Resveratrol selectively remodels soluble oligomers and fibrils of amyloid Abeta into off-pathway conformers

Techniques for Monitoring Protein Misfolding and Aggregation in Vitro and in Living Cells

Protein misfolding and aggregation have been considered important in understanding many neurodegenerative diseases and recombinant biopharmaceutical production. Therefore, various traditional and modern techniques have been utilized to monitor protein aggregation in vitro and in living cells. Fibril formation, morphology and secondary structure content of amyloidogenic proteins in vitro have been monitored by molecular probes, TEM/AFM, and CD/FTIR analyses, respectively. Protein aggregation in living cells has been qualitatively or quantitatively monitored by numerous molecular folding reporters based on either fluorescent protein or enzyme. Aggregation of a target protein is directly correlated to the changes in fluorescence or enzyme activity of the folding reporter fused to the target protein, which allows non-invasive monitoring aggregation of the target protein in living cells. Advances in the techniques used to monitor protein aggregation in vitro and in living cells have greatly facilitated the understanding of the molecular mechanism of amyloidogenic protein aggregation associated with neurodegenerative diseases, optimizing culture conditions to reduce aggregation of biopharmaceuticals expressed in living cells, and screening of small molecule libraries in the search for protein aggregation inhibitors.

Biochemistry of amyloid β-protein and amyloid deposits in Alzheimer disease

Progressive cerebral deposition of the amyloid β-protein (Aβ) in brain regions serving memory and cognition is an invariant and defining feature of Alzheimer disease. A highly similar but less robust process accompanies brain aging in many nondemented humans, lower primates, and some other mammals. The discovery of Aβ as the subunit of the amyloid fibrils in meningocerebral blood vessels and parenchymal plaques has led to innumerable studies of its biochemistry and potential cytotoxic properties. Here we will review the discovery of Aβ, numerous aspects of its complex biochemistry, and current attempts to understand how a range of Aβ assemblies, including soluble oligomers and insoluble fibrils, may precipitate and promote neuronal and glial alterations that underlie the development of dementia. Although the role of Aβ as a key molecular factor in the etiology of Alzheimer disease remains controversial, clinical trials of amyloid-lowering agents, reviewed elsewhere in this book, are poised to resolve the question of its pathogenic primacy.

Conformational differences between two amyloid β oligomers of similar size and dissimilar toxicity

Several protein conformational disorders (Parkinson and prion diseases) are linked to aberrant folding of proteins into prefibrillar oligomers and amyloid fibrils. Although prefibrillar oligomers are more toxic than their fibrillar counterparts, it is difficult to decouple the origin of their dissimilar toxicity because oligomers and fibrils differ both in terms of structure and size. Here we report the characterization of two oligomers of the 42-residue amyloid β (Aβ42) peptide associated with Alzheimer disease that possess similar size and dissimilar toxicity. We find that Aβ42 spontaneously forms prefibrillar oligomers at Aβ concentrations below 30 μm in the absence of agitation, whereas higher Aβ concentrations lead to rapid formation of fibrils. Interestingly, Aβ prefibrillar oligomers do not convert into fibrils under quiescent assembly conditions but instead convert into a second type of oligomer with size and morphology similar to those of Aβ prefibrillar oligomers. Strikingly, this alternative Aβ oligomer is non-toxic to mammalian cells relative to Aβ monomer. We find that two hydrophobic peptide segments within Aβ (residues 16-22 and 30-42) are more solvent-exposed in the more toxic Aβ oligomer. The less toxic oligomer is devoid of β-sheet structure, insoluble, and non-immunoreactive with oligomer- and fibril-specific antibodies. Moreover, the less toxic oligomer is incapable of disrupting lipid bilayers, in contrast to its more toxic oligomeric counterpart. Our results suggest that the ability of non-fibrillar Aβ oligomers to interact with and disrupt cellular membranes is linked to the degree of solvent exposure of their central and C-terminal hydrophobic peptide segments.

