Prevention of transthyretin amyloid disease by changing protein misfolding energetics

Efficient refolding of aggregation-prone citrate synthase by polyol osmolytes: how well are protein folding and stability aspects coupled?

Efficient refolding of proteins and prevention of their aggregation during folding are of vital importance in recombinant protein production and in finding cures for several diseases. We have used citrate synthase (CS) as a model to understand the mechanism of aggregation during refolding and its prevention using several known structure-stabilizing cosolvent additives of the polyol series. Interestingly, no parallel correlation between the folding effect and the general stabilizing effect exerted by polyols was observed. Although increasing concentrations of polyols increased protein stability in general, the refolding yields for CS decreased at higher polyol concentrations, with erythritol reducing the folding yields at all concentrations tested. Among the various polyols used, glycerol was the most effective in enhancing the CS refolding yield, and a complete recovery of enzymatic activity was obtained at 7 m glycerol and 10 mug/ml protein, a result superior to the action of the molecular chaperones GroEL and GroES in vitro. A good correlation between the refolding yields and the suppression of protein aggregation by glycerol was observed, with no aggregation detected at 7 m. The polyols prevented the aggregation of CS depending on the number of hydroxyl groups in them. Stopped-flow fluorescence kinetics experiments suggested that polyols, including glycerol, act very early in the refolding process, as no fast and slow phases were detectable. The results conclusively demonstrate that both the thermodynamic and kinetic aspects are critical in the folding process and that all structure-stabilizing molecules need not always help in productive folding to the native state. These findings are important for the rational design of small molecules for efficient refolding of various aggregation-prone proteins of commercial and medical relevance.

Protein aggregation determinants from a simplified model: cooperative folders resist aggregation

Two-chain aggregation simulations using minimalist models of proteins G, L, and mutants were used to investigate the fundamentals of protein aggregation. Mutations were selected to break up repeats of hydrophobic beads in the sequence while maintaining native topology and folding ability. Data are collected under conditions in which all chain types have similar folded populations and after equilibrating the separated chains to minimize competition between folding and aggregation. Folding cooperativity stands out as the best single-chain determinant under these conditions and for these simple models. It can be experimentally measured by the width of the unfolding transition during thermal denaturation and loosely related to population of intermediate-like states during folding. Additional measures of cooperativity and other properties such as radius of gyration fluctuations and patterning of hydrophobic residues are also examined. Initial contact system states with transition-state characteristics can be identified and are more expanded than average initial contact states. Two-chain minimalist model aggregates are considerably less structured than their native states and have minimal domain-swapping features.

Design, synthesis, and evaluation of oxazole transthyretin amyloidogenesis inhibitors

Ten oxazoles bearing a C(4) carboxyl group were synthesized and evaluated as transthyretin (TTR) amyloid fibril inhibitors. Substituting aryls at the C(2) position of the oxazole ring reveals that a 3,5-dichlorophenyl substituent significantly reduced amyloidogenesis. The efficacy of these inhibitors was enhanced further by installing an ethyl, a propyl, or a CF(3) group at the C(5) position. The CF(3) substitution at C(5) also improves the TTR binding selectivity over all the other proteins in human blood.

Selective binding to transthyretin and tetramer stabilization in serum from patients with familial amyloidotic polyneuropathy by an iodinated diflunisal derivative

In familial amyloidotic polyneuropathy, TTR (transthyretin) variants are deposited as amyloid fibrils. It is thought that this process involves TTR tetramer dissociation, which leads to partially unfolded monomers that aggregate and polymerize into amyloid fibrils. This process can be counteracted by stabilization of the tetramer. Several small compounds, such as diclofenac, diflunisal and flufenamic acid, have been reported to bind to TTR in vitro, in the T4 (thyroxine) binding channel that runs through the TTR tetramer, and consequently are considered to stabilize TTR. However, if these agents bind plasma proteins other than TTR, decreased drug availability will occur, compromising their use as therapeutic agents for TTR amyloidosis. In the present work, we compared the action of these compounds and of new derivatives designed to increase both selectivity of binding to TTR and inhibitory potency in relation to TTR amyloid fibril formation. We found two diflunisal derivatives that, in contrast with diclofenac, flufenamic acid and diflunisal, displaced T4 from TTR in plasma preferentially over binding to albumin and thyroxine binding globulin. The same diflunisal derivatives also had a stabilizing effect on TTR tetramers in plasma, as studied by isoelectric focusing of whole plasma under semi-denaturing conditions. In addition, by transmission electron microscopy, we demonstrated that, in contrast with other proposed TTR stabilizers (namely diclofenac, flufenamic acid and diflunisal), one of the diflunisal derivatives tested efficiently inhibited TTR aggregation. Taken together, our ex vivo and in vitro studies present evidence for the selectivity and efficiency of novel diflunisal derivates as TTR stabilizers and as inhibitors of fibril formation.

