Specific domains of Aβ facilitate aggregation on and association with lipid bilayers

The polyglutamine domain is the primary driver of seeding in huntingtin aggregation

Huntington's Disease (HD) is a fatal, neurodegenerative disease caused by aggregation of the huntingtin protein (htt) with an expanded polyglutamine (polyQ) domain into amyloid fibrils. Htt aggregation is modified by flanking sequences surrounding the polyQ domain as well as the binding of htt to lipid membranes. Upon fibrillization, htt fibrils are able to template the aggregation of monomers into fibrils in a phenomenon known as seeding, and this process appears to play a critical role in cell-to-cell spread of HD. Here, exposure of C. elegans expressing a nonpathogenic N-terminal htt fragment (15-repeat glutamine residues) to preformed htt-exon1 fibrils induced inclusion formation and resulted in decreased viability in a dose dependent manner, demonstrating that seeding can induce toxic aggregation of nonpathogenic forms of htt. To better understand this seeding process, the impact of flanking sequences adjacent to the polyQ stretch, polyQ length, and the presence of model lipid membranes on htt seeding was investigated. Htt seeding readily occurred across polyQ lengths and was independent of flanking sequence, suggesting that the structured polyQ domain within fibrils is the key contributor to the seeding phenomenon. However, the addition of lipid vesicles modified seeding efficiency in a manner suggesting that seeding primarily occurs in bulk solution and not at the membrane interface. In addition, fibrils formed in the presence of lipid membranes displayed similar seeding efficiencies. Collectively, this suggests that the polyQ domain that forms the amyloid fibril core is the main driver of seeding in htt aggregation.

Phosphomimetic Mutations Impact Huntingtin Aggregation in the Presence of a Variety of Lipid Systems

Huntington's disease (HD) is a neurodegenerative disorder caused by the abnormal expansion of a polyglutamine (polyQ) tract in the first exon of the htt protein (htt). PolyQ expansion triggers the aggregation of htt into a variety of structures, including oligomers and fibrils. This aggregation is impacted by the first 17 N-terminal amino acids (Nt17) of htt that directly precedes the polyQ domain. Beyond impacting aggregation, Nt17 associates with lipid membranes by forming an amphipathic α-helix. Post-translational modifications within Nt17 are known to modify HD pathology, and in particular, phosphorylation at T3, S13, and/or S16 retards fibrillization and ameliorates the phenotype in HD models. Due to Nt17's propensity to interact with lipid membranes, the impact of introducing phosphomimetic mutations (T3D, S13D, and S16D) into htt-exon1 on aggregation in the presence of a variety of model lipid membranes (total brain lipid extract, 1-palmitoyl-2-oleoyl-glycero-3-phosphatidylcholine, and 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-1'-rac-glycerol) was investigated. Phosphomimetic mutations altered htt's interaction with and aggregation in the presence of lipids; however, this was dependent on the lipid system.

Glymphatic failure as a final common pathway to dementia

Sleep is evolutionarily conserved across all species, and impaired sleep is a common trait of the diseased brain. Sleep quality decreases as we age, and disruption of the regular sleep architecture is a frequent antecedent to the onset of dementia in neurodegenerative diseases. The glymphatic system, which clears the brain of protein waste products, is mostly active during sleep. Yet the glymphatic system degrades with age, suggesting a causal relationship between sleep disturbance and symptomatic progression in the neurodegenerative dementias. The ties that bind sleep, aging, glymphatic clearance, and protein aggregation have shed new light on the pathogenesis of a broad range of neurodegenerative diseases, for which glymphatic failure may constitute a therapeutically targetable final common pathway.

The Role of Lipid Environment in Ganglioside GM1-Induced Amyloid β Aggregation

Ganglioside GM1 is the most common brain ganglioside enriched in plasma membrane regions known as lipid rafts or membrane microdomains. GM1 participates in many modulatory and communication functions associated with the development, differentiation, and protection of neuronal tissue. It has, however, been demonstrated that GM1 plays a negative role in the pathophysiology of Alzheimer's disease (AD). The two features of AD are the formation of intracellular neurofibrillary bodies and the accumulation of extracellular amyloid β (Aβ). Aβ is a peptide characterized by intrinsic conformational flexibility. Depending on its partners, Aβ can adopt different spatial arrangements. GM1 has been shown to induce specific changes in the spatial organization of Aβ, which lead to enhanced peptide accumulation and deleterious effect especially on neuronal membranes containing clusters of this ganglioside. Changes in GM1 levels and distribution during the development of AD may contribute to the aggravation of the disease.

