EGCG remodels mature alpha-synuclein and amyloid-beta fibrils and reduces cellular toxicity

Anti-amyloid compounds inhibit α-synuclein aggregation induced by protein misfolding cyclic amplification (PMCA)

Filaments made of α-synuclein form the characteristic Lewy pathology in Parkinson and other diseases. The formation of α-synuclein filaments can be reproduced in vitro by incubation of recombinant protein, but the filament growth is very slow and highly variable and so unsuitable for fast high throughput anti-aggregation drug screening. To overcome this obstacle we have investigated whether the protein misfolding cyclic amplification (PMCA) technique, used for fast amplification of prion protein aggregates, could be adapted for growing α-synuclein aggregates and thus suitable for screening of drugs to affect α-synuclein aggregation for the treatment of the yet incurable α-synucleinopathies. Circular dichroism, electron microscopy, and native and SDS-polyacrylamide gels were used to demonstrate α-synuclein aggregate formation by PMCA, and the strain imprint of the α-synuclein fibrils was studied by proteinase K digestion. We also demonstrated that α-synuclein fibrils are able to seed new α-synuclein PMCA reactions and to enter and aggregate in cells in culture. In particular, we have generated a line of “chronically infected” cells, which transmit α-synuclein aggregates even after multiple passages. To evaluate the sensitivity of the PMCA system as an α-synuclein anti-aggregating drug screening assay a panel of 10 drugs was tested. Anti-amyloid compounds proved efficient in inhibiting α-synuclein fibril formation induced by PMCA. Our results show that α-synuclein PMCA is a fast and reproducible system that could be used as a high throughput screening method for finding new α-synuclein anti-aggregating compounds.

Flavonoids as therapeutic compounds targeting key proteins involved in Alzheimer's disease

Alzheimer's disease is characterized by pathological aggregation of protein tau and amyloid-β peptides, both of which are considered to be toxic to neurons. Naturally occurring dietary flavonoids have received considerable attention as alternative candidates for Alzheimer's therapy taking into account their antiamyloidogenic, antioxidative, and anti-inflammatory properties. Experimental evidence supports the hypothesis that certain flavonoids may protect against Alzheimer's disease in part by interfering with the generation and assembly of amyloid-β peptides into neurotoxic oligomeric aggregates and also by reducing tau aggregation. Several mechanisms have been proposed for the ability of flavonoids to prevent the onset or to slow the progression of the disease. Some mechanisms include their interaction with important signaling pathways in the brain like the phosphatidylinositol 3-kinase/Akt and mitogen-activated protein kinase pathways that regulate prosurvival transcription factors and gene expression. Other processes include the disruption of amyloid-β aggregation and alterations in amyloid precursor protein processing through the inhibition of β-secretase and/or activation of α-secretase, and inhibiting cyclin-dependent kinase-5 and glycogen synthase kinase-3β activation, preventing abnormal tau phosphorylation. The interaction of flavonoids with different signaling pathways put forward their therapeutic potential to prevent the onset and progression of Alzheimer's disease and to promote cognitive performance. Nevertheless, further studies are needed to give additional insight into the specific mechanisms by which flavonoids exert their potential neuroprotective actions in the brain of Alzheimer's disease patients.

Biophysical groundwork as a hinge to unravel the biology of α-synuclein aggregation and toxicity

Alpha-synuclein (aS) and its aggregation properties are central in the development and spread of Parkinson's disease. Point mutations and multiplications of the SNCA gene encoding aS cause autosomal dominant forms of the disorder. Moreover, protein inclusions found in the surviving neurons of parkinsonian brains consist mainly of a fibrillar form of aS. Aggregates of aS, which form a transient, complex and heterogeneous ensemble, participate in a wide variety of toxic mechanisms that may be amplified by aS spreading among neighbouring neurons. Recently, significant effort has been directed into the study of the aS aggregation process and the impact of aS aggregates on neuron survival. In this review, we present state-of-the-art biophysical studies on the aS aggregation process in vitro and in cellular models. We comprehensively review the new insights generated by the recent biophysical investigations, which could provide a solid basis from which to design future biomedical studies. The diverse cellular models of aS toxicity and their potential use in the biophysical investigation are also discussed.

Ion mobility spectrometry-mass spectrometry defines the oligomeric intermediates in amylin amyloid formation and the mode of action of inhibitors

The molecular mechanisms by which different proteins assemble into highly ordered fibrillar deposits and cause disease remain topics of debate. Human amylin (also known as islet amyloid polypeptide/hIAPP) is found in vivo as amyloid deposits in the pancreatic islets of sufferers of type II diabetes mellitus, and its self-aggregation is thought to be a pathogenic factor in disease and to contribute to the failure of islet transplants. Here, electrospray ionization-ion mobility spectrometry-mass spectrometry (ESI-IMS-MS) has been used to monitor oligomer formation from IAPP. The detection, identification and characterization of oligomers from both human and rat amylin (rIAPP) are described. Oligomers up to and including hexamers have been detected for both peptides. From ESI-IMS-MS derived collision cross sections (CCS), these species are shown to be elongated in conformation. Collision-induced dissociation (CID-MS/MS) revealed differences in the gas-phase stability of the oligomers formed from hIAPP and rIAPP, which may contribute to their differences in amyloid propensity. Using ESI-IMS-MS, the mode of inhibition of amyloid formation from hIAPP using small molecules or co-incubation with rIAPP was also investigated. We show that the polyphenolic compounds epigallocatechin gallate (EGCG) and silibinin bind to specific conformers within a dynamic ensemble of hIAPP monomers, altering the progress of oligomerization and fibril assembly. Hetero-oligomer formation also occurs with rIAPP but leads only to inefficient inhibition. The results indicate that although different small molecules can be effective inhibitors of hIAPP self-assembly, their modes of action are distinct and can be distinguished using ESI-IMS-MS.

Therapeutic approaches against common structural features of toxic oligomers shared by multiple amyloidogenic proteins

Impaired proteostasis is one of the main features of all amyloid diseases, which are associated with the formation of insoluble aggregates from amyloidogenic proteins. The aggregation process can be caused by overproduction or poor clearance of these proteins. However, numerous reports suggest that amyloid oligomers are the most toxic species, rather than insoluble fibrillar material, in Alzheimer's, Parkinson's, and Prion diseases, among others. Although the exact protein that aggregates varies between amyloid disorders, they all share common structural features that can be used as therapeutic targets. In this review, we focus on therapeutic approaches against shared features of toxic oligomeric structures and future directions.

