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

Familial Hyperkalemic Hypertension

The rare disease Familial Hyperkalemic Hypertension (FHHt) is caused by mutations in the genes encoding Cullin 3 (CUL3), Kelch-Like 3 (KLHL3), and two members of the With-No-Lysine [K] (WNK) kinase family, WNK1 and WNK4. In the kidney, these mutations ultimately cause hyperactivation of NCC along the renal distal convoluted tubule. Hypertension results from increased NaCl retention, and hyperkalemia by impaired K + secretion by downstream nephron segments. CUL3 and KLHL3 are now known to form a ubiquitin ligase complex that promotes proteasomal degradation of WNK kinases, which activate downstream kinases that phosphorylate and thus activate NCC. For CUL3, potent effects on the vasculature that contribute to the more severe hypertensive phenotype have also been identified. Here we outline the in vitro and in vivo studies that led to the discovery of the molecular pathways regulating NCC and vascular tone, and how FHHt-causing mutations disrupt these pathways. Potential mechanisms for variability in disease severity related to differential effects of each mutation on the kidney and vasculature are described, and other possible effects of the mutant proteins beyond the kidney and vasculature are explored. © 2024 American Physiological Society. Compr Physiol 14:5839-5874, 2024.

Renal upregulation of NCC counteracts empagliflozin-mediated NHE3 inhibition in normotensive but not in hypertensive male rat

Sodium-glucose cotransporter-2 inhibitors (SGLT2i) reduce blood pressure (BP) in patients with hypertension, yet the precise molecular mechanisms remain elusive. SGLT2i inhibits proximal tubule (PT) NHE3-mediated sodium reabsorption in normotensive rodents, yet no hypotensive effect is observed under this scenario. This study examined the effect of empagliflozin (EMPA) on renal tubular sodium transport in normotensive and spontaneously hypertensive rats (SHRs). It also tested the hypothesis that EMPA-mediated PT NHE3 inhibition in normotensive rats is associated with upregulation of distal nephron apical sodium transporters. EMPA administration for 14 days reduced BP in 12-wk-old SHRs but not in age-matched Wistar rats. PT NHE3 activity was inhibited by EMPA treatment in both Wistar and SHRs. In Wistar rats, EMPA increased NCC activity, mRNA expression, protein abundance, and phosphorylation levels, but not in SHRs. SHRs showed higher NKCC2 activity and an abundance of cleaved ENaC α and γ subunits compared with Wistar rats, none of which were affected by EMPA. Another set of male Wistar rats was treated with EMPA, the NCC inhibitor hydrochlorothiazide (HCTZ), and EMPA combined with HCTZ or vehicle for 14 days. In these rats, BP reduction was observed only with combined EMPA and HCTZ treatment, not with either drug alone. These findings suggest that NCC upregulation counteracts EMPA-mediated inhibition of PT NHE3 in male normotensive rats, maintaining their baseline BP. Moreover, the reduction of NHE3 activity without further upregulation of major apical sodium transporters beyond the PT may contribute to the BP-lowering effect of SGLT2i in experimental models and patients with hypertension.NEW & NOTEWORTHY This study suggests that reduced NHE3-mediated sodium reabsorption in the renal proximal tubule may account, at least in part, for the BP-lowering effect of SGLT2 inhibitors in the setting of hypertension. It also demonstrates that chronic treatment with SGLT2 inhibitors upregulates NCC activity, phosphorylation, and expression in the distal tubule of normotensive but not hypertensive rats. SGLT2 inhibitor-mediated upregulation of NCC seems crucial to counteract proximal tubule natriuresis in subjects with normal BP.

Dysregulation of the WNK4-SPAK/OSR1 pathway has a minor effect on baseline NKCC2 phosphorylation

The with-no-lysine kinase 4 (WNK4)-sterile 20/SPS-1-related proline/alanine-rich kinase (SPAK)/oxidative stress-responsive kinase 1 (OSR1) pathway mediates activating phosphorylation of the furosemide-sensitive Na+-K+-2Cl- cotransporter (NKCC2) and the thiazide-sensitive NaCl cotransporter (NCC). The commonly used pT96/pT101-pNKCC2 antibody cross-reacts with pT53-NCC in mice on the C57BL/6 background due to a five amino acid deletion. We generated a new C57BL/6-specific pNKCC2 antibody (anti-pT96-NKCC2) and tested the hypothesis that the WNK4-SPAK/OSR1 pathway strongly regulates the phosphorylation of NCC but not NKCC2. In C57BL/6 mice, anti-pT96-NKCC2 detected pNKCC2 and did not cross-react with NCC. Abundances of pT96-NKCC2 and pT53-NCC were evaluated in Wnk4-/-, Osr1-/-, Spak-/-, and Osr1-/-/Spak-/- mice and in several models of the disease familial hyperkalemic hypertension (FHHt) in which the CUL3-KLHL3 ubiquitin ligase complex that promotes WNK4 degradation is dysregulated (Cul3+/-/Δ9, Klhl3-/-, and Klhl3R528H/R528H). All mice were on the C57BL/6 background. In Wnk4-/- mice, pT53-NCC was almost absent but pT96-NKCC2 was only slightly lower. pT53-NCC was almost absent in Spak-/- and Osr1-/-/Spak-/- mice, but pT96-NKCC2 abundance did not differ from controls. pT96-NKCC2/total NKCC2 was slightly lower in Osr1-/- and Osr1-/-/Spak-/- mice. WNK4 expression colocalized not only with NCC but also with NKCC2 in Klhl3-/- mice, but pT96-NKCC2 abundance was unchanged. Consistent with this, furosemide-induced urinary Na+ excretion following thiazide treatment was similar between Klhl3-/- and controls. pT96-NKCC2 abundance was also unchanged in the other FHHt mouse models. Our data show that disruption of the WNK4-SPAK/OSR1 pathway only mildly affects NKCC2 phosphorylation, suggesting a role for other kinases in NKCC2 activation. In FHHt models NKCC2 phosphorylation is unchanged despite higher WNK4 abundance, explaining the thiazide sensitivity of FHHt.NEW & NOTEWORTHY The renal cation cotransporters NCC and NKCC2 are activated following phosphorylation mediated by the WNK4-SPAK/OSR1 pathway. While disruption of this pathway strongly affects NCC activity, effects on NKCC2 activity are unclear since the commonly used phospho-NKCC2 antibody was recently reported to cross-react with phospho-NCC in mice on the C57BL/6 background. Using a new phospho-NKCC2 antibody specific for C57BL/6, we show that inhibition or activation of the WNK4-SPAK/OSR1 pathway in mice only mildly affects NKCC2 phosphorylation.

