TCDB is operated by the Saier Lab Bioinformatics Group

1.H.1 The Claudin Tight Junction (Claudin1) Family

Epithelia form boundaries of biological compartments, creating specialized absorptive and secretive surfaces such as the kidney tubules, the intestinal tract, and the mammary gland. The ability of epithelial cells to regulate absorption and secretion of essential ions such as sodium, chloride, calcium, and magnesium is critical for the maintenance of electrolyte balance (Van Itallie and Anderson, 2006). Ion transport across an epithelial layer can be either transcellular or paracellular (Shen et al., 2011). The transcellular pathway involves the movement of ions across the cytoplasm via plasma membrane channels, carriers, and exchangers (Muto et al., 2011). The paracellular pathway involves the movement of ions through the intercellular spaces between epithelial cells. The transmembrane proteins of tight junctions include claudins, junctional adhesion molecules (JAMs), occludin and tricellulin. Chiba et al. (2008) have provided an overview of these proteins, highlighting their roles and regulation, as well as their functional significance in human diseases. 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 (Krause et al., 2008).  Claudins have been reviewed from structural/functional standpoints (Krause et al. 2015).

The architecture of tight junctions can be conceptualized into compartments with the transmembrane barrier proteins (claudins, occludin, JAM-A, etc.), linked to peripheral scaffolding proteins (such as ZO-1, afadin, MAGI1, etc.) which are in turned linked to actin and microtubules through numerous linkers (cingulin, myosins, protein 4.1, etc.) (Van Itallie and Anderson 2014). Within this complex network are associated many signaling proteins that affect the barrier and broader cell functions. The PDZ domain is a commonly used motif to specifically link individual junction protein pairs. Van Itallie and Anderson 2014 reviewed some of the key proteins defining the tight junction as well as their detailed architecture and subcompartments. Claudins 1 and 3 can form homo- and heterophilic cis and trans interactions, and at least two different cis-interaction interfaces within claudin-3 homopolymers as well as within claudin-1/claudin-3 heteropolymers have been documented (Milatz et al. 2015).

Two TJ protein families can be distinguished, claudins, comprising 27 members in mammals, and TJ-associated MARVEL proteins (TAMP), comprising occludin, tricellulin, and MarvelD3 (Krug et al. 2014). They are linked to a multitude of TJ-associated regulatory and scaffolding proteins. The major TJ proteins are classified according to the physiological role they play in enabling or preventing paracellular transport. Many TJ proteins have sealing functions (claudins 1, 3, 5, 11, 14, 19, and tricellulin). In contrast, a significant number of claudins form channels across TJs which feature selectivity for cations (claudins 2, 10b, and 15), anions (claudin-10a and -17), or are permeable to water (claudin-2). For several TJ proteins, function is yet unclear as their effects on epithelial barriers are inconsistent (claudins 4, 7, 8, 16, and occludin). TJs undergo physiological and pathophysiological regulation by altering protein composition or abundance. Major pathophysiological conditions which involve changes in TJ protein composition are (1) effects of pathogens binding to TJ proteins, (2) altered TJ protein composition during inflammation and infection, and (3) altered TJ protein expression in cancers (Krug et al. 2014).

The gatekeeper of the paracellular pathway is the tight junction, which is located at apical cell-cell interactions of adjacent epithelial cells. Three known inherited disorders, familial hypomagnesemia (Simon et al., 1999), hypertension (Wilson et al., 2001), and autosomal recessive deafness (Wilcox et al. 2001) have been linked to proteins that localize at the tight junction. Transmembrane proteins of tight junctions include claudins, junctional adhesion molecules (JAMS), occludin and tricellulin. The cytoplasmic scaffolding proteins include Z0-1, -2 and -3 (Hartsock and Nelson, 2008).Their study has led to insights into the molecular nature of tight junctions (Chiba et al., 2008). Neurological diseases (Bednarczyk and Lukasiuk, 2011) and renal diseases (Li et al., 2011) have been reviewed. 

