Characterization of myelin basic protein thyroid hormone response element and its function in the context of native and heterologous promoter

Thyroid, neurodevelopmental, and kidney toxicities of common organic UV filters in embryo-larval zebrafish (Danio rerio), and their potential links

Organic UV filters (OUVFs) have been commonly used in sunscreen and many consumer products. Following dermal application, these compounds can enter circulation and may cause systemic effects in humans. In the present study, we chose four OUVFs frequently detected in the environment, i.e., avobenzone (AVB), benzophenone-3 (BP-3), octocrylene (OC), and octyl methoxycinnamate (OMC), and evaluated their thyroid, neurodevelopmental, and kidney toxicities. For this purpose, zebrafish embryos (<4 h post fertilization, hpf) were exposed to sublethal concentrations of AVB, BP-3, OC, or OMC until 120 hpf. Exposure to all OUVFs decreased thyroid hormone (TH) levels, probably by enhanced metabolism and excretion of THs (ugt1ab and/or sult1 st5) in the larval fish. Exposure to the OUVFs also induced hypoactivities and/or anxiety-like behaviors: Regulatory changes of mbp, gfap, c-fos, syn2a, sty1a, and stxbp1b genes, support the changes in normal neurobehavior of the larval fish. Moreover, the OUVFs exposure caused increased proteinuria in the fish, along with transcriptional changes of wt1, nephrin, podocin, and cdh17 genes, which could explain the observed reduction in kidney functions. Principal component analysis (PCA) implied the potential interplay of THs with neurogenesis, or podocyte differentiation of the larval fish. Toxicological consequences of altered TH homeostasis, neurobehavior, and kidney function at the early life stage warrant further investigations not only in humans but also in aquatic ecosystems.

Targeting central nervous system extracellular vesicles enhanced triiodothyronine remyelination effect on experimental autoimmune encephalomyelitis

The lack of targeted and high-efficiency drug delivery to the central nervous system (CNS) nidus is the main problem in the treatment of demyelinating disease. Extracellular vesicles (EVs) possess great promise as a drug delivery vector given their advanced features. However, clinical applications are limited because of their inadequate targeting ability and the “dilution effects” after systemic administration. Neural stem cells (NSCs) supply a plentiful source of EVs on account of their extraordinary capacity for self-renewal. Here, we have developed a novel therapeutic system using EVs from modified NSCs with high expressed ligand PDGF-A (EVPs) and achieve local delivery. It has been demonstrated that EVPs greatly enhance the target capability on oligodendrocyte lineage. Moreover, EVPs are used for embedding triiodothyronine (T3), a thyroid hormone that is critical for oligodendrocyte development but has serious side effects when systemically administered. Our results demonstrated that systemic injection of EVPs + T3, versus EVPs or T3 administration individually, markedly alleviated disease development, enhanced oligodendrocyte survival, inhibited myelin damage, and promoted myelin regeneration in the lesions of experimental autoimmune encephalomyelitis mice. Taken together, our findings showed that engineered EVPs possess a remarkable CNS lesion targeting potential that offers a potent therapeutic strategy for CNS demyelinating diseases as well as neuroinflammation.

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NSC-derived EV-PDGFA dramatically increased targeting efficiency to the lineage of OLGs and the demyelinated area in the CNS.

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EVPs-T3 exert the therapeutic ability in the lesion suppressed the disease development and protected myelin loss.

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EVPs-T3 increased numbers of OLGs in the lesion and TEM data evidenced that EVPs-T3 promotes myelin regeneration in vivo.

Thyroid hormone deficiency during zebrafish development impairs central nervous system myelination

Thyroid hormones are messengers that bind to specific nuclear receptors and regulate a wide range of physiological processes in the early stages of vertebrate embryonic development, including neurodevelopment and myelogenesis. We here tested the effects of reduced T3 availability upon the myelination process by treating zebrafish embryos with low concentrations of iopanoic acid (IOP) to block T4 to T3 conversion. Black Gold II staining showed that T3 deficiency reduced the myelin density in the forebrain, midbrain, hindbrain and the spinal cord at 3 and 7 dpf. These observations were confirmed in 3 dpf mbp:egfp transgenic zebrafish, showing that the administration of IOP reduced the fluorescent signal in the brain. T3 rescue treatment restored brain myelination and reversed the changes in myelin-related gene expression induced by IOP exposure. NG2 immunostaining revealed that T3 deficiency reduced the amount of oligodendrocyte precursor cells in 3 dpf IOP-treated larvae. Altogether, the present results show that inhibition of T4 to T3 conversion results in hypomyelination, suggesting that THs are part of the key signaling molecules that control the timing of oligodendrocyte differentiation and myelin synthesis from very early stages of brain development.

Development of cerebral mitochondrial respiratory function is impaired by thyroid hormone deficiency before birth in a region-specific manner

Thyroid hormones regulate adult metabolism partly through actions on mitochondrial oxidative phosphorylation (OXPHOS). They also affect neurological development of the brain, but their role in cerebral OXPHOS before birth remains largely unknown, despite the increase in cerebral energy demand during the neonatal period. Thus, this study examined prepartum development of cerebral OXPHOS in hypothyroid fetal sheep. Using respirometry, Complex I (CI), Complex II (CII), and combined CI&CII OXPHOS capacity were measured in the fetal cerebellum and cortex at 128 and 142 days of gestational age (dGA) after surgical thyroidectomy or sham operation at 105 dGA (term ~145 dGA). Mitochondrial electron transfer system (ETS) complexes, mRNA transcripts related to mitochondrial biogenesis and ATP production, and mitochondrial density were quantified using molecular techniques. Cerebral morphology was assessed by immunohistochemistry and stereology. In the cortex, hypothyroidism reduced CI-linked respiration and CI abundance at 128 dGA and 142 dGA, respectively, and caused upregulation of PGC1α (regulator of mitochondrial biogenesis) and thyroid hormone receptor β at 128 dGA and 142 dGA, respectively. In contrast, in the cerebellum, hypothyroidism reduced CI&II- and CII-linked respiration at 128 dGA, with no significant effect on the ETS complexes. In addition, cerebellar glucocorticoid hormone receptor and adenine nucleotide translocase (ANT1) were downregulated at 128 dGA and 142 dGA, respectively. These alterations in mitochondrial function were accompanied by reduced myelination. The findings demonstrate the importance of thyroid hormones in the prepartum maturation of cerebral mitochondria and have implications for the etiology and treatment of the neurodevelopmental abnormalities associated with human prematurity and congenital hypothyroidism.

The Current Challenges for Drug Discovery in CNS Remyelination

The myelin sheath wraps around axons, allowing saltatory currents to be transmitted along neurons. Several genetic, viral, or environmental factors can damage the central nervous system (CNS) myelin sheath during life. Unless the myelin sheath is repaired, these insults will lead to neurodegeneration. Remyelination occurs spontaneously upon myelin injury in healthy individuals but can fail in several demyelination pathologies or as a consequence of aging. Thus, pharmacological intervention that promotes CNS remyelination could have a major impact on patient’s lives by delaying or even preventing neurodegeneration. Drugs promoting CNS remyelination in animal models have been identified recently, mostly as a result of repurposing phenotypical screening campaigns that used novel oligodendrocyte cellular models. Although none of these have as yet arrived in the clinic, promising candidates are on the way. Many questions remain. Among the most relevant is the question if there is a time window when remyelination drugs should be administrated and why adult remyelination fails in many neurodegenerative pathologies. Moreover, a significant challenge in the field is how to reconstitute the oligodendrocyte/axon interaction environment representative of healthy as well as disease microenvironments in drug screening campaigns, so that drugs can be screened in the most appropriate disease-relevant conditions. Here we will provide an overview of how the field of in vitro models developed over recent years and recent biological findings about how oligodendrocytes mature after reactivation of their staminal niche. These data have posed novel questions and opened new views about how the adult brain is repaired after myelin injury and we will discuss how these new findings might change future drug screening campaigns for CNS regenerative drugs.

