Birmingham Veterans Affairs Medical Center, and Departments of Medicine, Cell Biology, and Microbiology, University of Alabama at Birmingham, Birmingham, Alabama
Address correspondence to: Dr. Daniel F. Balkovetz, 668 LHRB, 1530 3rd Avenue South, Birmingham, AL 35294-0007. Phone: 205-934-3589; Fax: 205-975-6288; E-mail: balkovet{at}uab.edu
The epithelial tight junction (TJ) is one of the epithelialcell–cell junctional complexes and is critical for themaintenance of epithelial cell polarity and control of paracellulartransport across epithelial tissues. In many renal physiologyand nephrology textbooks, the renal epithelial TJ is simplisticallydepicted in cartoons as a box between epithelial cells withan arrow going through to illustrate the role of the TJ in paracellulartransport (Figure 1). The understanding of the TJ protein compositionand function is a rapidly advancing field. Major breakthroughsin the characterization of the TJ architecture came from thelaboratory of the late Shoichiro Tsukita (1). Tsukita's groupidentified the first transmembrane protein components of theTJ: Occludin and a family of proteins named claudins. The firstclaudins were reported in 1998 (2). To date, 24 claudin isoformshave been identified, and the composition of particular claudinisoforms within a TJ seems to determine its paracellular transportproperties. In the kidney, the different claudin isoforms areexpressed in a nephron segment–specific pattern (3). Theparacellular transport properties vary in these different segmentsof the nephron and correlate with the composition of claudinisoforms in the TJ of the different nephron segments (4). Forexample, claudin-16 (also known as paracellin) is expressedin the TJ in the thick ascending limb and distal convolutedtubule and seems to mediate paracellular resorption of bothMg2+ and Ca2+. Mutations in claudin-16 are a cause of familialhypomagnesemia-hypercalciuria syndrome in humans (5).
Figure 1. Illustration depicting paracellular transport across the tight junction (TJ) in an epithelial cell monolayer.
In this issue of JASN, Feldman et al. (6) provide evidence fora unique and unappreciated effect of cyclosporin on the modulationof epithelial TJ barrier function. They report that cyclosporinincreases TGF- production and secretion and the expression ofthe TGF- receptor II. TGF- activates extracellular signal–regulatedkinase 1/2 (ERK1/2) and increases TJ transepithelial resistance(TER), a surrogate marker of TJ "tightness." The increase inTJ TER correlates with increases in TJ-associated proteins occludin,claudin-1, claudin-3, and zona occludens-2 (ZO-2) (althoughthe investigators looked only at TJ-associated proteins occludin,claudins 1 to 4, claudin-16, ZO-1, and ZO-2). Collectively,this study clearly demonstrates that cyclosporin modifies thecomposition of proteins in the TJ as well as TJ function.
The cellular mechanism by which cyclosporin increases TER ofthe TJ involves the TGF-/ERK1/2 signaling cascade, but the exactstructural changes in the TJ protein composition that lead tothe increase in the TER remain to be defined. Claudin-2 is animportant determinant of TER in epithelia, including two strainsof MDCK cells. MDCK strain II cells have a low TER (approximately100 /cm2) and abundantly express claudin-2 in the TJ. Conversely,MDCK strain I cells have a high TER (approximately 4000 to 10,000/cm2) and do not express claudin-2. Furose et al. (7) showedthat transfection of claudin-2 into MDCK strain I cells resultsin a TER value very similar to that observed in MDCK strainII cells. Two recent studies showed that activation of ERK1/2by either epidermal growth factor (8) or hepatocyte growth factor(9) results in a significant rise in TER accompanied by a dramaticloss of claudin-2 expression in MDCK strain II cells. Curiously,the study by Feldman et al. in this issue of JASN demonstratesactivation of ERK1/2 by cyclosporin, a dramatic increase inTER, but no changes in expression of claudin-2 (6). Their datasuggest that changes in other TJ-associated proteins, otherthan claudin-2, can also increase TJ barrier function as measuredby TER. For example, a recent report from the Yu laboratoryshowed that introduction of claudin-19 into the renal epithelialTJ increases the TER of renal epithelial TJ (10). Clearly, theregulation of TJ composition and function by cytokines and drugswill prove to be complex. Additional work is needed to clarifythe role of ERK1/2 signaling in the regulation of both the expressionof various claudin isoforms in the TJ and the paracellular permeabilityof the TJ in various epithelial tissues.
How could cyclosporin-specific modification of the renal epithelialTJ contribute to the adverse effect of nephrotoxicity? The studyof Feldman et al. (6) demonstrates an increase in TER; no effectin paracellular flux of dextran; and increases in occludin,claudin-1, claudin-3, and ZO-1 in MDCK cells during a 72-h period.From a pathophysiologic standpoint, acutely increasing the TJTER along the nephron would not provide an obvious cause ofcyclosporin-related nephrotoxicity, such as a reduction in glomerularfiltration and/or hyperkalemia. However, the long-term consequencesof cyclosporin exposure in vitro and in vivo on the renal TJcomposition and function are not known. In the rat liver, cyclosporinacutely increases the paracellular permeability across the TJ(11). If cyclosporin also increases in vivo TJ paracellularpermeability in renal tubular epithelial cells, then a reductionof GFR could occur from increased tubular backleak of glomerularfiltrate. In addition, increasing paracellular permeabilityto potassium ions in the distal nephron would cause cyclosporin-inducedhyperkalemia. The mechanism of tubular potassium backleak thatleads to cyclosporin-induced hyperkalemia is supported by studiesthat showed a reduction in the transtubular potassium gradientin patients who took cyclosporin (12). Another way by whichcyclosporin-induced alteration in TJ permeability could contributeto reduced renal function may occur at the level of ultrafiltrationformation within the glomerulus. It has been shown that cyclosporinalters the expression of ZO-1 in the glomerular podocyte slitdiaphragm (13). Alteration of TJ composition and function inthe glomerular podocytes is likely to modulate the ultrafiltrationbarrier in the glomerulus and subsequently change GFR.
