Expression, solubilization, and biochemical characterization of the tight junction transmembrane protein claudin-4

Physiology and function of the tight junction

Understanding of tight junctions has evolved from their historical perception as inert solute barriers to recognition of their physiological and biochemical complexity. Many proteins are specifically localized to tight junctions, including cytoplasmic actin-binding proteins and adhesive transmembrane proteins. Among the latter are claudins, which are critical barrier proteins. Current information suggests that the paracellular barrier is most usefully modeled as having two physiologic components: a system of charge-selective small pores, 4 A in radius, and a second pathway created by larger discontinuities in the barrier, lacking charge or size discrimination. The first pathway is influenced by claudin expression patterns and the second is likely controlled by different proteins and signals. Recent information on claudin function and disease-causing mutations have led to a more complete understanding of their role in barrier formation, but progress is impeded by lack of high resolution structural information.

Protein kinase C activation has distinct effects on the localization, phosphorylation and detergent solubility of the claudin protein family in tight and leaky epithelial cells

We have previously shown that protein kinase C (PKC) activation has distinct effects on the structure and barrier properties of cultured epithelial cells (HT29 and MDCK I). Since the claudin family of tight junction (TJ)-associated proteins is considered to be crucial for the function of mature TJ, we assessed their expression patterns and cellular destination, detergent solubility and phosphorylation upon PKC stimulation for 2 or 18 h with phorbol myristate acetate (PMA). In HT29 cells, claudins 1, 3, 4 and 5 and possibly claudin 2 were redistributed to apical cell–cell contacts after PKC activation and the amounts of claudins 1, 3 and 5, but not of claudin 2, were increased in cell lysates. By contrast, in MDCK I cells, PMA treatment resulted in redistribution of claudins 1, 3, 4 and 5 from the TJ and in reorganization of the proteins into more insoluble complexes. Claudins 1 and 4 were phosphorylated in both MDCK I and HT29 cells, but PKC-induced changes in claudin phosphorylation state were detected only in MDCK I cells. A major difference between HT29 and MDCK I cells, which have low and high basal transepithelial electrical resistance, respectively, was the absence of claudin 2 in the latter. Our findings show that PKC activation targets in characteristic ways the expression patterns, destination, detergent solubility and phosphorylation state of claudins in epithelial cells with different capacities to form an epithelial barrier.

Specificity of interaction between clostridium perfringens enterotoxin and claudin-family tight junction proteins

Clostridium perfringens enterotoxin (CPE), a major cause of food poisoning, forms physical pores in the plasma membrane of intestinal epithelial cells. The ability of CPE to recognize the epithelium is due to the C-terminal binding domain, which binds to a specific motif on the second extracellular loop of tight junction proteins known as claudins. The interaction between claudins and CPE plays a key role in mediating CPE toxicity by facilitating pore formation and by promoting tight junction disassembly. Recently, the ability of CPE to distinguish between specific claudins has been used to develop tools for studying roles for claudins in epithelial barrier function. Moreover, the high affinity of CPE to selected claudins makes CPE a useful platform for targeted drug delivery to tumors expressing these claudins.

Epithelial barriers in homeostasis and disease

Epithelia form barriers that are essential to life. This is particularly true in the intestine, where the epithelial barrier supports nutrient and water transport while preventing microbial contamination of the interstitial tissues. Along with plasma membranes, the intercellular tight junction is the primary cellular determinant of epithelial barrier function. Disruption of tight junction structure, as a result of specific protein mutations or aberrant regulatory signals, can be both a cause and an effect of disease. Recent advances have provided new insights into the extracellular signals and intracellular mediators of tight junction regulation in disease states as well as into the interactions of intestinal barrier function with mucosal immune cells and luminal microbiota. In this review, we discuss the critical roles of the tight junction in health and explore the contributions of barrier dysfunction to disease pathogenesis.

A claudin-4 modulator enhances the mucosal absorption of a biologically active peptide

Biologics, such as peptides, proteins and nucleic acids, are emerging pharmaceuticals. Passage across the epithelium is the first step in the absorption of biologics. Tight junctions (TJ) function as seals between adjacent epithelial cells, preventing free movement of solutes across the epithelium. We previously found that modulation of a key TJ component, claudin-4, is a potent method to enhance jejunal absorption when we used dextran as a model drug and the C-terminal fragment of Clostridium perfringens enterotoxin (C-CPE) as a claudin-4 modulator. Here, we investigated whether the claudin-4 modulator enhances jejunal, nasal and pulmonary absorption of a biologics human parathyroid hormone derivative, hPTH(1-34). The claudin-4 modulator enhanced nasal but not jejunal and pulmonary absorption of hPTH(1-34). C-CPE is hydrophobic with low solubility of less than 0.3mg/ml, but deletion of 10 amino acids at the N-terminal of C-CPE increased its solubility by 30-fold. Moreover, the N-terminal truncated C-CPE bound to claudin-4, modulated the TJ-barrier and enhanced jejunal absorption of dextran. The N-terminal-truncated C-CPE also enhanced jejunal and pulmonary absorption of hPTH(1-34). This report is the first to indicate that a claudin-4 modulator may be a promising enhancer of the jejunal, pulmonary and nasal absorption of a peptide drug.

The claudins

Claudins are crucial components of tight junctions and are important in regulating permeability and maintaining cell polarity in cell sheets.

