4,4(')-Dianilino-1,1(')-binaphthyl-5,5(')-disulfonate: report on non-beta-sheet conformers of Alzheimer's peptide beta(1-40)

Immobilized Amyloid Hexamer Fragments to Map Active Sites of Amyloid-Targeting Chemicals

As amyloid-β (Aβ) peptide is considered a biomarker and pathological culprit of Alzheimer's disease, Aβ-targeting compounds have been investigated for diagnostics development and drug discovery of the disorder. Unlike amyloid plaque targeting agents, such as clinically available amyloid radiotracers intercalating into the β-sheet structures of the aggregates, monomer and oligomer targeting chemicals are difficult to develop, as the transient and polymorphic nature of these peptides impedes their structural understanding. Here, we report a mapping approach to explore targeting residues of Aβ-imaging probes and Aβ-regulating drug candidates by utilizing a set of fragmented Aβ hexamers immobilized on a 96-well microplate in combination with fluorescent full-length Aβ for on-plate aggregation. To evaluate the mapping potential of the peptide plate, we tested previously reported fluorescent imaging agents (CRANAD-28, bis-ANS), aggregation inhibitors (curcumin, scyllo-inositol), and aggregate dissociators (necrostatin-1, sunitinib) targeting Aβ. Our approach enabled mechanistic understanding of compounds targeting nonfibrillar Aβ on an interacting sequence level.

Development of β-sheet structure in Aβ aggregation intermediates diminishes exposed hydrophobic surface area and enhances proinflammatory activity

Three decades of research, both in vitro and in vivo, have demonstrated the conformational heterogeneity that is displayed by the amyloid β peptide (Aβ) in Alzheimer's disease (AD). Understanding the distinct properties between Aβ conformations and how conformation may impact cellular activity remain open questions, yet still continue to provide new insights into protein misfolding and aggregation. In particular, there is interest in the group of soluble oligomeric prefibrillar Aβ species comprising lower molecular weight oligomers up to larger protofibrils. In the current study, a number of strategies were utilized to separate Aβ protofibrils and oligomers and show that the smaller Aβ oligomers have a much different conformation than Aβ protofibrils. The differences were consistent for both Aβ40 and Aβ42. Protofibrils bound thioflavin T to a greater extent than oligomers, and were highly enriched in β-sheet secondary structure. Aβ oligomers possessed a more open structure with significant solvent exposure of hydrophobic domains as determined by tryptophan fluorescence and bis-ANS binding, respectively. The protofibril-selective antibody AbSL readily discerned conformational differences between protofibrils and oligomers. The more developed structure for Aβ protofibrils ultimately proved critical for provoking the release of tumor necrosis factor α from microglial cells. The findings demonstrated a dependency on β-sheet structure for soluble Aβ aggregates to cause a microglial inflammatory response. The Aβ aggregation process yields many conformationally-varied species with different levels of β-structure and exposed hydrophobicity. The conformation elements likely determine biological activity and pathogenicity.

Defining the Neuropathological Aggresome across in Silico, in Vitro, and ex Vivo Experiments

The loss of proteostasis over the life course is associated with a wide range of debilitating degenerative diseases and is a central hallmark of human aging. When left unchecked, proteins that are intrinsically disordered can pathologically aggregate into highly ordered fibrils, plaques, and tangles (termed amyloids), which are associated with countless disorders such as Alzheimer's disease, Parkinson's disease, type II diabetes, cancer, and even certain viral infections. However, despite significant advances in protein folding and solution biophysics techniques, determining the molecular cause of these conditions in humans has remained elusive. This has been due, in part, to recent discoveries showing that soluble protein oligomers, not insoluble fibrils or plaques, drive the majority of pathological processes. This has subsequently led researchers to focus instead on heterogeneous and often promiscuous protein oligomers. Unfortunately, significant gaps remain in how to prepare, model, experimentally corroborate, and extract amyloid oligomers relevant to human disease in a systematic manner. This Review will report on each of these techniques and their successes and shortcomings in an attempt to standardize comparisons between protein oligomers across disciplines, especially in the context of neurodegeneration. By standardizing multiple techniques and identifying their common overlap, a clearer picture of the soluble neuropathological aggresome can be constructed and used as a baseline for studying human disease and aging.

Alzheimer risk factors age and female sex induce cortical Aβ aggregation by raising extracellular zinc

Aging and female sex are the major risk factors for Alzheimer's disease and its associated brain amyloid-β (Aβ) neuropathology, but the mechanisms mediating these risk factors remain uncertain. Evidence indicates that Aβ aggregation by Zn²⁺ released from glutamatergic neurons contributes to amyloid neuropathology, so we tested whether aging and sex adversely influences this neurophysiology. Using acute hippocampal slices, we found that extracellular Zn²⁺-elevation induced by high K⁺ stimulation was significantly greater with older (65 weeks vs 10 weeks old) rats, and was exaggerated in females. This was driven by slower reuptake of extracellular Zn²⁺, which could be recapitulated by mitochondrial intoxication. Zn²⁺:Aβ aggregates were toxic to the slices, but Aβ alone was not. Accordingly, high K⁺ caused synthetic human Aβ added to the slices to form soluble oligomers as detected by bis-ANS, attaching to neurons and inducing toxicity, with older slices being more vulnerable. Age-dependent energy failure impairing Zn²⁺ reuptake, and a higher maximal capacity for Zn²⁺ release by females, could contribute to age and sex being major risk factors for Alzheimer's disease.

