Eclosion hormone
How important is the C-terminus of Bombyx EH for biological activity? A molecule lacking the four amino acids at the C-terminus is 100-fold less active than native EH. By contrast, a peptide lacking the six N-terminal amino acids retain full activity. Thus, the C-terminus plays an important part in the biological function of EH. This domain could potentially function in binding to a receptor in the target cells. Another notable feature is the conservation of the six cysteine residues. These form intramolecular bonds. The positions of these disulfide bonds has been determined in Manduca and Bombyx EH (Kataoka, 1992 and Kono, 1990).
The locations of the three disulfide bonds of eclosion hormone (EH) isolated from
Manduca sexta were assigned by sequence analysis of thermolysin fragments and by
comparison of a key heterodimeric fragment to regiospecifically synthesized parallel
and antiparallel isomers. The complete structure of Manduca EH is a
62-residue peptide that has three disulfide bonds between Cys14-Cys38,
Cys18-Cys34, and Cys21-Cys49 (Kataoka, 1992).
The cDNAs encoding eclosion hormone in the silkworm Bombyx mori were
isolated and sequenced. The pre-EH molecule contains a
26-amino acid signal peptide and a 62-amino acid mature EH. A leucine residue at the carboxyl terminal of EH had not been
detected directly by the peptide analysis. Primer extension and Northern
hybridization analyses reveal that 0.9 kb mRNA is transcribed with a
66-nucleotide non-translated sequence at the 5'-end region. In situ hybridization
shows that the EH gene is expressed in two pairs of nuerosecretory cells in the brain
of 5th instar larva (Kamito, 1992).
Recombinant silkworm eclosion hormone was produced for the first time in yeast,
which was transformed with a shuttle plasmid containing a construct coding a signal
peptide and the mature sequence of the silkworm eclosion hormone. Successfully
transformed yeast process recombinant silkworm eclosion hormone I (EH-I) was
transported to periplasm at a concentration of 60 micrograms per liter of culture.
The biological activity of the purified recombinant silkworm eclosion hormone
exhibits the ED50 value of 0.2 ng, which is the same as that of the authentic hormone
isolated from the silkworm brain (Hayashi, 1990).
A gene encoding eclosion hormone (EH) from the silkworm Bombyx mori was
chemically synthesized, inserted into a secretion vector and expressed in Escherichia
coli, leading to the production of biologically active EH. Sequence analysis of
cystine-containing peptides in a thermolysin digest of this EH established the locations
of 3 disulfide bonds in the molecule. The 6 residues at
the NH2-terminus are dispensable but 4 residues at the COOH-terminal play an
important role in EH activity (Kono, 1990).
The EH-encoding gene of the tobacco hornworm Manduca sexta
was isolated by using a designed 72-mer oligonucleotide probe. Sequence analysis of this gene and its corresponding cDNA show that the EH gene is 7.8 kilobases and consists of three exons. Exon I is totally nontranslated; exon II contains a 26-amino acid signal peptide and amino acids 1-4 of the EH peptide, and exon III encodes the remainder of the peptide. The EH gene is present in a single copy per haploid genome and transcribes an 0.8-kb mRNA that is expressed in larval, diapausing pupal, and developing adult brains but not in the ventral nerve cord or in nonneural tissues. In situ hybridization shows that the EH gene is expressed in two pairs of ventromedial neurosecretory cells in brains of both larvae and developing adults (Horodyski, 1989).
A successful ecdysis in insects requires the precise coordination of behaviour with the
developmental changes that occur late in a molt. This coordination involves two sets
of endocrine cells: the peripherally located Inka cells, which release ecdysis triggering hormone (ETH), and the centrally located neurosecretory neurons, the VM neurons,
which release eclosion hormone (EH). These two sets of endocrine cells mutually
excite one another: EH acts on the Inka cells to cause the release of ETH. ETH, in
turn, acts on the VM neurons to cause the release of EH. This positive-feedback
relationship allows the Inka cells and the VM neurons to be the peripheral and
central halves, respectively, of a decision-making circuit. Once conditions for both
halves have been satisfied, their reciprocal excitation results in a massive EH/ETH
surge in the blood as well as a release of EH within the central nervous system. This
phasic signal then causes the tonic activation of a distributed network of peptidergic
neurons that contain crustacean cardioactive peptide (Ewer, 1997).
