Assembly of Alzheimer's Aβ peptide into nanostructured amyloid fibrils

Polyoxometalates Mediated Amyloid Fibrillation Dynamics and Restoration of Enzyme Activity of Hen Egg White Lysozyme Treated under Cold Atmospheric Pressure Plasma

Neurodegenerative disorders are one of the most devastating disorders worldwide. Although a definite mechanistic pathway of neurodegenerative disorders is still not clear, it is almost clear that these diseases are initiated by protein misfolding. Hen Egg White Lysozyme (Lyz) can be converted to highly arranged amyloid fibrils and is therefore considered a good model protein for studying protein aggregation in connection to neurodegeneration. In this study, Lyz has been converted to fibrils using He-air gas fed single jet cold atmospheric plasma (CAP). The reactive oxygen species and the reactive nitrogen species produced by the plasma jet interact with the protein molecules and enhance the fibril formation. We monitored the fibrillation kinetics with the Thioflavin T (ThT) assay and observed that fibrils are formed when the samples are treated for 10 min with He-air gas fed CAP. Further, we studied the role of a special class of inorganic nanomaterials called polyoxometalates (POMs) in the process of the Lyz fibrillation using various biophysical techniques. The Keggin POMs used in this study are phosphomolybdic acid (PMA) and silico molybdic acid (SMA). Keggin POMs bring in structural self-assembly of the protein and disrupt the fibrils as evidenced in the ThT assay and TEM analysis. Molecular docking studies together with electrokinetic potential studies show the interactions between POMs and Lyz dominated via hydrogen bonding and electrostatic interactions. The enzyme activity of Lyz was assessed using the substrate Micrococcus lysodeikticus and after treatment with POMs results showed a significant increase in the activity. This study could pave way for looking into Keggin POMs for possible application in neurodegeneration.

Effects of Dissolved Gases on the Amyloid Fibril Morphology

The onset or progression of numerous neurodegenerative diseases occurs due to aggregation of proteins that ultimately form fibrils. The assembly and morphology of fibrils are susceptible to environmental factors. In this work, we used atomic force microscopy (AFM) to investigate the effects of dissolved nitrogen and oxygen molecules on the morphology of fibrils formed by a hydrophobic amyloid peptide implicated in amyotrophic lateral sclerosis, 15 repeats of glycine-alanine, on a highly oriented pyrolytic graphite substrate. We started with preformed fibril solutions that were then diluted with buffers of different gas conditions, resulting in the aggregation of the fibrils into different morphologies that were revealed by AFM after adsorption on the substrate. Straight fibrils were observed in both degassed and ambient buffers, but a stronger lateral association was seen in degassed buffers. Smaller and softer fibrils were observed in O₂-supersaturated buffers, and plaque-like fibril aggregates of considerably large size were evident in N₂-supersaturated buffers. In overnight incubation experiments, we observed changes in both the morphology and height of the fibril aggregates, and their evolution varied with different gas conditions. These findings indicate that the gas type and concentration affect the aggregation of amyloid fibrils and may facilitate the development of biomaterial applications and treatments for amyloid-related diseases.

In Vitro Effects of (+)MK-801 (dizocilpine) and Memantine on β-Amyloid Peptides Linked to Alzheimer's Disease

An in vitro effect of (+)MK-801 (dizocilpine), an inhibitor of the glutamate/NMDA and nicotinic acetylcholine receptors, on the Aβ[1-42] and Aβ[1-40] peptides is described and compared to that of memantine. Memantine has been approved by the U.S. Food and Drug Administration for the treatment of mild-moderate Alzheimer's disease. Both compounds accelerated the formation of a β-sheet structure by Aβ[1-42], (+)MK-801 more rapidly than memantine, as observed in a thioflavin T fluorescence assay. The acceleration was followed by a decrease in the fluorescence signal that was not observed when the ligand was absent. Nuclear magnetic resonance spectra of the soluble peptides in the presence and absence of (+)MK-801 demonstrated that the monomeric form did not bind (+)MK-801 and that in the presence of (+)MK-801 the concentration of the monomeric form progressively decreased. Small angle X-ray scattering confirmed that the presence of (+)MK-801 resulted in a more rapid and characteristic transition to an insoluble form. These results suggest that (+)MK-801 and memantine accelerate the transition of Aβ[1-42] and Aβ[1-40] to ThT-negative insoluble forms.

Amylin in Alzheimer's disease: Pathological peptide or potential treatment?

