum AGR1487 decreases in vitro intestinal barrier integrity. The hypothesis was that cell surface structures of AGR1487 are responsible for the observed in vitro effect. AGR1487 was compared to another human oral L. fermentum strain, AGR1485, which does not cause the same effect. The examination of phenotypic traits associated with the composition of cell surface structures showed that compared to AGR1485, AGR1487 had a smaller genome, utilized different sugars, and had greater tolerance to acid and bile. The effect of the two strains on intestinal barrier integrity was determined using two independent measures of paracellular permeability of the intestinal epithelial Caco-2 cell line. The transepithelial electrical resistance (TEER) assay specifically measures ion permeability, whereas the mannitol flux assay measures the passage of uncharged molecules. Both live and UV-inactivated AGR1487 decreased TEER across Caco-2 cells implicating the cell surfaces structures in the effect. However, only live AGR1487, and not UV-inactivated AGR1487, increased the rate of passage of mannitol, implying that a secreted component(s) is responsible for this effect. These differences in barrier integrity results are likely due to the TEER and mannitol flux assays measuring different characteristics of the epithelial barrier, and therefore imply that there are multiple mechanisms involved in the effect of AGR1487 on barrier integrity.

Epithelial function and dysfunction in asthma

Asthma was previously defined as an allergic Th2-mediated inflammatory immune disorder. Recently, this paradigm has been challenged because not all pathological changes observed in the asthmatic airways are adequately explained simply as a result of Th2-mediated processes. Contemporary thought holds that asthma is a complex immune disorder involving innate as well as adaptive immune responses, with the clinical heterogeneity of asthma perhaps a result of the different relative contribution of these two systems to the disease. Epidemiological studies show that exposure to certain environmental substances is strongly associated with the risk of developing asthma. The airway epithelium is first barrier to interact with, and respond to, environmental agents (pollution, viral infection, allergens), suggesting that it is a key player in the pathology of asthma. Epithelial cells play a key role in the regulation of tissue homeostasis by the modulation of numerous molecules, from antioxidants and lipid mediators to growth factors, cytokines, and chemokines. Additionally, the epithelium is also able to suppress mechanisms involved in, for example, inflammation in order to maintain homeostasis. An intrinsic alteration or defect in these regulation mechanisms compromises the epithelial barrier, and therefore, the barrier may be more prone to environmental substances and thus more likely to exhibit an asthmatic phenotype. In support of this, polymorphisms in a number of genes that are expressed in the bronchial epithelium have been linked to asthma susceptibility, while environmental factors may affect epigenetic mechanisms that can alter epithelial function and response to environmental insults. A detailed understanding of the regulatory role of the airway epithelium is required to develop new therapeutic strategies for asthma that not only address the symptoms but also the underlining pathogenic mechanism(s) and prevent airway remodelling.

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.

β-Catenin Signaling Biases Multipotent Lingual Epithelial Progenitors to Differentiate and Acquire Specific Taste Cell Fates

Continuous taste bud cell renewal is essential to maintain taste function in adults; however, the molecular mechanisms that regulate taste cell turnover are unknown. Using inducible Cre-lox technology, we show that activation of β-catenin signaling in multipotent lingual epithelial progenitors outside of taste buds diverts daughter cells from a general epithelial to a taste bud fate. Moreover, while taste buds comprise 3 morphological types, β-catenin activation drives overproduction of primarily glial-like Type I taste cells in both anterior fungiform (FF) and posterior circumvallate (CV) taste buds, with a small increase in Type II receptor cells for sweet, bitter and umami, but does not alter Type III sour detector cells. Beta-catenin activation in post-mitotic taste bud precursors likewise regulates cell differentiation; forced activation of β-catenin in these Shh⁺ cells promotes Type I cell fate in both FF and CV taste buds, but likely does so non-cell autonomously. Our data are consistent with a model where β-catenin signaling levels within lingual epithelial progenitors dictate cell fate prior to or during entry of new cells into taste buds; high signaling induces Type I cells, intermediate levels drive Type II cell differentiation, while low levels may drive differentiation of Type III cells.

Taste is a fundamental sense that helps the body determine whether food can be ingested. Taste dysfunction can be a side effect of cancer therapies, can result from an alteration of the renewal capacities of the taste buds, and is often associated with psychological distress and malnutrition. Thus, understanding how taste cells renew throughout adult life, i.e. how newly born cells replace old cells as they die, is essential to find potential therapeutic targets to improve taste sensitivity in patients suffering taste dysfunction. Here we show that a specific molecular pathway, Wnt/β-catenin signaling, controls renewal of taste cells by regulating separate stages of taste cell turnover. We show that activating this pathway directs the newly born cells to become primarily a specific taste cell type whose role is to support the other taste cells and help them work efficiently.

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.

Mir-203-mediated tricellulin mediates lead-induced in vitro loss of blood-cerebrospinal fluid barrier (BCB) function

The blood-cerebrospinal fluid barrier (BCB) plays a critical role in the maintenance of optimal brain function. Tricellulin (TRIC), a protein localized at the tricellular contact sites of epithelial cells is involved in the formation of tight junctions in various epithelial barriers. However, little is known about its expression in the choroidal epithelial cells. It is well established that lead (Pb) exposure increases the leakage of the BCB. The purpose of this study is to investigate the expression and localization of TRIC in choroidal epithelial cells in vitro and whether altered TRIC expression mediates Pb-induced loss of barrier function. We found that TRIC protein and mRNA were expressed in choroidal epithelial cells in vitro and TRIC was localized at the tricellular contacts, colocalizing with occludin. Downregulation of TRIC by siRNA increased the BCB permeability corroborated by altered transendothelial electrical resistance (TEER) and FITC-dextran flux. Treatment with 10μM Pb reduced TRIC protein expression, but overexpression of TRIC alleviated the Pb-induced increase in BCB permeability. Bioinformatics analysis showed that mir-203 was a potential microRNA (miRNA) binding motif on TRIC 3'UTR, and that Pb exposure increased the expression of mir-203. Treatment with a mir-203 inhibitor increased TRIC protein expression and attenuated the Pb-induced BCB leakage. Our results establish that TRIC plays an important role in regulating BCB function.