Amyloid-Forming Segment Induces Aggregation of FUS-LC Domain from Phase Separation Modulated by Site-Specific Phosphorylation

The RNA-binding protein fused in sarcoma (FUS) forms physiological granules and pathological fibrils, which facilitate RNA functions and cause neurodegenerative diseases, respectively. Phosphorylation at Ser/Thr residues may regulate the functional assembly of FUS and prevent pathological aggregation in cells. However, the low-complexity nature of the FUS sequence makes it challenging to characterize how phosphorylation of specific sites within the core amyloid-forming segment affects aggregation. Taking advantage of the recently solved molecular structures of the fibrillar core of the FUS low-complexity (FUS-LC) domain, we systematically investigated the aggregation of repeated segments within the core. We identified a segment with a strong amyloid-forming tendency that induced the aggregation of FUS-LC domain in phase-separated liquid droplets and further seeded the aggregation of full-length FUS. The aggregation propensity and seeding ability of this amyloid-forming segment were modulated by site-specific phosphorylation. Solid-state nuclear magnetic resonance (NMR) spectroscopy and computational modeling implied that site-specific phosphorylation at Ser61 plays key roles in FUS assembly by disrupting both intra- and intermolecular interactions that maintain the amyloid core structure.

Zinc ion rapidly induces toxic, off-pathway amyloid-β oligomers distinct from amyloid-β derived diffusible ligands in Alzheimer's disease

Alzheimer’s disease (AD) is the most prevalent neurodegenerative disease in the elderly. Zinc (Zn) ion interacts with the pathogenic hallmark, amyloid-β (Aβ), and is enriched in senile plaques in brain of AD patients. To understand Zn-chelated Aβ (ZnAβ) species, here we systematically characterized ZnAβ aggregates by incubating equimolar Aβ with Zn. We found ZnAβ40 and ZnAβ42 both form spherical oligomers with a diameter of ~12–14 nm composed of reduced β-sheet content. Oligomer assembly examined by analytical ultracentrifugation, hydrophobic exposure by BisANS spectra, and immunoreactivity of ZnAβ and Aβ derived diffusible ligands (ADDLs) are distinct. The site-specific ¹³C labeled solid-state NMR spectra showed that ZnAβ40 adopts β-sheet structure as in Aβ40 fibrils. Interestingly, removal of Zn by EDTA rapidly shifted the equilibrium back to fibrillization pathway with a faster kinetics. Moreover, ZnAβ oligomers have stronger toxicity than ADDLs by cell viability and cytotoxicity assays. The ex vivo study showed that ZnAβ oligomers potently inhibited hippocampal LTP in the wild-type C57BL/6JNarl mice. Finally, we demonstrated that ZnAβ oligomers stimulate hippocampal microglia activation in an acute Aβ-injected model. Overall, our study demonstrates that ZnAβ rapidly form toxic and distinct off-pathway oligomers. The finding provides a potential target for AD therapeutic development.

Oil Palm Phenolics Inhibit the In Vitro Aggregation of β-Amyloid Peptide into Oligomeric Complexes

Alzheimer's disease is a severe neurodegenerative disease characterized by the aggregation of amyloid-β peptide (Aβ) into toxic oligomers which activate microglia and astrocytes causing acute neuroinflammation. Multiple studies show that the soluble oligomers of Aβ42 are neurotoxic and proinflammatory, whereas the monomers and insoluble fibrils are relatively nontoxic. We show that Aβ42 aggregation is inhibited in vitro by oil palm phenolics (OPP), an aqueous extract from the oil palm tree (Elaeis guineensis). The data shows that OPP inhibits stacking of β-pleated sheets, which is essential for oligomerization. We demonstrate the inhibition of Aβ42 aggregation by (1) mass spectrometry; (2) Congo Red dye binding; (3) 2D-IR spectroscopy; (4) dynamic light scattering; (5) transmission electron microscopy; and (6) transgenic yeast rescue assay. In the yeast rescue assay, OPP significantly reduces the cytotoxicity of aggregating neuropeptides in yeast genetically engineered to overexpress these peptides. The data shows that OPP inhibits (1) the aggregation of Aβ into oligomers; (2) stacking of β-pleated sheets; and (3) fibrillar growth and coalescence. These inhibitory effects prevent the formation of neurotoxic oligomers and hold potential as a means to reduce neuroinflammation and neuronal death and thereby may play some role in the prevention or treatment of Alzheimer's disease.

Toxicity of serum albumin on microglia upon seeding effect of amyloid peptide

We demonstrate in vitro cross-seeding of bovine serum albumin (BSA) in the presence of Aβ_25-35 and their cytotoxic effects on microglial cells. To investigate the cross-seeding of BSA in the presence of Aβ_25-35 fibrils, we examined how Aβ_25-35 fibrils can function as seeds to trigger and accelerate BSA fibrillogenesis using ThT, intrinsic fluorescence, ANS fluorescence and transmission electron microscopy (TEM). Moreover, the effects of these fibrils on microglial viability were measured using MTT and Annexin V/propidium iodide (PI) staining. Although Aβ_25-35 is toxic against microglia, it acted as seed and affected the aggregation pathway and accelerated the fibrillogenesis of BSA in vitro, resulted in an enhanced cytotoxic effect in comparison with Aβ_25-35 or BSA alone. These observations thought to be helpful to understand the molecular mechanism of enhanced toxicity due to the coexistence of the aggregation prone proteins/peptides,. then cross-seeding effect on microglial cells that may involve in neurodegenerative diseases such as Alzheimer's disease (AD).

