Postnatal age at onset of hyperoxic exposure influences developmentally regulated tropoelastin gene expression in the neonatal rat lung

Perinatal origins of bronchopulmonary dysplasia-deciphering normal and impaired lung development cell by cell

Bronchopulmonary dysplasia (BPD) is a multifactorial disease occurring as a consequence of premature birth, as well as antenatal and postnatal injury to the developing lung. BPD morbidity and severity depend on a complex interplay between prenatal and postnatal inflammation, mechanical ventilation, and oxygen therapy as well as associated prematurity-related complications. These initial hits result in ill-explored aberrant immune and reparative response, activation of pro-fibrotic and anti-angiogenic factors, which further perpetuate the injury. Histologically, the disease presents primarily by impaired lung development and an arrest in lung microvascular maturation. Consequently, BPD leads to respiratory complications beyond the neonatal period and may result in premature aging of the lung. While the numerous prenatal and postnatal stimuli contributing to BPD pathogenesis are relatively well known, the specific cell populations driving the injury, as well as underlying mechanisms are still not well understood. Recently, an effort to gain a more detailed insight into the cellular composition of the developing lung and its progenitor populations has unfold. Here, we provide an overview of the current knowledge regarding perinatal origin of BPD and discuss underlying mechanisms, as well as novel approaches to study the perturbed lung development.

Aberrant signaling pathways of the lung mesenchyme and their contributions to the pathogenesis of bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is a chronic lung disease in infants born extremely preterm, typically before 28 weeks' gestation, characterized by a prolonged need for supplemental oxygen or positive pressure ventilation beyond 36 weeks postmenstrual age. The limited number of autopsy samples available from infants with BPD in the postsurfactant era has revealed a reduced capacity for gas exchange resulting from simplification of the distal lung structure with fewer, larger alveoli because of a failure of normal lung alveolar septation and pulmonary microvascular development. The mechanisms responsible for alveolar simplification in BPD have not been fully elucidated, but mounting evidence suggests that aberrations in the cross-talk between growth factors of the lung mesenchyme and distal airspace epithelium have a key role. Animal models that recapitulate the human condition have expanded our knowledge of the pathology of BPD and have identified candidate matrix components and growth factors in the developing lung that are disrupted by conditions that predispose infants to BPD and interfere with normal vascular and alveolar morphogenesis. This review focuses on the deviations from normal lung development that define the pathophysiology of BPD and summarizes the various candidate mesenchyme-associated proteins and growth factors that have been identified as being disrupted in animal models of BPD. Finally, future areas of research to identify novel targets affected in arrested lung development and recovery are discussed.

In utero and postnatal exposure to arsenic alters pulmonary structure and function

In addition to cancer endpoints, arsenic exposures can also lead to non-cancerous chronic lung disease. Exposures during sensitive developmental time points can contribute to the adult disease. Using a mouse model, in utero and early postnatal exposures to arsenic (100 ppb or less in drinking water) were found to alter airway reactivity to methacholine challenge in 28 day old pups. Removal of mice from arsenic exposure 28 days after birth did not reverse the alterations in sensitivity to methacholine. In addition, adult mice exposed to similar levels of arsenic in drinking water did not show alterations. Therefore, alterations in airway reactivity were irreversible and specific to exposures during lung development. These functional changes correlated with protein and gene expression changes as well as morphological structural changes around the airways. Arsenic increased the whole lung levels of smooth muscle actin in a dose dependent manner. The level of smooth muscle mass around airways was increased with arsenic exposure, especially around airways smaller than 100 microm in diameter. This increase in smooth muscle was associated with alterations in extracellular matrix (collagen, elastin) expression. This model system demonstrates that in utero and postnatal exposure to environmentally relevant levels of arsenic can irreversibly alter pulmonary structure and function in the adults.

