Model Organisms in Drug Discovery

A CASE STUDY FOR ALZHEIMER’S DISEASE DRUG DISCOVERY 163
composed of amyloid (Ab) peptide. This peptide is generated by the sequential
processing of amyloid precursor protein (APP). The N-terminus of Ab is
generated by the processing activity named beta-secretase, followed by
proteolytic cleavage at the C-terminus by an activity called gamma-secretase.
Inhibition of gamma secretase activity is an obvious therapeutic goal for the
treatment of AD, but until recently the molecular components of gamma
secretase were unknown (Esler and Wolfe, 2001). Presenilin proteins (PS1 and
PS2 in mammals) were originally implicated in the etiology of AD based on
the finding that patients with genetic mutations in these genes were
predisposed to an early-onset form of AD (Levy-Lahad et al., 1995;
Sherrington et al., 1995). Further studies linked PS proteins, and components
of the membrane complex that they form, to gamma-secretase activity. The PS
proteins are highly conserved through evolution and also have been
implicated in the processing of Notch receptor (Levitan and Greenwald,
1995; Struhl and Greenwald, 1999).
Compound BMS AG6B was identified in a cell-based screen for compounds
that altered the Ab40/Ab42 peptide ratio produced in Ab processing
(unpublished data). In order to identify candidate molecular targets for
BMS AG6B, the drug was applied to Drosophila and C. elegans and the
resulting phenotypes were analyzed. These experiments were carried out with
the identity and biological activity unknown to the testers, such that the
hypotheses generated were based solely on the information from the
phenotypes.
Adult flies and their progeny were exposed to 10–40 mM BMS AG6B
dissolved in dimethylsulfoxide (DMSO) and administered in the flies’ food.
Treated adults had grossly normal behavior and morphology at all the
concentrations tested. However, exposure to 40 mM BMS AG6B was lethal to
larvae (second instar stage). At lower doses (10 mM) some (510%) of the flies
did survive to adulthood. When examined closely, the surviving adults flies
were found to have morphological defects in a number of tissues, including
notched wing margins, rough eyes, missing or fused leg segments and missing
abdominal bristles (Figure 6.2). This combination of phenotypes is
characteristic of mutations in members of the Notch signaling pathway
(Shellenbarger and Mohler, 1978). When the phenotypes of treated flies were
compared directly to those in a Notch hypomorph, they were found to be very
similar. Overall, the Drosophila phenotypes suggested that the drug was
disrupting the action of the Notch pathway. Similar Notch phenotypes in
Drosophila were seen with other gamma-secretase inhibitors (Micchelli et al.,
2002).
Wild-type C. elegans were treated with 0.1–2.0 mM BMS AG6B (added to
the bacterial lawn) throughout larval development and as adults. Worms
treated as adults exhibited no discernable changes in morphology or behavior,
and their progeny also appeared normal. All worms treated as larvae