A short guide to the tight junction.

Tight junctions (TJs) are specialized regions of contact between cells of epithelial and endothelial tissues that form selective semipermeable paracellular barriers that establish and maintain body compartments with different fluid compositions. As such, the formation of TJs represents a critical step in metazoan evolution, allowing the formation of multicompartmental organisms and true, barrier-forming epithelia and endothelia. In the six decades that have passed since the first observations of TJs by transmission electron microscopy, much progress has been made in understanding the structure, function, molecular composition and regulation of TJs. The goal of this Perspective is to highlight the key concepts that have emerged through this research and the future challenges that lie ahead for the field.

Expression and Localization Profiles of Tight Junction Proteins in Immune Cells Depend on Their Activation Status.

The ability of the immune system to combat pathogens relies on processes like antigen sampling by dendritic cells and macrophages migrating through endo- and epithelia or penetrating them with their dendrites. In addition, other immune cell subtypes also migrate through the epithelium after activation. For paracellular migration, interactions with tight junctions (TJs) are necessary, and previous studies reported TJ protein expression in several immune cells. Our investigation aimed to characterize, in more detail, the expression profiles of TJ proteins in different immune cells in both naïve and activated states. The mRNA expression analysis revealed distinct expression patterns for TJ proteins, with notable changes, mainly increases, upon activation. At the protein level, LSR appeared predominant, being constitutively present in naïve cell membranes, suggesting roles as a crucial interaction partner. Binding experiments suggested the presence of claudins in the membrane only after stimulation, and claudin-8 translocation to the membrane occurred after stimulation. Our findings suggest a dynamic TJ protein expression in immune cells, implicating diverse functions in response to stimulation, like interaction with TJ proteins or regulatory roles. While further analysis is needed to elucidate the precise roles of TJ proteins, our findings indicate important non-canonical functions of TJ proteins in immune response.

Measurement of tear resistance after manual capsulorhexis and femtosecond laser-assisted capsulotomy of crystalline lenses.

BACKGROUND: In this study, the tear resistance of porcine lens capsules after continuous curvilinear capsulorhexis (CCC) and femtosecond (fs)-laser-assisted capsulotomy for cataract surgery (FLC) with different laser parameters is measured with a custom-made testing setup. METHODS: Forty-five fresh porcine lenses were randomly chosen for CCC (n = 15) or FLC 1 (n = 15) and FLC 2 (n = 15). The FLC 1-group was treated with smaller spot distances than the FLC 2-group. The force necessary to break the opening of the anterior capsule and the maximum displacement were measured. RESULTS: The mean tear resistance of the CCC-group (150 ± 70 mN) was higher than that of the FLC 1-group (60 ± 20 mN) and the FLC 2-group (30 ± 20 mN). CONCLUSION: It could be shown that CCC leads to a significantly higher tear resistance of the opening than FLC in porcine lenses. The femtosecond laser group demonstrated that smaller spot distances lead to a higher tear resistance.

Clinical Significance of Claudin Expression in Oral Squamous Cell Carcinoma.

A change in claudin expression has been demonstrated in various tumors. The present study specifically compares claudin expression in oral squamous cell carcinoma (OSCC) with healthy oral epithelium from the same individual and analyzes the association between claudin expression and the clinically relevant course parameters. Our study includes tissue samples and clinically relevant follow-up data from 60 patients with primary and untreated OSCC. The oral mucosa was analyzed via Western blot for the expression of claudin-1, -2, -3, -4, -5, and -7. Importantly, the tumor and healthy tissues were obtained pairwise from patients, allowing for intraindividual comparisons. Both the healthy and tumor epithelium from the oral cavity did not express the claudin-3 protein. The intraindividual comparison revealed that, in OSCC, claudin-2 expression was higher, and the expression of claudin-4, -5, and -7 was lower than in healthy epithelium. An association was found between increased claudin-2 expression and shorter relapse-free survival. In addition, the reduced expression of claudin-4 had a negative impact on relapse-free survival. Furthermore, associations between the reduced expression of claudin-7 and the stage of a tumor, or the presence of lymph node metastases, were found. Thus, the expression level of claudin-2, -4, and -7 appears to be predictive of the diagnosis and prognosis of OSCC.

