Basic pattern of fluctuation in hemolymph PTTH titers during larval-pupal and pupal-adult development of the silkworm, Bombyx mori

Investigation of the effects of starvation stress in the midgut of the silkworm Bombyx mori

During their evolution, organisms have developed various mechanisms to adapt to changing nutritional conditions such as mobilization of storage molecules and activation of autophagy. In this study, the mechanism of adaptive responses in the midgut of the silkworm Bombyx mori L., 1758 (Lepidoptera: Bombycidae) larvae, which were starved for different days, was investigated. The study was carried out at the Insect Physiology Research Laboratory and Silkworm Culture Laboratory at Ege University between 2018 and 2020. For this purpose, the histological structure of the midgut was examined using hematoxylin&eosin staining and its protein, sugar, glycogen, and lipid contents were determined. As autophagy markers, lysosomal enzyme activities were measured and expressions of autophagy-related genes (mTOR, ATG8, and ATG12) were analyzed by qRT-PCR. The results showed that, depending on the time of onset of starvation stress, autophagy plays no role as an adaptive response under starvation conditions or occurs at a much more moderate level than autophagy which happens as part of cell death during larval-pupal metamorphosis.

Time- and temperature-dependent dynamics of prothoracicotropic hormone and ecdysone sensitivity co-regulate pupal diapause in the green-veined white butterfly Pieris napi

Diapause, a general shutdown of developmental pathways, is a vital adaptation allowing insects to adjust their life cycle to adverse environmental conditions such as winter. Diapause in the pupal stage is regulated by the major developmental hormones prothoracicotropic hormone (PTTH) and ecdysone. Termination of pupal diapause in the butterfly Pieris napi depends on low temperatures; therefore, we study the temperature-dependence of PTTH secretion and ecdysone sensitivity dynamics throughout diapause, with a focus on diapause termination. While PTTH is present throughout diapause in the cell bodies of two pairs of neurosecretory cells in the brain, it is absent in the axons, and the PTTH concentration in the haemolymph is significantly lower during diapause than during post diapause development, indicating that the PTTH signaling is reduced during diapause. The sensitivity of pupae to ecdysone injections is dependent on diapause stage. While pupae are sensitive to ecdysone during early diapause initiation, they gradually lose this sensitivity and become insensitive to non-lethal concentrations of ecdysone about 30 days into diapause. At low temperatures, reflecting natural overwintering conditions, diapause termination propensity after ecdysone injection is precocious compared to controls. In stark contrast, at high temperatures reflecting late summer and early autumn conditions, sensitivity to ecdysone does not return. Thus, here we show that PTTH secretion is reduced during diapause, and additionally, that the low ecdysone sensitivity of early diapause maintenance is lost during termination in a temperature dependent manner. The link between ecdysone sensitivity and low-temperature dependence reveals a putative mechanism of how diapause termination operates in insects that is in line with adaptive expectations for diapause.

Neurotransmitters Affect Larval Development by Regulating the Activity of Prothoracicotropic Hormone-Releasing Neurons in Drosophila melanogaster

Ecdysone, an essential insect steroid hormone, promotes larval metamorphosis by coordinating growth and maturation. In Drosophila melanogaster, prothoracicotropic hormone (PTTH)-releasing neurons are considered to be the primary promoting factor in ecdysone biosynthesis. Recently, studies have reported that the regulatory mechanisms of PTTH release in Drosophila larvae are controlled by different neuropeptides, including allatostatin A and corazonin. However, it remains unclear whether neurotransmitters provide input to PTTH neurons and control the metamorphosis in Drosophila larvae. Here, we report that the neurotransmitters acetylcholine (ACh) affect larval development by modulating the activity of PTTH neurons. By downregulating the expression of different subunits of nicotinic ACh receptors in PTTH neurons, pupal volume was significantly increased, whereas pupariation timing was relatively unchanged. We also identified that PTTH neurons were excited by ACh application ex vivo in a dose-dependent manner via ionotropic nicotinic ACh receptors. Moreover, in our Ca2+ imaging experiments, relatively low doses of OA caused increased Ca2+ levels in PTTH neurons, whereas higher doses led to decreased Ca2+ levels. We also demonstrated that a low dose of OA was conveyed through OA β-type receptors. Additionally, our electrophysiological experiments revealed that PTTH neurons produced spontaneous activity in vivo, which provides the possibility of the bidirectional regulation, coming from neurons upstream of PTTH cells in Drosophila larvae. In summary, our findings indicate that several different neurotransmitters are involved in the regulation of larval metamorphosis by altering the activity of PTTH neurons in Drosophila.

Induction of diapause by clock proteins period and timeless via changes in PTTH and ecdysteroid titer in the sugar beet moth, Scrobipalpa ocellatella (Lepidoptera: Gelechiidae)

The sugar beet moth, Scrobipalpa ocellatella (Boyd), one of the most severe sugar beet pests, causes quantitative and qualitative yield losses late in the autumn. Previously, it was shown that low temperature and short-day photoperiod together cause diapause induction in pupae. Here, the interaction of the critical elements of the diapause induction, including the period (PER), timeless (TIM), prothoracicotropic hormone (PTTH), and ecdysteroid titer, were investigated. Immunohistochemistry results showed that the number of period immunoreactivity (PER-ir) and TIM-ir cells in nondiapause pupae (NDP) was lower than in the brain of the diapause pupae (DP). Moreover, the number of PER-ir and TIM-ir cells in the protocerebrum and optic lobe (OL) of NDP was lower than DP. Moreover, lower PTTH content in the brain and hemolymph of DP was confirmed by competitive enzyme-linked immunosorbent assay. Enzyme immunoassay showed a lower 20-hydroxyecdysone (20E) titer in the hemolymph of the DP compared with the NDP. Within a short-day condition, PER and TIM titers increased in the brain leading to decreasing PTTH titers in the brain and hemolymph that caused decreasing 20E titer in the hemolymph, leading to the induction of diapause. This study suggests that PER and TIM could be one of the brain factors that play an essential role in regulating diapause in S. ocellatella.

