Protein synthesis and secretion by cultured retinal pigment epithelia

Oxidative Stress in Transthyretin-Mediated Amyloidosis: An Exploratory Study

Transthyretin-mediated amyloidosis (ATTR) is a systemic disease with protein precipitation in many tissues, mainly the peripheral nerve and heart. Both genetic (ATTRv, "v" for variant) and wild-type (ATTRwt) forms are known. Beyond the steric encumbrance, precipitated transthyretin seems to have a toxic effect. In this study carried out in men, we recruited 15 ATTRv patients, 7 ATTRv asymptomatic carriers, 14 ATTRwt patients and 10 young and 13 old healthy controls to evaluate the oxidative stress using FORD (Free Oxygen Radicals Defense) and FORT (Free Oxygen Radicals Test) analyses. ATTRv patients showed reduced FORD compared to ATTRwt and ATTRv asymptomatic carriers. FORD independently predicted the disease stage, with the early stages characterized by the highest consumption. These findings suggest a role for oxidative stress in the early stages of ATTRv.

Does Patisiran Reduce Ocular Transthyretin Synthesis? A Pilot Study of Two Cases

BACKGROUND

Variant transthyretin-mediated amyloidosis (ATTR-v) is a well-characterized disease affecting the neurologic and cardiovascular systems. Patisiran has been approved for neurologic involvement as it reduces hepatic synthesis of transthyretin (TTR). Eye involvement is a lateonset feature increasing the risk of glaucoma and cataracts in patients.

AIMS

The aim of this case series was to assess whether patisiran can effectively reduce TTR synthesis in such a barrier-protected organ as the eye.

METHODS

Two patisiran-treated ATTR-v patients underwent serum and aqueous humor sampling to measure TTR levels detected by SDS-PAGE and immunoblotting. Serum samples were compared to healthy control (HC), whereas aqueous humor samples were compared to non-amyloidotic subjects affected by cataracts and glaucoma.

RESULTS

Serum TTR levels representative of hepatic synthesis were sharply lower in treated patients if compared to the HC (-87.5% and -93.75%, respectively). Aqueous humor TTR levels showed mild-tono reduction in treated patients compared to non-amyloidotic subjects with cataracts (-34.9% and +8.1%, respectively) and glaucoma (-41.1% and -2.1%).

CONCLUSION

Patisiran does not seem to be as effective in inhibiting ocular TTR synthesis as it is in inhibiting hepatic synthesis. Re-engineering the envelope could allow the drug to target RPE cells thus avoiding any ocular involvement.

Scavenger Receptor Class B Member 1 Independent Uptake of Transthyretin by Cultured Hepatocytes Is Regulated by High Density Lipoprotein

Thyroid hormone (thyroxine, T4) is essential for the normal function of all cell types and is carried in serum bound to several proteins including transthyretin. Recently, evidence has emerged of alternate pathways for hormone entry into cells that are dependent on hormone binding proteins. Transthyretin and transthyretin bound T4 are endocytosed by placental trophoblasts through the high-density lipoprotein receptor, Scavenger Receptor Class B Type 1 (SR-B1). High density lipoprotein (HDL) affects the expression and function of SR-B1 in trophoblast cells. SR-B1 is also expressed in hepatocytes and we sought to determine if hepatocyte SR-B1 was involved in transthyretin or transthyretin-T4 uptake and whether uptake was affected by HDL. Transthyretin and transthyretin-T4 uptake by hepatocytes is not dependent on SR-B1. HDL treatment reduced SR-B1 expression. However, pretreatment of hepatocytes with HDL increased uptake of transthyretin-T4. Knockdown of SR-B1 expression using siRNA also increased transthyretin-T4 uptake. Coaddition of HDL to transthyretin uptake experiments blocked both transthyretin and transthyretin-T4 uptake. Hepatocyte uptake of transthyretin-T4 uptake is influenced by, but is not dependent on, SR-B1 expression. HDL also decreases transthyretin-T4 uptake and therefore diet or drugs may interfere with this process. This suggests that multiple lipoprotein receptors may be involved in the regulation of uptake of transthyretin-T4 in a cell-type specific manner. Further study is required to understand this important process.

