Influence of Proteolytic Cleavage of ENaC's Gamma Subunit upon Na+ and K+ Handling

The ENaC γ subunit is essential for homeostasis of Na+, K+, and body fluid. Dual γ subunit cleavage before and after a short inhibitory tract allows dissociation of this tract, increasing channel open probability (PO), in vitro. Cleavage proximal to the tract occurs at a furin recognition sequence (143RKRR146, in the mouse γ subunit). Loss of furin-mediated cleavage prevents in vitro activation of the channel by proteolysis at distal sites. We hypothesized that 143RKRR146 mutation to 143QQQQ146 (γQ4) in 129/Sv mice would reduce ENaC PO, impair flow-stimulated flux of Na+ (JNa) and K+ (JK) in perfused collecting ducts, reduce colonic amiloride-sensitive short circuit current (ISC), and impair Na+, K+, and body fluid homeostasis. Immunoblot of γQ4/Q4 mouse kidney lysates confirmed loss of a band consistent in size with the furin-cleaved proteolytic fragment. However, γQ4/Q4 male mice on a low Na+ diet did not exhibit altered ENaC PO or flow-induced JNa, though flow-induced JK modestly decreased. Colonic amiloride-sensitive ISC in γQ4/Q4 mice was not altered. γQ4/Q4 males, but not females, exhibited mildly impaired fluid volume conservation when challenged with a low Na+ diet. Blood Na+ and K+ were unchanged on a regular, low Na+, or high K+ diet. These findings suggest that biochemical evidence of γ subunit cleavage should be used in isolation to evaluate ENaC activity. Further, factors independent of γ subunit cleavage modulate channel PO and the influence of ENaC on Na+, K+, and fluid volume homeostasis in 129/Sv mice, in vivo.

The Human Gut and Dietary Salt: The Bacteroides/Prevotella Ratio as a Potential Marker of Sodium Intake and Beyond

The gut microbiota is a dynamic ecosystem that plays a pivotal role in maintaining host health. The perturbation of these microbes has been linked to several health conditions. Hence, they have emerged as promising targets for understanding and promoting good health. Despite the growing body of research on the role of sodium in health, its effects on the human gut microbiome remain under-explored. Here, using nutrition and metagenomics methods, we investigate the influence of dietary sodium intake and alterations of the human gut microbiota. We found that a high-sodium diet (HSD) altered the gut microbiota composition with a significant reduction in Bacteroides and inverse increase in Prevotella compared to a low-sodium diet (LSD). However, there is no clear distinction in the Firmicutes/Bacteroidetes (F/B) ratio between the two diet types. Metabolic pathway reconstruction revealed the presence of sodium reabsorption genes in the HSD, but not LSD. Since it is currently difficult in microbiome studies to confidently associate the F/B ratio with what is considered healthy (e.g., low sodium) or unhealthy (e.g., high sodium), we suggest that the use of a genus-based ratio such as the Bacteroides/Prevotella (B/P) ratio may be more beneficial for the application of microbiome studies in health.

Inhibition the ubiquitination of ENaC and Na,K-ATPase with erythropoietin promotes alveolar fluid clearance in sepsis-induced acute respiratory distress syndrome

Sepsis-induced acute respiratory distress syndrome (ARDS) causes significant fatalities worldwide and lacks pharmacological intervention. Alveolar fluid clearance (AFC) plays a pivotal role in the remission of ARDS and is markedly impaired in the pathogenesis of ARDS. Here, we demonstrated that erythropoietin could effectively ameliorate lung injury manifestations and lethality, restore lung function and promote AFC in a rat model of lipopolysaccharide (LPS)-induced ARDS. Moreover, it was proven that EPO-induced restoration of AFC occurs through triggering the total protein expression of ENaC and Na,K-ATPase channels, enhancing their protein abundance in the membrane, and suppressing their ubiquitination for degeneration. Mechanistically, the data indicated the possible involvement of EPOR/JAK2/STAT3/SGK1/Nedd4-2 signaling in this process, and the pharmacological inhibition of the pathway markedly eliminated the stimulating effects of EPO on ENaC and Na,K-ATPase, and subsequently reversed the augmentation of AFC by EPO. Consistently, in vitro studies of alveolar epithelial cells paralleled with that EPO upregulated the expression of ENaC and Na,K-ATPase, and patch-clamp studies further demonstrated that EPO substantially strengthened sodium ion currents. Collectively, EPO could effectively promote AFC by improving ENaC and Na,K-ATPase protein expression and abundance in the membrane, dependent on inhibition of ENaC and Na,K-ATPase ubiquitination, and resulting in diminishing LPS-associated lung injuries.

Dietary sodium alters aldosterone's effect on renal sodium transporter expression and distal convoluted tubule remodelling

Aldosterone is responsible for maintaining volume and potassium homeostasis. Although high salt consumption should suppress aldosterone production, individuals with hyperaldosteronism lose this regulation, leading to a state of high aldosterone despite dietary sodium consumption. The present study examines the effects of elevated aldosterone, with or without high salt consumption, on the expression of key Na+ transporters and remodelling in the distal nephron. Epithelial sodium channel (ENaC) α-subunit expression was increased with aldosterone regardless of Na+ intake. However, ENaC β- and γ-subunits unexpectedly increased at both a transcript and protein level with aldosterone when high salt was present. Expression of total and phosphorylated Na+ Cl- cotransporter (NCC) significantly increased with aldosterone, in association with decreased blood [K+ ], but the addition of high salt markedly attenuated the aldosterone-dependent NCC increase, despite equally severe hypokalaemia. We hypothesized this was a result of differences in distal convoluted tubule length when salt was given with aldosterone. Imaging and measurement of the entire pNCC-positive tubule revealed that aldosterone alone caused a shortening of this segment, although the tubule had a larger cross-sectional diameter. This was not true when salt was given with aldosterone because the combination was associated with a lengthening of the tubule in addition to increased diameter, suggesting that differences in the pNCC-positive area are not responsible for differences in NCC expression. Together, our results suggest the actions of aldosterone, and the subsequent changes related to hypokalaemia, are altered in the presence of high dietary Na+ . KEY POINTS: Aldosterone regulates volume and potassium homeostasis through effects on transporters in the kidney; its production can be dysregulated, preventing its suppression by high dietary sodium intake. Here, we examined how chronic high sodium consumption affects aldosterone's regulation of sodium transporters in the distal nephron. Our results suggest that high sodium consumption with aldosterone is associated with increased expression of all three epithelial sodium channel subunits, rather than just the alpha subunit. Aldosterone and its associated decrease in blood [K+ ] lead to an increased expression of Na-Cl cotransporter (NCC); the addition of high sodium consumption with aldosterone partially attenuates this NCC expression, despite similarly low blood [K+ ]. Upstream kinase regulators and tubule remodelling do not explain these results.

KDM6A Demethylase Regulates Renal Sodium Excretion and Blood Pressure

BACKGROUND

KDM6A (Lysine-Specific Demethylase 6A) is a specific demethylase for histone 3 lysine (K) 27 trimethylation (H3K27me3). The purpose of this study is to investigate whether KDM6A in renal tubule cells plays a role in the regulation of kidney function and blood pressure.

METHODS

We first crossed Ksp-Cre+/- and KDM6Aflox/flox mice for generating inducible kidney-specific deletion of KDM6A gene.

RESULTS

Notably, conditional knockout of KDM6A gene in renal tubule cells (KDM6A-cKO) increased H3K27me3 levels which leads to a decrease in Na excretion and elevation of blood pressure. Further analysis showed that the expression of NKCC2 (Na-K-2Cl cotransporter 2) and NCC (Na-Cl cotransporters) was upregulated which contributes to impaired Na excretion in KDM6A-cKO mice. The expression of AQP2 (aquaporin 2) was also increased in KDM6A-cKO mice, which may facilitate water reabsorption in KDM6A-cKO mice. The expression of Klotho was downregulated while expression of aging markers including p53, p21, and p16 was upregulated in kidneys of KDM6A-cKO mice, indicating that deletion of KDM6A in the renal tubule cells promotes kidney aging. Interestingly, KDM6A-cKO mice developed salt-sensitive hypertension which can be rescued by treatment with Klotho. KDM6A deficiency induced salt-sensitive hypertension likely through downregulation of the Klotho/ERK (extracellular signal-regulated kinase) signaling and upregulation of the WNK (with-no-lysine kinase) signaling.

CONCLUSIONS

This study provides the first evidence that KDM6A plays an essential role in maintaining normal tubular function and blood pressure. Renal tubule cell specific KDM6A deficiency causes hypertension due to increased H3K27me3 levels and the resultant downregulation of Klotho gene expression which disrupts the Klotho/ERK/NCC/NKCC2 signaling.

Collecting duct NCOR1 controls blood pressure by regulating mineralocorticoid receptor

INTRODUCTION

Nuclear receptor corepressor 1(NCOR1) is reported to play crucial roles in cardiovascular diseases, but its function in the kidney has remained obscure.

OBJECTIVE

We aim to elucidate the role of collecting duct NCOR1 in blood pressure (BP) regulation.

