Production and functional analysis of normal and variant recombinant human transthyretin proteins

Directional thyroid hormone distribution via the blood stream to target sites

Thyroid hormones are bound to three major serum transport proteins, thyroxin-binding globulin (TBG), transthyretin (TTR) and human serum albumin (HSA). TBG has the strongest affinity for thyroid hormones, TTR is also found in the cerebrospinal fluid and HSA is the most abundant protein in plasma. Combination defects of either a high affinity TTR or HSA variant do not compensate TBG deficiency, underscoring the dominant role of TBG among the thyroid hormone transport proteins. On the other hand, coexistence of raised affinity TTR and HSA variants causes an augmented hyperthyroxinemia. Variations in thyroid hormone transport proteins may alter thyroid function tests to mimic hypo- or hyperthyroidism. As affected individuals are clinically euthyroid and do not require treatment, identification of thyroid hormone transport protein defects is important to avoid unnecessary diagnostic and therapeutic interventions. Mammals share the multilayered system of thyroid hormone binding proteins with humans. Some of them, especially carnivores, do not express TBG. In dogs, this defect has been shown to be caused by a defective hepatocyte nuclear factor-1 binding site in the TBG promoter, preventing TBG synthesis in the liver. The major endogenous thyroid hormone metabolite 3-iodothyronamine (3-T1AM) exerts marked cryogenic, metabolic, cardiac and central nervous system actions. It is bound to apolipoproteinB-100 (ApoB100), possibly facilitating its cellular uptake via interaction with the low density lipoprotein-receptor. This review summarizes the handling of hydrophobic charged thyroid hormone signaling molecules and their metabolite 3-T1AM in aqueous body fluids and the advantages and limits of their serum distributor proteins.

Inherited defects of thyroxine-binding proteins

Thyroid hormones (TH) are bound to three major serum transport proteins, thyroxine-binding globulin (TBG), transthyretin (TTR) and human serum albumin (HSA). TBG has the strongest affinity for TH, whereas HSA is the most abundant protein in plasma. Individuals harboring genetic variations in TH transport proteins present with altered thyroid function tests, but are clinically euthyroid and do not require treatment. Clinical awareness and early recognition of these conditions are important to prevent unnecessary therapy with possible untoward effects. This review summarizes the gene, molecular structure and properties of these TH transport proteins and provides an overview of their inherited abnormalities, clinical presentation, genetic background and pathophysiologic mechanisms.

Advantageous uses of mass spectrometry for the quantification of proteins

Quantitative protein measurements by mass spectrometry have gained wide acceptance in research settings. However, clinical uptake of mass spectrometric protein assays has not followed suit. In part, this is due to the long-standing acceptance by regulatory agencies of immunological assays such as ELISA assays. In most cases, ELISAs provide highly accurate, sensitive, relatively inexpensive, and simple assays for many analytes. The barrier to acceptance of mass spectrometry in these situations will remain high. However, mass spectrometry provides solutions to certain protein measurements that are difficult, if not impossible, to accomplish by immunological methods. Cases where mass spectrometry can provide solutions to difficult assay development include distinguishing between very closely related protein species and monitoring biological and analytical variability due to sample handling and very high multiplexing capacity. This paper will highlight several examples where mass spectrometry has made certain protein measurements possible where immunological techniques have had a great difficulty.

Operative risks of domino liver transplantation for the familial amyloid polyneuropathy liver donor and recipient: a double analysis

Although domino liver transplantation (LT) is an established procedure, data about the operative risks are limited. This study aimed at evaluating the operative risks of domino LT. Two retrospective analyses were conducted (comparison of familial amyloid polyneuropathy [FAP] liver donors [61 patients] vs. FAP nondonors [39 patients] and FAP liver recipients [61 patients] vs. deceased donor liver recipients [61 patients]). First analysis showed a 60-day mortality of 6.6% for FAP donors and 7.7% for FAP nondonors (p = 1.0). No patient developed primary graft nonfunction. Acute rejection was higher in FAP nondonors compared to FAP donors (38.5% vs. 13.1%). Both groups had similar vascular and biliary complication rates. ICU stay was similar, whereas total hospitalization was longer for FAP nondonors. Both groups had similar 1- and 5-year patient and graft survival rates (83.4% vs. 87.2%, and 79.8% vs. 71.8%, p = 0.7) and (83.3% vs. 87.2%, and 79.1% vs.71.8%, p = 0.7). The second analysis showed a 1.6% mortality for FAP liver recipients vs. 3.2% of the control group (p = 1). Both groups had similar morbidity and technical complication rates (18.0% vs. 13.1%, p = 0.45) and (0.18 vs. 0.15, p = 0.65). The domino procedure does not add any risk to FAP donor or recipient. It increases the organ pool allowing transplantation of marginal recipients who otherwise are denied deceased donor liver transplantation.

