P4–032: Preferential amyloidogenic processing of KPI–containing amyloid precursor protein isoforms in neuroblastoma cells

One of the major pathologies observed in the brains of Alzheimer's disease patients is the accumulation of senile plaques composed of amyloid β (Aβ) peptides. These peptides are produced through the sequential proteolysis of the amyloid precursor protein (APP) by β–secretase (BACE1) and γ–secretase. Alternatively, a third secretase known as α–secretase (an ADAM protease) can preclude Aβ peptide formation by cleaving APP within the Aβ region. Three alternatively spliced APP isoforms are expressed in the brain; APP695, APP751 and APP770. The larger two isoforms contain a Kunitz–like protease inhibitor (KPI) region, and APP770 also contains a short region known as the OX2 domain. Oxidative stress increases the levels of KPI–containing APP isoforms and there is increased expression of these isoforms in the brains of AD patients. To determine whether the KPI–containing isoforms of APP are the source of the majority of sAPPβ secreted from neuroblastoma cells. Neuroblastoma cell lines have been used to investigate potential differences in the proteolysis of the different APP isoforms. The mRNAs for all three isoforms were expressed at a similar level in SH–SY5Y cells, but differences in how the proteins of each isoform were processed was observed. There was no difference in the amount of α–secretase cleavage product (sAPPα) released from the different isoforms. However, significant differences were observed in the β–secretase cleavage of the different isoforms. Our data show that sAPPβ is almost completely derived from the larger isoforms of APP and suggests that increased expression of these isoforms in the brain may lead to enhanced levels of Aβ peptide production.