Epigenetics of epithelial Na^+ channel-dependent sodium uptake and blood pressure regulation

The epithelial Na^＋ channel （ENaC） consists of α, β, γ subunits. Its expression and function are regulated by aldosterone at multiple levels including transcription. ENaC plays a key role in Na^＋ homeostasis and blood pressure. Mutations in ENaC subunit genes result in hypertension or hypotension, depending on the nature of the mutations. Transcription of αENaC is considered as the rate-limiting step in the formation of functional ENaC. As an aldosterone target gene, αENaC is activated upon aldosterone- mineralocorticoid receptor binding to the cis-elements in the αENaC promoter, which is packed into chromatin. However, how aldosterone alters chromatin structure to induce changes in transcription is poorly understood. Studies by others and us suggest that Dot1a-Af9 complex represses αENaC by directly binding and regulating targeted histone H3 K79 hypermethylation at the specific subregions of αENaC promoter. Aldosterone decreases Dot1a-Af9 formation by impairing expression of Dot1a and Af9 and by inducing Sgk1, which, in turn, phosphorylates Af9 at S435 to weaken Dot1a-Af9 interaction. MR attenuates Dot1a-Af9 effect by competing with Dot1a for binding Af9. Af17 relieves repression by interfering with Dot1a-Af9 interaction and promoting Dot1a nuclear export. Af17^-/- mice exhibit defects in ENaC expression, renal Na^＋ retention, and blood pressure control. This review gives a brief summary of these novel fndings.

Changes in urinary excretion of water and sodium transporters during amiloride and bendroflumethiazide treatment

AIM： To quantify changes in urinary excretion of aquaporin2 water channels （u-AQP2）, the sodium-potassium-chloride co-transporter （u-NKCC2） and the epithelial sodium channels （u-ENaC） during treatment with bendrofumethiazide （BFTZ）, amiloride and placebo.METHODS： In a randomized, double-blinded, placebo-controlled, 3-way crossover study we examined 23 healthy subjects on a standardized diet and fuid intake. The subjects were treated with amiloride 5 mg, BFTZ 1.25 mg or placebo twice a day for 4.5 d before each examination day. On the examination day, glomerular filtration rate was measured by the constant infusion clearance technique with 51Cr-EDTA as reference substance. To estimate the changes in water transport via AQP2 and sodium transport via NKCC2 and ENaC, u-NKCC2, the gamma fraction of ENaC （u-ENaCγ）, and u-AQP2 were measured at baseline and after infusion with 3% hypertonic saline. U-NKCC2, u-ENaCγ, u-AQP2 and plasma concentrations of vasopressin （p-AVP）, renin （PRC）, angiotensin Ⅱ （p-ANG Ⅱ） and aldosterone （p-Aldo） were measured, by radioimmunoassay. Central blood pressure was estimated by applanation tonometry and body fuid volumes were estimated by bio-impedance spectroscopy. General linear model with repeated measures or related samples Friedman’s two-way analysis was used to compare differences. Post hoc Bonferroni correction was used for multiple comparisons of post infusion periods to baseline within each treatment group.RESULTS： At baseline there were no differences in u-NKCC2, u-ENaCγ and u-AQP2. PRC, p-Ang Ⅱ and p-Aldo were increased during active treatments （P 〈 0.001）. After hypertonic saline, u-NKCC2 increased during amiloride （6% ± 34%; P = 0.081） and increased significantly during placebo （17% ± 24%; P = 0.010）. U-AQP2 increased signifcantly during amiloride （31% ± 22%; P 〈 0.001） and placebo （34% ± 27%; P 〈 0.001）, while u-NKCC2 and u-AQP2 did not change signifcantly during BFTZ （-7% ± 28%; P = 0.257 and 5% ± 16%; P = 0.261）. U- ENaCγ increased in all three groups （ P 〈 0.050）. PRC, AngⅡ and p-Aldo decreased to the same extent, while AVP increased, but to a smaller degree during BFTZ （ P = 0.048）. cDBP decreased significantly during BFTZ （P 〈 0.001）, but not during amiloride or placebo. There were no significant differences in body fuid volumes.CONCLUSION： After hypertonic saline, u-NKCC2 and u-AQP2 increased during amiloride, but not during BFTZ. Lower p-AVP during BFTZ potentially caused less stimulation of NKCC2 and AQP2 and subsequent lower reabsorption of water and sodium.

