Disease-causing mutations in KLHL3 impair its effect on WNK4 degradation

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.

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.

KLHL3-dependent WNK4 degradation affected by potassium through the neddylation and autophagy pathway

BACKGROUND

Studies reported that kelch-like protein 3 (KLHL3)-Cullin3(CUL3) E3 ligase ubiquitinated with-no-lysine kinase 4 (WNK4). Impaired WNK4 ubiquitination plays a key role in Familial hyperkalemic hypertension (FHHt, also called pseudohypoaldosteronism type II) which results from overaction of thiazide-sensitive sodium chloride cotransport (NCC). In addition, researchers have also found that dietary potassium deficiency activates NCC along the renal distal convoluted tubule (DCT). However, the underlying mechanism remains unclear about the relationship between potassium and WNK4.

METHODS

In the present study, we conducted in vitro and in vivo experiments to confirm that KLHL3-dependent WNK4 degradation is affected by potassium through the neddylation and autophagy pathway. In vitro, the WNK4 and KLHL3 plasmids were cotransfected into HEK293 cell lines by lipofectamine 2000, and then incubated with different potassium concentrations (1mmol/L and 10mmol/L) for 24 h, and further treated with MLN4924 or the autophagy inhibitor or both of MLN4924 and the autophagy inhibitor for another 24 h respectively. In vivo, we created mice that were fed with low or high potassium diets and then were injected MLN4924 in the experimental groups. The expression of WNK4, pWNK4, KLHL3, NEDD8, LC3 ,and P62 was detected by western blotting in vitro and vivo experiments.

RESULTS

We found that the abundance and phosphorylation of WNK4 increase when neddylation is inhibited both in vitro and vivo. Furthermore, the abundance of pWNK4, WNK4, NEDD8, and KLHL3 was increased in the low potassium (LK) group. Inhibiting autophagy can ameliorate the effect of potassium on the abundance and activity of WNK4 to some extent.

CONCLUSION

These findings suggest a complex regulation of potassium in the degradation of WNK4. Low potassium can activate WNK4, which may be related to neddylation and autophagy, but the mechanism needs to be further studied.

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.

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

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

The roles of KLHL family members in human cancers

The Kelch-like (KLHL) family members consist of three domains: bric-a-brac, tramtrack, broad complex/poxvirus and zinc finger domain, BACK domain and Kelch domain, which combine and interact with Cullin3 to form an E3 ubiquitin ligase. Research has indicated that KLHL family members ubiquitinate target substrates to regulate physiological and pathological processes, including tumorigenesis and progression. KLHL19, a member of the KLHL family, is associated with tumorigenesis and drug resistance. However, the regulation and cross talks of other KLHL family members, which also play roles in cancer, are still unclear. Our review mainly explores studies concerning the roles of other KLHL family members in tumor-related regulation to provide novel insights into KLHL family members.

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.

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

BACKGROUND AND OBJECTIVE

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

METHODS

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

KEY CONTENT AND FINDINGS

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

CONCLUSIONS

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

UBR5 is a novel regulator of WNK1 stability

The with no lysine (K) 1 (WNK1) protein kinase maintains cellular ion homeostasis in many tissues through actions on ion cotransporters and channels. Increased accumulation of WNK1 protein leads to pseudohypoaldosteronism type II (PHAII), a form of familial hypertension. WNK1 can be degraded via its adaptor-dependent recruitment to the Cullin3-RBX1 E3 ligase complex by the ubiquitin-proteasome system. Disruption of this process also leads to disease. To determine if this is the primary mechanism of WNK1 turnover, we examined WNK1 protein stability and degradation by measuring its rate of decay after blockade of translation. Here, we show that WNK1 protein degradation exhibits atypical kinetics in Hela cells. Consistent with this apparent complexity, we found that multiple degradative pathways can modulate cellular WNK1 protein amount. WNK1 protein is degraded not only by the proteasome, but also by the lysosome. Non-lysosomal cysteine proteases calpain and caspases also influence WNK1 degradation, as inhibitors of these proteases modestly increased WNK1 protein expression. Importantly, we discovered that the E3 ubiquitin ligase UBR5 interacts with WNK1 and its deficiency results in increased WNK1 protein. Our results further demonstrate that increased WNK1 in UBR5-depleted cells is attributable to reduced lysosomal degradation of WNK1 protein. Taken together, our findings provide insights into the multiplicity of degradative pathways involved in WNK1 turnover and uncover UBR5 as a previously unknown regulator of WNK1 protein stability that leads to lysosomal degradation of WNK1 protein.

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.

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.

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.

High glucose-induced effects on Na+-K+-2Cl- cotransport and Na+/H+ exchange of blood-brain barrier endothelial cells: involvement of SGK1, PKCβII, and SPAK/OSR1

Hyperglycemia exacerbates edema formation and worsens neurological outcome in ischemic stroke. Edema formation in the early hours of stroke involves transport of ions and water across an intact blood-brain barrier (BBB), and swelling of astrocytes. We showed previously that high glucose (HG) exposures of 24 hours to 7 days increase abundance and activity of BBB Na⁺-K⁺-2Cl^- cotransport (NKCC) and Na⁺/H⁺ exchange 1 (NHE1). Further, bumetanide and HOE-642 inhibition of these transporters significantly reduces edema and infarct following middle cerebral artery occlusion in hyperglycemic rats, suggesting that NKCC and NHE1 are effective therapeutic targets for reducing edema in hyperglycemic stroke. The mechanisms underlying hyperglycemia effects on BBB NKCC and NHE1 are not known. In the present study we investigated whether serum-glucocorticoid regulated kinase 1 (SGK1) and protein kinase C beta II (PKCβII) are involved in HG effects on BBB NKCC and NHE1. We found transient increases in phosphorylated SGK1 and PKCβII within the first hour of HG exposure, after 5-60 min for SGK1 and 5 min for PKCβII. However, no changes were observed in cerebral microvascular endothelial cell SGK1 or PKCβII abundance or phosphorylation (activity) after 24 or 48 h HG exposures. Further, we found that HG-induced increases in NKCC and NHE1 abundance were abolished by inhibition of SGK1 but not PKCβII, whereas the increases in NKCC and NHE activity were abolished by inhibition of either kinase. Finally, we found evidence that STE20/SPS1-related proline/alanine-rich kinase and oxidative stress-responsive kinase-1 (SPAK/OSR1) participate in the HG-induced effects on BBB NKCC.

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.