Site specific interaction of the polyphenol EGCG with the SEVI amyloid precursor peptide PAP(248-286)

Recently, a 39 amino acid peptide fragment from prostatic acid phosphatase has been isolated from seminal fluid that can enhance infectivity of the HIV virus by up to 4-5 orders of magnitude. PAP(248-286) is effective in enhancing HIV infectivity only when it is aggregated into amyloid fibers termed SEVI. The polyphenol EGCG (epigallocatechin-3-gallate) has been shown to disrupt both SEVI formation and HIV promotion by SEVI, but the mechanism by which it accomplishes this task is unknown. Here, we show that EGCG interacts specifically with the side chains of monomeric PAP(248-286) in two regions (K251-R257 and N269-I277) of primarily charged residues, particularly lysine. The specificity of interaction to these two sites is contrary to previous studies on the interaction of EGCG with other amyloidogenic proteins, which showed the nonspecific interaction of EGCG with exposed backbone sites of unfolded amyloidogenic proteins. This interaction is specific to EGCG as the related gallocatechin (GC) molecule, which shows greatly decreased antiamyloid activity, exhibits minimal interaction with monomeric PAP(248-286). The EGCG binding was shown to occur in two steps, with the initial formation of a weakly bound complex followed by a pH dependent formation of a tightly bound complex. Experiments in which the lysine residues of PAP(248-286) have been chemically modified suggest the tightly bound complex is created by Schiff-base formation with lysine residues. The results of this study could aid in the development of small molecule inhibitors of SEVI and other amyloid proteins.

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

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

Experimental inhibition of peptide fibrillogenesis by synthetic peptides, carbohydrates and drugs

Peptide fibrillogenesis generally begins by the transformation of normally soluble proteins into elongated aggregates which are called as amyloid. These fibrils mainly consist of ß-sheets. They share certain common characteristics such as a cross-ß x-ray diffraction pattern, association with other common proteins and typical staining by the dye Congo Red. The individual form of the deposit consists of a disease-specific peptide/protein. The disease-specific protein serves as the basis for the classification of the amyloids. The association of fibril-forming peptides/proteins with diseases makes them primary disease-targets. Understanding the molecular interactions involved in the fibril formation becomes the foremost requirement to characterize the target. Interference with these interactions of ß-sheets in vitro prevents and sometimes reverses the fibril assembly. A small molecule capable of interfering with the formation of fibril could have therapeutic applications in these diseases. This anti-aggregation approach appears to be a viable treatment option. A search for such a molecule is pursued actively world over. All types of compounds and approaches to slow down or prevent the aggregation process have been described in the literature. These efforts are reviewed in this chapter.

Monitoring insulin aggregation via capillary electrophoresis

Early stages of insulin aggregation, which involve the transient formation of oligomeric aggregates, are an important aspect in the progression of Type II diabetes and in the quality control of pharmaceutical insulin production. This study is the first to utilize capillary electrophoresis (CE) with ultraviolet (UV) detection to monitor insulin oligomer formation at pH 8.0 and physiological ionic strength. The lag time to formation of the first detected species in the aggregation process was evaluated by UV-CE and thioflavin T (ThT) binding for salt concentrations from 100 mM to 250 mM. UV-CE had a significantly shorter (5–8 h) lag time than ThT binding (15–19 h). In addition, the lag time to detection of the first aggregated species via UV-CE was unaffected by salt concentration, while a trend toward an increased lag time with increased salt concentration was observed with ThT binding. This result indicates that solution ionic strength impacts early stages of aggregation and β-sheet aggregate formation differently. To observe whether CE may be applied for the analysis of biological samples containing low insulin concentrations, the limit of detection using UV and laser induced fluorescence (LIF) detection modes was determined. The limit of detection using LIF-CE, 48.4 pM, was lower than the physiological insulin concentration, verifying the utility of this technique for monitoring biological samples. LIF-CE was subsequently used to analyze the time course for fluorescein isothiocyanate (FITC)-labeled insulin oligomer formation. This study is the first to report that the FITC label prevented incorporation of insulin into oligomers, cautioning against the use of this fluorescent label as a tag for following early stages of insulin aggregation.

Structure-based design of conformation- and sequence-specific antibodies against amyloid β

Conformation-specific antibodies that recognize aggregated proteins associated with several conformational disorders (e.g., Parkinson and prion diseases) are invaluable for diagnostic and therapeutic applications. However, no systematic strategy exists for generating conformation-specific antibodies that target linear sequence epitopes within misfolded proteins. Here we report a strategy for designing conformation- and sequence-specific antibodies against misfolded proteins that is inspired by the molecular interactions governing protein aggregation. We find that grafting small amyloidogenic peptides (6-10 residues) from the Aβ42 peptide associated with Alzheimer's disease into the complementarity determining regions of a domain (V(H)) antibody generates antibody variants that recognize Aβ soluble oligomers and amyloid fibrils with nanomolar affinity. We refer to these antibodies as gammabodies for grafted amyloid-motif antibodies. Gammabodies displaying the central amyloidogenic Aβ motif (18VFFA21) are reactive with Aβ fibrils, whereas those displaying the amyloidogenic C terminus (34LMVGGVVIA42) are reactive with Aβ fibrils and oligomers (and weakly reactive with Aβ monomers). Importantly, we find that the grafted motifs target the corresponding peptide segments within misfolded Aβ conformers. Aβ gammabodies fail to cross-react with other amyloidogenic proteins and scrambling their grafted sequences eliminates antibody reactivity. Finally, gammabodies that recognize Aβ soluble oligomers and fibrils also neutralize the toxicity of each Aβ conformer. We expect that our antibody design strategy is not limited to Aβ and can be used to readily generate gammabodies against other toxic misfolded proteins.