Hydroxylated Polychlorinated Biphenyls Selectively Bind Transthyretin in Blood and Inhibit Amyloidogenesis: Rationalizing Rodent PCB Toxicity

Polychlorinated biphenyls (PCBs) and their hydroxylated metabolites (OH-PCBs) are known to bind to transthyretin (TTR) in vitro, possibly explaining their bioaccumulation, rodent toxicity, and presumed human toxicity. Herein, we show that several OH-PCBs bind selectively to TTR in blood plasma; however, only one of the PCBs tested binds TTR in plasma. Some of the OH-PCBs displace thyroid hormone (T4) from TTR, rationalizing the toxicity observed in rodents, where TTR is the major T4 transporter. Thyroid binding globulin and albumin are the major T4 carriers in humans, making it unlikely that enough T4 could be displaced from TTR to be toxic. OH-PCBs are excellent TTR amyloidogenesis inhibitors in vitro because they bind to the TTR tetramer, imparting kinetic stability under amyloidogenic denaturing conditions. Four OH-PCB/TTR cocrystal structures provide further insight into inhibitor binding interactions.

The Crystal Structure of Transthyretin in Complex with Diethylstilbestrol

Transthyretin (TTR) is a homotetrameric plasma protein that, in conditions not yet completely understood, may aggregate, forming the fibrillar material associated with TTR amyloidosis. A number of reported experiments indicate that dissociation of the TTR tetramer occurs prior to fibril formation, and therefore, studies aiming at the discovery of compounds that stabilize the protein quaternary structure, thereby acting as amyloid inhibitors, are being performed. The ability of diethylstilbestrol (DES) to act as a competitive inhibitor for the thyroid hormone binding to TTR indicated a possible stabilizing effect of DES upon binding. Here we report the crystallographic study of DES binding to TTR. The structural data reveal two different binding modes, both located in the thyroxine binding channel. In both cases, DES binds deeply in the channel and establishes interactions with the equivalent molecule present in the adjacent binding site. The most remarkable features of DES interaction with TTR are its hydrophobic interactions within the protein halogen binding pockets, where its ethyl groups are snugly fitted, and the hydrogen bonds established at the center of the tetramer with Ser-117. Experiments concerning amyloid formation in vitro suggest that DES is effectively an amyloid inhibitor in acid-mediated fibrillogenesis and may be used for the design of more powerful drugs. The present study gave us further insight in the molecular mechanism by which DES competes with thyroid hormone binding to TTR and highlights key interactions between DES and TTR that oppose amyloid formation.

Anthraquinones inhibit tau aggregation and dissolve Alzheimer's paired helical filaments in vitro and in cells

The abnormal aggregation of tau protein into paired helical filaments (PHFs) is one of the hallmarks of Alzheimer's disease. Aggregation takes place in the cytoplasm and could therefore be cytotoxic for neurons. To find inhibitors of PHF aggregation we screened a library of 200,000 compounds. The hits found in the PHF inhibition assay were also tested for their ability to dissolve preformed PHFs. The results were obtained using a thioflavin S fluorescence assay for the detection and quantification of tau aggregation in solution, a tryptophan fluorescence assay using tryptophan-containing mutants of tau, and confirmed by a pelleting assay and electron microscopy of the products. Here we demonstrate the feasibility of the approach with several compounds from the family of anthraquinones, including emodin, daunorubicin, adriamycin, and others. They were able to inhibit PHF formation with IC50 values of 1-5 microm and to disassemble preformed PHFs at DC50 values of 2-4 microm. The compounds had a similar activity for PHFs made from different tau isoforms and constructs. The compounds did not interfere with the stabilization of microtubules by tau. Tau-inducible neuroblastoma cells showed the formation of tau aggregates and concomitant cytotoxicity, which could be prevented by inhibitors. Thus, small molecule inhibitors could provide a basis for the development of tools for the treatment of tau pathology in AD and other tauopathies.

Prevention of Alzheimer's Disease-associated Aβ Aggregation by Rationally Designed Nonpeptidic β-Sheet Ligands

A new concept is introduced for the rational design of beta-sheet ligands, which prevent protein aggregation. Oligomeric acylated aminopyrazoles with a donor-acceptor-donor (DAD) hydrogen bond pattern complementary to that of a beta-sheet efficiently block the solvent-exposed beta-sheet portions in Abeta-(1-40) and thereby prevent formation of insoluble protein aggregates. Density gradient centrifugation revealed that in the initial phase, the size of Abeta aggregates was efficiently kept between the trimeric and 15-meric state, whereas after 5 days an additional high molecular weight fraction appeared. With fluorescence correlation spectroscopy (FCS) exactly those two, i.e. a dimeric aminopyrazole with an oxalyl spacer and a trimeric head-to-tail connected aminopyrazole, of nine similar aminopyrazole ligands were identified as efficient aggregation retardants whose minimum energy conformations showed a perfect complementarity to a beta-sheet. The concentration dependence of the inhibitory effect of a trimeric aminopyrazole derivative allowed an estimation of the dissociation constant in the range of 10(-5) m. Finally, electrospray ionization mass spectrometry (ESI-MS) was used to determine the aggregation kinetics of Abeta-(1-40) in the absence and in the presence of the ligands. From the comparable decrease in Abeta monomer concentration, we conclude that these beta-sheet ligands do not prevent the initial oligomerization of monomeric Abeta but rather block further aggregation of spontaneously formed small oligomers. Together with the results from density gradient centrifugation and fluorescence correlation spectroscopy it is now possible to restrict the approximate size of soluble Abeta aggregates formed in the presence of both inhibitors from 3- to 15-mers.

