Expression of ENaC subunits, chloride channels, and aquaporins in ovine fetal lung: ontogeny of expression and effects of altered fetal cortisol concentrations

Preterm birth affects both surfactant synthesis and lung liquid resorption actors in fetal sheep

INTRODUCTION

After birth, the lungs must resorb the fluid they contain. This process involves multiple actors such as surfactant, aquaporins and ENaC channels. Preterm newborns often exhibit respiratory distress syndrome due to surfactant deficiency, and transitory tachypnea caused by a delay in lung liquid resorption. Our hypothesis is that surfactant, ENaC and aquaporins are involved in respiratory transition to extrauterine life and altered by preterm birth. We compared these candidates in preterm and term fetal sheeps.

MATERIALS AND METHODS

We performed cesarean sections in 8 time-dated pregnant ewes (4 at 100 days and 4 at 140 days of gestation, corresponding to 24 and 36 weeks of gestation in humans), and obtained 13 fetal sheeps in each group. We studied surfactant synthesis (SP-A, SP-B, SP-C), lung liquid resorption (ENaC, aquaporins) and corticosteroid regulation (glucocorticoid receptor, mineralocorticoid receptor and 11-betaHSD2) at mRNA and protein levels.

RESULTS

The mRNA expression level of SFTPA, SFTPB and SFTPC was higher in the term group. These results were confirmed at the protein level for SP-B on Western Blot analysis and for SP-A, SP-B and SP-C on immunohistochemical analysis. Regarding aquaporins, ENaC and receptors, mRNA expression levels for AQP1, AQP3, AQP5, ENaCα, ENaCβ, ENaCγ and 11βHSD2 mRNA were also higher in the term group.

DISCUSSION

Expression of surfactant proteins, aquaporins and ENaC increases between 100 and 140 days of gestation in an ovine model. Further exploring these pathways and their hormonal regulation could highlight some new explanations in the pathophysiology of neonatal respiratory diseases.

Lack of cystic fibrosis transmembrane conductance regulator disrupts fetal airway development in pigs

Loss of cystic fibrosis transmembrane conductance regulator (CFTR) function causes cystic fibrosis (CF), predisposing the lungs to chronic infection and inflammation. In young infants with CF, structural airway defects are increasingly recognized before the onset of significant lung disease, which suggests a developmental origin and a possible role in lung disease pathogenesis. The role(s) of CFTR in lung development is unclear and developmental studies in humans with CF are not feasible. Young CF pigs have structural airway changes and develop spontaneous postnatal lung disease similar to humans; therefore, we studied lung development in the pig model (non-CF and CF). CF trachea and proximal airways had structural lesions detectable as early as pseudoglandular development. At this early developmental stage, budding CF airways had smaller, hypo-distended lumens compared to non-CF airways. Non-CF lung explants exhibited airway lumen distension in response to forskolin/IBMX as well as to fibroblast growth factor (FGF)-10, consistent with CFTR-dependent anion transport/secretion, but this was lacking in CF airways. We studied primary pig airway epithelial cell cultures and found that FGF10 increased cellular proliferation (non-CF and CF) and CFTR expression/function (in non-CF only). In pseudoglandular stage lung tissue, CFTR protein was exclusively localized to the leading edges of budding airways in non-CF (but not CF) lungs. This discreet microanatomic localization of CFTR is consistent with the site, during branching morphogenesis, where airway epithelia are responsive to FGF10 regulation. In summary, our results suggest that the CF proximal airway defects originate during branching morphogenesis and that the lack of CFTR-dependent anion transport/liquid secretion likely contributes to these hypo-distended airways.

Factors relating caesarean section to persistent pulmonary hypertension of the newborn

BACKGROUND

Several studies have clearly demonstrated a significantly higher incidence of persistent pulmonary hypertension of the newborn (PPHN) in neonates delivered by caesarean section (CS) compared to those delivered vaginally. The pathophysiological factors underlying the link between CS and PPHN are still poorly understood. In this review, we describe the mechanisms that could explain the association between CS delivery and subsequent PPHN, as well as potential preventive measures.

DATA SOURCES

A literature search was conducted by electronic scanning of databases such as PubMed and Web of Science using the key words "persistent pulmonary hypertension of the newborn", "caesarean section", "iatrogenic prematurity", "oxidative stress", "late preterm", "labor" and "vasoactive agents".

RESULTS

Iatrogenic prematurity, higher rates of late preterm delivery and lack of physiological changes of labor play an important role in the association between CS and PPHN. CS delivery also results in limited endogenous pulmonary vasodilator synthesis and lower levels of protective anti-oxidants in the neonates. In addition, CS delivery exposes infants to a higher risk of respiratory distress syndrome and its concomitant increase in endothelin-1 levels, which might indirectly lead to a higher risk of developing PPHN. We believe that neonates delivered by CS are exposed to a combination of these pathophysiological events, culminating in an endpoint of respiratory distress, hypoxia, acidosis, and delayed transition and thereby increased risks of PPHN. The use of antenatal corticosteroids prior to elective CS in late preterm deliveries, promoting accurate informedconsent process, delaying elective CS to 39 weeks of gestation or beyond and antenatal maternal anti-oxidant supplementation could potentially mitigate the effects of CS delivery and minimize CS-related PPHN.

