Diapause hormone of the silkworm, Bombyx mori: Structure, gene expression and function

Abstract

Diapause hormone (DH) is a neuropeptide hormone which is secreted from the suboesophageal ganglion (SG) and is responsible for induction of embryonic diapause of the silkworm, Bombyx mori. DH is isolated from SGs and determined to be a 24 amino acid peptide amide. The cDNA encodes the polyprotein precursor from which DH, pheromone biosynthesis activating neuropeptide (PBAN) and three other neuropeptides are released to become mature. The C-terminal FXPRL-NH 2 sequence of DH is essential but not sufficient, and the two N-terminal regions are needed as a complementary sequence for expression of full activity. The DH-PBAN gene is composed of 6 exons interrupted by 5 introns and is expressed in 12 neurosecretory cells of the SG. The incubation of eggs at 25 °C, which induced embryonic diapause in the progeny, caused DH-PBAN mRNA content to increase at 5 different stages in the life cycle. By contrast, a 15 °C incubation induced expression of the gene in the late pharate adult stage. The temperature-controlled expression of DH-PBAN gene is closely correlated to the incidence of diapause, indicating that DH-PBAN gene expression is the initial event leading to diapause induction. DH acts to stimulate trehalase activity in developing ovaries to bring about hyperglycogenism in mature eggs, a prerequisite for diapause initiation. The availability of synthetic DH and the trehalase cDNA clone facilitates the study of molecular mechanisms of DH action. Using in vivo and in vitro systems, DH clearly induces expression of the trehalase gene in developing ovaries. New protein synthesis is not needed for this process, but a Ca 2+-dependent proteinkinase seems to be involved. Here, I review these topics to explore molecular mechanisms of diapause, a fundamental event that is critical for the success of insect life.