In the kidney, inwardly rectifying K+ channels of the Kir (KCNJ) family are essential for the control of salt transport. In the basolateral membrane of distal convoluted tubules (DCT), Kir4.1 (KCNJ10) together with Kir5.1 (KCNJ16) appears to be relevant for salt transport and adaptation of DCT to plasma K+ levels, so called K+ sensing. Loss-of-function mutations in KCNJ10 are known to result in EAST/SeSAME syndrome, a complex disorder with a renal salt wasting tubulopathy and alkalosis. Also for Kir5.1 a role in renal transport is likely: Paulais et al. observed hypokalemia, acidosis and hypercalciuria in a Kir5.1-/- mouse model and our collaborator Prof. Konrad identified in a patient with hypokalemia and acidosis a homozygous mutation of Kir5.1 (R35Q).
We hypothesized that the mutation R35Q leads to impaired channel function and disturbed salt transport in the kidney, especially in the DCT. To address this question, I characterized the mutant Kir5.1 channel and studied the phenotype of Kir5.1-/- mice.
The effect of the Kir5.1R35Q mutant on protein function was analyzed using immunofluorescence and patch-clamp experiments. Although computer algorithms had predicted a functional deficit, I found the conductance of cells transfected with Kir4.1-Kir5.1R35Q heteromers unaltered compared to Kir4.1-Kir5.1wt. Also on the single channel level, the properties of the Kir5.1R35Q mutant were normal and the subcellular localization appeared unaltered. Taken together, the function of the Kir5.1R35Q mutant seems at least largely preserved or normal. However, smaller functional deficits or modified channel regulation might still have escaped our notice.
To gain further insights into the role of Kir5.1 in the kidney, I investigated the renal phenotype of another Kir5.1-/- mouse model (not identical to the one of Paulais et al.). Using sorted tubules, I observed high Kir5.1 mRNA expression in DCT and in proximal tubules. Analysis of the Kir5.1-/- mice corroborated the hypokalemia phenotype, but the acidosis appeared compensated. Kir5.1-/- mice displayed a pathologically increased renal salt loss when fed a high NaCl diet pointing to a reduced capacity to adapt to dietary salt intake. To elucidate the role of Kir5.1 for K+ sensing in the DCT, isolated DCTs were exposed to changes of basolateral and apical K+. Fura-2 measurements revealed that only basolateral, but not apical, application of low K+* solution increased intracellular Ca2+ levels. However, deletion of Kir5.1 did not alter the Ca2+ signals suggesting that Kir5.1 channels are not essential for this component of the K+ sensing mechanism.
Taken together, this study provided evidence that the rare Kir5.1 variant R35Q is probably not (or not alone) causative for acidosis and hypokalemia observed in a single patient carrying this mutation. Nevertheless, our Kir5.1 knockout showed similar symptoms. Clearly, the role of Kir5.1 in the kidney is very complex and still incompletely understood. Further studies and the data from additional patients are required to decipher the precise role of Kir5.1 in the kidney and its possible role for human diseases
