Thyroid hormones and brain development

Understanding the hypothalamus-pituitary-thyroid axis in mct8 deficiency

Thyroid hormone (TH) metabolism and action via binding to nuclear receptors are intracellular events that require the passage of TH across the plasma membrane. This process is mediated by specific TH transporters of which the monocarboxylate transporter 8 (Mct8) has received major attention. Mct8 is highly expressed in different tissues such as liver, kidney, thyroid, pituitary and brain. In humans, inactivating mutations of the MCT8 gene (SLC16A2) are associated with severe forms of psychomotor retardation and abnormal TH serum levels (Allan-Herndon-Dudley syndrome). Surprisingly, Mct8 knockout (ko) mice do not exhibit overt neurological symptoms but fully replicate the unusual serum TH profile with highly increased serum T3 in the presence of low serum T4. In order to evaluate the underlying mechanisms for these abnormalities, TH transport and metabolism have been intensively studied in different tissues of Mct8 ko mice. Here, we summarize the observed changes within the hypothalamus-pituitary-thyroid axis that result in altered TH production and secretion. Although analysis of Mct8 ko mice has greatly expanded our knowledge, many open questions still remain to be addressed in order to define the tissue- and cell-specific role of this important TH transporter.

Hypothalamic gene expression rapidly changes in response to photoperiod in juvenile Siberian hamsters (Phodopus sungorus)

Siberian hamsters are seasonal mammals that survive a winter climate by making adaptations in physiology and behaviour. This includes gonadal atrophy, reduced food intake and body weight. The underlying central mechanisms responsible for the physiological adaptations are not fully established but involve reducing hypothalamic tri-iodthyronine (T3) levels. Juvenile Siberian hamsters born or raised in short days (SD) respond in a similar manner, although with an inhibition of gonadal development and growth instead of reversing an established long day (LD) phenotype. Using juvenile male hamsters, the present study aimed to investigate whether the central mechanisms are similar before the establishment of the mature LD phenotype. By in situ hybridisation, we examined the response of genes involved in thyroid hormone (Dio2 and Dio3, which determine hypothalamic T3 levels) and glucose/glutamate metabolism in the ependymal layer, histamine H3 receptor and VGF as representatives of the highly responsive dorsomedial posterior arcuate nucleus (dmpARC), and somatostatin, a hypothalamic neuropeptide involved in regulating the growth axis. Differential gene expression of type 2 and type 3 deiodinase in the ependymal layer, histamine H3 receptor in the dmpARC and somatostatin in the ARC was established by the eighth day in SD. These changes are followed by alterations in glucose metabolism related genes in the ependymal layer by day 16 and increased secretogranin expression in the dmpARC by day 32. In conclusion, our data demonstrate similar but rapid and highly responsive changes in gene expression in the brain of juvenile Siberian hamsters in response to a switch from LD to SD. The data also provide a temporal definition of gene expression changes relative to physiological adaptations of body weight and testicular development and highlight the likely importance of thyroid hormone availability as an early event in the adaptation of physiology to a winter climate in juvenile Siberian hamsters.

Critical role of types 2 and 3 deiodinases in the negative regulation of gene expression by T₃in the mouse cerebral cortex

Thyroid hormones regulate brain development and function through the control of gene expression, mediated by binding of T(3) to nuclear receptors. Brain T(3) concentration is tightly controlled by homeostatic mechanisms regulating transport and metabolism of T(4) and T(3). We have examined the role of the inactivating enzyme type 3 deiodinase (D3) in the regulation of 43 thyroid hormone-dependent genes in the cerebral cortex of 30-d-old mice. D3 inactivation increased slightly the expression of two of 22 positively regulated genes and significantly decreased the expression of seven of 21 negatively regulated genes. Administration of high doses of T(3) led to significant changes in the expression of 12 positive genes and three negative genes in wild-type mice. The response to T(3) treatment was enhanced in D3-deficient mice, both in the number of genes and in the amplitude of the response, demonstrating the role of D3 in modulating T(3) action. Comparison of the effects on gene expression observed in D3 deficiency with those in hypothyroidism, hyperthyroidism, and type 2 deiodinase (D2) deficiency revealed that the negative genes are more sensitive to D2 and D3 deficiencies than the positive genes. This observation indicates that, in normal physiological conditions, D2 and D3 play critical roles in maintaining local T(3) concentrations within a very narrow range. It also suggests that negatively and positively regulated genes do not have the same physiological significance or that their regulation by thyroid hormone obeys different paradigms at the molecular or cellular levels.

Revisiting thyroid hormones in schizophrenia

Thyroid hormones are crucial during development and in the adult brain. Of interest, fluctuations in the levels of thyroid hormones at various times during development and throughout life can impact on psychiatric disease manifestation and response to treatment. Here we review research on thyroid function assessment in schizophrenia, relating interrelations between the pituitary-thyroid axis and major neurosignaling systems involved in schizophrenia's pathophysiology. These include the serotonergic, dopaminergic, glutamatergic, and GABAergic networks, as well as myelination and inflammatory processes. The available evidence supports that thyroid hormones deregulation is a common feature in schizophrenia and that the implications of thyroid hormones homeostasis in the fine-tuning of crucial brain networks warrants further research.