Expression of tricellular tight junction proteins and the paracellular macromolecule barrier are recovered in remission of ulcerative colitis.

BACKGROUND: Ulcerative colitis (UC) has a relapsing and remitting pattern, wherein the underlying mechanisms of the relapse might involve an enhanced uptake of luminal antigens which stimulate the immune response. The tricellular tight junction protein, tricellulin, takes charge of preventing paracellular passage of macromolecules. It is characterized by downregulated expression in active UC and its correct localization is regulated by angulins. We thus analyzed the tricellulin and angulin expression as well as intestinal barrier function and aimed to determine the role of tricellulin in the mechanisms of relapse. METHODS: Colon biopsies were collected from controls and UC patients who underwent colonoscopy at the central endoscopy department of Campus Benjamin Franklin, Charité - Universitätsmedizin Berlin. Remission of UC was defined basing on the clinical appearance and a normal Mayo endoscopic subscore. Intestinal barrier function was evaluated by electrophysiological and paracellular flux measurements on biopsies mounted in Ussing chambers. RESULTS: The downregulated tricellulin expression in active UC was recovered in remission UC to control values. Likewise, angulins were in remission UC at the same levels as in controls. Also, the epithelial resistance which was decreased in active UC was restored in remission to the same range as in controls, along with the unaltered paracellular permeabilities for fluorescein and FITC-dextran 4 kDa. CONCLUSIONS: In remission of UC, tricellulin expression level as well as intestinal barrier functions were restored to normal, after they were impaired in active UC. This points toward a re-sealing of the impaired tricellular paracellular pathway and abated uptake of antigens to normal rates in remission of UC.

Angulin-1 (LSR) Affects Paracellular Water Transport, However Only in Tight Epithelial Cells.

Water transport in epithelia occurs transcellularly (aquaporins) and paracellularly (claudin-2, claudin-15). Recently, we showed that downregulated tricellulin, a protein of the tricellular tight junction (tTJ, the site where three epithelial cells meet), increased transepithelial water flux. We now check the hypothesis that another tTJ-associated protein, angulin-1 (alias lipolysis-stimulated lipoprotein receptor, LSR) is a direct negative actuator of tTJ water permeability depending on the tightness of the epithelium. For this, a tight and an intermediate-tight epithelial cell line, MDCK C7 and HT-29/B6, were stably transfected with CRISPR/Cas9 and single-guide RNA targeting angulin-1 and morphologically and functionally characterized. Water flux induced by an osmotic gradient using 4-kDa dextran caused water flux to increase in angulin-1 KO clones in MDCK C7 cells, but not in HT-29/B6 cells. In addition, we found that water permeability in HT-29/B6 cells was not modified after either angulin-1 knockout or tricellulin knockdown, which may be related to the presence of other pathways, which reduce the impact of the tTJ pathway. In conclusion, modulation of the tTJ by knockout or knockdown of tTJ proteins affects ion and macromolecule permeability in tight and intermediate-tight epithelial cell lines, while the transepithelial water permeability was affected only in tight cell lines.

Mosaic expression of claudins in thick ascending limbs of Henle results in spatial separation of paracellular Na+ and Mg2+ transport.

The thick ascending limb (TAL) of Henle's loop drives paracellular Na+, Ca2+, and Mg2+ reabsorption via the tight junction (TJ). The TJ is composed of claudins that consist of four transmembrane segments, two extracellular segments (ECS1 and -2), and one intracellular loop. Claudins interact within the same (cis) and opposing (trans) plasma membranes. The claudins Cldn10b, -16, and -19 facilitate cation reabsorption in the TAL, and their absence leads to a severe disturbance of renal ion homeostasis. We combined electrophysiological measurements on microperfused mouse TAL segments with subsequent analysis of claudin expression by immunostaining and confocal microscopy. Claudin interaction properties were examined using heterologous expression in the TJ-free cell line HEK 293, live-cell imaging, and Förster/FRET. To reveal determinants of interaction properties, a set of TAL claudin protein chimeras was created and analyzed. Our main findings are that (i) TAL TJs show a mosaic expression pattern of either cldn10b or cldn3/cldn16/cldn19 in a complex; (ii) TJs dominated by cldn10b prefer Na+ over Mg2+, whereas TJs dominated by cldn16 favor Mg2+ over Na+; (iii) cldn10b does not interact with other TAL claudins, whereas cldn3 and cldn16 can interact with cldn19 to form joint strands; and (iv) further claudin segments in addition to ECS2 are crucial for trans interaction. We suggest the existence of at least two spatially distinct types of paracellular channels in TAL: a cldn10b-based channel for monovalent cations such as Na+ and a spatially distinct site for reabsorption of divalent cations such as Ca2+ and Mg2.