Tight junctions of epithelial cells exclude macromolecules but allows permeation of ions. It has not been clear whether this ion-conducting property is mediated by aqueous pores or by ion channels. To investigate the permeability properties of the tight junction, Tang & Goodenough (2003) developed paracellular ion flux assays for four major extracellular ions, Na+, Cl-, Ca2+, and Mg2+. Tight junctions share biophysical properties with conventional ion channels, including size and charge selectivity, dependency of permeability on ion concentration, competition between permeant molecules, anomalous mole-fraction effects, and sensitivity to pH. Their results support the hypothesis that discrete ion channels are present at the tight junction. Unlike conventional ion channels, which mediate ion transport across lipid bilayers, the tight junction channels must orient parallel to the plane of the plasma membranes to support paracellular ion movements. This new class of paracellular-tight junction channels facilitates the transport of ions between separate extracellular compartments (Balkovetz, 2009). Claudin-2 forms highly cation-selective paracellular pores (Yu, 2009). The basis of this charge selectivity is likely to be the presence of a negatively charged binding site within the lumen of the pore.  Paracellin-1 may be a Mg2+ transporter (Brandao et al. 2012).

Heterotypic (head-to-head) binding between different claudin isoforms plays a role in regulating paracellular permeability. Claudin-3 and claudin-4 do not heterotypically interact despite having highly conserved extracellular loop (EL) domains (Daugherty et al., 2007). Claudin-1 and -5, which are heterotypically compatible with claudin-3, do not bind to claudin-4. In contrast, claudin-4 chimeras containing either the first EL domain or the second EL domain of claudin-3, do bind to claudin-1, claudin-3, and claudin-5. Moreover, a single point mutation in the first extracellular loop domain of claudin-3, converting Asn44 to the corresponding amino acid in claudin-4 (Thr) produced a claudin capable of heterotypic binding to claudin-4 while still retaining the ability to bind to claudin-1 and -5. Thus, control of heterotypic claudin-claudin interactions is sensitive to small changes in the EL domains (Daugherty et al., 2007).

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is a human disorder caused by mutations in the tight junction protein claudin-16 (Hou et al., 2007). Claudin-16 plays a key role in maintaining the paracellular cation selectivity of the thick ascending limbs of the nephron. Claudin-16-deficient mice exhibit chronic renal wasting of magnesium and calcium and develop renal nephrocalcinosis. Claudin-16 apparently forms a non-selective paracellular cation channel, rather than a selective Mg2+/Ca2+channel as previously proposed (Hou et al., 2007). Claudin-16 increases transepithelial electrical resistance and transepithelial magnesium transport (Ikari et al., 2008). The activation of polyvalent extracellular cation (Ca2+)-sensing receptor (CaSR; P41180) decreased the resistance and magnesium transport, which were recovered by co-treatment with dibutyryl cAMP. Activation of CaSR may thus decrease PKA activity, resulting in a decrease in phosphorylated claudin-16, the translocation of claudin-16 to lysosomes and a consequent decrease in magnesium reabsorption (Ikari et al., 2008).

Claudins comprise the primary constituents of tight junctions and determine paracellular permeability. Ion selectivity of the paracellular conductance is a complex function of claudin subtype and cellular context (Hou et al., 2007). These 4 TMS proteins have been characterized from structural standpoints and may have arisen from an early intragenic duplication event (Hua et al., 2003).  There are 27 claudin paralogues in mice and humans (Mineta et al. 2011).  Permselective paracellular claudin channels are specific for certain ions and non-ionic solutes. Recent studies using claudin knockout mice revealed that the loss of claudins' specific paracellular barrier and/or channel functions affects particular biological functions and leads to pathological states (Tamura and Tsukita 2014).

