The Na+:Cl- cotransporter is activated and phosphorylated at the amino-terminal domain upon intracellular chloride depletion

Pre-steady-state and reverse transport currents in the GABA transporter GAT1

The role of internal substrates in the biophysical properties of the GABA transporter GAT1 has been investigated electrophysiologically in Xenopus oocytes heterologously expressing the cotransporter. Increments in Cl(-) and/or Na(+) concentrations caused by intracellular injections did not produce significant effects on the pre-steady-state currents, while a positive shift of the charge-voltage (Q-V) and decay time constant (τ)-voltage (τ-V) curves, together with a slowing of τ at positive potentials, was observed following treatments producing cytosolic Cl(-) depletion. Activation of the reverse transport mode by injections of GABA caused a reduction in the displaced charge. In the absence of external Cl(-), a stronger reduction in the displaced charge, together with a significant increase in reverse transport current, was observed. Therefore, complementarity between pre-steady-state and transport currents, observed in the forward mode, is preserved in the reverse mode. All these findings can be qualitatively reproduced by a kinetic scheme in which, in the forward mode, the Cl(-) ion is released first, after the inward charge movement, while the two Na(+) ions can be released only after binding of external GABA. In the reverse mode, internal GABA must bind first to the empty transporter, followed by internal Na(+) and Cl(-).

A minor role of WNK3 in regulating phosphorylation of renal NKCC2 and NCC co-transporters in vivo

Mutations in WNK1 and WNK4 kinase genes have been shown to cause a human hereditary hypertensive disease, pseudohypoaldosteronism type II (PHAII). We previously discovered that WNK kinases phosphorylate and activate OSR1/SPAK kinases that regulate renal SLC12A family transporters such as NKCC2 and NCC, and clarified that the constitutive activation of this cascade causes PHAII. WNK3, another member of the WNK kinase family, was reported to be a strong activator of NCC/NKCC2 when assayed in Xenopus oocytes, suggesting that WNK3 also plays a major role in regulating blood pressure and sodium reabsorption in the kidney. However, it remains to be determined whether WNK3 is in fact involved in the regulation of these transporters in vivo. To clarify this issue, we generated and analyzed WNK3 knockout mice. Surprisingly, phosphorylation and expression of OSR1, SPAK, NKCC2 and NCC did not decrease in knockout mouse kidney under normal and low-salt diets. Similarly, expression of epithelial Na channel and Na/H exchanger 3 were not affected in knockout mice. Na⁺ and K⁺ excretion in urine in WNK3 knockout mice was not affected under different salt diets. Blood pressure in WNK3 knockout mice was not lower under normal diet. However, lower blood pressure was observed in WNK3 knockout mice fed low-salt diet. WNK4 and WNK1 expression was slightly elevated in the knockout mice under low-salt diet, suggesting compensation for WNK3 knockout by these WNKs. Thus, WNK3 may have some role in the WNK-OSR1/SPAK-NCC/NKCC2 signal cascade in the kidney, but its contribution to total WNK kinase activity may be minimal.

GABA reverse transport by the neuronal cotransporter GAT1: influence of internal chloride depletion

The role of intracellular ions on the reverse GABA transport by the neuronal transporter GAT1 was studied using voltage-clamp and [(3)H]GABA efflux determinations in Xenopus oocytes transfected with heterologous mRNA. Reverse transport was induced by intracellular GABA injections and measured in terms of the net outward current generated by the transporter. Changes in various intracellular ionic conditions affected the reverse current: higher concentrations of Na(+) enhanced the ratio of outward over inward transport current, while a considerable decrease of the outward current and a parallel reduction of the transporter-mediated GABA efflux were observed after treatments causing a diminution of the intracellular Cl(-) concentration. Particularly interesting was the impairment of the reverse transport observed after depletion of internal Cl(-) generated by the activity of a coexpressed K(+)-Cl(-) exporter KCC2. This finding suggests that reverse GABA transport may be physiologically regulated during early neuronal development, similarly to the functional alterations seen in GABA receptors caused by KCC2 activity.

Novel NCC mutants and functional analysis in a new cohort of patients with Gitelman syndrome

Gitelman syndrome (GS) is an autosomal recessive disorder characterized by hypokalemic metabolic alkalosis in conjunction with significant hypomagnesemia and hypocalciuria. The GS phenotype is caused by mutations in the solute carrier family 12, member 3 (SLC12A3) gene that encodes the thiazide-sensitive NaCl cotransporter (NCC). We analyzed DNA samples of 163 patients with a clinical suspicion of GS by direct sequencing of all 26 exons of the SLC12A3 gene. In total, 114 different mutations were identified, 31 of which have not been reported before. These novel variants include 3 deletions, 18 missense, 6 splice site and 4 nonsense mutations. We selected seven missense mutations to investigate their effect on NCC activity and plasma membrane localization by using the Xenopus laevis oocyte expression system. The Thr392Ile mutant did not display transport activity (probably class 2 mutation), while the Asn442Ser and Gln1030Arg NCC mutants showed decreased plasma membrane localization and consequently function, likely due to impaired trafficking (class 3 mutation). Even though the NaCl uptake was hampered for NCC mutants Glu121Asp, Pro751Leu, Ser475Cys and Tyr489His, the transporters reached the plasma membrane (class 4 mutation), suggesting an effect on NCC regulation or ion affinity. The present study shows the identification of 38 novel mutations in the SLC12A3 gene and provides insight into the mechanisms that regulate NCC.

Exploring the intricate regulatory network controlling the thiazide-sensitive NaCl cotransporter (NCC)

The thiazide-sensitive NaCl cotransporter (NCC) plays key roles in renal electrolyte transport and blood pressure maintenance. Regulation of this cotransporter has received increased attention recently, prompted by the discovery that mutations in the with-no-lysine (WNK) kinases are the molecular explanation for pseudohypoaldosteronism type II (PHAII). Studies suggest that WNK4 regulates NCC via two distinct pathways, depending on its state of activation. Furthermore, an intact STE20-related proline-alanine-rich kinase (SPAK)/oxidative stress response 1 kinase (OSR1) pathway was found to be necessary for a WNK4 PHAII mutation to increase NCC phosphorylation and blood pressure in mice. The mouse protein 25α is a novel regulator of the SPAK/OSR1 kinase family, which greatly increases their activity. The phosphorylation status of NCC and the WNK is regulated by the serum- and glucocorticoid-inducible kinase 1, suggesting novel mechanisms whereby aldosterone modulates NCC activity. Dephosphorylation of NCC by protein phosphatase 4 strongly influences the activity of the cotransporter, confirming an important role for NCC phosphorylation. Finally, γ-adducin increases NCC activity. This stimulatory effect is dependent on the phosphorylation status of the cotransporter. γ-Adducin only binds the dephosphorylated cotransporter, suggesting that phosphorylation of NCC causes the dissociation of γ-adducin. Since γ-adducin is not a kinase, it is tempting to speculate that the protein exerts its function by acting as a scaffold between the dephosphorylated cotransporter and the regulatory kinase. As more molecular regulators of NCC are identified, the system-controlling NCC activity is becoming increasingly complex. This intricacy confers an ability to integrate a variety of stimuli, thereby regulating NCC transport activity and ultimately blood pressure.

