Overexpression of the sodium chloride cotransporter is not sufficient to cause familial hyperkalemic hypertension

The sodium chloride cotransporter (NCC) is the primary target of thiazides diuretics, drugs used commonly for long-term hypertension therapy. Thiazides also completely reverse the signs of familial hyperkalemic hypertension (FHHt), suggesting that the primary defect in FHHt is increased NCC activity. To test whether increased NCC abundance alone is sufficient to generate the FHHt phenotype, we generated NCC transgenic mice; surprisingly, these mice did not display an FHHt-like phenotype. Systolic blood pressures of NCC transgenic mice did not differ from those of wild-type mice, even after dietary salt loading. NCC transgenic mice also did not display hyperkalemia or hypercalciuria, even when challenged with dietary electrolyte manipulation. Administration of fludrocortisone to NCC transgenic mice, to stimulate NCC, resulted in an increase in systolic blood pressure equivalent to that of wild-type mice (approximately 20 mm Hg). Although total NCC abundance was increased in the transgenic animals, phosphorylated (activated) NCC was not, suggesting that the defect in FHHt involves either activation of ion transport pathways other than NCC, or else direct activation of NCC, in addition to an increase in NCC abundance.

Through a glass darkly: salt transport by the distal tubule

The distal convoluted tubule (DCT) plays a central role in blood pressure and potassium homeostasis, as evidenced by diseases that occur when its function is modified. The paper by van der Lubbe and colleagues makes clear that angiotensin II itself increases the activity and abundance of the thiazide-sensitive Na-Cl cotransporter (NCC), independent of changes in circulating aldosterone. This Commentary provides additional perspective on that work.

The WNK kinase network regulating sodium, potassium, and blood pressure

The relationship between renal salt handling and hypertension is intertwined historically. The discovery of WNK kinases (With No lysine = K) now offers new insight to this relationship because WNKs are a crucial molecular pathway connecting hormones such as angiotensin II and aldosterone to renal sodium and potassium transport. To fulfill this task, the WNKs also interact with other important kinases, including serum and glucocorticoid-regulated kinase 1, STE20/SPS1-related, proline alanine-rich kinase, and oxidative stress responsive protein type 1. Collectively, this kinase network regulates the activity of the major sodium and potassium transporters in the distal nephron, including thiazide-sensitive Na-Cl cotransporters and ROMK channels. Here we show how the WNKs modulate ion transport through two distinct regulatory pathways, trafficking and phosphorylation, and discuss the physiologic and clinical relevance of the WNKs in the kidney. This ranges from rare mutations in WNKs causing familial hyperkalemic hypertension to acquired forms of hypertension caused by salt sensitivity or diabetes mellitus. Although many questions remain unanswered, the WNKs hold promise for unraveling the link between salt and hypertension, potentially leading to more effective interventions to prevent cardiorenal damage.

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.

Altered renal FGF23-mediated activity involving MAPK and Wnt: effects of the Hyp mutation

Fibroblast growth factor-23 (FGF23), a hormone central to renal phosphate handling, is elevated in multiple hypophosphatemic disorders. Initial FGF23-dependent Erk1/2 activity in the kidney localizes to the distal convoluted tubule (DCT) with the co-receptor α-Klotho (KL), distinct from Npt2a in proximal tubules (PT). The Hyp mouse model of X-linked hypophosphatemic rickets (XLH) is characterized by hypophosphatemia with increased Fgf23, and patients with XLH elevate FGF23 following combination therapy of phosphate and calcitriol. The molecular signaling underlying renal FGF23 activity, and whether these pathways are altered in hypophosphatemic disorders, is unknown. To examine Npt2a in vivo, mice were injected with FGF23. Initial p-Erk1/2 activity in the DCT occurred within 10 min; however, Npt2a protein was latently reduced in the PT at 30-60  min, and was independent of Npt2a mRNA changes. KL-null mice had no DCT p-Erk1/2 staining following FGF23 delivery. Under basal conditions in Hyp mice, c-Fos and Egr1, markers of renal Fgf23 activity, were increased; however, KL mRNA was reduced 60% (P<0.05). Despite the prevailing hypophosphatemia and elevated Fgf23, FGF23 injections into Hyp mice activated p-Erk1/2 in the DCT. FGF23 injection also resulted in phospho-β-catenin (p-β-cat) co-localization with KL in wild-type mice, and Hyp mice demonstrated strong p-β-cat staining under basal conditions, indicating potential crosstalk between mitogen-activated protein kinase and Wnt signaling. Collectively, these studies refine the mechanisms for FGF23 bioactivity, and demonstrate novel suppression of Wnt signaling in a KL-dependent DCT-PT axis, which is likely altered in XLH. Finally, the current treatment of phosphate and calcitriol for hypophosphatemic disorders may increase FGF23 activity.

