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

Senile plaques composed of amyloid-β protein (Aβ) 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 Aβ aggregates. The observations that microglia activation in vivo is selectively stimulated by distinct Aβ deposits led us to examine what specific form of Aβ is the most effective proinflammatory mediator in vitro. We report here that soluble prefibrillar species of Aβ(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 Aβ(1-42) in NaOH followed by dilution into F-12 media and isolation with size exclusion chromatography (SEC) revealed classic curvilinear β-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 Aβ(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 Aβ(1-40) protofibrils exhibited significantly less activity than concentration-matched Aβ(1-42). This report is the first to demonstrate microglial activation by SEC-purified protofibrils, and the overall findings indicate that small, soluble Aβ(1-42) protofibrils induce much greater microglial activation than mature insoluble fibrils.