Comparison of WNK4 and WNK1 kinase and inhibiting activities

WNK1/HSN2 mediates neurite outgrowth and differentiation via a OSR1/GSK3β-LHX8 pathway

With no lysine kinase 1 (WNK1) phosphorylates and activates STE20/SPS1-related proline-alanine-rich protein kinase (SPAK) and oxidative stress responsive kinase 1 (OSR1) to regulate ion homeostasis in the kidney. Mutations in WNK1 result in dysregulation of the WNK1-SPAK/OSR1 pathway and cause pseudohypoaldosteronism type II (PHAII), a form of hypertension. WNK1 is also involved in the autosomal recessive neuropathy, hereditary sensory and autonomic neuropathy type II (HSANII). Mutations in a neural-specific splice variant of WNK1 (HSN2) cause HSANII. However, the mechanisms underlying HSN2 regulation in neurons and effects of HSN2 mutants remain unclear. Here, we found that HSN2 regulated neurite outgrowth through OSR1 activation and glycogen synthase kinase 3β (GSK3β). Moreover, HSN2-OSR1 and HSN2-GSK3β signalling induced expression of LIM homeobox 8 (Lhx8), which is a key regulator of cholinergic neural function. The HSN2-OSR1/GSK3β-LHX8 pathway is therefore important for neurite outgrowth. Consistently, HSN2 mutants reported in HSANII patients suppressed SPAK and OSR1 activation and LHX8 induction. Interestingly, HSN2 mutants also suppressed neurite outgrowth by preventing interaction of between wild-type HSN2 and GSK3β. These results indicate that HSN2 mutants cause dysregulation of neurite outgrowth via GSK3β in the HSN2 and/or WNK1 pathways.

Clinical and Molecular Perspectives of Monogenic Hypertension

Advances in molecular research techniques have enabled a new frontier in discerning the mechanisms responsible for monogenic diseases. In this review, we discuss the current research on the molecular pathways governing blood pressure disorders with a Mendelian inheritance pattern, each presenting with a unique pathophysiology. Glucocorticoid Remediable Aldosteronism (GRA) and Apparent Mineralocorticoid Excess (AME) are caused by mutations in regulatory enzymes that induce increased production of mineralocorticoids or inhibit degradation of glucocorticoids, respectively. Geller syndrome is due to a point mutation in the hormone responsive element of the promotor for the mineralocorticoid receptor, rendering the receptor susceptible to activation by progesterone, leading to hypertension during pregnancy. Pseudohypoaldosteronism type II (PHA-II), also known as Gordon's syndrome or familial hyperkalemic hypertension, is a more variable disorder typically characterized by hypertension, high plasma potassium and metabolic acidosis. Mutations in a variety of intracellular enzymes that lead to enhanced sodium reabsorption have been identified. In contrast, hypertension in Liddle's syndrome, which results from mutations in the Epithelial sodium Channel (ENaC), is associated with low plasma potassium and metabolic alkalosis. In Liddle's syndrome, truncation of one the ENaC protein subunits removes a binding site necessary protein for ubiquitination and degradation, thereby promoting accumulation along the apical membrane and enhanced sodium reabsorption. The myriad effects due to mutation in phosphodiesterase 3A (PDE3A) lead to severe hypertension underlying sodium-independent autosomal dominant hypertension with brachydactyly. How mutations in PDE3A result in the phenotypic features of this disorder are discussed. Understanding the pathologies of these monogenic hypertensive disorders may provide insight into the causes of the more prevalent essential hypertension and new avenues to unravel the complexities of blood pressure regulation.

Nudge-nudge, WNK-WNK (kinases), say no more?

Contents Summary 35 I Overview of animal and plant WNK kinases 35 II. Structure: domains and topology 36 III. Phylogeny-evolutionary relationships 41 IV. Plant WNK kinase distribution and regulation of WNK expression and activity 41 V. Functions of WNK family members in physiology and development 41 VI. Say no more? Still many questions to be answered 45 Acknowledgements 46 References 46 SUMMARY: WITH NO LYSINE (WNK) kinases are serine/threonine kinases uniquely characterized by an anomalous placement of a catalytic lysine, hence their moniker. In animals, WNK protein kinases play critical roles in protein trafficking of components that mediate renal ion transport processes and regulate osmoregulation of cell volume. In plants, the WNK kinase gene family is larger and more diverse. Recent studies revealed WNK kinase roles in orchestrating the trafficking of an ion channel, a lipid kinase complex in animals, and a heterotrimeric G protein signaling component in plants that is necessary for signal transduction. For this reason, new attention is geared toward investigating the mechanisms adopted by WNK kinases to nudge intracellular proteins to their destinations. In this review, the functions of WNK kinases in protein trafficking are derived from what we have learned from the model organism Arabidopsis thaliana. To place this new idea in context, we provide the predicted WNK kinase structures, their predicted expression patterns, a speculated evolutionary pathway, and the regulatory roles of plant WNKs in transport processes and other physiologies. We brazenly predict that the WNK kinases in both plants and animals will soon be recognized as a nexus for trafficking-based signal transduction.

