Spatiotemporal patterns of IGF-like peptide expression in the silkmoth Bombyx mori predict its pleiotropic actions

Dichotomous sperm in Lepidopteran insects: a biorational target for pest management

Lepidoptera are unusual in possessing two distinct kinds of sperm, regular nucleated (eupyrene) sperm and anucleate (apyrene) sperm ('parasperm'). Sperm of both types are transferred to the female and are required for male fertility. Apyrene sperm play 'helper' roles, assisting eupyrene sperm to gain access to unfertilized eggs and influencing the reproductive behavior of mated female moths. Sperm development and behavior are promising targets for environmentally safer, target-specific biorational control strategies in lepidopteran pest insects. Sperm dimorphism provides a wide window in which to manipulate sperm functionality and dynamics, thereby impairing the reproductive fitness of pest species. Opportunities to interfere with spermatozoa are available not only while sperm are still in the male (before copulation), but also in the female (after copulation, when sperm are still in the male-provided spermatophore, or during storage in the female's spermatheca). Biomolecular technologies like RNAi, miRNAs and CRISPR-Cas9 are promising strategies to achieve lepidopteran pest control by targeting genes directly or indirectly involved in dichotomous sperm production, function, or persistence.

Insulin-Like Peptides and Cross-Talk With Other Factors in the Regulation of Insect Metabolism

The insulin-like peptide (ILP) and insulin-like growth factor (IGF) signalling pathways play a crucial role in the regulation of metabolism, growth and development, fecundity, stress resistance, and lifespan. ILPs are encoded by multigene families that are expressed in nervous and non-nervous organs, including the midgut, salivary glands, and fat body, in a tissue- and stage-specific manner. Thus, more multidirectional and more complex control of insect metabolism can occur. ILPs are not the only factors that regulate metabolism. ILPs interact in many cross-talk interactions of different factors, for example, hormones (peptide and nonpeptide), neurotransmitters and growth factors. These interactions are observed at different levels, and three interactions appear to be the most prominent/significant: (1) coinfluence of ILPs and other factors on the same target cells, (2) influence of ILPs on synthesis/secretion of other factors regulating metabolism, and (3) regulation of activity of cells producing/secreting ILPs by various factors. For example, brain insulin-producing cells co-express sulfakinins (SKs), which are cholecystokinin-like peptides, another key regulator of metabolism, and express receptors for tachykinin-related peptides, the next peptide hormones involved in the control of metabolism. It was also shown that ILPs in Drosophila melanogaster can directly and indirectly regulate AKH. This review presents an overview of the regulatory role of insulin-like peptides in insect metabolism and how these factors interact with other players involved in its regulation.

Identification of Gonadulin and Insulin-Like Growth Factor From Migratory Locusts and Their Importance in Reproduction in Locusta migratoria

Many insect species have several genes coding for insulin-related peptides (IRPs), but so far only a single IRP gene has been identified in migratory locusts. Here, we report and characterize two other genes coding for peptides that are related to insulin, namely gonadulin and arthropod insulin-like growth factor (aIGF); peptides postulated to be orthologs of Drosophila melanogaster insulin-like peptides 8 and 6 respectively. In Locusta migratoria the aIGF transcript is expressed in multiple tissues as was previously reported for IRP in both L. migratoria and Schistocerca gregaria, but there are significant differences in expression patterns between the two species. The gonadulin transcript, however, seems specific to the ovary, whereas its putative receptor transcript is expressed most abundantly in the ovary, fat body and the central nervous system. Since the central nervous system-fat body-ovary axis is essential for successful reproduction, we studied the influence of gonadulin on vitellogenesis and oocyte growth. A reduction in the gonadulin transcript (via RNA interference) led to a significant reduction in vitellogenin mRNA levels in the fat body and a strong oocyte growth inhibition, thus suggesting an important role for gonadulin in reproduction in this species.

Signal transducer and activator of transcription is involved in the expression regulation of ecdysteroid-induced insulin-like growth factor-like peptide in the pupal wing disc of silkworm, Bombyx mori

