Regulation of NKCC2 by a chloride-sensing mechanism involving the WNK3 and SPAK kinases

Admission and discharge sodium: Chloride ratio in acute heart failure - The importance of electrolytes

BACKGROUND

Chloride has prognostic implications in heart failure (HF). The sodium:chloride (Na:Cl) ratio gathers information of both ions.

OBJECTIVES

To study the prognostic impact of Na:Cl ratio in acute HF.

METHODS

We retrospectively analysed patients included in an acute HF cohort in 2009-2010.

PRIMARY ENDPOINT

all-cause mortality.

FOLLOW-UP

1-year from admission to study the impact of admission Na:Cl ratio; 1-year post-discharge to study the discharge ratio impact. ROC curves of the association of Na, Cl, and the Na:Cl ratio with 1-year mortality were determined and the AUC compared. The association of Na:Cl ratio with mortality was assessed in multivariate analyses.

RESULTS

We studied 616 patients. Median admission Na:Cl ratio = 1.34 (1.31-1.38). During 1-year since admission, 229 patients died, AUC for death-association of admission Na, Cl and Na:Cl ratio = 0.42 (0.38-0.47), 0.39 (0.35-0.44) and 0.58 (0.53-0.63), respectively, with significant difference between Na:Cl ratio curve and the others. When admission Na:Cl ratio ≥ 1.34, the multivariate-adjusted death-risk was 1.41 (1.04-1.89); 1.32 (1.04-1.68), per each 0.1 increase in ratio. In-hospital death rate was 4.1 %, median discharge Na:Cl ratio = 1.40 (1.37-1.45). During 1-year post-discharge, 205 patients (34.9 %) died; AUC for Na, Cl and Na:Cl ratio: 0.45 (0.40-0.50), 0.41 (0.36-0.46) and 0.57 (0.52-0.62), with differences between Na:Cl ratio curve and the others. When discharge Na:Cl ratio ≥ 1.43 (percentile 66.7), the adjusted-HR of death was 1.43 (1.04-1.97), p = 0.03, 1.54 (1.23-1.92) per 0.1 increase.

CONCLUSIONS

Elevated Na:Cl ratio is independently associated with all-cause death. Per each 0.1 increase in Na:Cl ratio at admission and discharged, there was a 32 % and 54 % higher risk, respectively.

Diuretic strategies in acute decompensated heart failure

Hospitalization for ADHF is linked to poor outcomes, with residual congestion at discharge significantly increasing the risk of HF readmissions and mortality. Diuretic resistance is a major contributor to inadequate decongestion during ADHF treatment. In this review, we discuss various decongestive strategies, emphasizing the management of diuretic resistance. Additionally, we examine the limitations of current decongestion trials and highlight key priorities for future research.

With No Lysine (K) Kinases and Sodium Transporter Function in Solute Exchange with Implications for BP Regulation as Elucidated through Drosophila

Like other multicellular organisms, the fruit fly Drosophila melanogaster must maintain homeostasis of the internal milieu, including the maintenance of constant ion and water concentrations. In mammals, the with no lysine (K) (WNK)-Ste20-proline/alanine rich kinase/oxidative stress response 1 kinase cascade is an important regulator of epithelial ion transport in the kidney. This pathway regulates SLC12 family cotransporters, including sodium-potassium-2-chloride, sodium chloride, and potassium chloride cotransporters. The WNK-Ste20-proline/alanine rich kinase/oxidative stress response 1 kinase cascade also regulates epithelial ion transport via regulation of the Drosophila sodium-potassium-2-chloride cotransporter in the Malpighian tubule, the renal epithelium of the fly. Studies in Drosophila have contributed to the understanding of multiple regulators of WNK pathway signaling, including intracellular chloride and potassium, the scaffold protein Mo25, hypertonic stress, hydrostatic pressure, and macromolecular crowding. These will be discussed together, with implications for mammalian kidney function and BP control.

Function and regulation of the insect NaCCC2 sodium transport proteins

NaCCC2 transport proteins, including those from Drosophila melanogaster (Ncc83) and Aedes aegypti (aeCCC2), are an insect-specific clade with sequence similarity to Na+-K+-2Cl- cotransporters. Whereas the Na+-K+-2Cl- cotransporters and other cation-chloride cotransporters are electroneutral, recent work indicates that Ncc83 and aeCCC2 carry charge across membranes. Here, we further characterize the regulation and transport properties of Ncc83 and aeCCC2 expressed in Xenopus oocytes. In cation uptake experiments, Li+ was used as a tracer for Na+ and Rb+ was used as a tracer for K+. Li+ uptake of oocytes expressing either aeCCC2 or Ncc83 was greater than uptake in water-injected controls, activated by hypotonic swelling, and not inhibited by ouabain or ethyl cinnamate. Rb+ uptake of oocytes expressing either aeCCC2 or Ncc83 was not different than water injected controls. In oocytes expressing either aeCCC2 or Ncc83, Li+ uptake plateaued with increasing Li+ concentrations, with apparent Km values in the range of 10 to 20 mM. Following exposure to ouabain, intracellular [Na+] was greater in oocytes expressing aeCCC2 than in controls. Elevating intracellular cAMP (via 8-bromo-cAMP) in Ncc83 oocytes significantly stimulated both Li+ uptake and membrane conductances. Elevating intracellular cAMP in aeCCC2 oocytes did not affect Li+ uptake, but stimulated membrane conductances. Overall, these results confirm that the NaCCC2s resemble other cation-chloride cotransporters in their regulation and some transport properties. However, unlike other cation-chloride cotransporters, they carry charge across membranes.

The role of uromodulin in cardiovascular disease: a review

Uromodulin, also referred to as Tamm Horsfall protein (THP), is a renal protein exclusively synthesized by the kidneys and represents the predominant urinary protein under normal physiological conditions. It assumes a pivotal role within the renal system, contributing not only to ion transport and immune modulation but also serving as a critical factor in the prevention of urinary tract infections and kidney stone formation. Emerging evidence indicates that uromodulin may serve as a potential biomarker extending beyond renal function. Recent clinical investigations and Mendelian randomization studies have unveiled a discernible association between urinary regulatory protein levels and cardiovascular events and mortality. This review primarily delineates the intricate relationship between uromodulin and cardiovascular disease, elucidates its predictive utility as a novel biomarker for cardiovascular events, and delves into its involvement in various physiological and pathophysiological facets of the cardiovascular system, incorporating recent advancements in corresponding genetics.

Prognostic value of hypochloremia on mortality in patients with heart failure: a systematic review and meta-analysis

AIMS

Electrolyte imbalances are common in patients with heart failure. Several studies have shown that a low serum chloride level is associated with adverse outcomes in hospitalized patients with acute heart failure and in outpatients with chronic heart failure. We performed a systematic review and meta-analysis to assess the association of hypochloremia with all-cause mortality in patients with heart failure.

METHODS

Data search was conducted from inception through 1 February 2023, using the following MeSH terms: ('chloride' OR 'hypochloremia') AND 'heart failure'. Studies evaluating the association between serum chloride and all-cause mortality in patients with heart failure were included. The predefined primary outcome was all-cause mortality. Pooled hazard ratios and 95% confidence intervals (CIs) were used as effect estimates and calculated with a random-effects model; fixed-effects model and leave-one-out sensitivity analyses were also performed.

RESULTS

A total of 15 studies, involving 25 848 patients, were included. The prevalence of hypochloremia ranged from 8.6 to 31.5%. Follow-up time ranged from 6 to 67 months. Hypochloremia as a categorical variable was associated with an increased risk of all-cause mortality [hazard ratio 1.56; 95% confidence interval (CI) 1.38-1.75; P  < 0.001]. As a continuous variable, serum chloride was associated with all-cause mortality (hazard ratio per mmol/l decrease in serum chloride: 1.06; 95% CI 1.05-1.07; P  < 0.001). Results were confirmed by using several sensitivity analyses.

CONCLUSION

Hypochloremia exhibits a significant prognostic value in patients with heart failure. Serum chloride can be used as an effective tool for risk stratifying in patients with heart failure.

