Molecular Mechanisms Underlying the Flavonoid-Induced Inhibition of α-Synuclein Fibrillation

Flavonoid intake and risk of Parkinson's disease

BACKGROUND

Flavonoids have been proposed to reduce the risk of Parkinson's disease (PD). However, results from epidemiological studies have been inconclusive.

OBJECTIVE

To prospectively examine the association between the intake of flavonoids and their subclasses and the risk of PD and how pesticides may confound or modify that association.

METHODS

The study population comprised 80 701 women (1984-2016) and 48 782 men (1986-2016) from two large US cohorts. Flavonoid intake was ascertained at baseline and every 4 years thereafter using a semiquantitative Food Frequency Questionnaire. We conducted multivariable-adjusted Cox regression models to estimate HRs and 95% CIs of PD according to quintiles of baseline and cumulative average intakes of flavonoids and subclasses. We repeated the analyses, adjusting for intakes of high-pesticide-residue fruits and vegetables (FVs) and stratifying by servings/day of high-pesticide-residue FV intake.

RESULTS

We identified 676 incident PD cases in women and 714 in men after 30-32 years of follow-up. Higher total flavonoid intake at baseline was not associated with a lower PD risk, neither in men (HR comparing highest to lowest quintile: 0.89, 95% CI: 0.69 to 1.14) nor in women (HR comparing highest to lowest quintile: 1.27, 95% CI: 0.98 to 1.64). Similar results were observed for cumulative average intakes and flavonoid subclasses. Results remained similar after adjustment for and stratification by high-pesticide-residue FV and when analyses were restricted to younger PD cases.

CONCLUSION

These results do not support a protective effect of flavonoid intake on PD risk. Pesticide residues do not confound or modify the association.

Development of an α-synuclein positron emission tomography tracer for imaging synucleinopathies

Synucleinopathies are characterized by the accumulation of α-synuclein (α-Syn) aggregates in the brain. Positron emission tomography (PET) imaging of synucleinopathies requires radiopharmaceuticals that selectively bind α-Syn deposits. We report the identification of a brain permeable and rapid washout PET tracer [18F]-F0502B, which shows high binding affinity for α-Syn, but not for Aβ or Tau fibrils, and preferential binding to α-Syn aggregates in the brain sections. Employing several cycles of counter screenings with in vitro fibrils, intraneuronal aggregates, and neurodegenerative disease brain sections from several mice models and human subjects, [18F]-F0502B images α-Syn deposits in the brains of mouse and non-human primate PD models. We further determined the atomic structure of the α-Syn fibril-F0502B complex by cryo-EM and revealed parallel diagonal stacking of F0502B on the fibril surface through an intense noncovalent bonding network via inter-ligand interactions. Therefore, [18F]-F0502B is a promising lead compound for imaging aggregated α-Syn in synucleinopathies.

Danshensu inhibits Aβ aggregation and neurotoxicity as one of the main prominent features of Alzheimer's disease

It has been found that the main cause of neurodegenerative proteinopathies, especially Alzheimer's disease (AD) is the formation of Aβ amyloid plaques, which can be regulated by application of potential small molecules. In the present study, we aimed to investigate the inhibitory effect of danshensu on Aβ(1-42) aggregation and relevant apoptotic pathway in neurons. A broad range of spectroscopic, theoretical, and cellular assays were done to investigate the anti-amyloidogenic characteristics of danshensu. It was found that danshensu triggers its inhibitory effect against Aβ(1-42) aggregation through modulation of hydrophobic patches as well as structural and morphological changes through a stacking interaction. Furthermore, it was observed that incubation of Aβ(1-42) samples with danshensu during aggregation process recovered the cell viability and mitigated the expression of caspase-3 mRNA and protein as well caspase-3 activity deregulated by Aβ(1-42) amyloid fibrils alone. In general, obtained data showed that danshensu potentially inhibits Aβ(1-42) aggregation and associated proteinopathies through regulation of apoptotic pathway in a concentration-dependent manner. Therefore, danshensu may be used as a promising biomolecule against the Aβ aggregation and associated proteinopathies, which can be further analyzed in the future studies for the treatment of AD.

Mechanistic Insights into Polyphenols' Aggregation Inhibition of α-Synuclein and Related Peptides

While several polyphenols were found to either inhibit or modulate the aggregation of proteins implicated in neurodegenerative diseases, such as Parkinson's disease (PD), discrepant action mechanisms have been reported. This, in addition to some polyphenols' pan-assay interference compounds' reputation, casts some doubts concerning their therapeutic relevance. Here, we studied, through molecular dynamics and enhanced sampling methods, the aggregation of 11-mer peptides from the non-amyloid-β component, an aggregation-prone domain of α-synuclein (α-syn) implicated in PD and other synucleinopathies, in neat water and aqueous solutions of resveratrol (RSV) and gallic acid (GA). Further, simulations of the complete protein were carried out in aqueous urea, RSV, and GA solutions. Our results show that peptide aggregation is not disrupted by either phenolic compound. Thus, instead, intrusion of RSV and GA in the inter-peptide region induces a peptide-peptide re-orientation, favoring terminal interactions that manifest in the formation of barrierless solvent-separated configurations. Moreover, although the (poly)phenols induce a pronounced peptide dewetting at high concentrations, β-sheet-rich regions, a hallmark of α-syn aggregation, are not disrupted. Thus, our results indicate that, if anything, RSV and GA delay or modulate peptide aggregation at high concentrations via the stabilization of solvent-separated conformations as opposed to aggregation inhibition. Structural analysis of the full protein, however, shows that the (poly)phenols induce more extended conformations of α-syn, similar to urea, possibly also influencing its aggregation propensity. However, opposite to urea, the (poly)phenols reduce α-syn's conformational space, likely due to steric effects and a slowdown of the solvent dynamics. These effects are concentration-dependent and possibly unattainable at therapeutic-relevant concentrations. These results suggest that the aggregation inhibition activity of RSV and GA in vitro should involve, instead, either the non-covalent binding to oligomeric intermediates or the stabilization of the monomer and/or oligomers through the formation of covalent bonds of the respective quinones with α-syn. In addition, the enhanced aggregation tendency of the peptides observed here could be associated with the formation of non-toxic oligomers, reported for some polyphenols.

Analysis of phenolic compounds in Parkinson's disease: a bibliometric assessment of the 100 most cited papers

Objective

The aim of this study was to identify and characterize the 100 most cited articles on Parkinson's disease (PD) and phenolic compounds (PCs).

Methods

Articles were selected in the Web of Science Core Collection up to June 2022 based on predetermined inclusion criteria, and the following bibliometric parameters were extracted: the number of citations, title, keywords, authors, year, study design, tested PC and therapeutic target. MapChart was used to create worldwide networks, and VOSviewer software was used to create bibliometric networks. Descriptive statistical analysis was used to identify the most researched PCs and therapeutic targets in PD.

Results

The most cited article was also the oldest. The most recent article was published in 2020. Asia and China were the continent and the country with the most articles in the list (55 and 29%, respectively). In vitro studies were the most common experimental designs among the 100 most cited articles (46%). The most evaluated PC was epigallocatechin. Oxidative stress was the most studied therapeutic target.

Conclusion

Despite the demonstrations in laboratorial studies, the results obtained point to the need for clinical studies to better elucidate this association.

