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Thyroid hormones (THs) play important roles in development, metamorphosis, and metabolism in vertebrates. During the past century, TH functions were regarded as a synapomorphy of vertebrates. More recently, accumulating evidence has gradually convinced us that TH functions also occur in invertebrate chordates. To date, however, TH-related studies in non-chordate invertebrates have been limited. In this study, THs were qualitatively detected by two reliable methods (HPLC and LC/MS) in a well-studied molluscan species, the Pacific oyster  Crassostrea gigas . Quantitative measurement of THs during the development of  C .  gigas  showed high TH contents during embryogenesis and that oyster embryos may synthesize THs endogenously. As a first step in elucidating the TH signaling cascade, an ortholog of vertebrate TH receptor (TR), the most critical gene mediating TH effects, was cloned in  C .  gigas . The sequence of  CgTR  has conserved DNA-binding and ligand-binding domains that normally characterize these receptors. Experimental results demonstrated that  CgTR  can repress gene expression through binding to promoters of target genes and can interact with oyster retinoid X receptor. Moreover,  CgTR  mRNA expression was activated by T4 and the transcriptional activity of  CgTR  promoter was repressed by unliganded  CgTR  protein. An atypical thyroid hormone response element (CgDR5) was found in the promoter of  CgTR , which was verified by electrophoretic mobility shift assay (EMSA). These results indicated that some of the  CgTR  function is conserved. However, the EMSA assay showed that DNA binding specificity of  CgTR  was different from that of the vertebrate TR and experiments with two dual-luciferase reporter systems indicated that  l -thyroxine, 3,3′,5-triiodothyronine, and triiodothyroacetic acid failed to activate the transcriptional activity of  CgTR . This is the first study to functionally characterize TR in mollusks. The presence of THs and the functions of  CgTR  in mollusks contribute to better understanding of the evolution of the TH system.

plos one

Identification of Thyroid Hormones and Functional Characterization of Thyroid Hormone Receptor in the Pacific Oyster Crassostrea gigas Provide Insight into Evolution of the Thyroid Hormone System