Cell-specific effects of thyroid hormone on RC3/neurogranin expression in rat brain

Neurogranin-like immunoreactivity in the zebrafish brain during development

Neurogranin (Nrgn) is a neural protein that is enriched in the cerebral cortex and is involved in synaptic plasticity via its interaction with calmodulin. Recently we reported its expression in the brain of the adult zebrafish (Alba-González et al. J Comp Neurol 530:1569–1587, 2022). In this study we analyze the development of Nrgn-like immunoreactivity (Nrgn-like-ir) in the brain and sensory structures of zebrafish embryos and larvae, using whole mounts and sections. First Nrgn-like positive neurons appeared by 2 day post-fertilization (dpf) in restricted areas of the brain, mostly in the pallium, epiphysis and hindbrain. Nrgn-like populations increased noticeably by 3 dpf, reaching an adult-like pattern in 6 dpf. Most Nrgn-like positive neurons were observed in the olfactory organ, retina (most ganglion cells, some amacrine and bipolar cells), pallium, lateral hypothalamus, thalamus, optic tectum, torus semicircularis, octavolateralis area, and viscerosensory column. Immunoreactivity was also observed in axonal tracts originating in Nrgn-like neuronal populations, namely, the projection of Nrgn-like immunopositive primary olfactory fibers to olfactory glomeruli, that of Nrgn-like positive pallial cells to the hypothalamus, the Nrgn-like-ir optic nerve to the pretectum and optic tectum, the Nrgn-like immunolabeled lateral hypothalamus to the contralateral region via the horizontal commissure, the octavolateralis area to the midbrain via the lateral lemniscus, and the viscerosensory column to the dorsal isthmus via the secondary gustatory tract. The late expression of Nrgn in zebrafish neurons is probably related to functional maturation of higher brain centers, as reported in the mammalian telencephalon. The analysis of Nrgn expression in the zebrafish brain suggests that it may be a useful marker for specific neuronal circuitries.

All-trans Retinoic Acid has Limited Therapeutic Effects on Cognition and Hippocampal Protein Expression After Controlled Cortical Impact

Traumatic brain injury (TBI) is known to impair synaptic function, and subsequently contribute to observed cognitive deficits. Retinoic Acid (RA) signaling modulates expression of synaptic plasticity proteins and is involved in hippocampal learning and memory. All trans-retinoic acid (ATRA), a metabolite of Vitamin A, has been identified as a potential pharmacotherapeutic for other neurological disorders due to this role. This study conducted an ATRA dose response to determine its therapeutic effects on cognitive behaviors and expression of hippocampal markers of synaptic plasticity and RA signaling proteins after experimental TBI. Under isoflurane anesthesia, adult male Sprague Dawley rats received either controlled cortical impact (CCI, 2.5 mm deformation, 4 m/s) or control surgery. Animals received daily intraperitoneal injection of 0.5, 1, 5, or 10 mg/kg of ATRA or vehicle for 2 weeks. Animals underwent motor and spatial learning and memory testing. Hippocampal expression of synaptic plasticity proteins neurogranin (Ng), and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor GluA1 sub-unit, as well as RA signaling proteins STRA6, ADLH1a1, CYP26A1 and CYP26B1 were evaluated by western blot at 2-weeks post-injury. ATRA treatment significantly recovered Ng synaptic protein expression, while having no effect on motor performance, spatial learning, and memory, and GluA1 expression after TBI. RA signaling protein expression is unchanged 2 weeks after TBI. Overall, ATRA administration after TBI showed limited therapeutic benefits compared to the vehicle.

Hypothyroidism and Diabetes-Related Dementia: Focused on Neuronal Dysfunction, Insulin Resistance, and Dyslipidemia

The incidence of dementia is steadily increasing worldwide. The risk factors for dementia are diverse, and include genetic background, environmental factors, sex differences, and vascular abnormalities. Among the subtypes of dementia, diabetes-related dementia is emerging as a complex type of dementia related to metabolic imbalance, due to the increase in the number of patients with metabolic syndrome and dementia worldwide. Thyroid hormones are considered metabolic regulatory hormones and affect various diseases, such as liver failure, obesity, and dementia. Thyroid dysregulation affects various cellular mechanisms and is linked to multiple disease pathologies. In particular, hypothyroidism is considered a critical cause for various neurological problems-such as metabolic disease, depressive symptoms, and dementia-in the central nervous system. Recent studies have demonstrated the relationship between hypothyroidism and brain insulin resistance and dyslipidemia, leading to diabetes-related dementia. Therefore, we reviewed the relationship between hypothyroidism and diabetes-related dementia, with a focus on major features of diabetes-related dementia such as insulin resistance, neuronal dysfunction, and dyslipidemia.

Perinatal exposure to the thyroperoxidase inhibitors methimazole and amitrole perturbs thyroid hormone system signaling and alters motor activity in rat offspring

Disruption of the thyroid hormone system during development can impair brain development and cause irreversible damage. Some thyroid hormone system disruptors act by inhibiting the thyroperoxidase (TPO) enzyme, which is key to thyroid hormone synthesis. For the potent TPO-inhibiting drug propylthiouracil (PTU) this has been shown to result in thyroid hormone system disruption and altered brain development in animal studies. However, an outstanding question is which chemicals beside PTU can cause similar effects on brain development and to what degree thyroid hormone insufficiency must be induced to be able to measure adverse effects in rats and their offspring. To start answering these questions, we performed a perinatal exposure study in pregnant rats with two TPO-inhibitors: the drug methimazole (MMI) and the triazole herbicide amitrole. The study involved maternal exposure from gestational day 7 through to postnatal day 22, to MMI (8 and 16 mg/kg body weight/day) or amitrole (25 and 50 mg/kg body weight/day). Both MMI and amitrole reduced serum T4 concentrations in a dose-dependent manner in dams and offspring, with a strong activation of the hypothalamic-pituitary-thyroid axis. This reduction in serum T4 led to decreased thyroid hormone-mediated gene expression in the offspring's brains and caused adverse effects on brain function, seen as hyperactivity and decreased habituation in preweaning pups. These dose-dependent effects induced by MMI and amitrole are largely the same as those observed with PTU. This demonstrates that potent TPO-inhibitors can induce effects on brain development in rats and that these effects are driven by T4 deficiency. This knowledge will aid the identification of TPO-inhibiting thyroid hormone system disruptors in a regulatory context and can serve as a starting point in search of more sensitive markers of developmental thyroid hormone system disruption.

Endocrine disrupting chemicals and thyroid hormone action

Thyroid hormones (predominantly thyroxine, T4, and triiodothyronine, T3) are essential for normal development and for adult physiology. There are several challenges, however, that make identifying chemicals that produce adverse effects by interfering with the thyroid system difficult. First, individual variability in serum concentrations of thyroid hormones represent only about 10% of the population reference range that is considered to be "normal." This means that populations studies evaluating the relationship between chemical exposure and serum thyroid hormones must be large enough to overcome this internal variance. In addition, we know that there are chemicals that do not produce changes in thyroid hormone levels, but nevertheless impact thyroid signaling in target tissues. A good example is that of polychlorinated biphenyls (PCBs). PCB exposure during development are clearly associated with cognitive deficits in humans. But PCB exposure isn't uniformly associated with a reduction in serum thyroid hormone in human populations despite mechanistic studies showing that PCBs reduce serum T4 in animals. In contrast, perchlorate is a chemical that inhibits iodide uptake, thereby reducing thyroid hormone synthesis and serum hormone levels. Human studies have been variable in identifying a relationship between thyroid hormone and perchlorate exposure, but studies also show that dietary iodine, cigarette smoking and other factors can modify this relationship. The conclusion is that identifying chemicals that interfere with thyroid hormone could depend on in vitro analysis of chemicals that interact with different proteins important for thyroid hormone to function properly.

