Excess APP O-glycosylation by GalNAc-T6 decreases Aβ production.

Alterations of the structure and/or amount of glycans present on proteins are associated with many diseases. We previously demonstrated that changes in N-glycans alter Aβ production. In the present study, we focused on the relationship between Alzheimer's disease (AD) and O-glycan, another type of glycan. The UDP-N-acetylgalactosamine:polypeptide N-acetylgalactosaminyltransferase (GalNAc-T) family functions in the first step of mucin-type O-glycan synthesis. Analysis of the expression of GalNAc-Ts in the human brain using real-time PCR revealed that the expression of several GalNAc-Ts was altered with sporadic AD progression. Three of these GalNAc-Ts (GalNAc-T1, GalNAc-T4 and GalNAc-T6) were transfected into HEK293T cells to examine their impact on Aβ production. Transfection of GalNAc-T6 significantly reduced both Aβ1-40 and Aβ1-42 generation, but GalNAc-T1 and GalNAc-T4 only reduced Aβ1-40 generation. Although these three GalNAc-Ts exhibited enzymatic activities on soluble amyloid precursor protein (APP), the GalNAc transferase activity of GalNAc-T6 to APP was most prominent. The expression of α-secretase and β-secretase was slightly altered in the transfected cells, but the activities of α-secretase and β-secretase were not significantly altered. These data suggest that excess O-glycosylation on APP by GalNAc-T6 inhibits Aβ production.