APP SUMOylation prevents BACE1 cleavage of APP and increases BACE1 degradation to promote the nonamyloidogenic pathway

Amyloid precursor protein (APP) plays a key role in the pathogenesis of Alzheimer’s disease (AD). APP undergoes different posttranslational modifications, but the role of SUMOylation modification of APP in the pathogenesis of AD is not known. The molecular mechanism and functional significance of APP SUMOylation have not been studied either. Using in vitro SUMOylation assay, plasmid DNA transfection and lentiviral vector transduction to the mouse hippocampus, we have found that APP is SUMO-modified by Ubc9 at Lys-587 and Lys-595 in the hippocampus endogenously. APP SUMOylation decreases the association between APP and β-secretase (BACE1), reduces amyloid-beta (Aβ), sAPPβ and BACE1 expression, but increases sAPPα expression in APP/PS1 mice. APP SUMOylation also facilitates the degradation of BACE1. Lenti-EGFP-SUMO1 vector transduction to APP/PS1 mice rescues spatial memory and recognition memory deficits, decreases the amount of Aβ and the accumulation of amyloid plaque compared with APP/PS1 mice receiving Lenti-EGFP vector transduction, whereas Lenti-EGFP-SUMO1ΔGG mutant vector transduction to APP/PS1 mice produces an opposite effect for these measures. Melatonin increases Ubc9 expression and enhances APP SUMOylation. In addition, blockade of APP phosphorylation at Thr-668 facilitates APP SUMOylation. These results together suggest that APP SUMOylation promotes the nonamyloidogenic processing of APP and functions as an endogenous protection mechanism against Aβ toxicity. Further, melatonin is an endogenous stimulus that enhances APP SUMOylation.