Dihydroxybenzoic acid isomers differentially dissociate soluble biotinyl-Aβ(1-42) oligomers

Polyphenolic compounds including a number of natural products such as resveratrol, curcumin, catechin derivatives, and nordihydroguaiaretic acid have effects on the assembly of Aβ fibrils and oligomers as well as on fibril morphology. Based on a lead structure obtained from a screen of a small molecule diversity library, simple benzoic acid derivatives distinguished by the number and position of hydroxyls on the aromatic ring displayed different abilities to dissociate preformed biotinyl-Aβ(1-42) oligomers. The 2,3-, 2,5-, and 3,4-dihydroxybenzoic acid (DHBA) isomers were active oligomer dissociators. The remaining DHBA isomers and the monohydroxy and unsubstituted benzoic acids were inactive and did not compete with the active compounds to block oligomer dissociation. None of the compounds blocked oligomer assembly, indicating that they do not interact with monomeric Aβ to shift the oligomer-monomer equilibrium. Dissociating activity was not associated with quinone redox cycling capacity of the compounds. Gallic acid (3,4,5-trihydroxybenzoic acid) stabilized biotinyl-Aβ(1-42) oligomers against intrinsic dissociation and blocked the effects of the active dissociators, independent of the concentration of dissociator. A model for the mechanism of action of the DHBA dissociators proposes that these compounds destabilize oligomer structure promoting progressive monomer dissociation rather than fissioning oligomers into smaller, but still macromolecular, species. Gallic acid blocks dissociation by stabilizing oligomers against this process.

Heat shock transcription factor 1 as a therapeutic target in neurodegenerative diseases

Neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis and prion-based neurodegeneration are associated with the accumulation of misfolded proteins, resulting in neuronal dysfunction and cell death. However, current treatments for these diseases predominantly address disease symptoms, rather than the underlying protein misfolding and cell death, and are not able to halt or reverse the degenerative process. Studies in cell culture, fruitfly, worm and mouse models of protein misfolding-based neurodegenerative diseases indicate that enhancing the protein-folding capacity of cells, via elevated expression of chaperone proteins, has therapeutic potential. Here, we review advances in strategies to harness the power of the natural cellular protein-folding machinery through pharmacological activation of heat shock transcription factor 1--the master activator of chaperone protein gene expression--to treat neurodegenerative diseases.

A safe, blood-brain barrier permeable triphenylmethane dye inhibits amyloid-β neurotoxicity by generating nontoxic aggregates

Growing evidence suggests that on-pathway amyloid-β (Aβ) oligomers are primary neurotoxic species and have a direct correlation with the onset of Alzheimer's disease (AD). One promising therapeutic strategy to block AD progression is to reduce the levels of these neurotoxic Aβ species using small molecules. While several compounds have been shown to modulate Aβ aggregation, compounds with such activity combined with safety and high blood-brain barrier (BBB) permeability have yet to be reported. Brilliant Blue G (BBG) is a close structural analogue of a U.S. Food and Drug Administration (FDA)-approved food dye and has recently garnered prominent attention as a potential drug to treat spinal cord injury due to its neuroprotective effects along with BBB permeability and high degree of safety. In this work, we demonstrate that BBG is an effective Aβ aggregation modulator, which reduces Aβ-associated cytotoxicity in a dose-dependent manner by promoting the formation of off-pathway, nontoxic aggregates. Comparative studies of BBG and three structural analogues, Brilliant Blue R (BBR), Brilliant Blue FCF (BBF), and Fast Green FCF (FGF), revealed that BBG is most effective, BBR is moderately effective, and BBF and FGF are least effective in modulating Aβ aggregation and cytotoxicity. Therefore, the two additional methyl groups of BBG and other structural differences between the congeners are important in the interaction of BBG with Aβ leading to formation of nontoxic Aβ aggregates. Our findings support the hypothesis that generating nontoxic aggregates using small molecule modulators is an effective strategy for reducing Aβ cytotoxicity. Furthermore, key structural features of BBG identified through structure-function studies can open new avenues into therapeutic design for combating AD.

