Intermolecular disulfide linkages are not required for transthyretin amyloid fibril formation in vitro

Drivers of α-Sheet Formation in Transthyretin under Amyloidogenic Conditions

Amyloid diseases make up a set of fatal disorders in which proteins aggregate to form fibrils that deposit in tissues throughout the body. Amyloid-associated diseases are challenging to study because amyloid formation occurs on time scales that span several orders of magnitude and involve heterogeneous, interconverting protein conformations. The development of more effective technologies to diagnose and treat amyloid disease requires both a map of the conformations sampled during amyloidogenesis and an understanding of the molecular mechanisms that drive this process. In prior molecular dynamics simulations of amyloid proteins, we observed the formation of a nonstandard type of secondary structure, called α-sheet, that we proposed is associated with the pathogenic conformers in amyloid disease, the soluble oligomers. However, the detailed molecular interactions that drive the conversion to α-sheet remain elusive. Here we use molecular dynamics simulations to interrogate a critical event in transthyretin aggregation, the formation of aggregation-competent, monomeric species. We show that conformational changes in one of the two β-sheets in transthyretin enable solvent molecules and polar side chains to form electrostatic interactions with main-chain peptide groups to facilitate and modulate conversion to α-sheet secondary structure. Our results shed light on the early conformational changes that drive transthyretin toward the α-sheet structure associated with toxicity. Delineation of the molecular events that lead to aggregation at atomic resolution can aid strategies to target the early, critical toxic soluble oligomers.

Structural Stabilization of Human Transthyretin by Centella asiatica (L.) Urban Extract: Implications for TTR Amyloidosis

Transthyretin is responsible for a series of highly progressive, degenerative, debilitating, and incurable protein misfolding disorders known as transthyretin (TTR) amyloidosis. Since dissociation of the homotetrameric protein to its monomers is crucial in its amyloidogenesis, stabilizing the native tetramer from dissociating using small-molecule ligands has proven a viable therapeutic strategy. The objective of this study was to determine the potential role of the medicinal herb Centella asiatica on human transthyretin (huTTR) amyloidogenesis. Thus, we investigated the stability of huTTR with or without a hydrophilic fraction of C. asiatica (CAB) against acid/urea-mediated denaturation. We also determined the influence of CAB on huTTR fibrillation using transmission electron microscopy. The potential binding interactions between CAB and huTTR was ascertained by nitroblue tetrazolium redox-cycling and 8-anilino-1-naphthalene sulfonic acid displacement assays. Additionally, the chemical profile of CAB was determined by liquid chromatography quadruple time-of-flight mass spectrometry (HPLC-QTOF-MS). Our results strongly suggest that CAB bound to and preserved the quaternary structure of huTTR in vitro. CAB also prevented transthyretin fibrillation, although aggregate formation was unmitigated. These effects could be attributable to the presence of phenolics and terpenoids in CAB. Our findings suggest that C. asiatica contains pharmaceutically relevant bioactive compounds which could be exploited for therapeutic development against TTR amyloidosis.

Ageing and amyloid fibrillogenesis: lessons from apolipoprotein AI, transthyretin and islet amyloid polypeptide

The age-associated (or senile) amyloidoses encompass a heterogeneous group of systemic or localized forms of amyloidosis. In this paper we present an overview of three age-associated amyloid forms derived from transthyretin, apolipoprotein AI and islet amyloid polypeptide. Mutations in the respective genes give rise to transthyretin and apolipoprotein AI forms of familial amyloidosis while senile forms of amyloid are associated with the wild-type proteins. Different mechanisms are probably of importance in the fibrillogenesis associated with these three amyloid types. It is also possible that different amyloidogenic pathways exist for a single amyloidogenic protein. Thus, limited proteolysis may be necessary in the fibrillogenesis in senile transthyretin amyloidosis but not in most familial transthyretin amyloidoses. Other factors in the pathogenesis of amyloidosis such as local concentration, nidus formation and glycation are also discussed.

