The Crystal Structure of Transthyretin in Complex with Diethylstilbestrol

Inhibitors of Transthyretin Amyloidosis: How to Rank Drug Candidates Using X-ray Crystallography Data

Amyloidosis is a group of protein misfolding diseases, which include spongiform encephalopathies, Alzheimer's disease and transthyretin (TTR) amyloidosis; all of them are characterized by extracellular deposits of an insoluble fibrillar protein. TTR amyloidosis is a highly debilitating and life-threatening disease. Patients carry less stable TTR homotetramers that are prone to dissociation into non-native monomers, which in turn rapidly self-assemble into oligomers and, ultimately, amyloid fibrils. Liver transplantation to induce the production of wild-type TTR was the only therapeutic strategy until recently. A promising approach to ameliorate transthyretin (TTR) amyloidosis is based on the so-called TTR kinetic stabilizers. More than 1000 TTR stabilizers have already been tested by many research groups, but the diversity of experimental techniques and conditions used hampers an objective prioritization of the compounds. One of the most reliable and unambiguous techniques applied to determine the structures of the TTR/drug complexes is X-ray diffraction. Most of the potential inhibitors bind in the TTR channel and the crystal structures reveal the atomic details of the interaction between the protein and the compound. Here we suggest that the stabilization effect is associated with a compaction of the quaternary structure of the protein and propose a scoring function to rank drugs based on X-ray crystallography data.

PITB: A high affinity transthyretin aggregation inhibitor with optimal pharmacokinetic properties

The aggregation of wild-type transthyretin (TTR) and over 130 genetic TTR variants underlies a group of lethal disorders named TTR amyloidosis (ATTR). TTR chemical chaperones are molecules that hold great promise to modify the course of ATTR progression. In previous studies, we combined rational design and molecular dynamics simulations to generate a series of TTR selective kinetic stabilizers displaying exceptionally high affinities. In an effort to endorse the previously developed molecules with optimal pharmacokinetic properties, we conducted structural design optimization, leading to the development of PITB. PITB binds with high affinity to TTR, effectively inhibiting tetramer dissociation and aggregation of both the wild-type protein and the two most prevalent disease-associated TTR variants. Importantly, PITB selectively binds and stabilizes TTR in plasma, outperforming tolcapone, a drug currently undergoing clinical trials for ATTR. Pharmacokinetic studies conducted on mice confirmed that PITB exhibits encouraging pharmacokinetic properties, as originally intended. Furthermore, PITB demonstrates excellent oral bioavailability and lack of toxicity. These combined attributes position PITB as a lead compound for future clinical trials as a disease-modifying therapy for ATTR.

Natural compounds as inhibitors of transthyretin amyloidosis and neuroprotective agents: analysis of structural data for future drug design

Natural compounds, such as plant and fruit extracts have shown neuroprotective effect against neurodegenerative diseases. It has been reported that several natural compounds binding to transthyretin (TTR) can be useful in amyloidosis prevention. TTR is a transporter protein that under physiological condition carries thyroxine (T4) and retinol in plasma and in cerebrospinal fluid (CSF); it also has a neuroprotective role against Alzheimer's disease (AD). However, TTR also is an amyloidogenic protein responsible for familial amyloid polyneuropathy (FAP) and familial amyloid cardiomyopathy (FAC). The TTR amyloidogenic potential is speeded up by several point mutations. One therapeutic strategy against TTR amyloidosis is the stabilisation of the native tetramer by natural compounds and small molecules. In this review, we examine the natural products that, starting from 2012 to present, have been studied as a stabiliser of TTR tetramer. In particular, we discussed the chemical and structural features which will be helpful for future drug design of new TTR stabilisers.

Undiscovered Roles for Transthyretin: From a Transporter Protein to a New Therapeutic Target for Alzheimer's Disease

Transthyretin (TTR), an homotetrameric protein mainly synthesized by the liver and the choroid plexus, and secreted into the blood and the cerebrospinal fluid, respectively, has been specially acknowledged for its functions as a transporter protein of thyroxine and retinol (the latter through binding to the retinol-binding protein), in these fluids. Still, this protein has managed to stay in the spotlight as it has been assigned new and varied functions. In this review, we cover knowledge on novel TTR functions and the cellular pathways involved, spanning from neuroprotection to vascular events, while emphasizing its involvement in Alzheimer’s disease (AD). We describe details of TTR as an amyloid binding protein and discuss its interaction with the amyloid Aβ peptides, and the proposed mechanisms underlying TTR neuroprotection in AD. We also present the importance of translating advances in the knowledge of the TTR neuroprotective role into drug discovery strategies focused on TTR as a new target in AD therapeutics.

