Thyroid hormones and 5'-deiodinase in the rat fetus late in gestation: effects of maternal hypothyroidism

Towards a science-based testing strategy to identify maternal thyroid hormone imbalance and neurodevelopmental effects in the progeny - part II: how can key events of relevant adverse outcome pathways be addressed in toxicological assessments?

The current understanding of thyroid-related adverse outcome pathways (AOPs) with adverse neurodevelopmental outcomes in mammals has been reviewed. This served to establish if standard rodent toxicity test methods and in vitro assays allow identifying thyroid-related modes-of-action potentially leading to adverse neurodevelopmental outcomes, and the human relevance of effects - in line with the European Commission's Endocrine Disruptor Criteria. The underlying hypothesis is that an understanding of the key events of relevant AOPs provides insight into differences in incidence, magnitude, or species sensitivity of adverse outcomes. The rodent studies include measurements of serum thyroid hormones, thyroid gland pathology and neurodevelopmental assessments, but do not directly inform on specific modes-of-action. Opportunities to address additional non-routine parameters reflecting critical events of AOPs in toxicological assessments are presented. These parameters appear relevant to support the identification of specific thyroid-related modes-of-action, provided that prevailing technical limitations are overcome. Current understanding of quantitative key event relationships is often weak, but would be needed to determine if the triggering of a molecular initiating event will ultimately result in an adverse outcome. Also, significant species differences in all processes related to thyroid hormone signalling are evident, but the biological implications thereof (including human relevance) are often unknown. In conclusion, careful consideration of the measurement (e.g. timing, method) and interpretation of additional non-routine parameters is warranted. These findings will be used in a subsequent paper to propose a testing strategy to identify if a substance may elicit maternal thyroid hormone imbalance and potentially also neurodevelopmental effects in the progeny.

NOD1 deficiency promotes an imbalance of thyroid hormones and microbiota homeostasis in mice fed high fat diet

The contribution of the nucleotide-binding oligomerization domain protein NOD1 to obesity has been investigated in mice fed a high fat diet (HFD). Absence of NOD1 accelerates obesity as early as 2 weeks after feeding a HFD. The obesity was due to increases in abdominal and inguinal adipose tissues. Analysis of the resting energy expenditure showed an impaired function in NOD1-deficient animals, compatible with an alteration in thyroid hormone homeostasis. Interestingly, free thyroidal T4 increased in NOD1-deficient mice fed a HFD and the expression levels of UCP1 in brown adipose tissue were significantly lower in NOD1-deficient mice than in the wild type animals eating a HFD, thus contributing to the observed adiposity in NOD1-deficient mice. Feeding a HFD resulted in an alteration of the proinflammatory profile of these animals, with an increase in the infiltration of inflammatory cells in the liver and in the white adipose tissue, and an elevation of the circulating levels of TNF-α. In addition, alterations in the gut microbiota in NOD1-deficient mice correlate with increased vulnerability of their ecosystem to the HFD challenge and affect the immune-metabolic phenotype of obese mice. Together, the data are compatible with a protective function of NOD1 against low-grade inflammation and obesity under nutritional conditions enriched in saturated lipids. Moreover, one of the key players of this early obesity onset is a dysregulation in the metabolism and release of thyroid hormones leading to reduced energy expenditure, which represents a new role for these hormones in the metabolic actions controlled by NOD1.

Thyroid hormone action in the developing testis: intergenerational epigenetics

Male fertility involves the successful transmission of the genetic code to the next generation. It requires appropriately timed cellular processes during testis development, adequate support of spermatogenesis by hormonal cues from the reproductive axis and cellular cross-talk between germ and somatic cells. In addition to being the vessel of the father’s genome, increasing evidence shows that the mature sperm carries valuable epigenetic information – the epigenome – that, after fecundation, influences the development of the next generation, affecting biological traits and disease susceptibility. The epigenome of the germ line is susceptible to environmental factors, including exogenous chemicals and diet, but it is also affected by endogenous molecules and pathophysiological conditions. Factors affecting testis development and the epigenetic information of the germ line are critical for fertility and of relevance to the non-genetic but heritable component in the etiology of complex conditions. Thyroid hormones are one of those factors and their action, when untimely, produces profound effects on the developing testis, affecting spermatogenesis, steroidogenesis, testis size, reproductive hormones and fertility. Altered thyroid hormone states can also change the epigenetic information of the male germ line, with phenotypic consequences for future generations. In the context of past literature concerning the consequences of altered thyroid hormone action for testis development, here we review recent findings about the pathophysiological roles of the principal determinants of testicular thyroid hormone action. We also discuss limited work on the effects of thyroid hormone on the male germ line epigenome and the implications for the intergenerational transmission of phenotypes via epigenetic mechanisms.

Thyroid Hormone Availability and Action during Brain Development in Rodents

Thyroid hormones (THs) play an essential role in the development of all vertebrates; in particular adequate TH content is crucial for proper neurodevelopment. TH availability and action in the brain are precisely regulated by several mechanisms, including the secretion of THs by the thyroid gland, the transport of THs to the brain and neural cells, THs activation and inactivation by the metabolic enzymes deiodinases and, in the fetus, transplacental passage of maternal THs. Although these mechanisms have been extensively studied in rats, in the last decade, models of genetically modified mice have been more frequently used to understand the role of the main proteins involved in TH signaling in health and disease. Despite this, there is little knowledge about the mechanisms underlying THs availability in the mouse brain. This mini-review article gathers information from findings in rats, and the latest findings in mice regarding the ontogeny of TH action and the sources of THs to the brain, with special focus on neurodevelopmental stages. Unraveling TH economy and action in the mouse brain may help to better understand the physiology and pathophysiology of TH signaling in brain and may contribute to addressing the neurological alterations due to hypo and hyperthyroidism and TH resistance syndromes.

Is the Intrinsic Genomic Activity of Thyroxine Relevant In Vivo? Effects on Gene Expression in Primary Cerebrocortical and Neuroblastoma Cells

BACKGROUND

The possibility that the intrinsic genomic activity of thyroxine (T4) is of physiological relevance has been frequently hypothesized. It might explain gene expression patterns in the brain found in type 2-deiodinase (Dio2)-deficient mice. These mice display normal expression of most thyroid hormone-dependent genes, despite decreased brain triiodothyronine (T3).

METHODS

The relative effects of T4 and T3 on gene expression were analyzed in mouse neuro-2a (N2a) cells stably expressing the thyroid hormone receptor α1, and in primary mouse cerebrocortical cells enriched in astrocytes or in neurons. Cortical cells were derived from Dio2-deficient mice to prevent conversion of T4 to T3. T4 and T3 were measured in the media at the beginning and end of incubation, and T4 and T3 antibodies were used to block T4 and T3 action.

RESULTS

In all cell types, T4 had intrinsic genomic activity. In N2a cells, T4 activity was higher on negative regulation (1/5th of T3 activity) than on positive regulation (1/40th of T3 activity). T4 activity on positive regulation was dependent on the cell context, and was higher in primary cells than in N2a cells.

CONCLUSION

T4 has intrinsic genomic activity. Positive regulation depends on the cell context, and primary cells appear much more sensitive than neuroblastoma cells. In all cells, negative regulation is more sensitive to T4 than positive regulation. These properties may explain the mostly normal gene expression in the brain of Dio2-deficient mice.

Abnormal motor phenotype at adult stages in mice lacking type 2 deiodinase

Background

Thyroid hormones have a key role in both the developing and adult central nervous system and skeletal muscle. The thyroid gland produces mainly thyroxine (T4) but the intracellular concentrations of 3,5,3′-triiodothyronine (T3; the transcriptionally active hormone) in the central nervous system and skeletal muscle are modulated by the activity of type 2 deiodinase (D2). To date no neurological syndrome has been associated with mutations in the DIO2 gene and previous studies in young and juvenile D2-knockout mice (D2KO) did not find gross neurological alterations, possibly due to compensatory mechanisms.

Aim

This study aims to analyze the motor phenotype of 3-and-6-month-old D2KO mice to evaluate the role of D2 on the motor system at adult stages in which compensatory mechanisms could have failed.

Results

Motor abilities were explored by validated tests. In the footprint test, D2KO showed an altered global gait pattern (mice walked slower, with shorter strides and with a hindlimb wider base of support than wild-type mice). No differences were detected in the balance beam test. However, a reduced latency to fall was found in the rotarod, coat-hanger and four limb hanging wire tests indicating impairment on coordination and prehensile reflex and a reduction of muscle strength. In histological analyses of cerebellum and skeletal muscle, D2KO mice did not present gross structural abnormalities. Thyroid hormones levels and deiodinases activities were also determined. In D2KO mice, despite euthyroid T3 and high T4 plasma levels, T3 levels were significantly reduced in cerebral cortex (48% reduction) and skeletal muscle (33% reduction), but not in the cerebellum where other deiodinase (type 1) is expressed.

