Thyroid hormone regulation of RC3, a brain-specific gene encoding a protein kinase-C substrate

Suppression of neurogranin expression by disruption of epigenetic DNA methylation in hippocampal mature granule cells after developmental exposure to neurotoxicants in rats

We previously performed comprehensive analyses of genes hypermethylated promoter regions and downregulated transcripts in the hippocampal dentate gyrus (DG) of rats upon weaning at postnatal day (PND) 21 after developmental exposure to 6-propyl-2-thiouracil (PTU), valproic acid, and glycidol (GLY), all of which are known to show irreversible effects on hippocampal neurogenesis in adulthood on PND 77. Here, we selected neurotransmitter and neurogenesis-related genes for validation analysis of methylation and expression. As a result, Nrgn by GLY and Shisa7, Agtpbp1, and Cyp46a1 by PTU underwent DNA hypermethylation and sustained downregulation. Immunohistochemical analysis of candidate gene products revealed that the number of neurogranin (NRGN)+ granule cells was decreased in the ventral DG by GLY on PND 21 and 77 and by PTU on PND 21. Among the samples of developmental or 28-day young adult-age exposure to known developmental neurotoxicants in humans, i.e., lead acetate, ethanol, and aluminum chloride, a decrease of NRGN+ cells by ethanol was also observed on PND 77 after developmental exposure. Double immunohistochemistry analysis revealed that NRGN was expressed in mature granule cells, and a similar immunoreactive cell distribution was found for phosphorylated calcium/calmodulin-activated protein kinase, a NRGN downstream molecule. After developmental PTU exposure, the number of activity-regulated cytoskeleton-associated protein+ granule cells was also profoundly decreased in the ventral DG in parallel with the decrease in NRGN+ cells on PND 21. These results suggest that NRGN is a potential marker for suppression of synaptic plasticity in mature granule cells in the ventral DG.

A Fine Regulation of the Hippocampal Thyroid Signalling Pro-Tects Hypothyroid Mice against Glial Cell Activation

Adult-onset hypothyroidism is associated with learning and cognitive dysfunctions, which may be related to alterations in synaptic plasticity. Local reduced levels of thyroid hormones (THs) may impair glia morphology and activity, and promote the increase of pro-inflammatory cytokine levels mainly in the hippocampus. Given that neuroinflammation induces memory impairments, hypothyroidism-related glia dysfunction may participate in brain disorders. Thus, we investigated the mechanisms linking hypothyroidism and neuroinflammation, from a protective perspective. We induced hypothyroidism in adult C57BL/6J and wild-derived WSB/EiJ male mice by a seven-week propylthiouracil (PTU) treatment. We previously showed that WSB/EiJ mice were resistant to high-fat diet (HFD)-induced obesity, showing no neuroinflammatory response through adaptive abilities, unlike C57BL/6J. As PTU and HFD treatments are known to induce comparable inflammatory responses, we hypothesized that WSB/EiJ mice might also be protected against hypothyroidism-induced neuroinflammation. We showed that hypothyroid WSB/EiJ mice depicted no hippocampal neuroinflammatory response and were able to maintain their hippocampal thyroid signalling despite low circulatisng TH levels. In contrast, C57BL/6J mice exhibited disturbed hippocampal TH signalling, accompanied by neuroinflammation and memory impairment. Our results reinforce the preponderance of the hippocampal TH regulatory system over TH circulating levels in the hippocampal glial reactivity.

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.

Regulation of Thyroid-disrupting Chemicals to Protect the Developing Brain

Synthetic chemicals with endocrine disrupting properties are pervasive in the environment and are present in the bodies of humans and wildlife. As thyroid hormones (THs) control normal brain development, and maternal hypothyroxinemia is associated with neurological impairments in children, chemicals that interfere with TH signaling are of considerable concern for children's health. However, identifying thyroid-disrupting chemicals (TDCs) in vivo is largely based on measuring serum tetraiodothyronine in rats, which may be inadequate to assess TDCs with disparate mechanisms of action and insufficient to evaluate the potential neurotoxicity of TDCs. In this review 2 neurodevelopmental processes that are dependent on TH action are highlighted, neuronal migration and maturation of gamma amino butyric acid-ergic interneurons. We discuss how interruption of these processes by TDCs may contribute to abnormal brain circuitry following developmental TH insufficiency. Finally, we identify issues in evaluating the developmental neurotoxicity of TDCs and the strengths and limitations of current approaches designed to regulate them. It is clear that an enhanced understanding of how THs affect brain development will lead to refined toxicity testing, reducing uncertainty and improving our ability to protect children's health.

COUP-TF1 Modulates Thyroid Hormone Action in an Embryonic Stem-Cell Model of Cortical Pyramidal Neuronal Differentiation

BACKGROUND

Thyroid hormone is critical for normal brain development and acts in a spatial and temporal specific pattern. Thyroid hormone excess, or deficiency, can lead to irreversible impairment of brain and sensory development. Chicken ovalbumin upstream-transcription factor 1 (COUP-TF1), expressed early in neuronal development, is essential to achieve normal brain structure. Thyroid hormone stimulation of gene expression is inversely correlated with the level of COUP-TF1 expression.

METHODS

An in vitro method of differentiating mouse embryonic stem (mES) cells into cortical neurons was utilized to study the influence of COUP-TF1 on thyroid hormone signaling in brain development. mES cells were cultured and differentiated in specific conditioned media, and a high percentage of nestin-positive progenitor neurons in the first stage, and cortical neurons in the second stage, was obtained with characteristic neuronal firing.

