Evaluation of amyloid precursor protein and ß-amyloid as biomarkers for Alzheimer s disease

Abstract

Alzheimer s disease (AD) is a neurodegenerative disorder characterised by accumulations of neurofibrillary tangles and amyloid plaques. The major components of the amyloid deposits are the ß-amyloid (Aß) peptides derived from the amyloid precursor protein (APP). The subsequential cleavages by beta- and gamma-secretase of APP generate Aß and, simultaneously, soluble ß-APP (ß-sAPP). In contrast, alpha-secretase cleavage precludes Aß formation and generates soluble alpha-APP (alpha-sAPP). Abnormal processing of the amyloid precursor protein (APP) with deposition of Aß into senile plaques is regarded as the key pathogenic event in AD. However, evidence for this hypothesis comes almost exclusively from studies on the very rare familial form of AD with mutations in the APP or presenilin genes. In the sporadic form of AD, it has not been established whether APP/Aß overexpression/mismetabolism is a primary event leading to neuronal degeneration or whether neuronal degeneration leads to secondary disturbances in APP/Aß metabolism. The overall aim of the thesis was to increase our knowledge of APP processing in sporadic AD, by investigating APP metabolites in cerebrospinal fluid (CSF) and plasma with immunological analyses. To study the secreted forms of APP in CSF, enzyme-linked immunosorbent assay (ELISA) methods were developed for quantification of alpha- and ß-sAPP. Investigations of alpha- and ß-sAPP in sporadic AD show that there is no aberrant APP metabolism in these patients. In contrast, after severe traumatic brain injury (TBI) the levels of the secreted forms of APP and Aß peptides were found to be increased. The most frequent damage in TBI is axonal injury. Thus, increased APP/Aß expression may occur as a secondary phenomenon after TBI with axonal damage. The role of Aß as a peripheral marker for sporadic AD was investigated. After TBI, the plasma-Aß level remained unaltered, despite a marked increase of Aß in ventricular CSF. These results support the suggestion that plasma-Aß does not reflect Aß metabolism in the brain, and thus is not a suitable marker for AD. Also, the plasma levels of Aß were not changed after platelet activation. Thus, sampling techniques affecting platelet activation do not seem to confound Aß measurement. To study several biomarkers simultaneously, the xMAPTM technology was used to develop a multiparametric bead-based assay for quantification Aß(1-42), total tau and phosphorylated tau in CSF. These biomarkers have proven to be useful as aid for the clinical diagnosis of AD. The new method provides a reproducible and highly accurate system for distinguishing AD from other neurological disorders and/or controls. Thus, the new method can replace the corresponding ELISA methods.