C-terminal tetrapeptides inhibit Aβ42-induced neurotoxicity primarily through specific interaction at the N-terminus of Aβ42

Synthesis and mechanistic study of ultrashort peptides that inhibits Alzheimer's Aβ-aggregation-induced neurotoxicity

Misfolding/aggregation of β-amyloid peptide lead to the formation of toxic oligomers or accumulation of amyloid plaques, which is a seminal step in the progression of Alzheimer's disease (AD). Despite continuous efforts in the development of therapeutic agents, the cure for AD remains a major challenge. Owing to specific binding affinity of structure-based peptides, we report the synthesis of new peptide-based inhibitors derived from the C-terminal sequences, Aβ38-40 and Aβ40-42. Preliminary screening using MTT cell viability assay and corroborative results from ThT fluorescence assay revealed a tripeptide showing significantly effective inhibition towards Aβ1-42 aggregation and induced toxicity. Peptide 3 exhibited excellent cell viability of 94.3 % at 2 μM and of 100 % at 4 μM and 10 μM. CD study showed that peptide 3 restrict the conformation transition of Aβ1-42 peptide towards cross-β-sheet structure and electron microscopy validated the absence of Aβ aggregates as indicated by the altered morphology of Aβ1-42 in the presence of peptide 3. The HRMS-ESI, DLS and ANS studies were performed to gain mechanistic insights into the effect of inhibitor against Aβ aggregation. This Aβ-derived ultrashort motif provides impetus for the development of peptide-based anti-AD agents.

IM-MS and ECD-MS/MS Provide Insight into Modulation of Amyloid Proteins Self-Assembly by Peptides and Small Molecules

Neurodegenerative proteinopathies are characterized by formation and deposition of misfolded, aggregated proteins in the nervous system leading to neuronal dysfunction and death. It is widely believed that metastable oligomers of the offending proteins, preceding the fibrillar aggregates found in the tissue, are the proximal neurotoxins. There are currently almost no disease-modifying therapies for these diseases despite an active pipeline of preclinical development and clinical trials for over two decades, largely because studying the metastable oligomers and their interaction with potential therapeutics is notoriously difficult. Mass spectrometry (MS) is a powerful analytical tool for structural investigation of proteins, including protein-protein and protein-ligand interactions. Specific MS tools have been useful in determining the composition and conformation of abnormal protein oligomers involved in proteinopathies and the way they interact with drug candidates. Here, we analyze critically the utilization of ion-mobility spectroscopy-MS (IM-MS) and electron-capture dissociation (ECD) MS/MS for analyzing the oligomerization and conformation of multiple amyloidogenic proteins. We also discuss IM-MS investigation of their interaction with two classes of compounds developed by our group over the last two decades: C-terminal fragments derived from the 42-residue form of amyloid β-protein (Aβ42) and molecular tweezers. Finally, we review the utilization of ECD-MS/MS for elucidating the binding sites of the ligands on multiple proteins. These approaches are readily applicable to future studies addressing similar questions and hold promise for facilitating the development of successful disease-modifying drugs against neurodegenerative proteinopathies.

Biophysical characterization as a tool to predict amyloidogenic and toxic properties of amyloid-β42 peptides

Amyloid-β42 (Aβ42) peptides are central to the amyloid pathology in Alzheimer's disease (AD). As biological mimetics, properties of synthetic Aβ peptides usually vary between vendors and batches, thus impacting the reproducibility of experimental studies. Here, we tested recombinantly expressed Aβ42 (Asp1 to Ala42) against synthetic Aβ42 from different suppliers using matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), circular dichroism (CD) spectroscopy, thioflavin T aggregation, surface plasmon resonance and MTT cell viability assays. Overall, our recombinant Aβ42 provided a reproducible mimetic of desired properties. Across experimental approaches, the combined detection of Aβ42 dimers and random coil to β-sheet transition only correlated with aggregation-prone and cytotoxic peptides. Conclusively, combining MALDI-MS with CD appears to provide a rapid, reliable means to predict the "bioactivity" of Aβ42.