In insects, 20-hydroxyecdysone (20E) and insulin-like growth factor-like peptides (IGFLPs) regulate the development of imaginal discs. However, how IGFLPs are up-regulated to impact the development of the pupal wing disc is still unclear. In this study, we investigated the expression regulation of IGFLP in the pupal wing disc of silkworm, Bombyx mori. We confirmed that B. mori IGFLP (BmIGFLP) was mainly expressed in the pupal wing disc and the expression of BmIGFLP could be significantly induced by 20E. Bioinformatics analysis of BmIGFLP promoter sequence revealed three cis-regulation elements (CREs) of signal transducer and activator of transcription (STAT), which is a key component in the Janus-activated kinase / STAT pathway. Luciferase activity assays showed that two CREs enhanced the transcriptional activity of BmIGFLP. Electrophoretic mobility shift and chromatin immunoprecipitation assays demonstrated that BmSTAT proteins in the nuclear extracts of B. mori pupal wing discs and BmN cells could only bind to the STAT CRE3, indicating that STAT CRE3 activated by BmSTAT enhances BmIGFLP expression at pupal stages. Although 20E could not enhance the expression of BmSTAT, 20E enhanced the nucleus translocation of BmSTAT to bind with the STAT CRE3 in the BmIGFLP promoter. The increase of transcriptional activity of the STAT CRE3 by overexpression of BmSTAT and addition of 20E in BmN cells confirmed this result. Taken together, all data indicate that BmSTAT is one of the transcription factors activating 20E-induced BmIGFLP expression in the pupal wing disc.

An insulin-like growth factor-like peptide promotes ovarian development in the silkmoth Bombyx mori

Insulin family peptides are known to be key regulators of growth and metabolism in insects and vertebrates. Insects have two types of insulin family peptides: insulin-like peptides and insulin-like growth factor (IGF)-like peptides (IGFLPs). We recently demonstrated that an IGFLP in the silkmoth, Bombyx mori (BIGFLP) promotes the growth of the genital imaginal disc ex vivo. However, the role of BIGFLP in the regulation of insect growth remains unclear because no in vivo study has been performed. Therefore, we analysed the functions of BIGFLP in vivo by constructing BIGFLP knock-out (KO) B. mori using the clustered regularly interspaced palindromic repeats (CRISPR) and CRISPR-associated protein 9 (CRISPR-Cas9) system. The KO moths exhibited decreased body weights and size of the appendages compared wild-type (wt) moths. Interestingly, KO females also had drastically lower ovary weights and number of eggs than wt females. However, mutant ovaries that were transplanted into wt host pupae reached a similar weight to wt ovaries that were transplanted into the wt hosts, suggesting that IGFLP in the haemolymph promotes ovarian development. These findings show that BIGFLP regulates the growth and development of adult organs, particularly the ovaries, in B. mori.

Bombyxin/Akt signaling in relation to the embryonic diapause process of the silkworm, Bombyx mori

Our previous study showed that phosphorylation of glycogen synthase kinase (GSK)-3β is related to the embryonic diapause process in Bombyx. However, the upstream signaling pathway was not clearly understood. In the present study, we examined bombyxin/Akt signaling in relation to the embryonic diapause process of B. mori. Results showed that GSK-3β phosphorylation stimulated by dechorionation was blocked by LY294002, a specific phosphatidylinositol 3-kinase (PI3K) inhibitor, indicating involvement of PI3K in GSK-3β phosphorylation in dechorionated eggs. Direct determination of Akt phosphorylation showed that dechorionation stimulated Akt phosphorylation. The Akt phosphorylation was blocked by LY294002. Temporal changes in Akt phosphorylation showed that different changing patterns exist between diapause and developing eggs. Relatively higher phosphorylation levels of Akt were detected between days 3 and 5 after oviposition in non-diapause eggs compared to those at the same stages in diapause eggs. Upon treatment with HCl, which prevents diapause initiation, Akt phosphorylation levels exhibited a later and much broader peak compared to diapause eggs. Examination of expression levels of the bombyxin-Z1 gene showed that in diapause eggs, a major peak occurred 1 day after oviposition, and its level then sharply decreased on day 2. However, in both non-diapause and HCl-treated eggs, a major broad peak was detected between days 1 and 4 after oviposition. These temporal changes in bombyxin-Z1 gene expression levels during embryonic stages coincided with changes in Akt phosphorylation, indicating that bombyxin-Z1 is likely an upstream signaling component for Akt phosphorylation. Taken together, our results indicated that PI3K/Akt is an upstream signaling pathway for GSK-3β phosphorylation and is associated with the diapause process of B. mori eggs. To our knowledge, this is the first study to demonstrate the potential correlation between bombyxin/Akt signaling and the embryonic diapause process.

