Wnk4 controls blood pressure and potassium homeostasis via regulation of mass and activity of the distal convoluted tubule

An overview of nutritional factors in the aetiopathogenesis of myocardial fibrosis in great apes

The main cause of mortality in great apes in zoological settings is cardiovascular disease (CVD), affecting all four taxa: chimpanzee (Pan troglodytes), bonobo (Pan paniscus), gorilla (Gorilla spp.) and orangutan (Pongo spp.). Myocardial fibrosis, the most typical histological characterisation of CVD in great apes, is non-specific, making it challenging to understand the aetiopathogenesis. A multifactorial origin of disease is assumed whereby many potential causative factors are directly or indirectly related to the diet, which in wild-living great apes mainly consists of high-fibre, low-carbohydrate and very low-sodium components. Diets of great apes housed in zoological settings are often different compared with the situation in the wild. Moreover, low circulating vitamin D levels have recently been recognised in great apes housed in more northern regions. Evaluation of current supplementation guidelines shows that, despite implementation of different dietary strategies, animals stay vitamin D insufficient. Therefore, recent hypotheses designate vitamin D deficiency as a potential underlying factor in the pathogenesis of myocardial fibrosis. The aim of this literature review is to: (i) examine important differences in nutritional factors between zoological and wild great ape populations; (ii) explain the potential detrimental effects of the highlighted dietary discrepancies on cardiovascular function in great apes; and (iii) elucidate specific nutrition-related pathophysiological mechanisms that may underlie the development of myocardial fibrosis. This information may contribute to understanding the aetiopathogenesis of myocardial fibrosis in great apes and pave the way for future clinical studies and a more preventive approach to great ape CVD management.

Cas12a-knock-in mice for multiplexed genome editing, disease modelling and immune-cell engineering

The pleiotropic effects of human disease and the complex nature of gene-interaction networks require knock-in mice allowing for multiplexed gene perturbations. Here we describe a series of knock-in mice with a C57BL/6 background and with the conditional or constitutive expression of LbCas12a or of high-fidelity enhanced AsCas12a, which were inserted at the Rosa26 locus. The constitutive expression of Cas12a in the mice did not lead to discernible pathology and enabled efficient multiplexed genome engineering. We used the mice for the retrovirus-based immune-cell engineering of CD4+ and CD8+ T cells, B cells and bone-marrow-derived dendritic cells, for autochthonous cancer modelling through the delivery of multiple CRISPR RNAs as a single array using adeno-associated viruses, and for the targeted genome editing of liver tissue using lipid nanoparticles. We also describe a system for simultaneous dual-gene activation and knockout (DAKO). The Cas12a-knock-in mice and the viral and non-viral delivery vehicles provide a versatile toolkit for ex vivo and in vivo applications in genome editing, disease modelling and immune-cell engineering, and for the deconvolution of complex gene interactions.

Evidence for Functional Regulation of the KLHL3/WNK Pathway by O-GlcNAcylation

The 42-member Kelch-like (KLHL) protein family are adaptors for ubiquitin E3 ligase complexes, governing the stability of a wide range of substrates. KLHL proteins are critical for maintaining proteostasis in a variety of tissues and are mutated in human diseases, including cancer, neurodegeneration, and familial hyperkalemic hypertension. However, the regulation of KLHL proteins remains incompletely understood. Previously, we reported that two KLHL family members, KEAP1 and gigaxonin, are regulated by O-linked β-N-acetylglucosamine (O-GlcNAc), an intracellular form of glycosylation. Interestingly, some ubiquitination targets of KEAP1 and gigaxonin are themselves also O-GlcNAcylated, suggesting that multi-level control by this posttranslational modification may influence many KLHL pathways. To test this hypothesis, we examined KLHL3, which ubiquitinates with-no-lysine (WNK) kinases to modulate downstream ion channel activity. Our biochemical and glycoproteomic data demonstrate that human KLHL3 and all four WNK kinases (WNK1-4) are O-GlcNAcylated. Moreover, our results suggest that O-GlcNAcylation affects WNK4 function in both osmolarity control and ferroptosis, with potential implications ranging from blood pressure regulation to neuronal health and survival. This work demonstrates the functional regulation of the KLHL3/WNK axis by O-GlcNAcylation and supports a broader model of O-GlcNAc serving as a general regulator of KLHL signaling and proteostasis.

The evolving concepts of KS-WNK1 effect on NCC activity

The field of the with-no-lysine kinases (WNKs) regulation of the thiazide-sensitive NaCl cotransporter (NCC) began at the start of the century with the discovery that mutations in two members of the family, WNK1 and WNK4, resulted in a condition known as familial hyperkalemic hypertension (FHHt). Since FHHt is the mirror image of Gitelman's syndrome that is caused by inactivating mutations of the SLC12A3 gene encoding NCC, it was expected that WNKs modulated NCC activity and that the increased function of the cotransporter is the pathophysiological mechanism of FFHt. This turned out to be the case. However, experiments over the first years generated unexpected observations that confused the field. Although most has been clarified, one issue still under a certain level of controversy is the role of an isoform of WNK1 that is only expressed in the kidney, almost entirely in the distal convoluted tubule, known as KS-WNK1. In this work, we present an overview of how the knowledge about the physiology of KS-WNK1 evolved over the years and propose explanations to understand its role in renal physiology.

Familial Hyperkalemic Hypertension

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

NRBP1 and TSC22D proteins impact distal convoluted tubule physiology through modulation of the WNK pathway

The With No lysine (WNK) kinases regulate processes such as cell volume and epithelial ion transport through the modulation of Cation Chloride Cotransporters such as the NaCl cotransporter, NCC, present in the distal convoluted tubule (DCT) of the kidney. Recently, the interaction of WNKs with Nuclear Receptor Binding Protein 1 (NRBP1) and Transforming Growth Factor β-Stimulated Clone 22 Domain (TSC22D) proteins was reported. Here we explored the effect of NRBP1 and TSC22Ds on WNK signaling in vitro and in the DCT. TSC22D1.1, TSC22D2, and NRBP1 are localized in DCT WNK bodies, which are cytoplasmic biomolecular condensates associated with WNK activation. In HEK293 cells, long TSC22D isoforms and NRBP1 increase WNK4 activity. DCT-specific NRBP1 knockout mice have reduced NCC phosphorylation and activate a compensatory response. Thus, NRBP1 and long TSC22D proteins are positive modulators of WNK signaling and modulate Na+ reabsorption in the kidney. NRBP1 and TSC22Ds likely influence WNK signaling in other tissues, impacting various physiological processes.