Pyrogallol, Corilagin and Chebulagic acid target the "fuzzy coat" of alpha-synuclein to inhibit the fibrillization of the protein

The accumulation of the intrinsically disordered protein alpha-synuclein (αSyn) in the form of insoluble fibrillar aggregates in the central nervous system is linked to a variety of neurodegenerative disorders such as Parkinson's disease, Lewy body dementia, and multiple system atrophy. Here we show that Pyrogallol, Corilagin and Chebulagic acid, compounds containing a different number of catechol rings, are independently capable of delaying and reducing the extent of αSyn fibrillization. The efficiency of inhibition was found to correlate with the number of catechol rings. Further, our NMR studies reveal that these compounds interact with the N-terminal region of αSyn which is unstructured even in the fibrillar form of the protein and is known as the "fuzzy coat" of fibrils. Thus, Corilagin and Chebulagic acid target the fuzzy coat of αSyn and not the amyloid core which is a common target for the inhibition of protein fibrillization. Our results indicate that the N-terminus also plays a key role in the fibrillization of αSyn.

The Interplay between Gut Microbiota and Parkinson's Disease: Implications on Diagnosis and Treatment

The bidirectional interaction between the gut microbiota (GM) and the Central Nervous System, the so-called gut microbiota brain axis (GMBA), deeply affects brain function and has an important impact on the development of neurodegenerative diseases. In Parkinson’s disease (PD), gastrointestinal symptoms often precede the onset of motor and non-motor manifestations, and alterations in the GM composition accompany disease pathogenesis. Several studies have been conducted to unravel the role of dysbiosis and intestinal permeability in PD onset and progression, but the therapeutic and diagnostic applications of GM modifying approaches remain to be fully elucidated. After a brief introduction on the involvement of GMBA in the disease, we present evidence for GM alterations and leaky gut in PD patients. According to these data, we then review the potential of GM-based signatures to serve as disease biomarkers and we highlight the emerging role of probiotics, prebiotics, antibiotics, dietary interventions, and fecal microbiota transplantation as supportive therapeutic approaches in PD. Finally, we analyze the mutual influence between commonly prescribed PD medications and gut-microbiota, and we offer insights on the involvement also of nasal and oral microbiota in PD pathology, thus providing a comprehensive and up-to-date overview on the role of microbial features in disease diagnosis and treatment.

Catechol-containing compounds are a broad class of protein aggregation inhibitors: Redox state is a key determinant of the inhibitory activities

A range of neurodegenerative and related aging diseases, such as Alzheimer's disease and type 2 diabetes, are linked to toxic protein aggregation. Yet the mechanisms of protein aggregation inhibition by small molecule inhibitors remain poorly understood, in part because most protein targets of aggregation assembly are partially unfolded or intrinsically disordered, which hinders detailed structural characterization of protein-inhibitor complexes and structural-based inhibitor design. Herein we employed a parallel small molecule library-screening approach to identify inhibitors against three prototype amyloidogenic proteins in neurodegeneration and related proteinopathies: amylin, Aβ and tau. One remarkable class of inhibitors identified from these screens against different amyloidogenic proteins was catechol-containing compounds and redox-related quinones/anthraquinones. Secondary assays validated most of the identified inhibitors. In vivo efficacy evaluation of a selected catechol-containing compound, rosmarinic acid, demonstrated its strong mitigating effects of amylin amyloid deposition and related diabetic pathology in transgenic HIP rats. Further systematic investigation of selected class of inhibitors under aerobic and anaerobic conditions revealed that the redox state of the broad class of catechol-containing compounds is a key determinant of the amyloid inhibitor activities. The molecular insights we gained not only explain why a large number of catechol-containing polyphenolic natural compounds, often enriched in healthy diet, have anti-neurodegeneration and anti-aging activities, but also could guide the rational design of therapeutic or nutraceutical strategies to target a broad range of neurodegenerative and related aging diseases.

The potential role of ischaemia-reperfusion injury in chronic, relapsing diseases such as rheumatoid arthritis, Long COVID, and ME/CFS: evidence, mechanisms, and therapeutic implications

Ischaemia-reperfusion (I-R) injury, initiated via bursts of reactive oxygen species produced during the reoxygenation phase following hypoxia, is well known in a variety of acute circumstances. We argue here that I-R injury also underpins elements of the pathology of a variety of chronic, inflammatory diseases, including rheumatoid arthritis, ME/CFS and, our chief focus and most proximally, Long COVID. Ischaemia may be initiated via fibrin amyloid microclot blockage of capillaries, for instance as exercise is started; reperfusion is a necessary corollary when it finishes. We rehearse the mechanistic evidence for these occurrences here, in terms of their manifestation as oxidative stress, hyperinflammation, mast cell activation, the production of marker metabolites and related activities. Such microclot-based phenomena can explain both the breathlessness/fatigue and the post-exertional malaise that may be observed in these conditions, as well as many other observables. The recognition of these processes implies, mechanistically, that therapeutic benefit is potentially to be had from antioxidants, from anti-inflammatories, from iron chelators, and via suitable, safe fibrinolytics, and/or anti-clotting agents. We review the considerable existing evidence that is consistent with this, and with the biochemical mechanisms involved.

Neuroprotection by Mucuna pruriens in Neurodegenerative Diseases

The medicinal plant Mucuna pruriens (Fabaceae) is widely known for its anti-oxidative and anti-inflammatory properties. It is a well-established drug in Ayurveda and has been widely used for the treatment of neurological disorders and male infertility for ages. The seeds of the plant have potent medicinal value and its extract has been tested in different models of neurodegenerative diseases, especially Parkinson's disease (PD). Apart from PD, Mucuna pruriens is now being studied in models of other nervous systems disorders such as Alzheimer's disease (AD), Amyotrophic lateral sclerosis (ALS) and stroke because of its neuroprotective importance. This review briefly discusses the pathogenesis of PD, AD, ALS and stroke. It aims to summarize the medicinal importance of Mucuna pruriens in treatment of these diseases, and put forward the potential targets where Mucuna pruriens can act for therapeutic interventions. In this review, the effect of Mucuna pruriens on ameliorating the neurodegeneration evident in PD, AD, ALS and stroke is briefly discussed. The potential targets for neuroprotection by the plant are delineated, which can be studied further to validate the hypothesis regarding the use of Mucuna pruriens for the treatment of these diseases.

Alpha-Synuclein Aggregation Pathway in Parkinson's Disease: Current Status and Novel Therapeutic Approaches

Following Alzheimer’s, Parkinson’s disease (PD) is the second-most common neurodegenerative disorder, sharing an unclear pathophysiology, a multifactorial profile, and massive social costs worldwide. Despite this, no disease-modifying therapy is available. PD is tightly associated with α-synuclein (α-Syn) deposits, which become organised into insoluble, amyloid fibrils. As a typical intrinsically disordered protein, α-Syn adopts a monomeric, random coil conformation in an aqueous solution, while its interaction with lipid membranes drives the transition of the molecule part into an α-helical structure. The central unstructured region of α-Syn is involved in fibril formation by converting to well-defined, β-sheet rich secondary structures. Presently, most therapeutic strategies against PD are focused on designing small molecules, peptides, and peptidomimetics that can directly target α-Syn and its aggregation pathway. Other approaches include gene silencing, cell transplantation, stimulation of intracellular clearance with autophagy promoters, and degradation pathways based on immunotherapy of amyloid fibrils. In the present review, we sum marise the current advances related to α-Syn aggregation/neurotoxicity. These findings present a valuable arsenal for the further development of efficient, nontoxic, and non-invasive therapeutic protocols for disease-modifying therapy that tackles disease onset and progression in the future.