Conclusions

The motor alterations observed in D2KO mice indicate an important role for D2 in T3 availability to maintain motor function and muscle strength. Our results suggest a possible implication of D2 in motor disorders.

The targeted inactivation of TRβ gene in thyroid follicular cells suggests a new mechanism of regulation of thyroid hormone production

Thyroid epithelial cells, or thyrocytes, express functional thyroid hormone receptors but no precise role has yet been assigned to either TRα or TRβ in the thyroid gland. In this study, we analyzed the impact of inactivating the TRβ gene in the thyroid of mice. First, we generated a mouse line named Thyr-Cre, expressing the Cre recombinase under the control of the thyroglobulin gene promoter, which led to a complete recombination of floxed genes in thyrocytes. Thyr-Cre mice were then crossed with TRβ floxed mice (TRβ(flox/flox)) to obtain a thyrocyte-selective deletion of TRβ. Thyr-TRβ(-/-) mice were characterized by a decrease in the size and functional activity of the thyroid gland. These alterations were associated with a decrease in plasma TSH concentration. Surprisingly, Thyr-TRβ(-/-) displayed elevated serum T(4) and rT(3) concentrations with no significant change in serum T(3) levels. Their intrathyroidal free T(4) and rT(3) contents were also elevated, whereas the ratio of serum T(4) to thyroid free T(4) was decreased by comparison with wild-type littermates. Also, within the thyroid, deiodinases D1 and D2 were reduced as well as the expression levels of genes encoding monocarboxylate transporters (Mct8 and Mct10). Such a decrease in intrathyroidal deiodination of T(4) and in the expression of genes encoding thyroid hormone transporters may contribute to the primary overproduction of T(4) observed in Thyr-TRβ(-/-) mice. In conclusion, these data show that the control of thyroid hormone production involves not only TRβ-dependent mechanisms acting at the level of hypothalamus and pituitary but also TRβ-dependent mechanisms acting at the thyroid level.

Septic shock non-thyroidal illness syndrome causes hypothyroidism and conditions for reduced sensitivity to thyroid hormone

Non-thyroidal illness syndrome (NTIS) is part of the neuroendocrine response to stress, but the significance of this syndrome remains uncertain. The aim of this study was to investigate the effect of lipopolysaccharide (LPS)-induced NTIS on thyroid hormone (TH) levels and TH molecular targets, as well as the relationship between septic shock nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB) activation and TH receptor β (THRB) gene expression at a multi-tissue level in a pig model. Prepubertal domestic pigs were given i.v. saline or LPS for 48 h. Serum and tissue TH was measured by chemiluminescence and RIA. Expression of THRs and cofactors was measured by real-time PCR, and deiodinase (DIO) activity was measured by enzyme assays. Tissue NF-kB nuclear binding activity was evaluated by EMSA. LPS-treated pigs had decreased TH levels in serum and most tissues. DIO1 expression in liver and kidney and DIO1 activity in kidney decreased after LPS. No changes in DIO2 activity were observed between groups. LPS induced an increase in hypothalamus, thyroid, and liver DIO3 activity. Among the other studied genes, monocarboxylate transporter 8 and THRB were the most commonly repressed in endotoxemic pigs. LPS-induced NF-kB activation was associated with a decrease in THRB gene expression only in frontal lobe, adrenal gland, and kidney cortex. We conclude that LPS-induced NTIS in pigs is characterized by hypothyroidism and tissue-specific reduced TH sensitivity. The role of NF-kB in regulating THRB expression during endotoxemia, if any, is restricted to a limited number of tissues.

Deiodinase activities in thyroids and tissues of iodine-deficient female rats

Severe iodine deficiency is characterized by goiter, preferential synthesis, and secretion of T(3) in thyroids, hypothyroxinemia in plasma and tissues, normal or low plasma T(3), and slightly increased plasma TSH. We studied changes in deiodinase activities and mRNA in several tissues of rats maintained on low-iodine diets (LIDs) or LIDs supplemented with iodine (LID+I). T(4) and T(3) concentrations decreased in plasma, tissues, and thyroids of LID rats, and T(4) decreased more than T(3) (50%). The highest type 1 iodothyronine deiodinase (D1) activities were found in the thyroid, kidney, and the liver; pituitary, lung, and ovary had lower D1 activities; but the lowest levels were found in the heart and skeletal muscle. D1 activity decreased in all tissues of LID rats (10-40% of LID+I rats), except for ovary and thyroids, which D1 activity increased 2.5-fold. Maximal type 2 iodothyronine deiodinase (D2) activities were found in thyroid, brown adipose tissue, and pituitary, increasing 6.5-fold in thyroids of LID rats and about 20-fold in the whole gland. D2 always increased in response to LID, and maximal increases were found in the cerebral cortex (19-fold), thyroid, brown adipose tissue, and pituitary (6-fold). Lower D2 activities were found in the ovary, heart, and adrenal gland, which increased in LID. Type 3 iodothyronine deiodinase activity was undetectable. Thyroidal Dio1 and Dio2 mRNA increased in the LID rats, and Dio1 decreased in the lung, with no changes in mRNA expression in other tissues. Our data indicate that LID induces changes in deiodinase activities, especially in the thyroid, to counteract the low T(4) synthesis and secretion, contributing to maintain the local T(3) concentrations in the tissues with D2 activity.

Maternal hypothyroidism decreases progesterone receptor expression in the cortical subplate of foetal rat brain

Steroid hormones exert profound effects on the development of brain areas controlling complex cognitive function in adulthood. One class, progestins, may contribute by acting on the progestin receptor (PR), which is transiently expressed in a critical layer of developing cortex: the subplate. PR expression in the subplate coincides with the establishment of ongoing cortical connectivity and may play an important organisational role. Identification of the factor(s) that regulate the precise timing of PR expression within subplate may help elucidate the function of PR. Thyroid hormone may interact with hormone response elements within the PR gene. The present study examined the effects of maternal hypothyroidism on levels of PR immunoreactivity (PR-IR) within the foetal subplate. Pregnant rats were made hypothyroid by the administration of methimazole and potassium perchlorate in drinking water. Maternal hypothyroidism significantly decreased PR-IR within the foetal subplate. Using the incorporation of 5-bromo-2'-deoxyuridine (BrDU) during subplate cell neurogenesis (embryonic day 13.5) to determine subplate cell survival in hypothyroid animals, we found that decreases in PR-IR cannot be attributed to significant subplate cell loss but are more likely the result of altered PR expression. Gestational thyroxine replacement to hypothyroid dams prevented the decrease in PR-IR within the subplate. These results identify thyroid hormone as a potential factor in the regulation of PR expression in the developing brain. These results are consistent with the idea that endocrine cross-talk between progesterone and thyroid hormone may be one mechanism by which maternal hypothyroidism alters normal cortical development.

Presence and regulation of D1 and D2 deiodinases in rat white adipose tissue

Thyroid hormones regulate adipogenic differentiation, lipogenic and lipolytic metabolism, and mitochondrial activity in adipose tissue. Triiodothyronine (T3) levels in tissues are regulated by the deiodinase enzymes. The objective was to study the activity and messenger RNA (mRNA) expression of the 5' outer-ring deiodinases (type 1 [D1] and type 2 [D2] deiodinase) and thyroid hormone concentrations in rat white adipose tissue (WAT), where only D1 activity had been described. Control, thyroidectomized, and thyroid hormone-treated rats were used. Type 1 and type 2 deiodinase mRNAs were determined in WAT by quantitative real-time polymerase chain reaction using Taqman probes; D1 and D2 activities were determined using reverse T3 and thyroxine (T4) as substrates. Thyroxine and T3 were measured by radioimmunoassay in plasma, liver, and adipose tissue. Type 1 and type 2 deiodinase mRNAs are present in epididymal rat WAT with similar abundance, which is 7% of the D2 mRNA levels in brown adipose tissue and 1% of D1 in liver. The Michaelis-Menten constants in WAT are 40 nmol/L T4 for D2 and 0.35 μmol/L reverse T3 for D1. Both D1 and D2 are regulated in rat epididymal WAT by thyroidal status. Thyroxine and T3 concentrations in plasma, liver, and WAT decreased after thyroidectomy and recovered after treatment with T4 + T3. Both D1 and D2 mRNAs increased in WAT from thyroidectomy rats; and T4 + T3 treatment inhibited them, especially D2 mRNA. Type 1 deiodinase activity did not change with thyroidal status, whereas D2 activity was inhibited by T4 + T3. The presence of both deiodinases in WAT suggests important roles in regulating T3 bioavailability for adipose tissue function and regulation of lipid metabolism and thermogenesis.