The effects of flanking sequences in the interaction of polyglutamine peptides with a membrane bilayer

Huntington's disease (HD) is caused by the presence of an extended polyglutamine (polyQ) region at the N-terminus of the huntingtin (htt) protein. The presence of flanking sequences adjacent to the polyQ region has been reported to modulate the effects of potentially toxic protein-membrane interactions. In this study, we consider four peptide systems with various combinations of flanking sequences (KKQ35KK, KKQ35P11KK, N17Q35KK, N17Q35P11KK) and use atomistic molecular dynamics simulations to study the interactions with a DOPC lipid bilayer. We observe significant membrane thinning, disorderliness of lipid molecules, and compensation effects between the top and the bottom leaflets of the bilayer depending on the presence of particular flanking sequences. Overall, we find that the presence of the N-17 flanking sequence is crucial for membrane interactions. Polyproline decreases the interaction with the membrane in the absence of N-17, but enhances it when present along N-17.

Cannabinoid effects on β amyloid fibril and aggregate formation, neuronal and microglial-activated neurotoxicity in vitro

Cannabinoid (CB) ligands have demonstrated neuroprotective properties. In this study we compared the effects of a diverse set of CB ligands against β amyloid-mediated neuronal toxicity and activated microglial-conditioned media-based neurotoxicity in vitro, and compared this with a capacity to directly alter β amyloid (Aβ) fibril or aggregate formation. Neuroblastoma (SH-SY5Y) cells were exposed to Aβ1-42 directly or microglial (BV-2 cells) conditioned media activated with lipopolysaccharide (LPS) in the presence of the CB1 receptor-selective agonist ACEA, CB2 receptor-selective agonist JWH-015, phytocannabinoids Δ(9)-THC and cannabidiol (CBD), the endocannabinoids 2-arachidonoyl glycerol (2-AG) and anandamide or putative GPR18/GPR55 ligands O-1602 and abnormal-cannabidiol (Abn-CBD). TNF-α and nitrite production was measured in BV-2 cells to compare activation via LPS or albumin with Aβ1-42. Aβ1-42 evoked a concentration-dependent loss of cell viability in SH-SY5Y cells but negligible TNF-α and nitrite production in BV-2 cells compared to albumin or LPS. Both albumin and LPS-activated BV-2 conditioned media significantly reduced neuronal cell viability but were directly innocuous to SH-SY5Y cells. Of those CB ligands tested, only 2-AG and CBD were directly protective against Aβ-evoked SH-SY5Y cell viability, whereas JWH-015, THC, CBD, Abn-CBD and O-1602 all protected SH-SY5Y cells from BV-2 conditioned media activated via LPS. While CB ligands variably altered the morphology of Aβ fibrils and aggregates, there was no clear correlation between effects on Aβ morphology and neuroprotective actions. These findings indicate a neuroprotective action of CB ligands via actions at microglial and neuronal cells.

Cellular targets for the beneficial actions of tea polyphenols

Green and black teas contain different biologically active polyphenolic compounds that might offer protection against a variety of human diseases. Although promising experimental and clinical data have shown protective effects, limited information is available on how these beneficial effects of tea polyphenols are mediated at the cellular level. Evidence is accumulating that catechins in green tea as well as theaflavins and thearubigins from black tea are the substances responsible for the physiologic effects of tea in vitro. The green tea catechin epigallocatechin-3-gallate (EGCG) is generally considered to be the biologically most active compound in vitro. The changes in the activities of various protein kinases, growth factors, and transcription factors represent a common mechanism involved in cellular effects of tea polyphenols. In addition to modification of intracellular signaling by activation of cellular receptors, it was shown that, at least for EGCG, tea polyphenols can enter the cells and directly interact with their molecular targets within cells. There, they frequently result in opposite effects in primary compared with tumor cells. Although tea polyphenols were long regarded as antioxidants, research in recent years has uncovered their prooxidant properties. The use of high nonphysiologic concentrations in many cell culture studies raises questions about the biological relevance of the observed effects for the in vivo situation. Efforts to attribute functional effects in vivo to specific molecular targets at the cellular level are still ongoing.

Modulation of human α-synuclein aggregation by a combined effect of calcium and dopamine

Parkinson's disease is characterized by the deposition of aggregated α-syn and its familial mutants into Lewy bodies leading to death of dopaminergic neurons. α-syn is involved in Ca(II) and dopamine (DA) signaling and their adequate balance inside neuronal cytoplasm is essential for maintaining healthy dopaminergic neurons. We have probed the binding energetics of Ca(II) and DA to human α-syn and its familial mutants A30P, A53T and E46K using isothermal titration calorimetry and have investigated the conformational and aggregation aspects using circular dichroism and fluorescence spectroscopy. While binding of Ca(II) to α-syn and its familial mutants was observed to be endothermic in nature, interaction of DA with α-syn was not detectable. Ca(II) enhanced fibrillation of α-syn and its familial mutants while DA promoted the formation of oligomers. However, Ca(II) and DA together critically favored the formation of protofibrils that are more cytotoxic than the mature fibrils. Using fluorescently labeled cysteine mutant A90C, we have shown that different aggregating species of α-syn formed in the presence of Ca(II) and DA are internalized into the human neuroblastoma cells with different rates and are responsible for the differential cytotoxicity depending on their nature. The findings put together suggest that an interplay between the concentrations of Ca(II), DA and α-syn can critically regulate the formation of various aggregating species responsible for the survival of dopaminergic neurons. Modulating this balance leading to either complete suppression of α-syn aggregation or promoting the formation of mature fibrils could be used as a strategy for the development of drugs to cure Parkinson's disease.

