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Tight junction (TJ) strands between epithelial or endothelial cells are formed by claudins, a protein family comprising up to 27 members in mammals. Although many more proteins are involved in the formation of TJ complexes, claudins are the only TJ proteins that are able to form TJ-like strands when overexpressed in cells that are normally devoid of TJs (e.g., fibroblasts). Within the paracellular cleft, the extracellular domains of claudins provide the matrix that seals the paracellular pathway. However, within this matrix, some claudins act as channels that specifically allow certain ions to cross this barrier. Barrier-forming claudins predominate in epithelia that enclose compartments containing harmful ion concentrations (e.g., H + in the stomach, K + in the inner ear endolymph) or high pressures (e.g., in blastocoel or brain ventricle formation during development). Here, even seemingly minor alterations in TJ composition may be detrimental to the organism. In contrast, in many transporting epithelia, channel-forming claudins are essential for transcellular and paracellular transport coupling. Mutation or knockout of channel-forming claudins in these tissues brings both transcellular and paracellular transports to a standstill. The present review will present examples to illustrate the importance of single members of the claudin family in general epithelial transport physiology.

Claudins are the main determinants of barrier properties of the tight junction. Many claudins have been shown to act by tightening the paracellular pathway, but several function as paracellular channels. While some depend on the endogenous claudin background of the analyzed cell line, for other claudins, a distinct charge-selectivity has been shown. This paper portrays cation-selective (claudin-2, claudin-10b, claudin-15) and anion-selective (claudin-10a, claudin-17) claudins and claudins with debatable channel properties (claudin-4, claudin-7, claudin-16). It also describes molecular properties determining the observed charge-selectivity and pore properties in general. In leaky tissues, they widely determine overall transport characteristics by providing paracellular ion-selective pathways. In small intestine, claudin-2 and claudin-15 replace each other in the developing gut. In kidney proximal tubules, claudin-2, claudin-10, and claudin-17 allow for paracellular reabsorption of sodium, chloride, and water., (© 2012 New York Academy of Sciences.)

Skin barrier function is indispensable to prevent the uncontrolled loss of water and solutes and to protect the body from external assaults. To fulfil this function, keratinocytes undergo a complex pathway of differentiation that terminates in the formation of the stratum corneum. Additionally, tight junctions (TJs), which are cell-cell junctions localized in the stratum granulosum, are involved in the barrier function of the skin. Important biological and clinical roles of TJs are strongly suggested by altered TJ protein levels and distribution in skin diseases like psoriasis, ichthyosis and atopic dermatitis. Because these skin diseases show alterations in differentiation and TJs, it was suggested that changes in TJs might simply be a consequence of altered differentiation. However, in this viewpoint, we like to argue that the situation is not as simple and depends on the specific microenvironment. We discuss three hypotheses regarding the interplay between TJs/TJ proteins and differentiation: (1) TJs/TJ proteins are influenced by differentiation, (2) differentiation is influenced by TJs/TJ proteins, and (3) TJs/TJ proteins and differentiation are independent of each other. In addition, the concept is introduced that both processes are going on at the same time, which means that while one specific TJ protein/barrier component might be influenced by differentiation, the other may influence differentiation., (© 2011 John Wiley & Sons A/S.)

Background: Epithelial barrier defects are well known in coeliac disease, but the mechanisms are only poorly defined. It is unclear, whether barrier disturbance reflects upregulated epithelial transcytosis or paracellular leakage., Objective: To characterise the molecular structure and function of the epithelial tight junction (TJ) and mechanisms of its dysregulation., Methods: Molecular analysis of proteins involved in TJ assembly and their regulation was performed by western blotting and confocal microscopy correlated to electrophysiology., Results: A complex alteration of the composition of epithelial TJ proteins (with more pore-forming claudins like claudin-2 and a reduction in tightening claudins like claudin-3, -5 and -7) was found for protein expression and subcellular localisation, responsible for an increase in paracellular biotin-NHS uptake. In contrast, epithelial apoptosis was only moderately elevated (accounting for a minor portion of barrier defects) and epithelial gross lesions--for example, at cell extrusion zones, were absent. This TJ alteration was linked to an altered localisation/expression of proteins regulating TJ assembly, the polarity complex protein Par-3 and the serine-/threonine phosphatase PP-1., Conclusions: Changes in cell polarity proteins Par-3 and PP-1 are associated with altered expression and assembly of TJ proteins claudin-2, -3, -5 and -7 and ZO-1, causing paracellular leakage in active coeliac disease.

