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Endocytic traffic jam in Niemann Pick type C disease - a cause of increased Alzheimer's amyloid-β formation

Accumulation of cholesterol upon NPC1 dysfunction in lipid storage disorder Niemann Pick Type C (NPC) causes increased formation of intracellular amyloid-β peptide (Aβ), a causative factor of Alzheimer’s disease (AD), supporting a link between cholesterol and AD. We have recently shown that Aβ increase upon cholesterol accumulation in NPC model cells is due to increased β-secretase processing of APP caused by decreased APP expression at the cell surface (Malnar et al. BBA 2010). The goal of this work was to elucidate APP trafficking defect(s) leading to increased formation of Aβ in NPC cells. We hypothesize that increased Aβ generation upon NPC1 dysfunction is caused by cholesterol-mediated accumulation of APP within early endosome compartments. Endosome fractionation revealed increased APP C-terminal fragment (CTFs) localization within early and late endosome fractions in CHO-NPC1-/- (NPC) vs. CHOwt cells. APP accumulation in early but not in late endosomes was further confirmed by immunostaining and confocal microscopy which also showed increased APP colocalization with recycling endosomes in NPC vs. wt cells. We detected that APP accumulation within early endosomes in NPC cells is dependent on high cholesterol levels, since cholesterol depletion in NPC cells reversed altered APP trafficking to that as in wt cells. Furthermore, we observed decreased recycling of APP in NPC cells vs. CHOwt, supporting its accumulation within early/recycling endosomes upon NPC1 dysfunction. Since cholesterol accumulation upon NPC1 dysfunction has recently shown to cause increased autophagy via Beclin-1 expression, we tested whether blocking autophagy would reduce Aβ in NPC cells to the levels as in CHOwt. However, bafilomycin A-treatment did not lower Aβ levels in NPC cells, suggesting that increased autophagy is not involved in formation of Aβ in NPC cells. In summary, our results show that cholesterol accumulation upon NPC1 dysfunction causes decreased recycling of APP to the cells surface and its accumulation within early endosome compartments leading to increased formation of Alzheimer's amyloid-β peptide.

Alzheimer's disease; amyloid-beta; APP; BACE1; cholesterol; endocytosis; Niemann-Pick type C disease; NPC1

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