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.

Lipid membranes catalyse the fibril formation of the amyloid-β (1-42) peptide through lipid-fibril interactions that reinforce secondary pathways

Alzheimer's disease is associated with the aggregation of amyloid-β (Aβ) peptides into oligomers and fibrils. We have explored how model lipid membranes modulate the rate and mechanisms of Aβ(1-42) self-assembly, in order to shed light on how this pathological reaction may occur in the lipid-rich environments that the peptide encounters in the brain. Using a combination of in vitro biophysical experiments and theoretical approaches, we show that zwitterionic DOPC lipid vesicles accelerate the Aβ(1-42) fibril growth rate by interacting specifically with the growing fibrils. We probe this interaction with help of a purpose-developed Förster resonance energy transfer assay that monitors the proximity between a fibril-specific dye and fluorescent lipids in the lipid vesicle membrane. To further rationalise these findings we use mathematical models to fit the aggregation kinetics of Aβ(1-42) and find that lipid vesicles alter specific mechanistic steps in the aggregation reaction; they augment monomer-dependent secondary nucleation at the surface of existing fibrils and facilitate monomer-independent catalytic processes consistent with fibril fragmentation. We further show that DOPC vesicles have no effect on primary nucleation. This finding is consistent with experiments showing that Aβ(1-42) monomers do not directly bind to the lipid bilayer. Taken together, our results show that plain lipid membranes with charge and composition that is representative of outer cell membranes can significantly augment autocatalytic steps in the self-assembly of Aβ(1-42) into fibrils. This new insight suggests that strategies to reduce fibril-lipid interactions in the brain may have therapeutic value.