Early exposure to hyperoxia or hypoxia adversely impacts cardiopulmonary development

Controlled oxygen delivery to power tissue regeneration

Oxygen plays a crucial role in human embryogenesis, homeostasis, and tissue regeneration. Emerging engineered regenerative solutions call for novel oxygen delivery systems. To become a reality, these systems must consider physiological processes, oxygen release mechanisms and the target application. In this review, we explore the biological relevance of oxygen at both a cellular and tissue level, and the importance of its controlled delivery via engineered biomaterials and devices. Recent advances and upcoming trends in the field are also discussed with a focus on tissue-engineered constructs that could meet metabolic demands to facilitate regeneration.

Cardioprotective Effects of Dexmedetomidine in an Oxidative-Stress In Vitro Model of Neonatal Rat Cardiomyocytes

Preterm birth is a risk factor for cardiometabolic disease. The preterm heart before terminal differentiation is in a phase that is crucial for the number and structure of cardiomyocytes in further development, with adverse effects of hypoxic and hyperoxic events. Pharmacological intervention could attenuate the negative effects of oxygen. Dexmedetomidine (DEX) is an α2-adrenoceptor agonist and has been mentioned in connection with cardio-protective benefits. In this study, H9c2 myocytes and primary fetal rat cardiomyocytes (NRCM) were cultured for 24 h under hypoxic condition (5% O₂), corresponding to fetal physioxia (pO₂ 32-45 mmHg), ambient oxygen (21% O₂, pO₂ ~150 mmHg), or hyperoxic conditions (80% O₂, pO₂ ~300 mmHg). Subsequently, the effects of DEX preconditioning (0.1 µM, 1 µM, 10 µM) were analyzed. Modulated oxygen tension reduced both proliferating cardiomyocytes and transcripts (CycD2). High-oxygen tension induced hypertrophy in H9c2 cells. Cell-death-associated transcripts for caspase-dependent apoptosis (Casp3/8) increased, whereas caspase-independent transcripts (AIF) increased in H9c2 cells and decreased in NRCMs. Autophagy-related mediators (Atg5/12) were induced in H9c2 under both oxygen conditions, whereas they were downregulated in NRCMs. DEX preconditioning protected H9c2 and NRCMs from oxidative stress through inhibition of transcription of the oxidative stress marker GCLC, and inhibited the transcription of both the redox-sensitive transcription factors Nrf2 under hyperoxia and Hif1α under hypoxia. In addition, DEX normalized the gene expression of Hippo-pathway mediators (YAP1, Tead1, Lats2, Cul7) that exhibited abnormalities due to differential oxygen tensions compared with normoxia, suggesting that DEX modulates the activation of the Hippo pathway. This, in the context of the protective impact of redox-sensitive factors, may provide a possible rationale for the cardio-protective effects of DEX in oxygen-modulated requirements on survival-promoting transcripts of immortalized and fetal cardiomyocytes.

Oxygen toxicity: cellular mechanisms in normobaric hyperoxia

In clinical settings, oxygen therapy is administered to preterm neonates and to adults with acute and chronic conditions such as COVID-19, pulmonary fibrosis, sepsis, cardiac arrest, carbon monoxide poisoning, and acute heart failure. In non-clinical settings, divers and astronauts may also receive supplemental oxygen. In addition, under current standard cell culture practices, cells are maintained in atmospheric oxygen, which is several times higher than what most cells experience in vivo. In all the above scenarios, the elevated oxygen levels (hyperoxia) can lead to increased production of reactive oxygen species from mitochondria, NADPH oxidases, and other sources. This can cause cell dysfunction or death. Acute hyperoxia injury impairs various cellular functions, manifesting ultimately as physiological deficits. Chronic hyperoxia, particularly in the neonate, can disrupt development, leading to permanent deficiencies. In this review, we discuss the cellular activities and pathways affected by hyperoxia, as well as strategies that have been developed to ameliorate injury.

Intratracheal administration of umbilical cord-derived mesenchymal stem cells attenuates hyperoxia-induced multi-organ injury via heme oxygenase-1 and JAK/STAT pathways

BACKGROUND

Bronchopulmonary dysplasia (BPD) is not merely a chronic lung disease, but a systemic condition with multiple organs implications predominantly associated with hyperoxia exposure. Despite advances in current management strategies, limited progress has been made in reducing the BPD-related systemic damage. Meanwhile, although the protective effects of human umbilical cord-derived mesenchymal stem cells (hUC-MSCs) or their exosomes on hyperoxia-induced lung injury have been explored by many researchers, the underlying mechanism has not been addressed in detail, and few studies have focused on the therapeutic effect on systemic multiple organ injury.

AIM

To investigate whether hUC-MSC intratracheal administration could attenuate hyperoxia-induced lung, heart, and kidney injuries and the underlying regulatory mechanisms.

METHODS

Neonatal rats were exposed to hyperoxia (80% O₂), treated with hUC-MSCs intratracheal (iT) or intraperitoneal (iP) on postnatal day 7, and harvested on postnatal day 21. The tissue sections of the lung, heart, and kidney were analyzed morphometrically. Protein contents of the bronchoalveolar lavage fluid (BALF), myeloperoxidase (MPO) expression, and malondialdehyde (MDA) levels were examined. Pulmonary inflammatory cytokines were measured via enzyme-linked immunosorbent assay. A comparative transcriptomic analysis of differentially expressed genes (DEGs) in lung tissue was conducted via RNA-sequencing. Subsequently, we performed reverse transcription-quantitative polymerase chain reaction and western blot analysis to explore the expression of target mRNA and proteins related to inflammatory and oxidative responses.

RESULTS

iT hUC-MSCs administration improved pulmonary alveolarization and angiogenesis (P < 0.01, P < 0.01, P < 0.001, and P < 0.05 for mean linear intercept, septal counts, vascular medial thickness index, and microvessel density respectively). Meanwhile, treatment with hUC-MSCs iT ameliorated right ventricular hypertrophy (for Fulton’s index, P < 0.01), and relieved reduced nephrogenic zone width (P < 0.01) and glomerular diameter (P < 0.001) in kidneys. Among the beneficial effects, a reduction of BALF protein, MPO, and MDA was observed in hUC-MSCs groups (P < 0.01, P < 0.001, and P < 0.05 respectively). Increased pro-inflammatory cytokines tumor necrosis factor-alpha, interleukin (IL)-1β, and IL-6 expression observed in the hyperoxia group were significantly attenuated by hUC-MSCs administration (P < 0.01, P < 0.001, and P < 0.05 respectively). In addition, we observed an increase in anti-inflammatory cytokine IL-10 expression in rats that received hUC-MSCs iT compared with rats reared in hyperoxia (P < 0.05). Transcriptomic analysis showed that the DEGs in lung tissues induced by hyperoxia were enriched in pathways related to inflammatory responses, epithelial cell proliferation, and vasculature development. hUC-MSCs administration blunted these hyperoxia-induced dysregulated genes and resulted in a shift in the gene expression pattern toward the normoxia group. hUC-MSCs increased heme oxygenase-1 (HO-1), JAK2, and STAT3 expression, and their phosphorylation in the lung, heart, and kidney (P < 0.05). Remarkably, no significant difference was observed between the iT and iP administration.

