Spontaneous incorporation of β-amyloid peptide into neutral liposomes

Nanomaterials for Modulating the Aggregation of β-Amyloid Peptides

The aberrant aggregation of amyloid-β (Aβ) peptides in the brain has been recognized as the major hallmark of Alzheimer's disease (AD). Thus, the inhibition and dissociation of Aβ aggregation are believed to be effective therapeutic strategiesforthe prevention and treatment of AD. When integrated with traditional agents and biomolecules, nanomaterials can overcome their intrinsic shortcomings and boost their efficiency via synergistic effects. This article provides an overview of recent efforts to utilize nanomaterials with superior properties to propose effective platforms for AD treatment. The underlying mechanismsthat are involved in modulating Aβ aggregation are discussed. The summary of nanomaterials-based modulation of Aβ aggregation may help researchers to understand the critical roles in therapeutic agents and provide new insight into the exploration of more promising anti-amyloid agents and tactics in AD theranostics.

Mixed Phospholipid Vesicles Catalytically Inhibit and Reverse Amyloid Fibril Formation

While many approaches to reduce fibrillation of amyloid-β (Aβ) have been aimed at slowing fibril formation, the degradation of fibrils remains challenging. We provide insight into fibril degradation as well as the inhibition of fiber formation by lipid vesicles composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine and 1,2-dioleoyl-sn-glycero-3-phospho-(1'-rac-glycerol). In the presence of vesicles with the optimal lipid composition, fibril formation was inhibited up to 76%. Additionally, by tuning the lipid composition, mature fibril content decreased up to 74% and the β-sheet content of Aβ was significantly reduced. The reduction in fibril and β-sheet content was consistent with a decrease in fibril diameter and could be attributed to the chaperone-like activity of the mixed vesicles. While demonstrating this remarkable activity, our findings present new evidence that lipid composition has a significant effect on the strength of the interaction between lipid bilayers and Aβ peptides/fibrils. This understanding has intriguing therapeutic implications in treating protein misfolding diseases.

Far from Inert: Membrane Lipids Possess Intrinsic Reactivity That Has Consequences for Cell Biology

In this article, it is hypothesized that a fundamental chemical reactivity exists between some non-lipid constituents of cellular membranes and ester-based lipids, the significance of which is not generally recognized. Many peptides and smaller organic molecules have now been shown to undergo lipidation reactions in model membranes in circumstances where direct reaction with the lipid is the only viable route for acyl transfer. Crucially, drugs like propranolol are lipidated in vivo with product profiles that are comparable to those produced in vitro. Some compounds have also been found to promote lipid hydrolysis. Drugs with high lytic activity in vivo tend to have higher toxicity in vitro. Deacylases and lipases are proposed as key enzymes that protect cells against the effects of intrinsic lipidation. The toxic effects of intrinsic lipidation are hypothesized to include a route by which nucleation can occur during the formation of amyloid fibrils.

Amyloid Beta Peptide Folding in Reverse Micelles

Previously published experimental studies have suggested that when the 40-residue amyloid beta peptide is encapsulated in a reverse micelle, it folds into a structure that may nucleate amyloid fibril formation (Yeung, P. S.-W.; Axelsen, P. H. J. Am. Chem. Soc. 2012, 134, 6061 ). The factors that induce the formation of this structure have now been identified in a multi-microsecond simulation of the same reverse micelle system that was studied experimentally. Key features of the polypeptide-micelle interaction include the anchoring of a hydrophobic residue cluster into gaps in the reverse micelle surface, the formation of a beta turn at the anchor point that brings N- and C-terminal segments of the polypeptide into proximity, high ionic strength that promotes intramolecular hydrogen bond formation, and deformation of the reverse micelle surface to facilitate interactions with the surface along the entire length of the polypeptide. Together, these features cause the simulation-derived vibrational spectrum to red shift in a manner that reproduces the red-shift previously reported experimentally. On the basis of these findings, a new mechanism is proposed whereby membranes nucleate fibril formation and facilitate the in-register alignment of polypeptide strands that is characteristic of amyloid fibrils.

