Biomedical Engineering Reference
In-Depth Information
potential AD therapeutics. However, since most AD patients express WT APP,
it is essential to investigate drug candidates in models expressing WT
APP because different proteases may process wild-type (WT) compared to
mutant APP substrates.
Many have viewed the aspartyl protease BACE1 as the primary, if not only,
b-secretase.
10-12
However, BACE1 was originally identified as a b-secretase in
cell and animal models expressing APP containing the rare Swedish mutant
(Swe) b-secretase site sequence (K670N/M671L) found in only one family of
AD patients.
13
While BACE1 readily cleaves the Swe b-secretase sequence
in vitro, it barely cleaves the WT b-secretase sequence. Further, there are no
published reports showing that BACE1 affects brain Ab in a transgenic mouse
model expressing WT APP (no mutations). The lack of data is conspicuous,
considering the well-known importance of WT APP processing in AD and the
extraordinary resources expended on BACE1 target validation.
These observations led us to search for another protease with WT b-secretase
activity whereas others had not, in the regulated secretory pathway of neurons.
Most of the peptides, including Ab, secreted by neurons are released from the
regulated secretory pathway.
14,15
Using neuroendocrine primary bovine chro-
man cells, which express APP containing the human WT sequence, we
determined that the cysteine protease, cathepsin B, has WT b-secretase activity
in the regulated secretory pathway. We subsequently confirmed that brain Ab
levels are reduced by cathepsin B inhibitors and by deleting the cathepsin B gene
in a transgenic mouse model expressing WT APP. Interestingly, we found that
cathepsin B prefers cleavage of the WT b-secretase site of APP but shows
essentially no cleavage of the Swe b-secretase site. Furthermore, inhibitors
of cathepsin B are effective in AD mice expressing human APP with the WT
b-secretase site of APP, but cathepsin B inhibitors have no effect in animal
models expressing APP containing the Swe mutation.
These novel pharmacogenetic features of inhibitors to cathepsin B are
explained in this chapter as being due to the unique cleavage specificity of
cathepsin B for the WT b-secretase site of APP, but not the Swedish mutant
b-secretase site. These new findings about cathepsin B suggest that it may
participate as a b-secretase. Together with findings in the field showing that
BACE1 participates in Ab production as a b-secretase activity,
11,16-22
cathepsin
B may function with BACE1 in Ab production. Significantly, the pharmaco-
genetic properties of the class of molecules that inhibit cathepsin B predict
that such inhibitors may be therapeutically ecacious for the majority of AD
patients expressing WT APP. The emerging role of cathepsin B as a drug target
for AD may benefit future development of therapeutic agents for AD.
6.2 Strategies to Identify Protease(s) Cleaving at the
Wild-Type b-Secretase Site of APP
The wild-type (WT) b-secretase site of APP is cleaved at Met-
k
Asp at the
N-terminal side of the Ab peptide sequence (Figure 6.2a). The amino acid