Expression of A2V-mutated Aβ in Caenorhabditis elegans results in oligomer formation and toxicity

Although Alzheimer's disease (AD) is usually sporadic, in a small proportion of cases it is familial and can be linked to mutations in β-amyloid precursor protein (APP). Unlike the other genetic defects, the mutation [alanine-673→valine-673] (A673V) causes the disease only in the homozygous condition with enhanced amyloid β (Aβ) production and aggregation; heterozygous carriers remain unaffected. It is not clear how misfolding and aggregation of Aβ is affected in vivo by this mutation and whether this correlates with its toxic effects. No animal models over-expressing the A673V–APP gene or alanine-2-valine (A2V) mutated human Aβ protein are currently available. Using the invertebrate Caenorhabditis elegans, we generated the first transgenic animal model to express the human Aβ_1–40 wild-type (WT) in neurons or possess the A2V mutation (Aβ_1–40A2V). Insertion of an Aβ-mutated gene into this nematode reproduced the homozygous state of the human pathology. Functional and biochemical characteristics found in the A2V strain were compared to those of transgenic C. elegans expressing Aβ_1–40WT. The expression of both WT and A2V Aβ_1–40 specifically reduced the nematode's lifespan, causing behavioral defects and neurotransmission impairment which were worse in A2V worms. Mutant animals were more resistant than WT to paralysis induced by the cholinergic agonist levamisole, indicating that the locomotor defect was specifically linked to postsynaptic dysfunctions. The toxicity caused by the mutated protein was associated with a high propensity to form oligomeric assemblies which accumulate in the neurons, suggesting this to be the central event involved in the postsynaptic damage and early onset of the disease in homozygous human A673V carriers.

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We generated the first transgenic animal model expressing in neurons the human Aβ_1–40 wild-type or has the A2V mutation.

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Aβ_1–40 expression reduced the worm's lifespan, caused behavioral and neuronal defects which were worse in the A2V strain.

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The behavioral defects of mutant worms were specifically linked to postsynaptic dysfunctions.

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The toxicity of Aβ_1–40A2V was associated with its high propensity to form oligomers which accumulate in the neurons.

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These transgenic strains represent an attractive tools for an in vivo screening of compounds interfering with oligomers.

Ion mobility spectrometry reveals the mechanism of amyloid formation of Aβ(25-35) and its modulation by inhibitors at the molecular level: epigallocatechin gallate and scyllo-inositol

Amyloid cascades leading to peptide β-sheet fibrils and plaques are central to many important diseases. Recently, intermediate assemblies of these cascades were identified as the toxic agents that interact with the cellular machinery. The relationship between the transformation from natively unstructured assembly to the β-sheet oligomers to disease is important in understanding disease onset and the development of therapeutic agents. Research on this early oligomeric region has largely been unsuccessful since traditional techniques measure only ensemble average oligomer properties. Here, ion mobility methods are utilized to deduce the modulation of peptide self-assembly pathways in the amyloid-β protein fragment Aβ(25-35) by two amyloid inhibitors (epigallocatechin gallate and scyllo-inositol) that are currently in clinical trials for Alzheimer's Disease. We provide evidence that suppression of β-extended oligomers from the onset of the conversion into β-oligomer conformations is essential for effective attenuation of β-structured amyloid oligomeric species often associated with oligomer toxicity. Furthermore, we demonstrate the ease with which ion mobility spectrometry-mass spectrometry can guide the development of therapeutic agents and drug evaluation by providing molecular level insight into the amyloid formation process and its modulation by small molecule assembly modulators.

The role of iron-induced fibrin in the pathogenesis of Alzheimer's disease and the protective role of magnesium

Amyloid hypothesis of Alzheimer's disease (AD) has recently been challenged by the increasing evidence for the role of vascular and hemostatic components that impair oxygen delivery to the brain. One such component is fibrin clots, which, when they become resistant to thrombolysis, can cause chronic inflammation. It is not known, however, why some cerebral thrombi are resistant to the fibrinolytic degradation, whereas fibrin clots formed at the site of vessel wall injuries are completely, although gradually, removed to ensure proper wound healing. This phenomenon can now be explained in terms of the iron-induced free radicals that generate fibrin-like polymers remarkably resistant to the proteolytic degradation. It should be noted that similar insoluble deposits are present in AD brains in the form of aggregates with Abeta peptides that are resistant to fibrinolytic degradation. In addition, iron-induced fibrin fibers can irreversibly trap red blood cells (RBCs) and in this way obstruct oxygen delivery to the brain and induce chronic hypoxia that may contribute to AD. The RBC-fibrin aggregates can be disaggregated by magnesium ions and can also be prevented by certain polyphenols that are known to have beneficial effects in AD. In conclusion, we argue that AD can be prevented by: (1) limiting the dietary supply of trivalent iron contained in red and processed meat; (2) increasing the intake of chlorophyll-derived magnesium; and (3) consumption of foods rich in polyphenolic substances and certain aliphatic and aromatic unsaturated compounds. These dietary components are present in the Mediterranean diet known to be associated with the lower incidence of AD and other degenerative diseases.

Blockade of the formation of insoluble ubiquitinated protein aggregates by EGCG3"Me in the alloxan-induced diabetic kidney

BACKGROUND

Renal accumulation of reactive carbonyl compounds (RCCs) has been linked to the progression of diabetic nephropathy. We previously demonstrated that carbonyl stress induces the formation of amino-carbonyl cross-links and sharply increases the content of β-sheet-rich structures, which is the seed of insoluble aggregates formation, and tea catechin (-)-epigallocatechin 3-gallate (EGCG) can reverse this process in vitro and in vivo. In this study, methylated derivative (-)-epigallocatechin-3-O-(3-O-methyl)-gallate (EGCG3"Me) was hypothesized to neutralize carbonyl stress mediating the formation of insoluble ubiquitinated protein (IUP) aggregates, and reduce the early development of diabetic nephropathy.

METHODS AND RESULTS

Diabetes was induced in mice by intraperitoneally injecting alloxan monohydrate (200 mg/kg/d) twice and administering EGCG3"Me by gavage for 15 d. Reagent case and western blot results showed that, in diabetic kidneys, the carbonyl proteins in the serum increased; and in insoluble protein fraction, 4-hydroxynonenal-modified proteins, IUP aggregates and p62 accumulated; FT-IR study demonstrated that the lipid content, anti-parallel β-sheet structure and aggregates increased. EGCG3"Me treatment could effectively reverse this process, even better than the negative control treatment.

CONCLUSIONS

EGCG3"Me exhibiting anti-β-sheet-rich IUP aggregate properties, maybe represents a new strategy to impede the progression of diabetic nephropathy and other diabetic complications.

