A beta deposition inhibitor screen using synthetic amyloid

Accelerating protein aggregation and amyloid fibrillation for rapid inhibitor screening

The accumulation and deposition of amyloid fibrils, also known as amyloidosis, in tissues and organs of patients has been found to be linked to numerous devastating neurodegenerative diseases. The aggregation of proteins to form amyloid fibrils, however, is a slow pathogenic process, and is a major issue for the evaluation of the effectiveness of inhibitors in new drug discovery and screening. Here, we used microdroplet reaction technology to accelerate the amyloid fibrillation process, monitored the process to shed light on the fundamental mechanism of amyloid self-assembly, and demonstrated the value of the technology in the rapid screening of potential inhibitor drugs. Proteins in microdroplets accelerated to form fibrils in milliseconds, enabling an entire cycle of inhibitor screening for Aβ40 within 3 minutes. The technology would be of broad interest to drug discovery and therapeutic design to develop treatments for diseases associated with protein aggregation and fibrillation.

Alzheimer's Disease Model System Using Drosophila

Alzheimer's disease (AD) is the most epidemic neuronal dysfunctions among elderly people. It is accompanied by neuronal disorders along with learning and memory defects, as well as massive neurodegeneration phenotype. The presence of intracellular neurofibrillary tangles (NFTs) and extracellular amyloid plaques, called senile plaques (SPs), and brain atrophy are typically observed in the brains of AD patients. It has been over 20 years since the discovery that small peptide, called beta-amyloid (Aβ), has pivotal role for the disease formation. Since then, a variety of drugs have been developed to cure AD; however, there is currently no effective drug for the disorder. This therapeutic void reflects lacks of ideal model system, which can evaluate the progression of AD in a short period. Recently, large numbers of AD model system have been established using Drosophila melanogaster by overproducing Aβ molecules in the brain. These systems successfully reflect some of the symptoms along with AD. In this review, we would like to point out "pros and cons" of Drosophila AD models.

Phytomedicines as potential inhibitors of β amyloid aggregation: significance to Alzheimer's disease

Throughout the history of drug development, plants have been an important source for the discovery of novel therapeutically active compounds for many diseases. The ethnopharmacological approach has provided several leads to identify potential new drugs from plant sources, including those for memory disorders. For the treatment of Alzheimer's disease the drug discovery focus shifted from cholinesterase inhibitors, to other targets primarily based on two key neuropathological hallmarks, namely the hyperphosphorylation of the tau protein resulting in the formation of neurofibrillary tangles (NFTs), and the increased formation and aggregation of amyloid-beta peptide (Aβ) derived from amyloid precursor protein (APP). The present article aims to provide a comprehensive literature survey of plants and their constituents that have been tested for Aβ aggregation, thus possibly relieving several features of Alzheimer's disease (AD).

Anti-amyloidogenic properties of some phenolic compounds

A family of 21 polyphenolic compounds consisting of those found naturally in danshen and their analogues were synthesized and subsequently screened for their anti-amyloidogenic activity against the amyloid beta peptide (Aβ₄₂) of Alzheimer’s disease. After 24 h incubation with Aβ₄₂, five compounds reduced thioflavin T (ThT) fluorescence, indicative of their anti-amyloidogenic propensity (p < 0.001). TEM and immunoblotting analysis also showed that selected compounds were capable of hindering fibril formation even after prolonged incubations. These compounds were also capable of rescuing the yeast cells from toxic changes induced by the chemically synthesized Aβ₄₂. In a second assay, a Saccharomyces cerevisiae AHP1 deletant strain transformed with GFP fused to Aβ₄₂ was treated with these compounds and analyzed by flow cytometry. There was a significant reduction in the green fluorescence intensity associated with 14 compounds. We interpret this result to mean that the compounds had an anti-amyloid-aggregation propensity in the yeast and GFP-Aβ₄₂ was removed by proteolysis. The position and not the number of hydroxyl groups on the aromatic ring was found to be the most important determinant for the anti-amyloidogenic properties.

A plant cell-based system that predicts aβ42 misfolding: potential as a drug discovery tool for Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by the accumulation of amyloid β (Aβ) peptides and the failure of mechanisms to clear toxic aggregates. The Aβ42 peptide is considered to be a causative factor that underlies the pathophysiology of AD, in part due to its propensity for misfolding and aggregation; the small oligomers that result represent toxic species. Thus agents that prevent Aβ42 misfolding/aggregation or, alternatively improve Aβ42 oligomer clearance, may have significant therapeutic value. We have developed the basis for a drug screening system based on transgenic plant cells that express Aβ42 fusion proteins to serve as the reliable indicators of the general conformational status of Aβ42. Within cells of transgenic tobacco and Nicotiana benthamiana, misfolding of Aβ42 causes the misfolding of a GFP fusion partner, and consequently there is a loss of fluorescence associated with the native GFP protein. In a similar fusion consisting of Aβ42 linked to hygromycin phosphotransferase II (Hpt II), a hygromycin-resistance marker, misfolding of Aβ42 leads to a misfolded Hpt II, and consequently the transgenic cells are unable to grow on media containing hygromycin. Importantly, substitution of the 'aggregation-prone' Aβ42 with a missense mutant of Aβ42 (F19S/L34F) that is not prone to misfolding/aggregation, 'rescues' both fusion partners. Several 'positive control' chemicals that represent inhibitors of Aβ42 aggregation, including curcumin, epigallocatechin-3-gallate (EGCG), and resveratrol show efficacy in preventing the Aβ42-fusion proteins from misfolding/aggregating in the transgenic plant cells. We discuss the potential of the two fusion protein systems to serve as the basis for an inexpensive, selective, and efficient screening system in which a plant cell can fluoresce or survive only in the presence of drug candidates that are able to prevent Aβ42 misfolding/aggregation.

A survey of peptides with effective therapeutic potential in Alzheimer's disease rodent models or in human clinical studies

Alzheimer’s disease (AD) is a devastating neurodegenerative disorder and the most common cause of dementia. Today, only palliative therapies are available. The pathological hallmarks of AD are the presence of neurofibrillary tangles and amyloid plaques, mainly composed of the amyloid-β peptide (Aβ), in the brains of the patients. Several lines of evidence suggest that the increased production and/or decreased cleavage of Aβ and subsequent accumulation of Aβ oligomers and aggregates play a fundamental role in the disease progress. Therefore, substances which bind to Aβ and influence aggregation thereof are of great interest. A wide range of Aβ binding peptides were investigated to date for therapeutic purposes. Only very few were shown to be effective in rodent AD models or in clinical studies. Here, we review those peptides and discuss their possible mechanisms of action.

