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

Juxtaglomerular apparatus-mediated homeostatic mechanisms: therapeutic implication for chronic kidney disease

INTRODUCTION

Juxtaglomerular apparatus (JGA)-mediated homeostatic mechanism links to how sodium-glucose cotransporter 2 inhibitors (SGLT2is) slow progression of chronic kidney disease (CKD) and may link to how tolvaptan slows renal function decline in autosomal dominant polycystic kidney disease (ADPKD).

AREA COVERED

JGA-mediated homeostatic mechanism has been hypothesized based on investigations of tubuloglomerular feedback and renin-angiotensin system. We reviewed clinical trials of SGLT2is and tolvaptan to assess the relationship between this mechanism and these drugs.

EXPERT OPINION

When sodium load to macula densa (MD) increases, MD increases adenosine production, constricting afferent arteriole (Af-art) and protecting glomeruli. Concurrently, MD signaling suppresses renin secretion, increases urinary sodium excretion, and counterbalances reduced sodium filtration. However, when there is marked increase in sodium load per-nephron, as in advanced CKD, MD adenosine production increases, relaxing Af-art and maintaining sodium homeostasis at the expense of glomeruli. The beneficial effects of tolvaptan on renal function in ADPKD may also depend on the JGA-mediated homeostatic mechanisms since tolvaptan inhibits sodium reabsorption in the thick ascending limb.The JGA-mediated homeostatic mechanism regulates Af-arts, constricting to relaxing according to homeostatic needs. Understanding this mechanism may contribute to the development of pharmacotherapeutic compounds and better care for patients with CKD.

Magnesium in hypertension: mechanisms and clinical implications

Hypertension is associated with increased risk of cardiovascular disease and death. Evidence suggests that Mg2+ depletion contributes to hypertension. It is estimated that 25% or more of the United States population experiences chronic, latent Mg2+ depletion. This review explores mechanisms by which Mg2+ influences blood pressure, modifying risk of hypertension and complicating its treatment. Mechanisms addressed include effects upon i) sympathetic tone, via the modulation of N-methyl-D-aspartate (NMDA) receptor and N-type Ca2+ channel activity, influencing catecholamine release from sympathetic nerve endings; ii) vascular tone, via alteration of L-type Ca2+ and endothelial nitric oxide synthase (eNOS) activity and prostacyclin release; iii) renal K+ handling, influencing systemic K+ balance and potentially indirectly influencing blood pressure; iv) aldosterone secretion from the adrenal cortex; and v) modulation of pro-hypertensive inflammatory processes in dendritic cells and macrophages, including activation of the NLR family pyrin domain containing 3 (NLRP3) inflammasome and stimulation of isolevuglandin (IsoLG) production. Discovery of these mechanisms has furthered our understanding of the pathogenesis of hypertension, with implications for treatment and has highlighted the role of Mg2+ balance in hypertension and cardiovascular disease.

KDM6A Demethylase Regulates Renal Sodium Excretion and Blood Pressure

BACKGROUND

KDM6A (Lysine-Specific Demethylase 6A) is a specific demethylase for histone 3 lysine (K) 27 trimethylation (H3K27me3). The purpose of this study is to investigate whether KDM6A in renal tubule cells plays a role in the regulation of kidney function and blood pressure.

METHODS

We first crossed Ksp-Cre+/- and KDM6Aflox/flox mice for generating inducible kidney-specific deletion of KDM6A gene.

RESULTS

Notably, conditional knockout of KDM6A gene in renal tubule cells (KDM6A-cKO) increased H3K27me3 levels which leads to a decrease in Na excretion and elevation of blood pressure. Further analysis showed that the expression of NKCC2 (Na-K-2Cl cotransporter 2) and NCC (Na-Cl cotransporters) was upregulated which contributes to impaired Na excretion in KDM6A-cKO mice. The expression of AQP2 (aquaporin 2) was also increased in KDM6A-cKO mice, which may facilitate water reabsorption in KDM6A-cKO mice. The expression of Klotho was downregulated while expression of aging markers including p53, p21, and p16 was upregulated in kidneys of KDM6A-cKO mice, indicating that deletion of KDM6A in the renal tubule cells promotes kidney aging. Interestingly, KDM6A-cKO mice developed salt-sensitive hypertension which can be rescued by treatment with Klotho. KDM6A deficiency induced salt-sensitive hypertension likely through downregulation of the Klotho/ERK (extracellular signal-regulated kinase) signaling and upregulation of the WNK (with-no-lysine kinase) signaling.

CONCLUSIONS

This study provides the first evidence that KDM6A plays an essential role in maintaining normal tubular function and blood pressure. Renal tubule cell specific KDM6A deficiency causes hypertension due to increased H3K27me3 levels and the resultant downregulation of Klotho gene expression which disrupts the Klotho/ERK/NCC/NKCC2 signaling.

Arginine vasopressin regulates the renal Na+-Cl- and Na+-K+-Cl- cotransporters through with-no-lysine kinase 4 and inhibitor 1 phosphorylation

Vasopressin regulates water homeostasis via the V2 receptor in the kidney at least in part through protein kinase A (PKA) activation. Vasopressin, through an unknown pathway, upregulates the activity and phosphorylation of Na+-Cl- cotransporter (NCC) and Na+-K+-2Cl- cotransporter 2 (NKCC2) by Ste20-related proline/alanine-rich kinase (SPAK) and oxidative stress-responsive kinase 1 (OSR1), which are regulated by the with-no-lysine kinase (WNK) family. Phosphorylation of WNK4 at PKA consensus motifs may be involved. Inhibitor 1 (I1), a protein phosphatase 1 (PP1) inhibitor, may also play a role. In human embryonic kidney (HEK)-293 cells, we assessed the phosphorylation of WNK4, SPAK, NCC, or NKCC2 in response to forskolin or desmopressin. WNK4 and cotransporter phosphorylation were studied in desmopressin-infused WNK4-/- mice and in tubule suspensions. In HEK-293 cells, only wild-type WNK4 but not WNK1, WNK3, or a WNK4 mutant lacking PKA phosphorylation motifs could upregulate SPAK or cotransporter phosphorylation in response to forskolin or desmopressin. I1 transfection maximized SPAK phosphorylation in response to forskolin in the presence of WNK4 but not of mutant WNK4 lacking PP1 regulation. We observed direct PP1 regulation of NKCC2 dephosphorylation but not of NCC or SPAK in the absence of WNK4. WNK4-/- mice with desmopressin treatment did not increase SPAK/OSR1, NCC, or NKCC2 phosphorylation. In stimulated tubule suspensions from WNK4-/- mice, upregulation of pNKCC2 was reduced, whereas upregulation of SPAK phosphorylation was absent. These findings suggest that WNK4 is a central node in which kinase and phosphatase signaling converge to connect cAMP signaling to the SPAK/OSR1-NCC/NKCC2 pathway.NEW & NOTEWORTHY With-no-lysine kinases regulate the phosphorylation and activity of the Na+-Cl- and Na+-K+-2Cl- cotransporters. This pathway is modulated by arginine vasopressin (AVP). However, the link between AVP and WNK signaling remains unknown. Here, we show that AVP activates WNK4 through increased phosphorylation at putative protein kinase A-regulated sites and decreases its dephosphorylation by protein phosphatase 1. This work increases our understanding of the signaling pathways mediating AVP actions in the kidney.

