Identification and bioactivity analysis of transthyretin-like protein in amphioxus: A case demonstrating divergent evolution from an enzyme to a hormone distributor

Evolution of thyroid hormone distributor proteins in fish

In plasma, thyroid hormone (TH) is bound to several TH distributor proteins (THDPs), constituting a TH delivery/distribution network. Extensive studies of THDPs from tetrapods has proposed an evolutionary scenario concerning structural and functional changes in THDPs, especially for transthyretin (TTR). When assessing, in an evolutionary context, the roles of THDPs as a component constituting part of the vertebrate thyroid system, the data from fish THDPs are critical. In this review the phylogenetic distributions, spatiotemporal expression patterns and binding properties of THDPs in fish are described, and the question of whether the evolutionary hypotheses proposed in tetrapod THDPs can be applied to fish THDPs is assessed. The phylogenetic distributions of THDPs are highly variable among fish groups. Analysis in this review reveals that the evolutionary hypotheses proposed in tetrapod THDPs cannot be applied to fish THDPs, and that the role of plasma lipoproteins as THDPs grows in importance in fish groups. In primitive fish, zinc is an import factor in TH binding to TTR, and high zinc content may facilitate the acquisition of high TH binding activity during the early evolution of TTR. Finally, the possible roles of THDPs in the vertebrate thyroid system are discussed.

The evolutionary road to invertebrate thyroid hormone signaling: Perspectives for endocrine disruption processes

Thyroid hormones (THs) are the only iodine-containing hormones that play fundamental roles in chordates and non-chordates. The chemical nature, mode of action and the synthesis of THs are well established in mammals and other vertebrates. Although thyroid-like hormones have been detected in protostomes and non-chordate deuterostomes, TH signaling is poorly understood as compared to vertebrates, particularly in protostomes. Therefore, the central objective of this article is to review TH system components and TH-induced effects in non-vertebrate chordates, non-chordate deuterostomes and protostomes based on available genomes and functional information. To accomplish this task, we integrate here the available knowledge on the THs signaling across non-vertebrate chordates, non-chordate deuterostomes and protostomes by considering studies encompassing TH system components and physiological actions of THs. We also address the possible interactions of thyroid disrupting chemicals and their effects in protostomes and non-chordate deuterostomes. Finally, the perspectives on current and future challenges are discussed.

Sequential Molecular Events of Functional Trade-Offs in 5-Hydroxyisourate Hydrolase Before and After Gene Duplication Led to the Evolution of Transthyretin During Chordate Diversification

Transthyretin (TTR), a plasma thyroid hormone distributor protein (THDP), emerged from 5-hydroxyisourate hydrolase (HIUHase), an enzyme involved in urate metabolism, by gene duplication at a stage of chordate evolution. Comparison of amino acid sequences revealed the presence of two His-rich segments in the primitive TTRs. Using several HIUHase and TTR mutants, we investigated 5-hydroxyisourate (HIU) hydrolysis activity and thyroid hormone (TH) binding activity to elucidate how a novel function as a THDP arose. Lancelet HIUHase was found to have higher enzyme activity than trout HIUHase. Two amino acid substitutions, R54E/Y119T, at the active sites of HIUHase, exerted weak [¹²⁵I]-3,3',5-triiodo-L-thyronine ([¹²⁵I]T3) binding activity with a concomitant loss of HIU hydrolysis activity. Addition of 3×His (3×H) to the N-terminal end weakened HIU hydrolysis activity of both lancelet and trout HIUHases, whereas it enhanced T3-binding activity of HIUHase R54E/Y119T. Trout HIUHase 3×H R54E/Y119T had higher [¹²⁵I]T3-binding activity than that of lancelet HIUHase 3×H R54E/Y119T, with a K_d of 143 nM, and displayed metal dependency and no TH binding specificity. Deletion of the N-terminal His-rich segment from lamprey TTR decreased T3-binding activity, while addition of 3×H to trout TTR increased T3-binding activity, while maintaining TH binding specificity. Our results suggest that functional trade-offs of HIU hydrolysis activity with TH binding activity might have sequentially occurred before and after gene duplication, and that TH binding specificity and high-affinity sites may have been acquired later in the course of TTR evolution.

Evolution of thyroid hormone distributor proteins

Thyroid hormones (THs) are evolutionarily old hormones, having effects on metabolism in bacteria, invertebrates and vertebrates. THs bind specific distributor proteins (THDPs) to ensure their efficient distribution through the blood and cerebrospinal fluid in vertebrates. Albumin is a THDP in the blood of all studied species of vertebrates, so may be the original vertebrate THDP. However, albumin has weak affinity for THs. Transthyretin (TTR) has been identified in the blood across different lineages in adults vs juveniles. TTR has intermediate affinity for THs. Thyroxine-binding globulin has only been identified in mammals and has high affinity for THs. Of these THDPs, TTR is the only one known to be synthesised in the brain and is involved in moving THs from the blood into the cerebrospinal fluid. We analysed the rates of evolution of these three THDPs: TTR has been most highly conserved and albumin has had the highest rate of divergence.