Congo red, an amyloid-inhibiting compound, alleviates various types of cellular dysfunction triggered by mutant protein kinase cγ that causes spinocerebellar ataxia type 14 (SCA14) by inhibiting oligomerization and aggregation

Several missense mutations in the protein kinase Cγ (γPKC) gene have been found to cause spinocerebellar ataxia type 14 (SCA14), an autosomal dominant neurodegenerative disease. We previously demonstrated that the mutant γPKC found in SCA14 is susceptible to aggregation that induces apoptotic cell death. Congo red is widely used as a histological dye for amyloid detection. Recent evidence has revealed that Congo red has the property to inhibit amyloid oligomers and fibril formation of misfolded proteins. In the present study, we examine whether Congo red inhibits aggregate formation and cytotoxicity of mutant γPKC. Congo red likely inhibits aggregate formation of mutant γPKC – green fluorescent protein (GFP) without affecting its expression level in SH-SY5Y cells. Congo red counteracts the insolubilization of recombinant mutant γPKC, suggesting that the dye inhibits aggregation of mutant γPKC by a direct mechanism. Congo red also inhibits aggregation and oligomerization of mutant γPKC-GFP in primary cultured cerebellar Purkinje cells. Moreover, the dye reverses the improper development of dendrites and inhibits apoptotic cell death in Purkinje cells that express mutant γPKC-GFP. These results indicate that amyloid-inhibiting compounds like Congo red may be novel therapeutics for SCA14.

Small molecule microarrays enable the discovery of compounds that bind the Alzheimer's Aβ peptide and reduce its cytotoxicity

The amyloid-β (Aβ) aggregation pathway is a key target in efforts to discover therapeutics that prevent or delay the onset of Alzheimer's disease. Efforts at rational drug design, however, are hampered by uncertainties about the precise nature of the toxic aggregate. In contrast, high-throughput screening of compound libraries does not require a detailed understanding of the structure of the toxic species, and can provide an unbiased method for the discovery of small molecules that may lead to effective therapeutics. Here, we show that small molecule microarrays (SMMs) represent a particularly promising tool for identifying compounds that bind the Aβ peptide. Microarray slides with thousands of compounds immobilized on their surface were screened for binding to fluorescently labeled Aβ. Seventy-nine compounds were identified by the SMM screen, and then assayed for their ability to inhibit the Aβ-induced killing of PC12 cells. Further experiments focused on exploring the mechanism of rescue for one of these compounds: Electron microscopy and Congo red binding showed that the compound enhances fibril formation, and suggest that it may rescue cells by accelerating Aβ aggregation past an early toxic oligomer. These findings demonstrate that the SMM screen for binding to Aβ is effective at identifying compounds that reduce Aβ toxicity, and can reveal potential therapeutic leads without the biases inherent in methods that focus on inhibitors of aggregation.

Inhibition of Abeta42 aggregation using peptides selected from combinatorial libraries

Increasing evidence suggests that the aggregation of the small peptide Abeta42 plays an important role in the development of Alzheimer's disease. Inhibiting the initial aggregation of Abeta42 may be an effective treatment for preventing, or slowing, the onset of the disease. Using an in vivo screen based on the enzyme EGFP, we have searched through two combinatorially diverse peptide libraries to identify peptides capable of inhibiting Abeta42 aggregation. From this initial screen, three candidate peptides were selected and characterized. ThT studies indicated that the selected peptides were capable of inhibiting amyloid aggregation. Additional ThT studies showed that one of the selected peptides was capable of disaggregating preformed Abeta42 fibers.

Discovery of amyloid-beta aggregation inhibitors using an engineered assay for intracellular protein folding and solubility

Genetic and biochemical studies suggest that Alzheimer's disease (AD) is caused by a series of events initiated by the production and subsequent aggregation of the Alzheimer's amyloid beta peptide (Abeta), the so-called amyloid cascade hypothesis. Thus, a logical approach to treating AD is the development of small molecule inhibitors that either block the proteases that generate Abeta from its precursor (beta- and gamma-secretases) or interrupt and/or reverse Abeta aggregation. To identify potent inhibitors of Abeta aggregation, we have developed a high-throughput screen based on an earlier selection that effectively paired the folding quality control feature of the Escherichia coli Tat protein export system with aggregation of the 42-residue AD pathogenesis effecter Abeta42. Specifically, a tripartite fusion between the Tat-dependent export signal ssTorA, the Abeta42 peptide and the beta-lactamase (Bla) reporter enzyme was found to be export incompetent due to aggregation of the Abeta42 moiety. Here, we reasoned that small, cell-permeable molecules that inhibited Abeta42 aggregation would render the ssTorA-Abeta42-Bla chimera competent for Tat export to the periplasm where Bla is active against beta-lactam antibiotics such as ampicillin. Using a fluorescence-based version of our assay, we screened a library of triazine derivatives and isolated four nontoxic, cell-permeable compounds that promoted efficient Tat-dependent export of ssTorA-Abeta42-Bla. Each of these was subsequently shown to be a bona fide inhibitor of Abeta42 aggregation using a standard thioflavin T fibrillization assay, thereby highlighting the utility of our bacterial assay as a useful screen for antiaggregation factors under physiological conditions.

Alginate-cellulose sulphate-oligocation microcapsules: Optimization of mass transport and mechanical properties

Microcapsules based on polyelectrolyte complexation, where the inner phase involves a blend of alginate and sodium cellulose sulphate (SCS), have mechanical and transport properties which are relatively insensitive to the chemical composition of the rigid polyanion. Specifically, the bursting force of 400- and 1000 microm microcapsules increase slightly with the degree of substitution of the SCS, though the molar mass of the SCS appears to influence the transport properties more strongly than its composition. The concentration of the sodium chloride in the gelling batch can be varied rather extensively, with optimum properties at approximately half (i.e. 0.5 M) the level typically employed for the formation of cell-containing microcapsules. This indicates that the microcapsule properties can be tuned for biocompatability, without concern that changes to the polymer microstructure or reaction process conditions would adversely influence the bursting force or molar mass cut-off of the capsules. The alginate-SCS blend, which is typical equimass, can be slightly increased in favour of the SCS (to 55 wt%) if one seeks to mechanically optimize the system. The substitution of the oligocation polymethylene-co-guanidine with pDADMAC seems strongly undesirable. Similarly, the replacement of SCS with sulphoethylcellulose, while possible, offers no important advantages. The overall optimum conditions appear to be for a SCS with a DS of 2, prepared at 1.2 wt% of total cation with alginate. The ideal ratio, for mechanical and transport properties, of SCS to alginate is 55:45 (wt:wt), which represents a subtle modification from the classical formulation with very good biocompatability.