A diselenide bond-containing ROS-responsive ruthenium nanoplatform delivers nerve growth factor for Alzheimer's disease management by repairing and promoting neuron regeneration

Alzheimer's disease (AD) is an incurable neurodegenerative disease. Repairing damaged nerves and promoting nerve regeneration are key ways to relieve AD symptoms. However, due to the lack of effective strategies to deliver nerve growth factor (NGF) to the brain, achieving neuron regeneration is a major challenge for curing AD. Herein, a ROS-responsive ruthenium nanoplatform (R@NGF-Se-Se-Ru) drug delivery system for AD management by promoting neuron regeneration and Aβ clearance was investigated. Under near-infrared (NIR) irradiation, nanoclusters have good photothermal properties, which can effectively inhibit the aggregation of Aβ and disaggregate Aβ fibrils. Interestingly, the diselenide bond in the nanoclusters is broken, and the nanoclusters are degraded into small ruthenium nanoparticles in the high reactive oxygen species (ROS) environment of the diseased area. Besides, NGF can promote neuronal regeneration and repair damaged nerves. Furthermore, R@NGF-Se-Se-Ru efficiently crosses the blood-brain barrier (BBB) owing to the covalently grafted target peptides of RVG (R). In vivo studies demonstrate that R@NGF-Se-Se-Ru nanoclusters decrease Aβ deposits, inhibit Aβ-induced cytotoxicity, and promote neurite outgrowth. The study confirms that promoting both Aβ clearance and neuron regeneration is an important therapeutic target for anti-AD drugs and provides a novel insight for AD therapy.

Effects of Aβ-derived peptide fragments on fibrillogenesis of Aβ

Amyloid β (Aβ) peptide aggregation plays a central role in Alzheimer's disease (AD) etiology. AD drug candidates have included small molecules or peptides directed towards inhibition of Aβ fibrillogenesis. Although some Aβ-derived peptide fragments suppress Aβ fibril growth, comprehensive analysis of inhibitory potencies of peptide fragments along the whole Aβ sequence has not been reported. The aim of this work is (a) to identify the region(s) of Aβ with highest propensities for aggregation and (b) to use those fragments to inhibit Aβ fibrillogenesis. Structural and aggregation properties of the parent Aβ_1-42 peptide and seven overlapping peptide fragments have been studied, i.e. Aβ_1-10 (P1), Aβ_6-15 (P2), Aβ_11-20 (P3), Aβ_16-25 (P4), Aβ_21-30 (P5), Aβ_26-36 (P6), and Aβ_31-42 (P7). Structural transitions of the peptides in aqueous buffer have been monitored by circular dichroism and Fourier transform infrared spectroscopy. Aggregation and fibrillogenesis were analyzed by light scattering and thioflavin-T fluorescence. The mode of peptide-peptide interactions was characterized by fluorescence resonance energy transfer. Three peptide fragments, P3, P6, and P7, exhibited exceptionally high propensity for β-sheet formation and aggregation. Remarkably, only P3 and P6 exerted strong inhibitory effect on the aggregation of Aβ_1-42, whereas P7 and P2 displayed moderate inhibitory potency. It is proposed that P3 and P6 intercalate between Aβ_1-42 molecules and thereby inhibit Aβ_1-42 aggregation. These findings may facilitate therapeutic strategies of inhibition of Aβ fibrillogenesis by Aβ-derived peptides.

Breaker peptides against amyloid-β aggregation: a potential therapeutic strategy for Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder, for which blocking the early steps of extracellular misfolded amyloid-β (Aβ) aggregation is a promising therapeutic approach. However, the pathological features of AD progression include the accumulation of intracellular tau protein, membrane-catalyzed cell death and the abnormal deposition of Aβ. Here, we focus on anti-amyloid breaker peptides derived from the Aβ sequence and non-Aβ-based peptides containing both natural and modified amino acids. Critical aspects of the breaker peptides include N-methylation, conformational restriction through cyclization, incorporation of unnatural amino acid, fluorinated molecules, polymeric nanoparticles and PEGylation. This review confers a general idea of such breaker peptides with in vitro and in vivo studies, which may advance our understanding of AD pathology and develop an effective treatment strategy against AD.

Mechanoenzymology: State of the Art and Challenges towards Highly Sustainable Biocatalysis

Global awareness of the importance of developing environmentally friendlier and more sustainable methods for the synthesis of valuable chemical compounds has led to the design of novel synthetic strategies, involving bio- and organocatalysis as well as the application of novel efficient and ground-breaking technologies such as present-day solvent-free mechanochemistry. In this regard, the evaluation of biocatalytic protocols mediated by the combination of mechanical activation and enzymatic catalysis has recently attracted the attention of the chemical community. Such mechanoenzymatic strategy represents an innovative and promising "green" approach in chemical synthesis that poses nevertheless new paradigms regarding the relative resilience of biomolecules to the mechanochemical stress and to the apparent high energy, at least in so-called hot-spots, during the milling process. Herein, relevant comments on the conceptualization of such mechanoenzymatic approach as a sustainable option in chemical synthesis, recent progress in the area, and associated challenges are discussed.

