OS9 Protein Interacts with Na-K-2Cl Co-transporter (NKCC2) and Targets Its Immature Form for the Endoplasmic Reticulum-associated Degradation Pathway

Elie Seaayfan,Nadia Defontaine,Sylvie Demaretz,Nancy Zaarour,Kamel Laghmani

doi:10.1074/jbc.m115.702514

Abstract

Mutations in the renal specific Na-K-2Cl co-transporter (NKCC2) lead to type I Bartter syndrome, a life-threatening kidney disease featuring arterial hypotension along with electrolyte abnormalities. We have previously shown that NKCC2 and its disease-causing mutants are subject to regulation by endoplasmic reticulum-associated degradation (ERAD). The aim of the present study was to identify the protein partners specifically involved in ERAD of NKCC2. To this end, we screened a kidney cDNA library through a yeast two-hybrid assay using NKCC2 C terminus as bait. We identified OS9 (amplified in osteosarcomas) as a novel and specific binding partner of NKCC2. Co-immunoprecipitation assays in renal cells revealed that OS9 association involves mainly the immature form of NKCC2. Accordingly, immunocytochemistry analysis showed that NKCC2 and OS9 co-localize at the endoplasmic reticulum. In cells overexpressing OS9, total cellular NKCC2 protein levels were markedly decreased, an effect blocked by the proteasome inhibitor MG132. Pulse-chase and cycloheximide-chase assays demonstrated that the marked reduction in the co-transporter protein levels was essentially due to increased protein degradation of the immature form of NKCC2. Conversely, knockdown of OS9 by small interfering RNA increased NKCC2 expression by increasing the co-transporter stability. Inactivation of the mannose 6-phosphate receptor homology domain of OS9 had no effect on its action on NKCC2. In contrast, mutations of NKCC2 N-glycosylation sites abolished the effects of OS9, indicating that OS9-induced protein degradation is N-glycan-dependent. In summary, our results demonstrate the presence of an OS9-mediated ERAD pathway in renal cells that degrades immature NKCC2 proteins. The identification and selective modulation of ERAD components specific to NKCC2 and its disease-causing mutants might provide novel therapeutic strategies for the treatment of type I Bartter syndrome.

Highlights

NKCC2 belongs to the superfamily of electroneutral cationcoupled chloride co-transporters, which contains the Na-Cl and K-Cl co-transporters [13]
Similar to ENaC [22, 23] and CFTR [20, 21], we previously demonstrated that NKCC2 is a substrate for the endoplasmic reticulum (ER)-associated protein degradation (ERAD) quality control system [25, 26]
We describe results obtained from the distal region of NKCC2 C terminus. 29 positive clones were selected by activation of three reporter genes, ADE2, HIS3, and MEL1

Summary

Introduction

NKCC2 belongs to the superfamily of electroneutral cationcoupled chloride co-transporters, which contains the Na-Cl and K-Cl co-transporters [13]. Folded proteins may be retained in the ER, and if folding cannot be achieved, they may form aggregates or be targeted for ER-associated protein degradation (ERAD) [18, 19] Based on their size and complex topologies, even the wildtype forms of integral membrane proteins such as cation-coupled chloride co-transporters are expected to encounter a significant number of hurdles during synthesis [19]. We show that OS9 interacts with the ER-resident form of NKCC2 and promotes its degradation by the proteasome pathway, revealing a new molecular pathway in the regulation of the co-transporter These findings may open up new avenues in studying the regulation of the ER-associated degradation of cation-coupled chloride co-transporters in general and in particular of renal Na-Cl co-transporters, proteins that are necessary for normal blood pressure homeostasis

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