Unique chloride-sensing properties of WNK4 permit the distal nephron to modulate potassium homeostasis

Clinical importance of potassium intake and molecular mechanism of potassium regulation

Introduction

Potassium (K⁺) intake is intrinsically linked to blood pressure. High-K⁺ intake decreases hypertension and associated lower mortality. On the other hand, hyperkalemia causes sudden death with fatal cardiac arrhythmia and is also related to higher mortality. Renal sodium (Na⁺)–chloride (Cl^‒) cotransporter (NCC), expressed in the distal convoluted tubule, is a key molecule in regulating urinary K⁺ excretion. K⁺ intake affects the activity of the NCC, which is related to salt-sensitive hypertension. A K⁺-restrictive diet activates NCC, and K⁺ loading suppresses NCC. Hyperpolarization caused by decreased extracellular K⁺ concentration ([K⁺]_ex) increases K⁺ and Cl^‒ efflux, leading to the activation of Cl^‒-sensitive with-no-lysine (WNK) kinases and their downstream molecules, including STE20/SPS1-related proline/alanine-rich kinase (SPAK) and NCC.

Results

We investigated the role of the ClC-K2 Cl^‒ channel and its β-subunit, barttin, using barttin hypomorphic (Bsnd^neo/neo) mice and found that these mice did not show low-K⁺-induced NCC activation and salt-sensitive hypertension. Additionally, we discovered that the suppression of NCC by K⁺ loading was regulated by another mechanism, whereby tacrolimus (a calcineurin [CaN] inhibitor) inhibited high-K⁺-induced NCC dephosphorylation and urinary K⁺ excretion. The K⁺ loading and the tacrolimus treatment did not alter the expression of WNK4 and SPAK. The depolarization induced by increased [K⁺]_ex activated CaN, which dephosphorylates NCC.

Conclusions

We concluded that there were two independent molecular mechanisms controlling NCC activation and K⁺ excretion. This review summarizes the clinical importance of K⁺ intake and explains how NCC phosphorylation is regulated by different molecular mechanisms between the low- and the high-K⁺ condition.

Electronic supplementary material

The online version of this article (10.1007/s10157-019-01766-x) contains supplementary material, which is available to authorized users.

The WNK-SPAK/OSR1 Kinases and the Cation-Chloride Cotransporters as Therapeutic Targets for Neurological Diseases

In recent years, cation-chloride cotransporters (CCCs) have drawn attention in the medical neuroscience research. CCCs include the family of Na⁺-coupled Cl^- importers (NCC, NKCC1, and NKCC2), K⁺-coupled Cl^- exporters (KCCs), and possibly polyamine transporters (CCC9) and CCC interacting protein (CIP1). For decades, CCCs have been the targets of several commonly used diuretic drugs, including hydrochlorothiazide, furosemide, and bumetanide. Genetic mutations of NCC and NKCC2 cause congenital renal tubular disorders and lead to renal salt-losing hypotension, secondary hyperreninemia, and hypokalemic metabolic alkalosis. New studies reveal that CCCs along with their regulatory WNK (Kinase with no lysine (K)), and SPAK (Ste20-related proline-alanine-rich kinase)/OSR1(oxidative stress-responsive kinase-1) are essential for regulating cell volume and maintaining ionic homeostasis in the nervous system, especially roles of the WNK-SPAK-NKCC1 signaling pathway in ischemic brain injury and hypersecretion of cerebrospinal fluid in post-hemorrhagic hydrocephalus. In addition, disruption of Cl^- exporter KCC2 has an effect on synaptic inhibition, which may be involved in developing pain, epilepsy, and possibly some neuropsychiatric disorders. Interference with KCC3 leads to peripheral nervous system neuropathy as well as axon and nerve fiber swelling and psychosis. The WNK-SPAK/OSR1-CCCs complex emerges as therapeutic targets for multiple neurological diseases. This review will highlight these new findings.

CHOLESTEROL AND CHLORIDE IN ACUTE HEART FAILURE

Detecting predictors of poor outcome is crucial for understanding the underlying pathophysiology of heart failure (HF) and thus creating new therapeutic concepts. It is well established that low serum lipid levels are associated with unfavorable outcomes in HF patients. Several studies examined the association between serum lipids and established predictors of mortality in HF patients. The aim of the present study was to examine the association of serum lipid and chloride concentrations, as well as their impact on survival in acute heart failure (AHF). The present study was performed as a prospective, single-centre, observational research. The study included 152 patients with AHF. Spearman's correlation coefficient revealed a significant positive correlation of serum chloride levels with serum levels of total cholesterol (ρ 0.221, p=0.006), low-density lipoprotein cholesterol (LDL-c) (ρ 0.187, p=0.015) and high-density lipoprotein-cholesterol (HDL-c) (ρ 0.169, p=0.038). Binary logistic regression revealed a significant association of chloride, total cholesterol and LDL-c serum levels measured at admission with hospital survival (OR 1.077, CI 1.01-1.154, p=0.034), (OR 1.731, CI 1.090-2.748, p=0.020) and (OR 1.839, CI 1.033-3.274, p=0.038), respectively, as well as with 3-month survival (OR 1.065, CI 1.002-1.131, p=0.042), (OR 1.625, CI 1.147-2.303, p=0.006) and (OR 1.711, CI 1.117-2.622, p=0.014), respectively. In conclusion, positive statistical association between serum cholesterol (total cholesterol, LDL-c and HDL-c) and chloride levels may suggest their similar modulation by AHF pathophysiology. Serum levels of total cholesterol, LDL-c and chloride contribute to patient survival.

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

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

Ciliary beating amplitude controlled by intracellular Cl- and a high rate of CO2 production in ciliated human nasal epithelial cells

The ciliary transport is controlled by two parameters of the ciliary beating, frequency (CBF) and amplitude. In this study, we developed a novel method to measure both CBF and ciliary bend distance (CBD, an index of ciliary beating amplitude) in ciliated human nasal epithelial cells (cHNECs) in primary culture, which are prepared from patients contracting allergic rhinitis and chronic sinusitis. An application of Cl^--free NO₃^- solution or bumetanide (an inhibitor of Na⁺/K⁺/2Cl^- cotransport), which decreases intracellular Cl^- concentration ([Cl^-]_i), increased CBD, not CBF, at 37 °C; however, it increased both CBD and CBF at 25 °C. Conversely, addition of Cl^- channel blockers (5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) and 4-[[4-Oxo-2-thioxo-3-[3-trifluoromethyl]phenyl]-5-thiazolidinylidene]methyl] benzoic acid (CFTR(inh)-172)), which increase [Cl^-]_i, decreased both CBD and CBF, suggesting that CFTR plays a crucial role for maintaining [Cl^-]_i in these cells. We speculate that Cl^- modulates activities of the molecular motors regulating both CBD and CBF in cHNECs. Moreover, application of the CO₂/HCO₃^--free solution did not change intracellular pH (pH_i), and addition of an inhibitor of carbonic anhydrase (acetazolamide) sustained pH_i increase induced by the NH₄⁺ pulse, which transiently increased pH_i in the absence of acetazolamide. These results indicate that the cHNEC produces a large amount of CO₂, which maintains a constant pH_i even under the CO₂/HCO₃^--free condition.

