AP‐2 reduces amyloidogenesis by promoting BACE1 trafficking and degradation in neurons

Cleavage of amyloid precursor protein (APP) by BACE‐1 (β‐site APP cleaving enzyme 1) is the rate‐limiting step in amyloid‐β (Aβ) production and a neuropathological hallmark of Alzheimer's disease (AD). Despite decades of research, mechanisms of amyloidogenic APP processing remain highly controversial. Here, we show that in neurons, APP processing and Aβ production are controlled by the protein complex‐2 (AP‐2), an endocytic adaptor known to be required for APP endocytosis. Now, we find that AP‐2 prevents amyloidogenesis by additionally functioning downstream of BACE1 endocytosis, regulating BACE1 endosomal trafficking and its delivery to lysosomes. AP‐2 is decreased in iPSC‐derived neurons from patients with late‐onset AD, while conditional AP‐2 knockout (KO) mice exhibit increased Aβ production, resulting from accumulation of BACE1 within late endosomes and autophagosomes. Deletion of BACE1 decreases amyloidogenesis and mitigates synapse loss in neurons lacking AP‐2. Taken together, these data suggest a mechanism for BACE1 intracellular trafficking and degradation via an endocytosis‐independent function of AP‐2 and reveal a novel role for endocytic proteins in AD.