L-citrulline Prevents Alveolar and Vascular Derangement in a Rat Model of Moderate Hyperoxia-induced Lung Injury

L-citrulline attenuates lipopolysaccharide-induced inflammatory lung injury in neonatal rats

BACKGROUND

Prenatal or postnatal lung inflammation and oxidative stress disrupt alveolo-vascular development leading to bronchopulmonary dysplasia (BPD) with and without pulmonary hypertension. L-citrulline (L-CIT), a nonessential amino acid, alleviates inflammatory and hyperoxic lung injury in preclinical models of BPD. L-CIT modulates signaling pathways mediating inflammation, oxidative stress, and mitochondrial biogenesis-processes operative in the development of BPD. We hypothesize that L-CIT will attenuate lipopolysaccharide (LPS)-induced inflammation and oxidative stress in our rat model of neonatal lung injury.

METHODS

Newborn rats during the saccular stage of lung development were used to investigate the effect of L-CIT on LPS-induced lung histopathology and pathways involved in inflammatory, antioxidative processes, and mitochondrial biogenesis in lungs in vivo, and in primary culture of pulmonary artery smooth muscle cells, in vitro.

RESULTS

L-CIT protected the newborn rat lung from LPS-induced: lung histopathology, ROS production, NFκB nuclear translocation, and upregulation of gene and protein expression of inflammatory cytokines (IL-1β, IL-8, MCP-1α, and TNF-α). L-CIT maintained mitochondrial morphology, increased protein levels of PGC-1α, NRF1, and TFAM (transcription factors involved in mitochondrial biogenesis), and induced SIRT1, SIRT3, and superoxide dismutases protein expression.

CONCLUSION

L-CIT may be efficacious in decreasing early lung inflammation and oxidative stress mitigating progression to BPD.

IMPACT

The nonessential amino acid L-citrulline (L-CIT) mitigated lipopolysaccharide (LPS)-induced lung injury in the early stage of lung development in the newborn rat. This is the first study describing the effect of L-CIT on the signaling pathways operative in bronchopulmonary dysplasia (BPD) in a preclinical inflammatory model of newborn lung injury. If our findings translate to premature infants, L-CIT could decrease inflammation, oxidative stress and preserve mitochondrial health in the lung of premature infants at risk for BPD.

COVID-19 Metabolomic-Guided Amino Acid Therapy Protects from Inflammation and Disease Sequelae

The outbreak of coronavirus disease 2019 (COVID-19) has caused a worldwide pandemic since 2019. A metabolic disorder is a contributing factor to deaths from COVID-19. However, the underlying mechanism of metabolic dysfunction in COVID-19 patients and the potential interventions are not elucidated. Here targeted plasma metabolomic is performed, and the metabolite profiles among healthy controls, and asymptomatic, moderate, and severe COVID-19 patients are compared. Among the altered metabolites, arachidonic acid and linolenic acid pathway metabolites are profoundly up-regulated in COVID-19 patients. Arginine biosynthesis, alanine, aspartate, and glutamate metabolism pathways are significantly disturbed in asymptomatic patients. In the comparison of metabolite variances among the groups, higher levels of l-citrulline and l-glutamine are found in asymptomatic carriers and moderate or severe patients at the remission stage. Furthermore, l-citrulline and l-glutamine combination therapy is demonstrated to effectively protect mice from coronavirus infection and endotoxin-induced sepsis, and is observed to efficiently prevent the occurrence of pulmonary fibrosis and central nervous system damage. Collectively, the data reveal the metabolite profile of asymptomatic COVID-19 patients and propose a potential strategy for COVID-19 treatment.

Pharmacotherapy for Pulmonary Hypertension in Infants with Bronchopulmonary Dysplasia: Past, Present, and Future

Approximately 8-42% of premature infants with chronic lung disease of prematurity, bronchopulmonary dysplasia (BPD), develop pulmonary hypertension (PH). Infants with BPD-PH carry alarmingly high mortality rates of up to 47%. Effective PH-targeted pharmacotherapies are desperately needed for these infants. Although many PH-targeted pharmacotherapies are commonly used to treat BPD-PH, all current use is off-label. Moreover, all current recommendations for the use of any PH-targeted therapy in infants with BPD-PH are based on expert opinion and consensus statements. Randomized Control Trials (RCTs) are needed to determine the efficacy of PH-targeted treatments in premature infants with or at risk of BPD-PH. Prior to performing efficacy RCTs, studies need to be conducted to obtain pharmacokinetic, pharmacodynamic, and safety data for any pharmacotherapy used in this understudied and fragile patient population. This review will discuss current and needed treatment strategies, identify knowledge deficits, and delineate both challenges to be overcome and approaches to be taken to develop effective PH-targeted pharmacotherapies that will improve outcomes for premature infants with or at risk of developing BPD-PH.