Anti-TTR Nanobodies Allow the Identification of TTR Neuritogenic Epitope Associated with TTR-Megalin Neurotrophic Activities

Transthyretin (TTR) has intrinsic neurotrophic physiological activities independent from its thyroxine ligands, which involve activation of signaling pathways through interaction with megalin. Still, the megalin binding motif on TTR is unknown. Nanobodies (Nb) have the ability to bind "hard to reach" epitopes being useful tools for protein/structure function. In this work, we characterize two anti-TTR Nanobodies, with similar mouse TTR binding affinities, although only one is able to block its neuritogenic activity (169F7_Nb). Through epitope mapping, we identified amino acids 14-18, at the entrance of the TTR central channel, to be important for interaction with megalin, and a stable TTR K15N mutant in that region was constructed. The TTR K15N mutant lacks neuritogenic activity, indicating that K15 is critical for TTR neuritogenic activity. Thus, we identify the putative binding site for megalin and describe two Nanobodies that will allow research and clarification of TTR physiological properties, regarding its neurotrophic effects.

Case series: clinical outcomes of the transthyretin valine-to-isoleucine mutation in a brother–sister pair

Background

Approximately 4% of the African-American population possess a valine-to-isoleucine (V122I) substitution within the transthyretin protein that results in a tendency for a normally tetrameric protein to dissociate into misfolded, monomeric subunits. These misfolded proteins can then accumulate pathologically and cause an autosomal dominant amyloid cardiomyopathy. Homozygous patients are infrequently documented in case reports, and though there are larger studies among heterozygous patients, there is a lack of studies or reports comparing disease within a family.

Case summary

In this case series, we discuss a 61-year-old African-American male who succumbed to heart failure secondary to cardiac amyloidosis while awaiting orthotopic heart transplantation. We compare his case with that of his sister, a 65-year-old African-American woman with a history of recurrent supraventricular tachycardia requiring radiofrequency ablation, and intermittent chest pain with chronically elevated troponin despite no evidence of coronary artery disease. The sister in question was found to be homozygous for the transthyretin (TTR) V122I mutation with evidence of infiltrative process on cardiac magnetic resonance imaging, while clinical testing verified a heterozygous genotype in the brother. Here, we compare the clinical course and imaging data for the aforementioned brother–sister pair in the context of the amyloidogenic transthyretin V122I gene variant.

Discussion

Through this familial report, we aim to highlight the variations in expression both within this family and in comparison, to the population. We also hope to emphasize the importance of genetic testing of families at risk for this specific transthyretin variant within the African-American community especially as novel therapies begin to emerge.

Enthalpy-Driven Stabilization of Transthyretin by AG10 Mimics a Naturally Occurring Genetic Variant That Protects from Transthyretin Amyloidosis

Transthyretin (TTR) amyloid cardiomyopathy (ATTR-CM) is a fatal disease with no available disease-modifying therapies. While pathogenic TTR mutations (TTRm) destabilize TTR tetramers, the T119M variant stabilizes TTRm and prevents disease. A comparison of potency for leading TTR stabilizers in clinic and structural features important for effective TTR stabilization is lacking. Here, we found that molecular interactions reflected in better binding enthalpy may be critical for development of TTR stabilizers with improved potency and selectivity. Our studies provide mechanistic insights into the unique binding mode of the TTR stabilizer, AG10, which could be attributed to mimicking the stabilizing T119M variant. Because of the lack of animal models for ATTR-CM, we developed an in vivo system in dogs which proved appropriate for assessing the pharmacokinetics-pharmacodynamics profile of TTR stabilizers. In addition to stabilizing TTR, we hypothesize that optimizing the binding enthalpy could have implications for designing therapeutic agents for other amyloid diseases.

Tetrabromobisphenol A Is an Efficient Stabilizer of the Transthyretin Tetramer

Amyloid formation of the human plasma protein transthyretin (TTR) is associated with several human disorders, including familial amyloidotic polyneuropathy (FAP) and senile systemic amyloidosis. Dissociation of TTR's native tetrameric assembly is the rate-limiting step in the conversion into amyloid, and this feature presents an avenue for intervention because binding of an appropriate ligand to the thyroxin hormone binding sites of TTR stabilizes the native tetrameric assembly and impairs conversion into amyloid. The desired features for an effective TTR stabilizer include high affinity for TTR, high selectivity in the presence of other proteins, no adverse side effects at the effective concentrations, and a long half-life in the body. In this study we show that the commonly used flame retardant tetrabromobisphenol A (TBBPA) efficiently stabilizes the tetrameric structure of TTR. The X-ray crystal structure shows TBBPA binding in the thyroxine binding pocket with bromines occupying two of the three halogen binding sites. Interestingly, TBBPA binds TTR with an extremely high selectivity in human plasma, and the effect is equal to the recently approved drug tafamidis and better than diflunisal, both of which have shown therapeutic effects against FAP. TBBPA consequently present an interesting scaffold for drug design. Its absorption, metabolism, and potential side-effects are discussed.

