Kerry Richard

Thyroid hormones and fetal neurological development

The development of fetal thyroid function is dependent on the embryogenesis, differentiation, and maturation of the thyroid gland. This is coupled with evolution of the hypothalamic-pituitary-thyroid axis and thyroid hormone metabolism, resulting in the regulation of thyroid hormone action, production, and secretion. Throughout gestation there is a steady supply of maternal thyroxine (T(4)) which has been observed in embryonic circulation as early as 4 weeks post-implantation. This is essential for normal early fetal neurogenesis. Triiodothyronine concentrations remain very low during gestation due to metabolism via placental and fetal deiodinase type 3. T(4) concentrations are highly regulated to maintain low concentrations, essential for protecting the fetus and reaching key neurological sites such as the cerebral cortex at specific developmental stages. There are many known cell membrane thyroid hormone transporters in fetal brain that play an essential role in regulating thyroid hormone concentrations in key structures. They also provide the route for intracellular thyroid hormone interaction with associated thyroid hormone receptors, which activate their action. There is a growing body of experimental evidence from rats and humans to suggest that even mild maternal hypothyroxinemia may lead to abnormalities in fetal neurological development. Our review will focus on the ontogeny of thyroid hormone in fetal development, with a focus on cell membrane transporters and TR action in the brain.

Regulation of hypoxia inducible factors (HIF) in hypoxia and normoxia during placental development.

During the first trimester of pregnancy the human placenta develops in an hypoxic environment caused by the occlusion of uterine spiral arterioles by extravillous trophoblasts (EVT). This period of low oxygen tension is crucial for successful pregnancy. In low oxygen environments, Hypoxia Inducible Factors (HIF) are the main regulators in the transcription of a number of genes. Target genes can induce anaerobic processes, reducing oxygen consumption, or promote angiogenesis, which establishes and enhances the vascular environment. The HIFs can function throughout all stages of placental differentiation and growth both in normal and pathological pregnancies (compromised by hypoxia/ischemia). Interestingly, HIFs respond to a multitude of changes during pregnancy, including 1) low oxygen, 2) renin-angiotensin system (RAS), 3) cytokines, and 4) growth factors, all of which regulate placental function. This review explores oxygen-dependent and oxygen-independent regulation and the role of HIF in placental development and differentiation.

Delivery of maternal thyroid hormones to the fetus

Thyroid hormones (THs) play an essential role in ensuring normal fetal development, particularly that of the central nervous system. Before 16 weeks gestation, the fetus relies solely on transplacental delivery of maternal T(4), and clinical studies suggest that even mild maternal thyroid hormone deficiency adversely affects the intellectual function of offspring. Maternofetal TH transfer is regulated by trophoblast cell membrane transporters, which mediate influx and efflux of THs, placental deiodinases D3 and D2, which control intraplacental TH levels, and TH-binding proteins (transthyretin), which provide transport roles in the placenta. This review discusses new information about mechanisms of transplacental delivery of T(4) to the fetus, providing insight into complex processes that are vitally important for normal fetal development.

Carrier-Mediated Thyroid Hormone Transport into Placenta by Placental Transthyretin

CONTEXT The serum protein transthyretin (TTR) plays an important role in the transport of thyroid hormone and retinol, which are critical for normal development of the human fetus. TTR is not only synthesized and secreted into the circulation by the liver and other tissues but is also synthesized by placental trophoblasts, which separate the maternal and fetal circulations. Whether it is secreted or taken up by these cells and whether it carries thyroid hormone is unknown. OBJECTIVE AND METHODS Our objective was to study placental handling of TTR and determine whether TTR participates in placental thyroid hormone transport. We investigated the capacity of human placenta and choriocarcinoma cell lines to secrete and internalize TTR and its ligands by Western blotting, immunofluorescence, and uptake of radiolabeled TTR. RESULTS Human placental explants and TTR expressing JEG-3 cells secrete TTR. JEG-3 cells grown in bicameral chambers secrete TTR, predominantly from the apical surface. Human placental explants and JEG-3 cells internalize Alexa Fluor488-labeled TTR and (125)I-TTR. Furthermore, binding to thyroid hormones (T(4), T(3)) increases (125)I-TTR uptake by enhancing tetramer formation. Cross-linking experiments confirm internalization of the TTR-(125)I-T(4) complex. CONCLUSIONS Our results suggest that human placenta and choriocarcinoma cells secrete transthyretin, which binds extracellular T(4), and that T(4) binding results in increased internalization of TTR-T(4) complex. TTR production by trophoblasts may represent a mechanism to allow transfer of maternal thyroid hormone to the fetal circulation that could have important implications for fetal development.

hSSB1 rapidly binds at the sites of DNA double-strand breaks and is required for the efficient recruitment of the MRN complex

hSSB1 is a newly discovered single-stranded DNA (ssDNA)-binding protein that is essential for efficient DNA double-strand break signalling through ATM. However, the mechanism by which hSSB1 functions to allow efficient signalling is unknown. Here, we show that hSSB1 is recruited rapidly to sites of double-strand DNA breaks (DSBs) in all interphase cells (G1, S and G2) independently of, CtIP, MDC1 and the MRN complex (Rad50, Mre11, NBS1). However expansion of hSSB1 from the DSB site requires the function of MRN. Strikingly, silencing of hSSB1 prevents foci formation as well as recruitment of MRN to sites of DSBs and leads to a subsequent defect in resection of DSBs as evident by defective RPA and ssDNA generation. Our data suggests that hSSB1 functions upstream of MRN to promote its recruitment at DSBs and is required for efficient resection of DSBs. These findings, together with previous work establish essential roles of hSSB1 in controlling ATM activation and activity, and subsequent DSB resection and homologous recombination (HR).

