Eclosion hormone: Biological Overview | Evolutionary Homologs | Regulation | Developmental Biology | References

Gene name - Eclosion hormone

Synonyms -

Cytological map position - 90B1-2

Function - neuropeptide

Keywords - hormone

Symbol - Eh

FlyBase ID: FBgn0000564

Genetic map position - 3-[60]

Classification - eclosion hormone

Cellular location - cytoplasmic and secreted



NCBI links: Precomputed BLAST | Entrez Gene
BIOLOGICAL OVERVIEW

Eclosion hormone (EH) was first isolated in moths and later shown to be a 62 amino acid peptide produced by two pairs of ventromedial neurosecretory cells in the brain. In moths it evinces marked behavioral and developmental actions. It is known to act directly on the CNS to release the stereotyped motor programs that trigger the shedding of the old cuticle at the end of each molt. Circulating peptide also acts on peripheral tissue to cause a diverse set of effects, such as an increase in plasticity of wing cuticle, discharge of dermal gland secretions, and programmed degeneration of ecdysial muscles (Horodyski, 1993 and references).

A successful ecdysis requires that EH release occurs during a precise phase of the molt cycle. For larval and pupal molts in the tobacco hornworm Manduca, ecdysis is linked to the developmental time table of the animal, occurring a fixed number of hours after the molt is initiated by the actions of the prothoracicotropic hormone and the ecdysteroids (see Ecdysone receptor). Ecdysis in adults is responsive to circadian rhythms as well.

The developmental cue appears to be supplied by the the ecdysteroids, steroid hormones that drive the molt. Each release of EH is preceded by a major release of ecdysteroids. The cue for EH release is not the time of appearance of the ecdysteroid but rather the time that ecdysteroid titer subsequently declines. Thus, if steroid withdrawal is delayed there is a dose-dependent delay in the time of the subsequent ecdysis. The effect of ecdysteroids is apparently through direct action on the brain since ecdysis in debrained animals with implanted brains is normal (Truman, 1992 and references).

Insects are restricted in the times that they are responsive to EH. Manduca becomes responsive to EH about 8 hours prior to pupal ecdysis. It is thought that steroid decline triggers some event at about 13 hours prior to ecdysis that results in the subsequent appearance of EH responsiveness. EH exposure results in a rapid elevation in the levels of guanosine 3',5' cyclic monophosphate (cGMP). cGMP, working through a cGMP-dependent protein kinase, then causes the phosphorylation of two endogenous phosphoprotein substrates of unknown function, the EGPs. EGPs are not detected during most of the molt and intermolt period and first become apparent in the CNS of Manduca at about 8 hours prior to ecdysis, the time when behavioral responsiveness also appears. The appearance of the EGPs is linked to ecdysteroids but requires their withdrawal (Morton, 1988a and b and Truman, 1992).

The hormone triggers a sequence of behaviors that must occur in a stereotyped sequence for the animal to molt successfully. In the Cecropia silkmoth, Hyalophora cecropia, EH triggers a sequence of three discrete behaviors. (1) The pre-eclosion behavior begins about 10-15 min after EH treatment and involves abdominal movements that apparently loosen the connections between the old and the new cuticles. (2) The eclosion behavior starts at about 75-90 min and culminates in the actual shedding of the old cuticle. The principle movements consist of waves of peristalsis that move from posterior to anterior, up the abdomen. (3). Wing inflation behavior begins at about 120 minutes. It also has a stereotyped duration and involves a tonic abdominal contraction, to aid the movement of blood into the thorax and wings, associated with a stereotyped series of wing movements (Truman, 1992 and references).

Abdomens isolated from animals prior to adult ecdysis do not show spontaneous ecdysis behavior but can be induced to show pre-eclosion and eclosion behaviors by EH injection. Thus the CNS contains pattern generators that encode not only the form of the various motor bursts (for rotary vs. peristaltic movements) but also the long term temporal organization of the behavior and the progression from one phase to the next. Since the presence of EH in the bath for less than 5 minutes is sufficient to trigger the entire motor sequence, it is concluded that the peptide provides a phasic signal in triggering the complex program (Truman, 1992 and references).

Centrally released EH (EH released into the brain and CNS and acting centrally on brain neurons), as distinguished from EH release into the circulation, is sufficient to cause the normal onset of ecdysis behavior. The axons of the ventral medial cells of the brain, the only cells in insects to synthesize EH, project the entire length of the CNS; hence, they are in proximity to the circuits that respond to EH. Thus, there is the potential for neurons of the CNS to be exposed to EH released locally within the segmental ganglia. What then is the role for circulating EH? At pupal ecdysis, a set of dermal glands, the Verson's glands, secrete a "cement" layer that covers the surface of the newly exposed cuticle. These glands fail to discharge in ligatured animals that ecdyse in the absence of circulating EH (Truman, 1992 and references).