Modifications of the 7-Hydroxyl Group of the Transthyretin Ligand Luteolin Provide Mechanistic Insights into Its Binding Properties and High Plasma Specificity

Amyloid formation of the plasma protein transthyretin (TTR) has been linked to familial amyloid polyneuropathy and senile systemic amyloidosis. Binding of ligands within its natural hormone binding site can stabilize the tetrameric structure and impair amyloid formation. We have recently shown that the flavonoid luteolin stabilizes TTR in human plasma with a very high selectivity. Luteolin, however, is inactivated in vivo via glucuronidation for which the preferred site is the hydroxy group at position 7 on its aromatic A-ring. We have evaluated the properties of two luteolin variants in which the 7-hydroxy group has been exchanged for a chlorine (7-Cl-Lut) or a methoxy group (7-MeO-Lut). Using an in vitro model, based on human liver microsomes, we verified that these modifications increase the persistence of the drug. Crystal structure determinations show that 7-Cl-Lut binds similarly to luteolin. The larger MeO substituent cannot be accommodated within the same space as the chlorine or hydroxy group and as a result 7-MeO-Lut binds in the opposite direction with the methoxy group in position 7 facing the solvent. Both 7-Cl-Lut and 7-MeO-Lut qualify as high-affinity binders, but in contrast to luteolin, they display a highly non-specific binding to other plasma components. The binding of the two conformations and the key-interactions to TTR are discussed in detail. Taken together, these results show a proof-of-concept that the persistence of luteolin towards enzymatic modification can be increased. We reveal two alternative high-affinity binding modes of luteolin to TTR and that modification in position 7 is restricted only to small substituents if the original orientation of luteolin should be preserved. In addition, the present work provides a general and convenient method to evaluate the efficacy of TTR-stabilizing drugs under conditions similar to an in vivo environment.

A new crystal form of human transthyretin obtained with a curcumin derived ligand

Transthyretin (TTR), a 54kDa homotetrameric protein that transports thyroxine (T4), has been associated with clinical cases of TTR amyloidosis for its tendency to aggregate to form fibrils. Many ligands with a potential to inhibit fibril formation have been studied by X-ray crystallography in complex with TTR. Unfortunately, the ligand is often found in ambiguous electron density that is difficult to interpret. The ligand validation statistics suggest over-interpretation, even for the most active compounds like diflunisal. The primary technical reason is its position on a crystallographic 2-fold axis in the most common crystal form. Further investigations with the use of polyethylene glycol (PEG) to crystallize TTR complexes have resulted in a new trigonal polymorph with two tetramers in the asymmetric unit. The ligand used to obtain this new polymorph, 4-hydroxychalcone, is related to curcumin. Here we evaluate this crystal form to understand the contribution it may bring to the study of TTR ligands complexes, which are often asymmetric.

The flavonoid luteolin, but not luteolin-7-O-glucoside, prevents a transthyretin mediated toxic response

Transthyretin (TTR) is a homotetrameric plasma protein with amyloidogenic properties that has been linked to the development of familial amyloidotic polyneuropathy (FAP), familial amyloidotic cardiomyopathy, and senile systemic amyloidosis. The in vivo role of TTR is associated with transport of thyroxine hormone T4 and retinol-binding protein. Loss of the tetrameric integrity of TTR is a rate-limiting step in the process of TTR amyloid formation, and ligands with the ability to bind within the thyroxin binding site (TBS) can stabilize the tetramer, a feature that is currently used as a therapeutic approach for FAP. Several different flavonoids have recently been identified that impair amyloid formation. The flavonoid luteolin shows therapeutic potential with low incidence of unwanted side effects. In this work, we show that luteolin effectively attenuates the cytotoxic response to TTR in cultured neuronal cells and rescues the phenotype of a Drosophila melanogaster model of FAP. The plant-derived luteolin analogue cynaroside has a glucoside group in position 7 of the flavone A-ring and as opposed to luteolin is unable to stabilize TTR tetramers and thus prevents a cytotoxic effect. We generated high-resolution crystal-structures of both TTR wild type and the amyloidogenic mutant V30M in complex with luteolin. The results show that the A-ring of luteolin, in contrast to what was previously suggested, is buried within the TBS, consequently explaining the lack of activity from cynaroside. The flavonoids represent an interesting group of drug candidates for TTR amyloidosis. The present investigation shows the potential of luteolin as a stabilizer of TTR in vivo. We also show an alternative orientation of luteolin within the TBS which could represent a general mode of binding of flavonoids to TTR and is of importance concerning the future design of tetramer stabilizing drugs.