Interaction of transthyretin with amyloid beta-protein: binding and inhibition of amyloid formation

Aggregated amyloid beta-protein (A beta) is a key component of the amyloid depositions found in the brains of patients with Alzheimer's disease. In contrast, in cerebrospinal fluid (CSF), A beta is found in a soluble form. The analysis of complexes of A beta with CSF proteins in a KBr gradient revealed an association of A beta only with free proteins and not with lipoprotein particles. Transthyretin (TTR), a second major CSF protein, formed SDS-stable complexes with A beta and significantly decreased the rate of A beta fibril formation. In physiological buffers and CSF, TTR exclusively decreased the level of A beta pentamers. Endogenous TTR-A beta complexes were detected in human CSF by immunoprecipitation. Using site-directed mutagenesis and computer-assisted modelling, we identified amino acid residues on the surface of the TTR monomer that interact with A beta. Specific TTR immunoreactivity was detected in multiple cortical neurons and astrocytes in the human brain. We propose that A beta binding proteins play a key role in the modulation of A beta aggregation and normally prevent amyloid formation in biological fluids and in the brain.

Chromium(III) ion and thyroxine cooperate to stabilize the transthyretin tetramer and suppress in vitro amyloid fibril formation

Transthyretin (TTR) amyloid fibril formation, which is triggered by the dissociation of tetrameric TTR, appears to be the causative factor in familial amyloidotic polyneuropathy and senile systemic amyloidosis. Binding of thyroxine (T(4)), a native ligand of TTR, stabilizes the tetramer, but the bioavailability of T(4) for TTR binding is limited due to the preferential binding of T(4) to globulin, the major T(4) carrier in plasma. Here, we show that Cr(3+) increased the T(4)-binding capacity of wild-type (WT) and amyloidogenic V30M-TTR. Moreover, we demonstrate that Cr(3+) and T(4) cooperatively suppressed in vitro fibril formation due to the stabilization of WT-TTR and V30M-TTR.

Dysprealbuminemic hyperthyroxinemia in a patient with hyperthyroid graves disease

Rare mutant forms of circulating albumin and prealbumin [transthyretin (TTR)] have increased binding affinity for thyroxine (T4). Patients with these variant plasma proteins, as a result of inherited mutations or as a paraneoplastic phenomenon, typically present with increased serum total T4 and, by some assay methodologies, an increased free T4 as well. Although these individuals are, in fact, euthyroid, nonspecific symptoms may lead to inappropriate treatment for hyperthyroidism. We present a 34-year-old woman in whom a mutant form of TTR with increased T4 binding affinity and coexisting Graves disease was present. Subsequent 131I therapy led to development of postablative hypothyroidism, which was obscured by her higher serum free T4 concentration. Circulating thyroid-binding globulin (TBG), albumin, and TTR concentrations were all within their respective reference limits. A T4-binding protein panel confirmed that TTR-bound T4 was significantly increased, whereas TBG- and albumin-bound T4 was normal, indicating that this patient had euthyroid dysprealbuminemic hyperthyroxinemia, which had been masked by the initial presentation of hyperthyroidism. These findings indicate that hypothyroidism can be masked by coexisting euthyroid dysprealbuminemic hyperthyroxinemia.

Amyloidogenic and anti-amyloidogenic properties of recombinant transthyretin variants

Most transthyretin (TTR) mutations lead to TTR amyloid depositions in patients with familial amyloidotic polyneuropathy and familial amyloidotic cardiomyopathy. However, though an amyloidogenic protein itself, TTR inhibits aggregation of Alzheimer's amyloid beta protein (A beta) in vitro and in vivo. The pathogenic relationship between two amyloidogenic processes remains unclear. To understand how TTR mutations influence the ability of TTR to inhibit A beta amyloidosis, forty-seven recombinant TTR variants were produced and analyzed. We showed that all recombinant proteins formed tetramers and were functional in thyroxine binding. Acid denaturation at pH 3.8 resulted in aggregation and fibril formation of all TTR variants. However, only TTR G42 and TTR P55 formed fibrils at pH 6.8. Most TTR variants bound to A beta and inhibited A beta aggregation in vitro. TTR variants S64, A71, Q89, V107, H114 and I122 revealed decreased binding to A beta and decreased inhibition of A beta aggregation. Only TTR G42 and TTR P55 completely failed to bind A beta and to inhibit A beta aggregation. We suggest that TTR variants characterized by decreased binding to A beta or by decreased inhibition of A beta aggregation in vitro may contribute to A beta amyloid formation in vivo. These TTR variants might be important targets for epidemiological studies in Alzheimer's disease.

