Ontogeny of apoptosis during lung development

An Experimental Model of Bronchopulmonary Dysplasia Features Long-Term Retinal and Pulmonary Defects but Not Sustained Lung Inflammation

Bronchopulmonary dysplasia (BPD) is a severe lung disease that affects preterm infants receiving oxygen therapy. No standardized, clinically-relevant BPD model exists, hampering efforts to understand and treat this disease. This study aimed to evaluate and confirm a candidate model of acute and chronic BPD, based on exposure of neonatal mice to a high oxygen environment during key lung developmental stages affected in preterm infants with BPD. Neonatal C57BL/6 mouse pups were exposed to 75% oxygen from postnatal day (PN)-1 for 5, 8, or 14 days, and their lungs were examined at PN14 and PN40. While all mice showed some degree of lung damage, mice exposed to hyperoxia for 8 or 14 days exhibited the greatest septal wall thickening and airspace enlargement. Furthermore, when assessed at PN40, mice exposed for 8 or 14 days to supplemental oxygen exhibited augmented septal wall thickness and emphysema, with the severity increased with the longer exposure, which translated into a decline in respiratory function at PN80 in the 14-day model. In addition to this, mice exposed to hyperoxia for 8 days showed significant expansion of alveolar epithelial type II cells as well as the greatest fibrosis when assessed at PN40 suggesting a healing response, which was not seen in mice exposed to high oxygen for a longer period. While evidence of lung inflammation was apparent at PN14, chronic inflammation was absent from all three models. Finally, exposure to high oxygen for 14 days also induced concurrent outer retinal degeneration. This study shows that early postnatal exposure to high oxygen generates hallmark acute and chronic pathologies in mice that highlights its use as a translational model of BPD.

Time-restricted feeding ameliorates maternal high-fat diet-induced fetal lung injury

Maternal inflammation ensuing from high-fat diet (HFD) intake during pregnancy is related to spontaneous preterm birth and respiratory impairment among premature infants. Recently, a circadian aligned dietary intervention referred to as Time-restricted feeding (TRF) has been reported to have beneficial metabolic effects. This study aimed to assess the effects of maternal TRF on fetal lung injury caused by maternal HFD intake. Female Wistar rats were kept on following three dietary regimens; Ad libitum normal chow diet (NCD-AL), Ad libitum HFD (HFD-AL) and Time-restricted fed HFD (HFD-TRF) from 5 months before mating and continued through pregnancy. Fetal lung samples were collected on the embryonic day 18.5, and apoptotic and inflammatory markers were assessed using TUNEL assay, western blotting, and qRT-PCR. Our results showed that TRF considerably prevented maternal HFD-induced apoptosis in fetal lung tissue that corroborated with a reduction in caspase activation and increased levels of anti-apoptotic BCL2 family proteins together with a lower level of ER-stress and autophagy markers including ATF6, CHOP and LC3-II. Besides, fetal lungs from HFD-TRF dams exhibited reduced expression of inflammatory genes that correlated with reduction and apoptotic injury throughout fetal development. Our results thus put forth TRF as a unique non-pharmacological approach to boost perinatal health beneath metabolic stress.

GORAB promotes embryonic lung maturation through antagonizing AKT phosphorylation, versican expression, and mesenchymal cell migration

Embryonic development of the alveolar sac of the lung is dependent upon multiple signaling pathways to coordinate cell growth, migration, and the formation of the extracellular matrix. Here, we identify GORAB as a regulator of embryonic alveolar sac formation as genetically disrupting the Gorab gene in mice resulted in fatal saccular maturation defects characterized by a thickened lung mesenchyme. This abnormality is not associated with impairments in cellular proliferation and death, but aberrantly increased protein kinase B (AKT) phosphorylation, elevated Vcan transcription, and enhanced migration of mesenchymal fibroblasts. Genetically augmenting PDGFRα, a potent activator of AKT in lung mesenchymal cells, recapitulated the alveolar phenotypes, whereas disrupting PDGFRα partially rescued alveolar phenotypes in Gorab-deficient mice. Overexpressing or suppressing Vcan in primary embryonic lung fibroblasts could, respectively, mimic or attenuate alveolar sac-like phenotypes in a co-culture model. These findings suggest a role of GORAB in negatively regulating AKT phosphorylation, the expression of Vcan, and the migration of lung mesenchyme fibroblasts, and suggest that alveolar sac formation resembles a patterning event that is orchestrated by molecular signaling and the extracellular matrix in the mesenchyme.

β1 Integrin regulates adult lung alveolar epithelial cell inflammation

Integrins, the extracellular matrix receptors that facilitate cell adhesion and migration, are necessary for organ morphogenesis; however, their role in maintaining adult tissue homeostasis is poorly understood. To define the functional importance of β1 integrin in adult mouse lung, we deleted it after completion of development in type 2 alveolar epithelial cells (AECs). Aged β1 integrin-deficient mice exhibited chronic obstructive pulmonary disease-like (COPD-like) pathology characterized by emphysema, lymphoid aggregates, and increased macrophage infiltration. These histopathological abnormalities were preceded by β1 integrin-deficient AEC dysfunction such as excessive ROS production and upregulation of NF-κB-dependent chemokines, including CCL2. Genetic deletion of the CCL2 receptor, Ccr2, in mice with β1 integrin-deficient type 2 AECs impaired recruitment of monocyte-derived macrophages and resulted in accelerated inflammation and severe premature emphysematous destruction. The lungs exhibited reduced AEC efferocytosis and excessive numbers of inflamed type 2 AECs, demonstrating the requirement for recruited monocytes/macrophages in limiting lung injury and remodeling in the setting of a chronically inflamed epithelium. These studies support a critical role for β1 integrin in alveolar homeostasis in the adult lung.

Vascular Niche in Lung Alveolar Development, Homeostasis, and Regeneration

Endothelial cells (ECs) constitute small capillary blood vessels and contribute to delivery of nutrients, oxygen and cellular components to the local tissues, as well as to removal of carbon dioxide and waste products from the tissues. Besides these fundamental functions, accumulating evidence indicates that capillary ECs form the vascular niche. In the vascular niche, ECs reciprocally crosstalk with resident cells such as epithelial cells, mesenchymal cells, and immune cells to regulate development, homeostasis, and regeneration in various organs. Capillary ECs supply paracrine factors, called angiocrine factors, to the adjacent cells in the niche and orchestrate these processes. Although the vascular niche is anatomically and functionally well-characterized in several organs such as bone marrow and neurons, the effects of endothelial signals on other resident cells and anatomy of the vascular niche in the lung have not been well-explored. This review discusses the role of alveolar capillary ECs in the vascular niche during development, homeostasis and regeneration.

Autophagy is required for lung development and morphogenesis

Bronchopulmonary dysplasia (BPD) remains a major respiratory illness in extremely premature infants. The biological mechanisms leading to BPD are not fully understood, although an arrest in lung development has been implicated. The current study aimed to investigate the occurrence of autophagy in the developing mouse lung and its regulatory role in airway branching and terminal sacculi formation. We found 2 windows of epithelial autophagy activation in the developing mouse lung, both resulting from AMPK activation. Inhibition of AMPK-mediated autophagy led to reduced lung branching in vitro. Conditional deletion of beclin 1 (Becn1) in mouse lung epithelial cells (Becn1Epi-KO), either at early (E10.5) or late (E16.5) gestation, resulted in lethal respiratory distress at birth or shortly after. E10.5 Becn1Epi-KO lungs displayed reduced airway branching and sacculi formation accompanied by impaired vascularization, excessive epithelial cell death, reduced mesenchymal thinning of the interstitial walls, and delayed epithelial maturation. E16.5 Becn1Epi-KO lungs had reduced terminal air sac formation and vascularization and delayed distal epithelial differentiation, a pathology similar to that seen in infants with BPD. Taken together, our findings demonstrate that intrinsic autophagy is an important regulator of lung development and morphogenesis and may contribute to the BPD phenotype when impaired.