Modulating Aβ Fibrillogenesis with 'Trojan' peptides

Abnormal aggregation of beta-amyloid (Aβ) peptide into amyloid plaques in the brain has been identified as one of the key factors in instigating AD pathogenesis. Inhibition of Aβ aggregation can be an important therapeutic strategy in disease management. In this work, we demonstrate the application of structure-based design of short peptides ('trojan peptides'), intended to intervene in the aggregation of the core recognition domain of amyloid-beta peptide, a known malefactor in Alzheimer's disease. The modulatory effect of trojan peptides has been assessed using ThT fluorescence assay, FETEM imaging, IR, and toxicity assays on model neuronal cell lines. Experimental results suggest that designed trojan peptides could impede the aggregation of the core amyloid fibril forming segment of Aβ peptide, arrest the formation of toxic fibrillar assemblies, and reduce cytotoxicity of the neuronal cell lines.

SUMOylation Prevents Huntingtin Fibrillization and Localization onto Lipid Membranes

Huntington's disease (HD), a genetic neurodegenerative disease, is caused by an expanded polyglutamine (polyQ) domain in the first exon of the huntingtin protein (htt). PolyQ expansion destabilizes protein structure, resulting in aggregation into a variety of oligomers, protofibrils, and fibrils. Beyond the polyQ domain, adjacent protein sequences influence the aggregation process. Specifically, the first 17 N-terminal amino acids (Nt17) directly preceding the polyQ domain promote the formation of α-helix-rich oligomers that represent intermediate species associated with fibrillization. Due to its propensity to form an amphipathic α-helix, Nt17 also facilitates lipid binding. Three lysine residues (K6, K9, and K15) within Nt17 can be SUMOylated, which modifies htt's accumulation and toxicity within cells in a variety of HD models. The impact of SUMOylation on htt aggregation and direct interaction with lipid membranes was investigated. SUMOylation of htt-exon1 inhibited fibril formation while promoting larger, amorphous aggregate species. These amorphous aggregates were SDS soluble but nonetheless exhibited levels of β-sheet structure similar to that of htt-exon1 fibrils. In addition, SUMOylation prevented htt binding, aggregation, and accumulation on model lipid bilayers comprised of total brain lipid extract. Collectively, these observations demonstrate that SUMOylation promotes a distinct htt aggregation pathway that may affect htt toxicity.

Acetylation of Aβ40 Alters Aggregation in the Presence and Absence of Lipid Membranes

A hallmark of Alzheimer's disease (AD) is the formation of senile plaques comprised of the β-amyloid (Aβ) peptide. Aβ fibrillization is a complex nucleation-dependent process involving a variety of metastable intermediate aggregates and features the formation of inter- and intramolecular salt bridges involving lysine residues, K16 and K28. Cationic lysine residues also mediate protein-lipid interactions via association with anionic lipid headgroups. As several toxic mechanisms attributed to Aβ involve membrane interactions, the impact of acetylation on Aβ₄₀ aggregation in the presence and absence of membranes was determined. Using chemical acetylation, varying mixtures of acetylated and nonacetylated Aβ₄₀ were produced. With increasing acetylation, fibril and oligomer formation decreased, eventually completely arresting fibrillization. In the presence of total brain lipid extract (TBLE) vesicles, acetylation reduced the interaction of Aβ₄₀ with membranes; however, fibrils still formed at near complete levels of acetylation. Additionally, the combination of TBLE and acetylated Aβ promoted annular aggregates. Finally, toxicity associated with Aβ₄₀ was reduced with increasing acetylation in a cell culture assay. These results suggest that in the absence of membranes that the cationic character of lysine plays a major role in fibril formation. However, acetylation promotes unique aggregation pathways in the presence of lipid membranes.

Effect of lipid saturation on amyloid-beta peptide partitioning and aggregation in neuronal membranes: molecular dynamics simulations

