Barttin Regulates the Subcellular Localization and Posttranslational Modification of Human Cl - /H + Antiporter ClC-5.

Dent disease 1 (DD1) is a renal salt-wasting tubulopathy associated with mutations in the Cl-/H+ antiporter CIC-5. The disease typically manifests with proteinuria, hypercalciuria, nephrocalcinosis, and nephrolithiasis but is characterized by large phenotypic variability of no dear origin. Several DD1 cases have been reported lately with additional atypical hypokalemic metabolic alkalosis and hyperaldosteronism, symptoms usually associated with another renal disease termed Bartter syndrome (BS). Expression of the Bartter-like DD1 mutant CIC-5 G261E in HEK293T cells showed that it is retained in the ER and lacks the complex glycosylation typical for CIC-5 WT. Accordingly, the mutant abolished CLC ionic transport. Such phenotype is not unusual and is often observed also in DD1 CIC-5 mutants not associated with Bartter like phenotype. We noticed, therefore, that one type of BS is associated with mutations in the protein barttin that serves as an accessory subunit regulating the function and subcellular localization of CIC-K channels. The overlapping symptomatology of DD1 and BS, together with the homology between the proteins of the CLC family, led us to investigate whether barttin might also regulate CIC-5 transport. In HEK293T cells, we found that barttin cotransfection impairs the complex glycosylation and arrests CIC-5 in the endoplasmic reticulum. As barttin and CIC-5 are both expressed in the thin and thick ascending limbs of the Henle's loop and the collecting duct, interactions between the two proteins could potentially contribute to the phenotypic variability of DD1. Pathologic barttin mutants differentially regulated trafficking and processing of CIC-5, suggesting that the interaction between the two proteins might be relevant also for the pathophysiology of BS. Our findings show that barttin regulates the subcellular localization not only of kidney CIC-K channels but also of the CIC-5 transporter, and suggest that CIC-5 might potentially play a role not only in kidney proximal tubules but also in tubular kidney segments expressing barttin. In addition, they demonstrate that the spectrum of clinical, genetic and molecular pathophysiology investigation of DD1 should be extended.