Potassium Effects on NCC Are Attenuated during Inhibition of Cullin E3-Ubiquitin Ligases

The thiazide-sensitive sodium chloride cotransporter (NCC) plays a vital role in maintaining sodium (Na⁺) and potassium (K⁺) homeostasis. NCC activity is modulated by with-no-lysine kinases 1 and 4 (WNK1 and WNK4), the abundance of which is controlled by the RING-type E3 ligase Cullin 3 (Cul3) and its substrate adapter Kelch-like protein 3. Dietary K⁺ intake has an inverse correlation with NCC activity, but the mechanism underlying this phenomenon remains to be fully elucidated. Here, we investigated the involvement of other members of the cullin family in mediating K⁺ effects on NCC phosphorylation (active form) and abundance. In kidneys from mice fed diets varying in K⁺ content, there were negative correlations between NCC (phosphorylated and total) and active (neddylated) forms of cullins (Cul1, 3, 4, and 5). High dietary K⁺ effects on phosphorylated NCC were attenuated in Cul3 mutant mice (CUL3-Het/Δ9). Short-term (30 min) and long-term (24 h) alterations in the extracellular K⁺ concentration did not affect cullin neddylation levels in ex vivo renal tubules. In the short term, the ability of high extracellular K⁺ to decrease NCC phosphorylation was preserved in the presence of MLN4924 (pan-cullin inhibitor), but the response to low extracellular K⁺ was absent. In the long term, MLN4924 attenuated the effects of high extracellular K⁺ on NCC phosphorylation, and responses to low extracellular K⁺ were absent. Our data suggest that in addition to Cul3, other cullins are involved in mediating the effects of K⁺ on NCC phosphorylation and abundance.

L-WNK1 is required for BK channel activation in intercalated cells

Large-conductance K+ (BK) channels expressed in intercalated cells (ICs) in the aldosterone-sensitive distal nephron (ASDN) mediate flow-induced K+ secretion. In the ASDN of mice and rabbits, IC BK channel expression and activity increase with a high-K+ diet. In cell culture, the long isoform of with-no-lysine kinase 1 (L-WNK1) increases BK channel expression and activity. Apical L-WNK1 expression is selectively enhanced in ICs in the ASDN of rabbits on a high-K+ diet, suggesting that L-WNK1 contributes to BK channel regulation by dietary K+. We examined the role of IC L-WNK1 expression in enhancing BK channel activity in response to a high-K+ diet. Mice with IC-selective deletion of L-WNK1 (IC-L-WNK1-KO) and littermate control mice were placed on a high-K+ (5% K+, as KCl) diet for 10 or more days. IC-L-WNK1-KO mice exhibited reduced IC apical + subapical α-subunit expression and BK channel-dependent whole cell currents compared with controls. Six-hour urinary K+ excretion in response a saline load was similar in IC-L-WNK1-KO mice and controls. The observations that IC-L-WNK1-KO mice on a high-K+ diet have higher blood K+ concentration and reduced IC BK channel activity are consistent with impaired urinary K+ secretion, demonstrating that IC L-WNK1 has a role in the renal adaptation to a high-K+ diet.NEW & NOTEWORTHY When mice are placed on a high-K+ diet, genetic disruption of the long form of with no lysine kinase 1 (L-WNK1) in intercalated cells reduced relative apical + subapical localization of the large-conductance K+ channel, blunted large-conductance K+ channel currents in intercalated cells, and increased blood K+ concentration. These data confirm an in vivo role of L-WNK1 in intercalated cells in adaptation to a high-K+ diet.

Effects of extreme potassium stress on blood pressure and renal tubular sodium transport

We characterized mouse blood pressure and ion transport in the setting of commonly used rodent diets that drive K⁺ intake to the extremes of deficiency and excess. Male 129S2/Sv mice were fed either K⁺-deficient, control, high-K⁺ basic, or high-KCl diets for 10 days. Mice maintained on a K⁺-deficient diet exhibited no change in blood pressure, whereas K⁺-loaded mice developed an ~10-mmHg blood pressure increase. Following challenge with NaCl, K⁺-deficient mice developed a salt-sensitive 8 mmHg increase in blood pressure, whereas blood pressure was unchanged in mice fed high-K⁺ diets. Notably, 10 days of K⁺ depletion induced diabetes insipidus and upregulation of phosphorylated NaCl cotransporter, proximal Na⁺ transporters, and pendrin, likely contributing to the K⁺-deficient NaCl sensitivity. While the anionic content with high-K⁺ diets had distinct effects on transporter expression along the nephron, both K⁺ basic and KCl diets had a similar increase in blood pressure. The blood pressure elevation on high-K⁺ diets correlated with increased Na⁺-K⁺-2Cl^- cotransporter and γ-epithelial Na⁺ channel expression and increased urinary response to furosemide and amiloride. We conclude that the dietary K⁺ maneuvers used here did not recapitulate the inverse effects of K⁺ on blood pressure observed in human epidemiological studies. This may be due to the extreme degree of K⁺ stress, the low-Na⁺-to-K⁺ ratio, the duration of treatment, and the development of other coinciding events, such as diabetes insipidus. These factors must be taken into consideration when studying the physiological effects of dietary K⁺ loading and depletion.

The Molecular Genetics of Gordon Syndrome

Gordon syndrome is a rare inherited monogenic form of hypertension, which is associated with hyperkalaemia and metabolic acidosis. Since the recognition of this predominantly autosomal dominant condition in the 1960s, the study of families with Gordon syndrome has revealed four genes WNK1, WNK4, KLHL3, and CUL3 to be implicated in its pathogenesis after a phenotype–genotype correlation was realised. The encoded proteins Kelch-like 3 and Cullin 3 interact to form a ring-like complex to ubiquitinate WNK-kinase 4, which, in normal circumstances, interacts with the sodium chloride co-symporter (NCC), the epithelial sodium channel (ENaC), and the renal outer medullary potassium channel (ROMK) in an inhibitory manner to maintain normokalaemia and normotension. WNK-kinase 1 has an inhibitory action on WNK-kinase 4. Mutations in WNK1, WNK4, KLHL3, and CUL3 all result in the accumulation of WNK-kinase 4 and subsequent hypertension, hyperkalaemia, and metabolic acidosis. This review explains the clinical aspects, disease mechanisms, and molecular genetics of Gordon syndrome.

WNK bodies cluster WNK4 and SPAK/OSR1 to promote NCC activation in hypokalemia

K⁺ deficiency stimulates renal salt reuptake via the Na⁺-Cl^- cotransporter (NCC) of the distal convoluted tubule (DCT), thereby reducing K⁺ losses in downstream nephron segments while increasing NaCl retention and blood pressure. NCC activation is mediated by a kinase cascade involving with no lysine (WNK) kinases upstream of Ste20-related proline-alanine-rich kinase (SPAK) and oxidative stress-responsive kinase-1 (OSR1). In K⁺ deficiency, WNKs and SPAK/OSR1 concentrate in spherical cytoplasmic domains in the DCT termed "WNK bodies," the significance of which is undetermined. By feeding diets of varying salt and K⁺ content to mice and using genetically engineered mouse lines, we aimed to clarify whether WNK bodies contribute to WNK-SPAK/OSR1-NCC signaling. Phosphorylated SPAK/OSR1 was present both at the apical membrane and in WNK bodies within 12 h of dietary K⁺ deprivation, and it was promptly suppressed by K⁺ loading. In WNK4-deficient mice, however, larger WNK bodies formed, containing unphosphorylated WNK1, SPAK, and OSR1. This suggests that WNK4 is the primary active WNK isoform in WNK bodies and catalyzes SPAK/OSR1 phosphorylation therein. We further examined mice carrying a kidney-specific deletion of the basolateral K⁺ channel-forming protein Kir4.1, which is required for the DCT to sense plasma K⁺ concentration. These mice displayed remnant mosaic expression of Kir4.1 in the DCT, and upon K⁺ deprivation, WNK bodies developed only in Kir4.1-expressing cells. We postulate a model of DCT function in which NCC activity is modulated by plasma K⁺ concentration via WNK4-SPAK/OSR1 interactions within WNK bodies.

