The Effect of WNK4 on the Na+-Cl- Cotransporter Is Modulated by Intracellular Chloride

Navigating the multifaceted intricacies of the Na+-Cl- cotransporter, a highly regulated key effector in the control of hydromineral homeostasis

The Na+-Cl- cotransporter (NCC; SLC12A3) is a highly regulated integral membrane protein that is known to exist as three splice variants in primates. Its primary role in the kidney is to mediate the cosymport of Na+ and Cl- across the apical membrane of the distal convoluted tubule. Through this role and the involvement of other ion transport systems, NCC allows the systemic circulation to reclaim a fraction of the ultrafiltered Na+, K+, Cl-, and Mg+ loads in exchange for Ca2+ and [Formula: see text]. The physiological relevance of the Na+-Cl- cotransport mechanism in humans is illustrated by several abnormalities that result from NCC inactivation through the administration of thiazides or in the setting of hereditary disorders. The purpose of the present review is to discuss the molecular mechanisms and overall roles of Na+-Cl- cotransport as the main topics of interest. On reading the narrative proposed, one will realize that the knowledge gained in regard to these themes will continue to progress unrelentingly no matter how refined it has now become.

Effect of low sodium and high potassium diet on lowering blood pressure and cardiovascular events

Incorporating aggressive lifestyle modifications along with antihypertensive medication therapy is a crucial treatment strategy to enhance the control rate of hypertension. Dietary modification is one of the important lifestyle interventions for hypertension, and it has been proven to have a clear effect. Among food ingredients, sodium and potassium have been found to have the strongest association with blood pressure. The blood pressure-lowering effect of a low sodium diet and a high potassium diet has been well established, especially in hypertensive population. A high intake of potassium, a key component of the Dietary Approaches to Stop Hypertension (DASH) diet, has also shown a favorable impact on the risk of cardiovascular events. Additionally, research conducted with robust measurement methods has shown cardiovascular benefits of low-sodium intake. In this review, we aim to discuss the evidence regarding the relationship between the low sodium and high potassium diet and blood pressure and cardiovascular events.

Tubular TMEM16A promotes tubulointerstitial fibrosis by suppressing PGC-1α-mediated mitochondrial homeostasis in diabetic kidney disease

Tubulointerstitial fibrosis (TIF) plays a crucial role in the progression of diabetic kidney disease (DKD). However, the underlying molecular mechanisms remain obscure. The present study aimed to examine whether transmembrane member 16A (TMEM16A), a Ca2+-activated chloride channel, contributes to the development of TIF in DKD. Interestingly, we found that TMEM16A expression was significantly up-regulated in tubule of murine model of DKD, which was associated with development of TIF. In vivo inhibition of TMEM16A channel activity with specific inhibitors Ani9 effectively protects against TIF. Then, we found that TMEM16A activation induces tubular mitochondrial dysfunction in in vivo and in vitro models, with the evidence of the TMEM16A inhibition with specific inhibitor. Mechanically, TMEM16A mediated tubular mitochondrial dysfunction through inhibiting PGC-1α, whereas overexpression of PGC-1α could rescue the changes. In addition, TMEM16A-induced fibrogenesis was dependent on increased intracellular Cl-, and reducing intracellular Cl- significantly blunted high glucose-induced PGC-1α and profibrotic factors expression. Taken together, our studies demonstrated that tubular TMEM16A promotes TIF by suppressing PGC-1α-mediated mitochondrial homeostasis in DKD. Blockade of TMEM16A may serve as a novel therapeutic approach to ameliorate TIF.

Dietary potassium stimulates Ppp1Ca-Ppp1r1a dephosphorylation of kidney NaCl cotransporter and reduces blood pressure

Consumption of low dietary potassium, common with ultraprocessed foods, activates the thiazide-sensitive sodium chloride cotransporter (NCC) via the with no (K) lysine kinase/STE20/SPS1-related proline-alanine-rich protein kinase (WNK/SPAK) pathway to induce salt retention and elevate blood pressure (BP). However, it remains unclear how high-potassium "DASH-like" diets (dietary approaches to stop hypertension) inactivate the cotransporter and whether this decreases BP. A transcriptomics screen identified Ppp1Ca, encoding PP1A, as a potassium-upregulated gene, and its negative regulator Ppp1r1a, as a potassium-suppressed gene in the kidney. PP1A directly binds to and dephosphorylates NCC when extracellular potassium is elevated. Using mice genetically engineered to constitutively activate the NCC-regulatory kinase SPAK and thereby eliminate the effects of the WNK/SPAK kinase cascade, we confirmed that PP1A dephosphorylated NCC directly in a potassium-regulated manner. Prior adaptation to a high-potassium diet was required to maximally dephosphorylate NCC and lower BP in constitutively active SPAK mice, and this was associated with potassium-dependent suppression of Ppp1r1a and dephosphorylation of its cognate protein, inhibitory subunit 1 (I1). In conclusion, potassium-dependent activation of PP1A and inhibition of I1 drove NCC dephosphorylation, providing a mechanism to explain how high dietary K+ lowers BP. Shifting signaling of PP1A in favor of activation of WNK/SPAK may provide an improved therapeutic approach for treating salt-sensitive hypertension.

Expression of the kidney anion exchanger 1 affects WNK4 and SPAK phosphorylation and results in claudin-4 phosphorylation

In the renal collecting ducts, chloride reabsorption occurs through both transcellular and paracellular pathways. Recent literature highlights a functional interplay between both pathways. We recently showed that in polarized inner medullary collecting duct cells, expression of the basolateral kidney anion exchanger 1 (kAE1) results in a decreased transepithelial electrical resistance (TEER), in a claudin-4 dependent pathway. Claudin-4 is a paracellular sodium blocker and chloride pore. Here, we show that kAE1 expression in mouse inner medullary collecting duct cells triggers WNK4, SPAK and claudin-4 phosphorylation. Expression of a functionally dead kAE1 E681Q mutant has no effect on phosphorylation of these proteins. Expression of a catalytically inactive WNK4 D321A or chloride-insensitive WNK4 L319F mutant abolishes kAE1 effect on TEER, supporting a contribution of WNK4 to the process. We propose that variations of the cytosolic pH and chloride concentration upon kAE1 expression alter WNK4 kinase activity and tight junction properties.

Thirty years of the NaCl cotransporter: from cloning to physiology and structure

The primary structure of the thiazide-sensitive NaCl cotransporter (NCC) was resolved 30 years ago by the molecular identification of the cDNA encoding this cotransporter, from the winter's flounder urinary bladder, following a functional expression strategy. This review outlines some aspects of how the knowledge about thiazide diuretics and NCC evolved, the history of the cloning process, and the expansion of the SLC12 family of electroneutral cotransporters. The diseases associated with activation or inactivation of NCC are discussed, as well as the molecular model by which the activity of NCC is regulated. The controversies in the field are discussed as well as recent publication of the three-dimensional model of NCC obtained by cryo-electron microscopy, revealing not only the amino acid residues critical for Na+ and Cl- translocation but also the residues critical for polythiazide binding to the transporter, opening the possibility for a new era in thiazide diuretic therapy.

Prognosis value of serum chloride on 1-year mortality in cirrhotic patients receiving transjugular intrahepatic portosystemic shunt

BACKGROUND AND PURPOSE

Emerging researches have regarded serum chloride as a capable predictor of mortality in liver cirrhosis. We aim to investigate the clinical role of admission chloride in cirrhotic patients with esophagogastric varices receiving transjugular intrahepatic portosystemic shunt (TIPS), which is unclear.

METHODS

We retrospectively analyzed data of cirrhotic patients with esophagogastric varices undergoing TIPS in Zhongnan Hospital of Wuhan University. Mortality outcome was obtained by following up for 1-year after TIPS. Univariate and multivariate Cox regression were used to identify independent predictors of 1-year mortality post-TIPS. The receiver operating characteristic (ROC) curves were adopted to assess the predictive ability of the predictors. In addition, log-rank test and Kaplan-Meier (KM) analyses were employed to evaluate the prognostic value of predictors in the survival probability.

RESULTS

A total of 182 patients were included ultimately. Age, fever symptom, platelet-to lymphocyte-ratio (PLR), lymphocyte-to-monocyte ratio (LMR), total bilirubin, serum sodium, chloride, and Child-Pugh score were related to 1-year follow-up mortality. In multivariate Cox regression analysis, serum chloride (HR = 0.823, 95%CI = 0.757-0.894, p < 0.001) and Child-Pugh score (HR = 1.401, 95%CI = 1.151-1.704, p = 0.001) were identified as independent predictors of 1-year mortality. Patients with serum chloride <107.35 mmol/L showed worse survival probability than those with serum chloride ≥107.35 mmol/L no matter with or without ascites (p < 0.05).

