LAUSR

Thyroid hormones (THs) are indispensable for each phyla in Chordata, while their functions in the non-chordate invertebrates are indistinct. Studies on the TH system in non-chordate invertebrates are important for understanding the evolution of TH system and may be applied in aquaculture or biofouling control at the same time. Iodothyronine deiodinases are keys to studying the TH system, as they are critical enzymes in maintaining TH homeostasis by catalyzing the initiation and termination of the effects of thyroid hormone in vertebrates. Here, we report the primary physiological effects of T4, the outer ring deiodinase activity, and a similar transcription regulation of two oyster deiodinases by TH receptor (CgTR) in an invertebrate, Pacific oyster Crassostrea gigas. L-thyroxine (T4) may have an important physiological function in the oyster, suggested by the growth retardation effect of excessive T4 in umbo larvae stage. The outer ring deiodinase activity transforming T4 to T3 (3, 3′, 5-triiodothyronine) was then detected in the Pacific oyster in vivo, which may be conducted by two oyster deiodinases (CgDx and CgDy). Transcription regulation of CgTR onto these two deiodinase genes was also verified by electrophoretic mobility shift assay and dual luciferase reporter assay in mammalian cells. These results contribute to a better understanding of the evolution of the TH system.