Methylene blue inhibits amyloid Abeta oligomerization by promoting fibrillization

Amyloid plaques are hallmark neuropathological lesions in Alzheimer's disease, which consist of abnormally aggregated Abeta protein. Multiple Abeta aggregated species have been identified, and neurotoxicity appears to be correlated with the amount of nonfibrillar oligomers. Therefore, selective inhibition of Abeta oligomer formation has emerged as an attractive means of therapeutic intervention. To investigate whether small molecules can modulate aggregation to achieve selective inhibition of neurotoxic amyloid oligomers, Abeta aggregation was assayed in vitro in the presence of methylene blue, using immunoreactivity with the prefibrillar oligomer-specific antibody A11, transmission electron microscopy, and turbidity assays. Methylene blue inhibited oligomerization when used at substoichiometric concentrations relative to that of the Abeta monomer. Inhibition of Abeta oligomerization was achieved concomitant with promotion of fibrillization, suggesting that oligomer and fibril formation are distinct and competing pathways. Methylene blue-mediated promotion of fiber formation occurred via a dose-dependent decrease in the lag time and an increase in the fibrillization rate, consistent with promotion of both filament nucleation and elongation. Addition of methylene blue to preformed oligomers resulted in oligomer loss and promotion of fibrillization. The data show that Abeta oligomer formation is inhibited by promoting fibril formation, which suggests that the relative pathological significance of oligomers and fibrils may be tested in vivo using methylene blue. If Abeta oligomers represent the primary pathogenic species, then inhibition of this highly toxic species via promotion of formation of less toxic aggregates may be therapeutically useful.     

Hsp104 targets multiple intermediates on the amyloid pathway and suppresses the seeding capacity of Abeta fibrils and protofibrils

The heat shock protein Hsp104 has been reported to possess the ability to modulate protein aggregation and toxicity and to “catalyze” the disaggregation and recovery of protein aggregates, including amyloid fibrils, in yeast, Escherichia coli, mammalian cell cultures, and animal models of Huntington's disease and Parkinson's disease. To provide mechanistic insight into the molecular mechanisms by which Hsp104 modulates aggregation and fibrillogenesis, the effect of Hsp104 on the fibrillogenesis of amyloid beta (Aβ) was investigated by characterizing its ability to interfere with oligomerization and fibrillogenesis of different species along the amyloid-formation pathway of Aβ. To probe the disaggregation activity of Hsp104, its ability to dissociate preformed protofibrillar and fibrillar aggregates of Aβ was assessed in the presence and in the absence of ATP. Our results show that Hsp104 inhibits the fibrillization of monomeric and protofibrillar forms of Aβ in a concentration-dependent but ATP-independent manner. Inhibition of Aβ fibrillization by Hsp104 is observable up to Hsp104/Aβ stoichiometric ratios of 1:1000, suggesting a preferential interaction of Hsp104 with aggregation intermediates (e.g., oligomers, protofibrils, small fibrils) on the pathway of Aβ amyloid formation. This hypothesis is consistent with our observations that Hsp104 (i) interacts with Aβ protofibrils, (ii) inhibits conversion of protofibrils into amyloid fibrils, (iii) arrests fibril elongation and reassembly, and (iv) abolishes the capacity of protofibrils and sonicated fibrils to seed the fibrillization of monomeric Aβ. Together, these findings suggest that the strong inhibition of Aβ fibrillization by Hsp104 is mediated by its ability to act at different stages and target multiple intermediates on the pathway to amyloid formation.     