Mg2+ restriction downregulates NCC through NEDD4-2 and prevents its activation by hypokalemia

Hypomagnesemia is associated with reduced kidney function and life-threatening complications and sustains hypokalemia. The distal convoluted tubule (DCT) determines final urinary Mg²⁺ excretion and, via activity of the Na⁺-Cl^- cotransporter (NCC), also plays a key role in K⁺ homeostasis by metering Na⁺ delivery to distal segments. Little is known about the mechanisms by which plasma Mg²⁺ concentration regulates NCC activity and how low-plasma Mg²⁺ concentration and K⁺ concentration interact to modulate NCC activity. To address this, we performed dietary manipulation studies in mice. Compared with normal diet, abundances of total NCC and phosphorylated NCC (pNCC) were lower after short-term (3 days) or long-term (14 days) dietary Mg²⁺ restriction. Altered NCC activation is unlikely to play a role, since we also observed lower total NCC abundance in mice lacking the two NCC-activating kinases, STE20/SPS-1-related proline/alanine-rich kinase and oxidative stress response kinase-1, after Mg²⁺ restriction. The E3 ubiquitin-protein ligase NEDD4-2 regulates NCC abundance during dietary NaCl loading or K⁺ restriction. Mg²⁺ restriction did not lower total NCC abundance in inducible nephron-specific neuronal precursor cell developmentally downregulated 4-2 (NEDD4-2) knockout mice. Total NCC and pNCC abundances were similar after short-term Mg²⁺ or combined Mg²⁺-K⁺ restriction but were dramatically lower compared with a low-K⁺ diet. Therefore, sustained NCC downregulation may serve a mechanism that enhances distal Na⁺ delivery during states of hypomagnesemia, maintaining hypokalemia. Similar results were obtained with long-term Mg²⁺-K⁺ restriction, but, surprisingly, NCC was not activated after long-term K⁺ restriction despite lower plasma K⁺ concentration, suggesting significant differences in distal tubule adaptation to acute or chronic K⁺ restriction.

Podocyte Injury Augments Intrarenal Angiotensin II Generation and Sodium Retention in a Megalin-Dependent Manner

We have previously shown that podocyte injury increases the glomerular filtration of liver-derived Agt (angiotensinogen) and the generation of intrarenal Ang II (angiotensin II) and that the filtered Agt is reabsorbed by proximal tubules in a manner dependent on megalin. In the present study, we aimed to study the role of megalin in the generation of renal Ang II and sodium handling during nephrotic syndrome. We generated proximal tubule-specific megalin KO (knockout) mice and crossed these animals with NEP25 mice, in which podocyte-specific injury can be induced by injection of the immunotoxin LMB2. Without podocyte injury, renal Agt staining was markedly diminished and urinary Agt increased in KO mice. However, renal Ang II was similar between KO and control mice on average: 117 (95% CI, 101-134) versus 101 (95% CI, 68-133) fmol/g tissue. We next tested the effect of megalin KO on intrarenal Ang II generation with podocyte injury. Control NEP25 mice showed markedly increased renal Agt staining and renal Ang II levels: 450 (336-565) fmol/g tissue. Megalin KO/NEP25 mice showed markedly diminished Agt reabsorption and attenuated renal Ang II: 199 (156-242) fmol/g tissue (P<0.001). Compared with control NEP25 mice, megalin KO/NEP25 mice excreted 5-fold more sodium in the urine. Western blot analysis showed that megalin KO decreased NHE3 and the cleaved α and γ forms of Epithelial Na Channel. These data indicate that Agt reabsorbed by proximal tubules via megalin in nephrotic syndrome is converted to Ang II, which may contribute to sodium retention and edema formation by activating NHE3 and Epithelial Na Channel.

High dietary salt-induced dendritic cell activation underlies microbial dysbiosis-associated hypertension

Excess dietary salt contributes to inflammation and hypertension via poorly understood mechanisms. Antigen presenting cells including dendritic cells (DCs) play a key role in regulating intestinal immune homeostasis in part by surveying the gut epithelial surface for pathogens. Previously, we found that highly reactive γ-ketoaldehydes or isolevuglandins (IsoLGs) accumulate in DCs and act as neoantigens, promoting an autoimmune-like state and hypertension. We hypothesized that excess dietary salt alters the gut microbiome leading to hypertension and this is associated with increased immunogenic IsoLG-adduct formation in myeloid antigen presenting cells. To test this hypothesis, we performed fecal microbiome analysis and measured blood pressure of healthy human volunteers with salt intake above or below the American Heart Association recommendations. We also performed 16S rRNA analysis on cecal samples of mice fed normal or high salt diets. In humans and mice, high salt intake was associated with changes in the gut microbiome reflecting an increase in Firmicutes, Proteobacteria and genus Prevotella bacteria. These alterations were associated with higher blood pressure in humans and predisposed mice to vascular inflammation and hypertension in response to a sub-pressor dose of angiotensin II. Mice fed a high salt diet exhibited increased intestinal inflammation including the mesenteric arterial arcade and aorta, with a marked increase in the B7 ligand CD86 and formation of IsoLG-protein adducts in CD11c+ myeloid cells. Adoptive transfer of fecal material from conventionally housed high salt-fed mice to germ-free mice predisposed them to increased intestinal inflammation and hypertension. These findings provide novel insight into the mechanisms underlying inflammation and hypertension associated with excess dietary salt and may lead to interventions targeting the microbiome to prevent and treat this important disease.

Electrolyte transport in the renal collecting duct and its regulation by the renin-angiotensin-aldosterone system

Distal nephron of the kidney plays key roles in fluid volume and electrolyte homeostasis by tightly regulating reabsorption and excretion of Na⁺, K⁺, and Cl^- Studies to date demonstrate the detailed electrolyte transport mechanisms in principal cells of the cortical collecting duct, and their regulation by renin-angiotensin-aldosterone system (RAAS). In recent years, however, accumulating data indicate that intercalated cells, another cell type that is present in the cortical collecting duct, also play active roles in the regulation of blood pressure. Notably, pendrin in β-intercalated cells not only controls acid/base homeostasis, but is also one of the key components controlling salt and K⁺ transport in distal nephron. We have recently shown that pendrin is regulated by the co-ordinated action of angiotensin II (AngII) and aldosterone, and at the downstream of AngII, mammalian target of rapamycin (mTOR) signaling regulates pendrin through inhibiting the kinase unc51-like-kinase 1 and promoting dephosphorylation of mineralocorticoid receptor (MR). In this review, we summarize recent advances in the current knowledge on the salt transport mechanisms in the cortical collecting duct, and their regulation by the RAAS.