Role of the Ubiquitin Proteasome System in the Regulation of Blood Pressure: A Review

The kidney and the vasculature play crucial roles in regulating blood pressure. The ubiquitin proteasome system (UPS), a multienzyme process mediating covalent conjugation of the 76-amino acid polypeptide ubiquitin to a substrate protein followed by proteasomal degradation, is involved in multiple cellular processes by regulating protein turnover in various tissues. Increasing evidence demonstrates the roles of UPS in blood pressure regulation. In the kidney, filtered sodium is reabsorbed through diverse sodium transporters and channels along renal tubules, and studies conducted till date have provided insights into the complex molecular network through which ubiquitin ligases modulate sodium transport in different segments. Components of these pathways include ubiquitin ligase neuronal precursor cell-expressed developmentally downregulated 4-2, Cullin-3, and Kelch-like 3. Moreover, accumulating data indicate the roles of UPS in blood vessels, where it modulates nitric oxide bioavailability and vasoconstriction. Cullin-3 not only regulates renal salt reabsorption but also controls vascular tone using different adaptor proteins that target distinct substrates in vascular smooth muscle cells. In endothelial cells, UPS can also contribute to blood pressure regulation by modulating endothelial nitric oxide synthase. In this review, we summarize current knowledge regarding the role of UPS in blood pressure regulation, focusing on renal sodium reabsorption and vascular function.

The WNK signaling pathway and salt-sensitive hypertension

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

Renal TNFα activates the WNK phosphorylation cascade and contributes to salt-sensitive hypertension in chronic kidney disease

The inappropriate over-activation of the with-no-lysine kinase (WNK)-STE20/SPS1-related proline/alanine-rich kinase (SPAK)-sodium chloride cotransporter (NCC) phosphorylation cascade increases sodium reabsorption in distal kidney nephrons, resulting in salt-sensitive hypertension. Although chronic kidney disease (CKD) is a common cause of salt-sensitive hypertension, the involvement of the WNK phosphorylation cascade is unknown. Moreover, the effect of immune systems on WNK kinases has not been investigated despite the fact that immune systems are important for salt sensitivity. Here we demonstrate that the protein abundance of WNK1, but not of WNK4, was increased at the distal convoluted tubules in the aristolochic acid nephropathy mouse model of CKD. Accordingly, the phosphorylation of both SPAK and NCC was also increased. Moreover, a high-salt diet did not adequately suppress activation of the WNK1-SPAK-NCC phosphorylation cascade in this model, leading to salt-sensitive hypertension. WNK1 also was increased in adenine nephropathy, but not in subtotal nephrectomy, models of CKD. By comparing the transcripts of these three models focusing on immune systems, we hypothesized that tumor necrosis factor (TNF)-α regulates WNK1 protein expression. In fact, TNF-α increased WNK1 protein expression in cultured renal tubular cells by reducing the transcription and protein levels of NEDD4-2 E3-ligase, which degrades WNK1 protein. Furthermore, the TNF-α inhibitor etanercept reversed the reduction of NEDD4-2 expression and upregulation of the WNK1-SPAK-NCC phosphorylation cascade in distal convoluted tubules in vivo in the aristolochic acid nephropathy model. Thus, salt-sensitive hypertension is induced in CKD via activation of the renal WNK1- SPAK-NCC phosphorylation cascade by TNF-α, reflecting a link with the immune system.

Genome Resequencing Reveals Congenital Causes of Embryo and Nestling Death in Crested Ibis (Nipponia nippon)

The crested ibis (Nipponia nippon) is endangered worldwide. Although a series of conservation measures have markedly increased the population size and distribution area of these birds, the high mortality of embryos and nestlings considerably decreases the survival potential of this bird species. High-throughput sequencing technology was utilized to compare whole genomes between ten samples from dead crested ibises (including six dead embryos and four dead nestlings aged 0-45 days) and 32 samples from living birds. The results indicated that the dead samples all shared the genetic background of a specific ancestral subpopulation. Furthermore, the dead individuals were less genetically diverse and suffered higher degrees of inbreeding compared with these measures in live birds. Several candidate genes (KLHL3, SETDB2, TNNT2, PKP1, AK1, and EXOSC3) associated with detrimental diseases were identified in the genomic regions that differed between the alive and dead samples, which are likely responsible for the death of embryos and nestlings. In addition, in these regions, we also found several genes involved in the protein catabolic process (UBE4A and LONP1), lipid metabolism (ACOT1), glycan biosynthesis and metabolism (HYAL1 and HYAL4), and the immune system (JAM2) that are likely to promote the normal development of embryos and nestlings. The aberrant conditions of these genes and biological processes may contribute to the death of embryos and nestlings. Our data identify congenital factors underlying the death of embryos and nestlings at the whole genome level, which may be useful toward informing more effective conservation efforts for this bird species.

Unveiling the Distinct Mechanisms by which Disease-Causing Mutations in the Kelch Domain of KLHL3 Disrupt the Interaction with the Acidic Motif of WNK4 through Molecular Dynamics Simulation

Kelch-like 3 (KLHL3) is a substrate adaptor of an E3 ubiquitin ligase complex that regulates the degradation of its substrates, including with-no-lysine [K] kinase 4 (WNK4). Mutations in KLHL3 are associated with pseudohypoaldosteronism type II (PHAII), a hereditary form of hypertension. Many PHAII-causing mutations are located in the Kelch domain of KLHL3 that binds with WNK4; however, detailed mechanisms by which these mutations disrupt the binding are not well-understood. In the present study we use molecular dynamics simulations and Western blot analyses to examine the effects of these mutations on the interaction between the Kelch domain of KLHL3 and the acidic motif (AM) of WNK4. The simulation results correlated well with those from Western blot analyses with the exception of the L387P mutation, which led to deregulation of AM degradation by KLHL3 but not recapitulated by simulations. On the basis of the simulation results, a mutation on the binding surface of the Kelch domain affected the Kelch-AM interaction through two major mechanisms: altering the electrostatic potential of the AM binding site and disrupting the Kelch-AM hydrogen bonds. The mutations buried inside the Kelch domain were predicted by our simulations to have no or modest effects on the Kelch-AM interaction. Buried mutations R384Q and S410L disrupted intramolecular hydrogen bonds within the Kelch domain and affected the Kelch-AM interaction indirectly. No significant effect of buried mutation A340V or A494T on the AM degradation or Kelch-AM interaction was observed, implying these mutations may disrupt mechanisms other than Kelch-AM interaction.

Severe Arterial Hypertension from Cullin 3 Mutations Is Caused by Both Renal and Vascular Effects

BACKGROUND

Mutations in four genes, WNK lysine deficient protein kinase 1 and 4 (WNK1 and WNK4), kelch like family member 3 (KLHL3), or Cullin 3 (CUL3), can result in familial hyperkalemic hypertension (FHHt), a rare Mendelian form of human arterial hypertension. Although all mutations result in an increased abundance of WNK1 or WNK4, all FHHt-causing CUL3 mutations, resulting in the skipping of exon 9, lead to a more severe phenotype.