Different Inhibitors of Aβ42-Induced Toxicity Have Distinct Metal-Ion Dependency

Oligomers of amyloid β-protein (Aβ) are thought to be the proximal toxic agents initiating the neuropathologic process in Alzheimer's disease (AD). Therefore, targeting the self-assembly and oligomerization of Aβ has been an important strategy for designing AD therapeutics. In parallel, research into the metallobiology of AD has shown that Zn²⁺ can strongly modulate the aggregation of Aβ in vitro and both promote and inhibit the neurotoxicity of Aβ, depending on the experimental conditions. Thus, successful inhibitors of Aβ self-assembly may have to inhibit the toxicity not only of Aβ oligomers themselves but also of Aβ-Zn²⁺ complexes. However, there has been relatively little research investigating the effects of Aβ self-assembly and toxicity inhibitors in the presence of Zn²⁺. Our group has characterized previously a series of Aβ42 C-terminal fragments (CTFs), some of which have been shown to inhibit Aβ oligomerization and neurotoxicity. Here, we asked whether three CTFs shown to be potent inhibitors of Aβ42 toxicity maintained their activity in the presence of Zn²⁺. Biophysical analysis showed that the CTFs had different effects on oligomer, β-sheet, and fibril formation by Aβ42-Zn²⁺ complexes. However, cell viability experiments in differentiated PC-12 cells incubated with Aβ42-Zn²⁺ complexes in the absence or presence of these CTFs showed that the CTFs completely lost their inhibitory activity in the presence of Zn²⁺ even when applied at 10-fold excess relative to Aβ42. In light of these results, we tested another inhibitor, the molecular tweezer CLR01, which coincidentally had been shown to have a high affinity for Zn²⁺, suggesting that it could disrupt both Aβ42 oligomerization and Aβ42-Zn²⁺ complexation. Indeed, we found that CLR01 effectively inhibited the toxicity of Aβ42-Zn²⁺ complexes. Moreover, it did so at a lower concentration than needed for inhibiting the toxicity of Aβ42 alone. In agreement with these results, CLR01 inhibited β-sheet and fibril formation in Aβ42-Zn²⁺ complexes. Our data suggest that, for the development of efficient therapeutic agents, inhibitors of Aβ self-assembly and toxicity should be examined in the presence of relevant metal ions and that molecular tweezers may be particularly attractive candidates for therapy development.

Real-Time BODIPY-Binding Assay To Screen Inhibitors of the Early Oligomerization Process of Aβ1-42 Peptide

Misfolding and aggregation of amyloid β1–42 peptide (Aβ1–42) play a central role in the pathogenesis of Alzheimer's disease (AD). Targeting the highly cytotoxic oligomeric species formed during the early stages of the aggregation process represents a promising therapeutic strategy to reduce the toxicity associated with Aβ1–42. Currently, the thioflavin T (ThT) assay is the only established spectrofluorometric method to screen aggregation inhibitors. The success of the ThT assay is that it can detect Aβ1–42 aggregates with high β‐sheet content, such as protofibrils or fibrils, which appear in the late aggregation steps. Unfortunately, by using the ThT assay, the detection of inhibitors of early soluble oligomers that present a low β‐sheet character is challenging. Herein, a new, facile, and robust boron‐dipyrromethene (BODIPY) real‐time assay suitable for 96‐well plate format, which allows screening of compounds as selective inhibitors of the formation of Aβ1–42 oligomers, is reported. These inhibitors decrease the cellular toxicity of Aβ1–42, although they fail in the ThT assay. The findings have been confirmed and validated by structural analysis and cell viability assays under comparable experimental conditions. It is demonstrated that the BODIPY assay is a convenient method to screen and discover new candidate compounds that slow down or stop the pathological early oligomerization process and are active in the cellular assay. Therefore, it is a suitable complementary screening method of the current ThT assay.

