Modulatory effects of bombyxin on ecdysteroidogenesis in Bombyx mori prothoracic glands

Sublethal effects of nitenpyram on the development of silkworm

The utilization of nitenpyram for aphid and whitefly control may induce environmental contamination and negative repercussions on non-target organisms. Formerly, we found that nitenpyram would pollute the peripheral and sub-peripheral areas of the adjacent mulberry orchard. Under acute toxicity conditions, nitenpyram induced oxidative damage in silkworms, affected biological metabolism, synthesis, immunity, and signal transduction. Considering the impact of nitenpyram mist drift on mulberry leaves, we investigated the effects of low concentrations of nitenpyram on silkworms. The results showed that silkworms exposed to 0.17 mg/L, 0.35 mg/L and 0.70 mg/L of nitenpyram (1/40 LC50, 1/20 LC50 and 1/10 LC50) showed obvious poisoning symptoms. The cocoon weight and cocoon shell weight decreased gradually with increases in the concentration, and these decreases prolonged the growth and development time of silkworms and induced the detoxification enzymes carboxylesterase (CarE) and glutathione-S-transferase (GST) to cope with the stress damage caused by nitenpyram. Exposure to low concentrations of nitenpyram downregulates genes involved in the drug metabolism-other enzymes and peroxisome pathway in silkworms. Additionally, through injection of miRNA mimics and inhibitors, we discovered that detoxifying enzyme pathway genes are influenced by bmo-miR-3382-3P, bmo-miR-3213-5P and bmo-miR-133, regulating the immune response of silkworms. This study provides an overall view of the toxicity and detoxification metabolism of nitenpyram in silkworm, and provides a reference for environmental assessment.

The interaction between 20-hydroxyecdysone and AMPK through PI3K activation in Chinese mitten crab, Eriocheir sinensis

In crustaceans, the steroid hormone 20-hydroxyecdysone (20E) initiates molting, and the molting process is also regulated by energy metabolism. AMPK is an energy sensor and plays a critical role in systemic energy balance. Here, the regulatory mechanism in the interaction between 20E and AMPK was investigated in Chinese mitten crab, Eriocheir sinensis. The results showed that the 20E concentration and the mRNA expression levels of 20E receptors in hepatopancreas were down-regulated post AMPK activator (AICAR) treatment, and were up-regulated after AMPK inhibitor (Compound C) injection in crabs. Besides, the molt-inhibiting hormone (MIH) gene expression in eyestalk showed the opposite patterns in response to the AICAR and Compound C treatment, respectively. Further investigation found that there was a significant reduction in 20E concentration post PI3K inhibitor (LY294002) treatment, and the phosphorylation level of PI3K was increased in hepatopancreas after AMPK inhibitor injection. On the other hand, the positive regulation of PI3K-mediated activation of AMPK was also observed, the phosphorylation levels of AMPKα, AMPKβ and PI3K in hepatopancreas were significantly increased post 20E injection. In addition, the phosphorylation levels of AMPKα and AMPKβ induced by 20E were decreased after the injection of PI3K inhibitor. Taken together, these results suggest that the regulatory cross-talk between 20E and AMPK is likely to act through PI3K pathway in E. sinensis, which appeared to be helpful for a better understanding in molting regulation.

Signaling in cAMP-stimulated ecdysteroidogenesis in prothoracic glands of the silkworm, Bombyx mori