Potassium conservation is impaired in mice with reduced renal expression of Kir4.1

To better understand the role of the inward-rectifying K channel Kir4.1 (KCNJ10) in the distal nephron, we initially studied a global Kir4.1 knockout mouse (gKO), which demonstrated the hypokalemia and hypomagnesemia seen in SeSAME/EAST syndrome and was associated with reduced Na/Cl cotransporter (NCC) expression. Lethality by ~3 wk, however, limits the usefulness of this model, so we developed a kidney-specific Kir4.1 "knockdown" mouse (ksKD) using a cadherin 16 promoter and Cre-loxP methodology. These mice appeared normal and survived to adulthood. Kir4.1 protein expression was decreased ~50% vs. wild-type (WT) mice by immunoblotting, and immunofluorescence showed moderately reduced Kir4.1 staining in distal convoluted tubule that was minimal or absent in connecting tubule and cortical collecting duct. Under control conditions, the ksKD mice showed metabolic alkalosis and relative hypercalcemia but were normokalemic and mildly hypermagnesemic despite decreased NCC expression. In addition, the mice had a severe urinary concentrating defect associated with hypernatremia, enlarged kidneys with tubulocystic dilations, and reduced aquaporin-3 expression. On a K/Mg-free diet for 1 wk, however, ksKD mice showed marked hypokalemia (serum K: 1.5 ± 0.1 vs. 3.0 ± 0.1 mEq/l for WT), which was associated with renal K wasting (transtubular K gradient: 11.4 ± 0.8 vs. 1.6 ± 0.4 in WT). Phosphorylated-NCC expression increased in WT but not ksKD mice on the K/Mg-free diet, suggesting that loss of NCC adaptation underlies the hypokalemia. In conclusion, even modest reduction in Kir4.1 expression results in impaired K conservation, which appears to be mediated by reduced expression of activated NCC.

Regulation of the renal NaCl cotransporter by the WNK/SPAK pathway: lessons learned from genetically altered animals

The renal thiazide-sensitive NaCl cotransporter (NCC) is the major salt transport pathway in the distal convoluted tubule of the mammalian nephron. NCC activity is critical for modulation of arterial blood pressure and serum potassium levels. Reduced activity of NCC in genetic diseases results in arterial hypotension and hypokalemia, while increased activity results in genetic diseases featuring hypertension and hyperkalemia. Several hormones and physiological conditions modulate NCC activity through a final intracellular complex pathway involving kinases and ubiquitin ligases. A substantial amount of work has been conducted to understand this pathway in the last 15 yr, but advances over the last 3 yr have helped to begin to understand how these regulatory proteins interact with each other and modulate the activity of this important cotransporter. In this review, we present the current model of NCC regulation by the Cullin 3 protein/Kelch-like 3 protein/with no lysine kinase/STE20-serine-proline alanine-rich kinase (CUL3/KELCH3-WNK-SPAK) pathway. We present a review of all genetically altered mice that have been used to translate most of the proposals made from in vitro experiments into in vivo observations that have helped to elucidate the model at the physiological level. Many questions have been resolved, but some others will require further models to be constructed. In addition, unexpected observations in mice have raised new questions and identified regulatory pathways that were previously unknown.

A mouse model of pseudohypoaldosteronism type II reveals a novel mechanism of renal tubular acidosis

Pseudohypoaldosteronism type II (PHAII) is a genetic disease characterized by association of hyperkalemia, hyperchloremic metabolic acidosis, hypertension, low renin, and high sensitivity to thiazide diuretics. It is caused by mutations in the WNK1, WNK4, KLHL3 or CUL3 gene. There is strong evidence that excessive sodium chloride reabsorption by the sodium chloride cotransporter NCC in the distal convoluted tubule is involved. WNK4 is expressed not only in distal convoluted tubule cells but also in β-intercalated cells of the cortical collecting duct. These latter cells exchange intracellular bicarbonate for external chloride through pendrin, and therefore, account for renal base excretion. However, these cells can also mediate thiazide-sensitive sodium chloride absorption when the pendrin-dependent apical chloride influx is coupled to apical sodium influx by the sodium-driven chloride/bicarbonate exchanger. Here we determine whether this system is involved in the pathogenesis of PHAII. Renal pendrin activity was markedly increased in a mouse model carrying a WNK4 missense mutation (Q562E) previously identified in patients with PHAII. The upregulation of pendrin led to an increase in thiazide-sensitive sodium chloride absorption by the cortical collecting duct, and it caused metabolic acidosis. The function of apical potassium channels was altered in this model, and hyperkalemia was fully corrected by pendrin genetic ablation. Thus, we demonstrate an important contribution of pendrin in renal regulation of sodium chloride, potassium and acid-base homeostasis and in the pathophysiology of PHAII. Furthermore, we identify renal distal bicarbonate secretion as a novel mechanism of renal tubular acidosis.

With no lysine kinase 4 modulates sodium potassium 2 chloride cotransporter activity in vivo

With no lysine kinase 4 (WNK4) is essential to activate the thiazide-sensitive NaCl cotransporter (NCC) along the distal convoluted tubule, an effect central to the phenotype of familial hyperkalemic hypertension. Although effects on potassium and sodium channels along the connecting and collecting tubules have also been documented, WNK4 is typically believed to have little role in modulating sodium chloride reabsorption along the thick ascending limb of the loop of Henle. Yet wnk4^-/- mice (knockout mice lacking WNK4) do not demonstrate the hypocalciuria typical of pure distal convoluted tubule dysfunction. Here, we tested the hypothesis that WNK4 also modulates bumetanide-sensitive Na-K-2Cl cotransporter (NKCC2) function along the thick ascending limb. We confirmed that w nk4^-/- mice are hypokalemic and waste sodium chloride, but are also normocalciuric. Results from Western blots suggested that the phosphorylated forms of both NCC and NKCC2 were in lower abundance in wnk4^-/- mice than in controls. This finding was confirmed by immunofluorescence microscopy. Although the initial response to furosemide was similar in wnk4^-/- mice and controls, the response was lower in the knockout mice when reabsorption along the distal convoluted tubule was inhibited. Using HEK293 cells, we showed that WNK4 increases the abundance of phosphorylated NKCC2. More supporting evidence that WNK4 may modulate NKCC2 emerges from a mouse model of WNK4-mediated familial hyperkalemic hypertension in which more phosphorylated NKCC2 is present than in controls. These data indicate that WNK4, in addition to modulating NCC, also modulates NKCC2, contributing to its physiological function in vivo.

Gender difference in kidney electrolyte transport. I. Role of AT1a receptor in thiazide-sensitive Na+-Cl- cotransporter activity and expression in male and female mice

We studied gender differences in Na⁺-Cl^- cotransporter (NCC) activity and expression in wild-type (WT) and AT_1a receptor knockout (KO) mice. In renal clearance experiments, urine volume (UV), glomerular filtration rate, absolute Na⁺ (E_Na) and K⁺ (E_K), and fractional Na⁺ (FE_Na) and K⁺ excretion were measured and compared at peak changes after bolus intravenous injection of hydrochlorothiazide (HCTZ; 30 mg/kg). In WT, females responded more strongly than males to HCTZ, with larger fractional increases of UV (7.8- vs. 3.4-fold), E_Na (11.7- vs. 5.7-fold), FE_Na (7.9- vs. 4.9-fold), and E_K (2.8- vs. 1.4-fold). In contrast, there were no gender differences in the responses to the diuretic in KO mice; HCTZ produced greater effects on male KO than on WT but similar effects on females. In WT, total (tNCC) and phosphorylated (pNCC) NCC protein expressions were 1.8- and 4.6-fold higher in females compared with males (P < 0.05), consistent with the larger response to HCTZ. In KO mice, tNCC and pNCC increased significantly in males to levels not different from those in females. There were no gender differences in the expression of the Na⁺/H⁺ exchanger (NHE3) in WT; NHE3 protein decreased to similar extents in male and female KO animals, suggesting AT_1a-mediated NHE3 expression in proximal tubules. The resulting increase in delivery of NaCl to the distal nephron may underlie increased NCC expression and activity in mice lacking the AT_1a receptor.