Hypertension: the missing WNKs

The With no Lysine [K] (WNK) family of enzymes are central in the regulation of blood pressure. WNKs have been implicated in hereditary hypertension disorders, mainly through control of the activity and levels of ion cotransporters and channels. Actions of WNKs in the kidney have been heavily investigated, and recent studies have provided insight into not only the regulation of these enzymes but also how mutations in WNKs and their interacting partners contribute to hypertensive disorders. Defining the roles of WNKs in the cardiovascular system will provide clues about additional mechanisms by which WNKs can regulate blood pressure. This review summarizes recent developments in the regulation of the WNK signaling cascade and its role in regulation of blood pressure.

Unique chloride-sensing properties of WNK4 permit the distal nephron to modulate potassium homeostasis

Dietary potassium deficiency activates thiazide-sensitive sodium chloride cotransport along the distal nephron. This may explain, in part, the hypertension and cardiovascular mortality observed in individuals who consume a low-potassium diet. Recent data suggest that plasma potassium affects the distal nephron directly by influencing intracellular chloride, an inhibitor of the with-no-lysine kinase (WNK)-Ste20p-related proline- and alanine-rich kinase (SPAK) pathway. As previous studies used extreme dietary manipulations, we sought to determine whether the relationship between potassium and NaCl cotransporter (NCC) is physiologically relevant and clarify the mechanisms involved. We report that modest changes in both dietary and plasma potassium affect NCC in vivo. Kinase assay studies showed that chloride inhibits WNK4 kinase activity at lower concentrations than it inhibits activity of WNK1 or WNK3. Also, chloride inhibited WNK4 within the range of distal cell chloride concentration. Mutation of a previously identified WNK chloride-binding motif converted WNK4 effects on SPAK from inhibitory to stimulatory in mammalian cells. Disruption of this motif in WNKs 1, 3, and 4 had different effects on NCC, consistent with the three WNKs having different chloride sensitivities. Thus, potassium effects on NCC are graded within the physiological range, which explains how unique chloride-sensing properties of WNK4 enable it to mediate effects of potassium on NCC in vivo.

Electroneutral absorption of NaCl by the aldosterone-sensitive distal nephron: implication for normal electrolytes homeostasis and blood pressure regulation

Sodium absorption by the distal part of the nephron, i.e., the distal convoluted tubule, the connecting tubule, and the collecting duct, plays a major role in the control of homeostasis by the kidney. In this part of the nephron, sodium transport can either be electroneutral or electrogenic. The study of electrogenic Na(+) absorption, which is mediated by the epithelial sodium channel (ENaC), has been the focus of considerable interest because of its implication in sodium, potassium, and acid-base homeostasis. However, recent studies have highlighted the crucial role played by electroneutral NaCl absorption in the regulation of the body content of sodium chloride, which in turn controls extracellular fluid volume and blood pressure. Here, we review the identification and characterization of the NaCl cotransporter (NCC), the molecule accounting for the main part of electroneutral NaCl absorption in the distal nephron, and its regulators. We also discuss recent work describing the identification of a novel "NCC-like" transport system mediated by pendrin and the sodium-driven chloride/bicarbonate exchanger (NDCBE) in the β-intercalated cells of the collecting system.

First WNK4-hypokalemia animal model identified by genome-wide association in Burmese cats

Burmese is an old and popular cat breed, however, several health concerns, such as hypokalemia and a craniofacial defect, are prevalent, endangering the general health of the breed. Hypokalemia, a subnormal serum potassium ion concentration ([K⁺]), most often occurs as a secondary problem but can occur as a primary problem, such as hypokalaemic periodic paralysis in humans, and as feline hypokalaemic periodic polymyopathy primarily in Burmese. The most characteristic clinical sign of hypokalemia in Burmese is a skeletal muscle weakness that is frequently episodic in nature, either generalized, or sometimes localized to the cervical and thoracic limb girdle muscles. Burmese hypokalemia is suspected to be a single locus autosomal recessive trait. A genome wide case-control study using the illumina Infinium Feline 63K iSelect DNA array was performed using 35 cases and 25 controls from the Burmese breed that identified a locus on chromosome E1 associated with hypokalemia. Within approximately 1.2 Mb of the highest associated SNP, two candidate genes were identified, KCNH4 and WNK4. Direct sequencing of the genes revealed a nonsense mutation, producing a premature stop codon within WNK4 (c.2899C>T), leading to a truncated protein that lacks the C-terminal coiled-coil domain and the highly conserved Akt1/SGK phosphorylation site. All cases were homozygous for the mutation. Although the exact mechanism causing hypokalemia has not been determined, extrapolation from the homologous human and mouse genes suggests the mechanism may involve a potassium-losing nephropathy. A genetic test to screen for the genetic defect within the active breeding population has been developed, which should lead to eradication of the mutation and improved general health within the breed. Moreover, the identified mutation may help clarify the role of the protein in K⁺ regulation and the cat represents the first animal model for WNK4-associated hypokalemia.

