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MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

Heightened epithelial Na⁺ channel-mediated Na⁺ absorption in a murine polycystic kidney disease model epithelium lacking apical monocilia

Departments of ¹Physiology and Biophysics and of ²Cell Biology and Divisions of ³Nephrology and Genetics and ⁴Translational Medicine, Department of Medicine, University of Alabama at Birmingham, Birmingham, Alabama; and ⁵Division of Nephrology, Department of Pediatrics and Medicine, Mount Sinai School of Medicine, New York, New York

Submitted 10 July 2005 ; accepted in final form 29 September 2005

The Tg737°^rpk autosomal recessive polycystic kidney disease (ARPKD) mouse carries a hypomorphic mutation in the Tg737 gene. Because of the absence of its protein product Polaris, the nonmotile primary monocilium central to the luminal membrane of ductal epithelia, such as the cortical collecting duct (CCD) principal cell (PC), is malformed. Although the functions of the renal monocilium remain elusive, primary monocilia or flagella on neurons act as sensory organelles. Thus we hypothesized that the PC monocilium functions as a cellular sensor. To test this hypothesis, we assessed the contribution of Polaris and cilium structure and function to renal epithelial ion transport electrophysiology. Properties of Tg737°^rpk mutant CCD PC clones were compared with clones genetically rescued with wild-type Tg737 cDNA. All cells were grown as polarized cell monolayers with similarly high transepithelial resistance on permeable filter supports. Three- to fourfold elevated transepithelial voltage (V_te) and short-circuit current (I_sc) were measured in mutant orpk monolayers vs. rescued controls. Pharmacological and cell biological examination of this enhanced electrical end point in mutant monolayers revealed that epithelial Na⁺ channels (ENaCs) were upregulated. Amiloride, ENaC-selective amiloride analogs (benzamil and phenamil), and protease inhibitors (aprotinin and leupeptin) attenuated heightened V_te and I_sc. Higher concentrations of additional amiloride analogs (ethylisopropylamiloride and dimethylamiloride) also revealed inhibition of V_te. Cell culture requirements and manipulations were also consistent with heightened ENaC expression and function. Together, these data suggest that ENaC expression and/or function are upregulated in the luminal membrane of mutant, cilium-deficient orpk CCD PC monolayers vs. cilium-competent controls. When the genetic lesion causes loss or malformation of the monocilium, ENaC-driven Na⁺ hyperabsorption may explain the rapid emergence of severe hypertension in a majority of patients with ARPKD.

cilia; hypertension; ion transport; epithelial cells

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