Rescue of retinal morphology and function in a humanized mouse at the mouse retinol-binding protein locus

Retinol-binding protein RBP4 is the specific carrier for retinol in the blood. We previously produced a Rbp4-deficient (Rbp4^-/-) mouse that showed electroretinogram (ERG) abnormalities, accompanied by histological and electron-microscopic changes such as fewer synapses in the inner plexiform layer in the central retina. To address whether human RBP4 gene expression can rescue the phenotypes observed in Rbp4^-/- mice, we produced a humanized (Rbp4^{hRBP4orf/ hRBP4orf}) mouse with a human RBP4 open reading frame in the mouse Rbp4 locus using a Cre-mutant lox recombination system. In Rbp4^{hRBP4orf/hRBP4orf} mice, the tissue-specific expression pattern of hRBP4orf was roughly the same as that of mouse Rbp4. ERG and morphological abnormalities observed in Rbp4^-/- mice were rescued in Rbp4^{hRBP4orf/hRBP4orf} mice as early as 7 weeks of age. The temporal expression pattern of hRBP4orf in the liver of Rbp4^{hRBP4orf/hRBP4orf} mice was similar to that of mouse Rbp4 in Rbp4^+/+mice. In contrast, hRBP4orf expression levels in eyes were significantly lower at 6 and 12 weeks of age compared with mouse Rbp4 but were restored to the control levels at 24 weeks. The serum hRBP4 levels in Rbp4^{hRBP4orf/hRBP4orf} mice were approximately 30% of those in Rbp4^+/+ at all ages examined. In accordance with this finding, the plasma retinol levels remained low in Rbp4^{hRBP4orf/hRBP4orf} mice. Retinol accumulation in the liver occurred in control and Rbp4^{hRBP4orf/hRBP4orf} mice but was higher in Rbp4^{hRBP4orf/hRBP4orf} mice at 30 weeks of age. Mouse transthyretin expression was not altered in Rbp4^-/- or Rbp4^{hRBP4orf/hRBP4orf} mice. Taken together, 30% of the serum RBP4 level was sufficient to correct the abnormal phenotypes observed in Rbp4^-/- mice.

Ophthalmic manifestations in a Chinese family with familial amyloid polyneuropathy due to a TTR Gly83Arg mutation

PURPOSE

To describe the characteristic ophthalmic phenotypes of a large Chinese family with familial amyloid polyneuropathy due to a missense mutation in transthyretin (TTR) (c.307 C>G).

METHODS

Twenty-seven individuals (12 affected, 15 unaffected) from a five-generation Chinese family underwent general medical examination and comprehensive ophthalmic examination, including best correct visual acuity, intraocular pressure measurements, Schirmer test, slitlamp examination, fundoscopy, and ocular ultrasonography. Histological examination of vitreous biopsies using Congo red staining and immunohistochemistry was performed. Cardiovascular magnetic resonance (CMR), electrocardiogram, and echocardiogram were used to evaluate cardiac amyloidosis. Electromyography was used to evaluate nerve function. All four exons of TTR were amplified by PCR, sequenced using a Bigdye terminator v3.1 cycle sequencing kit and analyzed on an ABI 3700XL Genetic Analyzer.

RESULTS

All 12 affected individuals in the family had ocular manifestations, including severe vitreous opacities, secondary glaucoma, xerophthalmia, dyscoria, and attenuated retinal arteries. Congo red staining demonstrated amyloid deposits in the vitreous, and immunohistochemical staining confirmed the deposition of TTR proteins in the vitreous. Twelve individuals had polyneuropathy, and electromyography detected functional damage in peripheral nerves. One individual was diagnosed with cardiac amyloidosis by CMR. Direct sequencing revealed the heterozygous missense mutation in TTR (c.307 C>G p.Gly83Arg) in all 12 affected individuals. The mutation co-segregated with the disease phenotype and was absent in 100 normal controls.

