Biology Reference
In-Depth Information
pulses are not simply a consequence of modulating mRNA levels of the
Halloween genes. In line with this, none of the Halloween genes appeared
to be affected in ring glands of
DHR4
mutants or
DHR4
RNAi animals.
Through ring gland-specific microarrays, a hitherto uncharacterized cyto-
chrome P450 gene,
Cyp6t3
, was identified as a candidate downstream target
of DHR4 (
Fig. 2.4
B). Several lines of evidence link
Cyp6t3
to
DHR4
and
ecdysone synthesis: (1) decreasing
Cyp6t3
expression in the prothoracic
gland results in molting defects and developmental delays, which can be res-
cued by 20E, suggesting that these animals are ecdysone deficient; (2)
Cyp6t3 activity can be provisionally placed into the “Black Box” of the
pathway: The ecdysone intermediate 5
b
-ketodiol can partially rescue the
Cyp6t3
RNAi phenotype, however, precursors upstream of the “Black
Box” cannot; (3)
Cyp6t3
expression is significantly increased in the ring
gland of both
DHR4
mutant and
DHR4
RNAi animals, indicating the gene
is normally repressed by DHR4; (4) knockdown of
Cyp6t3
expression in a
DHR4
mutant background can reverse the precocious pupariation pheno-
type; (5)
Cyp6t3
mRNA levels oscillate, where lower levels correlate with
times when DHR4 is nuclear. Taken together, these data demonstrate that
Cyp6t3
is negatively regulated by DHR4 to downregulate ecdysone pro-
duction, but whether the gene is a direct target of
DHR4
needs to be de-
termined by further experiments.
10. DHR3/E75-MEDIATED NO SIGNALING IS A CRITICAL
PLAYER OF ECDYSTEROIDOGENESIS
The recent report by
Caceres et al. (2011)
that NO plays a key role in
the production of ecdysone comes undoubtedly as a surprise. In brief, it was
shown that NO signaling regulates the interaction of DHR3 and E75 to
control the transcriptional activation of
b
ftz-f1
, all well-characterized players
of the ecdysone hierarchy (
Fig. 2.4
B).
NO is produced by nitric oxide synthase (NOS) and is a short-lived
diatomic molecule that acts as an intra- and transcellular messenger that reg-
ulates many physiological functions in both vertebrates and invertebrates
(
Bredt & Snyder, 1994
). The
Drosophila
genome harbors only one nitric ox-
ide synthase gene called
NOS
, which is expressed throughout
Drosophila
de-
velopment (
Stasiv, Regulski, Kuzin, Tully, & Enikolopov, 2001
). Previous
studies have shown that NO plays important roles in imaginal disk develop-
ment, synaptogenesis, formation of retinal projection patterns, response to
hypoxia, and behavioral responses (
Suman, Seth, & Chandna, 2008
).