New class of inhibitors of amyloid-beta fibril formation. Implications for the mechanism of pathogenesis in Alzheimer's disease

The amyloid hypothesis suggests that the process of amyloid-beta protein (Abeta) fibrillogenesis is responsible for triggering a cascade of physiological events that contribute directly to the initiation and progression of Alzheimer's disease. Consequently, preventing this process might provide a viable therapeutic strategy for slowing and/or preventing the progression of this devastating disease. A promising strategy to achieve prevention of this disease is to discover compounds that inhibit Abeta polymerization and deposition. Herein, we describe a new class of small molecules that inhibit Abeta aggregation, which is based on the chemical structure of apomorphine. These molecules were found to interfere with Abeta1-40 fibrillization as determined by transmission electron microscopy, Thioflavin T fluorescence and velocity sedimentation analytical ultracentrifugation studies. Using electron microscopy, time-dependent studies demonstrate that apomorphine and its derivatives promote the oligomerization of Abeta but inhibit its fibrillization. Preliminary structural activity studies demonstrate that the 10,11-dihydroxy substitutions of the D-ring of apomorphine are required for the inhibitory effectiveness of these aporphines, and methylation of these hydroxyl groups reduces their inhibitory potency. The ability of these small molecules to inhibit Abeta amyloid fibril formation appears to be linked to their tendency to undergo rapid autoxidation, suggesting that autoxidation product(s) acts directly or indirectly on Abeta and inhibits its fibrillization. The inhibitory properties of the compounds presented suggest a new class of small molecules that could serve as a scaffold for the design of more efficient inhibitors of Abeta amyloidogenesis in vivo.     

Distinct early folding and aggregation properties of Alzheimer amyloid-beta peptides Abeta40 and Abeta42: stable trimer or tetramer formation by Abeta42

The amyloid beta peptide (Abeta), composed of 40 or 42 amino acids, is a critical component in the etiology of the neurodegenerative Alzheimer disease. Abeta is prone to aggregate and forms amyloid fibrils progressively both in vitro and in vivo. To understand the process of amyloidogenesis, it is pivotal to examine the initial stages of the folding process. We examined the equilibrium folding properties, assembly states, and stabilities of the early folding stages of Abeta40 and Abeta42 prior to fibril formation. We found that Abeta40 and Abeta42 have different conformations and assembly states upon refolding from their unfolded ensembles. Abeta40 is predominantly an unstable and collapsed monomeric species, whereas Abeta42 populates a stable structured trimeric or tetrameric species at concentrations above approximately 12.5 microm. Thermodynamic analysis showed that the free energies of Abeta40 monomer and Abeta42 trimer/tetramer are approximately 1.1 and approximately 15/ approximately 22 kcal/mol, respectively. The early aggregation stages of Abeta40 and Abeta42 contain different solvent-exposed hydrophobic surfaces that are located at the sequences flanking its protease-resistant segment. The amyloidogenic folded structure of Abeta is important for the formation of spherical beta oligomeric species. However, beta oligomers are not an obligatory intermediate in the process of fibril formation because oligomerization is inhibited at concentrations of urea that have no effect on fibril formation. The distinct initial folding properties of Abeta40 and Abeta42 may play an important role in the higher aggregation potential and pathological significance of Abeta42.     

Defining the pathway of worm-like amyloid fibril formation by the mouse prion protein by delineation of the productive and unproductive oligomerization reactions

Aggregation reactions of proteins leading to amyloid fibril formation are often characterized by early transient accumulation of a heterogeneous population of soluble oligomers differing in size and structure. Delineating the kinetic roles of the different oligomeric forms in fibril formation has been a major challenge. The aggregation of the mouse prion protein to form worm-like amyloid fibrils at low pH is known to proceed via a β-rich oligomer ensemble, which is shown here to be comprised of two subpopulations of oligomers that differ in size and internal structure. The relative populations of the two oligomers can be tuned by varying the concentration of NaCl present. By demonstrating that the apparent rate constant for the formation of fibrils is dependent linearly on the concentration of the larger oligomer and is independent of the concentration of the smaller oligomer, we show that the larger oligomer is a productive intermediate that accumulates on the direct pathway of aggregation from monomer to worm-like fibrils. The smaller oligomer is shown to be populated off the pathway of the larger oligomer and, hence, is not directly productive for fibril formation. The relative populations of the two oligomers can also be tuned by single-amino acid residue changes in the sequence of the protein. The different protein variants yield worm-like fibrils of different lengths, and the apparent rate of formation of the fibrils by the mutant variants is also shown to be dependent on the concentration of the larger but not of the smaller oligomer formed.     

