Prenatal development of the pulmonary surfactant system and the influence of hypoxia

Association between Endotype of Prematurity and Mortality: A Systematic Review, Meta-Analysis, and Meta-Regression

INTRODUCTION

Preterm birth represents the leading cause of neonatal mortality. Pathophysiological pathways, or endotypes, leading to prematurity can be clustered into infection/inflammation and dysfunctional placentation. We aimed to perform a systematic review and meta-analysis exploring the association between these endotypes and risk of mortality during first hospital admission Methods: PROSPERO ID: CRD42020184843. PubMed and Embase were searched for observational studies examining infants with gestational age (GA) ≤34 weeks. Chorioamnionitis represented the infectious-inflammatory endotype, while dysfunctional placentation proxies were hypertensive disorders of pregnancy (HDP) and small for GA (SGA)/intrauterine growth restriction (IUGR). A random-effects model was used to calculate odds ratios (ORs) and 95% confidence intervals. Heterogeneity was studied using random-effects meta-regression analysis.

RESULTS

Of 4,322 potentially relevant studies, 150 (612,580 infants) were included. Meta-analysis showed positive mortality odds for chorioamnionitis (OR: 1.43, 95% confidence interval: 1.25-1.62) and SGA/IUGR (OR: 1.68, 95% confidence interval: 1.38-2.04) but negative mortality odds for HDP (OR 0.74, 95% confidence interval: 0.64-0.86). Chorioamnionitis was associated with a lower GA, while HDP and SGA/IUGR were associated with a higher GA. Meta-regression showed a significant correlation between these differences in GA and mortality odds.

CONCLUSION

Our data suggest that the infectious/inflammatory endotype of prematurity has a greater overall impact on mortality risk as it is the most frequent endotype in the lower GAs. However, when the endotype of placental dysfunction is severe enough to induce growth restriction, it is strongly associated with higher mortality rates even though newborns are more mature.

Impact of maternal late gestation undernutrition on surfactant maturation, pulmonary blood flow and oxygen delivery measured by magnetic resonance imaging in the sheep fetus

Restriction of fetal substrate supply has an adverse effect on surfactant maturation in the lung and thus affects the transition from in utero placental oxygenation to pulmonary ventilation ex utero. The effects on surfactant maturation are mediated by alteration in mechanisms regulating surfactant protein and phospholipid synthesis. This study aimed to determine the effects of late gestation maternal undernutrition (LGUN) and LGUN plus fetal glucose infusion (LGUN+G) compared to Control on surfactant maturation and lung development, and the relationship with pulmonary blood flow and oxygen delivery ( D O 2 ) measured by magnetic resonance imaging (MRI) with molecules that regulate lung development. LGUN from 115 to 140 days' gestation significantly decreased fetal body weight, which was normalized by glucose infusion. LGUN and LGUN+G resulted in decreased fetal plasma glucose concentration, with no change in fetal arterial P O 2 compared to control. There was no effect of LGUN and LGUN+G on the mRNA expression of surfactant proteins (SFTP) and genes regulating surfactant maturation in the fetal lung. However, blood flow in the main pulmonary artery was significantly increased in LGUN, despite no change in blood flow in the left or right pulmonary artery and D O 2 to the fetal lung. There was a negative relationship between left pulmonary artery flow and D O 2 to the left lung with SFTP-B and GLUT1 mRNA expression, while their relationship with VEGFR2 was positive. These results suggest that increased pulmonary blood flow measured by MRI may have an adverse effect on surfactant maturation during fetal lung development. KEY POINTS: Maternal undernutrition during gestation alters fetal lung development by impacting surfactant maturation. However, the direction of change remains controversial. We examined the effects of maternal late gestation maternal undernutrition (LGUN) on maternal and fetal outcomes, signalling pathways involved in fetal lung development, pulmonary haemodynamics and oxygen delivery in sheep using a combination of molecular and magnetic resonance imaging (MRI) techniques. LGUN decreased fetal plasma glucose concentration without affecting arterial P O 2 . Surfactant maturation was not affected; however, main pulmonary artery blood flow was significantly increased in the LGUN fetuses. This is the first study to explore the relationship between in utero MRI measures of pulmonary haemodynamics and lung development. Across all treatment groups, left pulmonary artery blood flow and oxygen delivery were negatively correlated with surfactant protein B mRNA and protein expression in late gestation.