As reviewed by Angelow et al. (2007;2008), the structure of claudin-based paracellular pores is largely unknown, but it is probably composed of homo- and hetero-typic claudin digomers. Both the proteins involved and the cell type determine the selectivity of paracellular transport. Claudins 2, 106 and 15 act preferentially as cation pores while claudins 10a and 7 are the only claudins that have significant anion pore properties (Angelow et al., 2008). However, claudins 4 and 7 have been reported to act as cation pores in MDCK II cells but as anion pores in LLC-PK 1 cells (Hou et al., 2006). They can pass neutral as well as charged small molecules. Their pore diameters are 8-15 Å. The first extracellular loop may line the paracellular pathways and determine the charge selectivity, but the C-terminal tail, which is modified by phosphorylation and palmitoylation and interacts with cytoskeletal proteins, may also play a role.

Claudin-2 pores are narrow, fluid filled, and cation selective (Yu et al., 2009). Charge selectivity is 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.

The control of claudin assembly into tight junctions requires a complex interplay between several classes of claudins, other transmembrane proteins and scaffold proteins (Findley and Koval, 2009). Claudins are also subject to regulation by post-translational modifications including phosphorylation and palmitoylation. Several human diseases have been linked to claudin mutations. Roles for claudins in regulating cell phenotype and growth control suggest a multifaceted role for claudins in regulation of cells beyond serving as a simple structural element of tight junctions. 

Epithelial transport relies on the proper function and regulation of the tight junction (TJ); otherwise, uncontrolled paracellular leakage of solutes and water would occur. They also act as a fence against mixing of membrane proteins of the apical and basolateral side. The proteins determining paracellular transport consist of four transmembrane regions, intracellular N and C terminals, one intracellular and two extracellular loops (ECLs). The ECLs interact laterally and with counterparts of the neighboring cell and thereby achieve a general sealing function. Two TJ protein families can be distinguished, claudins, comprising 27 members in mammals, and TJ-associated MARVEL proteins (TAMP), comprising occludin, tricellulin, and MarvelD3. They are linked to a multitude of TJ- associated regulatory and scaffolding proteins (Günzel and Fromm 2012). The major TJ proteins are classified according to the physiological role they play in enabling or preventing paracellular transport. Many TJ proteins have sealing functions (claudins 1, 3, 5, 11, 14, 19, and tricellulin). In contrast, a significant number of claudins form channels across TJs which feature selectivity for cations (claudins 2, 10b, and 15), anions (claudin-10a and -17), or are permeable to water (claudin-2). For several TJ proteins, their functions are unclear as their effects on epithelial barriers are inconsistent (claudins 4, 7, 8, 16, and occludin). TJs undergo physiological and pathophysiological regulation by altering protein composition or abundance. Major pathophysiological conditions which involve changes in TJ protein composition are (1) effects of pathogens binding to TJ proteins, (2) altered TJ protein composition during inflammation and infection, and (3) altered TJ protein expression in cancers (Günzel and Fromm 2012). 

The electric property of claudin pertains to two important organ functions: the renal and sensorineural functions. The kidney consists of three major segments of epithelial tubules with different paracellular permeabilities: the proximal tubule (PT), the thick acending limb of Henle's loop (TALH) and the collecting duct (CD). Claudins act as ion channels allowing selective permeation of Na+ in the PT, Ca2+ and Mg2+ in the TALH and Cl- in the CD. The inner ear, on the other hand, expresses claudins as a barrier to block K+ permeation between endolymph and perilymph. The permeability properties of claudins in different organs can be attributed to claudin interactions within the cell membrane and between neighboring cells. The first extracellular loop of claudins contains determinants of paracellular ionic permeability (Hou 2013).

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 with two extracellular segments (ECS1 and -2), and one intracellular loop. Claudins interact within the same (cis) and opposing (trans) plasma membranes. Claudins Cldn10b, -16, and -19 facilitate cation reabsorption in the TAL, and their absence leads to disturbances of renal ion homeostasis. Milatz et al. 2017 showed 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. Milatz et al. 2017 suggested 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+.

