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Pendrin—A New Target dominant polycystic disease, and hyponatremic syn- dromes. Moreover, the efficacy of blocking salt for Therapy? absorptive pathways is often limited by the plasticity of the renal tubule and its ability to stimulate the growth and Carsten A. Wagner activity of downstream segments contributing to treatment- National Center for Competence in Research Kidney.CH and resistant hypertension.3,6 Then, combinations of diuretics act- Institute of Physiology, University of Zurich, Zurich, Switzerland ing in subsequent segments and on distinct pathways may help fi J Am Soc Nephrol 27: 3499–3501, 2016. to improve diuretic ef cacy. doi: 10.1681/ASN.2016070720 The collecting duct is the last site of salt reabsorption and contributes to up to 2% of absorption of the filtered salt load. Renal salt reabsorption in the collecting duct depends Diuretics are among the most frequently used drugs to treat hy- on several factors and transport pathways. Distal delivery of pertension, congestive , and edema and play a role and chloride is determined by GFR and the rate of – in the prevention of some forms of recurrent nephrolithiasis.1 3 reabsorption of both ions in upstream nephron segments. Most commonly used diuretics act by directly inhibiting tubular Particularly, NaCl reabsorption by the thick ascending limb of transport processes, such as the loop diuretics blocking the fl 1 1 2 the loop of Henle and the in uences Na /K /2Cl (NKCC2) in the thick ascending the delivery of sodium and chloride to the collecting duct. limb of the loop of Henle, the and chlortalidones inhib- 1 1 2 There, sodium is reabsorbed in part by the epithelial Na iting the Na /Cl cotransporter (NCC) in the distal convoluted channel ENaC, which is stimulated by angiotensin II and al- tubule, or the potassium-sparing diuretics of the or dosterone. Chloride absorption occurs via different routes, in type blocking the epithelial sodium channel (ENaC) part through the paracellular pathway requiring claudins 4 and in the connecting tubule and collecting duct. Other classes 8 and through intercalated cells.7,8 In nontype A–intercalated of diuretics interfere with metabolic or endocrine mecha- cells, chloride can be directly reabsorbed in exchange for nisms linked to tubular transport processes, such as in the cases bicarbonate, a process mediated by the anion exchanger pen- of the rather infrequently used carbonic anhydrase inhibitors 1 1 drin, or chloride absorption can be coupled to sodium absorp- (e.g., ) that mostly reduce Na /H exchanger tion involving a more complex scheme. Secretion of two activity and the more widely used mineralocorticoid receptor bicarbonate ions and absorption of two chloride ions by blockers (e.g., and ) reducing the are driving the activity of another transporter, the 1 stimulatory effect of aldosterone on ENaC–mediated salt ab- Na –dependent bicarbonate-chloride exchanger (NDBCE), sorption. Two novel classes of substances increase diuresis but exchanging one intracellular chloride for one extracellular do not act primarily on salt reabsorptive pathways: the aqua- bicarbonate plus one sodium (Figure 1). This results in the retics blocking the V2 receptor reducing AQP2 netabsorptionofNaCl.8 water channel activation by the antidiuretic hormone (e.g., 1 Insights into the relevanceof pendrin in renal salt handling tolvaptan) and the inhibitors of the Na / come from human genetics studying patients with the rare 2 (SGLT2) (e.g., canagliflozin, dapagliflozin, and empagliflo- Pendred syndrome and various mouse models. Patients with zin) primarily reducing proximal tubular glucose absorption Pendred syndrome caused by mutations in pendrin causing osmotic diuresis.4,5 (SLC26A4) suffer most notably from sensorineural deafness The use of diuretics from the