Overview and outcome of functional screening for novel Aß-lowering proteases. A, Cartoon illustrating the overall design of the screen. Briefly, an arrayed collection of 352 protease-encoding cDNAs was cotransfected into CHO cells together with an APP-AP fusion construct. Following a medium change and overnight incubation, Aß40 levels, AP activity, and cytotoxicity (via MTT assay) were analyized in the resulting conditioned media. B, Results of the screen, expressed as [Aß40]/AP ratios normalized to intraplate controls. Data are mean of 4 replicates. Note that the largest decrease in Aß levels by far was acheived by BACE2. C, Confirmation of the results of the screen. Following scale-up and sequence verification, cDNAs encoding BACE2 and its homolog, BACE1, were cotransfected together with APP-AP into CHO cells. Consistent with the outcome of the medium-throughput screen, BACE2, but not BACE1, expression resulted in significant decreases in the levels of both Aß40 and Aß42. Data are mean ± SEM of 4 replicates, and are normalized to vector-only controls.