Detailed structural analysis of amyloidogenic wild-type transthyretin using a novel purification strategy and mass spectrometry

Wild-type transthyretin (TTR), normally a soluble plasma-circulating protein, can be amyloidogenic, i.e., form tissue-deposited fibrillar material in the extracellular matrix of various organs throughout the body. Senile systemic amyloidosis (SSA) is one such pathology and features TTR-containing amyloid deposits that are found primarily in the heart. The cause for this transition from soluble to insoluble protein in SSA is yet to be determined as specific structural features that might favor TTR fibrillogenesis have not yet been identified. The precise characterization of ex vivo fibril deposits might provide insight, but structural analyses of TTR from amyloid deposits have been hindered thus far by the lack of purification strategies that overcome the insolubility of the tissue-derived protein without degrading it. Consequently, the true biochemical nature of deposited TTR remains in question. In this study, we provide detailed analyses of both the soluble (serum) and deposited (tissue) forms of TTR from cases of SSA. In the serum, a distribution of mixed disulfides, specifically S-sulfonated and S-cysteinylated forms of TTR, as well as the unmodified protein were identified. The relative levels of the three TTR species in the SSA group were comparable to amounts present in sera from age-matched control groups. For characterization of the amyloid deposited TTR, we investigated cardiac tissue samples obtained from three separate cases of SSA. We report a novel chromatographic purification strategy performed under nonreducing conditions (to maintain cysteine disulfide status) and the use of this procedure in conjunction with detailed mass spectrometric analysis of TTR from the amyloid deposits. A series of C-terminal TTR fragments with N-termini ranging from amino acids 46 to 55 were identified. We also determined that the deposits in all samples contained Cys10 disulfide-linkedhomodimers composed of full-length TTR monomers. This last finding suggests an important role for Cys10 conjugation in the transition from soluble TTR to the pathological amyloid fibril.

Trypsin-based monolithic bioreactor coupled on-line with LC/MS/MS system for protein digestion and variant identification in standard solutions and serum samples

The applicability of a trypsin-based monolithic bioreactor coupled on-line with LC/MS/MS for rapid proteolytic digestion and protein identification is here described. Dilute samples are passed through the bioreactor for generation of proteolytic fragments in less than 10 min. After digestion and peptide separation, electrospray ionization tandem mass spectrometry is used to generate a peptide map and to identify proteolytic peptides by correlating their fragmentation spectra with amino acid sequences from a protein database. By digesting picomoles of proteins sufficient data from ESI and MS/MS were obtained to unambiguously identify proteins alone and in serum samples. This approach was also extended to locate mutation sites in beta-lactoglobulin A and B variants.

The β-strand D of transthyretin trapped in two discrete conformations

Conformational changes in native and variant forms of the human plasma protein transthyretin (TTR) induce several types of amyloid diseases. Biochemical and structural studies have mapped the initiation site of amyloid formation onto residues at the outer C and D beta-strands and their connecting loop. In this study, we characterise an engineered variant of transthyretin, Ala108Tyr/Leu110Glu, which is kinetically and thermodynamically more stable than wild-type transthyretin, and as a consequence less amyloidogenic. Crystal structures of the mutant were determined in two space groups, P2(1)2(1)2 and C2, from crystals grown in the same crystallisation set-up. The structures are identical with the exception for residues Leu55-Leu58, situated at beta-strand D and the following DE loop. In particular, residues Leu55-His56 display large shifts in the C2 structure. There the direct hydrogen bonding between beta-strands D and A has been disrupted and is absent, whereas the beta-strand D is present in the P2(1)2(1)2 structure. This difference shows that from a mixture of metastable TTR molecules, only the molecules with an intact beta-strand D are selected for crystal growth in space group P2(1)2(1)2. The packing of TTR molecules in the C2 crystal form and in the previously determined amyloid TTR (ATTR) Leu55Pro crystal structure is close-to-identical. This packing arrangement is therefore not unique in amyloidogenic mutants of TTR.

Cysteine 10 is a key residue in amyloidogenesis of human transthyretin Val30Met

Type I familial amyloidotic polyneuropathy (FAP), a systemic amyloidosis, is characterized by aggregation of variant transthyretin (TTR Val30Met) into stable, insoluble fibrils. This aggregation is caused by genetic and environmental factors. Genetic factors have been studied extensively. However, little is known about environmental or physiological factors involved in the disease process, and their identification may be important for development of effective treatment. X-ray crystallography of normal and amyloidogenic human TTR Val30Met in type I FAP showed that the -SH side chain of cysteine at position 10 (Cys10) forms a hydrogen bond with Gly57 in normal TTR but not in TTR Val30Met. This result suggests a crucial role for the free Cys10 residue and possible involvement of physiological factors affecting Cys residue reactivity in TTR amyloidogenesis. To analyze amyloidogenesis in vivo, our group generated murine FAP models by transgenic technology, with human TTR Val30Met. The three lines of transgenic mice expressed amyloidogenic mutant TTR (Cys10/Met30), wild-type TTR (Cys10/Val30), and artificial Cys-free mutant TTR (Ser10/Met30). Histochemical investigation showed deposition of amyloid derived from human TTR only in amyloidogenic mutant TTR (Cys10/Met30) mice. Thus, the -SH residue in Cys10 plays a crucial role in TTR Val30Met amyloidogenesis in vivo. These data suggest the possibility of innovative treatment via physiological factors modulating Cys10 residue reactivity.

