Studies of amyloid precursor protein trafficking and processing

Abstract: Alzheimer’s disease (AD) is the most common form of neurodegenerative disease that slowly impairs memory and thinking abilities. Abnormal accumulation and aggregation of amyloid β peptides (Aβ), derived from proteolytic cleavage of the amyloid precursor protein (APP), is thought to be one of the critical triggers in the pathogenesis. The trafficking and processing of APP is a complex process which can be regulated by many proteins. This thesis aims to expand our understanding of mechanisms that regulate processing and trafficking of APP. Study I aims to characterize the synaptic distribution of APP. We found that under normal conditions APP primarily distributed as fragments at synapses. Deletion of BACE1 led to an accumulation of APP C-terminal epitopes at synapses where APP N-terminal epitopes are present. A similar co-localization between APP N-terminals and C-terminals was also induced by pharmacological BACE1 inhibition. Moreover, proximity ligation assay revealed that the accumulation of APP C-terminals occurred in the presynaptic compartment. These results suggest that APP is primarily delivered to synapses as cleaved fragments, while full-length APP can be transported to the presynaptic compartment under conditions of reduced BACE1 activity. Study II aims to evaluate mechanisms for how exercise and BDNF regulate α-cleavage of APP. It has been suggested that regular exercise can reduce Aβ production. Moreover, exercise was suggested to enhance cognitive capacity, in which BDNF may contribute. We found that while the production of Aβ was reduced, the levels of sAPPα and BDNF were increased by exercise in a transgenic AD mouse model. Moreover, BDNF treatment was found to reduce Aβ production by regulating α-secretase activity. These results suggest that exercise and BDNF reduce Aβ production by enhancing α-secretase processing of APP. Study III and IV involve two members from the protein family of sorting nexins (SNXs), which have diverse functions in protein sorting and trafficking. Study III aims to elucidate the role of SNX3 in APP trafficking and processing as well as in Aβ production. In this study, the expression of SNX3 was manipulated by overexpression. We found that overexpression of SNX3 reduced the formation of Aβ. Moreover, SNX3 overexpression disturbed the association between APP and BACE1, and reduced APP internalization. While no change in BACE1 levels was found, APP levels were significantly decreased by SNX3 overexpression. These results suggest that overexpression of SNX3 decreases Aβ production by reducing internalization of APP. Study IV aims to test the involvement of SNX7 in Aβ production and APP processing. We found that overexpression of SNX7 decreased the levels of APP derived fragments, including Aβ. The cell surface and steady-state levels of APP, but not BACE1 were decreased by SNX7 overexpression. Moreover, the reduction of steady-state levels of APP was prevented by lysosomal inhibitors. Taken together, these results suggest that overexpression of SNX7 reduces Aβ production by enhancing lysosomal degradation of APP. In summary, we have examined mechanisms that regulate APP trafficking and processing, which may help develop novel therapeutic strategies for AD.