Augmentation of thyroid hormone receptor-mediated transcription by Ca2+/calmodulin-dependent protein kinase type IV

Effects of Gadolinium Deposits in the Cerebellum: Reviewing the Literature from In Vitro Laboratory Studies to In Vivo Human Investigations

Gadolinium (Gd)-based contrast agents (GBCAs) are chemicals injected intravenously during magnetic resonance imaging (MRI) to enhance the diagnostic yield. The repeated use of GBCAs can cause their deposition in the brain, including the cerebellum. Such deposition may affect various cell subsets in the brain and consequently cause behavioral alterations due to neurotoxicity. Caution should thus be exercised in using these agents, particularly in patients who are more likely to have repeated enhanced MRIs during their lifespan. Further studies are required to clarify the toxicity of GBCAs, and potential mechanisms causing neurotoxicity have recently been reported. This review introduces the effects of GBCAs in the cerebellum obtained from in vitro and in vivo studies and considers the possible mechanisms of neurotoxicity involved.

The Effects of Gadolinium-Based Contrast Agents on the Cerebellum: from Basic Research to Neurological Practice and from Pregnancy to Adulthood

Gadolinium (Gd)-based contrast agents (GBCAs) are used in magnetic resonance imaging (MRI) to increase the diagnostic yield. Current reports using animal models or human subjects have shown that GBCAs may be deposited in brain including the cerebellum. Although further studies may be required to clarify the toxicity of GBCAs, we should be more cautious to use these agents particularly in patients who more likely to have repeated enhanced MRI along their lifespan. In this editorial, current studies to clarify the toxicity of GBCAs in the cerebellum are introduced.

Repercussions of hypo and hyperthyroidism on the heart circadian clock

Myocardial gene expression and metabolism fluctuate over the course of the day in association with changes in energy supply and demand. Time-of-day-dependent oscillations in myocardial processes have been linked to the intrinsic cardiomyocyte circadian clock. Triiodothyronine (T3) is an important modulator of heart metabolism and function. Recently, our group has reported time-of-day-dependent rhythms in cardiac T3 sensitivity, as well as, T3-mediated acute alterations on core clock components. Hypo and hyperthyroidism are the second most prevalent endocrine disease worldwide. Considering the importance of the cardiomyocyte circadian clock and T3 to cardiac physiology, the aim of this study was to investigate the consequences of chronic hypo and hyperthyroidism on 24-h rhythms of circadian clock genes in the heart. Hypo and hyperthyroidism was induced in rats by thyroidectomy (Tx) and i.p. injections of supraphysiological dose of T3, respectively. Here we report alterations in mRNA levels of the major core clock components (Bmal1, Per2, Nr1d1, and Rora) for both experimental conditions (with the exception of Per2 during hyperthyroid condition). Oscillations in mRNA levels of key glucose and fatty-acid metabolism genes known to be clock controlled (Pdk4, Ucp3, Acot1, and Cd36) were equally affected by the experimental conditions, especially during the hypothyroid state. These findings suggest that chronic alterations in thyroid status significantly impacts 24-h rhythms in circadian clock and metabolic genes in the heart. Whether these perturbations contribute toward the pathogenesis of cardiac dysfunction associated with hypo and hyperthyroidism requires further elucidation.

Effects of Gadolinium-Based Contrast Agents on Thyroid Hormone Receptor Action and Thyroid Hormone-Induced Cerebellar Purkinje Cell Morphogenesis