CONCLUSION

Admission hypochloremia and increasing Child-Pugh score are independent predictors of 1-year mortality in cirrhotic patients with esophagogastric varices receiving TIPS.

Cullin 3 and Blood Pressure Regulation: Insights From Familial Hyperkalemic Hypertension

The study of rare monogenic forms of hypertension has led to the elucidation of important physiological pathways controlling blood pressure. Mutations in several genes cause familial hyperkalemic hypertension (also known as Gordon syndrome or pseudohypoaldosteronism type II). The most severe form of familial hyperkalemic hypertension is caused by mutations in CUL3, encoding CUL3 (Cullin 3)-a scaffold protein in an E3 ubiquitin ligase complex that tags substrates for proteasomal degradation. In the kidney, CUL3 mutations cause accumulation of the substrate WNK (with-no-lysine [K]) kinase and ultimately hyperactivation of the renal NaCl cotransporter-the target of the first-line antihypertensive thiazide diuretics. The precise mechanisms by which mutant CUL3 causes WNK kinase accumulation have been unclear, but several functional defects are likely to contribute. The hypertension seen in familial hyperkalemic hypertension also results from effects exerted by mutant CUL3 on several pathways in vascular smooth muscle and endothelium that modulate vascular tone. This review summarizes the mechanisms by which wild type and mutant CUL3 modulate blood pressure through effects on the kidney and vasculature, potential effects in the central nervous system and heart, and future directions for investigation.

Consideration of Kinase Inhibitors for the Treatment of Hydrocephalus

Hydrocephalus is a devastating condition characterized by excess cerebrospinal fluid (CSF) in the brain. Currently, the only effective treatment is surgical intervention, usually involving shunt placement, a procedure prone to malfunction, blockage, and infection that requires additional, often repetitive, surgeries. There are no long-term pharmaceutical treatments for hydrocephalus. To initiate an intelligent drug design, it is necessary to understand the biochemical changes underlying the pathology of this chronic condition. One potential commonality in the various forms of hydrocephalus is an imbalance in fluid–electrolyte homeostasis. The choroid plexus, a complex tissue found in the brain ventricles, is one of the most secretory tissues in the body, producing approximately 500 mL of CSF per day in an adult human. In this manuscript, two key transport proteins of the choroid plexus epithelial cells, transient receptor potential vanilloid 4 and sodium, potassium, 2 chloride co-transporter 1, will be considered. Both appear to play key roles in CSF production, and their inhibition or genetic manipulation has been shown to affect CSF volume. As with most transporters, these proteins are regulated by kinases. Therefore, specific kinase inhibitors are also potential targets for the development of pharmaceuticals to treat hydrocephalus.

Trajectory of serum chloride levels during decongestive therapy in acute heart failure

BACKGROUND

Hypochloremia is a risk factor for poor outcomes in patients with acute heart failure (AHF). However, the changes in serum chloride levels during decongestion therapy and their impact on prognosis remain unknown.

METHODS

In total, 2798 patients with AHF were retrospectively studied and divided into four groups according to their admission and discharge serum chloride levels: (1) normochloremia (n=2,192, 78%); (2) treatment-associated hypochloremia, defined as admission normochloremia with a subsequent decrease (<98 mEq/L) during hospitalization (n=335, 12%); (3) resolved hypochloremia, defined as admission hypochloremia that disappeared at discharge (n=128, 5%); (4) persistent hypochloremia, defined as chloride <98 mEq/L at admission and discharge (n = 143, 5%). The primary outcome was all-cause death, and the secondary outcomes were cardiovascular death and a composite of cardiovascular death and rehospitalization for heart failure after discharge.

RESULTS

The mean age was 76 ± 12 years and 1584 (57%) patients were men. The mean left ventricular ejection fraction was 46 ± 16%. During a median follow-up period of 365 days, persistent hypochloremia was associated with an increased risk of all-cause death (adjusted hazard ratio [95% confidence interval]: 2.27 [1.53-3.37], p < 0.001), cardiovascular death (2.38 [1.46-3.87], p < 0.001), and a composite of cardiovascular death and heart failure rehospitalization (1.47 [1.06-2.06], p = 0.022). However, the outcomes were comparable between patients with resolved hypochloremia and normochloremia.

CONCLUSIONS

Persistent hypochloremia was associated with worse clinical outcomes, while resolved hypochloremia and normochloremia showed a comparable prognosis. Changes in serum chloride levels can help identify patients with poor prognoses and can be used to determine subsequent treatment strategies.

Etiology and Management of Edema: A Review

The development of peripheral edema can often pose a significant diagnostic and therapeutic challenge for practitioners due to its association with a wide variety of underlying disorders ranging in severity. Updates to the original Starling's principle have provided new mechanistic insights into edema formation. Additionally, contemporary data highlighting the role of hypochloremia in the development of diuretic resistance provide a possible new therapeutic target. This article reviews the pathophysiology of edema formation and discusses implications for treatment.

Control of sodium and potassium homeostasis by renal distal convoluted tubules

Distal convoluted tubules (DCT), which contain the Na-Cl cotransporter (NCC) inhibited by thiazide diuretics, undergo complex modulation to preserve Na+ and K+ homeostasis. The lysine kinases 1 and 4 (WNK1 and WNK4), identified as hyperactive in the hereditary disease pseudohypoaldosteronism type 2, are responsible for activation of NCC and consequent hypokalemia and hypertension. WNK4, highly expressed in DCT, activates the SPAK/OSR1 kinases, which phosphorylate NCC and other regulatory proteins and transporters in the distal nephron. WNK4 works as a chloride sensor through a Cl- binding site, which acts as an on/off switch at this kinase in response to changes of basolateral membrane electrical potential, the driving force of cellular Cl- efflux. High intracellular Cl- in hyperkalemia decreases NCC phosphorylation and low intracellular Cl- in hypokalemia increases NCC phosphorylation and activity, which makes plasma K+ concentration a central modulator of NCC and of K+ secretion. The WNK4 phosphorylation by cSrc or SGK1, activated by angiotensin II or aldosterone, respectively, is another relevant mechanism of NCC, ENaC, and ROMK modulation in states such as volume reduction, hyperkalemia, and hypokalemia. Loss of NCC function induces upregulation of electroneutral NaCl reabsorption by type B intercalated cells through the combined activity of pendrin and NDCBE, as demonstrated in double knockout mice (KO) animal models, Ncc/pendrin or Ncc/NDCBE. The analysis of ks-Nedd-4-2 KO animal models introduced the modulation of NEDD4-2 by intracellular Mg2+ activity as an important regulator of NCC, explaining the thiazide-induced persistent hypokalemia.

Intracellular Ion Control of WNK Signaling

The with no lysine (K) (WNK) kinases are an evolutionarily ancient group of kinases with atypical placement of the catalytic lysine and diverse physiological roles. Recent studies have shown that WNKs are directly regulated by chloride, potassium, and osmotic pressure. Here, we review the discovery of WNKs as chloride-sensitive kinases and discuss physiological contexts in which chloride regulation of WNKs has been demonstrated. These include the kidney, pancreatic duct, neurons, and inflammatory cells. We discuss the interdependent relationship of osmotic pressure and intracellular chloride in cell volume regulation. We review the recent demonstration of potassium regulation of WNKs and speculate on possible physiological roles. Finally, structural and mechanistic aspects of intracellular ion and osmotic pressure regulation of WNKs are discussed.

NCC regulation by WNK signal cascade

With-no-lysine (K) (WNK) kinases have been identified as the causal genes for pseudohypoaldosteronism type II (PHAII), a rare hereditary hypertension condition characterized by hyperkalemia, hyperchloremic metabolic acidosis, and thiazide-hypersensitivity. We thought that clarifying the link between WNK and NaCl cotransporter (NCC) would bring us new mechanism(s) of NCC regulation. For the first time, we were able to produce a knock-in mouse model of PHAII and anti-phosphorylated NCC antibodies against the putative NCC phosphorylation sites and discover that constitutive activation of NCC and increased phosphorylation of NCC are the primary pathogenesis of the disease in vivo. We have since demonstrated that this regulatory mechanism is mediated by the kinases oxidative stress-response protein 1 (OSR1) and STE20/SPS1-related proline/alanine-rich kinase (SPAK) (WNK-OSR1/SPAK-NCC signaling cascade) and that the signaling is not only important in the pathological condition of PHAII but also plays a crucial physiological role in the regulation of NCC.

