Amyloid [beta]-peptide and tau in the diagnosis and pathogenesis of Alzheimer's disease

Abstract

Alzheimer's disease (AD) is a devastating disorder, for which no cure yet has been found. The search pathogenic mechanisms and early diagnostic markers of the disease is in the focus of today's research. The hallmarks of AD pathology are senile plaques and neurofibrillary tangles, of which the main components are the amyloid [beta] (A[beta]) and tau proteins, respectively. The work presented in this thesis describes investigations of soluble A[beta] and tau in cerebrospinal fluid (CSF), blood plasma and conditioned media from cultured primary skin fibroblasts. Specifically, the main isoforms of the A[beta] peptide, A[beta]40 and A[beta]42, were studied.

The aim of the thesis was to study whether the levels of soluble A[beta] and tau could reflect the pathogenetic mechanisms in the brain, thus acting as markers of the disease process. Further, it was investigated whether levels of A[beta] and tau were affected by the Apolipoprotein E (APOE) genotype, as carriers of the APOE [epsilon]4 allele are at a higher risk of developing AD than non-carriers.

Papers I and II describe investigations of tau in CSF. It was concluded that tau is a potential diagnostic marker of AD, as AD exhibited increased levels compared to healthy controls, patients with other neurological diseases and patients with mild cognitive impairment. Increased tau levels were detected in presymptomatic. As well as in affected carriers of an AD-causing double mutation at codons 670/671 of the APP gene, suggesting that increased tau is an early event in AD pathogenesis. It was also shown that tau increased with disease progression in carriers of the APOE [epsilon]4 allele.

The main finding of paper III was that elevated levels of A[beta]42 in plasma and conditioned media from skin fibroblasts specifically differentiated carriers of the various APP, PSI and PS2 mutations from non-carriers. In addition, this was seen several years before predicted onset of disease. The increase was not present in plasma from sporadic AD. An important conclusion from this study was that A[beta]42 is a key protein in the pathogenesis of AD, as several different mutations in three genes all resulted in increased release of A[beta]42. The other major finding of paper III was the indication that a systemic increase of A[beta]42 is not necessary for development of AD pathogenesis, as this was not found in sporadic AD. The latter finding led to further investigations of A[beta]40 and 42 in CSF, which in contrast to blood, is in direct contact with the brain.

Paper IV describes the development of new sandwich ELISA systems for quantification of A[beta]40 and 42 in CSF. This work included thorough characterization of the antibodies. The epitopes were determined with mapping experiments and with immunoprecipitation-mass spectrometry. The antigen recognition properties were investigated with various experiments, including studies of synthetic peptides in different states of aggregation and peptides dissolved in CSF and water, stored under different conditions.

In papers V and VI it was shown that the levels of CSF A[beta]42 in sporadic and familial AD were increased in early stages of the disease, and that A[beta]42 decreased with disease progression. The hypothesis derived from the findings presented in papers III, V and VI was therefore that an early increase in A[beta]42 leads to early A[beta] deposition in the brain in AD, as reflected in decreasing CSF levels of A[beta]42. In papers V and VI, it was also shown that A[beta]42 was increased in other disease conditions affecting the CNS, suggesting increased A[beta]42 to be a marker of general cell damage rather than an AD specific feature. If the high levels of CSF A[beta]42 found in other conditions occurred relatively late, the decrease with disease progression, rather than the initial increase, would thereby be the AD-specific step.

A[beta] and tau are key proteins in AD pathogenesis, and it is of great importance to further elucidate their functions as well as the relationship between them. In addition, the mechanisms of APP processing need to be determined for a better understanding of the causes of the early increase of A[beta]42 in AD, as well as to further investigate the processes of A[beta] aggregation and plaque formation.