The paracellular barrier of epithelia and endothelia is established by several tight junction proteins including claudin-3. Although claudin-3 is present in many epithelia including skin, lung, kidney, and intestine and in endothelia, its function is unresolved as yet. We therefore characterized claudin-3 by stable transfection of MDCK II kidney tubule cells with human claudin-3 cDNA. Two clone systems were analyzed, exhibiting high or low claudin-2 expression, respectively. Expression of other claudins was unchanged. Ultrastructurally, tight junction strands were changed toward uninterrupted and rounded meshwork loops. Functionally, the paracellular resistance of claudin-3-transfected monolayers was strongly elevated, causing an increase in transepithelial resistance compared to vector controls. Permeabilities for mono- and divalent cations and for anions were decreased. In the high-claudin-2 system, claudin-3 reduced claudin-2-induced cation selectivity, while in the low-claudin-2 system no charge preference was observed, the latter thus reflecting the "intrinsic" action of claudin-3. Furthermore, the passage of the paracellular tracers fluorescein (332Da) and FD-4 (4kDa) was decreased, whereas the permeability to water was not affected. We demonstrate that claudin-3 alters the tight junction meshwork and seals the paracellular pathway against the passage of small ions of either charge and uncharged solutes. Thus, in a kidney model epithelium, claudin-3 acts as a general barrier-forming protein., (Copyright © 2010 Elsevier B.V. All rights reserved.)

Whether or not significant amounts of water pass the tight junction (TJ) of leaky epithelia is still unresolved, because it is difficult to separate transcellular water flux from TJ-controlled paracellular water flux. Using an approach without differentiating technically between the transcellular and paracellular route, we measured transepithelial water flux with and without selective molecular perturbation of the TJ to unequivocally attribute changes to the paracellular pathway. To this end, MDCK C7 cells were stably transfected with either claudin-2 or claudin-10b, two paracellular cation-channel-forming TJ proteins that are not endogenously expressed in this cell line. Claudin-2 is typical of leaky, water-transporting epithelia, such as the kidney proximal tubule, whereas claudin-10b is present in numerous epithelia, including water-impermeable segments of the loop of Henle. Neither transfection altered the expression of endogenous claudins or aquaporins. Water flux was induced by an osmotic gradient, a Na(+) gradient or both. Under all conditions, water flux in claudin-2-transfected cells was elevated compared with vector controls, indicating claudin-2-mediated paracellular water permeability. Na(+)-driven water transport in the absence of an osmotic gradient indicates a single-file mechanism. By contrast, claudin-10b transfection did not alter water flux. We conclude that claudin-2, but not claudin-10b, forms a paracellular water channel and thus mediates paracellular water transport in leaky epithelia.

