Potentiation of surfactant release in fetal lung by thyroid hormone action

Thyroid and Corticosteroid Signaling in Amphibian Metamorphosis

In multicellular organisms, development is based in part on the integration of communication systems. Two neuroendocrine axes, the hypothalamic–pituitary–thyroid and the hypothalamic–pituitary–adrenal/interrenal axes, are central players in orchestrating body morphogenesis. In all vertebrates, the hypothalamic–pituitary–thyroid axis controls thyroid hormone production and release, whereas the hypothalamic–pituitary–adrenal/interrenal axis regulates the production and release of corticosteroids. One of the most salient effects of thyroid hormones and corticosteroids in post-embryonic developmental processes is their critical role in metamorphosis in anuran amphibians. Metamorphosis involves modifications to the morphological and biochemical characteristics of all larval tissues to enable the transition from one life stage to the next life stage that coincides with an ecological niche switch. This transition in amphibians is an example of a widespread phenomenon among vertebrates, where thyroid hormones and corticosteroids coordinate a post-embryonic developmental transition. The review addresses the functions and interactions of thyroid hormone and corticosteroid signaling in amphibian development (metamorphosis) as well as the developmental roles of these two pathways in vertebrate evolution.

Interdependence of Thyroid and Corticosteroid Signaling in Vertebrate Developmental Transitions

Post-embryonic acute developmental processes mainly allow the transition from one life stage in a specific ecological niche to the next life stage in a different ecological niche. Metamorphosis, an emblematic type of these post-embryonic developmental processes, has occurred repeatedly and independently in various phylogenetic groups throughout metazoan evolution, such as in cnidarian, insects, molluscs, tunicates, or vertebrates. This review will focus on metamorphoses and developmental transitions in vertebrates, including typical larval metamorphosis in anuran amphibians, larval and secondary metamorphoses in teleost fishes, egg hatching in sauropsids and birth in mammals. Two neuroendocrine axes, the hypothalamic-pituitary-thyroid and the hypothalamic-pituitary-adrenal/interrenal axes, are central players in the regulation of these life transitions. The review will address the molecular and functional evolution of these axes and their interactions. Mechanisms of integration of internal and environmental cues, and activation of these neuroendocrine axes represent key questions in an “eco-evo-devo” perspective of metamorphosis. The roles played by developmental transitions in the innovation, adaptation, and plasticity of life cycles throughout vertebrates will be discussed. In the current context of global climate change and habitat destruction, the review will also address the impact of environmental factors, such as global warming and endocrine disruptors on hypothalamic-pituitary-thyroid and hypothalamic-pituitary-adrenal/interrenal axes, and regulation of developmental transitions.

Manipulating plasma thyroid hormone levels at hatching alters development of endothermy and ventilation in Pekin duck (Anas platyrhynchos domestica)

At hatching in precocial birds, there are rapid physiological and metabolic phenotypic changes associated with attaining endothermy. During the transition to ex ovo life, thyroid hormone levels naturally increase, peaking at hatching, and then decline. To better understand the role of the natural increase in thyroid hormone at hatching in regulating the developmental trajectory of the Pekin duck's endothermic phenotype, we examined development of O2 consumption (V̇O2 ) and ventilation (frequency, tidal volume and minute ventilation) while inhibiting the developmental increase in thyroid hormones that occurs at hatching via administration of the thyroid-peroxidase inhibitor methimazole (MMI) or accelerating the developmental increase via triiodothyronine (T3) supplementation. Animals were dosed only on day 24 of a 28-day incubation period and studied on incubation day 25, during external pipping (EP) and 1 day post-hatching (dph). On day 25, there was an increase in V̇O2  in the hyperthyroid treatment compared with the other two treatments. During the EP stage, there was a significant effect of thyroid status on V̇O2 , with hyperthyroid V̇O2  being highest and hypothyroid V̇O2  the lowest. By 1 dph, the supplemented T3 and control animals had similar V̇O2  responses to cooling with comparable thermal neutral zones followed by increased V̇O2 Hypothyroid 1 dph hatchlings had a lower resting V̇O2  that did not increase to the same extent as the supplemented T3 and control animals during cooling. During EP, inhibiting the rise in T3 resulted in embryos with lower ventilation frequency and tidal volume than control and supplemented T3 embryos. At 1 dph, ventilation frequency of all animals increased during cooling, but tidal volume only increased in supplemented T3 and control hatchlings. Our data support the role of the late incubation increase in T3 in regulating the systemic development of endothermic metabolic capacity and associated control of ventilation occurring at hatching of the Pekin duck.

