Calcineurin inhibitors block sodium-chloride cotransporter dephosphorylation in response to high potassium intake

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.

Role of calcineurin in regulating renal potassium (K+) excretion: Mechanisms of calcineurin inhibitor-induced hyperkalemia

Calcineurin, protein phosphatase 2B (PP2B) or protein phosphatase 3 (PP3), is a calcium-dependent serine/threonine protein phosphatase. Calcineurin is widely expressed in the kidney and regulates renal Na+ and K+ transport. In the thick ascending limb, calcineurin plays a role in inhibiting NKCC2 function by promoting the dephosphorylation of the cotransporter and an intracellular sorting receptor, called sorting-related-receptor-with-A-type repeats (SORLA), is involved in modulating the effect of calcineurin on NKCC2. Calcineurin also participates in regulating thiazide-sensitive NaCl-cotransporter (NCC) in the distal convoluted tubule. The mechanisms by which calcineurin regulates NCC include directly dephosphorylation of NCC, regulating Kelch-like-3/CUL3 E3 ubiquitin-ligase complex, which is responsible for WNK (with-no-lysin-kinases) ubiquitination, and inhibiting Kir4.1/Kir5.1, which determines NCC expression/activity. Finally, calcineurin is also involved in regulating ROMK (Kir1.1) channels in the cortical collecting duct and Cyp11 2 expression in adrenal zona glomerulosa. In summary, calcineurin is involved in the regulation of NKCC2, NCC, and inwardly rectifying K+ channels in the kidney, and it also plays a role in modulating aldosterone synthesis in adrenal gland, which regulates epithelial-Na+-channel expression/activity. Thus, application of calcineurin inhibitors (CNIs) is expected to abrupt calcineurin-mediated regulation of transepithelial Na+ and K+ transport in the kidney. Consequently, CNIs cause hypertension, compromise renal K+ excretion, and induce hyperkalemia.

mTORc2 in Distal Convoluted Tubule and Renal K + Excretion during High Dietary K + Intake

Key Points

Background

Renal mechanistic target of rapamycin complex 2 (mTORc2) plays a role in regulating renal K+ excretion (renal-EK) and K+ homeostasis. Inhibition of renal mTORc2 causes hyperkalemia due to suppressing epithelial Na+ channel and renal outer medullary K+ (Kir1.1) in the collecting duct. We now explore whether mTORc2 of distal convoluted tubules (DCTs) regulates basolateral Kir4.1/Kir5.1, Na-Cl cotransporter (NCC), and renal-EK.

Methods

We used patch-clamp technique to examine basolateral Kir4.1/Kir5.1 in early DCT, immunoblotting, and immunofluorescence to examine NCC expression and in vivo measurement of urinary K+ excretion to determine baseline renal-EK in mice treated with an mTORc2 inhibitor and in DCT-specific rapamycin-insensitive companion of mTOR knockout (DCT-RICTOR-KO) mice.

Results

Inhibition of mTORc2 with AZD8055 abolished high-K+–induced inhibition of Kir4.1/Kir5.1 in DCT, high potassium–induced depolarization of the DCT membrane, and high potassium–induced suppression of phosphorylated Na-Cl cotransporter (pNCC) expression. AZD8055 stimulated the 40-pS inwardly rectifying K+ channel (Kir4.1/Kir5.1-heterotetramer) in early DCT in the mice on overnight high potassium intake; this effect was absent in the presence of protein kinase C inhibitors, which also stimulated Kir4.1/Kir5.1. AZD8055 treatment decreased renal-EK in animals on overnight high-potassium diet. Deletion of RICTOR in the DCT increased the Kir4.1/Kir5.1-mediated K+ currents, hyperpolarized the DCT membrane, and increased the expression of pWNK4 and pNCC. Renal-EK was lower and plasma K+ was higher in DCT-RICTOR-KO mice than corresponding control mice. In addition, overnight high-potassium diet did not inhibit Kir4.1/Kir5.1 activity in the DCT and failed to inhibit the expression of pNCC in DCT-RICTOR-KO mice. Overnight high potassium intake stimulated renal-EK in control mice, but this effect was attenuated in DCT-RICTOR-KO mice. Thus, overnight high potassium intakeinduced hyperkalemia in DCT-RICTOR-KO mice but not in control mice.

Conclusions

mTORc2 of the DCT inhibits Kir4.1/Kir5.1 activity and NCC expression and stimulates renal-EK during high potassium intake.

Defective natriuresis contributes to hyperkalemia in db/db mice during potassium supplementation

OBJECTIVES

Potassium supplementation reduces blood pressure and the occurrence of cardiovascular diseases, with K+-induced natriuresis playing a potential key role in this process. However, whether these beneficial effects occur in diabetes remains unknown.

METHODS

In this study, we examined the impact of high-K+ intake on renal Na+/K+ transport by determining the expression of major apical Na+ transporters, diuretics responses (as a proxy for specific Na+ transporter function), urinary Na+/K+ excretion, and plasma Na+/K+ concentrations in db/db mice, a model of type 2 diabetes mellitus.

RESULTS

Although db/m mice exhibited increased fractional excretion of sodium (FENa) and fractional excretion of potassium (FEK) under high-K+ intake, these responses were largely blunted in db/db mice, suggesting impaired K+-induced natriuresis and kaliuresis in diabetes. Consequently, high-K+ intake increased plasma K+ levels in db/db mice, which could be attributed to the abnormal activity of sodium-hydrogen exchanger 3 (NHE3), sodium-chloride cotransporter (NCC), and epithelial Na+ channel (ENaC), as high-K+ intake could not effectively decrease NHE3 and NCC and increase ENaC expression and activity in the diabetic group. Inhibition of NCC by hydrochlorothiazide could correct the hyperkalemia in db/db mice fed a high-K+ diet, indicating a key role for NCC in K+-loaded diabetic mice. Treatment with metformin enhanced urinary Na+/K+ excretion and normalized plasma K+ levels in db/db mice with a high-K+ diet, at least partially, by suppressing NCC activity.

CONCLUSION

Collectively, the impaired K+-induced natriuresis in diabetic mice under high-K+ intake may be primarily attributed to impaired NCC-mediated renal K+ excretion, despite the role of NHE3.