Hormonal Regulation of Oligodendrogenesis I: Effects across the Lifespan

The brain’s capacity to respond to changing environments via hormonal signaling is critical to fine-tuned function. An emerging body of literature highlights a role for myelin plasticity as a prominent type of experience-dependent plasticity in the adult brain. Myelin plasticity is driven by oligodendrocytes (OLs) and their precursor cells (OPCs). OPC differentiation regulates the trajectory of myelin production throughout development, and importantly, OPCs maintain the ability to proliferate and generate new OLs throughout adulthood. The process of oligodendrogenesis, the creation of new OLs, can be dramatically influenced during early development and in adulthood by internal and environmental conditions such as hormones. Here, we review the current literature describing hormonal regulation of oligodendrogenesis within physiological conditions, focusing on several classes of hormones: steroid, peptide, and thyroid hormones. We discuss hormonal regulation at each stage of oligodendrogenesis and describe mechanisms of action, where known. Overall, the majority of hormones enhance oligodendrogenesis, increasing OPC differentiation and inducing maturation and myelin production in OLs. The mechanisms underlying these processes vary for each hormone but may ultimately converge upon common signaling pathways, mediated by specific receptors expressed across the OL lineage. However, not all of the mechanisms have been fully elucidated, and here, we note the remaining gaps in the literature, including the complex interactions between hormonal systems and with the immune system. In the companion manuscript in this issue, we discuss the implications of hormonal regulation of oligodendrogenesis for neurological and psychiatric disorders characterized by white matter loss. Ultimately, a better understanding of the fundamental mechanisms of hormonal regulation of oligodendrogenesis across the entire lifespan, especially in vivo, will progress both basic and translational research.

Developmental gene expression patterns in the brain and liver of Xenopus tropicalis during metamorphosis climax

Thyroid hormones (THs) induce metamorphosis in amphibians, causing dynamic changes, whereas mammalian newborns undergo environmental transition from placenta to open air at birth. The similarity between amphibian metamorphosis and the mammalian perinatal periods has been repeatedly discussed. However, a corresponding developmental gene expression analysis has not yet been reported. In this study, we examined the developmental gene expression profiles in the brain and liver of Xenopus tropicalis during metamorphosis climax and compared them to the respective gene expression profiles of newborn rodents. Many upregulated genes identified in the tadpole brain during metamorphosis are also upregulated in the rodent brain during the first three postnatal weeks when the TH surge occurs. The upregulation of some genes in the brain was inhibited in thyroid hormone receptor α (TRα) knockout tadpoles but not in TRβ-knockout tadpoles, implying that brain metamorphosis is mainly mediated by TRα. The expression of some genes was also increased in the liver during metamorphosis climax. Our data suggest that the rodent brain undergoes TH-dependent remodeling during the first three postnatal weeks as observed in X. tropicalis during the larva-to-adult metamorphosis.

Enteric Microbiota⁻Gut⁻Brain Axis from the Perspective of Nuclear Receptors

Nuclear receptors (NRs) play a key role in regulating virtually all body functions, thus maintaining a healthy operating body with all its complex systems. Recently, gut microbiota emerged as major factor contributing to the health of the whole organism. Enteric bacteria have multiple ways to influence their host and several of them involve communication with the brain. Mounting evidence of cooperation between gut flora and NRs is already available. However, the full potential of the microbiota interconnection with NRs remains to be uncovered. Herewith, we present the current state of knowledge on the multifaceted roles of NRs in the enteric microbiota–gut–brain axis.

Influence of maternal thyroid hormones during gestation on fetal brain development

Thyroid hormones (THs) play an obligatory role in many fundamental processes underlying brain development and maturation. The developing embryo/fetus is dependent on maternal supply of TH. The fetal thyroid gland does not commence TH synthesis until mid gestation, and the adverse consequences of severe maternal TH deficiency on offspring neurodevelopment are well established. Recent evidence suggests that even more moderate forms of maternal thyroid dysfunction, particularly during early gestation, may have a long-lasting influence on child cognitive development and risk of neurodevelopmental disorders. Moreover, these observed alterations appear to be largely irreversible after birth. It is, therefore, important to gain a better understanding of the role of maternal thyroid dysfunction on offspring neurodevelopment in terms of the nature, magnitude, time-specificity, and context-specificity of its effects. With respect to the issue of context specificity, it is possible that maternal stress and stress-related biological processes during pregnancy may modulate maternal thyroid function. The possibility of an interaction between the thyroid and stress systems in the context of fetal brain development has, however, not been addressed to date. We begin this review with a brief overview of TH biology during pregnancy and a summary of the literature on its effect on the developing brain. Next, we consider and discuss whether and how processes related to maternal stress and stress biology may interact with and modify the effects of maternal thyroid function on offspring brain development. We synthesize several research areas and identify important knowledge gaps that may warrant further study. The scientific and public health relevance of this review relates to achieving a better understanding of the timing, mechanisms and contexts of thyroid programing of brain development, with implications for early identification of risk, primary prevention and intervention.

Thyroid Hormone Signaling in Oligodendrocytes: from Extracellular Transport to Intracellular Signal

Thyroid hormone plays an important role in central nervous system (CNS) development, including the myelination of variable axonal calibers. It is well-established that thyroid hormone is required for the terminal differentiation of oligodendrocyte precursor cells (OPCs) into myelinating oligodendrocytes by inducing rapid cell-cycle arrest and constant transcription of pro-differentiation genes. This is well supported by the hypomyelinating phenotypes exhibited by patients with congenital hypothyroidism, cretinism. During development, myelinating oligodendrocytes only appear after the formation of neural circuits, indicating that the timing of oligodendrocyte differentiation is important. Since fetal and post-natal serum thyroid hormone levels peak at the stage of active myelination, it is suspected that the timing of oligodendrocyte development is finely controlled by thyroid hormone. The essential machinery for thyroid hormone signaling such as deiodinase activity (utilized by cells to auto-regulate the level of thyroid hormone), and nuclear thyroid hormone receptors (for gene transcription) are expressed on oligodendrocytes. In this review, we discuss the known and potential thyroid hormone signaling pathways that may regulate oligodendrocyte development and CNS myelination. Moreover, we evaluate the potential of targeting thyroid hormone signaling for white matter injury or disease.

Histone deacetylase inhibition reduces hypothyroidism-induced neurodevelopmental defects in rats

Thyroid hormone (TH) through its receptor (TRα/β) influences spatio-temporal regulation of its target gene repertoire during brain development. Though hypothyroidism in WT rodent models of perinatal hypothyroidism severely impairs neurodevelopment, its effect on TRα/β knockout mice is less severe. An explanation to this paradox is attributed to a possible repressive action of unliganded TRs during development. Since unliganded TRs suppress gene expression through the recruitment of histone deacetylase (HDACs) via co-repressor complexes, we tested whether pharmacological inhibition of HDACs may prevent the effects of hypothyroidism on brain development. Using valproate, an HDAC inhibitor, we show that HDAC inhibition significantly blocks the deleterious effects of hypothyroidism on rat cerebellum, evident by recovery of TH target genes like Bdnf, Pcp2 and Mbp as well as improved dendritic structure of cerebellar Purkinje neurons. Together with this, HDAC inhibition also rescues hypothyroidism-induced motor and cognitive defects. This study therefore provides an insight into the role of HDACs in TH insufficiency during neurodevelopment and their inhibition as a possible therapeutics for treatment.

