Relation between alpha, beta, and gamma human amiloride- sensitive epithelial Na+ channel mRNA levels and nasal epithelial potential difference in healthy men

Systematic review and meta-analysis of nasal potential difference in hypoxia-induced lung injury

Nasal potential difference (NPD), a well-established in vivo clinical test for cystic fibrosis, reflects transepithelial cation and anion transport in the respiratory epithelium. To analyze whether NPD can be applied to diagnose hypoxic lung injury, we searched PubMed, EMBASE, Scopus, Web of Science, Ovid MEDLINE, and Google Scholar, and analyzed data retrieved from eleven unbiased studies for high altitude pulmonary edema (HAPE) and respiratory distress syndrome (RDS) using the software RevMan and R. There was a significant reduction in overall basal (WMD -5.27 mV, 95% CI: -6.03 to -4.52, P < 0.00001, I(2) = 42%), amiloride-sensitive (ENaC) (-2.87 mV, 95% CI: -4.02 to -1.72, P < 0.00001, I(2) = 51%), and -resistant fractions (-3.91 mV, 95% CI: -7.64 to -0.18, P = 0.04, I(2) = 95%) in lung injury patients. Further analysis of HAPE and RDS separately corroborated these observations. Moreover, SpO2 correlated with ENaC-associated NPD positively in patients only, but apparently related to CFTR-contributed NPD level inversely. These correlations were confirmed by the opposite associations between NPD values and altitude, which had a negative regression with SpO2 level. Basal NPD was significantly associated with amiloride-resistant but not ENaC fraction. Our analyses demonstrate that acute lung injury associated with systemic hypoxia is characterized by dysfunctional NPD.

Chronic Hypoxemia in Children With Congenital Heart Defect Impairs Airway Epithelial Sodium Transport

OBJECTIVE

Ambient hypoxia impairs the airway epithelial Na transport, which is crucial in lung edema reabsorption. Whether chronic systemic hypoxemia affects airway Na transport has remained largely unknown. We have therefore investigated whether chronic systemic hypoxemia in children with congenital heart defect affects airway epithelial Na transport, Na transporter-gene expression, and short-term lung edema accumulation.

DESIGN

Prospective, observational study.

SETTING

Tertiary care medical center responsible for nationwide pediatric cardiac surgery.

PATIENTS

Ninety-nine children with congenital heart defect or acquired heart disease (age range, 6 d to 14.8 yr) were divided into three groups based on their level of preoperative systemic hypoxemia: 1) normoxemic patients (SpO2% ≥ 95%; n = 44), 2) patients with cyanotic congenital heart defect and moderate hypoxemia (SpO2 86-94%; n = 16), and 3) patients with cyanotic congenital heart defect and profound systemic hypoxemia (SpO2 ≤ 85%; n = 39).

MEASUREMENTS AND MAIN RESULTS

Nasal transepithelial potential difference served as a surrogate measure for epithelial Na transport of the respiratory tract. Profoundly hypoxemic patients had 29% lower basal nasal transepithelial potential difference (p = 0.02) and 55% lower amiloride-sensitive nasal transepithelial potential difference (p = 0.0003) than normoxemic patients. In profoundly hypoxemic patients, nasal epithelial messenger RNA expressions of two airway Na transporters (amiloride-sensitive epithelial Na channel and β1- Na-K-ATPase) were not attenuated, but instead α1-Na-K-ATPase messenger RNA levels were higher (p = 0.03) than in the normoxemic patients, indicating that posttranscriptional factors may impair airway Na transport. The chest radiograph lung edema score increased after congenital cardiac surgery in profoundly hypoxemic patients (p = 0.0004) but not in patients with normoxemia or moderate hypoxemia.

CONCLUSIONS

The impaired airway epithelial amiloride-sensitive Na transport activity in profoundly hypoxemic children with cyanotic congenital heart defect may hinder defense against lung edema after cardiac surgery.

ENaCs and ASICs as therapeutic targets

The epithelial Na(+) channel (ENaC) and acid-sensitive ion channel (ASIC) branches of the ENaC/degenerin superfamily of cation channels have drawn increasing attention as potential therapeutic targets in a variety of diseases and conditions. Originally thought to be solely expressed in fluid absorptive epithelia and in neurons, it has become apparent that members of this family exhibit nearly ubiquitous expression. Therapeutic opportunities range from hypertension, due to the role of ENaC in maintaining whole body salt and water homeostasis, to anxiety disorders and pain associated with ASIC activity. As a physiologist intrigued by the fundamental mechanics of salt and water transport, it was natural that Dale Benos, to whom this series of reviews is dedicated, should have been at the forefront of research into the amiloride-sensitive sodium channel. The cloning of ENaC and subsequently the ASIC channels has revealed a far wider role for this channel family than was previously imagined. In this review, we will discuss the known and potential roles of ENaC and ASIC subunits in the wide variety of pathologies in which these channels have been implicated. Some of these, such as the role of ENaC in Liddle's syndrome are well established, others less so; however, all are related in that the fundamental defect is due to inappropriate channel activity.

