Hormone affinity and fibril formation of piscine transthyretin: the role of the N-terminal

Reenacting the Birth of a Function: Functional Divergence of HIUases and Transthyretins as Inferred by Evolutionary and Biophysical Studies

Transthyretin was discovered in the 1940s, named after its ability to bind thyroid hormones and retinol. In the genomic era, transthyretins were found to be part of a larger family with homologs of no obvious function, then called transthyretin-related proteins. Thus, it was proposed that the transthyretin gene could be the result of gene duplication of an ancestral of this newly identified homolog, later found out to be an enzyme involved in uric acid degradation, then named HIUase (5-hydroxy-isourate hydrolase). Here, we sought to re-enact the evolutionary history of this protein family by reconstructing, from a phylogeny inferred from 123 vertebrate sequences, three ancestors corresponding to key moments in their evolution-before duplication; the common transthyretin ancestor after gene duplication and the common ancestor of Eutheria transthyretins. Experimental and computational characterization showed the reconstructed ancestor before duplication was unable to bind thyroxine and likely presented the modern HIUase reaction mechanism, while the substitutions after duplication prevented that activity and were enough to provide stable thyroxine binding, as confirmed by calorimetry and x-ray diffraction. The Eutheria transthyretin ancestor was less prone to characterization, but limited data suggested thyroxine binding as expected. Sequence/structure analysis suggests an early ability to bind the Retinol Binding Protein. We solved the X-ray structures from the two first ancestors, the first at 1.46 resolution, the second at 1.55 resolution with well-defined electron density for thyroxine, providing a useful tool for the understanding of structural adaptation from enzyme to 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.

Transport of maternal transthyretin to the fetus in the viviparous teleost Neoditrema ransonnetii (Perciformes, Embiotocidae)

The molecular basis of viviparity in non-mammalian species has not been widely studied. Neoditrema ransonnetii, a surfperch, is a matrotrophic teleost whose fetuses grow by ovarian cavity fluid (OCF) ingestion and by nutrient absorption via their enlarged hindgut. We performed a proteomics analysis of N. ransonnetii plasma protein and found proteins specific to pregnant females; one of these was identified as transthyretin (TTR), a thyroid hormone distributor protein. We synthesized recombinant protein rNrTTR and raised an antibody, anti-rNrTTR, against it. Semi-quantitative analysis by western blotting using the antibody demonstrated that plasma TTR levels were significantly greater in pregnant fish than in non-pregnant fish. OCF and fetal plasma also contained high TTR levels. Immunohistochemical staining showed that large amounts of maternal TTR were taken up by fetal intestinal epithelial cells. These results indicate that maternal TTR is secreted into OCF and taken up by fetal enterocytes, presumably to deliver thyroid hormones to developing fetuses.

Interspecies Variation between Fish and Human Transthyretins in Their Binding of Thyroid-Disrupting Chemicals

Thyroid-disrupting chemicals (TDCs) are xenobiotics that can interfere with the endocrine system and cause adverse effects in organisms and their offspring. TDCs affect both the thyroid gland and regulatory enzymes associated with thyroid hormone homeostasis. Transthyretin (TTR) is found in the serum and cerebrospinal fluid of vertebrates, where it transports thyroid hormones. Here, we explored the interspecies variation in TDC binding to human and fish TTR (exemplified by Gilthead seabream ( Sparus aurata)). The in vitro binding experiments showed that TDCs bind with equal or weaker affinity to seabream TTR than to the human TTR, in particular, the polar TDCs (>500-fold lower affinity). Crystal structures of the seabream TTR-TDC complexes revealed that all TDCs bound at the thyroid binding sites. However, amino acid substitution of Ser117 in human TTR to Thr117 in seabream prevented polar TDCs from binding deep in the hormone binding cavity, which explains their low affinity to seabream TTR. Molecular dynamics and in silico alanine scanning simulation also suggested that the protein backbone of seabream TTR is more rigid than the human one and that Thr117 provides fewer electrostatic contributions than Ser117 to ligand binding. This provides an explanation for the weaker affinities of the ligands that rely on electrostatic interactions with Thr117. The lower affinities of TDCs to fish TTR, in particular the polar ones, could potentially lead to milder thyroid-related effects in fish.

