Relationship of structural to functional impairment during alveolar-capillary membrane development

Loss of Tbx4 Affects Postnatal Lung Development and Predisposes to Pulmonary Hypertension

Pulmonary arterial hypertension (PAH) is a progressive vascular disease characterized by remodeling of the precapillary pulmonary arteries. Genomic variation within the T-box 4 (TBX4) transcription factor is the second most common genetic cause of PAH, and can also cause severe lung developmental disorders with neonatal PH. Currently, the effect of TBX4 loss-of-function on later stages of lung development and predisposition to lung disease, including PH, is not well understood. Therefore, we have generated Tbx4 conditional knockout ( Tbx4-CKO ) mice in which Cre recombinase deletes exon 5 of Tbx4 within the embryonic lung mesenchyme to create a null allele. We harvested lungs from these mice at various timepoints to examine alveologenesis, vascularization, vascular remodeling, lung cellular composition, and disruption of transcriptional activity compared with control lungs. Right ventricular systolic pressure (RVSP) was measured in six-month-old mice to evaluate for PH. Tbx4-CKO lungs show enlargement of airspaces, as confirmed by an increase in mean linear intercept at P14 (24.9%), P36 (31.5%), and P180 (49.6%). These lungs also show a 39.3% decrease in von Willebrand Factor-positive vessels and a 14.2% increase in vessel wall thickness. Consistent with these results, Tbx4-CKO mice show a statistically significant increase of 15.7% in RVSP and 16.3% in the Fulton index. Bulk-RNA sequencing analysis revealed enrichment of pathways and genes relevant to lung alveologenesis, angiogenesis, and PH. Our results show that disruption of Tbx4 expression during early lung development is sufficient to disrupt postnatal lung development and circulation.

Effect of estrogen receptor α on cardiopulmonary adaptation to chronic developmental hypoxia in a rat model

Humans living at high-altitude (HA) have adapted to this environment by increasing pulmonary vascular and alveolar growth. RNA sequencing data from a novel murine model that mimics this phenotypical response to HA suggested estrogen signaling via estrogen receptor alpha (ERα) may be involved in this adaptation. We hypothesized ERα was a key mediator in the cardiopulmonary adaptation to chronic hypoxia and sought to delineate the mechanistic role ERα contributes to this process by exposing novel loss-of-function ERα mutant (ERαMut) rats to simulated HA. ERα mutant or wild-type (wt) rats were exposed to normoxia or hypoxia starting at conception and continued postnatally until 6 wk of age. Both wt and ERαMut animals born and raised in hypoxia exhibited lower body mass and higher hematocrits, total alveolar volumes (Va), diffusion capacities of carbon monoxide (DLCO), pulmonary arteriole (PA) wall thickness, and Fulton indices than normoxia animals. Right ventricle adaptation was maintained in the setting of hypoxia. Although no major physiologic differences were seen between wt and ERαMut animals at either exposure, ERαMut animals exhibited smaller mean linear intercepts (MLI) and increased PA total and lumen areas. Hypoxia exposure or ERα loss-of-function did not affect lung mRNA abundance of vascular endothelial growth factor, angiopoietin 2, or apelin. Sexual dimorphisms were noted in PA wall thickness and PA lumen area in ERαMut rats. In summary, in room air-exposed rats and rats with peri- and postnatal hypoxia exposure, ERα loss-of-function was associated with decreased alveolar size (primarily driven by hypoxic animals) and increased PA remodeling.NEW & NOTEWORTHY By exposing novel loss-of-function estrogen receptor alpha (Erα) mutant rats to a novel model of human high-altitude exposure, we demonstrate that ERα has subtle but inconsistent effects on endpoints relevant to cardiopulmonary adaptation to chronic hypoxia. Given that we observed some histologic, sex, and genotype differences, further research into cell-specific effects of ERα during hypoxia-induced cardiopulmonary adaptation is warranted.