Neuroendocrine control of ovulation

Modern methods of diagnosis have made the distinction between hypothalamic failure and ovarian failure routine. Failure of the orderly progression of hypothalamic gonadotrophin-releasing hormone (GnRH) → pituitary gonadotrophins → ovarian steroids and inhibin → hypothalamus/pituitary results in anovulation/amenorrhea. The hypothalamic connections that regulate the pattern and amplitude of GnRH pulses are plastic and respond to external/psychological conditions and internal/metabolic factors that may affect the hypothalamic substrate on which estrogen levels can act. We trace the neuroendocrine regulation of the ovarian cycle, concentrating on hypothalamic connections that underlie negative and positive feedback control of GnRH and the complementary role of the adenohypophysis. The main hormone regulating this "central axis" and the development of the endometrium is estradiol which is exported from the developing ovarian follicles and thereby closes the feedback loop with follicle development. Progesterone and inhibin are also involved. Neuroendocrine responses to internal and external factors can cause anovulation and amenorrhea. Generally, these are accompanied by abnormal negative feedback between estradiol and the gonadotrophins; coexistence of low estradiol and luteinizing hormone/follicle-stimulating hormone. There are three main causes: (1) genetic diseases that interfere with the migration of GnRH cells into the brain or result in misfolding of GnRH; (2) input from the brain that interrupts normal feedback (e.g. stress and weight loss amenorrhea); and (3) the effect of agents which alter central neurotransmission and hypothalamic function (e.g. elevated prolactin and psychotropic medications). All types of hypothalamic insufficiency result in insufficient stimulation of the ovaries. In addition to amenorrhea, this central alteration also results in other complications (downstream disease) that make hypothalamic amenorrhea of greater consequence than simply reproductive failure. Thus, there may be more at stake in the diagnosis and treatment of hypothalamic failure than brings the patient to her caregiver.

Expression profile and thyroid hormone responsiveness of transporters and deiodinases in early embryonic chicken brain development

We used the chick embryo to study the mechanisms regulating intracellular TH availability in developing brain. TH-transporters OATP1C1 and MCT8, and deiodinases D1, D2, and D3 were expressed in a region-specific way, well before the onset of endogenous TH secretion. Between day 4 and 10 of development MCT8 and D2 mRNA levels increased, while OATP1C1 and D3 mRNA levels decreased. D2 and D3 mRNAs were translated into active protein, while no D1 activity was detectable. Injection of THs into the yolk 24h before sampling increased TH levels in the brain and resulted in decreased OATP1C1 and increased MCT8 expression in 4-day-old embryos. A compensatory response in deiodinase activity was only observed at day 8. We conclude that THs are active in the early embryonic brain and TH-transporters and deiodinases can regulate their availability. However, the absence of clear compensatory mechanisms at day 4 makes the brain more vulnerable for changes in maternal TH supply.

Formation of excess sublobules in the cerebellum of hypothyroid rats

Cerebellar folia may increase in number in hypothyroid rats (Lauder et al., 1974; Hasebe et al., 2008a). In this study, we aimed to confirm the formation of an excess sublobule and to determine whether excess sublobules are consistently formed in conserved positions in hypothyroid rats. Instead of the foliation pattern partitioned by cerebellar fissures, we employed the bifurcation pattern of the internal granular layer for investigation of complexity of the cerebellar cortex in hypothyroid rats. The basic foliation pattern of the cerebellum was intact in hypothyroid rats, but lobules III to IX frequently showed an increase in the number of sublobules. The excess sublobules were mainly found in the folia and along the shallow region of the fissure. In other words, the excess sublobules were not located in random locations but rather in specific locations. The area in the internal granular layer of lobules V to IX was significantly larger than that in control rats. From the increased area of the internal granular layer it may be inferred that internal granular cells increase in number than those in normal rats. In our study, regions within the cerebellum that show an excess of sublobules correlate with regions that show an intermediate to late-forming internal granular layer (Altman, 1969). Our observations fit with the view that excess sublobules are formed by the external granular layer showing prolonged cell proliferation and hypothyroidism predominantly has an adverse impact on the intermediate to late phases in development of the internal granular layer.

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

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

Endocrine disrupters: a review of some sources, effects, and mechanisms of actions on behaviour and neuroendocrine systems

Some environmental contaminants interact with hormones and may exert adverse consequences as a result of their actions as endocrine disrupting chemicals (EDCs). Exposure in people is typically a result of contamination of the food chain, inhalation of contaminated house dust or occupational exposure. EDCs include pesticides and herbicides (such as dichlorodiphenyl trichloroethane or its metabolites), methoxychlor, biocides, heat stabilisers and chemical catalysts (such as tributyltin), plastic contaminants (e.g. bisphenol A), pharmaceuticals (i.e. diethylstilbestrol; 17α-ethinylestradiol) or dietary components (such as phytoestrogens). The goal of this review is to address the sources, effects and actions of EDCs, with an emphasis on topics discussed at the International Congress on Steroids and the Nervous System. EDCs may alter reproductively-relevant or nonreproductive, sexually-dimorphic behaviours. In addition, EDCs may have significant effects on neurodevelopmental processes, influencing the morphology of sexually-dimorphic cerebral circuits. Exposure to EDCs is more dangerous if it occurs during specific 'critical periods' of life, such as intrauterine, perinatal, juvenile or puberty periods, when organisms are more sensitive to hormonal disruption, compared to other periods. However, exposure to EDCs in adulthood can also alter physiology. Several EDCs are xenoestrogens, which can alter serum lipid concentrations or metabolism enzymes that are necessary for converting cholesterol to steroid hormones. This can ultimately alter the production of oestradiol and/or other steroids. Finally, many EDCs may have actions via (or independent of) classic actions at cognate steroid receptors. EDCs may have effects through numerous other substrates, such as the aryl hydrocarbon receptor, the peroxisome proliferator-activated receptor and the retinoid X receptor, signal transduction pathways, calcium influx and/or neurotransmitter receptors. Thus, EDCs, from varied sources, may have organisational effects during development and/or activational effects in adulthood that influence sexually-dimorphic, reproductively-relevant processes or other functions, by mimicking, antagonising or altering steroidal actions.