Claudin-16 (paracellin-1) is a tight junction protein localized mainly in the thick ascending limb of Henle's loop and also in the distal nephron. Its defect causes familial hypomagnesaemia with hypercalciuria and nephrocalcinosis. This had been taken as an indication that claudin-16 conveys paracellular Mg(2+) and Ca(2+) transport; however, evidence is still conflicting. We studied paracellular ion permeabilities as well as effects of claudin-16 on the driving forces for passive ion movement. MDCK-C7 cells were stably transfected with wild-type (wt) and mutant (R146T, T233R) claudin-16. Results indicated that paracellular permeability to Mg(2+) but not to Ca(2+) is increased in cells transfected with wt compared to mutant claudin-16 and control cells. Increased basolateral Mg(2+) concentration activated a transcellular Cl(-) current which was greatly enhanced in cells transfected with wt and T233R claudin-16, as compared to R146T claudin-16-transfected or control cells. This current was triggered by the basolateral calcium-sensing receptor causing Ca(2+) release from internal stores, thus activating apical Ca(2+)-sensitive Cl(-) channels and basolateral Ca(2+)-sensitive K(+) channels. Immunohistochemical data suggest that the Cl(-) channel involved is bestrophin. We conclude that claudin-16 itself possesses only moderate paracellular Mg(2+) permeability but governs transcellular Cl(-) currents by interaction with apical Ca(2+)-activated Cl(-) channels, presumably bestrophin. As the transepithelial voltage generated by such a current alters the driving force for all ions, this may be the major mechanism to regulate Mg(2+) and Ca(2+) absorption in the kidney.

Tricellulin is a tight junction protein localized in tricellular tight junctions (tTJs), the meeting points of three cells, but also in bicellular tight junctions (bTJs). To investigate its specific barrier functions in bTJs and tTJs, TRIC-a was expressed in low-level tricellulin-expressing cells, and MDCK II, either in all TJs or only in tTJs. When expressed in all TJs, tricellulin increased paracellular electrical resistance and decreased permeability to ions and larger solutes, which are associated with enhanced ultrastructural integrity of bTJs toward enhanced strand linearity. In tTJs in contrast, ultrastructure was unchanged and tricellulin minimized permeability to macromolecules but not to ions. This paradox is explained by properties of the tTJ central tube which is wide enough for passage of macromolecules, but too rare to contribute significantly to ion permeability. In conclusion, at low tricellulin expression the tTJ central tube forms a pathway for macromolecules. At higher expression, tricellulin forms a barrier in tTJs effective only for macromolecules and in bTJs for solutes of all sizes.

The tight junction protein claudin-10 is known to exist in two isoforms, resulting from two alternative exons, 1a and 1b (Cldn10a, Cldn10b). Here, we identified and characterized another four claudin-10 splice variants in mouse and human. One (Cldn10a_v1) results from an alternative splice donor site, causing a deletion of the last 57 nucleotides of exon 1a. For each of these three variants one further splice variant was identified (Cldn10a_v2, Cldn10a_v3, Cldn10b_v1), lacking exon 4. When transfected into MDCK cells, Cldn10a, Cldn10a_v1 and Cldn10b were inserted into the tight junction, whereas isoforms of splice variants lacking exon 4 were retained in the endoplasmic reticulum. Cldn10a transfection into MDCK cells confirmed the previously described increase in paracellular anion permeability. Cldn10a_v1 transfection had no direct effect, but modulated Cldn10a-induced organic anion permeability. At variance with previous reports in MDCK-II cells, transfection of high-resistance MDCK-C7 cells with Cldn10b dramatically decreased transepithelial resistance, increased cation permeability, and changed monovalent cation selectivity from Eisenman sequence IV to X, indicating the presence of a high field-strength binding site that almost completely removes the hydration shell of the permeating cations. The extent of all these effects strongly depended on the endogenous claudins of the transfected cells.

The thick ascending limb (TAL) of Henle mediates transcellular reabsorption of NaCl while generating a lumen-positive voltage that drives passive paracellular reabsorption of divalent cations. Disturbance of paracellular reabsorption leads to Ca(2+) and Mg(2+) wasting in patients with the rare inherited disorder of familial hypercalciuric hypomagnesemia with nephrocalcinosis (FHHNC). Recent work has shown that the claudin family of tight junction proteins form paracellular pores and determine the ion selectivity of paracellular permeability. Importantly, FHHNC has been found to be caused by mutations in two of these genes, claudin-16 and claudin-19, and mice with knockdown of claudin-16 reproduce many of the features of FHHNC. Here, we review the physiology of TAL ion transport, present the current view of the role and mechanism of claudins in determining paracellular permeability, and discuss the possible pathogenic mechanisms responsible for FHHNC.

