Alzheimer's disease (AD) is characterized pathologically by amyloid beta (A beta)-containing plaques. Generation of A beta from amyloid precursor protein (APP) by two enzymes, beta- and gamma-secretase, has therefore been in the AD research spotlight for decades. Despite this, how the physical interaction of APP with the secretases influences APP processing is not fully understood. Herein, we compared two genetically identical human iPSC-derived neuronal cell types: low A beta-secreting neuroprogenitor cells (NPCs) and high A beta-secreting mature neurons, as models of low versus high A beta production. We investigated levels of substrate, enzymes and products of APP amyloidogenic processing and correlated them with the proximity of APP to beta- and gamma-secretase in endo-lysosomal organelles. In mature neurons, increased colocalization of full-length APP with the beta-secretase BACE1 correlated with increased beta-cleavage product sAPP beta. Increased flAPP/BACE1 colocalization was mainly found in early endosomes. In the same way, increased colocalization of APP-derived C-terminal fragment (CTF) with presenilin-1 (PSEN1), the catalytic subunit of gamma-secretase, was seen in neurons as compared to NPCs. Furthermore, most of the interaction of APP with BACE1 in low A beta-secreting NPCs seemed to derive from CTF, the remaining APP part after BACE1 cleavage, indicating a possible novel product-enzyme inhibition. In conclusion, our results suggest that interaction of APP and APP cleavage products with their secretases can regulate A beta production both positively and negatively. beta- and gamma-Secretases are difficult targets for AD treatment due to their ubiquitous nature and wide range of substrates. Therefore, targeting APP-secretase interactions could be a novel treatment strategy for AD.