Interactions with WNK (with no lysine) family members regulate oxidative stress response 1 and ion co-transporter activity

Two of the four WNK (with no lysine (K)) protein kinases are associated with a heritable form of ion imbalance culminating in hypertension. WNK1 affects ion transport in part through activation of the closely related Ste20 family protein kinases oxidative stress-responsive 1 (OSR1) and STE20/SPS1-related proline-, alanine-rich kinase (SPAK). Once activated by WNK1, OSR1 and SPAK phosphorylate and stimulate the sodium, potassium, two chloride co-transporters, NKCC1 and NKCC2, and also affect other related ion co-transporters. We find that WNK1 and OSR1 co-localize on cytoplasmic puncta in HeLa and other cell types. We show that the C-terminal region of WNK1 including a coiled coil is sufficient to localize the fragment in a manner similar to the full-length protein, but some other fragments lacking this region are mislocalized. Photobleaching experiments indicate that both hypertonic and hypotonic conditions reduce the mobility of GFP-WNK1 in cells. The four WNK family members can phosphorylate the activation loop of OSR1 to increase its activity with similar kinetic constants. C-terminal fragments of WNK1 that contain three RFXV interaction motifs can bind OSR1, block activation of OSR1 by sorbitol, and prevent the OSR1-induced enhancement of ion co-transporter activity in cells, further supporting the conclusion that association with WNK1 is required for OSR1 activation and function at least in some contexts. C-terminal WNK1 fragments can be phosphorylated by OSR1, suggesting that OSR1 catalyzes feedback phosphorylation of WNK1.

SPAK/OSR1 regulate NKCC1 and WNK activity: analysis of WNK isoform interactions and activation by T-loop trans-autophosphorylation

Mutations in the WNK [with no lysine (K) kinase] family instigate hypertension and pain perception disorders. Of the four WNK isoforms, much of the focus has been on WNK1, which is activated in response to osmotic stress by phosphorylation of its T-loop residue (Ser382). WNK isoforms phosphorylate and activate the related SPAK (SPS1-related proline/alanine-rich kinase) and OSR1 (oxidative stress-responsive kinase 1) protein kinases. In the present study, we first describe the generation of double-knockin ES (embryonic stem) cells, where SPAK and OSR1 cannot be activated by WNK1. We establish that NKCC1 (Na+/K+/2Cl- co-transporter 1), a proposed target of the WNK pathway, is not phosphorylated or activated in a knockin that is deficient in SPAK/OSR1 activity. We also observe that activity of WNK1 and WNK3 are markedly elevated in the knockin cells, demonstrating that SPAK/OSR1 significantly influences WNK activity. Phosphorylation of another regulatory serine residue, Ser1261, in WNK1 is unaffected in knockin cells, indicating that this is not phosphorylated by SPAK/OSR1. We show that WNK isoforms interact via a C-terminal CCD (coiled-coil domain) and identify point mutations of conserved residues within this domain that ablate the ability of WNK isoforms to interact. Employing these mutants, we demonstrate that interaction of WNK isoforms is not essential for their T-loop phosphorylation and activation, at least for overexpressed WNK isoforms. Moreover, we finally establish that full-length WNK1, WNK2 and WNK3, but not WNK4, are capable of directly phosphorylating Ser382 of WNK1 in vitro. This supports the notion that T-loop phosphorylation of WNK isoforms is controlled by trans-autophosphorylation. These results provide novel insights into the WNK signal transduction pathway and provide genetic evidence confirming the essential role that SPAK/OSR1 play in controlling NKCC1 function. They also reveal a role in which the downstream SPAK/OSR1 enzymes markedly influence the activity of the upstream WNK activators. The knockin ES cells lacking SPAK/OSR1 activity will be useful in validating new targets of the WNK signalling pathway.

