Protein synthesis at the blood-brain barrier. The major protein secreted by amphibian choroid plexus is a lipocalin

Form and Function of the Vertebrate and Invertebrate Blood-Brain Barriers

The need to protect neural tissue from toxins or other substances is as old as neural tissue itself. Early recognition of this need has led to more than a century of investigation of the blood-brain barrier (BBB). Many aspects of this important neuroprotective barrier have now been well established, including its cellular architecture and barrier and transport functions. Unsurprisingly, most research has had a human orientation, using mammalian and other animal models to develop translational research findings. However, cell layers forming a barrier between vascular spaces and neural tissues are found broadly throughout the invertebrates as well as in all vertebrates. Unfortunately, previous scenarios for the evolution of the BBB typically adopt a classic, now discredited 'scala naturae' approach, which inaccurately describes a putative evolutionary progression of the mammalian BBB from simple invertebrates to mammals. In fact, BBB-like structures have evolved independently numerous times, complicating simplistic views of the evolution of the BBB as a linear process. Here, we review BBBs in their various forms in both invertebrates and vertebrates, with an emphasis on the function, evolution, and conditional relevance of popular animal models such as the fruit fly and the zebrafish to mammalian BBB research.

Thyroid Hormone Distributor Proteins During Development in Vertebrates

Thyroid hormones (THs) are ancient hormones that not only influence the growth, development and metabolism of vertebrates but also affect the metabolism of (at least some) bacteria. Synthesized in the thyroid gland (or follicular cells in fish not having a discrete thyroid gland), THs can act on target cells by genomic or non-genomic mechanisms. Either way, THs need to get from their site of synthesis to their target cells throughout the body. Despite being amphipathic in structure, THs are lipophilic and hence do not freely diffuse in the aqueous environments of blood or cerebrospinal fluid (in contrast to hydrophilic hormones). TH Distributor Proteins (THDPs) have evolved to enable the efficient distribution of THs in the blood and cerebrospinal fluid. In humans, the THDPs are albumin, transthyretin (TTR), and thyroxine-binding globulin (TBG). These three proteins have distinct patterns of regulation in both ontogeny and phylogeny. During development, an additional THDP with higher affinity than those in the adult, is present during the stage of peak TH concentrations in blood. Although TTR is the only THDP synthesized in the central nervous system (CNS), all THDPs from blood are present in the CSF (for each species). However, the ratio of albumin to TTR differs in the CSF compared to the blood. Humans lacking albumin or TBG have been reported and can be asymptomatic, however a human lacking TTR has not been documented. Conversely, there are many diseases either caused by TTR or that have altered levels of TTR in the blood or CSF associated with them. The first world-wide RNAi therapy has just been approved for TTR amyloidosis.

The affinity of transthyretin for T3 or T4 does not determine which form of the hormone accumulates in the choroid plexus

Normal development of the brain is dependent on the required amounts of thyroid hormones (THs) reaching specific regions of the brain during each stage of ontogeny. Many proteins are involved with regulation of TH bioavailability in the brain: the TH distributor protein transthyretin (TTR), TH transmembrane transporters (e.g. MCT8, MCT10, LAT1, OATP1C1) and deiodinases (D1, D2 and D3) which either activate or inactivate THs. Previous studies revealed that in mammals, T₄, but not T₃, accumulated in the choroid plexus and then entered the cerebrospinal fluid. In all mammalian species studied so far, TTR binds T₄ with higher affinity than T₃, whereas TTR in non-mammalian vertebrates binds T₃ with higher affinity than T₄. We investigated if the form of TH preferentially bound by TTR influenced the form of the TH that accumulated in the choroid plexus and consequently other areas of the brain. We measured the mRNA levels corresponding to TTR, MCT8, MCT10, LAT1, OATP1C1, D1, D2 and D3 in the brains of chickens at 11days post-hatching. TTR, D3 and OATP1C1 expression were found to be highly concentrated in the choroid plexus. D1, MCT8 and MCT10 mRNA levels were slightly greater in the choroid plexus than in other areas of the brain while D2 mRNA levels were lower. LAT1 mRNA was evenly expressed throughout the brain. Therefore, the choroid plexus appears to be a structure which exhibits sophisticated control of TH levels within the brain. We also measured the uptake of intravenously injected ¹²⁵I-T₃ and ¹²⁵I-T₄ into brains of chickens of the same age. ¹²⁵I-T₄ but not ¹²⁵I-T₃ accumulated in the choroid plexus and optic lobes. Therefore, the form of TH preferentially bound by TTR does not determine the form of TH that accumulates in the choroid plexus and other areas of the brain. As for mammals, T₃ present in the avian brain therefore seems mainly produced locally by conversion of T₄ into T₃ by D2.

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.

The diversity of mechanisms influenced by transthyretin in neurobiology: development, disease and endocrine disruption

Transthyretin (TTR) is a protein that binds and distributes thyroid hormones (THs). TTR synthesised in the liver is secreted into the bloodstream and distributes THs around the body, whereas TTR synthesised in the choroid plexus is involved in movement of thyroxine from the blood into the cerebrospinal fluid and the distribution of THs in the brain. This is important because an adequate amount of TH is required for normal development of the brain. Nevertheless, there has been heated debate on the role of TTR synthesised by the choroid plexus during the past 20 years. We present both sides of the debate and how they can be reconciled by the discovery of TH transporters. New roles for TTR have been suggested, including the promotion of neuroregeneration, protection against neurodegeneration, and involvement in schizophrenia, behaviour, memory and learning. Recently, TTR synthesis was revealed in neurones and peripheral Schwann cells. Thus, the synthesis of TTR in the central nervous system (CNS) is more extensive than previously considered and bolsters the hypothesis that TTR may play wide roles in neurobiological function. Given the high conservation of TTR structure, function and tissue specificity and timing of gene expression, this implies that TTR has a fundamental role, during development and in the adult, across vertebrates. An alarming number of 'unnatural' chemicals can bind to TTR, thus potentially interfering with its functions in the brain. One role of TTR is delivery of THs throughout the CNS. Reduced TH availability during brain development results in a reduced IQ. The combination of the newly discovered sites of TTR synthesis in the CNS, the increasing number of neurological diseases being associated with TTR, the newly discovered functions of TTR and the awareness of the chemicals that can interfere with TTR biology render this a timely review on TTR in neurobiology.

