The kidney-specific WNK1 isoform is induced by aldosterone and stimulates epithelial sodium channel-mediated Na+ transport

Tubuloid differentiation to model the human distal nephron and collecting duct in health and disease

Organoid technology is rapidly gaining ground for studies on organ (patho)physiology. Tubuloids are long-term expanding organoids grown from adult kidney tissue or urine. The progenitor state of expanding tubuloids comes at the expense of differentiation. Here, we differentiate tubuloids to model the distal nephron and collecting ducts, essential functional parts of the kidney. Differentiation suppresses progenitor traits and upregulates genes required for function. A single-cell atlas reveals that differentiation predominantly generates thick ascending limb and principal cells. Differentiated human tubuloids express luminal NKCC2 and ENaC capable of diuretic-inhibitable electrolyte uptake and enable disease modeling as demonstrated by a lithium-induced tubulopathy model. Lithium causes hallmark AQP2 loss, induces proliferation, and upregulates inflammatory mediators, as seen in vivo. Lithium also suppresses electrolyte transport in multiple segments. In conclusion, this tubuloid model enables modeling of the human distal nephron and collecting duct in health and disease and provides opportunities to develop improved therapies.

Single-cell transcriptomics and chromatin accessibility profiling elucidate the kidney-protective mechanism of mineralocorticoid receptor antagonists

Mineralocorticoid excess commonly leads to hypertension (HTN) and kidney disease. In our study, we used single-cell expression and chromatin accessibility tools to characterize the mineralocorticoid target genes and cell types. We demonstrated that mineralocorticoid effects were established through open chromatin and target gene expression, primarily in principal and connecting tubule cells and, to a lesser extent, in segments of the distal convoluted tubule cells. We examined the kidney-protective effects of steroidal and nonsteroidal mineralocorticoid antagonists (MRAs), as well as of amiloride, an epithelial sodium channel inhibitor, in a rat model of deoxycorticosterone acetate, unilateral nephrectomy, and high-salt consumption-induced HTN and cardiorenal damage. All antihypertensive therapies protected against cardiorenal damage. However, finerenone was particularly effective in reducing albuminuria and improving gene expression changes in podocytes and proximal tubule cells, even with an equivalent reduction in blood pressure. We noted a strong correlation between the accumulation of injured/profibrotic tubule cells expressing secreted posphoprotein 1 (Spp1), Il34, and platelet-derived growth factor subunit b (Pdgfb) and the degree of fibrosis in rat kidneys. This gene signature also showed a potential for classifying human kidney samples. Our multiomics approach provides fresh insights into the possible mechanisms underlying HTN-associated kidney disease, the target cell types, the protective effects of steroidal and nonsteroidal MRAs, and amiloride.

Association study of WNK1 genetic variants and essential hypertension risk in the Northern Han Chinese in Beijing

Background: Essential hypertension (EH) is a complex disorder resulting from interaction of genetic and environmental factors. Lysine deficient protein kinase 1 (WNK1) plays a very important role in maintaining renal potassium, sodium and chlorine ions balance as well as the regulation of blood pressure, so the WNK1 gene is considered a key gene for EH. This study thus sought to evaluate possible genetic associations between the WNK1 genetic variants and EH risk in the Northern Han Chinese population in Beijing. Methods: This study included 476 hypertensive subjects and 491 normotensive subjects. A total of 12 tag SNVs of WNK1 gene were genotyped successfully by TaqMan assay. Comparisons of the genotypic and allelic frequency between cases and controls were made by using the chi-square test. Logistic regression analyses were performed under different genetic models, and haplotype analysis was also conducted. Results: A total of 12 SNVs were identified as the tag SNVs for WNK1 gene. Significant associations were observed between WNK1 gene rs7305099 variant and EH risk, and T allele influenced hypertension risk in a protective manner. After correcting for multiple testing using Bonferroni, the significance remained for the SNV of rs7305099 in three genetic models [allele comparison, p < 0.0002, OR = 0.627, 95%CI (0.491-0.801); homozygote comparison, p < 0.0003, OR = 0.278, 95%CI (0.140-0.552); additive model, p < 0.0003, OR = 0.279, 95%CI (0.140-0.553)]. In the haplotype analyses, we found that the haplotype A-A-A-C-G-G-G was significantly associated with increased risk for EH (p = 0.043, OR = 1.23). Conclusion: Our data suggested that the rs7305099 genetic variant and the haplotype A-A-A-C-G-G-G on WNK1 gene might be associated with the susceptibility of EH in the Northern Han Chinese population. These could provide evidences to the risk assessment, early prevention and individualized therapy of EH to some extent.

Aldosterone Synthase Inhibitors and Dietary Interventions: A Combined Novel Approach for Prevention and Treatment of Cardiovascular Disease

Systemic hypertension (HTN) is the hallmark of cardiovascular disease and the forerunner of heart failure. These associations have been established over decades of research on essential HTN. Advancements in the treatment of patients diagnosed with HTN, consisting of alpha- or beta-adrenergic receptor blockers, calcium channel blockers, angiotensin-converting enzyme inhibitors, thiazide, or aldosterone receptor blockers known as anti-mineralocorticoids, in the presence or absence of low sodium salt diets, often fail to control blood pressure adequately to prevent morbidity and mortality. Low sodium diets have had limited success in controlling HTN because low sodium intake is associated with renin-angiotensin-aldosterone system upregulation. Therefore, upregulating aldosterone secretion, sodium, and water retention which, in turn, moves the blood pressure back toward the range of HTN dictated by the baroreceptor reset value, as a compensatory mechanism, especially in resistant HTN. These impediments to blood pressure control in HTN may have been effectively circumvented by the advent of a new class of drugs known as aldosterone synthase inhibitors, represented by baxdrostat. The mechanism of action of baxdrostat as an aldosterone synthase inhibitor demonstrates the inextricable linkage between sodium and blood pressure regulation. Theoretically, combining a low sodium diet with the activity of this aldosterone synthesis inhibitor should alleviate the adverse effect of renin-angiotensin-aldosterone system upregulation. Aldosterone synthesis inhibition should also decrease the oxidative stress and endothelial dysfunction associated with HTN, causing more endothelial nitric oxide synthesis, release, and vasorelaxation. To the best of our knowledge, this is the first systematic review to summarize evidence-based articles relevant to the use of a novel drug (aldosterone synthase inhibitor) in the treatment of HTN and cardiovascular disease. Making the current database of relevant information on baxdrostat and other aldosterone synthase inhibitors readily available will, no doubt, aid physicians and other medical practitioners in their decision-making about employing aldosterone synthase inhibitors in the treatment of patients.

Spatial charting of single-cell transcriptomes in tissues

Single-cell RNA sequencing methods can profile the transcriptomes of single cells but cannot preserve spatial information. Conversely, spatial transcriptomics assays can profile spatial regions in tissue sections, but do not have single-cell resolution. Here, we developed a computational method called CellTrek that combines these two datasets to achieve single-cell spatial mapping through coembedding and metric learning approaches. We benchmarked CellTrek using simulation and in situ hybridization datasets, which demonstrated its accuracy and robustness. We then applied CellTrek to existing mouse brain and kidney datasets and showed that CellTrek can detect topological patterns of different cell types and cell states. We performed single-cell RNA sequencing and spatial transcriptomics experiments on two ductal carcinoma in situ tissues and applied CellTrek to identify tumor subclones that were restricted to different ducts, and specific T cell states adjacent to the tumor areas. Our data show that CellTrek can accurately map single cells in diverse tissue types to resolve their spatial organization.