CONCLUSIONS

Vitreous opacity is very common in patients with the TTR Gly83Arg mutation; other clinical characteristics associated with the mutation include polyneuropathy and cardiac amyloidosis.

Carrier-mediated thyroid hormone transport into placenta by placental transthyretin

CONTEXT

The serum protein transthyretin (TTR) plays an important role in the transport of thyroid hormone and retinol, which are critical for normal development of the human fetus. TTR is not only synthesized and secreted into the circulation by the liver and other tissues but is also synthesized by placental trophoblasts, which separate the maternal and fetal circulations. Whether it is secreted or taken up by these cells and whether it carries thyroid hormone is unknown.

OBJECTIVE AND METHODS

Our objective was to study placental handling of TTR and determine whether TTR participates in placental thyroid hormone transport. We investigated the capacity of human placenta and choriocarcinoma cell lines to secrete and internalize TTR and its ligands by Western blotting, immunofluorescence, and uptake of radiolabeled TTR.

RESULTS

Human placental explants and TTR expressing JEG-3 cells secrete TTR. JEG-3 cells grown in bicameral chambers secrete TTR, predominantly from the apical surface. Human placental explants and JEG-3 cells internalize Alexa Fluor488-labeled TTR and (125)I-TTR. Furthermore, binding to thyroid hormones (T(4), T(3)) increases (125)I-TTR uptake by enhancing tetramer formation. Cross-linking experiments confirm internalization of the TTR-(125)I-T(4) complex.

CONCLUSIONS

Our results suggest that human placenta and choriocarcinoma cells secrete transthyretin, which binds extracellular T(4), and that T(4) binding results in increased internalization of TTR-T(4) complex. TTR production by trophoblasts may represent a mechanism to allow transfer of maternal thyroid hormone to the fetal circulation that could have important implications for fetal development.

The mechanism underlying defective Fcgamma receptor-mediated phagocytosis by HIV-1-infected human monocyte-derived macrophages

Clearance of IgG-opsonized erythrocytes is impaired in HIV-1-infected patients, suggesting defective FcgammaR-mediated phagocytosis in vivo. We have previously shown defective FcgammaR-mediated phagocytosis in HIV-1-infected human monocyte-derived macrophages (MDM), establishing an in vitro model for defective tissue macrophages. Inhibition was associated with decreased protein expression of FcR gamma-chain, which transduces immune receptor signals via ITAM motifs. FcgammaRI and FcgammaRIIIa signal via gamma-chain, whereas FcgammaRIIa does not. In this study, we showed that HIV-1 infection inhibited FcgammaRI-, but not FcgammaRIIa-dependent Syk activation in MDM, showing that inhibition was specific for gamma-chain-dependent signaling. HIV-1 infection did not impair gamma-chain mRNA levels measured by real-time PCR, suggesting a posttranscriptional mechanism of gamma-chain depletion. HIV-1 infection did not affect gamma-chain degradation (n = 7, p = 0.94) measured in metabolic labeling/chase experiments, whereas gamma-chain biosynthesis was inhibited (n = 12, p = 0.0068). Using an enhanced GFP-expressing HIV-1 strain, we showed that FcgammaR-mediated phagocytosis inhibition is predominantly due to a bystander effect. Experiments in which MDM were infected in the presence of the antiretroviral drug 3TC suggest that active viral replication is required for inhibition of phagocytosis in MDM. These data suggest that HIV-1 infection may affect only gamma-chain-dependent FcgammaR functions, but that this is not restricted to HIV-1-infected cells.

Synthesis of thyroid hormone binding proteins transthyretin and albumin by human trophoblast

CONTEXT

Mechanisms regulating materno-fetal transfer of thyroid hormone are not well understood. Modulation of trophoblast type 3 iodothyronine deiodinase (D3) may play an important role.

OBJECTIVE

The objective of this study was to investigate trophoblast thyroid hormone binding proteins that may modulate interactions between D3 and T4.