Binding site asymmetry in human transthyretin: insights from a joint neutron and X-ray crystallographic analysis using perdeuterated protein

Human transthyretin has an intrinsic tendency to form amyloid fibrils and is heavily implicated in senile systemic amyloidosis. Here, detailed neutron structural studies of perdeuterated transthyretin are described. The analyses, which fully exploit the enhanced visibility of isotopically replaced hydrogen atoms, yield new information on the stability of the protein and the possible mechanisms of amyloid formation. Residue Ser117 may play a pivotal role in that a single water molecule is closely associated with the γ-hydrogen atoms in one of the binding pockets, and could be important in determining which of the two sites is available to the substrate. The hydrogen-bond network at the monomer-monomer interface is more extensive than that at the dimer-dimer interface. Additionally, the edge strands of the primary dimer are seen to be favourable for continuation of the β-sheet and the formation of an extended cross-β structure through sequential dimer couplings. It is argued that the precursor to fibril formation is the dimeric form of the protein.

Stability of the transthyretin molecule as a key factor in the interaction with a-beta peptide--relevance in Alzheimer's disease

Transthyretin (TTR) protects against A-Beta toxicity by binding the peptide thus inhibiting its aggregation. Previous work showed different TTR mutations interact differently with A-Beta, with increasing affinities correlating with decreasing amyloidogenecity of the TTR mutant; this did not impact on the levels of inhibition of A-Beta aggregation, as assessed by transmission electron microscopy. Our work aimed at probing differences in binding to A-Beta by WT, T119M and L55P TTR using quantitative assays, and at identifying factors affecting this interaction. We addressed the impact of such factors in TTR ability to degrade A-Beta. Using a dot blot approach with the anti-oligomeric antibody A11, we showed that A-Beta formed oligomers transiently, indicating aggregation and fibril formation, whereas in the presence of WT and T119M TTR the oligomers persisted longer, indicative that these variants avoided further aggregation into fibrils. In contrast, L55PTTR was not able to inhibit oligomerization or to prevent evolution to aggregates and fibrils. Furthermore, apoptosis assessment showed WT and T119M TTR were able to protect against A-Beta toxicity. Because the amyloidogenic potential of TTR is inversely correlated with its stability, the use of drugs able to stabilize TTR tetrameric fold could result in increased TTR/A-Beta binding. Here we showed that iododiflunisal, 3-dinitrophenol, resveratrol, [2-(3,5-dichlorophenyl)amino] (DCPA) and [4-(3,5-difluorophenyl)] (DFPB) were able to increase TTR binding to A-Beta; however only DCPA and DFPB improved TTR proteolytic activity. Thyroxine, a TTR ligand, did not influence TTR/A-Beta interaction and A-Beta degradation by TTR, whereas RBP, another TTR ligand, not only obstructed the interaction but also inhibited TTR proteolytic activity. Our results showed differences between WT and T119M TTR, and L55PTTR mutant regarding their interaction with A-Beta and prompt the stability of TTR as a key factor in this interaction, which may be relevant in AD pathogenesis and for the design of therapeutic TTR-based therapies.

Flavonoid interactions with human transthyretin: combined structural and thermodynamic analysis

Transthyretin (TTR) is a carrier protein involved in human amyloidosis. The development of small molecules that may act as TTR amyloid inhibitors is a promising strategy to treat these pathologies. Here we selected and characterized the interaction of flavonoids with the wild type and the V30M amyloidogenic mutant TTR. TTR acid aggregation was evaluated in vitro in the presence of the different flavonoids. The best TTR aggregation inhibitors were studied by Isothermal Titration Calorimetry (ITC) in order to reveal their thermodynamic signature of binding to TTRwt. Crystal structures of TTRwt in complex with the top binders were also obtained, enabling us to in depth inspect TTR interactions with these flavonoids. The results indicate that changing the number and position of hydroxyl groups attached to the flavonoid core strongly influence flavonoid recognition by TTR, either by changing ligand affinity or its mechanism of interaction with the two sites of TTR. We also compared the results obtained for TTRwt with the V30M mutant structure in the apo form, allowing us to pinpoint structural features that may facilitate or hamper ligand binding to the V30M mutant. Our data show that the TTRwt binding site is labile and, in particular, the central region of the cavity is sensible for the small differences in the ligands tested and can be influenced by the Met30 amyloidogenic mutation, therefore playing important roles in flavonoid binding affinity, mechanism and mutant protein ligand binding specificities.