Aggregation of amyloid-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\beta $$\end{document}β (Aβ) peptides, cleaved from the amyloid precursor protein, is known as a precursor of the Alzheimer’s disease (AD). It is also known that Alzheimer’s disease is characterized by a substantial decrease of the amount of polyunsaturated lipids in the neuronal membranes of the frontal gray matter. To get insight into possible interconnection of these phenomena, we have carried out molecular dynamics simulations of two fragments of A\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\beta $$\end{document}β peptide, A\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\beta $$\end{document}β\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{1-28}$$\end{document}1-28 and A\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\beta $$\end{document}β\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{26-40}$$\end{document}26-40, in four different lipid bilayers: two monocomponent ones (14:0-14:0 PC, 18:0-22:6 PC), and two bilayers containing mixtures of 18:0-18:0 PE, 22:6-22:6 PE, 16:0-16:0 PC and 18:1-18:1 PC lipids of composition mimicking neuronal membranes in a “healthy” and “AD” brain. The simulations showed that the presence of lipids with highly unsaturated 22:6cis fatty acids chains strongly affects the interaction of amyloid-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\beta $$\end{document}β peptides with lipid membranes. The polyunsaturated lipids cause stronger adsorption of A\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\beta $$\end{document}β-peptides by the membrane and lead to weaker binding between peptides when the latter form aggregates. This difference in the behaviour observed in monocomponent bilayers is propagated in a similar fashion to the mixed membranes mimicking composition of neuronal membranes in “healthy” and “AD” brains, with “healthy” membrane having higher fraction of polyunsaturated lipids. Our simulations give strong indication that it can be physical–chemical background of the interconnection between amyloid fibrillization causing Alzheimer’s disease, and content of polyunsaturated lipids in the neuronal membranes.

Lipid Membranes Influence the Ability of Small Molecules To Inhibit Huntingtin Fibrillization

Several diseases, including Alzheimer's disease, Parkinson's disease, and Huntington's disease (HD), are associated with specific proteins aggregating and depositing within tissues and/or cellular compartments. The aggregation of these proteins is characterized by the formation of extended, β-sheet rich fibrils, termed amyloid. In addition, a variety of other aggregate species also form, including oligomers and protofibrils. Specifically, HD is caused by the aggregation of the huntingtin (htt) protein that contains an expanded polyglutamine domain. Due to the link between protein aggregation and disease, small molecule aggregation inhibitors have been pursued as potential therapeutic agents. Two such small molecules are epigallocatechin 3-gallate (EGCG) and curcumin, both of which inhibit the fibril formation of several amyloid-forming proteins. However, amyloid formation is a complex process that is strongly influenced by the protein's environment, leading to distinct aggregation pathways. Thus, changes in the protein's environment may alter the effectiveness of aggregation inhibitors. A well-known modulator of amyloid formation is lipid membranes. Here, we investigated if the presence of lipid vesicles altered the ability of EGCG or curcumin to modulate htt aggregation and influence the interaction of htt with lipid membranes. The presence of 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine or total brain lipid extract vesicles prevented the curcumin from inhibiting htt fibril formation. In contrast, EGCG's inhibition of htt fibril formation persisted in the presence of lipids. Collectively, these results highlight the complexity of htt aggregation and demonstrate that the presence of lipid membranes is a key modifier of the ability of small molecules to inhibit htt fibril formation.

The Amyloid-β Oligomer Hypothesis: Beginning of the Third Decade

The amyloid-β oligomer (AβO) hypothesis was introduced in 1998. It proposed that the brain damage leading to Alzheimer’s disease (AD) was instigated by soluble, ligand-like AβOs. This hypothesis was based on the discovery that fibril-free synthetic preparations of AβOs were potent CNS neurotoxins that rapidly inhibited long-term potentiation and, with time, caused selective nerve cell death (Lambert et al., 1998). The mechanism was attributed to disrupted signaling involving the tyrosine-protein kinase Fyn, mediated by an unknown toxin receptor. Over 4,000 articles concerning AβOs have been published since then, including more than 400 reviews. AβOs have been shown to accumulate in an AD-dependent manner in human and animal model brain tissue and, experimentally, to impair learning and memory and instigate major facets of AD neuropathology, including tau pathology, synapse deterioration and loss, inflammation, and oxidative damage. As reviewed by Hayden and Teplow in 2013, the AβO hypothesis “has all but supplanted the amyloid cascade.” Despite the emerging understanding of the role played by AβOs in AD pathogenesis, AβOs have not yet received the clinical attention given to amyloid plaques, which have been at the core of major attempts at therapeutics and diagnostics but are no longer regarded as the most pathogenic form of Aβ. However, if the momentum of AβO research continues, particularly efforts to elucidate key aspects of structure, a clear path to a successful disease modifying therapy can be envisioned. Ensuring that lessons learned from recent, late-stage clinical failures are applied appropriately throughout therapeutic development will further enable the likelihood of a successful therapy in the near-term.

