Transthyretin mutations in health and disease

S-Homocysteinylation of transthyretin is detected in plasma and serum of humans with different types of hyperhomocysteinemia

While the association of homocystinuria with disease is known for more than four decades, mild hyperhomocysteinemia has been detected more recently as a risk factor for a number of diseases. However, the mechanism which apparently renders (even mild) hyperhomocystenemia harmful is not known. Following reports on N-homocysteinylation of proteins by the homocysteine derivative homocysteine thiolactone, it has been suggested that homocysteinylation of proteins may contribute to the induction of biological effects by homocysteine. This has prompted us to study by electrospray ionization mass spectrometry homocysteinylation of transthyretin (TTR) in plasma and serum of humans with different types of hyperhomocysteinemia. We did not detect any N-homocysteinylation, but found pronounced S-homocysteinylation of TTR, if the concentration of total homocysteine was high. Our findings support a possible role of S-homocysteinylation of proteins in the mediation of detrimental effects of hyperhomocysteinemia. Careful study of posttranslational modifications of individual proteins may contribute to a better understanding of diseases associated with hyperhomocysteinemia.

The transthyretin-related protein family

A number of proteins related to the homotetrameric transport protein transthyretin (TTR) forms a highly conserved protein family, which we present in an integrated analysis of data from different sources combined with an initial biochemical characterization. Homologues of the transthyretin-related protein (TRP) can be found in a wide range of species including bacteria, plants and animals, whereas transthyretins have so far only been identified in vertebrates. A multiple sequence alignment of 49 TRP sequences from 47 species to TTR suggests that the tertiary and quaternary features of the three-dimensional structure are most likely preserved. Interestingly, while some of the TRP orthologues show as little as 30% identity, the residues at the putative ligand-binding site are almost entirely conserved. RT/PCR analysis in Caenorhabditis elegans confirms that one TRP gene is transcribed, spliced and predominantly expressed in the worm, which suggests that at least one of the two C. elegans TRP genes encodes a functional protein. We used double-stranded RNA-mediated interference techniques in order to determine the loss-of-function phenotype for the two TRP genes in C. elegans but detected no apparent phenotype. The cloning and initial characterization of purified TRP from Escherichia coli reveals that, while still forming a homotetramer, this protein does not recognize thyroid hormones that are the natural ligands of TTR. The ligand for TRP is not known; however, genomic data support a functional role involving purine catabolism especially linked to urate oxidase (uricase) activity.

Sequence-dependent denaturation energetics: A major determinant in amyloid disease diversity

Several misfolding diseases commence when a secreted folded protein encounters a partially denaturing microenvironment, enabling its self assembly into amyloid. Although amyloidosis is modulated by numerous environmental and genetic factors, single point mutations within the amyloidogenic protein can dramatically influence disease phenotype. Mutations that destabilize the native state predispose an individual to disease; however, thermodynamic stability alone does not reliably predict disease severity. Here we show that the rate of transthyretin (TTR) tetramer dissociation required for amyloid formation is strongly influenced by mutation (V30M, L55P, T119M, V122I), with rapid rates exacerbating and slow rates reducing amyloidogenicity. Although these rates are difficult to predict a priori, they notably influence disease penetrance and age of onset. L55P TTR exhibits severe pathology because the tetramer both dissociates quickly and is highly destabilized. Even though V30M and L55P TTR are similarly destabilized, the V30M disease phenotype is milder because V30M dissociates more slowly, even slower than wild type (WT). Although WT and V122I TTR have nearly equivalent tetramer stabilities, V122I cardiomyopathy, unlike WT cardiomyopathy, has nearly complete penetrance-presumably because of its 2-fold increase in dissociation rate. We show that the T119M homotetramer exhibits kinetic stabilization and therefore dissociates exceedingly slowly, likely explaining how it functions to protect V30MT119M compound heterozygotes from disease. An understanding of how mutations influence both the kinetics and thermodynamics of misfolding allows us to rationalize the phenotypic diversity of amyloid diseases, especially when considered in concert with other genetic and environmental data.

Inhibition of transthyretin amyloid fibril formation by 2,4-dinitrophenol through tetramer stabilization

Transthyretin (TTR), a homotetrameric thyroxine transport protein found in the plasma and cerebrospinal fluid, circulates normally as a innocuous soluble protein. In some individuals, TTR polymerizes to form insoluble amyloid fibrils. TTR amyloid fibril formation and deposition have been associated with several diseases like familial amyloid polyneuropathy and senile systemic amyloidosis. Inhibition of the fibril formation is considered a potential strategy for the therapeutic intervention. The effect of small water-soluble, hydrophobic ligand 2,4-dinitrophenol (2,4-DNP) on TTR amyloid formation has been tested. 2,4-DNP binds to TTR both at acidic and physiological pH, as shown by the quenching of TTR intrinsic fluorescence. Interestingly, 2,4-DNP not only binds to TTR at acidic pH but also inhibits amyloid fibril formation as shown by the light scattering and Congo red-binding assay. Inhibition of fibril formation by 2,4-DNP appears to be through the stabilization of TTR tetramer upon binding to the protein, which includes active site. These findings may have implications for the development of mechanism based small molecular weight compounds as therapeutic agents for the prevention/inhibition of the amyloid diseases.

Enhanced in vitro production of amyloid-like fibrils from mutant (S20G) islet amyloid polypeptide

Islet amyloid polypeptide (IAPP, "amylin") is the amyloid-fibril-forming polypeptide in the islets of Langerhans associated with type 2 diabetes mellitus. A missense mutation in the IAPP gene associated with early-onset type 2 diabetes has been identified in the Japanese population. This mutation results in a glycine for serine substitution at position 20 of the mature IAPP molecule. Whether or not formation of islet amyloid with resulting destruction of islet tissue is the cause of this diabetes is yet not known. The present in vitro study was performed in order to investigate any influence of the amino acid substitution on the fibril formation capacity. Synthetic full-length wild type (IAPPwt) and mutant (IAPPS20G) as well as corresponding truncated peptides (position 18-29) were dissolved in dimethylsulfoxide (DMSO) or in 10% acetic acid at a concentration of 10 mg/mL and their fibril forming capacity was checked by Congo red staining, electron microscopy, a Congo red affinity assay and Thioflavine Tfluorometric assay. It was found that full-length and truncated IAPPS20G both formed more amyloid-like fibrils and did this faster compared to IAPPwt. The fibril morphology differed slightly between the preparations.