Self-Assembly of Aβ40, Aβ42 and Aβ43 Peptides in Aqueous Mixtures of Fluorinated Alcohols

Fluorinated alcohols such as hexafluoroisopropanol (HFIP) and trifluoroethanol (TFE) have the ability to promote α-helix and β-hairpin structure in proteins and peptides. HFIP has been used extensively to dissolve various amyloidogenic proteins and peptides including Aβ, in order to ensure their monomeric status. In this paper, we have investigated the self-assembly of Aβ40, Aβ42, and Aβ43 in aqueous mixtures of fluorinated alcohols from freshly dissolved stock solutions in HFIP. We have observed that formation of fibrillar and non-fibrillar structures are dependent on the solvent composition. Peptides form fibrils with ease when reconstituted in deionized water from freshly dissolved HFIP stocks. In aqueous mixtures of fluorinated alcohols, either predominant fibrillar structures or clustered aggregates were observed. Aqueous mixtures of 20% HFIP are more favourable for Aβ fibril formation as compared to 20% TFE. When Aβ40, Aβ42, and Aβ43 stocks in HFIP are diluted in 50% aqueous mixtures in phosphate buffer or deionized water followed by slow evaporation of HFIP, Aβ peptides form fibrils in phosphate buffer and deionized water. The clustered structures could be off-pathway aggregates. Aβ40, Aβ42, and Aβ43 showed significant α-helical content in freshly dissolved HFIP stocks. The α-helical conformational intermediate in Aβ40, Aβ42, and Aβ43 could favour the formation of both fibrillar and non-fibrillar aggregates depending on solvent conditions and rate of α-helical to β-sheet transition.

Disulfide-bond scrambling promotes amorphous aggregates in lysozyme and bovine serum albumin

Disulfide bonds are naturally formed in more than 50% of amyloidogenic proteins, but the exact role of disulfide bonds in protein aggregation is still not well-understood. The intracellular reducing agents and/or improper use of antioxidants in extracellular environment can break proteins disulfide bonds, making them unstable and prone to misfolding and aggregation. In this study, we report the effect of disulfide-reducing agent dithiothreitol (DTT) on hen egg white lysozyme (lysozyme) and bovine serum albumin (BSA) aggregation at pH 7.2 and 37 °C. BSA and lysozyme proteins treated with disulfide-reducing agents form very distinct amorphous aggregates as observed by scanning electron microscope. However, proteins with intact disulfide bonds were stable and did not aggregate over time. BSA and lysozyme aggregates show unique but measurable differences in 8-anilino-1-naphthalenesulfonic acid (ANS) and 4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfonic acid (bis-ANS) fluorescence, suggesting a loose and flexible aggregate structure for lysozyme but a more compact aggregate structure for BSA. Scrambled disulfide-bonded protein aggregates were observed by nonreducing sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) for both proteins. Similar amorphous aggregates were also generated using a nonthiol-based reducing agent, tris(2-carboxyethyl)phosphine (TCEP), at pH 7.2 and 37 °C. In summary, formation of distinct amorphous aggregates by disulfide-reduced BSA and lysozyme suggests an alternate pathway for protein aggregation that may be relevant to several proteins.

Chiral metallohelical complexes enantioselectively target amyloid β for treating Alzheimer's disease

Stereochemistry is a very important issue for the pharmaceutical industry and can determine drug efficacy. The design and synthesis of small molecules, especially chiral molecules, which selectively target and inhibit amyloid-β (Aβ) aggregation, represent valid therapeutic strategies for treatment of Alzheimer's disease (AD). Herein we report that two triple-helical dinuclear metallosupramolecular complexes can act as a novel class of chiral amyloid-β inhibitors. Through targeting α/β-discordant stretches at the early steps of aggregation, these metal complexes can enantioselectively inhibit Aβ aggregation, which is demonstrated using fluorescent living cell-based screening and multiple biophysical and biochemical approaches. To the best of our knowledge, this is the first report of enantioselective inhibition of Aβ aggregation. Intriguingly, as a promising candidate for AD treatment, the chiral metal complex can cross the blood-brain barrier and have superoxide dismutase activity. It is well-known that chiral discrimination is important for understanding chiral drug action. Generally, one enantiomer is pharmaceutically active while the other is inactive or exerts severe side effects. Chiral discrimination should be important for AD treatment. Our work provides new insights into chiral inhibition of Aβ aggregation and opens a new avenue for design and screening of chiral agents as Aβ inhibitors against AD.

The peculiar role of the A2V mutation in amyloid-β (Aβ) 1-42 molecular assembly

We recently reported a novel Aβ precursor protein mutation (A673V), corresponding to position 2 of Aβ1-42 peptides (Aβ1-42A2V), that caused an early onset AD-type dementia in a homozygous individual. The heterozygous relatives were not affected as an indication of autosomal recessive inheritance of this mutation. We investigated the folding kinetics of native unfolded Aβ1-42A2V in comparison with the wild type sequence (Aβ1-42WT) and the equimolar solution of both peptides (Aβ1-42MIX) to characterize the oligomers that are produced in the early phases. We carried out the structural characterization of the three preparations using electron and atomic force microscopy, fluorescence emission, and x-ray diffraction and described the soluble oligomer formation kinetics by laser light scattering. The mutation promoted a peculiar pathway of oligomerization, forming a connected system similar to a polymer network with hydrophobic residues on the external surface. Aβ1-42MIX generated assemblies very similar to those produced by Aβ1-42WT, albeit with slower kinetics due to the difficulties of Aβ1-42WT and Aβ1-42A2V peptides in building up of stable intermolecular interaction.

Neuroprotective peptide–macrocycle conjugates reveal complex structure–activity relationships in their interactions with amyloid β

Novel neuroprotective peptide–macrocycle conjugates exhibit complex, multifaceted structure–activity relationships in their interactions with amyloid β.

