Preliminary Study On The Structure And Function Of Iodothyronine Deiodinase From Scallop Chlamys Farreri

Thyroid hormone system is one of the most important components ofNeuroendocrine-immune regulatory network, and it plays crucial roles in theimmunomodulation and homeostasis maintenance in vertebrate. Iodothyroninedeiodinase, a kind of key enzyme on the regulation of active thyroid hormone, alsoinvolves in the immunomodulation of vertebtates. The knowledge about thyroidhormone system and iodothyronine deiodinase is mainly from vertebrates, while it isquite meagre in invertebrates so far. In the present study, the cDNA ofiodothyronine deiodinase from scallop was cloned and it features of structure andfunction were investigated by the techniques of molecular biology and bioinformatics.The full-length cDNA of an iodothyronine deiodinase (designated as CfDx) wascloned from scallop Chlamys farreri. The complete cDNA sequence of CfDx was of1404bp, and it contained an open reading frame of900bp encoding a polypeptide of299amino acids. The deduced amino acid sequence of CfDx contained an in-frameTGA stop codon probably encoding an essential selenocysteine (SeC), and there was aconserved region of about15amino acids surrounding the SeC residue. CfDx sharedlow similarity of19.1%-23.9%with iodothyronine deiodinase identified fromvertebrates, and the similarity with the only one invertebrate iodothyronine deiodinasewas27.3%. The CfDx mRNA transcripts were detected in all the tested tissues,including haemocytes, hepatopancreas, kidney, adductor muscle, gonad, gill andmantle, with the higher expression level in hepatopancreas and kidney.After LPS stimulation, the CfDx mRNA expression level in haemocytes increasedsignificantly at12h (25.35-fold, P <0.05), and the concentration of T3in haemolymph increased significantly at12h (3.62ng dL-1, P <0.05) even theconcentration of T4did not change significantly. After an injection of50μg CfDxdsRNA, the expression level of CfDx mRNA in haemocytes of scallops began todecrease significantly at36h and maintained the relative low level (about0.3-fold ofthe PBS control group) from36to72h. The ratio of T4/T3in haemolymph began toincrease at36h (2.31-fold, P <0.05) and kept increasing from36to72h comparingwith that in the PBS control group.All the results suggested that iodothyronine deiodinase from scallop CfDx was animportant enzyme in the thyroid hormone system, which could transform thyroxineT4to the triiodothyronine T3. The iodothyronine deiodinase CfDx shared similarityon the structure and function of iodothyronine deiodinase from vertebtates, suggestingthat CfDx from scallop might be the molecular ancestor of iodothyronine deiodinasefrom vertebtates. When the immune system of scallop wass actived, the expression ofiodothyronine deiodinase was provoked which subsequently enhanced T3concentration to release more immune mediators. The research above can providemore fundamental information about the invertebtate thyroid hormone system as wellas the neuro-endocrine-immune network of invertebrate.