Inhibitors of Cathepsin B Improve Memory and Reduce β-Amyloid in Transgenic Alzheimer Disease Mice Expressing the Wild-type, but Not the Swedish Mutant, β-Secretase Site of the Amyloid Precursor Protein

Abstract

Elucidation of A␤-lowering agents that inhibit processing of the wild-type (WT) ␤-secretase amyloid precursor protein (APP) site, present in most Alzheimer disease (AD) patients, is a logical approach for improving memory deficit in AD. The cysteine protease inhibitors CA074Me and E64d were selected by inhibition of ␤-secretase activity in regulated secretory vesicles that produce ␤-amyloid (A␤). The regulated secretory vesicle activity, represented by cathepsin B, selectively cleaves the WT ␤-secretase site but not the rare Swedish mutant ␤-secretase site. In vivo treatment of London APP mice, expressing the WT ␤-secretase site, with these inhibitors resulted in substantial improvement in memory deficit assessed by the Morris water maze test. After inhibitor treatment, the improved memory function was accompanied by reduced amyloid plaque load, decreased A␤40 and A␤42, and reduced C-terminal ␤-secretase fragment derived from APP by ␤-secretase. However, the inhibitors had no effects on any of these parameters in mice expressing the Swedish mutant ␤-secretase site of APP. The notable efficacy of these inhibitors to improve memory and reduce A␤ in an AD animal model expressing the WT ␤-secretase APP site present in the majority of AD patients provides support for CA074Me and E64d inhibitors as potential AD therapeutic agents. Alzheimer disease (AD)2 is an age-related neurodegenerative disorder that results in loss of memory and accumulation of neurotoxic ␤-amyloid (A␤) peptides in brain (1-4). Expression of mutant forms of human amyloid precursor protein (APP) in mouse models of AD results in increased A␤ and amyloid plaques in brain, with memory deficits that resemble AD (2, 3, 5-7). Such studies demonstrate that overproduction of A␤ peptides participates as a major factor in the development of AD.A␤ peptides are produced by proteolytic processing of APP, resulting in production of A␤40 and A␤42 (1-40 and 1-42 residues, respectively). Proteases referred to as ␤-secretase and ␥-secretase cleave at the N-and C termini of A␤ within APP to generate A␤ peptides. A␤ peptides then undergo secretion to provide extracellular A␤ that produces neurotoxic effects with aggregation and accumulation in amyloid plaques of AD brains. Compounds that inhibit ␤-secretase activity are particularly attractive as potential therapeutic agents for AD.The majority of AD patients express the wild-type (WT) ␤-secretase site of APP. Therefore, compounds that inhibit cleavage of the WT ␤-secretase site are relevant to the AD population. Significantly, findings in this study showed that the endogenous ␤-secretase activity in A␤-containing regulated secretory vesicles (RSV) possesses high selectivity for cleaving the WT ␤-secretase site but does not cleave the Swedish (Swe) mutant ␤-secretase site. The RSV provide production and synthesis of a major portion of secreted A␤ (8, 9). Although many previous studies of ␤-secretase have utilized substrates containing the Swe mutant ␤-secretase site (10 -14) expressed in one extended family (15), it i...

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