Nedd4-2 modulates renal Na+-Cl- cotransporter via the aldosterone-SGK1-Nedd4-2 pathway

Regulation of renal Na(+) transport is essential for controlling blood pressure, as well as Na(+) and K(+) homeostasis. Aldosterone stimulates Na(+) reabsorption by the Na(+)-Cl(-) cotransporter (NCC) in the distal convoluted tubule (DCT) and by the epithelial Na(+) channel (ENaC) in the late DCT, connecting tubule, and collecting duct. Aldosterone increases ENaC expression by inhibiting the channel's ubiquitylation and degradation; aldosterone promotes serum-glucocorticoid-regulated kinase SGK1-mediated phosphorylation of the ubiquitin-protein ligase Nedd4-2 on serine 328, which prevents the Nedd4-2/ENaC interaction. It is important to note that aldosterone increases NCC protein expression by an unknown post-translational mechanism. Here, we present evidence that Nedd4-2 coimmunoprecipitated with NCC and stimulated NCC ubiquitylation at the surface of transfected HEK293 cells. In Xenopus laevis oocytes, coexpression of NCC with wild-type Nedd4-2, but not its catalytically inactive mutant, strongly decreased NCC activity and surface expression. SGK1 prevented this inhibition in a kinase-dependent manner. Furthermore, deficiency of Nedd4-2 in the renal tubules of mice and in cultured mDCT(15) cells upregulated NCC. In contrast to ENaC, Nedd4-2-mediated inhibition of NCC did not require the PY-like motif of NCC. Moreover, the mutation of Nedd4-2 at either serine 328 or 222 did not affect SGK1 action, and mutation at both sites enhanced Nedd4-2 activity and abolished SGK1-dependent inhibition. Taken together, these results suggest that aldosterone modulates NCC protein expression via a pathway involving SGK1 and Nedd4-2 and provides an explanation for the well-known aldosterone-induced increase in NCC protein expression.

Antagonistic regulation of cystic fibrosis transmembrane conductance regulator cell surface expression by protein kinases WNK4 and spleen tyrosine kinase

Members of the WNK (with-no-lysine [K]) subfamily of protein kinases regulate various ion channels involved in sodium, potassium, and chloride homeostasis by either inducing their phosphorylation or regulating the number of channel proteins expressed at the cell surface. Here, we describe findings demonstrating that the cell surface expression of the cystic fibrosis transmembrane conductance regulator (CFTR) is also regulated by WNK4 in mammalian cells. This effect of WNK4 is independent of the presence of kinase and involves interaction with and inhibition of spleen tyrosine kinase (Syk), which phosphorylates Tyr512 in the first nucleotide-binding domain 1 (NBD1) of CFTR. Transfection of catalytically active Syk into CFTR-expressing baby hamster kidney cells reduces the cell surface expression of CFTR, whereas that of WNK4 promotes it. This is shown by biotinylation of cell surface proteins, immunofluorescence microscopy, and functional efflux assays. Mutation of Tyr512 to either glutamic acid or phenylalanine is sufficient to alter CFTR surface levels. In human airway epithelial cells, downregulation of endogenous Syk and WNK4 confirms their roles as physiologic regulators of CFTR surface expression. Together, our results show that Tyr512 phosphorylation is a novel signal regulating the prevalence of CFTR at the cell surface and that WNK4 and Syk perform an antagonistic role in this process.

Activation of the bumetanide-sensitive Na+,K+,2Cl- cotransporter (NKCC2) is facilitated by Tamm-Horsfall protein in a chloride-sensitive manner

Active transport of NaCl across thick ascending limb (TAL) epithelium is accomplished by Na(+),K(+),2Cl(-) cotransporter (NKCC2). The activity of NKCC2 is determined by vasopressin (AVP) or intracellular chloride concentration and includes its amino-terminal phosphorylation. Co-expressed Tamm-Horsfall protein (THP) has been proposed to interact with NKCC2. We hypothesized that THP modulates NKCC2 activity in TAL. THP-deficient mice (THP(-/-)) showed an increased abundance of intracellular NKCC2 located in subapical vesicles (+47% compared with wild type (WT) mice), whereas base-line phosphorylation of NKCC2 was significantly decreased (-49% compared with WT mice), suggesting reduced activity of the transporter in the absence of THP. Cultured TAL cells with low endogenous THP levels and low base-line phosphorylation of NKCC2 displayed sharp increases in NKCC2 phosphorylation (+38%) along with a significant change of intracellular chloride concentration upon transfection with THP. In NKCC2-expressing frog oocytes, co-injection with THP cRNA significantly enhanced the activation of NKCC2 under low chloride hypotonic stress (+112% versus +235%). Short term (30 min) stimulation of the vasopressin V2 receptor pathway by V2 receptor agonist (deamino-cis-D-Arg vasopressin) resulted in enhanced NKCC2 phosphorylation in WT mice and cultured TAL cells transfected with THP, whereas in the absence of THP, NKCC2 phosphorylation upon deamino-cis-D-Arg vasopressin was blunted in both systems. Attenuated effects of furosemide along with functional and structural adaptation of the distal convoluted tubule in THP(-/-) mice supported the notion that NaCl reabsorption was impaired in TAL lacking THP. In summary, these results are compatible with a permissive role for THP in the modulation of NKCC2-dependent TAL salt reabsorptive function.

Regulation of the NKCC2 ion cotransporter by SPAK-OSR1-dependent and -independent pathways

Ion cotransporters, such as the Na(+)/Cl(-) cotransporter (NCC), control renal salt re-absorption and are regulated by the WNK-signalling pathway, which is over-stimulated in patients suffering from Gordon's hypertension syndrome. Here, we study the regulation of the NKCC2 (SLC12A1) ion cotransporter that contributes towards ~25% of renal salt re-absorption and is inhibited by loop-diuretic hypertensive drugs. We demonstrate that hypotonic low-chloride conditions that activate the WNK1-SPAK and OSR1 pathway promote phosphorylation of NKCC2 isoforms (A, B and F) at five residues (Ser91, Thr95, Thr100, Thr105 and Ser130). We establish that the SPAK and OSR1 kinases activated by WNK interact with an RFQV motif on NKCC2 and directly phosphorylate Thr95, Thr100, Thr105 and, possibly, Ser91. Our data indicate that a SPAK-OSR1-independent kinase, perhaps AMP-activated protein kinase (AMPK), phosphorylates Ser130 and that phosphorylation of Thr105 and Ser130 plays the most important roles in stimulating NKCC2 activity. In contrast with NCC, whose membrane translocation is triggered by SPAK-OSR1 phosphorylation, NKCC2 appears to be constitutively at the membrane. Our findings provide new insights into how NKCC2 is regulated and suggest that inhibitors of SPAK and/or OSR1 for the treatment of hypertension would be therapeutically distinct from thiazide or loop diuretics, as they would suppress the activity of both NCC and NKCC2.

Rare mutations in SLC12A1 and SLC12A3 protect against hypertension by reducing the activity of renal salt cotransporters

OBJECTIVES

Screening for variants in SLC12A1 and SLC12A3 genes, encoding the renal Na:Cl (NCC) and Na:K:2Cl (NKCC2) cotransporters, respectively, in 3125 members of the Framingham Heart Study (FHS) revealed that carrying a rare mutation in one of these genes was associated with a significant reduction in blood pressure, in the risk of arterial hypertension, and of death due to cardiovascular disease. Because near 60% of the rare mutations identified have not been related to Bartter's or Gitelman's disease, the consequence of such mutations on cotransporter activity is unknown.

METHODS

We used the heterologous expression system of Xenopus laevis oocytes, microinjected with wild-type or mutant NCC or NKCC2 cRNAs, to examine the effect of these inferred NCC and NKCC2 mutations on the cotransporters' functional properties. Cotransporter activity was defined as the diuretic-sensitive radioactive tracer uptake and response to known modulators was assessed.