ClC-Kb pore mutation disrupts glycosylation and triggers distal tubular remodeling

Mutations in the CLCNKB gene (1p36), encoding the basolateral chloride channel ClC-Kb, cause type 3 Bartter syndrome. We identified a family with a mixed Bartter/Gitelman phenotype and early-onset kidney failure and by employing a candidate gene approach, identified what we believe is a novel homozygous mutation (CLCNKB c.499G>T [p.Gly167Cys]) in exon 6 of CLCNKB in the index patient. We then validated these results with Sanger and whole-exome sequencing. Compared with wild-type ClC-Kb, the Gly167Cys mutant conducted less current and exhibited impaired complex N-linked glycosylation in vitro. We demonstrated that loss of Gly-167, rather than gain of a mutant Cys, impairs complex glycosylation, but that surface expression remains intact. Moreover, Asn-364 was necessary for channel function and complex glycosylation. Morphologic evaluation of human kidney biopsies revealed typical basolateral localization of mutant Gly167Cys ClC-Kb in cortical distal tubular epithelia. However, we detected attenuated expression of distal sodium transport proteins, changes in abundance of distal tubule segments, and hypokalemia-associated intracellular condensates from the index patient compared with control nephrectomy specimens. The present data establish what we believe are novel regulatory mechanisms of ClC-Kb activity and demonstrate nephron remodeling in humans, caused by mutant ClC-Kb, with implications for renal electrolyte handling, blood pressure control, and kidney disease.

Single-Nucleus RNA Sequencing Reveals Loss of Distal Convoluted Tubule 1 Renal Tubules in HIV Viral Protein R Transgenic Mice

Although hyponatremia and salt wasting are common in patients with HIV/AIDS, the understanding of their contributing factors is limited. HIV viral protein R (Vpr) contributes to HIV-associated nephropathy. To investigate the effects of Vpr on the distal tubules and on the expression level of the Slc12a3 gene, encoding the sodium-chloride cotransporter (which is responsible for sodium reabsorption in distal nephron segments), single-nucleus RNA sequencing was performed on kidney cortices from three wild-type (WT) and three Vpr transgenic (Vpr Tg) mice. The percentage of distal convoluted tubule (DCT) cells was significantly lower in Vpr Tg mice compared with WT mice (P < 0.05); in Vpr Tg mice, Slc12a3 expression was not significantly different in DCT cells. The Pvalb+ DCT1 subcluster had fewer cells in Vpr Tg mice compared with those in WT mice (P < 0.01). Immunohistochemistry revealed fewer Slc12a3+Pvalb+ DCT1 segments in Vpr Tg mice. Differential gene expression analysis between Vpr Tg and WT samples in the DCT cluster showed down-regulation of the Ier3 gene, which is an inhibitor of apoptosis. The in vitro knockdown of Ier3 by siRNA transfection induced apoptosis in mouse DCT cells. These observations suggest that the salt-wasting effect of Vpr in Vpr Tg mice is likely mediated by Ier3 down-regulation in DCT1 cells and loss of Slc12a3+Pvalb+ DCT1 segments.

Inhibition of WNK Kinases in NK Cells Disrupts Cellular Osmoregulation and Control of Tumor Metastasis

INTRODUCTION

The serine/threonine with-no-lysine (WNK) kinase family function in blood pressure control, electrolyte homeostasis, and cellular osmoregulation. These kinases and their downstream effectors are considered promising therapeutic targets in hypertension and stroke. However, the role of WNK kinases in immune cells remains poorly understood.

METHODS

Using the small-molecule WNK kinase inhibitors WNK463 and WNK-IN-11, we investigated how WNK kinase inhibition affects natural killer (NK) cell physiology.

RESULTS

WNK kinase inhibition with WNK463 or WNK-IN-11 significantly decreased IL-2-activated NK cell volume, motility, and cytolytic activity. Treatment of NK cells with these inhibitors induced autophagy by activating AMPK and inhibiting mTOR signaling. Moreover, WNK kinase inhibition increased phosphorylation of Akt and c-Myc by misaligning activity of activating kinases and inhibitory phosphatases. Treatment of tumor-bearing mice with WNK463 impaired tumor metastasis control by adoptively transferred NK cells.

CONCLUSION

The catalytic activity of WNK kinases has a critical role of multiple aspects of NK cell physiology and their pharmacologic inhibition negatively impacts NK cell function.

Regulation of ClC-K/barttin by endocytosis influences distal convoluted tubule hyperplasia

ClC-K/barttin channels are involved in the transepithelial transport of chloride in the kidney and inner ear. Their physiological role is crucial in humans because mutations in CLCNKB or BSND, encoding ClC-Kb and barttin, cause Bartter's syndrome types III and IV, respectively. In vitro experiments have shown that an amino acid change in a proline-tyrosine motif in the C-terminus of barttin stimulates ClC-K currents. The molecular mechanism of this enhancement and whether this potentiation has any in vivo relevance remains unknown. We performed electrophysiological and biochemical experiments in Xenopus oocytes and kidney cells co-expressing ClC-K and barttin constructs. We demonstrated that barttin possesses a YxxØ motif and, when mutated, increases ClC-K plasma membrane stability, resulting in larger currents. To address the impact of mutating this motif in kidney physiology, we generated a knock-in mouse. Comparing wild-type (WT) and knock-in mice under a standard diet, we could not observe any difference in ClC-K and barttin protein levels or localization, either in urinary or plasma parameters. However, under a high-sodium low-potassium diet, known to induce hyperplasia of distal convoluted tubules, knock-in mice exhibit reduced hyperplasia compared to WT mice. In summary, our in vitro and in vivo studies demonstrate that the previously identified PY motif is indeed an endocytic YxxØ motif in which mutations cause a gain of function of the channel. KEY POINTS: It is revealed by mutagenesis and functional experiments that a previously identified proline-tyrosine motif regulating ClC-K plasma membrane levels is indeed an endocytic YxxØ motif. Biochemical characterization of mutants in the YxxØ motif in Xenopus oocytes and human embryonic kidney cells indicates that mutants showed increased plasma membrane levels as a result of an increased stability, resulting in higher function of ClC-K channels. Mutation of this motif does not affect barttin protein expression and subcellular localization in vivo. Knock-in mice with a mutation in this motif, under conditions of a high-sodium low-potassium diet, exhibit less hyperplasia in the distal convoluted tubule than wild-type animals, indicating a gain of function of the channel in vivo.