Synthesis of alginate-curcumin nanocomposite and its protective role in transgenic Drosophila model of Parkinson's disease

The genetic models in Drosophila provide a platform to understand the mechanism associated with degenerative diseases. The model for Parkinson's disease (PD) based on normal human alpha-synuclein (αS) expression was used in the present study. The aggregation of αS in brain leads to the formation of Lewy bodies and selective loss of dopaminergic neurons due to oxidative stress. Polyphenols generally have the reduced oral bioavailability, increased metabolic turnover, and lower permeability through the blood brain barrier. In the present study, the effect of synthesized alginate-curcumin nanocomposite was studied on the climbing ability of the PD model flies, lipid peroxidation, and apoptosis in the brain of PD model flies. The alginate-curcumin nanocomposite at final doses of 10⁻⁵, 10⁻³, and 10⁻¹ g/mL was supplemented with diet, and the flies were allowed to feed for 24 days. A significant dose-dependent delay in the loss of climbing ability and reduction in the oxidative stress and apoptosis in the brain of PD model flies were observed. The results suggest that alginate-curcumin nanocomposite is potent in delaying the climbing disability of PD model flies and also reduced the oxidative stress as well as apoptosis in the brain of PD model flies.

Targeting dysregulation of brain iron homeostasis in Parkinson's disease by iron chelators

Brain iron accumulation has been implicated in a host of chronic neurological diseases, including Parkinson's disease (PD). The elevated iron levels observed in the substantia nigra of PD subjects have been suggested to incite the generation of reactive oxygen species and intracellular α-synuclein aggregation, terminating in the oxidative neuronal destruction of this brain area. Thus, elucidation of the molecular mechanisms involved in iron dysregulation and oxidative stress-induced neurodegeneration is a crucial step in deciphering PD pathology and in developing novel iron-complexing compounds aimed at restoring brain iron homeostasis and attenuating neurodegeneration. This review discusses the involvement of dysregulation of brain iron homeostasis in PD pathology, with an emphasis on the potential effectiveness of naturally occurring compounds and novel iron-chelating/antioxidant therapeutic hybrid molecules, exerting a spectrum of neuroprotective interrelated activities: antioxidant/monoamine oxidase inhibition, activation of the hypoxia-inducible factor (HIF)-1 signaling pathway, induction of HIF-1 target iron-regulatory and antioxidative genes, and inhibition of α-synuclein accumulation and aggregation.

The role of polyphenols in the modulation of sirtuins and other pathways involved in Alzheimer's disease

Alzheimer's disease (AD) is characterised by extracellular amyloid deposits, neurofibrillary tangles, synaptic loss, inflammation and extensive oxidative stress. Polyphenols, which include resveratrol, epigallocatechin gallate and curcumin, have gained considerable interest for their ability to reduce these hallmarks of disease and their potential to slow down cognitive decline. Although their antioxidant and free radical scavenging properties are well established, more recently polyphenols have been shown to produce other important effects including anti-amyloidogenic activity, cell signalling modulation, effects on telomere length and modulation of the sirtuin proteins. Brain accessible polyphenols with multiple effects on pathways involved in neurodegeneration and ageing may therefore prove efficacious in the treatment of age-related diseases such as AD, although the evidence for this so far is limited. This review aims to explore the known effects of polyphenols from various natural and synthetic sources on brain ageing and neurodegeneration, and to examine their multiple mechanisms of action, with an emphasis on the role that the sirtuin pathway may play and the implications this may have for the treatment of AD.

A combination cocktail improves spatial attention in a canine model of human aging and Alzheimer's disease

Alzheimer's disease (AD) involves multiple pathological processes in the brain, including increased inflammation and oxidative damage, as well as the accumulation of amyloid-β (Aβ) plaques. We hypothesized that a combinatorial therapeutic approach to target these multiple pathways may provide cognitive and neuropathological benefits for AD patients. To test this hypothesis, we used a canine model of human aging and AD. Aged dogs naturally develop learning and memory impairments, human-type Aβ deposits, and oxidative damage in the brain. Thus, 9 aged beagles (98-115 months) were treated with a medical food cocktail containing (1) an extract of turmeric containing 95% curcuminoids; (2) an extract of green tea containing 50% epigallocatechingallate; (3) N-acetyl cysteine; (4) R-alpha lipoic acid; and (5) an extract of black pepper containing 95% piperine. Nine similarly aged dogs served as placebo-treated controls. After 3 months of treatment, 13 dogs completed a variable distance landmark task used as a measure of spatial attention. As compared to placebo-treated animals, dogs receiving the medical food cocktail had significantly lower error scores (t11 = 4.3, p = 0.001) and were more accurate across all distances (F(1,9) = 20.7, p = 0.001), suggesting an overall improvement in spatial attention. Measures of visual discrimination learning, executive function and spatial memory, and levels of brain and cerebrospinal fluid Aβ were unaffected by the cocktail. Our results indicate that this medical food cocktail may be beneficial for improving spatial attention and motivation deficits associated with impaired cognition in aging and AD.

A computational exploration of the interactions of the green tea polyphenol (-)-Epigallocatechin 3-Gallate with cardiac muscle troponin C

Thanks to its polyphenols and phytochemicals, green tea is believed to have a number of health benefits, including protecting from heart disease, but its mechanism of action at the molecular level is still not understood. Here we explore, by means of atomistic simulations, how the most abundant of the green tea polyphenols, (-)-Epigallocatechin 3-Gallate (EGCg), interacts with the structural C terminal domain of cardiac muscle troponin C (cCTnC), a calcium binding protein that plays an important role in heart contractions. We find that EGCg favourably binds to the hydrophobic cleft of cCTnC consistently with solution NMR experiments. It also binds to cCTnC in the presence of the anchoring region of troponin I (cTnI(34-71)) at the interface between the E and H helices. This appears to affect the strength of the interaction between cCTnC and cTnI(34-71) and also counter-acts the effects of the Gly159Asp mutation, related to dilated cardiomyopathy. Our simulations support the picture that EGCg interacting with the C terminal domain of troponin C may help in regulating the calcium signalling either through competitive binding with the anchoring domain of cTnI or by affecting the interaction between cCTnC and cTnI(34-71).