In the present study, we investigated downstream pathways of cyclic adenosine monophosphate (cAMP) signaling (which is related to prothoracicotropic hormone (PTTH)-stimulated ecdysteroidogenesis) in Bombyx mori prothoracic glands (PGs). Results showed that treatment with either dibutyryl cAMP (dbcAMP) or 1-methyl-3-isobutylxanthine (MIX) inhibited phosphorylation of adenosine 5'-monophosphate-activated protein kinase (AMPK) and activated phosphorylation of the translational repressor, 4E-binding protein (4E-BP), a marker of target of rapamycin (TOR) signaling. A chemical activator of AMPK (5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside, AICAR) increased dbcAMP-inhibited AMPK phosphorylation and blocked dbcAMP-stimulated phosphorylation of 4E-BP, indicating that inhibition of AMPK phosphorylation lies upstream of dbcAMP-stimulated TOR signaling. Treatment of PGs with dbcAMP and MIX also stimulated phosphorylation of a 37-kDa protein, as recognized by a protein kinase C (PKC) substrate antibody, indicating that cAMP activates PKC signaling. Treatment with either LY294002 or AICAR did not affect dbcAMP-stimulated phosphorylation of the PKC-dependent 37-kDa protein, indicating that cAMP-stimulated PKC signaling is not related to phosphoinositide 3-kinase (PI3K) or AMPK. In addition, dbcAMP-stimulated ecdysteroidogenesis in PGs was partially inhibited by pretreatment with either LY294002, AICAR, or calphostin C. From these results, we concluded that AMPK/TOR/4E-BP and PKC pathways are involved in ecdysteroidogenesis of PGs stimulated by cAMP signaling in B. mori.

Expression of tyrosine phosphatases in relation to PTTH-stimulated ecdysteroidogenesis in prothoracic glands of the silkworm, Bombyx mori

Protein tyrosine phosphorylation is a reversible, dynamic process regulated by the activities of tyrosine kinases and tyrosine phosphatases. Although the involvement of tyrosine kinases in the prothoracicotropic hormone (PTTH)-stimulated ecdysteroidogenesis in insect prothoracic glands (PGs) has been documented, few studies have been conducted on the involvement of protein tyrosine phosphatases (PTPs) in PTTH-stimulated ecdysteroidogenesis. In the present study, we investigated the correlation between PTPs and PTTH-stimulated ecdysteroidogenesis in Bombyx mori PGs. Our results showed that the basal PTP enzymatic activities exhibited development-specific changes during the last larval instar and pupation stage, with high activities being detected during the later stages of the last larval instar. PTP enzymatic activity was stimulated by PTTH treatment both in vitro and in vivo. Pretreatment with phenylarsine oxide (PAO) and benzylphosphonic acid (BPA), two chemical inhibitors of tyrosine phosphatase, reduced PTTH-stimulated enzymatic activity. Determination of ecdysteroid secretion showed that treatment with PAO and BPA did not affect basal ecdysteroid secretion, but greatly inhibited PTTH-stimulated ecdysteroid secretion, indicating that PTTH-stimulated PTP activity is indeed involved in ecdysteroid secretion. PTTH-stimulated phosphorylation of the extracellular signal-regulated kinase (ERK) and 4E-binding protein (4E-BP) was partially inhibited by pretreatment with either PAO or BPA, indicating the potential link between PTPs and phosphorylation of ERK and 4E-BP. In addition, we also found that in vitro treatment with 20-hydroxyecdysone did not affect PTP enzymatic activity. We further investigated the expressions of two important PTPs (PTP 1B (PTP1B) and the phosphatase and tension homologue (PTEN)) in Bombyx PGs. Our immunoblotting analysis showed that B. mori PGs contained the proteins of PTP1B and PTEN, with PTP1B protein undergoing development-specific changes. Protein levels of PTP1B and PTEN were not affected by PTTH treatment. The gene expression levels of PTP1B and PTEN showed development-specific changes. From these results, we suggest that PTTH-regulated PTP signaling may crosstalk with ERK and target of rapamycin (TOR) signaling pathways and is a necessary component for stimulation of ecdysteroid secretion.

