Recent Studies on Prothoracic Gland Cell Growth and Ecdysteroidogenesis in the Silkworm, Bombyx mori
The prothoracic glands of developing insects are the sources of ecdysteroids that elicit molting and metamorphosis. Using silkworms as our model system, recent studies from our laboratory have focused on the following aspects: (1) stage-specific changes in prothoracictropic hormone (PTTH) signal transduction; (2) the activation of extracellular signal-regulated kinase (ERK) by PTTH in prothoracic gland cells; (3) correlation between cell growth and ecdysteroidogenesis during development; (4) a new autocrine regulatory mechanism underlying cell growth and ecdysteroidogenesis; (5) nutrition-dependent cell growth and ecdysteroidgenesis. Our results showed that development-specific PTTH-cAMP signal transduction, which showed different patterns between the fourth and last larval instars, may play an important role in regulating changes in prothoracic gland activity. Development-specific changes in ERK phosphorylation may also play a role in PTTH stimulation of ecdysteroidogenesis. DNA synthesis in the prothoracic gland cells undergoes a specific developmental change: the dramatic increases in DNA synthesis during the early (for 3rd and 4th instars) or middle (for last instar) stages precede the major increase in ecdysteroidogenesis during the later stages. We have also demonstrated that a novel autocrine factor activates both DNA synthesis of the gland cells and ecdysteroidogenesis by silkworm prothoracic glands. Moreover, our results showed that upon starvation on day 3 of the last instar, larvae increased ecdysteroid production rate to enhance the rate of survive. This review summarized our recent advance in studies on silkworm prothoracic glands.
KeywordsBombyx mori prothoracic gland PTTH signal transduction cell growth ecdysteroid secretion starvation
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