RESULTS

The number of nestin-positive progenitors, as determined by fluorescence-activated cell sorting analysis, was significantly greater with triiodothyronine (T3) treatment compared to control (p < 0.05). T3 enhanced the expression of cortical neuron marker (Tbr1 and Rc3) mRNAs. After COUP-TF1 knockdown, the number of nestin-positive progenitors was reduced compared to control (p < 0.05), but the number increased with T3 treatment. The mRNA of cortical neuronal gene markers was measured after COUP-TF1 knockdown. In the presence of T3, the peak expression of neuron markers Emx1, Tbr1, Camkiv, and Rc3 mRNA was earlier, at day 18 of differentiation, compared to control cells, at day 22. Furthermore, after COUP-TF1 knockdown, T3 induction of Rc3 and Tbr1 mRNA was significantly enhanced compared to cells expressing COUP-TF1.

CONCLUSION

These results indicate that COUP-TF1 plays an important role in modulating the timing and magnitude of T3-stimulated gene expression required for normal corticogenesis.

Thyroid hormone modulates neuroglobin and cytoglobin in rat brain

Thyroid hormones (THs) are essential and crucial for brain development, playing a role in growth and differentiation. Two globins named neuroglobin (Ngb) and cytoglobin (Cygb) are located in the brain, and each one has different distribution and function: They seem to have similar action by providing O(2) for respiratory chain, and detoxification of reactive oxygen species (ROS) and nitric oxide (NO) protecting tissues against irreversible lesions. We aimed to investigate the influence of thyroid state in Ngb and Cygb metabolism in different brain regions and evaluate their responses in cerebellum, hippocampus and cerebral cortex (hereafter called as cortex) after supraphysiological doses at different time points of TH administration. Experiments were carried out in rats, divided in eight experimental groups Control (C), thyroidectomy (Tx), and thyroidectomy treated with jugular intravenous injection (i.v). T3 (100 μl/100 g) injection and sacrificed after 30, 60, 120 min and 6, 12 and 24 h. In cortex, we found increase in Ngb gene and protein expression in different time points compared to C group, however Cygb gene and protein expression were decreased. In hippocampus, Ngb and Cygb protein expression increased 24 h after i.v. T3 injection in comparison to Tx. In cerebellum, we found increased Ngb gene expression after 120 min, 6, 12 and 24 h after T3 administration compared to Tx, and in contrast, protein expression was found to be significantly increased only 12 and 24 h compared to Tx. Ngb and Cygb expression in brain is influenced by thyroid hormone state both by its lack or excess.

Increased prefrontal cortex neurogranin enhances plasticity and extinction learning

Increasing plasticity in neurons of the prefrontal cortex (PFC) has been proposed as a possible therapeutic tool to enhance extinction, a process that is impaired in post-traumatic stress disorder, schizophrenia, and addiction. To test this hypothesis, we generated transgenic mice that overexpress neurogranin (a calmodulin-binding protein that facilitates long-term potentiation) in the PFC. Neurogranin overexpression in the PFC enhanced long-term potentiation and increased the rates of extinction learning of both fear conditioning and sucrose self-administration. Our results indicate that elevated neurogranin function within the PFC can enhance local plasticity and increase the rate of extinction learning across different behavioral tasks. Thus, neurogranin can provide a molecular link between enhanced plasticity and enhanced extinction.

Cocaine induces nuclear export and degradation of neuronal retinoid X receptor-γ via a TNF-α/JNK- mediated mechanism

Cocaine abuse represents an immense societal health and economic burden for which no effective treatment currently exists. Among the numerous intracellular signaling cascades impacted by exposure to cocaine, increased and aberrant production of pro-inflammatory cytokines in the CNS has been observed. Additionally, we have previously reported a decrease in retinoid-X-receptor-gamma (RXR-γ) in brains of mice chronically exposed to cocaine. Through obligate heterodimerization with a number of nuclear receptors, RXRs serve as master regulatory transcription factors, which can potentiate or suppress expression of a wide spectrum of genes. Little is known about the regulation of RXR levels, but previous studies indicate cellular stressors such as cytokines negatively regulate levels of RXRs in vitro. To evaluate the mechanism underlying the cocaine-induced decreases in RXR-γ levels observed in vivo, we exposed neurons to cocaine in vitro and examined pathways which may contribute to disruption in RXR signaling, including activation of stress pathways by cytokine induction. In these studies, we provide the first evidence that cocaine exposure disrupts neuronal RXR-γ signaling in vitro by promoting its nuclear export and degradation. Furthermore, we demonstrate this effect may be mediated, at least in part, by cocaine-induced production of TNF-α and its downstream effector c-Jun-NH-terminal kinase (JNK). Findings from this study are therefore applicable to both cocaine abuse and to pathological conditions characterized by neuroinflammatory factors, such as neurodegenerative disease.

A SAGE based approach to human glomerular endothelium: defining the transcriptome, finding a novel molecule and highlighting endothelial diversity

Background

Large scale transcript analysis of human glomerular microvascular endothelial cells (HGMEC) has never been accomplished. We designed this study to define the transcriptome of HGMEC and facilitate a better characterization of these endothelial cells with unique features. Serial analysis of gene expression (SAGE) was used for its unbiased approach to quantitative acquisition of transcripts.

Results

We generated a HGMEC SAGE library consisting of 68,987 transcript tags. Then taking advantage of large public databases and advanced bioinformatics we compared the HGMEC SAGE library with a SAGE library of non-cultured ex vivo human glomeruli (44,334 tags) which contained endothelial cells. The 823 tags common to both which would have the potential to be expressed in vivo were subsequently checked against 822,008 tags from 16 non-glomerular endothelial SAGE libraries. This resulted in 268 transcript tags differentially overexpressed in HGMEC compared to non-glomerular endothelia. These tags were filtered using a set of criteria: never before shown in kidney or any type of endothelial cell, absent in all nephron regions except the glomerulus, more highly expressed than statistically expected in HGMEC. Neurogranin, a direct target of thyroid hormone action which had been thought to be brain specific and never shown in endothelial cells before, fulfilled these criteria. Its expression in glomerular endothelium in vitro and in vivo was then verified by real-time-PCR, sequencing and immunohistochemistry.