The paracellular transport reactions proposed to be catalyzed by claudinins are:

Ions (Lumen) Ions (Tissues).

References associated with 1.H.1 family:

Angelow, S. and A.S. Yu. (2007). Claudins and paracellular transport: an update. Curr Opin Nephrol Hypertens 16: 459-464. 17693762
Angelow, S., R. Ahlstrom, and A.S. Yu. (2008). Biology of claudins. Am. J. Physiol. Renal Physiol 295: F867-876. 18480174
Balkovetz, D.F. (2009). Tight junction claudins and the kidney in sickness and in health. Biochim. Biophys. Acta. 1788: 858-863. 18675779
Bednarczyk, J. and K. Lukasiuk. (2011). Tight junctions in neurological diseases. Acta Neurobiol Exp (Wars) 71: 393-408. 22237490
Brandao K., Deason-Towne F., Perraud AL. and Schmitz C. (2013). The role of Mg2+ in immune cells. Immunol Res. 55(1-3):261-9. 22990458
Chen, Y.H., J.J. Lin, B.G. Jeansonne, R. Tatum, and Q. Lu. (2009). Analysis of claudin genes in pediatric patients with Bartter's syndrome. Ann. N.Y. Acad. Sci. 1165: 126-134. 19538297
Chiba, H., M. Osanai, M. Murata, T. Kojima, and N. Sawada. (2008). Transmembrane proteins of tight junctions. Biochim. Biophys. Acta. 1778: 588-600. 17916321
Daugherty, B.L., C. Ward, T. Smith, J.D. Ritzenthaler, and M. Koval. (2007). Regulation of heterotypic claudin compatibility. J. Biol. Chem. 282: 30005-30013. 17699514
De Benedetto, A., N.M. Rafaels, L.Y. McGirt, A.I. Ivanov, S.N. Georas, C. Cheadle, A.E. Berger, K. Zhang, S. Vidyasagar, T. Yoshida, M. Boguniewicz, T. Hata, L.C. Schneider, J.M. Hanifin, R.L. Gallo, N. Novak, S. Weidinger, T.H. Beaty, D.Y. Leung, K.C. Barnes, and L.A. Beck. (2011). Tight junction defects in patients with atopic dermatitis. J Allergy Clin Immunol 127: 773-86.e1-7. 21163515
Findley, M.K. and M. Koval. (2009). Regulation and roles for claudin-family tight junction proteins. IUBMB Life 61: 431-437. 19319969
Gong Y., Renigunta V., Zhou Y., Sunq A., Wang J., Yang J., Renigunta A., Baker LA. and Hou J. (2015). Biochemical and biophysical analyses of tight junction permeability made of claudin-16 and claudin-19 dimerization. Mol Biol Cell. 26(24):4333-46. 26446843
Günzel, D. and M. Fromm. (2012). Claudins and other tight junction proteins. Compr Physiol 2: 1819-1852. 23723025
Hartsock, A. and W.J. Nelson. (2008). Adherens and tight junctions: structure, function and connections to the actin cytoskeleton. Biochim. Biophys. Acta. 1778: 660-669. 17854762
Hou J., A.S. Gomes, D.L. Paul, and D.A. Goodenough. (2006). Study of claudin function by RNA interference.  J. Biol. Chem. 281: 36117-36123. 
Hou J., Q. Shan, T. Wang, A.S. Gomes, Q. Yan, D.L. Paul, M. Bleich, D.A. Goodenough. (2007). Transgenic RNAi depletion of claudin-16 and the renal handling of magnesium. J Biol Chem. 282: 17114-17122. 17442678
Hou, J. (2013). A connected tale of claudins from the renal duct to the sensory system. Tissue Barriers 1: e24968. 24533254
Hou, J., A. Renigunta, J. Yang, and S. Waldegger. (2010). Claudin-4 forms paracellular chloride channel in the kidney and requires claudin-8 for tight junction localization. Proc. Natl. Acad. Sci. USA 107: 18010-18015. 20921420
