WD domain similar to Apaf-1, and this form binds to cytochrome c. Provocatively,
a short form of Ark has been reported by one group, and this form lacks the WD
repeats, making it more similar to ced-4 (Kanuka et al., 1999). This suggests that Ark
may be activated independently of cytochrome c binding. Cytochrome c appears to
undergo conformational changes during apoptosis in Drosophila cells. However, it
does not appear to be released in large amounts from the mitochondria, as in
mammalian cells (Varkey et al., 1999; Dorstyn et al., 2002; Zimmermann et al.,
2002). Some data suggest that cytochrome c is not required for all apoptosis in
Drosophila cells (Zimmermann et al., 2002).
Ark is broadly expressed during development, with higher levels of expression in
a limited number of cells in the embryo (Kanuka et al., 1999; Rodriguez et al., 1999;
Zhou et al., 1999). Ark-mutant embryos have reduced levels of cell death, particularly
in the central nervous system (CNS) and epidermal regions, and the larval nervous
systems are substantially larger than those of the wild type and have excess cells
(Kanuka et al., 1999; Rodriguez et al., 1999; Zhou et al., 1999). Adults show a range
of phenotypes, including abnormal wings, extra bristles, extra photoreceptors, and
male sterility. The CNS of the larvae is also enlarged, a phenotype similar to that seen
in Rpr mutants (see below). These phenotypes suggest that Ark is required for some,
but not all, developmental apoptosis in the fly. RNA interference experiments also
suggest a role for Ark in stress-induced apoptosis in S2 cells (Zimmermann et al.,
2002). In this study, Rpr—and Grim-induced apoptosis was not affected by the loss
of Ark.
It is not yet known how (or whether) the fly Bcl-2 family members regulate
mitochondrial function or Ark activity. Two Bcl-2 family members have been
described in Drosophila, Debcl/dBorg-1/dRob-1 (Brachmann et al., 2000; Colussi et
al., 2000; Igaki et al., 2000) and Buffy (Flybase, 1994; Brachmann et al., 2000;
Colussi et al., 2000). Buffy has not been well characterized. Debcl contains BH1,
BH2, and BH3 domains, as well as a C-terminal transmembrane domain.
Overexpression studies indicate that Debcl is able to induce apoptosis in a variety of
tissues, while loss of function studies indicate that Debcl is required for some
developmental apoptosis.
There is some evidence that Debcl has both pro- and antiapoptotic activities.
Expression of low levels of Debcl protects cells from apoptosis induced by serum
withdrawal (Brachmann et al., 2000). However, expression of Debcl in the eye
sensitizes cells to apoptosis induced by UV (Brachmann et al., 2000). Some Debcl
overexpression phenotypes are suppressed by coexpression of the broad-spectrum
caspase inhibitor p35, while others are not (Colussi et al., 2000; Igaki et al., 2000;
Zimmermann et al., 2002). This suggests that in some situations Debcl may induce
cell death by both caspase-dependent and caspase-independent mechanisms.
The mechanism of Debcl function is unclear. Although Debcl can bind to a
number of the antiapoptotic Bcl-2 family members from other species (Colussi et al.,
2000), there is no evidence for an antiapoptotic Bcl-2 family member in flies. Genetic
data indicate that decreased DIAP1 function enhances the ability of Debcl to induce
apoptosis, while decreased Ark function suppresses Debcl-induced apoptosis (Colussi
APOPTOSIS IN DROSOPHILA 191