The role of alpha-, beta-, and gamma-ENaC subunits in distal lung epithelial fluid absorption induced by pulmonary edema fluid

Edema fluid (EF) increases epithelial Na(+) transport by rat fetal distal lung epithelia (FDLE) and induces net lung fluid absorption in fetal mouse lung explants [Rafii B, Gillie DJ, Sulowski C, Hannam V, Cheung T, Otulakowski G, Barker PM, O'Brodovich H. J Physiol (Lond) 544: 537-548, 2002]. We now show that EF increases fluid absorption across monolayers of rat FDLE in a dose-dependent manner. To study the role of subunits of the epithelial Na(+) channel (ENaC) in the phenomena, we cultured explants from the distal lungs of 16-day gestational age wild-type (WT) or alpha-, beta-, or gamma-ENaC knockout or heterozygote (HT) mice. WT explants cultured in media continuously expanded over time as a result of net fluid secretion. In contrast, when explants were exposed to EF for 24 h, net fluid absorption occurred. EF-exposed explants had normal histology, but marked changes were seen after Triton X-100 or staurosporine exposure. Transmission electron microscopy showed EF promoted lamellar body formation and abundant surfactant in the explants' lumens. EF-induced changes in explant size were similar in alpha-ENaC knockout, WT, and HT littermate fetal lung explants (P > 0.05). In contrast, EF's effect was attenuated in beta- and gamma-ENaC knockouts (P < 0.05) vs. WT and HT littermate fetal lung explants. EF exposure slightly decreased or had no effect on mRNA levels for alpha-ENaC in various mouse genotypes but decreased expression of beta- and gamma-ENaC subunit mRNAs (P < 0.01) across all genotype groups. We conclude that beta- and gamma-, but not alpha-, ENaC subunits are essential for EF to exert its maximal effect on net fluid absorption by distal lung epithelia.

Role of alveolar epithelial sodium transport in high altitude pulmonary edema (HAPE)

Alveolar edema results from an imbalance between fluid filtration into the alveolar space and removal by reabsorption. Hypoxia increases filtration by raising pulmonary capillary pressure and increasing endothelial and epithelial permeability allowing fluid and blood cells to access the alveoli. Active Na-reabsorption drives the fluid reabsorption from the alveolar space, but hypoxia inhibits reabsorption by inhibition of epithelial Na-channels (ENaC) and Na/K-ATPase. A (genetically determined) low activity of alveolar reabsorption in normoxia and further inhibition by hypoxia might cause HAPE-susceptibility, since at some point the depressed reabsorption may not keep pace with increased filtration. Na-reabsorption might even prove totally inefficient in the presence of large leaks of the alveolar barrier. Alveolar Na-reabsorption has not been measured in HAPE. Nasal epithelial Na-transport has been used as surrogate marker based on similarities in subunit expression of ENaC in nasal, airway, and alveolar epithelium. At high altitude cold, dryness, and nasal infections affect the nasal potential making any extrapolation to processes at the alveolar epithelium unreliable. The variability in nasal Na- and Cl-transport reduces the usefulness of nasal potentials to diagnose HAPE-susceptibility.

Improvement of lung compliance during postnatal adaptation correlates with airway sodium transport

RATIONALE

Fetal lung liquid secretion is coupled with chloride transport into the lung lumen. The postnatal clearance of lung liquid is dependent on osmotic force generated by active sodium absorption.

OBJECTIVE

To study the interaction between airway epithelial sodium transport and postnatal lung function.

METHODS

We determined lung compliance and nasal transepithelial potential difference as a measure of airway ion transport and epithelial sodium channel gene expression in 41 healthy newborn infants during the first 50 h after birth.

MEASUREMENTS AND MAIN RESULTS

Lung compliance improved significantly during the study period, whereas nasal potential difference remained constant. There was a significant decrease in the expressions of beta and gamma subunits of the epithelial sodium channel. A positive correlation existed between amiloride-sensitive nasal potential difference measured at 1-4 h of age and lung compliance at 21-27 h of age. We found no correlation between the molecular data and functional measurements.