Small molecule inhibitors of Abeta assembly

The Abeta peptide assembles into a variety of distinct types of structures in vitro and in the brain which have different biological consequences. Differential effects of inhibitory small molecules suggest that a sequential monomer - oligomer - fibril mechanism is overly simplistic and that soluble toxic oligomers and fibrils can be formed in common or separate pathways depending on the local environment. As a result, the effects of inhibitors are often assay-dependent because multiple pathways are operating. This review discusses strategies for teasing apart the intricate protein-protein interactions that result in Abeta assembly.

Small Molecule Inhibitors of Aggregation Indicate That Amyloid β Oligomerization and Fibrillization Pathways Are Independent and Distinct

Alzheimer disease is characterized by the abnormal aggregation of amyloid beta peptide into extracellular fibrillar deposits known as amyloid plaques. Soluble oligomers have been observed at early time points preceding fibril formation, and these oligomers have been implicated as the primary pathological species rather than the mature fibrils. A significant issue that remains to be resolved is whether amyloid oligomers are an obligate intermediate on the pathway to fibril formation or represent an alternate assembly pathway that may or may not lead to fiber formation. To determine whether amyloid beta oligomers are obligate intermediates in the fibrillization pathway, we characterized the mechanism of action of amyloid beta aggregation inhibitors in terms of oligomer and fibril formation. Based on their effects, the small molecules segregated into three distinct classes: compounds that inhibit oligomerization but not fibrillization, compounds that inhibit fibrillization but not oligomerization, and compounds that inhibit both. Several compounds selectively inhibited oligomerization at substoichiometric concentrations relative to amyloid beta monomer, with some active in the low nanomolar range. These results indicate that oligomers are not an obligate intermediate in the fibril formation pathway. In addition, these data suggest that small molecule inhibitors are useful for clarifying the mechanisms underlying protein aggregation and may represent potential therapeutic agents that target fundamental disease mechanisms.

A high-throughput screen for compounds that inhibit aggregation of the Alzheimer's peptide

Aggregation of the Alzheimer's peptide Abeta produces toxic multimeric species that play a key role in the development of Alzheimer's disease. Compounds that inhibit this aggregation may prove useful as therapeutic agents for the prevention or treatment of Alzheimer's disease. Although aggregation inhibitors may already exist in combinatorial libraries, finding these compounds in a cost-effective high-throughput manner poses an enormous challenge. To meet this challenge, we have developed a novel high-throughput screen capable of isolating inhibitors of Abeta aggregation from large libraries of inactive candidates. The screen uses a fusion of Abeta42 to GFP. In the absence of inhibition, the rapid misfolding and aggregation of Abeta42 causes the entire fusion protein to misfold, thereby preventing fluorescence. Compounds that inhibit Abeta42 aggregation enable GFP to fold into its native structure and be identified by the resulting fluorescent signal. By implementing the screen on a pilot library of triazine derivatives, we have identified several putative inhibitors. One of the selected compounds was studied in detail by a series of biochemical and biophysical methods. These studies confirmed that the selected compound inhibits aggregation of synthetic Abeta42 peptide. The fluorescence-based method described here is rapid and inexpensive and can be used to screen large libraries for inhibitors of Abeta42 aggregation and/or amyloidogenesis.

Ex situ atomic force microscopy analysis of beta-amyloid self-assembly and deposition on a synthetic template

The beta-amyloid (Abeta) deposition, which is the conversion of soluble Abeta peptides to insoluble plaques on a surface, is an essential pathological process in Alzheimer's disease (AD). The identification and characterization of possible environmental factors that may influence amyloid deposition in vivo are important to unveil the underlying etiology of AD. According to the amyloid cascade hypothesis, diffuse plaques are initial and visual deposits in the early event of AD, leading to amyloid plaques. To study amyloid deposition and growth in vitro, we prepared a synthetic template by immobilizing Abeta seeds on an N-hydroxysuccinimide ester-activated solid surface. According to our analysis with an ex situ atomic force microscope, the formation of amyloid plaque-like aggregates was mediated by the interaction between Abeta in a solution and on a synthetic template, suggesting that Abeta oligomers function well as seeds for amyloid deposition. It was observed that insoluble amyloid aggregates formed on the template surface serve as a sink of soluble Abeta in a solution as well as mediate the formation of intermediates in the pathway of amyloid fibrillization in a solution. Relative seeding efficiencies of fresh monomers, oligomers, and fully grown fibrils were analyzed by measuring the deposited plaque volume and its height distribution through atomic force microscopy. The result revealed that oligomeric forms of Abeta act more efficiently as seeds than monomers or fibrils do. Fluorescence spectroscopy with thioflavin T confirmed that amyloid aggregate formation proceeds in a concentration-dependent manner. Analysis with Fourier transform infrared spectroscopy indicated a progressive transition of soluble Abeta42 monomer to amyloid fibrils having antiparallel beta-sheet structure on the template. Furthermore, studies on the interaction between Abeta40 and 42, two major variants of Abeta derived from the amyloid precursor protein, showed that amyloid aggregate formation on the surface was accelerated further by the homogeneous association of soluble Abeta42 onto Abeta42 seeds than by other combinations. A slightly acidic condition was found to be unfavorable for amyloid formation. This study gives insight into understanding the effects of environmental factors on amyloid formation via the use of a synthetic template system.

Structural Differences in Aβ Amyloid Protofibrils and Fibrils Mapped by Hydrogen Exchange – Mass Spectrometry with On-line Proteolytic Fragmentation

We report here structural differences between Abeta(1-40) protofibrils and mature amyloid fibrils associated with Alzheimer's disease as determined using hydrogen-deuterium exchange-mass spectrometry (HX-MS) coupled with on-line proteolysis. Specifically, we have identified regions of the Abeta(1-40) peptide containing backbone amide hydrogen atoms that are protected from HX or exposed when this peptide is incorporated into protofibrils or amyloid fibrils formed in phosphate-buffered saline without stirring at 37 degrees C. Study of protofibrils was facilitated by use of the protofibril-stabilizing agent calmidazolium chloride. Our data clearly show that both the C-terminal segment 35-40 and the N-terminal segment 1-19 are highly exposed to HX in both fibrils and protofibrils. In contrast, the internal fragment 20-34 is highly protected from exchange in fibrils but much less so in protofibrils. The data suggest that the beta-sheet elements comprising the amyloid fibril are already present in protofibrils, but that they are expanded into some adjacent residues upon the formation of mature amyloid. The N-terminal approximately ten residues appear to be unstructured in both protofibrils and fibrils. The 20-30 segment of Abeta(1-40) is more ordered in fibrils than in protofibrils, suggesting that, if protofibrils are a mechanistic precursor of fibrils, the transition from protofibril to fibril involves substantial ordering of this region of the Abeta peptide.