Aldosterone mediates activation of the thiazide-sensitive Na-Cl cotransporter through an SGK1 and WNK4 signaling pathway

Aldosterone regulates volume homeostasis and blood pressure by enhancing sodium reabsorption in the kidney's distal nephron (DN). On the apical surface of these renal epithelia, aldosterone increases expression and activity of the thiazide-sensitive Na-Cl cotransporter (NCC) and the epithelial sodium channel (ENaC). While the cellular mechanisms by which aldosterone regulates ENaC have been well characterized, the molecular mechanisms that link aldosterone to NCC-mediated Na+/Cl- reabsorption remain elusive. The serine/threonine kinase with-no-lysine 4 (WNK4) has previously been shown to reduce cell surface expression of NCC. Here we measured sodium uptake in a Xenopus oocyte expression system and found that serum and glucocorticoid-induced kinase 1 (SGK1), an aldosterone-responsive gene expressed in the DN, attenuated the inhibitory effect of WNK4 on NCC activity. In addition, we showed--both in vitro and in a human kidney cell line--that SGK1 bound and phosphorylated WNK4. We found one serine located within an established SGK1 consensus target sequence, and the other within a motif that was, to our knowledge, previously uncharacterized. Mutation of these target serines to aspartate, in order to mimic phosphorylation, attenuated the effect of WNK4 on NCC activity in the Xenopus oocyte system. These data thus delineate what we believe to be a novel mechanism for aldosterone activation of NCC through SGK1 signaling of WNK4 kinase.

WNK4 diverts the thiazide-sensitive NaCl cotransporter to the lysosome and stimulates AP-3 interaction

With-no-lysine kinase 4 (WNK4) inhibits electroneutral sodium chloride reabsorption by attenuating the cell surface expression of the thiazide-sensitive NaCl cotransporter (NCC). The underlying mechanism for this effect remains poorly understood. Here, we explore how WNK4 affects the trafficking of NCC through its interactions with intracellular sorting machinery. An analysis of NCC cell surface lifetime showed that WNK4 did not alter the net rate of cotransporter internalization. In contrast, direct measurements of forward trafficking revealed that WNK4 attenuated the rate of NCC surface delivery, inhibiting the anterograde movement of cotransporters traveling to the plasma membrane from the trans-Golgi network. The response was paralleled by a dramatic reduction in NCC protein abundance, an effect that was sensitive to the lysosomal protease inhibitor leupeptin, insensitive to proteasome inhibition, and attenuated by endogenous WNK4 knockdown. Subcellular localization studies performed in the presence of leupeptin revealed that WNK4 enhanced the accumulation of NCC in lysosomes. Moreover, NCC immunoprecipitated with endogenous AP-3 complexes, and WNK4 increased the fraction of cotransporters that associate with this adaptor, which facilitates cargo transport to lysosomes. WNK4 expression also increased LAMP-2-positive lysosomal content, indicating that the kinase may act by a general AP-3-dependent mechanism to promote cargo delivery into the lysosomal pathway. Taken together, these findings indicate that WNK4 inhibits NCC activity by diverting the cotransporter to the lysosome for degradation by way of an AP-3 transport carrier.

