Probing APP Cleavage and Amyloid-β Assembly via Synthetic MUC-Type <i>O</i> -Glycosylated APP Glycopeptides

Aberrant proteolytic processing of amyloid precursor protein (APP) can alter amyloid-β (Aβ) peptide trafficking, with recent studies implicating MUC1-type <i>O</i>-glycosylation as a modulatory factor. In this study, we synthesized native and Swedish-mutated (Lys670Asn/Met671Leu) APP glycopeptides spanning the Aβ(1-23) region, including the β- and α-secretase cleavage sites, and introduced <i>O</i>-GalNAc moieties at Thr⁶⁶³, Ser⁶⁶⁷, and/or Tyr⁶⁸¹. Circular dichroism (CD) revealed conformational changes governed by the glycosylation site and glycan density. Increased glycan valency favored the stabilization of β-turn-rich structures typically associated with oligomeric and prefibrillar intermediates. The Swedish mutation enhanced β-secretase (BACE1) cleavage, especially when Ser⁶⁶⁷ was glycosylated, while additional glycans favored α-secretase (ADAM10) processing. However, this shift was not sufficient to counterbalance the amyloidogenic pathway. Similarly, Ser⁶⁶⁷ glycosylation promoted fibril formation in coincubation assays with Aβ40, while di- and triglycosylated peptides disrupted fibril architecture and favored oligomer formation, as confirmed by ThT kinetics, AFM/TEM imaging, and dynamic light scattering. These findings highlight the critical role of mutation and site-specific glycosylation in shaping APP proteolytic processing, secondary structure, and aggregation behavior, underscoring their importance for understanding APP function in both healthy and diseased states.