Α-synuclein misfolding and Parkinson's disease

Substantial evidence links α-synuclein, a small highly conserved presynaptic protein with unknown function, to both familial and sporadic Parkinson's disease (PD). α-Synuclein has been identified as the major component of Lewy bodies and Lewy neurites, the characteristic proteinaceous deposits that are the hallmarks of PD. α-Synuclein is a typical intrinsically disordered protein, but can adopt a number of different conformational states depending on conditions and cofactors. These include the helical membrane-bound form, a partially-folded state that is a key intermediate in aggregation and fibrillation, various oligomeric species, and fibrillar and amorphous aggregates. The molecular basis of PD appears to be tightly coupled to the aggregation of α-synuclein and the factors that affect its conformation. This review examines the different aggregation states of α-synuclein, the molecular mechanism of its aggregation, and the influence of environmental and genetic factors on this process.

Xanthene food dye, as a modulator of Alzheimer's disease amyloid-beta peptide aggregation and the associated impaired neuronal cell function

Background

Alzheimer's disease (AD) is the most common form of dementia. AD is a degenerative brain disorder that causes problems with memory, thinking and behavior. It has been suggested that aggregation of amyloid-beta peptide (Aβ) is closely linked to the development of AD pathology. In the search for safe, effective modulators, we evaluated the modulating capabilities of erythrosine B (ER), a Food and Drug Administration (FDA)-approved red food dye, on Aβ aggregation and Aβ-associated impaired neuronal cell function.

Methodology/Principal Findings

In order to evaluate the modulating ability of ER on Aβ aggregation, we employed transmission electron microscopy (TEM), thioflavin T (ThT) fluorescence assay, and immunoassays using Aβ-specific antibodies. TEM images and ThT fluorescence of Aβ samples indicate that protofibrils are predominantly generated and persist for at least 3 days. The average length of the ER-induced protofibrils is inversely proportional to the concentration of ER above the stoichiometric concentration of Aβ monomers. Immunoassay results using Aβ-specific antibodies suggest that ER binds to the N-terminus of Aβ and inhibits amyloid fibril formation. In order to evaluate Aβ-associated toxicity we determined the reducing activity of SH-SY5Y neuroblastoma cells treated with Aβ aggregates formed in the absence or in the presence of ER. As the concentration of ER increased above the stoichiometric concentration of Aβ, cellular reducing activity increased and Aβ-associated reducing activity loss was negligible at 500 µM ER.

Conclusions/Significance

Our findings show that ER is a novel modulator of Aβ aggregation and reduces Aβ-associated impaired cell function. Our findings also suggest that xanthene dye can be a new type of small molecule modulator of Aβ aggregation. With demonstrated safety profiles and blood-brain permeability, ER represents a particularly attractive aggregation modulator for amyloidogenic proteins associated with neurodegenerative diseases.

Lysine-specific molecular tweezers are broad-spectrum inhibitors of assembly and toxicity of amyloid proteins

Amyloidoses are diseases characterized by abnormal protein folding and self-assembly, for which no cure is available. Inhibition or modulation of abnormal protein self-assembly, therefore, is an attractive strategy for prevention and treatment of amyloidoses. We examined Lys-specific molecular tweezers and discovered a lead compound termed CLR01, which is capable of inhibiting the aggregation and toxicity of multiple amyloidogenic proteins by binding to Lys residues and disrupting hydrophobic and electrostatic interactions important for nucleation, oligomerization, and fibril elongation. Importantly, CLR01 shows no toxicity at concentrations substantially higher than those needed for inhibition. We used amyloid β-protein (Aβ) to further explore the binding site(s) of CLR01 and the impact of its binding on the assembly process. Mass spectrometry and solution-state NMR demonstrated binding of CLR01 to the Lys residues in Aβ at the earliest stages of assembly. The resulting complexes were indistinguishable in size and morphology from Aβ oligomers but were nontoxic and were not recognized by the oligomer-specific antibody A11. Thus, CLR01 binds already at the monomer stage and modulates the assembly reaction into formation of nontoxic structures. The data suggest that molecular tweezers are unique, process-specific inhibitors of aberrant protein aggregation and toxicity, which hold promise for developing disease-modifying therapy for amyloidoses.

