The effect of oxidative stress and sumoylation on APP processing in Alzheimer's disease

Alzheimer's disease (AD) is an age-associated dementia caused in part by neuronal death associated with amyloid beta (Abeta) deposition in brain parenchyma. Elucidating the steps in amyloid precursor protein (APP) processing into Abeta is key to understanding the pathogenesis of senile plaques and cerebral amyloid angiopathy (CAA), lesions that progressively accumulate in AD. The mediators of APP processing into Abeta or a non-amyloidogenic protein are: alpha-secretase (TACE), beta-secretase (BACE), and gamma-secretase. Oxidative stress induced by hydrogen peroxide treatment and sumoylation abnormally affect APP, BACE, TACE, and Presenilin-1 protein levels leading to the generation of Abeta. The effects of hydrogen peroxide on the processing of the amyloid precursor protein (APP) were studied to investigate the microvascular smooth muscle cells (MV-SMC) response to oxidative stress and its possible role in the pathogenesis of CAA. MV-SMCs exposed to hydrogen peroxide resulted in a decrease in mature cytoplasmic APP protein levels. We further examined the protein levels and activity of the APP processing enzymes alpha- and beta-secretase. Given that only beta-secretase levels changed in response to oxidative stress, we conclude that an increase in beta-secretase leads to a loss of mature APP as a result of beta-secretase processing. An increase in intracellular levels of Abeta, examined with laser scanning cytometry, demonstrated that APP decrease associated with oxidative stress resulted from increased processing of APP into Abeta. These results demonstrate that oxidative stress may contribute to increased Abeta deposition and MV-SMC cell death in CAA.; Sumoylation is a new post-translational modification system that controls stability, localization or activity of intracellular proteins. We show that hippocampal tissue microarrays (TMA) reveal a direct correlation between SUMO-1 immunopositive neurons and Abeta senile plaques in AD brain samples, while SUMO-2/3 immunopositive neurons exhibit a negative correlation with Abeta senile plaques. We further overexpressed SUMO-1, -2, or -3 in HEK-293T cells to determine the role of sumoylation in APP processing. Only SUMO-1 was shown to directly bind TACE, but SUMO-1, -2 and -3 caused an increase in both mature TACE and ADAM-10 protein levels and activity. Overexpressed SUMO increased the activity of BACE, which led to elevated levels of C99, the precursor to Abeta. We used immunoflourescence and laser scanning cytometry (LSC) to determine that there was an increase in intracellular levels of Abeta in HEK-293T cells with overexpressed SUMO. These findings suggest a strong link between sumoylation and APP processing and suggest that the sumoylation pathway may be critical in AD onset or progression.