RESULTS

Basal NCC activity was significantly reduced in all NCC mutants and, excluding NCC-S186F, response to WNK3, WNK4, or intracellular chloride depletion was conserved. Similarly, basal activity was reduced in six out of nine NKCC2 mutants and response to WNK3 was maintained. No effect on protein expression was seen, except for NCC-S186F, which was significantly reduced.

CONCLUSIONS

The rare NCC or NKCC2 mutations found in the FHS significantly reduced the basal activity of the cotransporters. This observation supports that even a small, but chronic reduction of NCC or NKCC2 function results in a lower blood pressure and decreased risk of hypertension in otherwise healthy individuals in the general population.

SPAK and WNK kinases: a new target for blood pressure treatment?

PURPOSE OF REVIEW

The regulation of sodium reabsorption by the distal kidney is fundamental to blood pressure control. The clinical success of thiazide diuretics as antihypertensive drugs underscores the importance of its target, the thiazide-sensitive sodium/chloride cotransporter (NCC), in this process. However, thiazides are often ineffective as monotherapy and have significant side-effects. An understanding of NCC regulation at a molecular level may allow the design of better tolerated and more effective antihypertensive agents. The aim of this review is to provide an overview of the recent developments in the regulation of NCC, highlighting a potential new therapeutic target for the treatment of hypertension.

RECENT FINDINGS

It has been appreciated for several years that WNK kinases affect NCC expression, following the discovery that mutations in WNK genes cause Gordon syndrome (pseudohypoaldosteronism type II), although the precise molecular mechanisms were unclear. What has emerged from further in-vitro work is a WNK signalling cascade with the STE20 kinases SPAK and OSR1 as the 'missing' intermediate kinases that are activated by WNKs. Confirmation that this WNK-SPAK cascade operates in vivo comes from work on the phenotype of a kinase-dead SPAK knockin mouse. This mouse is markedly hypotensive, salt wasting, and almost all of its NCC protein in the distal kidney is dephosphorylated. Finally, a genome-wide association study has identified an intronic SPAK polymorphism that associates with human blood pressure.

SUMMARY

SPAK is a recently identified regulator of NCC and, hence, sodium reabsorption in the distal nephron. SPAK gene variants may also be important players in essential hypertension. If the phenotype of the kinase-dead SPAK mouse mimics pharmacological inhibition of this kinase, then SPAK is a potentially very interesting new antihypertensive drug target.

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.

Phenotypes of pseudohypoaldosteronism type II caused by the WNK4 D561A missense mutation are dependent on the WNK-OSR1/SPAK kinase cascade

We recently reported increased phosphorylation of the NaCl cotransporter (NCC) in Wnk4(D561A/+) knock-in mice, an ideal model of the human hereditary hypertensive disease pseudohypoaldosteronism type II (PHAII). Although previous in vitro studies had suggested the existence of a phosphorylation cascade involving the WNK, OSR1 and SPAK kinases, whether the WNK-OSR1/SPAK cascade is in fact fully responsible for NCC phosphorylation in vivo and whether the activation of this cascade is the sole mediator of PHAII remained to be determined. To clarify these issues, we mated the Wnk4(D561A/+) knock-in mice with Spak and Osr1 knock-in mice in which the T-loop threonine residues in SPAK and OSR1 (243 and 185, respectively) were mutated to alanine to prevent activation by WNK kinases. We found that NCC phosphorylation was almost completely abolished in Wnk4(D561A/+)Spak(T)(243A/T243A)Osr1(T185A/+) triple knock-in mice, clearly demonstrating that NCC phosphorylation in vivo is dependent on the WNK-OSR1/SPAK cascade. In addition, the high blood pressure, hyperkalemia and metabolic acidosis observed in Wnk4(D561A/+) mice were corrected in the triple knock-in mice. These results clearly establish that PHAII caused by the WNK4 D561A mutation is dependent on the activation of the WNK-OSR1/SPAK-NCC cascade and that the contribution of other mechanisms to PHAII (independent of the WNK-OSR1/SPAK cascade) could be minimal.

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.

The activity of the thiazide-sensitive Na(+)-Cl(-) cotransporter is regulated by protein phosphatase PP4

Cation transport in the distal mammalian nephron relies on the SLC12 family of membrane cotransporters that include the thiazide-sensitive Na(+)-Cl⁻ cotransporter (NCC). NCC is regulated through a scaffold of interacting proteins, including the WNK kinases, WNK 1 and WNK 4, which are mutated in the hypertensive Gordon's syndrome. Dynamic regulation of NCC function by kinases must involve dephosphorylation by phosphatases, as illustrated by the role of PP1 and PP2B in the regulation of KCC members of the SLC12 family. There are 2 phosphorylation-controlled regulatory pathways for NCC: type 1, mediated by WNK4 and affecting trafficking to the surface membrane, and type 2, affecting intrinsic transporter kinetics by phosphorylation of conserved N-terminal S/T amino acids. Using the Xenopus oocyte expression system, we show that PP4 inhibits NCC activity - but not trafficking to the surface membrane - by a mechanism that requires phosphatase activity and a conserved N-terminal amino acid of NCC, threonine 58. This action is distinct from WNK4 regulation of membrane trafficking. In the mouse kidney, PP4 is selectively expressed in the distal nephron, including cells of the distal convoluted tubule cells, suggesting that PP4 may have a physiological role in regulating NCC and hence NaCl reabsorption in vivo.

Hypertension, dietary salt intake, and the role of the thiazide-sensitive sodium chloride transporter NCCT

A high salt intake in industrialized countries is an important cardiovascular risk factor. It remains at typically twice the maximum recommended levels of 5-6 g/day, and halving this would have enormous public health benefit in preventing stroke and cardiovascular disease. Salt homeostasis can also be affected pharmacologically by diuretic drugs that target mechanisms within the distal kidney nephron to cause salt wasting. Indeed, thiazide-type diuretics are among the most widely used agents in the management of hypertension and work by blocking NCCT, the NaCl-transporter in the distal nephron. The biology of this membrane transporter was not previously well understood until the discovery of the molecular basis of a rare familial form of hypertension called Gordon syndrome (pseudohypoaldosteronism type 2, PHAII). This has established that the NCCT transporter is dynamically regulated in the kidney by WNK kinases and a signaling cascade using a second kinase called SPAK. Common polymorphisms in the SPAK gene have recently been shown to affect blood pressure in human cohorts and removing its function lowers blood pressure in mice. The SPAK-deficient mouse actually has a phenotype reminiscent of Gitelman syndrome. This suggests that specific inhibitors of SPAK kinase may provide a novel class of diuretic drugs to lower blood pressure through salt wasting. The expectation is that SPAK inhibitors would mimic the on-target effects of thiazides but not their adverse off-target effects. An antihypertensive drug that could lower blood pressure with the efficacy of a thiazide without producing metabolic side effects such as hyperuricaemia or impaired glucose tolerance is therapeutically very attractive. It also exemplifies how data coming from the rare monogenic hypertension syndromes can together with genome-wide association studies in hypertension deliver novel druggable targets.