CONCLUSION

iT hUC-MSCs administration ameliorates hyperoxia-induced lung, heart, and kidney injuries by activating HO-1 expression and JAK/STAT signaling. The therapeutic benefits of local iT and iP administration are equivalent.

The CD146-HIF-1α axis regulates epithelial cell migration and alveolar maturation in a mouse model of bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is the most common challenge in preterm neonates. Retardation of alveolar development characterizes the pulmonary pathology in BPD. In the present study, we explored the roles of the CD146-HIF-1α axis in BPD. We demonstrated that the levels of reactive oxygen species (ROS) and soluble CD146 (sCD1146) were increased in the peripheral blood of preterm neonates with BPD. In alveolar epithelial cells, hyperoxia promoted the expression of HIF-1α and CD146, which reinforced each other. In a mouse model of BPD, by exposing pups to 65% hyperoxia, HIF-1α and CD146 were increased in the pulmonary tissues. Mechanistically, CD146 hindered the migration of alveolar epithelial cells; in contrast, movement was significantly enhanced in CD146-knockout alveolar epithelial cells. As expected, CD146-knockout ameliorated alveolarization and improved BPD disease severity. Taken together, our findings imply that the CD146-HIF-1α axis contributes to alveolarization and that CD146 may be a novel candidate in BPD therapy.

A Preconception Paternal Fish Oil Diet Prevents Toxicant-Driven New Bronchopulmonary Dysplasia in Neonatal Mice

New bronchopulmonary dysplasia is a developmental lung disease associated with placental dysfunction and impaired alveolarization. Risk factors for new BPD include prematurity, delayed postnatal growth, the dysregulation of epithelial-to-mesenchymal transition (EMT), and parental exposure to toxicants. Our group previously reported that a history of paternal toxicant exposure increased the risk of prematurity and low birth weight in offspring. A history of paternal toxicant exposure also increased the offspring's risk of new BPD and disease severity was increased in offspring who additionally received a supplemental formula diet, which has also been linked to poor lung development. Risk factors associated with new BPD are well-defined, but it is unclear whether the disease can be prevented. Herein, we assessed whether a paternal fish oil diet could attenuate the development of new BPD in the offspring of toxicant exposed mice, with and without neonatal formula feeding. We investigated the impact of a paternal fish oil diet preconception because we previously reported that this intervention reduces the risk of TCDD associated placental dysfunction, prematurity, and low birth weight. We found that a paternal fish oil diet significantly reduced the risk of new BPD in neonatal mice with a history of paternal toxicant exposure regardless of neonatal diet. Furthermore, our evidence suggests that the protective effects of a paternal fish oil diet are mediated in part by the modulation of small molecules involved in EMT.

Bronchopulmonary Dysplasia and Pulmonary Hypertension. The Role of Smooth Muscle adh5

Bronchopulmonary dysplasia (BPD) is characterized by alveolar simplification, airway hyperreactivity, and pulmonary hypertension. In our BPD model, we have investigated the metabolism of the bronchodilator and pulmonary vasodilator GSNO (S-nitrosoglutathione). We have shown the GSNO catabolic enzyme encoded by adh5 (alcohol dehydrogenase-5), GSNO reductase, is epigenetically upregulated in hyperoxia. Here, we investigated the distribution of GSNO reductase expression in human BPD and created an animal model that recapitulates the human data. Blinded comparisons of GSNO reductase protein expression were performed in human lung tissues from infants and children with and without BPD. BPD phenotypes were evaluated in global (adh5^-/-) and conditional smooth muscle (smooth muscle/adh5^-/-) adh5 knockout mice. GSNO reductase was prominently expressed in the airways and vessels of human BPD subjects. Compared with controls, expression was greater in BPD smooth muscle, particularly in vascular smooth muscle (2.4-fold; P = 0.003). The BPD mouse model of neonatal hyperoxia caused significant alveolar simplification, airway hyperreactivity, and right ventricular and vessel hypertrophy. Global adh5^-/- mice were protected from all three aspects of BPD, whereas smooth muscle/adh5^-/- mice were only protected from pulmonary hypertensive changes. These data suggest adh5 is required for the development of BPD. Expression in the pulmonary vasculature is relevant to the pathophysiology of BPD-associated pulmonary hypertension. GSNO-mimetic agents or GSNO reductase inhibitors, both of which are currently in clinical trials for other conditions, could be considered for further study in BPD.

Electrical remodeling and cardiotoxicity precedes structural and functional remodeling of mouse hearts under hyperoxia treatment

Clinical reports suggest a high incidence of ICU mortality with the use of hyperoxia during mechanical ventilation in patients. Our laboratory is pioneer in studying effect of hyperoxia on cardiac pathophysiology. In this study for the first time, we are reporting the sequence of cardiac pathophysiological events in mice under hyperoxic conditions in time-dependent manner. C57BL/6J male mice, aged 8-10 weeks, were treated with either normal air or >90% oxygen for 24, 48, and 72 h. Following normal air or hyperoxia treatment, physical, biochemical, functional, electrical, and molecular parameters were analyzed. Our data showed that significant reduction of body weight observed as early as 24 h hyperoxia treatment, whereas, no significant changes in heart weight until 72 h. Although we do not see any fibrosis in these hearts, but observed significant increase in cardiomyocyte size with hyperoxia treatment in time-dependent manner. Our data also demonstrated that arrhythmias were present in mice at 24 h hyperoxia, and worsened comparatively after 48 and 72 h. Echocardiogram data confirmed cardiac dysfunction in time-dependent manner. Dysregulation of ion channels such as Kv4.2 and KChIP2; and serum cardiac markers confirmed that hyperoxia-induced effects worsen with each time point. From these observations, it is evident that electrical remodeling precedes structural remodeling, both of which gets worse with length of hyperoxia exposure, therefore shorter periods of hyperoxia exposure is always beneficial for better outcome in ICU/critical care units.

Hypoxemia in infants with trisomy 21 in the neonatal intensive care unit

OBJECTIVE

Newborns with trisomy 21 (T21) often require NICU hospitalization. Oxygen desaturations are frequently observed in these infants, even in the absence of congenital heart defects (CHD). We hypothesized that NICU patients with T21 have more hypoxemia than those without T21.

