Transcriptional regulation of the PTTH receptor in prothoracic glands of the silkworm, Bombyx 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.

Protein kinase C signalling involved in prothoracicotropic hormone-stimulated prothoracic glands in the silkworm, Bombyx mori

In the present study, the participation of protein kinase C (PKC) signalling in prothoracicotropic hormone (PTTH)-stimulated ecdysteroidogenesis in Bombyx prothoracic glands (PGs) is demonstrated and characterized. PTTH stimulated phosphorylation of a 37-kDa protein in Bombyx PGs both in vitro and in vivo, as recognized by a PKC substrate antibody. Treatment with either A23187 or thapsigargin also stimulated this 37-kDa protein phosphorylation. PTTH-stimulated phosphorylation of the 37-kDa protein was markedly attenuated in the absence of Ca²⁺ . The phospholipase C (PLC) inhibitor, U73122, greatly inhibited PTTH-stimulated phosphorylation of this protein, indicating the involvement of Ca²⁺ and PLC. A mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) kinase (MEK) inhibitor (U0126), a phosphoinositide 3-kinase (PI3K) inhibitor (LY294002) and a chemical activator of adenosine 5'-monophosphate-activated protein kinase (AMPK) (5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside) did not affect PTTH-stimulated phosphorylation of the 37-kDa protein, implying that ERK and PI3K/AMPK are not the upstream signalling pathways for PKC-dependent protein phosphorylation. The mitochondrial oxidative phosphorylation inhibitors (the uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone and diphenylene iodonium) inhibited PTTH-stimulated phosphorylation of the 37-kDa protein, indicating its redox regulation. Treatment with PKC inhibitors (either calphostin C, chelerythrine C or rottlerin) reduced PTTH-stimulated phosphorylation of the 37-kDa protein. PTTH-stimulated ecdysteroidogenesis was also inhibited by treatment with rottlerin, thus further confirming participation of PKC-dependent phosphorylation in PTTH signalling. From these results, we demonstrated that redox-regulated PTTH-stimulated PKC signalling is involved in ecdysteroid secretion in Bombyx PGs.

Involvement of RSK phosphorylation in PTTH-stimulated ecdysone secretion in prothoracic glands of the silkworm, Bombyx mori

It is well known that phosphorylation of extracellular signal-regulated kinase (ERK) is involved in prothoracicotropic hormone (PTTH)-stimulated ecdysteroidogenesis in insect prothoracic glands (PGs). In the present study, we further investigated the downstream signalling pathways. Our results showed that PTTH stimulated p90 ribosomal S6 kinase (RSK) phosphorylation at Thr573 in Bombyx mori PGs both in vitro and in vivo. The in vitro PTTH stimulation was stage- and dose-dependent. The absence of Ca²⁺ reduced PTTH-stimulated RSK phosphorylation. Stimulation of RSK phosphorylation was also observed after treatment with either A23187 or thapsigargin. A phospholipase C (PLC) inhibitor, U73122, blocked PTTH-stimulated RSK phosphorylation. These results indicate the involvement of Ca²⁺ and PLC. Treatment with diphenylene iodonium (DPI), a mitochondrial oxidative phosphorylation inhibitor, blocked PTTH-regulated RSK phosphorylation, indicating its redox regulation. A mitogen-activated protein kinase (MAPK)/ERK kinase (MEK) inhibitor, U0126, but not a phosphatidylinositol 3-kinase (PI3K) inhibitor, LY294002, decreased PTTH-stimulated RSK phosphorylation, indicating that ERK is an upstream signalling. A protein kinase C (PKC) inhibitor, chelerythrine C, inhibited PTTH-stimulated RSK phosphorylation, and a PKC activator, phorbol 12-myristate acetate (PMA) stimulated RSK phosphorylation, indicating the involvement of PKC. BI-D1870, a specific RSK inhibitor, partly prevented PTTH-stimulated RSK phosphorylation and significantly inhibited PTTH-stimulated ecdysteroid secretion, indicating that PTTH-stimulated RSK phosphorylation is involved in ecdysteroidogenesis. Taken together, these data indicate that PTTH activates RSK phosphorylation which plays important roles in PTTH-stimulated ecdysteroidogenesis.

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.