Bombyxin-stimulated ecdysteroidogenesis in relation to sugar transporter/trehalase expressions in Bombyx prothoracic glands

Our previous studies showed that bombyxin stimulated ecdysteroidogenesis in Bombyx mori prothoracic glands (PGs) during a long-term incubation period in a phosphatidylinositol 3-kinase (PI3K)/Akt-dependent manner. In the present study, we further investigated the downstream signaling cascade in bombyxin-stimulated PGs. Our results showed that upon treatment with bombyxin, expression levels of the sugar transport 1 (St1) and St4 genes and trehalase 1 (Treh1) gene, but not ecdysteroid biosynthesis genes were greatly enhanced compared to the controls. Treatment with LY294002 (an inhibitor of PI3K) reduced the enhanced St1 and Treh1 expression levels, clearly indicating the involvement of PI3K. Treatment with 1 mM of mpV(pic) (a potent inhibitor of protein phosphotyrosine phosphatase and activator of insulin receptor (InR) kinase) also stimulated expression levels of the St1 and Treh1 genes, thus further confirming the involvement of the InR. Determining Treh enzyme activity showed that bombyxin treatment stimulated Treh enzyme activity in time- and PI3K-dependent manners. Validamycin A (a Treh inhibitor) blocked bombyxin-stimulated Treh enzyme activity and partly decreased bombyxin-stimulated ecdysteroidogenesis. A specific sugar transport inhibitor (cytochalasin B) and a glycolysis inhibitor (2-deoxy-D-glucose (2-DG)) also reduced bombyxin-stimulated ecdysteroidogenesis. Taken together, these results indicated that increased expressions of Sts and Treh1 and enhanced Treh enzyme activity downstream of InR/PI3K are involved in bombyxin-stimulated ecdysteroidogenesis in B. mori PGs.

Signaling Pathways That Regulate the Crustacean Molting Gland

A pair of Y-organs (YOs) are the molting glands of decapod crustaceans. They synthesize and secrete steroid molting hormones (ecdysteroids) and their activity is controlled by external and internal signals. The YO transitions through four physiological states over the molt cycle, which are mediated by molt-inhibiting hormone (MIH; basal state), mechanistic Target of Rapamycin Complex 1 (mTORC1; activated state), Transforming Growth Factor-β (TGFβ)/Activin (committed state), and ecdysteroid (repressed state) signaling pathways. MIH, produced in the eyestalk X-organ/sinus gland complex, inhibits the synthesis of ecdysteroids. A model for MIH signaling is organized into a cAMP/Ca2+-dependent triggering phase and a nitric oxide/cGMP-dependent summation phase, which maintains the YO in the basal state during intermolt. A reduction in MIH release triggers YO activation, which requires mTORC1-dependent protein synthesis, followed by mTORC1-dependent gene expression. TGFβ/Activin signaling is required for YO commitment in mid-premolt. The YO transcriptome has 878 unique contigs assigned to 23 KEGG signaling pathways, 478 of which are differentially expressed over the molt cycle. Ninety-nine contigs encode G protein-coupled receptors (GPCRs), 65 of which bind a variety of neuropeptides and biogenic amines. Among these are putative receptors for MIH/crustacean hyperglycemic hormone neuropeptides, corazonin, relaxin, serotonin, octopamine, dopamine, allatostatins, Bursicon, ecdysis-triggering hormone (ETH), CCHamide, FMRFamide, and proctolin. Contigs encoding receptor tyrosine kinase insulin-like receptor, epidermal growth factor (EGF) receptor, and fibroblast growth factor (FGF) receptor and ligands EGF and FGF suggest that the YO is positively regulated by insulin-like peptides and growth factors. Future research should focus on the interactions of signaling pathways that integrate physiological status with environmental cues for molt control.

Larval feeding status regulates the transcript levels of genes encoding PTTH and allatoregulatory peptides in silkworm Bombyx mori

In Bombyx mori, two dorsolateral neurosecretory cells (NSCs) in each of the two brain lobes have been identified as prothoracicotropic hormone (PTTH) producing cells. This neuropeptide in insects stimulates the prothoracic gland for the synthesis and release of ecdysone, responsible for the molting events. Allatotropin (AT) and allatostatin (AST) are allatoregulatory neuropeptides that regulate juvenile hormone biosynthesis. Here, by using RT-qPCR, we showed that in B. mori, nutritional stress modulates the mRNA expression of AT and AST-C (allatostain type C) in the central nervous system consisting of the brain lobes and all the associated ganglia. Using whole-mount in situ hybridization, we showed that the feeding status of Bombyx larvae also influences the expression of PTTH in the NSCs of the brain. Food deprivation significantly decreased the mRNA expression levels of PTTH in larvae at active or terminal growth period. Further, we showed that insulin modulates the expression level of PTTH. However, its action was dependent on the feeding status of the larvae. At feeding, the insulin decreased the PTTH expression level, while at food deprivation, the insulin increased the PTTH expression level. The data thus indicates that larval feeding status plays an important role in altering the mRNA expression levels of allatoregulatory peptide genes and PTTH.