CONCLUSIONS

An important part of pulmonary adaptation takes place during the first hour after birth. The improvement of lung compliance continues over the first postnatal days and coincides with down-regulation of epithelial sodium channel beta and gamma subunit expression.

Low expression of human epithelial sodium channel in airway epithelium of preterm infants with respiratory distress

OBJECTIVE

Active ion transport is critical to postnatal clearance of lung fluid. The importance of epithelial sodium channel (ENaC) in this clearance has been demonstrated in animal studies in which alpha-ENaC knockout mice died postnatally as a result of respiratory insufficiency. In animals, the expression of alpha-ENaC in respiratory epithelium is dependent on gestational age, but when assessed by in situ hybridization in the human (h), the mRNA is present from the earliest stages of pulmonary development. Therefore, the purpose of the present investigation was to quantify mRNA of the alpha-, beta-, and gamma-hENaC subunits of newborn preterm infants with respiratory distress and compare the gene expression data against those detected in healthy term infants. In addition, the effect of systemic dexamethasone therapy on the 3 hENaC subunits was studied in 4 preterm infants who received prolonged assisted ventilation.

METHODS

The expression of subunits of hENaC was determined in samples taken from nasal respiratory epithelium of 7 healthy term infants (gestation age: 39.3 +/- 0.9 weeks [mean +/- standard deviation) and 5 preterm infants (gestational age: 27.2 +/- 0.9 weeks) with respiratory distress syndrome within 5 hours of birth. Betamethasone had been given to all mothers of preterm infants. In 4 additional preterm infants who still required assisted ventilation at 43 +/- 6 days postnatal age, the expression of alpha-hENaC was determined in samples taken before and during treatment with dexamethasone.

RESULTS

Preterm infants with respiratory distress syndrome had low expression of all hENaC subunits relative to healthy term infants (alpha-hENaC: 5.38 +/- 2.01 [amol/fmol cytokeratin 18] vs 9.13 +/- 2.26; beta-hENaC: 2.44 +/- 1.43 vs 4.25 +/- 1.10; gamma-hENaC: 2.43 +/- 0.11 vs 6.81 +/- 3.24). Each of the 4 preterm infants who were treated with dexamethasone at approximately 1 month of age showed an increase in expression of alpha-hENaC and beta-hENaC subunit normalized to cytokeratin 18.

CONCLUSION

All 3 subunits of the hENaC are low in preterm relative to full-term infants. alpha-hENaC mRNA in respiratory epithelium is increased by therapeutic doses of glucocorticosteroid. Low expression of alpha-hENaC in human respiratory epithelium may play a role in the pathogenesis of respiratory distress in preterm infants.

Translational activation and repression by distinct elements within the 5'-UTR of ENaC alpha-subunit mRNA

The rat amiloride-sensitive epithelial Na(+) channel (rENaC), the rate-limiting step in epithelial Na(+) transport, consists of three subunits, alpha, beta, and gamma. We hypothesized that alpha-rENaC translation is regulated via its 5'-untranslated region (UTR). Transient transfections of alpha-rENaC promoter-reporter constructs in representative epithelial cell lines demonstrated up to fivefold differences in activity among constructs containing different amounts of the alpha-rENaC 5'-UTR sequence. Differences in reporter protein activity did not parallel differences in reporter mRNA, demonstrating that 5'-UTR regulation must be at the level of translation. Specifically, translation was enhanced by a region extending from +53 to +211 bp downstream from the transcription start site and repressed by the region between +367 and +499 bp. Examination of the 5'-UTR sequence revealed an out-of-frame initiation codon within the repressive region, 43 bp upstream from the start of the alpha-rENaC open reading frame. Mutational analysis of this upstream start codon indicated that it plays, at most, a minor role in impeding translation both in vitro and in vivo, suggesting that additional mechanisms of translational regulation are operative.

Expression of alpha-, beta-, and gamma-hENaC mRNA in the human nasal, bronchial, and distal lung epithelium