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.

L-WNK1 is required for BK channel activation in intercalated cells

Large-conductance K+ (BK) channels expressed in intercalated cells (ICs) in the aldosterone-sensitive distal nephron (ASDN) mediate flow-induced K+ secretion. In the ASDN of mice and rabbits, IC BK channel expression and activity increase with a high-K+ diet. In cell culture, the long isoform of with-no-lysine kinase 1 (L-WNK1) increases BK channel expression and activity. Apical L-WNK1 expression is selectively enhanced in ICs in the ASDN of rabbits on a high-K+ diet, suggesting that L-WNK1 contributes to BK channel regulation by dietary K+. We examined the role of IC L-WNK1 expression in enhancing BK channel activity in response to a high-K+ diet. Mice with IC-selective deletion of L-WNK1 (IC-L-WNK1-KO) and littermate control mice were placed on a high-K+ (5% K+, as KCl) diet for 10 or more days. IC-L-WNK1-KO mice exhibited reduced IC apical + subapical α-subunit expression and BK channel-dependent whole cell currents compared with controls. Six-hour urinary K+ excretion in response a saline load was similar in IC-L-WNK1-KO mice and controls. The observations that IC-L-WNK1-KO mice on a high-K+ diet have higher blood K+ concentration and reduced IC BK channel activity are consistent with impaired urinary K+ secretion, demonstrating that IC L-WNK1 has a role in the renal adaptation to a high-K+ diet.NEW & NOTEWORTHY When mice are placed on a high-K+ diet, genetic disruption of the long form of with no lysine kinase 1 (L-WNK1) in intercalated cells reduced relative apical + subapical localization of the large-conductance K+ channel, blunted large-conductance K+ channel currents in intercalated cells, and increased blood K+ concentration. These data confirm an in vivo role of L-WNK1 in intercalated cells in adaptation to a high-K+ diet.

Function and Regulation of the Epithelial Na+ Channel ENaC

The Epithelial Na⁺ Channel, ENaC, comprised of 3 subunits (αβγ, or sometimes δβγENaC), plays a critical role in regulating salt and fluid homeostasis in the body. It regulates fluid reabsorption into the blood stream from the kidney to control blood volume and pressure, fluid absorption in the lung to control alveolar fluid clearance at birth and maintenance of normal airway surface liquid throughout life, and fluid absorption in the distal colon and other epithelial tissues. Moreover, recent studies have also revealed a role for sodium movement via ENaC in nonepithelial cells/tissues, such as endothelial cells in blood vessels and neurons. Over the past 25 years, major advances have been made in our understanding of ENaC structure, function, regulation, and role in human disease. These include the recently solved three-dimensional structure of ENaC, ENaC function in various tissues, and mutations in ENaC that cause a hereditary form of hypertension (Liddle syndrome), salt-wasting hypotension (PHA1), or polymorphism in ENaC that contributes to other diseases (such as cystic fibrosis). Moreover, great strides have been made in deciphering the regulation of ENaC by hormones (e.g., the mineralocorticoid aldosterone, glucocorticoids, vasopressin), ions (e.g., Na⁺ ), proteins (e.g., the ubiquitin-protein ligase NEDD4-2, the kinases SGK1, AKT, AMPK, WNKs & mTORC2, and proteases), and posttranslational modifications [e.g., (de)ubiquitylation, glycosylation, phosphorylation, acetylation, palmitoylation]. Characterization of ENaC structure, function, regulation, and role in human disease, including using animal models, are described in this article, with a special emphasis on recent advances in the field. © 2021 American Physiological Society. Compr Physiol 11:1-29, 2021.

Real World Use of Hypertonic Saline in Refractory Acute Decompensated Heart Failure: A U.S. Center's Experience

OBJECTIVES

The purpose of this study was to investigate real world safety and efficacy of hypertonic saline therapy in cases of refractory acute decompensated heart failure (ADHF) at a large U.S. academic medical center.

BACKGROUND

Hypertonic saline therapy has been described as a potential management strategy for refractory ADHF, but experience in the United States is limited.

METHODS

A retrospective analysis was performed in all patients receiving hypertonic saline for diuretic therapy-resistant ADHF at the authors' institution since March 2013. The primary analytic approach was a comparison of the trajectory of clinical variables prior to and after administration of hypertonic saline, with secondary focus on predictors of treatment response.

RESULTS

A total of 58 hypertonic saline administration episodes were identified across 40 patients with diuretic-therapy refractory ADHF. Prior to hypertonic saline administration, serum sodium, chloride, and creatinine concentrations were worsening but improved after hypertonic saline administration (p < 0.001, all). Both total urine output and weight loss significantly improved with hypertonic saline (p = 0.01 and <0.001, respectively). Diuretic efficiency, defined as change in urine output per doubling of diuretic dose, also improved over this period (p < 0.01). There were no significant changes in respiratory status or overcorrection of serum sodium with the intervention.

CONCLUSIONS

In a cohort of patients who were refractory to ADHF, hypertonic saline administration was associated with increased diuretic efficiency, fluid and weight loss, and improvement of metabolic derangements, and no adverse respiratory or neurological signals were identified. Additional study of hypertonic saline as a diuretic adjuvant is warranted.

The Molecular Genetics of Gordon Syndrome

Gordon syndrome is a rare inherited monogenic form of hypertension, which is associated with hyperkalaemia and metabolic acidosis. Since the recognition of this predominantly autosomal dominant condition in the 1960s, the study of families with Gordon syndrome has revealed four genes WNK1, WNK4, KLHL3, and CUL3 to be implicated in its pathogenesis after a phenotype–genotype correlation was realised. The encoded proteins Kelch-like 3 and Cullin 3 interact to form a ring-like complex to ubiquitinate WNK-kinase 4, which, in normal circumstances, interacts with the sodium chloride co-symporter (NCC), the epithelial sodium channel (ENaC), and the renal outer medullary potassium channel (ROMK) in an inhibitory manner to maintain normokalaemia and normotension. WNK-kinase 1 has an inhibitory action on WNK-kinase 4. Mutations in WNK1, WNK4, KLHL3, and CUL3 all result in the accumulation of WNK-kinase 4 and subsequent hypertension, hyperkalaemia, and metabolic acidosis. This review explains the clinical aspects, disease mechanisms, and molecular genetics of Gordon syndrome.

