Discovery of the Cysteine Protease Cathepsin B as a Drug Target for Alzheimer’s Disease - Proteinases as Drug Targets - page 154

6.4 Inhibition or Knockout of Cathepsin B

Reduces Ab Production

6.4.1 Inhibition of Cathepsin B Reduces Ab in Regulated
Secretory Vesicles of Neurons
Incubation of the purified secretory vesicles for several hours at 37 1C results in
production of Ab (Figure 6.4a). Notably, incubation with the specific cathepsin
B inhibitor CA074, as well as with the methylated CA074Me form, results
in blockade of Ab production (Figure 6.4a). 41 Also, incubation with E64c,
which inhibits cathepsin B and cysteine proteases, results in blockade of Ab
production in the purified regulated secretory vesicles. These data illustrate a
role for cathepsin B in Ab production in secretory vesicles.
The effect of the cathepsin B inhibitor was also evaluated in neuronal chro-
man cells (in primary culture). CA074Me reduced the amount of Ab (Ab40) in
regulated secretory vesicles, whose content of Ab was measured in the culture
media after KCl stimulation of the regulated secretory pathway (Figure 6.4b).
The CA074Me inhibitor, however, had no effect on the amount of Ab secreted
from the constitutive secretory pathway. These results indicate the cellular role
of cathepsin B for production of Ab in the regulated secretory pathway.

6.4.2 Cathepsin B Inhibitor Treatment of Guinea Pigs,
Expressing Human Wild-Type APP, Reduces Brain Ab
Treatment of guinea pigs with CA074Me, a selective inhibitor of cathepsin B, by
intracerebroventricular administration resulted in significant reduction in brain
levels of Ab40 and Ab42 (Figure 6.5a,b, respectively). Furthermore, analyses of
synaptosomes isolated from guinea pig brains demonstrated that Ab (total) was
reduced in the nerve terminals of the synaptosome preparations (Figure 6.5c). 44
CA074Me also reduced brain levels of CTFb derived from APP by b-secretase,
suggesting that b-secretase was inhibited. In addition, the inhibitor resulted in
elevated brain levels of sAPPa, consistent with enhanced availability of APP for
a-secretase cleavage. The guinea pig expresses APP with the WT b-secretase
site 45 and is, therefore, a model for WT b-secretase function in Ab production.

6.4.3 Inhibition of Cathepsin B Reduces Brain Ab and Improves

Memory Deficits in a Mouse Model Expressing Human
APP with the Wild-Type b-Secretase Site, But Not in Mice
Expressing APP with the Swedish Mutant Site

6.4.3.1 Cathepsin B Inhibitor Treatment of AD Mice Expressing
Human APP with the Wild-Type b-Secretase Site
Results in Improved Memory
Importantly, inhibitors of cathepsin B improve memory and reduce Ab in
transgenic mice expressing human APP with the WT b-secretase site, expressed