Role of protein phosphatase 2A in PTTH-stimulated prothoracic glands of the silkworm, Bombyx mori

In the present study, the roles of a major serine/threonine protein phosphatase 2A (PP2A) in prothoracicotropic hormone (PTTH)-stimulated prothoracic glands (PGs) of Bombyx mori were evaluated. Immunoblotting analysis showed that Bombyx PGs contained a structural A subunit (A), a regulatory B subunit (B), and a catalytic C subunit (C), with each subunit undergoing development-specific changes. The protein levels of each subunit were not affected by PTTH treatment. However, the highly conserved tyrosine dephosphorylation of PP2A C subunit (PP2Ac), which appears to be related to activity, was increased by PTTH treatment in a time-dependent manner. We further demonstrated that phospholipase C (PLC), Ca²⁺, and reactive oxygen species (ROS) are upstream signaling for the PTTH-stimulated dephosphorylation of PP2Ac. The determination of PP2A enzymatic activity showed that PP2A enzymatic activity was stimulated by PTTH treatment both in vitro and in vivo. Okadaic acid (OA), a specific PP2A inhibitor, prevented the PTTH-stimulated dephosphorylation of PP2Ac and reduced both basal and PTTH-stimulated PP2A enzymatic activity. The determination of ecdysteroid secretion showed that treatment with OA did not affect basal ecdysteroid secretion but did significantly inhibit PTTH-stimulated ecdysteroid secretion, indicating that PTTH-stimulated PP2A activity is involved in ecdysteroidogenesis. Treatment with OA stimulated the basal phosphorylation of the extracellular signal-regulated kinase (ERK) and 4E-binding protein (4E-BP) without affecting PTTH-stimulated ERK and 4E-BP phosphorylation. From these results, we hypothesize that PTTH-regulated PP2A signaling is a necessary component for the stimulation of ecdysteroidogenesis, potentially by mediating the link between ERK and TOR signaling pathways.

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.

Stimulation of orphan nuclear receptor HR38 gene expression by PTTH in prothoracic glands of the silkworm, Bombyx mori

A complex signaling network appears to be involved in prothoracicotropic hormone (PTTH)-stimulated ecdysteroidogenesis in insect prothoracic glands (PGs). Less is known about the genomic action of PTTH signaling. In the present study, we investigated the effect of PTTH on the expression of Bombyx mori HR38, an immediate early gene (IEG) identified in insect systems. Our results showed that treatment of B. mori PGs with PTTH in vitro resulted in a rapid increase in HR38 expression. Injection of PTTH into day-5 last instar larvae also greatly increased HR38 expression, verifying the in vitro effect. Cycloheximide did not affect induction of HR38 expression, suggesting that protein synthesis is not required for PTTH's effect. A mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase (MEK) inhibitor (U0126), and a phosphoinositide 3-kinase (PI3K) inhibitor (LY294002), partially inhibited PTTH-stimulated HR38 expression, implying the involvement of both the ERK and PI3K signaling pathways. When PGs were treated with agents that directly elevate the intracellular Ca(2+) concentration (either A23187 or thapsigargin), an increase in HR38 expression was also detected, indicating that Ca(2+) is involved in PTTH-stimulated HR38 gene expression. A Western blot analysis showed that PTTH treatment increased the HR38 protein level, and protein levels showed a dramatic increase during the later stages of the last larval instar. Expression of HR38 transcription in response to PTTH appeared to undergo development-specific changes. Treatment with ecdysone in vitro did not affect HR38 expression. However, 20-hydroxyecdysone treatment decreased HR38 expression. Taken together, these results demonstrate that HR38 is a PTTH-stimulated IEG that is, at least in part, induced through Ca(2+)/ERK and PI3K signaling. The present study proposes a potential cross talk mechanism between PTTH and ecdysone signaling to regulate insect development and lays a foundation for a better understanding of the mechanisms of PTTH's actions.

The effect of silencing 20E biosynthesis relative genes by feeding bacterially expressed dsRNA on the larval development of Chilo suppressalis

RNA interference (RNAi) is a robust tool to study gene functions as well as potential for insect pest control. Finding suitable target genes is the key step in the development of an efficient RNAi-mediated pest control technique. Based on the transcriptome of Chilo suppressalis, 24 unigenes which putatively associated with insect hormone biosynthesis were identified. Amongst these, four genes involved in ecdysteroidogenesis i.e., ptth, torso, spook and nm-g were evaluated as candidate targets for function study. The partial cDNA of these four genes were cloned and their bacterially expressed dsRNA were fed to the insects. Results revealed a significant reduction in mRNA abundance of target genes after 3 days. Furthermore, knocked down of these four genes resulted in abnormal phenotypes and high larval mortality. After 15 days, the survival rates of insects in dsspook, dsptth, dstorso, and dsnm-g groups were significantly reduced by 32%, 38%, 56%, and 67% respectively, compared with control. Moreover, about 80% of surviving larvae showed retarded development in dsRNA-treated groups. These results suggest that oral ingestion of bacterially expressed dsRNA in C. suppressalis could silence ptth, torso, spook and nm-g. Oral delivery of bacterially expressed dsRNA provides a simple and potential management scheme against C. suppressalis.