γ-Adducin stimulates the thiazide-sensitive NaCl cotransporter

The thiazide-sensitive NaCl cotransporter (NCC) plays a key role in renal salt reabsorption and the determination of systemic BP, but the molecular mechanisms governing the regulation of NCC are not completely understood. Here, through pull-down experiments coupled to mass spectrometry, we found that γ-adducin interacts with the NCC transporter. γ-Adducin colocalized with NCC to the distal convoluted tubule. (22)Na(+) uptake experiments in the Xenopus laevis oocyte showed that γ-adducin stimulated NCC activity in a dose-dependent manner, an effect that occurred upstream from With No Lysine (WNK) 4 kinase. The binding site of γ-adducin mapped to the N terminus of NCC and encompassed three previously reported phosphorylation sites. Supporting this site of interaction, competition with the N-terminal domain of NCC abolished the stimulatory effect of γ-adducin on the transporter. γ-Adducin failed to increase NCC activity when these phosphorylation sites were constitutively inactive or active. In addition, γ-adducin bound only to the dephosphorylated N terminus of NCC. Taken together, our observations suggest that γ-adducin dynamically regulates NCC, likely by amending the phosphorylation state, and consequently the activity, of the transporter. These data suggest that γ-adducin may influence BP homeostasis by modulating renal NaCl transport.

Downregulation of NCC and NKCC2 cotransporters by kidney-specific WNK1 revealed by gene disruption and transgenic mouse models

WNK1 (with-no-lysine[K]-1) is a protein kinase of which mutations cause a familial hypertension and hyperkalemia syndrome known as pseudohypoaldosteronism type 2 (PHA2). Kidney-specific (KS) WNK1 is an alternatively spliced form of WNK1 kinase missing most of the kinase domain. KS-WNK1 downregulates the Na(+)-Cl(-) cotransporter NCC by antagonizing the effect of full-length WNK1 when expressed in Xenopus oocytes. The physiological role of KS-WNK1 in the regulation of NCC and potentially other Na(+) transporters in vivo is unknown. Here, we report that mice overexpressing KS-WNK1 in the kidney exhibited renal Na(+) wasting, elevated plasma levels of angiotensin II and aldosterone yet lower blood pressure relative to wild-type littermates. Immunofluorescent staining revealed reduced surface expression of total and phosphorylated NCC and the Na(+)-K(+)-2Cl(-) cotransporter NKCC2 in the distal convoluted tubule and the thick ascending limb of Henle's loop, respectively. Conversely, mice with targeted deletion of exon 4A (the first exon for KS-WNK1) exhibited Na(+) retention, elevated blood pressure on a high-Na(+) diet and increased surface expression of total and phosphorylated NCC and NKCC2 in respective nephron segments. Thus, KS-WNK1 is a negative regulator of NCC and NKCC2 in vivo and plays an important role in the control of Na(+) homeostasis and blood pressure. These results have important implications to the pathogenesis of PHA2 with WNK1 mutations.

The nanopeptide hormone vasopressin is a new player in the modulation of renal Na(+)-Cl(-) cotransporter activity

Vasopressin is a modulator of salt and water reabsorption, with known effects in the thick ascending limb and the collecting duct. Pedersen et al. present evidence that vasopressin administration increases the phosphorylation of the apical thiazide-sensitive Na(+)-Cl(-) cotransporter in the distal convoluted tubule. These effects appear to be independent of the renin-angiotensin system and to be mediated by the intracellular kinase SPAK. These observations expand the vasopressin-sensitive region of the nephron.

SPAK-knockout mice manifest Gitelman syndrome and impaired vasoconstriction

Polymorphisms in the gene encoding sterile 20/SPS1-related proline/alanine-rich kinase (SPAK) associate with hypertension susceptibility in humans. SPAK interacts with WNK kinases to regulate the Na(+)-K(+)-2Cl(-) and Na(+)-Cl(-) co-transporters [collectively, N(K)CC]. Mutations in WNK1/4 and N(K)CC can cause changes in BP and dyskalemia in humans, but the physiologic role of SPAK in vivo is unknown. We generated and analyzed SPAK-null mice by targeting disruption of exons 9 and 10 of SPAK. Compared with SPAK(+/+) littermates, SPAK(+/-) mice exhibited hypotension without significant electrolyte abnormalities, and SPAK(-/-) mice not only exhibited hypotension but also recapitulated Gitelman syndrome with hypokalemia, hypomagnesemia, and hypocalciuria. In the kidney tissues of SPAK(-/-) mice, the expression of total and phosphorylated (p-)NCC was markedly decreased, but that of p-OSR1, total NKCC2, and p-NKCC2 was significantly increased. We observed a blunted response to thiazide but normal response to furosemide in SPAK(-/-) mice. In aortic tissues, total NKCC1 expression was increased but p-NKCC1 was decreased in SPAK-deficient mice. Both SPAK(+/-) and SPAK(-/-) mice had impaired responses to the selective α(1)-adrenergic agonist phenylephrine and the NKCC1 inhibitor bumetanide, suggesting that impaired aortic contractility may contribute to the hypotension of SPAK-null mice. In summary, SPAK-null mice have defects of NCC in the kidneys and NKCC1 in the blood vessels, leading to hypotension through renal salt wasting and vasodilation. SPAK may be a promising target for antihypertensive therapy.

Pathophysiological roles of WNK kinases in the kidney

Since the discovery of mutations in the WNK1 and WNK4 genes in pseudohypoaldosteronism type II (PHAII), the pathophysiological role of WNK kinases in hypertension and renal ion transport has been a hot topic for investigation. Analyses from a mouse model carrying the same mutation as seen in PHAII patients, reveal a new signal cascade in the kidney that regulates NaCl and K balance in the body. WNK kinases phosphorylate and activate oxidative stress responsive kinase 1 (OSR1) and STE20-like proline and alanine-rich kinase (SPAK), and OSR1 and SPAK phosphorylate and activate the thiazide-sensitive Na-Cl cotransporter (NCC). Furthermore, this cascade is regulated by aldosterone, indicating that WNK-OSR1/SPAK-NCC cooperates with this system including the epithelial Na channel (ENaC) to conserve NaCl. With regard to K excretion, however, both systems work in opposite directions whereby PHAII and Liddle syndrome show hyperkalemia and hypokalemia, respectively. Thus, the identification of such aldosterone effecters other than ENaC, will reveal a novel regulatory mechanism of K excretion in the distal nephron, and also provides basic evidence for the therapeutic use of thiazide in various clinical situations.

Vasopressin induces phosphorylation of the thiazide-sensitive sodium chloride cotransporter in the distal convoluted tubule

The thiazide-sensitive Na(+)-Cl(-) cotransporter (NCC) is important for renal electrolyte balance and its phosphorylation causes an increase in its transport activity and cellular localization. Here, we generated phospho-specific antibodies against two conserved N-terminal phosphorylation sites (Thr53, Thr58 and Thr53/Thr58) to assess the role of arginine vasopressin (AVP) in regulating NCC in rodent kidney in vivo. Immunohistochemistry showed distinct staining of phosphorylated NCC (pNCC) at the apical plasma membrane domain of distal convoluted tubule (DCT) cells. Unlike total NCC, pNCC was localized only to the apical plasma membrane as determined by immunogold electron microscopy. In AVP-deficient Brattleboro rats, acute deamino-Cys-1, d-Arg-8 vasopressin (dDAVP) exposure significantly increased pNCC abundance at the apical plasma membrane by about threefold, whereas total NCC and its cellular distribution were not affected. dDAVP significantly increased the abundance of phosphorylated STE20/SPS1-related proline-alanine-rich kinase and oxidative stress-response kinase (SPAK and OSR1), kinases implicated in NCC phosphorylation. Intracellular calcium levels in early and late DCTs were increased in response to 1 min superfusion of dDAVP, confirming that these segments are AVP responsive. In rats fed a high-salt diet with angiotensin (ANG) type 1-receptor blockade, similar increases in pNCC and active SPAK and OSR1 were detected following chronic or acute dDAVP, thus indicating the effects of AVP are independent of ANGII. Our results show that AVP is a potent regulator of NCC activity.