Predictive and Experimental Motif Interaction Analysis Identifies Functions of the WNK-OSR1/SPAK Pathway

The WNK-OSR1/SPAK protein kinase signaling pathway regulates ion homeostasis and cell volume, but its other functions are poorly understood. To uncover undefined signaling functions of the pathway we analyzed the binding specificity of the conserved C-terminal (CCT) domains of OSR1 and SPAK to find all possible interaction motifs in human proteins. These kinases bind the core consensus sequences R-F-x-V/I and R-x-F-x-V/I. Motifs were ranked based on sequence, conservation, cellular localization, and solvent accessibility. Out of nearly 3,700 motifs identified, 90% of previously published motifs were within the top 2% of those predicted. Selected candidates (TSC22D1, CAVIN1, ATG9A, NOS3, ARHGEF5) were tested. Upstream kinases WNKs 1-4 and their close relatives, the pseudokinases NRBP1/2, contain CCT-like domains as well. We identified additional distinct motif variants lacking the conserved arginine previously thought to be required, and found that the NRBP1 CCT-like domain binds TSC22D1 via the same motif as OSR1 and SPAK. Our results further highlight the rich and diverse functionality of CCT and CCT-like domains in connecting WNK signaling to cellular processes.

Astrocytic chloride regulates brain function in health and disease

Chloride ions (Cl-) play a pivotal role in synaptic inhibition in the central nervous system, primarily mediated through ionotropic mechanisms. A recent breakthrough emphathizes the significant influence of astrocytic intracellular chloride concentration ([Cl-]i) regulation, a field still in its early stages of exploration. Typically, the [Cl-]i in most animal cells is maintained at lower levels than the extracellular chloride [Cl-]o, a critical balance to prevent cell swelling due to osmotic pressure. Various Cl- transporters are expressed differently across cell types, fine-tuning the [Cl-]i, while Cl- gradients are utilised by several families of Cl- channels. Although the passive distribution of ions within cells is governed by basic biophysical principles, astrocytes actively expend energy to sustain [Cl-]i at much higher levels than those achieved passively, and much higher than neuronal [Cl-]i. Beyond the role in volume regulation, astrocytic [Cl-]i is dynamically linked to brain states and influences neuronal signalling in actively behaving animals. As a vital component of brain function, astrocytic [Cl-]i also plays a role in the development of disorders where inhibitory transmission is disrupted. This review synthesises the latest insights into astrocytic [Cl-]i, elucidating its role in modulating brain function and its implications in various pathophysiological conditions.

Arginine vasopressin regulates the renal Na+-Cl- and Na+-K+-Cl- cotransporters through with-no-lysine kinase 4 and inhibitor 1 phosphorylation

Vasopressin regulates water homeostasis via the V2 receptor in the kidney at least in part through protein kinase A (PKA) activation. Vasopressin, through an unknown pathway, upregulates the activity and phosphorylation of Na+-Cl- cotransporter (NCC) and Na+-K+-2Cl- cotransporter 2 (NKCC2) by Ste20-related proline/alanine-rich kinase (SPAK) and oxidative stress-responsive kinase 1 (OSR1), which are regulated by the with-no-lysine kinase (WNK) family. Phosphorylation of WNK4 at PKA consensus motifs may be involved. Inhibitor 1 (I1), a protein phosphatase 1 (PP1) inhibitor, may also play a role. In human embryonic kidney (HEK)-293 cells, we assessed the phosphorylation of WNK4, SPAK, NCC, or NKCC2 in response to forskolin or desmopressin. WNK4 and cotransporter phosphorylation were studied in desmopressin-infused WNK4-/- mice and in tubule suspensions. In HEK-293 cells, only wild-type WNK4 but not WNK1, WNK3, or a WNK4 mutant lacking PKA phosphorylation motifs could upregulate SPAK or cotransporter phosphorylation in response to forskolin or desmopressin. I1 transfection maximized SPAK phosphorylation in response to forskolin in the presence of WNK4 but not of mutant WNK4 lacking PP1 regulation. We observed direct PP1 regulation of NKCC2 dephosphorylation but not of NCC or SPAK in the absence of WNK4. WNK4-/- mice with desmopressin treatment did not increase SPAK/OSR1, NCC, or NKCC2 phosphorylation. In stimulated tubule suspensions from WNK4-/- mice, upregulation of pNKCC2 was reduced, whereas upregulation of SPAK phosphorylation was absent. These findings suggest that WNK4 is a central node in which kinase and phosphatase signaling converge to connect cAMP signaling to the SPAK/OSR1-NCC/NKCC2 pathway.NEW & NOTEWORTHY With-no-lysine kinases regulate the phosphorylation and activity of the Na+-Cl- and Na+-K+-2Cl- cotransporters. This pathway is modulated by arginine vasopressin (AVP). However, the link between AVP and WNK signaling remains unknown. Here, we show that AVP activates WNK4 through increased phosphorylation at putative protein kinase A-regulated sites and decreases its dephosphorylation by protein phosphatase 1. This work increases our understanding of the signaling pathways mediating AVP actions in the kidney.

Low-Chloride Diet Prevents the Development of Arterial Hypertension and Protects Kidney Function in Angiotensin II-Infused Mice

INTRODUCTION

A comprehensive pathophysiological mechanism to explain the relationship between high-salt intake and hypertension remains undefined. Evidence suggests that chloride, as the accompanying anion of sodium in dietary salt, is necessary to develop hypertension. We evaluated whether reducing dietary Cl- while keeping a standard Na+ intake modified blood pressure, cardiac hypertrophy, renal function, and vascular contractility after angiotensin II (AngII) infusion.

METHODS

C56BL/6J mice fed with standard Cl- diet or a low-Cl- diet (equimolar substitution of Cl- by a mixture of Na+ salts, both diets with standard Na+ content) received AngII (infusion of 1.5 mg/kg/day) or vehicle for 14 days. We measured systolic blood pressure (SBP), glomerular filtration rate (GFR), natriuretic response to acute saline load, and contractility of aortic rings from mice infused with vehicle and AngII, in standard and low-Cl- diet.

RESULTS

The mice fed the standard diet presented increased SBP and cardiac hypertrophy after AngII infusion. In contrast, low-Cl- diet prevented the increase of SBP and cardiac hypertrophy. AngII-infused mice fed a standard diet presented hampered natriuretic response to saline load, meanwhile the low-Cl- diet preserved natriuretic response in AngII-infused mice, without change in GFR. Aortic rings from mice fed with standard diet or low-Cl- diet and infused with AngII presented a similar contractile response.

CONCLUSION

We conclude that the reduction in dietary Cl- as the accompanying anion of sodium in salt is protective from AngII pro-hypertensive actions due to a beneficial effect on kidney function and preserved natriuresis.

Prognostic impact of serum chloride concentrations in acute heart failure patients: A systematic Rreview and meta-analysis

OBJECTIVE

Acute heart failure (AHF) is a common disease in the emergency departments. Its occurrence is often accompanied by electrolyte disorders, but little attention is paid to chloride ion. Recent studies have shown that hypochloremia was associated with poor prognosis of AHF. Therefore, this meta-analysis aimed to assess the incidence of hypochloremia and the impact of the reduction of serum chloride on the prognosis of AHF patients.

METHODS

We searched Cochrane Library, Web of science, PubMed, Embase databases and searched the relevant studies on chloride ion and AHF prognosis. The search time is from the establishment of the database to December 29, 2021. Two researchers screened the literature and extracted data independently. The quality of the included literature was evaluated using Newcastle-Ottawa Scale (NOS) scale. The effect amount is expressed as hazard ratio (HR) or relative risk (RR) and 95% confidence interval (CI). Review Manager 5.4.1 software for was used to perform the meta-analysis.

RESULTS

Seven studies involving 6787 AHF patients were included in meta-analysis. Meta-analysis revealed that the incidence of hypochloremia in AHF patients at admission was 17% (95% CI: 0.11-0.22); One mmol /L decrease in serum chloride at admission was associated with 6% increased risk of all-cause death of AHF patients (HR = 1.06, 95% CI: 1.04-1.08, P < 0.00001); Compared with the non-hypochloremia group, the risk of all-cause death in the hypochloremia group increased by 1.71 times (RR = 1.71, 95% CI: 1.45-2.02, P < 0.00001), the risk of all-cause death in the progressive hypochloremia(development of hypochloremia after admission) group increased by 2.24 times (HR = 2.24, 95% CI: 1.72-2.92, P < 0.00001), and the risk of all-cause death in the persistent hypochloremia (hypochloremia both on admission and at discharge) group increased by 2.80 times (HR = 2.80, 95% CI: 2.10-3.72, P < 0.00001).

CONCLUSION

The available evidence shows that the decrease of chloride ion at admission is associated with poor prognosis of AHF patients, and the prognosis of persistent hypochloremia is worse.Some outcome indicators(progressive hypochloremia, persistent hypochloremia, and composite of death + HF hospitalization)are as few as 2 studies in the literature, and the results should be interpreted carefully.