CONCLUSIONS

The link between CS delivery and PPHN is complex. In view of the rising rates of CS worldwide, there is an urgent need to further explore the mechanisms linking CS to PPHN and experimentally test therapeutic options in order to allow effective targeted interventions.

Differential effects of late gestation maternal overnutrition on the regulation of surfactant maturation in fetal and postnatal life

KEY POINTS

Offspring of overweight and obese women are at greater risk for respiratory complications at birth. We determined the effect of late gestation maternal overnutrition (LGON) in sheep on surfactant maturation, glucose transport and fatty acid metabolism in the lung in fetal and postnatal life. There were significant decreases in surfactant components and numerical density of surfactant producing cells in the alveolar epithelium due to LGON in the fetal lung. However, there were no differences in the levels of these surfactant components between control and LGON lambs at 30 days of age. The reduced capacity for surfactant production in fetuses as a result of LGON may affect the transition to air breathing at birth. There was altered glucose transport and fatty acid metabolism in the lung as a result of LGON in postnatal life. However, there is a normalisation of surfactant components that suggests accelerated maturation in the lungs after birth.

ABSTRACT

With the increasing incidence of obesity worldwide, the proportion of women entering pregnancy overweight or obese has increased dramatically. The fetus of an overnourished mother experiences numerous metabolic changes that may modulate lung development and hence successful transition to air breathing at birth. We used a sheep model of maternal late gestation overnutrition (LGON; from 115 days' gestation, term 147 ± 3 days) to determine the effect of exposure to an increased plane of nutrition in late gestation on lung development in the fetus (at 141 days' gestation) and the lamb (30 days after birth). We found a decrease in the numerical density of surfactant protein positive cells, as well as a reduction in mRNA expression of surfactant proteins (SFTP-A, -B and -C), a rate limiting enzyme in surfactant phospholipid synthesis (phosphate cytidylyltransferase 1, choline, α; PCYT1A), and glucose transporters (SLC2A1 and SLC2A4) in the fetal lung. In lambs at 30 days after birth, there were no differences between Control and LGON groups in the surfactant components that were downregulated in the LGON fetuses. However, mRNA expression of SFTP-A, PCYT1A, peroxisome proliferator activated receptor-γ, fatty acid synthase and fatty acid transport protein were increased in LGON lambs compared to controls. These results indicate a reduced capacity for surfactant production in late gestation. While these deficits are normalised by 30 days after birth, the lungs of LGON lambs exhibited altered glucose transport and fatty acid metabolism, which is consistent with an enhanced capacity for surfactant synthesis and restoration of surfactant maturity in these animals.

Maternal chronic hypoxia increases expression of genes regulating lung liquid movement and surfactant maturation in male fetuses in late gestation

KEY POINTS

Chronic fetal hypoxaemia is a common pregnancy complication associated with intrauterine growth restriction that may influence respiratory outcome at birth. We investigated the effect of maternal chronic hypoxia for a month in late gestation on signalling pathways regulating fetal lung maturation and the transition to air-breathing at birth using isobaric hypoxic chambers without alterations to maternal food intake. Maternal chronic hypoxia in late gestation increases fetal lung expression of genes regulating hypoxia signalling, lung liquid reabsorption and surfactant maturation, which may be an adaptive response in preparation for the successful transition to air-breathing at birth. In contrast to other models of chronic fetal hypoxaemia, late gestation onset fetal hypoxaemia promotes molecular regulation of fetal lung maturation. This suggests a differential effect of timing and duration of fetal chronic hypoxaemia on fetal lung maturation, which supports the heterogeneity observed in respiratory outcomes in newborns following exposure to chronic hypoxaemia in utero.

