PULMONARY ELASTIN SYNTHESIS AND DEPOSITION IN DEVELOPING AND MATURE SHEEP: EFFECTS OF INTRAUTERINE GROWTH RESTRICTION

Perinatal Undernutrition, Metabolic Hormones, and Lung Development

Maternal and perinatal undernutrition affects the lung development of litters and it may produce long-lasting alterations in respiratory health. This can be demonstrated using animal models and epidemiological studies. During pregnancy, maternal diet controls lung development by direct and indirect mechanisms. For sure, food intake and caloric restriction directly influence the whole body maturation and the lung. In addition, the maternal food intake during pregnancy controls mother, placenta, and fetal endocrine systems that regulate nutrient uptake and distribution to the fetus and pulmonary tissue development. There are several hormones involved in metabolic regulations, which may play an essential role in lung development during pregnancy. This review focuses on the effect of metabolic hormones in lung development and in how undernutrition alters the hormonal environment during pregnancy to disrupt normal lung maturation. We explore the role of GLP-1, ghrelin, and leptin, and also retinoids and cholecalciferol as hormones synthetized from diet precursors. Finally, we also address how metabolic hormones altered during pregnancy may affect lung pathophysiology in the adulthood.

Novel functional role of GH/IGF-I in neonatal lung myofibroblasts and in rat lung growth after intrauterine growth restriction

Intrauterine growth restriction (IUGR) is a risk factor for neonatal chronic lung disease (CLD) characterized by reduced alveoli and perturbed matrix remodeling. Previously, our group showed an activation of myofibroblasts and matrix remodeling in rat lungs after IUGR. Because growth hormone (GH) and insulin-like growth factor I (IGF-I) regulate development and growth, we queried 1) whether GH/IGF-I signaling is dysregulated in lungs after IUGR and 2) whether GH/IGF-I signaling is linked to neonatal lung myofibroblast function. IUGR was induced in Wistar rats by isocaloric low-protein diet during gestation. Lungs were obtained at embryonic day (E) 21, postnatal day (P) 3, P12, and P23. Murine embryonic fibroblasts (MEF) or primary neonatal myofibroblasts from rat lungs of control (pnF^Co) and IUGR (pnF^IUGR) were used for cell culture studies. In the intrauterine phase (E21), we found a reduction in GH receptor (GH-R), Stat5 signaling and IGF-I expression in lungs after IUGR. In the postnatal phase (P3-P23), catchup growth after IUGR was linked to increased GH mRNA, GH-R protein, activation of proliferative Stat5/Akt signaling, cyclin D1 and PCNA in rat lungs. On P23, a thickening of the alveolar septae was related to increased vimentin and matrix deposition, indicating fibrosis. In cell culture studies, nutrient deprivation blocked GH-R/IGF-IR signaling and proliferation in MEFs; this was reversed by IGF-I. Proliferation and Stat5 activation were increased in pnF^IUGR. IGF-I and GH induced proliferation and migration of pnF^Co; only IGF-I had these effects on pnF^IUGR. Thus, we show a novel mechanism by which the GH/IGF-I axis in lung myofibroblasts could account for structural lung changes after IUGR.

Developmental programming of cardiovascular disease following intrauterine growth restriction: findings utilising a rat model of maternal protein restriction

Over recent years, studies have demonstrated links between risk of cardiovascular disease in adulthood and adverse events that occurred very early in life during fetal development. The concept that there are embryonic and fetal adaptive responses to a sub-optimal intrauterine environment often brought about by poor maternal diet that result in permanent adverse consequences to life-long health is consistent with the definition of "programming". The purpose of this review is to provide an overview of the current knowledge of the effects of intrauterine growth restriction (IUGR) on long-term cardiac structure and function, with particular emphasis on the effects of maternal protein restriction. Much of our recent knowledge has been derived from animal models. We review the current literature of one of the most commonly used models of IUGR (maternal protein restriction in rats), in relation to birth weight and postnatal growth, blood pressure and cardiac structure and function. In doing so, we highlight the complexity of developmental programming, with regards to timing, degree of severity of the insult, genotype and the subsequent postnatal phenotype.

[Intrauterine growth retardation and lung development]

Epidemiological studies have shown that intrauterine growth restriction is associated with increased respiratory morbidity in the neonatal period with an increased risk of bronchopulmonary dysplasia. Respiratory consequences of environmental intrauterine changes extend into childhood and adulthood with abnormal lung function tests. In animal models, changes in surfactant and alveolarization disorders vary from one study to another. Moreover, the molecular mechanisms involved are poorly understood. Fetal adaptations to intrauterine malnutrition result in permanent changes in lung structure, raising the question of lung "programming".