Oxidative stress and NF-κB signaling are involved in LPS induced pulmonary dysplasia in chick embryos

Inflammation or dysbacteriosis-derived lipopolysaccharides (LPS) adversely influence the embryonic development of respiratory system. However, the precise pathological mechanisms still remain to be elucidated. In this study, we demonstrated that LPS exposure caused lung maldevelopment in chick embryos, including higher embryo mortality, increased thickness of alveolar gas exchange zone, and accumulation of PAS⁺ immature pulmonary cells, accompanied with reduced expression of alveolar epithelial cell markers and lamellar body count. Upon LPS exposure, pulmonary cell proliferation was significantly altered and cell apoptosis was inhibited as well, indicating a delayed progress of pulmonary development. LPS treatment also resulted in reduced CAV-1 expression and up-regulation of Collagen I, suggesting increased lung fibrosis, which was verified by Masson staining. Moreover, LPS induced enhanced Nrf2 expression in E18 lungs, and the increased reactive oxygen species (ROS) production was confirmed in MLE-12 cells in vitro. Antioxidant vitamin C restored the LPS induced down-regulation of ABCA3, SP-C and GATA-6 in MLE-12 cells. Furthermore, LPS induced activation of NF-κB signaling in MLE-12 cells, and the LPS-induced decrease in SP-C expression was partially abrogated by blocking NF-κB signaling with Bay-11-7082. Bay-11-7082 also inhibited LPS-induced increases of ROS and Nrf2 expression. Taken together, we have demonstrated that oxidative stress and NF-κB signaling are involved in LPS induced disruption of pulmonary cell development in chick embryos.

Lipidomics reveals dramatic lipid compositional changes in the maturing postnatal lung

Lung immaturity is a major cause of morbidity and mortality in premature infants. Understanding the molecular mechanisms driving normal lung development could provide insights on how to ameliorate disrupted development. While transcriptomic and proteomic analyses of normal lung development have been previously reported, characterization of changes in the lipidome is lacking. Lipids play significant roles in the lung, such as dipalmitoylphosphatidylcholine in pulmonary surfactant; however, many of the roles of specific lipid species in normal lung development, as well as in disease states, are not well defined. In this study, we used liquid chromatography-mass spectrometry (LC-MS/MS) to investigate the murine lipidome during normal postnatal lung development. Lipidomics analysis of lungs from post-natal day 7, day 14 and 6–8 week mice (adult) identified 924 unique lipids across 21 lipid subclasses, with dramatic alterations in the lipidome across developmental stages. Our data confirmed previously recognized aspects of post-natal lung development and revealed several insights, including in sphingolipid-mediated apoptosis, inflammation and energy storage/usage. Complementary proteomics, metabolomics and chemical imaging corroborated these observations. This multi-omic view provides a unique resource and deeper insight into normal pulmonary development.

Alveologenesis: key cellular players and fibroblast growth factor 10 signaling

Background

Alveologenesis is the last stage in lung development and is essential for building the gas-exchanging units called alveoli. Despite intensive lung research, the intricate crosstalk between mesenchymal and epithelial cell lineages during alveologenesis is poorly understood. This crosstalk contributes to the formation of the secondary septae, which are key structures of healthy alveoli.

Conclusions

A better understanding of the cellular and molecular processes underlying the formation of the secondary septae is critical for the development of new therapies to protect or regenerate the alveoli. This review summarizes briefly the alveologenesis process in mouse and human. Further, it discusses the current knowledge on the epithelial and mesenchymal progenitor cells during early lung development giving rise to the key cellular players (e.g., alveolar epithelial cell type I, alveolar epithelial cell type II, alveolar myofibroblast, lipofibroblast) involved in alveologenesis. This review focusses mainly on the role of fibroblast growth factor 10 (FGF10), one of the most important signaling molecules during lung development, in epithelial and mesenchymal cell lineage formation.

Proliferation, apoptosis and expression of matrix metalloproteinase-9 in human fetal lung

Expression pattern of the Ki-67, caspase-3 and matrix metalloproteinases-9 (MMP-9) factors were immunohistochemically analyzed in 48 human fetal lungs from 12 to 40 weeks of gestation. The number of Ki-67 positive cells in the epithelium of canaliculare (88cells/mm(2)) and sacculare stage (93cells/mm(2)) were significantly higher than in the epithelium of pseudoglandular stage (12cells/mm(2)) (p=0.0008 vs. p=0.003). The number of Ki-67 positive cells in the mesenchyme of canaliculare stage (132cells/mm(2)) was significantly higher than in the mesenchyme of pseudoglandular stage (37cells/mm(2)) (p=0.001). The proliferation of mesenchymal cells was higher than the epithelial cells in all developmental stages, especially in the canaliculare stage (p=0.007). Similarly, the number of caspase-3 positive cells in the epithelium of canalicular stage (13cells/mm(2)) was significantly higher than in the epithelium of pseudoglandular stage (6cells/mm(2)) (p=0.002) with peaks in the conductive epithelium of canalicular stage. The number of caspase-3 positive cells in the mesenchyme of canaliculare stage (3cells/mm(2)) was significantly higher than in the mesenchyme of saccular stage (0cells/mm(2)) (p=0.05). There were no caspase-3 positive cells in the mesenchyme of pseudoglandular stage. However, unlike the Ki-67 expression, mesenchymal cells in comparison to epithelial cells express substantially less caspase-3 in all developmental stages. Up to the saccular stage, the expression of MMP-9 in mesenchymal cells showed a linear increase with most pronounced expression in that stage. The number of MMP-9 positive cells in the mesenchyme of canaliculare (20cells/mm(2)) and sacculare (39cells/mm(2)) stage were significantly higher than in the mesenchyme of pseudoglandular stage (12cells/mm(2)) (p=0.04 vs. p=0.004). The first epithelial cells that express MMP-9 were present only at the alveolar stage. Increased proliferation and apoptosis of the mesenchymal cells of canalicular stage is important for formation of definite structures within the stroma of the lung parenchyma. Although apoptosis in the epithelium is not pronounced as proliferation, it is important for thinning of the epithelium and consequent spread of respiratory tract. However in the saccular stage when mesenchyme disappears, MMP-9 expression is more important for primitive alveoli differentiation.

A breath of fresh air on the mesenchyme: impact of impaired mesenchymal development on the pathogenesis of bronchopulmonary dysplasia

The early mouse embryonic lung, with its robust and apparently reproducible branching pattern, has always fascinated developmental biologists. They have extensively used this embryonic organ to decipher the role of mammalian orthologs of Drosophila genes in controlling the process of branching morphogenesis. During the early pseudoglandular stage, the embryonic lung is formed mostly of tubes that keep on branching. As the branching takes place, progenitor cells located in niches are also amplified and progressively differentiate along the proximo-distal and dorso-ventral axes of the lung. Such elaborate processes require coordinated interactions between signaling molecules arising from and acting on four functional domains: the epithelium, the endothelium, the mesenchyme, and the mesothelium. These interactions, quite well characterized in a relatively simple lung tubular structure remain elusive in the successive developmental and postnatal phases of lung development. In particular, a better understanding of the process underlying the formation of secondary septa, key structural units characteristic of the alveologenesis phase, is still missing. This structure is critical for the formation of a mature lung as it allows the subdivision of saccules in the early neonatal lung into alveoli, thereby considerably expanding the respiratory surface. Interruption of alveologenesis in preterm neonates underlies the pathogenesis of chronic neonatal lung disease known as bronchopulmonary dysplasia. De novo formation of secondary septae appears also to be the limiting factor for lung regeneration in human patients with emphysema. In this review, we will therefore focus on what is known in terms of interactions between the different lung compartments and discuss the current understanding of mesenchymal cell lineage formation in the lung, focusing on secondary septae formation.