METHODS AND RESULTS

Collecting duct NCOR1 knockout (KO) mice manifested increased BP and aggravated vascular and renal injury in an angiotensin II (Ang II)-induced hypertensive model. KO mice also showed significantly higher BP than littermate control (LC) mice in deoxycorticosterone acetate (DOCA)-salt model. Further study showed that collecting duct NCOR1 deficiency aggravated volume and sodium retention after saline challenge. Among the sodium transporter in the collecting duct, the expression of the three epithelial sodium channel (ENaC) subunits was markedly increased in the renal medulla of KO mice. Consistently, BP in Ang II-infused KO mice decreased significantly to the similar level as those in LC mice after amiloride treatment. ChIP analysis revealed that NCOR1 deficiency increased the enrichment of mineralocorticoid receptor (MR) on the promoters of the three ENaC genes in primary inner medulla collecting duct (IMCD) cells. Co-IP results showed interaction between NCOR1 and MR, and luciferase reporter results demonstrated that NCOR1 inhibited the transcriptional activity of MR. Knockdown of MR eliminated the increased ENaC expression in primary IMCD cells isolated from KO mice. Finally, BP was significantly decreased in Ang II-infused KO mice after treatment of MR antagonist spironolactone and the difference between LC and KO mice was abolished.

CONCLUSIONS

NCOR1 interacts with MR to control ENaC activity in the collecting duct and to regulate sodium reabsorption and ultimately BP. Targeting NCOR1 might be a promising tactic to interrupt the volume and sodium retention of the collecting duct in hypertension.

Activation of renal epithelial Na+ channels (ENaC) in infants with congenital heart disease

Introduction

This study was designed to measure the concentration and activity of urinary proteases that activate renal epithelial sodium channel (ENaC) mediated Na+ transport in infants with congenital heart disease, a potential mechanism for fluid retention.

Methods

Urine samples from infants undergoing cardiac surgery were collected at three time points: T1) pre-operatively, T2) 6-8 h after surgery, and T3) 24 h after diuretics. Urine was collected from five heathy infant controls. The urine was tested for four proteases and whole-cell patch-clamp testing was conducted in renal collecting duct M-1 cells to test whether patient urine increased Na+ currents consistent with ENaC activation.

Results

Heavy chain of plasminogen, furin, and prostasin were significantly higher in cardiac patients prior to surgery compared to controls. There was no difference in most proteases before and after surgery. Urine from cardiac patients produced a significantly greater increase in Na+ inward currents compared to healthy controls.

Conclusion

Urine from infants with congenital heart disease is richer in proteases and has the potential to increase activation of ENaC in the nephron to enhance Na+ reabsorption, which may lead to fluid retention in this population.

Increased ENaC-mediated liquid absorption across vitamin-D deficient human airway epithelia

Vitamin D deficiency is a risk factor for exacerbation of obstructive airway disease, a hallmark of which is mucus dehydration and plugging. Calcitriol (the active form of vitamin D) deficiency in cultured human airway epithelia resulted in increased SCNN1G and ATP1B1 mRNAs encoding subunits of ENaC and the Na-K pump compared with supplemented epithelia. These drive the absorption of airway surface liquid. Consistently, calcitriol-deficient epithelia absorbed liquid faster than supplemented epithelia. Calcitriol deficiency also increased amiloride-sensitive Isc and Gt without altering Na-K pump activity, indicating the changes in amiloride-sensitivity arose from ENaC. ENaC activity can be regulated by trafficking, proteases, and channel abundance. We found the effect was likely not induced by changes to endocytosis of ENaC given that calcitriol did not affect the half-lives of amiloride-sensitive Isc and Gt. Furthermore, trypsin nominally increased Isc produced by epithelia ± calcitriol, suggesting calcitriol did not affect proteolytic activation of ENaC. Consistent with mRNA and functional data, calcitriol deficiency resulted in increased γENaC protein. These data indicate that the vitamin D receptor response controls ENaC function and subsequent liquid absorption, providing insight into the relationship between vitamin D deficiency and respiratory disease.NEW & NOTEWORTHY It is unknown why calcitriol (active vitamin D) deficiency worsens pulmonary disease outcomes. Results from mRNA, immunoblot, Ussing chamber, and absorption experiments indicate that calcitriol deficiency increases ENaC activity in human airway epithelia, decreasing apical hydration. Given that epithelial hydration is required for mucociliary transport and airway innate immune function, the increased ENaC activity observed in calcitriol-deficient epithelia may contribute to respiratory pathology observed in vitamin D deficiency.

Role of paraoxonase 3 in regulating ENaC-mediated Na+ transport in the distal nephron

Paraoxonase 3 (PON3) is expressed in the aldosterone-sensitive distal nephron, where filtered Na+ is reabsorbed mainly via the epithelial Na+ channel (ENaC) and Na+ -coupled co-transporters. We previously showed that PON3 negatively regulates ENaC through a chaperone mechanism. The present study aimed to determine the physiological role of PON3 in renal Na+ and K+ homeostasis. Pon3 knockout (KO) mice had higher amiloride-induced natriuresis and lower plasma [K+ ] at baseline. Single channel recordings in split-open tubules showed that the number of active channels per patch was significantly higher in KO mice, resulting in a higher channel activity in the absence of PON3. Although whole kidney abundance of ENaC subunits was not altered in Pon3 KOs, ENaC gamma subunit was more apically distributed within the connecting tubules and cortical collecting ducts of Pon3 KO kidneys. Additionally, small interfering RNA-mediated knockdown of PON3 in cultured mouse cortical collecting duct cells led to an increased surface abundance of ENaC gamma subunit. As a result of lower plasma [K+ ], sodium chloride co-transporter phosphorylation was enhanced in the KO kidneys, a phenotype that was corrected by a high K+ diet. Finally, PON3 expression was upregulated in mouse kidneys under dietary K+ restriction, potentially providing a mechanism to dampen ENaC activity and associated K+ secretion. Taken together, our results show that PON3 has a role in renal Na+ and K+ homeostasis through regulating ENaC functional expression in the distal nephron. KEY POINTS: Paraoxonase 3 (PON3) is expressed in the distal nephron of mouse kidneys and functions as a molecular chaperone to reduce epithelial Na+ channel (ENaC) expression and activity in heterologous expression systems. We examined the physiological role of PON3 in renal Na+ and K+ handling using a Pon3 knockout (KO) mouse model. At baseline, Pon3 KO mice had lower blood [K+ ], more functional ENaC in connecting tubules/cortical collecting ducts, higher amiloride-induced natriuresis, and enhanced sodium chloride co-transporter (NCC) phosphorylation. Upon challenge with a high K+ diet, Pon3 KO mice had normalized blood [K+ ] and -NCC phosphorylation but lower circulating aldosterone levels compared to their littermate controls. Kidney PON3 abundance was altered in mice under dietary K+ loading or K+ restriction, providing a potential mechanism for regulating ENaC functional expression and renal Na+ and K+ homeostasis in the distal nephron.

Proteolytic Activation of the Epithelial Sodium Channel (ENaC): Its Mechanisms and Implications

Epithelial sodium channel (ENaC) are integral to maintaining salt and water homeostasis in various biological tissues, including the kidney, lung, and colon. They enable the selective reabsorption of sodium ions, which is a process critical for controlling blood pressure, electrolyte balance, and overall fluid volume. ENaC activity is finely controlled through proteolytic activation, a process wherein specific enzymes, or proteases, cleave ENaC subunits, resulting in channel activation and increased sodium reabsorption. This regulatory mechanism plays a pivotal role in adapting sodium transport to different physiological conditions. In this review article, we provide an in-depth exploration of the role of proteolytic activation in regulating ENaC activity. We elucidate the involvement of various proteases, including furin-like convertases, cysteine, and serine proteases, and detail the precise cleavage sites and regulatory mechanisms underlying ENaC activation by these proteases. We also discuss the physiological implications of proteolytic ENaC activation, focusing on its involvement in blood pressure regulation, pulmonary function, and intestinal sodium absorption. Understanding the mechanisms and consequences of ENaC proteolytic activation provides valuable insights into the pathophysiology of various diseases, including hypertension, pulmonary disorders, and various gastrointestinal conditions. Moreover, we discuss the potential therapeutic avenues that emerge from understanding these mechanisms, offering new possibilities for managing diseases associated with ENaC dysfunction. In summary, this review provides a comprehensive discussion of the intricate interplay between proteases and ENaC, emphasizing the significance of proteolytic activation in maintaining sodium and fluid balance in both health and disease.

Epithelial N-methyl-D-aspartate (NMDA) receptors mediate renal vasodilation by affecting kidney autoregulation

Background

N-methyl-D-aspartate receptor (NMDAR) are amino acid receptors that are well studied in brain physiology; however, their role in kidney is poorly understood. Nonetheless, NMDAR inhibitors can increase serum K+ and reduce GFR, which suggests they have an important physiological role in the kidney. We hypothesized that NMDARs in the distal nephron induce afferent-arteriole vasodilation through the vasodilator mechanism connecting-tubule-glomerular feedback (CNTGF) that involves ENaC activation.