In insects, the shedding of the old cuticle at the end of a molt involves a stereotyped
sequence of distinct behaviors. Two peptides, ecdysis-triggering hormone (ETH) and
crustacean cardioactive peptide (CCAP), elicit the first two motor behaviors, the
pre-ecdysis and ecdysis behaviors, respectively. Exposing isolated abdominal ganglia
to ETH results in the generation of sustained pre-ecdysis bursts. By contrast,
exposing the entire isolated CNS to ETH results in the sequential appearance of
pre-ecdysis and ecdysis motor outputs. Previous research has shown that ETH
activates neurons within the brain that then release eclosion hormone within the CNS.
The latter elevates cGMP levels within target neurons and increases the excitability of a group of
neurons containing CCAP. The ETH-induced onset of ecdysis
bursts is always associated with a rise in intracellular cGMP within these CCAP
neurons. CCAP immunoreactivity decreases centrally during
normal ecdysis. Isolated, desheathed abdominal ganglia respond to CCAP by
generating rhythmical ecdysis bursts. These ecdysis motor bursts persist as long as
CCAP is present and can be reinduced by successive application of the peptide.
CCAP exposure also actively terminates pre-ecdysis bursts from the abdominal CNS,
even in the continued presence of ETH. Thus, the sequential performance of the two
behaviors arises from one modulator activating the first behavior and also initiating the
release of the second modulator. The second modulator then turns off the first
behavior while activating the second (Gammie, 1997).
Three insect peptide hormones, eclosion hormone (EH), ecdysis-triggering hormone (ETH) and crustacean cardioactive peptide (CCAP), have been implicated in controlling ecdysis behavior in insects. This study examines the interactions among these three peptides in the regulation of the ecdysis sequence. Using intracellular recordings, it has been found that ETH is a potent activator of the EH neurons, causing spontaneous action potential firing, broadening of the action potential and an increase in spike peak amplitude. In turn, electrical stimulation of the EH neurons or bath application of EH to desheathed ganglia results in the elevation of cyclic GMP (cGMP) levels within the Cell 27/704 group (which contains CCAP). This cGMP production increases the excitability of these neurons, thereby facilitating CCAP release and the generation of the ecdysis motor program. Extracellular recordings from isolated nervous systems show that EH has no effect on nervous systems with an intact sheath. In desheathed preparations, in contrast, EH causes only the ecdysis motor output. The latency from EH application to ecdysis is longer than that after CCAP application, but shorter than that when ETH is applied to a whole central nervous system. These data support a model in which ETH causes pre-ecdysis behavior and at higher concentrations stimulates the EH neurons. EH release then facilitates the onset of ecdysis by enhancing the excitability of the CCAP neurons (Gammie, 1999).
Ecdysis, or molting behavior, in insects requires the sequential action of high levels of ecdysteroids, which induce accumulation of ecdysis-triggering hormone (ETH) in Inka cells, followed by low levels of ecdysteroids, permissive for the onset of the behavior. High ecdysteroid levels suppress the onset of the behavioral sequence by inhibiting the development of competence to secrete ETH. In pharate pupae of Manduca sexta, Inka cells in the epitracheal glands normally develop competence to secrete ETH in response to eclosion hormone (EH) 8 h before pupation. Injection of 20-hydroxyecdysone (20E) into precompetent insects prevents this acquisition of competence, but does not affect EH-evoked accumulation of the second messenger cyclic GMP. Precompetent glands acquire competence in vitro after overnight culture, and this can be prevented by the inclusion of 20E at concentrations greater than 0.1 microg ml(-1)in the culture medium. Actinomycin D completely inhibits the acquisition of competence, demonstrating that it is dependent on transcriptional events. Cultured epitracheal glands become refractory to the inhibitory effects of 20E in the acquisition of competence at least 3 h earlier than for Actinomycin D, indicating that 20E acts on an early step in a sequence of nuclear events leading to transcription of a structural gene. These findings suggest that declining ecdysteroid levels permit a late event in transcription, the product of which is downstream of EH receptor activation and cyclic GMP accumulation in the cascade leading to ETH secretion (Kingan, 2000).