Much work is needed to facilitate our conceptual understandingof the regulation of epithelial TJ composition and functionduring normal physiologic as well as pathologic conditions.Recent advances in the identification of the architectural proteincomposition of the TJ provide us with needed information aboutwhat is in the TJ box and how to study TJ function/regulation.As with other discoveries, opening the TJ box will reveal theformidable complexities of this fascinating organelle and potentiallyrelease apparent evils, such as confusion and controversy regardingthe function and regulation of the TJ. Like Pandora's box, theopening of the TJ box will also provide humankind with hopethat we will ultimately understand how the TJ works and is regulated.The light to study TJ structure, function, and regulation isbecoming brighter!
Takeichi M, Shoichiro Tsukita: A life exploring the molecular architecture of the tight junction.
J Cell Biol 172
: 321
–323, 2006[Abstract/Free Full Text]
Furuse M, Fujita K, Hiiragi T, Fujimoto K, Tsukita S: Claudin-1 and -2: Novel integral membrane proteins localizing at tight junctions with no sequence similarity to occludin.
J Cell Biol 141
: 1539
–1550, 1998[Abstract/Free Full Text]
Kiuchi-Saishin Y, Gotoh S, Furuse M, Takasuga A, Tano Y, Tsukita S: Differential expression patterns of claudins, tight junction membrane proteins, in mouse nephron segments.
J Am Soc Nephrol 13
: 875
–886, 2002[Abstract/Free Full Text]
Balkovetz DF: Claudins at the gate: Determinants of renal epithelial tight junction paracellular permeability.
Am J Physiol Renal Physiol 290
: F572
–F579, 2006[Abstract/Free Full Text]
Simon DB, Lu Y, Choate KA, Velazquez H, Al-Sabban E, Praga M, Casari G, Bettinelli A, Colussi G, Rodriguez-Soriano J, McCredie D, Milford D, Sanjad S, Lifton RP: Paracellin-1, a renal tight junction protein required for paracellular Mg2+ resorption.
Science 285
: 103
–106, 1999[Abstract/Free Full Text]
Feldman G, Kiely B, Martin N, Gavin R, McMorrow T, Ryan MP: A role for TGF-beta in cyclosporine-induced modulation of renal epithelial barrier function.
J Am Soc Nephrol 18
: 1662
–1671, 2007
Furuse M, Furuse K, Sasaki H, Tsukita S: Conversion of zonulae occludentes from tight to leaky strand type by introducing claudin-2 into Madin-Darby canine kidney I cells.
J Cell Biol 153
: 263
–272, 2001[Abstract/Free Full Text]
Lipschutz JH, Li S, Arisco A, Balkovetz DF: Extracellular signal-regulated kinases 1/2 control claudin-2 expression in Madin-Darby canine kidney strain I and II cells.
J Biol Chem 280
: 3780
–3788, 2005[Abstract/Free Full Text]
Angelow S, El-Husseini R, Kanzawa SA, Yu AS: Renal localization and function of the tight junction protein, claudin-19.
Am J Physiol Renal Physiol March 27, 2007
[epub ahead of print]
Lora L, Mazzon E, Billington D, Milanesi C, Naccarato R, Martines D: Effects of cyclosporin A on paracellular and transcellular transport of horseradish peroxidase in perfused rat livers.
Dig Dis Sci 42
: 514
–521, 1997[CrossRef][Medline]
Kamel KS, Ethier JH, Quaggin S, Levin A, Albert S, Carlisle EJ, Halperin ML: Studies to determine the basis for hyperkalemia in recipients of a renal transplant who are treated with cyclosporine.
J Am Soc Nephrol 2
: 1279
–1284, 1992[Abstract]
Kim BS, Park HC, Kang SW, Choi KH, Ha SK, Han DS, Lee HY: Impact of cyclosporin on podocyte ZO-1 expression in puromycin aminonucleoside nephrosis rats.
Yonsei Med J 46
: 141
–148, 2005[Medline]
Related Article
Role for TGF- in Cyclosporine-Induced Modulation of Renal Epithelial Barrier Function
Gemma Feldman, Breda Kiely, Natalia Martin, Gavin Ryan, Tara McMorrow, and Michael P. Ryan
J. Am. Soc. Nephrol. 2007 18: 1662-1671.
[Abstract][Full Text][PDF]
Top
Introduction
Disclosures
production and secretion and the expression of the TGF-
/cm2) and abundantly express claudin-2 in the TJ. Conversely, MDCK strain I cells have a high TER (approximately 4000 to 10,000 
