Neonatal oxygen adversely affects lung function in adult mice without altering surfactant composition or activity

Abstract

Despite its potentially adverse effects on lung development and function, supplemental oxygen is often used to treat premature infants in respiratory distress. To understand how neonatal hyperoxia can permanently disrupt lung development, we previously reported increased lung compliance, greater alveolar simplification, and disrupted epithelial development in adult mice exposed to 100% inspired oxygen fraction between postnatal days 1 and 4. Here, we investigate whether oxygen-induced changes in lung function are attributable to defects in surfactant composition and activity, structural changes in alveolar development, or both. Newborn mice were exposed to room air or 40%, 60%, 80%, or 100% oxygen between postnatal days 1 and 4 and allowed to recover in room air until 8 wk of age. Lung compliance and alveolar size increased, and airway resistance, airway elastance, tissue elastance, and tissue damping decreased, in mice exposed to 60 -80% oxygen; changes were even greater in mice exposed to 100% oxygen. These alterations in lung function were not associated with changes in total protein content or surfactant phospholipid composition in bronchoalveolar lavage. Moreover, surface activity and total and hydrophobic protein content were unchanged in large surfactant aggregates centrifuged from bronchoalveolar lavage compared with control. Instead, the number of type II cells progressively declined in 60 -100% oxygen, whereas levels of T1␣, a protein expressed by type I cells, were comparably increased in mice exposed to 40 -100% oxygen. Thickened bundles of elastin fibers were also detected in alveolar walls of mice exposed to Ն60% oxygen. These findings support the hypothesis that changes in lung development, rather than surfactant activity, are the primary causes of oxygen-altered lung function in children who were exposed to oxygen as neonates. Furthermore, the disruptive effects of oxygen on epithelial development and lung mechanics are not equivalently dose dependent. bronchopulmonary dysplasia; epithelium; hyperoxia; type II cells BRONCHOPULMONARY DYSPLASIA (BPD) is a chronic lung disease often seen in premature infants with very low birth weight (21). At autopsy, lungs of infants who die from BPD are less vascularized, with fewer and larger alveoli (7). Although the pathophysiology of BPD is complex and related in part to gestational age, neonatal hyperoxia is recognized as an important contributing factor to this disease in many infants (see Refs. 3,12,17, 37 for review). Premature infants with BPD have low plasma levels of glutathione (59), and hyperoxia in the context of an immature antioxidant defense increases the potential for oxidative stress injury. The use of exogenous surfactant, antenatal steroids, and milder ventilation strategies has markedly increased survival and other improved outcomes for premature infants over the past two decades. However, many patients continue to show decreased lung capacity, even as young adolescents (19,20,55). Moreover, these children are often rehospitalized following r...

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

“…Like children and adolescents born prematurely who suffer from abnormalities in airflow obstruction and air trapping, 22-26 adult 8-week-old mice exposed to Ն60% oxygen between birth and postnatal day 4 have increased lung compliance attributed to alveolar simplification, increased elastin, and changes in the proportion of alveolar epithelial cells. 27,32 Female C57Bl6/J females exposed to hyperoxia also show spontaneous airway reactivity in response to methacholine challenge, perhaps modeling the increased risk for asthma often seen in children born prematurely (unpublished observations). And like children born prematurely who are often rehospitalized when infected with respiratory syncytial virus, 20,21 adult mice exposed to neonatal hyperoxia show increased susceptibility to influenza A virus infection.…”

“…In contrast, room air-and hyperoxia-exposed mice had identical tissue damping (a measure of energy dissipated into lung tissues) and airway resistance at 67 weeks; these measures had been significantly elevated in hyperoxia-exposed mice at 8 weeks. 32 Similarly, as defined by quantifying mean alveolar chord length, the alveolar simplification seen at 8 weeks was no longer evident; in fact, mean chord length was slightly, but significantly, reduced in aged mice previously exposed to neonatal hyperoxia ( Figure 2F). 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.…”

“…Equivalent amounts of protein were resolved on Tris-HCl SDS-PAGE gel, transferred to a polyvinylidene difluoride (PVDF) membrane, and blocked in 5% nonfat dried milk. Membranes were incubated in rabbit antiprosurfactant protein C (SP-C) ( 32 Stained sections were visualized with a Nikon E800 fluorescence microscope (Nikon), and images were captured with a SPOT-RT digital camera (Diagnostic Instruments, Sterling Heights, MI).…”

“…However, thickened bundles of elastin lining alveolar epithelial walls observed at 8 weeks of age were still evident in the aged mice, implying that this might contribute to the increased alveolar compliance when alveolar simplification was not readily apparent. 32 Pulmonary vascular imaging using micro-CT was used to quantify changes in small vessels in an unbiased, threedimensional fashion. Lungs from room air mice displayed uniform perfusion of the distal arterial tree, whereas lungs exposed to neonatal oxygen showed significant vascular rarefaction ( Figure 5, A and B).…”

“…[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.…”