METHODS

We created and compared two mouse models, one expressing the mutant Cul3 protein ubiquitously (pgk-Cul3∆9) and the other specifically in vascular smooth muscle cells (SM22-Cul3∆9). We conducted pharmacologic investigations on isolated aortas and generated stable and inducible HEK293 cell lines that overexpress the wild-type Cul3 or mutant Cul3 (Cul3∆9) protein.

RESULTS

As expected, pgk-Cul3∆9 mice showed marked hypertension with significant hyperkalemia, hyperchloremia and low renin. BP increased significantly in SM22-Cul3∆9 mice, independent of any measurable effect on renal transport. Only pgk-Cul3∆9 mice displayed increased expression of the sodium chloride cotransporter and phosphorylation by the WNK-SPAK kinases. Both models showed altered reactivity of isolated aortas to phenylephrine and acetylcholine, as well as marked acute BP sensitivity to the calcium channel blocker amlodipine. Aortas from SM22-Cul3∆9 mice showed increased expression of RhoA, a key molecule involved in regulation of vascular tone, compared with aortas from control mice. We also observed increased RhoA abundance and t _1/2 in Cul3∆9-expressing cells, caused by decreased ubiquitination.

CONCLUSIONS

Mutations in Cul3 cause severe hypertension by affecting both renal and vascular function, the latter being associated with activation of RhoA.

Renal COP9 Signalosome Deficiency Alters CUL3-KLHL3-WNK Signaling Pathway

BACKGROUND

The familial hyperkalemic hypertension (FHHt) cullin 3 (CUL3) mutant does not degrade WNK kinases normally, thereby leading to thiazide-sensitive Na-Cl cotransporter (NCC) activation. CUL3 mutant (CUL3Δ9) does not bind normally to the COP9 signalosome (CSN), a deneddylase involved in regulating cullin-RING ligases. CUL3Δ9 also caused increased degradation of the CUL3-WNK substrate adaptor kelch-like 3 (KLHL3). Here, we sought to determine how defective CSN action contributes to the CUL3Δ9 phenotype.

METHODS

The Pax8/LC1 mouse system was used to generate mice in which the catalytically active CSN subunit, Jab1, was deleted only along the nephron, after full development (KS-Jab1-/-).

RESULTS

Western blot analysis demonstrated that Jab1 deletion increased the abundance of neddylated CUL3. Moreover, total CUL3 expression was reduced, suggesting decreased CUL3 stability. KLHL3 was almost completely absent in KS-Jab1-/- mice. Conversely, the protein abundances of WNK1, WNK4, and SPAK kinases were substantially higher. Activation of WNK4, SPAK, and OSR1 was indicated by higher phosphorylated protein levels and translocation of the proteins into puncta, as observed by immunofluorescence. The ratio of phosphorylated NCC to total NCC was also higher. Surprisingly, NCC protein abundance was low, likely contributing to hypokalemia and Na+ and K+ wasting. Additionally, long-term Jab1 deletion resulted in kidney damage.

CONCLUSIONS

Together, the results indicate that deficient CSN binding contributes importantly to the FHHt phenotype. Although defective CUL3Δ9-faciliated WNK4 degradation likely contributes, dominant effects on KLHL3 may be a second factor that is necessary for the phenotype.

Dual gain and loss of cullin 3 function mediates familial hyperkalemic hypertension

Familial hyperkalemic hypertension is caused by mutations in with-no-lysine kinases (WNKs) or in proteins that mediate their degradation, kelch-like 3 (KLHL3) and cullin 3 (CUL3). Although the mechanisms by which WNK and KLHL3 mutations cause the disease are now clear, the effects of the disease-causing CUL3Δ403-459 mutation remain controversial. Possible mechanisms, including hyperneddylation, altered ubiquitin ligase activity, decreased association with the COP9 signalosome (CSN), and increased association with and degradation of KLHL3 have all been postulated. Here, we systematically evaluated the effects of Cul3Δ403-459 using cultured kidney cells. We first identified that the catalytically active CSN subunit jun activation domain-binding protein-1 (JAB1) does not associate with the deleted Cul3 4-helix bundle domain but instead with the adjacent α/β1 domain, suggesting that altered protein folding underlies the impaired binding. Inhibition of deneddylation with JAB1 siRNA increased Cul3 neddylation and decreased KLHL3 abundance, similar to the Cul3 mutant. We next determined that KLHL3 degradation has both ubiquitin ligase-dependent and -independent components. Proteasomal KLHL3 degradation was enhanced by Cul3Δ403-459; however, autophagic degradation was also upregulated by this Cul3 mutant. Finally, to evaluate whether deficient substrate adaptor was responsible for the disease, we restored KLHL3 to wild-type (WT) Cul3 levels. In the absence of WT Cul3, WNK4 was not degraded, demonstrating that Cul3Δ403-459 itself cannot degrade WNK4; conversely, when WT Cul3 was present, as in diseased humans, WNK4 degradation was restored. In conclusion, deletion of exon 9 from Cul3 generates a protein that is itself ubiquitin-ligase defective but also capable of enhanced autophagocytic KLHL3 degradation, thereby exerting dominant-negative effects on the WT allele.

Generation and analysis of a mouse model of pseudohypoaldosteronism type II caused by KLHL3 mutation in BTB domain

The Kelch-like 3 ( KLHL3) mutations contributed to the most common causative genes in patients with pseudohypoaldosteronism type II (PHAII); however, the molecular mechanisms of PHAII-causing mutations in BTB domain of KLHL3 in vivo have not been investigated. We generated and analyzed Klhl3 knock-in (KI) mice carrying a missense M131V mutation in the BTB domain (corresponding to human KLHL3 M78V mutation). Klhl3^M131V/+ KI mice exhibited typical PHAII phenotype with an exaggerated diuretic response to hydrochlorothiazide. Their kidney tissues showed an unchanged KLHL3, decreased cullin 3 (Cul3), and increased with-no-lysine kinases (WNKs) WNK1 and WNK4 along with an enhanced downstream ste20-related proline/alanine-rich kinase/oxidative stress response kinase 1-N(K)CC phosphorylation. Their Cul3 protein in the cytosol of distal convoluted tubule cells was also significantly attenuated on immunogold-labeling electron microscopy. In microdissected renal tubules, Klhl3^M131V/+ KI mice expressed high levels of Wnk4 mRNA in the distal nephron. In vitro coimmunoprecipitation showed the KLHL3 BTB domain mutation retained intact interaction with WNKs but reduced binding to Cul3, thus leading to the increased abundance of total WNKs. In summary, Klhl3^M131V/+ KI mice feature typical PHAII with a simultaneous increase of WNK1 and WNK4 through the impaired KLHL3 BTB domain binding to Cul3.-Lin, C.-M., Cheng, C.-J., Yang, S.-S., Tseng, M.-H., Yen, M.-T., Sung, C.-C., Lin, S.-H. Generation and analysis of a mouse model of pseudohypoaldosteronism type II caused by KLHL3 mutation in BTB domain.