De novo Transcriptome Analysis of Chinese Citrus Fly, Bactrocera minax (Diptera: Tephritidae), by High-Throughput Illumina Sequencing

The Chinese citrus fly, Bactrocera minax (Enderlein), is one of the most devastating pests of citrus in the temperate areas of Asia. So far, studies involving molecular biology and physiology of B. minax are still scarce, partly because of the lack of genomic information and inability to rear this insect in laboratory. In this study, de novo assembly of a transcriptome was performed using Illumina sequencing technology. A total of 20,928,907 clean reads were obtained and assembled into 33,324 unigenes, with an average length of 908.44 bp. Unigenes were annotated by alignment against NCBI non-redundant protein (Nr), Swiss-Prot, Clusters of Orthologous Groups (COG), Gene Ontology (GO), and Kyoto Encyclopedia of Genes and Genomes Pathway (KEGG) database. Genes potentially involved in stress tolerance, including 20 heat shock protein (Hsps) genes, 26 glutathione S-transferases (GSTs) genes, and 2 ferritin subunit genes, were identified. These genes may play roles in stress tolerance in B. minax diapause stage. It has previously been found that 20E application on B. minax pupae could avert diapause, but the underlying mechanisms remain unknown. Thus, genes encoding enzymes in 20E biosynthesis pathway, including Neverland, Spook, Phantom, Disembodied, Shadow, Shade, and Cyp18a1, and genes encoding 20E receptor proteins, ecdysone receptor (EcR) and ultraspiracle (USP), were identified. The expression patterns of 20E-related genes among developmental stages and between 20E-treated and untreated pupae demonstrated their roles in diapause program. In addition, 1,909 simple sequence repeats (SSRs) were detected, which will contribute to molecular marker development. The findings in this study greatly improve our genetic understanding of B. minax, and lay the foundation for future studies on this species.

Transcriptional regulation of insect steroid hormone biosynthesis and its role in controlling timing of molting and metamorphosis

The developmental transition from juvenile to adult is often accompanied by many systemic changes in morphology, metabolism, and reproduction. Curiously, both mammalian puberty and insect metamorphosis are triggered by a pulse of steroid hormones, which can harmonize gene expression profiles in the body and thus orchestrate drastic biological changes. However, understanding of how the timing of steroid hormone biosynthesis is regulated at the molecular level is poor. The principal insect steroid hormone, ecdysteroid, is biosynthesized from dietary cholesterol in the specialized endocrine organ called the prothoracic gland. The periodic pulses of ecdysteroid titers determine the timing of molting and metamorphosis. To date, at least nine families of ecdysteroidogenic enzyme genes have been identified. Expression levels of these genes correlate well with ecdysteroid titers, indicating that the transcriptional regulatory network plays a critical role in regulating the ecdysteroid biosynthesis pathway. In this article, we summarize the transcriptional regulation of ecdysteroid biosynthesis. We first describe the development of prothoracic gland cells during Drosophila embryogenesis, and then provide an overview of the transcription factors that act in ecdysteroid biosynthesis and signaling. We also discuss the external signaling pathways that target these transcriptional regulators. Furthermore, we describe conserved and/or diverse aspects of steroid hormone biosynthesis in insect species as well as vertebrates.

RNA interference suppression of the receptor tyrosine kinase Torso gene impaired pupation and adult emergence in Leptinotarsa decemlineata