The serine-threonine protein phosphatases that regulate the thiazide-sensitive NaCl cotransporter

The activity of the Na⁺-Cl^- cotransporter (NCC) in the distal convoluted tubule (DCT) is finely tuned by phosphorylation networks involving serine/threonine kinases and phosphatases. While much attention has been paid to the With-No-lysine (K) kinase (WNK)- STE20-related Proline Alanine rich Kinase (SPAK)/Oxidative Stress Responsive kinase 1 (OSR1) signaling pathway, there remain many unanswered questions regarding phosphatase-mediated modulation of NCC and its interactors. The phosphatases shown to regulate NCC's activity, directly or indirectly, are protein phosphatase 1 (PP1), protein phosphatase 2A (PP2A), calcineurin (CN), and protein phosphatase 4 (PP4). PP1 has been suggested to directly dephosphorylate WNK4, SPAK, and NCC. This phosphatase increases its abundance and activity when extracellular K⁺ is increased, which leads to distinct inhibitory mechanisms towards NCC. Inhibitor-1 (I1), oppositely, inhibits PP1 when phosphorylated by protein kinase A (PKA). CN inhibitors, like tacrolimus and cyclosporin A, increase NCC phosphorylation, giving an explanation to the Familial Hyperkalemic Hypertension-like syndrome that affects some patients treated with these drugs. CN inhibitors can prevent high K⁺-induced dephosphorylation of NCC. CN can also dephosphorylate and activate Kelch-like protein 3 (KLHL3), thus decreasing WNK abundance. PP2A and PP4 have been shown in in vitro models to regulate NCC or its upstream activators. However, no studies in native kidneys or tubules have been performed to test their physiological role in NCC regulation. This review focuses on these dephosphorylation mediators and the transduction mechanisms possibly involved in physiological states that require of the modulation of the dephosphorylation rate of NCC.

Blood pressure effects of sodium transport along the distal nephron

The mammalian distal nephron is a target of highly effective antihypertensive drugs. Genetic variants that alter its transport activity are also inherited causes of high or low blood pressure, clearly establishing its central role in human blood pressure regulation. Much has been learned during the past 25 years about salt transport along this nephron segment, spurred by the cloning of major transport proteins and the discovery of disease-causing genetic variants. Recognition is increasing that substantial cellular and segmental heterogeneity is present along this segment, with electroneutral sodium transport dominating more proximal segments and electrogenic sodium transport dominating more distal segments. Coupled with recent insights into factors that modulate transport along these segments, we now understand one important mechanism by which dietary potassium intake influences sodium excretion and blood pressure. This finding has solved the aldosterone paradox, by demonstrating how aldosterone can be both kaliuretic, when plasma potassium is elevated, and anti-natriuretic, when extracellular fluid volume is low. However, what also has become clear is that aldosterone itself only stimulates a portion of the mineralocorticoid receptors along this segment, with the others being activated by glucocorticoid hormones instead. These recent insights provide an increasingly clear picture of how this short nephron segment contributes to blood pressure homeostasis and have important implications for hypertension prevention and treatment.

Hypochloremia as a novel adverse prognostic factor in acute liver failure

BACKGROUND AND AIMS

Emerging evidence has identified hypochloremia as an independent predictor for mortality in multiple conditions including cirrhosis. Acute liver failure (ALF) is frequently complicated by electrolyte abnormalities. We investigated the prognostic value of hypochloremia in a large cohort of ALF patients from North America.

METHODS

The Acute Liver Failure Study Group (ALFSG) registry is a longitudinal cohort study involving 2588 ALF patients enrolled prospectively from 32 North American academic centres. The primary outcome was a composite of 21-day all-cause mortality or requirement for liver transplantation (death/LT).

RESULTS

Patients with hypochloremia (<98 mEq/L) had a significantly higher 21-day mortality rate (42.1%) compared with those with normal (27.5%) or high (>107 mEq/L) chloride (28.0%) (p < .001). There was lower transplant-free cumulative survival in the hypochloremic group than in the normo- or hyper-chloremic groups (log-rank, χ2 24.2, p < .001). Serum chloride was inversely associated with the hazard of 21-day death/LT with multivariable adjustment for known prognostic factors (adjusted hazard ratio [aHR]: 0.977; 95% CI: 0.969-0.985; p < .001). Adding chloride to the ALFSG Prognostic Index more accurately predicted risk of death/LT in 19% of patients (net reclassification improvement [NRI] = 0.19, 95% CI: 0.13-0.25) but underestimated the probability of transplant-free survival in 34% of patients (NRI = -0.34, 95% CI: -0.39 to -0.28).

CONCLUSIONS

Hypochloremia is a novel independent adverse prognostic factor in ALF. A new ALFSG-Cl Prognostic Index may improve the sensitivity to identify patients at risk for death without LT.

Multiple molecular mechanisms are involved in the activation of the kidney sodium-chloride cotransporter by hypokalemia

Low potassium intake activates the kidney sodium-chloride cotransporter (NCC) whose phosphorylation and activity depend on the With-No-Lysine kinase 4 (WNK4) that is inhibited by chloride binding to its kinase domain. Low extracellular potassium activates NCC by decreasing intracellular chloride thereby promoting chloride dissociation from WNK4 where residue L319 of WNK4 participates in chloride coordination. Since the WNK4-L319F mutant is constitutively active and chloride-insensitive in vitro, we generated mice harboring this mutation that displayed slightly increased phosphorylated NCC and mild hyperkalemia when on a 129/sv genetic background. On a low potassium diet, upregulation of phosphorylated NCC was observed, suggesting that in addition to chloride sensing by WNK4, other mechanisms participate which may include modulation of WNK4 activity and degradation by phosphorylation of the RRxS motif in regulatory domains present in WNK4 and KLHL3, respectively. Increased levels of WNK4 and kidney-specific WNK1 and phospho-WNK4-RRxS were observed in wild-type and WNK4L319F/L319F mice on a low potassium diet. Decreased extracellular potassium promoted WNK4-RRxS phosphorylation in vitro and ex vivo as well. These effects might be secondary to intracellular chloride depletion, as reduction of intracellular chloride in HEK293 cells increased phospho-WNK4-RRxS. Phospho-WNK4-RRxS levels were increased in mice lacking the Kir5.1 potassium channel, which presumably have decreased distal convoluted tubule intracellular chloride. Similarly, phospho-KLHL3 was modulated by changes in intracellular chloride in HEK293 cells. Thus, our data suggest that multiple chloride-regulated mechanisms are responsible for NCC upregulation by low extracellular potassium.

Arterial Blood Pressure, Neuronal Excitability, Mineral Metabolism and Cell Volume Regulation Mechanisms Revealed by Xenopus laevis oocytes

Xenopus laevis oocytes have been an invaluable tool to discover and explore the molecular mechanisms and characteristics of many proteins, in particular integral membrane proteins. The oocytes were fundamental in many projects designed to identify the cDNA encoding a diversity of membrane proteins including receptors, transporters, channels and pores. In addition to being a powerful tool for cloning, oocytes were later used to experiment with the functional characterization of many of the identified proteins. In this review I present an overview of my personal 30-year experience using Xenopus laevis oocytes and the impact this had on a variety of fields such as arterial blood pressure, neuronal excitability, mineral metabolism and cell volume regulation.