Lipid membranes catalyse the fibril formation of the amyloid-β (1-42) peptide through lipid-fibril interactions that reinforce secondary pathways

Alzheimer's disease is associated with the aggregation of amyloid-β (Aβ) peptides into oligomers and fibrils. We have explored how model lipid membranes modulate the rate and mechanisms of Aβ(1-42) self-assembly, in order to shed light on how this pathological reaction may occur in the lipid-rich environments that the peptide encounters in the brain. Using a combination of in vitro biophysical experiments and theoretical approaches, we show that zwitterionic DOPC lipid vesicles accelerate the Aβ(1-42) fibril growth rate by interacting specifically with the growing fibrils. We probe this interaction with help of a purpose-developed Förster resonance energy transfer assay that monitors the proximity between a fibril-specific dye and fluorescent lipids in the lipid vesicle membrane. To further rationalise these findings we use mathematical models to fit the aggregation kinetics of Aβ(1-42) and find that lipid vesicles alter specific mechanistic steps in the aggregation reaction; they augment monomer-dependent secondary nucleation at the surface of existing fibrils and facilitate monomer-independent catalytic processes consistent with fibril fragmentation. We further show that DOPC vesicles have no effect on primary nucleation. This finding is consistent with experiments showing that Aβ(1-42) monomers do not directly bind to the lipid bilayer. Taken together, our results show that plain lipid membranes with charge and composition that is representative of outer cell membranes can significantly augment autocatalytic steps in the self-assembly of Aβ(1-42) into fibrils. This new insight suggests that strategies to reduce fibril-lipid interactions in the brain may have therapeutic value.

Interaction of Aβ(1-42) amyloids with lipids promotes "off-pathway" oligomerization and membrane damage

The toxicity of amyloids, as Aβ(1-42) involved in Alzheimer disease, is a subject under intense scrutiny. Many studies link their toxicity to the existence of various intermediate structures prior to fiber formation and/or their specific interaction with membranes. In this study we focused on the interaction between membrane models and Aβ(1-42) peptides and variants (L34T, mG37C) produced in E. coli and purified in monomeric form. We evaluated the interaction of a toxic stable oligomeric form (oG37C) with membranes as comparison. Using various biophysical techniques as fluorescence and plasmon waveguide resonance, we clearly established that the oG37C interacts strongly with membranes leading to its disruption. All the studied peptides destabilized liposomes and accumulated slowly on the membrane (rate constant 0.02 min(-1)). Only the oG37C exhibited a particular pattern of interaction, comprising two steps: the initial binding followed by membrane reorganization. Cryo-TEM was used to visualize the peptide effect on liposome morphologies. Both oG37C and mG37C lead to PG membrane fragmentation. The PG membrane promotes peptide oligomerization, implicated in membrane disruption. WT (Aβ(1-42)) also perturbs liposome organization with membrane deformation rather than disruption. For all the peptides studied, their interaction with the membranes changes their fibrillization process, with less fibers and more small aggregates being formed. These studies allowed to establish, a correlation between toxicity, fiber formation, and membrane disruption.

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

A hallmark of Alzheimer's disease, a late-onset neurodegenerative disease, is the deposition of neuritic amyloid plaques composed of aggregated forms of the β-amyloid peptide (Aβ). Aβ forms a variety of nanoscale, toxic aggregate species ranging from small oligomers to fibrils. Aβ and many of its aggregate forms strongly interact with lipid membranes, which may represent an important step in several toxic mechanisms. Understanding the role that specific regions of Aβ play in regulating its aggregation and interaction with lipid membranes may provide insights into the fundamental interaction between Aβ and cellular surfaces. We investigated the interaction and aggregation of several Aβ fragments (Aβ1-11, Aβ1-28, Aβ10-26, Aβ12-24, Aβ16-22, Aβ22-35, and Aβ1-40) in the presence of supported model total brain lipid extract (TBLE) bilayers. These fragments represent a variety of chemically unique domains within Aβ, that is, the extracellular domain, the central hydrophobic core, and the transmembrane domain. Using scanning probe techniques, we elucidated aggregate morphologies for these different Aβ fragments in free solution and in the presence of TBLE bilayers. These fragments formed a variety of oligomeric and fibrillar aggregates under free solution conditions. Exposure to TBLE bilayers resulted in distinct aggregate morphologies compared to free solution and changes in bilayer stability dependent on the Aβ sequence. Aβ10-26, Aβ16-22, Aβ22-35, and Aβ1-40 aggregated into a variety of distinct fibrillar aggregates and disrupted the bilayer structure, resulting in altered mechanical properties of the bilayer. Aβ1-11, Aβ1-28, and Aβ12-24 had minimal interaction with lipid membranes, forming only sparse oligomers.