Natural compounds may open new routes to treatment of amyloid diseases

Protein misfolding disorders, such as Alzheimer's disease and Parkinson's disease, have in common that a protein accumulates in an insoluble form in the affected tissue. The process of aggregation follows a mechanism of seeded polymerization. Although the toxic species is still not well defined, the process, rather than the end product, of fibril formation is likely the main culprit in amyloid toxicity. These findings suggest that therapeutic strategies directed against the protein misfolding cascade should focus on depleting aggregation intermediates rather than on large fibrillar aggregates. Recent studies involving natural compounds have suggested new intervention strategies. The polyphenol epi-gallocatechine-3-gallate (EGCG), the main polyphenol in Camilla sinensis, binds directly to a large number of proteins that are involved in protein misfolding diseases and inhibits their fibrillization. Instead, it promotes the formation of stable, spherical aggregates. These spherical aggregates are not cytotoxic, have a lower β-sheet content than fibrils, and do not catalyze fibril formation. Correspondingly, epi-gallocatechine-3-gallate remodels amyloid fibrils into aggregates with the same properties. Derivatives of Orcein, which is a phenoxazine dye that can be isolated from the lichen Roccella tinctoria, form a second promising class of natural compounds. They accelerate fibril formation of the Alzheimer's disease-related amyloid-beta peptide. At the same time these compounds deplete oligomeric and protofibrillar forms of the peptide. These compounds may serve as proof-of-principle for the strategies of promoting and redirecting fibril formation. Both may emerge as two promising new therapeutic approaches to intervening into protein misfolding processes.

Inhibiting toxic aggregation of amyloidogenic proteins: a therapeutic strategy for protein misfolding diseases

BACKGROUND

The deposition of self-assembled amyloidogenic proteins is associated with multiple diseases, including Alzheimer's disease, Parkinson's disease and type 2 diabetes mellitus. The toxic misfolding and self-assembling of amyloidogenic proteins are believed to underlie protein misfolding diseases. Novel drug candidates targeting self-assembled amyloidogenic proteins represent a potential therapeutic approach for protein misfolding diseases.

SCOPE OF REVIEW

In this perspective review, we provide an overview of the recent progress in identifying inhibitors that block the aggregation of amyloidogenic proteins and the clinical applications thereof.

MAJOR CONCLUSIONS

Compounds such as polyphenols, certain short peptides, and monomer- or oligomer-specific antibodies, can interfere with the self-assembly of amyloidogenic proteins, prevent the formation of oligomers, amyloid fibrils and the consequent cytotoxicity.

GENERAL SIGNIFICANCE

Some inhibitors have been tested in clinical trials for treating protein misfolding diseases. Inhibitors that target the aggregation of amyloidogenic proteins bring new hope to therapy for protein misfolding diseases.

Mitochondrial membrane permeabilisation by amyloid aggregates and protection by polyphenols

Alzheimer's disease and Parkinson's disease are neurodegenerative disorders characterised by the misfolding of proteins into soluble prefibrillar aggregates. These aggregate complexes disrupt mitochondrial function, initiating a pathophysiological cascade leading to synaptic and neuronal degeneration. In order to explore the interaction of amyloid aggregates with mitochondrial membranes, we made use of two in vitro model systems, namely: (i) lipid vesicles with defined membrane compositions that mimic those of mitochondrial membranes, and (ii) respiring mitochondria isolated from neuronal SH-SY5Y cells. External application of soluble prefibrillar forms, but not monomers, of amyloid-beta (Aβ42 peptide), wild-type α-synuclein (α-syn), mutant α-syn (A30P and A53T) and tau-441 proteins induced a robust permeabilisation of mitochondrial-like vesicles, and triggered cytochrome c release (CCR) from isolated mitochondrial organelles. Importantly, the effect on mitochondria was shown to be dependent upon cardiolipin, an anionic phospholipid unique to mitochondria and a well-known key player in mitochondrial apoptosis. Pharmacological modulators of mitochondrial ion channels failed to inhibit CCR. Thus, we propose a generic mechanism of thrilling mitochondria in which soluble amyloid aggregates have the intrinsic capacity to permeabilise mitochondrial membranes, without the need of any other protein. Finally, six small-molecule compounds and black tea extract were tested for their ability to inhibit permeation of mitochondrial membranes by Aβ42, α-syn and tau aggregate complexes. We found that black tea extract and rosmarinic acid were the most potent mito-protectants, and may thus represent important drug leads to alleviate mitochondrial dysfunction in neurodegenerative diseases.

Amyloid-β-neuropeptide interactions assessed by ion mobility-mass spectrometry

Recently, small peptides have been shown to modulate aggregation and toxicity of the amyloid-β protein (Aβ). As such, these new scaffolds may help discover a new class of biotherapeutics useful in the treatment of Alzheimer's disease. Many of these inhibitory peptide sequences have been derived from natural sources or from Aβ itself (e.g., C-terminal Aβ fragments). In addition, much earlier work indicates that tachykinins, a broad class of neuropeptides, display neurotrophic properties, presumably through direct interactions with either Aβ or its receptors. Based on this work, we undertook a limited screen of neuropeptides using ion mobility-mass spectrometry to search for similar such peptides with direct Aβ binding properties. Our results reveal that the neuropeptides leucine enkephalin (LE) and galanin interact with both the monomeric and small oligomeric forms of Aβ(1-40) to create a range of complexes having diverse stoichiometries, while some tachyknins (i.e., substance P) do not. LE interacts with Aβ more strongly than galanin, and we utilized ion mobility-mass spectrometry, molecular dynamics simulations, gel electrophoresis/Western blot, and transmission electron microscopy to study the influence of this peptide on the structure of Aβ monomer, small Aβ oligomers, as well as the eventual formation of Aβ fibrils. We find that LE binds selectively within a region of Aβ between its N-terminal tail and hydrophobic core. Furthermore, our data indicate that LE modulates fibril generation, producing shorter fibrillar aggregates when added in stoichiometric excess relative to Aβ.

Biological activity of sym-triazines with acetylcholine-like substitutions as multitarget modulators of Alzheimer's disease

The bioactivities of two novel compounds (TAE-1 and TAE-2) that contain a sym-triazine scaffold with acetylcholine-like substitutions are examined as promising candidate agents against Alzheimer's disease. Inhibition of amyloid-β fibril formation in the presence of Aβ1-42, evaluated by Thioflavin T fluorescence, demonstrated comparable or improved activity to a previously reported pentapeptide-based fibrillogenesis inhibitor, iAβ5p. Destabilization of Aβ1-42 assemblies by TAE-1 and TAE-2 was confirmed by scanning electron microscopy imaging. sym-Triazine inhibition of acetylcholinesterase (AChE) activity was observed in cytosol extracted from differentiated human SH-SY5Y neuronal cells and also using human erythrocyte AChE. The sym-triazine derivatives were well tolerated by these cells and promoted beneficial effects on human neurons, upregulating expression of synaptophysin, a synaptic marker protein, and MAP2, a neuronal differentiation marker.