Methods and results

Using a tubule-specific transcriptome database combined with molecular biology and microscopy techniques, we showed kidney expression of NMDAR subunits along the nephron and specifically in ENaC-positive cells. This receptor is expressed in both male and female mice, with higher abundance in females (p=0.02). Microperfusing NMDAR agonists into the connecting tubule induced afferent-arteriole vasodilation (EC50 10.7 vs. 24.5 mM; p<0.001) that was blunted or eliminated with the use of NMDAR blocker MK-801 or with the ENaC inhibitor Benzamil, indicating a dependence on CNTGF of the NMDAR-induced vasodilation. In vivo, we confirmed this CNTGF-associated vasodilation using kidney micropuncture (Stop-flow pressure 37.9±2.6 vs. 28.6±1.9 mmHg, NMDAR agonist vs vehicle; p<0.01). We explored NMDAR and ENaC channel interaction by using mpkCCD cells and split-open connecting tubules. We observed increased amiloride-sensitive current following NMDAR activation that was prevented by MK-801 (1.14 vs. 0.4 μAmp; p=0.03). In split-open tubules, NMDAR activation increased ENaC activity (Npo Vehicle vs. NMDA; p=0.04).

Conclusion

NMDARs are expressed along the nephron, including ENaC-positive cells, with higher expression in females. Epithelial NMDAR mediates renal vasodilation through the connecting-tubule-glomerular feedback, by increasing ENaC activity.

Drosophila melanogaster as an organism model for studying cystic fibrosis and its major associated microbial infections

Cystic fibrosis (CF) is a human genetic disease caused by mutations in the cystic fibrosis transmembrane conductance regulator gene that encodes a chloride channel. The most severe clinical manifestation is associated with chronic pulmonary infections by pathogenic and opportunistic microbes. Drosophila melanogaster has become the invertebrate model of choice for modeling microbial infections and studying the induced innate immune response. Here, we review its contribution to the understanding of infections with six major pathogens associated with CF (Staphylococcus aureus, Pseudomonas aeruginosa, Burkholderia cepacia, Mycobacterium abscessus, Streptococcus pneumoniae, and Aspergillus fumigatus) together with the perspectives opened by the recent availability of two CF models in this model organism.

Vaping-Induced Proteolysis Causes Airway Surface Dehydration

Proteases such as neutrophil elastase cleave and activate the epithelial sodium channel (ENaC), causing airway dehydration. Our current study explores the impact of increased protease levels in vapers' airways on ENaC activity and airway dehydration. Human bronchial epithelial cultures (HBECs) were exposed to bronchoalveolar lavage fluid (BALF) from non-smokers, smokers and vapers. Airway surface liquid (ASL) height was measured by confocal microscopy as a marker of hydration. ENaC cleavage was measured by Western blotting. Human peripheral blood neutrophils were treated with a menthol-flavored e-liquid (Juul), and the resulting secretions were added to HBECs. BALF from smokers and vapers significantly and equally increased ENaC activity and decreased ASL height. The ASL height decrease was attenuated by protease inhibitors. Non-smokers' BALF had no effect on ENaC or ASL height. BALF from smokers and vapers, but not non-smokers, induced ENaC cleavage. E-liquid-treated neutrophil secretions cleaved ENaC and decreased ASL height. Our study demonstrated that elevated protease levels in vapers' airways have functional significance since they can activate ENaC, resulting in airway dehydration. Lung dehydration contributes to diseases like cystic fibrosis (CF), chronic obstructive pulmonary disease (COPD) and asthma. Thus, our data predict that vaping, like smoking, will cause airway surface dehydration that likely leads to lung disease.

Decaying kidney function during cirrhosis correlates with remodeling of distal colon aldosterone target gene expression

Liver cirrhosis is associated to circulatory abnormalities leading to hypovolemia and stimulation of the renin-angiotensin-aldosterone system (RAAS). Advanced stages of the disease cause renal failure, impairing K+ and Na+ homeostasis. It has been proposed that the distal colon undergoes functional remodeling during renal failure, in particular by aldosterone-driven increased K+ excretion. In this study, we compared the transcriptional response of aldosterone target genes in the rat distal colon under two models of increased circulating aldosterone (one with concomitant RAAS activation) and in a model of secondary hyperaldosteronism induced by cirrhosis. The expression of a subset of these genes was also tested in distal colon biopsies from control subjects or patients with cirrhosis with varying levels of disease progression and treated or not with mineralocorticoid receptor inhibitor spironolactone. We examined known aldosterone-regulated transcripts involved in corticosteroid signaling and transepithelial ion transport. In addition, we included aldosterone-regulated genes related to cell proliferation. Our comparison revealed multiple aldosterone target genes upregulated in the rat distal colon during decompensated cirrhosis. Epithelial Na+ channel β and γ subunit expression correlated positively with plasma aldosterone concentration and negatively with glomerular filtration rate. Patients with cirrhosis showed increased expression of 11-β-hydroxysteroid-dehydrogenase 2 (11βHSD2), which was reverted by spironolactone treatment, suggesting a sensitization of the distal colon to aldosterone action. In summary, our data show that decaying kidney function during cirrhosis progression toward a decompensated state with hypovolemia correlates with remodeling of distal colon ion transporter expression, supporting a role for aldosterone in the process.NEW & NOTEWORTHY Liver cirrhosis progression significantly alters ion transporter subunit expression in the rat distal colon, a change that correlated well with declining kidney function and the severity of the disease. Our data suggest that the steroid hormone aldosterone participates in this homeostatic response to maintain electrolyte balance.

Renal Medullary Overexpression of Sphingosine-1-Phosphate Receptor 1 Transgene Attenuates Deoxycorticosterone Acetate (DOCA)-Salt Hypertension

BACKGROUND

Our previous studies showed that renal medullary sphingosine-1-phosphate receptor 1 (S1PR1) mediated sodium excretion, high salt intake increased S1PR1 level, deoxycorticosterone acetate (DOCA) blocked high salt-induced S1PR1 in the renal medulla, and that conditional knockout of S1PR1 in the collecting duct aggravated DOCA-salt hypertension. The present study tested the hypothesis that overexpression of S1PR1 transgene in the renal medulla attenuates the sodium retention and hypertension in DOCA-salt mouse model.

METHODS

Male C57BL/6J mice received renal medullary transfection of control or S1PR1-expressing plasmids and then DOCA-salt treatment. Renal sodium excretion and arterial pressure were compared between control and S1PR1-overexpressed mice in response to high salt loading or pressure natriuresis.

RESULTS

S1PR1-transfected mice showed significantly enhanced urinary sodium excretion in response to acute sodium loading (0.93 ± 0.27 in control vs. 4.72 ± 1.12 µmol/min/gKW in S1PR1-overexpressed mice, P < 0.05) and the pressure natriuresis (3.58 ± 1.77 vs. 9.52 ± 1.38, P < 0.05), less positive sodium balance in response to chronic high-salt intake (3.05 ± 0.39 vs. 1.65 ± 0.39 mmol/72 hr, P < 0.05), and consequently, the attenuation of DOCA-salt hypertension (134.2 ± 6.79 vs. 109.8 ± 3.54 mm Hg, P < 0.05). The αENaC protein amount in the renal medulla was not changed, however, the βENaC was significantly decreased and the γENaC was significantly increased in S1PR1-overexpressed mice. The immunostaining showed apical membrane translocation of γENaC, while no change of αENaC and βENaC in control mice, and that the apical membrane translocation of γENaC was blocked in S1PR1-treasffected mice.

CONCLUSIONS

These results suggested that activation of S1PR1 in the renal medulla attenuates DOCA-induced sodium retention and salt-sensitive hypertension associated with inhibition of ENaC.

30-Min Exposure to Tobacco Smoke Influences Airway Ion Transport-An In Vitro Study

Introduction: Smoking is one of the most important causes of cancer in humans. However, it has not been proven how long exposure to cigarette smoke is sufficient to induce cancerogenesis. Cigarette smoke can cause changes in ion and water transport and the maintenance of mucociliary transport. The conducted research concerned the assessment of changes in ion transport in rabbit tracheal specimens after 30 min of exposure to cigarette smoke. Materials and Methods: A modified Ussing chamber was used to measure the transepithelial electrical potential under stationary conditions (PD) and during mechanical stimulation (PDmin), and the transepithelial electrical resistance (R) in control and cigarette smoke-exposed tracheal fragments. Results: Significant changes in PD (-2.53 vs. -3.92 mV) and PDmin (-2.74 vs. -0.39 mV) were noted for the samples exposed to smoke, which can be associated with a rise in reactivity after applying a mechanical stimulus. In addition, the measured R (108 vs. 136 Ω/cm²) indicated no changes in the vitality of the samples, but an increase in their permeability to ions in the experimental conditions. Conclusions: A single 30-min exposure to cigarette smoke has been shown to be associated with increased permeability of the tracheal epithelium to ions and thus to substances emitted during smoking, which might be sufficient to create the possibility of initiating procarcinogenic processes.