Hypothyroid states mitigate the diabetes-induced reduction of calbindin D-28k, calretinin, and parvalbumin immunoreactivity in type 2 diabetic rats

In this study, we investigated the differences in calbindin D-28k (CB), calretinin, (CR) and parvalbumin (PV) immunoreactivity in the hippocampus of Zucker diabetic fatty (ZDF) rats and Zucker lean control (ZLC) rats. In addition, we observed the effects of hypothyroidism on the levels of immunoreactivity of these proteins in ZDF rats. For this study, 7-week-old ZDF rats were used, and methimazole treatment was continued for 5 weeks to induce hypothyroidism. The animals were sacrificed at 12 weeks of age. ZDF rats showed increased blood glucose levels compared to those in ZLC rats. Methimazole intervention significantly reduced total and free T3 levels, and it ameliorated the increase of blood glucose levels in ZDF rats. In ZLC rats, CB, CR, and PV immunoreactivity was detected in regions of the hippocampus proper. In vehicle-treated ZDF rats, CB, CR, and PV immunoreactivity was significantly decreased in the hippocampus. However, in the methimazole-treated rats, CB, CR, and PV immunoreactivity was significantly increased compared to that in the vehicle-treated rats. These results suggest that hypothyroidism ameliorated the diabetes-induced reduction of CB, CR, and PV immunoreactivity in the hippocampus.

Detection and treatment of congenital hypothyroidism

Congenital hypothyroidism is the most frequent endocrine disorder in neonates. Controversy exists regarding the necessity to adjust current screening programs to also diagnose patients with central hypothyroidism or those with mild forms of congenital hypothyroidism, who have high TSH levels but normal T(4) and normal T(3) levels (also known as 'subclinical hypothyroidism'). Thyroid hormone replacement should start as soon as the diagnosis is confirmed by measurement of elevated TSH and low serum thyroid hormone levels. Further diagnostic approaches, such as ultrasonography, scintigraphy and measurement of thyroglobulin levels, to determine the subtype of congenital hypothyroidism, should not delay initiation of treatment. Recommendations regarding the initial dosage of levothyroxine vary considerably, and no general accepted guideline exists with regards to initial dosage or optimal time point for dose adjustment according to biochemical parameters. More than 30 years after the introduction of the first neonatal screening programs, mental retardation can be prevented in the majority of children (>90%) with congenital hypothyroidism if therapy is commenced within the first 2 weeks of life, making neonate screening for this disorder the most successful population-based screening test in pediatrics.

A modifier gene alleviates hypothyroidism-induced hearing impairment in Pou1f1dw dwarf mice

Thyroid hormone has pleiotropic effects on cochlear development, and genomic variation influences the severity of associated hearing deficits. DW/J-Pou1f1dw/dw mutant mice lack pituitary thyrotropin, which causes severe thyroid hormone deficiency and profound hearing impairment. To assess the genetic complexity of protective effects on hypothyroidism-induced hearing impairment, an F1 intercross was generated between DW/J-Pou1f1dw/+ carriers and an inbred strain with excellent hearing derived from Mus castaneus, CAST/EiJ. Approximately 24% of the (DW/J×CAST/EiJ) Pou1f1dw/dw F2 progeny had normal hearing. A genome scan revealed a locus on chromosome 2, named modifier of dw hearing, or Mdwh, that rescues hearing despite persistent hypothyroidism. This chromosomal region contains the modifier of tubby hearing 1 (Moth1) locus that encodes a protective allele of the microtubule-associated protein MTAP1A. DW/J-Pou1f1dw/+ carriers were crossed with the AKR strain, which also carries a protective allele of Mtap1a, and we found that AKR is not protective for hearing in the (DW/J×AKR) Pou1f1dw/dw F2 progeny. Thus, protective alleles of Mtap1a are not sufficient to rescue DW/J-Pou1f1dw/dw hearing. We expect that identification of protective modifiers will enhance our understanding of the mechanisms of hypothyroidism-induced hearing impairment.

The role of iodine in human growth and development

Iodine is an essential component of the hormones produced by the thyroid gland. Thyroid hormones, and therefore iodine, are essential for mammalian life. Iodine deficiency is a major public health problem; globally, it is estimated that two billion individuals have an insufficient iodine intake. Although goiter is the most visible sequelae of iodine deficiency, the major impact of hypothyroidism due to iodine deficiency is impaired neurodevelopment, particularly early in life. In the fetal brain, inadequate thyroid hormone impairs myelination, cell migration, differentiation and maturation. Moderate-to-severe iodine deficiency during pregnancy increases rates of spontaneous abortion, reduces birth weight, and increases infant mortality. Offspring of deficient mothers are at high risk for cognitive disability, with cretinism being the most severe manifestation. It remains unclear if development of the offspring is affected by mild maternal iodine deficiency. Moderate-to-severe iodine deficiency during childhood reduces somatic growth. Correction of mild-to-moderate iodine deficiency in primary school aged children improves cognitive and motor function. Iodine prophylaxis of deficient populations with periodic monitoring is an extremely cost effective approach to reduce the substantial adverse effects of iodine deficiency throughout the life cycle.

Thyroid hormone suppression of β-amyloid precursor protein gene expression in the brain involves multiple epigenetic regulatory events

Thyroid hormone (T3) suppresses cerebral gene expression of the β-amyloid precursor protein (APP), an integral membrane protein that plays a key role in the onset and progression of Alzheimer's disease. However, the mechanisms by which T3 signaling pathways inhibit APP gene transcription in the brain remain unclear. By carrying out chromatin immunoprecipitation with neuroblastoma cells and primary rat brain tissue, we show for the first time that thyroid hormone receptors (TRs) directly bind at the APP gene in vivo at a promoter region containing a negative T3-response element. We further show that T3 treatment decreases both histone H3 acetylation and histone H3 lysine 4 methylation at the APP promoter and that chemical inhibitors of histone deacetylases and histone lysine demethylase abrogate T3-dependent APP silencing. Our findings thus suggest that TRs actively facilitate T3-dependent silencing of APP gene expression via the recruitment of distinct histone modifying enzymes associated with transcriptional repression.