PGC-1α activity and mitochondrial dysfunction in preterm infants

Extremely low gestational age neonates (ELGANs) are born in a relatively hyperoxic environment with weak antioxidant defenses, placing them at high risk for mitochondrial dysfunction affecting multiple organ systems including the nervous, respiratory, ocular, and gastrointestinal systems. The brain and lungs are highly affected by mitochondrial dysfunction and dysregulation in the neonate, causing white matter injury (WMI) and bronchopulmonary dysplasia (BPD), respectively. Adequate mitochondrial function is important in providing sufficient energy for organ development as it relates to alveolarization and axonal myelination and decreasing oxidative stress via reactive oxygen species (ROS) and reactive nitrogen species (RNS) detoxification. Peroxisome proliferator-activated receptor gamma coactivator-1 alpha (PGC-1α) is a master regulator of mitochondrial biogenesis and function. Since mitochondrial dysfunction is at the root of WMI and BPD pathobiology, exploring therapies that can regulate PGC-1α activity may be beneficial. This review article describes several promising therapeutic agents that can mitigate mitochondrial dysfunction through direct and indirect activation and upregulation of the PGC-1α pathway. Metformin, resveratrol, omega 3 fatty acids, montelukast, L-citrulline, and adiponectin are promising candidates that require further pre-clinical and clinical studies to understand their efficacy in decreasing the burden of disease from WMI and BPD in preterm infants.

Citrulline protects against LPS‑induced acute lung injury by inhibiting ROS/NLRP3‑dependent pyroptosis and apoptosis via the Nrf2 signaling pathway

Acute lung injury (ALI) is a common complication in patients with sepsis and is accompanied by high mortality. The present study aimed to investigate if the organic compound citrulline has a protective against lipopolysaccharide (LPS)-stimulated ALI and its potential mechanisms. ALI was induced in mice by intraperitoneal (i.p.) injection of LPS (10 mg/kg). Citrulline (1 g/kg/day) was administrated i.p. 7 days prior to LPS injection. Mouse lung vascular endothelial cells (MLVECs) were divided into five groups: Control, LPS, LPS + Cit, LPS + N-acetyl-L-cysteine (NAC) and LPS + Cit + ML385. Lung injury was determined by morphology changes. Apoptosis and pyroptosis were detected using western blot analysis and immunofluorescence. The present results indicated that citrulline can significantly attenuate ALI. Citrulline pretreatment decreased the expression of NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) inflammasome and decreased pyroptosis and apoptosis. Intervention with the total reactive oxygen species (ROS) scavenger N-acetyl-L-cysteine attenuated NLRP3 inflammasome-associated pyroptosis and apoptosis in LPS-treated MLVECs. Citrulline pretreatment inhibited pyroptotic cell death and apoptosis induced by LPS. Citrulline decreased accumulation of intracellular ROS and activated the nuclear factor erythroid 2-related factor 2 (Nrf2) signaling pathway. Furthermore, the Nrf2 inhibitor ML385 reversed ROS generation, NLRP3 inflammasome-mediated pyroptosis and apoptosis suppressed by citrulline. In summary, the present data demonstrated that citrulline may confer protection against ALI via inhibition of ROS/NLRP3 inflammasome-dependent pyroptosis and apoptosis via the Nrf2 signaling pathway.

Stem-Cell Therapy for Bronchopulmonary Dysplasia (BPD) in Newborns

Premature newborns are at a higher risk for the development of respiratory distress syndrome (RDS), acute lung injury (ALI) associated with lung inflammation, disruption of alveolar structure, impaired alveolar growth, lung fibrosis, impaired lung angiogenesis, and development of bronchopulmonary dysplasia (BPD) with severe long-term developmental adverse effects. The current therapy for BPD is limited to supportive care including high-oxygen therapy and pharmacotherapy. Recognizing more feasible treatment options to improve lung health and reduce complications associated with BPD is essential for improving the overall quality of life of premature infants. There is a reduction in the resident stem cells in lungs of premature infants with BPD, which strongly suggests a critical role of stem cells in BPD pathogenesis; this warrants the exploration of the potential therapeutic use of stem-cell therapy. Stem-cell-based therapies have shown promise for the treatment of many pathological conditions including acute lung injury and BPD. Mesenchymal stem cells (MSCs) and MSC-derived extracellular vesicles (EVs) including exosomes are promising and effective therapeutic modalities for the treatment of BPD. Treatment with MSCs and EVs may help to reduce lung inflammation, improve pulmonary architecture, attenuate pulmonary fibrosis, and increase the survival rate.