Lack of action of exogenously administered T3 on the fetal rat brain despite expression of the monocarboxylate transporter 8

Mutations of the monocarboxylate transporter 8 gene (MCT8, SLC16A2) cause the Allan-Herndon-Dudley syndrome, an X-linked syndrome of severe intellectual deficit and neurological impairment. Mct8 transports thyroid hormones (T4 and T3), and the Allan-Herndon-Dudley syndrome is likely caused by lack of T3 transport to neurons during critical periods of fetal brain development. To evaluate the role of Mct8 in thyroid hormone action in the fetal brain we administered T4 or T3 to thyroidectomized pregnant dams treated with methyl-mercapto-imidazol to produce maternal and fetal hypothyroidism. Gene expression was then measured in the fetal cerebral cortex. T4 increased Camk4, Sema3c, and Slc7a3 expression, but T3 was without effect. To investigate the cause for the lack of T3 action we analyzed the expression of organic anion transport polypeptide (Oatp14, Slco1c1), a T4 transporter, and Mct8 (Slc16a2), a T4 and T3 transporter, by confocal microscopy. Both proteins were present in the brain capillaries forming the blood-brain barrier and in the epithelial cells of the choroid plexus forming the blood-cerebrospinal fluid barrier. It is concluded that T4 from the maternal compartment influences gene expression in the fetal cerebral cortex, possibly after transport via organic anion transporter polypeptide and/or Mct8, and conversion to T3 in the astrocytes. On the other hand, T3 does not reach the target neurons despite the presence of Mct8. The data indicate that T4, through local deiodination, provides most T3 in the fetal rat brain. The role of Mct8 as a T3 transporter in the fetal rat brain is therefore uncertain.

Subacute toxicity of p,p'-DDT on rat thyroid: Hormonal and histopathological changes

The purpose of this study is to assess the effect of p,p'-DDT on thyroid activity of male Wistar rats. Pesticide was administered intraperitoneally (i.p.) for 10 consecutive days at doses of 50 and 100mg/kg/day. At the end of the treatment, the endpoints examined included serum total levels of triiodothyronine (T(3)), total thyroxine (T(4)), and thyroid stimulating hormone (TSH). Thyroid gland histopathology and tissue metabolism of thyroid hormone (T(4) UDP-glucuronyltransferase UDP-GT and 5'-deiodinases) were determined. DDT treatment altered thyroid function namely by increasing hepatic excretion of T(4) glucuronide. At the dose of 50mg/kg it decreased T(4) circulating levels and increased thyroid 5'-deiodinase type I (5'-D-I) and brown adipose tissue (BAT) 5'-deiodinase type II (5'-D-II) activities but it did not affect liver 5'-D-I activity which might contribute to the maintenance of the serum T(3) level. Treatment with 100mgDDT/kg decreased serum thyroid hormone concentration and tissue 5'-D-I activity without affecting BAT 5'-D-II activity. Gland histomorphological analysis showed hyperplasia and squamous metaplasia with abundant colloid. These observations associated to the elevated serum TSH levels and gland hypertrophy suggest that DDT exposure induced an hypothyroidism state with a colloid goiter in rats.

Regional decrease of subcutaneous adipose tissue in patients with type 2 familial partial lipodystrophy is associated with changes in thyroid hormone metabolism

BACKGROUND

Familial partial lipodystrophy of the Dunnigan type (FPLD2) presents with a decrease of subcutaneous adipose tissue (SAT) in the limbs and trunk. As thyroid hormones (TH) play an important role in adipogenesis, we studied if SAT from subjects with FPLD2 have changes in the gene expression levels of monocarboxylate transporter 8 (MCT8), a TH transporter, and TH nuclear receptors and in iodothyronine deiodinases (DIOs) expression and activities that could affect TH bioavailability and action in white adipose tissue.

METHODS

Seven subjects with FPLD2 and 10 healthy controls were studied. Two biopsies of SAT were obtained from each subject, one near the umbilicus and the other from the thigh. Expression of MCT8, DIO2, DIO3, THRA1, THRB1, and RXRG mRNAs were quantified by real-time polymerase chain reaction. DIO1 and DIO2 activities in adipose tissue homogenates were determined. Serum thyroid-stimulating hormone and TH levels were measured by chemiluminescence.

RESULTS

Subjects with FPLD2 had lower levels of MCT8 mRNA expression in the thigh than in the abdomen SAT, and lower than in the abdomen and thigh SAT from control subjects. FPLD2 subjects also had higher DIO2 expression and activity in the thigh than in the abdomen SAT and higher than in controls.

CONCLUSIONS

Thigh SAT from subjects with FPLD2 has lower expression of MCT8 and higher DIO2 expression and activity than abdominal SAT, suggesting that changes in local TH metabolism may occur in areas with lipoatrophy. DIO2 expression and activity in SAT suggest that DIO2 can regulate the metabolism and action of TH in human white adipose tissue.

Thyroid hormone regulation of gene expression in the developing rat fetal cerebral cortex: prominent role of the Ca2+/calmodulin-dependent protein kinase IV pathway

Thyroid hormones influence brain development through regulation of gene expression mediated by nuclear receptors. Nuclear receptor concentration increases rapidly in the human fetus during the second trimester, a period of high sensitivity of the brain to thyroid hormones. In the rat, the equivalent period is the last quarter of pregnancy. However, little is known about thyroid hormone action in the fetal brain, and in rodents, most thyroid hormone-regulated genes have been identified during the postnatal period. To identify potential targets of thyroid hormone in the fetal brain, we induced maternal and fetal hypothyroidism by maternal thyroidectomy followed by antithyroid drug (2-mercapto-1-methylimidazole) treatment. Microarray analysis identified differentially expressed genes in the cerebral cortex of hypothyroid fetuses on d 21 after conception. Gene function analysis revealed genes involved in the biogenesis of the cytoskeleton, neuronal migration and growth, and branching of neurites. Twenty percent of the differentially expressed genes were related to each other centered on the Ca(2+) and calmodulin-activated kinase (Camk4) pathway. Camk4 was regulated directly by T(3) in primary cultured neurons from fetal cortex, and the Camk4 protein was also induced by thyroid hormone. No differentially expressed genes were recovered when euthyroid fetuses from hypothyroid mothers were compared with fetuses from normal mothers. Although the results do not rule out a specific contribution from the mother, especially at earlier stages of pregnancy, they indicate that the main regulators of thyroid hormone-dependent, fetal brain gene expression near term are the fetal thyroid hormones.

Effects of thyroid hormone replacement on associative learning and hippocampal synaptic plasticity in adult hypothyroid rats

Activity-dependent changes taking place at the hippocampal perforant pathway-dentate gyrus synapse during classical eyeblink conditioning were recorded in adult thyroidectomized (hypothyroid) and control (euthyroid) rats, and in animals treated with thyroid hormones 20 days after thyroidectomy (recovery rats). The aim was to determine the contribution of thyroid hormones and the consequences of adult-onset hypothyroidism to both associative learning and the physiological potentiation of hippocampal synapses during the actual learning process in alert behaving animals. Control and recovery rats presented similar learning curves, whereas hypothyroid animals presented lower values. A single pulse presented to the perforant pathway during the conditioned-unconditioned inter-stimulus interval evoked a monosynaptic field excitatory postsynaptic potential in dentate granule cells (whose slope was linearly related to the rate of acquisition in the control group), but not in hypothyroid and recovery animals. Input-output relationships and long-term potentiation evoked by train stimulation of the perforant pathway were significantly depressed in hypothyroid animals. Thyroid hormone treatment failed to normalize these two neurophysiological abnormalities observed in hypothyroid animals. In contrast, paired-pulse facilitation was not affected by thyroidectomy. The results indicate that thyroid hormone treatment after a short period of adult hypothyroidism helps to restore some hippocampally dependent functions, such as classical conditioning, but not other hippocampal properties, such as the synaptic plasticity evoked during associative learning and during experimentally induced long-term potentiation. The present results have important clinical implications for the handling of patients with adult-onset thyroid diseases.

Deiodinase-mediated thyroid hormone inactivation minimizes thyroid hormone signaling in the early development of fetal skeleton

Thyroid hormone (TH) plays a key role on post-natal bone development and metabolism, while its relevance during fetal bone development is uncertain. To study this, pregnant mice were made hypothyroid and fetuses harvested at embryonic days (E) 12.5, 14.5, 16.5 and 18.5. Despite a marked reduction in fetal tissue concentration of both T4 and T3, bone development, as assessed at the distal epiphyseal growth plate of the femur and vertebra, was largely preserved up to E16.5. Only at E18.5, the hypothyroid fetuses exhibited a reduction in femoral type I and type X collagen and osteocalcin mRNA levels, in the length and area of the proliferative and hypertrophic zones, in the number of chondrocytes per proliferative column, and in the number of hypertrophic chondrocytes, in addition to a slight delay in endochondral and intramembranous ossification. This suggests that up to E16.5, thyroid hormone signaling in bone is kept to a minimum. In fact, measuring the expression level of the activating and inactivating iodothyronine deiodinases (D2 and D3) helped understand how this is achieved. D3 mRNA was readily detected as early as E14.5 and its expression decreased markedly ( approximately 10-fold) at E18.5, and even more at 14 days after birth (P14). In contrast, D2 mRNA expression increased significantly by E18.5 and markedly ( approximately 2.5-fold) by P14. The reciprocal expression levels of D2 and D3 genes during early bone development along with the absence of a hypothyroidism-induced bone phenotype at this time suggest that coordinated reciprocal deiodinase expression keeps thyroid hormone signaling in bone to very low levels at this early stage of bone development.

