KLHL3 mutations cause familial hyperkalemic hypertension by impairing ion transport in the distal nephron

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

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

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

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

Targeting kelch-like (KLHL) proteins: achievements, challenges and perspectives

Kelch-like proteins (KLHLs) are a large family of BTB-containing proteins. KLHLs function as the substrate adaptor of Cullin 3-RING ligases (CRL3) to recognize substrates. KLHLs play pivotal roles in regulating various physiological and pathological processes by modulating the ubiquitination of their respective substrates. Mounting evidence indicates that mutations or abnormal expression of KLHLs are associated with various human diseases. Targeting KLHLs is a viable strategy for deciphering the KLHLs-related pathways and devising therapies for associated diseases. Here, we comprehensively review the known KLHLs inhibitors to date and the brilliant ideas underlying their development.

Hereditary causes of hypertension due to increased sodium transport

PURPOSE OF REVIEW

Hypertension, commonly known as high blood pressure, is a widespread health condition affecting a large number of individuals across the globe. Although lifestyle choices and environmental factors are known to have a significant impact on its development, there is growing recognition of the influence of genetic factors in the pathogenesis of hypertension. This review specifically focuses on the hereditary causes of hypertension that are associated with increased sodium transport through the thiazide-sensitive NaCl cotransporter (NCC) or amiloride-sensitive epithelial sodium channel (ENaC), crucial mechanisms involved in regulating blood pressure in the kidneys. By examining genetic mutations and signaling molecules linked to the dysregulation of sodium transport, this review aims to deepen our understanding of the hereditary causes of hypertension and shed light on potential therapeutic targets.

RECENT FINDINGS

Liddle syndrome (LS) is a genetic disorder that typically manifests early in life and is characterized by hypertension, hypokalemic metabolic alkalosis, hyporeninemia, and suppressed aldosterone secretion. This condition is primarily caused by gain-of-function mutations in ENaC. In contrast, Pseudohypoaldosteronism type II (PHAII) is marked by hyperkalemia and hypertension, alongside other clinical features such as hyperchloremia, metabolic acidosis, and suppressed plasma renin levels. PHAII results from overactivations of NCC, brought about by gain-of-function mutations in its upstream signaling molecules, including WNK1 (with no lysine (K) 1), WNK4, Kelch-like 3 (KLHL3), and cullin3 (CUL3).

SUMMARY

NCC and ENaC are integral components, and their malfunctions lead to disorders like LS and PHAII, hereditary causes of hypertension. Current treatments for LS involve ENaC blockers (e.g., triamterene and amiloride) in conjunction with low-sodium diets, effectively normalizing blood pressure and potassium levels. In PHAII, thiazide diuretics, which inhibit NCC, are the mainstay treatment, albeit with some limitations and potential side effects. Ongoing research in developing alternative treatments, including small molecules targeting key regulators, holds promise for more effective and tailored hypertension solutions.

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.

Insights into the diverse mechanisms and effects of variant CUL3-induced familial hyperkalemic hypertension

Familial hyperkalemic hypertension (FHHt), also known as Pseudohypoaldosteronism type II (PHAII) or Gordon syndrome is a rare Mendelian disease classically characterized by hyperkalemia, hyperchloremic metabolic acidosis, and high systolic blood pressure. The most severe form of the disease is caused by autosomal dominant variants in CUL3 (Cullin 3), a critical subunit of the multimeric CUL3-RING ubiquitin ligase complex. The recent identification of a novel FHHt disease variant of CUL3 revealed intricacies within the underlying disease mechanism. When combined with studies on canonical CUL3 variant-induced FHHt, these findings further support CUL3's role in regulating renal electrolyte transport and maintaining systemic vascular tone. However, the pathophysiological effects of CUL3 variants are often accompanied by diverse systemic disturbances in addition to classical FHHt symptoms. Recent global proteomic analyses provide a rationale for these systemic disturbances, paving the way for future mechanistic studies to reveal how CUL3 variants dysregulate processes outside of the renovascular axis. Video Abstract.