Recording Sodium Self-Inhibition of Epithelial Sodium Channels Using Automated Electrophysiology in Xenopus Oocytes

The epithelial sodium channel (ENaC) is a key regulator of sodium homeostasis that contributes to blood pressure control. ENaC open probability is adjusted by extracellular sodium ions, a mechanism referred to as sodium self-inhibition (SSI). With a growing number of identified ENaC gene variants associated with hypertension, there is an increasing demand for medium- to high-throughput assays allowing the detection of alterations in ENaC activity and SSI. We evaluated a commercially available automated two-electrode voltage-clamp (TEVC) system that records transmembrane currents of ENaC-expressing Xenopus oocytes in 96-well microtiter plates. We employed guinea pig, human and Xenopus laevis ENaC orthologs that display specific magnitudes of SSI. While demonstrating some limitations over traditional TEVC systems with customized perfusion chambers, the automated TEVC system was able to detect the established SSI characteristics of the employed ENaC orthologs. We were able to confirm a reduced SSI in a gene variant, leading to C479R substitution in the human α-ENaC subunit that has been reported in Liddle syndrome. In conclusion, automated TEVC in Xenopus oocytes can detect SSI of ENaC orthologs and variants associated with hypertension. For precise mechanistic and kinetic analyses of SSI, optimization for faster solution exchange rates is recommended.

Pendrin abundance, subcellular distribution, and function are unaffected by either αENaC gene ablation or by increasing ENaC channel activity

The intercalated cell Cl-/HCO3- exchanger, pendrin, modulates ENaC subunit abundance and function. Whether ENaC modulates pendrin abundance and function is however unknown. Because αENaC mRNA has been detected in pendrin-positive intercalated cells, we hypothesized that ENaC, or more specifically the αENaC subunit, modulates intercalated cell function. The purpose of this study was therefore to determine if αENaC is expressed at the protein level in pendrin-positive intercalated cells and to determine if αENaC gene ablation or constitutively upregulating ENaC activity changes pendrin abundance, subcellular distribution, and/or function. We observed diffuse, cytoplasmic αENaC label in pendrin-positive intercalated cells from both mice and rats, with much lower label intensity in pendrin-negative, type A intercalated cells. However, while αENaC gene ablation within principal and intercalated cells of the CCD reduced Cl- absorption, it did not change pendrin abundance or subcellular distribution in aldosterone-treated mice. Further experiments used a mouse model of Liddle's syndrome to explore the effect of increasing ENaC channel activity on pendrin abundance and function. The Liddle's variant did not increase either total or apical plasma membrane pendrin abundance in aldosterone-treated or in NaCl-restricted mice. Similarly, while the Liddle's mutation increased total Cl- absorption in CCDs from aldosterone-treated mice, it did not significantly affect the change in Cl- absorption seen with pendrin gene ablation. We conclude that in rats and mice, αENaC localizes to pendrin-positive ICs where its physiological role remains to be determined. While pendrin modulates ENaC abundance, subcellular distribution, and function, ENaC does not have a similar effect on pendrin.

Critical contribution of the intracellular C-terminal region to TRESK channel activity is revealed by the epithelial Na+ current ratio (ENaR) method

TRESK (K_2P18.1) possesses unique structural proportions within the K_2P background potassium channel family. The previously described TRESK regulatory mechanisms are based on the long intracellular loop between the second and third transmembrane segments (TMS). However, the functional significance of the exceptionally short intracellular C-terminal region (iCtr) following the fourth TMS has not yet been examined. In the present study, we investigated TRESK constructs modified at the iCtr by two-electrode voltage clamp and the newly developed epithelial sodium current ratio (ENaR) method in Xenopus oocytes. The ENaR method allowed the evaluation of channel activity by exclusively using electrophysiology, and provided data that are otherwise not readily available under whole-cell conditions. TRESK homodimer was connected with two ENaC (epithelial Na⁺ channel) heterotrimers and the Na⁺ current was measured as an internal reference, proportional to the number of channels in the plasma membrane. Modifications of TRESK iCtr resulted in diverse functional effects, indicating a complex contribution of this region to K⁺ channel activity. Mutations of positive residues in proximal iCtr locked TRESK in a low activity, calcineurin-insensitive state, although this phosphatase binds to distant motifs in the loop region. Accordingly, mutations in proximal iCtr may prevent the transmission of modulation to the gating machinery. Replacing distal iCtr with a sequence designed to interact with the inner surface of the plasma membrane increased the activity of the channel to unprecedented levels, as indicated by ENaR and single channel measurements. In conclusion, the distal iCtr is a major positive determinant of TRESK function.

Resveratrol downregulates ENaCs through the activation of AMPK in human colon cancer cells

Epithelial sodium channels (ENaCs) are critically engaged in a number of hallmarks of cancer progression, including proliferation, migration, invasion and apoptosis. Thus, the inhibition of ENaCs possesses therapeutic potential in cancer. Resveratrol, a natural polyphenol with anti-carcinogenic activity, is a potent activator of 5' AMP-activated protein kinase (AMPK) which reduces the abundance of ENaCs in the cell membrane by causing the internalization of the β subunit. However, the effect of resveratrol on ENaCs in cancer cells is unknown. Therefore, in this study, we aimed to investigate the effects of resveratrol on ENaCs in human colon cancer cells HCT116 and HT29. The influences of resveratrol either alone or together with AMPK inhibitor compound C (CC), and ENaC inhibitor amiloride on cell viability were examined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The expression levels of phospho-AMPK Thr¹⁷² and βENaC in cells were determined by immunofluorescence staining, and the expression of apoptotic markers Caspase-3 and Caspase-9 were analyzed by qRT-PCR. Resveratrol was found to activate AMPK in a dose-dependent manner. Both AMPK activation by resveratrol and ENaC inhibition by amiloride decreased cell viability and increased apoptosis significantly. AMPK activation also reduced βENaC expression in cells. Our results suggest that ENaC inhibition through AMPK activation might be a potential mechanism underlying the anti-cancer effects of resveratrol.

Salt Taste and Salt Sensitive Hypertension in HIV

PURPOSE OF REVIEW

To provide a summary of current literature and propose potential mechanistic models to help us understand the role of HIV infection/antiretroviral therapy (ART), salt taste sensitivity (STS), and salt sensitivity of blood pressure (SSBP) in hypertension development.

RECENT FINDINGS

The epithelial sodium channel (ENaC) is the main protein/sodium channel for recognizing Na + in the tongue and mediates preference to low-medium salt concentrations in animals and humans. Considering the pressor response to oral salt in individuals with SSBP, poor STS may worsen blood pressure. Specific genetic variants in ENaC are linked to salt taste perception and hypertension. HIV infection, some ART, and specific antihypertensive drugs are associated with reduced STS and an increased liking for salty foods. Persons with HIV (PWH) on ART may have a decreased STS and are at a higher risk of developing salt-sensitive hypertension. Inflammation mediated by dietary salt is one of the drivers of poor STS and salt-sensitive hypertension among PWH.

Metabolic Characterization and Glyceraldehyde-3-Phosphate Dehydrogenase-Dependent Regulation of Epithelial Sodium Channels in hPheo1 Wild-type and SDHB Knockdown Cells

Pheochromocytomas (PCC) and paragangliomas (PGL) are rare neuroendocrine tumors with limited curative treatment options outside of surgical resection. Patients with mutations in succinate dehydrogenase subunit B (SDHB) are at an increased risk of malignant and aggressive disease. As cation channels are associated with tumorigenesis, we studied the expression and activity of cation channels from the Degenerin superfamily in a progenitor cell line derived from a human PCC. hPheo1 wild-type (WT) and SDHB knockdown (KD) cells were studied to investigate whether epithelial sodium channels (ENaC) and acid-sensing ion channels (ASIC) are regulated by the activity of glyceraldehyde-3-phosphate dehydrogenase (GAPDH). First, we performed targeted metabolomic studies and quantified changes in glycolysis pathway intermediates and citric acid cycle intermediates using hPheo1 WT cells and SDHB KD cells. Next, we performed protein biochemistry and electrophysiology studies to characterize the protein expression and activity, respectively, of these ion channels. Our western blot experiments show both ENaC alpha and ASIC1/2 are expressed in both hPheo1 WT and SDHB KD cells, with lower levels of a cleaved 60 kDa form of ENaC in SDHB KD cells. Single-channel patch clamp studies corroborate these results and further indicate channel activity is decreased in SDHB KD cells. Additional experiments showed a more significant decreased membrane potential in SDHB KD cells, which were sensitive to amiloride compared to WT cells. We provide evidence for the differential expression and activity of ENaC and ASIC hybrid channels in hPheo1 WT and SDHB KD cells, providing an important area of investigation in understanding SDHB-related disease.

Molecular and Cellular Mechanisms of Salt Taste

Salt taste, the taste of sodium chloride (NaCl), is mechanistically one of the most complex and puzzling among basic tastes. Sodium has essential functions in the body but causes harm in excess. Thus, animals use salt taste to ingest the right amount of salt, which fluctuates by physiological needs: typically, attraction to low salt concentrations and rejection of high salt. This concentration-valence relationship is universally observed in terrestrial animals, and research has revealed complex peripheral codes for NaCl involving multiple taste pathways of opposing valence. Sodium-dependent and -independent pathways mediate attraction and aversion to NaCl, respectively. Gustatory sensors and cells that transduce NaCl have been uncovered, along with downstream signal transduction and neurotransmission mechanisms. However, much remains unknown. This article reviews classical and recent advances in our understanding of the molecular and cellular mechanisms underlying salt taste in mammals and insects and discusses perspectives on human salt taste.