In Drosophila melanogaster prothoracic gland (PG) cells, Torso mediates prothoracicotropic hormone (PTTH)-triggered mitogen activated protein kinase (MAPK) pathway (consisting of four core components Ras, Raf, MEK and ERK) to stimulate ecdysteroidogenesis. In this study, LdTorso, LdRas, LdRaf and LdERK were cloned in Leptinotarsa decemlineata. The four genes were highly or moderately expressed in the larval prothoracic glands. At the first- to third-instar stages, their expression levels were higher just before and right after the molt, and were lower in the mid instars. At the fourth-instar stage, their transcript levels were higher before prepupal stage. RNA interference-mediated knockdown of LdTorso delayed larval development, increased pupal weight, and impaired pupation and adult emergence. Moreover, knockdown of LdTorso decreased the mRNA levels of LdRas, LdRaf and LdERK, repressed the transcription of two ecdysteroidogenesis genes (LdPHM and LdDIB), lowered 20E titer, and downregulated the expression of several 20E-response genes (LdEcR, LdUSP, LdHR3 and LdFTZ-F1). Furthermore, silencing of LdTorso induced the expression of a JH biosynthesis gene LdJHAMT, increased JH titer, and activated the transcription of a JH early-inducible gene LdKr-h1. Thus, our results suggest that Torso transduces PTTH-triggered MAPK signal to regulate ecdysteroidogenesis in the PGs in a non-drosophiline insect.

The Systemic Control of Growth

Growth is a complex process that is intimately linked to the developmental program to form adults with proper size and proportions. Genetics is an important determinant of growth, as exemplified by the role of local diffusible molecules setting up organ proportions. In addition, organisms use adaptive responses allowing modulating the size of individuals according to environmental cues, for example, nutrition. Here, we describe some of the physiological principles participating in the determination of final individual size.

Initiation of metamorphosis and control of ecdysteroid biosynthesis in insects: The interplay of absence of Juvenile hormone, PTTH, and Ca(2+)-homeostasis

The paradigm saying that release of the brain neuropeptide big prothoracicotropic hormone (PTTH) initiates metamorphosis by activating the Torso-receptor/ERK pathway in larval prothoracic glands (PGs) is widely accepted nowadays. Upon ligand-receptor interaction Ca(2+) enters the PG cells and acts as a secondary messenger. Ecdysteroidogenesis results, later followed by apoptosis. Yet, some data do not fit in this model. In some species decapitated animals can still molt, even repeatedly, and metamorphose. PTTH does not universally occur in all insect species. PGs may also have other functions; PGs as counterpart of the vertebrate thymus? There are also small PTTHs. Finally, PTTH remains abundantly present in adults and plays a role in control of ecdysteroidogenesis (=sex steroid production) in gonads. This is currently documented only in males. This urges a rethinking of the PTTH-PG paradigm. The key question is: Why does PTTH-induced Ca(2+) entry only result in ecdysteroidogenesis and apoptosis in specific cells/tissues, namely the PGs and gonads? Indeed, numerous other neuropeptides also use Ca(2+) as secondary messenger. The recent rediscovery that in both invertebrates and vertebrates at least some isoforms of Ca(2+)-ATPase need the presence of an endogenous farnesol/juvenile hormone(JH)-like sesquiterpenoid for keeping cytosolic [Ca(2+)]i below the limit of apoptosis-induction, triggered the idea that it is not primarily PTTH, but rather the drop to zero of the JH titer that acts as the primordial initiator of metamorphosis by increasing [Ca(2+)]i. PTTH likely potentiates this effect but only in cells expressing Torso. PTTH: an evolutionarily ancient gonadotropin?

Modulatory effects of bombyxin on ecdysteroidogenesis in Bombyx mori prothoracic glands

In the present study, we investigated the modulatory effects of ecdysteroidogenesis of prothoracic glands (PGs) by bombyxin, an endogenous insulin-like peptide in the silkworm, Bombyx mori. The results showed that bombyxin stimulated ecdysteroidogenesis during a long-term incubation period and in a dose-dependent manner. Moreover, the injection of bombyxin into day 4-last instar larvae increased ecdysteroidogenesis 24h after the injection, indicating its possible in vivo function. Phosphorylation of the insulin receptor and Akt, and the target of rapamycin (TOR) signaling were stimulated by bombyxin, and stimulation of Akt phosphorylation and TOR signaling appeared to be dependent on phosphatidylinositol 3-kinase (PI3K). Bombyxin inhibited the phosphorylation of adenosine 5'-monophosphate-activated protein kinase (AMPK), and the inhibition appeared to be PI3K-independent. Bombyxin-stimulated ecdysteroidogenesis was blocked by either an inhibitor of PI3K (LY294002) or a chemical activator of AMPK (5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside, AICAR), indicating involvement of the PI3K/Akt and AMPK signaling pathway. Bombyxin did not stimulate extracellular signal-regulated kinase (ERK) signaling of PGs. Bombyxin, but not prothoracicotropic hormone (PTTH) stimulated cell viability of PGs. In addition, bombyxin treatment also affected mRNA expression levels of insulin receptor, Akt, AMPKα, -β, and -γ in time-dependent manners. These results suggest that bombyxin modulates ecdysteroidogenesis in B. mori PGs during development.