Data integration, analysis, and interpretation of eight academic CLARITY-BPA studies

"Consortium Linking Academic and Regulatory Insights on BPA Toxicity" (CLARITY-BPA) was a comprehensive "industry-standard" Good Laboratory Practice (GLP)-compliant 2-year chronic exposure study of bisphenol A (BPA) toxicity that was supplemented by hypothesis-driven independent investigator-initiated studies. The investigator-initiated studies were focused on integrating disease-associated, molecular, and physiological endpoints previously found by academic scientists into an industry standard guideline-compliant toxicity study. Thus, the goal of this collaboration was to provide a more comprehensive dataset upon which to base safety standards and to determine whether industry-standard tests are as sensitive and predictive as molecular and disease-associated endpoints. The goal of this report is to integrate the findings from the investigator-initiated studies into a comprehensive overview of the observed impacts of BPA across the multiple organs and systems analyzed. For each organ system, we provide the rationale for the study, an overview of methodology, and summarize major findings. We then compare the results of the CLARITY-BPA studies across organ systems with the results of previous peer-reviewed studies from independent labs. Finally, we discuss potential influences that contributed to differences between studies. Developmental exposure to BPA can lead to adverse effects in multiple organs systems, including the brain, prostate gland, urinary tract, ovary, mammary gland, and heart. As published previously, many effects were at the lowest dose tested, 2.5μg/kg /day, and many of the responses were non-monotonic. Because the low dose of BPA affected endpoints in the same animals across organs evaluated in different labs, we conclude that these are biologically - and toxicologically - relevant.

Adult onset of type 3 deiodinase deficiency in mice alters brain gene expression and increases locomotor activity

Constitutive loss of the type 3 deiodinase (DIO3) causes abnormally increased levels of thyroid hormone action in the developing and adult brain, leading to an array of behavioral abnormalities. To determine to what extent those phenotypes derive from a lack of DIO3 in the adult brain, versus developmental consequences, we created a mouse model of conditional DIO3 inactivation. Mice carrying "floxed" Dio3 alleles and a tamoxifen-inducible cre transgene were injected with tamoxifen at two months of age. Compared to oil-injected controls, the brain tissue of these mice showed a 75-80% decrease in DIO3 activity and 85-95% Dio3 mRNA was expressed from recombinant alleles. Mice with adult DIO3 deficiency did not show significant differences in growth, serum thyroid hormone parameters or behaviors related to anxiety and depression. However, female mice exhibited elevated locomotor activity and increased marble-burying behavior. They also manifested relatively modest alterations in the expression of T3-dependent genes and genes related to hyperactivity in a sex- and region-specific manner. Upon thyroid hormone treatment, the expression response of T3-regulated genes was generally more pronounced in DIO3-deficient female mice than in female controls, while the opposite effect of altered genotype was noticed in males. The extent of the molecular and behavioral phenotypes of adult-onset DIO3 deficiency suggests that a substantial proportion of the neurological abnormalities caused by constitutive DIO3 deficiency has a developmental origin. However, our results show that DIO3 in the adult brain also influences behavior and sensitivity to thyroid hormone action in a sexually dimorphic fashion.

Thyroid Hormones in the Brain and Their Impact in Recovery Mechanisms After Stroke

Thyroid hormones are of fundamental importance for brain development and essential factors to warrant brain functions throughout life. Their actions are mediated by binding to specific intracellular and membranous receptors regulating genomic and non-genomic mechanisms in neurons and populations of glial cells, respectively. Among others, mechanisms include the regulation of neuronal plasticity processes, stimulation of angiogenesis and neurogenesis as well modulating the dynamics of cytoskeletal elements and intracellular transport processes. These mechanisms overlap with those that have been identified to enhance recovery of lost neurological functions during the first weeks and months after ischemic stroke. Stimulation of thyroid hormone signaling in the postischemic brain might be a promising therapeutic strategy to foster endogenous mechanisms of repair. Several studies have pointed to a significant association between thyroid hormones and outcome after stroke. With this review, we will provide an overview on functions of thyroid hormones in the healthy brain and summarize their mechanisms of action in the developing and adult brain. Also, we compile the major thyroid-modulated molecular pathways in the pathophysiology of ischemic stroke that can enhance recovery, highlighting thyroid hormones as a potential target for therapeutic intervention.

CLARITY-BPA: Bisphenol A or Propylthiouracil on Thyroid Function and Effects in the Developing Male and Female Rat Brain

The CLARITY-BPA experiment, a large collaboration between the National Institute of Environmental Health Sciences, the National Toxicology Program, and the US Food and Drug Administration, is designed to test the effects of bisphenol A (BPA) on a variety of endocrine systems and end points. The specific aim of this subproject was to test the effect of BPA exposure on thyroid functions and thyroid hormone action in the developing brain. Timed-pregnant National Center for Toxicological Research Sprague-Dawley rats (strain code 23) were dosed by gavage with vehicle control (0.3% carboxymethylcellulose) or one of five doses of BPA [2.5, 25, 250, 2500, or 25,000 µg/kg body weight (bw) per day] or ethinyl estradiol (EE) at 0.05 or 0.50 µg/kg bw/d (n = 8 for each group) beginning on gestational day 6. Beginning on postnatal day (PND) 1 (day of birth is PND 0), the pups were directly gavaged with the same dose of vehicle, BPA, or EE. We also obtained a group of animals treated with 3 ppm propylthiouracil in the drinking water and an equal number of concordant controls. Neither BPA nor EE affected serum thyroid hormones or thyroid hormone‒sensitive end points in the developing brain at PND 15. In contrast, propylthiouracil (PTU) reduced serum T4 to the expected degree (80% reduction) and elevated serum TSH. Few effects of PTU were observed in the male brain and none in the female brain. As a result, it is difficult to interpret the negative effects of BPA on the thyroid in this rat strain because the thyroid system appears to respond differently from that of other rat strains.

Neurogranin Expression Is Regulated by Synaptic Activity and Promotes Synaptogenesis in Cultured Hippocampal Neurons

Neurogranin (Ng) is a calmodulin (CaM)-binding protein that is phosphorylated by protein kinase C (PKC) and is highly enriched in the dendrites and spines of telencephalic neurons. It is proposed to be involved in regulating CaM availability in the post-synaptic environment to modulate the efficiency of excitatory synaptic transmission. There is a close relationship between Ng and cognitive performance; its expression peaks in the forebrain coinciding with maximum synaptogenic activity, and it is reduced in several conditions of impaired cognition. We studied the expression of Ng in cultured hippocampal neurons and found that both protein and mRNA levels were about 10% of that found in the adult hippocampus. Long-term blockade of NMDA receptors substantially decreased Ng expression. On the other hand, treatments that enhanced synaptic activity such as long-term bicuculline treatment or co-culture with glial cells or cholesterol increased Ng expression. Chemical long-term potentiation (cLTP) induced an initial drop of Ng, with a minimum after 15 min followed by a slow recovery during the next 2-4 h. This effect was most evident in the synaptosome-enriched fraction, thus suggesting local synthesis in dendrites. Lentiviral expression of Ng led to increased density of both excitatory and inhibitory synapses in the second and third weeks of culture. These results indicate that Ng expression is regulated by synaptic activity and that Ng promotes the synaptogenesis process. Given its relationship with cognitive function, we propose targeting of Ng expression as a promising strategy to prevent or alleviate the cognitive deficits associated with aging and neuropathological conditions.