Gadolinium (Gd)-based contrast agents (GBCAs) are used in diagnostic imaging to enhance the quality of magnetic resonance imaging or angiography. After intravenous injection, GBCAs can accumulate in the brain. Thyroid hormones (THs) are critical for the development and functional maintenance of the central nervous system. TH actions in brain are mainly exerted through nuclear TH receptors (TRs). We examined the effects of GBCAs on TR-mediated transcription in CV-1 cells using transient transfection-based reporter assay and TH-mediated cerebellar Purkinje cell morphogenesis in primary culture. We also measured the cellular accumulation and viability of Gd after representative GBCA treatments in cultured CV-1 cells. Both linear (Gd-diethylene triamine pentaacetic acid-bis methyl acid, Gd-DTPA-BMA) and macrocyclic (Gd-tetraazacyclododecane tetraacetic acid, Gd-DOTA) GBCAs were accumulated without inducing cell death in CV-1 cells. By contrast, Gd chloride (GdCl3) treatment induced approximately 100 times higher Gd accumulation and significantly reduced the number of cells. Low doses of Gd-DTPA-BMA (10(-8) to 10(-6)M) augmented TR-mediated transcription, but the transcription was suppressed at higher dose (10(-5) to 10(-4)M), with decreased β-galactosidase activity indicating cellular toxicity. TR-mediated transcription was not altered by Gd-DOTA or GdCl3, but the latter induced a significant reduction in β-galactosidase activity at high doses, indicating cellular toxicity. In cerebellar cultures, the dendrite arborization of Purkinje cells induced by 10(-9)M T4 was augmented by low-dose Gd-DTPA-BMA (10(-7)M) but was suppressed by higher dose (10(-5)M). Such augmentation by low-dose Gd-DTPA-BMA was not observed with 10(-9)M T3, probably because of the greater dendrite arborization by T3; however, the arborization by T3 was suppressed by a higher dose of Gd-DTPA-BMA (10(-5)M) as seen in T4 treatment. The effect of Gd-DOTA on dendrite arborization was much weaker than that of the other compounds. These results indicate that exposure to specific GBCAs may, at least in part, cause toxic effects in the brain by disrupting the action of THs on TRs. The toxic effects of GBCAs may depend on the chemical structure of GBCA and the dose. Thus, it is very important to choose appropriate GBCAs for imaging to prevent adverse side effects.

An evo-devo approach to thyroid hormones in cerebral and cerebellar cortical development: etiological implications for autism

The morphological alterations of cortical lamination observed in mouse models of developmental hypothyroidism prompted the recognition that these experimental changes resembled the brain lesions of children with autism; this led to recent studies showing that maternal thyroid hormone deficiency increases fourfold the risk of autism spectrum disorders (ASD), offering for the first time the possibility of prevention of some forms of ASD. For ethical reasons, the role of thyroid hormones on brain development is currently studied using animal models, usually mice and rats. Although mammals have in common many basic developmental principles regulating brain development, as well as fundamental basic mechanisms that are controlled by similar metabolic pathway activated genes, there are also important differences. For instance, the rodent cerebral cortex is basically a primary cortex, whereas the primary sensory areas in humans account for a very small surface in the cerebral cortex when compared to the associative and frontal areas that are more extensive. Associative and frontal areas in humans are involved in many neurological disorders, including ASD, attention deficit-hyperactive disorder, and dyslexia, among others. Therefore, an evo-devo approach to neocortical evolution among species is fundamental to understand not only the role of thyroid hormones and environmental thyroid disruptors on evolution, development, and organization of the cerebral cortex in mammals but also their role in neurological diseases associated to thyroid dysfunction.

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

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

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.

Decreased cyclin-dependent kinase activity promotes thyroid hormone-dependent tail regression in Rana catesbeiana

The thyroid hormone (TH), 3,5,3'-triiodothyronine (T(3)), is an important regulator of diverse cellular processes including cell proliferation, differentiation, and apoptosis, with increasing evidence that the modulation of the phosphoproteome is an important factor in the TH-mediated response. However, little is understood regarding the mechanisms whereby phosphorylation may contribute to T(3)-mediated cellular outcomes during development. The cyclin-dependent kinases (Cdks) and mitogen-activated protein kinases (MAPK/ERK) have been implicated in TH signaling in mammalian cells. In this study, we have investigated, in frogs, the possible role that these kinases may have in the promotion of tail regression during tadpole metamorphosis, an important postembryonic process that is completely TH-dependent. Cdk2 steady state levels and activity increase in the tail concurrent with progression through the growth phase of metamorphosis, followed by a precipitous decrease coinciding with tail regression. Cyclin-A-associated kinase activity also follows a similar trend except that its associated kinase activity is maintained longer before a decrease in activity. Protein steady state levels of ERK1 and ERK2 remain relatively constant, and their kinase activities do not decrease until much later during tail regression. Tail tips cultured in serum-free medium in the presence of T(3) undergo regression, which is accelerated by coincubation with a specific Cdk2 inhibitor. Coincubation with PD098059, a MAPK inhibitor, has no effect. Thus, T(3)-dependent tail regression does not require MAPKs, but a decrease in Cdk2 activity promotes tail regression.