Synergistic Machine Learning Accelerated Discovery of Nanoporous Inorganic Crystals as Non-Absorbable Oral Drugs

Machine learning (ML) has taken drug discovery to new heights, where effective ML training requires vast quantities of high-quality experimental data as input. Non-absorbable oral drugs (NODs) have unique safety advantage for chronic diseases due to their zero systemic exposure, but their empirical discovery is still time-consuming and costly. Here, a synergistic ML method, integrating small data-driven multi-layer unsupervised learning, in silico quantum-mechanical computations, and minimal wet-lab experiments is devised to identify the finest NODs from massive inorganic materials to achieve multi-objective function (high selectivity, large capacity, and stability). Based on this method, a NH4-form nanoporous zeolite with merlinoite (MER) framework (NH4-MER) is discovered for the treatment of hyperkalemia. In three different animal models, NH4-MER shows a superior safety and efficacy profile in reducing blood K+ without Na+ release, which is an unmet clinical need in chronic kidney disease and Gordon's syndrome. This work provides a synergistic ML method to accelerate the discovery of NODs and other shape-selective materials.

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.

KS-WNK1 is required for the renal response to extreme changes in potassium intake

Kidney-specific with-no-lysine kinase 1 (KS-WNK1) is an isoform of WNK1 kinase that is predominantly found in the distal convoluted tubule of the kidney. The precise physiological function of KS-WNK1 remains unclear. Some studies have suggested that it could play a role in regulating potassium renal excretion by modulating the activity of the Na+-Cl- cotransporter (NCC). However, changes in the potassium diet from normal to high failed to reveal a role for KS-WNK1, but under a normal-potassium diet, the expression of KS-WNK1 is negligible. It is only detectable when mice are exposed to a low-potassium diet. In this study, we investigated the role of KS-WNK1 in regulating potassium excretion under extreme changes in potassium intake. After following a zero-potassium diet (0KD) for 10 days, KS-WNK1-/- mice had lower plasma levels of K+ and Cl- while exhibiting higher urinary excretion of Na+, Cl-, and K+ compared with KS-WNK1+/+ mice. After 10 days of 0KD or normal-potassium diet (NKD), all mice were challenged with a high-potassium diet (HKD). Plasma K+ levels markedly increased after the HKD challenge only in mice previously fed with 0KD, regardless of genotype. KSWNK1+/+ mice adapt better to HKD challenge than KS-WNK1-/- mice after a potassium-retaining state. The difference in the phosphorylated NCC-to-NCC ratio between KS-WNK1+/+ and KS-WNK1-/- mice after 0KD and HKD indicates a role for KS-WNK1 in both NCC phosphorylation and dephosphorylation. These observations show that KS-WNK1 helps the distal convoluted tubule to respond to extreme changes in potassium intake, such as those occurring in wildlife.NEW & NOTEWORTHY The findings of this study demonstrate that kidney-specific with-no-lysine kinase 1 plays a role in regulating urinary electrolyte excretion during extreme changes in potassium intake, such as those occurring in wildlife.  .

Ion homeostasis in diabetic kidney disease

The complications of type 2 diabetes are a major global public health problem with high incidence and mortality, affecting almost all individuals with diabetes worldwide. Diabetic kidney disease (DKD) is one such primary complication and has become a leading cause of end-stage renal disease in patients with diabetes. Progression from diabetes to DKD is a complex process typically involving multiple mechanisms. Recent remarkable clinical benefits of sodium-glucose cotransporter 2 (SGLT2) inhibitors in diabetes and DKD highlight the critical impact of renal ion homeostasis on disease progression. This review comprehensively examines the impact of ion homeostasis on the transition from diabetes to DKD, outlining possible therapeutic interventions and addressing the ongoing challenges in this rapidly developing field.

HIV viral protein R induces loss of DCT1-type renal tubules

Hyponatremia and salt wasting is a common occurance in patients with HIV/AIDS, however, the understanding of its contributing factors is limited. HIV viral protein R (Vpr) contributes to HIV-associated nephropathy. To investigate the effects of Vpr on the expression level of the Slc12a3 gene, encoding the Na-Cl cotransporter, which is responsible for sodium reabsorption in distal nephron segments, we performed single-nucleus RNA sequencing of kidney cortices from three wild-type (WT) and three Vpr-transgenic (Vpr Tg) mice. The results showed that the percentage of distal convoluted tubule (DCT) cells was significantly lower in Vpr Tg mice compared with WT mice (P < 0.05), and that in Vpr Tg mice, Slc12a3 expression was not different in DCT cell cluster. The Pvalb+ DCT1 subcluster had fewer cells in Vpr Tg mice compared with WT (P < 0.01). Immunohistochemistry demonstrated fewer Slc12a3+ Pvalb+ DCT1 segments in Vpr Tg mice. Differential gene expression analysis comparing Vpr Tg and WT in the DCT cluster showed Ier3, an inhibitor of apoptosis, to be the most downregulated gene. These observations demonstrate that the salt-wasting effect of Vpr in Vpr Tg mice is mediated by loss of Slc12a3+ Pvalb+ DCT1 segments via apoptosis dysregulation.

Genetic Diagnosis and Treatment of Inherited Renal Tubular Acidosis

Background

Renal tubular acidosis (RTA) is caused by various disruptions to the secretion of H+ by distal renal tubules and/or dysfunctional reabsorption of HCO3- by proximal renal tubules, which causes renal acidification dysfunction, ultimately leading to a clinical syndrome characterized by hyperchloremic metabolic acidosis with a normal anion gap. With the development of molecular genetics and gene sequencing technology, inherited RTA has also attracted attention, and an increasing number of RTA-related pathogenic genes have been discovered and reported.