Ecdysone-dependent feedback regulation of prothoracicotropic hormone controls the timing of developmental maturation

The activation of a neuroendocrine system that induces a surge in steroid production is a conserved initiator of the juvenile-to-adult transition in many animals. The trigger for maturation is the secretion of brain-derived neuropeptides, yet the mechanisms controlling the timely onset of this event remain ill-defined. Here, we show that a regulatory feedback circuit controlling the Drosophila neuropeptide Prothoracicotropic hormone (PTTH) triggers maturation onset. We identify the Ecdysone Receptor (EcR) in the PTTH-expressing neurons (PTTHn) as a regulator of developmental maturation onset. Loss of EcR in these PTTHn impairs PTTH signaling, which delays maturation. We find that the steroid ecdysone dose-dependently affects Ptth transcription, promoting its expression at lower concentrations and inhibiting it at higher concentrations. Our findings indicate the existence of a feedback circuit in which rising ecdysone levels trigger, via EcR activity in the PTTHn, the PTTH surge that generates the maturation-inducing ecdysone peak toward the end of larval development. Because steroid feedback is also known to control the vertebrate maturation-inducing hypothalamic-pituitary-gonadal axis, our findings suggest an overall conservation of the feedback-regulatory neuroendocrine circuitry that controls the timing of maturation initiation.

Superoxide dismutase down-regulation and the oxidative stress is required to initiate pupation in Bombyx mori

Perhaps, oxidative stress progresses pupation in some Lepidopteran insects; however, the reasons for this remain obscure. In our previous study, we clarified Bombyx mori SOD1 (BmSOD1) and B. mori SOD2 (BmSOD2) proteins respond in common to ultraviolet irradiation (UV) oxidative stress and metamorphosis. This result strongly suggested pupation initiates by oxidative stress and might mediate by down-regulation of expression of BmSOD1 and BmSOD2 proteins. Thus, we examined about these relationships in B. mori in this study. In the microarray data reanalysis, we found the Notch signaling pathways as the common pathways in pupation and UV oxidative stress in B. mori. Also, we showed a molting hormone, 20-hydroxyecdysone, leads not only generation of superoxide but also downregulation of the expression of BmSOD proteins during pupation in B. mori. Our findings can contribute to a deeper understanding of how biological defense systems work against environmental oxidative stress.

Genome-wide identification of chitin-binding proteins and characterization of BmCBP1 in the silkworm, Bombyx mori

The insect cuticle plays important roles in numerous physiological functions to protect the body from invasion of pathogens, physical injury and dehydration. In this report, we conducted a comprehensive genome-wide search for genes encoding proteins with peritrophin A-type (ChtBD2) chitin-binding domain (CBD) in the silkworm, Bombyx mori. One of these genes, which encodes the cuticle protein BmCBP1, was additionally cloned, and its expression and location during the process of development and molting in B. mori were investigated. In total, 46 protein-coding genes were identified in the silkworm genome, including those encoding 15 cuticle proteins analogous to peritrophins with one CBD (CPAP1s), nine cuticle proteins analogous to peritrophins with three CBD (CPAP3s), 15 peritrophic membrane proteins (PMPs), four chitinases, and three chitin deacetylases, which contained at least one ChtBD2 domain. Microarray analysis indicated that CPAP-encoding genes were widely expressed in various tissues, whereas PMP genes were highly expressed in the midgut. Quantitative polymerase chain reaction and western blotting showed that the cuticle protein BmCBP1 was highly expressed in the epidermis and head, particularly during molting and metamorphosis. An immunofluorescence study revealed that chitin co-localized with BmCBP1 at the epidermal surface during molting. Additionally, BmCBP1 was notably up-regulated by 20-hydroxyecdysone treatment. These results provide a genome-level view of the chitin-binding protein in silkworm and suggest that BmCBP1 participates in the formation of the new cuticle during molting.

Refining a steroidogenic model: an analysis of RNA-seq datasets from insect prothoracic glands

BACKGROUND

The prothoracic gland (PG), the principal steroidogenic organ of insects, has been proposed as a model for steroid hormone biosynthesis and regulation.

RESULTS

To validate the robustness of the model, we present an analysis of accumulated transcriptomic data from PGs of two model species, Drosophila melanogaster and Bombyx mori. We identify that the common core components of the model in both species are encoded by nine genes. Five of these are Halloween genes whose expression differs substantially between the PGs of these species.

CONCLUSIONS

We conclude that the PGs can be a model for steroid hormone synthesis and regulation within the context of mitochondrial cholesterol transport and steroid biosynthesis but beyond these core mechanisms, gene expression in insect PGs is too diverse to fit in a context-specific model and should be analysed within a species-specific framework.

