María Jesús Obregón

Developmental changes in rat brain 5'-deiodinase and thyroid hormones during the fetal period: The effects of fetal hypothyroidism and maternal thyroid hormones

ABSTRACT: We have studied the ontogenesis of 5′-deiodinase (5′D) activity in rat brain during fetal life, its capacity to respond to maternal or fetal hypothyroidism, and its regulation by maternal thyroid hormones. Type II 5′D (5′ D-II) activity increases 4-fold during the period studied (17 to 22 days of gestation), mainly between days 19 and 21. Fetal brain T4 concentrations increase in parallel with fetal plasma T4, whereas fetal brain T3 concentrations increase 18 times (days 17–21), six times more than would have been expected from the small increase in fetal plasma T3 levels. Maternal thyroidectomy did not affect 5′D-II activity or thyroid hormone concentrations in fetal brain (except brain T4 at 18 days of gestation). Fetal hypothyroidism, induced by giving a goitrogen (methimazole) to the mothers, depleted all fetal tissues studied, including the fetal thyroid, from thyroid hormones. By 19 days of gestation, the fetal brain was able to respond to hypothyroidism with a 3− to 5-fold increase in 5′D-II activity. Earlier onset of treatment with methimazole led to 2− to 3-fold increases in 5′D already at 17 and 18 days of gestation, showing that when fetal thyroid secretion starts the fetal brain 5′D-II is able to respond to hypothyroidism. Replacement of methimazole-treated mothers with physiological doses of T4, given by constant infusion, increased T4 and T3 concentrations in fetal brain, and inhibited fetal, as well as maternal, brain 5′D-II activity. But treatment of the mothers with T3 did not change T3 concentrations in the fetal brain, despite the increase in fetal plasma T3, and actually increased 5′D-II in fetal brain. Maternal cerebral 5′D-II was not inhibited by T3 treatment. Inverse relationships were found between the 5′D-II and thyroid hormone concentrations in the fetal brain. These correlations were not identical for fetuses from thyroidectomized and control mothers. In fetuses from thyroidectomized dams, brain 5′D-II is more sensitive to a decrease in brain T4 than in the progeny of control dams. The present results describe the developmental changes in rat cerebral 5′D-II activity and its regulation by thyroid hormones. Although fetal plasma T3 is 10% of adult levels, T3 concentrations in fetal brain increase almost to adult levels, suggesting an important role of local T3 production from T4, and thus, of 5′D-II in fetal brain. In addition, brain 5′D-II responds to thyroid hormone deficiency and can be modulated by maternal thyroid hormones when the fetus is hypothyroid.

Iodine supplementation during pregnancy: a public health challenge

Iodine deficiency remains the most frequent cause worldwide, after starvation, of preventable mental retardation in children. It causes maternal hypothyroxinemia, which affects pregnant women even in apparently iodine-sufficient areas, and often goes unnoticed because L-thyroxine (T4) levels remain within the normal range, and thyroid-stimulating hormone (TSH) is not increased. Even a mild hypothyroxinemia during pregnancy increases the risk of neurodevelopmental abnormalities, and experimental data clearly demonstrate that it damages the cortical cytoarchitecture of the fetal brain. The American Thyroid Association (ATA) recommends a supplement of 150 microg iodine/day during pregnancy and lactation, in addition to the use of iodized salt. We discuss the importance of iodine supplementation to ensure adequate T4 levels in all women who are considering conception and throughout pregnancy and lactation.

Fetal and Maternal Thyroid Hormones

It is well known that insufficient production of thyroid hormones during the fetal and neonatal period of development may result in permanent brain damage unless treatment with thyroid hormone is instituted very soon after birth. But congenital hypothyroidism is not the only situation in which brain damage may be related to insufficient thyroid function. Cretinism is the most severe manifestation of iodine deficiency disorders found in areas where iodine intake is greatly reduced. Some of the manifestations of cretinism suggest that the insult to the developing brain starts earlier than in the case of congenital hypothyroidism. Hypothyroxinemia of mothers with adequate iodine intake may also leave permanent, though less severe, mental retardation. For these reasons the possible role of maternal transfer of thyroid hormones during early fetal development have been reinvestigated, using the rat to obtain various experimental models. It has been shown that thyroid hormones are found in embryonic tissues before onset of fetal thyroid function and that thyroidectomy of the mother results in delayed development of the concepta. The concentrations of T4 and T3 in embryonic tissues from thyroidectomized dams were undetectable before the onset of fetal thyroid function, and still reduced in some tissues near term, despite the onset of fetal thyroid function. Treatment of control and thyroidectomized dams with methyl-mercaptoimidazole to block fetal thyroid function reduced thyroid hormone concentrations in fetal tissues near term, but this decrease could be partially avoided by infusion of physiological doses of thyroxine to the mothers. Iodine deficiency of the mothers resulted in thyroid hormone deficiency of the developing embryo, which was very marked until term in all tissues including the brain. The results strongly support a role of maternal thyroid hormones in fetal thyroid hormone economy both before and after the onset of the fetal thyroid function, at least in the rat. They also support a role of the hypothyroxinemia of iodine-deficient mothers in initiating the brain damage of the endemic cretin, a damage which would not be corrected once the fetal thyroid becomes active, as iodine-deficiency of the fetus would impair adequate production of hormones by its own thyroid, and maternal transfer would continue to be low.

