Renal Tubule Nedd4-2 Deficiency Stimulates Kir4.1/Kir5.1 and Thiazide-Sensitive NaCl Cotransporter in Distal Convoluted Tubule
Authors: Wu Peng , Su Xiao-Tong , Gao Zhong-Xiuzi , Zhang Dan-Dan , Duan Xin-Peng , Xiao Yu , Staub Olivier , Wang Wen-Hui , Lin Dao-Hong 
Published: April 2020

Significance Statement
The potassium channel Kir4.1 forms the Kir4.1/Kir5.1 heterotetramer in the basolateral membrane of the distal convoluted tubule (DCT) and plays an important role in regulating the thiazide-sensitive NaCl cotransporter (NCC). Deletion of the ubiquitin ligase Nedd4-2 has been shown to increase the expression of NCC and to cause salt-sensitive hypertension. The authors demonstrated that kidney-specific deletion of Nedd4-2 in mice also stimulates Kir4.1/Kir5.1 activity in the DCT and hyperpolarizes the DCT membrane. They also found that NCC activity/expression is largely inhibited in double-knockout mice deficient in both Kir4.1 and Nedd4-2 and that NCC activity/expression is higher in these double-knockout mice compared with mice lacking only Kir4.1. These findings suggest that Nedd4-2 regulates NCC expression through modulation of basolateral Kir4.1/Kir5.1 activity and through Kir4.1-independent regulation of NCC retrieval.

Background
The potassium channel Kir4.1 forms the Kir4.1/Kir5.1 heterotetramer in the basolateral membrane of the distal convoluted tubule (DCT) and plays an important role in the regulation of the thiazide-sensitive NaCl cotransporter (NCC). Kidney-specific deletion of the ubiquitin ligase Nedd4-2 increases expression of NCC, and coexpression of Nedd4-2 inhibits Kir4.1/Kir5.1 in vitro. Whether Nedd4-2 regulates NCC expression in part by regulating Kir4.1/Kir5.1 channel activity in the DCT is unknown.

Methods
We used electrophysiology studies, immunoblotting, immunostaining, and renal clearance to examine Kir4.1/Kir5.1 activity in the DCT and NCC expression/activity in wild-type mice and mice with kidney-specific knockout of Nedd4-2, Kir4.1, or both.

Results
Deletion of Nedd4-2 increased the activity/expression of Kir4.1 in the DCT and also, hyperpolarized the DCT membrane. Expression of phosphorylated NCC/total NCC and thiazide-induced natriuresis were significantly increased in the Nedd4-2 knockout mice, but these mice were normokalemic. Double-knockout mice lacking both Kir4.1/Kir5.1 and Nedd4-2 in the kidney exhibited increased expression of the epithelial sodium channel α-subunit, largely abolished basolateral potassium ion conductance (to a degree similar to that of kidney-specific Kir4.1 knockout mice), and depolarization of the DCT membrane. Compared with wild-type mice, the double-knockout mice displayed inhibited expression of phosphorylated NCC and total NCC and had significantly blunted thiazide-induced natriuresis as well as renal potassium wasting and hypokalemia. However, NCC expression/activity was higher in the double-knockout mice than in Kir4.1 knockout mice.

Conclusions
Nedd4-2 regulates Kir4.1/Kir5.1 expression/activity in the DCT and modulates NCC expression by Kir4.1-dependent and Kir4.1-independent mechanisms. Basolateral Kir4.1/Kir5.1 activity in the DCT partially accounts for the stimulation of NCC activity/expression induced by deletion of Nedd4-2.