Thyroid hormone effects on Krox-24 transcription in the post-natal mouse brain are developmentally regulated but are not correlated with mitosis

Thyroid hormone regulates the expression of SNAP-25 during rat brain development

Thyroid hormones are major regulators of postnatal brain development. Thyroid hormones act through nuclear receptors to modulate the expression of specific genes in the brain. We have used microarray analysis to identify novel responsive genes in 14-day-old hypothyroid rat brains, and discovered that synaptosomal-associated protein of 25 kDa (SNAP-25) was one of the thyroid hormone-responsive genes. SNAP-25 is a presynaptic plasma membrane protein and an integral component of the vesicle docking and fusion machinery mediating secretion of neurotransmitters and is required for neuritic outgrowth and synaptogenesis. Using microarray analysis we have shown that SNAP-25 was down-regulated in the hypothyroid rat brain compared with the age-matched controls. Real-time RT-PCR and western blotting analysis confirmed that SNAP-25 mRNA and protein levels decreased significantly in the developing hypothyroid rat brain. Our data suggest that in the developing rat brain, SNAP-25 expression is regulated by thyroid hormone, and thyroid hormone deficiency can cause decreased expression of SNAP-25 and this may on some level account for the impaired brain development seen in hypothyroidism.

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.

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.

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 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.

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.

Brain glucose utilization in mice with a targeted mutation in the thyroid hormone alpha or beta receptor gene

Brain glucose utilization is markedly depressed in adult rats made cretinous after birth. To ascertain which subtype of thyroid hormone (TH) receptors, TRalpha1 or TRbeta, is involved in the regulation of glucose utilization during brain development, we used the 2-[(14)C]deoxyglucose method in mice with a mutation in either their TRalpha or TRbeta gene. A C insertion produced a frameshift mutation in their carboxyl terminus. These mutants lacked TH binding and transactivation activities and exhibited potent dominant negative activity. Glucose utilization in the homozygous TRbetaPV mutant mice and their wild-type siblings was almost identical in 19 brain regions, whereas it was markedly reduced in all brain regions of the heterozygous TRalpha1PV mice. These suggest that the alpha1 receptor mediates the TH effects in brain. Inasmuch as local cerebral glucose utilization is closely related to local synaptic activity, we also examined which thyroid hormone receptor is involved in the expression of synaptotagmin-related gene 1 (Srg1), a TH-positively regulated gene involved in the formation and function of synapses [Thompson, C. C. (1996) J. Neurosci. 16, 7832-7840]. Northern analysis showed that Srg1 expression was markedly reduced in the cerebellum of TRalpha(PV/+) mice but not TRbeta(PV/PV) mice. These results show that the same receptor, TRalpha1, is involved in the regulation by TH of both glucose utilization and Srg1 expression.

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.

Effect of altered thyroid status on neurotrophin gene expression during postnatal development of the mouse cerebellum

Thyroid hormone (TH) plays an important role in brain development. The rodent cerebellum has been an excellent model for the study of the molecular mechanisms of TH action in brain. However, most studies have utilized the rat rather than the mouse. Considering the usefulness of mice with regard to diverse genetic models, the study of TH effect on mouse cerebellar development is needed. Thus, we examined the effect of perinatal hypothyroidism on the expression of neurotrophin-3 (NT-3) and brain-derived neurotrophic factor (BDNF) genes, which play critical roles in cerebellar development. Newborn mice were rendered hypothyroid by administering methimazole and perchlorate in drinking water to their mothers. The growth of hypothyroid mice was retarded, which was reversed by daily thyroxine administration. NT-3 and BDNF gene expression was depressed in the perinatal hypothyroid cerebellum. Furthermore, the expression of retinoid-receptor-related orphan nuclear hormone receptor-alpha (RORalpha), an orphan nuclear receptor that plays critical roles in Purkinje cell development, was also decreased. Morphologically, disappearance of the external granule cell layer was retarded and arborization of Purkinje cell dendrite was decreased, events that were also observed in hypothyroid rats. These results indicate that the mouse cerebellum is comparable to the rat cerebellum as a model for the examination of the molecular mechanisms of TH action in brain development.

Postnatal handling increases the expression of cAMP-inducible transcription factors in the rat hippocampus: the effects of thyroid hormones and serotonin