Role of the WNK-activated SPAK kinase in regulating blood pressure

Mutations within the with-no-K(Lys) (WNK) kinases cause Gordon's syndrome characterized by hypertension and hyperkalaemia. WNK kinases phosphorylate and activate the STE20/SPS1-related proline/alanine-rich kinase (SPAK) protein kinase, which phosphorylates and stimulates the key Na⁺:Cl⁻ cotransporter (NCC) and Na⁺:K⁺:2Cl⁻ cotransporters (NKCC2) cotransporters that control salt reabsorption in the kidney. To define the importance of this pathway in regulating blood pressure, we generated knock-in mice in which SPAK cannot be activated by WNKs. The SPAK knock-in animals are viable, but display significantly reduced blood pressure that was salt-dependent. These animals also have markedly reduced phosphorylation of NCC and NKCC2 cotransporters at the residues phosphorylated by SPAK. This was also accompanied by a reduction in the expression of NCC and NKCC2 protein without changes in messenger RNA (mRNA) levels. On a normal Na⁺-diet, the SPAK knock-in mice were normokalaemic, but developed mild hypokalaemia when the renin–angiotensin system was activated by a low Na⁺-diet. These observations establish that SPAK plays an important role in controlling blood pressure in mammals. Our results imply that SPAK inhibitors would be effective at reducing blood pressure by lowering phosphorylation as well as expression of NCC and NKCC2. See accompanying Closeup by Maria Castañeda-Bueno and Gerald Gamba (DOI 10.1002/emmm.200900059).

Effect of angiotensin II on the WNK-OSR1/SPAK-NCC phosphorylation cascade in cultured mpkDCT cells and in vivo mouse kidney

In our recent study using Wnk4(D561A/+) knockin mice, we determined that the WNK-OSR1/SPAK-NaCl cotransporter (NCC) phosphorylation cascade is important for regulating NCC function in vivo. Phosphorylation of NCC was necessary for its plasma membrane localization. Previously, angiotensin II infusion was shown to increase apical membrane expression of NCC in rats. Therefore, we investigated whether angiotensin II was an upstream regulator for the WNK-OSR1/SPAK-NCC cascade in cultured cells and in vivo kidney. In mpkDCT cells, the phosphorylation of OSR1 and NCC was increased 30 min after the addition of angiotensin II (10(-9)-10(-7)M) but returned to baseline after 18 h. In mice, a 5-min infusion of angiotensin II (5 ng/g/min) increased NCC phosphorylation in the kidney at 30 min and 2h after the injection but returned to baseline 24h later. This increase was inhibited by angiotensin II receptor blocker (valsartan) but not by aldosterone receptor blocker (eplerenone). Ten-day infusions of angiotensin II (720 ng/day) also increased phosphorylation of OSR1 and NCC in the mouse kidney, and both valsartan and eplerenone inhibited the increased phosphorylation. Although angiotensin II is identified as an upstream regulator for the WNK-OSR1/SPAK-NCC cascade in vivo, aldosterone appears to be the major regulator of this signal cascade in the long-term regulation by angiotensin II.

Short-term stimulation of the thiazide-sensitive Na+-Cl- cotransporter by vasopressin involves phosphorylation and membrane translocation

Vasopressin influences salt and water transport in renal epithelia. This is coordinated by the combined action of V2 receptor-mediated effects along distinct nephron segments. Modulation of NaCl reabsorption by vasopressin has been established in the loop of Henle, but its role in the distal convoluted tubule (DCT), an effective site for fine regulation of urinary electrolyte composition and the target for thiazide diuretics, is largely unknown. The Na+-Cl- cotransporter (NCC) of DCT is activated by luminal trafficking and phosphorylation at conserved NH2-terminal residues. Here, we demonstrate the effects of short-term vasopressin administration (30 min) on NCC activation in Brattleboro rats with central diabetes insipidus (DI) using the V2 receptor agonist desmopressin (dDAVP). The fraction of NCC abundance in the luminal plasma membrane was significantly increased upon dDAVP as shown by confocal microscopy, immunogold cytochemistry, and Western blot, suggesting increased apical trafficking of the transporter. Changes were paralleled by augmented phosphorylation of NCC as detected by antibodies against phospho-threonine and phospho-serine residues (2.5-fold increase at Thr53 and 1.4-fold increase at Ser71). dDAVP-induced phosphorylation of NCC, studied in tubular suspensions in the absence of systemic effects, was enhanced as well (1.7-fold increase at Ser71), which points to the direct mode of action of vasopressin in DCT. Changes were more pronounced in early (DCT1) than in late DCT as distinguished by the distribution of 11beta-hydroxysteroid dehydrogenase 2 in DCT2. These results suggest that the vasopressin-V(2) receptor-NCC signaling cascade is a novel effector system to adjust transepithelial NaCl reabsorption in DCT.

WNK4 enhances the degradation of NCC through a sortilin-mediated lysosomal pathway

WNK kinase is a serine/threonine kinase that plays an important role in electrolyte homeostasis. WNK4 significantly inhibits the surface expression of the sodium chloride co-transporter (NCC) by enhancing the degradation of NCC through a lysosomal pathway, but the mechanisms underlying this trafficking are unknown. Here, we investigated the effect of the lysosomal targeting receptor sortilin on NCC expression and degradation. In Cos-7 cells, we observed that the presence of WNK4 reduced the steady-state amount of NCC by approximately half. Co-transfection with truncated sortilin (a dominant negative mutant) prevented this WNK4-induced reduction in NCC. NCC immunoprecipitated with both wild-type sortilin and, to a lesser extent, truncated sortilin. Immunostaining revealed that WNK4 increased the co-localization of NCC with the lysosomal marker cathepsin D, and NCC co-localized with wild-type sortilin, truncated sortilin, and WNK4 in the perinuclear region. These findings suggest that WNK4 promotes NCC targeting to the lysosome for degradation via a mechanism involving sortilin.

Molecular physiology of the thiazide-sensitive sodium-chloride cotransporter

PURPOSE OF REVIEW

This review summarizes recent advances in the understanding of the molecular physiology and regulation of the thiazide-sensitive sodium-chloride cotransporter (NCC).

RECENT FINDINGS

Mutations of with-no-lysine (WNK) kinases 1 and 4 result in hyperactivity of NCC and familial hyperkalemic hypertension, a genetic syndrome of hypertension. Recent studies have shown that WNK1 and WNK4 activate the STE20 family protein kinases Ste20-related proline/alanine-rich kinase and odd-skipped-related 1, resulting in phosphorylation and activation of NCC. Additionally, a mouse knock-in model for a WNK4 familial hyperkalemic hypertension mutant demonstrated increased Ste20-related proline/alanine-rich kinase/odd-skipped-related 1 and NCC phosphorylation. It is unclear how these studies fit with the data indicating that WNK4 inhibits NCC, and the familial hyperkalemic hypertension mutations of WNK4 are loss-of-function mutations. Another WNK kinase, WNK3, also regulates NCC, activating NCC and antagonizing the effect of WNK4. Extracellular signal-related kinase 1/2 mitogen-activated protein kinase activation by Ras guanyl nucleotide-releasing protein 1 is another kinase pathway that appears to be a potent regulator of NCC. Other studies have described a role for angiotensin II in pressure natriuresis via actions on NCC. Recent studies examining the hormonal regulation of NCC have implicated angiotensin II and aldosterone in regulation of the WNK4-Ste20-related proline/alanine-rich kinase-NCC pathway.