The serine-threonine kinase PIM3 is an aldosterone-regulated protein in the distal nephron

The mineralocorticoid hormone aldosterone plays a crucial role in the control of Na+ and K+ balance, blood volume, and arterial blood pressure, by acting in the aldosterone-sensitive distal nephron (ASDN) and stimulating a complex transcriptional, translational, and cellular program. Because the complexity of the aldosterone response is still not fully appreciated, we aimed at identifying new elements in this pathway. Here, we demonstrate that the expression of the proto-oncogene PIM3 (Proviral Integration Site of Moloney Murine Leukemia Virus 3), a serine/threonine kinase belonging to the calcium/calmodulin-regulated group of kinases, is stimulated by aldosterone in vitro (mCCDcl1 cells), ex vivo (mouse kidney slices), and in vivo in mice. Characterizing a germline Pim3-/- mouse model, we found that these mice have an upregulated Renin-Angiotensin-Aldosterone System (RAAS), with high circulating aldosterone and plasma renin activity levels on both standard or Na+ -deficient diet. Surprisingly, we did not observe any obvious salt-losing phenotype in Pim3 KO mice as shown by normal blood pressure, plasma and urinary electrolytes, as well as unchanged expression levels of the major Na+ transport proteins. These observations suggest that the potential effects of the loss of the Pim3 gene are physiologically compensated. Indeed, the 2 other family members of the PIM kinase family, PIM1 and PIM2 are upregulated in the kidney of Pim3-/- mice, and may therefore be involved in such compensation. In conclusion, our data demonstrate that the PIM3 kinase is a novel aldosterone-induced protein, but its precise role in aldosterone-dependent renal homeostasis remains to be determined.

Glucocorticoid Receptor-Tethered Mineralocorticoid Receptors Increase Glucocorticoid-Induced Transcriptional Responses

Mineralocorticoid and glucocorticoid receptors (MRs and GRs) constitute a functionally important dual receptor system detecting and transmitting circulating corticosteroid signals. High expression of MRs and GRs occurs in the same cells in the limbic system, the primary site of glucocorticoid action on cognition, behavior, and mood; however, modes of interaction between the receptors are poorly characterized. We used chromatin immunoprecipitation with nucleotide resolution using exonuclease digestion, unique barcode, and single ligation (ChIP-nexus) for high-resolution genome-wide characterization of MR and GR DNA binding profiles in neuroblastoma cells and demonstrate recruitment to highly similar DNA binding sites. Expressed MR or GR showed differential regulation of endogenous gene targets, including Syt2 and Ddc, whereas coexpression produced augmented transcriptional responses even when MRs were unable to bind DNA (MR-XDBD). ChIP confirmed that MR-XDBD could be tethered to chromatin by GR. Our data demonstrate that MR can interact at individual genomic DNA sites in multiple modes and suggest a role for MR in increasing the transcriptional response to glucocorticoids.

Annexin II Light Chain p11 Interacts With ENaC to Increase Functional Activity at the Membrane

The epithelial Na⁺ channel (ENaC) provides for Na⁺ absorption in various types of epithelia including the kidney, lung, and colon where ENaC is localized to the apical membrane to enable Na⁺ entry into the cell. The degree of Na⁺ entry via ENaC largely depends on the number of active channels localized to the cell membrane, and is tightly controlled by interactions with ubiquitin ligases, kinases, and G-proteins. While regulation of ENaC endocytosis has been well-studied, relatively little is understood of the proteins that govern ENaC exocytosis. We hypothesized that the annexin II light chain, p11, could participate in the transport of ENaC along the exocytic pathway. Our results demonstrate that all three ENaC channel subunits interacted with p11 in an in vitro binding assay. Furthermore, p11 was able to immunoprecipitate ENaC in epithelial cells. Quantitative mass spectrometry of affinity-purified ENaC-p11 complexes recovered several other trafficking proteins including HSP-90 and annexin A6. We also report that p11 exhibits a robust protein expression in cortical collecting duct epithelial cells. However, the expression of p11 in these cells was not influenced by either short-term or long-term exposure to aldosterone. To determine whether the p11 interaction affected ENaC function, we measured amiloride sensitive Na⁺ currents in Xenopus oocytes or mammalian epithelia co-expressing ENaC and p11 or a siRNA to p11. Results from these experiments showed that p11 significantly augmented ENaC current, whereas knockdown of p11 decreased current. Further, knockdown of p11 reduced ENaC cell surface population suggesting p11 promotes membrane insertion of ENaC. Overall, our findings reveal a novel protein interaction that controls the number of ENaC channels inserted at the membrane via the exocytic pathway.

MST3 (mammalian Ste20-like protein kinase 3), a novel gene involved in ion homeostasis and renal regulation of blood pressure in spontaneous hypertensive rats

Defective renal salt and water excretion, together with increased salt intake, frequently contributes to hypertension. Recent studies indicate that Ste20 family kinases, such as proline-alanine-rich Ste20-related kinase (SPAK) and oxidative stress-response protein 1 (OSR1), are regulators of cell volume, ion transport, and hypertension. The aim of this study was to investigate whether mammalian sterile 20-like protein kinase 3 (MST3), which is also a stress-regulated kinase, is involved in the development of hypertension. MST3 expression was compared in Wistar-Kyoto (WKY) and spontaneously hypertensive rat (SHR) kidneys. MST3 expression was markedly reduced in principal cells of the collecting ducts from the renal inner medulla of SHR. The downregulation of MST3 expression was observed before and after the onset of hypertension in SHR. Mice fed high-salt diets (HS) exhibited a significant increase in MST3 protein level. This is the first study reporting that MST3, a Ste20-like kinase, exerts a conserved regulatory role in sodium homeostasis after high-salt diet and in the development of hypertension.

Potassium Homeostasis, Oxidative Stress, and Human Disease

Potassium is the most abundant cation in the intracellular fluid and it plays a vital role in the maintenance of normal cell functions. Thus, potassium homeostasis across the cell membrane, is very critical because a tilt in this balance can result in different diseases that could be life threatening. Both Oxidative stress (OS) and potassium imbalance can cause life threatening health conditions. OS and abnormalities in potassium channel have been reported in neurodegenerative diseases. This review highlights the major factors involved in potassium homeostasis (dietary, hormonal, genetic, and physiologic influences), and discusses the major diseases and abnormalities associated with potassium imbalance including hypokalemia, hyperkalemia, hypertension, chronic kidney disease, and Gordon's syndrome, Bartter syndrome, and Gitelman syndrome.

WNK signalling pathways in blood pressure regulation

Hypertension (high blood pressure) is a major public health problem affecting more than a billion people worldwide with complications, including stroke, heart failure and kidney failure. The regulation of blood pressure is multifactorial reflecting genetic susceptibility, in utero environment and external factors such as obesity and salt intake. In keeping with Arthur Guyton's hypothesis, the kidney plays a key role in blood pressure control and data from clinical studies; physiology and genetics have shown that hypertension is driven a failure of the kidney to excrete excess salt at normal levels of blood pressure. There is a number of rare Mendelian blood pressure syndromes, which have shed light on the molecular mechanisms involved in dysregulated ion transport in the distal kidney. One in particular is Familial hyperkalemic hypertension (FHHt), an autosomal dominant monogenic form of hypertension characterised by high blood pressure, hyperkalemia, hyperchloremic metabolic acidosis, and hypercalciuria. The clinical signs of FHHt are treated by low doses of thiazide diuretic, and it mirrors Gitelman syndrome which features the inverse phenotype of hypotension, hypokalemic metabolic alkalosis, and hypocalciuria. Gitelman syndrome is caused by loss of function mutations in the thiazide-sensitive Na/Cl cotransporter (NCC); however, FHHt patients do not have mutations in the SCL12A3 locus encoding NCC. Instead, mutations have been identified in genes that have revealed a key signalling pathway that regulates NCC and several other key transporters and ion channels in the kidney that are critical for BP regulation. This is the WNK kinase signalling pathway that is the subject of this review.

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.