Transport, Metabolism, and Function of Thyroid Hormones in the Developing Mammalian Brain

Ever since the discovery of thyroid hormone deficiency as the primary cause of cretinism in the second half of the 19th century, the crucial role of thyroid hormone (TH) signaling in embryonic brain development has been established. However, the biological understanding of TH function in brain formation is far from complete, despite advances in treating thyroid function deficiency disorders. The pleiotropic nature of TH action makes it difficult to identify and study discrete roles of TH in various aspect of embryogenesis, including neurogenesis and brain maturation. These challenges notwithstanding, enormous progress has been achieved in understanding TH production and its regulation, their conversions and routes of entry into the developing mammalian brain. The endocrine environment has to adjust when an embryo ceases to rely solely on maternal source of hormones as its own thyroid gland develops and starts to produce endogenous TH. A number of mechanisms are in place to secure the proper delivery and action of TH with placenta, blood-brain interface, and choroid plexus as barriers of entry that need to selectively transport and modify these hormones thus controlling their active levels. Additionally, target cells also possess mechanisms to import, modify and bind TH to further fine-tune their action. A complex picture of a tightly regulated network of transport proteins, modifying enzymes, and receptors has emerged from the past studies. TH have been implicated in multiple processes related to brain formation in mammals-neuronal progenitor proliferation, neuronal migration, functional maturation, and survival-with their exact roles changing over developmental time. Given the plethora of effects thyroid hormones exert on various cell types at different developmental periods, the precise spatiotemporal regulation of their action is of crucial importance. In this review we summarize the current knowledge about TH delivery, conversions, and function in the developing mammalian brain. We also discuss their potential role in vertebrate brain evolution and offer future directions for research aimed at elucidating TH signaling in nervous system development.

Sobetirome and its Amide Prodrug Sob-AM2 Exert Thyromimetic Actions in Mct8-Deficient Brain

BACKGROUND

Loss of function mutations in the thyroid hormone (TH)-specific cell membrane transporter, the monocarboxylate transporter 8 (MCT8), lead to profound psychomotor retardation and abnormal TH serum levels, with low thyroxine (T4) and high triiodothyronine (T3). Several studies point to impaired TH transport across brain barriers as a crucial pathophysiological mechanism resulting in cerebral hypothyroidism. Treatment options for MCT8-deficient patients are limited and are focused on overcoming the brain barriers. The aim of this study was to evaluate the ability of the TH analog sobetirome and its prodrug Sob-AM2 to access the brain and exert thyromimetic actions in the absence of Mct8.

METHODS

Juvenile wild-type (Wt) mice and mice lacking Mct8 and deiodinase type 2 (Mct8/Dio2KO) were treated systemically with daily injections of vehicle, 1 mg of sobetirome/kg body weight/day, or 0.3 mg of Sob-AM2/kg body weight/day for seven days. Sobetirome content was measured using liquid chromatography-tandem mass spectrometry, and T4 and T3 levels by specific radioimmunoassays. The effect of sobetirome treatment in the expression of T3-dependent genes was measured in the heart, liver, and cerebral cortex by real-time polymerase chain reaction.

RESULTS

Sob-AM2 treatment in Mct8/Dio2KO animals led to 1.8-fold more sobetirome content in the brain and 2.5-fold less in plasma in comparison to the treatment with the parent drug sobetirome. Both sobetirome and Sob-AM2 treatments in Mct8/Dio2KO mice greatly decreased plasma T4 and T3 levels. Dio1 and Ucp2 gene expression was altered in the liver of Mct8/Dio2KO mice and was not affected by the treatments. In the heart, Hcn2 but not Atp2a2 expression was increased after treatment with the analogs. Interestingly, both sobetirome and Sob-AM2 treatments increased the expression of several T3-dependent genes in the brain such as Hr, Abcd2, Mme, and Flywch2 in Mct8/Dio2KO mice.

CONCLUSIONS

Sobetirome and its amide prodrug Sob-AM2 can access the brain in the absence of Mct8 and exert thyromimetic actions modulating the expression of T3-dependent genes. At the peripheral level, the administration of these TH analogs results in the depletion of circulating T4 and T3. Therefore, sobetirome and Sob-AM2 have the potential to address the cerebral hypothyroidism and the peripheral hyperthyroidism characteristic of MCT8 deficiency.

Assessing dose-response relationships for endocrine disrupting chemicals (EDCs): a focus on non-monotonicity

The fundamental principle in regulatory toxicology is that all chemicals are toxic and that the severity of effect is proportional to the exposure level. An ancillary assumption is that there are no effects at exposures below the lowest observed adverse effect level (LOAEL), either because no effects exist or because they are not statistically resolvable, implying that they would not be adverse. Chemicals that interfere with hormones violate these principles in two important ways: dose-response relationships can be non-monotonic, which have been reported in hundreds of studies of endocrine disrupting chemicals (EDCs); and effects are often observed below the LOAEL, including all environmental epidemiological studies examining EDCs. In recognition of the importance of this issue, Lagarde et al. have published the first proposal to qualitatively assess non-monotonic dose response (NMDR) relationships for use in risk assessments. Their proposal represents a significant step forward in the evaluation of complex datasets for use in risk assessments. Here, we comment on three elements of the Lagarde proposal that we feel need to be assessed more critically and present our arguments: 1) the use of Klimisch scores to evaluate study quality, 2) the concept of evaluating study quality without topical experts' knowledge and opinions, and 3) the requirement of establishing the biological plausibility of an NMDR before consideration for use in risk assessment. We present evidence-based logical arguments that 1) the use of the Klimisch score should be abandoned for assessing study quality; 2) evaluating study quality requires experts in the specific field; and 3) an understanding of mechanisms should not be required to accept observable, statistically valid phenomena. It is our hope to contribute to the important and ongoing debate about the impact of NMDRs on risk assessment with positive suggestions.

Hippocampal Functioning and Verbal Associative Memory in Adolescents with Congenital Hypothyroidism

Thyroid hormone (TH) is essential for normal development of the hippocampus, which is critical for memory and particularly for learning and recalling associations between visual and verbal stimuli. Adolescents with congenital hypothyroidism (CH), who lack TH in late gestation and early life, demonstrate weak verbal recall abilities, reduced hippocampal volumes, and abnormal hippocampal functioning for visually associated material. However, it is not known if their hippocampus functions abnormally when remembering verbal associations. Our objective was to assess hippocampal functioning in CH using functional magnetic resonance imaging (fMRI). Fourteen adolescents with CH and 14 typically developing controls (TDC) were studied. Participants studied pairs of words and then, during fMRI acquisition, made two types of recognition decisions: in one they judged whether the pairs were the same as when seen originally and in the other, whether individual words were seen before regardless of pairing. Hippocampal activation was greater for pairs than items in both groups, but this difference was only significant in TDC. When we directly compared the groups, the right anterior hippocampus was the primary region in which the TDC and CH groups differed for this pair memory effect. Results signify that adolescents with CH show abnormal hippocampal functioning during verbal memory processing.

Transporters MCT8 and OATP1C1 maintain murine brain thyroid hormone homeostasis

Allan-Herndon-Dudley syndrome (AHDS), a severe form of psychomotor retardation with abnormal thyroid hormone (TH) parameters, is linked to mutations in the TH-specific monocarboxylate transporter MCT8. In mice, deletion of Mct8 (Mct8 KO) faithfully replicates AHDS-associated endocrine abnormalities; however, unlike patients, these animals do not exhibit neurological impairments. While transport of the active form of TH (T3) across the blood-brain barrier is strongly diminished in Mct8 KO animals, prohormone (T4) can still enter the brain, possibly due to the presence of T4-selective organic anion transporting polypeptide (OATP1C1). Here, we characterized mice deficient for both TH transporters, MCT8 and OATP1C1 (Mct8/Oatp1c1 DKO). Mct8/Oatp1c1 DKO mice exhibited alterations in peripheral TH homeostasis that were similar to those in Mct8 KO mice; however, uptake of both T3 and T4 into the brains of Mct8/Oatp1c1 DKO mice was strongly reduced. Evidence of TH deprivation in the CNS of Mct8/Oatp1c1 DKO mice included highly decreased brain TH content as well as altered deiodinase activities and TH target gene expression. Consistent with delayed cerebellar development and reduced myelination, Mct8/Oatp1c1 DKO mice displayed pronounced locomotor abnormalities. Intriguingly, differentiation of GABAergic interneurons in the cerebral cortex was highly compromised. Our findings underscore the importance of TH transporters for proper brain development and provide a basis to study the pathogenic mechanisms underlying AHDS.

