cDNA cloning and expression analysis of thyroid hormone receptor in the coho salmon Oncorhynchus kisutch during smoltification

Two types of cDNA clones encoding thyroid hormone receptor (TR) genes, TRα and TRβ, were isolated from the coho salmon Oncorhynchus kisutch. The two genes each consists of multiple isoforms, derived from different loci or by an alternative splicing. RT-PCR revealed a broad localization of the TRα and TRβ transcripts that both expressed in brain, gill, liver, muscle, kidney, ovary, skin and eyeball. During the early stages of the smoltification period, the mRNA level of the TR transcripts showed a large individual variability at the maximum of 10-fold. In contrast, as the plasma thyroid hormone level elevated at the middle of the period, the variation in the expression level became much smaller at 2-fold. After the completion of the smoltification, the mRNA accumulation level of the TR genes rapidly decreased. These findings suggest that the dynamics of the receptor (TR) should be taken account in addition to the availability of the hormone ligands (T3 and T4) to understand the underlying mechanisms of the smoltification causing numerous physiological and morphological changes in salmonids.