Special Issue on "The Tight Junction and Its Proteins: More than Just a Barrier".

For a long time, the tight junction (TJ) was known to form and regulate the paracellular barrier between epithelia and endothelial cell sheets. Starting shortly after the discovery of the proteins forming the TJ-mainly, the two families of claudins and TAMPs-several other functions have been discovered, a striking one being the surprising finding that some claudins form paracellular channels for small ions and/or water. This Special Issue covers numerous dedicated topics including pathogens affecting the TJ barrier, TJ regulation via immune cells, the TJ as a therapeutic target, TJ and cell polarity, the function of and regulation by proteins of the tricellular TJ, the TJ as a regulator of cellular processes, organ- and tissue-specific functions, TJs as sensors and reactors to environmental conditions, and last, but not least, TJ proteins and cancer. It is not surprising that due to this diversity of topics and functions, the still-young field of TJ research is growing fast. This Editorial gives an introduction to all 43 papers of the Special Issue in a structured topical order.

Leptin Downregulates Angulin-1 in Active Crohn's Disease via STAT3.

Crohn's disease (CD) has an altered intestinal barrier function, yet the underlying mechanisms remain to be disclosed. The tricellular tight junction protein tricellulin is involved in the maintenance of the paracellular macromolecule barrier and features an unchanged expression level in CD but a shifted localization. As angulins are known to regulate the localization of tricellulin, we hypothesized the involvement of angulins in CD. Using human biopsies, we found angulin-1 was downregulated in active CD compared with both controls and CD in remission. In T84 and Caco-2 monolayers, leptin, a cytokine secreted by fat tissue and affected in CD, decreased angulin-1 expression. This effect was completely blocked by STAT3 inhibitors, Stattic and WP1066, but only partially by JAK2 inhibitor AG490. The effect of leptin was also seen at a functional level as we observed in Caco-2 cells an increased permeability for FITC-dextran 4 kDa indicating an impaired barrier against macromolecule uptake. In conclusion, we were able to show that in active CD angulin-1 expression is downregulated, which leads to increased macromolecule permeability and is inducible by leptin via STAT3. This suggests that angulin-1 and leptin secretion are potential targets for intervention in CD to restore the impaired intestinal barrier.

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.

Tricellulin Effect on Paracellular Water Transport.

In epithelia, large amounts of water pass by transcellular and paracellular pathways, driven by the osmotic gradient built up by the movement of solutes. The transcellular pathway has been molecularly characterized by the discovery of aquaporin membrane channels. Unlike this, the existence of a paracellular pathway for water through the tight junctions (TJ) was discussed controversially for many years until two molecular components of paracellular water transport, claudin-2 and claudin-15, were identified. A main protein of the tricellular TJ (tTJ), tricellulin, was shown to be downregulated in ulcerative colitis leading to increased permeability to macromolecules. Whether or not tricellulin also regulates water transport is unknown yet. To this end, an epithelial cell line featuring properties of a tight epithelium, Madin-Darby canine kidney cells clone 7 (MDCK C7), was stably transfected with small hairpin RNA (shRNA) targeting tricellulin, a protein of the tTJ essential for the barrier against passage of solutes up to 10 kDa. Water flux was induced by osmotic gradients using mannitol or 4 and 40 kDa-dextran. Water flux in tricellulin knockdown (KD) cells was higher compared to that of vector controls, indicating a direct role of tricellulin in regulating water permeability in a tight epithelial cell line. We conclude that tricellulin increases water permeability at reduced expression.

