Hormonal regulation of alveolarization: structure-function correlation

BACKGROUND: Dexamethasone (Dex) limits and all-trans-retinoic acid (RA) promotes alveolarization. While structural changes resulting from such hormonal exposures are known, their functional consequences are unclear. METHODS: Neonatal rats were treated with Dex and/or RA during the first two weeks of life or were given RA after previous exposure to Dex. Morphology was assessed by light microscopy and radial alveolar counts. Function was evaluated by plethysmography at d13, pressure volume curves at d30, and exercise swim testing and arterial blood gases at both d15 and d30. RESULTS: Dex-treated animals had simplified lung architecture without secondary septation. Animals given RA alone had smaller, more numerous alveoli. Concomitant treatment with Dex + RA prevented the Dex-induced changes in septation. While the results of exposure to Dex + RA were sustained, the effects of RA alone were reversed two weeks after treatment was stopped. At d13, Dex-treated animals had increased lung volume, respiratory rate, tidal volume, and minute ventilation. On d15, both RA- and Dex-treated animals had hypercarbia and low arterial pH. By d30, the RA-treated animals resolved this respiratory acidosis, but Dex-treated animals continued to demonstrate blood gas and lung volume abnormalities. Concomitant RA treatment improved respiratory acidosis, but failed to normalize Dex-induced changes in pulmonary function and lung volumes. No differences in exercise tolerance were noted at either d15 or d30. RA treatment after the period of alveolarization also corrected the effects of earlier Dex exposure, but the structural changes due to RA alone were again lost two weeks after treatment. CONCLUSION: We conclude that both RA- and corticosteroid-treatments are associated with respiratory acidosis at d15. While RA alone-induced changes in structure andrespiratory function are reversed, Dex-treated animals continue to demonstrate increased respiratory rate, minute ventilation, tidal and total lung volumes at d30. Concomitant treatment with Dex + RA prevents decreased septation induced by Dex alone and results in correction of hypercarbia. However, these animals continue to have abnormal pulmonary function and lung volumes. Increased septation as a result of RA treatment alone is reversed upon discontinuation of treatment. These data suggest that Dex + RA treatment results in improved gas exchange likely secondary to normalized septation

A High-Throughput Screening Platform for In Vitro Elastic Fiber Production and the Mass Transport Properties of the Elastic Fiber Compromised Arterial Wall

Elastin comprises nearly 50% of the wall in large elastic arteries and has a broad variety of physiological roles. As a structural extracellular matrix protein, elastin is responsible for the reversible elasticity in large arties that dampens pulsatile flow and ultimately reduces the workload on the heart. Structural compromise to the elastic fiber network is apparent in the elastin genetic disorders, supravalvular aortic stenosis and autosomal dominant cutis laxa-1, and acquired elastin disorders including hypertension, atherosclerosis, artery calcification, aneurysms, diabetes, and obesity. All of these disorders lead to an increased incidence of cardiovascular related death and the compromised elastic fiber network plays an important role in the degeneration of cardiovascular function. Elastin also serves as an important biological signal in both the development of the arterial vasculature and the progression of several of the previously mentioned cardiovascular disorders. Elastin’s physiological role is often overlooked in strategies being developed to treat these cardiovascular disorders. The work of this thesis has focused on two areas in particular that elastic fibers have been underrepresented; the generation of elastic fibers in vitro and the importance elastic fiber network on determining the mass transport properties of the arterial wall. Tissue engineered arteries lack a proper elastic fiber network, in part because elastin content is difficult to quantify and because inducing elastic fiber formation in vitro is challenging. We developed a platform for measuring elastic fiber production in vitro. We used a competitive ELISA for desmosine, an amino acid unique to elastic fibers, to detect elastic fiber production. We made adjustments to the cell culture conditions of rat lung fibroblast cells to improve their output of elastic fibers. We used this platform to perform a high-throughput screen on a small molecule library to search for molecules that could induce elastic fiber production. We also used our platform to screen the effect of minodixil and diazoxide on elastic fiber production as these antihypertensive drugs have been shown by other researchers to induce elastin gene expression but their effect on mature elastic fiber production was undetermined. Drug delivery in pharmaceutical strategies for treating aneurysm formation, arterial stiffness, and atherosclerosis is a rapidly developing area. However, current models of mass transport in the arterial wall make numerous assumptions that either diminish the contribution of the elastic fiber network or ignore it completely and there is a lack of empirical investigation on the transport properties of the arterial wall. We hypothesized that the elastic fiber network serves to limit transport across the wall. We developed ex vivo methods for measuring transmural advective transport of solute and fluid in mouse carotid arteries at physiological pressure and axial stretch. We investigated the effect of disrupting the elastic fiber network on arterial wall transport using a genetic knockout of Fibulin-5 (Fbln5-/-) or treatment with elastase. Fibulin-5 is an important director of elastic fiber assembly. Arteries from Fbln5-/- mice have a loose, non-continuous elastic fiber network. The changes in transport properties of elastic fiber compromised arteries we observed have important implications for the kinetics of biomolecules and pharmaceuticals in arterial tissue following elastic fiber degradation due to aging or vascular disease