CONCLUSION

The amino acid substitution (S20G) is situated close to the region of the IAPP molecule implicated in the IAPP fibrillogenesis. The significantly increased formation of amyloid-like fibrils by IAPPS20G is highly interesting and may be associated with an increased islet amyloid formation in vivo and of fundamental importance in the pathogenesis of this specific form of diabetes.

Capture of a dimeric intermediate during transthyretin amyloid formation

Point mutations in the human plasma protein transthyretin are associated with the neurological disorder familial amyloidosis with polyneuropathy type 1. The disease is characterized by amyloid fibril deposits causing damage at the site of deposition. Substitution of two amino acids in the hydrophobic core of transthyretin lead to a mutant that was very prone to form amyloid. In addition, this mutant has also been shown to induce a toxic response on a neuroblastoma cell line. Renaturation of the transthyretin mutant at low temperature facilitated the isolation of an amyloid-forming intermediate state having the apparent size of a dimer. Increasing the temperature effectively enhanced the rate of interconversion from a partly denatured protein to mature amyloid. Using circular dichroism the beta-sheet content of the formed mature fibrils was significantly lower than that of the native fold of transthyretin. Morphology studies using electron microscopy also indicated a temperature-dependent transformation from amorphous aggregates toward mature amyloid fibrils. In addition, 1-anilino-8-naphtalenesulfonate fluorescence studies suggested the loss of the thyroxin-binding channel within both the isolated intermediate and the mature fibrils.

Structural distribution of mutations associated with familial amyloidotic polyneuropathy in human transthyretin

The human plasma protein transthyretin (TTR) is a highly stable soluble homotetrameric protein. Still, conformational changes in the wild type protein can lead to self-assembly into insoluble amyloid fibrils. In addition, 74 point mutations are known to enhance amyloid formation causing familial amyloidotic polyneuropathy (PAP). Alignment of TTR sequences from twenty different species shows that only six of these mutations occur as natural amino acids in other organisms. In this paper we analyse the distribution of FAP mutations within the three-dimensional structure of TTR. Contradictory to what might be expected from protein stability studies, the mutations are not restricted to structurally rigid parts of the molecule, nor are they concentrated at the monomer interaction sites.

Transthyretin slowly exchanges subunits under physiological conditions: A convenient chromatographic method to study subunit exchange in oligomeric proteins

Transthyretin (TTR) subunits were labeled with a charge-modifying tag to evaluate the possibility of subunit exchange between tetramers under physiological conditions. Starting with a mixture of two TTR homotetramers, one having all subunits tagged at the N termini and the other composed of untagged subunits, heterotetramer formation as a function of time and temperature was evaluated using ion exchange chromatography. The data indicate that the subunit exchange can occur under native conditions at physiological pH in vitro, albeit slowly. Wild-type TTR exchanges subunits on a timescale of days at 37 degrees C and on a timescale of hours at 4 degrees C. The familial amyloid polyneuropathy-associated variant V30M exchanges subunits at the same rate as wild-type TTR at 4 degrees C but slower and less efficiently at 37 degrees C. Small molecule tetramer stabilizers abolish TTR subunit exchange, supporting a dissociative mechanism.

Evaluating the binding selectivity of transthyretin amyloid fibril inhibitors in blood plasma

Transthyretin (TTR) tetramer dissociation and misfolding facilitate assembly into amyloid fibrils that putatively cause senile systemic amyloidosis and familial amyloid polyneuropathy. We have previously discovered more than 50 small molecules that bind to and stabilize tetrameric TTR, inhibiting amyloid fibril formation in vitro. A method is presented here to evaluate the binding selectivity of these inhibitors to TTR in human plasma, a complex biological fluid composed of more than 60 proteins and numerous small molecules. Our immunoprecipitation approach isolates TTR and bound small molecules from a biological fluid such as plasma, and quantifies the amount of small molecules bound to the protein by HPLC analysis. This approach demonstrates that only a small subset of the inhibitors that saturate the TTR binding sites in vitro do so in plasma. These selective inhibitors can now be tested in animal models of TTR amyloid disease to probe the validity of the amyloid hypothesis. This method could be easily extended to evaluate small molecule binding selectivity to any protein in a given biological fluid without the necessity of determining or guessing which other protein components may be competitors. This is a central issue to understanding the distribution, metabolism, activity, and toxicity of potential drugs.

A proposed structural model for amyloid fibril elongation: domain swapping forms an interdigitating beta-structure polymer

We propose a model illustrating how proteins, which differ in their overall sequences and structures, can form the propagating, twisted beta-sheet conformations, characteristic of amyloids. Some cases of amyloid formation can be explained through a "domain swapping" event, where the swapped segment is either a beta-hairpin or an unstable conformation which can partially unfold and assume a beta-hairpin structure. As in domain swapping, here the swapped beta-hairpin is at the edge of the structure, has few (if any) salt bridges and hydrogen bonds connecting it to the remainder of the structure and variable extents of buried non-polar surface areas. Additionally, in both cases the swapped piece constitutes a transient "building block" of the structure, with a high population time. Whereas in domain swapping the swapped fragment has been shown to be an alpha-helix, loop, strand or an entire domain, but so far not a beta-hairpin, despite the large number of cases in which it was already detected, here swapping may involve such a structural motif. We show how the swapping of beta-hairpins would form an interdigitated, twisted beta-sheet conformation, explaining the remarkable high stability of the protofibril in vitro. Such a swapping mechanism is attractive as it involves a universal mechanism in proteins, critical for their function, namely hinge-bending motions. Our proposal is consistent with structural superpositioning of mutational variants. While the overall r.m.s.d.s of the wild-type and mutants are small, the proposed hinge-bending region consistently shows larger deviations. These larger deviations illustrate that this region is more prone to respond to the mutational changes, regardless of their location in the sequence or in the structure. Nevertheless, above all, we stress that this proposition is hypothetical, since it is based on assumptions lacking definitive experimental support.

