Distinct functions for thyroid hormone receptors alpha and beta in brain development indicated by differential expression of receptor genes.

Thyroid hormones are essential for correct brain development, and since vertebrates express two thyroid hormone receptor genes (TR alpha and beta), we investigated TR gene expression during chick brain ontogenesis. In situ hybridization analyses showed that TR alpha mRNA was widely expressed from early embryonic stages, whereas TR beta was sharply induced after embryonic day 19 (E19), coinciding with the known hormone-sensitive period. Differential expression of TR mRNAs was striking in the cerebellum: TR beta mRNA was induced in white matter and granule cells after the migratory phase, suggesting a main TR beta function in late, hormone-dependent glial and neuronal maturation. In contrast, TR alpha mRNA was expressed in the earlier proliferating and migrating granule cells, and in the more mature granular and Purkinje cell layers after hatching, indicating a role for TR alpha in both immature and mature neural cells. Surprisingly, both TR genes were expressed in early cerebellar outgrowth at E9, before known hormone requirements, with TR beta mRNA restricted to the ventricular epithelium of the metencephalon and TR alpha expressed in migrating cells and the early granular layer. The results implicate TRs with distinct functions in the early embryonic brain as well as in the late phase of hormone requirement.     

Contrasting skeletal phenotypes in mice with an identical mutation targeted to thyroid hormone receptor alpha1 or beta

Thyroid hormone (T(3)) regulates bone turnover and mineralization in adults and is essential for skeletal development. Surprisingly, we identified a phenotype of skeletal thyrotoxicosis in T(3) receptor beta(PV) (TRbeta(PV)) mice in which a targeted frameshift mutation in TRbeta results in resistance to thyroid hormone. To characterize mechanisms underlying thyroid hormone action in bone, we analyzed skeletal development in TRalpha1(PV) mice in which the same PV mutation was targeted to TRalpha1. In contrast to TRbeta(PV) mice, TRalpha1(PV) mutants exhibited skeletal hypothyroidism with delayed endochondral and intramembranous ossification, severe postnatal growth retardation, diminished trabecular bone mineralization, reduced cortical bone deposition, and delayed closure of the skull sutures. Skeletal hypothyroidism in TRalpha1(PV) mutants was accompanied by impaired GH receptor and IGF-I receptor expression and signaling in the growth plate, whereas GH receptor and IGF-I receptor expression and signaling were increased in TRbeta(PV) mice. These data indicate that GH receptor and IGF-I receptor are physiological targets for T(3) action in bone in vivo. The divergent phenotypes observed in TRalpha1(PV) and TRbeta(PV) mice arise because the pituitary gland is a TRbeta-responsive tissue, whereas bone is TRalpha responsive. These studies provide a new understanding of the complex relationship between central and peripheral thyroid status.     

Resistance to thyroid hormone mediated by defective thyroid hormone receptor alpha

Background

Thyroid hormone acts via receptor subtypes (TRα1, TRβ1, TRβ2) with differing tissue distributions, encoded by distinct genes (THRA, THRB). THRB mutations cause a disorder with central (hypothalamic–pituitary) resistance to thyroid hormone action with markedly elevated thyroid hormone and normal TSH levels.

Scope of review

This review describes the clinical features, genetic and molecular pathogenesis of a homologous human disorder mediated by defective THRA. Clinical features include growth retardation, skeletal dysplasia and constipation associated with low-normal T4 and high-normal T3 levels and a low T4/T3 ratio, together with subnormal reverse T3 levels. Heterozygous TRa1 mutations in affected individuals generate defective mutant receptors which inhibit wild-type receptor action in a dominant negative manner.

Major conclusions

Mutations in human TRα1 mediate RTH with features of hypothyroidism in particular tissues (e.g. skeleton, gastrointestinal tract), but are not associated with a markedly dysregulated pituitary–thyroid axis.

General significance

Human THRA mutations could be more common but may have eluded discovery due to the absence of overt thyroid dysfunction. Nevertheless, in the appropriate clinical context, a thyroid biochemical signature (low T4/T3 ratio, subnormal reverse T3 levels), may enable future identification of cases.This article is part of a Special Issue entitled Thyroid hormone signalling.     

Early expression of thyroid hormone receptor beta and retinoid X receptor gamma in the Xenopus embryo

The role of thyroid hormone in Xenopus metamorphosis is particularly well understood as it plays an essential role in that process. However, recent evidence suggests that thyroid hormone may play an earlier role in amphibian embryogenesis. We demonstrate that Xenopus thyroid hormone receptor beta (XTR beta) is expressed shortly after neural fold closure, and that its expression is localized to the developing retina. Retinoid X receptor gamma (RXR gamma), a potential dimerization partner for XTR beta, was also found to be expressed in the retina at early stages, and at later stages RXR gamma was also expressed in the liver diverticulum. Addition of either thyroid hormone or 9-cis retinoic acid, the ligands for XTR beta and RXR gamma, respectively, did not alter the expression of their receptors. However, the addition of thyroid hormone and 9-cis retinoic acid did alter rhodopsin mRNA expression. Addition of thyroid hormone generates a small expansion of the rhodopsin expression domain. When 9-cis retinoic acid or a combination of thyroid hormone and 9-cis retinoic acid was administered, there was a decrease in the expression domain of rhodopsin in the developing retina. These results provide evidence for an early role for XTR beta and RXR gamma in the developing Xenopus retina.     