Refinement of ectopic protein expression through the GAL4/UAS system in Bombyx mori: application to behavioral and developmental studies

Silkmoth, Bombyx mori, is one of the important model insects in which transgenic techniques and the GAL4/UAS system are applicable. However, due to cytotoxicity and low transactivation activity of GAL4, effectiveness of the GAL4/UAS system and its application in B. mori are still limited. In the present study, we refined the previously reported UAS vector by exploiting transcriptional and translational enhancers, and achieved 200-fold enhancement of reporter GFP fluorescence in the GAL4/UAS system. Enhanced protein expression of membrane-targeted GFP and calcium indicator protein (GCaMP5G) drastically improved visualization of fine neurite structures and neural activity, respectively. Also, with the refined system, we generated a transgenic strain that expresses tetanus toxin light chain (TeTxLC), which blocks synaptic transmission, under the control of GAL4. Ectopic TeTxLC expression in the sex pheromone receptor neurons inhibited male courtship behavior, proving effectiveness of TeTxLC on loss-of-function analyses of neural circuits. In addition, suppression of prothoracicotropic hormone (PTTH) or insulin-like peptide (bombyxin) secretion impaired developmental timing and growth rate, respectively. Furthermore, we revealed that larval growth is sex-differentially regulated by these peptide hormones. The present study provides important technical underpinnings of transgenic approaches in silkmoths and insights into mechanisms of postembryonic development in insects.

Arylalkylamine N-acetyltransferase 1 gene (TcAANAT1) is required for cuticle morphology and pigmentation of the adult red flour beetle, Tribolium castaneum

In the insect cuticle tanning pathway (sclerotization and pigmentation), the enzyme arylalkylamine N-acetyltransferase (AANAT) catalyzes the acetylation of dopamine to form N-acetyldopamine (NADA), which is one of the major precursors for quinone-mediated tanning. In this study we characterized and investigated the function of TcAANAT1 in cuticle pigmentation of the red flour beetle, Tribolium castaneum. We isolated a full length TcAANAT1 cDNA that encodes a protein of 256 amino acid residues with a predicted GCN5-related acetyltransferase domain containing an acetyl-CoA binding motif. TcAANAT1 transcripts were detected at all stages of development with lowest expressions at the embryonic and pharate pupal stages. We expressed and purified the encoded recombinant TcAANAT1 protein (rTcAANAT1) that exhibited highest activity at slightly basic pH values (for example, pH 7.5 to 8.5 using dopamine as the substrate). In addition, rTcAANAT1 acts on a wide range of substrates including tryptamine, octopamine and norepinephrine with similar substrate affinities with K_m values in the range of 0.05-0.11 mM except for tyramine (K_m = 0.56 mM). Loss of function of TcAANAT1 caused by RNAi had no effect on larval and pupal development. The tanning of pupal setae, gin traps and urogomphi proceeded normally. However, the resulting adults (∼70%) exhibited a roughened exoskeletal surface, separated elytra and improperly folded hindwings. The body wall, elytra and veins of the hindwing of the mature adults were significantly darker than those of control insects probably due to the accumulation of dopamine melanin. A dark pigmentation surrounding the bristles located on the inter-veins of the elytron was evident primarily because of the underlying darkly pigmented trabeculae that partition the dorsal and ventral layers of the elytron. These results support the hypothesis that TcAANAT1 acetylates dopamine and plays a role in development of the morphology and pigmentation of T. castaneum adult cuticle.

Combinatory annotation of cell membrane receptors and signalling pathways of Bombyx mori prothoracic glands

The cells of prothoracic glands (PG) are the main site of synthesis and secretion of ecdysteroids, the biochemical products of cholesterol conversion to steroids that shape the morphogenic development of insects. Despite the availability of genome sequences from several insect species and the extensive knowledge of certain signalling pathways that underpin ecdysteroidogenesis, the spectrum of signalling molecules and ecdysteroidogenic cascades is still not fully comprehensive. To fill this gap and obtain the complete list of cell membrane receptors expressed in PG cells, we used combinatory bioinformatic, proteomic and transcriptomic analysis and quantitative PCR to annotate and determine the expression profiles of genes identified as putative cell membrane receptors of the model insect species, Bombyx mori, and subsequently enrich the repertoire of signalling pathways that are present in its PG cells. The genome annotation dataset we report here highlights modules and pathways that may be directly involved in ecdysteroidogenesis and aims to disseminate data and assist other researchers in the discovery of the role of such receptors and their ligands.

Integrative Proteomics and Metabolomics Analysis of Insect Larva Brain: Novel Insights into the Molecular Mechanism of Insect Wandering Behavior

Before metamorphosis, most holometabolous insects, such as the silkworm studied here, undergo a special phase called the wandering stage. Insects in this stage often display enhanced locomotor activity (ELA). ELA is vital because it ensures that the insect finds a safe and suitable place to live through the pupal stage. The physiological mechanisms of wandering behavior are still unclear. Here, we integrated proteomics and metabolomics approaches to analyze the brain of the lepidopteran insect, silkworm, at the feeding and wandering stages. Using LC-MS/MS and GC-MS, in all we identified 3004 proteins and 37 metabolites at these two stages. Among them, 465 proteins and 22 metabolites were changed. Neural signal transduction proteins and metabolites, such as neurofilament, dopaminergic synapse related proteins, and glutamic acid, were significantly altered, which suggested that active neural conduction occurred in the brain at the wandering stage. We also found decreased dopamine degradation at the wandering stage. The proposed changes in active neural conduction and increased dopamine concentration might induce ELA. In addition, proteins involved in the ubiquitin proteasome system and lysosome pathway were upregulated, revealing that the brain experiences morphological remodeling during metamorphosis. These findings yielded novel insights into the molecular mechanism underlying insect wandering behavior.