Changes in expressions of ecdysteroidogenic enzyme and ecdysteroid signaling genes in relation to Bombyx embryonic development

Although the role of ecdysteroids in regulating egg diapause process in Bombyx mori is well documented, temporal changes in expression levels of genes involved in ecdysteroid biosynthesis and its downstream signaling are less well understood. In the present study, we studied changes in expression levels of genes involved in ecdysteroid biosynthesis and its downstream signaling during embryonic development of B. mori. Results showed that in diapause eggs, the expression of ecdysteroid-phosphate phosphatase (EPPase) gene and Halloween genes (Spook [Spo] and Shade [Shd]) remained at very low levels. However, in eggs whose diapause initiation was prevented by HCl, significant increases in the messenger RNA (mRNA) levels of EPPase, Spo, and Shd were detected during embryonic development. Other Halloween genes (Neverland [Nvd] and Phantom [Phm]) also showed different changes between diapause and HCl-treated eggs. However, genes of Disembodied (Dib) and Shadow (Sad) showed similar changes in both diapause and HCl-treated eggs. We further investigated changes in expression levels of ecdysone receptor genes (EcRA, EcRB1, and USP) and downstream signaling genes (E75A, E75B, E74A, E74B, Br-C, HR3, HR4, KR-H1, and FTZ-F1). Results showed that genes of EcRA and the other nuclear receptors (E75A, E75B, E74A, HR3, HR4, KR-H1, and FTZ-F1) exhibited significant differential patterns between diapause and HCl-treated eggs, with increased levels being detected during later stages of embryonic development in HCl-treated eggs. Differential temporal changes in expressions of genes involved ecdysteroid biosynthesis and its downstream signaling found between diapause and HCl-treated eggs were further confirmed using nondiapause eggs. Our results showed that nondiapause eggs exhibited the same changing patterns as those in HCl-treated eggs, thus clearly indicating potential correlations between expressions of these genes and embryonic development in B. mori. To our knowledge, this is the first comprehensive report to study the transcriptional regulation of ecdysteroidogenic and ecdysteroid signaling genes, thus providing useful information for a clearer understanding of insect egg diapause mechanisms.

Control of the insect metamorphic transition by ecdysteroid production and secretion

Ecdysteroids are a class of steroid hormones that controls molting and metamorphic transitions in Ecdysozoan species including insects, in which ecdysteroid biosynthesis and its regulation have been extensively studied. Insect ecdysteroids are produced from dietary sterols by a series of reduction-oxidation reactions in the prothoracic gland and in Drosophila they are released into the hemolymph via vesicle-mediated secretion at the time of metamorphosis. To initiate precisely controlled ecdysteroid pulses, the prothoracic gland functions as a central node integrating both intrinsic and extrinsic signals to control ecdysteroid biosynthesis and secretion. In this review, we outline recent progress in the characterization of ecdysone biosynthesis and steroid trafficking pathways and the discoveries of novel factors regulating prothoracic gland function.

