Molecular cloning and characterization of thyroid hormone receptors in teleost fish

Thyroid hormones are pleiotropic factors important for many developmental a nd physiological functions in vertebrates. Their effects are mediated by tw o specific receptors (TR alpha and TR beta) which are members of the nuclea r hormone receptor super-family. To clarify the function of these receptors , our laboratory has started a comparative study of their role in teleost f ish. This type of approach has been hampered by the isolation of specific c lones for each fish species studied. In this report, we describe an efficie nt reverse transcription/PCR procedure that allows the isolation of large f ragments corresponding to TR alpha and TR beta of a wide range of teleost f ish. Phylogenetic analysis of these receptors revealed a placement consiste nt with their origin, sequences from teleost fish being clearly monophyleti c for both TR alpha and TR beta. Interestingly, this approach allowed us to isolate (from tilapia and salmon) several new TR alpha or TR beta isoforms resulting from alternative splicing. These isoforms correspond to expresse d transcripts and thus may have an important physiological function. In add ition, we isolated a cDNA encoding TR beta in the Atlantic salmon (Salmo sa lar) encoding a functional thyroid hormone receptor which binds specific th yroid hormone response elements and regulates transcription in response to thyroid hormones.