DESIGN

All infants with T21 without significant CHD discharged home from the NICU between 2009 and 2018 were included (n = 23). Controls were matched 20:1 for gestational age and length of stay. We compared daily severe hypoxemia events (SpO₂ < 80% for ≥10 s) for the whole NICU stay and the pre-discharge week.

RESULTS

Infants with T21 showed significantly more daily hypoxemia events during their entire NICU stay (median 10 versus 7, p = 0.0064), and more so in their final week (13 versus 7, p = 0.0008).

CONCLUSION

NICU patients with T21 without CHD experience more severe hypoxemia events than controls, particularly in the week before discharge. Whether this hypoxemia predicts or contributes to adverse outcomes is unknown.

Neonatal hyperoxia inhibits proliferation and survival of atrial cardiomyocytes by suppressing fatty acid synthesis

Preterm birth increases the risk for pulmonary hypertension and heart failure in adulthood. Oxygen therapy can damage the immature cardiopulmonary system and may be partially responsible for the cardiovascular disease in adults born preterm. We previously showed that exposing newborn mice to hyperoxia causes pulmonary hypertension by 1 year of age that is preceded by a poorly understood loss of pulmonary vein cardiomyocyte proliferation. We now show that hyperoxia also reduces cardiomyocyte proliferation and survival in the left atrium and causes diastolic heart failure by disrupting its filling of the left ventricle. Transcriptomic profiling showed that neonatal hyperoxia permanently suppressed fatty acid synthase (Fasn), stearoyl-CoA desaturase 1 (Scd1), and other fatty acid synthesis genes in the atria of mice, the HL-1 line of mouse atrial cardiomyocytes, and left atrial tissue explanted from human infants. Suppressing Fasn or Scd1 reduced HL-1 cell proliferation and increased cell death, while overexpressing these genes maintained their expansion in hyperoxia, suggesting that oxygen directly inhibits atrial cardiomyocyte proliferation and survival by repressing Fasn and Scd1. Pharmacologic interventions that restore Fasn, Scd1, and other fatty acid synthesis genes in atrial cardiomyocytes may, thus, provide a way of ameliorating the adverse effects of supplemental oxygen on preterm infants.

Characterization of the innate immune response in a novel murine model mimicking bronchopulmonary dysplasia

BACKGROUND

Bronchopulmonary dysplasia (BPD), the most common complication of prematurity, arises from various factors that compromise lung development, including oxygen and inflammation. Hyperoxia has been used to mimic the disease in newborn rodents. The use of a second hit to induce systemic inflammation has been suggested as an added strategy to better mimic the inflammatory aspect of BPD. Here we report a novel 2 hit (2HIT) BPD model with in-depth characterization of the innate immune response, enabling mechanistic studies of therapies with an immunomodulatory component.

METHODS

C57BL/6N mice were exposed to 85% O₂ from postnatal day (P)1 to P7, and received postnatally (P3) Escherichia coli LPS. At various timepoints, immune activation in the lung and at the systemic level was analyzed by fluorescence-activated cell sorting (FACS), and gene and protein expressions.

RESULTS

2HIT mice showed fewer alveoli, increased lung compliance, and right ventricular hypertrophy. A transient proinflammatory cytokine response was observed locally and systemically. Type 2 anti-inflammatory cytokine expression was decreased in the lung together with the number of mature alveolar macrophages. Simultaneously, a Siglec-F intermediate macrophage population emerged.

CONCLUSION

This study provides long-term analysis of the 2HIT model, suggesting impairment of type 2 cytokine environment and altered alveolar macrophage profile in the lung.

IMPACT

We have developed a novel 2HIT mouse BPD model with postnatal LPS and hyperoxia exposure, which enables mechanistic studies of potential therapeutic strategies with an immunomodulatory component. This is the first report of in-depth characterization of the lung injury and recovery describing the evolution of the innate immune response in a standardized mouse model for experimental BPD with postnatal LPS and hyperoxia exposure. The 2HIT model has the potential to help understand the link between inflammation and impaired lung development, and will enable testing of new therapies in a short and more robust manner.

Endothelial to mesenchymal transition during neonatal hyperoxia-induced pulmonary hypertension

Bronchopulmonary dysplasia (BPD), a chronic lung disease in premature infants, results from mechanical ventilation and hyperoxia, amongst other factors. Although most BPD survivors can be weaned from supplemental oxygen, many show evidence of cardiovascular sequelae in adulthood, including pulmonary hypertension and pulmonary vascular remodeling. Endothelial-mesenchymal transition (EndoMT) plays an important role in mediating vascular remodeling in idiopathic pulmonary arterial hypertension. Whether hyperoxic exposure, a known mediator of BPD in rodent models, causes EndoMT resulting in vascular remodeling and pulmonary hypertension remains unclear. We hypothesized that neonatal hyperoxic exposure causes EndoMT, leading to the development of pulmonary hypertension in adulthood. To test this hypothesis, newborn mice were exposed to hyperoxia and then allowed to recover in room air until adulthood. Neonatal hyperoxic exposure gradually caused pulmonary vascular and right ventricle remodeling as well as pulmonary hypertension. Male mice were more susceptible to developing pulmonary hypertension compared to female mice, when exposed to hyperoxia as newborns. Hyperoxic exposure induced EndoMT in mouse lungs as well as in cultured lung microvascular endothelial cells (LMVECs) isolated from neonatal mice and human fetal donors. This was augmented in cultured LMVECs from male donors compared to those from female donors. Using primary mouse LMVECs, hyperoxic exposure increased phosphorylation of both Smad2 and Smad3, but reduced Smad7 protein levels. Treatment with a selective TGF-β inhibitor SB431542 blocked hyperoxia-induced EndoMT in vitro. Altogether, we show that neonatal hyperoxic exposure caused vascular remodeling and pulmonary hypertension in adulthood. This was associated with increased EndoMT. These novel observations provide mechanisms underlying hyperoxia-induced vascular remodeling and potential approaches to prevent BPD-associated pulmonary hypertension by targeting EndoMT. © 2020 The Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Hyperoxia Injury in the Developing Lung Is Mediated by Mesenchymal Expression of Wnt5A

Rationale: Bronchopulmonary dysplasia (BPD) is a leading complication of preterm birth that affects infants born in the saccular stage of lung development at <32 weeks of gestation. Although the mechanisms driving BPD remain uncertain, exposure to hyperoxia is thought to contribute to disease pathogenesis.Objectives: To determine the effects of hyperoxia on epithelial-mesenchymal interactions and to define the mediators of activated Wnt/β-catenin signaling after hyperoxia injury.Methods: Three hyperoxia models were used: A three-dimensional organotypic coculture using primary human lung cells, precision-cut lung slices (PCLS), and a murine in vivo hyperoxia model. Comparisons of normoxia- and hyperoxia-exposed samples were made by real-time quantitative PCR, RNA in situ hybridization, quantitative confocal microscopy, and lung morphometry.Measurements and Main Results: Examination of an array of Wnt ligands in the three-dimensional organotypic coculture revealed increased mesenchymal expression of WNT5A. Inhibition of Wnt5A abrogated the BPD transcriptomic phenotype induced by hyperoxia. In the PCLS model, Wnt5A inhibition improved alveolarization following hyperoxia exposure, and treatment with recombinant Wnt5a reproduced features of the BPD phenotype in PCLS cultured in normoxic conditions. Chemical inhibition of NF-κB with BAY11-7082 reduced Wnt5a expression in the PCLS hyperoxia model and in vivo mouse hyperoxia model, with improved alveolarization in the PCLS model.Conclusions: Increased mesenchymal Wnt5A during saccular-stage hyperoxia injury contributes to the impaired alveolarization and septal thickening observed in BPD. Precise targeting of Wnt5A may represent a potential therapeutic strategy for the treatment 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.