Review Article on Molecular Mechanism of Regulation of Hypertension by Macro-elements (Na, K, Ca and Mg), Micro-elements/Trace Metals (Zn and Cu) and Toxic Elements (Pb and As)

Hypertension (HT) is a medical condition arising due to increase in blood pressure (BP) prevalent worldwide. The balanced dietary intakes of macro-elements and micro-elements including Na, K, Ca, Mg, Zn, and Cu have been described to maintain BP in humans by regulating the osmolarity of blood, cells/tissues, prevention of generation of oxidative and nitrosative stress (OANS), and endothelial damage through their functioning as important components of renin-angiotensin-aldosterone system (RAAS), antioxidant enzyme defense system, and maintenance of blood vascular-endothelial and vascular smooth muscle cell (VSMC) functions. However, inadequate/excess dietary intakes of Na/K, Ca/Mg, and Zn/Cu along with higher Pb and As exposures recognized to induce HT through common mechanisms including the followings: endothelial dysfunctions due to impairment of vasodilatation, increased vasoconstriction and arterial stiffness, blood clotting, inflammation, modification of sympathetic activity and higher catecholamine release, increased peripheral vascular resistance, and cardiac output; increased OANS due to reduced and elevated activities of extracellular superoxide dismutase and NAD(P)H oxidase, less nitric oxide bioavailability, decrease in cGMP and guanylate cyclase activity, increase in intracellular Ca2+ ions in VSMCs, and higher pro-inflammatory cytokines; higher parathyroid and calcitriol hormones; activation/suppression of RAAS resulting imbalance in blood Na+, K+, and water regulated by renin, angiotensin II, and aldosterone through affecting natriuresis/kaliuresis/diuresis; elevation in serum cholesterol and LDL cholesterol, decrease in HDL cholesterol due to defect in lipoprotein metabolism. The present study recommends the need to review simple dietary mineral intervention studies/supplementation trials before keeping their individual dietary excess intakes/exposures in consideration because their interactions lead to elevation and fall of their concentrations in body affecting onset of HT.

PIEZO1 is a distal nephron mechanosensor and is required for flow-induced K+ secretion

Ca2+-activated BK channels in renal intercalated cells (ICs) mediate luminal flow-induced K+ secretion (FIKS), but how ICs sense increased flow remains uncertain. We examined whether PIEZO1, a mechanosensitive Ca2+-permeable channel expressed in the basolateral membranes of ICs, is required for FIKS. In isolated cortical collecting ducts (CCDs), the mechanosensitive cation-selective channel inhibitor GsMTx4 dampened flow-induced increases in intracellular Ca2+ concentration ([Ca2+]i), whereas the PIEZO1 activator Yoda1 increased [Ca2+]i and BK channel activity. CCDs from mice fed a high-K+ (HK) diet exhibited a greater Yoda1-dependent increase in [Ca2+]i than CCDs from mice fed a control K+ diet. ICs in CCDs isolated from mice with a targeted gene deletion of Piezo1 in ICs (IC-Piezo1-KO) exhibited a blunted [Ca2+]i response to Yoda1 or increased flow, with an associated loss of FIKS in CCDs. Male IC-Piezo1-KO mice selectively exhibited an increased blood [K+] in response to an oral K+ bolus and blunted urinary K+ excretion following a volume challenge. Whole-cell expression of BKα subunit was reduced in ICs of IC-Piezo1-KO mice fed an HK diet. We conclude that PIEZO1 mediates flow-induced basolateral Ca2+ entry into ICs, is upregulated in the CCD in response to an HK diet, and is necessary for FIKS.

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.

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.

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.

Potassium homeostasis - Physiology and pharmacology in a clinical context

Membrane voltage controls the function of excitable cells and is mainly a consequence of the ratio between the extra- and intracellular potassium concentration. Potassium homeostasis is safeguarded by balancing the extra-/intracellular distribution and systemic elimination of potassium to the dietary potassium intake. These processes adjust the plasma potassium concentration between 3.5 and 4.5 mmol/L. Several genetic and acquired diseases but also pharmacological interventions cause dyskalemias that are associated with increased morbidity and mortality. The thresholds at which serum K+ not only associates but also causes increased mortality are hotly debated. We discuss physiologic, pathophysiologic, and pharmacologic aspects of potassium regulation and provide informative case vignettes. Our aim is to help clinicians, epidemiologists, and pharmacologists to understand the complexity of the potassium homeostasis in health and disease and to initiate appropriate treatment strategies in dyskalemic patients.

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.

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].

Increased intrarenal post-glomerular blood flow is a key condition for the development of calcineurin inhibitor-induced renal tubular acidosis in kidney transplant recipients

BACKGROUND

Hyperchloremic metabolic acidosis (HCMA) from renal tubular acidosis (RTA) is common in kidney transplant (KT) recipients. Calcineurin inhibitors (CNIs) are a potential cause of RTA, and whether HCMA is a determinant of poor graft prognosis is controversial.

METHODS

The subjects were living-donor KT recipients (LDKTRs, n = 47) and matched donors (n = 43). All cases of rejection, extrarenal causes, and respiratory disorders were excluded. HCMA was defined as having a [Na+] - [Cl-] value of ≤ 34 or starting alkalization. We determined the potential causes of HCMA in LDKTRs at 3 months (m) and 1 year (y) post-KT. We examined renal hemodynamic parameters in 26 LDKTRs at 1 y post-KT: namely, glomerular filtration rate (GFR), renal plasma flow (RPF), filtration fraction (FF; GFR/RPF) and pre-/post-glomerular vascular resistance (pre-/postVR).

RESULTS

The HCMA incidence in the 3-m post-KT LDKTR group was higher than that of the donors (51.0% vs. 6.9%, p<0.001, adjusted odds ratio: 6.7-15.7). Among adjusted factors, the most dominant HCMA contributor was low hemoglobin concentration (Hb ≤12 g/dL). Compared to non-HCMA cases, HCMA patients had low FF and low post-VR (p = 0.008, 0.003, respectively) suggesting increased intrarenal post-glomerular blood flow. The high pathological score of alternative arteriolar hyalinosis (aah) ≥2 was a significant HCMA risk. The tacrolimus trough level was not high in HCMA but was significantly high in HCMA in the low post-VR setting (p = 0.002).