ABSTRACT

Chronic fetal hypoxaemia is a common pregnancy complication that may arise from maternal, placental and/or fetal factors. Respiratory outcome of the infant at birth likely depends on the duration, timing and severity of the hypoxaemic insult. We have isolated the effect of maternal chronic hypoxia (MCH) for a month in late gestation on fetal lung development. Pregnant ewes were exposed to normoxia (21% O2 ) or hypoxia (10% O2 ) from 105 to 138 days of gestation (term ∼145 days). At 138 days, gene expression in fetal lung tissue was determined by quantitative RT-PCR. Cortisol concentrations were determined in fetal plasma and lung tissue. Numerical density of surfactant protein positive cells was determined by immunohistochemistry. MCH reduced maternal PaO2 (106 ± 2.9 vs. 47 ± 2.8 mmHg) and fetal body weight (4.0 ± 0.4 vs. 3.2 ± 0.9 kg). MCH increased fetal lung expression of the anti-oxidant marker CAT and decreased expression of the pro-oxidant marker NOX-4. MCH increased expression of genes regulating hypoxia signalling and feedback (HIF-3α, KDM3A, SLC2A1, EGLN-3). There was no effect of MCH on fetal plasma/lung tissue cortisol concentrations, nor genes regulating glucocorticoid signalling (HSD11B-1, HSD11B-2, NR3C1, NR3C2). MCH increased expression of genes regulating sodium (SCNN1-B, ATP1-A1, ATP1-B1) and water (AQP-4) movement in the fetal lung. MCH promoted surfactant maturation (SFTP-B, SFTP-D, ABCA3) at the molecular level, but did not alter the numerical density of surfactant positive cells in lung tissue. MCH in late gestation promotes molecular maturation of the fetal lung, which may be an adaptive response in preparation for the successful transition to air-breathing at birth.

The interaction of glucocorticoids and progesterone distinctively affects epithelial sodium transport

PURPOSE

Glucocorticoids and progesterone exert stimulatory effects on epithelial Na(+) transport, including increased mRNA expression of the participating ion transporters (epithelial Na(+) channels [ENaC] and Na,K-ATPases) and their electrophysiological activity. Fetuses threatened by preterm labor may receive high doses of glucocorticoids to stimulate lung maturation and are naturally exposed to high levels of female sex steroids. However, it is still unknown how the combination of both hormones influences the epithelial Na(+) transport, which is crucial for alveolar fluid clearance.

METHODS

Fetal distal lung epithelial cells were incubated in media supplemented with dexamethasone and progesterone. Real-time qPCR and Ussing chamber analysis were used to determine the effects on ENaC mRNA expression and channel activity. In addition, the specific progesterone receptor antagonist (PF-02367982) and the glucocorticoid receptor antagonist mifepristone were used to identify the involved hormone receptors.

RESULTS

Both dexamethasone and progesterone increased ENaC subunit expression and channel activity. However, the combination of dexamethasone and progesterone reduced the α- and γ-ENaC subunit expression compared to the effect of dexamethasone alone. Furthermore, higher dexamethasone concentrations in combination with progesterone also significantly reduced Na(+) transport in Ussing chamber measurements. Hormone receptor antagonists showed that inhibition of the progesterone receptor increased the mRNA expression of α- and γ-ENaC, whereas mifepristone decreased mRNA expression of all ENaC subunits.

CONCLUSION

Glucocorticoids and progesterone individually increase ENaC mRNA expression; however, the combination of both hormones decreases the stimulatory effects of dexamethasone on Na(+) transport and ENaC mRNA expression.

Mineralocorticoid effects in the late gestation ovine fetal lung

This study was designed to determine the effects of corticosteroids at MR in the late‐gestation fetal lung. Since both the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR) are expressed at relatively high levels in the fetal lung, endogenous corticosteroids may act at MR as well as GR in the preterm fetal lung. The GR agonist, betamethasone, the MR agonist, aldosterone, or both were infused intravenously for 48 h in ovine fetuses of approximately 130 days gestation. Effects on airway pressures during stepwise inflation of the in situ lung, expression of ENaC alpha (SCNN1A), ENaC beta (SCNN1B), and Na,K ATPase (ATP1A1), and elastin and collagen content were determined after the infusions. We found that aldosterone significantly reduced the airway pressure measured during the initial step in inflation of the lung, although aldosterone had no overall effect on lung compliance, nor did aldosterone induce expression of ENaCα, ENaCβ or Na,K ATPaseα1. Betamethasone significantly increased expression of the epithelial sodium channel (ENaC) subunit mRNAs, and collagen and elastin content in the lungs, although this dose of betamethasone also had no effect on lung compliance. There was no synergy between effects of the MR and GR agonists. Transcriptomic analysis suggested that although aldosterone did not alter genes in pathways related to epithelial sodium transport, aldosterone did alter genes in pathways involved in cell proliferation in the lungs. The results are consistent with corticosteroid‐induced fluid reabsorption at birth through GR rather than MR, but suggest that MR facilitates lung maturation, and may contribute to inflation with the first breaths via mechanisms distinct from known aldosterone effects in other epithelia.

Infusion of the mineralocorticoid receptor agonist, aldosterone, to the ovine fetus resulted in reduced airway pressures with initial lung inflation. However, aldosterone did not alter lung surfactant or epithelial sodium transport genes which are classical MR gene targets. Transcriptomic analysis revealed an aldosterone effect on genes related to cell cycle, suggesting that MR have a role distinct form that of GR in the maturing lung.

