Processing of amyloid precursor protein in human primary neuron and astrocyte cultures.

Increased production of amyloid beta peptide (A beta) is highly suspected to play a major role in Alzheimer's disease (AD) pathogenesis. Because A beta deposits in AD senile plaques appear uniquely in the brain and are fairly restricted to humans, we assessed amyloid precursor protein (APP) metabolism in primary cultures of the cell types associated with AD senile plaques: neurons, astrocytes, and microglia. We find that neurons secrete 40% of newly synthesized APP, whereas glia secrete only 10%. Neuronal and astrocytic APP processing generates five C-terminal fragments similar to those observed in human adult brain, of which the most amyloidogenic higher-molecular-weight fragments are more abundant. The level of amyloidogenic 4-kDa A beta exceeds that of nonamyloidogenic 3-kDa A beta in both neurons and astrocytes. In contrast, microglia make more of the smallest C-terminal fragment and no detectable A beta. We conclude that human neurons and astrocytes generate higher levels of amyloidogenic fragments than microglia and favor amyloidogenic processing compared with previously studied culture systems. Therefore, we propose that the higher amyloidogenic processing of APP in neurons and astrocytes, combined with the extended lifespan of individuals, likely promotes AD pathology in aging humans.