Recent advances in neuropeptide signaling in Drosophila, from genes to physiology and behavior

This review focuses on neuropeptides and peptide hormones, the largest and most diverse class of neuroactive substances, known in Drosophila and other animals to play roles in almost all aspects of daily life, as w;1;ell as in developmental processes. We provide an update on novel neuropeptides and receptors identified in the last decade, and highlight progress in analysis of neuropeptide signaling in Drosophila. Especially exciting is the huge amount of work published on novel functions of neuropeptides and peptide hormones in Drosophila, largely due to the rapid developments of powerful genetic methods, imaging techniques and innovative assays. We critically discuss the roles of peptides in olfaction, taste, foraging, feeding, clock function/sleep, aggression, mating/reproduction, learning and other behaviors, as well as in regulation of development, growth, metabolic and water homeostasis, stress responses, fecundity, and lifespan. We furthermore provide novel information on neuropeptide distribution and organization of peptidergic systems, as well as the phylogenetic relations between Drosophila neuropeptides and those of other phyla, including mammals. As will be shown, neuropeptide signaling is phylogenetically ancient, and not only are the structures of the peptides, precursors and receptors conserved over evolution, but also many functions of neuropeptide signaling in physiology and behavior.

Hormonal control of growth in the wing imaginal disks of Junonia coenia: the relative contributions of insulin and ecdysone

The wing imaginal disks of Lepidoptera can be grown in tissue culture, but require both insulin and ecdysone to grow normally. Here we investigate the contributions the two hormones make to growth. Ecdysone is required to maintain mitoses, whereas in the presence of insulin alone mitoses stop. Both ecdysone and insulin stimulate protein synthesis, but only ecdysone stimulates DNA synthesis. Insulin stimulates primarily cytoplasmic growth and an increase in cell size, whereas ecdysone, by virtue of its stimulation of DNA synthesis and mitosis, stimulates growth by an increase in cell number. Although both hormones stimulate protein synthesis they do so in different spatial patterns. Both hormones stimulate protein synthesis in the inter-vein regions, but ecdysone stimulates synthesis more strongly in the veins and in the margin of the wing disk. We propose that the balance of insulin and ecdysone signaling must be regulated to maintain normal growth, and when growth appears to be due primarily to an increase in cell number, or an increase in cell size, this may indicate growth occurred under conditions that favored a stronger role for ecdysone, or insulin, respectively.

Insulin-like growth factor (IGF)-like peptide and 20-hydroxyecdysone regulate the growth and development of the male genital disk through different mechanisms in the silkmoth, Bombyx mori

It is well established that ecdysteroids play pivotal roles in the regulation of insect molting and metamorphosis. However, the mechanisms by which ecdysteroids regulate the growth and development of adult organs after pupation are poorly understood. Recently, we have identified insulin-like growth factor (IGF)-like peptides (IGFLPs), which are secreted after pupation under the control of 20-hydroxyecdysone (20E). In the silkmoth, Bombyx mori, massive amounts of Bombyx-IGFLP (BIGFLP) are present in the hemolymph during pupal-adult development, suggesting its importance in the regulation of adult tissue growth. Thus, we hypothesized that the growth and development of adult tissues including imaginal disks are regulated by the combined effects of BIGFLP and 20E. In this study, we investigated the growth-promoting effects of BIGFLP and 20E using the male genital disks of B. mori cultured ex vivo, and further analyzed the cell signaling pathways mediating hormone actions. We demonstrate that 20E induces the elongation of genital disks, that both hormones stimulate protein synthesis in an additive manner, and that BIGFLP and 20E exert their effects through the insulin/IGF signaling pathway and mitogen-activated protein kinase pathway, respectively. These results show that the growth and development of the genital disk are coordinately regulated by both BIGFLP and 20E.

Insulin/IGF signaling in Drosophila and other insects: factors that regulate production, release and post-release action of the insulin-like peptides

Insulin, insulin-like growth factors (IGFs) and insulin-like peptides (ILPs) are important regulators of metabolism, growth, reproduction and lifespan, and mechanisms of insulin/IGF signaling (IIS) have been well conserved over evolution. In insects, between one and 38 ILPs have been identified in each species. Relatively few insect species have been investigated in depth with respect to ILP functions, and therefore we focus mainly on the well-studied fruitfly Drosophila melanogaster. In Drosophila eight ILPs (DILP1-8), but only two receptors (dInR and Lgr3) are known. DILP2, 3 and 5 are produced by a set of neurosecretory cells (IPCs) in the brain and their biosynthesis and release are controlled by a number of mechanisms differing between larvae and adults. Adult IPCs display cell-autonomous sensing of circulating glucose, coupled to evolutionarily conserved mechanisms for DILP release. The glucose-mediated DILP secretion is modulated by neurotransmitters and neuropeptides, as well as by factors released from the intestine and adipocytes. Larval IPCs, however, are indirectly regulated by glucose-sensing endocrine cells producing adipokinetic hormone, or by circulating factors from the intestine and fat body. Furthermore, IIS is situated within a complex physiological regulatory network that also encompasses the lipophilic hormones, 20-hydroxyecdysone and juvenile hormone. After release from IPCs, the ILP action can be modulated by circulating proteins that act either as protective carriers (binding proteins), or competitive inhibitors. Some of these proteins appear to have additional functions that are independent of ILPs. Taken together, the signaling with multiple ILPs is under complex control, ensuring tightly regulated IIS in the organism.

