title: Effects of membrane lipids on the activity and processivity of purified γ-secretase.
abstract: The 19-transmembrane multisubunit γ-secretase complex generates the amyloid β-peptide (Aβ) of Alzheimer's disease (AD) by intramembrane proteolysis of the β-amyloid precursor protein (APP). Despite substantial advances in elucidating how this protein complex functions, the effect of the local membrane lipid microenvironment on γ-secretase cleavage of substrates is still poorly understood. Using detergent-free proteoliposomes to reconstitute purified human γ-secretase, we examined the effects of fatty acyl (FA) chain length, saturation and double-bond isomerization, and membrane lipid polar headgroups on γ-secretase function. We analyzed γ-secretase activity and processivity [i.e., sequential cleavages in the APP transmembrane domain that convert longer Aβ species (e.g., Aβ(46)) into shorter ones (e.g., Aβ(40))] by quantifying the APP intracellular domain (AICD) and various Aβ peptides, including via a bicine/urea gel system that detects multiple Aβ lengths. These assays revealed several trends. (1) Switching from a cis to a trans isomer of a monounsaturated FA chain in phosphatidylcholine (PC) increased γ-activity, did not affect Aβ(42):Aβ(40) ratios, but decreased the ratio of long (≥42) versus short (≤41) Aβ peptides. (2) Increasing the FA carbon chain length (14, 16, 18, and 20) increased γ-activity, reduced longer Aβ species, and reduced the Aβ(42):Aβ(40) ratio. (3) Shifting the position of the double bond in 18:1(Δ9-cis) PC to the Δ6 position substantially reduced activity. (4) Gangliosides increased γ-activity but decreased processivity, thus elevating the Aβ(42):Aβ(40) ratio. (5) Phosphatidylserine decreased γ-activity but increased processivity. (6) Phosphatidylinositol strongly inhibited γ-activity. Overall, our results show that subtle changes in membrane lipid composition can greatly influence γ-secretase activity and processivity, suggesting that relatively small changes in lipid membrane composition may affect the risk of AD at least as much as presenilin or APP mutations do.
authors.affiliation: Center for Neurologic Diseases, Brigham and Women's Hospital, Boston, Massachusetts 02115, United States.
authors.forename: Oliver
authors.initials: O
authors.lastname: Holmes
authors.forename: Swetha
authors.initials: S
authors.lastname: Paturi
authors.forename: Wenjuan
authors.initials: W
authors.lastname: Ye
authors.forename: Michael S
authors.initials: MS
authors.lastname: Wolfe
authors.forename: Dennis J
authors.initials: DJ
authors.lastname: Selkoe
mesh_terms: D053829:Amyloid Precursor Protein Secretases / Q000302:isolation & purification* / Q000378:metabolism*; D016229:Amyloid beta-Peptides / Q000737:chemistry / Q000378:metabolism; D000818:Animals; D016466:CHO Cells; D006224:Cricetinae; D004789:Enzyme Activation; D005227:Fatty Acids / Q000737:chemistry; D006801:Humans; D008051:Lipid Bilayers / Q000737:chemistry; D008563:Membrane Lipids / Q000737:chemistry* / Q000378:metabolism; D010446:Peptide Fragments / Q000737:chemistry / Q000378:metabolism; D011499:Protein Processing, Post-Translational; D059748:Proteolysis; D013379:Substrate Specificity
chemical_list: D016229:Amyloid beta-Peptides; D005227:Fatty Acids; D008051:Lipid Bilayers; D008563:Membrane Lipids; D010446:Peptide Fragments; C078592:amyloid beta-protein (1-40); C538897:amyloid beta-protein (40-42); D053829:Amyloid Precursor Protein Secretases