Identification of novel N-terminal fragments of amyloid precursor protein in cerebrospinal fluid

Alzheimer's disease (AD) is a progressive neurodegenerative disorder of the central nervous system. Two pathological hallmarks in the brain of AD patients are neurofibrillary tangles and senile plaques. The plaques consist mainly of β-amyloid (Aβ) peptides that are produced from the amyloid precursor protein (APP), by sequential cleavage by β- and γ-secretase. Most previous studies have been focused on the C-terminal fragments of APP, where the Aβ sequence is localized. The purpose of this study was to search for N-terminal fragments of APP in cerebrospinal fluid (CSF) using mass spectrometry (MS). By using immunoprecipitation (IP) combined with matrix-assisted laser desorption/ionization time-of-flight MS as well as nanoflow liquid chromatography coupled to high resolution tandem MS we were able to detect and identify six novel N-terminal APP fragments [APP(18–119), APP(18–121), APP(18–122), APP(18–123), APP(18–124) and APP(18–126)], having molecular masses of approximately 12 kDa. The presence of these APP derivatives in CSF was also verified by Western blot analysis. Two pilot studies using either IP-MS or Western blot analysis indicated slightly elevated levels of N-terminal APP fragments in CSF from AD patients compared with controls, which are in need of replications in independent and larger patient materials.