Influence of dendrimer’s structure on its activity against amyloid fibril formation

Inhibition of fibril assembly is a potential therapeutic strategy in neurodegenerative disorders such as prion and Alzheimer's diseases. Highly branched, globular polymers-dendrimers-are novel promising inhibitors of fibril formation. In this study, the effect of polyamidoamine (PAMAM) dendrimers (generations 3rd, 4th, and 5th) on amyloid aggregation of the prion peptide PrP 185-208 and the Alzheimer's peptide Abeta 1-28 was examined. Amyloid fibrils were produced in vitro and their formation was monitored using the dye thioflavin T (ThT). Fluorescence studies were complemented with electron microscopy. The results show that the higher the dendrimer generation, the larger the degree of inhibition of the amyloid aggregation process and the more effective are dendrimers in disrupting the already existing fibrils. A hypothesis on dendrimer-peptide interaction mechanism is presented based on the dendrimers' molecular structure.

Kinetics and Thermodynamics of Amyloid Assembly Using a High‐Performance Liquid Chromatography–Based Sedimentation Assay

Nonnative protein aggregation has been classically treated as an amorphous process occurring by colloidal coagulation kinetics and proceeding to an essentially irreversible endpoint often ascribed to a chaotic tangle of unfolded chains. However, some nonnative aggregates, particularly amyloid fibrils, exhibit ordered structures that appear to assemble according to ordered mechanisms. Some of these fibrils, as illustrated here with the Alzheimer's plaque peptide amyloid beta, assemble to an endpoint that is a dynamic equilibrium between monomers and fibrils exhibiting a characteristic equilibrium constant with an associated free energy of formation. Some fibrils, as illustrated here with the polyglutamine repeat sequences associated with Huntington's disease, assemble via highly regular mechanisms exhibiting nucleated growth polymerization kinetics. Here, we describe a series of linked methods for quantitative analysis of such aggregation kinetics and thermodynamics, focusing on a robust high-performance liquid chromatography (HPLC)-based sedimentation assay. An integrated group of protocols is provided for peptide disaggregation, setting up the HPLC sedimentation assay, the preparation of fibril seed stocks and determination of the average functional molecular weight of the fibrils, elongation and nucleation kinetics analysis, and the determination of the critical concentration describing the thermodynamic endpoint of fibril elongation.

Selection of peptides binding to the amyloid b-protein reveals potential inhibitors of amyloid formation

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by extracellular amyloid plaques, cerebrovascular amyloid deposits, intracellular neurofibrillary tangles, and neuronal loss. Amyloid deposits are composed of insoluble fibers of a 39-43 amino acid peptide named the amyloid beta-protein (A beta). Neuropathological and genetic studies provide strong evidence of a key role for A beta amyloidosis in the pathogenesis of AD. Therefore, an obvious pharmacological target for treatment of AD is the inhibition of amyloid growth and/or inhibition of amyloid function. We took an unbiased approach to generate new inhibitors of amyloid formation by screening a FliTrx combinatorial peptide library for A beta binding peptides and identified four groups of peptides with different A beta binding motifs. In addition, we designed and examined peptides mimicking the A beta binding domain of transthyretin (TTR). Our results showed that A beta binding peptides selected from FliTrx peptide library and from TTR-peptide analogs are capable of inhibiting A beta aggregation and A beta deposition in vitro. These properties demonstrate that binding of selected peptides to the amyloid beta-protein may provide potent therapeutic compounds for the treatment AD.

polyglutamine aggregation nucleation: thermodynamics of a highly unfavorable protein folding reaction

Polyglutamine (polyGln) aggregation is implicated in the disease progression of Huntington's disease and other expanded CAG repeat diseases. PolyGln aggregation in vitro follows a simple nucleated growth polymerization pathway without apparent complications such as populated intermediates, alternative assembly pathways, or secondary nucleation phenomena. Previous analysis of the aggregation of simple polyGln peptides revealed that the critical nucleus (the number of monomeric units involved in the formation of an energetically unfavorable aggregation nucleus) is equal to one, suggesting that polyGln nucleation can be viewed as an unfavorable protein folding reaction. We provide here a method for experimentally determining the number of elongation growth sites per unit weight for any polyGln aggregate preparation, a key parameter required for completion of the nucleation kinetics analysis and determination of the thermodynamics of nucleation. We find that, for the polyGln peptide Q(47), the second-order rate constant for fibril elongation is 11,400 liters/mol per s, whereas K(n*)), the equilibrium constant for nucleation of aggregation, is remarkably small, equal to 2.6 x 10(-9). The latter value corresponds to a free energy of nucleus formation of +12.2 kcal/mol, a value consistent with a highly unfavorable folding reaction. The methods introduced here should allow further analysis of the energetics of polyGln nucleus formation and accurate comparisons of the seeding capabilities of different fibril preparations, a task of increasing importance in the amyloid field.

Template-directed self-assembly and growth of insulin amyloid fibrils

The formation of amyloid aggregates in tissue is a pathological feature of many neurodegenerative diseases and type II diabetes. Amyloid deposition, the process of amyloid growth by the association of individual soluble amyloid molecules with a pre-existing amyloid template (i.e., plaque), is known to be critical for amyloid formation in vivo. The requirement for a natural amyloid template, however, has made amyloid deposition study difficult and cumbersome. In the present work, we developed a novel, synthetic amyloid template by attaching amyloid seeds covalently onto an N-hydroxysuccinimide-activated surface, where insulin was chosen as a model amyloidogenic protein. According to ex situ atomic force microscopy observations, insulin monomers in solution were deposited onto the synthetic amyloid template to form fibrils, like hair growth. The fibril formation on the template occurred without lag time, and its rate was highly accelerated than in the solution. The fibrils were long, over 2 mum, and much thinner than those in the solution, which was caused by limited nucleation sites on the template surface and lack of lateral twisting between fibrils. According to our investigations using thioflavin T-induced fluorescence, birefringent Congo red binding, and circular dichroism, fibrils grown on the template were identified to be amyloids that formed through a conformational rearrangement of insulin monomers upon interaction with the template. The amyloid deposition rate followed saturation kinetics with respect to insulin concentration in the solution. The characteristics of amyloid deposition on the synthetic template were in agreement with previous studies performed with human amyloid plaques. It is demonstrated that the synthetic amyloid template can be used for the screening of inhibitors on amyloid deposition in vitro.

