Type II epithelial cells are critical target for hyperoxia-mediated impairment of postnatal lung development

Abstract

-Type II epithelial cells are essential for lung development and remodeling, as they are precursors for type I cells and can produce vascular mitogens. Although type II cell proliferation takes place after hyperoxia, it is unclear why alveolar remodeling occurs normally in adults whereas it is permanently disrupted in newborns. Using a line of transgenic mice whose type II cells could be identified by their expression of enhanced green fluorescent protein and endogenous expression of surfactant proteins, we investigated the age-dependent effects of hyperoxia on type II cell proliferation and alveolar repair. In adult mice, type II cell proliferation was low during room air and hyperoxia exposure but increased during recovery in room air and then declined to control levels by day 7. Eight weeks later, type II cell number and alveolar compliance were indistinguishable from those in room air controls. In newborn mice, type II cell proliferation markedly increased between birth and postnatal day 7 before declining by postnatal day 14. Exposure to hyperoxia between postnatal days 1 and 4 inhibited type II cell proliferation, which resumed during recovery and was aberrantly elevated on postnatal day 14. Eight weeks later, recovered mice had 70% fewer type II cells and 30% increased lung compliance compared with control animals. Recovered mice also had higher levels of T1␣, a protein expressed by type I cells, with minimal changes detected in genes expressed by vascular cells. These data suggest that perinatal hyperoxia adversely affects alveolar development by disrupting the proper timing of type II cell proliferation and differentiation into type I cells. alveoli; cell proliferation; differentiation; enhanced green fluorescent protein; proliferating cell nuclear antigen THE ALVEOLUS IS COMPOSED OF two epithelial cell types that can be identified by their distinct morphology and expression of unique genes. Type I cells are thin, flat cells that cover pulmonary vascular endothelial cells and comprise 95% of the alveolar surface (56). These cells are important for gas exchange, regulation of alveolar fluid levels, and stretch-induced modulation of surfactant secretion. Type I cells can be identified by their expression of T1␣ (also known as RTI 40 ), aquaporin-5, caveolin-1, or the cyclin-dependent kinase inhibitor p15 (41,42). Type II cells, on the other hand, are large,

Neonatal Hyperoxia Causes Pulmonary Vascular Disease and Shortens Life Span in Aging Mice

“…Alveolar simplification and increased compliance observed at 8 weeks of age is associated with a loss of type II epithelial cells as defined by expression of proSP-C, an increase in type I cells as defined by expression of T1␣, and minimal changes in vascular endothelial cells as defined by expression of PECAM. 27,40 Expression of airway CCSP was also reduced. To determine whether these changes persist in aging mice, the expression of these proteins was investigated by Western blot analysis.…”

“…Although two-dimensional histology sections did not reveal an apparent vascular pathology, 27 we had not previously explored vascular density with the micro-CT technique in mice 8 weeks after exposure to neonatal hyperoxia. We evaluated 8-week-old mice using the micro-CT technique to determine whether the vascular changes might be present at this early time point.…”

“…Dams were rotated every 24 hours between pups exposed to room air or hyperoxia until morning of the fourth day when oxygen-exposed mice were returned to room air. 27 Animals were then analyzed at 67 weeks of age, except for mRNA expression and heart weight measurements, which were done at 94 weeks. Data were predominantly collected from 47 mice exposed to room air and 53 mice exposed to oxygen, split between two separate exposures with similar numbers of mice in each group.…”

“…[22][23][24][25][26] Similarly, newborn animals exposed to Ն65% oxygen and recovered in room air develop long-term changes in airway responsiveness and increased lung volumes. [27][28][29][30][31] We previously reported that 8-week-old adult mice exposed to Ն60% oxygen for the first 4 days of life have simplified alveoli attributed to changes in elastin expression and an imbalance in alveolar epithelial type I and II cells. 27,32 Expression of vascular signaling genes was not significantly altered at this time, implying that longer exposures to hyperoxia might be needed to disrupt short-term vascular development and cause more severe disease.…”

“…[27][28][29][30][31] We previously reported that 8-week-old adult mice exposed to Ն60% oxygen for the first 4 days of life have simplified alveoli attributed to changes in elastin expression and an imbalance in alveolar epithelial type I and II cells. 27,32 Expression of vascular signaling genes was not significantly altered at this time, implying that longer exposures to hyperoxia might be needed to disrupt short-term vascular development and cause more severe disease. On the other hand, these mice were highly susceptible to influenza A virus infection, with greater inflammation, fibrosis, and mortality compared to siblings exposed to room air.…”