Transcriptome characterization analysis of Bactrocera minax and new insights into its pupal diapause development with gene expression analysis

Bactrocera minax is a major citrus pest distributed in China, Bhutan and India. The long pupal diapause duration of this fly is a major bottleneck for artificial rearing and underlying mechanisms remain unknown. Genetic information on B. minax transcriptome and gene expression profiles are needed to understand its pupal diapause. High-throughput RNA-seq technology was used to characterize the B. minax transcriptome and to identify differentially expressed genes during pupal diapause development. A total number of 52,519,948 reads were generated and assembled into 47,217 unigenes. 26,843 unigenes matched to proteins in the NCBI database using the BLAST search. Four digital gene expression (DGE) libraries were constructed for pupae at early diapause, late diapause, post-diapause and diapause terminated developmental status. 4,355 unigenes showing the differences expressed across four libraries revealed major shifts in cellular functions of cell proliferation, protein processing and export, metabolism and stress response in pupal diapause. When diapause was terminated by 20-hydroxyecdysone (20E), many genes involved in ribosome and metabolism were differentially expressed which may mediate diapause transition. The gene sets involved in protein and energy metabolisms varied throughout early-, late- and post-diapause. A total of 15 genes were selected to verify the DGE results through quantitative real-time PCR (qRT-PCR); qRT-PCR expression levels strongly correlated with the DGE data. The results provided the extensive sequence resources available for B. minax and increased our knowledge on its pupal diapause development and they shed new light on the possible mechanisms involved in pupal diapause in this species.

Ectopic expression of the male BmDSX affects formation of the chitin plate in female Bombyx mori

Mating structures are involved in successful copulation, intromission, and/or insemination. These structures enable tight coupling between external genitalia of two sexes. During Bombyx mori copulation, the double harpagones in the external genitalia of males clasp the female chitin plate, which is derived from the larval eighth abdominal segment; abnormal development of the female chitin plate affects copulation. We report that ERK phosphorylation (p-ERK) and expression of Abdominal-B (Abd-B) in the posterior abdomen of the female adult is lower than in the male. Ectopic expression of the male-specific spliced form of B. mori doublesex (Bmdsx(M)) in females, however, up-regulates Abd-B and spitz (spi) expression, increasing EGFR signaling activity, and thus forming an abnormal chitin plate and reduced female copulation. These findings indicate that Bmdsx affects the development of the eighth abdominal segment by regulating the activity of EGFR signaling and the expression of Abd-B, resulting in an extra eighth abdominal segment (A8) in males versus the loss of this segment in adult females.

Advances in silkworm studies accelerated by the genome sequencing of Bombyx mori

Significant progress has been achieved in silkworm (Bombyx mori) research since the last review on this insect was published in this journal in 2005. In this article, we review the new and exciting progress and discoveries that have been made in B. mori during the past 10 years, which include the construction of a fine genome sequence and a genetic variation map, the evolution of genomes, the advent of functional genomics, the genetic basis of silk production, metamorphic development, immune response, and the advances in genetic manipulation. These advances, which were accelerated by the genome sequencing project, have promoted B. mori as a model organism not only for lepidopterans but also for general biology.

Involvement of phosphorylation of adenosine 5'-monophosphate-activated protein kinase in PTTH-stimulated ecdysteroidogenesis in prothoracic glands of the silkworm, Bombyx mori