In vitro fluorescence displacement investigation of thyroxine transport disruption by bisphenol A

Bisphenol A (BPA) is a chemical with high production volume and wide applications in many industries. Although BPA is known as an endocrine disruptor, its toxic mechanisms have not been fully characterized. Due to its structural similarity to thyroid hormones thyroxine (T4) and triiodothyronine (T3), one possible mechanism of BPA toxicity is disruption of hormone transport by competitive binding with the transport proteins. In this study, the binding interactions of BPA, T4, and T3 with three thyroid hormone transport proteins, human serum albumin (HSA), transthyretin (TTR), and thyroxine-binding globulin (TBG) were investigated by fluorescence measurement. Using two site-specific fluorescence probes dansylamide and dansyl-L-proline, the binding constants of BPA with HSA at drug site I and site II were determined as 2.90 x 10(4) and 3.14 x 10(4) L/mol, respectively. By monitoring the intrinsic fluorescence of tryptophan, a binding constant of 4.70 x 10(3) L/mol was obtained. Similarly, by employing 8-anilino-1-naphthalenesulfonic acid as fluorescence probe, the binding affinity of BPA with TTR and TBG was measured to be 3.10 x 10(5) and 5.90 x 10(5) L/mol, respectively. In general, BPA showed lower binding affinity with the proteins than T3 did, and even lower affinity than T4. Using these binding constants, the amount of BPA which would bind to the transport proteins in human plasma was estimated. These results suggest that the concentrations of BPA commonly found in human plasma are probably not high enough to interfere with T4 transport.

The binding of synthetic triiodo l-thyronine analogs to human transthyretin: molecular basis of cooperative and non-cooperative ligand recognition

Transthyretin (TTR) is a tetrameric β-sheet-rich transporter protein directly involved in human amyloid diseases. Several classes of small molecules can bind to TTR delaying its amyloid fibril formation, thus being promising drug candidates to treat TTR amyloidoses. In the present study, we characterized the interactions of the synthetic triiodo L-thyronine analogs and thyroid hormone nuclear receptor TRβ-selective agonists GC-1 and GC-24 with the wild type and V30M variant of human transthyretin (TTR). To achieve this aim, we conducted in vitro TTR acid-mediated aggregation and isothermal titration calorimetry experiments and determined the TTR:GC-1 and TTR:GC-24 crystal structures. Our data indicate that both GC-1 and GC-24 bind to TTR in a non-cooperative manner and are good inhibitors of TTR aggregation, with dissociation constants for both hormone binding sites (HBS) in the low micromolar range. Analysis of the crystal structures of TTRwt:GC-1(24) complexes and their comparison with the TTRwt X-ray structure bound to its natural ligand thyroxine (T4) suggests, at the molecular level, the basis for the cooperative process displayed by T4 and the non-cooperative process provoked by both GC-1 and GC-24 during binding to TTR.

Conformational differences between the wild type and V30M mutant transthyretin modulate its binding to genistein: implications to tetramer stability and ligand-binding

Transthyretin (TTR) is a tetrameric beta-sheet-rich transporter protein directly involved in human amyloid diseases. It was recently found that the isoflavone genistein (GEN) potently inhibits TTR amyloid fibril formation (Green et al., 2005) and is therefore a promising candidate for TTR amyloidosis treatment. Here we used structural and biophysical approaches to characterize genistein binding to the wild type (TTRwt) and to its most frequent amyloidogenic variant, the V30M mutant. In a dose-dependent manner, genistein elicited considerable increases in both mutant and TTRwt stability as demonstrated by high hydrostatic pressure (HHP) and acid-mediated dissociation/denaturation assays. TTR:GEN crystal complexes and isothermal titration calorimetry (ITC) experiments showed that the binding mechanisms of genistein to the TTRwt and to V30M are different and are dependent on apoTTR structure conformations. Furthermore, we could also identify potential allosteric movements caused by genistein binding to the wild type TTR that explains, at least in part, the frequently observed negatively cooperative process between the two sites of TTRwt when binding ligands. These findings show that TTR mutants may present different ligand recognition and therefore are of value in ligand design for inhibiting TTR amyloidosis.