Serum Chloride and Heart Failure

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

Trajectory of serum chloride levels during decongestive therapy in acute heart failure

BACKGROUND

Hypochloremia is a risk factor for poor outcomes in patients with acute heart failure (AHF). However, the changes in serum chloride levels during decongestion therapy and their impact on prognosis remain unknown.

METHODS

In total, 2798 patients with AHF were retrospectively studied and divided into four groups according to their admission and discharge serum chloride levels: (1) normochloremia (n=2,192, 78%); (2) treatment-associated hypochloremia, defined as admission normochloremia with a subsequent decrease (<98 mEq/L) during hospitalization (n=335, 12%); (3) resolved hypochloremia, defined as admission hypochloremia that disappeared at discharge (n=128, 5%); (4) persistent hypochloremia, defined as chloride <98 mEq/L at admission and discharge (n = 143, 5%). The primary outcome was all-cause death, and the secondary outcomes were cardiovascular death and a composite of cardiovascular death and rehospitalization for heart failure after discharge.

RESULTS

The mean age was 76 ± 12 years and 1584 (57%) patients were men. The mean left ventricular ejection fraction was 46 ± 16%. During a median follow-up period of 365 days, persistent hypochloremia was associated with an increased risk of all-cause death (adjusted hazard ratio [95% confidence interval]: 2.27 [1.53-3.37], p < 0.001), cardiovascular death (2.38 [1.46-3.87], p < 0.001), and a composite of cardiovascular death and heart failure rehospitalization (1.47 [1.06-2.06], p = 0.022). However, the outcomes were comparable between patients with resolved hypochloremia and normochloremia.

CONCLUSIONS

Persistent hypochloremia was associated with worse clinical outcomes, while resolved hypochloremia and normochloremia showed a comparable prognosis. Changes in serum chloride levels can help identify patients with poor prognoses and can be used to determine subsequent treatment strategies.

Etiology and Management of Edema: A Review

The development of peripheral edema can often pose a significant diagnostic and therapeutic challenge for practitioners due to its association with a wide variety of underlying disorders ranging in severity. Updates to the original Starling's principle have provided new mechanistic insights into edema formation. Additionally, contemporary data highlighting the role of hypochloremia in the development of diuretic resistance provide a possible new therapeutic target. This article reviews the pathophysiology of edema formation and discusses implications for treatment.

Lower serum chloride concentrations are associated with an increased risk of death in ICU patients with acute kidney injury: an analysis of the MIMIC-IV database

BACKGROUND

The association between baseline serum chloride levels and mortality in patients with severe acute kidney injury (AKI) is unknown. Our aim was to investigate the relationship between baseline blood chloride levels and 28-day mortality in patients with AKI admitted to the ICU and to detect possible effect modifiers in this population.

METHODS

AKI patients with severe critical illness were extracted from the MIMIC-IV. During ICU admission, chloride levels were measured for the first time. Our primary outcome was 28-day mortality in patients with AKI after 24 hours in the ICU. Multivariable logistic regression was used to examine the association between three groups of chloride levels and 28-day mortality, and logistic regression with restricted cubic spline was applied to detect the non-linear trendy.

RESULTS

A total of 24,166 patients with critically ill AKI were included in this retrospective cohort study. The total 28-day mortality rate in the ICU was 15.9%. Overall, there was a U-shaped relationship between baseline serum chloride levels and 28-day mortality (non-linear P<0.001). Accordingly, patients with low serum chloride (<96 mEq/L) had a significantly increased risk of death compared to patients with normal serum chloride (96-108 meq/L) (adjusted OR=1.94, 95% CI: 1.68-2.24, P<0.001). None of the variables, including age, gender, 24-hour fluid intake, continuous renal replacement therapy, ventilation, Atrial fibrillation, Sequential Organ Failure Assessment score, whether to measure lactate and AKI stage, significantly modified the association between lower chloride levels and 28-day mortality.

CONCLUSIONS

Low serum chloride levels at baseline were associated with death at 28 days in intensive illnesses with AKI.

Intracellular Ion Control of WNK Signaling

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

Persistent Hypochloremia Is Associated with Adverse Prognosis in Patients Repeatedly Hospitalized for Heart Failure

BACKGROUND

Hypochloremia reflects neuro-hormonal activation in patients with heart failure (HF). However, the prognostic impact of persistent hypochloremia in those patients remains unclear.

METHODS

We collected the data of patients who were hospitalized for HF at least twice between 2010 and 2021 (n = 348). Dialysis patients (n = 26) were excluded. The patients were divided into four groups based on the absence/presence of hypochloremia (<98 mmol/L) at discharge from their first and second hospitalizations: Group A (patients without hypochloremia at their first and second hospitalizations, n = 243); Group B (those with hypochloremia at their first hospitalization and without hypochloremia at their second hospitalization, n = 29); Group C (those without hypochloremia at their first hospitalization and with hypochloremia at their second hospitalization, n = 34); and Group D (those with hypochloremia at their first and second hospitalizations, n = 16).

RESULTS

a Kaplan-Meier analysis revealed that all-cause mortality and cardiac mortality were the highest in Group D compared to the other groups. A multivariable Cox proportional hazard analysis revealed that persistent hypochloremia was independently associated with both all-cause death (hazard ratio 3.490, p < 0.001) and cardiac death (hazard ratio 3.919, p < 0.001).

CONCLUSIONS

In patients with HF, prolonged hypochloremia over two hospitalizations is associated with an adverse prognosis.

Lateral Diffusion of NKCC1 Contributes to Chloride Homeostasis in Neurons and Is Rapidly Regulated by the WNK Signaling Pathway

An upregulation of the Na⁺-K⁺-2Cl^- cotransporter NKCC1, the main chloride importer in mature neurons, can lead to depolarizing/excitatory responses mediated by GABA type A receptors (GABA_ARs) and, thus, to hyperactivity. Understanding the regulatory mechanisms of NKCC1 would help prevent intra-neuronal chloride accumulation that occurs in pathologies with defective inhibition. The cell mechanisms regulating NKCC1 are poorly understood. Here, we report in mature hippocampal neurons that GABAergic activity controls the membrane diffusion and clustering of NKCC1 via the chloride-sensitive WNK lysine deficient protein kinase 1 (WNK1) and the downstream Ste20 Pro-line Asparagine Rich Kinase (SPAK) kinase that directly phosphorylates NKCC1 on key threonine residues. At rest, this signaling pathway has little effect on intracellular Cl^- concentration, but it participates in the elevation of intraneuronal Cl^- concentration in hyperactivity conditions associated with an up-regulation of NKCC1. The fact that the main chloride exporter, the K⁺-Cl^- cotransporter KCC2, is also regulated in mature neurons by the WNK1 pathway indicates that this pathway will be a target of choice in the pathology.

Prognostic Significance of Serum Chloride Level Reduction in Patients with Chronic Heart Failure with Different Ejection Fractions

Little is known regarding the prognostic value of serum chloride in patients with chronic heart failure (CHF) with different ejection fractions. We sought to determine the postdischarge outcomes associated with lower serum chloride between different CHF types.We reviewed the medical records of 1221 consecutive patients with CHF admitted to the First Affiliated Hospital of Kunming Medical University from January 2017 to October 2021. After excluding patients with in-hospital death, missing follow-up data, missing serum chloride level data, or chronic dialysis therapy, 791 patients were included. Of these patients, 343 had heart failure with reduced ejection fraction (HFrEF; i.e., left ventricular ejection fraction (LVEF) < 40%), and 448 had heart failure with preserved ejection fraction (HFpEF) or heart failure with median ejection fraction (HFmrEF; HFpEF plus HFmrEF; i.e., LVEF ≥40%). Over a median follow-up of 750 days, 344 patients (43.5%) had all-cause mortality. In the univariate analysis, serum sodium and chloride were strongly associated with mortality in both HF subgroups (P < 0.0001). A multivariable model including both serum sodium and chloride showed the highly significant association between serum chloride and survival (P < 0.0001), whereas the association between serum sodium and mortality was not reported (HFpEF plus HFmrEF, hazard ratio (HR) 0.975, 95% confidence interval [CI] 0.942-1.010, P = 0.158; HFrEF, HR 1.007, 95% CI 0.966-1.051, P = 0.734). Kaplan-Meier survival curve analysis revealed a significant difference in mortality risk with decreasing chloride levels in all patients with CHF. The optimal cutoff value of chloride in predicting all-cause mortality was 102.95 mmol/L with area under the curve value of 0.76 [HR 0.760, 95% CI 0.727-0.793, P < 0.0001], sensitivity of 60.2%, and specificity of 78.3%.Lower serum chloride is an independent predictor of death in CHF, regardless of heart failure subtype.