Relation of Low Chloride Concentration to Diuretic Efficiency and Transplant-Free Survival in Children Hospitalized With Heart Failure

Serum chloride plays an important role in fluid homeostasis and is associated with impaired diuretic responsiveness and mortality in adults with heart failure (HF). We sought to characterize the relationship of serum chloride and diuretic efficiency (DE) and to determine its prognostic importance in children hospitalized with acute decompensated HF (ADHF). We studied DE, defined as net fluid output/kg+constant per mg of loop diuretic/kg, in 200 children hospitalized with ADHF. Median serum chloride at admission was 102 mmol/L (interquartile range 99 to 105 mmol/L), and hypochloremia (chloride ≤96 mmol/L) was present in 16% of the population at admission. Serum chloride correlated with serum sodium (r = 0.66; p < 0.001) and bicarbonate (r = -0.39; p < 0.001). In the adjusted analysis, lower chloride was associated with reduced DE (p < 0.001). Serum sodium was associated with DE on the unadjusted analysis; however, the association was eliminated when added to the model with chloride (p = 0.442). Lower chloride was also associated with features of inadequate decongestion during hospitalization: a positive fluid balance (p = 0.003), greater cumulative loop diuretic dose per weight (p = 0.001), addition of a thiazide diuretic during hospitalization (p < 0.001), less weight loss (p = 0.025), and longer length of stay (p = 0.003). Chloride concentration was independently associated with death or transplant 1 year after admission (hazard ratio 0.94; p < 0.001). As a dichotomous variable, hypochloremia was independently associated with reduced DE (p < 0.001) and decreased 1-year transplant-free survival (hazard ratio 2.3, p < 0.001). Lower serum chloride at hospital admission is strongly and independently associated with impaired DE and reduced transplant-free survival in children hospitalized with ADHF.

The E3 ubiquitin-protein ligase Nedd4-2 regulates the sodium chloride cotransporter NCC but is not required for a potassium-induced reduction of NCC expression

Na⁺ and K⁺ balance is influenced by the activity of the sodium chloride cotransporter NCC in the distal convoluted tubule. NCC activity and abundance are reduced by high extracellular K⁺. The E3 ubiquitin ligase neural precursor cell expressed developmentally downregulated 4–2 (Nedd4-2) has been proposed as a modulator of NCC abundance. Here, we examined the functional role of Nedd4-2 on NCC regulation and whether Nedd4-2 is important for the effects of high extracellular K⁺ on NCC. Total and plasma membrane levels of ubiquitylated NCC were lower in NCC-expressing MDCKI cells after Nedd4-2 deletion. NCC and phosphorylated NCC (pT58-NCC) levels were higher after Nedd4-2 deletion, and NCC levels on the plasma membrane were elevated. No significant changes were seen after Nedd4-2 knockdown in the levels of SPAK and phosphorylated SPAK (pS373-SPAK), the major NCC regulatory kinase. Nedd4-2 deficiency had no effect on the internalization rate of NCC from the plasma membrane, but NCC protein half-life was increased. In ex vivo experiments with kidney tubule suspensions from Nedd4-2 knockout (KO) mice, high K⁺ reduced total and pT58-NCC regardless of genotype. We conclude that Nedd4-2 is involved in ubiquitylation of NCC and modulating its plasma membrane levels and degradation. However, Nedd4-2 does not appear to be important for K⁺ induced reductions in NCC abundance.

The Post-Translational Modification Networking in WNK-Centric Hypertension Regulation and Electrolyte Homeostasis

The with-no-lysine (WNK) kinase family, comprising four serine-threonine protein kinases (WNK1-4), were first linked to hypertension due to their mutations in association with pseudohypoaldosteronism type II (PHAII). WNK kinases regulate crucial blood pressure regulators, SPAK/OSR1, to mediate the post-translational modifications (PTMs) of their downstream ion channel substrates, such as sodium chloride co-transporter (NCC), epithelial sodium chloride (ENaC), renal outer medullary potassium channel (ROMK), and Na/K/2Cl co-transporters (NKCCs). In this review, we summarize the molecular pathways dysregulating the WNKs and their downstream target renal ion transporters. We summarize each of the genetic variants of WNK kinases and the small molecule inhibitors that have been discovered to regulate blood pressure via WNK-triggered PTM cascades.

Regulation of Distal Nephron Transport by Intracellular Chloride and Potassium

BACKGROUND

Low potassium increases the phosphorylation and activity of the sodium chloride cotransporter (NCC) in the distal convoluted tubule of the nephron, which contributes to the hypertensive effect of the modern low potassium/high sodium diet. A central mediator of potassium regulation of NCC is the chloride-sensitive With No Lysine [K] (WNK) kinase.

SUMMARY

Chloride directly inhibits WNKs by binding to the active site. The mechanisms underlying WNK regulation by extracellular potassium are reviewed, as well as the modulatory effect of kidney-specific-WNK1. WNK1, but not WNK1 kinase activity, is also required for the aldosterone-independent regulation of the epithelial sodium channel by potassium. Whether intracellular chloride could be involved in this process is discussed. Recent studies demonstrating direct regulation of WNKs by intracellular potassium are also reviewed, and the potential physiological relevance to renal epithelial ion transport is discussed.

KEY MESSAGES

WNKs are sensors of the intracellular ionic milieu. In the nephron, changes in extracellular ion concentrations, resulting in changes in intracellular ion concentration, regulate WNK activity and downstream transporters and channels to maintain total body ion homeostasis.

Potassium homeostasis: sensors, mediators, and targets

Transmembrane potassium (K) gradients are key determinants of membrane potential that can modulate action potentials, control muscle contractility, and influence ion channel and transporter activity. Daily K intake is normally equal to the amount of K in the entire extracellular fluid (ECF) creating a critical challenge - how to maintain ECF [K] and membrane potential in a narrow range during feast and famine. Adaptations to maintain ECF [K] include sensing the K intake, sensing ECF [K] vs. desired set-point and activating mediators that regulate K distribution between ECF and ICF, and regulate renal K excretion. In this focused review, we discuss the basis of these adaptions, including (1) potential mechanisms for rapid feedforward signaling to kidney and muscle after a meal (before a rise in ECF [K]), (2) how skeletal muscles sense and respond to changes in ECF [K], (3) effects of K on aldosterone biosynthesis, and (4) how the kidney responds to changes in ECF [K] to modify K excretion. The concepts of sexual dimorphisms in renal K handling adaptation are introduced, and the molecular mechanisms that can account for the benefits of a K-rich diet to maintain cardiovascular health are discussed. Although the big picture of K homeostasis is becoming more clear, we also highlight significant pieces of the puzzle that remain to be solved, including knowledge gaps in our understanding of initiating signals, sensors and their connection to homeostatic adjustments of ECF [K].

Strategy for sodium-salt substitution: On the relationship between hypertension and dietary intake of cations

Chronic diseases, especially cardiovascular diseases (CVD), have become one of the main causes affecting human health. Hypertension is a prominent representative of CVD. The formation and development of hypertension is closely related to people's daily diet. A large number of studies have shown that excessive intake of salt (NaCl) could increase the risk of hypertension. In recent years, more and more investigations have focused on other cations that may be contained in edible salt, exploring whether they have an effect on hypertension and the underlying mechanism. This article focuses on the relationship between four metal elements (potassium, calcium, magnesium, and zinc) and hypertension, by discussing the main metabolic pathway, the impact of diet intake on blood pressure, and especially the regulation mechanisms on blood pressure in detail. At the same time, some opinions and suggestions are put forward, combined with the current hot topics "salt reduction" and "salt substitution".

A narrative review of Hyporeninemic hypertension-an indicator for monogenic forms of hypertension

BACKGROUND AND OBJECTIVE

While the role of the renin-angiotensin-aldosterone system (RAAS) in the development of hypertension is well known, the significance and contribution of low renin hypertension is often overlooked. RAAS stimulation results in more tubular absorption of sodium and water along the nephron, contributing to a higher circulating vascular volume. In addition, members of the RAAS system, such as angiotensin II, have direct effects on vascular vasoconstriction, the heart, aldosterone synthesis in the adrenal glands, the sympathetic nervous system, and the central nervous system. This has resulted in a line of antihypertensive therapeutics targeting RAAS with angiotensin converting enzyme (ACE) inhibitors, angiotensin receptor blockers (ARBs), and renin inhibitors, which prevent conversion of angiotensinogen to angiotensin. While general practitioners and nephrologists are well aware of the causes and the long-term consequences of elevated renin and aldosterone levels, the opposite situation with low renin and/or low aldosterone levels is frequently underappreciated. The objective of this review is to provide insight to the less common forms of hyporeninemic hypertension.

METHODS

We searched the PubMed online library for keywords related to hyporeninemic hypertension and focused on the pediatric population. For pathophysiology we focused on literature of the last 5 years.

KEY CONTENT AND FINDINGS

The low renin and aldosterone levels may be indicators of inherited (especially when associated with hypokalemia), monogenic forms of hypertension stimulating excessive tubular sodium and water absorption which subsequently results in plasma volume expansion and hypertension. These forms of hypertension require frequently specific forms of therapy. This underlines the importance of the practitioner to be familiar with these rare diseases.