Evidence for the role of megalin in renal uptake of transthyretin

The kidney is a major organ for uptake of the thyroid hormone thyroxine (T(4)) and its conversion to the active form, triiodothyronine. In the plasma, one of the T(4) carriers is transthyretin (TTR). In the present study we observed that TTR, the transporter of both T(4) and retinol-binding protein, binds to megalin, the multiligand receptor expressed on the luminal surface of various epithelia including the renal proximal tubules. In the kidney, megalin plays an important role in tubular uptake of macromolecules filtered through the glomerulus. To evaluate the importance of megalin for renal uptake of TTR, we performed binding/uptake assays using immortalized rat yolk sac cells with high expression levels of megalin. Radiolabeled TTR, free as well as in complex with thyroxine or retinol-binding protein, was rapidly taken up by the cells, and the uptake was strongly inhibited by a polyclonal megalin antibody and by the receptor-associated protein, a chaperone-like protein inhibiting ligand binding to megalin. In cell culture, different TTR mutations presented different levels of cell association and degradation, suggesting that the structure of TTR is important for megalin recognition. Both the apo form and the T(4)-bound form were taken up by the cells. Analysis of urine from patients with Dent's disease, a renal tubular disorder that alters receptor-mediated endocytic reabsorption of proteins, identified TTR as an abundant excreted protein. Furthermore, analysis of kidney sections of megalin-deficient mice revealed no immunohistochemical TTR labeling in intracellular vesicles in the proximal tubule cells when compared with wild type control littermates. Taken together, the present data indicate that TTR represents a novel megalin ligand of importance in the thyroid hormone homeostasis.

Postmortem findings in two familial amyloidosis patients with transthyretin variant Asp38Ala

Postmortem findings in 2 familial amyloidosis patients with the transthyretin variant (ATTR), Asp38Ala, are described Both showed cardiac failure, and progressive peripheral and autonomic neuropathy and died at the ages 82 and 57, respectively. TTR immunoreactive amyloid deposition was observed to be extensive in the myocardium, peripheral nerves, sympathetic ganglia and gastrointestinal tract. The pulmonary parenchyma was also diffusely involved, but renal glomeruli, follicular tissues of the thyroid, and the leptomeninges and subarachnoidal vessels of the central nervous system showed little deposition. The latter findings are not usually seen in the patients with ATTR Val30Met, the most common form of familial amyloidosis. Additionally, the clinicopathological findings of familial amyloidosis with ATTR Asp38Ala seem to vary in the different individuals.

Deuterium-proton exchange on the native wild-type transthyretin tetramer identifies the stable core of the individual subunits and indicates mobility at the subunit interface

Transthyretin is a human protein capable of amyloid formation that is believed to cause several types of amyloid disease, depending on the sequence deposited. Previous studies have demonstrated that wild-type transthyretin (TTR), although quite stable, forms amyloid upon dissociation from its native tetrameric form into monomers with an altered conformation. Many naturally occurring single-site variants of TTR display decreased stability in vitro, manifested by the early onset familial amyloid diseases in vivo. Only subtle structural changes were observed in X-ray crystallographic structures of these disease associated variants. In this study, the stability of the wild-type TTR tetramer was investigated at the residue-resolution level by monitoring (2)H-H exchange via NMR spectroscopy. The measured protection factors for slowly-exchanging amide hydrogen atoms reveal a stable core consisting of strands A, B, E, F, and interestingly, the loop between strands A and B. In addition, the faster exchange of amide groups from residues at the subunit interfaces suggests unexpected mobility in these regions. This information is crucial for future comparisons between disease-associated and wild-type tetramers. Such studies can directly address the regions of TTR that become destabilized as a consequence of single amino acid substitutions, providing clues to aspects of TTR amyloidogenesis.

The beta-slip: a novel concept in transthyretin amyloidosis

Transthyretin is a tetrameric plasma protein associated with two forms of amyloid disease. The structure of the highly amyloidogenic transthyretin triple mutant TTRG53S/E54D/L55S determined at 2.3 A resolution reveals a novel conformation: the beta-slip. A three-residue shift in beta strand D places Leu-58 at the position normally occupied by Leu-55 now mutated to serine. The beta-slip is best defined in two of the four monomers, where it makes new protein-protein interactions to an area normally involved in complex formation with retinol-binding protein. This interaction creates unique packing arrangements, where two protein helices combine to form a double helix in agreement with fiber diffraction and electron microscopy data. Based on these findings, a novel model for transthyretin amyloid formation is presented.

The transthyretin-retinol-binding protein complex

Transthyretin (TTR, formerly called prealbumin), one of the transporters of the hormone thyroxine and the lipocalin retinol-binding protein (RBP), the specific carrier of the vitamin, are known to form, under physiological conditions, a macromolecular complex that is believed to play an important physiological role: prevention of glomerular filtration of the low molecular weight RBP in the kidneys. The physiological significance of complex formation is discussed first, followed by a brief description of the three-dimensional structure of the two participating proteins. The two X-ray models of the complex available are subsequently discussed and compared and finally the non-crystallographic evidence that supports these models is reviewed.

The molecular interaction of 4'-iodo-4'-deoxydoxorubicin with Leu-55Pro transthyretin 'amyloid-like' oligomer leading to disaggregation

The crystal structure of the amyloidogenic Leu-55Pro transthyretin (TTR) variant has revealed an oligomer structure that may represent a putative amyloid protofibril [Sebastião, Saraiva and Damas (1998) J. Biol. Chem. 273, 24715-24722]. Here we report biochemical evidence that corroborates the isolation of an intermediate structure, an 'amyloid-like' oligomer, which is most probably present in the biochemical pathway that leads to amyloid deposition and that was isolated by the crystallization of the Leu-55Pro TTR variant. 4'-Iodo-4'-deoxydoxorubicin (IDOX) is a compound that interacts with amyloid fibrils of various compositions and it has been reported to reduce the amyloid load in immunoglobulin light chain amyloidosis [Merlini, Ascari, Amboldi, Bellotti, Arbustini, Perfetti, Ferrari, Zorzoli, Marinone, Garini et al. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 2959-2963]. In this work, we observed that the monoclinic Leu-55Pro TTR crystals, soaked with IDOX, undergo rapid dissociation. Moreover, under the same conditions, the orthorhombic wild-type TTR crystals are quite stable. This is explained by the different TTR conformations isolated upon crystallization of the two proteins; while the Leu-55Pro TTR exhibits the necessary conformation for IDOX binding, the same structure is not present in the crystallized wild-type protein. A theoretical model concerning the interaction of Leu-55Pro TTR with IDOX, which is consistent with the dissociation of the amyloid-like oligomer, is presented. In this model the IDOX iodine atom is buried in a pocket located between the two beta-sheets of the Leu-55Pro TTR monomer with the IDOX aromatic-moiety long axis nearly perpendicular to the direction of the beta-sheets.

