Biophysical comparison of soluble amyloid-ß(1-42) protofibrils, oligomers, and protofilaments.

Some of the pathological hallmarks of the Alzheimer's disease brain are senile plaques composed of insoluble amyloid-ß protein (Aß) fibrils. However, much of the recent emphasis in research has been on soluble Aß aggregates in response to a growing body of evidence that shows that these species may be more neurotoxic than fibrils. Within this subset of soluble aggregated Aß are protofibrils and oligomers. Although each species has been widely investigated separately, few studies have directly compared and contrasted their physical properties. In this work, we examined well-recognized preparations of Aß(1-42) oligomers and protofibrils with multiangle (MALS) and dynamic (DLS) light scattering in line with, or following, size-exclusion chromatography (SEC). Multiple SEC-MALS analyses of protofibrils revealed molecular weight (Mw) gradients ranging from 200 to 2600 kDa. Oligomeric Aß species are generally considered to be a smaller and more nascent than protofibrils. However, oligomer Mw values ranged from 225 to 3000 kDa, larger than that for protofibrils. Root-mean-square radius (Rg) values correlated with the Mw trends with protofibril Rg values ranging from 16 to 35 nm, while oligomers produced one population at 40-43 nm with a more disperse population from 22 to 39 nm. Hydrodynamic radius (RH) measurements by DLS and thioflavin T fluorescence measurements indicated that protofibrils and oligomers had commonalities, yet electron microscopy revealed morphological differences between the two. SEC-purified Aß(1-42) monomer at lower concentrations was slower to nucleate but formed protofibrils (1500 kDa) or soluble protofilaments (3000 kDa) depending on the buffer type. The findings from these studies shed new light on the similarities and differences between distinct soluble aggregated Aß species.

Aß42 Protofibrils Interact with and Are Trafficked through Microglial-Derived Microvesicles.

Microvesicles (MVs) and exosomes comprise a class of cell-secreted particles termed extracellular vesicles (EVs). These cargo-holding vesicles mediate cell-to-cell communication and have recently been implicated in neurodegenerative diseases such as Alzheimer's disease (AD). The two types of EVs are distinguished by the mechanism of cell release and their size, with the smaller exosomes and the larger MVs ranging from 30 to 100 nm and 100 nm to 1 µm in diameter, respectively. MV numbers are increased in AD and appear to interact with amyloid-ß peptide (Aß), the primary protein component of the neuritic plaques in the AD brain. Because microglial cells play such an important role in AD-linked neuroinflammation, we sought to characterize MVs shed from microglial cells, better understand MV interactions with Aß, and determine whether internalized Aß may be incorporated into secreted MVs. Multiple strategies were used to characterize MVs shed from BV-2 microglia after ATP stimulation. Confocal images of isolated MVs bound to fluorescently labeled annexin-V via externalized phosphatidylserine revealed a polydisperse population of small spherical structures. Dynamic light scattering measurements yielded MV diameters ranging from 150 to 600 nm. Electron microscopy of resin-embedded MVs cut into thin slices showed well-defined uranyl acetate-stained ring-like structures in a similar diameter range. The use of a fluorescently labeled membrane insertion probe, NBD C6-HPC, effectively tracked MVs in binding experiments, and an Aß ELISA confirmed a strong interaction between MVs and Aß protofibrils but not Aß monomers. Despite the lesser monomer interaction, MVs had an inhibitory effect on monomer aggregation. Primary microglia rapidly internalized Aß protofibrils, and subsequent stimulation of the microglia with ATP resulted in the release of MVs containing the internalized Aß protofibrils. The role of MVs in neurodegeneration and inflammation is an emerging area, and further knowledge of MV interaction with Aß may shed light on extracellular spread and influence on neurotoxicity and neuroinflammation.

Isolated amyloid-ß(1-42) protofibrils, but not isolated fibrils, are robust stimulators of microglia.

Senile plaques composed of amyloid-ß protein (Aß) are an unshakable feature of the Alzheimer's disease (AD) brain. Although there is significant debate on the role of the plaques in AD progression, there is little disagreement on their role in stimulating a robust inflammatory response within the context of the disease. Significant inflammatory markers such as activated microglia and cytokines are observed almost exclusively surrounding the plaques. However, recent evidence suggests that the plaque exterior may contain a measurable level of soluble Aß aggregates. The observations that microglia activation in vivo is selectively stimulated by distinct Aß deposits led us to examine what specific form of Aß is the most effective proinflammatory mediator in vitro. We report here that soluble prefibrillar species of Aß(1-42) were better than fibrils at inducing microglial tumor necrosis factor α (TNFα) production in either BV-2 and primary murine microglia. Reconstitution of Aß(1-42) in NaOH followed by dilution into F-12 media and isolation with size exclusion chromatography (SEC) revealed classic curvilinear ß-sheet protofibrils 100 nm in length. The protofibrils, but not monomers, markedly activated BV-2 microglia. Comparisons were also made between freshly isolated protofibrils and Aß(1-42) fibrils prepared from SEC-purified monomer. Surprisingly, while isolated fibrils had a much higher level of thioflavin T fluorescence per mole, they were not effective at stimulating either primary or BV-2 murine microglia compared to protofibrils. Furthermore, SEC-isolated Aß(1-40) protofibrils exhibited significantly less activity than concentration-matched Aß(1-42). This report is the first to demonstrate microglial activation by SEC-purified protofibrils, and the overall findings indicate that small, soluble Aß(1-42) protofibrils induce much greater microglial activation than mature insoluble fibrils.