Ontogeny and effects of thyroid hormone on beta1-adrenergic receptor mRNA expression in ovine fetal kidney cortex

Developmental changes of the fetal and neonatal thyroid gland and functional consequences on the cardiovascular system

Thyroid hormones play an important role in the development and function of the cardiac myocyte. Dysregulation of the thyroid hormone milieu affects the fetal cardiac cells via complex molecular mechanisms, either by altering gene expression or directly by affecting post-translational processes. This review offers a comprehensive summary of the effects of thyroid hormones on the developing cardiovascular system and its adaptation. Furthermore, we will highlight the gaps in knowledge and provide suggestions for future research.

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.

Renin expression in large renal vessels during fetal development depends on functional beta1/beta2-adrenergic receptors

During nephrogenesis, renin expression shifts from large renal arteries toward smaller vessels in a defined spatiotemporal pattern, finally becoming restricted to the juxtaglomerular position. Chronic stimulation in adult kidneys leads to a recruitment of renin expression in the upstream vasculature. The mechanisms that control this characteristic switch-on and switch-off in the immature and adult kidney are not well-understood. Previous studies in mice with juxtaglomerular cell-specific deletion of the adenylyl cyclase-stimulatory G protein Gsα suggested that signaling along the cAMP pathway plays an essential role for renin expression during nephrogenesis and in the adult kidney. To identify the Gsα-dependent receptor that might be involved in activating this pathway, the present studies were performed to compare renin expression in wild types with that in mice with targeted deletions of β(1) and β(2)-adrenoceptors. The sympathetic nervous system is an important regulator of the renin system in the adult kidney so that activation of β-adrenenoceptors may also participate in the activation of renin expression along the developing arterial tree and in upstream vasculature in adulthood. Compared with wild-types, renin expression was found to be significantly lower at all developmental stages in the kidneys of β(1)/β(2) Adr(-/-) mice. Three-dimensional analysis showed reduced renin expression in all segments of the vascular tree in mutants and a virtual absence of renin expression in the large arcuate arteries. Adult mutant kidneys showed the typical upstream renin expression after chronic stimulation. Tyrosine hydroxylase staining in fetal and postnatal kidneys revealed that sympathetic innervation of renin-producing cells occurs early in fetal development. Our data indicate that genetic disruption of β-adrenergic receptors reduces basal renin expression along the developing preglomerular tree and in adult kidneys. Furthermore, β-adrenergic receptor input is critical for the expression of renin in large renal vessels during early fetal development.

Development of vascular renin expression in the kidney critically depends on the cyclic AMP pathway

During metanephric kidney development, renin expression in the renal vasculature begins in larger vessels, shifting to smaller vessels and finally remaining restricted to the terminal portions of afferent arterioles at the entrance into the glomerular capillary network. The mechanisms determining the successive expression of renin along the vascular axis of the kidney are not well understood. Since the cAMP signaling cascade plays a central role in the regulation of both renin secretion and synthesis in the adult kidney, it seemed feasible that this pathway might also be critical for renin expression during kidney development. In the present study we determined the spatiotemporal development of renin expression and the development of the preglomerular arterial tree in mouse kidneys with renin cell-specific deletion of G(s)alpha, a core element for receptor activation of adenylyl cyclases. We found that in the absence of the G(s)alpha protein, renin expression was largely absent in the kidneys at any developmental stage, accompanied by alterations in the development of the preglomerular arterial tree. These data indicate that the maintenance of renin expression following a specific spatiotemporal pattern along the preglomerular vasculature critically depends on the availability of G(s)alpha. We infer from our data that the cAMP signaling pathway is not only critical for the regulation of renin synthesis and secretion in the mature kidney but that it also is critical for establishing the juxtaglomerular expression site of renin during development.