Principles of protein folding, misfolding and aggregation

This review summarises our current understanding of the underlying and universal mechanism by which newly synthesised proteins achieve their biologically functional states. Protein molecules, however, all have a finite tendency either to misfold, or to fail to maintain their correctly folded states, under some circumstances. This article describes some of the consequences of such behaviour, particularly in the context of the aggregation events that are frequently associated with aberrant folding. It focuses in particular on the emerging links between protein aggregation and the increasingly prevalent forms of debilitating disease with which it is now known to be associated.

Role of quality control pathways in human diseases involving protein misfolding

Advances in connecting phenotype to genotype have led to new insights regarding the basis of human disease. Many inherited diseases are now known to arise due to specific mutations within a gene that then lead to a protein product unable to assume a stable conformation within the cell. Cellular machineries serving as "quality control monitors" recognize and target such abnormally folded proteins for rapid destruction. As a consequence, specific biochemical pathways requiring the protein of interest are adversely affected and lead to the disease phenotype. Yet in other cases, upon its misfolding the particular protein quickly aggregates, leading to the formation of inclusion bodies that eventually lead to cell demise. In what follows I discuss some classic examples of human diseases known to arise due to mutations that lead to altered protein folding, abnormal protein maturation and/or protein aggregation. In many cases simply altering the protein folding environment within the cell, via molecular or pharmacological approaches, can effectively rescue the maturation and stability of the mutant protein and thereby reduce the onset and/or progression of the disease phenotype. These new insights regarding the mechanisms underlying the disease phenotype, as well as new approaches to correct the protein folding defect, will undoubtedly prove to have a tremendous impact on clinical medicine.

Amyloidosis: a clinico-pathophysiological synopsis

Amyloidosis encompasses a spectrum of diseases in which there is disordered folding of certain proteins that leads to them being deposited as insoluble fibrils in the extracellular space. The result of this process is impaired tissue structure and function. Amyloidosis may be acquired or hereditary and local or systemic, and is defined according to the identity of the fibril precursor protein. Over 20 unrelated proteins can form amyloid fibrils in vivo, which all share a lamellar cross-beta-sheet structure composed of non-covalently associated protein or peptide subunits. Glycosaminoglycans and the pentraxin protein, serum amyloid P component, are universal non-fibrillar constituents of amyloid deposits that are believed to play a role in fibrillogenesis and fibril persistence. Greater understanding of the processes underlying amyloidogenesis, at all levels from cellular to clinical, has led to improvements in diagnosis, monitoring and treatment of this group of diseases, as well as pointing to possible future therapies.

Unfolding a folding disease: folding, misfolding and aggregation of the marble brain syndrome-associated mutant H107Y of human carbonic anhydrase II

Most loss-of-function diseases are caused by aberrant folding of important proteins. These proteins often misfold due to mutations. The disease marble brain syndrome (MBS), known also as carbonic anhydrase II deficiency syndrome (CADS), can manifest in carriers of point mutations in the human carbonic anhydrase II (HCA II) gene. One mutation associated with MBS entails the His107Tyr substitution. Here, we demonstrate that this mutation is a remarkably destabilizing folding mutation. The loss-of-function is clearly a folding defect, since the mutant shows 64% of CO(2) hydration activity compared to that of the wild-type at low temperature where the mutant is folded. On the contrary, its stability towards thermal and guanidine hydrochloride (GuHCl) denaturation is highly compromised. Using activity assays, CD, fluorescence, NMR, cross-linking, aggregation measurements and molecular modeling, we have mapped the properties of this remarkable mutant. Loss of enzymatic activity had a midpoint temperature of denaturation (T(m)) of 16 degrees C for the mutant compared to 55 degrees C for the wild-type protein. GuHCl-denaturation (at 4 degrees C) showed that the native state of the mutant was destabilized by 9.2kcal/mol. The mutant unfolds through at least two equilibrium intermediates; one novel intermediate that we have termed the molten globule light state and, after further denaturation, the classical molten globule state is populated. Under physiological conditions (neutral pH; 37 degrees C), the His107Tyr mutant will populate the molten globule light state, likely due to novel interactions between Tyr107 and the surroundings of the critical residue Ser29 that destabilize the native conformation. This intermediate binds the hydrophobic dye 8-anilino-1-naphthalene sulfonic acid (ANS) but not as strong as the molten globule state, and near-UV CD reveals the presence of significant tertiary structure. Notably, this intermediate is not as prone to aggregation as the classical molten globule. As a proof of concept for an intervention strategy with small molecules, we showed that binding of the CA inhibitor acetazolamide increases the stability of the native state of the mutant by 2.9kcal/mol in accordance with its strong affinity. Acetazolamide shifts the T(m) to 34 degrees C that protects from misfolding and will enable a substantial fraction of the enzyme pool to survive physiological conditions.

Amyloid Fibril Formation by a Partially Structured Intermediate State of α-Chymotrypsin

Here we investigated the effects of 2,2,2-trifluoroethanol (TFE) on the structure of alpha-chymotrypsin. The protein aggregates maximally in 35% (v/v) TFE. Congo red and thioflavin-T binding experiments suggest that the aggregates induced by TFE have amyloid-like properties, and transmission electron microscopy data show that these aggregates have a fibrilar morphology. Fluorescence, circular dichroism, anilino-8-napthalene sulfonate binding, and Fourier-transformed infrared spectroscopy data suggest that formation of a partially structured intermediate state precedes the onset of the aggregation process. The native beta-barrel structure of alpha-chymotrypsin appears to be disrupted in the partially structured intermediate state in favour of a non-native extended beta-sheet conformation with exposed hydrophobic surfaces. The protein becomes "sticky" under these conditions and aggregates into amyloid-like structures. The data support the hypothesis that amyloid formation involves the ordered self-assembly of partially folded species that are critical soluble precursors of fibrilar aggregates.