Arterial Blood Pressure, Neuronal Excitability, Mineral Metabolism and Cell Volume Regulation Mechanisms Revealed by Xenopus laevis oocytes

Xenopus laevis oocytes have been an invaluable tool to discover and explore the molecular mechanisms and characteristics of many proteins, in particular integral membrane proteins. The oocytes were fundamental in many projects designed to identify the cDNA encoding a diversity of membrane proteins including receptors, transporters, channels and pores. In addition to being a powerful tool for cloning, oocytes were later used to experiment with the functional characterization of many of the identified proteins. In this review I present an overview of my personal 30-year experience using Xenopus laevis oocytes and the impact this had on a variety of fields such as arterial blood pressure, neuronal excitability, mineral metabolism and cell volume regulation.

Relation of Low Chloride Concentration to Diuretic Efficiency and Transplant-Free Survival in Children Hospitalized With Heart Failure

Serum chloride plays an important role in fluid homeostasis and is associated with impaired diuretic responsiveness and mortality in adults with heart failure (HF). We sought to characterize the relationship of serum chloride and diuretic efficiency (DE) and to determine its prognostic importance in children hospitalized with acute decompensated HF (ADHF). We studied DE, defined as net fluid output/kg+constant per mg of loop diuretic/kg, in 200 children hospitalized with ADHF. Median serum chloride at admission was 102 mmol/L (interquartile range 99 to 105 mmol/L), and hypochloremia (chloride ≤96 mmol/L) was present in 16% of the population at admission. Serum chloride correlated with serum sodium (r = 0.66; p < 0.001) and bicarbonate (r = -0.39; p < 0.001). In the adjusted analysis, lower chloride was associated with reduced DE (p < 0.001). Serum sodium was associated with DE on the unadjusted analysis; however, the association was eliminated when added to the model with chloride (p = 0.442). Lower chloride was also associated with features of inadequate decongestion during hospitalization: a positive fluid balance (p = 0.003), greater cumulative loop diuretic dose per weight (p = 0.001), addition of a thiazide diuretic during hospitalization (p < 0.001), less weight loss (p = 0.025), and longer length of stay (p = 0.003). Chloride concentration was independently associated with death or transplant 1 year after admission (hazard ratio 0.94; p < 0.001). As a dichotomous variable, hypochloremia was independently associated with reduced DE (p < 0.001) and decreased 1-year transplant-free survival (hazard ratio 2.3, p < 0.001). Lower serum chloride at hospital admission is strongly and independently associated with impaired DE and reduced transplant-free survival in children hospitalized with ADHF.

The Post-Translational Modification Networking in WNK-Centric Hypertension Regulation and Electrolyte Homeostasis

The with-no-lysine (WNK) kinase family, comprising four serine-threonine protein kinases (WNK1-4), were first linked to hypertension due to their mutations in association with pseudohypoaldosteronism type II (PHAII). WNK kinases regulate crucial blood pressure regulators, SPAK/OSR1, to mediate the post-translational modifications (PTMs) of their downstream ion channel substrates, such as sodium chloride co-transporter (NCC), epithelial sodium chloride (ENaC), renal outer medullary potassium channel (ROMK), and Na/K/2Cl co-transporters (NKCCs). In this review, we summarize the molecular pathways dysregulating the WNKs and their downstream target renal ion transporters. We summarize each of the genetic variants of WNK kinases and the small molecule inhibitors that have been discovered to regulate blood pressure via WNK-triggered PTM cascades.

WNK1 in the kidney

PURPOSE OF REVIEW

The aim of this manuscript was to review recent evidence uncovering the roles of the With No lysine (K) kinase 1 (WNK1) in the kidney.

RECENT FINDINGS

Analyses of microdissected mouse nephron segments have revealed the abundance of long-WNK1 and kidney-specific-WNK1 transcripts in different segments. The low levels of L-WNK1 transcripts in the distal convoluted tubule (DCT) stand out and support functional evidence on the lack of L-WNK1 activity in this segment. The recent description of familial hyperkalaemic hypertension (FHHt)-causative mutations affecting the acidic domain of WNK1 supports the notion that KS-WNK1 activates the Na+:Cl- cotransporter NCC. The high sensitivity of KS-WNK1 to KLHL3-targeted degradation and the low levels of L-WNK1 in the DCT, led to propose that this type of FHHt is mainly due to increased KS-WNK1 protein in the DCT. The observation that KS-WNK1 renal protein expression is induced by low K+ diet and recent reassessment of the phenotype of KS-WNK1-/- mice suggested that KS-WNK1 may be necessary to achieve maximal NCC activation under this condition. Evidences on the regulation of other renal transport proteins by WNK1 are also summarized.

SUMMARY

The diversity of WNK1 transcripts in the kidney has complicated the interpretation of experimental data. Integration of experimental data with the knowledge of isoform abundance in renal cell types is necessary in future studies about WNK1 function in the kidney.

Classic and Novel Mechanisms of Diuretic Resistance in Cardiorenal Syndrome

Despite the incompletely understood multiple etiologies and underlying mechanisms, cardiorenal syndrome is characterized by decreased glomerular filtration and sodium avidity. The underlying level of renal sodium avidity is of primary importance in driving a congested heart failure phenotype and ultimately determining the response to diuretic therapy. Historically, mechanisms of kidney sodium avidity and resultant diuretic resistance were primarily extrapolated to cardiorenal syndrome from non-heart failure populations. Yet, the mechanisms appear to differ between these populations. Recent literature in acute decompensated heart failure has refuted several classically accepted diuretic resistance mechanisms and reshaped how we conceptualize diuretic resistance mechanisms in cardiorenal syndrome. Herein, we propose an anatomically based categorization of diuretic resistance mechanisms to establish the relative importance of specific transporters and translate findings toward therapeutic strategies. Within this categorical structure, we discuss classic and novel mechanisms of diuretic resistance.

Anatomophysiology of the Henle's Loop: Emphasis on the Thick Ascending Limb

The loop of Henle plays a variety of important physiological roles through the concerted actions of ion transport systems in both its apical and basolateral membranes. It is involved most notably in extracellular fluid volume and blood pressure regulation as well as Ca²⁺ , Mg²⁺ , and acid-base homeostasis because of its ability to reclaim a large fraction of the ultrafiltered solute load. This nephron segment is also involved in urinary concentration by energizing several of the steps that are required to generate a gradient of increasing osmolality from cortex to medulla. Another important role of the loop of Henle is to sustain a process known as tubuloglomerular feedback through the presence of specialized renal tubular cells that lie next to the juxtaglomerular arterioles. This article aims at describing these physiological roles and at discussing a number of the molecular mechanisms involved. It will also report on novel findings and uncertainties regarding the realization of certain processes and on the pathophysiological consequences of perturbed salt handling by the thick ascending limb of the loop of Henle. Since its discovery 150 years ago, the loop of Henle has remained in the spotlight and is now generating further interest because of its role in the renal-sparing effect of SGLT2 inhibitors. © 2022 American Physiological Society. Compr Physiol 12:1-21, 2022.

Cation-coupled chloride cotransporters: chemical insights and disease implications

Cation-coupled chloride cotransporters (CCCs) modulate the transport of sodium and/or potassium cations coupled with chloride anions across the cell membrane. CCCs thus help regulate intracellular ionic concentration and consequent cell volume homeostasis. This has been largely exploited in the past to develop diuretic drugs that act on CCCs expressed in the kidney. However, a growing wealth of evidence has demonstrated that CCCs are also critically involved in a great variety of other pathologies, motivating most recent drug discovery programs targeting CCCs. Here, we examine the structure–function relationship of CCCs. By linking recent high-resolution cryogenic electron microscopy (cryo-EM) data with older biochemical/functional studies on CCCs, we discuss the mechanistic insights and opportunities to design selective CCC modulators to treat diverse pathologies.

The structural topology and function of all cation-coupled chloride cotransporters (CCCs) have been continuously investigated over the past 40 years, with great progress also thanks to the recent cryogenic electron microscopy (cryo-EM) resolution of the structures of five CCCs. In particular, such studies have clarified the structure–function relationship for the Na-K-Cl cotransporter NKCC1 and K-Cl cotransporters KCC1–4.

The constantly growing evidence of the crucial involvement of CCCs in physiological and various pathological conditions, as well as the evidence of their wide expression in diverse body tissues, has promoted CCCs as targets for the discovery and development of new, safer, and more selective/effective drugs for a plethora of pathologies.

Post-translational modification anchor points on the structure of CCCs may offer alternative strategies for small molecule drug discovery.