DESIGN

Placentas were obtained by informed consent from women delivering normal infants by repeat cesarean section at 38-40 wk gestation. T4 and T3 binding was examined in human placenta. Serum thyroid hormone binding proteins were identified by Western blotting, and their mRNA was examined by RT-PCR. Presence of these proteins in trophoblast was determined by immunocytochemistry and immunofluorescence. Cytosol was progressively purified to reveal additional thyroid hormone binding proteins that were identified by matrix-assisted laser desorption/ionization time of flight mass spectrometry. Effects of mefenamic acid on placental deiodination were examined by HPLC.

RESULTS

We detected high-affinity T4 and T3 binding in human placental cytosol. All three major serum-binding proteins, T4 binding globulin (TBG), transthyretin (TTR), and albumin, were present in cytosol. TTR mRNA and albumin mRNA were detected in human placenta, and TTR and albumin were identified histochemically in syncytiotrophoblasts. Neither TBG mRNA nor TBG was detected, suggesting that plasma TBG had contaminated the cytosol preparation. Low-affinity thyroid hormone binding proteins alpha-1-antitrypsin and alpha-1-acid glycoprotein were also identified. Addition of mefenamic acid, a potent inhibitor of thyroid hormone binding, to placental cytosol significantly enhanced deiodination of T4 by D3.

CONCLUSIONS

Placenta produces a series of thyroid hormone binding proteins that may modify thyroid hormone deiodination and materno-fetal thyroid hormone transport.

Retinal pigment epithelial cells exhibit unique expression and localization of plasma membrane syntaxins which may contribute to their trafficking phenotype

The SNARE membrane fusion machinery controls the fusion of transport vesicles with the apical and basolateral plasma-membrane domains of epithelial cells and is implicated in the specificity of polarized trafficking. To test the hypothesis that differential expression and localization of SNAREs may be a mechanism that contributes to cell-type-specific polarity of different proteins, we studied the expression and distribution of plasma-membrane SNAREs in the retinal pigment epithelium (RPE), an epithelium in which the targeting and steady-state polarity of several plasma membrane proteins differs from most other epithelia. We show here that retinal pigment epithelial cells both in vitro and in vivo differ significantly from MDCK cells and other epithelial cells in their complement of expressed t-SNAREs that are known - or suggested - to be involved in plasma membrane trafficking. Retinal pigment epithelial cells lack expression of the normally apical-specific syntaxin 3. Instead, they express syntaxins 1A and 1B, which are normally restricted to neurons and neuroendocrine cells, on their apical plasma membrane. The polarity of syntaxin 2 is reversed in retinal pigment epithelial cells, and it localizes to a narrow band on the lateral plasma membrane adjacent to the tight junctions. In addition, syntaxin 4 and the v-SNARE endobrevin/VAMP-8 localize to this sub-tight junctional domain, which suggests that this is a region of preferred vesicle exocytosis. Altogether, these data suggest that the unique polarity of many retinal pigment epithelial proteins results from differential expression and distribution of SNAREs at the plasma membrane. We propose that regulation of the expression and subcellular localization of plasma membrane SNAREs may be a general mechanism that contributes to the establishment of distinct sorting phenotypes among epithelial cell types.

The polarity of the retinal pigment epithelium

The diversity of epithelia in the body permits a multitude of organ-specific functions. One of the foremost examples of this is the retinal pigment epithelium. Located between the photoreceptors of the retina and their principal blood supply, the choriocapillaris, the retinal pigment epithelium is critical for the survival and function of retinal photoreceptors. To serve this purpose, the retinal pigment epithelium cell has adapted the classic Golgi-to-cell-surface targeting pathways first described in such prototypic epithelial cell models as the Madin-Darby canine kidney cell, to arrive at a unique distribution of membrane and secreted proteins. More recent data suggest that the retinal pigment epithelium also takes advantage of its inherent asymmetry to augment the classical pathways of Golgi-to-cell-surface traffic. As retinal pigment epithelium transplants and gene therapy represent potential cures for retinal degenerative diseases, understanding the basis of the unique polarity properties of retinal pigment epithelium cells will be a critical issue for the development of future therapies.

