Soluble amyloid precursor protein α and ß in CSF in Alzheimer's disease.

OBJECTIVE: Cerebral accumulation of amyloid ß (Aß) is a pathological hallmark of Alzheimer's disease (AD). Proteolytic processing of amyloid precursor protein (APP) by α- or ß-secretase results in two soluble metabolites, sAPPα and sAPPß, respectively. However, previous data have shown that both α- and ß-secretase have multiple cleavage sites. The aim of this study was to characterize the C-termini of sAPPα and sAPPß in cerebrospinal fluid (CSF) by mass spectrometry (MS) and to evaluate whether different combinations of these fragments better separate between AD patients and controls by comparing two different sAPP immunoassays. METHODS: Using immunoprecipitation and high resolution MS, the APP species present in CSF were investigated. CSF levels of sAPPα and sAPPß from patients with AD (n=43) and from non-demented controls (n=44) were measured using AlphaLISA and MSD immunoassays that employ different antibodies for C-terminal recognition of sAPPα. RESULTS: Four different C-terminal forms of sAPP were identified, sAPPß-M671, sAPPß-Y681, sAPPα-Q686, and sAPPα-K687 (APP770 numbering). Neither immunoassay for the sAPP species could separate the two patient groups. The correlation (R(2)) between the two immunoassays was 0.41 for sAPPα and 0.45 for sAPPß. CONCLUSION: Using high resolution MS, we show here for the first time that sAPPα in CSF ends at Q686 and K687. The findings also support the conclusion from several previous studies that sAPPα and sAPPß levels are unaltered in AD.