Constitutively Active SPAK Causes Hyperkalemia by Activating NCC and Remodeling Distal Tubules

Aberrant activation of with no lysine (WNK) kinases causes familial hyperkalemic hypertension (FHHt). Thiazide diuretics treat the disease, fostering the view that hyperactivation of the thiazide-sensitive sodium-chloride cotransporter (NCC) in the distal convoluted tubule (DCT) is solely responsible. However, aberrant signaling in the aldosterone-sensitive distal nephron (ASDN) and inhibition of the potassium-excretory renal outer medullary potassium (ROMK) channel have also been implicated. To test these ideas, we introduced kinase-activating mutations after Lox-P sites in the mouse Stk39 gene, which encodes the terminal kinase in the WNK signaling pathway, Ste20-related proline-alanine-rich kinase (SPAK). Renal expression of the constitutively active (CA)-SPAK mutant was specifically targeted to the early DCT using a DCT-driven Cre recombinase. CA-SPAK mice displayed thiazide-treatable hypertension and hyperkalemia, concurrent with NCC hyperphosphorylation. However, thiazide-mediated inhibition of NCC and consequent restoration of sodium excretion did not immediately restore urinary potassium excretion in CA-SPAK mice. Notably, CA-SPAK mice exhibited ASDN remodeling, involving a reduction in connecting tubule mass and attenuation of epithelial sodium channel (ENaC) and ROMK expression and apical localization. Blocking hyperactive NCC in the DCT gradually restored ASDN structure and ENaC and ROMK expression, concurrent with the restoration of urinary potassium excretion. These findings verify that NCC hyperactivity underlies FHHt but also reveal that NCC-dependent changes in the driving force for potassium secretion are not sufficient to explain hyperkalemia. Instead, a DCT-ASDN coupling process controls potassium balance in health and becomes aberrantly activated in FHHt.

Potassium Supplementation Prevents Sodium Chloride Cotransporter Stimulation During Angiotensin II Hypertension

Angiotensin II (AngII) hypertension increases distal tubule Na-Cl cotransporter (NCC) abundance and phosphorylation (NCCp), as well as epithelial Na(+) channel abundance and activating cleavage. Acutely raising plasma [K(+)] by infusion or ingestion provokes a rapid decrease in NCCp that drives a compensatory kaliuresis. The first aim tested whether acutely raising plasma [K(+)] with a single 3-hour 2% potassium meal would lower NCCp in Sprague-Dawley rats after 14 days of AngII (400 ng/kg per minute). The potassium-rich meal neither decreased NCCp nor increased K(+) excretion. AngII-infused rats exhibited lower plasma [K(+)] versus controls (3.6±0.2 versus 4.5±0.1 mmol/L; P<0.05), suggesting that AngII-mediated epithelial Na(+) channel activation provokes K(+) depletion. The second aim tested whether doubling dietary potassium intake from 1% (A1K) to 2% (A2K) would prevent K(+) depletion during AngII infusion and, thus, prevent NCC accumulation. A2K-fed rats exhibited normal plasma [K(+)] and 2-fold higher K(+) excretion and plasma [aldosterone] versus A1K. In A1K rats, NCC, NCCpS71, and NCCpT53 abundance increased 1.5- to 3-fold versus controls (P<0.05). The rise in NCC and NCCp abundance was prevented in the A2K rats, yet blood pressure did not significantly decrease. Epithelial Na(+) channel subunit abundance and cleavage increased 1.5- to 3-fold in both A1K and A2K; ROMK (renal outer medulla K(+) channel abundance) abundance was unaffected by AngII or dietary K(+) In summary, the accumulation and phosphorylation of NCC seen during chronic AngII infusion hypertension is likely secondary to potassium deficiency driven by epithelial Na(+) channel stimulation.

Na+-sensitive elevation in blood pressure is ENaC independent in diet-induced obesity and insulin resistance

The majority of patients with obesity, insulin resistance, and metabolic syndrome have hypertension, but the mechanisms of hypertension are poorly understood. In these patients, impaired sodium excretion is critical for the genesis of Na(+)-sensitive hypertension, and prior studies have proposed a role for the epithelial Na(+) channel (ENaC) in this syndrome. We characterized high fat-fed mice as a model in which to study the contribution of ENaC-mediated Na(+) reabsorption in obesity and insulin resistance. High fat-fed mice demonstrated impaired Na(+) excretion and elevated blood pressure, which was significantly higher on a high-Na(+) diet compared with low fat-fed control mice. However, high fat-fed mice had no increase in ENaC activity as measured by Na(+) transport across microperfused cortical collecting ducts, electrolyte excretion, or blood pressure. In addition, we found no difference in endogenous urinary aldosterone excretion between groups on a normal or high-Na(+) diet. High fat-fed mice provide a model of metabolic syndrome, recapitulating obesity, insulin resistance, impaired natriuresis, and a Na(+)-sensitive elevation in blood pressure. Surprisingly, in contrast to previous studies, our data demonstrate that high fat feeding of mice impairs natriuresis and produces elevated blood pressure that is independent of ENaC activity and likely caused by increased Na(+) reabsorption upstream of the aldosterone-sensitive distal nephron.

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.

KLHL3 regulates paracellular chloride transport in the kidney by ubiquitination of claudin-8

A rare Mendelian syndrome--pseudohypoaldosteronism type II (PHA-II)--features hypertension, hyperkalemia, and metabolic acidosis. Genetic linkage studies and exome sequencing have identified four genes--with no lysine kinase 1 (wnk1), wnk4, Kelch-like 3 (KLHL3), and Cullin 3 (Cul3)--mutations of which all caused PHA-II phenotypes. The previous hypothesis was that the KLHL3-Cul3 ubiquitin complex acted on the wnk4-wnk1 kinase complex to regulate Na(+)/Cl(-) cotransporter (NCC) mediated salt reabsorption in the distal tubules of the kidney. Here, we report the identification of claudin-8 as a previously unidentified physiologic target for KLHL3 and provide an alternative explanation for the collecting duct's role in PHA-II. Using a tissue-specific KO approach, we have found that deletion of claudin-8 in the collecting duct of mouse kidney caused hypotension, hypokalemia, and metabolic alkalosis, an exact mirror image of PHA-II. Mechanistically, the phenotypes in claudin-8 KO animals were caused by disruption of the claudin-8 interaction with claudin-4, the paracellular chloride channel, and delocalization of claudin-4 from the tight junction. In mouse collecting duct cells, knockdown of KLHL3 profoundly increased the paracellular chloride permeability. Mechanistically, KLHL3 was directly bound to claudin-8, and this binding led to the ubiquitination and degradation of claudin-8. The dominant PHA-II mutation in KLHL3 impaired claudin-8 binding, ubiquitination, and degradation. These findings have attested to the concept that the paracellular pathway is physiologically regulated through the ubiquitination pathway, and its deregulation may lead to diseases of electrolyte and blood pressure imbalances.

