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

Amyloid-beta (A beta) peptides originating from beta-amyloid precursor protein (APP) are critical in Alzheimer's disease (AD). Cellular cholesterol levels/distribution can regulate production and clearance of A beta peptides, albeit with contradictory outcomes. To better understand the relationship between cholesterol homeostasis and APP/A beta 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 A beta production, on APP/A beta metabolism and its relation to neuronal viability. Levels of APP C-terminal fragments (alpha-CTF/beta-CTF) and A beta peptides, but not APP mRNA/protein or soluble APP alpha/APP beta, 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 gamma-secretase, suggesting that accumulation of APP-CTFs is due to decreased turnover, whereas increased A beta levels may result from a combination of increased production and decreased turnover. APP-CTFs and A beta 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/A beta peptides. Moreover, markers of oxidative stress were increased in vulnerable brain regions of ANPC mice and enhanced beta-CTF/A beta 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/A beta metabolism and increasing neuronal vulnerability to oxidative stress in AD-related pathology.