Epithelial-mesenchymal co-culture model for studying alveolar morphogenesis

Division of large, immature alveolar structures into smaller, more numerous alveoli increases the surface area available for gas exchange. Alveolar division requires precise epithelial-mesenchymal interactions. However, few experimental models exist for studying how these cell-cell interactions produce changes in 3-dimensional structure. Here we report an epithelial-mesenchymal cell co-culture model where 3-dimensional peaks form with similar cellular orientation as alveolar structures in vivo. Co-culturing fetal mouse lung mesenchyme with A549 epithelial cells produced tall peaks of cells covered by epithelia with cores of mesenchymal cells. These structures did not form when using adult lung fibroblasts. Peak formation did not require localized areas of cell proliferation or apoptosis. Mesenchymal cells co-cultured with epithelia adopted an elongated cell morphology closely resembling myofibroblasts within alveolar septa in vivo. Because inflammation inhibits alveolar formation, we tested the effects of E. coli lipopolysaccharide on 3-dimensional peak formation. Confocal and time-lapse imaging demonstrated that lipopolysaccharide reduced mesenchymal cell migration, resulting in fewer, shorter peaks with mesenchymal cells present predominantly at the base. This epithelial-mesenchymal co-culture model may therefore prove useful in future studies of mechanisms regulating alveolar morphogenesis.

Caspase 3 activity in isolated fetal rat lung fibroblasts and rat periodontal ligament fibroblasts: cigarette smoke induced alterations

Background

Cigarette smoking is the leading cause of preventable death and has been implicated in pathogenesis of pulmonary, oral and systemic diseases. Smoking during pregnancy is a risk factor for the developing fetus and may be a major cause of infant mortality. Moreover, the oral cavity, and all cells within are the first to be exposed to cigarette smoke and may be a possible source for the spread of toxins to other organs of the body. Fibroblasts in general are morphologically heterogeneous connective tissue cells with diverse functions. Apoptosis or programmed cell death is a crucial process during embryogenesis and for the maintenance of homeostasis throughout life. Deregulation of apoptosis has been implicated in abnormal lung development in the fetus and disease progression in adults. Caspases are proteases which belong to the family of cysteine aspartic acid proteases and are key components for downstream amplification of intracellular apoptotic signals. Of 14 known caspases, caspase-3 is the key executioner of apoptosis. In the present study we explored the hypothesis that cigarette smoke (CS) extract activates caspase-3 in two types of fibroblasts, both of which would be exposed directly to cigarette smoke, isolated fetal rat lung fibroblasts and adult rat periodontal ligament (PDL) fibroblasts.

Methods

Isolated fetal rat lung fibroblasts and adult PDLs were used. Cells were exposed to different concentrations of CS for 60 min. Caspase-3 activity and its inhibition by Z-VAD-fmk were measured by caspase-3 fluorometric assay. The effect of CSE on cellular viability was measured using the MTT formazan assay. Caspase-3 expression was detected by western blot analysis and cellular localization of caspase-3 was determined by immunofluorescence using fluorescence microscopy.

Results

It was observed in fetal rat lung fibroblast cells that CSE extract significantly (p<0.05) increased caspase-3 activity and decrease cell proliferation. However, no significant changes in activity or viability were observed in PDLs.

Conclusions

This indicates CS activates caspase-3 the key regulatory point in apoptosis in fetal rat lung fibroblast cells suggesting that smoking during pregnancy may alter the developmental program of fetal lung, jeopardizing the establishment of critical cellular mechanisms necessary to expedite pulmonary maturation at birth.of critical cellular mechanisms necessary to expedite pulmonary maturation at birth.

Mechanical ventilation-induced apoptosis in newborn rat lung is mediated via FasL/Fas pathway

Mechanical ventilation induces pulmonary apoptosis and inhibits alveolar development in preterm infants, but the molecular basis for the apoptotic injury is unknown. The objective was to determine the signaling mechanism(s) of ventilation (stretch)-induced apoptosis in newborn rat lung. Seven-day-old rats were ventilated with room air for 24 h using moderate tidal volumes (8.5 ml/kg). Isolated fetal rat lung epithelial and fibroblast cells were subjected to continuous cyclic stretch (5, 10, or 17% elongation) for up to 12 h. Prolonged ventilation significantly increased the number of apoptotic alveolar type II cells (i.e., terminal deoxynucleotidyl transferase dUTP-mediated nick-end labeling and anti-cleaved caspase-3 immunochemistry) and was associated with increased expression of the apoptotic mediator Fas ligand (FasL). Fetal lung epithelial cells, but not fibroblasts, subjected to maximal (i.e., 17%, but not lesser elongation) cyclic stretch exhibited increased apoptosis (i.e., nuclear fragmentation and DNA laddering), which appeared to be mediated via the extrinsic pathway (increased expression of FasL and cleaved caspase-3, -7, and -8). The intrinsic pathway appeared not to be involved [minimal mitochondrial membrane depolarization (JC-1 flow analysis) and no activation of caspase-9]. Universal caspases inhibition and neutralization of FasL abrogated the stretch-induced apoptosis. Prolonged mechanical ventilation induces apoptosis of alveolar type II cells in newborn rats and the mechanism appears to involve activation of the extrinsic death pathway via the FasL/Fas system.

IL-1β expression in the distal lung epithelium disrupts lung morphogenesis and epithelial cell differentiation in fetal mice

Perinatal inflammation and the inflammatory cytokine IL-1 can modify lung morphogenesis. To examine the effects of antenatal expression of IL-1β in the distal airway epithelium on fetal lung morphogenesis, we studied lung development and surfactant expression in fetal mice expressing human IL-1β under the control of the surfactant protein (SP)-C promoter. IL-1β-expressing pups suffered respiratory failure and died shortly after birth. IL-1β caused fetal lung inflammation and enhanced the expression of keratinocyte-derived chemokine (KC/CXCL1) and monocyte chemoattractant protein 3 (MCP-3/CCL7), the calgranulins S100A8 and S100A9, the acute-phase protein serum amyloid A3, the chitinase-like proteins Ym1 and Ym2, and pendrin. IL-1β decreased the percentage of the total distal lung area made up of air saccules and the number of air saccules in the lungs of fetal mice. IL-1β inhibited the expression of VEGF-A and its receptors VEGFR-1 and VEGFR-2. The percentage of the cellular area of the distal lung made up of capillaries was decreased in IL-1β-expressing fetal mice. IL-1β suppressed the production of SP-B and pro-SP-C and decreased the amount of phosphatidylcholine and the percentage of palmitic acid in the phosphatidylcholine fraction of lung phospholipids, indicating that IL-1β prevented the differentiation of type II epithelial cells. The production of Clara cell secretory protein in the nonciliated bronchiolar (Clara) cells was likewise suppressed by IL-1β. In conclusion, expression of IL-1β in the epithelium of the distal airways disrupted the development of the airspaces and capillaries in the fetal lung and caused fatal respiratory failure at birth.