Transthyretin related familial amyloid polyneuropathy

Familial amyloid polyneuropathy (FAP) applies to a group of dominantly inherited severe diseases with endoneurial and polyvisceral deposition of amyloidosis. The transthyretin, essentially produced by the liver, is the main protein involved in FAP. Up to 80 different mutations of the transthyretin gene are identified, many of them being associated with small fibres sensory-motor and autonomic polyneuropathy and/or cardiomyopathy. Variable age of onset, clinical expression and penetrance are largely reported. However, phenotypic-genotypic correlations remain unclear and the genetic or environmental modifying factors are unknown. The liver transplantation is proposed as a curative treatment of FAP resulting in an improvement of the general condition and a stabilization of the neuropathy, in a majority of patients. At present, the ratio benefit/risk seems acceptable when the procedure is performed early in the course of the disease.

Interaction of the receptor for advanced glycation end products (RAGE) with transthyretin triggers nuclear transcription factor kB (NF-kB) activation

Mutated transthyretin (TTR) fibrils are associated with the pathology of familial amyloidotic polyneuropathy (FAP), in which extracellular amyloid deposits lead to degeneration of cells and tissues, in particular neurons of the peripheral nerve. Here we present evidence that the receptor for advanced glycation end products (RAGE), previously associated with Alzheimer's disease, acts as a selective cell surface acceptor site for both soluble and fibrillar TTR. Immunohistochemical studies demonstrating increased expression of RAGE in FAP tissues suggested the relevance of this receptor to TTR-induced fibrillar pathology. In vitro studies using soluble RAGE showed saturable specific interaction with soluble and fibrillar TTR with a K(d) of approximately 120 nM. However, no binding was observed when soluble TTR was combined with retinol-binding protein, which represents the form in which TTR normally circulates in plasma. Specific binding of TTR to RAGE-transfected Chinese hamster ovary cells (which was completely blocked by anti-RAGE) was observed, confirming that RAGE could mediate TTR binding to cellular surfaces. RAGE-dependent activation of nuclear transcription factor kB (NF-kB) by TTR fibrils was shown in PC-12 cells stably transfected to overexpress the receptor. Furthermore, FAP nerves showed up-regulation of p50, one of the NF-kB subunits, when compared with age-matched controls. From these observations we predict that, in vivo, the presence of TTR fibrils associated with cellular surfaces of FAP patients, by contributing to NF-kB activation, leads to the pathogenesis of neurodegeneration. Further insights into the consequences of the interaction of fibrillar TTR with RAGE may therefore provide a better understanding of neurodegeneration associated with FAP.

New transthyretin variants SER 91 and SER 116 associated with familial amyloidotic polyneuropathy. Mutations in brief no. 151. Online

Mutations of the transthyretin (TTR) gene are associated with familial amyloidotic polyneuropathy (FAP). Two new mutations were detected in French patients with TTR amyloidosis. The first patient was a 72 year old man who presented with severe and rapidly evolving sensory motor polyneuropathy of the 4 limbs, a bilateral carpal tunnel syndrome and a restrictive cardiomyopathy. His father died after a clinical history suggestive in retrospect of TTR amyloidosis. The second patient was a 75 year old man who presented with axonal sensory neuropathy of the 4 limbs and a bilateral carpal tunnel syndrome. In both cases immunohistochemistry performed on a nerve biopsy reveled TTR positive amyloid. Direct genomic sequencing of the full TTR gene coding region indicated two heterozygous transversions encoding Ser for Ala 91 substitution in the third exon of the gene in patient 1 and Ser for Tyr 116 substitution in the fourth exon of the gene in patient 2. The mutations were confirmed by digesting PCR products with restriction enzymes and were not found in a control population of 100 unrelated individuals. The Ser 116 substitution was also detected in the daughter and the 70 year old sister of the proband. However the absence of symptomatology suggestive of TTR amyloidosis may be related to the late onset of the disease. The clinical immunohistochemical and molecular studies in both patients are highly suggestive of an association between the Ser 91 and Ser 116 TTR variants with amyloidosis.

Screening and biochemical characterization of transthyretin variants in the Portuguese population

The study of pathogenic and nonpathogenic transthyretin (TTR) variants is very important for the understanding of such TTR-related diseases as hereditary amyloidosis and also to establish a relationship between the structure and function of the molecule. Variants with clinical manifestations can be easily detected, but clinically silent variants can be detected only by population screening programs using specialized techniques. Hybrid isoelectric focusing (HIEF) in extremely flattened immobilized pH gradients (IPG) allows the detection of even neutral amino acid substitutions and has been used to analyze approximately 5,000 samples from the Portuguese population. Comparison with samples from carriers of three known TTR mutations (Met 30 associated with hereditary amyloidosis, Met 119, and Asn 90) was also made. In this study we detected: (1) 8 individuals carriers of TTR Met 30, (2) 35 carriers of TTR Met 119, (3) 12 carriers of TTR Asn 90, (4) 1 compound heterozygote for TTR Met 30/Met 119, and (5) 5 variants that presented a different pattern from the controls used. We also performed DNA sequencing analyses of two of the variants with the different band pattern in HIEF. The individuals were found to be carriers of TTR Ile 122 and TTR Thr 190, respectively. All the mutations detected, except for Asn 90, result from substitutions in CpG hot spots and thus can be rather frequent in the populations. Studies on the clinical evolution of the compound heterozygotes and on the physical-chemical properties of these hybrid TTRs will help to understand the pathogenicity associated with TTR.

