Mucin-Type O-Glycosylation Proximal to beta-Secretase Cleavage Site Affects APP Processing and Aggregation Fate

The amyloid-beta precursor protein (APP) undergoes proteolysis by beta- and gamma-secretases to form amyloid-beta peptides (A beta), which is a hallmark of Alzheimer's disease (AD). Recent findings suggest a possible role of O-glycosylation on APP's proteolytic processing and subsequent fate for AD-related pathology. We have previously reported that Tyr(681)-O-glycosylation and the Swedish mutation accelerate cleavage of APP model glycopeptides by beta-secretase (amyloidogenic pathway) more than alpha-secretase (non-amyloidogenic pathway). Therefore, to further our studies, we have synthesized additional native and Swedish-mutated (glyco)peptides with O-GalNAc moiety on Thr(663) and/or Ser(667) to explore the role of glycosylation on conformation, secretase activity, and aggregation kinetics of A beta 40. Our results show that conformation is strongly dependent on external conditions such as buffer ions and solvent polarity as well as internal modifications of (glyco)peptides such as length, O-glycosylation, and Swedish mutation. Furthermore, the level of beta-secretase activity significantly increases for the glycopeptides containing the Swedish mutation compared to their nonglycosylated and native counterparts. Lastly, the glycopeptides impact the kinetics of A beta 40 aggregation by significantly increasing the lag phase and delaying aggregation onset, however, this effect is less pronounced for its Swedish-mutated counterparts. In conclusion, our results confirm that the Swedish mutation and/or O-glycosylation can render APP model glycopeptides more susceptible to cleavage by beta-secretase. In addition, this study sheds new light on the possible role of glycosylation and/or glycan density on the rate of A beta 40 aggregation.