Metabolic analysis of infants with bronchopulmonary dysplasia under early nutrition therapy: An observational cohort study

To assess the amino acid and fatty acid metabolite patterns between infants with and without bronchopulmonary dysplasia in different nutritional stages after birth and identify metabolic indicators of bronchopulmonary dysplasia. This was an observational cohort of preterm infants born at a gestational age ≤32 + 6 weeks and with a body weight ≤2000 g. Amino acid and carnitine profiles were measured in dried blood spots (DBSs) during the early nutrition transitional phase using tandem mass spectrometry. Bronchopulmonary dysplasia was defined as oxygen dependence at 36 weeks of postmenstrual age or 28 days after birth. Metabolomic analysis was employed to define metabolites with significant differences, map significant metabolites into pathways, and identify metabolic indicators of bronchopulmonary dysplasia. We evaluated 45 neonates with and 40 without bronchopulmonary dysplasia. Four amino acids and three carnitines showed differences between the groups. Three carnitines (C0, C2, and C6:1) were high in the bronchopulmonary dysplasia group mostly; conversely, all four amino acids (threonine, arginine, methionine, and glutamine (Gln)) were low in the bronchopulmonary dysplasia group. Pathway analysis of these metabolites revealed two pathways with significant changes (p < 0.05). ROC analysis showed Gln/C6:1 at total parenteral nutrition phase had both 80% sensitivity and specificity for predicting the development of bronchopulmonary dysplasia, with an area under the curve of 0.81 (95% confidence interval 0.71-0.89). Amino acid and fatty acid metabolite profiles changed in infants with bronchopulmonary dysplasia after birth during the nutrition transitional period, suggesting that metabolic dysregulation may participate in the development of bronchopulmonary dysplasia. Our findings demonstrate that metabolic indicators are promising for forecasting the occurrence of bronchopulmonary dysplasia among preterm neonates.

Targeting the lung endothelial niche to promote angiogenesis and regeneration: A review of applications

Lung endothelial cells comprise the pulmonary vascular bed and account for the majority of cells in the lungs. Beyond their role in gas exchange, lung ECs form a specialized microenvironment, or niche, with important roles in health and disease. In early development, progenitor ECs direct alveolar development through angiogenesis. Following birth, lung ECs are thought to maintain their regenerative capacity despite the aging process. As such, harnessing the power of the EC niche, specifically to promote angiogenesis and alveolar regeneration has potential clinical applications. Here, we focus on translational research with applications related to developmental lung diseases including pulmonary hypoplasia and bronchopulmonary dysplasia. An overview of studies examining the role of ECs in lung regeneration following acute lung injury is also provided. These diseases are all characterized by significant morbidity and mortality with limited existing therapeutics, affecting both young children and adults.

Intratracheal administration of mesenchymal stem cell-derived extracellular vesicles reduces lung injuries in a chronic rat model of bronchopulmonary dysplasia

Early therapeutic effect of intratracheally (IT)-administered extracellular vesicles secreted by mesenchymal stem cells (MSC-EVs) has been demonstrated in a rat model of bronchopulmonary dysplasia (BPD) involving hyperoxia exposure in the first 2 postnatal weeks. The aim of this study was to evaluate the protective effects of IT-administered MSC-EVs in the long term. EVs were produced from MSCs following GMP standards. At birth, rats were distributed in three groups: (a) animals raised in ambient air for 6 weeks (n = 10); and animals exposed to 60% hyperoxia for 2 weeks and to room air for additional 4 weeks and treated with (b) IT-administered saline solution (n = 10), or (c) MSC-EVs (n = 10) on postnatal days 3, 7, 10, and 21. Hyperoxia exposure produced significant decreases in total number of alveoli, total surface area of alveolar air spaces, and proliferation index, together with increases in mean alveolar volume, mean linear intercept and fibrosis percentage; all these morphometric changes were prevented by MSC-EVs treatment. The medial thickness index for <100 µm vessels was higher for hyperoxia-exposed/sham-treated than for normoxia-exposed rats; MSC-EV treatment significantly reduced this index. There were no significant differences in interstitial/alveolar and perivascular F4/8-positive and CD86-positive macrophages. Conversely, hyperoxia exposure reduced CD163-positive macrophages both in interstitial/alveolar and perivascular populations and MSC-EV prevented these hyperoxia-induced reductions. These findings further support that IT-administered EVs could be an effective approach to prevent/treat BPD, ameliorating the impaired alveolarization and pulmonary artery remodeling also in a long-term model. M2 macrophage polarization could play a role through anti-inflammatory and proliferative mechanisms.