Conclusions

Our results represent an extensive molecular characterization of HGMEC beyond a mere database, underline the endothelial heterogeneity, and propose neurogranin as a potential link in the kidney-thyroid axis.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-725) contains supplementary material, which is available to authorized users.

Genetic and functional analysis of the gene encoding neurogranin in schizophrenia

OBJECTIVES

Schizophrenia is a highly heritable disorder, but many aspects of its etiology and pathophysiology remain poorly understood. Recently, a SNP rs12807809 located upstream of the neurogranin (NRGN) gene achieved genome-wide significance in this disorder.

METHODS

In order to find the causal variants of NRGN gene in schizophrenia, we searched for genetic variants in the promoter region and all the exons (including both UTR ends and rs12807809) using direct sequencing in a sample of patients with schizophrenia (n=346) and non-psychotic controls (n=345), both being Han Chinese from Taiwan, and conducted an association and functional study.

RESULTS

We identified 7 common polymorphisms in the NRGN gene. SNP and haplotype-based analyses displayed no associations with schizophrenia. Additionally, we identified 5 rare variants in 6 out of 346 patients, including 3 rare variants located at the promoter region (g.-620A>G, g.-578C>G, and g.-344G>A) and 2 rare variants located at 5' UTR (c.-74C>G, and c.-41G>A). No rare variants were found in the control subjects. The results of the reporter gene assay demonstrated that the regulatory activity of construct containing g.-620G, g.-578G, g.-344A, c.-74G, and c.-41A was significantly lower as compared to the wild type construct (P<0.01 for g.-578G; P<0.001 for the other constructs). In silico analysis also demonstrated their influences on the regulatory function of NRGN gene.

CONCLUSIONS

Our study lends support to the hypothesis of multiple rare mutations in schizophrenia, and provides genetic clues that indicate the involvement of NRGN in this disorder.

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.

Effects of polychlorinated biphenyls on the development of neuronal cells in growth period; structure-activity relationship

Polychlorinated biphenyls (PCBs) are accumulated in our body through food chain and cause a variety of adverse health effects including neurotoxicities such as cognitive deficits and motor dysfunction. In particular, neonates are considered as a high risk group for the neurotoxicity of PCBs exposure. The present study attempted to analyze the structure-activity relationship among PCB congeners and the mechanism of PCBs-induced neurotoxicity. We measured total protein kinase C (PKC) activities, PKC isoforms, reactive oxygen species (ROS), and induction of neurogranin (RC-3) and growth associated protein-43 (GAP-43) mRNA in cerebellar granule cells of neonatal rats with phorbol 12, 13-dibutyrate ([(3)H]PDBu) binding assay, western blot, ROS assay, and reverse transcription PCR (RT-PCR) analysis respectively following the different structural PCBs exposure. Only non-coplanar PCBs showed a significant increase of total PKC-α and βII activity as measured with [(3)H]PDBu binding assay. ROS were more increased with non-coplanar PCBs than coplanar PCBs. The mRNA levels of RC-3 and GAP-43 were more induced with non-coplanar PCBs than coplanar PCBs, indicating that these factors may be useful biomarkers for differentiating non-coplanar PCBs from coplanar PCBs. Non-coplanar PCBs may be more potent neurotoxic congeners than coplanar PCBs. This study provides evidences that non-coplanar PCBs, which have been neglected in the risk assessment processes, should be added in the future to improve the quality and accuracy of risk assessment on the neuroendocrinal adverse effects of PCBs exposures.

Interaction of PFOS and BDE-47 co-exposure on thyroid hormone levels and TH-related gene and protein expression in developing rat brains

Perfluorooctane sulfonate (PFOS) and 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) are two persistent environmental contaminants that are toxic to developing nervous systems, particularly via their disruption of thyroid hormone (TH) function. To investigate whether an interaction existed between PFOS and BDE-47 on TH-mediated pathways, adult female Wistar rats were exposed to 3.2 and 32 mg/kg of PFOS or BDE-47 in their diet and co-exposed to a combination of each chemical (3.2 mg/kg) from gestational day 1 to postnatal day (PND) 14. Serum and brain tissues from both male and female neonates were collected on PNDs 1, 7, and 14 to examine TH-regulated gene and protein expression. The results revealed that (1) a significant accumulation difference occurred between the two chemicals; (2) On a equimolar basis, BDE-47 and PFOS affected serum total triiodothyronine and total thyroxine differently in adults and offspring; (3) there were region-specific and exposure- and time-dependent alterations in TH concentrations and tested gene and protein expression levels; and (4) interaction for the combined chemicals was only observed for brain-derived neurotrophic factor (BDNF), which exhibited a synergistic effect on PND 1 in the cortex and an antagonistic effect on PND 14 in the hippocampus. Our results suggest a complex TH-mediated gene and protein response to BDE-47 and/or PFOS exposure that seems little related to TH homeostasis and that little combined interaction of co-exposures was observed except on BDNF. The underlying mechanisms remain uncertain but seem to involve more actions than just TH-regulated pathway.

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.

Neurogranin phosphorylation fine-tunes long-term potentiation

Learning-related potentiation of synaptic strength at Cornu ammonis subfield 1 (CA1) hippocampal excitatory synapses is dependent on neuronal activity and the activation of glutamate receptors. However, molecular mechanisms that regulate and fine-tune the expression of long-term potentiation (LTP) are not well understood. Recently it has been indicated that neurogranin (Ng), a neuron-specific, postsynaptic protein that is phosphorylated by protein kinase C, potentiates synaptic transmission in an LTP-like manner. Here, we report that a Ng mutant that is unable to be phosphorylated cannot potentiate synaptic transmission in rat CA1 hippocampal neurons and results in a submaximal expression of LTP. Our results provide the first evidence that the phosphorylation of Ng can regulate LTP expression.