CONCLUSIONS

In this review article, we outline the different forms of hypertension characterized by low renin/low aldosterone and low renin/high aldosterone levels, how to diagnose these forms of hypertension, and how to treat them.

Renal calcium and magnesium handling in Gitelman syndrome

Gitelman syndrome (GS) is an autosomal recessive salt-losing tubulopathy caused by biallelic inactivating mutations in the SLC12A3 gene. This gene encodes the thiazide-sensitive sodium-chloride cotransporter (NCC) which is exclusively expressed in the distal convoluted tubules (DCT). GS patients classically present with hypokalemic metabolic alkalosis with hypocalciuria and hypomagnesemia. While hypokalemia and metabolic alkalosis are easily explained by effects of the genotypic defect in GS, the mechanisms by which hypomagnesemia and hypocalciuria develop in GS are poorly understood. In this review, we aim to achieve three major objectives. First, present a concise discussion about current understanding on physiologic calcium and magnesium handling in the DCT. Second, integrate expression data from studies on calciotropic and magnesiotropic proteins relevant to the GS disease state. Lastly, provide insights into the possible mechanisms of calcium-magnesium crosstalk relating to the co-occurrence of hypocalciuria and hypomagnesemia in GS models. Our analyses highlight specific areas of study that are valuable in elucidating possible molecular pathways of hypocalciuria and hypomagnesemia in GS.

Potassium and the kidney: a reciprocal relationship with clinical relevance

By controlling urinary potassium excretion, the kidneys play a key role in maintaining whole-body potassium homeostasis. Conversely, low urinary potassium excretion (as a proxy for insufficient dietary intake) is increasingly recognized as a risk factor for the progression of kidney disease. Thus, there is a reciprocal relationship between potassium and the kidney: the kidney regulates potassium balance but potassium also affects kidney function. This review explores this relationship by discussing new insights into kidney potassium handling derived from recently characterized tubulopathies and studies on sexual dimorphism. These insights reveal a central but non-exclusive role for the distal convoluted tubule in sensing potassium and subsequently modifying the activity of the sodium-chloride cotransporter. This is another example of reciprocity: activation of the sodium-chloride cotransporter not only reduces distal sodium delivery and therefore potassium secretion but also increases salt sensitivity. This mechanism helps explain the well-known relationship between dietary potassium and blood pressure. Remarkably, in children, blood pressure is related to dietary potassium but not sodium intake. To explore how potassium deficiency can cause kidney injury, we review the mechanisms of hypokalemic nephropathy and discuss if these mechanisms may explain the association between low dietary potassium intake and adverse kidney outcomes. We discuss if potassium should be repleted in patients with kidney disease and what role dietary potassium plays in the risk of hyperkalemia. Supported by data and physiology, we reach the conclusion that we should view potassium not only as a potentially dangerous cation but also as a companion in the battle against kidney disease.

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.

Molecular mechanisms for the modulation of blood pressure and potassium homeostasis by the distal convoluted tubule

Epidemiological and clinical observations have shown that potassium ingestion is inversely correlated with arterial hypertension prevalence and cardiovascular mortality. The higher the dietary potassium, the lower the blood pressure and mortality. This phenomenon is explained, at least in part, by the interaction between salt reabsorption in the distal convoluted tubule (DCT) and potassium secretion in the connecting tubule/collecting duct of the mammalian nephron: In order to achieve adequate K⁺ secretion levels under certain conditions, salt reabsorption in the DCT must be reduced. Because salt handling by the kidney constitutes the basis for the long‐term regulation of blood pressure, losing salt prevents hypertension. Here, we discuss how the study of inherited diseases in which salt reabsorption in the DCT is affected has revealed the molecular players, including membrane transporters and channels, kinases, and ubiquitin ligases that form the potassium sensing mechanism of the DCT and the processes through which the consequent adjustments in salt reabsorption are achieved.

Chloride sensing by WNK1 regulates NLRP3 inflammasome activation and pyroptosis

The NLRP3 inflammasome mediates the production of proinflammatory cytokines and initiates inflammatory cell death. Although NLRP3 is essential for innate immunity, aberrant NLRP3 inflammasome activation contributes to a wide variety of inflammatory diseases. Understanding the pathways that control NLRP3 activation will help develop strategies to treat these diseases. Here we identify WNK1 as a negative regulator of the NLRP3 inflammasome. Macrophages deficient in WNK1 protein or kinase activity have increased NLRP3 activation and pyroptosis compared with control macrophages. Mice with conditional knockout of WNK1 in macrophages have increased IL-1β production in response to NLRP3 stimulation compared with control mice. Mechanistically, WNK1 tempers NLRP3 activation by balancing intracellular Cl- and K+ concentrations during NLRP3 activation. Collectively, this work shows that the WNK1 pathway has a critical function in suppressing NLRP3 activation and suggests that pharmacological inhibition of this pathway to treat hypertension might have negative clinical implications.

High dietary potassium causes ubiquitin-dependent degradation of the kidney sodium-chloride cotransporter

The thiazide-sensitive sodium-chloride cotransporter (NCC) in the renal distal convoluted tubule (DCT) plays a critical role in regulating blood pressure (BP) and K⁺ homeostasis. During hyperkalemia, reduced NCC phosphorylation and total NCC abundance facilitate downstream electrogenic K⁺ secretion and BP reduction. However, the mechanism for the K⁺-dependent reduction in total NCC levels is unknown. Here, we show that NCC levels were reduced in ex vivo renal tubules incubated in a high-K⁺ medium for 24–48 h. This reduction was independent of NCC transcription, but was prevented using inhibitors of the proteasome (MG132) or lysosome (chloroquine). Ex vivo, high K⁺ increased NCC ubiquitylation, but inhibition of the ubiquitin conjugation pathway prevented the high K⁺-mediated reduction in NCC protein. In tubules incubated in high K⁺ media ex vivo or in the renal cortex of mice fed a high K⁺ diet for 4 days, the abundance and phosphorylation of heat shock protein 70 (Hsp70), a key regulator of ubiquitin-dependent protein degradation and protein folding, were decreased. Conversely, in similar samples the expression of PP1α, known to dephosphorylate Hsp70, was also increased. NCC coimmunoprecipitated with Hsp70 and PP1α, and inhibiting their actions prevented the high K⁺-mediated reduction in total NCC levels. In conclusion, we show that hyperkalemia drives NCC ubiquitylation and degradation via a PP1α-dependent process facilitated by Hsp70. This mechanism facilitates K⁺-dependent reductions in NCC to protect plasma K⁺ homeostasis and potentially reduces BP.

Prognostic Value of Hypochloremia in Critically Ill Patients With Decompensated Cirrhosis

OBJECTIVES

Cirrhosis is frequently complicated by electrolyte disturbances, with prior studies primarily focused on the importance of hyponatremia. Emerging evidence on patients with chronic heart failure and chronic kidney disease has identified hypochloremia as an independent predictor for mortality. This study aimed to investigate the prognostic value of serum chloride and its association with mortality in cirrhotic patients.

DESIGN

Retrospective cohort study.

SETTING

The medical ICU at Parkland Memorial Hospital, a tertiary care public health system in Dallas, Texas.

PATIENTS

Adult patients with confirmed diagnosis of decompensated cirrhosis who were admitted to the ICU between March 2015 and March 2017.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Kaplan-Meier analysis and multivariable Cox proportional hazard ratio models were performed to determine the impact of hypochloremia on 180-day mortality. Of the 389 enrolled patients, 133 (34.2%) died within 180 days of ICU admission. Patients with hypochloremia had higher 180-day mortality than those with normochloremia (45.2% vs 26.7%; p < 0.0001). Cumulative survival via the Kaplan-Meier method was significantly lower in the hypochloremic group. Serum chloride was independently associated with 180-day mortality with multivariable adjustment (hazard ratio, 0.95; 95% CI, 0.93-0.98; p = 0.001) or after adjusting for Model for End-stage Liver Disease or Sequential Organ Failure Assessment. Contrarily, the inverse association between serum sodium and mortality no longer existed in all multivariable models.

CONCLUSIONS

Serum chloride is independently and inversely associated with short-term mortality in critically ill cirrhotic patients. Hypochloremia, but not hyponatremia, remained associated with mortality with multivariable analyses, suggesting that hypochloremia may account for the mortality risk previously attributed to hyponatremia. These findings signify the prognostic value of serum chloride and potential inclusion of chloride into future cirrhosis prognostic scores.