Regulation of blood pressure and renal function by NCC and ENaC: lessons from genetically engineered mice

The activity of the thiazide-sensitive Na(+)/Cl(-) cotransporter (NCC) and of the amiloride-sensitive epithelial Na(+) channel (ENaC) is pivotal for blood pressure regulation. NCC is responsible for Na(+) reabsorption in the distal convoluted tubule (DCT) of the nephron, while ENaC reabsorbs the filtered Na(+) in the late DCT and in the cortical collecting ducts (CCD) providing the final renal adjustment to Na(+) balance. Here, we aim to highlight the recent advances made using transgenic mouse models towards the understanding of the regulation of NCC and ENaC function relevant to the control of sodium balance and blood pressure. We thus like to pave the way for common mechanisms regulating these two sodium-transporting proteins and their potential implication in structural remodeling of the nephron segments and Na(+) and Cl(-) reabsorption.

Paradoxical activation of the sodium chloride cotransporter (NCC) without hypertension in kidney deficient in a regulatory subunit of Na,K-ATPase, FXYD2

Na,K‐ATPase generates the driving force for sodium reabsorption in the kidney. Na,K‐ATPase functional properties are regulated by small proteins belonging to the FXYD family. In kidney FXYD2 is the most abundant: it is an inhibitory subunit expressed in almost every nephron segment. Its absence should increase sodium pump activity and promote Na⁺ retention, however, no obvious renal phenotype was detected in mice with global deletion of FXYD2 (Arystarkhova et al. 2013). Here, increased total cortical Na,K‐ATPase activity was documented in the Fxyd2^−/− mouse, without increased α1β1 subunit expression. We tested the hypothesis that adaptations occur in distal convoluted tubule (DCT), a major site of sodium adjustments. Na,K‐ATPase immunoreactivity in DCT was unchanged, and there was no DCT hypoplasia. There was a marked activation of thiazide‐sensitive sodium chloride cotransporter (NCC; Slc12a3) in DCT, predicted to increase Na⁺ reabsorption in this segment. Specifically, NCC total increased 30% and NCC phosphorylated at T53 and S71, associated with activation, increased 4‐6 fold. The phosphorylation of the closely related thick ascending limb (TAL) apical NKCC2 (Slc12a1) increased at least twofold. Abundance of the total and cleaved (activated) forms of ENaC α‐subunit was not different between genotypes. Nonetheless, no elevation of blood pressure was evident despite the fact that NCC and NKCC2 are in states permissive for Na⁺ retention. Activation of NCC and NKCC2 may reflect an intracellular linkage to elevated Na,K‐ATPase activity or a compensatory response to Na⁺ loss proximal to the TAL and DCT.

We discovered a substantial activation of renal NCC cotransporter in mice genetically depleted for the regulatory inhibitory subunit of Na,K‐ATPase, FXYD2. Surprisingly, no significant changes in urine output as well as elevation of blood pressure were detected suggesting compensatory adaptation elsewhere in nephron

Ubiquitylation and control of renal Na+ balance and blood pressure

Ubiquitylation is crucial for regulating numerous cellular functions. In the kidney, ubiquitylation regulates the epithelial Na(+) channel ENaC. The importance of this process is highlighted in Liddle's syndrome, where mutations interfere with ENaC ubiquitylation, resulting in constitutive Na(+) reabsorption and hypertension. There is emerging evidence that NCC, involved in hypertensive diseases, is also regulated by ubiquitylation. Here, we discuss the current knowledge and recent findings in this field.

WNK3 abrogates the NEDD4-2-mediated inhibition of the renal Na+-Cl- cotransporter

The serine/threonine kinase WNK3 and the ubiquitin-protein ligase NEDD4-2 are key regulators of the thiazide-sensitive Na+-Cl- cotransporter (NCC), WNK3 as an activator and NEDD2-4 as an inhibitor. Nedd4-2 was identified as an interacting partner of WNK3 through a glutathione-S-transferase pull-down assay using the N-terminal domain of WNK3, combined with LC-MS/MS analysis. This was validated by coimmunoprecipitation of WNK3 and NEDD4-2 expressed in HEK293 cells. Our data also revealed that the interaction between Nedd4-2 and WNK3 does not involve the PY-like motif found in WNK3. The level of WNK3 ubiquitylation did not change when NEDD4-2 was expressed in HEK293 cells. Moreover, in contrast to SGK1, WNK3 did not phosphorylate NEDD4-2 on S222 or S328. Coimmunoprecipitation assays showed that WNK3 does not regulate the interaction between NCC and NEDD4-2. Interestingly, in Xenopus laevis oocytes, WNK3 was able to recover the SGK1-resistant NEDD4-2 S222A/S328A-mediated inhibition of NCC and further activate NCC. Furthermore, elimination of the SPAK binding site in the kinase domain of WNK3 (WNK3-F242A, which lacks the capacity to bind the serine/threonine kinase SPAK) prevented the WNK3 NCC-activating effect, but not the Nedd4-2-inhibitory effect. Together, these results suggest that a novel role for WNK3 on NCC expression at the plasma membrane, an effect apparently independent of the SPAK kinase and the aldosterone-SGK1 pathway.

Impaired degradation of WNK1 and WNK4 kinases causes PHAII in mutant KLHL3 knock-in mice

Pseudohypoaldosteronism type II (PHAII) is a hereditary disease characterized by salt-sensitive hypertension, hyperkalemia and metabolic acidosis, and genes encoding with-no-lysine kinase 1 (WNK1) and WNK4 kinases are known to be responsible. Recently, Kelch-like 3 (KLHL3) and Cullin3, components of KLHL3-Cullin3 E3 ligase, were newly identified as responsible for PHAII. We have reported that WNK4 is the substrate of KLHL3-Cullin3 E3 ligase-mediated ubiquitination. However, WNK1 and Na-Cl cotransporter (NCC) were also reported to be a substrate of KLHL3-Cullin3 E3 ligase by other groups. Therefore, it remains unclear which molecule is the target(s) of KLHL3. To investigate the pathogenesis of PHAII caused by KLHL3 mutation, we generated and analyzed KLHL3(R528H/+) knock-in mice. KLHL3(R528H/+) knock-in mice exhibited salt-sensitive hypertension, hyperkalemia and metabolic acidosis. Moreover, the phosphorylation of NCC was increased in the KLHL3(R528H/+) mouse kidney, indicating that the KLHL3(R528H/+) knock-in mouse is an ideal mouse model of PHAII. Interestingly, the protein expression of both WNK1 and WNK4 was significantly increased in the KLHL3(R528H/+) mouse kidney, confirming that increases in these WNK kinases activated the WNK-OSR1/SPAK-NCC phosphorylation cascade in KLHL3(R528H/+) knock-in mice. To examine whether mutant KLHL3 R528H can interact with WNK kinases, we measured the binding of TAMRA-labeled WNK1 and WNK4 peptides to full-length KLHL3 using fluorescence correlation spectroscopy, and found that neither WNK1 nor WNK4 bound to mutant KLHL3 R528H. Thus, we found that increased protein expression levels of WNK1 and WNK4 kinases cause PHAII by KLHL3 R528H mutation due to impaired KLHL3-Cullin3-mediated ubiquitination.

