Molecular cloning of cDNA encoding thyroid stimulating hormone beta subunit of bighead carp Aristichthys nobilis and regulation of its gene expression.

The complementary DNA (cDNA) encoding pituitary thyroid stimulating hormone beta subunit (TSH-beta) of bighead carp was cloned and regulation of its gene expression was investigated for understanding phylogenetic divergence and evolution of TSH molecule. The cDNA was obtained from bighead carp pituitary total RNA by reverse transcription and polymerase chain reaction. Oligonucleotide primers were designed from the sequence of common carp. The full length sequence was then obtained by 3' and 5' rapid amplification of cDNA ends (RACE). The full-length sequence consisting of 3' and 5' untranslated regions was 585 bp long. The predicted amino acid sequence consisted of a signal peptide of 19 amino acid residues and a mature TSH beta subunit protein of 131 residues. The coding sequences of the cDNAs showed variable percentage homologies with those of other teleosts and vertebrate species. The predicted amino acid sequence shared 71% identity with rainbow trout and salmon, 90% with goldfish, 50% with eel and 94% with common carp in the mature protein region. The percentages of identity in the same region in comparison with bovine, porcine, rat, mouse, human and chicken were only 39, 42, 41, 40, 45 and 46%, respectively. TSH beta mRNA expression was found only in the pituitary tissue out of other tissues tested as testis, muscle, brain and heart. For the first time, thyrotropin releasing hormone (TRH) and thyroxine (T4) effects on pituitary TSH mRNA expression were tested in teleosts under in vitro conditions. TRH treatment on pituitary cells increased TSH beta mRNA level, while T4 treatment decreased TSH beta mRNA level. The present study provides a direct evidence, for the first time that TRH directly upregulates TSH beta gene expression in teleosts.