Protein misfolding in neurodegenerative diseases

A common pathogenic mechanism shared by diverse neurodegenerative disorders, like Alzheimer's disease, Parkinson's disease, Huntington's disease and transmissible spongiform encephalopathies, may be altered protein homeostasis leading to protein misfolding and aggregation of a wide variety of different proteins in the form of insoluble fibrils. Mutations in the genes encoding protein constituents of these aggregates have been linked to the corresponding diseases, thus a reasonable scenario of pathogenesis was based on misfolding of a neurone-specific protein that forms insoluble fibrils that subsequently kill neuronal cells. However, during the past 5 years accumulating evidence has revealed the neurotoxic role of prefibrillar intermediate forms (soluble oligomers and protofibrils) produced during fibril formation. Many think these may be the predominant neurotoxic species, whereas microscopically visible fibrillar aggregates may not be toxic. Large protein aggregates may rather be simply inactive, or even represent a protective state that sequesters and inactivates toxic oligomers and protofibrils. Further understanding of the biochemical mechanisms involved in protein misfolding and fibrillization may optimize the planning of common therapeutic approaches for neurodegenerative diseases, directed towards reversal of protein misfolding, blockade of protein oligomerization and interference with the action of toxic proteins.

Chemical inhibition of N-WASP by stabilization of a native autoinhibited conformation

Current drug discovery efforts focus primarily on proteins with defined enzymatic or small molecule binding sites. Autoregulatory domains represent attractive alternative targets for small molecule inhibitors because they also occur in noncatalytic proteins and because allosteric inhibitors may avoid specificity problems inherent in active site-directed inhibitors. We report here the identification of wiskostatin, a chemical inhibitor of the neural Wiskott-Aldrich syndrome protein (N-WASP). Wiskostatin interacts with a cleft in the regulatory GTPase-binding domain (GBD) of WASP in the solution structure of the complex. Wiskostatin induces folding of the isolated, unstructured GBD into its autoinhibited conformation, suggesting that wiskostatin functions by stabilizing N-WASP in its autoinhibited state. The use of small molecules to bias conformational equilibria represents a potentially general strategy for chemical inhibition of autoinhibited proteins, even in cases where such sites have not been naturally evolved in a target.

Pharmacological chaperones: potential treatment for conformational diseases

Increasing numbers of inherited diseases are found to result from mutations that lead to misfolded proteins. In many cases, the changes in conformation are relatively modest and the function of the protein would not be predicted to be affected. Yet, these proteins are recognized as "misfolded" and degraded prematurely. Recently, small molecules known as chemical and pharmacological chaperones were found to stabilize such mutant proteins and facilitate their trafficking to their site of action. Here, we review the recent published evidence suggesting that pharmacological chaperones represent promising avenues for the treatment of endocrine and metabolic diseases such as hyperinsulinemic hypoglycemia, hypogonadotropic hypogonadism and nephrogenic diabetes insipidus, and might become a general therapeutic strategy for the treatment of conformational diseases.

Conformational constraints for amyloid fibrillation: the importance of being unfolded

Recent reports give strong support to the idea that amyloid fibril formation and the subsequent development of protein deposition diseases originate from conformational changes in corresponding amyloidogenic proteins. In this review, recent findings are surveyed to illustrate that protein fibrillogenesis requires a partially folded conformation. This amyloidogenic conformation is relatively unfolded, and shares many structural properties with the pre-molten globule state, a partially folded intermediate frequently observed in the early stages of protein folding and under some equilibrium conditions. The inherent flexibility of such an intermediate is essential in allowing the conformational rearrangements necessary to form the core cross-beta structure of the amyloid fibril.

Protein s-glutathionylation in retinal pigment epithelium converts heat shock protein 70 to an active chaperone

A disulfide bond between key redox-sensitive cysteine residues and glutathione is one mechanism by which redox related allosteric effectors can regulate protein structure and function. Here we test the hypothesis that glutaredoxin-1 (Grx-1), a member of the oxidoreductase family of enzymes, may be a critical component of redox-sensitive molecular switches by mediating reversible protein S-glutathionylation and enzymatic catalysis of thiol/disulfide exchange. Deglutathionylation of a 70 kDa protein by Grx-1 was detected using a monoclonal antibody specific to protein S-glutathionylation. Heat shock cognate protein 70 (Hsc70) was identified as a substrate of Grx-1 through mass spectrometry. Recombinant Hsc70 was glutathionylated in vitro, and protein S-glutathionylation reversed by Grx-1. Glutathionylated Hsc70 was more effective in preventing luciferase aggregation at 43 degrees C than reduced Hsc70 in a dose dependent fashion. ATP did not effect the chaperone activity of Hsc70-SG but did increase the activity of reduced Hsc70-SG. Reversible glutathionylation of Hsc70 may provide a mechanism for post-translation regulation of chaperone activity.