Recent advances in the design and applications of amyloid-β peptide aggregation inhibitors for Alzheimer's disease therapy

Alzheimer's disease (AD) is an irreversible neurological disorder that progresses gradually and can cause severe cognitive and behavioral impairments. This disease is currently considered a social and economic incurable issue due to its complicated and multifactorial characteristics. Despite decades of extensive research, we still lack definitive AD diagnostic and effective therapeutic tools. Consequently, one of the most challenging subjects in modern medicine is the need for the development of new strategies for the treatment of AD. A large body of evidence indicates that amyloid-β (Aβ) peptide fibrillation plays a key role in the onset and progression of AD. Recent studies have reported that amyloid hypothesis-based treatments can be developed as a new approach to overcome the limitations and challenges associated with conventional AD therapeutics. In this review, we will provide a comprehensive view of the challenges in AD therapy and pathophysiology. We also discuss currently known compounds that can inhibit amyloid-β (Aβ) aggregation and their potential role in advancing current AD treatments. We have specifically focused on Aβ aggregation inhibitors including metal chelators, nanostructures, organic molecules, peptides (or peptide mimics), and antibodies. To date, these molecules have been the subject of numerous in vitro and in vivo assays as well as molecular dynamics simulations to explore their mechanism of action and the fundamental structural groups involved in Aβ aggregation. Ultimately, the aim of these studies (and current review) is to achieve a rational design for effective therapeutic agents for AD treatment and diagnostics.

Contributions of Mass Spectrometry to the Identification of Low Molecular Weight Molecules Able to Reduce the Toxicity of Amyloid-β Peptide to Cell Cultures and Transgenic Mouse Models of Alzheimer's Disease

Alzheimer's Disease affects approximately 33 million people worldwide and is characterized by progressive loss of memory at the cognitive level. The formation of toxic amyloid oligomers, extracellular amyloid plaques and amyloid angiopathy in brain by amyloid beta peptides are considered a part of the identified mechanism involved in disease pathogenesis. The optimal treatment approach leads toward finding a chemical compound able to form a noncovalent complex with the amyloid peptide thus blocking the process of amyloid aggregation. This direction gained an increasing interest lately, many studies demonstrating that mass spectrometry is a valuable method useful for the identification and characterization of such molecules able to interact with amyloid peptides. In the present review we aim to identify in the scientific literature low molecular weight chemical compounds for which there is mass spectrometric evidence of noncovalent complex formation with amyloid peptides and also there are toxicity reduction results which verify the effects of these compounds on amyloid beta toxicity towards cell cultures and transgenic mouse models developing Alzheimer's Disease.

Peptides, Peptidomimetics, and Carbohydrate-Peptide Conjugates as Amyloidogenic Aggregation Inhibitors for Alzheimer's Disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder accounting for 60-80% of dementia cases. For many years, AD causality was attributed to amyloid-β (Aβ) aggregated species. Recently, multiple therapies that target Aβ aggregation have failed in clinical trials, since Aβ aggregation is found in AD and healthy patients. Attention has therefore shifted toward the aggregation of the tau protein as a major driver of AD. Numerous inhibitors of tau-based pathology have recently been developed. Diagnosis of AD has shifted from measuring late stage senile plaques to early stage biomarkers, amyloid-β and tau monomers and oligomeric assemblies. Synthetic peptides and some derivative structures are being explored for use as theranostic tools as they possess the capacity both to bind the biomarkers and to inhibit their pathological self-assembly. Several studies have demonstrated that O-linked glycoside addition can significantly alter amyloid aggregation kinetics. Furthermore, natural O-glycosylation of amyloid-forming proteins, including amyloid precursor protein (APP), tau, and α-synuclein, promotes alternative nonamyloidogenic processing pathways. As such, glycopeptides and related peptidomimetics are being investigated within the AD field. Here we review advancements made in the last 5 years, as well as the arrival of sugar-based derivatives.

Toward a Rational Design to Regulate β-Amyloid Fibrillation for Alzheimer's Disease Treatment

The last decades have witnessed a growing global burden of Alzheimer's disease (AD). Evidence indicates that the onset and progression of AD is associated with β-amyloid (Aβ) peptide fibrillation. As such, there is a strong passion with discovering potent Aβ fibrillation inhibitors that can be developed into anti-amyloiddogenic agents for AD treatment. Current challenges that have arisen with this development involve with Aβ oligomer toxicity suppression and Blood Brain Barrier penetration capability. Considering most natural or biological events, one would observe that there is usually a "seed" to direct natural materials to assemble in response to a certain stimulation. Inspired by this, several materials or compounds, including nanoparticle, peptide or peptide mimics, and organic molecules, have been designed for the purpose of redirecting or impeding Aβ aggregation. Achieving these tasks requires comprehensive understanding on (1) initial Aβ assembly into insoluble deposits, (2) main concerns with fibrillation inhibition, and (3) current major methodologies to disrupt the aggregation. Herein, the objective of this review is to address these three areas, and enable the pathway for a promising therapeutic agent design for AD treatment.

Peptide synthesis: ball-milling, in solution, or on solid support, what is the best strategy?

