O-GalNAc glycosylation determines intracellular trafficking of APP and A production

A primary pathology of Alzheimer's disease (AD) is amyloid beta (A beta) deposition in brain parenchyma and blood vessels, the latter being called cerebral amyloid angiopathy (CAA). Parenchymal amyloid plaques presumably originate from neuronal A beta precursor protein (APP). Although vascular amyloid deposits' origins remain unclear, endothelial APP expression in APP knock -in mice was recently shown to expand CAA pathology, highlighting endothelial APP's importance. Furthermore, two types of endothelial APP-highly O-glycosylated APP and hypoO-glycosylated APP-have been biochemically identified, but only the former is cleaved for A beta production, indicating the critical relationship between APP O-glycosylation and processing. Here, we analyzed APP glycosylation and its intracellular trafficking in neurons and endothelial cells. Although protein glycosylation is generally believed to precede cell surface trafficking, which was true for neuronal APP, we unexpectedly observed that hypo-O-glycosylated APP is externalized to the endothelial cell surface and transported back to the Golgi apparatus, where it then acquires additional O-glycans. Knockdown of genes encoding enzymes initiating APP O-glycosylation significantly reduced A beta production, suggesting this nonclassical glycosylation pathway contributes to CAA pathology and is a novel therapeutic target.