Reactive oxygen species-mediated bombyxin signaling in Bombyx mori

In the present study, we demonstrated that bombyxin, an insect insulin-like peptide, modulated ecdysteroidogenesis in Bombyx mori prothoracic glands (PGs) through redox signaling. Our results showed that bombyxin treatment resulted in a transient increase in intracellular reactive oxygen species (ROS) concentration, as measured using 2',7'-dichlorofluorescin diacetate (DCFDA), an oxidation-sensitive fluorescent probe. The antioxidant N-acetylcysteine (NAC) abolished the bombyxin-induced increase in fluorescence in Bombyx PGs. Furthermore, bombyxin-induced ROS production was inhibited by mitochondrial oxidative phosphorylation inhibitors (rotenone and antimycin A), indicating mitochondria-mediated ROS production. The stimulation of ROS production in response to bombyxin appears to undergo development-specific changes. We further investigated the action mechanism of bombyxin-stimulated ROS signaling. Results showed that in the presence of either NAC, rotenone, or antimycin A, bombyxin-stimulated phosphorylation of insulin receptor, Akt, and 4E-binding protein (4E-BP) was blocked and bombyxin-stimulated ecdysteroidogenesis in PGs was greatly inhibited. From these results, we conclude that ROS signaling appears to be involved in bombyxin-stimulated ecdysteroidogenesis of PGs in B. mori by modulating the phosphorylation of insulin receptor, Akt, and 4E-BP. To our knowledge, this is the first demonstration of redox regulation in insulin signaling in an insect system.

Ecdysteroidogenesis and development in Heliothis virescens (Lepidoptera: Noctuidae): Focus on PTTH-stimulated pathways

Post-embryonic development and molting in insects are regulated by endocrine changes, including prothoracicotropic hormone (PTTH)-stimulated ecdysone secretion by the prothoracic glands (PGs). In Lepidoptera, two pathways are potentially involved in PTTH-stimulated ecdysteroidogenesis, mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase/protein kinase B/target of rapamycin (PI3K/Akt/TOR). We investigated the potential roles of both these pathways in Heliothis virescens ecdysteroidogenesis. We identified putative proteins belonging to MAPK and PI3K/Akt/TOR signaling cascades, using transcriptomic analyses of PGs from last (fifth) instar larvae. Using western blots, we measured the phosphorylation of 4E-BP and S6K proteins, the main targets of TOR, following the in vitro exposure of PGs to brain extract containing PTTH (hereafter referred to as PTTH) and/or the inhibitors of MAPK (U0126), PI3K (LY294002) or TOR (rapamycin). Next, we measured ecdysone production, under the same experimental conditions, by enzyme immunoassay (EIA). We found that in Heliothis virescens last instar larvae, both pathways modulated PTTH-stimulated ecdysteroidogenesis. Finally, we analyzed the post-embryonic development of third and fourth instar larvae fed on diet supplemented with rapamycin, in order to better understand the role of the TOR pathway in larval growth. When rapamycin was added to the diet of larvae, the onset of molting was delayed, the growth rate was reduced and abnormally small larvae/pupae with high mortality rates resulted. In larvae fed on diet supplemented with rapamycin, the growth of PGs was suppressed, and ecdysone production and secretion were inhibited. Overall, the in vivo and in vitro results demonstrated that, similarly to Bombyx mori, MAPK and PI3K/Akt/TOR pathways are involved in PTTH signaling-stimulated ecdysteroidogenesis, and indicated the important role of TOR protein in H. virescens systemic growth.

Isolation of an insulin-like peptide from the Asian malaria mosquito, Anopheles stephensi, that acts as a steroidogenic gonadotropin across diverse mosquito taxa

Many insulin-like peptides (ILPs) have been identified in insects, yet only a few were isolated in their native form for structural and functional studies. Antiserum produced to ILP3 in Aedes aegypti was used in a radioimmunoassay to monitor the purification of an ILP from heads of adult An. stephensi and recognized the ILP in other immunoassays. The structure of the purified peptide matched that predicted for the ILP3 in this species. The native form stimulated ecdysteroid production by ovaries isolated from non-blood fed females. Synthetic forms of An. stephensi ILP3 and ILP4 similarly activated this process in a dose responsive manner. This function was first established for ILP3 and ILP4 homologs in Aedes aegypti, thus suggesting their structural and functional conservation in mosquitoes. We tested the extent of conservation by treating ovaries of An. gambiae, Ae. aegypti, and Culex quinquefasciatus with the An. stephensi ILPs, and both the native and synthetic ILP3 were stimulatory, as was the ILP4. Taken together, these results offer the first evidence for ILP functional conservation across the Anophelinae and Culicinae subfamilies.