Summary

This paper focuses on the latest progress in the research of inherited RTA and systematically reviews the pathogenic genes, protein functions, clinical manifestations, internal relationship between genotypes and clinical phenotypes, diagnostic clues, differential diagnosis, and treatment strategies associated with inherited RTA. This paper aims to deepen the understanding of inherited RTA and reduce the missed diagnosis and misdiagnosis of RTA.

Key Messages

This review systematically summarizes the pathogenic genes, pathophysiological mechanisms, differential diagnosis, and treatment of different types of inherited RTA, which has good clinical value for guiding the diagnosis and treatment of inherited RTA.

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

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

Serum Chloride and Heart Failure

Despite significant advances in management, heart failure continues to impose a significant epidemiologic burden with high prevalence and mortality rates. For decades, sodium has been the serum electrolyte most commonly associated with outcomes; however, challenging the conventional paradigm of sodium's influence, recent studies have identified a more prominent role in serum chloride in the pathophysiology of heart failure. More specifically, hypochloremia is associated with neurohumoral activation, diuretic resistance, and a worse prognosis in patients with heart failure. This review examines basic science, translational research, and clinical studies to better characterize the role of chloride in patients with heart failure and additionally discusses potential new therapies targeting chloride homeostasis that may impact the future of heart failure care.

Structure and thiazide inhibition mechanism of the human Na-Cl cotransporter

The sodium-chloride cotransporter (NCC) is critical for kidney physiology1. The NCC has a major role in salt reabsorption in the distal convoluted tubule of the nephron2,3, and mutations in the NCC cause the salt-wasting disease Gitelman syndrome4. As a key player in salt handling, the NCC regulates blood pressure and is the target of thiazide diuretics, which have been widely prescribed as first-line medications to treat hypertension for more than 60 years5-7. Here we determined the structures of human NCC alone and in complex with a commonly used thiazide diuretic using cryo-electron microscopy. These structures, together with functional studies, reveal major conformational states of the NCC and an intriguing regulatory mechanism. They also illuminate how thiazide diuretics specifically interact with the NCC and inhibit its transport function. Our results provide critical insights for understanding the Na-Cl cotransport mechanism of the NCC, and they establish a framework for future drug design and for interpreting disease-related mutations.

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.

Molecular Mechanisms of Na-Cl Cotransporter in Relation to Hypertension in Chronic Kidney Disease

Chronic kidney disease (CKD) is a common clinical disease with an increasing incidence, affecting 10 to 15% of the world's population. Hypertension is the most common and modifiable risk factor for preventing adverse cardiovascular outcomes in patients with CKD. A survey from developed countries shows that 47% of hypertensive patients over the age of 20 have uncontrolled blood pressure (BP), and the control rate is even lower in developing countries. CKD is both a common cause of uncontrolled hypertension and a risk factor for altered sequelae. In particular, studies have demonstrated that abnormal blood-pressure patterns in CKD patients, such as non-dipping-blood-pressure patterns, are associated with a significantly increased risk of cardiovascular (CV) disease. The distal convoluted tubule (DCT) is a region of the kidney, and although only 5-10% of the sodium (Na⁺) filtered by the glomerulus is reabsorbed by DCT, most studies agree that Na-Cl cotransporter (NCC) in human, rabbit, mouse, and rat kidneys is the most important route of sodium reabsorption across the DCT for maintaining the homeostasis of sodium. The regulation of NCC involves a large and complex network structure, including certain physiological factors, kinases, scaffold proteins, transporter phosphorylation, and other aspects. This regulation network includes various levels. Naturally, cross-talk between the components of this system must occur in order to relay the important signals to the transporter to play its role. Knowledge of the mechanisms regulating NCC activation is critical for understanding and treating hypertension and CKD. Previous studies from our laboratory have investigated the mechanisms through which NCC is activated in several different models. In the following sections, we review the literature on the mechanisms of NCC in relation to hypertension in CKD.

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

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

Pathophysiology and genetics of salt-sensitive hypertension

Most hypertensive cases are primary and heavily associated with modifiable risk factors like salt intake. Evidence suggests that even small reductions in salt consumption reduce blood pressure in all age groups. In that regard, the ACC/AHA described a distinct set of individuals who exhibit salt-sensitivity, regardless of their hypertensive status. Data has shown that salt-sensitivity is an independent risk factor for cardiovascular events and mortality. However, despite extensive research, the pathogenesis of salt-sensitive hypertension is still unclear and tremendously challenged by its multifactorial etiology, complicated genetic influences, and the unavailability of a diagnostic tool. So far, the important roles of the renin-angiotensin-aldosterone system, sympathetic nervous system, and immune system in the pathogenesis of salt-sensitive hypertension have been studied. In the first part of this review, we focus on how the systems mentioned above are aberrantly regulated in salt-sensitive hypertension. We follow this with an emphasis on genetic variants in those systems that are associated with and/or increase predisposition to salt-sensitivity in humans.

Role of inwardly rectifying K+ channel 5.1 (Kir5.1) in the regulation of renal membrane transport

PURPOSE OF REVIEW

Kir5.1 interacts with Kir4.2 in proximal tubule and with Kir4.1 in distal convoluted tubule (DCT), connecting tubule (CNT) and cortical collecting duct (CCD) to form basolateral-K+-channels. Kir4.2/Kir5.1 and Kir4.1/Kir5.1 play an important role in regulating Na+/HCO3--transport of the proximal tubule and Na+/K+ -transport in the DCT/CNT/CCD. The main focus of this review is to provide an overview of the recent development in the field regarding the role of Kir5.1 regulating renal electrolyte transport in the proximal tubule and DCT.