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

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

The role of SLC12A family of cation-chloride cotransporters and drug discovery methodologies

The solute carrier family 12 (SLC12) of cation-chloride cotransporters (CCCs) comprises potassium chloride cotransporters (KCCs, e.g. KCC1, KCC2, KCC3, and KCC4)-mediated Cl- extrusion, and sodium potassium chloride cotransporters (N[K]CCs, NKCC1, NKCC2, and NCC)-mediated Cl- loading. The CCCs play vital roles in cell volume regulation and ion homeostasis. Gain-of-function or loss-of-function of these ion transporters can cause diseases in many tissues. In recent years, there have been considerable advances in our understanding of CCCs' control mechanisms in cell volume regulations, with many techniques developed in studying the functions and activities of CCCs. Classic approaches to directly measure CCC activity involve assays that measure the transport of potassium substitutes through the CCCs. These techniques include the ammonium pulse technique, radioactive or nonradioactive rubidium ion uptake-assay, and thallium ion-uptake assay. CCCs' activity can also be indirectly observed by measuring γ-aminobutyric acid (GABA) activity with patch-clamp electrophysiology and intracellular chloride concentration with sensitive microelectrodes, radiotracer 36Cl-, and fluorescent dyes. Other techniques include directly looking at kinase regulatory sites phosphorylation, flame photometry, 22Na+ uptake assay, structural biology, molecular modeling, and high-throughput drug screening. This review summarizes the role of CCCs in genetic disorders and cell volume regulation, current methods applied in studying CCCs biology, and compounds developed that directly or indirectly target the CCCs for disease treatments.

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.

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.

WNK1 in the kidney

PURPOSE OF REVIEW

The aim of this manuscript was to review recent evidence uncovering the roles of the With No lysine (K) kinase 1 (WNK1) in the kidney.

RECENT FINDINGS

Analyses of microdissected mouse nephron segments have revealed the abundance of long-WNK1 and kidney-specific-WNK1 transcripts in different segments. The low levels of L-WNK1 transcripts in the distal convoluted tubule (DCT) stand out and support functional evidence on the lack of L-WNK1 activity in this segment. The recent description of familial hyperkalaemic hypertension (FHHt)-causative mutations affecting the acidic domain of WNK1 supports the notion that KS-WNK1 activates the Na+:Cl- cotransporter NCC. The high sensitivity of KS-WNK1 to KLHL3-targeted degradation and the low levels of L-WNK1 in the DCT, led to propose that this type of FHHt is mainly due to increased KS-WNK1 protein in the DCT. The observation that KS-WNK1 renal protein expression is induced by low K+ diet and recent reassessment of the phenotype of KS-WNK1-/- mice suggested that KS-WNK1 may be necessary to achieve maximal NCC activation under this condition. Evidences on the regulation of other renal transport proteins by WNK1 are also summarized.

SUMMARY

The diversity of WNK1 transcripts in the kidney has complicated the interpretation of experimental data. Integration of experimental data with the knowledge of isoform abundance in renal cell types is necessary in future studies about WNK1 function in the kidney.

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.

STK39 enhances the progression of Cholangiocarcinoma via PI3K/AKT pathway

Serine/threonine kinase 39 (STK39) is overexpressed in various tumor tissues and plays an essential role in tumor progression. In this study, we investigated the clinical value, as well as the potential functions and mechanisms of STK39 in cholangiocarcinoma (CCA). The results showed that STK39 was overexpressed in CCA and negatively associated with the prognosis of patients with CCA. Functionally, STK39 knockdown suppressed cell proliferation, migration, and invasion, while STK39 overexpression facilitated tumor aggressiveness. The tumor-promoting effects of STK39 in CCA were also validated by in vivo experiments. Mechanistically, RNA-seq analysis identified that STK39 enhanced the progression of CCA by activating PI3K/AKT signaling pathway. Furthermore, overexpression of STK39 could induce gemcitabine resistance in CCA cells. Moreover, the increased expression of STK39 may be mediated by the dysregulation of miR-26a-5p. In summary, STK39 could be served as a valuable prognostic candidate and a potential therapeutic target of CCA.

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STK39 was overexpressed in CCA, negatively associated with the prognosis of patients with CCA

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STK39 knockdown suppressed cell proliferation and invasion. STK39 overexpression facilitated tumor aggressiveness

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STK39 mediates oncogenic effects on CCA cells by activating the PI3K/AKT signaling pathway

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STK39 reduces CCA sensitivity to gemcitabine. Increased expression of STK39 may be mediated by dysregulation of miR-26a-5p

With no lysine kinases: the key regulatory networks and phytohormone cross talk in plant growth, development and stress response

With No Lysine kinases (WNKs) are a distinct family of Serine/Threonine protein kinase with unique arrangement of catalytic residues in kinase domain. In WNK, an essential catalytic lysine requisite for attaching ATP and phosphorylation reaction is located in subdomain I, instead of subdomain II, which is essentially a typical feature of other Ser/Thr kinases. WNKs are identified in diverse organisms including multicellular and unicellular organisms. Mammalian WNKs are well characterized at structural and functional level, while plant WNKs are not explored much except few recent studies. Plant WNKs role in various physiological processes viz. ion maintenance, osmotic stress, pH homeostasis, circadian rhythms, regulation of flowering time, proliferation and organ development, and abiotic stresses are known, but the mechanisms involved are unclear. Plant WNKs are known to be involved in enhanced drought and salt stress response via ABA-signaling pathway, but the complete signaling cascade is yet to be elucidated. The current review will discuss the interplay between WNKs and growth regulators and their cross talks in plant growth and development. We have also highlighted the link between the stress phytohormones and WNK members in regulating abiotic stress responses in plants. The present review will provide an overall known mechanism on the involvement of WNKs in plant growth and development and abiotic stress response and highlight its role/applications in the development of stress-tolerant plants.

Sex-specific adaptations to high-salt diet preserve electrolyte homeostasis with distinct sodium transporter profiles

Kidneys continuously filter an enormous amount of sodium and adapt kidney Na+ reabsorption to match Na+ intake to maintain circulatory volume and electrolyte homeostasis. Males (M) respond to high-salt (HS) diet by translocating proximal tubule Na+/H+ exchanger isoform 3 (NHE3) to the base of the microvilli, reducing activated forms of the distal NaCl cotransporter (NCC) and epithelial Na+ channel (ENaC). Males (M) and females (F) on normal-salt (NS) diet present sex-specific profiles of "transporters" (cotransporters, channels, pumps, and claudins) along the nephron, e.g., F exhibit 40% lower NHE3 and 200% higher NCC abundance than M. We tested the hypothesis that adaptations to HS diet along the nephron will, likewise, exhibit sexual dimorphisms. C57BL/6J mice were fed for 15 days with 4% NaCl diet (HS) versus 0.26% NaCl diet (NS). On HS, M and F exhibited normal plasma [Na+] and [K+], similar urine volume, Na+, K+, and osmolal excretion rates normalized to body weight. In F, like M, HS lowered abundance of distal NCC, phosphorylated NCC, and cleaved (activated) forms of ENaC. The adaptations associated with achieving electrolyte homeostasis exhibit sex-dependent and independent mechanisms. Sex differences in baseline "transporters" abundance persist during HS diet, yet the fold changes during HS diet (normalized to NS) are similar along the distal nephron and collecting duct. Sex-dependent differences observed along the proximal tubule during HS show that female kidneys adapt differently from patterns reported in males, yet achieve and maintain fluid and electrolyte homeostasis.

Cation-coupled chloride cotransporters: chemical insights and disease implications

Cation-coupled chloride cotransporters (CCCs) modulate the transport of sodium and/or potassium cations coupled with chloride anions across the cell membrane. CCCs thus help regulate intracellular ionic concentration and consequent cell volume homeostasis. This has been largely exploited in the past to develop diuretic drugs that act on CCCs expressed in the kidney. However, a growing wealth of evidence has demonstrated that CCCs are also critically involved in a great variety of other pathologies, motivating most recent drug discovery programs targeting CCCs. Here, we examine the structure–function relationship of CCCs. By linking recent high-resolution cryogenic electron microscopy (cryo-EM) data with older biochemical/functional studies on CCCs, we discuss the mechanistic insights and opportunities to design selective CCC modulators to treat diverse pathologies.

The structural topology and function of all cation-coupled chloride cotransporters (CCCs) have been continuously investigated over the past 40 years, with great progress also thanks to the recent cryogenic electron microscopy (cryo-EM) resolution of the structures of five CCCs. In particular, such studies have clarified the structure–function relationship for the Na-K-Cl cotransporter NKCC1 and K-Cl cotransporters KCC1–4.