Thermogenic effect of triiodothyroacetic acid at low doses in rat adipose tissue without adverse side effects in the thyroid axis

Triiodothyroacetic acid (TRIAC) is a physiological product of triiodothyronine (T(3)) metabolism, with high affinity for T(3) nuclear receptors. Its interest stems from its potential thermogenic effects. Thus this work aimed 1) to clarify these thermogenic effects mediated by TRIAC vs. T(3) in vivo and 2) to determine whether they occurred predominantly in adipose tissues. To examine this, control rats were infused with equimolar T(3) or TRIAC doses (0.8 or 4 nmolx100 g body wt(-1) x day(-1)) or exposed for 48 h to cold. Both T(3) doses and only the highest TRIAC dose inhibited plasma and pituitary thyroid-stimulating hormone (TSH) and thyroxine (T(4)) in plasma and tissues. Interestingly, the lower TRIAC dose marginally inhibited plasma T(4). T(3) infusion increased plasma and tissue T(3) in a tissue-specific manner. The highest TRIAC dose increased TRIAC concentrations in plasma and tissues, decreasing plasma T(3). TRIAC concentrations in tissues were <10% those of T(3). Under cold exposure or high T(3) doses, TRIAC increased only in white adipose tissue (WAT). Remarkably, only the lower TRIAC dose activated thermogenesis, inducing ectopic uncoupling protein (UCP)-1 expression in WAT and maximal increases in UCP-1, UCP-2, and lipoprotein lipase (LPL) expression in brown adipose tissue (BAT), inhibiting UCP-2 in muscle and LPL in WAT. TRIAC, T(3), and cold exposure inhibited leptin secretion and mRNA in WAT. In summary, TRIAC, at low doses, induces thermogenic effects in adipose tissues without concomitant inhibition of TSH or hypothyroxinemia, suggesting a specific role regulating energy balance. This selective effect of TRIAC in adipose tissues might be considered a potential tool to increase energy metabolism.

Generation and characterization of dickkopf3 mutant mice

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.

Regulation of 5'-promoter activity of the rat growth hormone and growth hormone-releasing hormone receptor genes in the MtT/S and MtT/E cells

The MtT/E and MtT/S cells have been established from a mammotrophic pituitary tumor, and postulated to be progenitor and premature growth hormone (GH) cells, respectively. The difference in the regulation of GH and GH-releasing hormone (GHRH) receptor gene transcription in relation to the developmental stage of GH cells were examined in these two cell lines. In MtT/S cells, triiodothyronine (T3), all-trans retinoic acid (RA) and 9-cis retinoic acid (9cRA) stimulated GH promoter activity but dexamethasone (DEX) did not. On the other hand, DEX stimulated GHRH-receptor promoter alone. T3, RA and 9cRA showed little effect alone but each of them augmented the effect of DEX when used together with DEX. In MtT/E cells, DEX, RA and 9cRA showed similar effect as observed in MtT/S cells on both GH and GHRH-receptor promoter activity. However, T3 neither stimulated GH promoter activity nor augmented the DEX-induced GHRH-receptor gene transcription in MtT/E cells. RT-PCR analyses revealed that both cell types expressed TRalpha1, TRbeta1 and TRalpha2, but expression of TRbeta2, a pituitary specific isoform of TR, was only detected in MtT/S cells. However, the deficiency of TRbeta2 for its own sake does not appear to be a reason why T3 action was not observed in MtT/E cells, because co-transfection of expression plasmids for TRbeta2 and RXRalpha failed in conferring on the cells an ability to respond to T3 by increased GH or GHRH-receptor promoter activity. These results suggest that the acquisition of mechanisms responsible for the regulation of GH or GHRH-receptor transcription by T3 may be involved in the process of functional development of GH cells.

The effects of iodine deficiency on thyroid hormone deiodination

Iodine deficiency induces multiple intrathyroidal autoregulatory changes leading to an increased triiodothyronine (T(3)) production and secretion, at the expense of thyroxine (T(4)). It is characterized by low serum T(4), normal or slightly elevated T(3), and as a consequence of the latter, normal thyrotropin (TSH). Tissues are also hypothyroxinemic, but their T(3) concentrations are mostly normal and ensure clinical euthyroidism, except for those that depend to a high degree on local generation from T(4) by extrathyroidal mechanisms involving the iodothyronine deiodinases isoenzymes. Thus, unless iodine deficiency is so severe and chronic that intrathyroidal and extrathyroidal mechanisms are no longer sufficient to maintain a normal T(3) in most tissues, individuals are clinically and biochemically euthyroid, but some tissues may be selectively hypothyroid (i.e., the brain). In adults both the intrathyroidal and the extrathyroidal mechanisms reacting to the iodine deficiency are fully operative even when the latter is mild. They contribute jointly to the maintenance of elevated or normal T(3) in those tissues deriving most of it from the plasma, until iodine deficiency becomes very severe. Those depending to a large extent from local generation from T(4), mostly by an interplay between type 2 iodothyronine deiodinase (D2) and type 3 (D3), may already be T(3)-deficient (and hypothyroid) with mild iodine deficiency. Therefore, thyroid status of the iodine-deficient individual not only depends on the degree of iodine shortage, but is mostly tissue-specific, and is difficult to define for the individual as a whole: elevated, normal, and low concentrations of T(3) are found simultaneously in different tissues of the same animal, even with severe deficiencies. Most effects of iodine deficiency are reversed in the adults with an adequate iodine prophylaxis, but the absence of T(4) during early fetal life leads to irreversible brain damage (neurologic cretinism). Thyroid hormones of maternal origin are available to the embryo early in development and continue contributing to fetal thyroid hormone status, even after onset of fetal thyroid secretion. In the case of congenital hypothyroidism and normal maternal T(4), the transfer of the latter, together with increased D2 activity, protects the fetal brain from T(3) deficiency, even when it may be insufficient to maintain euthyroidism in other fetal tissues. Practically all of the T(3) found in the fetal brain is derived locally from T(4), and not from circulating T(3). In the case of severe iodine deficiency, both the embryo and the mother are T(4)-deficient; therefore, the fetal brain is exposed to T(3)-deficiency, both before and after onset of fetal thyroid function. This leads to irreversible alterations and damage to the central nervous system (i.e. abnormal corticogenesis). Moreover, because intrathyroidal autoregulatory mechanisms are not yet operative in the fetus, both T(4) and T(3) continue to be very low until birth, and the fetus is not only hypothyroxinemic, similar to its mother, but also clinically and biochemically hypothyroid.

The roles of the iodothyronine deiodinases in mammalian development

The thyroid hormones (TH) are essential for normal development in vertebrate species. This review considers the roles that the three deiodinases, types 1, 2 and 3 (D1, D2, and D3), play in regulating intracellular levels of TH during this critical period. The focus is on rodents and humans with emphasis on brain development. There is little evidence to suggest that the D1 plays a significant role in development and this is substantiated by the absence of any obvious developmental impairment in a D1-deficient mouse model. There is, however, compelling indirect evidence pertaining to the importance of the D2 in development, particularly with respect to that of the brain. However, surprisingly, a D2-deficient mouse model exhibits a very mild phenotype. This, together with the fact that D2 activity is increased in hypothyroidism, suggests that this deiodinase may be of greater importance in development when supplies of thyroxine are limited. The D3 is clearly essential for development in the euthyroid mammal. Information, both indirect and that obtained from a D3-deficient mouse model, strongly suggests that its presence in placenta, uterus and some fetal tissues are critical for limiting exposure of fetal tissues to inappropriate levels of TH.

Structure and function of the type 3 deiodinase gene

Thyroid hormones (TH) are essential for normal growth and development in vertebrates, and are important for the maintenance of normal metabolic activity in most tissues of the body. Because the actions of TH result from the binding of 3,3',5'-triiodothyronine (T(3)) to specific nuclear receptors in the target cell, the extent of TH action in a given cell is dependent in part on the intracellular concentration of T(3). The type 3 deiodinase (D3) is a selenoenzyme that inactivates TH by catalyzing their conversion to biologically inactive metabolites. The findings that D3 activity is very high in the pregnant uterus and fetoplacental unit, and that D3-deficient mice exhibit deficits in growth, viability, and fertility strongly suggest that D3 plays an important role in development. The D3 gene (Dio3) is preferentially expressed from the paternally inherited allele and is associated with an overlapping gene transcribed from the opposite DNA strand (Dio3os). D3 mRNA expression and D3 activity are regulated by a number of hormones and growth factors as well as by genomic imprinting. Although some genomic structures appear to mediate some of these effects, many details concerning the function of the Dio3 gene are unresolved. These include the full characterization of the Dio3 and Dio3os genes, the elucidation of the mechanisms responsible for the developmental and tissue-specific patterns observed in Dio3 allelic expression, and the response of the genes to hormones and growth factors. Knowledge of these details will be important for understanding the physiologic function of an enzyme that appears to be critical for normal mammalian development.