Disease-causing mutations in the acidic motif of WNK4 impair the sensitivity of WNK4 kinase to calcium ions

WNK4 is a serine/threonine protein kinase that is involved in pseudohypoaldosteronism type II (PHAII), a Mendelian form disorder featuring hypertension and hyperkalemia. Most of the PHAII-causing mutations are clustered in an acidic motif rich in negatively charged residues. It is unclear, however, whether these mutations affect the kinase activity in any way. In this study, we isolated kinase domain of WNK4 produced by Escherichia coli, and demonstrated its ability to phosphorylate the oxidative stress-responsive kinase-1 (OSR1) and the thiazide-sensitive Na(+)-Cl(-) cotransporter (NCC) in vitro. Threonine 48 was identified as the WNK4 phosphorylation site at mouse NCC. The phospho-mimicking T48D mutant of mouse NCC increased its protein abundance and Na(+) uptake, and also enhanced the phosphorylation at the N-terminal region of NCC by OSR1. When the acidic motif was included in the WNK4 kinase construct, the kinase activity of WNK4 exhibited sensitivity to Ca(2+) ions with the highest activity at Ca(2+) concentration around 1 μM using kinase-inactive OSR1 as a substrate. All tested PHAII-causing mutations at the acidic motif exhibited impaired Ca(2+) sensitivity. Our results suggest that these PHAII-causing mutations disrupt a Ca(2+)-sensing mechanism around the acidic motif necessary for the regulation of WNK4 kinase activity by Ca(2+) ions.

The WNKs: atypical protein kinases with pleiotropic actions

WNKs are serine/threonine kinases that comprise a unique branch of the kinome. They are so-named owing to the unusual placement of an essential catalytic lysine. WNKs have now been identified in diverse organisms. In humans and other mammals, four genes encode WNKs. WNKs are widely expressed at the message level, although data on protein expression is more limited. Soon after the WNKs were identified, mutations in genes encoding WNK1 and -4 were determined to cause the human disease familial hyperkalemic hypertension (also known as pseudohypoaldosteronism II, or Gordon's Syndrome). For this reason, a major focus of investigation has been to dissect the role of WNK kinases in renal regulation of ion transport. More recently, a different mutation in WNK1 was identified as the cause of hereditary sensory and autonomic neuropathy type II, an early-onset autosomal disease of peripheral sensory nerves. Thus the WNKs represent an important family of potential targets for the treatment of human disease, and further elucidation of their physiological actions outside of the kidney and brain is necessary. In this review, we describe the gene structure and mechanisms regulating expression and activity of the WNKs. Subsequently, we outline substrates and targets of WNKs as well as effects of WNKs on cellular physiology, both in the kidney and elsewhere. Next, consequences of these effects on integrated physiological function are outlined. Finally, we discuss the known and putative pathophysiological relevance of the WNKs.

Emerging roles for WNK kinases in cancer

The subfamily of WNK protein kinases is composed of four human genes and is characterised by a typical sequence variation within the conserved catalytic domain. Although most research has focussed on the role of WNK1, WNK3 and WNK4 in regulating different ion transporters in both the kidney and extrarenal tissues, there is growing evidence for additional roles of WNK kinases in various signalling cascades related to cancer. Here, we review the connection between WNK kinases and tumorigenesis and describe existing experimental evidence as well as potential new links to major aspects of tumour biology. In particular, we discuss their role in G1/S cell cycle progression, metabolic tumour cell adaptation, evasion of apoptosis and metastasis.

Characterization of the kinase activity of a WNK4 protein complex

Mutations in WNK4 protein kinase cause pseudohypoaldosteronism type II (PHAII), a genetic disorder that is characterized by renal NaCl and K(+) retention leading to hypertension and hyperkalemia. Consistent with this, WNK4 is known to regulate several renal tubule transporters, including the NaCl cotransporter, NCC, and the K(+) channel, ROMK, but the mechanisms are incompletely understood, and the role of the kinase activity in its actions is highly controversial. To assay WNK4 kinase activity, we have now succeeded in expressing and purifying full-length, enzymatically active WNK4 protein from HEK293 cells. We show that full-length wild-type WNK4 phosphorylates oxidative stress response kinase 1 (OSR1) and Ste20/SPS1-related proline/alanine-rich kinase (SPAK) in vitro. Introducing the PHAII-associated mutations, E559K, D561A, and Q562E, into our protein had no significant effect on this phosphorylation. We conclude that PHAII is unlikely to be caused by abnormal WNK4 kinase activity. We also made the intriguing observation that inactivating mutations of the WNK4 kinase domain did not completely abolish in vitro phosphorylation of OSR1/SPAK. Led by this, we identified a novel 40-kDa kinase that associates specifically with the COOH-terminal half of WNK4 and is able to phosphorylate both WNK4 and SPAK/OSR1. We suggest that this 40-kDa kinase functions in the WNK4 signal transduction pathway and may mediate some of the physiological actions attributed to WNK4.

