BRIEF REVIEW www.jasn.org Claudins and the Kidney Alan S.L. Yu Division of Nephrology and Hypertension, and the Kidney Institute, University of Kansas Medical Center, Kansas City, Kansas ABSTRACT Claudins are tight-junction membrane proteins that function as both pores and barriers dimensional structure of claudin-15 has in the paracellular pathway in epithelial cells. In the kidney, claudins determine the recently been solved at a resolution of 2.4 permeability and selectivity of different nephron segments along the renal tubule. In Å.17 The structure reveals a characteris- the proximal tubule, claudins have a role in the bulk reabsorption of salt and water. In the tic b-sheet fold comprising the two ex- thick ascending limb, claudins are important for the reabsorption of calcium and tracellular segments, which is anchored magnesium and are tightly regulated by the calcium-sensing receptor. In the distal to a transmembrane four-helix bundle. nephron, claudins need to form cation barriers and chloride pores to facilitate This b-sheet forms a palm-like region electrogenic sodium reabsorption and potassium and acid secretion. Aldosterone and that likely lines the paracellular pore the with-no-lysine (WNK) proteins likely regulate claudins to fine-tune distal nephron salt (Figure 1). transport. Genetic mutations in claudin-16 and -19 cause familial hypomagnesemic hypercalciuria with nephrocalcinosis, whereas polymorphisms in claudin-14 are associ- ated with kidney stone risk. It is likely that additional roles for claudins in the EXPRESSION OF CLAUDINS IN pathogenesis of other types of kidney diseases have yet to be uncovered. THE KIDNEY J Am Soc Nephrol 26: 11–19, 2015. doi: 10.1681/ASN.2014030284 Most claudins are expressed in the renal tubule. Each segment and cell expresses multiple isoforms (Figure 2, Table 1). It The renal tubule efficiently reabsorbs the are not completely settled.6,7 Claudin is widely believed that the specificsetof bulk of filtered salt and water, and proteins have four predicted transmem- claudins expressed by each nephron accurately fine-tunes the concentrations brane helical domains with a short in- segment determines the unique paracel- of many different solutes in the urine. tracellular N-terminus, two extracellular lular permeability properties of that seg- This challenging task is accomplished loops, and a long C-terminal tail. Clau- ment. In addition, the glomerulus also by a combination of transcellular and dins are thought to polymerize to form expresses claudins. Parietal epithelial paracellular transport. The paracellular continuous strands along the lateral cells express claudin-1.18,19 Mature po- pathway is a route for passive transport membrane of one cell while the extracel- docytes form slit diaphragms, which that passes between tubule epithelial lular domains of claudins on adjacent are a specialized form of intercellular cells,1 with the tight junction constitut- cells bridge the paracellular space to in- junction, but tight junctions are also ing the primary permeability barrier.2–4 teract with each other, much like the present during fetal development and Claudins are members of a family of teeth of a zipper. The first extracellular reappear during podocyte injury.20,21 tight-junction membrane proteins that loop appears to line the paracellular pore Claudin-5 and -6 have both been detected act simultaneously as paracellular pores and determine its selectivity,8,9 while the in podocytes.22,23 and barriers and determine the selectiv- second extracellular loop mediates trans ity to small ions and neutral solutes.5 interactions.10,11 The C-terminal tail plays roles in protein trafficking to the tight junction and protein stability,12 Published online ahead of print. Publication date available at www.jasn.org. STRUCTURE AND FUNCTION OF contains phosphorylation13 and palmi- CLAUDINS toylation sites,14 and has a conserved hy- Correspondence: Dr. Alan S.L. Yu, Kidney In- stitute, University of Kansas Medical Center, 3901 drophobic dipeptide motif that binds Rainbow Boulevard, 6018 WHE, Kansas City, KS There are 27 mammalian claudin genes, to PDZ domains on tight-junction 66160-3018. Email: [email protected] although the homology and nomencla- scaffolding proteins, including ZO1, Copyright © 2015 by the American Society of ture of the more distantly related genes ZO2, ZO3,15 and MUPP1.16 The three- Nephrology J Am Soc Nephrol 26: 11–19, 2015 ISSN : 1046-6673/2601-11 11 BRIEF REVIEW www.jasn.org erties of the host cell line. For example, reabsorption in the late superficial PT claudin-15 decreases Cl permeability have been well studied37,38 (Figure 3A). when overexpressed in Na-selective, leaky Late PT fluid has relatively high concen- 2 2 MDCK II cells,9 whereas in Cl -selective trations of Cl and low concentrations of 2 and less leaky LLC-PK1 cells, claudin-15 HCO3 because of early PT transcellular increases Na+ permeability.24 reabsorption of Na, coupled largely to 2 In general, claudins regulate a pore HCO3 or