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.

ACBD3Acyl-CoA binding domain containing 3 ADBE Activity-dependent bulk endocytosis ADP Adenosine di-phosphate ALS Amyotrophic lateral sclerosis Accepted Article This article is protected by copyright. All rights reserved AMPA α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid AP-2 Adaptor protein complex 2 APP Amyloid-beta precursor protein ARF6 ADP-ribosylation factor 6 ARH Autosomal recessive hypercholesterolemia ARP2/3 Actin related protein 2/3 complex BACE1 Beta-secretase 1 BAR Bin1/amphiphysin/RVS167 BDNF Brain-derived neurotrophic factor BIN1 Bridging Integrator-1 BMP Bone morphogenetic protein CALM Clathrin assembly lymphoid myeloid leukemia cAMP Cyclic adenosine monophosphate CaV1 Caveolin 1 CD14 Cluster of differentiation 14 CDC42 Cell Division Cycle 42 CIE Clathrin-independent endocytosis CLC Clathrin light chain CLTCL1 Clathrin Heavy Chain Like 1 CME Clathrin -mediated endocytosis CNS Central nervous system CRMP2 Collapsin response mediator protein 2 DAB Disabled protein DAG Diacylglycerol DCX Doublecortin DNM2 Dynamin 2 EGF Epidermal growth factor EPS15 Epidermal growth factor receptor pathway substrate 15 ER Endoplasmic reticulum GABA Gamma aminobutyric acid Accepted Article This article is protected by copyright. All rights reserved GPCR G protein-coupled receptors GPR161 G protein-coupled receptor 161 GSK3β Glycogen synthase kinase 3 beta GTP Guanosine-5'-triphosphate ITSN Intersectin 1 L1-CAM Neural cell adhesion molecule L1 precursor LC3 Microtubule-associated proteins 1A/1B light chain 3 LDL Low-density lipoproteins LRP6 LDL receptor related protein 6 LTD Long-term depression LTP Long-term potentiation MAG Myelin-associated glycoprotein MOG Myelin oligodendrocyte glycoprotein mTOR Mechanistic target of rapamycin NAV1 Neuron navigator 1 NG2 Neural/glial antigen 2 NGF Nerve growth factor NMDA N-Methyl-D-aspartate NPC Neural progenitor cells OCRL Inositol polyphosphate 5-phosphatase

Disrupted neuronal plasticity due to subtle inflammation is considered to play a fundamental role in the pathogenesis of major depressive disorder. Interferon-α (IFN-α) potentiates immune responses against viral pathogens that induce toll-like receptor-3 (TLR3) activation but evokes severe major depressive disorder in humans by mechanisms that remain insufficiently described. By using a previously established mouse model of depression induced by combined delivery of IFN-α and polyinosinic:polycytidylic acid (poly(I:C)), a TLR3 agonist, we provide evidence that IFN-α and poly(I:C) reduce apical dendritic spine density in the hippocampal CA1 area ex vivo via mechanisms involving decreased TrkB signaling. In vitro, IFN-α and poly(I:C) treatments required neuronal activity to reduce dendritic spine density and TrkB signaling. The levels of presynaptic protein vesicular glutamate transporter (VGLUT)-1 and postsynaptic protein postsynaptic density-95 (PSD95) were specifically decreased, whereas the expression of both synaptic and extrasynaptic α-amino-3-hydroxy-5-methyl-4isoxazolepropionic acid receptor 1 (AMPAR1) was increased by IFN-α and poly(I:C) delivery. Patch clamp recordings in primary hippocampal neurons revealed that morphological changes at the synapse induced by IFN-α and poly(I:C) costimulation were accompanied by an increased action potential threshold and action potential frequency, indicative of impaired neuronal excitability. Taken together, IFN-α and poly(I:C) delivery leads to structural and functional alterations at the synapse indicating that compromised neuroplasticity may play an integral role in the pathogenesis of immune response-induced depression.