Α-mangostin, a polyphenolic xanthone derivative from mangosteen, attenuates β-amyloid oligomers-induced neurotoxicity by inhibiting amyloid aggregation

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by the accumulation of β-sheet-rich amyloid oligomers or fibrils which are associated with cellular toxicity in the brain. Inhibition of Aβ aggregation could be a viable therapeutic strategy for slowing and/or preventing the progress of AD. Here we reported that α-mangostin (α-M), a polyphenolic xanthone derivative from mangosteen, concentration-dependently attenuated the neurotoxicity induced by Aβ-(1-40) or Aβ-(1-42) oligomers (EC(50) = 3.89 nM, 4.14 nM respectively) as observed by decreased cell viability and impaired neurite outgrowth in primary rat cerebral cortical neurons. Molecular docking and dynamics simulations demonstrated that α-M could potentially bind to Aβ and stabilize α-helical conformation. α-M was found to directly dissociate Aβ-(1-40) and Aβ-(1-42) oligomers by blotting with oligomer-specific antibodies. ThioflavinT fluorescence assay and electron microscopy imaging further demonstrated that α-M blocked the fibril formation as well as disturbed the pre-formed fibrils. Taken together, our results indicate that α-M is capable to inhibit and dissociate the Aβ aggregation, which could contribute to its effect of attenuating Aβ oligomers-induced neurotoxicity. Thus, α-M could be a great potential candidate for AD treatment. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Isolating toxic insulin amyloid reactive species that lack β-sheets and have wide pH stability

Amyloid diseases, including Alzheimer's disease, are characterized by aggregation of normally functioning proteins or peptides into ordered, β-sheet rich fibrils. Most of the theories on amyloid toxicity focus on the nuclei or oligomers in the fibril formation process. The nuclei and oligomers are transient species, making their full characterization difficult. We have isolated toxic protein species that act like an oligomer and may provide the first evidence of a stable reactive species created by disaggregation of amyloid fibrils. This reactive species was isolated by dissolving amyloid fibrils at high pH and it has a mass >100 kDa and a diameter of 48 ± 15 nm. It seeds the formation of fibrils in a dose dependent manner, but using circular dichroism and deep ultraviolet resonance Raman spectroscopy, the reactive species was found to not have a β-sheet rich structure. We hypothesize that the reactive species does not decompose at high pH and maintains its structure in solution. The remaining disaggregated insulin, excluding the toxic reactive species that elongated the fibrils, returned to native structured insulin. This is the first time, to our knowledge, that a stable reactive species of an amyloid reaction has been separated and characterized by disaggregation of amyloid fibrils.

A mimotope peptide of Aβ42 fibril-specific antibodies with Aβ42 fibrillation inhibitory activity induces anti-Aβ42 conformer antibody response by a displayed form on an M13 phage in mice

In Alzheimer's disease (AD), amyloid-β (Aβ) peptides accumulate in the brain in different forms, including fibrils and oligomers. Recently, we established three distinct conformation-dependent human single-chain Fv (scFv) antibodies, including B6 scFv, which bound to Aβ42 fibril but not to soluble-form Aβ, inhibiting Aβ42 fibril formation. In this study, we determined the mimotopes of these antibodies and found a common mimotope sequence, B6-C15, using the Ph.D.-C7C phage library. The B6-C15 showed weak homology to the C-terminus of Aβ42 containing GXXXG dimerization motifs. We synthesized the peptide of B6-C15 fused with biotinylated TAT at the N-terminus (TAT-B6-C15) and characterized its biochemical features on an Aβ42-fibrillation reaction in vitro. We demonstrated that, first, TAT-B6-C15 inhibited Aβ42 fibril formation; secondly, TAT-B6-C15 bound to prefibril Aβ42 oligomers but not to monomers, trimers, tetramers, fibrils, or ultrasonicated fragments; thirdly, TAT-B6-C15 inhibited Aβ42-induced cytotoxicity against human SH-SY5Y neuroblastoma cells; and, fourthly, when mice were administered B6-C15-phages dissolved in phosphate-buffered saline, the anti-Aβ42 conformer IgG antibody response was induced. These results suggested that the B6-C15 peptide might provide unique opportunities to analyze the Aβ42 fibrillation pathway and develop a vaccine vehicle for Alzheimer's disease.

Ligand binding to distinct states diverts aggregation of an amyloid-forming protein

Although small molecules that modulate amyloid formation in vitro have been identified, significant challenges remain in determining precisely how these species act. Here we describe the identification of rifamycin SV as a potent inhibitor of β(2) microglobulin (β(2)m) fibrillogenesis when added during the lag time of assembly or early during fibril elongation. Biochemical experiments demonstrate that the small molecule does not act by a colloidal mechanism. Exploiting the ability of electrospray ionization-ion mobility spectrometry-mass spectrometry (ESI-IMS-MS) to resolve intermediates of amyloid assembly, we show instead that rifamycin SV inhibits β(2)m fibrillation by binding distinct monomeric conformers, disfavoring oligomer formation and diverting the course of assembly to the formation of spherical aggregates. The results demonstrate the power of ESI-IMS-MS to identify specific protein conformers as targets for intervention in fibrillogenesis using small molecules and reveal a mechanism of action in which ligand binding diverts unfolded protein monomers toward alternative assembly pathways.