Maternal nutrient deprivation induces sex-specific changes in thyroid hormone receptor and deiodinase expression in the fetal guinea pig brain

Thyroid hormone deprivation during fetal life has been implicated in neurodevelopmental morbidity. In humans, poor growth in utero is also associated with fetal hypothyroxinaemia. In guinea pigs, a short period (48 h) of maternal nutrient deprivation at gestational day (gd) 50 results in fetuses with hypothyroxinaemia and increased brain/body weight ratios. Thyroid hormone action is mediated by nuclear thyroid hormone receptors (TRs) and is dependent upon the prereceptor regulation of supply of triiodothyronine (T3) by deiodinase enzymes. Examination of fetal guinea pig brains using in situ hybridization demonstrated widespread expression of mRNAs encoding TRalpha1, alpha2 and beta1, with regional colocalization of deiodinase type 2 (D2) mRNA in the developing forebrain, limbic structures, brainstem and cerebellum at gd52. With maternal nutrient deprivation, TRalpha1 and beta1 mRNA expression was significantly increased in the male, but decreased in the female fetal hippocampus and cerebellum and other areas showing high TR expression under euthyroid conditions. Maternal nutrient deprivation resulted in elevated D2 mRNA expression in males and females. Deiodinase type 3 (D3) mRNA expression was confined to the shell of the nucleus accumbens, the posterior amygdalohippocampal area, brainstem and cerebellum, and did not change with maternal nutrient deprivation. In conclusion, maternal nutrient deprivation resulted in sex-specific changes in TR mRNA expression and a generalized increase in D2 mRNAs within the fetal brain. These changes may represent a protective mechanism to maintain appropriate thyroid hormone action in the face of fetal hypothyroxinaemia in order to optimize brain development.

The role of type I and type II 5′ deiodinases on hexachlorobenzene-induced alteration of the hormonal thyroid status

Treatment of male Wistar rats with hexachlorobenzene (HCB) (1000 mg/kg b.w.) for 3-30 days decreases circulating levels of thyroxine (T4) but does not affect triiodothyronine (T3). Time courses were determined for 5' deiodinase type I (5' D-I) activity in thyroid, liver, and kidney and 5' deiodinase type II (5' D-II) activity in brown adipose tissue (BAT) to test the possibility that increased deiodinase activity might contribute to the maintenance of the serum T3 level. Specific 5' D-I activity was increased in the thyroid at 21 days and thereafter. No significant changes were observed in the liver, however, total 5' D-I activity in this tissue was increased at 30 days of treatment as a consequence of liver weight enhancement. HCB decreased kidney 5' D-I activity after 15 days, and BAT 5' D-II activity after 21 days of treatment. Total body 5' D-I activity was significantly increased by 30 days of HCB-treatment. HCB increased the activity of hepatic T4 uridine diphosphoglucuronosyl transferase (UDPGT) in a time-dependent manner, without changes in T3 UDPGT. We propose that increased T4 to T3 conversion in the thyroid and in the greatly enlarged liver may account for the maintenance of serum T3 concentration in hypothyroxinemic HCB-treated rats.

Iodothyronine levels in the human developing brain: major regulatory roles of iodothyronine deiodinases in different areas

Thyroid hormones are required for human brain development, but data on local regulation are limited. We describe the ontogenic changes in T(4), T(3), and rT(3) and in the activities of the types I, II, and III iodothyronine deiodinases (D1, D2, and D3) in different brain regions in normal fetuses (13-20 wk postmenstrual age) and premature infants (24-42 wk postmenstrual age). D1 activity was undetectable. The developmental changes in the concentrations of the iodothyronines and D2 and D3 activities showed spatial and temporal specificity but with divergence in the cerebral cortex and cerebellum. T(3) increased in the cortex between 13 and 20 wk to levels higher than adults, unexpected given the low circulating T(3). Considerable D2 activity was found in the cortex, which correlated positively with T(4) (r = 0.65). Cortex D3 activity was very low, as was D3 activity in germinal eminence and choroid plexus. In contrast, cerebellar T(3) was very low and increased only after midgestation. Cerebellum D3 activities were the highest (64 fmol/min.mg) of the regions studied, decreasing after midgestation. Other regions with high D3 activities (midbrain, basal ganglia, brain stem, spinal cord, hippocampus) also had low T(3) until D3 started decreasing after midgestation. D3 was correlated with T(3) (r = -0.682) and rT(3)/T(3) (r = 0.812) and rT(3)/T(4) (r = 0.889). Our data support the hypothesis that T(3) is required by the human cerebral cortex before midgestation, when mother is the only source of T(4). D2 and D3 play important roles in the local bioavailability of T(3). T(3) is produced from T(4) by D2, and D3 protects brain regions from excessive T(3) until differentiation is required.

Dynamics and regulation of intracellular thyroid hormone concentrations in embryonic chicken liver, kidney, brain, and blood

The intracellular thyroid hormone (TH) availability is influenced by different metabolic pathways. We investigated the relationship between tissue and plasma TH levels as well as the correlation with changes of deiodination and sulfation during chicken embryonic development. From day 14 until day 19, T3 remains unchanged in liver and kidney in spite of increasing plasma T4 and T3 levels and a slightly increased T4 availability in these tissues. During this period, the T3 breakdown capacity by type III deiodinase (D3) is high in liver but low in kidney. The TH inactivation capacity of type I deiodinase (D1), with production of inactive rT3 instead of T3, in kidney seems to be potentiated by the sulfation pathway. A sharp rise in T3 and T4 is detected in all tissues examined when the embryo switches to lung respiration. The same day, T4 content in liver is sharply enhanced and sulfation activity is decreased. So, T4 availability in liver is increased while a declined D3 activity allows for the accumulation of hepatic T3. The increase in renal T3 and T4 are more closely related to plasma TH profiles and a lack of correlation with the changes in renal D1 and D3 activity suggests that T4 and T3 content in this organ is strongly dependent on direct uptake from the blood. Despite much lower T4 levels, T3 levels in brain are in the same range as in liver and kidney and intracellular T3 even exceeds the T4 levels towards the end of development. The rise in TH content coincides with a drop in D3 activity, low sulfation activity and an increased T3 production capacity via type II deiodinase (D2). In conclusion, the current study describes the dynamics of intracellular TH concentrations in liver, kidney, and brain during chicken development and investigates their relationship with circulating TH levels and changes of deiodinases and sulfotransferases. The clear differences in intracellular TH profiles among the different tissues demonstrate that circulating levels are not necessarily representative for the local TH changes. Some of the changes in intracellular TH availability can be linked to changes in local deiodination and sulfation capacities, but the importance of these enzyme systems in relation to other factors, such as hormone uptake, differs between liver, kidney, and brain.

Age-dependent adaptation of the liver thyroid status and recovery of serum levels and hepatic insulin-like growth factor-I expression in neonatal and adult diabetic rats

The effect of treatment with thyroxine (T(4)) on the hepatic deiodinase (5'D-I) activity and triiodothyronine (T(3)) content and on insulin-like growth factor-I (IGF-I) secretion and mRNA hepatic expression were studied in neonatal and adult diabetic (D) rats and compared with 4 thyroidectomized (Tx) groups: neonatal and adult Tx rats treated or not with T(4). Serum T(3) and T(4) decreased by 92% in both Tx populations and by 80% to 70% in D adults according to the severity of diabetes: -70 mg/kg body weight (BW) (D(70)) or 50 mg/kg BW (D(50)) of streptozotocin (STZ) injected, whereas only a 30% to 33% decrease was found in D neonates. A similar decrease of liver 5'D-I activity and T(3) concentrations was found in neonatal and adult Tx rats, whereas a significant reduction in those parameters was observed only in adult diabetics, either D(70) or D(50), but not in D neonates. Serum levels and liver mRNA expression of IGF-I determined by ribonuclease protection assay, plasma and pituitary growth hormone (GH), plasma insulin, and glycemia were also measured in both D populations. A decrease in circulating IGF-I, previously reported for Tx adult rats, was also found in both D populations. T(4) treatment recovered IGF-I and liver T(3) in both Tx groups and D neonates, but not in D adults. These results show an age-dependent adaptation of the liver thyroid economy in diabetes, as hepatic 5'D-I does not respond to diabetes in neonates and IGF-I is insensitive to T(4) treatment in adult diabetics and suggest a positive correlation between hepatic T(3) content and IGF-I expression in conditions of diabetes and Tx.