In this study, we investigated inhibition of the phosphorylation of adenosine 5′-monophosphate-activated protein kinase (AMPK) by prothoracicotropic hormone (PTTH) in prothoracic glands of the silkworm, Bombyx mori. We found that treatment with PTTH in vitro inhibited AMPK phosphorylation in time- and dose-dependent manners, as seen on Western blots of glandular lysates probed with antibody directed against AMPKα phosphorylated at Thr172. Moreover, in vitro inhibition of AMPK phosphorylation by PTTH was also verified by in vivo experiments: injection of PTTH into day 7 last instar larvae greatly inhibited glandular AMPK phosphorylation. PTTH-inhibited AMPK phosphorylation appeared to be partially reversed by treatment with LY294002, indicating involvement of phosphatidylinositol 3-kinase (PI3K) signaling. A chemical activator of AMPK (5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside, AICAR) increased both basal and PTTH-inhibited AMPK phosphorylation. Treatment with AICAR also inhibited PTTH-stimulated ecdysteroidogenesis of prothoracic glands. The mechanism underlying inhibition of PTTH-stimulated ecdysteroidogenesis by AICAR was further investigated by determining the phosphorylation of eIF4E-binding protein (4E-BP) and p70 ribosomal protein S6 kinase (S6K), two known downstream signaling targets of the target of rapamycin complex 1 (TORC1). Upon treatment with AICAR, decreases in PTTH-stimulated phosphorylation of 4E-BP and S6K were detected. In addition, treatment with AICAR did not affect PTTH-stimulated extracellular signal-regulated kinase (ERK) phosphorylation, indicating that AMPK phosphorylation is not upstream signaling for ERK phosphorylation. Examination of gene expression levels of AMPKα, β, and γ by quantitative real-time PCR (qRT-PCR) showed that PTTH did not affect AMPK transcription. From these results, it is assumed that inhibition of AMPK phosphorylation, which lies upstream of PTTH-stimulated TOR signaling, may play a role in PTTH stimulation of ecdysteroidogenesis.

Developmental checkpoints and feedback circuits time insect maturation

The transition from juvenile to adult is a fundamental process that allows animals to allocate resource toward reproduction after completing a certain amount of growth. In insects, growth to a species-specific target size induces pulses of the steroid hormone ecdysone that triggers metamorphosis and reproductive maturation. The past few years have seen significant progress in understanding the interplay of mechanisms that coordinate timing of ecdysone production and release. These studies show that the neuroendocrine system monitors complex size-related and nutritional signals, as well as external cues, to time production and release of ecdysone. Based on results discussed here, we suggest that developmental progression to adulthood is controlled by checkpoints that regulate the genetic timing program enabling it to adapt to different environmental conditions. These checkpoints utilize a number of signaling pathways to modulate ecdysone production in the prothoracic gland. Release of ecdysone activates an autonomous cascade of both feedforward and feedback signals that determine the duration of the ecdysone pulse at each developmental transitions. Conservation of the genetic mechanisms that coordinate the juvenile-adult transition suggests that insights from the fruit fly Drosophila will provide a framework for future investigation of developmental timing in metazoans.

TOR signaling is involved in PTTH-stimulated ecdysteroidogenesis by prothoracic glands in the silkworm, Bombyx mori

The prothoracicotropic hormone (PTTH) is a stimulator of ecdysteroidogenesis in prothoracic gland of larval insects. Our recent studies showed that phosphoinositide 3-kinase (PI3K)/Akt signaling was involved in PTTH-stimulated ecdysteroidogenesis by Bombyx mori prothoracic glands. In the present study, downstream signaling of PI3K/Akt was further investigated. Results showed that PTTH rapidly enhanced the phosphorylation of translational repressor 4E-binding protein (4E-BP) and p70 ribosomal protein S6 kinase (S6K), two known downstream signaling targets of the target of rapamycin complex 1 (TORC1). PTTH stimulated 4E-BP phosphorylation in time- and dose-dependent manners. Injection of PTTH into day-6 last instar larvae greatly increased 4E-BP phosphorylation, verifying the in vitro effect. PTTH-stimulated 4E-BP phosphorylation was blocked by both LY294002 and wortmannin, indicating the involvement of PI3K. Mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase (MEK) inhibitors (PD 98059 and U0126), did not inhibit PTTH-stimulated 4E-BP phosphorylation, implying that ERK signaling is not related to PTTH-stimulated 4E-BP phosphorylation. The phosphorylation of S6K was also stimulated by PTTH both in vitro and in vivo. PI3K signaling appears to be involved in PTTH-stimulated phosphorylation of S6K. Rapamycin, a specific inhibitor of mammalian TOR signaling attenuated PTTH-stimulated phosphorylation of 4E-BP and S6K of the glands, and greatly inhibited PTTH-stimulated ecdysteroidogenesis. Examination of gene expression levels of 4E-BP and S6K showed that PTTH inhibited mRNA levels of both 4E-BP and S6K, indicating that PTTH may exert its action at both the transcriptional and phosphorylation levels. These results suggest that PTTH/PI3K/TOR/4E-BP (S6K) signaling is involved in PTTH-stimulated ecdysteroidogenesis by prothoracic glands in B. mori.