Sphingomyelin and GM1 Influence Huntingtin Binding to, Disruption of, and Aggregation on Lipid Membranes

Huntington disease (HD) is an inherited neurodegenerative disease caused by the expansion beyond a critical threshold of a polyglutamine (polyQ) tract near the N-terminus of the huntingtin (htt) protein. Expanded polyQ promotes the formation of a variety of oligomeric and fibrillar aggregates of htt that accumulate into the hallmark proteinaceous inclusion bodies associated with HD. htt is also highly associated with numerous cellular and subcellular membranes that contain a variety of lipids. As lipid homeostasis and metabolism abnormalities are observed in HD patients, we investigated how varying both the sphingomyelin (SM) and ganglioside (GM1) contents modifies the interactions between htt and lipid membranes. SM composition is altered in HD, and GM1 has been shown to have protective effects in animal models of HD. A combination of Langmuir trough monolayer techniques, vesicle permeability and binding assays, and in situ atomic force microscopy (AFM) were used to directly monitor the interaction of a model, synthetic htt peptide and a full-length htt-exon1 recombinant protein with model membranes comprised of total brain lipid extract (TBLE) and varying amounts of exogenously added SM or GM1. The addition of either SM or GM1 decreased htt insertion into the lipid monolayers. However, TBLE vesicles with an increased SM content were more susceptible to htt-induced permeabilization, whereas GM1 had no effect on permeablization. Pure TBLE bilayers and TBLE bilayers enriched with GM1 developed regions of roughened, granular morphologies upon exposure to htt-exon1, but plateau-like domains with a smoother appearance formed in bilayers enriched with SM. Oligomeric aggregates were observed on all bilayer systems regardless of induced morphology. Collectively, these observations suggest that the lipid composition and its subsequent effects on membrane material properties strongly influence htt binding and aggregation on lipid membranes.

Investigation of the interaction of amyloid β peptide (11–42) oligomers with a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membrane using molecular dynamics simulation

The mechanism of pore formation in model neural cell membranes by β amyloid (Aβ) peptides was investigated using molecular dynamics simulation which indicated that Aβ oligomers of size equal or greater than 3 has a higher tendency for pore formation than monomers and that cholesterol tends to retard Aβ binding and insertion into the membrane.

2-Arachidonoylglycerol metabolism is differently modulated by oligomeric and fibrillar conformations of amyloid beta in synaptic terminals

Alzheimer's disease (AD) is the most prevalent disorder of senile dementia mainly characterized by amyloid-beta peptide (Aβ) deposits in the brain. Cannabinoids are relevant to AD as they exert several beneficial effects in many models of this disease. Still, whether the endocannabinoid system is either up- or down-regulated in AD has not yet been fully elucidated. Thus, the aim of the present paper was to analyze endocannabinoid 2-arachidonoylglycerol (2-AG) metabolism in cerebral cortex synaptosomes incubated with Aβ oligomers or fibrils. These Aβ conformations were obtained by "aging" the 1-40 fragment of the peptide under different agitation and time conditions. A diminished availability of 2-AG resulting from a significant decrease in diacylglycerol lipase (DAGL) activity was observed in the presence of large Aβ_1-40 oligomers along with synaptosomal membrane damage, as judged by transmission electron microscopy and LDH release. Conversely, a high availability of 2-AG resulting from an increase in DAGL and lysophosphatidic acid phosphohydrolase activities occurred in the presence of Aβ_1-40 fibrils although synaptosomal membrane disruption was also observed. Interestingly, neither synaptosomal mitochondrial viability assayed by MTT reduction nor membrane lipid peroxidation assayed by TBARS formation measurements were altered by Aβ_1-40 oligomers or fibrils. These results show a differential effect of Aβ_1-40 peptide on 2-AG metabolism depending on its conformation.

Molecular Dynamics Simulations of Amyloid β-Peptide (1-42): Tetramer Formation and Membrane Interactions

The aggregation cascade and peptide-membrane interactions of the amyloid β-peptide (Aβ) have been implicated as toxic events in the development and progression of Alzheimer's disease. Aβ42 forms oligomers and ultimately plaques, and it has been hypothesized that these oligomeric species are the main toxic species contributing to neuronal cell death. To better understand oligomerization events and subsequent oligomer-membrane interactions of Aβ42, we performed atomistic molecular-dynamics (MD) simulations to characterize both interpeptide interactions and perturbation of model membranes by the peptides. MD simulations were utilized to first show the formation of a tetramer unit by four separate Aβ42 peptides. Aβ42 tetramers adopted an oblate ellipsoid shape and showed a significant increase in β-strand formation in the final tetramer unit relative to the monomers, indicative of on-pathway events for fibril formation. The Aβ42 tetramer unit that formed in the initial simulations was used in subsequent MD simulations in the presence of a pure POPC or cholesterol-rich raft model membrane. Tetramer-membrane simulations resulted in elongation of the tetramer in the presence of both model membranes, with tetramer-raft interactions giving rise to the rearrangement of key hydrophobic regions in the tetramer and the formation of a more rod-like structure indicative of a fibril-seeding aggregate. Membrane perturbation by the tetramer was manifested in the form of more ordered, rigid membranes, with the pure POPC being affected to a greater extent than the raft membrane. These results provide critical atomistic insight into the aggregation pathway of Aβ42 and a putative toxic mechanism in the pathogenesis of Alzheimer's disease.