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.

Claudins: Gatekeepers of lung epithelial function

The lung must maintain a proper barrier between airspaces and fluid filled tissues in order to maintain lung fluid balance. Central to maintaining lung fluid balance are epithelial cells which create a barrier to water and solutes. The barrier function of these cells is mainly provided by tight junction proteins known as claudins. Epithelial barrier function varies depending on the different needs within the segments of the respiratory tree. In the lower airways, fluid is required to maintain mucociliary clearance, whereas in the terminal alveolar airspaces a thin layer of surfactant enriched fluid lowers surface tension to prevent airspace collapse and is critical for gas exchange. As the epithelial cells within the segments of the respiratory tree differ, the composition of claudins found in these epithelial cells is also different. Among these differences is claudin-18 which is uniquely expressed by the alveolar epithelial cells. Other claudins, notably claudin-4 and claudin-7, are more ubiquitously expressed throughout the respiratory epithelium. Claudin-5 is expressed by both pulmonary epithelial and endothelial cells. Based on in vitro and in vivo model systems and histologic analysis of lungs from human patients, roles for specific claudins in maintaining barrier function and protecting the lung from the effects of acute injury and disease are being identified. One surprising finding is that claudin-18 and claudin-4 control lung cell phenotype and inflammation beyond simply maintaining a selective paracellular permeability barrier. This suggests claudins have more nuanced roles for the control of airway and alveolar physiology in the healthy and diseased lung.

Mast cell activation is involved in stress-induced epithelial barrier dysfunction in the esophagus

OBJECTIVE

We aimed to investigate the role of mast cell in stress-induced barrier dysfunction in the esophagus and its possible pathway involved using mast cell-deficient (Ws/Ws) rats.

METHODS

Ws/Ws rats and normal (+/+) rats were submitted to chronic restraint stress (CRS) 2 h/day for 7 days. Tissues were obtained from distal esophagus. Mast cells were counted under Alcian blue-safranin O stain. Activation of mast cells was assessed using transmission electron microscope. Esophageal epithelial barrier dysfunction was evaluated by measuring intercellular spaces (ICS) and by quantifying tight junction (TJ) proteins. The localization and expression of mast cell-derived tryptase and proteinase activated receptor 2 (PAR-2) were assessed.

RESULTS

A higher number of mast cells and higher proportion of activated mast cells were observed in CRS +/+ rats compared with non-stress controls. Increased ICS and decreased expression of some TJ proteins were observed in the CRS +/+ rats but not in the CRS Ws/Ws rats. Tryptase and its receptor PAR-2 were found elevated concomitantly by nearly 100% in CRS +/+ rats, but not in CRS Ws/Ws rats.

CONCLUSIONS

Mast cells play an important role in stress-induced epithelial barrier dysfunction in esophagus. The mechanism may involve the activation of PAR-2 by mast cell-derived tryptase, causing proinflammatory responses and the subsequent disruption of the epithelial TJ proteins and a disturbed cytoskeleton function, resulting in dilated intercellular spaces.

Proteomic analysis of proteins surrounding occludin and claudin-4 reveals their proximity to signaling and trafficking networks

Tight junctions are complex membrane structures that regulate paracellular movement of material across epithelia and play a role in cell polarity, signaling and cytoskeletal organization. In order to expand knowledge of the tight junction proteome, we used biotin ligase (BioID) fused to occludin and claudin-4 to biotinylate their proximal proteins in cultured MDCK II epithelial cells. We then purified the biotinylated proteins on streptavidin resin and identified them by mass spectrometry. Proteins were ranked by relative abundance of recovery by mass spectrometry, placed in functional categories, and compared not only among the N- and C- termini of occludin and the N-terminus of claudin-4, but also with our published inventory of proteins proximal to the adherens junction protein E-cadherin and the tight junction protein ZO-1. When proteomic results were analyzed, the relative distribution among functional categories was similar between occludin and claudin-4 proximal proteins. Apart from already known tight junction- proteins, occludin and claudin-4 proximal proteins were enriched in signaling and trafficking proteins, especially endocytic trafficking proteins. However there were significant differences in the specific proteins comprising the functional categories near each of the tagging proteins, revealing spatial compartmentalization within the junction complex. Taken together, these results expand the inventory of known and unknown proteins at the tight junction to inform future studies of the organization and physiology of this complex structure.

Use of cell-based screening to identify small-molecule compounds that modulate claudin-4 expression

Claudins constitute a family of at least 27 proteins with four transmembrane domains, and play a pivotal role in maintaining tight-junctions seals in diverse epithelial tissues. The expression of claudin-4 often changes in intestinal tissues of inflammatory bowel disease and various human cancers. Therefore, claudin-4 is a promising target for treatment of these diseases. In our previous study, we established a reporter cell line to monitor claudin-4 expression on the basis of a functional claudin-4 promoter. Using this cell line, we have performed a cell-based screen of a library containing 2642 biologically active small-molecule compounds to identify modulators of claudin-4 expression. The screen identified 24 potential modulators of the claudin-4 promoter activity. Fourteen of these compounds (12 of them novel) induced endogenous claudin-4 expression. The identified compounds might serve as lead compounds targeting aberrant gene expression in inflammatory bowel disease.