Epithelial Dysfunction in Lung Diseases: Effects of Amino Acids and Potential Mechanisms

Lung diseases affect millions of individuals all over the world. Various environmental factors, such as toxins, chemical pollutants, detergents, viruses, bacteria, microbial dysbiosis, and allergens, contribute to the development of respiratory disorders. Exposure to these factors activates stress responses in host cells and disrupt lung homeostasis, therefore leading to dysfunctional epithelial barriers. Despite significant advances in therapeutic treatments for lung diseases in the last two decades, novel interventional targets are imperative, considering the side effects and limited efficacy in patients treated with currently available drugs. Nutrients, such as amino acids (e.g., arginine, glutamine, glycine, proline, taurine, and tryptophan), peptides, and bioactive molecules, have attracted more and more attention due to their abilities to reduce oxidative stress, inhibit apoptosis, and regulate immune responses, thereby improving epithelial barriers. In this review, we summarize recent advances in amino acid metabolism in the lungs, as well as multifaceted functions of amino acids in attenuating inflammatory lung diseases based on data from studies with both human patients and animal models. The underlying mechanisms for the effects of physiological amino acids are likely complex and involve cell signaling, gene expression, and anti-oxidative reactions. The beneficial effects of amino acids are expected to improve the respiratory health and well-being of humans and other animals. Because viruses (e.g., coronavirus) and environmental pollutants (e.g., PM2.5 particles) induce severe damage to the lungs, it is important to determine whether dietary supplementation or intravenous administration of individual functional amino acids (e.g., arginine-HCl, citrulline, N-acetylcysteine, glutamine, glycine, proline and tryptophan) or their combinations to affected subjects may alleviate injury and dysfunction in this vital organ.

Present and Future of Bronchopulmonary Dysplasia

Bronchopulmonary dysplasia (BPD) is the most common respiratory disorder among infants born extremely preterm. The pathogenesis of BPD involves multiple prenatal and postnatal mechanisms affecting the development of a very immature lung. Their combined effects alter the lung's morphogenesis, disrupt capillary gas exchange in the alveoli, and lead to the pathological and clinical features of BPD. The disorder is ultimately the result of an aberrant repair response to antenatal and postnatal injuries to the developing lungs. Neonatology has made huge advances in dealing with conditions related to prematurity, but efforts to prevent and treat BPD have so far been only partially effective. Seeing that BPD appears to have a role in the early origin of chronic obstructive pulmonary disease, its prevention is pivotal also in long-term respiratory outcome of these patients. There is currently some evidence to support the use of antenatal glucocorticoids, surfactant therapy, protective noninvasive ventilation, targeted saturations, early caffeine treatment, vitamin A, and fluid restriction, but none of the existing strategies have had any significant impact in reducing the burden of BPD. New areas of research are raising novel therapeutic prospects, however. For instance, early topical (intratracheal or nebulized) steroids seem promising: they might help to limit BPD development without the side effects of systemic steroids. Evidence in favor of stem cell therapy has emerged from several preclinical trials, and from a couple of studies in humans. Mesenchymal stromal/stem cells (MSCs) have revealed a reparatory capability, preventing the progression of BPD in animal models. Administering MSC-conditioned media containing extracellular vesicles (EVs) have also demonstrated a preventive action, without the potential risks associated with unwanted engraftment or the adverse effects of administering cells. In this paper, we explore these emerging treatments and take a look at the revolutionary changes in BPD and neonatology on the horizon.

Intratracheal administration of clinical-grade mesenchymal stem cell-derived extracellular vesicles reduces lung injury in a rat model of bronchopulmonary dysplasia

Mesenchymal stem cells (MSCs) prevent the onset of bronchopulmonary dysplasia (BPD) in animal models, an effect that seems to be mediated by their secreted extracellular vesicles (EVs). The aim of this study was to compare the protective effects of intratracheally (IT) administered MSCs versus MSC-EVs in a hyperoxia-induced rat model of BPD. At birth, rats were distributed as follows: animals raised in ambient air for 2 wk ( n = 10), and animals exposed to 60% oxygen for 2 wk and treated with IT-administered physiological solution ( n = 10), MSCs ( n = 10), or MSC-EVs ( n = 10) on postnatal days 3, 7, and 10. The sham-treated hyperoxia-exposed animals showed reductions in total surface area of alveolar air spaces, and total number of alveoli ( Nalv), and an increased mean alveolar volume (Valv). EVs prompted a significant increase in Nalv ( P < 0.01) and a significant decrease in Valv ( P < 0.05) compared with sham-treated animals, whereas MSCs only significantly improved Nalv ( P < 0.05). Small pulmonary vessels of the sham-treated hyperoxia-exposed rats also showed an increase in medial thickness, which only EVs succeeded in preventing significantly ( P < 0.05). In conclusion, both EVs and MSCs reduce hyperoxia-induced damage, with EVs obtaining better results in terms of alveolarization and lung vascularization parameters. This suggests that IT-administered EVs could be an effective approach to BPD treatment.