Respiratory disorders in moderately preterm, late preterm, and early term infants

Even when it is just a few weeks before term gestation, early birth has consequences, resulting in higher morbidity and mortality. Respiratory issues related to moderate prematurity include delayed neonatal transition to air breathing, respiratory distress resulting from delayed fluid clearance (transient tachypnea of the newborn), surfactant deficiency (respiratory distress syndrome), and pulmonary hypertension. Management approaches emphasize appropriate respiratory support to facilitate respiratory transition and minimize iatrogenic injury. Studies are needed to determine the impact of respiratory distress coupled with mild-moderate prematurity on long-term outcome.

The fetal sheep lung does not respond to cortisol infusion during the late canalicular phase of development

The prepartum surge in plasma cortisol concentrations in humans and sheep promotes fetal lung and surfactant system maturation in the support of air breathing after birth. This physiological process has been used to enhance lung maturation in the preterm fetus using maternal administration of betamethasone in the clinical setting in fetuses as young as 24 weeks gestation (term = 40 weeks). Here, we have investigated the impact of fetal intravenous cortisol infusion during the canalicular phase of lung development (from 109- to 116-days gestation, term = 150 ± 3 days) on the expression of genes regulating glucocorticoid (GC) activity, lung liquid reabsorption, and surfactant maturation in the very preterm sheep fetus and compared this to their expression near term. Cortisol infusion had no impact on mRNA expression of the corticosteroid receptors (GC receptor and mineralocorticoid receptor) or HSD11B-2, however, there was increased expression of HSD11B-1 in the fetal lung. Despite this, cortisol infusion had no effect on the expression of genes involved in lung sodium (epithelial sodium channel -α, -β, or -γ subunits and sodium–potassium ATPase-β1 subunit) or water (aquaporin 1, 3, and 5) reabsorption when compared to the level of expression during exposure to the normal prepartum cortisol surge. Furthermore, in comparison to late gestation, cortisol infusion does not increase mRNA expression of surfactant proteins (SFTP-A, -B, and -C) or the number of SFTP-B-positive cells present in the alveolar epithelium, the cells that produce pulmonary surfactant. These data suggest that there may be an age before which the lung is unable to respond biochemically to an increase in fetal plasma cortisol concentrations.

Airway expression of the epithelial sodium channel α-subunit correlates with cortisol in term newborns

BACKGROUND

Glucocorticoids have profound effects on lung maturation and function. In in vitro and animal models, they induce epithelial sodium channels (ENaCs) in the airway epithelium, a process that is important to perinatal lung fluid clearance.

OBJECTIVE

The objective of this study was to determine whether, in newborn infants, airway ENaC expression is associated with cortisol concentrations.

METHODS

Cord blood, saliva, and cells from nasal epithelium were obtained from 69 infants delivered at term. Epithelial and saliva sampling was repeated 3 times: <3, 22 to 29, and 40 to 54 hours postnatally. Cortisol, thyrotropin, and free triiodothyronine concentrations were measured with immunoassays, and expression of α-ENaC and β-ENaC was quantified with real-time reverse-transcriptase polymerase chain reaction.

RESULTS

Expression of α-ENaC <30 minutes postnatally correlated with cord plasma cortisol in infants delivered by elective cesarean delivery. In addition, in the total study population <2 hours postnatally, α-ENaC expression correlated with salivary cortisol concentrations. β-ENaC expression, in contrast, showed no association with cortisol concentrations. A significant decrease in β-ENaC expression during the first postnatal day was revealed, whereas timing of the peak in α-ENaC expression seemed to depend on mode of delivery.

CONCLUSIONS

These results support a role in humans for endogenous glucocorticoids in the regulation of airway ion transport. This finding may be a physiologic mechanism mediating pulmonary adaptation in the newborn infant.

A role for mineralocorticoid receptors in the physiology of the ovine fetus: effects on ACTH and lung liquid composition

In the human and ovine fetus, the presence of 11β-hydroxysteroid dehydrogenase 1 allows cortisol and other corticosteroids to act at mineralocorticoid receptors (MRs) in lung and brain. To test the physiologic role of MRs in the late gestation fetus, fetal lambs were infused with a specific MR antagonist for 12 h. Infusion of the MR antagonist significantly increased plasma ACTH and cortisol concentrations. Infusion of the MR antagonist also significantly increased fetal Pco2 and hematocrit, and decreased fetal pH, but did not alter fetal heart rate or blood pressure. Infusion of the MR antagonist altered the ratio of Na⁺ to K⁺ in lung fluid but did not alter the rate of production of lung liquid or the expression of the epithelial sodium channel α or of the Na,K ATPaseα1 in lung. These results suggest that corticosteroids act at MR to regulate ACTH and blood volume and modulate lung fluid composition in the fetus, but basal levels of corticosteroids do not alter lung liquid production rate through effects on MR.