WNK4 is the major WNK positively regulating NCC in the mouse kidney

By analysing the pathogenesis of a hereditary hypertensive disease, PHAII (pseudohypoaldosteronism type II), we previously discovered that WNK (with-no-lysine kinase)–OSR1/SPAK (oxidative stress-responsive 1/Ste20-like proline/alanine-rich kinase) cascade regulates NCC (Na–Cl co-transporter) in the DCT (distal convoluted tubules) of the kidney. However, the role of WNK4 in the regulation of NCC remains controversial. To address this, we generated and analysed WNK4^−/− mice. Although a moderate decrease in SPAK phosphorylation and a marked increase in WNK1 expression were evident in the kidneys of WNK4^−/− mice, the amount of phosphorylated and total NCC decreased to almost undetectable levels, indicating that WNK4 is the major WNK positively regulating NCC, and that WNK1 cannot compensate for WNK4 deficiency in the DCT. Insulin- and low-potassium diet-induced NCC phosphorylation were abolished in WNK4^−/− mice, establishing that both signals to NCC were mediated by WNK4. As shown previously, a high-salt diet decreases phosphorylated and total NCC in WNK4^+/+ mice via AngII (angiotensin II) and aldosterone suppression. This was not ameliorated by WNK4 knock out, excluding the negative regulation of WNK4 on NCC postulated to be active in the absence of AngII stimulation. Thus, WNK4 is the major positive regulator of NCC in the kidneys.

The analyses of WNK4 (with-no-lysine kinase 4) knockout mice help to end a long-standing controversy about the role of WNK4 on NCC (Na–Cl co-transporter) regulations in the kidney. WNK4 is a strong positive regulator of NCC.

Mechanism of salt-sensitive hypertension: focus on adrenal and sympathetic nervous systems

A central role for the kidney among the systems contributing to BP regulation and the development of hypertension has been proposed. Both the aldosterone/mineralocorticoid receptor pathway and the renal sympathetic nervous system have important roles in the regulation of renal excretory function and BP control, but the mechanisms underlying these processes have remained unclear. However, recent studies revealed the activation of two pathways in salt-sensitive hypertension. Notably, Rac1, a member of the Rho-family of small GTP binding proteins, was identified as a novel ligand-independent modulator of mineralocorticoid receptor activity. Furthermore, these studies point to crucial roles for the Rac1-mineralocorticoid receptor-NCC/ENaC and the renal β-adrenergic stimulant-glucocorticoid receptor-WNK4-NCC pathways in certain rodent models of salt-sensitive hypertension. The nuclear mineralocorticoid and glucocorticoid receptors may contribute to impaired renal excretory function and the resulting salt-sensitive hypertension by increasing sodium reabsorption at different tubular segments. This review provides an in-depth discussion of the evidence supporting these conclusions and considers the significance with regard to treating salt-sensitive hypertension and salt-induced cardiorenal injury.

The sodium chloride cotransporter SLC12A3: new roles in sodium, potassium, and blood pressure regulation

SLC12A3 encodes the thiazide-sensitive sodium chloride cotransporter (NCC), which is primarily expressed in the kidney, but also in intestine and bone. In the kidney, NCC is located in the apical plasma membrane of epithelial cells in the distal convoluted tubule. Although NCC reabsorbs only 5 to 10% of filtered sodium, it is important for the fine-tuning of renal sodium excretion in response to various hormonal and non-hormonal stimuli. Several new roles for NCC in the regulation of sodium, potassium, and blood pressure have been unraveled recently. For example, the recent discoveries that NCC is activated by angiotensin II but inhibited by dietary potassium shed light on how the kidney handles sodium during hypovolemia (high angiotensin II) and hyperkalemia. The additive effect of angiotensin II and aldosterone maximizes sodium reabsorption during hypovolemia, whereas the inhibitory effect of potassium on NCC increases delivery of sodium to the potassium-secreting portion of the nephron. In addition, great steps have been made in unraveling the molecular machinery that controls NCC. This complex network consists of kinases and ubiquitinases, including WNKs, SGK1, SPAK, Nedd4-2, Cullin-3, and Kelch-like 3. The pathophysiological significance of this network is illustrated by the fact that modification of each individual protein in the network changes NCC activity and results in salt-dependent hypotension or hypertension. This review aims to summarize these new insights in an integrated manner while identifying unanswered questions.

WNK1-related Familial Hyperkalemic Hypertension results from an increased expression of L-WNK1 specifically in the distal nephron

Large deletions in the first intron of the With No lysine (K) 1 (WNK1) gene are responsible for Familial Hyperkalemic Hypertension (FHHt), a rare form of human hypertension associated with hyperkalemia and hyperchloremic metabolic acidosis. We generated a mouse model of WNK1-associated FHHt to explore the consequences of this intronic deletion. WNK1(+/FHHt) mice display all clinical and biological signs of FHHt. This phenotype results from increased expression of long WNK1 (L-WNK1), the ubiquitous kinase isoform of WNK1, in the distal convoluted tubule, which in turn, stimulates the activity of the Na-Cl cotransporter. We also show that the activity of the epithelial sodium channel is not altered in FHHt mice, suggesting that other mechanisms are responsible for the hyperkalemia and acidosis in this model. Finally, we observe a decreased expression of the renal outer medullary potassium channel in the late distal convoluted tubule of WNK1(+/FHHt) mice, which could contribute to the hyperkalemia. In summary, our study provides insights into the in vivo mechanisms underlying the pathogenesis of WNK1-mediated FHHt and further corroborates the importance of WNK1 in ion homeostasis and blood pressure.

Overexpression of pendrin in intercalated cells produces chloride-sensitive hypertension

Inherited and acquired disorders that enhance the activity of transporters mediating renal tubular Na(+) reabsorption are well established causes of hypertension. It is unclear, however, whether primary activation of an Na(+)-independent chloride transporter in the kidney can also play a pathogenic role in this disease. Here, mice overexpressing the chloride transporter pendrin in intercalated cells of the distal nephron (Tg(B1-hPDS) mice) displayed increased renal absorption of chloride. Compared with normal mice, these transgenic mice exhibited a delayed increase in urinary NaCl and ultimately, developed hypertension when exposed to a high-salt diet. Administering the same sodium intake as NaHCO3 instead of NaCl did not significantly alter BP, indicating that the hypertension in the transgenic mice was chloride-sensitive. Moreover, excessive chloride absorption by pendrin drove parallel absorption of sodium through the epithelial sodium channel ENaC and the sodium-driven chloride/bicarbonate exchanger (Ndcbe), despite an appropriate downregulation of these sodium transporters in response to the expanded vascular volume and hypertension. In summary, chloride transport in the distal nephron can play a primary role in driving NaCl transport in this part of the kidney, and a primary abnormality in renal chloride transport can provoke arterial hypertension. Thus, we conclude that the chloride/bicarbonate exchanger pendrin plays a major role in controlling net NaCl absorption, thereby influencing BP under conditions of high salt intake.