Evolution, Development, and Function of the Pulmonary Surfactant System in Normal and Perturbed Environments

Surfactant lipids and proteins form a surface active film at the air-liquid interface of internal gas exchange organs, including swim bladders and lungs. The system is uniquely positioned to meet both the physical challenges associated with a dynamically changing internal air-liquid interface, and the environmental challenges associated with the foreign pathogens and particles to which the internal surface is exposed. Lungs range from simple, transparent, bag-like units to complex, multilobed, compartmentalized structures. Despite this anatomical variability, the surfactant system is remarkably conserved. Here, we discuss the evolutionary origin of the surfactant system, which likely predates lungs. We describe the evolution of surfactant structure and function in invertebrates and vertebrates. We focus on changes in lipid and protein composition and surfactant function from its antiadhesive and innate immune to its alveolar stability and structural integrity functions. We discuss the biochemical, hormonal, autonomic, and mechanical factors that regulate normal surfactant secretion in mature animals. We present an analysis of the ontogeny of surfactant development among the vertebrates and the contribution of different regulatory mechanisms that control this development. We also discuss environmental (oxygen), hormonal and biochemical (glucocorticoids and glucose) and pollutant (maternal smoking, alcohol, and common "recreational" drugs) effects that impact surfactant development. On the adult surfactant system, we focus on environmental variables including temperature, pressure, and hypoxia that have shaped its evolution and we discuss the resultant biochemical, biophysical, and cellular adaptations. Finally, we discuss the effect of major modern gaseous and particulate pollutants on the lung and surfactant system.

Thyroid hormones in fetal growth and prepartum maturation

The thyroid hormones, thyroxine (T4) and triiodothyronine (T3), are essential for normal growth and development of the fetus. Their bioavailability in utero depends on development of the fetal hypothalamic-pituitary-thyroid gland axis and the abundance of thyroid hormone transporters and deiodinases that influence tissue levels of bioactive hormone. Fetal T4 and T3 concentrations are also affected by gestational age, nutritional and endocrine conditions in utero, and placental permeability to maternal thyroid hormones, which varies among species with placental morphology. Thyroid hormones are required for the general accretion of fetal mass and to trigger discrete developmental events in the fetal brain and somatic tissues from early in gestation. They also promote terminal differentiation of fetal tissues closer to term and are important in mediating the prepartum maturational effects of the glucocorticoids that ensure neonatal viability. Thyroid hormones act directly through anabolic effects on fetal metabolism and the stimulation of fetal oxygen consumption. They also act indirectly by controlling the bioavailability and effectiveness of other hormones and growth factors that influence fetal development such as the catecholamines and insulin-like growth factors (IGFs). By regulating tissue accretion and differentiation near term, fetal thyroid hormones ensure activation of physiological processes essential for survival at birth such as pulmonary gas exchange, thermogenesis, hepatic glucogenesis, and cardiac adaptations. This review examines the developmental control of fetal T4 and T3 bioavailability and discusses the role of these hormones in fetal growth and development with particular emphasis on maturation of somatic tissues critical for survival immediately at birth.

The role of extrinsic and intrinsic factors in the evolution of the control of pulmonary surfactant maturation during development in the amniotes

Pulmonary surfactant is a mixture of lipids and proteins that is secreted by alveolar Type II cells. It reduces alveolar surface tension and hence the work of breathing. Despite the tremendous diversity of lung structures amongst the vertebrates, the composition of surfactant is highly conserved. Conserved elements of the surfactant system amongst distantly related species are likely to be crucial factors for successful lung development. Understanding the mechanisms by which the surfactant system becomes operational in animals with dramatically different birthing strategies and in distantly related species will provide important information about the role of the surfactant system in the commencement of air breathing and the processes regulating surfactant maturation and secretion. In mammals, the embryonic maturation of the surfactant system is controlled by a host of factors, including glucocorticoids, thyroid hormones, and autonomic neurotransmitters. Here we review the mechanisms controlling the maturation of surfactant production, including birthing strategy, phylogeny, lung structure, and posthatching environment. Using four species of egg-laying amniote (chicken, dragon lizard, sea turtle, and crocodile) previously described in detail and the large amount of information available for mammals, we examine the hypothesis that the control of surfactant production is dependent on glucocorticoids (dexamethasone [Dex]), thyroid hormones (T3), and autonomic neurotransmitters (epinephrine and carbachol). We also examine whether the overall intrinsic pattern of the control of surfactant maturation is conserved throughout the vertebrate radiation and then how the environment (extrinsic factors) may account for the observed differences in the patterns of development. We also discuss the utility of a coculture system of embryonic Type II cells and fibroblasts to determine the evolutionary pattern behind the control of surfactant and to demonstrate that the surfactant system matures under multihormonal control. We demonstrate that Dex and T3 are stimulators of surfactant production during embryonic development, but they lose their efficacy closer to hatching or birth. Epinephrine stimulates surfactant secretion beyond 75% of development and also after hatching or birth. Carbachol stimulates surfactant secretion in the bearded dragon and saltwater crocodile but not in the sea turtle, chicken, or mammals. It is likely that the differences in control of surfactant development are likely to be primarily related to metabolic activity and the duration of incubation (i.e., the "speed" of development). Moreover, the hormones examined appear important in promoting development and therefore appear conserved within the amniotes. However, the autonomic neurotransmitters induced different responses in different species. Hence, some factors are crucial for the proper maturation of the surfactant system, whereas others vary throughout evolution without being detrimental to the overall function of the system.

