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

Levothyroxin restores hypothyroidism-induced impairment of LTP of hippocampal CA1: Electrophysiological and molecular studies

Hypothyroidism impairs synaptic plasticity as well as learning and memory. Clinical reports are conflicting about the ability of thyroid hormone replacement therapy to fully restore the hypothyroidism-induced learning and memory impairment. Recently, we have shown that hypothyroidism impairs LTP and cognition in adult rats. We have studied the effect of thyroxin replacement therapy on hypothyroidism-induced LTP impairment using electrophysiological and molecular approaches. Recording from CA1 region of the hippocampus in anesthetized adult rat indicated that 6 weeks of thyroxin replacement therapy (20 microg/kg/day) fully restored LTP impaired by hypothyroidism. Western blotting showed reduction in phosphorylated (P)-CAMKII, total-CaMKII, neurogranin, and calmodulin basal levels in the CA1 region of the hippocampus of hypothyroid rats. The levels of these molecules were normalized by thyroxin replacement therapy. The hypothyroid-induced elevation of basal calcineurin levels and activity was also normalized by thyroxin treatment. However, thyroxin replacement therapy did not restore hypothyroidism-induced reduction in PKCgamma basal protein levels. Additionally, real-time PCR, showed a reduction in basal neurogranin mRNA level that was normalized by thyroxin replacement therapy. In the sham (control) rats, induction of LTP by high-frequency stimulation increases P-CaMKII, and total CaMKII levels as well as CaMKII phosphotransferase activity. However, in hypothyroid rats, the same stimulation protocol induced an increase only in total-CaMKII. Thyroxin treatment normalized the levels and activity of these molecules. The results demonstrated that thyroxin therapy normalized the electrophysiological and molecular effects of hypothyroidism on the CA1 region and emphasized the critical role P-CaMKII plays in hypothyroidism-induced LTP impairment.

Anxiety, memory impairment, and locomotor dysfunction caused by a mutant thyroid hormone receptor alpha1 can be ameliorated by T3 treatment

The transcriptional properties of unliganded thyroid hormone receptors are thought to cause the misdevelopment during hypothyroidism of several functions essential for adult life. To specifically determine the role of unliganded thyroid hormone receptor alpha1 (TRalpha1) in neuronal tissues, we introduced a mutation into the mouse TRalpha1 gene that lowers affinity to thyroid hormone (TH) 10-fold. The resulting heterozygous mice exhibit several distinct neurological abnormalities: extreme anxiety, reduced recognition memory, and locomotor dysfunction. The anxiety and memory deficiencies were relieved by treatment with high levels of TH in adulthood, an effect that correlated with a normalization of GABAergic inhibitory interneurons in the hippocampal CA1 region. In contrast, a post-natal TH treatment was necessary and sufficient for ameliorating the adult locomotor dysfunction. Here, the hormone treatment normalized the otherwise delayed cerebellar development. The data thus identify two novel and distinct functions of an unliganded TRalpha1 during development and adulthood, respectively.

Molecular insight into the effects of hypothyroidism on the developing cerebellum

Despite the recognized importance of thyroid hormones for normal brain development, little is known about the critical molecular events underlying this role. We investigated the molecular basis of thyroid hormone action on the developing brain by comparing genome-wide gene expression patterns in the cerebellum between euthyroid and hypothyroid juvenile mice using microarrays. Pregnant dams were treated with 0.1% or 0.04% 6-propyl-2-thiouracil (PTU) in drinking water continuously from day 13 post conception until weaning to produce hypothyroid pups. Cerebella were collected from vehicle and 0.1% PTU treated pups at post-natal day (PND) 15, and mRNA from these was subjected to microarray analysis using Agilent high-density oligonucleotide chips. Statistical analysis (MAANOVA) revealed significant differential expression in 2940 genes including 1357 up- and 1583 down-regulated genes. Further analysis (combined MAANOVA and ANOVA) identified 204 significantly altered genes. Hypothyroidism had a greater effect on gene expression in male than in female pups. Transcriptional changes in several genes [Syt12 (Synaptotagmin 12), Rcor (RE1-silencing transcription factor co-repressor), Bag3 (Bcl-associated athanogene 3), p21, cyclin D, Bax (Bcl2-associated X protein), and Pcp2 (Purkinje cell protein 2)] were confirmed using real-time (RT) PCR analysis. Significantly altered expression of Bag3 in cerebella from PND 15 and PND 60 pups exposed to PTU suggests permanent functional alterations in the hypothyroid brain. The thyroid hormone negative regulation of Rcor expression was confirmed in vitro using HepG2 cells. In addition to Rcor, expression of several other genes that code for critical components of the REST (RE1-silencing transcription factor) pathway was shown to be altered in hypothyroid animals. These results suggest that modification of this pathway may have a significant role in causing impaired development in the hypothyroid brain.