Effects of novel acylhydrazones derived from 4-quinolone on the acetylcholinesterase activity and Aβ42 peptide fibrils formation

Acetylcholinesterase inhibitors and compounds that trigger Aβ amyloid oligomerization and fibrillization represent an opportunity to discover new drug candidates to treat Alzheimer’s disease. In this work, we synthesized nine new acylhydrazones and a known one, both employing 3-carboethoxy-4-quinolone derivatives as starting materials with chemical yields ranging from 63% to 90%. We evaluated the effect of these compounds on the acetylcholinesterase (AChE) activity and the fibrillization of Aβ₄₂ peptide. Except for one acylhydrazone, the compounds exhibited good inhibitory effect on AChE (1.2?μM?<?IC₅₀ values?<?17?μM). They also showed a significant decrease in the thioflavin-T fluorescence emission, suggesting an inhibitory effect on the Aβ₄₂ fibril formation.     

Reduction of the amyloidogenicity of a protein by specific binding of ligands to the native conformation

It is known that human muscle acylphosphatase (AcP) is able, under appropriate conditions in vitro, to aggregate and form amyloid fibrils of the type associated with human diseases. A number of compounds were tested for their ability to bind specifically to the native conformation of AcP under conditions favoring denaturation and subsequent aggregation and fibril formation. Compounds displaying different binding affinities for AcP were selected and their ability to inhibit protein fibrillization in vitro was evaluated. We found that compounds displaying a relatively high affinity for AcP are able to significantly delay protein fibrillization, mimicking the effect of stabilizing mutations; in addition, the effectiveness of such outcome correlates positively to both ligand concentration and affinity to the native state of AcP. By contrast, the inhibitory effect of ligands on AcP aggregation disappears in a mutant protein in which such binding affinity is lost. These results indicate that the stabilization of the native conformation of amyloidogenic proteins by specific ligand binding can be a strategy of general interest to inhibit amyloid formation in vivo.     

Inhibition of human IAPP fibril formation does not prevent beta-cell death: evidence for distinct actions of oligomers and fibrils of human IAPP

Type 2 diabetes mellitus (T2DM) is characterized by an approximately 60% deficit in beta-cell mass, increased beta-cell apoptosis, and islet amyloid derived from islet amyloid polypeptide (IAPP). Human IAPP (hIAPP) forms oligomers, leading to either amyloid fibrils or toxic oligomers in an aqueous solution in vitro. Either application of hIAPP on or overexpression of hIAPP in cells induces apoptosis. It remains controversial whether the fibrils or smaller toxic oligomers induce beta-cell apoptosis. Rifampicin prevents hIAPP amyloid fibril formation and has been proposed as a potential target for prevention of T2DM. We examined the actions of rifampicin on hIAPP amyloid fibril and toxic oligomer formation as well as its ability to protect beta-cells from either application of hIAPP or endogenous overexpression of hIAPP (transgenic rats and adenovirus-transduced beta-cells). We report that rifampicin (Acocella G. Clin Pharmacokinet 3: 108-127, 1978) prevents hIAPP fibril formation, but not formation of toxic hIAPP oligomers (Bates G. Lancet 361: 1642-1644, 2003), and does not protect beta-cells from apoptosis induced by either overexpression or application of hIAPP. These data emphasize that toxic hIAPP oligomers, rather than hIAPP fibrils, initiate beta-cell apoptosis and that screening tools to identify inhibitors of amyloid fibril formation are likely to be less useful than those that identify inhibitors of toxic oligomer formation. Finally, rifampicin and related molecules do not appear to be useful as candidates for prevention of T2DM.     