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

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

Identification of a novel KLHL3-interacting motif in the C-terminal region of WNK4

Mutations in with-no-lysine [K] kinase 4 (WNK4) and kelch-like 3 (KLHL3) are linked to pseudohypoaldosteronism type 2 (PHAII, also known as familial hyperkalemic hypertension or Gordon's syndrome). WNK4 is degraded by a ubiquitin E3 ligase with KLHL3 as the substrate adaptor for WNK4. Several PHAII-causing mutations, e.g. those in the acidic motif (AM) of WNK4 and in the Kelch domain of KLHL3, impair the binding between WNK4 and KLHL3. This results in a reduction in WNK4 degradation and an increase in WNK4 activity, leading to PHAII. Although the AM is important in interacting with KLHL3, it is unclear whether this is the only motif in WNK4 responsible for KLHL3-interacting. In this study, a novel motif of WNK4 that is capable of mediating the degradation of the protein by KLHL3 was identified. This C-terminal motif (termed as CM) is located in amino acids 1051-1075 of WNK4 and is rich in negatively charged residues. Both AM and CM responded to the PHAII mutations in the Kelch domain of KLHL3 in a similar manner, but AM is dominant among the two motifs. The presence of this motif likely allows WNK4 protein to respond to the KLHL3-mediated degradation when the AM is dysfunctional due to a PHAII mutation. This may be one of the reasons why PHAII is less severe when WNK4 is mutated compared to KLHL3 is mutated.

Semisynthesis, Biological Evaluation and Molecular Docking Studies of Barbatic Acid Derivatives as Novel Diuretic Candidates

Barbatic acid, a compound isolated from lichen, has demonstrated a variety of biological activities. In this study, a series of esters based on barbatic acid (6a-q') were designed, synthesized, and evaluated for their diuretic and litholytic activity at a concentration of 100 μmol/L in vitro. All target compounds were characterized using ¹H NMR, ¹³C NMR, and HRMS, and the spatial structure of compound 6w was confirmed using X-ray crystallography. The biological results showed that some derivatives, including 6c, 6b', and 6f', exhibited potent diuretic activity, and 6j and 6m displayed promising litholytic activity. Molecular docking studies further suggested that 6b' had an optimal binding affinity to WNK1 kinases related to diuresis, while 6j could bind to the bicarbonate transporter CaSR through a variety of forces. These findings indicate that some barbatic acid derivatives could be further developed into novel diuretic agents.

Genomic and Transcriptional Profiles of Kelch-like (klhl) Gene Family in Polyploid Carassius Complex

Genome duplication supplies raw genetic materials and has been thought to be essential for evolutionary innovation and ecological adaptation. Here, we select Kelch-like (klhl) genes to study the evolution of the duplicated genes in the polyploid Carassius complex, including amphidiploid C. auratus and amphitriploid C. gibelio. Phylogenetic, chromosomal location and read coverage analyses indicate that most of Carassius klhl genes exhibit a 2:1 relationship with zebrafish orthologs and confirm two rounds of polyploidy, an allotetraploidy followed by an autotriploidy, occurred during Carassius evolution. The lineage-specific expansion and biased retention/loss of klhl genes are also found in Carassius. Transcriptome analyses across eight adult tissues and seven embryogenesis stages reveal varied expression dominance and divergence between the two species. The expression of klhls in response to Carassius herpesvirus 2 infection shows different expression changes corresponding to distinct herpesvirus resistances in three C. gibelio gynogenetic clones. Finally, we find that most C. gibelio klhl genes possess three alleles except eight genes that have lost one or two alleles due to genome rearrangement. The allele expression bias is prosperous for Cgklhl genes and varies during embryogenesis owning to the sequential expression manner of the alleles. The current study provides global insights into the genomic and transcriptional evolution of duplicated genes in a given superfamily resulting from multiple rounds of polyploidization.

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.

Structure-function relationships in the sodium chloride cotransporter

The thiazide sensitive Na+:Cl− cotransporter (NCC) is the principal via for salt reabsorption in the apical membrane of the distal convoluted tubule (DCT) in mammals and plays a fundamental role in managing blood pressure. The cotransporter is targeted by thiazide diuretics, a highly prescribed medication that is effective in treating arterial hypertension and edema. NCC was the first member of the electroneutral cation-coupled chloride cotransporter family to be identified at a molecular level. It was cloned from the urinary bladder of the Pseudopleuronectes americanus (winter flounder) 30 years ago. The structural topology, kinetic and pharmacology properties of NCC have been extensively studied, determining that the transmembrane domain (TM) coordinates ion and thiazide binding. Functional and mutational studies have discovered residues involved in the phosphorylation and glycosylation of NCC, particularly on the N-terminal domain, as well as the extracellular loop connected to TM7-8 (EL7-8). In the last decade, single-particle cryogenic electron microscopy (cryo-EM) has permitted the visualization of structures at high atomic resolution for six members of the SLC12 family (NCC, NKCC1, KCC1-KCC4). Cryo-EM insights of NCC confirm an inverted conformation of the TM1-5 and TM6-10 regions, a characteristic also found in the amino acid-polyamine-organocation (APC) superfamily, in which TM1 and TM6 clearly coordinate ion binding. The high-resolution structure also displays two glycosylation sites (N-406 and N-426) in EL7-8 that are essential for NCC expression and function. In this review, we briefly describe the studies related to the structure-function relationship of NCC, beginning with the first biochemical/functional studies up to the recent cryo-EM structure obtained, to acquire an overall view enriched with the structural and functional aspects of the cotransporter.