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

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

The thyroid hormone receptor beta-specific agonist GC-1 selectively affects the bone development of hypothyroid rats

We investigated the effects of GC-1, a TRbeta-selective thyromimetic, on bone development of hypothyroid rats. Whereas T3 reverted the IGF-I deficiency and the skeletal defects caused by hypothyroidism, GC-1 had no effect on serum IGF-I or on IGF-I protein expression in the epiphyseal growth plate of the femur, but induced selective effects on bone development. Our findings indicate that T3 exerts some essential effects on bone development that are mediated by TRbeta1.

INTRODUCTION

We investigated the role of the thyroid hormone receptor beta1 (TRbeta1) on skeletal development of rats using the TRbeta-selective agonist GC-1.

MATERIALS AND METHODS

Twenty-one-day-old female rats (n = 6/group) were rendered hypothyroid (Hypo) and treated for 5 weeks with 0.3 ug/100 g BW/day of T3 (1xT3), 5xT3, or equimolar doses of GC-1 (1xGC-1 and 5xGC-1). Serum triiodothyronine (T3), thyroxine (T4), thyroid-stimulating hormone (TSH), and insulin-like growth factor (IGF)-I concentrations were determined by radioimmunoassay (RIA). BMD and longitudinal bone growth were determined by DXA. Trabecular bone histomorphometry and epiphyseal growth plate (EGP) morphometry were performed in the distal femur. Expressions of IGF-I protein and of collagen II and X mRNA were evaluated by immunohistochemistry and in situ hybridization, respectively. To determine hormonal effects on ossification, skeletal preparations of hypothyroid-, 5xGC-1-, and 5xT3-treated neonatal rats were compared.

RESULTS

Hypothyroidism impaired longitudinal body growth and BMD gain, delayed ossification, reduced the number of hypertrophic chondrocytes (HCs; 72% versus Euthyroid [Eut] rats; p < 0.001), and resulted in disorganized columns of EGP chondrocytes. Serum IGF-I was 67% reduced versus Eut rats (p < 0.001), and the expression of IGF-I protein and collagen II and X mRNA were undetectable in the EGP of Hypo rats. T3 completely or partially normalized all these parameters. In contrast, GC-1 did not influence serum concentrations or EGP expression of IGF-I, failed to reverse the disorganization of proliferating chondrocyte columns, and barely affected longitudinal growth. Nevertheless, GC-1 induced ossification, HC differentiation, and collagen II and X mRNA expression and increased EGP thickness to Eut values. GC-1-treated rats had higher BMD gain in the total tibia, total femur, and in the femoral diaphysis than Hypo animals (p < 0.05). These changes were associated with increased trabecular volume (48%, p < 0.01), mineralization apposition rate (2.3-fold, p < 0.05), mineralizing surface (4.3-fold, p < 0.01), and bone formation rate (10-fold, p < 0.01).

CONCLUSIONS

Treatment of hypothyroid rats with the TRbeta-specific agonist GC-1 partially reverts the skeletal development and maturation defects resultant of hypothyroidism. This finding suggests that TRbeta1 has an important role in bone development.

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.

Aberrant maturation of astrocytes in thyroid hormone receptor alpha 1 knockout mice reveals an interplay between thyroid hormone receptor isoforms

Although the effects of thyroid hormones on the development of neurons and oligodendrocytes are well documented, less is known about the hormonal effects on astrocytes. Our analyses of cerebellar slices from 2-month-old T(3) receptor protein (TR)alpha1-deficient mice show that mature astrocytes, Golgi epithelial cells, and their Bergmann processes had strongly reduced glial fibrillary acidic protein (GFAP) and nestin immunoreactivity, in contrast to wild-type mice. Furthermore, the Bergmann processes exhibited an irregular GFAP staining. A similar expression of nestin and GFAP was observed in 11-d-old (P11) mutant pups. Surprisingly, however, hypothyroidism normalized the appearance of these markers in the P11 mutants, suggesting that liganded TR beta is detrimental to astroglial cell differentiation in the absence of TR alpha 1. To test this hypothesis, hypothyroid mice were treated from birth until P11 with the TR beta-selective ligand GC-1. This treatment was devastating in the TR alpha 1(-/-) mice, causing little if any nestin or GFAP immunoreactivity, whereas the wild-type mice were normal. The results thus indicate an important interplay between thyroid hormone receptor isoforms in astroglial cell maturation.