The claudin multigene family encodes tetraspan membrane proteins that are crucial structural and functional components of tight junctions, which have important roles in regulating paracellular permeability and maintaining cell polarity in epithelial and endothelial cell sheets. In mammals, the claudin family consists of 24 members, which exhibit complex tissue-specific patterns of expression. The extracellular loops of claudins from adjacent cells interact with each other to seal the cellular sheet and regulate paracellular transport between the luminal and basolateral spaces. The claudins interact with multiple proteins and are intimately involved in signal transduction to and from the tight junction. Several claudin mouse knockout models have been generated and the diversity of phenotypes observed clearly demonstrates their important roles in the maintenance of tissue integrity in various organs. In addition, mutation of some claudin genes has been causatively associated with human diseases and claudin genes have been found to be deregulated in various cancers. The mechanisms of claudin regulation and their exact roles in normal physiology and disease are being elucidated, but much work remains to be done. The next several years are likely to witness an explosion in our understanding of these proteins, which may, in turn, provide new approaches for the targeted therapy of various diseases.

Claudin-16 and claudin-19 interaction is required for their assembly into tight junctions and for renal reabsorption of magnesium

Claudins are tight junction integral membrane proteins that are key regulators of the paracellular pathway. Defects in claudin-16 (CLDN16) and CLDN19 function result in the inherited human renal disorder familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC). Previous studies showed that siRNA knockdown of CLDN16 in mice results in a mouse model for FHHNC. Here, we show that CLDN19-siRNA mice also developed the FHHNC symptoms of chronic renal wasting of magnesium and calcium together with defective renal salt handling. siRNA knockdown of CLDN19 caused a loss of CLDN16 from tight junctions in the thick ascending limb (TAL) without a decrease in CLDN16 expression level, whereas siRNA knockdown of CLDN16 produced a similar effect on CLDN19. In both mouse lines, CLDN10, CLDN18, occludin, and ZO-1, normal constituents of TAL tight junctions, remained correctly localized. CLDN16- and CLDN19-depleted tight junctions had normal barrier function but defective ion selectivity. These data, together with yeast two-hybrid binding studies, indicate that a heteromeric CLDN16 and CLDN19 interaction was required for assembling them into the tight junction structure and generating cation-selective paracellular channels.

Structure-function studies of claudin extracellular domains by cysteine-scanning mutagenesis

Claudins form size- and charge-selective pores in the tight junction that control the paracellular flux of inorganic ions and small molecules. However, the structural basis for ion selectivity of paracellular pores is poorly understood. Here we applied cysteine scanning to map the paracellular pathway of ion permeation across claudin-2-transfected Madin-Darby canine kidney type I cells. Four potential pore-lining amino acid residues in the first extracellular loop were mutated to cysteine and screened for their accessibility to thiol-reactive reagents. All mutants were functional except D65C, which formed dimers by intermolecular disulfide bonding, leading to a loss of charge and size selectivity. This suggests that claudin-2 pores are multimeric and that Asp(65) lies close to a protein-protein interface. Methanethiosulfonate reagents of different size and charge and the organic mercury derivate, p-(chloromercuri)benzenesulfonic acid, significantly decreased paracellular ion permeation across I66C-transfected cells by a mechanism that suggests steric blocking of the pore. The conductance of wild-type claudin-2 and the other cysteine mutants was only weakly affected. The rate of reaction with I66C decreased dramatically with increasing size of the reagent, suggesting that Ile(66) is buried deep within a narrow segment of the pore with its side group facing into the lumen. Furthermore, labeling with N-biotinoylaminoethyl methanethiosulfonate showed that I66C was weakly reactive, whereas Y35C was strongly reactive, suggesting that Tyr(35) is located at the protein surface outside of the pore.

Structure and function of extracellular claudin domains

Most claudins are tight junction (TJ)-forming proteins. However, their interaction on the molecular level remains unresolved. It is hypothesized that the extracellular loops specify these claudin functions. It is assumed that the first extracellular loop (ECL1) is critical for determining the paracellular tightness and the selective paracellular ion permeability, and that the second extracellular loop may cause narrowing of the paracellular cleft. Using a combination of site-directed mutagenesis and homology modeling for the second extracellular loop (ECL2) of claudin-5, we found several amino acids important for claudin folding and/or trans-interaction to claudins in neighboring cells. These sensitive residues are highly conserved within one group of claudins, whereas the corresponding positions in the remaining claudins show a large sequence variety. Further functional data and analysis of sequence similarity for all claudins has led to their differentiation into two groups, designated as classic claudins (1-10, 14, 15, 17, 19) and nonclassic claudins (11-13, 16, 18, 20-24). This also corresponds to conserved structural features at ECL1 for classic claudins. Based on this, we propose a hypothesis for different pore-forming claudins. Pore formation or tightness is supported by the spatial encounter of a surplus of repulsing or attracting amino acid types at ECL1. A pore is likely opened by repulsion of equally charged residues, while an encounter of unequally charged residues leads to tight interaction. These considerations may reveal the ECLs of claudins as decisive submolecular determinants that specify the function of a claudin.

Cysteine mutagenesis to study the structure of claudin-2 paracellular pores

The structure and transport mechanism of paracellular pores are only poorly understood. Here we describe for the first time how the substituted cysteine accessibility method (SCAM), previously developed to study transmembrane transport, can be applied to analyze the pathway of paracellular ion permeation. Using stable transfected Madin Darby canine kidney type I cells, induced to express claudin-2, we show that paracellular cation transport can be blocked by sulfhydryl-specific methanethiosulfonate (MTS) and that SCAM can be used to identify residues that line paracellular pores.