Thyroid hormone deficiency disrupts rat eye neurodevelopment

Clinical and experimental studies have highlighted the role played by thyroid hormones (TH) in neural and neuro-sensorial development. However, knowledge on TH mechanisms on the developing visual system is still incomplete. To uncover TH actions on the eyes and vision we carried out a microscopical study on the role of TH in the developing retina and optic nerve, in a rat model of controlled TH deficiency (THD). Morphometric and stereological analyses of the retina and optic nerve showed a reduction in the volume of the eye (p<0.001) and optic nerve cross-sectional area (p<0.001), and thinning of the retinal layers (p<0.001). Glial development and myelination was significantly delayed in the THD optic nerves (p<0.001), as compared to controls. The data indicate that TH play an essential role in neuro-retinogenesis. Substitutive TH therapy in critical periods, should be considered in hypothyroidism-related eye disorders as well as neurodegenerative retinal processes.

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.

Mammalian forebrain ketimine reductase identified as μ-crystallin; potential regulation by thyroid hormones

Ketimine reductase (E.C. 1.5.1.25) was purified to apparent homogeneity from lamb forebrain by means of a rapid multi-step chromatography protocol. The purified enzyme was identified by MS/MS (mass spectrometry) as μ-crystallin. The identity was confirmed by heterologously expressing human μ-crystallin in Escherichia coli and subsequent chromatographic purification of the protein. The purified human μ-crystallin was confirmed to have ketimine reductase activity with a maximum specific activity similar to that of native ovine ketimine reductase, and was found to catalyse a sequential reaction. The enzyme substrates are putative neuromodulator/transmitters. The thyroid hormone 3,5,3'-l-triiodothyronine (T3) was found to be a strong reversible competitive inhibitor, and may have a novel role in regulating their concentrations. μ-Crystallin is also involved in intracellular T3 storage and transport. This research is the first to demonstrate an enzyme function for μ-crystallin. This newly demonstrated enzymatic activity identifies a new role for thyroid hormones in regulating mammalian amino acid metabolism, and a possible reciprocal role of enzyme activity regulating bioavailability of intracellular T3.

Maternal thyroid hormones are transcriptionally active during embryo-foetal development: results from a novel transgenic mouse model

Even though several studies highlighted the role of maternal thyroid hormones (THs) during embryo-foetal development, direct evidence of their interaction with embryonic thyroid receptors (TRs) is still lacking. We generated a transgenic mouse model ubiquitously expressing a reporter gene tracing TH action during development. We engineered a construct (TRE2×) containing two TH-responsive elements controlling the expression of the LacZ reporter gene, which encodes β-galactosidase (β-gal). The specificity of the TRE2× activation by TH was evaluated in NIH3T3 cells by cotransfecting TRE2× along with TRs, retinoic or oestrogen receptors in the presence of their specific ligands. TRE2× transgene was microinjected into the zygotes, implanted in pseudopregnant BDF1 (a first-generation (F1) hybrid from a cross of C57BL/6 female and a DBA/2 male) mice and transgenic mouse models were developed. β-gal expression was assayed in tissue sections of transgenic mouse embryos at different stages of development. In vitro, TRE2× transactivation was observed only following physiological T3 stimulation, mediated exclusively by TRs. In vivo, β-gal staining, absent until embryonic day 9.5-10.5 (E9.5-E10.5), was observed as early as E11.5-E12.5 in different primordia (i.e. central nervous system, sense organs, intestine, etc.) of the TRE2× transgenic embryos, while the foetal thyroid function (FTF) was still inactive. Immunohistochemistry for TRs essentially colocalized with β-gal staining. No β-gal staining was detected in embryos of hypothyroid transgenic mice. Importantly, treatment with T3 in hypothyroid TRE2× transgenic mice rescued β-gal expression. Our results provide in vivo direct evidence that during embryonic life and before the onset of FTF, maternal THs are transcriptionally active through the action of embryonic TRs. This model may have clinical relevance and may be employed to design end-point assays for new molecules affecting THs action.

Endocrine disruptors and childhood social impairment

Prenatal exposure to endocrine disruptors has the potential to impact early brain development. Neurodevelopmental toxicity in utero may manifest as psychosocial deficits later in childhood. This study investigates prenatal exposure to two ubiquitous endocrine disruptors, the phthalate esters and bisphenol A (BPA), and social behavior in a sample of adolescent inner-city children. Third trimester urines of women enrolled in the Mount Sinai Children's Environmental Health Study between 1998 and 2002 (n=404) were analyzed for phthalate metabolites and BPA. Mother-child pairs were asked to return for a follow-up assessment when the child was between the ages of 7 and 9 years. At this visit, mothers completed the Social Responsiveness Scale (SRS) (n=137), a quantitative scale for measuring the severity of social impairment related to Autistic Spectrum Disorders (ASD) in the general population. In adjusted general linear models increasing log-transformed low molecular weight (LMW) phthalate metabolite concentrations were associated with greater social deficits (β=1.53, 95% CI 0.25-2.8). Among the subscales, LMWP were also associated with poorer Social Cognition (β=1.40, 95% CI 0.1-2.7); Social Communication (β=1.86, 95% CI 0.5-3.2); and Social Awareness (β=1.25, 95% CI 0.1-2.4), but not for Autistic Mannerisms or Social Motivation. No significant association with BPA was found (β=1.18, 95% CI -0.75, 3.11). Prenatal phthalate exposure was associated with childhood social impairment in a multiethnic urban population. Even mild degrees of impaired social functioning in otherwise healthy individuals can have very important adverse effects over a child's lifetime. These results extend our previous finding of atypical neonatal and early childhood behaviors in relation to prenatal phthalate exposure.

