Olivier Chassande

The T3Rα gene encoding a thyroid hormone receptor is essential for post‐natal development and thyroid hormone production

The diverse functions of thyroid hormones are thought to be mediated by two nuclear receptors, T3Rα1 and T3Rβ, encoded by the genes T3Rα and T3Rβ respectively. The T3Rα gene also produces a non‐ligand‐binding protein T3Rα2. The in vivo functions of these receptors are still unclear. We describe here the homozygous inactivation of the T3Rα gene which abrogates the production of both T3Rα1 and T3Rα2 isoforms and that leads to death in mice within 5 weeks after birth. After 2 weeks of life, the homozygous mice become progressively hypothyroidic and exhibit a growth arrest. Small intestine and bones showed a strongly delayed maturation. In contrast to the negative regulatory function of the T3Rβ gene on thyroid hormone production, our data show that the T3Rα gene products are involved in up‐regulation of thyroid hormone production at weaning time. Thus, thyroid hormone production might be balanced through a positive T3Rα and a negative T3Rβ pathway. The abnormal phenotypes observed on the homozygous mutant mice strongly suggest that the T3Rα gene is essential for the transformation of a mother‐dependent pup to an ‘adult’ mouse. These data define crucial in vivo functions for thyroid hormones through a T3Rα pathway during post‐natal development.

Genetic Analysis Reveals Different Functions for the Products of the Thyroid Hormone Receptor α Locus

ABSTRACT Thyroid hormone receptors are encoded by the TRα(NR1A1) and TRβ (NR1A2) loci. These genes are transcribed into multiple variants whose functions are unclear. Analysis by gene inactivation in mice has provided new insights into the functional complexity of these products. Different strategies designed to modify the TRα locus have led to strikingly different phenotypes. In order to analyze the molecular basis for these alterations, we generated mice devoid of all known isoforms produced from the TRα locus (TRα0/0). These mice are viable and exhibit reduced linear growth, bone maturation delay, moderate hypothermia, and reduced thickness of the intestinal mucosa. Compounding TRα0 and TRβ− mutations produces viable TRα0/0β−/− mice, which display a more severe linear growth reduction and a more profound hypothermia as well as impaired hearing. A striking phenotypic difference is observed between TRα0/0 and the previously described TRα−/− mice, which retain truncated TRΔα isoforms arising from a newly described promoter in intron 7. The lethality and severe impairment of the intestinal maturation in TRα−/− mice are rescued in TRα0/0animals. We demonstrate that the TRΔα protein isoforms, which are natural products of the TRα locus, are the key determinants of these phenotypical differences. These data reveal the functional importance of the non-T3-binding variants encoded by theTRα locus in vertebrate postnatal development and homeostasis.

Effects of ligand and thyroid hormone receptor isoforms on hepatic gene expression profiles of thyroid hormone receptor knockout mice.

Little is known about the overall patterns of thyroid hormone (Th)‐mediated gene regulation by the main Th receptor (Tr) isoforms, Tr‐α and Tr‐β, in vivo. We used 48 complementary DNA microarrays to examine hepatic gene expression profiles of wild‐type and Thra and Thrb knockout mice under different Th conditions: no treatment, treatment with 3,3′,5‐triiodothyronine (T3), Th‐deprivation using propylthiouracil (PTU), and treatment with a combination of PTU and T3. Hierarchical clustering analyses showed that positively regulated genes fit into three main expression patterns. In addition, only a subpopulation of target genes repressed basal transcription in the absence of ligand. Interestingly, Thra and Thrb knockout mice showed similar gene expression patterns to wild‐type mice, suggesting that these isoforms co‐regulate most hepatic target genes. Differences in the gene expression patterns of Thra/Thrb double‐knockout mice and Th‐deprived wild‐type mice show that absence of receptor and of hormone can have different effects. This large‐scale study of hormonal regulation reveals the functions of Th and of Tr isoforms in the regulation of gene expression patterns.

Regulation of expression of thyroid hormone receptor isoforms and coactivators in liver and heart by thyroid hormone

Autoregulation of thyroid hormone (TH) receptors (TRs) is a mechanism whereby a cell can regulate its responsiveness to TH. Nuclear coactivators (NCoAs) modulate TH action and may also be important for regulation of TR expression. We have determined the effect of TH withdrawal and treatment on the expression of different isoforms of TR as well as expression of the NCoAs SRC-1, TIF-2 and SRC-3 using quantitative real time polymerase chain reaction. In order to identify the effect that each TR isoform exerts over the expression of the other, NCoA and TR transcripts were measured in liver and heart tissue from wild type mice or mice with deletion of either TR isoform or SRC-1 genes. In liver, regulation of TR beta1 and TR alpha2 subtype expression is inversely related to TH levels and the regulation of TR beta expression is, in part, controlled by TR alpha. In the heart, the opposite is the case, regulation of TR alpha2 and TR beta1 isoform expression is directly related to TH levels and this regulation is primarily controlled by TR alpha. Although NCoAs are, in general, increased in response to hypothyroidism or in states of TH resistance, SRC-1 specifically does not regulate TR isoform expression. We have demonstrated that TR isoforms and NCoAs are autoregulated transcription factors with tissue specificity.