PrPC 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 (PrPC) 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 PrPC 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 PrPC modulates APP processing to affect Aβ42 release and if any effect of PrPC 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 PrPC in APPswe and wild-type APP (APPwt) N2a cells. We found that PrPC 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. PrPC overexpression led to an increase in Aβ42 in HEK cells and sAPPβ in both N2a cells and HEK cells expressing APPswe, confirming that PrPC 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 PrPC knockdown. We propose that PrPC 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.