P1‐322: Differential uptake of amyloid‐β soluble oligomers and fibrils by human microglia

The presence of activated microglia is a constant feature of fibrillar amyloid-β (fAβ) deposits in Alzheimer's disease (AD). In vivo experiments at our laboratory suggest differential response of microglia to various conformations of Aβ. In support of this suggestion, we have observed substantially greater production of reactive oxygen species in cultured human microglia exposed to fAβ when compared to soluble Aβ oligomers (oAβ). In the present experiment we investigated potential differences in microglial response to fAβ and oAβ using another primary function of human microglia, namely phagocytosis. Primary microglia were obtained from postmortem human brains and cultured in media supplemented with GM-CSF. Cultures derived from five normal or AD cases were treated with various concentrations of 5-FAM labeled oAβ or fAβ. Uptake of the two species of Aβ was monitored at different time-points. Optical density of 5-FAM fluorescence resulting from Aβ phagocytosis was determined using Image Pro Plus software. oAβ and fAβ prepared using 5-FAM labeled peptide retained appropriate conformations and biological activity as measured by production of reactive oxygen species by microglia. Both oAβ and fAβ were bound to microglia membranes. While oAβ appeared to bind across the entire cell membrane, fAβ aggregates remained clustered at infrequent sites. oAβ was readily phagocytosed by human microglia in a dose and time dependent manner. fAβ appeared to remain on microglia membranes and was phagocytosed at a considerably slower rate. Optical density measurements of 5-FAM labeled Aβ revealed significantly greater oAβ uptake by human microglia after four hours of incubation when compared to fAβ (p < 0.0001). These observations demonstrate substantially greater ability of human microglia to phagocytose oAβ when compared to fAβ. They support our earlier observations demonstrating differential response of microglia to various conformations of Aβ. While microglia appear to exhibit a pro-inflammatory and potentially destructive response to fAβ, they exhibit a protective response to oAβ by phagocytosis and clearance of this conformation from the brain.