The preaggregated state of an amyloidogenic protein: hydrostatic pressure converts native transthyretin into the amyloidogenic state

Protein misfolding and aggregation cause several diseases, by mechanisms that are poorly understood. The formation of amyloid aggregates is the hallmark of most of these diseases. Here, the properties and formation of amyloidogenic intermediates of transthyretin (TTR) were investigated by the use of hydrostatic pressure and spectroscopic techniques. Native TTR tetramers (T(4)) were denatured by high pressure into a conformation that exposes tryptophan residues to the aqueous environment. This conformation was able to bind the hydrophobic probe bis-(8-anilinonaphthalene-1-sulfonate), indicating persistence of elements of secondary and tertiary structure. Lowering the temperature facilitated the pressure-induced denaturation of TTR, which suggests an important role of entropy in stabilizing the native protein. Gel filtration chromatography showed that after a cycle of compression-decompression at 1 degrees C, the main species present was a tetramer, with a small population of monomers. This tetramer, designated T(4)*, had a non-native conformation: it bound more bis-(8-anilinonaphthalene-1-sulfonate) than native T(4), was less stable under pressure, and on decompression formed aggregates under mild acidic conditions (pH 5-5.6). Our data show that hydrostatic pressure converts native tetramers of TTR into an altered state that shares properties with a previously described amyloidogenic intermediate, and it may be an intermediate that lies on the aggregation pathway. This "preaggregated" state, which we call T(4)*, provides insight into the question of how a correctly folded protein may degenerate into the aggregation pathway in amyloidogenic diseases.

Review: amyloidogenesis-unquestioned answers and unanswered questions

Current assumptions and conclusions in several active areas of amyloid research are examined to see how consistent the data from chosen in vitro and in vivo model systems are with clinical and anatomic observations. These areas include the assembly of amyloid-like fibrils in vitro, the nucleation phenomenon, amyloid fibril structure in vivo and in vitro, common structural components of the amyloids, and the regression of tissue amyloid and proteolysis of amyloid proteins. Divergences and congruencies are highlighted, which in turn suggests caution in the interpretation of present data, greater collaboration and communication among investigators, and, additional areas and techniques for investigation.

Apolipoprotein AI and transthyretin as components of amyloid fibrils in a kindred with apoAI Leu178His amyloidosis

We found a new C-terminal amyloidogenic variant of apolipoprotein AI (apoAI), Leu178His in a French kindred, associated with cardiac and larynx amyloidosis and skin lesions with onset during the fourth decade. This single-point mutation in exon 4 of the apoAI gene was detected by DNA sequencing of polymerase chain reaction amplified material and restriction fragment length polymorphism analysis in two siblings. Blood, larynx, and skin biopsies were available from one sibling. Anti-apoAI immunoblotting of isoelectric focusing of plasma showed a +1 alteration in the charge of the protein. Extraction of fibrils from the skin biopsy revealed both full-length and N-terminal fragments of apoAI and transthyretin (TTR). ApoAI and TTR co-localized in amyloid deposits as demonstrated by immunohistochemistry. The present report, together with the first recently described C-terminal amyloidogenic variant of apoAI, Arg173Pro, shows that amyloidogenicity of apoAI is not a feature exclusive to N-terminal variants. The most striking characteristic of amyloid fibrils in Leu178His is that wild-type TTR is co-localized with apoAI in the fibrils. We have previously determined that a fraction of plasma TTR circulates in plasma bound to high-density lipoprotein and that this interaction occurs through binding to apoAI. Therefore we hypothesize that nonmutated TTR might influence deposition of apoAI as amyloid.

Comparative studies of two transthyretin variants with protective effects on familial amyloidotic polyneuropathy: TTR R104H and TTR T119M

Recently, a new nonpathogenic transthyretin (TTR) variant-TTR R104H (TTR H104)-has been described in heterozygotic and compound heterozygotic individuals from a Japanese family with familial amyloidotic polyneuropathy (FAP). The compound heterozygotic individual, a carrier of TTR V30M (TTR M30) and TTR R104H (TTR M30/H104) presented a very mild form of FAP with slow progression of the disease. TTR and retinol binding protein (RBP) levels were found to be increased in serum from TTR H104 carriers. These characteristics are very similar to those found in compound heterozygotic carriers of TTR V30M-T119M (TTR M30/M119). To structurally compare these variants, we performed stability and thyroxine (T(4)) binding studies. TTR M30/H104 showed an increased resistance to dissociation into monomers similar to TTR M30/M119. This suggests that the His104 substitution has the same stabilizing effect on tetrameric TTR as the Met119 substitution. Concerning T(4) binding, TTR H104 presents a T(4) binding affinity lower than that of TTR M119, but still higher than normal TTR. However, TTR from the compound heterozygotic carrier of TTR M30/H104 presented a T(4) binding affinity lower than normal. The results indicate that the His 104 substitution induces structural alterations that increase the stability of the tetramer in compound heterozygotes for TTR M30 despite a lower affinity for T(4) binding. Thus, stability of TTR and binding affinity for T(4) may not be related. More detailed characterization of these variants is needed to clarify the structural alterations responsible for their increased stability.

Chemical dissection and reassembly of amyloid fibrils formed by a peptide fragment of transthyretin

We have examined the chemical dissection and subsequent reassembly of fibrils formed by a ten-residue peptide to probe the forces that drive the formation of amyloid. The peptide, TTR(10-19), encompasses the A strand of the inner beta-sheet structure that lines the thyroid hormone binding site of the human plasma protein transthyretin. When dissolved in water under low pH conditions the peptide readily forms amyloid fibrils. Electron microscopy of these fibrils indicates the presence of long (>1000 nm) rigid structures of uniform diameter (approximately 14 nm). Addition of urea (3 M) to preformed fibrils disrupts these rigid structures. The partially disrupted fibrils form flexible ribbon-like arrays, which are composed of a number of clearly visible protofilaments (3-4 nm diameter). These protofilaments are highly stable, and resist denaturation in 6 M urea at 75 degrees C over a period of hours. High concentrations (>50%, v/v) of 2,2,2-trifluoroethanol also dissociate TTR(10-19) fibrils to the constituent protofilaments, but these slowly dissociate to monomeric, soluble peptides with extensive alpha-helical structure. Dilution of the denaturant or co-solvent at the stage when dissociation to protofilaments has occurred results in the efficient reassembly of fibrils. These results indicate that assembly of fibrils from protofilaments involves relatively weak and predominantly hydrophobic interactions, whereas assembly of peptides into protofilaments involves both electrostatic and hydrophobic forces, resulting in a highly stable and compact structures.

