Sheep thyroxine-binding globulin: cDNA sequence and expression

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.

Thyroid hormones in the cerebrospinal fluid of the third ventricle of adult female sheep during different periods of reproductive activity

Thyroid hormones (THs) are obligatory for transition from breeding season to anestrus in sheep. In this process, THs act during a very limited time of the year and primarily within the brain. In ewes chronically equipped for sampling cerebrospinal fluid (CSF) from the third ventricle, we have characterized the concentrations of total and free thyroxine (T4), triiodothyronine (T3), and total reverse T3 (rT3) in the CSF during breeding season, anestrus and during a critical period required for transition to anestrus (December-March). The total T4, T3, rT3 and free T3 average concentrations (+/- SEM) in CSF were 1.5 +/- 0.07 ng/ml, 14.5 +/- 1.2 pg/ml, 43 +/- 7.4 pg/ml, and 0.6 +/- 0.05 pg/ml, respectively, and all were significantly lower (p < 0.001) than in blood plasma except free T4 (12.6 +/- 1.1 pg/ml), which was similar to that in plasma. There was a seasonal trend (p < 0.05) in the concentration of total T3 (highest in December) and free T4 (highest in November) in the CSF that does not follow that in blood plasma. During the period of transition to anestrus the CSF total T3/TT4 molar ratio and free T3/T4 ratio were significantly lower (p < 0.05 and p < 0.01, respectively) than in blood plasma, while the total rT3/T4 ratio was significantly higher (p < 0.01) at the end of this period (March). Additionally, the CSF total rT3 concentrations were also significantly correlated with the CSF total T4 levels (r = 0.57; p < 0.05). In conclusion, the CSF in sheep may serve as a considerable source of thyroid hormones for neuroendocrine events. The lack of significant changes in THs concentrations in the CSF during the period of transition to anestrus indicate that neither seasonal changes of THs circulating in the blood plasma nor THs circulating in the CSF actively drive the transition to anestrus.

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.

Role of thyroid hormones in human and laboratory animal reproductive health

The highly conserved nature of the thyroid gland and the thyroid system among mammalian species suggests it is critical to species survival. Studies show the thyroid system plays a critical role in the development of several organ systems, including the reproductive tract. Despite its highly conserved nature, the thyroid system can have widely different effects on reproduction and reproductive tract development in different species. The present review focuses on assessing the role of thyroid hormones in human reproduction and reproductive tract development and comparing it to the role of thyroid hormones in laboratory animal reproduction and reproductive tract development. The review also assesses the effects of thyroid dysfunction on reproductive tract development and function in humans and laboratory animals. Consideration of such information is important in designing, conducting, and interpreting studies to assess the potential effects of thyroid toxicants on reproduction and development.

Characterization and primary structures of bovine and porcine thyroxine-binding globulin

Thyroxine-binding globulin (TBG) is the major serum transport protein for iodothyronines in most of the large, omni- or herbivorous mammals. Characterization of human TBG (hTBG), including its 20 known natural variants, allowed the identification of the ligand-binding site and a correlation of diminished synthesis or loss of function with mutations in the TBG gene. Further refinement of the structure-function correlation, especially the high binding affinity and heat stability, requires characterization of other mammalian TBGs, of which only rat and sheep TBG were available. We now present some of the chemical and physical properties of bovine TBG (bTBG) and porcine TBG (pTBG) and their primary structures deduced from their cDNA sequences. The serum concentrations of bTBG and pTBG estimated by Scatchard analysis of T(4)-binding were similar to hTBG. The T(4)-binding affinity of human, bovine and porcine TBGs were all similar, at 1.2x10(10) M(-1). However, heat stability of the animal TBGs was reduced, with a half life of denaturation of 7 min (bTBG) and 5 min (pTBG) at 55 degreeC, compared with 21 min for hTBG. Nucleotide alignment revealed identity with hTBG of 85.5% (bTBG) and 83.7% (pTBG) and amino acid identity of 82.8% (bTBG) and 82.6% (pTBG). As expected, the relevant parts of the ligand-binding domain (amino acids 215-291, and 363-395) were highly conserved at more than 95% similarity. Comparison of the five known mammalian TBGs allows focusing of future mutagenesis experiments to further characterize the properties of the molecule.