Design and synthesis of curcumin analogues for in vivo fluorescence imaging and inhibiting copper-induced cross-linking of amyloid beta species in Alzheimer's disease

In this article, we first designed and synthesized curcumin-based near-infrared (NIR) fluorescence imaging probes for detecting both soluble and insoluble amyloid beta (Aβ) species and then an inhibitor that could attenuate cross-linking of Aβ induced by copper. According to our previous results and the possible structural stereohindrance compatibility of the Aβ peptide and the hydrophobic/hydrophilic property of the Aβ13-20 (HHQKLVFF) fragment, NIR imaging probe CRANAD-58 was designed and synthesized. As expected CRANAD-58 showed significant fluorescence property changes upon mixing with both soluble and insoluble Aβ species in vitro. In vivo NIR imaging revealed that CRANAD-58 was capable of differentiating transgenic and wild-type mice as young as 4 months old, the age that lacks apparently visible Aβ plaques and Aβ is likely in its soluble forms. According to our limited studies on the interaction mechanism between CRANAD-58 and Aβ, we also designed CRANAD-17 to attenuate the cross-linking of Aβ42 induced by copper. It is well-known that the coordination of copper with imidazoles on Histidine-13 and 14 (H13, H14) of Aβ peptides could initialize covalent cross-linking of Aβ. In CRANAD-17, a curcumin scaffold was used as an anchoring moiety to usher the designed compound to the vicinity of H13 and H14 of Aβ, and imidazole rings were incorporated to compete with H13/H14 for copper binding. The results of SDS-PAGE gel and Western blot indicated that CRANAD-17 was capable of inhibiting Aβ42 cross-linking induced by copper. This raises a potential for CRANAD-17 to be considered for AD therapy.

Amyloid-beta (Aβ) D7H mutation increases oligomeric Aβ42 and alters properties of Aβ-zinc/copper assemblies

Amyloid precursor protein (APP) mutations associated with familial Alzheimer's disease (AD) usually lead to increases in amyloid β-protein (Aβ) levels or aggregation. Here, we identified a novel APP mutation, located within the Aβ sequence (Aβ_D7H), in a Taiwanese family with early onset AD and explored the pathogenicity of this mutation. Cellular and biochemical analysis reveal that this mutation increased Aβ production, Aβ42/40 ratio and prolonged Aβ42 oligomer state with higher neurotoxicity. Because the D7H mutant Aβ has an additional metal ion-coordinating residue, histidine, we speculate that this mutation may promote susceptibility of Aβ to ion. When co-incubated with Zn²⁺ or Cu²⁺, Aβ_D7H aggregated into low molecular weight oligomers. Together, the D7H mutation could contribute to AD pathology through a “double punch” effect on elevating both Aβ production and oligomerization. Although the pathogenic nature of this mutation needs further confirmation, our findings suggest that the Aβ N-terminal region potentially modulates APP processing and Aβ aggregation, and further provides a genetic indication of the importance of Zn²⁺ and Cu²⁺ in the etiology of AD.

Distinct effects of Zn2+, Cu2+, Fe3+, and Al3+ on amyloid-beta stability, oligomerization, and aggregation: amyloid-beta destabilization promotes annular protofibril formation

Abnormally high concentrations of Zn(2+), Cu(2+), and Fe(3+) are present along with amyloid-β (Aβ) in the senile plaques in Alzheimer disease, where Al(3+) is also detected. Aβ aggregation is the key pathogenic event in Alzheimer disease, where Aβ oligomers are the major culprits. The fundamental mechanism of these metal ions on Aβ remains elusive. Here, we employ 4,4'-Bis(1-anilinonaphthalene 8-sulfonate) and tyrosine fluorescence, CD, stopped flow fluorescence, guanidine hydrochloride denaturation, and photo-induced cross-linking to elucidate the effect of Zn(2+), Cu(2+), Fe(3+), and Al(3+) on Aβ at the early stage of the aggregation. Furthermore, thioflavin T assay, dot blotting, and transmission electron microscopy are utilized to examine Aβ aggregation. Our results show that Al(3+) and Zn(2+), but not Cu(2+) and Fe(3+), induce larger hydrophobic exposures of Aβ conformation, resulting in its significant destabilization at the early stage. The metal ion binding induces Aβ conformational changes with micromolar binding affinities and millisecond binding kinetics. Cu(2+) and Zn(2+) induce similar assembly of transiently appearing Aβ oligomers at the early state. During the aggregation, we found that Zn(2+) exclusively promotes the annular protofibril formation without undergoing a nucleation process, whereas Cu(2+) and Fe(3+) inhibit fibril formation by prolonging the nucleation phases. Al(3+) also inhibits fibril formation; however, the annular oligomers co-exist in the aggregation pathway. In conclusion, Zn(2+), Cu(2+), Fe(3+), and Al(3+) adopt distinct folding and aggregation mechanisms to affect Aβ, where Aβ destabilization promotes annular protofibril formation. Our study facilitates the understanding of annular Aβ oligomer formation upon metal ion binding.

Folding stability of amyloid-beta 40 monomer is an important determinant of the nucleation kinetics in fibrillization

Amyloid formation is initiated by protein misfolding, followed by self-association to ultimately form amyloid fibrils. The discovery of toxic prefibrillar oligomers in many amyloidosis underscores the importance of understanding the folding mechanism prior to such aggregation. Here, we investigated the folding properties of the natively unfolded amyloid-β (Aβ) peptide and the familial variants (A21G, E22Q, E22G, E22K, and D23N) in Alzheimer's disease (AD). In combinations of native electrophoresis, analytical ultracentrifugation, fluorescence emission, and far-UV circular dichroism, we showed that all Aβ40 variants are predominantly monomeric with similar residual secondary structures, but distinct hydrophobic-exposed protein surfaces. Guanidine hydrochloride (GdnHCl) denaturation in the absence and presence of trifluoroethanol (TFE) showed that Aβ variants adopt an apparent 2-state equilibrium model with different stabilities, in which wild type is less stable than A21G but more stable than D23N and E22 mutants. By correlating the folding stability with the nucleation phase in fibrillization, we found the more stable the variant, the slower the nucleation, except for D23N. Besides, the unfolding of Aβ conformation leads to reduced formation of mature fibrils, but an increase in nonfibrillar, amorphous type of aggregates. Overall, we demonstrated that folding stability of Aβ is an important determinant of the nucleation kinetics.