Metformin Alleviates Diabetes-Associated Hypertension by Attenuating the Renal Epithelial Sodium Channel

Diabetic nephropathy is the primary cause of morbidity in type 2 diabetes mellitus (T2DM) patients. New data indicate that hypertension, a common comorbidity in T2DM, can worsen outcomes of diabetic nephropathy. While metformin is a commonly prescribed drug for treating type 2 diabetes, its blood pressure regulating ability is not well documented. The aim of this study was to investigate the effect of metformin on normalizing blood pressure in salt-loaded hypertensive diabetic db/db mice. Sixteen-week-old male and female diabetic db/db mice were individually placed in metabolic cages and then randomized to a control vehicle (saline) or metformin treatment group. We evaluated the blood pressure reducing ability of metformin in salt-induced hypertension and progression of nephropathy in db/db mice. We observed that metformin- normalized systolic blood pressure in hypertensive diabetic mice. Mechanistically, metformin treatment reduced renal cathepsin B expression. Low cathepsin B expression was associated with reduced expression and activity of the epithelial sodium channel (ENaC), sodium retention, and thus control of hypertension. In addition, we identified that urinary extracellular vesicles (EVs) from the diabetic mice are enriched in cathepsin B. Compared to treatment with urinary EVs of vehicle-treated hypertensive diabetic mice, the amiloride-sensitive transepithelial current was significantly attenuated upon exposure of renal collecting duct cells to urinary EVs isolated from metformin-treated db/db mice or cathepsin B knockout mice. Collectively, our study identifies a novel blood pressure reducing role of metformin in diabetic nephropathy by regulating the cathepsin B-ENaC axis.

Impact of KvLQT1 potassium channel modulation on alveolar fluid homeostasis in an animal model of thiourea-induced lung edema

Alveolar ion and fluid absorption is essential for lung homeostasis in healthy conditions as well as for the resorption of lung edema, a key feature of acute respiratory distress syndrome. Liquid absorption is driven by active transepithelial sodium transport, through apical ENaC Na⁺ channels and basolateral Na⁺/K⁺-ATPase. Our previous work unveiled that KvLQT1 K⁺ channels also participate in the control of Na⁺/liquid absorption in alveolar epithelial cells. Our aim was to further investigate the function of KvLQT1 channels and their interplay with other channels/transporters involved in ion/liquid transport in vivo using adult wild-type (WT) and KvLQT1 knock-out (KO) mice under physiological conditions and after thiourea-induced lung edema. A slight but significant increase in water lung content (WLC) was observed in naïve KvLQT1-KO mice, relative to WT littermates, whereas lung function was generally preserved and histological structure unaltered. Following thiourea-induced lung edema, KvLQT1-KO did not worsen WLC or lung function. Similarly, lung edema was not aggravated by the administration of a KvLQT1 inhibitor (chromanol). However, KvLQT1 activation (R-L3) significantly reduced WLC in thiourea-challenged WT mice. The benefits of R-L3 were prevented in KO or chromanol-treated WT mice. Furthermore, R-L3 treatment had no effect on thiourea-induced endothelial barrier alteration but restored or enhanced the levels of epithelial alveolar AQP5, Na⁺/K⁺-ATPase, and ENaC expressions. Altogether, the results indicate the benefits of KvLQT1 activation in the resolution of lung edema, probably through the observed up-regulation of epithelial alveolar channels/transporters involved in ion/water transport.

Validation of commercially available antibodies directed against subunits of the epithelial Na+ channel

The epithelial Na+ channel (ENaC) is traditionally composed of three subunits, although non-canonical expression has been found in various tissues including the vasculature, brain, lung, and dendritic cells of the immune system. Studies of ENaC structure and function have largely relied on heterologous expression systems, often with epitope-tagged channel subunits. Relevant in vivo physiological studies have used ENaC inhibitors, mice with global or tissue specific knockout of subunits, and anti-ENaC subunit antibodies generated by investigators or by commercial sources. Availability of well-characterized, specific antibodies is imperative as we move forward in understanding the role of ENaC in non-epithelial tissues where expression, subunit organization, and electrophysiological characteristics may differ from epithelial tissues. We report that a commonly used commercial anti-α subunit antibody recognizes an intense non-specific band on mouse whole kidney and lung immunoblots, which migrates adjacent to a less intense, aldosterone-induced full length α-subunit. This antibody localizes to the basolateral membrane of aquaporin 2 negative cells in kidney medulla. We validated antibodies against the β- and γ-subunits from the same commercial source. Our work illustrates the importance of validation studies when using popular, commercially available anti-ENaC antibodies.

Pharmacological inhibitors of the cystic fibrosis transmembrane conductance regulator exert off-target effects on epithelial cation channels

The cystic fibrosis transmembrane conductance regulator (CFTR) anion channel and the epithelial Na⁺ channel (ENaC) play essential roles in transepithelial ion and fluid transport in numerous epithelial tissues. Inhibitors of both channels have been important tools for defining their physiological role in vitro. However, two commonly used CFTR inhibitors, CFTR_inh-172 and GlyH-101, also inhibit non-CFTR anion channels, indicating they are not CFTR specific. However, the potential off-target effects of these inhibitors on epithelial cation channels has to date not been addressed. Here, we show that both CFTR blockers, at concentrations routinely employed by many researchers, caused a significant inhibition of store-operated calcium entry (SOCE) that was time-dependent, poorly reversible and independent of CFTR. Patch clamp experiments showed that both CFTR_inh-172 and GlyH-101 caused a significant block of Orai1-mediated whole cell currents, establishing that they likely reduce SOCE via modulation of this Ca²⁺ release-activated Ca²⁺ (CRAC) channel. In addition to off-target effects on calcium channels, both inhibitors significantly reduced human αβγ-ENaC-mediated currents after heterologous expression in Xenopus oocytes, but had differential effects on δβγ-ENaC function. Molecular docking identified two putative binding sites in the extracellular domain of ENaC for both CFTR blockers. Together, our results indicate that caution is needed when using these two CFTR inhibitors to dissect the role of CFTR, and potentially ENaC, in physiological processes.

Analysis of the rat chorda tympani nerve response to "super salty" sodium carbonate

In behavioral experiments, rats perceive sodium carbonate (Na2CO3) as super salty. In fact, when the dissociated Na+ ions are accounted for, rats perceive Na2CO3 as 5× saltier than equinormal concentrations of NaCl. The chorda tympani nerve (CT) responds to salts through at least two receptor mechanisms and is a model system for understanding how salt taste is transmitted to the brain. Here, we recorded CT nerve activity to a broad range of NaCl (3-300 mM) and Na2CO3 (3-300 mN) to investigate why Na2CO3 tastes so salty to rats. Benzamil, a specific epithelial sodium channel (ENaC) antagonist, was used to determine the relative contribution of apical ENaCs in Na2CO3 transduction. The benzamil-insensitive component of CT nerve responses was enhanced by increasing the adapted tongue temperature from 23°C to 30°C. Na2CO3 solutions are alkaline, so we compared neural responses (with and without benzamil) to 100 mM NaCl alone (6.2 pH) and at a pH (11.2 pH) that matched 100 mN Na2CO3. As expected, NaCl responses increased progressively with increasing concentration and temperature. Responses to 3 mN Na2CO3 were greater than 3 mM NaCl with and without benzamil, but the shape of the first log-fold range of was relatively flat. Adjusting the pH of NaCl to 11.2 abolished the thermal enhancement of 100 mN NaCl through the benzamil-insensitive pathway. Rinsing Na2CO3 off the tongue resulted in robust aftertaste that was concentration dependent, thermally sensitive, and benzamil-insensitive. Responses to alkaline NaCl did not recapitulate Na2CO3 responses or aftertaste, suggesting multiple transduction mechanisms for the cations (2Na+) and anion (CO3-2).

Roles of early events in the modifications undergone by bovine corneal endothelial cells during wound healing

A mechanical injury in bovine corneal endothelial (BCE) cells in culture induces: (1) a fast calcium wave (FCW); (2) slow increases in cytosolic sodium and calcium, critical for the healing process, and (3) a rise in the apoptotic rate with respect to quiescent cells. In order to investigate the nature of the stimuli that determine the ionic changes and apoptotic response, we performed here studies on a non-injury model of tissue restitution in BCE monolayers. For this, we employed cell cultures grown to confluence in the presence of a Parafilm strip. We observed that, previously to strip removal, most of the border cells had already developed the slow ionic modifications, while in the scratch wounds these changes gradually occur after several hours of healing. This finding suggests that, in BCE cells, the presence of a free edge is sufficient to trigger ionic modifications necessary for wound healing and to elicit an augmented apoptotic response. The apoptotic index of the migrating cells in the Parafilm model (PF) was determined to be approximately two-fold the one of scratch wounds, a result that, in agreement with our previous observations, we attributed to the absence of the FCW in the PF experiments. The findings of this work further contribute to the understanding of epithelial wound healing, a crucial adaptive, and homeostatic response.