Sensitive fluorescence polarization technique for rapid screening of alpha-synuclein oligomerization/fibrillization inhibitors

Parkinson's disease (PD) is characterized by the accumulation of fibrillar alpha-synuclein (alpha-Syn) inclusions known as Lewy bodies (LBs) and Lewy neurites. Mutations in the alpha-Syn gene or extra copies thereof cause familial PD or dementia with LBs (DLB) in rare kindreds, but abnormal accumulations of wildtype alpha-Syn also are implicated in the pathogenesis of sporadic PD, the most common movement disorder. Insights into mechanisms underlying alpha-Syn mediated neurodegeneration link alpha-Syn oligomerization and fibrillization to the onset and progression of PD. Thus, inhibiting alpha-Syn oligomer or fibril formation is a compelling target for discovering disease modifying therapies for PD, DLB, and related synucleinopathies. Although amyloid dyes recognize alpha-Syn fibrils, efficient detection of soluble oligomers remains a challenge. Here, we report a novel fluorescence polarization (FP) technique for examining alpha-Syn assembly by monitoring changes in its relative molecular mass during progression of normal alpha-Syn from highly soluble monomers to higher order multimers and thence insoluble amyloid fibrils. We report that FP is more sensitive than conventional amyloid dye methods for the quantification of mature fibrils, and that FP is capable of detecting oligomeric alpha-Syn, allowing for rapid automated screening of potential inhibitors of alpha-Syn oligomerization and fibrillization. Furthermore, FP can be combined with an amyloid dye in a single assay that simultaneously provides two independent biophysical readouts for monitoring alpha-Syn fibrillization. Thus, this FP method holds potential to accelerate discovery of disease modifying therapies for LB PD, DLB, and related neurodegenerative synucleinopathies.     

Inhibition of α-synuclein fibril assembly by small molecules: Analysis using epitope-specific antibodies

The conversion of soluble peptides and proteins into amyloid fibrils and/or intermediate oligomers is believed to be the central event in the pathogenesis of most human neurodegenerative diseases. Existing treatments are at best symptomatic. Accordingly, small molecule inhibitors of amyloid fibril formation and their mechanisms are of great interest. Here we report that the conformational changes undergone by α -synuclein as it assembles into amyloid fibrils can be detected by epitope-specific antibodies. We show that the conformations of polyphenol-bound α-synuclein monomers and dimers differ from those of unbound monomers and resemble amyloid fibrils. This strongly suggests that small molecule inhibitors bind and stabilize intermediates of amyloid fibril formation, consistent with the view that inhibitor-bound molecular species are on-pathway intermediates.     

What Drives Amyloid Molecules To Assemble into Oligomers and Fibrils?

We develop a theory for three states of equilibrium of amyloid peptides: the monomer, oligomer, and fibril. We assume that the oligomeric state is a disordered micellelike collection of a few peptide chains held together loosely by hydrophobic interactions into a spherical hydrophobic core. We assume that fibrillar amyloid chains are aligned and further stabilized by steric zipper interactions—hydrogen bonding, steric packing, and specific hydrophobic side-chain contacts. The model makes a broad set of predictions that are consistent with experimental results: 1), Similar to surfactant micellization, amyloid oligomerization should increase with peptide concentration in solution. 2), The onset of fibrillization limits the concentration of oligomers in the solution. 3), The extent of Aβ fibrillization increases with peptide concentration. 4), The predicted average fibril length versus monomer concentration agrees with data on α-synuclein. 5), Full fibril length distributions agree with data on α-synuclein. 6), Denaturants should melt out fibrils. And finally, 7), added salt should stabilize fibrils by reducing repulsions between amyloid peptide chains. It is of interest that small changes in solvent conditions can tip the equilibrium balance between oligomer and fibril and cause large changes in rates through effects on the transition-state barrier. This model may provide useful insights into the physical processes underlying amyloid diseases.     