Resveratrol acts not through anti-aggregative pathways but mainly via its scavenging properties against Aβ and Aβ-metal complexes toxicity

It has been recently suggested that resveratrol can be effective in slowing down Alzheimer's disease (AD) development. As reported in many biochemical studies, resveratrol seems to exert its neuro-protective role through inhibition of β-amyloid aggregation (Aβ), by scavenging oxidants and exerting anti-inflammatory activities. In this paper, we demonstrate that resveratrol is cytoprotective in human neuroblastoma cells exposed to Aβ and or to Aβ-metal complex. Our findings suggest that resveratrol acts not through anti-aggregative pathways but mainly via its scavenging properties.

Resveratrol inhibits the formation of multiple-layered β-sheet oligomers of the human islet amyloid polypeptide segment 22-27

The abnormal self-assembly of a number of proteins or peptides is a hallmark of >20 amyloidogenic diseases. Recent studies suggest that the pathology of amyloidogenesis can be attributed primarily to cytotoxic, soluble, intermediate oligomeric species rather than to mature amyloid fibrils. Despite the lack of available structural information regarding these transient species, many therapeutic efforts have focused on inhibiting the formation of these aggregates. One of the most successful approaches has been to use small molecules, many of which have been found to inhibit toxic species with high efficacy. A significant issue that remains to be resolved is the mechanism underlying the inhibitory effects of these molecules. In this article, we present extensive replica-exchange molecular dynamics simulations to study the early aggregation of the human islet amyloid polypeptide segment 22-27 in the presence and absence of the small-molecule inhibitor resveratrol. The simulations indicate that aggregation of these peptides was hindered by resveratrol via a mechanism of blocking the lateral growth of a single-layered β-sheet oligomer (rather than preventing growth by elongation along the fibril axis). Intersheet side-chain stacking, especially stacking of the aromatic rings, was blocked by the presence of resveratrol molecules, and the overall aggregation level was reduced.

Polyphenolic glycosides and aglycones utilize opposing pathways to selectively remodel and inactivate toxic oligomers of amyloid β

Substantial evidence suggests that soluble prefibrillar oligomers of the Aβ42 peptide associated with Alzheimer's disease are the most cytotoxic aggregated Aβ isoform. Limited previous work has revealed that aromatic compounds capable of remodeling Aβ oligomers into nontoxic conformers typically do so by converting them into off-pathway aggregates instead of dissociating them into monomers. Towards identifying small-molecule antagonists capable of selectively dissociating toxic Aβ oligomers into soluble peptide at substoichiometric concentrations, we have investigated the pathways used by polyphenol aglycones and their glycosides to remodel Aβ soluble oligomers. We find that eleven polyphenol aglycones of variable size and structure utilize the same remodeling pathway whereby Aβ oligomers are rapidly converted into large, off-pathway aggregates. Strikingly, we find that glycosides of these polyphenols all utilize a distinct remodeling pathway in which Aβ oligomers are rapidly dissociated into soluble, disaggregated peptide. This disaggregation activity is a synergistic combination of the aglycone and glycone moieties because combinations of polyphenols and sugars fail to disaggregate Aβ oligomers. We also find that polyphenolic glycosides and aglycones use the same opposing pathways to remodel Aβ fibrils. Importantly, both classes of polyphenols fail to remodel nontoxic Aβ oligomers (which are indistinguishable in size and morphology to Aβ soluble oligomers) or promote aggregation of freshly disaggregated Aβ peptide; thus revealing that they are specific for remodeling toxic Aβ conformers. We expect that these and related small molecules will be powerful chemical probes for investigating the conformational and cellular underpinnings of Aβ-mediated toxicity.

Suppression of amyloid beta A11 antibody immunoreactivity by vitamin C: possible role of heparan sulfate oligosaccharides derived from glypican-1 by ascorbate-induced, nitric oxide (NO)-catalyzed degradation