During ecdysis EH induces an increase in cyclic GMP (cGMP). Using antibodies against this second messenger, it has been shown
that this increase is confined to a network of 50 peptidergic neurons distributed throughout the CNS. Increases appeared 30 min after EH treatment, spread rapidly throughout these neurons, and are extremely long lived. This response is synaptically driven, and does not involve the soluble, nitric oxide (NO)-activated, guanylate cyclase. Stereotyped variations in the duration of the cGMP response among neurons suggest a role in coordinating responses having different latencies and durations (Ewer, 1994).
Additional studies were carried out to test whether nitric oxide is involved in increases in cGMP caused by the activity of eclosion hormone. In Manduca, the eclosion hormone-stimulated increase in cGMP is independent of either nitric oxide or carbon monoxide. In addition, a wide variety of inhibitors of lipid metabolism block the eclosion hormone-stimulated cGMP increase. This supports the hypothesis that the activation of the gyanylate cyclase is mediated by a lipid messenger. Eclosion hormone stimulates an increase in the levels of inositol(1,4,5)trisphosphate. The time course of this increase is consistent with the hyposthesis that eclosion hormone stimulation of a phospholipase C is an early event in the cascade that results in an increase in cGMP. Receptor-mediated lipid hydrolysis is often mediated by G protein-coupled receptors. Experiments using pertussis toxin show that the eclosion hormone-stimulated increase in cGMP is not mediated by a pertussis toxin-sensitive G protein (Morton, 1995b)
Eclosion hormone stimulates the formation of two
intracellular second messengers, cGMP and inositol(1,4,5)trisphosphate in the
abdominal ganglia of Bombyx mori. In order to elucidate the intracellular signaling
pathway involving these second messengers, the eclosion
hormone-mediated signal transduction was studied using saponin-treated abdominal ganglia. Eclosion hormone activates nitric oxide synthase. The eclosion hormone-induced cGMP increase is inhibited by various enzyme
inhibitors: these include NG-nitro-arginine, a nitric oxide synthase inhibitor (EGTA), a calcium
chelating reagent (W-5), a calmodulin inhibitor and compound 48/80 (a phospholipase C
inhibitor). The inositol(1,4,5)trisphosphate stimulates the formation of cGMP in
the Bombyx abdominal ganglia. Based on these findings a pathway is proposed: the signal initially triggered by eclosion hormone and eclosion
hormone receptor complex induces activation of phospholipase C, which produces
inositol(1,4,5)trisphosphate. Inositol(1,4,5)trisphosphate increases intracellular Ca2+,
followed by subsequent activation of nitric oxide synthase through the formation of
Ca(2+)-calmodulin complex (note: contradictory evidence above in Ewer [1994]). The reaction product, nitric oxide, acts on soluble
guanylate cyclase to stimulate cGMP formation that induces the ecdysis behavior in
Bombyx pharate adults (Shibanaka, 1994).
Eclosion hormone stimulates phosphatidylinositol
(PtdIns) hydrolysis in abdominal ganglia isolated from Bombyx mori at a specific stage in adult development. In the presence of EH, incubation of abdominal ganglia from silkworm pharate adults
leads to an increase in formation of inositol(1,4,5)trisphosphate, but this increase
takes place transiently, maximum increase being observed 30 s after the addition of
EH. PtdIns hydrolysis is stimulated by exogenous EH in a dose-dependent fashion
and is completely abolished by the phospholipase C inhibitors: neomycin and
compound 48/80. The EH-induced PtdIns hydrolysis develops in parallel with the
EH-induced eclosion behaviour during development of the adult. These results suggest
that the EH-stimulated PtdIns hydrolysis plays an important role in EH-mediated signal
transduction during adult development of B. mori (Shibanaka, 1993).
