Thyroid Hormone Receptors in Two Model Species for Vertebrate Embryonic Development: Chicken and Zebrafish

Chicken and zebrafish are two model species regularly used to study the role of thyroid hormones in vertebrate development. Similar to mammals, chickens have one thyroid hormone receptor alpha (TR alpha) and one TR beta gene, giving rise to three TR isoforms: TRa, TR beta 2, and TR beta 0, the latter with a very short amino terminal domain. Zebrafish also have one TR beta gene, providing two TR beta 1 variants. The zebrafish TRa gene has been duplicated, and at least three TRa isoforms are expressed: TRaA1-2 and TRaB are very similar, while TR alpha A1 has a longer carboxy-terminal ligand-binding domain. All these TR isoforms appear to be functional, ligand-binding receptors. As in other vertebrates, the different chicken and zebrafish TR isoforms have a divergent spatiotemporal expression pattern, suggesting that they also have distinct functions. Several isoforms are expressed from the very first stages of embryonic development and early chicken and zebrafish embryos respond to thyroid hormone treatment with changes in gene expression. Future studies in knockdown and mutant animals should allow us to link the different TR isoforms to specific processes in embryonic development.