Identification of potential biomarkers in the peripheral blood of neonates with bronchopulmonary dysplasia using WGCNA and machine learning algorithms

Bronchopulmonary dysplasia (BPD) is often seen as a pulmonary complication of extreme preterm birth, resulting in persistent respiratory symptoms and diminished lung function. Unfortunately, current diagnostic and treatment options for this condition are insufficient. Hence, this study aimed to identify potential biomarkers in the peripheral blood of neonates affected by BPD. The Gene Expression Omnibus provided the expression dataset GSE32472 for BPD. Initially, using this database, we identified differentially expressed genes (DEGs) in GSE32472. Subsequently, we conducted gene set enrichment analysis on the DEGs and employed weighted gene co-expression network analysis (WGCNA) to screen the most relevant modules for BPD. We then mapped the DEGs to the WGCNA module genes, resulting in a gene intersection. We conducted detailed functional enrichment analyses on these overlapping genes. To identify hub genes, we used 3 machine learning algorithms, including SVM-RFE, LASSO, and Random Forest. We constructed a diagnostic nomogram model for predicting BPD based on the hub genes. Additionally, we carried out transcription factor analysis to predict the regulatory mechanisms and identify drugs associated with these biomarkers. We used differential analysis to obtain 470 DEGs and conducted WGCNA analysis to identify 1351 significant genes. The intersection of these 2 approaches yielded 273 common genes. Using machine learning algorithms, we identified CYYR1, GALNT14, and OLAH as potential biomarkers for BPD. Moreover, we predicted flunisolide, budesonide, and beclomethasone as potential anti-BPD drugs. The genes CYYR1, GALNT14, and OLAH have the potential to serve as diagnostic biomarkers for BPD. This may prove beneficial in clinical diagnosis and prevention of BPD.

Premature infants born <28 weeks with acute kidney injury have increased bronchopulmonary dysplasia rates

BACKGROUND

Despite a growing understanding of bronchopulmonary dysplasia (BPD) and advances in management, BPD rates remain stable. There is mounting evidence that BPD may be due to a systemic insult, such as acute kidney injury (AKI). Our hypothesis was that severe AKI would be associated with BPD.

METHODS

We conducted a secondary analysis of premature infants [24-27 weeks gestation] in the Recombinant Erythropoietin for Protection of Infant Renal Disease cohort (N = 885). We evaluated the composite outcome of Grade 2/3 BPD or death using generalized estimating equations. In an exploratory analysis, urinary biomarkers of angiogenesis (ANG1, ANG2, EPO, PIGF, TIE2, FGF, and VEGFA/D) were analyzed.

RESULTS

594 (67.1%) of infants had the primary composite outcome of Grade 2/3 BPD or death. Infants with AKI (aOR: 1.69, 95% CI: 1.16-2.46) and severe AKI (aOR: 2.05, 95% CI: 1.19-3.54). had increased risk of the composite outcome after multivariable adjustment Among 106 infants with urinary biomarkers assessed, three biomarkers (VEGFA, VEGFD, and TIE2) had AUC > 0.60 to predict BPD.

CONCLUSIONS

Infants with AKI had a higher likelihood of developing BPD/death, with the strongest relationship seen in those with more severe AKI. Three urinary biomarkers of angiogenesis may have potential to predict BPD development.

IMPACT

AKI is associated with lung disease in extremely premature infants, and urinary biomarkers may predict this relationship. Infants with AKI and severe AKI have higher odds of BPD or death. Three urinary angiogenesis biomarkers are altered in infants that develop BPD. These findings have the potential to drive future work to better understand the mechanistic pathways of BPD, setting the framework for future interventions to decrease BPD rates. A better understanding of the mechanisms of BPD development and the role of AKI would have clinical care, cost, and quality of life implications given the long-term effects of BPD.

Maladaptive functional changes in alveolar fibroblasts due to perinatal hyperoxia impair epithelial differentiation