1H-NMR structural studies of a cystine-linked peptide containing residues 71-93 of transthyretin and effects of a Ser84 substitution implicated in familial amyloidotic polyneuropathy

The Ile-->Ser84 substitution in the thyroid hormone transport protein transthyretin is one of over 50 variations found to be associated with familial amyloid polyneuropathy, a hereditary type of lethal amyloidosis. Using a peptide analogue of the loop containing residue 84 in transthyretin, we have examined the putative local structural effects of this substitution using 1H-NMR spectroscopy. The peptide, containing residues 71-93 of transthyretin with its termini linked via a disulfide bond, was found to possess the same helix-turn motif as in the corresponding region of the crystallographically derived structure of transthyretin in 20% trifluoroethanol (TFE) solution. It therefore, represents a useful model with which to examine the effects of amyloidogenic substitutions. In a peptide analogue containing the Ile84-->Ser substitution it was found that the substitution does not greatly disrupt the overall three-dimensional structure, but leads to minor local differences at the turn in which residue 84 is involved. Coupling constant and NOE measurements indicate that the helix-turn motif is still present, but differences in chemical shifts and amide-exchange rates reflect a small distortion. This is in keeping with observations that several other mutant forms of transthyretin display similar subunit interactions and those that have been structurally analysed possess a near native structure. We propose that the Ser84 mutation induces only subtle perturbations to the transthyretin structure which predisposes the protein to amyloid formation.

Exposure of cryptic epitopes on transthyretin only in amyloid and in amyloidogenic mutants

The structural requirements for generation of amyloid from the plasma protein transthyretin (TTR) are not known, although it is assumed that TTR is partly misfolded in amyloid. In a search for structural determinants important for amyloid formation, we generated a TTR mutant with high potential to form amyloid. We demonstrated that the mutant represents an intermediate in a series of conformational changes leading to amyloid. Two monoclonal antibodies were generated against this mutant; each displayed affinity to ex vivo TTR and TTR mutants with amyloidogenic folding but not to wild-type TTR or mutants exhibiting the wild-type fold. Two cryptic epitopes were mapped to a domain of TTR, where most mutations associated with amyloidosis occur and which we propose is displaced at the initial phase of amyloid formation, opening up new surfaces necessary for autoaggregation of TTR monomers. The results provide direct biochemical evidence for structural changes in an amyloidogenic intermediate of TTR.

Ultrastructure of familial amyloid polyneuropathy amyloid fibrils: examination with high-resolution electron microscopy

The ultrastructure of familial amyloid polyneuropathy (FAP) amyloid fibrils was examined with high-resolution electron microscopy and immunolabeling. Sural nerve biopsies from FAP (Met 30) patients as well as control tissues were prepared for thin-section observations. Extracellular spaces in the vicinity of myelinated and unmyelinated peripheral nerves were found to be filled with amyloid fibrils as well as with deposits of an "amorphous" material. The fibril was composed of a surface layer and a core. The surface layer was made up of heparan sulfate proteoglycan and was externally associated with a loose assembly of 0.5- to 1-nm-wide filaments. The core was a microfibril-like structure in which amyloid P component was enclosed in a tight helical structure by chondroitin sulfate proteoglycan. Immunogold labeling showed that the peripheral fine filaments were composed of transthyretin. The dimensions of the transthyretin filament suggest that its basic unit is a modified monomer. The deposited amorphous material was a mixture of individual components of the fibril. These results suggest that the main body of FAP amyloid fibrils is similar to that of recently observed fibrils of experimental murine AA and hemodialysis-associated amyloid as well as of connective tissue microfibrils. The differences in the fibrils of these various types of amyloid are in the peripheral filaments which are composed of a protein specific to each type of amyloid.

A novel variant of transthyretin (Glu42Asp) associated with sporadic late-onset cardiac amyloidosis

A sixty-three year old French man presented with isolated late-onset amyloid cardiomyopathy proven by endomyocardial biopsy. There was no known family history of amyloidosis. Immunohistochemistry of cardiac deposits suggested that amyloi fibrils were derived from transthyretin. DNA sequencing revealed a point mutation in exon 2 of the transthyretin gene responsible for a novel amyloidogenic variant Asp42.

Transthyretin Tyr77 familial amyloid polyneuropathy: a clinicopathological study of a large kindred

More than 40 point mutations (producing different clinical manifestations) have been described in diverse points of the plasma protein transthyretin (TTR). The Met30 is considered the most common mutation, the Tyr77 mutation being the second most prevalent. However, data from patients with this late mutation are scarce, and usually come from isolated case reports or tables. The Tyr77 mutation is not as well characterized as the Met30 mutation, especially with respect to such aspects as prognosis or possible treatment by liver transplantation. We therefore present the clinical and pathological features of an extensive family with the Tyr77 TTR mutation, comprising 12 affected individuals over four generations. Six living individuals were followed over a 10-year period. Retrospective data were obtained with regard to the deceased family members. We found that an initial and sometimes prolonged carpal tunnel syndrome, beginning between the 6th and 7th decades, characterizes the Tyr77 mutation. In most cases this evolved to generalized peripheral nerve involvement, restrictive cardiomyopathy, and intestinal malabsortion. Although survival is usually high, there are progressive cases that should be candidates for liver transplant, before severe impairment has developed.

The crystal structure of amyloidogenic Leu55 --> Pro transthyretin variant reveals a possible pathway for transthyretin polymerization into amyloid fibrils

The x-ray crystal structure of the amyloidogenic Leu55 --> Pro transthyretin (TTR) variant, implicated as the causative agent in early-onset familial amyloidotic polyneuropathy (Jacobson, D. R., McFarlin, D. E., Kane, I., and Buxbaum, J. N. (1992) Hum. Genet. 89, 353-356), has been solved by molecular replacement, refined at 2.7 A to a Rcryst value of 0.190 (Fobs > 2.0sigma), and compared with wild-type transthyretin to understand the molecular mechanism(s) involved in amyloidogenesis. Leu55 --> Pro TTR crystallizes in space group C2, with eight monomers in the asymmetric unit, and the observed packing contacts are considerably different from those described for the wild-type protein. Refinement of the crystal structure shows that the proline for leucine substitution disrupts the hydrogen bonds between strands D and A, resulting in different interface contacts. Based on the assumption that the observed packing contacts may be significant for amyloidogenesis, a model for the TTR amyloid is proposed. It consists of a tubular structure with inner and outer diameters approximately of 30 and 100 A and four monomers per cross-section.

