Salt handling in the distal nephron: lessons learned from inherited human disorders

Abstract

The molecular basis of inherited salt-losing tubular disorders with secondary hypokalemia has become much clearer in the past two decades. Two distinct segments along the nephron turned out to be affected, the thick ascending limb of Henle's loop and the distal convoluted tubule, accounting for two major clinical phenotypes, hyperprostaglandin E syndrome and Bartter-Gitelman syndrome. To date, inactivating mutations have been detected in six different genes encoding for proteins involved in renal transepithelial salt transport. Careful examination of genetically defined patients ("human knockouts") allowed us to determine the individual role of a specific protein and its contribution to the overall process of renal salt reabsorption. The recent generation of several genetically engineered mouse models that are deficient in orthologous genes further enabled us to compare the human phenotype with the animal models, revealing some unexpected interspecies differences. As the first line treatment in hyperprostaglandin E syndrome includes cyclooxygenase inhibitors, we propose some hypotheses about the mysterious role of PGE(2) in the etiology of renal salt-losing disorders.

A Highly Conserved Motif at the COOH Terminus Dictates Endoplasmic Reticulum Exit and Cell Surface Expression of NKCC2

“…The initial rate of intracellular pH recovery (dpH i /dt) was measured over the first 20 s of records, as reported earlier (23,25). The ⌬pH i caused by NH 4 Cl addition was used to calculate the cell buffer capacity, which was not different between the studied groups (data not shown). There-fore, the Na-K-2Cl co-transporter activity is expressed as dpH i /dt.…”

“…Even more impressive, inactivating mutations of the NKCC2 gene in humans causes Bartter syndrome type 1 (BS1), a life-threatening renal tubular disorder for which the diagnosis is usually made in the antenatal-neonatal period, due to the presence of polyhydramnios, premature delivery, salt loss, hypokalemia, metabolic alkalosis, hypercalciuria, and nephrocalcinosis (3). Without appropriate treatment, patients with BS1 will not survive the early neonatal period (4). In congruence with the severity of the symptoms and the uniformity of the clinical picture, functional analysis of diverse NKCC2 mutants consistently revealed a loss of function effect of the tested mutations (5,6).…”

“…Cells were first bathed at 37°C in a CO 2 -free Hepes/Tris-buffered medium containing 135 mM NaCl, 10 mM Hepes (pH 7.4), 5 mM KCl, 1 mM MgCl 2 , 0.8 mM KH 2 PO 4 , 0.2 mM K 2 HOP 4 , 1 mM CaCl 2 , and 10 mM BaCl 2 to measure base-line pH i . 20 mM NH 4 Cl was then added to the medium, inducing a very rapid initial cellular alkalinization followed by a pH i recovery. The initial rate of intracellular pH recovery (dpH i /dt) was measured over the first 20 s of records, as reported earlier (23,25).…”

“…Ϫ transport activity was assessed by estimating the rate of intracellular acidification caused by entry into the cells of NH 4 ϩ via this transport mechanism after abrupt application of NH 4 Cl to the cells (23,24). As illustrated in Fig.…”

“…In the present work, we mainly focused on the effect of Y998X, the most distal mutation described to date in the NKCC2 COOH terminus. B, measurement of Na-K-2Cl co-transport activity in OKP cells expressing NKCC2 or Y998X proteins, mean initial rate of pH i recovery (dpH i /dt) from NH 4 ϩ -induced alkaline load. Each bar represents the mean Ϯ S.E.…”