Transferrin and thyroid hormone converge in the control of myelinogenesis

Myelination is a concerted mechanism tightly regulated in the brain. Although several factors are known to participate during this process, the complete sequence of events is far from being fully elucidated. Separate effects of apotransferrin (aTf) and thyroid hormone (TH) are well documented on rat myelin formation. TH promotes the maturation of oligodendrocyte progenitors (OPCs) into myelinating oligodendrocytes (OLGs), while aTf is able to induce the commitment of neural stem cells (NSCs) toward the oligodendroglial linage and favors OLG maturation. We have also demonstrated that Tf mRNA exhibited a seven-fold increase in hyperthyroid animals. These observations have led us to hypothesize that both factors may interplay during oligodendrogenesis. To assess the combined effects of aTf and TH on proper myelination in the rat brain, Tf expression and oligodendroglial maturation were evaluated at postnatal days 10 (P10) and 20 (P20) in several experimental groups. At P10, an up-regulation of both Tf mRNA and protein, as well as myelination, was found in hyperthyroid animals, while a decrease in Tf mRNA levels and myelin formation was detected in the hypothyroid group. At P20, no differences were found either in Tf mRNA or protein levels between hyperthyroid and control (Ctrol) rats, although differences in OLG differentiation remained. Also at P20, hypothyroid animals showed decreased Tf mRNA and protein levels accompanied with a less mature myelinating phenotype. Moreover, TH and aTf differentially regulate the expression of KLF9 transcription factor as well as TRα and TRβ at P10 and P20. Our results suggest that TH is necessary early in OLG development for aTf action, as exogenous aTf administration was unable to counteract the effect of low TH levels in the hypothyroid state in all the time points analyzed. Furthermore, the fact that hyperthyroidism induced an increase in Tf expression and aTf-dependent regulation of TRα strongly suggests that Tf could be involved in some of TH later effects on OLG maturation. Here we describe the possible relationship between TH and aTf and its implication in oligodendrogenesis.

Role of thyroid hormones in different aspects of nervous system regeneration in vertebrates

Spontaneous functional recovery from injury in the adult human nervous system is rare and trying to improve recovery remains a clinical challenge. Nervous system regeneration is a complicated sequence of events involving cell death or survival, cell proliferation, axon extension and remyelination, and finally reinnervation and functional recovery. Successful recovery depends on the cell-specific and time-dependent activation and repression of a wide variety of growth factors and guidance molecules. Thyroid hormones (THs), well known for their regulatory role in neurodevelopment, have recently emerged as important modulators of neuroregeneration. This review focuses on the endogenous changes in the proteins regulating TH availability and action in different cell types of the adult mammalian nervous system during regeneration as well as the impact of TH supplementation on the consecutive steps in this process. It also addresses possible differences in TH involvement between different vertebrate classes, early or late developmental stages and peripheral or central nervous system. The available data show that THs are able to stimulate many signaling pathways necessary for successful neurogeneration. They however also suggest that supplementation with T4 and/or T3 may have beneficial or detrimental influences depending on the dose and more importantly on the specific phase of the regeneration process.

A selective thyroid hormone β receptor agonist enhances human and rodent oligodendrocyte differentiation

Nerve conduction within the mammalian central nervous system is made efficient by oligodendrocyte-derived myelin. Historically, thyroid hormones have a well described role in regulating oligodendrocyte differentiation and myelination during development; however, it remains unclear which thyroid hormone receptors are required to drive these effects. This is a question with clinical relevance since nonspecific thyroid receptor stimulation can produce deleterious side-effects. Here we report that GC-1, a thyromimetic with selective thyroid receptor β action and a potentially limited side-effect profile, promotes in vitro oligodendrogenesis from both rodent and human oligodendrocyte progenitor cells. In addition, we used in vivo genetic fate tracing of oligodendrocyte progenitor cells via PDGFαR-CreER;Rosa26-eYFP double-transgenic mice to examine the effect of GC-1 on cellular fate and find that treatment with GC-1 during developmental myelination promotes oligodendrogenesis within the corpus callosum, occipital cortex and optic nerve. GC-1 was also observed to enhance the expression of the myelin proteins MBP, CNP and MAG within the same regions. These results indicate that a β receptor selective thyromimetic can enhance oligodendrocyte differentiation in vitro and during developmental myelination in vivo and warrants further study as a therapeutic agent for demyelinating models.

RXRs: collegial partners

Retinoid X Receptors (RXR) were initially identified as nuclear receptors binding with stereo-selectivity the vitamin A derivative 9-cis retinoic acid, although the relevance of this molecule as endogenous activator of RXRs is still elusive. Importantly, within the nuclear receptor superfamily, RXRs occupy a peculiar place, as they are obligatory partners for a number of other nuclear receptors, thus integrating the corresponding signaling pathways. In this chapter, we describe the structural features allowing RXR to form homo- and heterodimers, and the functional consequences of this unique ability. Furthermore, we discuss the importance of studying RXR activity at a genome-wide level in order to comprehensively address the biological implications of their action that is fundamental to understand to what extent RXRs could be exploited as new therapeutic targets.

Identification of thyroid hormone receptor binding sites in developing mouse cerebellum

Background

Thyroid hormones play an essential role in early vertebrate development as well as other key processes. One of its modes of action is to bind to the thyroid hormone receptor (TR) which, in turn, binds to thyroid response elements (TREs) in promoter regions of target genes. The sequence motif for TREs remains largely undefined as does the precise chromosomal location of the TR binding sites. A chromatin immunoprecipitation on microarray (ChIP-chip) experiment was conducted using mouse cerebellum post natal day (PND) 4 and PND15 for the thyroid hormone receptor (TR) beta 1 to map its binding sites on over 5000 gene promoter regions. We have performed a detailed computational analysis of these data.

Results

By analysing a recent spike-in study, the optimal normalization and peak identification approaches were determined for our dataset. Application of these techniques led to the identification of 211 ChIP-chip peaks enriched for TR binding in cerebellum samples. ChIP-PCR validation of 25 peaks led to the identification of 16 true positive TREs. Following a detailed literature review to identify all known mouse TREs, a position weight matrix (PWM) was created representing the classic TRE sequence motif. Various classes of promoter regions were investigated for the presence of this PWM, including permuted sequences, randomly selected promoter sequences, and genes known to be regulated by TH. We found that while the occurrence of the TRE motif is strongly correlated with gene regulation by TH for some genes, other TH-regulated genes do not exhibit an increased density of TRE half-site motifs. Furthermore, we demonstrate that an increase in the rate of occurrence of the half-site motifs does not always indicate the specific location of the TRE within the promoter region. To account for the fact that TR often operates as a dimer, we introduce a novel dual-threshold PWM scanning approach for identifying TREs with a true positive rate of 0.73 and a false positive rate of 0.2. Application of this approach to ChIP-chip peak regions revealed the presence of 85 putative TREs suitable for further in vitro validation.