Insulysin: an allosteric enzyme as a target for Alzheimer's disease

That the zinc metalloendopeptidase insulysin (insulin-degrading enzyme IDE) is a major b-amyloid (A(beta)) peptide-degrading enzyme in vivo is shown by the higher A(beta) peptide levels in the brain of an insulysin-deficient mouse. Insulysin was shown to initially cleave A(beta)1-40and A(beta)1-42 at His13-Gln14, His14-Gln15, and Phe19-Phe20. The insulysin-dependent cleavage of A(beta) prevents both the neurotoxic effects of the peptide as well as the ability of A(beta) to deposit onto synthetic amyloid plaques. The kinetics of the reaction of insulysin with the synthetic peptide substrate Abz-G-G-F-L-R-K-H-G-Q-EDDnp displays allosteric properties indicative of a regulated enzyme. Small peptide substrates increase the activity of insulysin toward the hydrolysis of A(beta)1-40 without affecting the activity of the enzyme toward insulin. These studies indicate that insulysin is a target for drug development in which small-molecule peptide analogs can be used to increase the rate of A(beta) clearance without affecting insulin levels.

Controlling {beta}-amyloid oligomerization by the use of naphthalene sulfonates: trapping low molecular weight oligomeric species

Aggregation of proteins and peptides has been shown to be responsible for several diseases known as amyloidoses, which include Alzheimer disease (AD), prion diseases, among several others. AD is a neurodegenerative disorder caused primarily by the aggregation of beta-amyloid peptide (Abeta). Here we describe the stabilization of small oligomers of Abeta by the use of sulfonated hydrophobic molecules such as AMNS (1-amino-5-naphthalene sulfonate); 1,8-ANS (1-anilinonaphthalene-8-sulfonate) and bis-ANS (4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfonate). The experiments were performed with either Abeta-1-42 or with Abeta-13-23, a shorter version of Abeta that is still able to form amyloid fibrils in vitro and contains amino acid residues 16-20, previously shown to be essential to peptide-peptide interaction and fibril formation. All sulfonated molecules tested were able to prevent Abeta aggregation in a concentration dependent fashion in the following order of efficacy: 1,8-ANS < AMNS < bis-ANS. Size exclusion chromatography revealed that in the presence of bis-ANS, Abeta forms a heterogeneous population of low molecular weight species that proved to be toxic to cell cultures. Since the ANS compounds all have apolar rings and negative charges (sulfonate groups), both hydrophobic and electrostatic interactions may contribute to interpeptide contacts that lead to aggregation. We also performed NMR experiments to investigate the structure of Abeta-13-23 in SDS micelles and found features of an alpha-helix from Lys(16) to Phe(20). 1H TOCSY spectra of Abeta-13-23 in the presence of AMNS displayed a chemical-shift dispersion quite similar to that observed in SDS, which suggests that in the presence of AMNS this peptide might adopt a conformation similar to that reported in the presence of SDS. Taken together, our studies provide evidence for the crucial role of small oligomers and their stabilization by sulfonate hydrophobic compounds.

Parenteral antioxidant treatment preserves temporal discrimination following intrahippocampal aggregated Aβ(1–42) injections

There is evidence that oxidative stress may play a role in the neuropathology of Alzheimer's disease (AD). This study used an aggregated beta-amyloid (Abeta) injection model of AD in the rat, and a recycling conjunctive schedule of food reinforcement to examine the effects of bilateral intrahippocampal injections of aggregated Abeta(1-42) (5.0 microl/side) on temporal discrimination, and the efficacy of the antioxidant alpha-tocopherol (150 mg/kg daily p.o.) in alleviating these effects. The results indicated that bilateral intrahippocampal injections of aggregated Abeta(1-42) detrimentally affected temporal discrimination from five-day block 31-35 post-injections until the end of the study (90 days post-injections). Daily treatment with alpha-tocopherol improved temporal discrimination under the recycling conjunctive schedule following aggregated Abeta(1-42) injections from the five-day block 61-65 days until the end of the study.

Structural properties of Abeta protofibrils stabilized by a small molecule

Metastable oligomeric and protofibrillar forms of amyloidogenic proteins have been implicated as on-pathway assembly intermediates in amyloid formation and as the major toxic species in a number of amyloid diseases including Alzheimer's disease. We describe here a chemical biology approach to structural analysis of Abeta protofibrils. Library screening yielded several molecules that stimulate Abeta aggregation. One of these compounds, calmidazolium chloride (CLC), rapidly and efficiently converts Abeta(1-40) monomers into clusters of protofibrils. As monitored by electron microscopy, these protofibrils persist for days when incubated in PBS at 37 degrees C, with a slow transition to fibrillar structures apparent only after several weeks. Like normal protofibrils, the CLC-Abeta aggregates exhibit a low thioflavin T response. Like Abeta fibrils, the clustered protofibrils bind the anti-amyloid Ab WO1. The CLC-Abeta aggregates exhibit the same protection from hydrogen-deuterium exchange as do protofibrils isolated from a spontaneous Abeta fibril formation reaction: approximately 12 of the 39 Abeta(1-40) backbone amide protons are protected from exchange in the protofibril, compared with approximately twice that number in amyloid fibrils. Scanning proline mutagenesis analysis shows that the Abeta molecule in these protofibrillar assemblies exhibits the same flexible N and C termini as do mature amyloid fibrils. The major difference in Abeta conformation between fibrils and protofibrils is added structural definition in the 22-29 segment in the fibril. Besides aiding structural analysis, compounds capable of facilitating oligomer and protofibril formation might have therapeutic potential, if they act to sequester Abeta in a form and/or location that cannot engage the toxic pathway.

Spectroscopic studies on native and protofibrillar insulin

The structure of insulin in amyloid fibrillar form has been recently shown as a well folded conformation using cryoelectron microscopy [Jimenez, J.L., Nettleton, E.J., Bouchard, M., Robinson, C.V., Dobson, C.M., Saibil H.R., 2002. The protofilament structure of insulin amyloid fibrils. Proc. Natl. Acad. Sci. USA. 99 9196-9201.]. Most of the amyloid aggregates elicit maximum toxicity in the protofibrillar (PF) intermediate state. Here, we describe PF intermediates of insulin are made-up monomers with flexible conformers. We also show protofibrils have three-dimensionally extended hydrophobic cavity to bind with 1-anilinonaphthalene-8-sulphonate (ANS) molecules. Energy transfer measurement revealed that ANS dye binding site of PF is within the range of FRET distance of insulin tyrosine residues. Significant proportion of beta-sheet, helical, and turn structures in the PF form indicate conformational dynamics in the folded chain of insulin in the PF assembled form. Though the conformational flexibility is noticeably present in the assembly, addition of GdnHCl could completely unfold PF into disordered structure suggesting structural "zipping" in the PF form. We have also shown that helical conformer inducing solvent 2,2,2-trifluoroethanol (TFE) could dissociate the PF aggregate indicating possible involvement of beta-sheets in contributing to PF stability.