Selected natural and synthetic agents effective against Parkinson's disease with diverse mechanisms

Similar to other neurodegenerative diseases, Parkinson's disease (PD) has been extensively investigated with respect to its neuropathological background and possible treatment options. Since the symptomatic outcomes are generally related to dopamine deficiency, the current treatment strategies towards PD mainly employ dopaminergic agonists as well as the compounds acting on dopamine metabolism. These drugs do not provide disease modifying properties; therefore alternative drug discovery studies focus on targets involved in the progressive neurodegenerative character of PD. This study has aimed to present the pathophysiology of PD concomitant to the representation of drugs and promising molecules displaying activity against the validated and non-validated targets of PD.

Disaggregation Behavior of Amyloid β Fibrils by Anthocyanins Studied by Total-Internal-Reflection-Fluorescence Microscopy Coupled with a Wireless Quartz-Crystal Microbalance Biosensor

Amyloid fibrils are formed from various proteins, some of which cause the corresponding neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases. It has been reported that many compounds inhibit the formation of amyloid fibrils. Anthocyanins are flavonoid pigments present in fruits and vegetables, which are known to suppress symptoms related with Alzheimer's disease. However, the influence of anthocyanins on the amyloid fibril remains unclear. Here, we succeeded in the direct monitoring of the disaggregation reaction of single amyloid β (Aβ) fibrils by anthocyanins using total-internal-reflection-fluorescence microscopy with a quartz-crystal microbalance (TIRFM-QCM). It is found that the disassembly activity to the Aβ fibrils depends on the number of hydroxyl groups in six-membered ring B of anthocyanin, and only delphinidin-3-galactoside, possessing three hydroxyl groups there, shows high disassembly activity. Our results show the importance of the number of hydroxyl groups and demonstrate the usefulness of TIRFM-QCM as a powerful tool in studying interactions between amyloid fibrils and compounds.

Ellagic Acid Inhibits α-Synuclein Aggregation at Multiple Stages and Reduces Its Cytotoxicity

α-Synuclein is a natively unfolded protein and its deposition in the Lewy body and Lewy neurites in the substantia nigra region of the brain is linked to Parkinson's disease (PD). The molecular mechanisms of α-synuclein aggregation and its clearance have not been well understood. Until now, several strategies have been designed to inhibit α-synuclein aggregation and related cytotoxicity. Polyphenols, small molecules, synthetic peptides, and peptide-derived molecules have been considered as potential candidates that inhibit α-synuclein oligomerization and its fibrillation, and a few of them are in clinical trials. We have identified a polyphenolic compound ellagic acid (EA) that inhibits α-synuclein aggregation. Our results demonstrated that EA inhibits primary nucleation, seeded aggregation, and membrane-induced aggregation. The cytotoxicity of α-synuclein oligomers and fibers treated with EA has been investigated and we found that EA treated oligomers and fibrils showed reduced cytotoxicity. Additionally, we also observed inhibition of membrane binding of α-synuclein by EA in SH-SY5Y cells. In conclusion, the present study suggests that small molecules such as ellagic acid have anti-amyloidogenic properties and may have therapeutic potential for Parkinson's disease and other proteinopathies.

Boosting the Photodynamic Degradation of Islet Amyloid Polypeptide Aggregates Via a "Bait-Hook-Devastate" Strategy

Photosensitizers that can generate reactive oxygen species (ROS) upon irradiation have emerged as promising agents for photodynamic degradation of toxic amyloid aggregates that are linked to many amyloidogenic diseases. However, due to the ultrastable β-sheet structure in amyloid aggregates and inefficient utilization of the generated ROS, it usually requires high stoichiometric concentration of the photosensitizer and/or intensive light irradiation to fully dissociate aggregates. In this work, we have developed a "bait-hook-devastate" strategy to boost the efficiency of the photodynamic degradation of amyloid aggregates. This strategy employs anionic polyacrylic acid as a bait to accumulate cationic human islet amyloid polypeptide (IAPP) aggregates and positively charged photosensitizer TPCI in a confined area through electronic interactions. Multiple characterization studies proved that the utilization rate of ROS generated by TPCI was remarkably improved via this strategy, which amplified the ability of TPCI to dissociate IAPP aggregates. Rapid and complete degradation of IAPP aggregates could be achieved by irradiating the system under very mild conditions for less than 30 min, and the IAPP-mediated cytotoxicity was also largely alleviated, providing a new paradigm to accelerate photodynamic degradation of amyloid aggregates for further practical applications.

Insights Into Peptide Inhibition of Alpha-Synuclein Aggregation

α-Synuclein (aSyn) aggregation is an attractive target for therapeutic development for a range of neurodegenerative conditions, collectively termed synucleinopathies. Here, we probe the mechanism of action of a peptide 4554W, (KDGIVNGVKA), previously identified through intracellular library screening, to prevent aSyn aggregation and associated toxicity. We utilize NMR to probe association and identify that 4554W associates with a "partially aggregated" form of aSyn, with enhanced association occurring over time. We also report the ability of 4554W to undergo modification through deamidation of the central asparagine residue, occurring on the same timescale as aSyn aggregation in vitro, with peptide modification enhancing its association with aSyn. Additionally, we report that 4554W can act to reduce fibril formation of five Parkinson's disease associated aSyn mutants. Inhibitory peptide binding to partially aggregated forms of aSyn, as identified here, is particularly attractive from a therapeutic perspective, as it would eliminate the need to administer the therapy at pre-aggregation stages, which are difficult to diagnose. Taken together the data suggest that 4554W could be a suitable candidate for future therapeutic development against wild-type, and most mutant aSyn aggregation.

Natural and Synthetic Derivatives of Hydroxycinnamic Acid Modulating the Pathological Transformation of Amyloidogenic Proteins

This review presents the main properties of hydroxycinnamic acid (HCA) derivatives and their potential application as agents for the prevention and treatment of neurodegenerative diseases. It is partially focused on the successful use of these compounds as inhibitors of amyloidogenic transformation of proteins. Firstly, the prerequisites for the emergence of interest in HCA derivatives, including natural compounds, are described. A separate section is devoted to synthesis and properties of HCA derivatives. Then, the results of molecular modeling of HCA derivatives with prion protein as well as with α-synuclein fibrils are summarized, followed by detailed analysis of the experiments on the effect of natural and synthetic HCA derivatives, as well as structurally similar phenylacetic and benzoic acid derivatives, on the pathological transformation of prion protein and α-synuclein. The ability of HCA derivatives to prevent amyloid transformation of some amyloidogenic proteins, and their presence not only in food products but also as natural metabolites in human blood and tissues, makes them promising for the prevention and treatment of neurodegenerative diseases of amyloid nature.