Abundant expression and purification of biologically active mitochondrial citrate carrier in baculovirus-infected insect cells

Heterologous expression of recombinant proteins is an essential technology for protein characterization. A major obstacle to investigating the biochemical properties of membrane proteins is the difficulty in obtaining sufficient amounts of functional protein. Here we report the successful expression of the tricarboxylate (or citrate) carrier (CIC) of eel (Anguilla anguilla) from Spodoptera frugiperda (Sf9) cells using the baculovirus expression system. The recombinant CIC was purified by affinity chromatography on Ni(2+)-NTA agarose; the yield of the purified active protein was 0.4-0.5 mg/l of culture. The transport characteristics of the recombinant CIC and the effects of inhibitors on transport are similar to those determined for eel liver mitochondrial CIC. Because the CIC is one member of an extensive family of mitochondrial transport proteins, it is likely that the procedure used in this study to express and purify this carrier can be successfully applied to other mitochondrial transport proteins, thus providing sufficient protein for functional characterization.

Regulation and roles for claudin-family tight junction proteins

Transmembrane proteins known as claudins play a critical role in tight junctions by regulating paracellular barrier permeability. The control of claudin assembly into tight junctions requires a complex interplay between several classes of claudins, other transmembrane proteins and scaffold proteins. Claudins are also subject to regulation by post-translational modifications including phosphorylation and palmitoylation. Several human diseases have been linked to claudin mutations, underscoring the physiologic function of these proteins. Roles for claudins in regulating cell phenotype and growth control also are beginning to emerge, suggesting a multifaceted role for claudins in regulation of cells beyond serving as a simple structural element of tight junctions.

Occludin oligomeric assemblies at tight junctions of the blood-brain barrier are altered by hypoxia and reoxygenation stress

Hypoxic (low oxygen) and reperfusion (post-hypoxic reoxygenation) phases of stroke promote an increase in microvascular permeability at tight junctions (TJs) of the blood-brain barrier (BBB) that may lead to cerebral edema. To investigate the effect of hypoxia (Hx) and reoxygenation on oligomeric assemblies of the transmembrane TJ protein occludin, rats were subjected to either normoxia (Nx, 21% O(2), 60 min), Hx (6% O(2), 60 min), or hypoxia/reoxygenation (H/R, 6% O(2), 60 min followed by 21% O(2), 10 min). After treatment, cerebral microvessels were isolated, fractionated by detergent-free density gradient centrifugation, and occludin oligomeric assemblies associated with plasma membrane lipid rafts were solubilized by perfluoro-octanoic acid (PFO) exclusively as high molecular weight protein complexes. Analysis by non-reducing and reducing sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis/western blot of PFO-solubilized occludin revealed that occludin oligomeric assemblies co-localizing with 'TJ-associated' raft domains contained a high molecular weight 'structural core' that was resistant to disassembly by either SDS or a hydrophilic reducing agent ex vivo, and by Hx and H/R conditions in vivo. However, exposure of PFO-solubilized occludin oligomeric assemblies to SDS ex vivo revealed the non-covalent association of a significant amount of dimeric and monomeric occludin isoforms to the disulfide-bonded inner core, and dispersal of these non-covalently attached occludin subunits to lipid rafts of higher density in vivo was differentially promoted by Hx and H/R. Our data suggest a model of isoform interaction within occludin oligomeric assemblies at the BBB that enables occludin to simultaneously perform a structural role in inhibiting paracellular diffusion, and a signaling role involving interactions of dimeric and monomeric occludin isoforms with a variety of regulatory molecules within different plasma membrane lipid raft domains.

Molecular determinants of the interaction between Clostridium perfringens enterotoxin fragments and claudin-3

Clostridium perfringens enterotoxin (CPE) binds to the extracellular loop 2 of a subset of claudins, e.g. claudin-3. Here, the molecular mechanism of the CPE-claudin interaction was analyzed. Using peptide arrays, recombinant CPE-(116-319) bound to loop 2 peptides of mouse claudin-3, -6, -7, -9, and -14 but not of 1, 2, 4, 5, 8, 10-13, 15, 16, 18-20, and 22. Substitution peptide mapping identified the central motif (148)NPL(150)VP, supposed to represent a turn region in the loop 2, as essential for the interaction between CPE and murine claudin-3 peptides. CPE-binding assays with claudin-3 mutant-transfected HEK293 cells or lysates thereof demonstrated the involvement of Asn(148) and Leu(150) of full-length claudin-3 in the binding. CPE-(116-319) and CPE-(194-319) bound to HEK293 cells expressing claudin-3, whereas CPE-(116-319) bound to claudin-5-expressing HEK293 cells, also. This binding was inhibited by substitutions T151A and Q156E in claudin-5. In contrast, removal of the aromatic side chains in the loop 2 of claudin-3 and -5, involved in trans-interaction between claudins, increased the amount of CPE-(116-319) bound. These findings and molecular modeling indicate different molecular mechanisms of claudin-claudin trans-interaction and claudin-CPE interaction. Confocal microscopy showed that CPE-(116-319) and CPE-(194-319) bind to claudin-3 at the plasma membrane, outside cell-cell contacts. Together, these findings demonstrate that CPE binds to the hydrophobic turn and flanking polar residues in the loop 2 of claudin-3 outside tight junctions. The data can be used for the specific design of CPE-based modulators of tight junctions, to improve drug delivery, and as chemotherapeutics for tumors overexpressing claudins.

Molecular basis for cation selectivity in claudin-2-based paracellular pores: identification of an electrostatic interaction site

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.

Biology of claudins

Claudins are a family of tight junction membrane proteins that regulate paracellular permeability of epithelia, likely by forming the lining of the paracellular pore. Claudins are expressed throughout the renal tubule, and mutations in two claudin genes are now known to cause familial hypercalciuric hypomagnesemia with nephrocalcinosis. In this review, we discuss recent advances in our understanding of the physiological role of various claudins in normal kidney function, and in understanding the fundamental biology of claudins, including the molecular basis for selectivity of permeation, claudin interactions in tight junction formation, and regulation of claudins by protein kinases and other intracellular signals.

