Evolutionary Insights from Fish Transthyretin

Thyroid hormones (THs) synthesised and released by the thyroid follicles in fish circulate bound to TH distributor proteins (THDPs). Transthyretin (TTR) is a tetrameric THDP with a conserved structure in vertebrates and functions as a high affinity T4 binder in tetrapods and as a dual T3/T4 binder in fish. The difference in TH affinity appears to be linked to the longer N-terminal region in piscine TTR. Relatively few studies exist on gene regulation in fish, but those that do indicate that it may be similar to those described in mammals. Recently, a family of transthyretin-like proteins was identified and shown to correspond to the enzyme 5-hydroxyisourate hydrolase (HIUase) of purine metabolism in zebra fish. A combination of in silico and laboratory-based approaches indicate that TTR has a widespread distribution but is most abundant in liver, while HIUase expression seems to be mainly in the liver. Gene transcripts encoding TTR and HIUase genes are described in representatives of the orders Cypriniformes, Salmoniformes, Pleuronectiformes, Perciformes, Tetraodontiformes, Beloniformes, Gasterosteiformes, Gadiformes and Petromyzontiformes. The evolution of TTR and HIUase genes in fish is discussed.