The
presenilin-containing γ-
secretase complex produces the
amyloid β-
peptide (Aβ) through intramembrane proteolysis, and >100
presenilin mutations are associated with familial
early-onset Alzheimer disease (AD). The question of whether these mutations result in AD through a gain or a loss of function remains highly controversial. Mutations in
presenilins increase ratios of 42- to 40-residue Aβ critical to pathogenesis, but other Aβs of 38-49 residues are also formed by γ-
secretase. Evidence in cells suggests the
protease first cleaves substrate within the transmembrane domain at the ϵ site to form 48- or 49-residue Aβ. Subsequent cleavage almost every three residues from the C terminus is thought to occur along two pathways toward shorter secreted forms of Aβ: Aβ49 → Aβ46 → Aβ43 → Aβ40 and Aβ48 → Aβ45 → Aβ42 → Aβ38. Here we show that the addition of synthetic long Aβ
peptides (Aβ45-49) directly into purified preparations of γ-
secretase leads to the formation of Aβ40 and Aβ42 whether the
protease complex is
detergent-solubilized or reconstituted into
lipid vesicles, and the ratios of products Aβ42 to Aβ40 follow a pattern consistent with the dual-pathway hypothesis. Kinetic analysis of five different AD-causing mutations in
presenilin-1 revealed that all result in drastic reduction of normal
carboxypeptidase function. Altered trimming of long Aβ
peptides to Aβ40 and Aβ42 by mutant
proteases occurs at multiple levels, independent of the effects on initial endoproteolysis at the ϵ site, all conspiring to increase the critical Aβ42/Aβ40 ratio implicated in AD pathogenesis. Taken together, these results suggest that specific reduction of
carboxypeptidase function of γ-
secretase leads to the gain of toxic Aβ42/Aβ40.