organic solutes, together with pathway that is selectively permeable to isosmotic water reabsorption. The late PT 2 2 smallionsandneutralsolutes.Unlikemost is more permeable to Cl than HCO3 transmembrane channels, claudins do not (PCl/PHCO3 ratio ranging from 2 to tend to be highly selective but they do 1839,40). This permits net passive reabsorp- 2 exhibit charge preference. Thus, claudins tion of Cl , presumably via paracellular that are more cation-selective preferen- diffusion, and generates a lumen-positive tially permeate Na+,K+, and other mono- electrical potential.41 This voltage in turn valent cations but also divalent cations provides the driving force for passive reab- such as Ca2+.25,26 Anion-selective claudins sorption of Na+,againpresumablyvia the 2 permeate Cl but also other halides and paracellular pathway. An estimated 32%– small anions.27 Claudins differ in the mag- 64% of superficial PT NaCl reabsorption is nitude of their permeability, with some passive and presumably paracellular (re- predominantly acting as pore-formers viewed in reference 42). and others acting mostly as barrier-formers. Claudin-2 is the main claudin respon- The underlying mechanism for this is sible for reabsorption of Na+. Claudin-2 is Figure 1. Structure of a claudin-15 mono- unknown but likely involves intrinsic highly expressed in the PT,18 with highest mer, viewed from just above the plane of the differences in the size and conformation levels in the late PT and early segment of membrane. Four transmembrane ahelices (blue) traverse the lipid bilayer. The first oftheporeformedbyeachclaudin the thin descending limb of long loops of 43 In vitro 25,26,44 extracellular domain contributes four b isoform. Henle. overexpression 45 strands (yellow) to the b-sheet structure and The pore diameter has been estimated and knockdown studies have shown a short extracellular helix (red), while the for claudin-2 and is 6.5–8Å,26,28 making that claudin-2 forms high-conductance, second extracellular domain contributes it permeable to small organic ions, neu- cation-selective paracellular pores. Muto the fifth b strand (green). The b sheet forms tral molecules, and even water.29 In ad- et al. generated a constitutive knockout an inward-facing palm that likely lines the dition to this pore pathway, the tight of claudin-2 in mice and showed a signif- pore pathway. Image of protein databank junction exhibits a low-capacity, size- icant decrease in net Na and water reab- ID: 4P79 (ref. 17) created with visual mo- independent permeability to uncharged sorption in the PT S2 segments and loss of lecular dynamics 1.9.1 (ref. 115). macromolecules, such as dextrans. The Na+ selectivity.46 The mice had normal latter has been dubbed the “leak path- fractional excretion of Na+ on a normal way”30 and appears to be dependent on diet but had excessive natriuresis in re- IN VITRO PERMEABILITY occludin31,32 and ZO-133 but not on sponse to a saline load. Thus, claudin-2 PROPERTIES OF CLAUDINS claudins. Table 1 summarizes the func- likely plays an important role in PT para- tional properties of claudins of particular cellular salt reabsorption. The function of individual claudins in relevance to the kidney. A more detailed The PT also reabsorbs a substantial determining paracellular permeabilityand and comprehensive catalog of claudins portion of filtered K+47,48 and Ca2+49,50 selectivity has been investigated primarily and their properties may be found in re- and this is thought to be mostly pas- byoverexpression and knockdown experi- cent general reviews.34,35 sive. There are two proposed mecha- ments in epithelial cell lines. The inter- nisms: (1) passive diffusion, driven by a pretation of such studies is complicated lumen-to-bath K+ and Ca2+ concentra- because, unlike transmembrane transport ROLE OF CLAUDINS IN THE tion gradient (generated by net water re- proteins, claudins must simultaneously PROXIMAL TUBULE absorption) and lumen-positive electrical function as both the barrier and the pores. gradient in the mid-late PT, or (2)convection Moreover, all epithelia already express The primary role of the proximal tubule (solvent drag). It has always been assumed multiple endogenous claudins, so the (PT) is bulk reabsorption of filtered sol- (as for NaCl) that the route of this passive 2 function of a heterologously expressed utes, including Na+,K+,Cl ,andCa2+, reabsorption is paracellular, but this has (or a knocked down) claudin gene must and water. The PT is the leakiest neph- never been definitively proven. Claudin-2 be superimposed on this background. As a ron segment in the renal tubule, with is highly permeable to K+ and also mod- consequence, the apparent function of transepithelial resistances of 5–7 V.cm2.36 erately permeable to Ca2+.26 Claudin-2 claudin is highly dependent on the prop- The driving forces for paracellular NaCl null mice do not have any abnormality in 12 Journal of the American Society of Nephrology J Am Soc Nephrol 26: 11–19, 2015 www.jasn.org BRIEF REVIEW Figure 2.