Infants born prematurely worldwide have up to a 50% chance of developing bronchopulmonary dysplasia (BPD), a clinical morbidity characterized by dysregulated lung alveolarization and microvascular development. It is known that PDGFR alpha-positive (PDGFRA+) fibroblasts are critical for alveolarization and that PDGFRA+ fibroblasts are reduced in BPD. A better understanding of fibroblast heterogeneity and functional activation status during pathogenesis is required to develop mesenchymal population-targeted therapies for BPD. In this study, we utilized a neonatal hyperoxia mouse model (90% O2 postnatal days 0-7, PN0-PN7) and performed studies on sorted PDGFRA+ cells during injury and room air recovery. After hyperoxia injury, PDGFRA+ matrix and myofibroblasts decreased and PDGFRA+ lipofibroblasts increased by transcriptional signature and population size. PDGFRA+ matrix and myofibroblasts recovered during repair (PN10). After 7 days of in vivo hyperoxia, PDGFRA+ sorted fibroblasts had reduced contractility in vitro, reflecting loss of myofibroblast commitment. Organoids made with PN7 PDGFRA+ fibroblasts from hyperoxia in mice exhibited reduced alveolar type 1 cell differentiation, suggesting reduced alveolar niche-supporting PDGFRA+ matrix fibroblast function. Pathway analysis predicted reduced WNT signaling in hyperoxia fibroblasts. In alveolar organoids from hyperoxia-exposed fibroblasts, WNT activation by CHIR increased the size and number of alveolar organoids and enhanced alveolar type 2 cell differentiation.

Capillary Changes Precede Disordered Alveolarization in a Mouse Model of Bronchopulmonary Dysplasia

Bronchopulmonary dysplasia (BPD), the most common sequela of preterm birth, is a severe disorder of the lung that is often associated with long-lasting morbidity. A hallmark of BPD is the disruption of alveolarization, whose pathogenesis is incompletely understood. Here, we tested the vascular hypothesis that disordered vascular development precedes the decreased alveolarization associated with BPD. Neonatal mouse pups were exposed to 7, 14, or 21 days of normoxia (21% O₂) or hyperoxia (85% O₂) with n = 8-11 for each group. The right lungs were fixed by vascular perfusion and investigated by design-based stereology or three-dimensional reconstruction of data sets obtained by serial block-face scanning EM. The alveolar capillary network of hyperoxia-exposed mice was characterized by rarefaction, partially altered geometry, and widening of capillary segments as shown by three-dimensional reconstruction. Stereology revealed that the development of alveolar epithelium and capillary endothelium was decreased in hyperoxia-exposed mice; however, the time course of these effects was different. That the surface area of the alveolar epithelium was smaller in hyperoxia-exposed mice first became evident at Day 14. In contrast, the surface area of the endothelium was reduced in hyperoxia-exposed mouse pups at Day 7. The thickness of the air-blood barrier decreased during postnatal development in normoxic mice, whereas it increased in hyperoxic mice. The endothelium and the septal connective tissue made appreciable contributions to the thickened septa. In conclusion, the present study provides clear support for the idea that the stunted alveolarization follows the disordered microvascular development, thus supporting the vascular hypothesis of BPD.

Transcriptomic modifications in developmental cardiopulmonary adaptations to chronic hypoxia using a murine model of simulated high-altitude exposure

Mechanisms driving adaptive developmental responses to chronic high-altitude (HA) exposure are incompletely known. We developed a novel rat model mimicking the human condition of cardiopulmonary adaptation to HA starting at conception and spanning the in utero and postnatal timeframe. We assessed lung growth and cardiopulmonary structure and function and performed transcriptome analyses to identify mechanisms facilitating developmental adaptations to chronic hypoxia. To generate the model, breeding pairs of Sprague-Dawley rats were exposed to hypobaric hypoxia (equivalent to 9,000 ft elevation). Mating, pregnancy, and delivery occurred in hypoxic conditions. Six weeks postpartum, structural and functional data were collected in the offspring. RNA-Seq was performed on right ventricle (RV) and lung tissue. Age-matched breeding pairs and offspring under room air (RA) conditions served as controls. Hypoxic rats exhibited significantly lower body weights and higher hematocrit levels, alveolar volumes, pulmonary diffusion capacities, RV mass, and RV systolic pressure, as well as increased pulmonary artery remodeling. RNA-Seq analyses revealed multiple differentially expressed genes in lungs and RVs from hypoxic rats. Although there was considerable similarity between hypoxic lungs and RVs compared with RA controls, several upstream regulators unique to lung or RV were identified. We noted a pattern of immune downregulation and regulation patterns of immune and hormonal mediators similar to the genome from patients with pulmonary arterial hypertension. In summary, we developed a novel murine model of chronic hypoxia exposure that demonstrates functional and structural phenotypes similar to human adaptation. We identified transcriptomic alterations that suggest potential mechanisms for adaptation to chronic HA.