Serum- and glucocorticoid-inducible kinase sgk2 stimulates the transport activity of human organic anion transporters 1 by enhancing the stability of the transporter

Human organic anion transporter 1 (hOAT1) belongs to a family of organic anion transporters that play critical roles in the body disposition of clinically important drugs, including anti-viral therapeutics, anti-cancer drugs, antibiotics, antihypertensives, and anti-inflammatories. hOAT1 is abundantly expressed in the kidney and brain. In the current study, we examined the regulation of hOAT1 by serum- and glucocorticoid-inducible kinase 2 (sgk2) in the kidney COS-7 cells. We showed that sgk2 stimulated hOAT1 transport activity. Such stimulation mainly resulted from an increased cell surface expression of the transporter, kinetically revealed as an increased maximal transport velocity V max without significant change in substrate-binding affinity K m. We further showed that stimulation of hOAT1 activity by sgk2 was achieved by preventing hOAT1 degradation. Our co-immunoprecipitation experiment revealed that the effect of sgk2 on hOAT1 was through a direct interaction between these two proteins. In conclusion, our study demonstrated that sgk2 stimulates hOAT1 transport activity by enhancing the stability of the transporter. This study provides the insights into sgk2 regulation of hOAT1-mediated transport in normal physiology and disease.

Hypochloraemia is strongly and independently associated with mortality in patients with chronic heart failure

AIMS

Hyponatraemia is strongly associated with adverse outcomes in heart failure. However, accumulating evidence suggests that chloride may play an important role in renal salt sensing and regulation of neurohormonal and sodium-conserving pathways. Our objective was to determine the prognostic importance of hypochloraemia in patients with heart failure.

METHODS AND RESULTS

Patients in the BEST trial with baseline serum chloride values were evaluated (n = 2699). Hypochloraemia was defined as a serum chloride ≤96 mmol/L and hyponatraemia as serum sodium ≤135 mmol/L. Hypochloraemia was present in 13.0% and hyponatraemia in 13.7% of the population. Chloride and sodium were only modestly correlated (r = 0.53), resulting in only 48.7% of hypochloraemic patients having concurrent hyponatraemia. Both hyponatraemia and hypochloraemia identified a population with greater disease severity; however, renal function tended to be worse and loop diuretic doses higher with hypochloraemia. In univariate analysis, lower serum sodium or serum chloride as continuous parameters were each strongly associated with mortality (P < 0.001). However, when both parameters were included in the same model, serum chloride remained strongly associated with mortality [hazard ratio (HR) 1.3 per standard deviation decrease, 95% confidence interval (CI) 1.18-1.42, P < 0.001], whereas sodium was not (HR 0.97 per standard deviation decrease, 95% CI 0.89-1.06, P = 0.52).

CONCLUSION

Serum chloride is strongly and independently associated with worsened survival in patients with chronic heart failure and accounted for the majority of the risk otherwise attributable to hyponatraemia. Given the critical role of chloride in a number of regulatory pathways central to heart failure pathophysiology, additional research is warranted in this area.

(WNK)ing at death: With-no-lysine (Wnk) kinases in neuropathies and neuronal survival

Members of With-no-lysine (WNK) family of serine-threonine kinase are key regulators of chloride ion transport in diverse cell types, controlling the activity and the surface expression of cation-chloride (Na(+)/K(+)-Cl(-)) co-transporters. Mutations in WNK1 and WNK4 are linked to a hereditary form of hypertension, and WNKs have been extensively investigated pertaining to their roles in renal epithelial ion homeostasis. However, some members of the WNK family and their splice isoforms are also expressed in the mammalian brain, and have been implicated in aspects of hereditary neuropathy as well as neuronal and glial survival. WNK2, which is exclusively enriched in neurons, is well known as an anti-proliferative tumor suppressor. WNK3, on the other hand, appears to promote cell survival as its inhibition enhances neuronal apoptosis. However, loss of WNK3 has been recently shown to reduce ischemia-associated brain damage. In this review, I surveyed the potentially context-dependent roles of WNKs in neurological disorders and neuronal survival.

Achieving the Benefits of a High-Potassium, Paleolithic Diet, Without the Toxicity

The average US dietary intake of K(+) is well below the current recommended nutritional requirements. This deficiency is even more striking when comparing our current intake with that of our ancestors, who consumed large amounts of dietary K(+). K(+) deficiency has been implicated in many diseases including cardiovascular disease, kidney stones, and osteoporosis. Importantly, dietary supplementation of K(+) has favorable effects on reducing blood pressure, decreasing the risk of stroke, improving bone health, and reducing the risk of nephrolithiasis. For this comprehensive review, we scanned the literature using PubMed and MEDLINE using the following search terms: potassium intake, renal potassium excretion, and prevention of hyperkalemia. Articles were selected for inclusion if they represented primary data or review articles published between 1980 and 2015 in high-impact journals. The normal kidney has the capacity to tightly regulate K(+) homoeostasis. We discuss new findings with respect to sensing mechanisms by which the kidney maintains K(+) homeostasis in the gastrointestinal tract and distal tubule. There are widely prescribed hypertensive medications that cause hyperkalemia and thus require dietary K(+) restriction. We conclude by discussing newly approved drugs capable of binding K(+) in the gastrointestinal tract and speculate that this new pharmacology might allow diet liberalization in patients at risk for hyperkalemia, affording them the numerous benefits of a K(+)-rich diet.

Serum- and glucocorticoid-inducible kinase SGK2 regulates human organic anion transporters 4 via ubiquitin ligase Nedd4-2

Human organic anion transporter 4 (hOAT4) belongs to a family of organic anion transporters that play critical roles in the body disposition of clinically important drugs, including anti-viral therapeutics, anti-cancer drugs, antibiotics, antihypertensives, and anti-inflammatories. hOAT4 is abundantly expressed in the kidney and placenta. In the current study, we examined the regulation of hOAT4 by serum- and glucocorticoid-inducible kinase 2 (sgk2) in the kidney COS-7 cells. We showed that sgk2 stimulated hOAT4 transport activity. Such stimulation mainly resulted from an increased cell surface expression of the transporter, kinetically revealed as an increased maximal transport velocity Vmax without significant change in substrate-binding affinity Km. We further showed that regulation of hOAT4 activity by sgk2 was mediated by ubiquitin ligase Nedd4-2. Overexpression of Nedd4-2 enhanced hOAT4 ubiquitination, and inhibited hOAT4 transport activity, whereas overexpression of ubiquitin ligase-dead mutant Nedd4-2/C821A or siRNA knockdown of endogenous Nedd4-2 had opposite effects on hOAT4. Our co-immunoprecipitation experiment revealed that sgk2 weakened the association between hOAT4 and Nedd4-2. In conclusion, our study demonstrated for the first time that sgk2 stimulated hOAT4 transport activity by abrogating the inhibitory effect of Nedd4-2 on the transporter.

