Programmed cell death contributes to postnatal lung development

Three-dimensional characteristics of the alveolar capillary network in infant and adult human lungs

BACKGROUND

A comprehensive understanding of vascular development in the human lung is still missing.

METHODS

Therefore, samples of infant (n = 5, 26 days to 18 months postnatally) and adult (n = 5, 20 to 40 years) human lungs were subjected to unbiased stereological estimation of the total number of capillary loops. Serial sections were segmented to visualize the alveolar capillary network (ACN) in 3D.

RESULTS

The number of capillary loops increased in parallel to lung volume from 26 days to 18 months, while in adults, it was not correlated to lung volume. In infant lungs, two capillary layers were separated by a connective tissue sheet with a growing number of interconnections. In adults, the mature ACN was almost, but not completely, single-layered. Here, the connective tissue was thinner but still centrally positioned, suggesting the persistence of interconnected parts of both layers of the previously double-layered ACN.

CONCLUSIONS

Small parts of the capillaries remain double-layered and seem to be grouped around the thin connective tissue sheet, suggesting a different mechanism of microvascular maturation than simple fusion of the two layers. These spots are a potential basis for further alveolarization after completion of bulk formation.

IMPACT

The 3D data offer a new conceptual approach to microvascular maturation of the lung. Microvascular maturation rather results from reduction than simple fusion of capillary fragments. Adult lungs maintain small double-layered capillary spots. These could offer a potential source of regeneration. The data are important to better understand normal and pathological lung development.

Dedifferentiated early postnatal lung myofibroblasts redifferentiate in adult disease

Alveolarization ensures sufficient lung surface area for gas exchange, and during bulk alveolarization in mice (postnatal day [P] 4.5-14.5), alpha-smooth muscle actin (SMA)+ myofibroblasts accumulate, secrete elastin, and lay down alveolar septum. Herein, we delineate the dynamics of the lineage of early postnatal SMA+ myofibroblasts during and after bulk alveolarization and in response to lung injury. SMA+ lung myofibroblasts first appear at ∼ P2.5 and proliferate robustly. Lineage tracing shows that, at P14.5 and over the next few days, the vast majority of SMA+ myofibroblasts downregulate smooth muscle cell markers and undergo apoptosis. Of note, ∼8% of these dedifferentiated cells and another ∼1% of SMA+ myofibroblasts persist to adulthood. Single cell RNA sequencing analysis of the persistent SMA- cells and SMA+ myofibroblasts in the adult lung reveals distinct gene expression profiles. For instance, dedifferentiated SMA- cells exhibit higher levels of tissue remodeling genes. Most interestingly, these dedifferentiated early postnatal myofibroblasts re-express SMA upon exposure of the adult lung to hypoxia or the pro-fibrotic drug bleomycin. However, unlike during alveolarization, these cells that re-express SMA do not proliferate with hypoxia. In sum, dedifferentiated early postnatal myofibroblasts are a previously undescribed cell type in the adult lung and redifferentiate in response to injury.

Impaired cell-cell communication and axon guidance because of pulmonary hypoperfusion during postnatal alveolar development

BACKGROUND

Pulmonary hypoperfusion is common in children with congenital heart diseases (CHDs) or pulmonary hypertension (PH) and causes adult pulmonary dysplasia. Systematic reviews have shown that some children with CHDs or PH have mitigated clinical outcomes with COVID-19. Understanding the effects of pulmonary hypoperfusion on postnatal alveolar development may aid in the development of methods to improve the pulmonary function of children with CHDs or PH and improve their care during the COVID-19 pandemic, which is characterized by cytokine storm and persistent inflammation.

METHODS AND RESULTS

We created a neonatal pulmonary hypoperfusion model through pulmonary artery banding (PAB) surgery at postnatal day 1 (P1). Alveolar dysplasia was confirmed by gross and histological examination at P21. Transcriptomic analysis of pulmonary tissues at P7(alveolar stage 2) and P14(alveolar stage 4) revealed that the postnatal alveolar development track had been changed due to pulmonary hypoperfusion. Under the condition of pulmonary hypoperfusion, the cell-cell communication and axon guidance, which both determine the final number of alveoli, were lost; instead, there was hyperactive cell cycle activity. The transcriptomic results were further confirmed by the examination of axon guidance and cell cycle markers. Because axon guidance controls inflammation and immune cell activation, the loss of axon guidance may explain the lack of severe COVID-19 cases among children with CHDs or PH accompanied by pulmonary hypoperfusion.

CONCLUSIONS

This study suggested that promoting cell-cell communication or supplementation with guidance molecules may treat pulmonary hypoperfusion-induced alveolar dysplasia, and that COVID-19 is less likely to cause a cytokine storm in children with CHD or PH accompanied by pulmonary hypoperfusion.

Alveologenesis: What Governs Secondary Septa Formation

The simplification of alveoli leads to various lung pathologies such as bronchopulmonary dysplasia and emphysema. Deep insight into the process of emergence of the secondary septa during development and regeneration after pneumonectomy, and into the contribution of the drivers of alveologenesis and neo-alveolarization is required in an efficient search for therapeutic approaches. In this review, we describe the formation of the gas exchange units of the lung as a multifactorial process, which includes changes in the actomyosin cytoskeleton of alveocytes and myofibroblasts, elastogenesis, retinoic acid signaling, and the contribution of alveolar mesenchymal cells in secondary septation. Knowledge of the mechanistic context of alveologenesis remains incomplete. The characterization of the mechanisms that govern the emergence and depletion of αSMA will allow for an understanding of how the niche of fibroblasts is changing. Taking into account the intense studies that have been performed on the pool of lung mesenchymal cells, we present data on the typing of interstitial fibroblasts and their role in the formation and maintenance of alveoli. On the whole, when identifying cell subpopulations in lung mesenchyme, one has to consider the developmental context, the changing cellular functions, and the lability of gene signatures.