An inducible transgenic mouse model for familial hypertension with hyperkalaemia (Gordon's syndrome or pseudohypoaldosteronism type II)

Mutations in the novel serine/threonine WNK [With No lysine (=K)] kinases WNK1 and WNK4 cause PHAII (pseudohypoaldosteronism type II or Gordon's syndrome), a rare monogenic syndrome which causes hypertension and hyperkalaemia on a background of a normal glomerular filtration rate. Current animal models for PHAII recapitulate some aspects of the disease phenotype, but give no clues to how rapidly the phenotype emerges or whether it is reversible. To this end we have created an inducible PHAII transgenic animal model that expresses a human disease-causing WNK4 mutation, WNK4 Q565E, under the control of the Tet-On system. Several PHAII inducible transgenic mouse lines were created, each with differing TG (transgene) copy numbers and displaying varying degrees of TG expression (low, medium and high). Each of these transgenic lines demonstrated similar elevations of BP (blood pressure) and plasma potassium after 4 weeks of TG induction. Withdrawal of doxycycline switched off mutant TG expression and the disappearance of the PHAII phenotype. Western blotting of microdissected kidney nephron segments confirmed that expression of the thiazide-sensitive NCC (Na⁺-Cl⁻ co-transporter) was increased, as expected, in the distal convoluted tubule when transgenic mice were induced with doxycycline. The kidneys of these mice also do not show the morphological changes seen in the previous transgenic model expressing the same mutant form of WNK4. This inducible model shows, for the first time, that in vivo expression of a mutant WNK4 protein is sufficient to cause the rapid and reversible appearance of a PHAII disease phenotype in mice.

Renal tubular NEDD4-2 deficiency causes NCC-mediated salt-dependent hypertension

The E3 ubiquitin ligase NEDD4-2 (encoded by the Nedd4L gene) regulates the amiloride-sensitive epithelial Na+ channel (ENaC/SCNN1) to mediate Na+ homeostasis. Mutations in the human β/γENaC subunits that block NEDD4-2 binding or constitutive ablation of exons 6-8 of Nedd4L in mice both result in salt-sensitive hypertension and elevated ENaC activity (Liddle syndrome). To determine the role of renal tubular NEDD4-2 in adult mice, we generated tetracycline-inducible, nephron-specific Nedd4L KO mice. Under standard and high-Na+ diets, conditional KO mice displayed decreased plasma aldosterone but normal Na+/K+ balance. Under a high-Na+ diet, KO mice exhibited hypercalciuria and increased blood pressure, which were reversed by thiazide treatment. Protein expression of βENaC, γENaC, the renal outer medullary K+ channel (ROMK), and total and phosphorylated thiazide-sensitive Na+Cl- cotransporter (NCC) levels were increased in KO kidneys. Unexpectedly, Scnn1a mRNA, which encodes the αENaC subunit, was reduced and proteolytic cleavage of αENaC decreased. Taken together, these results demonstrate that loss of NEDD4-2 in adult renal tubules causes a new form of mild, salt-sensitive hypertension without hyperkalemia that is characterized by upregulation of NCC, elevation of β/γENaC, but not αENaC, and a normal Na+/K+ balance maintained by downregulation of ENaC activity and upregulation of ROMK.

Effects of ACE inhibition and ANG II stimulation on renal Na-Cl cotransporter distribution, phosphorylation, and membrane complex properties

The renal distal tubule Na-Cl cotransporter (NCC) reabsorbs <10% of the filtered Na(+) but is a key control point for blood pressure regulation by angiotensin II (ANG II), angiotensin-converting enzyme inhibitors (ACEI), and thiazide diuretics. This study aimed to determine whether NCC phosphorylation (NCCp) was regulated by acute (20-30 min) treatment with the ACEI captopril (12 μg/min × 20 min) or by a sub-pressor dose of ANG II (20 ng·kg(-1)·min(-1)) in Inactin-anesthetized rats. By immuno-EM, NCCp was detected exclusively in or adjacent to apical plama membranes (APM) in controls and after ACEI or ANG II treatment, while NCC total was detected in both APM and subapical cytoplasmic vesicles (SCV) in all conditions. In renal homogenates, neither ACEI nor ANG II treatment altered NCCp abundance, assayed by immunoblot. However, by density gradient fractionation we identified a pool of low-density APM in which NCCp decreased 50% in response to captopril and was restored during ANG II infusion, and another pool of higher-density APM that responded reciprocally, indicative of regulated redistribution between two APM pools. In both pools, NCCp was preferentially localized to Triton-soluble membranes. Blue Native gel electrophoresis established that APM NCCp localized to ~700 kDa complexes (containing γ-adducin) while unphosphorylated NCC in intracellular membranes primarily localized to ~400 kDa complexes: there was no evidence for native monomeric or dimeric NCC or NCCp. In summary, this study demonstrates that phosphorylated NCC, localized to multimeric complexes in the APM, redistributes in a regulated manner within the APM in response to ACEI and ANG II.

Regulation of the renal Na+-Cl- cotransporter by phosphorylation and ubiquitylation

The activity of the renal thiazide-sensitive NaCl cotransporter (NCC) in the distal convoluted tubule plays a key role in defining arterial blood pressure levels. Increased or decreased activity of the NCC is associated with arterial hypertension or hypotension, respectively. Thus it is of major interest to understand the activity of NCC using in vivo models. Phosphorylation of certain residues of the amino-terminal domain of NCC has been shown to be associated with its activation. The development of phospho-specific antibodies against these sites provides a powerful tool that is helping to increase our understanding of the molecular physiology of NCC. Additionally, NCC expression in the plasma membrane is modulated by ubiquitylation, which represents another major mechanism for regulating protein activity. This work presents a review of our current knowledge of the regulation of NCC activity by phosphorylation and ubiquitylation.

Phosphorylation of Na-Cl cotransporter by OSR1 and SPAK kinases regulates its ubiquitination

Na-Cl cotransporter (NCC) is phosphorylated in its amino terminus based on salt intake under the regulation of the WNK-OSR1/SPAK kinase cascade. We have observed that total protein abundance of NCC and its apical membrane expression varies in the kidney based on the phosphorylation status. To clarify the mechanism, we examined NCC ubiquitination status in mice fed low, normal and high salt diets, as well as in a model mouse of pseudohypoaldosteronism type II (PHAII) where NCC phosphorylation is constitutively elevated. Low-salt diet decreased NCC ubiquitination, while high-salt diet increased NCC ubiquitination in the kidney, and this was inversely correlated with total and phosphorylated NCC abundance. In the PHAII model, the ubiquitination of NCC in kidney was also lower when compared to that in wild-type littermates. To evaluate the relationship between phosphorylation and ubiquitination of NCC, we expressed wild-type, phospho-deficient and -mimicking NCC in COS7 cells, and the ubiquitination of immunoprecipitated total and biotinylated surface NCC was evaluated. NCC ubiquitination was increased in the phospho-deficient NCC and decreased in phospho-mimicking NCC in both total and surface NCC. Thus, we demonstrated that NCC phosphorylation decreased NCC ubiquitination, which may contribute to the increase of NCC abundance mostly on plasma membranes.

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.