Structure-based design of kinetic stabilizers that ameliorate the transthyretin amyloidoses

Small molecules that bind to normally unoccupied thyroxine (T(4)) binding sites within transthyretin (TTR) in the blood stabilize the tetrameric ground state of TTR relative to the dissociative transition state and dramatically slow tetramer dissociation, the rate-limiting step for the process of amyloid fibril formation linked to neurodegeneration and cell death. These so-called TTR kinetic stabilizers have been designed using structure-based principles and one of these has recently been shown to halt the progression of a human TTR amyloid disease in a clinical trial, providing the first pharmacologic evidence that the process of amyloid fibril formation is causative. Structure-based design has now progressed to the point where highly selective, high affinity TTR kinetic stabilizers that lack undesirable off-target activities can be produced with high frequency.

Alanine-to-threonine substitutions and amyloid diseases: butyrylcholinesterase as a case study

Alanine-to-threonine (A to T) substitutions caused by single nucleotide polymorphisms (SNPs) occur in diverse proteins, and in certain cases these substitutions induce self-aggregation into amyloid fibrils or aggregation in other amyloidogenic proteins. This is compatible with the inverse preferences of alanine to form helices and of threonine to support beta-sheet structures, which are crucial for amyloid fibrils formation. Our interest in these mutations was initiated by studying the potential effects of the A539T substitution in the butyrylcholinesterase BChE-K variant on amyloid fibrils formation in Alzheimer's disease. Other examples are, Parkinson's disease (PD), where A53T alpha-synuclein occurs in Lewy bodies and familial amyloid polyneuropathy (FAP), where an A25T substitution appears in transthyretin (TTR). In peripheral organs, an A34T substitution is found in the light chain immunoglobulin genes of patients with systemic amyloidosis and in familial hypercholesterolemia, an A370T substitution occurs in the LDLR regulator of cholesterol homeostasis. That such substitutions appear in proteins with important cellular functions suggests that they confer antagonistic pleiotropy, providing added value at an earlier age but causing damages and inducing amyloid diseases later on. This, in turn, may explain the evolutionary selection and preservation of these substitutions. The structural effect of residue substitutions and in particular A to T substitutions in amyloidogenic diseases thus merits further attention.

Comparative neuroprotective properties of stilbene and catechin analogs: action via a plasma membrane receptor site?

Various studies have reported on the neuroprotective effects of polyphenols, widely present in food, beverages, and natural products. For example, we have shown that resveratrol, a polyphenol enriched in red wine and other foods such as peanuts, protects hippocampal cells against beta-amyloid (Abeta)-induced toxicity, a key protein involved in the neuropathology of Alzheimer disease. This effect involves, at least in part, the capacity of resveratrol to activate the phosphorylation of delta isoform of protein kinase C (PKC-delta). The neuroprotective action of resveratrol is shared by piceatannol, a stilbene derivative, as well as by tea-derived catechin gallate esters. The thioflavin T assay indicated that all these polyphenols inhibited the formation of Abeta fibrils, suggesting that this action likely also contributes to their neuroprotective effects. Binding and autoradiographic studies revealed that the effects of polyphenols might involve specific binding sites that are particularly enriched in the choroid plexus in the rat brain. Interestingly, the choroid plexus secretes transthyretin, a protein that has been shown to modulate Abeta aggregation and that may be critical to the maintenance of normal learning capacities in aging. Taken together, these data suggest that polyphenols target multiple enzymes/proteins, leading to their neuroprotective actions, possibly through action via specific plasma membrane binding sites.

Hormone affinity and fibril formation of piscine transthyretin: the role of the N-terminal

Transthyretin (TTR) transports thyroid hormones (THs), thyroxine (T4) and triiodothyronine (T3) in the blood of vertebrates. TH-binding sites are highly conserved in vertebrate TTR, however, piscine TTR has a longer N-terminus which is thought to influence TH-binding affinity and may influence TTR stability. We produced recombinant wild type sea bream TTR (sbTTRWT) plus two mutants in which 6 (sbTTRM6) and 12 (sbTTRM12) N-terminal residues were removed. Ligand-binding studies revealed similar affinities for T3 (Kd=10.6+/-1.7nM) and T4 (Kd=9.8+/-0.97nM) binding to sbTTRWT. Affinity for THs was unaltered in sbTTRM12 but sbTTRM6 had poorer affinity for T4 (Kd=252.3+/-15.8nM) implying that some residues in the N-terminus can influence T4 binding. sbTTRM6 inhibited acid-mediated fibril formation in vitro as shown by fluorometric measurements using thioflavine T. In contrast, fibril formation by sbTTRM12 was significant, probably due to decreased stability of the tetramer. Such studies also suggested that sbTTRWT is more resistant to fibril formation than human TTR.

