A novel Aβ isoform pattern in CSF reflects γ-secretase inhibition in Alzheimer disease

Abstract

IntroductionLY450139 (semagacestat) inhibits γ-secretase, a key enzyme for generation of amyloid β (Aβ), the peptide deposited in plaques in Alzheimer disease (AD). Previous data have shown that LY450139 lowers plasma Aβ, but has no clear effect on Aβ1-40 or Aβ1-42 levels in cerebrospinal fluid (CSF). By using targeted proteomics techniques, we recently identified several shorter Aβ isoforms, such as Aβ1-16, that in experimental settings increase during γ-secretase inhibitor treatment, and thus may serve as sensitive biochemical indices of the treatment effect. Here, we test the hypothesis that these shorter Aβ isoforms may be biomarkers of γ-secretase inhibitor treatment in clinical trials.MethodsIn a phase II clinical trial, 35 individuals with mild to moderate AD were randomized to placebo (n = 10) or LY450139 (100 mg (n = 15) or 140 mg (n = 10)) and underwent lumbar puncture at baseline and after 14 weeks of treatment. The CSF Aβ isoform pattern was analyzed with immunoprecipitation combined with MALDI-TOF mass spectrometry.ResultsThe CSF levels of Aβ1-14, Aβ1-15, and Aβ1-16 showed a dose-dependent increase by 57% and 74%, 21% and 35%, and 30% and 67%, respectively in the 100-mg and 140-mg treatment groups. Aβ1-40 and Aβ1-42 were unaffected by treatment.ConclusionsCSF Aβ1-14, Aβ1-15, and Aβ1-16 increase during γ-secretase inhibitor treatment in AD, even at doses that do not affect Aβ1-42 or Aβ1-40, probably because of increased substrate availability of the C99 APP stub (APP β-CTF) induced by γ-secretase inhibition. These Aβ isoforms may be novel sensitive biomarkers to monitor the biochemical effect in clinical trials.Trial registrationClinical Trials.gov NCT00244322

Site-specific characterization of threonine, serine, and tyrosine glycosylations of amyloid precursor protein/amyloid β-peptides in human cerebrospinal fluid

“…Altogether 33 different Aβ peptides were identified (SI Appendix, Table S1) some of which, e.g., Aβ1-15, previously identified as a positive biomarker of γ-secretase inhibition (22,23), constituted less than 1% of the total ion current chromatogram. No sign of any glycosylated derivatives of Aβ1-40 (SI Appendix, Fig.…”

Characterization of Intermediate Steps in Amyloid Beta (Aβ) Production under Near-native Conditions

“…Second, for the first time we show that ␥-secretase catalyzed processing at every third or fourth amino acid continues beyond A␤40 and A␤38 and we show how this mechanism results in major shorter A␤ peptides including A␤30, A␤31, A␤34, A␤37, and A␤39. Third, we identify A␤34 as a major convergence point for the product lines, which may also explain the particular sensitivity of this A␤ species in recent human trials with two different ␥-secretase inhibitors (19,24). Fourth, and perhaps most importantly, we identify cross-talks between the major A␤ product lines, and several novel reactions resulting in A␤46 -43, A␤42-40, A␤40 -38, and A␤40 -37 and that directly impact A␤40 and A␤42 production opening up new opportunities for modulation of A␤42 production and the A␤42/40 ratio.…”

“…Minor cleavage events result in cross-talks between the major routes and alternative pathways to modulate A␤42 production and the A␤42/40 production ratio (hatched arrows). The major processing routes converge at A␤34, which is further hydrolyzed by ␥-secretase resulting in A␤30, which is the shortest BACE and ␥-secretase-dependent A␤1-x peptide identified in human CSF (19). All peptides (arrows) are written in CàN-terminal direction.…”

Mechanism of protein phosphatase-2Aregulating phosphorylation of amyloid precursor proteosome and Aβ generation

“…PP-2A can reduce the phosphorylation level of APP and it was reported that the rise of the phosphorylation level of APP could increase the Aβ generated (26). Up to now, people still have not been well aware of the pathogenesis of AD but the majority of the scholars have stated that Aβ deposition was the common approach for all factors causing AD (27).…”