Early maternal hypothyroxinemia alters histogenesis and cerebral cortex cytoarchitecture of the progeny

Epidemiological studies from both iodine-sufficient and -deficient human populations strongly suggest that early maternal hypothyroxinemia (i.e., low circulating free thyroxine before onset of fetal thyroid function at midgestation) increases the risk of neurodevelopmental deficits of the fetus, whether or not the mother is clinically hypothyroid. Rat dams on a low iodine intake are hypothyroxinemic without being clinically hypothyroid because, as occurs in pregnant women, their circulating 3,5,3'-triiodothyronine level is usually normal. We studied cell migration and cytoarchitecture in the somatosensory cortex and hippocampus of the 40-day-old progeny of the iodine-deficient dams and found a significant proportion of cells at locations that were aberrant or inappropriate with respect to their birth date. Most of these cells were neurons, as assessed by single- and double-label immunostaining. The cytoarchitecture of the somatosensory cortex and hippocampus was also affected, layering was blurred, and, in the cortex, normal barrels were not formed. We believe that this is the first direct evidence of an alteration in fetal brain histogenesis and cytoarchitecture that could only be related to early maternal hypothyroxinemia. This condition may be 150-200 times more common than congenital hypothyroidism and ought to be prevented both by mass screening of free thyroxine in early pregnancy and by early iodine supplementation to avoid iodine deficiency, however mild.

Early maternal hypothyroxinemia alters histogenesis and cerebral cortex cytoarchitecture of the progeny

Epidemiological studies from both iodine-sufficient and -deficient human populations strongly suggest that early maternal hypothyroxinemia (i.e., low circulating free thyroxine before onset of fetal thyroid function at midgestation) increases the risk of neurodevelopmental deficits of the fetus, whether or not the mother is clinically hypothyroid. Rat dams on a low iodine intake are hypothyroxinemic without being clinically hypothyroid because, as occurs in pregnant women, their circulating 3,5,3'-triiodothyronine level is usually normal. We studied cell migration and cytoarchitecture in the somatosensory cortex and hippocampus of the 40-day-old progeny of the iodine-deficient dams and found a significant proportion of cells at locations that were aberrant or inappropriate with respect to their birth date. Most of these cells were neurons, as assessed by single- and double-label immunostaining. The cytoarchitecture of the somatosensory cortex and hippocampus was also affected, layering was blurred, and, in the cortex, normal barrels were not formed. We believe that this is the first direct evidence of an alteration in fetal brain histogenesis and cytoarchitecture that could only be related to early maternal hypothyroxinemia. This condition may be 150-200 times more common than congenital hypothyroidism and ought to be prevented both by mass screening of free thyroxine in early pregnancy and by early iodine supplementation to avoid iodine deficiency, however mild.

Neonatal thyroid disorders

Hypothyroxinaemia, which is common in the preterm infant, and thyrotoxicosis, which is rare, are important neonatal thyroid disorders. Their causes and treatment are discussed.

Mitochondrial biogenesis and thyroid status maturation in brown fat require CCAAT/enhancer-binding protein alpha

Brown fat differentiation in mice is fully achieved in fetuses at term and entails the acquisition of not only adipogenic but also thermogenic and oxidative mitochondrial capacities. The present study of the mice homozygous for a deletion in the gene for CCAAT/enhancer-binding protein alpha (C/EBPalpha-null mice) demonstrates that C/EBPalpha is essential for all of these processes. Developing brown fat from C/EBPalpha-null mice showed a lack of uncoupling protein-1 expression, impaired adipogenesis, and reduced size and number of mitochondria per cell when compared with wild-type mice. Furthermore, immature mitochondrial morphology was found in brown fat, but not in liver or heart, from C/EBPalpha-null mice. Concordantly, expression of both nuclear and mitochondrial genome-encoded genes for mitochondrial proteins was reduced in C/EBPalpha-null brown fat, although expression of mitochondrial rRNA and mitochondrial DNA content were unaltered. Expression of nuclear respiratory factor-2, thyroid hormone nuclear receptors, and peroxisome proliferator-activated receptor gamma coactivator-1, was delayed in C/EBPalpha-null brown fat. Iodothyronine 5'-deiodinase activity and thyroid hormone content were also reduced in brown fat from C/EBPalpha-null mice, indicating for the first time a crucial role for C/EBPalpha in controlling thyroid status in developing brown fat, which may contribute to impaired mitochondrial biogenesis and cell differentiation. When survival of C/EBPalpha-null mice was achieved by transgenically expressing C/EBPalpha only in the liver, a substantial recovery in brown fat differentiation was found by day 7 of postnatal age, which is associated with a compensatory overexpression of C/EBPdelta and C/EBPbeta.

Fetal tissues are exposed to biologically relevant free thyroxine concentrations during early phases of development

Maternal hypothyroxinemia in early pregnancy is often associated with irreversible effects on neuropsychomotor development. To evaluate fetal tissue exposure to maternal thyroid hormones up to midgestation, we measured total T(4) and free T(4) (FT(4)), T(3), rT(3), TSH, and possible binding proteins in first trimester coelomic and amniotic fluids and in amniotic fluid and fetal serum up to 17 wk. Samples were obtained before interruption of maternal-fetal connections. The concentrations in fetal compartments of T(4) and T(3) are more than 100-fold lower than those in maternal serum, and their biological relevance for fetal development might be questioned. We found, however, that in all fetal fluids the concentrations of T(4) available to developing tissues, namely FT(4), reach values that are at least one third of those biologically active in their euthyroid mothers. FT(4) levels in fetal fluids are determined by both their T(4)-binding protein composition and the T(4) or FT(4) in maternal serum. The binding capacity is determined ontogenically, is independent of maternal thyroid status, and is far in excess of the T(4) in fetal fluids. Thus, the availability of FT(4) for embryonic and fetal tissues would decrease in hypothyroxinemic women, even if they were euthyroid. A decrease in the availability of FT(4), a major precursor of intracellular nuclear receptor-bound T(3), may result in adverse effects on the timely sequence of developmental events in the human fetus. These findings ought to influence our present approach to maternal hypothyroxinemia in early pregnancy regardless of whether TSH is increased or whether overt or subclinical hypothyroidism is detected.

Influence of type II 5' deiodinase on TSH content in diabetic rats

The influence of hypothalamic and pituitary type II 5'deiodinase (5'D-II) activities and T3 content on pituitary TSH content was investigated in streptozotocin (STZ)-induced diabetic rats (D). The results show, first, that hypothalamic and pituitary 5'D-II activities were lower in neonatal D rats versus control (C) rats, and the normal developmental pattern was altered. Secondly, when D and C rats were thyroidectomized (Tx) at 25 days of age (D+Tx, C+Tx), pituitary and hypothalamic 5'D-II activities increased ten days later in both populations vs. intact rats, but the percentage of increase was smaller in D+Tx than in C+Tx. The hypothalamic T3 to T4 ratios were also decreased in D+Tx animals (0.38) as compared to C+Tx rats (1.64). The hypothalamic T3 content was reduced by 30% in D as compared to C rats and by 80% in D+Tx as compared to C+Tx rats, showing a defect in hypothalamic T4 deiodination. Pituitary TSH content increased after Tx in D+Tx, but not in C+Tx. These results in diabetic rats indicate that the hypothalamic and pituitary 5'D-II activity and hypothalamic T3 content are affected by diabetes and play a role in the regulation of pituitary TSH content.

Hypothyroxinemia in premature infants: is thyroxine treatment necessary?

The increased survival of very low birth weight (VLBW) premature infants has been associated with an increased prevalence of transient disorders of thyroid function during the early weeks of life. All VLBW infants have relatively low, and gestation age-dependent, thyroxine-binding globulin (TBG) concentrations associated with variably low total thyroxine (T4) concentrations. These infants also have a high prevalence (30%-60%) of nonthyroidal illnesses that impact thyroid function, including total and free iodothyronine concentrations. Finally, thyroid gland hormone biosynthesis and the hypothalamic-pituitary axis are relatively immature and thyroid glandular iodine stores are low. As a result, VLBW infants manifest a high prevalence of transient primary hypothyroidism (0.41%) and transient hypothalamic-pituitary (thyrotropin [TSH] deficiency) hypothyroidism (5%-10%?). Thyroid function should be monitored in VLBW infants during the first 1-4 weeks of life to detect these disorders as well as the much less common permanent congenital hypothyroidism. An elevated serum TSH level (> 20 mU/L) is diagnostic of primary hypothyroidism and a low free T4 concentration with a normal or low serum TSH level suggests hypothalamic-pituitary hypothyroidism. Treatment of transient or permanent primary hypothyroidism and of congenital TSH deficiency is indicated. Preliminary data suggests that 30-60-day treatment of transient hypothalamic-pituitary hypothyroidism improves IQ at 2 years of age. However, further information confirming the benefit of treatment, the threshold free T4 values for diagnosis, and the optimal dose of T4 are necessary.