The role of glycoprotein 130 family of cytokines in fetal rat lung development

The glycoprotein 130 (gp130) dependent family of cytokines comprises interleukin-6 (IL-6), IL-11, leukemia inhibitory factor (LIF), cardiotrophin-like cytokine (CLC), ciliary neurotrophic factor (CNTF), cardiotrophin-1 (CT-1) and oncostatin M (OSM). These cytokines share the membrane gp130 as a common signal transducer. Recently, it was demonstrated that IL-6 promotes, whereas LIF inhibits fetal lung branching. Thus, in this study, the effects on fetal lung morphogenesis of the other classical members of the gp130-type cytokines (IL-11, CLC, CNTF, CT-1 and OSM) were investigated. We also provide the first description of these cytokines and their common gp130 receptor protein expression patterns during rat lung development. Fetal rat lung explants were cultured in vitro with increasing concentrations of IL-11, CLC, CNTF, CT-1 and OSM. Treated lung explants were morphometrically analyzed and assessed for MAPK, PI3K/AKT and STAT3 signaling modifications. IL-11, which similarly to IL-6 acts through a gp130 homodimer receptor, significantly stimulated lung growth via p38 phosphorylation. On the other hand, CLC, CNTF, CT-1 and OSM, whose receptors are gp130 heterodimers, inhibited lung growth acting in different signal-transducing pathways. Thus, the present study demonstrated that although cytokines of the gp130 family share a common signal transducer, there are specific biological activities for each cytokine on lung development. Indeed, cytokine signaling through gp130 homodimers stimulate, whereas cytokine signaling through gp130 heterodimers inhibit lung branching.

Group B Streptococcus induces a caspase-dependent apoptosis in fetal rat lung interstitium

Group B Streptococcus (GBS) is an important pathogen and is associated with sepsis and meningitis in neonates and infants. An ex vivo model that facilitates observations of GBS interactions with multiple host cell types over time was used to study its pathogenicity. GBS infections were associated with profound reductions in fetal lung; explant size, and airway branching. Elevated levels of apoptosis subsequent to GBS infections were observed by whole-mount confocal immunofluorescence using activated-caspase-3-antibodies and terminal deoxynucleotidyl transferase dUTP nick end-labeling (TUNEL) assays. The caspase inhibitor Z-VAD-FMK abolished the increase in TUNEL-positive cells associated with GBS infections, indicating that the GBS-induced apoptosis was caspase-dependent. Digital image analyses revealed that both GBS and the active form of caspase-3 were distributed primarily within the lung interstitium, suggesting that these tissues are important targets for GBS. Antibodies to the active form of caspase-3 colocalized with both macrophage- and erythroblast-markers, suggesting that these hematopoietic cells are vulnerable to GBS-mediated pathogenesis. These studies suggest that GBS infections profoundly alter lung morphology and caspase-dependent hematopoietic cell apoptosis within the lung interstitium play roles in GBS pathophysiology in this model.

Lipid hydroperoxide formation regulates postnatal rat lung cell apoptosis and alveologenesis

An acute increase in oxygen tension after birth imposes an oxidative stress upon the lung. We hypothesized that the resultant increase in reactive oxygen species, specifically lipid hydroperoxides, would trigger postnatal alveologenesis and physiological lung cell apoptosis in the neonatal rat. Neonatal rats were either untreated or treated daily with subcutaneous vehicle or diphenyl phenyl diamine, a scavenger of lipid hydroperoxides and inhibitor of lipid peroxidation, from day 1 to 6 of life. Alveolar formation and physiological lung cell apoptosis were assessed by morphometry, immunohistochemistry, and Western blot analyses on day 7 samples. Substitution experiments were conducted using the prototypic lipid hydroperoxide t-butylhydroperoxide. At a minimum effective dose of 15μg/g body wt, treatment with diphenyl phenyl diamine resulted in a significant increase in tissue fraction and mean linear intercept and significant reductions in small peripheral blood vessels, secondary crest formation, lung and secondary crest cell DNA synthesis, and estimated alveolar number. Decreased numbers of apoptotic type II pneumocytes and mesenchymal cells, and decreased contents of proapoptotic cleaved caspase-3 and -7 and cytoplasmic cytochrome c, and an increase in antiapoptotic Bcl-xL were found in lungs treated with diphenyl phenyl diamine. A prevention of selected changes induced by diphenyl phenyl diamine was observed with concurrent treatment with intraperitoneal t-butylhydroperoxide, at a minimally effective dose of 187μg/g body wt. We conclude that oxidative stress after birth induces lipid hydroperoxide formation, which, in turn, triggers postnatal alveologenesis and physiological lung cell apoptosis in the neonatal rat.

Antenatal imatinib treatment reduces pulmonary vascular remodeling in a rat model of congenital diaphragmatic hernia

The pathophysiology of congenital diaphragmatic hernia (CDH) is constituted by pulmonary hypoplasia and pulmonary hypertension (PH). We previously reported successful treatment with imatinib of a patient with CDH. This study examines the effect of antenatal imatinib administration on the pulmonary vasculature in a rat model of CDH. Pregnant rats were given nitrofen to induce CDH. Controls were given olive oil. Half of the CDH fetuses and half of the controls were treated with imatinib antenatally E17-E21, rendering four groups: Control, Control+Imatinib, CDH, and CDH+Imatinib. Lung sections were obtained for morphometry and immunohistochemistry, and protein was purified for Western blot. Effects of nitrofen and imatinib on Ki-67, caspase-3, PDGF-B, and PDGF receptors were analyzed. Imatinib significantly reduced medial wall thickness in pulmonary arteries of rats with CDH. It also normalized lumen area and reduced the proportion of fully muscularized arteries. Imatinib also caused medial thinning in the control group. Cell proliferation was increased in CDH, and this proliferation was significantly reduced by imatinib. PDGF-B and PDGFR-β were upregulated in CDH, and imatinib treatment resulted in a downregulation. PDGFR-α remained unchanged in CDH but was significantly downregulated by imatinib. Antenatal imatinib treatment reduces development of medial wall thickness and restores lumen area in pulmonary arteries in nitrofen-induced CDH. The mechanism is reduced cell proliferation. Imatinib is an interesting candidate for antenatal therapy for PH in CDH, but potential side effects need to be investigated and more specific targeting of PDGF signaling is needed.

Fetal hyperglycemia alters lung structural development in neonatal rat

Maternal diabetes is associated with increased risk for abnormal fetal organogenesis, but its effects on the developing lungs are still insufficiently known. To determine the effect of maternal hyperglycemia on postnatal lung development, we studied lung structural and cellular changes in newborn rats exposed to intrauterine hyperglycemia. We induced hyperglycemia in Sprague-Dawley rats with i.p. streptozotocin before pregnancy and allowed the hyperglycemic and control dams deliver at term. Lungs were obtained on postnatal day (d) 0, d7, and d14 and analyzed for lung weight and morphology, as well as cellular apoptosis (TUNEL staining) and proliferation (PCNA staining). Quantitative micro-CT analysis of the lung vasculature was additionally performed at d14. At birth, maternal hyperglycemia resulted in decreased relative lung weight, thinner alveolar septa and increased cellular apoptosis and proliferation, when compared to controls. At 1 and 2 weeks of age pulmonary cell apoptosis and alveolar chord length remained unchanged, but cell proliferation and number of secondary crests were increased in the hyperglycemia-exposed neonatal lungs in comparison with the controls. Density of small arterioles on histological examination and the structure of pulmonary arterial vasculature in micro-CT analysis of the neonatal lungs were not influenced by maternal hyperglycemia. Our results suggest, that maternal hyperglycemia is related to developmental structural alterations in postnatal rat lungs. These early changes may reflect aberrant maturational adaptation in response to the hyperglycemic fetal environment.

Mechanostasis in apoptosis and medicine

Mechanostasis describes a complex and dynamic process where cells maintain equilibrium in response to mechanical forces. Normal physiological loading modes and magnitudes contribute to cell proliferation, tissue growth, differentiation and development. However, cell responses to abnormal forces include compensatory apoptotic mechanisms that may contribute to the development of tissue disease and pathological conditions. Mechanotransduction mechanisms tightly regulate the cell response through discrete signaling pathways. Here, we provide an overview of links between pro- and anti-apoptotic signaling and mechanotransduction signaling pathways, and identify potential clinical applications for treatments of disease by exploiting mechanically-linked apoptotic pathways.