CONCLUSION

Among LDKTRs, low hemoglobin level is an important contributor to the manifestation of HCMA in the induction period, and increased intrarenal post-glomerular blood flow is a key condition for the development of CNI-induced RTA. This article is protected by copyright. All rights reserved.

Dietary K+ acts as a genuine diuretic

K⁺ balance in mammals relies on regulated renal K⁺ excretion matching unregulated fluctuating K⁺ intake. Upon a K⁺ rich meal, rapid and powerful K⁺ excretion is needed. Renal K⁺ secretion is stimulated by the increased tubular flow. We speculated that high K⁺ intake acutely increases urinary flow to stimulate K⁺ excretion.

METHODS

Mice were K⁺ challenged through diets or gavage. Post K⁺ loading urinary output, osmolarity, [K⁺ ]_u , [Na⁺ ]_u , plasma osmolarity, [copeptin]_p , [K⁺ ]_p , and [Na⁺ ]_p were measured. To locate the mechanism of K⁺ -induced diuresis in the glomerular/tubular system we measured creatinine excretion and assessed functional transport in isolated perfused TALs and CDs during an acute [K⁺ ]_bl switch from 3.6 to 6.5 mM. Molecular adaptations of transport proteins involved in water reabsorption were investigated by immunoblotting.

RESULTS

(1) Mice switched from a 1% to 2% K⁺ diet increased diuresis within 12 hours and reciprocally reduced diuresis when switched from 1% to 0.01% K⁺ diet. (2) A single K⁺ gavage load, corresponding to 25%-50% of daily K⁺ intake, induced 100% increase in diuresis within 30 minutes. This occurred despite augmented plasma osmolarity and AVP synthesis. (3) K⁺ gavage did not change GFR. (4) In isolated perfused TALs, shifting [K⁺ ]_bl from 3.6 to 6.5 mM did not affect AVP-induced NaCl transport. (5) In sharp contrast, in isolated perfused CDs, shifting [K⁺ ]_bl from 3.6 to 6.5 mM markedly reduced CD AVP sensitivity, ie inhibited water absorption.

CONCLUSION

Dietary K⁺ loading induces a rapidly on-setting diuresis. The mechanism of K⁺ -induced diuresis involves desensitization of the CD to AVP.

Paracrine and endocrine regulation of renal potassium secretion

The seminal studies conducted by Giebisch and colleagues in the 1960s paved the way for understanding the renal mechanisms involved in K+ homeostasis. It was demonstrated that differential handling of K+ in the distal segments of the nephron is crucial for proper K+ balance. Although aldosterone had been classically ascribed as the major ion transport regulator in the distal nephron, thereby contributing to K+ homeostasis, it became clear that aldosterone per se could not explain the kidney's ability to modulate kaliuresis in both acute and chronic settings. The existence of alternative kaliuretic and antikaliuretic mechanisms was suggested by physiological studies in the 1980s but only gained form and shape with the advent of molecular biology. It is now established that the kidneys recruit several endocrine and paracrine mechanisms for adequate kaliuretic response. These mechanisms include the direct effects of peritubular K+, a gut-kidney regulatory axis sensing dietary K+ levels, the kidney secretion of kallikrein during postprandial periods, the upregulation of angiotensin II receptors in the distal nephron during chronic changes in the K+ diet, and the local increase of prostaglandins by low K+ diet. This review discusses recent advances in the understanding of endocrine and paracrine mechanisms underlying the modulation of K+ secretion and how these mechanisms impact kaliuresis and K+ balance. We also highlight important unknowns about the regulation of renal K+ excretion under physiological circumstances.

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.

Specific disruption of calcineurin-signaling in the distal convoluted tubule impacts the transcriptome and proteome, and causes hypomagnesemia and metabolic acidosis

Adverse effects of calcineurin inhibitors (CNI), such as hypertension, hyperkalemia, acidosis, hypomagnesemia and hypercalciuria, have been linked to dysfunction of the distal convoluted tubule (DCT). To test this, we generated a mouse model with an inducible DCT-specific deletion of the calcineurin regulatory subunit B alpha (CnB1-KO). Three weeks after CnB1 deletion, these mice exhibited hypomagnesemia and acidosis, but no hypertension, hyperkalemia or hypercalciuria. Consistent with the hypomagnesemia, CnB1-KO mice showed a downregulation of proteins implicated in DCT magnesium transport, including TRPM6, CNNM2, SLC41A3 and parvalbumin but expression of calcium channel TRPV5 in the kidney was unchanged. The abundance of the chloride/bicarbonate exchanger pendrin was increased, likely explaining the acidosis. Plasma aldosterone levels, kidney renin expression, abundance of phosphorylated sodium chloride-cotransporter and abundance of the epithelial sodium channel were similar in control and CnB1-KO mice, consistent with a normal sodium balance. Long-term potassium homeostasis was maintained in CnB1-KO mice, but in-vivo and ex-vivo experiments indicated that CnB1 contributes to acute regulation of potassium balance and sodium chloride-cotransporter. Tacrolimus treatment of control and CnB1-KO mice demonstrated that CNI-related hypomagnesemia is linked to impaired calcineurin-signaling in DCT, while hypocalciuria and hyponatremia occur independently of CnB1 in DCT. Transcriptome and proteome analyses of isolated DCTs demonstrated that CnB1 deletion impacts the expression of several DCT-specific proteins and signaling pathways. Thus, our data support a critical role of calcineurin for DCT function and provide novel insights into the pathophysiology of CNI side-effects and involved molecular players in the DCT.

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.

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.