Club Cell Heme Oxygenase-1 Deletion: Effects in Hyperoxia-Exposed Adult Mice

Thioredoxin reductase-1 (TXNRD1) inhibition activates nuclear factor (erythroid-derived 2)-like 2 (Nrf2) responses and prevents acute lung injury (ALI). Heme oxygenase-1 (HO-1) induction following TXNRD1 inhibition is Nrf2-dependent in airway epithelial (club) cells in vitro. The influence of club cell HO-1 on lung development and lung injury responses is poorly understood. The present studies characterized the effects of hyperoxia on club cell-specific HO-1 knockout (KO) mice. These mice were generated by crossing Hmox1 flox mice with transgenic mice expressing cre recombinase under control of the club cell-specific Scgb1a1 promoter. Baseline analyses of lung architecture and function performed in age-matched adult wild-type and KO mice indicated an increased alveolar size and airway resistance in HO-1 KO mice. In subsequent experiments, adult wild-type and HO-1 KO mice were either continuously exposed to >95% hyperoxia or room air for 72 h or exposed to >95 hyperoxia for 48 h followed by recovery in room air for 48 h. Injury was quantitatively assessed by calculating right lung/body weight ratios (g/kg). Analyses indicated an independent effect of hyperoxia but not genotype on right lung/body weight ratios in both wild-type and HO-1 KO mice. The magnitude of increases in right lung/body weight ratios was similar in mice of both genotypes. In the recovery model, an independent effect of hyperoxia but not genotype was also detected. In contrast to the continuous exposure model, right lung/body weight ratio mice were significantly elevated in HO-1 KO but not wild-type mice. Though club cell HO-1 does not alter hyperoxic sensitivity in adult mice, it significantly influences lung development and resolution of lung injury following acute hyperoxic exposure.

Oxygen injury in neonates: which is worse? hyperoxia, hypoxia, or alternating hyperoxia/hypoxia

Premature birth results in an increased risk of respiratory distress and often requires oxygen therapy. While the supplemental oxygen has been implicated as a cause of bronchopulmonary dysplasia (BPD), in clinical practice this supplementation usually only occurs after the patient's oxygen saturation levels have dropped. The effect of hyperoxia on neonates has been extensively studied. However, there is an unanswered fundamental question: which has the most impact-hyperoxia, hypoxia or fluctuating oxygen levels? In this review, we will summarize the reported effect of hypoxia, hyperoxia or a fluctuation of oxygen levels (hypoxia/hyperoxia cycling) in preterm neonates, with special emphasis on the lungs.

Severe but not moderate hyperoxia of newborn mice causes an emphysematous lung phenotype in adulthood without persisting oxidative stress and inflammation

Background

Preterm newborns typically require supplemental oxygen but hyperoxic conditions also damage the premature lung. Oxygen-induced lung damages are mainly studied in newborn mouse models using oxygen concentrations above 75% and looking at short-term effects. Therefore, we aimed at the investigation of long-term effects and their dependency on different oxygen concentrations.

Methods

Newborn mice were exposed to moderate vs. severe hyperoxic air conditions (50 vs. 75% O₂) for 14 days followed by a longer period of normoxic conditions. Lung-related parameters were collected at an age of 60 or 120 days.

Results

Severe hyperoxia caused lower alveolar density, enlargement of parenchymal air spaces and fragmented elastic fibers as well as higher lung compliance with peak airflow limitations and higher sensitivity to ventilation-mediated damages in later life. However, these long-term lung structural and functional changes did not restrict the voluntary physical activity. Also, they were not accompanied by ongoing inflammatory processes, increased formation of reactive oxygen species (ROS) or altered expressions of antioxidant enzymes (superoxide dismutases, catalase) and lung elasticity-relevant proteins (elastin, pro-surfactant proteins) in adulthood. In contrast to severe hyperoxia, moderate hyperoxia was less lung damaging but also not free of long-term effects (higher lung compliance without peak airflow limitations, increased ROS formation).

Conclusions

Severe but not moderate neonatal hyperoxia causes emphysematous lungs without persisting oxidative stress and inflammation in adulthood. As the existing fragmentation of the elastic fibers seems to play a pivotal role, it indicates the usefulness of elastin-protecting compounds in the reduction of long-term oxygen-related lung damages.

Normobaric oxygen treatment improves neuronal survival functional recovery and axonal plasticity after newborn hypoxia-ischemia

BACKGROUND

Newborn hypoxia ischemia (HI) is one of the most prevalent cases in the emergency and can result from fetal hypoxia during delivery. In HI, restricted blood supply to the fetal brain may cause epilepsy or mental disorders.

METHODS

In the present study, seven-day-old pups were subjected HI and treated with different normobaric oxygen (NBO) concentrations (21%, 70% or 100%). In the acute phase, we analyzed infarct area, disseminate neuronal injury and surviving neurons. In addition, we studied the regulation of PTEN and MMP-9 proteins which were suggested to be activated by HI in the ischemic tissue. Moreover, long-term effects of NBO treatments were evaluated with open field, rotarod and Barnes maze tests. We also examined axonal plasticity with EGFP-AAV injection.

RESULTS

Here, we demonstrate that hyperoxic NBO concentration causes an increase in cellular survival and a decrease in the number of apoptotic cells, meanwhile inhibiting the proteins involved in cellular death mechanisms. Moreover, we found that hyperoxia decreases anxiety, promotes motor coordination and improve spatial learning and memory. Notably that axonal sprouting was promoted by hyperoxia.

CONCLUSION

Our data suggest that NBO is a promising approach for the treatment of newborn HI, which encourage proof-of-concept studies in newborn.

