γ-Secretase Activating Protein (GSAP) regulates Amyloid Precursor Protein (APP) trafficking
Peng Xu1, Jerry Chang1, Eitan Wong2, Yue-Ming Li2, Paul Greengard1,*
1The Rockefeller University, 1230 York Avenue, New York, NY 10065, USA; 2Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York, NY 10065, USA
Amyloid precursor protein (APP) is processed sequentially in different subcellular compartments to form Amyloid-β (Aβ), Aβ is thought to play a central role in Alzheimer's disease (AD). Extensive studies have demonstrated that Aβ production can be regulated by modulating APP intracellular trafficking. Previously we identified γ-secretase activating protein (GSAP) as a key modulator for γ-secretase. GSAP specifically facilitates APP amyloidogenic processing without influencing Notch cleavage. Here, we demonstrate that GSAP also regulates APP trafficking.
The majority of APP resides within the Golgi. However, it has been proposed that cell surface APP is highly correlated with Aβ production. After GSAP depletion, we found that APP level is significantly reduced at the cell surface while protein levels of total APP as well as Golgi APP are not changed. To obtain better spatial and temporal resolution, we utilized advanced live cell imaging tools to investigate trafficking of APP in single vesicles. We observed that GSAP depletion affects APP vesicle movement at both cell surface and intracellular domains. APP vesicles ordinarily have a very confined movement. However, GSAP depletion reduces APP confinement and allows it to move more dynamically. Furthermore, we observed that GSAP co-localizes with APP-CTF and γ-secretase in the cholesterol-enriched detergent resistant membrane microdomain (DRM), which has been thought to be the key compartment for amyloidogenesis. APP phosphorylation plays an important role in regulating its association with DRM. We found that GSAP depletion affects the levels of phosphorylated APP as well as APP-CTF in the DRM fraction. Finally, we observed that GSAP is associated with a protein complex comprising APP and δCOP. Interestingly, our lab has demonstrated recently that APP trafficking and Aβ production are modulated by δCOP. In summary, the present work has demonstrated that GSAP associates with APP and regulates its trafficking to the DRM where it is converted to Aβ.