X-ray absorption spectroscopy reveals a substantial increase of sulfur oxidation in transthyretin (TTR) upon fibrillization

Transthyretin (TTR) amyloid fibrils are the main component of the amyloid deposits occurring in Familial Amyloidotic Polyneuropathy patients. This is 1 of 20 human proteins leading to protein aggregation disorders such as Alzheimer's and Creutzfeldt-Jakob diseases. The structural details concerning the association of the protein molecules are essential for a better understanding of the disease and consequently the design of new strategies for diagnosis and therapeutics. Disulfide bonds are frequently considered essential for the stability of protein aggregates and since in the TTR monomers there is one cysteine residue, it is important to determine unambiguously the redox state of sulfur present in the fibrils. In this work we used x-ray spectroscopy to further characterize TTR amyloid fibrils. The sulfur K-edge absorption spectra for the wild type and some amyloidogenic TTR variants in the soluble and fibrillar forms were analyzed. Whereas in the soluble proteins the thiol group from cysteine (R-SH) and the thioether group from methionine (R-S-CH(3)) are the most abundant forms, in the TTR fibrils there is a significant oxidation of sulfur to the sulfonate form in the cysteine residue and a partial oxidation of sulfur to sulfoxide in the methionine residues. Further interpretation of the data reveals that there are no disulfide bridges in the fibrillar samples and suggest conformational changes in the TTR molecule, namely in strand A and/or in its vicinity, upon fibril formation.

Native state hydrogen exchange study of suppressor and pathogenic variants of transthyretin

Transthyretin (TTR) is an amyloidogenic protein whose aggregation is responsible for numerous familial amyloid diseases, the exact phenotype being dependent on the sequence deposited. Many familial disease variants display decreased stability in vitro, and early onset pathology in vivo. Only subtle structural differences were observed upon crystallographic comparison of the disease-associated variants to the T119M interallelic trans-suppressor. Herein three human TTR single amino acid variant homotetramers including two familial amyloidotic polyneuropathy (FAP) causing variants (V30M and L55P), and a suppressor variant T119M (known to protect V30M carriers from disease by trans-suppression) were investigated in a residue-specific fashion by monitoring (2)H-(1)H exchange employing NMR spectroscopy. The measured protection factors for slowly exchanging amide hydrogen atoms reveal destabilization of the protein core in the FAP variants, the core consisting of strands A, B, E and G and the loop between strands A and B. The same core exhibits much slower exchange in the suppressor variant. Accelerated exchange rates were observed for residues at the subunit interfaces in L55P, but not in the T119M or V30M TTR. The correlation between destabilization of the TTR core strands and the tendency for amyloid formation supports the view that these strands are involved in amyloidogenicity, consistent with previous (2)H-(1)H exchange analysis of the WT-TTR amyloidogenic intermediate.

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.

A comparative analysis of 23 structures of the amyloidogenic protein transthyretin

Self-assembly of the human plasma protein transthyretin (TTR) into unbranched insoluble amyloid fibrils occurs as a result of point mutations that destabilize the molecule, leading to conformational changes. The tertiary structure of native soluble TTR and many of its disease-causing mutants have been determined. Several independent studies by X-ray crystallography have suggested structural differences between TTR variants which are claimed to be of significance for amyloid formation. As these changes are minor and not consistent between the studies, we have compared all TTR structures available at the protein data bank including three wild-types, three non-amyloidogenic mutants, seven amyloidogenic mutants and nine complexes. The reference for this study is a new 1.5 A resolution structure of human wild-type TTR refined to an R-factor/R-free of 18.6 %/21.6 %. The present findings are discussed in the light of the previous structural studies of TTR variants, and show the reported structural differences to be non-significant.