Physiochemical characterization of the Alzheimer's disease-related peptides A beta 1-42Arctic and A beta 1-42wt

The amyloid beta peptide (A beta) is crucial for the pathogenesis of Alzheimer's disease. Aggregation of monomeric A beta into insoluble amyloid fibrils proceeds through several soluble A beta intermediates, including protofibrils, which are believed to be central in the disease process. The main reason for this is their implication in familial Alzheimer's disease with the Arctic amyloid precursor protein mutation (E693G). This mutation gives rise to early onset Alzheimer's disease, and synthetic A beta 1-40Arctic displays an enhanced rate of protofibril formation in vitro[Nilsberth C, Westlind-Danielsson A, Eckman CB, Condron MM, Axelman K, Forsell C, Stenh C, Luthman J, Teplow DB, Younkin SG, Naslund J & Lannfelt L. (2001) Nat Neurosci4, 887-893]. To increase our understanding of the mechanisms involved in A beta aggregation, especially A beta monomer oligomerization into protofibrils and protofibril fibrillization into fibrils, the kinetics of A beta 1-42wt and A beta 1-42Arctic aggregation were examined under different physiochemical conditions, such as concentration, temperature, ionic strength and pH. We used size exclusion chromatography for this purpose, where monomers are separated from protofibrils, and fibrils are separated from protofibrils in a centrifugation step. The Arctic mutation significantly accelerated both A beta 1-42wt protofibril formation and protofibril fibrillization. In addition, we demonstrated that two distinct chemical processes - monomer oligomerization and protofibril fibrillization - were affected differently by changes in the micro-environment and that the Arctic mutation alters the peptide response to such changes.     

Nucleation of β-rich oligomers and β-barrels in the early aggregation of human islet amyloid polypeptide

The self-assembly of human islet amyloid polypeptide (hIAPP) into β-sheet rich amyloid aggregates is associated with pancreatic β-cell death in type 2 diabetes (T2D). Prior experimental studies of hIAPP aggregation reported the early accumulation of α-helical intermediates before the rapid conversion into β-sheet rich amyloid fibrils, as also corroborated by our experimental characterizations with transmission electron microscopy and Fourier transform infrared spectroscopy. Although increasing evidence suggests that small oligomers populating early hIAPP aggregation play crucial roles in cytotoxicity, structures of these oligomer intermediates and their conformational conversions remain unknown, hindering our understanding of T2D disease mechanism and therapeutic design targeting these early aggregation species. We further applied large-scale discrete molecule dynamics simulations to investigate the oligomerization of full-length hIAPP, employing multiple molecular systems of increasing number of peptides. We found that the oligomerization process was dynamic, involving frequent inter-oligomeric exchanges. On average, oligomers had more α-helices than β-sheets, consistent with ensemble-based experimental measurements. However, in ~4–6% independent simulations, β-rich oligomers expected as the fibrillization intermediates were observed, especially in the pentamer and hexamer simulations. These β-rich oligomers could adopt β-barrel conformations, recently postulated to be the toxic oligomer species but only observed computationally in the aggregates of short amyloid protein fragments. Free-energy analysis revealed high energies of these β-rich oligomers, supporting the nucleated conformational changes of oligomers in amyloid aggregation. β-barrel oligomers of full-length hIAPP with well-defined three-dimensional structures may play an important pathological role in T2D etiology and may be a therapeutic target for the disease.     

Heparin binds 8 kDa gelsolin cross-β-sheet oligomers and accelerates amyloidogenesis by hastening fibril extension