first group directly blocking and frequently develop hypothyroidism and goiter. However, salt reabsorption can be limited by hypo- or hyperkalemia or their renal phenotype has been studied very little to date. hypercalcemia. In patients with CKD and increasingly lower Hypochloremic alkalosis was reported in two patients with GFR, the efficacy of drugs can be reduced, requiring escalating Pendred syndrome; however, the condition occurred only doses or alternative drugs.The indications for SGLT2inhibitors during acute illness or therapy with thiazides primarily caus- and antagonists are distinct and currently, ing volume depletion.9,10 Next to these incidental reports, de- mostly restricted to the treatment of type 2 diabetes, autosomal tailed studies in mice suggest an important role of pendrin in renal control of salt excretion and BP. Salt depletion or aldo- Published online ahead of print. Publication date available at www.jasn.org. sterone stimulates pendrin expression and activity in parallel to other chloride and sodium transport proteins, such as NaCl Correspondence: Prof. Carsten A. Wagner, National Center for Compe- 8 tence in Research Kidney.CH and Institute of Physiology, University of Zurich, cotransporter NCC or the epithelial sodium channel ENaC. Winterthurerstrasse 190, CH-8057 Zurich, Switzerland. Email: Wagnerca@ Mice lacking pendrin lose more salt during salt depletion and access.uzh.ch have lower BP, suggesting that pendrin is required for the re- Copyright © 2016 by the American Society of Nephrology nal capacity to conserve salt and maintain BP. Moreover,

J Am Soc Nephrol 27: 3499–3510, 2016 ISSN : 1046-6673/2712-3499 3499 EDITORIALS www.jasn.org

Figure 1. Transport pathways involved in salt absorption by the distal convoluted tubule (DCT) and collecting system as targets of diuretics. Scheme of a nephron depicting the major transport pathways targeted by commonly used diuretics. In the thick ascending limb of the loop of Henle (TAL), and chlortalidon inhibit the NKCC2 cotransporter, and in the DCT, thiazides blockthe NCC cotransporter. In the collecting system (first cellsstartingin thelate DCT), potassium-sparingdiureticsinhibitENaCactivity inprincipal cells (PCs). Chlorideabsorption occursin part through a pendrin-dependent process located in type B intercalated cells (B-ICs; basolateral V-ATPase) and nontype A/nontype B intercalated cells (non-A/ non-B ICs; apical V-ATPase). V-ATPase–driven proton secretion energizes chloride/bicarbonate exchange by pendrin. Coupling of pendrin activity to the NDCBE allows for NaCl absorption. NDBCE may also be targeted by thiazides, whereas pendrin can be blocked by novel inhibitors. 2 1 1 1 AE1, anion exchanger 1; CIC-K, ClC-K chloride channels; Cl , chloride; ENaC, epithelial sodium channel; H ,proton;K ,potassium;Na ,sodium; 1 NCC, NaCl-cotransporter; Pds, Pendrin; ROMK, renal outer medullary K -channel; V-ATPase, V-type proton pump. pendrin-deficient mice are partly protected from developing Verkman and colleagues14 identified novel compounds se- hypertension during high-salt/aldosterone treatment. Vice lectively inhibiting pendrin with high affinity using a func- versa, mice overexpressing pendrin develop chloride-sensitive tional chemical library screen. As reported here in this issue hypertension.11 of the Journal of the American Society of Nephrology,these Because salt absorption by the collecting duct is influenced compounds given to mice had, per se, no effect on diuresis or by salt delivery, the role of pendrin in compensating for reduced acid-base parameters in urine and blood.15 However, when salt absorption by earlier segments has been studied. On the one given in combination with furosemide, pendrin inhibitors hand, chronic inhibition of NCC activity with thiazides increases potentiated the diuretic effect of