Roles of WNK4 and SPAK in K+-mediated dephosphorylation of the NaCl cotransporter

Phosphorylation of the thiazide-sensitive NaCl cotransporter (NCC) in the distal convoluted tubule (DCT) is altered rapidly in response to changes in extracellular K+ concentration ([K+]). High extracellular [K+] is believed to activate specific phosphatases to dephosphorylate NCC, thereby reducing its activity. This process is defective in the human disease familial hyperkalemic hypertension, in which extracellular [K+] fails to dephosphorylate NCC, suggesting an interplay between NCC-activating and NCC-inactivating switches. Here, we explored the role of STE20/SPS1-related proline-alanine-rich protein kinase (SPAK) and intracellular Cl- concentration in the rapid effects of extracellular K+ on NCC phosphorylation. SPAK was found to be rapidly dephosphorylated in vitro in human embryonic kidney cells and ex vivo in kidney slices by high [K+]. Acute high-K+ challenge resulted in DCT1-specific SPAK dephosphorylation in vivo and dissolution of SPAK puncta. In line with the postulate of interplay between activating and inactivating switches, we found that the "on" switch, represented by with no lysine kinase 4 (WNK4)-SPAK, must be turned off for rapid NCC dephosphorylation by high [K+]. Longer-term WNK-SPAK-mediated stimulation, however, altered the sensitivity of the system, as it attenuated rapid NCC dephosphorylation due to acute K+ loading. Although blockade of protein phosphatase (PP)1 increased NCC phosphorylation at baseline, neither PP1 nor PP3, singly or in combination, was essential for NCC dephosphorylation. Overall, our data suggest that NCC phosphorylation is regulated by a dynamic equilibrium between activating kinases and inactivating phosphatases, with kinase inactivation playing a key role in the rapid NCC dephosphorylation by high extracellular K+.NEW & NOTEWORTHY Although a great deal is known about mechanisms by which thiazide-sensitive NaCl cotransporter is phosphorylated and activated, much less is known about dephosphorylation. Here, we show that rapid dephosphorylation by high K+ depends on the Cl- sensitivity of with no lysine kinase 4 and the rapid dephosphorylation of STE20/SPS1-related proline-alanine-rich protein kinase, primarily along the early distal convoluted tubule.

Role of KLHL3 and dietary K+ in regulating KS-WNK1 expression

The physiological role of the shorter isoform of with no lysine kinase (WNK)1 that is exclusively expressed in the kidney (KS-WNK1), with particular abundance in the distal convoluted tubule, remains elusive. KS-WNK1, despite lacking the kinase domain, is nevertheless capable of stimulating the NaCl cotransporter, apparently through activation of WNK4. It has recently been shown that a less severe form of familial hyperkalemic hypertension featuring only hyperkalemia is caused by missense mutations in the WNK1 acidic domain that preferentially affect cullin 3 (CUL3)-Kelch-like protein 3 (KLHL3) E3-induced degradation of KS-WNK1 rather than that of full-length WNK1. Here, we show that full-length WNK1 is indeed less impacted by the CUL3-KLHL3 E3 ligase complex compared with KS-WNK1. We demonstrated that the unique 30-amino acid NH2-terminal fragment of KS-WNK1 is essential for its activating effect on the NaCl cotransporter and recognition by KLHL3. We identified specific amino acid residues in this region critical for the functional effect of KS-WNK1 and KLHL3 sensitivity. To further explore this, we generated KLHL3-R528H knockin mice that mimic human mutations causing familial hyperkalemic hypertension. These mice revealed that the KLHL3 mutation specifically increased expression of KS-WNK1 in the kidney. We also observed that in wild-type mice, the expression of KS-WNK1 was only detectable after exposure to a low-K+ diet. These findings provide new insights into the regulation and function of KS-WNK1 by the CUL3-KLHL3 complex in the distal convoluted tubule and indicate that this pathway is regulated by dietary K+ levels.NEW & NOTEWORTHY In this work, we demonstrated that the kidney-specific isoform of with no lysine kinase 1 (KS-WNK1) in the kidney is modulated by dietary K+ and activity of the ubiquitin ligase protein Kelch-like protein 3. We analyzed the role of different amino acid residues of KS-WNK1 in its activity against the NaCl cotransporter and sensitivity to Kelch-like protein 3.

Activation of the kidney sodium chloride cotransporter by the β2-adrenergic receptor agonist salbutamol increases blood pressure

The thiazide-sensitive sodium-chloride-cotransporter (NCC) in the kidney distal convoluted tubule (DCT) plays an essential role in sodium and potassium homeostasis. Here, we demonstrate that NCC activity is increased by the β2-adrenoceptor agonist salbutamol, a drug prevalently used to treat asthma. Relative to β1-adrenergic receptors, the β2-adrenergic receptors were greatly enriched in mouse DCT cells. In mice, administration of salbutamol increased NCC phosphorylation (indicating increased activity) within 30 minutes but also caused hypokalemia, which also increases NCC phosphorylation. In ex vivo kidney slices and isolated tubules, salbutamol increased NCC phosphorylation in the pharmacologically relevant range of 0.01-10 μM, an effect observed after 15 minutes and maintained at 60 minutes. Inhibition of the inwardly rectifying potassium channel (Kir) 4.1 or the downstream with-no-lysine kinases (WNKs) and STE20/SPS1-related proline alanine-rich kinase (SPAK) pathway greatly attenuated, but did not prevent, salbutamol-induced NCC phosphorylation. Salbutamol increased cAMP in tubules, kidney slices and mpkDCT cells (model of DCT). Phosphoproteomics indicated that protein phosphatase 1 (PP1) was a key upstream regulator of salbutamol effects. A role for PP1 and the PP1 inhibitor 1 (I1) was confirmed in tubules using inhibitors of PP1 or kidney slices from I1 knockout mice. On normal and high salt diets, salbutamol infusion increased systolic blood pressure, but this increase was normalized by thiazide suggesting a role for NCC. Thus, β2-adrenergic receptor signaling modulates NCC activity via I1/PP1 and WNK-dependent pathways, and chronic salbutamol administration may be a risk factor for hypertension.

Genetic and drug-induced hypomagnesemia: different cause, same mechanism

Magnesium (Mg2+) plays an essential role in many biological processes. Mg2+ deficiency is therefore associated with a wide range of clinical effects including muscle cramps, fatigue, seizures and arrhythmias. To maintain sufficient Mg2+ levels, (re)absorption of Mg2+ in the intestine and kidney is tightly regulated. Genetic defects that disturb Mg2+ uptake pathways, as well as drugs interfering with Mg2+ (re)absorption cause hypomagnesemia. The aim of this review is to provide an overview of the molecular mechanisms underlying genetic and drug-induced Mg2+ deficiencies. This leads to the identification of four main mechanisms that are affected by hypomagnesemia-causing mutations or drugs: luminal transient receptor potential melastatin type 6/7-mediated Mg2+ uptake, paracellular Mg2+ reabsorption in the thick ascending limb of Henle's loop, structural integrity of the distal convoluted tubule and Na+-dependent Mg2+ extrusion driven by the Na+/K+-ATPase. Our analysis demonstrates that genetic and drug-induced causes of hypomagnesemia share common molecular mechanisms. Targeting these shared pathways can lead to novel treatment options for patients with hypomagnesemia.

Deletion of Kir5.1 abolishes the effect of high Na+ intake on Kir4.1 and Na+-Cl- cotransporter

High sodium (HS) intake inhibited epithelial Na⁺ channel (ENaC) in the aldosterone-sensitive distal nephron and Na⁺-Cl^- cotransporter (NCC) by suppressing basolateral Kir4.1/Kir5.1 in the distal convoluted tubule (DCT), thereby increasing renal Na⁺ excretion but not affecting K⁺ excretion. The aim of the present study was to explore whether deletion of Kir5.1 compromises the inhibitory effect of HS on NCC expression/activity and renal K⁺ excretion. Patch-clamp experiments demonstrated that HS failed to inhibit DCT basolateral K⁺ channels and did not depolarize K⁺ current reversal potential of the DCT in Kir5.1 knockout (KO) mice. Moreover, deletion of Kir5.1 not only increased the expression of Kir4.1, phospho-NCC, and total NCC but also abolished the inhibitory effect of HS on the expression of Kir4.1, phospho-NCC, and total NCC and thiazide-induced natriuresis. Also, low sodium-induced stimulation of NCC expression/activity and basolateral K⁺ channels in the DCT were absent in Kir5.1 KO mice. Deletion of Kir5.1 decreased ENaC currents in the late DCT, and HS further inhibited ENaC activity in Kir5.1 KO mice. Finally, measurement of the basal renal K⁺ excretion rate with the modified renal clearance method demonstrated that long-term HS inhibited the renal K⁺ excretion rate and steadily increased plasma K⁺ levels in Kir5.1 KO mice but not in wild-type mice. We conclude that Kir5.1 plays an important role in mediating the effect of HS intake on basolateral K⁺ channels in the DCT and NCC activity/expression. Kir5.1 is involved in maintaining renal ability of K⁺ excretion during HS intake. NEW & NOTEWORTHY Kir5.1 plays an important role in mediating the effect of high sodium intake on basolateral K⁺ channels in the distal convoluted tubule and Na⁺-Cl^- cotransporter activity/expression.