Cathepsin K deficiency aggravates lung injury in hyperoxia-exposed newborn mice

Cathepsin K (CatK) is a potent collagenase and elastase and may be involved in the development of neonatal bronchopulmonary dysplasia. The authors evaluated the effects of CatK deletion on neonatal lung development and response to prolonged hyperoxic challenge. CatK deficiency resulted in thinner alveolar walls than wild-type littermates on postnatal day (PN) 7. However, no morphological difference could be detected between CatK-deficient and control groups on PN 14. Exposure to 90% oxygen for 7 days after birth caused intensive CatK expression in the bronchial epithelium and alveolar macrophages of wild-type mice. Hyperoxia caused fatal respiratory distress in both groups of mice. However, whereas ∼20% of wild-type mice survived for 2 weeks in hyperoxia, all CatK-deficient mice died within the first 9 postnatal days. Hyperoxia-exposed lungs of CatK-deficient mice contained high number of macrophages and multinucleated giant cells and had increased content of reduced glutathione, indicating intensified pulmonary oxidative stress. These results suggest that CatK is involved in pulmonary development and it may be an important host-defence protease in the oxygen-stressed newborn lung.

Mesenchymal nuclear factor I B regulates cell proliferation and epithelial differentiation during lung maturation

The Nuclear factor I (NFI) transcription factor family consists of four genes (Nfia, Nfib, Nfic and Nfix) that regulate the development of multiple organ systems in mice and humans. Nfib is expressed in both lung mesenchyme and epithelium and mice lacking Nfib have severe lung maturation defects and die at birth. Here we continue our analysis of the phenotype of Nfib⁻/⁻ lungs and show that Nfib specifically in lung mesenchyme controls late epithelial and mesenchymal cell proliferation and differentiation. There are more PCNA, BrdU, PHH3 and Ki67 positive cells in Nfib⁻/⁻ lungs than in wild type lungs at E18.5 and this increase in proliferation marker expression is seen in both epithelial and mesenchymal cells. The loss of Nfib in all lung cells decreases the expression of markers for alveolar epithelial cells (Aqp5 and Sftpc), Clara cells (Scgb1a1) and ciliated cells (Foxj1) in E18.5 lungs. To test for a specific role of Nfib in lung mesenchyme we generated and analyzed Nfib(flox/flox), Dermo1-Cre mice. Loss of Nfib only in mesenchyme results in decreased Aqp5, Sftpc and Foxj1 expression, increased cell proliferation, and a defect in sacculation similar to that seen in Nfib⁻/⁻ mice. In contrast, mesenchyme specific loss of Nfib had no effect on the expression of Scgb1a1 in the airway. Microarray and QPCR analyses indicate that the loss of Nfib in lung mesenchyme affects the expression of genes associated with extracellular matrix, cell adhesion and FGF signaling which could affect distal lung maturation. Our data indicate that mesenchymal Nfib regulates both mesenchymal and epithelial cell proliferation through multiple pathways and that mesenchymal NFI-B-mediated signals are essential for the maturation of distal lung epithelium.

Early alterations in cytokine expression in adult compared to developing lung in mice after radiation exposure

To assess early changes in the lung after low-dose radiation exposure that may serve as targets for mitigation of lung injury in the aftermath of a terrorist event, we analyzed cytokine expression after irradiation. Adult mice were studied after whole-lung or total-body irradiation. Mouse pups of different ages were also investigated after total-body irradiation. mRNA abundance was analyzed in tissue and plasma, and pathological changes were assessed. In lung tissue, dose-related changes were seen in IL1B, IL1R2 and CXCR2 mRNA expression at 1 and 6 h after irradiation, concurrent with increases in plasma protein levels of KC/CXCL1 and IL6. However, in the pups, changes in IL1 abundance were not detected until 28 days of age, coincident with the end of postnatal lung growth, although apoptosis was detected at all ages. In conclusion, although cytokines were expressed after low doses of radiation, their role in the progression of tissue response is yet to be determined. They may be candidates for use in marker-based biodosimetry. However, the lack of cytokine induction in early life suggests that different end points (and mitigating treatments) may be required for children.

Spatiotemporal alteration in phosphatidylinositide 3-kinase-serine/threonine protein kinase B signaling in the nitrofen-induced hypoplastic lung

PURPOSE

The pathogenesis of pulmonary hypoplasia in congenital diaphragmatic hernia (CDH) is not fully understood. The serine/threonine protein kinase B (AKT) plays important roles for lung morphogenesis through epithelial-mesenchymal interaction in phosphatidylinositide 3-kinase (PI3K)-dependent manner. It has been reported that the lung explant morphogenesis in mice is interfered by inhibitors of the PI3K-AKT pathway. We hypothesized that PI3K and AKT gene and protein expression/distribution are altered during epithelial morphogenesis in the nitrofen-induced hypoplastic lung.

METHODS

Pregnant rats were exposed to either olive oil or nitrofen on day 9 of gestation (D9). Fetal lungs were harvested on D15, D18, and D21 and divided into 3 groups as follows: control, nitrofen with CDH (CDH[-]), and nitrofen without CDH (CDH[+]) (n = 8 at each time-point, respectively). Reverse transcription polymerase chain reaction and immunohistochemistry were performed.

RESULTS

Messenger RNA expression levels of PI3K at D21 was significantly decreased in CDH(-) and CDH(+) group (5.71 +/- 0.85 and 6.80 +/- 0.88, respectively) compared to controls (8.95 +/- 3.22; P < .05). Messenger RNA levels of AKT were also significantly decreased at D18 in CDH(-) and CDH(+) lungs (1.21 +/- 0.16 and 1.20 +/- 0.32, respectively) compared to controls (1.62 +/- 0.14; P < .01). The PI3K immunoreactivity was diminished in the distal epithelium at D18 and decreased in the overall intensity at D21 in hypoplastic lungs compared to controls. The AKT immunoreactivity was decreased in mesenchyme at D18 and decreased overall intensity at D21 in CDH lungs compared to controls.

CONCLUSION

Spatiotemporal alteration of pulmonary PI3K and AKT gene and protein expression during epithelial morphogenesis may interfere with epithelial-mesenchymal interaction, causing pulmonary hypoplasia in CDH by disrupting PI3K-AKT signaling pathway.

Androgen receptor and 17beta-HSD type 2 regulation in neonatal mouse lung development

A QPCR analysis of androgen receptor and several androgen metabolizing genes was performed during the saccular and alveolar stages of mouse lung development. Androgen receptor expression showed a statistically significant increase during the alveolar stage while levels of 17beta-hydroxysteroid dehydrogenase type 2 (17beta-HSD 2) expression significantly decreased at the end of the saccular stage and remained low throughout the alveolar period. 17beta-HSD 1, 17beta-HSD 5, 5alpha-reductase type 1, and mouse 3alpha-HSD did not present such a regulation. The androgen receptor protein was primarily detected in the nucleus of airway epithelial cells and of a subset of respiratory epithelial cells. 17beta-HSD 2 mRNA co-localized with androgen receptor protein during saccularization, but was absent from airway epithelium during alveolarization. Taken together, our results demonstrate temporal and spatial regulation of androgen receptor and 17beta-HSD 2 during the sacculo-alveolar transition period of mouse lung development suggesting control of androgen action.

Histologic resolution of pulmonary interstitial glycogenosis

Pulmonary interstitial glycogenosis (PIG) is an enigmatic lung disorder of unknown etiology that presents with neonatal respiratory distress. Despite its dramatic clinical presentation, the diagnosis of PIG has a favorable prognosis with rare mortality in the absence of comorbid conditions. In this report, we describe changes in successive lung biopsies in a neonate who presented with respiratory failure and pulmonary hypertension. Diagnostic lung biopsy at 10 days of age exhibited classic histologic and ultrastructural findings of PIG with diffuse expansion of the alveolar interstitium by glycogenated mesenchymal cells. Subsequent to the patient's clinical improvement, a repeat biopsy at 49 days of age showed significant resolution of the disorder. Colocalization of vimentin-immunopositive cells with both phospho-histone H3 and cleaved caspase-3 demonstrated prominent attenuation of mesenchymal cell proliferation and apoptosis in the second biopsy. Although the self-limited nature of PIG has been described clinically, it has never been documented histologically. We present this case to illustrate the clinical and pathologic resolution of the disorder and speculate that the lesional mesenchymal cells may have transient proliferative capacity.

