in the mixed neuronal culture, the presence of inhibitory neurons balances neuronal activities, as seen under a physiological condition, making Aβ production more physiologically controlled. Second, in the pure excitatory neuron study, conditioned media containing 294 nM (10 µg/ml) of apoE3 or apoE4 were collected from transfected HEK293 cells and were then used to treat pure excitatory neurons in culture43. This apoE concentration is >800-folds higher than the physiological concentrations of apoE (~0.35 nM) in mouse brain ISF34. In addition, it has been reported that HEK293 cell-secreted apoE differs from astrocytes-secreted apoE, including their poor lipidation status, self-aggregation, and not binding well with the LDL receptor46. In our study, however, we used astrocyte-secreted apoE3 or apoE4 at concentrations (0.35 or 1.47 nM) similar to those in mouse brain ISF34.