While presenting particularly interesting advantages, peptide synthesis by ball-milling was never compared to the two traditional strategies, namely peptide syntheses in solution and on solid support (solid-phase peptide synthesis, SPPS). In this study, the challenging VVIA tetrapeptide was synthesized by ball-milling, in solution, and on solid support. The three strategies were then compared in terms of yield, purity, reaction time and environmental impact. The results obtained enabled to draw some strengths and weaknesses of each strategy, and to foresee what will have to be implemented to build more efficient and sustainable peptide syntheses in the near future.

Identification of the primary peptide contaminant that inhibits fibrillation and toxicity in synthetic amyloid-β42

Understanding the pathophysiology of Alzheimer disease has relied upon the use of amyloid peptides from a variety of sources, but most predominantly synthetic peptides produced using t-butyloxycarbonyl (Boc) or 9-fluorenylmethoxycarbonyl (Fmoc) chemistry. These synthetic methods can lead to minor impurities which can have profound effects on the biological activity of amyloid peptides. Here we used a combination of cytotoxicity assays, fibrillation assays and high resolution mass spectrometry (MS) to identify impurities in synthetic amyloid preparations that inhibit both cytotoxicity and aggregation. We identify the Aβ42Δ39 species as the major peptide contaminant responsible for limiting both cytotoxicity and fibrillation of the amyloid peptide. In addition, we demonstrate that the presence of this minor impurity inhibits the formation of a stable Aβ42 dimer observable by MS in very pure peptide samples. These results highlight the critical importance of purity and provenance of amyloid peptides in Alzheimer’s research in particular, and biological research in general.

Alzheimer's Protective Cross-Interaction between Wild-Type and A2T Variants Alters Aβ42 Dimer Structure

Whole genome sequencing has recently revealed the protective effect of a single A2T mutation in heterozygous carriers against Alzheimer's disease (AD) and age-related cognitive decline. The impact of the protective cross-interaction between the wild-type (WT) and A2T variants on the dimer structure is therefore of high interest, as the Aβ dimers are the smallest known neurotoxic species. Toward this goal, extensive atomistic replica exchange molecular dynamics simulations of the solvated WT homo- and A2T hetero- Aβ_1-42 dimers have been performed, resulting into a total of 51 μs of sampling for each system. Weakening of a set of transient, intrachain contacts formed between the central and C-terminal hydrophobic residues is observed in the heterodimeric system. The majority of the heterodimers with reduced interaction between central and C-terminal regions lack any significant secondary structure and display a weak interchain interface. Interestingly, the A2T N-terminus, particularly residue F4, is frequently engaged in tertiary and quaternary interactions with central and C-terminal hydrophobic residues in those distinct structures, leading to hydrophobic burial. This atypical involvement of the N-terminus within A2T heterodimer revealed in our simulations implies possible interference on Aβ₄₂ aggregation and toxic oligomer formation, which is consistent with experiments. In conclusion, the present study provides detailed structural insights onto A2T Aβ₄₂ heterodimer, which might provide molecular insights onto the AD protective effect of the A2T mutation in the heterozygous state.

Rationally Designed Peptides and Peptidomimetics as Inhibitors of Amyloid-β (Aβ) Aggregation: Potential Therapeutics of Alzheimer's Disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease with no clinically accepted treatment to cure or halt its progression. The worldwide effort to develop peptide-based inhibitors of amyloid-β (Aβ) aggregation can be considered an unplanned combinatorial experiment. An understanding of what has been done and achieved may advance our understanding of AD pathology and the discovery of effective therapeutic agents. We review here the history of such peptide-based inhibitors, including those based on the Aβ sequence and those not derived from that sequence, containing both natural and unnatural amino acid building blocks. Peptide-based aggregation inhibitors hold significant promise for future AD therapy owing to their high selectivity, effectiveness, low toxicity, good tolerance, low accumulation in tissues, high chemical and biological diversity, possibility of rational design, and highly developed methods for analyzing their mode of action, proteolytic stability (modified peptides), and blood-brain barrier (BBB) permeability.

Modulation in the conformational and stability attributes of the Alzheimer's disease associated amyloid-beta mutants and their favorable stabilization by curcumin: molecular dynamics simulation analysis

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by progressive accumulation of amyloid-beta (Aβ) peptides in brain. In the present study, two familial Aβ42 mutations, namely A2V (harmful) and A2T (protective) have been analyzed and compared with the wild-type (WT) by performing all-atom molecular dynamics (MD) simulations in the absence and presence of curcumin, a well-known inhibitor of Aβ plaque formation. Mutant A2V was found to exhibit highest stability followed by WT and mutant A2T in the absence of curcumin. This stability trend was found to be reversed in the presence of curcumin, suggesting a significant change in the conformational landscape of Aβ42 folding. Due to significant differences in the folding and interaction patterns of the mutants A2V and A2T, curcumin exhibited higher binding affinity for mutant A2T as compared to that of A2V. To the best of our knowledge, this is the first report on the effect of curcumin binding on structural landscapes of the two contrasting point mutants providing an understanding of the basis of Aβ plaque formation and its prevention by curcumin.