The thiazide-sensitive Na-Cl cotransporter is regulated by a WNK kinase signaling complex

The pathogenesis of essential hypertension remains unknown, but thiazide diuretics are frequently recommended as first-line treatment. Recently, familial hyperkalemic hypertension (FHHt) was shown to result from activation of the thiazide-sensitive Na-Cl cotransporter (NCC) by mutations in WNK4, although the mechanism for this effect remains unknown. WNK kinases are unique members of the human kinome, intimately involved in maintaining electrolyte balance across cell membranes and epithelia. Previous work showed that WNK1, WNK4, and a kidney-specific isoform of WNK1 interact to regulate NCC activity, suggesting that WNK kinases form a signaling complex. Here, we report that WNK3, another member of the WNK kinase family expressed by distal tubule cells, interacts with WNK4 and WNK1 to regulate NCC in both human kidney cells and Xenopus oocytes, further supporting the WNK signaling complex hypothesis. We demonstrate that physiological regulation of NCC in oocytes results from antagonism between WNK3 and WNK4 and that FHHt-causing WNK4 mutations exert a dominant-negative effect on wild-type (WT) WNK4 to mimic a state of WNK3 excess. The results provide a mechanistic explanation for the divergent effects of WT and FHHt-mutant WNK4 on NCC activity, and for the dominant nature of FHHt in humans and genetically modified mice.

WNK1 is a novel regulator of Munc18c-syntaxin 4 complex formation in soluble NSF attachment protein receptor (SNARE)-mediated vesicle exocytosis

Defects in soluble NSF attachment protein receptor (SNARE)-mediated granule exocytosis occur in islet beta cells, adipocytes, and/or skeletal muscle cells correlate with increased susceptibility to insulin resistance and diabetes. The serine/threonine kinase WNK1 (with no K (lysine)) has recently been implicated in exocytosis and is expressed in all three of these cell types. To search for WNK1 substrates related to exocytosis, we conducted a WNK1 two-hybrid screen, which yielded Munc18c. Munc18c is known to be a key regulator of accessibility of the target membrane (t-SNARE) protein syntaxin 4 to participate in SNARE core complex assembly, although a paucity of Munc18c-binding factors has precluded discovery of its precise functions. To validate WNK1 as a new Munc18c-interacting partner, the direct interaction between WNK1 and Munc18c was confirmed using in vitro binding analysis, and endogenous WNK1-Munc18c complexes were detected in the cytosolic and plasma membrane compartments of the islet beta cell line MIN6. This binding interaction is mediated through the N-terminal 172 residues of Munc18c and the kinase domain residues of WNK1 (residues 159-491). Expression of either of these two minimal interaction domains resulted in inhibition of glucose-stimulated insulin secretion, consistent with a functional importance for the endogenous WNK1-Munc18c complex in exocytosis. Interestingly, Munc18c failed to serve as a WNK1 substrate in kinase activity assays, suggesting that WNK1 functions in SNARE complex assembly outside its role as a kinase. Taken together, these data support a novel role for WNK1 and a new mechanism for the regulation of SNARE complex assembly by WNK1-Munc18c complexes.

Cotransporters, WNKs and hypertension: important leads from the study of monogenetic disorders of blood pressure regulation

Major advances are being made in identifying the structure and behaviour of regulatory cascades that control the activity of cation-Cl(-) cotransporters and certain Na(+), K(+) and Cl(-) channels. These transporters play key roles in regulating arterial blood pressure as they are not only responsible for NaCl reabsorption in the thick ascending limb and distal tubule of the kidney, but are also involved in regulating smooth muscle Ca(2+) levels. It is now apparent that defects in these transporters, and particularly in the regulatory cascades, cause some monogenetic forms of hypertension and may contribute to essential hypertension and problems with K(+) homoeostasis. Two families of kinases are prominent in these processes: the Ste-20-related kinases [OSR1 (oxidative stress-responsive kinase 1) and SPAK (Ste20/SPS1-related proline/alanine-rich kinase)] and the WNKs [with no lysine kinases]. These kinases affect the behaviour of their targets through both phosphorylation and by acting as scaffolding proteins, bringing together regulatory complexes. This review analyses how these kinases affect transport by activating or inhibiting individual transporters at the cell surface, or by changing the surface density of transporters by altering the rate of insertion or removal of transporters from the cell surface, and perhaps through controlling the rate of transporter degradation. This new knowledge should not only help us target antihypertensive therapy more appropriately, but could also provide the basis for developing new therapeutic approaches to essential hypertension.