Oligomeric structure and minimal functional unit of the electrogenic sodium bicarbonate cotransporter NBCe1-A

The electrogenic sodium bicarbonate cotransporter NBCe1-A mediates the basolateral absorption of sodium and bicarbonate in the proximal tubule. In this study the oligomeric state and minimal functional unit of NBCe1-A were investigated. Wild-type (wt) NBCe1-A isolated from mouse kidney or heterologously expressed in HEK293 cells was predominantly in a dimeric state as was shown using fluorescence energy transfer, pulldown, immunoprecipitation, cross-linking experiments, and nondenaturing perfluorooctanoate-PAGE. NBCe1-A monomers were found to be covalently linked by S-S bonds. When each of the 15 native cysteine residues were individually removed on a wt-NBCe1-A backbone, dimerization of the cotransporter was not affected. In experiments involving multiple native cysteine residue removal, both Cys(630) and Cys(642) in extracellular loop 3 were shown to mediate S-S bond formation between NBCe1-A monomers. When native NBCe1-A cysteine residues were individually reintroduced into a cysteineless NBCe1-A mutant backbone, the finding that a Cys(992) construct that lacked S-S bonds functioned normally indicated that stable covalent linkage of NBCe1-A monomers was not a necessary requirement for functional activity of the cotransporter. Studies using concatameric constructs of wt-NBCe1-A, whose activity is resistant to methanesulfonate reagents, and an NBCe1-A(T442C) mutant, whose activity is completely inhibited by methanesulfonate reagents, confirmed that NBCe1-A monomers are functional. Our results demonstrate that wt-NBCe1-A is predominantly a homodimer, dependent on S-S bond formation that is composed of functionally active monomers.

Claudin-16 and claudin-19 interact and form a cation-selective tight junction complex

Tight junctions (TJs) play a key role in mediating paracellular ion reabsorption in the kidney. Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is an inherited disorder caused by mutations in the genes encoding the TJ proteins claudin-16 (CLDN16) and CLDN19; however, the mechanisms underlying the roles of these claudins in mediating paracellular ion reabsorption in the kidney are not understood. Here we showed that in pig kidney epithelial cells, CLDN19 functioned as a Cl(-) blocker, whereas CLDN16 functioned as a Na(+) channel. Mutant forms of CLDN19 that are associated with FHHNC were unable to block Cl(-) permeation. Coexpression of CLDN16 and CLDN19 generated cation selectivity of the TJ in a synergistic manner, and CLDN16 and CLDN19 were observed to interact using several criteria. In addition, disruption of this interaction by introduction of FHHNC-causing mutant forms of either CLDN16 or CLDN19 abolished their synergistic effect. Our data show that CLDN16 interacts with CLDN19 and that their association confers a TJ with cation selectivity, suggesting a mechanism for the role of mutant forms of CLDN16 and CLDN19 in the development of FHHNC.

Structural characterization of recombinant human CD81 produced in Pichia pastoris

Human CD81 (hCD81) protein has been recombinantly produced in the methylotrophic yeast Pichia pastoris. The purified protein, produced at a yield of 1.75 mg/L of culture, was shown to interact with Hepatitis C virus E2 glycoprotein. Immunofluorescent and flow cytometric staining of P. pastoris protoplasts with monoclonal antibodies specific for the second extracellular loop (EC2) of hCD81 confirmed the antigenicity of the recombinant molecule. Full-length hCD81 was solubilized with an array of detergents and subsequently characterized using circular dichroism (CD) and analytical ultracentrifugation. These biophysical techniques confirmed that the protein solution comprises a homogenous species possessing a highly-defined alpha-helical secondary structure. The predicted alpha-helical content of the protein from CD analysis (77.1%) fits remarkably well with what would be expected (75.2%) from knowledge of the protein sequence together with the data from the crystal structure of the second extracellular loop. This study represents the first biophysical characterization of a full-length recombinant tetraspanin, and opens the way for structure-activity analyses of this ubiquitous family of transmembrane proteins.

Structure and function of claudins

Claudins are tetraspan transmembrane proteins of tight junctions. They determine the barrier properties of this type of cell-cell contact existing between the plasma membranes of two neighbouring cells, such as occurring in endothelia or epithelia. Claudins can completely tighten the paracellular cleft for solutes, and they can form paracellular ion pores. It is assumed that the extracellular loops specify these claudin functions. It is hypothesised that the larger first extracellular loop is critical for determining the paracellular tightness and the selective ion permeability. The shorter second extracellular loop may cause narrowing of the paracellular cleft and have a holding function between the opposing cell membranes. Sequence analysis of claudins has led to differentiation into two groups, designated as classic claudins (1-10, 14, 15, 17, 19) and non-classic claudins (11-13, 16, 18, 20-24), according to their degree of sequence similarity. This is also reflected in the derived sequence-structure function relationships for extracellular loops 1 and 2. The concepts evolved from these findings and first tentative molecular models for homophilic interactions may explain the different functional contribution of the two extracellular loops at tight junctions.

Claudins and paracellular transport: an update

PURPOSE OF REVIEW

Claudins are tight junction proteins that form paracellular barriers and pores. The purpose of this timely review is to provide an update on the exciting new advances in our understanding of claudin biology and their relevance to renal physiology and pathophysiology.