The hereditary amyloidoses

Hereditary amyloidosis is, in general, a systemic condition related to multiple organ system involvement by beta-structured protein deposits. As such, it often mimics the more common forms of systemic amyloidosis: immunoglobulin light chain (AL, primary) and reactive (AA, secondary). The challenge diagnostically is to recognize hereditary amyloidosis as a distinct entity and then to determine the specific type of genetic disease. There are several types of hereditary amyloidosis and precise diagnosis is essential for proper therapy and genetic counselling. This chapter strives to present the subject of hereditary amyloidosis in a way which facilitates understanding of the disease, of the means for diagnosis, of the present and possible future therapies, and of the importance of combined basic and medical research.

Deposition of transthyretin amyloid is not accelerated by the same amyloid in vivo

Acceleration of amyloid deposition by administration of amyloid fibrils and transmissibility of disease have been reported in several types of amyloidoses. Families with a variant transthyretin (TTR V30M)-associated familial amyloidotic polyneuropathy (FAP) exhibit genetic anticipation, with TTR V30M-amyloid depositing at an earlier age in successive generations. The molecular bases of anticipation in FAP have remained to be determined. We asked if administration of TTR-amyloid fibrils (ATTR) extracted from the heart of an FAP TTR V30M patient would accelerate ATTR deposition in transgenic mice expressing the human mutant ttr gene responsible for FAP TTR V30M and indeed the administration did accelerate deposition of apolipoprotein A-II-amyloid fibrils (AApoAII), and not A TTR. Our experiments present, for the first time, evidence that the degree of inducibility of ATTR is low relative to AApoAII and we suggest that administration of ATTR may not explain the genetic anticipation which occurs in FAP.

Formation of amyloid aggregates from human lysozyme and its disease‐associated variants using hydrostatic pressure

Formation of amyloid deposits from the Ile56Thr or Asp67His variants of human lysozyme is a hallmark of autosomal hereditary systemic amyloidosis. It has recently been shown that amyloid fibrils can be formed in vitro from wild-type (WT), I56T, or D67H lysozyme variants upon prolonged incubation at acidic pH and elevated temperatures (1). Here, we have used hydrostatic pressure as a tool to generate amyloidogenic states of WT and variant lysozymes at physiological pH. WT or variant lysozyme samples were initially compressed to 3.5 kbar (at 57 degrees C, pH 7.4). Decompression led to the formation of amyloid fibrils, protofibrils, or globular aggregates, as indicated by light scattering, thioflavin T fluorescence, and transmission electron microscopy analysis. Increased 1-anilinonaphthalene-8-sulfonate binding to the proteins was also observed, indicating exposure of hydrophobic surface area. Thus, pressure appears to induce a conformational state of lysozyme that aggregates readily upon decompression. These results support the notion that amyloid aggregation results from the formation of partially unfolded protein conformations and suggest that pressure may be a useful tool for the generation of the amyloidogenic conformations of lysozyme and other proteins.

Native state stabilization by NSAIDs inhibits transthyretin amyloidogenesis from the most common familial disease variants

Transthyretin (TTR) tetramer dissociation and misfolding affords a monomeric amyloidogenic intermediate that misassembles into aggregates including amyloid fibrils. Amyloidogenesis of wild-type (WT) TTR causes senile systemic amyloidosis (SSA), whereas fibril formation from one of the more than 80 TTR variants leads to familial amyloidosis, typically with earlier onset than SSA. Several nonsteroidal anti-inflammatory drugs (NSAIDs) stabilize the native tetramer, strongly inhibiting TTR amyloid fibril formation in vitro. Structure-based designed NSAID analogs are even more potent amyloid inhibitors. The effectiveness of several NSAIDs, including diclofenac, diflunisal, and flufenamic acid, as well as the diclofenac analog, 2-[(3,5-dichlorophenyl) amino] benzoic acid (inhibitor 1), has been demonstrated against WT TTR amyloidogenesis. Herein, the efficacy of these compounds at preventing acid-induced fibril formation and urea-induced tetramer dissociation of the most common disease-associated TTR variants (V30M, V122I, T60A, L58H, and I84S) was evaluated. Homotetramers of these variants were employed for the studies within, realizing that the tetramers in compound heterozygote patients are normally composed of a mixture of WT and variant subunits. The most common familial TTR variants were stabilized substantially by flufenamic acid and inhibitor 1, and to a lesser extent by diflunisal, against acid-mediated fibril formation and chaotrope denaturation, suggesting that this chemotherapeutic option is viable for patients with familial transthyretin amyloidosis.

The elongation of yeast prion fibers involves separable steps of association and conversion

A self-perpetuating change in the conformation of the translation termination factor Sup35p is the basis for the prion [PSI+], a protein-based genetic element of Saccharomyces cerevisiae. In a process closely allied to in vivo conversion, the purified soluble, prion-determining region of Sup35p (NM) converts to amyloid fibers by means of nucleated conformational conversion. First, oligomeric species convert to nuclei, and these nuclei then promote polymerization of soluble protein into amyloid fibers. To elucidate the nature of the polymerization step, we created single-cysteine substitution mutants at different positions in NM to provide unique attachment sites for various probes. In vivo, the mutants behaved like wild-type protein in both the [psi-] and [PSI+] states. In vitro, they assembled with wild-type kinetics and formed fibers with the same morphologies. When labeled with fluorescent probes, two mutants, NMT158C and NME167C, exhibited a change in fluorescence coincident with amyloid assembly. These mutants provided a sensitive measure for the kinetics of fiber elongation, and the lag phase in conversion. The cysteine in the mutant NMK184C remained exposed after assembly. When labeled with biotin and bound to streptavidin beads, it was used to capture radiolabeled soluble NM in the process of conversion. This process established the existence of a detergent-susceptible intermediate in fiber elongation. Thus, the second stage of nucleated conformational conversion, fiber elongation, itself contains at least two steps: the association of soluble protein with preformed fibers to form an assembly intermediate, followed by conformational conversion into amyloid.

