Enzymatic Characteristics of I213T Mutant Presenilin-1/γ-Secretase in Cell Models and Knock-in Mouse Brains FAMILIAL ALZHEIMER DISEASE-LINKED MUTATION IMPAIRS γ-SITE CLEAVAGE OF AMYLOID PRECURSOR PROTEIN C-TERMINAL FRAGMENT β

Abstract Presenilin (PS)/γ-secretase-mediated intramembranous proteolysis of amyloid precursor protein produces amyloid β (Aβ) peptides in which Aβ species of different lengths are generated through multiple cleavages at the γ-, ζ-, and ϵ-sites. An increased Aβ42/Aβ40 ratio is a common characteristic of most cases of familial Alzheimer disease (FAD)-linked PS mutations. However, the molecular mechanisms underlying amyloid precursor protein proteolysis leading to increased Aβ42/Aβ40 ratios still remain unclear. Here, we report our findings on the enzymatic analysis of γ-secretase derived from I213T mutant PS1-expressing PS1/PS2-deficient (PS–/–) cells and from the brains of I213T mutant PS1 knock-in mice. Kinetics analyses revealed that the FAD mutation reduced de novo Aβ generation, suggesting that mutation impairs the total catalytic rate of γ-secretase. Analysis of each Aβ species revealed that the FAD mutation specifically reduced Aβ40 levels more drastically than Aβ42 levels, leading to an increased Aβ42/Aβ40 ratio. By contrast, the FAD mutation increased the generation of longer Aβ species such as Aβ43, Aβ45, and >Aβ46. These results were confirmed by analyses of γ-secretase derived from I213T knock-in mouse brains, in which the reduction of de novo Aβ generation was mutant allele dose-dependent. Our findings clearly indicate that the mechanism underlying the increased Aβ42/Aβ40 ratio observed in cases of FAD mutations is related to the differential inhibition of γ-site cleavage reactions, in which the reaction producing Aβ40 is subject to more inhibition than that producing Aβ42. Our results also provide novel insight into how enhancing the generation of longer Aβs may contribute to Alzheimer disease onset.