Inhibitory effect of tumor necrosis factor-α on the basolateral Kir4.1/Kir5.1 channels in the thick ascending limb during diabetes

Diabetic nephropathy is a major contributor to the morbidity and mortality of patients with diabetes. TNF-α expression is elevated during diabetes and is implicated in the pathogenesis of diabetic nephropathy; however, its underlying molecular mechanisms remain unclear. The present study aimed to investigate the effect and molecular mechanism of TNF-α on the basolateral inwardly rectifying potassium (Kir)4.1/Kir5.1 channels in the thick ascending limb (TAL) of rat kidneys using western blotting and the patch clamp technique to provide a theoretical basis for the cause of the decrease in kidney concentrating capacity during diabetes. The results demonstrated that urinary TNF-α excretion and protein TNF-α expression in the TAL increased and basolateral Kir4.1/Kir5.1 channel activity decreased during diabetes; however, diabetic rats exhibited amelioration of Kir4.1/Kir5.1 activity with a soluble TNF-α antagonist, TNF receptor fusion protein (TNFR:Fc). These results suggested that TNF-α inhibited the activity of the basolateral Kir4.1/Kir5.1 channel in the TAL of rat kidneys during diabetes. In addition, the protein expression levels of phospholipase A₂ (PLA₂) and cyclooxygenase-₂ (COX₂) increased in diabetic rats, the effects of which deceased following treatment with TNFR:Fc compared with the diabetic group. Furthermore, an agonist of PLA₂ (melittin) and COX₂ production [prostaglandin E₂ (PGE₂)] inhibited the basolateral Kir4.1/Kir5.1 channels. Taken together, the results of the present study suggested that the inhibitory effect of TNF-α on the basolateral Kir4.1/Kir5.1 channels in the TAL during diabetes is mediated by the PLA₂/COX₂/PGE₂ signaling pathway.

A five amino acids deletion in NKCC2 of C57BL/6 mice affects analysis of NKCC2 phosphorylation but does not impact kidney function

Aim

The phosphorylation level of the furosemide‐sensitive Na⁺‐K⁺‐2Cl⁻ cotransporter (NKCC2) in the thick ascending limb (TAL) is used as a surrogate marker for NKCC2 activation and TAL function. However, in mice, analyses of NKCC2 phosphorylation with antibodies against phosphorylated threonines 96 and 101 (anti‐pT96/pT101) give inconsistent results. We aimed (a) to elucidate these inconsistencies and (b) to develop a phosphoform‐specific antibody that ensures reliable detection of NKCC2 phosphorylation in mice.

Methods

Genetic information, molecular biology, biochemical techniques and mouse phenotyping was used to study NKCC2 and kidney function in two commonly used mouse strains (ie 129Sv and in C57BL/6 mice). Moreover, a new phosphoform‐specific mouse NKCC2 antibody was developed and characterized.

Results

Amino acids sequence alignment revealed that C57BL/6 mice have a strain‐specific five amino acids deletion (ΔF97‐T101) in NKCC2 that diminishes the detection of NKCC2 phosphorylation with previously developed pT96/pT101 NKCC2 antibodies. Instead, the antibodies cross‐react with the phosphorylated thiazide‐sensitive NaCl cotransporter (NCC), which can obscure interpretation of results. Interestingly, the deletion in NKCC2 does not impact on kidney function and/or expression of renal ion transport proteins as indicated by the analysis of the F2 generation of crossbred 129Sv and C57BL/6 mice. A newly developed pT96 NKCC2 antibody detects pNKCC2 in both mouse strains and shows no cross‐reactivity with phosphorylated NCC.

Conclusion

Our work reveals a hitherto unappreciated, but essential, strain difference in the amino acids sequence of mouse NKCC2 that needs to be considered when analysing NKCC2 phosphorylation in mice. The new pNKCC2 antibody circumvents this technical caveat.

Chloride in Heart Failure Syndrome: Its Pathophysiologic Role and Therapeutic Implication

Until recently, most studies of heart failure (HF) focused on body fluid dynamics through control of the sodium and water balance in the body. Chloride has remained largely ignored in the medical literature, and in clinical practice, chloride is generally considered as an afterthought to the better-known electrolytes of sodium and potassium. In recent years, however, the important role of chloride in the distribution of body fluid has emerged in the field of HF pathophysiology. Investigation of HF pathophysiology according to the dynamics of serum chloride is rational considering that chloride is an established key electrolyte for tubulo-glomerular feedback in the kidney and a possible regulatory electrolyte for body fluid distribution. The present review provides a historical overview of HF pathophysiology, followed by descriptions of the recent attention to the electrolyte chloride in the cardiovascular field, the known role of chloride in the human body, and recent new findings regarding the role of chloride leading to the proposed ‘chloride theory’ hypothesis in HF pathophysiology. Next, vascular and organ congestion in HF is discussed, and finally, a new classification and potential therapeutic strategy are proposed according to the ‘chloride theory’.

Mechanistic interactions of uromodulin with the thick ascending limb: perspectives in physiology and hypertension

Hypertension is a significant risk factor for cardiovascular disease and mortality worldwide. The kidney is a major regulator of blood pressure and electrolyte homeostasis, with monogenic disorders indicating a link between abnormal ion transport and salt-sensitive hypertension. However, the association between salt and hypertension remains controversial. Thus, there is continued interest in deciphering the molecular mechanisms behind these processes. Uromodulin (UMOD) is the most abundant protein in the normal urine and is primarily synthesized by the thick ascending limb epithelial cells of the kidney. Genome-wide association studies have linked common UMOD variants with kidney function, susceptibility to chronic kidney disease and hypertension independent of renal excretory function. This review will discuss and provide predictions on the role of the UMOD protein in renal ion transport and hypertension based on current observational, biochemical, genetic, pharmacological and clinical evidence.

Clinical relevance of serum electrolytes in dogs and cats with acute heart failure: A retrospective study

Background

Hypochloremia is a strong negative prognostic factor in humans with congestive heart failure (CHF), but the implications of electrolyte abnormalities in small animals with acute CHF are unclear.

Objectives

To document electrolyte abnormalities present upon admission of small animals with acute CHF, and to assess the relationship between electrolyte concentrations and diuretic dose, duration of hospitalization and survival time.

Animals

Forty‐six dogs and 34 cats with first onset of acute CHF.

Methods

Retrospective study. The associations between electrolyte concentrations and diuretic doses were evaluated with Spearman rank correlation coefficients. Relationship with duration of hospitalization and survival were assessed by simple linear regression and Cox proportional hazard regression, respectively.

Results

The most commonly encountered electrolyte anomaly was hypochloremia observed in 24% (9/46 dogs and 10/34 cats) of cases. In dogs only, a significant negative correlation was identified between serum chloride concentrations at admission (median 113 mmol/L [97‐125]) and furosemide doses both at discharge (median 5.2 mg/kg/day [1.72‐9.57]; r = −0.59; P < .001) and at end‐stage heart failure (median 4.7 mg/kg/day [2.02‐7.28]; r = −0.62; P = .005). No significant hazard ratios were found for duration of hospitalization nor survival time for any of the electrolyte concentrations.

Conclusions and Clinical Importance

The observed association between serum chloride concentrations and diuretic doses suggests that hypochloremia could serve as a marker of disease severity and therapeutic response in dogs with acute CHF.

Lower serum chloride concentrations are associated with increased risk of mortality in critically ill cirrhotic patients: an analysis of the MIMIC-III database

Background

Cirrhosis can be complicated by electrolyte abnormalities, but the major focus has been concentrated on the clinical significance of serum sodium levels. Emerging studies have identified hypochloremia as an independent prognostic marker in patients with chronic heart failure and chronic kidney disease. The aim of this study was to investigate whether serum chloride levels were associated with mortality of critically ill cirrhotic patients.

Methods

Critically ill cirrhotic patients were identified from the Multi-parameter Intelligent Monitoring in Intensive Care III Database. The primary outcome was ICU mortality. Logistic regression was used to explore the association between serum chloride levels and ICU mortality. The area under the receiver operating characteristic curves (AUC) was used to assess the performance of serum chloride levels for predicting ICU mortality.

Results

A total of 1216 critically ill cirrhotic patients were enrolled in this study. The overall ICU mortality rate was 18.8%. Patients with hypochloremia had a higher ICU mortality than those with non-hypochloremia (34.2% vs. 15.8%; p < 0.001). After multivariable risk adjustment for age, gender, ethnicity, chloride, sodium, Model for End-stage Liver Disease score, Sequential Organ Failure Assessment score, Elixhauser comorbidity index, mechanical ventilation, vasopressors, renal replacement therapy, acute kidney injury, hemoglobin, platelet, and white blood cell, serum chloride levels remained independently associated with ICU mortality (OR 0.94; 95% CI 0.91–0.98; p = 0.002) in contrast to serum sodium levels, which were no longer significant (OR 1.03; 95% CI 0.99–1.08; p = 0.119). The AUC of serum chloride levels (AUC, 0.600; 95% CI 0.556–0.643) for ICU mortality was statistically higher than that of serum sodium levels (AUC, 0.544; 95% CI 0.499–0.590) (p < 0.001).