Alzheimer's disease (AD) is a neurodegenerative disease for which we currently lack effective treatments or a cure. The pancreatic peptide hormone amylin has recently garnered interest as a potential pharmacological target for the treatment of AD. A number of studies have demonstrated that amylin and amylin analogs like the FDA-approved diabetes drug pramlintide can reduce amyloid burden in the brain and improve cognitive symptoms of AD. However, other data suggest that amylin may have pathological effects in AD due to its propensity to misfold and aggregate under certain conditions. Here, the literature supporting a beneficial versus harmful role of amylin in AD is reviewed. Additionally, several critical gaps in the literature are discussed, such as our limited understanding of the amylin system during aging and in disease states, as well as complexities of amylin receptor signaling and of changing pathophysiology during AD progression that might underlie the seemingly conflicting or contradictory results in the amylin/AD literature. This article is part of the Special Issue entitled 'Metabolic Impairment as Risk Factors for Neurodegenerative Disorders.'

Attenuation of β-Amyloid Toxicity In Vitro and In Vivo by Accelerated Aggregation

Accumulation and aggregation of β-amyloid (Aβ) peptides result in neuronal death, leading to cognitive dysfunction in Alzheimer's disease. The self-assembled Aβ molecules form various intermediate aggregates including oligomers that are more toxic to neurons than the mature aggregates, including fibrils. Thus, one strategy to alleviate Aβ toxicity is to facilitate the conversion of Aβ intermediates to larger aggregates such as fibrils. In this study, we designed a peptide named A3 that significantly enhanced the formation of amorphous aggregates of Aβ by accelerating the aggregation kinetics. Thioflavin T fluorescence experiments revealed an accelerated aggregation of Aβ monomers, accompanying reduced Aβ cytotoxicity. Transgenic Caenorhabditis elegans over-expressing amyloid precursor protein exhibited paralysis due to the accumulation of Aβ oligomers, and this phenotype was attenuated by feeding the animals with A3 peptide. These findings suggest that the Aβ aggregation-promotion effect can potentially be useful for developing strategies to reduce Aβ toxicity.

Evidence against a role of P-glycoprotein in the clearance of the Alzheimer's disease Aβ1-42 peptides

It has been proposed that the amyloid-β peptides (Aβ) cause the neuronal degeneration in the Alzheimer's disease brain. An imbalance between peptide production at the neuronal level and their elimination across the blood-brain-barrier (BBB) results in peptide accumulation inside the brain. The identification and functional characterization of the transport systems in the BBB with the capacity to transport Aβ is crucial for the understanding of Aβ peptide traffic from the brain to the blood. In this context, it has been suggested that the P-glycoprotein (P-gp), expressed in endothelial cells of the BBB, plays a role in the elimination of Aβ. However, there is little, if any, experimental evidence to support this; therefore, the aim of this investigation was to determine whether P-gp is capable of transporting Aβ peptides. Our results show that ATPase activity measured in plasma membrane vesicles of K562 cells overexpressing P-gp is not increased by the presence of Aβ42, suggesting that Aβ42 is not a P-gp substrate. Similarly, P-gp of pirarubicin was unaffected by Aβ42. Moreover, the overexpression of P-gp does not protect cells against Aβ42 toxicity. Taken together, our results support the conclusion that Aβ42 is not transported by P-gp.

Neuropathology and biochemistry of Aβ and its aggregates in Alzheimer's disease

Alzheimer's disease (AD) is characterized by β-amyloid plaques and intraneuronal τ aggregation usually associated with cerebral amyloid angiopathy (CAA). Both β-amyloid plaques and CAA deposits contain fibrillar aggregates of the amyloid β-peptide (Aβ). Aβ plaques and CAA develop first in neocortical areas of preclinical AD patients and, then, expand in a characteristic sequence into further brain regions with end-stage pathology in symptomatic AD patients. Aβ aggregates are not restricted to amyloid plaques and CAA. Soluble and several types of insoluble non-plaque- and non-CAA-associated Aβ aggregates have been described. Amyloid fibrils are products of a complex self-assembly process that involves different types of transient intermediates. Amongst these intermediate species are protofibrils and oligomers. Different variants of Aβ peptides may result from alternative processing or from mutations that lead to rare forms of familial AD. These variants can exhibit different self-assembly and aggregation properties. In addition, several post-translational modifications of Aβ have been described that result, for example, in the production of N-terminal truncated Aβ with pyroglutamate modification at position 3 (AβN3pE) or of Aβ phosphorylated at serine 8 (pSer8Aβ). Both AβN3pE and pSer8Aβ show enhanced aggregation into oligomers and fibrils. However, the earliest detectable soluble and insoluble Aβ aggregates in the human brain exhibit non-modified Aβ, whereas AβN3pE and pSer8Aβ are detected in later stages. This finding indicates the existence of different biochemical stages of Aβ aggregate maturation with pSer8Aβ being related mainly to cases with symptomatic AD. The conversion from preclinical to symptomatic AD could thereby be related to combined effects of increased Aβ concentration, maturation of aggregates and spread of deposits into additional brain regions. Thus, the inhibition of Aβ aggregation and maturation before entering the symptomatic stage of the disease as indicated by the accumulation of pSer8Aβ may represent an attractive treatment strategy for preventing disease progression.