Electrophysiology of Sodium Receptors in Taste Cells

Sodium intake is important to maintain proper osmolarity and volume of extracellular fluid in vertebrates. The ability to find sources of sodium ions for managing electrolyte homeostasis relies on the activity of the taste system to sense salt. Several studies have been performed to understand the mechanisms underlying Na+ reception in taste cells, the peripheral detectors for food chemicals. It is now generally accepted that Na+ interacts with specific ion channels in taste cell membrane, called sodium receptors. As ion channels, these proteins mediate transmembrane ion fluxes (that is, electrical currents) during their operation. Thus, a lot of information on the functional properties of sodium receptors has been obtained by using electrophysiological techniques. Here, I review our current knowledge on the biophysical and physiological features of these receptors obtained by applying the patch-clamp recording techniques to single taste cells.

Sodium channels in the apical membrane of human nasal epithelial cells

Objective To study the electrophysiological properties of sodium channels in the apical membrane of human nasal epithelial cells Method Nasal epithelial cells of human inferior turbinate from patients with obstructive sleep apnea syndrome were cultured in serum free medium on collagen gel coated membranes at an air liquid interface and studied by a patch clamp technique Results In cell attached patches, a typical single channel current with a conductance of 21 09?pS and reversal potential of -50 96 were recorded The permeability ratio P Na /P K was more than 5 80 In the presence of 10 4 mmol/L amiloride in the pipette, the incidence of sodium channels decreased from 26 67% to 5 13% This revealed that a population of channels were inhibited by amiloride at a dose of 10 4 mmol/L Ca 2+ at dose of 10 3 mmol/L did not influence the incidence of sodium channels There was no obvious association between voltage and the open probability of the channels Conclusions Our results indicate that most Na + channels in cell attached patches of human nasal epithelial cells are amiloride sensitive and Na + selective Only a few channels are amiloride insensitive The channels were not activated by extracellular Ca 2+ and the open probability followed a voltage independent manner

Epithelial Sodium and Chloride Channels and Asthma

Objective：To focus on the asthmatic pathogenesis and clinical manifestations related to epithelial sodium channel （ENaC）/chlorine ion channel.Data Sources：The data analyzed in this review were the English articles from 1980 to 2015 from journal databases,primarily PubMed and Google Scholar.The terms used in the literature search were：（1） ENaCs;cystic fibrosis （CF） transmembrane conductance regulator （CFTR）;asthma/asthmatic,（2） ENaC/sodium salt;CF;asthma/asthmatic,（3） CFTR/chlorine ion channels;asthma/asthmatic,（4） ENaC/sodium channel/scnn1a/scnn1b/scnn1g/scnn1d/amiloride-sensitive/amiloride-inhibtable sodium channels/sodium salt;asthma/asthmatic,lung/pulmonary/respiratory/tracheal/alveolar,and （5） CFTR;CF;asthma/asthmatic （ti）.Study Selection：These studies included randomized controlled trials or studies covering asthma pathogenesis and clinical manifestations related to ENaC/chlorine ion channels within the last 25 years （from 1990 to 2015）.The data involving chronic obstructive pulmonary disease and CF obtained from individual studies were also reviewed by the authors.Results：Airway surface liquid dehydration can cause airway inflammation and obstruction.ENaC and CFTR are closely related to the airway mucociliary clearance.Ion transporters may play a critical role in pathogenesis of asthmatic exacerbations.Conclusions：Ion channels have been the center of many studies aiming to understand asthmatic pathophysiological mechanisms or to identify therapeutic targets for better control of the disease.

