The Concerted Regulation of Intracellular Signaling by Amyloid Precursor Protein and Aβ Peptide

It is widely accepted that A-beta (Aβ) generated from amyloid precursor protein (APP) oligomerizes and fibrillizes to form neuritic plaques in the Alzheimer’s disease (AD) brain, yet little is known about the contribution of APP preceding AD pathogenesis. Our data presented here suggest that APP has a functional role in cell cycle regulation and proliferation. First, we demonstrat that APP is pathologically phosphorylated at Thr668 and that P-APP localizes to the centrosomes. Furthermore, P-APP is proteolytically processed in a cell cycle -dependent manner to generate its pathogenic metabolites. Using Stable Isotope Labeling by Amino Acids in Culture (SILAC) and mass spectrometry analyses, we also show that expression of APP results in the expression of proliferation-associated proteins and the phosphorylation of proteins associated with cell cycle regulation and transcription. Here, we demonstrate that APP expression and oligomeric Aβ42 elicit Ras/ERK signaling cascade and glycogen synthase kinase3 (GSK3) activation. Both ERK and GSK3 are known to induce hyperphosphorylation of tau and of APP at Thr668, and our findings suggest that aberrant signaling by APP facilitates these events. Supporting this notion, analysis of human brain samples show increased expression of Ras, activation of GSK3 and phosphorylation of APP and tau, which correlate with Aβ levels in the AD brains. Furthermore, treatment of primary rat neurons with Aβ recapitulate these events and show enhanced Ras-ERK signaling, GSK3 activation, upregulation of cyclin D1, and phosphorylation of APP and tau. The finding that Aβ induces Thr668 phosphorylation on APP, which we show enhances APP proteolysis and Aβ generation, denotes a vicious feed-forward mechanism by which APP and Aβ promote tau hyperphosphorylation and neurodegeneration in AD. Based on these results we hypothesize that aberrant proliferative signaling by APP plays a fundamental role in AD neurodegeneration and an inhibition of this would impede the mitotic catastrophe and neurodegeneration observed in AD