The constantly growing evidence of the crucial involvement of CCCs in physiological and various pathological conditions, as well as the evidence of their wide expression in diverse body tissues, has promoted CCCs as targets for the discovery and development of new, safer, and more selective/effective drugs for a plethora of pathologies.

Post-translational modification anchor points on the structure of CCCs may offer alternative strategies for small molecule drug discovery.

Knockdown of STK39 suppressed cell proliferation, migration, and invasion in hepatocellular carcinoma by repressing the phosphorylation of mitogen-activated protein kinase p38

Hepatocellular carcinoma (HCC) is a serious malignant tumor of the liver. It has been reported that serine/threonine kinase 39 (STK39) participates in tumorigenesis. However, the role of STK39 in HCC remains unknown. In this study, the qRT-PCR and western blot assay demonstrated that STK39 expression was enhanced in HCC patients and tissues. Moreover, CCK-8 and colony formation assays confirmed that knockdown of STK39 suppressed SK-HEP-1 and Huh7 cells proliferation. Furthermore, wound healing assay and transwell assay revealed that knockdown of STK39 repressed SK-HEP-1 and Huh7 cells migration and invasion. Interestingly, knockdown of STK39 reduced p-p38/p38 ratio and levels of c-Myc. Consistently, knockdown of STK39 inhibited the HCC tumor growth in vivo. In summary, knockdown of STK39 suppressed the proliferation, migration, and invasion of HCC cells by inducing the lower levels of p-p38, which might provide a novel therapeutic target for HCC.

A five amino acids deletion in NKCC2 of C57BL/6 mice affects analysis of NKCC2 phosphorylation but does not impact kidney function

Aim

The phosphorylation level of the furosemide‐sensitive Na⁺‐K⁺‐2Cl⁻ cotransporter (NKCC2) in the thick ascending limb (TAL) is used as a surrogate marker for NKCC2 activation and TAL function. However, in mice, analyses of NKCC2 phosphorylation with antibodies against phosphorylated threonines 96 and 101 (anti‐pT96/pT101) give inconsistent results. We aimed (a) to elucidate these inconsistencies and (b) to develop a phosphoform‐specific antibody that ensures reliable detection of NKCC2 phosphorylation in mice.

Methods

Genetic information, molecular biology, biochemical techniques and mouse phenotyping was used to study NKCC2 and kidney function in two commonly used mouse strains (ie 129Sv and in C57BL/6 mice). Moreover, a new phosphoform‐specific mouse NKCC2 antibody was developed and characterized.

Results

Amino acids sequence alignment revealed that C57BL/6 mice have a strain‐specific five amino acids deletion (ΔF97‐T101) in NKCC2 that diminishes the detection of NKCC2 phosphorylation with previously developed pT96/pT101 NKCC2 antibodies. Instead, the antibodies cross‐react with the phosphorylated thiazide‐sensitive NaCl cotransporter (NCC), which can obscure interpretation of results. Interestingly, the deletion in NKCC2 does not impact on kidney function and/or expression of renal ion transport proteins as indicated by the analysis of the F2 generation of crossbred 129Sv and C57BL/6 mice. A newly developed pT96 NKCC2 antibody detects pNKCC2 in both mouse strains and shows no cross‐reactivity with phosphorylated NCC.

Conclusion

Our work reveals a hitherto unappreciated, but essential, strain difference in the amino acids sequence of mouse NKCC2 that needs to be considered when analysing NKCC2 phosphorylation in mice. The new pNKCC2 antibody circumvents this technical caveat.

Molecular Mechanism of Inhibiting WNK Binding to OSR1 by Targeting the Allosteric Pocket of the OSR1-CCT Domain with Potential Antihypertensive Inhibitors: An In Silico Study

The oxidative-stress-responsive kinase 1 (OSR1) and the STE20/SPS1-related proline-alanine-rich kinase (SPAK) are physiological substrates of the with-no-lysine (WNK) kinase. They are the master regulators of cation Cl^- cotransporters that could be targeted for discovering novel antihypertensive agents. Both kinases have a conserved carboxy-terminal (CCT) domain that recognizes a unique peptide motif (Arg-Phe-Xaa-Val) present in their upstream kinases and downstream substrates. Here, we have combined molecular docking with molecular dynamics simulations and free-energy calculations to identify potential inhibitors that can bind to the allosteric pocket of the OSR1-CCT domain and impede its interaction with the WNK peptide. Our study revealed that STOCK1S-14279 and Closantel bound strongly to the allosteric pocket of OSR1 and displaced the WNK peptide from the primary pocket of OSR1. We showed that primarily Arg1004 and Gln1006 of the WNK4-peptide motif were involved in strong H-bond interactions with Glu453 and Arg451 of OSR1. Besides, our study revealed that atoms of Arg1004 were solvent-exposed in cases of STOCK1S-14279 and Closantel, implying that the WNK4 peptide was moved out of the pocket. Overall, the predicted potential inhibitors altogether abolish the OSR1-WNK4-peptide interaction, suggesting their potency as a prospective allosteric inhibitor against OSR1.

Mutation affecting the conserved acidic WNK1 motif causes inherited hyperkalemic hyperchloremic acidosis

Gain-of-function mutations in with no lysine (K) 1 (WNK1) and WNK4 genes are responsible for familial hyperkalemic hypertension (FHHt), a rare, inherited disorder characterized by arterial hypertension and hyperkalemia with metabolic acidosis. More recently, FHHt-causing mutations in the Kelch-like 3-Cullin 3 (KLHL3-CUL3) E3 ubiquitin ligase complex have shed light on the importance of WNK's cellular degradation on renal ion transport. Using full exome sequencing for a 4-generation family and then targeted sequencing in other suspected cases, we have identified new missense variants in the WNK1 gene clustering in the short conserved acidic motif known to interact with the KLHL3-CUL3 ubiquitin complex. Affected subjects had an early onset of a hyperkalemic hyperchloremic phenotype, but normal blood pressure values"Functional experiments in Xenopus laevis oocytes and HEK293T cells demonstrated that these mutations strongly decrease the ubiquitination of the kidney-specific isoform KS-WNK1 by the KLHL3-CUL3 complex rather than the long ubiquitous catalytically active L-WNK1 isoform. A corresponding CRISPR/Cas9 engineered mouse model recapitulated both the clinical and biological phenotypes. Renal investigations showed increased activation of the Ste20 proline alanine-rich kinase-Na+-Cl- cotransporter (SPAK-NCC) phosphorylation cascade, associated with impaired ROMK apical expression in the distal part of the renal tubule. Together, these new WNK1 genetic variants highlight the importance of the KS-WNK1 isoform abundance on potassium homeostasis.

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.

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.

Characterizing an allosteric inhibitor-induced inactive state in with-no-lysine kinase 1 using Gaussian accelerated molecular dynamics simulations

The binding of an allosteric inhibitor in WNK1 leads to the inactive state.

STK39 is a novel kinase contributing to the progression of hepatocellular carcinoma by the PLK1/ERK signaling pathway

Rationale: Protein kinases are critical therapeutic targets for curing hepatocellular carcinoma (HCC). As a serine/threonine kinase, the potential roles of serine/threonine kinase 39 (STK39) in HCC remain to be explored. Methods: The expression of STK39 was examined by RT-qPCR, western blotting and immunohistochemistry. Cell proliferation and apoptosis were detected by CCK8 and TUNEL kit. Cell migration and invasion assays were performed using a transwell system with or without Matrigel. RNA-seq, mass spectrometry and luciferase reporter assays were used to identify STK39 binding proteins. Results: Here, we firstly report that STK39 was highly overexpressed in clinical HCC tissues compared with adjacent tissues, high expression of STK39 was induced by transcription factor SP1 and correlated with poor patient survival. Gain and loss of function assays revealed that overexpression of STK39 promoted HCC cell proliferation, migration and invasion. In contrast, the depletion of STK39 attenuated the growth and metastasis of HCC cells. Moreover, knockdown of STK39 induced the HCC cell cycle arrested in the G2/M phase and promoted apoptosis. In mechanistic studies, RNA-seq revealed that STK39 positively regulated the ERK signaling pathway. Mass spectrometry identified that STK39 bound to PLK1 and STK39 promoted HCC progression and activated ERK signaling pathway dependent on PLK1. Conclusions: Thus, our study uncovers a novel role of STK39/PLK1/ERK signaling axis in the progress of HCC and suggests STK39 as an indicator for prognosis and a potential drug target of HCC.

