ABSTRACT
Finding inhibitors of Aß42 generation is a major goal of Alzheimer’s disease drug development. Two target protease activities, ß-and γ-secretases, were operationally defined more than 10 years ago, but progress in this area has been slow because the actual enzymes were not identified. Using an expression cloning strategy we have identified a novel membrane bound aspartic protease, BACE1, as ß-secretase. This finding has been con firmed and BACE1 and its homologue BACE2 have been characterized in detail by many groups. Major progress has been made in two areas: First, the x-ray crystal structure, which is critical for rational inhibitor design, has been solved and shown to be similar to that of other pepsin family members. Second, knockout studies show that BACE1 is critical for Aß genera tion, but the knockout mice show an otherwise normal phenotype, raising the possibility that therapeutic BACE1 inhibition could be accomplished without major mechanism-based toxic ity. However, target-mediated toxicity of ß-secretase inhibition cannot be ruled out based on the currently available data alone. While various peptidic ß-secretase inhibitors have been pub lished, the key challenge now is the generation of more drug-like compounds that could be developed for therapeutic purposes. Other current areas of investigation, including identifica tion of BACE1 substrates and the potential role of BACE1 overexpression in AD, are discussed.
from the middle of its large precursor protein,1 the two necessary proteolytic cleavage events, one at the N-terminus by an enzyme termed “ß-secretase” and one at the C-terminus by an enzyme termed “γ-secretase”, have attracted a lot of attention. This is understandable because Aß formation is the initial step in the hypothetical amyloid cascade2 and is thus supposed to be ultimately responsible for the pathology of Alzheimer’s disease (AD). Moreover, the necessity of proteolytic cleavage for Aß generation immediately suggested the existence of two potential therapeutic intervention targets which could be addressed using standard protease inhibition approaches. Consequently, APP processing and Aß generation have been studied in a variety of systems by many investigators and their results are summarized in Figure 2.1. At least three distinct protease activities are involved in processing the membrane protein APP along two major pathways, the a-secretase and the amyloid forming ß-secretase pathway. A relatively small minority of APP molecules enters the ß-secretase pathway in which ß-secretase cleaves APP and releases a soluble fragment, ß-APPs. The C-terminal membrane bound C99 peptide is then cleaved by γ-secretase within the transmembrane domain and two major isoforms of 40 and 42 amino acid length with different C-termini, Aß4o and Aß42, are generated. In the a-secretase pathway, a-secretase cleaves in the middle of the Aß region (thus precluding Aß