Lung development and emerging roles for type 2 immunity

Lung development is a complex process mediated through the interaction of multiple cell types, factors and mediators. In mice, it starts as early as embryonic day 9 and continues into early adulthood. The process can be separated into five different developmental stages: embryonic, pseudoglandular, canalicular, saccular, and alveolar. Whilst lung bud formation and branching morphogenesis have been studied extensively, the mechanisms of alveolarisation are incompletely understood. Aberrant lung development can lead to deleterious consequences for respiratory health such as bronchopulmonary dysplasia (BPD), a disease primarily affecting preterm neonates, which is characterised by increased pulmonary inflammation and disturbed alveolarisation. While the deleterious effects of type 1-mediated inflammatory responses on lung development have been well established, the role of type 2 responses in postnatal lung development remains poorly understood. Recent studies indicate that type 2-associated immune cells, such as group 2 innate lymphoid cells and alveolar macrophages, are increased in number during postnatal alveolarisation. Here, we present the current state of understanding of the postnatal stages of lung development and the key cell types and mediators known to be involved. We also provide an overview of how stem cells are involved in lung development and regeneration, and the negative influences of respiratory infections. Copyright © 2018 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Neonatal hyperoxia promotes asthma-like features through IL-33-dependent ILC2 responses

BACKGROUND

Premature infants often require oxygen supplementation and, therefore, are exposed to oxidative stress. Following oxygen exposure, preterm infants frequently develop chronic lung disease and have a significantly increased risk of asthma.

OBJECTIVE

We sought to identify the underlying mechanisms by which neonatal hyperoxia promotes asthma development.

METHODS

Mice were exposed to neonatal hyperoxia followed by a period of room air recovery. A group of mice was also intranasally exposed to house dust mite antigen. Assessments were performed at various time points for evaluation of airway hyperresponsiveness, eosinophilia, mucus production, inflammatory gene expression, and TH and group 2 innate lymphoid cell (ILC2) responses. Sera from term- and preterm-born infants were also collected and levels of IL-33 and type 2 cytokines were measured.

RESULTS

Neonatal hyperoxia induced asthma-like features including airway hyperresponsiveness, mucus hyperplasia, airway eosinophilia, and type 2 pulmonary inflammation. In addition, neonatal hyperoxia promoted allergic TH responses to house dust mite exposure. Elevated IL-33 levels and ILC2 responses were observed in the lungs most likely due to oxidative stress caused by neonatal hyperoxia. IL-33 receptor signaling and ILC2s were vital for the induction of asthma-like features following neonatal hyperoxia. Serum IL-33 levels correlated significantly with serum levels of IL-5 and IL-13 but not IL-4 in preterm infants.

CONCLUSIONS

These data demonstrate that an axis involving IL-33 and ILC2s is important for the development of asthma-like features following neonatal hyperoxia and suggest therapeutic potential for targeting IL-33, ILC2s, and oxidative stress to prevent and/or treat asthma development related to prematurity.

Bronchopulmonary dysplasia: A review of pathogenesis and pathophysiology

Bronchopulmonary dysplasia (BPD) is a chronic lung disease of primarily premature infants that results from an imbalance between lung injury and repair in the developing lung. BPD is the most common respiratory morbidity in preterm infants, which affects nearly 10, 000 neonates each year in the United States. Over the last two decades, the incidence of BPD has largely been unchanged; however, the pathophysiology has changed with the substantial improvement in the respiratory management of extremely low birth weight (ELBW) infants. Here we have attempted to comprehensively review and summarize the current literature on the pathogenesis and pathophysiology of BPD. Our goal is to provide insight to help further progress in preventing and managing severe BPD in the ELBW infants.