The serine protease plasmin plays detrimental roles in epithelial sodium channel activation and podocyte injury in Dahl salt-sensitive rats

Salt-sensitive hypertension is associated with poor clinical outcomes. The epithelial sodium channel (ENaC) in the kidney plays pivotal roles in sodium reabsorption and blood pressure regulation, in which its γ subunit is activated by extracellular serine proteases. In proteinuric nephropathies, plasmin filtered through injured glomeruli reportedly activates γENaC in the distal nephron and causes podocyte injury. We previously reported that Dahl salt-sensitive (DS) rats fed a high-salt (HS) diet developed hypertension and proteinuria along with γENaC activation and that a synthetic serine protease inhibitor, camostat mesilate, mitigated these changes. However, the role of plasmin in DS rats remained unclear. In this study, we evaluated the relationship between plasmin and hypertension as well as podocyte injury and the effects of plasmin inhibitors in DS rats. Five-week-old DS rats were divided into normal-salt diet, HS diet, and HS+plasmin inhibitor (either tranexamic acid [TA] or synthetic plasmin inhibitor YO-2) groups. After blood pressure measurement and 24 h urine collection over 5 weeks, rats were sacrificed for biochemical analyses. The HS group displayed severe hypertension and proteinuria together with activation of plasmin in urine and γENaC in the kidney, which was significantly attenuated by YO-2 but not TA. YO-2 inhibited the attachment of plasmin(ogen) to podocytes and alleviated podocyte injury by inhibiting apoptosis and inflammatory/profibrotic cytokines. YO-2 also suppressed upregulation of protease-activated receptor-1 and phosphorylated ERK1/2. These results indicate an important role of plasmin in the development of salt-sensitive hypertension and related podocyte injury, suggesting plasmin inhibition as a potential therapeutic strategy.

Human Alpha-1 Antitrypsin Attenuates ENaC and MARCKS and Lowers Blood Pressure in Hypertensive Diabetic db/db Mice

Hypertension may develop before or after the onset of diabetes and it is known to increase the risk of developing diabetic nephropathy. Alpha-1 antitrypsin (AAT) is a multi-functional protein with beneficial effects in various diseases but its role in reducing blood pressure in the diabetic kidney has not been thoroughly studied. Like blood pressure, epithelial sodium channels (ENaC) and its adaptor protein myristoylated alanine-rich C-kinase substrate (MARCKS) are regulated by circadian rhythms. Our hypothesis is that administration of human AAT (hAAT) reduces blood pressure in hypertensive diabetic mice by attenuating membrane expression of ENaC and its association with the actin cytoskeleton. First, we show hAAT administration results in reduced blood pressure in diabetic db/db mice compared to vehicle treatment in both the inactive and active cycles. Western blotting and immunohistochemistry analyses showed a reduction of ENaC and the actin cytoskeleton protein, MARCKS in the kidneys of diabetic db/db mice treated with hAAT compared to vehicle. hAAT treatment resulted in elevated amounts of extracellular vesicles present in the urine of diabetic db/db mice compared to vehicle treatment both in the inactive and active cycles. Multiple hexosylceramides, among other lipid classes increased in urinary EVs released from hAAT treated hypertensive diabetic mice compared to vehicle treated mice. Taken together, these data suggest hAAT treatment could normalize blood pressure in the diabetic kidney in a mechanism involving attenuation of renal ENaC and MARCKS protein expression and possibly ceramide metabolism to hexosylceramide in kidney cells.

The molecular makeup of peripheral and central baroreceptors: stretching a role for Transient Receptor Potential (TRP), Epithelial Sodium Channel (ENaC), Acid Sensing Ion Channel (ASIC), and Piezo channels

The autonomic nervous system maintains homeostasis of cardiovascular, respiratory, gastrointestinal, urinary, immune, and thermoregulatory function. Homeostasis involves a variety of feedback mechanisms involving peripheral afferents, many of which contain molecular receptors sensitive to mechanical deformation, termed mechanosensors. Here, we focus on the molecular identity of mechanosensors involved in the baroreflex control of the cardiovascular system. Located within the walls of the aortic arch and carotid sinuses, and/or astrocytes in the brain, these mechanosensors are essential for the rapid moment-to-moment feedback regulation of blood pressure (BP). Growing evidence suggests that these mechanosensors form a co-existing system of peripheral and central baroreflexes. Despite the importance of these molecules in cardiovascular disease and decades of research, their precise molecular identity remains elusive. The uncertainty surrounding the identity of these mechanosensors presents a major challenge in understanding basic baroreceptor function and has hindered the development of novel therapeutic targets for conditions with known arterial baroreflex impairments. Therefore, the purpose of this review is to (i) provide a brief overview of arterial and central baroreflex control of BP, (ii) review classes of ion channels currently proposed as the baroreflex mechanosensor, namely Transient Receptor Potential (TRP), Epithelial Sodium Channel (ENaC), Acid Sensing Ion Channel (ASIC), and Piezo, along with additional molecular candidates that serve mechanotransduction in other organ systems, and (iii) summarize the potential clinical implications of impaired baroreceptor function in the pathophysiology of cardiovascular disease.

Lipid Profiles of Urinary Extracellular Vesicles Released during the Inactive and Active Phases of Aged Male Mice with Spontaneous Hypertension

Hypertension remains a major problem, especially in the elderly, as it increases the risk for cardiovascular, coronary artery, cerebrovascular, and kidney diseases. Extracellular vesicles (EVs) play a role in the aging process and contribute to pathophysiology. Our goal was to examine differences in lipid profiles of urinary EVs (uEVs) collected during the inactive and active phases of aged mice and investigate whether these EVs regulate the density of lipid rafts in mouse cortical collecting duct (mpkCCD) principal cells. Here, we demonstrate the epithelial sodium channel (ENaC) inhibitor benzyl amiloride reduced systolic blood pressure in aged male mice during the inactive and active phases. Lipidomics data demonstrate differential enrichment of lipids between the two groups. For example, there are more phosphatidylethanolamine plasmalogens, particularly in the form of alkyl phosphatidylethanolamines, that are enriched in active phase uEVs compared to inactive phase uEVs from the same mice. Amiloride-sensitive transepithelial current increased more in mpkCCD cells challenged with uEVs from the active phase group. Moreover, more ENaC alpha protein was distributed to lipid raft fractions of mpkCCD cells challenged with active phase uEVs. Taken together, the identification of bioactive lipids associated with lipid rafts that are enriched in EVs released during the active phase of aged mice may offer clues to help understand lipid raft organization in recipient principal cells after EV uptake and increased renal ENaC activity, leading to a time-of-day dependent regulation of blood pressure in an aging model.

WNK1 is a chloride-stimulated scaffold that regulates mTORC2 activity and ion transport

Mammalian (or mechanistic) target of rapamycin complex 2 (mTORC2) is a kinase complex that targets predominantly Akt family proteins, SGK1 and protein kinase C (PKC), and has well-characterized roles in mediating hormone and growth factor effects on a wide array of cellular processes. Recent evidence suggests that mTORC2 is also directly stimulated in renal tubule cells by increased extracellular K+ concentration, leading to activation of the Na+ channel, ENaC, and increasing the electrical driving force for K+ secretion. We identify here a signaling mechanism for this local effect of K+. We show that an increase in extracellular [K+] leads to a rise in intracellular chloride (Cl-), which stimulates a previously unknown scaffolding activity of the protein 'with no lysine-1' (WNK1) kinase. WNK1 interacts selectively with SGK1 and recruits it to mTORC2, resulting in enhanced SGK1 phosphorylation and SGK1-dependent activation of ENaC. This scaffolding effect of WNK1 is independent of its own kinase activity and does not cause a generalized stimulation of mTORC2 kinase activity. These findings establish a novel WNK1-dependent regulatory mechanism that harnesses mTORC2 kinase activity selectively toward SGK1 to control epithelial ion transport and electrolyte homeostasis.

PIP2 Interacts Electrostatically with MARCKS-like Protein-1 and ENaC in Renal Epithelial Cells

We examined the interaction of a membrane-associated protein, MARCKS-like Protein-1 (MLP-1), and an ion channel, Epithelial Sodium Channel (ENaC), with the anionic lipid, phosphatidylinositol 4, 5-bisphosphate (PIP₂). We found that PIP₂ strongly activates ENaC in excised, inside-out patches with a half-activating concentration of 21 ± 1.17 µM. We have identified 2 PIP₂ binding sites in the N-terminus of ENaC β and γ with a high concentration of basic residues. Normal channel activity requires MLP-1's strongly positively charged effector domain to electrostatically sequester most of the membrane PIP₂ and increase the local concentration of PIP₂. Our previous data showed that ENaC covalently binds MLP-1 so PIP₂ bound to MLP-1 would be near PIP₂ binding sites on the cytosolic N terminal regions of ENaC. We have modified the charge structure of the PIP₂ -binding domains of MLP-1 and ENaC and showed that the changes affect membrane localization and ENaC activity in a way consistent with electrostatic theory.