Potassium acts through mTOR to regulate its own secretion

Potassium (K+) secretion by kidney tubule cells is central to electrolyte homeostasis in mammals. In the K+ secretory "principal" cells of the distal nephron, electrogenic Na+ transport by the epithelial sodium channel (ENaC) generates the electrical driving force for K+ transport across the apical membrane. Regulation of this process is attributable in part to aldosterone, which stimulates the gene transcription of the ENaC-regulatory kinase, SGK1. However, a wide range of evidence supports the conclusion that an unidentified aldosterone-independent pathway exists. We show here that in principal cells, K+ itself acts through the type 2 mTOR complex (mTORC2) to activate SGK1, which stimulates ENaC to enhance K+ excretion. The effect depends on changes in K+ concentration on the blood side of the cells, and requires basolateral membrane K+-channel activity. However, it does not depend on changes in aldosterone, or on enhanced distal delivery of Na+ from upstream nephron segments. These data strongly support the idea that K+ is sensed directly by principal cells to stimulate its own secretion by activating the mTORC2-SGK1 signaling module, and stimulate ENaC. We propose that this local effect acts in concert with aldosterone and increased Na+ delivery from upstream nephron segments to sustain K+ homeostasis.

Kir4.1/Kir5.1 in the DCT plays a role in the regulation of renal K+ excretion

The aim of this mini review is to provide an overview regarding the role of inwardly rectifying potassium channel 4.1 (Kir4.1)/Kir5.1 in regulating renal K+ excretion. Deletion of Kir4.1 in the kidney inhibited thiazide-sensitive NaCl cotransporter (NCC) activity in the distal convoluted tubule (DCT) and slightly suppressed Na-K-2Cl cotransporter (NKCC2) function in the thick ascending limb (TAL). Moreover, increased dietary K+ intake inhibited, whereas decreased dietary K+ intake stimulated, the basolateral potassium channel (a Kir4.1/Kir5.1 heterotetramer) in the DCT. The alteration of basolateral potassium conductance is essential for the effect of dietary K+ intake on NCC because deletion of Kir4.1 in the DCT abolished the effect of dietary K+ intake on NCC. Since potassium intake-mediated regulation of NCC plays a key role in regulating renal K+ excretion and potassium homeostasis, the deletion of Kir4.1 caused severe hypokalemia and metabolic alkalosis under control conditions and even during increased dietary K+ intake. Finally, recent studies have suggested that the angiotensin II type 2 receptor (AT2R) and bradykinin-B2 receptor (BK2R) are involved in mediating the effect of high dietary K+ intake on Kir4.1/Kir5.1 in the DCT.

Reciprocal Regulation of KCC2 Trafficking and Synaptic Activity

The main inhibitory neurotransmitter receptors in the adult central nervous system (CNS) are type A γ-aminobutyric acid receptors (GABA_ARs) and glycine receptors (GlyRs). Synaptic responses mediated by GlyR and GABA_AR display a hyperpolarizing shift during development. This shift relies mainly on the developmental up-regulation of the K⁺-Cl^- co-transporter KCC2 responsible for the extrusion of Cl^-. In mature neurons, altered KCC2 function-mainly through increased endocytosis-leads to the re-emergence of depolarizing GABAergic and glycinergic signaling, which promotes hyperexcitability and pathological activities. Identifying signaling pathways and molecular partners that control KCC2 surface stability thus represents a key step in the development of novel therapeutic strategies. Here, we present our current knowledge on the cellular and molecular mechanisms governing the plasma membrane turnover rate of the transporter under resting conditions and in response to synaptic activity. We also discuss the notion that KCC2 lateral diffusion is one of the first parameters modulating the transporter membrane stability, allowing for rapid adaptation of Cl^- transport to changes in neuronal activity.

WNK4 kinase is a physiological intracellular chloride sensor

With-no-lysine (WNK) kinases regulate renal sodium-chloride cotransporter (NCC) to maintain body sodium and potassium homeostasis. Gain-of-function mutations of WNK1 and WNK4 in humans lead to a Mendelian hypertensive and hyperkalemic disease pseudohypoaldosteronism type II (PHAII). X-ray crystal structure and in vitro studies reveal chloride ion (Cl^-) binds to a hydrophobic pocket within the kinase domain of WNKs to inhibit its activity. The mechanism is thought to be important for physiological regulation of NCC by extracellular potassium. To test the hypothesis that WNK4 senses the intracellular concentration of Cl^- physiologically, we generated knockin mice carrying Cl^--insensitive mutant WNK4. These mice displayed hypertension, hyperkalemia, hyperactive NCC, and other features fully recapitulating human and mouse models of PHAII caused by gain-of-function WNK4. Lowering plasma potassium levels by dietary potassium restriction increased NCC activity in wild-type, but not in knockin, mice. NCC activity in knockin mice can be further enhanced by the administration of norepinephrine, a known activator of NCC. Raising plasma potassium by oral gavage of potassium inactivated NCC within 1 hour in wild-type mice, but had no effect in knockin mice. The results provide compelling support for the notion that WNK4 is a bona fide physiological intracellular Cl^- sensor and that Cl^- regulation of WNK4 underlies the mechanism of regulation of NCC by extracellular potassium.

Furosemide reduces BK-αβ4-mediated K+ secretion in mice on an alkaline high-K+ diet

Special high-K diets have cardioprotective effects and are often warranted in conjunction with diuretics such as furosemide for treating hypertension. However, it is not understood how a high-K diet (HK) influences the actions of diuretics on renal K⁺ handling. Furosemide acidifies the urine by increasing acid secretion via the Na⁺-H⁺ exchanger 3 (NHE3) in TAL and vacuolar H⁺-ATPase (V-ATPase) in the distal nephron. We previously found that an alkaline urine is required for large conductance Ca²⁺-activated K⁺ (BK)-αβ4-mediated K⁺ secretion in mice on HK. We therefore hypothesized that furosemide could reduce BK-αβ4-mediated K⁺ secretion by acidifying the urine. Treating with furosemide (drinking water) for 11 days led to decreased urine pH in both wild-type (WT) and BK-β4-knockout mice (BK-β4-KO) with increased V-ATPase expression and elevated plasma aldosterone levels. However, furosemide decreased renal K⁺ clearance and elevated plasma [K⁺] in WT but not BK-β4-KO. Western blotting and immunofluorescence staining showed that furosemide treatment decreased cortical expression of BK-β4 and reduced apical localization of BK-α in connecting tubules. Addition of the carbonic anhydrase inhibitor, acetazolamide, to furosemide water restored urine pH along with renal K⁺ clearance and plasma [K⁺] to control levels. Acetazolamide plus furosemide also restored the cortical expression of BK-β4 and BK-α in connecting tubules. These results indicate that in mice adapted to HK, furosemide reduces BK-αβ4-mediated K⁺ secretion by acidifying the urine.