Role of thyroid hormone receptor subtypes α and β on gene expression in the cerebral cortex and striatum of postnatal mice

The effects of thyroid hormones (THs) on brain development and function are largely mediated by the control of gene expression. This is achieved by the binding of the genomically active T3 to transcriptionally active nuclear TH receptors (TRs). T3 and the TRs can either induce or repress transcription. In hypothyroidism, the reduction of T3 lowers the expression of a set of genes, the positively regulated genes, and increases the expression of negatively regulated genes. Two mechanisms may account for the effect of hypothyroidism on genes regulated directly by T3: first, the loss of T3 signaling and TR transactivation, and second, an intrinsic activity of the unliganded TRs directly responsible for repression of positive genes and enhancement of negative genes. To analyze the contribution of the TR subtypes α and β, we have measured by RT-PCR the expression of a set of positive and negative genes in the cerebral cortex and the striatum of TR-knockout male and female mice. The results indicate that TRα1 exerts a predominant but not exclusive role in the regulation of positive and negative genes. However, a fraction of the genes analyzed are not or only mildly affected by the total absence of TRs. Furthermore, hypothyroidism has a mild effect on these genes in the absence of TRα1, in agreement with a role of unliganded TRα1 in the effects of hypothyroidism.

Tetrac can replace thyroid hormone during brain development in mouse mutants deficient in the thyroid hormone transporter mct8

The monocarboxylate transporter 8 (MCT8) plays a critical role in mediating the uptake of thyroid hormones (THs) into the brain. In patients, inactivating mutations in the MCT8 gene are associated with a severe form of psychomotor retardation and abnormal serum TH levels. Here, we evaluate the therapeutic potential of the TH analog 3,5,3',5'-tetraiodothyroacetic acid (tetrac) as a replacement for T(4) in brain development. Using COS1 cells transfected with TH transporter and deiodinase constructs, we could show that tetrac, albeit not being transported by MCT8, can be metabolized to the TH receptor active compound 3,3',5-triiodothyroacetic acid (triac) by type 2 deiodinase and inactivated by type 3 deiodinase. Triac in turn is capable of replacing T(3) in primary murine cerebellar cultures where it potently stimulates Purkinje cell development. In vivo effects of tetrac were assessed in congenital hypothyroid Pax8-knockout (KO) and Mct8/Pax8 double-KO mice as well as in Mct8-KO and wild-type animals after daily injection of tetrac (400 ng/g body weight) during the first postnatal weeks. This treatment was sufficient to promote TH-dependent neuronal differentiation in the cerebellum, cerebral cortex, and striatum but was ineffective in suppressing hypothalamic TRH expression. In contrast, TSH transcript levels in the pituitary were strongly down-regulated in response to tetrac. Based on our findings we propose that tetrac administration offers the opportunity to provide neurons during the postnatal stage with a potent TH receptor agonist, thereby eventually reducing the neurological damage in patients with MCT8 mutations without deteriorating the thyrotoxic situation in peripheral tissues.

The pathophysiological consequences of thyroid hormone transporter deficiencies: Insights from mouse models

BACKGROUND

As a prerequisite for thyroid hormone (TH) metabolism and action TH has to be transported into cells where TH deiodinases and receptors are located. The trans-membrane passage of TH is facilitated by TH transporters of which the monocarboxylate transporter MCT8 has been most intensively studied. Inactivating mutations in the gene encoding MCT8 are associated with a severe form of psychomotor retardation and abnormal serum TH levels (Allan-Herndon-Dudley syndrome). In order to define the underlying pathogenic mechanisms, Mct8 knockout mice have been generated and intensively studied. Most surprisingly, Mct8 ko mice do not show any neurological symptoms but fully replicate the abnormal serum thyroid state.

SCOPE OF REVIEW

We will summarize the findings of these mouse studies that shed light on various aspects of Mct8 deficiency and unambiguously demonstrated the pivotal role of Mct8 in mediating TH transport in various tissues. These studies have also revealed the presence of the complex interplay between different pathogenic mechanisms that contribute to the generation of the abnormal TH serum profile.

MAJOR CONCLUSIONS

Most importantly, studies of Mct8 ko mice indicated the presence of additional TH transporters that act in concert with Mct8. Interesting candidates for such a function are the L-type amino acid transporters Lat1 and Lat2 as well as the organic anion transporting polypeptide Oatp1c1.

GENERAL SIGNIFICANCE

Overall, the analysis of Mct8 deficient mice has greatly expanded our knowledge about the (patho-) physiological function of this transporter and established a sound basis for the characterization of additional TH transporter candidates. This article is part of a Special Issue entitled Thyroid hormone signalling.

Impact of Oatp1c1 deficiency on thyroid hormone metabolism and action in the mouse brain

Organic anion-transporting polypeptide 1c1 (Oatp1c1) (also known as Slco1c1 and Oatp14) belongs to the family of Oatp and has been shown to facilitate the transport of T(4). In the rodent brain, Oatp1c1 is highly enriched in capillary endothelial cells and choroid plexus structures where it may mediate the entry of T(4) into the central nervous system. Here, we describe the generation and first analysis of Oatp1c1-deficient mice. Oatp1c1 knockout (KO) mice were born with the expected frequency, were not growth retarded, and developed without any overt neurological abnormalities. Serum T(3) and T(4) concentrations as well as renal and hepatic deiodinase type 1 expression levels were indistinguishable between Oatp1c1 KO mice and control animals. Hypothalamic TRH and pituitary TSH mRNA levels were not affected, but brain T(4) and T(3) content was decreased in Oatp1c1-deficient animals. Moreover, increased type 2 and decreased type 3 deiodinase activities indicate a mild hypothyroid situation in the brain of Oatp1c1 KO mice. Consequently, mRNA expression levels of gene products positively regulated by T(3) in the brain were down-regulated. This central nervous system-specific hypothyroidism is presumably caused by an impaired passage of T(4) across the blood-brain barrier and indicates a unique function of Oatp1c1 in facilitating T(4) transport despite the presence of other thyroid hormone transporters such as Mct8.

Adult-onset hypothyroidism enhances fear memory and upregulates mineralocorticoid and glucocorticoid receptors in the amygdala

Hypothyroidism is the most common hormonal disease in adults, which is frequently accompanied by learning and memory impairments and emotional disorders. However, the deleterious effects of thyroid hormones deficiency on emotional memory are poorly understood and often underestimated. To evaluate the consequences of hypothyroidism on emotional learning and memory, we have performed a classical Pavlovian fear conditioning paradigm in euthyroid and adult-thyroidectomized Wistar rats. In this experimental model, learning acquisition was not impaired, fear memory was enhanced, memory extinction was delayed and spontaneous recovery of fear memory was exacerbated in hypothyroid rats. The potentiation of emotional memory under hypothyroidism was associated with an increase of corticosterone release after fear conditioning and with higher expression of glucocorticoid and mineralocorticoid receptors in the lateral and basolateral nuclei of the amygdala, nuclei that are critically involved in the circuitry of fear memory. Our results demonstrate for the first time that adult-onset hypothyroidism potentiates fear memory and also increases vulnerability to develop emotional memories. Furthermore, our findings suggest that enhanced corticosterone signaling in the amygdala is involved in the pathophysiological mechanisms of fear memory potentiation. Therefore, we recommend evaluating whether inappropriate regulation of fear in patients with post-traumatic stress and other mental disorders is associated with abnormal levels of thyroid hormones, especially those patients refractory to treatment.