Recently, it has become clear that a second hormone is involved in ecdysis. Manduca sexta Ecdysis-triggering hormone, Mas-ETH (see Drosophila ETH), was described in the tobacco hornworm. It has been suggested that a peripheral step involving ETH is interposed between the secretion of EH and the subsequent triggering of the ecdysial motor programs from the CNS. In this model ETH would be the direct trigger for CNS activation in ecdysis. Mas-ETH contains 26 amino acids and is produced by a segmentally distributed endocrine system of epitracheal glands (EGs). The EGs, whose most prominent component is a large white Inka cell, are situated on ventral tracheal trunks near each spiracle. The EGs undergo a marked reduction in volume, appearance, and immunohistochemical staining during ecdysis, at which time Mas-ETH is found in the hemolymph. Injection of EG extract or synthetic Mas-ETH into pharate larvae, pupae, or adults initiates preecdysis within 2 to 10 minutes, followed by ecdysis. Sensitivity to injected Mas-ETH appears much earlier before ecdysis and occurs with shorter latency than that reported for eclosion hormone. The isolated central nervous system responds to Mas-ETH, but not to Eclosion hormone, with patterned motor bursting corresponding to in vivo preecdysis and ecdysis. Mas-ETH may be an immediate blood-borne trigger for ecdysis through a direct action on the nervous system (Zitnan, 1996).

What then are the roles of EH and ETH? The location of the Inka cells is especially intriguing: they are found on the trachea near each spiracle, the openings through which the tracheal linings are withdrawn during ecdysis. These linings are the most fragile parts of the old cuticle: if they are torn and left behind during ecdysis, the tracheae to that region are obstructed and oxygen delivery is impaired. Thus the Inka cells are in an excellent position to monitor the changes in the tracheae during the molt. Sitting at this most vunerable site on the periphery, they might serve as a final checkpoint to ensure that the old cuticle is ready to be sloughed off before the insect irreversibly commits itself to ecdysis. Although EH is likely to trigger the release of ETH from Inka cells, some observations remain unexplained. As described above, ecdysis still occurs when the secretion of EH into the circulation is prevented by removal of the peripheral release sites. The occurrence of the ecdysal behaviors under these conditions is thought to be due to a local release of EH within the CNS. Under these circumstances, peripheral EH targets, such as dermal glands, should not secrete their products because of the lack of circulating EH. Thus lack of blood borne EH should result in a failure of the Inka cells to release their peptide. Nevertheless, ecdysis occurs in the absence of circulating EH. Thus the relation of EH and ETH is not yet clear (Truman, 1996).

The role of the two EH-producing neurons in Drosophila was examined by using an EH cell-specific enhancer to activate cell death genes reaper and head involution defective to ablate the EH cells. In the EH cell knockout flies, larval and adult ecdyses are disrupted, yet a third of the knockouts emerge as adults, demonstrating that EH has a significant but nonessential role in ecdysis. The EH cell knockouts have discrete behavioral deficits, including slow, uncoordinated eclosion and an insensitivity to ecdysis-triggering hormone. The knockouts lack the lights-on eclosion response despite having a normal circadian eclosion rhythm. This study represents a novel approach to the dissection of neuropeptide regulation of a complex behavioral program (McNabb, 1997).


GENE STRUCTURE

cDNA clone length - 702 bases

Bases in 5' UTR - 164

Exons - 4

Bases in 3' UTR - 250


PROTEIN STRUCTURE

Amino Acids - 97

Structural Domains

A portion of the Drosophila EH DNA sequence is 67% identical to that encoding amino acids 13-62 of Manduca EH. The amino acid identity is 68% in this region. The DNA sequence encoding the N-terminus of the peptide is located on the upstream exon number 2. Exon number 1 is untranslated. Exons 1 and 2 have no similarity to Menduca sequences. The N-terminal 24 amino acids constitute a hydrophobic signal peptide found in secreted proteins. The most probable site of signal peptide cleavage is immediately following Ala 24. The size of the Drosophila peptide is 11 amino acids longer than that of Manduca and Bombyx, with 10 additional amino acids at the N-terminus and 1 at the C-terminus. A potential endoproteolytic cleavage site in the Drosophila peptide raises the possibility that two peptides are derived from a single precursor which may regulate related or distinct physiological processes (Horodyski, 1993).


Eclosion hormone: Evolutionary Homologs | Regulation | Developmental Biology | References

date revised: 20 MAR 97 

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