P4-070: Amyloid precursor-like proteins 1 and 2 influence proteolytic processing of the amyloid precursor protein by alpha- and beta-secretase

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The ectodomain of the amyloid precursor protein (APP) is shed by two different proteases called α– and β–secretase. α–secretase cleavage takes place at the plasma membrane and leads to the secretion of the neuroprotective sAPPα. In contrast, β–secretase cleavage occurs after APP is internalized from the plasma membrane into endosomes and β–secretase cleavage leads to the generation of the pathogenic Aβ peptide. The molecular mechanisms underlying the control of APP cleavage by either protease remain little understood. We found that the APP homolog APLP1 (amyloid precursor–like protein 1) influences APP shedding and this study aims to analyze the underlying molecular mechanism. In human embryonic kidney 293 cells expression of APLP1 strongly activated APP shedding by α–secretase and slightly reduced β–secretase cleavage. As revealed by domain deletion analysis, the increase in APP shedding required the GYENPTY amino acid motif within the cytoplasmic domain of APLP1. This motif is conserved among the APP family members and is essential for endocytosis and binding of APP to cytosolic interaction partners like the adaptor protein FE65. It is known that FE65 links APP to the LDL receptor–related protein (LRP), which is an endocytic receptor required for efficient internalization of APP. Using LRP knock–out cells, we found that APLP1 induces APP shedding only in the presence of LRP, suggesting that APLP1 influences the LRP–dependent APP endocytosis. In addition, an antibody uptake assay revealed that APLP1 reduced the rate of APP endocytosis. As a consequence more APP becomes available for α–cleavage at the plasma membrane. Our study provides a novel mechanism for APP shedding. We propose that proteins, such as APLP1, which can functionally replace APP in terms of complex formation with FE65 and LRP, affect APP endocytosis and shedding by α– and β–secretase. Moreover, we provide a model of how the different members of the APP gene family may mutually regulate their shedding.