Conclusions

In critically ill cirrhotic patients, serum chloride levels are independently and inversely associated with ICU mortality, thus highlighting the prognostic role of serum chloride levels which are largely overlooked.

The Role of Serum Chloride in Acute and Chronic Heart Failure: A Narrative Review

Clinical guidelines include diuretics for the treatment of heart failure (HF), not to decrease mortality but to decrease symptoms and hospitalizations. More attention has been paid to the worse outcomes, including mortality, associated with continual diuretic therapy due to hypochloremia. Studies have revealed a pivotal role for serum chloride in the pathophysiology of HF and is now a target of treatment to decrease mortality. The prognostic value of serum chloride in HF has been the subject of much attention. Mechanistically, the macula densa, a region in the renal juxtaglomerular apparatus, relies on chloride levels to sense salt and volume status. The recent discovery of with-no-lysine (K) (WNK) protein kinase as an intracellular chloride sensor sheds light on the possible reason of diuretic resistance in HF. The action of chloride on WNKs results in the upregulation of the sodium-potassium-chloride cotransporter and sodium-chloride cotransporter receptors, which could lead to increased electrolyte and fluid reabsorption. Genetic studies have revealed that a variant of a voltage-sensitive chloride channel (CLCNKA) gene leads to almost a 50% decrease in current amplitude and function of the renal chloride channel. This variant increases the risk of HF. Several trials exploring the prognostic value of chloride in both acute and chronic HF have shown mostly positive results, some even suggesting a stronger role than sodium. However, so far, interventional trials exploring serum chloride as a therapeutic target have been largely inconclusive. This study is a review of the pathophysiologic effects of hypochloremia in HF, the genetics of chloride channels, and clinical trials that are underway to investigate novel approaches to HF management.

Molecular characteristics and physiological roles of Na+ -K+ -Cl- cotransporter 2

Na+ -K+ -Cl- cotransporter 2 (NKCC2; SLC12A1) is an integral membrane protein that comes as three splice variants and mediates the cotranslocation of Na+ , K+ , and Cl- ions through the apical membrane of the thick ascending loop of Henle (TALH). In doing so, and through the involvement of other ion transport systems, it allows this nephron segment to reclaim a large fraction of the ultrafiltered Na+ , Cl- , Ca2+ , Mg2+ , and HCO3- loads. The functional relevance of NKCC2 in human is illustrated by the many abnormalities that result from the inactivation of this transport system through the use of loop diuretics or in the setting of inherited disorders. The following presentation aims at discussing the physiological roles and molecular characteristics of Na+ -K+ -Cl- cotransport in the TALH and those of the individual NKCC2 splice variants more specifically. Many of the historical and recent data that have emerged from the experiments conducted will be outlined and their larger meaning will also be placed into perspective with the aid of various hypotheses.

Baseline and incident hypochloremia in chronic heart failure outpatients: Clinical correlates and prognostic role

BACKGROUND

Electrolyte serum disorders are associated with poor outcome in chronic heart failure. The aim of this study sought to identify the main driver of incident hypochloremia in chronic HF (CHF) outpatients and to determine the prognostic value of baseline and incident hypochloremia.

METHODS

Consecutive CHF outpatients were enrolled and clinical, laboratoristic and echocardiographic evaluations were performed at baseline and repeated yearly in a subgroup of patients. Baseline and incident hypochloremia were evaluated. During an up to 5-year follow-up, all-cause mortality was the primary end-point for outcome.

RESULTS

Among 506 patients enrolled, 120 patients died during follow-up. At baseline, hypochloremia was present in 10% of patients and it was associated with mortality at univariate (HR: 3.25; 95%CI: 2.04-5.18; p<0.001) and at multivariate analysis (HR 2.14; 95%CI: 1.23-3.63; p: 0.005) after correction for well-established CHF prognostic markers. Among patients with repeated evaluations and without baseline hypochloremia, in 13% of these, incident hypochloremia occurred during follow-up and furosemide equivalent daily dose was its first determinant (HR for 1 mg/die: 1.008; 95%CI: 1.004-1.013; p<0.001) at forward stepwise logistic regression analysis. Finally, incident hypochloremia was associated with mortality at univariate (HR: 4.69; 95%CI: 2.69-8.19; p<0.001) as well as at multivariate analysis (HR: 2.97; 95%CI: 1.48-5.94; p: 0.002).

CONCLUSIONS

In CHF outpatients baseline and incident hypochloremia are independently associated with all-cause mortality, thus highlighting the prognostic role of serum chloride levels which are generally unconsidered. Future studies should evaluate if the strict monitoring and correction of hypochloremia could exert a beneficial effect on prognosis.

WNK4 kinase: from structure to physiology

With no lysine kinase-4 (WNK4) belongs to a serine-threonine kinase family characterized by the atypical positioning of its catalytic lysine. Despite the fact that WNK4 has been found in many tissues, the majority of its study has revolved around its function in the kidney, specifically as a positive regulator of the thiazide-sensitive NaCl cotransporter (NCC) in the distal convoluted tubule of the nephron. This is explained by the description of gain-of-function mutations in the gene encoding WNK4 that causes familial hyperkalemic hypertension. This disease is mainly driven by increased downstream activation of the Ste20/SPS1-related proline-alanine-rich kinase/oxidative stress responsive kinase-1-NCC pathway, which increases salt reabsorption in the distal convoluted tubule and indirectly impairs renal K⁺ secretion. Here, we review the large volume of information that has accumulated about different aspects of WNK4 function. We first review the knowledge on WNK4 structure and enumerate the functional domains and motifs that have been characterized. Then, we discuss WNK4 physiological functions based on the information obtained from in vitro studies and from a diverse set of genetically modified mouse models with altered WNK4 function. We then review in vitro and in vivo evidence on the different levels of regulation of WNK4. Finally, we go through the evidence that has suggested how different physiological conditions act through WNK4 to modulate NCC activity.

WNK4 kinase: role of chloride sensing in the distal convoluted tubule

PURPOSE OF REVIEW

This review focuses on recent efforts in identifying with-no-lysine kinase 4 (WNK4) as a physiological intracellular chloride sensor and exploring regulators of intracellular chloride concentration ([Cl-]i) in the distal convoluted tubule (DCT).

RECENT FINDINGS

The discovery of WNK1's chloride-binding site provides the mechanistic details of the chloride-sensing regulation of WNK kinases. The subsequent in-vitro studies reveal that the chloride sensitivities of WNK kinases were variable. Because of its highest chloride sensitivity and dominant expression, WNK4 emerges as the leading candidate of the chloride sensor in DCT. The presentation of hypertension and increased sodium-chloride cotransporter (NCC) activity in chloride-insensitive WNK4 mice proved that WNK4 is inhibitable by physiological [Cl-]i in DCT. The chloride-mediated WNK4 regulation is responsible for hypokalemia-induced NCC activation but unnecessary for hyperkalemia-induced NCC deactivation. This chloride-sensing mechanism requires basolateral potassium and chloride channels or cotransporters, including Kir4.1/5.1, ClC-Kb, and possibly KCCs, to modulate [Cl-]i in response to the changes of plasma potassium.

SUMMARY

WNK4 is both a master NCC stimulator and an in-vivo chloride sensor in DCT. The understanding of chloride-mediated regulation of WNK4 explains the inverse relationship between dietary potassium intake and NCC activity.

High blood pressure induced by vitamin D deficiency is associated with renal overexpression and hyperphosphorylation of Na+-K+-2Cl- cotransporter type 2

OBJECTIVES

Clinical and epidemiological studies have suggested a correlation between vitamin D deficiency (VDD) and high blood pressure (BP). This study aimed to test the hypothesis that high BP induced by VDD is associated with altered expression and covalent modification of apical sodium transporters along the nephron. The contributions of the intrarenal renin-angiotensin system (RAS) and oxidative stress were also investigated.

METHODS

Male Wistar rats were fed a vitamin D-free (n = 26) or standard diet (n = 25) for 30 days. BP was recorded using noninvasive and invasive procedures. The expression levels of total and phosphorylated apical sodium transporters in rat renal cortex and medulla were evaluated by immunoblotting. Intrarenal RAS components were assessed by immunoblotting and ELISA. Renal oxidative stress was analyzed by measuring the concentrations of thiobarbituric acid reactive substances and reduced glutathione.