RECENT FINDINGS

Loss-of-function-mutations of KCNJ16 cause a new form of tubulopathy, characterized by hypokalaemia, Na+-wasting, acid-base-imbalance and metabolic-acidosis. Abnormal bicarbonate transport induced by loss-of-function of KCNJ16-mutants is recapitulated in Kir4.2-knockout-(Kir4.2 KO) mice. Deletion of Kir5.1 also abolishes the effect of dietary Na+ and K+-intakes on the basolateral membrane voltage and NCC expression/activity. Long-term high-salt intake or high-K+-intake causes hyperkalaemic in Kir5.1-deficient mice.

SUMMARY

Kir4.2/Kir5.1 activity in the proximal tubule plays a key role in regulating Na+, K+ and bicarbonate-transport through regulating electrogenic-Na+-bicarbonate-cotransporter-(NBCe1) and type 3-Na+/H+-exchanger-(NHE3). Kir4.1/Kir5.1 activity of the DCT plays a critical role in mediating the effect of dietary-K+ and Na+-intakes on NCC activity/expression. As NCC determines the Na+ delivery rate to the aldosterone-sensitive distal nephron (ASDN), defective regulation of NCC during high-salt and high-K+ compromises renal K+ excretion and K+ homeostasis.

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.

Inwardly rectifying K+ channels 4.1 and 5.1 (Kir4.1/Kir5.1) in the renal distal nephron

The inwardly rectifying potassium channel (Kir) 4.1 (encoded by KCNJ10) interacts with Kir5.1 (encoded by KCNJ16) to form a major basolateral K⁺ channel in the renal distal convoluted tubule (DCT), connecting tubule (CNT), and the cortical collecting duct (CCD). Kir4.1/Kir5.1 heterotetramer plays an important role in regulating Na⁺ and K⁺ transport in the DCT, CNT, and CCD. A recent development in the field has firmly established the role of Kir4.1/Kir5.1 heterotetramer of the DCT in the regulation of thiazide-sensitive Na-Cl cotransporter (NCC). Changes in Kir4.1/Kir5.1 activity of the DCT are an essential step for the regulation of NCC expression/activity induced by dietary K⁺ and Na⁺ intakes and play a role in modulating NCC by type 2 angiotensin II receptor (AT2R), bradykinin type II receptor (BK2R), and β-adrenergic receptor. Since NCC activity determines the Na⁺ delivery rate to the aldosterone-sensitive distal nephron (ASDN), a distal nephron segment from late DCT to CCD, Kir4.1/Kir5.1 activity plays a critical role not only in the regulation of renal Na⁺ absorption but also in modulating renal K⁺ excretion and maintaining K⁺ homeostasis. Thus, Kir4.1/Kir5.1 activity serves as an important component of renal K⁺ sensing mechanism. The main focus of this review is to provide an overview regarding the role of Kir4.1 and Kir5.1 of the DCT and CCD in the regulation of renal K⁺ excretion and Na⁺ absorption.

Low Salt Delivery Triggers Autocrine Release of Prostaglandin E2 From the Aldosterone-Sensitive Distal Nephron in Familial Hyperkalemic Hypertension Mice

Aberrant activation of with-no-lysine kinase (WNK)-STE20/SPS1-related proline-alanine-rich protein kinase (SPAK) kinase signaling in the distal convoluted tubule (DCT) causes unbridled activation of the thiazide-sensitive sodium chloride cotransporter (NCC), leading to familial hyperkalemic hypertension (FHHt) in humans. Studies in FHHt mice engineered to constitutively activate SPAK specifically in the DCT (CA-SPAK mice) revealed maladaptive remodeling of the aldosterone sensitive distal nephron (ASDN), characterized by decrease in the potassium excretory channel, renal outer medullary potassium (ROMK), and epithelial sodium channel (ENaC), that contributes to the hyperkalemia. The mechanisms by which NCC activation in DCT promotes remodeling of connecting tubule (CNT) are unknown, but paracrine communication and reduced salt delivery to the ASDN have been suspected. Here, we explore the involvement of prostaglandin E2 (PGE2). We found that PGE2 and the terminal PGE2 synthase, mPGES1, are increased in kidney cortex of CA-SPAK mice, compared to control or SPAK KO mice. Hydrochlorothiazide (HCTZ) reduced PGE2 to control levels, indicating increased PGE2 synthesis is dependent on increased NCC activity. Immunolocalization studies revealed mPGES1 is selectively increased in the CNT of CA-SPAK mice, implicating low salt-delivery to ASDN as the trigger. Salt titration studies in an in vitro ASDN cell model, mouse CCD cell (mCCD-CL1), confirmed PGE2 synthesis is activated by low salt, and revealed that response is paralleled by induction of mPGES1 gene expression. Finally, inhibition of the PGE2 receptor, EP1, in CA-SPAK mice partially restored potassium homeostasis as it partially rescued ROMK protein abundance, but not ENaC. Together, these data indicate low sodium delivery to the ASDN activates PGE2 synthesis and this inhibits ROMK through autocrine activation of the EP1 receptor. These findings provide new insights into the mechanism by which activation of sodium transport in the DCT causes remodeling of the ASDN.

ROMK channels are inhibited in the aldosterone-sensitive distal nephron of renal tubule Nedd4-2-deficient mice

