PrP^C limits APP delivery to the plasma membrane and alters its processing to facilitate aβ release

Several lines of evidence suggest that the cellular isoform of prion protein (PrP^C) plays one or more roles in Alzheimer's Disease (AD). We previously found, in mouse neuroblastoma N2a cells that express human APP carrying the Swedish mutation (N2a-APPswe), that PrP^C expression correlated with the secretion of Aβ₄₂ peptide, the product of proteolytic processing of the amyloid precursor protein (APP) that is central to AD. To determine whether PrP^C modulates APP processing to affect Aβ₄₂ release and if any effect of PrP^C is selective for APPswe, we applied the MesoScale Discovery (MSD) platform to assess APP processing and Aβ secretion before and after siRNA-induced knockdown of PrP^C in APPswe and wild-type APP (APPwt) N2a cells. We found that PrP^C knockdown reduced the major isoforms of secreted Aβ peptides in both cell lines (with the exception of Aβ₄₂ in N2a-APPwt cells) in the absence of a reduction in either APP expression or enhanced Aβ degradation. Additionally, the soluble ectodomain of β-secretase cleavage (sAPPβ) was reduced in both cell lines whereas the ectodomain of α-secretase cleavage (sAPPα) was increased only in N2a-APPwt cells. PrP^C overexpression led to an increase in Aβ₄₂ in HEK cells and sAPPβ in both N2a cells and HEK cells expressing APPswe, confirming that PrP^C promotes the amyloidogenic processing pathway independent of cell type. Biotinylation and immunofluorescence studies in the N2a cell lines revealed an increase in APP labeling at the plasma membrane, an increase in APP endocytosis, and an accumulation of APP C-terminal fragments (CTFs) after PrP^C knockdown. We propose that PrP^C limits delivery of APP to the plasma membrane, which acts to promote the amyloidogenic pathway by increasing its exposure to BACE1 within early secretory compartments.