Residual structure in islet amyloid polypeptide mediates its interactions with soluble insulin

Islet amyloid polypeptide (IAPP), a 37-amino acid polypeptide hormone of the calcitonin family, is colocalized and cosecreted with insulin in secretory granules of pancreatic islet beta cells. IAPP can assemble into toxic oligomers and amyloid fibrils, a hallmark of type 2 diabetes. Its interactions with insulin in the secretory granules might influence the formation of cytotoxic oligomers and amyloid fibrils. Presented NMR analysis shows that IAPP, free in solution and in complex with insulin, retains elements of residual secondary structure. NMR chemical shifts and (15)N relaxation data as well as 49 ns replica exchange molecular dynamic simulations indicate that the transiently populated helical structure in residues 11-18 is essential for interactions with insulin. These interactions are mediated by salt bridges between positively charged residues Arg11 or Arg18 of rat IAPP and Glu13 of insulin B chain as well as by hydrophobic interactions flanking the salt bridges. The insulin binding region is composed of the same amino acids in amyloidogenic human IAPP and soluble rat IAPP (with the sole exception of His/Arg-18), implying the same binding mode for both hormones. This His/Arg-18 mutation results in reduced affinity binding of human IAPP to insulin in comparison to rat IAPP as it is detected by surface plasmon resonance biosensor analysis. Implications of the described interactions between soluble forms of IAPP and insulin in preventing oligomerization of human IAPP are discussed.

A chemical screening approach reveals that indole fluorescence is quenched by pre-fibrillar but not fibrillar amyloid-beta

Aggregated amyloid-beta (Abeta) peptide is implicated in the pathology of Alzheimer's disease. In vitro and in vivo, these aggregates are found in a variety of morphologies, including globular oligomers and linear fibrils, which possess distinct biological activities. However, known chemical probes, including the dyes thioflavin T and Congo Red, appear to lack selectivity for specific amyloid structures. To identify molecules that might differentiate between these architectures, we employed a fluorescence-based interaction assay to screen a collection of 68 known Abeta ligands against pre-formed oligomers and fibrils. In these studies, we found that the fluorescence of five indole-based compounds was selectively quenched ( approximately 15%) in the presence of oligomers, but remained unchanged after addition of fibrils. These results suggest that indoles might be complementary to existing chemical probes for studying amyloid formation in vitro.

Electrochemical and conformational consequences of copper (Cu(I) and Cu(II)) binding to beta-amyloid(1-40)

Copper-induced structural rearrangements of Abeta40 structure and its redox properties are described in this study. Electrochemical and fluorescent methods are used to characterise the behaviour of Abeta-Cu species. The data suggest that time-dependent folding of Abeta-Cu species may cause changes in the redox potentials.Extracellular deposits of beta-amyloid (Abeta) into senile plaques are the major features observed in brains of Alzheimer's disease (AD) patients. A high concentration of copper has been associated with insoluble amyloid plaques. It is known that Abeta(1-40) can bind copper with high affinity, but electrochemical properties of Abeta(1-40)-Cu complexes are not well-characterised. In this study we demonstrate that complexation of copper (both as Cu(I) and Cu(II)) by Abeta(1-40) reduces the metal electrochemical activity. Formation of copper-Abeta(1-40) complexes is associated with alteration of the redox potential. The data reveal significant redox activity of fresh Abeta-copper solutions. However, copper-induced structural rearrangements of the peptide, documented by CD, correspond with time-dependent changes of formal reduction potentials (E(0')) of the complex. Fluorescent and electrochemical (cyclic voltammetry and differential pulse voltammetry) techniques suggest that reduction of the redox activity by Abeta-Cu complexes could be attributed to conformational changes that diminished copper accessibility to the external environment. According to our evidence, conformational rearrangements, induced by copper binding to amyloid, elongate the time necessary to attain the same beta-sheet content as for the metal-free peptide. Although the redox activity of Abeta-Cu complexes diminishes in a time-dependent manner, they are not completely devoid of toxicity as they destabilize red blood cells osmotic fragility, even after prolonged incubation.

High resolution absorption spectrum of dianilino in the vapor phase

Photophysical and photochemical properties of diradical in the first excited state is recorded for the very first time with the IR, band structure for dianilino molecule at room temperature, in the vapor phase. In this experiment high resolution absorption spectra of anilino free radical, dianilino, aniline in the vapor phase and silicon dioxide in the solid state were recorded by flash photolysis technique photographically. Silicon dioxide absorption band between 250 and 255 nm were also observed for the reaction cell, because the cell and windows of the cell material were spectrosilica grade fused quartz. And this absorption band also used as wavelength calibration for all the photoproducts formed in the reaction cell during optical pumping.

