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cleaves Atg8 from PE once the autophagosome has fused with the lysosome
(
Ichimura et al., 2000
). Since Atg8 remains on the membrane throughout
autophagosome maturation,
it
is a useful marker of autophagosomes
(
Klionsky et al., 2008
).
3. DROSOPHILA AS A MODEL FOR STUDYING THE
INTERFACE BETWEEN STEROID SIGNALING, NUTRITION,
AND GROWTH DURING DEVELOPMENT
Drosophila
development provides a useful system for studying the
coordination of cell growth, division, and death that is necessary for the
animal to reach its proper size. Fly development is regulated by the steroid
20-hydroxyecdysone (ecdysone), and insulin and insulin-like growth factor
signaling. These pathways are also known to regulate autophagy in differ-
ent contexts; however, the coordination of steroid, insulin signaling, and
autophagy is poorly understood. Recent studies have investigated the
relationship between ecdysone and growth factor signaling in flies
(
Colombani et al., 2005; Layalle, Arquier, & L´opold, 2008
), and under-
standing how these two pathways coordinate with each other may provide
insight into how autophagy fits into this dynamic to facilitate animal
homeostasis.
3.1. Steroid signaling
During development,
Drosophila
transitions through many different stages,
and these transitions are signaled by pulses of the steroid hormone, ecdysone
(
Riddiford, Cherbas, & Truman, 2000; Thummel, 2001
).
Drosophila
begins
life as an embryo, and approximately 1 day after egg lay, they hatch as first
instar larvae. The larvae feed and grow for approximately 3.5 days, and they
molt twice during this period to become second instar larvae 24 h after
hatching and third instar larvae 48 h after hatching. After the larval period,
the animal stops feeding and a high titer pulse of ecdysone triggers puparium
formation. This ecdysone pulse also induces the programmed cell death of
the larval midgut (
Lee, Cooksey, & Baehrecke, 2002
). Prepupal develop-
ment lasts for 12 h, and another peak in ecdysone titer triggers the prepu-
pal-pupal transition and initiates programmed cell death of the larval
salivary glands (
Lee et al., 2003
). Pupal development lasts for 3.5 days, after
which the adult animal ecloses. A remarkable transformation occurs during
this final developmental period; the tissues necessary to the feeding larva