Type 3 deiodinase deficiency causes spatial and temporal alterations in brain T3 signaling that are dissociated from serum thyroid hormone levels

The type 3 deiodinase (D3) is an enzyme that inactivates thyroid hormones (TH) and is highly expressed during development and in the central nervous system. D3-deficient (D3KO) mice develop markedly elevated serum T(3) level in the perinatal period. In adulthood, circulating T(4) and T(3) levels are reduced due to functional deficits in the thyroid axis and peripheral tissues (i.e. liver) show evidence of decreased TH action. Given the importance of TH for brain development, we aimed to assess TH action in the brain of D3KO mice at different developmental stages and determine to what extent it correlates with serum TH parameters. We used a transgenic mouse model (FINDT3) that expresses the reporter gene β-galactosidase (β-gal) in the central nervous system as a readout of local TH availability. Together with experiments determining expression levels of TH-regulated genes, our results show that after a state of thyrotoxicosis in early development, most regions of the D3KO brain show evidence of decreased TH action at weaning age. However, later in adulthood and in old age, the brain again manifests a thyrotoxic state, despite reduced serum TH levels. These region-specific changes in brain TH status during the life span of the animal provide novel insight into the important role of the D3 in the developing and adult brain. Our results suggest that, even if serum concentrations of TH are normal or low, impaired D3 activity may result in excessive TH action in multiple brain regions, with potential consequences of altered neural function that may be of clinical relevance to neurological and neuroendocrine disorders.

Analysis of thyroid response element activity during retinal development

Thyroid hormone (TH) signaling components are expressed during retinal development in dynamic spatial and temporal patterns. To probe the competence of retinal cells to mount a transcriptional response to TH, reporters that included thyroid response elements (TREs) were introduced into developing retinal tissue. The TREs were placed upstream of a minimal TATA-box and two reporter genes, green fluorescent protein (GFP) and human placental alkaline phosphatase (PLAP). Six of the seven tested TREs were first tested in vitro where they were shown to drive TH-dependent expression. However, when introduced into the developing retina, the TREs reported in different cell types in both a TH-dependent and TH-independent manner, as well as revealed specific spatial patterns in their expression. The role of the known thyroid receptors (TR), TRα and TRβ, was probed using shRNAs, which were co-electroporated into the retina with the TREs. Some TREs were positively activated by TR+TH in the developing outer nuclear layer (ONL), where photoreceptors reside, as well as in the outer neuroblastic layer (ONBL) where cycling progenitor cells are located. Other TREs were actively repressed by TR+TH in cells of the ONBL. These data demonstrate that non-TRs can activate some TREs in a spatially regulated manner, whereas other TREs respond only to the known TRs, which also read out activity in a spatially regulated manner. The transcriptional response to even simple TREs provides a starting point for understanding the regulation of genes by TH, and highlights the complexity of transcriptional regulation within developing tissue.

Thyroid hormones regulate selenoprotein expression and selenium status in mice

Impaired expression of selenium-containing proteins leads to perturbed thyroid hormone (TH) levels, indicating the central importance of selenium for TH homeostasis. Moreover, critically ill patients with declining serum selenium develop a syndrome of low circulating TH and a central downregulation of the hypothalamus-pituitary-thyroid axis. This prompted us to test the reciprocal effect, i.e., if TH status would also regulate selenoprotein expression and selenium levels. To investigate the TH dependency of selenium metabolism, we analyzed mice expressing a mutant TH receptor α1 (TRα1+m) that confers a receptor-mediated hypothyroidism. Serum selenium was reduced in these animals, which was a direct consequence of the mutant TRα1 and not related to their metabolic alterations. Accordingly, hyperthyroidism, genetically caused by the inactivation of TRβ or by oral TH treatment of adult mice, increased serum selenium levels in TRα1+m and controls, thus demonstrating a novel and specific role for TRα1 in selenium metabolism. Furthermore, TH affected the mRNA levels for several enzymes involved in selenoprotein biosynthesis as well as serum selenoprotein P concentrations and the expression of other antioxidative selenoproteins. Taken together, our results show that TH positively affects the serum selenium status and regulates the expression of several selenoproteins. This demonstrates that selenium and TH metabolism are interconnected through a feed-forward regulation, which can in part explain the rapid parallel downregulation of both systems in critical illness.

Structural and functional alterations in the hippocampus due to hypothyroidism

Thyroid hormones (THs) exert a broad spectrum of effects on the central nervous system (CNS). Hypothyroidism, especially during CNS development, can lead to structural and functional changes (mostly resulting in mental retardation). The hippocampus is considered as one of the most important CNS structures, while the investigation and understanding of its direct and indirect interactions with the THs could provide crucial information on the neurobiological basis of the (frequently-faced in clinical practice) hypothyroidism-induced mental retardation and neurobehavioral dysfunction. THs-deficiency during the fetal and/or the neonatal period produces deleterious effects for neural growth and development (such as reduced synaptic connectivity, delayed myelination, disturbed neuronal migration, deranged axonal projections, decreased synaptogenesis and alterations in neurotransmitters' levels). On the other hand, the adult-onset thyroid dysfunction is usually associated with neurological and behavioural abnormalities. In both cases, genomic and proteomic changes seem to occur. The aim of this review is to provide an up-to-date synopsis of the available knowledge regarding the aforementioned alterations that take place in the hippocampus due to fetal-, neonatal- or adult-onset hypothyroidism.