SPAK and OSR1 kinases bind and phosphorylate the β2-Adrenergic receptor

SPAK and OSR1 are two cytoplasmic serine/threonine protein kinases that regulate the function of a series of sodium, potassium and chloride co-transporters via phosphorylation. Over recent years, it has emerged that these two kinases may have diverse function beyond the regulation of ion co-transporters. Inspired by this, we explored whether SPAK and OSR1 kinases impact physically and phosphorylate the β₂-adrenergic receptor (β₂ADR). Herein, we report that the amino acid sequence of the human β₂ADR displays a SPAK/OSR1 consensus binding motif and using a series of pulldown and in vitro kinase assays we show that SPAK and OSR1 bind the β₂ADR and phosphorylate it in vitro. This work provides a notable example of SPAK and OSR1 kinases binding to a G-protein coupled receptor and taps into the potential of these protein kinases in regulating membrane receptors beyond ion co-transporters.

Geniposide in Gardenia jasminoides var. radicans Makino modulates blood pressure via inhibiting WNK pathway mediated by the estrogen receptors

Objectives

To investigate the effects of geniposide in an iridoid found in Gardenia jasminoides var. radicans Makino (GJRM) in spontaneous hypertensive rat (SHR) and explore the possible mechanisms.

Methods

In this study, we detected the content of geniposide in GJRM by high-performance liquid chromatography (HPLC). Then, we used acute diuretic experiments to determine whether geniposide has diuretic effect. Moreover, we carried out experiments on SHR to further study the mechanism of hypertension, while real-time PCR, Western blot and immunohistochemistry were used for the experiments in vivo test. Hypotonic model was used for in vitro test.

Key findings

Our data showed that the content of geniposide in the extract of GJRM is 27.54%. Meanwhile, 50 mg/kg geniposide showed the strongest effect on promoting urine volume. Further study indicated that the extract of GJRM and geniposide could significantly reduce blood pressure and promote the excretion of urine and Na+ in SHR. In addition, geniposide significantly inhibited the activation of the with-no-lysine kinase (WNK) signalling pathway and significantly increases the protein expressions of estrogen receptor α (ERα), estrogen receptor β (ERβ) and G protein-coupled receptor 30 (GPR30) in SHR. In hypotonic model, geniposide significantly inhibits the phosphorylation of NKCC and NCC and could be antagonistic to estrogen receptor antagonists.

Conclusions

Collectively, we would suggest that geniposide may potentially be utilized as an adjunct to existing thiazide and thiazide-like diuretics to control hypertension, mainly through inhibiting the activation of the WNK signalling pathway mediated by the estrogen receptor.

Molecular characteristics and physiological roles of Na+ -K+ -Cl- cotransporter 2

Na⁺–K⁺–Cl⁻ cotransporter 2 (NKCC2; SLC12A1) is an integral membrane protein that comes as three splice variants and mediates the cotranslocation of Na⁺, K⁺, and Cl⁻ ions through the apical membrane of the thick ascending loop of Henle (TALH). In doing so, and through the involvement of other ion transport systems, it allows this nephron segment to reclaim a large fraction of the ultrafiltered Na⁺, Cl⁻, Ca²⁺, Mg²⁺, and HCO₃⁻ loads. The functional relevance of NKCC2 in human is illustrated by the many abnormalities that result from the inactivation of this transport system through the use of loop diuretics or in the setting of inherited disorders. The following presentation aims at discussing the physiological roles and molecular characteristics of Na⁺–K⁺–Cl⁻ cotransport in the TALH and those of the individual NKCC2 splice variants more specifically. Many of the historical and recent data that have emerged from the experiments conducted will be outlined and their larger meaning will also be placed into perspective with the aid of various hypotheses.

Bicarbonate permeation through anion channels: its role in health and disease

Many anion channels, frequently referred as Cl^- channels, are permeable to different anions in addition to Cl^-. As the second-most abundant anion in the human body, HCO₃^- permeation via anion channels has many important physiological roles. In addition to its classical role as an intracellular pH regulator, HCO₃^- also controls the activity and stability of dissolved proteins in bodily fluids such as saliva, pancreatic juice, intestinal fluid, and airway surface liquid. Moreover, HCO₃^- permeation through these channels affects membrane potentials that are the driving forces for transmembrane transport of solutes and water in epithelia and affect neuronal excitability in nervous tissue. Consequently, aberrant HCO₃^- transport via anion channels causes a number of human diseases in respiratory, gastrointestinal, genitourinary, and neuronal systems. Notably, recent studies have shown that the HCO₃^- permeabilities of several anion channels are not fixed and can be altered by cellular stimuli, findings which may have both physiological and pathophysiological significance. In this review, we summarize recent progress in understanding the molecular mechanisms and the physiological roles of HCO₃^- permeation through anion channels. We hope that the present discussions can stimulate further research into this very important topic, which will provide the basis for human disorders associated with aberrant HCO₃^- transport.

The Drosophila Malpighian tubule as a model for mammalian tubule function

PURPOSE OF REVIEW

Studies of the genetic model organism, Drosophila melanogaster, have unraveled molecular pathways relevant to human physiology and disease. The Malpighian tubule, the Drosophila renal epithelium, is described here, including tools available to study transport; conserved transporters, channels, and the signaling pathways regulating them; and fly models of kidney stone disease.

RECENT FINDINGS

Tools to measure Malpighian tubule transport continue to advance, including use of a transgenic sensor to quantify intracellular pH and proton fluxes. A recent study generated an RNA-sequencing-based atlas of tissue-specific gene expression, with resulting insights into Malpighian tubule gene expression of transporters and channels. Advances have been made in understanding the molecular physiology of the With No Lysine kinase-Ste20-related proline/alanine rich kinase/oxidative stress response kinase cascade that regulates epithelial ion transport in flies and mammals. New studies in Drosophila kidney stone models have characterized zinc transporters and used Malpighian tubules to study the efficacy of a plant metabolite in decreasing stone burden.

SUMMARY

Study of the Drosophila Malpighian tubule affords opportunities to better characterize the molecular physiology of epithelial transport mechanisms relevant to mammalian renal physiology.

Molecular regulation of NKCC2 in blood pressure control and hypertension

PURPOSE OF REVIEW

The apical Na/K/2Cl cotransporter (NKCC2) mediates NaCl reabsorption by the thick ascending limb, contributing to maintenance of blood pressure (BP). Despite effective NKCC2 inhibition by loop diuretics, these agents are not viable for long-term management of BP due to side effects. Novel molecular mechanisms that control NKCC2 activity reveal an increasingly complex picture with interacting layers of NKCC2 regulation. Here, we review the latest developments that shine new light on NKCC2-mediated control of BP and potential new long-term therapies to treat hypertension.

RECENT FINDINGS

Emerging molecular NKCC2 regulators, often binding partners, reveal a complex overlay of interacting mechanisms aimed at fine tuning NKCC2 activity. Different factors achieve this by shifting the balance between trafficking steps like exocytosis, endocytosis, recycling and protein turnover, or by balancing phosphorylation vs. dephosphorylation. Further molecular details are also emerging on previously known pathways of NKCC2 regulation, and recent in-vivo data continues to place NKCC2 regulation at the center of BP control.