Alternatively spliced proline-rich cassettes link WNK1 to aldosterone action

The thiazide-sensitive NaCl cotransporter (NCC) is important for renal salt handling and blood-pressure homeostasis. The canonical NCC-activating pathway consists of With-No-Lysine (WNK) kinases and their downstream effector kinases SPAK and OSR1, which phosphorylate NCC directly. The upstream mechanisms that connect physiological stimuli to this system remain obscure. Here, we have shown that aldosterone activates SPAK/OSR1 via WNK1. We identified 2 alternatively spliced exons embedded within a proline-rich region of WNK1 that contain PY motifs, which bind the E3 ubiquitin ligase NEDD4-2. PY motif-containing WNK1 isoforms were expressed in human kidney, and these isoforms were efficiently degraded by the ubiquitin proteasome system, an effect reversed by the aldosterone-induced kinase SGK1. In gene-edited cells, WNK1 deficiency negated regulatory effects of NEDD4-2 and SGK1 on NCC, suggesting that WNK1 mediates aldosterone-dependent activity of the WNK/SPAK/OSR1 pathway. Aldosterone infusion increased proline-rich WNK1 isoform abundance in WT mice but did not alter WNK1 abundance in hypertensive Nedd4-2 KO mice, which exhibit high baseline WNK1 and SPAK/OSR1 activity toward NCC. Conversely, hypotensive Sgk1 KO mice exhibited low WNK1 expression and activity. Together, our findings indicate that the proline-rich exons are modular cassettes that convert WNK1 into a NEDD4-2 substrate, thereby linking aldosterone and other NEDD4-2-suppressing antinatriuretic hormones to NCC phosphorylation status.

Epithelial sodium transport and its control by aldosterone: the story of our internal environment revisited

Transcription and translation require a high concentration of potassium across the entire tree of life. The conservation of a high intracellular potassium was an absolute requirement for the evolution of life on Earth. This was achieved by the interplay of P- and V-ATPases that can set up electrochemical gradients across the cell membrane, an energetically costly process requiring the synthesis of ATP by F-ATPases. In animals, the control of an extracellular compartment was achieved by the emergence of multicellular organisms able to produce tight epithelial barriers creating a stable extracellular milieu. Finally, the adaptation to a terrestrian environment was achieved by the evolution of distinct regulatory pathways allowing salt and water conservation. In this review we emphasize the critical and dual role of Na(+)-K(+)-ATPase in the control of the ionic composition of the extracellular fluid and the renin-angiotensin-aldosterone system (RAAS) in salt and water conservation in vertebrates. The action of aldosterone on transepithelial sodium transport by activation of the epithelial sodium channel (ENaC) at the apical membrane and that of Na(+)-K(+)-ATPase at the basolateral membrane may have evolved in lungfish before the emergence of tetrapods. Finally, we discuss the implication of RAAS in the origin of the present pandemia of hypertension and its associated cardiovascular diseases.

Lack of family-based association between common variations in WNK1 and blood pressure level

BACKGROUND

WNK1 (With No-lysine Kinase 1) modulates numerous sodium transport-related ion channels involved in regulation of blood pressure. Several studies have indicated associations between the common variants of the WNK1 gene and hypertension or blood pressure levels. However, little data exists on Asian populations and normotensive or pre-hypertensive subjects. Our aim was to detect whether the common variations in the WNK1 gene are potential contributors to individual variations in blood pressure in a family-based sample.

MATERIAL AND METHODS

525 individuals from 116 families were selected from a rural community of Northern China. Five single-nucleotide polymorphisms were selected from the WNK1 gene. Single-marker and haplotype analyses were conducted using the Family-Based Association Test program.

RESULTS

Regretful, no associations for the 5 WNK1 SNPs and the constructed haplotype blocks of WNK1 with blood pressure level reached nominal statistical significance.

CONCLUSIONS

We conclude that although multiple candidate genes are involved in development of hypertension, the genetic polymorphism in WNK1 is not a major contributor to the observed variability in blood pressure and familial clustering risk of hypertension.

The WNK-SPAK/OSR1 pathway: master regulator of cation-chloride cotransporters

The WNK-SPAK/OSR1 kinase complex is composed of the kinases WNK (with no lysine) and SPAK (SPS1-related proline/alanine-rich kinase) or the SPAK homolog OSR1 (oxidative stress-responsive kinase 1). The WNK family senses changes in intracellular Cl(-) concentration, extracellular osmolarity, and cell volume and transduces this information to sodium (Na(+)), potassium (K(+)), and chloride (Cl(-)) cotransporters [collectively referred to as CCCs (cation-chloride cotransporters)] and ion channels to maintain cellular and organismal homeostasis and affect cellular morphology and behavior. Several genes encoding proteins in this pathway are mutated in human disease, and the cotransporters are targets of commonly used drugs. WNKs stimulate the kinases SPAK and OSR1, which directly phosphorylate and stimulate Cl(-)-importing, Na(+)-driven CCCs or inhibit the Cl(-)-extruding, K(+)-driven CCCs. These coordinated and reciprocal actions on the CCCs are triggered by an interaction between RFXV/I motifs within the WNKs and CCCs and a conserved carboxyl-terminal docking domain in SPAK and OSR1. This interaction site represents a potentially druggable node that could be more effective than targeting the cotransporters directly. In the kidney, WNK-SPAK/OSR1 inhibition decreases epithelial NaCl reabsorption and K(+) secretion to lower blood pressure while maintaining serum K(+). In neurons, WNK-SPAK/OSR1 inhibition could facilitate Cl(-) extrusion and promote γ-aminobutyric acidergic (GABAergic) inhibition. Such drugs could have efficacy as K(+)-sparing blood pressure-lowering agents in essential hypertension, nonaddictive analgesics in neuropathic pain, and promoters of GABAergic inhibition in diseases associated with neuronal hyperactivity, such as epilepsy, spasticity, neuropathic pain, schizophrenia, and autism.

Regulation of Potassium Homeostasis

Potassium is the most abundant cation in the intracellular fluid, and maintaining the proper distribution of potassium across the cell membrane is critical for normal cell function. Long-term maintenance of potassium homeostasis is achieved by alterations in renal excretion of potassium in response to variations in intake. Understanding the mechanism and regulatory influences governing the internal distribution and renal clearance of potassium under normal circumstances can provide a framework for approaching disorders of potassium commonly encountered in clinical practice. This paper reviews key aspects of the normal regulation of potassium metabolism and is designed to serve as a readily accessible review for the well informed clinician as well as a resource for teaching trainees and medical students.

Genome-wide analysis of murine renal distal convoluted tubular cells for the target genes of mineralocorticoid receptor

BACKGROUND AND OBJECTIVE

Mineralocorticoid receptor (MR) is a member of nuclear receptor family proteins and contributes to fluid homeostasis in the kidney. Although aldosterone-MR pathway induces several gene expressions in the kidney, it is often unclear whether the gene expressions are accompanied by direct regulations of MR through its binding to the regulatory region of each gene. The purpose of this study is to identify the direct target genes of MR in a murine distal convoluted tubular epithelial cell-line (mDCT).

METHODS

We analyzed the DNA samples of mDCT cells overexpressing 3xFLAG-hMR after treatment with 10(-7)M aldosterone for 1h by chromatin immunoprecipitation with deep-sequence (ChIP-seq) and mRNA of the cell-line with treatment of 10(-7)M aldosterone for 3h by microarray.

RESULTS

3xFLAG-hMR overexpressed in mDCT cells accumulated in the nucleus in response to 10(-9)M aldosterone. Twenty-five genes were indicated as the candidate target genes of MR by ChIP-seq and microarray analyses. Five genes, Sgk1, Fkbp5, Rasl12, Tns1 and Tsc22d3 (Gilz), were validated as the direct target genes of MR by quantitative RT-qPCR and ChIP-qPCR. MR binding regions adjacent to Ctgf and Serpine1 were also validated.