Perinatal iron and copper deficiencies alter neonatal rat circulating and brain thyroid hormone concentrations

Copper (Cu), iron (Fe), and iodine/thyroid hormone (TH) deficiencies lead to similar defects in late brain development, suggesting that these micronutrient deficiencies share a common mechanism contributing to the observed derangements. Previous studies in rodents (postweanling and adult) and humans (adolescent and adult) indicate that Cu and Fe deficiencies affect the hypothalamic-pituitary-thyroid axis, leading to altered TH status. Importantly, however, relationships between Fe and Cu deficiencies and thyroidal status have not been assessed in the most vulnerable population, the developing fetus/neonate. We hypothesized that Cu and Fe deficiencies reduce circulating and brain TH levels during development, contributing to the defects in brain development associated with these deficiencies. To test this hypothesis, pregnant rat dams were rendered Cu deficient (CuD), FeD, or TH deficient from early gestation through weaning. Serum thyroxine (T(4)) and triiodothyronine (T(3)), and brain T(3) levels, were subsequently measured in postnatal d 12 (P12) pups. Cu deficiency reduced serum total T(3) by 48%, serum total T(4) by 21%, and whole-brain T(3) by 10% at P12. Fe deficiency reduced serum total T(3) by 43%, serum total T(4) by 67%, and whole-brain T(3) by 25% at P12. Brain mRNA analysis revealed that expression of several TH-responsive genes were altered in CuD or FeD neonates, suggesting that reduced TH concentrations were sensed by the FeD and CuD neonatal brain. These results indicate that at least some of the brain defects associated with neonatal Fe and Cu deficiencies are mediated through reductions in circulating and brain TH levels.

Hypothyroxinemia: a subclinical condition affecting neurodevelopment

Hypothyroxinemia with low levels of circulating free thyroxine and normal levels of thyrotropin, which is usually caused by iodine deficiency, may affect pregnant women even in apparently iodine-sufficient areas, and it is debated whether it increases the risk of neurodevelopmental abnormalities in children born to them. Epidemiological observations indeed indicate that this is the case. Animal models show abnormal brain cortical cytoarchitecture in pups born to mildly hypothyroxinemic dams. In regions where the availability and use of iodized salt is inadequate (where <90% of households use iodized salt), the WHO and the International Council for Control of Iodine Deficiency Disorders (ICCIDD) recommend iodine supplementation so that the total iodine intake is 250 µg/day to prevent iodine deficiency during gestation and lactation.

Functional identification of neural stem cell-derived oligodendrocytes by means of calcium transients elicited by thrombin

Current immunosuppressive treatments for central nervous system demyelinating diseases fail to prevent long-term motor and cognitive decline in patients. Excitingly, glial cell transplantation arises as a promising complementary strategy to challenge oligodendrocytes loss occurring in myelination disorders. A potential source of new oligodendrocytes is the subventricular zone (SVZ) pool of multipotent neural stem cells. However, this approach has been handicapped by the lack of functional methods for identification and pharmacological analysis of differentiating oligodendrocytes, prior to transplantation. In this study, we questioned whether SVZ-derived oligodendrocytes could be functionally discriminated due to intracellular calcium level ([Ca(2+)](i)) variations following KCl, histamine, and thrombin stimulations. Previously, we have shown that SVZ-derived neurons and immature cells can be discriminated on the basis of their selective [Ca(2+)](i) rise upon KCl and histamine stimulation, respectively. Herein, we demonstrate that O4+ and proteolipid protein-positive (PLP+) oligodendrocytes do not respond to these stimuli, but display a robust [Ca(2+)](i) rise following thrombin stimulation, whereas other cell types are thrombin-insensitive. Thrombin-induced Ca(2+) increase in oligodendrocytes is mediated by protease-activated receptor-1 (PAR-1) activation and downstream signaling through G(q/11) and phospholipase C (PLC), resulting in Ca(2+) recruitment from intracellular compartments. This method allows the analysis of functional properties of oligodendrocytes in living SVZ cultures, which is of major interest for the development of effective grafting strategies in the demyelinated brain. Additionally, it opens new perspectives for the search of new pro-oligodendrogenic factors to be used prior grafting.

Thyroid hormone-regulated mouse cerebral cortex genes are differentially dependent on the source of the hormone: a study in monocarboxylate transporter-8- and deiodinase-2-deficient mice

Thyroid hormones influence brain development through the control of gene expression. The concentration of the active hormone T(3) in the brain depends on T(3) transport through the blood-brain barrier, mediated in part by the monocarboxylate transporter 8 (Mct8/MCT8) and the activity of type 2 deiodinase (D2) generating T(3) from T(4). The relative roles of each of these pathways in the regulation of brain gene expression is not known. To shed light on this question, we analyzed thyroid hormone-dependent gene expression in the cerebral cortex of mice with inactivated Mct8 (Slc16a2) and Dio2 genes, alone or in combination. We used 34 target genes identified to be controlled by thyroid hormone in microarray comparisons of cerebral cortex from wild-type control and hypothyroid mice on postnatal d 21. Inactivation of the Mct8 gene (Mct8KO) was without effect on the expression of 31 of these genes. Normal gene expression in the absence of the transporter was mostly due to D2 activity because the combined disruption of Mct8 and Dio2 led to similar effects as hypothyroidism on the expression of 24 genes. Dio2 disruption alone did not affect the expression of positively regulated genes, but, as in hypothyroidism, it increased that of negatively regulated genes. We conclude that gene expression in the Mct8KO cerebral cortex is compensated in part by D2-dependent mechanisms. Intriguingly, positive or negative regulation of genes by thyroid hormone is sensitive to the source of T(3) because Dio2 inactivation selectively affects the expression of negatively regulated genes.