SUMMARY

Several layers of emerging molecular mechanisms that control NKCC2 activity may operate simultaneously, but they can also be controlled independently. This provides an opportunity to identify new pharmacological targets to fine-tune NKCC2 activity for BP management.

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 PI3k-WNK Pathway by Ethyl Acetate Partition Fraction of Gardenia jasminoides var. radicans Makino in SHR, NRK52e Cells, and IMCD3 Cells

The antihypertensive mechanism was studied of the ethyl acetate fraction of the ethanol extract (EAPF) of Gardenia jasminoides var. radicans Makino (GJRM). GJRM is a fake product of Gardenia jasminoides Ellis ( G. jasminoides), but in China’s Henan province, the production of GJRM is much more than G. jasminoides’s, but its traditional usage is as a dye. Gardenia jasminoides can be used to reduce blood pressure and blood glucose levels. The chemical compositions of GJRM and G. jasminoides are similar, and we previously confirmed that GJRM can also reduce blood pressure. Here, we report that the EAPF of GJRM could activate the phosphoinositide 3-kinases (PI3K) pathway in the kidneys of spontaneously hypertensive rats, thus increasing the content of nitric oxide and bradykinin in sera and decreasing endothelin-1 content. EAPF can also decrease the levels of with-no-lysine kinase 1 (WNK1) expression, WNK4 and oxidative stress-responsive kinase 1 messenger ribonucleic acid (mRNA), and Na-K-2Cl cotransporter and sodium chloride cotransporters mRNA and phosphorylation. To investigate the antihypertensive effects of the EAPF of GJRM, 5 monoterpenoids isolated from EAPF were studied for their effects on NRK52e and IMCD3 cells. These compounds inhibited the PI3K-WNK signaling pathway to varying degrees under hypotonic conditions; 4-methoxyrehmapicrogenin had the best effect.

Modulation of brain cation-Cl- cotransport via the SPAK kinase inhibitor ZT-1a

The SLC12A cation-Cl^- cotransporters (CCC), including NKCC1 and the KCCs, are important determinants of brain ionic homeostasis. SPAK kinase (STK39) is the CCC master regulator, which stimulates NKCC1 ionic influx and inhibits KCC-mediated efflux via phosphorylation at conserved, shared motifs. Upregulation of SPAK-dependent CCC phosphorylation has been implicated in several neurological diseases. Using a scaffold-hybrid strategy, we develop a novel potent and selective SPAK inhibitor, 5-chloro-N-(5-chloro-4-((4-chlorophenyl)(cyano)methyl)-2-methylphenyl)-2-hydroxybenzamide ("ZT-1a"). ZT-1a inhibits NKCC1 and stimulates KCCs by decreasing their SPAK-dependent phosphorylation. Intracerebroventricular delivery of ZT-1a decreases inflammation-induced CCC phosphorylation in the choroid plexus and reduces cerebrospinal fluid (CSF) hypersecretion in a model of post-hemorrhagic hydrocephalus. Systemically administered ZT-1a reduces ischemia-induced CCC phosphorylation, attenuates cerebral edema, protects against brain damage, and improves outcomes in a model of stroke. These results suggest ZT-1a or related compounds may be effective CCC modulators with therapeutic potential for brain disorders associated with impaired ionic homeostasis.

WNK1 phosphorylation sites in TBC1D1 and TBC1D4 modulate cell surface expression of GLUT1

Glucose uptake by mammalian cells is a key mechanism to maintain cell and tissue homeostasis and relies mostly on plasma membrane-localized glucose transporter proteins (GLUTs). Two main cellular mechanisms regulate GLUT proteins in the cell: first, expression of GLUT genes is under dynamic transcriptional control and is used by cancer cells to increase glucose availability. Second, GLUT proteins are regulated by membrane traffic from storage vesicles to the plasma membrane (PM). This latter process is triggered by signaling mechanisms and well-studied in the case of insulin-responsive cells, which activate protein kinase AKT to phosphorylate TBC1D4, a RAB-GTPase activating protein involved in membrane traffic regulation. Previously, we identified protein kinase WNK1 as another kinase able to phosphorylate TBC1D4 and regulate the surface expression of the constitutive glucose transporter GLUT1. Here we describe that downregulation of WNK1 through RNA interference in HEK293 cells led to a 2-fold decrease in PM GLUT1 expression, concomitant with a 60% decrease in glucose uptake. By mass spectrometry, we identified serine (S) 704 in TBC1D4 as a WNK1-regulated phosphorylation site, and also S565 in the paralogue TBC1D1. Transfection of the respective phosphomimetic or unphosphorylatable TBC1D mutants into cells revealed that both affected the cell surface abundance of GLUT1. The results reinforce a regulatory role for WNK1 in cell metabolism and have potential impact for the understanding of cancer cell metabolism and therapeutic options in type 2 diabetes.

Impact of angiotensin II-mediated stimulation of sodium transporters in the nephron assessed by computational modeling

Angiotensin II (ANG II) raises blood pressure partly by stimulating tubular Na⁺ reabsorption. The effects of ANG II on tubular Na⁺ transporters (i.e., channels, pumps, cotransporters, and exchangers) vary between short-term and long-term exposure. To better understand the physiological impact, we used a computational model of transport along the rat nephron to predict the effects of short- and long-term ANG II-induced transporter activation on Na⁺ and K⁺ reabsorption/secretion, and to compare measured and calculated excretion rates. Three days of ANG II infusion at 200 ng·kg^-1·min^-1 is nonpressor, yet stimulates transporter accumulation. The increase in abundance of Na⁺/H⁺ exchanger 3 (NHE3) or activated Na⁺-K⁺-2Cl^- cotransporter-2 (NKCC2-P) predicted significant reductions in urinary Na⁺ excretion, yet there was no observed change in urine Na⁺. The lack of antinatriuresis, despite Na⁺ transporter accumulation, was supported by Li⁺ and creatinine clearance measurements, leading to the conclusion that 3-day nonpressor ANG II increases transporter abundance without proportional activation. Fourteen days of ANG II infusion at 400 ng·kg^-1·min^-1 raises blood pressure and increases Na⁺ transporter abundance along the distal nephron; proximal tubule and medullary loop transporters are decreased and urine Na⁺ and volume output are increased, evidence for pressure natriuresis. Simulations indicate that decreases in NHE3 and NKCC2-P contribute significantly to reducing Na⁺ reabsorption along the nephron and to pressure natriuresis. Our results also suggest that differential regulation of medullary (decrease) and cortical (increase) NKCC2-P is important to preserve K⁺ while minimizing Na⁺ retention during ANG II infusion. Lastly, our model indicates that accumulation of active Na⁺-Cl^- cotransporter counteracts epithelial Na⁺ channel-induced urinary K⁺ loss.

Regulation of CFTR Bicarbonate Channel Activity by WNK1: Implications for Pancreatitis and CFTR-Related Disorders

BACKGRAOUD & AIMS

Aberrant epithelial bicarbonate (HCO₃^-) secretion caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene is associated with several diseases including cystic fibrosis and pancreatitis. Dynamically regulated ion channel activity and anion selectivity of CFTR by kinases sensitive to intracellular chloride concentration ([Cl^-]_i) play an important role in epithelial HCO₃^- secretion. However, the molecular mechanisms of how [Cl^-]_i-dependent mechanisms regulate CFTR are unknown.

METHODS

We examined the mechanisms of the CFTR HCO₃^- channel regulation by [Cl^-]_i-sensitive kinases using an integrated electrophysiological, molecular, and computational approach including whole-cell, outside-out, and inside-out patch clamp recordings and molecular dissection of WNK1 and CFTR proteins. In addition, we analyzed the effects of pancreatitis-causing CFTR mutations on the WNK1-mediated regulation of CFTR.