RECENT FINDINGS

Accumulating evidence from numerous studies indicates that the primary role of claudins is to determine the permeability and charge selectivity of the paracellular pathway to small ions. Studies in which claudins are overexpressed in cell lines have potential limitations and need to be interpreted cautiously. Ribonucleic acid interference is a novel approach to functional characterization. Claudins are believed to assemble into multimers by homophilic and heterophilic side-by-side and head-to-head interaction; however, there is still limited evidence for this. The roles of a few claudins in the renal tubule, including claudins 2, 8, 10, 16 and 19, have now been elucidated.

SUMMARY

These findings reveal tantalizing clues to claudin biology and function. Much remains unknown, however, and these findings will hopefully encourage further research in this important area.

Compositional and stoichiometric analysis of Clostridium perfringens enterotoxin complexes in Caco-2 cells and claudin 4 fibroblast transfectants

Clostridium perfringens enterotoxin (CPE) binds to host cell receptors, forming a small complex precursor for two large complexes reportedly having molecular masses of approximately 155 or approximately 200 kDa. Formation of the approximately 155 kDa complex causes a Ca(2+) influx that leads to apoptosis or oncosis. CPE complex composition is currently poorly understood, although occludin was identified in the approximately 200 kDa complex. The current study used heteromer gel shift analysis to show both CPE large complexes contain six CPE molecules. Ferguson plots and size exclusion chromatography re-sized the approximately 155 and approximately 200 kDa complexes as approximately 425-500 kDa and approximately 550-660 kDa respectively. Co-immunoprecipitation and electroelution studies demonstrated both CPE-binding and non-CPE-binding claudins are associated with all three CPE complexes in Caco-2 cells and with small complex and approximately 425-500 kDa complex of claudin 4 transfectants. Fibroblast transfectants expressing claudin 4 or C-terminal truncated claudin 4 were CPE-sensitive and formed the approximately 425 kDa complex, indicating claudin-induced cell signalling is not required for CPE action and that expression of a single receptor claudin suffices for approximately 425-500 kDa CPE complex formation. These results identify CPE as a unique toxin that combines with tight junction proteins to form high-molecular-mass hexameric pores and alter membrane permeability.

Oligomerization domain of the multidrug resistance-associated transporter ABCG2 and its dominant inhibitory activity

Overexpression of human ATP-binding cassette transporter ABCG2 in cancer cells causes multidrug resistance by effluxing anticancer drugs. ABCG2 is considered as a half transporter and is thought to function as a homodimer. However, recent evidence suggests that it may exist as a higher form of oligomer consisting of 12 subunits. In this study, we mapped the oligomerization domain of human ABCG2 to its transmembrane domain consisting of TM5-loop-TM6. This oligomerization domain, when expressed alone in HEK293 cells, also forms a homododecamer. Furthermore, this domain has activity that inhibits drug efflux and resistance function of the full-length ABCG2 likely by disrupting the formation of the homo-oligomeric full-length ABCG2. These findings suggest that human ABCG2 may exist and work as a homo-oligomer by interactions located in TM5-loop-TM6, and that ABCG2 oligomerization may be used as a target for therapeutic development to circumvent ABCG2-mediated drug resistance in cancer treatment.

Regulation of function by dimerization through the amino-terminal membrane-spanning domain of human ABCC1/MRP1

Overexpression of some ATP-binding cassette (ABC) membrane transporters such as ABCB1/P-glycoprotein/MDR1 and ABCC1/MRP1 causes multidrug resistance in cancer chemotherapy. It has been thought that half-ABC transporters with one nucleotide-binding domain and one membrane-spanning domain (MSD) likely work as dimers, whereas full-length transporters with two nucleotide-binding domains and two or three MSDs function as monomers. In this study, we examined the oligomeric status of the human full-length ABC transporter ABCC1/MRP1 using several biochemical approaches. We found 1) that it is a homodimer, 2) that the dimerization domain is located in the amino-terminal MSD0L0 (where L0 is loop 0) region, and 3) that MSD0L0 has a dominant-negative function when coexpressed with wild-type ABCC1/MRP1. These findings suggest that ABCC1/MRP1 may exist and function as a dimer and that MSD0L0 likely plays some structural and regulatory functions. It is also tempting to propose that the MSD0L0-mediated dimerization may be targeted for therapeutic development to sensitize ABCC1/MRP1-mediated drug resistance in cancer chemotherapy.

Activation of an SP binding site is crucial for the expression of claudin 1 in rat epididymal principal cells

Claudin 1 (CLDN1) is a tight junctional protein present in the epididymis. Limited information exists regarding the regulation of Cldn1 transcription. In the epididymis, the regulation of the 5' flanking region of genes coding for tight junctional proteins is unknown. The present objectives were to investigate the transcriptional regulation of the Cldn1 gene in the rat epididymis. A 1.8-kb sequence of the 5' flanking region of the rat Cldn1 gene was cloned. The transcriptional start site is an adenine located at the -198 position relative to the first codon, and 26 bp downstream of the putative TATA box. It is the only start site for the Cldn1 gene transcription in the rat epididymis. The Cldn1 promoter was inserted into a luciferase gene expression vector and transfected into a rat caput epididymal cell line (RCE-1). Sequential deletion analysis revealed that minimal promoter activity was achieved with the construct containing -61 to +164 bp of the promoter. This sequence contained a TATA box and two consensus SP1 binding sites. Electrophoretic mobility shift and supershift assays confirmed that SP1 and SP3 were present in RCE-1 cells and epididymal nuclear extracts, and that they bind to the 5' SP1 binding motif of the promoter. Site-directed mutagenesis of the 5' SP1 binding site resulted in a 4-fold decrease in transactivation of the minimal promoter sequence. These findings indicate that SP1 and SP3 bind to the Cldn1 promoter region, and that this interaction influences the expression of Cldn1 in the rat epididymis.