The signal transduction of the peptide Eclosion hormone in the silkworm Bombyx
mori appears to be mediated via the second messenger cyclic GMP throughout the
life cycle. Injection of 8-bromo-cGMP induces the ecdysis behavior in pharate adults
with similar latency to eclosion hormone-induced ecdysis; the moulting occurs 50-70
min after the injection. The potency of 8Br-cGMP is 10(2) fold higher than that of
cGMP; the efficacy is increased by the co-injection of the phosphodiesterase
inhibitor IBMX. In silkworm pupal ecdysis, both Eclosion
hormone and 8Br-cGMP induce the molting behavior in a dose-dependent
manner. The adult development of the ability to respond to 8Br-cGMP takes place
concomitantly with the response to Eclosion hormone. Both the developmental time
courses are shifted by a shift in the light and dark cycles. Accordingly, the sensitivities
to the peptide and cyclic nucleotide develop correspondingly with shifts in circadian rhythm of light and dark. Thus throughout the silkworm life cycle, eclosion hormone is
effective to trigger the ecdysis behavior; cGMP plays a crucial role as the second
messenger in the eclosion hormone-mediated signal transduction (Shibanaka, 1991).
The neuropeptide eclosion hormone acts directly on the nervous system of the tobacco
hornworm Manduca sexta to trigger ecdysis behavior at the end of each molt. The action of eclosion hormone is mediated via the
intracellular messenger cyclic GMP. No
stimulation of guanylate cyclase is seen in homogenized nervous tissue, suggesting
that eclosion hormone does not directly stimulate a membrane-bound form of
guanylate cyclase. Nitric oxide synthase inhibitors, N-methylarginine and nitroarginine,
have no effect on eclosion hormone-stimulated cyclic GMP levels. By contrast,
4-bromophenacyl bromide, an inhibitor of arachidonic acid release, and
nordihydroguaiaretic acid, an inhibitor of arachidonic acid metabolism, almost
completely abolish the eclosion hormone-stimulated cyclic GMP increase. It is
hypothesized that eclosion hormone receptors are coupled to a lipase, activation of
which causes the release of arachidonic acid. Either the arachidonic acid directly
stimulates the soluble guanylate cyclase or further metabolism of arachidonic acid
yields compounds that activate guanylate cyclase (Morton, 1992).
Two phosphoproteins, both with an apparent molecular
weight of 54 kDa, are described in the CNS of the tobacco hornworm, Manduca sexta. Their
phosphorylation is regulated by the neuropeptide Eclosion hormone, and the
second messenger cGMP. The phosphoprotein thus have been named the EGPs (Eclosion hormone-
and cGMP-regulated phosphoproteins). Although cAMP is more effective than
cGMP at stimulating the phosphorylation of the EGPs in CNS homogenates, cGMP is more effective in the
intact CNS. Since cGMP mediates the action of EH, this
strongly suggests that cGMP is the second messenger that stimulates the
phosphorylation of the EGPs in vivo. The EGPs can only be phosphorylated in vitro
during discrete time periods in the life of Manduca. During the larval and pupal
molts, the EGPs can first be phosphorylated just prior to ecdysis. Their ability to be
phosphorylated is correlated with the time when the insect is sensitive to EH. This
close temporal correlation suggests that the ability to phosphorylate the EGPs
determines when the insect can first respond to EH. During adult development, the
EGPs first appeared 6 d before sensitivity to EH, suggesting that at
this stage other factors may also be involved in the regulation of sensitivity. For the
ecdyses of all 3 stages, EH appears to stimulate the phosphorylation of the EGPs at
ecdysis. The EGPs are found in all regions of the prepupal nervous system, but only in the abdominal and pterothoracic ganglia of the
developing adult. Fractionation of nervous system homogenates by ultracentrifugation
reveals that one of the EGPs is present only in the pellet fraction, whereas the
other is approximately equally distributed between pellet and supernatant.
Furthermore, the EGPs in the pellet fraction can be partially solubilized with
detergents and high salt concentrations (Morton, 1988a).