RESULTS

Among the WNK1, SPAK, and OSR1 kinases that constitute a [Cl^-]_i-sensitive kinase cascade, the expression of WNK1 alone was sufficient to increase the CFTR bicarbonate permeability (P_HCO3/P_Cl) and conductance (G_HCO3) in patch clamp recordings. Molecular dissection of the WNK1 domains revealed that the WNK1 kinase domain is responsible for CFTR P_HCO3/P_Cl regulation by direct association with CFTR, while the surrounding N-terminal regions mediate the [Cl^-]_i-sensitivity of WNK1. Furthermore, the pancreatitis-causing R74Q and R75Q mutations in the elbow helix 1 of CFTR hampered WNK1-CFTR physical associations and reduced WNK1-mediated CFTR P_HCO3/P_Cl regulation.

CONCLUSION

The CFTR HCO₃^- channel activity is regulated by [Cl^-]_i and a WNK1-dependent mechanism. Our results provide new insights into the regulation of the ion selectivity of CFTR and the pathogenesis of CFTR-related disorders.

Endocytic recycling of Na+ -K+ -Cl- cotransporter type 2: importance of exon 4

KEY POINTS

Na⁺ -K⁺ -Cl^- cotransporter type 2 (NKCC2) is a 27-exon membrane protein that is expressed in the thick ascending limb (TAL) of Henle where it is involved in reabsorption of the ultrafiltered NaCl load. It comes as three splice variants that are identical to each other except for the residue composition of exon 4 and that differ in their transport characteristics, functional roles and distributions along the TAL. In this report, it is shown that the variants also differ in their trafficking properties and that two residues in exon 4 play a key role in this regard. One of these residues was also shown to sustain carrier internalization. Through these results, a novel function for the alternatively spliced exon of NKCC2 has been identified and a domain that is involved in carrier trafficking has been uncovered for the first time in a cation-Cl^- cotransporter family member.

ABSTRACT

Na⁺ -K⁺ -Cl^- cotransporter type 2 (NKCC2) is a 12-transmembrane (TM) domain cell surface glycoprotein that is expressed in the thick ascending limb (TAL) of Henle and stimulated during cell shrinkage. It comes as three splice variants (A, B and F) that are identical to each other except for TM2 and the following connecting segment (CS2). Yet, these variants do not share the same localization, transport characteristics and physiological roles along the TAL. We have recently found that while cell shrinkage could exert its activating effect by increasing NKCC2 expression at the cell surface, the variants also responded differentially to this stimulus. In the current work, a mutagenic approach was exploited to determine whether CS2 could play a role in carrier trafficking and identify the residues potentially involved. We found that when the residue of position 238 in NKCC2A (F) and NKCC2B (Y) was replaced by the corresponding residue in NKCC2F (V), carrier activity increased by over 3-fold and endocytosis decreased concomitantly. We also found that when the residue of position 230 in NKCC2F (M) was replaced by the one in NKCC2B (T), carrier activity and affinity for ions both increased substantially whereas expression at the membrane decreased. Taken together, these results suggest that CS2 is involved in carrier trafficking and that two of its residues, those of positions 238 and 230, are part of an internalization motif. They also indicate that the divergent residue of position 230 plays the dual role of specifying ion affinity and sustaining carrier internalization.

Superoxide increases surface NKCC2 in the rat thick ascending limbs via PKC

The apical Na⁺-K⁺-2Cl^- cotransporter (NKCC2) mediates NaCl reabsorption by the thick ascending limb (TAL). The free radical superoxide ( O2- ) stimulates TAL NaCl absorption by enhancing NKCC2 activity. In contrast, nitric oxide (NO) scavenges O2- and inhibits NKCC2. NKCC2 activity depends on the number of NKCC2 transporters in the TAL apical membrane and its phosphorylation. We hypothesized that O2- stimulates NKCC2 activity by enhancing apical surface NKCC2 expression. We measured surface NKCC2 expression in rat TALs by surface biotinylation and Western blot analysis. Treatment of TALs with O2- produced by exogenous xanthine oxidase (1 mU/ml) and hypoxanthine (500 µM) stimulated surface NKCC2 expression by ~18 ± 5% (P < 0.05). O2- -stimulated surface NKCC2 expression was blocked by the O2- scavenger tempol (50 µM). Scavenging H₂O₂ with 100 U/ml catalase did not block the stimulatory effect of xanthine oxidase-hypoxanthine (22 ± 8% increase from control, P < 0.05). Inhibition of endogenous NO production with N^ω-nitro-l-arginine methyl ester enhanced surface NKCC2 expression by 21 ± 6% and, when added together with xanthine oxidase-hypoxanthine, increased surface NKCC2 by 41 ± 10% (P < 0.05). Scavenging O2- with superoxide dismutase (300 U/ml) decreased this stimulatory effect by 60% (39 ± 4% to 15 ± 10%, P < 0.05). Protein kinase C inhibition with Gö-6976 (100 nM) blocked O2- -stimulated surface NKCC2 expression (P < 0.05). O2- did not affect NKCC2 phosphorylation at Thr^96/101 or its upstream kinases STE20/SPS1-related proline/alanine-rich kinase-oxidative stress-responsive kinase 1. We conclude that O2- increases surface NKCC2 expression by stimulating protein kinase C and that this effect is blunted by endogenous NO. O2- -stimulated apical trafficking of NKCC2 may be involved in the enhanced surface NKCC2 expression observed in Dahl salt-sensitive rats.

Tyrosine phosphorylation modulates cell surface expression of chloride cotransporters NKCC2 and KCC3

Cellular chloride transport has a fundamental role in cell volume regulation and renal salt handling. Cellular chloride entry or exit are mediated at the plasma membrane by cotransporter proteins of the solute carrier 12 family. For example, NKCC2 resorbs chloride with sodium and potassium ions at the apical membrane of epithelial cells in the kidney, whereas KCC3 releases chloride with potassium ions at the basolateral membrane. Their ion transport activity is regulated by protein phosphorylation in response to signaling pathways. An additional regulatory mechanism concerns the amount of cotransporter molecules inserted into the plasma membrane. Here we describe that tyrosine phosphorylation of NKCC2 and KCC3 regulates their plasma membrane expression levels. We identified that spleen tyrosine kinase (SYK) phosphorylates a specific N-terminal tyrosine residue in each cotransporter. Experimental depletion of endogenous SYK or pharmacological inhibition of its kinase activity increased the abundance of NKCC2 at the plasma membrane of human embryonic kidney cells. In contrast, overexpression of a constitutively active SYK mutant decreased NKCC2 membrane abundance. Intriguingly, the same experimental approaches revealed the opposite effect on KCC3 abundance at the plasma membrane, compatible with the known antagonistic roles of NKCC and KCC cotransporters in cell volume regulation. Thus, we identified a novel pathway modulating the cell surface expression of NKCC2 and KCC3 and show that this same pathway has opposite functional outcomes for these two cotransporters. The findings have several biomedical implications considering the role of these cotransporters in regulating blood pressure and cell volume.

KLHL3 single-nucleotide polymorphism is associated with essential hypertension in Chinese Han population

Hypertension, including secondary and essential hypertension (EH) variants, is a multifactorial disease, affecting more than one billion people worldwide. Secondary hypertension results from mutations in the putative gene KLHL3 (Kelch-like protein 3); however, it has not been reported whether the KLHL3 gene polymorphisms are associated with EH. Here, we investigated the association between KLHL3 (rs2301708 and rs7444370) polymorphisms and EH in the Chinese Han population.This case-control study included 522 subjects-260 patients with EH and 262 normotensive controls matched for age, gender, body mass index (BMI), hemoglobin A1c (HbA1c), total cholesterol (TC), triglyceride (TG), and levels of Na, K, and Cl. The distribution of functional rs2301708 and rs7444370 polymorphisms within the KLHL3 gene was assessed through polymerase chain reaction (PCR) and restriction-fragment length polymorphism (RFLP).There was no significant difference in allelic and genotypic frequencies of KLHL3 rs2301708 between the EH and normotensive groups; however, the rs7444370 T allele and CT genotype in females was significantly associated with a protective effect against EH (P = .001, P = .002; P = .019, P = .052), and the haplotype CT of rs2301708 and rs7444370 among females in the EH group was less than in the normotensive group (P = .000; P = .007).The KLHL3 rs7444370 variant could be a protective factor in the pathogenesis of females' EH.