furosemide. Importantly, pendrin expression, whereas on the other hand, concomitant the pendrin inhibitor even further increased diuresis in mice genetic ablation of NCC and pendrin causes a severe syndrome givenchronicallyfurosemide.However,micetreatedwith of renal salt loss and volume depletion.8,12 Similarly, deletion of plus pendrin inhibitor showed a paradox- NCC and the NDBCE transporter working in conjunction with ical reduction in diuresis.15 pendrin also leads to salt wasting.13 Taken together, the biology Several issues are of major interest, and some questions of pendrin and its interactions with other salt–transporting remain unanswered. Next to tests of safety and pharmacoki- pathways make pendrin an attractive target for novel inhibitors netics in humans, the question remains of whether pendrin that may be useful as diuretics standing alone or used in com- inhibitor will have a similar diuretic potency in humans as in bination with existing drugs to enhance their efficacy. rodents. The relative number of pendrin-expressing cells may

3500 Journal of the American Society of Nephrology J Am Soc Nephrol 27: 3499–3510, 2016 www.jasn.org EDITORIALS be relatively lower in human kidney compared with rodent 2. Roush GC, Kaur R, Ernst ME: Diuretics: A review and update. JCardio- kidney and thus, limit the efficacy of pendrin inhibitors.16,17 vasc Pharmacol Ther 19: 5–13, 2014 Next to kidney, pendrin (SLC26A4) is expressed in various 3. Vongpatanasin W: Resistant hypertension: A review of diagnosis and management. JAMA 311: 2216–2224, 2014 other organs, including thyroid glands, inner ear, adrenal glands, 4. Oliva RV, Bakris GL: Blood pressure effects of sodium-glucose co- and airways, which will affect its side effects as well as modulate transport 2 (SGLT2) inhibitors. J Am Soc Hypertens 8: 330–339, its diuretic and antihypertensive effects. Although pendrin func- 2014 tion in inner ear and thyroid glands may be problematic and 5. Torres VE: Vasopressin receptor antagonists, heart failure, and poly- cystic kidney disease. Annu Rev Med 66: 195–210, 2015 cause side effects, the inhibition of adrenal gland pendrin may JClinInvest fi 6. Knepper MA: Systems biology of diuretic resistance. 125: add to the bene cial effects of pendrin blockade in kidney. 1793–1795, 2015 Pendrin function in adrenal glands seems to support aldoste- 7. Hou J: The kidney tight junction (review). Int J Mol Med 34: 1451–1457, 2014 rone secretion, and inhibition may help to reduce renal salt 8. Wall SM, Lazo-Fernandez Y: The role of pendrin in renal physiology. reabsorption as well as other extrarenal effects of aldosterone Annu Rev Physiol 77: 363–378, 2015 (e.g., in heart and vasculature).18 9. Pela I, Bigozzi M, Bianchi B: Profound hypokalemia and hypochloremic metabolic alkalosis during therapy in a child with Pendred Chronic treatment with loop diuretics as well as thiazides and syndrome. Clin Nephrol 69: 450–453, 2008 chlortalidones often causes hypokalemia because of the excessive 10. Kandasamy N, Fugazzola L, Evans M, Chatterjee K, Karet F: Life- stimulation of potassium secretion in the collecting duct by in- threatening metabolic alkalosis in Pendred syndrome. Eur J Endocrinol creased urinary flow and stimulation of ENaC activity, which in 165: 167–170, 2011 1 1 turn, increasesrenal outermedullary K –mediated K secretion. 11. Jacques T, Picard N, Miller RL, Riemondy KA, Houillier P, Sohet F, Ramakrishnan SK, Büsst CJ, Jayat M, Cornière N, Hassan H, The combination of the pendrin inhibitor with furosemide fur- 1 Aronson PS, Hennings JC, Hübner CA, Nelson RD, Chambrey R, ther increased urinary K excretion, pointing to another possible Eladari D: Overexpression of pendrin in intercalated cells pro- side effect of this class of inhibitors. Also, metabolic alkalosis was duces chloride-sensitive hypertension. JAmSocNephrol24: aggravated by the combination of furosemide and the pendrin 1104–1113, 2013 inhibitor, further underlining the potency of this combination. 