Regulatory control of the Na-Cl co-transporter NCC and its therapeutic potential for hypertension

Hypertension is the largest risk factor for cardiovascular disease, the leading cause of mortality worldwide. As blood pressure regulation is influenced by multiple physiological systems, hypertension cannot be attributed to a single identifiable etiology. Three decades of research into Mendelian forms of hypertension implicated alterations in the renal tubular sodium handling, particularly the distal convoluted tubule (DCT)-native, thiazide-sensitive Na-Cl cotransporter (NCC). Altered functions of the NCC have shown to have profound effects on blood pressure regulation as illustrated by the over activation and inactivation of the NCC in Gordon's and Gitelman syndromes respectively. Substantial progress has uncovered multiple factors that affect the expression and activity of the NCC. In particular, NCC activity is controlled by phosphorylation/dephosphorylation, and NCC expression is facilitated by glycosylation and negatively regulated by ubiquitination. Studies have even found parvalbumin to be an unexpected regulator of the NCC. In recent years, there have been considerable advances in our understanding of NCC control mechanisms, particularly via the pathway containing the with-no-lysine [K] (WNK) and its downstream target kinases, SPS/Ste20-related proline-alanine-rich kinase (SPAK) and oxidative stress responsive 1 (OSR1), which has led to the discovery of novel inhibitory molecules. This review summarizes the currently reported regulatory mechanisms of the NCC and discusses their potential as therapeutic targets for treating hypertension.

MST3 Involvement in Na+ and K+ Homeostasis with Increasing Dietary Potassium Intake

K⁺ loading inhibits NKCC2 (Na-K-Cl cotransporter) and NCC (Na-Cl cotransporter) in the early distal tubules, resulting in Na⁺ delivery to the late distal convoluted tubules (DCTs). In the DCTs, Na⁺ entry through ENaC (epithelial Na channel) drives K⁺ secretion through ROMK (renal outer medullary potassium channel). WNK4 (with-no-lysine 4) regulates the NCC/NKCC2 through SAPK (Ste20-related proline-alanine-rich kinase)/OSR1 (oxidative stress responsive). K⁺ loading increases intracellular Cl⁻, which binds to the WNK4, thereby inhibiting autophosphorylation and downstream signals. Acute K⁺ loading-deactivated NCC was not observed in Cl⁻-insensitive WNK4 mice, indicating that WNK4 was involved in K⁺ loading-inhibited NCC activity. However, chronic K⁺ loading deactivated NCC in Cl⁻-insensitive WNK4 mice, indicating that other mechanisms may be involved. We previously reported that mammalian Ste20-like protein kinase 3 (MST3/STK24) was expressed mainly in the medullary TAL (thick ascending tubule) and at lower levels in the DCTs. MST3^−/− mice exhibited higher ENaC activity, causing hypernatremia and hypertension. To investigate MST3 function in maintaining Na⁺/K⁺ homeostasis in kidneys, mice were fed diets containing various concentrations of Na⁺ and K⁺. The 2% KCl diets induced less MST3 expression in MST3^−/− mice than that in wild-type (WT) mice. The MST3^−/− mice had higher WNK4, NKCC2-S130 phosphorylation, and ENaC expression, resulting in lower urinary Na⁺ and K⁺ excretion than those of WT mice. Lower urinary Na⁺ excretion was associated with elevated plasma [Na⁺] and hypertension. These results suggest that MST3 maintains Na⁺/K⁺ homeostasis in response to K⁺ loading by regulation of WNK4 expression and NKCC2 and ENaC 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.

Sympathetic Regulation of the NCC (Sodium Chloride Cotransporter) in Dahl Salt-Sensitive Hypertension

Increased sympathoexcitation and renal sodium retention during high salt intake are hallmarks of the salt sensitivity of blood pressure. The mechanism(s) by which excessive sympathetic nervous system release of norepinephrine influences renal sodium reabsorption is unclear. However, studies demonstrate that norepinephrine can stimulate the activity of the NCC (sodium chloride cotransporter) and promote the development of SSH (salt-sensitive hypertension). The adrenergic signaling pathways governing NCC activity remain a significant source of controversy with opposing studies suggesting a central role of upstream α1- and β-adrenoceptors in the canonical regulatory pathway involving WNKs (with-no-lysine kinases), SPAK (STE20/SPS1-related proline alanine-rich kinase), and OxSR1 (oxidative stress response 1). In our previous study, α1-adrenoceptor antagonism in norepinephrine-infused male Sprague-Dawley rats prevented the development of norepinephrine-evoked SSH in part by suppressing NCC activity and expression. In these studies, we used selective adrenoceptor antagonism in male Dahl salt-sensitive rats to test the hypothesis that norepinephrine-mediated activation of the NCC in Dahl SSH occurs via an α1-adrenoceptor dependent pathway. A high-salt diet evoked significant increases in NCC activity, expression, and phosphorylation in Dahl salt-sensitive rats that developed SSH. Increases were associated with a dysfunctional WNK1/4 dynamic and a failure to suppress SPAK/OxSR1 activity. α1-adrenoceptor antagonism initiated before high-salt intake or following the establishment of SSH attenuated blood pressure in part by suppressing NCC activity, expression, and phosphorylation. Collectively, our findings support the existence of a norepinephrine-activated α1-adrenoceptor gated pathway that relies on WNK/SPAK/OxSR1 signaling to regulate NCC activity in SSH.

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.

Sodium-calcium exchanger 1 is the key molecule for urinary potassium excretion against acute hyperkalemia

The sodium (Na⁺)-chloride cotransporter (NCC) expressed in the distal convoluted tubule (DCT) is a key molecule regulating urinary Na⁺ and potassium (K⁺) excretion. We previously reported that high-K⁺ load rapidly dephosphorylated NCC and promoted urinary K⁺ excretion in mouse kidneys. This effect was inhibited by calcineurin (CaN) and calmodulin inhibitors. However, the detailed mechanism through which high-K⁺ signal results in CaN activation remains unknown. We used Flp-In NCC HEK293 cells and mice to evaluate NCC phosphorylation. We analyzed intracellular Ca²⁺ concentration ([Ca²⁺]_in) using live cell Ca²⁺ imaging in HEK293 cells. We confirmed that high-K⁺-induced NCC dephosphorylation was not observed without CaN using Flp-In NCC HEK29 cells. Extracellular Ca²⁺ reduction with a Ca²⁺ chelator inhibited high-K⁺-induced increase in [Ca²⁺]_in and NCC dephosphorylation. We focused on Na⁺/Ca²⁺ exchanger (NCX) 1, a bidirectional regulator of cytosolic Ca²⁺ expressed in DCT. We identified that NCX1 suppression with a specific inhibitor (SEA0400) or siRNA knockdown inhibited K⁺-induced increase in [Ca²⁺]_in and NCC dephosphorylation. In a mouse study, SEA0400 treatment inhibited K⁺-induced NCC dephosphorylation. SEA0400 reduced urinary K⁺ excretion and induced hyperkalemia. Here, we identified NCX1 as a key molecule in urinary K⁺ excretion promoted by CaN activation and NCC dephosphorylation in response to K⁺ load.