Tilivalline- and Tilimycin-Independent Effects of Klebsiella oxytoca on Tight Junction-Mediated Intestinal Barrier Impairment.

Klebsiella oxytoca causes antibiotic-associated hemorrhagic colitis and diarrhea. This was attributed largely to its secreted cytotoxins tilivalline and tilimycin, inductors of epithelial apoptosis. To study whether Klebsiella oxytoca exerts further barrier effects, T84 monolayers were challenged with bacterial supernatants derived from tilivalline/tilimycin-producing AHC6 or its isogeneic tilivalline/tilimycin-deficient strain Mut-89. Both preparations decreased transepithelial resistance, enhanced fluorescein and FITC-dextran-4kDa permeabilities, and reduced expression of barrier-forming tight junction proteins claudin-5 and -8. Laser scanning microscopy indicated redistribution of both claudins off the tight junction region in T84 monolayers as well as in colon crypts of mice infected with AHC6 or Mut-89, indicating that these effects are tilivalline/tilimycin-independent. Furthermore, claudin-1 was affected, but only in a tilivalline/tilimycin-dependent manner. In conclusion, Klebsiella oxytoca induced intestinal barrier impairment by two mechanisms: the tilivalline/tilimycin-dependent one, acting by increasing cellular apoptosis and a tilivalline/tilimycin-independent one, acting by weakening the paracellular pathway through the tight junction proteins claudin-5 and -8.

A cytochrome P450 CYP71 enzyme expressed in Sorghum bicolor root hair cells participates in the biosynthesis of the benzoquinone allelochemical sorgoleone.

Sorgoleone, a major component of the hydrophobic root exudates of Sorghum spp., is probably responsible for many of the allelopathic properties attributed to members of this genus. Much of the biosynthetic pathway for this compound has been elucidated, with the exception of the enzyme responsible for the catalysis of the addition of two hydroxyl groups to the resorcinol ring. A library prepared from isolated Sorghum bicolor root hair cells was first mined for P450-like sequences, which were then analyzed by quantitative reverse transcription-polymerase chain reaction (RT-qPCR) to identify those preferentially expressed in root hairs. Full-length open reading frames for each candidate were generated, and then analyzed biochemically using both a yeast expression system and transient expression in Nicotiana benthamiana leaves. RNA interference (RNAi)-mediated repression in transgenic S. bicolor was used to confirm the roles of these candidates in the biosynthesis of sorgoleone in planta. A P450 enzyme, designated CYP71AM1, was found to be capable of catalyzing the formation of dihydrosorgoleone using 5-pentadecatrienyl resorcinol-3-methyl ether as substrate, as determined by gas chromatography-mass spectroscopy (GC-MS). RNAi-mediated repression of CYP71AM1 in S. bicolor resulted in decreased sorgoleone contents in multiple independent transformant events. Our results strongly suggest that CYP71AM1 participates in the biosynthetic pathway of the allelochemical sorgoleone.

In Colon Epithelia, Clostridium perfringens Enterotoxin Causes Focal Leaks by Targeting Claudins Which are Apically Accessible Due to Tight Junction Derangement.

Clostridium perfringens enterotoxin (CPE) causes food poisoning and antibiotic-associated diarrhea. It uses some claudin tight junction proteins (eg, claudin-4) as receptors to form Ca2+-permeable pores in the membrane, damaging epithelial cells in small intestine and colon. We demonstrate that only a subpopulation of colonic enterocytes which are characterized by apical dislocation of claudins are CPE-susceptible. CPE-mediated damage was enhanced if paracellular barrier was impaired by Ca2+ depletion, proinflammatory cytokine tumor necrosis factor α, or dedifferentiation. Microscopy, Ca2+ monitoring, and electrophysiological data showed that CPE-mediated cytotoxicity and barrier disruption was limited by extent of CPE-binding. The latter was restricted by accessibility of non-junctional claudin molecules such as claudin-4 at apical membranes. Focal-leaks detected in HT-29/B6 colonic monolayers were verified for native tissue using colon biopsies. These mechanistic findings indicate how CPE-mediated effects may turn from self-limiting diarrhea into severe clinical manifestation such as colonic necrosis-if intestinal barrier dysfunction, eg, during inflammation facilitates claudin accessibility.