Prenatal inflammation enhances antenatal corticosteroid-induced fetal lung maturation

Respiratory complicˆations are the major cause of morbidity and mortality among preterm infants, which is partially prevented by the administration of antenatal corticosteroids (ACS). Most very preterm infants are exposed to chorioamnionitis, but short- and long-term effects of ACS treatment in this setting are not well defined. In low-resource settings, ACS increased neonatal mortality by perhaps increasing infection. We report that treatment with low-dose ACS in the setting of inflammation induced by intraamniotic lipopolysaccharide (LPS) in rhesus macaques improves lung compliance and increases surfactant production relative to either exposure alone. RNA sequencing shows that these changes are mediated by suppression of proliferation and induction of mesenchymal cellular death via TP53. The combined exposure results in a mature-like transcriptomic profile with inhibition of extracellular matrix development by suppression of collagen genes COL1A1, COL1A2, and COL3A1 and regulators of lung development FGF9 and FGF10. ACS and inflammation also suppressed signature genes associated with proliferative mesenchymal progenitors similar to the term gestation lung. Treatment with ACS in the setting of inflammation may result in early respiratory advantage to preterm infants, but this advantage may come at a risk of abnormal extracellular matrix development, which may be associated with increased risk of chronic lung disease.

Tenascin-C deficiency impairs alveolarization and microvascular maturation during postnatal lung development

After the airways have been formed by branching morphogenesis the gas exchange area of the developing lung is enlarged by the formation of new alveolar septa (alveolarization). The septa themselves mature by a reduction of their double-layered capillary networks to single-layered ones (microvascular maturation). Alveolarization in mice is subdivided into a first phase (postnatal days 4-21, classical alveolarization), where new septa are lifted off from immature preexisting septa, and a second phase (day 14 to adulthood, continued alveolarization), where new septa are formed from mature septa. Tenascin-C (TNC) is a multidomain extracellular matrix protein contributing to organogenesis and tumorigenesis. It is highly expressed during classical alveolarization, but afterward its expression is markedly reduced. To study the effect of TNC deficiency on postnatal lung development, the formation and maturation of the alveolar septa were followed stereologically. Furthermore, the number of proliferating (Ki-67-positive) and TUNEL-positive cells was estimated. In TNC-deficient mice for both phases of alveolarization a delay and catch-up were observed. Cell proliferation was increased at days 4 and 6; at day 7, thick septa with an accumulation of capillaries and cells were observed; and the number of TUNEL-positive cells (dying cells or DNA repair) was increased at day 10. Whereas at days 15 and 21 premature microvascular maturation was detected, the microvasculature was less mature at day 60 compared with wild type. No differences were observed in adulthood. We conclude that TNC contributes to the formation of new septa, to microvascular maturation, and to cell proliferation and migration during postnatal lung development.NEW & NOTEWORTHY Previously, we showed that the extracellular matrix protein tenascin-C takes part in prenatal lung development by controlling branching morphogenesis. Now we report that tenascin-C is also important during postnatal lung development, because tenascin-C deficiency delays the formation and maturation of the alveolar septa during not only classical but also continued alveolarization. Adult lungs are indistinguishable from wild type because of a catch-up formation of new septa.

Identification of a FGF18-expressing alveolar myofibroblast that is developmentally cleared during alveologenesis

Alveologenesis is an essential developmental process that increases the surface area of the lung through the formation of septal ridges. In the mouse, septation occurs postnatally and is thought to require the alveolar myofibroblast (AMF). Though abundant during alveologenesis, markers for AMFs are minimally detected in the adult. After septation, the alveolar walls thin to allow efficient gas exchange. Both loss of AMFs or retention and differentiation into another cell type during septal thinning have been proposed. Using a novel Fgf18:CreERT2 allele to lineage trace AMFs, we demonstrate that most AMFs are developmentally cleared during alveologenesis. Lung mesenchyme also contains other poorly described cell types, including alveolar lipofibroblasts (ALF). We show that Gli1:CreERT2 marks both AMFs as well as ALFs, and lineage tracing shows that ALFs are retained in adult alveoli while AMFs are lost. We further show that multiple immune cell populations contain lineage-labeled particles, suggesting a phagocytic role in the clearance of AMFs. The demonstration that the AMF lineage is depleted during septal thinning through a phagocytic process provides a mechanism for the clearance of a transient developmental cell population.

Developmental milestones in neonatal and juvenile C57Bl/6 mouse - Indications for the design of juvenile toxicity studies

There is a growing demand for wild type mice and mouse models of disease that may be more representative of human conditions but there is little information on neonatal and juvenile mouse anatomy. This project produces sound and comprehensive histology background data on the developing neonatal mouse at different time points from Day 0 until Day 28. The work describes optimal methods for tissue harvesting, fixation and processing from the neonatal and juvenile mice which can be used in routine toxicology studies. A review of the available literature revealed inconsistencies in the developmental milestones reported in the mouse. Although it is true that the sequence of events during the development is virtually the same in mice and rats, important developmental milestones in the mouse often happen earlier than in the rat, and these species should not be used interchangeably.

How high resolution 3-dimensional imaging changes our understanding of postnatal lung development

During the last 10 + years biologically and clinically significant questions about postnatal lung development could be answered due to the application of modern cutting-edge microscopic and quantitative histological techniques. These are in particular synchrotron radiation based X-ray tomographic microscopy (SRXTM), but also ³Helium Magnetic Resonance Imaging, as well as the stereological estimation of the number of alveoli and the length of the free septal edge. First, the most important new finding may be the following: alveolarization of the lung does not cease after the maturation of the alveolar microvasculature but continues until young adulthood and, even more important, maybe reactivated lifelong if needed to rescue structural damages of the lungs. Second, the pulmonary acinus represents the functional unit of the lung. Because the borders of the acini could not be detected in classical histological sections, any investigation of the acini requires 3-dimensional (imaging) methods. Based on SRXTM it was shown that in rat lungs the number of acini stays constant, meaning that their volume increases by a factor of ~ 11 after birth. The latter is very important for acinar ventilation and particle deposition.

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.