Local cellular sources of apolipoprotein E in the human retina and retinal pigmented epithelium: implications for the process of drusen formation

PURPOSE

The inheritance of specific apolipoprotein E allelles has been linked to atherosclerosis, Alzheimer disease, and, most recently, to the incidence of age-related macular degeneration. Apolipoprotein E is a common component of the extracellular plaques and deposits characteristic of these disorders, including drusen, which are a hallmark of age-related macular degeneration. Accordingly, we assessed the potential biosynthetic contribution of local ocular cell types to the apolipoprotein E found in drusen.

METHODS

We measured apolipoprotein E mRNA levels in human donor tissues using a quantitative assay of apolipoprotein E transcription, and we localized apolipoprotein E protein to specific cell types and compartments in the neural retina, retinal pigmented epithelium, and choroid using laser scanning confocal immunofluorescence microscopy.

RESULTS

Apolipoprotein E immunoreactivity is associated with photoreceptor outer segments, the retinal ganglion cell layer, the retinal pigmented epithelium basal cytoplasm and basal lamina, and with both collagenous layers of Bruch membrane. Apolipoprotein E appears to be a ubiquitous component of drusen, irrespective of clinical phenotype. It also accumulates in the cytoplasm of a subpopulation of retinal pigmented epithelial cells, many of which overlie or flank drusen. Mean levels of apolipoprotein E mRNA in the adult human retina are 45% and 150% of the levels measured in liver and adult brain, the two most abundant biosynthetic sources of apolipoprotein E. Apolipoprotein E mRNA levels are highest in the inner retina, and lowest in the outer retina where photoreceptors predominate. Significant levels of apolipoprotein E mRNA are also present in the retinal pigmented epithelium/choroid complex and in cultured human retinal pigmented epithelial cells.

CONCLUSIONS

Apolipoprotein E protein is strategically located at the same anatomic locus where drusen are situated, and the retinal pigmented epithelium is the most likely local biosynthetic source of apolipoprotein E at that location. Age-related alteration of lipoprotein biosynthesis and/or processing at the level of the retinal pigmented epithelium and/or Bruch membrane may be a significant contributing factor in drusen formation and age-related macular degeneration pathogenesis.

Developmental expression, tissue distribution and hormonal regulation of fish (Sparus aurata) serum retinol-binding protein

Retinol-binding protein (RBP) is the specific carrier of retinol in vertebrates and forms a 1:1 complex with transthyretin (TTR). A cDNA encoding serum RBP was cloned from liver and 7-day larvae of the marine fish Sparus aurata. The mature protein is 176 amino acids long and shows sequence identity of 77-78%, 56%, 63% and 62% with rainbow trout, Xenopus, chicken and human RBP, respectively. Northern blot analysis of hepatic RBP revealed two transcripts: a major one of approximately 1.4-1.5 kb and a minor of approximately 0.7 kb. Distribution of RBP mRNA in various tissues was studied by RT-PCR and showed high expression in liver and skin, and low expression in brain, kidney and gill filament (20-35% of the level in liver). RBP expression in intestine, pyloric caeca, muscle and pituitary was estimated to be approximately 7-14% of the level in liver. The ontogeny of RBP expression in S. aurata was examined in unfertilized eggs, embryos and larvae by using RT-PCR followed by hybridization with a specific probe. RBP transcript was found in all larval stages studied. Very low levels of RBP mRNA were detected in unfertilized eggs and in embryos 8 h after fertilization with a gradual increase at 12 h and 15-16 h post-fertilization. A single injection of estradiol-17beta to S. aurata immature, bisexual fish or to adult males reduced steady-state levels of hepatic RBP by 37 and 25%, respectively. The same treatment induced vitellogenin expression. The present data suggest that in fish, liver is the main site of RBP synthesis, but that RBP may have an important function in fish skin. RBP is expressed early in embryonic development and in fish its expression can be down regulated by estrogen.

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.