RESULTS

Higher BP levels in VDD rats than controls were accompanied by overexpression and hyperphosphorylation of renal cortical and medullary Na+-K+-2Cl- cotransporter type 2, enhanced levels of phosphorylated Na+/H+ exchanger type 3, and reduced expression levels of total and phosphorylated Na+/Cl- cotransporter. Changes in intrarenal RAS induced by VDD vs. controls included the marked elevation of medullary renin expression, higher expression of cortical angiotensinogen, higher urinary angiotensinogen excretion, and higher cortical and medullary angiotensin II content. VDD rats displayed higher thiobarbituric acid reactive substances/glutathione ratios in the renal cortex and medulla than controls.

CONCLUSION

These results suggest that the molecular mechanisms underlying the effects of VDD on BP may include the upregulation of Na+-K+-2Cl- cotransporter type 2 and activation of intrarenal RAS and oxidative stress.

Hypochloraemia in Patients with Heart Failure: Causes and Consequences

Hypochloraemia is a common electrolyte abnormality in patients with heart failure (HF). It has a strong association with adverse outcome regardless of HF phenotype and independent of other prognostic markers. How hypochloraemia develops in a patient with HF and how it might influence outcome are not clear, and in this review we explore the possible mechanisms. Patients with HF and hypochloraemia almost invariably take higher doses of loop diuretic than patients with normal chloride levels. However, renal chloride and bicarbonate homeostasis are closely linked, and the latter may be influenced by neurohormonal activation: it is likely that the etiology of hypochloraemia in patients with HF is multifactorial and due to more than just diuretic-induced urinary losses. There are multiple proposed mechanisms by which low chloride concentrations may lead to an adverse outcome in patients with HF: by increasing renin release; by a stimulatory effect on the with-no-lysine kinases which might increase renal sodium-chloride co-transporter activity; and by an adverse effect on myocardial conduction and contractility. None of these proposed mechanisms are proven in humans with HF. However, if true, it might suggest that hypochloraemia is a therapeutic target that might be amenable to treatment with acetazolamide or chloride supplementation.

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

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

Co-Existence of Congenital Adrenal Hyperplasia and Bartter Syndrome due to Maternal Uniparental Isodisomy of HSD3B2 and CLCNKB Mutations

INTRODUCTION

We present a patient with co-existence of 3β-hydroxysteroid dehydrogenase type 2 (HSD3B2) deficiency and Bartter syndrome, a unique dual combination of opposing pathologies that has not been reported previously in the literature.

CASE

A female infant (46,XX) born at 34/40 weeks' gestation, weighing 2.67 kg (-1.54 standard deviation score) to non-consanguineous parents presented on day 4 of life with significant weight loss. Subsequent investigations revealed hyponatraemia, hypochloraemia, metabolic alkalosis, elevated 17-hydroxyprogesterone, ACTH, and renin. Urine steroid profile suggested HSD3B2 deficiency, which was confirmed by the identification of a homozygous HSD3B2 mutation. Due to the persistence of the hypochlo-raemic and hypokalemic alkalosis, an underlying renal tubulopathy was suspected. Sequence analysis of a targeted tubulopathy gene panel revealed a homozygous deletion in CLCNKB, consistent with Bartter syndrome type 3. The mother was found to be heterozygous for both mutations in -HSD3B2 and CLCNKB, and the father was negative for both. Single-nucleotide polymorphism microarray analysis confirmed 2 segments of homozygosity on chromosome 1 of maternal ancestry, encompassing both HSD3B2 and CLCKNB.

DISCUSSION

Identification of a homozygous rare mutation in an offspring of non-consanguineous parents should raise suspicion of uniparental disomy, especially if the phenotype is unusual, potentially encompassing more than one disorder. The persistence of hypokalemic alkalosis, the biochemical fingerprint of hyperaldosteronism in a child with a form of CAH in which aldosterone production is severely impaired, challenges our current understanding of mineralocorticoid-mediated effects in the collecting duct.

Molecular regulation of NKCC2 in blood pressure control and hypertension

PURPOSE OF REVIEW

The apical Na/K/2Cl cotransporter (NKCC2) mediates NaCl reabsorption by the thick ascending limb, contributing to maintenance of blood pressure (BP). Despite effective NKCC2 inhibition by loop diuretics, these agents are not viable for long-term management of BP due to side effects. Novel molecular mechanisms that control NKCC2 activity reveal an increasingly complex picture with interacting layers of NKCC2 regulation. Here, we review the latest developments that shine new light on NKCC2-mediated control of BP and potential new long-term therapies to treat hypertension.

RECENT FINDINGS

Emerging molecular NKCC2 regulators, often binding partners, reveal a complex overlay of interacting mechanisms aimed at fine tuning NKCC2 activity. Different factors achieve this by shifting the balance between trafficking steps like exocytosis, endocytosis, recycling and protein turnover, or by balancing phosphorylation vs. dephosphorylation. Further molecular details are also emerging on previously known pathways of NKCC2 regulation, and recent in-vivo data continues to place NKCC2 regulation at the center of BP control.

SUMMARY

Several layers of emerging molecular mechanisms that control NKCC2 activity may operate simultaneously, but they can also be controlled independently. This provides an opportunity to identify new pharmacological targets to fine-tune NKCC2 activity for BP management.

Transcriptomic profiles of Dunaliella salina in response to hypersaline stress

Background

Dunaliella salina is a good model organism for studying salt stress. In order to have a global understanding of the expression profiles of Dunaliella salina in response to hypersaline stress, we performed quantitative transcriptomic analysis of Dunaliella salina under hypersaline stress (2.5 M NaCl) of different time duration by the second and third generation sequencing method.

Results

Functional enrichment of the up-regulated genes was used to analyze the expression profiles. The enrichment of photosynthesis was observed, accompanied by enrichments of carbon fixation, pigment biosynthetic process and heme biosynthetic process, which also imply the enhancement of photosynthesis. Genes responsible for starch hydrolysis and glycerol synthesis were significantly up-regulated. The enrichment of biosynthesis of unsaturated fatty acids implies the plasma membrane undergoes changes in desaturation pattern. The enrichment of endocytosis implies the degradation of plasma membrane and might help the synthesis of new glycerophospholipid with unsaturated fatty acids. Co-enrichments of protein synthesis and degradation imply a higher protein turnover rate. The enrichments of spliceosome and protein processing in endoplasmic reticulum imply the enhancement of regulations at post-transcriptional and post-translational level. No up-regulation of any Na⁺ or Cl⁻ channels or transporters was detected, which implies that the extra exclusion of the ions by membrane transporters is possibly not needed. Voltage gated Na⁺ and Cl⁻ channels, mechanosensitive ion channel are possible signal receptors of salt stress, and Ca²⁺ and MAP kinase pathways might play a role in signal transduction.

Conclusion

At global transcriptomic level, the response of Dunaliella salina to hypersaline stress is a systematic work, possibly involving enhancements of photosynthesis, carbon fixation, and heme biosynthetic process, acceleration of protein turnover, spliceosome, protein processing in endoplasmic reticulum, and endocytosis, as well as degradation of starch, synthesis of glycerol, membrane lipid desaturation. Altogether, the changes of these biological processes occurred at trancriptomic level will help understand how a new intracellular balance achieved in Dunaliella salina to adapt to hypersaline environment, which are worth being confirmed at the physiological levels.

Cl- as a bona fide signaling ion

Cl^- is the major extracellular (Cl^-_out) and intracellular (Cl^-_in) anion whose concentration is actively regulated by multiple transporters. These transporters generate Cl^- gradients across the plasma membrane and between the cytoplasm and intracellular organelles. [Cl^-]_in changes rapidly in response to cell stimulation and influences many physiological functions, as well as cellular and systemic homeostasis. However, less appreciated is the signaling function of Cl^-. Cl^- interacts with multiple proteins to directly modify their activity. This review highlights the signaling function of Cl^- and argues that Cl^- is a bona fide signaling ion, a function deserving extensive exploration.

Cardiogenic Shock Due to End-Stage Heart Failure and Acute Myocardial Infarction: Characteristics and Outcome of Temporary Mechanical Circulatory Support

BACKGROUND

Mechanical circulatory support (MCS) is increasingly used in cardiogenic shock, but outcomes may differ between patients with acute myocardial infarction (AMI) or end-stage heart failure (ESHF). This study aimed to describe the characteristics of patients with cardiogenic shock due to AMI and ESHF.