Positive feedback regulation of prothoracicotropic hormone secretion by ecdysteroid--a mechanism that determines the timing of metamorphosis

When insect larvae have fully grown, prothoracicotropic hormone (PTTH) is released from the brain, triggering the initiation of metamorphic development through stimulation of ecdysteroid secretion by the prothoracic glands. The present study analyzes the mechanism that regulates the occurrence of this PTTH surge. In the silkworm Bombyx mori, the PTTH surge occurs on day 6 of the fifth instar and is preceded by a small rise in hemolymph ecdysteroid titer, which occurs late on day 5. We therefore hypothesized that this rise of ecdysteroid titer is involved in the induction of the PTTH surge. To test this hypothesis, two experiments were conducted. First, a small amount of 20-hydroxyecdysone was injected on day 4, two days before the expected day of the PTTH surge, to simulate the small rise in hemolymph ecdysteroid titer on day 5. This injection led to a precocious surge of PTTH the next day. Next, the hemolymph ecdysteroid titer on day 5 was artificially lowered by injecting ecdysteroid-22-oxidase, which inactivates 20-hydroxyecdysone. After this treatment, the PTTH surge did not occur on day 6 in 80% of the animals. These results indicate that a small rise of the hemolymph ecdysteroid titer plays a critical role in the induction of the PTTH surge. Since basal ecdysteroidogenic activity of the prothoracic glands increases with larval growth, a circulating level of ecdysteroids may convey information about larval maturity to the brain, to coordinate larval growth and metamorphosis. This is the first report in invertebrates to demonstrate positive feedback regulation of the surge of a tropic hormone by a downstream steroid hormone.

How clocks and hormones act in concert to control the timing of insect development

During the last century, insect model systems have provided fascinating insights into the endocrinology and developmental biology of all animals. During the insect life cycle, molts and metamorphosis delineate transitions from one developmental stage to the next. In most insects, pulses of the steroid hormone ecdysone drive these developmental transitions by activating signaling cascades in target tissues. In holometabolous insects, ecdysone triggers metamorphosis, the remarkable remodeling of an immature larva into a sexually mature adult. The input from another developmental hormone, juvenile hormone (JH), is required to repress metamorphosis by promoting juvenile fates until the larva has acquired sufficient nutrients to survive metamorphosis. Ecdysone and JH act together as key endocrine timers to precisely control the onset of developmental transitions such as the molts, pupation, or eclosion. In this review, we will focus on the role of the endocrine system and the circadian clock, both individually and together, in temporally regulating insect development. Since this is not a coherent field, we will review recent developments that serve as examples to illuminate this complex topic. First, we will consider studies conducted in Rhodnius that revealed how circadian pathways exert temporal control over the production and release of ecdysone. We will then take a look at molecular and genetic data that revealed the presence of two circadian clocks, located in the brain and the prothoracic gland, that regulate eclosion rhythms in Drosophila. In this context, we will also review recent developments that examined how the ecdysone hierarchy delays the differentiation of the crustacean cardioactive peptide (CCAP) neurons, an event that is critical for the timing of ecdysis and eclosion. Finally, we will discuss some recent findings that transformed our understanding of JH function.

Relationship between the expression of Rab family GTPases and neuropeptide hormones in the brain of Bombyx mori

Rab proteins are small GTPases that play essential roles in vesicle transport. In this study, we examined the expression of Rab proteins and neuropeptide hormones in the brain of the silkworm, Bombyx mori. We produced antibodies against B. mori Rab1 and Rab14 in rabbits. Immunoblotting of samples of brain tissue from B. mori revealed a single band for each antibody. Rab1 and Rab14 immunohistochemical labeling in the brain of B. mori was restricted to neurons of the pars intercerebralis and dorsolateral protocerebrum. Rab1, Rab7 and Rab14 co-localized with bombyxin. Rab1 and Rab7 co-localized with eclosion hormone. Rab1 co-localized with prothoracicotropic hormone. These results suggest that Rab1, Rab7 and Rab14 may be involved in neuropeptide transport in the brain of B. mori. This is the first report on the specificity of Rab proteins for the secretion of different neuropeptides in insects.

Molecular cloning and characterization of Ecdysone oxidase and 3-dehydroecdysone-3α-reductase involved in the ecdysone inactivation pathway of silkworm, Bombyx mori

Molting hormone (ecdysteroid) is one of the most important hormones in insects. The synthesis and inactivation of the ecdysteroid regulate the developmental process of insects. A major pathway of ecdysone inactivation is that ecdysone is converted to 3-dehydroecdysone, and then further to 3-epiecdysone in insects. Two enzymes (ecdysone oxidase: EO and 3DE-3α-reductase) participate in this pathway. In this study, based on the previously characterized cDNAs in Spodoptera littoralis, we cloned and characterized EO and 3DE-3α-reductase genes in the silkworm, Bombyx mori. The heterologously expressed proteins of the two genes in yeast showed the ecdysone oxidase and 3DE-3α-reductase activities, respectively. Expression of BmEO was only detected in the midgut at transcriptional and translational levels. We also localized EO within the midgut goblet cell cavities. For Bm3DE-3α-reductase gene, RT-PCR and western blot showed that it was expressed in the midgut and the Malpighian tubules. Moreover, we localized 3DE-3α-reductase within the midgut goblet cell cavities and the cytosol of principal cells of the Malpighian tubules. These two genes have similar expression profiles during different developmental stages. Both genes were highly expressed at the beginning of the 5^th instar, and remained a relative low level during the feeding stage, and then were highly expressed at the wandering stage. All these results showed that the profiles of the two genes were well correlated with the ecdysteroid titer. The functional characterization of the enzymes participating in ecdysone inactivation in the silkworm provides hints for the artificial regulation of the silkworm development and biological control of pests.

