Romk1 Knockout Mice Do Not Produce Bartter Phenotype but Exhibit Impaired K Excretion

Abstract

Romk knock-out mice show a similar phenotype to Bartter syndrome of salt wasting and dehydration due to reduced Na-K2Cl-cotransporter activity. At least three ROMK isoforms have been identified in the kidney; however, unique functions of any of the isoforms in nephron segments are still poorly understood. We have generated a mouse deficient only in Romk1 by selective deletion of the Romk1-specific first exon using an ES cell CreLoxP strategy and examined the renal phenotypes, ion transporter expression, ROMK channel activity, and localization under normal and high K intake. The renal outer medullary potassium channel (ROMK) 3 is an ATP-dependent potassium channel (Kir1.1) that forms apical K channels that play an important role in K ϩ recycling to support sodium and chloride absorption in the thick ascending limb (TAL), and in regulation of K ϩ secretion in the collecting duct (CD). Mutations in the ROMK channel cause Type II Bartter syndrome that presents with polyhydramnios and postnatal life threatening volume depletion caused by loss of salt reabsorbing capacity by the thick ascending limb (1). Romk knock-out mice, originally generated from Gary Shull's lab at the University of Cincinnati, exhibited the same phenotypes as the Bartter syndrome in humans (2). By using the Romk Bartter mouse model, we have further confirmed that ROMK forms both small conductance K ϩ channels (SK) and 70 pS K channels in apical membranes of the thick ascending limb and cortical collecting duct (3, 4). We have demonstrated: 1) absence of both SK activity and 70 pS K ϩ channels in apical membranes of thick ascending limb and cortical collecting duct in Romk knock-out mice; 2) Romk knock-out mice produced similar phenotypes to Bartter syndrome consisting of salt and water wasting due to reduced NKCC2 activity and expression (3, 5); 3) the salt and water wasting from the kidney is compensated by increased thiazide-sensitive NaCl transport expression and activity, distal tubule hypertrophy, elevated renin-Ang II, and aldosterone levels (6). 4) The reduced K ϩ secretion in Romk null mice is compensated by increased maxi-K channel activity in the collecting tubule (7).However, three ROMK isoforms (ROMK1, -2, and -3) have been identified in the rat kidney (8 -10), but unique functions of any of the ROMK isoforms in nephron segments are still poorly understood. ROMK1 is expressed in the distal and collecting tubules and uniquely regulated by PTK/PTP-dependent endocytosis, which may be the mechanism for high-K-mediated enhanced ROMK channel activity in the collecting tubule (11,12). To study the functional role of ROMK1 in the regulation of Na and K homeostasis, we have generated a mouse deficient only in Romk1 by selective deletion of the Romk1 exon, using an ES cell Cre-LoxP strategy. We have examined the phenotypes of these mice by metabolic and renal clearance, and examined K channel activities by the patch clamp. The ion transporters, Nkcc2, Ncc, and Romk2, expression were measured by Q-PCR and ROMK localization was examined by immun...

The Biological Role of Klotho Protein in the Development of Cardiovascular Diseases

“…Moreover, high K + intake increases the activity of ROMK1 in the apical membrane of cortical collecting duct cells, to secrete more K + into the urine and maintain K + balance. Experimental data showed a defect in secretion of K + and hyperkalemia in Romk1 −/− mice under high K + diet [28]. The regulation of K + homeostasis by Klotho was examined by Cha et al (2009).…”

Net K + secretion in the thick ascending limb of mice on a low-Na, high-K diet

“…al. demonstrated that ROMK1 isoform, which is expressed in the CCD, but not TAL, was required for ROMK trafficking in response to high K intake 47 . This study suggests that the ROMK2 isoform in the TAL is regulated by different signaling pathways than the ROMK1 in the ASDN.…”

The Renal Outer Medullary Potassium Channel Inhibitor, MK-7145, Lowers Blood Pressure, and Manifests Features of Bartters Syndrome Type II Phenotype

“…These isoforms differ at their cytoplasmic amino acid terminus and are differentially expressed in the nephron. Although ROMK knockout mice and rats lacking the three isoforms recapitulate the phenotype of type II Bartter's syndrome (Lu et al, 2002;Lorenz et al, 2002;Zhou et al, 2013), unique functions of the ROMK1 isoform have only been recently elucidated (Dong et al, 2016). ROMK1 knockout did not produce the Bartter's phenotype, and there was no functional coupling between ROMK1 and the Na 1 /K 1 /2Cl 2 cotransporter at the TALH.…”

“…Three different ROMK isoforms are present in human and rat kidney (Dong et al, 2016). These isoforms differ at their cytoplasmic amino acid terminus and are differentially expressed in the nephron.…”