Effects of perfluoroalkyl compounds on mRNA expression levels of thyroid hormone-responsive genes in primary cultures of avian neuronal cells

There is growing interest in assessing the neurotoxic and endocrine disrupting potential of perfluoroalkyl compounds (PFCs). Several studies have reported in vitro and in vivo effects related to neuronal development, neural cell differentiation, prenatal and postnatal development and behavior. PFC exposure altered hormone levels and the expression of hormone-responsive genes in mammalian and aquatic species. This study is the first to assess the effects of PFCs on messenger RNA (mRNA) expression in primary cultures of neuronal cells in two avian species: the domestic chicken (Gallus domesticus) and herring gull (Larus argentatus). The following thyroid hormone (TH)–responsive genes were examined using real-time reverse transcription-PCR: type II iodothyronine 5′-deiodinase (D2), D3, transthyretin (TTR), neurogranin (RC3), octamer motif–binding factor (Oct-1), and myelin basic protein. Several PFCs altered the mRNA expression levels of genes associated with the TH pathway in avian neuronal cells. Short-chained PFCs (less than eight carbons) altered the expression of TH-responsive genes (D2, D3, TTR, and RC3) in chicken embryonic neuronal cells to a greater extent than long-chained PFCs (more than or equal to eight carbons). Variable transcriptional changes were observed in herring gull embryonic neuronal cells exposed to short-chained PFCs; mRNA levels of Oct-1 and RC3 were upregulated. This is the first study to report that PFC exposure alters mRNA expression in primary cultures of avian neuronal cells and may provide insight into the possible mechanisms of action of PFCs in the avian brain.

Developmental iodine deficiency and hypothyroidism reduce phosphorylation of calcium/calmodulin-dependent kinase II in the rat entorhinal cortex

Iodine is essential for the synthesis of triiodothyronine (T₃) and thyroxine (T₄). Iodine deficiency leads to inadequate thyroid hormone. Hypothyroidism induced by iodine deficiency during gestation and postnatal period leads to cognitive deficits in learning and memory. However, the mechanism underlying these deficits is unclear. Calcium-dependent calmodulin kinase II (CaMKII) known as a potential memory molecule regulates important neuronal functions including learning and memory. Recent studies have shown that hypothyroidism alters phosphorylation of CaMKII in hippocampus or even in sympathetic ganglia of rats. Though the entorhinal cortex (EC) is an important functional structure within the neuronal network responsible for learning and memory, little is known about the effect of hypothyroidism on phosphorylation of CaMKII in the EC. Here, we report that iodine deficiency and propylthiouracil treatment through gestation and lactation reduce phosphorylation of CaMKII in the EC of pups. The increase of calcineurin, as well as reduction of neurogranin and calmodulin, may account for the reduced phosphorylation of CaMKII induced by developmental iodine deficiency and hypothyroidism. These findings in the EC may contribute to understanding the mechanisms that underlie impairment of learning and memory induced by developmental iodine deficiency and 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.

The nature of the compensatory response to low thyroid hormone in the developing brain

Thyroid hormone is essential for normal brain development, although the degree to which the developing brain is sensitive to small perturbations in serum thyroxin is not clear. An important concept related to this is that the developing brain possesses potent mechanisms to compensate for low serum thyroid hormone, and this concept is routinely employed in discussions concerning clinical treatments or public health. However, experimental studies have not directly tested whether (or the degree to which) putative compensatory mechanisms can ameliorate the consequences of small reductions in serum thyroxin (T(4)). To formally test this concept, we employed a model of graded T(4) reductions using doses of propylthiouracil (PTU) that were 200- to 67-fold lower than the dose traditionally used to produce hypothyroidism in rats. PTU produced a stepwise decrease in serum total T(4), and a stepwise increase in serum thyroid-stimulating hormone (TSH), in type 2 deiodinase mRNA expression and enzyme activity in the brain, and in the expression of the mRNA encoding the tri-iodothyronine (T(3)) transporter MCT8 in the postnatal day (P) 15 cortex. However, the mRNA encoding RC3/neurogranin, a direct target of T(3) action, exhibited a strong negative linear correlation with serum total T(4) despite these adaptive responses. In addition, single-cell analysis of RC3 mRNA levels in cortical neurones demonstrated that the co-expression of MCT8 did not alter the relationship between RC3 mRNA and serum T(4). These findings do not support the currently envisioned concept of the developing brain being capable of compensating for low T(4).

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.

Polychlorinated biphenyls (Aroclor 1254) do not uniformly produce agonist actions on thyroid hormone responses in the developing rat brain

Thyroid hormone (TH) is essential for normal brain development, and polychlorinated biphenyls (PCBs) are known to interfere with TH action in the developing brain. Thus, it is possible that the observed neurotoxic effects of PCB exposure in experimental animals and humans are mediated in part by their ability to interfere with TH signaling. PCBs may interfere with TH signaling by reducing circulating levels of TH, acting as TH receptor analogs, or both. If PCBs act primarily by reducing serum TH levels, then their effects should mimic those of low TH. In contrast, if PCBs act primarily as TH agonists in the developing brain, then they should mimic the effect of T(4) in hypothyroid animals. We used a two-factor design to test these predictions. Both hypothyroidism (Htx) and/or PCB treatment reduced serum free and total T(4) on postnatal d 15. However, only Htx increased pituitary TSHbeta expression. RC3/neurogranin expression was decreased by Htx and increased by PCB treatment. In contrast, Purkinje cell protein-2 expression was reduced in hypothyroid animals and restored by PCB treatment. Finally, PCB treatment partially ameliorated the effect of Htx on the thickness of the external granule layer of the cerebellum. These studies demonstrate clearly that PCB exposure does not mimic the effect of low TH on several important TH-sensitive measures in the developing brain. However, neither did PCBs mimic T(4) in hypothyroid animals on all end points measured. Thus, PCBs exert a complex action on TH signaling in the developing brain.

Hypothyroxinemia of prematurity: cause, diagnosis and management

Infants born at extreme prematurity are at a high risk of developmental disability. A major risk factor for disability is having a low level of thyroid hormone, described as hypothyroxinemia, which is recognized to be a frequent phenomenon in these infants. At present, there is uncertainty among clinicians regarding the most appropriate method of managing hypothyroxinemia of prematurity. The literature suggests that some, but not all, forms of thyroid supplementation may reduce the incidence of disability in infants born at extreme prematurity. There is a pressing need to confirm the benefit of treatment and to establish the optimal way to treat transient hypothyroxinemia in these infants.

TIPIT: a randomised controlled trial of thyroxine in preterm infants under 28 weeks gestation: magnetic resonance imaging and magnetic resonance angiography protocol

Background

Infants born at extreme prematurity are at high risk of developmental disability. A major risk factor for disability is having a low level of thyroid hormone described as hypothyroxinaemia, which is recognised to be a frequent phenomenon in these infants. Derangements of critical thyroid function during the sensitive window in prematurity when early development occurs, may have a range of long term effects for brain development. Further research in preterm infants using neuroimaging techniques will increase our understanding of the specificity of the effects of hypothyroxinaemia on the developing foetal brain. This is an explanatory double blinded randomised controlled trial which is aimed to assess the effect of thyroid hormone supplementation on brain size, key brain structures, extent of myelination, white matter integrity and vessel morphology, somatic growth and the hypothalamic-pituitary-adrenal axis.

Methods

The study is a multi-centred double blinded randomised controlled trial of thyroid hormone supplementation in babies born below 28 weeks' gestation. All infants will receive either levothyroxine or placebo until 32 weeks corrected gestational age. The primary outcomes will be width of the sub-arachnoid space measured using cranial ultrasound and head circumference at 36 weeks corrected gestational age. The secondary outcomes will be thyroid hormone concentrations, the hypothalamic pituitary axis status and auxological data between birth and expected date of delivery; thyroid gland volume, brain size, volumes of key brain structures, extent of myelination and brain vessel morphology at expected date of delivery and markers of morbidity which include duration of mechanical ventilation and/or oxygen requirement and chronic lung disease.

