Analysis of Aβ cell interactions and uptake in hippocampal culture

In Alzheimer’s disease, an over-­accumulation of amyloid-­β (Aβ) protein causes amyloid plaque formation, neurodegeneration and cognitive decline by an unknown mechanism. In late-­onset Alzheimer’s disease, accumulation of Aβ may be due to defects in the normal clearance of the protein. To better understand how cells in the brain normally degrade Aβ, a comparative analysis of Aβ cell interaction and uptake was undertaken in cultures of primary hippocampal cells. Cultures were prepared from neonatal mice and incubated with fluorescein-­labelled human-­sequence Aβ for various times (0 -­ 24 hours) and concentrations (10 nM -­ 1 μM). The ability of cells to bind and internalise Aβ was assessed by live-­cell microscopy and by immunocytochemistry. Aβ was found to be rapidly internalised by microglia (n = 6 experiments). In these cells, Aβ was partially localised with early-endosomal markers EEA1 and Rab5, and with the lysosomal marker LAMP1 (n = 3 experiments). This suggests microglia may traffic Aβ to lysosomes for degradation. Aβ was found to bind rapidly to neurons in a punctate fashion, but Aβ was not internalised by neurons (n > 20 experiments). No co-­localisation of Aβ was observed with EEA1, Rab5 or LAMP1 in hippocampal neurons. Optical slicing and live-­cell washout experiments revealed that Aβ remained bound to the neuron surface for up to 18 hours of incubation. No binding or internalisation of Aβ was observed in astrocytes. The data suggest that Aβ has a strong affinity for primary hippocampal neurons, but most of the Aβ is not rapidly internalised. Under the same conditions Aβ was internalised by microglia, which may constitute a degradation pathway for Aβ.