Knockout of microRNA‑26a promotes lung development and pulmonary surfactant synthesis

Normal formation and function of the lungs are essential for the transition of the fetus to an air‑breathing environment at birth. The synthesis of pulmonary surfactant (PS), which is produced by type II alveolar epithelial cells (AECIIs), is required for proper lung development. Previous in vitro studies have suggested that PS synthesis is regulated by microRNA (miR)‑26a in fetal rat AECIIs. The present study explored the potential role of miR‑26a in lung development and PS synthesis by using a miR‑26a‑1/miR‑26a‑2 double knockout mouse model. Hematoxylin and eosin staining and transmission electron microscopy were used to observe the morphology of fetal lungs. Reverse transcription‑quantitative polymerase chain reaction and western blot analysis were performed to examine the mRNA and protein levels of surfactant‑associated proteins. The results demonstrated that the lung formation in the knockout mice was more mature, and that there were more mature lamellar bodies inside AECIIs in miR‑26a knockout mice at late stages of lung development. The findings further demonstrated that knockout of miR‑26a increased surfactant‑associated mRNA and protein expression levels. The results indicated that knockout of miR‑26a promotes lung development and PS synthesis.

Influence of prenatal hypoxia and postnatal hyperoxia on morphologic lung maturation in mice

Background

Oxygen supply as a lifesaving intervention is frequently used to treat preterm infants suffering additionally from possible prenatal or perinatal pathogen features. The impact of oxygen and/or physical lung injury may influence the morphological lung development, leading to a chronic postnatal lung disease called bronchopulmonary dysplasia (BPD). At present different experimental BPD models are used. However, there are no systematic comparative studies regarding different influences of oxygen on morphological lung maturation.

Objective

We investigated the influence of prenatal hypoxia and/or postnatal hyperoxia on morphological lung maturation based on stereological parameters, to find out which model best reflects morphological changes in lung development comparable with alterations found in BPD.

Methods

Pregnant mice were exposed to normoxia, the offspring to normoxia (No/No) or to hyperoxia (No/Hyper). Furthermore, pregnant mice were exposed to hypoxia and the offspring to normoxia (Hypo/No) or to hyperoxia (Hypo/Hyper). Stereological investigations were performed on all pups at 14 days after birth.

Results

Compared to controls (No/No) 1) the lung volume was significantly reduced in the No/Hyper and Hypo/Hyper groups, 2) the volume weighted mean volume of the parenchymal airspaces was significantly higher in the Hypo/Hyper group, 3) the total air space volume was significantly lower in the No/Hyper and Hypo/Hyper groups, 4) the total septal surface showed significantly lower values in the No/Hyper and Hypo/Hyper groups, 5) the wall thickness of septa showed the highest values in the Hypo/Hyper group without reaching significance, 6) the volume density and the volume weighted mean volume of lamellar bodies in alveolar epithelial cells type II (AEII) were significantly lower in the Hypo/Hyper group.

Conclusion

Prenatal hypoxia and postnatal hyperoxia differentially influence the maturation of lung parenchyma. In 14 day old mice a significant retardation of morphological lung development leading to BPD-like alterations indicated by different parameters was only seen after hypoxia and hyperoxia.