1alpha,25(OH)2D3 and its 3-epimer promote rat lung alveolar epithelial-mesenchymal interactions and inhibit lipofibroblast apoptosis

Although alveolar wall thinning has been attributed to apoptosis of interstitial lung lipofibroblasts (LFs), the underlying molecular mechanism(s) remains unknown. Although the physiological vitamin D steroid hormone 1alpha,25(OH)(2)D(3) (1,25D) has been suggested as a local paracrine/autocrine effector of fetal lung maturation and is known to affect fibroblast apoptosis, its effects on LF apoptosis are unknown. We determined the role of 1,25D and its metabolite, C-3-epimer (3-epi-1,25D), on LF and alveolar type II (ATII) cell differentiation, proliferation, and apoptosis. Embryonic day 19 Sprague-Dawley fetal rat lung LFs and ATII cells were treated with 1,25D or 3-epi-1,25D (1 x 10(-10) to 1 x 10(-8) M) for 24 h, and cell proliferation, apoptosis, and differentiation were assessed. Both 1,25D and 3-epi-1,25D exhibited dose-dependent increases in expression of the key homeostatic epithelial-mesenchymal differentiation markers, increased LF and ATII cell proliferation, and decreased apoptosis. Furthermore, rat pups administered 1,25D from postnatal days 0 to 14 showed increased expressions of key LF and ATII cell differentiation markers, increased Bcl-2-to-Bax ratio as an index of decreased spontaneous alveolar LF and ATII cell apoptosis, increased alveolar count, and a paradoxical increase in septal thickness. We conclude that spatial- and temporal-specific actions of vitamin D play a critical role in perinatal lung maturation by stimulating key alveolar epithelial-mesenchymal interactions and by modulating LF proliferation/apoptosis. These data not only provide the biological rationale for the presence of an alveolar vitamin D paracrine system, but also provide the first integrated molecular mechanism for increased surfactant synthesis and alveolar septal thinning during perinatal lung maturation.

Pulmonary effects of keratinocyte growth factor in newborn rats exposed to hyperoxia

Acute lung injury and compromised alveolar development characterize bronchopulmonary dysplasia (BPD) of the premature neonate. High levels of keratinocyte growth factor (KGF), a cell-cell mediator with pleiotrophic lung effects, are associated with low BPD risk. KGF decreases mortality in hyperoxia-exposed newborn rodents, a classic model of injury-induced impaired alveolarization, although the pulmonary mechanisms of this protection are poorly defined. These were explored through in vitro and in vivo approaches in the rat. Hyperoxia decreased by 30% the rate of wound closure of a monolayer of fetal alveolar epithelial cells, due to cell death, which was overcome by recombinant human KGF (100 ng/ml). In rat pups exposed to >95% O2 from birth, increased viability induced by intraperitoneal injection of KGF (2 microg/g body wt) every other day was associated with prevention of neutrophil influx in bronchoalveolar lavage (BAL), prevention of decreases in whole lung DNA content and cell proliferation rate, partial prevention of apoptosis increase, and a markedly increased proportion of surfactant protein B-immunoreactive cells in lung parenchyma. Increased lung antioxidant capacity is likely to be due in part to enhanced CAAT/enhancer binding protein alpha expression. By contrast, KGF neither corrected changes induced by hyperoxia in parameters of lung morphometry that clearly indicated impaired alveolarization nor had any significant effect on tissue or BAL surfactant phospholipids. These findings evidence KGF alveolar epithelial cell protection, enhancing effects on alveolar repair capacity, and anti-inflammatory effects in the injured neonatal lung that may account, at least in part, for its ability to reduce mortality. They argue in favor of a therapeutic potential of KGF in the injured neonatal lung.

Dysfunctional cystic fibrosis transmembrane conductance regulator inhibits phagocytosis of apoptotic cells with proinflammatory consequences

Cystic fibrosis (CF) is caused by mutated CF transmembrane conductance regulator (CFTR) and is characterized by robust airway inflammation and accumulation of apoptotic cells. Phagocytosis of apoptotic cells (efferocytosis) is a pivotal regulator of inflammation, because it prevents postapoptotic necrosis and actively suppresses release of a variety of proinflammatory mediators, including IL-8. Because CF is associated with accumulation of apoptotic cells, inappropriate levels of IL-8, and robust inflammation, we sought to determine whether CFTR deficiency specifically impairs efferocytosis and its regulation of inflammatory mediator release. Here we show that CFTR deficiency directly interferes with efferocytosis by airway epithelium, an effect that is not due to altered binding of apoptotic cells to epithelial cells or altered expression of efferocytosis receptors. In contrast, expression of RhoA, a known negative regulator of efferocytosis, is substantially increased in CFTR-deficient cells, and inhibitors of RhoA or its downstream effector Rho kinase normalize efferocytosis in these cells. Impaired efferocytosis appears to be mediated through an amiloride-sensitive ion channel, because amiloride restores phagocytic competency in CFTR-deficient cells. Finally, ineffective efferocytosis in CFTR-deficient cells appears to have proinflammatory consequences, because apoptotic cells enhance IL-8 release by these cells, but not by wild-type controls. Therefore, in CF, dysregulated efferocytosis may lead to accumulation of apoptotic cells and impaired regulation of the inflammatory response and, ultimately, may suggest a new therapeutic target.

Heat shock protein 27 protects lung epithelial cells from hyperoxia-induced apoptotic cell death

Oxygen toxicity or hyperoxia is one of the major contributing factors in the development of bronchopulmonary dysplasia. Heat shock protein 27 (Hsp27) is an important chaperone protein in the postnatal lung development. However, the role of Hsp27 in lung epithelial cells during hyperoxia is unclear. Our studies by cDNA array and immunohistochemistry revealed that hyperoxia decreased Hsp27 expression in newborn rat lungs. Western blot showed that hyperoxic treatment significantly decreased Hsp27 protein expression in cultured human lung epithelial cells (A549). The expression of Hsp27 was decreased approximately twofold after 24-h and threefold after 48- and 72-h hyperoxic exposure compared with that of the A549 cells exposed to normoxia (p < 0.05, n = 3). Knockdown of Hsp27 expression by siRNA resulted in more apoptotic cell death in A549 cells. Overexpression of Hsp27 reduced hyperoxia-induced apoptotic cell death to 9.2% in Hsp27 overexpressing A549 cells from 12.6% in control A549 cells after 72-h hyperoxic exposure (p < 0.01, n = 8-9). Overexpression of Hsp27 also diminished hyperoxia-induced caspase-9 activation in A549 cells. Our results demonstrated that hyperoxia decreased Hsp27 expression in newborn rat lung and cultured human lung epithelial cells. Overexpression of Hsp27 could reduce hyperoxia-induced apoptosis in cultured human lung epithelial cells.