Modulation of Amyloid β-Protein (Aβ) Assembly by Homologous C-Terminal Fragments as a Strategy for Inhibiting Aβ Toxicity

Self-assembly of amyloid β-protein (Aβ) into neurotoxic oligomers and fibrillar aggregates is a key process thought to be the proximal event leading to development of Alzheimer's disease (AD). Therefore, numerous attempts have been made to develop reagents that disrupt this process and prevent the formation of the toxic oligomers and aggregates. An attractive strategy for developing such reagents is to use peptides derived from Aβ based on the assumption that such peptides would bind to full-length Aβ, interfere with binding of additional full-length molecules, and thereby prevent formation of the toxic species. Guided by this rationale, most of the studies in the last two decades have focused on preventing formation of the core cross-β structure of Aβ amyloid fibrils using β-sheet-breaker peptides derived from the central hydrophobic cluster of Aβ. Though this approach is effective in inhibiting fibril formation, it is generally inefficient in preventing Aβ oligomerization. An alternative approach is to use peptides derived from the C-terminus of Aβ, which mediates both oligomerization and fibrillogenesis. This approach has been explored by several groups, including our own, and led to the discovery of several lead peptides with moderate to high inhibitory activity. Interestingly, the mechanisms of these inhibitory effects have been found to be diverse, and only in a small percentage of cases involved interference with β-sheet formation. Here, we review the strategy of using C-terminal fragments of Aβ as modulators of Aβ assembly and discuss the relevant challenges, therapeutic potential, and mechanisms of action of such fragments.

Toxicity inhibitors protect lipid membranes from disruption by Aβ42

Although the precise molecular factors linking amyloid β-protein (Aβ) to Alzheimer's disease (AD) have not been deciphered, interaction of Aβ with cellular membranes has an important role in the disease. However, most therapeutic strategies targeting Aβ have focused on interfering with Aβ self-assembly rather than with its membrane interactions. Here, we studied the impact of three toxicity inhibitors on membrane interactions of Aβ42, the longer form of Aβ, which is associated most strongly with AD. The inhibitors included the four-residue C-terminal fragment Aβ(39-42), the polyphenol (-)-epigallocatechin-3-gallate (EGCG), and the lysine-specific molecular tweezer, CLR01, all of which previously were shown to disrupt different steps in Aβ42 self-assembly. Biophysical experiments revealed that incubation of Aβ42 with each of the three modulators affected membrane interactions in a distinct manner. Interestingly, EGCG and CLR01 were found to have significant interaction with membranes themselves. However, membrane bilayer disruption was reduced when the compounds were preincubated with Aβ42, suggesting that binding of the assembly modulators to the peptide attenuated their membrane interactions. Importantly, our study reveals that even though the three tested compounds affect Aβ42 assembly differently, membrane interactions were significantly inhibited upon incubation of each compound with Aβ42, suggesting that preventing the interaction of Aβ42 with the membrane contributes substantially to inhibition of its toxicity by each compound. The data suggest that interference with membrane interactions is an important factor for Aβ42 toxicity inhibitors and should be taken into account in potential therapeutic strategies, in addition to disruption or remodeling of amyloid assembly.

Mechanism of C-Terminal Fragments of Amyloid β-Protein as Aβ Inhibitors: Do C-Terminal Interactions Play a Key Role in Their Inhibitory Activity?