Decreased KLHL3 expression is involved in the pathogenesis of pseudohypoaldosteronism type II caused by cullin 3 mutation in vivo

BACKGROUND

Pseudohypoaldosteronism type II (PHAII) is a hereditary hypertensive disease caused by mutations in four genes: WNK1, WNK4, Kelch-like3 (KLHL3), and cullin3 (CUL3). Recently, it was revealed that CUL3-KLHL3 E3 ligase complex ubiquitinates WNK1 and WNK4, leading to their degradation, and that a common pathogenesis of PHAII is defective WNK degradation due to CUL3-KLHL3 E3 ligase complex impairment. PHAII-causing CUL3 mutations mediate exon9 skipping, producing a CUL3 protein with a 57-amino acid deletion (Δ403-459). However, the pathogenic effects of KLHL3, an adaptor protein that links WNKs with CUL3, in PHAII caused by CUL3 mutation remain unclear.

METHODS

To clarify detailed pathophysiological mechanisms underlying PHAII caused by CUL3 mutation in vivo, we generated and analyzed knock-in mice carrying the same CUL3 exon9 deletion (CUL3^WT/Δex9) as that reported in PHAII patients.

RESULTS

CUL3^WT/Δex9 mice exhibited a PHAII-like phenotype. Interestingly, we confirmed markedly decreased KLHL3 expression in CUL3^WT/Δex9 mice by confirming the true KLHL3 band in vivo. However, the expression of other KLHL family proteins, such as KLHL2, was comparable between WT and mutant mice.

CONCLUSION

KLHL3 expression was decreased in CUL3^WT/Δex9 mice. However, expression levels of other KLHL family proteins were comparable between the wild-type and mutant mice. These findings indicate that the decreased abundance of KLHL3 is a specific phenomenon caused by mutant CUL3 (Δexon9). Our findings would improve our understanding of the pathogenesis of PHAII caused by CUL3 mutation in vivo.

Kidney-specific WNK1 isoform (KS-WNK1) is a potent activator of WNK4 and NCC

Familial hyperkalemic hypertension (FHHt) can be mainly attributed to increased activity of the renal Na⁺:Cl^- cotransporter (NCC), which is caused by altered expression and regulation of the with-no-lysine (K) 1 (WNK1) or WNK4 kinases. The WNK1 gene gives rise to a kidney-specific isoform that lacks the kinase domain (KS-WNK1), the expression of which occurs primarily in the distal convoluted tubule. The role played by KS-WNK1 in the modulation of the WNK/STE20-proline-alanine rich kinase (SPAK)/NCC pathway remains elusive. In the present study, we assessed the effect of human KS-WNK1 on NCC activity and on the WNK4-SPAK pathway. Microinjection of oocytes with human KS-WNK1 cRNA induces remarkable activation and phosphorylation of SPAK and NCC. The effect of KS-WNK1 was abrogated by eliminating a WNK-WNK-interacting domain and by a specific WNK inhibitor, WNK463, indicating that the activation of SPAK/NCC by KS-WNK1 is due to interaction with another WNK kinase. Under control conditions in oocytes, the activating serine 335 of the WNK4 T loop is not phosphorylated. In contrast, this serine becomes phosphorylated when the intracellular chloride concentration ([Cl^-]_i) is reduced or when KS-WNK1 is coexpressed with WNK4. KS-WNK1-mediated activation of WNK4 is not due to a decrease of the [Cl^-]_i. Coimmunoprecipitation analysis revealed that KS-WNK1 and WNK4 interact with each other and that WNK4 becomes autophosphorylated at serine 335 when it is associated with KS-WNK1. Together, these observations suggest that WNK4 becomes active in the presence of KS-WNK1, despite a constant [Cl^-]_i.

Overexpression of WNK1 in POMC-expressing neurons reduces weigh gain via WNK4-mediated degradation of Kir6.2

"With no lysine" (WNK) kinases have been shown to regulate various ion transporters in various tissues, but studies on the function of WNK kinases in the brain have been limited. In this study, we discovered that WNK1 and WNK4 in POMC-expressing neuronal cells in WNK1 overexpressed transgenic mice (WNK1 TG) decrease appetite via degradation of Kir6.2. Weight gain after 20 weeks of age was delayed in WNK1 TG mice as a result of reduced food intake. Expression of WNK1 and proopiomelanocortin (POMC) was higher in POMC-expressing neurons in the hypothalamus of WNK1 TG mice than in WT mice. Immunostaining of serial sections of the hypothalamus revealed that POMC-expressing neurons were smaller in WNK1 TG mice than in WT mice. In addition, expression of Kir6.2 was significantly reduced in WNK1 TG mice. Overexpression and knockdown of WNK4 demonstrated that WNK4 regulates protein expression of Kir6.2 via protein-protein interaction. Accordingly, reduced age-dependent weight gain of WNK1 TG mice seems to be related with the decreased Kir6.2 expression via WNK1- and WNK4-regulated protein stability of Kir6.2.

The regulation of Na+Cl- cotransporter by with-no-lysine kinase 4

PURPOSE OF REVIEW

Abundant evidence supports that the NaCl cotransporter (NCC) activity is tightly regulated by the with-no-lysine (WNK) kinases. Here, we summarize the data regarding NCC regulation by WNKs, with a particular emphasis on WNK4.

RECENT FINDINGS

Several studies involving in-vivo and in-vitro models have provided paradoxical data regarding WNK4 regulation of the NCC. Although some studies show that WNK4 can activate the NCC, other equally compelling studies show that WNK4 inhibits the NCC. Recent studies have shown that WNK4 is regulated by the intracellular chloride concentration ([Cl]i), which could account for these paradoxical results. In conditions of high [Cl]i, WNK4 could act as an inhibitor via heterodimer formation with other WNKs. In contrast, when [Cl]i is low, WNK4 can activate Ste20-related, proline-alanine-rich kinase (SPAK)/oxidative stress responsive kinase 1 (OSR1) and thus the NCC. Modulation of WNK4 by [Cl]i has been shown to account for the potassium-sensing properties of the distal convoluted tubule. Other regulators of WNK4 include hormones and ubiquitination.