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.

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).

Regulation of CaMKII by phospho-Thr253 or phospho-Thr286 sensitive targeting alters cellular function

Calcium/calmodulin-stimulated protein kinase II (CaMKII) is an important mediator of synaptic function that is regulated by multi-site phosphorylation and targeting through interactions with proteins. A new phosphorylation site at Thr253 has been identified in vivo, that does not alter CaMKII activity, but does alter CaMKII function through interactions with binding proteins. To identify these proteins, as well as to examine the specific effects following Thr253 or Thr286 phosphorylation on these interactions, we developed an in vitro overlay binding assay. We demonstrated that the interaction between CaMKII and its binding proteins was altered by the phosphorylation state of both the CaMKII and the partner, and identified a CaMKII-specific sequence that was responsible for the interaction between CaMKII and two interacting proteins. By comparing CaMKII binding profiles in tissue and cell extracts, we demonstrated that the CaMKII binding profiles varied with cell type, and also showed that overexpression of a CaMKII Thr253 phospho-mimic mutant in human neuroblastoma and breast cancer cells dramatically altered the morphology and growth rates when compared to overexpression of non-phosphorylated CaMKII. This data highlights the importance of the microenvironment in regulating CaMKII function, and describes a potentially new mechanism by which the functions of CaMKII can be regulated.

Neurogranin enhances synaptic strength through its interaction with calmodulin

Learning-correlated plasticity at CA1 hippocampal excitatory synapses is dependent on neuronal activity and NMDA receptor (NMDAR) activation. However, the molecular mechanisms that transduce plasticity stimuli to postsynaptic potentiation are poorly understood. Here, we report that neurogranin (Ng), a neuron-specific and postsynaptic protein, enhances postsynaptic sensitivity and increases synaptic strength in an activity- and NMDAR-dependent manner. In addition, Ng-mediated potentiation of synaptic transmission mimics and occludes long-term potentiation (LTP). Expression of Ng mutants that lack the ability to bind to, or dissociate from, calmodulin (CaM) fails to potentiate synaptic transmission, strongly suggesting that regulated Ng-CaM binding is necessary for Ng-mediated potentiation. Moreover, knocking-down Ng blocked LTP induction. Thus, Ng-CaM interaction can provide a mechanistic link between induction and expression of postsynaptic potentiation.

Endocrine disrupting polyhalogenated organic pollutants interfere with thyroid hormone signalling in the developing brain

Persistent polyhalogenated organic pollutants are present worldwide and accumulate along the food chain. They interfere with human and animal health and are particularly harmful for pre- and perinatal neurodevelopment. The mechanisms behind the observed effects vary depending on the specific compound investigated. Co-planar polychlorinated biphenyls (PCBs) can act via the arylhydrocarbon receptor while many ortho-substituted PCBs disrupt intracellular Ca(2+) homeostasis. A common mechanism for a wide variety of PCBs is interference with thyroid hormone (TH) signalling in developing brain, by changing intracellular TH availability or by interacting directly at the level of the TH receptors. Studies on gene expression in cortex and cerebellum revealed both hypothyroid- and hyperthyroid-like effects. However, since THdependent gene expression plays a crucial role in the coordination of neuronal proliferation, migration, synaptogenesis, myelination, etc., both reduced/delayed and increased/premature expression may result in permanent structural changes in neuronal communication networks, leading to lifelong deficits in cognitive performance, motor functions, and psychobehavior. In a similar way, PCBs are able to interfere with estrogen- and androgen-dependent brain development and in some studies neurobehavioral outcome was shown to be gender-specific. Other persistent organohalogens like polychlorinated dibenzo-p-dioxins (PCDDs) and polybrominated diphenyl ethers (PBDEs) also act as endocrine disrupters in the developing brain. Several of the mechanisms involved are similar to those of PCBs, but each group also works via own specific pathways. The fact that persistent organohalogens can amplify the neurotoxic effects of other environmental pollutants, such as heavy metals, further increases their risk for human and animal neurodevelopment.

Abnormal growth of the corticospinal axons into the lumbar spinal cord of the hyt/hyt mouse with congenital hypothyroidism

Thyroid hormone deficiency may cause severe neurological disorders resulting from developmental deficits of the central nervous system. The mutant hyt/hyt mouse, characterized by fetal-onset, life-long hypothyroidism resulting from a point mutation of the thyroid-stimulating hormone receptor of the thyroid gland, displays a variety of abnormalities in motor behavior that are likely associated with dysfunctions of specific brain regions and a defective corticospinal tract (CST). To test the hypothesis that fetal and neonatal hypothyroidism cause abnormal CST development, the growth of the CST was investigated in hypothyroid hyt/hyt mice and their euthyroid progenitors, the BALB/cByJ mice. Anterograde labeling with biotinylated dextran amine demonstrated a decrease in the number of CST axons in the hyt/hyt mouse at the first lumbar level at postnatal day (P) 10. After retrograde tracing with fast blue (FB), fewer FB-labeled neurons were found in the motor cortex, the red nucleus, and the lateral vestibular nucleus of the hyt/hyt mouse. At the fourth lumbar level, the hyt/hyt mouse also showed smaller CST cross-sectional areas and significantly lower numbers of unmyelinated axons, myelinated axons, and growth cones within the CST during postnatal development. At P10, the hyt/hyt mouse demonstrated significantly lower immunoreactivity of embryonic neural cell adhesion molecule in the CST at the seventh cervical level, whereas the expression of growth-associated protein 43 remained unchanged. Our study demonstrated an abnormal development of the CST in the hyt/hyt mouse, manifested by reduced axon quantity and retarded growth pattern at the lumbar spinal cord.