Stimulation of JNK Phosphorylation by the PTTH in Prothoracic Glands of the Silkworm, Bombyx mori

In this study, phosphorylation of c-Jun N-terminal kinase (JNK) by the prothoracicotropic hormone (PTTH) was investigated in prothoracic glands (PGs) of the silkworm, Bombyx mori. Results showed that JNK phosphorylation was stimulated by the PTTH in time- and dose-dependent manners. In vitro activation of JNK phosphorylation in PGs by the PTTH was also confirmed in an in vivo experiment, in which a PTTH injection greatly increased JNK phosphorylation in PGs of day-6 last instar larvae. JNK phosphorylation caused by PTTH stimulation was greatly inhibited by U73122, a potent and specific inhibitor of phospholipase C (PLC) and an increase in JNK phosphorylation was also detected when PGs were treated with agents (either A23187 or thapsigargin) that directly elevated the intracellular Ca²⁺ concentration, thereby indicating involvement of PLC and Ca²⁺. Pretreatment with an inhibitor (U0126) of mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase (MEK) and an inhibitor (LY294002) of phosphoinositide 3-kinase (PI3K) failed to significantly inhibit PTTH-stimulated JNK phosphorylation, indicating that ERK and PI3K were not related to JNK. We further investigated the effect of modulation of the redox state on JNK phosphorylation. In the presence of either an antioxidant (N-acetylcysteine, NAC) or diphenylene iodonium (DPI), PTTH-stimulated JNK phosphorylation was blocked. The JNK kinase inhibitor, SP600125, markedly inhibited PTTH-stimulated JNK phosphorylation and ecdysteroid synthesis. The kinase assay of JNK in PGs confirmed its stimulation by PTTH and inhibition by SP600125. Moreover, PTTH treatment did not affect JNK or Jun mRNA expressions. Based on these findings, we concluded that PTTH stimulates JNK phosphorylation in Ca²⁺- and PLC-dependent manners and that the redox-regulated JNK signaling pathway is involved in PTTH-stimulated ecdysteroid synthesis in B. mori PGs.

The impact of pollen consumption on honey bee (Apis mellifera) digestive physiology and carbohydrate metabolism

Carbohydrate-active enzymes play an important role in the honey bee (Apis mellifera) due to its dietary specialization on plant-based nutrition. Secretory glycoside hydrolases (GHs) produced in worker head glands aid in the processing of floral nectar into honey and are expressed in accordance with age-based division of labor. Pollen utilization by the honey bee has been investigated in considerable detail, but little is known about the metabolic fate of indigestible carbohydrates and glycosides in pollen biomass. Here, we demonstrate that pollen consumption stimulates the hydrolysis of sugars that are toxic to the bee (xylose, arabinose, mannose). GHs produced in the head accumulate in the midgut and persist in the hindgut that harbors a core microbial community composed of approximately 10⁸ bacterial cells. Pollen consumption significantly impacted total and specific bacterial abundance in the digestive tract. Bacterial isolates representing major fermentative gut phylotypes exhibited primarily membrane-bound GH activities that may function in tandem with soluble host enzymes retained in the hindgut. Additionally, we found that plant-originating β-galactosidase activity in pollen may be sufficient, in some cases, for probable physiological activity in the gut. These findings emphasize the potential relative contributions of host, bacteria, and pollen enzyme activities to carbohydrate breakdown, which may be tied to gut microbiome dynamics and associated host nutrition.

Myosuppressin is involved in the regulation of pupal diapause in the cabbage army moth Mamestra brassicae

Diapause, a programmed developmental arrest, is common in insects, enabling them to survive adverse seasons. It is well established that pupal diapause is regulated by ecdysteroids secreted by the prothoracic glands (PGs), with cessation of ecdysteroid secretion after pupal ecdysis leading to pupal diapause. A major factor regulating the gland activity is prothoracicotropic hormone (PTTH) secreted from the brain. In our previous study, we demonstrated that the cessation of PTTH release after pupal ecdysis resulted in the inactivation of the PGs, leading to pupal diapause in the cabbage army moth Mamestra brassicae. Here we show that a neuropeptide myosuppressin also contributes to the inactivation of PGs at the initiation of diapause. Myosuppressin suppresses PTTH-stimulated activation of the PGs in vitro. Concentrations of myosuppressin in the hemolymph after pupal ecdysis are higher in diapause pupae than in nondiapause pupae.

