The exocyst is an insulin-sensitive regulator of amyloid precursor protein trafficking and amyloid-beta generation in neurons

Intracellular trafficking of amyloid precursor protein (APP) critically influences amyloidogenic processing, yet the mechanisms regulating this pathway remain incompletely defined. The exocyst is a highly conserved, insulin-responsive, eight-protein Rab effector complex that directs intracellular transport vesicle targeting and docking. We identified APP in a proteomics screen of neuronal cell surface proteins altered after chemical inhibition of exocyst activity. In SH-SY5Y cells expressing a mutant APP that enhances amyloidogenic processing, RNAi-mediated silencing of exocyst subunits significantly decreased sAPP and Aβ secretion, leading to significant intracellular APP accumulation. We found high-resolution co-localization of APP with exocyst subunits in soma and neurites of differentiated human SH-SY5Y neurons and mouse primary hippocampal neurons, and live-cell TIRF microscopy identified highly coordinated movement between fluorescently-tagged exocyst and APP proteins. These interactions were confirmed in these cells and in mouse brain histological sections by proximity ligation assays (PLAs) demonstrating close (<40nm) APP-EXOC5 association. To examine if exocyst activity in neurons is regulated by insulin, as it is in adipocytes and muscle, we generated a SH-SY5Y cell line with pHluorin-tagged GLUT4. Inhibition of the exocyst prevented exocytosis of GLUT4 to the plasma membrane in response to insulin. Additionally, using PLAs in mouse primary hippocampal neurons and SH-SY5Y neurons, we found that GLUT4-EXOC5 associations were increased by insulin signaling, but APP-EXOC5 associations were markedly reduced, indicating insulin-dependent retargeting of the exocyst complex away from APP+ vesicles towards GLUT+ vesicles. All together, these data identify the exocyst as a novel insulin-regulated mediator of neuronal APP trafficking and Aβ secretion.