Acetylation within the First 17 Residues of Huntingtin Exon 1 Alters Aggregation and Lipid Binding

Huntington's disease (HD) is a genetic neurodegenerative disorder caused by an expanded polyglutamine (polyQ) domain near the N-terminus of the huntingtin (htt) protein. Expanded polyQ leads to htt aggregation. The first 17 amino acids (Nt(17)) in htt comprise a lipid-binding domain that undergoes a number of posttranslational modifications that can modulate htt toxicity and subcellular localization. As there are three lysines within Nt(17), we evaluated the impact of lysine acetylation on htt aggregation in solution and on model lipid bilayers. Acetylation of htt-exon1(51Q) and synthetic truncated htt-exon 1 mimicking peptides (Nt(17)-Q35-P10-KK) was achieved using a selective covalent label, sulfo-N-hydroxysuccinimide (NHSA). With this treatment, all three lysine residues (K6, K9, and K15) in Nt(17) were significantly acetylated. N-terminal htt acetylation retarded fibril formation in solution and promoted the formation of larger globular aggregates. Acetylated htt also bound lipid membranes and disrupted the lipid bilayer morphology less aggressively compared with the wild-type. Computational studies provided mechanistic insights into how acetylation alters the interaction of Nt(17) with lipid membranes. Our results highlight that N-terminal acetylation influences the aggregation of htt and its interaction with lipid bilayers.

Potential Artifacts in Sample Preparation Methods Used for Imaging Amyloid Oligomers and Protofibrils due to Surface-Mediated Fibril Formation

Accurate imaging of nanometer-sized structures and morphologies is essential to characterizing amyloid species formed at various stages of amyloid aggregation. In this article, we examine the effect of different drying procedures on the final morphology of surface-mediated fibrils formed during the incubation period, which may then be mistaken as oligomers or protofibrils intentionally formed in solution for a particular study. Atomic force microscopy results show that some artifacts, such as globules, flakelike structures, and even micrometer-long fibrils, can be produced under various drying conditions. We also demonstrate that one can prevent drying artifacts by using an appropriate spin-coating procedure to dry amyloid samples. This procedure can bypass the wetting/dewetting transition of the liquid layer during the drying process and preserve the structure of interest on the substrate without generating drying artifacts.

Proteins Containing Expanded Polyglutamine Tracts and Neurodegenerative Disease

Several hereditary neurological and neuromuscular diseases are caused by an abnormal expansion of trinucleotide repeats. To date, there have been 10 of these trinucleotide repeat disorders associated with an expansion of the codon CAG encoding glutamine (Q). For these polyglutamine (polyQ) diseases, there is a critical threshold length of the CAG repeat required for disease, and further expansion beyond this threshold is correlated with age of onset and symptom severity. PolyQ expansion in the translated proteins promotes their self-assembly into a variety of oligomeric and fibrillar aggregate species that accumulate into the hallmark proteinaceous inclusion bodies associated with each disease. Here, we review aggregation mechanisms of proteins with expanded polyQ-tracts, structural consequences of expanded polyQ ranging from monomers to fibrillar aggregates, the impact of protein context and post-translational modifications on aggregation, and a potential role for lipid membranes in aggregation. As the pathogenic mechanisms that underlie these disorders are often classified as either a gain of toxic function or loss of normal protein function, some toxic mechanisms associated with mutant polyQ tracts will also be discussed.

Reduced Lipid Bilayer Thickness Regulates the Aggregation and Cytotoxicity of Amyloid-β