α-Synuclein in Parkinson's disease

α-Synuclein is a presynaptic neuronal protein that is linked genetically and neuropathologically to Parkinson's disease (PD). α-Synuclein may contribute to PD pathogenesis in a number of ways, but it is generally thought that its aberrant soluble oligomeric conformations, termed protofibrils, are the toxic species that mediate disruption of cellular homeostasis and neuronal death, through effects on various intracellular targets, including synaptic function. Furthermore, secreted α-synuclein may exert deleterious effects on neighboring cells, including seeding of aggregation, thus possibly contributing to disease propagation. Although the extent to which α-synuclein is involved in all cases of PD is not clear, targeting the toxic functions conferred by this protein when it is dysregulated may lead to novel therapeutic strategies not only in PD, but also in other neurodegenerative conditions, termed synucleinopathies.

Protein folding and aggregation into amyloid: the interference by natural phenolic compounds

Amyloid aggregation is a hallmark of several degenerative diseases affecting the brain or peripheral tissues, whose intermediates (oligomers, protofibrils) and final mature fibrils display different toxicity. Consequently, compounds counteracting amyloid aggregation have been investigated for their ability (i) to stabilize toxic amyloid precursors; (ii) to prevent the growth of toxic oligomers or speed that of fibrils; (iii) to inhibit fibril growth and deposition; (iv) to disassemble preformed fibrils; and (v) to favor amyloid clearance. Natural phenols, a wide panel of plant molecules, are one of the most actively investigated categories of potential amyloid inhibitors. They are considered responsible for the beneficial effects of several traditional diets being present in green tea, extra virgin olive oil, red wine, spices, berries and aromatic herbs. Accordingly, it has been proposed that some natural phenols could be exploited to prevent and to treat amyloid diseases, and recent studies have provided significant information on their ability to inhibit peptide/protein aggregation in various ways and to stimulate cell defenses, leading to identify shared or specific mechanisms. In the first part of this review, we will overview the significance and mechanisms of amyloid aggregation and aggregate toxicity; then, we will summarize the recent achievements on protection against amyloid diseases by many natural phenols.

Progress and developments in tau aggregation inhibitors for Alzheimer disease

Pharmacological approaches directed toward Alzheimer disease are diversifying in parallel with a growing number of promising targets. Investigations on the microtubule-associated protein tau yielded innovative targets backed by recent findings about the central role of tau in numerous neurodegenerative diseases. In this review, we summarize the recent evolution in the development of nonpeptidic small molecules tau aggregation inhibitors (TAGIs) and their advancement toward clinical trials. The compounds are classified according to their chemical structures, providing correlative insights into their pharmacology. Overall, shared structure-activity traits are emerging, as well as specific binding modes related to their ability to engage in hydrogen bonding. Medicinal chemistry efforts on TAGIs together with encouraging in vivo data argue for successful translation to the clinic.

Advances in ion mobility spectrometry-mass spectrometry reveal key insights into amyloid assembly

Interfacing ion mobility spectrometry to mass spectrometry (IMS-MS) has enabled mass spectrometric analyses to extend into an extra dimension, providing unrivalled separation and structural characterization of lowly populated species in heterogeneous mixtures. One biological system that has benefitted significantly from such advances is that of amyloid formation. Using IMS-MS, progress has been made into identifying transiently populated monomeric and oligomeric species for a number of different amyloid systems and has led to an enhanced understanding of the mechanism by which small molecules modulate amyloid formation. This review highlights recent advances in this field, which have been accelerated by the commercial availability of IMS-MS instruments. This article is part of a Special Issue entitled: Mass spectrometry in structural biology.

Toward the molecular mechanism(s) by which EGCG treatment remodels mature amyloid fibrils

Protein misfolding and/or aggregation has been implicated as the cause of several human diseases, such as Alzheimer's and Parkinson's diseases and familial amyloid polyneuropathy. These maladies are referred to as amyloid diseases, named after the cross-β-sheet amyloid fibril aggregates or deposits common to these disorders. Epigallocatechin-3-gallate (EGCG), the principal polyphenol present in green tea, has been shown to be effective at preventing aggregation and is able to remodel amyloid fibrils comprising different amyloidogenic proteins, although the mechanistic underpinnings are unclear. Herein, we work toward an understanding of the molecular mechanism(s) by which EGCG remodels mature amyloid fibrils made up of Aβ(1-40), IAPP(8-24), or Sup35NM(7-16). We show that EGCG amyloid remodeling activity in vitro is dependent on auto-oxidation of the EGCG. Oxidized and unoxidized EGCG binds to amyloid fibrils, preventing the binding of thioflavin T. This engagement of the hydrophobic binding sites in Aβ(1-40), IAPP(8-24), or Sup35NM(Ac7-16) Y→F amyloid fibrils seems to be sufficient to explain the majority of the amyloid remodeling observed by EGCG treatment, although how EGCG oxidation drives remodeling remains unclear. Oxidized EGCG molecules react with free amines within the amyloid fibril through the formation of Schiff bases, cross-linking the fibrils, which may prevent dissociation and toxicity, but these aberrant post-translational modifications do not appear to be the major driving force for amyloid remodeling by EGCG treatment. These insights into the molecular mechanism of action of EGCG provide boundary conditions for exploring amyloid remodeling in more detail.

Molecular mechanism of the inhibition of EGCG on the Alzheimer Aβ(1-42) dimer

Growing evidence supports that amyloid β (Aβ) oligomers are the major causative agents leading to neural cell death in Alzheimer's disease. The polyphenol (-)-epigallocatechin gallate (EGCG) was recently reported to inhibit Aβ fibrillization and redirect Aβ aggregation into unstructured, off-pathway oligomers. Given the experimental challenge to characterize the structures of Aβ/EGCG complexes, we performed extensive atomistic replica exchange molecular dynamics simulations of Aβ1-42 dimer in the present and absence of EGCG in explicit solvent. Our equilibrium Aβ dimeric structures free of EGCG are consistent with the collision cross section from ion-mobility mass spectrometry and the secondary structure composition from circular dichroism experiment. In the presence of EGCG, the Aβ structures are characterized by increased inter-center-of-mass distances, reduced interchain and intrachain contacts, reduced β-sheet content, and increased coil and α-helix contents. Analysis of the free energy surfaces reveals that the Aβ dimer with EGCG adopts new conformations, affecting therefore its propensity to adopt fibril-prone states. Overall, this study provides, for the first time, insights on the equilibrium structures of Aβ1-42 dimer in explicit aqueous solution and an atomic picture of the EGCG-mediated conformational change on Aβ dimer.