The WNK signaling pathway and salt-sensitive hypertension

The distal nephron of the kidney has a central role in sodium and fluid homeostasis, and disruption of this homeostasis due to mutations of with-no-lysine kinase 1 (WNK1), WNK4, Kelch-like 3 (KLHL3), or Cullin 3 (CUL3) causes pseudohypoaldosteronism type II (PHAII), an inherited hypertensive disease. WNK1 and WNK4 activate the NaCl cotransporter (NCC) at the distal convoluted tubule through oxidative stress-responsive gene 1 (OSR1)/Ste20-related proline-alanine-rich kinase (SPAK), constituting the WNK-OSR1/SPAK-NCC phosphorylation cascade. The level of WNK protein is regulated through degradation by the CUL3-KLHL3 E3 ligase complex. In the normal state, the activity of WNK signaling in the kidney is physiologically regulated by sodium intake to maintain sodium homeostasis in the body. In patients with PHAII, however, because of the defective degradation of WNK kinases, NCC is constitutively active and not properly suppressed by a high salt diet, leading to abnormally increased salt reabsorption and salt-sensitive hypertension. Importantly, recent studies have demonstrated that potassium intake, insulin, and TNFα are also physiological regulators of WNK signaling, suggesting that they contribute to the salt-sensitive hypertension associated with a low potassium diet, metabolic syndrome, and chronic kidney disease, respectively. Moreover, emerging evidence suggests that WNK signaling also has some unique roles in metabolic, cardiovascular, and immunological organs. Here, we review the recent literature and discuss the molecular mechanisms of the WNK signaling pathway and its potential as a therapeutic target.

Intracellular chloride: a regulator of transepithelial transport in the distal nephron

PURPOSE OF REVIEW

This review focuses on the role of intracellular chloride in regulating transepithelial ion transport in the distal convoluted tubule (DCT) in response to perturbations in plasma potassium homeostasis.

RECENT FINDINGS

Low dietary potassium increases the phosphorylation and activity of the sodium chloride cotransporter (NCC) in the DCT, and vice versa, affecting sodium-dependent potassium secretion in the downstream aldosterone-sensitive distal nephron. In cells, NCC phosphorylation is increased by lowering of intracellular chloride, via activation of the chloride-sensitive with no lysine (WNK)-SPAK/OSR1 (Ste20-related proline/alanine-rich kinase/oxidative stress response) kinase cascade. In-vivo studies have demonstrated pathway activation in the kidney in response to low dietary potassium. A possible mechanism is lowering of DCT intracellular chloride in response to low potassium because of parallel basolateral potassium and chloride channels. Recent studies support a role for these channels in the response of NCC to varying potassium. Studies examining chloride-insensitive WNK mutants, in the Drosophila renal tubule and in the mouse, lend further support to a role for chloride in regulating WNK activity and transepithelial ion transport. Caveats, alternatives, and future directions are also discussed.

SUMMARY

Chloride sensing by WNK kinase provides a mechanism to allow coupling of extracellular potassium with NCC phosphorylation and activity to maintain potassium homeostasis.

Regulation of the Renal NaCl Cotransporter and Its Role in Potassium Homeostasis

Daily dietary potassium (K+) intake may be as large as the extracellular K+ pool. To avoid acute hyperkalemia, rapid removal of K+ from the extracellular space is essential. This is achieved by translocating K+ into cells and increasing urinary K+ excretion. Emerging data now indicate that the renal thiazide-sensitive NaCl cotransporter (NCC) is critically involved in this homeostatic kaliuretic response. This suggests that the early distal convoluted tubule (DCT) is a K+ sensor that can modify sodium (Na+) delivery to downstream segments to promote or limit K+ secretion. K+ sensing is mediated by the basolateral K+ channels Kir4.1/5.1, a capacity that the DCT likely shares with other nephron segments. Thus, next to K+-induced aldosterone secretion, K+ sensing by renal epithelial cells represents a second feedback mechanism to control K+ balance. NCC’s role in K+ homeostasis has both physiological and pathophysiological implications. During hypovolemia, NCC activation by the renin-angiotensin system stimulates Na+ reabsorption while preventing K+ secretion. Conversely, NCC inactivation by high dietary K+ intake maximizes kaliuresis and limits Na+ retention, despite high aldosterone levels. NCC activation by a low-K+ diet contributes to salt-sensitive hypertension. K+-induced natriuresis through NCC offers a novel explanation for the antihypertensive effects of a high-K+ diet. A possible role for K+ in chronic kidney disease is also emerging, as epidemiological data reveal associations between higher urinary K+ excretion and improved renal outcomes. This comprehensive review will embed these novel insights on NCC regulation into existing concepts of K+ homeostasis in health and disease.

Hyperkalemia and blood pressure regulation

Hypertension is common in the general population. Management of hypertensive patients at risk of hyperkalemia is challenging due to potential life-threatening complications such as cardiac arrest. Chronic hyperkalemia is often associated with impaired renal ability to excrete excessive potassium ions (K+). This may refer to chronic kidney disease or certain pharmacological interventions, including broadly used renin-angiotensin-aldosterone system and calcineurin inhibitors. Understanding the intrinsic mechanisms permitting kidney adaptations to hyperkalemia is critical for choosing therapeutic strategies. Valuable insights were obtained from the analysis of familial hyperkalemic hypertension (FHHt) syndrome, which became a classic model for coincidence of high blood pressure and hyperkalemia. FHHt can be caused by mutations in several genes, all of them resulting in excessive activity of with-no-lysine kinases (WNKs) in the distal nephron of the kidney. WNKs have been increasingly recognized as key signalling enzymes in the regulation of renal sodium ions (Na+) and K+ handling, enabling adaptive responses to systemic shifts of potassium homoeostasis consequent to variations in dietary potassium intake or disease. The WNK signalling pathway recruits a complex protein network mediating catalytic and non-catalytic effects of distinct WNK isoforms on relevant Na+- or K+-transporting proteins. In this review article, we summarize recent progress in understanding WNK signalling. An update of available models for renal adaptation to hyperkalemic conditions is presented. Consequences for blood pressure regulation are discussed. Pharmacological targeting of WNKs or their substrates offers promising options to manage hypertension while preventing hyperkalemia.