Transport of thyroid hormones via the choroid plexus into the brain: the roles of transthyretin and thyroid hormone transmembrane transporters

Thyroid hormones are key players in regulating brain development. Thus, transfer of appropriate quantities of thyroid hormones from the blood into the brain at specific stages of development is critical. The choroid plexus forms the blood-cerebrospinal fluid barrier. In reptiles, birds and mammals, the main protein synthesized and secreted by the choroid plexus is a thyroid hormone distributor protein: transthyretin. This transthyretin is secreted into the cerebrospinal fluid and moves thyroid hormones from the blood into the cerebrospinal fluid. Maximal transthyretin synthesis in the choroid plexus occurs just prior to the period of rapid brain growth, suggesting that choroid plexus-derived transthyretin moves thyroid hormones from blood into cerebrospinal fluid just prior to when thyroid hormones are required for rapid brain growth. The structure of transthyretin has been highly conserved, implying strong selection pressure and an important function. In mammals, transthyretin binds T4 (precursor form of thyroid hormone) with higher affinity than T3 (active form of thyroid hormone). In all other vertebrates, transthyretin binds T3 with higher affinity than T4. As mammals are the exception, we should not base our thinking about the role of transthyretin in the choroid plexus solely on mammalian data. Thyroid hormone transmembrane transporters are involved in moving thyroid hormones into and out of cells and have been identified in many tissues, including the choroid plexus. Thyroid hormones enter the choroid plexus via thyroid hormone transmembrane transporters and leave the choroid plexus to enter the cerebrospinal fluid via either thyroid hormone transmembrane transporters or via choroid plexus-derived transthyretin secreted into the cerebrospinal fluid. The quantitative contribution of each route during development remains to be elucidated. This is part of a review series on ontogeny and phylogeny of brain barrier mechanisms.

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: evolution of transthyretin biosynthesis

Thyroid hormones are involved in growth and development, particularly of the brain. Thus, it is imperative that these hormones get from their site of synthesis to their sites of action throughout the body and the brain. This role is fulfilled by thyroid hormone distributor proteins. Of particular interest is transthyretin, which in mammals is synthesized in the liver, choroid plexus, meninges, retinal and ciliary pigment epithelia, visceral yolk sac, placenta, pancreas and intestines, whereas the other thyroid hormone distributor proteins are synthesized only in the liver. Transthyretin is synthesized by all classes of vertebrates; however, the tissue specificity of transthyretin gene expression varies widely between classes. This review summarizes what is currently known about the evolution of transthyretin synthesis in vertebrates and presents hypotheses regarding tissue-specific synthesis of transthyretin in each vertebrate class.

The Hypothalamic-Pituitary-Thyroid (HPT) Axis in Frogs and Its Role in Frog Development and Reproduction

Metamorphosis of the amphibian tadpole is a thyroid hormone (TH)-dependent developmental process. For this reason, the tadpole is considered to be an ideal bioassay system to identify disruption of thyroid function by environmental contaminants. Here we provide an in-depth review of the amphibian thyroid system with particular focus on the role that TH plays in metamorphosis. The amphibian thyroid system is similar to that of mammals and other tetrapods. We review the amphibian hypothalamic-pituitary-thyroid (HPT) axis, focusing on thyroid hormone synthesis, transport, and metabolism. We also discuss the molecular mechanisms of TH action, including the role of TH receptors, the actions of TH on organogenesis, and the mechanisms that underlie the pleiotropic actions of THs. Finally, we discuss methods for evaluating thyroid disruption in frogs, including potential sites of action, relevant endpoints, candidate protocols for measuring thyroid axis disruption, and current gaps in our knowledge. The utility of amphibian metamorphosis as a model for evaluating thyroid axis disruption has recently led to the development of a bioassay using Xenopus laevis.

Cell and Molecular Biology of Transthyretin and Thyroid Hormones

Advances in four areas of transthyretin (TTR) research result in this being a timely review. Developmental studies have revealed that TTR is synthesized in all classes of vertebrates during development. This leads to a new hypothesis on selection pressure for hepatic TTR synthesis during development only, changing the previous hypotheses from "onset" of hepatic TTR synthesis in adulthood to "maintaining" hepatic TTR synthesis into adulthood. Evolutionary studies have revealed the existence of TTR-like proteins (TLPs) in nonvertebrate species and elucidated some of their functions. Consequently, TTR is an excellent model for the study of the evolution of protein structure, function, and localization. Studies of human diseases have demonstrated that TTR in the cerebrospinal fluid can form amyloid, but more recently there has been recognition of the roles of TTR in depression and Alzheimer's disease. Furthermore, amyloid mutations in human TTR that are the normal residues in other species result in cardiac deposition of TTR amyloid in humans. Finally, a revised model for TTR-thyroxine entry into the cerebrospinal fluid via the choroid plexus, based on data from studies in TTR null mice, is presented. This review concentrates on TTR and its thyroid hormone binding, in development and during evolution, and summarizes what is currently known about TLPs and the role of TTR in diseases affecting the brain.

Thyroid system-disrupting chemicals: interference with thyroid hormone binding to plasma proteins and the cellular thyroid hormone signaling pathway

In vertebrates, thyroid hormones are essential for post-embryonic development, such as establishing the central nervous system in mammals and metamorphosis in amphibians. The present paper summarizes the possible extra-thyroidal processes that environmental chemicals are known to or suspected to target in the thyroid hormone-signaling pathway. We describe how such chemicals interfere with thyroid-hormone-binding protein functions in plasma, thyroid-hormone-uptake system, thyroid-hormone-metabolizing enzymes, and activation or suppression of thyroid-hormone-responsive genes through thyroid-hormone receptors in mammals and amphibian tadpoles. Several organohalogens affect different aspects of the extra-thyroidal thyroid-hormone-signaling pathway but hardly affect thyroid hormone binding to receptors. Rodents and amphibian tadpoles are most sensitive to the effects of environmental chemicals during specific thyroid-hormone-related developmental windows. Possible mechanisms by which environmental chemicals exert multipotent activities beyond one hormone-signaling pathway are discussed.