Seeding specificity in amyloid growth induced by heterologous fibrils

Over residues 15-36, which comprise the H-bonded core of the amyloid fibrils it forms, the Alzheimer's disease plaque peptide amyloid beta (Abeta) possesses a very similar sequence to that of another short, amyloidogenic peptide, islet amyloid polypeptide (IAPP). Using elongation rates to quantify seeding efficiency, we inquired into the relationship between primary sequence similarity and seeding efficiency between Abeta-(1-40) and amyloid fibrils produced from IAPP as well as other proteins. In both a solution phase and a microtiter plate elongation assay, IAPP fibrils are poor seeds for Abeta-(1-40) elongation, exhibiting weight-normalized efficiencies of only 1-2% compared with Abeta-(1-40) fibrils. Amyloid fibrils of peptides with sequences completely unrelated to Abeta also exhibit poor to negligible seeding ability for Abeta elongation. Fibrils from a number of point mutants of Abeta-(1-40) exhibit intermediate seeding abilities for wild-type Abeta elongation, with differing efficiencies depending on whether or not the mutation is in the amyloid core region. The results suggest that amyloid fibrils from different proteins exhibit structural differences that control seeding efficiencies. Preliminary results also suggest that identical sequences can grow into different conformations of amyloid fibrils as detected by seeding efficiencies. The results have a number of implications for amyloid structure and biology.

The neuroprotective peptide NAP inhibits the aggregation of the beta-amyloid peptide

Alzheimer's disease (AD) is characterized by brain plaques containing the beta-amyloid peptide (Abeta). One approach for treating AD is by blocking Abeta aggregation. Activity-dependent neuroprotective protein contains a peptide, NAP that protects neurons in culture against Abeta toxicity. Here, NAP was shown to inhibit Abeta aggregation using: (1) fluorimetry; (2) electron microscopy; (3) high-throughput screening of Abeta deposition onto a synthetic template (synthaloid); and (4) Congo Red staining of neurons. Further assays showed biotin-NAP binding to Abeta. These results suggest that part of the neuroprotective mechanism exerted by NAP is through modulation of toxic protein folding in the extracellular milieu.

Congo red populates partially unfolded states of an amyloidogenic protein to enhance aggregation and amyloid fibril formation

Congo red (CR) has been reported to inhibit or enhance amyloid fibril formation by several proteins. To gain insight into the mechanism(s) for these apparently paradoxical effects, we studied as a model amyloidogenic protein, a dimeric immunoglobulin light chain variable domain. With a range of molar ratios of CR, i.e. r = [CR]/[protein dimer], we investigated the aggregation kinetics, conformation, hydrogen-deuterium exchange, and thermal stability of the protein. In addition, we used isothermal titration calorimetry to characterize the thermodynamics of CR binding to the protein. During incubation at 37 degrees C or during thermal scanning, with CR at r = 0.3, 1.3, and 4.8, protein aggregation was greatly accelerated compared with that measured in the absence of the dye. In contrast, with CR at r = 8.8, protein unfolding was favored over aggregation. The aggregates formed with CR at r = 0 or 0.3 were typical amyloid fibrils, but mixtures of amyloid fibrils and amorphous aggregates were formed at r = 1.3 and 4.8. CR decreased the apparent thermal unfolding temperature of the protein. Furthermore, CR perturbed the tertiary structure of the protein without significantly altering its secondary structure. Consistent with this result, CR also increased the rate of hydrogen-deuterium exchange by the protein. Isothermal titration calorimetry showed that CR binding to the protein was enthalpically driven, indicating that binding was mainly the result of electrostatic interactions. Overall, these results demonstrate that at low concentrations, CR binding to the protein favors a structurally perturbed, aggregation-competent species, resulting in acceleration of fibril formation. At high CR concentration, protein unfolding is favored over aggregation, and fibril formation is inhibited. Because low concentrations of CR can promote amyloid fibril formation, the therapeutic utility of this compound or its analogs to inhibit amyloidoses is questionable.

Identification of benzothiazoles as potential polyglutamine aggregation inhibitors of Huntington's disease by using an automated filter retardation assay

Preventing the formation of insoluble polyglutamine containing protein aggregates in neurons may represent an attractive therapeutic strategy to ameliorate Huntington's disease (HD). Therefore, the ability to screen for small molecules that suppress the self-assembly of huntingtin would have potential clinical and significant research applications. We have developed an automated filter retardation assay for the rapid identification of chemical compounds that prevent HD exon 1 protein aggregation in vitro. Using this method, a total of 25 benzothiazole derivatives that inhibit huntingtin fibrillogenesis in a dose-dependent manner were discovered from a library of approximately 184,000 small molecules. The results obtained by the filter assay were confirmed by immunoblotting, electron microscopy, and mass spectrometry. Furthermore, cell culture studies revealed that 2-amino-4,7-dimethyl-benzothiazol-6-ol, a chemical compound similar to riluzole, significantly inhibits HD exon 1 aggregation in vivo. These findings may provide the basis for a new therapeutic approach to prevent the accumulation of insoluble protein aggregates in Huntington's disease and related glutamine repeat disorders.

Behavioural and histopathological analyses of ibuprofen treatment on the effect of aggregated Abeta(1-42) injections in the rat

It has been suggested that inflammatory processes may play a role in the development of Alzheimer's disease (AD), and that nonsteroidal anti-inflammatory drug treatments may provide protection against the onset of AD. In the current study male Wistar rats were trained in two-lever operant chambers under an alternating lever cyclic-ratio ratio (ALCR) schedule. When responding showed no trends, subjects were divided into groups. One group was bilaterally injected into the CA3 area of the hippocampus with 5 microl of aggregated beta-amyloid (Abeta) suspension, and one group was bilaterally injected into the CA3 area of the hippocampus with 5 microl of sterile saline. Subgroups were treated twice daily with 0.1 ml (40 mg/kg) ibuprofen administered orally. The results indicated that chronic administration of ibuprofen protected against detrimental behavioural effects following aggregated Abeta injections. Withdrawal of ibuprofen treatment from aggregated Abeta-injected subjects produced a decline in behavioural performance to the level of the non-treated aggregated Abeta-injected group. Ibuprofen treatment reduced the numbers of reactive astrocytes following aggregated Abeta injection, and withdrawal of ibuprofen resulted in an increase of reactive astrocytes. These results suggest that induced inflammatory processes may play a role in AD, and that ibuprofen treatment may protect against some of the symptoms seen in AD.