Modified HEK cells simulate DCT cells in their sensitivity and response to changes in extracellular K

A potassium (K+ ) rich diet is known to have an antihypertensive effect that has been embodied by the NHLBI in the DASH diet. However, the molecular basis for this blood pressure-lowering effect has been unclear, until a recent study proposed a model in which the DCT cells of the kidney regulate their salt transport in response to variations in intracellular chloride ([Cl- ]i ), which are directly regulated by serum K+ . With the knowledge that WNK proteins are Cl- sensors, and are a part of the WNK/SPAK/NCC signaling cascade which regulates the NCC, the main salt transporter in the distal nephron, we examined the effect of serum K+ on the ([Cl- ]i ) and, in turn its effect on the WNK4 signaling pathway in a "modified HEK 293T" cell line. Using a fluorescence-based approach in this cell line, we have shown that the membrane potential of the cell membrane is sensitive to the small changes in external KCl within the physiological range (2-5 mM), thus functioning as a K+ electrode. When the extracellular K+ was progressively increased (2-5 mM), the membrane depolarization lead to a subsequent increase in [Cl- ]i measured by fluorescence quenching of an intracellular chloride sensor. Increase in extracellular [K] resulted in a decrease in the phosphorylation of the WNK4 protein and its downstream targets, SPAK and NCC. This confirms that small changes in serum K can affect WNK4/SPAK/NCC signaling and transcellular Na+ flux through the DCT and provide a possible mechanism by which a K-rich DASH diet could reduce blood pressure.

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.

Tacrolimus ameliorates the phenotypes of type 4 Bartter syndrome model mice through activation of sodium-potassium-2 chloride cotransporter and sodium-chloride cotransporter

Type 4 Bartter syndrome (BS) is caused by genetic mutations in barttin, which is coded for by BSND. Barttin serves as the β-subunit of the ClC-K chloride (Cl^-) channel, which is widely expressed in distal nephrons. Type 4 BS is characterized by severely impaired reabsorption of salt, which may cause polyuria, hypokalemia, and metabolic alkalosis. Calcineurin inhibitors reportedly induce renal salt retention and hyperkalemia by enhancing the phosphorylation of the sodium (Na⁺)-potassium (K⁺)-2Cl^- cotransporter (NKCC2) and Na⁺-Cl^- cotransporter (NCC). In addition, we have previously reported that tacrolimus, a calcineurin inhibitor, increases the levels of phosphorylated NCC. In this study, we administered tacrolimus to barttin hypomorphic (Bsnd^neo/neo) mice, a murine model of type 4 BS that exhibits polyuria, hypokalemia, and metabolic alkalosis. Administration of tacrolimus increased the serum K⁺ level and suppressed urinary K⁺ excretion. Furthermore, after treatment with tacrolimus, Bsnd^neo/neo mice increased levels of phosphorylated NCC and NKCC2. We conclude that tacrolimus partially improves clinical phenotypes of Bsnd^neo/neo mice, and that calcineurin inhibitors might be effective for treating type 4 BS.

Cullin-Ring ubiquitin ligases in kidney health and disease

PURPOSE OF REVIEW

Members of the Cullin family act as scaffolds in E3 ubiquitin ligases and play a central role in mediating protein degradation. Interactions with many different substrate-binding adaptors permit Cullin-containing E3 ligases to participate in diverse cellular functions. In the kidney, one well established target of Cullin-mediated degradation is the transcription factor Nrf2, a key player in responses to oxidative stress. The goal of this review is to discuss more recent findings revealing broader roles for Cullins in the kidney.

RECENT FINDINGS

Cullin 3 acts as the scaffold in the E3 ligase regulating Nrf2 abundance, but was more recently shown to be mutated in the disease familial hyperkalemic hypertension. Studies seeking to elucidate the molecular mechanisms by which Cullin 3 mutations lead to dysregulation of renal sodium transport will be discussed. Disruption of Cullin 3 in mice unexpectedly causes polyuria and fibrotic injury suggesting it has additional roles in the kidney. We will also review recent transcriptomic data suggesting that other Cullins are also likely to play important roles in renal function.

SUMMARY

Cullins form a large and diverse family of E3 ubiquitin ligases that are likely to have many important functions in the kidney.

Clinical importance of potassium intake and molecular mechanism of potassium regulation

Introduction

Potassium (K⁺) intake is intrinsically linked to blood pressure. High-K⁺ intake decreases hypertension and associated lower mortality. On the other hand, hyperkalemia causes sudden death with fatal cardiac arrhythmia and is also related to higher mortality. Renal sodium (Na⁺)–chloride (Cl^‒) cotransporter (NCC), expressed in the distal convoluted tubule, is a key molecule in regulating urinary K⁺ excretion. K⁺ intake affects the activity of the NCC, which is related to salt-sensitive hypertension. A K⁺-restrictive diet activates NCC, and K⁺ loading suppresses NCC. Hyperpolarization caused by decreased extracellular K⁺ concentration ([K⁺]_ex) increases K⁺ and Cl^‒ efflux, leading to the activation of Cl^‒-sensitive with-no-lysine (WNK) kinases and their downstream molecules, including STE20/SPS1-related proline/alanine-rich kinase (SPAK) and NCC.

Results

We investigated the role of the ClC-K2 Cl^‒ channel and its β-subunit, barttin, using barttin hypomorphic (Bsnd^neo/neo) mice and found that these mice did not show low-K⁺-induced NCC activation and salt-sensitive hypertension. Additionally, we discovered that the suppression of NCC by K⁺ loading was regulated by another mechanism, whereby tacrolimus (a calcineurin [CaN] inhibitor) inhibited high-K⁺-induced NCC dephosphorylation and urinary K⁺ excretion. The K⁺ loading and the tacrolimus treatment did not alter the expression of WNK4 and SPAK. The depolarization induced by increased [K⁺]_ex activated CaN, which dephosphorylates NCC.