Zebrafish and chicken lipocalin-type prostaglandin D synthase homologues: Conservation of mammalian gene structure and binding ability for lipophilic molecules, and difference in expression profile and enzyme activity

Lipocalin-type prostaglandin (PG) D synthase (L-PGDS) is a bifunctional protein possessing both the ability to synthesize PGD(2) and to serve as a carrier protein for lipophilic molecules. L-PGDS has been extensively studied in mammalian species, whereas little is known about non-mammalian forms. Here, we identified and characterized the L-PGDS homologues from non-mammals such as zebrafish and chicken. Phylogenetic analysis revealed that L-PGDSs of mammalian and non-mammalian organisms form a "L-PGDS sub-family" that has been evolutionally separated from other lipocalin gene family proteins. The genes for zebrafish and chicken L-PGDS homologues consisted of 6 exons, and all of the exon/intron boundaries were completely identical to those of mammalian L-PGDS genes. Zebrafish and chicken L-PGDS genes were clustered with several lipocalin genes in the chromosome, as in the case of mouse and human genes. Gene expression profiles were different among chicken, mouse, human, except for conservation of abundant expression in the brain and heart. The chicken L-PGDS homologue carried weak PGDS activity, whereas the zebrafish protein did not show any of the activity. However, when the amino-terminal region of the zebrafish L-PGDS homologue was exchanged for that of mouse L-PGDS carrying the Cys residue essential for PGDS activity, this chimeric protein showed weak PGDS activity. Both zebrafish and chicken L-PGDS homologues bound thyroxine and all-trans retinoic acid, like mammalian L-PGDSs and other lipocalin gene family proteins. These results indicate that non-mammalian and mammalian L-PGDS genes evolved from the same ancestral gene and that the non-mammalian L-PGDS homologue was the primordial form of L-PGDS but whose major function was and is to serve as a carrier protein for lipophilic molecules. During molecular evolution, the mammalian L-PGDS protein might have acquired effective PGDS activity through substitution of several amino acid residues, especially in the amino-terminal region including the Cys residue, which is essential for PGDS activity.

Synthesis of transthyretin by the ependymal cells of the subcommissural organ

Transthyretin (TTR) is a protein involved in the transport of thyroid hormones in blood and cerebrospinal fluid (CSF). The only known source of brain-produced TTR is the choroid plexus. In the present investigation, we have identified the subcommissural organ (SCO) as a new source of brain TTR. The SCO is an ependymal gland that secretes glycoproteins into the CSF, where they aggregate to form Reissner's fibre (RF). Evidence exists that the SCO also secretes proteins that remain soluble in the CSF. To investigate the CSF-soluble compounds secreted by the SCO further, antibodies were raised against polypeptides partially purified from fetal bovine CSF. One of these antibodies (against a 14-kDa compound) reacted with secretory granules in cells of fetal and adult bovine SCO, organ-cultured bovine SCO and the choroid plexus of several mammalian species but not with RF. Western blot analyses with this antibody revealed two polypeptides of 14 kDa and 40 kDa in the bovine SCO, in the conditioned medium of SCO explants, and in fetal and adult bovine CSF. Since the monomeric and tetrameric forms of TTR migrate as bands of 14 kDa and 40 kDa by SDS-polyacrylamide gel electrophoresis, a commercial preparation of human TTR was run, with both bands being reactive with this antibody. Bovine SCO was also shown to synthesise mRNA encoding TTR under in vivo and in vitro conditions. We conclude that the SCO synthesises TTR and secretes it into the CSF. Colocalisation studies demonstrated that the SCO possessed two populations of secretory cells, one secreting both RF glycoproteins and TTR and the other secreting only the former. TTR was also detected in the SCO of bovine embryos suggesting that this ependymal gland is an important source of TTR during brain development.

The evolution of transthyretin synthesis in the choroid plexus

Choroid plexus has the highest concentration of transthyretin (TTR) mRNA in the body, 4.4 microg TTR mRNA/g wet weight tissue, compared with 0.39 microg in the liver. The proportion of TTR to total protein synthesis in choroid plexus is 12%. All newly synthesized TTR is secreted towards the ventricles. Net transfer of T4 occurs only towards the ventricle and depends on ongoing protein synthesis. Thyroxine-binding globulin (TBG), TTR and albumin form a "buffering" system for plasma [T4] because of their overlapping affinities and on/off rates for L-thyroxine (T4)-binding. The individual components of this network determining T4 distribution are functionally highly redundant. Absence of TBG (humans), or TTR (mice), or albumin (humans, rats) is not associated with hypothyroidism. Natural selection is based on small, inheritable alterations improving function. The study of these alterations can identify function. TTR genes were cloned and sequenced for a large number of vertebrate species. Systematic, stepwise changes during evolution occurred only in the N-terminal region, which became shorter and more hydrophilic. Simultaneously, a change in function occurred: TTR affinities for T4 are higher in mammals than in reptiles and birds. L-triiodothyronine (T3) affinities show the opposite trend. This favors site-specific regulation of thyroid hormones by tissue-specific deiodinases in the brain.

The evolution of transthyretin synthesis in vertebrate liver, in primitive eukaryotes and in bacteria

Thyroid hormones are evolutionarily old signal molecules, which can partition between compartments by partitioning into lipid membranes. The role of thyroid hormone distributor proteins is to ensure that sufficient thyroid hormone remains in the bloodstream. Of particular interest is the role of transthyretin, synthesised by the liver and secreted into the blood. In this review, three hypotheses are presented, suggesting the selection pressures leading to the onset of transthyretin synthesis in the liver during evolution. A thyroid hormone distribution network would be a selection advantage over a single protein performing this function. Similarly to the situation in eutherians, hepatic transthyretin synthesis in marsupials is under negative acute phase regulation. The overall three-dimensional structure of transthyretin did not change appreciably during vertebrate evolution. The region of the primary sequence which evolved most was the N-terminal region of the subunit. The N-termini of transthyretin changed from longer and more hydrophobic in reptiles/birds, to shorter and more hydrophilic in eutherians. These changes are correlated with a change in preference from binding of triiodothyronine, to binding thyroxine. As the rest of the molecule had not changed significantly during vertebrate evolution, the gene coding for transthyretin must have evolved prior to the divergence of the vertebrates from the non-vertebrates. Five open reading frames in the genomes of C. elegans (2), S. dublin, S. pombe and E. coli were identified. The protein products are predicted to form tetramers similar to transthyretins. Two possible functions of these proteins are suggested.