Retinoic acid attenuates O2-induced inhibition of lung septation

Exposure of the newborn lung to hyperoxia is associated with impaired alveolar development. In newborn rats exposed to hyperoxia and studied at day 14 of life, retinoic acid (RA) treatment improved survival and increased lung collagen but did not improve alveolar development. To determine whether RA treatment during exposure to hyperoxia results in late improvement in alveolarization, we treated newborn rats with RA and hyperoxia from day 3 to day 14 and then weaned O2 to room air by day 20, and studied the animals on day 42. O2-exposed animals had larger mean lung volumes, larger alveoli, and decreased gas-exchange tissue relative to air-exposed animals, whereas RA-treated O2-exposed animals were not statistically different from air-exposed controls. Relative to control animals, elastin staining at day 14 was decreased in hyperoxia-exposed lung independent of RA treatment, and, at day 42, elastin staining was similar in all treatment groups. At day 14, elastin gene expression was similar in all treatment groups, whereas at day 42 lung previously exposed to hyperoxia showed increased elastin signal independent of RA treatment. These results indicate that RA treatment during hyperoxia exposure promotes septal formation without evidence of effects on elastin gene expression after 4 wk of recovery.

Syntaxin 1A is transiently expressed in fetal lung mesenchymal cells: potential developmental roles

Lung development is a complex process in which epithelial-mesenchymal interactions play a key role. A conserved secretory apparatus, the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, is essential for exocytosis in many cell types. Syntaxins, located on the terminal plasma membrane (T-SNAREs), are a critical component of the secretosomal complex involved in vesicular docking, fusion, and exocytosis. We analyzed syntaxin 1A mRNA and protein in fetal rat lung ontogeny, demonstrating peak expression on about day 19 of embryonic development, immediately preceding type II pneumocyte differentiation. Syntaxin 1A is predominantly expressed by lipofibroblasts, which are required for bombesin-like peptide-induced surfactant phospholipid synthesis (choline uptake) by isolated type II cells. In organ cultures, anti-syntaxin 1A antibody HPC-1 blocks choline uptake both at baseline and when induced by bombesin-like peptide or dexamethasone. HPC-1 also promotes thymidine uptake in parallel in a dose-dependent fashion. These observations indicate a potential role for syntaxin 1A during fetal lung development, possibly through involvement in secretion of mesenchymal cell-derived factors that induce terminal type II cell differentiation.

Transcriptional regulation of tropoelastin expression in rat lung fibroblasts: changes with age and hyperoxia

Elastic fibers are thought to provide structural support for secondary septa as the lung undergoes the transition from the saccular to the alveolar stage. The synthesis of the soluble precursor of elastin, tropoelastin, occurs during a finite developmental period. We have investigated the developmental regulation of tropoelastin gene transcription and mRNA expression in fetal and postnatal rat lung fibroblasts and have assessed the changes in tropoelastin gene expression caused by hyperoxic exposure during secondary septal development. With the use of an RT-PCR assay and intron-specific primers to detect heterogeneous nuclear RNA (hnRNA) and intron-spanning primers to detect mRNA in freshly isolated rat lung fibroblasts, tropoelastin gene expression was found to be upregulated late in gestation. From days 18 to 21 of gestation, there was a 4.5-fold increase in tropoelastin hnRNA (P < 0.0001) and a 6-fold increase in mRNA (P = 0.002). After birth, tropoelastin expression was downregulated. Signals decreased from fetal day 21 to postnatal day 2 for both tropoelastin hnRNA (P = 0. 021) and mRNA (P = 0.043). Tropoelastin hnRNA decreased further from days 2 to 6 (P = 0.04). Both tropoelastin hnRNA and mRNA were again upregulated during alveolarization from days 9 to 11, indicating that, once upregulated, transcription of the tropoelastin gene is not constant but varies with fetal and postnatal age. Exposure to >95% oxygen, when initiated on postnatal day 2 or 3 and continued until day 11, significantly diminished the developmental increase in tropoelastin hnRNA (P < 0.005) and mRNA (P < 0.05) normally seen on days 9-11, indicating that the postnatal upregulation of tropoelastin gene expression is inhibited by hyperoxic exposure in the early postnatal period.