Intracellular Ion Control of WNK Signaling

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

NCC regulation by WNK signal cascade

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

A genomic deep field view of hypertension

Blood pressure is regulated by a complex neurohumoral system including the renin-angiotensin-aldosterone system, natriuretic peptides, endothelial pathways, the sympathetic nervous system, and the immune system. This review charts the evolution of our understanding of the genomic basis of hypertension at increasing resolution over the last 5 decades from monogenic causes to polygenic associations, spanning ∼30 monogenic rare variants and >1500 single nucleotide variants. Unexpected early wins from blood pressure genomics include deepening of our understanding of the complex causation of hypertension; refinement of causal estimates bidirectionally between blood pressure, risk factors, and outcomes through Mendelian randomization; risk stratification using polygenic risk scores; and opportunities for precision medicine and drug repurposing.

Glucose/Fructose Delivery to the Distal Nephron Activates the Sodium-Chloride Cotransporter via the Calcium-Sensing Receptor

BACKGROUND

The calcium-sensing receptor (CaSR) in the distal convoluted tubule (DCT) activates the NaCl cotransporter (NCC). Glucose acts as a positive allosteric modulator of the CaSR. Under physiologic conditions, no glucose is delivered to the DCT, and fructose delivery depends on consumption. We hypothesized that glucose/fructose delivery to the DCT modulates the CaSR in a positive allosteric way, activating the WNK4-SPAK-NCC pathway and thus increasing salt retention.

METHODS

We evaluated the effect of glucose/fructose arrival to the distal nephron on the CaSR-WNK4-SPAK-NCC pathway using HEK-293 cells, C57BL/6 and WNK4-knockout mice, ex vivo perfused kidneys, and healthy humans.

RESULTS

HEK-293 cells exposed to glucose/fructose increased SPAK phosphorylation in a WNK4- and CaSR-dependent manner. C57BL/6 mice exposed to fructose or a single dose of dapagliflozin to induce transient glycosuria showed increased activity of the WNK4-SPAK-NCC pathway. The calcilytic NPS2143 ameliorated this effect, which was not observed in WNK4-KO mice. C57BL/6 mice treated with fructose or dapagliflozin showed markedly increased natriuresis after thiazide challenge. Ex vivo rat kidney perfused with glucose above the physiologic threshold levels for proximal reabsorption showed increased NCC and SPAK phosphorylation. NPS2143 prevented this effect. In healthy volunteers, cinacalcet administration, fructose intake, or a single dose of dapagliflozin increased SPAK and NCC phosphorylation in urinary extracellular vesicles.

CONCLUSIONS

Glycosuria or fructosuria was associated with increased NCC, SPAK, and WNK4 phosphorylation in a CaSR-dependent manner.

Blood pressure effects of sodium transport along the distal nephron

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

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.

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.

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.

Monogenic forms of low-renin hypertension: clinical and molecular insights

Monogenic disorders of hypertension are a distinct group of diseases causing dysregulation of the renin-angiotensin-aldosterone system and are characterized by low plasma renin activity. These can chiefly be classified as causing (i) excessive aldosterone synthesis (familial hyperaldosteronism), (ii) dysregulated adrenal steroid metabolism and action (apparent mineralocorticoid excess, congenital adrenal hyperplasia, activating mineralocorticoid receptor mutation, primary glucocorticoid resistance), and (iii) hyperactivity of sodium and chloride transporters in the distal tubule (Liddle syndrome and pseudohypoaldosteronism type 2). The final common pathway is plasma volume expansion and catecholamine/sympathetic excess that causes urinary potassium wasting; hypokalemia and early-onset refractory hypertension are characteristic. However, several single gene defects may show phenotypic heterogeneity, presenting with mild hypertension with normal electrolytes. Evaluation is based on careful attention to family history, physical examination, and measurement of blood levels of potassium, renin, and aldosterone. Genetic sequencing is essential for precise diagnosis and individualized therapy. Early recognition and specific management improves prognosis and prevents long-term sequelae of severe hypertension.