Crocodile transthyretin: structure, function, and evolution

Structure and function were studied for Crocodylus porosus transthyretin (crocTTR), an important intermediate in TTR evolution. The cDNA for crocTTR mRNA was cloned and sequenced and the amino acid sequence of crocTTR was deduced. In contrast to mammalian TTRs, but similar to avian and lizard TTRs, the subunit of crocTTR had a long and hydrophobic NH(2)-terminal region. Different from the situation in mammals, triiodothyronine (T(3)) was bound by crocTTR with higher affinity than thyroxine (T(4)). Recombinant crocTTR and a chimeric construct, with the NH(2)-terminal region of crocTTR being replaced by that of Xenopus laevis TTR, were synthesized in the yeast Pichia pastoris. Analysis of the affinity of the chimeric TTRs showed that the NH(2)-terminal region modulates T(4) and T(3) binding characteristics of TTR. The structural differences of the NH(2)-terminal regions of reptilian and amphibian TTRs were caused by a shift in splice sites at the 5' end of exon 2. The comparison of crocodile and other vertebrate TTRs shows that TTR evolution is an example for positive Darwinian evolution and identifies its molecular mechanism.

Structure and expression of the transthyretin gene in the choroid plexus: a model for the study of the mechanism of evolution

Thyroid hormones are key regulators of brain differentiation and function. They permeate strongly into lipid membranes. However, a substantial portion of thyroid hormone is retained in the intravascular/extracellular compartments by binding to plasma proteins. In the brain, transthyretin is the most important of these proteins. This transthyretin is synthesized in the epithelial cells of the choroid plexus and exclusively secreted towards the brain. A net movement of thyroid hormones from the blood to the brain ensues. During evolution, transthyretin synthesis in the choroid plexus and the beginnings of a neocortex first appeared at the stage of the stem reptiles. The affinity of transthyretin for thyroxine increased and that for triiodothyronine decreased during evolution. This could augment the importance of deiodination for regulation of metabolism and gene expression by thyroid hormones in the brain. Successive shifts of the splice site at the 5' end of exon 2 of transthyretin precursor mRNA in the 3' direction led to a shortening of the N-terminal sections and to an increase in hydrophilicity of the N-terminal regions of transthyretin. This shift can be explained by a sequence of single base mutations. It could be an example for a molecular mechanism of positive Darwinian evolution. The selection pressure, which led to the expression of the transthyretin gene in the choroid plexus during evolution, might have been the maintenance of thyroid hormone homeostasis in the extracellular compartment of the brain in the presence of the greatly increasing volume of the lipid phase.

Evolution of structure, ontogeny of gene expression, and function of Xenopus laevis transthyretin

Xenopus laevis transthyretin (xTTR) cDNA was cloned and sequenced. The derived amino acid sequence was very similar to those of other vertebrate transthyretins (TTR). TTR gene expression was observed during metamorphosis in X. laevis tadpole liver but not in tadpole brain nor adult liver. Recombinant xTTR was synthesized in Pichia pastoris and identified by amino acid sequence, subunit molecular mass, tetramer formation, and binding to retinol-binding protein. Contrary to mammalian xTTRs, the affinity of xTTR was higher for L-triiodothyronine than for L-thyroxine. The regions of the TTR genes coding for the NH(2)-terminal sections of the polypeptide chains of TTR seem to have evolved by stepwise shifts of mRNA splicing sites between exons 1 and 2, resulting in shorter and more hydrophilic NH(2) termini. This may be one molecular mechanism of positive Darwinian evolution. Open reading frames with xTTR-like sequences in the genomes of C. elegans and several microorganisms suggested evolution of the TTR gene from ancestor TTR gene-like "DNA modules." Increasing preference for binding of L-thyroxine over L-triiodothyronine may be associated with evolving tissue-specific regulation of thyroid hormone action by deiodination.

Amphibian choroid plexus lipocalin, Cpl1

Choroid plexus lipocalin 1 (Cpl1) has been isolated from the African clawed toad (Xenopus laevis) and the cane toad (Bufo marinus). Xcpl1 has been used as a marker for studying early neural development. Due to its retinoid binding properties and the fact that it causes dysmorphogenesis when overexpressed in the early embryo, the protein product is considered to be part of the retinoic acid signalling pathway. Later in development and during adulthood, the epithelial cell sheet of the choroid plexus which forms the blood-cerebrospinal fluid barrier expresses cpl1 as the predominant secretory protein. These data, the similarity of Cpl1 to prostaglandin D(2) synthase and its functional homology to transthyretin will be discussed.

Biochemical, structural, genetic, physiological, and pathophysiological features of lipocalin-type prostaglandin D synthase

Lipocalin-type prostaglandin (PG) D synthase (PGDS) catalyzes the isomerization of PGH(2), a common precursor of various prostanoids, to produce PGD(2), a potent endogenous somnogen and nociceptive modulator, in the presence of sulfhydryl compounds. PGDS is an N-glycosylated monomeric protein with an M(r) of 20000-31000 depending on the size of the glycosyl moiety. PGDS is localized in the central nervous system and male genital organs of various mammals and in the human heart and is secreted into the cerebrospinal fluid, seminal plasma, and plasma, respectively, as beta-trace. The PGDS concentrations in these body fluids are useful for the diagnosis of several neurological disorders, dysfunction of sperm formation, and cardiovascular and renal diseases. The cDNA and gene for PGDS have been isolated from several animal species, and the tissue distribution and cellular localization have also been determined. This enzyme is considered to be a dual functional protein; i.e. it acts as a PGD(2)-producing enzyme and also as a lipophilic ligand-binding protein, because the enzyme binds biliverdin, bilirubin (K(d)=30 nM), retinaldehyde, retinoic acid (K(d)=80 nM) with high affinities. X-ray crystallographic analyses revealed that PGDS possesses a beta-barrel structure with a hydrophobic pocket in which an active thiol, Cys(65), the active center for the catalytic reaction, was located facing to the inside of the pocket. Gene-knockout and transgenic mice for PGDS were generated and found to have abnormalities in the regulation of nociception and sleep.