Tight junctions form the paracellular barrier for ions and uncharged solutes not only in "tight" but also in "leaky" epithelia. In the premolecular era of tight junction research, this was believed to be achieved in a perfect or less perfect way, depending mainly on the amount of horizontally oriented tight junction strands. During the past decade it emerged that tight junction molecules, such as claudin-1 and many others, strengthen the barrier, while a few claudins, such as claudin-2 or -10, weaken it. This report focuses on three claudins: one channel former and two barrier builders. Claudin-2 represents the prototype of a paracellular, channel-forming, tight junction protein responsible for specific transfer of solutes across the epithelium without entering the cells. This channel is selective for small cations but nearly impermeable to anions and uncharged solutes of any size. In contrast, claudin-5, a tight junction protein typical for all endothelia but also found in some epithelia, was characterized as a potent barrier builder. Claudin-8, another barrier builder, was demonstrated to be regulated by Na(+) uptake in surface epithelial cells of human colon. Here, aldosterone enhanced Na(+) absorption by dual action: transcellularly by inducing the epithelial sodium channel and paracellularly by preventing back leakage of absorbed Na(+) by upregulating claudin-8.

During the past decade, claudins have been established as major determinants of paracellular permeablilty in epithelia. In the kidney, each nephron segment expresses a distinct pattern of claudins. Cells of the thick ascending limb of Henle's loop (TAL), which is characterized by high paracellular cation permeability, co-express an unusually large number of different claudins: claudin-10, -16, and -19 and, depending on the species, also claudin-3, -4, -8, and/or -11. The function of most of these claudins has been investigated in vitro. We present a summary of their function with special emphasis on claudin-16 and -19. Mutations in the corresponding human genes lead to severely impaired renal Ca(2+) and Mg(2+) handling. To date, 42 different claudin-16 mutations and three claudin-19 mutations have been reported. These mutations prevent the claudins from reaching the surface membrane, decrease membrane residence time, or render them functionless. In spite of the clear clinical symptoms such as hypomagnesemia, hypercalciuria, nephrocalcinosis, and renal insufficiency, mechanisms that link claudin-16 and -19 to these symptoms are still unknown. Depending on the cell type used in overexpression studies, claudin-16 appears to cause a mild increase in paracellular Mg(2+)-permeability or a pronounced increase in Na(+) permeability. Claudin-19 selectively decreases Cl(-) permeability, thus synergistically increasing relative cation permeability, or indiscriminately decreases paracellular permeability. In the light of these results it is hypothesized that the renal Mg(2+)/Ca(2+) waste may not be solely due to reduced resorption in the TAL but at least in part to paracellular back-leak of Mg(2+)/Ca(2+) into the tubular lumen of the distal convoluted tubule.

Paracellular ion transport in epithelia is mediated by pores formed by members of the claudin family. The degree of selectivity and the molecular mechanism of ion permeation through claudin pores are poorly understood. By expressing a high-conductance claudin isoform, claudin-2, in high-resistance Madin-Darby canine kidney cells under the control of an inducible promoter, we were able to quantitate claudin pore permeability. Claudin-2 pores were found to be narrow, fluid filled, and cation selective. Charge selectivity was mediated by the electrostatic interaction of partially dehydrated permeating cations with a negatively charged site within the pore that is formed by the side chain carboxyl group of aspartate-65. Thus, paracellular pores use intrapore electrostatic binding sites to achieve a high conductance with a high degree of charge selectivity.