Epithelial sodium channels in macrophage migration and polarization: role of proinflammatory cytokines TNFα and IFNγ

Migration of monocytes-macrophages plays an important role in phagocytosis of pathogens and cellular debris in a variety of pathophysiological conditions. Although epithelial Na+ channels (ENaCs) are required for normal migratory responses in other cell types, their role in macrophage migration signaling is unknown. To address this possibility, we determined whether ENaC message is present in several peripheral blood monocyte cell populations and tissue-resident macrophages in healthy humans using the Human Protein Atlas database (www.proteinatlas.org) and the mouse monocyte cell line RAW 264.7 using RT-PCR. We then determined that selective ENaC inhibition with amiloride inhibited chemotactic migration (∼50%), but not phagocytosis, of the mouse monocyte-macrophage cell line RAW 264.7. Furthermore, we generated a cell line stably expressing an NH2-terminal truncated αENaC to interrupt normal channel trafficking and found it suppressed migration. Prolonged exposure (48 h) of RAW 264.7 cells to proinflammatory cytokines interferon γ (IFNγ) and/or tumor necrosis factor α (TNFα) inhibited RAW 264.7 migration and abolished the amiloride (1 µM)-sensitive component of migration, a finding consistent with ENaC downregulation. To determine if proinflammatory cytokines regulate αENaC protein expression, cells were exposed to proinflammatory cytokines IFNγ (10 ng/mL, last 48 h) and TNFα (10 ng/mL, last 24 h). By Western blot analysis, we found whole cell αENaC protein is reduced ≥50%. Immunofluorescence demonstrated heterogeneous αENaC inhibition. Finally, we found that overnight exposure to amiloride stimulated morphological changes and increased polarization marker expression. Our findings suggest that ENaC may be a critical molecule in macrophage migration and polarization.

Effects of Chronic Kidney Disease on Nanomechanics of the Endothelial Glycocalyx Are Mediated by the Mineralocorticoid Receptor

Endothelial mechanics control vascular reactivity and are regulated by the mineralocorticoid receptor (MR) and its downstream target, the epithelial Na⁺ channel (ENaC). Endothelial dysfunction is a hallmark of chronic kidney disease (CKD), but its mechanisms are poorly understood. We hypothesized that CKD disrupts endothelial mechanics in an MR/ENaC-dependent process.

METHODS

Primary human endothelial cells were cultured with uremic serum derived from children with stage 3-5 (predialysis) CKD or adult hemodialysis (HD) patients or healthy controls. The height and stiffness of the endothelial glycocalyx (eGC) and cortex were monitored by atomic force microscopy (AFM) using an ultrasensitive mechanical nanosensor.

RESULTS

In a stage-dependent manner, sera from children with CKD induced a significant increase in eGC and cortex stiffness and an incremental reduction of the eGC height. AFM measurements were significantly associated with individual pulse wave velocity and serum concentrations of gut-derived uremic toxins. Serum from HD patients increased MR expression and mechanical stiffness of the endothelial cortex, an effect reversed by MR and ENaC antagonists, decreased eNOS expression and NO bioavailability, and augmented monocyte adhesion.

CONCLUSION

These data indicate progressive structural damage of the endothelial surface with diminishing kidney function and identify the MR as a mediator of CKD-induced endothelial dysfunction.

Albumin Stimulates Epithelial Na+ Transport and Barrier Integrity by Activating the PI3K/AKT/SGK1 Pathway

Albumin is a major serum protein and is frequently used as a cell culture supplement. It is crucially involved in the regulation of osmotic pressure and distribution of fluid between different compartments. Alveolar epithelial Na⁺ transport drives alveolar fluid clearance (AFC), enabling air breathing. Whether or not albumin affects AFC and Na⁺ transport is yet unknown. We therefore determined the acute and chronic effects of albumin on Na⁺ transport in fetal distal lung epithelial (FDLE) cells and the involved kinase pathways. Chronic BSA treatment strongly increased epithelial Na⁺ transport and barrier integrity in Ussing chambers. BSA did not elevate mRNA expression of Na⁺ transporters in FDLE cells after 24 h. Moreover, acute BSA treatment for 45 min mimicked the chronic effects. The elevated Na⁺ transport was caused by an increased maximal ENaC activity, while Na,K-ATPase activity remained unchanged. Acute and chronic BSA treatment lowered membrane permeability, confirming the increased barrier integrity observed in Ussing chambers. Western blots demonstrated an increased phosphorylation of AKT and SGK1, and PI3K inhibition abolished the stimulating effect of BSA. BSA therefore enhanced epithelial Na⁺ transport and barrier integrity by activating the PI3K/AKT/SGK1 pathway.

Elevated intracellular Na(+) and osmolarity stimulate catalytic activity of the ubiquitin ligase Nedd4-2

Regulation of catalytic activity of E3 ubiquitin ligases is critical for their cellular functions. We identified an unexpected mode of regulation of E3 catalytic activity by ions and osmolarity; enzymatic activity of the HECT family E3 Nedd4-2/Nedd4L is enhanced by increased intracellular Na⁺ ([Na⁺]_i) and by hyperosmolarity. This stimulated activity is mediated by activation of p38-MAPK and is inhibited by WNKs. Moreover, protease (Furin)-mediated activation of the epithelial Na⁺ channel ENaC (a bona fide Nedd4-2 substrate), which leads to increased [Na⁺]_i and osmolarity, results in enhanced Nedd4-2 catalytic activity. This enhancement is inhibited by a Furin inhibitor, by a protease-resistant ENaC mutant, or by treatment with the ENaC inhibitor amiloride. Moreover, WNK inhibition, which stimulates catalytic activity of Nedd4-2, leads to reduced levels of cell-surface ENaC and reduced channel activity. ENaC activity does not affect Nedd4-2:ENaC binding. Therefore, these results demonstrate activation of a ubiquitin ligase by Na⁺ and osmotic changes. Importantly, they reveal a negative feedback loop in which active ENaC leads to stimulation of catalytic activity of its own suppressor, Nedd4-2, to protect cells from excessive Na⁺ loading and hyperosmotic stress and to protect the animal from hypertension.

Effects of Hypoxia on Respiratory Diseases: Perspective View of Epithelial Ion Transport

The balance of gas exchange and lung ventilation is essential for the maintenance of body homeostasis. There are many ion channels and transporters in respiratory epithelial cells, including epithelial sodium channel, Na,K-ATPase, cystic fibrosis transmembrane conductance regulator, and some transporters. These ion channels/transporters maintain the capacity of liquid layer on the surface of respiratory epithelial cells, and provide an immune barrier for the respiratory system to clear off foreign pathogens. However, in some harmful external environment and/or pathological conditions, the respiratory epithelium is prone to hypoxia, which would destroy the ion transport function of the epithelium and unbalance the homeostasis of internal environment, triggering a series of pathological reactions. Many respiratory diseases associated with hypoxia manifest an increased expression of hypoxia-inducible factor-1, which mediates the integrity of the epithelial barrier and affects epithelial ion transport function. It is important to study the relationship between hypoxia and ion transport function, whereas the mechanism of hypoxia-induced ion transport dysfunction in respiratory diseases is not clear. This review focuses on the relationship of hypoxia and respiratory diseases, as well as dysfunction of ion transport and tight junctions in respiratory epithelial cells under hypoxia.

Rodent models to study sodium retention in experimental nephrotic syndrome

Sodium retention and edema are hallmarks of nephrotic syndrome (NS). Different experimental rodent models have been established for simulating NS, however, not all of them feature sodium retention which requires proteinuria to exceed a certain threshold. In rats, puromycin aminonucleoside nephrosis (PAN) is a classic NS model introduced in 1955 that was adopted as doxorubicin-induced nephropathy (DIN) in 129S1/SvImJ mice. In recent years, mice with inducible podocin deletion (Nphs2^Δipod ) or podocyte apoptosis (POD-ATTAC) have been developed. In these models, sodium retention is thought to be caused by activation of the epithelial sodium channel (ENaC) in the distal nephron through aberrantly filtered serine proteases or proteasuria. Strikingly, rodent NS models follow an identical chronological time course after the development of proteinuria featuring sodium retention within days and spontaneous reversal thereafter. In DIN and Nphs2^Δipod mice, inhibition of ENaC by amiloride or urinary serine protease activity by aprotinin prevents sodium retention, opening up new and promising therapeutic approaches that could be translated into the treatment of nephrotic patients. However, the essential serine protease(s) responsible for ENaC activation is (are) still unknown. With the use of nephrotic rodent models, there is the possibility that this (these) will be identified in the future. This review summarizes the various rodent models used to study experimental nephrotic syndrome and the insights gained from these models with regard to the pathophysiology of sodium retention.

Effects of amiloride on acetylcholine-dependent arterial vasodilation evolve over time in mice on a high salt diet

The maintenance of endothelial health is required for normal vascular function and blood pressure regulation. The epithelial Na⁺ channel (ENaC) in endothelial cells has emerged as a new molecular player in the regulation of endothelial nitric oxide production and vascular stiffness. While ENaC expression in the kidney is negatively regulated by high [Na⁺ ], ENaC expression in isolated endothelial cells has been shown to increase in response to a high extracellular [Na⁺ ]. In culture, this increased expression leads to cellular stiffening and decreased nitric oxide release. In vivo, the effects of high salt diet on endothelial ENaC expression and activity have varied depending on the animal model utilized. Our aim in the present study was to examine the role of endothelial ENaC in mediating vasorelaxation in the C57Bl/6 mouse strain. We utilized pressure myography to test the responsiveness of thoracodorsal arteries to acetylcholine in mice with increased sodium consumption both in the presence and absence of increased aldosterone. ENaC's contribution was assessed with the use of the specific inhibitor amiloride. We found that while aldosterone had very little effect on ENaC's contribution to acetylcholine sensitivity, a high salt diet led to an amiloride-dependent shift in the acetylcholine response of vessels. However, the direction of this shift was dependent on the length of high salt diet administration. Overall, our studies reveal that ENaC's role in the endothelium may be more complicated than previously thought. The channel does not simply inhibit nitric oxide generation, but instead helps preserve a homeostatic response.