Tissue damage in the amyloidoses: Transthyretin monomers and nonnative oligomers are the major cytotoxic species in tissue culture

The transthyretin (TTR) amyloidoses are human diseases in which the misfolded TTR protein aggregates in tissues with subsequent visceral, peripheral, and autonomic nerve dysfunction. Recent reports have stressed the importance of oligomeric intermediates as major cytotoxic species in various forms of amyloidogenesis. We have examined the cytotoxic effects of several quaternary structural states of wild-type and variant TTR proteins on cells of neural lineage. TTR amyloid fibrils and soluble aggregates >100 kDa were not toxic. Incubation of TTR under the conditions of the cell assay and analysis by size-exclusion chromatography and SDS/PAGE reveal that monomeric TTR or relatively small, rapidly formed aggregates of a maximum size of six subunits were the major cytotoxic species. Small molecules that stabilize the native tetrameric state were shown to prevent toxicity. The studies are consistent with a model in which the misfolded TTR monomer rapidly aggregates to form transient low molecular mass assemblies (<100 kDa) that are highly cytotoxic in tissue culture.

Therapeutic approaches to protein-misfolding diseases

Several sporadic and genetic diseases are caused by protein misfolding. These include cystic fibrosis and other devastating diseases of childhood as well as Alzheimer's, Parkinson's and other debilitating maladies of the elderly. A unified view of the molecular and cellular pathogenesis of these conditions has led to the search for chemical chaperones that can slow, arrest or revert disease progression. Molecules are now emerging that link our biophysical insights with our therapeutic aspirations.

The systemic amyloidoses

PURPOSE OF REVIEW

Clinical management of the amyloidoses has historically been the province of rheumatologists, because of the relation to long-standing inflammation in rheumatoid arthritis, ankylosing spondylitis, and juvenile chronic arthritis. Currently, nephrologists, hematologist-oncologists, neurologists, and transplant surgeons all have a diagnostic or therapeutic interest. Current advances, using the tools of physical biochemistry, cell biology, and genetics, have begun to impact the diagnosis and clinical management of these disorders and raise questions regarding our notions of protein conformation in vivo and how nonnatively folded proteins may produce disease.

RECENT FINDINGS

It appears that all amyloidogenic precursors undergo some degree of misfolding that allows them to populate an immediate precursor pool from which they rapidly aggregate. Depending on the particular protein, a variety of mechanisms appear operative, some of which involve nonphysiologic proteolysis, defective physiologic proteolysis, mutations involving changes in thermodynamic or kinetic properties, and pathways that are yet to be defined. Whatever the particular process, the result is a tendency toward oligomeric aggregation followed by the assembly of higher order structures that become insoluble under physiologic conditions. Detailed analyses have been described for transthyretin (senile systemic amyloidosis and familial amyloid polyneuropathy), immunoglobulin light chains (light-chain amyloid), beta2 microglobulin (dialysis-related amyloid), and apolipoprotein A1, and are in process for others. SUMMARY Therapies have been proposed based on precursor stabilization (transthyretin), elimination of the synthesizing cell (light-chain amyloid), fibril disruption and immunization to induce host-mediated aggregate clearance (Alzheimer disease, light-chain amyloid, prions), and aggressive therapy of a primary inflammatory process (amyloid A). During the next decade, the value of these therapies, and others, suggested by studies on the basic properties of cells and proteins, will become clear.

Monitoring protein stability and aggregation in vivo by real-time fluorescent labeling

In vivo fluorescent labeling of an expressed protein has enabled the observation of its stability and aggregation directly in bacterial cells. Mammalian cellular retinoic acid-binding protein I (CRABP I) was mutated to incorporate in a surface-exposed omega loop the sequence Cys-Cys-Gly-Pro-Cys-Cys, which binds specifically to a biarsenical fluorescein dye (FlAsH). Unfolding of labeled tetra-Cys CRABP I is accompanied by enhancement of FlAsH fluorescence, which made it possible to determine the free energy of unfolding of this protein by urea titration in cells and to follow in real time the formation of inclusion bodies by a slow-folding, aggregationprone mutant (FlAsH-labeled P39A tetra-Cys CRABP I). Aggregation in vivo displayed a concentration-dependent apparent lag time similar to observations of protein aggregation in purified in vitro model systems.