Conclusions

This study further elucidates TRβ gene regulation in mouse cerebellum, with 211 promoter regions identified to bind to TR. While we have identified 85 putative TREs within these regions, future work will study other mechanisms of action that may mediate the remaining observed TR-binding activity.

mTOR: a link from the extracellular milieu to transcriptional regulation of oligodendrocyte development

Oligodendrocyte development is controlled by numerous extracellular signals that regulate a series of transcription factors that promote the differentiation of oligodendrocyte progenitor cells to myelinating cells in the central nervous system. A major element of this regulatory system that has only recently been studied is the intracellular signalling from surface receptors to transcription factors to down-regulate inhibitors and up-regulate inducers of oligodendrocyte differentiation and myelination. The current review focuses on one such pathway: the mTOR (mammalian target of rapamycin) pathway, which integrates signals in many cell systems and induces cell responses including cell proliferation and cell differentiation. This review describes the known functions of mTOR as they relate to oligodendrocyte development, and its recently discovered impact on oligodendrocyte differentiation and myelination. A potential model for its role in oligodendrocyte development is proposed.

Biological significance of a thyroid hormone-regulated secretome

The thyroid hormone, 3,3,5-triiodo-L-thyronine (T3), modulates several physiological processes, including cellular growth, differentiation, metabolism and proliferation, via interactions with thyroid hormone response elements (TREs) in the regulatory regions of target genes. Several intracellular and extracellular protein candidates are regulated by T3. Moreover, T3-regulated secreted proteins participate in physiological processes or cellular transformation. T3 has been employed as a marker in several disorders, such as cardiovascular disorder in chronic kidney disease, as well as diseases of the liver, immune system, endocrine hormone metabolism and coronary artery. Our group subsequently showed that T3 regulates several tumor-related secretory proteins, leading to cancer progression via alterations in extracellular matrix proteases and tumor-associated signaling pathways in hepatocellular carcinomas. Therefore, elucidation of T3/thyroid hormone receptor-regulated secretory proteins and their underlying mechanisms in cancers should facilitate the identification of novel therapeutic targets. This review provides a detailed summary on the known secretory proteins regulated by T3 and their physiological significance. This article is part of a Special Issue entitled: An Updated Secretome.

Revisiting thyroid hormones in schizophrenia

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

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

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

Effect of hypothyroxinemia on thyroid hormone responsiveness and action during rat postnatal neocortical development

Neurological deficits due to maternal and neonatal hypothyroxinemia under mild-moderate iodine deficiency are a major preventable health problem worldwide. The present study assesses the impact of hypothyroxinemia on postnatal neocortical development and also compares it to the known effects of severe hypothyroidism. Our results strongly suggest that even within elevated circulating triiodothyronine (T3) levels, hypothyroxinemia significantly impairs thyroid hormone responsiveness in developing rat neocortex. The significant compensatory alteration in deiodinase levels with unaltered monocarboxylate transporter 8 (MCT8) and thyroid hormone receptors (TRs), although found to be similar in hypothyroxinemic and hypothyroid condition, is more pronounced under later condition. The resultant downregulation of nuclear myelin binding protein (MBP) and mitochondrial transcripts Cytochrome oxidase III (Cox III) as well as significantly enhanced mitochondrial localization of Bax and reduced Bcl-2 and Bcl-xL accompanied by enhanced release of Cytochrome c and Smac with activation of caspase-3 indicates pronounced apoptosis leading to compromised cellular survival. The similarities of this responsiveness albeit with difference in degree under hypothyroidism and hypothyroxinemic state with adequate availability of T3 are suggestive of an independent role of thyroxine in neocortex development. Taken together, this study brings forth the neurophysiological aspects of hypothyroxinemia and underscores the importance of adequate iodine nutrition along with mandatory thyroxin monitoring during pregnancy and after birth.

Developmental PCB exposure induces hypothyroxinemia and sex-specific effects on cerebellum glial protein levels in rats

Polychlorinated biphenyls (PCBs) are persistent lipophilic environmental contaminants which are found in fatty tissues of humans and wild-life alike. Maternal transfer of PCBs to offspring is easily achieved across the placenta and via lactation. In male rats, perinatal PCB exposure induces behavioral abnormalities, in addition to hypothyroxinemia and white matter changes. There are sex differences in white matter volume synthesis and density in adult and aged rodents. Yet whether PCB exposure effects on white matter are sex-specific is unclear, because the previous studies were conducted in male offspring. Furthermore, although hypothyroxinemia induced by PCB exposure is thought to trigger white matter changes, PCBs also affect interleukin-6 (IL-6) expression, and IL-6 regulates white matter growth. We hypothesized that perinatal PCB exposure would have sex-specific effects on white matter development associated with altered IL-6 levels. We found that female offspring had higher levels of myelin basic protein (MBP) than males did, at postnatal day (PND) 7, 18 and 21. PCB exposure induced hypothyroxinemia in males and females at PND7, 14, 21, and 42. PCB exposure also increased MBP and reduced glial fibrillary acidic protein (GFAP) levels in males at PND21, but had the opposite effect in females. In addition, at PND14 and 21, PCB exposure elevated IL-6 levels in male offspring only. The induction of sex-specific changes in white matter proteins, in the absence of sex differences in thyroxine levels after PCB exposure, suggests that serum thyroxine levels do not directly contribute to the white matter alterations. Instead, IL-6 may contribute to increased MBP levels in males, whereas in females estromimetic and thyromimetic PCB metabolites may affect white matter development. This data adds to an increasing body of literature showing that perinatal insults induce sex-specific effects in offspring.

An overview of nuclear receptor coregulators involved in cerebellar development

Nuclear receptors (NRs) precisely control the gene regulation throughout the development of the central nervous system, including the cerebellum. Functionally, the full activity of NRs requires their cognate coregulators to be recruited by NRs and modulate the activation or repression of target gene expression. Recent progress of in vitro studies of NR coregulators has revealed that NR coregulators form large complexes in a cyclic manner and subsequently exert genetic and epigenetic influence via various intrinsic enzyme activities. Moreover, NR coregulators physiologically provide a combinatorial code for time- and gene-specific responses depending on their expression levels, relative affinities for individual receptors, and posttranslational modification. Since expression of many cerebellar genes is known to be regulated by NRs critical in a specific period for cerebellar development, their partnership with cognate coregulators may be an important factor for normal cerebellar development. This review summarizes current findings regarding the molecular structures, molecular mechanisms, temporal and spatial expression patterns, and possible biological functions of NR coregulators related to cerebellar development.

Minireview: Defining the roles of the iodothyronine deiodinases: current concepts and challenges

As is typical of other hormone systems, the actions of the thyroid hormones (TH) differ from tissue to tissue depending upon a number of variables. In addition to varying expression levels of TH receptors and transporters, differing patterns of TH metabolism provide a critical mechanism whereby TH action can be individualized in cells depending on the needs of the organism. The iodothyronine deiodinases constitute a family of selenoenzymes that selectively remove iodide from thyroxine and its derivatives, thus activating or inactivating these hormones. Three deiodinases have been identified, and much has been learned regarding the differing structures, catalytic activities, and expression patterns of these proteins. Because of their differing properties, the deiodinases appear to serve varying functions that are important in regulating metabolic processes, TH action during development, and feedback control of the thyroid axis. This review will briefly assess these functional roles and others proposed for the deiodinases and examine some of the current challenges in expanding our knowledge of these important components of the thyroid homeostatic system.