Claudin-related intestinal diseases.

With up to 200 m(2) the human intestine is the organ with the largest absorptive surface of the body. It is lined by a single layer of epithelial cells that separates the host from the environment. The intestinal epithelium provides both, selective absorption of nutrients, ions, and water but also a highly effective barrier function which includes the first line of defense against environmental antigens. The paracellular part of this barrier function is provided by tight junction (TJ) proteins, especially the large family of claudins. Changes in abundance or molecular structure of claudins can generally result in three typical effects, (i) decreased absorptive passage, (ii) increased secretory passage of small solutes and water causing leak flux diarrhea and (iii) increased absorptive passage of macromolecules which may induce inflammatory processes. Several intestinal diseases are associated with such changes that can result in intestinal inflammation and symptoms like weight loss, abdominal pain or diarrhea. This review summarizes our current knowledge on barrier dysfunction and claudin dysregulation in several intestinal diseases gastroenterologists are often faced with, like inflammatory bowel disease, microscopic colitis, celiac disease, irritable bowel syndrome, gallstones and infectious diseases like HIV enteropathy, Campylobacter jejuni and Clostridium perfringens infection.

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.

Tight junctions of the proximal tubule and their channel proteins.

The renal proximal tubule achieves the majority of renal water and solute reabsorption with the help of paracellular channels which lead through the tight junction. The proteins forming such channels in the proximal tubule are claudin-2, claudin-10a, and possibly claudin-17. Claudin-2 forms paracellular channels selective for small cations like Na+ and K+. Independently of each other, claudin-10a and claudin-17 form anion-selective channels. The claudins form the paracellular "pore pathway" and are integrated, together with purely sealing claudins and other tight junction proteins, in the belt of tight junction strands surrounding the tubular epithelial cells. In most species, the proximal tubular tight junction consists of only 1-2 (pars convoluta) to 3-5 (pars recta) horizontal strands. Even so, they seal the tubule very effectively against leak passage of nutrients and larger molecules. Remarkably, claudin-2 channels are also permeable to water so that 20-25% of proximal water absorption may occur paracellularly. Although the exact structure of the claudin-2 channel is still unknown, it is clear that Na+ and water share the same pore. Already solved claudin crystal structures reveal a characteristic ß-sheet, comprising ß-strands from both extracellular loops, which is anchored to a left-handed four-transmembrane helix bundle. This allowed homology modeling of channel-forming claudins present in the proximal tubule. The surface of cation- and anion-selective claudins differ in electrostatic potentials in the area of the proposed ion channel, resulting in the opposite charge selectivity of these claudins. Presently, while models of the molecular structure of the claudin-based oligomeric channels have been proposed, its full understanding has only started.

Zinc strengthens the jejunal barrier by reversibly tightening the paracellular route.

During the postweaning period, piglets are prone to gastrointestinal infections. The resulting impairment of intestinal barrier function may cause diarrhea associated with growth retardation or even death of piglets. Orally applied Zn is commonly used to prevent and treat diarrhea, but its mode of action still needs to be elucidated. To analyze the molecular mechanism whereby Zn acts on porcine intestinal barrier function, ex vivo studies on piglet jejunum and accompanying in vitro studies on a porcine jejunal epithelial cell line, IPEC-J2/PS, were performed with electrophysiological tools. Feeding pharmacological Zn doses exerted no significant electrophysiologically ascertainable short- and long-term effects on jejunal barrier function ex vivo. However, in IPEC-J2/PS, basolateral Zn was cytotoxic since its application caused a release of lactate dehydrogenase and an irreversible breakdown of the epithelial barrier. In contrast, apical Zn application caused an immediate increase in paracellular resistance and a decrease in permeability to the paracellular marker fluorescein, reflecting overall barrier strengthening in vitro. Apical effects were fully reversible upon washout. This indicates that Zn supplemented to feed was completely washed out during ex vivo jejunum preparation. We conclude that there is no evidence for long-term barrier effects through prophylactic Zn supplementation and that extracellular Zn acts acutely and reversibly from the apical side via tightening the paracellular route, thus counteracting leak-flux diarrhea.NEW & NOTEWORTHY Therapeutically administered Zn successfully treats diarrhea in veterinary and human medicine. Here we present data that Zn strengthens the porcine jejunal epithelial barrier by reversibly tightening the paracellular route for inorganic ions and small solutes. Acute or long-lasting Zn effects on transcellular transport (Cl- secretion) were not detected. We therefore conclude that Zn is useful for acutely treating leak-flux diarrhea rather than secretory diarrhea. Suitability as prophylactic feed supplement, however, is questionable.

