NKCC2 activity is inhibited upon the expression of the gain-of-function CASR mutant A843E in renal cells

The mutation gain-of-function A843E of the Calcium Sensing Receptor (CASR) causes a Bartter syndrome type V. Patients carrying this CASR variant showed a remarkable reduced renal NaCl reabsorption in Thick Ascendent Limb (TAL) resulting in renal loss of NaCl in the absence of mutations in renal Na+ and Cl- ion transporters. The molecular mechanisms underlying this clinical phenotype are still unknown. Indeed, we investigated, in LLC-PK1 epithelial renal cells, the possible functional cross-talk of CASR-A843E mutant with NKCC2, the main transporter involved in the NaCl reabsorption in the TAL. The steady-state NKCC2 phosphorylation and activity were inhibited by about 60% in cells transfected with CASR-A843E mutant compared to CASR WT-transfected cells, used as control. Of note, the low-chloride-dependent NKCC2 activation was also strongly inhibited upon the expression of CASR-A843E mutant. Interestingly the maximal activation of CASR WT mimicked the effect of CASR-A843E on NKCC2 regulation. The expression of the CASR mutant did not alter the apical localization of NKCC2 in LLC-PK1 cells, suggesting that the CASR-A843E affected intracellular pathway/s modulating NKCC2 activity rather than NKCC2 intracellular trafficking. Our findings opened new avenues not only to gain more insight in the role of CASR in the physiopathology of the kidney, but also to conceive novel therapeutic strategies for the treatment of the Bartter syndrome type V.