Dietary retinoic acid affects song maturation and gene expression in the song system of the zebra finch

Vitamin A, an essential nutrient, is required in its acidic form (retinoic acid) for normal embryogenesis and neuronal development, typically within well-defined concentration ranges. In zebra finches, a songbird species, localized retinoic acid synthesis in the brain is important for the development of song, a learned behavior sharing significant commonalities with speech acquisition in humans. We tested how dietary retinoic acid affects the development of song behavior and the brain's system for song control. Supplemental doses of retinoic acid given to juveniles during the critical period for song learning resulted in more variable or plastic-like songs when the birds reached adulthood, compared to the normal songs of vehicle-fed controls. We also observed that several genes (brinp1, nrgn, rxr-alpha, and sdr2/scdr9) had altered levels of expression in specific nuclei of the song system when comparing the experimental and control diet groups. Interestingly, we found significant correlations between gene expression levels in nuclei of the anterior forebrain pathway (lMAN and area X) and the degree of variability in the recorded songs. We observed, however, no major morphological effects such as changes in the volumes of song nuclei. Overall, our results lend further support to a fundamental role of retinoic acid in song maturation and point to possible molecular pathways associated with this action. The data also demonstrate that dietary content of Vitamin A can affect the maturation of a naturally learned complex behavior.

Identification of genes mediating thyroid hormone action in the developing mouse cerebellum

Despite the indispensable role thyroid hormone (TH) plays in brain development, only a small number of genes have been identified to be directly regulated by TH and its precise mechanism of action remains largely unknown, partly because most of the previous studies have been carried out at postnatal day 15 or later. In the present study, we screened for TH-responsive genes in the developing mouse cerebellum at postnatal day 4 when morphological alterations because of TH status are not apparent. Among the new candidate genes selected by comparing gene expression profiles of experimentally hypothyroid, hypothyroid with postnatal thyroxine replacement, and control animals using oligoDNA microarrays, six genes were confirmed by real-time PCR to be positively (orc1l, galr3, sort1, nlgn3, cdk5r2, and zfp367) regulated by TH. Among these, sort1, cdk5r2, and zfp367 were up-regulated already at 1 h after a single injection of thyroxine to the hypothyroid or control animal, suggesting them to be possible primary targets of the hormone. Cell proliferation and apoptosis examined by BrdU incorporation and terminal deoxynucleotide transferase-mediated dUTP nick-end labeling assay revealed that hypothyroidism by itself did not enhance apoptosis at this stage, but rather increased cell survival, possibly through regulation of these newly identified genes.

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.

Molecular studies on the protective effect of nicotine in adult-onset hypothyroidism-induced impairment of long-term potentiation

We have recently shown that chronic nicotine treatment reverses hypothyroidism-induced learning and memory impairment. Chronic nicotine treatment also reverses the hypothyroidism-induced impairment of long-term potentiation (LTP). Analysis of LTP associated key signaling molecules revealed that chronic nicotine treatment prevented the hypothyroidism-induced reduction of the basal phosphotransferase activity of CaMKII and protein levels of P-CaMKII. In addition, the failure of high frequency stimulation to increase the levels of P-CaMKII in hypothyroid rats was reversed by nicotine treatment, suggesting that the neuroprotective effect of nicotine during hypothyroidism involved activation of CaMKII. Furthermore, chronic nicotine treatment reverses the hypothyroidism-induced elevated phosphatase activity and protein levels of calcineurin, a phosphatase that regulates CaMKII activation. We conclude that the neuroprotective effects of nicotine in adult-onset hypothyroidism may result from restoration of CaMKII and calcineurin activity.

Behavioral inhibition and impaired spatial learning and memory in hypothyroid mice lacking thyroid hormone receptor alpha

Thyroid hormone insufficiency leads to impaired neurogenesis, behavioral alterations and cognitive deficits. Thyroid hormone receptors, expressed in brain regions involved in these behaviors, mediate the effects of thyroid hormone deficiency or excess. To determine the contribution of thyroid hormone receptor alpha (TRalpha) in these behaviors, we examined the behavior of euthyroid as well as hypo- and hyperthyroid mice lacking all isoforms of the TRalpha (TRalpha(o/o)). The hypothyroxinemic TRalpha(o/o) mice demonstrated behavioral inhibition, manifested in decreased activity and increased anxiety/fear in the open field test (OFT) and increased immobility in the forced swim test (FST) compared to C57BL/6J mice. TRalpha(o/o) mice also showed learning and recall impairments in the Morris water maze (MWM), which were exaggerated by hypothyroidism in TRalpha(o/o) mice. These impairments were concurrent with increased thigmotaxis, suggesting an increased anxiety-like state of the TRalpha(o/o) mice in the MWM. Expression of genes, known to be involved in processes modulating learning and memory, such as glucocorticoid receptor (GR), growth-associated protein 43 (GAP-43) and neurogranin (RC3), were significantly decreased in the hippocampus of TRalpha(o/o) mice. GR expression was also decreased in the frontal cortex and amygdala of TRalpha(o/o) mice, indicating that expression of GR is regulated, probably developmentally, by one or more isoforms of TRalpha in the mouse brain. Taken together these data demonstrate behavioral alterations in the TRalpha(o/o) mice, indicating the functional role of TRalpha, and a delicate interaction between TRalpha and TRbeta-regulated genes in these behaviors. Thyroid hormone-regulated genes potentially responsible for the learning deficit found in TRalpha(o/o) mice include GR, RC3 and GAP-43.

