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2.4.3 AST suppresses JH biosynthesis without stage specificity
Among the peptides, ASTs are the most well-studied JH-regulatory peptides
which reversibly inhibit JH biosynthesis by the CA (see reviews by
Goodman & Granger, 2005; Stay & Tobe, 2007; Weaver & Audsley,
2009
). ASTs have three different types by their structures. AST-A peptides
were the first to be isolated from the cockroach
D. punctata
and have been
found in many different insect species, but some AST-A peptides stimulate
JH biosynthesis in a stage-specific fashion (
Clark, Lange, Zhang, & Tobe,
2008; Stay, Zhang, & Tobe, 2002
). AST-B family peptides have been iden-
tified in crickets and locusts, and AST-C peptides are members of the PISCF
family found in Lepidoptera (
Goodman & Granger, 2005
). ASTs are pro-
duced in the brain and are considered to be transferred to the CA through
nerve axons. AST-C suppressed JH biosynthesis by 50-80% in
Bombyx
in all
the stages we studied, from the fourth to fifth instar stadium, and did not
show any stage-specific variation in effect (
Hiruma et al., 2010
; unpublished
data). The lower amount of JH synthesis by an intact brain-CC-CA com-
plex (
Fig. 3.3
A) is probably due to the inhibitory action of AST-C synthe-
sized in the brain and supplied to the CA via the CC. The role of AST-C
might be to adjust the total amount of JH synthesis by preventing the over-
production of JH, although the timing of the release of this peptide is
unknown.
2.4.4 Mode of action of sNPF and AST
Both sNPF and AST-C suppress JH biosynthesis by the CA, but the stage-
dependent suppressive action of sNPF is distinct from that of AST-C. When
the CA of day 0 fifth instar larvae were incubated with these peptides,
MevK
and
HMGR
expressions were prevented by sNPF, although not only
MevK
and
HMGR
but also
IPPI
mRNA levels were depressed by AST-C
(unpublished data); therefore, the modes of action of these peptides must
be different from each other. AST-Cwas initially believed to inhibit JH syn-
thesis by the transfer of citrate to the cytoplasm across the mitochondrial
membrane or by the cleavage of citrate to yield cytoplasmic acetyl-CoA
rather than by the inhibition of the enzymes in the mevalonate pathway
(
Sutherland & Feyereisen, 1996
), but it is apparent that inhibition of some
JH synthetic enzymes is also involved.
2.4.5 ETH activates JH biosynthesis at the time of the final larval ecdysis
ETH is a peptide first found in
Manduca
which is responsible for initiating and
regulating insect ecdysis, along with eclosion hormone (EH), corazonin, and
crustacean cardioactive peptide (CCAP). ETH is produced by epitracheal