Hyperoxia-induced NF-kappaB activation occurs via a maturationally sensitive atypical pathway

NF-kappaB activation is exaggerated in neonatal organisms after oxidant and inflammatory insults, but the reason for this and the downstream effects are unclear. We hypothesized that specific phosphorylation patterns of IkappaBalpha could account for differences in NF-kappaB activation in hyperoxia-exposed fetal and adult lung fibroblasts. After exposure to hyperoxia (>95% O(2)), nuclear NF-kappaB binding increased in fetal, but not adult, lung fibroblasts. Unique to fetal cells, phosphorylation of IkappaBalpha on tyrosine 42, rather than serine 32/36 as seen in TNF-alpha-exposed cells, preceded NF-kappaB nuclear translocation. In fetal cells stably transfected with an NF-kappaB-driven luciferase reporter, hyperoxia significantly suppressed reporter activity, in contrast to increased reporter activity after TNF-alpha incubation. Targeted gene profiling analysis showed that hyperoxia resulted in decreased expression of multiple genes, including proapoptotic factors. Transfection with a dominant-negative IkappaBalpha (Y42F), which cannot be phosphorylated on tyrosine 42, resulted in upregulation of multiple proapoptotic genes. In support of this finding, caspase-3 activity and DNA laddering were specifically increased in fetal lung fibroblasts expressing Y42F after exposure to hyperoxia. These data demonstrate a unique pathway of NF-kappaB activation in fetal lung fibroblasts after exposure to hyperoxia, whereby these cells are protected against apoptosis. Activation of this pathway in fetal cells may prevent the normal pattern of fibroblast apoptosis necessary for normal lung development, resulting in aberrant lung morphology in vivo.

Apoptosis of mesenchymal cells during the pseudoglandular stage of lung development affects branching morphogenesis

Apoptosis has been implicated as an important process in the development of several organ systems, including limbs, kidneys, and the heart. In developing murine lungs, the authors found that apoptosis was more predominant during the pseudoglandular stage of lung development than during the saccular stage, with 93% of the apoptotic structures in the mesenchyme. Murine lung explants cultured in the presence of zinc chloride and aurintricarboxylic acid, 2 blockers of endonuclease function, showed decreased branching. These observations suggest that apoptosis predominates in mesenchymal cells during the pseudoglandular stage of lung development, and may be important for normal progression of lung branching morphogenesis.

Fetal tracheal occlusion for severe congenital diaphragmatic hernia in humans: a morphometric study of lung parenchyma and muscularization of pulmonary arterioles

OBJECTIVE

The aim of this study is to examine structure of lung parenchyma (Pp) and muscularization of pulmonary arterioles (PAs) in human fetuses that underwent tracheal occlusion (TO) therapy for severe congenital diaphragmatic hernia (CDH).

MATERIAL

Fifteen fetuses underwent TO, with 5 survivors (Am J Obstet Gynecol. 2000:183;1059-1066). Paraffin-embedded lung specimens from 7 of 10 nonsurvivors (CDH-TO) and 6 age-matched fetuses (CDH) were available for morphometric analysis, which included measurements of point fraction of lung Pp and surface density. The PAs were categorized according to external diameter (<70 microm and 70 <or= 140 microm). Percent medial wall thickness (%MWT) was calculated by dividing raw measurements of MWT by external diameter.

RESULTS

Gestational age at TO was 27.6 +/- 0.9 weeks with a mean duration of TO of 32.6 +/- 6.8 days. Gestational age at delivery (CDH-TO 31.9 +/- 0.9 vs CDH 35.4 +/- 1.8 weeks; P = .18) and postnatal survival time (CDH-TO 20.5 +/- 6.0 vs CDH 18.6 +/- 7.8 days; P = .85) were not significantly different between groups. Tracheal occlusion significantly increased the lung-to-body weight ratio (CDH-TO 13.0 +/- 2.2 vs CDH 6.6 +/- 0.9; P = .02). Tracheal occlusion tended to decrease right-lung Pp (CDH-TO 54.6% +/- 2.6% vs CDH 65.7% +/- 5.9%; P = .05), whereas left-lung Pp was not different between groups (CDH-TO 63.0% +/- 3.5% vs CDH 66.7% +/- 4.1%; P = .51). Surface density of airspaces was not different between groups in either left (CDH-TO 171.3 +/- 16.1 cm(-1) vs CDH 151.1 +/- 8.1 cm(-1); P = .34) or right (CDH-TO 172.0 +/- 10.6 cm(-1) vs CDH 160.8 +/- 3.6 cm(-1); P = .33) lungs. The %MWT in small and large PA was similar between groups.

CONCLUSIONS

Open prenatal TO in human fetuses increased lung growth, as evidenced by an increase in lung weight, but did not improve parenchymal structure or muscularization of PAs.

Nasal ventilation alters mesenchymal cell turnover and improves alveolarization in preterm lambs

RATIONALE

Bronchopulmonary dysplasia (BPD) is a frequent cause of morbidity in preterm infants that is characterized by prolonged need for ventilatory support in an intensive care environment. BPD is characterized histopathologically by persistently thick, cellular distal airspace walls. In normally developing lungs, by comparison, remodeling of the immature parenchymal architecture is characterized by thinning of the future alveolar walls, a process predicated on cell loss through apoptosis.

OBJECTIVES

We hypothesized that minimizing lung injury, using high-frequency nasal ventilation to provide positive distending pressure with minimal assisted tidal volume displacement, would increase apoptosis and decrease proliferation among mesenchymal cells in the distal airspace walls compared with a conventional mode of support (intermittent mandatory ventilation).

METHODS

Accordingly, we compared two groups of preterm lambs: one group managed by high-frequency nasal ventilation and a second group managed by intermittent mandatory ventilation. Each group was maintained for 3 days.

MEASUREMENTS AND MAIN RESULTS

Oxygenation and ventilation targets were sustained with lower airway pressures and less supplemental oxygen in the high-frequency nasal ventilation group, in which alveolarization progressed. Thinning of the distal airspace walls was accompanied by more apoptosis, and less proliferation, among mesenchymal cells of the high-frequency nasal ventilation group, based on morphometric, protein abundance, and mRNA expression indices of apoptosis and proliferation.

CONCLUSIONS

Our study shows that high-frequency nasal ventilation preserves the balance between mesenchymal cell apoptosis and proliferation in the distal airspace walls, such that alveolarization progresses.

Fas-ligand-induced apoptosis of respiratory epithelial cells causes disruption of postcanalicular alveolar development

Premature infants are at risk for bronchopulmonary dysplasia, a complex condition characterized by impaired alveolar development and increased alveolar epithelial apoptosis. The functional involvement of pulmonary apoptosis in bronchopulmonary dysplasia- associated alveolar disruption remains undetermined. The aims of this study were to generate conditional lung-specific Fas-ligand (FasL) transgenic mice and to determine the effects of FasL-induced respiratory epithelial apoptosis on alveolar remodeling in postcanalicular lungs. Transgenic (TetOp)(7)-FasL responder mice, generated by pronuclear microinjection, were bred with Clara cell secretory protein (CCSP)-rtTA activator mice. Doxycycline (Dox) was administered from embryonal day 14 to postnatal day 7, and lungs were studied between embryonal day 19 and postnatal day 21. Dox administration induced marked respiratory epithelium-specific FasL mRNA and protein up-regulation in double-transgenic CCSP-rtTA(+)/(TetOp)(7)-FasL(+) mice compared with single-transgenic CCSP-rtTA(+) littermates. The Dox-induced FasL up-regulation was associated with dramatically increased apoptosis of alveolar type II cells and Clara cells, disrupted alveolar development, decreased vascular density, and increased postnatal lethality. These data demonstrate that FasL-induced alveolar epithelial apoptosis during postcanalicular lung remodeling is sufficient to disrupt alveolar development after birth. The availability of inducible lung-specific FasL transgenic mice will facilitate studies of the role of apoptosis in normal and disrupted alveologenesis and may lead to novel therapeutic approaches for perinatal and adult pulmonary diseases characterized by dysregulated apoptosis.