Trial registration

Current Controlled Trials ISRCTN89493983

T3 administration in adult hypothyroid mice modulates expression of proteins involved in striatal synaptic plasticity and improves motor behavior

Adult-onset hypothyroidism is associated with neurological changes such as cognitive dysfunction and impaired learning, which may be related to alterations of synaptic plasticity. We investigate the consequence of adult-onset hypothyroidism on thyroid-mediated transcription events in striatal synaptic plasticity, and the effect of triiodothyronine (T3) replacement. We used hypothyroid mice, treated with propylthiouracil (PTU) and methimazole (MMI), with or without subsequent administration of T3. We evaluated the amount of T3 nuclear receptors (TRalpha1, TRbeta) and striatal plasticity indicators: neurogranin (RC3), Ras homolog enriched in striatum (Rhes), Ca2+/calmodulin-dependent protein kinase (CaMKII), and dopamine- and cAMP-regulated phosphoprotein (DARPP-32). In addition, we assessed hypothyroid mice motor behavior as related to striatum synaptic functions. Hypothyroid mice exhibited significantly reduced TRbeta, RC3 and Rhes expression. T3 administration reversed the expression of TRbeta, RC3, and up-regulated CaMKII levels as well as motor behavior, and decreased DARPP-32 protein phosphorylation. We suggest that thyroid hormone modulation had a major impact on striatal synaptic plasticity of adult mice which produced in turn motor behavior modifications.

General background on the hypothalamic-pituitary-thyroid (HPT) axis

This article reviews the thyroid system, mainly from a mammalian standpoint. However, the thyroid system is highly conserved among vertebrate species, so the general information on thyroid hormone production and feedback through the hypothalamic-pituitary-thyroid (HPT) axis should be considered for all vertebrates, while species-specific differences are highlighted in the individual articles. This background article begins by outlining the HPT axis with its components and functions. For example, it describes the thyroid gland, its structure and development, how thyroid hormones are synthesized and regulated, the role of iodine in thyroid hormone synthesis, and finally how the thyroid hormones are released from the thyroid gland. It then progresses to detail areas within the thyroid system where disruption could occur or is already known to occur. It describes how thyroid hormone is transported in the serum and into the tissues on a cellular level, and how thyroid hormone is metabolized. There is an in-depth description of the alpha and beta thyroid hormone receptors and their functions, including how they are regulated, and what has been learned from the receptor knockout mouse models. The nongenomic actions of thyroid hormone are also described, such as in glucose uptake, mitochondrial effects, and its role in actin polymerization and vesicular recycling. The article discusses the concept of compensation within the HPT axis and how this fits into the paradigms that exist in thyroid toxicology/endocrinology. There is a section on thyroid hormone and its role in mammalian development: specifically, how it affects brain development when there is disruption to the maternal, the fetal, the newborn (congenital), or the infant thyroid system. Thyroid function during pregnancy is critical to normal development of the fetus, and several spontaneous mutant mouse lines are described that provide research tools to understand the mechanisms of thyroid hormone during mammalian brain development. Overall this article provides a basic understanding of the thyroid system and its components. The complexity of the thyroid system is clearly demonstrated, as are new areas of research on thyroid hormone physiology and thyroid hormone action developing within the field of thyroid endocrinology. This review provides the background necessary to review the current assays and endpoints described in the following articles for rodents, fishes, amphibians, and birds.

Abnormal thyroid hormone metabolism in mice lacking the monocarboxylate transporter 8

In humans, inactivating mutations in the gene of the thyroid hormone transporter monocarboxylate transporter 8 (MCT8; SLC16A2) lead to severe forms of psychomotor retardation combined with imbalanced thyroid hormone serum levels. The MCT8-null mice described here, however, developed without overt deficits but also exhibited distorted 3,5,3'-triiodothyronine (T3) and thyroxine (T4) serum levels, resulting in increased hepatic activity of type 1 deiodinase (D1). In the mutants' brains, entry of T4 was not affected, but uptake of T3 was diminished. Moreover, the T4 and T3 content in the brain of MCT8-null mice was decreased, the activity of D2 was increased, and D3 activity was decreased, indicating the hypothyroid state of this tissue. In the CNS, analysis of T3 target genes revealed that in the mutants, the neuronal T3 uptake was impaired in an area-specific manner, with strongly elevated thyrotropin-releasing hormone transcript levels in the hypothalamic paraventricular nucleus and slightly decreased RC3 mRNA expression in striatal neurons; however, cerebellar Purkinje cells appeared unaffected, since they did not exhibit dendritic outgrowth defects and responded normally to T3 treatment in vitro. In conclusion, the circulating thyroid hormone levels of MCT8-null mice closely resemble those of humans with MCT8 mutations, yet in the mice, CNS development is only partially affected.

Making the gradient: thyroid hormone regulates cone opsin expression in the developing mouse retina

Most mammals have two types of cone photoreceptors, which contain either medium wavelength (M) or short wavelength (S) opsin. The number and spatial organization of cone types varies dramatically among species, presumably to fine-tune the retina for different visual environments. In the mouse, S- and M-opsin are expressed in an opposing dorsal-ventral gradient. We previously reported that cone opsin patterning requires thyroid hormone beta2, a nuclear hormone receptor that regulates transcription in conjunction with its ligand, thyroid hormone (TH). Here we show that exogenous TH inhibits S-opsin expression, but activates M-opsin expression. Binding of endogenous TH to TRbeta2 is required to inhibit S-opsin and to activate M-opsin. TH is symmetrically distributed in the retina at birth as S-opsin expression begins, but becomes elevated in the dorsal retina at the time of M-opsin onset (postnatal day 10). Our results show that TH is a critical regulator of both S-opsin and M-opsin, and suggest that a TH gradient may play a role in establishing the gradient of M-opsin. These results also suggest that the ratio and patterning of cone types may be determined by TH availability during retinal development.

Northern blot and in situ hybridization analyses for the neurogranin mRNA in the developing monkey cerebral cortex

Neurogranin is a postsynaptic substrate for protein kinase C, and its expression is related to dendritic spine development and postsynaptic plasticity. Using both Northern blot analysis and in situ hybridization techniques, we investigated the developmental changes of neurogranin expression in the monkey cerebral cortex. In each of four neocortical areas examined, i.e., the prefrontal area (area FD of von Bonin and Bailey), the temporal association area (TE), the primary somatosensory area (PB), and the primary visual area (OC), the Northern blot analysis showed that the amount of neurogranin mRNA was low during the prenatal and perinatal periods until postnatal day 8. It increased during postnatal development and reached its peak value at postnatal day 70 (in area OC) or postnatal month 6 (in area FD, TE, and PB). After that, the amount of neurogranin mRNA in the cerebral neocortex decreased gradually until postnatal years 2-3. The in situ hybridization experiments also showed a transient increase of neurogranin mRNA in the neocortex during postnatal day 70 to postnatal month 6. The transient increase was prominent in layers II and III of areas FD and TE; deep in layer III of area PB; and in layers II, III, and IV of area OC. In the hippocampus, in contrast to the results in the neocortex, the expression of neurogranin mRNA was decreased almost continuously during the postnatal period. The transiently increased expression of neurogranin in the postnatal neocortex may be a molecular basis for the postsynaptic modification of afferent inputs possibly from subcortical structures.

Iodine supplementation improves cognition in iodine-deficient schoolchildren in Albania: a randomized, controlled, double-blind study

BACKGROUND

Iodine is required for the production of thyroid hormones, which are necessary for normal brain development and cognition. Although several randomized trials examined the effect of iodine supplementation on cognitive performance in schoolchildren, the results were equivocal.

OBJECTIVE

We aimed to ascertain whether providing iodized oil to iodine-deficient children would affect their cognitive and motor performance.

DESIGN

In a double-blind intervention trial, 10-12-y-old children (n = 310) in primary schools in rural southeastern Albania were randomly assigned to receive 400 mg I (as oral iodized oil) or placebo. We measured urinary iodine (UI), thyroid-stimulating hormone (TSH), and total thyroxine (TT4) concentrations and thyroid gland volume (by ultrasound). The children were given a battery of 7 cognitive and motor tests, which included measures of information processing, working memory, visual problem solving, visual search, and fine motor skills. Thyroid ultrasound and the biochemical and psychological tests were repeated after 24 wk.

