Distribution and pathophysiological role of amyloid precursor protein and presenilin 1 : characterization in rats and in vitro studies on the pathogenic arctic mutation

Abstract

Alzheimer's disease is the most common form of dementia that presents a growing dilemma worldwide. Alzheimer's disease is neuropathologically characterized by cerebral atrophy accompanied by deposition of senile plaques and neurofibrillary tangles. The plaques are primarily composed of the amyloid beta peptide (AP), which is produced following processing of the amyloid precursor protein (APP) by the proteases beta- and gamma-secretase. A familial link can be found in approximately half of all Alzheimer's disease cases. In a few families the disease is caused by pathogenic mutations located on three different genes encoding presenilin (PS) 1, PS2 and APP.

PS is an important component of the complex executing gamma-secretase processing of APP. A detailed characterization of the distribution of PS1 mRNA revealed that PS1 was found both in neurons and peripheral tissues such as testis, kidney, spleen, adrenal gland and thymus. This distribution indicated that the function of PS1 was likely also systemic, and not only found in processes involved with Alzheimer's disease. This idea has been confirmed more recently with the realization that the proteolytic activities of the gamma-secretase complex act on both Notch signaling pathways as well as APP processing.

Neuronal cell-death in Alzheimer's disease has been suggested to involve both inflammatory and apoptotic reactions. One way of investigating apoptotic cell death in the hippocampus, a region gravely affected in AD, is by treating rats with the neurotoxic agent trimethyltin (TMT). Using this model, hippocampal neurons may be studied in order to investigate relationships in the neurodegenerative pathway. Following TMT-induced intoxication, a significant reduction of mRNA encoding the APP isoform consisting of 695 amino acids (APP695) was observed. In contrast mRNA encoding APP containing a Kuniz protease inhibitor domain (APP-KPI) as well as PS1 were unaltered despite neuronal loss. This might be explained by mRNA expression in invading astrocytes. One week after TMT-treatment, neuronal cells died by apoptosis. In addition, interleukin 1beta (IL-1beta), an inflammatory marker, could be detected indicating dual effects of TMT on cell-death involving both apoptosis and necrosis.

A novel mutation (E693G - the Arctic mutation) was identified in a Swedish family with Alzheimer's disease. Plasma AP levels were reduced in Arctic mutation carriers. Levels of A beta were also reduced in media from Arctic transfected cells. This is contrast to the effects seen with other Alzheimer's disease mutations, which all increase AP production. Biochemical characterization of the Arctic peptide revealed that Arctic AP displayed altered kinetic properties leading to the formation of more stable protofibrils, an intermediary species in the fibrillization pathway. Further investigations revealed that Arctic APP was processed differently compared to wt APP. The (alpha secretase cleavage was reduced and the beta-secretase cleavage increased. The high beta-secretase processing indicates an increased AP production, contradictory to previous findings on AP levels in plasma and media. One possible explanation could be that A beta with the Arctic substitution rapidly forms protofibrils. The protofibrils may be retained within the cells causing intracellular accumulation of AP and as a consequence less AP will be secreted from the cells. An increased intracellular load of AP, would lead to neuronal cell stress eventually resulting in neurodegeneration. This was also seen in stably transfected SH-SY5Y cells. Arctic APP reduced the viability of the cells, which underwent apoptotic cell-death. In addition, cell death inducers further aggravated the apoptotic effects of the Arctic mutation.

Protofibril formation is central in the pathogenic process leading to Alzheimer's disease in the Arctic family. Protofibrils have also been suggested to be involved in the pathogenic events leading to Alzheimer's disease in non-familial cases. The Arctic mutation could therefore be a good model for protofibrillar disease and a valuable tool for future drug design.