Flavonoids with Vicinal Hydroxyl Groups Inhibit Human Calcitonin Amyloid Formation

Human calcitonin (hCT) is a 32-residue peptide hormone that can aggregate into amyloid fibrils and cause cellular toxicity. In this study, we investigated the inhibition effects of a group of polyphenolic molecules on hCT amyloid formation. Our results suggest that the gallate moiety in epigallocatechin-3-gallate (EGCG), a well-recognized amyloid inhibitor, is not critical for its inhibition function in the hCT amyloid formation. Our results demonstrate that flavonoid compounds, such as myricetin, quercetin, and baicalein, that contain vicinal hydroxyl groups on the phenyl ring effectively prevent hCT fibrillization. This structural feature may also be applied to non-flavonoid polyphenolic inhibitors. Moreover, our results indicate a plausible mechanistic role of these vicinal hydroxyl groups which might include the oxidation to form a quinone and the subsequent covalent linkage with amino acid residues such as lysine or histidine in hCT. This may further disrupt the crucial electrostatic and aromatic interactions involved in the process of hCT amyloid fibril formation. The inhibition activity of the polyphenolic compounds against hCT fibril formation may likely be attributed to a combination of factors such as covalent linkage formation, aromatic stacking, and hydrogen bonding interactions.

Tryptophan-galactosylamine conjugates inhibit and disaggregate amyloid fibrils of Aβ42 and hIAPP peptides while reducing their toxicity

Self-assembly of proteins into amyloid fibrils is a hallmark of various diseases, including Alzheimer's disease (AD) and Type-2 diabetes Mellitus (T2DM). Aggregation of specific peptides, like Aβ42 in AD and hIAPP in T2DM, causes cellular dysfunction resulting in the respective pathology. While these amyloidogenic proteins lack sequence homology, they all contain aromatic amino acids in their hydrophobic core that play a major role in their self-assembly. Targeting these aromatic residues by small molecules may be an attractive approach for inhibiting amyloid aggregation. Here, various biochemical and biophysical techniques revealed that a panel of tryptophan-galactosylamine conjugates significantly inhibit fibril formation of Aβ42 and hIAPP, and disassemble their pre-formed fibrils in a dose-dependent manner. They are also not toxic to mammalian cells and can reduce the cytotoxicity induced by Aβ42 and hIAPP aggregates. These tryptophan-galactosylamine conjugates can therefore serve as a scaffold for the development of therapeutics towards AD and T2DM.

Targeting α-synuclein for PD Therapeutics: A Pursuit on All Fronts

Parkinson's Disease (PD) is characterized both by the loss of dopaminergic neurons in the substantia nigra and the presence of cytoplasmic inclusions called Lewy Bodies. These Lewy Bodies contain the aggregated α-synuclein (α-syn) protein, which has been shown to be able to propagate from cell to cell and throughout different regions in the brain. Due to its central role in the pathology and the lack of a curative treatment for PD, an increasing number of studies have aimed at targeting this protein for therapeutics. Here, we reviewed and discussed the many different approaches that have been studied to inhibit α-syn accumulation via direct and indirect targeting. These analyses have led to the generation of multiple clinical trials that are either completed or currently active. These clinical trials and the current preclinical studies must still face obstacles ahead, but give hope of finding a therapy for PD with time.

Multifaceted Influences of Melanin-Like Particles on Amyloid-beta Aggregation

The properties of eumelanin-like particles (EMPs) and pheomelanin-like particles (PMPs) in regulating the process of amyloid formation of amyloid-beta 42 (Aβ42) were examined. EMPs and PMPs are effective both in interfering with amyloid aggregation of Aβ42 and in remodeling matured Αβ42 fibers. The results suggest that some (but not all) molecular species consisting of melanin-like particles (MPs) are responsible for their inhibiting property toward amyloid formation, and the influence is likely manifested by long-range interactions. Incubating preformed Aβ42 fibers with catechols or MPs leads to the formation of mesh-like, interconnected Aβ42 fibers encapsulated with melanin-like material. MPs are kinetically more effective than catechol monomers in this process, and a detailed investigation reveals that 4,5-dihydroxyindole, a major intermediate in the formation of melanin-like species, and its derivatives are mainly responsible for remodeling amyloid fibers.

Insight into the molecular mechanism underlying the inhibition of α-synuclein aggregation by hydroxytyrosol

Parkinson's disease (PD) is the second most prevalent neurodegenerative disease in the elderly people. To date, drugs able to reverse the disease are not available; the gold standard is levodopa that only relieves clinical symptoms, yet with severe side effects after prolonged administration. Many efforts are underway to find alternative targets for PD prevention or treatment, the most promising being α-synuclein (Syn). Recently, we reported that oleuropein aglycone (OleA) interferes with amyloid aggregation of Syn both stabilizing its monomeric state and inducing the formation of harmless, off-pathway oligomers. This study is focused at describing the interaction between Syn and hydroxytyrosol (HT), the phenolic moiety and main metabolite of OleA, and the interferences with Syn aggregation by using biophysical and biological techniques. Our results show that HT dose-dependently inhibits Syn aggregation and that covalent and non-covalent binding mediate HT-Syn interaction. HT does not modify the natively unfolded structure of Syn, rather, it stabilizes specific regions of the molecule leading to inhibition of protein fibrillation. Cellular assays showed that HT reduces the toxicity of Syn aggregates. Moreover, Syn aggregates interaction with the cell membrane, an important factor for prion-like properties of Syn on-pathway oligomers, was reduced in cells exposed to Syn aggregates grown in the presence of HT.

Nonpharmacological Modulation of Chronic Inflammation in Parkinson's Disease: Role of Diet Interventions

Neuroinflammation is increasingly recognized as an important pathophysiological feature of neurodegenerative diseases such as Parkinson's disease (PD). Recent evidence suggests that neuroinflammation in PD might originate in the intestine and the bidirectional communication between the central and enteric nervous system, the so-called "gut-brain axis," has received growing attention due to its contribution to the pathogenesis of neurological disorders. Diet targets mediators of inflammation with various mechanisms and combined with dopaminergic treatment can exert various beneficial effects in PD. Food-based therapies may favorably modulate gut microbiota composition and enhance the intestinal epithelial integrity or decrease the proinflammatory response by direct effects on immune cells. Diets rich in pre- and probiotics, polyunsaturated fatty acids, phenols including flavonoids, and vitamins, such as the Mediterranean diet or a plant-based diet, may attenuate chronic inflammation and positively influence PD symptoms and even progression of the disease. Dietary strategies should be encouraged in the context of a healthy lifestyle with physical activity, which also has neuroimmune-modifying properties. Thus, diet adaptation appears to be an effective additive, nonpharmacological therapeutic strategy that can attenuate the chronic inflammation implicated in PD, potentially slow down degeneration, and thereby modify the course of the disease. PD patients should be highly encouraged to adopt corresponding lifestyle modifications, in order to improve not only PD symptoms, but also general quality of life. Future research should focus on planning larger clinical trials with dietary interventions in PD in order to obtain hard evidence for the hypothesized beneficial effects.

Characterizing the inhibition of α-synuclein oligomerization by a pharmacological chaperone that prevents prion formation by the protein PrP

Aggregation of the disordered protein α-synuclein into amyloid fibrils is a central feature of synucleinopathies, neurodegenerative disorders that include Parkinson's disease. Small, pre-fibrillar oligomers of misfolded α-synuclein are thought to be the key toxic entities, and α-synuclein misfolding can propagate in a prion-like way. We explored whether a compound with anti-prion activity that can bind to unfolded parts of the protein PrP, the cyclic tetrapyrrole Fe-TMPyP, was also active against α-synuclein aggregation. Observing the initial stages of aggregation via fluorescence cross-correlation spectroscopy, we found that Fe-TMPyP inhibited small oligomer formation in a dose-dependent manner. Fe-TMPyP also inhibited the formation of mature amyloid fibrils in vitro, as detected by thioflavin T fluorescence. Isothermal titration calorimetry indicated Fe-TMPyP bound to monomeric α-synuclein with a stoichiometry of 2, and two-dimensional heteronuclear single quantum coherence NMR spectra revealed significant interactions between Fe-TMPyP and the C-terminus of the protein. These results suggest commonalities among aggregation mechanisms for α-synuclein and the prion protein may exist that can be exploited as therapeutic targets.