Conclusions

We concluded that there were two independent molecular mechanisms controlling NCC activation and K⁺ excretion. This review summarizes the clinical importance of K⁺ intake and explains how NCC phosphorylation is regulated by different molecular mechanisms between the low- and the high-K⁺ condition.

Electronic supplementary material

The online version of this article (10.1007/s10157-019-01766-x) contains supplementary material, which is available to authorized users.

Potassium acts through mTOR to regulate its own secretion

Potassium (K+) secretion by kidney tubule cells is central to electrolyte homeostasis in mammals. In the K+ secretory "principal" cells of the distal nephron, electrogenic Na+ transport by the epithelial sodium channel (ENaC) generates the electrical driving force for K+ transport across the apical membrane. Regulation of this process is attributable in part to aldosterone, which stimulates the gene transcription of the ENaC-regulatory kinase, SGK1. However, a wide range of evidence supports the conclusion that an unidentified aldosterone-independent pathway exists. We show here that in principal cells, K+ itself acts through the type 2 mTOR complex (mTORC2) to activate SGK1, which stimulates ENaC to enhance K+ excretion. The effect depends on changes in K+ concentration on the blood side of the cells, and requires basolateral membrane K+-channel activity. However, it does not depend on changes in aldosterone, or on enhanced distal delivery of Na+ from upstream nephron segments. These data strongly support the idea that K+ is sensed directly by principal cells to stimulate its own secretion by activating the mTORC2-SGK1 signaling module, and stimulate ENaC. We propose that this local effect acts in concert with aldosterone and increased Na+ delivery from upstream nephron segments to sustain K+ homeostasis.

Protein Phosphatase 1 Inhibitor-1 Mediates the cAMP-Dependent Stimulation of the Renal NaCl Cotransporter

BACKGROUND

A number of cAMP-elevating hormones stimulate phosphorylation (and hence activity) of the NaCl cotransporter (NCC) in the distal convoluted tubule (DCT). Evidence suggests that protein phosphatase 1 (PP1) and other protein phosphatases modulate NCC phosphorylation, but little is known about PP1's role and the mechanism regulating its function in the DCT.

METHODS

We used ex vivo mouse kidney preparations to test whether a DCT-enriched inhibitor of PP1, protein phosphatase 1 inhibitor-1 (I1), mediates cAMP's effects on NCC, and conducted yeast two-hybrid and coimmunoprecipitation experiments in NCC-expressing MDCK cells to explore protein interactions.

RESULTS

Treating isolated DCTs with forskolin and IBMX increased NCC phosphorylation via a protein kinase A (PKA)-dependent pathway. Ex vivo incubation of mouse kidney slices with isoproterenol, norepinephrine, and parathyroid hormone similarly increased NCC phosphorylation. The cAMP-induced stimulation of NCC phosphorylation strongly correlated with the phosphorylation of I1 at its PKA consensus phosphorylation site (a threonine residue in position 35). We also found an interaction between NCC and the I1-target PP1. Moreover, PP1 dephosphorylated NCC in vitro, and the PP1 inhibitor calyculin A increased NCC phosphorylation. Studies in kidney slices and isolated perfused kidneys of control and I1-KO mice demonstrated that I1 participates in the cAMP-induced stimulation of NCC.

CONCLUSIONS

Our data suggest a complete signal transduction pathway by which cAMP increases NCC phosphorylation via a PKA-dependent phosphorylation of I1 and subsequent inhibition of PP1. This pathway might be relevant for the physiologic regulation of renal sodium handling by cAMP-elevating hormones, and may contribute to salt-sensitive hypertension in patients with endocrine disorders or sympathetic hyperactivity.

Molecular mechanisms for the regulation of blood pressure by potassium

It has been well documented that the amount of potassium in the diet is associated with blood pressure levels in the population: the higher the potassium consumption, the lower the blood pressure and the cardiovascular mortality. In the last few years certain mechanisms for potassium regulation of salt reabsorption in the kidney have been elucidated at the molecular level. In this work we discuss the evidence demonstrating the relationship between potassium intake and blood pressure levels in human populations and in animal models, as well as the experimental data that reveal the effects of potassium on transepithelial Na⁺ reabsorption in different nephron segments. We also discuss the physiological relevance of K⁺-induced natriuresis, and finally, we focus on the molecular mechanisms by which extracellular potassium modulates the activity of the renal NaCl cotransporter, which is the mechanism that has been best dissected so far.

WNK4 kinase is a physiological intracellular chloride sensor

With-no-lysine (WNK) kinases regulate renal sodium-chloride cotransporter (NCC) to maintain body sodium and potassium homeostasis. Gain-of-function mutations of WNK1 and WNK4 in humans lead to a Mendelian hypertensive and hyperkalemic disease pseudohypoaldosteronism type II (PHAII). X-ray crystal structure and in vitro studies reveal chloride ion (Cl^-) binds to a hydrophobic pocket within the kinase domain of WNKs to inhibit its activity. The mechanism is thought to be important for physiological regulation of NCC by extracellular potassium. To test the hypothesis that WNK4 senses the intracellular concentration of Cl^- physiologically, we generated knockin mice carrying Cl^--insensitive mutant WNK4. These mice displayed hypertension, hyperkalemia, hyperactive NCC, and other features fully recapitulating human and mouse models of PHAII caused by gain-of-function WNK4. Lowering plasma potassium levels by dietary potassium restriction increased NCC activity in wild-type, but not in knockin, mice. NCC activity in knockin mice can be further enhanced by the administration of norepinephrine, a known activator of NCC. Raising plasma potassium by oral gavage of potassium inactivated NCC within 1 hour in wild-type mice, but had no effect in knockin mice. The results provide compelling support for the notion that WNK4 is a bona fide physiological intracellular Cl^- sensor and that Cl^- regulation of WNK4 underlies the mechanism of regulation of NCC by extracellular potassium.

