Cell surface proteoglycan-mediated uptake and accumulation of the Alzheimer's disease peptide Aβ(1-42)

Proteoglycans (PGs) have been found in Alzheimer's disease amyloid-β (Aβ) plaques and their glycosaminoglycan chains reportedly influence Aβ aggregation, neurotoxicity and intracellular accumulation in cell and animal models, but their exact pathophysiological role(s) remain unclear. We have studied the cellular uptake of fluorescently labelled Aβ(1-42) and Aβ(1-40) peptides in normal CHO cells (K1) and the mutant cell line (pgsA-745) which lacks all protein-attached heparan and chondroitin sulfate chains. After 24 h of incubation, CHO-K1 accumulates more Aβ(1-42) and Aβ(1-40) compared with CHO-pgsA-745, consistent with the suggested role of PGs in Aβ uptake. However, after short incubation times (≤3 h) there was no difference; moreover, the time evolution of Aβ(1-42) accumulation in CHO-K1 followed an unusual sigmoidal-like trend, indicating a possible involvement of PG-mediated peptide aggregation in Aβ endocytosis. Neither Aβ(1-42) nor Aβ(1-40) could stimulate uptake of a 10 kDa dextran (a general endocytosis marker) suggesting that Aβ-induced upregulation of endocytosis does not occur. CHO-K1 cells contained a higher number of Aβ(1-42)-positive vesicles, but the intensity difference per vesicle was only marginal suggesting that the superior accumulation of Aβ(1-42) stems from a higher number of endocytic events. FRET imaging support that intracellular Aβ(1-42) is aggregated in both cell types. We also report that CHO-pgsA-745 cells perform less endocytosis than CHO-K1 and, albeit this does not explain their difference in Aβ internalisation, we discuss a general method for data compensation. Altogether, this study contributes new insights into the mechanisms of PG-mediated Aβ uptake that may be relevant for our understanding of their role in AD pathology.