Human tear lipocalin

Human tear prealbumin, now called tear lipocalin, was originally described as a major protein of human tear fluid, which was thought to be tear specific. However, recent investigations demonstrated that it is identical with lingual von Ebner's gland protein, and is also produced in prostate, nasal mucosa and tracheal mucosa. Homologous proteins have been found in rat, pig and probably dog and horse. Tear lipocalin is an unusual lipocalin member, because of its high promiscuity for relative insoluble lipids and binding characteristics that differ from other members. In addition, it shows inhibitory activity on cysteine proteinases similar to cystatins, a feature unique among lipocalins. Although it acts as the principal lipid binding protein in tear fluid, a more general physiological function has to be proposed due to its wide distribution and properties. It would be ideally suited for scavenging of lipophilic, potentially harmful substances and thus might act as a general protection factor of epithelia.

Evolution of the thyroid hormone-binding protein, transthyretin

Transthyretin (TTR) belongs to a group of proteins, which includes thyroxine-binding globulin and albumin, that bind to and transport thyroid hormones in the blood. TTR is also indirectly implicated in the carriage of vitamin A through the mediation of retinol-binding protein (RBP). It was first identified in 1942 in human serum and cerebrospinal fluid and was formerly called prealbumin for its ability to migrate faster than serum albumin on electrophoresis of whole plasma. It is a single polypeptide chain of 127 amino acids (14,000 Da) and is present in the plasma as a tetramer of noncovalently bound monomers. The major sites of synthesis of TTR in eutherian mammals, marsupials, and birds are the liver and choroid plexus but in reptiles it is synthesised only in the choroid plexus. The observation that TTR is strongly expressed in the choroid plexus but not in the liver of the stumpy-tailed lizard and the strong conservation of expression in the choroid plexus from reptiles to mammals have been taken as evidence to suggest that extrahepatic synthesis of TTR evolved first. The identification and cloning of TTR from the liver of an amphibian, Rana catesbeiana, and a teleost fish, Sparus aurata, and its absence from the choroid plexus of both species suggest an alternative model for its evolution. Protein modelling studies are presented that demonstrate differences in the electrostatic characteristics of the molecule in human, rat, chicken, and fish, which may explain why, in contrast to TTR from human and rat, TTR from fish and birds preferentially binds triiodo-l-thyronine.

The evolution of the thyroid hormone distributor protein transthyretin in the order insectivora, class mammalia

Thyroid hormones are involved in the regulation of growth and metabolism in all vertebrates. Transthyretin is one of the extracellular proteins with high affinity for thyroid hormones which determine the partitioning of these hormones between extracellular compartments and intracellular lipids. During vertebrate evolution, both the tissue pattern of expression and the structure of the gene for transthyretin underwent characteristic changes. The purpose of this study was to characterize the position of Insectivora in the evolution of transthyretin in eutherians, a subclass of Mammalia. Transthyretin was identified by thyroxine binding and Western analysis in the blood of adult shrews, hedgehogs, and moles. Transthyretin is synthesized in the liver and secreted into the bloodstream, similar to the situation for other adult eutherians, birds, and diprotodont marsupials, but different from that for adult fish, amphibians, reptiles, monotremes, and Australian polyprotodont marsupials. For the characterization of the structure of the gene and the processing of mRNA for transthyretin, cDNA libraries were prepared from RNA from hedgehog and shrew livers, and full-length cDNA clones were isolated and sequenced. Sections of genomic DNA in the regions coding for the splice sites between exons 1 and 2 were synthesized by polymerase chain reaction and sequenced. The location of splicing was deduced from comparison of genomic with cDNA nucleotide sequences. Changes in the nucleotide sequence of the transthyretin gene during evolution are most pronounced in the region coding for the N-terminal region of the protein. Both the derived overall amino sequences and the N-terminal regions of the transthyretins in Insectivora were found to be very similar to those in other eutherians but differed from those found in marsupials, birds, reptiles, amphibians, and fish. Also, the pattern of transthyretin precursor mRNA splicing in Insectivora was more similar to that in other eutherians than to that in marsupials, reptiles, and birds. Thus, in contrast to the marsupials, with a different pattern of transthyretin gene expression in the evolutionarily "older" polyprotodonts compared with the evolutionarily "younger" diprotodonts, no separate lineages of transthyretin evolution could be identified in eutherians. We conclude that transthyretin gene expression in the liver of adult eutherians probably appeared before the branching of the lineages leading to modern eutherian species.

Prostaglandin D synthase: structure and function

Prostaglandin (PG) D synthase catalyzes the isomerization of PGH2, a common precursor of various prostanoids, to produce PGD2 in the presence of sulfhydryl compounds. PGD2 induces sleep, regulates nociception, inhibits platelet aggregation, acts as an allergic mediator, and is further converted to 9 alpha, 11 beta-PGF2 or the J series of prostanoids, such as PGJ2, delta 12-PGJ2, and 15-deoxy-delta 12,14-PGJ2. We have purified two distinct types of PGD synthase; one is the lipocalin-type enzyme and the other is the hematopoietic enzyme. We isolated the cDNA and the gene for each enzyme and determined the tissue distribution profile and the cellular localization in several animal species. Lipocalin-type PGD synthase is localized in the central nervous system and male genital organs of various mammals and the human heart and is secreted into cerebrospinal fluid, seminal plasma, and plasma, respectively. The human enzyme was identified as beta-trace, which is a major protein in human cerebrospinal fluid. This enzyme is considered to be a dual-function protein; it acts as a PGD2-producing enzyme and also as a lipophilic ligand-binding protein, because the enzyme binds retinoids, thyroids, and bile pigments, with high affinities. Hematopoietic PGD synthase is widely distributed in the peripheral tissues and localized in the antigen-presenting cells, mast cells, and megakaryocytes. The hematopoietic enzyme is the first recognized vertebrate homolog of the sigma class of glutathione S-transferase. X-ray crystallographic analyses and generation of gene-knockout and transgenic mice for each enzyme have been performed.

