Insect Developmental Hormones Ecdysteroids

Ecdysteroids are relatively polar steroid hormones released by the prothoracic glands in immature stages and the gonads in adults. They are the chief regulators of gene expression in insect development and reproduction. The first structure to be elucidated was a-ecdysone (aE), the immediate precursor to 20-hydroxyecdysone (20HE), accepted as the main protagonist in ecdysteroid actions. While 20HE indeed is associated with the majority of ecdysteroid actions, there is evidence to support aE as a signaling molecule in certain instances. Other suspected ecdysteroids are 20,26HE and makisterones.

Juvenile Hormones

JHs, sesquiterpenoid derivatives from the sterol synthesis pathway, are released by the corpora allata. Six types are known in insects, with JH III being the predominant form. The chief action of JHs is to modulate ecdysteroid-mediated gene expression. No receptors for JH have been clearly identified, but it seems likely that the hormone interacts with intracellular receptors or proteins that modulate ecdysteroid signaling. The signature of JH action is to promote expression of the immature phenotype.

Prothoracicotropic Hormone

Prothoracicotropic hormone (PTTH) is a large peptide hormone released by brain neurosecretory cells from terminals in the corpora cardiaca or corpora allata. PTTH stimulates the prothoracic gland to synthesize and release aE or, in some instances, 3-dehydroecdysone, which are both then converted to 20HE in the hemolymph or by target tissues. PTTH is a homodimer consisting of two 12-kDa subunits joined by a disulfide bond. Release of PTTH is regulated by sensory inputs to the brain, which convey information about body size and nutritional state.

Bursicon

Bursicon is a 30- to 40-kDa peptide that accelerates scleroti-zation of cuticle. It is released from neurosecretory cells in the brain and ventral nerve cord after each ecdysis. Since insects are particularly vulnerable to predation during and after ecdysis, rapid hardening of the cuticle maximizes survival.

Eclosion Hormone

Eclosion hormone (EH) is a 62-amino-acid peptide hormone that mediates circadian-mediated eclosion to the adult stage and each larval ecdysis. Eclosion hormone is released into the blood from ventral median neurosecretory cells of the brain, causing release of ecdysis-triggering hormone (ETH) from Inka cells of the epitracheal endocrine system. It is also implicated in the elevation of cyclic GMP in a subset of neurons in the central nervous system that initiate ecdysis behavior.

Ecdysis-Triggering Hormones

ETHs are peptide hormones synthesized and released from Inka cells at the end of the molt. ETHs act directly on the central nervous system (CNS) to cause a behavioral sequence that leads to shedding of the cuticle. ETH release is caused by eclosion hormone, and the action of ETH on the CNS leads to release of eclosion hormone from VM neurons. It has been proposed that ETH and EH engage in a positivefeedback signaling pathway that results in depletion of ETH from Inka cells, which is necessary for the transition from preecdysis to ecdysis behaviors.

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