Lipids in Amyloid-β Processing, Aggregation, and Toxicity

Aggregation of amyloid-beta (Aβ) peptide is the major event underlying neuronal damage in Alzheimer's disease (AD). Specific lipids and their homeostasis play important roles in this and other neurodegenerative disorders. The complex interplay between the lipids and the generation, clearance or deposition of Aβ has been intensively investigated and is reviewed in this chapter. Membrane lipids can have an important influence on the biogenesis of Aβ from its precursor protein. In particular, increased cholesterol in the plasma membrane augments Aβ generation and shows a strong positive correlation with AD progression. Furthermore, apolipoprotein E, which transports cholesterol in the cerebrospinal fluid and is known to interact with Aβ or compete with it for the lipoprotein receptor binding, significantly influences Aβ clearance in an isoform-specific manner and is the major genetic risk factor for AD. Aβ is an amphiphilic peptide that interacts with various lipids, proteins and their assemblies, which can lead to variation in Aβ aggregation in vitro and in vivo. Upon interaction with the lipid raft components, such as cholesterol, gangliosides and phospholipids, Aβ can aggregate on the cell membrane and thereby disrupt it, perhaps by forming channel-like pores. This leads to perturbed cellular calcium homeostasis, suggesting that Aβ-lipid interactions at the cell membrane probably trigger the neurotoxic cascade in AD. Here, we overview the roles of specific lipids, lipid assemblies and apolipoprotein E in Aβ processing, clearance and aggregation, and discuss the contribution of these factors to the neurotoxicity in AD.

Targeting the proper amyloid-beta neuronal toxins: a path forward for Alzheimer's disease immunotherapeutics

Levels of amyloid-beta monomer and deposited amyloid-beta in the Alzheimer’s disease brain are orders of magnitude greater than soluble amyloid-beta oligomer levels. Monomeric amyloid-beta has no known direct toxicity. Insoluble fibrillar amyloid-beta has been proposed to be an in vivo mechanism for removal of soluble amyloid-beta and exhibits relatively low toxicity. In contrast, soluble amyloid-beta oligomers are widely reported to be the most toxic amyloid-beta form, both causing acute synaptotoxicity and inducing neurodegenerative processes. None of the amyloid-beta immunotherapies currently in clinical development selectively target soluble amyloid-beta oligomers, and their lack of efficacy is not unexpected considering their selectivity for monomeric or fibrillar amyloid-beta (or both) rather than soluble amyloid-beta oligomers. Because they exhibit acute, memory-compromising synaptic toxicity and induce chronic neurodegenerative toxicity and because they exist at very low in vivo levels in the Alzheimer’s disease brain, soluble amyloid-beta oligomers constitute an optimal immunotherapeutic target that should be pursued more aggressively.

Progress in the direct structural characterization of fibrous amphiphilic supramolecular assemblies in solution by transmission electron microscopic techniques

The self-assembly of amphiphilic molecules into fibrous structures has been the subject of numerous studies over past decades due to various current and promising technical applications. Although very different in their head group chemistry many natural as well as synthetic amphiphilic compounds derived from carbohydrates, carbocyanine dyes, or amino acids tend to form fibrous structures by molecular self-assembly in water predominantly twisted ribbons or tubes. Often a transition between these assembly structures is observed, which is a phenomenon already theoretically approached by Wolfgang Helfrich and still focus point in current research. With the development of suitable sample preparation and electron optical imaging techniques, cryogenic transmission electron microscopy (cryo-TEM) in combination with three-dimensional (3D) reconstruction techniques has become a particular popular direct characterization technique for supramolecular assemblies in general. Here we review the recent progress in deriving precise structural information from cryo-TEM data of particularly fibrous structures preferably in three dimensions.

