Abstract

Production of amyloid β-protein (Aβ) is carried out by the membrane-embedded γ-secretase complex. Mutations in the transmembrane domain of amyloid β-protein precursor (APP) associated with early-onset familial Alzheimer's disease (FAD) can alter the ratio of aggregation-prone 42-residue Aβ (Aβ42) to 40-residue Aβ (Aβ40). However, APP substrate is proteolyzed processively by γ-secretase along two pathways: Aβ49→Aβ46→Aβ43→Aβ40 and Aβ48→Aβ45→Aβ42→Aβ38. Effects of FAD mutations on each proteolytic step are unknown, largely due to difficulties in detecting and quantifying longer Aβ peptides. To address this, we carried out systematic and quantitative analyses of all tri- and tetrapeptide coproducts from proteolysis of wild-type and 14 FAD-mutant APP substrates by purified γ-secretase. These small peptides, including FAD-mutant forms, were detected by tandem mass spectrometry and quantified by establishing concentration curves for each of 32 standards. APP intracellular domain (AICD) coproducts were quantified by immunoblot, and the ratio of AICD products corresponding to Aβ48 and Aβ49 was determined by mass spectrometry. Levels of individual Aβ peptides were determined by subtracting levels of peptide coproducts associated with degradation from those associated with production. This method was validated for Aβ40 and Aβ42 by specific ELISAs and production of equimolar levels of Aβ and AICD. Not all mutant substrates led to increased Aβ42/40. However, all 14 disease-causing mutations led to inefficient processing of longer forms of Aβ ≥ 45 residues. In addition, the effects of certain mutations provided insight into the mechanism of processive proteolysis: intermediate Aβ peptides apparently remain bound for subsequent trimming and are not released and reassociated.

Highlights

  • Cerebral plaques composed of the amyloid β-protein (Aβ) are a defining pathological feature of Alzheimer’s disease [1]

  • The relative effects of the familial Alzheimer’s disease (FAD) mutations compared with WT substrate on both Aβ40 and Aβ42 are closely similar between the two conditions

  • Aβ42 has been long accepted to be the pathogenic Aβ variant in Alzheimer’s disease. This particular Aβ variant is the major component of the cerebral extraneuronal amyloid plaques that characterize the disease, and FAD mutations in amyloid β-protein precursor (APP) and the presenilins can elevate the ratio of Aβ42 to Aβ40

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Summary

RESEARCH ARTICLE

Familial Alzheimer’s disease mutations in amyloid protein precursor alter proteolysis by γ-secretase to increase amyloid β-peptides of ≥45 residues. Wolfe1,* From the 1Department of Medicinal Chemistry, 2Mass Spectrometry Laboratory, University of Kansas, Lawrence, Kansas, USA

Edited by Paul Fraser
Results
Discussion
Total AICD
Experimental procedures
Mass spectrometric detection of AICD products
Immunoblotting of AICD products
Full Text
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