Characterization of the thyroxine-binding site of thyroxine-binding globulin by site-directed mutagenesis

The principal transport protein for T4 in human blood, thyroxine-binding globulin (TBG), binds T4 with an exceptionally high affinity (Ka = 10(10) M(-1)). Its homology to the superfamily of the serpins has recently been used in the design of chimeric proteins, providing experimental evidence that an eight-stranded beta-barrel domain encompasses the ligand-binding site. We have now characterized the T4 binding site by site-directed mutagenesis. Sequence alignment of TBG from several species revealed a phylogenetically highly conserved stretch of amino acids comprising strands 2B and 3B of the beta-barrel motif. Mutations within this region (Val228Glu, Cys234Trp, Thr235Trp, Thr235Gln, Lys253Ala, and Lys253Asp), designed to impose steric hindrance or restriction of its mobility, had no significant influence on T4 binding. However, binding affinity was 20-fold reduced by introduction of an N-linked glycosylation site at the turn between strands 2B and 3B (Leu246Thr) without compromising the proper folding of this mutant as assessed by immunological methods. In most other serpins, this glycosylation site is highly conserved and has been shown to be crucial for cortisol binding of corticosteroid-binding globulin, the only other member of the serpins with a transport function. The ligand-binding site could thus be located to a highly aromatic environment deep within the beta-barrel. The importance of the binding site's aromatic character was investigated by exchanging phenylalanines with alanines. Indeed, these experiments revealed that substitution of Phe249 in the middle of strand 3B completely abolished T4 binding, while the substitution of several other phenylalanines had no effect.

Evolution of thyroid hormone binding by transthyretins in birds and mammals

Transthyretin, a protein synthesized and secreted by the choroid plexus and liver, binds thyroid hormones in extracellular compartments. This binding prevents accumulation of thyroid hormones in the lipids of membranes, establishing extracellular thyroid hormone pools for the distribution of the hormones throughout the body and brain. The N-termini of the transthyretin subunits are longer and more hydrophobic in chicken than in eutherian transthyretins. Here, we show that this is a general structural feature of avian transthyretins. Systematic changes of protein structure during evolution result from selection pressure leading to changes in function. The evolution of transthyretin function, namely, the binding of thyroid hormones, was studied in nine vertebrate species. The affinity of thyroxine binding to transthyretin is lowest in avians (mean Kd of about 30 nm), intermediate in metatherians (mean Kd of about 17 nm) and highest in eutherians (mean Kd of about 11 nm). The affinity for 3,5,3'-triiodothyronine shows an opposite trend, being four times higher for avian transthyretins than for mammalian transthyretins.

Evolution of shorter and more hydrophilic transthyretin N-termini by stepwise conversion of exon 2 into intron 1 sequences (shifting the 3' splice site of intron 1)

Transthyretin cDNA was cloned from Eastern Grey Kangaroo liver and its nucleotide sequence determined. Analysis of the derived amino acid sequence of kangaroo transthyretin, together with data obtained previously for transthyretins from other vertebrate species [Duan, W., Richardson, S. J., Babon, J. J., Heyes, R. J., Southwell, B. R., Harms, P. J., Wettenhall, R. E. H., Dziegielewska, K. M., Selwood, L., Bradley, A. J., Brack, C. M. & Schreiber, G. (1995) Eur. J. Biochem. 227, 396-406], showed that the N-terminus is the region which changes most distinctly during evolution. It has been shown for human, mouse and rat transthyretins, that this region is encoded by DNA at the border of exon 1 and exon 2. Therefore, this section of transthyretin genomic DNA was amplified by PCR and directly sequenced for the Buffalo Rat, Tammar Wallaby, Eastern Grey Kangaroo, Stripe-faced Dunnart, Short-tailed Grey Opossum and White Leghorn Chicken. The splice sites at both ends of intron 1 were identified by comparison with the cDNA sequences. The obtained data suggest that the N-termini of transthyretin evolved by successive shifts of the 3' splice site of intron 1 in the 3' direction, resulting in successive shortening of the 5' end of exon 2. At the protein level, this resulted in a shorter and more hydrophilic N-terminal region of transthyretin. Successive shifts in splice sites may be an evolutionary mechanism of general importance, since they can lead to stepwise changes in the properties of proteins. This could be a molecular mechanism for positive Darwinian selection.

The evolution of gene expression, structure and function of transthyretin

Thyroxine, the most abundant thyroid hormone in blood, partitions into lipid membranes. In a network-like system, thyroxine-binding plasma proteins counteract this partitioning and establish intravascular, protein-bound thyroxine pools. These are far larger than the free thyroxine pools. In larger eutherians, proteins specifically binding thyroxine are albumin, transthyretin, and thyroxine-binding globulin. Some binding of thyroxine can also occur to lipoproteins. During evolution, transthyretin synthesis first appeared in the choroid plexus of the stem reptiles, about 300 million years ago. Transthretin synthesis in the liver evolved much later, independently, in birds, eutherians and some marsupial species. Analysis of 57 human transthyretin variants suggests that most mutations in transthyretin are not compatible with its normal metabolism and lead to its deposition as amyloid. Analysis of transthyretin or its gene in 20 different species shows that evolutionary changes of transthyretin predominantly occurred near the N-termini. A change in RNA splicing between exon 1 and exon 2 led to a decrease in hydrophobicity and length of the N-termini. It is proposed that the selection pressure producing these changes was the need for a more effective prevention of thyroxine partitioning into lipids. Lipid pools increased during evolution with the increases in relative sizes of brains and internal organs and changes in lipid composition of membranes in ectothermic and endothermic species.