Regulation of the Na-K-2Cl cotransporter NKCC2 by ubiquitylation

Abstract

NKCC2, localized to the apical membrane of thick ascending limb epithelial cells, is essential for renal salt handling and systemic electrolyte homeostasis. NKCC2 undergoes extensive ubiquitylation, with the E3 protein ligase Nedd4-2 implicated as a key regulator. However, progress has been limited by challenges expressing NKCC2 in mammalian cell lines, hindering mechanistic studies of NKCC2 ubiquitylation. Therefore, the aims of this study were to develop a mammalian cell model enabling mechanistic investigations of NKCC2 ubiquitylation, including the role of Nedd4-2 and the functional consequences of site-specific modification. A tetracycline-inducible MDCKI cell line was generated expressing human NKCC2 and used to assess Nedd4-2-dependent and site-specific ubiquitylation of NKCC2 using biochemical, imaging, and functional assays. The MDCKI cell line demonstrated stable, inducible expression of full-length human NKCC2. In this cell line, mutating the ubiquitylation site at K871 increased membrane abundance and uptake activity, without altering internalization rates. Nedd4-2 co-immunoprecipitated with NKCC2, and Nedd4-2 deletion increased total, but not membrane NKCC2 abundance. In summary, ubiquitylation on NKCC2 at K871 plays a key role in controlling NKCC2 membrane localization and thus function. Although Nedd4-2 can modulate NKCC2 abundance, it is not involved in NKCC2 trafficking. We conclude that the generated cell line provides a robust model for mechanistic studies of NKCC2 and will aid studies examining posttranslational regulation of NKCC2.