Molecular and thyroid hormone binding properties of lamprey transthyretins: The role of an N-terminal histidine-rich segment in hormone binding with high affinity

Transthyretin (TTR) is a plasma thyroid hormone (TH) binder that emerged from an ancient hydroxyisourate hydrolase by gene duplication. To know how an ancient TTR had high affinity for THs, molecular and TH binding properties of lamprey TTRs were investigated. In adult serum, the lipoprotein LAL was a major T3 binder with low affinity. Lamprey TTRs had an N-terminal histidine-rich segment, and had two classes of binding sites for 3,3',5-triiodo-L-thyronine (T3): a high-affinity and a low-affinity site. Mutant TTRΔ_3-11, lacking the N-terminal histidine-rich segment, lost the high-affinity T3 binding site. [¹²⁵I]T3 binding to wild type TTR and mutant TTRΔ_3-11, was differentially modulated by Zn²⁺. Zn²⁺ contents of wild type TTR were 7-10/TTR (mol/mol). Our results demonstrate that lamprey TTR is a Zn²⁺-dependent T3 binder. The N-terminal histidine-rich segment may be essential for neo-functionalization (i.e., high-affinity T3 binding activity) of an ancient TTR after gene duplication.

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.

Increasing the length and hydrophobicity of the C-terminal sequence of transthyretin strengthens its binding affinity to retinol binding protein

Transthyretin (TTR) is a transporter for thyroid hormone (TH) and retinol, the latter via binding with retinol binding protein (RBP). Both the N‐terminal and C‐terminal regions of the TTR subunit are located in close proximity to the central binding channel for ligands. During the evolution of vertebrates, these regions changed in length and hydropathy. The changes in the N‐terminal sequence were demonstrated to affect the binding affinities for THs and RBP. Here, the effects of changes in the C‐terminal sequence were determined. Three chimeric TTRs, namely pigC/huTTR (human TTR with the C‐terminal sequence changed to that of Sus scrofa TTR), xenoN/pigC/huTTR (human TTR with the N‐terminal and C‐terminal sequences changed to those of Xenopus laevis and S. scrofa, respectively), and pigC/crocTTR (Crocodylus porosus TTR with the C‐terminal sequence changed to that of S. scrofa TTR), were constructed and their binding affinities for human RBP were determined at low TTR/RBP molar ratio using chemiluminescence immunoblotting. The binding dissociation constant (K_d) values of pigC/huTTR, xenoN/pigC/huTTR and pigC/crocTTR were 3.20 ± 0.35, 1.53 ± 0.38 and 0.31 ± 0.04 μm, respectively, and the K_d values of human and C. porosus TTR were 4.92 ± 0.68 and 1.42 ± 0.45 μm, respectively. These results demonstrate chimeric TTRs bound RBP with a higher strength than wild‐type TTRs, and the changes in the C‐terminal sequence of TTR had a positive effect on its binding affinity for RBP. In addition, changes to the N‐terminal and C‐terminal sequences showed comparable effects on the binding affinity.

The goitrogenic efficiency of thioamides in a marine teleost, sea bream (Sparus auratus)

Studies on the role of thyroid hormones (THs) in teleost fish physiology have deployed the synthetic goitrogens, methimazol (MMI), propilthiouracil (PTU) and thiourea (TU) that are used to treat human hyperthyroidism. However, the action of the goitrogens, MMI, PTU and TU at different levels of the hypothalamic-pituitary-thyroid (HPT) axis in teleosts is largely unknown. The central importance of the hypothalamus and pituitary in a number of endocrine regulated systems and the cross-talk that occurs between different endocrine axes makes it pertinent to characterize the effects of MMI, PTU and TU, on several endpoints of the thyroid system. The marine teleost, sea bream (Sparus auratus) was exposed to MMI, PTU and TU (1mg/kg wet weight per day), via the diet for 21days. Radioimmunoassays (RIA) of plasma THs and ELISA of the TH carrier transthyretin (TTR) revealed that MMI was the only chemical that significantly reduced plasma TH levels (p<0.05), although both MMI and PTU significantly (p<0.05) reduced plasma levels of circulating TTR (p<0.05). Histological analysis of the thyroid tissue revealed modifications in thyrocyte activity that explain the reduced circulating levels of THs. MMI also significantly (p<0.05) up-regulated transcript abundance of liver deiodinase 1 and 2 while significantly (p<0.05) decreasing TRβ expression in the pituitary, all hallmarks of HPT axis action of goitrogens in vertebrates. The results indicate that in the sea bream MMI is the most effective goitrogen followed by PTU and that TU (1mg/kg wet weight for 21days) failed to have a goitrogenic effect. The study highlights the non-uniform effect of goitrogens on the thyroid axis of sea bream and provides the basis for future studies of thyroid disrupting pollutants.