Identification of thyroid hormone receptor binding sites and target genes using ChIP-on-chip in developing mouse cerebellum

Thyroid hormone (TH) is critical to normal brain development, but the mechanisms operating in this process are poorly understood. We used chromatin immunoprecipitation to enrich regions of DNA bound to thyroid receptor beta (TRβ) of mouse cerebellum sampled on post natal day 15. Enriched target was hybridized to promoter microarrays (ChIP-on-chip) spanning −8 kb to +2 kb of the transcription start site (TSS) of 5000 genes. We identified 91 genes with TR binding sites. Roughly half of the sites were located in introns, while 30% were located within 1 kb upstream (5′) of the TSS. Of these genes, 83 with known function included genes involved in apoptosis, neurodevelopment, metabolism and signal transduction. Two genes, MBP and CD44, are known to contain TREs, providing validation of the system. This is the first report of TR binding for 81 of these genes. ChIP-on-chip results were confirmed for 10 of the 13 binding fragments using ChIP-PCR. The expression of 4 novel TH target genes was found to be correlated with TH levels in hyper/hypothyroid animals providing further support for TR binding. A TRβ binding site upstream of the coding region of myelin associated glycoprotein was demonstrated to be TH-responsive using a luciferase expression system. Motif searches did not identify any classic binding elements, indicating that not all TR binding sites conform to variations of the classic form. These findings provide mechanistic insight into impaired neurodevelopment resulting from TH deficiency and a rich bioinformatics resource for developing a better understanding of TR binding.

Regulation of fibroblast growth factor receptor-1 (FGFR1) by thyroid hormone: identification of a thyroid hormone response element in the murine Fgfr1 promoter

T(3) is essential for normal skeletal development, acting mainly via the TRalpha1 nuclear receptor. Nevertheless, the mechanisms of T(3) action in bone are poorly defined. Fibroblast growth factor receptor-1 (FGFR1) is also essential for bone formation. Fgfr1 expression and activity are positively regulated by T(3) in osteoblasts, and in mice that harbor a dominant negative PV mutation targeted to TRalpha1 or TRbeta, Fgfr1 expression is sensitive to skeletal thyroid status. To investigate mechanisms underlying T(3) regulation of FGFR1, we obtained primary calvarial osteoblasts from wild-type and TRbeta(PV/PV) littermate mice. T(3) treatment increased Fgfr1 expression 2-fold in wild-type cells, but 8-fold in TRbeta(PV/PV) osteoblasts. The 4-fold increased T(3) sensitivity of TRbeta(PV/PV) osteoblasts was associated with a markedly increased ratio of TRalpha1:TRbeta1 expression that resulted from reduced TRbeta1 expression in TRbeta(PV/PV) osteoblasts compared with wild-type. Bioinformatics and gel shift studies, and mutational analysis, identified a specific TR binding site 279-264 nucleotides upstream of the murine Fgfr1 promoter transcription start site. Transient transfection analysis of a series of Fgfr1 promoter 5'-deletion constructs, of a mutant reporter construct, and a series of heterologous promoter constructs, confirmed that this region of the promoter mediates a TR-dependent transcriptional response to T(3). Thus, in addition to indirect regulation of FGFR1 expression by T(3) reported previously, T(3) also activates the Fgfr1 promoter directly via a thyroid hormone response element located at positions -279/-264.

Effects of perinatal exposure to low-dose cadmium on thyroid hormone-related and sex hormone receptor gene expressions in brain of offspring

Perinatal cadmium (Cd) exposure has been shown to alter behaviors and reduce learning ability of offspring. A few studies have shown that Cd reduced serum thyroid hormones (THs), which are important for brain development during the perinatal period. Brain specific genes, neurogranin (RC3) and myelin basic protein (BMP), are known to be regulated by TH through TH receptors (TR). It has been suggested that RC3 may play roles in memory and learning. In addition, Cd has been suggested to have estrogen-like activity. To evaluate the effects of perinatal low-dose exposure to Cd on thyroid hormone-related gene (RC3, TR-beta1, MBP, RAR-beta) and sex hormone receptor gene (ER-alpha, ER-beta and PgR) expressions in the brain and on behaviors of offspring, mice were administered with 10ppm Cd (from gestational day 1 to postnatal day 10) and/or 0.025% methimazole (MMI; anti-thyroid drug) (from gestational day 12 to postnatal day 10) in drinking water. Also, 0.1% MMI was administered as a positive control (high MMI group). RC3 mRNA expression was reduced in the female brain of combined exposure and high MMI groups and was negatively correlated with the activity in the open-field. ER-alpha, ER-beta and PgR mRNA expressions were decreased in male and female Cd, and female Cd+MMI groups, respectively; among these changes the reduced expression of PgR was opposite to estrogenic action. These results suggested that perinatal exposure to Cd disrupted the gene expressions of sex hormone receptors, which could not be considered to be a result of estrogenic action. Our study indicates that alteration in the gene expressions of RC3 and sex hormone receptors in the brain induced by perinatal Cd and MMI exposure might be one mechanism of developmental toxicity of Cd.

New insights into thyroid hormone action

Thyroid hormones (THs) have important effects on cellular development, growth, and metabolism. They bind to thyroid hormone receptors (TRs), TRalpha and TRbeta, which belong to the nuclear hormone receptor superfamily. These receptors also bind to enhancer elements in the promoters of target genes, and can regulate both positive and negative transcription. Recent emerging evidence has characterized some of the molecular mechanisms by which THs regulate transcription as co-repressors, and co-activators have been identified and their effects on histone acetylation examined. THs also have rapid effects that do not require transcription. These can occur via TRs or other cellular proteins, and typically occur outside the nucleus. It appears that THs regulate multiple cellular functions using a diverse array of receptors and signaling systems. TR isoform- or pathway-specific drugs might provide the therapeutic benefits of TH action such as decreasing obesity or lowering cholesterol levels without some of the side effects of hyperthyroidism.

Loss of Testicular Orphan Receptor 4 Impairs Normal Myelination in Mouse Forebrain

Testicular orphan nuclear receptor 4 (TR4) has been suggested to play important roles in the development and functioning of the central nervous system (CNS). We find reduced myelination in TR4 knockout (TR4(-/-)) mice, which is particularly obvious in forebrains and in early developmental stages. Further analysis reveals that CC-1-positive (CC-1+) oligodendrocytes are decreased in TR4(-/-) forebrains. The O4+ signals are also reduced in TR4(-/-) forebrains when examined at postnatal d 7. However, the number and proliferation rate of platelet-derived growth factor receptor alpha-positive (PDGFalphaR+) oligodendrocyte precursor cells (OPCs) remain unaffected in these regions, suggesting that loss of TR4 interrupts oligodendrocyte differentiation. This is further supported by the observation that CC-1+ oligodendrocytes derived from 5-bromo-2'-deoxyuridine incorporating OPCs are significantly reduced in TR4(-/-) forebrains. We also find higher Jagged1 expression levels in axon fiber-enriched regions in TR4(-/-) forebrains, suggesting a more activated Notch signaling in these regions that correlates with previous reports showing that Notch activation inhibits oligodendrocyte differentiation. Together, our results suggest that TR4 is required for proper myelination in the CNS and is particularly important for oligodendrocyte differentiation and maturation in the forebrain regions. The altered Jagged1-Notch signaling in TR4(-/-) forebrain underlies a potential mechanism that contributes to the reduced myelination in the forebrain.