Retardation of Abeta fibril formation by phospholipid vesicles depends on membrane phase behavior

An increasing amount of evidence suggests that in several amyloid diseases, the fibril formation in vivo and the mechanism of toxicity both involve membrane interactions. We have studied Alzheimer's disease related amyloid beta peptide (Abeta). Recombinant Abeta(M1-40) and Abeta(M1-42) produced in Escherichia coli, allows us to carry out large scale kinetics assays with good statistics. The amyloid formation process is followed in means of thioflavin T fluorescence at relatively low (down to 380 nM) peptide concentration approaching the physiological range. The lipid membranes are introduced in the system as large and small unilamellar vesicles. The aggregation lagtime increases in the presence of lipid vesicles for all situations investigated and the phase behavior of the membrane in the vesicles has a large effect on the aggregation kinetics. By comparing vesicles with different membrane phase behavior we see that the solid gel phase dipalmitoylphosphatidylcholine bilayers cause the largest retardation of Abeta fibril formation. The membrane-induced retardation reaches saturation and is present when the vesicles are added during the lag time up to the nucleation point. No significant difference is detected in lag time when increasing amount of negative charge is incorporated into the membrane.

Impact of phospholipid bilayer saturation on amyloid-beta protein aggregation intermediate growth: a quartz crystal microbalance analysis

Evidence that membrane-associated amyloid aggregate growth can impart membrane damage represents one possible mechanism for the neurodegeneration associated with deposited amyloid-beta protein (Abeta) aggregates in the brains of Alzheimer's disease (AD) patients. This potential pathogenic event necessitates an understanding of the impact that cellular membrane composition may have on Abeta aggregate growth. In the current study, a quartz crystal microbalance (QCM) was employed to examine the growth of Abeta(1-40) aggregation intermediates on supported phospholipid bilayers (SPBs) assembled at the crystal surface. These surface-specific measurements illustrate that zwitterionic SPBs selectively bind aggregated but not monomeric protein, and these bound aggregates are capable of supporting nonsaturable reversible growth via monomer addition. Growth-capable Abeta(1-40) aggregation intermediates more readily bind SPBs composed of phospholipids with a greater degree of carbon saturation. Furthermore, kinetic analysis afforded by the quantitative real-time QCM measurements reveals that SPBs with greater saturation also better support the growth of bound Abeta(1-40) aggregation intermediates as a result of the slower dissociation of bound monomer rather than more efficient recognition between aggregate and monomeric protein. These findings correlate with epidemiological and experimental evidence that links increased dietary intake of polyunsaturated fatty acids to a reduced risk of AD.

Kinetics of amyloid formation and membrane interaction with amyloidogenic proteins

Interest in amyloidogenesis has exploded in recent years, as scientists recognize the role of amyloid protein aggregates in degenerative diseases such as Alzheimer's and Parkinson's disease. Assembly of proteins or peptides into mature amyloid fibrils is a multistep process initiated by conformational changes, during which intermediate aggregation states such as oligomers, protofibrils, and filaments are sampled. Although once it was assumed that the mature fibril was the biologically toxic species, more recently it has been widely speculated that soluble intermediates are the most damaging. Because of its relevance to mechanism of disease, the paths traversed during fibrillogenesis, and the kinetics of the process, are of considerable interest. In this review we discuss various kinetic models used to describe amyloidogenesis. Although significant advances have been made, construction of rigorous, detailed, and experimentally validated quantitative models remains a work in progress. We briefly review recent literature that illustrates the interplay between kinetics and amyloid-membrane interactions: how do different intermediates interact with lipid bilayers, and how does the lipid bilayer affect kinetics of amyloidogenesis?