APP overexpression in the absence of NPC1 exacerbates metabolism of amyloidogenic proteins of Alzheimer's disease.

ABSTRACT

Amyloid-ß (Aß) peptides originating from ß-amyloid precursor protein (APP) are critical in Alzheimer's disease (AD). Cellular cholesterol levels/distribution can regulate production and clearance of Aß peptides, albeit with contradictory outcomes. To better understand the relationship between cholesterol homeostasis and APP/Aß metabolism, we have recently generated a bigenic ANPC mouse line overexpressing mutant human APP in the absence of Niemann-Pick type C-1 protein required for intracellular cholesterol transport. Using this unique bigenic ANPC mice and complementary stable N2a cells, we have examined the functional consequences of cellular cholesterol sequestration in the endosomal-lysosomal system, a major site of Aß production, on APP/Aß metabolism and its relation to neuronal viability. Levels of APP C-terminal fragments (α-CTF/ß-CTF) and Aß peptides, but not APP mRNA/protein or soluble APPα/APPß, were increased in ANPC mouse brains and N2a-ANPC cells. These changes were accompanied by reduced clearance of peptides and an increased level/activity of γ-secretase, suggesting that accumulation of APP-CTFs is due to decreased turnover, whereas increased Aß levels may result from a combination of increased production and decreased turnover. APP-CTFs and Aß peptides were localized primarily in early-/late-endosomes and to some extent in lysosomes/autophagosomes. Cholesterol sequestration impaired endocytic-autophagic-lysosomal, but not proteasomal, clearance of APP-CTFs/Aß peptides. Moreover, markers of oxidative stress were increased in vulnerable brain regions of ANPC mice and enhanced ß-CTF/Aß levels increased susceptibility of N2a-ANPC cells to H2O2-induced toxicity. Collectively, our results show that cellular cholesterol sequestration plays a key role in APP/Aß metabolism and increasing neuronal vulnerability to oxidative stress in AD-related pathology.