Oligomer-targeting with a conformational antibody fragment promotes toxicity in Aβ-expressing flies

Introduction

The self-assembly of Aβ peptides into a range of conformationally heterogeneous amyloid states represents a fundamental event in Alzheimer’s disease. Within these structures oligomeric intermediates are considered to be particularly pathogenic. To test this hypothesis we have used a conformational targeting approach where particular conformational states, such as oligomers or fibrils, are recognized in vivo by state-specific antibody fragments.

Results

We show that oligomer targeting with the KW1 antibody fragment, but not fibril targeting with the B10 antibody fragment, affects toxicity in Aβ-expressing Drosophila melanogaster. The effect of KW1 is observed to occur selectively with flies expressing Aβ(1–40) and not with those expressing Aβ(1–42) or the arctic variant of Aβ(1–42) This finding is consistent with the binding preference of KW1 for Aβ(1–40) oligomers that has been established in vitro. Strikingly, and in contrast to the previously demonstrated in vitro ability of this antibody fragment to block oligomeric toxicity in long-term potentiation measurements, KW1 promotes toxicity in the flies rather than preventing it. This result shows the crucial importance of the environment in determining the influence of antibody binding on the nature and consequences of the protein misfolding and aggregation.

Conclusions

While our data support to the pathological relevance of oligomers, they highlight the issues to be addressed when developing inhibitory strategies that aim to neutralize these states by means of antagonistic binding agents.

Misfolding of amyloidogenic proteins and their interactions with membranes

In this paper, we discuss amyloidogenic proteins, their misfolding, resulting structures, and interactions with membranes, which lead to membrane damage and subsequent cell death. Many of these proteins are implicated in serious illnesses such as Alzheimer’s disease and Parkinson’s disease. Misfolding of amyloidogenic proteins leads to the formation of polymorphic oligomers and fibrils. Oligomeric aggregates are widely thought to be the toxic species, however, fibrils also play a role in membrane damage. We focus on the structure of these aggregates and their interactions with model membranes. Study of interactions of amlyoidogenic proteins with model and natural membranes has shown the importance of the lipid bilayer in protein misfolding and aggregation and has led to the development of several models for membrane permeabilization by the resulting amyloid aggregates. We discuss several of these models: formation of structured pores by misfolded amyloidogenic proteins, extraction of lipids, interactions with receptors in biological membranes, and membrane destabilization by amyloid aggregates perhaps analogous to that caused by antimicrobial peptides.

Single-particle characterization of Aβ oligomers in solution

Determining the pathological role of amyloids in amyloid-associated diseases will require a method for characterizing the dynamic distributions in size and shape of amyloid oligomers with high resolution. Here, we explored the potential of resistive-pulse sensing through lipid bilayer-coated nanopores to measure the size of individual amyloid-β oligomers directly in solution and without chemical modification. This method classified individual amyloid-β aggregates as spherical oligomers, protofibrils, or mature fibers and made it possible to account for the large heterogeneity of amyloid-β aggregate sizes. The approach revealed the distribution of protofibrillar lengths (12- to 155 -mer) as well as the average cross-sectional area of protofibrils and fibers.

Molecular basis of β-amyloid oligomer recognition with a conformational antibody fragment

Oligomers are intermediates of the β-amyloid (Aβ) peptide fibrillogenic pathway and are putative pathogenic culprits in Alzheimer's disease (AD). Here we report the biotechnological generation and biochemical characterization of an oligomer-specific antibody fragment, KW1. KW1 not only discriminates between oligomers and other Aβ conformations, such as fibrils or disaggregated peptide; it also differentiates between different types of Aβ oligomers, such as those formed by Aβ (1-40) and Aβ (1-42) peptide. This high selectivity of binding contrasts sharply with many other conformational antibodies that interact with a large number of structurally analogous but sequentially different antigens. X-ray crystallography, NMR spectroscopy, and peptide array measurements imply that KW1 recognizes oligomers through a hydrophobic and significantly aromatic surface motif that includes Aβ residues 18-20. KW1-positive oligomers occur in human AD brain samples and induce synaptic dysfunctions in living brain tissues. Bivalent KW1 potently neutralizes this effect and interferes with Aβ assembly. By altering a specific step of the fibrillogenic cascade, it prevents the formation of mature Aβ fibrils and induces the accumulation of nonfibrillar aggregates. Our data illuminate significant mechanistic differences in oligomeric and fibril recognition and suggest the considerable potential of KW1 in future studies to detect or inhibit specific types of Aβ conformers.