Glycosaminoglycans (GAGs) are highly sulfated linear polysaccharides prevalent in the extracellular matrix, and they associate with virtually all amyloid deposits in vivo. GAGs accelerate the aggregation of many amyloidogenic peptides in vitro, but little mechanistic evidence is available to explain why. Herein, spectroscopic methods demonstrate that GAGs do not affect the secondary structure of the monomeric 8 kDa amyloidogenic fragment of human plasma gelsolin. Moreover, monomerized 8 kDa gelsolin does not bind to heparin under physiological conditions. In contrast, 8 kDa gelsolin cross-β-sheet oligomers and amyloid fibrils bind strongly to heparin, apparently because of electrostatic interactions between the negatively charged polysaccharide and a positively charged region of the 8 kDa gelsolin assemblies. Our observations are consistent with a scaffolding mechanism whereby cross-β-sheet oligomers, upon formation, bind to GAGs, accelerating the fibril extension phase of amyloidogenesis, possibly by concentrating and orienting the oligomers to more efficiently form amyloid fibrils. Notably, heparin decreases the 8 kDa gelsolin concentration necessary for amyloid fibril formation, likely a consequence of fibril stabilization through heparin binding. Because GAG overexpression, which is common in amyloidosis, may represent a strategy for minimizing cross-β-sheet oligomer toxicity by transforming them into amyloid fibrils, the mechanism described herein for GAG-mediated acceleration of 8 kDa gelsolin amyloidogenesis provides a starting point for therapeutic strategy development. The addition of GAG mimetics, small molecule sulfonates shown to reduce the amyloid load in animal models of amyloidosis, to a heparin-accelerated 8 kDa gelsolin aggregation reaction mixture neither significantly alters the rate of amyloidogenesis nor prevents oligomers from binding to GAGs, calling into question their commonly accepted mechanism.     

The effects of the flavonoid baicalein and osmolytes on the Mg 2+ accelerated aggregation/fibrillation of carboxymethylated bovine 1SS-alpha-lactalbumin

Many protein conformational diseases arise when proteins form alternative stable conformations, resulting in aggregation and accumulation of the protein as fibrillar deposits, or amyloids. Interestingly, numerous proteins implicated in amyloid protein formation show similar structural and functional properties. Given this similarity, we tested the notion that carboxymethylated bovine alpha-lactalbumin (1SS-alpha-lac) could serve as a general amyloid fibrillation/aggregation model system. Like most amyloid forming systems, Mg2+ ions accelerate 1SS-alpha-lac amyloid fibril formation. While osmolytes such as trimethylamine N-oxide (TMAO), and sucrose enhanced thioflavin T detected aggregation, a mixture of trehalose and TMAO substantially inhibited aggregation. Most importantly however, the flavonoid, baicalein, known to inhibit alpha-synuclein amyloid fibril formation, also inhibits 1SS-alpha-lac amyloid with the same apparent efficacy. These data suggest that the easily obtainable 1SS-alpha-lac protein can serve as a general amyloid model and that some small molecule amyloid inhibitors may function successfully with many different amyloid systems.     

Amyloid Protofibrils of Lysozyme Nucleate and Grow Via Oligomer Fusion

The mechanisms linking deposits of insoluble amyloid fibrils to the debilitating neuronal cell death characteristic of neurodegenerative diseases remain enigmatic. Recent findings implicate transiently formed intermediates of mature amyloid fibrils as the principal toxic agent. Hence, determining which intermediate aggregates represent on-pathway precursors or off-pathway side branches is critical for understanding amyloid self-assembly, and for devising therapeutic approaches targeting relevant toxic species. We examined amyloid fibril self-assembly in acidic solutions, using the model protein hen egg-white lysozyme. Combining in situ dynamic light scattering with calibrated atomic-force microscopy, we monitored the nucleation and growth kinetics of multiple transient aggregate species, and characterized both their morphologies and physical dimensions. Upon incubation at elevated temperatures, uniformly sized oligomers formed at a constant rate. After a lag period of several hours, protofibrils spontaneously nucleated. The nucleation kinetics of protofibrils and the tight match of their widths and heights with those of oligomers imply that protofibrils both nucleated and grew via oligomer fusion. After reaching several hundred nanometers in length, protofibrils assembled into mature fibrils. Overall, the amyloid fibril assembly of lysozyme followed a strict hierarchical aggregation pathway, with amyloid monomers, oligomers, and protofibrils forming on-pathway intermediates for assembly into successively more complex structures.     