Iodination of salicylic acid improves its binding to transthyretin

Transthyretin (TTR) is a plasma homotetrameric protein associated with senile systemic amyloidosis and familial amyloidotic polyneuropathy. In theses cases, TTR dissociation and misfolding induces the formation of amyloidogenic intermediates that assemble into toxic oligomeric species and lead to the formation of fibrils present in amyloid deposits. The four TTR monomers associate around a central hydrophobic channel where two thyroxine molecules can bind simultaneously. In each thyroxine binding site there are three pairs of symmetry related halogen binding pockets which can accommodate the four iodine substituents of thyroxine. A number of structurally diverse small molecules that bind to the TTR channel increasing the protein stability and thereafter inhibiting amyloid fibrillogenesis have been tested. In order to take advantage of the high propensity to interactions between iodine substituents and the TTR channel we have identified two iodinated derivatives of salicylic acid, 5-iodosalicylic acid and 3,5-diiodosalicylic acid, available commercially. We report in this paper the relative binding affinities of salicylic acid and the two iodinated derivatives and the crystal structure of TTR complexed with 3,5-diiodosalicylic acid, to elucidate the higher binding affinity of this compound towards TTR.

Comparative in vitro and ex vivo activities of selected inhibitors of transthyretin aggregation: relevance in drug design

Destabilization of the tetrameric fold of TTR (transthyretin) is important for aggregation of the protein which culminates in amyloid fibril formation. Many TTR mutations interfere with tetramer stability, increasing the amyloidogenic potential of the protein. The vast majority of proposed TTR fibrillogenesis inhibitors are based on in vitro assays with isolated protein, limiting their future use in clinical assays. In the present study we investigated TTR fibrillogenesis inhibitors using a cellular system that produces TTR intermediates/aggregates in the medium. Plasmids carrying wild-type TTR, V30M or L55P cDNA were transfected into a rat Schwannoma cell line and TTR aggregates were investigated in the medium using a dot-blot filter assay followed by immunodetection. Results showed that, in 24 h, TTR L55P forms aggregates in the medium, whereas, up to 72 h, wild-type TTR and V30M do not. A series of 12 different compounds, described in the literature as in vitro TTR fibrillogenesis inhibitors, were tested for their ability to inhibit L55P aggregate formation; in this system, 2-[(3,5-dichlorophenyl) amino] benzoic acid, benzoxazole, 4-(3,5-difluorophenyl) benzoic acid and tri-iodophenol were the most effective inhibitors, as compared with the reference iododiflunisal, previously shown by ex vivo and in vitro procedures to stabilize TTR and inhibit fibrillogenesis. Among these drugs, 2-[(3,5-dichlorophenyl) amino] benzoic acid and tri-iodophenol stabilized TTR from heterozygotic carriers of V30M in the same ex vivo conditions as those used previously for iododiflunisal. The novel cellular-based test herein proposed for TTR fibrillogenesis inhibitor screens avoids not only lengthy and cumbersome large-scale protein isolation steps but also artefacts associated with most current in vitro first-line screening methods, such as those associated with acidic conditions and the absence of serum proteins.

Models for binding cooperativities of inhibitors with transthyretin

Here, molecular dynamics (MD) simulations are performed to study the differences of binding channel shapes of TTR with two inhibitors, flufenamic acid (FLU) and one kind of N-phenyl phenoxazine (BPD). The asymmetries of global structure including the central binding channel are found to be intrinsic. Moreover, the conformational changes of the binding channel are responsible for negative cooperativity (NC) or independent cooperativity (IC) of ligands. The results suggested a possible binding mechanism addressing NC of FLU and IC of BPD. For FLU, when the first ligand binds with TTR, it leads to expansion of the second binding site which may weaken the interaction of the second FLU with TTR. But for BPD, the first ligand's binding changes the second site's shape slightly, the second ligand has similar binding ability with TTR in the second site like the first binding event.

Novel therapeutic strategies for the treatment of protein-misfolding diseases

Most proteins in the cell adopt a compact, globular fold that determines their stability and function. Partial protein unfolding under conditions of cellular stress results in the exposure of hydrophobic regions normally buried in the interior of the native structure. Interactions involving the exposed hydrophobic surfaces of misfolded protein conformers lead to the formation of toxic aggregates, including oligomers, protofibrils and amyloid fibrils. A significant number of human disorders (e.g. Alzheimer disease, Parkinson disease, Huntington disease, amyotrophic lateral sclerosis and type II diabetes) are characterised by protein misfolding and aggregation. Over the past five years, outstanding progress has been made in the development of therapeutic strategies targeting these diseases. Three promising approaches include: (1) inhibiting protein aggregation with peptides or small molecules identified via structure-based drug design or high-throughput screening; (2) interfering with post-translational modifications that stimulate protein misfolding and aggregation; and (3) upregulating molecular chaperones or aggregate-clearance mechanisms. Ultimately, drug combinations that capitalise on more than one therapeutic strategy will constitute the most effective treatment for patients with these devastating illnesses.