Vitreous amyloidosis after liver transplantation in patients with familial amyloid polyneuropathy: ocular synthesis of mutant transthyretin

Vitreous amyloidosis has been reported in patients with familial amyloidotic polyneuropathy (FAP) who are carriers of different mutant transthyretins (TTR). The mutant TTR constitutes the majority of the amyloid vitreous fibrils in heterozygous Val30Met patients. Due to the ocular synthesis of TTR, it is possible that the retina constitutes the source of vitreous amyloid fibrils, if so, orthotopic liver transplantation (OLT) performed to remove the mutant TTR from circulation might not be effective in treating/avoiding vitreous amyloid. We present vitreous amyloidosis in a FAP patient from Maiorca with ATTR Val30Met who underwent OLT at age 38. Progressive impairment of visual acuity (VA) appeared bilaterally 2 years after OLT due to vitreous opacities consistent with amyloid; successful bilateral vitrectomy was performed. Amyloid was demonstrated in the vitrectomy material by Congo red staining, immunohistochemistry and Western blotting analyses were positive with an antibody for human TTR. Mass spectrometry of TTR revealed the presence of the mutant in approximately 20% of the TTR. Future structural studies on vitreous material with different proportions of normal/versus mutant TTR might shed some light on TTR fibrillogenesis. These results show that vitreous deposition of TTR amyloidfibrils occurs after OLT, suggesting that ongoing intraocular synthesis of mutant TTR might contribute to this process. We also present the progression after OLT of vitreous amyloidosis previously diagnosed in three patients with TTR Val71Ala.

Plasma retinol binding protein: structure and function of the prototypic lipocalin

In terms of both structure and biological function, retinol binding protein (RBP) is one of the best characterized members of the lipocalin superfamily. The molecular interactions in which RBP participates are described herein.

Transthyretin localization in cultured and native human retinal pigment epithelium

The aim of this study was to determine transthyretin subcellular localization in cultured and native human retinal pigment epithelium. Monoclonal and polyclonal antibodies directed against human plasma transthyretin were used to detect transthyretin-specific immunoreactivity in cultured human retinal pigment epithelium. At the light microscopic level, transthyretin-specific immunoreactivity was observed throughout the cytosol with intense perinuclear staining. Nuclear staining was faint, but detectable. Both monoclonal and polyclonal antibodies exhibited similar staining patterns. An electron microscopic immunogold labeling protocol detected transthyretin-specific immunoreactivity in cultured and native human retinal pigment epithelium. Transthyretin-specific immunogold labeling within mitochondrial, apical, basal, nuclear and cytosolic subcellular compartments was quantitated and statistically analyzed. The pattern of transthyretin labeling was similar for each antibody, and comparable between cultured and native human retinal pigment epithelium. Transthyretin labeling was observed in mitochondrial and nuclear compartments, and in close apposition to both apical and basal membranes. Transthyretin labeling density was highest in the mitochondrial compartment and was significantly greater than labeling in all other compartments. Detection of transthyretin labeling in mitochondrial and nuclear compartments suggests an intracellular role for transthyretin in human retinal pigment epithelium, possibly as a cytoplasmic carrier protein for thyroxine.

Perfluorodecalin modifies the pattern of cell arrangement and induces loss of neurites in rat retinal cultures

Perfluorodecalin (PFD), a high specific weight, water-immiscible perfluorocarbon, previously studied as a potential blood substitute, now is used widely in the field of ophthalmic surgery as a tool for maneuvering intraocular tissues and as a short- or medium-term vitreous substitute. In in vivo experiments, several types of lesions in retinal tissue have been described in conjunction with long-term PFD treatment. To better evaluate the biological effects of PFD on retinal cells, we tested it on primary cultures of rat retina seeded on special cyclopore wells that allow the culture to be fed from the bottom side while the top side is in contact with the water-immiscible compound. We found that PFD changed the pattern of cell arrangement and induced loss of neurites. The modification of cell arrangement was less evident at the periphery of the wells where the amount of PFD, and consequently the pressure exerted, was lower. This observation suggests that the changes may be due more to a physical than to a toxic effect of PFD.

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.