Regulatory mechanism of silkworm hemocyte adhesion to organs

Circulating hemocytes in the body fluid of the silkworm are increased during the larval-larval molting period. We investigated hemocyte adhesion to organs mediating the selectin-selectin ligands during the feeding period and the larval-larval molting period using the lectin staining method, sugar chain digestion test with glycoside hydrolases, and the hemocyte adhesion inhibition test using monosaccharides. The results of these tests suggested that the selectin ligand involved in hemocyte adhesion was the Sialyl Lewis x-type, and the structure was changed from the feeding period to the larval-larval molting period. Beta-galactosidase appears to be an enzyme that eliminates N-acetylgalactosamine and sialylated N-acetylgalactosamine from the terminal of Sialyl Lewis x. Beta-galactosidase activation in skin basement membranes, muscle, fat bodies, midguts, and hemocytes increased markedly during the larval-larval molting period, and at that time, hemocytes were detached from organs. Adding 20-hydroxyecdysone or its analog, tebufenozide to cultured fat bodies increased β-galactosidase activity in these tissues. Therefore, 20-hydroxyecdysone may induce a structural change in Sialyl Lewis x type sugar chains on the cell surface of silkworm's organs by increasing the β-galactosidase activity to detach hemocytes from organs and increase the number of circulating hemocytes during the larval-larval molting period.

Gene encoding the prothoracicotropic hormone of a moth is expressed in the brain and gut

The molts of lepidopteran insects are typically controlled by the brain-derived prothoracicotropic hormone (PTTH) that stimulates ecdysteroidogenesis in the prothoracic glands (PGs). We report here that the larvae and pupae of the moth Sesamia nonagrioides can molt without brain (PGs must be present), suggesting that there might be a secondary source of PTTH. We addressed this issue by characterizing spatial and temporal expression patterns of the PTTH gene. To this end we identified a major part of the corresponding cDNA. Protein deduced from this cDNA fragment consisted of 128 amino acids and showed 48-85% homology with the matching regions of PTTHs known from other Lepidoptera. Quantification of PTTH expression in major body organs of the last instar larvae revealed high expression in the brain (fading in post-feeding larvae) and considerable expression in the gut (with a maximum in post-feeding larvae). The content of PTTH message in the gut was enhanced after decapitation. It is concluded that the molts of S. nonagrioides larvae are driven by PTTH gene expression in the gut.

Mutations of an arylalkylamine-N-acetyltransferase, Bm-iAANAT, are responsible for silkworm melanism mutant

Coloration is one of the most variable characters in animals and provides rich material for studying the developmental genetic basis of pigment patterns. In the silkworm, more than 100 gene mutation systems are related to aberrant color patterns. The melanism (mln) is a rare body color mutant that exhibits an easily distinguishable phenotype in both larval and adult silkworms. By positional cloning, we identified the candidate gene of the mln locus, Bm-iAANAT, whose homologous gene (Dat) converts dopamine into N-acetyldopamine, a precursor for N-acetyldopamine sclerotin in Drosophila. In the mln mutant, two types of abnormal Bm-iAANAT transcripts were identified, whose expression levels are markedly lower than the wild type (WT). Moreover, dopamine content was approximately twice as high in the sclerified tissues (head, thoracic legs, and anal plate) of the mutant as in WT, resulting in phenotypic differences between the two. Quantitative reverse transcription PCR analyses showed that other genes involved in the melanin metabolism pathway were regulated by the aberrant Bm-iAANAT activity in mln mutant in different ways and degrees. We therefore propose that greater accumulation of dopamine results from the functional deficiency of Bm-iAANAT in the mutant, causing a darker pattern in the sclerified regions than in the WT. In summary, our results indicate that Bm-iAANAT is responsible for the color pattern of the silkworm mutant, mln. To our knowledge, this is the first report showing a role for arylalkylamine-N-acetyltransferases in color pattern mutation in Lepidoptera.

Control of ecdysteroidogenesis in prothoracic glands of insects: a review

The very first step in the study of the endocrine control of insect molting was taken in 1922. Stefan Kopec characterized a factor in the brain of the gypsy moth, Lymantria dispar which appeared to be essential for metamorphosis. This factor was later identified as the neuropeptide prothoracicotropic hormone (PTTH), the first discovery of a series of factors involved in the regulation of ecdysteroid biosynthesis in insects. It is now accepted that PTTH is the most important regulator of prothoracic gland (PG) ecdysteroidogenesis. The periodic increases in ecdysteroid titer necessary for insect development can basically be explained by the episodic activation of the PGs by PTTH. However, since the characterization of the prothoracicostatic hormone (PTSH), it has become clear that in addition to 'tropic factors', also 'static factors', which are responsible for the 'fine-tuning' of the hemolymph ecdysteroid titer, are at play. Many of these regulatory factors are peptides originating from the brain, but also other, extracerebral factors both of peptidic and non-peptidic nature are able to affect PG ecdysteroidogenesis, such as the 'classic' insect hormones, juvenile hormone (JH) and the molting hormone (20E) itself. The complex secretory pattern of ecdysteroids as observed in vivo is the result of the delicate balance and interplay between these ecdysiotropic and ecdysiostatic factors.