Nutrition-dependent control of insect development by insulin-like peptides

In metazoans, members of the insulin-like peptide (ILP) family play a role in multiple physiological functions in response to the nutritional status. ILPs have been identified and characterized in a wide variety of insect species. Insect ILPs that are mainly produced by several pairs of medial neurosecretory cells in the brain circulate in the hemolymph and act systemically on target tissues. Physiological and biochemical studies in Lepidoptera and genetic studies in the fruit fly have greatly expanded our knowledge of the physiological functions of ILPs. Here, we outline the recent progress of the structural classification of insect ILPs and overview recent studies that have elucidated the physiological functions of insect ILPs involved in nutrient-dependent growth during development.

Insulin-like and IGF-like peptides in the silkmoth Bombyx mori: discovery, structure, secretion, and function

A quarter of a century has passed since bombyxin, the first insulin-like peptide identified in insects, was discovered in the silkmoth Bombyx mori. During these years, bombyxin has been studied for its structure, genes, distribution, hemolymph titers, secretion control, as well as physiological functions, thereby stimulating a wide range of studies on insulin-like peptides in other insects. Moreover, recent studies have identified a new class of insulin family peptides, IGF-like peptides, in B. mori and Drosophila melanogaster, broadening the base of the research area of the insulin-related peptides in insects. In this review, we describe the achievements of the studies on insulin-like and IGF-like peptides mainly in B. mori with short histories of their discovery. Our emphasis is that bombyxins, secreted by the brain neurosecretory cells, regulate nutrient-dependent growth and metabolism, whereas the IGF-like peptides, secreted by the fat body and other peripheral tissues, regulate stage-dependent growth of tissues.

Eat to reproduce: a key role for the insulin signaling pathway in adult insects

Insects, like all heterotrophic organisms, acquire from their food the nutrients that are essential for anabolic processes that lead to growth (larval stages) or reproduction (adult stage). In adult females, this nutritional input is processed and results in a very specific output, i.e., the production of fully developed eggs ready for fertilization and deposition. An important role in this input-output transition is attributed to the insulin signaling pathway (ISP). The ISP is considered to act as a sensor of the organism's nutritional status and to stimulate the progression of anabolic events when the status is positive. In several insect species belonging to different orders, the ISP has been demonstrated to positively control vitellogenesis and oocyte growth. Whether or not ISP acts herein via a mediator action of lipophilic insect hormones (ecdysteroids and juvenile hormone) remains debatable and might be differently controlled in different insect orders. Most likely, insulin-related peptides, ecdysteroids and juvenile hormone are involved in a complex regulatory network, in which they mutually influence each other and in which the insect's nutritional status is a crucial determinant of the network's output. The current review will present an overview of the regulatory role of the ISP in female insect reproduction and its interaction with other pathways involving nutrients, lipophilic hormones and neuropeptides.

Insulin-producing cells and their regulation in physiology and behavior ofDrosophila1This review is part of a virtual symposium on recent advances in understanding a variety of complex regulatory processes in insect physiology and endocrinology, including development, metabolism, cold hardiness, food intake and digestion, and diuresis, through the use of omics technologies in the postgenomic era

Insulin-like peptide signaling regulates development, growth, reproduction, metabolism, stress resistance, and life span in a wide spectrum of animals. Not only the peptides, but also their tyrosine kinase receptors and the downstream signaling pathways are conserved over evolution. This review summarizes roles of insulin-like peptides (DILPs) in physiology and behavior of Drosophila melanogaster Meigen, 1830. Seven DILPs (DILP1–7) and one receptor (dInR) have been identified in Drosophila. These DILPs display cell and stage specific expression patterns. In the adult, DILP2, 3, and 5 are expressed in insulin-producing cells (IPCs) among the median neurosecretory cells of the brain, DILP7 in 20 neurons of the abdominal ganglion, and DILP6 in the fat body. The DILPs of the IPCs regulate starvation resistance, responses to oxidative and temperature stress, and carbohydrate and lipid metabolism. Furthermore, the IPCs seem to regulate feeding, locomotor activity, sleep and ethanol sensitivity, but the mechanisms are not elucidated. Insulin also alters the sensitivity in the olfactory system that affects food search behavior, and regulates peptidergic neurons that control aspects of feeding behavior. Finally, the control of insulin production and release by humoral and neuronal factors is discussed. This includes a fat body derived factor and the neurotransmitters GABA, serotonin, octopamine, and two neuropeptides.