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 (TRalpha-1 and TRbeta) and two novel forms including a transcript resembling TRalpha with variation in the LBD as well as a TRalpha-truncated (TRalpha-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, TRalpha-1, TRbeta, or TRalpha-t was specifically knocked down and thyroid hormone-induced D3 gene expression was measured. Short interfering RNA against TRalpha-1 or TRbeta reduced the T3 induction of deiodinase gene expression to 50% or less than 25% of control (T3 treated) cells, respectively. Knocking down TRalpha-t alone, however, increased D3 expression 500-fold supporting the hypothesis that TRalpha-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.