Manduca sexta prothoracicotropic hormone: evidence for a role beyond steroidogenesis

Prothoracicotropic hormone (PTTH) is a homodimeric brain peptide hormone that positively regulates the production of ecdysteroids by the prothoracic gland of Lepidoptera and probably other insects. PTTH was first purified from heads of adult domestic silkworms, Bombyx mori. Prothoracic glands of Bombyx and Manduca sexta undergo apoptosis well before the adult stage is reached, raising the recurring question of PTTH function at these later stages. Because Bombyx has been domesticated for thousands of years, the possibility exists that the presence of PTTH in adult animals is an accidental result of domestication for silk production. In contrast, Manduca has been raised in the laboratory for only five or six decades. The present study found that Manduca brains contain PTTH at all stages examined post-prothoracic gland apoptosis, i.e., pharate adult and adult life, and that PTTH-dependent changes in protein phosphorylation and protein synthesis were observed in several reproductive and reproduction-associated organs. The data indicate that PTTH indeed plays a role in non-steroidogenic tissues and suggest possible future avenues for determining which cellular processes are being so regulated.

EcR expression in the prothoracicotropic hormone-producing neurosecretory cells of the Bombyx mori brain

The steroid hormone 20-hydroxyecdysone (20E) initiates insect molting and metamorphosis through binding with a heterodimer of two nuclear receptors, the ecdysone receptor (EcR) and ultraspiracle (USP). Expression of the specific isoforms EcR-A and EcR-B1 governs steroid-induced responses in the developing cells of the silkworm Bombyx mori. Here, analysis of EcR-A and EcR-B1 expression during larval-pupal development showed that both genes were up-regulated by 20E in the B. mori brain. Whole-mount in situ hybridization and immunohistochemistry revealed that EcR-A and EcR-B1 mRNAs and proteins were exclusively located in two pairs of lateral neurosecretory cells in the larval brain known as the prothoracicotropic hormone (PTTH)- producing cells (PTPCs). In the pupal brain, EcR-A and EcR-B1 expression was detected in tritocerebral cells and optic lobe cells in addition to PTPCs. As PTTH controls ecdysone secretion by the prothoracic gland, these results indicate that 20E-responsive PTPCs are the master cells of insect metamorphosis.

Circadian orchestration of developmental hormones in the insect, Rhodnius prolixus

This review presents a new perspective on the circadian regulation and functions of insect developmental hormones. In Rhodnius prolixus (Hemiptera), the brain neuropeptide prothoracicotropic hormone (PTTH) is released with a circadian rhythm that is controlled by paired photosensitive clocks in the brain. These clocks comprise the dorsal and lateral PER/TIM clock neurons known to regulate behavioral rhythms in Drosophila. Axons of PTTH and clock cells make close contact. Photosensitive PER/TIM clocks also reside in the paired prothoracic glands (PGs), which generate rhythmic synthesis and release of the ecdysteroid molting hormones. The PG clocks are entrained by both light and PTTH. These four clocks are coupled together by both nerves and hormones into a timing system whose primary regulated output is the circadian rhythm of ecdysteroids in the hemolymph. This complex timing system appears necessary to ensure circadian organization of the gene expression that is induced in target cells by ecdysteroids via circadian cycling of the nuclear ecdysteroid receptor (EcR). This multioscillator system serves to transduce 'the day outside' into endocrine rhythms that orchestrate 'the day inside'. It has many functional similarities with vertebrate circadian systems.

Silk Moth Neuropeptide Hormones: Prothoracicotropic Hormone and Others

The silkworm, Bombyx mori, is a very useful model species, especially in genetics, biochemistry, physiology, and molecular biology, helping researchers unravel the many mysteries involved in the insect life process. The present review describes our early contributions as chemists to the study of the molting and metamorphosis of B. mori. We also present research by Japanese scientists that contributed to the isolation and characterization of peptide hormones from B. mori.

Neurohormones in insect stress: A review

The neurohormones are the master regulators of all life processes in insects and they create a strategy of stress protecting events. Neurohormones are synthesized mainly in insect brain neurosecretory neurons. Various stressors of different intensity cause specific changes which influence on neurosecretory neurons activity and synthesis of neurohormones (biogene amines, ecdysiotropins, ecdysiostatins, allatoregulatory neurohormones, adipokinetic neurohormones, etc.). Biogene amines in insects may function as neurohormones controlling carbohydrate and lipid metabolism as the primary response of the insects to the effect of stressors. Intermediary metabolism in insects is mainly regulated by adipokinetic hormones which supply organism by energy especially in extreme conditions. Stress induces changes in release of ecdysioregulatory and allatoregulatory neurohormones and modificates ecdysones and juvenile hormones synthesis in prothoracic gland and corpora allata. The involvement of hormones of an ecdysteroid or JH type in response to stress creates the danger of an untimely induction of morphogenetic process in target cells. Limiting the quantity of secreted hormones and shortening the period when target cells are sensitive to morphogenetic stimuli removes this danger.

hormone nuclear receptor (EcR) exhibits circadian cycling in certain tissues, but not others, during development in Rhodnius prolixus (Hemiptera)

The insect moulting hormones, viz. the ecdysteroids, regulate gene expression during development by binding to an intracellular protein, the ecdysteroid receptor (EcR). In the insect Rhodnius prolixus, circulating levels of ecdysteroids exhibit a robust circadian rhythm. This paper demonstrates associated circadian rhythms in the abundance and distribution of EcR in several major target tissues of ecdysteroids, but not in others. Quantitative analysis of immunofluorescence images obtained by confocal laser-scanning microscopy following the use of anti-EcR has revealed a marked daily rhythm in the nuclear abundance of EcR in cells of the abdominal epidermis, brain, fat body, oenocytes and rectal epithelium of Rhodnius. This EcR rhythm is synchronous with the rhythm of circulating hormone levels. It free-runs in continuous darkness for several cycles, showing that EcR nuclear abundance is under circadian control. Circadian control of a nuclear receptor has not been shown previously in any animal. We infer that the above cell types detect and respond to the temporal signals in the rhythmic ecdysteroid titre. In several cell types, the rhythm in cytoplasmic EcR peaks several hours prior to the EcR peak in the nucleus each day, thereby implying a daily migration of EcR from the cytoplasm to the nucleus. This finding shows that EcR is not a constitutive nuclear receptor, as has previously been assumed. In the brain, rhythmic nuclear EcR has been found in peptidergic neurosecretory cells, indicating a potential pathway for feedback regulation of the neuroendocrine system by ecdysteroids, and also in regions containing circadian clock neurons, suggesting that the circadian timing system in the brain is also sensitive to rhythmic ecdysteroid signals.

