Channel Function of Polycystin-2 in the Endoplasmic Reticulum Protects against Autosomal Dominant Polycystic Kidney Disease
Authors: Padhy Biswajit , Xie Jian , Wang Runping , Lin Fang , Huang Chou-Long 
Published: July 2022

Significance Statement
Mutations of PKD2 encoding polycystin-2 cause autosomal dominant polycystic kidney disease (ADPKD). The prevailing hypothesis is that polycystin-2–mediated calcium ion influx in the primary cilia plays a central role in anticystogenesis. In this hypothesis, polycystin-2’s role in endoplasmic reticulum (ER) is to facilitate trafficking polycystin-1–polycystin-2 complexes to the cilia to inhibit cystogenesis. The trimeric intracellular cation (TRIC) channel TRIC-B is an ER-resident potassium ion channel that mediates potassium-calcium counterion exchange for inositol trisphosphate–mediated calcium ion release. Using TRIC-B as an experimental tool, the authors demonstrate that channel function of ER-localized polycystin-2 is important for anticystogenesis, presumably acting as a potassium channel to facilitate calcium ion release. The results help illuminate ADPKD pathogenesis and provide proof of principle for pharmacotherapy by TRIC-B activators.

Background
Mutations of PKD2, which encodes polycystin-2, cause autosomal dominant polycystic kidney disease (ADPKD). The prevailing view is that defects in polycystin-2–mediated calcium ion influx in the primary cilia play a central role in the pathogenesis of cyst growth. However, polycystin-2 is predominantly expressed in the endoplasmic reticulum (ER) and more permeable to potassium ions than to calcium ions.

Methods
The trimeric intracellular cation (TRIC) channel TRIC-B is an ER-resident potassium channel that mediates potassium–calcium counterion exchange for inositol trisphosphate–mediated calcium ion release. Using TRIC-B as a tool, we examined the function of ER-localized polycystin-2 and its role in ADPKD pathogenesis in cultured cells, zebrafish, and mouse models.

Results
Agonist-induced ER calcium ion release was defective in cells lacking polycystin-2 and reversed by exogenous expression of TRIC-B. Vice versa, exogenous polycystin-2 reversed an ER calcium-release defect in cells lacking TRIC-B. In a zebrafish model, expression of wild-type but not nonfunctional TRIC-B suppressed polycystin-2–deficient phenotypes. Similarly, these phenotypes were suppressed by targeting the ROMK potassium channel (normally expressed on the cell surface) to the ER. In cultured cells and polycystin-2–deficient zebrafish phenotypes, polycystin-2 remained capable of reversing the ER calcium release defect even when it was not present in the cilia. Transgenic expression of Tric-b ameliorated cystogenesis in the kidneys of conditional Pkd2-inactivated mice, whereas Tric-b deletion enhanced cystogenesis in Pkd2-heterozygous kidneys.

Conclusions
Polycystin-2 in the ER appears to be critical for anticystogenesis and likely functions as a potassium ion channel to facilitate potassium–calcium counterion exchange for inositol trisphosphate–mediated calcium release. The results advance the understanding of ADPKD pathogenesis and provides proof of principle for pharmacotherapy by TRIC-B activators.