A murine model of acute lung injury identifies growth factors to promote tissue repair and their biomarkers

Type II alveolar epithelial cells (AEC2s) play a crucial role in the regeneration of type I AECs after acute lung injury. The mechanisms underlying the regeneration of AEC2s are not fully understood. To address this issue, here, we investigated a murine model of acute lung injury using mice expressing human Diphtheria Toxin Receptor (DTR) under the control of Lysozyme M promoter (LysM-DTR). DT injection induced the depletion of AEC2s, alveolar macrophages, and bone marrow (BM)-derived myeloid cells in LysM-DTR mice, and the mice died within 6 days after DT injection. Apoptotic AEC2s and bronchiolar epithelial cells appeared at 24 hr, whereas Ki67-positive proliferating cells appeared in the alveoli and bronchioles in the lung of LysM-DTR mice at 72-96 hr after DT injection. Transfer of wild-type BM cells into LysM-DTR mice accelerated the regeneration of AEC2s along with the up-regulation of several growth factors. Moreover, several metabolites were significantly decreased in the sera of LysM-DTR mice compared with WT mice after DT injection, suggesting that these metabolites might be biomarkers to predict AEC2s injury. Together, LysM-DTR mice might be useful to identify growth factors to promote lung repair and the metabolites to predict the severity of lung injury.

Bronchopulmonary Dysplasia: An Update of Current Pharmacologic Therapies and New Approaches

Bronchopulmonary dysplasia (BPD) remains the most prevalent long-term morbidity of surviving extremely preterm infants and is associated with significant health care utilization in infancy and beyond. Recent advances in neonatal care have resulted in improved survival of extremely low birth weight (ELBW) infants; however, the incidence of BPD has not been substantially impacted by novel interventions in this vulnerable population. The multifactorial cause of BPD requires a multi-pronged approach for prevention and treatment. New approaches in assisted ventilation, optimal nutrition, and pharmacologic interventions are currently being evaluated. The focus of this review is the current state of the evidence for pharmacotherapy in BPD. Promising future approaches in need of further study will also be reviewed.

Association of the proliferation of lung fibroblasts with the ERK1/2 signaling pathway in neonatal rats with hyperoxia-induced lung fibrosis

Bronchopulmonary dysplasia (BPD) is a common, serious complication occurring in premature infants. Although clinical characteristics and pathologic changes are well described, the pathogenesis of alveolar dysplasia and interstitial fibrosis is less clear. Lung fibroblasts (LFs) are present in the extracellular matrix and serve essential roles during pulmonary epithelial injury and in response to fibrosis development in BPD. The current study investigated hyperoxia-induced proliferation of primary LFs in vitro and mechanisms that may be involved. Newborn rats were exposed to 90% oxygen, while control rats were kept in normal atmosphere. Primary LFs were isolated on postnatal day 3, 7 and 14. Hyperoxia-induced proliferation of LFs isolated on day 7 and 14 by accelerating the cell cycle progression from G1 to S phase. Collagen type I protein secretion and mRNA expression on day 7 and 14 were increased by hyperoxia compared with the controls. Hyperoxia significantly increased the phosphorylation of extracellular signal-regulated kinase (ERK) and significantly increased collagen type I expression compared with the room air control group. The findings indicated that an increase in LF proliferation in response to hyperoxia was associated with ERK1/2 phosphorylation. This mechanism may contribute to over-proliferation of LFs leading to disturbed formation of normal alveoli.

Potential Nutrients for Preventing or Treating Bronchopulmonary Dysplasia

Bronchopulmonary dysplasia (BPD) is a frequent complication occurring in extremely preterm infants. Despite recent advances in newborn medicine, the incidence of BPD does not appear to have changed markedly, and specific treatments and prevention strategies are still lacking. Nutrition plays an important role in normal lung development and maturation. Malnutrition may delay somatic growth and new alveoli development, thus aggravating pulmonary injury involved in the pathogenesis of BPD. However, few nutrients have been investigated for their potential to mitigate the pathogenesis of BPD. In this article, we reviewed the recent progress in research on potential nutrients useful for the prevention or treatment of BPD, including glutamine, cysteine and N-acetylcysteine, L-arginine and L-citrulline, long chain polyunsaturated fatty acids (LCPUFAs), inositol, selenium, and some antioxidant vitamins including vitamin A. Current evidence shows that vitamin A and LCPUFA can prevent BPD, and that L-citrulline might provide a new method to treat chronic pulmonary hypertension associated with BPD in premature infants. The effects of other nutrients on BPD prevention need to be further studied.