CONCLUSIONS

We, for the first time, captured the genome-wide distribution of MR in mDCT cells and, furthermore, identified five MR target genes in the cell-line. These results will contribute to further studies on the mechanisms of kidney diseases.

Functional coupling of renal K+ and Na+ handling causes high blood pressure in Na+ replete mice

A network of kinases, including WNKs, SPAK and Sgk1, is critical for the independent regulation of K+ and Na+ transport in the distal nephron. Angiotensin II is thought to act as a key hormone in orchestrating these kinases to switch from K+ secretion during hyperkalaemia to Na+ reabsorption during intravascular volume depletion, thus keeping disturbances in electrolyte and blood pressure homeostasis at a minimum. It remains unclear, however, how K+ and Na+ transport are regulated during a high Na+ intake, which is associated with suppressed angiotensin II levels and a high distal tubular Na+ load. We therefore investigated the integrated blood pressure, renal, hormonal and gene and protein expression responses to large changes of K+ intake in Na+ replete mice. Both low and high K+ intake increased blood pressure and caused Na+ retention. Low K+ intake was accompanied by an upregulation of the sodium-chloride cotransporter (NCC) and its activating kinase SPAK, and inhibition of NCC normalized blood pressure. Renal responses were unaffected by angiotensin AT1 receptor antagonism, indicating that low K+ intake activates the distal nephron by an angiotensin-independent mode of action. High K+ intake was associated with elevated plasma aldosterone concentrations and an upregulation of the epithelial sodium channel (ENaC) and its activating kinase Sgk1. Surprisingly, high K+ intake increased blood pressure even during ENaC or mineralocorticoid receptor antagonism, suggesting the contribution of aldosterone-independent mechanisms. These findings show that in a Na+ replete state, changes in K+ intake induce specific molecular and functional adaptations in the distal nephron that cause a functional coupling of renal K+ and Na+ handling, resulting in Na+ retention and high blood pressure when K+ intake is either restricted or excessively increased.

Vasopressin and the regulation of aquaporin-2

Water excretion is regulated in large part through the regulation of osmotic water permeability of the renal collecting duct epithelium. Water permeability is controlled by vasopressin through regulation of the water channel, aquaporin-2 (AQP2). Two processes contribute: (1) regulation of AQP2 trafficking to the apical plasma membrane; and (2) regulation of the total amount of the AQP2 protein in the cells. Regulation of AQP2 abundance is defective in several water-balance disorders, including many polyuric disorders and the syndrome of inappropriate antidiuresis. Here we review vasopressin signaling in the renal collecting duct that is relevant to the two modes of water permeability regulation.

Loss of WNK3 is compensated for by the WNK1/SPAK axis in the kidney of the mouse

WNK3 kinase is expressed throughout the nephron and acts as a positive regulator of NKCC2 and NCC in vitro. Here we addressed the in vivo relevance of WNK3 using WNK3-deficient mice. WNK3−/− mice were viable and showed no gross abnormalities. The net tubular function was similar in wild-type (WT) and WNK3−/− mice as assessed by determination of 24-h urine output (1.63 ± .06 in WT and 1.55 ± .1 ml in WNK3−/−, n=16; P=0.42) and ambient urine osmolarity (1,804 ± 62 in WT vs. 1,819 ± 61 mosmol/kg in WNK3−/−, n=40; P=0.86). Water restriction (48 h) increased urine osmolarity similarly in both genotypes to 3,440 ± 220 and 3,200 ± 180 mosmol/kg in WT and WNK3−/− mice, respectively ( n=11; P=0.41). The glomerular filtration rate (343 ± 22 vs. 315 ± 13 ml/min), renal blood flow (1.35 ± 0.1 vs. 1.42 ± 0.04 ml), and plasma renin concentration (94 ± 18 vs. 80 ± 13 ng ANG I·ml−1·h−1) were similar between WT and WNK3−/− mice ( n=13; P=0.54). WNK1 was markedly upregulated in WNK3-deficient mice, whereas the expression of WNK4 was similar in both genotypes. When the mice were fed a salt-restricted diet [0.02% NaCl (wt/wt)] the levels of pSPAK/OSR1, pNKCC2, and pNCC were enhanced in both genotypes compared with the baseline conditions, with the levels in WNK3−/− exceeding those in WT mice. The upregulation of pSPAK/OSR1, pNKCC2, and pNCC in WNK3−/− mice relative to the levels in WT mice when fed a low-salt diet was paralleled by an increased diuresis in response to hydrochlorothiazide. In summary, the overall relevance of WNK3 for the renal reabsorption of NaCl appears to be limited and can be largely compensated for by the activation of WNK3-independent pathways. Consequently, our data suggest that WNK3 may serve as a member of a kinase network that facilitates the fine-tuning of renal transepithelial NaCl transport.

Kidney-specific WNK1 regulates sodium reabsorption and potassium secretion in mouse cortical collecting duct

Kidney-specific with-no-lysine kinase 1 (KS-WNK1) is a kinase-deficient variant of WNK1 that is expressed exclusively in the kidney. It is abundantly expressed in the distal convoluted tubule (DCT) and to a lesser extent in the cortical thick ascending limb (cTAL), connecting tubule, and cortical collecting duct (CCD). KS-WNK1 inhibits Na(+)-K(+)-2Cl(-)- and sodium chloride cotransporter-mediated Na(+) reabsorption in cTAL and DCT, respectively. Here, we investigated the role of KS-WNK1 in regulating Na(+) and K(+) transport in CCD using in vitro microperfusion of tubules isolated from KS-WNK1 knockout mice and control wild-type littermates. Because baseline K(+) secretion and Na(+) reabsorption were negligible in mouse CCD, we studied tubules isolated from mice fed a high-K(+) diet for 2 wk. Compared with that in wild-type tubules, K(+) secretion was reduced in KS-WNK1 knockout CCD perfused at a low luminal fluid rate of ~1.5 nl/min. Na(+) reabsorption and the lumen-negative transepithelial potential difference were also lower in the KS-WNK1 knockout CCD compared with control CCD. Increasing the perfusion rate to ~5.5 nl/min stimulated K(+) secretion in the wild-type as well as knockout CCD. The magnitudes of flow-stimulated increase in K(+) secretion were similar in wild-type and knockout CCD. Maxi-K(+) channel inhibitor iberiotoxin had no effect on K(+) secretion when tubules were perfused at ~1.5 nl/min, but completely abrogated the flow-dependent increase in K(+) secretion at ~5.5 nl/min. These findings support the notion that KS-WNK1 stimulates ROMK-mediated K(+) secretion, but not flow-dependent K(+) secretion mediated by maxi-K(+) channels in CCD. In addition, KS-WNK1 plays a role in regulating Na(+) transport in the CCD.

Glucocorticoid-induced hypertension

Glucocorticoid-induced hypertension is a common clinical problem that is poorly understood, thus rendering treatment strategies sub-optimal. This form of hypertension has been commonly thought to be mediated by excess sodium and water reabsorption by the renal mineralocorticoid receptor. However, experimental and clinical data in both humans and animal models suggest important roles for the glucocorticoid receptor as well, in both the pathogenesis and maintenance of this hypertension. The glucocorticoid receptor is widely expressed in a number of organ systems relevant to blood pressure regulation, including the kidney, the brain and the vasculature. In vitro studies in isolated kidney tissues as well as in vascular smooth muscle and vascular endothelial cells have attempted to elucidate the molecular physiology of glucocorticoid-induced hypertension, but have generally been limited by the inability to study signaling pathways in an intact organism. More recently, the power of mouse genetics has been employed to examine the tissue-specific contributions of vascular and extra-vascular tissues to this form of hypertension. Here we review recent developments in our understanding of the pathogenesis of glucocorticoid-induced hypertension.