Functional redundancy of NSCL-1 and NeuroD during development of the petrosal and vestibulocochlear ganglia

To study the role of different members of the bHLH gene family for sensory organ development we have generated NSCL-1 and NeuroD compound-mutant mice. Double homozygous animals were characterized by a more severe reduction of the petrosal and vestibulocochlear ganglia than NeuroD-knockout mice. The more severe reduction of the petrosal and vestibulocochlear ganglia in double-knockout mice indicates overlapping functions of the two genes during neuronal development. Interestingly, we also found that the two genes are jointly regulated by thyroid hormone during sensory hair cell development. We further present a detailed expression analysis of NSCL-1 and NSCL-2 during sensory neuron development. NSCL-1 expression was detected in all developing cranial ganglia including the petrosal and vestibulocochlear ganglion, in inner and outer hair cells of the organ of Corti and in hair cells of the vestibular system. Expression domains in other sensory structures include the retina, Merkel cells of the developing skin and sensory cells of the tongue. The expression of NSCL-2 was restricted to developing cranial ganglia, the retina and the vestibular nerve. Both NSCL-1 and NSCL-2 genes are active only in postmitotic neurons, indicating a role for neuronal cell migration and/or differentiation within the sensory system.

Thyroid hormones and retinoids: a possible link between genes and environment in schizophrenia

Phenotypic discordance for schizophrenia in monozygotic twins clearly indicates involvement of environmental factors as key determinants in disease development. Positive findings from genome scans, linkage and association studies apply in only a minority of those affected, while post-mortem brain investigations reveal altered expression of genes and proteins involved in numerous neurodevelopmental, metabolic and neurotransmitter pathways. Such altered expressions could result, on the one hand, from mutations in coding regions or polymorphisms in the promoter and regulatory regions in genes within those areas identified by gene searches or, on the other hand, from inadequate amounts of modulators, transporters and synthesizers of transcription factors necessary for regulation of the putative genes. Hormones and vitamins are such modulators. They could serve as bridges between genes and environment in schizophrenia. Multiple evidence supports the suggestion of retinoids and thyroid hormones as plausible actors in these roles. Both are not only essential for normal development of the central nervous system but also regulate the expression of many neurotransmitters, their synthesizing enzymes and receptors, and other genes in broader signaling transduction cascades affecting pathways that are altered in response to treatment. Functional and positional candidate genes include retinoic acid and thyroid hormone receptors, retinaldehyde dehydrogenases and deiodinases, which synthesize the powerful morphogens, retinoic acid and triiodothyronine, and the enzymes involved in their inactivation. This review highlights selective evidence supporting the retinoid and thyroid hormone hypotheses of schizophrenia.

Expression of myelin basic protein in two oligodendroglial cell lines is modulated by apotransferrin through different transcription factors

We have shown that apotransferrin (aTf) promotes the differentiation of two oligodendroglial cell (OLGc) lines, N19 and N20.1, representing different stages of OLGc maturation. Although in both cell lines aTf promoted myelin basic protein (MBP) expression, an increase in cAMP levels and CREB phosphorylation was observed only in the less mature cells (N19), suggesting that the maturation induced by aTf is achieved probably through different signaling pathways. We transfected both cell lines with the proximal region of the human MBP promoter fused to the lacZ reporter gene. In both transfected cell lines, addition of aTf produced an activation of the promoter. To elucidate the mechanisms involved in this action, Western blot analysis, EMSAs, and RT-PCR were performed for different transcription factors involved in mbp regulation. In the N20.1 line, treatment with aTf increased the expression and the DNA-binding capacity of thyroid hormone (TH) receptors, Sp1, and nuclear factor-kappaB (NFkappaB). For these cells we found that an inductor of NFkappaB (tumor necrosis factor-alpha) promoted MBP messenger synthesis, whereas mithramycin, a specific inibitor of Sp1, and a cAMP analog (db-cAMP) inhibited its transcription. In the N19 cell line, aTf stimulated NF-I and NFkappaB activation, but, aside from aTf, only db-cAMP induced mbp transcription. These data suggest that, depending on the OLGc maturational stage, aTf modulates MBP expression and OLGc differentiation through different signaling pathways and different transcription factors.

Thyroid hormone deiodinases in the central and peripheral nervous system

Thyroid hormones play a critical role in development and functioning of the nervous system. Deiodinases (type 2 [D2] and type 3 [D3]) contribute to the control of thyroid hormone action in the nervous system by regulating the local concentrations of triiodothyronine (T(3)), the main active thyroid hormone. Most brain T(3) is indeed locally formed by deiodination of thyroxine (T(4)). This reaction is catalyzed by D2 expressed in astrocytes throughout the brain and in tanycytes in the mediobasal hypothalamus. D3, which inactivates both T(4) and T(3), is mainly expressed in neurons also throughout the brain, with high expression in hippocampus and pyriform cortex. The regulation of deiodinases by many factors in addition to the thyroid hormones indicate that their role is not limited to mitigate the fluctuations in plasma T(4) and T(3). In contrast to the brain, deiodinases are not expressed in the adult peripheral nerve. Nerve lesions induce D2 in peripheral nerve sheaths and D3 in the endoneurial compartment containing Schwann cells. On the basis of available data summarized in this review, D2 and D3 clearly contribute to determine T(3) concentrations depending on the area of the nervous system, the state of development, and the pathophysiologic conditions.

Thyroid hormones and brain development

The action of thyroid hormones (thyroxine, T4; triiodothyronine, T3) on brain development and function is gaining renewed interest. It has been known for many years that thyroid hormones are very important in mammalian brain maturation, influencing many aspects related to neural cell migration, differentiation, and signaling. In the last 10 years, genes regulated by thyroid hormones have been identified in the rodent brain, and understanding of the role of thyroid hormone nuclear receptors has been facilitated with the analysis of the phenotype of mutant mice for the different receptor isoforms. The general picture that emerges is that T4 and T3 may enter the brain through specific transporters. T4 is converted to the active hormone, T3, in glial cells, astrocytes, and tanycytes, although the main target cells are neurons and maturing oligodendrocytes. T3, acting through the nuclear receptors, controls the expression of genes involved in myelination, cell differentiation, migration, and signaling. In addition to transducing the T3 signal, the nuclear receptors also have activity in the unliganded state (i.e., as aporeceptors), mainly as repressors of transcription. The physiological meaning of aporreceptor action is not known, but they may play a role in the genesis of the hypothyroid phenotype. Among the questions that remain to be explored in more detail is the role of thyroid hormones and the T3 receptors, both liganded and unliganded, in the fetal brain, especially before onset of fetal thyroid gland function. These questions are relevant for human health and the management of thyroid diseases during pregnancy.

Thyroid hormone regulates oligodendrocyte accumulation in developing rat brain white matter tracts

Thyroid hormone (TH) is necessary for normal axonal myelination. Myelin basic protein (MBP) is a structural protein essential for myelin function. In this study, we demonstrate that perinatal hypothyroidism regulates MBP mRNA levels via indirect mechanisms. We observed decreased MBP mRNA accumulation in the hypothyroid rat brain at postnatal (PN) d 10 and 50. Acute TH replacement did not rescue hypothyroid MBP mRNA levels at PN5, 10, or 50. TH is necessary for normal intrahemispheric commissure development including the anterior commissure (AC) and the corpus callosum (CC). We determined that perinatal hypothyroidism decreases AC area and cellularity in the developing rat brain by PN10 and 50. In the developing CC, hypothyroidism initially increases area and cellularity by PN5, but then ultimately decreases area and cellularity by PN50. MBP-expressing oligodendrocytes are a recognized target of TH and are responsible for myelination within intrahemispheric commissures. We found that hypothyroidism reduces the number of mature oligodendrocytes within both the AC and CC. This reduction is noted at PN5, 10, and 50 in the AC and by PN10 and 50 in the CC. Together, these data suggest that TH regulates MBP mRNA levels through indirect mechanisms. These data demonstrate the complex mechanisms whereby TH regulates myelination in the developing brain.