The voltage-gated K+ channel subunit Kv1.1 links kidney and brain

Analysis of Mendelian Mg2+ wasting disorders helps us to unravel the mechanisms of Mg2+ homeostasis. In this issue of the JCI, Glaudemans andcolleagues show that mutations in voltage-gated K+ channel subtype 1.1(Kv1.1) cause autosomal dominant hypomagnesemia in humans (see the related article beginning on page 936). Interestingly, other mutations in the same protein cause the neurological disease episodic ataxia type 1. The authors show, using cells with heterologous expression of the wild-type and mutant channels, that the mutant channel is dysfunctional and speculate that Mg2+ wasting results from changes in apical membrane voltage along the nephron. Mechanisms by which the apical voltage is generated and howKv1.1 fits within this context are discussed herein.

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.

The continuous erythropoietin receptor activator (C.E.R.A.) corrects anemia at extended administration intervals in patients with chronic kidney disease not on dialysis: results of a phase II study

AIM

This study was designed to assess the potential of the continuous erythropoietin receptor activator (C.E.R.A.) to correct anemia at extended administration intervals in erythropoiesis-stimulating agent-naīve patients with chronic kidney disease (CKD) not on dialysis and to determine its optimal starting dose.

METHODS

Patients were assigned to subcutaneous C.E.R.A. at 3 doses: 0.15, 0.30 and 0.60 microg/kg/wk. During the first 6 weeks, dose adjustments for efficacy were not permitted in order to assess dose response. Within each of the 3 dose groups, patients were randomized to receive C.E.R.A. QW, Q2W or Q3W; the total dose during the first 6 weeks was the same for a particular dose group across the frequency subgroups. During the next 12 weeks, dose was adjusted according to predefined hemoglobin (Hb) criteria. The primary efficacy parameter was change in Hb over 6 weeks, estimated from regression analysis between baseline and the point at which the patient received a dose change or blood transfusion. It therefore provided an estimate of Hb increase based on starting dose. Other endpoints included Hb response rate (proportion of patients with a Hb increase > 1.0 g/dl on 2 consecutive occasions). A 1-year extension period investigated long term tolerability and efficacy.

RESULTS

A dose-dependent relationship was noted in the mean change in Hb from baseline over 6 weeks (p < 0.0001), independent of administration schedule (p = 0.9201). There was also a significant relationship between Hb change and median serum C.E.R.A. concentration (p < 0.0001). Erythropoietic responses were sustained in all groups with mean changes from baseline in Hb > 1.2 g/dl observed at doses > or = 0.30 microg/kg/wk. Hb response rate increased with increasing dose: 67, 72 and 90% with C.E.R.A. 0.15, 0.30 and 0.60 microg/kg/wk, respectively. Generally, the median Hb response time was faster with increasing dose (89, 43 and 31 days, respectively). Response was unrelated to administration frequency. Stable Hb concentrations were maintained throughout the 1-year extension period. C.E.R.A. was generally well tolerated, and the most common adverse events were hypertension, urinary tract infection and renal failure.

CONCLUSIONS

C.E.R.A. corrected anemia and maintained sustained and stable control of Hb over 1 year. These results suggest that 0.60 microg/kg subcutaneous C.E.R.A. given twice monthly is a suitable starting dose for further investigation in Phase III studies in patients with CKD not on dialysis.