Sonic Hedgehog Signaling Regulates Myofibroblast Function during Alveolar Septum Formation in Murine Postnatal Lung

Sonic Hedgehog (Shh) signaling regulates mesenchymal proliferation and differentiation during embryonic lung development. In the adult lung, Shh signaling maintains mesenchymal quiescence and is dysregulated in diseases such as idiopathic pulmonary fibrosis and chronic obstructive pulmonary disease. Our previous data implicated a role for Shh in postnatal lung development. Here, we report a detailed analysis of Shh signaling during murine postnatal lung development. We show that Shh pathway expression and activity during alveolarization (postnatal day [P] 0-P14) are distinct from those during maturation (P14-P24). This biphasic pattern is paralleled by the transient presence of Gli1⁺;α-smooth muscle actin (α-SMA)⁺ myofibroblasts in the growing alveolar septal tips. Carefully timed inhibition of Hedgehog (Hh) signaling during alveolarization defined mechanisms by which Shh influences the mesenchymal compartment. First, interruption of Hh signaling at earlier time points results in increased lung compliance and wall structure defects of increasing severity, ranging from moderately enlarged alveolar airspaces to markedly enlarged airspaces and fewer secondary septa. Second, Shh signaling is required for myofibroblast differentiation: Hh inhibition during early alveolarization almost completely eliminates Gli1⁺;α-SMA⁺ cells at the septal tips, and Gli1-lineage tracing revealed that Gli1⁺ cells do not undergo apoptosis after Hh inhibition but remain in the alveolar septa and are unable to express α-SMA. Third, Shh signaling is vital to mesenchymal proliferation during alveolarization, as Hh inhibition decreased proliferation of Gli1⁺ cells and their progeny. Our study establishes Shh as a new alveolarization-promoting factor that might be affected in perinatal lung diseases that are associated with impaired alveolarization.

Development of the lung

To fulfill the task of gas exchange, the lung possesses a huge inner surface and a tree-like system of conducting airways ventilating the gas exchange area. During lung development, the conducting airways are formed first, followed by the formation and enlargement of the gas exchange area. The latter (alveolarization) continues until young adulthood. During organogenesis, the left and right lungs have their own anlage, an outpouching of the foregut. Each lung bud starts a repetitive process of outgrowth and branching (branching morphogenesis) that forms all of the future airways mainly during the pseudoglandular stage. During the canalicular stage, the differentiation of the epithelia becomes visible and the bronchioalveolar duct junction is formed. The location of this junction stays constant throughout life. Towards the end of the canalicular stage, the first gas exchange may take place and survival of prematurely born babies becomes possible. Ninety percent of the gas exchange surface area will be formed by alveolarization, a process where existing airspaces are subdivided by the formation of new walls (septa). This process requires a double-layered capillary network at the basis of the newly forming septum. However, in parallel to alveolarization, the double-layered capillary network of the immature septa fuses to a single-layered network resulting in an optimized setup for gas exchange. Alveolarization still continues, because, at sites where new septa are lifting off preexisting mature septa, the required second capillary layer will be formed instantly by angiogenesis. The latter confirms a lifelong ability of alveolarization, which is important for any kind of lung regeneration.

Early and late effects of prenatal corticosteroid treatment on the microRNA profiles of lung tissue in rats

Glucocorticoids have been administered to mothers at risk of premature delivery to induce maturation of preterm fetal lungs and prevent the development of respiratory distress syndrome. Micro (mi)RNAs serve various crucial functions in cell proliferation, differentiation and organ development; however, few studies have demonstrated an association between miRNAs and lung development. The aim of the present study was to investigate alterations in the miRNA profiles of rat lung tissue following prenatal glucocorticoid therapy for fetal lung development. The differences in miRNA expression profiles were compared between postnatal days 7 (D7) and 120 (D120) rat lung tissues, followed by validation using reverse transcription-quantitative polymerase chain reaction. The miRNA profiles of rat lung tissues following prenatal dexamethasone (DEX) therapy were also investigated. miRNAs with 2-fold changes were selected for further analysis. At D120, 6 upregulated and 6 downregulated miRNAs were detected, compared with D7. Among these differentially expressed miRNAs, miR-101-3p and miR-99b-5p were associated with the lowest and highest expressions of miRNA at D7, respectively. A limited impact on the miRNA profiles of rat lung tissues was observed following prenatal DEX treatment, which may help to further clarify the mechanisms underlying normal lung development. However, the results of the present study cannot entirely elucidate the effects of prenatal DEX treatment on the lung development of premature infants, and further studies investigating the impact of prenatal corticosteroids on fetal lung miRNA profiles are required.

The development and plasticity of alveolar type 1 cells

Alveolar type 1 (AT1) cells cover >95% of the gas exchange surface and are extremely thin to facilitate passive gas diffusion. The development of these highly specialized cells and its coordination with the formation of the honeycomb-like alveolar structure are poorly understood. Using new marker-based stereology and single-cell imaging methods, we show that AT1 cells in the mouse lung form expansive thin cellular extensions via a non-proliferative two-step process while retaining cellular plasticity. In the flattening step, AT1 cells undergo molecular specification and remodel cell junctions while remaining connected to their epithelial neighbors. In the folding step, AT1 cells increase in size by more than 10-fold and undergo cellular morphogenesis that matches capillary and secondary septa formation, resulting in a single AT1 cell spanning multiple alveoli. Furthermore, AT1 cells are an unexpected source of VEGFA and their normal development is required for alveolar angiogenesis. Notably, a majority of AT1 cells proliferate upon ectopic SOX2 expression and undergo stage-dependent cell fate reprogramming. These results provide evidence that AT1 cells have both structural and signaling roles in alveolar maturation and can exit their terminally differentiated non-proliferative state. Our findings suggest that AT1 cells might be a new target in the pathogenesis and treatment of lung diseases associated with premature birth.

DHA suppresses chronic apoptosis in the lung caused by perinatal inflammation

We have previously shown that an adverse perinatal environment significantly alters lung growth and development and results in persistently altered cardiopulmonary physiology in adulthood. Our model of maternal LPS treatment followed by 14 days of neonatal hyperoxia exposure causes severe pulmonary disease characterized by permanent decreases in alveolarization and diffuse interstitial fibrosis. The current investigations tested the hypothesis that dysregulation of Notch signaling pathways contributes to the permanently altered lung phenotype in our model and that the improvements we have observed previously with maternal docosahexaenoic acid (DHA) supplementation are mediated through normalization of Notch-related protein expression. Results indicated that inflammation (IL-6 levels) and oxidation (F2a-isoprostanes) persisted through 8 wk of life in mice exposed to LPS/O2 perinatally. These changes were attenuated by maternal DHA supplementation. Modest but inconsistent differences were observed in Notch-pathway proteins Jagged 1, DLL 1, PEN2, and presenilin-2. We detected substantial increases in markers of apoptosis including PARP-1, APAF-1, caspase-9, BCL2, and HMGB1, and these increases were attenuated in mice that were nursed by DHA-supplemented dams during the perinatal period. Although Notch signaling is not significantly altered at 8 wk of age in mice with perinatal exposure to LPS/O2, our findings indicate that persistent apoptosis continues to occur at 8 wk of age. We speculate that ongoing apoptosis may contribute to persistently altered lung development and may further enhance susceptibility to additional pulmonary disease. Finally, we found that maternal DHA supplementation prevented sustained inflammation, oxidation, and apoptosis in our model.

