title: Loss of cleavage at β'-site contributes to apparent increase in β-amyloid peptide (Aβ) secretion by β-secretase (BACE1)-glycosylphosphatidylinositol (GPI) processing of amyloid precursor protein.
abstract: Several lines of evidence implicate lipid raft microdomains in Alzheimer disease-associated β-amyloid peptide (Aβ) production. Notably, targeting β-secretase (β-site amyloid precursor protein (APP)-cleaving enzyme 1 (BACE1)) exclusively to lipid rafts by the addition of a glycosylphosphatidylinositol (GPI) anchor to its ectodomain has been reported to elevate Aβ secretion. Paradoxically, Aβ secretion is not reduced by the expression of non-raft resident S-palmitoylation-deficient BACE1 (BACE1-4C/A (C474A/C478A/C482A/C485A)). We addressed this apparent discrepancy in raft microdomain-associated BACE1 processing of APP in this study. As previously reported, we found that expression of BACE1-GPI elevated Aβ secretion as compared with wild-type BACE1 (WTBACE1) or BACE1-4C/A. However, this increase occurred without any difference in the levels of APP ectodomain released following BACE1 cleavage (soluble APPβ), arguing against an overall increase in BACE1 processing of APP per se. Further analysis revealed that WTBACE1 cleaves APP at β- and β'-sites, generating +1 and +11 β-C-terminal fragments and secreting intact as well as N-terminally truncated Aβ. In contrast, three different BACE1-GPI chimeras preferentially cleaved APP at the β-site, mainly generating +1 β-C-terminal fragment and secreting intact Aβ. As a consequence, cells expressing BACE1-GPI secreted relatively higher levels of intact Aβ without an increase in BACE1 processing of APP. Markedly reduced cleavage at β'-site exhibited by BACE1-GPI was cell type-independent and insensitive to subcellular localization of APP or the pathogenic KM/NL mutant. We conclude that the apparent elevation in Aβ secretion by BACE1-GPI is mainly attributed to preferential cleavage at the β-site and failure to detect +11 Aβ species secreted by cells expressing WTBACE1.
authors.affiliation: Department of Neurobiology, The University of Chicago, Chicago, Illinois 60637, USA.
authors.forename: Kulandaivelu S
authors.initials: KS
authors.lastname: Vetrivel
authors.forename: Arghya
authors.initials: A
authors.lastname: Barman
authors.forename: Ying
authors.initials: Y
authors.lastname: Chen
authors.forename: Phuong D
authors.initials: PD
authors.lastname: Nguyen
authors.forename: Steven L
authors.initials: SL
authors.lastname: Wagner
authors.forename: Rajeev
authors.initials: R
authors.lastname: Prabhakar
authors.forename: Gopal
authors.initials: G
authors.lastname: Thinakaran
mesh_terms: D000595:Amino Acid Sequence; D053829:Amyloid Precursor Protein Secretases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D016229:Amyloid beta-Peptides / Q000737:chemistry / Q000378:metabolism*; D000818:Animals; D016282:Aspartic Acid Endopeptidases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D002462:Cell Membrane / Q000378:metabolism; D015971:Gene Expression Regulation, Enzymologic; D017261:Glycosylphosphatidylinositols / Q000378:metabolism*; D057809:HEK293 Cells; D006801:Humans; D051379:Mice; D056004:Molecular Dynamics Simulation; D008969:Molecular Sequence Data; D016296:Mutagenesis; D009154:Mutation; D010446:Peptide Fragments / Q000378:metabolism; D017434:Protein Structure, Tertiary; D012995:Solubility; D013379:Substrate Specificity
chemical_list: D016229:Amyloid beta-Peptides; D017261:Glycosylphosphatidylinositols; D010446:Peptide Fragments; D053829:Amyloid Precursor Protein Secretases; D016282:Aspartic Acid Endopeptidases; C509282:Bace1 protein, mouse