Slower clearance of human SAA1.5 in mice: implications for allele specific variation of SAA concentration in human

Serum amyloid A (SAA), the serum precursor of the fibrillar component (AA proteins) in reactive amyloid deposits, is a multigene product. SAA1, the prominent acute phase isotype in serum and also the dominant fibril precursor, has several allelic variants. In Japan, each of the three major alleles (1.1, 1.3 and 1.5) appears with approximately equal frequency. Recent research suggested that allele 1.5 has a positive influence on the serum SAA concentration. To clarify this, in the present study, recombinant human SAA1.1, SAA1.3 and SAA1.5 were produced in an E. coli expression system and those species of reconstituted high density lipoprotein were injected into mice to examine plasma clearance. SAA1.5 disappeared from plasma more slowly than the other two isotypes. This may account for the positive influence of allele 1.5 on the serum SAA concentration.

Expression of a synthetic gene encoding human transthyretin in Escherichia coli

Transthyretin is an amyloidogenic protein that causes human amyloid polyneuropathy and senile systemic amyloidosis as a result of the deposition of normal and/or mutant transthyretin in the form of amyloid fibrils. A high-expression plasmid of human transthyretin was constructed in order to facilitate the study of amyloid fibril formation of this protein. The transthyretin gene was constructed by an assembly of eight chemically synthesized oligonucleotides and amplified by polymerase chain reaction, and the amplified gene was inserted into an Escherichia coli expression vector. The expression plasmid was transformed into M15 cells and the gene product was expressed as a polyhistidine-tagged fusion protein. Purified recombinant transthyretin was obtained by one-step nickel chelation affinity chromatography and the production level of the protein was 130mg per 1L of culture. Furthermore, the expressed protein showed the same characteristics in terms of tetramer formation at neutral pH and amyloid formation at acidic pH as did the authentic human transthyretin. This system will enable biophysical and structural studies of this protein to be advanced.

Biophysical analysis of normal transthyretin: implications for fibril formation in senile systemic amyloidosis

Transthyretin (TTR) is a plasma protein that transports thyroid hormone and retinol binding protein-vitamin A complex. Eighty-four variants of TTR have been identified and seventy-four are associated with familial amyloidotic polyneuropathy. Normal TTR is the major protein found in the fibrillar deposits in the heart at time of autopsy of individuals with senile systemic amyloidosis. The mechanism by which normally soluble TTR deposits as organ-damaging, insoluble, pathological fibrils late in life is unknown. Understanding the mechanism of fibrillogenesis of normal TTR is critical to the design of clinical treatments aimed at retardation, prevention, or reversal of fibril deposition. We have employed a biophysical approach to explore the hypothesis that an instability in a particular secondary or tertiary structure plays a role in the ability of normal TTR to form fibrils at physiological pH. Using far UV circular dichroic (CD) spectroscopy as a function of temperature we have identified simultaneous, cooperative, reversible structural changes in the beta-sheet and alpha-helical regions. The flexible short, surface-located loops undergo an irreversible conformational change at a lower temperature. Spectra before and after heating are different, particularly in the wavelength region associated with these loops, strongly suggesting that the major portion of TTR returns to its initial conformation while the loops do not. Near UV CD reveals partially reversible and irreversible changes in tertiary structure. Using calorimetry to directly measure the enthalpy associated with these changes, two peaks are observed, with further analysis suggesting conformational intermediates. Precipitates from heated samples reveal pre-fibrillar morphology by negative stain electron microscopy. These biophysical studies suggest that heat-induced conformational rearrangements enable normal TTR to assemble into pre-fibrils at physiological pH.

Domino liver transplants for metabolic disorders: experience with familial amyloidotic polyneuropathy

BACKGROUND

Shortage of liver donors means that new methods of liver procurement must be explored. In domino transplantation, organs explanted during transplantation in one patient are transplanted into a second patient. Domino procedures can be performed with livers from patients having transplantation for hepatic metabolic disorders that cause systemic disease without affecting other liver functions. Familial amyloidotic polyneuropathy (FAP) type I is one of these.

STUDY DESIGN

We reviewed the Paul Brousse experience with a domino liver transplant program for FAP, hoping to extend the approach to other metabolic disorders.

RESULTS

Livers from 10 patients transplanted for FAP type 1 were used for domino transplants to patients with unresectable primary or metastatic liver cancers. There was no perioperative mortality. Neuropathy or cardiomyopathy did not increase the morbidity of the domino liver explant and transplant procedures. Morbidity for the domino recipients did not appear to be increased. Variant transthyretin was detected in the serum in FAP liver recipients, with no immediate clinical consequences.