WNK1 and WNK4 modulate CFTR activity

The cystic fibrosis transmembrane conductance regulator (CFTR) is an ATP-gated chloride channel. WNK kinases are widely expressed modulators of ion transport. WNK1 and WNK4, two WNK kinases that are mutated in familial hyperkalemic hypertension (FHHt), are co-expressed with CFTR in several organs, raising the possibility that WNK kinases might alter CFTR activity in vivo or that CFTR could be involved in the pathogenesis of FHHt. Here, we report that WNK1 co-localizes with CFTR protein in pulmonary epithelial cells. Co-expression of WNK1 or WNK4 with CFTR in Xenopus laevis oocytes suppresses chloride channel activity. The effect of WNK4 is dose dependent and occurs, at least in part, by reducing CFTR protein abundance at the plasma membrane. This effect is independent of WNK4 kinase activity. In contrast, the effect of WNK1 on CFTR activity requires intact WNK1 kinase activity. Moreover WNK1 and WNK4 exhibit additive CFTR inhibition. Previous reports suggest that patients with FHHt exhibit mild changes in nasal potential difference that resemble the more severe changes that occur in cystic fibrosis. We report that the FHHt-causing mutant WNK4 Q562E is a more potent inhibitor of CFTR activity than is the wild-type WNK4. Taken together, these results suggest that WNK1 and WNK4 may modulate CFTR activity; they further suggest that WNK kinases may be potential therapeutic targets for cystic fibrosis.

Wnk4 controls blood pressure and potassium homeostasis via regulation of mass and activity of the distal convoluted tubule

The mechanisms that govern homeostasis of complex systems have been elusive but can be illuminated by mutations that disrupt system behavior. Mutations in the gene encoding the kinase WNK4 cause pseudohypoaldosteronism type II (PHAII), a syndrome featuring hypertension and hyperkalemia. We show that physiology in mice transgenic for genomic segments harboring wild-type (TgWnk4(WT)) or PHAII mutant (TgWnk4(PHAII)) Wnk4 is changed in opposite directions: TgWnk4(PHAII) mice have higher blood pressure, hyperkalemia, hypercalciuria and marked hyperplasia of the distal convoluted tubule (DCT), whereas the opposite is true in TgWnk4(WT) mice. Genetic deficiency for the Na-Cl cotransporter of the DCT (NCC) reverses phenotypes seen in TgWnk4(PHAII) mice, demonstrating that the effects of the PHAII mutation are due to altered NCC activity. These findings establish that Wnk4 is a molecular switch that regulates the balance between NaCl reabsorption and K+ secretion by altering the mass and function of the DCT through its effect on NCC.

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.

WNK1 kinase isoform switch regulates renal potassium excretion

Members of the WNK family of serine/threonine kinases have been implicated as important modulators of salt homeostasis, regulating the balance between renal sodium reabsorption and potassium excretion. Gain-of-expression mutations in the WNK1 gene uncouple Na(+) and K(+) balance and cause a familial disorder of diminished renal potassium excretion, excessive sodium retention, and hypertension (pseudohypoaldosteronism type II or Gordon's syndrome). Alternative splicing of the WNK1 gene produces a kidney-specific short form of WNK1 (KS-WNK1) and a more ubiquitous long form (L-WNK1), but it is not clear how either of these isoforms influence renal potassium excretion. Here we demonstrate that KS-WNK1 and L-WNK1 converge in a pathway to regulate the renal outer-medullary K(+) channel, Kir1.1. Reconstitution studies in Xenopus oocytes reveal that L-WNK1 significantly inhibits Kir1.1 by reducing cell surface localization of the channel. A catalytically inactive L-WNK1 mutant has no inhibitory effect on Kir1.1, indicating that channel inhibition depends on kinase activity. KS-WNK1, lacking an intact kinase domain, does not directly alter Kir1.1. Instead, KS-WNK1 negatively regulates L-WNK1 to release Kir1.1 from inhibition. Acute dietary potassium loading increases the relative abundance of KS-WNK1 to L-WNK1 transcript and protein in the kidney, indicating that physiologic up-regulation of Kir1.1 activity involves a WNK1 isoform switch and KS-WNK1-mediated release from L-WNK1 inhibition. Thus, these observations provide evidence for the physiological regulation of Na(+) and K(+) balance by a kinase isoform switch mechanism.

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.

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.