Regulation of thyroid hormone metabolism during fetal development

Compared with adults, plasma T3 concentrations in the human fetus are decreased, whereas levels of rT3 and the different iodothyronine sulfates, T4S, T3S, rT3S and 3,3'-T2S, are increased. The low T3 and high rT3 concentrations reflect the preponderance of inner ring versus outer ring deiodinase activity due to high type III iodothyronine deiodinase (D3) expression in fetal tissues, such as liver and brain, the placenta, and perhaps also the uterus, in combination with still incomplete expression of hepatic type I iodothyronine deiodinase (D1) expression. In contrast to humans, D3 is hardly expressed in the fetal rat liver. However, high D3 expression is observed in the embryonic chicken liver which decreases dramatically towards the end of incubation, resulting in a marked increase in plasma T3. Thyroid hormone is essential for the development of the brain, in which local conversion of the prohormone T4 to the active hormone T3 by the type II iodothyronine deiodinase (D2) plays a very important role. In contrast to the rat, however, little is known about the ontogeny of D2 in different human brain areas. The cause of the high concentrations of sulfated iodothyronines in fetal plasma is unknown. In adults, the liver is an important site for the clearance of these conjugates, where they are rapidly degraded by D1. Although fetal human liver expresses significant D1 activity, clearance of iodothyronine sulfates may be defective due to the lack of transporters mediating their hepatic uptake. However, production of iodothyronine sulfates may also be increased in the human fetus, although the responsible sulfotransferases and their location remain to be identified. Sulfation may be a reversible pathway of thyroid hormone inactivation, depending on the recovery of free hormone by sulfatases. However, little is known at present about the characteristics and regulation of these enzymes in fetal human tissues. Further studies are required to increase our understanding of the tissue-specific and stage-dependent regulation of thyroid hormone bioactivity during human development.

The influence of low doses 131I-induced maternal hypothyroidism on the development of rat embryos

The aim of our research was to create and verify a model for studying the effects of a low dose of 131I and 131I-induced maternal hypothyroidism on the development of the embryo's thyroid gland and brain. The given dose (150 microCi) corresponds to the absorbed dose of 0.5 Gy. This dose is similar to that dose received by large numbers of the population of the C.I.S. regions polluted by radioactive isotopes of iodine as a result of the Chernobyl accident in 1985. Thirty-five female Wistar rats and their 168 newborn pups were used for observation. The females were divided into a control group and four experimental groups (each distinguished by the time of 131I injection: group I - no less than 12 days before mating; groups II, III and IV - on 5th, 10th and 16th days of gestation, respectively). In all the experimental female groups the incorporate dose of 131I led to hypothyroidism accompanied by a 43% reduction in the thyroxin level and by a nearly 8-fold increase in the TSH level. However, the influence of maternal hypothyroidism on the development of the thyroid gland and brain of embryos depends on the time when 131I took effect. There is a reduction in the weight of the newborns' brain and thyroid gland, total body mass. The hormonal status of the newborns' thyroid gland also changes. The proposed model will allow us to study many aspects of induced changes in the brain and thyroid gland of the embryos which develop under conditions of maternal hypothyroidism resulting from a low dose of 131I, administered at the critical times of development.

Immunohistochemical study of fibronectin and thyroglobulin in the thyroid gland of female rats after exposure to radioactive iodine

The aim of this work was to study the effect of a dose of 150 microCi 131I on the barrier properties of the thyroid epithelium in pregnant female rats. Thirty-five female Wistar rats were divided into a control and four experimental groups (each distinguished by the time of 131I injection: group I--no less then 12 days before mating; groups II, III, and IV--on 5th, 10th, and 16th days of gestation, respectively). The thyroid glands were fixed in Bouin's fluid, embedded in paraffin, and stained immunohistochemically for thyroglobulin and fibronectin. In group IV the appearance of follicles with fibronectin-positive colloid demonstrates the penetration of blood plasma into the follicular lumen. There are more fibronectin positive follicles in group III. Regardless of the nature of the follicles' contents, numerous thyrocytes with an intensive fibronectin positive reaction begin to appear in the follicles. In group II the number of fibronectin positive follicles and thyrocytes is clearly reduced, and in group I only a few remain. In group IV there is a noticeable reduction in the quantity of colloid inside the follicles and often an absence of any thyroglobulin positive reaction. There are thyrocytes in which thyroglobulin positive granules localized in the basal zone. There is thyroglobulin positive staining in the stroma and blood vessels. In group II thyroglobulin is no longer found in the stroma. Small doses of 131I provoke a serious breakdown in the thyroid epithelium's barrier properties, although these changes are of a transient nature. The central zone of the thyroid gland reacts more actively and dynamically to exposure to radioactive iodine than the peripheral zone.

Thyroid hormones in human tumoral and normal nervous tissues

We have studied T4 and T3 concentrations, DNA and protein concentrations and 5' and 5 deiodinases in samples of brain tumors obtained at surgery from 49 patients, and, in most cases, also from surrounding normal tissue. T4 concentrations in normal cortical tissue (6.19+/-0.45 ng/g) were lower than in white matter, but the difference disappeared when referred to the DNA content (2.26+/-0.27 ng/mg DNA). No other differences were found between cortical and white matter, or among cortical lobes. T4 in normal tissue was higher than previously reported, mostly from autopsy samples, whereas T3 (0.99+/-0.07 ng/g) was similar. 5'D-I activity was negligible as compared to 5'D-II (8.11+/-1.09 fmol/h/mg protein). When expressed in relation to the different DNA contents of normal vs. tumoral tissue, 5'D-II activities were the same for both. 5D activity was highly variable in the tumoral tissue, with negligible activities in meningiomas and pituitary adenomas. When referred to the DNA content, T4 and 5'D-II were the same, but T3 concentrations were lower in the tumor (0.24+/-0.03 ng/mg DNA) as compared to normal (0.35+/-0.04 ng/mg DNA) tissue samples. Whether or not this decrease of T3 affects the expression of T3-sensitive processes remains to be studied.

Ontogeny of iodothyronine deiodinases in human liver

The role of the deiodinases D1, D2, and D3 in the tissue-specific and time-dependent regulation of thyroid hormone bioactivity during fetal development has been investigated in animals but little is known about the ontogeny of these enzymes in humans. We analyzed D1, D2, and D3 activities in liver microsomes from 10 fetuses of 15-20 weeks gestation and from 8 apparently healthy adult tissue transplant donors, and in liver homogenates from 2 fetuses (20 weeks gestation), 5 preterm infants (27-32 weeks gestation), and 13 term infants who survived up to 39 weeks postnatally. D1 activity was determined using 1 microM [3',5'-125I]rT3 as substrate and 10 mM dithiothreitol (DTT) as cofactor, D2 activity using 1 nM [3',5'-125I]T4 and 25 mM DTT in the presence of 1 mM 6-propyl-2-thiouracil (to block D1 activity) and 1 microM T3 (to block D3 activity), and D3 activity using 10 nM [3,5-125I]T3 and 50 mM DTT, by quantitation of the release of 125I. The assays were validated by high performance liquid chromatography of the products, and kinetic analysis [Michaelis-Menten constant (Km) of rT3 for D1: 0.5 microM; Km of T3 for D3: 2 nM]. In liver homogenates, D1 activity was not correlated with age, whereas D3 activity showed a strong negative correlation with age (r -0.84), with high D3 activities in preterm infants and (except in 1 infant of 35 weeks) absent D3 activity in full-term infants. In microsomes, D1 activities amounted to 4.3-60 pmol/min/mg protein in fetal livers and to 170-313 pmol/min/mg protein in adult livers, whereas microsomal D3 activities were 0.15-1.45 pmol/min/mg protein in fetuses and <0.1 pmol/min/mg protein in all but one adult. In the latter sample, D3 activity amounted to 0.36 pmol/min/mg protein. D2 activity was negligible in both fetal and adult livers. These findings indicate high D1 and D3 activities in fetal human liver, and high D1 and mostly absent D3 activities in adult human liver. Therefore, the low serum T3 levels in the human fetus appear to be caused by high hepatic (and placental) D3 activity rather than caused by low hepatic D1 activity. The occasional expression of D3 in adult human liver is intriguing and deserves further investigation.

The effects of thyroid hormone level and action in developing brain: are these targets for the actions of polychlorinated biphenyls and dioxins?

