The kidney-specific WNK1 isoform is induced by aldosterone and stimulates epithelial sodium channel-mediated Na ⁺ transport

Abstract

WNK1 belongs to a unique family of Ser͞Thr kinases that have been implicated in the control of blood pressure. Intronic deletions in the WNK1 gene result in its overexpression and lead to pseudohypoaldosteronism type II, a disease with salt-sensitive hypertension and hyperkalemia. How overexpression of WNK1 leads to Na ؉ retention and hypertension is not entirely clear. Similarly, there is no information on the hormonal regulation of expression of WNK kinases. There are two main WNK1 transcripts expressed in the kidney: the originally described ''long'' WNK1 and a shorter transcript that is specifically expressed in the kidney (KS-WNK1). The goal of this study was to determine the effect of aldosterone, the main hormonal regulator of Na ؉ homeostasis, on the transcription of WNK1 isoforms in renal target cells, by using an unique mouse cortical collecting duct cell line that stably expresses functional mineralocorticoid receptors. Our results demonstrate that aldosterone, at physiological concentrations, rapidly induces the expression of the KS-WNK1 but not that of the long-WNK1 in these cells. Importantly, stable overexpression of KS-WNK1 significantly increases transepithelial Na ؉ transport in cortical collecting duct cells. Similarly, coexpression of KS-WNK1 and the epithelial Na ؉ channel in Fischer rat thyroid epithelial cells also stimulates Na ؉ current, suggesting that KS-WNK1 affects the subcellular location or activity but not the expression of epithelial Na ؉ channel. These observations suggest that stimulation of KS-WNK1 expression might be an important element of aldosteroneinduced Na ؉ retention and hypertension.hypertension ͉ collecting duct H igh blood pressure affects 20-25% of the adult population in industrialized countries; yet the etiology of the majority of cases is poorly understood. In recent years, however, the molecular mechanism of several monogenic forms of hypertension showing Mendelian inheritance has been identified. Interestingly, the mutations in these forms of hypertension converge on a common pathway, i.e., the aldosterone-regulated renal sodium reabsorption. For instance, Liddle's syndrome, a disease mimicking hyperaldosteronism, is caused by gain-of-function mutations in the epithelial sodium channel (ENaC), which leads to abnormally high renal Na ϩ reabsorption, low-renin hypertension, and hypokalemic alkalosis (1). In contrast, the features of another autosomal dominant form of familial hypertension, pseudohypoaldosteronism type II (PHAII) or Gordon's syndrome are more complex than Liddle's syndrome. In PHAII, the hypertension is associated with chloride-dependent sodium retention, hyperkalemia, and metabolic acidosis despite normal glomerular filtration rate (2). PHAII patients are markedly sensitive to thiazide diuretics (3). Despite the fact that these features indicate an overactive thiazide-sensitive Na-Cl cotransporter (NCC), no mutations in this transporter have been found in affected patients.WNKs (with no lysine [K]) are Ser͞Thr kinases that lack a key lysine in t...

WNK1 kinase isoform switch regulates renal potassium excretion

“…Consistent with a switch mechanism, L-WNK1 blocks the inhibitory effect of WNK4 on the NCC transporter, restoring Na transport activity to near normal levels (5). In addition, both forms of WNK1 have been reported to stimulate the epithelial sodium channel (ENaC) (2,4). Because PHAII-causing mutations in the WNK1 gene are large intronic deletions that increase WNK1 expression, the stimulatory effects of WNK1 isoforms on the two sodium transporters in the distal nephron explain the exaggerated sodium reabsorption in PHAII, but the action of WNK1 isoforms on potassium handling is unclear.…”

“…KS-WNK1 (2) and L-WNK1 (4) have been reported to independently stimulate ENaC. Interestingly, KS-WNK1 also has been found to be increased by aldosterone (2). Consequently, after dietary potassium loading, well established actions of elevated aldosterone and KS-WNK1 would be expected to activate ENaC, depolarize the apical membrane potential, and thereby provide enhanced driving Fig.…”

“…T he WNK family of serine͞threonine kinases has been shown to function as important regulators of salt homeostasis (1)(2)(3)(4)(5)(6)(7)(8)(9). Mutations in the WNK4 or WNK1 genes cause a familial disorder [pseudohypoaldosteronism type II, (PHAII)] of diminished renal potassium excretion, excessive sodium retention, and hypertension (10).…”

WNK3 kinase is a positive regulator of NKCC2 and NCC, renal cation-Cl - cotransporters required for normal blood pressure homeostasis

“…4), kinaseactive WNK3's activities in the TAL and DCT are inferred to promote increased renal NaCl reabsorption in nephron segments that normally mediate the reabsorption of Ϸ30% and Ϸ7% of the filtered NaCl load, respectively. The absence of an effect of WNK3 on ENaC and paracellular Cl Ϫ flux, processes that occur in the more distal CD, suggest that WNK3 activity can shift NaCl reabsorption toward more proximal nephron segments, a key difference from WNK1 and WNK4, whose action has been shown in part to lie downstream of aldosterone in more distal parts of the nephron (14). Nonetheless, the absence of effect on these latter pathways must be interpreted with some caution, because it is possible that additional cellular components required for a regulatory effect of WNK3 are absent from the oocyte and Madin-Darby canine kidney cell systems used in this study.…”

“…Mutations that increase expression of WNK1 cause a similar phenotype (5). Recent evidence suggests WNK1 is an upstream regulator of WNK4 at NCC (7) and may also regulate ENaC through SGK1 (14,15). WNK kinases are characterized by the substitution of cysteine for lysine at a highly conserved residue in the catalytic domain; they are found in both animal and plant species (16,17).…”