SUMMARY

NCC is subject to a complex regulatory network of kinases, which appear quite sensitive to alterations of the hormonal and physiologic milieu.

Targeted disruption of the Wnk4 gene decreases phosphorylation of Na-Cl cotransporter, increases Na excretion and lowers blood pressure

We recently generated Wnk4(D561A/+) knockin mice and found that a major pathogenesis of pseudohypoaldosteronism type II was the activation of the OSR1/SPAK kinase-NaCl cotransporter (NCC) phosphorylation cascade by the mutant WNK4. However, the physiological roles of wild-type WNK4 on the regulation of Na excretion and blood pressure, and whether wild-type WNK4 functions positively or negatively in this cascade, remained to be determined. In the present study, we generated WNK4 hypomorphic mice by deleting exon 7 of the Wnk4 gene. These mice did not show hypokalemia and metabolic alkalosis, but they did exhibit low blood pressure and increased Na and K excretion under low-salt diet. Phosphorylation of OSR1/SPAK and NCC was significantly reduced in the mutant mice as compared with their wild-type littermates. Protein levels of ROMK and Maxi K were not changed, but epithelial Na channel appeared to be activated as a compensatory mechanism for the reduced NCC function. Thus, wild-type WNK4 is a positive regulator for the WNK-OSR1/SPAK-NCC cascade, and WNK4 is a potential target of anti-hypertensive drugs.

Expression and phosphorylation of the Na+-Cl- cotransporter NCC in vivo is regulated by dietary salt, potassium, and SGK1

The Na-Cl cotransporter NCC is expressed in the distal convoluted tubule, activated by phosphorylation, and has been implicated in renal NaCl and K(+) homeostasis. The serum and glucocorticoid inducible kinase 1 (SGK1) contributes to renal NaCl retention and K(+) excretion, at least in part, by stimulating the epithelial Na(+) channel and Na(+)-K(+)-ATPase in the downstream segments of aldosterone-sensitive Na(+)/K(+) exchange. In this study we confirmed in wild-type mice (WT) that dietary NaCl restriction increases renal NCC expression and its phosphorylation at Thr(53), Thr(58), and Ser(71), respectively. This response, however, was attenuated in mice lacking SGK1 (Sgk1(-/-)), which may contribute to impaired NaCl retention in those mice. Total renal NCC expression and phosphorylation at Thr(53), Thr(58), and Ser(71) in WT were greater under low- compared with high-K(+) diet. This finding is consistent with a regulation of NCC to modulate Na(+) delivery to downstream segments of Na(+)/K(+) exchange, thereby modulating K(+) excretion. Dietary K(+)-dependent variation in renal expression of total NCC and phosphorylated NCC were not attenuated in Sgk1(-/-) mice. In fact, high-K(+) diet-induced NCC suppression was enhanced in Sgk1(-/-) mice. The hyperkalemia induced in Sgk1(-/-) mice by a high-K(+) diet may have augmented NCC suppression, thereby increasing Na(+) delivery and facilitating K(+) excretion in downstream segments of impaired Na(+)/K(+) exchange. In summary, changes in NaCl and K(+) intake altered NCC expression and phosphorylation, an observation consistent with a role of NCC in NaCl and K(+) homeostasis. The two maneuvers dissociated plasma aldosterone levels from NCC expression and phosphorylation, implicating additional regulators. Regulation of NCC expression and phosphorylation by dietary NaCl restriction appears to involve SGK1.

The thiazide-sensitive Na+-Cl- cotransporter: molecular biology, functional properties, and regulation by WNKs

The thiazide-sensitive Na+-Cl(-) cotransporter is the major salt reabsorption pathway in the distal convoluted tubule, which is located just after the macula densa at the beginning of the aldosterone-sensitive nephron. This cotransporter was identified at the molecular level in the early 1990s by the pioneering work of Steven C. Hebert and coworkers, opening the molecular area, not only for the Na+-Cl(-) cotransporter but also for the family of electroneutral cation-coupled chloride cotransporters that includes the loop diuretic-sensitive Na+-K+-2Cl(-) cotransporter of the thick ascending limb of Henle's loop. This work honoring the memory of Steve Hebert presents a brief review of our current knowledge about salt and water homeostasis generated as a consequence of cloning the cotransporter, with particular emphasis on the molecular biology, physiological properties, human disease due to decreased or increased activity of the cotransporter, and regulation of the cotransporter by a family of serine/threonine kinases known as WNK. Thus one of the legacies of Steve Hebert is a better understanding of salt and water homeostasis.

Renal and brain isoforms of WNK3 have opposite effects on NCCT expression

Mutations in the WNK kinases WNK1 and WNK4 cause a rare familial form of hypertension (Gordon syndrome) by increasing expression of the thiazide-sensitive co-transporter NCCT in the kidney. Regulation of NCCT expression involves a scaffold of proteins composed of several kinases, including the third member of the WNK kinase family, WNK3. This protein, expressed in several tissues including kidney and brain, displays splice variation around exons 18 and 22. We expressed these proteins in Xenopus oocytes and found that the renal isoform of WNK3 increased but the brain isoform decreased NCCT expression and activity. Introduction of a kinase-inactivating mutation into renal WNK3 reversed its action on NCCT, and the same mutation in the brain isoforms led to loss of function. We also studied the effect of phosphorylation of a key NCCT threonine (T58) on the effects of WNK3/4 coexpression; NCCT mutants with a T58A or T58D substitution had the same surface expression as T58 but had significantly altered transporter activity; however, both isoforms of WNK3 as well as WNK4 still modulated expression of these NCCT mutants. Finally, experiments using kinase-dead STE20/SPS1-related proline/alanine-rich kinase (SPAK), a putative downstream target for WNKs, revealed that brain WNK3 acts in tandem with SPAK, whereas renal WNK3 seems to upregulate NCCT through a SPAK-independent pathway. Taken together, these results suggest that the C-terminal motifs contributed by exons 18 and 22 play an important role in the actions of WNK3 isoforms on NCCT.

Angiotensin II signaling increases activity of the renal Na-Cl cotransporter through a WNK4-SPAK-dependent pathway

Mutations in the kinase WNK4 cause pseudohypoaldosteronism type II (PHAII), a syndrome featuring hypertension and high serum K(+) levels (hyperkalemia). WNK4 has distinct functional states that regulate the balance between renal salt reabsorption and K(+) secretion by modulating the activities of renal transporters and channels, including the Na-Cl cotransporter NCC and the K(+) channel ROMK. WNK4's functions could enable differential responses to intravascular volume depletion (hypovolemia) and hyperkalemia. Because hypovolemia is uniquely associated with high angiotensin II (AngII) levels, AngII signaling might modulate WNK4 activity. We show that AngII signaling in Xenopus oocytes increases NCC activity by abrogating WNK4's inhibition of NCC but does not alter WNK4's inhibition of ROMK. This effect requires AngII, its receptor AT1R, and WNK4, and is prevented by the AT1R inhibitor losartan. NCC activity is also increased by WNK4 harboring mutations found in PHAII, and this activity cannot be further augmented by AngII signaling, consistent with PHAII mutations providing constitutive activation of the signaling pathway between AT1R and NCC. AngII's effect on NCC is also dependent on the kinase SPAK because dominant-negative SPAK or elimination of the SPAK binding motif in NCC prevent activation of NCC by AngII signaling. These effects extend to mammalian cells. AngII increases phosphorylation of specific sites on SPAK and NCC that are necessary for activation of each in mpkDCT cells. These findings place WNK4 in the signaling pathway between AngII and NCC, and provide a mechanism by which hypovolemia maximizes renal salt reabsoprtion without concomitantly increasing K(+) secretion.