Altered vasoreactivity in neonatal rats with pulmonary hypertension associated with bronchopulmonary dysplasia: Implication of both eNOS phosphorylation and calcium signaling

Bronchopulmonary dysplasia (BPD) consists of an arrest of pulmonary vascular and alveolar growth, with persistent hypoplasia of the pulmonary microvasculature and alveolar simplification. In 25 to 40% of the cases, BPD is complicated by pulmonary hypertension (BPD-PH) that significantly increases the risk of morbidity. In vivo studies suggest that increased pulmonary vascular tone could contribute to late PH in BPD. Nevertheless, an alteration in vasoreactivity as well as the mechanisms involved remain to be confirmed. The purpose of this study was thus to assess changes in pulmonary vascular reactivity in a murine model of BPD-PH. Newborn Wistar rats were exposed to either room air (normoxia) or 90% O₂ (hyperoxia) for 14 days. Exposure to hyperoxia induced the well-known features of BPD-PH such as elevated right ventricular systolic pressure, right ventricular hypertrophy, pulmonary vascular remodeling and decreased pulmonary vascular density. Intrapulmonary arteries from hyperoxic pups showed decreased endothelium-dependent relaxation to acetylcholine without any alteration of relaxation to the NO-donor sodium nitroprusside. This functional alteration was associated with a decrease of lung eNOS phosphorylation at the Ser1177 activating site. In pups exposed to hyperoxia, serotonin and phenylephrine induced exacerbated contractile responses of intrapulmonary arteries as well as intracellular calcium response in pulmonary arterial smooth muscle cells (PASMC). Moreover, the amplitude of the store-operated Ca²⁺ entry (SOCE), induced by store depletion using a SERCA inhibitor, was significantly greater in PASMC from hyperoxic pups. Altogether, hyperoxia-induced BPD-PH alters the pulmonary arterial reactivity, with effects on both endothelial and smooth muscle functions. Reduced activating eNOS phosphorylation and enhanced Ca²⁺ signaling likely account for alterations of pulmonary arterial reactivity.

Nitric oxide and hyperoxic acute lung injury

Hyperoxic acute lung injury (HALI) refers to the damage to the lungs secondary to exposure to elevated oxygen partial pressure. HALI has been a concern in clinical practice with the development of deep diving and the use of normobaric as well as hyperbaric oxygen in clinical practice. Although the pathogenesis of HALI has been extensively studied, the findings are still controversial. Nitric oxide (NO) is an intercellular messenger and has been considered as a signaling molecule involved in many physiological and pathological processes. Although the role of NO in the occurrence and development of pulmonary diseases including HALI has been extensively studied, the findings on the role of NO in HALI are conflicting. Moreover, inhalation of NO has been approved as a therapeutic strategy for several diseases. In this paper, we briefly summarize the role of NO in the pathogenesis of HALI and the therapeutic potential of inhaled NO in HALI.

Fractal analysis of alveolarization in hyperoxia-induced rat models of bronchopulmonary dysplasia

No papers are available about potentiality of fractal analysis in quantitative assessment of alveolarization in bronchopulmonary dysplasia (BPD). Thus, we here performed a comparative analysis between fractal [fractal dimension ( D) and lacunarity] and stereological [mean linear intercept ( Lm), total volume of alveolar air spaces, total number of alveoli, mean alveolar volume, total volume and surface area of alveolar septa, and mean alveolar septal thickness] parameters in experimental hyperoxia-induced models of BPD. At birth, rats were distributed between the following groups: 1) rats raised in ambient air for 2 wk; 2) rats exposed to 60% oxygen for 2 wk; 3) rats raised in normoxia for 6 wk; and 4) rats exposed to 60% hyperoxia for 2 wk and to room air for further 4 wk. Normoxic 6-wk rats showed increased D and decreased lacunarity with respect to normoxic 2-wk rats, together with changes in all stereological parameters except for mean alveolar volume. Hyperoxia-exposed 2-wk rats showed significant changes only in total number of alveoli, mean alveolar volume, and lacunarity with respect to equal-in-age normoxic rats. In the comparison between 6-wk rats, the hyperoxia-exposed group showed decreased D and increased lacunarity, together with changes in all stereological parameters except for septal thickness. Analysis of receiver operating characteristic curves showed a comparable discriminatory power of D, lacunarity, and total number of alveoli; Lmand mean alveolar volume were less discriminative. D and lacunarity did not show significant changes when different segmentation thresholds were applied, suggesting that the fractal approach may be fit to automatic image analysis.

Role of Nitric Oxide Isoforms in Vascular and Alveolar Development and Lung Injury in Vascular Endothelial Growth Factor Overexpressing Neonatal Mice Lungs

Background

The role of vascular endothelial growth factor (VEGF)-induced 3 different nitric oxide synthase (NOS) isoforms in lung development and injury in the newborn (NB) lung are not known. We hypothesized that VEGF-induced specific NOS pathways are critical regulators of lung development and injury.