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.

Perspectives in the study of thyroid hormone action on brain development and function

The purpose of this review is to provide an up-to-date report on the molecular and physiologic processes involved in the role of thyroid hormone as an epigenetic factor in brain maturation. We summarize the available data on the control of brain gene expression by thyroid hormone, the correlation between gene expression and physiologic effects, and the likely mechanisms of action of thyroid hormone on brain gene expression. In addition we propose a role for unliganded thyroid hormone receptors in the pathogenesis of hypothyroidism. Finally, we review recent data indicating that thyroid hormone receptors have an impact on behavior.

Thyroid hormone upregulates ecto-5'-nucleotidase/CD73 in C6 rat glioma cells

Thyroid hormones have profound effects on the central nervous system, such as proliferation, secretion of growth factors and gene expression regulation. Ecto-NTPDases and ecto-5'-nucleotidase can control the extracellular ATP/adenosine levels, which have been described as proliferation factors. Here, we investigated the influence of T(3) on the enzyme cascade which catalyzes interconversion of purine nucleotides in rat C6 glioma cells. Exposure of C6 cells to T(3) caused a dose dependent increase of 30% in the AMP hydrolysis up to 0.25 nM, which was suppressed by actinomycin. No significant alteration was observed on ATP/ADP hydrolysis and T(4) at higher concentrations (10-1000 nM) promoted an increase in AMP hydrolysis that was not dose dependent. T(3) treatment also increased the expression of CD73 mRNA. Besides the importance of the ecto-5'-NT in the cell proliferation and differentiation, its overexpression can enhance extracellular adenosine levels, which could also be an important proliferation signal.

Paradoxical triiodothyronine suppression of S14 transcription in permanent hepatic cell lines

Both in vivo and in primary rat hepatocyte culture, carbohydrate and triiodothyronine (T(3)) rapidly induce transcription of the rat S14 gene. To determine if regulation of this gene by T(3) is similar in human liver cells, we transfected the S14 upstream region into HepG2 cells. We chose this cell line because many others have used this cell line to study the effect of thyroid hormone on hepatic gene expression. We found that changing media glucose concentration did not affect S14 transcription. Furthermore, addition of T(3) to HepG2 cells caused a marked reduction of rat S14 transcription. This paradoxical reduction was dependent on cotransfection of the T(3) receptor. We obtained similar results in the other human hepatoma cell lines, HuH-7 and Hep3B. The paradoxical response was not limited to human cells. We found a similar response in the nonmalignant permanent mouse liver cell line, AML-12. This paradoxical response was specific to the S14 gene because transfection of all the cell lines with a CAT or luciferase reporter driven by a mouse mammary tumor virus promoter containing 1 or 4 copies of a palindromic thyroid hormone response element (TRE) showed marked induction by T(3). Our results show that T(3) abnormally regulates the S14 gene in proliferating liver cell lines of diverse origins. This paradoxical regulation by T(3) is caused by an interaction between T(3) and the thyroid hormone receptor. The factors that lead to this paradoxical response are not active in primary hepatocytes and normal intact liver.

Ethanol specifically decreases peroxisome proliferator activated receptor beta in B12 oligodendrocyte-like cells

Peroxisome proliferator activated receptors (PPARs) are nuclear receptors that control important genes involved in lipid metabolism. Their role in nerve cells is uncertain, although anomalous myelination of the corpus callosum has been described in the PPARbeta-null mouse, and abnormalities of this tissue have been documented in fetal alcohol syndrome in humans. We report here that ethanol treatment of B12 oligodendrocyte-like cells induces a concentration- and time-dependent decrease in the mRNA and protein levels of PPARbeta, with no effect on PPARalpha or PPARgamma. The effect on PPARbeta is seen as an increase in mRNA degradation, as assessed by run-off assays, due to a significant decrease in PPARbeta mRNA half-life, with no observed changes in intracellular localization. Our results suggest a possible link between PPARbeta function and ethanol-induced abnormal myelination in oligodendrocytes.

Nuclear receptor superfamily: Principles of signaling

Nuclear receptors (NRs) comprise a family of 49 members that share a common structural organization and act as ligand-inducible transcription factors with major (patho)physiological impact. For some NRs (“orphan receptors”), cognate ligands have not yet been identified or may not exist. The principles of DNA recognition and ligand binding are well understood from both biochemical and crystal structure analyses. The 3D structures of several DNA-binding domains (DBDs),in complexes with a variety of cognate response elements, and multiple ligand-binding domains (LBDs), in the absence (apoLBD)and presence (holoLBD) of agonist, have been established and reveal canonical structural organization. Agonist binding induces a structural transition in the LBD whose most striking feature is the relocation of helix H12, which is required for establishing a coactivator complex, through interaction with members of the p160 family (SRC1, TIF2, AIB1) and/or the TRAP/DRIP complex. The p160-dependent coactivator complex is a multiprotein complex that comprises histone acetyltransferases (HATs), such as CBP,methyltransferases, such as CARM1, and other enzymes (SUMO ligase,etc.). The agonist-dependent recruitment of the HAT complex results in chromatin modification in the environment of the target gene promoters, which is requisite to, or may in some cases be sufficient for, transcription activation. In the absence of ligands, or in the presence of some antagonists, certain NRs are bound to distinct multiprotein complexes through the interaction with corepressors, such as NCoR and SMRT. Corepressor complexes comprise histone deacetylases (HDACs) that have the capacity to condense chromatin over target gene promoters. Ligands have been designed that selectively modulate the interaction between NRs and their coregulators. Both HATs and HDACs can also modify the acetylation status of nonhistone proteins, but the significance in the context of NR signaling is unclear. NRs communicate with other intracellular signaling pathways on a mutual basis, and their functionality may be altered, positively or negatively, by post-translational modification. The majority of NRs act as retinoid X receptor (RXR) heterodimers in which RXR cannot a priori respond autonomously to its cognate ligand to activate target gene transcription. This RXR subordination allows signaling pathway identity for the RXR partner. The corresponding mechanism is understood and reveals cell and NR selectivity, indicating that RXR can, under certain conditions, act autonomously. NRs are regulators of cell life and death,and NR malfunction can be at the basis of both disease and therapy, as is impressively documented in the case of acute promyelocytic leukemia. Recently, several pathways have been uncovered that link NR action with cell proliferation and apoptosis.

Thyroid hormone regulates the expression of NeuroD/BHF1 during the development of rat cerebellum

During the postnatal development of the rat cerebellum, there is an extensive proliferation of granular neurones in the external layer, followed by their migration and differentiation in the internal layer. These processes are impaired by neonatal hypothyroidism and can be restored by thyroid hormone therapy. They are also abolished in transgenic mice in which the neuroD gene is not expressed. This gene encodes a basic helix-loop-helix (bHLH) transcription factor (NeuroD), which induces the differentiation of neuronal precursors. We studied the expression of neuroD/BHF1-A mRNA during the postnatal development of euthyroid and hypothyroid rats, and compared it with that of neurotrophin-3 (NT-3), a marker of granular neurone differentiation. In euthyroid animals, the neuroD/BHF1-A mRNA increases 6-fold between days 4 and 15 after birth, and then decreases to 50% of this level in the adult. NT-3 mRNA expression followed a similar pattern, although it was increased only 3-fold. Hypothyroidism reduced both mRNA levels by 35-45%, depending on the postnatal stage. In hypothyroid pups, the injection of triiodothyronine (T3) restored normal levels of both mRNAs within 6 h. In 15-day old hypothyroid rats, the amount of NeuroD protein was reduced by about 35%. It increased about 2-fold 24 h after T3 injection. In conclusion, our results indicate that thyroid hormones (TH) regulate the expression of NeuroD during the "critical period" of cerebellum development. This regulation may constitute an early event in the control of differentiation of the cerebellar granular neurones by TH.

