Truncated Variant of ASIC2b Confers Epithelial Sodium Channel Properties to ASIC2: Role in Nephrotic Syndrome

Abstract

Sodium retention responsible for nephrotic edema stems from epithelial sodium channel (ENaC) and Na,K‐ATPase induction in cortical collecting ducts (CCD) of puromycin aminonucleoside (PAN) nephrotic rats. However PAN rats show hyper‐aldosteronemia whereas most nephrotic patients do not. Aldosterone‐clamped (AC) PAN rats still retain sodium in ASDN although ENaC is no longer expressed. We searched for an alternate sodium entry pathway in CCD from AC‐PAN rats, a more relevant model of human disease.By RACE we isolated from AC‐PAN rat kidney a cDNA identical to that of ASIC2b except that it lacks the 207 3′‐most nucleotides. This truncated sequence contains an ATG within the reading frame of the full length sequence. Heterologous expression in OKP cells demonstrated that it is translated in a protein lacking the 71 N‐ter amino acids. Two‐electrode voltage clamp analysis in Xenopus ovocytes showed that co‐expression of truncated ASIC2b and ASIC2a induces an acid‐stimulated cation current which, instead of being transient, lasts as long as the stimulus is maintained. ASIC2a and truncated‐ASIC2b thus constitute heteromeric channels potentially supporting epithelial sodium transport. ASIC2b expression is induced in CCD of AC‐PAN rats where it localizes at the apical pole. ASIC2b was detected in kidney biopsies from 5/8 patients with idiopathic nephrotic syndrome but not in 26/26 patients with other renal diseases. In ASIC2b‐positive nephrotic patients, ASIC2b was located in CCD.Results suggest that truncated ASIC2b/ASIC2a mediates sodium retention in nephrotic syndrome and appears as a new therapeutic target.