PTTH was one of the earliest hormones discovered in
insects. PTTH regulates insect molting and metamorphosis
by promoting the biosynthesis and release of the molting
hormone ecdysone from the prothoracic glands.
The mature PTTH forms a homodimer linked by an
interchain disulfide bond while a cystine knot motif composed
of three intrachain disulfide bridges is likely
formed within each chain.Silkworm PTTH has been
reported to have a carbohydrate chain at Asn,although its recombinant protein lacking the sugar chain
also exhibits activity to induce adult development.Although the seven Cys residues that form inter- and
intrachain disulfide bonds are highly conserved among
PTTHs of various insect species, their primary structures
are much less conserved.
The primary function of PTTH is to promote the biosynthesis
of the molting hormone ecdysone in the prothoracic
glands in response to environmental stimuli. This is
best demonstrated by its regulatory role in pupal diapause;
for example, in the cabbage army moth Mamestra
brassicae, the hemolymph PTTH titer is reduced during
the diapause period, but it is released into the hemolymph
to promote ecdysone biosynthesis when the environmental
condition allows for adult development.
Regulation of synthesis and release
The silkworm PTTHproduced in the brain neurosecretory
cells is transported through their axons and stored in
the corpora allata. In response to environmental stimuli
such as the light-dark cycle, PTTH is released from the
corpora allata into the hemolymph and the transient
increase of its concentration in the hemolymph stimulates
ecdysone biosynthesis in the prothoracic glands.In contrast,
Drosophila PTTH is secreted in proximity of the prothoracic
gland cells within the ring gland from axon
terminals that directly contact the gland.Upstream neurons
that innervate the PTTH-producing neurons and
regulate their activity are beginning to be identified in
D. melanogaster.