Organization of the ENaC-regulatory machinery

The control of fluid and electrolyte homeostasis in vertebrates requires the integration of a diverse set of signaling inputs, which control epithelial Na(+) transport, the principal ionic component of extracellular fluid. The key site of regulation is a segment of the kidney tubules, frequently termed the aldosterone-sensitive distal nephron, wherein the epithelial Na(+) channel (or ENaC) mediates apical ion entry. Na(+) transport in this segment is strongly regulated by the salt-retaining hormone, aldosterone, which acts through the mineralocorticoid receptor (MR) to influence the expression of a selected set of target genes, most notably the serine-threonine kinase SGK1, which phosphorylates and inhibits the E3 ubiquitin ligase Nedd4-2. It has long been known that ENaC activity is tightly regulated in vertebrate epithelia. Recent evidence suggests that SGK1 and Nedd4-2, along with other ENaC-regulatory proteins, physically associate with each other and with ENaC in a multi-protein complex. The various components of the complex are regulated by diverse signaling networks, including steroid receptor-, PI3-kinase-, mTOR-, and Raf-MEK-ERK-dependent pathways. In this review, we focus on the organization of the targets of these pathways by multi-domain scaffold proteins and lipid platforms into a unified complex, thereby providing a molecular basis for signal integration in the control of ENaC.

The role of the ENaC-regulatory complex in aldosterone-mediated sodium transport

The mineralocorticoid aldosterone is indispensable for the control of blood pressure and fluid volume in mammals. It acts in large part to increase the abundance and activity of the epithelial Na(+) channel (ENaC), which mediates apical Na(+) entry in the distal parts of the kidney tubules. Aldosterone acts through the mineralocorticoid receptor to alter the transcription of specific genes, including SGK1 and GILZ1. Recent evidence suggests that these key aldosterone-regulated factors function within a unique multi-protein ENaC-regulatory-complex that governs the net cell surface expression and activity of the channel. Another aldosterone-induced protein, CNK3 (connector enhancer of kinase suppressor of Ras 3), also stimulates ENaC and has all of the features of a scaffolding protein. With these observations in mind, we discuss the possibility that CNK3 coordinates the dynamic assembly of the ENaC-regulatory-complex, and promotes context-appropriate aldosterone signal transduction in the regulation of epithelial Na(+) transport.

Syndromes of impaired ion handling in the distal nephron: pseudohypoaldosteronism and familial hyperkalemic hypertension

The distal nephron, which is the site of the micro-regulation of water absorption and ion handling in the kidneys, is under the control of aldosterone. Impairment of the mineralocorticoid signal transduction pathway results in resistance to the action of aldosterone and of mineralocorticoids in general. Herein, we review two syndromes in which ion handling in the distal nephron is impaired: pseudohypoaldosteronism (PHA) and familial hyperkalemic hypertension (FHH). PHA is a rare inherited syndrome characterized by mineralocorticoid resistance, which leads to salt loss, hypotension, hyperkalemia and metabolic acidosis. There are two types of this syndrome: a renal (autosomal dominant) type due to mutations of the mineralocorticoid receptor (MR), and a systemic (autosomal recessive) type due to mutations of the epithelial sodium channel (ENaC). There is also a transient form of PHA, which may be due to urinary tract infections, obstructive uropathy or several medications. FHH is a rare autosomal dominant syndrome, characterized by salt retention, hypertension, hyperkalemia and metabolic acidosis. In FHH, mutations of WNK (with-no-lysine kinase) 4 and 1 alter the activity of several ion transportation systems in the distal nephron. The study of the pathophysiology of PHA and FHH greatly elucidated our understanding of the renin-angiotensin-aldosterone system function and ion handling in the distal nephron. The physiological role of the distal nephron and the pathophysiology of diseases in which the renal tubule is implicated may hence be better understood and, based on this understanding, new drugs can be developed.

Independent regulation of Na+ and K+ balance by the kidney

The understanding of the independent regulation of sodium and potassium by the kidney has remained elusive. Recent evidence now points to dissimilar regulatory mechanisms in ion handling, dependent on the presence of either aldosterone alone or angiotensin II with aldosterone among other factors. This review summarizes past and present information in an attempt to reconcile the current concepts of differential regulation of sodium and potassium balance through the with-no-lysine (K) kinase (WNK) system and the previous knowledge regarding ion transport mechanisms in the distal nephron.

Aldosterone paradox: differential regulation of ion transport in distal nephron

The mechanisms through which aldosterone promotes apparently opposite effects like salt reabsorption and K(+) secretion remain poorly understood. The identification, localization, and physiological analysis of ion transport systems in distal nephron have revealed an intricate network of interactions between several players, revealing the complex mechanism behind the aldosterone paradox. We review the mechanisms involved in differential regulation of ion transport that allow the fine tuning of salt and K(+) balance.

Mechanisms underlying rapid aldosterone effects in the kidney

The steroid hormone aldosterone is a key regulator of electrolyte transport in the kidney and contributes to both homeostatic whole-body electrolyte balance and the development of renal and cardiovascular pathologies. Aldosterone exerts its action principally through the mineralocorticoid receptor (MR), which acts as a ligand-dependent transcription factor in target tissues. Aldosterone also stimulates the activation of protein kinases and secondary messenger signaling cascades that act independently on specific molecular targets in the cell membrane and also modulate the transcriptional action of aldosterone through MR. This review describes current knowledge regarding the mechanisms and targets of rapid aldosterone action in the nephron and how aldosterone integrates these responses into the regulation of renal physiology.

The kidneys and aldosterone/mineralocorticoid receptor system in salt-sensitive hypertension

Strong evidence supports the ability of the aldosterone/mineralocorticoid receptor (MR) system to dominate long-term blood pressure control. It is also increasingly recognized as an important mediator of cardiovascular and renal diseases, particularly in the presence of excessive salt intake. In a subgroup of individuals with metabolic syndrome, adipocyte-derived aldosterone-releasing factors cause inappropriate secretion of aldosterone in the adrenal glands during salt loading, resulting in the development of salt-induced hypertension and cardiac and renal damage. On the other hand, emerging data reveal that aldosterone is not a sole regulator of MR activity. We have identified the signaling crosstalk between MR and small GTPase Rac1 as a novel pathway to facilitate MR signaling. Such a local control system for MR can also be relevant to the pathogenesis of salt-sensitive hypertension, and future studies will clarify the detailed mechanism for the intricate regulation of the aldosterone/MR cascade.

