Molecular and genetic studies suggest that thyroid hormone receptor is both necessary and sufficient to mediate the developmental effects of thyroid hormone

Abstract

Thyroid hormone (TH) affects diverse biological processes and can exert its effects through both gene regulation via binding the nuclear TH receptors (TRs) and non-genomic actions via binding to cell surface and cytoplasmic proteins. The critical importance of TH in vertebrate development has long been established, ranging from the formation of human cretins to the blockage of frog metamorphosis due the TH deficiency. How TH affects vertebrate development has been difficult to study in mammals due to the complications associated with the uterus-enclosed mammalian embryos. Anuran metamorphosis offers a unique opportunity to address such an issue. Using Xenopus as a model, we and others have shown that the expression of TRs and their heterodimerization partners RXRs (9-cis retinoic acid receptors) correlates temporally with metamorphosis in different organs in two highly related species, Xenopuslaevis and Xenopus tropicalis. In vivo molecular studies have shown that TR and RXR are bound to the TH response elements (TREs) located in TH-inducible genes in developing tadpoles of both species. More importantly, transgenic studies in X. laevis have demonstrated that TR function is both necessary and sufficient for mediating the metamorphic effects of TH. Thus, the non-genomic effects of TH have little or no roles during metamorphosis and likely during vertebrate development in general.

Fig. 1

Mechanisms of transcriptional regulation by TR. For TH-inducible genes, TR heterodimerized with RXR constitutively binds the TREs in their promoters or enhancers. In the absence of TH, TR binds corepressor complexes, such as those containing histone deacetylase HDAC3 and the highly related protein N-CoR or SMRT to inhibit transcription from the promoters. This is accomplished in part through deacetylation of lysine residues of histone H3 and H4 to induce a “closed” chromatin state, as suggested by the folding of histone tails (red beaded structure) on to the DNA helix, because of the charge-charge interaction between the positively charged histone tails and negatively charged DNA. The binding by TH induces a conformational change in TR, leading to the binding of coactivator complexes, such as those containing coactivators SRC and p300, which are histone acetyltransferases (HATs). They will acetylate histones H3 and H4, facilitating the formation of an “open” chromatin state, as diagramed by the unfolding of histone tails (red beaded structure) away from the DNA helix due to the neutralization of the positive charges on the histone tails by acetylation. Liganded TR can also recruit other coactivator complexes, such as chromatin remodeling complexes and mediator complex (also known as DRIP/TRAP complex), with the latter directly contacting RNA polymerase, to activate transcription.

Fig. 2

A dual function model of TR in frog development. During embryogenesis, TH response genes are expressed at basal levels in the absence of TR and TH to facilitate embryonic organ development. After tadpole hatching at stage 35/36, TRα expression increases, reaching high levels by stage 45 when tadpole feeding begins (Yaoita and Brown, 1990). RXRα is also highly expressed by this time (Wong and Shi, 1995), the TR/RXR heterodimers bind to TH response genes to repress their expression due to the lack of TH, thus ensuring proper tadpole growth and preventing premature metamorphosis. When endogenous TH level rises after stage 55 (Leloup and Buscaglia, 1977), the TH-bound TR/RXR heterodimers then activate TH response genes, such as the TRβ genes, leading to metamorphosis.

Fig. 3

TR is necessary for the metamorphic effects of TH. Transgenic expression of a dominant negative TR (dnTR) blocks TH-induced metamorphosis. Wild type animals treated with TH underwent characteristic changes, including gill resorption and limb morphogenesis (compared the middle panels to the ones on the left). The TH-treatment failed to induce such changes in the sibling transgenic animals (right panels, which resemble the left but not the middle ones) (Buchholz et al., 2003).

Fig. 4

TR is sufficient to mediate the metamorphic effects of TH. Wild type tadpoles and sibling tadpoles transgenic for a dominant positive TR (dpTR) under the control of a heat shock-inducible promoter were reared together in methimazole to block endogenous T3 synthesis and were heat-shocked daily for 8 days. For comparison, wild type tadpoles were treated with TH for 3 days. Note that the heat shock induction of dpTR expression resulted in metamorphic events, including gill resorption (bracket) and limb outgrowth (arrowhead) in the transgenic (middle) but not wild type (top) animals, just like TH-treatment of wild type sibling animals (bottom) (Buchholz et al., 2004).