title: Changed membrane integration and catalytic site conformation are two mechanisms behind the increased Aβ42/Aβ40 ratio by presenilin 1 familial Alzheimer-linked mutations.
abstract: The enzyme complex γ-secretase generates amyloid β-peptide (Aβ), a 37-43-residue peptide associated with Alzheimer disease (AD). Mutations in presenilin 1 (PS1), the catalytical subunit of γ-secretase, result in familial AD (FAD). A unifying theme among FAD mutations is an alteration in the ratio Aβ species produced (the Aβ42/Aβ40 ratio), but the molecular mechanisms responsible remain elusive. In this report we have studied the impact of several different PS1 FAD mutations on the integration of selected PS1 transmembrane domains and on PS1 active site conformation, and whether any effects translate to a particular amyloid precursor protein (APP) processing phenotype. Most mutations studied caused an increase in the Aβ42/Aβ40 ratio, but via different mechanisms. The mutations that caused a particular large increase in the Aβ42/Aβ40 ratio did also display an impaired APP intracellular domain (AICD) formation and a lower total Aβ production. Interestingly, seven mutations close to the catalytic site caused a severely impaired integration of proximal transmembrane/hydrophobic sequences into the membrane. This structural defect did not correlate to a particular APP processing phenotype. Six selected FAD mutations, all of which exhibited different APP processing profiles and impact on PS1 transmembrane domain integration, were found to display an altered active site conformation. Combined, our data suggest that FAD mutations affect the PS1 structure and active site differently, resulting in several complex APP processing phenotypes, where the most aggressive mutations in terms of increased Aβ42/Aβ40 ratio are associated with a decrease in total γ-secretase activity.
authors.affiliation: Department of NVS, Center for Alzheimer Research, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Johanna
authors.initials: J
authors.lastname: Wanngren
authors.affiliation: Department of Biochemistry & Biophysics, Stockholm University, Stockholm, Sweden.
authors.forename: Patricia
authors.initials: P
authors.lastname: Lara
authors.affiliation: Department of Biochemistry & Biophysics, Stockholm University, Stockholm, Sweden.
authors.forename: Karin
authors.initials: K
authors.lastname: Ojemalm
authors.affiliation: Department of NVS, Center for Alzheimer Research, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Silvia
authors.initials: S
authors.lastname: Maioli
authors.affiliation: Department of Biochemistry & Biophysics, Stockholm University, Stockholm, Sweden.
authors.forename: Nasim
authors.initials: N
authors.lastname: Moradi
authors.affiliation: Department of Biochemistry & Biophysics, Stockholm University, Stockholm, Sweden.
authors.forename: Lu
authors.initials: L
authors.lastname: Chen
authors.affiliation: Department of NVS, Center for Alzheimer Research, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Lars O
authors.initials: LO
authors.lastname: Tjernberg
authors.affiliation: AlzeCure, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Johan
authors.initials: J
authors.lastname: Lundkvist
authors.affiliation: Department of Biochemistry & Biophysics, Stockholm University, Stockholm, Sweden.
authors.forename: IngMarie
authors.initials: I
authors.lastname: Nilsson
authors.affiliation: Department of NVS, Center for Alzheimer Research, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Helena
authors.initials: H
authors.lastname: Karlström
keywords: AD, Alzheimer disease; AICD, amyloid precursor protein intracellular domain; APP, amyloid precursor protein; Alzheimer disease; Amyloid β-peptide; Aβ, amyloid-β peptide; BD8, blastocyst-derived embryonic stem cells; Bis-Tris, 2-(bis(2-hydroxyethyl)amino)-2-(hydroxymethyl)propane-1,3-diol; CHAPSO, 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonic acid; CRM, column-washed dog pancreas rough microsomes; CTF, C-terminal fragment; ER, endoplasmic reticulum; Endo H, endoglycosidase H; FAD, familial AD; FLIM/FRET, Fluorescence Lifetime Imaging/ Fluorescence Resonance Energy Transfer; GCB, γ-secretase inhibitor coupled to biotin; GVP, Gal4VP16; Lep, leader peptidase; MGD, minimal glycosylation distance; MSD, Meso Scale Discovery; Membrane integration; NTF, N-terminal fragment; PS, presenilin; Protein structure; RM, rough microsomes; TMD, transmembrane domains; WT, wild type; γ-Secretase