Association of WNK1 exon 1 polymorphisms with essential hypertension in Hani and Yi minorities of China

The association of polymorphisms in exon 1 of the WNK1 gene with essential hypertension in the minority groups of Hani and Yi of China was investigated in the case-control study. The sequence of 1257bp containing the WNK1 gene exon 1 was determined in 1307 individuals (649 essential hypertension subjects and 658 controls) to identify SNPs in Hani and Yi minority groups. Four of eleven previously known SNPs (rs3168640, rs11885, rs11554421 and rs34880640) were identified. The SNP analysis indicated that SNPs rs11885 and rs11554421 were significantly associated with hypertension in both Hani and Yi populations, and rs34880640 was significantly associated with hypertension in Hani but not in Yi population, adjusted for covariates. Haplotype analysis indicated that the haplotype H1 significantly decreased the risk of hypertension in both populations. These results suggested that WNK1 polymorphisms were involved in the predisposition of essential hypertension in Hani and Yi populations and its effects showed a clear population specificity. This finding supported the importance of population specificity in determining the genetic factors associated with diseases and thus disease treatment.

The WNK kinase network regulating sodium, potassium, and blood pressure

The relationship between renal salt handling and hypertension is intertwined historically. The discovery of WNK kinases (With No lysine = K) now offers new insight to this relationship because WNKs are a crucial molecular pathway connecting hormones such as angiotensin II and aldosterone to renal sodium and potassium transport. To fulfill this task, the WNKs also interact with other important kinases, including serum and glucocorticoid-regulated kinase 1, STE20/SPS1-related, proline alanine-rich kinase, and oxidative stress responsive protein type 1. Collectively, this kinase network regulates the activity of the major sodium and potassium transporters in the distal nephron, including thiazide-sensitive Na-Cl cotransporters and ROMK channels. Here we show how the WNKs modulate ion transport through two distinct regulatory pathways, trafficking and phosphorylation, and discuss the physiologic and clinical relevance of the WNKs in the kidney. This ranges from rare mutations in WNKs causing familial hyperkalemic hypertension to acquired forms of hypertension caused by salt sensitivity or diabetes mellitus. Although many questions remain unanswered, the WNKs hold promise for unraveling the link between salt and hypertension, potentially leading to more effective interventions to prevent cardiorenal damage.

The WNKs: atypical protein kinases with pleiotropic actions

WNKs are serine/threonine kinases that comprise a unique branch of the kinome. They are so-named owing to the unusual placement of an essential catalytic lysine. WNKs have now been identified in diverse organisms. In humans and other mammals, four genes encode WNKs. WNKs are widely expressed at the message level, although data on protein expression is more limited. Soon after the WNKs were identified, mutations in genes encoding WNK1 and -4 were determined to cause the human disease familial hyperkalemic hypertension (also known as pseudohypoaldosteronism II, or Gordon's Syndrome). For this reason, a major focus of investigation has been to dissect the role of WNK kinases in renal regulation of ion transport. More recently, a different mutation in WNK1 was identified as the cause of hereditary sensory and autonomic neuropathy type II, an early-onset autosomal disease of peripheral sensory nerves. Thus the WNKs represent an important family of potential targets for the treatment of human disease, and further elucidation of their physiological actions outside of the kidney and brain is necessary. In this review, we describe the gene structure and mechanisms regulating expression and activity of the WNKs. Subsequently, we outline substrates and targets of WNKs as well as effects of WNKs on cellular physiology, both in the kidney and elsewhere. Next, consequences of these effects on integrated physiological function are outlined. Finally, we discuss the known and putative pathophysiological relevance of the WNKs.

Activation of PI3-kinase stimulates endocytosis of ROMK via Akt1/SGK1-dependent phosphorylation of WNK1

WNK kinases stimulate endocytosis of ROMK channels to regulate renal K+ handling. Phosphatidylinositol 3-kinase (PI3K)-activating hormones, such as insulin and IGF 1, phosphorylate WNK1, but how this affects the regulation of ROMK abundance is unknown. Here, serum starvation of ROMK-transfected HEK cells led to an increase of ROMK current density; subsequent addition of insulin or IGF1 inhibited ROMK currents in a PI3K-dependent manner. Serum and insulin also increased phosphorylation of the downstream kinases Akt1 and SGK1 as well as WNK1. A biotinylation assay suggested that insulin and IGF1 inhibit ROMK by enhancing its endocytosis, a process that WNK1 may mediate. Knockdown of WNK1 with siRNA or expression of a phospho-deficient WNK1 mutant (T58A) both prevented insulin-induced inhibition of ROMK currents, suggesting that phosphorylation at Threonine-58 of WNK1 is important to mediate the inhibition of ROMK by PI3K-activating hormones or growth factors. In vitro and in vivo kinase assays supported the notion that Akt1 and SGK1 can phosphorylate WNK1 at this site, and we established that Akt1 and SGK1 synergistically inhibit ROMK through WNK1. We used dominant-negative intersectin and dynamin constructs to show that SGK1-mediated phosphorylation of WNK1 inhibits ROMK by promoting its endocytosis. Taken together, these results suggest that PI3K-activating hormones inhibit ROMK by enhancing its endocytosis via a mechanism that involves phosphorylation of WNK1 by Akt1 and SGK1.

Downregulation of NCC and NKCC2 cotransporters by kidney-specific WNK1 revealed by gene disruption and transgenic mouse models

WNK1 (with-no-lysine[K]-1) is a protein kinase of which mutations cause a familial hypertension and hyperkalemia syndrome known as pseudohypoaldosteronism type 2 (PHA2). Kidney-specific (KS) WNK1 is an alternatively spliced form of WNK1 kinase missing most of the kinase domain. KS-WNK1 downregulates the Na(+)-Cl(-) cotransporter NCC by antagonizing the effect of full-length WNK1 when expressed in Xenopus oocytes. The physiological role of KS-WNK1 in the regulation of NCC and potentially other Na(+) transporters in vivo is unknown. Here, we report that mice overexpressing KS-WNK1 in the kidney exhibited renal Na(+) wasting, elevated plasma levels of angiotensin II and aldosterone yet lower blood pressure relative to wild-type littermates. Immunofluorescent staining revealed reduced surface expression of total and phosphorylated NCC and the Na(+)-K(+)-2Cl(-) cotransporter NKCC2 in the distal convoluted tubule and the thick ascending limb of Henle's loop, respectively. Conversely, mice with targeted deletion of exon 4A (the first exon for KS-WNK1) exhibited Na(+) retention, elevated blood pressure on a high-Na(+) diet and increased surface expression of total and phosphorylated NCC and NKCC2 in respective nephron segments. Thus, KS-WNK1 is a negative regulator of NCC and NKCC2 in vivo and plays an important role in the control of Na(+) homeostasis and blood pressure. These results have important implications to the pathogenesis of PHA2 with WNK1 mutations.

Mechanisms of type I and type II pseudohypoaldosteronism

Pseudohypoaldosteronism (PHA) types I and II are curious genetic disorders that share hyperkalemia as a predominant finding. Together they have become windows to understanding new molecular physiology in the kidney. Autosomal recessive PHAI results from mutations in the epithelial sodium channel (ENaC), whereas autosomal dominant PHAI is characterized by mutations in the mineralocorticoid receptor. PHAII is the result of mutations in a family of serine-threonine kinases called with-no-lysine kinases (WNK)1 and WNK4. WNK4 negatively regulates the NaCl cotransporter (NCC), and PHAII mutations in WNK4 abrogate this affect. WNK4 also regulates the expression or function of renal outer medullary potassium (ROMK) channels, ENaCs, and Cl transporters. WNK1 also regulates NCC and ROMK. Aldosterone inactivates WNK1 and WNK4 activity. Whether angiotensin II can fine tune the actions of aldosterone is still unclear.