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ecdysone biosynthetic enzymes (
Keightley, Lou, & Smith, 1990; Niwa et al.,
2005; Yamanaka et al., 2007
). High expression of the ecdysone biosynthetic
enzymes coincides with the ecdysone peaks, suggesting that upregulation of
the components in the pathway is required to increase the steroidogenic
capacity of the gland (
Rewitz, Rybczynski, Warren, & Gilbert, 2006;
Yamanaka et al., 2007
). The importance of the transcriptional regulation
is highlighted by evidence suggesting that low expression of these genes,
including
spook
, presumably prevents PTTH-stimulated ecdysone synthesis
despite PTTH being able to induce a glandular phosphorylation response in
the early fifth instar
Bombyx
(
Gu &Chow, 2005; Lin &Gu, 2007; Yamanaka
et al., 2007
). Reduced expression of the ecdysone biosynthetic enzymes in
PTTH-ablated larvae indicates that PTTH plays a role in the transcriptional
up-regulation of these genes in
Drosophila
(
McBrayer et al., 2007
). However,
the moderate effect of PTTH does not fully explain the dramatic increase in
expression of these genes during development (
Yamanaka et al., 2007
).
Thus, other mechanisms must be involved in the transcriptional regulation
of the Halloween genes. Consistent with this notion, the transcriptional
repressor DHR4, a key target of PTTH signaling, has only minor effect
on Halloween gene expression (
Ou et al., 2011
). Other mechanisms might
involve feedforward regulation by the action of ecdysone itself, believed to
autonomously activate the PG in culture, or perhaps involve input from
TGF
b
/Activin and/or nitric oxide signaling (see discussion below:
Caceres et al., 2011; Gibbens et al., 2011; Yamanaka et al., 2007
).
In addition to the enzymes involved in ecdysone biosynthesis,
npc1
is also
expressed specifically in the PGwhere it is thought to be involved in supplying
cholesterol for steroidogenesis (
Huang, Suyama, Buchanan, Zhu, & Scott,
2005
). Normal expression of
npc1
and the Halloween genes requires Broad,
which is interesting considering that JH, which suppresses
broad
expression, also
inhibits
spook
expression in
Bombyx
(
Xiang et al., 2010; Yamanaka et al., 2007
).
3.2. TGF
/Activin and nitric oxide signaling are essential for
steroidogenesis
Although several factors have been shown to regulate PG activity (see
Marchal et al., 2010
for a recent review), recent reports have highlighted
the essential role of nitric oxide (NO) and TGF
b
/Activin (
Caceres et al.,
2011; Gibbens et al., 2011
). In peripheral tissues, ecdysone elicits a genetic
response through a set of primary ecdysone-inducible nuclear receptors
(
King-Jones & Thummel, 2005
). However, some of these nuclear receptors,
b