Purification and characterization of thyroid-hormone-binding protein from masu salmon serum. A homolog of higher-vertebrate transthyretin

We purified a thyroid-hormone-binding protein (THBP) from serum of masu salmon at the stage of smoltification when the concentrations of endogenous thyroid hormones in plasma reach the highest levels. All steps of sequential column chromatography suggest that this THBP is responsible for most L-3,5,3'-triiodothyronine-binding activity in serum at this stage. The molecular mass of this protein was estimated to be 60 kDa by gel filtration but only 15 kDa by SDS/PAGE, which suggests that it is comprised of four identical subunits. The amino acid sequence of its N-terminal portion was highly similar to those of vertebrate transthyretins. These molecular features indicate that masu salmon THBP is a homolog of transthyretins from tetrapods. However, in contrast with mammalian transthyretins, the affinity of masu salmon transthyretin for L-3,5,3'-triiodothyronine was three times greater than for L-thyroxine. This rank order affinity is similar to that of avian and frog transthyretins. Scatchard analysis revealed that masu salmon transthyretin possesses a single class of binding site for L-3,5,3'-triiodothyronine, with a Kd of 13.8 nM at 0 degrees C. Taken together with the data reported by Chang et al. [Eur. J. Biochem. (1999) 259, 534-542], these results suggest that transthyretin has changed from a L-3,5, 3'-triiodothyronine-carrier protein to a L-thyroxine-carrier protein during mammalian evolution.

Binding of biliverdin, bilirubin, and thyroid hormones to lipocalin-type prostaglandin D synthase

Lipocalin-type prostaglandin D synthase is a major protein of the cerebrospinal fluid and was originally known as beta-trace. We investigated the binding ability of prostaglandin D synthase toward bile pigments, thyroid hormones, steroid hormones, and fatty acids in this present study. We found that the recombinant enzyme binds bile pigments and thyroid hormones, resulting in quenching of the intrinsic tryptophan fluorescence, the appearance of induced circular dichroism of the lipophilic ligands, and a red shift of the absorption spectra of bilirubin and biliverdin. The binding of prostaglandin D synthase to lipophilic ligands was also demonstrated by the resonant mirror technique and surface plasmon resonance detection. The dissociation constants were calculated to be 33 nM, 37 nM, 660 nM, 820 nM, and 2.08 microM for biliverdin, bilirubin, L-thyroxine, 3,3',5'-triiodo-L-thyronine, and 3,3', 5-triiodo-L-thyronine, respectively. Biliverdin and bilirubin underwent a shift in their absorption peaks from 375 to 380 nm and from 439 to 446 nm, respectively, after binding to prostaglandin D synthase. Bilirubin bound to the enzyme showed a bisignate CD spectrum with a (-) Cotton effect at 422 nm and a (+) Cotton effect at 472 nm, indicating a right-handed chirality. The ligands also inhibited prostaglandin D synthase activity noncompetitively in a concentration-dependent manner, with IC50 values between 3.9 and 10. 9 microM. Epididymal retinoic acid-binding protein and beta-lactoglobulin, two other lipocalin proteins that bind retinoids such as prostaglandin D synthase, did not show any significant interaction with bile pigments or thyroid hormones. These results show that prostaglandin D synthase binds small lipophilic ligands with a specificity distinct from that of other lipocalins.

Prostaglandin D2 and sleep regulation

Prostaglandin (PG) D2 is recognized as the most potent endogenous sleep-promoting substance whose action mechanism is the best characterized among the various sleep-substances thus far reported. The PGD2 concentration in rat cerebrospinal fluid (CSF) shows a circadian change coupled to the sleep-wake cycle and elevates with an increase in sleep propensity during sleep deprivation. Lipocalin-type PGD synthase is dominantly produced in the arachnoid membrane and choroid plexus of the brain, and is secreted into the CSF to become beta-trace, a major protein component of the CSF. The PGD synthase as well as the PGD2 thus produced circulates in the ventricular system, subarachnoidal space, and extracellular space in the brain system. PGD2 then interacts with DP receptors in the chemosensory region of the ventro-medial surface of the rostral basal forebrain to initiate the signal to promote sleep probably via the activation of adenosine A2A receptive neurons. The activation of DP receptors in the PGD2-sensitive chemosensory region results in activation of a cluster of neurons within the ventrolateral preoptic area, which may promote sleep by inhibiting tuberomammillary nucleus, the source of the ascending histaminergic arousal system.

Abundant synthesis of transthyretin in the brain, but not in the liver, of turtles

The binding of thyroxine to proteins in the blood plasma of the turtle, Trachemys scripta, was analyzed by incubation with radioactive thyroxine, electrophoresis and autoradiography. Albumin and an alpha-globulin were found to bind thyroxine; no thyroxine-binding transthyretin was detected in the prealbumin region. In contrast to blood plasma, a thyroxine-binding prealbumin was observed in medium from T. scripta choroid plexus incubated in vitro. RNA was extracted from brain tissue containing choroid plexus and from liver of T. scripta and Chelydra serpentina and analyzed by hybridization with transthyretin cDNA from the lizard Tiliqua rugosa. The brain RNAs contained substantial amounts of transthyretin mRNA, whereas only trace amounts of transthyretin mRNA were detected in RNA from liver. No transthyretin mRNA was observed in RNA from kidney. The results support the hypothesis that the expression of the transthyretin gene first evolved in the choroid plexus of the brain at the stage of the stem reptiles, whereas abundant transthyretin synthesis in liver evolved much later, and independently, in mammals and birds.

Prostaglandin D synthase (beta-trace protein): a molecular clock to trace the origin of REM sleep?

In 1965, Zuckerkandl and Pauling proposed a novel concept that some important molecules termed semantides, which carry the information of the genes or a transcript thereof, can be used as molecular clocks to trace evolutionary history. According to this concept, enzymes are designated as tertiary semantides, following genes (primary semantides) and the mRNA (secondary semantides). Based on this idea, I propose that prostaglandin D synthase (which has been demonstrated recently as identical to the beta-trace protein present in the cerebrospinal fluid of mammals) may serve as a molecular clock to trace the origin and evolution of rapid eye movement sleep in the vertebrates.

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.