Curcumin modulates α-synuclein aggregation and toxicity

In human beings, Parkinson's disease (PD) is associated with the oligomerization and amyloid formation of α-synuclein (α-Syn). The polyphenolic Asian food ingredient curcumin has proven to be effective against a wide range of human diseases including cancers and neurological disorders. While curcumin has been shown to significantly reduce cell toxicity of α-Syn aggregates, its mechanism of action remains unexplored. Here, using a series of biophysical techniques, we demonstrate that curcumin reduces toxicity by binding to preformed oligomers and fibrils and altering their hydrophobic surface exposure. Further, our fluorescence and two-dimensional nuclear magnetic resonance (2D-NMR) data indicate that curcumin does not bind to monomeric α-Syn but binds specifically to oligomeric intermediates. The degree of curcumin binding correlates with the extent of α-Syn oligomerization, suggesting that the ordered structure of protein is required for effective curcumin binding. The acceleration of aggregation by curcumin may decrease the population of toxic oligomeric intermediates of α-Syn. Collectively; our results suggest that curcumin and related polyphenolic compounds can be pursued as candidate drug targets for treatment of PD and other neurological diseases.

Biophysical insights into how surfaces, including lipid membranes, modulate protein aggregation related to neurodegeneration

There are a vast number of neurodegenerative diseases, including Alzheimer’s disease (AD), Parkinson’s disease (PD), and Huntington’s disease (HD), associated with the rearrangement of specific proteins to non-native conformations that promotes aggregation and deposition within tissues and/or cellular compartments. These diseases are commonly classified as protein-misfolding or amyloid diseases. The interaction of these proteins with liquid/surface interfaces is a fundamental phenomenon with potential implications for protein-misfolding diseases. Kinetic and thermodynamic studies indicate that significant conformational changes can be induced in proteins encountering surfaces, which can play a critical role in nucleating aggregate formation or stabilizing specific aggregation states. Surfaces of particular interest in neurodegenerative diseases are cellular and subcellular membranes that are predominately comprised of lipid components. The two-dimensional liquid environments provided by lipid bilayers can profoundly alter protein structure and dynamics by both specific and non-specific interactions. Importantly for misfolding diseases, these bilayer properties can not only modulate protein conformation, but also exert influence on aggregation state. A detailed understanding of the influence of (sub)cellular surfaces in driving protein aggregation and/or stabilizing specific aggregate forms could provide new insights into toxic mechanisms associated with these diseases. Here, we review the influence of surfaces in driving and stabilizing protein aggregation with a specific emphasis on lipid membranes.

Insights into antiamyloidogenic properties of the green tea extract (-)-epigallocatechin-3-gallate toward metal-associated amyloid-β species

Despite the significance of Alzheimer's disease, the link between metal-associated amyloid-β (metal-Aβ) and disease etiology remains unclear. To elucidate this relationship, chemical tools capable of specifically targeting and modulating metal-Aβ species are necessary, along with a fundamental understanding of their mechanism at the molecular level. Herein, we investigated and compared the interactions and reactivities of the green tea extract, (-)-epigallocatechin-3-gallate [(2R,3R)-5,7-dihydroxy-2-(3,4,5-trihydroxyphenyl)-3,4-dihydro-2H-1-benzopyran-3-yl 3,4,5-trihydroxybenzoate; EGCG], with metal [Cu(II) and Zn(II)]-Aβ and metal-free Aβ species. We found that EGCG interacted with metal-Aβ species and formed small, unstructured Aβ aggregates more noticeably than in metal-free conditions in vitro. In addition, upon incubation with EGCG, the toxicity presented by metal-free Aβ and metal-Aβ was mitigated in living cells. To understand this reactivity at the molecular level, structural insights were obtained by ion mobility-mass spectrometry (IM-MS), 2D NMR spectroscopy, and computational methods. These studies indicated that (i) EGCG was bound to Aβ monomers and dimers, generating more compact peptide conformations than those from EGCG-untreated Aβ species; and (ii) ternary EGCG-metal-Aβ complexes were produced. Thus, we demonstrate the distinct antiamyloidogenic reactivity of EGCG toward metal-Aβ species with a structure-based mechanism.

sym-Triazines for directed multitarget modulation of cholinesterases and amyloid-β in Alzheimer's disease

Alzheimer's disease (AD) is a complex neurodegenerative disorder marked by numerous causative factors of disease progression, termed pathologies. We report here the synthesis of a small library of novel sym-triazine compounds designed for targeted modulation of multiple pathologies related to AD, specifically human acetylcholinesterase (AChE), butyrylcholinesterase (BuChE), and Aβ aggregation. Rational targeting of AChE was achieved by the incorporation of acetylcholine substrate analogues into a sym-triazine core in either a mono-, di-, or trisubstituted regime. A subset of these derivatives demonstrated improved activity compared to several commercially available cholinesterase inhibitors. High AChE/BuChE selectivity was characteristic of all derivatives, and AChE steady-state kinetics indicated a mixed-type inhibition mechanism. Further integration of multiple hydrophobic phenyl units allowed for improved β-sheet intercalation into amyloid aggregates. Several highly effective structures exhibited fibril inhibition greater than the previously reported β-sheet-disrupting penta-peptide, iAβ5p, evaluated by thioflavin T fluorescence spectroscopy and transmission electron microscopy. Highly effective sym-triazines were shown to be well tolerated by differentiated human neuronal cells, as demonstrated by the absence of adverse effects on cellular viability at a wide range of concentrations. Parallel targeting of multiple pathologies using sym-triazines is presented here as an effective strategy to address the complex, multifactorial nature of AD progression.

Epigallocatechin-3-gallate: a useful, effective and safe clinical approach for targeted prevention and individualised treatment of neurological diseases?