Environmental perchlorate: why it matters

The only known mechanism of toxicity for perchlorate is interference with iodide uptake at the sodium-iodide symporter (NIS). The NIS translocates iodide across basolateral membranes to the thyroid gland so it can be used to form thyroid hormones (TH). NIS is also expressed in the mammary gland during lactation, so that iodide can be transferred from a mother to her child. Without adequate iodide, an infant cannot produce sufficient TH to meet its developmental needs. Effects expected from perchlorate are those that would be seen in conditions of hypothyroidism or hypothyroxinemia. The probability of a permanent adverse effect is greatest during early life, as successful neurodevelopment is TH-dependent. Study of perchlorate risk is complicated by a number of factors including thyroid status of the mother during gestation, thyroid status of the fetus, maternal and infant iodine intake, and exposure of each to other TH-disrupting chemicals. Perhaps the greatest standing issue, and the issue most relevant to the field of analytical chemistry, is the simple fact that human exposure has not been quantified. This review will summarize perchlorate's potential to adversely affect neurodevelopment. Whether current environmental exposures to perchlorate contribute to neuro-impairment is unknown. Risks posed by perchlorate must be considered in conjunction with iodine intake.

Type 3 deiodinase is critical for the maturation and function of the thyroid axis

Developmental exposure to appropriate levels of thyroid hormones (THs) in a timely manner is critical to normal development in vertebrates. Among the factors potentially affecting perinatal exposure of tissues to THs is type 3 deiodinase (D3). This enzyme degrades THs and is highly expressed in the pregnant uterus, placenta, and fetal and neonatal tissues. To determine the physiological role of D3, we have generated a mouse D3 knockout model (D3KO) by a targeted inactivating mutation of the Dio3 gene in mouse ES cells. Early in life, D3KO mice exhibit delayed 3,5,3'-triiodothyronine (T3) clearance, a markedly elevated serum T3 level, and overexpression of T3-inducible genes in the brain. From postnatal day 15 to adulthood, D3KO mice demonstrate central hypothyroidism, with low serum levels of 3,5,3',5'-tetraiodothyronine (T4) and T3, and modest or no increase in thyroid-stimulating hormone (TSH) concentration. Peripheral tissues are also hypothyroid. Hypothalamic T3 content is decreased while thyrotropin-releasing hormone (TRH) expression is elevated. Our results demonstrate that the lack of D3 function results in neonatal thyrotoxicosis followed later by central hypothyroidism that persists throughout life. These mice provide a new model of central hypothyroidism and reveal a critical role for D3 in the maturation and function of the thyroid axis.

Increased phosphorylation of neurogranin in the brain of hypoxic preconditioned mice

Neurogranin/RC3 (Ng/rodent cortex-enriched mRNA clone #3), a postsynaptic neuronal protein kinase C (PKC) substrate, binds calmodulin (CaM) at low Ca(2+) levels. Neurotransmitters triggering influx calcium induce neurogranin phosphorylation by PKC in physiological or pathophysiological conditions. Phosphorylated Ng reduces the affinity of Ng to bind CaM, which may affect the activities of calmodulin-dependent downstream enzymes, such as nitric oxide synthase (NOS), CaM-dependent protein kinase II (CaMKII) and adenylate cyclase (AC). These protein enzymes have been reported to play key roles in the development of ischemic/hypoxic preconditioning (I/HPC). We previously demonstrated that activation of cPKCbetaII and gamma isoforms may be involved in the early phase of cerebral hypoxic preconditioning. However, as a substrate of PKC, the role of Ng in the onset of cerebral hypoxic preconditioning is unknown. In this study, we examined the effects of repetitive hypoxic exposure on the status of Ng phosphorylation in the cortex and hippocampus of mice. Using Western blot analysis, we found that the levels of Ng phosphorylation in the cortex and hippocampus of the hypoxic group of mice increased significantly from that of the normoxic group (p<0.05). These results suggest that neurogranin protein may be involved in the development of cerebral hypoxic preconditioning.

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.

Thyroid hormone responsive protein (THRP) mediates thyroid hormone-induced cytotoxicity in primary neuronal cultures

The thyroid hormone responsive protein (THRP) is a novel gene product that remains responsive to thyroid hormone (TH) in the cerebral cortex of adult rats. To study the effects of THRP on neuronal cell survival, primary neurons cultured from rats at embryonic day 19 were treated with either 10(-7) mol L(-1) 3,5,3'-triiodothyronine (T(3)), or 10(-7) mol L(-1) L: -thyroxine (T(4)). This resulted in decreasing neuronal cell number starting 48 h after treatment. T(3) -related cytotoxicity was also documented by measurement of lactate dehydrogenase release into the medium and by propidium iodide staining. Treatment of cells with 10(-7) mol L(-1) T(3) resulted in a significant increase in THRP mRNA levels as early as 24 h of treatment in a concentration-dependent manner. T(3) treatment did not alter glyceraldehyde 3-phosphate dehydrogenase (G3PDH) mRNA levels. Exogenous expression of THRP by transfecting cells with a THRP expression construct (pSVL-THRP) was associated with a significant increase in cell death as measured by the increased number of propidium iodide staining cells (18.0+/-2.1 cells per field) compared with mock-transfected cells (3.3+/-0.2), P<0.002. To further document THRP-induced cytotoxicity, the cells were either transfected with pSVL (empty vector)+pSV2neo (neomycin resistance vector for cell labeling), pSVL-THRP+pSV2neo, or pSVL-THRP+pc-Abl (cAbl tyrosine kinase expressing vector)+pSV2neo. After 24 h the cells were treated with 500 microg mL(-1) G418 (a congener of neomycin) to eliminate the non-transfected cells. Transfection with pSVL-THRP reduced neuronal survival relative to cells transfected with pSVL (356+/-15.6 compared with 145+/-16.9, P<0.05). Co-transfection of THRP with wild-type c-Abl did not alter the effect of THRP on cell survival. It is concluded that THRP is an important factor in TH-induced neuronal cell death.

