Thyroid hormone receptor α1 is involved in the anterior formation of zebrafish, Danio rerio, embryo

Retinoic acid (RA) is a well-documented morphogen for vertebrate embryonic development wheareas RA-induced gene transcription has been known to be downregulated by thyroid hormone receptors (TRs). Because TRα1, a TR isoform is expressed in various vertebrate embryos before the formation of thyroid gland, it has been thus suggested to regulate the RA-mediated body pattern formation in a ligand-independent manner. In this report, we found that the treatment of 6 × 10^-6 M of RA to 1∼8 cell stage embryos blocked the formation of head. zTRα1 is predominantly expressed in early-stage embryos and neurulas before the formation of thyroid gland. Finally, when zTRα1 anti-sense RNA was injected into one-cell stage embryos to block the endogenous expression of zTRα1, it hindered the head formation as the exogenous RA treatment did. Taken together with our previous report that zTRα1 downregulates the RA-induced gene transcription, these observations indicate that zTRα1 expressed in early-stage embryos may control the normal development of anterior portion by counteracting RA-induced gene transcriptions.