In utero exposure to tributyltin alters the expression of e-cadherin and localization of claudin-1 in intercellular junctions of the rat ventral prostate

Tributyltin (TBT) is an environmental contaminant, exhibiting well-established toxicity to reproductive systems in aquatic organisms. Little information exists regarding the effects of TBT on mammalian reproduction. Cellular junctions are crucial for sperm development and maturation. Intercellular tight junctions are formed by transmembrane proteins such as claudins (Cldns), while the formation of tight junctions involves signaling components of adhering junctions, comprised of cadherins. The objectives of this study were to determine the effects of in utero exposure to TBT on the rat ventral prostate. Pregnant Sprague-Dawley rats were given doses of TBT (2.5, 10, or 20 mg/kg) throughout gestation and sacrificed at Day 91. Ventral prostate weights of TBT-treated rats were decreased in all treatment groups. Results of gene expression macro-array analysis indicated that numerous genes related to cellular adhesion and cell polarity were affected. Cldn-1 mRNA levels decreased after exposure to TBT. Cldn-1 was immunolocalized to the apical lateral margins of adjacent prostatic epithelial cells in controls, but was increasingly dispersed along the lateral plasma membrane with increasing TBT dose, suggesting that the targeting of Cldn-1 or its localization to tight junctions was altered as a result of fetal TBT exposure. E-cadherin mRNA levels and immunolocalization were decreased in a dose-dependent manner. These data indicate that in utero TBT exposure results in permanent alterations in ventral prostate and that these are associated with alterations in the expression and distribution of cell adhesion and tight junctional proteins.

Study of Claudin Function by RNA Interference

Claudins are tight junction proteins that play a key selectivity role in the paracellular conductance of ions. Numerous studies of claudin function have been carried out using the overexpression strategy to add new claudin channels to an existing paracellular protein background. Here, we report the systematic knockdown of endogenous claudin gene expression in Madin-Darby canine kidney (MDCK) cells and in LLC-PK1 cells using small interfering RNA against claudins 1-4 and 7. In MDCK cells (showing cation selectivity), claudins 2, 4, and 7 are powerful effectors of paracellular Na+ permeation. Removal of claudin-2 depressed the permeation of Na+ and resulted in the loss of cation selectivity. Loss of claudin-4 or -7 expression elevated the permeation of Na+ and enhanced the proclivity of the tight junction for cations. On the other hand, LLC-PK1 cells express little endogenous claudin-2 and show anion selectivity. In LLC-PK1 cells, claudin-4 and -7 are powerful effectors of paracellular Cl- permeation. Knockdown of claudin-4 or -7 expression depressed the permeation of Cl- and caused the tight junction to lose the anion selectivity. In conclusion, claudin-2 functions as a paracellular channel to Na+ to increase the cation selectivity of the tight junction; claudin-4 and -7 function either as paracellular barriers to Na+ or as paracellular channels to Cl-, depending upon the cellular background, to decrease the cation selectivity of the tight junction.

Claudins--key pieces in the tight junction puzzle

Tight junctions restrict the flow of ions and aqueous molecules between cells by forming a selective barrier to the paracellular pathway. Permeability of the tight junction barrier is determined by a class of transmembrane proteins known as claudins. The relationship between claudins and paracellular permeability is complex and determined not only by the profile of claudin expression but also by the arrangement of claudins and other proteins into tight junction strands. This review summarizes progress in understanding how claudins are assembled into tight junctions and how they interact with other tight junction proteins.

Claudins and epithelial paracellular transport

Tight junctions form continuous intercellular contacts controlling solute movement through the paracellular pathway across epithelia. Paracellular barriers vary among epithelia in electrical resistance and behave as if they are lined with pores that have charge and size selectivity. Recent evidence shows that claudins, a large family (at least 24 members) of intercellular adhesion molecules, form the seal and its variable pore-like properties. This evidence comes from the study of claudins expressed in cultured epithelial cell models, genetically altered mice, and human mutants. We review information on the structure, function, and transcriptional and posttranslational regulation of the claudin family as well as of their evolutionarily distant relatives called the PMP22/EMP/MP20/claudin, or pfam00822, superfamily.

Analysis of the oligomeric structure of the motor protein prestin

Prestin, a member of the solute carrier family 26, is expressed in the basolateral membrane of outer hair cells. This protein provides the molecular basis for outer hair cell somatic electromotility, which is crucial for the frequency selectivity and sensitivity of mammalian hearing. It has long been known that there are abundantly expressed approximately 11-nM protein particles present in the basolateral membrane. These particles were hypothesized to be the motor proteins that drive electromotility. Because the calculated size of a prestin monomer is too small to form an approximately 11-nM particle, the possibility of prestin oligomerization was examined. We investigated possible quaternary structures of prestin by lithium dodecyl sulfate-PAGE, perfluoro-octanoate-PAGE, a membrane-based yeast two-hybrid system, and chemical cross-linking experiments. Prestin, obtained from different host or native cells, is resistant to dissociation by lithium dodecyl sulfate and behaves as a stable oligomer on lithium dodecyl sulfate-PAGE. In the membrane-based yeast two-hybrid system, homo-oligomeric interactions between prestin-bait/prestin-prey suggest that prestin molecules can associate with each other. Chemical cross-linking experiments, perfluoro-octanoate-PAGE/Western blot, and affinity purification experiments all indicate that prestin exists as a higher order oligomer, such as a tetramer, in prestin-expressing yeast, mammalian cell lines and native outer hair cells. Our data from experiments using hydrophobic and hydrophilic reducing reagents suggest that the prestin dimer is connected by a disulfide bond embedded in the prestin hydrophobic core. This stable dimer may act as the building block for producing the higher order oligomers that form the approximately 11-nM particles in the outer hair cell's basolateral membrane.