Effect of the N-terminal sequence on the binding affinity of transthyretin for human retinol-binding protein

During vertebrate evolution, the N-terminal region of transthyretin (TTR) subunit has undergone a change in both length and hydropathy. This was previously shown to change the binding affinity for thyroid hormones (THs). However, it was not known whether this change affects other functions of TTR. In the present study, the effect of these changes on the binding of TTR to retinol-binding protein (RBP) was determined. Two wild-type TTRs from human and Crocodylus porosus, and three chimeric TTRs, including a human chimeric TTR in which its N-terminal sequence was changed to that of C. porosus TTR (croc/huTTR) and two C. porosus chimeric TTRs (hu/crocTTR in which its N-terminal sequence was changed to that of human TTR and xeno/crocTTR in which its N-terminal sequence was changed to that of Xenopus laevis TTR), were analyzed for their binding to human RBP by native-PAGE followed by immunoblotting and a chemilluminescence assay. The K(d) of human TTR was 30.41 ± 2.03 μm, and was similar to that reported for the second binding site, whereas that of crocodile TTR was 2.19 ± 0.24 μm. The binding affinities increased in croc/huTTR (K(d) = 23.57 ± 3.54 μm) and xeno/crocTTR (K(d) = 0.61 ± 0.16 μm) in which their N-termini were longer and more hydrophobic, but decreased in hu/crocTTR (K(d) = 5.03 ± 0.68 μm) in which its N-terminal region was shorter and less hydrophobic. These results suggest an influence of the N-terminal primary structure of TTR on its function as a co-carrier for retinol with RBP.

Evolutionary changes to transthyretin: structure-function relationships

Transthyretin is one of the three major thyroid hormone-binding proteins in plasma and/or cerebrospinal fluid of vertebrates. It transports retinol via binding to retinol-binding protein, and exists mainly as a homotetrameric protein of approximately 55 kDa in plasma. The first 3D structure of transthyretin was an X-ray crystal structure from human transthyretin. Elucidation of the structure-function relationship of transthyretin has been of significant interest since its highly conserved structure was shown to be associated with several aspects of metabolism and with human diseases such as amyloidosis. Transthyretin null mice do not have an overt phenotype, probably because transthyretin is part of a network with other thyroid hormone distributor proteins. Systematic study of the evolutionary changes of transthyretin structure is an effective way to elucidate its function. This review summarizes current knowledge about the evolution of structural and functional characteristics of vertebrate transthyretins. The molecular mechanism of evolutionary change and the resultant effects on the function of transthyretin are discussed.

Stability and fibril formation properties of human and fish transthyretin, and of the Escherichia coli transthyretin-related protein

Human transthyretin (hTTR) is one of several proteins known to cause amyloid disease. Conformational changes in its native structure result in aggregation of the protein, leading to insoluble amyloid fibrils. The transthyretin (TTR)-related proteins comprise a protein family of 5-hydroxyisourate hydrolases with structural similarity to TTR. In this study, we tested the amyloidogenic properties, if any, of sea bream TTR (sbTTR) and Escherichia coli transthyretin-related protein (ecTRP), which share 52% and 30% sequence identity, respectively, with hTTR. We obtained filamentous structures from all three proteins under various conditions, but, interestingly, different structures displayed different tinctorial properties. hTTR and sbTTR formed thin, curved fibrils at low pH (pH 2-3) that bound thioflavin-T (thioflavin-T-positive) but did not stain with Congo Red (CR) (CR-negative). Aggregates formed at the slightly higher pH of 4.0-5.5 had different morphology, displaying predominantly amorphous structures. CR-positive material of hTTR was found in this material, in agreement with previous results. ecTRP remained soluble at pH 2-12 at ambient temperatures. By raising of the temperature, fibril formation could be induced at neutral pH in all three proteins. Most of these temperature-induced fibrils were thicker and straighter than the in vitro fibrils seen at low pH. In other words, the temperature-induced fibrils were more similar to fibrils seen in vivo. The melting temperature of ecTRP was 66.7 degrees C. This is approximately 30 degrees C lower than the melting temperatures of sbTTR and hTTR. Information from the crystal structures was used to identify possible explanations for the reduced thermostability of ecTRP.