Characterization of thyroid hormone receptors during early development of the Japanese eel (Anguilla japonica)

• Japanese eel larvae provide a good model for early development of Anguilliformes.
• Cloned TR genes encode proteins conserved in their active regions.
• TRs showed TH-dependent activation of transcription from a TH-responsive promoter.
• Expression of TRαs peaked during pre-metamorphic stage.
• Expression patterns during metamorphosis resembled those in other Anguilliformes.

We studied the profiles of thyroid hormone receptors (TRs) in Japanese eels (Anguilla japonica) during development from hatched larvae to juveniles. Two TRαs (TRαA and TRαB) and one TRβ (TRβA) cDNA clones were generated by RACE. The TRαA, TRαB and TRβA cDNAs encoded 416, 407 and 397 amino acid proteins with much higher homologies to the Japanese conger eel (Conger myriaster) TRs than to other fish TRs. In a transiently transfected Japanese eel cell line, Hepa-E1, the TRs showed thyroid hormone (TH)-dependent activation of transcription from the TH-responsive promoter. Four TR cDNA clones, including TRβB reported in a previous study, were analyzed by real-time RT-PCR. The TR mRNA levels in hatched larvae were determined. The two TRβ mRNAs were present at low levels but there was a peak in the TRαs during the larval stage before metamorphosis. During metamorphosis, the two TRαs both exhibited peaks and expression of the two TRβs was higher than during the early growth stage. This expression pattern is similar to that of the Japanese conger eel. It is possible that thyroid hormones control the early development of Japanese eels and Japanese conger eels through TRs. This is the first analysis of the expression sequence of TRs during early larval stages of Anguilliformes.