A relação entre a função tireoidiana e a depressão: uma revisão

OBJETIVO: O papel da função tireoidiana nas doenças depressivas é pouco claro. Embora existam algumas evidências de que discretas alterações tireoidianas predisponham a casos de depressão, as anormalidades específicas envolvendo a tireóide e os quadros depressivos permanecem pouco conhecidas. Serão destacados nesta revisão os principais achados envolvendo os quadros depressivos e a função tireoidiana, com especial atenção na participação das monoaminas cerebrais nesta relação. MÉTODO: Foram realizados levantamento no sistema Medline e na literatura. RESULTADOS: Existem evidências de atividade alterada do eixo hipotálamo-hipófise-tireóide (HHT) em alguns casos de depressão, que incluem: aumento dos níveis de T4, resposta alterada do TSH pós-estímulo com TRH, presença de anticorpos antitireoidianos e concentração elevada de TRH no LCR. A relação entre estas anormalidades, as principais monoaminas cerebrais e os subtipos de quadros depressivos é complexa e ainda não permite o estabelecimento de hipóteses diretas de compreensão. CONCLUSÕES: Após anos de pesquisas, permanece pouco esclarecida a importância da relação entre o eixo HHT e as depressões, assim como os mecanismos subjacentes às alterações tireoidianas encontradas nos pacientes deprimidos. Portanto, mais pesquisas serão necessárias para uma melhor compreensão do papel do eixo HHT na patogênese e no tratamento dos quadros depressivos.

Control of thyroid hormone action in the developing rat brain

Thyroid hormones play important roles in brain development. The physiologic function of thyroid hormones in the developing brain is to provide a timing signal that leads to the induction of differentiation and maturation programs during precise stages of development. Inappropriate initiation of these timing events leads to asynchrony in developmental processes and a deleterious outcome. The developing brain is protected from premature thyroid hormone signaling through a variety of measures. Firstly, local brain levels of both thyroxine and triiodothyronine are controlled by ontogenically regulated patterns of production and metabolism. Secondly, developmentally regulated expression of nuclear proteins involved with the nuclear TH response apparatus control the temporal response of brain genes to thyroid hormone. Finally, developmental regulation of TH action modulating transcription factor expression also controls TH action in the developing brain. Together these molecular mechanisms cooperatively act to temporally control TH action during brain development. A description of these controlling mechanisms is the subject of this review.

Action of thyroid hormone in brain

Among the most critical actions of thyroid hormone in man and other mammals are those exerted on brain development. Severe hypothyroidism during the neonatal period leads to structural alterations, including hypomyelination and defects of cell migration and differentiation, with long-lasting, irreversible effects on behavior and performance. A complex regulatory mechanism operates in brain involving regulation of the concentration of the active hormone, T3, and the control of gene expression. Most brain T3 is formed locally from its precursor, T4, by the action of type II deiodinase which is expressed in glial cells, tanycytes, and astrocytes. Type III deiodinase (DIII) is also involved in the regulation of T3 concentrations, especially during the embryonic and early post-natal periods. DIII is expressed in neurons and degrades T4 and T3 to inactive metabolites. The action of T3 is mediated through nuclear receptors, which are expressed mainly in neurons. The receptors are ligand-modulated transcription factors, and a number of genes have been identified as regulated by thyroid hormone in brain. The regulated genes encode proteins of myelin, mitochondria, neurotrophins and their receptors, cytoskeleton, transcription factors, splicing regulators, cell matrix proteins, adhesion molecules, and proteins involved in intracellular signaling pathways. The role of thyroid hormone is to accelerate changes of gene expression that take place during development. Surprisingly, null-mutant mice for the T3 receptors show almost no signs of central nervous system involvement, in contrast with the severe effects of hypothyroidism. The resolution of this paradox is essential to understand the role of thyroid hormone and its receptors in brain development and function.

Developmental exposure to polychlorinated biphenyls exerts thyroid hormone-like effects on the expression of RC3/neurogranin and myelin basic protein messenger ribonucleic acids in the developing rat brain

Polychlorinated biphenyls (PCBs) are a class of industrial compounds consisting of paired phenyl rings with various degrees of chlorination. They are now ubiquitous, persistent environmental contaminants that are routinely found in samples of human and animal tissues and are known to affect brain development. The effects of PCBs on brain development may be attributable, at least in part, to their ability to reduce circulating levels of thyroid hormone. However, the developmental effects of PCB exposure are not fully consistent with hypothyroidism. Because some individual PCB congeners interact strongly with various thyroid hormone binding proteins, several investigators have speculated that these congeners may be producing thyroid hormone-like effects on brain development. Therefore, we tested whether a mixture of PCBs, Aroclor 1254 (A1254), would produce an antithyroid or thyromimetic effect on the expression of known thyroid hormone-responsive genes in the developing brain. Pregnant female rats were fed various doses of A1254 (0, 1, 4, and 8 mg/kg) from gestational day 6 to weaning on postnatal day (P) 21. Pups derived from these dams were sampled on P5, P15, and P30. Total T4 was reduced by A1254 in a dose-dependent manner, but body weight of the pups or dams was not affected. The expression of RC3/Neurogranin and myelin basic protein was not affected by A1254 on P5 or P30. However, on P15, RC3/Neurogranin was elevated by A1254 in a dose-dependent manner, and myelin basic protein expression followed this general pattern. These data clearly demonstrate that the developmental effects of PCB exposure are not simply a function of PCB-induced hypothyroidism.