In vitro and in vivo studies on transthyretin oligomerization

The oligomerization of transthyretin has been studied in vitro and in vivo. The results showed that wild-type transthyretin synthesized in vitro in the absence of microsomes did not form dimers and tetramers, but in the presence of microsomes the mature transthyretin which had been translocated into the microsomal lumen formed dimers and a small amount of tetramers which could be detected only by using a cross-linking reagent. Efficiency of tetramer formation depends upon the source of microsomes; in fact the amount of tetramer formed in liver microsomes was much higher than that in pancreas microsomes. Transthyretin synthesized in HepG2 cells appeared after SDS-PAGE analysis in mostly tetrameric form, while that synthesized in transfected COS-1 cells appeared mainly as dimers. Brefeldin A treatment and pulse-chase experiments in HepG2 cells showed that transthyretin tetramer was formed in the endoplasmic reticulum. These results strongly indicate that transthyretin tetramer is formed most efficiently in the endoplasmic reticulum lumen of hepatocytes. Transthyretin without the signal peptide [S(-)] and transthyretin with a mutation that prevents processing of the signal peptide Msc failed to form dimers and tetramers in vitro. In the transfected COS-1 cells, however, S(-) transthyretin did form dimers while Msc transthyretin failed to oligomerize. These results show that the cleavage of the signal peptide and some cellular factors are required for transthyretin oligomerization.

Protein subunit interactions and structural integrity of amyloidogenic transthyretins: evidence from electrospray mass spectrometry

Wild-type and variant transthyretins form amyloid fibrils in two different diseases. The biologically active form of transthyretin is a tetramer but there is evidence that a monomeric species is the amyloidogenic intermediate. Using mass spectrometry we have developed an approach to monitor the proportions of monomer and tetramer in wild-type and variant transthyretins, and found a strong correlation between the instability of the tetramer in the gas phase and the amyloidogenicity of the protein variant. The presence of water molecules in the central channel has been found to be critical for maintaining intact the complex in the gas phase, with additional stability observed in the presence of excess thyroxine. The solution structure of monomeric transthyretin under fibril-forming conditions was studied using hydrogen exchange monitored by mass spectrometry. The results show that Val30Met transthyretin, the commonest amyloidogenic variant, exhibits loss of hydrogen exchange protection substantially more rapidly than the wild-type protein, suggesting partial unfolding of the beta-sheet structure. These results provide new insights into the correlation between tetramer stability and amyloidogenicity as well as supporting a possible route to fibril formation via transient unfolding of the transthyretin monomer.

Study of an anti-human transthyretin immunoadsorbent. Influence of coupling chemistry on binding capacity and ligand leakage

A variant of transthyretin (TTR Val30Met) has been identified as the main protein precursor of the amyloid fibrils deposited in familial amyloidotic polyneuropathy (FAP). Specific removal of TTR in an extracorporeal immunoadsorption procedure is currently under investigation as a possible treatment of FAP. Immunoadsorbents were constructed by immobilizing murine anti-TTR monoclonal antibody 88.6.BA9 onto agarose gel supports via several different coupling chemistries. The influence of coupling conditions such as pH and antibody density, and of perfusion variables, such as antigen concentration and applied flow-rate, on the TTR capture efficiency, was determined. Cyanogen bromide-, carbonyldiimidazole- and aldehyde-activated (ALD) supports conjugated with antibody at optimal pH, provided immunoadsorbents with comparable TTR binding capacities. Regarding stability, leakage was lowest for the ALD based immunoadsorbents, particularly at high pH.

Comparative stability and clearance of [Met30]transthyretin and [Met119]transthyretin

[Met119]Transthyretin has been described as a non-amyloidogenic transthyretin variant. In Portugal, it has also been found in compound heterozygotic individual carriers of [Met30]transthyretin, the most prevalent variant associated with familial amyloidotic polyneuropathy. In these individuals, the evolution of the disease seems to be more benign than in typical [Met30]transthyretin carriers, suggesting a protective effect of [Met119]transthyretin on the pathogenic effects of [Met30]transthyretin. To study the mechanisms of this protective effect, we performed comparative in vivo clearance studies. Heterotetrameric [Met119]transthyretin showed a slower clearance, whereas homotetrameric [Met30]transthyretin presented a faster clearance. These data correlate with the relative TTR levels present in carriers of these mutations. Comparative analyses of the resistance to dissociation into monomers of serum transthyretin by 4M urea isoelectric focusing suggested a higher tetrameric stability of transthyretin in [Met119]transthyretin carriers, in contrast to a lower stability in [Met30]transthyretin carriers. The compound heterozygotes presented a pattern similar to the normal individuals. Our results suggest that the protective clinical effect of the Met119 mutation possibly involves the stabilisation of the tetrameric structure of transthyretin. Whether this behaviour correlates with the different metabolism found for the two variants is not known. The approaches reported here open some possibilities for the study and development of future therapeutic agents of familial amyloidotic polyneuropathy.

Amyloid deposits in the gastrointestinal tract of aging dogs

Pathologic features, immunohistochemical characteristics, and incidence of gastrointestinal amyloid deposition were investigated in 78 canine necropsies, including 43 males and 35 females ranging from 2 months to 20 years of age. In 31 of 78 dogs, amyloid protein was accumulated in or around vessel walls located in submucosal and muscular layers of the alimentary tract and mesenteric tissues. These deposits did not seem to affect the function(s) of the digestive tract histologically. The deposits showed congophilia and green birefringence under polarized microscopy. Transmission electron microscopy revealed randomly arranged amyloid fibrils 6-10 nm in diameter around a blood vessel. Congophilia was retained even after oxidation with potassium permanganate, suggesting that this protein is not the AA form of amyloid. Although immunohistochemical studies were performed using antibodies to canine amyloid apolipoprotein AI and synthetic human beta amyloid peptide, the immunochemical nature of the protein was not determined. The mean ages of both male and female dogs with gastrointestinal amyloid deposition were higher than those of their normal counterparts (P < 0.001), and the incidence of deposition in elderly dogs was higher than that in young dogs (P < 0.005). However, there was no sex predilection with respect to both the mean age of the dogs with deposition and the incidence of occurrence. The mechanism(s) and importance of this form of amyloid deposition remain unclear.