Regulation of the Na+-Cl- cotransporter by dietary NaCl: a role for WNKs, SPAK, OSR1, and aldosterone

This Commentary aims to integrate or interrelate the available data with the current study by Chiga and co-workers, which defines an important influence of aldosterone in the phosphorylation and thus activation of the Na(+)-Cl(-) cotransporter (NCC) in response to changes in NaCl intake and implicates the involvement of SPAK/OSR1 kinases and WNKs.

The regulation of salt transport and blood pressure by the WNK-SPAK/OSR1 signalling pathway

It has recently been shown that the WNK [with-no-K(Lys)] kinases (WNK1, WNK2, WNK3 and WNK4) have vital roles in the control of salt homeostasis and blood pressure. This Commentary focuses on recent findings that have uncovered the backbone of a novel signal-transduction network that is controlled by WNK kinases. Under hyperosmotic or hypotonic low-Cl– conditions, WNK isoforms are activated, and subsequently phosphorylate and activate the related protein kinases SPAK and OSR1. SPAK and OSR1 phosphorylate and activate ion co-transporters that include NCC, NKCC1 and NKCC2, which are targets for the commonly used blood-pressure-lowering thiazide-diuretic and loop-diuretic drugs. The finding that mutations in WNK1, WNK4, NCC and NKCC2 cause inherited blood-pressure syndromes in humans highlights the importance of these enzymes. We argue that these new findings indicate that SPAK and OSR1 are promising drug targets for the treatment of hypertension, because inhibiting these enzymes would reduce NCC and NKCC2 activity and thereby suppress renal salt re-absorption. We also discuss unresolved and controversial questions in this field of research.

WNK3 and WNK4 amino-terminal domain defines their effect on the renal Na+-Cl- cotransporter

Loss of physiological regulation of the renal thiazide-sensitive Na+-Cl- cotransporter (NCC) by mutant WNK1 or WNK4 results in pseudohypoaldosteronism type II (PHAII) characterized by arterial hypertension and hyperkalemia. WNK4 normally inhibits NCC, but this effect is lost by eliminating WNK4 catalytic activity or through PHAII-type mutations. In contrast, another member of the WNK family, WNK3, activates NCC. The positive effect of WNK3 on NCC also requires its catalytic activity. Because the opposite effects of WNK3 and WNK4 on NCC were observed in the same expression system, sequences within the WNKs should endow these kinases with their activating or inhibiting properties. To gain insight into the structure-function relationships between the WNKs and NCC, we used a chimera approach between WNK3 and WNK4 to elucidate the domain of the WNKs responsible for the effects on NCC. Chimeras were constructed by swapping the amino or carboxyl terminus domains, which flank the central kinase domain, between WNK3 and WNK4. Our results show that the effect of chimeras toward NCC follows the amino-terminal domain. Thus the amino terminus of the WNKs contains the sequences that are required for their activating or inhibiting properties on NCC.

WNK kinases, renal ion transport and hypertension

Two members of a recently discovered family of protein kinases are the cause of an inherited disease known as pseudohypoaldosteronism type II (PHAII). These patients exhibit arterial hypertension together with hyperkalemia and metabolic acidosis. This is a mirror image of Gitelman disease that is due to inactivating mutations of the SLC12A3 gene that encodes the thiazide-sensitive Na(+):Cl(-) cotransporter. The uncovered genes causing PHAII encode for serine/threonine kinases known as WNK1 and WNK4. Physiological and biochemical studies have revealed that WNK1 and WNK4 modulate the activity of several transport pathways of the aldosterone-sensitive distal nephron, thus increasing our understanding of how diverse renal ion transport proteins are coordinated to regulate normal blood pressure levels. Observations discussed in the present work place WNK1 and WNK4 as genes involved in the genesis of essential hypertension and as potential targets for the development of antihypertensive drugs.

Regulation of NKCC2 by a chloride-sensing mechanism involving the WNK3 and SPAK kinases

The Na(+):K(+):2Cl(-) cotransporter (NKCC2) is the target of loop diuretics and is mutated in Bartter's syndrome, a heterogeneous autosomal recessive disease that impairs salt reabsorption in the kidney's thick ascending limb (TAL). Despite the importance of this cation/chloride cotransporter (CCC), the mechanisms that underlie its regulation are largely unknown. Here, we show that intracellular chloride depletion in Xenopus laevis oocytes, achieved by either coexpression of the K-Cl cotransporter KCC2 or low-chloride hypotonic stress, activates NKCC2 by promoting the phosphorylation of three highly conserved threonines (96, 101, and 111) in the amino terminus. Elimination of these residues renders NKCC2 unresponsive to reductions of [Cl(-)](i). The chloride-sensitive activation of NKCC2 requires the interaction of two serine-threonine kinases, WNK3 (related to WNK1 and WNK4, genes mutated in a Mendelian form of hypertension) and SPAK (a Ste20-type kinase known to interact with and phosphorylate other CCCs). WNK3 is positioned upstream of SPAK and appears to be the chloride-sensitive kinase. Elimination of WNK3's unique SPAK-binding motif prevents its activation of NKCC2, as does the mutation of threonines 96, 101, and 111. A catalytically inactive WNK3 mutant also completely prevents NKCC2 activation by intracellular chloride depletion. Together these data reveal a chloride-sensing mechanism that regulates NKCC2 and provide insight into how increases in the level of intracellular chloride in TAL cells, as seen in certain pathological states, could drastically impair renal salt reabsorption.

Cotransporters, WNKs and hypertension: an update

PURPOSE OF REVIEW

Studies of inherited conditions characterized by high or low blood pressure reveal the importance of a new signalling cascade, With no Lysine kinases (WNK) --> ste20/SPS1-related proline/alanine-rich kinase (SPAK)/oxidative stress-responsive kinase-1 (OSR1) --> Cation-Chloride Cotransporters (CCC), in regulating blood pressure and in the pathogenesis of essential hypertension. This review explores how these molecules interact to co-ordinate sodium homeostasis and how errors in these interactions may result in hypertension.

RECENT FINDINGS

Studies using transgenic animals and gene knockins have clarified the role of mutant WNK4 in hypertension, by revealing its main action to be increasing the expression and activity of sodium-chloride cotransporter (NCC) in the kidney. Functional studies show how phosphorylation of WNK1 regulates both its activity and ability to interact with SPAK/OSR1, and clearly place it upstream of SPAK/OSR1 in the cascade. The structural basis for the interactions between SPAK/OSR1 and targets has been identified.

SUMMARY

WNKs, activated by upstream kinases or autophosphorylation, bind and phosphorylate SPAK/OSR1, which in turn phosphorylate and activate NCCs and Na-K-Cl cotransporters (NKCCs). This increases sodium retention in the kidney (NKCC2, NCC) and vascular resistance (NKCC1), but decreases renin release (NKCC1). Hypertension-associated mutant WNKs increase surface expression and activation of renal tubular NKCC2 and NCC. Whether this adequately explains the hypertension awaits studies of these mutants in other tissues.

[Expression and regulation of renal sodium-cotransporters and -antiporters, and related-transport proteins]

The authors' researches have been focused on pathogenic, physiological and biochemical mechanisms in hypertension and diabetes. Studies on hypertension were performed using salt-sensitive hypertensive Dahl rats as compared with the corresponding normotensive rats. Especially, implication with mobilization of electrolytes such as sodium, potassium, calcium and magnesium in hypertension gave rise to provocative to the author. Furthermore, complications of diabetes with hypertension were themes for the authors' researches. Thus, sodium-dependent glucose transport has been studied on sodium-dependent glucose transporters such as SGLT1 and SGLT2 using cell lines of porcelain renal cell, LLC-PK(1), and murine renal cell, NRK-52E. Relationship between magnesium mobilization and NO in hypertension has been explored using renal epithelial cell-lines and salt-sensitive hypertensive Dahl rats in the latter half of the author's research life.