Regulation of epithelial sodium channel ^-subunit expression by adenosine receptor A2a in alveolar epithelial ceils

Background The amiloride-sensitive epithelial sodium channel a-subunit （a-ENaC） is an important factor for alveolar fluid clearance during acute lung injury. The relationship between adenosine receptor A2a （A2aAR） expressed in alveolar epithelial cells and aα-ENaC is poorly understood. We targeted the A2aAR in this study to investigate its role in the expression of αa-ENaC and in acute lung injury.Methods A549 cells were incubated with different concentrations of A2aAR agonist CGS-21680 and with 100 μmol/L CGS-21680 for various times. Rats were treated with lipopolysaccharide （LPS） after CGS-21680 was injected. Animals were sacrificed and tissue was harvested for evaluation of lung injury by analysis of the lung wet-to-dry weight ratio, lung permeability and myeloperoxidase activity. RT-PCR and Western blotting were used to determine the mRNA and protein expression levels of α-ENaC in A549 cells and alveolar type II epithelial cells.Results Both mRNA and protein levels of α-ENaC were markedly higher from 4 hours to 24 hours after exposure to 100μmol/L CGS-21680. There were significant changes from 0.1 umol/L to 100 μmol/L CGS-21680, with a positive correlation between increased concentrations of CGS-21680 and expression of α-ENaC. Treatment with CGS-21680during LPS induced lung injury protected the lung and promoted α-ENaC expression in the alveolar epithelial cells.Conclusion Activation of A2aAR has a protective effect during the lung injury, which may be beneficial to the prognosis of acute lung injury

Lipoxin A4 Ameliorates Lipopolysaccharide-lnduced A549 Cell Injury through Upregulation of N-myc Downstream-Regulated Gene-1

Background： Lipoxin A4 （LXA4） can alleviate lipopolysaccharide （LPS）-induced acute lung injury （ALl） and acute respiratory distress syndrome through promoting epithelial sodium channel （ENaC） expression in lung epithelial cells. However, how LXA4 promote ENaC expression is still largely elusive. The present study aimed to explore genes and signaling pathway involved in regulating ENaC expression induced by LXA4. Methods： A549 cells were incubated with LPS and LXA4, or in combination, and analyzed by quantitative real-time polymerase chain reaction （qRT-PCR） of ENaC-α/γ. Candidate genes affected by LXA4 were explored by transcriptome sequencing ofA549 cells. The critical candidate gene was validated by qRT-PCR and Western blot analysis ofA549 cells treated with LPS and LXA4 at different concentrations and time intervals. LXA4 receptor （ALX） inhibitor BOC-2 was used to test induction of candidate gene by LXA4. Candidate gene siRNA was adopted to analyze its influence on A549 viability and ENaC-α expression. Phosphoinositide 3-kinase （PI3K） inhibitor LY294002 was utilized to probe whether the PI3K signaling pathway was involved in LXA4 induction of candidate gene expression. Results： The A549 cell models of ALl were constrticted and subjected to transcriptome sequencing. Among candidate genes, N-myc downstream- regulated gent- 1 （NDRG 1 ） was validated by real-time-PCR and Western blot. NDRG 1 mRNA was elevated in a dose-dependent manner of LXA4, whereas BOC-2 antagonized NDRG 1 expression induced by LXA4. NDRG I siRNA suppressed viability of LPS-treated A549 cells （treatment vs. control, 0.605± 0.063 vs. 0.878 ± 0.083, P = 0.040） and ENaC-α expression （treatment vs. control, 0.458 ± 0.038 vs. 0.711 ± 0.035, P = 0.008）. LY294002 inhibited NDRG 1 （treatment vs. control, 0.459 ± 0.023 vs. 0.726 ± 0.020, P 0.001 ） and ENaC-α （treatment vs. control, 0.236 ± 0.021 vs. 0.814 ±0.025, P 〈 0.001 ） expressions and serum- and glucocorticoid-inducible kinase I phosphorylation （treatment vs. control, 0.442± 0.024 vs. 1.046 ± 0.082, P = 0.002）, indicating the PI3K signaling pathway was involved in regulating NDRG 1 expression induced by LXA4. Conclusion： Our research uncovered a critical role of NDRG1 in LXA4 alleviation of LPS-induced A549 cell injury through mediating PI3K signaling to restore ENaC expression.

Lung fluid during postnatal transition

Successful postnatal pulmonary adaptation is dependent on cessation of fetal fluid secretion into the luminal space of the lung and a switch to fluid absorption during the perinatal period. Most infants undergo this transformation rapidly, but some encounter difficulties.