Thyroid hormone and adipocyte differentiation

Thyroid hormones act as pleiotropic factors in many tissues during development, by regulating genes involved in differentiation. The adipose tissue, a target of thyroid hormones, is the main place for energy storage and acts as a regulator of energy balance, sending signals to keep metabolic control. Adipogenesis is a complex process that involves proliferation of preadipocytes and its differentiation into mature adipocytes. This process is regulated by several transcription factors (CCAAT/enhancer-binding proteins [C/EBPs], peroxisome proliferator-activated receptors [PPARs]) that act coordinately, activating adipocyte-specific genes that will provide the adipocytic phenotype. Thyroid hormones regulate many of those genes, markers of differentiation of adipocytes, those involved in lipogenesis, lipolysis, and thermogenesis in the brown adipose tissue (BAT). Triiodothyronine (T3) actions are achieved either directly through specific thyroid response elements (TREs), by regulating other key genes as PPARs, or through specific isoforms of the nuclear T3 receptors. The availability of T3 is regulated through the deiodinases D3, D2, and D1. D3 is activated by serum and mitogens during proliferation of preadipocytes, while D2 is linked to the differentiation program of adipocytes, through the C/EBPs that govern its functionality, providing the T3 required for thermogenesis and lipogenesis. The relationship between white adipose tissue (WAT) and BAT and the possible reactivation of WAT by activation of uncoupling protein-1 (UCP1) is discussed.

Ontogenesis of Thyroid Function and Interactions with Maternal Function

Fetal and neonatal development of thyroid function involves the embryogenesis, differentiation and maturation of the thyroid gland, of the hypothalamic-pituitary-thyroid axis and of the systems controlling thyroid hormone metabolism. We focus here on aspects related to neurodevelopment. Throughout gestation, thyroxine (T4) transferred from the mother, present in embryonic fluids by 4 weeks, protects the fetal brain. Free T4 (FT4) in fetal fluids increases rapidly, approaching adult levels by midgestation, in concentrations that are determined by the maternal serum T4. T3 remains very low throughout pregnancy. In the cerebral cortex T3, generated from T4, reaches adult values by midgestation and is partly bound to specific nuclear receptor isoforms. The iodothyronine deiodinases are important for the spatial and temporal presence of T3 in different fetal brain areas. After onset of fetal thyroid secretion at midgestation, maternal transfer of T4 continues to contribute importantly to fetal serum T4, protecting neurodevelopment until birth. In rats, even a transient period of maternal hypothyroxinemia disrupts neurodevelopment irreversibly, supporting epidemiological evidence for its negative role in human neurodevelopment. The prompt treatment of maternal hypothyroidism or hypothyroxinemia should mitigate negative effects on neurodevelopment. Neurodevelopmental deficits of preterm infants might also result from an untimely interruption of the maternal transfer of T4 [Morreale de Escobar et al: J Clin Endocrinol Metab 2000;85:3975-3987; Best Pract Res Clin Endocrinol Metab 2004;18:225-248; Eur J Endocrinol 2004;151(suppl 3):U25-U37].

Generation and Characterization of dickkopf3 Mutant Mice

ABSTRACT dickkopf (dkk) genes encode a small family of secreted Wnt antagonists, except for dkk3, which is divergent and whose function is poorly understood. Here, we describe the generation and characterization of dkk3 mutant mice. dkk3-deficient mice are viable and fertile. Phenotypic analysis shows no major alterations in organ morphology, physiology, and most clinical chemistry parameters. Since Dkk3 was proposed to function as thyroid hormone binding protein, we have analyzed deiodinase activities, as well as thyroid hormone levels. Mutant mice are euthyroid, and the data do not support a relationship of dkk3 with thyroid hormone metabolism. Altered phenotypes in dkk3 mutant mice were observed in the frequency of NK cells, immunoglobulin M, hemoglobin, and hematocrit levels, as well as lung ventilation. Furthermore, dkk3-deficient mice display hyperactivity.

