Endogenous APP accumulates in synapses after BACE1 inhibition

• Published in: Neuroscience research 2016-08, Vol.109, p.9-15
• Main Authors: Nigam, Saket Milind, Xu, Shaohua, Ackermann, Frauke, Gregory, Joshua A., Lundkvist, Johan, Lendahl, Urban, Brodin, Lennart
• Format: Article
• Language: English
• Subjects: Amyloid beta-Protein Precursor - metabolism; Amyloid Precursor Protein Secretases - antagonists & inhibitors; Amyloid Precursor Protein Secretases - genetics; Animals; APP; Aspartic Acid Endopeptidases - antagonists & inhibitors; Aspartic Acid Endopeptidases - genetics; BACE1; Cells, Cultured; Immunocytochemistry; Mice, Inbred C57BL; Mice, Knockout; Morpholines - pharmacology; Neurons - drug effects; Neurons - metabolism; Proximity ligation assay; Pyrimidines - pharmacology; Rats, Sprague-Dawley; Synapse; Synapses - drug effects; Synapses - metabolism; Synaptic vesicle
• ISSN: 0168-0102; 1872-8111
• DOI: 10.1016/j.neures.2016.02.002

•N- and C-terminal APP epitopes were largely separated at wild-type synapses.•In BACE 1 knockout neurons N- and C-terminal epitopes showed an increased overlap.•The amount of full-length APP was increased in brains of APP knockout mice.•Proximity of the APP C-terminus with SV2 increased after BACE1 inhibition. BACE1-mediated cleavage of APP is a pivotal step in the production of the Alzheimer related Aβ peptide and inhibitors of BACE1 are currently in clinical development for the treatment of Alzheimer disease (AD). While processing and trafficking of APP has been extensively studied in non-neuronal cells, the fate of APP at neuronal synapses and in response to reduced BACE1 activity has not been fully elucidated. Here we examined the consequence of reduced BACE1 activity on endogenous synaptic APP by monitoring N- and C-terminal APP epitopes by immunocytochemistry. In control rodent primary hippocampal neuron cultures, labeling with antibodies directed to N-terminal APP epitopes showed a significant overlap with synaptic vesicle markers (SV2 or synaptotagmin). In contrast, labeling with antibodies directed to C-terminal epitopes of APP showed only a limited overlap with these proteins. In neurons derived from BACE1-deficient mice, and in control neurons treated with a BACE1 inhibitor, both the N-terminal and the C-terminal APP labeling overlapped significantly with synaptic vesicle markers. Moreover, BACE1 inhibition increased the proximity between the APP C-terminus and SV2 as shown by a proximity ligation assay. These results, together with biochemical observations, indicate that BACE1 can regulate the levels of full-length APP at neuronal synapses.