Neurodegenerative disorders show an increasing prevalence in a number of highly developed countries. Often, these diseases require life-long treatment mostly with drugs which are costly and mostly accompanied by more or less serious side-effects. Their heterogeneous manifestation, severity and outcome pose the need for individualised treatment options. There is an intensive search for new strategies not only for treating but also for preventing these diseases. Green tea and green tea extracts seem to be such a promising and safe alternative. However, data regarding the beneficial effects and possible underlying mechanism, specifically in clinical trials, are rare and rather controversial or non-conclusive. This review outlines the existing evidence from preclinical studies (cell and tissue cultures and animal models) and clinical trials regarding preventive and therapeutic effects of epigallcatechin-3-gallate in neurodegenerative diseases and considers antioxidative vs. pro-oxidative properties of the tea catechin important for dosage recommendations.

Prion protein-mediated toxicity of amyloid-β oligomers requires lipid rafts and the transmembrane LRP1

Soluble oligomers of the amyloid-β (Aβ) peptide cause neurotoxicity, synaptic dysfunction, and memory impairments that underlie Alzheimer disease (AD). The cellular prion protein (PrP^C) was recently identified as a high affinity neuronal receptor for Aβ oligomers. We report that fibrillar Aβ oligomers recognized by the OC antibody, which have been shown to correlate with the onset and severity of AD, bind preferentially to cells and neurons expressing PrP^C. The binding of Aβ oligomers to cell surface PrP^C, as well as their downstream activation of Fyn kinase, was dependent on the integrity of cholesterol-rich lipid rafts. In SH-SY5Y cells, fluorescence microscopy and co-localization with subcellular markers revealed that the Aβ oligomers co-internalized with PrP^C, accumulated in endosomes, and subsequently trafficked to lysosomes. The cell surface binding, internalization, and downstream toxicity of Aβ oligomers was dependent on the transmembrane low density lipoprotein receptor-related protein-1 (LRP1). The binding of Aβ oligomers to cell surface PrP^C impaired its ability to inhibit the activity of the β-secretase BACE1, which cleaves the amyloid precursor protein to produce Aβ. The green tea polyphenol (−)-epigallocatechin gallate and the red wine extract resveratrol both remodeled the fibrillar conformation of Aβ oligomers. The resulting nonfibrillar oligomers displayed significantly reduced binding to PrP^C-expressing cells and were no longer cytotoxic. These data indicate that soluble, fibrillar Aβ oligomers bind to PrP^C in a conformation-dependent manner and require the integrity of lipid rafts and the transmembrane LRP1 for their cytotoxicity, thus revealing potential targets to alleviate the neurotoxic properties of Aβ oligomers in AD.

Dual-function triazole-pyridine derivatives as inhibitors of metal-induced amyloid-β aggregation

Dysregulated metal ions are hypothesized to play a role in the aggregation of the amyloid-β (Aβ) peptide, leading to Alzheimer's disease (AD) pathology. In addition to direct effects on Aβ aggregation, both Cu and Fe can catalyze the generation of reactive oxygen species (ROS), possibly contributing to significant neuronal toxicity. Therefore, disruption of metal-Aβ interactions has become a viable strategy for AD therapeutic development. Herein, we report a new series of dual-function triazole-pyridine ligands [4-(2-(4-(pyridin-2-yl)-1H-1,2,3-triazol-1-yl)ethyl)morpholine (L1), 3-(4-(pyridin-2-yl)-1H-1,2,3-triazol-1-yl)propan-1-ol (L2), 2-(4-(pyridin-2-yl)-1H-1,2,3-triazol-1-yl)acetic acid (L3), and 5-(4-(pyridin-2-yl)-1H-1,2,3-triazol-1-yl)pentan-1-amine (L4)] that interact with the Aβ peptide and modulate its aggregation in vitro. Metal chelation and Aβ interaction properties of these molecules were studied by UV-vis, NMR spectroscopy and X-ray crystallography. In addition, turbidity and transmission electron microscopy (TEM) were employed to determine the anti-aggregation properties of L1-L4. All compounds demonstrated an ability to limit metal-induced Aβ aggregation. Overall, our studies suggest the utility of the triazole-pyridine framework in the development of chemical reagents toward inhibitors for metal-triggered Aβ aggregation.

Modulators of amyloid protein aggregation and toxicity: EGCG and CLR01

Abnormal protein folding and self-assembly causes over 30 cureless human diseases for which no disease-modifying therapies are available. The common side to all these diseases is formation of aberrant toxic protein oligomers and amyloid fibrils. Both types of assemblies are drug targets, yet each presents major challenges to drug design, discovery, and development. In this review, we focus on two small molecules that inhibit formation of toxic amyloid protein assemblies — the green-tea derivative (−)-epigallocatechin-3-gallate (EGCG), which was identified through a combination of epidemiologic data and a compound library screen, and the molecular tweezer CLR01, whose inhibitory activity was discovered in our group based on rational reasoning, and subsequently confirmed experimentally. Both compounds act in a manner that is not specific to one particular protein and thus are useful against a multitude of amyloidogenic proteins, yet they act via distinct putative mechanisms. CLR01 disrupts protein aggregation through specific binding to lysine residues, whereas the mechanisms underlying the activity of EGCG are only recently beginning to unveil. We discuss current in vitro and, where available, in vivo literature related to EGCG and CLR01’s effects on amyloid β-protein, α-synuclein, transthyretin, islet amyloid polypeptide, and calcitonin. We also describe the toxicity, pharmacokinetics, and mechanism of action of each compound.

Natural biomolecules and protein aggregation: emerging strategies against amyloidogenesis

Biomolecular self-assembly is a fundamental process in all organisms. As primary components of the life molecular machinery, proteins have a vast array of resources available to them for self-assembly in a functional structure. Protein self-assembly, however, can also occur in an aberrant way, giving rise to non-native aggregated structures responsible for severe, progressive human diseases that have a serious social impact. Different neurodegenerative disorders, like Huntington's, Alzheimer's, and spongiform encephalopathy diseases, have in common the presence of insoluble protein aggregates, generally termed "amyloid," that share several physicochemical features: a fibrillar morphology, a predominantly beta-sheet secondary structure, birefringence upon staining with the dye Congo red, insolubility in common solvents and detergents, and protease resistance. Conformational constrains, hydrophobic and stacking interactions can play a key role in the fibrillogenesis process and protein-protein and peptide-peptide interactions-resulting in self-assembly phenomena of peptides yielding fibrils-that can be modulated and influenced by natural biomolecules. Small organic molecules, which possess both hydrophilic and hydrophobic moieties able to bind to peptide/protein molecules through hydrogen bonds and hydrophobic and aromatic interactions, are potential candidates against amyloidogenesis. In this review some significant case examples will be critically discussed.