The zebrafish thyroid hormone receptor α1 is expressed during early embryogenesis and can function in transcriptional repression

Nuclear receptors are a large family of ligand dependent transcription factors which participate in many diverse processes during development. In this report, we describe the cloning of the zebrafish thyroid hormone receptor alpha 1 (TR alpha 1) gene, the cellular counter-part of the viral oncogene v-erbA. TR alpha 1 is expressed during oogenesis and maternally supplied to the embryo. TR alpha 1 is expressed again after the mid blastula transition. By examining the effects of increased expression of TR alpha 1 on expression of a reporter gene which responds to both TR alpha 1 and retinoic acid receptors (RARs), we show that the zebrafish TR alpha 1 can act as a repressor during early zebrafish development before thyroid hormone is present in the embryo. In addition, our data suggest that TR alpha 1 can repress retinoic acid (RA)-signaling during early development. We propose that TR alpha 1 functions during early development as a transcriptional repressor, similar to the constitutive repressor activity of its viral counterpart v-erbA, which regulates anterior-posterior (A/P) patterning by repressing RA-signaling.