Protein misfolding, aggregation and mechanism of amyloid cytotoxicity: An overview and therapeutic strategies to inhibit aggregation

Protein and peptides are converted from their soluble forms into highly ordered fibrillar aggregates under various conditions inside the cell. Such transitions confer diverse neurodegenerative diseases including Alzheimer's disease, Huntington's disease Prion's disease, Parkinson's disease, polyQ and share abnormal folding of potentially cytotoxic protein species linked with degeneration and death of precise neuronal populations. Presently, major advances are made to understand and get detailed insight into the structural basis and mechanism of amyloid formation, cytotoxicity and therapeutic approaches to combat them. Here we highlight classifies and summarizes the detailed overview of protein misfolding and aggregation at their molecular level including the factors that promote protein aggregation under in vivo and in vitro conditions. In addition, we describe the recent technologies that aid the characterization of amyloid aggregates along with several models that might be responsible for amyloid induced cytotoxicity to cells. Overview on the inhibition of amyloidosis by targeting different small molecules (both natural and synthetic origin) have been also discussed, that provides important approaches to identify novel targets and develop specific therapeutic strategies to combat protein aggregation related neurodegenerative diseases.

In vitro models of synucleinopathies: informing on molecular mechanisms and protective strategies

Alpha-synuclein (α-Syn) is a central player in Parkinson's disease (PD) and in a spectrum of neurodegenerative diseases collectively known as synucleinopathies. The protein was first associated with PD just over 20 years ago, when it was found to (i) be a major component of Lewy bodies and (ii) to be also associated with familial forms of PD. The characterization of α-Syn pathology has been achieved through postmortem studies of human brains. However, the identification of toxic mechanisms associated with α-Syn was only achieved through the use of experimental models. In vitro models are highly accessible, enable relatively rapid studies, and have been extensively employed to address α-Syn-associated neurodegeneration. Given the diversity of models used and the outcomes of the studies, a cumulative and comprehensive perspective emerges as indispensable to pave the way for further investigations. Here, we subdivided in vitro models of α-Syn pathology into three major types: (i) models simulating α-Syn fibrillization and the formation of different aggregated structures in vitro, (ii) models based on the intracellular expression of α-Syn, reporting on pathogenic conditions and cellular dysfunctions induced, and (iii) models using extracellular treatment with α-Syn aggregated species, reporting on sites of interaction and their downstream consequences. In summary, we review the underlying molecular mechanisms discovered and categorize protective strategies, in order to pave the way for future studies and the identification of effective therapeutic strategies. This article is part of the Special Issue "Synuclein".

Dihydromyricetin Inhibits α-Synuclein Aggregation, Disrupts Preformed Fibrils, and Protects Neuronal Cells in Culture against Amyloid-Induced Cytotoxicity

Fibrillogenesis of α-synuclein (αSN) is associated with the onset and progression of Parkinson's disease (PD). Dihydromyricetin (DHM), a natural flavonoid compound extracted from Ampelopsis grossedentata, has proven antioxidative, antineuroinflammatory, and neuroprotective effects in dementia. However, it remains unclear if DHM can impede αSN fibrillogenesis and attenuate the corresponding cytotoxicity. Herein, we found that DHM could inhibit αSN fibrillogenesis and destabilize mature αSN fibrils in a dose-dependent manner. Moreover, DHM protected against αSN-induced cytotoxicity by improving the cell viability by 34.73 ± 3.68% at a 1:1 molar ratio of αSN to DHM. Molecular dynamics simulations showed that DHM interacts with the αSN trimer mainly via nonpolar mechanisms. The key residues by which αSN interacts with DHM were hydrophobic, and their side chains and main chains showed a synergistic effect via hydrophobic and hydrogen-bonding interactions. These findings suggest that DHM possesses great potential to be developed into a new aggregation inhibitor for αSN.

Pre-structured hydrophobic peptide β-strands: A universal amyloid trap?

Amyloid fibril formation has long been studied because of the variety of proteins that are capable of adopting this structure despite sharing little sequence homology. This makes amyloid fibrils a challenging focus for inhibition studies because the peptides and proteins that form amyloid fibrils cannot be targeted based on a sequence motif. Most peptide inhibitors that target specific amyloidogenic proteins rely heavily on sequence recognition to ensure that the inhibitory peptide is able to bind its target. This approach is limited to targeting one amyloidogenic protein at a time. However, there is increasing evidence of cross-reactivity between amyloid-forming polypeptides. It has therefore become more useful to study the similarities between these proteins that goes beyond their sequence homology. Indeed, the observation that amyloidogenic proteins adopt similar secondary structures along the pathway to fibril formation opens the way to an interesting investigation: the development of inhibitors that could be universal amyloid traps. The review below will analyze two specific amyloidogenic proteins, α-synuclein and human amylin, and introduce a small number of peptides that have been shown to be capable of inhibiting the amyloidogenesis of both of these very dissimilar polypeptides. Some of the inhibitory peptide motifs may indeed, be applicable to Aβ and other amyloidogenic systems.

Plant Extracts and Phytochemicals Targeting α-Synuclein Aggregation in Parkinson's Disease Models

α-Synuclein (α-syn) is a presynaptic protein that regulates the release of neurotransmitters from synaptic vesicles in the brain. α-Syn aggregates, including Lewy bodies, are features of both sporadic and familial forms of Parkinson's disease (PD). These aggregates undergo several key stages of fibrillation, oligomerization, and aggregation. Therapeutic benefits of drugs decline with disease progression and offer only symptomatic treatment. Novel therapeutic strategies are required which can either prevent or delay the progression of the disease. The link between α-syn and the etiopathogenesis and progression of PD are well-established in the literature. Studies indicate that α-syn is an important therapeutic target and inhibition of α-syn aggregation, oligomerization, and fibrillation are an important disease modification strategy. However, recent studies have shown that plant extracts and phytochemicals have neuroprotective effects on α-syn oligomerization and fibrillation by targeting different key stages of its formation. Although many reviews on the antioxidant-mediated, neuroprotective effect of plant extracts and phytochemicals on PD symptoms have been well-highlighted, the antioxidant mechanisms show limited success for translation to clinical studies. The identification of specific plant extracts and phytochemicals that target α-syn aggregation will provide selective molecules to develop new drugs for PD. The present review provides an overview of plant extracts and phytochemicals that target α-syn in PD and summarizes the observed effects and the underlying mechanisms. Furthermore, we provide a synopsis of current experimental models and techniques used to evaluate plant extracts and phytochemicals. Plant extracts and phytochemicals were found to inhibit the aggregation or fibril formation of oligomers. These also appear to direct α-syn oligomer formation into its unstructured form or promote non-toxic pathways and suggested to be valuable drug candidates for PD and related synucleinopathy. Current evidences from in vitro studies require confirmation in the in vivo studies. Further studies are needed to ascertain their potential effects and safety in preclinical studies for pharmaceutical/nutritional development of these phytochemicals or dietary inclusion of the plant extracts in PD treatment.