Recent advances in our understanding of the mechanisms of late lung development and bronchopulmonary dysplasia

The objective of lung development is to generate an organ of gas exchange that provides both a thin gas diffusion barrier and a large gas diffusion surface area, which concomitantly generates a steep gas diffusion concentration gradient. As such, the lung is perfectly structured to undertake the function of gas exchange: a large number of small alveoli provide extensive surface area within the limited volume of the lung, and a delicate alveolo-capillary barrier brings circulating blood into close proximity to the inspired air. Efficient movement of inspired air and circulating blood through the conducting airways and conducting vessels, respectively, generates steep oxygen and carbon dioxide concentration gradients across the alveolo-capillary barrier, providing ideal conditions for effective diffusion of both gases during breathing. The development of the gas exchange apparatus of the lung occurs during the second phase of lung development-namely, late lung development-which includes the canalicular, saccular, and alveolar stages of lung development. It is during these stages of lung development that preterm-born infants are delivered, when the lung is not yet competent for effective gas exchange. These infants may develop bronchopulmonary dysplasia (BPD), a syndrome complicated by disturbances to the development of the alveoli and the pulmonary vasculature. It is the objective of this review to update the reader about recent developments that further our understanding of the mechanisms of lung alveolarization and vascularization and the pathogenesis of BPD and other neonatal lung diseases that feature lung hypoplasia.

Cumulative effects of neonatal hyperoxia on murine alveolar structure and function

BACKGROUND

Bronchopulmonary dysplasia (BPD) results from alveolar simplification and abnormal development of alveolar and capillary structure. Survivors of BPD display persistent deficits in airflow and membrane and vascular components of alveolar gas diffusion. Despite being the defining feature of BPD, various neonatal hyperoxia models of BPD have not routinely assessed pulmonary gas diffusion.

METHODS

To simulate the most commonly-utilized neonatal hyperoxia models, we exposed neonatal mice to room air or ≥90% hyperoxia during key stages of distal lung development: through the first 4 (saccular), 7 (early alveolar), or 14 (bulk alveolar) postnatal days, followed by a period of recovery in room air until 8 weeks of age when alveolar septation is essentially complete. We systematically assessed and correlated the effects of neonatal hyperoxia on the degree of alveolar-capillary structural and functional impairment. We hypothesized that the degree of alveolar-capillary simplification would correlate strongly with worsening diffusion impairment.

RESULTS

Neonatal hyperoxia exposure, of any duration, resulted in alveolar simplification and impaired pulmonary gas diffusion. Mean Linear Intercept increased in proportion to the length of hyperoxia exposure while alveolar and total lung volume increased markedly only with prolonged exposure. Surprisingly, despite having a similar effect on alveolar surface area, only prolonged hyperoxia for 14 days resulted in reduced pulmonary microvascular volume. Estimates of alveolar and capillary structure, in general, correlated poorly with assessment of gas diffusion.

CONCLUSION

Our results help define the physiological and structural consequences of commonly-employed neonatal hyperoxia models of BPD and inform their clinical utility. Pediatr Pulmonol. 2017;52:616-624. © 2016 Wiley Periodicals, Inc.

Looking ahead: where to next for animal models of bronchopulmonary dysplasia?

Bronchopulmonary dysplasia (BPD) is the most common complication of preterm birth, with appreciable morbidity and mortality in a neonatal intensive care setting. Much interest has been shown in the identification of pathogenic pathways that are amenable to pharmacological manipulation (1) to facilitate the development of novel therapeutic and medical management strategies and (2) to identify the basic mechanisms of late lung development, which remains poorly understood. A number of animal models have therefore been developed and continue to be refined with the aim of recapitulating pathological pulmonary hallmarks noted in lungs from neonates with BPD. These animal models rely on several injurious stimuli, such as mechanical ventilation or oxygen toxicity and infection and sterile inflammation, as applied in mice, rats, rabbits, pigs, lambs and nonhuman primates. This review addresses recent developments in modeling BPD in experimental animals and highlights important neglected areas that demand attention. Additionally, recent progress in the quantitative microscopic analysis of pathology tissue is described, together with new in vitro approaches of value for the study of normal and aberrant alveolarization. The need to examine long-term sequelae of damage to the developing neonatal lung is also considered, as is the need to move beyond the study of the lungs alone in experimental animal models of BPD.

Membrane and Capillary Components of Lung Diffusion in Infants with Bronchopulmonary Dysplasia

RATIONALE

Autopsied lungs of infants with bronchopulmonary dysplasia (BPD) demonstrate impaired alveolar development with larger and fewer alveoli, which is consistent with our previous physiologic findings of lower pulmonary diffusing capacity of the lung for carbon monoxide (DL(CO)) in infants and toddlers with BPD compared with healthy controls born at full term (FT). However, it is not known whether the decreased DL(CO) in infants with BPD results from a reduction in both components of DL(CO): pulmonary membrane diffusing capacity (D(M)) and Vc.