Evolution, Development, and Function of the Pulmonary Surfactant System in Normal and Perturbed Environments

Surfactant lipids and proteins form a surface active film at the air-liquid interface of internal gas exchange organs, including swim bladders and lungs. The system is uniquely positioned to meet both the physical challenges associated with a dynamically changing internal air-liquid interface, and the environmental challenges associated with the foreign pathogens and particles to which the internal surface is exposed. Lungs range from simple, transparent, bag-like units to complex, multilobed, compartmentalized structures. Despite this anatomical variability, the surfactant system is remarkably conserved. Here, we discuss the evolutionary origin of the surfactant system, which likely predates lungs. We describe the evolution of surfactant structure and function in invertebrates and vertebrates. We focus on changes in lipid and protein composition and surfactant function from its antiadhesive and innate immune to its alveolar stability and structural integrity functions. We discuss the biochemical, hormonal, autonomic, and mechanical factors that regulate normal surfactant secretion in mature animals. We present an analysis of the ontogeny of surfactant development among the vertebrates and the contribution of different regulatory mechanisms that control this development. We also discuss environmental (oxygen), hormonal and biochemical (glucocorticoids and glucose) and pollutant (maternal smoking, alcohol, and common "recreational" drugs) effects that impact surfactant development. On the adult surfactant system, we focus on environmental variables including temperature, pressure, and hypoxia that have shaped its evolution and we discuss the resultant biochemical, biophysical, and cellular adaptations. Finally, we discuss the effect of major modern gaseous and particulate pollutants on the lung and surfactant system.

Increased lung prolyl hydroxylase and decreased glucocorticoid receptor are related to decreased surfactant protein in the growth-restricted sheep fetus

Experimental placental restriction (PR) by carunclectomy in fetal sheep results in intrauterine growth restriction (IUGR), chronic hypoxemia, increased plasma cortisol, and decreased lung surfactant protein (SP) expression. The mechanisms responsible for decreased SP expression are unknown but may involve decreased glucocorticoid (GC) action or changes in hypoxia signaling. Endometrial caruncles were removed from nonpregnant ewes to induce PR. Lungs were collected from control and PR fetuses at 130-135 (n = 19) and 139-145 (n = 28) days of gestation. qRT-PCR and Western blotting were used to quantify lung mRNA and protein expression, respectively, of molecular regulators and downstream targets of the GC and hypoxia-signaling pathways. We confirmed a decrease in SP-A, -B, and -C, but not SP-D, mRNA expression in PR fetuses at both ages. There was a net downregulation of GC signaling with a reduction in GC receptor (GR)-α and -β protein expression and a decrease in the cofactor, GATA-6. GC-responsive genes including transforming growth factor-β1, IL-1β, and β2-adrenergic receptor were not stimulated. Prolyl hydroxylase domain (PHD)2 mRNA and protein and PHD3 mRNA expression increased with a concomitant increase in hypoxia-inducible factor-1α (HIF-1α) and HIF-1β mRNA expression. There was an increase in mRNA expression of several, but not all, hypoxia-responsive genes. Hence, both GC and hypoxia signaling may contribute to reduced SP expression. Although acute hypoxia normally inactivates PHDs, chronic hypoxemia in the PR fetus increased PHD abundance, which normally prevents HIF signaling. This may represent a mechanism by which chronic hypoxemia contributes to the decrease in SP production in the IUGR fetal lung.

Antenatal corticosteroid therapy: current strategies and identifying mediators and markers for response

Landmark early work has led to the nearly universal use of antenatal corticosteroids to accelerate fetal lung maturity with pregnancies complicated by impending preterm birth. Antenatal corticosteroids clearly reduce respiratory morbidity, death, and other adverse neonatal outcomes. Limited pregnant human pharmacokinetic data and some animal data give clinicians some information as to the behavior of the drug in the body. However, there is controversy about the type, amount, and frequency of steroid to use for this therapy. This review article summarizes the history, clinical use, and pharmacology of antenatal steroids. In addition, the review highlights some potential mediators of steroid response and current research strategies aimed at possible optimization of this therapy.

Intravenous insulin therapy during lung resection does not affect lung function or surfactant proteins

Background

The surgical resection of lung disrupts glucose homeostasis and causes hyperglycemia, as in any other major surgery or critical illness. We performed a prospective study where we carefully lowered hyperglycemia by insulin administration during the surgery, and for the first time we monitored immediate insulin effects on lung physiology and gene transcription.

