Evolution of the Thyroid Hormone-Binding Protein, Transthyretin

Thyroid hormones have been identified in the plasma of all groups of vertebrates; they are pluripotent and are involved in growth, differentiation, metamorphosis, reproduction, hibernation, and thermogenesis

D.M. Power; N.P. Elias; S.J. Richardson; J. Mendes; C.M. Soares; C.R.A. Santos

2002

Scholarcy highlights

  • Thyroid hormones have been identified in the plasma of all groups of vertebrates; they are pluripotent and are involved in growth, differentiation, metamorphosis, reproduction, hibernation, and thermogenesis
  • The basic unit of all vertebrate thyroid glands has been conserved throughout evolution and consists of a follicle, formed by a single layer of epithelial cells enclosing a fluid-filled space or “colloid.” The follicle traps inorganic iodide, which is incorporated into thyroid hormone precursors, which are stored in the colloid
  • The organisation of the gland is characteristic for each vertebrate group; in mammals the thyroid is composed of two lobes connected by an isthmus across the ventral surface of the trachea; birds have two isolated, rounded lobes lying on either side of the trachea; in snakes and turtles there is a single discoid structure located anterior to the heart; the amphibian thyroid is bipartite; and in most bony fish there is no organised gland and the follicles are scattered singly or in small groups in the loose connective tissue of the pharynx
  • TH have a common chemical structure consisting of a hydrophobic thyronine nucleus, which accounts for their poor water solubility, a hydrophilic hydroxyl group attached to the phenolic ring and four iodines in positions 3, 5, 3Ј, and 5Ј in thyroxine and three at 3, 5, and 3Ј in triiodo-l-thyronine
  • In one study radiolabelled T4 was incubated with plasma from adult reptiles, amphibia, and fish and the resulting products were subsequently fractionated by SDS– PAGE followed by autoradiography
  • 4 1 in teleost fish and amphibia, TTR appears to have a higher affinity for T3 than for T4, the contradictory observations mentioned earlier may be explained by suggesting that albumin may be the principal T4-binding protein and TTR the principal T3-binding protein
  • Strong expression of TTR in the choroid plexus but not in the liver of the stumpy-tailed lizard, together with the strong conservation of TTR in the choroid plexus from reptiles to mammals, and the apparent absence from amphibians and fish, on the basis of ligand-binding studies with plasma from adults, led to the hypothesis that the expression of the TTR gene first arose in the brain of reptiles; it subsequently and independently arose in the liver of birds, eutherians, and diprotodont marsupials

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