Thyroid hormone receptor alpha 1 regulates expression of the Na+/H+ exchanger (NHE1)

In this paper we examine the role of thyroid hormone in regulating expression of the Na+/H+ exchanger. Thyroid hormone has been reported to regulate the activity of the Na+/H+ exchanger messenger RNA in some cell types. Treatment of cardiac myocytes with 3,5',3'-triiodothyronine results in an increased expression of Na+/H+ exchanger protein. Also, compared with euthyroid animals, hypothyroid rats express decreased amounts of the Na+/H+ exchanger protein. To examine the mechanisms involved in regulating expression of the Na+/H+ exchanger, we have characterized the regulation of a distal element of the NHE1 promoter by the thyroid hormone receptor. We have previously shown that a -1085/-800 nucleotide (nt) region of the promoter is a modular element with a -841/-800 nt activating element. Using electrophoretic mobility shift assay, we show that this element interacts with thyroid hormone receptor TRalpha(1), a nuclear hormone receptor. The addition of exogenous TRalpha increased transcriptional activity of the -841/-800 nt element of the Na+/H+ exchanger promoter. We show that TRalpha binds to a region on the -841/-800 nt element that is near, but not identical, to the previously identified chicken ovalbumin upstream promoter transcription factor-binding site. Our results are the first demonstration that thyroid hormone and the thyroid hormone receptor TRalpha(1) regulate expression of the Na+/H+ exchanger.

Coexpression of thyroid hormone receptor isoforms in mouse oligodendrocytes

Double and triple immunocytochemistry with stage-specific markers and specific antireceptor antibodies was used to study expression of nuclear thyroid hormone receptor (TR) isoforms in cultured mouse oligodendrocytes. To evaluate the coexpression of each TR isoform, antibodies were raised in rabbits and mice against specific regions of alpha1-TR and alpha2-TR common to both alpha isoforms and beta1-TR. Their specificities were assessed by Western blotting and by immunocytochemistry on rat hepatocytes. Oligodendrocyte subpopulations were found to coexpress the alpha- and beta1-TR epitopes at defined developmental stages. Both alpha- and beta1-TR isoforms are colocalized in oligodendrocytes during an early stage identified by the marker OL-1, before 2',3'-cyclic nucleotide 3'-phosphohydrolase is expressed. Expression of beta1-TR varies during maturation, and that of alpha-TR decreases during terminal maturation.

Early activation of transcription factor expression in Schwann cells by progesterone

Progesterone (PROG) promotes the myelination of sciatic nerves during regeneration after cryolesion. But, little is known about the molecular mechanisms by which the hormone exerts its effects. This could be initiated by the regulation of transcription factor expression in Schwann cells, which produce the myelin sheaths in the peripheral nervous system. We investigated by RT-PCR whether PROG activated expression of transcription factors: Egr-1 (Krox-24) Egr-2 (Krox-20), Egr-3, c-jun, jun B, jun D, c-Fos, Fos B, Fra-1, Fra-2, CREB, ATF 4, SCIP and Sox-10 in cultured Schwann cells. PROG triggered a quick (visible as soon as 15 min), strong (6 to 18-fold) and transient (1-2 h) stimulation of Egr-1, Egr-2, Egr-3 and Fos B genes expression. Expression of other genes remained unaffected by PROG treatment. The same expression pattern was obtained in the MSC 80 line (mouse Schwann cells), but not in the NIH-3T3 and CHO lines. Estradiol and testosterone induced different patterns of transcription factor gene activation in Schwann cells. Serum stimulated all genes activated by PROG in addition c-fos, fra-1 and fra-2. The PROG effects were blocked by Actinomycin D and by RU 486. This suggests that the activation of these genes occurs at the transcriptional level via the interaction of the hormone with its cognate receptor. Thus, PROG can regulate Schwann cell functions and differentiation by transiently activating specific transcription factors.

Induction of type 3 iodothyronine deiodinase by nerve injury in the rat peripheral nervous system

Thyroid hormones are essential for the development and repair of the peripheral nervous system. The type 2 deiodinase, which is responsible for the activation of T(4) into T(3), is induced in injured sciatic nerve. To obtain information on the type 3 deiodinase (D3) responsible for the degradation of thyroid hormones, we looked for its expression (mRNA and activity) in the sciatic nerve after injury. D3 was undetectable in the intact sciatic nerve of adult rats, but was rapidly and highly increased in the distal and proximal segments after nerve lesion. After cryolesion, D3 up-regulation disappeared after 3 d in the proximal segment, whereas it was sustained for 10 d in the distal segment, then declined to reach basal levels after 28 d, when functional recovery was completed. After a transsection preventing the nerve regeneration, up-regulation of D3 persisted up to 28 d at high levels in the distal segment. D3 was expressed in peripheral connective sheaths and in the internal endoneural compartment. D3 mRNA was inducible by 12-O-tetradecanoylphorbol-13-acetate in cultured fibroblasts or Schwann cells. In conclusion, induction of D3 in the peripheral nervous system after injury may play an important role during the regeneration process by adjusting intracellular T(3) levels.

Rapid effects of triiodothyronine on immediate-early gene expression in Schwann cells

In the peripheral nervous system, triiodothyronine (T3) plays an important role in the development and regeneration of nerve fibers and in myelin formation. However, the target genes of T3 in peripheral nerves remain to be identified. We investigated whether T3 activated genes of transcription factors in Schwann cells. Expression of egr-1 (krox-24), egr-2 (krox-20), egr-3, c-jun, junB, c-fos, fosB, fra-1, fra-2, and CREB genes was analyzed by reverse transcription-polymerase chain reaction (RT-PCR) in Schwann cells isolated from neonatal rat sciatic nerves and in the cell lines MSC-80 (mouse Schwann cells), NIH-3T3 (mouse fibroblasts), and CHO (Chinese hamster ovary cells). Some of these transcription factors have been shown to be involved in Schwann cell differentiation. T3 triggered a rapid (15-30 min), transient (1-2-h) and strong (6- to 15-fold) stimulation of Egr-1, Egr-2, Egr-3, Jun B, c-Fos, and Fos B mRNA expression in Schwann cells. In contrast, expression of c-Jun, Fra-1, Fra-2, and CREB mRNA was not affected by T3. The stimulatory effects of T3 could be abolished by adding actinomycin D. T3 triggered the same pattern of gene stimulation in the mouse Schwann cell line MSC80, but not in the NIH-3T3 and CHO cell lines. Serum activated all the genes that responded to T3 and in addition fra-1 and fra-2, but not c-jun and CREB. Immunoblotting showed that the increase in Egr-1 and c-Fos mRNA levels was accompanied by an increase in the corresponding proteins. In addition, shifts of the protein bands indicated a posttranslational modification of the two proteins. These effects of T3 are likely to be mediated by the intracellular T3 receptor, as the D-isomer RT3 and T0, which do not bind to T3 receptors, proved ineffective. The present data suggested that T3 may regulate Schwann cell functions and differentiation by transiently activating the expression of specific transcription factors.