Methods

The levels of blood gases (pH, pCO₂, pO₂, HCO_3-, HCO_3- std, base excess, FiO₂, and pO₂/FiO₂) were measured at the beginning of surgery, at the end of surgery, and two hours after. Samples of healthy lung tissue surrounding the tumour were obtained during the surgery, anonymized and sent for subsequent blinded qPCR analysis (mRNA levels of surfactant proteins A1, A2, B, C and D were measured). This study was done on a cohort of 64 patients who underwent lung resection. Patients were randomly divided, and half of them received insulin treatment during the surgery.

Results

We demonstrated for the first time that insulin administered intravenously during lung resection does not affect levels of blood gases. Furthermore, it does not induce immediate changes in the expression of surfactant proteins.

Conclusion

According to our observations, short insulin treatment applied intravenously during resection does not affect the quality of breathing.

Prenatal hypoxia downregulates the expression of pulmonary vascular endothelial growth factor and its receptors in fetal mice

BACKGROUND

Previous reports showed that prenatal hypoxia delays the process of lung maturation. Vascular endothelial growth factor (VEGF) and its receptors were important for lung development. However, the role of VEGF and VEGF receptors in altered fetal lung development and maturation induced by prenatal hypoxia remains unknown.

OBJECTIVES

To elucidate the role of VEGF and VEGF receptors in altered fetal lung development and maturation induced by prenatal hypoxia.

METHODS

Lung sections of control and maternal hypoxic fetal mice were used for the determination of lung development and total RNA isolated from lung homogenates were used for determination of the expression patterns of VEGF, Flt-1, Flk-1, hypoxia-inducible factor (HIF)-1α, HIF-2α, surfactant protein (SP)-A, SP-B, SP-C, and SP-D by quantitative real-time RT-PCR.

RESULTS

Prenatal hypoxia resulted in fetal mice body weight gain impairment, delayed fetal pulmonary aeration and maturation. Pulmonary SP-A, SP-B, SP-C, and SP-D mRNA were all decreased in the prenatal hypoxia group. In addition, we demonstrated that prenatal hypoxia inhibited the developmental increase of pulmonary HIF-1α and HIF-2α expression and resulted in decreasing VEGF and its receptors (Flt-1 and Flk-1) at the mRNA expression level and VEGF protein level in fetal lungs. These inhibitory effects persisted and progressed even when the dams were returned to air.

CONCLUSIONS

We suggest that prenatal hypoxia insults, at least in late gestation, influence pulmonary VEGF and VEGF receptor expression through the down-regulation of HIF pathways and impair fetal lung growth and maturation.

Antenatal steroids and the IUGR fetus: are exposure and physiological effects on the lung and cardiovascular system the same as in normally grown fetuses?

Glucocorticoids are administered to pregnant women at risk of preterm labour to promote fetal lung surfactant maturation. Intrauterine growth restriction (IUGR) is associated with an increased risk of preterm labour. Hence, IUGR babies may be exposed to antenatal glucocorticoids. The ability of the placenta or blood brain barrier to remove glucocorticoids from the fetal compartment or the brain is compromised in the IUGR fetus, which may have implications for lung, brain, and heart development. There is conflicting evidence on the effect of exogenous glucocorticoids on surfactant protein expression in different animal models of IUGR. Furthermore, the IUGR fetus undergoes significant cardiovascular adaptations, including altered blood pressure regulation, which is in conflict with glucocorticoid-induced alterations in blood pressure and flow. Hence, antenatal glucocorticoid therapy in the IUGR fetus may compromise regulation of cardiovascular development. The role of cortisol in cardiomyocyte development is not clear with conflicting evidence in different species and models of IUGR. Further studies are required to study the effects of antenatal glucocorticoids on lung, brain, and heart development in the IUGR fetus. Of specific interest are the aetiology of IUGR and the resultant degree, duration, and severity of hypoxemia.