Identification of multiple claudins in the rat epididymis

The luminal environment of the epididymis is highly specialized with specific proteins, ions, pH, etc. required for sperm maturation. Tight junctions between epididymal principal cells are responsible for the formation of the blood-epididymal barrier, which regulates this luminal environment. Claudins (Cldns) are a recently discovered family of transmembrane proteins and are essential components of tight junctions. Previous work from our laboratory has demonstrated the presence and localization of Cldn-1 in all regions of the rat epididymis. The objective of this study was to determine the presence and localization of other Cldns in the epididymis. Using RT-PCR we have identified mRNA transcripts for Cldn-3 through -9 in each region of the adult rat epididymis. Immunolocalization of Cldn-3, Cldn-4, and Cldn-5 were done in adult as well as in 42- and 14-day-old rats. Cldn-5 in adult rats was localized exclusively in blood vessels of the interstitium. Cldn-3 was localized apically in the epididymal epithelium between adjacent principal cells throughout the epididymis, where tight junctions have been reported histologically. There were no differences in the localization of Cldn-3 in epididymides of rats at the different ages. In 14-day-old rats, Cldn-4 was localized all along the lateral plasma membrane between adjacent principal cells. The immunostaining was more pronounced in the proximal regions of the epididymis. In both 42-day-old rats and adults, Cldn-4 was localized primarily to apical tight junctions between principal cells and staining was more pronounced in the proximal region of the epididymis. Cldn-16 transcripts were also identified by RT-PCR. These transcripts were present in both proximal and distal regions of the epididymis of young (Day 14 and 21) animals, but only in the proximal (initial segment) region of the adult epididymis. These data indicate that epididymal tight junctions are composed of several Cldns, suggestive of a complex regulation of the blood-epididymal barrier.

Oligomerization and interacellular localization of the glycoprotein receptor ERGIC-53 is independent of disulfide bonds

ERGIC-53 is a type I transmembrane lectin facilitating the efficient export of a subset of secretory glycoproteins from the endoplasmic reticulum. Previous results have shown that ERGIC-53 is present as reduction-sensitive homo-oligomers, i.e. as a balanced mixture of disulfide-linked hexamers and dimers, with the two cysteine residues located close to the transmembrane domain playing a crucial role in oligomerization. Here, we demonstrate, using sucrose gradient sedimentation, cross-linking analyses, and non-denaturing gel electrophoresis, that ERGIC-53 is present exclusively as a hexameric complex in cells. However, the hexamers exist in two forms, one as a disulfide-linked, Triton X-100, perfluoro-octanic acid, and SDS-resistant complex, and the other as a non-covalent, Triton X-100, perfluoro-octanoic acid-resistant, but SDS-sensitive, complex made up of three disulfide-linked dimers that are likely to interact through the coiled-coil domains present in the luminal part of the protein. In contrast to what was previously believed, neither of the membrane-proximal cysteine residues plays an essential role in the formation, or maintenance, of the latter form of hexamers. Subcellular fractionation revealed that the double-cysteine mutant was present in the endoplasmic reticulum-Golgi-intermediate compartment, indicating that the two cysteine residues are not essential for the intracellular distribution of ERGIC-53. Based on these results, we present a model for the formation of the two hexameric forms.

The cytoplasmic tails of claudins can influence tight junction barrier properties through effects on protein stability

The tight junction seal formed between epithelial cells varies among tissues in both tightness and ionic charge selectivity. We recently demonstrated that the extracellular domains of the claudin family of proteins are determinants of both characteristics, but in that study other unidentified domains in the claudins clearly contributed to their physiological potency. To investigate the importance of the cytoplasmic carboxyl-terminal domains in determining the degree to which a claudin can influence barrier properties, we constructed chimeras by exchanging the tails of claudin-2 and -4 and expressing them in MDCK II cells. Although swapping these domains had little effect on claudin localization, we found that the tail of claudin-2 could stabilize claudin-4, with a concomitant increase in both protein level and physiologic influence. This difference in stability was not an artifact of their chimeric structure, since metabolic radio-labeling experiments revealed that the half-life of endogenous claudin-2 is more than three times longer than claudin-4 (>12 h and approximately 4 h respectively). Further, half-life was not affected by removing the carboxyl-terminal three amino acids, which form a PDZ-binding motif. The finding that cytoplasmic tails of claudins strongly influence stability reveals a potential mechanism by which cells can establish their tight junction protein composition and thus function.

Effects of transdifferentiation and EGF on claudin isoform expression in alveolar epithelial cells

Rat alveolar epithelial type II cells grown on polycarbonate filters form high-resistance monolayers and concurrently acquire many phenotypic properties of type I cells. Treatment with EGF has previously been shown to increase transepithelial resistance across alveolar epithelial cell (AEC) monolayers. We investigated changes in claudin expression in primary cultured AEC during transdifferentiation to the type I cell-like phenotype ( days 0, 1, and 8), and on day 5 in culture ± EGF (10 ng/ml) from day 0 or day 4. Claudins 4 and 7 were increased, whereas claudins 3 and 5 were decreased, on later compared with earlier days in culture. Exposure to EGF led to increases in claudins 4 and 7 and decreases in claudins 3 and 5. Claudin 1 was only faintly detectable in freshly isolated type II cells and remained unchanged over time in culture and after exposure to EGF. These results suggest that increases in transepithelial resistance accompanying AEC transdifferentiation and/or EGF exposure are mediated, at least in part, by changes in the pattern of expression of specific claudin isoforms.

