Biology Reference
In-Depth Information
is not sufficient to rescue animals to develop beyond the first instar,
suggesting that
spookier
is not the only target of MLD. Similar to the
woc
mu-
tant, mutations in the
mld
gene also result in overgrowth of the ring gland,
reflecting a possibility that increasing the gland size serves as a common strat-
egy to compensate for the inability of the tissue to synthesize ecdysone
(
Neubueser et al., 2005
). However, it may be still worthwhile to examine
whether either WOC or MLD plays a role in the regulation of cell size/
growth.
11.3. DRE4 (dre4)
Loss-of-function mutations of the
Drosophila dre4
gene cause stage-specific
developmental arrests due to a reduction of ecdysone levels. Studies on a
temperature-sensitive
dre4
allele showed that mutant ring glands failed to
produce ecdysone shortly after a shift to the restrictive temperature, indicat-
ing a cell-autonomous function of the
dre4
gene in the ring gland (
Sliter &
Gilbert, 1992
). Some evidence suggests that the DRE4 functions by remo-
deling chromatin. A heterodimer of DRE4 and SSPR1 (known as FACT)
associates with the GAGA factor, a well-established chromatin remodeling
factor (
Shimojima et al., 2003
). While these results have not been obtained
for the prothoracic gland, it is still quite likely that DRE4 functions by rem-
odeling chromatin structure in the prothoracic gland, possibly facilitating
local access for transcription factors to regulate ecdysteroidogenic genes.
11.4. Smad2/Med (smox/medea)
Smad2, along with its heteromeric partner Medea (Smad4), serves as the
core downstream mediator of the TGF
b
/Activin signaling in
Drosophila
(
Brummel et al., 1999; Schmierer & Hill, 2007
), and a recent report shows
that this pathway plays an important role in ecdysone synthesis pathways
(
Gibbens et al., 2011
)(
Fig. 2.4
). Knocking down
Smad2
via RNAi specif-
ically in the prothoracic gland causes larvae to arrest in the third instar, and
this defect can be rescued by providing 20E in the medium. Knocking down
other key components of the TGF
b
/Activin pathway gave rise to the same
nonpupariating phenotype, suggesting that TGF
b
/Activin signaling is re-
quired for the proper onset of metamorphosis. This study has also shown
that transcript levels of
torso
(
Strecker, Yip, & Lipshitz, 1991
) and
Insulin
Receptor
(
InR
)(
Chen, Jack, & Garofalo, 1996
) are dramatically reduced
when
Smad2
is silenced via RNAi. In addition, two ecdysone biosynthetic
genes,
dib
and
spok
, show severely reduced transcript levels in
Smad2
RNAi