Fluorescent N-arylaminonaphthalene sulfonate probes for amyloid aggregation of alpha-synuclein

The deposition of fibrillar structures (amyloids) is characteristic of pathological conditions including Alzheimer's and Parkinson's diseases. The detection of protein deposits and the evaluation of their kinetics of aggregation are generally based on fluorescent probes such as thioflavin T and Congo red. In a search for improved fluorescence tools for studying amyloid formation, we explored the ability of N-arylaminonaphthalene sulfonate (NAS) derivatives to act as noncovalent probes of alpha-synuclein (AS) fibrillation, a process linked to Parkinson's disease and other neurodegenerative disorders. The compounds bound to fibrillar AS with micromolar K(d)s, and exhibited fluorescence enhancement, hyperchromism, and high anisotropy. We conclude that the probes experience a hydrophobic environment and/or restricted motion in a polar region. Time- and spectrally resolved emission intensity and anisotropy provided further information regarding structural features of the protein and the dynamics of solvent relaxation. The steady-state and time-resolved parameters changed during the course of aggregation. Compared with thioflavin T, NAS derivatives constitute more sensitive and versatile probes for AS aggregation, and in the case of bis-NAS detect oligomeric as well as fibrillar species. They can function in convenient, continuous assays, thereby providing useful tools for studying the mechanisms of amyloid formation and for high-throughput screening of factors inhibiting and/or reversing protein aggregation in neurodegenerative diseases.

Bis-styrylpyridine and bis-styrylbenzene derivatives as inhibitors for Aβ fibril formation

New bis-styrylpyridine and bis-styrylbenzene derivatives were designed and synthesized. These 34 compounds were evaluated by Abeta fibril formation inhibitory assay using thioflavin T as a dye (named ThT assay). Most of them showed excellent inhibitory activities for Abeta fibril formation at IC50 of 0.1-2.7 microM which is comparable to curcumin (IC50 of 0.8 microM). Among them, nine compounds were screened for their cytotoxicities on HT-22 cell by MTT assay at 1, 10, and 50 microM. In particular, I-7 and II-2 exhibited the best combination of inhibitory activity and compound cytotoxicity.

Discrete conformational changes as regulators of the hydrolytic properties of beta-amyloid (1-40)

Beta-amyloid (1-40) (Abeta), the main component of senile plaques seen in the brains of Alzheimer's disease patients, was found to be toxic both as fibrils and smaller soluble globular aggregates. The hydrolytic properties of Abeta, a new biochemical activity described previously [Brzyska M, Bacia A & Elbaum D (2001) Eur J Biochem 268, 3443-3454], may contribute to its overall toxicity. In this study, the hydrolysis of fluorescein ester series was studied under predetermined conditions affecting Abeta hydrophobicity and conformation. Reaction products of the most effectively decomposed ester (dibutyrate) were characterized using HPLC and ESI-MS. Hydrophobicity of Abeta, as measured by bis-8-anilinonaphthalene fluorescence, correlated with its hydrolytic abilities. FTIR and CD data analysis showed a relationship between enhanced hydrolytic abilities and Abeta structure. Seriously limited hydrolysis caused by higher peptide concentrations is consistent with monomeric/dimeric Abeta species participation in the process, confirmed by thioflavine T binding. Inhibition of hydrolysis was caused by beta-sheet breaker peptide (LPFFD), indicating that the Abeta central hydrophobic cluster (amino acids 17-21) participates in the process. The reported Abeta properties suggest that small conformational alterations of the peptide structure may have a pronounced effect on its functions and biological activity.

Amyloid accumulation and pathogensis of Alzheimer's disease: significance of monomeric, oligomeric and fibrillar Abeta

This chapter reviews recent findings that indicate that soluble amyloid oligomers may represent the primary pathological species in Alzheimer's and other degenerative diseases. Various amyloids share a number of common properties, including their structures and pathways for fibril formation and accumulation in disease. Recent findings suggest that toxic amyloid oligomers share a common structure, suggesting that they also share a common mechanism of pathogenesis

Distinct Early Folding and Aggregation Properties of Alzheimer Amyloid-β Peptides Aβ40 and Aβ42

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.

Butyrylcholinesterase attenuates amyloid fibril formation in vitro

In Alzheimer's disease, both acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) colocalize with brain fibrils of amyloid-beta (Abeta) peptides, and synaptic AChE-S facilitates fibril formation by association with insoluble Abeta fibrils. Here, we report that human BChE and BSP41, a synthetic peptide derived from the BChE C terminus, inversely associate with the soluble Abeta conformers and delay the onset and decrease the rate of Abeta fibril formation in vitro, at a 1:100 BChE/Abeta molar ratio and in a dose-dependent manner. The corresponding AChE synthetic peptide (ASP)40 peptide, derived from the homologous C terminus of synaptic human (h)AChE-S, failed to significantly affect Abeta fibril formation, attributing the role of enhancing this process to an AChE domain other than the C terminus. Circular dichroism and molecular modeling confirmed that both ASP40 and BChE synthetic peptide (BSP)41 are amphipathic alpha-helices. However, ASP40 shows symmetric amphipathicity, whereas BSP41 presented an aromatic tryptophan residue in the polar side of the C terminus. That this aromatic residue is causally involved in the attenuating effect of BChE was further supported by mutagenesis experiments in which (W8R) BSP41 showed suppressed capacity to attenuate fibril formation. In Alzheimer's disease, BChE may have thus acquired an inverse role to that of AChE by adopting imperfect amphipathic characteristics of its C terminus.