Alterations in thyroid hormone level or responsivity to thyroid hormone have significant neurologic sequelae throughout the life cycle. During fetal and early neonatal periods, disorders of thyroid hormone may lead to the development of motor and cognitive disorders. During childhood and adult life, thyroid hormone is required for neuronal maintenance as well as normal metabolic function. Those with an underlying disorder of thyroid hormone homeostasis or mitochondrial function may be at greater risk for developing cognitive, motor, or metabolic dysfunction upon exposure to substances which alter thyroid hormone economy. Polychlorinated biphenyls (PCBs) and dioxins have been argued to interfere with thyroid hormone action and thus may affect the developing and mature brain. Animal models provide useful tools for studying the effects of thyroid hormone disorders and the effects of environmental endocrine disruptors. The congenitally hypothyroid, hyt/hyt, mouse exhibits abnormalities in both the cognitive and motor systems. In this mouse and other animal models of thyroid hormone disorders, delayed somatic and reflexive development are noted, as are permanent deficits in hearing and locomotor and adaptive motor behavior. This animal's behavioral abnormalities are predicated on anatomic abnormalities in the nervous system. In turn, these abnormalities are correlated with differences in neuronal structural proteins. In normal mice, the expression of mRNAs coding for these proteins occurs temporally with the onset of autonomous thyroid hormone production. The hyt/hyt mouse has a mutation in the thyroid stimulating hormone receptor (TSHr) gene which renders it incapable of transducing the TSH signal in the thyrocyte to produce thyroid hormone. Some behavioral and possibly some biochemical abnormalities in mice exposed to PCBs are similar to those seen in the hyt/hyt mouse. In addition to direct effects on brain development and neuronal maintenance, thyroid hormone is necessary for maintaining metabolic functioning through its influence on mitochondria. Because the brain is particularly sensitive to inadequate energy generation, disorders of thyroid hormone economy also indirectly impair brain functioning. Alterations in thyroid hormone level result in differing expression of mitochondrial genes. Mutations in these mitochondrial genes lead to well-recognized syndromes of encephalomyopathy, myopathy, and multisystem disorder. Hence, PCBs and dioxins, by possibly altering the thyroid hormone milieu, may alter the functioning of mitochondria in the generation of adenosine triphosphate (ATP). The use of animal models of thyroid hormone deficiency for behavioral, anatomic, histologic, and molecular comparison will help elucidate the mechanisms of action of these putative endocrine-disrupting compounds. The study of thyroid hormone disorders provides a template for relating thyroid hormone mediated effects on the brain to these compounds.

Effects of thyroid hormone on embryonic oligodendrocyte precursor cell development in vivo and in vitro

The oligodendrocyte precursor cell divides a limited number of times before terminal differentiation. The timing of differentiation depends on both intracellular mechanisms and extracellular signals, including mitogens that stimulate proliferation and signals such as thyroid hormone (TH) and retinoic acid (RA) that help trigger the cells to stop dividing and differentiate. We show here that, both in vivo and in vitro, TH is required for the normal development of rodent optic nerve oligodendrocytes, although in its absence some oligodendrocyte development still occurs, perhaps promoted by signals from axons. We also demonstrate that TH from both mother and pup plays a part in oligodendrocyte development in vivo. Finally, we show that precursors in embryonic nerve cultures differ from those in postnatal cultures in two ways: they respond much better to TH than to RA, and they respond more slowly to TH, suggesting that oligodendrocyte precursor cells mature during their early development.

Regulation of glucose transport, and glucose transporters expression and trafficking in the heart: studies in cardiac myocytes

Cardiac muscle is characterized by a high rate of glucose consumption. In the absence of insulin, glucose transport into cardiomyocytes limits the rate of glucose utilization and therefore it is important to understand the regulation of glucose transporters. Cardiac muscle cells express 2 distinct glucose transporters, GLUT4 and GLUT1; although GLUT4 is quantitatively the more important glucose transporter expressed in heart, GLUT1 is also expressed at a substantial level. In isolated rat cardiomyocytes, insulin acutely stimulates glucose transport and translocates both GLUT4 and GLUT1 from an intracellular site to the cell surface. Recent evidence indicates the existence of at least 2 distinct intracellular membrane populations enriched in GLUT4 with a different protein composition. Elucidation of the intracellular location of these 2 GLUT4 vesicle pools in cardiac myocytes, their role in GLUT4 trafficking, and their relation to insulin-induced GLUT4 translocation needs to be addressed.

Changes in thyroid hormone concentrations and total contents through ontogeny in three anuran species: evidence for daily cycles

Three anuran species (Rana perezi, Xenopus laevis, and Bufo calamita) of different phylogenetic origins and ecological habitats have been studied during ontogeny with respect to day/night changes in whole-body concentrations and total content of extrathyroidal thyroxine (T4) and triiodothyronine (T3). There were no significant day/night changes in thyroid hormones (TH) during embryonic stages. Daily cycles in TH with higher nocturnal values appeared during premetamorphosis in R. perezi and X. laevis. Cyclicity disappears for T3, while it is reversed for T4, in prometamorphic R. perezi and X. laevis. In contrast, there were significantly higher T3 (0.74 +/- 0.13 ng/g) and T4 (2.08 +/- 0.54 ng/g) levels at night in prometamorphic B. calamita. Significant daily changes in T3 and T4 with higher nocturnal values (T3, 788.29 +/- 118.38 pg/g; T4, 1.95 +/- 0.4 ng/g) were again seen in X. laevis at the end of climax, while in B. calamita low TH values appeared at early scotophase and there were no significant changes in R. perezi at this time. Similar daily profiles were observed for TH whole-body concentrations and total contents.

Neonatal hypothyroxinemia: effects of iodine intake and premature birth

We have investigated the effects of iodine (I) intake on urinary I excretion in preterm (PT) babies up to 2 months after birth and its effect on serum T4, free T4 (FT4), T3, TSH, and thyroglobulin (Tg) levels compared to those in term (T) newborns. Very premature and very sick infants were in negative I balance for the first weeks after birth. Later, these same infants, as well as the other PT and T newborns, were in positive balance; 75- 80% of the ingested I was not accounted for in the urine. The urinary I levels of PT and T neonates cannot be equated to their I intakes. T4, FT4, and T3 levels in PT and T neonates increased with postmenstrual age, whereas Tg decreased and TSH did not change. Serum FT4, T3, Tg, and TSH levels in PT neonates were affected negatively, independently from age, by a low I intake. PT birth also affected T4, FT4, and Tg negatively, independently from I intake and postmenstrual age, for at least 6-8 weeks after birth. Care should be taken to avoid I deficiency in PT neonates. However, even when I intake is adequate, PT newborns are hypothyroxinemic compared to T babies during an important period of brain development. This suggests the possible convenience of interventions that might mimic the intrauterine hormone environment and accelerate maturation.

Maternal nonthyroidal illness and fetal thyroid hormone status, as studied in the streptozotocin-induced diabetes mellitus rat model

We have used the streptozotocin-induced diabetes mellitus pregnant rat as a model of maternal nonthyroidal illness. We measured the effects of different degrees of diabetes mellitus on maternal body weight, the outcome of pregnancy, circulating glucose, insulin, T4, T3, rT3, and TSH in mother and fetus, T4 and T3 in maternal and fetal tissues, and iodothyronine deiodinases in liver, lung, and brain. All of the changes in thyroid hormone status typical of nonthyroidal illnesses were observed in the mothers and were related to the degree of the metabolic imbalances. Most were controlled with a daily insulin dose of 0.5 U/100 g BW. Normalization of maternal placental T4, however, required higher insulin doses than in other maternal tissues. The number and body weight of the fetuses, their pituitary GH contents, and their thyroid hormone status were severely affected. The total extrathyroidal T4 and T3 pools decreased to one third of normal fetal values. T4 and T3 concentrations in the fetal brain were lower than normal, and the expected increase in type II 5'deiodinase activity was not observed. The low cerebral T3 only improved with adequate insulin treatment of the dams. It is concluded that maternal diabetes mellitus, and possibly other nonthyroidal illnesses that impair the availability of intracellular energy stores, may affect fetal brain T3 when thyroid hormones are essential for normal development.

Maternal diabetes mellitus, a rat model for nonthyroidal illness: correction of hypothyroxinemia with thyroxine treatment does not improve fetal thyroid hormone status

Maintenance of normal maternal thyroxinemia prevents severe triiodothyronine (T3) deficiency of the fetus with primary thyroid failure (1). We have studied whether thyroxine (T4) would also protect the fetal brain when maternal hypothyroxinemia is caused by nonthyroidal illnesses. We have used the streptozotocin-induced diabetes mellitus pregnant rat as a model of maternal nonthyroidal illness. We measured the effects of diabetes mellitus, and of correction of the ensuing maternal hypothyroxinemia with T4 as compared to insulin, on maternal body weight, the outcome of pregnancy, glucose, insulin, T4, T3, reverse T3, and thyrotropin levels in the maternal and fetal circulation, as well as T4 and T3 concentrations in tissues, and iodothyronine deiodinases in liver, lung, and brain. The diabetic mothers showed changes in thyroid hormone status typical of nonthyroidal illnesses. Thyroid hormone status of the fetuses was severely affected: the total T4 and T3 pools decreased to one-third of normal values. T4 and T3 concentrations in the fetal brain were lower than normal and the expected increase in 5'-deiodinase activity was not observed. Although insulin treatment avoided or mitigated these changes, the low cerebral T3 did not improve with T4 treatment of the maternal hypothyroxinemia. Several findings indicated that treatment of the severely ill dams with T4 was actually harmful for the outcome of pregnancy. These negative effects were observed without the expected increase in the maternal or fetal T3 pools.