Amyloid β (Aβ) is generated from the copper- and heparan sulfate (HS)-binding amyloid precursor protein (APP) by proteolytic processing. APP supports S-nitrosylation of the HS proteoglycan glypican-1 (Gpc-1). In the presence of ascorbate, there is NO-catalyzed release of anhydromannose (anMan)-containing oligosaccharides from Gpc-1-nitrosothiol. We investigated whether these oligosaccharides interact with Aβ during APP processing and plaque formation. anMan immunoreactivity was detected in amyloid plaques of Alzheimer (AD) and APP transgenic (Tg2576) mouse brains by immunofluorescence microscopy. APP/APP degradation products detected by antibodies to the C terminus of APP, but not Aβ oligomers detected by the anti-Aβ A11 antibody, colocalized with anMan immunoreactivity in Tg2576 fibroblasts. A 50-55-kDa anionic, sodium dodecyl sulfate-stable, anMan- and Aβ-immunoreactive species was obtained from Tg2576 fibroblasts using immunoprecipitation with anti-APP (C terminus). anMan-containing HS oligo- and disaccharide preparations modulated or suppressed A11 immunoreactivity and oligomerization of Aβ42 peptide in an in vitro assay. A11 immunoreactivity increased in Tg2576 fibroblasts when Gpc-1 autoprocessing was inhibited by 3-β[2(diethylamino)ethoxy]androst-5-en-17-one (U18666A) and decreased when Gpc-1 autoprocessing was stimulated by ascorbate. Neither overexpression of Gpc-1 in Tg2576 fibroblasts nor addition of copper ion and NO donor to hippocampal slices from 3xTg-AD mice affected A11 immunoreactivity levels. However, A11 immunoreactivity was greatly suppressed by the subsequent addition of ascorbate. We speculate that temporary interaction between the Aβ domain and small, anMan-containing oligosaccharides may preclude formation of toxic Aβ oligomers. A portion of the oligosaccharides are co-secreted with the Aβ peptides and deposited in plaques. These results support the notion that an inadequate supply of vitamin C could contribute to late onset AD in humans.

Structural basis for Aβ1–42 toxicity inhibition by Aβ C-terminal fragments: discrete molecular dynamics study

Amyloid β-protein (Aβ) is central to the pathology of Alzheimer's disease. Of the two predominant Aβ alloforms, Aβ(1-40) and Aβ(1-42), the latter forms more toxic oligomers. C-terminal fragments (CTFs) of Aβ were recently shown to inhibit Aβ(1-42) toxicity in vitro. Here, we studied Aβ(1-42) assembly in the presence of three effective CTF inhibitors and an ineffective fragment, Aβ(21-30). Using a discrete molecular dynamics approach that recently was shown to capture key differences between Aβ(1-40) and Aβ(1-42) oligomerization, we compared Aβ(1-42) oligomer formation in the absence and presence of CTFs or Aβ(21-30) and identified structural elements of Aβ(1-42) that correlated with Aβ(1-42) toxicity. CTFs co-assembled with Aβ(1-42) into large heterooligomers containing multiple Aβ(1-42) and inhibitor fragments. In contrast, Aβ(21-30) co-assembled with Aβ(1-42) into heterooligomers containing mostly a single Aβ(1-42) and multiple Aβ(21-30) fragments. The CTFs, but not Aβ(21-30), decreased the β-strand propensity of Aβ(1-42) in a concentration-dependent manner. CTFs and Aβ(21-30) had a high binding propensity to the hydrophobic regions of Aβ(1-42), but only CTFs were found to bind the Aβ(1-42) region A2-F4. Consequently, only CTFs but not Aβ(21-30) reduced the solvent accessibility of Aβ(1-42) in region D1-R5. The reduced solvent accessibility of Aβ(1-42) in the presence of CTFs was comparable to the solvent accessibility of Aβ(1-40) oligomers formed in the absence of Aβ fragments. These findings suggest that region D1-R5, which was more exposed to the solvent in Aβ(1-42) than in Aβ(1-40) oligomers, is involved in mediating Aβ(1-42) oligomer neurotoxicity.

Characteristics of amyloid-related oligomers revealed by crystal structures of macrocyclic β-sheet mimics

Protein amyloid oligomers have been strongly linked to amyloid diseases and can be intermediates to amyloid fibers. β-Sheets have been identified in amyloid oligomers. However, because of their transient and highly polymorphic properties, the details of their self-association remain elusive. Here we explore oligomer structure using a model system: macrocyclic peptides. Key amyloidogenic sequences from Aβ and tau were incorporated into macrocycles, thereby restraining them to β-strands, but limiting the growth of the oligomers so they may crystallize and cannot fibrillate. We determined the atomic structures for four such oligomers, and all four reveal tetrameric interfaces in which β-sheet dimers pair together by highly complementary, dry interfaces, analogous to steric zippers found in fibers, suggesting a common structure for amyloid oligomers and fibers. In amyloid fibers, the axes of the paired sheets are either parallel or antiparallel, whereas the oligomeric interfaces display a variety of sheet-to-sheet pairing angles, offering a structural explanation for the heterogeneity of amyloid oligomers.