Identification of a mammalian homologue of the fungal Tom70 mitochondrial precursor protein import receptor as a thyroid hormone-regulated gene in specific brain regions

Thyroid hormone is an important regulator of mammalian brain maturation. By differential display PCR, we isolated a cDNA clone (S2) that is specifically up-regulated in the striatum of neonatal hypothyroid rats. S2 was identified as KIAA0719, the first human gene distantly homologous to the fungal Tom70, which encodes a member of the translocase mitochondrial outer membrane complex involved in the import of preproteins into the mitochondria. By northern and in situ hybridization studies, KIAA0719 was found to be up-regulated in the striatum, nucleus accumbens, and discrete cortical layers of 15-day-old hypothyroid rats. In contrast, lower expression was found in the olfactory tubercle, whereas no differences were detected in other brain regions. Significantly, treatment of hypothyroid animals with single injections of thyroxine restored the normal levels of KIAA0719 expression. Moreover, treatment of control animals with thyroxine led to a reduced expression, demonstrating a negative hormonal regulation in vivo. Thus, KIAA0719 gene expression is regulated by thyroid hormone in the neonatal rat brain in a region-specific fashion. Given the role of the homologous Tom70 gene, the alteration of KIAA0719 expression may contribute to the changes in mitochondrial morphology and physiology caused by hypothyroidism in the developing rat brain.

Variation in hippocampal dynorphin b-immunoreactive mossy fiber terminal fields of apomorphine-(un)susceptible rats

The size of distinct hippocampal sub-fields were measured in the apomorphine-susceptible and apomorphine-unsusceptible rat lines. Mossy fiber terminal fields were delineated using dynorphin B immunoreactivity and area measurements were taken from (1) the supra-pyramidal mossy fiber terminal field; (2) the intra- and infra-pyramidal mossy fiber terminal field; (3) the hilus of the fascia dentata (4) the non dynorphin B immunoreactive area of the regio inferior and fascia dentata and (5) the total area of regio inferior and fascia dentata. The data indicate that statistically significant differences in the morphometry of the hippocampal subfields of the apomorphine susceptible and unsusceptible rats are confined to the intra- and infra terminal field: the relative size of the left and right intra- and infra terminal field of apomorphine unsusceptible rats are significantly larger than those of the apomorphine susceptible rats. These data explain at least in part the differential response of these rats to novelty.

The human RC3 gene homolog, NRGN contains a thyroid hormone-responsive element located in the first intron

NRGN is the human homolog of the neuron-specific rat RC3/neurogranin gene. This gene encodes a postsynaptic 78-amino acid protein kinase substrate that binds calmodulin in the absence of calcium, and that has been implicated in dendritic spine formation and synaptic plasticity. In the rat brain RC3 is under thyroid hormone control in specific neuronal subsets in both developing and adult animals. To evaluate whether the human gene is also a target of thyroid hormone we have searched for T3-responsive elements in NRGN cloned genomic fragments spanning the whole gene. Labeled DNA fragments were incubated with T3 receptors (T3R) and 9-cis-retinoic acid receptors and immunoprecipitated using an anti T3R antibody. A receptor-binding site was localized in the first intron, 3000 bp downstream from the origin of transcription. Footprinting analysis revealed the sequence GGATTAAATGAGGTAA, closely related to the consensus T3-responsive element of the direct repeat (DR4) type. This sequence binds the T3R-9-cis-retinoic acid receptors heterodimers, but not T3R monomers or homodimers, and is able to confer regulation by T3R and T3 when fused upstream of the NRGN or thymidine kinase promoters. The data reported in this work suggest that NRGN is a direct target of thyroid hormone in human brain, and that control of expression of this gene could underlay many of the consequences ofhypothyroidism on mental states during development as well as in adult subjects.

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

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

Cloning and characterization of a complementary DNA for a thyroid hormone-responsive protein in mature rat cerebral tissue

A gene responsive to thyroid hormone (TH) has been identified in the adult rat brain cerebral tissue. A cDNA probe differentially expressed in euthyroid, hypothyroid and hyperthyroid rat cerebral tissue, generated by reverse transcriptase-PCR differential display of mRNA, was used to screen the rat brain cDNA library. A 3.4 kb positive clone hybridized in Northern blots with a 3.8 kb mRNA that proved to be TH responsive (THR). The remaining coding sequence and a part of the 5' untranslated region of this cDNA were obtained by 5' rapid amplification of cDNA ends. The deduced amino acid sequence revealed that THR protein (THRP), a 68 kDa moiety, has 83% sequence similarity with c-Abl interactor protein (Abi-2), which is a substrate for tyrosine kinase activity of c-Abl. The extensive similarity between the two proteins suggests a potential role for THRP as a substrate for c-Abl. Northern analysis showed that the expression of THR mRNA in hyperthyroid rats is 6-fold that in euthyroid rats. There is also a 4-6-fold increase in the concentration of THRP, as analysed by Western analysis. Owing to the extensive similarity between rat THRP and human Abi-2, a polyclonal anti- (human Abi-2) antibody was successfully used for Western analysis of proteins from the rat tissues. The observed increase in both the mRNA and the protein did not decline after beta-adrenergic system blockade with propranolol, suggesting that the action of TH on the expression of this gene is not mediated through the beta-adrenergic system. Immunohistochemical studies revealed that neuronal cells were particularly rich in THRP. Both THR mRNA and THRP are rapidly induced in vivo after intravenous administration of thyroxine. Tissue distribution studies indicated that the cerebral tissue was particularly enriched with THR mRNA and 68 kDa THRP. A cDNA clone for a THR gene could provide a useful tool to study the molecular mechanisms of TH effects on cerebral tissue in adult animals.