OBJECTIVES

We hypothesized that impairment of alveolar development in BPD results in a decrease in both D(M) and Vc components of DlCO but that the D(M)/Vc ratio would not differ between the BPD and FT groups.

METHODS

DL(CO) was measured under conditions of room air and high inspired oxygen (90%), which enabled D(M) and Vc to be calculated.

MEASUREMENTS AND MAIN RESULTS

D(M) and Vc increased with increasing body length; however, infants with BPD had significantly lower D(M) and Vc than FT subjects after adjustment for race, sex, body length, and corrected age. In contrast to D(M) and Vc, the D(M)/Vc ratio remained constant with increasing body length and did not differ for infants with BPD and FT subjects.

CONCLUSIONS

Our findings are consistent with infants with BPD having impaired alveolar development with fewer but larger alveoli, as well as a reduced Vc.

Initial Suppression of Transforming Growth Factor-β Signaling and Loss of TGFBI Causes Early Alveolar Structural Defects Resulting in Bronchopulmonary Dysplasia

Septation of the gas-exchange saccules of the morphologically immature mouse lung requires regulated timing, spatial direction, and dosage of transforming growth factor (TGF)-β signaling. We found that neonatal hyperoxia acutely initially diminished saccular TGF-β signaling coincident with alveolar simplification. However, sustained hyperoxia resulted in a biphasic response and subsequent up-regulation of TGF-β signaling, ultimately resulting in bronchopulmonary dysplasia. Significantly, we found that the TGF-β-induced matricellular protein (TGFBI) was similarly biphasically altered in response to hyperoxia. Moreover, genetic ablation revealed that TGFBI was required for normal alveolar structure and function. Although the phenotype was not neonatal lethal, Tgfbi-deficient lungs were morphologically abnormal. Mutant septal tips were stunted, lacked elastin-positive tips, exhibited reduced proliferation, and contained abnormally persistent alveolar α-smooth muscle actin myofibroblasts. In addition, Tgfbi-deficient lungs misexpressed TGF-β-responsive follistatin and serpine 1, and transiently suppressed myofibroblast platelet-derived growth factor α differentiation marker. Finally, despite normal lung volume, Tgfbi-null lungs displayed diminished elastic recoil and gas exchange efficiency. Combined, these data demonstrate that initial suppression of the TGF-β signaling apparatus, as well as loss of key TGF-β effectors (like TGFBI), underlies early alveolar structural defects, as well as long-lasting functional deficits routinely observed in chronic lung disease of infancy patients. These studies underline the complex (and often contradictory) role of TGF-β and indicate a need to design studies to associate alterations with initial appearance of phenotypical changes suggestive of bronchopulmonary dysplasia.

Recent advances in the mechanisms of lung alveolarization and the pathogenesis of bronchopulmonary dysplasia

Alveolarization is the process by which the alveoli, the principal gas exchange units of the lung, are formed. Along with the maturation of the pulmonary vasculature, alveolarization is the objective of late lung development. The terminal airspaces that were formed during early lung development are divided by the process of secondary septation, progressively generating an increasing number of alveoli that are of smaller size, which substantially increases the surface area over which gas exchange can take place. Disturbances to alveolarization occur in bronchopulmonary dysplasia (BPD), which can be complicated by perturbations to the pulmonary vasculature that are associated with the development of pulmonary hypertension. Disturbances to lung development may also occur in persistent pulmonary hypertension of the newborn in term newborn infants, as well as in patients with congenital diaphragmatic hernia. These disturbances can lead to the formation of lungs with fewer and larger alveoli and a dysmorphic pulmonary vasculature. Consequently, affected lungs exhibit a reduced capacity for gas exchange, with important implications for morbidity and mortality in the immediate postnatal period and respiratory health consequences that may persist into adulthood. It is the objective of this Perspectives article to update the reader about recent developments in our understanding of the molecular mechanisms of alveolarization and the pathogenesis of BPD.