Characteristics of the brown hagfish Paramyxine atami transthyretin: Metal ion-dependent thyroid hormone binding

Transthyretin (TTR) is a vertebrate-specific protein involved in thyroid hormone distribution in plasma, and its gene is thought to have emerged by gene duplication from the gene for the ancient TTR-related protein, 5-hydroxyisourate hydrolase, at some early stage of chordate evolution. We investigated the molecular and hormone-binding properties of the brown hagfish Paramyxine atami TTR. The amino acid sequence deduced from the cloned hagfish TTR cDNA shared 33-50% identities with those of other vertebrate TTRs but less than 24% identities with those of vertebrate and deuterostome invertebrate 5-hydroxyisourate hydrolases. Hagfish TTR, as well as lamprey and little skate TTRs, had an N-terminal histidine-rich segment, allowing purification by metal-affinity chromatography. The affinity of hagfish TTR for 3,3',5-triiodo-L-thyronine (T3) was 190 times higher than that for L-thyroxine, with a dissociation constant of 1.5-3.9nM at 4°C. The high-affinity binding sites were strongly sensitive to metal ions. Zn²⁺ and Cu²⁺ decreased the dissociation constant to one-order of magnitude, whereas a chelator, o-phenanthroline, increased it four times. The number of metal ions (mainly Zn²⁺ and Cu²⁺) was approximately 12/TTR (mol/mol). TTR was also a major T3-binding protein in adult hagfish sera and its serum concentration was approximately 8μM. These results suggest that metal ions and the acquisition of N-terminal histidine-rich segment may cooperatively contribute to the evolution toward an ancient TTR with high T3 binding activity from either 5-hydroxyisourate hydrolase after gene duplication.

Characterization of little skate (Leucoraja erinacea) recombinant transthyretin: Zinc-dependent 3,3',5-triiodo-l-thyronine binding

Transthyretin (TTR) diverged from an ancestral 5-hydroxyisourate hydrolase (HIUHase) by gene duplication at some early stage of chordate evolution. To clarify how TTR had participated in the thyroid system as an extracellular thyroid hormone (TH) binding protein, TH binding properties of recombinant little skate Leucoraja erinacea TTR was investigated. At the amino acid level, skate TTR showed 37-46% identities with the other vertebrate TTRs. Because the skate TTR had a unique histidine-rich segment in the N-terminal region, it could be purified by Ni-affinity chromatography. The skate TTR was a 46-kDa homotetramer of 14.5kDa subunits, and had one order of magnitude higher affinity for 3,3',5-triiodo-l-thyronine (T3) and some halogenated phenols than for l-thyroxine. However, the skate TTR had no HIUHase activity. Ethylenediaminetetraacetic acid (EDTA) treatment inhibited [(125)I]T3 binding activity whereas the addition of Zn(2+) to the EDTA-treated TTR recovered [(125)I]T3 binding activity in a Zn(2+) concentration-dependent manner. Scatchard analysis revealed the presence of two classes of binding site for T3, with dissociation constants of 0.24 and 17nM. However, the high-affinity sites were completely abolished with 1mM EDTA, whereas the remaining low-affinity sites decreased binding capacity. The number of zinc per TTR was quantified to be 4.5-6.3. Our results suggest that skate TTR has tight Zn(2+)-binding sites, which are essential for T3 binding to at least the high-affinity sites. Zn(2+) binding to the N-terminal histidine-rich segment may play an important role in acquisition or reinforcement of TH binding ability during early evolution of TTR.

Tweaking the structure to radically change the function: the evolution of transthyretin from 5-hydroxyisourate hydrolase to triiodothyronine distributor to thyroxine distributor

Often, we elucidate evolutionary processes backwards, starting with eutherian mammals and gradually climbing down the evolutionary tree to those species who have survived since long before mammals evolved. This is also true for elucidating the evolution of specific proteins, in this case, the protein currently known as "transthyretin" (TTR). TTR was first described in eutherian mammals and was known as a thyroxine (T4) binding protein. However, mammals are the exception among vertebrates in respect to the function of TTR, as in teleost fish, amphibians, reptiles and birds TTR preferentially binds triiodothyronine (T3), which is the active form of thyroid hormone (TH). The TTR gene possibly arose as a duplication of the transthyretin-like protein (TLP) gene, around the stage of the agnathans. Some vertebrate species have both the TTR and TLP genes, while others have "lost" the TLP gene. TLP genes have been found in all kingdoms. The TLPs analyzed to date do not bind THs or their analogs, but are enzymes involved in uric acid metabolism; specifically, they are 5-hydroxyisourate hydrolases. A Salmonella TLP knock-out strain demonstrated that TLP was essential for the bacteria's survival in the high uric acid environment of the chicken alimentary tract. Many other TLPs are yet to be characterized for their function although several have been confirmed as 5-hydroxyisourate hydrolases. This review describes the evolution of TLP/TTR and how subtle changes in gene structure or amino acid substitution can drastically change the function of this protein, without altering its overall 3D conformation.