The genetics of the amyloidoses

The amyloidoses are diseases in which abnormalities in the secondary structure of precursor proteins result in decreased solubility under physiologic conditions, with subsequent organ compromise. A total of 18 proteins have been definitively identified as amyloid precursors associated with human disease. Mutations in the genes that encode some of these proteins produce autosomal dominant disease in mid to late adult life. Until recently, the late onset has obscured the familial nature of some of the disorders. This is especially true in the apparently sporadic disease-producing deposits found even later in life. In many instances, these deposits are derived from precursors encoded by wild-type genes (perhaps influenced by alleles that are polymorphic in the normal population); in other cases, they represent autosomal dominant disease with age-dependent penetrance. The genetic aspects of amyloid diseases produced by the deposition of four different proteins have been investigated in detail and provide insights into the particular diseases and amyloidogenesis in general.

Characterization of transthyretin mutants from serum using immunoprecipitation, HPLC/electrospray ionization and matrix-assisted laser desorption/ionization mass spectrometry

A mass spectrometry approach for the detection and identification of variants of the plasma protein transthyretin (TTR) is presented. The single amino acid substitutions found in TTR are closely associated with familial transthyretin amyloidosis (ATTR), a hereditary degenerative disease. A definitive diagnosis of ATTR relies on the detection and identification of TTR variants. The approach presented here is based on isolation of serum TTR using immunoprecipitation. The detection of the variant is achieved by mass measurement of the intact protein with electrospray ionization mass spectrometry (ESIMS). The liquid chromatography/ESIMS analysis of the tryptic digest of the protein followed by subsequent matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry and MALDI postsource decay of the relevant recovered chromatographic fraction containing the variant peptide allows the identification of unknown variants. The method was successfully tested using serum from ATTR patients with known variants (Val30-->Met and Val122-->Ile). A new TTR variant, Ser23-->Asn, was detected and identified using the above method where isoelectric focusing and restriction enzyme analysis failed to identify the nature of the variant.

Purification and characterization of amyloid-related transthyretin associated with familial amyloidotic cardiomyopathy

Analysis of amyloid fibril material associated with familial amyloidotic cardiomyopathy revealed that it contains a mixture of transthyretin-related polypeptides. The major protein band in SDS/polyacrylamide gel corresponding to a molecular mass of 14.5 kDa, consists of transthyretin fragments starting at positions 46, 49 and 59, the latter not previously identified, and one blocked fragment derived from the N-terminal part of transthyretin. In reverse-phase HPLC, the major fragment recovered was that starting at Thr49, indicating a trypsin-like cleavage (Lys at position 48). Two minor bands, corresponding to 17 kDa and 35 kDa, contained proteins with blocked N-termini, and migrated as monomeric and dimeric transthyretin, respectively. A 13-kDa protein band was found to contain transthyretin with a ragged N-terminus, mainly starting at positions 2 and 5. Three more bands, corresponding to 10, 25 and 29 kDa, consist of transthyretin molecules with blocked N-termini and most likely of aggregates of truncated molecules. A point mutation of amyloid transthyretin was identified at position 111 (Met instead of Leu in normal serum transthyretin) which confirms the mutation found for Danish siblings with familial amyloidotic cardiomyopathy. However, the presence of a non-variant amyloid transthyretin was also observed, indicating that the Danish kindred is heterozygous with respect to this point mutation. Isoelectric focusing of the amyloid fibril material resolved multiple protein bands ranging over pH 4.5-6.5, confirming heterogeneities. Methanol extraction of the cardiac amyloid fibril material prior to the purification steps reveals a methanol-soluble substance amounting to about 10% (by mass dry material) of the amyloid fibril material. A yellow substance in this fraction shows absorbance maxima (270, 280 and 430 nm) similar to those observed for transthyretin in normal serum. Gas chromatography/mass spectrometry of the methanol extract revealed the presence of saturated fatty acids (C14:0, C16:0 and C18:0 in the corresponding ratio 2:8:5) and polyunsaturated fatty acids (C16:1, C18:1, C18:2 and C20:4 in the corresponding ratio of 1:2:1:1) as further constituents of the amyloid fibril material.

Genetic abnormalities and pathogenesis of familial amyloidotic polyneuropathy

Familial amyloidotic polyneuropathy (FAP) is an autosomal inherited disease, characterized by extracellular amyloid deposits and by peripheral neuropathy. Amyloid fibrils derived from most types of FAP consist of variant transthyretin (TTR) with single amino acid substitutions, and methionine 30 TTR is the most common variant TTR. TTR is mainly produced in the liver and the choroid plexus. Biochemical and molecular biological techniques have been revealing the amyloidogenicity of variant TTR in vitro and in vivo using the transgenic mouse as a model. It will be important for the development of effective therapy to find out the factors, other than variant TTR, which affect amyloid deposition and define the tissue specificity of amyloid.