P4-265: BACE1 inhibition reduces Abeta production and alters APP processing in non-transgenic mice

β–site APP–cleaving enzyme 1 (BACE1) is a key enzyme initiating the generation of amyloid β (Aβ) peptides from amyloid precursor protein (APP); thus BACE1 is an attractive therapeutic target for Alzheimer's disease (AD). The effect of BACE1 inhibition has been conventionally tested in transgenic mice expressing familial AD (FAD) mutation. However, sporadic late–onset AD, the highest frequent type of AD, has no genetic mutation, and there is no evidence suggesting elevated APP expression in sporadic AD cases. Therefore, testing BACE1 therapeutic agents in APP transgenic mice, especially APP–β–site mutant, may not be relevant. We aimed to examine the feasibility of evaluation of BACE1 therapeutic agents using non–transgenic (wild type) mice. Moreover, we determined how pharmacological BACE1 inhibition alters Aβ production and APP processing in non–transgenic mice. We used a newly developed mouse Aβ ELISA, which utilizes N terminus (Aβ 1–4) and C terminus (Aβ 35–40) specific antibodies, clones 14F1 and 1A10, respectively. This ELISA detects full–length mouse Aβ 1–40 at the sub nM range, which is approximately 1/10th of the endogenous Aβ levels in non–transgenic mouse brain. We treated primary cultured neurons prepared from non–transgenic mice in vitro and non–transgenic mice in vivo with a BACE1 inhibitor (Inhibitor IV, Calbiochem), and investigated Aβ levels and APP processing using the ELISA and immunoblotting. Treatment with the BACE1 inhibitor, Inhibitor IV, reduced endogenous Aβ generation in primary cultured neurons prepared from non–transgenic mice in a dose–dependent manner; Aβ was fully abolished at 1 μM. Intracerebroventricular injection of this BACE1 inhibitor significantly reduced (30% compared to vehicle–treated mice, P<0.01) endogenous Aβ levels in non–transgenic mice. In response to BACE1 inhibition, the level of soluble APP (sAPP)β, the β–secretase–cleaved APP fragment, was significantly reduced, while sAPPα, the α–secretase–cleaved APP fragment, was significantly enhanced. This result indicates that β–secretase competes with α–secretase for APP processing in non–transgenic mice. This study shows the feasibility to evaluate BACE1 therapeutic agents in non–transgenic mice in vitro and in vivo. In addition, BACE1 inhibition reduces Aβ production in non–transgenic mice, shifting APP processing toward non–amyloidogenic alpha–secretase cleavage.