Structural basis for the protective role of sulfite against transthyretin amyloid formation

Transthyretin (TTR) is a plasma protein, which under conditions not yet completely understood, aggregates forming amyloid deposits that occur extracellularly. It is a protein composed of four identical subunits. Each monomer has a single cysteine residue (Cys10), which in the plasma is reduced (Cys-SH), oxidized (Cys-SO3-), sulfonated (Cys-S-SO3-) or bound to various sulfhydryls. There is evidence that these chemical modifications of the SH group alter the stability and the amyloidogenic potential of the protein. The sulfonated form was found to enhance the stability of the native conformation of TTR, avoiding misassembly of the protein leading to amyloid. Consequently, the potential treatment of TTR-type amyloidosis by sulfite has been suggested. The structure of TTR pre-incubated with sulfite at physiological pH, was determined by X-ray crystallography to provide structural insight for the stabilizing effect of sulfite. Each subunit has a beta-sandwich conformation, with two four stranded beta-pleated sheets (DAGH and CBEF) and a small alpha-helix between strands. The sulfonated cysteines have two sulfite oxygens involved in intramonomer hydrogen bonds that bridge Cys10, the amino acid immediately before beta-strand A, to the amino acids immediately after the edge beta-strand D. Implications of the newly observed interactions in the inhibition of fibril formation are discussed in light of the recent structural models of TTR amyloid fibrils.

Structural and functional analysis of PucM, a hydrolase in the ureide pathway and a member of the transthyretin-related protein family

The ureide pathway, which produces ureides from uric acid, is an essential purine catabolic process for storing and transporting the nitrogen fixed in leguminous plants and some bacteria. PucM from Bacillus subtilis was recently characterized and found to catalyze the second reaction of the pathway, hydrolyzing 5-hydroxyisourate (HIU), a product of uricase in the first step. PucM has 121 amino acid residues and shows high sequence similarity to the functionally unrelated protein transthyretin (TTR), a thyroid hormone-binding protein. Therefore, PucM belongs to the TTR-related proteins (TRP) family. The crystal structures of PucM at 2.0 A and its complexes with the substrate analogs 8-azaxanthine and 5,6-diaminouracil reveal that even with their overall structure similarity, homotetrameric PucM and TTR are completely different, both in their electrostatic potential and in the size of the active sites located at the dimeric interface. Nevertheless, the absolutely conserved residues across the TRP family, including His-14, Arg-49, His-105, and the C-terminal Tyr-118-Arg-119-Gly-120-Ser-121, indeed form the active site of PucM. Based on the results of site-directed mutagenesis of these residues, we propose a possible mechanism for HIU hydrolysis. The PucM structure determined for the TRP family leads to the conclusion that diverse members of the TRP family would function similarly to PucM as HIU hydrolase.

Self-Assembly of Amylin(20–29) Amide-Bond Derivatives into Helical Ribbons and Peptide Nanotubes rather than Fibrils

Uncontrolled aggregation of proteins or polypeptides can be detrimental for normal cellular processes in healthy organisms. Proteins or polypeptides that form these amyloid deposits differ in their primary sequence but share a common structural motif: the (anti)parallel beta sheet. A well-accepted approach for interfering with beta-sheet formation is the design of soluble beta-sheet peptides to disrupt the hydrogen-bonding network; this ultimately leads to the disassembly of the aggregates or fibrils. Here, we describe the synthesis, spectroscopic analysis, and aggregation behavior, imaged by electron microscopy, of several backbone-modified amylin(20-29) derivatives. It was found that these amylin derivatives were not able to form fibrils and to some extent were able to inhibit fibril growth of native amylin(20-29). However, two of the amylin peptides were able to form large supramolecular assemblies, like helical ribbons and peptide nanotubes, in which beta-sheet formation was clearly absent. This was quite unexpected since these peptides have been designed as soluble beta-sheet breakers for disrupting the characteristic hydrogen-bonding network of (anti)parallel beta sheets. The increased hydrophobicity and the presence of essential amino acid side chains in the newly designed amylin(20-29) derivatives were found to be the driving force for self-assembly into helical ribbons and peptide nanotubes. This example of controlled and desired peptide aggregation may be a strong impetus for research on bionanomaterials in which special shapes and assemblies are the focus of interest.