Methodology

We studied NB wild type (WT), lung epithelial cell-targeted VEGF165 doxycycline-inducible overexpressing transgenic (VEGFTG), VEGFTG treated with a NOS1 inhibitor (L-NIO), VEGFTG x NOS2^-/- and VEGFTG x NOS3^+/- mice in room air (RA) for 7 postnatal (PN) days. Lung morphometry (chord length), vascular markers (Ang1, Ang2, Notch2, vWF, CD31 and VE-cadherin), cell proliferation (Ki67), vascular permeability, injury and oxidative stress markers (hemosiderin, nitrotyrosine and 8-OHdG) were evaluated.

Results

VEGF overexpression in RA led to increased chord length and vascular markers at PN7, which were significantly decreased to control values in VEGFTG x NOS2^−/− and VEGFTG x NOS3^+/- lungs. However, we found no noticeable effect on chord length and vascular markers in the VEGFTG / NOS1 inhibited group. In the NB VEGFTG mouse model, we found VEGF-induced vascular permeability in the NB murine lung was partially dependent on NOS2 and NOS3-signaling pathways. In addition, the inhibition of NOS2 and NOS3 resulted in a significant decrease in VEGF-induced hemosiderin, nitrotyrosine- and 8-OHdG positive cells at PN7. NOS1 inhibition had no significant effect.

Conclusion

Our data showed that the complete absence of NOS2 and partial deficiency of NOS3 confers protection against VEGF-induced pathologic lung vascular and alveolar developmental changes, as well as injury markers. Inhibition of NOS1 does not have any modulating role on VEGF-induced changes in the NB lung. Overall, our data suggests that there is a significant differential regulation in the NOS-mediated effects of VEGF overexpression in the developing mouse lung.

Combined iNO and endothelial progenitor cells improve lung alveolar and vascular structure in neonatal rats exposed to prolonged hyperoxia

BACKGROUND

Stem cells or inhaled nitric oxide (iNO) are reported to improve lung structures in bronchopulmonary dysplasia (BPD) models. We hypothesized that combined iNO and transplanted endothelial progenitor cells (EPCs) might restore lung structure in rats after neonatal hyperoxia.

METHODS

Litters were separated into eight groups: room air, hyperoxia, hyperoxia + iNO, hyperoxia + iNO + L-NAME, hyperoxia + EPCs, hyperoxia + EPCs + L-NAME, hyperoxia + EPCs + iNO, and hyperoxia + EPCs + iNO + L-NAME. Litters were exposed to hyperoxia from the 21st day, then, sacrificed. EPCs were injected on the 21st day. L-NAME was injected daily for 7 d from the 21st day. Serum vascular endothelial growth factor (VEGF), radial alveolar count (RAC), VIII factor, EPCs engraftment, lung VEGF, VEGFR2, endothelial nitric oxide (eNOS) and SDF-1 expression, and NO production were examined.

RESULTS

Hyperoxia exposure led to air space enlargement, loss of lung capillaries, and low expression of VEGF and eNOS. Transplanted EPCs, when combined with iNO, had significantly increased engraftment in lungs, compared to EPCs alone, upon hyperoxia exposure. There was improvement in alveolarization, microvessel density, and upregulation of VEGF and eNOS proteins in the hyperoxia-exposed EPCs with iNO group, compared to hyperoxia alone.

CONCLUSION

Combined EPCs and iNO improved lung structures after neonatal hyperoxia. This was associated with the upregulation of VEGF and eNOS expression.

Animal models of bronchopulmonary dysplasia. The term rat models

Bronchopulmonary dysplasia (BPD) is the chronic lung disease of prematurity that affects very preterm infants. Although advances in perinatal care have enabled the survival of infants born as early as 23-24 wk of gestation, the challenge of promoting lung growth while protecting the ever more immature lung from injury is now bigger. Consequently, BPD remains one of the most common complications of extreme prematurity and still lacks specific treatments. Progress in our understanding of BPD and the potential of developing therapeutic strategies have arisen from large (baboons, sheep, and pigs) and small (rabbits, rats, and mice) animal models. This review focuses specifically on the use of the rat to model BPD and summarizes how the model is used in various research studies and the advantages and limitations of this particular model, and it highlights recent therapeutic advances in BPD by using this rat model.

L-citrulline provides a novel strategy for treating chronic pulmonary hypertension in newborn infants

Effective therapies are urgently needed for infants with forms of pulmonary hypertension that develop or persist beyond the first week of life. The L-arginine nitric oxide (NO) precursor, L-citrulline, improves NO signalling and ameliorates pulmonary hypertension in newborn animals. In vitro studies demonstrate that manipulating L-citrulline transport alters NO production.

CONCLUSION

Strategies that increase the supply and transport of L-citrulline merit pursuit as novel approaches to managing infants with chronic, progressive pulmonary hypertension.