Targeting the early oligomerization of amyloid β protein (Aβ) is a promising therapeutic strategy for Alzheimer's disease (AD). Recently, certain C-terminal fragments (CTFs) derived from Aβ42 were shown to be potent inhibitors of Aβ-induced toxicity. The shortest peptide studied, Aβ(39-42), has been shown to modulate Aβ oligomerization and inhibit Aβ toxicity. Understanding the mechanism of these CTFs, especially Aβ(39-42), is of significance for future therapeutic development of AD and peptidomimetic-based drug development. Here we used ion mobility spectrometry-mass spectrometry to investigate the interactions between two modified Aβ(39-42) derivatives, VVIA-NH2 and Ac-VVIA, and full-length Aβ42. VVIA-NH2 was previously shown to inhibit Aβ toxicity, whereas Ac-VVIA did not. Our mass spectrometry analysis revealed that VVIA-NH2 binds directly to Aβ42 monomer and small oligomers while Ac-VVIA binds only to Aβ42 monomer. Ion mobility studies showed that VVIA-NH2 modulates Aβ42 oligomerization by not only inhibiting the dodecamer formation but also disaggregating preformed Aβ42 dodecamer. Ac-VVIA also inhibits and removes preformed Aβ42 dodecamer. However, the Aβ42 sample with the addition of Ac-VVIA clogged the nanospray tip easily, indicating that larger aggregates are formed in the solution in the presence of Ac-VVIA. Molecular dynamics simulations suggested that VVIA-NH2 binds specifically to the C-terminal region of Aβ42 while Ac-VVIA binds dispersedly to multiple regions of Aβ42. This work implies that C-terminal interactions and binding to Aβ oligomers are important for C-terminal fragment inhibitors.

Alzheimer's protective A2T mutation changes the conformational landscape of the Aβ₁₋₄₂ monomer differently than does the A2V mutation

The aggregation of amyloid-β (Aβ) peptides plays a crucial role in the etiology of Alzheimer's disease (AD). Recently, it has been reported that an A2T mutation in Aβ can protect against AD. Interestingly, a nonpolar A2V mutation also has been found to offer protection against AD in the heterozygous state, although it causes early-onset AD in homozygous carriers. Since the conformational landscape of the Aβ monomer is known to directly contribute to the early-stage aggregation mechanism, it is important to characterize the effects of the A2T and A2V mutations on Aβ₁₋₄₂ monomer structure. Here, we have performed extensive atomistic replica-exchange molecular dynamics simulations of the solvated wild-type (WT), A2V, and A2T Aβ₁₋₄₂ monomers. Our simulations reveal that although all three variants remain as collapsed coils in solution, there exist significant structural differences among them at shorter timescales. A2V exhibits an enhanced double-hairpin population in comparison to the WT, similar to those reported in toxic WT Aβ₁₋₄₂ oligomers. Such double-hairpin formation is caused by hydrophobic clustering between the N-terminus and the central and C-terminal hydrophobic patches. In contrast, the A2T mutation causes the N-terminus to engage in unusual electrostatic interactions with distant residues, such as K16 and E22, resulting in a unique population comprising only the C-terminal hairpin. These findings imply that a single A2X (where X = V or T) mutation in the primarily disordered N-terminus of the Aβ₁₋₄₂ monomer can dramatically alter the β-hairpin population and switch the equilibrium toward alternative structures. The atomistically detailed, comparative view of the structural landscapes of A2V and A2T variant monomers obtained in this study can enhance our understanding of the mechanistic differences in their early-stage aggregation.

Capping of aβ42 oligomers by small molecule inhibitors

Aβ42 peptides associate into soluble oligomers and protofibrils in the process of forming the amyloid fibrils associated with Alzheimer's disease. The oligomers have been reported to be more toxic to neurons than fibrils, and have been targeted by a wide range of small molecule and peptide inhibitors. With single touch atomic force microscopy (AFM), we show that monomeric Aβ42 forms two distinct types of oligomers, low molecular weight (MW) oligomers with heights of 1-2 nm and high MW oligomers with heights of 3-5 nm. In both cases, the oligomers are disc-shaped with diameters of ~10-15 nm. The similar diameters suggest that the low MW species stack to form the high MW oligomers. The ability of Aβ42 inhibitors to interact with these oligomers is probed using atomic force microscopy and NMR spectroscopy. We show that curcumin and resveratrol bind to the N-terminus (residues 5-20) of Aβ42 monomers and cap the height of the oligomers that are formed at 1-2 nm. A second class of inhibitors, which includes sulindac sulfide and indomethacin, exhibit very weak interactions across the Aβ42 sequence and do not block the formation of the high MW oligomers. The correlation between N-terminal interactions and capping of the height of the Aβ oligomers provides insights into the mechanism of inhibition and the pathway of Aβ aggregation.