Hou, J., A. Renigunta, M. Konrad, A.S. Gomes, E.E. Schneeberger, D.L. Paul, S. Waldegger, and D.A. Goodenough. (2008). Claudin-16 and claudin-19 interact and form a cation-selective tight junction complex. J Clin Invest 118(2): 619-628. 18188451
Hua V.B., A.B. Chang, J.H. Tchieu, N.M. Kumar, P.A. Nielsen, M.H. Saier Jr. (2003). Sequence and phylogenetic analyses of 4 TMS junctional proteins of animals: connexins, innexins, claudins and occludins. J. Membr. Biol. 194: 59-76. 14502443
Ikari, A., C. Okude, H. Sawada, Y. Sasaki, Y. Yamazaki, J. Sugatani, M. Degawa, and M. Miwa. (2008). Activation of a polyvalent cation-sensing receptor decreases magnesium transport via claudin-16. Biochim. Biophys. Acta. 1778(1): 283-290. 17976367
Irudayanathan, F.J., J.P. Trasatti, P. Karande, and S. Nangia. (2015). Molecular Architecture of the Blood Brain Barrier Tight Junction Proteins-A Synergistic Computational and In Vitro Approach. J Phys Chem B. [Epub: Ahead of Print] 26654362
Krause G., Protze J. and Piontek J. (2015). Assembly and function of claudins: Structure-function relationships based on homology models and crystal structures. Semin Cell Dev Biol. 42:3-12. 25957516
Krause, G., L. Winkler, S.L. Mueller, R.F. Haseloff, J. Piontek, and I.E. Blasig. (2008). Structure and function of claudins. Biochim. Biophys. Acta. 1778: 631-645. 18036336
Krug, S.M., J.D. Schulzke, and M. Fromm. (2014). Tight junction, selective permeability, and related diseases. Semin Cell Dev Biol 36: 166-176. 25220018
Li, J., W. Ananthapanyasut, and A.S. Yu. (2011). Claudins in renal physiology and disease. Pediatr Nephrol 26: 2133-2142. 21365189
Magyar, J.P., C. Ebensperger, N. Schaeren-Wiemers, and U. Suter. (1997). Myelin and lymphocyte protein (MAL/MVP17/VIP17) and plasmolipin are members of an extended gene family. Gene 189: 269-275. 9168137
Milatz, S. and T. Breiderhoff. (2017). One gene, two paracellular ion channels-claudin-10 in the kidney. Pflugers Arch 469: 115-121. 27942952
Milatz, S., J. Piontek, J.D. Schulzke, I.E. Blasig, M. Fromm, and D. Günzel. (2015). Probing the cis-arrangement of prototype tight junction proteins claudin-1 and claudin-3. Biochem. J. 468: 449-458. 25849148
Milatz, S., N. Himmerkus, V.C. Wulfmeyer, H. Drewell, K. Mutig, J. Hou, T. Breiderhoff, D. Müller, M. Fromm, M. Bleich, and D. Günzel. (2017). Mosaic expression of claudins in thick ascending limbs of Henle results in spatial separation of paracellular Na+ and Mg2+ transport. Proc. Natl. Acad. Sci. USA 114: E219-E227. 28028216
Mineta, K., Y. Yamamoto, Y. Yamazaki, H. Tanaka, Y. Tada, K. Saito, A. Tamura, M. Igarashi, T. Endo, K. Takeuchi, and S. Tsukita. (2011). Predicted expansion of the claudin multigene family. FEBS Lett. 585: 606-612. 21276448
Muto S., Furuse M. and Kusano E. (2012). Claudins and renal salt transport. Clin Exp Nephrol. 16(1):61-7. 22038258
