Functional significance of a truncated thyroid receptor subtype lacking a hormone-binding domain in goldfish

Thyroid hormones are important mediators of growth and development in vertebrates and act by binding to a specific family of thyroid receptors (TRs). The TRs belong to the nuclear receptor superfamily, with two conserved regions, a DNA binding domain and a ligand binding domain (LBD). We recently demonstrated the presence of four TR subtypes in goldfish, two with complete DNA binding domains and LBDs (TRα-1 and TRβ) and two novel forms including a transcript resembling TRα with variation in the LBD as well as a TRα-truncated (TRα-t) form lacking a LBD. To study the functional significance of TR subtypes, we first investigated the regulation of hepatic goldfish deiodinase type 3 (D3) by T₃ and validated a bioassay in which D3 gene expression is up-regulated significantly in vivo and in vitro. Using short interfering RNA, TRα-1, TRβ, or TRα-t was specifically knocked down and thyroid hormone-induced D3 gene expression was measured. short interfering RNA against TRα-1 or TRβ reduced the T₃ induction of deiodinase gene expression to 50% or less than 25% of control (T₃ treated) cells, respectively. Knocking down TRα-t alone, however, increased D3 expression 500-fold supporting the hypothesis that TRα-t plays a modulatory role in thyroid hormone-induced gene expression. Our results provide important insight into thyroid receptor biology in goldfish and a framework for the better understanding of thyroid receptor function in all vertebrates.