Platelet-Derived Secreted Amyloid-Precursor Protein-β as a Marker for Diagnosing Alzheimer's Disease

A marker of Alzheimer's disease (AD) with a high sensitivity and specificity would facilitate a diagnosis at early stages. Blood platelets may be of particular interest in search of biomarkers, because they express amyloid-precursor protein (APP), and display a dysfunctional processing in AD. The aim of the present study is to establish and validate an assay for secreted amyloid-precursor protein (sAPP)-alpha and -beta in platelets of AD and mild cognitively impaired (MCI) subjects, compared to healthy young and old controls. Freshly isolated platelet extracts (25 mu g) were incubated with or without recombinant BACE1 (beta-site APP-Cleaving Enzyme; beta-secretase, 8U) at 37 degrees C and low pH and the levels of sAPP-alpha and sAPP-b were measured by specific ELISAs. Our data show that sAPP-alpha levels were not different between AD, MCI and control subjects. However, sAPP-beta levels in MCI and AD were significantly elevated relative to controls. When recombinant BACE1 was added, no changes were seen in sAPP-alpha levels, but the processed sAPP-beta levels were again markedly increased. The sAPP-beta processing was specific and selective after 2.5 hours at 37 degrees C, and was possibly mediated by exogenous BACE1, because it was blocked by a BACE1 inhibitor and BACE1 enzyme levels were enhanced in AD patients. Our data reveal that quantitive analysis of platelet sAPP-beta assay by ELISA may be a novel diagnostic biomarker for MCI and AD.