Biology Reference
In-Depth Information
Ecdysone
EcR-USP
Signaling genes
Cellular organization
genes
Apoptosis genes
Autophagy genes
BR-C
E74A
E93
Programmed
cell
death
b
FTZ-F1
Figure 4.2 Genetic regulation of ecdysone-induced autophagy in Drosophila salivary
glands. At 10 h after puparium formation, there is a rise in ecdysone titer, and ecdysone
binds to its heterodimeric receptor which consists of EcR and USP. The ecdysone recep-
tor complex functions together with bFTZ-F1 to induce transcription of the early genes;
BR-C, E74A, and E93. The early genes activate transcription of many late genes involved
in signaling, cellular organization, apoptosis, and autophagy.
degrade through histolysis and are replaced by growing tissues that will be
necessary to the walking, flying, and reproducing adult.
Ecdysone signaling has been studied extensively in the larval salivary
glands of
Drosophila
. The pulses of ecdysone regulate stage- and tissue-
specific developmental pathways through a transcriptional hierarchy
(
Thummel, 1995
)(
Fig. 4.2
). Ecdysone signals by binding its receptor which
is a heterodimer of two nuclear receptors, ecdysone receptor (EcR) and
ultraspiracle (USP) (
Koelle et al., 1991; Thomas, Stunnenberg, & Stewart,
1993; Yao, Segraves, Oro, McKeown, & Evans, 1992
). The EcR complex
activates transcription of the early genes; these include
Broad Complex
(
BR-C
),
E74A
,
E75
,and
E93
(
Baehrecke & Thummel, 1995; Burtis,
Thummel, Jones, Karim, & Hogness, 1990; DiBello, Withers, Bayer,
Fristrom, & Guild, 1991; Segraves & Hogness, 1990
). The early genes then
activate transcription of the late genes, which are thought to function more
directly in the regulation of developmental processes. In the salivary glands,
the
b
FTZ-F1 orphan nuclear receptor is expressed during the mid-prepupal
dip in ecdysone titer (
Lavorgna, Karim, Thummel, &Wu, 1993
). During the
ecdysone peak that triggers salivary gland degradation, the EcR complex and
b
FTZ-F1 function together to reinduce transcription of
BR-C
,
E74A
,and
E75
and to activate transcription of the stage-specific early gene,
E93
(
Baehrecke & Thummel, 1995; Broadus, McCabe, Endrizzi, Thummel, &
Woodard, 1999; Woodard, Baehrecke, & Thummel, 1994
).
b
FTZ-F1
,
BR-C
,
E74A
,and
E93
are all necessary for the proper degradation of larval
salivary glands (
Broadus et al., 1999; Jiang, Lamblin, Steller, & Thummel,
2000; Lee et al., 2000; Restifo and White, 1991
).
E93
mayhaveamore
prominent role in autophagic cell death than the other early genes as it
also appears to be required for autophagosome formation in the dying larval
midgut (
Lee, Cooksey, & Baehrecke, 2002
).
