Alzheimer Presenilin-1 Mutations Dramatically Reduce Trimming of Long Amyloid β-Peptides (Aβ) by γ-Secretase to Increase 42-to-40-Residue Aβ

The presenilin-containing gamma-secretase complex produces the amyloid beta-peptide (A beta) 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 A beta critical to pathogenesis, but other A beta s of 38 - 49 residues are also formed by gamma-secretase. Evidence in cells suggests the protease first cleaves substrate within the transmembrane domain at the E site to form 48- or 49-residue A beta. Subsequent cleavage almost every three residues from the C terminus is thought to occur along two pathways toward shorter secreted forms of A beta: A beta 49 -> A beta 46 -> A beta 43 -> A beta 40 and A beta 48 -> A beta 45 -> A beta 42 -> A beta 38. Here we show that the addition of synthetic long A beta peptides (A beta 45- 49) directly into purified preparations of gamma-secretase leads to the formation of A beta 40 and A beta 42 whether the protease complex is detergent-solubilized or reconstituted into lipid vesicles, and the ratios of products A beta 42 to A beta 40 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 A beta peptides to A beta 40 and A beta 42 by mutant proteases occurs at multiple levels, independent of the effects on initial endoproteolysis at the E site, all conspiring to increase the critical A beta 42/A beta 40 ratio implicated in A beta pathogenesis. Taken together, these results suggest that specific reduction of carboxypeptidase function of gamma-secretase leads to the gain of toxic A beta 42/A beta 40.