CONCLUSIONS

The domino approach is feasible and requires careful planning of the surgical procedures for liver explantation, particularly for the nature and site of vascular anastomoses. Domino transplantation of metabolically dysfunctional livers creates new categories of potential donors and potential recipients. It raises new ethical, technical, and societal issues. The domino approach could be used in several genetic or biochemical disorders now treated by liver transplantation. It has the potential to increase the number of liver grafts available for transplantation.

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.

Tertiary structures of amyloidogenic and non-amyloidogenic transthyretin variants: new model for amyloid fibril formation

The most common form of hereditary systemic amyloidosis is familial amyloidotic polyneuropathy associated with single amino acid changes in the plasma protein transthyretin. So far, high resolution structures of only three amyloidogenic variants (Met30, Ser84, Ile122) and one non-amyloidogenic variant (Thr109) have been reported complemented by X-ray fiber diffraction studies and image reconstruction from electron micrographs of amyloid fibrils. To investigate the role of structural factors in this disease, we extended our studies to other transthyretin variants. We report crystallization and structural investigations of three amyloidogenic (Arg10, Ala60, Tyr77) and two non-amyloidogenic variants (Ser6, Met119). The similarity of these structures to normal transthyretin does not give direct clues to the fibril forming process. Since transthyretin amyloid fibrils contain a major fragment starting at position 49, besides the intact molecule, we calculated the solvent accessibility of residue 48. Indeed, all amyloidogenic variants show an increased main chain solvent exposure when compared to normal transthyretin and non-amyloidogenic variants, which can be postulated to result in increased susceptibility to proteolysis. After limited proteolysis, dimers are incapable of reassociation to native tetramers. We present a model for amyloid fibril formation based on formation of fibrils from N-terminal truncated dimers as building blocks.

Thyroxine binding to transthyretin Met 119. Comparative studies of different heterozygotic carriers and structural analysis

The majority of the known transthyretin (TTR) variants are associated with amyloidosis, but there are also variants associated with euthyroid hyperthyroxinemia and others are apparently nonpathogenic. TTR Met 119 is a nonpathogenic variant found to be frequent in the Portuguese population. Previous studies on thyroxine (T4) binding to semi-purified TTR from heterozygotic carriers of TTR Met 119, reported by us and other groups, revealed different results. Therefore, to further characterize T4 binding to TTR Met 119 we performed T4-TTR binding studies in homotetrameric-recombinant TTR Met 119 variant and normal TTR. We also studied T4 binding to TTR purified from serum of different heterozygotic carriers of TTR Met 119 including compound heterozygotic individuals carriers of a TTR mutation in the other allele. We observed an increased T4 binding affinity to TTR Met 119 from heterozygotic individuals and compound heterozygotes and this effect of increasing T4 binding affinity was consistent and independent from the mutation present in the other allele. Recombinant homotetrameric TTR Met 119 and heterotetrameric protein from heterozygotic carriers of TTR Met 119 presented similar T4 binding affinity demonstrating the increased T4 binding affinity of TTR Met 119. X-ray crystallography studies performed on the recombinant TTR Met 119 variant revealed structural alterations mainly at the level of residue Leu 110 allowing a closer contact between the hormone and the mutant protein. These results are consistent with the observed T4 binding results.

Transport of serum transthyretin into chicken oocytes. A receptor-mediated mechanism

Transthyretin (TTR) is involved in the transport of thyroid hormones and, due to its interaction with serum retinol-binding protein, also of vitamin A. The importance of both ligands in vertebrate embryonic development has prompted us to investigate the molecular details of TTR transport function in a powerful germ cell system, the rapidly growing chicken oocytes. Yolk TTR is derived from the circulatory system, since biotinylated TTR was recovered by immunoaffinity chromatography of yolk obtained from a hen previously infused with in vitro biotinylated chicken serum proteins. In concordance with the intraoocytic localization in an endosomal compartment, ligand blotting and chemical cross-linking experiments revealed the presence of a approximately 115-kDa TTR-binding oocyte membrane protein. This putative TTR receptor was not detected in chicken ovarian granulosa cells or embryonic fibroblasts and was different from the previously described oocyte-specific receptor for two estrogen-induced chicken serum lipoproteins, vitellogenin and very low density lipoprotein (Barber, D. L., Sanders, E. J., Aebersold, R., and Schneider, W. J. (1991) J. Biol. Chem. 266, 18761-18770). Furthermore, in contrast to the serum levels of the yolk precursor lipoproteins, those of TTR were not significantly changed by estrogen; thus, TTR represents a newly defined, estrogen-independent class of yolk precursor proteins. These data strongly suggest that oocytic TTR is derived from the circulation, where it is a constitutive component, and deposited into yolk as a result of endocytosis mediated by a specific receptor.