WNK kinases regulate sodium chloride and potassium transport by the aldosterone-sensitive distal nephron

With-No-Lysine [K] (WNKs) are a recently discovered family of serine/threonine protein kinases that contain a uniquely structured catalytic domain. Mutations in the genes encoding two family members, WNK1 and WNK4, cause a chloride-dependent, thiazide-sensitive inherited syndrome of hypertension and hyperkalemia. Over the past 5 years, physiologic studies have demonstrated that these proteins regulate transcellular and paracellular epithelial ion flux. In this mini review, we discuss WNK1 and WNK4 gene products and their regulatory effects on sodium chloride and potassium handling in the aldosterone-sensitive distal nephron. Experimental observations regarding the effects of these proteins on transport processes mediated by the thiazide-sensitive Na-Cl co-transporter, the epithelial sodium channel, the renal outer medullary potassium channel, and the paracellular pathway integrate into a model that suggests an essential role for WNKs in coordinating renal Na-Cl reabsorption and K(+) secretion.

WNK kinases influence TRPV4 channel function and localization

TRPV4, a renally expressed nonselective cation channel of the transient receptor potential (TRP) family, is gated by hypotonicity. Kinases of the WNK family influence expression and function of the thiazide-sensitive Na+-Cl- cotransporter, and monogenic human hypertension has been linked to mutations in the gene coding for WNK4. Along with TRPV4, WNK isoforms are highly expressed in the distal nephron. We show here that coexpression of WNK4 downregulates TRPV4 function in human embryonic kidney (HEK-293) cells and that this effect is mediated via decreased cell surface expression of TRPV4; total abundance of TRPV4 in whole cell lysates is unaffected. The effect of the related kinase WNK1 on TRPV4 function and surface expression was similar to that of WNK4. Disease-causing point mutations in WNK4 abrogate, but do not eliminate, the inhibitory effect on TRPV4 function. In contrast to wild-type WNK4, a kinase-dead WNK4 point mutant failed to influence TRPV4 trafficking; however, deletion of the entire WNK4 kinase domain did not blunt the effect of WNK4 on localization of TRPV4. Deletion of the extreme COOH-terminal putative coiled-coil domain of WNK4 abolished its effect. In immunoprecipitation experiments, we were unable to detect direct interaction between TRPV4 and either WNK kinase. In aggregate, these data indicate that TRPV4 is functionally regulated by WNK family kinases at the level of cell surface expression. Because TRPV4 and WNK kinases are coexpressed in the distal nephron in vivo and because there is a tendency toward hypercalcemia in TRPV4-/- mice, we speculate that this pathway may impact systemic Ca2+ balance. In addition, because WNK kinases and TRPV4 are activated by anisotonicity, they may comprise elements of an osmosensing or osmotically responsive signal transduction cascade in the distal nephron.

Volume sensitivity of cation-Cl- cotransporters is modulated by the interaction of two kinases: Ste20-related proline-alanine-rich kinase and WNK4

In the present study, we have demonstrated functional interaction between Ste20-related proline-alanine-rich kinase (SPAK), WNK4 [with no lysine (K)], and the widely expressed Na+-K+-2Cl- cotransporter type 1 (NKCC1). NKCC1 function, which we measured in Xenopus laevis oocytes under both isosmotic (basal) and hyperosmotic (stimulated) conditions, was unaffected when SPAK and WNK4 were expressed alone. In contrast, expression of both kinases with NKCC1 resulted in a significant increase in cotransporter activity and an insensitivity to external osmolarity or cell volume. NKCC1 activation is dependent on the catalytic activity of SPAK and likely also of WNK4, because mutations in their catalytic domains result in an absence of cotransporter stimulation. The results of our yeast two-hybrid experiments suggest that WNK4 does not interact directly with NKCC1 but does interact with SPAK. Functional experiments demonstrated that the binding of SPAK to WNK4 was also required because a SPAK-interaction-deficient WNK4 mutant (Phe997Ala) did not increase NKCC1 activity. We also have shown that the transport function of K+-Cl- cotransporter type 2 (KCC2), a neuron-specific KCl cotransporter, was diminished by the expression of both kinases under both isosmotic and hyposmotic conditions. Our data are consistent with WNK4 interacting with SPAK, which in turn phosphorylates and activates NKCC1 and phosphorylates and deactivates KCC2.

Dominant-negative regulation of WNK1 by its kidney-specific kinase-defective isoform

With-no-lysine kinase-1 (WNK1) gene mutations cause familial hyperkalemic hypertension (FHHt), a Mendelian disorder of excessive renal Na+ and K+ retention. Through its catalytic activity, full-length kinase-sufficient WNK1 (L-WNK1) suppresses its paralog, WNK4, thereby upregulating thiazide-sensitive Na-Cl cotransporter (NCC) activity. The predominant renal WNK1 isoform, KS-WNK1, expressed exclusively and at high levels in distal nephron, is a shorter kinase-defective product; the function of KS-WNK1 must therefore be kinase independent. Here, we report a novel role for KS-WNK1 as a dominant-negative regulator of L-WNK1. Na+ transport studies in Xenopus laevis oocytes demonstrate that KS-WNK1 downregulates NCC activity indirectly, by inhibiting L-WNK1. KS-WNK1 also associates with L-WNK1 in protein complexes in oocytes and attenuates L-WNK1 kinase activity in vitro. These observations suggest that KS-WNK1 plays an essential role in the renal molecular switch regulating Na+ and K+ balance; they provide insight into the kidney-specific phenotype of FHHt.