Calcineurin dephosphorylates Kelch-like 3, reversing phosphorylation by angiotensin II and regulating renal electrolyte handling

Calcineurin inhibitors (CNIs) are potent immunosuppressants; hypertension and hyperkalemia are common adverse effects. Activation of the renal Na-Cl cotransporter (NCC) is implicated in this toxicity; however, the mechanism is unknown. CNIs’ renal effects mimic the hypertension and hyperkalemia resulting from mutations in WNK kinases or in KLHL3-CUL3 ubiquitin ligase. WNKs activate NCC and are degraded by ubiquitylation upon their binding to KLHL3. The binding of WNKs to KLHL3 is prevented by KLHL3 mutations or by PKC-mediated KLHL3 phosphorylation at serine 433. This work shows that calcineurin dephosphorylates KLHL3^S433, promoting WNK4 degradation. Conversely, CNIs inhibit KLHL3^S433 dephosphorylation, preventing WNK degradation. These findings implicate calcineurin in the normal regulation of KLHL3’s binding of WNK4 and identify a direct target causing CNI-induced pathology.

Calcineurin is a calcium/calmodulin-regulated phosphatase known for its role in activation of T cells following engagement of the T cell receptor. Calcineurin inhibitors (CNIs) are widely used as immunosuppressive agents; common adverse effects of CNIs are hypertension and hyperkalemia. While previous studies have implicated activation of the Na-Cl cotransporter (NCC) in the renal distal convoluted tubule (DCT) in this toxicity, the molecular mechanism of this effect is unknown. The renal effects of CNIs mimic the hypertension and hyperkalemia that result from germ-line mutations in with-no-lysine (WNK) kinases and the Kelch-like 3 (KLHL3)–CUL3 ubiquitin ligase complex. WNK4 is an activator of NCC and is degraded by binding to KLHL3 followed by WNK4’s ubiquitylation and proteasomal degradation. This binding is prevented by phosphorylation of KLHL3 at serine 433 (KLHL3^S433-P) via protein kinase C, resulting in increased WNK4 levels and increased NCC activity. Mechanisms mediating KLHL3^S433-P dephosphorylation have heretofore been unknown. We now demonstrate that calcineurin expressed in DCT is a potent KLHL3^S433-P phosphatase. In mammalian cells, the calcium ionophore ionomycin, a calcineurin activator, reduces KLHL3^S433-P levels, and this effect is reversed by the calcineurin inhibitor tacrolimus and by siRNA-mediated knockdown of calcineurin. In vivo, tacrolimus increases levels of KLHL3^S433-P, resulting in increased levels of WNK4, phosphorylated SPAK, and NCC. Moreover, tacrolimus attenuates KLHL3-mediated WNK4 ubiquitylation and degradation, while this effect is absent in KLHL3 with S433A substitution. Additionally, increased extracellular K⁺ induced calcineurin-dependent dephosphorylation of KLHL3^S433-P. These findings demonstrate that KLHL3^S433-P is a calcineurin substrate and implicate increased KLHL3 phosphorylation in tacrolimus-induced pathologies.

NaCl cotransporter abundance in urinary vesicles is increased by calcineurin inhibitors and predicts thiazide sensitivity

Animal studies have shown that the calcineurin inhibitors (CNIs) cyclosporine and tacrolimus can activate the thiazide-sensitive NaCl cotransporter (NCC). A common side effect of CNIs is hypertension. Renal salt transporters such as NCC are excreted in urinary extracellular vesicles (uEVs) after internalization into multivesicular bodies. Human studies indicate that CNIs also increase NCC abundance in uEVs, but results are conflicting and no relationship with NCC function has been shown. Therefore, we investigated the effects of CsA and Tac on the abundance of both total NCC (tNCC) and phosphorylated NCC at Thr60 phosphorylation site (pNCC) in uEVs, and assessed whether NCC abundance in uEVs predicts the blood pressure response to thiazide diuretics. Our results show that in kidney transplant recipients treated with cyclosporine (n = 9) or tacrolimus (n = 23), the abundance of both tNCC and pNCC in uEVs is 4–5 fold higher than in CNI-free kidney transplant recipients (n = 13) or healthy volunteers (n = 6). In hypertensive kidney transplant recipients, higher abundances of tNCC and pNCC prior to treatment with thiazides predicted the blood pressure response to thiazides. During thiazide treatment, the abundance of pNCC in uEVs increased in responders (n = 10), but markedly decreased in non-responders (n = 8). Thus, our results show that CNIs increase the abundance of both tNCC and pNCC in uEVs, and these increases correlate with the blood pressure response to thiazides. This implies that assessment of NCC in uEVs could represent an alternate method to guide anti-hypertensive therapy in kidney transplant recipients.

Responses of distal nephron Na+ transporters to acute volume depletion and hyperkalemia

We assessed effects of acute volume reductions induced by administration of diuretics in rats. Direct block of Na⁺ transport produced changes in urinary electrolyte excretion. Adaptations to these effects appeared as alterations in the expression of protein for the distal nephron Na⁺ transporters NCC and ENaC. Two hours after a single injection of furosemide (6 mg/kg) or hydrochlorothiazide (HCTZ; 30 mg/kg) Na⁺ and K⁺ excretion increased but no changes in the content of activated forms of NCC (phosphorylated on residue T53) or ENaC (cleaved γ-subunit) were detected. In contrast, amiloride (0.6 mg/kg) evoked a similar natriuresis that coincided with decreased pT53NCC and increased cleaved γENaC. Alterations in posttranslational membrane protein processing correlated with an increase in plasma K⁺ of 0.6-0.8 mM. Decreased pT53NCC occurred within 1 h after amiloride injection, whereas changes in γENaC were slower and were blocked by the mineralocorticoid receptor antagonist spironolactone. Increased γENaC cleavage correlated with elevation of the surface expression of the subunit as assessed by in situ biotinylation. Na depletion induced by 2 h of furosemide or HCTZ treatment increases total NCC expression without affecting ENaC protein. However, restriction of Na intake for 10 h (during the day) or 18 h (overnight) increased the abundance of both total NCC and of cleaved α- and γENaC. We conclude that the kidneys respond acutely to hyperkalemic challenges by decreasing the activity of NCC while increasing that of ENaC. They respond to hypovolemia more slowly, increasing Na⁺ reabsorptive capacities of both of these transporters.