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.

Amyloid fibril formation and protein misassembly: a structural quest for insights into amyloid and prion diseases

The assembly and misassembly of normally soluble proteins into fibrilar structures is thought to be a causative agent in a variety of human amyloid and prion diseases. Structural and mechanistic studies of this process are beginning to elucidate the conformational changes required for the conversion of a normally soluble and functional protein into a defined quaternary structure.

Inhibiting transthyretin amyloid fibril formation via protein stabilization

Transthyretin (TTR) amyloid fibril formation is observed systemically in familial amyloid polyneuropathy and senile systemic amyloidosis and appears to be the causative agent in these diseases. Herein, we demonstrate conclusively that thyroxine (10.8 microM) inhibits TTR fibril formation efficiently in vitro and does so by stabilizing the tetramer against dissociation and the subsequent conformational changes required for amyloid fibril formation. In addition, the nonnative ligand 2,4,6-triiodophenol, which binds to TTR with slightly increased affinity also inhibits TTR fibril formation by this mechanism. Sedimentation velocity experiments were employed to show that TTR undergoes dissociation (linked to a conformational change) to form the monomeric amyloidogenic intermediate, which self-assembles into amyloid in the absence, but not in the presence of thyroxine. These results demonstrate the feasibility of using small molecules to stabilize the native fold of a potentially amyloidogenic human protein, thus preventing the conformational changes, which appear to be the common link in several human amyloid diseases. This strategy and the compounds resulting from further development should prove useful for critically evaluating the amyloid hypothesis--i.e., the putative cause-and-effect relationship between TTR amyloid deposition and the onset of familial amyloid polyneuropathy and senile systemic amyloidosis.

A new isoleucine substitution of Val-20 in transthyretin tetramers selectively impairs dimer-dimer contacts and causes systemic amyloidosis

The most frequent form of inherited amyloidoses is associated with mutations in the transthyretin (TTR) gene coding for 127-amino acid residues of four identical, noncovalently linked subunits that form a pair of dimers in the plasma protein complex. Amyloid fibrils containing the variant and to a lesser extent the wild-type form of the TTR molecule are deposited in various organs, including peripheral nerves and the myocardium, with polyneuropathy and cardiomyopathy as major clinical manifestations. So far, more than 40 distinct amino acid substitutions distributed throughout the TTR sequence over 30 positions have been found to be correlated with an increased amyloidogenicity of TTR. Most of these amyloidogenic amino acid substitutions are suspected to alter the conformation and stability of the monomer. Here we identify and characterize by protein and DNA analysis a novel amyloidogenic Val-20 to Ile mutation in a German three-generation family. The index patient suffered from severe amyloid cardiomyopathy at the age of 60. Conformational stability and unfolding behavior of the Ile-20 monomer in urea gradients was found to be almost indistinguishable from that of wild-type TTR. In contrast, tetramer stability was significantly reduced in agreement with the expected change in the interactions between the two opposing dimers via the side chain of Ile-20. Our observations provide strong evidence for the view that amyloidogenic amino acid substitutions in TTR facilitate the conversion of tetrameric TTR complexes into those conformational intermediates of the TTR folding pathway that have an intrinsic amyloidogenic potential.

The crystal structure of transthyretin from chicken

The crystal structure of chicken transthyretin has been solved at 290-pm resolution by molecular-replacement techniques. Transthyretin is the protein component of the amyloid fibrils found in patients suffering from either familial amyloidotic polyneuropathy or senile systemic amyloidosis. Familial amyloidotic polyneuropathy is an autosomal dominant hereditary type of amyloidosis which involves transthyretin with either one or two amino acid substitutions. The three-dimensional structure of chicken transthyretin was determined in order to compare a non-amyloidogenic, species-variant transthyretin with wild-type and mutant transthyretin molecules. Of the 31 chicken-to-human residue differences, 9 occur at positions which in human transthyretin give rise to amyloidogenic variants although none corresponds to the appropriate side-chain substitutions. The model of chicken transthyretin has been refined to an R-factor of 19.9%. The overall fold of the protein is that of an all-beta protein. Compared with wild-type human transthyretin the avian transthyretin shows quite large differences in the region known to be involved in binding to retinol-binding protein, it has a much shorter helical component than the human protein and some of the monomer-monomer interactions are different.

Transthyretin gene in Alzheimer's disease patients

Amyloid beta (Abeta) is known to be the main component of Alzheimer's disease (AD) senile plaques. A homologous peptide is a normal component of biological fluids and is called soluble Abeta (sAbeta). Synthetic peptides homologous to Abeta form amyloid-like fibrils in vitro. This fibril formation can be inhibited by normal human cerebrospinal fluid (CSF) [Wisniewski et al., Ann. Neurol. 34 (1993)]. Furthermore, it has been proposed that normal transthyretin (TTR), which is a component of CSF, can itself bind sAbeta, preventing amyloid fibril formation, and that variants of TTR could be associated with AD [Schwarzman et al., Proc. Natl. Acad. Sci. USA, 91 (1994)]. Because of this possible association, we screened for TTR mutations from 47 sporadic and 19 early-onset familial AD patients using single strand conformation polymorphism analysis. Our results show no correlation between TTR variants and Alzheimer's disease in this group of patients.

Alternative conformations of amyloidogenic proteins govern their behavior

Recent publications strongly support the hypothesis that conformational changes in amyloidogenic proteins lead to amyloid fibril formation and cause disease. Biophysical studies on several amyloidogenic proteins provide insights into the conformational changes required for fibrilogenesis. In addition, newly available moderate to high resolution structural studies are bringing us closer to understanding the structure of amyloid.