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.

Mechanical load and mechanical integrity of lung cells - experimental mechanostimulation of epithelial cell- and fibroblast-monolayers

Experimental mechanostimulation of soft biologic tissue is widely used to investigate cellular responses to mechanical stress or strain. Reactions on mechanostimulation are investigated in terms of morphological changes, inflammatory responses and apoptosis/necrosis induction on a cellular level. In this context, the analysis of the mechanical characteristics of cell-layers might allow to indicate patho-physiological changes in the cell-cell contacts. Recently, we described a device for experimental mechanostimulation that allows simultaneous measurement of the mechanical characteristics of cell-monolayers. Here, we investigated how cultivated lung epithelial cell- and fibroblast-monolayers behave mechanically under different amplitudes of biaxial distension. The cell monolayers were sinusoidally deflected to 5%, 10% or 20% surface gain and their mechanical properties during mechanostimulation were analyzed. With increasing stimulation amplitudes more pronounced reductions of cell junctions were observed. These findings were accompanied by a substantial loss of monolayer rigidity. Pulmonary fibroblast monolayers were initially stiffer but were stronger effected by the mechanostimulation compared to epithelial cell-monolayers. We conclude that, according to their biomechanical function within the pulmonary tissue, epithelial cells and fibroblasts differ with respect to their mechanical characteristics and tolerance of mechanical load.

Neonatal hyperoxia stimulates the expansion of alveolar epithelial type II cells

Supplemental oxygen used to treat infants born prematurely disrupts angiogenesis and is a risk factor for persistent pulmonary disease later in life. Although it is unclear how neonatal oxygen affects development of the respiratory epithelium, alveolar simplification and depletion of type II cells has been observed in adult mice exposed to hyperoxia between postnatal Days 0 and 4. Because hyperoxia inhibits cell proliferation, we hypothesized that it depleted the adult lung of type II cells by inhibiting their proliferation at birth. Newborn mice were exposed to room air (RA) or hyperoxia, and the oxygen-exposed mice were recovered in RA. Hyperoxia stimulated mRNA expressed by type II (Sftpc, Abca3) and type I (T1α, Aquaporin 5) cells and inhibited Pecam expressed by endothelial cells. 5-Bromo-2'-deoxyuridine labeling and fate mapping with enhanced green fluorescence protein controlled statically by the Sftpc promoter or conditionally by the Scgb1a1 promoter revealed increased Sftpc and Abca3 mRNA seen on Day 4 reflected an increase in expansion of type II cells shortly after birth. When mice were returned to RA, this expanded population of type II cells was slowly depleted until few were detected by 8 weeks. These findings reveal that hyperoxia stimulates alveolar epithelial cell expansion when it disrupts angiogenesis. The loss of type II cells during recovery in RA may contribute to persistent pulmonary diseases such as those reported in children born preterm who were exposed to supplemental oxygen.

Delay in rat lung alveolarization after the combined exposure of maternal hyperglycemia and postnatal hyperoxia

BACKGROUND

Maternal diabetes interferes with fetal lung development and postnatal treatments may further disturb pulmonary growth. Therefore, we investigated the effect of postnatal oxygen exposure on alveolar development in neonatal rat lungs pre-exposed to intrauterine hyperglycemia.

METHODS

Diabetes was induced in Sprague-Dawley rats with streptozotocin injection before pregnancy. Hyperglycemia-exposed and control litters were randomized to breath room air or 85% oxygen for 7 days after birth. Lungs were analyzed on postnatal d7 for weight, morphology, apoptosis, proliferation, and biomarkers of oxidative stress.

RESULTS

Maternal hyperglycemia accelerated lung development as demonstrated by thinner alveolar walls and slightly increased secondary septation when compared to room air bred rats. Hyperoxia alone caused thin-walled and enlarged alveoli with few secondary septa. Interestingly, the dual exposure inhibited the thinning of alveolar walls and the disappearance of mesenchymal cells from the alveolar walls together with the delay in the formation of alveoli and secondary crests. While the lungs' oxidative stress was similar in all groups, pulmonary apoptosis and proliferation were altered.

CONCLUSION

Our results thus indicate that the hyperglycemic priming of the fetal lung modifies the deleterious effect of hyperoxia on alveolarization in neonatal rats.

Organ-specific development characterizes circadian clock gene Per2 expression in rats

To explore developmental changes in circadian organization of central and peripheral oscillators, circadian rhythms in clock gene expression were examined in 12 organs in transgenic rats carrying a bioluminescence reporter for Per2. Organ slices were obtained from different developmental stages starting at postnatal day 5 and tissue was cultured for more than 6 days. In addition, four organs were examined from embryonic day 20. Robust circadian rhythms in bioluminescence were detected in all organs examined. The circadian period in vitro was specific to each organ and remained essentially the same during development. The circadian peak phase on the first day of culture was significantly different not only among organs but also in the same organ. Three patterns in circadian phase were detected during development. Thus, during development, circadian phase did not change in the suprachiasmatic nucleus, adrenal gland, and liver, whereas delay shifts were seen in the pineal, lung, heart, kidney, spleen, thymus, and testis. Finally, circadian phase advanced at postnatal day 10–15 and subsequently delayed in skeletal muscle and stomach.Circadian amplitude also showed developmental changes in several organs. These findings indicate that the temporal orders of physiological functions of various organs change during development. Such age-dependent and organ-specific changes in the phase relationship among circadian clocks most likely reflect entrainment to organ-specific time cues at different developmental stages.

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.