Diflunisal Analogues Stabilize the Native State of Transthyretin. Potent Inhibition of Amyloidogenesis

Analogues of diflunisal, an FDA-approved nonsteroidal antiinflammatory drug (NSAID), were synthesized and evaluated as inhibitors of transthyretin (TTR) aggregation, including amyloid fibril formation. High inhibitory activity was observed for 26 of the compounds. Of those, eight exhibited excellent binding selectivity for TTR in human plasma (binding stoichiometry >0.50, with a theoretical maximum of 2.0 inhibitors bound per TTR tetramer). Biophysical studies reveal that these eight inhibitors dramatically slow tetramer dissociation (the rate-determining step of amyloidogenesis) over a duration of 168 h. This appears to be achieved through ground-state stabilization, which raises the kinetic barrier for tetramer dissociation. Kinetic stabilization of WT TTR by these eight inhibitors is further substantiated by the decreasing rate of amyloid fibril formation as a function of increasing inhibitor concentration (pH 4.4). X-ray cocrystal structures of the TTR.18(2) and TTR.20(2) complexes reveal that 18 and 20 bind in opposite orientations in the TTR binding site. Moving the fluorines from the meta positions in 18 to the ortho positions in 20 reverses the binding orientation, allowing the hydrophilic aromatic ring of 20 to orient in the outer binding pocket where the carboxylate engages in favorable electrostatic interactions with the epsilon-ammonium groups of Lys 15 and 15'. The hydrophilic aryl ring of 18 occupies the inner binding pocket, with the carboxylate positioned to hydrogen bond to the serine 117 and 117' residues. Diflunisal itself appears to occupy both orientations based on the electron density in the TTR.1(2) structure. Structure-activity relationships reveal that para-carboxylate substitution on the hydrophilic ring and dihalogen substitution on the hydrophobic ring afford the most active TTR amyloid inhibitors.

Protein aggregation and aggregate toxicity: new insights into protein folding, misfolding diseases and biological evolution

The deposition of proteins in the form of amyloid fibrils and plaques is the characteristic feature of more than 20 degenerative conditions affecting either the central nervous system or a variety of peripheral tissues. As these conditions include Alzheimer's, Parkinson's and the prion diseases, several forms of fatal systemic amyloidosis, and at least one condition associated with medical intervention (haemodialysis), they are of enormous importance in the context of present-day human health and welfare. Much remains to be learned about the mechanism by which the proteins associated with these diseases aggregate and form amyloid structures, and how the latter affect the functions of the organs with which they are associated. A great deal of information concerning these diseases has emerged, however, during the past 5 years, much of it causing a number of fundamental assumptions about the amyloid diseases to be re-examined. For example, it is now apparent that the ability to form amyloid structures is not an unusual feature of the small number of proteins associated with these diseases but is instead a general property of polypeptide chains. It has also been found recently that aggregates of proteins not associated with amyloid diseases can impair the ability of cells to function to a similar extent as aggregates of proteins linked with specific neurodegenerative conditions. Moreover, the mature amyloid fibrils or plaques appear to be substantially less toxic than the pre-fibrillar aggregates that are their precursors. The toxicity of these early aggregates appears to result from an intrinsic ability to impair fundamental cellular processes by interacting with cellular membranes, causing oxidative stress and increases in free Ca2+ that eventually lead to apoptotic or necrotic cell death. The 'new view' of these diseases also suggests that other degenerative conditions could have similar underlying origins to those of the amyloidoses. In addition, cellular protection mechanisms, such as molecular chaperones and the protein degradation machinery, appear to be crucial in the prevention of disease in normally functioning living organisms. It also suggests some intriguing new factors that could be of great significance in the evolution of biological molecules and the mechanisms that regulate their behaviour.

Synthesis and Characterization of Potent Bivalent Amyloidosis Inhibitors That Bind Prior to Transthyretin Tetramerization

The misfolding of transthyretin (TTR), including rate-limiting tetramer dissociation and partial monomer denaturation, is sufficient for TTR misassembly into amyloid and other abnormal quaternary structures associated with senile systemic amyloidosis, familial amyloid polyneuropathy, and familial amyloid cardiomyopathy. Monovalent small molecules that bind to one or both of the unoccupied thyroid hormone binding sites at the TTR quaternary structure interface stabilize the native state, raising the kinetic barrier for tetramer dissociation sufficiently that the rate of dissociation, and therefore amyloidosis, becomes slow. Bivalent amyloid inhibitors that bind to both binding sites simultaneously are reported herein. The candidate bivalent inhibitors are generally unable to bind to the native TTR tetramer and typically do not engage in monovalent binding owing to a strong inhibitor orientation preference. However, the TTR quaternary structure can assemble around several of the bivalent inhibitors if the inhibitor intercepts the protein before assembly occurs. Some of the wild-type TTR.bivalent inhibitor complexes prepared in this fashion retain a tetrameric structure when subjected to substantial denaturation stresses (8 M urea, 120 h). The best bivalent inhibitor reduced acid-mediated TTR (3.6 microM) amyloid fibril formation to 6% of that exhibited by TTR in the absence of inhibitor, a significant improvement over the approximately 30% observed for the best monovalent inhibitors (3.6 microM, 72 h). The apparent dissociation rate of the best bivalent inhibitor is effectively zero, consistent with the idea that TTR tetramer dissociation and inhibitor dissociation are linked-as a result of the inhibitor-templating tetramer assembly. X-ray cocrystal structures of two of the complexes demonstrate that the bivalent inhibitors simultaneously occupy both sites in TTR, consistent with the 1:1 binding stoichiometry derived from HPLC analysis. The purpose of this study was to demonstrate that bivalent inhibitors could be useful; what resulted are the best inhibitors produced to date. In this context, molecules capable of intercepting TTR during folding and assembly in the lumen of the endoplasmic reticulum would be of obvious interest.

