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NDT Advance Access originally published online on January 31, 2009
Nephrology Dialysis Transplantation 2009 24(4):1074-1077; doi:10.1093/ndt/gfp013
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© The Author [2009]. Published by Oxford University Press on behalf of ERA-EDTA. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org


The renal WNK kinase pathway: a new link to hypertension

Ewout J. Hoorn, Nils van der Lubbe and Robert Zietse

Department of Internal Medicine, Erasmus Medical Center, Rotterdam, The Netherlands

Correspondence and offprint requests to: Ewout J. Hoorn, Erasmus Medical Center, Department of Internal Medicine, Room D-406, PO Box 2040, 3000 CA Rotterdam, The Netherlands. Tel: +31-10-7040704; Fax: +31-10-4366372; E-mail: ejhoorn{at}gmail.com

Keywords: aldosterone; epithelial sodium channel; exosomes; pseudohypoaldosteronism; sodium-chloride cotransporter



   The renal WNK kinase pathway: a new link to hypertension
Top
 The renal WNK kinase...
The renal WNK kinase...
 WNK kinases and hypertension
 References
 
The discovery of the renal WNK kinase pathway is offering new insights into sodium, potassium and blood pressure regulation in the distal nephron. It has also largely explained the pathogenesis of a genetic form of hypertension called familial hyperkalaemic hypertension (FHHt, also known as pseudohypoaldosteronism type II or Gordon's syndrome), because it is caused by mutations in WNK kinases. However, the question is: do the renal WNK kinases have clinical significance beyond this rare syndrome? Here, we review the most recent data on renal WNK kinase physiology and discuss their potentially broader roles in electrolyte transport and hypertension.



   The renal WNK kinase pathway: current status
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 The renal WNK kinase...
 The renal WNK kinase...
WNK kinases and hypertension
 References
 
As so often in science, the identification of the WNK kinases was a serendipitous finding. In 2000, Xu et al. pursued a nested polymerase chain reaction cloning strategy to identify novel members of the mitogen-activated protein/extracellular signal-regulated protein kinase family [1]. Instead, they found a new member of the serine/ threonine kinase family. They named the new kinase WNK, which stands for With No K, with ‘K’ referring to the amino acid lysine. The name highlights their unique characteristic: the location of the catalytic lysine crucial for binding to ATP in subdomain I instead of II, as is the case in all other protein kinases. To date, five WNK kinases have been identified, namely WNK1 through WNK4 and kidney-specific WNK1 (KS-WNK1, a second transcript from the WNK1 gene). The physiological functions of the WNK kinases are diverse and include cell volume regulation, neurotransmission, cell proliferation, embryonic development, paracellular permeability and transepithelial ion transport [2]. Focusing on the latter, the WNK kinases were found to figure prominently in intracellular signalling cascades throughout the nephron, determining the activity or abundance of the major renal sodium and potassium transporters. In the distal nephron, the WNK kinases are involved in the regulation of the Na-Cl-cotransporter in the distal convoluted tubule (DCT) and the epithelial sodium channel (ENaC) in the connecting tubule (CNT) and collecting duct (CD) (Figure 1). In the DCT, the abundance and phosphorylation of the Na-Cl-cotransporter are determined by the ratio between WNK3 (stimulator) and WNK4 (inhibitor), i.e. they form a ‘molecular rheostat’ [3]. For added complexity, WNK1 inhibits WNK4, while KS-WNK1 inhibits WNK1. Recently, STE20/SPS1-related proline alanine-rich kinase and the oxidative stress-responsive kinase-1 (SPAK/OSR1) were also shown to be involved in the regulation of the Na-Cl-cotransporter [4] and possibly interact with WNK3 and WNK4 [5]. In the CNT and CD, the phosphorylation of ENaC is regulated by WNK1 and KS-WNK1, but inhibited by WNK4. Serum- and glucocorticoid-regulated kinase 1 (SGK1) plays a central role in these effects, because WNK1 indirectly activates ENaC due to its activation of SGK1 via phosphatidyl inositol 3-kinase (PI3K), whereas SGK1 reverses the inhibition of ENaC by WNK4. Finally, the renal outer medullary potassium channel (ROMK), which is expressed in all of these nephron segments, is inhibited by WNK1 and WNK4, because they interact with the scaffolding protein intersection to stimulate clathrin-mediated endocytosis [6]. The last 8 years, studies using Xenopus oocyte and HEK-293 cells have been instrumental in unravelling the molecular machinery of the renal WNK kinase pathway. Currently, the first animal studies are focusing on the relationship between aldosterone and WNK kinases. In wild-type mice and rats, potassium loading and aldosterone were shown to increase KS-WNK1 and WNK4 mRNA [7,8], whereas potassium restriction decreased KS-WNK1 and increased WNK1 [9]. Although KS-WNK1 mRNA was significantly higher in mice on a high sodium diet compared to those on a low sodium diet [7], no difference at the protein level of KS-WNK1 or WNK4 was observed [4]. These findings may shed light on what has been coined the ‘aldosterone paradox’, i.e. the long-standing physiological question how aldosterone can be both a sodium-retaining and potassium-secreting hormone [2,10]. Namely, if potassium loading (hyperkalaemia) activates KS-WNK1 and WNK4, this will inhibit the Na-Cl-cotransporter and favour electrogenic sodium reabsorption by ENaC, thereby increasing the transepithelial voltage and stimulating potassium secretion. The opposite occurs when a low sodium diet (hypovolaemia) does not affect or even decreases KS-WNK1 and WNK4, because this will activate the Na-Cl-cotransporter and favour electroneutral sodium reabsorption with a relative conservation of potassium.