An in vivo protein landscape of the mouse DCT during high dietary K+ or low dietary Na+ intake

The hormone aldosterone is essential for maintaining K⁺ and Na⁺ balance and controlling blood pressure. Aldosterone has different effects if it is secreted due to hypovolemia or hyperkalemia. The kidney distal convoluted tubule (DCT) is believed to play a central role in mediating the differential responses to aldosterone. To determine the alterations in the DCT that may be responsible for these effects, male mice with green fluorescent protein expression specifically in the DCT were maintained on diets containing low NaCl (hypovolemic state) or high potassium citrate (hyperkalemic state) for 4 days, and DCT cells were isolated using fluorescence-activated cell sorting. This pure population of DCT cells was subjected to analysis by liquid chromatography-coupled tandem mass spectrometry. Over 3,000 proteins were identified in the DCT, creating the first proteome of the mouse DCT. Of the identified proteins, 210 proteins were altered in abundance following a low-NaCl diet and 625 proteins following the high-K⁺ diet. Many of these changes were not detectable by analyzing whole kidney samples from the same animals. When comparing responses to high-K⁺ versus low-Na⁺ diets, protein translation, chaperone-mediated protein folding, and protein ubiquitylation were likely to be significantly altered in the DCT subsequent to a high-K⁺ diet. In conclusion, this study defines an in vivo protein landscape of the DCT in male mice following either a low-NaCl or a high-K⁺ diet and acts as an essential resource for the kidney research community.NEW & NOTEWORTHY The mineralocorticoid aldosterone, essential for maintaining body K⁺ and Na⁺ balance, has different effects if secreted due to hypovolemia or hyperkalemia. Here, we used proteomics to profile kidney distal convoluted tubule (DCT) cells isolated by a novel FACS approach from mice fed a low-Na⁺ diet (mimicking hypovolemia) or a high-K⁺ diet (mimicking hyperkalemia). The study provides the first in-depth proteome of the mouse DCT and insights into how it is physiologically regulated.

WNK4 kinase: from structure to physiology

With no lysine kinase-4 (WNK4) belongs to a serine-threonine kinase family characterized by the atypical positioning of its catalytic lysine. Despite the fact that WNK4 has been found in many tissues, the majority of its study has revolved around its function in the kidney, specifically as a positive regulator of the thiazide-sensitive NaCl cotransporter (NCC) in the distal convoluted tubule of the nephron. This is explained by the description of gain-of-function mutations in the gene encoding WNK4 that causes familial hyperkalemic hypertension. This disease is mainly driven by increased downstream activation of the Ste20/SPS1-related proline-alanine-rich kinase/oxidative stress responsive kinase-1-NCC pathway, which increases salt reabsorption in the distal convoluted tubule and indirectly impairs renal K⁺ secretion. Here, we review the large volume of information that has accumulated about different aspects of WNK4 function. We first review the knowledge on WNK4 structure and enumerate the functional domains and motifs that have been characterized. Then, we discuss WNK4 physiological functions based on the information obtained from in vitro studies and from a diverse set of genetically modified mouse models with altered WNK4 function. We then review in vitro and in vivo evidence on the different levels of regulation of WNK4. Finally, we go through the evidence that has suggested how different physiological conditions act through WNK4 to modulate NCC activity.

WNK4 kinase: role of chloride sensing in the distal convoluted tubule

PURPOSE OF REVIEW

This review focuses on recent efforts in identifying with-no-lysine kinase 4 (WNK4) as a physiological intracellular chloride sensor and exploring regulators of intracellular chloride concentration ([Cl-]i) in the distal convoluted tubule (DCT).

RECENT FINDINGS

The discovery of WNK1's chloride-binding site provides the mechanistic details of the chloride-sensing regulation of WNK kinases. The subsequent in-vitro studies reveal that the chloride sensitivities of WNK kinases were variable. Because of its highest chloride sensitivity and dominant expression, WNK4 emerges as the leading candidate of the chloride sensor in DCT. The presentation of hypertension and increased sodium-chloride cotransporter (NCC) activity in chloride-insensitive WNK4 mice proved that WNK4 is inhibitable by physiological [Cl-]i in DCT. The chloride-mediated WNK4 regulation is responsible for hypokalemia-induced NCC activation but unnecessary for hyperkalemia-induced NCC deactivation. This chloride-sensing mechanism requires basolateral potassium and chloride channels or cotransporters, including Kir4.1/5.1, ClC-Kb, and possibly KCCs, to modulate [Cl-]i in response to the changes of plasma potassium.

SUMMARY

WNK4 is both a master NCC stimulator and an in-vivo chloride sensor in DCT. The understanding of chloride-mediated regulation of WNK4 explains the inverse relationship between dietary potassium intake and NCC activity.

NaCl cotransporter activity and Mg2+ handling by the distal convoluted tubule

The genetic disease Gitelman syndrome, knockout mice, and pharmacological blockade with thiazide diuretics have revealed that reduced activity of the NaCl cotransporter (NCC) promotes renal Mg²⁺ wasting. NCC is expressed along the distal convoluted tubule (DCT), and its activity determines Mg²⁺ entry into DCT cells through transient receptor potential channel subfamily M member 6 (TRPM6). Several other genetic forms of hypomagnesemia lower the drive for Mg²⁺ entry by inhibiting activity of basolateral Na⁺-K⁺-ATPase, and reduced NCC activity may do the same. Lower intracellular Mg²⁺ may promote further Mg²⁺ loss by directly decreasing activity of Na⁺-K⁺-ATPase. Lower intracellular Mg²⁺ may also lower Na⁺-K⁺-ATPase indirectly by downregulating NCC. Lower NCC activity also induces atrophy of DCT cells, decreasing the available number of TRPM6 channels. Conversely, a mouse model with increased NCC activity was recently shown to display normal Mg²⁺ handling. Moreover, recent studies have identified calcineurin and uromodulin (UMOD) as regulators of both NCC and Mg²⁺ handling by the DCT. Calcineurin inhibitors paradoxically cause hypomagnesemia in a state of NCC activation, but this may be related to direct effects on TRPM6 gene expression. In Umod^-/- mice, the cause of hypomagnesemia may be partly due to both decreased NCC expression and lower TRPM6 expression on the cell surface. This mini-review discusses these new findings and the possible role of altered Na⁺ flux through NCC and ultimately Na⁺-K⁺-ATPase in Mg²⁺ reabsorption by the DCT.

Physiological Processes Modulated by the Chloride-Sensitive WNK-SPAK/OSR1 Kinase Signaling Pathway and the Cation-Coupled Chloride Cotransporters

The role of Cl^– as an intracellular signaling ion has been increasingly recognized in recent years. One of the currently best described roles of Cl^– in signaling is the modulation of the With-No-Lysine (K) (WNK) – STE20-Proline Alanine rich Kinase (SPAK)/Oxidative Stress Responsive Kinase 1 (OSR1) – Cation-Coupled Cl^–Cotransporters (CCCs) cascade. Binding of a Cl^– anion to the active site of WNK kinases directly modulates their activity, promoting their inhibition. WNK activation due to Cl^– release from the binding site leads to phosphorylation and activation of SPAK/OSR1, which in turn phosphorylate the CCCs. Phosphorylation by WNKs-SPAK/OSR1 of the Na⁺-driven CCCs (mediating ions influx) promote their activation, whereas that of the K⁺-driven CCCs (mediating ions efflux) promote their inhibition. This results in net Cl^– influx and feedback inhibition of WNK kinases. A wide variety of alterations to this pathway have been recognized as the cause of several human diseases, with manifestations in different systems. The understanding of WNK kinases as Cl^– sensitive proteins has allowed us to better understand the mechanistic details of regulatory processes involved in diverse physiological phenomena that are reviewed here. These include cell volume regulation, potassium sensing and intracellular signaling in the renal distal convoluted tubule, and regulation of the neuronal response to the neurotransmitter GABA.