Insulin and 20-hydroxyecdysone action in Bombyx mori: Glycogen content and expression pattern of insulin and ecdysone receptors in fat body

Insulin and ecdysone signaling play a critical role on the growth and development of insects including Bombyx mori. Our previous study showed that Bombyx larvae reached critical weight for metamorphosis between day 3.5 and 4 of the fifth larval instar. The present study showed that the effect of insulin on the accumulation of glycogen in fat body of Bombyx larvae depends on the critical growth period. When larvae are in active growth period (before reaching critical weight), insulin caused increased accumulation of glycogen, while its treatment in larvae at terminal growth period (after critical period) resulted in an increased mobilization of glycogen. During terminal growth period, insulin and 20-hydroxyecdysone (20E) showed an antagonistic effect on the accumulation of fat body glycogen in fed, food deprived and decapitated larvae as well as in isolated abdomens. Insulin treatment decreased the glycogen content, whereas, 20E increased it. Food deprivation and decapitation caused an increase in the transcript levels of insulin receptor (InR) and this increase in InR expression might be attributed to a decrease in synthesis/secretion of insulin-like peptides, as insulin treatment in these larvae showed a down-regulation in InR expression. However, insulin showed an up-regulation in InR in isolated abdomens and it suggests that in food deprived and decapitated larvae, the exogenous insulin may interact with some head and/or thoracic factors in modulating the expression of InR. Moreover, in fed larvae, insulin-mediated increase in InR expression indicates that its regulation by insulin-like peptides also depends on the nutritional status of the larvae. The treatment of 20E in fed larvae showed an antagonistic effect on the transcript levels since a down-regulation in InR expression was observed. 20E treatment also led to a decreased expression of InR in food deprived and decapitated larvae as well as in isolated abdomens. Insulin and 20E also modulated the expression level of ecdysone receptors (EcRB1 and USP1). 20E treatment showed an up-regulation in expression of ecdysone receptors, but only in fed larvae, whereas insulin treatment showed a down-regulation in the expression of EcRB1 and USP1 in all the experimental larvae studied. Further, the data indicates that an up-regulation of ecdysone receptors is associated with an increase in fat body glycogen content, whereas an up-regulation of insulin receptor expression causes glycogen mobilization. The study, therefore, suggests that the insulin and ecdysone signaling are linked to each other and that both insulin and ecdysone are involved in regulating the carbohydrate reserves in B. mori.

Calcium influx enhances neuropeptide activation of ecdysteroid hormone production by mosquito ovaries

A critical step in mosquito reproduction is the ingestion of a blood meal from a vertebrate host. In mosquitoes like Aedes aegypti, blood feeding stimulates the release of ovary ecdysteroidogenic hormone (OEH) and insulin-like peptide 3 (ILP3). This induces the ovaries to produce ecdysteroid hormone (ECD), which then drives egg maturation. In many immature insects, prothoracicotropic hormone (PTTH) stimulates the prothoracic glands to produce ECD that directs molting and metamorphosis. The receptors for OEH, ILP3 and PTTH are different receptor tyrosine kinases with OEH and ILP3 signaling converging downstream in the insulin pathway and PTTH activating the mitogen-activated protein kinase pathway. Calcium (Ca(2+)) flux and cAMP have also been implicated in PTTH signaling, but the role of Ca(2+) in OEH, ILP3, and cAMP signaling in ovaries is unknown. Here, we assessed whether Ca(2+) flux affects OEH, ILP3, and cAMP activity in A. aegypti ovaries and also asked whether PTTH stimulated ovaries to produce ECD. Results indicated that Ca(2+) flux enhanced but was not essential for OEH or ILP3 activity, whereas cAMP signaling was dependent on Ca(2+) flux. Recombinant PTTH from Bombyx mori fully activated ECD production by B. mori PTGs, but exhibited no activity toward A. aegypti ovaries. Recombinant PTTH from A. aegypti also failed to stimulate either B. mori PTGs or A. aegypti ovaries to produce ECD. We discuss the implications of these results in the context of mosquito reproduction and ECD biosynthesis by insects generally.