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.

Activation of the thiazide-sensitive Na+-Cl- cotransporter by the WNK-regulated kinases SPAK and OSR1

Mutations increasing WNK1 kinase expression in humans cause the pseudohypoaldosteronism type II hypertension syndrome. This condition is treated effectively by thiazide diuretics, which exert their effects by inhibiting the Na+-Cl(-) cotransporter (NCC), suggesting a link between WNK1 and NCC. Here, we demonstrate that the SPAK and OSR1 kinases that are activated by WNK1 phosphorylate human NCC at three conserved residues (Thr46, Thr55 and Thr60). Activation of the WNK1-SPAK/OSR1 signalling pathway by treatment of HEK293 or mpkDCT kidney distal-convoluted-tubule-derived cells with hypotonic low-chloride conditions induced phosphorylation of NCC at residues phosphorylated by SPAK/OSR1. Efficient phosphorylation of NCC was dependent upon a docking interaction between an RFXI motif in NCC and SPAK/OSR1. Mutation of Thr60 to Ala in NCC markedly inhibited phosphorylation of Thr46 and Thr55 as well as NCC activation induced by hypotonic low-chloride treatment of HEK293 cells. Our results establish that the WNK1-SPAK/OSR1 signalling pathway plays a key role in controlling the phosphorylation and activity of NCC. They also suggest a mechanism by which increased WNK1 overexpression could lead to hypertension and that inhibitors of SPAK/OSR1 might be of use in reducing blood pressure by suppressing phosphorylation and hence activity of NCC.

Reduced urinary excretion of thiazide-sensitive Na-Cl cotransporter in Gitelman syndrome: preliminary data

BACKGROUND

The relationship between SLC12A3 mutations and actual sodium-chloride (Na-Cl) cotransporter (NCC) expression in patients with Gitelman syndrome (GS) was rarely evaluated. Detection of urinary thiazide-sensitive NCC was not tried in patients with GS.

STUDY DESIGN

Case series.

SETTING & PARTICIPANTS

6 patients with GS and 1 patient with surreptitious vomiting.

OUTCOMES & MEASUREMENTS

Renal clearance study, mutation analysis using reverse-transcription polymerase chain reaction and direct sequencing for the SLC12A3 gene, and immunohistochemical staining for NCC, Na-K-2Cl-cotransporter, alpha1-subunit of Na(+),K(+)-ATPase, and calbindin-D(28K) of the renal biopsy specimens were performed. Membrane fractions of urine were obtained by using differential centrifugation and probed with antibodies against human NCC and aquaporin 2.

RESULTS

Results of clearance studies were consistent with GS, showing decreased distal fractional chloride reabsorption with only furosemide. SLC12A3 gene mutations were found in all patients with GS. Immunohistochemistry showed markedly decreased NCC expression in the distal convoluted tubule, whereas expression of other transporters remained intact. Urinary NCC excretion was markedly decreased in patients with GS, but not in the patient with surreptitious vomiting.

LIMITATIONS

Small number of patients and lack of mutation analysis of CLCNKB.

CONCLUSIONS

There were no relations between NCC expression and types of mutations. Detection of urinary NCC might be helpful for the differential diagnosis of GS.

WNK4-mediated regulation of renal ion transport proteins

Point mutations in WNK4 [for With No K (lysine)], a serine-threonine kinase that is expressed in the distal nephron of the kidney, are linked to familial hyperkalemic hypertension (FHH). The imbalanced electrolyte homeostasis in FHH has led to studies toward an understanding of WNK4-mediated regulation of ion transport proteins in the kidney. A growing number of ion transport proteins for Na(+), K(+), Ca(2+), and Cl(-), including ion channels and transporters in the transcellular pathway and claudins in the paracellular pathway, are shown to be regulated by WNK4 from studies using models ranging from Xenopus laevis oocytes to transgenic and knockin mice. WNK4 regulates these transport proteins in different directions and by different cellular mechanisms. The common theme of WNK4-mediated regulation is to alter the abundance of ion transport proteins at the plasma membrane, with the exception of claudins, which are phosphorylated in the presence of WNK4. The regulation of WNK4 can be blocked by the full-length WNK1, whose action is in turn antagonized by a kidney-specific WNK1 variant lacking the kinase domain. In addition, WNK4 also activates stress-related serine-threonine kinases to regulate members of the SLC12 family members of cation-chloride cotransporters. In many cases, the FHH-causing mutants of WNK4 exhibit differences from wild-type WNK4 in regulating ion transport proteins. These regulations well explain the clinical features of FHH and provide insights into the multilayered regulation of ion transport processes in the distal nephron.

Molecular pathogenesis of pseudohypoaldosteronism type II: generation and analysis of a Wnk4(D561A/+) knockin mouse model

WNK1 and WNK4 mutations have been reported to cause pseudohypoaldosteronism type II (PHAII), an autosomal-dominant disorder characterized by hyperkalemia and hypertension. To elucidate the molecular pathophysiology of PHAII, we generated Wnk4(D561A/+) knockin mice presenting the phenotypes of PHAII. The knockin mice showed increased apical expression of phosphorylated Na-Cl cotransporter (NCC) in the distal convoluted tubules. Increased phosphorylation of the kinases OSR1 and SPAK was also observed in the knockin mice. Apical localization of the ROMK potassium channel and transepithelial Cl(-) permeability in the cortical collecting ducts were not affected in the knockin mice, whereas activity of epithelial Na(+) channels (ENaC) was increased. This increase, however, was not evident after hydrochlorothiazide treatment, suggesting that the regulation of ENaC was not a genetic but a secondary effect. Thus, the pathogenesis of PHAII caused by a missense mutation of WNK4 was identified to be increased function of NCC through activation of the OSR1/SPAK-NCC phosphorylation cascade.

Regulation of activity and localization of the WNK1 protein kinase by hyperosmotic stress

Mutations within the WNK1 (with-no-K[Lys] kinase-1) gene cause Gordon's hypertension syndrome. Little is known about how WNK1 is regulated. We demonstrate that WNK1 is rapidly activated and phosphorylated at multiple residues after exposure of cells to hyperosmotic conditions and that activation is mediated by the phosphorylation of its T-loop Ser382 residue, possibly triggered by a transautophosphorylation reaction. Activation of WNK1 coincides with the phosphorylation and activation of two WNK1 substrates, namely, the protein kinases STE20/SPS1-related proline alanine–rich kinase (SPAK) and oxidative stress response kinase-1 (OSR1). Small interfering RNA depletion of WNK1 impairs SPAK/OSR1 activity and phosphorylation of residues targeted by WNK1. Hyperosmotic stress induces rapid redistribution of WNK1 from the cytosol to vesicular structures that may comprise trans-Golgi network (TGN)/recycling endosomes, as they display rapid movement, colocalize with clathrin, adaptor protein complex 1 (AP-1), and TGN46, but not the AP-2 plasma membrane–coated pit marker nor the endosomal markers EEA1, Hrs, and LAMP1. Mutational analysis suggests that the WNK1 C-terminal noncatalytic domain mediates vesicle localization. Our observations shed light on the mechanism by which WNK1 is regulated by hyperosmotic stress.