Claudin-16 (Cldn16) is selectively expressed at tight junctions (TJs) of renal epithelial cells of the thick ascending limb of Henle's loop, where it plays a central role in the reabsorption of divalent cations. Over 20 different mutations in the CLDN16 gene have been identified in patients with familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC), a disease of excessive renal Mg2+ and Ca2+ excretion. Here we show that disease-causing mutations can lead to the intracellular retention of Cldn16 or affect its capacity to facilitate paracellular Mg2+ transport. Nine of the 21 Cldn16 mutants we characterized were retained in the endoplasmic reticulum, where they underwent proteasomal degradation. Three mutants accumulated in the Golgi complex. Two mutants were efficiently delivered to lysosomes, one via clathrin-mediated endocytosis following transport to the cell surface and the other without appearing on the plasma membrane. The remaining 7 mutants localized to TJs, and 4 were found to be defective in paracellular Mg2+ transport. We demonstrate that pharmacological chaperones rescued surface expression of several retained Cldn16 mutants. We conclude that FHHNC can result from mutations in Cldn16 that affect intracellular trafficking or paracellular Mg2+ permeability. Knowledge of the molecular defects associated with disease-causing Cldn16 mutations may open new venues for therapeutic intervention.

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 β-sheet, comprising β-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. [ABSTRACT FROM AUTHOR]

The mechanism of TJ (tight junction) assembly and the structure of TJ strand-forming Cldns (claudins) are unclear. To identify determinants of assembly of blood-brain barrier-related Cldn3 and Cldn5, chimaeric mutants were analysed by cellular reconstitution of TJ strands and live-cell imaging. On the basis of the rescue of mutants deficient for strand formation, we identified Cldn5 residues (Cys128, Ala132, Ile142, Ala163, Ile166 and Leu174) involved in Cldn folding and assembly. Experimental results were combined with structural bioinformatics approaches. Initially the experimentally validated previous model of the ECL2 (extracellular loop 2) of Cldn5 was extended to the flanking transmembrane segments (TM3/TM4). A coiled-coil interface probably caused by alternating small and large residues is supported by concomitant knob-into-hole interactions including Cldn5-specific residues identified in the present paper. To address arrangement of the TMs in a four-helix bundle, data from evolutionary sequence couplings and comparative modelling of intramolecular interfaces in the transmembrane region of Cldns led to a complete Cldn5 model. Our suggested Cldn subtypespecific intramolecular interfaces that are formed by conserved coiled-coil motifs and non-conserved residues in distinct TM positions were confirmed by the recently released crystal structure of Cldn15. The identified molecular and structural determinants essentially contribute to assembly of Cldns into TJ strands. [ABSTRACT FROM AUTHOR]

It has been reported recently that the cystic fibrosis transmembrane conductance regulator (CFTR) besides transcellular chloride transport, also controls the paracellular permeability of bronchial epithelium. The aim of this study was to test whether overexpressing wtCFTR solely regulates paracellular permeability of cell monolayers. To answer this question we used a CFBE41o– cell line transfected with wtCFTR or mutant F508del-CFTR and compered them with parental line and healthy 16HBE14o– cells. Transepithelial electrical resistance (TER) and paracellular fluorescein flux were measured under control and CFTR-stimulating conditions. CFTR stimulation significant decreased TER in 16HBE14o– and also in CFBE41o– cells transfected with wtCFTR. In contrast, TER increased upon stimulation in CFBE41o– cells and CFBE41o– cells transfected with F508del-CFTR. Under non-stimulated conditions, all four cell lines had similar paracellular fluorescein flux. Stimulation increased only the paracellular permeability of the 16HBE14o– cell monolayers. We observed that 16HBE14o– cells were significantly smaller and showed a different structure of cell-cell contacts than CFBE41o– and its overexpressing clones. Consequently, 16HBE14o– cells have about 80% more cell-cell contacts through which electrical current and solutes can leak. Also tight junction protein composition is different in ‘healthy’ 16HBE14o– cells compared to ‘cystic fibrosis’ CFBE41o– cells. We found that claudin-3 expression was considerably stronger in 16HBE14o– cells than in the three CFBE41o– cell clones and thus independent of the presence of functional CFTR. Together, CFBE41o– cell line transfection with wtCFTR modifies transcellular conductance, but not the paracellular permeability. We conclude that CFTR overexpression is not sufficient to fully reconstitute transport in CF bronchial epithelium. Hence, it is not recommended to use those cell lines to study CFTR-dependent epithelial transport. [ABSTRACT FROM AUTHOR]