Thyroid hormone action during brain development: more questions than answers

Thyroid hormone is essential for proper brain development since it acts on processes such as neuronal migration and differentiation, myelination and synaptogenesis. In this review, we summarize the consequences of thyroid hormone deficiency for brain development with special focus on the cerebellum, an important target of thyroid action. In addition, we discuss the role of iodothyronine deiodinases and thyroid hormone transporters in regulating local thyroid hormone concentrations as well as current knowledge about the function of thyroid hormone receptors and their target genes during brain maturation. Despite considerable progress in recent years in deciphering thyroid hormone signaling pathways we still know very little on the molecular level by which mode of action thyroid hormone exerts its cell-specific effects. Hence, we will particularly address the open questions that remain to be addressed in order to better understand the role of thyroid hormone in brain development.

Stimulatory effects of thyroid hormone on brain angiogenesis in vivo and in vitro

Thyroid hormone is critical for the proper development of the central nervous system. However, the specific role of thyroid hormone on brain angiogenesis remains poorly understood. Treatment of rats from birth to postnatal day 21 (P21) with propylthiouracil (PTU), a reversible blocker of triiodothyronine (T3) synthesis, resulted in decreased brain angiogenesis, as indicated by reduced complexity and density of microvessels. However, when PTU was withdrawn at P22, these parameters were fully recovered by P90. These changes were paralleled by an altered expression of vascular endothelial growth factor A (Vegfa) and basic fibroblast growth factor (Fgf2). Physiologic concentrations of T3 and thyroxine (T4) stimulated proliferation and tubulogenesis of rat brain-derived endothelial (RBE4) cells in vitro. Protein and mRNA levels of VEGF-A and FGF-2 increased after T3 stimulation of RBE4 cells. The thyroid hormone receptor blocker NH-3 abolished T3-induced Fgf2 and Vegfa upregulation, indicating a receptor-mediated effect. Thyroid hormone inhibited the apoptosis in RBE4 cells and altered mRNA levels of apoptosis-related genes, namely Bcl2 and Bad. The present results show that thyroid hormone has a substantial impact on vasculature development in the brain. Pathologically altered vascularization could, therefore, be a contributing factor to the neurologic deficits induced by thyroid hormone deficiency.

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.

Human testicular orphan receptor 4 enhances thyroid hormone receptor signaling

The thyroid hormone receptor (TR) and human testicular orphan receptor 4 (TR4) belong to the nuclear hormone receptor superfamily. They are ligand-dependent transcription factors. TR and TR4 bind to a similar thyroid response element (TRE), known as a direct repeat with four nucleotide spacing (DR4). This study examined the possible interaction or cross-talking between those two receptors. We hypothesized that protein-protein interaction between TR4 and TR may promote TR-mediated transcriptional activity. Glutathione S-transferase pull-down and immunoprecipitation assays showed direct interaction between TR and TR4. Electrophoretic mobility-shift assay demonstrated that TR and TR4 could co-occupy the same TRE. The interaction between TR4 and TR may enhance regulation of genes targeted by TR, such as furin, fibrinogen, cdk2 and p21 expression. We found that TR4 function is similar with TR as TR4 alone could regulate expression of some TR target genes, and could increase cell migration or inhibit cell proliferation. Importantly, the TR-dependent inhibition of cell proliferation and stimulation of cell migration are more enhanced in the HepG2-TR cells stably over-expressing TR4. Overall, TR4 not only has modulation abilities similar to TR but also can cross-talk with TR and promote the TR signaling pathway.

Thyroxin regulates BDNF expression to promote survival of injured neurons

A growing amount of evidence indicates that neuronal trauma can induce a recapitulation of developmental-like mechanisms for neuronal survival and regeneration. Concurrently, ontogenic dependency of central neurons for brain-derived neurotrophic factor (BDNF) is lost during maturation but is re-acquired after injury. Here we show in organotypic hippocampal slices that thyroxin, the thyroid hormone essential for normal CNS development, induces up-regulation of BDNF upon injury. This change in the effect of thyroxin is crucial to promote survival and regeneration of damaged central neurons. In addition, the effect of thyroxin on the expression of the K-Cl cotransporter (KCC2), a marker of neuronal maturation, is changed from down to up-regulation. Notably, previous results in humans have shown that during the first few days after traumatic brain injury or spinal cord injury, thyroid hormone levels are often diminished. Our data suggest that maintaining normal levels of thyroxin during the early post-traumatic phase of CNS injury could have a therapeutically positive effect.

Endocrine disruptors and thyroid hormone physiology

Endocrine disruptors are man-made chemicals that can disrupt the synthesis, circulating levels, and peripheral action of hormones. The disruption of sex hormones was subject of intensive research, but thyroid hormone synthesis and signaling are now also recognized as important targets of endocrine disruptors. The neurological development of mammals is largely dependent on normal thyroid hormone homeostasis, and it is likely to be particularly sensitive to disruption of the thyroid axis. Here, we survey the main thyroid-disrupting chemicals, such as polychlorinated biphenyls, perchlorates, and brominated flame-retardants, that are characteristic disruptors of thyroid hormone homeostasis, and look at their suspected relationships to impaired development of the human central nervous system. The review then focuses on disrupting mechanisms known to be directly or indirectly related to the transcriptional activity of the thyroid hormone receptors.