Nephrotic Syndrome: Oedema Formation and Its Treatment With Diuretics

Oedema is a defining element of the nephrotic syndrome. Its' management varies considerably between clinicians, with no national or international clinical guidelines, and hence variable outcomes. Oedema may have serious sequelae such as immobility, skin breakdown and local or systemic infection. Treatment of nephrotic oedema is often of limited efficacy, with frequent side-effects and interactions with other pharmacotherapy. Here, we describe the current paradigms of oedema in nephrosis, including insights into emerging mechanisms such as the role of the abnormal activation of the epithelial sodium channel in the collecting duct. We then discuss the physiological basis for traditional and novel therapies for the treatment of nephrotic oedema. Despite being the cardinal symptom of nephrosis, few clinical studies guide clinicians to the rational use of therapy. This is reflected in the scarcity of publications in this field; it is time to undertake new clinical trials to direct clinical practice.

"I don't get no respect": the role of chloride in acute kidney injury

Acute kidney injury (AKI) is a major public health problem that complicates 10-40% of hospital admissions. Importantly, AKI is independently associated with increased risk of progression to chronic kidney disease, end-stage renal disease, cardiovascular events, and increased risk of in-hospital and long-term mortality. The chloride content of intravenous fluid has garnered much attention over the last decade, as well as its association with excess use and adverse outcomes, including AKI. Numerous studies show that changes in serum chloride concentration, independent of serum sodium and bicarbonate, are associated with increased risk of AKI, morbidity, and mortality. This comprehensive review details the complex renal physiology regarding the role of chloride in regulating renal blood flow, glomerular filtration rate, tubuloglomerular feedback, and tubular injury, as well as the findings of clinical research related to the chloride content of intravenous fluids, changes in serum chloride concentration, and AKI. Chloride is underappreciated in both physiology and pathophysiology. Although the exact mechanism is debated, avoidance of excessive chloride administration is a reasonable treatment option for all patients and especially in those at risk for AKI. Therefore, high-risk patients and those with "incipient" AKI should receive balanced solutions rather than normal saline to minimize the risk of AKI.

Renal potassium physiology: integration of the renal response to dietary potassium depletion

We summarize the current understanding of the physiology of the renal handling of potassium (K⁺), and present an integrative view of the renal response to K⁺ depletion caused by dietary K⁺ restriction. This renal response involves contributions from different nephron segments, and aims to diminish the rate of excretion of K⁺ as a result of: decreasing the rate of electrogenic (and increasing the rate of electroneutral) reabsorption of sodium in the aldosterone-sensitive distal nephron (ASDN), decreasing the abundance of renal outer medullary K⁺ channels in the luminal membrane of principal cells in the ASDN, decreasing the flow rate in the ASDN, and increasing the reabsorption of K⁺ in the cortical and medullary collecting ducts. The implications of this physiology for the association between K⁺ depletion and hypertension, and K⁺ depletion and formation of calcium kidney stones are discussed.

Modulation of Cl- signaling and ion transport by recruitment of kinases and phosphatases mediated by the regulatory protein IRBIT

IRBIT is a multifunctional protein that controls the activity of various epithelial ion transporters including NBCe1-B. Interaction with IRBIT increases NBCe1-B activity and exposes two cryptic Cl^--sensing GXXXP sites that enable regulation of NBCe1-B by intracellular Cl^- (Cl^- _in). Here, phosphoproteomic analysis revealed that IRBIT controlled five phosphorylation sites in NBCe1-B that determined both the active conformation of the transporter and its regulation by Cl^- _in Mutational analysis suggested that the phosphorylation status of Ser²³², Ser²³³, and Ser²³⁵ was regulated by IRBIT and determined whether NBCe1 transporters are in active or inactive conformations. The absence of phosphorylation at Ser²³², Ser²³³, or Ser²³⁵ produced NBCe1-B in the conformations pSer²³³/pSer²³⁵, pSer²³²/pSer²³⁵, or pSer²³²/pSer²³³, respectively. The activity of the pSer²³³/pSer²³⁵ form was similar to that of IRBIT-activated NBCe1-B, but it was insensitive to inhibition by Cl^- _in The properties of the pSer²³²/pSer²³⁵ form were similar to those of wild-type NBCe1-B, whereas the pSer²³²/pSer²³³ form was partially active, further activated by IRBIT, but retained inhibition by Cl^- _in Furthermore, IRBIT recruited the phosphatase PP1 and the kinase SPAK to control phosphorylation of Ser⁶⁵, which affected Cl^- _in sensing by the ³²GXXXP³⁶ motif. IRBIT also recruited the phosphatase calcineurin and the kinase CaMKII to control phosphorylation of Ser¹², which affected Cl^- _in sensing by the ¹⁹⁴GXXXP¹⁹⁸ motif. Ser²³², Ser²³³, and Ser²³⁵ are conserved in all NBCe1 variants and affect their activity. These findings reveal how multiple kinase and phosphatase pathways use phosphorylation sites to fine-tune a transporter, which have important implications for epithelial fluid and HCO₃ ^- secretion.

Nudge-nudge, WNK-WNK (kinases), say no more?

Contents Summary 35 I Overview of animal and plant WNK kinases 35 II. Structure: domains and topology 36 III. Phylogeny-evolutionary relationships 41 IV. Plant WNK kinase distribution and regulation of WNK expression and activity 41 V. Functions of WNK family members in physiology and development 41 VI. Say no more? Still many questions to be answered 45 Acknowledgements 46 References 46 SUMMARY: WITH NO LYSINE (WNK) kinases are serine/threonine kinases uniquely characterized by an anomalous placement of a catalytic lysine, hence their moniker. In animals, WNK protein kinases play critical roles in protein trafficking of components that mediate renal ion transport processes and regulate osmoregulation of cell volume. In plants, the WNK kinase gene family is larger and more diverse. Recent studies revealed WNK kinase roles in orchestrating the trafficking of an ion channel, a lipid kinase complex in animals, and a heterotrimeric G protein signaling component in plants that is necessary for signal transduction. For this reason, new attention is geared toward investigating the mechanisms adopted by WNK kinases to nudge intracellular proteins to their destinations. In this review, the functions of WNK kinases in protein trafficking are derived from what we have learned from the model organism Arabidopsis thaliana. To place this new idea in context, we provide the predicted WNK kinase structures, their predicted expression patterns, a speculated evolutionary pathway, and the regulatory roles of plant WNKs in transport processes and other physiologies. We brazenly predict that the WNK kinases in both plants and animals will soon be recognized as a nexus for trafficking-based signal transduction.