We used whole cell recording to examine the renal outer medullary K+ channel (ROMK or Kir1.1) and epithelial Na+ channel (ENaC) in the late distal convoluted tubule (DCT2)/initial connecting tubule (iCNT) and in the cortical collecting duct (CCD) of kidney tubule-specific neural precursor cell-expressed developmentally downregulated protein 4-2 (Nedd4-2) knockout mice (Ks-Nedd4-2 KO) and floxed neural precursor cell-expressed developmentally downregulated 4-like (Nedd4l) mice (control). Tertiapin Q (TPNQ)-sensitive K+ currents (ROMK) were smaller in both the DCT2/iCNT and CCD of Ks-Nedd4-2 KO mice on a normal diet than in control mice. Neither high dietary salt intake nor low dietary salt intake had a significant effect on ROMK activity in the DCT2/iCNT and CCD of control and Ks-Nedd4-2 KO mice. In contrast, high dietary K+ intake (HK) increased, whereas low dietary K+ intake (LK) decreased TPNQ-sensitive K+ currents in floxed Nedd4l mice. However, the effects of dietary K+ intake on ROMK channel activity were absent in Ks-Nedd4-2 KO mice since neither HK nor LK significantly affected TPNQ-sensitive K+ currents in the DCT2/iCNT and CCD. Moreover, TPNQ-sensitive K+ currents in the DCT2/iCNT and CCD of Ks-Nedd4-2 KO mice on HK were similar to those of control mice on LK. Amiloride-sensitive Na+ currents in the DCT2/iCNT and CCD were significantly higher in Ks-Nedd4-2 KO mice than in floxed Nedd4l mice on a normal K+ diet. HK increased ENaC activity of the DCT2/iCNT only in control mice, but HK stimulated ENaC of the CCD in both control and Ks-Nedd4-2 KO mice. Moreover, the HK-induced increase in amiloride-sensitive Na+ currents was larger in Ks-Nedd4-2 KO mice than in control mice. Deletion of Nedd4-2 increased with no lysine kinase 1 expression and abolished HK-induced inhibition of with no lysine kinase 1. We conclude that deletion of Nedd4-2 increases ENaC activity but decreases ROMK activity in the aldosterone-sensitive distal nephron and that HK fails to stimulate ROMK, but robustly increases ENaC activity in the CCD of Nedd4-2-deficient mice.NEW & NOTEWORTHY We demonstrate that renal outer medullary K+ (ROMK) channel activity is inhibited in the late distal convoluted tubule/initial connecting tubule and cortical collecting duct of neural precursor cell-expressed developmentally downregulated protein 4-2 (Nedd4-2)-deficient mice. Also, deletion of Nedd4-2 abolishes the stimulatory effect of dietary K+ intake on ROMK. The lack of high K+-induced stimulation of ROMK is associated with the absence of high K+-induced inhibition of with no lysine kinase 1.

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.

Deletion of renal Nedd4-2 abolishes the effect of high K+ intake on Kir4.1/Kir5.1 and NCC activity in the distal convoluted tubule

High-dietary K⁺ (HK) intake inhibits basolateral Kir4.1/Kir5.1 activity in the distal convoluted tubule (DCT), and HK-induced inhibition of Kir4.1/Kir5.1 is essential for HK-induced inhibition of NaCl cotransporter (NCC). Here, we examined whether neural precursor cell expressed developmentally downregulated 4-2 (Nedd4-2) deletion compromises the effect of HK on basolateral Kir4.1/Kir5.1 and NCC in the DCT. Single-channel recording and whole cell recording showed that neither HK decreased nor low-dietary K⁺ (LK) increased basolateral Kir4.1/Kir5.1 activity of the DCT in kidney tubule-specific Nedd4-2 knockout (Ks-Nedd4-2 KO) mice. In contrast, HK inhibited and LK increased Kir4.1/Kir5.1 activity in control mice [neural precursor cell expressed developmentally downregulated 4-like (Nedd4l)^flox/flox]. Also, HK intake decreased the negativity of K⁺ current reversal potential in the DCT (depolarization) only in control mice but not in Ks-Nedd4-2 KO mice. Renal clearance experiments showed that HK intake decreased, whereas LK intake increased, hydrochlorothiazide-induced renal Na⁺ excretion only in control mice, but this effect was absent in Ks-Nedd4-2 KO mice. Western blot analysis also demonstrated that HK-induced inhibition of phosphorylated NCC (Thr⁵³) and total NCC was observed only in control mice but not in Ks-Nedd4-2 KO mice. Furthermore, expression of all three subunits of the epithelial Na⁺ channel in Ks-Nedd4-2 KO mice on HK was higher than in control mice. Thus, plasma K⁺ concentrations were similar between Nedd4l^flox/flox and Ks-Nedd4-2 KO mice on HK for 7 days despite high NCC expression. We conclude that Nedd4-2 plays a role in regulating HK-induced inhibition of Kir4.1/Kir5.1 and NCC in the DCT.NEW & NOTEWORTHY Basolateral Kir4.1/Kir5.1 in the distal convoluted tubule plays an important role as a "K⁺ sensor" in the regulation of renal K⁺ excretion after high K⁺ intake. We found that neural precursor cell expressed developmentally downregulated 4-2 (Nedd4-2) a role in mediating the effect of K⁺ diet on Kir4.1/Kir5.1 and NaCl cotransporter because high K⁺ intake failed to inhibit basolateral Kir4.1/Kir5.1 and NaCl cotransporter in kidney tubule-specific Nedd4-2 knockout mice.

Loop Diuretics in Infants with Heart Failure

Tremendous advances have been made in the last 5 decades in the surgical management of congenital heart disease (CHD). Most infants affected by clinically significant CHD are at risk for developing heart failure (HF). Adult HF management is mostly evidence-based and our knowledge in this field has expanded significantly in the past decade. However, data on management approaches for HF in infants are limited. The indications and implications for various medications and interventions in patients with HF secondary to CHD are an upcoming area of interest. It is critical that we expand our ability to prevent, detect, and manage HF in the pediatric population.

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.

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

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

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

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

Deletion of renal Nedd4-2 abolishes the effect of high sodium intake (HS) on Kir4.1, ENaC, and NCC and causes hypokalemia during high HS