Retinoid-related orphan receptor alpha controls the early steps of Purkinje cell dendritic differentiation

Dendritic differentiation involves both regressive and growth events. The mechanisms controlling the regressive events are poorly understood. This study is aimed at determining the role of the nuclear receptor retinoid-related orphan receptor alpha (RORalpha) in Purkinje cell (PC) dendritic differentiation in organotypic cultures. As observed in vivo, in these cultures, fusiform PCs with embryonic bipolar shape undergo regression before the outgrowth of the ultimate dendritic tree. We show that lentiviral-mediated hRORalpha1 overexpression in fusiform PCs leads to a cell-autonomous accelerated progression of dendritic differentiation. In addition, RORalpha is necessary for the PC regressive events: whereas staggerer RORalpha-deficient PCs remain in the embryonic fusiform stage, replacement of hRORalpha1 restores normal dendritogenesis. These results demonstrate that RORalpha expression in fusiform PCs is crucial for the dendritic regression and progression of the following step of extension of dendritic processes. However, it does not seem to participate to the last stage of dendritic growth. This study identifies RORalpha as a nuclear receptor crucial for the control of dendritic remodeling during development.

Calmodulin-Dependent Kinase IV Stimulates Vitamin D Receptor-Mediated Transcription

1,25-Dihydroxyvitamin D3 [1,25-(OH)2D3] promotes intestinal absorption of calcium primarily by binding to the vitamin D receptor (VDR) and regulating gene expression. 1,25-(OH)2D3 also exerts rapid actions at the cell membrane that include increasing intracellular calcium levels and activating protein kinase cascades. To explore potential cross talk between calcium signaling elicited by the nongenomic actions of 1,25-(OH)2D3 and the genomic pathway mediated by VDR, we examined the effects of activated Ca2+/calmodulin-dependent kinases (CaMKs) on 1,25-(OH)2D3/VDR-mediated transcription. Expression of a constitutively active form of CaMKIV dramatically stimulated 1,25-(OH)2D3-activated reporter gene expression in COS-7, HeLa, and ROS17/2.8 cell lines. Metabolic labeling studies indicated that CaMKIV increased VDR phosphorylation levels. In addition, CaMKIV increased the independent transcription activity of the VDR coactivator SRC (steroid receptor coactivator) 1, and promoted ligand-dependent interaction between VDR and SRC coactivator proteins in mammalian two-hybrid studies. The functional consequences of this multifaceted mechanism of CaMKIV action were revealed by reporter gene studies, which showed that CaMKIV and select SRC coactivators synergistically enhanced VDR-mediated transcription. These studies support a model in which CaMKIV signaling stimulates VDR-mediated transcription by increasing phosphorylation levels of VDR and enhancing autonomous SRC activity, resulting in higher 1,25-(OH)2D3-dependent interaction between VDR and SRC coactivators.

Current perspectives on the role of thyroid hormone in growth and development of cerebellum

The thyroid hormone (TH) is essential for growth and development of brain, including the cerebellum. Deficiency of TH during the perinatal period results in abnormal cerebellar development, which is well documented in rodent animal models. TH exerts its major effect by binding to the nuclear TH receptor (TR), a ligand-regulated transcription factor. Although TR is highly expressed in many brain regions, including the cerebellum, TH-target genes that likely play critical roles in brain development have not yet been fully clarified. At present, however, expression of many cerebellar genes is known to be altered by perinatal hypothyroidism. Interestingly, after the critical period of TH action (first 2 weeks of postnatal life in rodent cerebellum), the activities of many genes that are altered by perinatal hypothyroidism return to the same levels as those of euthyroid animal despite morphological alterations. Several prominent candidate genes that may play key roles in TH-mediated cerebellar development are discussed in this review. On the other hand, TR-mediated transcription may be modulated by various substances. The nuclear hormone receptor superfamily contains more than 40 transcriptional factors and, most of these receptors are present in the brain. Possible interactions between TR and such transcription factors are also discussed. Further, several additional issues that need to be clarified are discussed. One such issue is the discrepancy of phenotypes among TR-knockout and perinatal hypothyroid mice. Recent studies have provided several important clues to address this issue. Another current area that needs attention is the effect of endocrine disruptors on brain development. Since the molecular structures of TH and several endocrine disrupting chemicals are similar, the effect of such chemicals on brain may be exerted at least in part through the TH system. Recent studies have shown the possible interaction between TR and such chemicals. Overall, this review provides current findings regarding molecular mechanisms on TH action in cerebellar development.