12. Soleimani M, Barone S, Xu J, Shull GE, Siddiqui F, Zahedi K, Amlal H: Double knockout of pendrin and Na-Cl cotransporter (NCC) causes The pendrin inhibitor reduced the diuretic response to thi- severe salt wasting, volume depletion, and renal failure. Proc Natl Acad azide treatment. At first glance, this is surprising, because the Sci U S A 109: 13368–13373, 2012 combined ablation of NCC and pendrin causes a massive diure- 13. Sinning A, Radionov N, Trepiccione F, López-Cayuqueo KI, Jayat M, sis and renal phenotype in mice.12 However, thiazides have also Baron S, Cornière N, Alexander RT, Hadchouel J, Eladari D, Hübner 1 been reported to block not only NCC activity but also, the CA, Chambrey R: Double knockout of the Na -driven Cl-/HCO3- ex- changer and Na1/Cl- cotransporter induces hypokalemia and volume NDCBE transporter working in conjunction with pendrin in depletion [published online ahead of print May 5, 2016]. JAmSoc the collecting duct. Thus, during acute treatment with thiazides, Nephrol doi:10.1681/ASN.2015070734 both NCC– and NDCBE/pendrin–dependent salt absorption 14. Haggie PM, Phuan PW, Tan JA, Zlock L, Finkbeiner WE, Verkman AS: may be blocked, and further blockade with a pendrin inhibitor Inhibitors of pendrin anion exchange identified in a small molecule screen increase airway surface liquid volume in cystic fibrosis. FASEB J does not cause more salt wasting. In contrast, pendrin inhibi- – fi 30: 2187 2197, 2016 tion in adrenal glands may reduce ltration and thereby, cause a 15. Cil O, Haggie PM, Phuan PW, Tan JA, Verkman AS: Small-molecule paradoxical reduction in diuresis. Clearly, these effects will re- inhibitors of pendrin potentiate the diuretic action of furosemide. JAm quire more clarification and may also provide novel insights Soc Nephrol 27: 3706–3714, 2016 into the renal handling of chloride and the role of pendrin. 16. Hiatt MJ, Ivanova L, Toran N, Tarantal AF, Matsell DG: Remodeling Am J Pathol – Pendrin inhibitors, if making the difficult way into clinics, of the fetal collecting duct epithelium. 176: 630 637, 2010 may represent another promising new class of drugs targeting a 17. Wagner S, Vogel R, Lietzke R, Koob R, Drenckhahn D: Immunochemical tubular transport pathway, like the more recently developed characterization of a band 3-like anion exchanger in collecting duct 1 inhibitors of transporters or the renal outer medullary K of human kidney. Am J Physiol 253: F213–F221, 1987 channel.19,20 Their specificapplicationmaybeinthecombi- 18. Lazo-Fernandez Y, Aguilera G, Pham TD, Park AY, Beierwaltes WH, nation with other diuretics, such as loop diuretics, to address Sutliff RL, Verlander JW, Pacak K, Osunkoya AO, Ellis CL, Kim YH, Shipley GL, Wynne BM, Hoover RS, Sen SK, Plotsky PM, Wall SM: some of the adaptive responses of the tubular transport ma- Pendrin localizes to the adrenal medulla and modulates catechol- chinery contributing to the resistance to diuretics. amine release. Am J Physiol Endocrinol Metab 309: E534–E545, 2015 19. Esteva-Font C, Anderson MO, Verkman AS: Urea transporter proteins as targets for small-molecule diuretics. Nat Rev Nephrol 11: 113–123, DISCLOSURES 2015 None. 20. Martelli A, Testai L, Breschi MC, Calderone V: Inhibitors of the renal outer medullary potassium channel: A patent review. Expert Opin Ther Pat 25: 1035–1051, 2015 REFERENCES

1. Xu H, Zisman AL, Coe FL, Worcester EM: Kidney stones: An update on current pharmacological management and future directions. Expert See related article, “Small-Molecule Inhibitors of Pendrin Potentiate the Diuretic Opin Pharmacother 14: 435–447, 2013 Action of Furosemide,” on pages 3706–3714.

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