Claudins are tight junction membrane proteins that are expressed in epithelia and endothelia and form paracellular barriers and pores that determine tight junction permeability. This review summarizes our current knowledge of this large protein family and discusses recent advances in our understanding of their structure and physiological functions. [ABSTRACT FROM AUTHOR]

Strong communication and interaction between the retinal pigment epithelium (RPE) and the photoreceptor (PR) cells is essential for vision. RPE cells are essential for supporting and maintaining PR cells by transporting nutrients, waste products and ions, and phagocytosing photoreceptor outer segments (POS). POS phagocytosis follows a circadian pattern, taking place in the morning in human, mice and other organisms. However, it remains unknown whether other RPE processes follow a daily rhythm. To study the daily rhythm of RPE cells, we isolated murine RPE cells at six different time points during a 24 h period, after which RNA was isolated and sequenced. Murine RPE flatmounts were isolated at four different time points to study daily rhythm in protein abundance and localisation. EnrichR pathway analysis resulted in 13 significantly-enriched KEGG pathways (p < 0.01) of which seven showed a large number of overlapping genes. Several genes were involved in intracellular trafficking, possibly playing a role in nutrient transport, POS phagocytosis or membrane protein trafficking, with different expression patterns during the day-night cycle. Other genes were involved in actin cytoskeleton building, remodelling and crosslinking and showed a high expression in the morning, suggesting actin cytoskeleton remodelling at this time point. Finally, tight junction proteins Cldn2 and Cldn4 showed a difference in RNA and protein expression and tight junction localisation over time. Our study suggests that several important processes in the RPE follow a day-night rhythm, including intracellular trafficking, and processes involving the actin cytoskeleton and tight junctions. The differential protein localisation of Cldn2 in the RPE during the day-night cycle suggest that Cldn2 may facilitate paracellular water and sodium transport during the day. • Cldn2 RNA and protein expression show clear differences over time in RPE cells. • Paracellular water and sodium transport in RPE might be facilitated by Cldn2 during the day. • Genes involved in intracellular trafficking show differential day-night expression. • Genes involved in cytoskeleton remodelling show high expression in the morning. [ABSTRACT FROM AUTHOR]

The mechanism of tight junction (TJ) assembly and the structure of claudins (Cldn) that form the TJ strands are unclear. This limits the molecular understanding of paracellular barriers and strategies for drug delivery across tissue barriers. Cldn3 and Cldn5 are both common in the blood-brain barrier but form TJ strands with different ultrastructures. To identify the molecular determinants of folding and assembly of these classic claudins, Cldn3/Cldn5 chimericmutants were generated and analyzed by cellular reconstitution of TJ strands, live cell confocal imaging, and freeze-fracture electron microscopy. A comprehensive screening was performed on the basis of the rescue of mutants deficient for strand formation. Cldn3/Cldn5 residues in transmembrane segment 3, TM3 (Ala-127/Cys-128, Ser-136/ Cys-137, Ser-138/Phe-139), and the transition of TM3 to extracellular loop 2, ECL2 (Thr-141/Ile-142) and ECL2 (Asn-148/ Asp-149, Leu-150/Thr-151, Arg-157/Tyr-158), were identified to be involved in claudin folding and/or assembly. Blue native PAGE and FRET assays revealed 1% n-dodecyl β-D-maltosideresistant cis-dimerization for Cldn5 but not for Cldn3. This homophilic interaction was found to be stabilized by residues in TM3.The resulting subtype-specific cis-dimer is suggested to be a subunit of polymeric TJ strands and contributes to the specific ultrastructure of the TJ detected by freeze-fracture electron microscopy. In particular, the Cldn5-like exoplasmic face-associated and particle-type strands were found to be related to cisdimerization. These results provide new insight into the mechanisms of paracellular barrier formation by demonstrating that defined non-conserved residues in TM3 and ECL2 of classic claudins contribute to the formation of TJ strands with differing ultrastructures. [ABSTRACT FROM AUTHOR]