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

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

Diuretics in the Management of Cardiorenal Syndrome

The leading cause of death worldwide is cardiovascular disease. The heart and the kidneys are functionally interdependent, such that dysfunction in one organ may cause dysfunction in the other. By one estimate, more than 60% of patients with congestive heart failure develop chronic kidney disease. Volume overload and congestion are hallmarks of heart failure, and these findings are associated with severe symptoms and poor outcomes. Given the importance of congestion, diuretics remain a cornerstone of heart failure management. However, diuretic treatment remains largely empirical, with little evidence currently available to guide decisions. In this review, we discuss the pathophysiology of cardiorenal syndrome, the pharmacology of loop diuretics, mechanisms of diuretic resistance, and evidence-based treatment paradigms.

Role of WNK Kinases in the Modulation of Cell Volume

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

Cell Volume Control in Healthy Brain and Neuropathologies

Regulation of cellular volume is a critical homeostatic process that is intimately linked to ionic and osmotic balance in the brain tissue. Because the brain is encased in the rigid skull and has a very complex cellular architecture, even minute changes in the volume of extracellular and intracellular compartments have a very strong impact on tissue excitability and function. The failure of cell volume control is a major feature of several neuropathologies, such as hyponatremia, stroke, epilepsy, hyperammonemia, and others. There is strong evidence that such dysregulation, especially uncontrolled cell swelling, plays a major role in adverse pathological outcomes. To protect themselves, brain cells utilize a variety of mechanisms to maintain their optimal volume, primarily by releasing or taking in ions and small organic molecules through diverse volume-sensitive ion channels and transporters. In principle, the mechanisms of cell volume regulation are not unique to the brain and share many commonalities with other tissues. However, because ions and some organic osmolytes (e.g., major amino acid neurotransmitters) have a strong impact on neuronal excitability, cell volume regulation in the brain is a surprisingly treacherous process, which may cause more harm than good. This topical review covers the established and emerging information in this rapidly developing area of physiology.

Potassium conservation is impaired in mice with reduced renal expression of Kir4.1

To better understand the role of the inward-rectifying K channel Kir4.1 (KCNJ10) in the distal nephron, we initially studied a global Kir4.1 knockout mouse (gKO), which demonstrated the hypokalemia and hypomagnesemia seen in SeSAME/EAST syndrome and was associated with reduced Na/Cl cotransporter (NCC) expression. Lethality by ~3 wk, however, limits the usefulness of this model, so we developed a kidney-specific Kir4.1 "knockdown" mouse (ksKD) using a cadherin 16 promoter and Cre-loxP methodology. These mice appeared normal and survived to adulthood. Kir4.1 protein expression was decreased ~50% vs. wild-type (WT) mice by immunoblotting, and immunofluorescence showed moderately reduced Kir4.1 staining in distal convoluted tubule that was minimal or absent in connecting tubule and cortical collecting duct. Under control conditions, the ksKD mice showed metabolic alkalosis and relative hypercalcemia but were normokalemic and mildly hypermagnesemic despite decreased NCC expression. In addition, the mice had a severe urinary concentrating defect associated with hypernatremia, enlarged kidneys with tubulocystic dilations, and reduced aquaporin-3 expression. On a K/Mg-free diet for 1 wk, however, ksKD mice showed marked hypokalemia (serum K: 1.5 ± 0.1 vs. 3.0 ± 0.1 mEq/l for WT), which was associated with renal K wasting (transtubular K gradient: 11.4 ± 0.8 vs. 1.6 ± 0.4 in WT). Phosphorylated-NCC expression increased in WT but not ksKD mice on the K/Mg-free diet, suggesting that loss of NCC adaptation underlies the hypokalemia. In conclusion, even modest reduction in Kir4.1 expression results in impaired K conservation, which appears to be mediated by reduced expression of activated NCC.

Regulation of the renal NaCl cotransporter by the WNK/SPAK pathway: lessons learned from genetically altered animals

The renal thiazide-sensitive NaCl cotransporter (NCC) is the major salt transport pathway in the distal convoluted tubule of the mammalian nephron. NCC activity is critical for modulation of arterial blood pressure and serum potassium levels. Reduced activity of NCC in genetic diseases results in arterial hypotension and hypokalemia, while increased activity results in genetic diseases featuring hypertension and hyperkalemia. Several hormones and physiological conditions modulate NCC activity through a final intracellular complex pathway involving kinases and ubiquitin ligases. A substantial amount of work has been conducted to understand this pathway in the last 15 yr, but advances over the last 3 yr have helped to begin to understand how these regulatory proteins interact with each other and modulate the activity of this important cotransporter. In this review, we present the current model of NCC regulation by the Cullin 3 protein/Kelch-like 3 protein/with no lysine kinase/STE20-serine-proline alanine-rich kinase (CUL3/KELCH3-WNK-SPAK) pathway. We present a review of all genetically altered mice that have been used to translate most of the proposals made from in vitro experiments into in vivo observations that have helped to elucidate the model at the physiological level. Many questions have been resolved, but some others will require further models to be constructed. In addition, unexpected observations in mice have raised new questions and identified regulatory pathways that were previously unknown.

CLC Chloride Channels and Transporters: Structure, Function, Physiology, and Disease

CLC anion transporters are found in all phyla and form a gene family of eight members in mammals. Two CLC proteins, each of which completely contains an ion translocation parthway, assemble to homo- or heteromeric dimers that sometimes require accessory β-subunits for function. CLC proteins come in two flavors: anion channels and anion/proton exchangers. Structures of these two CLC protein classes are surprisingly similar. Extensive structure-function analysis identified residues involved in ion permeation, anion-proton coupling and gating and led to attractive biophysical models. In mammals, ClC-1, -2, -Ka/-Kb are plasma membrane Cl−channels, whereas ClC-3 through ClC-7 are 2Cl−/H+-exchangers in endolysosomal membranes. Biological roles of CLCs were mostly studied in mammals, but also in plants and model organisms like yeast and Caenorhabditis elegans. CLC Cl−channels have roles in the control of electrical excitability, extra- and intracellular ion homeostasis, and transepithelial transport, whereas anion/proton exchangers influence vesicular ion composition and impinge on endocytosis and lysosomal function. The surprisingly diverse roles of CLCs are highlighted by human and mouse disorders elicited by mutations in their genes. These pathologies include neurodegeneration, leukodystrophy, mental retardation, deafness, blindness, myotonia, hyperaldosteronism, renal salt loss, proteinuria, kidney stones, male infertility, and osteopetrosis. In this review, emphasis is laid on biophysical structure-function analysis and on the cell biological and organismal roles of mammalian CLCs and their role in disease.