Neural precursor cell expressed developmentally downregulated protein 4-2 (Nedd4-2) regulates the expression of Kir4.1, thiazide-sensitive NaCl cotransporter (NCC), and epithelial Na⁺ channel (ENaC) in the aldosterone-sensitive distal nephron (ASDN), and Nedd4-2 deletion causes salt-sensitive hypertension. We now examined whether Nedd4-2 deletion compromises the effect of high-salt (HS) diet on Kir4.1, NCC, ENaC, and renal K⁺ excretion. Immunoblot analysis showed that HS diet decreased the expression of Kir4.1, Ca²⁺-activated large-conductance K⁺ channel subunit-α (BKα), ENaCβ, ENaCγ, total NCC, and phospho-NCC (at Thr⁵³) in floxed neural precursor cell expressed developmentally downregulated gene 4-like (Nedd4l^fl/fl) mice, whereas these effects were absent in kidney-specific Nedd4-2 knockout (Ks-Nedd4-2 KO) mice. Renal clearance experiments also demonstrated that Nedd4-2 deletion abolished the inhibitory effect of HS diet on hydrochlorothiazide-induced natriuresis. Patch-clamp experiments showed that neither HS diet nor low-salt diet had an effect on Kir4.1/Kir5.1 currents of the distal convoluted tubule in Nedd4-2-deficient mice, whereas we confirmed that HS diet inhibited and low-salt diet increased Kir4.1/Kir5.1 activity in Nedd4l^flox/flox mice. Nedd4-2 deletion increased ENaC currents in the ASDN, and this increase was more robust in the cortical collecting duct than in the distal convoluted tubule. Also, HS-induced inhibition of ENaC currents in the ASDN was absent in Nedd4-2-deficient mice. Renal clearance experiments showed that HS intake for 2 wk increased the basal level of renal K⁺ excretion and caused hypokalemia in Ks-Nedd4-2-KO mice but not in Nedd4l^flox/flox mice. In contrast, plasma Na⁺ concentrations were similar in Nedd4l^flox/flox and Ks-Nedd4-2 KO mice on HS diet. We conclude that Nedd4-2 plays an important role in mediating the inhibitory effect of HS diet on Kir4.1, ENaC, and NCC and is essential for maintaining normal renal K⁺ excretion and plasma K⁺ ranges during long-term HS diet.NEW & NOTEWORTHY The present study suggests that Nedd4-2 is involved in mediating the inhibitory effect of high salt (HS) diet on Kir4.1/kir5.1 in the distal convoluted tubule, NaCl cotransporter function, and epithelial Na⁺ channel activity and that Nedd4-2 plays an essential role in maintaining K⁺ homeostasis in response to a long-term HS diet. This suggests the possibility that HS intake could lead to hypokalemia in subjects lacking proper Nedd4-2 E3 ubiquitin ligase activity in aldosterone-sensitive distal nephron.

Sodium Transporters in Human Health and Disease

Sodium (Na+) electrochemical gradients established by Na+/K+ ATPase activity drives the transport of ions, minerals, and sugars in both excitable and non-excitable cells. Na+-dependent transporters can move these solutes in the same direction (cotransport) or in opposite directions (exchanger) across both the apical and basolateral plasma membranes of polarized epithelia. In addition to maintaining physiological homeostasis of these solutes, increases and decreases in sodium may also initiate, directly or indirectly, signaling cascades that regulate a variety of intracellular post-translational events. In this review, we will describe how the Na+/K+ ATPase maintains a Na+ gradient utilized by multiple sodium-dependent transport mechanisms to regulate glucose uptake, excitatory neurotransmitters, calcium signaling, acid-base balance, salt-wasting disorders, fluid volume, and magnesium transport. We will discuss how several Na+-dependent cotransporters and Na+-dependent exchangers have significant roles in human health and disease. Finally, we will discuss how each of these Na+-dependent transport mechanisms have either been shown or have the potential to use Na+ in a secondary role as a signaling molecule.

Mechanisms coupling sodium and magnesium reabsorption in the distal convoluted tubule of the kidney

Hypomagnesaemia is a common feature of renal Na+ wasting disorders such as Gitelman and EAST/SeSAME syndrome. These genetic defects specifically affect Na+ reabsorption in the distal convoluted tubule, where Mg2+ reabsorption is tightly regulated. Apical uptake via TRPM6 Mg2+ channels and basolateral Mg2+ extrusion via a putative Na+ -Mg2+ exchanger determines Mg2+ reabsorption in the distal convoluted tubule. However, the mechanisms that explain the high incidence of hypomagnesaemia in patients with Na+ wasting disorders of the distal convoluted tubule are largely unknown. In this review, we describe three potential mechanisms by which Mg2+ reabsorption in the distal convoluted tubule is linked to Na+ reabsorption. First, decreased activity of the thiazide-sensitive Na+ /Cl- cotransporter (NCC) results in shortening of the segment, reducing the Mg2+ reabsorption capacity. Second, the activity of TRPM6 and NCC are determined by common regulatory pathways. Secondary effects of NCC dysregulation such as hormonal imbalance, therefore, might disturb TRPM6 expression. Third, the basolateral membrane potential, maintained by the K+ permeability and Na+ -K+ -ATPase activity, provides the driving force for Na+ and Mg2+ extrusion. Depolarisation of the basolateral membrane potential in Na+ wasting disorders of the distal convoluted tubule may therefore lead to reduced activity of the putative Na+ -Mg2+ exchanger SLC41A1. Elucidating the interconnections between Mg2+ and Na+ transport in the distal convoluted tubule is hampered by the currently available models. Our analysis indicates that the coupling of Na+ and Mg2+ reabsorption may be multifactorial and that advanced experimental models are required to study the molecular mechanisms.

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.

Nondipping Blood Pressure: Predictive or Reactive Failure of Renal Sodium Handling?

Blood pressure follows a daily rhythm, dipping during nocturnal sleep in humans. Attenuation of this dip (nondipping) is associated with increased risk of cardiovascular disease. Renal control of sodium homeostasis is essential for long-term blood pressure control. Sodium reabsorption and excretion have rhythms that rely on predictive/circadian as well as reactive adaptations. We explore how these rhythms might contribute to blood pressure rhythm in health and disease.

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.

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

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

Distal convoluted tubule sexual dimorphism revealed by advanced 3D imaging

The thiazide-sensitive Na⁺-Cl^- cotransporter (NCC) is more abundant in kidneys of female subjects than of male subjects. Because morphological remodeling of the distal convoluted tubule (DCT) is dependent on NCC activity, it has been generally assumed that there is a corresponding sexual dimorphism in the structure of the DCT, leading to a larger female DCT. Until now, this has never been directly examined. Here, optical clearing techniques were combined with antibody labeling of DCT segment markers, state-of-the-art high-speed volumetric imaging, and analysis tools to visualize and quantify DCT morphology in male and female mice and study the DCT remodeling response to furosemide. We found an unexpected sex difference in the structure of the DCT. Compared with the male mice, female mice had a shorter DCT, a higher cellular density of NCC, and a greater capacity to elongate in response to loop diuretics. Our study revealed a sexual dimorphism of the DCT. Female mice expressed a greater density of NCC transporters in a shorter structure to protect Na⁺ balance in the face of greater basal distal Na⁺ delivery yet have a larger reserve and structural remodeling capacity to adapt to unique physiological stresses. These observations provide insight into mechanisms that may drive sex differences in the therapeutic responses to diuretics.