Synthesis of (-)-5,8-dihydroxy-3R-methyl-2R-(dipropylamino)-1,2,3,4-tetrahydronaphthalene: an inhibitor of beta-amyloid(1-42) aggregation

A concise synthesis of the beta-amyloid(1-42 )aggregation inhibitor (-)-5,8-dihydroxy-3R-methyl-2R-(dipropylamino)-1,2,3,4-tetrahydronaphthalene [(-)-2] has been developed. The key step is a regio- and diastereoselective hydroboration-amination sequence to convert alkene into amine. Enantiomeric resolution was achieved by recrystallization of amine as the dibenzoyl-D-tartaric acid salt. Hydroquinone is a potent inhibitor of the fibrillar aggregation of beta-amyloid as determined in two different assay systems.

Apolipoprotein E affects amyloid formation but not amyloid growth in vitro: mechanistic implications for apoE4 enhanced amyloid burden and risk for Alzheimer's disease

The transition from the partially folded soluble Abeta monomer to insoluble Abeta amyloidfibrils is seminal to the formation and growth of amyloid plaques in Alzheimer's disease (AD). A detailed understanding of the role of AD risk factors in these processes is essential to understanding the physiochemical nature of this conformational rearrangement. The apolipoprotein E epsilon4 allele, a risk factor for AD, affects AD pathology by increasing amyloid burden relative to the much more common epsilon3 allele. In the present study, in vitro models were employed to probe the effect of these proteins on kinetically distinct steps in Abeta fibrillogenesis. Formation of Abeta amyloid was faster in the presence of apoE4 than apoE3, while growth of existing plaques was unaffected by either isoform. Further, experiments with Abeta stereoisomers establish that this effect of apoE3 is mediated through interaction with oligomeric fibrillogenic intermediates rather than through specific contacts with monomeric Abeta. Consistent with the altered pathology and enhanced risk for AD associated with inheritance of the epsilon4 allele, we conclude that APOE epsilon4 is a risk factor for AD not due to a pathological gain of function of apoE4 but to a loss of protective function of apoE3.

Angiotensin-converting enzyme degrades Alzheimer amyloid beta-peptide (A beta ); retards A beta aggregation, deposition, fibril formation; and inhibits cytotoxicity

We have demonstrated that the angiotensin-converting enzyme (ACE) genotype is associated with Alzheimer's disease (AD) in the Japanese population (). To determine why ACE affects susceptibility to AD, we examined the effect of purified ACE on aggregation of the amyloid beta-peptide (A beta) in vitro. Surprisingly, ACE was found to significantly inhibit A beta aggregation in a dose response manner. The inhibition of aggregation was specifically blocked by preincubation of ACE with an ACE inhibitor, lisinopril. ACE was confirmed to retard A beta fibril formation with electron microscopy. ACE inhibited A beta deposits on a synthaloid plate, which was used to monitor A beta deposition on autopsied brain tissue. ACE also significantly inhibited A beta cytotoxicity on PC12 h. The most striking fact was that ACE degraded A beta by cleaving A beta-(1-40) at the site Asp(7)-Ser(8). This was proven with reverse-phase HPLC, amino acid sequence analysis, and MALDI-TOF/MS. Compared with A beta-(1-40), aggregation and cytotoxic effects of the degradation products A beta-(1-7) and A beta-(8-40) peptides were reduced or virtually absent. These findings led to the hypothesis that ACE may affect susceptibility to AD by degrading A beta and preventing the accumulation of amyloid plaques in vivo.

Amyloid fibril formation in microwell plates for screening of inhibitors

Fibril formation is the basis of amyloid production in a number of disease states, such as Alzheimer's disease, diabetes and immunocytic dyscrasias. Compounds that inhibit fibril formation could be directly relevant to the treatment of amyloid diseases, and may also provide a foundation for the development of interventions in other molecular condensation diseases ranging from sickle cell anemia to atherosclerosis. We developed an economical and convenient high-throughput method for screening compounds against fibril formation in microwell plates. Chalcones, flavonoids and biflavonoids were screened against fibril formation by a recombinant antibody variable domain (V1). Chalcones 6 and 14 were found to demonstrate inhibition at 0.1 microM in 79 microM of protein solution in both test tube and microwell plate assays. The concentration of protein in the microwell plate assay could be as low as 5 microM using ThT as a monitoring agent. Molecular modeling studies indicated that both compounds could be individually docked into a binding site at the monomer-monomer interface of the V(L) protein dimer. These studies suggested that these compounds could potentially stabilize the VL dimer and therefore reduce its tendency to form fibrils. These findings may provide the basis for a new therapeutic approach to prevent or treat amyloid diseases.

Insulysin hydrolyzes amyloid beta peptides to products that are neither neurotoxic nor deposit on amyloid plaques

Insulysin (EC. 3.4.22.11) has been implicated in the clearance of beta amyloid peptides through hydrolytic cleavage. To further study the action of insulysin on Abeta peptides recombinant rat insulysin was used. Cleavage of both Abeta(1-40) and Abeta(1-42) by the recombinant enzyme was shown to initially occur at the His(13)-His(14), His(14)-Gln(15), and Phe(19)-Phe(20) bonds. This was followed by a slower cleavage at the Lys(28)-Gly(29), Val(18)-Phe(19), and Phe(20)-Ala(21) positions. None of the products appeared to be further metabolized by insulysin. Using a rat cortical cell system, the action of insulysin on Abeta(1-40) and Abeta(1-42) was shown to eliminate the neurotoxic effects of these peptides. Insulysin was further shown to prevent the deposition of Abeta(1-40) onto a synthetic amyloid. Taken together these results suggest that the use of insulysin to hydrolyze Abeta peptides represents an alternative gene therapeutic approach to the treatment of Alzheimer's disease.

Energy landscape theory for Alzheimer's amyloid beta-peptide fibril elongation

Recent experiments on the kinetics of deposition and fibril elongation of the Alzheimer's beta-amyloid peptide on preexisting fibrils are analyzed. A mechanism is developed based on the dock-and-lock scheme recently proposed by Maggio and coworkers to organize their experimental observations of the kinetics of deposition of beta-peptide on preexisting amyloid fibrils and deposits. Our mechanism includes channels for (1) a one-step prion-like direct deposition on fibrils of activated monomeric peptide in solution, and (2) a two-step deposition of unactivated peptide on fibrils and subsequent reorganization of the peptide-fibril complex. In this way, the mechanism and implied "energy landscape" unify a number of schemes proposed to describe the process of fibril elongation. This beta-amyloid landscape mechanism (beta ALM) is found to be in good agreement with existing experimental data. A number of experimental tests of the mechanism are proposed. The mechanism leads to a clear definition of overall equilibrium or rate constants in terms of the energetics of the elementary underlying processes. Analysis of existing experimental data suggests that fibril elongation occurs through a two-step mechanism of nonspecific peptide absorption and reorganization. The mechanism predicts a turnover in the rate of fibril elongation as a function of temperature and denaturant concentration. Proteins 2001;42:217-229.