Saitoh, Y., H. Suzuki, K. Tani, K. Nishikawa, K. Irie, Y. Ogura, A. Tamura, S. Tsukita, and Y. Fujiyoshi. (2015). Tight junctions. Structural insight into tight junction disassembly by Clostridium perfringens enterotoxin. Science 347: 775-778. 25678664
Shen, L., C.R. Weber, D.R. Raleigh, D. Yu, and J.R. Turner. (2011). Tight junction pore and leak pathways: a dynamic duo. Annu. Rev. Physiol. 73: 283-309. 20936941
Simon, D.B., Y. Lu, K.A. Choate, H. Velazquez, E. Al-Sabban, M. Praga, G. Casari, A. Bettinelli, G. Colussi, J. Rodriguez-Soriano, D. McCredie, D. Milford, S. Sanjad, and R.P. Lifton. (1999). Paracellin-1, a renal tight junction protein required for paracellular Mg2+ resorption. Science 285: 103-106. 10390358
Suzuki, H., Y. Ito, Y. Yamazaki, K. Mineta, M. Uji, K. Abe, K. Tani, Y. Fujiyoshi, and S. Tsukita. (2013). The four-transmembrane protein IP39 of Euglena forms strands by a trimeric unit repeat. Nat Commun 4: 1766. 23612307
Tamura A. and Tsukita S. (2014). Paracellular barrier and channel functions of TJ claudins in organizing biological systems: advances in the field of barriology revealed in knockout mice. Semin Cell Dev Biol. 36:177-85. 25305579
Tang V.W., Goodenough D.A. (2003). Paracellular ion channel at the tight junction. Biophys J. 84: 1660-1673. 12609869
Van Itallie, C.M. and J.M. Anderson. (2006). Claudins and epithelial paracellular transport. Annu. Rev. Physiol. 68: 403-429. 16460278
Van Itallie, C.M. and J.M. Anderson. (2014). Architecture of tight junctions and principles of molecular composition. Semin Cell Dev Biol 36: 157-165. 25171873
Van Itallie, C.M., L.L. Mitic, and J.M. Anderson. (2011). Claudin-2 forms homodimers and is a component of a high molecular weight protein complex. J. Biol. Chem. 286: 3442-3450. 21098027
Wang, W., X. Tan, L. Zhou, F. Gao, and X. Dai. (2010). Involvement of the expression and redistribution of claudin-23 in pancreatic cancer cell dissociation. Mol Med Report 3: 845-850. 21472324
Wilcox, E.R., Q.L. Burton, S. Naz, S. Riazuddin, T.N. Smith, B. Ploplis, I. Belyantseva, T. Ben-Yosef, N.A. Liburd, R.J. Morell, B. Kachar, D.K. Wu, A.J. Griffith, S. Riazuddin, and T.B. Friedman. (2001). Mutations in the gene encoding tight junction claudin-14 cause autosomal recessive deafness DFNB29. Cell 104: 165-172. 11163249
Wilson, F.H., S. Disse-Nicodème, K.A. Choate, K. Ishikawa, C. Nelson-Williams, I. Desitter, M. Gunel, D.V. Milford, G.W. Lipkin, J.M. Achard, M.P. Feely, B. Dussol, Y. Berland, R.J. Unwin, H. Mayan, D.B. Simon, Z. Farfel, X. Jeunemaitre, and R.P. Lifton. (2001). Human hypertension caused by mutations in WNK kinases. Science 293: 1107-1112. 11498583
Yu, A.S. (2009). Molecular basis for cation selectivity in claudin-2-based pores. Ann. N.Y. Acad. Sci. 1165: 53-57. 19538287
Yu, A.S., M.H. Cheng, S. Angelow, D. Günzel, S.A. Kanzawa, E.E. Schneeberger, M. Fromm, and R.D. Coalson. (2009). Molecular basis for cation selectivity in claudin-2-based paracellular pores: identification of an electrostatic interaction site. J Gen Physiol 133: 111-127. 19114638