Cyclized NDGA modifies dynamic α-synuclein monomers preventing aggregation and toxicity

Growing evidence implicates α-synuclein aggregation as a key driver of neurodegeneration in Parkinson’s disease (PD) and other neurodegenerative disorders. Herein, the molecular and structural mechanisms of inhibiting α-synuclein aggregation by novel analogs of nordihydroguaiaretic acid (NDGA), a phenolic dibenzenediol lignan, were explored using an array of biochemical and biophysical methodologies. NDGA analogs induced modest, progressive compaction of monomeric α-synuclein, preventing aggregation into amyloid-like fibrils. This conformational remodeling preserved the dynamic adoption of α-helical conformations, which are essential for physiological membrane interactions. Oxidation-dependent NDGA cyclization was required for the interaction with monomeric α-synuclein. NDGA analog-pretreated α-synuclein did not aggregate even without NDGA-analogs in the aggregation mixture. Strikingly, NDGA-pretreated α-synuclein suppressed aggregation of naïve untreated aggregation-competent monomeric α-synuclein. Further, cyclized NDGA reduced α-synuclein-driven neurodegeneration in Caenorhabditis elegans. The cyclized NDGA analogs may serve as a platform for the development of small molecules that stabilize aggregation-resistant α-synuclein monomers without interfering with functional conformations yielding potential therapies for PD and related disorders.

Toward the discovery and development of effective modulators of α-synuclein amyloid aggregation

A host of human diseases, including Parkinson's disease and Dementia with Lewy bodies, are suspected to be directly linked to protein aggregation. Amyloid protein aggregates and oligomeric intermediates of α-synuclein are observed in synucleinopathies and considered to be mediators of cellular toxicity. Hence, α-synuclein has seen as one of the leading and most compelling targets and is receiving a great deal of attention from researchers. Nevertheless, there is no neuroprotective approach directed toward Parkinson's disease or other synucleinopathies so far. In this review, we summarize the available data concerning inhibitors of α-synuclein aggregation and their advancing towards clinical use. The compounds are grouped according to their chemical structures, providing respective insights into their mechanism of action, pharmacology, and pharmacokinetics. Overall, shared structure-activity elements are emerging, as well as specific binding modes related to the ability of the modulators to establish hydrophobic and hydrogen bonds interactions with the protein. Some molecules with encouraging in vivo data support the possibility of translation to the clinic.

Computational Study on the Role of γ-Synuclein in Inhibiting the α-Synuclein Aggregation

BACKGROUND

α-Synuclein (αS) is the precursor protein present in Lewy Bodies that helps in the formation of highly ordered amyloid fibrils that is associated with the occurrence of Parkinson's disease, a neuro-degenerative disorder. Many reports have now been focused on finding the probable targets to weaken this debilitating disease. Recently γ-synuclein (γS), a presynaptic protein, was highlighted to inhibit the aggregation propensity of αS both in vivo and in vitro. However the nature, location and specificity of molecular interactions existing between the αS and γS is not known in spite of the potential importance of γS as an inhibitor of αS.

OBJECTIVE

To understand the inhibition of αS aggregation by γS at the molecular level.

METHODS

Umbrella sampling method was used along with molecular dynamics simulation to investigate the conformational dynamics, degree of association and molecular interaction between the monomeric units in the αS/γS hetero-dimer.

RESULTS AND DISCUSSION

The dissociation energy barrier for αS/γS hetero-dimer was found to be higher than αS/αS homo-dimer. αS can therefore readily form a hetero-dimer by combining with γS than forming a homo-dimer. We also observed strong transient interactions involving hydrogen bonds, salt-bridges and non-bonded contacts between the monomeric units in αS/γS hetero-dimer.

CONCLUSION

Our findings suggest that γS may inhibit the aggregation propensity of αS.

Triphala inhibits alpha-synuclein fibrillization and their interaction study by NMR provides insights into the self-association of the protein

The process of assembly and accumulation of the intrinsically disordered protein (IDP), alpha-synuclein (αSyn) into amyloid fibrils is a pathogenic process leading to several neurodegenerative disorders such as Parkinson's disease, multiple system atrophy and others. Although several molecules are known to inhibit αSyn fibrillization, the mechanism of inhibition is just beginning to emerge. Here, we report the inhibition of fibrillization of αSyn by Triphala, a herbal preparation in the traditional Indian medical system of Ayurveda. Triphala was found to be a rich source of polyphenols which are known to act as amyloid inhibitors. ThT fluorescence and TEM studies showed that Triphala inhibited the fibrillization of αSyn. However, it was observed that Triphala does not disaggregate preformed αSyn fibrils. Further, native-PAGE showed that Triphala reduces the propensity of αSyn to oligomerize during the lag phase of fibrillization. Our NMR results showed that certain stretches of residues in the N-terminal and NAC regions of αSyn play an anchor role in the self-association process of the protein, thereby providing mechanistic insights into the early events during αSyn fibrillization.

Triphala inhibits αSyn self-association by interacting with anchoring regions which are responsible for αSyn oligomerization.

Protective effects of hydroxytyrosol against α-synuclein toxicity on PC12 cells and fibril formation

There is a considerable consensus that the increased production and/or aggregation of α-synuclein (αsyn) plays a central role in the pathogenesis of Parkinson's disease (PD). Therefore, a method of identifying molecules that block αsyn aggregation and prevent the loss of dopaminergic neurons is urgently needed in order to treat or slow the progression of PD. Hydroxytyrosol (HT), a well-known bioactive food compound present in olive oil, olives and wine, possesses demonstrated antioxidant and anti-inflammatory properties that can cross the Blood Brain Barrier (BBB). In the present work, the role of HT, tyrosol (TYR) and other tyrosine metabolites (hydroxyphenyl acetic acid (HPAA)) against αsyn aggregation, destabilisation and toxicity was evaluated through the use of different in vitro tests (Thioflavin T (ThT), Transmission Electronic Microscopy (TEM), electrophoresis and MTT assay). Results show that HT presents a strong inhibitory effect preventing αsyn aggregation and exercising a destabilising effect by disaggregating αsyn fibrils. Moreover, HT is able to counteract αsyn-induced toxicity. This is the first time that the effect of HT against αsyn toxicity and aggregation is evaluated. Thus, HT can be considered a promising compound for further approaches to tackling PD.

Exploring the Structural Diversity in Inhibitors of α-Synuclein Amyloidogenic Folding, Aggregation, and Neurotoxicity