Impact of early life AT1 blockade on adult cardiac morpho-functional changes and the renin-angiotensin system in a model of neonatal high oxygen-induced cardiomyopathy

We previously reported that neonatal blockade of angiotensin II AT₁ receptor prevents cardiac changes in 4 weeks rats with neonatal hyperoxia-induced cardiomyopathy, a recognized model of prematurity-related deleterious conditions. Considering the importance of AT₁ receptor and the renin angiotensin system (RAS) in normal development, the present study aimed to investigate the adult effects of neonatal AT₁ blockade on left ventricle (LV) in rats exposed to neonatal hyperoxia. Sprague-Dawley pups were exposed to 80% O₂ or room air from days 3-10. AT₁ blocker (losartan) or H₂O were given by gavage from day 8-10. LV function (echo and intraventricular pressure), histology and expression of RAS components were examined in 15-16 weeks old adult males. Losartan treatment prevented myocardial fibrosis, LV wall thickening and stroke volume reduction in rats exposed to high O₂ in the neonatal period. However, Losartan treatment of O₂-exposed pups led to reduced ejection fraction (EF) and fractional shortening (FS), and did not prevent changes in diastolic function. Losartan also did not prevent increased LV AT₂ and decreased angiotensin-(1-7) Mas receptors expression observed in high O₂-exposed rats. Neonatal Losartan attenuated long-term impact of neonatal hyperoxia but also led to decreased EF and FS. Increased AT₂ and decreased Mas receptor expression observed in O₂-exposed group were unaffected by Losartan treatment. Our results show that early life Losartan treatment aimed at preventing cardiac consequences of neonatal deleterious conditions may also comprise detrimental effects that require further investigation prior to clinical translation in developing children.

Lipoxin A4 reduces hyperoxia-induced lung injury in neonatal rats through PINK1 signaling pathway

Bronchopulmonary dysplasia (BPD) is a common chronic lung disease in premature infants and is mainly caused by hyperoxia exposure and mechanical ventilation. Alveolar simplification, pulmonary vascular abnormalities and pulmonary inflammation are the main pathological changes in hyperoxic lung injury animals. Lipoxin A4 (LXA4) is an important endogenous lipid that can mediate the regression of inflammation and plays a role in acute lung injury and asthma. The purpose of this study was to evaluate the effects of LXA4 on inflammation and lung function in neonatal rats with hyperoxic lung injury and to explore the mechanism of the PINK1 pathway. After 85% oxygen exposure in newborn rats for 7 days, the BPD model was established. We found that LXA4 could significantly reduce cell and protein infiltration and oxidative stress in rat lungs, improve pulmonary function and alveolar simplification, and promote weight gain. LXA4 inhibited the expression of TNF-α, MCP-1 and IL-1β in serum and BALF from hyperoxic rats. Moreover, we found that LXA4 could reduce the expression of the PINK1 gene and down-regulate the expression of PINK1, Parkin, BNIP3L/Nix and the autophagic protein LC3B.These protective effects of LXA4 could be partially reversed by addition of BOC-2.Thus, we concluded that LXA4 can alleviate the airway inflammatory response, reduce the severity of lung injury and improve lung function in a hyperoxic rat model of BPD partly through the PINK1 signaling pathway.

Effects of neonatal hyperoxia on the critical period of postnatal development of neurochemical expressions in brain stem respiratory-related nuclei in the rat

We have identified a critical period of respiratory development in rats at postnatal days P12‐13, when inhibitory influence dominates and when the response to hypoxia is at its weakest. This critical period has significant implications for Sudden Infant Death Syndrome (SIDS), the cause of which remains elusive. One of the known risk factors for SIDS is prematurity. A common intervention used in premature infants is hyperoxic therapy, which, if prolonged, can alter the ventilatory response to hypoxia and induce sustained inhibition of lung alveolar growth and pulmonary remodeling. The goal of this study was to test our hypothesis that neonatal hyperoxia from postnatal day (P) 0 to P10 in rat pups perturbs the critical period by altering the normal progression of neurochemical development in brain stem respiratory‐related nuclei. An in‐depth, semiquantitative immunohistochemical study was undertaken at P10 (immediately after hyperoxia and before the critical period), P12 (during the critical period), P14 (immediately after the critical period), and P17 (a week after the cessation of hyperoxia). In agreement with our previous findings, levels of cytochrome oxidase, brain‐derived neurotrophic factor (BDNF), TrkB (BDNF receptor), and several serotonergic proteins (5‐HT_1A and _2A receptors, 5‐HT synthesizing enzyme tryptophan hydroxylase [TPH], and serotonin transporter [SERT]) all fell in several brain stem respiratory‐related nuclei during the critical period (P12) in control animals. However, in hyperoxic animals, these neurochemicals exhibited a significant fall at P14 instead. Thus, neonatal hyperoxia delayed but did not eliminate the critical period of postnatal development in multiple brain stem respiratory‐related nuclei, with little effect on the nonrespiratory cuneate nucleus.

Impact of hyperoxia on cardiac pathophysiology

Mechanical ventilation with high oxygen therapy (hyperoxia) is widely implemented in critical care and ICU settings. Although supplemental oxygen is beneficial to treat hypoxia, its use is also associated with poor outcomes and high mortality in patients. Lung injury due to hyperoxia exposure has been well-documented in patients, including in adults and neonates. Thus, lung injury due to hyperoxia has been extensively researched in both preclinical and clinical studies. However, hyperoxia has also been shown to be associated with hemodynamic changes in patients in ICU, including reductions in heart rate, stroke volume, and cardiac output. In addition, certain experimental studies report that hyperoxia exposure in neonates results in cardiac dysfunction in later adult life. Despite this, until recently, the impact of hyperoxia within the heart has not been well studied, or reported, specifically in adult experimental models. To close this significant gap, our lab has sought to clarify hyperoxia-induced cardiac pathophysiology in adult murine models. This review discusses the current findings regarding the cardiovascular impact of hyperoxia exposure.

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.

Alterations in VASP phosphorylation and profilin1 and cofilin1 expression in hyperoxic lung injury and BPD

BACKGROUND

Hyperoxia is a frequently employed therapy for prematurely born infants, induces lung injury and contributes to development of bronchopulmonary dysplasia (BPD). BPD is characterized by decreased cellular proliferation, cellular migration, and failure of injury repair systems. Actin binding proteins (ABPs) such as VASP, cofilin1, and profilin1 regulate cell proliferation and migration via modulation of actin dynamics. Lung mesenchymal stem cells (L-MSCs) initiate repair processes by proliferating, migrating, and localizing to sites of injury. These processes have not been extensively explored in hyperoxia induced lung injury and repair.

METHODS

ABPs and CD146+ L-MSCs were analyzed by immunofluorescence in human lung autopsy tissues from infants with and without BPD and by western blot in lung tissue homogenates obtained from our murine model of newborn hyperoxic lung injury.