American Thyroid Association Guide to investigating thyroid hormone economy and action in rodent and cell models.

BACKGROUND An in-depth understanding of the fundamental principles that regulate thyroid hormone homeostasis is critical for the development of new diagnostic and treatment approaches for patients with thyroid disease. SUMMARY Important clinical practices in use today for the treatment of patients with hypothyroidism, hyperthyroidism, or thyroid cancer are the result of laboratory discoveries made by scientists investigating the most basic aspects of thyroid structure and molecular biology. In this document, a panel of experts commissioned by the American Thyroid Association makes a series of recommendations related to the study of thyroid hormone economy and action. These recommendations are intended to promote standardization of study design, which should in turn increase the comparability and reproducibility of experimental findings. CONCLUSIONS It is expected that adherence to these recommendations by investigators in the field will facilitate progress towards a better understanding of the thyroid gland and thyroid hormone dependent processes.

The Changing Role of Maternal Thyroid Hormone in Fetal Brain Development

This review briefly summarizes: (1) the changes in maternal thyroid function that are imposed by the presence of the fetus and the high concentrations of human chorionic gonadotropin essential for the maintenance of the pregnancy, which result in high first trimester free thyroxine and triiodothyronine, requiring doubling of the iodine intake; (2) the changes in the fetal compartment up to midgestation, which result in increasing concentrations of triiodothyronine in the cerebral cortex generated locally from thyroxine by high activities of type 2 iodothyronine deiodinase; (3) the important role of the maternal contribution of thyroxine to the fetal circulation after onset of secretion of hormones by the fetal thyroid; and (4) the consequences of the interruption of the maternal supply of thyroid hormones that occur with prematurity. Efforts to devise appropriate strategies to avoid or shorten the postnatal hypothyroxinemia of infants born prematurely may well result in fewer and less severe neurodevelopmental deficits.

Thyroxine treatment and recovery of hypothyroidism-induced pyramidal cell damage.

We have previously shown that changes occur in pyramids of area 17 of the rat visual cortex both after thyroidectomy (T) at 10 and at 40 days of age. To assess the effects of thyroxine treatment, instituted at different ages after T, two series of experiments were performed. A : rats were T at 10 days of age and either left untreated, or injected once daily with 1.5 micrograms thyroxine (T4)/100 g body weight. Treatment was started at 12, 15, 20, 30 or 40 days of age. Groups of untreated and T4-treated T rats and of age-paired intact controls were killed at different ages, ranging from 40 to 80 days. B : rats were T at 40 days of age, a group being treated with the same T4 dose starting 30 days after T. These animals, solvent-treated T rats and intact age-paired controls were killed at 90 days of age. The number and distribution of spines along the shaft of Golgi-stained pyramidal cells of the visual cortex were measured and fitted by a mathematical model developed previously. Body weights, pituitary growth hormone contents, plasma thyrotropic hormone, thyroxine and triiodothyronine levels were measured to assess the degree of hypothyroidism. It was found that treatment with T4 of rats T at 10 days of age prevented the alterations of pyramidal cells, provided treatment was started by 12 days of age and euthyroidism was maintained. In rats T at 40 days of age, treatment with T4 had an ameliorating effect despite a delay in onset of treatment of 30 days after T. Whatever the mechanisms which are involved, the present results stress once more the importance of very early treatment of hypothyroid newborns, if permanent cortical brain damage is to be prevented.

Brown adipocytes differentiated in vitro can express the gene for the uncoupling protein thermogenin: effects of hypothyroidism and norepinephrine.

Expression of the gene for the brown-fat specific uncoupling protein thermogenin was investigated in cell cultures by hybridization of isolated RNA with a cDNA clone corresponding to mouse thermogenin. The RNA was isolated 3-4 days after confluence from cells differentiated in culture from precursors isolated from the interscapular brown adipose tissue of 5-week-old mice. Very low thermogenin mRNA levels were found in cells derived from untreated mice, and there was only little effect of added norepinephrine on thermogenin gene expression in these cells. However, in cells derived from hypothyroid (methimazole-treated) mice there was a higher expression of thermogenin, and norepinephrine had a marked augmenting effect on the thermogenin mRNA level in these cells. These effects of thermogenin mRNA levels were specific, in that they contrasted with the effects of hypothyroidism and norepinephrine on the level of other mRNA species in these cells (coding for beta-actin, lipoprotein lipase, cytochrome-c oxidase, and glycerol-3-phosphate dehydrogenase). It was concluded that brown-fat cells in culture can reach a differentiated state, sufficiently advanced that the unique properties of these cells can be expressed, and that thermogenin gene expression (i.e., the level of thermogenin mRNA) is under direct control of norepinephrine.