The European and Japanese eel NaCl cotransporter β exhibit chloride currents and are resistant to thiazide type diuretics

The thiazide-sensitive Na⁺-Cl^- cotransporter (NCC) is the major pathway for salt reabsorption in the mammalian distal convoluted tubule, and the inhibition of its function with thiazides is widely used for the treatment of arterial hypertension. In mammals and teleosts, NCC is present as one ortholog that is mainly expressed in the kidney. One exception, however, is the eel, which has two genes encoding NCC. The eNCCa is located in the kidney and eNCCb, which is present in the apical membrane of the rectum. Interestingly, the European eNCCb functions as a NaCl cotransporter that is nevertheless resistant to thiazides and is not activated by low-chloride hypotonic stress. However, in the Japanese eel rectal sac, a thiazide-sensitive NaCl transport mechanism has been described. The protein sequences between eNCCβ and jNCCβ are 98% identical. Here, by site-directed mutagenesis, we transformed eNCCβ into jNCCβ. Our data showed that jNCCβ, similar to eNCCβ, is resistant to thiazides. In addition, both NCCβ proteins have high transport capacity with respect to their renal NCC orthologs, and in contrast to known NCCs, exhibit electrogenic properties that are reduced when residue I172 is substituted by A, G or M. This is considered a key residue for the chloride ion-binding sites of NKCC and KCC. We conclude that NCCb proteins are not sensitive to thiazides and have electrogenic properties dependent on Cl^-, and site I172 is important for the function of NCCβ.

Generation and analysis of pseudohypoaldosteronism type II knock-in mice caused by a nonsense KLHL3 mutation in the Kelch domain

Mutations in the Kelch-like 3 (KLHL3) gene are the most common cause of inherited pseudohypoaldosteronism type II (PHAII) featuring thiazide-sensitive hypertension and hyperkalemic metabolic acidosis. Although Klhl3^{R528H /+} knock-in (KI) mice carrying a missense mutation in the Kelch repeat domain have been reported, nonsense KLHL3 mutations in the same domain that cause PHAII have not been fully investigated in vivo. We generated and analyzed Klhl3 KI mice harboring a nonsense W523X mutation (corresponding to the human KLHL3 W470X mutation). Both heterozygous and homozygous Klhl3^{W523X /+} KI mice exhibited typical PHAII with low-renin hypertension, hyperkalemia with reduced renal potassium excretion, and hyperchloremic metabolic acidosis. Their kidney tissues showed the presence of Klhl3 mRNA and increased Klhl3 protein levels along with enhanced downstream Wnk1/4-Spak/Osr1-N(k)cc phosphorylation. Increased protein expression of total Spak, phosphor(p-)Spak, total Ncc, and p-Ncc from urinary extracellular vesicles (uEVs) also confirmed the activation of the Wnk-mediated Ncc pathway. In vitro studies showed that the human KLHL3 W470X mutation resulted in increased KLHL3 protein stability and disrupted its binding affinity for WNK1/4, leading to the attenuated degradation and increased abundance of total WNKs. In conclusion, nonsense Klhl3^{W523X /+} mice recapitulating PHAII phenotypes exhibit Klhl3 protein stability, abrogating its binding to Wnks, with enhanced Ncc expression in the kidney tissue and even in uEVs. Activation of the WNK-mediated Na⁺ -Cl^- co-transporter reiterated the in vivo pathogenic role of nonsense KLHL3 mutations in PHAII.

Kelch-like protein 3 in human disease and therapy

Kelch-like protein 3 (KLHL3) is a substrate adaptor of Cullin3-RING ubiquitin ligase (CRL3), and KLHL3-CUL3 complex plays a vital role in the ubiquitination of specific substrates. Mutations and abnormal post-translational modifications of KLHL3-CUL3 affect substrate ubiquitination and may related to the pathogenesis of Gordon syndrome (GS), Primary Hyperparathyroidism (PHPT), Diabetes Mellitus (DM), Congenital Heart Disease (CHD), Pre-eclampsia (PE) and even cancers. Therefore, it is essential to understand the function and molecular mechanisms of KLHL3-CUL3 for the treatment of related diseases. In this review, we summary the structure and function of KLHL3-CUL3, the effect of KLHL3-CUL3 mutations and aberrant modifications in GS, PHPT, DM, CHD and PE. Moreover, we noted a possible role of KLHL3-CUL3 in carcinogenesis and provided ideas for targeting KLHL3-CUL3 for related disease treatment.