RESULTS

Decreased F-actin content, ratio of VASPpS157/VASPpS239, and profilin 1 expression were observed in human lung tissues but this same pattern was not observed in lungs from hyperoxia-exposed newborn mice. Increases in cofilin1 expression were observed in both human and mouse tissues at 7d indicating a dysregulation in actin dynamics which may be related to altered growth. CD146 levels were elevated in human and newborn mice tissues (7d).

CONCLUSION

Altered phosphorylation of VASP and expression of profilin 1 and cofilin 1 in human tissues indicate that the pathophysiology of BPD involves dysregulation of actin binding proteins. Lack of similar changes in a mouse model of hyperoxia exposure imply that disruption in actin binding protein expression may be linked to interventions or morbidities other than hyperoxia alone.

Supraphysiological Levels of Oxygen Exposure During the Neonatal Period Impairs Signaling Pathways Required for Learning and Memory

Preterm infants often require prolonged oxygen supplementation and are at high risk of neurodevelopmental impairment. We recently reported that adult mice exposed to neonatal hyperoxia (postnatal day [P] 2 to 14) had spatial navigation memory deficits associated with hippocampal shrinkage. The mechanisms by which early oxidative stress impair neurodevelopment are not known. Our objective was to identify early hyperoxia-induced alterations in hippocampal receptors and signaling pathways necessary for memory formation. We evaluated C57BL/6 mouse pups at P14, exposed to either 85% oxygen or air from P2 to 14. We performed targeted analysis of hippocampal ligand-gated ion channels and proteins necessary for memory formation, and global bioinformatic analysis of differentially expressed hippocampal genes and proteins. Hyperoxia decreased hippocampal mGLU7, TrkB, AKT, ERK2, mTORC1, RPS6, and EIF4E and increased α3, α5, and ɤ2 subunits of GABA_A receptor and PTEN proteins, although changes in gene expression were not always concordant. Bioinformatic analysis indicated dysfunction in mitochondria and global protein synthesis and translational processes. In conclusion, supraphysiological oxygen exposure reduced proteins necessary for hippocampus-dependent memory formation and may adversely impact hippocampal mitochondrial function and global protein synthesis. These early hippocampal changes may account for memory deficits seen in preterm survivors following prolonged oxygen supplementation.

Pulmonary IL-1β expression in early life causes permanent changes in lung structure and function in adulthood

Chorioamnionitis, mechanical ventilation, oxygen therapy, and postnatal infection promote inflammation in the newborn lung. The long-term consequences of pulmonary inflammation during infancy have not been well characterized. The aim of this study was to examine the impact of inflammation during the late saccular to alveolar stages of lung development on lung structure and function in adulthood. To induce IL-1β expression in the pulmonary epithelium of mice with a tetracycline-inducible human IL-1β transgene, doxycycline was administered via intraperitoneal injections to bitransgenic pups and their littermate controls on postnatal days (PN) 0, 0.5, and 1. Lung structure, inflammation, and airway reactivity were studied in adulthood. IL-1β production in early life resulted in increased numbers of macrophages and neutrophils on PN21, but inflammation subsided by PN42. Permanent changes in alveolar structure, i.e., larger alveoli and thicker alveolar walls, were present from PN21 to PN84. Lack of alveolar septation thus persisted after IL-1β production and inflammation had ceased. Early IL-1β production caused goblet cell hyperplasia, enhanced calcium-activated chloride channel 3 (CLCA3) protein expression, and increased airway reactivity in response to methacholine on PN42. Lymphoid follicles were present adjacent to small airways in the lungs of adult bitransgenic mice, and levels of the B cell chemoattractant CXC-motif ligand (CXCL) 13 were elevated in the lungs of bitransgenic mice compared with controls. In conclusion, IL-1β-induced pulmonary inflammation in early life causes a chronic lung disease in adulthood.

A Shared Pattern of β-Catenin Activation in Bronchopulmonary Dysplasia and Idiopathic Pulmonary Fibrosis

Wnt/β-catenin signaling is necessary for normal lung development, and abnormal Wnt signaling contributes to the pathogenesis of both bronchopulmonary dysplasia (BPD) and idiopathic pulmonary fibrosis (IPF), fibrotic lung diseases that occur during infancy and aging, respectively. Using a library of human normal and diseased human lung samples, we identified a distinct signature of nuclear accumulation of β-catenin phosphorylated at tyrosine 489 and epithelial cell cytosolic localization of β-catenin phosphorylated at tyrosine 654 in early normal lung development and fibrotic lung diseases BPD and IPF. Furthermore, this signature was recapitulated in murine models of BPD and IPF. Image analysis of immunofluorescence colocalization demonstrated a consistent pattern of elevated nuclear phosphorylated β-catenin in the lung epithelium and surrounding mesenchyme in BPD and IPF, closely resembling the pattern observed in 18-week fetal lung. Nuclear β-catenin phosphorylated at tyrosine 489 associated with an increased expression of Wnt target gene AXIN2, suggesting that the observed β-catenin signature is of functional significance during normal development and injury repair. The association of specific modifications of β-catenin during normal lung development and again in response to lung injury supports the widely held concept that repair of lung injury involves the recapitulation of developmental programs. Furthermore, these observations suggest that β-catenin phosphorylation has potential as a therapeutic target for the treatment and prevention of both BPD and IPF.

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.

Exposure of neonatal mice to bromine impairs their alveolar development and lung function

The halogen bromine (Br₂) is used extensively in industry and stored and transported in large quantities. Its accidental or malicious release into the atmosphere has resulted in significant casualties. The pathophysiology of Br₂-induced lung injury has been studied in adult animals, but the consequences of Br₂ exposure to the developing lung are completely unknown. We exposed neonatal mouse littermates on postnatal day 3 (P3) to either Br₂ at 400 ppm for 30 min (400/30), to Br₂ at 600 ppm for 30 min (600/30), or to room air, then returned them to their dams and observed until P14. Mice exposed to Br₂ had decreased survival (S) and had decreased weight (W) at P14 in the 400/30 group (S = 63.5%, W = 6.67 ± 0.08) and in the 600/30 group (S = 36.1%, W = 5.13 ± 0.67) as compared with air breathing mice (S = 100%, W = 7.96 ± 0.30). Alveolar development was impaired, as evidenced by increased mean linear intercept at P14. At P14, Br₂ exposed mice also exhibited a decrease of arterial partial pressure of oxygen, decreased quasi-static lung compliance, as well as increased alpha smooth muscle actin mRNA and protein and increased mRNA for IL-1β, IL-6, CXCL1, and TNFα. Global gene expression, evaluated by RNA sequencing and Ingenuity Pathway Analysis, revealed persistent abnormalities in gene expression profiles at P14 involving pathways of "formation of lung" and "pulmonary development." The data indicate that Br₂ inhalation injury early in life results in severe lung developmental consequences, wherein persistent inflammation and global altered developmental gene expression are likely mechanistic contributors.