SUMMARY

Modulation of WNK4 activity by [Cl]i can account for its dual role on the NCC, and this has important physiological implications regarding the regulation of extracellular potassium concentration. Defective regulation of WNKs by ubiquitination explains most cases of familial hyperkalemic hypertension.

Longitudinal exome-wide association study to identify genetic susceptibility loci for hypertension in a Japanese population

Genome-wide association studies have identified various genetic variants associated with complex disorders. However, these studies have commonly been conducted in a cross-sectional manner. Therefore, we performed a longitudinal exome-wide association study (EWAS) in a Japanese cohort. We aimed to identify genetic variants that confer susceptibility to hypertension using ~244 000 single-nucleotide variants (SNVs) and physiological data from 6026 Japanese individuals who underwent annual health check-ups for several years. After quality control, the association of hypertension with SNVs was tested using a generalized estimating equation model. Finally, our longitudinal EWAS detected seven hypertension-related SNVs that passed strict criteria. Among these variants, six SNVs were densely located at 12q24.1, and an East Asian-specific motif (haplotype) 'CAAAA' comprising five derived alleles was identified. Statistical analyses showed that the prevalence of hypertension in individuals with the East Asian-specific haplotype was significantly lower than that in individuals with the common haplotype 'TGGGT'. Furthermore, individuals with the East Asian haplotype may be less susceptible to the adverse effects of smoking on hypertension. The longitudinal EWAS for the recessive model showed that a novel SNV, rs11917356 of COL6A5, was significantly associated with systolic blood pressure, and the derived allele at the SNV may have spread throughout East Asia in recent evolutionary time.

Impaired degradation of medullary WNK4 in the kidneys of KLHL2 knockout mice

Mutations in the with-no-lysine kinase 1 (WNK1), WNK4, Kelch-like 3 (KLHL3), and Cullin3 (CUL3) genes were identified as being responsible for hereditary hypertensive disease pseudohypoaldosteronism type II (PHAII). Normally, the KLHL3/CUL3 ubiquitin ligase complex degrades WNKs. In PHAII, the loss of interaction between KLHL3 and WNK4 increases levels of WNKs because of impaired ubiquitination, leading to abnormal over-activation of the WNK-OSR1/SPAK-NCC cascade in the kidney's distal convoluted tubules (DCT). KLHL2, which is highly homologous to KLHL3, was reported to ubiquitinate and degrade WNKs in vitro. Mutations in KLHL2 have not been reported in patients with PHAII, suggesting that KLHL2 plays a different physiological role than that played by KLHL3 in the kidney. To investigate the physiological roles of KLHL2 in the kidney, we generated KLHL2^-/- mice. KLHL2^-/- mice did not exhibit increased phosphorylation of the OSR1/SPAK-NCC cascade and PHAII-like phenotype. KLHL2 was predominantly expressed in the medulla compared with the cortex. Accordingly, medullary WNK4 protein levels were significantly increased in the kidneys of KLHL2^-/- mice. KLHL2 is indeed a physiological regulator of WNK4 in vivo; however, its function might be different from that of KLHL3 because KLHL2 mainly localized in medulla.

KLHL3 Knockout Mice Reveal the Physiological Role of KLHL3 and the Pathophysiology of Pseudohypoaldosteronism Type II Caused by Mutant KLHL3

Mutations in the with-no-lysine kinase 1 (WNK1), WNK4, kelch-like 3 (KLHL3), and cullin3 (CUL3) genes are known to cause the hereditary disease pseudohypoaldosteronism type II (PHAII). It was recently demonstrated that this results from the defective degradation of WNK1 and WNK4 by the KLHL3/CUL3 ubiquitin ligase complex. However, the other physiological in vivo roles of KLHL3 remain unclear. Therefore, here we generated KLHL3-/- mice that expressed β-galactosidase (β-Gal) under the control of the endogenous KLHL3 promoter. Immunoblots of β-Gal and LacZ staining revealed that KLHL3 was expressed in some organs, such as brain. However, the expression levels of WNK kinases were not increased in any of these organs other than the kidney, where WNK1 and WNK4 increased in KLHL3-/- mice but not in KLHL3+/- mice. KLHL3-/- mice also showed PHAII-like phenotypes, whereas KLHL3+/- mice did not. This clearly demonstrates that the heterozygous deletion of KLHL3 was not sufficient to cause PHAII, indicating that autosomal dominant type PHAII is caused by the dominant negative effect of mutant KLHL3. We further demonstrated that the dimerization of KLHL3 can explain this dominant negative effect. These findings could help us to further understand the physiological roles of KLHL3 and the pathophysiology of PHAII caused by mutant KLHL3.

Regulation of Renal Electrolyte Transport by WNK and SPAK-OSR1 Kinases

The discovery of four genes responsible for pseudohypoaldosteronism type II, or familial hyperkalemic hypertension, which features arterial hypertension with hyperkalemia and metabolic acidosis, unmasked a complex multiprotein system that regulates electrolyte transport in the distal nephron. Two of these genes encode the serine-threonine kinases WNK1 and WNK4. The other two genes [kelch-like 3 (KLHL3) and cullin 3 (CUL3)] form a RING-type E3-ubiquitin ligase complex that modulates WNK1 and WNK4 abundance. WNKs regulate the activity of the Na(+):Cl(-) cotransporter (NCC), the epithelial sodium channel (ENaC), the renal outer medullary potassium channel (ROMK), and other transport pathways. Interestingly, the modulation of NCC occurs via the phosphorylation by WNKs of other serine-threonine kinases known as SPAK-OSR1. In contrast, the process of regulating the channels is independent of SPAK-OSR1. We present a review of the remarkable advances in this area in the past 10 years.

Phosphorylation of KLHL3 at serine 433 impairs its interaction with the acidic motif of WNK4: a molecular dynamics study

Interaction between the acidic motif (AM) of protein kinase WNK4 and the Kelch domain of KLHL3 are involved in the pathogenesis of pseudohypoaldosteronism type II, a hereditary form of hypertension. This interaction is disrupted by some disease-causing mutations in either WNK4 or KLHL3, or by angiotensin II- and insulin-induced phosphorylation of KLHL3 at serine 433, which is also a site frequently mutated in patients. However, the mechanism by which this phosphorylation disrupts the interaction is unclear. In this study, we approached this problem using molecular dynamics simulation with structural, dynamical and energetic analyses. Results from independent simulations indicate that when S433 was phosphorylated, the electrostatic potential became more negative in the AM binding site of KLHL3 and therefore was unfavorable for binding with the negatively charged AM. In addition, the intermolecular hydrogen bond network that kept the AM stable in the binding site of KLHL3 was disrupted, and the forces for the hydrophobic interactions between the AM of WNK4 and KLHL3 were also reduced. As a result, the weakened interactions were no longer capable of holding the AM of WNK4 at its binding site in KLHL3. In conclusion, phosphorylation of KLHL3 at S433 disrupts the hydrogen bonds, hydrophobic and electrostatic interactions between the Kelch domain of KLHL3 and the AM of WNK4. This study provides a key molecular understanding of the KLHL3-mediated regulation of WNK4, which is an integrative regulator of electrolyte homeostasis and blood pressure regulation in the kidney. Significances Statement: WNK4 is an integrative regulator of electrolyte homeostasis, which is important in the blood pressure regulation by the kidney. Interaction between WNK4 and KLHL3 is a key physiological process that is impaired in a hereditary form of hypertension. This study provides substantial new insights into the role of phosphorylation of KLHL3 in regulating the interaction with WNK4, and therefore advances our understanding of molecular pathogenesis of hypertension and the mechanism of blood pressure regulation.