Recent advances in late lung development and the pathogenesis of bronchopulmonary dysplasia

In contrast to early lung development, a process exemplified by the branching of the developing airways, the later development of the immature lung remains very poorly understood. A key event in late lung development is secondary septation, in which secondary septa arise from primary septa, creating a greater number of alveoli of a smaller size, which dramatically expands the surface area over which gas exchange can take place. Secondary septation, together with architectural changes to the vascular structure of the lung that minimize the distance between the inspired air and the blood, are the objectives of late lung development. The process of late lung development is disturbed in bronchopulmonary dysplasia (BPD), a disease of prematurely born infants in which the structural development of the alveoli is blunted as a consequence of inflammation, volutrauma, and oxygen toxicity. This review aims to highlight notable recent developments in our understanding of late lung development and the pathogenesis of BPD.

Hedgehog signaling in neonatal and adult lung

Sonic Hedgehog (Shh) signaling is essential during embryonic lung development, but its role in postnatal lung development and adult lung are not known. Using Gli1(nlacZ) reporter mice to identify cells with active Hh signaling, we found that Gli1(nlacZ)-positive mesenchymal cells are densely and diffusely present up to 2 weeks after birth and decline in number thereafter. In adult mice, Gli1(nlacZ)-positive cells are present around large airways and vessels and are sparse in alveolar septa. Hh-stimulated cells are mostly fibroblasts; only 10% of Gli1(nlacZ)-positive cells are smooth muscle cells, and most smooth muscle cells do not have activation of Hh signaling. To assess its functional relevance, we influenced Hh signaling in the developing postnatal lung and adult injured lung. Inhibition of Hh signaling during early postnatal lung development causes airspace enlargement without diminished alveolar septation. After bleomycin injury in the adult lung, there are abundant Gli1(nlacZ)-positive mesenchymal cells in fibrotic lesions and increased numbers of Gli1(nlacZ)-positive cells in preserved alveolar septa. Inhibition of Hh signaling with an antibody against all Hedgehog isoforms does not reduce bleomycin-induced fibrosis, but adenovirus-mediated overexpression of Shh increases collagen production in this model. Our data provide strong evidence that Hh signaling can regulate lung stromal cell function in two critical scenarios: normal development in postnatal lung and lung fibrosis in adult lung.

Neonatal steroids induce a down-regulation of tenascin-C and elastin and cause a deceleration of the first phase and an acceleration of the second phase of lung alveolarization

Pre- and postnatal corticosteroids are often used in perinatal medicine to improve pulmonary function in preterm infants. To mimic this clinical situation, newborn rats were treated systemically with dexamethasone (Dex), 0.1-0.01 mg/kg/day on days P1-P4. We hypothesized that postnatal Dex may have an impact on alveolarization by interfering with extracellular matrix proteins and cellular differentiation. Morphological alterations were observed on 3D images obtained by high-resolution synchrotron radiation X-ray tomographic microscopy. Alveolarization was quantified stereologically by estimating the formation of new septa between days P4 and P60. The parenchymal expression of tenascin-C (TNC), smooth muscle actin (SMA), and elastin was measured by immunofluorescence and gene expression for TNC by qRT-PCR. After Dex treatment, the first phase of alveolarization was significantly delayed between days P6 and P10, whereas the second phase was accelerated. Elastin and SMA expressions were delayed by Dex treatment, whereas TNC expression was delayed and prolonged. A short course of neonatal steroids impairs the first phase of alveolarization, most likely by altering the TNC and elastin expression. Due to an overshooting catch-up during the second phase of alveolarization, the differences disappear when the animals reach adulthood.

Mechanisms of acute respiratory distress syndrome in children and adults: a review and suggestions for future research

OBJECTIVES

To provide a current overview of the epidemiology and pathophysiology of acute respiratory distress syndrome in adults and children, and to identify research questions that will address the differences between adults and children with acute respiratory distress syndrome.

DATA SOURCES

Narrative literature review and author-generated data.

DATA SELECTION

The epidemiology of acute respiratory distress syndrome in adults and children, lung morphogenesis, and postnatal lung growth and development are reviewed. The pathophysiology of acute respiratory distress syndrome is divided into eight categories: alveolar fluid transport, surfactant, innate immunity, apoptosis, coagulation, direct alveolar epithelial injury by bacterial products, ventilator-associated lung injury, and repair.

DATA EXTRACTION AND SYNTHESIS

Epidemiologic data suggest significant differences in the prevalence and mortality of acute respiratory distress syndrome between children and adults. Postnatal lung development continues through attainment of adult height, and there is overlap between the regulation of postnatal lung development and inflammatory, apoptotic, alveolar fluid clearance, and repair mechanisms. Therefore, there is a different biological baseline network of gene and protein expression in children as compared with adults.

CONCLUSIONS

There are significant obstacles to performing research on children with acute respiratory distress syndrome. However, epidemiologic, clinical, and animal studies suggest age-dependent differences in the pathophysiology of acute respiratory distress syndrome. In order to reduce the prevalence and improve the outcome of patients with acute respiratory distress syndrome, translational studies of inflammatory, apoptotic, alveolar fluid clearance, and repair mechanisms are needed. Understanding the differences in pathophysiologic mechanisms in acute respiratory distress syndrome between children and adults should facilitate identification of novel therapeutic interventions to prevent or modulate lung injury and improve lung repair.

Acute lung injury in preterm fetuses and neonates: mechanisms and molecular pathways

Acute lung injury (ALI) results in high morbidity and mortality among preterm neonates and efforts have therefore been devoted to both antenatal and postnatal prevention of the disease. ALI is the result of an inflammatory response which is triggered by a variety of different mechanisms. It mostly affects the fetal lung and, in particular, causes damage to the integrity of the lung's alveolar-capillary unit while weakening its cellular linings. Chemotactic activity and inflammatory products, such as proinflammatory cytokines TNF-α, IL-1, IL-6, IL-11, VEGF,TGF-α and TGF-β, provoke serious damage to the capillary endothelium and the alveolar epithelium, resulting in hyaline membrane formation and leakage of protein-rich edema fluid into the alveoli. Chorioamnionitis plays a major part in triggering fetal lung inflammation, while mechanical ventilation, the application of which is frequently necessary in preterm neonates, also causes ALI by inducing proinflammatory cytokines. Many different ventilation-strategies have been developed in order to reduce potential lung injury. Furthermore, tissue injury may occur as a result of injurious oxygen by-products (Reactive Oxygen Species, ROS), secondary to hyperoxia. Knowledge of the inflammatory pathways that connect intra-amniotic inflammation and ALI can lead to the formulation of novel interventional procedures. Future research should concentrate on the pathophysiology of ALI in preterm neonates and οn possible pharmaceutical interventions targeting prevention and/or resolution of ALI.