Effects of maternal food restriction on offspring lung extracellular matrix deposition and long term pulmonary function in an experimental rat model

Intrauterine growth restriction (IUGR) increases the risk of respiratory compromise throughout postnatal life. However, the molecular mechanism(s) underlying the respiratory compromise in offspring following IUGR is not known. We hypothesized that IUGR following maternal food restriction (MFR) would affect extracellular matrix deposition in the lung, explaining the long-term impairment in pulmonary function in the IUGR offspring. Using a well-established rat model of MFR during gestation to produce IUGR pups, we found that at postnatal day 21, and at 9 months (9M) of age the expression and abundance of elastin and alpha smooth muscle actin (αSMA), two key extracellular matrix proteins, were increased in IUGR lungs when compared to controls (P < 0.05, n = 6), as determined by both Western and immunohistochemistry analyses. Compared to controls, the MFR group showed no significant change in pulmonary resistance at baseline, but did have significantly decreased pulmonary compliance at 9M (P < 0.05 vs. control, n = 5). In addition, MFR lungs exhibited increased responsiveness to methacholine challenge. Furthermore, exposing cultured fetal rat lung fibroblasts to serum deprivation increased the expression of elastin and elastin-related genes, which was blocked by serum albumin supplementation, suggesting protein deficiency as the predominant mechanism for increased pulmonary elastin deposition in IUGR lungs. We conclude that accompanying the changes in lung function, consistent with bronchial hyperresponsiveness, expression of the key alveolar extracellular matrix proteins elastin and αSMA increased in the IUGR lung, thus providing a potential explanation for the compromised lung function in IUGR offspring.

Injury and repair in the very immature lung following brief mechanical ventilation

Mechanical ventilation (MV) of very premature infants contributes to lung injury and bronchopulmonary dysplasia (BPD), the effects of which can be long-lasting. Little is currently known about the ability of the very immature lung to recover from ventilator-induced lung injury. Our objective was to determine the ability of the injured very immature lung to repair in the absence of continued ventilation and to identify potential mechanisms. At 125 days gestational age (days GA, 0.85 of term), fetal sheep were partially exposed by hysterotomy under anesthesia and aseptic conditions; they were intubated and ventilated for 2 h with an injurious MV protocol and then returned to the uterus to continue development. Necropsy was performed at either 1 day (short-term group, 126 days GA, n = 6) or 15 days (long-term group, 140 days GA, n = 5) after MV; controls were unventilated (n = 7-8). At 1 day after MV, lungs displayed signs of injury, including hemorrhage, disorganized elastin and collagen deposition in the distal airspaces, altered morphology, significantly reduced secondary septal crest density, and decreased airspace. Bronchioles had thickened epithelium with evidence of injury and sloughing. Relative mRNA levels of early response genes (connective tissue growth factor, cysteine-rich 61, and early growth response-1) and proinflammatory cytokines [interleukins (IL)-1β, IL-6, IL-8, tumor necrosis factor-α, and transforming growth factor-β] were not different between groups 1 day after MV. At 15 days after MV, lung structure was normal with no evidence of injury. We conclude that 2 h of MV induces severe injury in the very immature lung and that these lungs have the capacity to repair spontaneously in the absence of further ventilation.

Partial pulmonary embolization disrupts alveolarization in fetal sheep

Background

Although bronchopulmonary dysplasia is closely associated with an arrest of alveolar development and pulmonary capillary dysplasia, it is unknown whether these two features are causally related. To investigate the relationship between pulmonary capillaries and alveolar formation, we partially embolized the pulmonary capillary bed.

Methods

Partial pulmonary embolization (PPE) was induced in chronically catheterized fetal sheep by injection of microspheres into the left pulmonary artery for 1 day (1d PPE; 115d gestational age; GA) or 5 days (5d PPE; 110-115d GA). Control fetuses received vehicle injections. Lung morphology, secondary septal crests, elastin, collagen, myofibroblast, PECAM1 and HIF1α abundance and localization were determined histologically. VEGF-A, Flk-1, PDGF-A and PDGF-Rα mRNA levels were measured using real-time PCR.

Results

At 130d GA (term ~147d), in embolized regions of the lung the percentage of lung occupied by tissue was increased from 29 ± 1% in controls to 35 ± 1% in 1d PPE and 44 ± 1% in 5d PPE fetuses (p < 0.001). Secondary septal crest density was reduced from 8 ± 0% in controls to 5 ± 0% in 1d PPE and 4 ± 0% in 5d PPE fetuses (p < 0.05), indicating impaired alveolar formation. The deposition of differentiated myofibroblasts (23 ± 1% vs 28 ± 1%; p < 0.001) and elastin fibres (3 ± 0% vs 4 ± 0%; p < 0.05) were also impaired in embolized lung regions of PPE fetuses compared to controls. PPE did not alter the deposition of collagen or PECAM1. At 116d GA in 5d PPE fetuses, markers of hypoxia indicated that a small and transient hypoxic event had occurred (hypoxia in 6.7 ± 1.4% of the tissue within embolized regions of 5d PPE fetuses at 116d compared to 0.8 ± 0.2% of tissue in control regions). There was no change in the proportion of tissue labelled with HIF1α. There was no change in mRNA levels of the angiogenic factors VEGF and Flk-1, although a small increase in PDGF-Rα expression at 116d GA, from 1.00 ± 0.12 in control fetuses to 1.61 ± 0.18 in 5d PPE fetuses may account for impaired differentiation of alveolar myofibroblasts and alveolar development.

Conclusions

PPE impairs alveolarization without adverse systemic effects and is a novel model for investigating the role of pulmonary capillaries and alveolar myofibroblasts in alveolar formation.