Two proteins (the EGPs) in the CNS of the tobacco hornworm Manduca sexta are
phosphorylated by the action of the neuropeptide eclosion hormone (EH), which
triggers ecdysis behavior. The onset of sensitivity to
EH requires prior exposure to the steroid hormone 20-hydroxyecdysone (20-HE). A
series of steroid removal and replacement experiments indicates that the EGPs are
also regulated by 20-HE with a time course that is similar to that seen for the 20-HE
regulation of behavioral sensitivity to EH. This suggests that the steroid regulation of
EH sensitivity is due to the regulation of the EGPs. The appearance of the EGPs
requires not only the presence of 20-HE, but also its subsequent removal. The
appearance of the EGPs can be blocked in vivo and in vitro by maintaining artificially
elevated levels of 20-HE, but only up to a particular time in development. The ending
of this steroid-sensitive period occurs 4 hr before the normal appearance of the EGPs,
consistent with the hypothesis that the EGPs are synthesized de novo in response to
the removal of 20-HE (Morton, 1988b).
The action of Eclosion hormone is mediated by an increase in cGMP and is associated with the phophorylation of two proteins, the EGPs. The ability of insects to responed to EH is developmentally regulated with sensitivity being first seen at about 8 hours prior to the normal time of ecdysis. The EGPs are also first detectable in the CNS at 8 hours prior to ecdysis, suggesting that it is their synthesis which determines EH sensitivity. The protein synthesis inhibitor cyclohexamide was used to study the development of the events leading to pupal ecdysis in Manduca. Protein synthesis is necessary about 10 hours before ecdysis for both the development of EH sensitivity and for the appearance of the EGPs (Morton, 1995a)
In the moth Manduca sexta, the declining ecdysteroid titer on the final day of the molt
from the fourth to the fifth larval instar acts on the ventromedial neurosecretory cells
(VM cells) to stimulate the release of eclosion hormone (EH). EH then triggers the
motor programs involved in ecdysis behavior. Intracellular recordings made
from the VM cells throughout the intermolt and molting stages show no spontaneous
action potentials until 0.9 hr before ecdysis (during the expected time of EH release),
when 50% of the VM cells fire tonically at rest. This change is associated with a
marked reduction in VM cell threshold without alteration of input resistance, resting
potential, or synaptic drive. The increase in VM cell excitability is dependent on the
declining ecdysteroid titer, because the injection of 20-hydroxyecdysone (20-HE) 11.5
hr before ecdysis significantly delays the expected decrease in VM cell threshold.
Since the same steroid treatment given 4.6 hr before ecydysis does not delay the
subsequent increase in VM cell excitability, the inhibitory actions of 20-HE appear not
to be mediated by a rapid membrane mechanism. The possible involvement of
genomic events in the steroid-dependent increase in VM cell excitability was
examined using the RNA synthesis inhibitor actinomycin D (AcD). When injected 6.3
hr before ecdysis, AcD blocks EH release without altering the response of the
nervous system to exogenous peptide. Such AcD treatments also prevents the
increase in VM cell excitability. These results suggest that the declining ecdysteroid
titer increases the excitability of the VM cells via a transcription-dependent process (Hewes, 1994).
The effects of removing or disconnecting portions of the central nervous system prior to the time of EH release was examined on the initiation of pre-ecdysis and ecdysis behaviors at the final larval moltof Manduca. The initiation of pre-ecdysis abdominal conpressions at the appropriate time requires the terminal abdominal ganglion but not the brain. The initiation of pre-ecdysis proleg retractions at the appropriate time requires neither the terminal abdominal ganglion nor the brain. The initiation of ecdysis at the appropriate time usually requires the brain but not the terminal abdominal ganglion. Premature pre-ecdysis (but not ecdysis) can be elicited in isolated abdomens by injection of eclosion hormone. Finally, pre-ecdysis behavior performed by brainless larvae is not associated with the normal elevation of eclosion hormone bioactivity in the hemolymph or the normal loss of of EH immunoreactivity from peripheral neurohemal release sites (Novicki, 1996).