Poly-(L-alanine) expansions form core β-sheets that nucleate amyloid assembly

Expansion to a total of 11-17 sequential alanine residues from the normal number of 10 in the polyadenine-binding protein nuclear-1 (PABPN1) results in formation of intranuclear, fibrillar inclusions in skeletal muscle and hypothalamic neurons in adult-onset, dominantly inherited oculopharyngeal muscular dystrophy (OPMD). To understand the role that homopolymeric length may play in the protein misfolding that leads to the inclusions, we analyzed the self-assembly of synthetic poly-(L-alanine) peptides having 3-20 residues. We found that the conformational transition and structure of polyalanine (polyAla) assemblies in solution are not only length-dependent but also are determined by concentration, temperature, and incubation time. No beta-sheet complex was detected for those peptides characterized by n < 8, where n is number of alanine residues. A second group of peptides with 7 < n < 15 showed varying levels of complex formation, while for those peptides having n > 15, the interconversion process from the monomeric to the beta-sheet complex was complete under any of the tested experimental conditions. Unlike the typical tinctorial properties of amyloid fibrils, polyalanine fibrils did not show fluorescence with thioflavin T or apple-green birefringence with Congo red; however, like amyloid, X-ray diffraction showed that the peptide chains in these fibrils were oriented normal to the fibril axis (i.e., in the cross-beta arrangement). Neighboring beta-sheets are quarter-staggered in the hydrogen-bonding direction such that the alanine side-chains were closely packed in the intersheet space. Strong van der Waals contacts between side-chains in this arrangement likely account for the high stability of the macromolecular fibrillar complex in solution over a wide range of temperature (5-85 degrees C), and pH (2-10.5), and its resistance to denaturant (< 8 M urea) and to proteases (protease K, trypsin). We postulate that a similar stabilization of an expanded polyalanine stretch could form a core beta-sheet structure that mediates the intermolecular association of mutant proteins into fibrillar inclusions in human pathologies.

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.

A hybrid molecule that prohibits amyloid fibrils and alleviates neuronal toxicity induced by beta-amyloid (1-42)

Inhibition of oligomeric amyloid beta (Abeta) peptide or fibril formation has emerged as a major therapeutic target for developing new drugs for Alzheimer's disease. We focused on developing inhibitors by synthesizing hybrid molecules of ferulic acid and styryl benzene, which has been known as a fibril binder. Initially, cell-based assay was carried out to evaluate the effective compound. A selected effector, 1, alleviated the Abeta-induced neuronal toxicity in differentiated SH-SY5Y human neuroblastoma cells. The effector could also inhibit Abeta fibril formation, monitored by thioflavin T fluorescence intensity assay and transmitted electron microscopic images. A strong binding affinity of 1 to non-fibrous monomer-like Abeta, which was immobilized to a surface chip, was measured using a surface plasmon resonance technique. The data suggest that the effector shifts the equilibrium of multimeric Abeta, inhibiting the pathogenic oligomer or fibril formation.

Amyloid-β aggregates formed at polar-nonpolar interfaces differ from amyloid-β protofibrils produced in aqueous buffers

The deposition of aggregated amyloid-beta (Abeta) peptides in the brain as senile plaques is a pathological hallmark of Alzheimer's disease (AD). Several lines of evidence indicate that fibrillar and, in particular, soluble aggregates of these 40- and 42-residue peptides are important in the etiology of AD. Recent studies also stress that amyloid aggregates are polymorphic and that a single polypeptide can fold into multiple amyloid conformations. Here we review our recent reports that Abeta(1-40) in vitro can form soluble aggregates with predominant beta-structures that differ in stability and morphology. One class of aggregates involved soluble Abeta protofibrils, prepared by vigorous overnight agitation of monomeric Abeta(1-40) in low ionic strength buffers. These aggregates were quite stable and disaggregated to only a limited extent on dilution. A second class of soluble Abeta aggregates was generated at polar-nonpolar interfaces. Aggregation in a two-phase system of buffer over chloroform occurred more rapidly than in buffer alone. In buffered 2% hexafluoroisopropanol (HFIP), microdroplets of HFIP were formed and the half-time for aggregation was less than 10 minutes. Like Abeta protofibrils, these interfacial aggregates showed increased thioflavin T fluorescence and were rich in beta-structure by circular dichroism. However, electron microscopy and atomic force microscopy revealed very different morphologies. The HFIP aggregates formed initial globular clusters that progressed over several days to soluble fibrous aggregates. When diluted out of HFIP these aggregates initially were very unstable and disaggregated completely within 2 minutes. However, their stability increased as they progressed to fibers. It is important to determine whether similar interfacial Abeta aggregates are produced in vivo.

Amyloid-β Protofibrils Differ from Amyloid-β Aggregates Induced in Dilute Hexafluoroisopropanol in Stability and Morphology

The brains of Alzheimer's disease (AD) patients contain large numbers of amyloid plaques that are rich in fibrils composed of 40- and 42-residue amyloid-beta (Abeta) peptides. Several lines of evidence indicate that fibrillar Abeta and especially soluble Abeta aggregates are important in the etiology of AD. Recent reports also stress that amyloid aggregates are polymorphic and that a single polypeptide can fold into multiple amyloid conformations. Here we demonstrate that Abeta-(1-40) can form soluble aggregates with predominant beta-structures that differ in stability and morphology. One class of aggregates involved soluble Abeta protofibrils, prepared by vigorous overnight agitation of monomeric Abeta-(1-40) at low ionic strength. Dilution of these aggregation reactions induced disaggregation to monomers as measured by size exclusion chromatography. Protofibril concentrations monitored by thioflavin T fluorescence decreased in at least two kinetic phases, with initial disaggregation (rate constant approximately 1 h(-1)) followed by a much slower secondary phase. Incubation of the reactions without agitation resulted in less disaggregation at slower rates, indicating that the protofibrils became progressively more stable over time. In fact, protofibrils isolated by size exclusion chromatography were completely stable and gave no disaggregation. A second class of soluble Abeta aggregates was generated rapidly (<10 min) in buffered 2% hexafluoroisopropanol (HFIP). These aggregates showed increased thioflavin T fluorescence and were rich in beta-structure by circular dichroism. Electron microscopy and atomic force microscopy revealed initial globular clusters that progressed over several days to soluble fibrous aggregates. When diluted out of HFIP, these aggregates initially were very unstable and disaggregated completely within 2 min. However, their stability increased as they progressed to fibers. Relative to Abeta protofibrils, the HFIP-induced aggregates seeded elongation by Abeta monomer deposition very poorly. The techniques used to distinguish these two classes of soluble Abeta aggregates may be useful in characterizing Abeta aggregates formed in vivo.