Regulation of renal Na transporters in response to dietary K

Changes in the expression of Na transport proteins were measured in the kidneys of mice with increased dietary K intake for 1 wk. The epithelial Na channel (ENaC) was upregulated, with enhanced expression of full-length and cleaved forms of α-ENaC and cleaved γ-ENaC. At the same time, the amount of the NaCl cotransporter NCC and its phosphorylated form decreased by ~50% and ~80%, respectively. The expression of the phosphorylated form of the Na-K-2Cl cotransporter NKCC2 also decreased, despite an increase in overall protein content. The effect was stronger in males (80%) than in females (40%). This implies that less Na⁺ is reabsorbed in the thick ascending limb of Henle's loop and distal convoluted tubule along with Cl^-, whereas more is reabsorbed in the aldosterone-sensitive distal nephron in exchange for secreted K⁺. The abundance of the proximal tubule Na/H exchanger NHE3 decreased by ~40%, with similar effects in males and females. Time-course studies indicated that NCC and NHE3 proteins decreased progressively over 7 days on a high-K diet. Expression of mRNA encoding these proteins increased, implying that the decreased protein levels resulted from decreased rates of synthesis or increased rates of degradation. The potential importance of changes in NHE3, NKCC2, and NCC in promoting K⁺ excretion was assessed with a mathematical model. Simulations indicated that decreased NHE3 produced the largest effect. Regulation of proximal tubule Na⁺ transport may play a significant role in achieving K homeostasis.

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

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

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.

Differential roles of WNK4 in regulation of NCC in vivo

The Na+-Cl- cotransporter (NCC) in distal convoluted tubule (DCT) plays important roles in renal NaCl reabsorption. The current hypothesis for the mechanism of regulation of NCC focuses on WNK4 and intracellular Cl- concentration ([Cl-]i). WNK kinases bind Cl-, and Cl- binding decreases the catalytic activity. It is believed that hypokalemia under low K+ intake decreases [Cl-]i to activate WNK4, which thereby phosphorylates and stimulates NCC through activation of SPAK. However, increased NCC activity and apical NaCl entry would mitigate the fall in [Cl-]i. Whether [Cl-]i in DCT under low-K+ diet is sufficiently low to activate WNK4 is unknown. Furthermore, increased luminal NaCl delivery also stimulates NCC and causes upregulation of the transporter. Unlike low K+ intake, increased luminal NaCl delivery would tend to increase [Cl-]i. Thus we investigated the role of WNK4 and [Cl-]i in regulating NCC. We generated Wnk4-knockout mice and examined regulation of NCC by low K+ intake and by increased luminal NaCl delivery in knockout (KO) and wild-type mice. Wnk4-KO mice have marked reduction in the abundance, phosphorylation, and functional activity of NCC vs. wild type. Low K+ intake increases NCC phosphorylation and functional activity in wild-type mice, but not in Wnk4-KO mice. Increased luminal NaCl delivery similarly upregulates NCC, which, contrary to low K+ intake, is not abolished in Wnk4-KO mice. The results reveal that modulation of WNK4 activity by [Cl-]i is not the sole mechanism for regulating NCC. Increased luminal NaCl delivery upregulates NCC via yet unknown mechanism(s) that may override inhibition of WNK4 by high [Cl-]i.

Role of WNK4 and kidney-specific WNK1 in mediating the effect of high dietary K+ intake on ROMK channel in the distal convoluted tubule

With-no-lysine kinase 4 (WNK4) and kidney-specific (KS)-WNK1 regulate ROMK (Kir1.1) channels in a variety of cell models. We now explore the role of WNK4 and KS-WNK1 in regulating ROMK in the native distal convoluted tubule (DCT)/connecting tubule (CNT) by measuring tertiapin-Q (TPNQ; ROMK inhibitor)-sensitive K⁺ currents with whole cell recording. TPNQ-sensitive K⁺ currents in DCT2/CNT of KS- WNK1^-/- and WNK4^-/- mice were significantly smaller than that of WT mice. In contrast, the basolateral K⁺ channels (a Kir4.1/5.1 heterotetramer) in the DCT were not inhibited. Moreover, WNK4^-/- mice were hypokalemic, while KS- WNK1^-/- mice had normal plasma K⁺ levels. High K⁺ (HK) intake significantly increased TPNQ-sensitive K⁺ currents in DCT2/CNT of WT and WNK4^-/- mice but not in KS- WNK1^-/- mice. However, TPNQ-sensitive K⁺ currents in the cortical collecting duct (CCD) were normal not only under control conditions but also significantly increased in response to HK in KS- WNK1^-/- mice. This suggests that the deletion of KS-WNK1-induced inhibition of ROMK occurs only in the DCT2/CNT. Renal clearance study further demonstrated that the deletion of KS-WNK1 did not affect the renal ability of K⁺ excretion under control conditions and during increasing K⁺ intake. Also, HK intake did not cause hyperkalemia in KS- WNK1^-/- mice. We conclude that KS-WNK1 but not WNK4 is required for HK intake-induced stimulation of ROMK activity in DCT2/CNT. However, KS-WNK1 is not essential for HK-induced stimulation of ROMK in the CCD, and the lack of KS-WNK1 does not affect net renal K⁺ excretion.

Intracellular Chloride and Scaffold Protein Mo25 Cooperatively Regulate Transepithelial Ion Transport through WNK Signaling in the Malpighian Tubule

Background With No Lysine kinase (WNK) signaling regulates mammalian renal epithelial ion transport to maintain electrolyte and BP homeostasis. Our previous studies showed a conserved role for WNK in the regulation of transepithelial ion transport in the Drosophila Malpighian tubule.Methods Using in vitro assays and transgenic Drosophila lines, we examined two potential WNK regulators, chloride ion and the scaffold protein mouse protein 25 (Mo25), in the stimulation of transepithelial ion flux.ResultsIn vitro, autophosphorylation of purified Drosophila WNK decreased as chloride concentration increased. In conditions in which tubule intracellular chloride concentration decreased from 30 to 15 mM as measured using a transgenic sensor, Drosophila WNK activity acutely increased. Drosophila WNK activity in tubules also increased or decreased when bath potassium concentration decreased or increased, respectively. However, a mutation that reduces chloride sensitivity of Drosophila WNK failed to alter transepithelial ion transport in 30 mM chloride. We, therefore, examined a role for Mo25. In in vitro kinase assays, Drosophila Mo25 enhanced the activity of the Drosophila WNK downstream kinase Fray, the fly homolog of mammalian Ste20-related proline/alanine-rich kinase (SPAK), and oxidative stress-responsive 1 protein (OSR1). Knockdown of Drosophila Mo25 in the Malpighian tubule decreased transepithelial ion flux under stimulated but not basal conditions. Finally, whereas overexpression of wild-type Drosophila WNK, with or without Drosophila Mo25, did not affect transepithelial ion transport, Drosophila Mo25 overexpressed with chloride-insensitive Drosophila WNK increased ion flux.Conclusions Cooperative interactions between chloride and Mo25 regulate WNK signaling in a transporting renal epithelium.