The aggregation of amyloid-β (Aβ) on lipid bilayers has been implicated as a mechanism by which Aβ exerts its toxicity in Alzheimer's disease (AD). Lipid bilayer thinning has been observed during both oxidative stress and protein aggregation in AD, but whether these pathological modifications of the bilayer correlate with Aβ misfolding is unclear. Here, we studied peptide-lipid interactions in synthetic bilayers of the short-chain lipid dilauroyl phosphatidylcholine (DLPC) as a simplified model for diseased bilayers to determine their impact on Aβ aggregate, protofibril, and fibril formation. Aβ aggregation and fibril formation in membranes composed of dioleoyl phosphatidylcholine (DOPC) or 1- palmitoyl-2-oleoyl phosphatidylcholine mimicking normal bilayers served as controls. Differences in aggregate formation and stability were monitored by a combination of thioflavin-T fluorescence, circular dichroism, atomic force microscopy, transmission electron microscopy, and NMR. Despite the ability of all three lipid bilayers to catalyze aggregation, DLPC accelerates aggregation at much lower concentrations and prevents the fibrillation of Aβ at low micromolar concentrations. DLPC stabilized globular, membrane-associated oligomers, which could disrupt the bilayer integrity. DLPC bilayers also remodeled preformed amyloid fibrils into a pseudo-unfolded, molten globule state, which resembled on-pathway, protofibrillar aggregates. Whereas the stabilized, membrane-associated oligomers were found to be nontoxic, the remodeled species displayed toxicity similar to that of conventionally prepared aggregates. These results provide mechanistic insights into the roles that pathologically thin bilayers may play in Aβ aggregation on neuronal bilayers, and pathological lipid oxidation may contribute to Aβ misfolding.

Cholesterol Modifies Huntingtin Binding to, Disruption of, and Aggregation on Lipid Membranes

Huntington's disease (HD) is an inherited neurodegenerative disease caused by abnormally long CAG-repeats in the huntingtin gene that encode an expanded polyglutamine (polyQ) domain near the N-terminus of the huntingtin (htt) protein. Expanded polyQ domains are directly correlated to disease-related htt aggregation. Htt is found highly associated with a variety of cellular and subcellular membranes that are predominantly comprised of lipids. Since cholesterol homeostasis is altered in HD, we investigated how varying cholesterol content modifies the interactions between htt and lipid membranes. A combination of Langmuir trough monolayer techniques, vesicle permeability and binding assays, and in situ atomic force microscopy were used to directly monitor the interaction of a model, synthetic htt peptide and a full-length htt-exon1 recombinant protein with model membranes comprised of total brain lipid extract (TBLE) and varying amounts of exogenously added cholesterol. As the cholesterol content of the membrane increased, the extent of htt insertion decreased. Vesicles containing extra cholesterol were resistant to htt-induced permeabilization. Morphological and mechanical changes in the bilayer associated with exposure to htt were also drastically altered by the presence of cholesterol. Disrupted regions of pure TBLE bilayers were grainy in appearance and associated with a large number of globular aggregates. In contrast, morphological changes induced by htt in bilayers enriched in cholesterol were plateau-like with a smooth appearance. Collectively, these observations suggest that the presence and amount of cholesterol in lipid membranes play a critical role in htt binding and aggregation on lipid membranes.

Validation and Characterization of a Novel Peptide That Binds Monomeric and Aggregated β-Amyloid and Inhibits the Formation of Neurotoxic Oligomers

Although the formation of β-amyloid (Aβ) deposits in the brain is a hallmark of Alzheimer disease (AD), the soluble oligomers rather than the mature amyloid fibrils most likely contribute to Aβ toxicity and neurodegeneration. Thus, the discovery of agents targeting soluble Aβ oligomers is highly desirable for early diagnosis prior to the manifestation of a clinical AD phenotype and also more effective therapies. We have previously reported that a novel 15-amino acid peptide (15-mer), isolated via phage display screening, targeted Aβ and attenuated its neurotoxicity (Taddei, K., Laws, S. M., Verdile, G., Munns, S., D'Costa, K., Harvey, A. R., Martins, I. J., Hill, F., Levy, E., Shaw, J. E., and Martins, R. N. (2010) Neurobiol. Aging 31, 203-214). The aim of the current study was to generate and biochemically characterize analogues of this peptide with improved stability and therapeutic potential. We demonstrated that a stable analogue of the 15-amino acid peptide (15M S.A.) retained the activity and potency of the parent peptide and demonstrated improved proteolytic resistance in vitro (stable to t = 300 min, c.f. t = 30 min for the parent peptide). This candidate reduced the formation of soluble Aβ42 oligomers, with the concurrent generation of non-toxic, insoluble aggregates measuring up to 25-30 nm diameter as determined by atomic force microscopy. The 15M S.A. candidate directly interacted with oligomeric Aβ42, as shown by coimmunoprecipitation and surface plasmon resonance/Biacore analysis, with an affinity in the low micromolar range. Furthermore, this peptide bound fibrillar Aβ42 and also stained plaques ex vivo in brain tissue from AD model mice. Given its multifaceted ability to target monomeric and aggregated Aβ42 species, this candidate holds promise for novel preclinical AD imaging and therapeutic strategies.