Dietary potassium and the kidney: lifesaving physiology

Potassium often has a negative connotation in Nephrology as patients with chronic kidney disease (CKD) are prone to develop hyperkalaemia. Approaches to the management of chronic hyperkalaemia include a low potassium diet or potassium binders. Yet, emerging data indicate that dietary potassium may be beneficial for patients with CKD. Epidemiological studies have shown that a higher urinary potassium excretion (as proxy for higher dietary potassium intake) is associated with lower blood pressure (BP) and lower cardiovascular risk, as well as better kidney outcomes. Considering that the composition of our current diet is characterized by a high sodium and low potassium content, increasing dietary potassium may be equally important as reducing sodium. Recent studies have revealed that dietary potassium modulates the activity of the thiazide-sensitive sodium-chloride cotransporter in the distal convoluted tubule (DCT). The DCT acts as a potassium sensor to control the delivery of sodium to the collecting duct, the potassium-secreting portion of the kidney. Physiologically, this allows immediate kaliuresis after a potassium load, and conservation of potassium during potassium deficiency. Clinically, it provides a novel explanation for the inverse relationship between dietary potassium and BP. Moreover, increasing dietary potassium intake can exert BP-independent effects on the kidney by relieving the deleterious effects of a low potassium diet (inflammation, oxidative stress and fibrosis). The aim of this comprehensive review is to link physiology with clinical medicine by proposing that the same mechanisms that allow us to excrete an acute potassium load also protect us from hypertension, cardiovascular disease and CKD.

Hypochloraemia in Patients with Heart Failure: Causes and Consequences

Hypochloraemia is a common electrolyte abnormality in patients with heart failure (HF). It has a strong association with adverse outcome regardless of HF phenotype and independent of other prognostic markers. How hypochloraemia develops in a patient with HF and how it might influence outcome are not clear, and in this review we explore the possible mechanisms. Patients with HF and hypochloraemia almost invariably take higher doses of loop diuretic than patients with normal chloride levels. However, renal chloride and bicarbonate homeostasis are closely linked, and the latter may be influenced by neurohormonal activation: it is likely that the etiology of hypochloraemia in patients with HF is multifactorial and due to more than just diuretic-induced urinary losses. There are multiple proposed mechanisms by which low chloride concentrations may lead to an adverse outcome in patients with HF: by increasing renin release; by a stimulatory effect on the with-no-lysine kinases which might increase renal sodium-chloride co-transporter activity; and by an adverse effect on myocardial conduction and contractility. None of these proposed mechanisms are proven in humans with HF. However, if true, it might suggest that hypochloraemia is a therapeutic target that might be amenable to treatment with acetazolamide or chloride supplementation.

Distal convoluted tubule Cl- concentration is modulated via K+ channels and transporters

Cl^--sensitive with-no-lysine kinase (WNK) plays a key role in regulating the thiazide-sensitive Na⁺-Cl^- cotransporter (NCC) in the distal convoluted tubule (DCT). Cl^- enters DCT cells through NCC and leaves the cell across the basolateral membrane via the Cl^- channel ClC-K2 or K⁺-Cl^- cotransporter (KCC). While KCC is electroneutral, Cl^- exit via ClC-K2 is electrogenic. Therefore, an alteration in DCT basolateral K⁺ channel activity is expected to influence Cl^- movement across the basolateral membrane. Although a role for intracellular Cl^- in the regulation of WNK and NCC has been established, intracellular Cl^- concentrations ([Cl^-]_i) have not been directly measured in the mammalian DCT. Therefore, to measure [Cl^-]_i in DCT cells, we generated a transgenic mouse model expressing an optogenetic kidney-specific Cl-Sensor and measured Cl^- fluorescent imaging in the isolated DCT. Basal measurements indicated that the mean [Cl^-]_i was ~7 mM. Stimulation of Cl^- exit with low-Cl^- hypotonic solutions decreased [Cl^-]_i, whereas inhibition of KCC by DIOA or inhibition of ClC-K2 by NPPB increased [Cl^-]_i, suggesting roles for both KCC and ClC-K2 in the modulation of [Cl^-]_i . Blockade of basolateral K⁺ channels (Kir4.1/5.1) with barium significantly increased [Cl^-]_i. Finally, a decrease in extracellular K⁺ concentration transiently decreased [Cl^-]_i, whereas raising extracellular K⁺ transiently increased [Cl^-]_i, further suggesting a role for Kir4.1/5.1 in the regulation of [Cl^-]_i. We conclude that the alteration in ClC-K2, KCC, and Kir4.1/5.1 activity influences [Cl^-]_i in the DCT.

WNK3 and WNK4 exhibit opposite sensitivity with respect to cell volume and intracellular chloride concentration

Cation-coupled chloride cotransporters (CCC) play a role in modulating intracellular chloride concentration ([Cl^-]_i) and cell volume. Cell shrinkage and cell swelling are accompanied by an increase or decrease in [Cl^-]_i, respectively. Cell shrinkage and a decrease in [Cl^-]_i increase the activity of NKCCs (Na-K-Cl cotransporters: NKCC1, NKCC2, and Na-Cl) and inhibit the activity of KCCs (K-Cl cotransporters: KCC1 to KCC4), wheras cell swelling and an increase in [Cl^-]_i activate KCCs and inhibit NKCCs; thus, it is unlikely that the same kinase is responsible for both effects. WNK1 and WNK4 are chloride-sensitive kinases that modulate the activity of CCC in response to changes in [Cl^-]_i. Here, we showed that WNK3, another member of the serine-threonine kinase WNK family with known effects on CCC, is not sensitive to [Cl^-]_i but can be regulated by changes in extracellular tonicity. In contrast, WNK4 is highly sensitive to [Cl^-]_i but is not regulated by changes in cell volume. The activity of WNK3 toward NaCl cotransporter is not affected by eliminating the chloride-binding site of WNK3, further confirming that the kinase is not sensitive to chloride. Chimeric WNK3/WNK4 proteins were produced, and analysis of the chimeras suggests that sequences within the WNK's carboxy-terminal end may modulate the chloride affinity. We propose that WNK3 is a cell volume-sensitive kinase that translates changes in cell volume into phosphorylation of CCC.

Epoxyeicosatrienoic acid metabolites inhibit Kir4.1/Kir5.1 in the distal convoluted tubule

Cytochrome P-450 (Cyp) epoxygenase-dependent metabolites of arachidonic acid (AA) have been shown to inhibit renal Na+ transport, and inhibition of Cyp-epoxygenase is associated with salt-sensitive hypertension. We used the patch-clamp technique to examine whether Cyp-epoxygenase-dependent AA metabolites inhibited the basolateral 40-pS K+ channel (Kir4.1/Kir5.1) in the distal convoluted tubule (DCT). Application of AA inhibited the basolateral 40-pS K+ channel in the DCT. The inhibitory effect of AA on the 40-pS K+ channel was specific because neither linoleic nor oleic acid was able to mimic the effect of AA on the K+ channel. Inhibition of Cyp-monooxygenase with N-methylsulfonyl-12,12-dibromododec-11-enamide or inhibition of cyclooxygenase with indomethacin failed to abolish the inhibitory effect of AA on the 40-pS K+ channel. However, the inhibition of Cyp-epoxygenase with N-methylsulfonyl-6-(propargyloxyphenyl)hexanamide abolished the effect of AA on the 40-pS K+ channel in the DCT. Moreover, addition of either 11,12-epoxyeicosatrienoic acid (EET) or 14,15-EET also inhibited the 40-pS K+ channel in the DCT. Whole cell recording demonstrated that application of AA decreased, whereas N-methylsulfonyl-6-(propargyloxyphenyl)hexanamide treatment increased, Ba2+-sensitive K+ currents in the DCT. Finally, application of 14,15-EET but not AA was able to inhibit the basolateral 40-pS K+ channel in the DCT of Cyp2c44-/- mice. We conclude that Cyp-epoxygenase-dependent AA metabolites inhibit the basolateral Kir4.1/Kir5.1 in the DCT and that Cyp2c44-epoxygenase plays a role in the regulation of the basolateral K+ channel in the mouse DCT.