Hyperkalemia in the Hypertensive Patient

PURPOSE OF REVIEW

Hyperkalemia develops in a patient with systemic arterial hypertension (HTN) if one or more risk factors are present, namely chronic kidney disease (CKD) (especially severe stage 4-5 CKD), diabetes mellitus (DM), heart failure (HF), or pharmacological therapies that interfere with potassium homeostasis, mainly through renin-angiotensin-aldosterone inhibition (RAASi). Hyperkalemia is a considerable reason of morbidity (emergency department (ED) visits and hospitalizations) and portends a higher mortality risk in patients at risk; for instance, hyperkalemia increases the risk of mortality within 1 day of a hyperkalemic event. This review aims to identify the risk factors for high-serum potassium, highlight the risk versus benefit of RAASi in certain patient populations, and outline preventive as well as therapeutic strategies for hyperkalemia.

RECENT FINDINGS

A growing body of evidence supports the safety and efficacy of cation-exchange resins, patiromer, or sodium zirconium cyclosilicate, in patients with a compelling indication for RAASi, yet in whom such therapy was complicated by hyperkalemia, allowing these patients to benefit from continued RAASi therapy. In summary, novel cation exchange polymers present the clinician with a new and safe strategy to address hyperkalemia in patients with a compelling indication for ongoing RAASi therapy instead of withdrawal of such therapy.

Salt-Losing Tubulopathies in Children: What's New, What's Controversial?

Renal tubulopathies provide insights into the inner workings of the kidney, yet also pose therapeutic challenges. Because of the central nature of sodium in tubular transport physiology, disorders of sodium handling may affect virtually all aspects of the homeostatic functions of the kidney. Yet, owing to the rarity of these disorders, little clinical evidence regarding treatment exists. Consequently, treatment can vary widely between individual physicians and centers and is based mainly on understanding of renal physiology, reported clinical observations, and individual experiences. Salt-losing tubulopathies can affect all tubular segments, from the proximal tubule to the collecting duct. But the more frequently observed disorders are Bartter and Gitelman syndrome, which affect salt transport in the thick ascending limb of Henle's loop and/or the distal convoluted tubule, and these disorders generate the greatest controversies regarding management. Here, we review clinical and molecular aspects of salt-losing tubulopathies and discuss novel insights provided mainly by genetic investigations and retrospective clinical reviews. Additionally, we discuss controversial topics in the management of these disorders to highlight areas of importance for future clinical trials. International collaboration will be required to perform clinical studies to inform the treatment of these rare disorders.

With no lysine kinase 4 modulates sodium potassium 2 chloride cotransporter activity in vivo

With no lysine kinase 4 (WNK4) is essential to activate the thiazide-sensitive NaCl cotransporter (NCC) along the distal convoluted tubule, an effect central to the phenotype of familial hyperkalemic hypertension. Although effects on potassium and sodium channels along the connecting and collecting tubules have also been documented, WNK4 is typically believed to have little role in modulating sodium chloride reabsorption along the thick ascending limb of the loop of Henle. Yet wnk4^-/- mice (knockout mice lacking WNK4) do not demonstrate the hypocalciuria typical of pure distal convoluted tubule dysfunction. Here, we tested the hypothesis that WNK4 also modulates bumetanide-sensitive Na-K-2Cl cotransporter (NKCC2) function along the thick ascending limb. We confirmed that w nk4^-/- mice are hypokalemic and waste sodium chloride, but are also normocalciuric. Results from Western blots suggested that the phosphorylated forms of both NCC and NKCC2 were in lower abundance in wnk4^-/- mice than in controls. This finding was confirmed by immunofluorescence microscopy. Although the initial response to furosemide was similar in wnk4^-/- mice and controls, the response was lower in the knockout mice when reabsorption along the distal convoluted tubule was inhibited. Using HEK293 cells, we showed that WNK4 increases the abundance of phosphorylated NKCC2. More supporting evidence that WNK4 may modulate NKCC2 emerges from a mouse model of WNK4-mediated familial hyperkalemic hypertension in which more phosphorylated NKCC2 is present than in controls. These data indicate that WNK4, in addition to modulating NCC, also modulates NKCC2, contributing to its physiological function in vivo.

Role of ClC-K and barttin in low potassium-induced sodium chloride cotransporter activation and hypertension in mouse kidney

The sodium chloride cotransporter (NCC) has been identified as a key molecule regulating potassium balance. The mechanisms of NCC regulation during low extracellular potassium concentrations have been studied in vitro. These studies have shown that hyperpolarization increased chloride efflux, leading to the activation of chloride-sensitive with-no-lysine kinase (WNK) kinases and their downstream molecules, including STE20/SPS1-related proline/alanine-rich kinase (SPAK) and NCC. However, this mechanism was not studied in vivo. Previously, we developed the barttin hypomorphic mouse (Bsnd^neo/neo mice), expressing very low levels of barttin and ClC-K channels, because barttin is an essential β-subunit of ClC-K. In contrast with Bsnd^−/− mice, Bsnd^neo/neo mice survived to adulthood. In Bsnd^neo/neo mice, SPAK and NCC activation after consuming a low-potassium diet was clearly impaired compared with that in wild-type (WT) mice. In ex vivo kidney slice experiment, the increase in pNCC and SPAK in low-potassium medium was also impaired in Bsnd^neo/neo mice. Furthermore, increased blood pressure was observed in WT mice fed a high-salt and low-potassium diet, which was not evident in Bsnd^neo/neo mice. Thus, our study provides in vivo evidence that, in response to a low-potassium diet, ClC-K and barttin play important roles in the activation of the WNK4-SPAK-NCC cascade and blood pressure regulation.