Physiological role of SLC12 family members in the kidney

The solute carrier family 12, as numbered according to Human Genome Organisation (HUGO) nomenclature, encodes the electroneutral cation-coupled chloride cotransporters that are expressed in many cells and tissues; they play key roles in important physiological events, such as cell volume regulation, modulation of the intracellular chloride concentration, and transepithelial ion transport. Most of these family members are expressed in specific regions of the nephron. The Na-K-2Cl cotransporter NKCC2, which is located in the thick ascending limb, and the Na-Cl cotransporter, which is located in the distal convoluted tubule, play important roles in salt reabsorption and serve as the receptors for loop and thiazide diuretics, respectively (Thiazide diuretics are among the most commonly prescribed drugs in the world.). The activity of these transporters correlates with blood pressure levels; thus, their regulation has been a subject of intense research for more than a decade. The K-Cl cotransporters KCC1, KCC3, and KCC4 are expressed in several nephron segments, and their role in renal physiology is less understood but nevertheless important. Evidence suggests that they are involved in modulating proximal tubule glucose reabsorption, thick ascending limb salt reabsorption and collecting duct proton secretion. In this work, we present an overview of the physiological roles of these transporters in the kidney, with particular emphasis on the knowledge gained in the past few years.

Hypertension: the missing WNKs

The With no Lysine [K] (WNK) family of enzymes are central in the regulation of blood pressure. WNKs have been implicated in hereditary hypertension disorders, mainly through control of the activity and levels of ion cotransporters and channels. Actions of WNKs in the kidney have been heavily investigated, and recent studies have provided insight into not only the regulation of these enzymes but also how mutations in WNKs and their interacting partners contribute to hypertensive disorders. Defining the roles of WNKs in the cardiovascular system will provide clues about additional mechanisms by which WNKs can regulate blood pressure. This review summarizes recent developments in the regulation of the WNK signaling cascade and its role in regulation of blood pressure.

Context-dependent mechanisms modulating aldosterone signaling in the kidney

The aldosterone-mineralocorticoid receptor (MR) system serves as the major regulator of fluid homeostasis, and is an important drug target for the treatment of hypertension, heart failure, and chronic kidney disease. While the ligand aldosterone plays a central role in facilitating MR activity, recent studies have revealed that MR signaling is modulated through distinct mechanisms at the levels of the receptor and the downstream targets. Notably, phosphorylation of the ligand-binding domain in MR regulates the ability of the receptor to bind to ligand in renal intercalated cells, providing an additional layer of regulation that allows the cell-selective control of MR signaling. These mechanisms are involved in the context-dependent effects of aldosterone in the distal nephron. In this article, the recent progress in the understanding of mechanisms regulating the action of aldosterone is discussed, focusing on the connecting tubules and collecting duct in the kidney.

Characterisation of the Cullin-3 mutation that causes a severe form of familial hypertension and hyperkalaemia

Deletion of exon 9 from Cullin-3 (CUL3, residues 403-459: CUL3(Δ403-459)) causes pseudohypoaldosteronism type IIE (PHA2E), a severe form of familial hyperkalaemia and hypertension (FHHt). CUL3 binds the RING protein RBX1 and various substrate adaptors to form Cullin-RING-ubiquitin-ligase complexes. Bound to KLHL3, CUL3-RBX1 ubiquitylates WNK kinases, promoting their ubiquitin-mediated proteasomal degradation. Since WNK kinases activate Na/Cl co-transporters to promote salt retention, CUL3 regulates blood pressure. Mutations in both KLHL3 and WNK kinases cause PHA2 by disrupting Cullin-RING-ligase formation. We report here that the PHA2E mutant, CUL3(Δ403-459), is severely compromised in its ability to ubiquitylate WNKs, possibly due to altered structural flexibility. Instead, CUL3(Δ403-459) auto-ubiquitylates and loses interaction with two important Cullin regulators: the COP9-signalosome and CAND1. A novel knock-in mouse model of CUL3(WT) (/Δ403-459) closely recapitulates the human PHA2E phenotype. These mice also show changes in the arterial pulse waveform, suggesting a vascular contribution to their hypertension not reported in previous FHHt models. These findings may explain the severity of the FHHt phenotype caused by CUL3 mutations compared to those reported in KLHL3 or WNK kinases.

Hypertension-causing Mutations in Cullin3 Protein Impair RhoA Protein Ubiquitination and Augment the Association with Substrate Adaptors

Cullin-Ring ubiquitin ligases regulate protein turnover by promoting the ubiquitination of substrate proteins, targeting them for proteasomal degradation. It has been shown previously that mutations in Cullin3 (Cul3) causing deletion of 57 amino acids encoded by exon 9 (Cul3Δ9) cause hypertension. Moreover, RhoA activity contributes to vascular constriction and hypertension. We show that ubiquitination and degradation of RhoA is dependent on Cul3 in HEK293T cells in which Cul3 expression is ablated by either siRNA or by CRISPR-Cas9 genome editing. The latter was used to generate a Cul3-null cell line (HEK293T(Cul3KO)). When expressed in these cells, Cul3Δ9 supported reduced ubiquitin ligase activity toward RhoA compared with equivalent levels of wild-type Cul3 (Cul3WT). Consistent with its reduced activity, binding of Cul3Δ9 to the E3 ubiquitin ligase Rbx1 and neddylation of Cul3Δ9 were impaired significantly compared with Cul3WT. Conversely, Cul3Δ9 bound to substrate adaptor proteins more efficiently than Cul3WT. Cul3Δ9 also forms unstable dimers with Cul3WT, disrupting dimers of Cul3WT complexes that are required for efficient ubiquitination of some substrates. Indeed, coexpression of Cul3WT and Cul3Δ9 in HEK293T(Cul3KO) cells resulted in a decrease in the active form of Cul3WT. We conclude that Cul3Δ9-associated ubiquitin ligase activity toward RhoA is impaired and suggest that Cul3Δ9 mutations may act dominantly by sequestering substrate adaptors and disrupting Cul3WT complexes.