Thyroid-hormone-dependent activation of the phosphoinositide 3-kinase/Akt cascade requires Src and enhances neuronal survival

We have reported previously a non-genomic action of T3 (3,3',5-tri-iodothyronine), which stimulates the PI3K (phosphoinositide 3-kinase)/Akt pathway via p85alpha, the regulatory subunit of PI3K, in human skin fibroblasts. The aim of the present study was to elucidate the mechanism by which T3 activates PI3K, and to investigate the physiological role of this T3 action in neuronal cells. We found that T3 activates PI3K/Akt through Src. First, T3 rapidly induced the activation of Src and Akt in N2a cells expressing TRalpha1 (thyroid hormone receptor alpha1; N2aTRalpha), and both were attenuated by either the addition of a Src inhibitor or Src siRNA. In contrast, a PI3K inhibitor could only block the activation of Akt. Secondly, T3 enhanced TRalpha1-p85alpha-Src complex formation, which was also abrogated by a Src inhibitor. The activation of Src and PI3K/Akt contributes to the anti-apoptotic effect of T3 in N2aTRalpha cells. Moreover, it was also observed in primary cerebral cortical neurons that T3 induced the activation of PI3K/Akt and suppressed serum-deprivation-induced apoptosis. Together, the findings of the present study demonstrate a novel non-genomic action of T3 on neuronal cell survival, and provide new insights into the mechanism underlying this action, which involves Src activation and TRalpha1-p85alpha-Src complex formation.

Thyroid hormone induces glial lineage of primary neurospheres derived from non-pathological and pathological rat brain: implications for remyelination-enhancing therapies

Thyroid hormone exerts a critical role in developmental myelination, acting on the production and maturation of oligodendrocyte, and on the expression of genes encoding for myelin protein. Since remyelination is considered a recapitulation of cellular and molecular events occurring during development, we tested the possibility of stimulating the oligodendroglial lineage and maturation in neurospheres derived from the subventricular zone of adult rats using 3,5,3'-L-triiodothyronine (T3). Both non-pathological and pathological brains derived from rats affected by the inflammatory-demyelinating disease experimental allergic encephalomyelitis (EAE) were included in the study. We investigated the effect of in vitro T3 exposure on: (i) the expression of nuclear thyroid hormone receptors; (ii) proliferation rate; (iii) differentiation into neurons, astrocytes and oligodendrocytes, focusing our attention on oligodendrocyte maturation. T3 reduced the proliferation rate of neurospheres when cultured in the presence of mitogens, shifting towards oligodendroglial lineage as indicated by increased expression of olig-1, and also favoring oligodendrocyte maturation, as indicated by the expression of antigens associated with different maturation stages. Neurospheres derived from EAE rats show a strong limitation in oligodendrocyte generation, which is completely restored by T3 treatment. These results indicate that T3 is a key factor in regulating neurosphere biology, when derived either from non-pathological or pathological adult brains, suggesting that T3 might be an important factor in favoring remyelination in demyelinating disorders.

Neuronal 3',3,5-triiodothyronine (T3) uptake and behavioral phenotype of mice deficient in Mct8, the neuronal T3 transporter mutated in Allan-Herndon-Dudley syndrome

Thyroid hormone transport into cells requires plasma membrane transport proteins. Mutations in one of these, monocarboxylate transporter 8 (MCT8), have been identified as underlying cause for the Allan-Herndon-Dudley syndrome, an X-linked mental retardation in which the patients also present with abnormally high 3',3,5-triiodothyronine (T(3)) plasma levels. Mice deficient in Mct8 replicate the thyroid hormone abnormalities observed in the human condition. However, no neurological deficits have been described in mice lacking Mct8. Therefore, we subjected Mct8-deficient mice to a comprehensive immunohistochemical, neurological, and behavioral screen. Several behavioral abnormalities were found in the mutants. Interestingly, some of these behavioral changes are compatible with hypothyroidism, whereas others rather indicate hyperthyroidism. We thus hypothesized that neurons exclusively dependent on Mct8 are in a hypothyroid state, whereas neurons expressing other T(3) transporters become hyperthyroid, if they are exposed directly to the high plasma T(3). The majority of T(3) uptake in primary cortical neurons is mediated by Mct8, but pharmacological inhibition suggested functional expression of additional T(3) transporter classes. mRNAs encoding six T(3) transporters, including L-type amino acid transporters (LATs), were coexpressed with Mct8 in isolated neurons. We then demonstrated Lat2 expression in cultured neurons and throughout murine brain development. In contrast, LAT2 is expressed in microglia in the developing human brain during gestation, but not in neurons. We suggest that lack of functional complementation by alternative thyroid hormone transporters in developing human neurons precipitates the devastating neurodevelopmental phenotype in MCT8-deficient patients, whereas Mct8-deficient mouse neurons are functionally complemented by other transporters, for possibly Lat2.

Thyroid hormone receptor alpha plays an essential role in the normalisation of adult-onset hypothyroidism-related hypoexpression of synaptic plasticity target genes in striatum

Thyroid hormone (TH) deficiency leads to molecular changes resulting in behavioural deficits. TH action is mediated by two types of nuclear receptors (TRs), TRalpha and TRbeta, which control target gene transcription. The relative contributions of the two TR products in mediating adult TH responses are poorly understood. As TRalpha1 transcripts are widely distributed in the brain, they presumably mediate most of the TH effects. This report examines the role and specific functions of T3 receptor isoforms on regulation of striatal synaptic plasticity indicators using adult hypothyroid mutant mice that fail to express single or multiple TR gene products. We then evaluated the effect of this hypothyroidism, with or without subsequent administration of T3, on T3 nuclear receptor (TRalpha1, TRbeta) and synaptic plasticity gene expression in TRalpha(0/0), TRbeta(-/-) and wild-type 129/SV mice. Hypothyroid wild-type mice exhibited reduced TRbeta, RC3, CaMKII and Rhes expression. The mRNA levels of Rhes and CaMKII were the same in all three hypothyroid substrains. By contrast, hypothyroid TRbeta(-/-) mice had higher RC3 mRNA levels than wild-type. T3 administration restored TRbeta, RC3 and CaMKII levels in hypothyroid wild-type mice, without significant Rhes upregulation. T3 administration normalised expression of all genes studied in hypothyroid TRbeta(-/-) but not TRalpha(0/0) mice. Thus, TRalpha apparently plays an essential role in restoring the expression of the TH-regulated genes potentially involved in striatal synaptic plasticity.