Transcriptome analysis of the molting gland (Y-organ) from the blackback land crab, Gecarcinus lateralis

In decapod crustaceans, arthropod steroid hormones or ecdysteroids regulate molting. These hormones are synthesized and released from a pair of molting glands called the Y-organs (YO). Cyclic nucleotide, mTOR, and TGFβ/Smad signaling pathways mediate molt cycle-dependent phase transitions in the YO. To further identify the genes involved in the regulation of molting, a YO transcriptome was generated from three biological replicates of intermolt blackback land crab, Gecarcinus lateralis. Illumina sequencing of cDNA libraries generated 227,811,829 100-base pair (bp) paired-end reads; following trimming, 90% of the reads were used for further analyses. The trimmed reads were assembled de novo using Trinity software to generate 288,673 contigs with a mean length of 872 bp and a median length of 1842 bp. Redundancy among contig sequences was reduced by CD-HIT-EST, and the output constituted the baseline transcriptome database. Using Bowtie2, 92% to 93% of the reads were mapped back to the transcriptome. Individual contigs were annotated using BLAST, HMMER, TMHMM, SignalP, and Trinotate, resulting in assignments of 20% of the contigs. Functional and pathway annotations were carried out via gene ontology (GO) and KEGG orthology (KO) analyses; 58% and 44% of the contigs with BLASTx hits were assigned to GO and KO terms, respectively. The gene expression profile was similar to a crayfish YO transcriptome database, and the relative abundance of each contig was highly correlated among the three G. lateralis replicates. Signal transduction pathway orthologs were well represented, including those in the mTOR, TGFβ, cyclic nucleotide, MAP kinase, calcium, VEGF, phosphatidylinositol, ErbB, Wnt, Hedgehog, Jak-STAT, and Notch pathways.

The homeodomain transcription factors antennapedia and POU-M2 regulate the transcription of the steroidogenic enzyme gene Phantom in the silkworm

The steroid hormone ecdysone, which controls insect molting and metamorphosis, is synthesized in the prothoracic gland (PG), and several steroidogenic enzymes that are expressed specifically in the PG are involved in ecdysteroidogenesis. In this study, we identified new regulators that are involved in the transcriptional control of the silkworm steroidogenic enzyme genes. In silico analysis predicted several potential cis-regulatory elements (CREs) for the homeodomain transcription factors Antennapedia (Antp) and POU-M2 in the proximal promoters of steroidogenic enzyme genes. Antp and POU-M2 are expressed dynamically in the PG during larval development, and their overexpression in silkworm embryo-derived (BmE) cells induced the expression of steroidogenic enzyme genes. Importantly, luciferase reporter analyses, electrophoretic mobility shift assays, and chromatin immunoprecipitation assays revealed that Antp and POU-M2 promote the transcription of the silkworm steroidogenic enzyme gene Phantom (Phm) by binding directly to specific motifs within overlapping CREs in the Phm promoter. Mutations of these CREs in the Phm promoter suppressed the transcriptional activities of both Antp and POU-M2 in BmE cells and decreased the activities of mutated Phm promoters in the silkworm PG. In addition, pulldown and co-immunoprecipitation assays demonstrated that Antp can interact with POU-M2. Moreover, RNA interference-mediated down-regulation of either Antp or POU-M2 during silkworm wandering not only decreased the ecdysone titer but also led to the failure of metamorphosis. In summary, our results suggest that Antp and POU-M2 coordinate the transcription of the silkworm Phm gene directly, indicating new roles for homeodomain proteins in regulating insect ecdysteroidogenesis.