Postnatal handling increases glucocorticoid receptor expression in the rat hippocampus, thus altering the regulation of hypothalamic synthesis of corticotropin-releasing hormone and the hypothalamic-pituitary-adrenal response to stress. The effect on glucocorticoid receptor gene expression represents one mechanism by which the early environment can exert a long-term effect on neural development. The handling effect on hippocampal glucocorticoid receptor expression is dependent on peripheral thyroid hormone release and the activation of ascending serotonergic pathways. In primary hippocampal cell cultures, serotonin (5-HT) increases glucocorticoid receptor expression, and this effect appears to be mediated by increased cAMP levels. In the current studies we examined the in vivo effects of handling on hippocampal cAMP-protein kinase A (PKA) activity. In 7-d-old rat pups, we found that (1) postnatal handling increased adenylyl cyclase activity and hippocampal cAMP levels, (2) the effect of handling on cAMP levels was completely blocked by treatment with either propylthiouracil (PTU), a thyroid hormone synthesis inhibitor, or the 5-HT receptor antagonist, ketanserin, and (3) handling also increased hippocampal PKA activity. We then examined the effects of handling on cAMP-inducible transcription factors. Handling rapidly increased levels of the mRNAs for nerve growth factor-inducible factor A (NGFI-A) (zif268, krox24) and activator protein-2 (AP-2) as well as for NGFI-A and AP-2 immunoreactivity throughout the hippocampus. Finally, we found that the effects of handling on NGFI-A and AP-2 expression were significantly reduced by concurrent treatment with either PTU or ketanserin, effects that paralleled those on cAMP formation. NGFI-A and AP-2 have been implicated in the regulation of glucocorticoid receptor expression during development. Thus, these findings suggest that postnatal handling might alter glucocorticoid receptor gene expression via cAMP-PKA pathways involving the activation of NGFI-A and AP-2.

Thyroid hormone action and brain development

Thyroid hormone (TH) plays a crucial role in brain development. Developing rodent cerebellum might be an excellent model for studying the molecular mechanisms of TH action in the brain because perinatal hypothyroidism greatly affects its ontogeny. Although the TH-regulated genes that play crucial roles in cerebellar development have not yet been fully characterized, recent studies have provided novel insights into TH action in brain development.

Transcriptional repression of TRH promoter function by T3: analysis by in vivo gene transfer

We consider how an integrated in vivo model can be used to study the specific transcriptional effects of specific receptors in neuroendocrine systems. Our example is the role of thyroid receptor (TR) isoforms in mediating negative feedback effects of T3 on TRH (thyrotropin releasing hormone) expression. The in vivo transfection method employed polyethylenimine (PEI) to introduce genes directly into specifc regions of the brains of mice, rats, and Xenopus tadpoles. In the mouse model, the technique has served to examine TR effects on TRH transcription and on the pituitary-thyroid axis end point: thyroid hormone secretion. When a TRH-luciferase construct is introduced into the hypothalami of newborn mice TRH-luciferase transcription is regulated physiologically, being significantly increased in hypothyroidism and decreased in T3-treated animals. When various T3-binding forms of TRβ or TRα are expressed in the hypothalamus, all TRβ isoforms give T3-dependent regulation of TRH transcription, whereas TRα isoforms block T3-dependent transcription. Moreover, TR transcriptional effects are correlated with physiological consequences on circulating T4. Thus, somatic gene transfer shows TR subtypes to have distinct, physiologically relevant effects on TRH transcription. The approach is an appealing alternative to germinal transgenesis for studying specific neuroendocrine regulations at defined developmental stages in different species.Key words: thyroid hormone, TRH, mouse central nervous system, non viral gene transfer, polyethylenimine.

Altered molecular response to adrenoreceptor-induced cardiac hypertrophy in Egr-1-deficient mice

Unmanipulated early growth response-1 (Egr-1)-deficient -/- mice have similar heart-to-body weight ratios but express lower amounts of atrial natriuretic factor (ANF), beta-myosin heavy chain (beta-MHC), skeletal actin, NGF1-A binding protein (NAB)-2, Sp1, c-fos, c-jun, GATA-4, and Nkx2.5 than +/+ or +/- mice. alpha-MHC, tubulin, and NAB-1 expression was similar. Isoproterenol (Iso) and phenylephrine (PE) infusion into +/+ and -/- mice increased heart weight, ANF, beta-MHC, skeletal actin, Sp1, NAB-2, c-fos, and c-jun expression, but induction in -/- mice was lower. Only Iso + PE-treated +/+ mice showed induction of NAB-1, GATA-4, and Nkx2.5. Foci of fibrosis were found in Iso + PE-treated -/- and +/+ mice. Surprisingly, vehicle-treated -/- mice displayed fibrosis and increased Sp1, skeletal actin, Nkx2.5, and GATA-4 expression without hypertrophy. Minipump removal caused the agonist-treated hearts and gene expression to regress to control or near-control levels. Thus Egr-1 deficiency caused a blunted catecholamine-induced hypertrophy response and increased sensitivity to stress.

Hypothyroidism prolongs mitotic activity in the post-natal mouse brain

Circulating T(4) and T(3) were measured during the first three post-natal weeks in the mouse and found to increase in a triphasic manner. The first increase occurred at post-natal day 6 and was simultaneous with a decrease in bromodeoxyuridine incorporation in areas showing post-natal mitosis. We investigated whether there was a causal relationship between increased thyroid hormone levels and decreased proliferation by inducing hypothyroidism in dams and progeny. Hypothyroidism prolonged mitotic activity in the olfactory bulb, hippocampus, subventricular zone and the cerebellar cortex. This suggests that the increase in T(3) at the end of the first postnatal week is implicated in terminating progenitor proliferation in many parts of the mouse brain.