Paradoxical activation of the sodium chloride cotransporter (NCC) without hypertension in kidney deficient in a regulatory subunit of Na,K-ATPase, FXYD2.

Elena Arystarkhova,Kathleen J Sweadner,Alicia A Mcdonough,Richard Bouley,Donna L Ralph,Yi Bessie Liu

doi:10.14814/phy2.12226

Abstract

Na,K‐ATPase generates the driving force for sodium reabsorption in the kidney. Na,K‐ATPase functional properties are regulated by small proteins belonging to the FXYD family. In kidney FXYD2 is the most abundant: it is an inhibitory subunit expressed in almost every nephron segment. Its absence should increase sodium pump activity and promote Na+ retention, however, no obvious renal phenotype was detected in mice with global deletion of FXYD2 (Arystarkhova et al. 2013). Here, increased total cortical Na,K‐ATPase activity was documented in the Fxyd2−/− mouse, without increased α1β1 subunit expression. We tested the hypothesis that adaptations occur in distal convoluted tubule (DCT), a major site of sodium adjustments. Na,K‐ATPase immunoreactivity in DCT was unchanged, and there was no DCT hypoplasia. There was a marked activation of thiazide‐sensitive sodium chloride cotransporter (NCC; Slc12a3) in DCT, predicted to increase Na+ reabsorption in this segment. Specifically, NCC total increased 30% and NCC phosphorylated at T53 and S71, associated with activation, increased 4‐6 fold. The phosphorylation of the closely related thick ascending limb (TAL) apical NKCC2 (Slc12a1) increased at least twofold. Abundance of the total and cleaved (activated) forms of ENaC α‐subunit was not different between genotypes. Nonetheless, no elevation of blood pressure was evident despite the fact that NCC and NKCC2 are in states permissive for Na+ retention. Activation of NCC and NKCC2 may reflect an intracellular linkage to elevated Na,K‐ATPase activity or a compensatory response to Na+ loss proximal to the TAL and DCT.

Highlights

FXYD proteins play essential roles in modulation of Na, K-ATPase activity
Only FXYD2a is found in proximal convoluted tubules (PT), while only FXYD2b is expressed in distal convoluted tubule (DCT) and connecting tubules (CNT) (Arystarkhova et al 2002b; Pu et al 2001)
No significant differences between wild-type and FXYD2-depleted mice were found in plasma concentration of major electrolytes or plasma osmolality under basal conditions (Table 1)

Summary

Introduction

FXYD proteins play essential roles in modulation of Na, K-ATPase activity. The seven members of the gene family (Sweadner and Rael 2000) exhibit tissue- and cell-specific distribution, and when associated with the Na,K-ATPase they differentially modulate kinetic properties of the pump either by changing affinity for the substrates or affecting the Vmax (Geering 2006). Four different FXYDs are expressed in a segment-specific manner (Wetzel and Sweadner 2001; Capurro et al 1996; Lubarski et al 2005; Wetzel and Sweadner 2003), with FXYD2 being the most abundant. It has two splice variants, FXYD2a and FXYD2b, which differ only in the first exon coding for the extracellular N-terminus of the molecule (Arystarkhova et al 2002b; Ku€ster et al 2000).

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