Plasma Potassium Determines NCC Abundance in Adult Kidney-Specific γENaC Knockout

The amiloride-sensitive epithelial sodium channel (ENaC) and the thiazide-sensitive sodium chloride cotransporter (NCC) are key regulators of sodium and potassium and colocalize in the late distal convoluted tubule of the kidney. Loss of the αENaC subunit leads to a perinatal lethal phenotype characterized by sodium loss and hyperkalemia resembling the human syndrome pseudohypoaldosteronism type 1 (PHA-I). In adulthood, inducible nephron-specific deletion of αENaC in mice mimics the lethal phenotype observed in neonates, and as in humans, this phenotype is prevented by a high sodium (HNa⁺)/low potassium (LK⁺) rescue diet. Rescue reflects activation of NCC, which is suppressed at baseline by elevated plasma potassium concentration. In this study, we investigated the role of the γENaC subunit in the PHA-I phenotype. Nephron-specific γENaC knockout mice also presented with salt-wasting syndrome and severe hyperkalemia. Unlike mice lacking αENaC or βΕΝaC, an HNa⁺/LK⁺ diet did not normalize plasma potassium (K⁺) concentration or increase NCC activation. However, when K⁺ was eliminated from the diet at the time that γENaC was deleted, plasma K⁺ concentration and NCC activity remained normal, and progressive weight loss was prevented. Loss of the late distal convoluted tubule, as well as overall reduced βENaC subunit expression, may be responsible for the more severe hyperkalemia. We conclude that plasma K⁺ concentration becomes the determining and limiting factor in regulating NCC activity, regardless of Na⁺ balance in γENaC-deficient mice.

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.

The signaling role for chloride in the bidirectional communication between neurons and astrocytes

It is well known that the electrical signaling in neuronal networks is modulated by chloride (Cl^-) fluxes via the inhibitory GABA_A and glycine receptors. Here, we discuss the putative contribution of Cl^- fluxes and intracellular Cl^- to other forms of information transfer in the CNS, namely the bidirectional communication between neurons and astrocytes. The manuscript (i) summarizes the generic functions of Cl^- in cellular physiology, (ii) recaps molecular identities and properties of Cl^- transporters and channels in neurons and astrocytes, and (iii) analyzes emerging studies implicating Cl^- in the modulation of neuroglial communication. The existing literature suggests that neurons can alter astrocytic Cl^- levels in a number of ways; via (a) the release of neurotransmitters and activation of glial transporters that have intrinsic Cl^- conductance, (b) the metabotropic receptor-driven changes in activity of the electroneutral cation-Cl^- cotransporter NKCC1, and (c) the transient, activity-dependent changes in glial cell volume which open the volume-regulated Cl^-/anion channel VRAC. Reciprocally, astrocytes are thought to alter neuronal [Cl^-]_i through either (a) VRAC-mediated release of the inhibitory gliotransmitters, GABA and taurine, which open neuronal GABA_A and glycine receptor/Cl^- channels, or (b) the gliotransmitter-driven stimulation of NKCC1. The most important recent developments in this area are the identification of the molecular composition and functional heterogeneity of brain VRAC channels, and the discovery of a new cytosolic [Cl^-] sensor - the Wnk family protein kinases. With new work in the field, our understanding of the role of Cl^- in information processing within the CNS is expected to be significantly updated.

Mutant Cullin 3 causes familial hyperkalemic hypertension via dominant effects

Mutations in the ubiquitin ligase scaffold protein Cullin 3 (CUL3) cause the disease familial hyperkalemic hypertension (FHHt). In the kidney, mutant CUL3 (CUL3-Δ9) increases abundance of With-No-Lysine [K] Kinase 4 (WNK4), with excessive activation of the downstream Sterile 20 (STE20)/SPS-1-related proline/alanine-rich kinase (SPAK) increasing phosphorylation of the Na+-Cl- cotransporter (NCC). CUL3-Δ9 promotes its own degradation via autoubiquitination, leading to the hypothesis that Cul3 haploinsufficiency causes FHHt. To directly test this, we generated Cul3 heterozygous mice (CUL3-Het), and Cul3 heterozygotes also expressing CUL3-Δ9 (CUL3-Het/Δ9), using an inducible renal epithelial-specific system. Endogenous CUL3 was reduced to 50% in both models, and consistent with autoubiquitination, CUL3-Δ9 protein was undetectable in CUL3-Het/Δ9 kidneys unless primary renal epithelia cells were cultured. Abundances of WNK4 and phosphorylated NCC did not differ between control and CUL3-Het mice, but they were elevated in CUL3-Het/Δ9 mice, which also displayed higher plasma [K+] and blood pressure. Abundance of phosphorylated Na+-K+-2Cl- cotransporter (NKCC2) was also increased, which may contribute to the severity of CUL3-Δ9-mediated FHHt. WNK4 and SPAK localized to puncta in NCC-positive segments but not in NKCC2-positive segments, suggesting differential effects of CUL3-Δ9. These results indicate that Cul3 haploinsufficiency does not cause FHHt, but dominant effects of CUL3-Δ9 are required.

Potassium-regulated distal tubule WNK bodies are kidney-specific WNK1 dependent

WNK bodies are large punctate membraneless cytosolic signaling foci that sequester WNK serine–threonine kinases and form in renal distal tubular epithelial cells during shifts in total body potassium balance. The assembly of these structures requires KS-WNK1, a truncated isoform of the WNK1 gene that is exclusively expressed in the distal tubule.

With-no-lysine (WNK) kinases coordinate volume and potassium homeostasis by regulating renal tubular electrolyte transport. In the distal convoluted tubule (DCT), potassium imbalance causes WNK signaling complexes to concentrate into large discrete foci, which we call “WNK bodies.” Although these structures have been reported previously, the mechanisms that drive their assembly remain obscure. Here, we show that kidney-specific WNK1 (KS-WNK1), a truncated kinase-defective WNK1 isoform that is highly expressed in the DCT, is critical for WNK body formation. While morphologically distinct WNK bodies were evident in the distal tubules of mice subjected to dietary potassium loading and restriction, KS-WNK1 knockout mice were deficient in these structures under identical conditions. Combining in vivo observations in kidney with reconstitution studies in cell culture, we found that WNK bodies are dynamic membraneless foci that are distinct from conventional organelles, colocalize with the ribosomal protein L22, and cluster the WNK signaling pathway. The formation of WNK bodies requires an evolutionarily conserved cysteine-rich hydrophobic motif harbored within a unique N-terminal exon of KS-WNK1. We propose that WNK bodies are not pathological aggregates, but rather are KS-WNK1–dependent microdomains of the DCT cytosol that modulate WNK signaling during physiological shifts in potassium balance.