RESULTS

At baseline, the children's median UI concentration was 43 microg/L; 87% were goitrous, and nearly one-third had low concentrations of circulating TT4. Treatment with iodine markedly improved iodine and thyroid status: at 24 wk, median UI in the treated group was 172 microg/L, mean TT4 was approximately 40% higher, and the prevalence of hypothyroxinemia was < 1%. In the placebo group after the intervention, these variables did not differ significantly from baseline. Compared with placebo, iodine treatment significantly improved performance on 4 of 7 tests: rapid target marking, symbol search, rapid object naming, and Raven's Coloured Progressive Matrices (P < 0.0001).

CONCLUSION

Information processing, fine motor skills, and visual problem solving are improved by iodine repletion in moderately iodine-deficient schoolchildren.

Neurogranin/RC3 enhances long-term potentiation and learning by promoting calcium-mediated signaling

In neurons, neurogranin (Ng) binds calmodulin (CaM), and its binding affinity is reduced by increasing Ca2+, phosphorylation by PKC, or oxidation by oxidants. Ng concentration in the hippocampus of adult mice varied broadly (Ng+/+, 160-370 and Ng+/-, approximately 70-230 pmol/mg); the level in Ng+/+ mice is one of the highest among all neuronal CaM-binding proteins. Among Ng+/- mice, but less apparent in Ng+/+, a significant relationship existed between their hippocampal levels of Ng and performances in the Morris water maze. Ng-/- mice performed poorly in this task; they also displayed deficits in high-frequency-induced long-term potentiation (LTP) in area CA1 of hippocampal slices, whereas low-frequency-induced long-term depression was enhanced. Thus, compared with Ng+/+ mice, the frequency-response curve of Ng-/- shifted to the right. Paired-pulse facilitation and synaptic fatigue during prolonged stimulation at 10 Hz (900 pulses) were unchanged in Ng-/- slices, indicating their normal presynaptic function. Measurements of Ca2+ transients in CA1 pyramidal neurons after weak and strong tetanic stimulations (100 Hz, 400 and 1000 msec, respectively) revealed a significantly greater intracellular Ca2+ ([Ca2+]i) response in Ng+/+ compared with Ng-/- mice, but the decay time constants did not differ. The diminished Ca2+ dynamics in Ng-/- mice are a likely cause of their decreased propensity to undergo LTP. Thus, Ng may promote a high [Ca2+]i by a "mass-action" mechanism; namely, the higher the Ng concentration, the more Ng-CaM complexes will be formed, which effectively raises [Ca2+]i at any given Ca2+ influx. This mechanism provides potent signal amplification in enhancing synaptic plasticity as well as learning and memory.

Bisphenol-A, an environmental contaminant that acts as a thyroid hormone receptor antagonist in vitro, increases serum thyroxine, and alters RC3/neurogranin expression in the developing rat brain

Considering the importance of thyroid hormone (TH) in brain development, it is of potential concern that a wide variety of environmental chemicals can interfere with thyroid function or, perhaps of greater concern, with TH action at its receptor (TR). Recently bisphenol-A (BPA, 4,4' isopropylidenediphenol) was reported to bind to the rat TR and act as an antagonist in vitro. BPA is a high production volume chemical, with more than 800 million kg of BPA produced annually in the United States alone. It is detectable in serum of pregnant women and cord serum taken at birth; is 5-fold higher in amniotic fluid at 15-18 wk gestation, compared with maternal serum; and was found in concentrations of up to 100 ng/g in placenta. Thus, the human population is widely exposed to BPA and it appears to accumulate in the fetus. We now report that dietary exposure to BPA of Sprague Dawley rats during pregnancy and lactation causes an increase in serum total T4 in pups on postnatal d 15, but serum TSH was not different from controls. The expression of the TH-responsive gene RC3/neurogranin, measured by in situ hybridization, was significantly up-regulated by BPA in the dentate gyrus. These findings suggest that BPA acts as a TH antagonist on the beta-TR, which mediates the negative feedback effect of TH on the pituitary gland, but that BPA is less effective at antagonizing TH on the alpha-TR, leaving TRalpha-mediated events to respond to elevated T4.

Impairment in short-term but enhanced long-term synaptic potentiation and ERK activation in adult hippocampal area CA1 following developmental thyroid hormone insufficiency

Thyroid hormones are critical for the development and maturation of the central nervous system. Insufficiency of thyroid hormones during development impairs performance on tasks of learning and memory that rely upon the hippocampus and impairs synaptic function in young hypothyroid animals. The present study was designed to determine if perturbations in synaptic function persist in adult euthyroid animals exposed developmentally to insufficient levels of hormone. Pre- and postnatal thyroid hormone insufficiency was induced by administration of 3 or 10 ppm propylthiouracil (PTU) to pregnant and lactating dams via the drinking water from gestation day (GD) 6 until postnatal day (PN) 30. This regimen produced a graded level of hormonal insufficiency in the dam and the offspring. Population spike and population excitatory postsynaptic potentials (EPSP) were recorded at the pyramidal cell layer and the stratum radiatum, respectively, in area CA1 of hippocampal slices from adult male offspring. PTU exposure increased baseline synaptic transmission, reduced paired-pulse facilitation, and increased the magnitude of the population spike long-term potentiation (LTP). Phosphorylation of the extracellular signal-regulated kinases (ERK1 and ERK2) was increased as a function of LTP stimulation in slices from PTU-exposed adult animals. On the other hand, no differences in the basal levels of synaptic proteins implicated in synaptic plasticity (total ERK, synapsin, growth-associated protein-43, and neurogranin) were detected. These results reinforce previous findings of persistent changes in synaptic function and, importantly extend these observations to moderate levels of thyroid hormone insufficiency that do not induce significant toxicity to the dams or the offspring. Such alterations in hippocampal synaptic function may contribute to persistent behavioral deficits associated with developmental hypothyroidism.

RC3/neurogranin is expressed in pyramidal neurons of motor and somatosensory cortex in normal and denervated monkeys

RC3/neurogranin is a neuron-specific calpacitin located in the cytoplasm and, especially, in dendrites and dendritic spines of cortical neurons, involved in many aspects of excitatory transmission and long-term potentiation. We investigated RC3 expression in pyramidal cortical neurons and interneurons of the motor and somatosensory cortex of normal Macaca fascicularis by means of double immunofluorescence and with techniques that combine immunohistochemistry and radioactive in situ hybridization. We show that RC3 is expressed in virtually all pyramidal neurons and spiny stellate neurons of neocortical areas 4, 3b, 1, 2, 5, 7, and SII, but not in the majority of cortical interneurons. RC3 protein and mRNA are tightly colocalized with the alpha subunit of CaM kinase II and the 200-kD, nonphosphorylated neurofilament, whereas they are absent from cells expressing the 27-kD, vitamin D-dependent calbindin and parvalbumin. In order to investigate possible activity-dependent regulation of the expression of RC3, we compared these results with those obtained from monkeys subjected to chronic peripheral cutaneous denervation of the first finger. We found that the pattern of distribution of RC3 in motor and somatosensory cortices after nerve cut did not differ from normal.