Aberrant cGMP signaling persists during recovery in mice with oxygen-induced pulmonary hypertension

Bronchopulmonary dysplasia (BPD), a common complication of preterm birth, is associated with pulmonary hypertension (PH) in 25% of infants with moderate to severe BPD. Neonatal mice exposed to hyperoxia for 14d develop lung disease similar to BPD, with evidence of associated PH. The cyclic guanosine monophosphate (cGMP) signaling pathway has not been well studied in BPD-associated PH. In addition, there is little data about the natural history of hyperoxia-induced PH in mice or the utility of phosphodiesterase-5 (PDE5) inhibition in established disease. C57BL/6 mice were placed in room air or 75% O₂ within 24h of birth for 14d, followed by recovery in room air for an additional 7 days (21d). Additional pups were treated with either vehicle or sildenafil for 7d during room air recovery. Mean alveolar area, pulmonary artery (PA) medial wall thickness (MWT), RVH, and vessel density were evaluated at 21d. PA protein from 21d animals was analyzed for soluble guanylate cyclase (sGC) activity, PDE5 activity, and cGMP levels. Neonatal hyperoxia exposure results in persistent alveolar simplification, RVH, decreased vessel density, increased MWT, and disrupted cGMP signaling despite a period of room air recovery. Delayed treatment with sildenafil during room air recovery is associated with improved RVH and decreased PA PDE5 activity, but does not have significant effects on alveolar simplification, PA remodeling, or vessel density. These data are consistent with clinical studies suggesting inconsistent effects of sildenafil treatment in infants with BPD-associated PH.

The Future of Bronchopulmonary Dysplasia: Emerging Pathophysiological Concepts and Potential New Avenues of Treatment

Yearly more than 15 million babies are born premature (<37 weeks gestational age), accounting for more than 1 in 10 births worldwide. Lung injury caused by maternal chorioamnionitis or preeclampsia, postnatal ventilation, hyperoxia, or inflammation can lead to the development of bronchopulmonary dysplasia (BPD), one of the most common adverse outcomes in these preterm neonates. BPD patients have an arrest in alveolar and microvascular development and more frequently develop asthma and early-onset emphysema as they age. Understanding how the alveoli develop, and repair, and regenerate after injury is critical for the development of therapies, as unfortunately there is still no cure for BPD. In this review, we aim to provide an overview of emerging new concepts in the understanding of perinatal lung development and injury from a molecular and cellular point of view and how this is paving the way for new therapeutic options to prevent or treat BPD, as well as a reflection on current treatment procedures.

Oxidative injury of the pulmonary circulation in the perinatal period: Short- and long-term consequences for the human cardiopulmonary system

Development of the pulmonary circulation is a complex process with a spatial pattern that is tightly controlled. This process is vulnerable for disruption by various events in the prenatal and early postnatal periods. Disruption of normal pulmonary vascular development leads to abnormal structure and function of the lung vasculature, causing neonatal pulmonary vascular diseases. Premature babies are especially at risk of the development of these diseases, including persistent pulmonary hypertension and bronchopulmonary dysplasia. Reactive oxygen species play a key role in the pathogenesis of neonatal pulmonary vascular diseases and can be caused by hyperoxia, mechanical ventilation, hypoxia, and inflammation. Besides the well-established short-term consequences, exposure of the developing lung to injurious stimuli in the perinatal period, including oxidative stress, may also contribute to the development of pulmonary vascular diseases later in life, through so-called "fetal or perinatal programming." Because of these long-term consequences, it is important to develop a follow-up program tailored to adolescent survivors of neonatal pulmonary vascular diseases, aimed at early detection of adult pulmonary vascular diseases, and thereby opening the possibility of early intervention and interfering with disease progression. This review focuses on pathophysiologic events in the perinatal period that have been shown to disrupt human normal pulmonary vascular development, leading to neonatal pulmonary vascular diseases that can extend even into adulthood. This knowledge may be particularly important for ex-premature adults who are at risk of the long-term consequences of pulmonary vascular diseases, thereby contributing disproportionately to the burden of adult cardiovascular disease in the future.

Bronchopulmonary dysplasia: new becomes old again!

Despite the many advances in neonatology, bronchopulmonary dysplasia (BPD) continues to be a frustrating disease of prematurity. BPD is a disease which is defined oddly by its treatment rather than its pathophysiology, leading to frequently changing nomenclature which has widespread implications on our ability to both understand and follow the progression of BPD. As various treatment modalities for BPD were developed and a larger number of extremely preterm infants survived, the "old" BPD based on lung injury from oxygen therapy and mechanical ventilation transitioned into a "new" BPD focused more on the interruption of normal development. However, the interruption of normal development does not solely apply to lung development. The effects of prematurity on vascular development cannot be overstated and pulmonary vascular disease has become the new frontier of BPD. As we begin to better understand the complex, multifactorial pathophysiology of BPD, it is necessary to again focus on appropriate, pathology-driven nomenclature that can effectively describe the multiple clinical phenotypes of BPD.

Endothelial Monocyte-Activating Polypeptide II Mediates Macrophage Migration in the Development of Hyperoxia-Induced Lung Disease of Prematurity

Myeloid cells are key factors in the progression of bronchopulmonary dysplasia (BPD) pathogenesis. Endothelial monocyte-activating polypeptide II (EMAP II) mediates myeloid cell trafficking. The origin and physiological mechanism by which EMAP II affects pathogenesis in BPD is unknown. The objective was to determine the functional consequences of elevated EMAP II levels in the pathogenesis of murine BPD and to investigate EMAP II neutralization as a therapeutic strategy. Three neonatal mouse models were used: (1) BPD (hyperoxia), (2) EMAP II delivery, and (3) BPD with neutralizing EMAP II antibody treatments. Chemokinic function of EMAP II and its neutralization were assessed by migration in vitro and in vivo. We determined the location of EMAP II by immunohistochemistry, pulmonary proinflammatory and chemotactic gene expression by quantitative polymerase chain reaction and immunoblotting, lung outcome by pulmonary function testing and histological analysis, and right ventricular hypertrophy by Fulton's Index. In BPD, EMAP II initially is a bronchial club-cell-specific protein-derived factor that later is expressed in galectin-3⁺ macrophages as BPD progresses. Continuous elevated expression corroborates with baboon and human BPD. Prolonged elevation of EMAP II levels recruits galectin-3⁺ macrophages, which is followed by an inflammatory state that resembles a severe BPD phenotype characterized by decreased pulmonary compliance, arrested alveolar development, and signs of pulmonary hypertension. In vivo pharmacological EMAP II inhibition suppressed proinflammatory genes Tnfa, Il6, and Il1b and chemotactic genes Ccl2 and Ccl9 and reversed the severe BPD phenotype. EMAP II is sufficient to induce macrophage recruitment, worsens BPD progression, and represents a targetable mechanism of BPD development.