Cytochrome P450 CYP307A1/Spook: A regulator for ecdysone synthesis in insects

The prothoracic gland (PG) has essential roles in synthesizing and secreting a steroid hormone called ecdysone that is critical for molting and metamorphosis of insects. However, little is known about the genes controlling ecdysteroidogenesis in the PG. To identify genes functioning in the PG of the silkworm, Bombyx mori, we used differential display PCR and focused on a cytochrome P450 gene designated Cyp307a1. Its expression level positively correlates with a change in the hemolymph ecdysteroid titer. In addition, Drosophila Cyp307a1 is encoded in the spook locus, one of the Halloween mutant family members showing a low ecdysone titer in vivo, suggesting that Cyp307a1 is involved in ecdysone synthesis. While Drosophila Cyp307a1 is expressed in the early embryos and adult ovaries, the expression is not observed in the PGs of embryos or third instar larvae. These results suggest a difference in the ecdysone synthesis pathways during larval development in these insects.

Endocrine changes associated with the starvation-induced premature metamorphosis in the yellow-spotted longicorn beetle, Psacothea hilaris

Under 25 degrees C and a long day photoperiod, about half of the fourth instar Psacothea hilaris larvae molt to the fifth instar on day 13 and pupate about 18 days later; the rest pupate without a further larval molt with a mean fourth instar period of 24 days. However, starvation of fourth instar larvae exceeding a threshold weight induces premature pupation, resulting in the formation of small but morphologically normal adults. To clarify the endocrine basis for this premature pupation, hemolymph juvenile hormone (JH) and ecdysteroid titers were quantified during the fed and the starved periods. Normally fed fourth instar larvae exhibited two populations with regard to JH and ecdysteroid titers, one having JH titers ranging from 1.2 to 2.1 ng/ml through to day 13, the other, similarly high titers in the early part of the instar but low titers reaching 0.1 ng/ml on day 13. One population had ecdysteroid titers with a peak of 43 ng/ml on day 10, coinciding well with the period when some larvae normally molt to the fifth instar (day 13), the other, a small peak of 14 ng/ml on day 14 and a large peak of 70 ng/ml on day 17 coinciding well with the period just before the prepupa stage. When fourth instar larvae were starved after 4 days of feeding, JH titers decreased sharply in the next 24h and never recovered, and a small but significant increase (to 21 ng/ml) in ecdysteroid titers was observed on day 6, followed by a large peak of 63 ng/ml on day 11. Altogether, these results suggest that starvation induces a rapid decline in the JH titer, and this cues the early occurrence of a small ecdysteroid peak that commits larvae to early metamorphosis.

The ecdysteroidogenic P450 Cyp302a1/disembodied from the silkworm, Bombyx mori, is transcriptionally regulated by prothoracicotropic hormone

During larval and pupal development of insects, ecdysone is synthesized in the prothoracic gland (PG). Although several Drosophila genes, including Halloween P450 genes, are known to be important for ecdysteroidogenesis in PG, little is known of the ecdysteroidogenic genes in other insects. Here we report on Cyp302a1/disembodied (dib-Bm), one of the Halloween P450s in the silkworm Bombyx mori that is a carbon-22 hydroxylase. dib-Bm is predominantly expressed in PG and its developmental expression profile is correlated with a change in the ecdysteroid titre in the haemolymph. Furthermore, dib-Bm expression in cultured PGs is significantly induced by treatment with prothoracicotropic hormone. This is the first report on the transcriptional induction of a steroidogenic gene by the tropic hormone in insects.

Myocyte enhancer factor 2 (MEF2) is a key modulator of the expression of the prothoracicotropic hormone gene in the silkworm, Bombyx mori

Prothoracicotropic hormone (PTTH) plays a central role in controlling molting, metamorphosis, and diapause termination in insects by stimulating the prothoracic glands to synthesize and release the molting hormone, ecdysone. Using Autographa californica nucleopolyhedrovirus (AcNPV)-mediated transient gene transfer into the central nervous sytem (CNS) of the silkworm, Bombyx mori, we identified two cis-regulatory elements that participate in the decision and the enhancement of PTTH gene expression in PTTH-producing neurosecretory cells (PTPCs). The cis-element mediating the enhancement of PTTH gene expression binds the transcription factor Bombyx myocyte enhancer factor 2 (BmMEF2). The BmMEF2 gene was expressed in various tissues including the CNS. In brain, the BmMEF2 gene was expressed at elevated levels in two types of lateral neurosecretory cells, namely PTPCs and corazonin-like immunoreactive lateral neurosecretory cells. Overexpression of BmMEF2 cDNA caused an increase in the transcription of PTTH. Therefore, BmMEF2 appears to be particularly important in the brain where it is responsible for the differentiation of lateral neurosecretory cells, including the enhancement of PTTH gene expression. This is the first report to identify a target gene of MEF2 in the invertebrate nervous system.