Properties of WNK1 and implications for other family members

WNKs are large serine/threonine protein kinases structurally distinct from all other members of the protein kinase superfamily. Of the four human WNK family members, WNK1 and WNK4 have been linked to a hereditary form of hypertension, pseudohypoaldosteronism type II. We characterized the biochemical properties and regulation of WNK1 that may contribute to its physiological activities and abnormal function in disease. We showed that WNK1 is activated by hypertonic stress in kidney epithelial cells and in breast and colon cancer cell lines. In addition, hypotonic stress also led to a modest increase in WNK1 activity. Gel filtration suggested that WNK1 exists as a tetramer, and yeast two-hybrid data showed that the N terminus of WNK1 (residues 1-222) interacts with residues 481-660, which includes the WNK1 autoinhibitory domain and a C-terminal coiled-coil domain. Although cell biological studies have suggested a functional interaction between WNK1 and WNK4, we found no evidence of stable interactions between these kinases. However, WNK1 phosphorylated both WNK4 and WNK2. In addition, the WNK1 autoinhibitory domain inhibited the catalytic activity of these WNKs. These findings suggest potential mechanisms for interconnected regulation of WNK family members.

Mechanisms of WNK1 and WNK4 interaction in the regulation of thiazide-sensitive NaCl cotransport

With-no-lysine (WNK) kinases are highly expressed along the mammalian distal nephron. Mutations in either WNK1 or WNK4 cause familial hyperkalemic hypertension (FHHt), suggesting that the protein products converge on a final common pathway. We showed previously that WNK4 downregulates thiazide-sensitive NaCl cotransporter (NCC) activity, an effect suppressed by WNK1. Here we investigated the mechanisms by which WNK1 and WNK4 interact to regulate ion transport. We report that WNK1 suppresses the WNK4 effect on NCC activity and associates with WNK4 in a protein complex involving the kinase domains. Although a kinase-dead WNK1 also associates with WNK4, it fails to suppress WNK4-mediated NCC inhibition; the WNK1 kinase domain alone, however, is not sufficient to block the WNK4 effect. The carboxyterminal 222 amino acids of WNK4 are sufficient to inhibit NCC, but this fragment is not blocked by WNK1. Instead, WNK1 inhibition requires an intact WNK4 kinase domain, the region that binds to WNK1. In summary, these data show that: (a) the WNK4 carboxyl terminus mediates NCC suppression, (b) the WNK1 kinase domain interacts with the WNK4 kinase domain, and (c) WNK1 inhibition of WNK4 is dependent on WNK1 catalytic activity and an intact WNK1 protein. These findings provide insight into the complex interrelationships between WNK1 and WNK4 and provide a molecular basis for FHHt.

Molecular Physiology and Pathophysiology of Electroneutral Cation-Chloride Cotransporters

Electroneutral cation-Cl−cotransporters compose a family of solute carriers in which cation (Na+or K+) movement through the plasma membrane is always accompanied by Cl−in a 1:1 stoichiometry. Seven well-characterized members include one gene encoding the thiazide-sensitive Na+−Cl−cotransporter, two genes encoding loop diuretic-sensitive Na+−K+−2Cl−cotransporters, and four genes encoding K+−Cl−cotransporters. These membrane proteins are involved in several physiological activities including transepithelial ion absorption and secretion, cell volume regulation, and setting intracellular Cl−concentration below or above its electrochemical potential equilibrium. In addition, members of this family play an important role in cardiovascular and neuronal pharmacology and pathophysiology. Some of these cotransporters serve as targets for loop diuretics and thiazide-type diuretics, which are among the most commonly prescribed drugs in the world, and inactivating mutations of three members of the family cause inherited diseases such as Bartter's, Gitelman's, and Anderman's diseases. Major advances have been made in the past decade as consequences of molecular identification of all members in this family. This work is a comprehensive review of the knowledge that has evolved in this area and includes molecular biology of each gene, functional properties of identified cotransporters, structure-function relationships, and physiological and pathophysiological roles of each cotransporter.