Regulation of blood pressure and renal electrolyte balance by Cullin-RING ligases

PURPOSE OF REVIEW

Efforts to explore the pathogenic mechanisms underlying hereditary hypertension caused by a single gene mutation have brought about conceptual advances in our understanding of blood pressure regulation. We here discuss a novel pathogenic mechanism underlying the hereditary hypertensive disease pseudohypoaldosteronism type II (PHAII), caused by mutations in three different genes encoding for Cullin-3, Kelch-like protein 3 (KLHL3), and with-no-lysine kinases (WNKs).

RECENT FINDINGS

In 2001, mutations in genes encoding for WNKs were identified as being responsible for PHAII. Recent advancements in genetics, in particular whole-exome sequencing, have revealed that mutations in two additional genes encoding for KLHL3 and Cyllin3 also cause PHAII. This discovery contributed to the clarification of the previously unknown regulatory mechanism of WNKs, namely WNK ubiquitination by the KLHL3-Cullin-3 E3 ligase complex.

SUMMARY

Levels of WNKs within cells are regulated via ubiquitination by the KLHL3-Cullin-3 E3 ligase complex and are important determinants of the activity of the WNK-oxidative stress-responsive gene 1 and Ste20-related proline-alanine-rich kinase-SLC12A transporter signaling cascade. The PHAII-causing mutations in WNK4, KLHL3, and Cullin-3 result in the decreased ubiquitination and increased abundance of WNK4 in the kidney, thereby activating the thiazide-sensitive NaCl cotransporter and causing PHAII.

Revisiting the NaCl cotransporter regulation by with-no-lysine kinases

The renal thiazide-sensitive Na(+)-Cl(-) cotransporter (NCC) is the salt transporter in the distal convoluted tubule. Its activity is fundamental for defining blood pressure levels. Decreased NCC activity is associated with salt-remediable arterial hypotension with hypokalemia (Gitelman disease), while increased activity results in salt-sensitive arterial hypertension with hyperkalemia (pseudohypoaldosteronism type II; PHAII). The discovery of four different genes causing PHAII revealed a complex multiprotein system that regulates the activity of NCC. Two genes encode for with-no-lysine (K) kinases WNK1 and WNK4, while two encode for kelch-like 3 (KLHL3) and cullin 3 (CUL3) proteins that form a RING type E3 ubiquitin ligase complex. Extensive research has shown that WNK1 and WNK4 are the targets for the KLHL3-CUL3 complex and that WNKs modulate the activity of NCC by means of intermediary Ste20-type kinases known as SPAK or OSR1. The understanding of the effect of WNKs on NCC is a complex issue, but recent evidence discussed in this review suggests that we could be reaching the end of the dark ages regarding this matter.

Kelch-Like Protein 2 Mediates Angiotensin II-With No Lysine 3 Signaling in the Regulation of Vascular Tonus

Recently, the kelch-like protein 3 (KLHL3)-Cullin3 complex was identified as an E3 ubiquitin ligase for with no lysine (WNK) kinases, and the impaired ubiquitination of WNK4 causes pseudohypoaldosteronism type II (PHAII), a hereditary hypertensive disease. However, the involvement of WNK kinase regulation by ubiquitination in situations other than PHAII has not been identified. Previously, we identified the WNK3-STE20/SPS1-related proline/alanine-rich kinase-Na/K/Cl cotransporter isoform 1 phosphorylation cascade in vascular smooth muscle cells and found that it constitutes an important mechanism of vascular constriction by angiotensin II (AngII). In this study, we investigated the involvement of KLHL proteins in AngII-induced WNK3 activation of vascular smooth muscle cells. In the mouse aorta and mouse vascular smooth muscle (MOVAS) cells, KLHL3 was not expressed, but KLHL2, the closest homolog of KLHL3, was expressed. Salt depletion and acute infusion of AngII decreased KLHL2 and increased WNK3 levels in the mouse aorta. Notably, the AngII-induced changes in KLHL2 and WNK3 expression occurred within minutes in MOVAS cells. Results of KLHL2 overexpression and knockdown experiments in MOVAS cells confirmed that KLHL2 is the major regulator of WNK3 protein abundance. The AngII-induced decrease in KLHL2 was not caused by decreased transcription but increased autophagy-mediated degradation. Furthermore, knockdown of sequestosome 1/p62 prevented the decrease in KLHL2, suggesting that the mechanism of KLHL2 autophagy could be selective autophagy mediated by sequestosome 1/p62. Thus, we identified a novel component of signal transduction in AngII-induced vascular contraction that could be a promising drug target.

Distal convoluted tubule

The distal convoluted tubule (DCT) is a short nephron segment, interposed between the macula densa and collecting duct. Even though it is short, it plays a key role in regulating extracellular fluid volume and electrolyte homeostasis. DCT cells are rich in mitochondria, and possess the highest density of Na+/K+-ATPase along the nephron, where it is expressed on the highly amplified basolateral membranes. DCT cells are largely water impermeable, and reabsorb sodium and chloride across the apical membrane via electroneurtral pathways. Prominent among this is the thiazide-sensitive sodium chloride cotransporter, target of widely used diuretic drugs. These cells also play a key role in magnesium reabsorption, which occurs predominantly, via a transient receptor potential channel (TRPM6). Human genetic diseases in which DCT function is perturbed have provided critical insights into the physiological role of the DCT, and how transport is regulated. These include Familial Hyperkalemic Hypertension, the salt-wasting diseases Gitelman syndrome and EAST syndrome, and hereditary hypomagnesemias. The DCT is also established as an important target for the hormones angiotensin II and aldosterone; it also appears to respond to sympathetic-nerve stimulation and changes in plasma potassium. Here, we discuss what is currently known about DCT physiology. Early studies that determined transport rates of ions by the DCT are described, as are the channels and transporters expressed along the DCT with the advent of molecular cloning. Regulation of expression and activity of these channels and transporters is also described; particular emphasis is placed on the contribution of genetic forms of DCT dysregulation to our understanding.