Regulation of alveologenesis: clinical implications of impaired growth

During its development that begins in intrauterine life, the lung is transformed from a simple epithelial lined sac that emerges from the foregut into a complex arrangement of blood vessels, airways, and alveoli that make up the mature lung structure. This remarkable transformation that continues for several years postnatally, is achieved by the influence of several genes, transcription factors, growth factors and hormones upon the cells and proteins of the lung bud. A seminal event in this process is the formation of the air-blood barrier within the alveolar wall, an evolutionary modification that permits independent air-breathing existence in mammals. Molecular biological techniques have enabled elucidation of the mechanistic pathways contributing to alveologenesis and have provided probable molecular bases for examples of impaired alveologenesis encountered by the paediatric pathologist.

Identification of glucocorticoid-regulated genes that control cell proliferation during murine respiratory development

Glucocorticoids play a vital role in fetal respiratory development and act via the intracellular glucocorticoid receptor (GR) to regulate transcription of key target genes. GR-null mice die at birth due to respiratory dysfunction associated with hypercellularity and atelectasis. To identify events associated with this lung phenotype we examined perinatal cellular proliferation rates and apoptotic indices. We demonstrate that compared to wild-type controls, day 18.5 postcoitum (p.c.) GR-null mouse lungs display significantly increased cell proliferation rates (1.8-fold P < 0.05) and no change in apoptosis. To examine underlying molecular mechanisms, we compared whole genome expression profiles by microarray analysis at 18.5 days p.c. Pathways relating to cell proliferation, division and cell cycle were significantly down-regulated while pathways relating to carbohydrate metabolism, kinase activities and immune responses were significantly up-regulated. Differential levels of gene expression were verified by quantitative-RT-PCR and/or Northern analysis. Key regulators of proliferation differentially expressed in the lung of 18.5 p.c. GR-null lungs included p21 CIP1 (decreased 2.9-fold, P < 0.05), a negative regulator of the cell cycle, and Mdk (increased 6.0-fold, P < 0.05), a lung growth factor. The more under-expressed genes in 18.5 p.c. GR-null lungs included Chi3l3 (11-fold, P < 0.05), a macrophage inflammatory response gene and Ela1 (9.4-fold, P < 0.05), an extracellular matrix remodeling enzyme. Our results demonstrate that GR affects the transcriptional status of a number of regulatory processes during late fetal lung development. Amongst these processes is cell proliferation whereby GR induces expression of cell cycle repressors while suppressing induction of a well characterized cell cycle stimulator.

Lung epithelial cell apoptosis during acute lung injury in infancy

CONTEXT

Apoptosis of lung epithelial cells is implicated in the pathogenesis of acute lung injury. Most research on this subject has focused on adults. Very little is known about a potential interaction of this process with lung development in children.

OBJECTIVE

To summarize the current literature on lung epithelial cell apoptosis and common causes of acute lung injury in infants and young children and to identify new areas of research.

DESIGN

A Medline-based literature search.

RESULTS AND CONCLUSIONS

Few studies have focused on lung epithelial cell apoptosis during common causes of acute lung injury in children. Nevertheless, the limited literature suggests that this may be an important mechanism during respiratory distress syndrome of infants and viral respiratory tract infection. Apoptosis is an essential process during lung development and maturation. Insufficient attention has been paid to potential consequences of this for the short- and long-term outcomes of acute lung injury.

Galectin-1 in secondary alveolar septae of neonatal mouse lung

In mice, alveolarization occurs during postnatal days 4 through 12, when secondary alveolar septae create thin-walled alveoli in the distal lung. We hypothesized that genes predominantly expressed in newly forming secondary alveolar septae influence the process of alveolarization. To address this hypothesis, tips of secondary alveolar septae were isolated from sections of postnatal day 6 mouse lung tissue using laser capture microdissection. Total RNA was isolated and amplified from the dissected alveolar septal tips and from intact postnatal day 6 lung tissue. Gene expression in the samples was characterized using Affymetrix mouse U74AN2 GeneChips. Galectin-1 was an abundantly expressed transcript that was enriched in the alveolar septal tips compared with levels in the whole lung tissue. Galectins are β-galactoside-binding proteins involved in the regulation of cell proliferation, differentiation, and apoptosis in fibroblasts, muscle cells and endothelial cells, cell types that are present in the alveolar wall. Immunostaining in postnatal day 6 lung tissue confirmed that galectin-1 protein is concentrated in the tips of secondary alveolar septae, predominantly in myofibroblasts. Fibroblasts isolated from day 6 neonatal mouse lung tissue contained galectin-1 protein. Real-time PCR demonstrated that galectin-1 mRNA levels in mouse lung tissue peak at postnatal day 6. Immunoblot analysis confirmed that peak levels of lung galectin-1 protein are found at postnatal days 6 to 12. The increased expression of galectin-1 at the site and time of ongoing alveolarization in the newborn mouse is suggestive that galectin-1 may play an important role in this critical aspect of lung development.

Impact of the loss of Hoxa5 function on lung alveogenesis

The involvement of genes controlling embryonic processes in the etiology of diseases often escapes attention because of the focus given to their inherent developmental role. Hoxa5 belongs to the Hox gene family encoding transcription factors known for their role in skeletal patterning. Hoxa5 is required for embryonic respiratory tract morphogenesis. We now show that the loss of Hoxa5 function has severe repercussions on postnatal lung development. Hoxa5-/- lungs present an emphysema-like morphology because of impaired alveogenesis. Chronic inflammation characteristics, including goblet cell hyperplasia, mucus hypersecretion, and recruitment of inflammatory cells, were also observed. Altered cell specification during lung morphogenesis triggered goblet cell anomalies. In addition, the defective motility of alveolar myofibroblast precursors in the embryonic lung led to the mispositioning of the alveolar myofibroblasts and to abnormal elastin deposition postnatally. Both goblet cell hyperplasia and elastic fiber abnormalities contributed to the chronic physiopathological features of Hoxa5-/- lungs. They constituted an attractive stimulus to recruit activated macrophages that in turn generated a positive feedback loop that perpetuated macrophage accumulation in the lung. The present work corroborates the notion that altered Hox gene expression may predispose to lung pathologies.

Fibroblast growth factor-2 and receptor-1alpha(IIIc) regulate postnatal rat lung cell apoptosis

RATIONALE

Fibroblast growth factor receptor-1alpha(IIIc) [FGF-R1alpha(IIIc)] regulates recovery of neonatal rat lung growth, after 95% oxygen-mediated growth arrest. Its role in normal postnatal alveologenesis is unknown.

OBJECTIVE

To determine if FGF-R1alpha(IIIc) regulates normal postnatal alveologenesis.

METHODS

Truncated soluble FGF-R1alpha(IIIc) or neutralizing antibodies to FGF-1 or FGF-2 were injected intraperitoneally into 3-d-old rats. The pups were killed at Day 7 for studies of alveolar development.

MEASUREMENTS AND MAIN RESULTS

Injected, truncated soluble FGF-R1alpha(IIIc) inhibited phosphorylation of the endogenous FGF-R1, and downstream pathway, and paradoxically increased lung DNA content and tissue fraction while inhibiting lung cell DNA synthesis. The increase in tissue thickness was due to reduced apoptosis, as indicated by reductions in cleaved effector caspases 3 and 7. Inhibition of the intrinsic apoptosis pathway was suggested by decreases in the proapoptotic protein Bax and mitochondrial cytochrome c release, and an increase in the antiapoptotic protein Bcl-x(L). Injected antibodies to FGF-1 and FGF-2 had no effect on DNA synthesis, but both increased Bcl-x(L) content and decreased cytochrome c release and cleaved caspase-7 protein expression. However, only injection of the antibody to FGF-2 replicated the increased tissue fraction and inhibited apoptosis observed with the injection of truncated soluble FGF-R1alpha(IIIc).

CONCLUSIONS

Inhibition of ligand binding, most likely of FGF-2, to the FGF-R1alpha(IIIc) inhibits normal postnatal lung cell apoptosis.