Cullin 3 Exon 9 Deletion in Familial Hyperkalemic Hypertension Impairs Cullin3-Ring-E3 Ligase (CRL3) Dynamic Regulation and Cycling

Cullin 3 (CUL3) is the scaffold of Cullin3 Ring E3-ligases (CRL3s), which use various BTB-adaptor proteins to ubiquitinate numerous substrates targeting their proteasomal degradation. CUL3 mutations, responsible for a severe form of familial hyperkalemia and hypertension (FHHt), all result in a deletion of exon 9 (amino-acids 403-459) (CUL3-∆9). Surprisingly, while CUL3-∆9 is hyperneddylated, a post-translational modification that typically activates CRL complexes, it is unable to ubiquitinate its substrates. In order to understand the mechanisms behind this loss-of function, we performed comparative label-free quantitative analyses of CUL3 and CUL3-∆9 interactome by mass spectrometry. It was observed that CUL3-∆9 interactions with COP9 and CAND1, both involved in CRL3 complexes’ dynamic assembly, were disrupted. These defects result in a reduction in the dynamic cycling of the CRL3 complexes, making the CRL3-∆9 complex an inactive BTB-adaptor trap, as demonstrated by SILAC experiments. Collectively, the data indicated that the hyperneddylated CUL3-∆9 protein is inactive as a consequence of several structural changes disrupting its dynamic interactions with key regulatory partners.

Sequence and structural variations determining the recruitment of WNK kinases to the KLHL3 E3 ligase

The BTB-Kelch protein KLHL3 is a Cullin3-dependent E3 ligase that mediates the ubiquitin-dependent degradation of kinases WNK1-4 to control blood pressure and cell volume. A crystal structure of KLHL3 has defined its binding to an acidic degron motif containing a PXXP sequence that is strictly conserved in WNK1, WNK2 and WNK4. Mutations in the second proline abrograte the interaction causing the hypertension syndrome pseudohypoaldosteronism type II. WNK3 shows a diverged degron motif containing four amino acid substitutions that remove the PXXP motif raising questions as to the mechanism of its binding. To understand this atypical interaction, we determined the crystal structure of the KLHL3 Kelch domain in complex with a WNK3 peptide. The electron density enabled the complete 11-mer WNK-family degron motif to be traced for the first time revealing several conserved features not captured in previous work, including additional salt bridge and hydrogen bond interactions. Overall, the WNK3 peptide adopted a conserved binding pose except for a subtle shift to accommodate bulkier amino acid substitutions at the binding interface. At the centre, the second proline was substituted by WNK3 Thr541, providing a unique phosphorylatable residue among the WNK-family degrons. Fluorescence polarisation and structural modelling experiments revealed that its phosphorylation would abrogate the KLHL3 interaction similarly to hypertension-causing mutations. Together, these data reveal how the KLHL3 Kelch domain can accommodate the binding of multiple WNK isoforms and highlight a potential regulatory mechanism for the recruitment of WNK3.

Roles of Cullin-RING Ubiquitin Ligases in Cardiovascular Diseases

Maintenance of protein homeostasis is crucial for virtually every aspect of eukaryotic biology. The ubiquitin-proteasome system (UPS) represents a highly regulated quality control machinery that protects cells from a variety of stress conditions as well as toxic proteins. A large body of evidence has shown that UPS dysfunction contributes to the pathogenesis of cardiovascular diseases. This review highlights the latest findings regarding the physiological and pathological roles of cullin-RING ubiquitin ligases (CRLs), an essential player in the UPS, in the cardiovascular system. To inspire potential therapeutic invention, factors regulating CRL activities are also discussed.