Transthyretin and the human placenta.

Since its discovery, transthyretin (TTR) has been regarded as an important hepatically derived protein carrier of thyroid hormones and retinol in blood. However, in more recent years it has been shown that TTR has other important functions. TTR is abundant in cerebrospinal fluid, where it may be involved in transport of thyroid hormones into the brain. TTR derived amyloid is associated with diseases such as senile systemic amyloidosis, familial amyloid polyneuropathy and familial amyloid cardiomyopathy. Recently, synthesis, secretion and uptake of TTR by human placenta have been reported. TTR appears to play an important role in the delivery of maternal thyroid hormone to the developing fetus. This review explores the various proposed roles of TTR and more recent findings on TTR synthesis and expression in the placenta.

Expression and uptake of the thyroxine-binding protein transthyretin is regulated by oxygen in primary trophoblast placental cells

Transplacental delivery of maternal thyroid hormones to the fetus, in particular thyroxine (T₄), is critical in ensuring normal fetal neurological development. The fetus relies on maternal T₄ till around 16 weeks gestation, but mechanisms of placental T₄ transport are not yet fully elucidated. Placenta produces, secretes and takes up the thyroid hormone-binding protein transthyretin (TTR). Many placental genes are regulated by oxygen levels, which are relatively low (1%) in the early first trimester, rising to 3% in the mid first trimester and 8% in the early second trimester and thereafter. We examined the expression and uptake of TTR in isolated primary human placental cytotrophoblast cells cultured under different oxygen concentrations (1, 3, 8, 21% O₂ and 200 μM desferrioxamine (DFO)) for 24 h. We observed sevenfold higher expression of TTR mRNA and protein levels at 1% O₂ than at 8 and 21% O₂. Significant increases were observed after culture at 3% O₂ and following DFO treatment. We observed significantly higher uptake of ¹²⁵I-TTR and Alexa-594-TTR when cells were cultured at 1 and 3% O₂ and in the presence of 200 μM DFO than at 8 and 21% O₂. When JEG-3 choriocarcinoma cells were transfected with TTR promoter reporter constructs, increased luciferase activity was measured in cells cultured at 1 and 3% O₂ in comparison to 8 and 21% O₂. We conclude that placental TTR expression and uptake is increased by the relative hypoxia observed in the first trimester of pregnancy, a time when materno-fetal T₄ transfer is the sole source of fetal T₄.

Ontogenic changes in human placental sodium iodide symporter expression

The human fetus requires a maternal supply of iodide to synthesize thyroid hormone from 16 weeks gestation. Placental iodide transport is regulated by the sodium iodide symporter (NIS). We studied the ontogeny of NIS in placentas from surgically terminated pregnancies and from normal term pregnancies. NIS mRNA was low at 6 weeks gestation and peaked at 12 weeks gestation. Placental NIS protein levels are significantly correlated with gestational age during early pregnancy and increase with increased placental vascularization. This would lead to increased iodide supply to meet increased fetal requirements for thyroid hormone synthesis as the pregnancy progresses.

Oxygen concentration regulates expression and uptake of transthyretin, a thyroxine binding protein, in JEG-3 choriocarcinoma cells

Maternal thyroid hormone is provided to the fetus before the onset of fetal thyroid function (at about 16 weeks) and is essential for normal neurologic development. Mechanisms of transport are uncertain but transthyretin (TTR), a thyroxine binding protein produced by the placenta may be involved. Placental oxygen concentrations in early pregnancy are low, about 1% early in the first trimester and rising to 8% over the next 12 weeks. This study investigated the regulation of TTR expression, secretion and uptake in JEG-3 placental cells cultured at different oxygen concentrations. TTR mRNA and protein expression and (125)I-TTR and Alexa-Fluor594-TTR uptake were significantly higher in cells cultured at 1% and 3% O(2), than at 8% O(2). This suggests that increased carrier mediated T(4) transport by placental TTR may be induced by the low oxygen environment of early pregnancy, a time when the fetus has its highest requirement for transport of maternal T(4).

Traversing barriers – How thyroid hormones pass placental, blood-brain and blood-cerebrospinal fluid barriers

Thyroid hormone is essential for normal human fetal growth and brain development. As the fetal thyroid does not secrete thyroid hormones until about 18 weeks gestation, early fetal brain development depends on passage of maternal hormone across the placenta into the fetal circulation. To reach the fetal brain, maternally derived and endogenously produced thyroid hormone has to cross the blood-brain and blood-cerebrospinal fluid barriers. In this review we will discuss the complex biological barriers (involving membrane transporters, enzymes and distributor proteins) that must be overcome to ensure that the developing human brain has adequate exposure to thyroid hormone.