Soluble amyloid-β oligomers as synaptotoxins leading to cognitive impairment in Alzheimer's disease

Alzheimer’s disease (AD) is the most common form of dementia in the elderly, and affects millions of people worldwide. As the number of AD cases continues to increase in both developed and developing countries, finding therapies that effectively halt or reverse disease progression constitutes a major research and public health challenge. Since the identification of the amyloid-β peptide (Aβ) as the major component of the amyloid plaques that are characteristically found in AD brains, a major effort has aimed to determine whether and how Aβ leads to memory loss and cognitive impairment. A large body of evidence accumulated in the past 15 years supports a pivotal role of soluble Aβ oligomers (AβOs) in synapse failure and neuronal dysfunction in AD. Nonetheless, a number of basic questions, including the exact molecular composition of the synaptotoxic oligomers, the identity of the receptor(s) to which they bind, and the signaling pathways that ultimately lead to synapse failure, remain to be definitively answered. Here, we discuss recent advances that have illuminated our understanding of the chemical nature of the toxic species and the deleterious impact they have on synapses, and have culminated in the proposal of an Aβ oligomer hypothesis for Alzheimer’s pathogenesis. We also highlight outstanding questions and challenges in AD research that should be addressed to allow translation of research findings into effective AD therapies.

Amyloid β oligomers in Alzheimer's disease pathogenesis, treatment, and diagnosis

Protein aggregation is common to dozens of diseases including prionoses, diabetes, Parkinson's and Alzheimer's. Over the past 15 years, there has been a paradigm shift in understanding the structural basis for these proteinopathies. Precedent for this shift has come from investigation of soluble Aβ oligomers (AβOs), toxins now widely regarded as instigating neuron damage leading to Alzheimer's dementia. Toxic AβOs accumulate in AD brain and constitute long-lived alternatives to the disease-defining Aβ fibrils deposited in amyloid plaques. Key experiments using fibril-free AβO solutions demonstrated that while Aβ is essential for memory loss, the fibrillar Aβ in amyloid deposits is not the agent. The AD-like cellular pathologies induced by AβOs suggest their impact provides a unifying mechanism for AD pathogenesis, explaining why early stage disease is specific for memory and accounting for major facets of AD neuropathology. Alternative ideas for triggering mechanisms are being actively investigated. Some research favors insertion of AβOs into membrane, while other evidence supports ligand-like accumulation at particular synapses. Over a dozen candidate toxin receptors have been proposed. AβO binding triggers a redistribution of critical synaptic proteins and induces hyperactivity in metabotropic and ionotropic glutamate receptors. This leads to Ca(2+) overload and instigates major facets of AD neuropathology, including tau hyperphosphorylation, insulin resistance, oxidative stress, and synapse loss. Because different species of AβOs have been identified, a remaining question is which oligomer is the major pathogenic culprit. The possibility has been raised that more than one species plays a role. Despite some key unknowns, the clinical relevance of AβOs has been established, and new studies are beginning to point to co-morbidities such as diabetes and hypercholesterolemia as etiological factors. Because pathogenic AβOs appear early in the disease, they offer appealing targets for therapeutics and diagnostics. Promising therapeutic strategies include use of CNS insulin signaling enhancers to protect against the presence of toxins and elimination of the toxins through use of highly specific AβO antibodies. An AD-dependent accumulation of AβOs in CSF suggests their potential use as biomarkers and new AβO probes are opening the door to brain imaging. Overall, current evidence indicates that Aβ oligomers provide a substantive molecular basis for the cause, treatment and diagnosis of Alzheimer's disease.

Investigation of temperature induced mechanical changes in supported bilayers by variants of tapping mode atomic force microscopy

Tapping mode atomic force microscopy (AFM) is an invaluable technique for examining topographical features of biological materials in solution, and there has been a growing interest in developing techniques to provide further compositional contrast and information concerning surface mechanical properties. Phase shifts, cantilever response at higher harmonic frequencies of the drive, and time-resolved tip/sample force reconstruction have all been shown to provide additional compositional contrast of surfaces, as compared to basic tapping mode AFM imaging. This study aimed to demonstrate the relative ability of these different imaging techniques to detect temperature induced changes in the elastic modulus of supported total brain lipid extract (TBLE) bilayer patches on mica. To aid in direct comparison between the different imaging techniques, all required data was obtained simultaneously while capturing traditional tapping mode AFM topography images. While all of the techniques were able to provide compositional contrast consistent with known temperature-induced changes in the bilayer patch, interpretation of the resulting contrast was not always straightforward. Phase imaging suffered from contrast inversion. Individual harmonics responded in a variety of ways to the temperature-induced changes in elastic modulus of the bilayer. The maximum tapping force (or peak force) associated with imaging the bilayer correctly reflected the changes in elastic modulus of the lipid bilayer. Importantly, as the required data can be obtained simultaneously, combining these different imaging techniques can lead to a more complete understanding of a sample's mechanical features.