Combined Kelch-like 3 and Cullin 3 Degradation is a Central Mechanism in Familial Hyperkalemic Hypertension in Mice

BACKGROUND

Mutations in the ubiquitin ligase scaffold protein Cullin 3 (CUL3) gene cause the disease familial hyperkalemic hypertension (FHHt). In the kidney, mutant CUL3 (CUL3-Δ9) increases abundance of With-No-Lysine (K) Kinase 4 (WNK4), inappropriately activating sterile 20/SPS-1-related proline/alanine-rich kinase (SPAK), which then phosphorylates and hyperactivates the Na⁺Cl^- cotransporter (NCC). The precise mechanism by which CUL3-Δ9 causes FHHt is unclear. We tested the hypothesis that reduced abundance of CUL3 and of Kelch-like 3 (KLHL3), the CUL3 substrate adaptor for WNK4, is mechanistically important. Because JAB1, an enzyme that inhibits CUL3 activity by removing the ubiquitin-like protein NEDD8, cannot interact with CUL3-Δ9, we also determined whether Jab1 disruption mimicked the effects of CUL3-Δ9 expression.

METHODS

We used an inducible renal tubule-specific system to generate several mouse models expressing CUL3-Δ9, mice heterozygous for both CUL3 and KLHL3 (Cul3^+/-/Klhl3^+/- ), and mice with short-term Jab1 disruption (to avoid renal injury associated with long-term disruption).

RESULTS

Renal KLHL3 was higher in Cul3^-/- mice, but lower in Cul3^-/-/Δ9 mice and in the Cul3^+/-/Δ9 FHHt model, suggesting KLHL3 is a target for both WT and mutant CUL3. Cul3^+/-/Klhl3^+/- mice displayed increased WNK4-SPAK activation and phospho-NCC abundance and an FHHt-like phenotype with increased plasma [K⁺] and salt-sensitive blood pressure. Short-term Jab1 disruption in mice lowered the abundance of CUL3 and KLHL3 and increased the abundance of WNK4 and phospho-NCC.

CONCLUSIONS

Jab1^-/- mice and Cul3^+/-/Klhl3^+/- mice recapitulated the effects of CUL3-Δ9 expression on WNK4-SPAK-NCC. Our data suggest degradation of both KLHL3 and CUL3 plays a central mechanistic role in CUL3-Δ9-mediated FHHt.

How Many Cell Types Are in the Kidney and What Do They Do?

The kidney maintains electrolyte, water, and acid-base balance, eliminates foreign and waste compounds, regulates blood pressure, and secretes hormones. There are at least 16 different highly specialized epithelial cell types in the mammalian kidney. The number of specialized endothelial cells, immune cells, and interstitial cell types might even be larger. The concerted interplay between different cell types is critical for kidney function. Traditionally, cells were defined by their function or microscopical morphological appearance. With the advent of new single-cell modalities such as transcriptomics, epigenetics, metabolomics, and proteomics we are entering into a new era of cell type definition. This new technological revolution provides new opportunities to classify cells in the kidney and understand their functions.

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.

Structures of the Human SPAK and OSR1 Conserved C-Terminal (CCT) Domains

STE20/SPS1-related proline/alanine-rich kinase (SPAK) and oxidative stress responsive 1 (OSR1) kinase are two serine/threonine protein kinases that regulate the function of ion co-transporters through phosphorylation. The highly conserved C-terminal (CCT) domains of SPAK and OSR1 bind to RFx[V/I] peptide sequences from their upstream 'With No Lysine Kinases (WNKs), facilitating their activation via phosphorylation. Thus, the inhibition of SPAK and OSR1 binding, via their CCT domains, to WNK kinases is a plausible strategy for inhibiting SPAK and OSR1 kinases. To facilitate structure-guided drug design of such inhibitors, we expressed and purified human SPAK and OSR1 CCT domains and solved their crystal structures. Interestingly, these crystal structures show a highly conserved primary pocket adjacent to a flexible secondary pocket. We also employed a biophysical strategy and determined the affinity of SPAK and OSR1 CCT domains to short peptides derived from WNK4 and NKCC1. Together, this work provides a platform that facilitates the design of CCT domain specific small molecule binders that inhibit SPAK- and OSR1-activation by WNK kinases, and these could be useful in treating hypertension and ischemic stroke.

Aldosterone-Regulated Sodium Transport and Blood Pressure

Aldosterone is a major mineralocorticoid steroid hormone secreted by glomerulosa cells in the adrenal cortex. It regulates a variety of physiological responses including those to oxidative stress, inflammation, fluid disruption, and abnormal blood pressure through its actions on various tissues including the kidney, heart, and the central nervous system. Aldosterone synthesis is primarily regulated by angiotensin II, K⁺ concentration, and adrenocorticotrophic hormone. Elevated serum aldosterone levels increase blood pressure largely by increasing Na⁺ re-absorption in the kidney through regulating transcription and activity of the epithelial sodium channel (ENaC). This review focuses on the signaling pathways involved in aldosterone synthesis and its effects on Na⁺ reabsorption through ENaC.