Diffusion-weighted MR imaging of fetal lung maturation in sheep: effect of prenatal cortisone administration on ADC values

OBJECTIVE

To assess changes in diffusion properties in the fetal lung after cortisone administration with diffusion-weighted imaging (DWI) in fetal sheep.

METHODS

DWI was performed on 11 pregnant sheep with singleton pregnancies on a 1.5-T MRI scanner. Four animals received cortisone injections before baseline imaging. Seven animals served as controls. Apparent diffusion coefficient (ADC) was measured on DWI in the fetal lungs by two independent readers. The Pearson test was used to correlate ADC and gestational age. A t-test was performed to compare differences in ADC values at the baseline and follow-up images within and between groups. Inter-rater reliability was calculated.

RESULTS

In the cortisone group, ADC values increased about 10 % between the baseline and follow-up images (P = 0.039). Comparing the cortisone and control groups, ADC values of the baseline images did not differ; whereas in the follow-up imaging, ADC values were significantly higher in the cortisone group (P = 0.024). Lung ADC values did not correlate with gestational age (P = 0.970). Inter-rater reliability was high (0.970, P = 0.000).

CONCLUSION

In this experimental model, MR-DWI can detect cortisone-induced changes in diffusion properties of the fetal lung.

KEY POINTS

• Corticosteroids are frequently administered antenatally to prevent fetal lung immaturity at birth • DWI can detect changes in the fetal lung after corticosteroid administration • Changes can be detected as early as 5 days after treatment • Fetal MRI may offer a non-invasive method of monitoring lung maturation.

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.

Transcription factor Klf4, induced in the lung by oxygen at birth, regulates perinatal fibroblast and myofibroblast differentiation

The fluid-filled lung exists in relative hypoxia in utero (∼25 mm Hg), but at birth fills with ambient air where the partial pressure of oxygen is ∼150 mm Hg. The impact of this change was studied in mouse lung with microarrays to analyze gene expression one day before, and 2, 6, 12 and 24 hours after birth into room air or 10% O₂. The expression levels of >150 genes, representing transcriptional regulation, structure, apoptosis and antioxidants were altered 2 hrs after birth in room air but blunted or absent with birth in 10% O₂. Kruppel-like factor 4 (Klf4), a regulator of cell growth arrest and differentiation, was the most significantly altered lung gene at birth. Its protein product was expressed in fibroblasts and airway epithelial cells. Klf4 mRNA was induced in lung fibroblasts exposed to hyperoxia and constitutive expression of Klf4 mRNA in Klf4-null fibroblasts induced mRNAs for p21^cip1/Waf1, smooth muscle actin, type 1 collagen, fibronectin and tenascin C. In Klf4 perinatal null lung, p21^cip1/Waf1mRNA expression was deficient prior to birth and associated with ongoing cell proliferation after birth; connective tissue gene expression was deficient around birth and smooth muscle actin protein expression was absent from myofibroblasts at tips of developing alveoli; p53, p21^cip1/Waf1 and caspase-3 protein expression were widespread at birth suggesting excess apoptosis compared to normal lung. We propose that the changing oxygen environment at birth acts as a physiologic signal to induce lung Klf4 mRNA expression, which then regulates proliferation and apoptosis in fibroblasts and airway epithelial cells, and connective tissue gene expression and myofibroblast differentiation at the tips of developing alveoli.

Development and remodeling of the vertebrate blood-gas barrier

During vertebrate development, the lung inaugurates as an endodermal bud from the primitive foregut. Dichotomous subdivision of the bud results in arborizing airways that form the prospective gas exchanging chambers, where a thin blood-gas barrier (BGB) is established. In the mammalian lung, this proceeds through conversion of type II cells to type I cells, thinning, and elongation of the cells as well as extrusion of the lamellar bodies. Subsequent diminution of interstitial tissue and apposition of capillaries to the alveolar epithelium establish a thin BGB. In the noncompliant avian lung, attenuation proceeds through cell-cutting processes that result in remarkable thinning of the epithelial layer. A host of morphoregulatory molecules, including transcription factors such as Nkx2.1, GATA, HNF-3, and WNT5a; signaling molecules including FGF, BMP-4, Shh, and TFG- β and extracellular proteins and their receptors have been implicated. During normal physiological function, the BGB may be remodeled in response to alterations in transmural pressures in both blood capillaries and airspaces. Such changes are mitigated through rapid expression of the relevant genes for extracellular matrix proteins and growth factors. While an appreciable amount of information regarding molecular control has been documented in the mammalian lung, very little is available on the avian lung.

Role of mechanical stretching and lipopolysaccharide in early apoptosis and IL-8 of alveolar epithelial type II cells A549

OBJECTIVE

To investigate the effects of mechanical stretching and lipopolysaccharide (LPS) on the early apoptosis and IL-8 production of alveolar epithelial type II cells A549.

METHODS

The experimental matrix consisted of three integrated studies. In the first study, A549 cells were subjected to different stretching strain frequency and duration time to see the effects on the early apoptosis. In the second study, A549 cells were subjected to mechanical stretch (15% 4 h, 0.5 Hz) and LPS (1 or 100 ng/mL) to see whether mechanical strain and LPS also have an addictive effect on the early apoptosis. In the third study to investigate whether this addictive effect could be induced by LPS and mechanical stretch on IL-8 production, A549 cells were subjected to LPS (100 ng/mL) and mechanical strain (15%, 0.5 Hz, 4 h). Real time PCR and enzyme linked immunosorbent assay were used to measure mRNA and protein level of IL-8. The early apoptosis was detected by flow cytometry.

RESULTS

Mechanical stretch induced the early apoptosis in a force and frequency and time-dependent manner. In the presence of LPS, mechanical stretch enhanced LPS-induced early apoptosis, especially in 100 ng/mL LPS group compared with 1 ng/mL LPS and the control group. Mechanical stretch increased IL-8 production and enhanced LPS-induced IL-8 screation both in mRNA and protein levels.

