Description
DH is synthesized by the subesophageal ganglion (SG).
It is released into the hemolymph via the corpus cardiacumcorpus
allatum (CC-CA) complex, and targets the developing
ovaries during pupal-adult development to induce
the embryonic diapause of the silkworm Bombyx mori.
Structure
DH is a C-terminally amidated peptide consisting of
24 aa residues with a C-terminal sequence of FGPRLNH2.Therefore, DH is a member of the FXPRL-NH2
family. DH is widely conserved among lepidopteran
insects.
Discovery
DH was first discovered as a hormone that is synthesized
by SG and released into the hemolymph to induce
embryonic diapause in B. mori.It was isolated from
B. mori SG, and its structure was determined in 1991.In addition, DH has been shown to induce the termination
of pupal diapause in the corn earworm, Heliothis/
Helicoverpa species.
Distribution
DH is detected in seven pairs of neurosecretory cells
(DH-PBAN-producing neurosecretory cells; DHPCs),which consists of the SMd, SMx, and SLb neuromeres
along the ventral midline and in SL cells located within
the B. mori SG.
Biological functions
B. mori DH acts on the developing ovaries during the
middle pupal stage and transfers the DH signal into the
oocyte via DHR. At 2–3 days after egg-laying, the eggs
reach the diapause stage, with the enlargement of both
extremities of the embryo and the completion of mesoderm
segregation. In developing ovaries, DH is known
to enhance trehalase activity on the oolemma, which
leads to the accelerated incorporation of hemolymph trehalose
by the oocyte and finally, to higher levels of glycogen
deposition, compared to the oocyte without the DH
signal. This glycogen is utilized for the production of sorbitol
and glycerol along with the diapause. DH promotes
trehalase activity via upregulation of the trehalase gene,
during the period when Ca
2+ and protein kinase C are
involved in signal transduction.In Orgyia thyellina, DH
induces seasonal reproductive polyphenism, including
the egg diapause.
Regulation of synthesis and release
B. mori DH is biosynthesized in the neurosecretory cells
in the SG. It is transported via nervus corporis cardiaci-III,
maxillary nervus, nervi corporis cardiaci-ventralis, and
nervi corporis cardiaci-nervus recurrens to the CC-CA
complex, and is then released into the hemolymph.The
expression of the B. mori DH-PBAN gene is switched on
by the interaction of a cis-element (-1117 to-1088nt)
on the 50 upstream region with a transcription factor
(B. mori Pitx1).In the pupae of diapause eggs, which
are the producers of the bivoltine strain (Kosetsu), the
DH titer was measured at 8.85 ± 2.71pM, which was twofold
higher than in the nondiapause egg producer.