At the end of metamorphosis, the intersegmental muscles of the moth Antheraea
polyphemus undergo rapid degeneration in response to the peptide eclosion hormone
(EH). Muscle death is preceded by a 22-fold increase in muscle
guanosine-3',5'-cyclic monophosphate (cGMP) titers, which peak 60 min after
peptide exposure. EH induces a dose-dependent increase in muscle cGMP content with a
threshold dose similar to that needed to induce cell death. Exogenous cGMP, but not
cAMP, mimic the action of EH. Sodium nitroprusside, a potent stimulator of
guanylate cyclase, and methylated xanthines, a class of 3',5'-cyclic-nucleotide
phosphodiesterase inhibitors, also induce the selective death of these muscles. It is
concluded that an elevation of cGMP level is involved in EH-induced muscle
degeneration. The intersegmental muscles become sensitive to EH at the end of adult
development in response to the declining titers of the steroid molting hormones, the
ecdysteroids. At earlier times, treatment with EH, exogenous cGMP, sodium
nitroprusside, or methylated xanthines is ineffective in causing cell death.
Nevertheless, treatment with EH at this time results in a marked increase in
intersegmental-muscle cGMP. Thus, the onset of physiological responsiveness to the
peptide hormone presumably results from biochemical changes distal to the EH
receptors and guanylate cyclase (Schwartz, 1984).
Ecdysis behavior in arthropods is driven by complex interactions among multiple neuropeptide signaling systems. To understand the roles of neuropeptides and their receptors in the red flour beetle, Tribolium castaneum, systemic RNA interference (RNAi) experiments were performed utilizing post-embryonic injections of double-stranded (ds) RNAs corresponding to ten gene products representing four different peptide signaling pathways: eclosion hormone (EH), ecdysis triggering hormone (ETH), crustacean cardioactive peptide (CCAP) and bursicon. Behavioral deficiencies and developmental arrests occurred as follows: RNAi of (1) eh or eth disrupted preecdysis behavior and prevented subsequent ecdysis behavior; (2) ccap interrupted ecdysis behavior; and (3) bursicon subunits resulted in wrinkled elytra due to incomplete wing expansion, but there was no effect on cuticle tanning or viability. RNAi of genes encoding receptors for those peptides produced phenocopies comparable to those of their respective cognate neuropeptides, except in those cases where more than one receptor was identified. The phenotypes resulting from neuropeptide RNAi in Tribolium differ substantially from phenotypes of the respective Drosophila mutants. Results from this study suggest that the functions of neuropeptidergic systems that drive innate ecdysis behavior have undergone significant changes during the evolution of arthropods (Arakane, 2008).
The earliest peptide signal for ecdysis behavior in Manduca so far identified is corazonin, which triggers the neuroendocrine cascade by inducing the release of ETH from the epitracheal glands. However, in Tribolium and also in other coleopteran species, corazonin is apparently absent because there has been no report of immunoreactivity with corazonin antiserum. Furthermore, no Tribolium sequences encoding this peptide or its receptor have been reported so far. Thus, the signal initiating ecdysis in this coleopteran must be something other than corazonin. In the case of the albino locust, which is believed to lack corazonin, there is no ecdysis deficiency reported, implying that corazonin may be lepidopteran-specific as a signal for ETH release (Arakane, 2008).
Severe deficiencies in preecdysis behavior were observed in Tribolium after treatment with either dseh, dseth, dsethr or dsethr-a. There were some occasional twitching-like D–V contractions in these insects, which may have been caused by incomplete suppression of the targeted mRNA. This deficiency in preecdysis behavior resulted in the failure of subsequent ecdysis behavior, which in turn resulted in failure to eclose and finally in death. The ETH signal has been found to be necessary and sufficient in Drosophila for both preecdysis and ecdysis behaviors. In Manduca, the sufficiency of ETH for inducing premature preecdysis and ecdysis behaviors also supports the conclusion that ETH is one of the earliest ecdysis-initiating molecules. This study also supports the notion of ETH being an early essential signal for ecdysis in Tribolium. In addition, dseth was found to cause deficiencies in larval and pupal ecdysis depending on the time of injection (Arakane, 2008).