Genomic organization and alternative promoter usage of the two thyroid hormone receptor beta genes in Xenopus laevis.

Each of the two Xenopus laevis thyroid hormone receptor beta genes is at least 70 kilobases in length with similar intron-exon organization. There are up to eight alternatively spliced exons in the 5'-untranslated region. Excluding the extreme amino terminus, each receptor is encoded by six exons spanning about 6 kilobases of the genome, in which each of the two zinc fingers that comprise the DNA-binding domain is encoded by a separate exon and the hormone-binding domain is split into three exons. The last exon of the coding region also contains at least 600 base pairs of the 3'-untranslated region, which is about 8 kilobases. Each of the receptor genes has two promoters and just one of them is up-regulated in tadpoles by the administration of thyroid hormone.     

Thyroid hormone alters the DNA binding properties of chicken thyroid hormone receptors alpha and beta.

The effects of thyroid hormone agonists on thyroid hormone receptor (TR)/DNA complex formation was investigated to elucidate the mechanism by which TRs transactivate genes in response to ligand. The data, obtained from gel shift experiments, indicate that thyroid hormones alter the conformation of TRs bound to DNA, irrespective of if the element is occupied by monomeric TR, homodimeric TR/TR, or heterodimeric complexes with the retinoid receptors RAR or RXR. Furthermore, triiodo-thyronine (T3) prevents 2 TR molecules from binding to oligonucleotides containing direct repeats or inverted palindromes of the consensus AGGTCA motif, an effect that was not detected with palindromic elements. Heterodimers bound to direct repeats were less affected: RXR/TR were fully and RAR/TR complexes partially resistant to thyroid hormone. The data suggest that a ligand-induced conformational change in TR prevents double TR occupancy of a response element containing 2 direct repeats of the consensus binding motif, possibly by steric hindrance, whereas such an event does not prevent TR/RXR heterodimers from binding to DNA. Finally, our data show that a monomeric, liganded TR bound preferentially to the second half site in a AGGTCActcaAGGTCA element, and therefore indicate that nucleotides adjacent to the consensus half site contribute to binding specificity.     

Characterization and developmental expression of beta tubulin genes in Drosophila melanogaster.

Genomic clones containing beta tubulin sequences were isolated from a lambda library of Drosophila melanogaster. In situ hybridization localized three genes to 56D and 60B on chromosome 2 as well as to 85D on chromosome 3. The latter was known through genetic analysis to be specifically expressed during spermatogenesis. The genomic clone, pTu85, derived from this region contains one complete beta tubulin coding region as well as the 3' end of an additional so far unidentified beta tubulin gene. Genomic Southern hybridizations reveal a total of five fragments with beta tubulin homology. Clone pTu56 codes for an RNA of 1.8 kb which is expressed in all developmental stages. Clone pTu60 codes for a 2.5-kb RNA expressed during embryogenesis and pupation. In testes RNA we detected a 2.2-kb message homologous to pTu85.     

Effects of bisphenol A on thyroid hormone-dependent up-regulation of thyroid hormone receptor alpha and beta and down-regulation of retinoid X receptor gamma in Xenopus tail culture

We investigated effects of different concentrations (10(-7) - 10(-5) M) of bisphenol A (BPA), which is known as an estrogenic and anti-thyroid hormonal endocrine disrupter, on the expression of thyroid hormone receptor (TR) alpha and beta and retinoid X receptor (RXR) gamma mRNA in tails of stage 52-54 Xenopus tadpoles in organ culture in the presence or absence of different concentrations of triiodo-thyronine (T(3)). In the absence of T(3), BPA at any concentration examined did not show remarkable effects on tail length but blocked 10(-7) M T(3)-induced tail resorption in a concentration-dependent manner. Semi-quantitative analyses of TRalpha and TRbeta mRNAs by RT-PCR in the tail specimens indicated that BPA shows an apparent antagonistic effect towards the receptors and reduced their mRNA levels relative to controls. When administered together with 10(-7) M T(3), the antagonistic effects of BPA were detected more clearly and dose-dependently. While BPA prevented the autoinduction of both TRalpha and TRbeta genes by T(3), the effect was less marked on TRalpha than on TRbeta. BPA also moderately suppressed RXRgamma gene expression. Gene expression of RXRgamma, a partner for heterodimer formation of TRs, was supressed by T(3) alone and also by BPA alone, but no additive effects were observed so far as studied. The present study indicates that a relatively low concentration of BPA, 10(-7) M, as compared with those examined previously (10(-5) to 10(-4) M) by us and other investigators, acts as an antagonist of T(3) through suppression of TRalpha and TRbeta gene expression in Xenopus tail in culture.