Constitutive secretion of beta-trace protein by cultivated porcine choroid plexus epithelial cells: elucidation of its complete amino acid and cDNA sequences

Primary porcine choroid plexus epithelial cells cultivated in chemically defined medium maintain their epithelial characteristics and form confluent monolayers. They produce a fluid the composition of which resembles cerebrospinal fluid. The present study demonstrates constitutive secretion of large amounts of beta-trace protein. This intrathecally synthesized protein is a prominent polypeptide constituent of natural cerebrospinal fluid. According to the identity of amino acid sequences it has previously been tentatively identified as a prostaglandin-D synthase and as a member of the lipocalin protein family. beta-Trace was purified from cell culture supernatants and was subjected to tryptic digestion and amino acid sequencing of the resulting peptides. The complete primary structure of the protein was obtained by additional isolation of the cDNA from cultured epithelial cells. The porcine 163-amino acid polypeptide showed 69% identity with the human beta-trace and contained two N-glycosylation sites occupied by complex-type oligosaccharides as is the case for the human protein. The amino acid sequences around the N-glycosylation sites of mammalian beta-trace proteins (porcine, human, murine, and rat) were highly conserved. The nucleotide sequence was found to be less conserved; the porcine cDNA had a strikingly high GC-content (67%). The constitutive secretion of beta-trace protein from the in vitro cultivated porcine choroid plexus epithelial cells demonstrates that the cells have retained their major in vivo physiological properties: secretion of cerebrospinal fluid proteins. Therefore, this in vitro culture system may be used as a versatile tool for studying the regulation of the formation of cerebrospinal fluid.

Molecular mechanism of sleep regulation by prostaglandin D2

Recent biochemical, molecular biological, and pharmacological experiments revealed that prostaglandin D synthase as well as prostaglandin D2 circulated in the ventricular system, subarachnoidal space, and extracellular space in the brain. Prostaglandin D2 then interacts with chemosensors or receptors on the ventro-medial surface of the rostral basal forebrain to initiate the signal to promote sleep. Prostaglandin D2 is, therefore, not a typical neurotransmitter but rather a 'neurohormone' or an 'informational substance' that circulates through the cerebrospinal fluid and transmits certain chemical messages to promote sleep. The mode of communication through the cerebrospinal fluid in the ventricular system and the extracellular space has advantages for global regulation of the brain to induce sleep.

The lipocalin Xlcpl1 expressed in the neural plate of Xenopus laevis embryos is a secreted retinaldehyde binding protein

The cellular and structural properties and binding capabilities of a lipocalin expressed in the early neural plate of Xenopus laevis embryos and the adult choroid plexus have been investigated. It was found that this lipocalin, termed Xlcpl1, binds retinal at a nanomolar concentration, retinoic acid in the micromolar range, but does not show binding to retinol. Furthermore, this protein also binds D/L thyroxine. The Xlcpl1 cDNA was expressed in cell culture using the vaccinia virus expression system. In AtT20 cells, Xlcpl1 was secreted via the constitutive secretory pathway. We therefore assume that cpl1 binds retinaldehyde during the transport through the compartments of the secretory pathway that are considered to be the storage compartments of retinoids. Therefore, cpl1-expressing cells will secrete the precursors of active retinoids such as retinoic acid isomers. These retinoids may enter the cytosol by diffusion or receptor-controlled mechanisms, as has been shown for exogenously applied retinoids. Based on these data, it is suggested that cpl1 is an integral member of the retinoid signaling pathway and, therefore, it plays a key role in pattern formation in early embryonic development.

Choroid plexus: the major site of mRNA expression for the beta-trace protein (prostaglandin D synthase) in human brain

Expression of beta-trace protein (beta-trace), recently identified as glutathion-independent prostaglandin D synthase (prostaglandin-H2 D-isomerase; EC 5.3.99.2), was localized in paraffin sections of the human brain by in situ hybridization using digoxigenin-labeled antisense cRNA probes. The mRNA for beta-trace was predominantly found in the epithelial cells of the choroid plexus. Hybridization signals were also obtained in some oligodendrocytes, particularly in the white matter. In the leptomeninges, specific signals were found in meningeal macrophages and in single cells of the arachnoid barrier layer. The cells exhibiting hybridization signals with the antisense cRNA probes for beta-trace were identified by counterstaining with antibodies directed against specific cell markers. Additionally, beta-trace mRNA was localized in tubular epithelial and basal cells of the human epididymis and in different cell types within the seminiferous epithelium of the testis.

Binding of thyroxine to pig transthyretin, its cDNA structure, and other properties

Thyroxine binding to proteins in pig plasma during electrophoresis was observed in the albumin, but not in the prealbumin and post-albumin regions. Transthyretin could be identified in medium from in vitro pig choroid plexus incubations by size and number of subunits and a very high rate of synthesis and secretion. Its electrophoretic mobility was intermediate between that of thyroxine-binding globulin and albumin. It bound thyroxine, retinol-binding protein, anti-(rat transthyretin) antibodies and behaved similarly to transthyretins from other vertebrate species when plasma was extracted with phenol. Inhibition experiments with the synthetic flavonoid F 21388, analysing the binding of thyroxine, suggested that transthyretin is not a major thyroxine carrier in the bloodstream of pigs. Cloning and sequencing of transthyretin cDNA from both choroid plexus and liver showed that the same transthyretin mRNA is expressed in pig choroid plexus and liver. The amino acid sequence derived from the nucleotide sequence revealed that pig transthyretin differs from the transthyretins of all other studied vertebrate species by an unusual C-terminal extension consisting of the amino acids glycine, alanine and leucine. This extension results from the mutation of a stop codon into a codon for glycine. The unusual C-terminal extensions do not seem to interfere with the access of thyroxine to its binding site in the central channel of transthyretin.

Dorsal-ventral patterning and differentiation of noggin-induced neural tissue in the absence of mesoderm

In Xenopus development, dorsal mesoderm is thought to play a key role in both induction and patterning of the nervous system. Previously, we identified a secreted factor, noggin, which is expressed in dorsal mesoderm and which can mimic that tissue's neural-inducing activity, without inducing mesoderm. Here the neural tissue induced in ectodermal explants by noggin is further characterized using four neural-specific genes: two putative RNA-binding proteins, nrp-1 and etr-1; the synaptobrevin sybII; and the lipocalin cpl-1. First we determine the expression domain of each gene during embryogenesis. Then we analyze expression of these genes in noggin-treated explants. All markers, including the differentiated marker sybII, are expressed in noggin-induced neural tissue. Furthermore, cpl-1, a marker of dorsal brain, and etr-1, a marker absent in much of the dorsal forebrain, are expressed in non-overlapping territories within these explants. We conclude that the despite the absence of mesoderm, noggin-induced neural tissue shows considerable differentiation and organization, which may represent dorsal-ventral patterning of the forebrain.