Two subtypes of ETH receptors, A and B, arising from mutually exclusive alternative exon usage, are highly conserved in insects. Studies with Manduca and Drosophila showed that ethr-a is expressed mainly in numerous peptidergic cells in the CNS, while ethr-b is expressed in poorly-characterized interneuron cells. Exon-specific dsRNA in Tribolium showed that dsethr-a-treated insects had significantly fewer D–V contractions, a phenotype identical to that obtained following treatment with dseth or dseh. However, eclosion of insects injected with dsethr-b occurred normally, with no substantial reduction in preecdysial D–V contractions. Therefore, ethr-a, which activates downstream peptidergic signals, is a necessary component in Tribolium eclosion, whereas the role of ethr-b remains unclear. Switching from one behavior to the next within a certain time interval in the behavioral sequence had been thought to involve inhibitory neurons, it was not possible to determine whether premature ecdysis behavior occurred in the dsethr-b-injected Tribolium as a result of defects in inhibitory neurons (Arakane, 2008).
Positive feedback between EH and ETH has been found in Manduca. Release of ETH is triggered by corazonin as the initiator of the EH-ETH feedback loop. EH-associated positive feedback induces a massive release of ETH for the initiation of ecdysis motor patterns. However, a positive feedback loop was not found in Drosophila. Rather, Drosophila EH apparently acts downstream of ETH and is the factor triggering ecdysis behavior, a conclusion based on the timing of the cellular response of EH cells, which show Ca2+ elevation upon treatment with ETH during pupal ecdysis. Surprisingly, the EH-cell-knockout in Drosophila resulted in only a partial impairment during adult eclosion, with a significant proportion of the insects dying before pupation. In Tribolium, EH was required for early preecdysis behavior. Thus, the ETH-EH feedback loop, if it occurs, probably occurs during preecdysis in Tribolium, as it does in Manduca (Arakane, 2008).
In Drosophila, the ccap null mutant did not show any abnormality during development or ecdysis, whereas ccap-cell ablation resulted in deficiencies in both pupal and adult ecdysis. Therefore, it was concluded that other neuropeptides, which are co-expressed in the CCAP cells, are probably responsible for the phenotypes of the ccap cell-knockout. Thus, the role of CCAP in Drosophila ecdysis remains unclear. The neuropeptides co-expressed in the CCAP cells with presumed functions in ecdysis are bursicon, partner of bursicon and myoinhibitory peptide (Arakane, 2008).
In contrast to Drosophila, ccap RNAi in Tribolium resulted in a lethal arrest during ecdysis. The ecdysis deficiency was associated with significantly weaker behaviors, including reverse-bending, wing air-filling and A–P contraction, whereas these insects underwent normal preecdysis behavior. The dsccapr-2 treatment resulted in the same phenotype as that of dsccap, whereas dsccapr-1 treatment did not produce any abnormalities. Therefore, CCAP and CCAPR-2 are in the signaling pathway for ecdysis behavior, while the function of CCAPR-1 remains unknown (Arakane, 2008).
The bur and pbur genes in Tribolium form a tandem pair in the genome, separated by only ~1.3 kb. This arrangement is similar to the bur/pbur gene structure in the honeybee. Previously, it was proposed that the honeybee bur/pbur gene consisted of one long open reading frame encoding a multi-domain protein including both bur and pbur. Subsequently, however, different transcription units for bur and pbur were reported. Using RT-PCR this study determined that Tribolium bur and pbur are probably separate transcription units. Results from gene-specific RNAi for both bur and pbur support the hypothesis that two different transcription units exist. In addition, whereas mosquito bur was found to undergo trans-splicing, this study found that the Tribolium bur and pbur genes contain complete open reading frames, excluding the possibility of trans-splicing (Arakane, 2008).
A heterodimeric complex Bur/pBur consisting of the products of the bur and pbur genes is a cysteine knot family hormone that has been reported to initiate two different functional activities in Drosophila, namely cuticle tanning and wing expansion after adult eclosion. Drosophila mutants for bur and receptor mutant rickets (rk) showed deficiencies in both tanning and wing expansion (Arakane, 2008).
