Inversin Modulates Transepithelial Ion Transport in Mouse Renal Cells

Abstract

Type II Nephronophthisis (NPHP2) is an autosomal recessive disorder caused by mutations in the NPHP2 gene that encodes inversin protein. Mice carrying mutations in inversin exhibit situs inversus and enlarged kidneys with dilated collecting ducts that rapidly develop into renal cysts. The mechanism of cyst growth in NPHP2 is not completely understood. We hypothesize that increased kidney size in NPHP2 is due to increased electrolyte secretion and fluid accumulation in the cysts. The present study explored the loss of inversin on ion transport in a mouse epithelial cortical collecting duct (mCCD) cell line in vitro. Knockdown of inversin in mCCD cells was performed using siRNA and the net transepithelial ion transport was measured by electrophysiology. Partial knockdown (45%) of inversin RNA decreased the basal net ion transport up to 90% as measured by short circuit current (SCC) in mCCD cells compared to control cells. A change in SCC could be due to an anion moving from basolateral to apical media (secretory) or a cation moving from apical to basolateral media (absorptive). Accordingly, addition of vasopressin, a stimulator of anion (Cl-, HCO3-) and cation (Na+) transport, to the basolateral side of inversin knockdown cells increased anion transport and decreased cation transport compared to control cells. Addition of amiloride, a specific blocker of the epithelial sodium channel ENaC, to the apical side decreased the net ion transport to zero in both inversin knockdown and control cells, indicating the activity of ENaC. These results suggest that inversin may play a role in the anion secretion and Na+ reabsorption in renal epithelial cells and its loss could lead to fluid accumulation contributing to the enlarged kidneys observed in NPHP2.