Inhibition of amyloid fibrillogenesis and toxicity by a peptide chaperone

Aggregation of proteins in tissues is associated with several diseases, including Alzheimer's disease. It is characterized by the accumulation of amyloid beta peptide (Abeta) in the extracellular spaces of the brain cells, resulting in neuronal death and other pathological changes. alpha-Crystallin, a small heat-shock protein in lens, and a peptide chaperone having the functional site sequence DFVIFLDVKHFSPEDLTVK of alphaA-crystallin may inhibit Abeta fibrillogenesis and toxicity. The peptide chaperone (mini-alphaA-crystallin), having an Abeta interacting domain and a complex solubilizing domain, was shown in previous studies to prevent aggregation of several proteins under denaturing conditions. In this in vitro study, using transmission electron microscopy and thioflavin T binding assay, we show that mini-alphaA-crystallin arrests the fibril formation of Abeta peptides. Mini-alphaA-crystallin also suppresses the toxic action of Abeta on rat pheochromocytoma (PC 12) cells. The wide chaperoning capability of the peptide and its ability to inhibit amyloid fibril formation and suppress toxicity suggest that mini-alphaA-crystallin may serve as a universal chaperone in controlling diseases of protein aggregation, including Alzheimer's disease.     

Amphotericin B binds to amyloid fibrils and delays their formation: a therapeutic mechanism?

The membrane-active antifungal agent amphotericin B (AmB) is one of the few agents shown to slow the course of prion diseases in animals. Congo Red and other small molecules have been reported to directly inhibit amyloidogenesis in both prion and Alzheimer peptide model systems via specific binding. We propose that it is possible that AmB may act similarly to physically prevent conversion of the largely alpha-helical prion protein (PrP) to the pathological beta-sheet aggregate protease-resistant isoform (PrP(res)) in prion disease and by analogy prevent fibrillization in amyloid diseases. To assess whether AmB is capable of binding specifically to amyloid fibrils as does Congo Red, we have used the insulin fibril and Abeta 25-35 amyloid model fibril system. We find that AmB does bind strongly to both insulin (K(d) = 1.1 microM) and Abeta 25-35 amyloid (K(d) = 6.4 microM) fibrils but not to native insulin. Binding is characterized by a red-shifted AmB spectrum indicative of a more hydrophobic environment. Thus AmB seems to have a complementary face for amyloid fibrils but not the native protein. In addition, AmB interacts specifically with Congo Red, a known fibril-binding agent. In kinetic fibril formation studies, AmB was able to significantly kinetically delay the formation of Abeta 25-35 fibrils at pH 7.4 but not insulin fibrils at pH 2.     

Novel fluorescent trimethine cyanine dye 7519 for amyloid fibril inhibition assay

Abstract

Fluorescence spectroscopy was used to study the ability of dye 7519 to follow the transition of monomeric insulin into fibrils and applicability of the dye to the insulin aggregation inhibition assay. The commercially available classic amyloid stain, thioflavin T, was used as the reference dye. For selecting potential inhibitors, the QSAR approach was applied. Dye 7519 appeared to be suitable for monitoring insulin aggregation into fibrils in vitro. The properties of the dye allowed us to test it as a potential probe in the screening assay of potential inhibitors of insulin fibrillization. One hundred forty-four flavonoids were tested as potential inhibitors of amyloid fibril formation using the quantitative structure activity relationship approach. Among them, 10 candidates with high indexes of inhibition were selected for tests in vitro using dye 7519 and the reference amyloid dye thioflavine T. Using dye 7519 fluorescence, we found that two compounds had inhibitory effects on insulin amyloid formation. These results agree with inhibition data using the thioflavine T assay. Our studies demonstrated that the fluorescent cyanine dye 7519 is a sensitive probe for quantitative detection of insulin amyloid formation and can be applied to screen agents capable of affecting aggregation of amyloid proteins.