LAUSR

Under physiological conditions, β-site amyloid precursor protein (APP)-cleaving enzyme 2 (BACE2) cleaves APP within Aβ sequence thereby functioning like a α-secretase. However, BACE2 could also function as a conditional β-secretase during aging, contributing to Alzheimer’s disease pathogenesis. To date the physiological functions of BACE2 in endothelium are largely unknown. The present study is therefore designed to investigate the role of BACE2 in APP metabolism in human BMECs. Cultured human BMECs (passage 5-6 or passage 22) were treated with BACE2siRNA (30 nM, for 3 days), levels of soluble APPα (sAPPα, a neurotrophic product of non-amyloidogenic processing of APP) and Aβ40 in the supernatant were measured. In human BMECs (passage 5-6), genetic inactivation of BACE2 significantly decreased production of sAPPα (n=12, P<0.05), but had no effect on production of Aβ40 (n=9, P>0.05). BACE2siRNA treatment significantly suppressed APP protein expression (n=7, P<0.05), but augmented protein levels of BACE1 (n=7, P<0.05). Genetic inactivation of BACE2 did not change protein levels of mature ADAM10 (n=7, P>0.05). Thus, reduced sAPPα secretion by BACE2siRNA treatment is likely caused not only by decreased α-secretase-like function of BACE2, but also by reduced APP expression. We further examined the effects of BACE2siRNA in senescent human BMECs. In cultured human BMECs (passage 22), protein expressions of senescent markers (p 21Cip1 and p 16INK4a ) were significantly increased (n=4, P<0.05). Genetic inactivation of BACE2 in senescent human BMECs also significantly suppressed secretion of sAPPα (n=8, P<0.05), but did not affect Aβ40 production (n=8, P>0.05). BACE2-siRNA treatment significantly inhibited protein expressions of APP and mature ADAM10 (n=7, P<0.05), but did not change BACE1 protein expression (n=7, P>0.05). Thus in senescent human BMECs, reduced APP expression and impaired α-processing may play important roles in the decreased sAPPα production. Since our previous studies have demonstrated that endothelial production of sAPPα significantly contributes to the sAPPα content in the hippocampus, our current findings suggests that inhibition of BACE2 could impair protective function of sAPPα in the hippocampus.