Phenotypic assessment of pulmonary hypertension using high-resolution echocardiography is feasible in neonatal mice with experimental bronchopulmonary dysplasia and pulmonary hypertension: a step toward preventing chronic obstructive pulmonary disease

Bronchopulmonary dysplasia (BPD) and chronic obstructive pulmonary disease (COPD) are chronic lung diseases of human infants and adults, respectively, that are characterized by alveolar simplification. One-third of the infants with severe BPD develop pulmonary hypertension (PH). More importantly, PH increases morbidity and mortality in BPD patients. Additionally, COPD is a common respiratory morbidity in former BPD patients. The lack of an appropriate small animal model wherein echocardiography (Echo) can demonstrate PH is one of the major barriers to understand the molecular mechanisms of the disease and, thereby, develop rational therapies to prevent and/or treat PH in BPD patients. Thus, the goal of this study was to establish a model of experimental BPD and PH and investigate the feasibility of Echo to diagnose PH in neonatal mice. Since hyperoxia-induced oxidative stress and inflammation contributes to the development of BPD with PH, we tested the hypothesis that exposure of newborn C57BL/6J mice to 70% O2 (hyperoxia) for 14 days leads to lung oxidative stress, inflammation, alveolar and pulmonary vascular simplification, pulmonary vascular remodeling, and Echo evidence of PH. Hyperoxia exposure caused lung oxidative stress and inflammation as evident by increased malondialdehyde adducts and inducible nitric oxide synthase, respectively. Additionally, hyperoxia exposure caused growth restriction, alveolar and pulmonary vascular simplification, and pulmonary vascular remodeling. At 14 days of age, Echo of these mice demonstrated that hyperoxia exposure decreased pulmonary acceleration time (PAT) and PAT/ejection time ratio and increased right ventricular free wall thickness, which are indicators of significant PH. Thus, we have demonstrated the feasibility of Echo to phenotype PH in neonatal mice with experimental BPD with PH, which can aid in discovery of therapies to prevent and/or treat BPD with PH and its sequelae such as COPD in humans.

Dose-dependent effects of glucocorticoids on pulmonary vascular development in a murine model of hyperoxic lung injury

BACKGROUND

Exposure of neonatal mice to hyperoxia results in pulmonary vascular remodeling and aberrant phosphodiesterase type 5 (PDE5) signaling. Although glucocorticoids are frequently utilized in the NICU, little is known about their effects on the developing pulmonary vasculature and on PDE5. We sought to determine the effects of hydrocortisone (HC) on pulmonary vascular development and on PDE5 in a neonatal mouse model of hyperoxic lung injury.

METHODS

C57BL/6 mice were placed in 21% O2 or 75% O2 within 24 h of birth and received HC (1, 5, or 10 mg/kg subcutaneously every other day) or vehicle. At 14 d, right ventricular hypertrophy (RVH), medial wall thickness (MWT), lung morphometry, and pulmonary artery (PA) PDE5 activity were assessed. PDE5 activity was measured in isolated pulmonary artery smooth muscle cells exposed to 21 or 95% O2 ± 100 nmol/l HC for 24 h.

RESULTS

Hyperoxia resulted in alveolar simplification, RVH, increased MWT, and increased PA PDE5 activity. HC decreased hyperoxia-induced RVH and attenuated MWT. HC had dose-dependent effects on alveolar simplification. HC decreased hyperoxia-induced PDE5 activity both in vivo and in vitro.

CONCLUSIONS

HC decreases hyperoxia-induced pulmonary vascular remodeling and attenuates PDE5 activity. These findings suggest that HC may protect against hyperoxic injury in the developing pulmonary vasculature.

Oxygen Use in Neonatal Care: A Two-edged Sword

In the neonatal period, the clinical use of oxygen should be taken into consideration for its beneficial and toxicity effects. Oxygen toxicity is due to the development of reactive oxygen species (ROS) such as OH^• that is one of the strongest oxidants in nature. Of note, generation of ROS is a normal occurrence in human and it is involved in a myriad of physiological reactions. Anyway an imbalance between production of oxidant species and antioxidant defenses, called oxidative stress, could affect various aspect of organisms' physiology and it could determine pathological consequences to living beings. Neonatal oxidative stress is essentially due to decreased antioxidants, increased ROS, or both. Studies have demonstrated that antioxidant capacity is lower in preterm newborns than term babies. This well-known deficiency of antioxidant factors is only a piece of a cohort of factors, which can be involved in the neonatal oxidative stress and the increased production of ROS may be a main factor. Mechanisms of ROS generation are: mitochondrial respiratory chain, free iron and Fenton reaction, inflammation, hypoxia and/or ischemia, reperfusion, and hyperoxia. Oxidative stress following hyperoxia has been recognized to be responsible for lung, central nervous system, retina, red blood cell injuries, and possibly generalized tissue damage. When supplemental oxygen is needed for care, it would be prudent to avoid changes and fluctuations in SpO₂. The definition of the safest level of oxygen saturations in the neonate remains an area of active research. Currently, on the basis of the published evidences, the most suitable approach would be to set alarm limits between 90 and 95%. It should allow to avoid SpO₂ values associated with potential hypoxia and/or hyperoxia. Although the usefulness of antioxidant protection in the neonatal period is still under investigation, the risk of tissue damage due to oxidative stress in perinatal period should not be underestimated.

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.

Can maternal DHA supplementation offer long-term protection against neonatal hyperoxic lung injury?

The effect of adverse perinatal environment (like maternal infection) has long-standing effects on many organ systems, including the respiratory system. Use of maternal nutritional supplements is an exciting therapeutic option that could be used to protect the developing fetus. In a recent issue of the journal, Ali and associates (Ali M, Heyob KM, Velten M, Tipple TE, Rogers LK. Am J Physiol Lung Cell Mol Physiol 309: L441-L448, 2015) specifically look at maternal docosahexaenoic acid (DHA) supplementation and its effect on chronic apoptosis in the lung in a mouse model of perinatal inflammation and postnatal hyperoxia. Strikingly, the authors show that pulmonary apoptosis was augmented even 8 wk after the hyperoxia-exposed mice had been returned to room air. This effect was significantly attenuated in mice that were subjected to maternal dietary DHA supplementation. These findings are novel, significantly advance our understanding of chronic effects of adverse perinatal and neonatal events on the developing lung, and thereby offer novel therapeutic options in the form of maternal dietary supplementation with DHA. This editorial reviews the long-term effects of adverse perinatal environment on postnatal lung development and the protective effects of dietary supplements such as DHA.