Control of the development of the pulmonary surfactant system in the saltwater crocodile, Crocodylus porosus

Pulmonary surfactant is a mixture of lipids and proteins that controls the surface tension of the fluid lining the inner lung. Its composition is conserved among the vertebrates. Here we hypothesize that the in ovo administration of glucocorticoids and thyroid hormones during late incubation will accelerate surfactant development in the saltwater crocodile, Crocodylus porosus. We also hypothesize that the increased maturation of the type II cells in response to hormone pretreatment will result in enhanced responsiveness of the cells to surfactant secretagogues. We sampled embryos at days 60, 68, and 75 of incubation and after hatching. We administered dexamethasone (Dex), 3,5,3'-triiodothyronine (T(3)), or a combination of both hormones (Dex + T(3)), 48 and 24 h before each prehatching time point. Lavage analysis indicated that the maturation of the phospholipids (PL) in the lungs of embryonic crocodiles occurs rapidly. Only T(3) and Dex + T(3) increased total PL in lavage at embryonic day 60, but Dex, T(3), and Dex + T(3) increased PL at day 75. The saturation of the PLs was increased by T(3) and Dex + T(3) at day 68. Swimming exercise did not increase the amount or alter the saturation of the surfactant PLs. Pretreatment of embryos with Dex, T(3), or Dex + T(3) changed the secretion profiles of the isolated type II cells. Dex + T(3) increased the response of the cells to agonists at days 60 and 68. Therefore, glucocorticoids and thyroid hormones regulate surfactant maturation in the crocodile.

Differential effects in vivo of thyroid hormone on the expression of surfactant phospholipid, surfactant protein mRNA and antioxidant enzyme mRNA in fetal rat lung

Antenatal administration of triiodo-L-thyronine (T3) to late gestation rats resulted in decreased lung antioxidant enzyme (AOE) activity but increased surfactant phospholipids. In fetal rat lung explant cultures, T3 decreased the expression of surfactant proteins (SP) A and B. There have been no reported studies of the simultaneous in vivo developmental influence of T3 on both pulmonary AOE and SP gene expression. We hypothesized that antenatal T3 treatment would cause differential regulation of surfactant phospholipid, SP, and AOE genes in the late gestation fetal rat. Timed pregnant rats received intramuscular injections of either T3 (7 mg/kg) or placebo on days 19 and 20 of gestation and fetuses were delivered on day 21. Fetal lung SP-A, SP-B, SP-C, and AOE mRNA levels were studied by Northern analysis. AOE mRNA levels were further quantitated by solution hybridization. Total lung phospholipids (TPL) and disaturated phosphatidylcholine (DSPC) content were quantitated by a phosphorus assay. T3 significantly increased TPL and DSPC content, and significantly decreased the expression of SP-A, SP-C, CuZnSOD, and catalase genes. Because of a crucial interplay of these factors for normal lung function at the time of birth, the molecular mechanisms by which these apparently opposing changes are accomplished warrant further investigation.

Growth retardation and the development of the respiratory system in fetal sheep

In an experimental model of fetal growth retardation which involves the reduction of placental mass in ewes, we have investigated the effects of intrauterine deprivation on aspects of structural development of the trachea and lungs of fetal sheep (140 days gestation). We have also measured the volume of luminal liquid aspirated from the lungs and the phospholipid content of this liquid as an index of pulmonary surfactant production. The effects of growth retardation are evident in the trachea where the structural development of the mucosal and submucosal layers has been affected. Abnormal aspects of development include the frequent lack of a ciliated border on epithelial cells in the mucosal layer and the reduction in the extent of the folds usually characteristic of this layer in near term fetal sheep. Although the fetal lungs are smaller in growth retardation (P less than 0.01) they are appropriate for fetal weight and their structural development does not appear to have been retarded. In contrast, lung liquid volume is significantly reduced in relation to lung weight in growth retarded fetuses and the concentration of phospholipids in lung liquids is also reduced (P less than 0.01).