Designing drugs to stop the formation of prion aggregates and other amyloids

Amyloid protein aggregates are implicated in many neurodegenerative diseases, including Alzheimer's disease and the prion diseases. Therapeutics to block amyloid formation are often tested in vitro, but it is not clear how to extrapolate from these experiments to a clinical setting, where the effective drug dose may be much lower. Here we address this question using a theoretical kinetic model to calculate the growth rate of protein aggregates as a function of the dose of each of three categories of drug. We find that therapeutics which block the growing ends of amyloids are the most promising, as alternative strategies may be ineffective or even accelerate amyloid formation at low drug concentrations. Our mathematical model can be used to identify and optimise an end-blocking drug in vitro. Our model also suggests an alternative explanation for data previously thought to prove the existence of an entity known as protein X.

Review: model peptides and the physicochemical approach to beta-amyloids

beta-Amyloid peptides are the main protein components of neuritic plaques and may be important in the pathogenesis of Alzheimer's Disease. The determination of the structure of beta-amyloid fibrils poses a challenge because of the limited solubility of beta-amyloid peptides and the noncrystalline nature of fibrils formed from these peptides. In this paper, we describe several physicochemical approaches which have been used to examine fibrils and the fibrillogenesis of peptide models of beta-amyloid. Recent advances in solid state NMR, such as the DRAWS pulse sequence, have made this approach a particularly attractive one for peptides such as beta-amyloid, which are not yet amenable to high-resolution solution phase NMR and crystallography. The application of solid state NMR techniques has yielded information on a model peptide comprising residues 10-35 of human beta-amyloid and indicates that in fibrils, this peptide assumes a parallel beta-strand conformation, with all residues in exact register. In addition, we discuss the use of block copolymers of Abeta peptides and polyethylene glycol as probes for the pathways of fibrillogenesis. These methods can be combined with other new methods, such as high-resolution synchrotron X-ray diffraction and small angle neutron and X-ray scattering, to yield structural data of relevance not only to disease, but to the broader question of protein folding and self-assembly.

Activation barriers to structural transition determine deposition rates of Alzheimer's disease a beta amyloid

Brain amyloid composed of the approximately 40-amino-acid human beta-amyloid peptide A beta is integral to Alzheimer's disease pathology. To probe the importance of a conformational transition in Abeta during amyloid growth, we synthesized and examined the solution conformation and amyloid deposition activity of A beta congeners designed to have similar solution structures but to vary substantially in their barriers to conformational transition. Although all these peptides adopt similar solution conformations, a covalently restricted Abeta congener designed to have a very high barrier to conformational rearrangement was inactive, while a peptide designed to have a reduced barrier to conformational transition displayed an enhanced deposition rate relative to wild-type A beta. The hyperactive peptide, which is linked to a heritable A beta amyloidosis characterized by massive amyloid deposition at an early age, displayed a reduced activation barrier to deposition consistent with a larger difference in activation entropy than in activation enthalpy relative to wild-type A beta. These results suggest that in Alzheimer's disease, as in the prion diseases, a conformational transition in the depositing peptide is essential for the conversion of soluble monomer to insoluble amyloid, and alterations in the activation barrier to this transition affect amyloidogenicity and directly contribute to human disease.

The Alzheimer's peptide a beta adopts a collapsed coil structure in water

The self-assembly of the soluble peptide Abeta into Alzheimer's disease amyloid is believed to involve a conformational change. Hence the solution conformation of Abeta is of significant interest. In contrast to studies in other solvents, in water Abeta is collapsed into a compact series of loops, strands, and turns and has no alpha-helical or beta-sheet structure. Conformational stabilization is primarily attributed to van der Waals and electrostatic forces. A large conspicuous uninterrupted hydrophobic patch covers approximately 25% of the surface. The compact coil structure appears meta-stable, and because fibrillization leads to formation of intermolecular beta-sheet secondary structure, a global conformational rearrangement is highly likely. A molecular hypothesis for amyloidosis includes at least two primary driving forces, changes in solvation thermodynamics during formation of amyloid deposits and relief of internal conformational stress within the soluble precursor during formation of lower-energy amyloid fibrils.

Direct measurement of islet amyloid polypeptide fibrillogenesis by mass spectrometry

A novel method for monitoring fibrillogenesis is developed and applied to the amyloidogenic peptide, islet amyloid polypeptide (IAPP). The approach, based on electrospray ionization mass spectrometry, is complementary to existing assays of fibril formation as it monitors directly the population of precursor rather than product molecules. We are able to monitor fiber formation in two modes: a quenched mode in which fibril formation is halted by dilution into denaturant and a real time mode in which fibril formation is conducted within the capillary of the electrospray source. Central to the method is the observation that fibrillar IAPP does not compromise the ionization of monomeric IAPP. Furthermore, under mild ionization conditions, fibrillar IAPP does not dissociate and contribute to the monomeric signal. Critically, we introduce an internal standard, rat IAPP, for analysis on the mass spectrometer. This standard is sufficiently similar in sequence in that it ionizes identically to human IAPP. Furthermore, the sequence is sufficiently different in that it does not form fibrils and is distinguishable on the basis of mass. Applied to IAPP fibrillogenesis, our technique reveals that precursor consumption in seeded reactions obeys first-order kinetics. Furthermore, a consistent level of monomer persists in both seeded and unseeded experiments after the fibril formation is complete. Given the inherent stability of fibrils, we expect this approach to be applicable to other amyloid systems.

Controlling polymerization of beta-amyloid and prion-derived peptides with synthetic small molecule ligands

The Alzheimer beta-amyloid peptide (Abeta) and a fragment of the prion protein have the capacity of forming amyloid-like fibrils when incubated under physiological conditions in vitro. Here we show that a small amyloid ligand, RO-47-1816/001, enhances this process severalfold by binding to amyloid molecules and apparently promote formation of the peptide-to-peptide bonds that join the monomers of the amyloid fibrils. This effect could be antagonized by other ligands, including analogues of RO-47-1816/001, as well as the structurally unrelated ligand Congo red. Analogues of RO-47-1816/001 with low affinity for amyloid did not display any antagonistic effect. In conclusion, these data suggest that synthetic molecules, and possibly also small natural substances present in the brain, may act in a chaperone-like fashion, promoting Abeta polymerization and growth of amyloid fibrils in vitro and possibly also in vivo. Furthermore, we demonstrate that small organic molecules can be used to inhibit the action of amyloid-enhancing compounds.