This study discovered that treatments with dsbur, dspbur or dsrk all produced similar postecdysis defects, namely weak postecdysis activity, wrinkled elytra and a failure to retract the hindwing, but none of these caused lethality within the observation time of 2–3 weeks after eclosion. In Drosophila, bursicon induces wing cell death and wing expansion after eclosion. The wrinkled elytra and the deficiency in proper folding of the hindwing after RNAi in Tribolium may be equivalent to the Drosophila phenotype. Interestingly, RNAi of these genes resulted in significantly diminished strengths in preecdysis behavior. The data imply that bur/pbur and their putative receptor rk in Tribolium are involved in the regulation of preecdysis behavior, and even more in postecdysis behavior. An additional unique phenotype was found only in insects injected with dsbur, which exhibited weaker A–P contractions during ecdysis and consequently an extended duration for completion of the shedding of the exuvium. These observations suggest an unknown but separate function for bursicon in addition to its role as a component of dimeric Bur/pBur acting through its receptor Rickets. Alternatively, the phenotypic variation could have been caused by different dosages of remaining transcripts in RNAi or by stability of the protein that had been produced earlier (Arakane, 2008).
Perhaps the most interesting observation in this study is that normal tanning occurs in beetles subjected to RNAi for the group of genes encoding the neuropeptides described in this study. Maturation of the cuticle is a gradual process of pigmentation and sclerotization during the first five days after eclosion. A recent study involving Drosophila has shown that bursicon acts through phosphorylation of tyrosine hydroxylase, which catalyzes an early step of catecholamine production for cuticle tanning. It has been shown previously that RNAi of Tribolium laccase 2, which is a phenoloxidase downstream of tyrosine hydroxylase in the same metabolic pathway, suppressed cuticle tanning. This result indicated that a similar cuticle tanning pathway exists in both Drosophila and Tribolium. It is concluded, therefore, that the Bur/pBur signaling pathway is required for proper wing expansion and folding in Tribolium but not for tyrosine hydroxylase/laccase-mediated tanning. Recent study in the silkworm also reported that RNAi of bur found no distinct tanning phenotype, while a deficiency in wing expansion was observed. In addition, the regulation of cuticle tanning in Tribolium appears to be different from that of Drosophila, even though the tanning pathway itself is probably conserved (Arakane, 2008).
This has been a study of key peptidergic signaling systems for insect ecdysis in T. castaneum, representing a more basal holometabolous order (Coleoptera) relative to the species of Lepidoptera and Diptera studied previously. RNAi in Tribolium followed by behavioral analysis revealed differences in the roles of EH, CCAP and bursicon compared to those found in Drosophila. Both ETH and EH are necessary for preecdysis and ecdysis behaviors in Tribolium, while an essential role of EH has not been found in Drosophila. CCAP is necessary for ecdysis behavior in Tribolium, whereas the Drosophila ccap null mutant shows normal ecdysis. In Tribolium Bur/pBur is necessary for postecdysis behavior, including wing expansion and folding, whereas, unlike the case in dipterans, it does not have a role in cuticle tanning (Arakane, 2008).
Bur/pBur signaling is involved in preecdysis behavior. Only bursicon appears to have an additional role in ecdysis behavior in Tribolium (Arakane, 2008).
The differences in the precise roles of each peptidergic component among Tribolium, Drosophila and Manduca in controlling innate ecdysis behavior and cuticle tanning can be interpreted as a consequence of evolution. The loss of essential roles for EH and CCAP as well as a gain in function for bursicon in Drosophila may be associated with modifications of the requirements of those neuropeptidergic signals in these processes, whereas the more ancestral Tribolium and possibly Manduca strictly require EH and CCAP signaling systems. A comparative analysis of the functions of peptidergic signals from additional taxa will provide further insights into the evolution and regulation of ecdysis and tanning in the Ecdysozoa (Arakane, 2008).
Home page: The Interactive Fly © 1995, 1996 Thomas B. Brody, Ph.D.
The Interactive Fly resides on the
Eclosion hormone:
Biological Overview
| Regulation
| Developmental Biology
| References
Society for Developmental Biology's Web server.