Bile acids and salt-sensitive hypertension: a role of the gut-liver axis

Salt-sensitivity of blood pressure (SSBP) affects 50% of the hypertensive and 25% of the normotensive populations. Importantly, SSBP is associated with increased risk for mortality in both populations independent of blood pressure. Despite its deleterious effects, the pathogenesis of SSBP is not fully understood. Emerging evidence suggests a novel role of bile acids in salt-sensitive hypertension and that they may play a crucial role in regulating inflammation and fluid volume homeostasis. Mechanistic evidence implicates alterations in the gut microbiome, the epithelial sodium channel (ENaC), the farnesoid X receptor, and the G protein-coupled bile acid receptor TGR5 in bile acid-mediated effects on cardiovascular function. The mechanistic interplay between excess dietary sodium-induced alterations in the gut microbiome and immune cell activation, bile acid signaling, and whether such interplay may contribute to the etiology of SSBP is still yet to be defined. The main goal of this review is to discuss the potential role of bile acids in the pathogenesis of cardiovascular disease with a focus on salt-sensitive hypertension.

Optogenetic Control of PIP2 Interactions Shaping ENaC Activity

The activity of the epithelial Na⁺ Channel (ENaC) is strongly dependent on the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2). PIP2 binds two distinct cationic clusters within the N termini of β- and γ-ENaC subunits (βN1 and γN2). The affinities of these sites were previously determined using short synthetic peptides, yet their role in sensitizing ENaC to changes in PIP2 levels in the cellular system is not well established. We addressed this question by comparing the effects of PIP2 depletion and recovery on ENaC channel activity and intracellular Na⁺ levels [Na⁺]_i. We tested effects on ENaC activity with mutations to the PIP2 binding sites using the optogenetic system CIBN/CRY2-OCRL to selectively deplete PIP2. We monitored changes of [Na⁺]_i by measuring the fluorescent Na⁺ indicator, CoroNa Green AM, and changes in channel activity by performing patch clamp electrophysiology. Whole cell patch clamp measurements showed a complete lack of response to PIP2 depletion and recovery in ENaC with mutations to βN1 or γN2 or both sites, compared to wild type ENaC. Whereas mutant βN1 also had no change in CoroNa Green fluorescence in response to PIP2 depletion, γN2 did have reduced [Na⁺]_i, which was explained by having shorter CoroNa Green uptake and half-life. These results suggest that CoroNa Green measurements should be interpreted with caution. Importantly, the electrophysiology results show that the βN1 and γN2 sites on ENaC are each necessary to permit maximal ENaC activity in the presence of PIP2.

A highly selective, cell-permeable furin inhibitor BOS-318 rescues key features of cystic fibrosis airway disease

In cystic fibrosis (CF), excessive furin activity plays a critical role in the activation of the epithelial sodium channel (ENaC), dysregulation of which contributes to airway dehydration, ineffective mucociliary clearance (MCC), and mucus obstruction. Here, we report a highly selective, cell-permeable furin inhibitor, BOS-318, that derives selectivity by eliciting the formation of a new, unexpected binding pocket independent of the active site catalytic triad. Using human ex vivo models, BOS-318 showed significant suppression of ENaC, which led to enhanced airway hydration and an ∼30-fold increase in MCC rate. Furin inhibition also protected ENaC from subsequent activation by neutrophil elastase, a soluble protease dominant in CF airways. Additional therapeutic benefits include protection against epithelial cell death induced by Pseudomonas aeruginosa exotoxin A. Our findings demonstrate the utility of selective furin inhibition as a mutation-agnostic approach that can correct features of CF airway pathophysiology in a manner expected to deliver therapeutic value.

Ala663Thr Gene Polymorphism With Sudden Sensorineural Hearing Loss

Objective: The etiology of sudden sensorineural hearing loss (SSNHL) is still unknown. It has been demonstrated that normal endolymph metabolism is essential for inner ear function and that epithelial sodium channels (ENaC) may play an important role in the regulation of endolymphatic Na⁺. This study aimed to explore the potential association between αENaC p. Ala663Thr gene polymorphism and SSNHL. Methods: Polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) was used to examine the genotype and allele frequency of the αENaC p. Ala663Thr polymorphism in 20 cases of low-frequency SSNHL (LF-SSNHL), 19 cases of high-frequency SSNHL (HF-SSNHL), 31 cases of all frequency SSNHL (AF-SSNHL), 42 cases of profound deafness SSNHL (PD-SSNHL), and 115 normal controls. Results: The T663 allele was found to be significantly associated with an increased risk of LF-SSNHL (p = 0.046, OR = 2.16, 95% CI = 1.01-4.62). The TT genotype and T663 allele, on the other hand, conferred a protective effect for PD-SSNHL (AA vs. TT: p = 0.012, OR = 0.25, 95% CI = 0.08-0.74; A vs. T: p = 0.001, OR = 0.36, 95% CI = 0.21-0.61). However, there was no statistically significant difference in genotype or allele frequency between the two groups (HF-SSNHL and AF-SSNHL) and the control group. Conclusion: The αENaC p. Ala663Thr gene polymorphism plays different roles in different types of SSNHL.

Aldosterone-Regulated Sodium Transport and Blood Pressure

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

Paraoxonase 2 is an ER chaperone that regulates the epithelial Na+ channel

The mammalian paraoxonases (PONs) have been linked to protection against oxidative stress. However, the physiological roles of members in this family (PON1, PON2, and PON3) are still being characterized. PON2 and PON3 are expressed in the aldosterone-sensitive distal nephron of the kidney and have been shown to negatively regulate expression of the epithelial sodium channel (ENaC), a trimeric ion channel that orchestrates salt and water homeostasis. To date, the nature of this phenomenon has not been explored. Therefore, to investigate the mechanism by which PON2 regulates ENaC, we expressed PON2 along with the ENaC subunits in fisher rat thyroid (FRT) cells, a system that is amenable to biochemical analyses of ENaC assembly and trafficking. We found that PON2 primarily resides in the endoplasmic reticulum (ER) in FRT cells, and its expression reduces the abundance of each ENaC subunit, reflecting enhanced subunit turnover. In contrast, no effect on the levels of mRNAs encoding the ENaC subunits was evident. Inhibition of lysosome function with chloroquine or NH4Cl did not alter the inhibitory effect of PON2 on ENaC expression. In contrast, PON2 accelerates ENaC degradation in a proteasome-dependent manner and acts before ENaC subunit ubiquitination. As a result of enhanced ENaC subunit ubiquitination and degradation, both channel surface expression and ENaC-mediated Na+ transport in FRT cells were reduced by PON2. Together, our data suggest that PON2 functions as an ER chaperone to monitor ENaC biogenesis and redirects the channel for ER-associated degradation.

Insulin regulates Nedd4-2 via a PKB-dependent mechanism in HEI-OC1 auditory cells-crosstalks with sphingolipid and cAMP signaling

BACKGROUND

The mechanisms of association between diabetes and inner ear dysfunction are unknown, although endolymphatic hydrops may be involved. We have previously shown that insulin signaling components are expressed in human saccule and that insulin signaling takes place in HEI-OC1 auditory cells.

AIM

To explore Nedd4-2 as a target for insulin signaling.

MATERIALS AND METHODS

Effects of insulin were analyzed using western blot and confocal microscopy in HEI-OC1 auditory cells.

RESULTS

Insulin induced phosphorylation of Nedd4-2 and increased the amount of ENaC at the plasma membrane. Also, protein kinase B (PKB) and NDRG1, a substrate for SGK1 (serum and glucocorticoid stimulated kinase), were phosphorylated in response to insulin. The SGK1 inhibitor GSK650394 prevented insulin-induced phosphorylation of NRDG1, but not of PKB and Nedd4-2, whereas the phosphatidylinositol 3-kinase (PI3K) inhibitor wortmannin and the PKB inhibitor MK2206 inhibited phosphorylation of all components. Ceramides prevented insulin-induced phosphorylation of PKB and NDRG1, but not of Nedd4-2. The ceramide metabolite sphingosine 1-phosphate induced phosphorylation of Nedd4-2.

CONCLUSIONS

Insulin induces phosphorylation of Nedd4-2, most likely involving PI3K/PKB signaling. Sphingosine 1-phosphate might protect Nedd4-2 against ceramide-induced insulin resistance.

SIGNIFICANCE

Insulin-mediated regulation of Nedd4-2 might impact on inner ear sodium homeostasis with implications for diabetes-induced inner ear damage.