Regulation of Na+-K+-Cl- cotransporter type 2 by the with no lysine kinase-dependent signaling pathway

Na⁺-K⁺-Cl^- cotransporter type 2 (NKCC2) is confined to the apical membrane of the thick ascending limb of Henle, where it reabsorbs a substantial fraction of the ultrafiltered NaCl load. It is expressed along this nephron segment as three main splice variants (called NKCC2A, NKCC2B, and NKCC2F) that differ in residue composition along their second transmembrane domain and first intracellular cytosolic connecting segment (CS2). NKCC2 is known to be activated by cell shrinkage and intracellular [Cl^-] reduction. Although the with no lysine (WNK) kinases could play a role in this response, the mechanisms involved are ill defined, and the possibility of variant-specific responses has not been tested thus far. In this study, we have used the Xenopus laevis oocyte expression system to gain further insight in these regards. We have found for the first time that cell shrinkage could stimulate NKCC2A- and NKCC2B-mediated ion transport by increasing carrier abundance at the cell surface and that this response was achieved (at least in part) by the enzymatic function of a WNK kinase. Interestingly, we have also found that the activity and cell surface abundance of NKCC2F were less affected by cell shrinkage compared with the other variants and that ion transport by certain variants could be stimulated through WNK kinase expression in the absence of carrier redistribution. Taken together, these results suggest that the WNK kinase-dependent pathway can affect both the trafficking as well as intrinsic activity of NKCC2 and that CS2 plays an important role in carrier regulation.

Thick Ascending Limb Sodium Transport in the Pathogenesis of Hypertension

The thick ascending limb plays a key role in maintaining water and electrolyte balance. The importance of this segment in regulating blood pressure is evidenced by the effect of loop diuretics or local genetic defects on this parameter. Hormones and factors produced by thick ascending limbs have both autocrine and paracrine effects, which can extend prohypertensive signaling to other structures of the nephron. In this review, we discuss the role of the thick ascending limb in the development of hypertension, not as a sole participant, but one that works within the rich biological context of the renal medulla. We first provide an overview of the basic physiology of the segment and the anatomical considerations necessary to understand its relationship with other renal structures. We explore the physiopathological changes in thick ascending limbs occurring in both genetic and induced animal models of hypertension. We then discuss the racial differences and genetic defects that affect blood pressure in humans through changes in thick ascending limb transport rates. Throughout the text, we scrutinize methodologies and discuss the limitations of research techniques that, when overlooked, can lead investigators to make erroneous conclusions. Thus, in addition to advancing an understanding of the basic mechanisms of physiology, the ultimate goal of this work is to understand our research tools, to make better use of them, and to contextualize research data. Future advances in renal hypertension research will require not only collection of new experimental data, but also integration of our current knowledge.

Area under the curve analysis of blood pressure reveals increased spontaneous locomotor activity in SPAK knock-in mice: relevance for hypotension induced by SPAK inhibition?

SPAK (Ste20/SPS1-related proline/alanine-rich kinase) has been recently identified as a protein kinase which targets the electroneutral cation-coupled chloride cotransporters and it stands out as a target for inhibition in novel anti-hypertensive agents. From this prospective, any information about physiological consequences of SPAK inhibition would be useful to better understand the efficacy and potential adverse effects of the SPAK-based anti-hypertensive therapy. Radiotelemetry was employed to continuously measure the parameters of blood pressure (mean arterial blood pressure, systolic blood pressure, and diastolic blood pressure), heart rate, and physical activity in SPAK knock-in (KI) mice and littermate controls for 24 h. For each parameter, the area under the curve (AUC) was calculated and compared between the SPAK KI mice and wild type mice. There was no statistically significant difference in the AUC of blood pressure parameters between SPAK KI and littermate mice. When mice were physically inactive, the AUCs for blood pressures were significantly lower in SPAK KI than in littermates. When physically active, blood pressures were significantly higher in SPAK KI than in littermates. The heart rate followed a similar pattern. The AUC of physical activity was significantly increased in SPAK KI mice when compared to littermates and the SPAK KI mice spent significantly less time in an inactive state and significantly more time in active states. Comparison between SPAK KI mice and unrelated wild type mice yielded similar results to the comparison between SPAK KI mice and littermates. We conclude that SPAK inhibition increases spontaneous locomotor activity, which has a significant effect on blood pressure.

From Molecules to Mechanisms: Functional Proteomics and Its Application to Renal Tubule Physiology

Classical physiological studies using electrophysiological, biophysical, biochemical, and molecular techniques have created a detailed picture of molecular transport, bioenergetics, contractility and movement, and growth, as well as the regulation of these processes by external stimuli in cells and organisms. Newer systems biology approaches are beginning to provide deeper and broader understanding of these complex biological processes and their dynamic responses to a variety of environmental cues. In the past decade, advances in mass spectrometry-based proteomic technologies have provided invaluable tools to further elucidate these complex cellular processes, thereby confirming, complementing, and advancing common views of physiology. As one notable example, the application of proteomics to study the regulation of kidney function has yielded novel insights into the chemical and physical processes that tightly control body fluids, electrolytes, and metabolites to provide optimal microenvironments for various cellular and organ functions. Here, we systematically review, summarize, and discuss the most significant key findings from functional proteomic studies in renal epithelial physiology. We also identify further improvements in technological and bioinformatics methods that will be essential to advance precision medicine in nephrology.

Fructose acutely stimulates NKCC2 activity in rat thick ascending limbs by increasing surface NKCC2 expression

The thick ascending limb (TAL) reabsorbs 25% of the filtered NaCl through the Na⁺-K⁺-2Cl^- cotransporter (NKCC2). NKCC2 activity is directly related to surface NKCC2 expression and phosphorylation. Higher NaCl reabsorption by TALs is linked to salt-sensitive hypertension, which is linked to consumption of fructose in the diet. However, little is known about the effects of fructose on renal NaCl reabsorption. We hypothesized that fructose, but not glucose, acutely enhances TAL-dependent NaCl reabsorption by increasing NKCC2 activity via stimulation of surface NKCC2 levels and phosphorylation at Thr^96/101. We found that fructose (5 mM) increased transport-related O₂ consumption in TALs by 11.1 ± 3.2% ( P < 0.05). The effect of fructose on O₂ consumption was blocked by furosemide. To study the effect of fructose on NKCC2 activity, we measured the initial rate of NKCC2-dependent thallium influx. We found that 20 min of treatment with fructose (5 mM) increased NKCC2 activity by 58.5 ± 16.9% ( P < 0.05). We then used surface biotinylation to measure surface NKCC2 levels in rat TALs. Fructose increased surface NKCC2 expression in a concentration-dependent manner (22 ± 5,  49 ± 10, and 101 ± 59% of baseline with 1, 5, and 10 mM fructose, respectively, P < 0.05), whereas glucose or a glucose metabolite did not. Fructose did not change NKCC2 phosphorylation at Thre^96/101 or total NKCC2 expression. We concluded that acute fructose treatment increases NKCC2 activity by enhancing surface NKCC2 expression, rather than NKCC2 phosphorylation. Our data suggest that fructose consumption could contribute to salt-sensitive hypertension by stimulating NKCC2-dependent NaCl reabsorption in TALs.