Aggregation of α-Synuclein (αS) protein to amyloid fibrils is a neuropathological hallmark of Parkinson's disease (PD). Growing evidence suggests that extracellular αS aggregation plays a pivotal role in neurodegeneration found in PD in addition to the intracellular αS aggregates in Lewy bodies (LB). Here, we identified and compared a diverse set of molecules capable of mitigating protein aggregation and exogenous toxicity of αSA53T, a more aggregation-prone αS mutant found in familial PD. For the first time, we investigated the αS anti-amyloid activity of semi-synthetic flavonoid 2', 3', 4' trihydroxyflavone or 2-D08, which was compared with natural flavones myricetin and transilitin, as well as such structurally diverse polyphenols as honokiol and punicalagin. Additionally, two novel synthetic compounds with a dibenzyl imidazolidine scaffold, Compound 1 and Compound 2, were also investigated as they exhibited favorable binding with αSA53T. All seven compounds inhibited αSA53T aggregation as demonstrated by Thioflavin T fluorescence assays, with modified fibril morphology observed by transmission electron microscopy. Ion mobility-mass spectrometry (IM-MS) was used to monitor the structural conversion of native αSA53T into amyloidogenic conformations and all seven compounds preserved the native unfolded conformations of αSA53T following 48 h incubation. The presence of each test compound in a 1:2 molar ratio was also shown to inhibit the neurotoxicity of preincubated αSA53T using phaeochromocytoma (PC12) cell viability assays. Among the seven tested compounds 2-D08, honokiol, and the synthetic Compound 2 demonstrated the highest inhibition of aggregation, coupled with neuroprotection from preincubated αSA53T in vitro. Molecular docking predicted that all compounds bound near the lysine-rich region of the N-terminus of αSA53T, where the flavonoids and honokiol predominantly interacted with Lys 23. Overall, these findings highlight that (i) restricted vicinal trihydroxylation in the flavone B-ring is more effective in stabilizing the native αS conformations, thus blocking amyloidogenic aggregation, than dihydroxylation aggregation in both A and B-ring, and (ii) honokiol, punicalagin, and the synthetic imidazolidine Compound 2 also inhibit αS amyloidogenic aggregation by stabilizing its native conformations. This diverse set of molecules acting on a singular pathological target with predicted binding to αSA53T in the folding-prone N-terminal region may contribute toward novel drug-design for PD.

Peptides as Potential Therapeutics for Alzheimer's Disease

Intracellular synthesis, folding, trafficking and degradation of proteins are controlled and integrated by proteostasis. The frequency of protein misfolding disorders in the human population, e.g., in Alzheimer's disease (AD), is increasing due to the aging population. AD treatment options are limited to symptomatic interventions that at best slow-down disease progression. The key biochemical change in AD is the excessive accumulation of per-se non-toxic and soluble amyloid peptides (Aβ(1-37/44), in the intracellular and extracellular space, that alters proteostasis and triggers Aβ modification (e.g., by reactive oxygen species (ROS)) into toxic intermediate, misfolded soluble Aβ peptides, Aβ dimers and Aβ oligomers. The toxic intermediate Aβ products aggregate into progressively less toxic and less soluble protofibrils, fibrils and senile plaques. This review focuses on peptides that inhibit toxic Aβ oligomerization, Aβ aggregation into fibrils, or stabilize Aβ peptides in non-toxic oligomers, and discusses their potential for AD treatment.

Natural polyphenols effects on protein aggregates in Alzheimer's and Parkinson's prion-like diseases

Alzheimer's and Parkinson's diseases are the most common neurodegenerative diseases. They are characterized by protein aggregates and so can be considered as prion-like disease. The major components of these deposits are amyloid peptide and tau for Alzheimer's disease, α-synuclein and synphilin-1 for Parkinson's disease. Drugs currently proposed to treat these pathologies do not prevent neurodegenerative processes and are mainly symptomatic therapies. Molecules inducing inhibition of aggregation or disaggregation of these proteins could have beneficial effects, especially if they have other beneficial effects for these diseases. Thus, several natural polyphenols, which have antioxidative, anti-inflammatory and neuroprotective properties, have been largely studied, for their effects on protein aggregates found in these diseases, notably in vitro. In this article, we propose to review the significant papers concerning the role of polyphenols on aggregation and disaggregation of amyloid peptide, tau, α-synuclein, synphilin-1, suggesting that these compounds could be useful in the treatments in Alzheimer's and Parkinson's diseases.

A study on the modulation of alpha-synuclein fibrillation by Scutellaria pinnatifida extracts and its neuroprotective properties

Aggregation of alpha-synuclein (α-SN) is a key pathogenic event in Parkinson's disease (PD) leading to dopaminergic degeneration. The identification of natural compounds inhibiting α-SN aggregation may have a major role in treating PD. Different Scutellaria species are known as valuable medicinal plants, primarily due to their high flavonoid levels. Scutellaria pinnatifida (S. pinnatifida) is endemic to Iran; however, the knowledge of its pharmaceutical properties is limited. Here we report that S. pinnatifida extracts have an anti-fibrillation effect on α-SN aggregation and neuroprotective properties on PC12 and primary dopaminergic neurons. Treatment during α-SN fibril formation with S. pinnatifida extracts showed that the extractions performed with dichloromethane (DCMEx) and n-butanol (BuOHEx) strongly inhibited α-SN fibrillation. TLC-based analysis revealed that S. pinnatifida contains a great amount of flavonoids with high antioxidant properties as shown using a radical scavenging assay. Further analysis using HPLC and Mass spectroscopy on the DCMEx revealed the presence of baicalein in this extract. We then selected the more efficient extracts based on cell viability and ROS scavenging on PC12 cells and tested their neuroprotective properties on primary dopaminergic neurons. Our results showed the extracts strongly protected against α-SN oligomers. Surprisingly, they also neutralized the severe toxicity of paraquat. Therefore, S. pinnatifida may be a potential valuable medicinal herb for further studies related to the treatment of PD.

Metal chelator EGCG attenuates Fe(III)-induced conformational transition of α-synuclein and protects AS-PC12 cells against Fe(III)-induced death

The fibrillation and aggregation of α-synuclein (AS), along with the conformational transition from random coil to β-sheet, are the critical steps in the development of Parkinson's disease (PD). It is acknowledged that iron accumulation in the brain may lead to the fibrillation of AS. However, (-)-epigallocatechin gallate (EGCG) can penetrate the blood-brain barrier, chelate metal ions, and inhibit the fibrillation of amyloid proteins. Therefore, EGCG is warranted to be investigated for its potential to cure amyloid-related diseases. In the present work, we sought to study the effects of EGCG on Fe(III)-induced fibrillation of AS on both molecular and cellular levels. We demonstrate that Fe(III) interacts with the amino residue of Tyr and Ala of AS, then accelerates the fibrillation of AS, and increases intracellular reactive oxygen species (ROS) in the AS transduced-PC12 cells (AS-PC12 cells). However, EGCG significantly inhibits this process by chelating Fe(III) and protects AS-PC12 cells against the toxicity induced by ROS and β-sheet-enriched AS fibrils. These findings yield useful information that EGCG might be a promising drug to prevent and treat the neurodegenerative diseases.

Molecular Mechanism for the (-)-Epigallocatechin Gallate-Induced Toxic to Nontoxic Remodeling of Aβ Oligomers

(-)-Epigallocatechin gallate (EGCG) effectively reduces the cytotoxicity of the Alzheimer's disease β-amyloid peptide (Aβ) by remodeling seeding-competent Aβ oligomers into off-pathway seeding-incompetent Aβ assemblies. However, the mechanism of EGCG-induced remodeling is not fully understood. Here we combine ¹⁵N and ¹H dark-state exchange saturation transfer (DEST), relaxation, and chemical shift projection NMR analyses with fluorescence, dynamic light scattering, and electron microscopy to elucidate how EGCG remodels Aβ oligomers. We show that the remodeling adheres to a Hill-Scatchard model whereby the Aβ(1-40) self-association occurs cooperatively and generates Aβ(1-40) oligomers with multiple independent binding sites for EGCG with a K_d ∼10-fold lower than that for the Aβ(1-40) monomers. Upon binding to EGCG, the Aβ(1-40) oligomers become less solvent exposed, and the β-regions, which are involved in direct monomer-protofibril contacts in the absence of EGCG, undergo a direct-to-tethered contact shift. This switch toward less engaged monomer-protofibril contacts explains the seeding incompetency observed upon EGCG remodeling and suggests that EGCG interferes with secondary nucleation events known to generate toxic Aβ assemblies. Unexpectedly, the N-terminal residues experience an opposite EGCG-induced shift from tethered to direct contacts, explaining why EGCG remodeling occurs without release of Aβ(1-40) monomers. We also show that upon binding Aβ(1-40) oligomers the relative positions of the EGCG B and D rings change with respect to that of ring A. These distinct structural changes occurring in both Aβ(1-40) oligomers and EGCG during remodeling offer a foundation for understanding the molecular mechanism of EGCG as a neurotoxicity inhibitor. Furthermore, the results reported here illustrate the effectiveness of DEST-based NMR approaches in investigating the mechanism of low-molecular-weight amyloid inhibitors.