Figure 1
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  		Fig. 1 The renal WNK kinase pathway in the distal nephron. The current model of the renal WNK kinase pathway in the distal nephron, including the effects of aldosterone, a low potassium diet (low K+) and insulin. Stimulatory effects are depicted as red arrows including a ‘+’ symbol, inhibitory effects are depicted as black arrows including a ‘–’ symbol. Phosphorylation is depicted with the symbol ‘P’.

 


   WNK kinases and hypertension
Top
 The renal WNK kinase...
 The renal WNK kinase...
 WNK kinases and hypertension
References
 
The WNK kinases have attracted most attention as the cause of FHHt. Positional cloning studies of patients with FHHt revealed two causes [11]: intronic deletions causing the overexpression of wild-type WNK1 (which inhibits WNK4) and missense mutations causing mutant WNK4 (which inhibits wild-type WNK4 and fails to inhibit WNK3). Mice transgenic for mutant WNK4 [12] and knock-in mice with one mutant and one wild-type WNK4 allele [13] recapitulate the phenotype of FHHt. The rarity of FHHt raises the question if the WNK kinases have clinical relevance beyond this syndrome. To answer this question, we first address two other questions: is the current management of hypertension suboptimal and, if so, what is needed to improve it? In our opinion, the answer to the first question is clearly ‘yes’, because ~90% of the cases of hypertension are still considered to be ‘essential’ and treatment is largely trial-and-error. The answer to the second question logically follows from the first: better management of hypertension requires a better understanding of its pathogenesis and markers to determine the patient's individual sensitivity to antihypertensive drugs. Can the renal WNK kinases contribute to this mission? We believe so and provide three examples. First, the renal WNK kinase pathway offers a potential mechanistic explanation for the association between potassium depletion and salt-sensitive hypertension [14]. Our diet has gradually changed from potassium-rich and sodium-poor in Paleolithic times to the opposite in modern times [15]. As noted, potassium restriction increases WNK1 and decreases KS-WNK1 in animals [7–9]. A decrease in KS-WNK1 will relieve its inhibition of WNK1, allowing it to either inhibit WNK4 and activate the Na-Cl-cotransporter (DCT) or to activate SGK1 and ENaC (CNT and CD, Figure 1). The result is increased sodium reabsorption at two nephron sites that will increase blood pressure. The second example is the association between hypertension and hyperinsulinaemia, a prominent feature of diseases such as diabetes mellitus and obesity. Song et al. showed that the rise in blood pressure in rats on chronic insulin treatment was likely due to enhanced sodium reabsorption by the Na-Cl-cotransporter and ENaC, because apical localization of ENaC subunits was increased and treatment with hydrochlorothiazide and amiloride resulted in increased natriuresis [16]. Interestingly, insulin reduced cortical WNK4 expression [16], which would indeed be expected to activate the Na-Cl-cotransporter and ENaC (Figure 1). The third example is that single nucleotide polymorphisms and haplotypes in WNK1 contribute to blood pressure variation in the general population [17], possibly mediated via effects on the gradient of blood pressure change with age [18]. Interestingly, the gene encoding for SPAK (STK39), which interacts with the WNK kinases (Figure 1), was also recently identified as a hypertension susceptibility gene in an Amish population [19]. The ability to predict individual predispositions to hypertension logically leads to the last unanswered question: are there biomarkers for hypertension? Screening for polymorphisms and haplotypes could be a useful clinical tool in the foreseeable future although the available tests have not found widespread use in clinical practice [20]. It has proven difficult to establish clear-cut associations between blood