Pre-eclampsia is associated with altered expression of the renal sodium transporters NKCC2, NCC and ENaC in urinary extracellular vesicles

Pre-eclampsia is a hypertensive disorder of pregnancy characterised by hypertension and sodium retention by the kidneys. To identify changes in sodium uptake proteins in the tubules of the distal nephron, we studied their expression in urinary extracellular vesicles or exosomes (uEVs). Urine was collected from women with pre-eclampsia or during normal pregnancy, and from healthy non-pregnant controls. uEVs were isolated by centrifugation and analyzed by Western blot. Expression, proteolytic cleavage and phosphorylation was determined by densitometric analysis normalized to the exosome marker CD9. Results showed a significant increase in phosphorylation of the activating S130 site in NKCC2, the drug target for frusemide, in women with pre-eclampsia compared with normal pregnant women. Phosphorylation of the activating sites T101/105 in NKCC2 was similar but the activating T60 site in NCC, the drug target for thiazide diuretics, showed significantly less phosphorylation in pre-eclampsia compared with normal pregnancy. Expression of the larger forms of the α subunit of ENaC, the drug target for amiloride, was significantly greater in pre-eclampsia, with more fragmentation of theγ subunit. The differences observed are predicted to increase the activity of NKCC2 and ENaC while reducing that of NCC. This will increase sodium reabsorption, and so contribute to hypertension in pre-eclampsia.

Role of WNK Kinases in the Modulation of Cell Volume

Ion Transport across the cell membrane is required to maintain cell volume homeostasis. In response to changes in extracellular osmolarity, most cells activate specific metabolic or membrane-transport pathways to respond to cell swelling or shrinkage and return their volume to its normal resting state. This process involves the rapid adjustment of the activities of channels and transporters that mediate flux of K⁺, Na⁺, Cl^-, and small organic osmolytes. Cation chloride cotransporters (CCCs) NKCCs and KCCs are a family of membrane proteins modulated by changes in cell volume and/or in the intracellular chloride concentration ([Cl^-]_i). Cell swelling triggers regulatory volume decrease (RVD), promoting solute and water efflux to restore normal cell volume. Swelling-activated KCCs mediate RVD in most cell types. In contrast, cell shrinkage triggers regulatory volume increase (RVI), which involves the activation of the NKCC1 cotransporter of the CCC family. Regulation of the CCCs during RVI and RVD by protein phosphorylation is a well-characterized mechanism, where WNK kinases and their downstream kinase substrates, SPAK and OSR1 constitute the essential phospho-regulators. WNKs-SPAK/OSR1-CCCs complex is required to regulate cell shrinkage-induced RVI or cell swelling-induced RVD via activating or inhibitory phosphorylation of NKCCs or KCCs, respectively. WNK1 and WNK4 kinases have been established as [Cl^-]_i sensors/regulators, while a role for WNK3 kinase as a cell volume-sensing kinase has emerged and is proposed in this chapter.

The Photosensitising Clinical Agent Verteporfin Is an Inhibitor of SPAK and OSR1 Kinases

STE20/SPS1-related proline/alanine-rich kinase (SPAK) and oxidative-stress-responsive kinase 1 (OSR1) are two serine/threonine protein kinases that play key roles in regulating ion homeostasis. Various SPAK and OSR1 mouse models exhibited reduced blood pressure. Herein, the discovery of verteporfin, a photosensitising agent used in photodynamic therapy, as a potent inhibitor of SPAK and OSR1 kinases is reported. It is shown that verteporfin binds the kinase domains of SPAK and OSR1 and inhibits their catalytic activity in an adenosine triphosphate (ATP)-independent manner. In cells, verteporfin was able to suppress the phosphorylation of the ion co-transporter NKCC1; a downstream physiological substrate of SPAK and OSR1 kinases. Kinase panel screening indicated that verteporfin inhibited a further eight protein kinases more potently than that of SPAK and OSR1. Although verteporfin has largely been studied as a modifier of the Hippo signalling pathway, this work indicates that the WNK-SPAK/OSR1 signalling cascade is also a target of this clinical agent. This finding could explain the fluctuation in blood pressure noted in patients and animals treated with this drug.

WNK-SPAK/OSR1 signaling: lessons learned from an insect renal epithelium

WNK [with no lysine (K)] kinases regulate renal epithelial ion transport to maintain homeostasis of electrolyte concentrations, extracellular volume, and blood pressure. The SLC12 cation-chloride cotransporters, including the sodium-potassium-2-chloride (NKCC) and sodium chloride cotransporters (NCC), are targets of WNK regulation via the intermediary kinases SPAK (Ste20-related proline/alanine-rich kinase) and OSR1 (oxidative stress response). The pathway is activated by low dietary potassium intake, resulting in increased phosphorylation and activity of NCC. Chloride regulates WNK kinases in vitro by binding to the active site and inhibiting autophosphorylation and has been proposed to modulate WNK activity in the distal convoluted tubule in response to low dietary potassium. WNK-SPAK/OSR1 regulation of NKCC-dependent ion transport is evolutionarily ancient, and it occurs in the Drosophila Malpighian (renal) tubule. Here, we review recent studies from the Drosophila tubule demonstrating cooperative roles for chloride and the scaffold protein Mo25 (mouse protein-25, also known as calcium-binding protein-39) in the regulation of WNK-SPAK/OSR1 signaling in a transporting renal epithelium. Insights gained from this genetically manipulable and physiologically accessible epithelium shed light on molecular mechanisms of regulation of the WNK-SPAK/OSR1 pathway, which is important in human health and disease.

Axial and cellular heterogeneity in electrolyte transport pathways along the thick ascending limb

The thick ascending limb (TAL) extends from the border of the inner medulla to the renal cortex, thus ascending through regions with wide differences in tissue solute and electrolyte concentrations. Structural and functional differences between TAL cells in the medulla (mTAL) and the cortex (cTAL) would therefore be useful to adapt TAL transport function to a changing external fluid composition. While mechanisms common to all TAL cells play a central role in the reclamation of about 25% of the NaCl filtered by the kidney, morphological features, Na⁺ / K⁺ -ATPase activity, NKCC2 splicing and phosphorylation do vary between segments and cells. The TAL contributes to K⁺ homeostasis and TAL cells with high or low basolateral K⁺ conductances have been identified which may be involved in K⁺ reabsorption and secretion respectively. Although transport rates for HCO3- do not differ between mTAL and cTAL, divergent axial and cellular expression of H⁺ transport proteins in TAL have been documented. The reabsorption of the divalent cations Ca²⁺ and Mg²⁺ is highest in cTAL and paralleled by differences in divalent cation permeability and the expression of select claudins. Morphologically, two cell types with different cell surface phenotypes have been described that still need to be linked to specific functional characteristics. The unique external environment and its change along the longitudinal axis require an axial functional heterogeneity for the TAL to optimally participate in conserving electrolyte homeostasis. Despite substantial progress in understanding TAL function, there are still considerable knowledge gaps that are just beginning to become bridged.

Trafficking and regulation of the NKCC2 cotransporter in the thick ascending limb

PURPOSE OF REVIEW

The kidney Na-K-2Cl cotransporter (NKCC2) is essential for urinary concentration and renal electrolyte handling. Loss of function mutations in the NKCC2 gene cause urinary salt and potassium wasting, whereas excessive NKCC2 function has been linked to high blood pressure. Loop diuretics, targeting the transporter, are instrumental for relieving edema or hypertension. This review focuses on intrinsic mechanisms regulating NKCC2 activity at the posttranslational level, namely its trafficking and phosphorylation.

RECENT FINDINGS

Protein networks mediating cellular turnover of NKCC2 have recently received major attention. Several key components of its apical trafficking were identified, including respective chaperones, SNARE protein family members and raft-associated proteins. NKCC2 internalization has been characterized qualitatively and quantitatively. Kinase and phosphatase pathways regulating NKCC2 activity have been clarified and links between NKCC2 phosphorylation and trafficking proposed. Constitutive and inducible NKCC2 trafficking and phosphorylation mechanisms have been specified with focus on endocrine control of thick ascending limb (TAL) function by vasopressin.

SUMMARY

Proper NKCC2 trafficking and phosphorylation are critical to the TAL function in the physiological context of urinary concentration and extracellular volume regulation. Clarification of the underlying mechanisms and respective protein networks may open new therapeutic perspectives for better management of renal electrolyte disorders and blood pressure control.