Sexual Dimorphic Pattern of Renal Transporters and Electrolyte Homeostasis

Compared with males, females have lower BP before age 60, blunted hypertensive response to angiotensin II, and a leftward shift in pressure natriuresis. This study tested the concept that this female advantage associates with a distinct sexual dimorphic pattern of transporters along the nephron. We applied quantitative immunoblotting to generate profiles of transporters, channels, claudins, and selected regulators in both sexes and assessed the physiologic consequences of the differences. In rats, females excreted a saline load more rapidly than males did. Compared with the proximal tubule of males, the proximal tubule of females had greater phosphorylation of Na+/H+ exchanger isoform 3 (NHE3), distribution of NHE3 at the base of the microvilli, and less abundant expression of Na+/Pi cotransporter 2, claudin-2, and aquaporin 1. These changes associated with less bicarbonate reabsorption and higher lithium clearance in females. The distal nephrons of females had a higher abundance of total and phosphorylated Na+/Cl- cotransporter (NCC), claudin-7, and cleaved forms of epithelial Na+ channel (ENaC) α and γ subunits, which associated with a lower baseline plasma K+ concentration. A K+-rich meal increased the urinary K+ concentration and decreased the level of renal phosphorylated NCC in females. Notably, we observed similar abundance profiles in female versus male C57BL/6 mice. These results define sexual dimorphic phenotypes along the nephron and suggest that lower proximal reabsorption in female rats expedites excretion of a saline load and enhances NCC and ENaC abundance and activation, which may facilitate K+ secretion and set plasma K+ at a lower level.

Dietary K+ and Cl- independently regulate basolateral conductance in principal and intercalated cells of the collecting duct

The renal collecting duct contains two distinct cell types, principal and intercalated cells, expressing potassium K_ir4.1/5.1 (KCNJ10/16) and chloride ClC-K2 (ClC-Kb in humans) channels on their basolateral membrane, respectively. Both channels are thought to play important roles in controlling systemic water-electrolyte balance and blood pressure. However, little is known about mechanisms regulating activity of K_ir4.1/5.1 and ClC-K2/b. Here, we employed patch clamp analysis at the single channel and whole cell levels in freshly isolated mouse collecting ducts to investigate regulation of K_ir4.1/5.1 and ClC-K2/b by dietary K⁺ and Cl^- intake. Treatment of mice with high K⁺ and high Cl^- diet (6% K⁺, 5% Cl^-) for 1 week significantly increased basolateral K⁺-selective current, single channel K_ir4.1/5.1 activity and induced hyperpolarization of basolateral membrane potential in principal cells when compared to values in mice on a regular diet (0.9% K⁺, 0.5% Cl^-). In contrast, basolateral Cl^--selective current and single channel ClC-K2/b activity was markedly decreased in intercalated cells under this condition. Substitution of dietary K⁺ to Na⁺ in the presence of high Cl^- exerted a similar inhibiting action of ClC-K2/b suggesting that the channel is sensitive to variations in dietary Cl^- per se. Cl^--sensitive with-no-lysine kinase (WNK) cascade has been recently proposed to orchestrate electrolyte transport in the distal tubule during variations of dietary K⁺. However, co-expression of WNK1 or its major downstream effector Ste20-related proline-alanine-rich kinase (SPAK) had no effect on ClC-Kb over-expressed in Chinese hamster ovary (CHO) cells. Treatment of mice with high K⁺ diet without concomitant elevations in dietary Cl^- (6% K⁺, 0.5% Cl^-) elicited a comparable increase in basolateral K⁺-selective current, single channel K_ir4.1/5.1 activity in principal cells, but had no significant effect on ClC-K2/b activity in intercalated cells. Furthermore, stimulation of aldosterone signaling by Deoxycorticosterone acetate (DOCA) recapitulated the stimulatory actions of high K⁺ intake on K_ir4.1/5.1 channels in principal cells but was ineffective to alter ClC-K2/b activity and basolateral Cl^- conductance in intercalated cells. In summary, we report that variations of dietary K⁺ and Cl^- independently regulate basolateral potassium and chloride conductance in principal and intercalated cells. We propose that such discrete mechanism might contribute to fine-tuning of urinary excretion of electrolytes depending on dietary intake.

Epithelial Na+ Channel Regulation by Extracellular and Intracellular Factors

Epithelial Na⁺ channels (ENaCs) are members of the ENaC/degenerin family of ion channels that evolved to respond to extracellular factors. In addition to being expressed in the distal aspects of the nephron, where ENaCs couple the absorption of filtered Na⁺ to K⁺ secretion, these channels are found in other epithelia as well as nonepithelial tissues. This review addresses mechanisms by which ENaC activity is regulated by extracellular factors, including proteases, Na⁺, and shear stress. It also addresses other factors, including acidic phospholipids and modification of ENaC cytoplasmic cysteine residues by palmitoylation, which enhance channel activity by altering interactions of the channel with the plasma membrane.

Regulation of Potassium Homeostasis in CKD

Disturbances of potassium homeostasis can cause either hyperkalemia or hypokalemia and result in serious consequences. Although the consequences of acute and chronic hyperkalemia and treatment of these conditions in CKD have been widely appreciated by nephrologists, more recent information has focused attention on the consequences of chronic hypokalemia. Several recent studies have documented a "U-shaped" relationship between the serum [K⁺] and higher mortality in several clinical studies. The causes of dyskalemias are placed into the unique perspective of patients with CKD and its evolution with progression of CKD to later stages and focuses on the pathophysiology of these disorders. Emphasis is placed on the high mortality associated with both low and high levels of potassium that are unique to patients with CKD. Recent information regarding sensors of changes in the serum [K⁺] that evoke changes in NaCl transport in the DCT1 and subsequent efferent responses by aldosterone-responsive cells in the DCT2 and cortical collecting duct to adjust K⁺ secretion by the renal outer medullary potassium channel is reviewed in detail. These sensing mechanisms can be interrupted by drugs, such as the calcineurin inhibitors to cause both hypertension and hyperkalemia in kidney transplant patients, or can be inherited as familial hypertensive hyperkalemia. The role and pathogenesis of angiotensin-converting enzyme inhibitors and angiotensin receptor blockers in causing hyperkalemia is a common stop point for cessation of these important drugs, but, and newer agents to lower the serum [K⁺] that might allow continuation of angiotensin-converting enzyme or angiotensin receptor blocker therapy are examined. Finally, the importance of emphasis on potassium-containing foods, such as fresh produce and fruit in the diets of patients with early-stage CKD, is examined as an under-appreciated area requiring more emphasis by nephrologists caring for these patients and may be unique to food-challenged patients with CKD.

Emerging Targets of Diuretic Therapy

Diuretics are commonly prescribed for treatment in patients with hypertension, edema, or heart failure. Studies on hypertensive and salt-losing disorders and on urea transporters have contributed to better understanding of mechanisms of renal salt and water reabsorption and their regulation. Proteins involved in the regulatory pathways are emerging targets for diuretic and aquaretic therapy. Integrative high-throughput screening, protein structure analysis, and chemical modification have identified promising agents for preclinical testing in animals. These include WNK-SPAK inhibitors, ClC-K channel antagonists, ROMK channel antagonists, and pendrin and urea transporter inhibitors. We discuss the potential advantages and side effects of these potential diuretics.