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.

Hyperkalemia: pathophysiology, risk factors and consequences

There have been significant recent advances in our understanding of the mechanisms that maintain potassium homoeostasis and the clinical consequences of hyperkalemia. In this article we discuss these advances within a concise review of the pathophysiology, risk factors and consequences of hyperkalemia. We highlight aspects that are of particular relevance for clinical practice. Hyperkalemia occurs when renal potassium excretion is limited by reductions in glomerular filtration rate, tubular flow, distal sodium delivery or the expression of aldosterone-sensitive ion transporters in the distal nephron. Accordingly, the major risk factors for hyperkalemia are renal failure, diabetes mellitus, adrenal disease and the use of angiotensin-converting enzyme inhibitors, angiotensin receptor blockers or potassium-sparing diuretics. Hyperkalemia is associated with an increased risk of death, and this is only in part explicable by hyperkalemia-induced cardiac arrhythmia. In addition to its well-established effects on cardiac excitability, hyperkalemia could also contribute to peripheral neuropathy and cause renal tubular acidosis. Hyperkalemia-or the fear of hyperkalemia-contributes to the underprescription of potentially beneficial medications, particularly in heart failure. The newer potassium binders could play a role in attempts to minimize reduced prescribing of renin-angiotensin inhibitors and mineraolocorticoid antagonists in this context.

Renal Tubule Nedd4-2 Deficiency Stimulates Kir4.1/Kir5.1 and Thiazide-Sensitive NaCl Cotransporter in Distal Convoluted Tubule

BACKGROUND

The potassium channel Kir4.1 forms the Kir4.1/Kir5.1 heterotetramer in the basolateral membrane of the distal convoluted tubule (DCT) and plays an important role in the regulation of the thiazide-sensitive NaCl cotransporter (NCC). Kidney-specific deletion of the ubiquitin ligase Nedd4-2 increases expression of NCC, and coexpression of Nedd4-2 inhibits Kir4.1/Kir5.1 in vitro. Whether Nedd4-2 regulates NCC expression in part by regulating Kir4.1/Kir5.1 channel activity in the DCT is unknown.

METHODS

We used electrophysiology studies, immunoblotting, immunostaining, and renal clearance to examine Kir4.1/Kir5.1 activity in the DCT and NCC expression/activity in wild-type mice and mice with kidney-specific knockout of Nedd4-2, Kir4.1, or both.

RESULTS

Deletion of Nedd4-2 increased the activity/expression of Kir4.1 in the DCT and also, hyperpolarized the DCT membrane. Expression of phosphorylated NCC/total NCC and thiazide-induced natriuresis were significantly increased in the Nedd4-2 knockout mice, but these mice were normokalemic. Double-knockout mice lacking both Kir4.1/Kir5.1 and Nedd4-2 in the kidney exhibited increased expression of the epithelial sodium channel α-subunit, largely abolished basolateral potassium ion conductance (to a degree similar to that of kidney-specific Kir4.1 knockout mice), and depolarization of the DCT membrane. Compared with wild-type mice, the double-knockout mice displayed inhibited expression of phosphorylated NCC and total NCC and had significantly blunted thiazide-induced natriuresis as well as renal potassium wasting and hypokalemia. However, NCC expression/activity was higher in the double-knockout mice than in Kir4.1 knockout mice.

CONCLUSIONS

Nedd4-2 regulates Kir4.1/Kir5.1 expression/activity in the DCT and modulates NCC expression by Kir4.1-dependent and Kir4.1-independent mechanisms. Basolateral Kir4.1/Kir5.1 activity in the DCT partially accounts for the stimulation of NCC activity/expression induced by deletion of Nedd4-2.

High-phytate/low-calcium diet is a risk factor for crystal nephropathies, renal phosphate wasting, and bone loss

Phosphate overload contributes to mineral bone disorders that are associated with crystal nephropathies. Phytate, the major form of phosphorus in plant seeds, is known as an indigestible and of negligible nutritional value in humans. However, the mechanism and adverse effects of high-phytate intake on Ca²⁺ and phosphate absorption and homeostasis are unknown. Here, we show that excessive intake of phytate along with a low-Ca²⁺ diet fed to rats contributed to the development of crystal nephropathies, renal phosphate wasting, and bone loss through tubular dysfunction secondary to dysregulation of intestinal calcium and phosphate absorption. Moreover, Ca²⁺ supplementation alleviated the detrimental effects of excess dietary phytate on bone and kidney through excretion of undigested Ca²⁺-phytate, which prevented a vicious cycle of intestinal phosphate overload and renal phosphate wasting while improving intestinal Ca²⁺ bioavailability. Thus, we demonstrate that phytate is digestible without a high-Ca²⁺ diet and is a risk factor for phosphate overloading and for the development of crystal nephropathies and bone disease.

Hereditary causes of primary aldosteronism and other disorders of apparent excess mineralocorticoid activity

Secondary hypertension is a common condition with a broad differential diagnosis. Identification of the true cause of hypertension can be critical for guiding appropriate management. Here, we review hereditary conditions underlying the most common cause of secondary hypertension, primary aldosteronism, as well as other disorders impacting various levels of mineralocorticoid action. Recently, several pathogenic variants of ion channels have been described as etiologies of familial aldosteronism. Defects in steroid hormone synthesis cause hypertension in 11β-hydroxylase deficiency and 17α-hydroxylase deficiency, two types of congenital adrenal hyperplasia. Inappropriate activation of mineralocorticoid receptors underlies the syndrome of apparent mineralocorticoid excess and constitutive activation of the mineralocorticoid receptor. Finally, Liddle syndrome and pseudohypoaldosteronism type 2 are disorders impacting the function of renal sodium channels, the endpoint of mineralocorticoid action. We discuss the pathophysiology, clinical presentation, diagnosis and management of these low renin hypertension states that ultimately result in apparent excess mineralocorticoid activity.