Characterization of thyroid hormone receptor α and β in the metamorphosing Japanese conger eel, Conger myriaster

Abstract

Two distinct TRα cDNA clones (TRαA and TRαB) were isolated from conger eel ( Conger myriaster). The deduced amino acid sequences of the conger eel TRαs showed higher homologies to the TRαs of other vertebrates than to TRβs. Determination of TR mRNA in metamorphosing eels was performed using competitive RT-PCR. Of the two TRα mRNAs identified, TRαA mRNA expression was shown to be relatively higher than that of TRαB, and there was a peak in the expression of each during metamorphic climax. We hypothesize that both TRαs play important roles in morphological differentiation during metamorphosis. The expression pattern of TRβ1 mRNA was also higher during metamorphic climax and high levels of expression continued after metamorphosis. This suggests that TRβ1 is the adult fish form which appears at high frequency after metamorphosis. It was also shown that TRβ2 is highly expressed specifically in the brain and pituitary gland in larvae and juvenile forms including during metamorphosis and there was a peak in TRβ2 mRNA in the elver after metamorphosis. Thus, we propose that TRβ2 plays an important role in the regulation of the hypothalamic–pituitary–thyroid axis.