Preparation protocols of aβ(1-40) promote the formation of polymorphic aggregates and altered interactions with lipid bilayers

The appearance of neuritic amyloid plaques comprised of β-amyloid peptide (Aβ) in the brain is a predominant feature in Alzheimer's disease (AD). In the aggregation process, Aβ samples a variety of potentially toxic aggregate species, ranging from small oligomers to fibrils. Aβ has the ability to form a variety of morphologically distinct and stable amyloid fibrils. Commonly termed polymorphs, such distinct aggregate species may play a role in variations of AD pathology. It has been well documented that polymorphic aggregates of Aβ can be produced by changes in the chemical environment and peptide preparations. As Aβ and several of its aggregated forms are known to interact directly with lipid membranes and this interaction may play a role in a variety of potential toxic mechanisms associated with AD, we determine how different Aβ(1-40) preparation protocols that lead to distinct polymorphic fibril aggregates influence the interaction of Aβ(1-40) with model lipid membranes. Using three distinct protocols for preparing Aβ(1-40), the aggregate species formed in the absence and presence of a lipid bilayers were investigated using a variety of scanning probe microscopy techniques. The three preparations of Aβ(1-40) promoted distinct oligomeric and fibrillar aggregates in the absence of bilayers that formed at different rates. Despite these differences in aggregation properties, all Aβ(1-40) preparations were able to disrupt supported total brain lipid extract bilayers, altering the bilayer's morphological and mechanical properties.

Surface effects mediate self-assembly of amyloid-β peptides

Here we present a label-free method for studying the mechanism of surface effects on amyloid aggregation. In this method, spin-coating is used to rapidly dry samples, in a homogeneous manner, after various incubation times. This technique allows the control of important parameters for self-assembly, such as the surface concentration. Atomic force microscopy is then used to obtain high-resolution images of the morphology. While imaging under dry conditions, we show that the morphologies of self-assembled aggregates of a model amyloid-β peptide, Aβ(12-28), are strongly influenced by the local surface concentration. On mica surfaces, where the peptides can freely diffuse, homogeneous, self-assembled protofibrils formed spontaneously and grew longer with longer subsequent incubation. The surface fibrillization rate was much faster than the rates of fibril formation observed in solution, with initiation occurring at much lower concentrations. These data suggest an alternative pathway for amyloid formation on surfaces where the nucleation stage is either bypassed entirely or too fast to measure. This simple preparation procedure for high-resolution atomic force microscopy imaging of amyloid oligomers and protofibrils should be applicable to any amyloidogenic protein species.

Curvature enhances binding and aggregation of huntingtin at lipid membranes

Huntington disease (HD) is a genetic neurodegenerative disease caused by an expanded polyglutamine (polyQ) domain in the first exon of the huntingtin (Htt) protein, facilitating its aggregation. Htt interacts with a variety of membraneous structures within the cell, and the first 17 amino acids (Nt17) of Htt directly flanking the polyQ domain comprise an amphiphathic α-helix (AH) lipid-binding domain. AHs are also known to detect membrane curvature. To determine if Htt exon 1 preferentially binds curved membranes, in situ atomic force microscopy (AFM) studies were performed. Supported lipid bilayers are commonly used as model membranes for AFM studies of protein aggregation. However, these supported bilayers usually lack curvature. By forming a bilayer on top of silica nanobeads (50 ± 10 nm) deposited on a silicon substrate, model supported lipid bilayers with flat and curved regions were developed for AFM studies. The presence of the bilayer over the beads was validated by continual imaging of the formation of the bilayer, height measurements, and spatially resolved mechanical measurements of the resulting bilayer using scanning probe acceleration microscopy. Interpretation of this data was facilitated by numerical simulations of the entire imaging process. The curved supported bilayers associated with the beads were found to be more compliant than flat supported bilayers, consistent with the altered packing density of lipids caused by the induced curvature. This model bilayer system was exposed to a synthetic truncated Htt exon 1 peptide (Nt17Q35P10KK), and this peptide preferentially accumulated on curved membranes, consistent with the ability of AHs to sense membrane curvature.

Misfolding of amyloidogenic proteins and their interactions with membranes

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