Neurotherapeutic applications of nanoparticles in Alzheimer's disease

A rapid increase in incidence of neurodegenerative disorders has been observed with the aging of the population. Alzheimer's disease (AD) is the most common neurodegenerative disorder among the elderly. It is characterized by memory dysfunction, loss of lexical access, spatial and temporal disorientation and impairment of judgement clinically. Unfortunately, clinical development of drugs for the symptomatic and disease-modifying treatment of AD has resulted in both promise and disappointment. Indeed, a large number of drugs with differing targets and mechanisms of action were investigated with only a few of them being clinically available. The targeted drug delivery to the central nervous system (CNS), for the diagnosis and treatment of neurodegenerative disorders such as AD, is restricted due to the limitations posed by the blood-brain barrier (BBB) as well as due to opsonization by plasma proteins in the systemic circulation and peripheral side-effects. Over the last decade, nanoparticle-mediated drug delivery represents one promising strategy to successfully increase the CNS penetration of several therapeutic moieties. Different nanocarriers are being investigated to treat and diagnose AD by delivering at a constant rate a host of therapeutics over times extending up to days, weeks or even months. This review provides a concise incursion on the current pharmacotherapies for AD besides reviewing and discussing the literature on the different drug molecules that have been successfully encapsulated in nanoparticles (NPs). Some of them have been shown to cross the BBB and have been tested either for diagnosis or treatment of AD. Finally, the route of NPs administration and the future prospects will be discussed.

Aromatic small molecules remodel toxic soluble oligomers of amyloid beta through three independent pathways

In protein conformational disorders ranging from Alzheimer to Parkinson disease, proteins of unrelated sequence misfold into a similar array of aggregated conformers ranging from small oligomers to large amyloid fibrils. Substantial evidence suggests that small, prefibrillar oligomers are the most toxic species, yet to what extent they can be selectively targeted and remodeled into non-toxic conformers using small molecules is poorly understood. We have evaluated the conformational specificity and remodeling pathways of a diverse panel of aromatic small molecules against mature soluble oligomers of the Aβ42 peptide associated with Alzheimer disease. We find that small molecule antagonists can be grouped into three classes, which we herein define as Class I, II, and III molecules, based on the distinct pathways they utilize to remodel soluble oligomers into multiple conformers with reduced toxicity. Class I molecules remodel soluble oligomers into large, off-pathway aggregates that are non-toxic. Moreover, Class IA molecules also remodel amyloid fibrils into the same off-pathway structures, whereas Class IB molecules fail to remodel fibrils but accelerate aggregation of freshly disaggregated Aβ. In contrast, a Class II molecule converts soluble Aβ oligomers into fibrils, but is inactive against disaggregated and fibrillar Aβ. Class III molecules disassemble soluble oligomers (as well as fibrils) into low molecular weight species that are non-toxic. Strikingly, Aβ non-toxic oligomers (which are morphologically indistinguishable from toxic soluble oligomers) are significantly more resistant to being remodeled than Aβ soluble oligomers or amyloid fibrils. Our findings reveal that relatively subtle differences in small molecule structure encipher surprisingly large differences in the pathways they employ to remodel Aβ soluble oligomers and related aggregated conformers.

Countering amyloid polymorphism and drug resistance with minimal drug cocktails

Several fatal, progressive neurodegenerative diseases, including various prion and prion-like disorders, are connected with the misfolding of specific proteins. These proteins misfold into toxic oligomeric species and a spectrum of distinct self-templating amyloid structures, termed strains. Hence, small molecules that prevent or reverse these protein-misfolding events might have therapeutic utility. Yet it is unclear whether a single small molecule can antagonize the complete repertoire of misfolded forms encompassing diverse amyloid polymorphs and soluble oligomers. We have begun to investigate this issue using the yeast prion protein Sup35 as an experimental paradigm. We have discovered that a polyphenol, (-)epigallocatechin-3-gallate (EGCG), effectively inhibited the formation of infectious amyloid forms (prions) of Sup35 and even remodeled preassembled prions. Surprisingly, EGCG selectively modulated specific prion strains and even selected for EGCG-resistant prion strains with novel structural and biological characteristics. Thus, treatment with a single small molecule antagonist of amyloidogenesis can select for novel, drug-resistant amyloid polymorphs. Importantly, combining EGCG with another small molecule, 4,5-bis-(4-methoxyanilino)phthalimide, synergistically antagonized and remodeled a wide array of Sup35 prion strains without producing any drug-resistant prions. We suggest that minimal drug cocktails, small collections of drugs that collectively antagonize all amyloid polymorphs, should be identified to besiege various neurodegenerative disorders.