CONCLUSIONS

Mechanical stretch can induce the early apoptosis and IL-8 secretion. Mechanical stretch and LPS have an addictive effect on the early apoptosis and IL-8 production in alveolar type 2 cells, which is one of the mechanisms of ventilator-induced lung injury.

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.

Conditional deletion of epithelial IKKβ impairs alveolar formation through apoptosis and decreased VEGF expression during early mouse lung morphogenesis

Background

Alveolar septation marks the beginning of the transition from the saccular to alveolar stage of lung development. Inflammation can disrupt this process and permanently impair alveolar formation resulting in alveolar hypoplasia as seen in bronchopulmonary dysplasia in preterm newborns. NF-κB is a transcription factor central to multiple inflammatory and developmental pathways including dorsal-ventral patterning in fruit flies; limb, mammary and submandibular gland development in mice; and branching morphogenesis in chick lungs. We have previously shown that epithelial overexpression of NF-κB accelerates lung maturity using transgenic mice. The purpose of this study was to test our hypothesis that targeted deletion of NF-κB signaling in lung epithelium would impair alveolar formation.

Methods

We generated double transgenic mice with lung epithelium-specific deletion of IKKβ, a known activating kinase upstream of NF-κB, using a cre-loxP transgenic recombination strategy. Lungs of resulting progeny were analyzed at embryonic and early postnatal stages to determine specific effects on lung histology, and mRNA and protein expression of relevant lung morphoreulatory genes. Lastly, results measuring expression of the angiogenic factor, VEGF, were confirmed in vitro using a siRNA-knockdown strategy in cultured mouse lung epithelial cells.

Results

Our results showed that IKKβ deletion in the lung epithelium transiently decreased alveolar type I and type II cells and myofibroblasts and delayed alveolar formation. These effects were mediated through increased alveolar type II cell apoptosis and decreased epithelial VEGF expression.

Conclusions

These results suggest that epithelial NF-κB plays a critical role in early alveolar development possibly through regulation of VEGF.

The pulmonary blood-gas barrier in the avian embryo: inauguration, development and refinement

In vertebrates, efficient gas exchange depends primarily on establishment of a thin blood-gas barrier (BGB). The primordial air conduits of the developing avian lung are lined with a cuboidal epithelium that is ultimately converted to a squamous one that participates in the formation of the BGB. In the early stages, cells form intraluminal protrusions (aposomes) then transcellular double membranes separating the aposome from the basal part of the cell establish, unzip and sever the aposome from the cell. Additionally, better endowed cells squeeze out adjacent cells or such cells constrict spontaneously thus extruding the squeezed out aposome. Formation of vesicles or vacuoles below the aposome and fusion of such cavities with their neighboring cognates results in severing of the aposome. Augmentation of cavities and their subsequent fusion with the apical plasma membranes results in formation of numerous microfolds separating concavities on the apical part of the cell. Abscission of such microfolds results in a smooth squamous epithelium just before hatching.

Life-long programming implications of exposure to tobacco smoking and nicotine before and soon after birth: evidence for altered lung development

Tobacco smoking during pregnancy remains common, especially in indigenous communities, and likely contributes to respiratory illness in exposed offspring. It is now well established that components of tobacco smoke, notably nicotine, can affect multiple organs in the fetus and newborn, potentially with life-long consequences. Recent studies have shown that nicotine can permanently affect the developing lung such that its final structure and function are adversely affected; these changes can increase the risk of respiratory illness and accelerate the decline in lung function with age. In this review we discuss the impact of maternal smoking on the lungs and consider the evidence that smoking can have life-long, programming consequences for exposed offspring. Exposure to maternal tobacco smoking and nicotine intake during pregnancy and lactation changes the genetic program that controls the development and aging of the lungs of the offspring. Changes in the conducting airways and alveoli reduce lung function in exposed offspring, rendering the lungs more susceptible to obstructive lung disease and accelerating lung aging. Although it is generally accepted that prevention of maternal smoking during pregnancy and lactation is essential, current knowledge of the effects of nicotine on lung development does not support the use of nicotine replacement therapy in this group.

Inhibition of apoptosis by 60% oxygen: a novel pathway contributing to lung injury in neonatal rats

During early postnatal alveolar formation, the lung tissue of rat pups undergoes a physiological remodeling involving apoptosis of distal lung cells. Exposure of neonatal rats to severe hyperoxia (≥95% O2) both arrests lung growth and results in increased lung cell apoptosis. In contrast, exposure to moderate hyperoxia (60% O2) for 14 days does not completely arrest lung cell proliferation and is associated with parenchymal thickening. On the basis of similarities in lung architecture observed following either exposure to 60% O2, or pharmacological inhibition of physiological apoptosis, we hypothesized that exposure to 60% O2 would result in an inhibition of physiological lung cell apoptosis. Consistent with this hypothesis, we observed that the parenchymal thickening induced by exposure to 60% O2 was associated with decreased numbers of apoptotic cells, increased expressions of the antiapoptotic regulator Bcl-xL, and the putative antiapoptotic protein survivin, and decreased expressions of the proapoptotic cleaved caspases-3 and -7. In summary, exposure of the neonatal rat lung to moderate hyperoxia results in an inhibition of physiological apoptosis, which contributes to the parenchymal thickening observed in the resultant lung injury.

Cleaved caspase-3 in lung epithelium of children who died with acute respiratory distress syndrome

OBJECTIVE

To investigate the extent of cleaved caspase-3 immunostaining in lung epithelial cells in children with acute respiratory distress syndrome.

DESIGN

Observational study in sixteen children who died with acute respiratory distress syndrome and diffuse alveolar damage.

SETTING

Pediatric intensive care unit.

PATIENTS

Sixteen children with fatal acute respiratory distress syndrome and diffuse alveolar damage.

INTERVENTION

None.

MEASUREMENTS AND MAIN RESULTS

Double immunohistochemistry for cleaved caspase-3 and (pan)cytokeratin in lung tissues obtained at autopsy. Spectral imaging was used for the quantification of immunohistochemistry colocalization of these markers. We found a wide range in the percentage of alveolar epithelial cell surface area with positive cleaved caspase-3 staining in the lungs of children with acute respiratory distress syndrome (from 1% to almost 20%). The degree of caspase-3 immunostaining in epithelial cells positively correlated with age.

CONCLUSION

There is a high variability in the extent of classic apoptosis in lung epithelial cells in pediatric acute respiratory distress syndrome, potentially in part dependent on age.