Therapeutic Potential of Baicalein in Alzheimer's Disease and Parkinson's Disease

Alzheimer's disease and Parkinson's disease are the two most common, progressive central neurodegenerative diseases affecting the population over the age of 60 years. Apart from treatments that temporarily improve symptoms, there is no medicine currently available to inhibit or reverse the progression of Alzheimer's disease and Parkinson's disease. In traditional Chinese medicine, the root of Scutellaria baicalensis Georgi is a classic compatible component in the decoction of herbal medicine used for treating central nervous system diseases. Modern pharmacokinetic studies have confirmed that baicalein (5,6,7-trihydroxyflavone) is a major bioactive flavone constituent root of S. baicalensis Georgi. Studies showed that baicalein possesses a range of key pharmacological properties, such as reducing oxidative stress, anti-inflammatory properties, inhibiting aggregation of disease-specific amyloid proteins, inhibiting excitotoxicity, stimulating neurogenesis and differentiation action, and anti-apoptosis effects. Based on these properties, baicalein shows therapeutic potential for Alzheimer's disease and Parkinson's disease. In this review, we summarize the pharmacological protective actions of baicalein that make it suitable for the treatment of Alzheimer's disease and Parkinson's disease, and discuss the potential mechanisms underlying the effects.

Natural product-based amyloid inhibitors

Many chronic human diseases, including multiple neurodegenerative diseases, are associated with deleterious protein aggregates, also called protein amyloids. One common therapeutic strategy is to develop protein aggregation inhibitors that can slow down, prevent, or remodel toxic amyloids. Natural products are a major class of amyloid inhibitors, and several dozens of natural product-based amyloid inhibitors have been identified and characterized in recent years. These plant- or microorganism-extracted compounds have shown significant therapeutic potential from in vitro studies as well as in vivo animal tests. Despite the technical challenges of intrinsic disordered or partially unfolded amyloid proteins that are less amenable to characterizations by structural biology, a significant amount of research has been performed, yielding biochemical and pharmacological insights into how inhibitors function. This review aims to summarize recent progress in natural product-based amyloid inhibitors and to analyze their mechanisms of inhibition in vitro. Major classes of natural product inhibitors and how they were identified are described. Our analyses comprehensively address the molecular interactions between the inhibitors and relevant amyloidogenic proteins. These interactions are delineated at molecular and atomic levels, which include covalent, non-covalent, and metal-mediated mechanisms. In vivo animal studies and clinical trials have been summarized as an extension. To enhance natural product bioavailability in vivo, emerging work using nanocarriers for delivery has also been described. Finally, issues and challenges as well as future development of such inhibitors are envisioned.

Screening of plants used in the European traditional medicine to treat memory disorders for acetylcholinesterase inhibitory activity and anti amyloidogenic activity

ETHNOPHARMACOLOGICAL RELEVANCE

Plants used in the traditional medicine of Europe to treat memory dysfunction and/or to enhance memory were investigated for activity against the underlying mechanisms of Alzheimer's disease.

AIM OF THE STUDY

To investigate 35 ethanolic extracts of plants, selected using an ethnopharmacological approach, for anti-amyloidogenic activity as well as an ability to inhibit the enzymatic activity of acetylcholinesterase.

MATERIALS AND METHODS

The anti-amyloidogenic activity of the extracts against amyloid beta was investigated by Thioflavin T fibrillation assays and the ability to inhibit the enzymatic activity of acetylcholinesterase was evaluated monitoring the hydrolysis of acetylthiocholine RESULTS: Under the experimental conditions investigated, extracts of two plants, Carum carvi and Olea sylvestris, inhibited amyloid beta fibrillation considerably, eight plant extracts inhibited amyloid beta fibrillation to some extent, 16 plant extracts had no effect on amyloid beta fibrillation and nine extracts accelerated fibrillation of amyloid beta. Furthermore, five plant extracts from Corydalis species inhibited the enzymatic activity of acetylcholinesterase considerably, one plant extract inhibited the enzymatic activity of acetylcholinesterase to some extent and 29 plant extract had no effect on the enzymatic activity of acetylcholinesterase.

CONCLUSIONS

An optimal extract in this study would possess acetylcholinesterase inhibitory activity as well as anti-amyloidogenic activity in order to address multiple facets of Alzheimer's disease, until the molecular origin of the disease is unraveled. Unfortunately no such extract was found.

Complexes between cationic pyridylphenylene dendrimers and ovine prion protein: do hydrophobic interactions matter?

MD simulation predicted the possible binding sites for the dendrimer interactions with protein while ITC data revealed both electrostatic and hydrophobic driving forces for the complexation.

Modulating the Amyloidogenesis of α-Synuclein

Alpha-Synuclein is found in the neuronal cells but its native function is not well known. While α -synuclein is an intrinsically disordered protein that adopts a helical conformation upon membrane binding, numerous studies have shown that oligomeric β-forms of this protein are cytotoxic. This response to misfolded species contributes to Parkinson's Disease etiology and symptoms. The resulting amyloid fibrils are an established diagnostic in Parkinson's Disease. In this review, we focus on strategies that have been used to inhibit the amyloidogenesis of α -synuclein either by stabilizing the native state, or by redirecting the pathway to less toxic aggregates. Small molecules such as polyphenols, peptides as well as large proteins have proven effective at protecting cells against the cytotoxicity of α-synuclein. These strategies may lead to the development of therapeutic agents that could prove useful in combating this disease.

Potential for brain accessibility and analysis of stability of selected flavonoids in relation to neuroprotection in vitro

Natural food sources constitute a promising source of new compounds with neuroprotective properties, once they have the ability to reach the brain. Our aim was to evaluate the brain accessibility of quercetin, epigallocatechin gallate (EGCG) and cyanidin-3-glucoside (C3G) in relation to their neuroprotective capability. Primary cortical neuron cultures were exposed to oxidative insult in the absence and presence of the selected compounds, and neuroprotection was assessed through evaluation of apoptotic-like and necrotic-like cell death. The brain accessibility of selected compounds was assessed using an optimised human blood-brain barrier model. The blood-brain barrier model was crossed rapidly by EGCG and more slowly by C3G, but not by quercetin. EGCG protected against oxidation-induced neuronal necrotic-like cell death by ~40%, and apoptosis by ~30%. Both quercetin and C3G were less effective, since only the lowest quercetin concentration was protective, and C3G only prevented necrosis by ~37%. Quercetin, EGCG and C3G effectively inhibited α-synuclein fibrillation over the relevant timescale applied here. Overall, EGCG seems to be the most promising neuroprotective compound. Thus, inclusion of this polyphenol in the diet might provide an affordable means to reduce the impact of neurodegenerative diseases.