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

It is widely recognized that the phenotype of familial hyperkalemic hypertension is mainly a consequence of increased activity of the renal Na(+)-Cl(-) cotransporter (NCC) because of altered regulation by with no-lysine-kinase 1 (WNK1) or WNK4. The effect of WNK4 on NCC, however, has been controversial because both inhibition and activation have been reported. It has been recently shown that the long isoform of WNK1 (L-WNK1) is a chloride-sensitive kinase activated by a low Cl(-) concentration. Therefore, we hypothesized that WNK4 effects on NCC could be modulated by intracellular chloride concentration ([Cl(-)]i), and we tested this hypothesis in oocytes injected with NCC cRNA with or without WNK4 cRNA. At baseline in oocytes, [Cl(-)]i was near 50 mM, autophosphorylation of WNK4 was undetectable, and NCC activity was either decreased or unaffected by WNK4. A reduction of [Cl(-)]i, either by low chloride hypotonic stress or coinjection of oocytes with the solute carrier family 26 (anion exchanger)-member 9 (SLC26A9) cRNA, promoted WNK4 autophosphorylation and increased NCC-dependent Na(+) transport in a WNK4-dependent manner. Substitution of the leucine with phenylalanine at residue 322 of WNK4, homologous to the chloride-binding pocket in L-WNK1, converted WNK4 into a constitutively autophosphorylated kinase that activated NCC, even without chloride depletion. Elimination of the catalytic activity (D321A or D321K-K186D) or the autophosphorylation site (S335A) in mutant WNK4-L322F abrogated the positive effect on NCC. These observations suggest that WNK4 can exert differential effects on NCC, depending on the intracellular chloride concentration.

Ubiquitylation and control of renal Na+ balance and blood pressure

Ubiquitylation is crucial for regulating numerous cellular functions. In the kidney, ubiquitylation regulates the epithelial Na(+) channel ENaC. The importance of this process is highlighted in Liddle's syndrome, where mutations interfere with ENaC ubiquitylation, resulting in constitutive Na(+) reabsorption and hypertension. There is emerging evidence that NCC, involved in hypertensive diseases, is also regulated by ubiquitylation. Here, we discuss the current knowledge and recent findings in this field.

Hyperkalemic hypertension-associated cullin 3 promotes WNK signaling by degrading KLHL3

Familial hyperkalemic hypertension (FHHt) is a monogenic disease resulting from mutations in genes encoding WNK kinases, the ubiquitin scaffold protein cullin 3 (CUL3), or the substrate adaptor kelch-like 3 (KLHL3). Disease-associated CUL3 mutations abrogate WNK kinase degradation in cells, but it is not clear how mutant forms of CUL3 promote WNK stability. Here, we demonstrated that an FHHt-causing CUL3 mutant (CUL3 Δ403-459) not only retains the ability to bind and ubiquitylate WNK kinases and KLHL3 in cells, but is also more heavily neddylated and activated than WT CUL3. In cells, activated CUL3 Δ403-459 depleted KLHL3, preventing WNK degradation, despite increased CUL3-mediated WNK ubiquitylation; therefore, CUL3 loss in kidney should phenocopy FHHt in murine models. As predicted, nephron-specific deletion of Cul3 in mice did increase WNK kinase levels and the abundance of phosphorylated Na-Cl cotransporter (NCC). Over time, however, Cul3 deletion caused renal dysfunction, including hypochloremic alkalosis, diabetes insipidus, and salt-sensitive hypotension, with depletion of sodium potassium chloride cotransporter 2 and aquaporin 2. Moreover, these animals exhibited renal inflammation, fibrosis, and increased cyclin E. These results indicate that FHHt-associated CUL3 Δ403-459 targets KLHL3 for degradation, thereby preventing WNK degradation, whereas general loss of CUL3 activity - while also impairing WNK degradation - has widespread toxic effects in the kidney.

Disorders of aldosterone synthesis, secretion, and cellular function

PURPOSE OF REVIEW

The purpose of this review is to describe the renin-angiotensin-aldosterone system and its regulatory control of sodium, potassium, chloride, hydrogen ion, and water homeostasis through its effects on the expression and activity of distal renal tubular cotransporter proteins and to discuss the gene mutations encoding these structures that disturb the function of this system.

RECENT FINDINGS

Primary hypoaldosteronism may be the result of acquired or congenital errors in renal juxtaglomerular function (the source of renin), angiotensin generation or activity, or aldosterone synthesis. Secondary hypoaldosteronism (pseudohypoaldosteronism) occurs as a consequence of mutations in genes encoding the mineralocorticoid receptor (MR), the three subunits of the aldosterone-responsive, amiloride-sensitive nonvoltage-gated sodium channel encoded by SCNN1A, SCNN1B, and SCNN1G, the gene that regulates posttranslational phosphorylation (encoded by WNK4) of the thiazide-sensitive sodium chloride cotransporter encoded by SLC12A3, and those that regulate phosphorylation and ubiquitination of cofactors encoded by WNK1, KLH3, and CUL3 that affect WNK4 function.

SUMMARY

Acquired disorders of renal function as well as mutations in many genes may adversely affect aldosterone-mediated mineral homeostasis.

Structural and biochemical characterization of the KLHL3-WNK kinase interaction important in blood pressure regulation

WNK1 [with no lysine (K)] and WNK4 regulate blood pressure by controlling the activity of ion co-transporters in the kidney. Groundbreaking work has revealed that the ubiquitylation and hence levels of WNK isoforms are controlled by a Cullin-RING E3 ubiquitin ligase complex (CRL3^KLHL3) that utilizes CUL3 (Cullin3) and its substrate adaptor, KLHL3 (Kelch-like protein 3). Loss-of-function mutations in either CUL3 or KLHL3 cause the hereditary high blood pressure disease Gordon's syndrome by stabilizing WNK isoforms. KLHL3 binds to a highly conserved degron motif located within the C-terminal non-catalytic domain of WNK isoforms. This interaction is essential for ubiquitylation by CRL3^KLHL3 and disease-causing mutations in WNK4 and KLHL3 exert their effects on blood pressure by disrupting this interaction. In the present study, we report on the crystal structure of the KLHL3 Kelch domain in complex with the WNK4 degron motif. This reveals an intricate web of interactions between conserved residues on the surface of the Kelch domain β-propeller and the WNK4 degron motif. Importantly, many of the disease-causing mutations inhibit binding by disrupting critical interface contacts. We also present the structure of the WNK4 degron motif in complex with KLHL2 that has also been reported to bind WNK4. This confirms that KLHL2 interacts with WNK kinases in a similar manner to KLHL3, but strikingly different to how another KLHL protein, KEAP1 (Kelch-like enoyl-CoA hydratase-associated protein 1), binds to its substrate NRF2 (nuclear factor-erythroid 2-related factor 2). The present study provides further insights into how Kelch-like adaptor proteins recognize their substrates and provides a structural basis for how mutations in WNK4 and KLHL3 lead to hypertension.

WNK kinases regulate mammalian blood pressure. The level of WNK protein in a cell is regulated by the KLHL3–CUL3 ubiquitin ligase. We define the interaction between KLHL3 and WNK, identifying the WNK degron, and present the crystal structure of the KLHL3–WNK degron complex.