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.

The variety of genetic defects explains the phenotypic heterogeneity of Familial Hyperkalemic Hypertension

Introduction

Familial hyperkalemic hypertension is a rare inherited form of arterial hypertension. Four genes are responsible for this disease, the variants of these genes cause disruption in the regulation of ion transport in the distal renal tubule. Whether the genotype explains the large phenotypic heterogeneity has not been fully explored.

Methods

We retrospectively analyzed clinical and genetic data of 153 cases (84 probands, 69 relatives) with familial hyperkalemic hypertension.

Results

Pathogenic variants (25 novel variants) were identified as follows: KLHL3 (n = 50), CUL3 (n = 16), WNK1 acidic motif (n = 11), WNK4 acidic motif (n = 4) and WNK1 intron 1 deletions (n = 3). De novo cases were mainly observed in the CUL3-related cases (9 of 12) and recessive cases were only observed in KLHL3-related cases (14 of 50). More severe forms were observed in recessive KLHL3 and CUL3 cases that were also associated with growth retardation. Patients with WNK1 acidic motif variants had a typical biological phenotype and lower frequency of hypertension conversely to WNK4 variants affecting the same motif. Patients with heterozygous KLHL3 and WNK1 deletions had milder forms. Familial screening in 178 relatives allowed detection and care for 69 positive cases. Blood pressure and hyperkalemia were improved by hydrochlorothiazide in all groups.

Conclusions

This study confirms the phenotypic variability ranging from the severe and early forms associated with CUL3 and recessive KLHL3 genotypes through intermediate forms associated with KLHL3 dominant, WNK4 and WNK1 deletion to mild form associated with WNK1 acidic motif genotype and reinforces the interest of genetic screening to better orientate medical care and genetic counseling.

Genotype-phenotype correlation in Gordon's syndrome: report of two cases carrying novel heterozygous mutations

Gordon's syndrome, known also as Pseudohypoaldosteronism type II is a rare inherited dominant form of low-renin hypertension associated with hyperkalemia and metabolic acidosis. Four genes related to the regulation of the NaCl co-symporter NCC have been discovered associated to Gordon phenotypes: WINK 1 and WINK4, which, along with WNK2 and WNK3, encode a family of WNK-kinases, and KLHL3 and CUL3 encoding respectively, Kelch-like 3 protein and cullin. Heterozygous mutations in these genes constitutively activate NCC leading to abnormally increased salt reabsorption and salt-sensitive hypertension. Thiazide diuretic is the recognized treatment for this condition. We report and discuss phenotypic and genetic heterogeneity of two patients with Gordon's syndrome carrying novel heterozygous mutations in the WNK1 and KLHL3 genes. A very rare variant in the SCNN1G gene encoding the γ subunit of epithelial sodium channel ENaC was also identified in one patient.

A case report of pseudohypoaldosteronism type II with a homozygous KLHL3 variant accompanied by hyperthyroidism

BACKGROUND

Pseudohypoaldosteronism type II (PHAII), also called Gordon syndrome, is a rare hereditary disease caused by variants in the WNK1, WNK4, KLHL3 and CUL3 genes. The combination of PHAII with hyperthyroidism and secondary hyperparathyroidism has not been reported previously.

CASE PRESENTATION

A 54-year-old female with recently diagnosed Graves' disease presented hyperkalemia, hypertension, hypercalciuria, elevated levels of parathyroid hormone (PTH) and normal renal function. PHAII was established based on the finding of a homozygous variant (c.328 A > G, T110A) in the KLHL3 gene. Low-dose thiazide diuretics normalized her potassium, calcium and PTH.

CONCLUSIONS

PHAII caused by a KLHL3 variant can affect adults later in life. This diagnosis should be considered in patients with hypertension, consistent hyperkalemia, and normal eGFR and can be corrected by thiazides. The patient also had hyperthyroidism and secondary hyperparathyroidism. The latter was also corrected by thiazide treatment. The hyperthyroidism was assumed to be unrelated to PHAII.

