Relationship between beta amyloid peptide generating molecules and neprilysin in Alzheimer disease and normal brain

β-Amyloid peptide (Aβ) is generated by two cleavages of amyloid precursor protein (APP). The initial cleavage by BACE is followed by γ-secretase cleavage of the C-terminal APP fragment. Presenilin-1 (PS-1) is intimately related to γ-secretase. Once formed, Aβ is mainly broken down by neprilysin. To estimate vulnerability to Aβ senile plaque formation, we measured the relative mRNA levels of APP695, APP751, APP770, BACE, presenilin-1 (PS-1) and neprilysin in nine brain areas and in heart, liver, spleen and kidney in a series of Alzheimer disease (AD) and control cases. Each of the mRNAs was expressed in every tissue examined. APP695 was the dominant APP isoform in brain. Compared with controls, APP695 and PS-1 mRNA levels were significantly elevated in high plaque areas of AD brain, while neprilysin mRNA levels were significantly reduced. BACE levels were not significantly different in AD compared with control brain. In peripheral organs, there were no significant differences in any of the mRNAs between AD and control cases. APP isoforms were differently expressed in the periphery than in brain, with APP 751>770>695. Neprilysin mRNA levels were much higher, while APP695 and PS-1 mRNA levels were much lower in the periphery than in brain. The data suggest that, in the periphery, the capacity to degrade Aβ is srong, accounting for the failure of Aβ deposits to form. In plaque prone areas of AD brain, the capacity to degrade Aβ is weak, while the capacity to generate Ab is upregulated. In plaque resistant areas of brain, a closer balance exists, but there is some tendency towards lower degrading and higher synthesizing capacity in AD brain compared with control brain. Overall, the data indicate that effectiveness of degradation by neprilysin may be a key factor in determining whether Aβ deposits develop.