Wallaby transthyretin

A cDNA library was constructed from liver RNA of the Australian diprotodont marsupial Macropus eugenii, the Tammar wallaby. A cloned full-length transthyretin cDNA was sequenced. The derived amino-acid sequence showed 68% overall similarity to that of human transthyretin, with 86% similarity in the thyroxine binding site. Comparisons of nucleotide and amino acid sequences from several vertebrate species indicated that the greatest differences were in the region corresponding to the disordered N-terminus of mature human transthyretin. The evolutionary trees deduced from parsimony analyses of amino acid and nucleotide sequences of transthyretins, are consistent with that derived from fossil records.

Structural and functional significance of cysteine residues of glutathione-independent prostaglandin D synthase. Identification of Cys65 as an essential thiol

Glutathione-independent prostaglandin D synthase in rat brain is composed of 189 amino acid residues and catalyzes the isomerization of prostaglandin H2 to prostaglandin D2, an endogenous sleep-promoting substance. This enzyme is the only enzyme among members of the lipocalin superfamily composed of various secretory lipophilic ligand-carrier proteins and is recently identified to be a beta-trace protein, a major constituent of human cerebrospinal fluid. We expressed the active enzyme in Escherichia coli and then systematically substituted all cysteine residues of the delta 1-29 enzyme at positions of 65, 89, and 186 with alanine or serine. The parent and mutant enzymes were purified to apparent homogeneity with a recovery of approximately 30% by chromatography with Sephadex G-50 and S-Sepharose, by which all the enzymes showed identical elution profiles. The purified enzymes, irrespective of the mutation, showed almost the same circular dichroism spectral characteristics as displayed by a highly ordered beta-structure. The recombinant enzymes containing Cys65 showed the activity comparable with that of the enzyme purified from rat brain (approximately 3 mumol/min/mg of protein) in the presence, but not in the absence, of sulfhydryl compounds. However, all of the single, double, and triple mutants without Cys65 lost the enzyme activity. The purified delta 1-29 Ala89,186 enzyme was inactivated reversibly by conjugation with glutathione at Cys65 and irreversibly by the stoichiometric chemical modification with N-ethylmaleimide. These results indicate that Cys65 is an essential thiol of the enzyme and that both the intrinsic and extrinsic sulfhydryl groups are necessary for nonoxidative rearrangement of 9,11-endoperoxide of prostaglandin H2 to produce prostaglandin D2 catalyzed by the enzyme.

Evolution of transthyretin in marsupials

The evolution of the expression and the structure of the gene for transthyretin, a thyroxine-binding plasma protein formerly called prealbumin, was studied in three marsupial species: the South American polyprotodont Monodelphis domestica, the Australian polyprotodont Sminthopsis macroura and the Australian diprotodont Petaurus breviceps. The transthyretin gene was found to be expressed in the choroid plexus of all three species. In liver it was expressed in P. breviceps and in M. domestica, but not in S. macroura. This, together with previous studies [Richardson, S. J., Bradley, A. J., Duan, W., Wettenhall, R. E. H., Harms, P. J., Babon, J. J., Southwell, B. R., Nicol, S., Donnellan, S. C. & Schreiber, G. (1994) Am. J. Physiol. 266, R1359-R1370], suggests the independent evolution of transthyretin synthesis in the liver of the American Polyprotodonta and the Australian Diprotodonta. The results obtained from cloning and sequencing of the cDNA for transthyretin from the three species suggested that, in the evolution of the structure of transthyretin in vertebrates, marsupial transthyretin structures are intermediate between bird/reptile and eutherian transthyretin structures. In marsupials, as in birds and reptiles, a hydrophobic tripeptide beginning with valine and ending with histidine was found in transthyretin at a position which has been identified in eutherians as the border between exon 1 and intron 1. In humans, rats and mice, the nine nucleotides, coding for this tripeptide in marsupials/reptiles/birds, are found at the 5' end of intron 1. They are no longer present in mature transthyretin mRNA. This results in a change in character of the N-termini of the subunits of transthyretin from hydrophobic to hydrophilic. This change might affect the accessibility of the thyroxine-binding site in the central channel of transthyretin, since, at least in humans, the N-termini of the subunits of transthyretin are located in the vicinity of the channel entrance [Hamilton, J. A., Steinrauf, L. K., Braden, B. C., Liepnieks, J., Benson, M. D., Holmgren, G., Sandgren, O. & Steen, L. (1993) J. Biol. Chem. 268, 2416-2424].

Ependymins and their potential role in neuroplasticity and regeneration: calcium-binding meningeal glycoproteins of the cerebrospinal fluid and extracellular matrix

1. Ependymins are unique, highly divergent secretory proteins of the fish endomeninx. Thus far, no homologous sequences have been characterized in mammals. 2. Soluble ependymins are the predominant constituents of the cerebrospinal fluid of many teleost fish. A bound form of these glycoproteins is associated with the extracellular matrix probably with collagen fibrils. The latter may be the functional form of ependymins. 3. Ependymins bind Ca2+ via N-linked sialic acid residues leading to a conformational transition. 4. The molecular function of ependymins seems to be related to cell contact phenomena involving the extracellular matrix. For example, adhesive or anti-adhesive interactions may possibly influence ingrowing axons.

Transthyretin expression evolved more recently in liver than in brain

1. Transthyretin was found to be synthesized and secreted by choroid plexus from rats, echidnas, and lizards, but not toads. 2. Transthyretin was observed in blood from placental mammals, birds, and marsupials, but not reptiles and monotremes. 3. The obtained data suggest that transthyretin synthesis by the liver evolved independently in the lineage leading to the placental mammals and marsupials and in that leading to the birds. 4. It is proposed that transthyretin gene expression in mammalian liver appeared about 200 million years later than its first occurrence in the choroid plexus of the stem reptiles.