pressure polymorphisms (e.g. {alpha}-adducin, angiotensinogen, angiotensin-converting enzyme) and the risk of hypertension, cardiovascular events or responsiveness to therapy [21,22]. However, the modest effect of variants in a single gene may be explained by their interactions with related genes. Indeed, when variants in the genes for WNK1, {alpha}-adducin (influences activity Na-K-ATPase) and Nedd4-2 (ubiquinates ENaC) were combined, a significant effect was found on renal salt handling, the blood pressure response to saline and thiazides and nocturnal systolic blood pressure [23]. This being said, genes are still far off from the actual biological work force, namely proteins. Therefore, one would wish for a measure of Na-Cl-cotransporter, ENaC or WNK activity in the distal nephron. Because renal biopsies are not regularly performed in hypertensive patients, a logical alternative would be urine, because it contains many disease-associated proteins. Studies that proved this principle have demonstrated increased urinary excretion of the Na-Cl-cotransporter in patients with FHHt [24] and a specific urinary pattern of the ENaC-activator prostasin in patients with primary aldosteronism [25]. These studies used whole urine, but a more targeted approach could be to use so-called urinary exosomes. Urinary exosomes are the internal vesicles of multivesicular bodies secreted by renal epithelial cells and contain the Na-Cl-cotransporter and ENaC (it is not known if WNK kinases are present in exosomes) [26]. Urinary exosomes have not been analysed in hypertensive disorders, but their utility is illustrated by the identification of exosomal biomarkers that are capable of predicting acute renal failure prior to a rise in serum creatinine [27,28].

Perspectives
The role of the renal WNK kinases and their interactions with sodium and potassium transporters in the rapidly evolving cell models of the DCT, CNT and CD is becoming increasingly clear. Nevertheless, the roles of WNK3 and especially WNK2 in the distal nephron are relatively unknown. In addition, biology is never as simple as a single protein family, and at least three kinase systems appear to coordinate signal transduction from receptor to transporter, including the WNK kinases, SGK1 and SPAK/OSR1 (Figure 1). Although aldosterone is an indisputable activator of WNK kinases, it is unknown if other hormones acting on the distal nephron such as vasopressin, angiotensin II and atrial natriuretic peptide are also capable of regulating the WNK kinases. The first animal studies have focused on aldosterone and WNK kinases, but a complete picture likely also requires the analysis of other circulating hormones, the related receptors and transporters and, of course, blood pressure. Apart from physiological insights, it seems logical to pursue the quest of finding urinary biomarkers for hypertension [29]. As of yet, the renal WNK kinases as drug targets is science fiction, but the example of the tyrosine kinase inhibitor imatinib for chronic myeloid leukaemia illustrates that it is not a priori impossible to selectively inhibit kinase systems [30,31]. The feasibility to inhibit WNK1 was also illustrated in mice heterozygous for the WNK1 mutation, which showed a marked reduction in blood pressure without apparent side effects [32]. Hypertension is obviously a multifactorial and complex disease, but the WNK kinase pathway is opening an attractive avenue to better understand and potentially diagnose and treat hypertension.



   Acknowledgments
 
EJH is supported by an Erasmus MC Fellowship 2008 (internal grant) and a Kolff Junior Postdoc grant (Dutch Kidney Foundation).

Conflict of interest statement. None declared.



   References
Top
 The renal WNK kinase...
 The renal WNK kinase...
 WNK kinases and hypertension
 References
 

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Received for publication: 23.12.08
Accepted in revised form: 6. 1.09


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