The amount of fluid covering the epithelium of the airways and alveolar space is modulated by active transport of Na+ from the lumen through the apical membrane Na+ permeant ion channels towards the interstitial space. We have measured the subunit expression of the amiloride-sensitive human Na+ channel (hENaC) by concomitant assessment of alpha-, beta-, and gamma-hENaC mRNA in the nasal, bronchial, and peripheral lung epithelia of adult patients undergoing lobectomy secondary to lung cancer. The study employed quantitative competitive reverse-transcriptase-polymerase chain reaction and qualitative in situ hybridization techniques. The hENaC mRNA content of each sample was normalized to the amount of epithelial cell-specific cytokeratin 18 (CK18) mRNA. Nasal epithelium contained significantly more (p < 0.05) alpha-hENaC mRNA (18 +/- 5 SD amol/fmol CK18), than bronchus (8 +/- 2 SD amol/fmol) and peripheral lung (9 +/- 2 SD amol/fmol). The ratio of gamma-hENaC/alpha-hENaC mRNA concentration was lowest in the nasal area, and it increased significantly towards the distal lung regions. The change in beta-hENaC mRNA was less profound. In situ hybridization studies of bronchial and peripheral lung sections selectively revealed expression of alpha-hENaC mRNA in superficial epithelium and submucosal glands of large airways, in bronchiolar epithelium, and in alveolar cells. We conclude that the relative expression of the hENaC subunit genes changes from the proximal to distal regions of the human respiratory tract.

Early expression of beta- and gamma-subunits of epithelial sodium channel during human airway development

The amiloride-sensitive epithelial Na(+) channel (ENaC) is an apical membrane protein complex involved in active Na(+) absorption and in control of fluid composition in airways. There are no data reporting the distribution of its pore-forming alpha-, beta-, and gamma-subunits in the developing human lung. With use of two different rabbit polyclonal antisera raised against beta- and gamma-ENaC, immunohistochemical localization of the channel was performed in fetal (10-35 wk) and in adult human airways. Both subunits were detected after 17 wk of gestation on the apical domain of bronchial ciliated cells, in glandular ducts, and in bronchiolar ciliated and Clara cells. After 30 wk, the distribution of beta- and gamma-subunits was similar in fetal and adult airways. In large airways, the two subunits were detected in ciliated cells, in cells lining glandular ducts, and in the serous gland cells. In the distal bronchioles, beta- and gamma-subunits were identified in ciliated and Clara cells. Ultrastructural immunogold labeling confirmed the identification of beta- and gamma-ENaC proteins in submucosal serous cells and bronchiolar Clara cells. Early expression of ENaC proteins in human fetal airways suggests that Na(+) absorption might begin significantly before birth, even if secretion is still dominant.

Hypoxia reduces airway epithelial sodium transport in rats

Ascent to high altitude leads to pulmonary edema formation in some individuals. Recent laboratory evidence supports the hypothesis that hypoxia may impair the function of the alveolar epithelium and thus augment edema accumulation via reduced clearance of lung liquid. We investigated the effect of hypobaric hypoxia on epithelial sodium transport in adult Sprague-Dawley rats by measuring the nasal transepithelial potential difference (PD) as an index of airway sodium transport. Baseline PDs were similar to those previously reported in other species. Administration of amiloride resulted in a significant fall in nasal PD, as did ouabain administration for 24 h (-27.8 vs. -18.8 mV; P = 0.001; n = 5 rats). Exposure to hypobaric hypoxia (0.5 atm) for 24 h caused a significant fall in nasal PD (-23.7 vs. -18.8 mV; P = 0.002; n = 15 rats), which was not additive to the changes in nasal PD produced by amiloride or ouabain. We conclude that subacute exposure to moderate hypobaric hypoxia can inhibit sodium transport by the airway epithelium in rats.

Structure and hormone responsiveness of the gene encoding the alpha-subunit of the rat amiloride-sensitive epithelial sodium channel

The rat amiloride-sensitive epithelial sodium channel (rENaC) is the rate-limiting step for vectorial transport of Na+ across tight epithelia. The complex is composed of three subunits, alpha, beta, and gamma. Expression of the subunits has been shown to be tissue-specific and developmentally and hormonally regulated. To study mechanisms involved in transcriptional regulation of alpharENaC, we determined the genomic organization of the alpharENaC gene. By 5' rapid amplification of cDNA ends and primer extension, two transcriptional start sites were detected 453 base pairs (bp) apart, resulting in alternative 5' untranslated region (UTR) lengths of 515 or 62 bp. The longer 5' UTR is more prevalent in fetal lung than in adult lung or kidney. The 5' untranslated and coding regions are contained within 12 exons, with the translation start site located within the first exon. Sequence analysis of approximately 1,500 bp of 5' flanking DNA identified putative binding sites for transcription factors PEA3, SP1, AP-1, nuclear factor-kappaB, and thyroid and glucocorticoid receptors. alpharENaC promoter-reporter gene constructs produced low levels of reporter gene activity in transiently transfected cells, which could be increased by dexamethasone (DEX) treatment. Tri-iodothyronine treatment alone had no effect but potentiated stimulation by DEX.