An F-domain introduced by alternative splicing regulates activity of the zebrafish thyroid hormone receptor α

Thyroid hormones (THs) play an important role in vertebrate development; however, the underlying mechanisms of their actions are still poorly understood. Zebrafish (Danio rerio) is an emerging vertebrate model system to study the roles of THs during development. In general, the response to THs relies on closely related proteins and mechanisms across vertebrate species, however some species-specific differences exist. In contrast to mammals, zebrafish has two TRα genes (thraa, thrab). Moreover, the zebrafish thraa gene expresses a TRα isoform (TRαA1) that differs from other TRs by containing additional C-terminal amino acids. C-terminal extensions, called "F domains", are common in other members of the nuclear receptor superfamily and modulate the response of these receptors to hormones. Here we demonstrate that the F-domain constrains the transcriptional activity of zebrafish TRα by altering the selectivity of this receptor for certain coactivator binding motifs. We found that the F-domain of zebrafish TRαA1 is encoded on a separate exon whose inclusion is regulated by alternative splicing, indicating a regulatory role of the F-domain in vivo. Quantitative expression analyses revealed that TRαA1 is primarily expressed in reproductive organs whereas TRαB and the TRαA isoform that lacks the F-domain (TRαA1-2) appear to be ubiquitous. The relative expression levels of these TRα transcripts differ in a tissue-specific manner suggesting that zebrafish uses both alternative splicing and differential expression of TRα genes to diversify the cellular response to THs.