Thyroglobulin Represents a Novel Molecular Architecture of Vertebrates

Abstract

Thyroid hormones modulate not only multiple functions in vertebrates (energy metabolism, central nervous system function, seasonal changes in physiology, and behavior) but also in some non-vertebrates where they control critical post-embryonic developmental transitions such as metamorphosis. Despite their obvious biological importance, the thyroid hormone precursor protein, thyroglobulin (Tg), has been experimentally investigated only in mammals. This may bias our view of how thyroid hormones are produced in other organisms. In this study we searched genomic databases and found Tg orthologs in all vertebrates including the sea lamprey (Petromyzon marinus). We cloned a full-size Tg coding sequence from western clawed frog (Xenopus tropicalis) and zebrafish (Danio rerio). Comparisons between the representative mammal, amphibian, teleost fish, and basal vertebrate indicate that all of the different domains of Tg, as well as Tg regional structure, are conserved throughout the vertebrates. Indeed, in Xenopus, zebrafish, and lamprey Tgs, key residues, including the hormonogenic tyrosines and the disulfide bond-forming cysteines critical for Tg function, are well conserved despite overall divergence of amino acid sequences. We uncovered upstream sequences that include start codons of zebrafish and Xenopus Tgs and experimentally proved that these are full-length secreted proteins, which are specifically recognized by antibodies against rat Tg. By contrast, we have not been able to find any orthologs of Tg among non-vertebrate species. Thus, Tg appears to be a novel protein elaborated as a single event at the base of vertebrates and virtually unchanged thereafter.

Development of the thyroid gland

“…Moreover, sequencing the amphioxus genome has revealed homologs to all genes involved in thyroid hormone synthesis and metabolism in non-vertebrate chordates except for that encoding thyroglobulin (Paris et al, 2008). A recent comparative study confirmed that thyroglobulin is confined to the vertebrate series including lamprey (Petromyzon marinus) (Holzer et al, 2016), further supporting an evolutionary link between the evolution of a novel prohormone and the appearance of a follicular thyroid. Interestingly, thyroglobulin is expressed in the larval lamprey endostyle (Monaco et al, 1978), indicating that the basal machinery for prohormone synthesis and accumulation is already in place before the endostyle metamorphoses into a follicular thyroid.…”

Relationship between the dimerization of thyroglobulin and its ability to form triiodothyronine

“…7C). Once again, these data may provide a rationale for why the unique tail sequence of TG bearing the conserved T 3 -forming site has traveled together in evolution with the conserved ChEL domain (14): ChEL provides homodimerization, allowing for MIT-DIT coupling at the antepenultimate Tyr residue of TG.…”

“…Understanding the structural basis for T 3 formation at the antepenultimate TG residue is of considerable physiological importance because a) this is the main site of T 3 hormonogenesis within TG (7); b) TSH stimulation of thyrocytes physiologically increases the T 3 hormonogenic capability of TG, including at the antepenultimate site (29); c) this provides a rationale for evolutionary conservation of the antepenultimate Tyr residue in vertebrate TG (14), and d) enhanced T 3 hormonogenesis within TG is an especially relevant problem in Graves' disease (5,30). However, there have been no publications to clearly suggest the MIT residue that serves as a primary donor to this evolutionarily conserved T 3 -forming site in TG.…”

“…Recent studies of the lamprey Tgn gene sequence indicate that the ChEL domain and the contiguous unique tail sequence of TG that together bear the three Tyr residues (noted above) that have been implicated in T 3 hormonogenesis are evolutionarily conserved throughout the vertebrates (14).…”

“…The Tgn gene encodes a single polypeptide synthesized in the thyrocyte endoplasmic reticulum as a ~330 kDa glycoprotein comprised of repeat domains within region I (bearing type 1 repeats plus linker and hinge segments), region II+III (bearing type 2 and type 3 repeats), the carboxyl-terminal Cholinesterase-Like (ChEL) domain (containing both of the potential donor MIT residues noted above), and a unique (~50 residue) tail sequence bearing the antepenultimate Tyr residue that can form T 3 and is conserved throughout all vertebrates from lamprey to human (14). Shortly after its synthesis, TG self-associates noncovalently into a ~660 kDa TG dimer (15).…”