An in vitro study on regulation of prothoracic gland activity in the early last-larval instar of the silkworm Bombyx mori

The endocrine mechanisms that regulate prothoracic gland (PG) activity in early stages of final larval instar of the silkworm Bombyx mori were investigated using a newly developed long-term cultivation system of the gland. The PGs dissected from day-0 fifth instar larvae did not secrete detectable amounts of ecdysone for the first 24 h in culture but started secretion within the next 2 days. The amount of secreted ecdysone increased day by day. When day-0 PGs were co-cultivated with corpora allata, however, they remained inactive for at least 8 days. PGs dissected from 1-day younger larvae (day-3 fourth instar larvae) secreted ecdysone for the first 24 h but stopped secretion for the next 24 h, followed by recovery of ecdysone secretory activity. By contrast, PGs from day-1 fourth instar larvae remained active throughout a cultivation period without any sign of inactivation. However, when the same glands were exposed to a high titer of 20-hydroxyecdysone for the second 24h in culture, they gradually lost their activity. These results indicate that PGs of fourth instar larvae are inactivated by ecdysteroid through a negative feedback mechanism and that thus inactivated PGs spontaneously recover ecdysone secretory activity in the early fifth instar unless inhibited by juvenile hormone.

Endocrine changes associated with metamorphosis and diapause induction in the yellow-spotted longicorn beetle, Psacothea hilaris

At 25 degrees C and under a long-day photoperiod, all 5th instar Psacothea hilaris larvae pupate at the next molt. Under a short-day photoperiod, in contrast, they undergo one or two additional larval molts and enter diapause; the 7th instar larvae enter diapause without further molt. The changes in hemolymph juvenile hormone (JH III) titers, JH esterase activity, and ecdysteroid titers in pupation-destined, pre-diapause, and diapause-destined larvae were examined. JH titers of the 5th instar pupation-destined larvae decreased continuously from 1.3 ng/ml and became virtually undetectable on day 13, when JH esterase activity peaked. Ecdysteroids exhibited a small peak on day 8, 1 day before gut purge, and a large peak on day 11, 2 days before the larvae became pre-pupae. The two ecdysteroid peaks are suggested to be associated with pupal commitment and pupation, respectively. JH titers of the 5th instar pre-diapause larvae were maintained at approximately 1.5 ng/ml for 5 days and then increased to form a peak (3.3 ng/ml) on day 11. JH esterase activity remained at a low level throughout. Ecdysteroid levels exhibited a large peak of 40 ng/ml on day 18, coincident with the larval molt to the 6th instar. JH titers of the 7th instar diapause-destined larvae peaked at 1.9 ng/ml on day 3, and a level of approximately 1.1 ng/ml was maintained even 30-60 days into the instar, when they were in diapause. Ecdysteroid titers remained approximately 0.02 ng/ml. Diapause induction in this species was suggested to be a consequence of high JH and low ecdysteroid titers.

Relationship between firing activity of bombyxin-producing neurosecretory cells and hemolymph bombyxin titer in the silkworm Bombyx mori

Isolated brain-retrocerebral neurohemal complex of the silkworm of Bombyx mori was stimulated electrically and the released bombyxin (an insulin-like neuropeptide) was measured using time-resolved fluoroimmunoassay. The amount of bombyxin release depended on the number of stimulus pulses delivered to the axonal tract of the bombyxin-producing (BP) neurosecretory cells, and 17 fg of bombyxin per pulse was released from a cell. The titer of bombyxin in the hemolymph of bombyxin-II injected pupae decreased exponentially, the half-life being 170 min. To relate firing activity of a population of BP cells to the hormone titer in the hemolymph, bombyxin titer and its change in the hemolymph were calculated numerically. We assumed that the amount of bombyxin release was proportional to the firing rate of BP cells and the released bombyxin was inactivated with the same time course of injected bombyxin. Our calculations suggested that the hemolymph bombyxin titer may fluctuate dynamically and the mean titer is 380 pg/ml, a level which is close to the actually determined bombyxin titer at middle stages of pupal-adult development.

The cycling and distribution of PER-like antigen in relation to neurons recognized by the antisera to PTTH and EH in Thermobia domestica

The cephalic nervous system of the firebrat contains antigens recognized by antisera to the clock protein period (PER), the prothoracicotropic hormone (PTTH) and the eclosion hormone (EH). The content of the 115 kDa PER-like antigen visualized on the western blots fluctuates in diurnal rhythm with a maximum in the night. The oscillations entrained in a 12:12 h light/dark (LD) cycle persist in the darkness and disappear in continuous light. They are detected by immunostaining in 14 pairs of the protocerebral neurons and are extreme in four suboesophageal neurons and two cells in each corpus cardiacum that contain PER only during the night phase. No circadian fluctuations occur in three lightly stained perikarya of the optic lobe. Five cell bodies located in each brain hemisphere between the deuto-and the tritocerebrum retain weak immunoreactivity under constant illumination. In all cells, the staining is confined to the cytoplasm and never occurs in the cell nuclei. The cells containing PER-like material do not react with the anti-PTTH and anti-EH antisera, which recognize antigens of about 50 and 20 kDa, respectively. The anti-PTTH antiserum stains in each brain hemisphere seven neurons in the protocerebrum, eight in the optic lobe, and 3-5 in the posterior region of the deutocerebrum. The antiserum to EH reacts in each hemisphere with just two cells located medially to the mushroom bodies. No cycling of the PTTH-like and EH-like antigens was detected.