Myrrh exerts barrier-stabilising and -protective effects in HT-29/B6 and Caco-2 intestinal epithelial cells.

PURPOSE: Myrrh, the oleo-gum resin of Commiphora molmol, is well known for its anti-inflammatory properties. In different animal models, it protected against DSS-, TNBS- and oxazolone-induced colitis. To date, no information concerning the effect of myrrh on barrier properties are available. Thus, this study investigates the effect of myrrh on paracellular barrier function in the absence or presence of the pro-inflammatory cytokine TNFα. METHODS: Monolayers of human colon cell lines HT-29/B6 and Caco-2 were incubated with myrrh under control conditions or after challenge with the pro-inflammatory cytokine TNFα. Barrier function was analysed by electrophysiological and permeability measurements, Western blotting, immunostaining in combination with confocal microscopy, and freeze-fracture electron microscopy. RESULTS: In Caco-2 cells, myrrh induced an increase in transepithelial resistance (TER) which was associated with downregulation of the channel-forming tight junction (TJ) protein claudin-2 via inhibition of the PI3 kinase signalling pathway. In HT-29/B6 cells, myrrh had no effect on barrier properties under basic conditions, but protected against barrier damage induced by TNFα, as indicated by a decrease in TER and an increase in fluorescein permeability. The TNFα effect was associated with a redistribution of the sealing TJ protein claudin-1, an increase in the expression of claudin-2 and a change in TJ ultrastructure. Most importantly, all TNFα effects were inhibited by myrrh. The effect of myrrh on claudin-2 expression in this cell line was mediated via inhibition of the STAT6 pathway. CONCLUSIONS: This study shows for the first time that myrrh exerts barrier-stabilising and TNFα-antagonising effects in human intestinal epithelial cell models via inhibition of PI3K and STAT6 signalling. This suggests therapeutic application of myrrh in intestinal diseases associated with barrier defects and inflammation.

Molecular basis of claudin-17 anion selectivity.

Claudin-17 is a paracellular channel-forming tight junction protein. Unlike the cation channels claudin-2 and -15, claudin-17 forms a distinct anion-selective channel. Aim of this study was to determine the molecular basis of channel formation and charge selectivity of this protein. To achieve this, residues located in the extracellular loops (ECL) 1 and 2 of claudin-17 were substituted, preferably those whose charges differed in claudin-17 and in claudin-2 or -15. The respective mutants were stably expressed in MDCK C7 cells and their ability to form charge-selective channels was analyzed by measuring ion permeabilities and transepithelial electrical resistance. The functional data were combined with homology modeling of the claudin-17 protomer using the structure of claudin-15 as template. In ECL1, K65, R31, E48, and E44 were found to be stronger involved in Cldn17 channel function than the clustered R45, R56, R59, and R61. For K65, not only charge but also stereochemical properties were crucial for formation of the anion-selective channel. In ECL2, both Y149 and H154 were found to contribute to constitution of the anion channel in a distinct manner. In conclusion, we provide insight into the molecular mechanism of the formation of charge- and size-selective paracellular ion channels. In detail, we propose a hydrophilic furrow in the claudin-17 protomer spanning from a gap between the ends of TM2 and TM3 along R31, E48, and Y67 to a gap between K65 and S68 lining the anion channel.