Rapid unfolding of a domain populates an aggregation-prone intermediate that can be recognized by GroEL

Some amino acid substitutions in phage P22 coat protein cause a temperature-sensitive folding (tsf) phenotype. In vivo, these tsf amino acid substitutions cause coat protein to aggregate and form intracellular inclusion bodies when folded at high temperatures, but at low temperatures the proteins fold properly. Here the effects of tsf amino acid substitutions on folding and unfolding kinetics and the stability of coat protein in vitro have been investigated to determine how the substitutions change the ability of coat protein to fold properly. The equilibrium unfolding transitions of the tsf variants were best fit to a three-state model, N if I if U, where all species concerned were monomeric, a result confirmed by velocity sedimentation analytical ultracentrifugation. The primary effect of the tsf amino acid substitutions on the equilibrium unfolding pathway was to decrease the stability (DeltaG) and the solvent accessibility (m-value) of the N if I transition. The kinetics of folding and unfolding of the tsf coat proteins were investigated using tryptophan fluorescence and circular dichroism (CD) at 222 nm. The tsf amino acid substitutions increased the rate of unfolding by 8-14-fold, with little effect on the rate of folding, when monitored by tryptophan fluorescence. In contrast, when folding or unfolding reactions were monitored by CD, the reactions were too fast to be observed. The tsf coat proteins are natural substrates for the molecular chaperones, GroEL/S. When native tsf coat protein monomers were incubated with GroEL, they bound efficiently, indicating that a folding intermediate was significantly populated even without denaturant. Thus, the tsf coat proteins aggregate in vivo because of an increased propensity to populate this unfolding intermediate.

A camelid antibody fragment inhibits the formation of amyloid fibrils by human lysozyme

Amyloid diseases are characterized by an aberrant assembly of a specific protein or protein fragment into fibrils and plaques that are deposited in various organs and tissues, often with serious pathological consequences. Non-neuropathic systemic amyloidosis is associated with single point mutations in the gene coding for human lysozyme. Here we report that a single-domain fragment of a camelid antibody raised against wild-type human lysozyme inhibits the in vitro aggregation of its amyloidogenic variant, D67H. Our structural studies reveal that the epitope includes neither the site of mutation nor most residues in the region of the protein structure that is destabilized by the mutation. Instead, the binding of the antibody fragment achieves its effect by restoring the structural cooperativity characteristic of the wild-type protein. This appears to occur at least in part through the transmission of long-range conformational effects to the interface between the two structural domains of the protein. Thus, reducing the ability of an amyloidogenic protein to form partly unfolded species can be an effective method of preventing its aggregation, suggesting approaches to the rational design of therapeutic agents directed against protein deposition diseases.

Rationalization of the effects of mutations on peptide and protein aggregation rates

In order for any biological system to function effectively, it is essential to avoid the inherent tendency of proteins to aggregate and form potentially harmful deposits. In each of the various pathological conditions associated with protein deposition, such as Alzheimer's and Parkinson's diseases, a specific peptide or protein that is normally soluble is deposited as insoluble aggregates generally referred to as amyloid. It is clear that the aggregation process is generally initiated from partially or completely unfolded forms of the peptides and proteins associated with each disease. Here we show that the intrinsic effects of specific mutations on the rates of aggregation of unfolded polypeptide chains can be correlated to a remarkable extent with changes in simple physicochemical properties such as hydrophobicity, secondary structure propensity and charge. This approach allows the pathogenic effects of mutations associated with known familial forms of protein deposition diseases to be rationalized, and more generally enables prediction of the effects of mutations on the aggregation propensity of any polypeptide chain.

Design strategies for anti-amyloid agents

Numerous diseases have been linked to a common pathogenic process called amyloidosis, whereby proteins or peptides clump together in the brain or body to form toxic soluble oligomers and/or insoluble fibres. An attractive strategy to develop therapies for these diseases is therefore to inhibit or reverse protein/peptide aggregation. A diverse range of small organic ligands have been found to act as aggregation inhibitors. Alternatively, the wild-type peptide can be derivatised so that it still binds to the amyloid target, but prevents further aggregation. This can be achieved by adding a bulky group or charged amino acid to either end of the peptide, or by incorporating proline residues or N-methylated amide groups.

Role of the N and C-terminal strands of beta 2-microglobulin in amyloid formation at neutral pH

Beta 2-microglobulin (beta(2)m) is known to form amyloid fibrils de novo in vitro under acidic conditions (below pH 4.8). Fibril formation at neutral pH, however, has only been observed by deletion of the N-terminal six residues; by the addition of pre-assembled seeds; or in the presence of Cu(2+). Based on these observations, and other structural data, models for fibril formation of beta(2)m have been proposed that involve the fraying of the N and C-terminal beta-strands and the consequent loss of edge strand protective features. Here, we examine the role of the N and C-terminal strands in the initiation of fibrillogenesis of beta(2)m by creating point mutations in strands A and G and comparing the properties of the resulting proteins with variants containing similar mutations elsewhere in the protein. We show that truncation of buried hydrophobic side-chains in strands A and G promotes rapid fibril formation at neutral pH, even in unseeded reactions, and increases the rate of fibril formation under acidic conditions. By contrast, similar mutations created in the remaining seven beta-strands of the native protein have little effect on the rate or pH dependence of fibril formation. The data are consistent with the view that perturbation of the N and C-terminal edge strands is an important feature in the generation of assembly-competent states of beta(2)m.