Insight into the inhibition effect of acidulated serum albumin on amyloid β-protein fibrillogenesis and cytotoxicity

Alzheimer's disease (AD) is the most prevalent form of dementia, and aggregation of amyloid β-proteins (Aβ) into soluble oligomers and fibrils has been implicated in the pathogenesis of AD. Herein we developed acidulated serum albumin for the inhibition of Aβ42 fibrillogenesis. Bovine serum albumin (BSA) was modified with diglycolic anhydride, leading to the coupling of 14.5 more negative charges (carboxyl groups) on average on each protein surface. The acidulated BSA (A-BSA) was characterized and confirmed to keep the tertiary structure and stability of BSA. Extensive biophysical and biological analyses showed that A-BSA significantly inhibited Aβ42 fibrillogenesis and mitigated amyloid cytotoxicity. As compared to the Aβ42-treated group (cell viability, 50%), the cell viability increased to 88% by the addition of equimolar A-BSA. The inhibitory effect was remarkably higher than that of BSA at the same concentration. On the basis of the experimental findings, a mechanistic model was proposed. The model considers that Aβ42 is bound to the A-BSA surface by hydrophobic interactions, but the widely distributed negative charges on the A-BSA surface give rise to electrostatic repulsions to the bound Aβ42 that is also negatively charged. The two well-balanced opposite forces make Aβ42 adopt extended conformations instead of the β-sheet structure that is necessary for the on-pathway fibrillogenesis, even when the protein is released off the surface. Thus, A-BSA greatly slows down the fibrillation and changes the fibrillogenesis pathway, leading to the formation of less toxic aggregates. The findings and the mechanistic model offer new insights into the development of more potent inhibitors of Aβ fibrillogenesis and cytotoxicity.

Modulating self-assembly of amyloidogenic proteins as a therapeutic approach for neurodegenerative diseases: strategies and mechanisms

Abnormal protein assembly causes multiple devastating disorders in the central nervous system (CNS), such as Alzheimer's, Parkinson's, Huntington's, and prion diseases. Due to the now extended human lifespan, these diseases have been increasing in prevalence, resulting in major public health problems and the associated financial difficulties worldwide. The wayward proteins that lead to disease self-associate into neurotoxic oligomers and go on to form fibrillar polymers through multiple pathways. Thus, a range of possible targets for pharmacotherapeutic intervention exists along these pathways. Many compounds have shown different levels of effectiveness in inhibiting aberrant self-assembly, dissociating existing aggregates, protecting cells against neurotoxic insults, and in some cases ameliorating disease symptoms in vivo, yet achieving efficient, disease-modifying therapy in humans remains a major unattained goal. To a large degree, this is because the mechanisms of action for these drugs are essentially unknown. For successful design of new effective drugs, it is crucial to elucidate the mechanistic details of their action, including the actual target(s) along the protein aggregation pathways, how the compounds modulate these pathways, and their effect at the cellular, tissue, organ, and organism level. Here, the current knowledge of major mechanisms by which some of the more extensively explored drug candidates work are discussed. In particular, we focus on three prominent strategies: 1) stabilizing the native fold of amyloidogenic proteins, 2) accelerating the aggregation pathways towards the fibrillar endpoint thereby reducing accumulation of toxic oligomers, and 3) modulating the assembly process towards nontoxic oligomers/aggregates. The merit of each strategy is assessed, and the key points to consider when analyzing the efficacy of possible drug candidates and their mechanism of action are discussed.

Aβ(39-42) modulates Aβ oligomerization but not fibril formation

Recently, certain C-terminal fragments (CTFs) of Aβ42 have been shown to be effective inhibitors of Aβ42 toxicity. Here, we examine the interactions between the shortest CTF in the original series, Aβ(39-42), and full-length Aβ. Mass spectrometry results indicate that Aβ(39-42) binds directly to Aβ monomers and to the n = 2, 4, and 6 oligomers. The Aβ42:Aβ(39-42) complex is further probed using molecular dynamics simulations. Although the CTF was expected to bind to the hydrophobic C-terminus of Aβ42, the simulations show that Aβ(39-42) binds at several locations on Aβ42, including the C-terminus, other hydrophobic regions, and preferentially in the N-terminus. Ion mobility-mass spectrometry (IM-MS) and electron microscopy experiments indicate that Aβ(39-42) disrupts the early assembly of full-length Aβ. Specifically, the ion-mobility results show that Aβ(39-42) prevents the formation of large decamer/dodecamer Aβ42 species and, moreover, can remove these structures from solution. At the same time, thioflavin T fluorescence and electron microscopy results show that the CTF does not inhibit fibril formation, lending strong support to the hypothesis that oligomers and not amyloid fibrils are the Aβ form responsible for toxicity. The results emphasize the role of small, soluble assemblies in Aβ-induced toxicity and suggest that Aβ(39-42) inhibits Aβ-induced toxicity by a unique mechanism, modulating early assembly into nontoxic hetero-oligomers, without preventing fibril formation.