Thioflavin T Promotes Aβ(1-40) Amyloid Fibrils Formation

Fibrillogenesis of the small peptide Aβ(1-40) is considered to be the hallmark of Alzheimer's disease. Some evidence indicates small oligomers, rather than mature fibrils, as the key cytotoxic agents. The fluorescent dye Thioflavin T (ThT) is often used to detect amyloid deposits in both in vivo and in vitro experiments, and it is used to study kinetic measurements, under the fundamental hypothesis that this probe does not influence the aggregation processes. We report experimental data showing that ThT may promote the Aβ(1-40) peptide amyloid aggregation changing solvent-peptide interactions and stabilizing more ordered β-like conformation. This finding has a two-fold importance: It is a fundamental warning in all fibrillation experiments where ThT is used as fluorescent probe, and it suggests that ThT, accelerating fibril formation, could be used to reduce the presence of transient small oligomers, thus interfering with the pathogenic impact of Aβ peptide.

Solid-state NMR reveals a close structural relationship between amyloid-β protofibrils and oligomers

We have studied tertiary contacts in protofibrils and mature fibrils of amyloid-β (Aβ) peptides using solid-state NMR spectroscopy. Although intraresidue contacts between Glu-22 and Ile-31 were found in Aβ protofibrils, these contacts were completely absent in mature Aβ fibrils. This is consistent with the current models of mature Aβ fibrils. As these intramolecular contacts have also been reported in Aβ oligomers, our measurements suggest that Aβ protofibrils are structurally more closely related to oligomers than to mature fibrils. This suggests that some structural alterations have to take place on the pathway from Aβ oligomers/protofibrils to mature fibrils, in agreement with a model that suggests a conversion of intramolecular hydrogen-bonded structures of Aβ oligomers to the intermolecular stabilized mature fibrils (Hoyer, W., Grönwall, C., Jonsson, A., Ståhl, S., and Härd, T. (2008) Proc. Natl. Acad. Sci. U.S.A. 105, 5099-5104).

The effect of Aβ on IAPP aggregation in the presence of an isolated β-cell membrane

Fibrillar aggregates of the islet amyloid polypeptide (IAPP) and amyloid-β (Aβ) are known to deposit at pancreatic β-cells and neuronal cells and are associated with the cell degenerative diseases type-2 diabetes mellitus (T2DM) and Alzheimer's disease (AD), respectively. Since IAPP is secreted by β-cells and a membrane-damaging effect of IAPP has been discussed as a reason for β-cell dysfunction and the development of T2DM, studies of the interaction of IAPP with the β-cell membrane are of high relevance for gaining a molecular-level understanding of the underlying mechanism. Recently, it has also been shown that patients suffering from T2DM exhibit an increased risk to develop AD and vice versa, and a molecular link between AD and T2DM has been suggested. In this study, membrane lipids from the rat insulinoma-derived INS-1E β-cell line were isolated, and their interaction with the amyloidogenic peptides IAPP and Aβ and a mixture of both peptides has been studied. To yield insight into the associated peptides' conformational changes and their effect on the membrane integrity during aggregation, we have carried out attenuated total reflection Fourier transform infrared spectroscopy, fluorescence microscopy, and atomic force microscopy experiments. The IAPP-Aβ heterocomplexes formed were shown to adsorb, aggregate, and permeabilize the isolated β-cell membrane significantly slower than pure IAPP, however, at a rate that is much faster than that of pure Aβ. In addition, it could be shown that isolated β-cell membranes cause similar effects on the kinetics of IAPP and IAPP-Aβ fibril formation as anionic heterogeneous model membranes.

Oligomeric intermediates in amyloid formation: structure determination and mechanisms of toxicity

Oligomeric intermediates are non-fibrillar polypeptide assemblies that occur during amyloid fibril formation and that are thought to underlie the aetiology of amyloid diseases, such as Alzheimer's disease, Parkinson's disease and Huntington's disease. Focusing primarily on the oligomeric states formed from Alzheimer's disease β-amyloid (Aβ) peptide, this review will make references to other polypeptide systems, highlighting common principles or sequence-specific differences. The covered topics include the structural properties and polymorphism of oligomers, the biophysical mechanism of peptide self-assembly and its role for pathogenicity in amyloid disease. Oligomer-dependent toxicity mechanisms will be explained along with recently emerging possibilities of interference.