Polychlorinated biphenyls (PCBs) exert thyroid hormone-like effects in the fetal rat brain but do not bind to thyroid hormone receptors

Polychlorinated biphenyls (PCBs) are ubiquitous environmental contaminants routinely found in human and animal tissues. Developmental exposure to PCBs is associated with neuropsychologic deficits, which may be related to effects on thyroid hormone (TH) signaling in the developing brain. However, PCBs may interfere with TH signaling solely by reducing circulating levels of TH, or they may exert direct effects on TH receptors (TRs). Therefore, we tested whether maternal exposure to a commercial PCB mixture, Aroclor 1254 (A1254), exerts effects in the fetal brain by one or both of these mechanisms. Dams were dosed daily with 0, 1, or 4 mg/kg A1254 from gestational day 6 (GD6) until they were sacrificed on GD16. A1254 significantly reduced circulating levels of triiodothyronine (T3) and thyroxine (T4) in pregnant rats but increased the expression of several TH-responsive genes in the fetal cortex, including neuroendocrine-specific protein A (NSP-A), RC3/neurogranin, and Oct-1. These findings are consistent with a direct action of PCBs on TRs. However, we did not identify parent PCB congeners or metabolites that bound to rat TRs isolated from hepatic nuclei. These findings indicate that PCBs can interfere with TH signaling in the fetal brain by direct actions on the fetus rather than by producing maternal hypothyroidism.

Differential effects of triiodothyronine and the thyroid hormone receptor beta-specific agonist GC-1 on thyroid hormone target genes in the b ain

The availability of synthetic thyroid hormone receptor agonists provides a valuable tool to analyze whether specific receptor isoforms mediate specific physiological responses to thyroid hormone. GC-1 is a thyroid hormone analog displaying selectivity for thyroid hormone receptor beta. We have analyzed the effect of GC-1 on expression of thyroid hormone target genes in the cerebrum and cerebellum. Congenitally hypothyroid rats were treated with single daily doses of either T3 or GC-1. Both compounds similarly induced Purkinje cell protein-2 (PCP-2) in the cerebellum. Expression of RC3 and Rhes in the caudate, and hairless, neurotrophin-3, Reelin, and Rev-ErbAalpha in the cerebellum, was analyzed by in situ hybridization on postnatal d 16. Hypothyroidism strongly decreased expression of RC3 and Rhes in the caudate, and hairless, Rev-ErbAalpha, and neurotrophin-3 in the cerebellum, and increased Reelin. T3 treatment normalized the expression of all genes. However, GC-1 effectively normalized expression of Rhes and Reelin only. The lack of a GC-1 effect on most cerebellar genes can be explained by the known distribution of thyroid hormone receptor alpha and beta isoforms. However, in the caudate, RC3 and Rhes are expressed in the same cells, and therefore, they may represent specific gene responses linked to specific thyroid hormone receptor isoforms.

Congenital hypothyroidism: a review of current diagnostic and treatment practices in relation to neuropsychologic outcome

Because thyroid hormone is essential for normal brain development, children born with congenital hypothyroidism who lack thyroid hormone during a circumscribed period of early development are at risk of brain damage and mental retardation. Since the advent of newborn screening programs in the 1980s, the diagnosis and treatment of this condition are now provided in the first 2-3 weeks of birth in most regions. While this is usually sufficient to prevent mental retardation, the children so identified attain mildly reduced IQs from expectation, and may still experience subtle and specific neurocognitive deficits. Their particular deficits are related to the brief period of thyroid hormone insufficiency they undergo, especially factors reflecting the severity of hypothyroidism at the time of diagnosis, the duration of hypothyroidism in infancy, and thyroid hormones at time of testing. In this article, we review the specific kinds of deficits demonstrated by children with congenital hypothyroidism who were diagnosed by screening and treated early, as well as the factors associated with their disease and its management that contribute to these deficits. The disease-related factors that will be reviewed will include the etiology of hypothyroidism and severity of disease at the time of diagnosis, while the treatment-related factors will include age at onset of therapy, starting and subsequent dose levels, compliance, and treatment-adequacy issues. Also examined will be the effects of hormone levels at the time of testing. In addition, the role of moderating variables such as social, genetic, and environmental influences, as well as the child's gender, will be discussed. Furthermore, several new issues including the quality of subsequent management, ultimate outcome, and pregnancy will be additionally reviewed. In conclusion, while outcome in congenital hypothyroidism is substantially improved by screening, affected children do still experience mild neuropsychologic deficits. To reduce the impact of persisting deficits, further research is needed to determine the optimal starting dose for the different etiologies, guidelines for subsequent management, and alternative therapies. Moreover, now that the original samples are reaching adulthood and, in females, childbearing age, further research is also needed regarding treatment during pregnancy in women with congenital hypothyroidism, as is research to determine how this population ultimately fares in adulthood.

Divergent expression of type 2 deiodinase and the putative thyroxine-binding protein p29, in rat brain, suggests that they are functionally unrelated proteins

Deiodinases (D1, D2, and D3) are selenoproteins involved in thyroid hormone metabolism. Generation of the active hormone T(3), from T(4), is carried out by D1 and D2, whereas D3 degrades both hormones. The identity of the cloned D2 as a selenoprotein is well supported by biochemical and physiological data. However, an alternative view has proposed that type 2 deiodinase is a nonselenoprotein complex containing a putative T(4) binding subunit called p29, with an almost identity in sequence with the Dickkopf protein Dkk3. To explore a possible functional relationship between p29 and D2, we have compared their mRNA expression patterns in the rat brain. In brain, parenchyma p29 was expressed in neurons. High expression levels were found in all the regions of the blood-cerebrospinal fluid (CSF) barrier. p29 was present in different types of cells than D2, with the exception of the tanycytes. Our data do not support that p29 has a functional relationship with D2. On the other hand, expression of p29 in the blood-CSF barrier suggests that it might be involved in T(4) transport to and from the CSF, but further studies are needed to substantiate this hypothesis.

Deletion of the thyroid hormone receptor alpha 1 prevents the structural alterations of the cerebellum induced by hypothyroidism

Thyroid hormone (T3) controls critical aspects of cerebellar development, such as migration of postmitotic granule cells and terminal differentiation of Purkinje cells. T3 acts through nuclear receptors (TR) of two types, TRalpha1 and TRbeta, that either repress or activate gene expression. We have analyzed the cerebellar structure of developing mice lacking the TRalpha1 isoform, which normally accounts for about 80% of T3 receptors in the cerebellum. Contrary to what was expected, granule cell migration and Purkinje cell differentiation were normal in the mutant mice. Even more striking was the fact that when neonatal hypothyroidism was induced, no alterations in cerebellar structure were observed in the mutant mice, whereas the wild-type mice showed delayed granule cell migration and arrested Purkinje cell growth. The results support the idea that repression by the TRalpha1 aporeceptor, and not the lack of thyroid hormone, is responsible for the hypothyroid phenotype. This conclusion was supported by experiments with the TRbeta-selective compound GC-1. Treatment of hypothyroid animals with T3, which binds to TRalpha1 and TRbeta, prevents any defect in cerebellar structure. In contrast, treatment with GC-1, which binds to TRbeta but not TRalpha1, partially corrects Purkinje cell differentiation but has no effect on granule cell migration. Our data indicate that thyroid hormone has a permissive effect on cerebellar granule cell migration through derepression by the TRalpha1 isoform.

Thyroid hormone-dependent regulation of Talpha1 alpha-tubulin during brain development

Thyroid hormone (T3) is essential for brain development and most of its actions are exerted at the gene expression level after interaction with nuclear receptors. In particular, genes encoding cytoskeletal proteins are influenced by the thyroidal status. Thyroid hormone is involved in the normal downregulation of the Talpha1 alpha-tubulin gene during postnatal growth. The action of T3 on Talpha1 tubulin expression is complex and is exerted at least at two levels. In cultured cells, T3 induces a transient and fast decrease of Talpha1 mRNA concentration. This effect is enhanced when transcription is blocked by actinomycin D, suggesting that T3 increases mRNA degradation. In transgenic animals T3 affects the expression of beta-galactosidase under control of the Talpha1 promoter in the same way as the endogenous gene, supporting an effect mediated through the Talpha1 promoter. However, the Talpha1 promoter is not regulated by T3 in transfected cells and, therefore, the effects of the hormone in vivo are likely to be indirect. It is concluded that regulation of Talpha1 alpha-tubulin by thyroid hormone is the result of multiple influences including effects on mRNA half life and indirect effects at the promoter level.