Postnatal hyperoxia exposure differentially affects hepatocytes and liver haemopoietic cells in newborn rats

Premature newborns are frequently exposed to hyperoxic conditions and experimental data indicate modulation of liver metabolism by hyperoxia in the first postnatal period. Conversely, nothing is known about possible modulation of growth factors and signaling molecules involved in other hyperoxic responses and no data are available about the effects of hyperoxia in postnatal liver haematopoiesis. The aim of the study was to analyse the effects of hyperoxia in the liver tissue (hepatocytes and haemopoietic cells) and to investigate possible changes in the expression of Vascular Endothelial Growth Factor (VEGF), Matrix Metalloproteinase 9 (MMP-9), Hypoxia-Inducible Factor-1α (HIF-1α), endothelial Nitric Oxide Synthase (eNOS), and Nuclear Factor-kB (NF-kB). Experimental design of the study involved exposure of newborn rats to room air (controls), 60% O₂ (moderate hyperoxia), or 95% O₂ (severe hyperoxia) for the first two postnatal weeks. Immunohistochemical and Western blot analyses were performed. Severe hyperoxia increased hepatocyte apoptosis and MMP-9 expression and decreased VEGF expression. Reduced content in reticular fibers was found in moderate and severe hyperoxia. Some other changes were specifically produced in hepatocytes by moderate hyperoxia, i.e., upregulation of HIF-1α and downregulation of eNOS and NF-kB. Postnatal severe hyperoxia exposure increased liver haemopoiesis and upregulated the expression of VEGF (both moderate and severe hyperoxia) and eNOS (severe hyperoxia) in haemopoietic cells. In conclusion, our study showed different effects of hyperoxia on hepatocytes and haemopoietic cells and differential involvement of the above factors. The involvement of VEGF and eNOS in the liver haemopoietic response to hyperoxia may be hypothesized.

Human amniotic fluid stem cells protect rat lungs exposed to moderate hyperoxia

BACKGROUND

Treatment of bronchopulmonary dysplasia (BPD) remains as yet an unmet clinical need and recently stem cells have been proposed as a therapeutic tool in animal models. We investigated the role of amniotic fluid stem cells (AFS) in an adult rat model of hyperoxia lung injury.

METHODS

Fifty Sprague-Dawley rats were, at birth, randomly exposed to moderate hyperoxia or room air for 14 days and a single dose of human amniotic fluid stem (hAFS) or human Fibroblasts (hF), cells was delivered intratracheally (P21). At P42 animals were euthanized and lung tissue examined using histology, immunohistochemistry, PCR, and ELISA. hAFS cells characterization and homing were studied by immunofluorescence.

RESULTS

In rats treated with hAFS and hF cells 16S human rRNA fragment was detected. Despite a low level of pulmonary hAFS cell retention (1.43 ± 0.2% anti-human-mitochondria-positive cells), the lungs of the treated animals revealed higher secondary crest numbers and lower mean linear intercept and alveolar size, than those exposed to hyperoxia, those left untreated or treated with hF cells. Except for those treated with hAFS cells, moderate hyperoxia induced an increase in protein content of IL-6, IL-1β, as well as IF-γ and TGF-1β in lung tissues. High VEGF expression and arrangement of capillary architecture in hAFS cell group were also detected.

CONCLUSIONS

Treatment with hAFS cells has a reparative potential through active involvement of cells in alveolarization and angiogenesis. A downstream paracrine action was also taken into account, in order to understand the immunodulatory response.

Effects of postnatal hyperoxia exposure on the rat dentate gyrus and subventricular zone

Premature newborns may be exposed to hyperoxia in the first postnatal period, but clinical and experimental works have raised the question of oxygen toxicity for the developing brain. However, specific analysis of hyperoxia exposure on neurogenesis is still lacking. Thus, the aim of the present study was to evaluate possible changes in the morphometric parameters of the main neurogenic sites in newborn rats exposed to 60 or 95 % oxygen for the first 14 postnatal days. The optical disector, a morphometric method based upon unbiased sampling principles of stereology, was applied to analyse cell densities, total volumes, and total cell numbers of the dentate gyrus (DG) and subventricular zone (SVZ). Apoptosis and proliferation were also studied by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling method and anti-ki67 immunohistochemistry, respectively. Severe hyperoxia increased the percentage of apoptotic cells in the DG. Moderate and severe hyperoxia induced a proliferative response both in the DG and SVZ, but the two neurogenic sites showed different changes in their morphometric parameters. The DG of both the hyperoxic groups showed lower volume and total cell number than that of the normoxic one. Conversely, the SVZ of newborn rats exposed to 95 % hyperoxia showed statistically significant higher volume and total cell number than SVZ of rats raised in normoxia. Our findings indicate that hyperoxia exposure in the first postnatal period affects both the neurogenic areas, although in different ways, i.e. reduction of DG and expansion of SVZ.