Fetal and postnatal lung defects reveal a novel and required role for Fgf8 in lung development

The fibroblast growth factor, FGF8, has been shown to be essential for vertebrate cardiovascular, craniofacial, brain and limb development. Here we report that Fgf8 function is required for normal progression through the late fetal stages of lung development that culminate in alveolar formation. Budding, lobation and branching morphogenesis are unaffected in early stage Fgf8 hypomorphic and conditional mutant lungs. Excess proliferation during fetal development disrupts distal airspace formation, mesenchymal and vascular remodeling, and Type I epithelial cell differentiation resulting in postnatal respiratory failure and death. Our findings reveal a previously unknown, critical role for Fgf8 function in fetal lung development and suggest that this factor may also contribute to postnatal alveologenesis. Given the high number of premature infants with alveolar dysgenesis and lung dysplasia, and the accumulating evidence that short-term benefits of available therapies may be outweighed by long-term detrimental effects on postnatal alveologenesis, the therapeutic implications of identifying a factor or pathway that can be targeted to stimulate normal alveolar development are profound.

Development, repair and fibrosis: what is common and why it matters

The complex structure of the lung is developed sequentially, initially by epithelial tube branching and later by septation of terminal air sacs with accompanying coordinated growth of a variety of lung epithelial and mesenchymal cells. Groups of transcriptional factors, peptide growth factors and their intracellular signaling regulators, as well as extracellular matrix proteins are programmed to be expressed at appropriate levels in the right place at the right time to control normal lung formation. Studies of lung development and lung repair/fibrosis to date have discovered that many of the same factors that control normal development are also key players in lung injury repair and fibrosis. Transforming growth factor-beta (TGF-beta) family peptide signaling is a prime example. Lack of TGF-beta signaling results in abnormal lung branching morphogenesis and alveolarization during development, whereas excessive amounts of TGF-beta signaling cause severe hypoplasia in the immature lung and fibrosis in mature lung. This leads us to propose the 'Goldilocks' hypothesis of regulatory signaling in lung development and injury repair that everything must be done just right!

Lung organogenesis

Developmental lung biology is a field that has the potential for significant human impact: lung disease at the extremes of age continues to cause major morbidity and mortality worldwide. Understanding how the lung develops holds the promise that investigators can use this knowledge to aid lung repair and regeneration. In the decade since the "molecular embryology" of the lung was first comprehensively reviewed, new challenges have emerged-and it is on these that we focus the current review. Firstly, there is a critical need to understand the progenitor cell biology of the lung in order to exploit the potential of stem cells for the treatment of lung disease. Secondly, the current familiar descriptions of lung morphogenesis governed by growth and transcription factors need to be elaborated upon with the reinclusion and reconsideration of other factors, such as mechanics, in lung growth. Thirdly, efforts to parse the finer detail of lung bud signaling may need to be combined with broader consideration of overarching mechanisms that may be therapeutically easier to target: in this arena, we advance the proposal that looking at the lung in general (and branching in particular) in terms of clocks may yield unexpected benefits.

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.

Epithelial ablation of Bcl-XL increases sensitivity to oxygen without disrupting lung development

Recent studies indicate that the antiapoptotic Bcl-X(L), one of five isoforms expressed by the Bcl-X gene, protects a variety of cell lines exposed to hyperoxia. However, its role in lung development and protection against oxidative stress in vivo is not known. Here, we show Bcl-X(L) is the predominant isoform expressed in the lung, and the only isoform detected in respiratory epithelium. Because loss of Bcl-X(L) is embryonically lethal, Bcl-X(L) was ablated throughout the respiratory epithelium by mating mice with a floxed exon II of the Bcl-X gene with mice expressing Cre under control of the surfactant protein-C promoter. Interestingly, the loss of Bcl-X(L) in respiratory epithelium was perinatally lethal in approximately 50% of the expected offspring. However, some adult mice lacking the gene were obtained. The epithelial-specific ablation of Bcl-X(L) did not disrupt pulmonary function, the expression of epithelial cell-specific markers, or lung development. However, it shifted the lung toward a proapoptotic state, defined by a reduction in antiapoptotic Mcl-1, an increase in proapoptotic Bak, and increased sensitivity of the respiratory epithelium to hyperoxia. Intriguingly, increased 8-oxoguanine lesions seen during hyperoxia were also evident as lungs transitioned to room air at birth, a time when perinatal lethality in some mice lacking Bcl-X(L) was observed. These findings reveal that the epithelial-specific expression of Bcl-X(L) is not required for proper lung development, but functions to protect respiratory epithelial cells against oxygen-induced toxicity, such as during hyperoxia and the lung's first exposure to ambient air.

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.

Are nicotine replacement therapy, varenicline or bupropion options for pregnant mothers to quit smoking? Effects on the respiratory system of the offspring

Nicotine occurs in tobacco smoke. It is a habit-forming substance and is prescribed by health professionals to assist smokers to quit smoking. It is rapidly absorbed from the lungs of smokers. It crosses the placenta and accumulates in the developing fetus. Nicotine induces formation of oxygen radicals and at the same time also reduces the antioxidant capacity of the lungs. Nicotine and the oxidants cause point mutations in the DNA molecule thereby changing the program that controls lung growth and maintenance of lung structure. The data available indicate that maternal nicotine exposure induces a persistent inhibition of glycolysis and a drastically increased AMP level. These metabolic changes are thought to contribute to the faster aging of the lungs of the offspring of mothers that are exposed to nicotine via the placenta and mother's milk. The lungs of these animals are more susceptible to damage as shown by the gradual deterioration of the lung parenchyma. The rapid metabolic and structural aging of the lungs of the animals exposed to nicotine via the placenta and mother's milk, and thus during phases of lung development characterized by rapid cell division, is likely due to 'programming' induced by nicotine. Since varenicline, a partial nicotine agonist, has basically the same structure as nicotine, and also binds to acetylcholine receptors in competition with nicotine (but with largely the same effect), it is not advisable to use nicotine or varenicline during gestation and lactation. Furthermore, the use of individual vitamin supplements is also not advisable because of the negative impact on the program that controls maintenance of lung structural and functional integrity and aging. A more appropriate smoking cessation program will also include a mixture of antioxidant nutrients such as in tomato juice.

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.