METHODS

Single-center study of consecutive patients with cardiogenic shock due to AMI (n = 26) and ESHF (n = 42) who underwent MCS (extracorporeal life support, Impella or temporary ventricular assist devices). Arterial and venous O2 content and CO2 tension (PCO2), O2-hemoglobin affinity (P50) were measured. Veno-arterial difference in PCO2/arterio-venous difference in O2 content ratio was derived. Acid-base balance was characterized by the Gilfix method. MCS-related complications that required intervention or surgery were collected.

RESULTS

Patients with ESHF had lower ejection fraction, higher right and left-sided filling pressures, pulmonary artery pressure and vascular resistance, lower oxygen delivery (DO2) compared with AMI, which was not fully compensated by the increased hemoglobin P50. As a result, patients with ESHF had higher veno-arterial difference in PCO2 relative to arterio-venous difference in O2 content. Despite greater anerobic metabolism, patients with ESHF had less severe metabolic acidosis and base deficit compared with AMI, predominantly due to differences in strong ions.

CONCLUSION

The cardiogenic shock phenotype in ESHF was distinct from AMI, characterized by higher filling and pulmonary artery pressures, lower DO2, greater anaerobic metabolism but less severe metabolic acidosis.

Diuretic Resistance in Cardio-Nephrology: Role of Pharmacokinetics, Hypochloremia, and Kidney Remodeling

Background: Diuretic resistance is among the most challenging problems that the cardio-nephrologist must address in daily clinical practice, with a considerable burden on hospital admissions and health care costs. Indeed, loop diuretics are the first-line therapy to overcome fluid overload in heart failure patients. The pathophysiological mechanisms of fluid and sodium retention are complex and depend on several neuro-hormonal signals mainly acting on sodium reabsorption along the renal tubule. Consequently, doses and administration modalities of diuretics must be carefully tailored to patients in order to overcome under- or overtreatment. The frequent and tricky development of diuretic resistance depends in part on post-diuretic sodium retention, reduced tubular secretion of the drug, and reduced sodium/chloride sensing. Sodium and chloride depletions have been recently shown to be major factors mediating these processes. Aquaretics and high-saline infusions have been recently suggested in cases of hyponatremic conditions. This review discusses the limitations and strengths of these approaches. Summary: Long-term diuretic use may lead to diuretic resistance in cardio-renal syndromes. To overcome this complication intravenous administration of loop diuretics and a combination of different diuretic classes have been proposed. In the presence of hyponatremia, high-saline solutions in addition to loop diuretics might be beneficial, whereas aquaretics require caution to avoid overcorrection. Key Messages: Diuretic resistance is a central theme for cardio-renal syndromes. Hyponatremia and hypochloremia may be part of the mechanisms for diuretic resistance. Aquaretics and high-saline solutions have been proposed as possible new therapeutic solutions.

Endocytic recycling of Na+ -K+ -Cl- cotransporter type 2: importance of exon 4

KEY POINTS

Na⁺ -K⁺ -Cl^- cotransporter type 2 (NKCC2) is a 27-exon membrane protein that is expressed in the thick ascending limb (TAL) of Henle where it is involved in reabsorption of the ultrafiltered NaCl load. It comes as three splice variants that are identical to each other except for the residue composition of exon 4 and that differ in their transport characteristics, functional roles and distributions along the TAL. In this report, it is shown that the variants also differ in their trafficking properties and that two residues in exon 4 play a key role in this regard. One of these residues was also shown to sustain carrier internalization. Through these results, a novel function for the alternatively spliced exon of NKCC2 has been identified and a domain that is involved in carrier trafficking has been uncovered for the first time in a cation-Cl^- cotransporter family member.

ABSTRACT

Na⁺ -K⁺ -Cl^- cotransporter type 2 (NKCC2) is a 12-transmembrane (TM) domain cell surface glycoprotein that is expressed in the thick ascending limb (TAL) of Henle and stimulated during cell shrinkage. It comes as three splice variants (A, B and F) that are identical to each other except for TM2 and the following connecting segment (CS2). Yet, these variants do not share the same localization, transport characteristics and physiological roles along the TAL. We have recently found that while cell shrinkage could exert its activating effect by increasing NKCC2 expression at the cell surface, the variants also responded differentially to this stimulus. In the current work, a mutagenic approach was exploited to determine whether CS2 could play a role in carrier trafficking and identify the residues potentially involved. We found that when the residue of position 238 in NKCC2A (F) and NKCC2B (Y) was replaced by the corresponding residue in NKCC2F (V), carrier activity increased by over 3-fold and endocytosis decreased concomitantly. We also found that when the residue of position 230 in NKCC2F (M) was replaced by the one in NKCC2B (T), carrier activity and affinity for ions both increased substantially whereas expression at the membrane decreased. Taken together, these results suggest that CS2 is involved in carrier trafficking and that two of its residues, those of positions 238 and 230, are part of an internalization motif. They also indicate that the divergent residue of position 230 plays the dual role of specifying ion affinity and sustaining carrier internalization.

Superoxide increases surface NKCC2 in the rat thick ascending limbs via PKC

The apical Na⁺-K⁺-2Cl^- cotransporter (NKCC2) mediates NaCl reabsorption by the thick ascending limb (TAL). The free radical superoxide ( O2- ) stimulates TAL NaCl absorption by enhancing NKCC2 activity. In contrast, nitric oxide (NO) scavenges O2- and inhibits NKCC2. NKCC2 activity depends on the number of NKCC2 transporters in the TAL apical membrane and its phosphorylation. We hypothesized that O2- stimulates NKCC2 activity by enhancing apical surface NKCC2 expression. We measured surface NKCC2 expression in rat TALs by surface biotinylation and Western blot analysis. Treatment of TALs with O2- produced by exogenous xanthine oxidase (1 mU/ml) and hypoxanthine (500 µM) stimulated surface NKCC2 expression by ~18 ± 5% (P < 0.05). O2- -stimulated surface NKCC2 expression was blocked by the O2- scavenger tempol (50 µM). Scavenging H₂O₂ with 100 U/ml catalase did not block the stimulatory effect of xanthine oxidase-hypoxanthine (22 ± 8% increase from control, P < 0.05). Inhibition of endogenous NO production with N^ω-nitro-l-arginine methyl ester enhanced surface NKCC2 expression by 21 ± 6% and, when added together with xanthine oxidase-hypoxanthine, increased surface NKCC2 by 41 ± 10% (P < 0.05). Scavenging O2- with superoxide dismutase (300 U/ml) decreased this stimulatory effect by 60% (39 ± 4% to 15 ± 10%, P < 0.05). Protein kinase C inhibition with Gö-6976 (100 nM) blocked O2- -stimulated surface NKCC2 expression (P < 0.05). O2- did not affect NKCC2 phosphorylation at Thr^96/101 or its upstream kinases STE20/SPS1-related proline/alanine-rich kinase-oxidative stress-responsive kinase 1. We conclude that O2- increases surface NKCC2 expression by stimulating protein kinase C and that this effect is blunted by endogenous NO. O2- -stimulated apical trafficking of NKCC2 may be involved in the enhanced surface NKCC2 expression observed in Dahl salt-sensitive rats.

Intracellular chloride: a regulator of transepithelial transport in the distal nephron

PURPOSE OF REVIEW

This review focuses on the role of intracellular chloride in regulating transepithelial ion transport in the distal convoluted tubule (DCT) in response to perturbations in plasma potassium homeostasis.

RECENT FINDINGS

Low dietary potassium increases the phosphorylation and activity of the sodium chloride cotransporter (NCC) in the DCT, and vice versa, affecting sodium-dependent potassium secretion in the downstream aldosterone-sensitive distal nephron. In cells, NCC phosphorylation is increased by lowering of intracellular chloride, via activation of the chloride-sensitive with no lysine (WNK)-SPAK/OSR1 (Ste20-related proline/alanine-rich kinase/oxidative stress response) kinase cascade. In-vivo studies have demonstrated pathway activation in the kidney in response to low dietary potassium. A possible mechanism is lowering of DCT intracellular chloride in response to low potassium because of parallel basolateral potassium and chloride channels. Recent studies support a role for these channels in the response of NCC to varying potassium. Studies examining chloride-insensitive WNK mutants, in the Drosophila renal tubule and in the mouse, lend further support to a role for chloride in regulating WNK activity and transepithelial ion transport. Caveats, alternatives, and future directions are also discussed.

SUMMARY

Chloride sensing by WNK kinase provides a mechanism to allow coupling of extracellular potassium with NCC phosphorylation and activity to maintain potassium homeostasis.