A study of b-secretase cleaved Alzheimer amyloid precursor protein

Sammanfattning: Alzheimer's disease (AD) is characterized by the degeneration and loss of neurons, intracellular neurofibrillary tangles and the accumulation of extracellular senile plaques consisting mainly of beta- amyloid (A-beta). A-beta is generated from the amyloid precursor protein (APP) through sequential cleavage by proteases P- and gamma-secretase. Alternatively, APP may be cleaved within the A-beta region by alpha-secretase, preventing intact A-beta formation. Both the alpha- and beta-secretase cleavages result in the release of large soluble APP fragments called alpha- or beta-sAPP, respectively. The work presented in this thesis describes the processing and secretion of differentially cleaved APP. The purpose of the study were to investigate the beta-secretase cleavage of APP. Paper 1 examined differentially cleaved APP as diagnostic markers for AD. It was concluded that soluble beta- secretase cleaved APP (beta-sAPP) levels in CSF do not change in AD, although soluble (alpha-secretase cleaved APP (alpha-sAPP) and total sAPP decreases. Paper 11 and 111 provided insights into the mechanisms of the alternative APP cleavages during apoptosis in two different cell systems. In a primary rat cortical culture system, calcium homeostasis and caspase actions proved to be important effectors of the beta-secretase cleavage. Expression of the Arctic APP mutation in human neuroblastoma cells increased the vulnerability to cell death and modified beta-sAPP secretion, stressing the role of FAD mutations in apoptosis and APP processing. The localization and content of beta-sAPP in brain was explored in paper IV. Altered beta-sAPP staining pattems indicated abnormal processing and transport of APP in AD brain. In summary, (i) beta-sAPP in CSF, (ii) beta-sAPP secretion from apoptotic neurons and (iii) beta-sAPP in brain were analysed. The results indicated that altered processing and transport of APP takes place in AD and during apoptosis. Since APP processing is considered a key event in the pathological cascade leading to AD, the proteases that cleave APP and the regulation mechanisms of those proteases are prime therapeutic targets.