RORalpha coordinates reciprocal signaling in cerebellar development through sonic hedgehog and calcium-dependent pathways

The cerebellum provides an excellent system for understanding how afferent and target neurons coordinate sequential intercellular signals and cell-autonomous genetic programs in development. Mutations in the orphan nuclear receptor RORalpha block Purkinje cell differentiation with a secondary loss of afferent granule cells. We show that early transcriptional targets of RORalpha include both mitogenic signals for afferent progenitors and signal transduction genes required to process their subsequent synaptic input. RORalpha acts through recruitment of gene-specific sets of transcriptional cofactors, including beta-catenin, p300, and Tip60, but appears independent of CBP. One target promoter is Sonic hedgehog, and recombinant Sonic hedgehog restores granule precursor proliferation in RORalpha-deficient cerebellum. Our results suggest a link between RORalpha and beta-catenin pathways, confirm that a nuclear receptor employs distinct coactivator complexes at different target genes, and provide a logic for early RORalpha expression in coordinating expression of genes required for reciprocal signals in cerebellar development.

Transcriptional regulation of human Rev-erbalpha gene expression by the orphan nuclear receptor retinoic acid-related orphan receptor alpha

The Rev-erb and retinoic acid-related orphan receptors (ROR) are two related families of orphan nuclear receptors that recognize similar response elements but have opposite effects on transcription. Recently, the Rev-erbalpha gene promoter has been characterized and shown to harbor a functional Rev-erbalpha-binding site known as Rev-DR2, responsible for negative feedback down-regulation of promoter activity by Rev-erbalpha itself. The present study aimed to investigate whether Rev-erbalpha gene expression is regulated by RORalpha. Gel shift analysis demonstrated that in vitro translated hRORalpha1 protein binds to the Rev-DR2 site, both as monomer and dimer. Chromatin immunoprecipitation assays demonstrated that binding of RORalpha to this site also occurred in vivo in human hepatoma HepG2 cells. The Rev-DR2 site was further shown to be functional as it conferred hRORalpha1 responsiveness to a heterologous promoter and to the natural human Rev-erbalpha gene promoter in these cells. Mutation of this site in the context of the natural Rev-erbalpha gene promoter abolished its activation by RORalpha, indicating that this site plays a key role in hRORalpha1 action. Finally, adenoviral overexpression of hRORalpha1 in HepG2 cells led to enhanced hRev-erbalpha mRNA accumulation, further confirming the physiological importance of RORalpha1 in the regulation of Rev-erbalpha expression.

Retinoic acid receptor-related orphan receptor (ROR) alpha4 is the predominant isoform of the nuclear receptor RORalpha in the liver and is up-regulated by hypoxia in HepG2 human hepatoma cells

The retinoic acid receptor-related orphan receptor alpha (RORalpha) is critically involved in many physiological functions in several organs. We find that the main RORalpha isoform in the mouse liver is the RORalpha4 isoform, in terms of both mRNA and protein levels, while the RORalpha1 isoform is less abundant. Because hypoxia is a major feature of liver physiology and pathology, we examined the effect of this stress on Rora gene expression and RORalpha transcriptional activity. HepG2 human hepatoma cells were cultured for 24 h under normoxia (20% O2) or hypoxia (10, 2, and 0.1% O2) and the abundance of the Rora transcripts measured by Northern blot and semi-quantitative RT-PCR. Hypoxic HepG2 cells contained more Rora mRNA than controls. This was also observed in rat hepatocytes in primary culture. Cobalt chloride and desferrioxamine also increased the amount of Rora mRNA in HepG2 cells. It is likely that these treatments increase the amount of the RORalpha4 protein in HepG2 cells as evidenced by Western blotting in the case of desferrioxamine. Transient transfection experiments indicated that hypoxia, cobalt chloride, and desferrioxamine all stimulate RORalpha transcriptional activity in HepG2 cells. Hence, we believe that RORalpha participates in the control of gene transcription in hepatic cells and modulates gene expression in response to hypoxic stress.

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.