One of the hallmarks of
Alzheimer's disease is the accumulation of
senile plaques in brain, extracellular lesions comprised mostly of aggregates of the
amyloid beta-peptide (Abeta). Abeta is proteolytically derived from the Alzheimer's
amyloid precursor
protein (APP). The generation of Abeta and nonamyloidogenic derivatives of APP involves utilization of alternative processing pathways and multiple subcellular compartments. To improve our understanding of the regulation of APP processing, we investigated the effects of
wortmannin, a
phosphatidylinositol 3-kinase (PI3-kinase) inhibitor, on APP processing. PI3-kinases form a multifaceted family of
enzymes that represent converging points for multiple signal transduction pathways and also act as key regulators of vesicular trafficking. In N2a
neuroblastoma cells expressing either wild-type APP or the "Swedish" familial
Alzheimer's disease-associated mutant variant of APP,
wortmannin treatment resulted in decreased release of both Abeta and soluble APPalpha. In parallel, full-length APP and both processed derivatives accumulated inside the cells. These effects were not present at nanomolar concentrations of
wortmannin, but only at micromolar concentrations, implying the possible involvement of a recently described trans-Golgi network (TGN)-associated
PI3-kinase that is resistant to nanomolar concentrations of the inhibitor, but sensitive to micromolar concentrations. All effects were reversible when the
drug was removed from the cell culture medium. Given the suspected site of action of this novel
PI3-kinase activity at the TGN, it is tempting to speculate that the unexpected increase in the levels of both intracellular soluble APPalpha and intracellular Abeta might be due to
wortmannin-induced covesiculation of APP together with its respective
secretase enzymes within the TGN, leading to the execution of alpha-, beta-, and
gamma-secretase reactions.