Targeted broad-based genetic testing by next-generation sequencing informs diagnosis and facilitates management in patients with kidney diseases

Background

The clinical diagnosis of genetic renal diseases may be limited by the overlapping spectrum of manifestations between diseases or by the advancement of disease where clues to the original process are absent. The objective of this study was to determine whether genetic testing informs diagnosis and facilitates management of kidney disease patients.

Methods

We developed a comprehensive genetic testing panel (KidneySeq) to evaluate patients with various phenotypes including cystic diseases, congenital anomalies of the kidney and urinary tract (CAKUT), tubulointerstitial diseases, transport disorders and glomerular diseases. We evaluated this panel in 127 consecutive patients ranging in age from newborns to 81 years who had samples sent in for genetic testing.

Results

The performance of the sequencing pipeline for single-nucleotide variants was validated using CEPH (Centre de’Etude du Polymorphism) controls and for indels using Genome-in-a-Bottle. To test the reliability of the copy number variant (CNV) analysis, positive samples were re-sequenced and analyzed. For patient samples, a multidisciplinary review board interpreted genetic results in the context of clinical data. A genetic diagnosis was made in 54 (43%) patients and ranged from 54% for CAKUT, 53% for ciliopathies/tubulointerstitial diseases, 45% for transport disorders to 33% for glomerulopathies. Pathogenic and likely pathogenic variants included 46% missense, 11% nonsense, 6% splice site variants, 23% insertion–deletions and 14% CNVs. In 13 cases, the genetic result changed the clinical diagnosis.

Conclusion

Broad genetic testing should be considered in the evaluation of renal patients as it complements other tests and provides insight into the underlying disease and its management.

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

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

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

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

WNK-SPAK/OSR1-NCC kinase signaling pathway as a novel target for the treatment of salt-sensitive hypertension

Hypertension is the most prevalent health condition worldwide, affecting ~1 billion people. Gordon's syndrome is a form of secondary hypertension that can arise due to a number of possible mutations in key genes that encode proteins in a pathway containing the With No Lysine [K] (WNK) and its downstream target kinases, SPS/Ste20-related proline-alanine-rich kinase (SPAK) and oxidative stress responsive kinase 1 (OSR1). This pathway regulates the activity of the thiazide-sensitive sodium chloride cotransporter (NCC), which is responsible for NaCl reabsorption in the distal nephron. Therefore, mutations in genes encoding proteins that regulate the NCC proteins disrupt ion homeostasis and cause hypertension by increasing NaCl reabsorption. Thiazide diuretics are currently the main treatment option for Gordon's syndrome. However, they have a number of side effects, and chronic usage can lead to compensatory adaptations in the nephron that counteract their action. Therefore, recent research has focused on developing novel inhibitory molecules that inhibit components of the WNK-SPAK/OSR1-NCC pathway, thereby reducing NaCl reabsorption and restoring normal blood pressure. In this review we provide an overview of the currently reported molecular inhibitors of the WNK-SPAK/OSR1-NCC pathway and discuss their potential as treatment options for Gordon's syndrome.

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.

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.

Control of craniofacial and brain development by Cullin3-RING ubiquitin ligases: Lessons from human disease genetics

Metazoan development relies on intricate cell differentiation, communication, and migration pathways, which ensure proper formation of specialized cell types, tissues, and organs. These pathways are crucially controlled by ubiquitylation, a reversible post-translational modification that regulates the stability, activity, localization, or interaction landscape of substrate proteins. Specificity of ubiquitylation is ensured by E3 ligases, which bind substrates and co-operate with E1 and E2 enzymes to mediate ubiquitin transfer. Cullin3-RING ligases (CRL3s) are a large class of multi-subunit E3s that have emerged as important regulators of cell differentiation and development. In particular, recent evidence from human disease genetics, animal models, and mechanistic studies have established their involvement in the control of craniofacial and brain development. Here, we summarize regulatory principles of CRL3 assembly, substrate recruitment, and ubiquitylation that allow this class of E3s to fulfill their manifold functions in development. We further review our current mechanistic understanding of how specific CRL3 complexes orchestrate neuroectodermal differentiation and highlight diseases associated with their dysregulation. Based on evidence from human disease genetics, we propose that other unknown CRL3 complexes must help coordinate craniofacial and brain development and discuss how combining emerging strategies from the field of disease gene discovery with biochemical and human pluripotent stem cell approaches will likely facilitate their identification.

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.