Role of WNK kinases in regulating tubular salt and potassium transport and in the development of hypertension

A recently discovered family of protein kinases is responsible for an autosomal-dominant disease known as Gordon's syndrome or pseudohypoaldosteronism type II (PHA-II) that features hyperkalemia and hyperchloremic metabolic acidosis, accompanied by hypertension and hypercalciuria. Four genes have been described in this kinase family, which has been named WNK, due to the absence of a key lysine in kinase subdomain II (with no K kinases). Two of these genes, WNK1 and WNK4 located in human chromosomes 12 and 17, respectively, are responsible for PHA-II. Immunohystochemical analysis revealed that WNK1 and WNK4 are predominantly expressed in the distal convoluted tubule and collecting duct. The physiological studies have shown that WNK4 downregulates the activity of ion transport pathways expressed in these nephron segments, such as the apical thiazide-sensitive Na+-Cl−cotransporter and apical secretory K+channel ROMK, as well as upregulates paracellular chloride transport and phosphorylation of tight junction proteins such as claudins. In addition, WNK4 downregulates other Cl−influx pathways such as the basolateral Na+-K+-2Cl−cotransporter and Cl−/HCO3−exchanger. WNK4 mutations behave as a loss of function for the Na+-Cl−cotransporter and a gain of function when it comes to ROMK and claudins. These dual effects of WNK4 mutations fit with proposed mechanisms for developing electrolyte abnormalities and hypertension in PHA-II and point to WNK4 as a multifunctional regulator of diverse ion transporters.

WNK1 et WNK4, nouveaux acteurs de l’homéostasie hydrosodée

Arterial hypertension is a complex trait influenced by a variety of environmental and genetic factors. Several approaches can be used to identify its susceptibility genes : one is to study rare monogenic forms of hypertension, like familial hyperkalemic hypertension (FHH). Also known as pseudohypoaldosteronism type 2 or Gordon syndrome, FHH is characterized by hypertension, hyperkalemia despite normal renal glomerular filtration rate, abnormalities which are particularly sensitive to thiazide diuretics. Mild hyperchloremia, metabolic acidosis, and suppressed plasma renin activity are associated findings. Despite its phenotypic and genetic heterogeneity, mutations in two related genes, WNK1 and WNK4, were recently identified. These genes belong to a newly identified family of serine-threonine (with no lysine [K]) kinases. Both are highly expressed in the kidney and in a variety of epithelia involved in chloride transport. It has thus been postulated that these two kinases could be implicated in a new pathway of ionic transport regulation. Several studies have very recently confirmed this hypothesis in vitro, in Xenopus oocytes or kidney cell lines. They have shown that, in the renal distal tubule, WNK4 inhibits sodium reabsorption and potassium secretion, via inhibition of NCC (thiazide-sensitive Na+-Cl- cotransporter) and K+ channel ROMK activity, respectively. Interestingly, FHH mutations have opposite effects : while they lead to loss of NCC inhibition, they increase ROMK inhibition. Moreover, they also increase paracellular permeability to chloride of MDCK cells. WNK4 also inhibits apical and basal chloride transporters present in extra-renal epithelia, such as CFEX and Na+-K+-2 Cl-, respectively. It is also interesting to note that the WNK4-mediated negative regulation of NCC activity is in turn inhibited by WNK1. By its role on several transporters, WNK4 appears as a putative key regulator of ionic transport and blood pressure.

WNK kinases: molecular regulators of integrated epithelial ion transport

PURPOSE OF REVIEW

The WNK kinases are a recently discovered family of serine-threonine kinases that have been shown to play an essential role in the regulation of electrolyte homeostasis. This review focuses on the recent evidence elucidating the functions of these kinases in normal and disease physiology.

RECENT FINDINGS

Mutations in WNK1 and WNK4 have been shown to cause pseudohypoaldosteronism type II, a disease featuring hypertension with hyperkalemia. Recent work has demonstrated that WNK4 is a potent inhibitor of diverse epithelial transporters including the thiazide-sensitive sodium chloride co-transporter (NCCT) and the renal outer medullary potassium ion channel. In addition, WNK4 activity promotes paracellular chloride ion flux. Importantly, mutations in WNK4 that cause disease have divergent effects on these transport pathways. WNK4 mutations relieve the inhibition of NCCT, increase the inhibition of the renal outer medullary potassium ion channel, and further increase paracellular chloride ion flux. These findings can explain the observed physiological abnormalities in patients with pseudohypoaldosteronism type II, and support a model in which WNK4 is a molecular switch that can alter the balance between chloride ion reabsorption and potassium ion secretion. The WNK kinases are also found in diverse epithelia throughout the body that are involved in chloride ion flux, suggesting that these kinases may play a general role in the regulation of chloride ion flux.

SUMMARY

The WNK kinases define a previously unrecognized signaling pathway that is essential for the integrated regulation of electrolyte homeostasis. Their function has implications for understanding the coordinated regulation of electrolyte homeostasis and blood pressure, and identifies WNKs as dynamic regulators of the paracellular flux pathway.