The effect of cholesterol and monosialoganglioside (GM1) on the release and aggregation of amyloid beta-peptide from liposomes prepared from brain membrane-like lipids

In order to investigate the influence of cholesterol (Ch) and monosialoganglioside (GM1) on the release and subsequent deposition/aggregation of amyloid beta peptide (Abeta)-(1-40) and Abeta-(1-42), we have examined Abeta peptide model membrane interactions by circular dichroism, turbidity measurements, and transmission electron microscopy (TEM). Model liposomes containing Abeta peptide and a lipid mixture composition similar to that found in the cerebral cortex membranes (CCM-lipid) have been prepared. In all, four Abeta-containing liposomes were investigated: CCM-lipid; liposomes with no GM1 (GM1-free lipid); those with no cholesterol (Ch-free lipid); liposomes with neither cholesterol nor GM1 (Ch-GM1-free lipid). In CCM liposomes, Abeta was rapidly released from membranes to form a well defined fibril structure. However, for the GM1-free lipid, Abeta was first released to yield a fibril structure about the membrane surface, then the membrane became disrupted resulting in the formation of small vesicles. In Ch-free lipid, a fibril structure with a phospholipid membrane-like shadow formed, but this differed from the well defined fibril structure seen for CCM-lipid. In Ch-GM1-free lipid, no fibril structure formed, possibly because of membrane solubilization by Abeta. The absence of fibril structure was noted at physiological extracellular pH (7.4) and also at liposomal/endosomal pH (5.5). Our results suggest a possible role for both Ch and GM1 in the membrane release of Abeta from brain lipid bilayers.

Direct Measurement of Antigen Binding Properties of CD1 Proteins Using Fluorescent Lipid Probes

CD1 proteins are antigen-presenting molecules that bind foreign and self-lipids and stimulate specific T cell responses. In the current study, we investigated ligand binding by CD1 proteins by developing a fluorescent probe binding approach using soluble recombinant human CD1 proteins. To increase stability and yield, soluble group 1 CD1 (CD1b and CD1c) and group 2 CD1 (CD1d) proteins were produced as single chain secreted CD1 proteins in which beta2-microglobulin was fused to the N termini of the CD1 heavy chains by a flexible peptide linker sequence. Analysis of ligand binding properties of single chain secreted CD1 proteins by using fluorescent lipid probes indicated significant differences in ligand preference and in pH dependence of binding by group 1 versus group 2 CD1 proteins. Whereas group 1 CD1 isoforms (CD1b and CD1c) show stronger binding of nitrobenzoxadiazole (NBD)-labeled dialkyl-based ligands (phosphatidylcholine, sphingomyelin, and ceramide), group 2 CD1 (CD1d) proteins were stronger binders of small hydrophobic probes such as 1-anilinonaphthalene-8-sulfonic acid and 4,4'-dianilino-1,1'-naphthyl-5,5'-disulfonic acid. Competition studies indicated that binding of fluorescent lipid probes involved association of the probe with the hydrophobic ligand binding groove of CD1 proteins. Analysis of selected alanine substitution mutants of human CD1b known to inhibit antigen presentation showed that NBD-labeled lipid probe binding could be used to distinguish mutations that interfere with ligand binding from those that affect T cell receptor docking. Our findings provide further evidence for the functional specialization of different CD1 isoforms and demonstrate the value of the fluorescent lipid probe binding method for assisting structure-based studies of CD1 function.

Y10W beta(1-40) fluorescence reflects epitope exposure in conformers of Alzheimer's beta-peptide

The Alzheimer's beta-peptide in neutral aqueous solution is characterized variously as a random coil or a heterogeneous mixture of conformers. Under conditions of lowered pH characteristic of intracellular compartments such as endosomes or lysosomes, a different conformation is favored, which is reflected in the biophysical and biological properties of the peptide. The reactivity of the epitope of the monoclonal antibody 6F/3D, encompassing residues 9-14, is drastically reduced. The fluorescence of human sequence beta(1-40) with the tyrosine at position 10 substituted with tryptophan (Y10W beta(1-40)) is quenched nearly 50% when the peptide is shifted to pH 4.6. The exposure of the 6F/3D epitope parallels Y10W beta(1-40) fluorescence changes induced by a variety of perturbations. The linkage of the sensitivity of immunological detection with the potential for monitoring rapid changes by fluorescence offers convergence of biology and biophysics in the study of beta-amyloid peptide conformation.

Common structure of soluble amyloid oligomers implies common mechanism of pathogenesis

Soluble oligomers are common to most amyloids and may represent the primary toxic species of amyloids, like the Abeta peptide in Alzheimer's disease (AD). Here we show that all of the soluble oligomers tested display a common conformation-dependent structure that is unique to soluble oligomers regardless of sequence. The in vitro toxicity of soluble oligomers is inhibited by oligomer-specific antibody. Soluble oligomers have a unique distribution in human AD brain that is distinct from fibrillar amyloid. These results indicate that different types of soluble amyloid oligomers have a common structure and suggest they share a common mechanism of toxicity.