Selective decrease in paracellular conductance of tight junctions: role of the first extracellular domain of claudin-5

Claudin-5 is a protein component of many endothelial tight junctions, including those at the blood-brain barrier, a barrier that limits molecular exchanges between the central nervous system and the circulatory system. To test the contribution of claudin-5 to this barrier function of tight junctions, we expressed murine claudin-5 in Madin-Darby canine kidney II cells. The result was a fivefold increase in transepithelial resistance in claudin-5 transductants and a reduction in conductance of monovalent cations. However, the paracellular flux of neither neutral nor charged monosaccharides was significantly changed in claudin-5 transductants compared to controls. Therefore, expression of claudin-5 selectively decreased the permeability to ions. Additionally, site-directed mutations of particular amino acid residues in the first extracellular domain of claudin-5 altered the properties of the tight junctions formed in response to claudin-5 expression. In particular, the conserved cysteines were crucial: mutation of either cysteine abolishted the ability of claudin-5 to increase transepithelial resistance, and mutation of Cys(64) strikingly increased the paracellular flux of monosaccharides. These new insights into the functions of claudin-5 at the molecular level in tight junctions may account for some aspects of the blood-brain barrier's selective permeability.

The enteric toxins of Clostridium perfringens

The Gram-positive pathogen Clostridium perfringens is a major cause of human and veterinary enteric disease largely because this bacterium can produce several toxins when present inside the gastrointestinal tract. The enteric toxins of C. perfringens share two common features: (1) they are all single polypeptides of modest (approximately 25-35 kDa) size, although lacking in sequence homology, and (2) they generally act by forming pores or channels in plasma membranes of host cells. These enteric toxins include C. perfringens enterotoxin (CPE), which is responsible for the symptoms of a common human food poisoning and acts by forming pores after interacting with intestinal tight junction proteins. Two other C. perfringens enteric toxins, epsilon-toxin (a bioterrorism select agent) and beta-toxin, cause veterinary enterotoxemias when absorbed from the intestines; beta- and epsilon-toxins then apparently act by forming oligomeric pores in intestinal or extra-intestinal target tissues. The action of a newly discovered C. perfringens enteric toxin, beta2 toxin, has not yet been defined but precedent suggests it might also be a pore-former. Experience with other clostridial toxins certainly warrants continued research on these C. perfringens enteric toxins to develop their potential as therapeutic agents and tools for cellular biology.

The tight junction: a multifunctional complex

Multicellular organisms are separated from the external environment by a layer of epithelial cells whose integrity is maintained by intercellular junctional complexes composed of tight junctions, adherens junctions, and desmosomes, whereas gap junctions provide for intercellular communication. The aim of this review is to present an updated overview of recent developments in the area of tight junction biology. In a relatively short time, our knowledge of the tight junction has evolved from a relatively simple view of it being a permeability barrier in the paracellular space and a fence in the plane of the plasma membrane to one of it acting as a multicomponent, multifunctional complex that is involved in regulating numerous and diverse cell functions. A group of integral membrane proteins-occludin, claudins, and junction adhesion molecules-interact with an increasingly complex array of tight junction plaque proteins not only to regulate paracellular solute and water flux but also to integrate such diverse processes as gene transcription, tumor suppression, cell proliferation, and cell polarity.

Characterization of oligomeric human half-ABC transporter ATP-binding cassette G2

Human ATP-binding cassette G2 (ABCG2, also known as mitoxantrone resistance protein, breast cancer-resistance protein, ABC placenta) is a member of the superfamily of ATP-binding cassette (ABC) transporters that have a wide variety of substrates. Overexpression of human ABCG2 in model cancer cell lines causes multidrug resistance by actively effluxing anticancer drugs. Unlike most of the other ABC transporters which usually have two nucleotide-binding domains and two transmembrane domains, ABCG2 consists of only one nucleotide-binding domain followed by one transmembrane domain. Thus, ABCG2 has been thought to be a half-transporter that may function as a homodimer. In this study, we characterized the oligomeric feature of human ABCG2 using non-denaturing detergent perfluoro-octanoic acid and Triton X-100 in combination with gel filtration, sucrose density gradient sedimentation, and gel electrophoresis. Unexpectedly, we found that human ABCG2 exists mainly as a tetramer, with a possibility of a higher form of oligomerization. Monomeric and dimeric ABCG2 did not appear to be the major form of the protein. Further immunoprecipitation analysis showed that the oligomeric ABCG2 did not contain any other proteins. Taken together, we conclude that human ABCG2 likely exists and functions as a homotetramer.

Role of claudin interactions in airway tight junctional permeability

Airway epithelial tight junctions (TJs) serve to separate the external and internal environments of the lung. However, the members of the claudin family that mediate this function have not been fully delineated. We characterized the claudin expression in normal airways removed from human donors during lung transplantation and determined the contribution of each claudin to airway barrier function. Stable cell lines in NIH/3T3 and human airway (IB3.1) cells were constructed expressing the claudin components found in the human airway, claudin-1, -3, or -5. The effects of claudin expression on transepithelial resistance, permeability coefficients, and claudin-claudin interactions were assessed. Claudin-1 and -3 decreased solute permeability, whereas claudin-5 increased permeability. We also detected oligomerization of claudin-5 in cell lines and in freshly excised human airways. Coimmunoprecipitation studies revealed heterophilic interactions between claudin species in both cell lines and human airway epithelium. These suggest that airway TJs are regulated by claudinclaudin interactions that confer the selectivity of the junction.